Complex seismic anisotropy in Madagascar revealed by shear-wave splitting measurements

Shear wave fast polarization directions (Phi) and delay times (delta t) obtained from SKS and SKKS splitting measurements are reported for 25 temporary and 2 permanent seismic stations distributed throughout Madagascar. Results show a complicated pattern of Phi that is not easily explained by a single source of anisotropy, such as fossil anisotropy in the lithosphere or mantle flow from the African superplume or absolute plate motion. To interpret the results, we divide the island into four areas. The northern part of the island is characterized by an similar to ENE-WSW Phi pattern, which can be attributed to NE-oriented mantle flow from the African superplume or plate motion modified by SE-oriented mantle flow from the Comoros hotspot. In the centre of the island, a roughly circular Phi pattern is observed and can be attributed to upwelling asthenosphere, caused by lithospheric delamination, interacting with mantle flow from the superplume or plate motion. Small-scale mantle flow can also be invoked to explain a separate semi-circular Phi pattern in the southwestern part of the island. In the south-central and southeastern parts of the island, Phi is parallel to the NW strike of shear zones at many stations and can be attributed to fossil anisotropy in the lithosphere. However, NE-oriented fast polarization directions along the northern edge of this area do not align with the orientation of shear zones and could instead be part of the circular Phi pattern that characterizes the central part of the island.