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Abstract
The Variscan belt of western Europe is part of a large Palaeozoic mountain system, 1000 km broad and 8000 km long, which extended from the Caucasus to the Appalachian and Ouachita mountains of northern America at the end of the Carboniferous. This system, built between 480 and 250 Ma, resulted from the diachronic collision of two continents: Laurentia–Baltica to the NW and Gondwana to the SE. Between these two continents, small, intermediate continental plates separated by oceanic sutures mainly have been defined (based on palaeomagnetism) as Avalonia and Armorica. They are generally assumed to have been detached from Gondwana during the early Ordovician and docked to Laurentia and Baltica before the Carboniferous collision between Gondwana and Laurentia–Baltica. Palaeomagnetic and palaeobiostratigraphic methods allow two main oceanic basins to be distinguished: the Iapetus ocean between Avalonia and Laurentia and between Laurentia and Baltica, with a lateral branch (Tornquist ocean) between Avalonia and Baltica, and the Rheic ocean between Avalonia and the so-called Armorica microplate. Closure of the Iapetus ocean led to the Caledonian orogeny: a belt resulting from collision between Laurentia and Baltica, and from softer collisions between Avalonia and Laurentia and between Avalonia and Baltica. Closure of the Rheic ocean led to the Variscan orogeny by collision of Avalonia plus Armorica with Gondwana. A tectonic approach allows this scenario to be further refined. Another important oceanic suture is defined: the Galicia–Southern Brittany suture, running through France and Iberia and separating the Armorica microplate into North Armorica and South Armorica. Its closure by northward (or/and westward?) oceanic and then continental subduction led to early Variscan (430–370 Ma) tectonism and metamorphism in the internal parts of the Variscan belt. As no Palaeozoic suture can be detected south of South Armorica, this latter microplate should be considered as part of Gondwana since early Palaeozoic times and during its Palaeozoic north-westward drift. Thus, the name Armorica should be restricted to the microplate included between the Rheic and the Galicia–Southern Brittany sutures.
... The development of the Variscan chain was also widely affected by the activity of crustal-scale strike-slip shear zones during the late-Carboniferous ca. 340-300 Ma (Arthaud and Matte 1977;Matte 2001;Carosi and Palmeri 2002;Di Vincenzo et al. 2004;Carreras and Druguet 2014;Franke et al. 2017;Simonetti 2021;Schulmann et al. 2022;Franke and Żelaźniewicz 2023). ...
... In contrast, Cruciani et al. (2016) interpreted this area to have developed in a transtensional regime at ca. 305-295 Ma, where exhumation was driven by the Monte Grighini shear zone (MGSZ). Owing to several similarities between the MGSZ and the transpressive Posada-Asinara shear zone (PASZ) in northern Sardinia and within the framework of the southern European Variscan belt (Matte 2001;Corsini and Rolland 2009;Carosi et al. 2020Carosi et al. , 2022; Simonetti 2021 for a review), a re-investigation of the tectonic regime associated with the MGSZ has been performed. Here we present an updated view on the tectono-metamorphic history of the Monte Grighini dome and its non-coaxial deformation, by integrating field observations, meso-and microstructural data, vorticity analysis, P-T estimates and in situ U-(Th)-Pb (texturally-and chemically-controlled) geochronology of monazite. ...
... In particular, similar features and ages of transpressive tectonics have been observed in Sardinia , as well as in the Maures-Tanneron Massif (MTM, France; Simonetti et al. 2020a;Bolle et al. 2023) and in the External Crystalline Massif (Italy and France; Simonetti et al. 2018Simonetti et al. , 2020bSimonetti et al. , 2021Jacob et al. 2021;Fréville et al. 2022;Vanardois et al. 2022a, b). All these have been associated with the East Variscan Shear Zone framework (EVSZ; Matte 2001;Corsini and Rolland 2009;Padovano et al. 2012Padovano et al. , 2014Simonetti 2021;Fig. 9b). ...
This work presents an integrated structural, kinematic, and petrochronological study of the Monte Grighini dome within the
Variscan hinterland–foreland transition zone of Sardinia (Italy). The area is characterised by dextral transpressive deformation
partitioned into low- and high-strain zones (Monte Grighini shear zone, MGSZ). Geothermobarometry of one sample
of sillimanite-bearing mylonitic metapelite indicates that the Monte Grighini shear zone developed under high-temperature
(~ 625 °C) and low-pressure (~ 0.4–0.6 GPa) conditions. In situ U–(Th)–Pb monazite geochronology reveals that the deformation
in the shear zone initiated at ca. 315 Ma. Although previous studies have interpreted the Monte Grighini shear zone
to have formed in a transtensional regime, our structural and kinematic results integrated with constraints on the relative
timing of deformation indicate that it shows similarities with other dextral ductile transpressive shear zones in the Southern
European Variscan belt (i.e., the East Variscan Shear Zone, EVSZ). However, dextral transpression in the Monte Grighini
shear zone started later than in other portions of the EVSZ within the framework of the Southern European Variscan Belt
due to the progressive migration and rejuvenation of deformation from the core to the external sectors of the belt.
... However, the number, palaeogeography, lifetime and even the existence of other oceanic domains are poorly constrained and these uncertainties lead to various geodynamic models of pre-Carboniferous convergence and subductions (compare Kroner andRomer, 2013 andFranke et al., 2017; or for the western branch, Lardeaux et al., 2014 andBallèvre et al., 2014). These models (see Vanderhaeghe et al., 2020 for a recent review) can be subdivided into those involving (i) one single ocean (generally the Rheic, see von Raumer and Stampfli, 2008;Kroner and Romer, 2013), (ii) several (up to three) oceanic basins (Matte, 2001;Franke et al., 2017), (iii) monocyclic continuous subduction zones (see Matte 1986Matte , 2001Matte , 2007Ballèvre et al., 2014) or (iv) polycyclic subduction-collision models involving northward subduction of an oceanic lithosphere until the Silurian-Lower Devonian (generating the so-called eo-variscan event/suture) and southward subduction of another oceanic domain during Middle Devonian to Lowermost Carboniferous times (Faure et al., 2005(Faure et al., , 2008Lardeaux et al., 2014). More specifically, the identification of the subducted ocean(s) located along the southern Variscan domain is debated. ...
... However, the number, palaeogeography, lifetime and even the existence of other oceanic domains are poorly constrained and these uncertainties lead to various geodynamic models of pre-Carboniferous convergence and subductions (compare Kroner andRomer, 2013 andFranke et al., 2017; or for the western branch, Lardeaux et al., 2014 andBallèvre et al., 2014). These models (see Vanderhaeghe et al., 2020 for a recent review) can be subdivided into those involving (i) one single ocean (generally the Rheic, see von Raumer and Stampfli, 2008;Kroner and Romer, 2013), (ii) several (up to three) oceanic basins (Matte, 2001;Franke et al., 2017), (iii) monocyclic continuous subduction zones (see Matte 1986Matte , 2001Matte , 2007Ballèvre et al., 2014) or (iv) polycyclic subduction-collision models involving northward subduction of an oceanic lithosphere until the Silurian-Lower Devonian (generating the so-called eo-variscan event/suture) and southward subduction of another oceanic domain during Middle Devonian to Lowermost Carboniferous times (Faure et al., 2005(Faure et al., , 2008Lardeaux et al., 2014). More specifically, the identification of the subducted ocean(s) located along the southern Variscan domain is debated. ...
... More specifically, the identification of the subducted ocean(s) located along the southern Variscan domain is debated. It is referred to as the Galicia, (South) Brittany and Massif central ocean (or a combination of these appellations; Matte, 1986;Matte et al., 2001Matte et al., , 2007, the Rheic ocean (von Raumer and Stampfli, 2008;Nance et al., 2010;Kroner and Romer, 2013), or a branch of the Rheic ocean (Paris and Robardet, 1998) or the Saxothuringian ocean (Franke et al., 2017). ...
This paper presents and discusses new geochronological and petrological data on a suite of calc-alkaline plutons composed predominantly of diorites and tonalites from the West Massif Central. Their petrochemical fingerprints are compatible with partial melting of a hydrous mantle wedge followed by fractional crystallization of amphibole and plagioclase before final emplacement between 5 and 8 kbar within the continental upper plate of a subduction system. In situ U-Pb zircon dating on tonalites yields a fairly narrow age range of 365−354 Ma (including uncertainties) for igneous crystallization. These calc-alkaline plutons imply active margin magmatism near the Devonian-Carboniferous boundary and are contemporaneous with the back-arc magmatism and HP metamorphism as dated by recent studies. However, such isolated igneous bodies do not form a transcrustal magmatic arc but rather represent dispersed plutons emplaced within less than 30 Myr when all data from the Variscan belt of France are considered. In Limousin, they intrude migmatitic paragneisses and retrogressed eclogites from the Upper Gneiss Unit (UGU), suggesting that the high pressure rocks were already exhumed at 19−30 km depth before 365 Ma. Moreover, the diorites and tonalites are never found within units below the UGU. It therefore suggests that these tectono-metamorphic units of the Western French Massif Central were piled up after 354 Ma. Altogether these results support the monocyclic model for Variscan geodynamics in the French Massif Central, with the transition between oceanic subduction and continental collision taking place between Upper Devonian and Lower Carboniferous.
... The main sutures ( Fig. 1) considered in this study are: (1) the Rheic and Rheno-Hercynian sutures (Matte, 2001;McKerrow et al., 2000;Linnemann et al., 2007;von Raumer et al., 2009;Nance et al., 2010), which marks the southern limit of Laurussia (Laurentia-Baltica-Avalonia) and its tectonic contact with the peri-Gondwana terranes of the Variscan belt; (2) the Saxo-Thuringian suture Matte, 2001;Mazur and Aleksandrowski, 2001;Willner et al., 2002;Schulmann et al., 2009), separating the Saxo-Thuringian zone/terrane (Saxothuringia) from the Teplá-Barrandian block in the northern part of the Bohemian Massif;and (3) the Moldanubian suture Medaris et al., 2005;Faryad et al., 2013b), which runs south-east of the Teplá-Brrandian block (called Bohemia in Franke et al., 2017) and separates the Armorica-type crustal elements from the northern Gondwana margin. ...
... The main sutures ( Fig. 1) considered in this study are: (1) the Rheic and Rheno-Hercynian sutures (Matte, 2001;McKerrow et al., 2000;Linnemann et al., 2007;von Raumer et al., 2009;Nance et al., 2010), which marks the southern limit of Laurussia (Laurentia-Baltica-Avalonia) and its tectonic contact with the peri-Gondwana terranes of the Variscan belt; (2) the Saxo-Thuringian suture Matte, 2001;Mazur and Aleksandrowski, 2001;Willner et al., 2002;Schulmann et al., 2009), separating the Saxo-Thuringian zone/terrane (Saxothuringia) from the Teplá-Barrandian block in the northern part of the Bohemian Massif;and (3) the Moldanubian suture Medaris et al., 2005;Faryad et al., 2013b), which runs south-east of the Teplá-Brrandian block (called Bohemia in Franke et al., 2017) and separates the Armorica-type crustal elements from the northern Gondwana margin. ...
... The Armorican Massif and the French Massif Central offer a complete cross-section of a basin corresponding to the Galicia-Moldanubian Ocean (e.g., Matte, 2001). The early Palaeozoic magmatic rocks and sedimentary basin fill have been preserved in metamorphic nappes transported towards the southern flank of the Variscan orogen (Matte, 1991(Matte, , 2001. ...
The high- to ultrahigh-pressure ((U)HP) metamorphic rocks are present within the European Variscan belt between the Bohemian and Iberian massifs (the Galicia-Moldanubian zone) and they are partly incorporated into the Alpine orogenic system. Due to their involvement in various allochthonous units, the affiliation of the (U)HP rocks to the suture zones that were the sites of their initial exhumation, is not always clear. The Bohemian Massif preserves the best evidence of Variscan sutures with clear relationships to the exposed (U)HP rocks. They are the Moldanubian and the Saxo-Thuringian sutures bounding the Tepl´a-Barrandian block from the SSE and NNE, respectively. The distribution of (U)HP rocks coincides with the boundaries of mantle lithosphere domains, delimited from large-scale seismic anisotropy, and reveals the NW-ward inclination of the Moldanubian mantle
lithosphere domain beneath the Tepl´a-Barrandian block and thus a subduction polarity to the NW. The eastern margin of the Tepl´a-Barrandian block contains a magmatic arc, which is in direct contact with the Moldanubian orogenic wedge, and both are penetrated by lamprophyre dykes (~340 Ma), which dates the cessation of the collision-related shortening and crustal consolidation. The overall crustal geometry of the Saxo-Thuringian suture implies the SE-ward polarity of subduction during its formation. However, based on seismic tomography and
anisotropy model, the suture at mantle depths appears as a sub-vertical boundary between the Saxo-Thuringian and the Tepl´a-Barrandian lithosphere domains. The Saxo-Thuringian zone bears evidence of blueschist facies metamorphism in the (para)autochthonous units, which are strongly retrogressed. Compared to the Moldanubian zone, (U)HP rocks are less common in the Saxo-Thuringian zone and occur as nappes and klippes, some of which are exposed near the Moldanubian suture. The similarities of the Saxo-Thuringian (U)HP rocks to those in the
Moldanubian zone and their allochthonous positions favour formation of some of the (U)HP rocks along the Moldanubian suture and their subsequent emplacement into the Saxothuringian zone. The Moldanubian suture appears to control the distribution of most of the (U)HP rocks exposed along the European Variscan Belt. They all show similarities regarding lithology, mainly fragments of mantle rocks included in felsic materials, and their
granulite-amphibolite facies thermal overprint.
