Constraining the exhumation history of the Greater Himalayan sequence, Kali Gandaki, Central Nepal

Abstract

Understanding how the South Tibetan Detachment System (STDS) evolved through time and space is necessary for understanding the evolution of the Himalayan orogen. We present new (with previously published) thermochronologic results from a transect in the footwall and ductile shear zone of the basal structure of the STDS in the Kali Gandaki region: the Annapurna detachment. The exhumation history is interpreted from observations using 1D thermal-kinematic models that invert for the exhumation rate of samples. Recently published data suggested that high-temperature slip on the detachment persisted until at least ca. 12 Ma, more recently than is commonly assumed for STDS deformation. Our new data and modelling support those findings and suggest that the cessation of slip coincided with a dramatic, > 50% decrease in the exhumation rate of the shear zone and its footwall at ca. 12-10 Ma. Exhumation rates remained low until ca. 3 Ma, after which they increased to levels comparable with those that characterised STDS activity. Plausible causes of this late pulse of exhumation include an intensification of the Asian Winter monsoon and establishment of today's Indian Summer Monsoon, glaciation, and/or an internal structural reorganisation of the Himalayan orogenic wedge driving localised rock uplift in the hinterland. Supplementary material: https://doi.org/10.6084/m9.figshare.c.6949467