The tectonic evolution of the North Qiangtang terrane at Mesozoic and Cenozoic determined by deformation in the Raggyorcaka area, Tibet

The Mesozoic and Cenozoic deformation of the North Qiangtang terrane reflects a portion of the tectonic evolution of the Tethys Ocean, involving Indosinian, Late Yanshanian, and Cenozoic intraplate structures of shallow-superficial tectonic levels. The structural mapping of the Permian-Triassic tectonic layer indicates that the Mesozoic tectonic style was dominated by Late Yanshanian horizontal contractional deformation characterised by NW-SE trending thrust faults inheriting and transforming early-formed folds and their accommodation faults. These folds and thrusts were superimposed on Indosinian buckle folds. Structural analysis on deformations of the Neogene tectonic layer indicates that there were two transitions in the tectonic evolution of North Qiangtang terrane dividing the Cenozoic deformation into three stages. In episode I (50-40 Ma to 8-4 Ma), the crust experienced large-scale N-S horizontal shortening, and buckle folds and thrust faults increased the vertical crustal thickness. In episode II (8-4 Ma to the Pleistocene), E-W passive stretching formed conjugate strike-slip fault systems and pull-apart basins. During this episode, the Qiangtang basin was extruded eastwards, and a number of superficial N-S trending buckle folds were formed. In episode III (the Pleistocene to the present), the stress field was reversed, and the Qiangtang basin experienced E-W active stretching, forming conjugate normal faults and graben structure.