Thermochronometric analysis of the North Lunggar Rift: Implications for the timing of extension initiation and structural style of deformation in southern Tibet

Abstract

Active north-trending rifts in southern Tibet are kinematically linked to conjugate sets of strike-slip faults along the Bangong-Nujiang Suture zone (BNS), but the initiation age of the strike-slip faults is currently unknown. Apatite and zircon (U-Th)/He thermochronometric analysis of the North Lunggar Rift shows the timing of onset of extension is 14 - 7 Ma, with a later episode of rapid extension and topography building exhumation at 5 - 3 Ma that continues into the present-day. Deformation of the North Lunggar Rift is driven primarily by isostatic rebound of the footwall at depth, as tectonic unloading and denudation exert a positive buoyancy force. This process causes the central portion of the North Lunggar Range to have the highest geothermal gradient and the greatest relief due to doming and increased uplift. Because the geothermal gradient is highest in the center of the footwall the amount of cumulative exhumation is lowest, but still exhibits the highest relief, as doming can cause uplift without denudation. Comparison between active local and regional structures and accumulated strain indicates that there was a regional change in lithospheric boundary conditions during the late Miocene - Pliocene that may have resulted in a switch from pure extension to constriction. The Tibetan Plateau may be a modern analogue to Basin and Range style extension demonstrating a causal linkage between thickened crust and the development of large magnitude extensional systems and metamorphic core complexes. Deformation accommodation in the Himalayan and Tibetan Plateau is best characterized by a combination of previously proposed models that rely on regional boundary conditions. Crustal thickening and orogenic collapse drove the initial onset of extension in the Himalaya and Tibet, and constriction in the upper crust and east directed flow of the lower crust resulted from the continued northward under-thrusting of the Indian slab that confined and forced ductile material eastward toward the free boundary, which once a threshold was reached caused a change in lithospheric boundary conditions driving an increase in rift activity, the initiation of strike-slip faulting along the BNS, and accelerated eastward extrusion of central Tibetan lithosphere.