Quantitative constraints on shortening deformation characteristics of the fold at the south-edge thrust fault of the Yanqi Basin, Southern Tien Shan

Abstract: The collision and compression between the Indian and Eurasian plates have resulted in intense crustal shortening and deformation in the Tian Shan since the Cenozoic, leading to its renewed uplift and making the Tian Shan one of the most intensely deformed intracontinental orogenic belts and seismically active regions in the world. Cenozoic deformation of the Tian Shan is characterized by north–south crustal shortening, which is mainly controlled by approximately E-W striking thrust faults, N-W striking dextral strike-slip faults, and N-E striking sinistral strike-slip faults. Concurrently, pronounced tectonic deformation occurred within the Tien Shan, forming a series of E-W trending intermontane basins, including the Turpan, Kumishi, Yanqi, and Yili basins. The Yanqi Basin, located in the southeastern Tian Shan, has experienced significant tectonic deformation due to the continuous uplift of the Tien Shan. Since the Late Quaternary, tectonic deformation has been mainly concentrated along the northern and southern margins of the basin. Two major active tectonic systems are developed along the northern margin, including the Yanqi Basin north-edge thrust fault and the piedmont thrust-fold belt. Along the southern margin, the Yanqi Basin south-edge thrust fault is developed, striking E-W and dipping southward, forming a complex thrust-fold belt. Within this fold belt on the southern margin of the basin, Quaternary geomorphic surfaces are well preserved and display fault scarps of variable heights. Multiple generations of alluvial fans are dissected along the fault scarps. In this study, high-resolution topographic data of faulted alluvial fan landforms along the southern margin thrust-fold belt were acquired using airborne LiDAR. Detailed geomorphic interpretation and quantitative analysis were conducted to identify multiple generations of landforms developed perpendicular to the fault strike. Based on comprehensive geomorphic interpretation and field investigations, deposits from different generations of alluvial fans were sampled for surface age determination. Furthermore, based on measurements of exposed strata and the construction of characteristic topographic profiles across the alluvial fans, we established the cross-sectional geometry and deformation model of the thrust fault-fold belt at the south-edge of the Yanqi Basin. By integrating the tri-shear fault-propagation fold model from fault-related fold theory, we constrain the shortening deformation characteristics of the thrust fault-fold belt.  This allows us to estimate the shortening amount and shortening rate for the belt at the southern margin of the Yanqi Basin.  Combined with analysis of surrounding fold deformation and fault slip rates, this work not only reveals the deep geometry and activity mechanism of the thrust fault at the south-edge of the Yanqi Basin but also provides constraints for understanding intracontinental deformation within the Southern Tian Shan.