Ondrej Lexa

STRUCTURAL CONSTRAINTS ON THE EVOLUTION OF THE CENTRAL ASIAN OROGENIC BELT IN SW MONGOLIA

We provide a detailed description of the structures along a 300 km long and 50 km wide transect across the Central Asian Orogenic Belt (CAOB) in southwestern Mongolia, covering the Precambrian Dzabkhan continental domain with overthrust Neoproterozoic ophiolites in the north (Lake Zone), a Silurian-Devonian passive margin association (Gobi-Altai Zone) and oceanic domain (Trans-Altai Zone) in the center, and a continental area (South Gobi Zone) in the south. Structural analysis suggests late Cambrian collapse of the thickened Lake Zone continental crust, leading to stretching of the lithosphere and followed by Silurian-Devonian formation of oceanic crust in the Trans-Altai domain. Subsequent emplacement of Devonian-Carboniferous and late Carboniferous magmatic arcs occurred on the Gobi-Altai and South Gobi Zone crusts, respectively, during E-W shortening. Finally, the entire system was affected by N-S convergence from the Permian to Jurassic, leading to heterogeneous shortening of the orogenic domain. The model best fitting these observations is one of generalized westward drift of the Tuva-Mongol-Dzabkhan-Baydrag ribbon continents during the Silurian-Devonian, associated with westward-subduction of the Mongol-Okhotsk Ocean and sequential growth of syn-convergent magmatic arcs. Back-arc basins opened during this period in the area of the western Paleoasian Ocean. The present-day shape of the CAOB in southern Mongolia was probably formed during Permian to Mesozoic anticlockwise rotation and folding of the Tuva-Mongol-Dzabkhan-Baydrag continental ribbons, combined with a strike-slip (transpressional) reactivation of ancient transform boundaries in the Paleoasian oceanic domain. All continental and oceanic crustal domains were reactivated and intensely deformed during this convergence in a style controlled by crustal rheology and a heterogeneous Permian magmatic-thermal input. The sequence of tectonic events is tested against published paleomagnetic data, paleogeographic reconstructions and tectonic models, leading to a revised model for the accretion of juvenile crust to a continental margin in the CAOB of southern Mongolia.

Lithostratigraphic and geochronological constraints on the evolution of the Central Asian Orogenic Belt in SW Mongolia: Early Paleozoic rifting followed by late Paleozoic accretion

New SHRIMP U-Pb and evaporation Pb-Pb zircon ages, together with a revision of the lithostratigraphy of “suspect” terranes in SW Mongolia, suggest that the collage of continental and oceanic units in this region resulted from recurrent magmatic reworking and deformation of Silurian–early Devonian proximal and distal passive margin sequences of the Paleo-Asian Ocean. The zircon ages from early Ordovician volcaniclastic rocks and syntectonic felsic dikes reveal an heterogeneous stretching of the Precambrian Dzabkhan microcontinent (Lake Zone basement) during the Ordovician, followed by the development of a carbonate platform on a proximal margin (Gobi-Altai Zone), serpentinite breccias and Silurian chert sequences on a distal margin and possibly also the formation of oceanic crust. The assumed early Neoproterozoic South Gobi continental zone may either represent an allochthonous block detached from Dzabkhan or, less likely, the conjugate margin of a Paleo-Asian continental rift. Early Devonian volcanism subsequently affected both types of margins with back-arc spreading centers and arcs located in the core of the future Trans-Altai Zone. During the late Devonian to early Carboniferous a Japan-type magmatic arc developed on the previously stretched continental crust of the Gobi-Altai Zone. This event was associated with shortening of the entire domain, exhumation of the deep arc core and formation of intramontane basins with Devonian and Carboniferous detrital zircons of the adjacent Lake Zone continent. Clastic, flysch-type sedimentation occurred on the former distal margin and in oceanic areas. During this early Carboniferous contraction event the continental and oceanic units were imbricated and accreted to the continent in the north. Subsequently, late Carboniferous volcanic arc sequences and a Japan-type magmatic arc developed on the Trans-Altai oceanic crust and the southern South Gobi Zone, respectively. Finally, a Permian thermal event was localized in the Gobi-Altai–Lake Zone contact domain and was responsible for formation of Permian grabens, bimodal volcanism and substantial melting of the accreted crust.

Anatomy of a diffuse cryptic suture zone: An example from the Bohemian Massif, European Variscides

The fate of the lower plate during continental collision can be examined in deeply eroded orogens such as the late Paleozoic Variscan belt in continental Europe. In particular, the Bohemian Massif at its eastern extremity preserves well the evolution of an Andean-type orogen involved in continental collision. This process included relamination of subducted light felsic material rich in radioactive elements underneath a dense mafic lower crust of the upper plate. This led to gravity-driven overturns and overprinting of the original suture by a broad zone of mixed upper and lower plate materials. In the studied example, this zone of interaction repeatedly reappears within the orogen, forming a so-called “diffuse cryptic suture zone.”

