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Abstract

The South Tibetan Detachment System (STDS) is a network of orogen-parallel, low-angle normal faults and shear zones traced along the entire length of the Himalayan orogen. It played a critical role in facilitating large-scale, normal-sense movement between the upper and middle crust during convergence. Despite its significance, key controls on the deformation processes of the STDS remain poorly understood. This study presents a time-resolved deformation history of the STDS in the Tsum Valley (Manaslu region, central Nepalese Himalaya), detailing its kinematics, deformation mechanisms, and stress history. In the Tsum Valley, the STDS is a ~2500 m wide zone of ductile deformation that includes Greater Himalayan Sequence (GHS) and Tethyan Sedimentary Sequence (TSS) rocks, without a discrete brittle detachment at higher levels. In situ Rb-Sr geochronology indicates that deformation and cooling within the STDS occurred between 17-13 Ma. Deformation occurred under monoclinic-dominant general shear flow. Distinct flow geometries between the rocks within the STDS and those at its boundaries imply that the boundary rocks record a different strain path, potentially linked to the onset of E-W extension in the region. STDS deformation was coupled with exhumation-driven cooling, resulting in the telescoping of isotherms from 500-600°C in the GHS to