Miocene North‐Directed Thrusting Along the Great Counter Thrust in Southeastern Tibet: Implications for Indian Lithosphere Underthrusting

Abstract The Great Counter Thrust (GCT), a prominent backthrust system in the Himalayan orogen, plays a critical role in testing first‐order tectonics models of Himalayan evolution. It juxtaposed Indian plate materials from the south directly against Eurasian rocks to the north. This study integrates structural and thermochronological analyses to investigate GCT activity in eastern Tibet, near the Eastern Himalayan Syntaxis. Structural analysis identifies three distinct deformation units in the hanging wall rocks from north to south: Unit 1, characterized by north‐directed thrusting; Unit 2, where early deformation fabrics were overprinted by late north‐directed thrusting; and Unit 3, controlled by south‐directed thrusting. New 40 Ar/ 39 Ar and zircon fission‐track data indicate that the GCT activity persisted until ∼13–12 Ma in eastern Tibet, lasting 3–4 Myr longer than in western Tibet. GCT activity coincided with the doming of the North Himalayan Gneiss Domes, driven by flat underthrusting of the Indian lithosphere beneath the Tibetan lithosphere without an intervening asthenospheric window. Subsequent slab tearing triggered mantle upwelling and horizontal flow between the underthrusted Indian lithosphere and overlying Tibetan lithosphere. The onset of horizontal mantle flow marked a tectonic shift from north–south contraction to east–west extension, ceasing GCT activity and initiating north–south rifting. This transition first occurred west of the Yadong‐Gulu Rift before 15 Ma. East of the rift, similar processes began around ∼12 Ma, as the Indian lithosphere underthrust farther northward in the eastern region.