Article

The Grenvillian–Sveconorwegian orogeny in Fennoscandia: Back-thrusting and extensional shearing along the "Mylonite Zone"

September 2011
Precambrian Research 189(3-4):368-388

September 2011
189(3-4):368-388

DOI:10.1016/j.precamres.2011.06.005

Authors:

Giulio Viola

University of Bologna

Iain Henderson

Norges geologiske undersøkelse

Bernard Bingen

Norges geologiske undersøkelse

Bart WH Hendriks

Statoil ASA

New structural and geochronological investigations of the "Mylonite Zone" (MZ), an arcuate terrane boundary in southwest Scandinavia, contribute to a refined conceptual model for the Grenvillian-Sveconorwegian tectonic evolution of the Mesoproterozoic Sveconorwegian orogenic belt. During late convergence, around 970Ma, the MZ acted as a top-to-the-SE thrust that accommodated crustal shortening in the eastern part of the orogen by juxtaposing the Idefjorden Terrane in the hanging wall in the west against the "Eastern Segment" footwall in the east. The eastward vergence of the MZ and of similarly oriented second-order nearby shear zones is interpreted as reflecting late back-thrusting within the overall W-vergent orogeny. Back-thrusting was possibly promoted by the backstop role played by the rigid block formed by the 1810-1650Ma Transscandinavian Igneous Belt. During subsequent E-W crustal extension, the MZ thrust-related fabrics were reactivated in an extensional fashion with bulk top-to-the-W kinematics. This was triggered by gravitational instabilities resulting from crustal overthickening during the shortening phase. 40Ar-39Ar biotite and white mica ages from a greenschist-facies and extension-related mylonite range between 922 and 860Ma. This long-lived episode is expressed by extensional structures that evolved continuously from purely ductile to brittle during progressive exhumation of the footwall. The "Eastern Segment" is interpreted as an immature asymmetric core complex, exhumed in the footwall of the extensional MZ, through the antithetic normal displacement of the MZ itself and of the top-to-the-E Sveconorwegian Frontal Deformation Zone farther to the east. The core complex is bound to the north by the transtensional Hammarö Shear Zone, characterized by penetrative constrictional fabrics, interpreted as indicative of an overall transtensional regime.

Subduction and loss of continental crust during the Mesoproterozoic Sveconorwegian Orogeny

Article

Full-text available

May 2024
PRECAMBRIAN RES

The late Mesoproterozoic Sveconorwegian Orogeny in SW Fennoscandia is characterized by tectonically bound units that record different metamorphic, magmatic, and deformation histories, interpreted to indicate separation by some unknown distance prior to orogeny. New zircon U–Pb and Lu–Hf isotope data from a 1200 km-long NE–SW transect including Archean to 1450 Ma rocks constrain the likely age and isotopic architecture of western Fennoscandia prior to the late Mesoproterozoic Sveconorwegian Orogeny. Zircon age and Hf-isotope patterns indicate that the units comprising the Sveconorwegian Province are both younger and isotopically more juvenile than the surrounding autochthonous Fennoscandian crust, and thus most likely derived from at west of the present-day Norwegian coastline. The Mylonite Zone defines a major tectonic structure separating allochthonous Sveconorwegian units in its hanging wall from autochthonous Fennoscandian crust in its footwall. New and compiled metamorphic age data demonstrate that the Mylonite Zone can be traced westward through the Western Gneiss Region, aligning with Nordfjord in western Norway, where it was reused later during Caledonian deformation. The proposed westward continuation of the Mylonite Zone accommodated several hundred kilometers of sinistral strike-slip movement. Eastward translation of crust probably took place sometime between 1020 and 990 Ma, coinciding with a magmatic lull, followed by a shift to more evolved isotopic compositions in the hanging wall (Telemark) and high-pressure eclogite-facies metamorphism in the footwall (Eastern Segment) to the Mylonite Zone. Following this relatively short period of compression, the entire orogen and its foreland underwent extension lasting until at least 930 Ma. The nature and fate of the ca. 500 km of crust originally separating the autochthonous and allochthonous units remain elusive. There is no evidence of arc magmatism related to Benioff-style subduction of oceanic crust, and thus we propose an amagmatic Ampferer-style subduction comprising spontaneous subduction of thinned continental crust, as proposed for the Western Alps. Subduction of continental crust and associated radioactive heat-producing elements could also account for the anomalously high temperatures in the lithospheric mantle under the Sveconorwegian Province, which cannot easily be accounted for by other mechanisms. The Sveconorwegian Province may be an anomalous feature in an otherwise larger scale orogen.

40Ar/39Ar constraints on the tectonic evolution of the central part of the Mesoproterozoic Sveconorwegian orogen

Article

Dec 2022
J STRUCT GEOL

Structural data combined with ⁴⁰Ar/³⁹Ar geochronology of hornblende, muscovite, biotite and plagioclase from 37 localities along three transects allow the time-constrained reconstruction of the Sveconorwegian deformation and cooling history of the Kongsberg lithotectonic unit and of the boundary zone to the adjacent Telemark lithotectonic unit in southern Norway. The Kongsberg lithotectonic unit consists of pervasively deformed, steeply dipping amphibolite-to granulite-facies gneisses, in which Mesoproterozoic ⁴⁰Ar/³⁹Ar ages of c. 1090-1070 Ma record cooling and exhumation. Ages scattering around 1000 Ma are interpreted as resetting of the K/Ar systems related to sinistral strike-slip deformation along c. N–S trending greenschist-to amphibolite facies shear zones and associated large-scale folding. Later, Neoproterozoic top-to-the-E normal shearing accommodated by the newly discovered Prestfoss detachment selectively exploiting the Kongsberg-Telemark boundary zone caused the exhumation of amphibolite-facies rocks of the Telemark lithotectonic unit and their juxtaposition against amphibolite-facies gneisses of the Kongsberg lithotectonic unit, which had cooled earlier in the orogenic history. ⁴⁰Ar/³⁹Ar ages constrain shearing and associated cooling to between c. 940 Ma and c. 900 Ma. Finally, a Silurian (420 ± 11 Ma) ⁴⁰Ar/³⁹Ar age may reflect localized partial to complete resetting due to Caledonian tectonics, and a Permian (288 ± 1 Ma) ⁴⁰Ar/³⁹Ar age constrains the intrusion of an Oslo Rift granite.

Repeated brittle reactivations of a pre-existing plastic shear zone: combined K–Ar and 40 Ar– 39 Ar geochronology of the long-lived (>700 Ma) Himdalen–Ørje Deformation Zone, SE Norway

Article

Full-text available

Oct 2022

Brittle reactivation of plastic shear zones is frequently observed in geologically old terranes. To better understand such deformation zones, we have studied the >700 Ma long structural history of the Himdalen–Ørje Deformation Zone (HØDZ) in SE Norway by K–Ar and ⁴⁰ Ar– ³⁹ Ar geochronology, and structural characterization. Several generations of mylonites make up the ductile part of HØDZ, the Ørje Shear Zone. A ⁴⁰ Ar– ³⁹ Ar white mica plateau age of 908.6 ± 7.0 Ma constrains the timing of extensional reactivation of the Ørje mylonite. The mylonite is extensively reworked during brittle deformation events by the Himdalen Fault. ⁴⁰ Ar– ³⁹ Ar plateau ages of 375.0 ± 22.7 Ma and 351.7 ± 4.4 Ma from pseudotachylite veins and K–Ar ages of authigenic illite in fault gouge at c. 380 Ma are interpreted to date initial brittle deformation, possibly associated with the Variscan orogeny. Major brittle deformation during the Early–Mid Permian Oslo Rift is documented by a ⁴⁰ Ar– ³⁹ Ar pseudotachylite plateau age of 294.6 ± 5.2 Ma and a K–Ar fault gouge age of c. 270 Ma. The last datable faulting event is constrained by the finest size fraction in three separate gouges at c. 200 Ma. The study demonstrates that multiple geologically significant K–Ar ages can be constrained from fault gouges within the same fault core by combining careful field sampling, structural characterization, detailed mineralogy and illite crystallinity analysis. We suggest that initial localization of brittle strain along plastic shear zones is controlled by mechanical anisotropy of parallel-oriented, throughgoing phyllosilicate-rich foliation planes within the mylonitic fabric.

Anorthosite formation and emplacement coupled with differential tectonic exhumation of ultrahigh-temperature rocks in a Sveconorwegian continental back-arc setting

Article

Full-text available

May 2022
PRECAMBRIAN RES

The tectonic setting and mechanisms and duration of emplacement of Proterozoic massif-type anorthosites and the significance of typically associated ultrahigh-temperature (UHT) host rocks have been debated for decades. This is particularly true of the Rogaland Anorthosite Province (RAP) in the SW Sveconorwegian Orogen. Earlier studies suggest that the RAP was emplaced over 1–3 Myr around 930 Ma towards the end of orogenesis, resulting in an up to 15–20 km-wide contact metamorphic aureole. However, our structural observations show that the RAP is located in the footwall of a 15 km-wide extensional detachment (Rogaland Extensional Detachment, RED), separating the intrusions and their UHT host rocks from weakly metamorphosed rocks in the hanging wall. U–Pb zircon dating of leucosome in extensional pull-aparts associated with the RED yields ages of 950–935 Ma, consistent with Re–Os molybdenite ages from brittle extensional structures in the hanging-wall block that range between 980 and 930 Ma. A metapelite in the immediate vicinity of the RAP yields a 950 Ma U–Pb age of matrix-hosted monazite, and part of the RAP was intruded by the Storgangen norite dike at ca. 950 Ma, providing a minimum age of emplacement. These ages are consistent with Ar–Ar hornblende and biotite ages that show rapid cooling of the footwall before 930 Ma, but slow cooling of the hanging wall. Field and geochronologic data suggest that the RAP formed and was emplaced over a long period of time, up to 100 Myr, with different emplacement mechanisms reflecting an evolving regional stress regime. The distribution of UHT rocks around the RAP reflects differential extensional exhumation between 980 and 930 Ma, not contact metamorphism. The duration and style of orogenic activity and externally (as opposed to gravitationally) driven extension suggest that the RAP formed in a continental back-arc setting.

The Sveconorwegian orogeny

Article

Nov 2020
GONDWANA RES

This article reviews the geology of the Sveconorwegian orogen in south Scandinavia and existing tectonic models for the Mesoproterozoic to Neoproterozoic Sveconorwegian orogeny. It proposes an updated geodynamic scenario of large, hot, long-duration continental collision starting at c. 1065 Ma between proto-Baltica and another plate, presumably Amazonia, in a Rodinia-forming context. An orogenic plateau formed at 1280 Ma as a back-arc Cordillera-style plateau, and then grew further stepwise after 1065 Ma, as a collisional Tibetan-style plateau. Voluminous mantle- and crustal-derived Sveconorwegian magmatism took place in the hinterland in the west of the orogen, mainly: (i) bimodal magmatism at 1280–1145 Ma, overlapping with extensional intramontane basin sedimentation, (ii) the calc-alkaline Sirdal magmatic belt at 1065–1020 Ma, (iii) the hydrous ferroan hornblende-biotite granite (HBG) suite at 985–925 Ma and (iv) the anhydrous ferroan massif-type anorthosite-mangerite-charnockite (AMC) suite at 935–915 Ma. High-alumina orthopyroxene megacrysts in anorthosite imply mafic underplating at 1040 Ma and remelting of the underplates at 930 Ma. Overlapping with magmatism, protracted low-pressure, granulite-facies metamorphism reached twice ultra-high temperature conditions, of 0.6 GPa-920 °C at 1030–1005 Ma and 0.4 GPa-920 °C at 930 Ma. These features imply shallow asthenosphere under the crust. Towards the foreland in the east, metamorphism shows increasing high-pressure signature eastwards with time, with peak P-T values of 1.15 GPa-850 °C at 1150–1120 Ma in the Bamble-Kongsberg lithotectonic units, 1.5 GPa-740 °C at c. 1050 Ma in the Idefjorden lithotectonic unit, and 1.8 GPa-870 °C at c. 990 Ma in the Eastern Segment under eclogite-facies conditions. These are attributed to retreating delamination of the dense sub-continental lithospheric mantle and growth of the orogenic plateau towards the foreland. After c. 930 Ma, convergence came to a halt, the orogenic plateau collapsed, and 16 km of overburden was removed by extension and erosion.

Magma-driven, high-grade metamorphism in the Sveconorwegian Province, southwest Norway, during the terminal stages of Fennoscandian Shield evolution

Article

Full-text available

Feb 2018
GEOSPHERE

Recently it has been argued that the Sveconorwegian orogeny in southwest Fennoscandia comprised a series of accretionary events between 1140 and 920 Ma, behind a long-lived, active continental margin characterized by voluminous magmatism and high-grade metamorphism. Voluminous magnesian granitic magmatism is recorded between 1070 and 1010 Ma (Sirdal Magmatic Belt, SMB), with an apparent drop in activity ca. 1010-1000 Ma. Granitic magmatism resumed ca. 1000-990 Ma, but with more ferroan (A type) compositions (hornblende-biotite granites). This ferroan granitic magmatism was continuous until 920 Ma, and included emplacement of an AMCG (anorthosite-mangerite-charnockite-granite) complex (Rogaland Igneous Complex). Mafic rocks with ages corresponding to the spatially associated granites suggest that heat from underplated mafic magma was the main driving force for lower crustal melting and long-lived granitic magmatism. The change from magnesian to ferroan compositions may reflect an increasingly depleted and dehydrated lower crustal source. High-grade metamorphic rocks more than ~20 km away from the Rogaland Igneous Complex yield metamorphic ages of 1070-1015 Ma, corresponding to SMB magmatism, whereas similar rocks closer to the Rogaland Igneous Complex yield ages between 1100 and 920 Ma, with an apparent age peak ca. 1000 Ma. Ti-in-zircon temperatures from these rocks increase from ~760 to 820 °C ca. 970 Ma, well before the inferred emplacement age of the Rogaland Igneous Complex (930 Ma), suggesting that long-lived, high-grade metamorphism was not directly linked to the emplacement of the latter, but rather to the same mafic underplating that was driving lower crustal melting. Structural data suggest that the present-day regional distribution of high- and low-grade rocks reflects late-stage orogenic doming.

Polyphasal foreland-vergent deformation in a deep section of the 1Ga Sveconorwegian orogen

Article

May 2015
PRECAMBRIAN RES

Metamorphic belts in Precambrian shields expose deep interiors of orogens and are often challenging to interpret in tectonic terms. The Eastern Segment of the 1.1-0.9 Ga Sveconorwegian orogen represents a metamorphic belt, which was metamorphosed at high-pressure granulite and upper amphibolite facies at 35-40 km depth and shows highly complex fold patterns. We use detailed structural analysis in combination with U-Pb SIMS dating of complex zircon to identify the structural and tectonic evolution in a composite migmatitic orthogneiss complex of the Eastern Segment. We link four fold phases to late-orogenic foreland-vergent flow, and date D2-D3 at 0.97-0.95 Ga. Leucosome and mesosome of felsic metasupracrustal migmatitic gneiss contain igneous zircon that dates the crystallization of the source rock or protolith at 1695 ± 8 Ma, and 1690 ± 8 Ma, respectively, demonstrating a temporal link to unmetamorphosed or little metamorphosed igneous rocks east of the Sveconorwegian orogen (the Transscandinavian Igneous Belt). Early migmatization attributed to Hallandian orogenesis is dated by formation of secondary zircon in two leucosome samples at 1402 ± 12 and 1386 ± 7. The pre-Sveconorwegian structure (Sc), which is strongly overprinted by Sveconorwegian deformation and migmatization is a composite coarse gneissic layering made up of a primary compositional layering and (variably present) Hallandian leucosome veins. The dominant foliation, a pervasive gneissic banding (S1), is axial planar to intrafolial F1 folds and developed as a result of tectonic overprint of Sc; S1 is associated with a strong ESE-trending aggregate stretching lineation (L1). S1 and L1 were folded by asymmetric SE-vergent F2 folds during foreland-vergent flow. Crystallisation of Sveconorwegian zircon in syn-F2 leucosome dates this phase at 970 ± 5 Ma. The sequence was subsequently deformed by symmetric and asymmetric F3 folds that are S- to SE-vergent. Syn-F3 leucosome, mineral parageneses and microtextures associated with D3 show that this deformation occurred under still high temperatures. The last ductile phase (D4) also involved the generation of leucosome synkinematic with N-S trending folds that deformed all previous structures under amphibolite facies conditions. K-feldspar-rich, originally coarse-grained and strongly deformed metapegmatite contain two generations of zircon: texturally old 1414 ± 5 Ma cores and fragments, and voluminous Sveconorwegian envelopes, and new grains that demonstrate the presence of melt as late as 958 ± 7 Ma. Ductile structures are similar in metasupracrustal and metaplutonic orthogneiss complexes. Likely, these units were tectonically juxtaposed during D1, while D2-D4 structures reflect a common tectonic evolution after their emplacement. We interpret D1-4 structures as recording WNW-ESE convergence (D1) and ESE-vergent flow (D2, D3), followed by E-W gentle upright folding (D4). Sveconorwegian 0.98-0.96 Ga foreland-vergent deformation, accompanied by migmatization at all four stages, was responsible for formation of the polyphasal deformation pattern in this part of the orogen.

Genesis and evolution of brittle structures in southwestern Finland and western South Africa - insights into fault reactivation, fluid flow and structural maturity in Precambrian cratons

Thesis

Full-text available

Apr 2015

Jussi Mattila

The bedrock of old crystalline cratons is characteristically saturated with brittle structures formed during successive superimposed episodes of deformation and under varying stress regimes. As a result, the crust effectively deforms through the reactivation of pre-existing structures rather than by through the activation, or generation, of new ones, and is said to be in a state of 'structural maturity'. By combining data from Olkiluoto Island, southwestern Finland, which has been investigated as the potential site of a deep geological repository for high-level nuclear waste, with observations from southern Sweden, it can be concluded that the southern part of the Svecofennian shield had already attained structural maturity during the Mesoproterozoic era. This indicates that the phase of activation of the crust,i.e. the time interval during which new fractures were generated, was brief in comparison to the subsequent reactivation phase. Structural maturity of the bedrock was also attained relatively rapidly in Namaqualand, western South Africa, after the formation of first brittle structures during Neoproterozoic time. Subsequent brittle deformation in Namaqualand was controlled by the reactivation of pre-existing strike-slip faults.In such settings, seismic events are likely to occur through reactivation of pre-existing zones that are favourably oriented with respect to prevailing stresses. In Namaqualand, this is shown for present day seismicity by slip tendency analysis, and at Olkiluoto, for a Neoproterozoic earthquake reactivating a Mesoproterozoic fault. By combining detailed field observations with the results of paleostress inversions and relative and absolute time constraints, seven distinct superimposed paleostress regimes have been recognized in the Olkiluoto region. From oldest to youngest these are: (1) NW-SE to NNW-SSE transpression, which prevailed soon after 1.75 Ga, when the crust had sufficiently cooled down to allow brittle deformation to occur. During this phase conjugate NNW-SSE and NE-SW striking strike-slip faults were active simultaneous with reactivation of SE-dipping low-angle shear zones and foliation planes. This was followed by (2) N-S to NE-SW transpression, which caused partial reactivation of structures formed in the first event; (3) NW-SE extension during the Gothian orogeny and at the time of rapakivi magmatism and intrusion of diabase dikes; (4) NE-SW transtension that occurred between 1.60 and 1.30 Ga and which also formed the NW-SE-trending Satakunta graben located some 20 km north of Olkiluoto. Greisen-type veins also formed during this phase. (5) NE-SW compression that postdates both the formation of the 1.56 Ga rapakivi granites and 1.27 Ga olivine diabases of the region; (6) E-W transpression during the early stages of the Mesoproterozoic Sveconorwegian orogeny and which also predated (7) almost coaxial E-W extension attributed to the collapse of the Sveconorwegian orogeny. The kinematic analysis of fracture systems in crystalline bedrock also provides a robust framework for evaluating fluid-rock interaction in the brittle regime; this is essential in assessment of bedrock integrity for numerous geo-engineering applications, including groundwater management, transient or permanent CO2 storage and site investigations for permanent waste disposal. Investigations at Olkiluoto revealed that fluid flow along fractures is coupled with low normal tractions due to in-situ stresses and thus deviates from the generally accepted critically stressed fracture concept, where fluid flow is concentrated on fractures on the verge of failure. The difference is linked to the shallow conditions of Olkiluoto - due to the low differential stresses inherent at shallow depths, fracture activation and fluid flow is controlled by dilation due to low normal tractions. At deeper settings, however, fluid flow is controlled by fracture criticality caused by large differential stress, which drives shear deformation instead of dilation. Keywords: brittle deformation, structural geology, structural maturity, fluid flow, seismicity, fault zones, paleostress analysis, slip tendency, dilation tendency, Olkiluoto, Namaqualand, Finland, South Africa

Mesoproterozoic continental growth: U–Pb–Hf–O zircon record in the Idefjorden Terrane, Sveconorwegian Orogen

Article

Full-text available

May 2015
PRECAMBRIAN RES

The Idefjorden Terrane of the Sveconorwegian Orogen, Fennoscandia, is known to be an area of comparatively juvenile Mesoproterozoic continental growth. Here we provide an improved model of crustal growth based on new coupled zircon U–Pb–O–Lu–Hf isotopic data on thirteen samples of mafic to intermediate plutonic rocks from different domains of the Idefjorden Terrane. The new data support a retreating volcanic arc system, with shorter pulses of accretion. A gradual increase of radiogenic Hf (mean ɛHf from 3.5 to 5.4) in plutonic rocks intruded between ca. 1630 Ma and 1560 Ma reflects an increase in juvenile mantle-derived magma in the genesis of the plutonic suites. This trend is consistent with development of an extensional back-arc rift geotectonic setting, accommodating deposition of the Stora Le-Marstrand greywacke dominated metasediment sequence. Combined isotopic information and the detrital zircon record of the Stora Le-Marstrand Formation support the interpretation that the Idefjorden Terrane was separated from the Fennoscandian Shield before the Sveconorwegian Orogeny.

