HR: 13:40h
AN: T52E-01    [PDF]
TI: Mantle Flow Beneath Slow-Spreading Ridges Constrained by Seismic Anisotropy in Atlantic Lithosphere
AU: * Gaherty, J
EM: gaherty@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AU: Dunn, R
EM: dunnr@hawaii.edu
AF: Dept. of Geology and Geophysics, SOEST University of Hawaii, Honolulu, HI 96822 United States
AB: Seismic anisotropy within the oceanic lithosphere provides one of the most direct means to study deformation associated with convection in the mantle. Advection beneath a mid-ocean ridge spreading center deforms the mantle rocks, and as the rocks cool to produce the oceanic lithosphere, they retain a record of this deformation in the form of lattice-preferred orientation (LPO) of olivine grains. LPO direction and strength can be estimated from directional and/or polarization dependence (anisotropy) of seismic wave speeds, and mid-ocean ridge mantle flow properties can be inferred. Mantle flow beneath the slow-spreading Mid-Atlantic Ridge (MAR) is suspected to be strongly three-dimensional due to the influence of hotspots and other thermal variations, and this thermal heterogeneity may be related to buoyancy-driven flow beneath the ridge. This notion is supported by two analyses of lithospheric anisotropy in the Atlantic, which until recently had not been well characterized. Radial anisotropy imaged near the hotspot-influenced Reykjanes Ridge implies a quasi-vertical (rather than horizontal) orientation of the lithospheric fabric. Azimuthal anisotropy within a narrow swatch of western Atlantic lithosphere that was formed via ultra-slow spreading is weaker than that found in the Pacific by a factor of two. Both can be interpreted in terms of buoyancy-driven flow beneath the MAR. Here we extend these results using regional surface-wave analyses of the Atlantic basin. Earthquakes from Atlantic source regions recorded at broad-band seismic instruments located on Atlantic islands and the surrounding margins provide excellent sensitivity to oceanic lithosphere structure, without contamination by continental heterogeneity. By characterizing such structure in both hotspot-influenced (e.g. Azores) and normal slow-spreading lithosphere, and comparing these structures to the Pacific, we evaluate the degree to which spreading rate and/or mantle source temperature control fabric and mantle flow beneath mid-ocean ridges.
DE: 3035 Midocean ridge processes
DE: 7218 Lithosphere and upper mantle
DE: 8121 Dynamics, convection currents and mantle plumes
SC: Tectonophysics [T]
MN: 2003 Fall Meeting