Early Cretaceous strata of the Nepal Himalayas: conjugate margins and rift volcanism during Gondwanan breakup

Abstract

Early Cretaceous sandstones, shales and marlstones (Chukh, Tangbe and Muding Formations, at least 625 m thick) crop out north of the Main Central Thrust in central Nepal. They were strongly deformed during Himalayan collision, with telescoping of facies transitions as a result of crustal shortening. The sediments show northerly palaeoflow and were deposited on the steadily subsiding northern (Tethyan) margin of Gondwana. Berriasian deltaic deposits pass upward into Valanginian-Albian, storm-dominated shelf deposits and into latest Albian pelagic slope carbonates. An unconformity, probably of Valanginian age, separates the Chukh and Tangbe formations locally, and is overlain by volcanic conglomerates. The Gondwanan margin succession matches well with that observed on the formerly conjugate margin of NW Australia, which separated from Greater India during the Valanginian to Hauterivian. Volcanic detritus, where suitable for analysis, shows within-plate geochemical affinity and reflects Gondwanan fragmentation. Abundant volcanic detritus in Aptian strata may have been derived from extensions of the Rajmahal Traps of northeast India. However, volcanism was active near central Nepal in the Valanginian and volcanic detritus, possibly far travelled, reached the area during the Berriasian. Early Cretaceous strata1 successions and relative sea-level changes show a first-order relationship to tectonism associated with Gondwanan break-up.