Vol.6 No.3 2014

Research paper : Marine geological mapping project in the Okinawa area (K. ARAI et al.)−161−Synthesiology - English edition Vol.6 No.3 (2013) An active back-arc basin, known as the Okinawa Trough, runs almost parallel to the Ryukyu Arc on its northwestern side.[11] The trough is 1,000 km long and 200 km wide, with a depth that gradually increases along the axis from north to south, reaching a maximum of 2,000 m. Although there is some disagreement over when formation of Okinawa Trough began, it is generally considered to have been formed during the Pleistocene, based on evidence from seismic stratigraphy.[12] A volcanic front, including submarine volcanoes, runs from Kyushu to the offshore area northwest of Okinawa-jima Island in the Okinawa Trough.[4][13] While several previous studies have examined the Cenozoic tectonics that formed the Ryukyu Arc and the back-arc basin,[11][14] there is a paucity of data from the northern region, and the tectonics of rifting are not well understood. Therefore, data were collected from the northern Okinawa Trough during the GH11 cruise. The Okinawa Trough is thought to be in an active rifting stage, in which the continental crust is being extended. The northern Okinawa Trough is filled with stratified sediments, with several normal faults cutting the trough (Fig. 6). Rotated fault blocks dip northwest on the northwest side of the trough and toward the southeast on the southeast side,[15] and the sedimentary layer is thinnest in the axial region. The axis of rifting does not necessarily correspond to the areas of deepest bathymetry. In general, the axis of rifting is northeast-southwest, crossing the Okinawa Trough, but it is intersected by en echelon faults.[16] These observations demonstrate the importance of examining underlying geological structure, in addition to bathymetry, to gain a full understanding of the geology of these marine areas. 4 Development of submarine mineral resourcesAs an example of the use of the marine geological maps, we shall discuss the development of submarine mineral resources. AIST has developed techniques and processes for the sampling and analysis of surface sediments and geological structure below the seafloor. It is believed that geological structures control the area in which minerals are deposited.[17] Therefore, these processes aid the organization of the geological information, which can then be used to identify potential areas of mineral deposits. In particular, marine geological surveys performed over uniform grids by AIST may help discover potential deposits in previously overlooked areas.Multiple national organizations complete submarine mineral resources assessments using a variety of equipment and technology. The Japan Agency for Marine Earth Science and Technology (JAMSTEC) and various university organizations have conducted submarine mineral surveys, focusing on particular resources and gaining an understanding of formation processes. For example, the Integrated Ocean Drilling Program (IODP) undertook scientific drilling for seabed bacteria in the Iheya-North hydrothermal field of the Okinawa Trough.[18] Japan Oil, Gas and Metals National Corporation (JOGMEC) completed core sampling using the Benthic Multicoring System (BMS), targeting prospective regions for resource development and evaluating deposit sizes. These studies have focused on target resources, whereas AIST studies focus on promising locations. To assess submarine mineral resouces effectively, both strategies are required, and must be conducted with mutual collaboration and cooperation.The following descriptions are the examples of surveys done in the Okinawa Project. The survey for the development of submarine mineral resources based on the marine geological surveys of AIST and the issues will be explained.Fig. 4 Scenario for the data collection, creation, and use for the 1:200,000 marine geological mapsFig. 5 Maps of the survey area covered in the Okinawa Project, and associated seafloor geography (from Reference [1])The blocks depict the scheduled survey area for geological maps. As part of the Okinawa Project, ten 1:200,000 geological maps will be created for areas around the island.databaseSeafloor sedimentation mapsMarine geological mapsGravity and magnetic anomaly mapsAssessment of submarine resourceAssessments of geological hazardsCreation of marine geology databaseUse of data and mapsMagnetic observationGravity observationSeismic reflection profilingGeographical surveyCollection of seawaterCollection of sediment samplesStationary observationGeophysical observationData collection and analysisSedimentologyGeochemistryOceanographyGeophysicsStructural geologyStratigraphyIntegration of elemental technologyEast China Sea continental shelfEast China Sea continental shelfOkinawa TroughOkinawa TroughYonakuni-jima IslandYonakuni-jima IslandIshigaki-jima IslandaIshigaki-jima IslandaMiyako-jima IslandMiyako-jima IslandKume-jima IslandKume-jima IslandOkinawa-jima IslandOkinawa-jima IslandTokuno-shima IslandTokuno-shima IslandAmami-oshima IslandAmami-oshima IslandOkinoerabu-jima IslandOkinoerabu-jima IslandTokara StraitTokara StraitKerama GapKerama GapYaku-shima IslandYaku-shima IslandRyukyu TrenchRyukyu Trench


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