ANISOTROPIC SEISMIC SCATTERING IN OLD ATLANTIC CRUST

An oblique seismic experiment (OSE) was carried out at Ocean Drilling Program (ODP) Hole 418A in the western North Atlantic to investigate v ertical, lateral, and azimuthal structural variations in the upper por tions of relatively old ocean crust. Original analysis of the data for travel time anisotropy did not show systematic azimuthal trends in se ismic structure except at shallow depths immediately adjacent to the b orehole. Subsequent analysis of the OSE data, presented here, reveals a correlation between scattering strengths versus azimuth and spreadin g direction of the crust. Analyses are performed on body and coda wave s and include coda energy and normalized coda energy calculations, spe ctral analysis, coda dropoff rates, and cross-correlation analysis. Al l of the analyses show trends indicating an increase in seismic scatte ring along directions parallel to the paleospreading direction. These results suggest that evidence of crustal formation processes is still present and seismically detectable after 108 m.y. of crustal evolution . Scattering is likely caused by crustal cracks and/or aligned heterog eneities indicating that either large crustal cracks are still present in the crust or that homogenization of the crust through hydrothermal deposition of secondary minerals is not complete at this site.