Eocene‐early Miocene foreland basin development and the history of Himalayan thrusting, western and central Nepal

Abstract

Sedimentologic, petrographic, and U‐Pb detrital zircon ages from middle Eocene through early Miocene sedimentary rocks in the Lesser Himalayan zone of western and central Nepal indicate that a peripheral foreland basin system had developed in the eastern Himalayan collision zone by middle Eocene time. The shallow‐marine, Eocene Bhainskati Formation accumulated in a back‐bulge depozone between a southward migrating forebulge and the Indian craton. Migration of the forebulge through this region during Eocene‐Oligocene time produced a regional unconformity that spans ∼15–20 Myr. By early Miocene time, the forebulge unconformity was onlapped by the distal fringes of the southward migrating foredeep depozone, represented by fluvial deposits of the Dumri Formation. Continued southward migration of the foredeep during the Neogene accommodated the fluvial Siwalik Group. Light mineral provenance data and U‐Pb detrital zircon ages suggest that the Bhainskati was derived partly from Tethyan sedimentary rocks of the Tibetan Himalayan zone during initial growth of the Himalayan fold‐thrust belt. The Dumri was derived from metasedimentary and crystalline rocks of the Greater Himalayan zone during emplacement of the Main Central thrust and contemporaneous tectonic unroofing by normal faulting along the South Tibetan detachment system. The Lesser Himalayan crystalline thrust sheets were emplaced soon after deposition of the Dumri Formation, ∼15–10 Ma. Paleocurrent and lithofacies data from the Dumri Formation indicate deposition by west‐southwestward flowing rivers that drained into the Indus portion of the Himalayan foreland basin system during the early Miocene. Thick channel sandstones in the lower Dumri may represent the early Miocene counterpart of the modern Ganges River. Eastward diversion of the Ganges drainage system to near its present location had occurred by ∼15 Ma, as the high‐standing Aravalli Range on the northern Indian shield approached the front of the fold‐thrust belt. Assuming reasonable values for the flexural rigidity of Indian lithosphere, the time span of the forebulge unconformity yields a velocity of ∼14–33 mm/yr for the southward migration of the fold‐thrust belt relative to India. This range of values is consistent with Neogene and present‐day estimates and suggests that only one third to one half of India‐Eurasia convergence has been accommodated by shortening in the Himalayan fold‐thrust belt since the onset of collision.