Geochronological constraints on Miocene structural reorganization of the South Tibetan Detachment, eastern Himalaya

In the eastern Himalaya (Bhutan) two distinct top-down-to-the-north segments of the South Tibetan Detachment (STD) system are exposed. New U-Pb and 40Ar/39Ar data provide precise age constraints for ductile deformation along these shear zones. The outer segment of the STD is a diffuse ductile shear zone preserved as the structural boundary to broad open synformal klippen of upper crustal rocks. LA-ICP-MS U-Pb in situ ages of monazite included in pre- to syn-kinematic garnet from a hanging-wall schist indicate that the outer shear zone became active after ca. 23 Ma. Shearing continued at least until ca. 15.5 Ma and the rocks cooled through ca. 425 °C by ca. 11.0 Ma, as constrained by SHRIMP U-Pb ages of magmatic zircon and 40Ar/39Ar ages of muscovite in leucogranite sills which have been ductily deformed by STD shearing. The inner STD segment is a more focused ductile shear zone active at least until ca. 11.0 Ma (SHRIMP U-Pb ages of magmatic zircon in deformed leucogranites) and overprinted by up to Recent brittle faulting. These data indicate that in the eastern Himalaya ductile deformation on the STD was coeval with the Main Central Thrust between <23 and ca. 13 Ma 1,2. Furthermore, STD-related deformation continued more recently in the eastern Himalaya than in central and western parts of the orogen. Our data allow for a more detailed reconstruction of continental collision in the eastern Himalaya in which the outer STD segment was abandoned in the mid-Miocene and passively transported southward in the hanging wall of the Main Himalayan Thrust (the basal detachment of the orogen), while top-to-the-north ductile to brittle shearing continued on the inner STD segment. These data combined with published geochronologic data for the eastern Himalaya demonstrate that foreland translation and exhumation of a mid-crustal dome (viscous wedge model) is a tenable mechanism for this tectonic reorganization. References 1Stüwe, K., and D. Foster (2001), J. Asian Earth Sci., 19(1-2), 85-95. 2Daniel, C. G., et al., (2003), J. Met. Geol., 21, 317-334.