HR: 1330h
AN: T43A-02    [Abstracts]
TI: Relationship Between Strong Seismic Reflectors In Young Igneous Oceanic Crust And Borehole/Core Measurements Of Physical Properties
AU: * Blackman, D K
EM: dblackman@ucsd.edu
AF: Scripps Institution of Oceanography, IGPP/SIO/UCSD, La Jolla, CA 92093 United States
AU: Karner, G D
EM: garry@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, P.O. Box 1000, Palisades, NY 10964 United States
AU: Shipboard Scientific Party, E
EM:
AF: IODP, JOIDES Resolution, College Station, TX 77845 United States
AB: Multichannel seismic data from a tectonic exposure of young oceanic crust, 30° N on the western flank of the Mid-Atlantic Ridge, display a prominent reflector at 0.2-0.5 s two-way travel-time. The reflector is continuous over several km and has a gently domal shape that is reminiscent, although not directly bottom-simulating, of the domal morphology of the Atlantis Massif. Expeditions 304 and 305 of the Integrated Ocean Drilling Program have drilled past 1000 m at this site. Physical properties measurements on core samples and in-situ density and velocity measurements provided by downhole logging provide a basis for interpreting the relationship between the local reflectivity at the site and physical properties of the section. Onboard density and compressional velocity measurements of core sample show a relationship to borehole logging estimates of seismic velocity with depth. Lithologically, the recovered section is essentially of near-constant density gabbros. These data can be used to define acoustic impedance and hence offer an explanation for reflectors in oceanic crust. In particular, laboratory-measured velocity measurements are systematically higher than logging sonic velocities from the seafloor to ~220 meters below seafloor (mbsf). From 220-780 mbsf, the laboratory and logging velocities are coincident. The pattern in the upper ~220 m can be explained by the closure of fractures, the rocks of which are not sampled by coring. Preliminary analysis suggests there may be a correspondence between the degree of alteration, which is related to variation in olivine content within gabbroic layers, the degree of fracture closing, and the velocity and density of the oceanic crust. The progressive closing of fractures within the upper few 100's of meters is consistent with caliper data. The velocity distribution with depth might be explained in terms of the closing of macro fractures shallower than 220 mbsf, the systematic closure of micro-cracks between 220-780 mbsf, followed by a systematic decrease below 780 mbsf in rock volume determined by Poisson's ratio and overburden stress. Derived synthetics underscore the importance of crack closure and potential effects of alteration in controlling seismic reflectivity.
DE: 3022 Marine sediments--processes and transport
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
DE: 8105 Continental margins and sedimentary basins
SC: Tectonophysics [T]
MN: 2005 Joint Assembly