THERMAL EVOLUTION OF THE YARLUNG SUTURE: LINKAGES TO TECTONICS, CLIMATE, AND RIVER INCISION

Thermochronologic data from Indian and Asian rocks juxtaposed along the Yarlung suture in southern Tibet are a key record of orogenic burial and subsequent exhumation, which might have been driven by tectonics, strengthening of the South Asian Monsoon, or river incision. New (n = 23 samples) and compiled (n = 132 samples) thermochronologic ages from a >1200-km-long swath of the Yarlung suture reveal apparently synchronous cooling of Gangdese magmatic arc rocks from temperatures above biotite ⁴⁰Ar/³⁹Ar closure (~350 ºC) and Xigaze forearc basin rocks from temperatures above zircon He closure (~180 ºC) beginning by 22 Ma. Younger conglomerate units deposited along the suture ~25 My after collision initiation were also heated to temperatures above zircon He closure (~180 ºC) prior to cooling onset at ~20 Ma. Composite age-probability curves separated by tectonic unit reveal a decrease in exhumation magnitude from north (Gangdese arc) to south (Xigaze forearc, Kailas Formation, and Liuqu Formation). The Great Counter thrust (GCT) system, which cuts Yarlung suture zone assemblages along the length of the suture, was active during exhumation. However, the observation that Gangdese arc and Xigaze forearc rocks are consistently located in the footwall of GCT strands precludes a simple structural interpretation involving hanging-wall exhumation and footwall heating. We explore three end-member models to explain the onset of cooling ca. 22-20 Ma, including 1) intensification of the South Asian monsoon resulting in erosional exhumation, 2) fluvial reorganization resulting in erosional exhumation along the Yarlung river valley, and 3) a structural interpretation in which GCT backthrusting is related to a major foreland-verging thrust fault carrying Gangdese batholith rocks the hanging wall. The youngest thermochronologic ages from the suture zone are 9-4 Ma and likely reflect local exhumation related to Miocene-to-recent, orogen-parallel (E-W) extension of southern Tibet.