Crustal influx, indentation, ductile thinning and gravity redistribution in a continental wedge: Building a Moldanubian mantled gneiss dome with underthrust Saxothuringian material (European Variscan belt)

[1] The contribution of lateral forces, vertical load, gravity redistribution and erosion to the origin of mantled gneiss domes in internal zones of orogens remains debated. In the Orlica-Snieznik dome (Moldanubian zone, European Variscan belt), the polyphase tectono-metamorphic history is initially characterized by the development of subhorizontal fabrics associated with medium- to high-grade metamorphic conditions in different levels of the crust. It reflects the eastward influx of a Saxothuringian-type passive margin sequence below a Tepla-Barrandian upper plate. The ongoing influx of continental crust creates a thick felsic orogenic root with HP rocks and migmatitic orthogneiss. The orogenic wedge is subsequently indented by the eastern Brunia microcontinent producing a multiscale folding of the orogenic infrastructure. The resulting kilometre-scale folding is associated with the variable burial of the middle crust in synforms and the exhumation of the lower crust in antiforms. These localized vertical exchanges of material and heat are coeval with a larger crustal-scale folding of the whole infrastructure generating a general uplift of the dome. It is exemplified by increasing metamorphic conditions and younging of 40Ar/39Ar cooling ages toward the extruded migmatitic subdomes cored by HP rocks. The vertical growth of the dome induces exhumation by pure shear-dominated ductile thinning laterally evolving to non-coaxial detachment faulting, while erosion feeds the surrounding sedimentary basins. Modeling of the Bouguer anomaly grid is compatible with crustal-scale mass transfers between a dense superstructure and a lighter infrastructure. The model implies that the Moldanubian Orlica-Snieznik mantled gneiss dome derives from polyphase recycling of Saxothuringian material.

The Moldanubian Zone in the French Massif Central, Vosges/Schwarzwald and Bohemian Massif revisited: differences and similarities

Abstract In order to portray the main differences and similarities between the Northeastern Variscan segments (French Massif Central (FMC), Vosges, Black Forest and Bohemian Massif (BM)), we review their crustal-scale architectures, the specific rock associations and lithotectonic sequences, as well as the ages of the main magmatic and metamorphic events. This review demonstrates significant differences between the ‘Moldanubian’ domains in the BM and the FMC. On this basis we propose distinguishing between the Eastern and Western Moldanubian zones, while the Vosges/Black Forest Mountains are an intermediate section between the BM and the FMC. The observed differences are the result of, first, the presence in the French segment of an early large-scale accretionary system prior to the main Variscan collision and, second, the duration of Saxothuringian/Armorican subduction, which generated long-lived magmatic arc and back-arc systems in the Bohemian segment, while the magmatic activity in the FMC was comparably short-lived.

Contrasting microstructures and deformation mechanisms in metagabbro mylonites contemporaneously deformed under different temperatures (c. 650 °C and c. 750 °C)

Abstract Deformation mechanisms of amphibole and plagioclase were investigated in two metagabbroic sheets (the eastern and western metagabbros from the Staré Město belt, eastern Bohemian Massif), using petrology, quantitative microstructural and electron back-scattered diffraction methods. After the gabbroic pyroxene was replaced by amphibole, both gabbroic bodies became progressively deformed. The eastern metagabbros were deformed under temperature of c. 650 °C and the western metagabbros under c. 750 °C. Subgrain rotation and dislocation creep, characterized by strong crystallographic and shape preferred orientations, operated in plagioclase of the eastern belt during the early stages of deformation. Subsequent randomizing of plagioclase crystallographic preferred orientation is interpreted to be due to grain boundary sliding in the mylonitic stage. Large (50–150 μm) grain sizes during the mylonitic stages are interpreted to be due to low strain rates. Amphibole is stronger and deforms cataclastically, leading to important grain size reduction when the bulk rock strength drops substantially. In the western belt, plagioclase deformed by dislocation creep accompanied by grain boundary migration (possibly chemically induced) while heterogeneous nucleation and syndeformational grain growth in conjunction with dislocation creep were typical for amphiboles.

Structural evolution of the central part of the Krušné hory (Erzgebirge) Mountains in the Czech Republic—evidence for changing stress regime during Variscan compression

Abstract In the central part of the Krusne hory (Erzgebirge) Mountains, the parautochthonous metasedimentary basement has been overthrust by a crustal nappe of fine- and coarse-grained orthogneisses. The thrust boundary is defined by the presence of mafic eclogites with preserved subduction-related fabric. Westward thrusting of an allochthonous unit is associated with the development of the main metamorphic foliation and lineation in non-eclogitic lithologies. Buttressing of the allochthonous body from the west is responsible for the development of late, large-scale folds with N–S trending hinges and vertical axial planes. Subsequent N–S compression leads to large-scale folding of both the parautochthonous and allochthonous units. This deformation produces km-scale antiforms with hinges plunging to the west and is associated with the development of the E–W stretching lineation as a result of complete reworking of earlier fabric in the limb zones. N–S shortening is also associated with the development of small-scale folds and brittle-ductile kink bands suggesting a decrease in temperature, and, thus, uplift of the whole studied area during this event. The last stage of deformation is characterised by the development of kink-band folds and a crenulation cleavage. These structures suggest a sub-vertical direction of principal compression, developed exclusively in those parts of the area in which the N–S compression produced steep planar fabric.

