Karel Schulmann

THERMAL EVOLUTION AND EXHUMATION IN OBLIQUELY CONVERGENT (TRANSPRESSIVE) OROGENS

Most P-T-t path models to date have considered a linear erosion rate for exhumation from burial depth, related to isostatic readjustment of crustal thickness. A few have discussed extension-enhanced exhumation. Erosional exhumation can only restore lower crustal rocks from the thickened mountain root to their previous original depth in the pre-collisional crust. One major assumption of all models to date is that the compressive forces responsible for crustal thickening cease before elevation and erosion begins. However, compression is often still active, even if the crustal thickening has stopped. Further compression of the thickened and strongly deformed orogenic root is responsible for forceful exhumation-extrusion of softened rocks upwards. Hence, the rate of exhumation is related to the rate of convergence of colliding plates. Extrusional exhumation can elevate buried rocks to any depth depending on the action of fault-shear systems. In the extrusional exhumation models examined here, the ascending rocks cool faster because they approach the zero temperature surface conditions more rapidly than by isostatic erosion. The exhumation rate is also governed by the angle between the plate boundary and the displacement vector (α), implying that the convergent plate boundaries are regarded as complex transpressive systems in which the degree of obliquity can be expressed by the ratio of pure to simple shear components. A low ratio of pure/simple shear typical for wrench-dominated plate boundaries, implies long-distance horizontal transport from the buried position in the original orogen, and longer periods during which metamorphic heating occurs. A high ratio of pure/simple shear characteristic for frontal-like convergence implies a rapid exhumation without significant heating. Thus granulites, Barrovian-type, and blueschist facies metamorphism are characterised by increasing angle of obliquity (α∼10°, α∼30°, α∼90°), respectively. The high-temperature limit at low ratio of pure/simple shear is the geotherm T∞ and lies at temperatures that would commonly be taken to indicate that addition of mantle heat was required to generate hot geotherms and P-T-t paths. Wrench faulting over 100′s of kilometres at reasonable slow convergence rates could lead to extensive dehydration melting and granulite formation in large-scale collisional orogens.

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.

Thermally softened continental extensional zones (arcs and rifts) as precursors to thickened orogenic belts

Abstract Intra-continental deformation of soft zones during continental collision requires weak continental lithosphere which is able to be shortened across considerable width during later convergence. This enables significant thickening with formation of an orogenic root. We have examined models with a history of lithospheric thinning by pure shear during an earlier phase of intra-continental extension with associated heating. Geologically this situation is appropriate to intra-continental rifts and back-arc basins. If thinning of elevated thermal structure is decoupled from the thinning of lithology then a weak (soft) lower crust and sub-arc/rift mantle result. This weak structure has a favoured rheology for subsequent convergent thickening while the lithosphere is still hot. These regions are associated with formation of granulites and metamorphic assemblages typical of high-temperature/low-pressure (HT/LP). If convergence starts while the heat input is still active then the failed rifts and arcs are shut off by lateral wedging of the hard lower crust and upper mantle of shoulder regions into the softened arc/rift domain. Such sites are ideal for the formation and for the exhumation of metamorphic core complexes. Subsequent thickening during convergence leads to HT eclogites when the previous arc/rift was hot and to medium-T eclogites for a thickened “standard” geotherm. These P–T paths are counterclockwise and their shapes are strongly dependent on the amount of previous thinning and type of initial geotherm. If the compression starts long after cessation of the extensional event and associated thermal anomaly, then the geotherm of the extended area relaxes and the whole region hardens. In this case, no homogeneous thickening occurs and deep continental roots cannot form.

A model for a continental accretionary wedge developed by oblique collision: the NE Bohemian Massif

Structural and metamorphic investigations of the northeastern margin of the Bohemian Massif indicate three main sequential Devonian–Carboniferous tectonic events: (1) Devonian rifting; (2) Early Carboniferous oblique underthrusting and formation of a continental accretionary wedge; (3) eduction of the wedge and Late Carboniferous transpression. Devonian rifting of the Brunia microcontinent resulted in the formation of two crustal‐scale boudins associated with the development of two syn‐rift Devonian basins. This extensional template strongly influenced the nature of the ensuing Variscan contractional deformation. Early Carboniferous (350–330 Ma), progressive, highly oblique underthrusting of the two crustal boudins beneath the Lugian terrane to the west, generated syn‐deformational Barrovian metamorphism and the formation of a continental accretionary wedge. The wedge was further compressed by continued underthrusting of Brunia which resulted in the successive vertical extrusion (eduction) of an upper and lower allochthon, derived from the more deeply underthrust crustal boudin. The eduction was terminated by a Late Carboniferous (330–310 Ma) transpressional event resulting from continued plate convergence. Release of mantle‐derived magma during late‐stage eduction thermally softened the transpressional zones in the more external parts of the wedge. The resultant differential displacements gave rise to extensional unroofing of the internal part of the wedge.

