HR: 08:15h
AN: T41G-02    [Abstracts]
TI: Dynamics of Slow Seafloor Spreading Constrained by Seismic Anisotropy in Atlantic Lithosphere
AU: * Gaherty, J B
EM: gaherty@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964 United States
AU: Dunn, R A
EM: dunnr@hawaii.edu
AF: Dept. of Geology and Geophysics, University of Hawaii - Manoa, Honolulu, HI 96822 United States
AU: Delorey, A A
EM: delorey@hawaii.edu
AF: Dept. of Geology and Geophysics, University of Hawaii - Manoa, Honolulu, HI 96822 United States
AB: Seismic anisotropy within the oceanic lithosphere provides one of the most direct means to study mantle deformation associated with mid-ocean ridge and hotspot volcanism. 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 of olivine. In the fast-spreading Pacific, observations of seismic anisotropy suggest that spreading-center deformation is quite simple, essentially 2-D corner-flow oriented in the spreading direction. While lithospheric anisotropy is less well characterized in slow-spreading regions such as the Atlantic, the segmented nature of slow-spreading ridges and the abundance of near-ridge hotspots suggest that shallow mantle deformation in these regions may be more complex than that found beneath fast-spreading ridges. This notion is supported by two analyses of lithospheric anisotropy in the Atlantic. First, radial anisotropy imaged near the Reykjanes Ridge implies a quasi-vertical (rather than horizontal) orientation of the lithospheric fabric, which suggests a buoyant (rather than passive) mode of spreading and melt extraction in this hotspot-influenced region. Second, azimuthal anisotropy within a swatch of western Atlantic lithosphere that was formed via ultra-slow spreading has a magnitude of 3%, nearly a factor of two weaker than that found in the Pacific. This observation suggests that shallow mantle deformation at slow-spreading ridges is accommodated in part by localized (brittle) mechanisms. 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 anisotropy in both hotspot-influenced (e.g. Iceland, 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: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 7218 Lithosphere and upper mantle
DE: 3035 Midocean ridge processes
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting