HR: 0800h
AN: V51B-0571    [Abstracts]
TI: Upper Mantle Structure Beneath the Gal\'{a}pagos Hotspot from Surface Wave Tomography
AU: * Villagomez, D R
EM: darwin@newberry.uoregon.edu
AF: Dept. of Geol. Sci., Univ. of Oregon, Eugene, OR 97403 United States
AU: Toomey, D R
EM: drt@newberry.uoregon.edu
AF: Dept. of Geol. Sci., Univ. of Oregon, Eugene, OR 97403 United States
AU: Hooft, E E
EM: emilie@newberry.uoregon.edu
AF: Dept. of Geol. Sci., Univ. of Oregon, Eugene, OR 97403 United States
AU: Solomon, S C
EM: scs@dtm.ciw.edu
AF: DTM, Carnegie Institution of Washington, Washington, DC 20015 United States
AB: To understand plume-lithosphere interaction in a near-ridge setting, we present a surface wave tomographic study of the upper mantle beneath the Gal\'{a}pagos Archipelago. We use Rayleigh waves recorded by a network of 10 broadband seismometers deployed from 1999 to 2003 for the IGUANA experiment and the GSN station PAYG. We analyze waves in 12 separate frequency bands (8-50 mHz), which are sensitive to shear wave velocity ({\it Vs}) structure in the upper 150 km. To account for non-great-circle propagation caused by multipathing we use the two-plane-wave approximation of Forsyth and others. Two-dimensional models of phase velocity obtained at each frequency are inverted for three-dimensional variations in {\it Vs}. Average one-dimensional phase velocities are 1-2% slower than for 0-4 My-old Pacific mantle, and phase velocities vary laterally by $\pm$3%. Inversions of phase velocities reveal that {\it Vs} varies regionally from 3.7 to 4.1 km/s, 3-15% slower than predicted along a 1300$\deg$C adiabat, and that there are two volumes of pronounced low velocity ($>$10% {\it Vs} reduction). Neither anomaly can be attributed to temperature alone; instead they require increased amounts of partial melt. The first anomaly, located beneath the volcanoes of the southwestern archipelago that erupt large volumes of enriched magmas, is most pronounced above 40 km depth and its magnitude increases toward the surface. This anomaly lies above an area of thinner-than-normal mantle transition zone and a cylindrical low-velocity body imaged by P and S wave tomography at depths of 100 to 250 km. This first anomaly may be the result of melt accumulation above a region of decompression melting driven by plume upwelling. The second low-velocity volume underlies the central archipelago, including the islands of Santiago and Marchena, and appears to be concentrated between 50 and 80 km depth. This anomaly is less pronounced near the surface, underlies a region that produces MORB, and coincides with a region of apparent isotropy as reported by Fontaine and others. This anomaly could indicate decompression melting of a depleted upper portion of the plume, possibly the result of modest local upwelling driven by a northward transition to thinner lithosphere. Our results, together with those from body wave tomography, suggest that geochemical patterns observed in the archipelago are in part the result of progressive melting of material in a plume conduit that rises from southwest to northeast. We are currently integrating results from surface and body wave imaging in an effort to constrain interactions at mantle depths between the hotspot and the Gal\'{a}pagos Spreading Center.
DE: 8180 Tomography
DE: 7255 Surface waves and free oscillations
DE: 8121 Dynamics, convection currents and mantle plumes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting