Tectonics of the Middle Triassic intracontinental Xuefengshan Belt, South China: new insights from structural and chronological constraints on the basal décollement zone

In orogenic belts, a basal décollement zone often develops at depth to accommodate the shortening due to folding and thrusting of the sedimentary cover. In the Early Mesozoic intracontinental Xuefengshan Belt of South China, such a décollement zone is exposed in the core of anticlines formed by the emplacement of the late-orogenic granitic plutons. The detailed, multi-scale structural analysis documents a synmetamorphic ductile deformation. In the basal décollement, the Neoproterozoic pelite and sandstone, and the intruding Early Paleozoic granites are changed into mylonites and orthogneiss, respectively. The metamorphic foliation contains a NW-SE stretching lineation associated with top-to-the-NW kinematic indicators. The ductile shearing of these high-strained rocks can be correlated with the NW-verging folds and thrusts recognized in the Neoproterozoic to Early Triassic sedimentary cover. Monazite U-Th-Pbtot chemical dating, and zircon SIMS U-Pb dating provide age constraints of the ductile shearing between 242 and 226 Ma, and late orogenic granite emplacement around 235-215 Ma. In agreement with recent geochronological data, these new results show that the Xuefengshan Belt is an Early Mesozoic orogen dominated by the NW-directed shearing and thrusting. At the scale of the South China block, as the southeastern boundary of the Xuefengshan Belt, the Chenzhou-Linwu fault separates the Early Mesozoic and the Early Paleozoic domains, to the NW and the SE, respectively. The architecture of this belt was possibly originated from the continental underthrusting to the SE of the South China block in response to the northwest-directed subduction of the Paleo-Pacific plate.