40Ar/ 39Ar laser dating of Zongwulong ductile shear zone in northeastern Tibetan Plateau ：Constrains on the closure time of the northmost Paleo-Tethys ocean

2021 ◽  
Author(s):  
Wanli Gao ◽  
Zongxiu Wang
Keyword(s):  
Shear Deformation ◽  
Shear Zone ◽  
Ductile Deformation ◽  
Low Grade ◽  
Ductile Shear Zone ◽  
The South ◽  
Oblique Collision ◽  
Ductile Shear ◽  
Tethys Ocean ◽  
Tectonic Belt

<p><strong><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.67d6c7216eff55356050161/sdaolpUECMynit/12UGE&app=m&a=0&c=5572aca4b392eef83f52919e1be673e9&ct=x&pn=gepj.elif&d=1" alt="">Abstract</strong>：The Zongwulong tectonic belt (ZTB) is located between the northern Qaidam tectonic belt and the south Qilian orogenic belt and contains Late Paleozoic and Early- Middle Triassic strata. Structural features and geochronology of Zongwulong ductile shear zone have key implications for the tectonic property of the ZTB. This study integrated field structure, microscopic structure and <sup>40</sup>Ar/<sup>39</sup>Ar laser probe analysis. The shear zone strikes ~NEE-SWW and dips at a high angle, with a NWW-SEE trending and WE stretching lineation, indicating the shear zone as a thrust- slip shear ductile shear. The asymmetric folds, rotating porphyroclast,structural lens and crenulation cleavage can be seen in the field. Mica fish, S − C fabrics, σ type quartz porphyroclastic and quartz wire drawing structure can also be observed under microscope, indicating that the strike- slip- related ductile deformation and mylonitization occurred under low- grade greenschist facies conditions at temperatures of <em>300° C − 400° C</em>.  The highly deformed<br>mylonite schist yielded <sup>40</sup>Ar/<sup>39</sup>Ar ages <em>(245.8±1.7)Ma </em>and <em>(238.5±2.6)</em>Ma for muscovite and biotite, respectively, indicating that the shear deformation occurred during the Early- Mid Triassic. Combined with comprehensive analysis of regional geology and petrology, the authors hold that the age of ductile shear deformation represents the time of Triassic orogeny in the ZTB. The oroginic activity was probably related to the oblique collision between the South Qilian block and the Oulongbuluke block after the closure of the northermost Paleo-Tethys ocean.</p>

scholarly journals Polyphase deformation along the South Bohemian Batholith-Moldanubian nappes boundary – The Freyenstein Fault System (Bohemian Massif/Austria)

2020 ◽  
Vol 113 (1-2) ◽  
pp. 139-153
Author(s):  
Gerit E. U. Griesmeier ◽  
Christoph Iglseder ◽  
Ralf Schuster ◽  
Konstantin Petrakakis
Keyword(s):  
Shear Zone ◽  
Bohemian Massif ◽  
Potassium Feldspar ◽  
Strike Slip ◽  
Greenschist Facies ◽  
Fault System ◽  
Ductile Shear Zone ◽  
The South ◽  
Shear Sense ◽  
Ductile Shear

AbstractThis work describes the Freyenstein Fault System, which extends over 45 km in the southeastern part of the Bohemian Massif (Lower Austria). It represents a ductile shear zone overprinted by a brittle fault located at the eastern edge of the South Bohemian Batholith towards the Moldanubian nappes. It affects Weinsberg- and a more “fine-grained” granite, interlayered aplitic granite and pegmatite dikes as well as paragneiss of the Ostrong Nappe System. The ductile shear zone is represented by approximately 500 m thick greenschist-facies mylonite dipping about 60° to the southeast. Shear-sense criteria like clast geometries, SCC`-type shear band fabrics as well as abundant microstructures show top to the south/ southsouthwest normal shearing with a dextral strike-slip component. Mineral assemblages in mylonitized granitoid consist of pre- to syntectonic muscovite- and biotite-porphyroclasts as well as dynamically recrystallized potassium feldspar, plagioclase and quartz. Dynamic recrystallization of potassium feldspar and the stability of biotite indicate upper green-schist-facies metamorphic conditions during the early phase of deformation. Fluid infiltration at lower greenschist-facies conditions led to local sericitization of feldspar and synmylonitic chloritisation of biotite during a later stage of ductile deformation. Finally, a brittle overprint by a north-south trending, subvertical, sinistral strike-slip fault that shows a normal component is observed. Ductile normal shearing along the Freyenstein Shear Zone is interpreted to have occurred between 320 Ma and c. 300 Ma. This time interval is indicated by literature data on the emplacement of the hostrock and cooling below c. 300°C inferred from two Rb-Sr biotite ages measured on undeformed granites close to the shear zone yielding 309.6 ± 3 Ma and 290.9 ± 2.9 Ma, respectively. Brittle sinistral strike-slip faulting at less than 300°C presumably took place not earlier than 300 Ma. Early ductile shearing along the Freyenstein Fault System may be genetically, but not kinematically linked to the Strudengau Shear Zone, as both acted in an extensional regime during late Variscan orogenic collapse. A relation to other major northeast-southwest trending faults of this part of the Bohemian Massif (e.g. the Vitis-Pribyslav Fault System) is indicated for the phase of brittle sinistral movement.

