Deep crustal structure controls the deformation style of the foreland fold-and-thrust belt in south Tian Shan

Despite extensive studies have documented the structural styles of shallow thrust-nappe belts in the upper crust, the role of deep-seated inherited structures in foreland thrust evolution remains poorly constrained. This knowledge gap hinders in-depth investigations into the deep-shallow structural relationships and contact patterns between tectonic units in basin-range junction belt. The southern Tian Shan margin features extensive nappe-thrust belts with diverse deformation styles over >1000 km, offering an ideal natural laboratory to investigate this issue. We conducted a high-resolution crustal structure study in the southern margin of the western Tianshan, revealing the deep-shallow deformation architecture of the foreland thrust-nappe system. Three distinct units are identified: (1) The northern Tarim Basin exhibits high seismic velocities with a crustal thickness of ~45-48 km, where its lower crust intrudes into the Kashi fold-and-thrust belt (FTB); (2) Kashi FTB exhibits a crustal thickness of 53-70 km and is characterized by pronounced mid-lower crustal low-velocity zones (LVZs), indicating crustal thickening. The lower crust beneath northern Kashi FTB underthrust northward beneath the Tian Shan. (3) The southern Tian Shan margin has a crustal thickness of ~60 km, with velocity and discontinuity structures distinct from those in the Kashi FTB. The LVZ beneath the Kashi FTB not only accommodates significant crustal shortening but also facilitates the development of piedmont thrust-nappe structures. By serving as a detachment zone, it drives southward propagation of the nappes toward the northern Tarim Basin, forming basement-cored anticlines. Our findings reveal the complex crustal architecture of the southwestern Tian Shan,  demonstrating that inherited structures control the formation and evolution of piedmont thrust-nappe tectonics.