HR: 1330h
AN: B12A-0743    [PDF]
TI: Mantle Structure Beneath the East Pacific Rise and its Relation to Tectonic Segmentation, Axial Morphology and Hydrothermal Activity
AU: * Toomey, D R
EM: drt@newberry.uoregon.edu
AF: Univ. of Oregon, 1272 Geol. Sci., Eugene, OR 97403
AU: Dunn, R A
EM: rdunn@hawaii.edu
AF: Univ. of Hawaii, Dept. of Geol. and Geophy., 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 Geophy., Woods Hole, MA 02543
AB: The UNDERSHOOT experiment was designed to test competing magma supply models for the East Pacific Rise (EPR) by mapping the pattern of magma delivery from the mantle to the crust along the entire length of a transform-bounded segment between the Siqueiros and Clipperton fracture zones. Here we present tomographic images of shallow mantle, seismic heterogeneity for the region of $9\deg$-$10\deg$15'N. These results, in combination with those obtained for the $9\deg$03'N overlapping spreading center (OSC) [Dunn et al., 2001] and the rise axis south of the OSC [Jousselin et al., 2003], document relations between the distribution of mantle melt and the characteristics of tectonic segmentation, the morphology of the axial high and the distribution of hydrothermal activity. At the segment scale, the mantle low-velocity zone (MLVZ) is comprised of two contiguous trends that lie sub-parallel to the axial high. These trends are shifted with respect to one another in a right-lateral sense at $9\deg$18'N, 30 km north of the OSC. South (north) of this shift the MLVZ is located more than 5 km west (east) of the axial high. North of $9\deg$18'N the trend of the MLVZ is rotated counterclockwise with respect to the axial high, such that the MLVZ lies east of the rise south of $9\deg$30'N and either beneath or slightly to the west of the rise north of $9\deg$30'N. It is an open question as to whether the segmentation of the MLVZ into two linear trends is a response to or cause of OSC propagation. Parallel to the rise the amplitude of the MLVZ is modulated at intervals of approximately 20 km. Sections of the rise associated with significant hydrothermal plume activity (e.g., $8\deg$48'-$8\deg$58'N and $9\deg$29'N-$10\deg$01'N), as defined by light attenuation measurements, are associated with larger amplitude, rise-centered MLVZs. Conversely, sections of the rise lacking continuous hydrothermal plume activity (e.g., $8\deg$58'N-$9\deg$29'N) are underlain by either relatively low-amplitude, rise-centered MLVZs or MLVZs located off axis. These relations may indicate that vigorously venting hydrothermal fields require abundant accumulations of melt at mantle depths which can frequently replenish the crustal magmatic system. Between the Clipperton and the Siqueiros transforms the largest amplitude MLVZ is located off axis near $9\deg$10'N, in a region where crust is anomalously thick, axial depth is relatively deep and the cross-sectional area of the axial summit is decreased. We thus infer that along-axis variations in axial depth and cross-sectional area of the axial high bear more relation to the cross-axis location of the MLVZ, than they do to magma supply. This is consistent with models that attribute axial high depth and morphology to a flexural response.
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 3025 Marine seismics (0935)
DE: 3035 Midocean ridge processes
DE: 4815 Ecosystems, structure and dynamics
DE: 7218 Lithosphere and upper mantle
SC: Biogeosciences [B]
MN: 2003 Fall Meeting