KINEMATIC EVOLUTION OF THE TANGRA YUMCO RIFT, SOUTH-CENTRAL TIBET

We evaluate spatiotemporal trends in fault displacement, extension onset, and exhumation rates to interpret the kinematic evolution of the Tangra Yumco (TYC) rift. We present new geologic mapping, zircon (U-Th)/He (ZHe) thermochronology, U-Pb geochronology, and HeFTy thermal modeling results. The TYC rift is bounded by two NNE striking (~N10°E-N35°E) high angle (~45-70°) active normal faults that alternate dominance along strike. Footwall granodiorites show pervasive northeast dipping foliation, with slip lineations and fault plane striations indicating northeast directed oblique sinistral-normal slip. Hanging wall deposits display synthetic graben structures potentially indicative of low-angle faulting in the central TYC rift. In North and South TYC, hanging wall deposits are cut by a series of active high-angle normal faults which likely sole into a master fault at depth. Analysis of ~50 samples collected across key structural relationships in and around TYC yield 14 mean U-Pb dates between ~59-49 Ma and ~190 single-grain ZHe dates between ~60-4 Ma with spatial trends in ZHe data correlating strongly with latitude. Samples from Gangdese latitudes show a concentration of ~28-16 Ma ages, while those north of ~29.8° latitude yield both younger (~8-4 Ma) and older (~60-48 Ma) ages. We interpret (1) Gangdese Range samples reflect exhumation during contraction and uplift along the GCT peaking at ~21-20 Ma, (2) ~8-4 Ma ages reveal extension timing along fault segments experiencing significant rift-related exhumation, and (3) ~60-48 Ma ages represent un-reset or partially-reset samples from segments of the TYC rift that have experienced lesser magnitudes of rift exhumation. HeFTy thermal models indicate a two-stage cooling history with initial slow cooling followed by accelerated cooling rates in Late Miocene-Pliocene time (~13-4 Ma) consistent with prior results from TYC and other Tibetan rifts. Our data are consistent with a segment linkage fault evolution model for the TYC rift, with underthrusting of Indian lithosphere likely related to the northward acceleration of rifting. Future work will utilize detailed HeFTy modeling including new U-Pb and apatite fission track (AFT) results to better constrain the exhumation history of TYC and discriminate between dynamic models of extension for southern Tibet.