HR: 1330h
AN: V12B-0580    [PDF]
TI: P and S wave delay times and SKS splitting results from the GLIMPSE experiment: comparison with gravity anomalies and recent volcanism.
AU: * Lamm, R A
AF: Brown University, Department of Geological Sciences, 324 Brook Street, Providence, RI 02912 United States
AU: Harmon, N
EM: Nicholas_Harmon@brown.edu
AF: Brown University, Department of Geological Sciences, 324 Brook Street, Providence, RI 02912 United States
AU: Forsyth, D W
EM: Donald_Forsyth@brown.edu
AF: Brown University, Department of Geological Sciences, 324 Brook Street, Providence, RI 02912 United States
AU: Webb, S C
EM: scw@ldeo.columbia.edu
AF: Lamont-Dogherty Earth Observatory, P.O. Box 1000, Palisades, NY 10964 United States
AB: The Gravity Lineations Intraplate Melting Petrologic and Seismologic Expedition (GLIMPSE) is investigating the origin of a series of intraplate en echelon volcanic ridge systems in the South Pacific that are not the product of hot spots. The long-term ocean bottom seismometer (OBS) deployment of the experiment has measured P and S wave delay times as well as shear wave splitting from SKS phases. Average P wave delay times at individual OBSs ranged from -.59 s to .45 s with an average standard deviation of .05 s. S wave delays ranged from -.96 s to 1.12 s with an average standard deviation of .16 s. The late arrivals (positive delay) are generally associated with regions of recent volcanism (as inferred from side scan sonar images of the seafloor) indicating that there is anomalous seismic structure beneath the Sojourn and Hotu Matua volcanic ridges. Estimates of lithospheric thinning required to produce the observed delay times yield upwards of 50 km of thinning, which seems very unlikely for even the oldest ($\sim$ 9 Ma) seafloor in the region. Therefore, the anomalous structure probably involves the asthenosphere and partial melting. The fastest P-wave anomaly lies within a region of positive residual gravity anomaly and the slower times generally fall within areas of negative residual anomalies. Splitting times ranged from 1.1 to 2.2 s with fast direction azimuths trending within error in the direction of absolute plate motion for the Pacific plate near the East Pacific Rise (EPR). Shear wave splitting in the GLIMPSE study region may be attributable to three components of anisotropy: lattice-preferred orientation (LPO) of olivine crystals in the shallow mantle due to spreading induced strain from the EPR; LPO due to deeper channelized return flow in the asthenosphere from the Pacific Superswell to the EPR; and melt filled lithospheric cracks aligned in the direction of absolute plate motion resulting in shape preferred orientation (SPO) anisotropy. However, calculations of splitting times for penny shaped melt inclusions indicate that a high melt volume fraction ($>$ .05) must be present for SPO in the lithosphere to be a major contributor to the observed splitting times, making the SPO component very unlikely. The greatest splitting delays in both GLIMPSE and the MELT Experiment were associated with areas of anomalously slow S wave velocity to the west of the East Pacific Rise, suggesting strong anisotropy in the shallow asthenosphere.
DE: 7220 Oceanic crust
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting