Phanerozoic tectonics of south China block: New insights from the polyphase deformation in the Yunkai massif

The Yunkai massif is a key region to decipher the tectonic evolution of the south China block since the contained high-grade metamorphic rocks experienced a polyphase deformation. The earliest event (D₁) corresponds to a top-to-the-northwest ductile shearing, coeval with amphibolite facies metamorphism, probably developed during a postorogenic synmigmatization extensional event occurred in early Paleozoic time. The main ductile deformation (D₂) is top-to-the-northeast shearing, occurred in early Mesozoic time, coeval with amphibolite to greenschist facies metamorphism and associated with the development of northeast verging recumbent folds within the Neoproterozoic-Paleozoic sedimentary cover of the south China block. The tectonic significance of D₂ is possibly linked with the subduction of Indochina beneath south China along the Song Ma belt of North Vietnam. Another compressional event (D₃) of Late Jurassic to Cretaceous age is indicated by SE verging recumbent folds with NE-SW trending axes developed both in the sedimentary cover and gneissic basement. The northwestward subduction of the Pacific plate or the SE thrusting of the Songpan-Ganzi fold belt upon the Yangtze craton can be considered as the geodynamic causes of this late Mesozoic event. Last, NE-SW trending brittle sinistral strike-slip and normal faulting (D₄) control the massif uplifting and the subsiding of continental half grabens. This structure, which is widespread in the eastern margin of the Eurasia continent, can be related either to the westward subduction of the Paleo-Pacific plate or to lithosphere removal during the late Mesozoic.