Seismic signatures of a hydrated mantle wedge from antigorite crystal-preferred orientation (CPO)

We report electron backscatter diffraction (EBSD) measurements of antigorite crystal preferred orientations (CPOs) from 7 samples of antigorite schist from serpentinite mélanges adjacent to the Motagua fault system in central Guatemala. The CPOs range from diffuse girdle to point distributions of (001)atg. Girdle distributions of (001)atg are not expected from deformation theory or experiments, suggesting that they are a result of growth rather than deformation, and may thus be topotactically related to the CPO of the olivine from which the antigorite grew. The calculated seismic anisotropy ranges from 6% to 28% in VP, and 5% to 33% in VS, and is highest for samples with a point maximum of (001)atg. For all samples the minimum VP corresponds to the pole to (001)atg, and the maximum VP occurs within the foliation for samples with a clearly defined foliation. Trench-parallel shear-wave splitting observations for subduction zones can best be explained by a combination of olivine B-type CPO and antigorite oriented with (001) parallel to the foliation; only a relatively thin (~20 km) zone of hydrated mantle is required to explain the observed splitting times.