HR: 17:00h
AN: V34A-05    [Abstracts]
TI: Seismic Expression of Flood-Basalt Lithospheric Refertilization
AU: * Savage, B
EM: savage@uri.edu
AF: University of Rhode Island, 317 Woodward Hall 9 East Alumni Ave, Kingston, RI 02881, United States
AU: Silver, P G
EM: silver@dtm.ciw.edu
AF: Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Rd, Washington, DC 20015, United States
AB: Analysis of S and P receiver functions from the Southern African Seismic experiment (SASE) for lithospheric discontinuities beneath the Kalahari craton reveals an abrupt reduction in seismic velocity (4%) at approximately 150 km depth, which we term the K-discontinuity. This feature is restricted to the northern half of the array, extending from the Zimbabwe craton south to the Thabazimbi-Murchison lineament, and from Botswana in the west to the edge of the Kalahari craton in the east. It thus spans several Archean sutures and is thus unlikely to be related to Archean tectonics. It does, however, appear to be related to subsequent magmatic episodes. In particular, the strongest expression of the anomaly is coincident with the most intense locale of volcanism associated with the Mesozoic Karoo flood basalt (at the intersection of 4 giant dyke swarms), and it extends to the northern edge of the Bushveld intrusion. From mantle xenoliths and xenocrysts, the lithosphere in this region appears to have experienced a long-term infiltration of basaltic melt and metasomatic fluids. We propose that this seismic discontinuity reflects the influence of this melt/metasomatic infiltration, which, over time, has intruded and refertilized the Archean Lithosphere. Based on kimberlites pipes that show obvious signs of melt metasomatism and likely Karoo influence, such as Lesotho, the observed reduction in seismic velocity is plausibly consistent with the observed major-element and trace-element enrichment at about 150 km depth in this kimberlite. We speculate that this depth represents a permeability barrier to the porous flow of basaltic magma and corresponds to the intersection of the volatile-rich basalt liquidus with the lithospheric geotherm.
DE: 1025 Composition of the mantle
DE: 3619 Magma genesis and partial melting (1037)
DE: 7203 Body waves
DE: 7218 Lithosphere (1236)
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting