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Abstract
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 crystals. The mechanical anisotropy that results from such
preferred orientation typically produces a direction of maximum seismic wave velocity
parallel to the plate motion direction. This has been explained by the existence of
an olivine preferred orientation with an 'a-axis' maximum parallel to the induced
mantle flow direction. In subduction zones, however, the olivine a axes have been
inferred to be arranged roughly perpendicular to plate motion, which has usually been
ascribed to localized complex mantle flow patterns. Recent experimental work 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.