Episodic exhumation of the Greater Himalayan Sequence since the Miocene constrained by fission track thermochronology in Nyalam, central Himalaya

Abstract

The Greater Himalayan Sequence (GHS), which makes up the core of the Himalayan orogen, has an uppermost tectonic contact defined by the South Tibetan Detachment System (STDS) and a lower tectonic contact defined by the Main Central Thrust (MCT). The GHS occurs as one of the most important tectostratigraphic units for deciphering processes related to tectonic and climatic exhumation across the orogen. Zircon and apatite fission track (ZFT, AFT) dating were carried out along a transect in Nyalam, central Himalaya in southern Tibet to constrain cooling driven by orogenic process since the middle Miocene. The hanging wall of the STDS yields an essentially unreset Jurassic ZFT age in the Jurassic strata. However, below the STDS within the GHS there is a clear and distinct thermal signal of cooling related to exhumation. In the footwall and within the GHS, the rocks have ZFT ages of middle Miocene to Pliocene, and AFT ages of late Miocene to Quaternary that get younger downward and away from the STDS. In combination with thermal structure modeling, a two-part episodic model, which is widely compatible with existing thermochronological data, is proposed for cooling and exhumation of the GHS since the middle Miocene: [1] middle Miocene; and [2] Pliocene to Quaternary (Recent). The middle Miocene cooling is suggested to have resulted from a rapid tectonic unroofing by down-to-the-north slip on the STDS. The tectonic exhumation was also recorded by several other thermochronological systems (e.g. biotite 40Ar/ 39Ar) with concordant middle Miocene cooling ages in different structural positions across the GHS. Post middle Miocene ZFT and AFT cooling ages in the lower part of the GHS suggest accelerated cooling by climate-enhanced erosional exhumation, which was initiated in the late Miocene to Pliocene and was dramatic in the Quaternary to Recent. Thermochronological data and modeling further imply that the present Himalayan topographic front may have been shaped essentially by surface erosion since the late Miocene, when the Himalayan divide might have been some 20–30 km to the south of its present position. However, these data do not preclude the possibility that the intense erosional exhumation may have triggered rock uplift to approach and/or maintain a steady topography in the GHS.