Seismic anisotropy of the lithosphere and asthenosphere beneath southern Madagascar from teleseismic shear wave splitting analysis and waveform modeling

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.

Numerical Modeling of Mantle Flow Beneath Madagascar to Constrain Upper Mantle Rheology Beneath Continental Regions

“…Another potential source of the seismic anisotropy in southern Madagascar is fossil anisotropy in the entire lithosphere as suggested by Reiss et al (). In the south central region and part of southeastern region, the geology is characterized by the Ranotsara shear zone that was active during the East African Orogeny 500 Ma (e.g., de Wit et al, ).…”

“…Comparison of synthetic splitting (white bars) derived from lithosphere‐mantle wind interactions models with SKS splitting measurement bars colored according to angular misfit (0–90°; Ramirez et al, ; Reiss et al, ) (a) for Behn et al () global mantle flow and (b) for (Forte et al, ) global mantle flow. Gray wedges represent confidence interval associated with the synthetic splitting.…”

“…Reiss et al () produced the first SKS splitting observations along the SELASOMA seismic profile (SEismological signatures in the Lithosphere/Asthenosphere System Of Southern Madagascar) that spans southern Madagascar in a NE‐SW orientation, while Ramirez et al () provided additional SKS splitting measurements across the entire continental island (Figure ). Azimuthal seismic anisotropy across Madagascar is quite complex.…”

“…Along the SELASOMA profile, anisotropy varies from a NW‐SE direction in the center of the profile to a SW‐NE direction along its eastern and western sections (Figure , yellow bars). Reiss et al () interpret seismic anisotropy in the central part of the profile as sourced from the lithosphere in association with Precambrian ductile deformation at an existing narrow Ranotsara shear zone (see Figure ) that strikes approximately NW‐SE. In contrast, they attribute the SW‐NE oriented observations to asthenospheric flow induced by absolute plate motion.…”

The structure of the crust and uppermost mantle beneath Madagascar

“…In fact, they found that the central part of Madagascar is thinner (∼39 km) than the eastern coast (∼44 km). Additional information on crustal structure comes from a recent study of SKS splitting along the SELASOMA profile showing strong crustal anisotropy of up to 12 per cent in a 150 km broad zone located along the shear zones in southern Madagascar (Reiss et al 2016).…”