New seismic reflection data in the eastern Atlantic have enabled two large-scale dipping reflectors imaged within Mesozoic ocean crust to be mapped in three dimensions and placed in their sea-floor-spreading context. 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 east and extends between 4 and 10 km below basement. Both reflectors are oriented approximately parallel to isochrons, lie just north of a 5-km-offset, third-order discontinuity, are planar in cross section, and dip at ∼30°. We interpret both these reflectors as lithospheric-scale faults formed within the median valley at the axis. We suggest that the lower part of the eastward-dipping fault was “captured” from the North American plate, possibly by rapid propagation of an active volcanic rift across the discontinuity.

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