HR: 1340h
AN: T33B-1358    [Abstracts]
TI: Shear velocities in the oceanic crust at the East Pacific Rise 9° 18' N to 10° 30' N from compliance measurements
AU: * Nooner, S L
EM: snooner@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Rt. 9W, Palisades, NY 10026, United States
AU: Webb, S C
EM: scw@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Rt. 9W, Palisades, NY 10026, United States
AU: Crawford, W C
EM: crawford@ipgp.jussieu.fr
AF: Institute de Physique du Globe de Paris, 4, Place Jussieu, Paris, 75252, France
AB: Compliance was measured at 21 sites along the East Pacific Rise (EPR) from 9° 18' N to 10° 30' N during the MADCAP (Melt And Diking from Compliance And Pressure) experiment on the R/V Atlantis from February 13 to March 19, 2007. Measurements at 10° 30' N across the ridge segment 22 km north of the Clipperton transform fault show a stiff lower crust, which suggests that there is little crustal melt. This is consistent with previous descriptions of this segment as "magmatically starved" based on its morphology. Most of the compliance measurements were made on the EPR segment south of the Clipperton transform fault. At the northern end of this ridge segment, a compliance transect at 10° 2' N spans the ridge axis and continues to a seamount 16 km east. These measurements indicate that shear velocities are low beneath the ridge axis but increase rapidly off axis to the east, suggesting no magmatic connection between the ridge axis and the Watchstander seamount chain. Shear velocities beneath the nearest (and most recently active) seamount are similar to other off axis sites, suggesting that there is little or no crustal melt there. A 26 km long compliance transect across the ridge axis near 9° 20' N suggests that the region of low crustal shear velocities is constrained to within 3-4 km of the ridge axis. The compliance measurements preclude the existence of a melt body 18-20 km east of the ridge axis as had been inferred from an apparent mid-crustal reflector observed in a recent OBS experiment. The compliance over that site show low shear velocities only in the uppermost crust associated with a thick layer 2A. A final compliance transect stretches northward along the ridge axis and across the Clipperton ridge-transform intersection (RTI). Measurements made north of where the ridge crosses the inferred location of the RTI show lower crustal shear velocities than normal for off-axis crust, but this observation is consistent with previous refraction work that suggested extensive alteration of the mid-crust and a thick layer 2A north of the transform. Within 9 km south of the RTI, lower crustal shear velocities on axis decrease to values comparable to other sites on-axis along this ridge segment suggesting the robust magma supply extends to within 9 km of the RTI.
DE: 3025 Marine seismics (0935, 7294)
DE: 3035 Midocean ridge processes
DE: 7220 Oceanic crust
DE: 7245 Mid-ocean ridges
DE: 7250 Transform faults
SC: Tectonophysics [T]
MN: 2007 Fall Meeting