Two Phase Transpressional Ductile Shearing In The Eastern North Qaidam Mountain: Response To Early Paleozoic-Early Mesozoic Oblique Convergence

The transpressional shear zone,commonly recognized in the convergence plate margins and in intraplate settings,has played a significant role for accommodating accretion,collision lateral extrusion parallel to orogens.The North Qaidam tectonic belt,which is located in northern Tibet,was regarded as resulting from closure of Proto-Tethys,continental deep subduction and post-orogenic extension collapse during early Paleozoic era.The Kunlun-Qaidam-Qinling blocks were subjected to northward subduction of Paleo-Tethys,closure of Paleo-Tethys and subsequent collision during Mesozoic era.Recently,two phases of NWW—SEE trending transpression ductile shear zone was newly recognized,which are early Paleozoic dextral ductile shear zone(DSZ)and Mesozoic sinistral ductile shear zones(SSZ)in the Chahanhe area,eastern North Qaidam Mountains.In this contribution,we present macro-microstructural observations,quartz C-axis fabric analyses,quantitative structural data(including kinematic vorticity,paleo-stress and strain rate),and combine with zircon U-Pb and biotite Ar-Ar dating data,to investigate the type of the transpression of the two phases of ductile shear zones and discuss their tectonic evolution.The macro-to micro-structural feature and quartz C-axis fabric in X-Z planes show that DSZ is characterized by dextral strike-slip shear sense.Finite strain and kinematic vorticity in deformed rocks indicate a slightly prolate ellipsoid near plane strain,suggesting a transpressional style.Mineral recrystallization mechanism,quartz C-axis fabrics slip system and opening-angle thermometer reveal that the DSZ developed under 500 to 553?,corresponding to a mid-crust level(16?18 km).Combining with dynamic recrystallization particle size of quartz,the differential stress and strain rate were estimated to be 29?30 Mpa and 10^-12s^-1,respectively.Zircon U-Pb dating of diorite involved in ductile shearing gave an age of 432±3 Ma.Combining with previously geological data suggests the timing of the dextral ductile shearing deformation between middle Silurian and early Devonian(432?396 Ma).The integrated data indicate that DSZ is characterized by combination of horizontal contraction perpendicular to the orogenic belt and lateral extrusion parallel to the orogenic belt,accompanied by a part of top-to-the south thrust component,resulted from oblique convergence and collision during late early Paleozoic orogeny.The macro-microstructure,quartz C-axis fabric and kinematic vorticity consistently show that SSZ is characterized by sinistral strike-slip shear sense.Finite strain in deformed rocks mainly indicate a prolate ellipsoid,suggesting a transpressional style.Mineral recrystallization mechanism,quartz C-axis fabrics slip system and opening-angle thermometer reveal that the deformation temperature of SSZ is between 400 to553?,which increases gradually from north to south in each of SSZ.Combining with zircon U-Pb dating and biotite Ar-Ar dating,suggests the timing of the sinistral ductile shearing deformation between early-middle Triassic(252?235 Ma).The integrated data indicate that SSZ are the response to the northward oblique subduction of Paleo-Tethys and collision.