HR: 1330h
AN: B12A-0738    [PDF]
TI: Sub-Moho Melt Accumulations Beneath the East Pacific Rise Between the Siqueiros Fracture Zone and the $9\deg$03'N Overlapping Spreading Center
AU: * Jousselin, D
EM: jousse@crpg.cnrs-nancy.fr
AF: INPL-ENSG-CRPG, 15 rue ND des Pauvres, Nancy, 54501 France
AU: Toomey, D R
EM: drt@newberry.uoregon.edu
AF: Univ. of Oregon, 1272 Geo. Sci., Eugene, OR 97403
AU: Dunn, R A
EM: dunnr@hawaii.edu
AF: Univ. of Hawaii, Dept. of Geol. and Geophys., Honolulu, HI 96822
AU: Wilcock, W S
EM: wilcock@u.washington.edu
AF: Univ. of Washington, School of Oceanography, Seattle, WA 98195
AU: Detrick, R S
EM: rdetrick@whoi.edu
AF: WHOI, Dept. Geol. and Geophys, Woods Hole, MA 02543
AB: We present the results of a tomographic investigation of the uppermost mantle structure along an 80-km-long section of the East Pacific Rise bounded by the Siqueiros fracture zone and the $9\deg$03'N overlapping spreading center (OSC). Mantle images were obtained by the undershooting technique, using delay times generated by 979 sources and recorded by 15 ocean bottom receivers. Both sources and receivers were located along rise-parallel lines. Off-axis crustal structure is well constrained by first and secondary arrivals (Pg, PmP and Pn waves). Mantle structure beneath the axial region is reconstructed from the delay times of Pn waves. We find that the data require an anisotropic upper mantle and significant isotropic heterogeneity. We do not directly invert for the intensity and orientation of seismic anisotropy. Instead, images are obtained for a range of anisotropic starting models. For a constant field of anisotropy, the minimum data misfit is obtained for 7% anisotropy with the fast axis oriented approximately parallel to the spreading direction. Examination of the delay time residuals after inversion suggests that seismic anisotropy may be heterogeneous at a scale of 5 to 10 km. The tomographic images of isotropic heterogeneity reveal that the axial high is underlain by a continuous low-velocity anomaly that decreases in magnitude from north to south. Parallel to the EPR the location of the upper-mantle, low-velocity anomaly is closely aligned with the sinuous morphology of rise. The magnitude of the sub-Moho anomaly is greatest between $8\deg$50'N and $9\deg$N, a region immediately south of the OSC where the cross-sectional area of the axial high is greatest and hydrothermal plume activity has been documented. Both the amplitude and the cross-axis width of the low-velocity anomaly are less to the south of ~$8\deg$45'N and toward the Siqueiros fracture zone, a result consistent with a relative decrease in the amount of melt present at sub-Moho depths. This region of EPR includes a section that has recently experienced anomalous seismic activity and where bathymetric data and dredge samples show the presence of volcanic ridges comprised of high-silica basalts.
DE: 3025 Marine seismics (0935)
DE: 3035 Midocean ridge processes
DE: 7220 Oceanic crust
SC: Biogeosciences [B]
MN: 2003 Fall Meeting