Exhumation of an eclogite terrane as a hot migmatitic nappe, Sveconorwegian orogen

Article

Full-text available

Dec 2014
LITHOS

We demonstrate a case of eclogite exhumation in a partially molten, low-viscosity fold nappe within high-grade metamorphosed crust in the Eastern Segment of the Sveconorwegian orogen. The nappe formed during tectonic extrusion, melt-weakening assisted exhumation and foreland-directed translation of eclogitized crust, and stalled at 35–40 km depth within the collisional belt. The eclogites are structurally restricted to a regional recumbent fold in which stromatic orthogneiss with pods of amphibolitized eclogite make up the core. High-temperature mylonitic gneiss with remnants of kyanite eclogite (P > 15 kbar) compose a basal shear zone 50 km long and < 4 km wide. Heterogeneously sheared and partly migmatized augen gneiss forms a tectonostratigraphic marker in front of and beneath the nappe, and is in turn structurally enveloped by a composite sequence of orthogneisses and metabasites. The entire tectonostratigraphic pile underwent near-pervasive deformation and recrystallization under high-pressure granulite and upper-amphibolite conditions. U-Pb SIMS metamorphic zircon ages of eclogite and stromatic orthogneiss constrain the time of eclogitization at 988 ± 6 Ma and 978 ± 7 Ma. Migmatization, concomitant deformation, and exhumation are dated at 976 ± 6 Ma, and crystallization of post-kinematic melt at 956 ± 7 Ma. Orthogneiss protoliths are dated at 1733 ± 11 and 1677 ± 10 Ma (stromatic gneiss) and 1388 ± 7 Ma (augen gneiss in footwall), demonstrating origins indigenous to the Eastern Segment. Eclogitization and exhumation were coeval with the Rigolet phase of the Grenvillian orogeny, reflecting the late stage of continental collision during construction of the supercontinent Rodinia.

Mesoproterozoic Strike-Slip Faulting within the Åland Rapakivi Batholith, Southwestern Finland

Article

Full-text available

Feb 2024

Paleostress inversion analysis of outcrop data from brittle fault structures within the Mesoproterozoic 1.58 Ga Åland rapakivi granite, southwestern Finland, revealed two separate strike-slip faulting stages. Stage 1 is dominated by dextral slip along E–W-trending faults under WNW–ESE to NNW–SSE compression, whereas Stage 2 displays less prominent faulting localized in an orthogonal network of N–S and E–W trending faults that developed under NE–SW compression. Relative age constraints indicate that faulting occurred between 1.58 and 0.5 Ga, and further correlation with previously published results indicate a 1.55–1.4 Ga age for Stage 1 faulting, while Stage 2 is compatible with previously described fault reactivations between 1.3–1.2 Ga. To place the results of the fault analyses in a wider framework, we conducted a regional structural interpretation using bathymetric, topographic, and geophysical datasets and reviewed previously published results. Based on the above, we attribute the emplacement of the 1.6–1.5 Ga rapakivi granites and the subsequent development of the Mesoproterozoic sedimentary basins to the reactivation of inherited Paleoproterozoic shear zones during Mesoproterozoic crustal extension. As such, this study contributes towards understanding the relationships between magmatism and strain localisation in continental (failed) rift settings.

The Central Indian Tectonic Zone: A Rodinia supercontinent-forming collisional zone and analogy with the Grenville and Sveconorwegian orogens

Article

Aug 2023
GEOSPHERE

In the paleogeographic reconstructions of the Rodinia supercontinent, the circum-global 1.1–0.9 Ga collisional belt is speculated to skirt the SE coast of India, incorporating the Rodinian-age Eastern Ghats Province. But the Eastern Ghats Province may not have welded with the Indian landmass until 550–500 Ma. Instead, the ~1500-km-long, E-striking Central Indian Tectonic Zone provides an alternate option for linking the 1.1–0.9 Ga circum-global collisional belt through India. The highly tectonized Central Indian Tectonic Zone formed due to the early Neoproterozoic collision of the North India and the South India blocks. Based on a summary of the recent findings in the different crustal domains within the Central Indian Tectonic Zone, we demonstrate that the 1.03–0.93 Ga collision involved thrusting that resulted in the emplacement of low-grade metamorphosed allochthonous units above the high-grade basement rocks; the development of crustal-scale, steeply dipping, orogen-parallel transpressional shear zones; syn-collisional felsic magmatism; and the degeneration of orogenesis by extensional exhumation. The features are analogous to those reported in the broadly coeval Grenville and Sveconorwegian orogens. We suggest that the 1.1–0.9 Ga circum-global collisional belt in Rodinia swings westward from the Australo-Antarctic landmass and passes centrally through the Greater India landmass, which for the most part welded at 1.0–0.9 Ga. It follows that the paleogeographic positions of India obtained from paleomagnetic data older than 1.1–0.9 Ga are likely to correspond to the positions of the North and South India blocks, respectively, and not to the Greater India landmass in its entirety.

Zircon U–Pb-Hf isotope data in eclogite and metagabbro from southern Sweden reveal a common long-lived evolution and enriched source

Article

Full-text available

Sep 2020

Several orogenies have shaped the bedrock of southern Sweden. While mafic intrusions represent significant sources of information for reconstructing geodynamics and crustal evolution, the characterization of the various generations of such intrusions in Sweden remains limited. We report in situ zircon U–Pb ages and Hf isotope data from a Fe-Ti eclogite and a coronitic metagabbro from the Eastern Segment in southern Sweden. Crystallisation ages at 1683 ± 17 Ma of the eclogite suggest affiliation with the surrounding 1730–1660 Ma Transscandinavian Igneous Belt intrusions that dominate the Eastern Segment. Secondary zircon growth and Pb-loss in the eclogite sample at 1459 ± 44 Ma and the crystallisation of the metagabbro at 1431 ± 26 Ma overlap and are related to magmatic activity during the Hallandian orogeny. Zircon Hf isotope signatures with chondritic and sub-chondritic values at ~1683 Ma and ~1431 Ma, respectively, correspond to an enriched (or mildly depleted) source in line with a “Mixed Svecofennian Crustal Reservoir”. These isotope signatures are more enriched than those in the surrounding gneisses. Zircon isotope data from the herein analysed zircon grains indicate that the eclogite and metagabbro had an enriched mafic source in the mid to lower crust, or within the subcontinental lithospheric mantle below Fennoscandia.

A Tonian age for the Visingsö Group in Sweden constrained by detrital zircon dating and biochronology: Implications for evolutionary events

Article

Full-text available

Mar 2017
GEOL MAG

Detrital zircon U–Pb ages from samples of the Neoproterozoic Visingsö Group, Sweden, yield a maximum depositional age of ≤ 886±9 Ma (2σ). A minimum depositional age is established biochronologically using organic-walled and vase-shaped microfossils present in the upper formation of the Visingsö Group; the upper formation correlates with the Kwagunt Formation of the 780–740 Ma Chuar Group in Arizona, USA, and the lower Mount Harper Group, Yukon, Canada, that is older than 740 Ma. Mineralized scale microfossils of the type recorded from the upper Fifteenmile Group, Yukon, Canada, where they occur in a narrow stratigraphic range and are younger than 788 Ma, are recognized for the first time outside Laurentia. The mineralized scale microfossils in the upper formation of the Visingsö Group seem to have a wider stratigraphic range, and are older than c . 740 Ma. The inferred age range of mineralized scale microfossils is 788–740 Ma. This time interval coincides with the vase-shaped microfossil range because both microfossil groups co-occur. The combined isotopic and biochronologic ages constrain the Visingsö Group to between ≤ 886 and 740 Ma, thus Tonian in age. This is the first robust age determination for the Visingsö Group, which preserves a rich microfossil assemblage of worldwide distribution. The organic and mineralized microorganisms preserved in the Visingsö Group and coeval successions elsewhere document global evolutionary events of auto- and heterotrophic protist radiations that are crucial to the reconstruction of eukaryotic phylogeny based on the fossil record and are useful for the Neoproterozoic chronostratigraphic subdivision.

Linking orogenesis across a supercontinent; the Grenvillian and Sveconorwegian margins on Rodinia

Article

Full-text available

Apr 2017
GONDWANA RES

The Sveconorwegian orogeny in SW Baltica comprised a series of geographically and tectonically discrete events between 1140 and 920 Ma. Thrusting and high-grade metamorphism at 1140–1080 Ma in central parts of the orogen were followed by arc magmatism and ultra-high-temperature metamorphism at 1060–920 Ma in the westernmost part of the orogen. In the eastern part of the orogen, crustal thickening and high-pressure metamorphism took place at 1050 in one terrane and at 980 Ma in another. These discrete tectonothermal events are incompatible with an evolution resulting from collision with another major, continental landmass, and better explained as accretion and re-amalgamation of fragmented and attenuated crustal blocks of the SW Baltica margin behind an evolving continental-margin arc. In contrast, the coeval, along-strike Grenvillian orogeny is typically ascribed to long-lived collision with Amazonia. Here we argue that coeval, but tectonically different events in the Sveconorwegian and Grenville orogens may be linked through the behavior of the Amazonia plate. Subduction of Amazonian oceanic crust, and consequent slab pull, beneath the Sveconorwegian may have driven long-lived collision in the Grenville. Conversely, the development of a major orogenic plateau in the Grenville may have slowed convergence, thereby affecting the rate of oceanic subduction and thus orogenic evolution in the Sveconorwegian. Convergence ceased in the Grenville at ca. 980 Ma, in contrast to the Sveconorwegian where convergence continued until ca. 920 Ma, and must have been accommodated elsewhere along the Grenville–Amazonia segment of the margin, for example in the Goiás Magmatic Arc which had been established along the eastern Amazonian margin by 930 Ma. Our model shows how contrasting but coeval orogenic behavior can be linked through geodynamic coupling along and across tectonic plates.

P–T evolution and high-temperature deformation of Precambrian eclogite, Sveconorwegian orogen

Thesis

Full-text available

Mar 2016

Lorraine Tual

The 1.1-0.9 Ga Sveconorwegian orogen is one of several Grenvillian-aged orogenic belts that mark the amalgamation of supercontinent Rodinia. The highest-pressure rocks in the Sveconorwegian orogen are eclogites in the Eastern Segment (SW Sweden). The eclogites occur in a nappe in the high-grade metamorphic level of the Eastern Segment that represents a window into the deepest part of this Precambrian mountain belt. The aim of this thesis is to reconstruct the metamorphic history of the eclogite-bearing nappe by characterizating the deformation associated with exhumation (Paper I) and by reconstructing the P–T evolution (pressure and temperature; papers II and III). Paper I focuses on the deformation structures in the basal shear zone of the eclogite-bearing nappe. These structures developed during exhumation at high-temperature conditions. Top-to-the-east shear and east-directed flow produced intense folding, interpreted as formed by a combination of simple and pure shear. The interplay of shearing, folding, and melt localization lead to localized shear, high-temperature brittle fracturing, and the formation of high-temperature chevron folds in high-strain zones. Paper II retraces the metamorphic evolution of the eclogite-bearing nappe by thermodynamic modelling (THERMOCALC©) and construction of P–T pseudosections for two different types of eclogite. One of the samples gave information on both the prograde and the retrograde paths, and an estimate of peak metamorphic conditions of 850–900 °C and ~18 kbar. The first stage of the prograde path, representing a medium P/T gradient, is recorded in the core of garnet grains. The second part of the prograde path and the retrograde path are both steep. The chemical growth zoning of garnet is preserved which, together with the shape of the P–T path, reflects short residence time at high temperatures. Paper III reports the results of two independent trace element thermometers, which are based on the Zr-contents in rutile and Ti-contents in quartz. The combination of these two methods confirmed the P–T evolution calculated in Paper II. In particular, Ti-in-quartz thermometry are in agreement with the pseudosection estimates at high temperatures, and the minerals appear unaffected by diffusional resetting. A pseudosection model, showing the changes in modal abundance of different phases along the P–T path, demonstrates that rutile grains in the matrix recrystallized from smaller-sized rutile grains, and that this process was simultaneous with the main dehydration reaction in the rock (continuous breakdown of hornblende and formation of clinopyroxene). This study illustrates that Zr-in-rutile and Ti-in-quartz thermometry cannot only robustly constrain a prograde evolution, but when combined with a pseudosection model can also yield information on recrystallization processes. In fact, the combination of these methods provides an unrivalled tool for petrologic interpretation. The data presented in this thesis testifies to westward tectonic burial of continental crust at ~65 km depth and 890 °C at a late stage of the Sveconorwegian orogenesis. The following foreland-directed tectonic exhumation of the eclogite-bearing nappe was associated with partial melting, ductile flow folding and shearing. The character of both prograde and retrograde P–T paths suggests rapid tectonic burial and exhumation consistent with collision at the end of the Sveconorwegian orogeny.

The structural, metamorphic and magmatic evolution of Mesoproterozoic orogens

Article

Aug 2015
PRECAMBRIAN RES

The Mesoproterozoic (1600–1000 Ma) is an Era of Earth history that has been defined in the literature as being quiescent in terms of both tectonics and the evolution of the biosphere and atmosphere (Holland, 2006, Piper, 2013b and Young, 2013). The ‘boring billion’ is an informal term that is given to a time period overlapping the Mesoproterozoic period, extending from 1.85 to 0.85 Ga (Holland, 2006). Orogenesis was not absent from this period however, with various continents featuring active accretionary orogenesis along their margins for the entire Mesoproterozoic (see Condie, 2013 and Roberts, 2013), and others featuring major continental collisional orogenesis that relates to the formation of the supercontinent Rodinia towards the end of the Mesoproterozoic. Looking at it another way, this period followed the formation of perhaps the first long-lived supercontinent, Columbia (a.k.a. Nuna), and then it prepared the ground for the momentous geological and biological events in the Neoproterozoic that paved the way for the Cambrian explosion of life. As such it is a very important period of Earth history to understand better. Do orogens formed in the Mesoproterozoic differ from those formed in the recent past, or those formed in early Earth history, and if so in what way? Do orogens in the Mesoproterozoic have distinct structural, metamorphic or magmatic characteristics? How are Mesoproterozoic orogens related geodynamically and kinematically? These are overarching questions that this collection of sixteen research papers aims to address. This introduction presents a brief discussion of the contribution of these papers to these questions and topics.

High-Temperature Deformation in the Basal Shear Zone of an Eclogite-Bearing Fold Nappe, Sveconorwegian Orogen, Sweden

Article

May 2015
PRECAMBRIAN RES

Ductile shear zones associated with emplacement of high-pressure nappes are key features to resolve exhumation mechanisms. The Eastern Segment of the Sveconorwegian orogen hosts an eclogite-bearing fold nappe, whose basal shear zone shows structures, associated with emplacement of the eclogite-bearing nappe and decompression under high to intermediate pressure granulite and upper amphibolite facies conditions. Based on detailed structural mapping of a 4 km well-exposed section of the basal shear zone, we describe two major phases of deformation. An early deformation stage (D1) formed a penetrative gneissic foliation and tectonic layering, including isoclinal folds (F1). The sequence was subsequently affected by up to km-scale tight south-vergent folds (F2) with sheared out limbs. At the outcrop scale, asymmetric F2 folds are commonly S-vergent, but symmetric folds with different degrees of tightness are also present. Melt was present at all stages of deformation and the structural relations demonstrate mutual feedback between melt localization and fold formation. F2 folds have shallowly E-plunging fold axis parallel to a stretching lineation defined by high-grade mineral aggregates. Both constitute prominent structures of the basal shear zone. F2-folds are associated with an axial planar fabric (S2), defined by upper-amphibolite- and locally granulite-facies mineral assemblages. D2 shear structures are associated with top-to-the-east kinematic indicators throughout the section. The D1 episode was responsible for emplacement of the eclogite-nappe into its present structural position. The subsequently developed lineation-parallel folds are interpreted to form by general shear, where the structures reflect the regional E-directed flow of the entire eclogite-nappe.

Multiple reactivation and strain localization along a Proterozoic orogen-scale deformation zone: The Kongsberg-Telemark boundary in southern Norway revisited

Article

Mar 2015
PRECAMBRIAN RES

Structural analysis defines a multiphase Sveconorwegian tectonic evolution for the boundary zonebetween the Kongsberg and Telemark lithotectonic units in S Norway, referred to as the Kongsberg-Telemark Boundary Zone (KTBZ). This large-scale weakness zone developed predominantly within andat the margin of a c. 110 km long granitic belt, the intrusion of which is dated between 1170 ± 11 and1146 ± 5 Ma by U–Pb SIMS zircon geochronology. The oldest KTBZ ductile fabric formed during the Sve-conorwegian orogenic cycle (c. 1140–900 Ma) as a penetrative top-to-the-W shear fabric, which wassubsequently reactivated selectively by sinistral transpression that formed characteristic mylonitic shearzones within the granitic belt. Later folding affected the area at the northern end of the Kongsberg litho-tectonic unit. Analysis of the subregional foliation trajectories unravels the occurrence of a large-scalefold structure, the “Norefjell-Hønefoss Fold”. All these structures are in turn cut by late-Sveconorwegian,E-dipping shear zones and normal faults, which accommodated a distinct phase of exhumation of theTelemark lithotectonic unit in the footwall of the KTBZ. This extensional detachment widens towardthe north, where it might have controlled the emplacement of the late-orogenic Flå granite. Since lateSveconorwegian times, the KTBZ was repeatedly reactivated in a brittle fashion forming complex faultpatterns, extensive quartz vein networks and leading to the generation of the so-called “Store Rivnings-breksje”, a 100 km long brittle fault zone that follows the trend of the KTBZ and that locally juxtaposesblocks with different ductile precursor histories. The newly established deformation history helps torefine existing models for the orogenic evolution of the central Sveconorwegian orogen. The characteri-zation of the Norefjell-Hønefoss fold structure provides a new perspective on Sveconorwegian geometriesand fabrics in the area. The reactivation history established for the KTBZ helps to better understand thedynamics of long-lived weakness zones of Precambrian origin in general.

