CLIMATE CHANGE, TECTONICS AND THE EVOLUTION OF THE HIMALAYAN RIVERS AND TOPOGRAPHY

How major rivers evolve during orogeny is a fundamental question in Earth Sciences. For example, mountain building can cause focused erosion that in turn may trigger thermal weakening and localized contraction, leading to positive feedback between erosion and uplift. One of the best places to address the coupled interactions among tectonics, climate change and surface processes is the active Himalayan orogen. There all these factors can be well quantified due to significantly larger and thus more readily detectable signals than most places on Earth. To address the how tectonics, surface processes, and climate may have interacted with one another, we investigated how the Himalayan drainage system has evolved in the Late Cenozoic as a result of interactions between north-trending Tibetan rifts and east-trending Himalayan uplift. We examined this complex yet scientifically stimulating problem via determining when and how the east-flowing Yalu River (Yarlung Tsangpo) was captured by the Subansiri River, the largest transverse drainage system in the eastern Himalaya. Understanding capture mechanisms of the Yalu River by major Himalayan transverse rivers has first-order implications for the Himalayan exhumation history, sediment-transport processes, and petrologic evolution of the Himalayan foreland basin. The main hypothesis we are testing is that a climate event marked by protracted glacier advances triggered the formation of a rift dam in SE Tibet via strongly coupled tectonic-climate processes. This rift-induced dam blocked and subsequently diverted the east-flowing Yalu River into the south-flowing Subansiri River. Accelerated Himalayan uplift, possibly energized by the capture event, eventually forced the Yalu to return to its original course. The possibility of Yalu-Subansir connection was first raised by our initial detrital-zircon study of Late Miocene-Pliocene foreland-basin sediments from the eastern Himalaya using coupled U-Pb zircon dating and Hf isotope analysis. Subsequent fieldwork in SE Tibet further bolsters this possibility by revealing a sequence of valley-fill sediments distributed across the Yalu-Subansir drainage divide. The valley fill, likely representing the paleo-Yalu-Subansiri river sediments, is located in the north-trending Cona-Zedong rift zone of SE Tibet and contains thick (>150 m locally) moraine deposits. These associations suggest that the capture event of the Yalu by the Subansir was controlled by rifting and occurred during major glacier advances. A key prediction of our climate-induced rift-dam hypothesis is a sequence of events starting with initial glacier advances followed by synchronous fast rift-induced uplift (relative to river incision), deposition of the paleo-Yalu-Subansi river sediments (represented by the valley fill in SE Tibet), and deposition of foreland sediments carrying Gangdese-batholith detritus from southern Tibet. The restoring mechanism of the Yalu River by Himalayan uplift predicts that the uplift rate of the eastern Himalaya was accelerated in the latest Pliocene.