Dating of the oldest continental sediments from the Himalayan foreland basin

A detailed knowledge of Himalayan development is important for our wider un derstanding of several global processes, ranging from models of plateau upl ift to changes in oceanic chemistry and climate(1-4). Continental sediments 55 Myr old found in a foreland basin in Pakistan(5) are, by more than 20 M yr, the oldest deposits thought to have been eroded from the Himalayan meta morphic mountain belt. This constraint on when erosion began has influenced models of the timing and diachrony of the India-Eurasia collision(6-8), ti ming and mechanisms of exhumation(9,10) and uplift(11), as well as our gene ral understanding of foreland basin dynamics(12). But the depositional age of these basin sediments was based on biostratigraphy from four intercalate d marl units(5). Here we present dates of 257 detrital grains of white mica from this succession, using the Ar-40-(39) Ar method, and find that the la rgest concentration of ages are at 36-40 Myr. These dates are incompatible with the biostratigraphy unless the mineral ages have been reset, a possibi lity that we reject on the basis of a number of lines of evidence. A more d etailed mapping of this formation suggests that the marl units are structur ally intercalated with the continental sediments and accordingly that biost ratigraphy cannot be used to date the clastic succession. The oldest contin ental foreland basin sediments containing metamorphic detritus eroded from the Himalaya orogeny therefore seem to be at least 15-20 Myr younger than p reviously believed, and models based on the older age must be re-evaluated.