HISTORY OF GEOSCIENCE AND A FUTURE PERSPECTIVE IN JAPANESE TRENCHES: FROM KAIKO AND DSDP STUDY THROUGH 21st CENTURY

Although the Nankai trough had been thought to be a normal faulted basin boundary until the end of 1960s, it now became one of the typical accretionary prism trenches after plate-tectonic ideas and many series of drilling and diving study associated with seismic profiling and other seismological and swath mapping researches. The submarine, ongoing processes were correlatable to the on land, past age examples. Now the accretionary processes with lateral variations were well elucidated by manned submersible study along two canyons. Japan trench, on the other hand, has been considered to be another type of a trench with subduction erosion of trench retreat and trench slope subsidence. However, now it was noticed rather conspicuous thrust-faulted slope of Miocene rocks with a present, local tiny accretionary prism on its toe. At the same time, large-scale submarine landslide is developed elsewhere in the landward slope, the strike of which is controlled by the oceanic plate side lineaments of normal faults on the subducting Pacific slab. The major differences of these two types could be attributed either to the sediment supplied trench floor without normal-faulted slab, or to the sediment starved trench with normal faulted slab, in the former the frontal thrust propagates into the trench turbidites, while in the latter the slope rocks are being slid into the grabens in the trench. One more type of trench-sediment accretion is proposed for the Sagami trough, where the Izu-arc derived volcaniclastic sediments have been accreted toward the Honshu arc side since Miocene. Its SE-end where the TTT-type Boso triple junction rests at the deepest part (9.4 km), unique deep-sea fan deposits are accreted or collapsed in the semi- or un-stable bottomless triangle depth. As a whole, swath mapping, multi-channel seismic profiling, and direct observation by submersibles with deep-towed profiling are effective methods in addition to drilling (coring). Asperity for which the seamount or another slab heterogeneity may work is a current topic. Further deep-drilling with tiltmeter installation should monitor the coming great earthquakes to mitigate the hazards.