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

Abstract:

The >2800-km-long Tian Shan consists of several NW-SE, W-E, or NE-SW striking high ranges, which are separated by the narrow intermontane basins. Faulting and folding on the margins of intermontane basins play important roles in accommodating the Cenozoic deformation of the Tian Shan. In this paper, we provide constraints on the deformation pattern of the eastern Tian Shan by estimating the slip history of the intermontane Kumysh Fault, on the southern margin of the Kumysh Basin. Abandoned alluvial fan surfaces are well preserved in the basin margins. We obtained the topographic data using the satellite stereo imagery and dated geomorphic units using cosmogenic nuclide (¹⁰Be) 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/a 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/a 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.