... In our study, we investigate the role that major episodes of mountain building may have played during mineralization in the Central African Copperbelt and the Kupferschiefer. Our results suggest that the deposits are closely associated with two of the largestknown mountain subaerial ranges in Earth's history: the Transgondwanan supermountain belt (Squire et al. 2006) for the Katangan Basin and the Mid-Pangean supermountain belt (Matte 2001;Torsvik and Cocks 2004) for the Permian Basin (Fig. 3). ...
... d-e Convergence between Gondwana and Laurentia-Baltica produced the ~ 8000-km-long Mid-Pangean supermountain belt. Modified after Matte (2001) and Torsvik and Cocks (2004) Content courtesy of Springer Nature, terms of use apply. Rights reserved. ...
... The Permian Basin was located near the base of the 8000-km-long Mid-Pangean supermountain belt (Fig. 3d-e). The Mid-Pangean supermountain belt developed when the continental landmasses of Gondwana and Laurentia-Baltica (plus several microplates) collided diachronously between about 480 and 250 Ma (Matte 2001;Torsvik and Cocks 2004). Climate models estimate that, at about 255 Ma, the section of the Mid-Pangean supermountain belt bounding the Permian Basin near southwestern Poland and southern Germany (present-day locations) had a mean altitude of at least 2000 m (Fluteau et al. 2001). ...
Sedimentary rock–hosted stratiform copper deposits are the world’s second largest source of copper and the largest source of cobalt, with about 73% of the copper occurring in two basins: the Katangan Basin (Central African Copperbelt) and the Permian Basin (Kupferschiefer). Why these two sedimentary basins are so highly endowed in copper is puzzling because sedimentary rock–hosted stratiform copper deposits have formed since the Paleoproterozoic and they all share remarkably similar ore mineralogy, host-rock characteristics and basin settings. We suggest that this discrepancy is due to the development of these two basins close to the bases of ~ 8000-km-long supermountain belts. The supermountain belts were instrumental in raising oxygen levels in Earth’s atmosphere, as well as providing a voluminous source of groundwater and a powerful and long-lived driver for the fluid-flow system. The elevated oxygen levels facilitated the diagenetic processes that converted copper-bearing labile minerals to amorphous iron-oxides and smectite and then in turn to hematite and illite. When oxidized brines flushed through the basin successions, the liberated copper was transported to units containing carbon-rich mudstone and the metals were deposited. For the Katangan Basin, development of the Transgondwanan supermountain belt along its margins between about 525 and 510 Ma explains the delay of several hundreds of millions of years between basin formation and mineralization in the Central African Copperbelt. In contrast, development of the Mid-Pangean supermountain belt formed penecontemporaneous with the Permian Basin explains the similarity in timing between basin formation and mineralization in the Kupferschiefer.
... In this study, we review the geology of a region of SW Iberia included in the European Variscan orogenic belt, formed at the Gondwana-Laurussia convergent margins during the Late Paleozoic assembly of the supercontinent Pangea (Matte, 2001;Nance and Murphy, 2013) which is an exceptional good natural laboratory for describing such tectono-thermal A C C E P T E D M A N U S C R I P T structures (Vanderhaeghe et al., 2020;Schulmann et al., 2022). At the westernmost tip of the Variscan belt, similar gneiss domes are exposed in the continental basement of Iberia (Fig. 1a), representing the North Gondwana continental margin. ...
... The diachronic collision between Gondwana and Laurussia started in the Middle-Upper Devonian, associated with the closure of the Rheic Ocean, and ended with the formation of the Variscan belt in Iberia (Matte, 2001;Martinez-Catalán et al., 2009). The structure of SW Iberia is complex, resulting from the interaction of successive contractional and extensional deformation events associated with the development of the Variscan orogeny. ...
... The existence of episodic subduction along Gondwanan and proto-Gondwanan margins, facing the West Africa Craton, Tuareg Shield, Sahara Metacraton and surrounding areas since at least 750 to ca. 500 Ma, is widely accepted (e.g., Pereira et al., 2012;Díez Fernández et al., 2019;Arenas et al., 2021;Avigad et al., 2022;Rojo-Pérez et al., 2022;Moreno-Martín et al., In press). The sedimentary rocks generated along these margins were derived from recycling specific crustal areas, preserving valuable geochemical information useful in paleoreconstructions. Subsequently, as a result of collision between Gondwana and Laurussia during Devonian-Carboniferous times (Matte, 2001;Martínez Catalán, 2011;Arenas et al., 2014;Díez Fernández et al., 2016), sedimentary series deposited at different positions along the Gondwanan margin experienced different Variscan tectonometamorphic evolution, defining several geotectonic zones along the Variscan Orogen, mainly a large allochthonous unit thrusting over the autochthonous Gondwanan margin (Fig. 1). ...
... (c) Schematic stratigraphic column of Cap de Creus Massif (after Casas et al., 2015), showing the approximate location of samples. Matte, 2001;Michard et al., 2010). The internal zones of the Variscan Orogen preserve ophiolitic units and distal sections of the Gondwana margin affected by high-P metamorphism (allochthonous complexes), whereas domains originally closer to cratonic Gondwana are mostly included in the Variscan Autochthon (e.g., Arenas et al., 2016a;Díez Fernández et al., 2016;Martínez Catalán et al., 2021;Novo-Fernández et al., In press). ...
... Following the prolonged Variscan orogenic cycle (c. 480-290 Ma; Matte, 2001), development of Neotethyan oceanic basins (including the Piedmont-Liguria and Meliata oceans) led to the separation of Africa from Europe during Late Triassic to Jurassic time, producing an intervening assemblage of continental microplates and ocean basins. Reconstructions of this complex tectonic mosaic remain subject to debate, but recent studies show consensus that the Adria microplate was the southernmost microplate, remained kinematically linked to Africa, and was separated from the adjacent microcontinent to the north, termed Alcapia, by a shear zone rather than an ocean basin (Handy et al., 2010(Handy et al., , 2015. ...
... A small number of ages >800 Ma are excluded for clarity (17 zircon; 1 apatite; 0 rutile; 1 garnet U-Pb; and 1 garnet Lu-Hf). (Schulz & von Raumer, 2011), probably caused by docking of Armorica and Gondwana (Matte, 2001). Preservation of pre-Alpine detrital garnet in sedimentary units incorporated into the Alpine orogen and subjected to blueschist facies metamorphism has also been documented (Manzotti & Ballèvre, 2013). ...
Detrital geochronology employing the widely‐used zircon U‐Pb proxy is biased toward igneous events and metamorphic anatexis; additionally, zircon is highly refractory and frequently polycyclic. Garnet, a rock‐forming and thus commonly occurring mineral, is predominantly metamorphic and much less refractory. Here, we report in situ U‐Pb and Lu‐Hf ages from detrital garnet hosted in ancient and modern sediments of the European Alps. Both geochronometers are biased toward the most recent garnet‐crystallizing metamorphic event in the source area, with fewer inherited ages. This likely reflects efficient removal of inherited garnet during diagenesis and metamorphism, and is in contrast to detrital zircon, apatite, and rutile U‐Pb data, which largely record pre‐Alpine ages. Neither the U‐Pb nor Lu‐Hf system in garnet exhibits a relationship between age recovery and composition. However, the Lu‐Hf system in garnet yields significantly better age recovery than the U‐Pb system. Estimated initial ²³⁸U/²⁰⁶Pbc values at the time of crystallization are near unity, suggesting that garnet does not significantly partition U from Pb during crystallization, at least for the generally almandine‐rich garnets analyzed in this study. Hence, Lu‐Hf geochronology of detrital garnet offers an effective method to detect and date the most recent phase of mid‐grade metamorphism in sub‐anatectic source areas, in which detrital zircon U‐Pb analysis may be of less utility.
... In this study, we review the geology of a region of SW Iberia included in the European Variscan orogenic belt, formed within the Gondwana-Laurussia convergent margin during the Late Paleozoic assembly of the supercontinent Pangaea (Matte 2001;Murphy and Nance 2013). This is an exceptionally good natural laboratory for describing such tectono-thermal structures (Vanderhaeghe et al. 2020;Schulmann et al. 2022). ...
... The diachronic collision between Gondwana and Laurussia started in the Middle-Upper Devonian, associated with the closure of the Rheic Ocean, and ended with the formation of the Variscan belt in Iberia (Matte 2001;Martínez Catalán et al. 2009). The structure of SW Iberia is complex, resulting from the interaction of successive contractional and extensional deformation events associated with the development of the Variscan orogeny. ...
A new structural study of a D2-M2 tectono-thermal structure in SW Iberia (Ponte de Sor-Seda gneiss dome) characterized by a spatial distribution of telescoping isograds providing a record of Buchan-type metamorphic conditions is presented. It comprises an infrastructure made up of the Lower Gneiss (LGU) and the Intermediate Schist (ISU) units separated by early D2 ductile extensional shear zones. The LGU and the ISU are composed of Ediacaran-Cambrian rocks that experienced the M2 highest-grade metamorphic conditions (amphibolite facies). Late Ediacaran-Early Terreneuvian and Late Miaolingian-Early Furongian protolith ages for LGU (496 ± 3 Ma) and ISU (539 ± 2 Ma) orthogneisses are reported. The superstructure made of Cambrian-Devonian rocks (Upper Slate Unit. USU) deformed under M2 greenschist facies, tectonically overlies the ISU across a D2 extensional shear zone. Kinematic criteria associated with D2-M2 fabrics indicate top-to-ESE-SE sense of shear. A late-D2 brittle-ductile high-angle extensional shear zone (Seda shear zone) crosscut the gneiss dome. D3 upright folds, thrusts and transpressive shear zones caused the steepening of D2 structures and the local crenulation of S2 foliation. The Mississippian D2-M2 event recorded in the OMZ may be regarded as a regional scale phenomenon that markedly influenced the crustal architecture of North Gondwana during the assembly of Pangea.
Supplementary material at https://doi.org/10.6084/m9.figshare.c.6828875
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... The evolution of the Variscan chain in Europe is characterized by the development of crustal-scale strike-slip shear zones during the Late Carboniferous (Arthaud & Matte, 1977;Matte, 2001;Carreras & Druguet, 2014;Franke et al., 2017;Cochelin et al., 2021;Faure & Ferrière, 2022). In particular, the southern European Variscan belt was widely affected by the development of a network of crustal-scale dextral transpressive shear zones called the East Variscan Shear Zone (EVSZ; Corsini & Rolland, 2009;Carosi et al., 2012;Padovano et al., 2012Padovano et al., , 2014Simonetti, 2021). ...
... The shape of the Variscan belt in Europe results from a Devonian-Carboniferous continent-continent collision between Laurentia-Baltica and Gondwana (Matte, 1986(Matte, , 2001. The metamorphic basement of the Sardinia island represents a fragment of the southern European Variscan belt and, due to the lack of the Alpine overprint, it represents a good locality to investigate the Paleozoic tectonometamorphic evolution (Carmignani et al., 1994;see Cruciani et al., 2015 for a critical review). ...
... Hence, minimum displacement probably attains the dimension of a few hundreds of km. Such long-distance displacement along the MT accords with a proposal of Matte (2001), which takes the MT far beyond the Bohemian Massif into the south-western Alps, the Maures-Tanneron Massif and the Corso-Sardic block, the latter in its pre-Permian position. A N-trending orogenic zonation in those areas comprises an equivalent of the TB to the E (present-day northern Corsica), followed westwards by equivalents of the Moldanubian allochthons and low-grade Palaeozoic rocks deposited on the N-Gondwana margin. ...
... A N-trending orogenic zonation in those areas comprises an equivalent of the TB to the E (present-day northern Corsica), followed westwards by equivalents of the Moldanubian allochthons and low-grade Palaeozoic rocks deposited on the N-Gondwana margin. Those Variscan zones were taken, by Matte (2001), to represent a former southward continuation of the north-eastern wing of the Bohemian Arc. That model has gained support by several papers on shear zones in the Maures-Tanneron/Corso-Sardic block and the alpine External Massifs (Carosi et al., 2020;Elter et al., 2020;Helbing et al., 2006;Padovano et al., 2012;Rossi et al., 2009;Simonetti et al., 2018Simonetti et al., , 2020. ...
