Collaborative Research: A Geochemical Approach to Quantifying the Magnitude of Strain and Fluid Flow along the Subduction Interface

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Earth’s largest earthquakes occur along the megathrusts (the subduction plate interfaces) that separate rigid tectonic plates in subduction zones. The accumulation and release of elastic strain along these plate interfaces is predicated on the “healing” of the interface after the large earthquakes. This project will characterize the chemical and mechanical processes that control this healing during the time between earthquakes. Observations of ancient subduction interfaces, now exposed on land, will be used to inform a numerical model for slip and fluid flow. Together, these approaches will allow the investigators to determine how subduction zone processes impact the spatial and temporal patterns of earthquakes—an important step in assessing the hazards associated with active subduction zones. The study includes support for an early-career faculty member, a Ph.D. student, and nine undergraduates. Six of the undergraduates will be recruited from University of the Incarnate Word, an undergraduate body that serves a local Hispanic community.This project comprises a structural and geochemical investigation of two exhumed terrains that preserve a record of the physical and chemical conditions relevant to the plate interface of the global array of active subduction zones. Combined, the Shimanto belt in Japan and the Kodiak Accretionary complex in Alaska, form a composite record spanning depths from 8 to 20 km and peak temperatures between 150 and 350˚C. Electron-microscope-based elemental mapping of samples collected during several field campaigns will enable quantification of mineral phases and their redistribution with progressive strain. Combined cathodoluminescence mapping, electron backscatter diffraction, and fluid-inclusion microthermometry will constrain paleotemperatures and finite strain, and identify deformation mechanisms. Quantitative estimates of strain rate will be derived by combining strain magnitudes with field observations of shear zone thickness and present-day estimates of plate convergence rate. These measurements and observations will be used to inform a numerical model that parameterizes the effects of seismic slip, fluid flow and interseismic restrengthening on the temporal and spatial distribution of earthquakes on the plate interface. The multidisciplinary workplan is designed to address the following fundamental questions: Q1) What are the magnitudes of volume strain and shear strain associated with interseismic deformation? Q2) Is there a balance between dissolution along the scaly fabric and precipitation of minerals within cracks in blocks? Q3) What are the relative contributions to quartz vein formation of local diffusive mass transfer of silica versus the influx of dissolved silica by an externally derived fluid? Q4) How does partitioning of deformation mechanisms vary from the updip to the downdip ends of the seismogenic zone?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.