Late Eocene crustal thickening followed by Early-Late Oligocene extension along the India-Asia suture zone: Evidence for cyclicity in the Himalayan orogen

Abstract

The timing of geologic events along the India-Asia suture in southern Tibet remains poorly understood because minimal denudation prevents widespread exposure of structurally deep rocks. In this study, we present geologic maps of two structurally deep domes, cored by mylonitic orthogneisses, across the India-Asia suture zone in southwestern Tibet. New U-Pb zircon ages and rock textures indicate that core orthogneisses are originally Gangdese arc rocks that experienced Late Eocene prograde metamorphism, probably during crustal thickening. Crosscutting leucogranite sills underwent northwest-southeast extension related to slip along a brittle ductile shear zone here designated the Ayi Shan detachment. The timing of shear along detachment is bracketed by zircon U-Pb ages of 26–32 Ma for these pre- to syn(?)-extensional leucogranites, and by a 40 Ar/ 39 Ar muscovite age of 18.10 ± 0.05 Ma for a rhyolitic dike. This rhyolite dike crosscuts a widespread siliciclastic unit that was deposited across the detachment, which we correlate to the Kailas Formation. The Great Counter thrust defines the surface trace of the India-Asia suture zone; it cuts the Kailas Formation, and is in turn cut by the Karakoram fault. A new 40 Ar/ 39 Ar muscovite age of 10.17 ± 0.04 Ma for the Karakoram fault footwall is consistent with published thermochronologic data that indicate Late Miocene transtension in southwestern Tibet.