SUMMARY OF SCIENTIFIC RESULTS

Chronostratigraphic Synthesis
Detailed bio- and magnetostratigraphic records were obtained at all sites facilitating correlation among sites. Paleomagnetic results for Site 1173 and 1174 show clear geomagnetic changes from the middle Miocene to Pleistocene (Fig. 34) and can be correlated to the polarity changes at Site 808. The basal age of the trench-wedge turbidites including the transition zone to the upper Shikoku Basin facies is estimated to be ~0.8 Ma. The boundary between the Upper and lower Shikoku Basin at Sites 1173 and 1174 is interpreted to be 3—5 Ma. The age of the décollement zone at Site 1174 is estimated to be from 5.894 Ma (beginning of C3A Chron) to 6.935 Ma (termination of C3B Chron, which corresponds with the age of décollement zone at Site 808). At Site 1177 located in the Ashizuri Transect, the lower Shikoku Basin facies extends from the Pliocene to early Miocene, although it is difficult to identify the geomagnetic records because of poor core recovery (Fig. 34). Because Site 1177 was drilled without coring to 300.2 mbsf, correlations could not be made between Site 1177 and Site 582.

Pleistocene to Pliocene paleomagnetic data are similar at Sites 1175 and 1176 and document a few geomagnetic excursions within the Brunhes Chron.

At Sites 1173, 1174, 1177, and 808, the trench-wedge turbidites and the Shikoku Basin show contrasting sedimentation rates in the age-depth plots (Fig. 35). The sedimentation rates of the trench turbidites within the transition zone of each site shows high values with the highest at Site 808 (204 m/m.y. at Site 1173; 698 m/m.y. at Site 1174; and 842 m/m.y. at Site 808). In contrast, sedimentation rates are slower in the upper and lower Shikoku Basin facies. Rates for the upper Shikoku Basin facies are 77.82 m/m.y. at Site 1173, 78.46 m/m.y. at Site 1174, and 81.62 m/m.y. at Site 808. The sedimentation rate of the upper Shikoku Basin facies at Site 1177, however, is lower (40.37 m/m.y.). Sedimentation rates for the lower Shikoku Basin facies are lower than those of the upper Shikoku Basin facies. The lower Shikoku Basin facies rates are 27.40 m/m.y. at Site 1173, 35.22 m/m.y. at Site 1174, 28.27 m/m.y. at Site 1177, and 31.67 m/m.y. at Site 808.

Magnetic susceptibilities proved a valuable tool for correlation among sites (Fig. 8A) and help constrain the correlation of the décollement and incipient décollement horizons at Sites 1173, 1174, 808, and 1177.