Based on GPS measurements conducted from 1992 to 2006, we present the current crustal movement velocity field for approximately 400 sites in the Tianshan Mountains and their adjacent areas, and estimate slip rates on the major faults using a 2-D elastic dislocation model. Our studies show slip rates within the range of 1―4 mm/a on the NW-SE trending strike-slip faults (such as Talas-Fergana fault) in the Tianshan Mountains. We also found the slip rates on the approximately WE-SN trending gently-dipping detachment fault vary from 10―13 mm/a for the southwest Tianshan Mountains to 2―5 mm/a for the eastern Tianshan Mountains, and to 6―12 mm/a for the Kyrgrz Tianshan. The GPS velocity field reveals that the total convergence is not uniformly distributed across the Tianshan Mountains, with 80%―90% of the N-S shortening absorbed along the southern and northern edges, and relatively little deformation accommodated within the interior. This first-order feature of strain pattern is explained best by underthrusting of adjacent blocks beneath the Tianshan Mountains along a basal detachment fault. We found the occurrence of historical M7―8 earthquakes somewhere in the locked ramp that connects the creeping and locking segments of the detachment, thereby resulting in elastic strain concentration and accumulation around it. The elastic strain confined in the upper crustal layer above the detachment ultimately releases through infrequent great earthquakes in the Tianshan Mountains, resulting in considerable folding and faulting at their margins. The Tianshan Mountains propagated outward and rose progressively as a wedge-shaped block.
YANG ShaoMin1,2, LI Jie1,3 & WANG Qi2,1 1 Research Center of Space Science and Technology, China University of Geosciences, Wuhan 430074, China
We simulate GPS horizontal velocity field in terms of rotations of crustal blocks to describe deformation behavior of the Chinese mainland and its neighboring areas.31 crustal blocks are bounded primarily by^30 Quaternary faults with distinct geometries and variable long-term rates of<20 mm/a,and 1 683 GPS velocities were determined from decade-long observations mostly with an averaged uncertainty of 1?2 mm/a.We define GPS velocity at a site by the combination of motion of rigid block and elastic strain induced by the fault that is locking during a seismic cycle.Model velocities predicted from the preferable block model match well with the GPS velocities to an uncertainty of^1.7 mm/a.The slip rates inferred from this model is in a range of 6?18 mm/a for the major faults in Tibet and its margins and 1?4 mm/a in eastern China,consistent with geological observations.Our numerical simulation suggests that the crustal blocks deform internally at a level of^10×10-9 /a,quite small in comparison with significant deformation localized along fault zones of 50?100 km wide. We conclude that the pattern of continental deformation is not continuous-like but block-like,and the tenet of plate tectonics may be applicable to characterize the active deformation in Asia.
