RAPID PLIO-PLEISTOCENE EXHUMATION ACROSS THE CENTRAL NEPALESE HIMALAYA

Eighty-two apatite and 7 zircon fission-track and 8 apatite (U-Th)/He analyses are used to constrain the bedrock cooling history in the central Nepalese Himalaya along the Marsyandi drainage. Samples include eight vertical relief profiles averaging more than 2 km in elevation range, as well as valley bottom samples. Apatite fission track (AFT) ages range from 3 to 0 Ma, zircon fission track (ZFT) ages from 1.9 to 0.8 Ma and apatite (U-Th)/He (He) ages from 0.9 to 0.3 Ma, with typical ZFT, AFT and He ages at river level of 0.8, 0.5, and 0.3 Ma, respectively. Exhumation rates calculated from these data show no discernable variation across the Greater Himalaya (25 km wide) and the southern Tethyan strata (20 km wide) in the study area. The arid hangingwall of the South Tibetan Detachment appears to have eroded as fast as the monsoon-drenched southern flank of the Greater Himalaya during the time period bracketed by the apatite fission track data. Most ridges have maximum ages of 1 to 0.8 Ma at 2 km above the valley bottom, whereas only one ridge crest (in the south-central zone of the field area) yielded significantly older ages of 2 Ma. Zircon and apatite fission-track ages from this vertical transect suggest slower erosion rates of 1.5 km/Myr from 2 to 0.8 Ma. Apatite He ages from the same samples indicate a faster erosion rate of 2.5 km/Myr over the last 0.8 Myr. We infer that a splay of the Main Central Thrust separates this southern ridge from the rest of the Greater Himalaya. We conclude that, despite strong spatial gradients in climate, the exhumation rate has been roughly uniform and very rapid (2.5 - 5 km/My) for the last 0.8 Myr across the Greater Himalaya and southern Tibet.