EVIDENCE FOR ASYMMETRIC ACCRETION AND LOW-ANGLE, PLANAR FAULTS IN SLOW-SPREADING OCEANIC-CRUST

New seismic reflection data in the eastern Atlantic have enabled two l arge-scale dipping reflectors imaged within Mesozoic ocean crust to be mapped in three dimensions and placed in their sea-floor-spreading co ntext. One of the reflectors dips to the west and extends from a step in the igneous basement to the Moho. The other reflector dips to the e ast and extends between 4 and 10 km below basement. Both reflectors ar e oriented approximately parallel to isochrons, lie just north of a 5- km-offset, third-order discontinuity, are planar in cross section, and dip at similar to 30 degrees. We interpret both these reflectors as l ithospheric-scale faults formed within the median valley at the axis. We suggest that the lower part of the eastward-dipping fault was ''cap tured'' from the North American plate, possibly by rapid propagation o f an active volcanic rift across the discontinuity.