Mantle upwelling beneath Madagascar: evidence from receiver function analysis and shear wave splitting
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Crustal receiver functions have been calculated from 128 events for two three-component broadband seismomenters located on the south coast (FOMA) and in the central High Plateaux (ABPO) of Madagascar. For each station, crustal thickness and V p /V s ratio were estimated from H- κ plots. Self-consistent receiver functions from a smaller back-azimuthal range were then selected, stacked and inverted to determine shear wave velocity structure as a function of depth. These results were corroborated by guided forward modeling and by Monte Carlo error analysis. The crust is found to be thinner (39 ± 0.7 km) beneath the highland center of Madagascar compared to the coast (44 ± 1.6 km), which is the opposite of what would be expected for crustal isostasy, suggesting that present-day long wavelength topography is maintained, at least in part, dynamically. This inference of dynamic support is corroborated by shear wave splitting analyses at the same stations, which produce an overwhelming majority of null results (>96 %), as expected for vertical mantle flow or asthenospheric upwelling beneath the island. These findings suggest a sub-plate origin for dynamic support.
KeywordsCrustal Thickness Moho Depth Shear Wave Splitting Mantle Upwelling Lattice Prefer Orientation
This research has benefited from discussions with A. Gilligan, G. Roberts, N. White and M. Davis, and from technical support from I. Frame. JDP is supported by BP Exploration. The comments of two anonymous reviewers greatly strengthened the manuscript. Figures were prepared using Inkscape and GMT4.2.0.
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