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7 Geological map of part of the South Urals foreland thrust and fold belt, the Magnitogorsk accretionary complex, and the western part of the Magnitogorsk Arc. The black line shows the location of the cross section in 8.
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The Uralide Orogen of Russia contains well-preserved examples of Paleozoic arc–continent collisions. The Tagil Arc in the
Middle Urals formed as an intra-oceanic arc from the Late Ordovician through the Devonian and appears to have collided with
the continental margin of Laurussia in the Early Carboniferous. The Magnitogorsk Arc in the South Urals...
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... The location of the area is given in Fig. 4. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Brown et al., 2006b) while the pre-existing basement structures at various orientations to the orogen played an important role in the partitioning of deformation through the foreland Brown et al., 2011). In the South Urals, the E-W oriented structure at the intersection of the Precaspian Basin north margin and the collisional front imposed a drastic change in the structural style of the deformed foreland (Brown et al., 2004). ...
The evolution of intraplate sedimentary basins located in the vicinity of an active convergent plate boundary is often controlled by the collisional dynamics of the adjacent orogen. The transfer of compression from the orogen to the platform's interior results in the formation of complex structural geometry and kinematics that often reactivate older crustal faults, focus far-field stresses and control the evolution of associated sedimentary basins. One place where this localisation can be optimally understood is the Precaspian Basin, situated at the SE periphery of the East European Craton and bordered to the east by the Uralian orogen. The Precaspian Basin and its northern margin experienced long-term extension and subsidence interrupted by several short-lived shortening episodes. To understand the impact and role of pre-existing basement structures on the geometry and kinematics of the subsequent deformation, we analysed subsurface data from the northern margin of the Precaspian Basin by the means of 3D seismic interpretation correlated with wells and structural modelling. The analysis results provide new insights into the kinematic effects of the Late Devonian contraction event. The superposition of stratigraphic units above and below the angular unconformity suggests an intra-Famennian age of the deformation event. Steeply dipping bi-verging reverse fault zones associated with variable amounts of sinistral strike-slip movement component display an arcuate geometry, trending from WSW-ENE to NW-SE. The distribution of deformation indicates that this complex kinematic pattern was driven by a NE-SW oriented contraction and transpression, where faults show the characteristics of a restraining bend area. This area is interpreted as part of a regional transcurrent fault system developed on the northern periphery of the Precaspian Basin. Furthermore, the study results suggest that intralithospheric stress localisation transmitted by the Paleouralian subduction zone resulted in the reactivation of pre-existing basement structures, propagation of faults, and localized short-term exhumation of the Precaspian Basin north margin. Generated by the collision of Magnitogorsk volcanic arc with East European Craton, far-field stress transfer produced a zone of oblique deformation. These data and interpretation demonstrate that the northern margin of the Precaspian Basin is an excellent natural setting to investigate and better understand mechanisms of far-field strain localisation and reactivation of deformational structures in stable platform areas resulting in the intracratonic mountain building process.
... Finally, an even more distant tephra source, NNE and NE (for Kowala Fig. 12. Correlation of the Variscan foreland developmental stages with the Devonian-Carboniferous tectonic and magmatic processes in the Sudetes, and Pripyat-Dnieper-Donets (PDD) and Donbas rift system (Narkiewicz, 2020, modified; see also Wilson and Lyashkevich, 1996;McCann et al., 2003;Alexandre et al., 2004;Yutkina et al., 2017), Kola (Arzamastsev et al., 2013) and South Urals (Magnitogorsk and Tagil volcanic arcs; Brown et al., 2011). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) ...
