A climatic trigger for a major Miocene unconformity in the Himalayan Foreland Basin

Subsidence in foreland basins is modulated by the size of the flexural load, the elastic thickness of the foreland lithosphere, the nature of the sedimentary fill, and the rate of convergence. Basal “forebulge unconformities” are well known from these basins, but here we focus on a major unconformity in the Himalayan foreland, which removed much of the Oligocene–lower Miocene. This is a critical time in the Himalaya because it spans the period of initial rapid exhumation of high-grade metamorphic rocks and the start of motion on the Main Central Thrust. We show that the synchronous timing of unconformity and rapid Greater Himalayan exhumation may be explained by the same trigger, enhanced erosion. We estimate that accelerating erosion between 24 and 20 Ma would have removed ~1.5 km more rock from the Greater Himalaya than would have been the case had erosion remained at the slower Paleogene rates. During this time erosion must have temporarily exceeded rock uplift rates. By transferring rock from the Himalaya to the Indian Ocean the load is reduced and the Indian Plate is flexed less, so that the depth of the foreland basin shallows to a new equilibrium state. We use a flexural model to estimate that erosional unloading could have caused uplift of 400–500 m of the basin at the range front, extending 70–150 km into the basin. Intensification of the Asian summer monsoon around the Oligo-Miocene boundary (~24 Ma) is the most likely trigger of the stronger erosion. Similar unconformities are seen at 15–16 Ma and in the Pleistocene also, linked to faster erosion at these times