THE SEISMIC STRUCTURE OF MADAGASCAR: A SYNTHESIS OF STUDIES USING BODY AND SURFACE WAVES

Data from the MACOMO (MAdagascar COmoros MOzambique) seismic experiment are used to examine the structure of the crust, lithosphere, and asthenosphere beneath Madagascar. The experiment deployed 31 broadband stations across Madagascar and Mozambique for up to two years. Seismic data are supplemented with those collected by the RHUM-RUM (Réunion Hotspot and Upper Mantle - Réunions Unterer Mantel) land stations and Madagascar Seismic Profile experiments that were deployed at the same time as MACOMO, as well as four permanent GSN, GEOSCOPE and GEOFON stations. Combined receiver functions and Rayleigh wave dispersion curves provide the first island-wide map of crustal thickness, which varies from 45-km-thick beneath the spine of the north-south trending central mountain range, to less than 20 km beneath the western basins. The crystalline continental crust is stretched as thin as 12 km here, a result of rifting from mainland Africa, and covered by basin sediments up to 9 km thick. The surface wave tomography was carried out using several different methods that include ambient noise tomography, 2-plane-wave tomography, and the Automated Surface Wave Phase Velocity Measuring System of Ge et al. [2014]. The body wave tomography was carried out using the adaptive-stacking method of Rawlinson and Kennett [2004]. Combined, they present good resolution of sub-crustal structures extending several hundred km beneath Madagascar, and most strikingly identify three upper mantle low-velocity regions that underlie three regions of Cenozoic intraplate volcanism. The larger two low-velocity anomalies, beneath the central and northern regions of Madagascar, appear to be largely independent in shallow imaging, but deeper body-wave imaging suggests a connection at depth. Likewise, the subsurface connection between the central and southern intraplate volcanic regions agrees with age and isotopic data, which suggest a similar source. Various possible interpretations for these observations will be discussed.