The growth, evolution, and destruction of continental crust during continental collision are fundamental processes that form great mountain belts such as the Himalaya, alter regional and global climate, and impact the transfer of mass between the Earth's surface and deep interior. Deeply buried rocks that have returned to the Earth's surface record these processes in their textures, chemistry, and isotopic composition, so analyzing such samples allows reconstruction of the continental collision processes, analogous to using a flight data recorder to reconstruct the events leading to a plane crash. Rocks buried to >100 km are termed ultrahigh-pressure (UHP) rocks, and are particularly useful for testing models of continental collision. Study of UHP rocks also aids understanding of the physical conditions relevant to the fluid-releasing mineral reactions that are linked to the deep earthquakes and melting that occur beneath volcanic chains such as the Cascade Range of the Pacific Northwest and the Aleutian Islands of Alaska.
This project focuses on the processes responsible for the deep (>100 km) burial and exhumation of continental crust, as recorded in rocks of the North Qaidam UHP terrane, western China, where UHP rocks and a long-lived (>30 Myr) collisional history are well documented. Ultrahigh-pressure metamorphism is now recognized as an integral component of continental collision for at least the past 600 Myr, but a growing data set indicates that some of the largest terranes remained at UHP conditions for an unexpectedly long time, challenging the commonly invoked interpretation that the complete burial + exhumation cycle must be brief (<5-10 Myr). Models developed to explain long UHP durations predict distinct spatial and temporal trends in temperature and pressure, but current data lack the necessary spatial distribution and temperature resolution to test the applicability of these models. This project will determine the thermal structure (spatial and time evolution of temperature, as well as pressure) of the southeastern segment of the North Qaidam UHP terrane by combining (1) recently developed trace element thermometers, forward thermodynamic modeling, and conventional approaches to improve the accuracy and precision of temperature estimates, and (2) systematic U-Pb geochronology, tied to pressure and temperature constraints using trace elements and inclusions, to place these results in a time context. The planned research will involve education and training of graduate and undergraduate students in fundamental analytical techniques as well as international collaborative research. Results of the project will provide the fundamental constraints necessary to evaluate processes leading to long UHP duration, to test existing model predictions as well as develop new models, and to make meaningful comparisons with other well-documented UHP terranes. These results should significantly advance our knowledge of general UHP formation/exhumation processes in ancient and modern collisional mountain belts.
