Vol.6 No.3 2014
Research paper : Marine geological mapping project in the Okinawa area (K. ARAI et al.)−162−Synthesiology - English edition Vol.6 No.3 (2013) 4.1 Investigating potential sites in the Okinawa TroughFor this study, we defined potential sites as areas in which the hydrothermal activities currently occur or may have occurred in the past. In the Okinawa Trough, hydrothermal activity may occur at the juncture of the rift axis and intercepting faults, as well as in the calderas of the submarine volcanic front associated with the arc-trench system. Previous surveys around Japan have noted the presence of hydrothermal deposits around submarine calderas and structural depressions in the volcanic fronts of back-arc rifting zones in the Izu-Ogasawara Arc and Okinawa Trough (Fig. 1). Examining the subsurface geological structure, the distribution of fault systems, and the distribution of submarine volcanoes and calderas, may reveal previously unknown hydrothermal deposits in the Okinawa Trough. Moreover, identifying oceanic regions with a similar geological history may help locate mineral deposits in areas that are no longer hydrothermally active. The survey area for the 1:200,000 marine geology maps (Fig. 5) does not extend to the trough surveyed in the Okinawa Project. Therefore, to identify potential sites, it is necessary to expand the survey area to include the back-arc basin of the Okinawa Trough.4.2 Significance of uniform grid data for discovering potential sitesMarine geophysical surveys are conducted along survey lines perpendicular to the strike of the geological structure or deformation. For the 1:200,000 marine geological maps, survey lines were spaced approximately two nautical miles apart. Data were collected using a narrow multi-beam echo sounder and parametric sub-bottom profiler (SBP), while simultaneously conducting seismic reflection profiling, gravity surveys, and magnetic surveys. The cross line was set at intervals of four nautical miles. By doing this, it was possible to capture geological structures such as faults and fold axes with lengths of 5 km or more. Sediment sampling was conducted at the intersections of survey lines and cross lines. The collection and analysis of such uniform data sets are essential for understanding the whole region, and in limiting the possibility of overlooked structures. 4.3 Current situation and issues affecting the survey of potential sites4.3.1 Geophysical surveysThe seismic reflection profiling survey is one of the basic methods to know the marine geological structure. The Izena Hole is one potential site for submarine hydrothermal deposits in the juncture of the rift axis and intersecting faults of the Okinawa Trough. Identification of the geological structure of such sites within the back-arc region can be completed using seismic reflection profiling. Using this technique, we succeeded in obtaining cross-sections showing the geological structure of several submarine calderas of the Okinawa Trough during GH09-12 cruises. By combining seismic reflection profiling with rock sampling, gravity profiling, and magnetic profiling, we were able to determine the age, tectonics, and formation mechanisms of the calderas.However, identifying hydrothermal deposits requires vertical and horizontal resolutions down to several meters, since deposits are composed of mixed sediments from the caldera wall and surrounding area. Also, potential deposits in back-arc basin are generally located in areas with complex topography near caldera walls, and imaging these conventional reflection profiling is difficult. Therefore, additional methods with greater accuracy, such as high-resolution acoustic surveys combined with gravity and magnetic profiling, are required to locate hydrothermal deposits.4.3.2 Stationary observation surveysFor the conventional sampling, the stationary surveys we conducted included surface sediment sampling over survey grids using a grab sampler, columnar core sampling, and rock sampling using a dredge in steep areas with exposed rocks. The surface sediment samples we collected have not previously been used to search for indications of hydrothermal activity. However, it can be achieved by leaching the sediment samples using a weak acid, and Two way travel time (sec)Trough axisOkinawa Trough2.52.01.51.00.5NWSE10 kmFig. 6 Cross-section of the seismic profile obtained in the GH11 cruise (from Reference )The seismic profile cross-section perpendicular to the Okinawa Trough. The internal reflectors clearly display stratified sediments, which are intersected by normal faults (dashed line).