Exploring the Variability of Late Cenozoic Exhumation Rates across the Himalayan Rain Shadow

Abstract:

The Himalayan ranges of South Asia form one of the world’s most impressive rain shadows. Data from the NASA – Japan Aerospace Exploration Agency Tropical Rainfall Measurement Mission (TRMM) suggest that precipitation in the central Himalaya drops from more than four meters per year along the southern flanks of the Higher Himalaya (falling mostly as rain during the summer monsoon) to about half a meter per year north of the range crest on the southern Tibetan Plateau. While a correlation between modern precipitation and erosion seems intuitive, important questions remain regarding how far backward in time the correlation might extend. Previous investigations of the relationships between precipitation patterns and thermochronologic cooling dates south of the Himalayan range crest have yielded discrepant results, partly due to the fact that many were conducted along deep trans-Himalayan gorges that serve to channel monsoon storms locally northward, sometimes obscuring broader trends. We are addressing this problem through the comparative studies of bedrock exhumation on million-year timescales north and south of the range crest in transects that are not along major trans-Himalayan gorges. Our low-temperature thermochronology dataset from Greater Himalayan Sequence leucogranites and sillimanite gneisses north of the range crest indicate cooling through the white mica ⁴⁰Ar/³⁹Ar closure isotherm between 15.5 – 14.5 Ma with zircon (U-Th)/He ages ranging from 14.5 – 11 Ma. Though (U-Th)/He systematics of apatites from these samples is complex, our data point to 9 – 8 Ma cooling through the ~70°C isotherm. Our developing dataset from the Khumbu Himal region south of the range crest thus far indicates cooling through ApHe closure occurred more recently at 3 – 2 Ma, about 5 – 6 million years later those from southern Tibet. We will be investigating this dataset further using the 1-D thermal model QTQt as well as the 1-D thermal-kinematic model Pecube.