HR: 0800h
AN: S51A-0996    [Abstracts]
TI: Rayleigh and Love Wave Tomographic Study of the Upper Mantle Beneath the Reykjanes Ridge
AU: Dunn, R A
EM: dunn@soest.hawaii.edu
AF: Dept. of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822 United States
AU: * Delorey, A
EM: delorey@hawaii.edu
AF: Dept. of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822 United States
AU: Gaherty, J
EM: gaherty@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AB: General insight into the existence and role of mantle plumes in mantle dynamics may be obtained by studying how melting and flow patterns beneath mid-ocean ridges are perturbed near hotspots. The Iceland-Reykjanes Ridge system is a well-studied example of this interaction. Extending southward from Iceland, the Reykjanes Ridge exhibits an anomalous character for its slow spreading rate suggesting southward spreading of a plume material beneath the ridge. One end-member model for this flow suggests that low-viscosity plume material is narrowly channeled beneath the ridge axis; an alternative model suggests that it spreads radially outward beneath the lithosphere. We analyzed Love and Rayleigh waves that were generated by earthquakes located to the south of Iceland and that were recorded by 46 broadband seismometers located across Iceland. Over 12000 measurements of the phase, group travel time, and amplitude of narrow-pass filtered waveforms (10-50s Love; 14-100s Rayleigh) were included in an inversion for mantle and crustal shear anisotropic velocity structure. Amplitudes of the waveforms are sensitive to the velocity structure across the ridge and are largest at stations located near the ridge axis. This is due to lateral refraction of surface wave energy into a wide low-velocity region beneath the ridge axis that acts as a lateral wave-guide, trapping the surface wave energy. The waveforms are sensitive to the magnitude and width of the low-velocity region and our surface wave inversion technique exploits this sensitivity. Tomographic images show a wide low-velocity region beneath the lithosphere (approx. 600 km across and centered on the ridge) that is consistent with high temperatures (<200\?) and a small amount of melt (<0.5%). Our results indicate that plume material spreads relatively broadly outwards to the south of Iceland, and is not restricted to a narrow, sub-ridge, channel. Our results also indicate that the spreading plume mixes with the greater part of the upper mantle, to at least 200 km depth.
DE: 3035 Midocean ridge processes
DE: 3037 Oceanic hotspots and intraplate volcanism
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7255 Surface waves and free oscillations
DE: 7270 Tomography (6982, 8180)
SC: Seismology [S]
MN: Fall Meeting 2005