ACCOMPLISHMENTS AND INTERESTING OBSERVATIONS
The principal objective of Leg 186 was to install two permanent borehole observatories with several seismic and deformation-measuring sensors. A strainmeter, tiltmeter, and two broad-frequency range seismometers were grouted in at the bottom of boreholes drilled deep enough to penetrate higher velocity, indurated rock. This objective was successfully achieved. Figures 8 and 9 show the seafloor installation as well as the arrangement of sensors in the borehole. Actual vibration isolated television camera views of the seafloor unit are shown in Figure 10. In addition, both sites were cored to instrument depth and have yielded scientific results of considerable interest. In all, we recovered 1742 m of core from the two sites, despite the major emphasis on instrument installation. The dominant lithology was diatomaceous silty clay or claystone with many ash and some dolomite layers (Fig. 11).Compared with results from earlier drilling along the Japan trench, the salinity and chlorinity variations with depth exhibit both similar behavior and significant differences. Sites along the deep-sea terrace‹Sites 1150, 1151, 438, and 439‹have chlorinity of 550-550 mM just below the seafloor, decreasing to ~300 mM at a depth of 1000 m. In contrast, sites far from the subducting slab, such as the ODP Leg 126 Sea of Japan Sites 798 and 799, and sites near the trench (Site 536) have chlorinity that does not decrease as much. Figure 12 shows a comparison of the deep-sea terrace data from Sites 1150, 1151, and 438, and data from the Sea of Japan Site 799. Site 1150 has a notable deviation from the monotonic behavior of other sites. Down to 600 m the chlorinity does not drop below 500 mM, but below 650 m, the values drop to those of the other terrace sites. One tentative explanation is that in the terrace sites, there is a supply of freshwater from depth that mixes (possibly continuously) with the saline sea bottom-derived water. In Site 1150, a less permeable layer interrupts this mixing, so that the upper part is isolated from freshwater. In other areas such as the Sea of Japan sites or the near trench sites, this freshwater supply is not available. A possible source of the freshwater is dehydration of slab interface components at depth where earthquakes occur (i.e., deeper than 10 km).
The age of the sediments of Sites 1150 and 1151, as well as those from Sites 438 and 584 farther to the north, are shown in Figure 13. The sedimentation rates derived from these data are shown in Figure 14. Aside from the generally lower rate of sedimentation from 0.5-3 Ma or so, a notable feature is the fairly widespread increase at 6-8 Ma. Many factors can be responsible for this increase and one of the important contributions of Leg 186 is to provide the samples and data that can be used postcruise to constrain these possibilities. The tantalizing range of causes include change in wind direction or increase in the rate of mountain building because of increased tectonic compression.
There have been major changes in the tectonics of Japan. Until 14 m.y. ago, the islands were subjected to east-west tension, and the Sea of Japan was opening. Today, there is strong east-west compression. It is possible that some of the change in sediment flux as well as volcanic output is affected by the changes in the force system at the subduction interface. From Figure 15 it is apparent that there was a major increase in volcanic deposits at Site 1150 ~3 Ma and a decrease in the most recent half a million years or so. At Site 1151, the increase starts at ~4 Ma. Further north (40.6°N) in Hole 438A, the volcanism increased from ~5 Ma until ~2 Ma. The cores collected during 186 will enable us to identify, quantify, and date the ash layers derived from great volcanic eruptions.
Whereas a major goal of Leg 186 is measuring the current deformation resulting from the subduction forces, the fractures and hole deformation data also bear on that problem. The logging of Hole 1150B indicates significant enlargement of the hole by 40% in the east-west direction. This means that the north-south compressive stresses are greater. This is not surprising because the bending of the upper plate caused by the subduction drag would indeed result in east-west tension in the upper layer. The faults observed in the cores from Hole 1150B are consistent with such an east-west tensional stress field (Fig. 16A). Hole 1151D was logged to depths less than 870 m. To that depth there were negligible breakouts, indicating that either stresses are much lower or the rock was much stronger. It is known that the rock at Site 1151 was more stable during drilling than that at Site 1150, but it is not clear what effect is dominant in governing the hole elongation. In addition, Figure 16B shows that the fracture directions are less well organized than those of Site 1150. Figure 17 shows a comparison of the dominantly normal faults in the two holes. The fault density is rather similar, though Site 1150 does have more fractures. Overall, the long-term deformation, as determined by number, orientation, and offset of faults, indicates that the two sites are broadly comparable (Figs. 16, 17). Normal faulting dominates in both holes with the extension direction being west-northwest-east-southeast in both cases. Interpreting the differences will be aided by postcruise analysis of the mechanical properties of the cores. In all the above fields and others not mentioned, postcruise analysis of the data and samples provided by Leg 186 will improve our understanding of the processes in this subduction zone.