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Abstract:
Madagascar occupies a key position in the assembly and breakup of the supercontinent
Gondwana. It has been used in numerous geological studies to reconstruct its original position within
Gondwana and to derive plate kinematics. Seismological observations in Madagascar to date have been
sparse. Using a temporary, dense seismic profile across southern Madagascar, we present the first published
study of seismic anisotropy from shear wave splitting analyses of teleseismic phases. The splitting parameters
obtained show significant small-scale variation of fast polarization directions and delay times across the
profile, with fast polarization rotating from NW in the center to NE in the east and west of the profile. The
delay times range between 0.4 and 1.5 s. A joint inversion of waveforms at each station is applied to derive
hypothetical one-layer splitting parameters. We use finite-difference, full-waveform modeling to test several
hypotheses about the origin and extent of seismic anisotropy. Our observations can be explained by
asthenospheric anisotropy with a fast polarization direction of 50°, approximately parallel to the absolute
plate motion direction, in combination with blocks of crustal anisotropy. Predictions of seismic anisotropy as
inferred from global mantle flow models or global anisotropic surface wave tomography are not in
agreement with the observations. Small-scale variations of splitting parameters require significant crustal
anisotropy. Considering the complex geology of Madagascar, we interpret the change in fast-axis directions
as a ~150 km wide zone of ductile deformation in the crust as a result of the intense reworking of lithospheric
material during the Pan-African orogeny. This fossil anisotropic pattern is underlain by asthenospheric
anisotropy induced by plate motion.
