Data quality throughout the survey was excellent, and it is anticipated that much can be learned from the profiles directly and through use of the profiles as they provide structural, lithologic, and hydrologic context for other data as discussed above. Two example profiles are provided here that reveal a number of interesting sedimentary lithologic and tectonic features. The profile shown in Figure 3A crosses a broad distributary channel not far from ODP Leg 168 Sites 1026 and 1027. Among the features seen in this profile are (1) locally low-reflection amplitudes in the vicinity of Shot 260 between 3.65 and 3.75 s, equivalent to the interval where unconsolidated massive sands were encountered at Site 1027; (2) the progressive onlap of channel deposits on the levee of the next channel system to the west (between Shots 300 and 400); (3) a normal fault or a differential-compaction growth fault in the lower half of the sedimentary section near Shot 240; and (4) a rugged basement surface comprising buried ridges and valleys, with local relief of up to 0.6 s, roughly 600 m. Lower basement relief and a stronger basement reflection amplitude is seen in the profile shown in Figure 3B. Here, some postdepositional deformation of the seismic stratigraphy is clearly related to differential compaction, but several disruptions (e.g., at Shot 820 and near Shot 960) are probably associated with relatively recent off-axis tectonic motion. This section also demonstrates the effectiveness of migration. Sharp offsets in the basement surface produced by normal faulting at the ridge axis or possibly along the edge of intrasedimentary sills or flows erupted off-axis are imaged very well (e.g., near Shots 900 and 1040).
In addition to local basement and sedimentary structures, the seismic data presented in this report provide constraints on regional basement topography and sediment thickness variations. To facilitate use of the data, we have provided files of manually picked reflection traveltime depths to the seafloor and to the top of seismic basement (format described above). Four "worse-case" examples are provided in Figure 4 to show the nature of potential errors associated with the picks.
Because the seafloor and basement picking were done independently, there are some instances in areas of no sediment cover where depths to the seafloor and to basement differ (Fig. 4A); this can lead to positive and negative small erroneous sediment thicknesses. In areas of rugged basement relief (i.e., where basement is rough, features are small, or the average slope is steep), basement reflections are often weak and difficult to distinguish (Figs. 4B-4D). In many of these instances, the basement surface has been chosen where coherent sedimentary reflectors terminate (Fig. 4B). In the lower parts of thick, older (pre-Pleistocene, nonturbidite) sections, internal reflections are weak, and this method does not work well (Fig. 4C). Where weak basement arrivals approach the seafloor, recognition of basement is made even more difficult by the strong seafloor reflection coda (Fig. 4D). At some locations (e.g., along lines in Fig. 4B and Fig. 4D), high-amplitude, reverberant basement reflections may originate from one or more off-axis massive flow units or sills; caution must be used in interpreting these horizons to represent the top of permeable "hydrologic basement" (e.g., Davis and Chapman, 1997).
One of the applications of the data has been to define the regional continuity of the sediment cover on the eastern Juan de Fuca Ridge flank and the local and regional variations in its thickness. Five long transects of seafloor and basement topography derived from the *.XYZ files are shown in Figure 5. The northernmost is a composite (provided for convenience in a single file named LEG_168.XYZ) that crosses the ODP Leg 168 drilling sites (Fig. 5A). Distances shown in the profiles and provided in the *.XYZ files have been determined by projection onto a line striking at N 107°E (perpendicular to the strike of local isochrons) and beginning at the Endeavour ridge axis at 47°58.46´N, 129°4.8´W. Whereas most of the crust along the profiles was created by Endeavour segment spreading, recent northward propagation of the Cobb offset (Johnson et al., 1983) causes complications to the simple age structure in the youngest parts of the southerly profiles (Fig. 5B). Approximate ages along the lines are shown for those portions having a simple spreading history.
Several things contribute to the observed basement topography and sediment distribution. On a large scale, thermal aging and subsidence of the Juan de Fuca Plate along with the effects of sediment loading cause the basement surface to deepen with age. Departures from a simple monotonic and consistent rate of subsidence are evident, both in the way of departures from a smooth basement depth vs. age increase and in a general trend of increasing basement depths to the south over the latitudinal range covered by the profiles. The cross-strike variations may be associated with long-term variations in effective axial lithospheric temperatures and crustal volcanic supply. The along-strike variations may be related to original variations in depth associated with ridge segmentation. On a small scale, topographic relief created by normal faulting and variations in volcanic supply at the rift axis is present (e.g., Kappel and Ryan, 1986). The amplitude of this relief is typically about 100 m, although relief of over 600 m is present in the eastern portion of the northern line. Small seamounts also create local relief, and at five locations, edifices rise above the sediment surface. With the exception of these seamounts and the elevated region lying within about 20 km of the ridge axis, the basement relief is fully buried by flat-lying turbidite sedimentary deposits derived from the nearby continental margin during the Pleistocene.