The formation of E-W extension in southern Tibet: from an integrated geodynamic model

Under continuous continental collision of the Indian and Eurasia plates, several N-S rifts are widely distributed in southern Tibet, indicating the predominant E-W extension. The E-W extension is one of the most remarkable tectonic features in the Tibetan Plateau. Hypotheses including gravitational collapse, India-Eurasia continental collision, underthrusting of Indian lower crust, and crustal flow are proposed. However, the relative roles of each driving force remain controversial due to lack of a unified model. Here, we have developed a high-resolution 3D numerical model, integrating multiple tectonic factors including India-Eurasia continental collision, underthrusting Indian lower crust, gravitational collapse as well as discontinuously distributed weak mid-crustal zones, to simulate the crustal deformation in southern Tibet. The integrated muti-force model produces crustal deformation consistent with GNSS horizontal velocities and strain rates in southern Tibet. In details, India-Eurasia continental collision can cause not only N-S compression but also smaller E-W extension in southern Tibet. The underthrusting process of rigid Indian lower crust could reduce the N-S compression in its overlying upper crust caused by continental collision. Gravitational collapse leads to pronounced extension in southern Tibet, which can not only generate E-W extension, but also resist N-S compression from continental collision along with Indian lower crust underthrusting. Notably, the local weak mid-crustal zones in the east part of southern Tibet flows faster driven by gravitational collapse, which enhances E-W extension in the upper crust and locally decouples the underthrusting Indian lower crust and Tibetan upper crust. Overall, the predominant E-W extension in southern Tibet is jointly controlled by gravitational collapse, India-Eurasia continental collision along with Indian lower crust underthrusting, and local weak mid-crustal zones. India-Eurasia continental collision has contributed to the regional E-W extension throughout the southern Tibet, while gravitational collapse has played a significant role in enhancing E-W extension rates in the east part.