Dynamics of intracontinental convergence between the western Tarim basin and central Tien Shan constrained by centroid moment tensors of regional earthquakes

Among the outstanding tectonic questions regarding the convergence between the Tien Shan and Tarim basin in northwestern China are the manner in which deformation is accommodated within their lithospheres, and the extent that the Tarim lithosphere underthrusts the Tien Shan. In particular, the amount and type of deformation within the Tarim basin is poorly understood. It is also uncertain if the convergence between the Tarim and the Tien Shan takes place mainly along a discrete boundary, or if the Tarim lithosphere simply indents into the Kazach shield, forming the Tien Shan through crustal thickening accommodated by a distributed series of thrust faults. In this study we use hypocentres from published earthquake catalogues and waveforms recorded by regional seismic networks to determine earthquake source parameters through regional centroid moment tensor inversion. The entire dataset consists of 160 earthquakes that occurred between 1969 and 2009 and with moment magnitudes between 3.5 and 7 distributed throughout the central Tien Shan and northwestern Tarim Basin. The estimated focal depths of these earthquakes range from the near-surface to about 44 km. Focal mechanisms throughout much of the Tien Shan indicate active deformation accommodated by thrust faults from at least the upper crust to 30 km depth. South of the Tien Shan, the Jia-shi earthquake sequence within the Tarim basin suggests that both crustal shortening and localized flexure are part of a complicated process involving rotational convergence. Inside the Tarim basin, two earthquakes with thrust faulting mechanisms near the crust-mantle boundary beneath the Bachu uplift imply a brittle rheology of the lower crust. High-angle thrust events occur broadly across the Tien Shan, suggesting that the Tarim lithosphere as a whole is strong and indents into the Kazach shield to create the mountain range.