Johann Genser

Eastern Alpine tectono-metamorphic evolution: Constraints from two-dimensional P-T-t modeling

We use two-dimensional (2-D) P-T-t modeling to constrain the thermal and rheological aspects of different scenarios for the late Mesozoic and Cenozoic tectonic evolution of the Eastern Alps, inferred from excellent data sets from the Tauern Window (TW). Models invoking subduction of the South Penninic (SP) oceanic lithosphere during thrusting and subsequent erosion of the Austro-Alpine (AA) upper plate nappe stack are inconsistent with the observed thermal evolution within the AA and Penninic units. In these models, predictions for the AA peak thermal conditions are lower than observed. After exhumation and cooling to midcrustal levels and subduction of the continental Middle Penninic (MP) block, the AA undergoes a phase of renewed heating to almost the previous peak temperatures. Simultaneously, the Penninic units experience a phase of heating upon subduction, followed by cooling after onset of subduction of the North Penninic (NP) basin. The model predictions are inconsistent with the observed nearly isothermal uplift path of the SP after subduction and cannot explain observed inverted metamorphic peak conditions in the deeper AA (amphibolite facies) down to the higher Penninic unit (greenschist facies). A model with the beginning of subduction of the SP occurring after crustal thickening of the AA and subsequent return to normal crustal thicknesses is compatible with the P-T-t data. In this model, peak temperature conditions are higher in the AA, followed by a phase of strong cooling in the AA upper plate with the onset of underthrusting. This model also explains successfully nearly isothermal exhumation of the MP and inverted metamorphic peak conditions in the deeper AA. Material accreted to the hanging wall from the oceanic crust (SP) experiences a phase of cooling during ongoing subduction of oceanic lithosphere and begins to heat up to its thermal climax after the subduction of trailing continental lithosphere. The subsequent PT path of the Penninic units strongly depends on the timing and rates of underthrusting by the foreland. Observed PT paths in the MP within the TW require continuous subduction of the NP and the trailing European foreland under the exhuming MP block. Documented rapid cooling in the final uplift phase of the Penninic units in the TW requires exhumation rates up to approximately 4 mm/yr. Predictions of slightly elevated present-day geothermal gradients in the TW area are consistent with available heat flow data. As a result of Mesozoic rifting followed by late Mesozoic crustal thickening of the AA, paleorheological reconstructions are characterized by a contrast between relatively strong oceanic lithosphere and adjacent weak continental lithosphere. Predicted decoupling of weak continental and oceanic lithosphere during subsequent subduction of the Penninic units can explain observed Late Cretaceous crustal extension in the AA units in terms of gravitational spreading. Ongoing subduction leads to an overall strength increase due to underthrusting of cool oceanic lithosphere, whereas subduction of continental lithosphere causes a strength decrease in the upper levels of the lithosphere. Continuous crustal thickening and relaxation of the depressed isotherms reduce the strength of the lower lithospheric mantle beneath the central orogen, further enhanced by rapid late-stage uplift. Predictions for the present-day rheological structure of the Eastern Alps support the existence of a strong upper crustal layer, two wedge-shaped strong upper mantle layers to the north and the south of the orogen, and a weak upper mantle underlying the central orogen.

40Ar/39Ar muscovite ages from the Penninic‐Austroalpine plate boundary, Eastern Alps

The Radstadt Mountains, Eastern Alps, expose the tectonic boundary between the base of the Austroalpine continental plate (hanging wall) and the Penninic oceanic units (foot wall). Parts of the Austroalpine basement were penetratively deformed because of ongoing rifting of the continental crust during the Permian (290–250 Ma). Austroalpine Permo-Mesozoic cover rocks were deformed during the Cretaceous (∼80 Ma) and the Paleogene (55–50 Ma). These ages are interpreted as representing two distinct events of nappe stacking. The younger ages represent the collision of Austroalpine and Penninic tectonic units. The Penninic nappe complex displays a successive decrease of ages from ∼37 to 25 Ma from high to deep tectonic levels. A second age group of ∼22 Ma was found both in low-temperature release steps and as plateau ages close to the Penninic-Austroalpine boundary. It is attributed to a thermal overprint due to ductile extension of the overthickened orogenic wedge.

