Giridas maiti, Arnab Roy , Nibir Mandal

The tectonic history of the Himalaya-Tibet Mountain Range records two important extensional tectonic events: 1) N-S extension in the Himalaya-Tibet transition zone, and 2) E-W extension in the southern and central Tibet, manifested in the form of east-west and north-south striking normal faults, respectively. The N-S extensional event (~22 Ma) started to commence earlier than the E-W extension (~18 Ma) and phased out by ~11 Ma, whereas the E-W extension continued to recent time (~4 Ma), suggesting a temporally overlapping period of ~7 Myr. This article addresses the question- did they originate from the same dynamical process? Using numerical and laboratory experiments, we show that a decreasing India-Asia convergence velocity induced gravitational collapse of the Tibetan plateau is the main driving force for both the extensional tectonic events, but they were controlled by two different mechanisms. Our results show that with a drop in the convergence velocity southern Tibet underwent gravitational collapse due to pressure relaxation in the underlying Himalayan wedge, and the collapse forced the deep crustal materials to extrude up, creating N-S extension (22 – 11 Ma) along the Himalaya-Tibet transition zone. On the other hand, presence of rigid Tarim block in north-western Tibet caused differential topographic uplifts from west to east, resulting in a first-order eastward topographic gradient of 0.1º during the initial fast-stage of India-Asia collision (> 22 Ma). Later on, this topographic slope prompted eastward crustal flows in the course of gravitational collapse, leading to E-W extensional tectonics (~18 - 4 Ma) in Tibet.