HR: 0800h
AN: V31B-1437    [Abstracts]
TI: Evidence for Topographic Swells Beneath Gravity Lineations in the South Pacific.
AU: * Harmon, N
EM: Nicholas_Harmon@brown.edu
AF: Brown University, Box 1846, Providence, RI 02912 United States
AU: Forsyth, D W
EM: Donald_Forsyth@brown.edu
AF: Brown University, Box 1846, Providence, RI 02912 United States
AU: Scheirer, D
EM: Daniel_Scheirer@brown.edu
AF: Brown University, Box 1846, Providence, RI 02912 United States
AB: The Gravity Lineations Intraplate Melting Petrologic and Seismologic Expedition (GLIMPSE) experiment is investigating the origins of the gravity lineations in the South Pacific and their relationship to intraplate volcanism at the Sojourn and Hotu Matua ridge systems. Using the satellite free air gravity anomaly and high quality shipboard bathymetry in the GLIMPSE study region, we have calculated the Mantle Bouguer Anomaly (MBA) and the residual Mantle Bouguer Anomaly (rMBA). In the MBA, we see strong negative anomalies (~20-40 mGals) beneath the Sojourn and Brown Ridges as well as beneath the Hotu Matua volcanic complex, indicating low-density material beneath the ridge systems in the form of thickened crust or lower density mantle material. We subtract the effects of surface loading of an elastic plate by the volcanic ridges to find the residual topography and the residual MBA. The residual seafloor topography shows a systematic ~200 km wavelength undulation in the seafloor with a strike roughly parallel to the spreading direction, producing residual bathymetric highs of 100-300 m beneath the Sojourn and Hotu Matua ridge systems. There are broad (100 km wide) negative anomaly bands in the rMBA of ~10-40 mGals beneath the entire length of the Sojourn Ridge to the East Pacific Rise and discontinuously from west of Hotu Matua to the EPR. Similar features are found in the extension of the Puka Puka ridge into the Rano Rahi seamount field. The swells beneath the intraplate volcanism and the gravity lineations require a contribution from subsurface positively buoyant low-density material. The seismic refraction and Rayleigh wave dispersion experiments corroborate the existence of anomalies at depth. The swells contradict lithospheric cracking models of the formation of the gravity lineations that predict the gravity lineations should be located over bathymetric troughs, and favor dynamic models such as small-scale convection.
DE: 8434 Magma migration
DE: 3010 Gravity
DE: 3035 Midocean ridge processes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting