Erosional variability along the northwest Himalaya

Abstract

Erosional exhumation and topography in mountain belts are temporally and spatially variable over million year timescales because of changes in both the location of deformation and climate. We investigate spatiotemporal variations in exhumation across a 150 × 250 km compartment of the NW Himalaya, India. Twenty‐four new and 241 previously published apatite and zircon fission track and white mica 40Ar/39Ar ages are integrated with a 1‐D numerical model to quantify rates and timing of exhumation alongstrike of several major structures in the Lesser, High, and Tethyan Himalaya. Analysis of thermochronometer data suggests major temporal variations in exhumation occurred in the early middle Miocene and at the Plio‐Pleistocene transition. (1) Most notably, exhumation rates for the northern High Himalayan compartments were high (2–3 mm a−1) between ∼23–19 and ∼3–0 Ma and low (0.5–0.7 mm a−1) in between ∼19–3 Ma. (2) Along the southern High Himalayan slopes, however, high exhumation rates of 1–2 mm a−1 existed since 11 Ma. (3) Our thermochronology data sets are poorly correlated with present‐day rainfall, local relief, and specific stream power which may likely result from (1) a lack of sensitivity of changes in crustal cooling to spatial variations in erosion at high exhumation rates (>∼1 mm a−1), (2) spatiotemporal variation in erosion not mimicking the present‐day topographic or climatic conditions, or (3) the thermochronometer samples in this region having cooled under topography that only weakly resembled the modern‐day topography.