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

Abstract

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, 40 Ar/ 39 Ar 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.