Spatial and Temporal Variations in Exhumation across the NW-Himalaya

Exhumation in mountain belts is temporally and spatially variable over long time scales (> 106 yr). The topography is a result of changes in both the location of deformation (mountain growth) and erosive climate (mountain decay). However, many studies are limited in the extent to which spatial variations in exhumation can be quantified due to the limited geographic extent over which samples are collected. We investigated spatial and temporal variations in exhumation and deformation across a 150x200 km region of the NW Himalaya, India. 25 new and 168 previously published apatite and zircon fission track and muscovite 40Ar/39Ar ages are integrate with a 1D model to quantify rates and timing of deformation and exhumation/erosion along strike of several major structures in the Lesser and Higher Himalaya. The model solves the advection-diffusion equation and predicts thermochronometer ages for variations in exhumation/erosion rates, thermophysical properties, and thermal gradients. The new and previously published apatite fission track ages range from 10 to 0.6 Ma. The sample area is characterized by 0.7-5 km-high-topography, high relief (about 4 km), and large (<0.3 to 2.5 m/yr) precipitation gradients. All the thermochronometer data indicate large temporal and spatial variations in exhumation. Erosion and exhumation rates are inversely correlated with local relief and specific stream power, but not with modern precipitation gradients. Results from thermokinematic modeling of all thermochronometer systems indicate: (1) High Himalaya exhumation rates were <0.5-0.8 between about 15 and 4 Ma and increased to 1.5-3 mm/yr after 4 Ma. (2) In the Lesser Himalaya exhumation rates are 1mm/yr over the last 10 Ma. The previous temporal variations in erosion in the Greater Himalaya are spatially consistent along 200 km of strike, regardless of structural variations. This suggests that tectonics as well as erosion, has controlled the exhumational evolution of this region. The correlation between erosion/exhumation rates derived from AFT ages and present day specific stream power suggest that the location of deformation and rock uplift has been consistent for the last 4Ma. Thus the High Himalaya has been exhumed rapidly since then, however ZFT and muscovite 40Ar/39Ar do not provide additional constrains, if exhumation have been consistent for longer time.