HR: 1340h
AN: T33A-1353    [Abstracts]
TI: Upper Mantle Structure Beneath the Eastern Pacific Ocean Ridges
AU: * Gu, Y J
EM: jgu@phys.ualberta.ca
AF: University of Alberta, Dept. of Physics, 535A Avadh Bhatia Physics Lab, Edmonton, Ab T6G2J1 Canada
AU: Webb, S C
AF: Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AU: Gaherty, J B
AF: Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AU: Lerner-Lam, A
AF: Lamont-Doherty Earth Observatory, Columbia University, 61 Route 9W, Palisades, NY 10964 United States
AB: We analyze vertical-component body and surface waves for ten Mw$>$5 earthquakes, recorded by ocean bottom seismometers at regional and teleseismic distances. Through waveform modeling we place new constraints on along-axis variation in temperature and partial melt beneath the Eastern Pacific Ridges. The resulting best-fit models show over 9% variation in average lithosphere shear velocities between different ridge segments. We demonstrate that the lid velocity correlates with the square root of plate age consistent with a conductive cooling process, but find a more rapid dependence on age close to the axial rifts. We map the average plate age into a mean lithospheric temperature for each of our models using a half-space cooling model, and the temperature derivatives (dVs/dT) determined from least squares fits are -1.1 m/s/deg and -0.26 m/s/deg, respectively, for temperatures above and below 1000C. The former estimate is more negative than values determined by earlier reports (-0.4 to -0.7 m/s/deg), using global or regional data from a much wider range of sea floor age but with less resolution at young ages. The high absolute dVs/dT value suggests the presence of partial melt at shallow mantle depths beneath young ocean crust out to an age of approximately 5 Myr. Our data also show a strong north-south difference in mantle structure. The surface waves that traverse through the southern EPR experience shear velocities that are as low as $\sim$3.75 km/s, more than 0.2 km/sec slower than the average mantle structure at comparable depths beneath the northern EPR and the Galapagos spreading center. This difference cannot be explained by simple conductive cooling or spreading rate variation between ridge segments. We hypothesize that more melt exists near the ridge axis of southern EPR, either due to higher melt production in the south or more efficient melt extraction in the north.
DE: 8123 Dynamics, seismotectonics
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8150 Plate boundary--general (3040)
DE: 7200 SEISMOLOGY
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting