Late Cenozoic rock exhumation of the Longmen Shan structural belt and their tectonic implications

The Cenozoic structural architecture and geodynamic mechanism of the Longmen Shan structural belt are essential for understanding the tectonic evolution of the eastern Tibetan Plateau, but remain controversial. Models of different manners of deformation in different lithospheric levels have been proposed. For example, the crustal extrusion model suggests that Late Cenozoic deformation of the Longmen Shan was dominated by crustal shortening, whereas the channel flow model proposes lower crustal thickening and flow, accommodated by frontal shortening and hinterland extension. This study compiles previously published low-temperature thermochronology data from the Longmen Shan to constrain spatial changes in rock exhumation and test previous geodynamic models. The results show that exhumation increases abruptly across the Yingxiu-Beichuan fault, suggesting the fault is the main thrust boundary between the Longmen Shan and the Sichuan Basin to the east. Further, younger Late Cenozoic cooling ages and faster exhumation are observed on the hinterland Wenchuan-Maowen fault, where a dichotomy of ages on the hanging wall versus footwall suggests Late Cenozoic thrust activity. In addition, towards the hinterland to the west, exhumation rates decrease threefold over a distance of about 80 km. Such an exhumation pattern indicates a westward decrease of tectonic uplift, providing the regional topography reached a steady-state, whereby exhumation is in balance with tectonic uplift. The observed spatial changes in exhumation support a listric crustal configuration where reverse faults in the Longmen Shan merge gradually into a gentle detachment seated at depth. Results of this study support a revised upper crustal shortening model, where the triangular Songpan-Ganzi terrane, bounded by two left-lateral strike-slip faults (Ganzi-Xianshuihe fault to the south and eastern Kunlun fault to the north), might have extruded eastward towards the western margin of the Sichuan Basin along the listric Longmen Shan faults.