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

Abstract

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/ À 10 Ma and~570 Ma, with metamorphic zircon growth at 23-29 Ma. 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/ À 3 and 660 Ma. Undeformed pegmatites near Galchhi constrain movement of the Galchhi shear zone to ≥22.5 ± 2.3 Ma, long before slip of the Main Central Thrust in the region (≤17 Ma). 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.

Petrological constraints on the tectonic setting of the Kathmandu Nappe in the Langtang–Gosainkund–Helambu regions, Central Nepal Himalaya

“…the STDS does not occur in the KN). Model C (Khanal et al ., ; see also Yin, ; although less explicit) correlates the KN to both the GHS (lower Bhimpedi Group) and the TSS (upper Bhimpedi Group + Phulchauki Group), thus implying that: (i) the STDS is located within the KN, separating the lower Bhimpedi Group from the upper Bhimpedi Group and (ii) that the GSZ is an intra‐GHS thrust juxtaposing two different GHS units. Khanal et al .…”

A review on out-of-sequence deformation in the Himalaya

“…Later Himalayan shortening (¼the D 2 deformation as referred to in Mukherjee & Koyi 2010b). Khanal et al (2015) included the Langtang Thrust, the Toijem Shear Zone, the Mangri Shear Zone, the Bhanwua/Bhanua Thrust, are plotted from previous publications. The map is reproduced from Zhang et al (2015), fig.…”

Zircon U‐Pb ages and Hf isotopes of the Askot klippe, Kumaun, northwest India: Implications for Paleoproterozoic tectonics, basin evolution and associated metallogeny of the northern Indian cratonic margin

“…Pronounced differences in depositional ages and spatial association with circa 1850 Ma versus circa 500 Ma granitoids can discriminate Paleoproterozoic lower LH rocks from Neoproterozoic to Cambrian GH and upper LH rocks [ Ahmad et al ., ; DeCelles et al ., ; Richards et al ., ; Tobgay et al ., ; McKenzie et al ., ; Long et al ., ; Khanal et al ., ]. The preferred crystallization ages of 1857 ± 19 and 1878 ± 19 Ma for orthogneisses in the Askot klippe correspond well with circa 1850 Ma granite‐granodiorite plutons and volcanic rocks associated with the lower LH [ Kohn et al ., , and references therein].…”