EVIDENCE OF TWO PARALLEL, OROGEN SCALE THRUST SHEETS BOUNDING THE BASE OF THE GREATER HIMALAYA IN CENTRAL NEPAL

The structural architecture of central Nepal is complicated by the presence of two major thrusts bounding the base of the Greater Himalaya (GH), the Main Central thrust (MCT) and Ramgarh thrust (RT). Rocks between the RT and MCT are lower Lesser Himalayan (LH) rock, whereas rocks on the hanging wall of the MCT are GH rock. In ~100 km of along strike distance, the thickness of the RT sheet ranges from ~0.2 km in Malekhu to ~3.5 km in Kodari-Tatopani (KT). All units carried by the RT sheet are within the Robang Formation. The RT sheet in Malekhu contains Dunga Quartzite (DQ), greenish gray phyllite with ~15 m of metabasic intrusive rock parallel to bedding, and sheared bands of metabasic rock. In Galchhi, the RT sheet contains DQ, greenish gray phyllite with ~30 m of metabasic intrusive rock and ~40 m of mylonitic diorite augen gneiss. In KT, the RT sheet contains mylonitic-phylonitic garnetiferous schist, DQ with gray green phyllite, ~30 m of granitic gneiss intruding the DQ, and garnet-kyanite schist. In Malekhu and Galchhi, the MCT juxtaposes GH garnetiferous schist with mylonitic texture of the Kathmandu klippe against lower LH quartzite and phyllite; whereas in KT, the MCT juxtaposes banded GH gneiss with elongated garnets showing top-to the south sense of shear over lower LH rock.

In Malekhu and KT, U-Pb detrital zircons age populations of the DQ yields a maximum depositional age of Paleoproterozoic time. In Galchhi, U-Pb analysis of igneous zircons from a mylonitic diorite gneiss in the Robang Formation yields a crystallization age of Paleoproterozoic time, similar to the age of a granitic gneiss intruding the DQ at KT. These ages are similar to the oldest unit of the LH, the Kunchha Formation (1900 Ma), and suggest that the Robang Formation is possibly a distal facies of the Kunchha Formation and is always carried on the hanging wall of the RT sheet on top of the younger LH rock. The thrust previously mapped as the MCT at KT is actually the RT. Both thrusts coexist in central Nepal but carry rocks with different protoliths with wide variations in lithology, thickness and metamorphism. Mapping the MCT on the basis of metamorphic criteria and presence of strained rock completely obliterates the presence of the orogenic scale RT. Such a mistake would lead to an incorrect kinematic evolution and limit our understanding of the Himalayan orogenic system.