The Concept of Lineaments in Geological Structural Analysis; Principles and Methods: A Review Based on Examples from Norway

Article

Full-text available

Jun 2024

Application of lineament analysis in structural geology gained renewed interest when remote sensing data and technology became available through dedicated Earth observation satellites like Landsat in 1972. Lineament data have since been widely used in general structural investigations and resource and geohazard studies. The present contribution argues that lineament analysis remains a useful tool in structural geology research both at the regional and local scales. However, the traditional “lineament study” is only one of several methods. It is argued here that structural and lineament remote sensing studies can be separated into four distinct strategies or approaches. The general analyzing approach includes general structural analysis and identification of foliation patterns and composite structural units (mega-units). The general approach is routinely used by most geologists in preparation for field work, and it is argued that at least parts of this should be performed manually by staff who will participate in the field activity. We argue that this approach should be a cyclic process so that the lineament database is continuously revised by the integration of data acquired by field data and supplementary data sets, like geophysical geochronological data. To ensure that general geological (field) knowledge is not neglected, it is our experience that at least a part of this type of analysis should be performed manually. The statistical approach conforms with what most geologists would regard as “lineament analysis” and is based on statistical scrutiny of the available lineament data with the aim of identifying zones of an enhanced (or subdued) lineament density. It would commonly predict the general geometric characteristics and classification of individual lineaments or groups of lineaments. Due to efficiency, capacity, consistency of interpretation methods, interpretation and statistical handling, this interpretative approach may most conveniently be performed through the use of automatized methods, namely by applying algorithms for pattern recognition and machine learning. The focused and dynamic approaches focus on specified lineaments or faults and commonly include a full structural geological analysis and data acquired from field work. It is emphasized that geophysical (potential field) data should be utilized in lineament analysis wherever available in all approaches. Furthermore, great care should be taken in the construction of the database, which should be tailored for this kind of study. The database should have a 3D or even 4D capacity and be object-oriented and designed to absorb different (and even unforeseen) data types on all scales. It should also be designed to interface with shifting modeling tools and other databases. Studies of the Norwegian mainland have utilized most of these strategies in lineament studies on different scales. It is concluded that lineament studies have revealed fracture and fault systems and the geometric relations between them, which would have remained unknown without application of remote sensing data and lineament analysis.

Petersson et al., 2023. The robustness of the Lu-Hf and Sm-Nd isotopic systems during metamorphism – A case study of the Åker metabasite in southern Sweden. Prec. Res

Article

Full-text available

Jun 2023
PRECAMBRIAN RES

While whole-rock Lu-Hf isotope analysis remains one of the only ways to obtain initial Hf isotope signatures of old mafic rocks, Hf isotope analyses of more robust accessory zircon in intermediate to silicic rocks have largely replaced whole-rock analyses during the last decade. This has led to a discrepancy in the amount of existing data from mafic and felsic lithologies. However, especially in mafic, Si-poor rocks with a metamorphic imprint, Hf isotope data rely on whole-rock analysis since baddeleyite, commonly used for U-Pb age analyses of mafic rocks, is sensitive to alteration and metamorphism. Hence, to accurately evaluate the trace element and isotope signatures of altered mafic rocks, it is important to understand the mechanisms of element mobility during metamorphism. Here, we report whole-rock trace element compositions, Lu-Hf and Sm-Nd isotope data from variably deformed and metamorphosed samples of a mafic intrusion in southern Sweden, the Åker metabasite. These data suggest that trace elements were undisturbed on a whole-rock sample scale during deformation at upper amphibolite facies (at least 1000 MPa and 600 ◦C) metamorphism under hydrated conditions. Despite redistribution of Zr associated with the breakdown of baddeleyite and other igneous phases, the Åker metabasite has retained its chemical and isotopic integrity since igneous crystallisation at ca. 1565 Ma. This study demonstrates and strengthens the feasibility of whole-rock analyses of (meta-)mafic rocks for determining initial εNd and εHf values, despite deformation and metamorphism under hydrated amphibolite-grade metamorphic conditions. Testing the coherence of the calculated initial Nd and Hf isotope ratios by examining variably deformed and metamorphosed varieties of a rock in a single outcrop, could be used as a model for research on more complex Archean rocks.

Rodinia without Baltica? Constraints from Sveconorwegian orogenic style and palaeomagnetic data

Article

Full-text available

May 2023

The core of the Rodinia supercontinent has long been considered to have consisted of three cratons - Baltica, Laurentia, and Amazonia - amalgamated along the late Mesoproterozoic Sveconorwegian, Grenville, and Sunsas orogens. In recent years, however, it has become increasingly clear that the metamorphic and magmatic evolution of the Sveconorwegian orogen is inconsistent with a collisional model. Although geological data alone do not rule out proximity to Rodinia, palaeomagnetic data indicate significant latitudinal separation of Baltica and Laurentia during supercontinent assembly. In this contribution, we briefly review two recently proposed and mutually exclusive tectonic models for the Sveconorwegian orogeny and present a compilation of previously published and new chemical and isotopic data. A lack of crustal thickening throughout much of the orogen and few if any changes in lower-crustal sources and melting conditions between 1.3 and 0.9 Ga suggest that the western part of the Sveconorwegian orogeny represents a change from a dominantly extensional to a compressional back-arc regime, but without a significant change in overall tectonic setting. This orogenic evolution is incompatible with amalgamation into Rodinia and suggests that Baltica may have been isolated until the Silurian Caledonian orogeny. Supplementary material at https://doi.org/10.6084/m9.figshare.c.6627988

Geochronological constraints for a two-stage history of the Sveconorwegian rare-element pegmatite province formation

Article

Jan 2023
PRECAMBRIAN RES

Most pegmatites of southern Norway seem to be derived from anatectic melting of metamorphic rocks during the Sveconorwegian orogeny rather than to be highly evolved residual melts derived from granites. We test this hypothesis by providing new age data for thirteen pegmatites and one granite. Based on these new age data, we distinguish two age groups of Sveconorwegian pegmatites (>1,000 m³ in size); Group 1: 1100–1030 Ma and Group 2: 930–890 Ma. All pegmatites except those from the Østfold area crystallized significantly earlier or later than adjacent granites. The Tørdal granite, yielding an age of 946 ± 4 Ma, is about 40 Ma older than the adjacent pegmatites. Field evidence and the age difference between pegmatites and granites supports an anatectic origin for these pegmatites. Sources of these pegmatite melts are biotite- and biotite-amphibole gneisses and amphibolites. Group 1 pegmatites formed in transpressional regimes after peak metamorphism, whereas Group 2 pegmatites formed in an extensional regime and the required heat for partial melting was provided by mafic magma underplating. Differences in the rheological behavior of amphibolite and granitic gneiss during extensional tectonics are the major reason why Group 2 pegmatites occur preferentially in large amphibolite bodies. Under mid-crustal conditions, amphibolite reacts brittle to semi-brittle forming open structures in an extensional tectonic regime where partial melts drained into. Granitic gneisses react in a ductile manner and do not have the ability to drain partial melt. Pegmatite formation in the Grenville Province, i.e., the Laurentian part of the Grenville–Sveconorwegian orogenic belt, formed between ca. 1090 and 980 Ma peaking at 1010 to 980 Ma. Thus, the Grenville peak postdates the Sveconorwegian Group 1 peak by about 30 Ma. These pegmatites formed in similar orogenic settings, implying that similar tectono-metamorphic developments along the Grenville–Sveconorwegian orogenic belt were diachronous. We conclude that local anatexis is the major pegmatite-melt forming process in the Sveconorwegian as well as Grenville orogen. Local anatexis also may be important in other pegmatite provinces.

Enigmatic Mid‐Proterozoic Orogens: Hot, Thin, and Low

Article

Full-text available

Aug 2021
GEOPHYS RES LETT

Since the Archean, secular change in orogenic style is demonstrated through evolution of metamorphic conditions and geochemical proxies. Linked to orogenic style is the amount of crustal thickening and elevation, whereas orogenic vigor is related to the supercontinent cycle. An array of Proterozoic orogens spanned the assembly of supercontinents Columbia and Rodinia, but the vigor of orogenesis is debated, with proposals for both Mesoproterozoic quiescence and climax. We show mid-Proterozoic orogenesis occurred globally and was broadly continuous; furthermore, orogens exhibit elevated metamorphic thermobaric ratios with large volumes of high-temperature felsic magmatic rocks. These features reflect higher mantle heat flux leading to increased mid-crustal flow and lower elevation. In this context, proposals that geochemical proxies for crustal thickness record orogenic quiescence are inconsistent with the geological record. Alternatively, secular change in crustal thickness is attributed to orogenic style, namely the prevalence of hot, thin, and low orogens in the mid-Proterozoic.

Tracing the Sveconorwegian orogen into the Caledonides of West Norway: Geochronological and isotopic studies on magmatism and migmatization

Article

Jul 2021
PRECAMBRIAN RES

The Sveconorwegian orogen represents a branch of Grenville-age (~1250–950 Ma) orogenic belts that formed during the construction of the supercontinent Rodinia. This study traces the Sveconorwegian records from its type-area in the Baltic Shield of South Norway into basement windows underneath Caledonian nappes, by combining zircon U–Pb geochronology and Hf–O isotopes. Samples along a N-S trending transect reveal multiple magmatic episodes during Gothian (ca. 1650 Ma), Telemarkian (ca. 1500 Ma) and Sveconorwegian (1050–1020 Ma vs. 980–930 Ma) orogenesis as well as Sveconorwegian migmatization (1050–950 Ma). Newly documented 1050–1020 Ma magmatism and migmatization extend the Sirdal Magmatic Belt to a 300 km-long, NNW-SSE trending crustal domain, with the northern boundary corresponding to the gradual transition from Telemarkian to Gothian crust. These Precambrian crustal heterogeneities largely controlled the development of Caledonian shear zones. The ca. 1050–1040 Ma granitic and mafic magmas show similar isotopic signatures with slightly negative or positive εHf(t) and moderate δ¹⁸O values (6–7‰), which indicates that crustal reworking was more dominant than juvenile inputs during their genesis. The generation of leucosomes and leucogranites at ca. 1030–1020 Ma, which have a more evolved Hf isotopic composition, probably reflects an even higher degree of remelting of older crust. The Hf–O isotopic patterns show that Sveconorwegian magmas differ from typical arc magmas by lower involvement of sedimentary components and juvenile material. This makes the 1050–930 Ma magmatism incompatible with a long-term subduction setting. The ca. 1650–1500 Ma samples, in contrast, generally have juvenile Hf isotopic compositions associated with varying δ¹⁸O values of 4.5–9‰, consistent with subduction-accretion processes involving significant sedimentary recycling. This accretionary margin was most likely transformed into the Sveconorwegian orogen through collisional interactions of Baltica, Laurentia and Amazonia in the context of Rodinia amalgamation.

The role of the Leba Ridge–Riga–Pskov Fault Zone in the tectonic evolution of the deep-facies Livonian Tongue within the Baltic Ordovician–Silurian sedimentary basin: a review

Article

Full-text available

Mar 2021
EST J EARTH SCI

Located in the interior of the East European Craton (EEC), the Baltic Ordovician–Silurian Basin hosts an elongated tongue-like deep-marine depression, the Livonian Tongue (LT), which extends from Sweden across Latvia and separates the Estonian and Lithuanian shallow-marine shelves. The tectonic origin of the LT has been suggested already since its discovery in the early 1960s. However, the nature of tectonic forces and mechanisms behind the evolution of this narrow intracratonic subsidence zone in the Ordovician–Silurian of the Baltic Basin has remained poorly understood. The origin of the LT can be related to an extensive intracratonic dislocation zone known as the Leba Ridge–Riga–Pskov Fault Zone (LeRPFZ) that coincides largely with the axis of the LT. The LeRPFZ reveals some heavily uplifted basement blocks and has, therefore, been considered as an up-warped anticline�type structure. Recent studies show that the LT has developed in highly complex and changing stress field conditions during the Caledonian orogeny. The subsidence and widening phase of the LT in the Ordovician and early Silurian coincides with, and was possibly governed by, the Avalonia collision with Baltica from the SW when high shear stress forced LeRPFZ blocks to move obliquely towards the NE. As Laurentia was approaching Baltica and finally collided with it in the mid-Silurian, the shear stress became progressively mingled with compression from the NW and the subsidence of the LeRPFZ became reversed, triggering LT withdrawal to the SW. Thus, being once the deep-water centre of the Baltic Ordovician–Silurian Basin, the LT became the most uplifted and intensely eroded EEC interior zone by the Devonian

In situ Rb-Sr dating of slickenfibres in deep crystalline basement faults

Article

Full-text available

Jan 2020

Establishing temporal constraints of faulting is of importance for tectonic and seismicity reconstructions and predictions. Conventional fault dating techniques commonly use bulk samples of syn-kinematic illite and other K-bearing minerals in fault gouges, which results in mixed ages of repeatedly reactivated faults as well as grain-size dependent age variations. Here we present a new approach to resolve fault reactivation histories by applying high-spatial resolution Rb-Sr dating to fine-grained mineral slickenfibres in faults occurring in Paleoproterozoic crystalline rocks. Slickenfibre illite and/or K-feldspar together with co-genetic calcite and/or albite were targeted with 50 µm laser ablation triple quadrupole inductively coupled plasma mass spectrometry analyses (LA-ICP-MS/MS). The ages obtained disclose slickenfibre growth at several occasions spanning over 1 billion years, from at least 1527 Ma to 349 ± 9 Ma. The timing of these growth phases and the associated structural orientation information of the kinematic indicators on the fracture surfaces are linked to far-field tectonic events, including the Caledonian orogeny. Our approach links faulting to individual regional deformation events by minimizing age mixing through micro-scale analysis of individual grains and narrow crystal zones in common fault mineral assemblages.

Tectonomagmatic evolution of the Sveconorwegian Orogen recorded in the chemical and isotopic compositions of 1070 – 920 Ma Sveconorwegian granitoids

Article

Full-text available

Nov 2019
PRECAMBRIAN RES

The Sveconorwegian Province in Southern Norway and Sweden hosts at least four granitoid suites, representing apparently continuous magmatism at the SW margin of the Fennoscandian Shield between 1070 and 920 Ma. This study presents a compilation of published and new zircon LA-ICP-MS U-Pb geochronology, whole-rock and zircon geochemistry and Sm-Nd isotope data for the granitoid suites and demonstrates the granitoids’ ability to record changes in the tectonomagmatic evolution of this orogenic Province. The Sirdal Magmatic Belt (SMB, ca. 1070–1010 Ma) represents the earliest magmatism, west in the Province, followed by two hornblende-biotite granitoid suites (HBG, ca. 1000–920 Ma) and the Flå–Iddefjord–Bohus suite (FIB, ca. 925 Ma), in central and eastern parts of the Province, respectively. The SMB and the HBG bodies located outside of the SMB (referred to as HBGout) are chemically similar, whereas the HBG bodies located in the same region as the SMB (referred to as HBGin) are more ferroan, enriched in incompatible elements and have higher zircon saturation temperatures. Isotopically, the SMB and both HBG suites fall on an evolutionary trend from widespread 1.5 Ga crust in the region, suggesting this was the dominant crustal contribution to magmatism. The FIB suite is more peraluminous, rich in inherited zircon, and has isotopic compositions suggesting a more evolved source than both the HBG suites and the SMB. Trace element modelling shows that the SMB and HBGout suites could have formed by 50% partial melting of 1.5 Ga crust, whereas 5–10% remelting of the dehydrated and depleted SMB residue accounts for the geochemical composition of the HBGin suite. The available data suggest a scenario where the 1.5 Ga lower crust underwent melting due to long-lived mafic underplating giving rise to the SMB suite. After ca. 1000 Ma, regional-scale extension may have led to more widespread mafic underplating causing remelting of the residue following SMB melt extraction, forming the HBGin suite, with lower-crustal melting farther east forming the HBGout suite. Changes in melt composition over this 150 Myr time interval may thus be ascribed to an evolving melt source rather than fundamental changes in tectonic regime. Deep continental subduction at ca. 990 Ma, east in the orogen, provided an isotopically evolved crustal source for the FIB suite. The data underline the difference in tectonic processes across the orogen, with long-lived, high temperatures in the western and central parts and colder, high-pressure events in the eastern parts of the orogen.

Breaking the Grenville-Sveconorwegian link in Rodinia reconstructions

Article

May 2019

The Grenville, Sveconorwegian, and Sunsas orogens are typically inferred to reflect collision between Laurentia, Baltica, and Amazonia at ca. 1.0 Ga, forming a central portion of the Rodinia supercontinent. This triple-junction configuration is often nearly identical in otherwise diverse Rodinia reconstructions. However, available geological data suggest that although the Grenville and Sveconorwegian provinces shared a similar tectonic evolution from pre-1.8 to ca. 1.5 Ga, they record distinctly different tectonic histories leading up to, during, and possibly following Grenville-Sveconorwegian orogenesis. Moreover, paleomagnetic data suggest the two continents were separated at peak orogenesis, further invalidating any direct correlation. A number of possible interpretations are permissible with available geological and paleomagnetic data, of which a 'classic' triple-junction configuration appears least likely. In contrast to the commonly inferred intertwined Proterozoic evolution of Baltica and Laurentia, the possibility remains that they were unrelated for a billion years between 1.5 and 0.45 Ga.

The eastern boundary of Sveconorwegian reworking in the Baltic Shield, defined by 40Ar/39Ar geochronology across the southernmost Sveconorwegian Province

Article

Feb 2018
PRECAMBRIAN RES

New 40Ar/39Ar dates of hornblende, biotite and muscovite from 13 localities along a 130 km transect in southern Sweden provide insight into the cooling and exhumation history during Sveconorwegian orogeny, from 970 to 880 Ma. The Eastern Segment represents Baltican continental crust underthrust below the western Sveconorwegian terranes at 990–970 Ma. The western part of the Eastern Segment consists of amphibolite- to granulite-facies pervasively deformed gneisses, in which hornblende and biotite 40Ar/39Ar dates between 901 and 889 Ma record the cooling and exhumation through 530–330 °C. In the easternmost part of the orogen is a c. 25 km wide boundary zone characterized by non-penetrative greenschist- to amphibolite-facies deformation zones. There, muscovite apparent ages range from 882 to 902 Ma, biotite from 892 to 906 Ma, and hornblende from 1.37 to 1.47 Ga, meaning that muscovite and partly biotite record Sveconorwegian overprint, while amphibole was disturbed. Several biotite samples record ages in the interval 0.9–1.4 Ga, reflecting excess Ar components. East of the Sveconorwegian Province, in the Blekinge-Bornholm Province, hornblende and mica apparent ages spread between 1.13 and 1.41 Ma, implying that neither mineral underwent complete Sveconorwegian resetting. The data pattern reflects that the Eastern Segment experienced slow cooling (∼3 °C/Ma) from peak metamorphism of 800–700 °C at 980–960 Ma to 900 Ma, when significant cooling though greenschist-facies conditions set in (∼26 °C/Ma). This cooling was related to tectonically driven extension, accommodated by greenschist-facies shear zones along the eastern boundary zone.

Zircon Growth during Progressive Recrystallization of Gabbro to Garnet Amphibolite, Eastern Segment, Sveconorwegian Orogen

Article

Apr 2017

The interpretation of whether a dated metamorphic zircon generation grew during the prograde, peak or retrograde stage of a metamorphic cycle is critical to geological interpretation. This study documents a case at Herrestad, in the eastern part of the 1±0 Ga Sveconorwegian Province, involving progressive metamorphic recrystallization of gabbro to garnet amphibolite and associated behaviour of Zr-bearing minerals. In this case, textures show that baddeleyite is by far the main source of Zr for metamorphic zircon growth. The amount of metamorphic zircon formed was primarily controlled by the degree of metamorphic recrystallization, which in turn was controlled by deformation and the presence of a fluid as a transport medium. Zircon in the Herrestad rocks shows a range of morphologies and internal textures at different degrees of metamorphic recrystallization. Igneous zircon occurs together with baddeleyite in coarse-grained olivine-free facies of the gabbro. Metamorphic polycrystalline zircon rims on baddeleyite and minute (<5 μm) bead-like zircon grains at Fe-Ti oxide boundaries characterize the transition to coronitic metagabbro. With increasing metamorphic recrystallization, polycrystalline zircon rims grow at the expense of baddeleyite and the amount of minute bead-like zircon increases, forming strings of zircon beads with increasing distance from Fe-Ti oxide grains. The progressive breakdown of baddeleyite results in polycrystalline zircon aggregates that become denser and finally form single grains in completely recrystallized garnet amphibolite. Late magmatic zircon crystallized at 156765 Ma, whereas metamorphic zircon dates amphibolite-facies metamorphic recrystallization at 97067 Ma. The Herrestad case illustrates a general rule that the bulk Zr budget in originally baddeleyite-bearing rocks will rapidly become locked into metamorphic zircon during the first event of metamorphic recrystallization, when silica and Zr are released from the igneous minerals. Incomplete metamorphic recrystallization and partial preservation of baddeleyite, however, also allows later stages of zircon formation. Thus, in incompletely reacted rocks the final result may be highly complex with microscale zircon of several age generations. © The Author 2017. Published by Oxford University Press. All rights reserved.

