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Seismotectonics in the Pamir:An oblique transpressional shear and south-directed deep-subduction model

Geoscience Frontiers 2011年第1期2卷 1-15页

作者： Jiasheng Zhang Xinjian Shan Xiongnan Huang Institute of Geology China Earthquake Administration Beijing 100029 China

The 3-D geometry of the seismicity in Hindu Kush-Pamir--western China region has been defined by seismic records for 1975-1999 from the National Earthquake Information Center, the U.S. Geological Survey, and over 16,000 relocated earthquakes since 1975 recorded by the Xinjiang seismic network of China. The results show that most Ms≥ 5.0 hypocenters in the area are confined to a major intracontinental seismic shear zone （MSSZ）. The MSSZ, which dips southwards in Pamir has a north- dipping counterpart in the Hindu Kush to the west; the two tectonic realms are separated by the sinistral Chaman transform fault of the India-Asia collisional zone. We demonstrate that the MSSZ constitutes the upper boundary of a south-dipping, actively subducting Pamir continental plate. Three seismic concentrations are recognized just above the Pamir MSSZ at depths between 45-65 km, 95-120 km, and 180-220 km, suggesting different structural relationships where each occurs. Results from focal mechanism solutions in all three seismological concentrations show orientations of the principal maximum stress to be nearly horizontal in an NNW-SSE direction. The south-dipping Pamir subduction slab is wedge-shaped with a wide upper top and a narrow deeper bottom; the slab has a gentle angle of dip in the upper part and steeper dips in the lower part below an elbow depth of ca. 80--120 km. Most of the deformation related to the earthquakes occurs within the hanging wall of the subducting Pamir slab. Published geologic data and repeated GPS measurements in the Pamir document a broad supra-subduc- tion, upper crustal zone of evolving antithetic （i.e. north-dipping） back-thrusts that contribute to northsouth crustal shortening and are responsible for exhumation of some ultrahigh-pressure rocks formed during earlier Tethyan plate convergence. An alternating occurrence in activity of Pamir and Chaman seismic zones indicates that there is interaction between strike-slip movement of the Chaman transform fa

关键词： Earthquake Seismotectonics Deep-subduction ofcontinental crust Transform fault ramp thrusting Central Asia

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