Detrital apatite (U-Th)/He constraints on the exhumational histories of the Arunachal Pradesh Himalaya and the Shillong Plateau

Erosion in the Himalaya is driven largely by a strongly coupled system of extreme climatic conditions and active tectonic processes. Spatial and temporal variations in erosion rates along strike are presumably controlled by differences in local climate, seismicity, deformation rates, and lithology. Quantifying the contribution of each of these parameters to the erosional budget of the Himalaya, however, is a nontrivial problem. The easternmost portion of the Himalayan arc offers a natural laboratory to explore the role of climatic influence on erosion rates. Deformation and uplift of the Shillong Plateau since ~8 Ma has created an orographic barrier ~400 km long that shields the eastern Himalaya, in Arunachal Pradesh, India, from a significant proportion of the precipitation carried by the South Asian Monsoon. Long-term exhumation rates derived from the Himalaya west and east of this orographic barrier have been shown to differ by a factor of ~2, a difference ascribed to reduced climatic forcing of erosion in the lee of the Shillong Plateau. Here we present apatite (U-Th)/He thermochronology data from modern detrital samples collected from northeast India. Between 18-20 single grain ages from each catchment were analyzed in order to calculate erosion rates on a 106 yr timescale. Recently developed Bayesian techniques for the inverse modeling of detrital data were used to derive time-temperature histories for each sample. Recent erosion rates modeled for a single south-facing catchment on the Shillong Plateau are modest, ~0.25 km Myr-1, and show a clear increase in exhumation rates at ~8 Ma from rates of <0.01 km Myr-1. These results are consistent with published estimates based on bedrock thermochronology sampling. Detrital data from the Arunachal Himalaya also reveal an increase in exhumation rate, from ~0.01 to ~0.55 km Myr-1, at ~7 Ma. While these rates are consistent with erosion rates derived from bedrock samples in the lee of the Shillong Plateau in the Bhutanese Himalaya, the increase in Himalayan exhumation rate coeval with the uplift of the Shillong Plateau is inconsistent with the hypothesis that a reduction in climatic forcing of erosion in the Himalaya accompanies uplift of the Shillong Plateau. Rather, taken together, these data indicate that the easternmost Himalaya, as a whole, experienced a significant increase in exhumation rate in the late Miocene, although the absolute rates are lower than observed throughout the Bhutanese and Nepalese Himalaya. The temporal correlation of this increase suggests a regional cause, possibly reflecting changes in the stress field across the India-Eurasia-Burma plate boundaries, or intensification of the South Asian monsoon.