Microstructural investigations along a blueschist facies paleomegathrust: Implications for deformation mechanisms for deep slow slip behavior

The subduction interface is characterized by downdip changes in seismic and aseismic behavior that is controlled by metamorphic reactions, deformation conditions, rheological properties, and fluid sources and pathways. Recent seismological evidence shows that a mixture of brittle-ductile deformation at the downdip limit of the seismogenic zone is responsible for producing slow slip and very low-frequency earthquakes. One way to address the possible mechanics and causes behind this dual deformation behavior is by observing exhumed rocks that provide us with the opportunity to make direct geological observations for microstructure and deformation mechanism analysis. Our project aims to understand the deformation mechanism hosted in the dominant mineral phases of metacherts and blueschist from an exhumed paleomegathrust in the Franciscan Subduction Complex. 

The Mesozoic Franciscan Subduction Complex consists mostly of underplated clastic sediment-rich terranes that are metamorphosed from prehnite-pumpellyite to blueschist facies. Angel Island, in the San Francisco Bay, is composed of blueschist facies metasedimentary and metabasic rocks containing potential paleo-megathrust faults that correlate to the source depths of slow slip earthquakes. Subduction-related faults and shear zones crop out in sea cliffs around the perimeter of the island and enable us to observe deformation features in characteristic subduction zone lithologies. We focus on a proposed subduction-related shear zone in blueschist-facies metachert juxtaposed against mafic/ultramafic rocks. Centimetric-scale field maps show coeval brittle-ductile features such as mutually cross-cutting extension fractures, kink folds, veins, and mylonitic foliations hosted in a wide variety of rheologically distinct lithologies. These lithologies contain synkinematic sodic amphibole and stilpnomelane demonstrating that this fault was active at blueschist facies. The main deforming mineral phases are sodic amphiboles, phyllosilicates, and quartz. The structural fabric comprises of shear fabrics, kink folding, dismembered veins, flattened grains, solution seams, and compositional layering. Amphiboles and phyllosilicates define the foliation of these rocks whereas quartz exhibits evidence of dislocation creep. Using EBSD-generated phase maps, we try to determine the deformation mechanisms hosted within the metacherts and blueschists along the paleomegathrust. Paleostress during megathrust creep will be constrained using EBSD-generated grain size statistics. Our study highlights the complexity of deformation within these lithologies while presenting evidence for possible deformation mechanisms witnessed at slow slip depths.