A SYSTEMATIC THERMOCHRONOLOGIC TRANSECT ACROSS THE HIMALAYAN RAIN SHADOW IS CONSISTENT WITH COUPLING OF PRECIPITATION PATTERNS AND PLIOCENE-RECENT EXHUMATION

Models of the exhumation histories of mountain ranges are often quantified through thermal-kinematic modeling of thermochronologic datasets. The Himalaya correspond with one of the world’s starkest rain shadows, with data from NASA’s Tropical Rainfall Measurement Mission (TRMM) suggesting that regions on the southern flank of the Himalaya may receive up to four meters of rainfall per year, while areas of southern Tibet north of the range crest obtain only about half a meter of rainfall annually. While the precise position of this transition is only generally defined by the spatial resolution of the TRMM data to within a few kilometers, the data clearly indicate that the transition occurs south of the range crest. New multichronometer datasets (⁴⁰Ar/³⁹Ar muscovite and (U-Th)/He zircon and apatite) for bedrock samples collected along a transect from north of Mount Everest, on the Tibetan Plateau, to the southern flank of the Himalaya in the Khumbu region of Nepal, show a dramatic break in cooling histories at a position well south of the range crest and within spatial uncertainty of the TRMM-derived position of the rainfall transition. 1D inverse thermal-kinematic and erosion modeling of the thermochronometer data for the range of possible transient denudation histories indicates that the thermochronometric transition marks a shift between two regions with dramatically different exhumation histories within the late Miocene-Pliocene, with high, presumably erosional exhumation of post-Miocene age constrained to the region of high modern precipitation related to the South Asian summer monsoon. These results support a model in which the current precipitation patterns across the rain shadow, which clearly correlate with exhumation rate patterns today, had developed by at least the late Miocene. Our results do not support the model of Carrapa et al. (2016, Geology), who used their thermochronologic data for samples collected north of Mount Everest, together with pre-existing datasets from farther south to argue for a continuous northward migration of the orographic precipitation front since the Miocene from far south of Everest to its modern position at the range crest.