Kinematics, non-coaxial flow and rheological constraints of the South Tibetan Detachment System in central Himalaya

2021 ◽  
Author(s):  
Nania Laura ◽  
Montomoli Chiara ◽  
Iaccarino Salvatore ◽  
Leiss Bernd ◽  
Carosi Rodolfo
Keyword(s):  
Shear Zone ◽  
Simple Shear ◽  
Regional Scale ◽  
Crystalline Lattice ◽  
Strain Rates ◽  
Central Himalaya ◽  
Ductile Shear Zone ◽  
The South ◽  
Ductile Shear ◽  
Structural Levels

<p>A challenge in tectonic studies concerns the attempt to relate deformation features at the microscale and the crystalline lattice scale of rock-forming minerals up to the regional scale. The South Tibetan Detachment System (STDS) in Himalaya is a natural laboratory for such correlations, being a prime example of regional-scale low-angle ductile extensional fault/shear zone systems within collisional settings, with a top-down-to-the-north sense of shear. The STDS shearing involves, with a thickness of c. 1-2 km, the uppermost part of the metamorphic core of the belt, the Greater Himalayan Sequence (GHS), and the basal part of the Tethyan Himalayan Sequence (THS), developing a mylonitic foliation and a nearly constant strike. Recurrent ideas on the STDS architecture and rheological behavior come from the clearly and well exposed 3D outcrops around the Everest area (Eastern Nepal), where it mostly developed in quartz-bearing lithologies with a lower ductile shear zone and an upper brittle fault. Vice versa, the location of the exact shear zone boundaries and structural evolution of the STDS are still under controversial discussions in Central-Western Nepal, where few kinematic indicators occur in the carbonate-bearing lithologies of both GHS and THS.</p><p>In this contribution, we examine a suite of over 20 field-oriented marble samples affected by the STDS, comparing the deformation recorded by calcite in two different areas in central Himalaya, where essentially only the ductile shear zone has been clearly identified. Calcite microstructures (e.g., grain size and shape) and crystallographic preferred orientations (textures) of impure marbles from the Lower Dolpo region and pure marbles from the Manaslu area (Western Nepal), coupled with petrographic observations, allowed us to conclude on temperature, paleo stress, strain rates, and kinematic of the flow. Our results support the idea of a complex history of the STDS in regard to different thermal and lithospheric stress regimes during deformation. Decreasing temperatures from an early-stage of shearing (at HT-MT condition) to a late-stage of shearing (LT conditions) are coupled with increasing differential stress recorded at comparable strain rates and decreasing simple shear conditions. We propose a progressive exhumation of the STDS towards shallower structural levels, with a temporal (rather than spatial) lowering of kinematic vorticity (“decelerating strain path”), in which progressively more general shear replaced high-temperature simple shear flow during cooling, strain hardening, and narrowing of the shear zone. Microstructural and texture analysis of pure and impure marble proved to be a useful approach to characterize the STDS location and architecture, supporting that, when the upper-brittle fault is not well developed, the ductile shearing proceeded at high structural levels.</p>

Application Of Electron Probe Analyses Of Minerals In Discussing The Relationship Between Ductile Deformation And Metamorphism

1990 ◽  
Vol 48 (2) ◽  
pp. 250-251
Author(s):  
Fan Guochuan ◽  
Sun Zhongshi
Keyword(s):  
Chemical Composition ◽  
Shear Deformation ◽  
Electron Probe ◽  
General Rule ◽  
Granulite Facies ◽  
Ductile Deformation ◽  
Higher Temperature ◽  
Ductile Shear ◽  
The Relationship ◽  
Ductile Shear Deformation

Under influence of ductile shear deformation, granulite facies mineral paragenesis underwent metamorphism and changes in chemical composition. The present paper discusses some changes in chemical composition of garnet in hypers thene_absent felsic gnesiss and of hypersthene in rock in early and late granulite facies undergone increasing ductile shear deformation .In garnet fetsic geniss, band structures were formed because of partial melting and resulted in zoning from massive⟶transitional⟶melanocrate zones in increasing deformed sequence. The electron-probe analyses for garnet in these zones are listed in table 1 . The Table shows that Mno, Cao contents in garnet decrease swiftly from slightly to intensely deformed zones.In slightly and moderately deformed zones, Mgo contents keep unchanged and Feo is slightly lower. In intensely deformed zone, Mgo contents increase, indicating a higher temperature. This is in accord with the general rule that Mgo contents in garnet increase with rising temperature.

Structural analysis of the Nanchang-Wanzai sinistral ductile shear zone (Jiangnan region, South China)

1991 ◽  
Vol 6 (1) ◽  
pp. 13-23 ◽  
Author(s):  
Liangshu Shu ◽  
Jacques Charvet ◽  
Yangshen Shi ◽  
Michel Faure ◽  
Dominique Cluzel ◽  
...  
Keyword(s):  
Structural Analysis ◽  
Shear Zone ◽  
South China ◽  
Ductile Shear Zone ◽  
Ductile Shear
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