Leucogranite geochronological constraints on the termination of the South Tibetan Detachment in eastern Himalaya

Abstract

The South Tibetan Detachment System (STDS) developed between the very-low-grade Tethyan Himalayan Sequence (THS) and the high-grade metamorphic Greater Himalayan Sequence (GHS) over an along-strike distance exceeding 2000km within the Himalayan orogen. Motion along the STDS has been associated with the exhumation of the GHS and the uplift of the Himalayan Mountains. Geological field mapping and U–Th–Pb geochronological analyses were performed to trace and date the STDS in the Yadong region of southern Tibet, where it is a top-to-the-north shear zone juxtaposing the Yadong klippe of the THS with the high-grade GHS. This shear zone is intruded by multiple generations of leucogranites emplaced mainly at ca. 23Ma, ca. 20–18Ma and ca. 17–16Ma. The undeformed textures of the leucogranites and their cross-cutting relationships with the shear zone indicate that ductile motion along the STDS terminated by ca. 20Ma. This is largely consistent with the deformation history of the STDS in central and western Himalaya. The termination of shearing along the STDS during the Early Miocene occurred well before the rapid cooling of the GHS during the Middle Miocene, suggesting that exhumation and decompression of the GHS was still ongoing after the cessation of motion along the STDS.