Geochronology of quartzo-feldspathic and amphibolitic migmatite gneisses in the Eastern Segment of the Sveconorwegian orogen, southwest Sweden

Article

Full-text available

Traces of the Transscandinavian Igneous Belt in the central Scandinavian Caledonides: U-Pb zircon dating and geochemistry of crystalline basement rocks in the Middle Allochthon

Article

Sep 2015

Basement-slices are frequent components in the lower nappes of the central Scandinavian Caledonides. New geochronological and geochemical data provide evidence that three of these basement-slices in the Middle Allochthon are derived from the Transscandinavian Igneous Belt. Dated samples in this study comprise a quartz monzonite from the Stalon Nappe Complex and a quartz monzodiorite and a monzonite from the Ammarna¨s Nappe Complex. A Ce/Yb-Ta/Yb-plot of the mafic rocks from the Ammarna¨s Nappe Complex suggests a calc-alkaline to shoshonitic geochemical character. Low Ni- and Cr-contents and low Mg# indicate that the mafic rocks originated from an already evolved magma. Enrichment of Fe-Ti oxides and V . 200 ppm indicate presence of cumulus phases to various degrees. These basement-derived rocks in the Middle Allochthon yielded concordant U-Pb zircon SIMS ages of 1799±10 Ma, 1787±6Ma and 1797±5 Ma. They are therefore interpreted to represent rocks detached from the Palaeoproterozoic Transscandinavian Igneous Belt and incorporated in the Middle Allochthon during Scandian orogeny.

The Late Mesoproterozoic Sirdal Magmatic Belt, SW Norway: Relationships between magmatism and metamorphism and implications for Sveconorwegian orogenesis

Article

May 2015
PRECAMBRIAN RES

The Late Mesoproterozoic Sveconorwegian Province is commonly correlated with the continent-collision related Grenville Province in eastern Canada. Recently, however, the evolution of the Sveconorwegian Province in SW Norway has been strongly debated, casting doubt on a direct correlation between these provinces.

Detrital Zircon Signatures of the Baltoscandian Margin along the Arctic Circle Caledonides in Sweden: The Sveconorwegian connection

Article

May 2015
PRECAMBRIAN RES

New evidence is presented here that the Sveconorwegian Orogen continued northwards from type areas in southwestern Scandinavia along the Baltoscandian outer margin into the high Arctic. The Silver Road (Silvervägen) profile through the Scandinavian Caledonides, located in Sweden along the Arctic Circle at 66-67° N, provides a full section through the tectonostratigraphy of the Baltoscandian margin from the Autochthon, via the Lower Allochthon to the upperment parts of the Middle Allochthon. Metamorphic grade increases upwards through the nappes, being low greenschist facies at lowest levels and increasing to eclogite grade in the highest parts of the Seve Nappe Complex, the latter being related to early Ordovician subduction of the Baltoscandian outermost margin. The sedimentary rocks range in age from Neoproterozoic to Ordovician and provide evidence of the changes of environment from the Baltoscandian platform, westwards out over the Cryogenian rifted margin to the continent-ocean transition zone; also the Ordovician foreland basin. Twelve samples of psammites from the different tectonostratigraphic levels have yielded U/Pb detrital zircon age-signatures that reflect the changing character of their provenance. Autochthonous sandstones are derived from late Paleoproterozoic (1800-1950 Ma) crystalline rocks in the vicinity to the east of the thrust front. Ediacaran-early Cambrian quartzites of the Lower Allochthon also yield mainly late Paleoproterozoic zircon signatures, but with subordinate Mesoproterozoic and late Archaean populations, whilst mid Ordovician, W-derived foreland basin turbidites are dominated by Sveconorwegian (950-1100 Ma) signatures, with subordinate older Mesoproterozoic to latest Paleoproterozoic populations. All samples from the lower parts of the Middle Allochthon (lacking dolerite dykes) have signatures that are dominated by latest Paleoproterozoic and early Mesoproterozoic ages, with subordinate populations down to Sveconorwegian ages; the latter dominate the overlying Särv nappes and also the Seve Nappe Complex, where c.945 Ma rhyodacites have been previously reported. This evidence of Sveconorwegian source rocks in the hinterland, taken together with previously published detrital zircon data farther south and north of the Arctic Circle, clearly favors the interpretation that the Sveconorwegian Orogen, during the Neoproterozoic, extended along the entire Baltoscandian outer margin into the high Arctic.

U–Pb geochronology of the syn-orogenic Knaben molybdenum deposits, Sveconorwegian Orogen, Norway

Article

Full-text available

May 2014

Paired isotope dilution – thermal ionization mass spectrometry (ID-TIMS) and secondary ion mass spectrometry (SIMS) zircon U–Pb data elucidate geochronological relations in the historically important Knaben molybdenum mining district, Sveconorwegian Orogen, south Norway. This polyphase district provided c . 8.5 Mt of ore with a grade of 0.2%. It consists of mineralized quartz veins, silica-rich gneiss, pegmatites and aplites associated with a heterogeneous, locally sulphide-bearing, amphibolites facies gneiss called Knaben Gneiss, and hosted in a regional-scale monotonous, commonly weakly foliated, granitic gneiss. An augen gneiss at the Knaben I deposit yields a 1257±6 Ma magmatic zircon age, dating the pre-Sveconorwegian protolith of the Knaben Gneiss. Mineralized and non-mineralized granitic gneiss samples at the Knaben II and Kvina deposits contain some 1488–1164 Ma inherited zircon and yield consistent intrusion ages of 1032±4, 1034±6 and 1036±6 Ma. This age links magmatism in the district to the regional 1050–1020 Ma Sirdal I-type granite suite, corresponding to voluminous crustal melting during the Sveconorwegian orogeny. A high-U, low-Th/U zircon rim is present in all samples. It defines several age clusters between 1039±6 and 1009±7 Ma, peaking at c . 1016 Ma and overlapping with a monazite age of 1013±5 Ma. The rim records protracted hydrothermal activity, which started during the main magmatic event and outlasted it. This process was coeval with regional high-grade Sveconorwegian metamorphism. Molybdenum deposition probably started during this event when silica-rich mineralizing fluids or hydrous magmas were released from granite magma batches. An analogy between the Knaben district and shallow, short-lived porphyry Mo deposits is inappropriate.

Zircon U-Pb and Hf - isotopes from the eastern part of the Sveconorwegian Orogen, SW Sweden: Implications for the growth of Fennoscandia

Article

Full-text available

Jan 2013

Current models for the growth of Fennoscandia, including the eastern part of the Sveconorwegian Province, are largely based on U–Pb data and do not discriminate between juvenile and reworked crust. Here we present new combined U–Pb and Hf isotopic data, from the Eastern Segment and the Idefjorden terrane of the Sveconorwegian Province, and suggest a revised model of crustal growth. Most of the crystalline basement in this part of the shield formed by mixing of a 2.1–1.9 Ga juvenile component and Archaean crust. Archaean reworking decreases between 1.9 and 1.7 Ga and a mixed Svecofennian crustal reservoir is generated. Succeeding magmatism between 1.7 and 1.4 Ga indicates reworking of this reservoir with little or no crust generation. At c. 1.2 Ga, an influx of juvenile magma is recorded by granite to quartz-syenite magmatism with mildly depleted (ε Hf 1.18 Ga of c. 3) signatures. The amount of recycled crust in the 1.9–1.7 Ga arc system is in contrast to previously proposed models for the growth of the southwestern part of the Fennoscandian Shield. This model agrees with long-term subduction along the western margin of Fennoscandia, but suggests substantial reworking of existing crust and decreasing amounts of <1.9 Ga crustal growth. Supplementary material The analytical method, U–Pb SIMS table, U–Pb LA-SF-ICP-MS table and Lu–Hf table are available at www.geolsoc.org.uk/SUP18648

Image analysis of acoustic data and interpretation of rock stress orientations for geothermal exploration in Gothenburg borehole GE-1, Southwest Sweden

Article

Feb 2024

This study contributes to geothermal exploration in 1660-1520 Ma old reworked bedrock in Sweden. Our primary objectives are to constrain the orientation of horizontal stresses, and to discuss implications for geothermal exploration. High-resolution acoustic televiewer image data reveal the downhole distribution of stress indicators (borehole breakouts, drilling induced fractures and petal centerline fractures), pre-existing structures (natural fractures, foliation). About 135 m of stress indicators are measured from 0.2-1.0 km. The results suggest a uniform NNW-SSE mean maximum horizontal stress orientation. A total of 1525 pre-existing structures (natural fractures, foliation) are mapped in borehole GE-1. The prevailing stress regime controls if natural fractures and foliation are well-oriented for stimulation. For strike-slip and normal faulting stress regimes, well-oriented fractures are steeply dipping towards WSW. For a reverse faulting stress regime, shallow dipping fractures are well-oriented for simulation. The downhole distribution of stress indicators and other stress measurements in the region and other parts of Fennoscandia tentatively suggest a strike-slip stress regime, but additional studies are needed to constrain the complete stress field at study depth and towards EGS reservoir target depth. Our secondary objective is to highlight that interpretation of high-resolution acoustic data particularly in metamorphic crystalline rocks are subjective, and that more guidelines for data interpretation are needed. The interactive interpretation of the images is based on visual analyses of complex pre-existing structures and stress indicators with highly variable shapes. The application of three methods for data analyses in the GE-1 borehole propose that drilling induced fractures are little influenced by the method applied. Interpretations on individual borehole breakout azimuths may, however result in over 10 differences in orientation. Supplementary material at https://doi.org/10.6084/m9.figshare.c.7082902

Long-distance transport of clastic material revealed by monazite and muscovite dating: Albian arenites, extra-Carpathian Poland

Article

Mar 2023
SEDIMENT GEOL

The absolute ages based on chemical U\\Pb dating ofmonazite (CHIME) and 40Ar/39Ar muscovite were used for provenance determination of the Albian arenites ofthe southern part of the extra-Carpathian Poland. The ages of detrital monazites allowed a division of the study area into two domains: western and eastern, related to the Miechów and Lublin areas respectively. The western domain is characterized by a unimodal distribution ofmon- azite ages – 330–380 Ma that unequivocally indicates a Variscan provenance of the detrital monazite. The mus- covite 40Ar/39Ar method applied to a western domain sample yielded a primary 360.0± 1.21Ma age, confirming a Variscan source of the detrital material. The eastern domain is characterized by a polymodal distribution of monazite ages and is further divided into two subdomains: northeastern and southeastern. Both subdomains are characterized by a polymodal distribution ofmonazite ages peaking at 390–490 Ma; 0.8–1.1 Ga; ca. 1.5 Ga and ca. 1.8 Ga, indicating the Caledonian, Sveconorwegian, Hallandian-Danopolonian plus Gothian and Svecofennian origins ofthe detrital monazite. The southeastern subdomain additionally has a significant amount of detrital monazite with 330–380 Ma ages, suggesting a notable influence of a Variscan source. The 40Ar/39Ar muscovite data from the northeastern subdomain sample, ca. 476.4 ± 1.27 and subordinate 496.4 ± 2.83 Ma, confirms that a Caledonian source area was active during sedimentation of the Albian deposits. Detrital muscovite from southeastern subdomain sample yielded two ages: 478.1 ± 3.71 Ma and 502.1 ± 0.65 Ma also indicating a Caledonian age of the detrital muscovite. The monazite and muscovite data indicate at least two independent main ultimate source areas: the Bohemian Massif for the Miechów area (western domain) and the Baltic Shield for the Lublin area (eastern domain). The results document an over 1200 km long transport of clastic material in the Central European Sedimentary Basin during the Albian.

Precambrian fault reactivation revealed by structural and K-Ar geochronological data from the spent nuclear fuel repository in Olkiluoto, southwestern Finland

Article

Full-text available

Jan 2022
TECTONOPHYSICS

Integrated structural/geochronological studies help unraveling complex brittle deformation histories. We have analysed the structural geological database of brittle faults from the ONKALO™ underground facility for spent nuclear fuel in Olkiluoto in southwestern Finland. Based on the structural geological data from eleven representative fault zones, we classify the Olkiluoto brittle structural features into four fault systems, referred to as Fault system I to IV. The classification is based on their structural properties and tectonic history, crosscutting relationships, fault rock mineralogical characterization and 3D modelling. Some constraints on the timing of faulting are provided by K-Ar dates on synkinematic illite from fault gouge samples. Our results show that the bedrock in southwestern Finland experienced numerous brittle deformation phases between ca. 1.75 and 0.9 Ga. N-S strike-slip faults (Fault systems I and II) formed at mid-crustal levels ca. 1.79–1.75 Ga ago in response to NW-SE/NNW-SSE compression soon after the Svecofennian orogeny. Later E-W striking oblique dextral/normal faults (Fault system III) are tentatively associated with the Gothian orogeny 1.6 Ga ago. These three fault systems were reactivated during NE-SW compression ca. 1.3–1.2 Ga ago, coeval with intrusion of a regional swarm of olivine diabase sills. E-W compression at the onset of the Sveconorwegian orogeny ca. 1.1–1.0 Ga ago resulted in the formation of SE dipping low-angle thrust faults (Fault system IV) and the selective reactivation of fault system II and III. Overall E-W extension during the collapse of the Sveconorwegian orogen ca. 0.97–0.87 Ga ago caused the localised reactivation of fault systems III and IV. Our research approach, which is integral to the siting process of repositories for spent nuclear fuel, demonstrates that the basement in southwestern Finland experienced repeated reactivation since the Mesoproterozoic, suggesting that future deformation localization is likely to be also accommodated by reactivation of existing brittle structures rather than formation of new faults.

The metamorphic and magmatic record of collisional orogens

Article

Oct 2021

The Cenozoic Himalaya-Tibet orogen is generally regarded as the archetypal continental collision zone and is often used as an analogue for interpreting ancient orogenic events. However, given the wide diversity observed in present-day collisional mountain belts, the extent to which such inferences can be made remains debated. In this Review, we compare the metamorphic and magmatic record of the Himalaya-Tibet orogen to four ancient orogens — the Palaeozoic Caledonian orogen, the Meso-Neoproterozoic Grenville and Sveconorwegian orogens, and the Palaeoproterozoic Trans-Hudson orogen — to establish the controls on the underlying dynamics and the nature of the resulting rock record. The similarities in rock records, and, thus, thermal conditions, are interpreted to result from comparable foreland strengths, resulting in similar maximum crustal thicknesses. Apparent differences in the records are mainly attributed to variation in exposed structural level rather than fundamentally different tectonic processes. We, therefore, suggest that foreland rheology is a critical factor in determining the effectiveness of orogen comparisons. Future research is required to investigate the causes and consequences of lateral variability in mountain belts, in particular, focussing on the record of orogens smaller than those considered here, and to understand if and why mountain building processes have varied through Earth history.

The Sveconorwegian orogeny – Reamalgamation of the fragmented southwestern margin of Fennoscandia

Article

Full-text available

Jul 2020
PRECAMBRIAN RES

The Sveconorwegian orogeny encompasses magmatic, metamorphic and deformational events between ca. 1140 and 920 Ma at the southwestern margin of Fennoscandia. In recent years, the tectonic setting of this nearly 200 Myr-long evolution has been debated, with some workers arguing for collision with an unknown continent off the present-day southwest coast of Norway, and others advocating accretionary processes inboard of an active margin. Recently, it has been suggested that orogeny may have been gravity-driven by delamination and foundering of heavy subcontinental lithospheric mantle in an intraplate setting, in some ways similar to proposed sagduction processes in the Archaean. Resolving the tectonic setting of the Sveconorwegian orogen has implications for correlation with other orogens and Rodinia supercontinent reconstructions and for assessments of the evolution of plate tectonics on Earth, from the Archaean to the present. Here, we present new mapping and geochronological data from the Bamble and Telemark lithotectonic units in the central and western Sveconorwegian orogen – the former representing a critical region separating western parts of the orogen that underwent long-lived high- to ultrahigh-temperature metamorphism and magmatism from parts closer to the orogenic foreland that underwent episodic high-pressure events. The data show that the units constituting the Sveconorwegian orogen most likely formed at the southwestern margin of Fennoscandia between ca. 1800 and 1480 Ma, followed by fragmentation during widespread extension between ca. 1340 and 1100 Ma marked by bimodal magmatism and sedimentation. A summary of Sveconorwegian magmatic, metamorphic and depositional events in the different units shows disparate histories prior to their assembly with adjacent units. The most likely interpretation of this record seems to be that episodic, Sveconorwegian metamorphic and deformational events in the central and eastern parts of the orogen represent accretion and assembly of these units. This process most likely took place behind an active margin to the southwest that sustained mafic underplating in the proximal back-arc, resulting in high- to ultrahigh-temperature metamorphism in the western parts. In this interpretation, all features of the Sveconorwegian orogen are readily explained by modern-style plate tectonic processes and hypotheses involving some form of vertical, intraplate tectonics are not supported.

Chapter 17 Accretionary orogens reworked in an overriding plate setting during protracted continent–continent collision, Sveconorwegian orogen, southwestern Sweden

Article

Jan 2020

The Eastern Segment in the Sveconorwegian orogen, southwestern Sweden, is dominated by 2.0–1.8, 1.7 and 1.5–1.4 Ga crust; and the overlying Idefjorden terrane by 1.6–1.5 Ga crust. Assuming reorganization of a subduction system prior to 1.5–1.4 Ga and applying a sinistral transpressive component of disruption during the subsequent Sveconorwegian orogeny (1.1–0.9 Ga), the Idefjorden terrane is inferred to be indigenous outboard rather than exotic with respect to the continental plate Fennoscandia (Baltica). The geological record then records successive westwards shift of accretionary orogens along a convergent plate boundary for at least 500 million years. Sveconorwegian foreland-younging tectonic cycles at c. 1.05 (or older)–1.02 Ga (Idefjorden terrane) and at c. 0.99–0.95 Ga (Eastern Segment) prevailed. Crustal thickening and exhumation during oblique convergence preceded migmatization, magmatic activity and a changeover to an extensional regime, possibly triggered by delamination of continental lithosphere, in each cycle. Convergence after 0.95 Ga involved antiformal doming with extensional deformation at higher crustal levels (Eastern Segment) and continued magmatic activity (Idefjorden terrane). An overriding plate setting is inferred during either accretionary orogeny or, more probably, protracted continent–continent collision. Continuity of the erosional fronts in the Grenville and Sveconorwegian orogens is questioned.

Chapter 14 Regional context and lithotectonic framework of the 1.1–0.9 Ga Sveconorwegian orogen, southwestern Sweden

Article

Jan 2020

The 1.1–0.9 Ga Sveconorwegian orogen in southwestern Scandinavia belongs to the global system of mountain belts established during the assembly of the supercontinent Rodinia. An overall north–south structural trend and five lithotectonic units bounded by crustal-scale shear zones characterize this orogen. In Sweden, the Eastern Segment abuts the orogen's cratonic foreland eastwards and is separated from the Idefjorden terrane westwards by a ductile shear zone, up to 5 km thick, displaying a sinistral transpressive component. These two lithotectonic units differ on the basis of their pre-Sveconorwegian accretionary tectonic evolution, and the timing of Sveconorwegian high-pressure metamorphism, anatexis and polyphase deformation. High-pressure granulites and migmatites formed at c. 1.05–1.02 Ga in the Idefjorden terrane; eclogites, high-pressure granulites and migmatites at c. 0.99–0.95 Ga in the Eastern Segment. Magmatic activity and crustal extension progressed westwards at c. 0.98–0.92 Ga. Prior to or at 0.93–0.91 Ga, greenschist facies shear deformation with top-to-the-foreland movement affected the frontal part of the orogen. Geodynamic uncertainties concern the affinity of the Idefjorden terrane relative to Fennoscandia (Baltica), the character of the Sveconorwegian orogenesis, and the contiguous or non-contiguous nature of the erosional fronts of the late Mesoproterozoic–early Neoproterozoic orogens in Sweden and Canada.

