Formation, reactivation and exhumation of the extruded basement wedge in the southern Longmen Shan, eastern Tibetan plateau

The Longmen Shan (LMS) fold-and-thrust belt in the eastern Tibetan plateau marks the boundary between the Songpan-Ganze terrane and the Sichuan basin. Deformation mechanisms and the mountain building process in the LMS remain unclear. Here we conducted field mapping, structural analysis, Ar-40/Ar-39 geochronology of biotite, and re-interpretation of the Wenchuan Earthquake Fault Scientific Drilling borehole data and a seismic reflection profile across the LMS. Our results indicate that a gently dipping LMS extruded wedge overlies the reactivated Yangtze craton. This wedge consists of several rigid Neoproterozoic complexes, bounded by the NW-dipping LMS detachment above and the Yingxiu-Beichuan thrust below. The Yingxiu-Beichuan thrust extends northwestwards to a depth of c. 8 km beneath the Songpan-Ganze terrane. Coeval activation of the top-to-the-NW LMS detachment and the top-to-the-SE Yingxiu-Beichuan thrust in the Early Cretaceous resulted in the extrusion of the LMS wedge from a depth of 10-15 km. The wedge rapidly exhumed through imbricated thrusting along low-strength series since 40-30 Ma. This Cretaceous basement wedge extrusion bridges the time gap between the Late Triassic-Early Jurassic crustal shortening and the Cenozoic rapid exhumation in the LMS, representing a transformation from north-south compression in the Late Triassic-Early Jurassic to east-west compression in the Cenozoic.