Pyroclastic horizons are common in the middle–upper Famennian carbonate and siliceous successions located near the Rhenohercynian margins of Laurussia and the Variscian Saxothuringian oceanic realm. We present a detailed geochronological, geochemical and mineralogical study of tuffites from three sites in southern Poland: Holy Cross Mountains (Kowala), Kraków–Silesia monocline (Czatkowice, Dębnik anticline), and Sudetes (Bardo, Bardo Mountains). Airborne volcanic ash in Czatkowice was deposited on a shallow carbonate platform, in Kowala in the deeper intrashelf basin and tephra in Bardo in the deep pelagic basin below the carbonate compensation depth. The LA-ICP-MS U-Pb data demonstrate nearly uniform ages of zircon 365.6 ± 2.9 Ma and 363.0 ± 3.5 Ma for Czatkowice samples and monazite 363.4 ± 5.8 Ma and 363.1 ± 4.8 Ma for samples from Czatkowice and Kowala, respectively. Mineralogical and geochemical data showed the occurrence of two groups of Famennian tuffites in southern Poland, which are genetically related to different geotectonic realms. The predominantly rhyolitic–rhyodacitic–dacitic volcanogenic material from Kowala and Czatkowice results from within-plate effusive pyroclastic activity. The basalt–trachyandesite tuffites from Bardo were formed in an active plate margin setting connected with the Gondwana–Laurussia convergence, where processes of subduction and accretion of new crust occurred. The Late Devonian magmatic center belonging to the Pripyat–Dnieper–Donets–Donbas rift could be the most probable proximal source area for volcanic ash deposits in the Holy Cross Mountains and the Kraków–Silesia monocline. More distal sources could be late Devonian rift-related magmatism in the Dacia megaterrane or the Maritime magmatic province. In contrast, the source of the older volcanism generating the pyroclastic plume and deposits in the Bardo Mountain Basin could be related to the eruptive activity in the Moravian–Silesian Zone and/or magmatic centers in some Variscan terranes, which are now incorporated into the Alpine orogen.
... For example, with a lower lithospheric strength difference between island arc and back-arc basin (Figure 4b), the lithosphere does not rupture and maintains a stable single subduction state (Figure 4b and Figure S7 in Supporting Information S1). The collision between the Magnitogorsk Arc and the Laurussia Margin in the Middle through Late Devonian may be an excellent case of the single subduction model, which only happens with accretion and no SPR (Brown et al., 2011). With a larger lithospheric strength difference (Figure 4c), the lithospheric mantle and crust of the island arc are more likely to be decoupled, resulting in a mantle detachment (Figure 4d). ...
It is generally accepted that the subduction polarity reversal (SPR) results from the strong collision of two plates. Yet, the SPR of the Solomon Back‐arc Basin is started in the “soft docking” stage, and the mechanism by which the “soft docking” induced subduction initiation (SI) remains elusive. We find that the mass depletion of the plateau influences the evolution of the subduction patterns during SI. And the island arc rheological strength affects the development of the shear zone between an island arc and back‐arc basin which favors SI. What's more, with the increase of the rheological strength difference, the SI is more easily to occur, and the contribution of the plateau collision to SI weakens. Hence, by combining the available geological evidence, we suggest that the Solomon Island Arc rheological strength and the Ontong‐Java plateau collision jointly controlled the SI during the “soft docking” stage.
... The Upper Paleozoic sedimentary formations of the Late Devonian, Carboniferous and Permian Systems occurring at the east edge of the East European Platform and in the Ural Foredeep, were mainly produced by the accumulation of terrigenous material removed from the Ural orogen, which was situated eastward and completely finished its development at the end of Late Paleozoic [22][23][24]. The thickness of Upper Paleozoic sedimentary sequences approaches 12-15 km, and these rocks comprise large and giant oil, gas and salt deposits [25,26]. ...
A chromite occurrence called the Sabantuy paleoplacer was discovered in the Southern Pre-Ural region, at the east edge of the East-European Platform in the transitional zone to the Ural Foredeep. A ca. 1 m-thick chromite-bearing horizon is traced at a depth of 0.7–1.5 m from the earth’s surface for the area of ca. 15,000 m2. The chromspinel content in sandstones reaches 30%–35%, maximum values of Cr2O3 are 16–17 wt. %. The grain size of detrital chromspinel ranges from 0.15 to 0.25 mm. Subangular octahedral crystals dominate; rounded grains and debris are rare. The composition of detrital chromspinel varies widely and is constrained by the substitution of Al3+ and Cr3+, Fe2+ and Mg2+ cations. Chemically, low-Al (Al2O3 = 12 wt. %) and high-Cr (Cr2O3 = 52–56 wt. %) chromspinel prevail. The compositional analysis using discrimination diagrams showed that most chromites correspond to mantle peridotites of subduction settings. Volcanic rocks could be an additional source for detrital chromites. It is confirmed by compositions of monomineralic, polymineralic and melt inclusions in chromspinels. The presented data indicates that ophiolite peridotites and related chromite ore associated with oceanic and island-arc volcanic rocks, widespread in the Ural orogen, could be the main sources of the detrital chromspinel of the Sabantuy paleoplacer.