... The Geneva Basin lays over a crystalline basement resulted from the Palaeozoic Variscan orogeny (c.a. 480-250 Ma, Matte, 2001). The latter stages of this orogeny related to the continental collision between the Gondwana to the southeast and Laurentia-Baltica to the northwest, forming the supercontinent of Pangaea (Matte, 2001 and reference therein). ...
... 480-250 Ma, Matte, 2001). The latter stages of this orogeny related to the continental collision between the Gondwana to the southeast and Laurentia-Baltica to the northwest, forming the supercontinent of Pangaea (Matte, 2001 and reference therein). After the main Variscan orogeny, the dextral translation of Gondwana and Laurussia and the reorganization of the asthenospheric flow patterns, caused the collapse of the orogeny and the thinning of the lithosphere and the setting of a transtensional and transpessional tectonic regime together with a strong regional thermal subsidence (Ziegler et al., 2004;Wilson et al., 2004). ...
A wide range of regional-and reservoir scale subsurface evaluation activities of geothermal energy resources and underground thermal energy storage potential have been carried in the Canton of Geneva area located in the Westernmost Swiss Molasse Basin. These activities promoted since 2012 through the 'GEothermie2020' program by the Canton authorities and SIG (Service Industriel de Genève), include both technical, regulatory and social acceptance aspects and it is aimed at implementing a nuclear-free 2050 energy strategy increasing the penetration of renewable energy sources into the energy system, and by reducing the energy consumption and the use of fossil fuels. A large data set of 2D seismic lines mostly acquired for hydrocarbon exploration carried out in the 70s and 80s, have been collected and in part reprocessed. To this, additional new 2D seismic and new walk-around and walk-above VSP were acquired in selected wells in order to establish a better controls on velocity model and reservoir heterogeneity and thus set the basis for future 3D seismic acquisition. Newly-acquired gravity data, calibrated with the available deep boreholes have been integrated in a single data base which served as basis for establishing a sound subsurface stratigraphic model. The latter spans from the Permo-Carboniferous to the Cenozoic age, including potential geothermal reservoirs throughout the stratigraphic succession. The mapping of fault system and geochemical analysis of associated mineralisation has been carried out to identify deeply rooted lineaments which may connect the crystalline basement to shallower formations and thus be important conduits of geothermal fluids. Over the 8 years of exploratory study aimed at 1) identifying and assessing the geothermal play elements 2) identifying a number of most suitable subsurface targets for both heat direct-use and storage, using a play fairway analysis approach and 3) supporting further evaluation and risk assessment (including the possible undesired occurrence of hydrocarbons and induced seismic). The encouraging results of the first shallow exploration well confirm the preliminary positive evaluation of the geothermal potential of the region. However, subsurface uncertainties related to structural and stratigraphic compartmentalization and the variability of reservoir properties remain large and will be targeted by further data acquisition such as downhole geophysics, extended dynamic tests, and 3D seismic survey. These data, together with knowledge transfer from the more mature HC industry will provide the geothermal players with the necessary best practice, knowledge and workflows to accomplish successfully the exploratory of geothermal resources.
... The tectonostratigraphic evolution of the WSB started with the accretion of the Southalpine with the Hanseatic terranes at ca. 430-450 Ma (Franz and Romer 2007), which were successively involved in the ca. 420-290 Ma Hercynian Orogeny due to the Laurussia-Gondwana collision (e.g., Boriani and Villa 1997;Franke et al. 2000;Matte et al. 2001;Faure et al. 2009;Spiess et al. 2010;Schulmann et al. 2014). Moreover, due to the Mesozoic heating (Spear 1995;Handy et al. 1999) and the Cenozoic Alpine tectonic events being predominantly recorded into macro shear zones within the Southalpine domains (Schmid et al. 1987;Zingg et al. 1990;Boriani et al. 1990b;Schmid 1967Schmid , 1993Vavra and Schaltegger 1999), the preserved Variscan WSB represents a key site to study the geodynamic evolution of Mid-to Late-Paleozoic structures, and thus to provide further constraints on the effects of a thermally perturbed crust during the gravitational collapse of a mature orogen. ...
The onset of the Cossato-Mergozzo-Brissago shear zone within the Strona Ceneri Border Zone in the W-Southalpine basement (Italy) and its role in the collapse of the Variscan crust have been the subject of considerable controversy. A set of new petrographic, geochemical and geochronological data from a suite of syn-kinematic migmatitic paragneiss and amphibolites in between the upper and lower crustal sections of the W-Southalpine basement provide new evidence on the thermo-mechanical role played by the middle crust in the evolution of the Permian Southalpine basement. The petrological investigation of these amphibolite-facies rocks and U–Pb ages from monazite crystals, occurring in distinct microstructural positions, provide new P–T-t constraints on the late-Paleozoic tectono-thermal evolution of the Variscan middle crust. The SCBZ units recorded tectonic events from a possible Early Silurian Cenerian (ca. 440 Ma) overprint onto the proto-sedimentary units of the Southalpine basement to the Mid-Permian (ca. 285 Ma) syn-kinematic partial melting event developed close to the CMB shear zone. Phase equilibria modeling is used to constrain the metamorphic conditions recorded by this section of the Variscan basement. Pressure–temperature (P–T) isochemical phase diagrams show that, after the ca. 330 Ma Variscan metamorphic peak at P ≅ 4 kbar and T < 630 °C, the SCBZ paragneiss experienced isobaric heating up to 700–720 °C at ca. 285 Ma, which led to the formation of a syn-kinematic partial melting event coeval to the emplacement of the Mafic Complex in the lower Ivrea-Verbano Zone. These new geochronological and petrological constraints on the SCBZ paragneiss seem to corroborate the hypothesis that the transition from the stage of mature Variscan orogen to the stage of its collapse developed in the Permian, at ca. 285 Ma. Thus, we argue that the orogenic collapse was probably driven by the rheological weakening of the mid-crustal SCBZ units induced by their syn-tectonic partial melting and, ultimately, by the coeval thermal perturbation of the crust due to the intrusion of the mafic igneous suite at the crust-mantle boundary.
... The genesis of these granitoids may be related to either the interaction of mantellic magma with crustal rocks or with the partial fusion of exclusively crustal sources. The present-day structure of the Moroccan Variscan belt is mainly the result of oblique convergence between the Gondwana and the Laurasia supercontinents during the late Palaeozoic (e.g., Ribeiro et al., 1979;Matte, 2001;Michard et al., 2010;Martínez-Catalán et al., 2021;Arenas et al., 2021;Chopin et al., 2023). This chain, located in northwest African Variscan-Alleghanian orogen, is subdivided into different segments (Fig. 1). ...
In the northern part of the Marrakech High Atlas (MHA), along the southern Variscan segment of the Western Meseta, a Variscan granitic intrusion crops out, intruding metasediments and meta-volcanosedimentary rocks of Early Cambrian to Ordovician age. A new whole-rock Rb-Sr isochron age of 268 ± 9 Ma for the granite, combined with a previously published whole-rock Rb-Sr radiometric dating (271 ± 3 Ma), reveals a post-kinematic (tectonic) character with regard to the main Variscan deformational event, belonging within the tectonic context of the Moroccan Variscan orogenic belt. Geochemically, the Azegour intrusion is metaluminous to peraluminous and exhibits a calc-alkaline affinity with a ferruginous composition. The massif shows an extremely differentiated character (SiO2 = 77.53–78.14 per cent), K2O and high total alkali contents, FeOt/(FeOt + MgO) and Ga/Al ratios, which have typical characteristics of an A-type granite. In addition, the granite contains high concentrations of LREE (LaN/SmN= 7.9–13.67) relative to HREE (LaN/YbN= 4.81–11.61) and a well-defined Eu negative anomaly (Eu/Eu* = 0.44–0.75). The granitic samples exhibit a strong enrichment of the most incompatible elements (RbN/YbN = 69.84–159.98) and a strong depletion of Ba, Sr, Eu, Nb, P and Ti. These characteristics are similar to those of A1-type granites. The absence of mineralogy typical of an S-type granite, combined with its weakly peraluminous character [A/CNK (molar Al2O3/CaO+Na2O+K2O) = 1,013–1,045], suggest that there is little or no significant involvement of supracrustal sources in the petrogenesis of the intrusion studied. Despite the strongly differentiated character of Azegour granitic rocks samples, their multi-element patterns shows many similarities to those of I-type granitoids, which has led to postulate that the parental liquids of A1-type were derived from partial melting of mafic magmas. The representative samples studied show less depleted εNd(t = 270 Ma)values of –0.94 to –4.85 and lower positive to slightly negative εSr(t = 270 Ma) values of –1.45 to 9.32. The isotopic data suggest that the Azegour granite was emplaced 270 myr ago, apparently generated by partial melting of a mafic/intermediate magma source in the lower crust as a result of the underplating of the asthenosphere mantle-derived Oceanic Island Basalt-like magmas. Alternatively, their isotopic signatures also can be attributed to the interaction and/or hybridisation of basaltic liquids derived from the mantle with these lower crust materials. The generated parental magma probably occurred at deep structural levels and involved fractional crystallisation processes by the separation of a mineralogical association composed of plagioclase + potassium feldspar ± biotite ± amphibole ± sphene ± apatite. The whole-rock Rb-Sr age of 268 ± 9 Ma, whole-rock geochemistry and Sr-Nd isotopic compositions of εNd(t = 270 Ma) and εSr(t = 270 Ma), combined with fieldwork data, suggest that the Azegour granite was emplaced.
... In this paper, the Carboniferous successions pertaining to the Culm facies outcropping in the Internal Zone of the Rif Belt (Northwestern Africa), in the northern Gondwana, are studied. The Culm sedimentary event is commonly referred to a Carboniferous regional syn-orogenic deposition, generally outlined in turbiditic or flysch basins displaced worldwide all over the Gondwanian-Laurussia area, that defines the acme of the Variscan (Hercynian) pre-collisional phase linked to the closure of the Rheic Ocean and the Paleotethysian collisions of the northern Gondwana plates in the Palaeozoic (Matte, 2001;Nance et al., 2010;Criniti et al., 2023). Remnant carboniferous siliciclastic deposits are usually located parallel to the Variscan front in the western-central Europe, both in outcrop and subsurface (Franke and Engel, 1988;Hartley and Warr, 1990;Burne, 1995;Oncken et al., 2000;Hartley and Otava, 2001). ...
... The Devonian-Carboniferous European Variscan Belt, or Variscides, formed due to the convergence between Laurussia to the north and Gondwana to the south (in presentday orientation). This convergence led to the closure of intermediate oceanic basins and the accretion of diverse terranes [1][2][3][4][5][6][7][8][9][10][11]. The geodynamic evolution of the Variscides and its geological implications can be summarized as follows: (i) during the Devonian to early Carboniferous periods, subduction and collision events occurred, leading to the stacking of nappes and subsequent thickening of the crust; (ii) in the late Carboniferous to early Permian periods, dextral wrenching ensued following oblique convergence between Gondwana and Laurussia, resulting in the reactivation or formation of large-scale strike-slip faults and shear zones within the crust, the emplacement of significant volumes of granitoids, and the formation of narrow intracontinental basins accompanied by bimodal magmatism; and (iii) during the middle Permian period, a generalized tectonic setting prevailed, which led to the development of gradually expanding sedimentary basins marking the onset of the Alpine sedimentary cycle. ...
Sardinia (Italy) represents one of the most comprehensive cross-sections of the Variscan orogen. The metamorphic and structural complexity characterizing its axial zone still presents many unresolved issues in the current state of knowledge. The data presented from the structural study of the entire axial zone of this area have allowed the authors to propose a subdivision into two new structural complexes. In particular, a younger complex is identified as the New Gneiss Complex, containing remnants of an older and higher-grade metamorphic complex defined as the Old Gneiss Complex. The structural and geometric relationships between the two complexes suggest the redefinition of the axial zone of Sardinia as part of the intracontinental East Variscan Shear Zone/medium-temperature Regional Mylonitic Complex. Comparable relationships are also highlighted in many other areas of the Variscan chain (e.g., Morocco, Corsica, Maures Massif, and Argentera Massif). Extending this new structural interpretation to other axial zones of the South European Variscan orogen could provide new hints for reconstructing the collision boundaries between Gondwana and Laurussia in the late Carboniferous to the early Permian periods.
... The Iberian Massif represents one of the largest exposures of the Variscan orogen formed at the end of the Palaeozoic, between ca. 370 and 290 Ma, through the convergence between the Avalonia microcontinent and the northern margin of Gondwanaland (e.g., Matte, 1986;2001;Kroner & Romer, 2013). ...
