Northward subduction-related orogenesis of Chinese SW Tianshan (NW China): Constraints from structural and metamorphic analysis of the HP/UHP accretionary complex
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The Chinese Tianshan belt of the southern Altaids has undergone a complicated geological evolution. Different theories have been proposed to explain its evolution and these are still hotly debated. The major subduction polarity and the way of accretion are the main problems. Southward, northward and multiple subduction models have been proposed. This study focuses on the structural geology of two main faults in the region, the South Tianshan Fault, which is investigated in the Muzhaerte and Kapuerte river valleys, and the Nikolaev Line, which is investigated in the Muzhaerte river valley. The South Tianshan Fault has a NW dip with NW-pointing lineations in the Muzhaerte river valley. Two shear sense motions have been observed within both of these fault zones, a sinistral one, and a dextral one. Structural analyses on the fault zones, supported by previous studies on petrology and seismic tomography, suggest that subduction has been northward rather than southward. The two opposing shear sense directions most likely are a result of the extensional regime occurring during the Permian, though they might also indicate that the Yili block was first dragged along towards the east due to the clockwise rotation of the Tarim block. After the Tarim block stopped rotating, the Yili block still kept going eastward, inducing the dextral shear senses within the fault zones.