Late Cretaceous reactivation of a Paleo-Tethyan suture zone in Xizang of western China: extensional collapse of the proto-Tibetan plateau?

The deformation of the Himalaya-Tibetan plateau remains one of the hottest examples of Earth's tectonics and dynamic evolution. What drives deformation and growth of the plateau, and how this is partitioned into diverse structural styles and mechanisms remain heated discussions. Mesozoic orogens also contributed to crustal thickening of the plateau prior to the Cenozoic India-Asia collision and notably, they were reactivated due to its structural inheritance and low viscosity since the collision, for example, the first uplift in Qiangtang and the Paleo-Tethyan suture zone in its interior. However, few attention was paid to pre-collision reactivation and structural superimposition of Mesozoic orogens. The newly discovered NWW-trending Ejiumai shear zone with biotite ⁴⁰Ar/³⁹Ar and zircon and monazite U-Pb ages of ca. 80-70 Ma flanks the northern border of the Paleo-Tethyan suture zone and mainly includes reactivated Triassic basement gneisses and syntectonic pegmatite. Combined with the oblique moving kinematics including both sinistral and normal-sense shear, a transtension regime with lower crustal anatexis can be concluded for the genesis of Ejiumai shear zone. Concurrent granitic plutons were also found in the suture hundreds of kilometers to the east. Based on these observations, we present a schematic model of extensional collapse of the proto-Tibetan plateau induced by far-field northward indentation of Neo-Tethyan suture zone to the south in Late Cretaceous.