Structural development at the leading edge of the salt-bearing Kuqa fold-and-thrust belt, southern Tian Shan, NW China

摘要

The Tian Shan is one of the largest and most active intracontinental orogenic belts in the world, which has experienced complex deformation, resulting in the formation of several fold-and-thrust belts (FTBs) in the piedmonts. Investigations on when and how the Cenozoic compressional deformation has reached the leading edge of the FTBs will improve our understanding of the characteristics and mechanism of deformation propagation in contractional orogenic wedges. In this study, we focus on the kinematic evolution of the Nanka (South Kalayuergun) and Yangtake anticlines at the southern limit of the western Kuqa FTB where the Paleocene-Eocene Kumugeliemu Formation (E(1-2)km) salt has been involved into deformation via the area-depth strain (ADS) analysis of petroleum seismic reflection profiles. The results indicate that these two detachment folds have experienced two-stage tectonic shortening almost synchronously since the late Miocene, which are in contrast to the previously inferred progressive eastward elongation. The initial age of folding is temporally consistent with that of the Yaken anticline in the eastern sector where the detachment is the Miocene Jidike Formation (N(1)j) salt, suggesting that salt probably has facilitated the basinward propagation of structural deformation to reach its external pinch-out rapidly. Meanwhile, the salt-lubricated detachment has also promoted the lateral fold propagation, and hence the present length was quickly reached at a very early stage. The simultaneous, episodic deformation at the leading edge demonstrated by our analysis is consistent with the results of analog and numerical modeling, which would further our understanding of structural development and topographic expression in salt-bearing FTBs.