Orogen-parallel extension and exhumation enhanced by denudation in the trans-Himalayan Arun River gorge, Ama Drime Massif, Tibet-Nepal

Abstract

Focused denudation and mid-crustal flow are coupled in many active tectonic settings, including the Himalaya, where exhumation of mid-crustal rocks accommodated by thrust faults and low-angle detachment systems during crustal shortening is well documented. New structural and (U-Th)/He apatite data from the Mount Everest region demonstrate that the trans-Himalayan Ama Drime Massif has been exhumed at a minimum rate of ~1 mm/yr between 1.5 and 3.0 Ma during orogen-parallel extension. The Ama Drime Massif offsets the South Tibetan detachment system, and therefore the South Tibetan detachment system is no longer capable of accommodating south-directed mid-crustal flow or coupling it with focused denudation. Previous investigations interpreted the NNE-SSW–striking shear zone on the west side of the Ama Drime Massif as the Main Central thrust zone; however, our data show that the Ama Drime Massif is bounded on either side by 100–300-m-thick normal-sense shear zone and detachment systems that are kinematically linked to young brittle faults that offset Quaternary deposits and record active orogen-parallel extension. When combined with existing data, these results suggest that the Ama Drime Massif was exhumed during orogen-parallel extension that was enhanced by, or potentially coupled with, denudation in the trans-Himalayan Arun River gorge. This model provides important insights into the mechanisms that exhumed trans-Himalayan antiformal structures during orogen-parallel extension along the southern margin of the Tibetan Plateau.