LOW-TEMPERATURE THERMOCHRONOLOGY OF THE NORTH LUNGGAR RIFT, WEST-CENTRAL TIBET

The tectonic regime in southern Tibet is dominantly east-west extension, as most of the significant deformation observed is related to kinematically linked north-trending rifts and v-shaped conjugate strike-slip fault systems accommodating coeval north-south contraction and eastward stretching. Estimates for the timing of onset for east-west extension vary from the middle to late Miocene, but the precise timing is debated, and spatial coverage of timing constraints across Tibet remains sparse. The North Lunggar Rift is located in west-central Tibet within the Lhasa terrane, and is an example of an active north-trending rift accommodating east-west extension. The North Lunggar Range is bounded on its east side by an east dipping low-angle (< 40° dipping) normal fault (detachment). Previous work suggests the rift system is an active metamorphic core-complex at an early stage in its evolution, and that a significant portion of uplift and deformation may result from active isostatic rebound of the detachment footwall at depth based on the following: A drainage divide in the hanging wall basin is undergoing active uplift and incision as a result of basinward migration of active faulting in the region of inferred maximum displacement. Here we present apatite and zircon (U-Th)/He thermochronological data from detachment parallel and perpendicular transects, in the context of fault geometry, fault kinematics, and petrographic data. Results show an along-strike variation in cooling age with the oldest ages in the north of the rift and the youngest ages to the south. The youngest cooling ages translate to the highest exhumation and detachment slip-rates, which are proximal to the greatest relief of the range and the drainage divide in the hanging wall basin. Zircon grains from the youngest samples in the central and southern sections of the range yield anomalously high U concentrations (up to 9000 ppm), which may have a significant effect on the thermal history of the North Lunggar Rift. Age-elevation plots from detachment perpendicular transects show rapid exhumation between 5 - 3 Ma. Additional analyses will determine if there are spatial and temporal differences for the onset of extension in southern Tibet.