Metamorphic Study along the Wenchuan Thrust (Longmen Shan, Sichuan, China); a Key to Understand the Two Phases of Thickening of the Eastern Tibetan Plateau

Abstract:

The Longmen Shan mountain belt, represents the eastern margin of Tibetan plateau (Sichuan, China) and culminated over 7000m. Despite very low convergent rate observed across his belt, it is active as attested by the Wenchuan earthquake Mw 7.9 (2008) that rupture and out of sequence thrust in this belt. The formation of this mountain bet results from two orogenic phase, one at the end of Trias (due to the closure of the Paleotethys), and one during Cenozoic time following the Indian Asia collision. This study aims to identify the deformation and metamorphism related to Mesozoic event and those relate to Cenozoic evolution. Structural, microstructural, metamorphic observations, PT estimates (graphitization of carbonaceous material, quantified X-ray images, chlorite-phengite-quartz-water multi-equilibrium and thermodynamic modelling) and U-Pb geochronology are used to describe the tectono-metamorphic evolution of the Xuelongbao area in the internal part of the Longmen Shan belt (eastern border of the Tibetan plateau, Sichuan, China). The Xuelongbao granite is dated at 765±7 Ma (in situ U/Pb dating on zircon), indicating that it forms part of the western Neoproterozoic South China block basement. The deformation in the western sedimentary cover above the Xuelongbao massif is intense, with step cleavage, twisted fold axes and CS structures with top to the SE thrusting vergence. Four stages of deformation are described, three of them are related to the Mesozoic wedge thickening, and the last one is due to the Cenozoic deformation. An inverted metamorphic gradient from 470°C, 8 kbar to 620°C, 13 kbar is identified in the cover above the Xuelongbao basement, suggesting a stack of sedimentary slices during the propagation of the Mesozoic accretionary wedge on the South China block margin. This decollement zone has been exhumed during the Cenozoic D4 exhumation of the Xuelongbao basement along the Wenchuan thrust, where greenschist overprint is observed.