HR: 14:40h
AN: V33G-05    [Abstracts]
TI: The Seismic Low Velocity Zone West of the EPR Where an Off-Axis Plume may be Feeding the Ridge
AU: * Weeraratne, D S
EM: Dayanthie$_$Weeraratne@brown.edu
AF: Brown University, P.O. Box 1846, Providence, RI 02912 United States
AU: Forsyth, D W
EM: Donald_Forsyth@brown.edu
AF: Brown University, P.O. Box 1846, Providence, RI 02912 United States
AU: Webb, S C
EM: scw@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AB: We present seismic Rayleigh wave data from the GLIMPSE experiment to investigate the upper mantle beneath intra-plate seamount chains on young (4 - 9 Ma) Pacific seafloor which may be the surface manifestation of mantle flow between an off-axis plume source (e.g. the hotspot Super Swell region) and the EPR. Shear wave inversion of phase velocity data collected from the study area surrounding the Sojourn ridge and Hotu Matua seamount complex indicate a high velocity lithosphere extending to 60 km $\pm$ 20 km is underlain by anomalous low velocities reaching a minumum of $\sim$3.9 km/s at 70 km depth. Sufficient long period data is available to resolve a steep positive velocity gradient at the base of the low velocity zone (LVZ) at about 110 km depth. Rayleigh waves also demonstrate significant attenuation for periods between 40 s and 70 s with a minimum quality factor, 65 ${ < Q_{LR} < }$ 110. Recent studies that suggest variations in the elastic properties of the oceanic LVZ can be explained exclusively by the effects of temperature and pressure are able to predict the seismic velocities that we observe but indicate stronger attenuation than is shown in our data. Conversely, models that consider only the effects of dehydration are consistent with the range of seismic attenuation but indicate seismic velocities which are too high. We propose a model beneath young seafloor where decompression melting begins at $\sim$110 km and "dries out" at $\sim$60 km. The low velocities and moderate attenuation can be explained by the combined effects of partial melt and dehydration. The unusually low seismic velocities may be associated with asthenospheric flow from a volatile-rich off-axis plume to the East Pacific Rise.
DE: 9355 Pacific Ocean
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 7218 Lithosphere and upper mantle
DE: 3035 Midocean ridge processes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting