Two Phases of Crustal Shortening in Northeastern Tibet as a Result of a Stronger Qaidam Lithosphere During the Cenozoic India-Asia Collision

Although two phases of Cenozoic crustal deformation in the Qilian Shan thrust belt of northeastern Tibet has been documented, their dynamic causes remain unclear. To address this issue, we investigate whether the mechanical strength of the Qaidam Basin, which is located between the Eastern Kunlun Range and the Qilian Shan, was a controlling factor for the observed deformation history by performing 2-D thermal-mechanical simulations. Our models consider the division of the Tibetan terranes (i.e., Qilian arc, Kunlun-Qaidam, Songpan-Ganzi, Qiangtang, and Lhasa, from north to south) bounded by suture zones. Simulation results show that weak suture zones in central Tibet can lead to peeling off and sinking of the mantle lithosphere beneath the Qiangtang and Songpan-Ganzi terranes. After lithospheric delamination, varying north-south width, thickness and strength of the Qaidam crust creates three end-member model results: (a) the mantle lithosphere of the Lhasa terrane delaminates while the mantle lithosphere beneath Eastern Kunlun-Qaidam terrane and the Qilian Shan thrust belt (KQQ) remains undeformed, (b) the Lhasa mantle lithosphere moves northward while the KQQ mantle lithosphere subducts southward, and (c) the Lhasa mantle lithosphere moves northward while lithospheric thickening occurs north of the Qaidam Basin. Our simulations show that pre-existing weaknesses in northeastern Tibet can be activated and deformed shortly after the onset of collision, and a second wave of deformation sweeps across northeastern Tibet after lithospheric delamination in northern Tibet.