Xue, Zhenhua et al---GEOLOGICAL SOCIETY OF AMERICA BULLETIN，2022

Abstrct: The Longmenshan Thrust Belt in Eastern Tibet resulted from a Mesozoic orogeny and Cenozoic reworking. It is generally believed that the Cenozoic tectonics along the Longmenshan Thrust Belt are mostly inherited from the Mesozoic. Reconstructing the Mesozoic tectonics of the Longmenshan Thrust Belt is therefore important for understanding its evolutionary history. On the basis of detailed structural analysis, we recognized a Main Central Boundary that divides the Longmenshan Thrust Belt into a Southeastern Zone and a Northwestern Zone. Both zones underwent a main D-1 event characterized by D-1E top-to-the-SE thrusting in the Southeastern Zone and D-1w top-to-the-NW/N thrusting in the Northwestern Zone. In the Southeastern Zone, a D-2 top-to-the-NW/N normal faulting that cuts the D-1E structures is developed along the NW boundary of the basement complexes. Newly obtained and previous geochronological data indicate that the D-1E and D-1w events occurred synchronously at ca. 224-219 Ma, and the D2 top-to-the-NW/N normal faulting was episodically activated at ca. 166-160 Ma, 141-120 Ma, 81-47 Ma, and 27-25 Ma. Episodic and synchronously activated top-to-the-NW normal faulting and top-to-the-SE thrusting along the northwestern and southeastern boundaries of the basement complexes, respectively, leads us to propose that the basement slices were episodically imbricated to the SE during the Late Jurassic-Early Cretaceous and Late Cretaceous-earliest Paleocene. The D-1w amphibolite facies metamorphic rocks above the basement complexes recorded fast exhumation during the Late Jurassic-Early Cretaceous. We propose that the early Mesozoic northwestward basement underthrusting along a crustal "weak zone" was responsible for the D-1w double-vergent thrusting and amphibolite facies metamorphism. Subsequent basement-slice imbrications reworked the Longmenshan Thrust Belt and exhumed the amphibolite facies rocks. Our results highlight the importance of basement underthrusting and imbrication in the formation and reworking of the intracontinental Longmenshan Thrust Belt in Eastern Tibet.