Evidence for a far-traveled thrust sheet in the Greater Himalayan thrust system, and an alternative model to building the Himalaya

The Galchhi shear zone underlies the Kathmandu klippe in central Nepal and has emerged as a key structure for discriminating competing models for the formation of the Himalayan orogenic wedge. New chronologic data from the Galchhi area suggest a new structural and orogenic interpretation. Zircons from quartzites and an orthogneiss restrict protolith deposition to between 467+7/-10Ma and similar to 570Ma, with metamorphic zircon growth at 23-29Ma. Comparable data from the Greater Himalayan Sequence (GHS) at the intra-GHS Langtang thrust, north of Galchhi, similarly restrict GHS deposition to between 475+7/ - 3 and similar to 660Ma. Undeformed pegmatites near Galchhi constrain movement of the Galchhi shear zone to 22.52.3Ma, long before slip of the Main Central Thrust in the region (17Ma). Shear sense indicators in the Galchhi area indicate both top-to-the-south and top-to-the-north shears. The old age of movement, Neoproterozoic youngest detrital zircons, occurrence of top-to-the-south shear sense indicators, and intrusive Paleozoic granites, all suggest that the Galchhi shear zone is an intra-GHS top-to-the-south thrust, rather than either a thrust involving Lesser Himalayan rocks, or a top-to-the-north shear zone that juxtaposed Tethyan and GHS rocks during coeval movement of the Main Central Thrust. The GHS in Nepal was not emplaced as a single body of rock but consists of at least two ductile thrust sheets, present in both the hinterland at Langtang and toward the foreland at Galchhi. GHS thrust sheets sequentially underplated during southward propagation of the thrust belt.