Intracontinental underthrusting and lower crustal duplexing drive the uplift of the Qilian Shan

The mechanism driving crustal shortening and thickening under the background of intracontinental orogeny has long been a focal point in plate tectonic theory. In particular, the uplift of the Qilian Shan, an intraplate orogenic system that experienced uplift due to the far-field effects of the India–Asia plate collision during the Cenozoic, remains a subject of ongoing debate. In this study, we first report two NE-trending deep seismic reflection profiles of large dynamite shots, totaling ~400 km across the entire Qilian Shan. These two profiles provide high-resolution imaging of the lithospheric architecture beneath the South Qilian Shan and North Qilian Shan. Our results reveal two sets of prominent south-dipping reflections within the middle-lower crust of the Qilian Shan, which are accommodated by two large northward thrust faults. These south-dipping reflections are interpreted as a thrust fault system accompanied by middle-crustal duplexes beneath the South Qilian Shan, while beneath the North Qilian Shan, they represent the middle-lower crustal duplex structures. Additionally, these reflections offset the Moho and extend from the lower crust into the upper mantle. By integrating our seismic findings with available chronological data and prior geological and geophysical research, we propose that the far-field effects of the India–Asia collision induced two distinct phases of passive southward underthrusting of the North China Craton beneath the Qilian Shan. These two phases, along with the formation of multiple duplex structures in the middle-lower crust, played a pivotal role in the Cenozoic crustal shortening and thickening of the Qilian Shan. The Qilian Shan experienced significant uplift as a whole during the mid-Miocene and subsequently expanded towards the Hexi Corridor between 1 and 4 Ma.