The Lusinde biotite granite constitutes a small late-post-tectonic Variscan massif, emplaced along the western termination of the Juzbado-Penalva Shear Zone (JPCSZ). The main objective of this work is to constrain the time of crystallisation of the Lusinde massif and to apply the technique of anisotropy of magnetic susceptibility to better understand pluton emplacement and internal structure. New LA–ICP–MS U–Pb zircon geochronological data for this intrusion yielded an emplacement age of 295 ± 1 Ma. The Lusinde granite shows relatively low values of magnetic susceptibility (Km = 152 × 10-6 to 271 × 10-6 SI), typical of granites of the ilmenite series in which biotite is the main carrier of magnetic signal. A pyroxene- and amphibole-bearing mafic microgranular enclave exhibits higher Km magnitudes (536 × 10-6 SI). Magnetic foliations are steeply plunging and strike subparallel to the pluton irregular margins. Magnetic lineations show rather variable directions and steep plunges, suggesting that the whole massif may correspond to a feeder zone. The studied pluton seems to belong to an E-W band of steep lineations traced along the northern border of the main mass of the Cota-Viseu late-post-tectonic granite and associated mafic and intermediate rocks, which has been interpreted as an alignment of feeder zones related to the extensional termination of the Juzbado-Penalva Shear Zone (JPCSZ). As a result, it is concluded that the emplacement of the Lusinde granite was tectonically controlled by the JPCSZ.
... The westernmost part of this belt corresponds to the Iberian Variscan Belt (IBV), which is subdivided into six palaeogeographic zones as follows, each having specific tectonic, geologic, metamorphic, and stratigraphic characteristics [20,31,32] Currently, two distinct models are in discussion to explain the tectonic evolution of the IVB and its corresponding magmatism. One model points out the existence of a single orocline, the Cantabrian orocline, defining a C-shape, continental-scale single bend [33][34][35]. In the second model, two oroclines are considered, as follows: a clear northern curvature, namely the Cantabrian Arc (or Cantabrian orocline) [36] and an opposite slightly defined southern curvature, named as the Central Iberian Arc (or Central Iberian Orocline) [37][38][39]. ...
In this paper, we have synthesized the information derived from more than 20 papers and PhD theses on the anisotropy of the magnetic susceptibility (AMS) of 19 Variscan granite plutons, spanning the period between 320 Ma and 296 Ma. The AMS data are obtained from 876 sampling sites with more than 7080 AMS measurements and a re-interpretation is proposed. The studied granites exhibit a magnetic susceptibility (Km) ranging from 30 to 10,436 × 10−6 SI units. Most granites typically exhibit Km values below 1000 × 10−6 SI, indicative of paramagnetic behavior. Biotite serves as the main carrier of iron (Fe), emphasizing the reduced conditions prevalent during the formation of granite melts in the Variscan orogeny. The AMS fabrics of the studied granite plutons record the magma strain, expressing the chronologic evolution of the stress field during the orogeny. This chronologic approach highlights the magmatic events between around 330 and 315 Ma, occurring in an extensional regime, in which the Borralha pluton is an example of a suite that recorded this extensional AMS fabric. Plutons with ages between 315 and 305 Ma show AMS fabrics, pointing out their emplacement in a compressional tectonic regime related to the Variscan collision. The plutons, younger than 305 Ma, record AMS fabrics indicating that the tectonic setting for emplacement changes from a wrench regime to an extensional one at the end of the collision stage. This is evident as there is a chronological overlap between the granites that exhibit AMS fabrics indicating extension and the ones that have AMS fabrics indicating a wrench regime.
... In the European Variscan belt, distinct microplates (or continental ribbons), separated by eclogite-bearing suture zones, are identified based on paleomagnetic, paleogeographic, and litho-tectonic constraints (see reviews with references therein in Franke, 1989;Matte, 2001;Stampfli et al., 2013;Domeier and Torsvik, 2014;Franke et al., 2017;Edel et al., 2018;Martínez Catalán et al., 2021;Schulmann et al., 2022;Roda et al., 2023). In this framework, a significant discovery of three decades of geological research is the similarity of both the lithostratigraphy and tectono-metamorphic evolution of the Argentera-Mercantour Massif, the Maures-Tanneron Massif, Variscan Corsica, and Sardinia (Carmignani et al., 1992;Crévola and Pupin, 1994;Carosi and Oggiano, 2002;Bellot et al., 2003;Franceschelli et al., 2005;Rossi et al., 2009;Corsini and Rolland, 2009;Compagnoni et al., 2010;Elter et al., 2010;Schneider et al., 2014;Gosso et al., 2019;Bolle et al., 2023;Tabaud et al., 2023). ...
The newfound Bois de Sélasse eclogite in the eastern Argentera-Mercantour Massif (External Crystalline Massifs, Western Alps) is crucial for better constraining the tectonic evolution of the southern part of the European Variscan belt. The whole-rock composition of this eclogite aligns with that of a basaltic protolith with a normal mid-oceanic-ridge affinity, and U-Pb dating on igneous zircon cores reveals an emplacement age of 524 ± 5 Ma. The emplacement may have occurred either in the oceanic lithosphere to the north of the active Gondwana margin or within a back-arc basin during the subduction beneath Gondwana. Exceptionally preserved prehnite–pumpellyite to eclogite facies minerals provide evidence of prograde metamorphism along a standard oceanic subduction geotherm (≤10 °C/km). Peak eclogite facies conditions are constrained at 610–660 °C and 1.9–2.3 GPa by thermodynamic modeling combined with Ti-in-zircon and Zr-in-rutile thermometry. A minimum age for eclogite facies metamorphism is established at 339 ± 6 Ma by U-Pb dating on metamorphic zircon rims. The protolith of the Bois de Sélasse eclogite is indeed older than the Variscan oceans, but it was similarly affected by Variscan subduction. We discuss the implications of this new finding in the context of the European Variscan belt.
... The basement exposed in Sardinia represents a segment of the Southern European Variscan belt formed between ∼380 Ma and ∼280 Ma (Arthaud & Matte, 1977;Carmignani et al., 1994;Matte, 2001;Di Vincenzo et al., 2004) because of the continent-continent collision involving Laurentia-Baltica and Gondwana. The portion of the belt exposed in Central and Western Europe is known as the Variscides and its south-eastern part underwent reworking and fragmentation during the Alpine Orogeny. ...
... The Iberian RMGs are located in the Variscan Belt, which is the basement that formed along the Devonian, Carboniferous, and Early Permian [17]. From a tectonic point of view, the Variscan Belt is the result of the Devonian and Carboniferous collision between Gondwana, as well as some peri-Gondwana terranes and the Avalonian margin of Laurentia [18]. The early imprint of this collision was recorded by the development of active margins and arcs and the subsequent accretion of arc units onto continental blocks, thereby triggering HP/HT eclogite units and the accretion of ophiolites formed in suprasubduction environments [19]. ...
The intensive variables, geochemical, mineralogical, and petrogenetic constraints of the Iberian peraluminous rare metal granites (RMGs), many of them unknown, are presented. The mineral chemistry of ore and gangue minerals, whole rock analyses, geothermobarometry, melt water and phosphorus contents, mass balance, and Rayleigh modeling were performed to achieve these objectives. These procedures allow us to distinguish two main contrasting granitic types: Nb-Ta-rich and Nb-Ta-poor granites. The former have lower crystallization temperatures, higher water contents, and lower emplacement pressures than Nb-Ta-poor granites. Nb-Ta-rich granites also have higher fluoride contents, strong fractionation into geochemical twins, higher Na contents, and different evolutionary trends. At the deposit scale, the fractional crystallization of micas properly explains the variation in the Ta/Nb ratio in both Nb-Ta-poor and Nb-Ta-rich RMGs, although in higher-grade granites, the variation is not as clear due to the action of fluids. Fluid phase separation processes especially occurred in the Nb-Ta rich granites, thus transporting halogens and metals that increased the grades in the top and sometimes in the core of granites. Gas-driven filter pressing processes facilitated the migration of fluid and melt near solidus melt in Nb-Ta-rich granites. The geochemical signature of the Iberian rare metal granites mainly follows the trends of two-mica granites and P-rich cordierite granites, but also of granodiorites.
... The Devonian-Carboniferous convergence of Gondwana and Laurussia including the Armorica and Avalonia microplates separated by the Rheic ocean resulted in the closure of the Rheic ocean and the Variscan orogeny across Central Europe (e.g. Matte, 2001). The closure of the Rheic ocean produced a suture that is traceable in central and southwestern Europe (e.g. ...
... Considering the Devonian plate-tectonic environment, differences appear when appreciating the plate-tectonic evolution. In the general juxtaposition of terrane assemblages during Devonian times, formerly being part of a long ribbon continent behind the subducting Rheic Ocean, Matte (2001) admitted an iteration of oceanic spaces and microplates, like the opening Rheic Ocean and the Mid-European Galician Ocean. Stampfli et al. (2013, their Fig. ...
... O ciclo Varisco é caracterizado pela colisão diacrónica entre dois supercontinentes, Laurásia e Gondwana (Matte, 1986;Matte, 2001). O orógeno Varisco abrange montanhas desde a América do Norte, de África e parte da Europa até à Ásia ocidental. ...
... The geological composition of this tectonic domain is mainly associated with the Variscan Orogeny of late Paleozoic age (480-290 Ma) [24]. This orogenic cycle was the result of the collision of the supercontinent Gondwana with Laurasia, and the closing of the Rheic Ocean [25,26], and was responsible for defining the structure of the Iberian Massif [27], in which rocks of Precambrian and Paleozoic age outcrop. The evolution of OMZ is essentially characterized by the development of sedimentary basins, during the Carboniferous, after the collision between Gondwana and Laurasia. ...
The Arraiolos Zone has been affected by the persistent superficial seismicity (focal depth < 20 km) of a weak magnitude (M < 4) and some events of a higher magnitude (M > 4), and is mainly located around the Aldeia da Serra village. On 15 January 2018, at 11:51 UTC, the largest instrumental earthquake recorded in that area occurred, with a magnitude (ML 4.9) located northeast of Arraiolos, near the Aldeia da Serra village. This event was followed by a sequence of aftershocks with a magnitude (ML) ≤ 3.5. This seismic sequence was monitored by the designated temporary seismic network of Arraiolos, comprising 12 broadband seismic stations (CMG 6TD, 30 s) from the ICT (Institute of Earth Sciences, Évora) and 21 short-period stations (CDJ 2.0 Hz) from the IDL (Instituto Dom Luiz), distributed around the epicenter, within a radius of approximately 25 km. To infer the structure and kinematics of faults at depth and to constrain the crustal stress field in which the earthquakes occur, we use the polarities of the first P-wave arrivals and the S/P amplitude ratios to better constrain the focal mechanisms of 54 events selected, and apply the HASH algorithm. Overall, the good-quality (defined by the HASH parameters) focal solutions are characterized by a mixture of reverse and strike-slip mechanisms in our study area (AZS). Our seismicity and focal mechanism results suggest that the horizontal stress is more dominant than the vertical one and oriented in the NW-SE direction, parallel with the strike of the main faults. This analysis leads us to affirm that the ASZ is an active right-lateral shear zone.
... Permian implies that this orogenic event can hardly be related to the Variscan Orogeny. In fact, Variscan pressurization events are related to Laurussia and Gondwana (and peri-Gondwana) convergence and collision and the closure of the Rheic Ocean and adjacent basins, that were all almost completed by Devonian to early-mid Carboniferous time (Matte 2001;Murphy et al. 2006Murphy et al. , 2010Martínez-Catalán et al. 2007Nance et al. 2010;Pereira et al. 2010Pereira et al. , 2012Díez Fernández et al. 2016;Arenas et al. 2021;Novo-Fernández et al. 2022). ...
... The Gondwanan antipodal Ibero-Armorican and SNEO oroclines likely have a common origin in Gondwanan kinematics (Klootwijk, 2009;Veevers, 2013) (Edel et al., 2015(Edel et al., , 2018, and by latest Carboniferous to mid Permian magmatic episodes (Paquette et al., 2003;Schaltegger and Brack, 2007;Visonà et al., 2007;Pereira et al., 2014;Gaggero et al., 2017;Secchi et al., 2022). The latest Carboniferous-Permian phase of dextral shearing was likely primed by late Carboniferous development of precursory lineaments such as the East Variscan Shear Zone, part of the Atlas-East Variscan-Elbe shear system (Matte, 2001;Corsini and Rolland, 2009;Ballèvre et al., 2018;Elter et al., 2020;Simonetti et al., 2020;Faure and Ferrière, 2022;Vanardois et al., 2022;Bolle et al., 2023). ...
