RECONSTRUCTION OF THE TECTONIC AND EXHUMATION HISTORY OF THE NORTH LUNGGAR RIFT, SOUTHERN TIBET, THROUGH INTEGRATED FOOTWALL AND DETRITAL HANGING WALL THERMOCHRONOMETRY

Despite major shortening during India-Asia convergence and uplift of the Tibetan plateau, the dominant middle Miocene to recent geological features across the southern and central plateau are north-trending rifts. While most Tibetan rift basins are characterized by high-angle faults, the Lunggar rift is characterized by a low-angle normal fault, forming an incipient metamorphic core complex. This study aims to constrain the deposition and exhumation history of the Lunggar rift basin and the bounding low-angle fault by combining the footwall exhumation history with the sedimentologic and detrital thermochronometric record preserved in coarse-clastic basin fill of the hanging wall. Upper Cenozoic supra-detachment basin sediments consist of debris-flow conglomerates, fine-grained lacustrine sandstones and siltstones grading upsection into fluvial sandstones and conglomerates. In this study, we systematically integrate detailed apatite and zircon (U-Th)/He thermochronometric results from both (1) the crystalline lower plate to elucidate the cooling and exhumation history of the Lunggar metamorphic core complex and (2) detrital samples from the exposed upper-plate stratigraphy to glean insights into the older exhumation history that has been eroded and re-deposited in the hanging wall. Detrital apatite (U-Th)/He ages also provide constraints on the maximum depositional age and confirm the synrift nature of the sediments. While bedrock (U-Th)/He ages exhibit elevation-independent Pliocene ages, the detrital signatures show progressive exhumation and erosion of a middle Miocene partial retention zone and significantly extend the thermal history back in time. This integrated approach allows for a more comprehensive reconstruction of the tectonic and thermal evolution of the Lunggar rift and supports a two-stage evolution of the rift, with earlier rapid exhumation in the middle Miocene (only preserved in the detrital record) and later rapid exhumation and topographic construction at 4-3 Ma that likely continues to present. This case study illustrates the power of integrated bedrock and detrital thermochronometry in a detailed structural and stratigraphic context in reconstructing the evolution of an extensional rift system.