Exhumation of the Tauern window, Eastern Alps

Abstract The exhumation of metamorphic domes within orogenic belts is exemplified by the Tauern window in the Eastern Alps. There, the exhumation is related to partitioning of final orogenic shortening into deep-seated thrusts, near-surface antiformal bending forming brachyanticlines, and almost orogen-parallel strike-slip faults due to oblique continental plate collision. Crustal thickening by formation of an antiformal stack within upper to middle crustal portions of the lower lithosphere is a prerequisite of late-stage orogenic window formation. Low-angle normal faults at releasing steps of crustal-scale strike-slip faults accomodate tectonic unloading of synchronously thickened crust and extension along strike of the orogen, forming pull-apart metamorphic domes. Initiation of low-angle normal faults is largely controlled by rock rheology, especially at the brittle-ductile transitional level within the lithosphere. Several mechanisms may contribute to uplift and exhumation of previously buried crust within such a setting: (1) Shortening along deep-seated blind thrusts results in the formation of brachyanticlines and bending of metamorphic isograds; (2) oversteps of strike-slip faults within the wrench zone control the final geometry of the window; (3) unloading by tectonic unroofing and erosional denudation; and (4) vertical extrusion of crustal scale wedges. Rapid decompression of previously buried crust results in nearly isothermal exhumation paths, and enhanced fluid circulation along subvertical tensile fractures (hydrothermal ore and silicate veins) that formed due to overall coaxial stretching of lower plate crust.

The Klagenfurt Basin in the Eastern Alps: an intra-orogenic decoupled flexural basin?

Abstract The Klagenfurt Basin is an E-W-trending narrow Sarmation to Quaternary flexural basin formed by flexure of the Austro-Alpine lithosphere by loading through the southerly adjacent Karawanken Mountains in the Eastern Alps (Austria). The tectonic evolution of the basin is kinematically connected to Miocene brittle transpressive dextral strike-slip deformation along the Periadriatic Fault which separates the South Alpine unit from the Austro-Alpine unit in the Eastern Alps. The final NW-directed overthrust of the Karawanken Mountains onto the foreland is characterised by a positive flower structure, which is kinematically linked to dextral transpressive shearing along the Periadriatic Fault. Only a small part of the lithosphere appears to support the regional isostatic response to the load by the Karawanken Mountains. The area is characterised by a crustal thickness between 40 and 45 km, elevated heat flow and a strength distribution which suggests that only the upper crust elastically supports the topographic load. Ductile flow of the lower crust is also supported by the absence of a crustal root beneath the Karawanken chain. Rheologic models for this lithospheric configuration indicate a mechanical decoupling of upper crust, lower crust and mantle and generally low strengths for the lower crust and mantle.

Structural evolution within an extruding wedge: model and application to the Alpine-Pannonian system

Continental escape or lateral extrusion often results from late-stage contraction within continental collision zones when convergence is partitioned into orthogonal contraction, crustal thickening, surface uplift, and sideward motion of fault-bounded blocks. Geometrical arguments suggest that each individual fault-bounded block suffers a specific sequence of deformation. The style of deformation also depends on the location within the block. This includes: (1) initial shortening at the continental couple (future zone of maximum shortening: ZMS); (2) formation of a conjugate shear fracture system and initiation of orogen-parallel displacement of the decoupled extruding block away from the ZMS; (3) because of the changing width of the escaping block away from the ZMS the style of internal deformation changes within the extruding block: (i) shortening (thrusting, folding), surface uplift at the ZMS; (ii) strike-slip faulting along confining wrench corridors and formation of pull-apart basins at oversteps of en echelon shear fractures; (iii) extension parallel and perpendicular to the displacement vector far away from the ZMS. (4) Finally, the extruding block is gradually overprinted by general, laterally expanding contraction that starts to develop from the ZMS. This inferred sequence of deformation is tested by the Oligocene to Recent development of the Alpine-Pannonian system where late stage formation and extrusion of an orogen-parallel block started during the Oligocene. Stages 2 and 3 developed during Early to Middle Miocene, and final general contraction occurred during Late Miocene to Recent.