... The exceptional case of Yaman Kasy, the best studied example in Ordovician-Silurian rocks of the Urals, yields key features to be expected in a well-preserved ancient vent system, including the sub-seafloor feeder stockwork system, massive sulfide mound, vent chimney/ conduit debris, including chimney fragments and reworked interlayered sulfide, and oxide degraded mound sediments (Maslennikov, 1991;Herrington et al., 1998). Devonian systems in the same tectonic domain of the southern Urals also show exceptional preservation resulting from the atypical evolution of the orogen (Brown et al., 2011). Comparative deposits in other tectonic units of the middle Urals are more highly deformed and to date have yielded no primary sulfide textures or fossils (Herrington et al., 2005b). ...
... The Uralide orogen (hereafter Urals) extends nearly 2500 km from the Aral Sea to Novaya Zemlya and records the Paleozoic collision of at least two intra-oceanic arcs with the margin of Laurussia and a final continent-continent collision with the Kazakh and Siberian plates (Brown et al., 2006(Brown et al., , 2011. The paleo-Uralian ocean basin developed during Late Cambrian to Early Ordovician rifting. ...
Hydrothermal vents are among the most fascinating environments that exist within the modern oceans, being home to highly productive communities of specially-adapted fauna, supported by chemical energy emanating from the Earth's subsurface. As hydrothermal vents have been a feature our planet since the Hadean, their history is intricately weaved into that of life on Earth. Despite an overall scant fossil record due to the improbabilities of preservation of vent deposits and organisms, recent fossil findings from ancient vent environments, accompanied by molecular data as well as fossils from ecologically-similar environments, have yielded invaluable new insights into the history of life at hydrothermal vents. Fossils from hydrothermal vents are among the earliest contenders for direct evidence of life on Earth, while a range of additional fossil finds indicate that vent habitats were readily exploited by microbes during the Precambrian. The first metazoans possibly appeared within vents during the Cambrian, and by the Ordovician-Silurian, hydrothermal vents in the deep ocean were colonised by mollusc, brachiopod and tubeworm taxa whose large abundances and sizes suggest these early animals were well-adapted to this setting. A transition in vent community composition occurred during the Mesozoic, as modern vent faunas began to occupy these environments and replace Paleozoic taxa. Molecular evidence indicates that many additional taxa radiated within vents during the Cenozoic, demonstrating that throughout Earth history, organisms were repeatedly able to overcome the challenges of adapting to the harsh conditions at vents to exploit their productivity. Targeting ancient vent deposits that have undergone low degrees of diagenetic or metamorphic change during mining-related exposure has great potential to provide further insights into the vent fossil record and fill existing gaps in knowledge.
... From the end of Frasnian, it formed subaerial mountain ranges that provided sediments to the foreland and forearc basins. However, at the end of Devonian, the Magnitogorsk arc subsided below sea-level and was covered by a Tournaisian carbonate platform (Brown et al. 2006(Brown et al. , 2011. In consequence, it could not be responsible for the positive shift in ε Nd (t) values on the Karatau platform at the beginning of the Carboniferous. ...
The neodymium isotope composition of micritic limestones from the Devonian–Carboniferous carbonate platform of the Greater Karatau (southern Kazakhstan) was investigated to test the ability of calcite micrite to archive Nd isotope signatures of seawater. The carbonate fraction that displays seawater-like rare earth element (REE + Y) signatures is often more radiogenic than the dispersed terrigenous material in the samples. Hence, its Nd isotope composition is interpreted to correspond to the seawater from which the micrite was precipitated. The seawater on the Karatau platform exhibited an extremely wide range of ε Nd (t) values from –9.3 to +4.3 (the most radiogenic value measured for past seawater to date) and very uniform Sm/Nd ratios, from 0.19 to 0.22, lying within the range characteristic for modern oceanic water. The temporal trend in ε Nd (t) values is interpreted to document the final closure of the Uralian–Turkestan Ocean. It shows that the subduction along Kazakhstan's active margin had already started at the beginning of the Tournaisian ( c . 355 Ma), at least 23 Myr earlier than previously thought. The application of Nd isotope time series on biostratigraphically dated carbonates opens a new direction for geotectonic studies. This approach has the potential to provide useful constraints for the precise dating of the duration of geotectonic and volcanic events.