Chapter 15 Polyphase (1.9–1.8, 1.5–1.4 and 1.0–0.9 Ga) deformation and metamorphism of Proterozoic (1.9–1.2 Ga) continental crust, Eastern Segment, Sveconorwegian orogen

Article

Jan 2020

The Eastern Segment in the Sveconorwegian orogen comprises Paleoproterozoic–Mesoproterozoic magmatic suites, which formed along an active continental margin, and Mesoproterozoic suites emplaced during intracratonic extension. Zn–Pb sulphide and Fe oxide mineralizations in 1.9 Ga metavolcanic rocks form a significant mineral resource cluster in the northeastern part. Deformation and metamorphism under low-pressure (≤5 kbar) and variable-temperature conditions, including anatexis and granulite facies, prevailed during 1.9–1.8 Ga (Svecokarelian) and 1.5–1.4 Ga (Hallandian) accretionary orogenies. Sveconorwegian tectonothermal reworking initiated at c. 0.99–0.98 Ga in structurally lower levels. Crustal shortening, underthrusting with eclogite facies metamorphism (18 kbar), exhumation by eastwards thrusting (D 1 ) during continued shortening and high-pressure granulite (8–12 kbar) to upper amphibolite facies metamorphism prevailed. Anatexis and folding around east–west axial surfaces with west-northwesterly constrictional strain (D 2 ) followed at c. 0.98–0.95 Ga, being consanguineous with crustal extension. Structurally higher levels, northwards and eastwards, consist of high-pressure (10–12 kbar) orthogneisses, not affected by anatexis but also showing polyphase deformation. Sveconorwegian convergence ceased with upright folding along north–south axial surfaces and, in the uppermost frontal part, greenschist facies shearing with top-to-the-foreland normal followed by reverse displacement after 0.95 Ga. The normal shearing detached the upper compartment from the underlying gneisses.

Chapter 16 Polyphase (1.6–1.5 and 1.1–1.0 Ga) deformation and metamorphism of Proterozoic (1.7–1.1 Ga) continental crust, Idefjorden terrane, Sveconorwegian orogen

Article

Jan 2020

Crust generated during an accretionary orogeny at 1.66–1.52 Ga (Gothian), and later during crustal extension at c. 1.51–1.49, c. 1.46, c. 1.34–1.30 Ga and after c. 1.33 Ga, dominate the Idefjorden terrane. Metamorphism under greenschist to, locally, high-pressure granulite facies, emplacement of syn-orogenic pegmatite and granite, and polyphase deformation followed at 1.05–1.02 Ga (Agder tectonothermal phase, Sveconorwegian orogeny). Sinistral transpressive deformation, including foreland-directed thrusting, preceded top-to-the-west movement and large-scale open folding along north–south axial trends during the younger orogeny. Crustal extension with emplacement of dolerite and lamprophyre dykes, norite–anorthosite, and a batholithic granite took place at c. 0.95–0.92 Ga (Dalane phase, Sveconorwegian orogeny). Ductile shear zones divide the Idefjorden terrane into segments distinguished by the character of the Gothian crustal component. Orthogneisses with c. 1.66 and c. 1.63–1.59 Ga protoliths occur in the Median segment; c. 1.59–1.52 Ga gneissic intrusive rocks and 1.6 Ga paragneisses with relicts of Gothian deformation and migmatization at c. 1.59 Ga and at c. 1.56–1.55 Ga occur in the Western segment. Mineral resources include stratabound Cu–Fe sulphides hosted by sandstone deposited after c. 1.33 Ga, and polymetallic quartz vein mineralization locally containing Au.

Breaking the Grenville–Sveconorwegian link in Rodinia reconstructions

Article

May 2019
TERRA NOVA

Trond Slagstad

The Grenville, Sveconorwegian, and Sunsas orogens are typically inferred to reflect collision between Laurentia, Baltica, and Amazonia at ca. 1.0 Ga, forming a central portion of the Rodinia supercontinent. This triple‐junction configuration is often nearly identical in otherwise diverse Rodinia reconstructions. However, available geological data suggest that although the Grenville and Sveconorwegian provinces shared a similar tectonic evolution from pre‐1.8 to ca. 1.5 Ga, they record distinctly different tectonic histories leading up to, during, and possibly following Grenville–Sveconorwegian orogenesis. Moreover, paleomagnetic data suggest the two continents were separated at peak orogenesis, further invalidating any direct correlation. A number of possible interpretations are permissible with available geological and paleomagnetic data, of which a ‘classic’ triple‐junction configuration appears least likely. In contrast to the commonly inferred intertwined Proterozoic evolution of Baltica and Laurentia, the possibility remains that they were unrelated for a billion years between 1.5 and 0.45 Ga. This article is protected by copyright. All rights reserved.

Foreland-directed gravitational collapse along curved thrust fronts: insights from a minor thrust-related shear zone in the Umbria-Marche belt, central-northern Italy

Article

Full-text available

Apr 2016
GEOL MAG

Gravitational collapse occurs during the mature evolution of orogenic belts, but its signature is difficult to discriminate in macroscopic structures from that of pre-, syn- or late-/post-orogenic extension, so reliable mesoscopic examples are particularly useful. A composite fabric developed along a lateral thrust ramp in the Apennines reveals mesoscopic normal faults that truncate the thrust surface, overprint the S-fabric and merge downwards in a foreland-directed splay, leaving the thrust footwall undeformed. These relationships indicate syn-/late-thrusting extension, which we interpret as induced by hanging-wall gravitational collapse. Our study provides critical constraints for reconstructing the kinematic evolution of collapsing thrust fronts.

40 Ar– 39 Ar biotite age of a lamprophyre dyke and constraints on the timing of ductile deformation inside the Idefjorden terrane and along the Mylonite Zone, Sveconorwegian orogen, south-west Sweden

Article

Jun 2015

A sharply cross-cutting lamprophyre dyke inside the Idefjorden terrane and along the Mylonite Zone in the Sveconorwegian orogen, Sweden, yields a plateau 40Ar-39Ar biotite age of 914.6 ± 1.2 Ma. This result confirms a published K-Ar age and is interpreted to record magmatic intrusion of the dykes. The lamprophyres are coeval to the similarly oriented but less potassic 916 ± 11 Ma noritic to anorthositic Hakefjorden Complex exposed west of the Göta Älv Shear Zone in the Idefjorden terrane. The lamprophyres are characterized by high magnetic susceptibility, chilled margins, biotite phenocrysts and deuteric alteration of the groundmass. They classify as kersantite and minette, and are interpreted to represent rare, minor explosive mantle-derived magmas, intruded along deep-reaching, east-west trending, extensional structures related to the waning stages of the Sveconorwegian orogeny. Since the host bedrock, at the current level of erosion, was responding to brittle deformation prior to the intrusion of the lamprophyre dykes, the age of 914.6 ± 1.2 Ma sets a minimum limit for the termination of Sveconorwegian ductile deformation inside the Idefjorden terrane and along the Mylonite Zone, north of Lake Vänern. This contrasts to the situation in the westernmost Rogaland-Vest Agder sector of the Sveconorwegian orogen, in Norway, where 930-920 Ma anorthosite to granite plutonism was spatially related to high temperature-low pressure metamorphism and ductile deformation during the waning stages of the Sveconorwegian orogeny. The new age underscores the diachronic exhumation of the orogen.

Hallandian 1.45 Ga high-temperature metamorphism in Baltica: P-T evolution and SIMS U-Pb zircon ages of aluminous gneisses, SW Sweden

Article

Apr 2015
PRECAMBRIAN RES

The southernmost Baltic Shield exposes polymetamorphic continental crust that was largely formed and accreted during a series of 1.92-1.66 Ga Paleoproterozoic orogenic events and later reworked during the 1.14-0.90 Ga Sveconorwegian orogeny. An intermediate period of metamorphism, deformation and magmatism at 1.47-1.38 Ga has been attributed to the Hallandian orogeny, but due to overprinting by Sveconorwegian high-grade metamorphism and deformation, the P-T-t evolution and deformation of the Hallandian event have remained obscure. This study presents the first quantitative P-T model of the Hallandian event using high-temperature aluminous gneisses in the south-easternmost marginal part of the Sveconorwegian orogen. The high-grade metamorphism and spatially associated granite magmatism are dated using U-Pb SIMS analysis of zircon. Petrography, bulk and mineral geochemistry, and pseudosection models demonstrate prograde staurolite-sillimanite-grade metamorphism reaching granulite-facies temperatures (700-750 °C) at low pressures (4-5 kbar), with the formation of Crd + Sil + Grt + K-fsp + Ilm + Melt ± Bt. The rocks followed a clockwise P-T path. Later stages involved the formation of sillimanite + biotite at the expense of garnet and cordierite. Local low-temperature and fluid-assisted retrogression also caused the formation of chlorite and muscovite at the expense of cordierite. Both granites and aluminous gneisses contain complex zircon with inherited 1.70 Ga igneous cores and high-U, secondary zircon, mainly formed by reworking of protolith cores. The latter date the Hallandian high-grade metamorphism at 1451 ± 6 Ma and the granite magmatism at 1445 ± 8 Ma. The presence of 1.70 Ga igneous zircon cores in both metamorphic and magmatic rocks suggest that they formed from similar protoliths. The protolith ages correlate with the youngest generation of magmatic rocks of the Transscandinavian Igneous Belt. The aluminous gneisses are of supracrustal origin, and may have formed by chemical alteration of magmatic rocks. Hallandian regional metamorphism took place under a strongly elevated geotherm and was associated with granitic magmatism, suggesting an accretionary orogenic setting. The Hallandian event may demonstrate an 1.47-1.38 Ga Andean-type continental margin at the SW margin of Baltica.

Ga ferro-potassic A-type granitoids of southern Norway by extreme differentiation from basic magmas

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Major and trace elements, Sr and Nd isotopic data as well as mineral compositions are presented for a selection of the 1.0–0.9 ferro-potassic A-type granitoids (Bessefjellet, Rustfjellet, Verhuskjerringi, Valle, Holum, Svöfjell, Handeland-Tveit, Åseral, Lyn-gdal gabbronorites) that occur close to the Mandal-Ustaoset Line (MUL) of southern Norway. These hornblende biotite granitoids (HBG) define an extensive differentiation trend ranging from gabbronorites (50 wt.% SiO 2) to granites (77 wt.% SiO 2). This trend is interpreted as resulting from extreme fractional crystallization of several basaltic magma batches with similar major and trace elements compositions. At 930 Ma, the HBG suite displays a narrower range in I Sr (0.7027–0.7056) than in ε Nd(t) (+1.97 down to −4.90) suggesting some assimilation of a Rb-depleted lower crust (AFC process) or/and source variability. An age of 929 ± 47 Ma is given by a Rb-Sr isochron on the Holum granite (Sr i = 0.7046 ± 0.0006, MSWD = 1.7). Geothermobarometers indicate a low pressure of emplacement (1.3–2.7 kbar) and an oxygen fugacity close to NNO. High liquidus temperatures are given by the apatite saturation thermometer (1005–1054 • C) and are in agreement with results from other studies. The basaltic parent magmas of the HBG suite are partial melts of an hydrous mafic, potassic source lying either in the lithospheric upper mantle or in the mafic lower crust derived from it. This contrasts with the 930 Ma anorthosite–mangerite–charnockite suite (AMC suite) of the Rogaland Province for which a depleted lower crustal anhydrous gabbronoritic source has been indicated. The present data imply the penecontemporaneous melting of two contrasting sources in southern Norway. The source duality could result from an increasing degree of metamorphism (amphibolite to granulite) from East to West, an horizontal stratification of the lower crust or from the stratification of the lithosphere (melting of the lower crust or upper mantle). It may also indicate that the AMC and HBG suites formed in two distinct crustal segments. The linear alignment of the HBG suite along the Mandal-Ustaoset shear zone suggests that a linear uprise of the asthenosphere, following a lithospheric delamination under this structure, could be the vector of the mantle heat.

Timing of continental building in the Sveconorwegian orogen, SW Scandinavia

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Jan 2005

The timing of continental building in the Sveconorwegian orogen of SW Scandinavia is evaluated with zircon U-Pb geochronology. ID-TIMS, LA-ICPMS and SIMS data are reported for 21 samples of orthogneiss, metarhyolite and metasandstone in S Norway, with emphasis on the Suldal area. The Sveconorwegian orogen is divided into a reworked Fennoscandian 1.80-1.64 Ga parautochthonous segment, the Eastern Segment, and two allochthonous terranes. The Idefjorden terrane is interpreted as a composite 1.66-1.52 Ga arc formed at the margin or near the margin of Fennoscandia. The western terrane, including the Telemark, Hardangervidda, Suldal and Rogaland-Vest Agder sectors, is named Telemarkia. U-Pb zircon data indicate that Telemarkia was built during a short magmatic event between 1.52 and 1.48 Ga, and was located at the margin of a Palaeoproterozoic craton, possibly Fennoscandia. No basement older than 1.5 Ga can be positively identified. In the early stage of the Sveconorwegian orogeny, Telemarkia collided with the Idefjorden terrane. The Bamble-Kongsberg sector, characterized by a mixed lithology and 1.13-1.10 Ga early-Sveconorwegian high-grade metamorphism, is interpreted as the original collision zone between these terranes.

Listric normal faulting during postorogenic extension revealed by 40Ar/39Ar thermochronology near the Robertson Lake shear zone, Grenville orogen, Canada

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Apr 1996

The Robertson Lake shear zone is a major plastic to brittle extensional shear zone in the Grenville orogen that bounds the Mazinaw and Sharbot Lake domains and provides information on the style of late extension and the unroofing history of the orogen. Argon isotope data were collected from hornblende and micas to determine 40Ar/39Ar ages, constrain the temperature-time histories of these two domains, and infer the unroofing history of the region. Hornblende cooling ages across the Mazinaw domain (footwall) show little variation, indicating uniform unroofing of the footwall since 950 Ma. Phlogopite, muscovite, and biotite cooling ages of the footwall are 924 to 890 Ma. The cooling history of the Mazinaw domain is characterized by slow cooling after peak metamorphism (circa 1000 Ma), accelerated cooling (4°-5°C/m.y.) from 950 Ma to 890 Ma, and an average cooling rate of ~1°C/m.y. to circa 590 Ma, when these rocks were at or near the surface. The cooling history of Sharbot Lake (hanging wall) domain is drastically different than that of the Mazinaw domain. Hornblende and biotite cooling ages in the central portion of the domain are 1009 and 969 Ma, respectively, indicating a cooling rate of 5°C/m.y. after slow cooling from metamorphic temperatures. Biotite and phlogopite cooling ages determined from samples located at different distances from the shear zone do not lie along the same cooling curve, indicating that the cooling history varied across the domain. Cooling rates in the hanging wall adjacent to the shear zone are low (2°C/m.y.). A biotite cooling age (1029 Ma) and preservation of an amphibole growth age (1205 Ma) in the hanging wall adjacent to the shear zone reflect shallow crustal levels for this sample since 1205 Ma. These data indicate that the hanging wall away from the shear zone was unroofed from deeper crustal levels faster and much later than the hanging wall adjacent to the shear zone. The varied cooling histories across the region are resolved by listric normal faulting that lead to uniform unroofing of the footwall and differential unroofing across the hanging wall due to rotation during fault displacement.

Crustal-scale shortening and extension across the Grenville Province of western Qu??bec

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Apr 1996

A deep seismic reflection survey shot in 1993 crosses almost the full width of the Grenville Province in western Québec. The seismic transect provides a very clear image of the crust-mantle boundary and the most precise definition to date of the various Grenvillian terranes. The crust is around 44 km thick beneath the Grenville Front but thins rapidly to 36 km some 60 km to the southeast; it is also notable that the greatest crustal thickness of 50 km occurs at the southeast end of the transect, far from the inferred location of the main Grenvillian collision. The Grenville Front zone, in which NW directed thrusting at about 1 Ga was followed by SE directed extension, is defined by discontinuous, SE dipping reflections, which extend down to the Moho. To the southeast, the overlying migmatitic Archean parautochthon, almost half of the transect, is characterised instead by NW dipping reflectors extending into the lower crust. These reflectors are in turn truncated to the southeast by a 12-km-thick zone of intense SE dipping reflections, the Baskatong crustal ramp. The base of the allochthonous terranes (Allochthon Boundary Thrust) is likely located at this ramp, which flattens out at around 30-km depth into the base of the relatively transparent intermediate crust. A highly reflective upper crustal deck corresponding to rocks of the Mont-Laurier terrane was thrust over the Baskatong crustal ramp and is represented further to the northwest by the klippelike Cabonga allochthon. The synformal, transparent Morin anorthosite-charnockite complex belongs to the same upper crustal level. Ramp anticlines and the overriding basal thrust of the upper allochthons demonstrate the NW directed propagation of tectonic transport during the Grenvillian orogeny, involving deformation and displacement along the Baskatong ramp with a relay into the Grenville Front zone. Postaccretional extension appears to have been primarily accommodated along the latter two crustal discontinuities thinning the crust immediately south of the Grenville Front and affecting the crust up to 350 km away from the front.

Subduction of continental crust, the origin of post-orogenic granitoids (and anorthosites?) and the evolution of Fennoscandia

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Jun 2009

Hannes Brueckner

Slices of silica-rich continental crust subducted into the mantle during collision may undergo metamorphism and exhumation towards the surface as coherent high-pressure or ultrahigh-pressure (HP or UHP) terranes or, if stalled in the mantle, melting and return towards the surface as magmas, or a combination of these two processes. Sonic exposed HP or UHP terranes contain anatectic granitoids demonstrating that melting does occur during exhumation. Therefore crust trapped in the mantle should also melt when radioactive heating and/or conductive heating raise temperatures to the appropriate solidus. Terranes with hydrous phases will melt readily through hydrate-breakdown reactions. Terranes lacking hydrous phases may require adiabatic decompression to melt, possibly as heated quartz-rich crust becomes ductile and rises diapirically. The magmas generated will intrude the overlying plate to form late-, post- and possibly anorogenic granitoids, depending oil the time required to reach solidus temperatures. Geochemical characteristics will depend on P-T conditions, the chemistry and mineralogy of the subducted terrane (especially the presence of hydrous phases), and the amount of melt interaction with the mantle. The removal of sialic upper crust may strand the denser malic lower crust, which subsequently could melt to generate anorogenic anorthosites, Fe-K granitoids and related rocks. The evolution of the Fennoscandian Shield documents a change from slab melting in the Mesoproterozoic to combined melting and exhumation in the Neoproterozoic to intact exhumation without significant melting in the Palaeozoic.

Eclogites in the central part of the Sveconorwegian Eastern Segment of the Baltic Shield: Support for an extensive eclogite terrane

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Sep 2005

Mafic boudins found at Viared in the central Sveconorwegian Eastern Segment of the Baltic Shield display both mineralogical and textural features demonstrating that these rocks were once eclogites. The mafic boudins are hornblende-plagioclase amphibolite, but the interiors show evidence of retrogression from eclogite: a grid-like pattern in clinopyroxene grains containing exsolved plagioclase from former omphacite, and garnets commonly surrounded by plagioclase coronas. Geothermobarometry was carried out on two samples and the peak pressure conditions were estimated by re-integrating the original clinopyroxene (Jd23) composition, giving 15.0-16.7 kbar and temperatures of 719 to 811°C. Calculations using the existing retrograde assemblage of clinopyroxene-gamet-plagioclase-quartz give values of 10.5-12.5 kbar and 700-770°C. A 974±3 Ma Re-Os age on molybdenite and 961±26 Ma from titanite represent a minimum age for boudin formation and retrogression. Pre-Sveconorwegian regional migmatization in the granodioritic country rock is represented by well developed CL-dark zircon rims seen in two samples, dated at 1426±18 and 1415±15 Ma. The protolith age of the country rock is 1701±10 Ma from zircon cores. Two related molybdenite samples gave ages at 957±4 and 949±4 Ma respectively, representing either protracted amphibolite facies conditions or a low grade 230-320°C alteration event. The textural and mineralogical features, together with the calculated P-T conditions show that the mafic boudins at Viared were subjected to eclogite facies conditions at ∼50 km depth. The evidence for high-P metamorphism at Viared, together with other known occurrences at Ullared, Skene and Kedum, shows that a significant part of the Eastern Segment was subjected to eclogite conditions.