A long and detailed pole path has been defined from ignimbritic successions across the Tamworth Belt forearc basin of a Carboniferous continental arc in the southern New England Orogen (SNEO) of eastern Australia. Stratigraphic successions spanning about 50 myr have been studied paleomagnetically in 400 sites covering 4500 samples. The Carboniferous SNEO path is thought representative for Australia and Gondwana. Its prominent from-south-over-east-to-north loop with a mid Carboniferous apex differs fundamentally from conventional Australian and Gondwanan Carboniferous pole paths featuring from-south-over-west-to-north loops. The eastern loop of the SNEO path is supported by poles from other workers on the Tamworth Belt. The western loop of conventional paths may reflect unrecognised overprinting and alternative polarity interpretation. Mid-to-latest Carboniferous segments of the SNEO path and of a Carboniferous-to-Permian pole path for the northern Variscan massifs of Armorica (AR) are comparable in shape and length, each spanning more than a hundred degrees of arc. Visual matching of the two pole path segments by rotation around an Euler pole, with the SNEO segment taken as representative for Gondwana and representation of the AR segment tentatively extended to Laurussia, locates Armorica off northwestern Gondwana, and with it Laurussia, in a mid-to-latest Carboniferous Pangea-B configuration. The two mid-to-latest-Carboniferous pole path segments are each bounded by prominent mid Carboniferous and latest Carboniferous-to-earliest/early Permian loops, likely reflecting global tectonic events, dating the Pangea-B configuration as extending from the Sudetic phase to the Asturian phase of the Variscan Orogeny, with transformation to Pangea-A starting likely therefrom. Such a Pangean evolution offers new insights in location of a northern Gondwanan Armorican Spur, causes of the late Carboniferous Hercynian Unconformity and early Permian Pangea-wide extension, and drivers for contemporaneous oroclinal deformation of the Ibero-Armorican Arc and the SNEO. Global movements described by the SNEO and AR pole paths suggest causality between a Visean northern excursion of Gondwana that likely disturbed the earth’s moment-of-inertia, a latest Visean-to-Serpukhovian inertial interchange true polar wander (IITPW) event that led to the Serpukhovian biodiversity crisis and latest Visean-Serpukhovian onset of continental glaciation consolidating the Late Paleozoic Ice Age (LPIA), and the Bashkirian start of the Permo-Carboniferous Reverse Superchron (PCRS).
... two main continents, Gondwana and Laurussia, as well as intervening micro-continents and oceanic basins, resulting in the formation of the Pangea supercontinent at the end of the Paleozoic (Kröner and Romer, 2013;Matte, 2001;Stampfli et al., 2013). One of the largest exposures of the Variscan Belt in Western Europe is preserved in the French Massif Central (FMC) (Vanderhaeghe et al., 2020). ...
... The so-called Hercynian basement rocks originated from the large-scale Variscan orogeny, and outcrops can now be found all over the Europe. In Western Europe, the maximum tectonic activity was roughly between 360 and 300 Ma and led to the 55 formation of several lithological units depending on the original plate, their position and their metamorphism degree during the orogenic collision (Matte, 2001;Nance et al., 2010;Skrzypek et al., 2014;Lardeaux et al., 2014). These rocks are now the basement of the West European large-scale Permian to Cretaceous basins and of the different grabens composing the European Cenozoic Rift System that formed since the late Eocene Epoch (Bourquin et al., 2007;Schumacher, 2002;Ziegler, 1990;Bourgeois et al., 2007). ...
Fracture networks linked to the brittle deformation of rocks are often hosts of fluid flows or geomechanical discontinuities which are important to model for rock mass stability analyses, reservoir assessment or storage capacity evaluation. However, the fractures can be mapped only poorly by subsurface geophysics or borehole imagery. 1D scan lines or 2D maps on outcrops analogues are thus one of the current methods used for the assessemet of the networks, but are very limited regarding the 3D distribution of the fractures. This paper shows the first tests and results for a new automated workflow of fracture properties characterization, using high resolution LiDAR and Photogrammetry data on outcrops. From the obtained 3d point clouds on different objects such as quarries, cliffs or road sides, we reconstruct the outcrop surface in a 3D meshed surface with the help of a Simple and Scalable Surface Reconstruction (SSSR). The fracture planes are automatically detected based on a region-growing segmentation approach based on the spatial distribution of the points. It allows to quickly and effectively extract fracture properties such as orientation, geometry and position in the 3d space, with a possibility for direct 3D density computation without using proxies from 1D or 2D data. This paper present the workflow methodology and the tests on 5 basement rock outcrops of different outcropping quality. We also compare the method with a manual fracture picking tool and a classical 1D scan line methodology on the field. It is a first step to get more precise, automatic, easier and faster modelling workflows on fractured rocks from outcrop analogues data.
... In this setting, regional scale transcurrent/transtensive shear zones were mainly active in central Pangaea (Arthaud and Matte 1977;Matte 2001), leading to the palaeogeographic configuration of the Palaeozoic units exposed in the central Mediterranean and in the Apennines (Molli et al. 2020). At that time in these areas, widespread magmatism was associated with the strike-slip tectonics (e.g. ...
... Continental plates collided in the Lower Paleozoic, which produced the Variscan Orogeny (Matte 2001;Ribeiro et al. 2007;Arenas et al. 2016;Veludo et al. 2017). The later Mesozoic rifting and breakup of Pangea into Laurasia and Gondwana had a profound effect on the continental crust of the western border of Iberia since the metasedimentary materials present in this area are the product of sedimentation in the Tethys Sea, which was between these two supercontinents . ...
Geoconservation, geotourism and geodiversity are concepts that should be taught at all educational levels. The entire society should preserve the natural resources in a sustainable way. Geological processes and quarrying created and still creates cultural heritage in landscapes, villages and cities worldwide. Rock outcrops, historical quarries, forms of extraction and buildings built with stones are a heritage that should safeguard. This work details a geotourism route that would bring new life to Tras-os-Montes e Alto Douro University (UTAD) campus. The proposed route connects a historical quarry located in the UTAD campus with the main church built with its granite and different rock outcrops that show the geological history of the city of Vila Real (Portugal). It envisages a three-points of interest tour from the Fernando Real Geology Museum (UTAD) to a church built with Prezandães granite from Folhadela.
... The Iberian Massif represents a piece of the Western European Variscan Orogen, which was formed after the collision of Laurussia, Gondwana and their peripheral terranes during the late Paleozoic (Figs. 1a and 1b;Kroner and Romer, 2013;Matte, 2001;Schulmann et al., 2014). This collision resulted in the amalgamation of A C C E P T E D M A N U S C R I P T continental terranes separated by two types of suture zones, those along which Gondwanaderived terranes are juxtaposed and those where Gondwana-derived terranes are juxtaposed against pieces of continental crust with Laurussian affinity. ...
Exhumation of high-pressure (P) rocks may require a long path and multiple deformation phases. During this journey, late faults and folds can introduce changes to the primary tectonic stacking and lead to misleading conclusions regarding subduction polarity and plate reconstructions. This hypothesis has been tested positively via mapping and structural analysis in the eastern section of the Central Unit (Eastern Ossa-Morena Complex, Iberian Massif), which comprises Devonian high-P rocks subducted during the Variscan Orogeny. Following subduction beneath Gondwana, exhumation was assisted by in-sequence underthrusting of the continental crust, along with thinning of the overlying and formerly accreted crust. Convergence persisted and was accommodated by Gondwana-directed, out-of-sequence thrusts. Subsequent extension favored erosion and basin inception during the Early Carboniferous, whereas further convergence produced late folding and faulting during Late Carboniferous sinistral transpression. Late faults duplicated the Devonian suture zone several times, producing a series of closely-spaced exposures of a single suture. The manner in which late faults affected the Devonian suture produced an outcome that could be mistaken for a collection of individual suture zones. Late faults may distort the primary relationships between upper and lower plates; however, they provide a geometry-based approach for restoring the primary geometry of suture zones.
... The Variscan chain is an 8000 km-long belt formed as the consequence of Palaeozoic subduction and collision events [Martínez Catalán et al., 2020, Matte, 2001. Successive rifting, subduction and collision of different continental microblocs and/or island arcs were identified before the final collision of Laurussia and Gondwana [Franke et al., 2017]. ...
... The Variscan-Alleghanian-Ouachita orogen is a long (∼10,000 km) mountain belt ranging from the Urals to the Ouachita Mountains (Nance et al., 2010). This mountain system formed between 480 and 250 Ma because of the convergence of Laurussia with Gondwana along with the amalgamation and incorporation of several microcontinents (Matte, 2001). Ireland lies in the northern Variscan foreland (Fig. 1A), where the Variscan orogeny primarily reworked preexisting Caledonian structures. ...
The Variscan orogen in southern Ireland and Britain is characterized by an intensely deformed, E-W−trending fold-and-thrust belt. Farther north in Ireland, the Carboniferous North Dublin Basin exhibits tight chevron folds and kinematically linked en echelon vein sets, along with bedding-parallel veins with slickenfibers. This deformation is assumed to be Variscan in age, despite lying 150 km north of the supposed Variscan “front.” The laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb dating of these calcite veins undertaken for this study showed that relict Variscan U-Pb ages are very poorly preserved. Instead, late Eocene ages were obtained from many calcite veins, including fold hinge breccias and bedding-parallel slickenfiber veins associated with N-S shortening (flexural slip). Also, U-Pb ages from one bedding-parallel vein showed protracted flexural slip over ∼5 m.y. during late Eocene times. Detection of the growth domains within this vein was facilitated by the imaging approach to LA-ICP-MS U-Pb dating adopted in this study, which can identify age-homogeneous domains by integrating spatial U-Pb isotopic information with maps of petrogenetically diagnostic major and trace elements. The late Eocene fold reactivation phase was hitherto undetected on the Irish mainland, but regional Cenozoic N-directed shortening has been documented in Mesozoic−Cenozoic sequences of the southern Irish Sea, Celtic Sea, southern England, and the Paris Basin. We attribute late Eocene fold reactivation to far-field, N-directed shortening associated with the Alpine/Pyrenean orogenies. It is likely that many Variscan or Caledonian folds, particularly in southern onshore Ireland, were reactivated during Eocene−Oligocene shortening, which has not been recognized to date because of the lack of post-Variscan markers (e.g., dikes, Mesozoic−Cenozoic cover sequences).
High‐temperature–low‐pressure metamorphism is commonly associated with intermediate to felsic magmatism in continental orogenic belts. The heat budgets and transfer mechanisms responsible for such elevated temperatures and partial melting of the upper crust are uncertain. The Trois Seigneurs massif, French Pyrenees, preserves a structurally continuous record of Variscan high‐temperature–low‐pressure metamorphism through a sequence of upper‐to‐mid‐crustal Paleozoic metasedimentary rocks. Conventional thermobarometry and phase equilibria calculations show that metamorphic conditions span ~2.5 kbar, 575°C to suprasolidus conditions of ~6 kbar, 700°C. Peak temperatures depend strongly on depth: temperature gradients of 50–60°C/km are present through the uppermost 12 km of the section; deeper portions (12–20 km) define restricted temperature conditions of ~650–700°C. The lowest‐grade metamorphic rocks preserve the largest spread in monazite ²⁰⁶ Pb*/ ²³⁸ U dates, from c. 325–285 Ma, while the spread in dates is restricted to c. 305–290 Ma in the highest‐grade rocks. Within this spread, each sample yields a well‐defined population of monazite ²⁰⁶ Pb*/ ²³⁸ U dates with peaks at c. 305 Ma in the andalusite schists, 295 Ma in the sillimanite schists, and 300 Ma in the migmatite sample. Monazite trace‐element compositions capture a systematic change with decreasing date and increasing metamorphic grade, including a more negative Eu‐anomaly and decreasing Sr concentrations, consistent with co‐crystallizing feldspar; increasing HREE and Y contents, consistent with xenotime breakdown; and decreasing Th/U, reflecting increasing U content during breakdown of inherited zircon. Zircon rims from a granite unit that formed via partial melting of the Paleozoic sedimentary package yields a ²⁰⁶ Pb/ ²³⁸ U‐ ²⁰⁷ Pb/ ²³⁵ U concordia age of 304.1 ± 3.73 Ma. These rims have trace‐element compositions reflecting cogenetic apatite and zircon growth during granite formation. Zircon from a calc‐alkaline granodiorite intrusion preserves a 40 Ma record of melt‐related activity in the lower crust that preceded the regional thermal climax. We interpret these petrochronological data to show that the Trois Seigneurs field gradient including andalusite schist and biotite granite samples represents a genuine geotherm through Variscan orogenic crust during the regional thermal climax at 305 Ma. When combined with constraints from other Pyrenean massifs, the form of the geotherm is consistent with a thermal scenario in which heat is advected to the upper crust by intermediate‐composition magmas generated in the lower crust. A simple thermal model for this process indicates that anatexis in the upper crust may plausibly occur within 10 Ma of the initiation of the lower‐crustal melting. Such a thermal scenario, however, requires focusing of melt through a fertile lower crust and an elevated Moho heat flux. We suggest that this process may have controlled the attainment of high‐temperature–low‐pressure metamorphic conditions along the Variscan belt and may currently be operating in zones of post‐orogenic continental extension.
