One major feature that stands out from the site survey is the difference in seismic "character" between the sites on 40-Ma crust and those surveyed on 56-Ma crust. Horizons are more continuous in the sediments along the 40-Ma line in both the east-west direction (roughly perpendicular to the abyssal hill strike) (Fig. F3) and in the north-south direction (almost parallel to the strike of the hills) (Fig. F8) than along the 56-Ma transect in either the east-west (Figs. F5, F6) or north-south (Fig. F9; Site 1217) direction. It takes only a brief examination to determine that sediments along the 56-Ma line are much more extensively faulted than those along the 40-Ma line.
The faulting along the 56-Ma line cuts through not only the older sediments but sometimes offsets the P3 horizon (Figs. F9, F10), suggesting that faults stayed active through the Eocene and developed offsets of 75-100 m. At Site 1220 and Site 1219, the P4 horizon (~30 Ma) may be offset by the faulting as well. In contrast, faulting is much less extensive on 40-Ma crust. On this younger crust offsets occasionally extend to P4 and P5, but they are usually on the order of 10 m and sediments appear to drape over the faults in low abyssal hills.
The difference in tectonism between the two transects is significant and deserves explanation. We hypothesize that the more highly faulted early Eocene crust may result from the significant reorganization of plate motion, ridge axes, and transform faults in this time period or shortly afterward (Rea and Dixon, 1983), when the early Eocene crust was relatively young (Menard, 1978; Rea and Dixon, 1983; Rosa and Molnar, 1988; Atwater, 1989). Better age control from drilling should help to better constrain this problem, and further surveys after drilling will better define the extent of anomalous faulting.