Segmentation and deformation mechanisms of the North Tuole Shan fault in the Qilian Mountains: Insights from channel steepness index (ksn) and fault activity

The North Tuole Shan Fault (NTF) is a significant thrust fault system in the Qilian Mountains. Understanding its activity patterns and differential deformation characteristics is essential for deciphering regional tectonic uplift, crustal deformation mechanisms, and geomorphic evolution. However, the segmentation characteristics of this fault and their controlling factors remain poorly understood, hindering a comprehensive understanding of major fault evolution in the region. In this study, we extracted k_sn values from 107 river channels along the northern slope of Tuole Shan in the central Qilian Mountains and compared them with previous findings on NTF activity. The results reveal a general consistency between fault activity variations and k_sn distribution, indicating that k_sn values serve as a reliable indicator for fault activity variations and their controlling factors. Further analysis suggests that k_sn variations between the East and West Jingtie segments are predominantly governed by fault activity differences, with secondary influences from bedrock erosion resistance. Higher k_sn values in the Qilian Segment are likely related to the transition of the Riyue Shan Fault (RYF) from dextral strike-slip motion to oblique thrusting at the fault tip. This transition intensifies tectonic loading, resulting in a simultaneous increase in k_sn values and vertical slip rates. Existing studies on the strike-slip rate of the RYF, along with new analytical results, suggest that the angular variation involved in this kinematic transition is likely ≤30°. This suggests that, in addition to faulting and lithology, lateral extrusion associated with strike-slip faulting may also play a significant role in controlling adjacent fault activity and shaping the tectonic landscape. These findings offer novel insights into the mechanisms regulating fault activity variations.