Tectonic plate motion is thought to cause solid-state plastic flow within the underlying upper mantle and accordingly lead to the development of a lattice preferred orientation of the constituent olivine crystals1,2,3. The mechanical anisotropy that results from such preferred orientation typically produces a direction of maximum seismic wave velocity parallel to the plate motion direction4,5. This has been explained by the existence of an olivine preferred orientation with an ‘a-axis’ maximum parallel to the induced mantle flow direction3,5,6,7,8. In subduction zones, however, the olivine a axes have been inferred to be arranged roughly perpendicular to plate motion9,10,11,12,13, which has usually been ascribed to localized complex mantle flow patterns10,11,12,13. Recent experimental work14 suggests an alternative explanation: under conditions of high water activity, a ‘B-type’ olivine preferred orientation may form, with the a-axis maximum perpendicular to the flow direction. Natural examples of such B-type preferred orientation are, however, almost entirely unknown. Here we document widespread B-type olivine preferred orientation patterns from a subduction-type metamorphic belt in southwest Japan and show that these patterns developed in the presence of water. Our discovery implies that mantle flow above subduction zones may be much simpler than has generally been thought.
