Monsoon Intensification Linked to Greater Himalayan Exhumation in the Early Miocene

The timing of Greater Himalaya exhumation after 23 Ma has long been enigmatic because while it substantially postdates initial India-Asia collision around 50 Ma it also pre-dates the apparent intensification of the monsoon at 8 Ma. Channel flow models for the formation of the Himalaya require focused erosion at the range front to explain exhumation, yet this is hard to achieve without a strong summer monsoon. How then did the Himalaya form prior to 8 Ma? We present new geochemical evidence that exploits the link between summer monsoon strength and the intensity of chemical weathering. Our data point to an Early Miocene intensification of the monsoon after ca. 24 Ma, with weakening again after 10 Ma. This pattern is found in both East and South Asia and suggests a common trigger, perhaps elevation of Tibet to a critical threshold that draws rain far inland in a way not seen during the Paleogene. The weakening of the monsoon after 10 Ma resulted in slower exhumation and a reorganization of Himalaya structure, most notably the initiation of the Main Boundary Thrust. Marine sediment records show a slowing erosional flux to the Indian Ocean and South China Sea after 10 Ma, suggesting a simple positive link between climate and rates of continental erosion. We suggest that while tectonic forces build the Tibetan Plateau, this affects the regional climate and this in turn drives mountain erosion and rock uplift. It is the onset of the monsoon, not plate tectonics allows the high-grade metamorphic rocks and granites of the Greater Himalaya to be exhumed.