Tracking the Growth of the Himalayan Fold-and-Thrust Belt From Lower Miocene Foreland Basin Strata: Dumri Formation, Western Nepal

Abstract

New data from the lower Miocene Dumri Formation of western Nepal document exhumation of the Himalayan fold-thrust belt and provenance of the Neogene foreland basin system. We employ U-Pb zircon, Th-Pb monazite, 40Ar/39Ar white mica, and zircon fission track chronometers to detrital minerals to constrain provenance, timing, and rate of exhumation of Himalayan source regions. Clusters of Proterozoic–early Paleozoic (900–400 Ma) Th-Pb monazite and 40Ar/39Ar white mica detrital ages provide evidence for erosion of a Greater Himalayan sequence protolith unaffected by high-grade Eohimalayan metamorphism. A small population of ~40 Ma cooling ages in detrital white mica grains shows exhumation of low-grade metamorphic Tethyan Himalayan sequence through the ~350 °C closure temperature along the Tethyan Frontal thrust (proto-South Tibetan detachment) during the late Eocene. Dumri Formation detritus shows a ~12 Myr time difference between cooling of its source rocks through the ~350 and ~240 °C closure temperatures as recorded by ~40–38 Ma youngest peak cooling ages in 40Ar/39Ar detrital white mica and ~28–24 Ma youngest populations in detrital zircon fission track. Exhumation between circa 40 and 28 Ma is consistent with slip and exhumation along the Main Central Thrust. Combined with similar data from northwestern India, our study suggests west-to-east spatially variable exhumation rates along strike of the Main Central Thrust. Our data also show an increase in exhumation during middle Miocene–Pliocene time, which is consistent with growth of the Lesser Himalaya duplex.