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 (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/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.

Late Quaternary Kinematics and Deformation Rate of the Huoyanshan Structure Derived From Deformed River Terraces in the South Piedmont of the Eastern Chinese Tian Shan

“…Many studies e.g., Deng et al (2000), Thompson et al (2002), Shen et al (2003), Hubert-Ferrari et al (2007), Yang et al (2008), Lu et al (2019) have been conducted in the Western Tian Shan region to reveal the kinematics and deformation rate of fault-related folding and have shown that shortening is relatively uniformly distributed across the major intermontane basin boundaries and basin interior structures along a N-S transect of the Western Tian Shan, rather than solely at its margins. The Eastern Chinese Tian Shan is also dominated by intense tectonic deformation, which is shown by the study results of widespread active faults Deng et al (2000), Lin et al (2002), Wu et al (2016), Huang et al (2018a), Huang et al (2018b), Ren et al (2019), Wang et al (2020a), paleoearthquake ruptures Feng (1997) and global positioning system (GPS) measurements (Zhang et al, 2004;Yang et al, 2008;Wang and Shen, 2020). However, in contrast to the Western Tian Shan, the deformation pattern and slip partitioning in the Eastern Chinese Tian Shan remain unclear because of the lack of quantitative study on the deformation rate and kinematics of the major structural belt.…”

“…The Eastern Chinese Tian Shan consists of the Bogda and Kuruktag ranges and several intermontane basins, such as the Turpan, Yanqi and Kumysh Basins (Figure 1B). Previous field investigations have recognized a series of active faults that bound the intermontane basins and accommodate N-S convergence (Deng et al, 2000;Lin et al, 2002;Fu et al, 2003;Wu et al, 2016;Wang et al, 2020a). The Eastern Chinese Tian Shan can be divided into two parts based on the Turpan basin (the largest intermontane basin).…”

“…By studying the typical offset landforms and lineaments of scarps on the eastern segment of the north-edge fault of the Yanqi Basin, Huang et al (2018a) constrained the shortening rate to 0.4-0.5 mm/ yr on the northern margin of the basin. Based on dating the alluvial fan surfaces, measuring the heights of the fault scarps, and investigating a paleoearthquake trench wall, Wang et al (2020a) estimated that a ∼0.3 mm/yr shortening rate was accommodated by the Kumysh Fault during the late Quaternary. The shortening rate was estimated to be ∼1 mm/yr since the late Quaternary at the Bogda foreland thrust fault through geologic and geomorphologic field surveys, trench excavation and OSL dating (Wu et al, 2016).…”

“…(C) Topography and major active faults in the Eastern Chinese Tian Shan (see Figure 1B for the location). Fault data are from Deng et al, 2000;Shen et al, 2003;and Wu et al, 2016; red dots indicate earthquakes (M > 5) from 1900-01 to 2020-11 (downloaded from the United States Geological Survey at https:// earthquake.usgs.gov/); shortening rates in yellow font in Figure 1C are from Wu et al, 2016 andWang et al, 2020a; and those in pink font are from this study; HSK, Hongshankou structure; YS, Yanshan structure; KDK, Kendeke structure; HYS, Huoyanshan structure; QKT, Qiketai structure.…”

Late Quaternary Slip Rate and Kinematics of the Baoertu Fault, Constrained by 10Be Exposure Ages of Displaced Surfaces within Eastern Tian Shan

“…interior of the range [12][13][14], which is mainly distributed along the thrust structures with late Quaternary slip rates of 0.1-3 mm/yr in the western Tian Shan [13].…”