Supplementary material: Nd isotope and REE concentration data, summary of stratigraphic and lithological data, field photographs and additional geochemical plots are available at: https://doi.org/10.6084/m9.figshare.c.5110163
... The Khabarny mafic-ultramafic Massif belongs to the upper unit of the much larger Sakmara Allochthon in the West Uralian Zone (Puchkov, 2000(Puchkov, , 2002(Puchkov, , 2013. This zone is described as the Magnitogorsk Accretionary Complex according to Brown et al. (2011), and it underwent intensive folding and thrusting, and includes klippe, that contain easterly-derived ophiolites and arc volcanics (Puchkov, 2009(Puchkov, , 2013. The Sakmara Allochthon consists of a series of deformed thrust sheets and can be divided in three tectonic units (Fig. 1). ...
... Regional tectonic settings are defined by the arc-continent collision in the Uralides (e.g. Brown et al., 2011). The negative ENdt of garnetites (Pushkarev et al., 2008) illustrates that the granulites of the metamorphic complex have a continental crust signature, and can be regarded as a highgrade metamorphic sedimentary or igneous rock. ...
High-grade metamorphic rocks underlying the intrusive layered dunite–pyroxenite–gabbronorite East-Khabarny Complex (EKC) are integrated in the complex Khabarny mafic–ultramafic Massif in the Sakmara Allochthon zone in the Southern Urals. These rocks are associated with high-temperature shear zones. Garnetites from the upper part of the metamorphic unit close to the contact with EKC gabbronorite are chemically and texturally analysed to estimate their formation conditions and fluid regime. Fluids provide crucial information of formation conditions and evolution of these garnetites during high-grade metamorphism, and are preserved in channel positions within Si6O1812- rings of cordierite, and in fluid inclusions in quartz and garnet. Minerals and fluid inclusions of the garnetites are studied with X-ray fluorescence spectrometry, electron microprobe analyses, Raman spectroscopy, and microthermometry. The garnetites mainly consist of garnet (up to 80 vol. %), cordierite and quartz. Accessory minerals are rutile, ilmenite, graphite, magnetite and cristobalite. Granulite-facies metamorphic conditions of the garnetites are estimated with the garnet–cordierite–sillimanite–quartz geothermobarometer: temperatures of 740 to 830 ˚C and pressures of 770–845 MPa. The average garnet composition in end-member concentrations is 48·5 mole % almandine (±3·9), 34·7 mole % pyrope (±3·3), 10·3 mole % spessartine (±1·1), 1·8 mole % grossular (±1·5), and 1·5 mole % andradite (±1·5). The cordierite electron microprobe analyses reveal an average Mg2+ fraction of 0·79 ± 0·01 in the octahedral site. Relicts of a strong positive temperature anomaly (up to 1000 ˚C) are evidenced by the preservation of cristobalite crystals in garnet and the high titanium content of quartz (0·031 ± 0·008 mass % TiO2) and garnet (0·31 ± 0·16 mole % end-member Schorlomite-Al). The fluid components H2O, CO2, N2 and H2S are detected in cordierite, which correspond to a relatively oxidized fluid environment that is common in granulites. In contrast, a highly reduced fluid environment is preserved in fluid inclusions in quartz nodules, which are mono-fluid phase at room temperature and composed of CH4 (>96 mole %) with locally minor amounts of C2H6, N2, H2S and graphite. The fluid inclusions occur in homogeneous assemblages with a density of 0·349 to 0·367 g·cm-3. The CH4-rich fluid may represent peak-temperature metamorphic conditions, and is consistent with temperature estimation (∼1000 ˚C) from Ti-in-garnet and Ti-in-quartz geothermometry. Tiny CH4-rich fluid inclusions (diameter 0·5 to 2 µm) are also detected by careful optical analyses in garnet and at the surface of quartz crystals that are included in garnet grains. Graphite in fluid inclusions precipitated at retrograde metamorphic conditions around 300–310 ± 27 ˚C. Aragonite was trapped simultaneously with CH4-rich fluids and is assumed to have crystallized at metastable conditions. The initial granulite facies conditions that led to the formation of a cordierite and garnet mineral assemblage must have occurred in a relative oxidized environment (QFM-buffered) with H2O–CO2-rich fluids. Abundant intrusions or tectonic emplacement of mafic to ultramafic melts from the upper mantle that were internally buffered at a WI-buffered (wüstite–iron) level must have released abundant hot CH4-rich fluids that flooded and subsequently dominated the system. The origin of the granulite-facies conditions is similar to peak-metamorphic conditions in the Salda complex (Central Urals) and the Ivrea–Verbano zone (Italian Alps) as a result of magmatic underplating that provided an appearance of a positive thermal anomaly, and further joint emplacement (magmatic and metamorphic rocks together) into upper crustal level as a high temperature plastic body (diapir).