Initiation of left-lateral deformation along the Ailao Shan–Red River shear zone: new microstructural, textural, and geochronological constraints from the Diancang Shan metamorphic massif, SW Yunnan, China

The Diancang Shan metamorphic massif, the northwestern extension of the Ailao Shan Massif, is a typical metamorphic complex situated along the NW–SE-trending Ailao Shan–Red River shear zone. Diancang Shan granitic and amphibolitic mylonites collected from sheared high-grade metamorphic rocks were studied using petrographic and electron-backscatter diffraction techniques. Sensitive high-resolution ion microprobe U–Pb dating of zircon grains from the granitic mylonites constrains the timing of shearing. Macro- and microstructural and textural analysis reveals intense plastic deformation of feldspar, quartz, and amphibole under amphibolite-facies conditions, all consistently document left-lateral shearing. Porphyritic monzogranitic mylonite within the shear zone possesses evidence supporting a sequential, progressive process from crystallization during magma emplacement, through submagmatic flow to solid-state plastic deformation. We suggest that the early-kinematic pluton subsequently underwent strong left-lateral strike–slip shearing. The development of complex textures of quartz, feldspar, and amphibole from the granitic and amphibolitic mylonites apparently records successive variation of conditions attending coherent, solid-state high-temperature ductile deformation during regional left-lateral shearing. All magmatic zircons from the mylonitized porphyritic monzogranite give U–Pb ages of 30.95 ± 0.61 million years for the crystallization of the granite. This age provides the timing of onset of left-lateral shearing along the Ailao Shan–Red River shear zone in the Diancang Shan high-grade metamorphic massif.

40Ar/39Ar age evidence for Altyn fault tectonic activities in western China

Four40Ar/39Ar age groups of mica, hornblende and K-feldspar were obtained from Proterozoic and early Paleozoic metamorphic rocks in the Aksay-Dangjin Pass area, western China. The samples away from the middle shear zone of the Altyn fault belt yield two plateau age groups in the range of 461-445.2 Ma and 414.9-342.8 Ma, respectively. They represent the tectono-thermal events that had been recorded in the rocks that were displaced by the Altyn strike-slip fault in late Ordovician-early Silurian and Devonian, respectively. These two age groups should be related to the closures of Northern and Southern Qilian Oceans. The deformed granitic gneiss from the northern belt gives a plateau age group of 178.4-137.5 Ma, which is interpreted as the active age of the Altyn fault in the middle-late Jurassicearly Cretaceous and should be related to the accretion of Lhasa block to the north. The sample from the middle shear zone of the Altyn fault belt yields two plateau ages of 36.4 and 26.3 Ma, respectively, suggesting the strike-slip movement with strong metamorphism at greenschist facies along the Altyn fault in the late Eocene. This event occurred in the most areas of the northern Tibet Plateau and should be in response in the north to the collision between Indian and Eurasian continents. The present study demonstrates that the Altyn fault is characterized by multiple pulse-style activities under the tectonic setting of convergence between the Indian and Eurasian continents.

Microstructural and textural development of calcite marbles during polyphase deformation of Penninic units within the Tauern Window (Eastern Alps)

