[M-IS18] Drilling Earth Science

[MIS18-P12] Seismic Propoerties of gabbroic sections in oceanic core complexes: constraints from seafloor drilling

*Natsue Abe^1,2, Donna K. Blackman³, Richard L. Carlson⁴, Gilles Guerin⁵, Benoit Ildefonse⁶, Amber Kumpf⁷ (1.Mantle Drilling Promotion Office, MarE3, Japan Agency for Marine-Earth Science and Technology , 2.Graduate School of Natural Science & Technology, Kanazawa University, 3.University of California Santa Cruz, 4.Texas A&M University, 5.Lamont Doherty Earth Observatory, Columbia University, 6.Geosciences Montpellier, CNRSm University of Montpellier, 7.Muskegon Community College)

Keywords:Oceanic core complex, seismic properties of ocean crust, IODP, JOIDES Resolution, Exp. 304, 305, 340T, 360

The physical properties of rock within oceanic core complexes (OCC) provide information about the history of magmatism, deformation and alteration associated with detachment faulting and unroofing of their gabbroic sequences. New core and logging data from Atlantis Bank (AB) OCC are compared with prior deep sea drilling data from this site on the Southwest Indian Ridge, as well as from Atlantis Massif (AM) OCC on the Mid-Atlantic Ridge. The average seismic velocity at each site is typical for gabbroic rock. Downhole core and log variations are found to depend on both porosity and alteration. Porosity at millimeter to meter scale impacts P-wave velocities (6.0 -6.4 km/s) in the upper several hundred meters at both OCC. Below ~600-800 m depth, in situ velocities are higher (6.6 6.9 km/s) for the remainder of the 1.5 km drilled to date, except in zones where alteration is significant. Variability of 0.5 – 1 km/s over depth intervals affected by little to no alteration. Seismic anisotropy is negligible overall, with just a few intervals showing consistent fast azimuth and magnitude of a few percent. The depth extent of alteration, probably related to the zone deformed below the detachment when active, is ~600 -800 meters at both AB and AM.