HR: 0800h
AN: V31B-1438    [Abstracts]
TI: Crustal Structure Beneath the Gravity Lineations in the South Pacific From Seismic Refraction Data
AU: * Holmes, R
EM: cholmes@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, P.O. Box 1000, 61 Route 9W, Palisades, 10964
AU: Webb, S C
EM: scw@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, P.O. Box 1000, 61 Route 9W, Palisades, 10964
AU: Forsyth, D W
EM: Donald_Forsyth@brown.edu
AF: Brown University, 324 Brook Street, Box 1846, Providence, 02912
AB: The GLIMPSE ({\bf G}ravity {\bf L}ineations, {\bf I}ntraplate {\bf M}elting, {\bf P}etrologic and {\bf S}eismic {\bf E}xpedition) Experiment explores the origin of cross-grain gravity lineations and associated volcanic ridges in the central South Pacific. The active-source seismic refraction component of GLIMPSE Experiment investigated the crustal thickness and velocity structure beneath the Sojourn Ridge and the Hotu Matua ridge system (~116.5-115.5{\deg}W, 16.0-12.5{\deg}S). Forward modeling of P-wave first arrival picks along a 450 km refraction line shot to 16 ocean bottom seismometers shows crust thickening beneath the ridges, ranging from the 5.3-5.5 km typical of oceanic crust along this segment of the Southern East Pacific Rise (SEPR) to a maximum of 9.3-9.8 km beneath the Sojourn Ridge. Several small volcanoes crosscut by the refraction line also exhibit root structures, suggesting a consistent pattern of periodic ridge-perpendicular crustal thickness variations with a scale length of 50-75 km. In addition, crustal thickening is consistently asymmetric beneath the ridges, extending further to the north than to the south beneath the associated bathymetric highs. Crustal thickening is primarily associated with variations in layers 2A, 2B and 2C, indicating increased dike emplacement and eruption of extrusives. Small-scale asthenospheric convection and channeling of low-density material from the South Pacific Superswell, models previously invoked to explain the observed free-air gravity pattern in the SEPR region, may be similarly responsible for the periodic changes in crustal thickness. Discrimination between the two models requires further research on upper mantle structure.
DE: 9355 Pacific Ocean
DE: 8015 Local crustal structure
DE: 3025 Marine seismics (0935)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting