HR: 0830h
AN: V11E-0546    [PDF]
TI: Shallow mantle and crustal structure beneath the Mid-Atlantic Ridge (35$\deg$N): melt supply and crustal construction
AU: * Lekic, V
EM: lekic@fas.harvard.edu
AF: Harvard Univ., Dept.of Earth and Planetary Sciences, Cambridge, MA 02138
AU: Dunn, R
EM: dunnr@hawaii.edu
AF: Univ. of Hawaii, Manoa, Dept.of Geology and Geophysics, Honolulu, HI 96822
AU: Toomey, D
EM: drt@newberry.uoregon.edu
AF: University of Oregon, Dept.of Geological Sciences, Eugene, OR 97403
AU: Detrick, R
EM: rdetrick@whoi.edu
AF: WHOI, Dept. of Geology and Geophysics, Woods Hole, MA 02543
AB: P-waves recorded during wide-angle refraction experiments provide a measure of crustal thickness and the velocity structure of the crust and uppermost mantle along the Mid-Atlantic Ridge near 35N. We use 29,605 crustal refractions, 14,682 reflections from the base of the crust, and 9322 mantle refractions to generate a three-dimensional anisotropic P-wave image of sub-ridge structure that extends 60 km along the ridge, 50 km across the ridge, and to 10 km depth. These data were compiled from six separate two- and three-dimensional experiments which, taken together, include 49 ocean-bottom instruments and 5118 airgun shots. This section of the Mid-Atlantic Ridge is bounded to the north by the Oceanographer fracture zone and to the south by a non-transform offset and previous studies suggest that more melt is supplied to the center of the ridge segment than the ends. Shallow isotropic velocities are generally low in a band that parallels the ridge axis and this band is punctuated by a series of small, 10-km-long, low velocity anomalies of up to -1 km/s magnitude. The variability of shallow crustal structure suggests strong spatial and temporal variability of the melt supply to the shallow crust. In addition, in the shallow crust we detect a small amount of azimuthal anisotropy that indicates the presence of vertically aligned cracks that are oriented parallel to the strike of the ridge. Such cracks indicate that the shallow crust is experiencing tensional forces generated by plate spreading. At the center of the ridge segment, where a line of seamounts intersects the ridge, a 10x10 km$^{2}$ low-velocity region (-0.6-0.8 km/s magnitude) extends downward through the crust and into the mantle where it widens to 20x20 km$^{2}$ in width. The seismic image is consistent with a region of high temperatures and perhaps a small amount of melt and thus indicates the current magma supply system of the ridge. Although the aperture of our imaging does not extend completely to the segment ends, the crust is generally thinner towards the segment ends (5 km thick) and thicker at the center of the segment (8.5 km thick). Our results indicate that mantle-derived melts rising beneath this ridge segment are focused at mantle depths towards the center of the segment resulting in a thicker crust and a high-temperature, partial melt zone near the segment center. Within the median valley, a volcanic ridge runs northward from the segment center, but is not underlain by low velocities in the lower crust nor mantle. Thus, this ridge may be similar to an Hawaiian type rift zone, with magma fed to it laterally from the ridge segment's center instead of from directly below.
DE: 3025 Marine seismics (0935)
DE: 3035 Midocean ridge processes
DE: 7220 Oceanic crust
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting