Constraints on brittle field exhumation of the Everest‐Makalu section of the Greater Himalayan Sequence: Implications for models of crustal flow

Abstract

New apatite and zircon fission track (FT) data from the summit slopes of Everest and along the Barun, Arun, Dudh Kosi, and Kangshung valleys that drain the Everest and Makalu massifs cover a vertical sample transect of almost 8000 m of the Eastern Nepal Greater Himalayan Sequence (GHS). Apatite FT ages range from 0.9 ± 0.3 Ma to 3.1 ± 0.3 Ma in the GHS with ages increasing systematically with elevation. Apatite FT ages in the Everest Series and summit Ordovician limestones are much older, up to 30.5 ± 5.1 Ma. Zircon FT ages from the GHS range from 3.8 ± 0.4 Ma to 16.3 ± 0.8 Ma. The brittle exhumation rates calculated from these data show the GHS was exhumed, since ∼9 Ma, at an average rate of 1.0 ± 0.2 mm/a. Pliocene exhumation rates are higher at 1.7 ± 0.3 mm/a. These values are not significantly different from the estimate of ductile exhumation rates of 1.8 mm/a recorded by metamorphic minerals undergoing decompression between 18.7 and 15.6 Ma but are well below the values (up to 10 mm/a) used by thermomechanical models for ductile channel flow in the GHS. If representative of the GHS these models will therefore require further ‘tuning’. Higher exhumation rates in the Pliocene have also been observed in other parts of the Himalaya and points to a regional cause, likely increased erosion due to the onset of late Pliocene–Pleistocene glaciation of the high Himalaya.