Abstract The evolution of calcite microstructures and crystallographic preferred orientations (CPOs) is well understood due to well constrained experimental studies. However, the interpretation of naturally deformed calcite marbles is more difficult because of less constrained strain paths, a multiphase deformation history, and variable P – T conditions. The Penninic units within the Tauern Window (Eastern Alps) have been affected by several deformation events and metamorphic overprint. Generally, three major deformational events can be distinguished. D 1 is related to underthrusting of Penninic units beneath the Austroalpine nappe complex, and top-to-the-N nappe stacking within the Penninic continental units. Deformation stage D 2 is interpreted as reflecting the subsequent continent collision between the Penninic continental units and the European foreland. D 3 is related to the formation of the dome structure of the Tauern Window. This polyphase deformation history can be partly reconstructed by the evolution of calcite microfabrics and CPOs. Three types of calcite-fabrics are distinguished within the Penninic units of the Tauern Window and the Lower Austroalpine unit. D 1 -fabrics are characterized by equilibrated microstructures and LT-CPOs. The CPOs are generally strong and symmetric, with one well developed cluster near the Z -axis of the finite strain ellipsoid. These fabrics have locally been overprinted by subsequent amphibolite to greenschist facies metamorphism. Generally, the occurrence of LT-fabrics coincides with the occurrence of amphibolite facies metamorphic mineral assemblages in the central part of the Tauern Window, while HT-fabrics have been observed outside this area. Fabrics from the central part of the Tauern Window have likely been strengthened during subsequent thermal equilibration, while the fabrics that have been observed at the peripheral parts have been less affected by subsequent metamorphic overprint. Therefore, D 1 -fabrics do not reflect D 1 conditions, but subsequent thermal equilibration. Similar observations have been made for the evolution of D 2 -fabrics. LT-fabrics dominate inside the amphibolite facies isograde, HT-fabrics occur outside (greenschist facies metamorphic conditions). The fabrics are characterized by high finite strains near the margins of the Tauern Window, the intensity of which decreases towards the central parts, where microstructures are characterized by recrystallization and thermal equilibration due to amphibolite facies metamorphic overprint. The strong CPOs document this influence. The HT-fabrics and microstructures within peripheral areas indicate that they have less been affected by this thermal event, and, therefore, are more indicative for the deformational conditions during D 2 . D 3 is restricted to distinct shear zones along the tectonic boundaries of the Tauern Window. From the central parts to the shear zone boundaries, a clear evolution of microfabrics can be observed. In internal parts, microstructures are characteristic for intracrystalline plasticity with a dominating activity of r − -glide. Twinning was less important during the final phases of deformation. On approaching the shear zone boundaries, the grain size decreases due to dynamic recrystallization and secondary grain size reduction until ultramylonites are formed. Within these domains grain boundary sliding seems to have been dominant. In conclusion, calcite CPOs from polyphase areas do not only include information on the deformation conditions, but also bear information about the thermal overprint subsequent to the main deformational event.

Geochemistry of metabasites in the north of the Shahrekord, Sanandaj-Sirjan Zone, Iran

Metabasites are exposed in the north of Shahrekord that is a part of the structural zone of Sanandaj-Sirjan in south- western Iran. The metabasites are composed mainly of eclogites, amphibolites and garnet amphibolites. We have conducted a geochemical study of metabasites in order to determine their protolith nature and geodynamic setting. The whole rock chemistry indicates a basalt to basaltic andesite composition for the metabasites. Our geochemical investigation strongly suggests that the metabasites are derived from tholeiitic basalt and were emplaced in a MORB tectonic setting.

Characteristics, timing, and offsets of the middle-southern segment of the western boundary strike-slip fault of the Songliao Basin in Northeast China

As the west boundary fault of the Songliao Basin and the eastern margin of the Da Hinggan Mountains, the Nenjiang-Balihan fault is located in the central part of Northeast China. It is traditionally considered to be a huge deep-seated NNE-striking fault, characterized by a normal fault or detachment fault displacement. The field investigation resulted in the finding of ductile shear zones in the Lingxia and Louzidian areas, the middle and southern sectors of the fault system. The authors conducted measurements of structural elements in the field, micro-structural studies, finite strain measurements, a study on preferred crystal orientations of quartz determined by Electron Back Scatter Diffraction and muscovite 40Ar/39Ar chronology of the deformed rocks in the ductile shear zone. The results show that the deformation features of the Lingxia and Louzidian ductile shear zones are similar, and that they represent one continuous fault, i.e., the middle-southern segment of the Nenjiang-Balihan Fault, which experienced a sinistral strike-slip ductile shearing in the Early Cretaceous (∼130 Ma). By measuring the displacement of the Xar Moron River suture and Wolegen Group on both sides of the Nenjiang-Balihan Fault, it is found that the cumulative strike-slip offset of the fault is about 40–50 km.