Teleseismic Body Wave Analysis of the Madagascan Asthenosphere, and the Relationship to Intraplate Volcanism

Thursday, 17 December 2015
Poster Hall (Moscone South)
Martin J Pratt1, Michael Edward Wysession2, Ghassan I Aleqabi1, Douglas Wiens1, Andrew Nyblade3, Patrick Shore4, Gérard Rambolamanana5, Rakotondraibe Tsiriandrimanana5 and Fenitra Sy Tanjona Andriampenomanana Ny Ony5, (1)Washington University in St Louis, Department of Earth and Planetary Sciences, St. Louis, MO, United States, (2)Washington Univ, Saint Louis, MO, United States, (3)Penn St Univ, University Park, PA, United States, (4)Washington University in St Louis, St. Louis, MO, United States, (5)Institute and Observatory of Geophysics Antananarivo-University of Antananarivo, Physics, Antananarivo, Madagascar
Recent intraplate volcanism in Madagascar (since 1 Ma) is currently unexplored through broadband seismic methods. The Madagascar continental crust fragment is located between the hotspots of Réunion and Comoros may allow for a possible deep source. However, it remains unclear what the origin may be or what pathway the rising asthenosphere may take. Geochemical analysis of recent basalts [e.g. Bardintzeff et al., 2010] appears to suggest that melting may only occur in the lower lithospheric mantle, although the enriched, alkaline volcanism in the north shows some similarities to a Comoros hotspot source with a continental crust influence. The stress regime of Madagascar is E–W extensional, so reactivated NNW–SSE oriented faults, remnant from Madagascar’s split from Africa, may provide potential pathways for rising magma. The MACOMO (MAdagascar COmoros MOzambique) seismic experiment deployed 25 broadband stations across the Madagascar and 6 stations in 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, 7 stations from the Madagascar Seismic Profile experiment, both deployed at the same time as MACOMO, as well as 4 permanent GSN, GEOSCOPE and GEOFON stations. Using the adaptive stacking method of Rawlinson and Kennett [2004], we analyze relative travel times of teleseismic P- and S-waves to generate the first tomographic models of the mantle beneath Madagascar. We combine the models with shallow structure information gained from surface wave and ambient noise tomography that shows low velocity zones beneath the central (Itasy/Ankaratra) and northern (Nosy Bé/Massif d’Ambre) volcanic regions extending to depths of at least 150 km. Preliminary P-wave finite-frequency tomography results (using the method of Schmandt and Humphreys, [2010]) from 181 events suggests that the central low velocity zone may extend even deeper into the asthenospheric mantle.