Shear-wave splitting in the Mariana trough—a relation between back-arc spreading and mantle flow?

A shear-wave splitting analysis was performed in the Mariana trench on data recorded by an array of semi broad-band Ocean Bottom Seismometers (OBS), with the aim of studying the Mariana back-arc spreading system and clarifying the nature of the back-arc process. We look for possible mechanisms relating anisotropy and mantle flow. Direct sampling of ray paths through the slab suggests that anisotropy in the slab is relatively small compared with that of the mantle wedge. Fast polarizations at most OBS are consistent with the direction of relative motion between the subducting Pacific and overriding Philippine plates. Exceptions are seismic stations located on the ridge spreading axis which show fast polarizations perpendicular to the ridge curvature. From simple synthetic modeling we estimate an upper limit for the depth of the layer of mantle flow due to the spreading ridge, but as the data show, its influence wanes quickly with distance from the axis. These results suggest that the “sea-anchor” or “extrusion” models are the most favorable scenarios of back-arc opening among other geodynamic models.