Collaborative Research: Structure and dynamics of the Alaska mantle wedge

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Subduction zones are the most active geologic features on the planet - oceanic plates descend into the earth's interior at geological speeds and carry with them fluids and materials from the sea floor. As this material heats, fluids such as water are released, lubricating faults at shallow depths and fluxing the warm Earth's deeper mantle to make magmas. As a result, subduction zones host the planet's largest earthquakes and most of the violent volcanic eruptions. In North America, the Alaska-Aleutian subduction system is by far the largest such system - Alaska has hosted the largest earthquake in North America, and the planet's largest 20th century eruption. The eastern end of this subduction zone lies directly beneath most of the population of the state so creates a major natural hazard. At the same time this eastern end is geologically complex, making the pathways of fluid difficult to understand and complicating the underlying theories of volcanism. This project aims to significantly advance our understanding of the nature of the mantle in this complex transition, using a revolutionary new data set. Over the last several years a vast amount of high-quality earthquake signals has been collected from new seismometers in Alaska - the entire state has been covered since 2015-16 by the EarthScope Transportable Array that places state-of-the-art instrumentation every 85 km, accompanied by a number of smaller, dense deployments over areas of interest. All these projects ensonify with earthquake signals the interior of the planet in this highly active and complex region. This specific project aims to capitalize on these data to address and test several hypotheses that will help better understand the ways in which large volcanoes form and more generally the variations in temperature of the Earth's mantle. The results will provide a framework for interpreting rocks that come from similar environments in the geologic record. This project focuses on three generic hypotheses regarding geodynamic process in subduction zones: 1) Variability in fluid release from subducting plates correlates with variability in the degree of melting in the overlying mantle wedge - tested through a variety of seismic proxies. 2) Rock fabric as revealed by seismic anisotropy is controlled by distance from the edge of the slab as expected if three-dimensional flow controls it. 3) The depth at which the mantle wedge transitions from cold forearc to hot subarc is globally constant. Measurements of seismic attenuation at mantle depths provides a proxy for temperature in all of these regions, and local-earthquake shear-wave splitting will complement new teleseismic (SKS) splitting measurements to infer anisotropy. Parallel observations of seismicity and high-frequency phases that interact with the slab surface then allow inferences about the mantle wedge to be compared with slab dehydration. High-frequency wavefield simulations of split shear waves will assess the maximum depth of a supra-slab anisotropic slow layer, a probable signature of slab-mantle coupling depth. At the same time, petrologically-driven models provide a framework for making predictions that test each hypothesis. These hypotheses will be tested via comparison of three distinct corridors within Alaska for which EarthScope and related projects provide unusually good sampling: (a) the Cook Inlet corridor where normal Pacific lithosphere subducts and the arc is robust; (b) the nearly amagmatic Denali corridor where the Yakutat oceanic plateau subducts and generates intermediate-depth earthquakes; and (c) the Wrangell Volcanic Field corridor where slab seismicity is nearly absent but there is very high volume volcanism. These comparisons take advantage of Alaska Transportable Array combined with several dense portable broadband experiments (BEAAR, SALMON, MOOS, WVLF), previous projects conducted by the PIs and which sample each of these corridors. This project addresses EarthScope science objectives and emphasizes interdisciplinary work at the interface between petrology, seismology, and geodynamics. It leverages education and outreach opportunities through the EarthScope National Office, notably those available through the EarthScope website and social media. All project participants - including graduate students supported at two institutions- will work with the EarthScope National Office to maximize scientific outreach of the project. The project will generate improved predictions of amplitudes of seismic waves in south-central Alaska, including within the Anchorage metropolitan region; therefore the project can contribute toward seismic hazard assessments and ground motion prediction.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.