Alpine burial and heterogeneous exhumation of Variscan crust in the West Carpathians: insight from thermodynamic and argon diffusion modelling

Phase equilibrium modelling of metagranitoids and metapelites in the MnO–Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O system was used to characterize Variscan and Alpine metamorphism in the crystalline basement of the Vepor Unit, West Carpathians. The calculated P–T conditions range between 570–670 °C and 6–8.5 kbar for the Variscan and 430–600 °C and 5–11 kbar for the Alpine event. These two events show contrasting metamorphic field gradients and P–T evolutions, indicating 22–27 °C km−1 during the retrograde Variscan metamorphism and 15–18 °C km−1 during the prograde Alpine metamorphism. The prograde Alpine metamorphism is associated with the Early Cretaceous overthrusting of the southern Gemer Unit, which resulted in apparently contemporaneous burial and horizontal ductile spreading of the underlying Vepor basement. The argon diffusion modelling was used to interpret the existing Variscan, Alpine and mixed 40Ar/39Ar cooling ages, and to constrain the T–t evolution and Alpine thermal overprint. Constructed Alpine metamorphic isograds and isotherms show horizontal P–T gradients resulting from heterogeneous exhumation of the deeper parts of the Vepor basement along two narrow belts during later folding. Here the structurally deeper metapelites form large-scale anticlinal cusp-like structures that separate structurally higher metagranitoids.

Anatexis of accretionary wedge, Pacific‐type magmatism, and formation of vertically stratified continental crust in the Altai Orogenic Belt

Granitoid magmatism and its role in differentiation and stabilization of the Paleozoic accretionary wedge in the Chinese Altai are evaluated in this study. Voluminous Silurian-Devonian granitoids intruded a greywacke-dominated Ordovician sedimentary succession (the Habahe Group) of the accretionary wedge. The close temporal and spatial relationship between the regional anatexis and the formation of granitoids, as well as their geochemical similarities including rather unevolved Nd isotopic signatures and the strong enrichment of large-ion lithophile elements relative to many of the high field strength elements, may indicate that the granitoids are product of partial melting of the accretionary wedge rocks. Whole-rock geochemistry and pseudosection modeling show that regional anatexis of fertile sediments could have produced a large amount of melts compositionally similar to the granitoids. Such process could have left a high-density garnet- and/or garnet-pyroxene granulite residue in the deep crust, which can be the major reason for the gravity high over the Chinese Altai. Our results show that melting and crustal differentiation can transform accretionary wedge sediments into vertically stratified and stable continental crust. This may be a key mechanism contributing to the peripheral continental growth worldwide.

Geophysical and geochemical nature of relaminated arc-derived lower crust underneath oceanic domain in southern Mongolia

The Central Asian Orogenic Belt (CAOB) in southern Mongolia consists of E-W trending Neoproterozoic cratons and Silurian-Devonian oceanic tectonic zones. Previous study revealed that the Early Paleozoic accretionary wedge and the oceanic tectonic zone are underlain by a layer giving a homogeneous gravity signal. Forward gravity modelling suggests that this layer is not formed of high-density material typical of lower oceanic crust but is composed of low- to intermediate-density rocks resembling continental crust. The nature of this lower crust is constrained by the whole-rock geochemistry and zircon Hf isotopic signature of abundant Late Carboniferous high-K calc-alkaline and Early Permian A-type granitoids intruding the two Early Paleozoic domains. It is possible to explain the genesis of these granitoids by anatexis of juvenile, metaigneous (tonalitic-gabbroic) rocks of Late Cambrian age, the source of which is presumed to lie in the “Khantaishir” arc (520–495 Ma) further north. In order to test this hypothesis, the likely modal composition and density of Khantaishir arc-like protoliths are thermodynamically modelled at granulite- and higher amphibolite-facies conditions. It is shown that the current average density of the lower crust inferred by gravity modelling (2730 ± 20 kg/m3) matches best metamorphosed leucotonalite to diorite. Based on these results, it is now proposed that Mongolian CAOB has an architecture in which the accretionary wedge and oceanic upper crust is underlain by allochthonous lower crust that originated in a Cambrian arc. A tectonic model explaining relamination of allochthonous felsic to intermediate lower crust beneath mafic upper crust is proposed.