Seismic anisotropy in the region of the Chile margin triple junction

We have measured shear wave splitting at three temporary three-component short period stations that were deployed in southern Chile above the subducted Chile Rise spreading centre (Taitao Peninsula and environs). Subduction of the Chile Rise has been occurring beneath South America for at least the past 14 m.y. Previously published models of the ridge subduction posit the existence of ‘slab windows’, asthenosphere-filled gaps between subducted lithosphere segments of the spreading ridge, through which mantle might flow. Our preliminary results include two consistent fast polarization directions of splitting in the study region. Delay times between fast and slow split shear waves average around 1.0 s for all phases (ScS, PcS, SKS, and SKKS) that we measured. Fast-axis azimuths vary systematically among the three stations: near the coast, fast axes are parallel to the spreading ridge segments of the Chile Rise (approximately N-trending). This splitting fast-axis direction probably reflects either along-axis asthenospheric flow or results from the preferential attenuation effects of aligned pockets of melt at the subducted ridge segment. At one inland station above the slab window, we find two splitting fast-axis directions, one parallel to the subducted Chile Rise ridge segments, and a second trending NW–SE. We infer that upper mantle deformation in the vicinity of a well developed slab window is complicated and probably involves two superposed directions of upper mantle deformation. One of these directions (NW–SE) may indicate anomalous flow of asthenospheric mantle in the vicinity of the slab window gap.