Active Thrusting in an Intermontane Basin: The Kumysh Fault, Eastern Tian Shan

Abstract Faulting and folding on the margins of intermontane basins play important roles in accommodating the Cenozoic deformation of the Tian Shan, an intracontinental orogen in central Asia. In the eastern Tian Shan, the southern margin of the elongated Kumysh Basin is bounded by a NE dipping thrust fault, the Kumysh Fault. We report the first investigation of the slip rate of this fault, which is constrained at two sites by profiling the faulted topography and dating the abandoned alluvial fan surfaces. We obtained the topographic data using the satellite stereo imagery and dated geomorphic units using cosmogenic nuclide ( 10 Be) exposure ages and optically stimulated luminescence methods. The results from Monte Carlo simulations of the data suggest an average dip‐slip rate of ~0.35 mm/yr on the Kumysh Fault since 50–60 ka. The “best fit” models indicate an increase after ~5 ka, which may be apparent or caused by the most recent earthquakes. Structural analysis of the displaced strata on a trench wall suggest at least 2.8 m dip offset caused by the late Holocene earthquakes, which is similar to the heights of fault scarps (1.8–2.9 m) on the second youngest alluvial fans (5.6–5.2 ka). Using the dip angle of ~25–40°, we estimate ~0.31 mm/yr shortening rate across the Kumysh Fault during the late Quaternary. By comparing our results with other estimates published in the easternmost Tian Shan, we suggest that deformation within intermontane basins may accommodate up to 50% of the total late Quaternary shortening rate between the Junggar and Tarim blocks, with only the remaining half accommodated by the main, range‐bounding thrusts.