The Variscan belts of North Africa are renowned for their diversity and richness in ore deposits of significant economic potential. The Variscan mineral resources, of polymetallic character, are geologically controlled by complex and polyphase tectonic-magmatic events driven by the Variscan orogeny, dating back to the late Paleozoic times. The Variscan (Hercynian) vein-type mineralization, along the northern fringe of the West African Craton, is mainly hosted within amalgamated Precambrian and Paleozoic terranes of the Moroccan Meseta domain and Algerian-Moroccan Atlas system. The development of large ductile to brittle shear zones accompanied or not by plutonic magmatic bodies (mantle, mixed, or crustal) led to the deposition of the main sulfidic and oxidized metallic mineralization as hydrothermal veins during the late Variscan orogeny. Mineralization is predominantly antimony Pb, Sb, Zn, (As, Sn) sulfides and Ag sulfosalts with free or sulfide-trapped gold commonly associated with As quartz structures sheltered along E-W to NE-SW or NW-SE reactivated basement faults. Two mineralization process phases have been identified; the first one with Sn-W-Mo±Au (Be) is associated with the Hercynian calc-alkaline to monzonitic granitoid emplacement, dated at 320 to 280 Ma. The second one with Pb-Zn-Ag accompanied or not by the leucogranites setting resulted from the reactivation of the pre-existing rooted shear zones, concomitant to the uplift and the erosion of the crust during the late Permian–early Triassic. The formation of mineralizing hydrothermal fluids can be linked either to the direct influence of basal-crustal plutonic magma (calc-alkaline granitoids) to purely crustal magma (Leucogranites), or to the circulation of hydrothermal fluids, of metamorphic origin, coming from the base of the upper crust (supracrustal) through active shear zones during the late Permian transpression phase. Hydrothermal fluids can also be originated from the process of dehydration of sediments by compaction, from the initiation of regional metamorphism resulting from the effect of lithostatic and hydrostatic pressures, and the circulations of meteoric and seawater. These hydrothermal fluids infiltrate in-depth, heat up, and create a geothermal circuit which, while going up toward the surface, deposits the mineralization of tin, tungsten, native gold associated with arsenopyrite, gold-bearing pyrite, silver-bearing galena, sphalerite, in zones made permeable by brecciation and opening of the pre-existing fractured or newly created and faulted systems. Undoubtedly, the Tighza-Aouam (Pb, Zn, Ag) deposit is also rich in gold and tungsten, the El Hammam fluorite deposit, not metalliferous, and the Achemach Tin deposit are economically the most prominent Variscan ore deposits, exploited exclusively in the Hercynian Central Massif of Morocco; while those of neighboring Algeria correspond to the polymetallic veins of the Boukais massif, Ougarta belt (Cu-Pb±Zn±Ag and Au) and of the Beni Snouss district (Au, As) in the GharRouban massif.
A summary of the geology of North Africa is given in this introduction. North Africa stands in a peculiar position that has enabled the record of a protracted history since the Archean. After the Proterozoic assembly of different cratonic nuclei, the newly formed supercontinent Gondwana underwent a Paleozoic–Mesozoic story that repeatedly weakened and dismembered its northern margin. Before Phanerozoic, the end of Neoproterozoic undoubtedly affected the northern Gondwana margin, but the extent of the Cadomian crustal reworking remains to be ascertained. Paleozoic cycles of Rheic and Paleo-Tethys openings rhythmed the deformation of this Gondwana margin until the opening of the Tethys (Maghrebian Tethys to the west or Neo-Tethys further east). Although the associated 3D plate kinematics of these paleo-configurations have been refined for a decade and provide significant constraints, unambiguous evidence is still lacking on many points. For instance: (1) the Variscan model for NW Africa is still debated on how it correlates with the Variscan-Alleghanian belt; (2) the age of Paleo-Tethys opening remains a matter of debate; (3) the Maghrebian/Neo-Tethys crustal natures and space extensions are still poorly known. These are a few of the questions that are still pending regarding this rich and fascinating geological story recorded in North Africa that will still provide exciting and stimulating works for decades to come. We hope that this short introduction will interest and stimulate the readers curious about this part of the world where there is still a lot to do.
The general configuration of the main continental blocks in the Gondwana supercontinent and the Ediacaran–early Paleozoic tectonic evolution of its northern margin are widely accepted. However, reconstruction of the original positions and the question of potential separation of the Gondwana-derived crustal segments that are now included in the Variscan and Alpine orogenic belts remain controversial. The Western Carpathians, part of the Alpine–Carpathian belt, represents an important crustal segment broken-off from northern Gondwana and later incorporated into both the Variscan and Alpine collisional orogens. The earliest tectonic evolution and paleogeography of the pre-Variscan basement of the Western Carpathians remains poorly known, due to insufficient age data and intense polyphase tectonometamorphic overprints, both Variscan and Alpine. This paper provides new results of Usingle bondPb dating and Hf isotopic analysis of detrital zircons as well as a whole-rock geochemical study from metavolcanic-sedimentary basement units of the Western Carpathians. The obtained age spectra suggest that the sedimentary succession was supplied dominantly by Ediacaran (c. 600 Ma) zircons, with a relatively minor role for Stenian–Tonian (c. 1.2–0.9 Ga) and Paleoproterozoic cratonic (c. 2.2–1.8 Ga) zircons. The mixed Hf isotopic signature (εHf(t) values ranging from −20 to +12) of the Ediacaran zircons indicates substantial mixing of mantle-derived magmas with mature crustal material, typical of continental magmatic arcs. In contrast, the mostly negative εHf(t) values (−15 to +4) of the cratonic zircons suggest recycling of an older continental crust. The presumably youngest part of the sequence is also characterised by high proportion of early Paleozoic zircons with generally negative εHf(t) values (−10 to −2). The zircon Usingle bondPb age spectra, Hf isotopic patterns and whole-rock geochemical signatures of the studied Western Carpathians sequences are interpreted as reflecting deposition at a progressively developing Cambrian–Silurian passive margin setting. The West Carpathian data have been correlated with a comprehensive detrital zircon Usingle bondPb age and Hf isotope data set compiled from possible source areas and other Gondwana-derived units to test the possibility of their primary linkages. These correlations indicate strong similarity in both zircon Usingle bondPb age spectra and Hf isotopic compositions to other parts of the Ediacaran (Cadomian) continental magmatic arc. Older, cratonic sources are linked to the Saharan or East African parts of northern Gondwana, whereas the early Paleozoic detritus must represent a local volcanic source. Taken together, our new data from the Western Carpathians provide constraints for a new paleogeographic model of the northern African part of the Gondwana passive margin.
A review of the synorogenic basins formed on the Gondwana side of the Variscan orogen of Iberia is presented, highlighting the widespread ocurrence of calciturbiditic formations and olistostromes containing reef-limestone olistoliths in Iberia's Variscan basins. Using key-examples from the Variscan orogen for comparison (Azrou-Khenifra and Rhenohercynian basins), the significance of these olistostromes and flyschoid deposits is discussed. Our tectonic models of the Variscan belt in Iberia propose possible drivers of synorogenic carbonate platform/reef destruction responsible for the origin of calciturbidites and olistostromes. One model proposes the formation of an orogenic plateau by lateral flow of partially molten orogenic roots in the context of Laurussia-Gondwana convergence, following the destruction of the Rheic Ocean and Gondwana (lower) plate slab retreat. An alternative model invokes subduction of Paleotethys oceanic lithosphere beneath the Gondwana (upper) plate. Both emphasize the Mississippian occurrence of a significant thermal anomaly beneath Gondwana that favored strong lithosphere thinning, creating ideal conditions for synorogenic carbonate platform/reef destruction, and for the formation of calciturbidite deposits and olistostromes in Iberia. Variscan paleotopography would look alike in both situations. Thus, distinguishing these models is not straightforward, with differences in the kinematics of the regional tectonic transport in the superstructure of Mississippian gneiss domes.
Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists
Available online: https://www.lyellcollection.org/doi/abs/10.1144/jgs2023-187
The Cadomian Orogeny produced a subduction‐related orogen along the periphery of Gondwana and configured the pre‐Variscan basement of the Iberian Massif. The architecture of the Cadomian Orogen requires detailed structural analysis for reconstruction because of severe tectonic reworking during the Paleozoic (Variscan cycle). Tectonometamorphic analysis and data compilation in SW Iberia (La Serena Massif, Spain) have allowed the identification of three Cadomian deformation phases and further constrained the global architecture and large‐scale processes that contributed to the Ediacaran building and early Paleozoic dismantling of the Cadomian Orogen. The first phase (DC1, prior to 573 Ma) favored tabular morphology in plutons that intruded during the building of a continental arc. The second phase (DC2, 573–535 Ma) produced an upright folding and contributed to further crustal thickening. The third phase of deformation (DC3, ranging between ∼535 and ∼480 Ma) resulted in an orogen‐parallel dome with oblique extensional flow. DC1 represents the crustal growth and thickening stage. DC2 is synchronous with a period of crustal thickening that affected most of the Gondwanan periphery, from the most external sections (Cadomian fore‐arc) to the inner ones (Cadomian back‐arc). We explain DC2 as a consequence of flat subduction, which was followed by a period dominated by crustal extension (DC3) upon roll‐back of the lower plate. The Ediacaran construction of the Cadomian Orogen (DC1 and DC2) requires ongoing subduction beneath Gondwana s.l., whereas its dismantlement during the Early Paleozoic is compatible with oblique, sinistral convergence.
The renaissance botanical garden of ‘El Bosque’ in Béjar (Salamanca, Spain) presents a pond bounded by a dam in its western part. The latter is formed by two masonry walls interconnected by buttresses. Cubic spaces in between are filled with a variable grain‐size material (silty sand) that allows limited water flow. In recent years the southern part of the dam has experienced localized and random subsidence that jeopardizes the entrance to part of the garden. To regain access, a proper and reliable diagnosis of the origin, magnitude and relevance of the subsidence must be made. In this regard, we have undertaken a microgravity survey in the dam area to identify places with an anomalous distribution of the filling material in order to foresee further sinking or potential collapsing areas. The precise positioning (2 mm resolution) and accurate terrain correction needed in this kind of high‐resolution gravity surveys (points every 1.5 m) was achieved by creating a detailed Digital Terrain Model (cm resolution) with a remotely piloted aircraft. In addition, we performed three electric resistivity tomography (ERT) profiles at different levels of the garden: i) on the dam itself, ii) right on the foot of the dam and parallel to it (5 m below and ∼17m to the W), and iii) a bit farther, but also parallel to the dam (8 m below and ∼27m to the W). The ERT profiles identified high conductivity in water‐saturated areas and determined the paths that rainfall and pond's seepage water follow in the dam and its underground, formed by granites. The geophysical studies were paired with geotechnical analyses of the sunk materials. The study concluded that the thinnest fraction of the dam's filling material (i.e., silts) is being washed away, leaving behind sand with less density and stability, susceptible to collapse. Thus, the observed sinking is related to soil piping, i.e. to soil internal erosion and compaction issues that force the soil material to re‐adjust geometrically and volumetrically.
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The Bohemian Massif in the eastern Variscan Orogen is the host of globally significant ultrahigh-pressure metamorphic rock suites that experienced enigmatic unroofing processes. In the eastern Bohemian Massif, the Moravosilesian Culm Basin (The Czech Republic) with ≤7.5 km thick Lower Carboniferous sili-
ciclastic turbidities has recorded crustal unroofing associated with the late Paleozoic amalgamation of Gondwana and Laurussia. This study presents data from sandstone petrography, heavy mineral assemblages, and detrital zircon U-Pb geochronology from Moravosilesian Culm sediments. Sandstone modal compositions imply continental/recycled orogen affinities. High-grade metamorphic lithic fragments and high garnet contents in upper Culm sediments indicate the exhumation of the middle to lower crustal materials, consistent with those of the Bohemian Massif. Detrital zircon U-Pb data constrain maximum depositional ages of sediments in the Drahany Upland to 335–326 Ma (late Viséan–middle Serpukhovian). Detrital, igneous and metamorphic zircon U-Pb data compiled from surrounding regions support that source regions for lower Culm sediments (Moldanubian Unit and Sudetes) remained the same. Whereas, upper Myslejovice sediments might have derived from exhumation and erosion of the root of the Moldanubian Unit and/or Moravosilesian Nappes. Rapid exhumation of the Moldanubian Unit within the Bohemian Massif during the deposition of upper Culm sediments is inferred.
