BOUNDARY CONDITIONS FOR THE INDIA-ASIA COLLISION IN WESTERN TIBET: COLLISION OBLIQUITY VS. UNDERTHRUSTING INDIA REVEALED (Invited Presentation)

A N-S transect along the Gurla Mandhata massif (GMM) in southwest Tibet and the Lunggar rift system to the north spans the western Lhasa block of Tibet. Rocks exposed in the footwalls of these extensional structures were analyzed for their low-temperature thermal history. The kinematic boundary conditions between the GMM and the Lunggar rift system are distinct. In the south, the GMM receives slip from the dextral Karakoram fault to the NW, acting as an extensional step-over along a west-dipping detachment, deflecting dextral slip into the High Himalaya. This kinematic relationship is consistent with the GMM acting to accommodate the collision obliquity between India and Asia at this western longitude.

To the north, the Lunggar rift has southern and northern segments separated in between by an E-striking dextral accommodation zone. Collectively, the entire rift system is ~200 km long N-S, with rift valleys up to ~20 km in width. Locally, where the active low-angle detachment shows maximum fault slip, the range-bounding detachment is overlain by undeformed moraines. However, several km into the rift basin, fault scarps with >70 m of vertical separation parallel the range front. This geometry is consistent with basin-ward migration of the low-angle structure as the footwall rebounds from tectonic unloading, back-rotates, and abandons the detachment. Consistent with active footwall rebound, active depocenters have migrated from the central rift segment toward the rift tips to the north and south. Zircon U-Th(He) thermochronometric data from footwall granitoids combined with 3D thermo-kinematic modeling are consistent with an anti-listric geometry for normal faults exhuming the Lunggar range. Finally, rift initiation began throughout the transect during the Miocene, followed by a distinct acceleration in rifting in the late Miocene-Pliocene beginning in the south, followed by a northward “sweeping” kinematic wave of accelerating E-W extension. The velocity of the northward “sweep” in accelerating extension is identical to the present day rate of northward underthrusting India. The spatial and temporal pattern of rapid E-W directed extension is well explained by lower crustal thickening that youngs to the north in response to the northward underthrusting of India, balanced by coeval upper crustal extension.