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.