Abstract

The development of crystal preferred orientation (CPO) of antigorite causes a strong mechanical anisotropy to develop in antigorite-rich serpentinite and hence to the hydrous wedge mantle in subduction zones where it is thought to be widely distributed. Several different types of antigorite CPO are known associated with distinct anisotropies, but the controls on the formation of different antigorite CPOs are not well known. Electron backscat-ered diffraction (EBSD) mapping of shear zone domains within antigorite serpentinite reveals several progressive microstructural changes with increasing strain: i) rotation of the antigorite c-axes towards the normal to the shear zone; ii) development of an antigorite CPO within the shear zone with the b-axes parallel to the extension direc-tion; and iii) rotation and increase in the strength of the antigorite shape-preferred orientation (SPO) consistent with the CPO. The similar grain-size and grain-shape distributions that are found within and outside of the shear zone irrespective of the degree of the strain suggest that the contribution of internal plastic deformation of individual grains to the microstructural changes is considered negligible, at least as a primary mechanism. The changes in ori-entation are consistent with theoretical models for passive rotation of elongated rigid grains. These features suggest that the studied antigorite CPO in the naturally deformed antigorite-rich serpentinite is formed mainly by mechanical rotation of elongate grains assisted by grain boundary sliding. The hydrated wedge mantle with this type of antigorite CPO suggests aseismic slip along the subduction interface.