Geologic and U-Th-Pb geochronologic evidence for early Paleozoic tectonism in the Kathmandu thrust sheet, central Nepal Himalaya

Abstract

The Kathmandu thrust sheet consists of Upper Proterozoic through mid-Paleozoic rocks that were emplaced over Lesser Himalayan strata (part of India's cratonal cover) during middle to late Tertiary time. Primary components of the thrust sheet include Upper Proterozoic metasedimentary rocks of the Bhimphedi Group, Cambrian-Ordovician granite bodies, and Ordovician (through Devonian?) conglomerate, sandstone, shale, and limestone of the Phulchauki Group. Deformation, metamorphism, uplift, and erosion accompanied Tertiary emplacement of the thrust sheet, but there is also evidence of widespread early Paleozoic tectonism, such as: (1) Rocks of the Bhimphedi Group were metamorphosed at ca. 490 Ma, as indicated by Th-Pb ages of monazite inclusions in garnet crystals. (2) Bhimphedi rocks are interpreted to have been imbricated along a south-vergent(?) thrust system during early Paleozoic time, with U-Pb ages recording ductile deformation through ca. 484 Ma but ending (at least locally) by ca. 473 Ma. (3) Cambrian-Ordovician granite bodies may have been generated by crustal melting during thrust loading, and at least some are interpreted to have been emplaced as syntectonic sills along the early Paleozoic thrust faults. (4) Ordovician conglomeratic strata were shed from the uplifted Bhimphedi Group and Cambrian-Ordovician granite bodies and are interpreted to have accumulated in a foreland basin setting with respect to the early Paleozoic thrust system. Our findings, together with evidence for early Paleozoic ductile deformation and metamorphism in adjoining regions, indicate that early Paleozoic tectonism has played an important role in shaping the Himalayan orogen. This early Paleozoic tectonism has been overlooked in most current models for the formation of the Himalayan orogen.