Emplacement mechanism and thermobarometry of the Sveconorwegian Bohus granite, SW Sweden

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Sep 2003

The 920 Ma, post-kinematic Sveconorwegian (Grenvillian) Bohus granite of south-western Sweden was principally emplaced along a gently easterly dipping weakness zone at mesozonal brittle crustal depth, in an extensional stress regime. The fusion of the protolith gneisses was facilitated by a Sveconorwegian post-orogenic uplift which accelerated the dehydration melting of biotite. The crystalrich magma surges crystallised predominantly as biotite monzogranites, locally rich in secondary muscovite. Biotite + muscovite assemblage became stable in the late magmatic stage, principally in the pegmatitic-aplitic facies. Amphibolite facies para and orthogneiss xenoliths are very frequent in the northern part of the granite massif, where they are interpreted to represent remnants of a giant, brecciated roof pendant. Contact metamorphic transformation of large paragneiss xenoliths, at a specific locality, Tjärnö island, enables garnet-biotite geothermometry and garnet-plagioclase geobarometry studies of physical emplacement conditions during intrusion of the granite. Garnet-bearing Bohus granite samples from four additional localities were also used for the geothermobarometric calculations. Element partitioning in garnet-biotite indicate that the temperature of intrusion was at least 715[ddot]C and the final crystallisation (solidus) temperature 670–680[ddot]C. Garnets in the granite and the granofelsic xenolith are low-calcium almandine-spessartines, in contrast to the generally calcium-rich almandinetype garnets of the metamorphic rocks of SW Sweden. Pressure is determined with the independant approaches of quartz + muscovite + biotite + plagioclase + garnet and garnet-only Gd/Dy geobarometry, supported by evaluation of emplacement features and mineral assemblages. Pressure is found to have been c. 4 kbar or slightly higher during the final crystallisation of the Bohus granite, corresponding to depths of c. 15 km.

Transtension in Arcs and Orogens

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May 2002

John F. Dewey

Transtension is the oblique divergence between bounding plates or blocks that combines a coaxial orthogonal extension with a deformation zone boundary-parallel, noncoaxial, component, which generates a bulk constrictional strain. The coaxial component determines the rate and amount of crustal/lithospheric thinning and part of the horizontal extension. The non-coaxial component controls the vorticity, the horizontal shortening, and part of the horizontal extension. The instantaneous stretching direction bisects the angle between the direction of divergence (transport direction) and the zone boundary orthogonal. In the brittle regime, normal fault arrays accommodate vertical shortening and horizontal extension whereas simultaneous wrench fault arrays allow horizontal shortening. All faults, except those that are vertical and parallel with the zone boundary, rotate either with or against vorticity, depending on their orientation with respect to the transport direction and the zone boundary. Block rotation controls fault slip-direction and sense. Where the transport direction is at greater than 19.5° to the zone boundary, the coaxial component, vertical shortening, normal faults, and horizontal foliation dominate. At angles less than 19.5°, the non-coaxial component, horizontal shortening, wrench faults, and vertical foliation dominate at small strains and for transport directions close to the zone boundary. For larger strains, the deformation path passes through pure prolate constriction and the two shortening axes (Z and Y) swap so that a horizontal foliation may be superposed on an earlier vertical foliation. In the brittle regime, normal-dominant-over-wrench fault systems are superposed on wrench-dominant-over-normal ones. Thus, polyphase deformation sequences may develop as a result of a continuous strain path.In the Late Cambrian/Early Ordovician oceanic island arc of western Newfoundland, dextral transtension generated a vertical NE-striking schistosity in pillow lavas, with a subhorizontal stretch and NW-striking dikes prior to the intrusion of the Twillingate Granite at 510 Ma. Continued dextral transtension led to the development, from 488 to 455 Ma, of one or more supra-subduction zone ophiolite complexes in dextral pull-apart basins, prior to the mid-Ordovician collision of the arc and obduction onto the Laurentian continental margin. In the Eocene Guam segment of the Marianas Arc, sinistral transtension generated a boninitic dike complex with orthogonal folds.In the Western Gneiss Region of southwest Norway, Late Silurian/Early Devonian sinistral transtension generated constrictional fabrics in gneisses exhumed beneath west-slipping extensional detachments carrying growing extensional basins that were experiencing simultaneous N-S shortening. In the Basin and Range, between the Lake Tahoe-Las Vegas seismic zone and the Garlock Fault, high-strain-rate dextral transtension between the Sierra Nevada and the Colorado Plateau resulted in bulk constriction with folded, "turtleback" detachments and a regionally low average elevation.

UPb dating of the post-kinematic Sveconorwegian (Grenvillian) Bohus granite, SW Sweden: evidence of restitic zircon

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Jun 1991
PRECAMBRIAN RES

U-Pb zircon isotopic studies of a representative granite type and a pegmatite-aplite belonging to the post-kinematic Sveconorwegian monzogranitic Bohus granite show that a part of the zircon material has preserved isotopic characteristics of the protolith and consequently yields erroneous ages. However, monazite and xenotime from the aplite-pegmatite yield almost concordant ages, at 919±5 Ma and 922±5 Ma, respectively, which is interpreted to be the crystallisation age of the major portion of the Bohus granite.The present investigation advocates, owing to the preservation of the protolith signature in the zircon U-Pb data, that restite unmixing is one mechanism which contributes to the chemical variation of the weakly peraluminous Bohus granite.The geochemical and petrographic characteristics of the Bohus granite indicate that the different magma surges constituted anatectic, crust-derived, water-undersaturated liquids with varying amounts of suspended crystals. Crystal fractionation and restite unmixing from this crystal mush, together with differential partial melting, were the principal magmamodifying mechanisms during the generation of the Bohus granite massif.

Grenvillian extensional tectonics in northwest Scotland: Reply

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Jan 1997
GEOLOGY

New structural data indicate that eclogite-bearing gneisses in the Glenelg-Attadale inlier of the Caledonian Moine thrust nappe, northwest Scotland, were involved in a major pre-Caledonian system of extensional top-to-the-east ductile shear zones. These shear zones coalesce upward to constitute a major extensional detachment below metasedimentary rocks of the Moine Supergroup. Both the eclogite-bearing gneisses and the Moinian metasedimentary rocks display evidence of extensional deformation, but the latter contain structures indicative of a lower intensity of bulk finite shear strain and may have experienced only the later of two discrete phases of extension that we recognize in the inlier. Coplanar Caledonian brittle-ductile, top-to-the-west thrusting has resulted in only localized and limited reactivation. Timing of the extensional deformation is bracketed by a published Sm/Nd age of 1.08 Ga for eclogite equilibration and by a minimum age of deposition for the Moine Supergroup of 840 Ma. We propose that the eclogite-bearing lower crust was exhumed as a result of collapse of the Grenville Orogen, and that the evolution of the Moinian basin was controlled by this extensional event.

Sveconorwegian extension-parallel deformation structures; an example from the Hammaro Shear Zone, SW Sweden

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Jun 1997

On the northern shore of Lake Vänern, southern Värmland, southwestern Sweden, various orthogneisses are affected by a major, steeply dipping shear zone, the Hammarö Shear Zone. It is several kilometres wide and comprises a network of individual shear zones (less than 100 m wide), which strike approximately E-W across the Eastern Segment of the Southwest Scandinavian Domain in the Fennoscandian Shield, from the major Mylonite Zone in the west to the northeastern shore of Lake Vänern. Three fold phases, extensive dynamic and static recrystallisation, and a conspicuous, gently plunging lineation are the main features of the Hammarö Shear Zone. Early deformation occurred under amphibolite-facies conditions, but narrow zones with a green-schist-facies assemblage demonstrate a lower grade of metamorphism during the late stages. Orthogneisses are variably overprinted by the shear deformation. No indisputable supracrustal rocks are found adjacent to the zone, and within the zone there is an extensive tectonic overprinting on primary and pre-shear zone structures. The shear zone is intimately related to large-scale regional folding (F4) which also occurs south of Lake Vänern. The steep mylonitic foliation and the horizontal stretching lineation are interpreted to be the result of the final stages of horizontal E-W extension of the crust during the Mesoproterozoic D4 event.

Nature and distribution of deep crustal reservoirs in the southwestern part of the Baltic Shield: Evidence for Nd, Sr and Pb isotope data on Late Sveconorwegian

Article

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Mar 2001

Late Sveconorwegian ('postorogenic') granites (c. 1.0-0.93 Ga) make up a voluminous and widespread suite of intrusions across south Norway. From radiogenic isotope data, three groups of late Sveconorwegian granites can be distinguished: (1) granite with more than 150 ppm Sr, 87Rb/ 86Sr<5, 87Sr/ 86Sr 0.93Ga<0.710 and ε Nd<0; (2) granite with less than 150 ppm Sr, 87Rb/ 86Sr>5, 87Sr/ 86Sr 0.93Ga>0.710 and ε Nd<0; (3) juvenile granite with 87Sr/ 86Sr 0.93Ga<0.705 and ε Nd>0. Granite plutons belonging to Group 1 ('normal-Sr concentration granite') occur all over south Norway and include the largest batholiths (Øtfold, Flå, Herefoss). Granite plutons of Group 2 ('low-Sr concentration granites') are restricted to north-central Telemark (Rjukan rift), and are associated with c. 1.5 Ga Rjukan Group rhyolite. Group 3 is represented by one intrusion only, but still suggests input of mantle-derived magma, or the presence of young, mantle derived rocks in the deep crust in the region at c. 0.93 Ga. The Group 1 granites are similar in Sr and Nd characteristics to some of the older (1.05 Ga) Sveconorwegian granitic intrusions ('augen gneisses') in the region in terms of radiogenic isotope systematics, but Pb isotopes suggest that the magmas did not form by simple remelting of augen gneiss. The Nd, Sr, and Pb isotopic systematics of the late Sveconorwegian granites indicate mixing between a depleted-mantle derived component and two or more major components with an extended crustal history. One of the crustal end members is present throughout south Norway and has an isotopic signature similar to older granitic rocks of the Trans Scandinavian Igneous Belt (TIB). The other crustal end member is indistinguishable from Rjukan Group rhyolites and slightly younger intrusions associated with these, and is restricted to areas within the mid-Proterozoic Rjukan rift. The available data suggest that the deep continental crust of south Norway, both east and west of the Permian Oslo Rift, is an integral part of the Baltic Shield, with a common history back to 1.7-1.9 Ga, that is, to the end of the Svecofennian orogeny and the TIB magmatism.

Derivation of the 1.0 - 0.9 Ga ferro-potassic A-type granitoids of S Norway by extreme differentiation from basic magmas

Article

Full-text available

Jul 2003
PRECAMBRIAN RES

Major and trace elements, Sr and Nd isotopic data as well as mineral compositions are presented for a selection of the 1.0–0.9 ferro-potassic A-type granitoids (Bessefjellet, Rustfjellet, Verhuskjerringi, Valle, Holum, Svöfjell, Handeland-Tveit, Åseral, Lyngdal gabbronorites) that occur close to the Mandal-Ustaoset Line (MUL) of southern Norway. These hornblende biotite granitoids (HBG) define an extensive differentiation trend ranging from gabbronorites (50wt.% SiO2) to granites (77wt.% SiO2). This trend is interpreted as resulting from extreme fractional crystallization of several basaltic magma batches with similar major and trace elements compositions. At 930Ma, the HBG suite displays a narrower range in ISr (0.7027–0.7056) than in εNd(t) (+1.97 down to −4.90) suggesting some assimilation of a Rb-depleted lower crust (AFC process) or/and source variability. An age of 929±47Ma is given by a Rb-Sr isochron on the Holum granite (Sri=0.7046±0.0006, MSWD=1.7). Geothermobarometers indicate a low pressure of emplacement (1.3–2.7kbar) and an oxygen fugacity close to NNO. High liquidus temperatures are given by the apatite saturation thermometer (1005–1054°C) and are in agreement with results from other studies. The basaltic parent magmas of the HBG suite are partial melts of an hydrous mafic, potassic source lying either in the lithospheric upper mantle or in the mafic lower crust derived from it. This contrasts with the 930Ma anorthosite–mangerite–charnockite suite (AMC suite) of the Rogaland Province for which a depleted lower crustal anhydrous gabbronoritic source has been indicated. The present data imply the penecontemporaneous melting of two contrasting sources in southern Norway. The source duality could result from an increasing degree of metamorphism (amphibolite to granulite) from East to West, an horizontal stratification of the lower crust or from the stratification of the lithosphere (melting of the lower crust or upper mantle). It may also indicate that the AMC and HBG suites formed in two distinct crustal segments. The linear alignment of the HBG suite along the Mandal-Ustaoset shear zone suggests that a linear uprise of the asthenosphere, following a lithospheric delamination under this structure, could be the vector of the mantle heat.

Tectonic setting of post‐orogenic granites within SW Fennoscandia based on deep seismic and gravity data

Article

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Jul 2007
TERRA NOVA

Within the Svecononvegian Province of SW Scandinavia granite intrusions are abundant. Prominent ones are the Bohus-Iddefjord and the Flå granites, the so- called Bohus-Flå Granite belt. The age of these granites, consistent at ≅ 900 Ma, coincides with the late extensional stage of the Svecononvegian-Grenvillian orogen. Gravity observations and deep seismic profiling lines are presented that also cover the Skagerrak Sea. The geophysical data suggest that the Bohus granite continues seaward for at least 80 km and its thickness, offshore, is estimated to be 2–4 km. Where the seismic profiling lines intersect the inferred seaward extension of the Bohus granite, a distinct reflection pattern is observed at ≅ 1.6 s. TWT. This coincides with a gravity modelled thickness of ≅ 4 km. The seismic profiles also show a large Moho offset beneath the modelled granite. It is proposed that this offset is related to Svecononvegian crustal underthrusting and that the granite melt could have formed by anatexis of mid-crustal rocks downthrusted to greater depths in the vicinity of the seismically observed Moho offset.

Beskrivning till provisoriska översiktliga berggrundskartan Borås

Article

Jan 1988

Irradiation of samples for 40Ar/39Ar dating using the Geological Survey TRIGA reactor

Article

Jan 1981

The data from 45 irradiations indicate that most samples require 10 to 40 MWH in the central thimble. The reactor is suitable for dating when the effects of flux gradients and interfering Ar isotopes derived from Ca and K are minimized.-K.A.R.

Geochronology of high-grade metamorphism in the Sveconorwegian belt, S. Norway: U-Pb, Th-Pb and Re-Os data

Article

Jan 2008
NORW J GEOL

The timing of Sveconorwegian bigh-grade metamorphism is evaluated in the four lithotectonic units/terranes exposed in the Sveconorwegian belt in South Norway. U-Pb, Th-Pb and Re-Os data were obtained from 21 samples of Mesoproterozoic ortho- and paragneisses. In the Bamble Terrane, SIMS monazite U-Pb data constrain peak amphibolite- to granulite-facies metamorphism between 1137 ±1 and 1127 ±6 Ma, and unroofing at 1107 ±9 Ma. In the Kongsherg Terrane, a molybdenite Re-Os date at 1112 ±4 Ma and a monazite U-Pb date at 1092 ±1 Ma bracket amphibolite-facies metamorphism. In the Idefjorden Terrane, west of the Oslo rift, a pulse of Gothian metamorphism at 1539 ±8 Ma and three pulses of Sveconorwegian metamorphism at 1091 ±18 Ma, 1052 ±4 to 1043 ±8 Ma and 1024 ±9 Ma are recorded by monazite, zircon and titanite data. Amphibolite-facies metamorphism peaked at 1052 ±4 Main the kyanite field (1.00-1.17 GPa, 688-780 °C). At the boundary between the Idefjorden and Telemarkia Terranes, SIMS zircon U-Pb data constrain amphibolite-facies metamorphism between 1012 ±7 and 1008 ±14 Ma and provide a maximum age for shearing along the Vardefjell Shear Zone between these two terranes. Amphibolite-facies metamorphism in the Telemarkia hanging wall of the Vardefjell Shear Zone is coeval at 1014 ±1 Ma. In the Suldal Sector of the Telemarkia Terrane, monazite data yield an age of 1005 ±7 Ma for amphibolite-facies migmatitization. In the Rogaland-Vest Agder Sector of the Telemarkia Terrane, paired U-Pb and Th-Pb ID-TIMS monazite analyses date monazite growth to have been between 1013 ±5 and 980 ±5 Ma during M1 intermediate-pressure regional metamorphism, and between 927 ±5 and 922 ±5 Ma. during M2 low-pressure high- to ultrahigh-temperature metamorphism. SIMS monazite dates at 1032 ±5 and 990 ±8 Ma in a granulite, situated outside the area affected by M2 metamorphism, demonstrate granulite-facies conditions during M1 metamorphism. Available data show that the four terranes in South Norway have distinct metamorphic histories. The Bamble and Kongsberg Terranes show evidence for early-Sveconorwegian metamorphism between 1140 and 1080 Ma. The Idefjorden Terrane, though locally affected by this early metamorphism, was mainly reworked by medium to high-pressure metamorphism between 1050 and 1025 Ma. The Telemarkia Terrane was reworked later between c. 1035 and 970 Ma. A high-temperature metamorphic phase at 930-920 Ma. is specific for the Telemarkia Terrane and is related to post-collisional intrusion of the Rogaland anorthosite-mangerite-charnockite (AMC) Complex.

A four-phase model for the Sveconorwegian orogeny, SW Scandinavia

Article

Jan 2008
NORW J GEOL

The Sveconorwegian orogenic belt resulted from collision between Fennoscandia and another major plate, possibly Amazonia, at the end of the Mesoproterozoic. The belt divides, from east to west, into a Paleoproterozoic Eastern Segment, and four mainly Mesoproterozoic terranes transported relative to Fennsocandia. These are the Idefjorden, Kongsberg, Bamble and Telemarkia Terranes. The Eastern Segment is lithologically related to the Transcandinavian Igneous Belt (TIB), in the Fennoscandian foreland of the belt. The terranes are possibly endemic to Fennoscandia, though an exotic origin for the Telemarkia Terrane is possible. A review of existing geological and geochronological data supports a four-phase Sveconorwegian assembly of these lithotectonic units. (1) At 1140-1080 Ma, the Arendal phase represents the collision between the ldefjorden and Telemarkia Terranes, which produced the Bamble and Kongsberg tectonic wedges. This phase involved closure of an oceanic basin, possibly marginal to Fennoscandia, accretion of a volcanic arc, high-grade metamorphism and deformation in the Bamble and Kongsberg Terranes peaking in granulite-facies conditions at 1140-1125 Ma, and thrusting of the Bamble Terrane onto the Telemarkia Terrane probably at c. 1090-1080 Ma. (2) At 1050-980 Ma, the Agder phase corresponds to the main Sveconorwegian oblique (?) continent-continent collision. It resulted in underthrusing and burial of the Idefjorden Terrane to high-pressure conditions at 1050 Ma, followed by exhumation. Crustal thickening in the Telemarkia Terrane led to protracted granite magmatism starting at 1050 Ma and to high-grade metamorphism starting at 1035 Ma. Metamorphism peaked in granulite-facies conditions in the Rogaland-Vest Agder Sector. (3) At 980-970 Ma, the Falkenberg phase reflects final convergence in the belt, shortly followed by divergence. Foreland propagation of the orogeny is indicated by underthrusting of the Eastern Segment to eclogites facies conditions at c. 970 Ma. (4) Between 970 and 900 Ma, the Dalane phase corresponds to gravitational collapse of the belt. It is associated with post-collisional magmatism increasing in volume westwards, exhumation of the southern part of the Eastern Segment as a core complex, and exhumation of the Rogaland-Vest Agder sector in the Telemarkia Terrane as a wide gneiss dome. Formation of the gneiss dome peaked at 930-920 Ma with low-pressure high-temperature granulite-facies metamorphism and intrusion of an anorthosite-mangerite-charnokite (AMC) complex.

