RAPID MIOCENE BURIAL, EXHUMATION AND UPLIFT OF THE SUTURE ZONE IN TIBET: EVIDENCE FROM MULTI-PROXY ISOTOPIC ANALYSES OF THE OLIGO-MIOCENE KAILAS FORMATION

The Oligocene- early Miocene Kailas Formation along the Indo-Asia suture zone in Tibet represents the record of geodynamic processes related to underthrusting of India beneath Asia following collision. Deposition of the Kailas Formation during the late Oligocene-early Miocene has been interpreted to have occurred at low elevation in an extensional setting related to southward rollback of India under Asia. Apatite fission track and zircon (U-Th)/He data from twelve samples of Kailas Formation and two samples of the underlying Gangdese batholith distributed along a lateral distance of more than 800 km between Mt. Kailas in the west and the Xigatze area to the east show early Miocene (17 ± 3 Ma) cooling, younger than the depositional ages of the Kailas Formation. Zircon (U-Th)/He data show partial resetting with the young age components being consistent with AFT ages, supporting rapid Miocene cooling. Apatite (U-Th)/He ages are Pliocene, indicating a second pulse of rapid cooling. Seven samples from the Rongbuk granite north of Mt. Everest, in the footwall of the South Tibetan Detachment (STD), show AFT ages of 18.5 ± 3.2 Ma. Together these data require heating of the Kailas Formation to temperatures sufficient to completely reset the AFT system and partially reset the ZHe system by burial (either by sediment accmulation or tectonic burial beneath the Great Counter thrust) between ~23 and 18 Ma, followed by regional rapid uplift and exhumation of 6-8 km of overburden by ~18-17 Ma. The Mt. Everest results suggest a connection between tectonic exhumation along the STD and regional exhumation along the suture zone. The large aerial and vertical scale of the early Miocene exhumation event suggests a plate-scale process is responsible. Candidate mechanisms include a return to rapid Indian plate underthrusting after Oligocene-early Miocene rollback, or mid-crustal flow. In any case the rapid regional erosional event (6-8 km of overburden removal) suggests that the paleo Tsangpo River must have been regionally integrated by the early Miocene in order to export the resulting detritus.