Paleogene crustal extension in the eastern segment of the NE Tibetan plateau

Abstract The Tibetan plateau experienced lateral growth in response to the India–Eurasia collision, but uncertainties remain about when and how its present-day margins were formed. It was commonly viewed that crustal shortening in the NE Tibetan plateau had commenced since the Eocene and led to reverse faulting and compressional basins. Our recent investigations of some Cenozoic intermontane basins reveal that synsedimentary normal faults occur widely in the eastern segment of the NE Tibetan plateau. Normal faulting at basin bases obviously controlled basin sedimentation and presumably resulted in footwall uplift and basement exhumation. Small-scale synsedimentary normal faults are very common in basin fill sequences although they decrease in intensity upwards and eventually vanish in the uppermost Paleogene sequences. Measurement of the attitudes of synsedimentary normal faults indicates that crustal stretching was in NW–SE direction. The NW–SE extension was consistent with Paleogene stress field of the whole North China craton, which were thought responsible for the NE–SW-trending rift basins in the interior of the North China craton, such as the Yinchuan, Weihe, and Bohai Bay rift basin systems. A broad unconformity separates Paleogene and Neogene sequences in the eastern segment of the NE Tibetan plateau, contrasting strikingly with conformable Oligocene to Middle Miocene sequences in the neighboring Ningnan and Yinchuan rift basins. It thus follows that the eastern segment of the NE Tibetan plateau had not been formed in Paleogene when it was basically dictated by crustal extension, possibly as a result of far-field effect of subduction of the western Pacific plate. End-Oligocene termination of normal faulting and the presence of a regional unconformity indicate that the eastern segment of the NE Tibetan plateau began rising at the beginning of the Miocene.