HR: 09:00h
AN: T11B-05    [PDF]
TI: Axial High Topography and Thermal Structure Along the Plume-Influenced Western Galapagos Spreading Center
AU: * Blacic, T M
EM: blacic@geology.ucdavis.edu
AF: University of California Davis, Geology Department 1 Shields Avenue, Davis, CA 95616 United States
AU: Ito, G
EM: gito@hawaii.edu
AF: SOEST, Department of Geology and Geophysics University of Hawaii, Honolulu, HI 96822 United States
AU: Canales, J P
EM: jpcanales@whoi.edu
AF: Woods Hole Oceanographic Institute, Department of Geology and Geophysics 360 Woods Hole Rd., Woods Hole, MA 02543 United States
AU: Shah, A
EM: ashah@qur.nrl.navy.mil
AF: Naval Research Laboratory, Marine Physics Branch Code 7420 455 Overlook Ave. SW, Washington, DC 20375 United States
AU: Detrick, R
EM: rdetrick@whoi.edu
AF: Woods Hole Oceanographic Institute, Department of Geology and Geophysics 360 Woods Hole Rd., Woods Hole, MA 02543 United States
AU: Sinton, J
EM: sinton@hawaii.edu
AF: SOEST, Department of Geology and Geophysics University of Hawaii, Honolulu, HI 96822 United States
AU: Lin, J
EM: jlin@whoi.edu
AF: Woods Hole Oceanographic Institute, Department of Geology and Geophysics 360 Woods Hole Rd., Woods Hole, MA 02543 United States
AB: Recent studies propose that the axial high topography observed at intermediate- and fast-spreading mid-ocean ridges (MORs) reflects the thermal structure and stress state of the MOR lithosphere. Hotspot related variations in thermal structure along the western Galapagos Spreading Center (GSC) thus offer an opportunity to test some of the proposed causes for axial high topography. Along the intermediate rate spreading western GSC we observe axial high topography from $\sim$$92.5\deg$W to $91\deg$W similar to that seen along portions of the fast-spreading East Pacific Rise. In this region we see large variations in the amplitude of the axial high with the height decreasing dramatically with distance away from the Galapagos hotspot. The decrease in amplitude of the axial high corresponds remarkably well to an increase in the depth of the seismically imaged axial magma lens. Eberle and Forsyth (1998) suggest that dike injections above the magma lens relieve tension in the shallow crust and generate large vertical variations in horizontal stress. The resulting bending moment thus uplifts the ridge axis. Contrary to our observations at the GSC, their model predicts increasing uplift with increasing depth of the magma lens. Alternatively, Shah and Buck (2001) propose that the crust is hot and partially molten directly beneath the ridge axis and that material rapidly cools and accretes to the plate as it moves off axis. As it cools, the crust becomes more dense and tends to sink relative to the level at which it accreted to the plate resulting in a topographic high at the axis. If the axial magma lens is deeper, then the change in temperature with distance from the axis is reduced and the near-axis lithosphere is strengthened, resulting in a smaller axial high. Thus it appears that Shah and Buck's (2001) model is more consistent with our observations of the western GSC than Eberle and Forsyth's (1998) hypothesis. We can constrain the width of the region of rapid cooling off axis using seismic refraction studies, which show a zone of low seismic wave velocity beneath the ridge axis. Using constraints from multichannel seismic imaging of the axial magma lens, seismic refraction data, gravity and bathymetry measurements we will investigate aspects of the thermal structure of the western GSC between $93\deg$W and $91.5\deg$W including width of the hydrothermal cooling region, crustal strength, and percent melt present in the lower crust to further our understanding of the causes of axial high topography at mid-ocean ridges.
DE: 3010 Gravity
DE: 3025 Marine seismics (0935)
DE: 3035 Midocean ridge processes
DE: 3045 Seafloor morphology and bottom photography
SC: Tectonophysics [T]
MN: 2003 Fall Meeting