Apatite and zircon fission-track thermochronology constraining the interplay between tectonics, topography and exhumation, Arunachal Himalaya

Thirty-eight new apatite and zircon fission-track ages from 26 bedrock samples vary from 2.0 ± 0.3 to 12.1 ± 1.2 Ma, and 3.3 ± 0.3 and 13.2 ± 0.7 Ma, respectively, along three transects of the Kurung, Subansiri, and Siyom Rivers, which flow across the major structures of the Arunachal Himalaya. These cooling ages reveal marked variations in millennial-scale (>105 yr) exhumation rates from 0.6 to 3.0 mm/yr. A distinct positive correlation is visible between local topographic relief, hill slopes, channel steepness, and exhumation rates. The cooling ages are younger in the northern antiformal domains and older within the synformal nappe along the mountain front. Thermal modelling and time–temperature paths suggest that zones of rapid exhumation are controlled by structural windows within the Lesser Himalaya that were developed between 8 and 6 Ma over blind Main Himalayan Thrust (MHT). This time of rapid rock uplift and major topographic change led to a two-fold increase in the exhumation rates in the northern antiformal domains than the southern front of Arunachal Himalaya. Variation in cooling ages does not correlate with the present-day precipitation pattern. Tectonics appears to be the leading factor in driving the exhumation rates and landscape evolution in the Arunachal Himalaya.