Seismic anisotropy in the legion of the Chile margin triple junction

We have measured shear wave splitting at three temporary three-component sh ort period stations that were deployed in southern Chile above the subducte d Chile Rise spreading centre (Taitao Peninsula and environs). Subduction o f 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 li thosphere segments of the spreading ridge, through which mantle might flow. Our preliminary results include two consistent fast polarization direction s 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 st ations: near the coast, fast axes are parallel to the spreading ridge segme nts of the Chile Rise (approximately N-trending). This splitting fast-axis direction probably reflects either along-axis asthenospheric flow or result s from the preferential attenuation effects of aligned pockets of melt at t he subducted ridge segment. At one inland station above the slab window, we find two splitting fast-axis directions, one parallel to the subducted Chi le Rise ridge segments, and a second trending NW-SE. We infer that upper ma ntle deformation in the vicinity of a well developed slab window is complic ated and probably involves two superposed directions of upper mantle deform ation. One of these directions (NW-SE) may indicate anomalous flow of asthe nospheric mantle in the vicinity of the slab window gap. (C) 1999 Elsevier Science Ltd. All rights reserved.