Structural deformation of Xinyang-Yuanlong ductile shear zone in Tianshui area，Western Qinling Mountains，and its geological significance

Abstract：The Xinyang-Yuanlong ductile shear zone on the north margin of Western Qinling Mountains is the main ductile tectonic boundary which separates the Western Qinling orogenic belt from the Northern Qilian orogenic belt．The deformation style in the ductile shear zone is complex and diverse, with multiphase structural superposition，and deformation varies in intensity in different parts．From the macro- and micro-structure，it is revealed that the deformation has both sinistral and dextral shear，but the major part of the deformation is right-handed．Macro-structure shows characteristics of oblique thrust from NNE to SSW; nevertheless, the structure is mainly reformed and superposed．According to EBSD fabric analysis, LPOs of quartz indicate non-coaxial deformation and reveal that the quartz grains displaying dextral shear are apparently characterized by medium-temperature prismatic fabric ，low-medium temperature rhombohedral fabric，and low-temperature basal fabric．The quartz grains displaying sinistral shear are indistinctly characterized by low-temperature basal fabric（occasionally medium-temperature prismatic fabric）；LPOs of calcite show that the grains are dominated by e1 twinning glide and r1 translation glide，and reveal both sinistral and dextral shear characteristics．All these data indicate that the shear zone probably experienced a complex deformation process from medium-temperature through low-medium temperature to low temperature，with the deformation environment from low greenschist facies through high greenschist facies to low amphibolite facies．The transformation of shear sense suggests that dextral strike-slip ductile shear is dominant，while sinistral thrusting shear is subordinate in the shear zone．According to regional comparative analysis，Xinyang-Yuanlong ductile shear zone has marked two main structural deformational records responding to the tectonic evolution in the Paleozoic: The early record is sinistral thrusting shear deformation that was the adjustment to the thrusting in a direction of NNE to SSW，caused by the continent-arc or continent-continent collision related to the closure of the Tianshui-Wushan Ocean in the Silurian；The later record is dextral shear deformation and the inversional S-type tectonic style caused by the intense dextral strike-slip at the conjunction of Qinling and Qilian orogen in the Late Devonian to Early Carboniferous period．