... The 2000-km-long Uralides consist of several longitudinal terranes that extend parallel to the eastern margin of Baltica. These tectonic domains are variably divided, from west to east, as the (1) undeformed foreland basin, (2) foreland fold-thrust belt, (3) Tagil-Magnitogorsk zone, and (4) East Uralian zone (Hamilton, 1970;Berzin et al., 1996;Maslov et al., 1997;Puchkov, 2009;Puchkov et al., 2013;Brown et al., 2011). These zones record Paleozoic subduction, arc magmatism, island-arc amalgamation, and the final collision of Baltica with Siberia and Kazakhstan. ...
... Preexisting structures within the Precambrian basement are at a high angle to these tectonics domains. A northwest-southeast structural grain within Archean and Proterozoic rocks is composed of several basement highs and lows, including from north to south respectively, the Mid-Russian aulacogen, Perm-Bashkir Arch, Kaltasin (or Kama-Belsk) aulacogen, Tatar Arch, Sernovodsk-Abdulino aulacogen, and Orenburg Arch (Bogdanova et al., 2008;Brown et al., 2011). The aulacogens are filled with as much as 15 km of Protero- Zuza and Yin | Balkatach hypothesis GEOSPHERE | Volume 13 | Number 5 zoic sediments that record ~200 m.y. of variably interrupted intracontinental rifting during the Neoproterozoic (Maslov et al., 1997;Brown et al., 1999). ...
... Baltica's eastern margin along the Ural Ocean remained passive throughout much of the Paleozoic, with the exception of several poorly understood arc accretion events (Fig. 7). The Tagil and Magnitogorsk oceanic arcs collided with the eastern margin of Baltica in the Devonian and Carboniferous, respectively ( Fig. 7) (e.g., Herrington et al., 2002;Brown et al., 2006Brown et al., , 2011. In the late Carboniferous, the Ural Ocean began to subduct underneath Kazakhstan to the east (Bea et al., 2002). ...
