EOCENE EARLY MIOCENE FORELAND BASIN DEVELOPMENT AND THE HISTORY OF HIMALAYAN THRUSTING, WESTERN AND CENTRAL NEPAL

Sedimentologic, petrographic, and U-Pb detrital zircon ages from middl e Eocene through early Miocene sedimentary rocks in the Lesser Himalay an zone of western and central Nepal indicate that a peripheral forela nd 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 fo rebulge and the Indian craton. Migration of the forebulge through this region during Eocene-Oligocene time produced a regional unconformity that spans similar to 15-20 Myr. By early Miocene time, the forebulge unconformity was onlapped by the distal fringes of the southward migra ting foredeep depozone, represented by fluvial deposits of the Dumri F ormation. Continued southward migration of the foredeep during the Neo gene accommodated the fluvial Siwalik Group. Light mineral provenance data and U-Pb detrital zircon ages suggest that the Bhainskati was der ived partly from Tethyan sedimentary rocks of the Tibetan Himalayan zo ne 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 conte mporaneous tectonic unroofing by normal faulting along the South Tibet an detachment system. The Lesser Himalayan crystalline thrust sheets w ere emplaced soon after deposition of the Dumri Formation, similar to 15-10 Ma. Paleocurrent and lithofacies data from the Dumri Formation i ndicate deposition by west-southwestward flowing rivers that drained i nto the Indus portion of the Himalayan foreland basin system during th e early Miocene. Thick channel sandstones in the lower Dumri may repre sent the early Miocene counterpart of the modern Ganges River. Eastwar d diversion of the Ganges drainage system to near its present location had occurred by similar to 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 velo city of similar to 14-33 mm/yr for the southward migration of the fold -thrust belt relative to India. This range of values is consistent wit h Neogene and present-day estimates and suggests that only one third t o one half of India-Eurasia convergence has been accommodated by short ening in the Himalayan fold-thrust belt since the onset of collision.