They have a ~20-km-wide вЂњdomalвЂќ morphology with shallow basement and increased upper crustal velocities. Within two tectonically dominated segments, we make the first wide-angle seismic identifications of buried oceanic core complexes in mature (>20В Ma) Atlantic Ocean crust. We estimate that the two crustal styles are present in equal proportions on the floor of the Atlantic. While velocity-depth profiles fit within previous envelopes for slow-spreading crust, our results suggest that such generalizations give a misleading impression of uniformity. We interpret these segments as tectonically dominated, with PmP representing a serpentinized (alteration) front. The second has a reduced contrast in velocity gradient between the upper and lower crust and PmP shallower than the 7.5В kmВ s в€’1 contour. We interpret these segments as magmatically robust, with PmP representing a petrological boundary between crust and mantle. Here, PmP coincides with the 7.5В kmВ s в€’1 contour, and velocity increases to >7.8В kmВ s в€’1 within 1В km below. The first displays a classic two-layer velocity structure with a high gradient Layer 2 (~0.9В s в€’1) above a lower gradient Layer 3 (0.2В s в€’1). We identify two types of crustal segment. All ridge discontinuities share similar primary characteristics, independent of the offset. The profile crosses five ridge segments separated by a transform and three nontransform offsets. We present a high-resolution 2-D P-wave velocity model from a 225-km-long active seismic profile, collected over ~60вЂ“75В Ma central Atlantic crust.

Angle of reflection seismic