Mesozoic Tethyan strata of Thakkhola, Nepal: evidence for the drift and breakup of Gondwana

Abstract

The Thakkhola region of central Nepal contains at least 1.5 km of coastal to neritic and (upper) slope deposits of Late Triassic to mid-Cretaceous (latest Albian) age. New paleomagnetic, paleobiogeographic and paleoflow data confirm that the strata were deposited on the northern Gondwana margin, bordering Tethys while Thakkhola lay at mid-latitudes (28–41° S). Late Triassic coastal deposits are overlain by Early-Mid Jurassic shelf units; thick Early Jurassic carbonates correspond with the most northerly paleolatitude recorded, probably reflecting Thakkhola's rapid drift into the subtropics. The Late Jurassic to Early Cretaceous drift again in higher latitudes resulted in deposition of terrigenous clastics. A hiatus spanning late Early Callovian through Early Oxfordian time corresponds to a global transgression that may reflect accelerated sea-floor spreading in both the Atlantic and Pacific Oceans. Late Jurassic dark shales, correlative with the regionally extensive Spiti Shale, are deep shelf to slope deposits; septal strength indices for cephalopods suggest depths > 250 m. The shales contain an Indo-SW Pacific ammonite assemblage but also a diversified agglutinated foraminiferal assemblage of Boreal affinity. Such deep-water foraminiferal assemblages probably had a large cosmopolitan component. The shale is intensely deformed and its overmature condition probably reflects in part Cenozoic orogenic events. Lower Cretaceous (Berrisian through Aptian) deltaic deposits overlying the shales contain volcanoclastic material that probably was derived from coeval volcanics of the Lesser Himalayas to the south, based on compositional, paleoflow and facies analysis. Volcanism reflects the onset of sea-floor spreading between Greater India and northwestern Australia in Late Valanginian time. At the top of the Thakkhola Mesozoic sequence are hemipelagic, slope, carbonates of latest Albian age. The Thakkhola succession generally correlates well with that observed on a formerly contiguous continental margin, offshore northwestern Australia.