Hakuyu Okada

Nature and development of Late Mesozoic and early cenozoic sedimentary basins in southwest Japan

Abstract The southwestern part of the Japanese Islands is a typical example of an active continental margin characterized by backarc, intra-arc and forearc sedimentary basins. The back-arc basins provide evidence of tectonic continuity with the Asian continent. The basins are genetically classified as half-graben, ridge and forearc basins. The half-graben and ridge basins are controlled by strike-slip faulting, although the half-graben basins are pull-apart basins on a much larger scale than the ridge basins and are in many instances closely associated with igneous activity. Forearc basins occupy the area from the deep-sea terrace to the trench floor. Forearc basin-fill sediments form accretionary prisms as a result of subduction processes. Good examples of these basins are well developed in the Cretaceous and Paleogene sediments of southwest Japan.

Plume-related sedimentary basins in East Asia during the Cretaceous

Abstract Many Cretaceous sedimentary basins of various sizes occur in East Asia, which are classified into four geotectonic settings: forearc, intra-arc, back-arc, and intra-continental basins. Among these, intra-arc and back-arc basins are characterized by rifting and pull-apart origins, and intra-continental basins by rifting related to large-scale magmatism or plume. The pull-apart basins in Southwest Japan are closely related to plate-subduction magmatism. The continental region rift basins in the Early Cretaceous, however, seem to have no direct relation to subduction-controlled magmatisms, because almost no accretionary prism was developed along the continental margin at that time. This means that plume-related magmatism played an important role in the eastern part of the Asian continent due to return flow effects from the 670 km thermal boundary layer. Such a plume magmatism explains systematically the origin and development of varieties of rift basins in East Asia in Cretaceous time.

Photographic evidence of variable bottom-current activity in the Suruga and Sagami Bays, central Japan

Abstract Complex patterns of bottom-current behaviour were clarified by studies of sedimentary features and orientations of benthic animals in the deep Suruga and Sagami Bays on the Pacific side of central Honshu, Japan. Both the Suruga and Sagami Bay measure about 60 km in length as well as in width at their mouths and are about 1500 m deep in their central portions. The size of each bay is comparable to that of ancient forearc basins. The bottom sediments are characterized by turbidites and slump deposits. At 32 stations on the bottom of the bays, deep-sea photographs were taken, most of which clearly indicate bottom-current activity. Current-induced bottom features are current-lineations, moat-like scours around resistant objects, crag-and-tail structures behind obstacles, ripple marks, sand ridges and deformed biogenic structures such as burrows, mounds, tracks and faeces. These features are produced by bottom currents with rather high velocities. Other important current indicators are some benthic organisms, which in general show a sensitive response to currents and adopt particular orientations. Typical examples of megabenthos identified in the bottom photographs as effective current indicators are the small deimatid holothurian Peniagone japonica , the benthic shrimp Glyphocrangon hastacauda , sea anemones, and sea pens. Among them, the orientation of Peniagone japonica shows abrupt changes of current direction with time, for example, from N (0°) to SW (240°) during 15 min and from N to S and back to N, a complete rotation during 40 min. The results of these observations indicate that the bottom currents in deep bays tend to fluctuate rapidly in velocity and direction, probably owing to strong internal tidal waves in the very steep embayments. Upslope currents appear to be present at the mouths of submarine canyons. Thus, it should be borne in mind that palaeocurrent analysis of ancient bottom-current deposits or contourites is limited in application.

Anatomy of trench-slope basins: Examples from the Nankai Trough

Abstract Along convergent margins, forearc basins and trenches are most important as major sedimentary basins of terrigenous materials. In addition to them, small basins are generally developed on the slope between the trench-slope break and the toe of the slope. In this study some characteristic features of the inner trench-slope basins of the Nankai Trough are shown on the basis of the sea-beam maps and single-channel seismic profiles obtained on board RV Jean Charcot on Leg 1 of the Kaiko Project Phase I (1984). The study areas cover the inner trench slope off Shikoku and off the Enshu-nada Sea, central Honshu. In both areas the inner trench-slope basins vary in size from a few kilometers to 10 km wide and 10–25 km long, being filled with turbidites 200–1000 m thick. These basins are controlled tectonically, indicating some contrasting features between those off Shikoku and those off the Enshu-nada Sea. The basins off Shikoku are developed in front of major thrusts (thrust-controlled), whereas the basins off the Enshu-nada Sea represent the synclinal axes of the intensely folded accretionary wedges (fold-controlled). In-filling sediments are characterized by turbidites transported mostly longitudinally and sometimes by slide deposits supplied laterally. Some of the sediments seem to have been subject to recent tectonic activities. These features are useful for a better understanding of ancient forearc basin development.