Lithospheric slices preserving pre-Alpine metamorphic imprints are widely described in the Alps. The Variscan parageneses recorded in continental, oceanic, and mantle rocks suggest a heterogeneous metamorphic evolution across the Alpine domains. In this contribution, we collect quantitative metamorphic imprints and ages of samples that document Variscan tectonometamor- phic evolution from 420 to 290 Ma. Based on age distribution and metamorphic imprint, three main stages can be identified for the Variscan evolution of the Alpine region: Devonian (early Variscan), late Devonian–late Carboniferous (middle Variscan), and late Carboniferous–early Per- mian (late Variscan). The dominant metamorphic imprint during Devonian times was recorded under eclogite and HP granulite facies conditions in the Helvetic–Dauphinois–Provençal, Penninic, and eastern Austroalpine domains and under Ep-amphibolite facies conditions in the Southalpine domain. These metamorphic conditions correspond to a mean Franciscan-type metamorphic field gradient. During the late Devonian–late Carboniferous period, in the Helvetic–Dauphinois–Provençal and central Austroalpine domains, the dominant metamorphic imprint developed under eclogite and HP granulite facies conditions with a Franciscan field gradient. Amphibolite facies condi- tions dominated in the Penninic and Southalpine domains and corresponded to a Barrovian-type metamorphic field gradient. At the Carboniferous–Permian transition, the metamorphic imprints mainly developed under amphibolite-LP granulite facies conditions in all domains of the Alps, corre- sponding to a mean metamorphic field gradient at the transition between Barrovian and Abukuma (Buchan) types. This distribution of the metamorphic imprints suggests a pre-Alpine burial of oceanic and continental crust underneath a continental upper plate, in a scenario of single or multiple oceanic subductions preceding the continental collision. Both scenarios are discussed and revised considering the consistency of collected data and a comparison with numerical models. Finally, the distribution of Devonian to Triassic geothermal gradients agrees with a sequence of events that starts with sub- duction, continues with continental collision, and ends with the continental thinning announcing the Jurassic oceanization.
The evolution of the Gondwana along the flank of the West African Craton was complex and is far from
understood. Subduction-related activity along this margin spanned between c. 750 and 500 Ma.
Sections close to African cratons record the earliest stages, while Autochthonous and Allochthonous
domains of the Variscan Belt preserve the latest stages of the arc system, essentially between c. 540 and
500 Ma. The geochemistry of the Ediacaran-early Cambrian siliciclastic series deposited along this
Cadomian active margin preserves the evolutionary history of their sources, which are related to activity
in the arc and nearby continental areas. In this sense, the SW Iberian Massif (Ossa-Morena Complex)
preserves a section of this Ediacaran-early Cambrian peri-Gondwanan arc. Its evolution can be tracked
through the characterization of the subduction-related magmatism (including the Mérida Massif) and
coeval metasedimentary record (Serie Negra Group and Malcocinado Formation) during a time interval
spanning almost 100 m.y., from pre-602 Ma to at least c. 534 Ma. This study reveals that arc magmatism
is closely linked with synorogenic deposition in a complex way so far unexplored. Arc recycling is
revealed by the isotopic equivalence of synorogenic strata to the first magmatic event (pre-602 Ma),
and by geochronological data of the arc-building pulses. The earliest magmatic pulses (c. 602–550 Ma)
are characterized by significant crustal input, likely favoured by subduction erosion. Subsequently,
magmatism evolved towards larger mantle involvement (c. 540–534 Ma), likely associated with progressive
variation in the slab angle. These slab-mantle-upper plate interactions generated changes in
the arc dynamics leading to an extensional setting with alkaline magmatism during the Cambrian. This
review proposes a model of petrogenetic and geodynamic arc evolution between the Ediacaran and
the Early Cambrian. The gathered data could improve the accuracy of future palaeogeographic
reconstructions for the northern margin of Gondwana.
The Aracena metamorphic belt, in the southwest Iberian Massif, is characterized by the presence of MORB-derived amphibolites
and continental rocks deformed and metamorphosed during the Hercynian orogeny. Geochemical relationships of these amphibolites
indicate the existence of a multiple fractionation process from a set of parental magmas, implying the existence of a multi-chamber
system beneath the ridge where the basalt protolith was extruded. Neodymium isotopic ratios are typical of MORB, and oxygen
isotopes indicate that these amphibolites have been derived from the uppermost part of the oceanic crust. Thermal evolution,
revealed from the study of chemical variations in the amphibole chemistry, is interpreted as resulting from subduction in
a low-pressure regime in which the thermal structure of the continental hanging-wall played an important role. This continental
wall was previously heated by subduction of a slab window resulting from migration of a triple junction along the continental
edge during plate convergence. Three petrologic arguments support this tectonic model. These are: (1) the low-pressure inverted
metamorphic gradient of amphibolites of the oceanic domain; (2) the high-temperature- low-pressure metamorphism of the continental
hanging wall; (3) the early intrusion of boninites into the continental domain.
Field characteristics indicate that the Badajoz-Cordóba shear zone has had a long, polyphase tectonothermal evolution. 40Ar/39Ar cooling ages recorded within central, ductile sectors of the Badajoz-Cordóba shear zone range from ca. 370 to 360 Ma (amphibole) and 340 to 330 Ma (muscovite). These are interpreted to date post-metamorphic cooling through contrasting temperatures required for intracrystalline retention of argon following a regionally significant Variscan tectonothermal overprint. These results combined with field relationships indicate that the Badajoz-Cordóba shear zone experienced at least 15 km of uplift relative to adjacent areas during late Paleozoic sinistral transpression. The data also indicate that individual structural units within the shear zone experienced contrasting uplift histories (both in total amount and rate). The 40Ar/39Ar cooling ages indicate complete Variscan rejuvenation of all older intracrystalline argon systems within central sectors of the Badajoz-Cordóba shear zone.
: We summarise the main events that mark the contraction and extension histories of the Scandinavian Caledonides and the European Variscides. We show that continental subduction may have developed similarly large and asymmetric thrust systems in both orogens. However whereas continent-continent collision developed in the Variscides, extension began in the Scandinavian Caledonides marking the end of continental subduction. This led extensional tectonics to affect two continental crusts with contrasted rheology and therefore led to contrasted extensional modes. We argue that plate divergence responsible for extension in the Scandinavian Caledonides was triggered by the Variscan collision between Laurasia and Gondwana. In contrast, horizontal buoyancy forces acting on a thermally softened thickened crust were more likely responsible for extension in the Variscan belt.
1990.04 We review the highlights of the 1988 symposium on Palaeozoic Biogeography and Palaeogeography, and present a revised set of 20 Palaeozoic base maps,that incorporate much,of the new data presented at the symposium. The maps include 5 major innovations: (1) A preliminary attempt has been made,to describe the motion of the Cathaysian terranes during the Palaeozoic; (2) a more detailed description of the events surrounding the Iapetus Ocean is presented; (3) an alternative apparent polar wandering,path for Gondwana,has been constructed using the changing distributions of palaeoclimatically restricted lithofacies; (4) new,palaeomagnetic,data have been incorporated that places Laurentia and Baltica at more southerly latitudes, and adjacent to Gondwana, during the Early Devonian; Siberia is also placed further south in the light of biogeographic data presented at the symposium; (5) Kazakhstan is treated as a westward extension of Siberia, rather than as a separate palaeocontinent. The relationships between climatic changes, sea level changes, evolutionary radiations and intercontinental migrations are discussed. A symposium on Palaeozoic Biogeography and Palaeogeography,
The Coimbra—Cordoba shear zone is a major lineament roughly parallel to the NW—SE Variscan structures of the southern Iberian Peninsula.Inside the shear zone evidence for a non-coaxial strain path exists from structural, microstructural and lattice-preferred orientation data on quartz. Plane strain is suggested by the study of microstructures in the (XY) and (YZ) sections in all formations occurring in the shear zone. With increasing strain, a mylonitic series develops.The deformation, contemporaneous with epizonal to mesozonal metamorphism, is responsible for: 1.(1) the regional subvertical foliation oblique to the shear zone2.(2) the well defined subhorizontal lineation striking NW—SE3.(3) the discrete planes of shear roughly parallel to the shear zone.The data are interpreted as resulting from a left-lateral slip motion with a displacement of at least 72 km. In the light of plate tectonics it is inferred that the Coimbra—Cordoba lineament could have been a suture zone which evolved into an intracontinental left-lateral shear zone during the Variscan orogeny of the Ibero-Armorican arc.
The first part of the paper gives a general and schematic overview of the palaeogeography of the present-day peri-Atlantic regions from the early Ordovician to the late Devonian. The second part focuses on the outstanding problems concerning the relations that have existed between the different parts of western Europe. In both cases, diverging interpretations are discussed by comparing the results of sedimentological and palaeontological studies with those coming from other branches of Earth Sciences. -English summary
Zusammenfassung Die Ossa-Morena Zone (OMZ) und die Südportugiesische Zone (SPZ) bauen die südlichsten Anteile des Iberischen Massivs auf (europäisches Variszikum). Sie unterscheiden sich auf lithostratigraphischer, petrographischer und struktureller Basis. Die Grenze zwischen OMZ und SPZ wird vom schmalen Streifen der Acebuches-Amphiboliten gebildet, Meta-Ophiliten tholeiitisch-ozeanischer Prägung (Bard &Moine, 1979;Munha et al. 1986). Sie lassen sich über mehr als 200 km verfolgen. Eine intensive Scherung überprägt sie im Süden. Eine Unterteilung des Südrandes der OMZ und des Nordrandes der SPZ konnte mittels struktureller und lithologischer Kriterien durchgeführt werden. Der südlichste Abschnitt der OMZ längs der Acebuches-Amphibolite, besteht aus hochgradig metamorphen Gesteinen; er ist gegen Norden durch eine weitere, größere Scherzone begrenzt. Die Grenze zwischen OMZ und SPZ muß somit als eine Sutur-Zone zwischen zwei kontinentalen Blöcken betrachtet werden, der OMZ im Norden (heutige Position) und der SPZ im Süden. Diese Sutur wird von einer sinistral transpressiven Konvergenz-Zone mit großräumig liegenden Falten überprägt. Sie kann durch den Ibero-Armorikanischer Bogen mit dem Lizard-Komplex in SW-England korreliert werden.
The mid-European segment of the Variscides is a tectonic collage consisting of (from north to south): Avalonia, a Silurian-early Devonian magmatic arc, members of the Armorican Terrane Assemblage (ATA: Franconia, Saxo-Thuringia, Bohemia) and Moldanubia (another member of the ATA or part of N Gondwana?).
The evolution on the northern flank of the Variscides is complex. Narrowing of the Rheic Ocean between Avalonia and the ATA occurred during the late Ordovician through early Emsian, and was accompanied by formation of an oceanic island arc. By the early Emsian, the passive margin of Avalonia, the island arc and some northern part of the ATA were closely juxtaposed, but there is no tectonometamorphic evidence of collision. Renewed extension in late Emsian time created the narrow Rheno-Hercynian Ocean whose trace is preserved in South Cornwall and at the southern margins of the Rhenish Massif and Harz Mts. Opening of this ‘successor ocean’ to the Rheic left Armorican fragments stranded on the northern shore. These were later carried at the base of thrust sheets over the Avalonian foreland. Closure of the Rheno-Hercynian Ocean in earliest Carboniferous time was followed by deformation of the foreland sequences during the late lower Carboniferous to Westphalian.
Closure of narrow oceanic realms on both sides of Bohemia occurred during the mid- and late Devonian by bilateral subduction under the Bohemian microplate. In both these belts (Saxo-Thuringian, Moldanubian), continental lithosphere was subducted to asthenospheric depths, and later partially obducted. Collisional deformation and metamorphism were active from the late Devonian to the late lower Carboniferous in a regime of dextral transpression. The orthogonal component of intra-continental shortening produced an anti-parallel pair of lithospheric mantle slabs which probably joined under the zone of structural parting and became detached. This allowed the ascent of asthenospheric material, with important thermal and rheological consequences. The strike slip displacements were probably in the order of hundreds of kilometres, since they have excised significant palaeogeographic elements.
Late Paleozoic wrench faulting in southern Europe and northern Africa is interpreted as a right-lateral shear zone induced by the relative motion of two plates - a northern one that includes the Canadian Shield, Greenland, and stable Europe and a southern one that includes the African Shield plus an unknown eastern extension. The relative movement of these two plates was transformed into shortening at both ends of the shear zone and led to the formation of late Paleozoic mountain belts: the Urals to the east and the southern Appalachians to the west. Theoretical and experimental models of the dynamics of faulting may account for the arrangement of the fractures in the shear zone and for the observed displacements.
