NEW INSIGHTS OF DEEP-SEA GEOLOGIC ARCHITECTURES USING SUBMERSIBLES: EXAMPLES FROM THE NANKAI TROUGH

This paper deals with insights gained from submersible observations of geologic architectures in accretionary prisms.

The concepts of accretionary prism evolution were established from on land studyiesin the Southern Uplands of Scotland by McKellow, Leggett et al. (1977). These studies implied that accretionary prisms are currently growing in the present deep-sea regions. Under the sea, many geologists and geophysicists surveyed such active accretionary prisms and related oceanic-plate-subduction phenomena in many regions. They clarified geologic architectures of accretionary prisms thoroughly using seismic images coupled with DSDP and ODP drill cores.

The combination survey using drilled cores and seismic images showed 2- to 3-D geologic architectures within a few kilometers of the sea floor. In contrast to the oceanic surveys, we commonly use three methods to understand the geologic architectures on land: drilling, geophysical exploration (including seismic surveys) and outcrop observations. The outcrop observations can clarify meter-scale geologic mesostructures. We then extract representative geologic architectures in that region from such mesostructures. It plays a key role in understanding of the geologic architectures that are shown by drilled cores and geophysical exploration data.

We will show mesoscopic geologic observations of the active submarine Nankai prism using submersibles. Prior to our surveys, manned-submersibles Nautile and Shinkai2000 dove into this region mainly during the Japanese-French KAIKO projects and subsequent KAIKO projects in the 1980’s. Our dive project was to observe the geologic architectures along deep-sea canyons using manned- and unmanned-submersibles. Those submersible studies disclosed detailed 3-D internal prism architectures and sub-surface architectures (e.g. submarine slides, seepages and so on).