Extrusion tectonics and elevation of lower crustal metamorphic rocks in convergent orogens

In the proposed model, weak zones of rheologically homogeneous crust, continually compressed between rigid lithospheric indenting plates, can be exhumed upward by extrusion. The rate of exhumation is governed by the rate of convergence of approaching plates and by the width of the weak deformable zone. Modeled extrusion results in near-isothermal decompression during the rapid elevation history in narrow orogenic belts for realistic plate velocities. Such pressure-temperature-time paths exhibit distinct collapsed geochronologies and record maximum metamorphic temperatures ( T max ) at the maximum burial depth ( P max ).

The behaviour of rigid triaxial ellipsoidal particles in viscous flows—modeling of fabric evolution in a multiparticle system

Abstract The behaviour of rigid triaxial ellipsoidal particles in slowly moving viscous flows has been studied by means of numerical solution of Jefferys (1922) equations. The motion of a single particle as well as a multiparticle system evolution in the combined pure shear and simple shear flow was modeled. The development of a preferred orientation depends on particle axial ratios and the flow geometry. Stable fabrics are produced only for sufficiently high components of pure shear. For lower pure-shear components oscillating fabrics develop, periodic in the case of spheroidal (biaxial) particles and irregularly oscillating in the case of triaxial ones. The fabric symmetries may not directly reflect the flow geometry in the case of triaxial particles.

Fabric evolution of rigid inclusions during mixed coaxial and simple shear flows

Abstract In a slowly moving viscous fluid, a population of non-interacting, rigid inclusions evolves in an oscillatory fashion or creates a stable fabric, depending on the flow geometry and the shape of the inclusions. Using Jefferys equations for the motion of a rigid ellipsoidal inclusion, a mixed coaxial and simple shear flow was investigated, both analytically and by numerical modeling. Exact results were obtained for spheroidal inclusions. Depending on the axial ratio of the inclusion and the amount of the coaxial component of flow, individual inclusions can move cyclically or rotate to a stable orientation in one of two mutually orthogonal directions. Equations were found which allow us to distinguish between these types of motion and to determine the stable inclusion orientation. The relative motions of individual inclusions determine the preferred orientation in a population of inclusions. In a heterogeneous population with diverse axial ratios, the fabric evolves as the superposition of subfabrics corresponding to subsets with different axial ratios. The orientations of stable subfabrics depend on axial ratios, and could thus be used as kinematic indicators. In cases of a sufficiently large coaxial component of flow, the fabric created by a heterogeneous population may comprise two stable, mutually orthogonal subfabrics. No exact results are available for general (i.e., non-spheroidal) ellipsoidal inclusions, and numerical modeling must be used. If the coaxial component of flow is sufficiently strong, the non-spheroidal inclusions move much like the spheroidal ones. Orthogonal subfabrics may appear as transient features in the evolution of a homogeneous population of non-spheroidal ellipsoidal inclusions.

Chronological constraints on the pre-Variscan evolution of the northeastern margin of the Bohemian Massif, Czech Republic

Abstract New single zircon ages enable us to provide an evolutionary scenario for the Neoproterozoic to Cambro-Ordovician tectonic history of part of the easternmost Sudetes along the northeastern margin of the Bohemian Massif. The easternmost crustal segment (Brunia) yields Neoproterozoic ages from both autochthonous and allochthonous Variscan units; these ages document a Cadomian (Pan-African) history that may be linked with the northern margin of Gondwana. A Cambro-Ordovician magmatic-thermal event in Brunia is represented by granitic to pegmatitic dykes intruding Neoproterozoic crust and by localized partial anatexis. Farther west a narrow zone of Cambro-Ordovician rifting is identified (Staré Městro belt), marked by gabbroic magmatism, bimodal volcanism and medium-pressure granulite facies metamorphism. The westernmost crustal domain (Orlica-Sniezník dome) is represented by Neoproterozoic crust intruded by Cambro-Ordovician plutons consisting of calk-alkaline granitoid rocks and affected by widespread Cambro-Ordovician anatexis. The geodynamic setting of the Neoproterozoic and Cambro-Ordovician domains is similar to that of the Western Sudetes, where both Cambro-Ordovician rifting and calc-alkaline magmatism were identified. We discuss the rifting mechanics in terms of sequential crustal thinning along the northern margin of Gondwana. The calc-alkaline magmatism, in conjunction with crustal rifting, is related to a back-arc geometry in front of a retreating south-dipping subduction zone during progressive closure of the Tornquist Ocean southeast of Avalonia.

Contrasting Early Carboniferous field geotherms: evidence for accretion of a thickened orogenic root and subducted Saxothuringian crust (Central European Variscides)

In the central part of the Erzgebirge, the age of granulite-facies metamorphism corresponds to the age of formation of mafic eclogites exhumed at the base of the recently defined Lower Crystalline nappe. Granulite-facies conditions in acidic continental crust and eclogite-facies conditions in mafic eclogites developed at the same time of about 342 Ma, suggesting the existence of two distinct tectonic units with contrasting initial geothermal gradients. Existing petrological and geochronological data are used to present a new model for Carboniferous collision at the western margin of the Bohemian Massif. We propose that juxtaposition of rocks with contrasting thermal histories is the result of subduction of the Saxothuringian crust and its later collision with a thickened continental orogenic root existing to the SE. A short time span between the formation ages of high-pressure rocks and cooling ages of the host lithologies suggests extremely fast exhumation of a coupled subduction zone–orogenic root system during continuing Variscan collision.