Onset of oblique extension in south-central Tibet by 15 Ma: implications for diachronous extension of the Tibetan Plateau

Despite active convergence of India at ~5 cm/y, the present-day style of deformation of the interior of the Tibetan Plateau is extensional. Extensional structures within the Tibetan Plateau often link to strike-slip faults and trend at high angles to the Himalayan front. When and how rifts developed remains a subject of debate. In the east, development of extension is thought to have started at ~8-4 Ma, whereas in the far west, extension was active by ~ 16-14 Ma. One difficulty in assessing extension may be that these structures are not restricted to the Plateau. Rifts are found in Qangtang (Shuang Hu) and across the Himalayas (Thakkhola). To assess the timing and style of extension in southern Tibet, we mapped areas of the Lopukangri rift at a scale of 1:100,000. The Lopukangri rift is one of 6 recognized wedge-shaped basins of a trailing extensional imbricate fan connected via the Lunggar Shan detachment to the Central Tibet Conjugate Fault Zone. The ~120 km long by 4-10-km wide rift is bounded by the Lopukangri fault system (LFS), which comprises a zone of west-dipping ~N15E-striking oblique-normal faults that offset the India-Asia suture zone ~15 km and juxtapose Tethyan Sedimentary Sequence rocks of greenschist facies in the hanging wall and amphibolite facies rocks in the footwall. We employed field mapping, 40Ar/39Ar mica thermochronology on hanging wall and footwall rocks to assess their thermal history, and U-Pb zircon geochronology on igneous intrusions. Based on our geochronology results and mapped cross-cutting relationships, we interpret footwall exhumation from a depth of 12-10 km to have occurred at ~15-14 Ma. Granitic intrusions in the footwall young northwards and are pre to synkinematic with slip along the LFS suggesting a genetic relationship between extension and magmatic emplacement throughout the early history of the fault system. The LFS has a complex geometry: 1) Metamorphic basement is exposed solely in the southern 20 km of the rift where ~10 km of slip have been calculated. 2) The active margin fault in the north is predominantly strike-slip with right-lateral offset of Quaternary terrace risers and streams of 700m -1 km. 3) Kinematics vary with slip N35W-directed south of the suture, and N30E-directed immediately north of the suture. 4) While younger faults in the north slip N10W, older faults slipped N25-60W. Our structural model describes at least 2 periods of extension. 40Ar/39Ar mica ages from the granitic intrusions reflect fast cooling rates (~ 175°C/My) from 700 to 350°C requiring the initial exhumation episode to slow down by Pliocene(?) time, leading to a second stage of extension. Our results suggest a Middle Miocene initiation of oblique extension, which challenges the notion that inception of extension is a young (≤8 Ma) phenomenon. Pure E-W extension may not fully describe the style of deformation across the Plateau.