Variations in the orientation of Cenozoic contractional strain in the northeastern Tibetan Plateau: Evidence from magnetic fabrics (western Qaidam Basin)

Recognizing variations in the orientation of contractional strain is essential for understanding the tectonic evolution of an orogenic system. Many studies on the northeastern (NE) Tibetan Plateau have analyzed the orientation of crustal shortening, but the timing and spatial distribution of this deformation are still debated. Here, we conduct detailed magnetic fabric analyses of post-Oligocene sedimentary rocks along the Xichagou profile in the western Qaidam Basin to determine the orientation of contractional strain in this key region. The results reveal two transitions in the orientation of contractional strain at Xichagou from NE-SW to NNE-SSW at -20 Ma and from NNE-SSW back to NE-SW after -7 Ma. The prevailing NE-SW-oriented contractional strain was most likely due to sinistral shear penetration of the Altyn Tagh Fault (ATF) since the early Oligocene. The first transition could be linked to the NNE-directed propagation of the Qimen Tagh Fault from the early Miocene to at least -7 Ma, and the second transition may reflect the dominantly NE-SW-oriented contractional strain associated with widespread sinistral strike-slip faulting and thrusting in the NE Tibetan Plateau. Combining these data with other geological evidence, we conclude that the sinistral shear along the ATF was transferred to major thrust belts in the NE Tibetan Plateau during the early Oligocene and was broadly distributed throughout the NE Tibetan Plateau after the middle Miocene. Both the directional variations in the contractional strain and tectonic deformation propagated southeastward and inward of the Qaidam Basin. This observation explains the southeastward migration of the depocenter of the Qaidam Basin since the middle Miocene.