Investigation of fluids and hydrated mantle in subduction zones, with a focus on Cascadia

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Earth's most damaging earthquakes and volcanic eruptions occur along subduction-zone plate boundaries where oceanic lithosphere descends into the mantle. For example, the magnitude (M) 9.0 subduction-zone earthquake that struck Tohoku, Japan on March 11, 2011 resulted in the loss of >19,000 lives and estimated damages of US$235B. The Cascadia subduction zone extends for >1,000 km along the west coast of North America from Vancouver Island to northern California. Rupture of the Cascadia plate boundary has resulted in great M9 earthquakes every ~500 years, with the last one occurring on January 26, 1700 AD. Damaging Cascadia earthquakes also occur within the subducting Juan de Fuca and overriding North American plates.     Subduction-zone earthquakes are linked to the generation, transport, and resorption of water-rich fluids. Understanding the mechanical behaviour of the plate boundary, and therefore the seismic potential, requires understanding the distribution of hydrous minerals and fluids along the boundary. Recent work has demonstrated that the distribution of earthquakes in the overriding plate is linked to the permeability of the plate boundary and the extent to which fluids generated at depth escape into the overlying crust. At deeper levels, fluids released from the subducting plate hydrate the overlying mantle forming serpentine, talc, and other weak minerals.     My five-year program of research supports my long-term objective to better understand subduction zones and leverages my previous modelling efforts. My short-term objectives focus on subduction-zone fluids that infiltrate the mantle wedge. Specifically, I propose to (1) construct mineralogical maps of the mantle wedge as a function of bulk composition and H2O content, (2) conduct field-based petrologic studies to constrain the spatial scale(s) of H2O equilibrium in hydrated mantle rocks, and (3) develop an integrated model of fluid flow in the Cascadia subduction zone as illuminated by seismicity. The mineralogical maps will be constructed by combining numerical heat transfer and thermodynamic models, integrating the results with seismological observations. Field work will focus on ultramafic rocks exposed in British Columbia and Washington state.     I propose to train a diverse HQP group consisting of three graduate students (1 PhD and 2 MSc) and one undergraduate student using equitable and inclusive recruitment and mentoring practices. Research training in thermal-petrologic modelling techniques and metamorphic petrology will provide a strong foundation for these students to pursue academic and technical careers.     The results of the proposed program of research will further our understanding of subduction zone fluids and earthquakes, potentially opening up new avenues to improve long-term forecasting of seismic hazards. Research results will be presented at international geoscience meetings and workshops, and published in high-quality peer-reviewed journals.