Figure F2. A. Seismic profile across the northern part of the 3-D seismic survey (1989 line 2). The frontal thrust, décollement surface, ocean basement, and approximate outline of the area shown in Figure 4 are marked. This profile is tentatively interpreted to show stepping down of the décollement and underplating of subducted sediments beneath Hydrate Ridge. B. Line 1 from the 1989 survey, which crosses Hydrate Ridge just south of the 3-D survey. Relatively gentle folding of sediments on the western flank of Hydrate Ridge suggest that vergence of the deformation front has recently changed from landward to seaward. A possible step in oceanic basement is also shown. Oceanic basement is ~7 km beneath the summit of Hydrate Ridge. SP = shotpoint, BSR = bottom-simulating reflector.