Ocean Drilling Program, United States

Magnetic measurements of samples from Ocean Drilling Program Leg 192 cores were made to characterize the magnetic fabric of oceanic basalts from the Ontong Java Plateau. Results of anisotropy of magnetic susceptibility measurements from both pillowed and more massive units indicate broad similarities in their general properties but several important differences in the orientation of their anisotropy axes. Few samples studied show evidence of strong anisotropy but many of those that do are associated with the same changes in characteristic remanent magnetization inclination that have been used to identify inclination groups. Such groups are thought to represent a record of the Earth's magnetic field over periods too short to average secular variation. The more strongly anisotropic samples are therefore interpreted to be located near major boundaries between discrete cooling units. The orientation of maximum magnetic susceptibility is dominantly subhorizontal and provides an estimate of local flow conditions. Azimuths of the maximum anisotropy axis of individual samples at each site have been used to identify a preferred azimuth that may reflect a sustained flow direction. These azimuths differ from site to site, suggesting that flow directions record local rather than regional conditions. Sites 1186 and 1187 exhibit the same strong directional bias along N155 degrees , which is parallel to local bathymetric patterns. Site 1185 has a predominantly east-west preferred azimuth that is interpreted to represent the dominant flow direction near the eastern edge of the plateau. Site 1183 lies closest to the crest of the plateau but displays a weakly developed azimuthal bias along two orthogonal directions-one along N120 degrees /300 degrees and one along N30 degrees /210 degrees . This ambiguity reflects the difficulty in reliably defining a dominant flow direction in weakly anisotropic samples. More detailed sampling and analysis are required to reliably identify individual flow units and to map changes in the flow regime across the plateau.