Sodic Amphibole Deformation Mechanisms: Comparing the Condrey Mountain Schist Rock Record with Experimental Data
Carolyn Tewksbury-Christle, Fort Lewis College
Whitney Behr, ETH Zurich
Abstract
Subduction interface rheology controls the spectrum of deformation observed in modern subduction zones, including complex seismic and aseismic slip. Slow slip and tremor (SST) events occur at 25-55 km deep in modern subduction zones, coincident with blueschist-facies metamorphism. Blueschist rheology may therefore play an important role in SST source mechanics, but characterizing sodic amphibole deformation mechanisms relies on both experimental and rock record evidence. Subduction complexes exhumed from SST depths record ductile deformation of mafic blueschists accommodated by sodic amphibole, and recent experiments on sodic amphibole deformation defined flow laws for both diffusion and dislocation creep. We examined deformation mechanisms in mafic blueschists from the Condrey Mountain Schist (CMS), an exhumed subduction complex in Northern California, using field structural mapping, microstructural analysis, and Electron Backscatter Diffraction (EBSD) analysis. The CMS provides a snapshot in P-T-X space to evaluate experimental flow laws.
The CMS blueschist-facies unit consists primarily of metasedimentary rocks with broadly folded m- to km-scale lenses of mafic blueschist and metaserpentinites. These lithologies subducted along the same prograde path to 460°C and ca. 35-40 km, overlapping with SST depths. Microstructures and crystallographic preferred orientations (CPOs) from EBSD data vary amongst sodic amphibole-bearing lithologies within the mafic blueschists. In nearly-pure sodic amphibole lithologies, amphiboles have boudinaged cores surrounded by overgrowths with distinctly different pleochroism and small fibrous crystals at mineral tips and have an L-type CPO (aligned c-axes; girdled a- and b-axes). These data are potentially consistent with dissolution precipitation creep (DPC), with the CPO resulting from the mineral habit. In epidote blueschists, sodic amphibole microstructures are characterized by small acicular grains amongst larger grains with sweeping undulose extinction and blocky tips and CPOs are SL-type (aligned a-, b-, and c-axes). These core-and-mantle microstructures and SL-type CPOs are potentially consistent with dislocation creep.
The CMS rock record suggests multiple sodic amphibole deformation mechanisms, depending on the presence of other phases. DPC microstructures in nearly pure sodic-amphibole layers from the CMS are similar to experimental diffusion creep microstructures. Those experiments, however, used CMS epidote blueschist as a starting material, which has dislocation creep microstructures in the rock record. These differences may be due to the presence of other phases or fluids, strain history, or potential small-scale variations in amphibole composition. Because diffusion and dislocation creep have different transient rheologies, resolving the controls on deformation mechanisms is key to modeling and understanding modern SST processes.
Session
Deformation of mafic or ultramafic rocks