Miocene structural evolution and exhumation of the Ximeng dome in Yunnan, southeastern Tibet: Implications for intraplate deformation during extrusion of the Sundaland block

Abstract

The Ximeng complex is located in the eastern Shai Thai block, a part of the extruding Sundaland block in southeastern Tibet. The complex is mainly composed of Neoproterozoic medium-grade metamorphic rocks and Early Paleozoic granite in the core surrounded by Paleozoic low-grade metamorphic rocks as a cover. The two structural units are separated by a tectonic contact (TDC). Detailed structural analysis reveals that the structural framework of the Ximeng complex is a dome structure and the complex experienced a primary and progressive shearing during doming. The rheological stratification led to the differential behavior of rocks from different stratigraphic and crustal levels during shearing. Zircon U-Pb ages of the Laojiezi granites in the core range from 465.50±2.70 to 430.37±0.93Ma. The cooling path from 40Ar-39Ar and apatite fission track data indicates that the complex experienced two stages of Cenozoic cooling, i.e., an early rapid cooling and a subsequent slow cooling. The early rapid cooling and exhumation of the complex are attributed to doming and accompanied by middle to upper crustal flow from ca. 23 to 20Ma.Shearing, cooling and exhumation of the middle to lower crustal rocks are widely distributed along the boundary fault zones (i.e., the Ailao Shan Red River shear zone and the Sagaing shear zone) of the extruding Sundaland block and within the triangular area between them. The doming, ductile shearing and exhumation of the Ximeng complex highlight the importance of intraplate deformation coeval with Indian-Eurasian collision. Localized crustal flow within the Sundaland block is possibly an important process during the extrusion of the Sundaland block.