Was Baltica right-way-up or upside-down in the Neoproterozoic?

Article

Sep 2006

Baltica is a progeny of Rodinia, born from the breakup of the supercontinent in the Neoproterozoic. Within Rodinia, Baltica is generally placed adjacent to NE Laurentia but in a variety of configurations, which vary by up to 3000 km along the strike of the Laurentian margin and include both right-way-up and upside-down orientations (current coordinates). Geological and palaeomagnetic data show that the only viable reconstruction juxtaposes the western Scandinavian margin of Baltica, in its right-way-up orientation, against the Rockall-Scotland-SE Greenland segment of Laurentia.

Left-lateral transpressive deformation and its tectonic implications, Sveconorwegian Orogen, Baltic Shield, southwestern Sweden

Article

Sep 1996
PRECAMBRIAN RES

The Mylonite Zone (MZ) is a major, ductile deformation zone in the Sveconorwegian orogen (Baltic Shield) of southwestern Sweden and southeastern Norway which has a strike length of over 400 km and an across-strike width which often exceeds 5 km. It is an orogen-parallel deformation zone which formed under retrogressive metamorphic conditions relative to the higher-grade structures in the surrounding crustal units. The MZ marks a conspicuous metamorphic break in the area south of lake Vänern and a distinct lithological break in the area north of this lake. Regional metamorphic considerations suggest that its surface exposure represents an oblique section through the crust with deeper levels exposed along the southern parts of the zone and shallower levels exposed farther north.

New tectonic divisions of the Grenville Province, Southeast Canadian Shield

Article

Feb 1989

On the basis of geological, geophysical, and geochronological data, the Grenville Province has been divided into three first-order longitudinal belts, the Parautochthonous Belt (PB), Allochthonous Polycyclic Belt (APB), and Allochthonous Monocyclic Belt (AMB). These are set apart by three first-order tectonic boundaries, the Grenville Front (GF), Allochthon Boundary Thrust (ABT), and Monocyclic Belt Boundary Zone (MBBZ). The belts are subdivided into terranes based on internal lithological character. The GF separates the Archean to Proterozoic foreland northwest of the orogen from reworked equivalents to the southeast. Continuous at the scale of the orogen, its main characteristic is that of a crustal-scale contraction fault. The PB, although less clearly identified along the length of the orogen, in most places represents upgraded and tectonically reworked rocks of the adjacent foreland. The boundary between the PB and the APB to the southeast, the ABT, is most clearly delineated in the eastern half of the province. It is the locus of major crustal delamination along which high-grade, mostly middle Proterozoic, polycyclic terranes were tectonically transported northwest toward and onto the PB. The AMB comprises two separate areas underlain by the Wakeham Supergroup and what is currently known as the Grenville Supergroup, respectively; its basal contact, the MBBZ, is a décollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks. This first-order zonation implies a tectonic polarity to the Grenville Province, superimposed on which are second-order features evident from contrasting tectonic styles and radiometric ages. These characteristics are consistent with a diachronous or oblique collisional model for the Grenville orogen.

Steiger, R. H. & Jaeger, E. Subcommission on geochronology: convention of the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359-362

Article

Oct 1977
EARTH PLANET SC LETT

Brittle tectonic evolution and paleostress field reconstruction in the southwestern part of the Fennoscandian Shield, Forsmark, Sweden

Article

Aug 2011

Six hundred fault slip data have provided robust paleostress fields within an approximately 35 km3 volume of Paleoproterozoic (1.9 Ga) rocks in the southwestern Fennoscandian Shield, Forsmark, Sweden. These rocks were affected by penetrative ductile strain from 1.87 to 1.86 Ga, folding, ductile strain along discrete zones around 1.8 Ga, and semibrittle or brittle deformation around and after 1.8 Ga. Compatible paleostress fields have been identified using site-by-site and merged data sets from outcrops and oriented drill cores. Transpressive deformation with a regional NNW-SSE 1 axis, associated with clockwise stress deviation inside a tectonic lens, resulted in dextral slip along regionally significant, steep WNW-ESE and NW-SE deformation zones. The semibrittle and most of the brittle structures, including specifically the epidote-bearing fractures, were established during this oldest regime around 1.8 Ga (latest Svecokarelian). A younger paleostress field with a NE-SW 1 axis, which was also transpressive in character, is inferred to have been active at 1.7-1.6 Ga. The best defined paleostress field is transpressive in character with a WNW-ESE 1 axis that resulted in sinistral reactivation along the WNW-ESE and NW-SE zones. The main set of laumontite-stepped faults developed at this stage at 1.1-0.9 Ga (Sveconorwegian). It is impossible to exclude fully the influence of reactivation during even younger Phanerozoic tectonic events. Subordinate extensional paleostress fields were related either to the latest Svecokarelian and Sveconorwegian transpressive regimes, due to 1 and 2 stress permutations, or to regional extensional tectonic regimes during the Meso- or Neoproterozoic or later during the Permo-Carboniferous and/or Mesozoic.

Precise U-Pb ages for Grenvillian and pre-Grenvillian thrusting of Proterozoic and Archean metamorphic assemblages in the Grenville Front tectonic zone, Canada

Article

Aug 1994

T.E. Krogh

While the position of the Grenville Front is placed at a structural feature associated with the onset of high-grade rocks when approached from the north, the time of first metamorphism in these rocks is, in general, pre-Grenvillian and highly variable. U-Pb isotopic results indicate that the Grenville Front zone is characterized along its length by partially reset U-Pb systems, high-pressure assemblages, local melting, and rapid cooling. Crustal loading and shearing, with lead loss related to mechanically induced recrystallization and local heat that was rapidly terminated by rebound and cooling, are required. -from Author

40Ar/39Ar geochronological constraints on the tectonothermal evolution of the Eastern Segment of the Sveconorwegian Orogen, south-central Sweden

Article

Jul 1996

A 40Ar/39Ar study to constrain the tectonothermal evolution across the Eastern Segment of the Sveconorwegian Orogen has been initiated in the area north and east of lake Vänern, south-central Sweden. This segment of the orogen is confined by two major deformation zones, the Sveconorwegian Frontal Deformation Zone (SFDZ) in the east and the Mylonite Zone in the west. Previous structural work and the prograde character of the metamorphism within the study area suggest that an older (< c. 1.57 Ga), regional foliation was formed by ductile shear deformation in a compressional tectonic regime. The orientation of this foliation was subsequently modified by later rotation along younger ductile shear zones in the easternmost, frontal part of the orogen (SFDZ). The 40Ar/39Ar ages for hornblende suggest that the regional foliation is Sveconorwegian. Furthermore, white mica ages demonstrate that the Sveconorwegian tectonothermal overprint continues at least 40 km east of the traditionally accepted limit situated along the 'Protogine Zone'. These results also provide age constraints for different phases of Sveconorwegian tectonothermal evolution with an older group of ages from 1009-965 Ma and a younger set from 930-905 Ma. The older ages are inferred to constrain a minimum age for crustal thickening during which the regional foliation and metamorphism developed, while the younger are associated with later compressional movement along the SFDZ.

Extensional collapse of thickened continental lithosphere: A working hypothesis for the Alboran Sea and Gibraltar arc

Article

Jan 1989
GEOLOGY

Several features of the Alboran Sea suggest that it may have been a high collisional ridge in Paleogene time that subsequently underwent extensional-collapse, driving radial thrusting around the Gibraltar arc. (1) The basin is underlain by thin (13-20 km) continental crust, has an east-west-trending horst and graben morphology, was the locus of Neogene volcanism, and has subsided 2-4 km since the middle Miocene. (2) Extension and subsidence in the basin coincided in time with outwardly directed thrusting in the surrounding mountain chains. (3) Africa and Europe were converging slowly during this period, so extension must have been driven by internally generated forces. (4) Onshore, rocks metamorphosed at 40 km depth are exposed beneath major low-angle normal faults that separate them from low-grade rocks above. (5) Emplacement of solid bodies of Iherzolite at asthenospheric temperature into the base of the collisional edifice in late Oligocene time suggests detachment of the lithospheric root beneath the collision zone. This would have increased the surface elevation and the potential energy of the system and would have favored extensional collapse of the ridge.

Is there evidence for magmatic underplating beneath the Oslo Rift?: Magmatic underplating beneath the Oslo Rift?

Article

Apr 2005

Abstract We challenge some of the long-standing beliefs related to the Permian Oslo Rift structure, often referred to as a case example/type locality for continental rifting. The crustal structure of the Oslo Rift was long presumed to be thinned Proterozoic crust overlying a Permian high-density layer, interpreted as magmatic underplating. New data support an alternative view of the crustal structure in the Oslo Rift region. The Bouguer gravity high in the region shows a strong asymmetry: a steep, westward-facing gradient to the west of the rift, and a much gentler eastern gradient. We present a 3D density model based on petrophysical and seismic information, which accounts for the Bouguer gravity high using an eastward extension of old Precambrian structures, without invoking a prominent magmatic underplated structure. Reactivation of old pre-rift structures appears to be an important feature, affecting the evolution and location of the Permo-Carboniferous Oslo Rift. Terra Nova, 17, 129–134, 2005

Geology of the Protogine Zone south of Lake Vattern, southern Sweden: A reinterpretation

Article

Sep 1992

Influential models describe the Protogine Zone (PZ) as a collisional suture and thrust front and, in particular, as the extension of the Grenvillian thrust front into Scandinavia. These models do not consider the significance of a rich variety of geological features related to Late Riphean-Vendian rifting of the craton. This paper calls attention to steep, northerly trending and anastomosing, narrow deformation zones of ductile or brittle-ductile character. The zones can be mapped out into the Vättern Graben of c. 700–800 Ma age. Uplift of southwestern Sweden after c. 910 Ma must have had a profound tectonic influence on the bedrock along the PZ. The effects of this late deformation must be identified and subtracted from the structural record before the significance of Sveconorwegian-Grenvillian deformation can be assessed. There is no other foliation south of Lake Vättern qualifying for the name “protogine zone foliation”. Rift-related magmatic and metallogenic features are remarkably restricted to the zone. In fact, most of the features along the southern part of the PZ traditionally ascribed to collisional tectonics may instead be extensional in nature. For instance, an asymmetrical rift with footwall rebound and unroofing could account for the normal and reverse faulting of the PZ and the contrasting metamorphism and structures of SW and SE Sweden.

UPb age of titanite in the Mylonite Zone, southwestern Sweden

Article

Mar 1993

U-Pb dating of titanite from a mafic lens in the Mylonite Zone in southwestern Sweden yields an almost concordant age of c.920 Ma. The titanite formed during retrogression of granulite- to amphibolite-facies rocks. The obtained U-Pb age of titanite is close to the Sm-Nd isochron ages of garnet-bearing granulite-facies mineral assemblages in the Varberg area just south of the Mylonite Zone. Since the blocking temperature is higher for Nd diffusion in garnet than for the Pb diffusion in titanite it is suggested that the rocks of the Mylonite Zone were undergoing amphibolite-facies deformation, uplift and cooling while the granulites of the Varberg area were still at a considerably higher temperature. -from Authors

The Precambrian development of southern Sweden

Article

Oct 1980

Roland Gorbatschev

The Precambrian of southern Sweden represents a gradual accretion of the Baltic Shield. The Svecofennian region in the east was stabilized by about 1750 Ma. In the west, an orogenic event between ca. 1600 and 1700 Ma added new continental crust to the shield, possibly also reworking a marginal segment of the Svecofennian. A continuous belt of granites and porphyries marks the boundary of the south-western orogen with the stable Svecofennian region. The subsequent development of southern Sweden was essentially ensialic, periods of granite intrusion and metamorphism interrupting anorogenic developments. In a final culmination of activity around 900–1100 Ma, southern Sweden played the role of a crustal segment marginal to the Grenvillian orogen.

Den centralvarmlandska mylonitzonen och dess fortsattning i Norge

Article

Jan 1937

Nils Magnusson

Geochronology of eclogite facies metamorphism in SW Sweden

Article

Mar 2001
PRECAMBRIAN RES

Decompressed eclogites in the Sveconorwegian Province, SW Sweden, have been dated using U-Pb geochronology. Zircons are common as inclusions in garnet and kyanite, and other minerals in the decompressed eclogites. Titanite inclusions are found exclusively in the core of garnets. The mineral inclusions and the chemical zoning of the garnets suggest inital growth under prograde amphibolite facies conditions followed by eclogite facies metamorphism and subsequent decompression through the high-pressure granulite and upper amphibolite facies. The zircon and titanite thus formed prior to the eclogite stage of the P-T path. The age of the eclogite forming event was determined by ion probe dating of zircon inclusions in garnets. The obtained age of 972±14 Ma is the maximum age of the eclogitisation. The age of the titanite inclusions in garnet is 945±4 Ma. This age is similar to other U-Pb ages of titanite in the region which suggest that the titanite has been isotopically reset and that the age reflects cooling.The mode of occurrence, textural relationships and the chemical homogeneity suggest that the zircons formed from Zr released from magmatitic Fe–Ti oxides and possibly amphiboles during breakdown of magmatic minerals at the onset of the Sveconorwegian metamorphism.Spot analyses of complex zircons from a granitic dyke in the eclogite yielded an age of 1403±15 Ma for magmatic cores and an age of 963±22 Ma for metamorphic rims. The older age is a minimum age of the eclogite protolith and correspond to the age of a generation of granites in the region. The rim age is within error identical to the age of eclogite metamorphism. The eclogite metamorphism in SW Sweden is younger than its Grenvillian counterparts in Scotland, Canada and USA.

Emplacement of polygeneration pegmatites in relation to Sveco-Norwegian contractional tectonics: Examples from southern Norway

Article

Aug 2004
PRECAMBRIAN RES

Recent studies have demonstrated that there is a strong link between sites of concentrated magmatism and crustal deformation zones. Magmatism is often described as being syn-kinematic where one or more increments of intrusion punctuate deformation with successive generations of injections being progressively deformed. Pegmatite formation in the Mesoproterozoic of south Norway has been considered as post-kinematic in nature relative to Sveconorwegian (Grenvillian) deformation (1.13Ga to ∼0.85Ga) during accretion of the SW margin of Baltica. This paper presents structural observations from mapping of some pegmatites in the Bamble Terrane of south Norway demonstrating that the pegmatites are structurally related to Sveconorwegian fold geometries associated with peak metamorphism at approximately 1.14Ga and are kinematically related to overthrust geometries associated with the initial overthrusting phase of the Porsgrunn-Kristiansand Fault when the Bamble Terrane docked with the underlying Telemark Terrane. Local fold geometries suggest a strain regime where the maximum contractional axis is sub-horizontal and oriented NW-SE consistent with the regional kinematics. The pegmatites display a systematic deformation pattern consistent with deformation in the isoclinal fold limbs. Pegmatites are also deformed into asymmetric antiformal folds above reverse-shear structures and are also cut by these structures thus showing a thrust geometry. The undeformed pegmatites are emplaced in sub-horizontal fractures and suggests sub-horizontal contractional axis related to overthrusting which is identical to the strain regime of the folding. The evidence suggests that the pegmatites are syn-tectonic and were injected into reverse shear-related fractures. Deformation was progressive and incremental with longer periods of ductile deformation at low strain rate punctuated by shorter periods of fracturing and pegmatite injection at high strain rate. This work therefore implies an intimate spatial and temporal relationship between deformation and magmatism during crustal accretion on the western margin of Fennoscandia.

Contrasting magmatic arcs in the Palaeoproterozoic of the south-western Baltic Shield

Article

Nov 1998
PRECAMBRIAN RES

Following the Svecofennian arc accretionary growth and extensive granitoid magmatism in the Transscandinavian Igneous Belt (TIB), new crustal growth occurred west of the TIB to form the Gothian orogen. An early stage is manifested by 1.69–1.65Ga subduction-related magmatism in the Ätran Terrane. A second stage, forming an 140km wide segment east of the Permian Oslo Rift, is recorded by three 1.66–1.59Ga metamorphosed volcano-sedimentary units, exposed in the Horred, Åmål and Stora Le-Marstrand formations in the Idefjorden Terrane. The 1.66Ga Horred Formation is dominated by felsic volcanics and has geochemical signatures indicative of formation in an island arc setting. In contrast, the lithologically similar volcanic sequences in the 1.61Ga Åmål Formation have geochemical signatures consistent with a continental-margin setting. The 1.60–1.59Ga Stora Le-Marstrand Formation is dominated by greywacke-type metasediments with subordinate metabasalts. These volcanics have markedly primitive trace element signatures and depleted Nd isotopic compositions, all consistent with their derivation in an oceanic island arc setting. The sediments document two provenances: a distal continental source and one with Nd isotopic compositions similar to the SLM volcanism. Many of the metasediments in the Stora Le-Marstrand formation have chemical signatures consistent with derivation from continental crust, suggesting that this volcanic arc developed in the vicinity of a continental massif, possibly in a setting similar to the Philippine Sea. Accretion of the Horred and Stora Le-Marstrand arc systems occurred prior to 1.61 and 1.59Ga, respectively, and was followed by voluminous, ca 1.59Ga calc-alkaline magmatism.

P-wave crustal structure of the Lake Vänern area, Sweden: EUGENO-S profile 6

Article

Aug 1988
TECTONOPHYSICS

To investigate the deep structure of the Precambrian basement, airgun shots in Lake Vänern, southern Sweden, were recorded on land by two 100 km lines of MARS stations to the south, two Geostore stations east and west of the lake and a 7 km multichannel reflection array on the Värmlandsnäs in the centre of the lake. Modelling of two profiles running roughly N-S, parallel to the major tectonic grain and on either side of the Mylonite Zone, shows variation in crustal seismic velocity structure from east to west. Comparison of these models with near vertical incidence stacked sections from the Värmlandsnäs experiment and with an E-W profile suggests continuity of reflecting structures in a N-S direction. This E-W profile shows shallowly west dipping, strongly reflective structures, probably associated with the Mylonite Zone and also a small-scale Moho structure, tentatively interpreted as the western limit of a wedge of overriden older Precambrian crust.

Geochronology and Thermochronology by the *???Ar/39Ar Method

Article

Jan 1999

This monograph has been produced for the benefit of the practising Earth scientist or student wishing to understand more fully the advantages and limitations of this method. A historical introduction is followed by a description of the basic principles of the method. A section covering technical aspects of the experimental method is then included. The interpretation of results is discussed, followed by a description of diffusion theory, experiments and thermochronology. The final chapter deals with applications and case studies. A bibliographic reference list and index are included. -A.W.Hall

Proterozoic crustal evolution in southcentral Fennoscandia

Article

Karin Appelquist

Article

Sep 2000

Results of whole-rock chemistry, mineral chemistry, and Nd and Sr isotope studies are reported on a newly identified dyke swarm in the Bamble sector, south Norway. The Blengsvatn dyke swarm consists of alkalic gabbros to monzogabbros and monzodiorites. Hornblende is the dominant mafic mineral in the alkalic gabbro to monzogabbro dykes. These dykes have Mg# between 47 and 62, Cr 90-132 ppm, Ni 43-222 ppm, Ba 56-151 ppm, and ΣREE between 45 and 116 ppm. LaCN/YBCN ratios vary from 1.5 to 3.7. With the exception of one sample, the dykes are both olivine and nepheline normative, some contain slightly K-deficient relict biotite phenocrysts. The dominant mafic mineral in the monzodiorite dykes is biotite. These dykes have Mg# between 25 and 43, Cr and Ni ≤10 ppm, and P2O5 (0.69-1.68 wt.%) and high Ba (>2100 ppm). They have ΣREE of about 755 ppm and LaCN/ YbCN ratios of about 10. The dykes are olivine normative. Both types of dykes together yield a Sm-Nd whole-rock regression line of 1.47±0.07 Ga (MSWD 8.2), with an initial εNd of +3.5. The alkalic gabbro to monzogabbro dykes alone yield a Sm-Nd regression line of 1.57±0.17 Ga (MSWD 5.8, εNd +3.9). The dykes are the first reported evidence of magmatic activity in this part of the Baltic Shield during this period (c. 1.5 Ga), and are interpreted as being the products of a phase of incipient rifting during the period prior to the Sveconorwegian orogeny. The Rb-Sr isotopic system has been disturbed, with data scattering around an 1.1 Ga reference line. As the Blengsvatn dykes intruded the Blengsvatn gabbro and supracrustals of the Nidelva quartzite complex, they provide independent evidence for the occurrence of a suite of pre-Sveconorwegian ('Gothian') gabbros, and demonstrate that the Nidelva quartzite complex was deposited prior to c. 1.5 Ga.

