Relationship between leucogranites and the Qomolangma detachment in the Rongbuk Valley, south Tibet

Central to understanding the exhumation history of the Himalaya is knowing the timing of slip and magnitude of displacement on the primary fault syste ms that bound the range. The widely accepted view that early Miocene deform ation in the Himalaya is characterized by simultaneous shortening along the Main Central thrust and extension at shallower crustal levels in part deve loped on the basis of knowledge of the age of the Rongbuk granite and its a pparent crosscutting relationship with the Qomolangma detachment. This key contact, however, has not previously been directly observed. Field mapping of the Qomolangma detachment and its footwall reveals that no leucogranite bodies crosscut the detachment. These observations together with Th-Pb mona zite dating of leucogranites exposed in the footwall suggest that slip was occurring across the Qomolangma detachment shear zone ca, 17 Ma. Although t here is no evidence that requires simultaneous shortening and extension in the High Himalaya, our observations are consistent with alternating periods of shortening and extension in the Himalaya since the early Miocene.