Late Quaternary normal faulting within the rigid Ili Basin: Insights into the deformation pattern of the northern Tian Shan, NW China

The deformation pattern of the Tian Shan orogenic belt associated with the ongoing India–Eurasia collision is poorly understood due to the lack of deformation of the intermontane basins embedded in the mountains. The Yamadu Fault is a NW-striking structure in the interior of the rigid Ili Basin and has clearly experienced tectonic deformation. In this study, we quantified its late Quaternary activity based on interpretations of detailed high-resolution remote sensing images and field investigations. Observations from four field sites along the ∼70-km-long structure indicate that the Yamadu fault is characterized by normal faulting and exhibits a southeastward decrease in deformation rate along its strike. By surveying the displaced geomorphic surfaces with an unmanned drone and dating the late Quaternary sediments via optically stimulated luminescence, we estimate a maximum late Quaternary vertical slip rate of 0.37 ± 0.08 mm/yr and an E–W extension rate of 0.19 ± 0.08 mm/yr at the northwesternmost extent of the fault. The Yamadu Fault represents a boundary structure accommodating counterclockwise rotation and relative divergence between the Ili block and Tian Shan Mountains caused by right-lateral faulting at the northern margin of the basin. The Cenozoic deformation of the Tian Shan area included continuous deformation via the overwhelming crustal shortening accommodated by the soft lithospheric structure and a block-like deformation pattern of lateral extrusion and/or rotation accommodated by the rigid intermontane basin.