The Gothian and Labradorian orogens: Variations in accretionary tectonism along a late Paleoproterozoic Laurentia-Baltica margin

Article

Jun 1997

Intermittent crustal growth characterised late Paleoproterozoic development in western Baltica during Gothian orogenesis, and in eastern Laurentia during Labradorian orogenesis. Both regions are inferred to have belonged to the same margin of a supercontinent, but they do not show identical tectonic histories. Long-lived convergent margin activity associated with successive, oceanward migrating stages of subduction characterized western Baltica during the late Paleoproterozoic, in contrast to the development of a pre-Labradorian, ca. 1.71 Ga sedimentary depocentre close to the margin of pre-Labradorian Laurentia that gave way to Labradorian 1.68-1.65 Ga calc-alkaline magmatism associated with subduction away from cratonic Laurentia. Continued Gothian, ca. 1.62-1.58 Ga continental-margin calc-alkaline magmatism and arc accretion has no recognized counterpart in eastern Laurentia, where collision of the outboard microcontinents/arcs resulted in voluminous granitoid magmatism caused by crustal thickening. Subduction either ceased at 1.65 Ga or northward subduction was initiated much farther south. The caveat to all interpretations is that some of the apparent differences may reflect inadequate geochronological databases of western Baltica and southeasternmost Laurentia.

Growth and collapse of a deeply eroded orogen: Insights from structural, geophysical, and geochronological constraints on the Pan-African evolution of NE Mozambique

Article

Oct 2008

This paper presents results of a large multidiciplinary geological mapping project in NE Mozambique, with a focus on the structural evolution of this part of the East African Orogen (EAO). It integrates field structural studies with geophysical interpretations and presents new geochronological data. The tectonic architecture of NE Mozambique can be subdivided into five megatectonic units on the basis of lithology, structure and geochronology: unit 1, Paleoproterozoic Ponta Messuli Complex in the extreme NW corner of NE Mozambique, which represents the local NW foreland to the EAO; unit 2, a collage of Mesoproterozoic metamorphic complexes, which forms the basement to unit 3, a stack of Neoproterozoic, NW directed imbricate thrust nappes named here the "Cabo Delgado Nappe Complex" (CDNC); unit 4, restricted Neoproterozoic metasedimentary basins; and unit 5, two exotic Neoproterozoic granulite mélange complexes. The units were assembled during a long and complex history of NW directed shortening, which commenced with nappe stacking and emplacement of the CDNC over the Mesoproterozoic basement terranes toward the NW foreland. It is proposed that the CDNC and the Eastern Granulites farther north in Tanzania are remnants of Neoproterozoic volcanic arcs and microcontinents formed "outboard" of the Mesoproterozoic continent after 596 ± 11 Ma. Field and potential field geophysical data show that the nappes were folded by regional-scale NE-SW trending folds that formed in response to a later stage of the same shortening episode and this episode gave rise to the Lurio Belt, a prominent structural feature of northern Mozambique and a key element (often as suture zone) in many Gondwana reconstructions. The Lurio Belt is here interpreted as a structure generated during folding of the CDNC during later stages of the progressive shortening event. It is, however, a repeatedly reactivated shear zone, probably at the site of an older (Mesoproterozoic?) discontinuity, with an intense pure shear deformation history. It is cored by strongly attenuated lenses of a granulitic tectonic mélange, the Ocua Complex (megatectonic unit 5) and is intruded by Late Pan-African granitoids of the Malema Suite. The compressional phase of the orogen was postdated by NW-SE directed extension. New U-Pb zircon and monazite dates show that extension was initiated at circa 540 Ma in the eastern Lurio Belt. It is argued that extension was the result of a major episode of orogenic collapse of the EAO, initiated by gravitational instabilities resulting from crustal thickening during the shortening phase.

Did Bushmanland extensionally unroof Namaqualand?

Article

Nov 2006
PRECAMBRIAN RES

The Namaqua-Natal Orogen evolved between about 2000 and 1000Ma as part of an orogenic system that assembled the Rodinian supercontinent, and “wraps” the Archaean Kaapvaal Craton to the south and west. The Orogen includes the western ultra-high-temperature sapphirine/hercynite-granulite-facies Namaqualand Terrane, structurally overlain, to the north, by the low-grade, high-level igneous, Richtersveld Terrane (1900–1700Ma) and, to the east, by the amphibolite-facies granite gneiss/Pb-Zn-rich supracrustal Bushmanland Terrane, which is overthrust, from the north along the Groothoek Thrust, by the high-pressure Hom Terrane. West of the dextral Tantalite Valley/Pofadder Shear Zone, the gently-dipping contacts among these terranes have been considered to be thrusts. With the exception of the Groothoek Thrust, the contacts are low-angle, top-down-to-the-east or northeast, folded extensional detachments with displacements of up to at least 100km. The Namaqualand part of the Namaqua-Natal Orogen comprises two distinct event sequences, a Kibaran phase of crustal shortening and thickening, and voluminous granitic sheets at about 1200Ma, then a Namaquan phase of mafic underplating, ultra-high-temperature metamorphism, granitic sheets, dextral transtension, constrictional fabrics, and crustal thinning from about 1060 to 1030Ma. A dominant constrictional LS fabric was developed in regionally sub-horizontal layered pink granite gneisses and metasediments deformed by ENE-trending folds. Extensional detachments developed late in the Namaquan phase when the ultra-high-temperature rocks were exhumed. In the Bushmanland Terrane, Aggeneys metasediments have mainly constrictional fabrics and are deformed by giant upward-facing, downward-closing sheath folds, developed during transtension, in which lie 1700Ma Pb/Zn SedEx deposits like those of Broken Hill and Flin Flon. The Namaqualand Terrane may be the largest and oldest extensional core complex on Earth. The Proterozoic Era was characterized by transtensional ultra-high-temperature, horizontally-layered, lineated, pink gneiss terrains with corrugating fold hinges roughly parallel with the finite stretching direction.

Ion probe dating of a migmatite in SW Sweden: The fate of zircon in crustal processes

Article

Apr 2004
PRECAMBRIAN RES

A migmatitic orthogneiss in the Western Segment in the Sveconorwegian Province of the Baltic Shield was dated using the ion-probe U–Pb method on zircon grains, which were also analysed for rare earth elements. Mesosome zircons have 1.605±0.010Ga magmatic cores, which places the gneiss protolith in the same 1.61–1.59Ga time bracket as continental arc-related gneisses, abundant in this part of the Sveconorwegian Province. These cores show REE profiles with strong HREE enrichment, positive Ce- and negative Eu-anomalies, typical of magmatic zircon. Migmatite leucosomes are folded and parallel with or slightly discordant to the fabric. They contain a small population of zircon with cores and metamorphic rims, which are interpreted as xenocrysts incorporated in the leucosome during melting of the mesosome. CL-bright metamorphic embayments and rims on xenocrysts reflect 1.01±0.05Ga Sveconorwegian metamorphic reworking. Ce-anomalies are nearly absent and Eu-anomalies are reduced relative to igneous spots. This is probably a feature of fluid controlled environments where Ce and Eu oxidation states are buffered by the metamorphic fluid. From this and discordant rims from the mesosome we also conclude that the rims formed by reworking of the older zircon where the Pb-loss was also fluid induced. In the leucosome veins, magmatic acicular zircon gives 0.92±0.01Ga, ascribed to the crystallisation of the veins. They originated by local melting, probably augmented by magma that formed at a deeper level. Widespread granitic and noritic late-Sveconorwegian magmatism close to 0.92Ga in other parts of the Western Segment has equivalents in the Norwegian sectors of the Sveconorwegian Province. Leucosome formation was therefore part of a regional event related to exhumation of the Sveconorwegian Eastern Segment. We also provide the first unequivocal evidence for ductile deformation related to late-Sveconorwegian magmatism.

Seismic images of eclogites, crustal-scale extension, and Moho relief in the eastern Grenville Province, Quebec

Article

Jan 1995
GEOLOGY

Seismic images from a 250 km Lithoprobe reflection profile in the interior of the eastern Grenville province provide important new constraints on the crustal architecture of this part of the orogen. Prominent upper-crustal reflections can be correlated with exposures of high-pressure metamorphic rocks in the Manicouagan shear belt, providing the first direct evidence for eclogite reflectivity in the Grenville province. The eclogites are cut by major late Grenvillian normal faults that penetrate the deep crust and preserve evidence of extensional collapse of the overthickened orogen. North-to-south crustal thinning, indicated by a change in Moho reflection time from 16 to 13 s, correlates well with regional Bouguer gravity trends and is accompanied by a dramatic increase in the reflectivity of the lower crust. These features underscore the significance of recently recognized along-strike variations in tectonic style within the Grenville province and point to the internal complexity that characterizes the root zones of collisional orogens.

Inclined transpression at the toe of an arcuate thrust: An example from the Precambrian 'Mylonite Zone' of the Sveconorwegian orogen

Article

Jun 2010

The 'Mylonite Zone' (MZ) forms a major, arcuate terrane boundary in the Precambrian Sveconorwegian orogen of SW Scandinavia. SE-directed thrusting along this curvilinear shear zone emplaced the higher-grade Idefjorden Terrane to the west onto the lower-grade Eastern Segment terrane to the east. Detailed structural characterization of the MZ mylonites in two different localities (Värmlandsnäs and Bua peninsulas) reveals a complex three-dimensional strain pattern. Inclined transpression is inferred on the basis of coexisting (and broadly coeval) foliation-parallel oblique shearing (resolvable in a strike-slip and dip-slip component) and acrossfoliation shortening. The former accommodated the transpressive component of the MZ, and its kinematics is either sinistral or dextral depending on the local strike of the MZ with respect to the regional thrust shortening vector. The latter led to pure-shear shortening perpendicular to the thrust sheet and subsequent lateral extrusion parallel to the mylonitic foliation via the development of antithetic displacements. No significant strain partitioning is observed at the meso-scale and strain is thus truly triclinic. The example described is an outstanding case of triclinic deformation, confirms theoretical analyses of complex strain models and adds valuable natural field constraints to our knowledge of deformation in the crust.

Unraveling 1.5 Ga of brittle deformation history in the Laxemar-Simpevarp area, southeast Sweden: A contribution to the Swedish site investigation study for the disposal of highly radioactive nuclear waste

Article

Dec 2009

The Swedish Nuclear Fuel and Waste Management Company (SKB) is undertaking site investigation at two locations in Sweden, Forsmark and Laxemar-Simpevarp, with the aim of identifying a suitable area for the construction of a deep repository for the disposal of highly radioactive nuclear waste. Fault slip data from outcrops and oriented drill cores were used to compute paleostress states and to unravel the sites' brittle deformation history. Results from the Laxemar-Simpevarp area show that its suggested brittle history results from multiple reactivation of fracture and fault sets caused by the many orogenic episodes that affected the region during at least 1.5 Ga of geological evolution in the brittle deformational regime. Two compressional, approximately NW/NNW-SE/SSE and NNE-SSW oriented shortening events generated sets of conjugate, steep strike-slip fractures. These sets formed during the late stages of the Svecokarelian and possibly also of the Gothian orogeny, soon after the region entered the brittle deformation domain. The Mesoproterozoic Sveconorwegian orogeny generated fractures and faults that are assigned to a third set of conjugate strike-slip faults, which constrain an approximately E-W σ1. The Caledonian shortening, oriented approximately NW-SE to E-W, reactivated the latter but also formed a new, similarly oriented set of subvertical strike-slip fractures. Permian transtension was oriented NW-SW and caused a prominent set of moderately dipping NW-SE trending normal faults in the Precambrian basement of the study area. Two other approximately NW-SW and NW-SE oriented shortening events are recorded in Ordovician limestones and can be tentatively linked to the far-field effects of the Laramide and Alpine orogenies.

Terrane analysis, regional nomenclature and crustal evolution in the Southwest Scandinavian Domain of the Fennoscandian Shield

Article

Jun 2005

Tom Andersen

Tectonostratigraphic terrane analysis of an orogenic belt involves the use of a well-defined nomenclature which is related to the internal structure and relative displacement of blocks in the orogenic "puzzle". The tectonostratigraphic terrane is the central concept in terrane analysis, and the term should be restricted to units fulfilling the definition, i.e. to fault-bounded crustal blocks which have distinct lithologic and stratigraphic successions and which have geologic histories different from neighbouring terranes. Application of terrane analysis to a complex, poly-orogenic region such as the Southwest Scandinavian Domain of the Fennoscandian Shield (SSD) must include information on the entire crustal evolution history of potential terranes, including petrological, geochemical and other non-geochronological information. At the present state of knowledge, no crustal block within the SSD fulfil the criteria as a Gothian or Sveconorwegian tectonostratigraphic terrane. Until basic evidence of terrane status is established for a given block of crust, the use of a descriptive regional nomenclature is therefore preferred. A revised, non-genetic nomenclature is proposed, which does not assign terrane status to any of its units, and which will hopefully help eliminating confusing differences in usage, and be helpful in future analysis of Gothian and Sveconorwegian tectonics and crustal evolution in the region, To reach a stage where terrane analysis of the Gothian and Sveconorwegian evolution of the SSD becomes meaningful, it is necessary to carry out both structural studies and isotopic dating of the ductile deformation zones which are potential terrane boundaries.

187Re-187Os evidence for crustal lenses of Svecofennian age within the Mylonite Zone, SW Sweden

Article

Mar 2002

Anders Scherstén

Re and Os concentrations and isotopic composition were analysed in four ultramatic cumulates from the Kedum tectonic lens within the Mylonite Zone, SW Sweden. An 1887+/-36 Ma isochron is obtained for six samples and interpreted to date the Kedum body. This is significantly older than the similar to1600-1700 Ma surrounding crust and the first rocks of Svecofennian age west of the Trans Scandinavian Igneous Belt. The initial gOs of 15.7+/-4.0 indicates contribution from a radiogenic source such as old crust.

Kinematics of a major fan-like structure in the eastern part of the Sveconorwegian orogen, Baltic Shield, south-central Sweden — comment

Article

Jun 1996
PRECAMBRIAN RES

The N-S-trending so-called Protogine Zone in the Baltic Shield of south-central Sweden is usually considered to mark a tectonic boundary between the rocks of the Transscandinavian Igneous Belt (TIB) in the east and the Sveconorwegian orogen in the west. Detailed structural mapping in the Karlskoga-Kristinehamn area has shown that an anastomosing network of ductile deformation zones with generally N-S strike extends ca. 40 km east of the traditional “Protogine Zone”. Furthermore, the western boundary of this ductile deformation and the TIB is not constrained in the Kristinehamn area. Reconnaissance studies indicate that they both extend westwards towards the so-called Mylonite Zone.

Decompressed eclogites in the Sveconorwegian (-Grenvillian) orogen of SW Sweden: Petrology and tectonic implications

Article

Oct 2004
J METAMORPH GEOL

Charlotte Möller

Relict eclogites and associated high-pressure rocks are present in the Eastern Segment of the SW Swedish gneiss region (the tectonic counterpart of the Parautochthonous Belt of the Canadian Grenville). These rocks give evidence of Sveconorwegian eclogite facies metamorphism and subsequent pervasive reworking and deformation at granulite and amphibolite facies conditions. The best-preserved eclogite relics suggest a clockwise P–T –t history, beginning in the amphibolite facies, progressing through the eclogite facies, decompressing and partially reequilibrating through the high- and medium-pressure granulite facies, before cooling through the amphibolite facies. Textures demonstrate the former coexistence of the plagioclase-free assemblages garnet+clinopyroxene+quartz+rutile+ilmenite, garnet+clinopyroxene+ kyanite+rutile, and garnet+kyanite+quartz+rutile. The former existence of omphacite is evidenced by up to 45 vol.% plagioclase expelled as small grains within large clinopyroxene. Matrix plagioclase is secondary and occurs expelled from clinopyroxene or in fine-grained, granulite facies reaction domains formed during resorption of garnet and kyanite. Garnet shows preserved prograde growth zoning with rimward increasing pyrope content, decreasing spessartine content and decreasing Fe/(Fe+Mg) ratio, but is partly resorbed and reequilibrated at the rims. P–T estimates from microdomains with clinopyroxene+plagioclase+quartz+garnet indicate pressures of 9.5–12 kbar and temperatures of 705–795 °C for a stage of the granulite facies decompression. The preservation of the prograde zoning suggests that the rocks did not reside at these high temperatures for more than a few million years, and chemical disequilibrium and ‘frozen’ reaction textures indicate heterogeneous reaction progress and overstepping of reactions during the decompression through the granulite facies. Together these features suggest a rapid tectonic exhumation. The eclogite relics occur within a high-grade deformation zone with WNW–ESE stretching and associated oblique normal-sense, top-to-the-east (sensu lato) displacement, suggesting that extension was a main cause for the decompression and exhumation. Probable tectonic scenarios for this deformation are Sveconorwegian late-orogenic gravitational collapse or overall WNW–ESE extension.

Petrology and geothermobarometry of granulite facies metapelites from the His??y-Torungen area, south Norway: New data on the Sveconorvegian P-T-t path of the Bamble sector

Article

Dec 2004
J METAMORPH GEOL

T.-L. Knudsen

ABSTRACT The high-grade rocks (metapelite, quartzite, metagabbro) of the Hisøy-Torungen area represent the south-westernmost exposures of granulites in the Proterozoic Bamble sector, south Norway. The area is isoclinally folded and a metamorphic P–T–t path through four successive stages (M1-M4) is recognized. Petrological evidence for a prograde metamorphic event (M1) is obtained from relict staurolite + chlorite + albite, staurolite + hercynite + ilmenite, cordierite + sillimanite, fine-grained felsic material + quartz and hercynite + biotite ± sillimanite within metapelitic garnet. The phase relations are consistent with a pressure of 3.6 ± 0.5 kbar and temperatures up to 750–850°C. M1 is connected to the thermal effect of the gabbroic intrusions prior to the main (M2) Sveconorwegian granulite facies metamorphism. The main M2 granulite facies mineral assemblages (quartz+ plagioclase + K-feldspar + garnet + biotite ± sillimanite) are best preserved in the several-metre-wide Al-rich metapelites, which represent conditions of 5.9–9.1 kbar and 790–884°C. These P–T conditions are consistent with a temperature increase of 80–100°C relative to the adjacent amphibolite facies terranes. No accompanying pressure variations are recorded. Up to 1-mm-wide fine-grained felsic veinlets appear in several units and represent remnants of a former melt formed by the reaction: Bt + Sil + Qtz→Grt + lq. This dehydration reaction, together with the absence of large-scale migmatites in the area, suggests a very reduced water activity in the rocks and XH2O = 0.25 in the C–O–H fluid system was calculated for a metapelitic unit. A low but variable water activity can best explain the presence or absence of fine-grained felsic material representing a former melt in the different granulitic metapelites. The strongly peraluminous composition of the felsic veinlets is due to the reaction: Grt +former melt ± Sil→Crd + Bt ± Qtz + H2O, which has given poorly crystalline cordierite aggregates intergrown with well-crystalline biotite. The cordierite- and biotite-producing reaction constrains a steep first-stage retrograde (relative to M2) uplift path. Decimetre- to metre-wide, strongly banded metapelites (quartz + plagioclase + biotite + garnet ± sillimanite) inter-layered with quartzites are retrograded to (M3) amphibolite facies assemblages. A P–T estimate of 1.7–5.6 kbar, 516–581°C is obtained from geothermobarometry based on rim-rim analyses of garnet–biotite–plagioclase–sillimanite–quartz assemblages, and can be related to the isoclinal folding of the rocks. M4 greenschist facies conditions are most extensively developed in millimetre-wide chlorite-rich, calcite-bearing veins cutting the foliation.

The Grenvillian–Sveconorwegian orogeny in Fennoscandia: Back-thrusting and extensional shearing along the "Mylonite Zone"

Abstract

No full-text available

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