The aim of this project is to better understand how the moving plates that compose the outer rigid shell of the solid Earth behave. These plates may be composed of dense oceanic rock that underlies the deep oceans and/or more buoyant continental rock that makes up landmasses. For decades, it has been recognized that oceanic plates readily sink (subduct) into the Earth along subduction zones, which produce planar zones of earthquakes at depth. More recently, it has been appreciated that continental plates can also subduct deep into the Earth, but how they do so and what drives this process are poorly understood. The best example of active continental subduction is where the Indian plate is moving northward and colliding with the Eurasian plate, resulting in continued development of the Himalaya Mountains, Tibetan Plateau, and Pamir Mountains to the west. East of the Pamir Mountains, there is growing evidence that India is subducting northward beneath Eurasia, but there are only rare earthquakes at depths commonly observed in oceanic subduction zones. The only subduction-like zone of seismicity within a continent on Earth is located beneath the Pamir Mountains, which is the focus of this study. Unlike the case to the east, however, the seismicity is suggestive of southward subduction of the Eurasian plate. In this study, the principal investigators will test two hypotheses for why the Eurasian plate is subducting southward. The first is that the Indian plate has inserted itself into the Eurasian plate, like a wedge splitting a piece of wood, forcing the lower part of the Eurasian plate to subduct. The second hypothesis is that one of several processes could have abruptly increased the density contrast between the the Eurasian plate and deeper Earth beneath it, causing it to subduct rapidly, perhaps initially at rates greater than the rate of the northward motion of India. These hypotheses make contrasting predictions for the geological history of the high Pamir in Tajikistan, which we will test by quantifying the history of faulting, determining the age of the rocks and when and how fast they were brought from depth to the surface, and constraining where the rocks came from. Both of the hypotheses are novel compared to those that have been raised previously for the Pamir or India-Eurasia collision zone to the east. Validating either would thus provide documentation of an underappreciated behavior of continental subduction. In addition to the scientific goals of the project, the proposal contains educational and outreach components that are multifaceted and of societal relevance. They include the training and mentoring of graduate and undergraduate students in a STEM discipline, which will contribute to workforce development in a field (geosciences) that is expanding to address important national needs and challenges. The project is also facilitating scientific exchange between people and institutions in the United States and Tajikistan?the most impoverished country in central Asia whose stability depends heavily on foreign investment. The project is promoting cross-cultural understanding between American and Tajiks and geographic and scientific awareness by giving presentations in multiple venues that encompass culture, geography, and Earth science to local communities. The project will also support outreach activities aimed at advancing scientific understanding and sustainability for Americans for the local Tucson community, including lectures and field trips into the mountains for secondary school students and teachers and constructing an exhibit to be displayed at the University of Arizona's Flandrau Science Center, and for the annual Tucson Gem and Mineral Show--the largest show of its kind in the world.
This project is investigating the metamorphic, structural, and magmatic evolution of Cenozoic gneiss domes in the Pamir to test their potential linkages with Miocene to Recent development of the Pamir salient, northward underthrusting of India, and southward subduction of Asian lithosphere. The Pamir orogen is distinguished by a pronounced, northward-convex salient and a spatially extensive, orogen-parallel suite of gneiss domes. Both the salient and gneiss domes are thought to have developed synchronously, largely since Oligo-Miocene time. The thick crust (greater than 65 kilometers) of the Pamir is underlain in the south by a high-velocity mantle interpreted to be northward underthrust Indian lithosphere, and in the north by a southward-dipping zone of intermediate-depth seismicity that has been attributed to intracontinental subduction of Asian lithosphere. This project is testing two end-member 'tectonic drivers' that may genetically link all of these features: (1) A short-lived phase of rapid northward rollback/retreat of a southward-subducting slab of Asian lithosphere, during which the Pamir gneiss domes were exhumed by significant North-South horizontal extension (approximately 140 kilometers) and growth of the Pamir salient. (2) A protracted phase of northward underthrusting/wedging of Indian lithosphere that forced vertical exhumation of Asian mid-crust above it and southward subduction of Asian lithosphere beneath it. These two end-member hypotheses are not mutually exclusive, nevertheless, they make contrasting orogen-scale predictions that can be tested with geologic investigations. We are testing the predictions for the kinematic, metamorphic, and magmatic evolution of the gneiss domes. Our approach integrates geologic mapping and structural analysis to constrain the kinematics of gneiss-dome exhumation; metamorphic petrology, U/Th-Pb geochronology + trace element analyses to quantify the history of prograde and retrograde metamorphism; moderate- and low-temperature thermochronology to quantify the history of exhumation; and U-Pb geochronology and isotope analysis of zircon (hafnium) and titanite (neodymium) to constrain the history and sources of Cenozoic magmatism.
