HIGH HIMALAYAN LEUCOGRANITES INDICATE BRIEF (≤3 MYR) DUCTILE EXTENSION OF THE STDS

The South Tibetan Detachment System (STDS) is an orogen-scale normal shear zone in the Himalaya that separates the high-grade Greater Himalayan Sequence (GHS) from the low-grade Tethyan Himalayan Sequence (THS). The STDS is key for Himalayan thermal-mechanical models because it is widely viewed as representing the upper structural bound to ductile flow of the GHS. Channel flow models imply long-term (c. 20 Myr) ductile shearing on the STDS, whereas other models such as critical taper imply brief shearing. The Malari leucogranite, in the Garhwal region of India, cuts ductile, normal-sense shear fabrics of the STDS, and has experienced relatively little subsolidus brittle deformation. Zircon U-Pb ages, collected by laser-ablation ICP-MS and corrected for initial U/Th disequilibrium, indicate pluton emplacement at 19.0±0.5 Ma. The ductile phase of STDS movement clearly must have ceased by 19 Ma. The small size of the Malari body additionally limits later brittle deformation between the THS and GHS to <1 km. Studies elsewhere in the Himalaya suggest initiation of STDS ductile movement no earlier than ~24 Ma and likely ~22 Ma. Thus, the duration of ductile extension along the STDS was certainly <5 Myr, and likely ≤3. A similarly short duration of extension is indicated by chronologic microanalysis of monazite from the external klippe of the STDS in Bhutan (Chambers et al., this session). These data are difficult to reconcile with the long durations implied by channel flow, and are instead more consistent with the models in which internal weakening of the wedge, likely from partial melting, caused a brief interval of flattening and ductile extension (Burchfiel and Royden, 1985, Geology; Northrup, 1996, Tectonics).