Poster Presentation:  AGU 2021 Fall Meeting, Thursday, December 16, 4-6 p.m.
Abstract:
Seismic anisotropy of mafic blueschists: constraints from exhumed glaucophane-
rich blueschists with implications for the subduction interface

Authors: Jason Ott, Cailey Condit, and Vera Schulte-Pelkum

     Subduction zones generate some of Earth’s most significant geological hazards including megathrust earthquakes, tsunami, and arc volcanism. The interconnected mechanical and chemical processes controlling the dynamics of these complex systems remain poorly understood in part because we have limited exhumed subduction exposures to provide geologic constraints on remote geophysical observations. One such geophysical tool, seismic anisotropy, informs our understanding of subduction zone structure and can link deep processes to their expression as hazards at the surface. Here we provide essential geologic constraints from the rock record to better interpret observations of slab seismic anisotropy. Mafic blueschists, the glaucophane-rich product of high-P/low-T metamorphism of hydrated oceanic crust, occur along the slab top between the seismogenic zone and the sub-arc and can develop significant seismic anisotropy. This observed anisotropy is likely produced by crystallographic preferred orientations (CPO) of glaucophane and white mica formed during ductile subduction deformation. We present results from a suite of electron backscatter diffraction (EBSD)-based seismic anisotropy values from an exhumed global blueschist collection to illustrate the bounds of the range of seismic anisotropy signatures along the deforming plate interface at blueschist facies conditions. Our suite of samples exhibit variable mineralogies and represent diverse P-T conditions of metamorphism and deformation, together spanning much of the blueschist facies stability field. EBSD- constrained modal mineralogies and CPO strength together with the elastic properties of each phase are used to calculate the bulk elastic properties and resultant seismic anisotropy (in Vp and Vs) of each sample. We present trends in calculated seismic anisotropy relating to the varying phase volumes and CPO strength. This compilation improves the interpretation and construction of seismic models of the subducting oceanic lithosphere in subduction zones, increases accuracy in imaging of blueschist-metamorphosed oceanic crust, and may better constrain the depth of the blueschist-eclogite transition in individual subduction zones.

Final Poster

DOI: 10.5281/ZENODO.5789490

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