LATE QUATERNARY DEFORMATION RATE OF THE WENSU FORELAND THRUST BELT IN THE SOUTHERN TIANSHAN MOUNTAINS
In response to the ongoing India-Eurasia collision in the late Cenozoic,the Tianshan orogenic belt was reactivated and experienced rapid uplift.Strong uplifted topography results in that the mountains propagate from the range front toward the foreland basin to form several fan-shaped foreland thrust belts both on its north and south sides.These foreland thrust belts accommodate the most north-south convergence strain and control the regional deformation pattern.However,in contrast to the well-studied foreland thrust belts,the kinematics and deformation rate of the transition area between different foreland thrust belts have not been well-documented.Therefore,it is still unclear how the crustal shortening in the foreland basins changes along the east-west direction.Further,the deformation characteristics and seismic hazard in this region are poorly understood because quanti-tative information on active deformation is lacking.The Wensu foreland thrust belt is located in the Kalpin and Kuqa foreland thrust systems'transition areas.In contrast to the Kuqa and Kalpin foreland thrust belts at its east and west sides,the Wensu foreland thrust belt propagated approximately 20km toward the basin and only developed one row of active thrust fault-anticline belts,namely the North Wensu thrust fault-anticline belt.The North Wensu structural belt shows clear evidence of tectonic solid activity because the late Quaternary sediments and river terraces have been faulted.However,this structural belt's kinematics and late Quaternary deformation rate remain poorly constrained.This study quantifies its deformation mode based on field geological mapping across the anticline.Our results indicate that the North Wensu structural belt is a fault-bending fold.Based on interpretations of detailed high-resolution remote sensing images and field investigations,five levels of river terraces can be identified along the Kekeya River valley.By surveying of the displaced terraces with an unmanned drone,the crustal shortening values of～20.7m、～35.3m and～46.9m are determined for the T3,T4 and T5 terraces,respec-tively.Our optically stimulated luminescence(OSL)dating yields a depositional age of(9.02±0.55)ka for the T3 terrace,(24.23±1.58)ka for the T4 terrace,and(40.43±3.07)ka for the T5 terrace.Thus,we estimate a crustal shortening of～1.31mm/a in the late Quaternary(since approxi-mately 40ka),and approximately 2.29mm/a in the Holocene for the North Wensu structural belt.Our results indicate that the deformation rate of the North Wensu structural belt exhibits an obvious increase in the Holocene.This phenomenon indicates that the strong earthquake activity on the North Wensu thrust belt has increased significantly in the Holocene,implying an irregular activity habit of the strong earthquake recurrence cycle on this tectonic belt.The propagation deformation toward the basin of the Wensu foreland thrust belt is very limited.Therefore,we suggest that the foreland thrust belt is a thick-skinned nappe structure and is dominated by high-angle thrust faulting.The tectonic deformation in the Wensu region seems to be characterized by considered vertical growth.Although the deformation rate is small,the uplift amplitude is significant in this region.
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