Collaborative Research: Fluid infiltration of the continental crust during Laramide flat-slab subduction: a unique tectonic setting

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This study investigates an ancient example of a continental margin setting that involves shallow-angle or flat-slab subduction, that is, where an oceanic plate dives beneath a continental margin at a shallow angle. Rocks from an ancient example, which was active 80 to 40 million years ago, are preserved in southern California and Arizona. Comparison of the ancient example with mathematical simulations of this setting shows that they do not accurately simulate important aspects. For example, studies have shown that rocks at the top of the oceanic plate got much hotter than predicted by the simulations, and that above the oceanic plate, where cooling is predicted, some rocks melted. The investigators will collect data from rocks in the ancient setting that were once located just above the descending oceanic plate. This will lead to improved simulations and a better understanding of similar modern tectonic settings, many of which pose significant seismic risk to human populations. The project supports the participation of undergraduates in the research at Northern Arizona University and the University of Wisconsin at Madison. In addition, the project supports four early career PhDs, three of whom are members of an underrepresented group. This project will investigate the conditions, timing, and fluid sources of deep-crustal fluid infiltration during Laramide flat-slab subduction in southeastern California (Big Maria Mountains) and western Arizona (Cemetery Ridge). Cemetery Ridge exposes metamorphosed oceanic sediments (Orocopia Schist) containing blocks of mantle peridotite transported ~300 km inland by Laramide low-angle subduction. The Big Maria Mountains exposes suprasubduction crustal rocks, also inferred to have been located ~300 km inboard of the paleotrench. Both areas display strong evidence for Laramide fluid-driven metamorphism and metasomatism. Chemical changes associated with metasomatism will be evaluated by comparing the bulk-rock chemistry of metasomatised rocks to unmetamorphosed protoliths. The pressure-temperature conditions of metasomatism will be investigated using Raman spectroscopy of carbonaceous material thermometry and quartz in garnet elastic barometry and thermodynamic modeling. The timing of fluid infiltration will be determined by laser ablation split stream mass spectrometry of titanite, monazite, and zircon. The source of fluids will be evaluated using stable isotopes of oxygen and boron. The new data will provide a basis for constraining model parameters needed for mathematical simulation of the system for which independent estimates are unavailable (frictional coefficient, subduction angle, suprasubduction viscous heating, and critical temperature for thermal weakening within the subduction channel). The project will achieve broader impacts through efforts that target the development of a diverse workforce in the geosciences and increasing public scientific literacy. Presentations of the research to the public will be made by undergraduate participants at research symposia hosted at both participating institutions. In addition, the project supports career advancement for four early career PhDs, three of whom are members of an underrepresented group.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.