Noriyuki Nasu

A summary of Cenozoic tectonic history along the IPOD Japan Trench transect

Along the International Program of Ocean Drilling (IPOD) Japan Trench transect, a mid-Cenozoic period of tectonism without arc volcanism separated an earlier Cretaceous to Paleogene and a later Neogene period of plate convergence and subduction. The Cretaceous to early Paleogene period of convergence is evidenced by andesitic volcanism associated with a large syncline, probably a forearc basin. The seaward flank of the basin is a tectonically thickened sediment sequence inferred to be an accretionary complex. The arc, the syncline, and the thickened sediment sequence resemble an arc-trench system of the size and structure of the Great Valley forearc basin and Franciscan accretionary complex of California, and they were probably continuations of the Yezo geosyncline and associated sequences exposed to the north on Hokkaido. The arc-trench system is contemporaneous with the adjacent Shimanto arc-trench system southwest of Tokyo Bay. In the early Paleogene, volcanism ceased, and part of the thickened sediment sequence of the Cretaceous-Paleogene margin was emergent as a 160-km-wide landmass which is recognized in seismic records as a later Paleogene sediment source. At the end of the Paleogene, this landmass began to subside and volcanism began again, but it was located first briefly on the east, and then on the west of the Cretaceous arc along the present arc of northern Honshu. The land-mass erosion surface is clearly visible as an angular unconformity in multichannel seismic-reflection records; the sequence of Neogene sediment, explosive volcanism, and benthic foraminiferal assemblages recording subsidence from subareal conditions to the present bathyal depths have been studied in Deep Sea Drilling Project (DSDP) cores. Cretaceous subduction appears to have resulted in a very extensive accretionary complex, whereas the Neogene subduction period appears to have been marked by little net accretion in the forearc area but massive subsidence and some erosion of the front of the convergent margin.

Rock fragments and pebbles dredged near Jimmu Seamount, northwestern Pacific

Abstract Fifty-five rock fragments and pebbles were dredged at 2,550–2,800 fathoms (4,650–5,100 metres) near Jimmu Seamount, 700 miles southeast of Kamchatka. Tuffaceous shale, andesitic and dacitic tuff, pyroxene-hornblende dacite-pumice, and augite-hypersthene andesite are the most abundant rock types. Minor constituents in the haul include augite-pigeonite basalt, augite-olivine basalt, olivine dolerite, quartz syenite porphyry, metabasalt, sandstone, and chert. Petrographic and chemical analyses indicate closer affinities with Kuril Islands and Kamchatka volcanic rocks than with Cenozoic Japanese or Pacific Basin types. Probably these rocks were dropped by Pleistocene icebergs carried south and east by ocean currents.

Neogene geological history of the Tohoku Island Arc system

The Pacific-type orogeny in the Tohoku Island Arc is discussed using marine geological and geophysical data from both Pacific and Japan Sea along the Tohoku region. The Tohoku Arc is divided into three belts; inner volcanic and sedimentary belt, intermediate uplifted belt and outer sedimentary trench belt. Thick Neogene sediments which are distinguished in several layers by continuous seismic reflection profiling occur on both sides of the intermediate belt. The dominant structural trend of the Neogene layers is approximately parallel to the coast line and to the axis of the Japan Trench and has a extension of approximately 100 km in each unit on the Pacific side. The trench slope break is an uplifted zone of Neogene layers. The structural trend of the upper continental slope and outer shelf is relative uplift of the landward side. Tilted block movement toward the west is the dominant structural trend on the Japan Sea side. Structural trends which can be seen in both the inner and outer belts may suggest horizontal compressional stress of east to west. Orogenesis and tectogenesis in the Tohoku Arc has been active since early Miocene or latest Oligocene. It may be implied that the Japan Trench was not present during Late Cretaceous to Paleogene, as is suggested by the volcanism of the Tohoku Arc. The basic framework of the present structure was formed during late Miocene to early Pliocene in both the inner and outer belts. Structural movements were reactivated during late Pleistocene.

Large-scale ripple marks on the shelf margin of the Northern Okinawa Trough

Trains of large-scale ripple marks (megaripples and sand waves) were found on the Amakusa and East China Sea shelves bordering the northern Okinawa Trough. Side-scan sonar surveys were carried out in 1974 and 1976 to investigate sea-floor features lying along a proposed submarine cable line. Megaripples were found on the outer margin of the Amakusa shelf between depths of 140 and 200 m. The megaripples were especially well developed at a depth of 167 m. They were typically straight-transverse crested with asymmetrical profiles, and measured 7 to 15 m in wavelength and 0.4 to 1.4 m in waveheight. Formation of the megaripples on the Amakusa shelf is probably controlled by relatively complex oceanographic conditions. A secondary circulation associated with the Gotô-nada clock-wise Current may be responsible for formation of the ripple marks. Local vorticities generated in the coastal boundary layer as a result of curvature of the Gotô-nada Current are known to cause the complex flow pattern at the Gotô and Amakusa shelf margins. The main semidiurnal (M2) tidal current may also interact with these fluid processes.On the East China Sea shelf, megaripples and sand waves were found between depths of 140 and 220 m. Sand waves (∼200 m in wavelength) were observed in seismic reflection profiles. Large-scale lunate megaripples were observed at a depth of 154 m by the side-scan sonar. They had wavelengths of 10 to 30 m and waveheights of 1 to as high as 3 m. It appears from the types and nature of distribution of the megaripples that they are responding to the present-day flow regime, and it is partly ascertained from our observations over an interval of two years that the megaripples appear to be short-term response elements compared wit hteh sand waves. We conclude that the megaripples on the East China Sea shelf are current-formed during peak typhoon flow in August to November. From their distribution, the long term path of the main flow of the Tsushima Current is inferred at the edge of the East China Sea shelf. An area of low sediment mud content (less than 20 per cent) coincides with this path giving further support to our interpretation.

New directions of oceanographic research and development

Interdisciplinary oceanographic studies at the Australian Institute of Marine Sciences, J. Baker the Bedfors Institute of Oceanography - current programme and future directions, S.B. MacPhee FIO marine research programme for the 1990s, Z. Chen IFREMER and the scientific and technological challenges of the ocean, P. Papon JAMSTEC - present and future, I. Uchida activities of the Ocean Research Institute, University of Tokyo and its future development, T. Asai Woods Hole Oceanographic Institution - status and plans, C.E. Dorman Scripps Institution of Oceanography - present and future, E.A. Frieman scientific research at the P.P. Shirshov Institute of Oceanology, V.S. Yastrebov.