The Phanerozoic history of the Paleo-Asian, Tethyan, and Pacific oceanic domains is important for unraveling the tectonic evolution of the Eurasian and Laurentian continents. The validity of existing models that account for the development and closure of the Paleo-Asian and Tethyan Oceans critically depends on the assumed initial configuration and relative positions of the Precambrian cratons that separate the two oceanic domains, including the North China, Tarim, Karakum, Turan, and southern Baltica cratons. Existing studies largely neglect the Phanerozoic tectonic modification of these Precambrian cratons (e.g., the effects of India-Arabia-Eurasia convergence and post-Rodinia rifting). In this work we systematically restore these effects and evaluate the tectonic relationships among these cratons to test the hypothesis that the Baltica, Turan, Karakum, Tarim, and North China cratons were linked in the Neoproterozoic as a single continental strip, with variable along-strike widths. Because most of the tectonic boundaries currently separating these cratons postdate the closure of the Paleo-Asian and Tethyan Oceans, we are able to establish a >6000-km-long Neoproterozoic contiguous continent referred to here as Balkatach (named from the Baltica–Karakum–Tarim–North China connection). By focusing on the regional geologic history of Balkatach’s continental margins, we propose the following tectonic model for the initiation and evolution of the Paleo-Asian, Tethyan, and Pacific oceanic domains and the protracted amalgamation and growth history of the Eurasian continent. (1) The early Neoproterozoic collision of the combined Baltica–Turan–Karakum–South Tarim continents with the linked North Tarim–North China cratons led to the formation of a coherent Balkatach continent. (2) Rifting along Balkatach’s margins in the late Neoproterozoic resulted in the opening of the Tethyan Ocean to the south and unified Paleo-Asian and Pacific Oceans to the north (present-day coordinates). This process led to the detachment of Balkatach-derived microcontinents that drifted into the newly formed Paleo-Asian Ocean. (3) The rifted microcontinents acted as nuclei for subduction systems whose development led to the eventual demise of the Paleo-Asian Ocean during the formation of the Central Asian Orogenic System (CAOS). Closure of this ocean within an archipelago-arc subduction system was accommodated by counterclockwise rotation of the Balkatach continental strip around the CAOS. (4) Initial collision of central Balkatach and the amalgamated arcs and microcontinents of the CAOS in the mid-Carboniferous was followed by a bidirectional propagation of westward and eastward suturing. (5) The closure of the Paleo-Asian Ocean in the early Permian was accompanied by a widespread magmatic flare up, which may have been related to the avalanche of the subducted oceanic slabs of the Paleo-Asian Ocean across the 660 km phase boundary in the mantle. (6) The closure of the Paleo-Tethys against the southern margin of Balkatach proceeded diachronously, from west to east, in the Triassic–Jurassic.
... However, Tessalina et al. (2016b) report radiometric dates from VMS ores from Mednogorsk across into the Magnitogorsk arc that yield similar, late Devonian ages across the entire orogen. This age coincides with the arc-continent collision event (Brown et al., 2011) rather than the age of the host sequences and suggests that this dates a later resetting hydrothermal event. A further overprinting hydrothermal event is also recorded at 345 Ma for two of the VMS deposits. ...
... East Baltica (Uralian margin) Late Cambrian to early Ordovician rifting along eastern Baltica established a passive margin that lasted until the Magnitogorsk island arc collided with the South Urals in the middleelate Devonian (Table 1; Fig. 1) (Bradley, 2008). The timing of that accretion is constrained by: an eastward shift in the locus of island arc magmatism in the Givetian, Frasnian UHP metamorphism of Baltica-derived crust and the Frasnian-Famennian deposition of westward-younging foreland basin sediments west of the arc (Brown et al., 2011). Together with the pre-late Devonian passive margin of Baltica, that indicates that subduction of the intervening ocean must have been eastward-directed, beneath the island arc. ...
... Together with the pre-late Devonian passive margin of Baltica, that indicates that subduction of the intervening ocean must have been eastward-directed, beneath the island arc. The oldest rocks of the Magnitogorsk island arc suggest that intraoceanic subduction commenced in the early Devonian, and consumption of the ocean basin continued until the middleelate Devonian collision (Brown et al., 2011). To the north, in the Middle and North Urals, the late Ordovician to Devonian Tagil island arc is preserved in a structural position similar to the Magnitogorsk arc (i.e. the Magnitogorsk-Tagil Zone) and is also inferred to have formed above an east-dipping subduction zone, but the timing of its collision with Baltica is less well-defined. ...
... To the north, in the Middle and North Urals, the late Ordovician to Devonian Tagil island arc is preserved in a structural position similar to the Magnitogorsk arc (i.e. the Magnitogorsk-Tagil Zone) and is also inferred to have formed above an east-dipping subduction zone, but the timing of its collision with Baltica is less well-defined. Broadly, Puchkov (2009a) and Brown et al. (2011) have determined that its accretion was underway in the early Carboniferous (Table 1). Yet further north, in the Polar Urals, Ordovician to Devonian passive margin sediments are found juxtaposed with SilurianeDevonian island arc volcanic rocks to the east. ...