Most of the Phanerozoic orogenic belts exhibit HP to UHP metamorphism. First discovered in the Alps and the Norwegian Caledonides, ultra-high pressure rocks are now described in various Paleozoic belts including the Kazakhstan Caledonides, the Uralides, and the Variscides. In these orogenic belts both oceanic and continental, crustal and mantle rocks underwent HP to UHP metamorphism in a variable range of temperatures. The PT conditions require subduction of these rocks to depths sometimes, in the case of continental crust, exceeding 100 km. In the Uralides as in the Variscides, the HP metamorphism occurred very early, 100 to 110 m.y. before the end of the orogeny. In the Uralides, there is a linear HP belt west of the main ophiolitic suture. The HP metamorphism (15-18 kbar/500-700°C) developed in supracrustal continental rocks outcropping beneath an ophiolitic nappe. This is the result of the eastward subduction of the East European continental margin beneath a Uralian oceanic lithosphere and an island arc. The island arc is well preserved and the volcanites underwent only low-grade metamorphism. The Variscides exhibit two kinds of HP rocks: (1) the lower allochthonous units, beneath ophiolitic nappes, are characterized by HP to UHP/LT metamorphism (12-25 kbar/400-700°C), developed in the supracrustal rocks of a thinned continental margin, (2) the upper unit includes oceanic rocks and arc crust and mantle with a HP/MT granulitic metamorphism (10-15 kbar/700-900°C). For the northern Iberian and Massif Central sections, this duality may be explained by the following model: HP/UHP rocks of the lower allochthonous units may be interpreted as parts of a thinned passive margin of the Gondwana supercontinent, deeply subducted (between 400 and 380 Ma) beneath an oceanic and island-arc lithosphere, part of a Galicia-Massif Central ocean. The plate tectonic setting is similar to that of the Urals and Oman. Granulitic rocks of the upper unit may represent the deep crust and mantle of an island arc, itself subducted at about the same time beneath the conjugate thinned passive margin (Avalon) of the Galicia-Massif Central ocean.
Palaeozoic palaeogeography, highlighting the North Atlantic Caledonian evolution and the destruction of the Iapetus Ocean and the Tornquist Sea, is recapitulated with reconstruction maps from Early Ordovician to Mid-Devonian times. In the Early Ordovician (Tremadoc-Arenig), Laurentia, Siberia, and the North China Block were positioned in equatorial latitudes, Baltica was located at intermediate southerly latitudes, whilst Avalonia and the European Massifs were located together with the North African part of Gondwana in high southerly latitudes. During the Ordovician, Baltica drifted northwards and approached Siberia while undergoing counter-clockwise rotations. Avalonia rifted away from Gondwana during Arenig-Llanvirn time, and the Tornquist Sea, separating Avalonia and Baltica, narrowed gradually during the Ordovician followed by Late Ordovician 'soft docking' of Eastern Avalonia and Baltica prior to their joint collision with Laurentia. The main collisional event between Baltica and Laurentia occurred at c. 425 Ma and was marked by deep subduction of Baltican crust beneath Laurentia with concomitant eastward translation of nappes over the Baltican margin. Deep subduction was a function both of rapid motion of Baltica (8-10 cm/year) toward a stationary Laurentia and precedence of prolonged subduction of large volumes of cold lithosphere. Shortly after collision, in Emsian times, these rocks were exhumed by extensional collapse.
Deep seismic reflection profiling (ECORS program vertical and wide-angle survey) coupled with field and subsurface geology are used to propose a complete NNE–SSW section of the Variscan Belt in western France from Belgium to southern Brittany. On this 800 km long profile, which probably crosses two Paleozoic sutures, the Variscan Belt appears as a broad fanlike orogen characterized by large (100 km) northward and southward overthrusts with polyphase deformation and metamorphism. The central part of the Belt (Central Armorican zone) suffered a simpler dextral shearing parallel to the strike of the belt with only green schist metamorphism. Deep reflection profiling shows that all the large thrusts visible at the surface root deeply in the lower crust or even crosscut the Moho. In the same way, all the large strike-slip faults parallel to the belt appear at depth as narrow discontinuities which crosscut the whole crust and, in some cases, the Moho. The seismically well-layered crust develops essentially at depth in the most deformed and metamorphic internal parts of the belt, in contrast with the relatively transparent Brabant foreland basement, which escaped the Variscan deformation. The Moho appears as a relatively flat boundary at an average depth of 35 km with different characters: It is sharp and strongly reflective both by wide angle and vertical seismics below the internal thrust zones, much less distinct in the central Armorican zone, and only defined by low frequency signals at wide angle below the Brabant foreland. All these characteristics led us to consider the layering of the lower crust and the Moho itself as initially tectonometamorphic features essentially produced by shearing and crust mantle decollement during the Variscan stacking of the crust by intracontinental lithospheric subduction.
Most of the Phanerozoic orogenic belts exhibit HP to UHP metamorphism. First discovered in the Alps and the Norwegian Caledonides, ultra-high pressure rocks are now described in various Paleozoic belts including the Kazakhstan Caledonides, the Uralides, and the Variscides. In these orogenic belts both oceanic and continental, crustal and mantle rocks underwent HP to UHP metamorphism in a variable range of temperatures. The PT conditions require subduction of these rocks to depths sometimes, in the case of continental crust, exceeding 100 km. In the Uralides as in the Variscides, the HP metamorphism occurred very early, 100 to 110 m.y. before the end of the orogeny. In the Uralides, there is a linear HP belt west of the main ophiolitic suture. The HP metamorphism (15-18 kbar/500-700 degrees C) developed in supracrustal continental rocks outcropping beneath an ophiolitic nappe. This is the result of the eastward subduction of the East European continental margin beneath a Uralian oceanic lithosphere and an island are. The island are is well preserved and the volcanites underwent only low-grade metamorphism. The Variscides exhibit two kinds of HP rocks: (1) the lower allochthonous units, beneath ophiolitic nappes, are characterized by HP to UHP/LT metamorphism (12-25 kbar/400-700 degrees C), developed in the supracrustal rocks of a thinned continental margin, (2) the upper unit includes oceanic rocks and are crust and mantle with a HP/MT granulitic metamorphism (10-15 kbar/700-900 degrees C). For the northern Iberian and Massif Central sections, this duality may be explained by the following model: HP/UHP rocks of the lower allochthonous units may be interpreted as parts of a thinned passive margin of the Gondwana supercontinent, deeply subducted (between 400 and 380 Ma) beneath an oceanic and island-are lithosphere, part of a Galicia-Massif Central ocean. The plate tectonic setting is similar to that of the Urals and Oman. Granulitic rocks of the upper unit may represent the deep crust and mantle of an island are, itself subducted at about the same time beneath the conjugate thinned passive margin (Avalon) of the Galicia-Massif Central ocean.
The new Geodynamic Map of Gondwana Supercontinent Assembly provides insight into the Neoproterozoic breakup of the Rodinia supercontinent that existed from 1000 to 725 Ma, and the subsequent amalgamation of Gondwanaland. Breakout of Laurentia from Rodinia at 725 Ma marks the reorganization of lithospheric plate motions that resulted in the Pan African-Brasiliano orogeny and assembly of Gondwanaland that lasted from 725 to 500 Ma.
A 5 m long permineralized trunk from the Upper Kellwasser member
of southern
Morocco represents
the first record of a large trunk of an identifiable species of
Callixylon, C.
erianum, from Gondwana.
This occurrence constitutes the most reliable evidence based on plant megafossils
for a floral connection
between Laurussia and Gondwana in late Devonian times and for a proximity
of
these continents in Famennian times. The potential of this trunk for studies
of the
architecture and growth patterns of the earliest
trees with a gymnospermous type of arborescent habit is discussed.
pThe Variscan fold belt of Europe resulted from the collision of Africa, Baltica, Laurentia and the intervening microplates in early Paleozoic times. Over the past few years, many geological, palaeobiogeographic and palaeomagnetic studies have led to significant improvements in our understanding of this orogenic belt. Whereas it is now fairly well established that Avalonia drifted from the northern margin of Gondwana in Early Ordovician times and collided with Baltica in the late Ordovician/early Silurian, the nature of the Gondwana derived Armorican microplate is more enigmatic. Geological and new palaeomagnetic data suggest Armorica comprises an assemblage of terranes or microblocks. Palaeobiogeographic data indicate that these terranes had similar drift histories, and the Rheic Ocean separating Avalonia from the Armorican Terrane Assemblage closed in late Silurian/early Devonian times. An early to mid Devonian phase of extensional tectonics along this suture zone resulted in formation of the relatively narrow Rhenohercynian basin which closed progressively between the late Devonian and early Carboniferous. In this contribution, we review the constraints provided by palaeomagnetic data, compare these with geological and palaeobiogeographic evidence, and present a sequence of palaeogeographic reconstructions for these circum-Atlantic plates and microplates from Ordovician through to Devonian times.
The Teisseyre–Tornquist transcurrent fault zone forms the southwestern margin of Baltica in central Europe. Across this fault zone Baltica is fringed by several tectonostratigraphic terranes, some with Cadomian basement. Five tectonostratigraphic terranes defined by dissimilar Cadomian basement and Paleozoic cover units are present in the Czech Republic and southern Poland between the Bohemian Massif and the Teisseyre–Tornquist fault zone. Two of these terranes, the Małopolska terrane and the Lubliniec–Zawiercie–Wieluń terrane, lodged in a restraining bend of the Teisseyre–Tornquist transcurrent fault zone during the sinistral transpression of Eastern Avalonia and Baltica, and were deformed during the Caledonian orogeny. The Łysogóry terrane, situated in a releasing bend of the Teisseyre–Tornquist transcurrent fault zone, has a thicker Paleozoic sedimentary succession than the neighboring terranes, and was deformed during the Variscan orogeny. The Moravian terrane is part of the Rhenohercynian zone of Armorica–Cadomia. The Upper Silesia terrane acted as an indentor and impinged on the assemblage of East Avalonian and Armorican–Cadomian terranes during the Early Carboniferous and deformed the polyorogenic Kraków mobile belt and the Moravian mobile belt. The Kraków mobile belt, deformed in both the Caledonian and Variscan orogenies, includes the western margin of the Małopolska terrane and the Lubliniec–Zawiercie–Wieluń terrane. It is also the locus of Early Carboniferous age magmatism with porphyry copper mineralization.
The Variscan belt of Western Europe is part of a large intra-Paleozoic belt extending on both sides of the Atlantic from the Ouachitas in the US and the Mauritanides in West Africa to the Bohemian Massif in Czechoslovakia and Poland. The belt was constructed between 500 and 250 Ma as a result of convergence and collision between two main continents, Laurentia-Baltica and Africa, after the closure of various oceanic basins (Iapetus, Rheic and Galicia-Massif Central) now recorded as discrete, in part cryptic, sutures which form the roots of large crystalline nappes in which are found dismembered remnants of oceanic crust and mantle. The accretionary history of the Variscan belt in western Europe is related to the progressive closure of probably two oceanic basins: the Rheic in the north, and another, the Galicia-Massif Central, in the south, by respectively southward and northward intraoceanic subduction followed by obduction and intracontinental lithospheric subduction. The result is a broad (700–800 km), fan-like, divergent belt which must be considered as a classical obduction-collision orogen with a long intracontinental history of about 100–150 Ma. Its arcuate shape is probably due to the northwestward impingement of a promontory of the African continent into Laurentia-Baltica.
A plate tectonics model is presented to explain the tectonometamorphic characteristics of the European Variscides. After the closing of two oceanic domains by two-sided subduction (500-420 Ma) and obduction (420-380 Ma), collision of the European and African continental plates occurred. We propose that the subsequent complex intracontinental deformation (380-290 Ma) is the result of a double subduction of the continental lithosphere accompanied by crust-mantle décollement. This mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the Variscan Belt. Accounting for this model and for the relationships between the European Variscides and the other Paleozoic peri-Atlantic belts (Caledonides, Appalachian, Mauritanides and Morocco), we infer the relative positions of Africa, America and Europe between the Silurian and the Permian.
The destruction of Iapetus and Tornquist's Oceans Tectonothermal evolution of the Bad-ajoz-Co rdoba shear zone (SW Iberia): characteristics and 40Ar/39Ar mineral age constraints
- Elf Aquitaine
- ±
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Pale oge ographie de l'Europe occi-dentale de l'Ordovicien au De vonien
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The allochthonous Upper Devonian of Mrirt (eastern Moroccan Meseta). North African continuation of a Montagne Noire carbonate platform? (Abstract)
- R.R. Becker
- M.R. House
- J.E.A. Marshall
P‐T metamorphic history of the Badajoz‐Cordoba medium grade shear belt (Iberian variscan Fold Belt, Southern Spain
- Abalos B.
Proceedings of the sixth International Graptolite Conference of the GWG (IPA)
- J.C. Guttierez-Marco
- M. Robardet
- J.M. Piçarra
Paleozoic assembly of Pangea: a new plate tectonic model for the Taconic, Caledonian and Hercynian orogenies
- Van der Voo R.
Démembrement anté‐mésozoïque de la chaîne varisque d’Europe occidentale et d’Afrique du Nord: rôle essentiel des grands décrochements transpressifs dextres accompagnant la rotation‐translation horaire de l’Afrique durant le Stéphanien
- Bard J.P.
Geochronology and geochemistry of eclogites from the Marianské Laznë complex, Czech Republic: Implications for Variscan orogenesis
- B.L. Beard
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La cinématique de la plaque Ibérique
- Olivet J.L.
North African continuation of a Montagne Noire carbonate platform? (Abstract)
- R R Becker
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Department of Geology
- C R Scotese
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Proceedings of the sixth International Graptolite Conference of the GWG (IPA)
- J C Guttierez-Marco
- M Robardet
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A la recherche des océans perdus: Les éclogites de Vendée
- Montigny R.