Geological and geophysical characters of the southern Okinawa Trough and adjacent area.

Abstract

The Hydrographic Department of Japan has conducted hydrographic, geological and geophysical surveys in the southwestern Okinawa Trough and adjacent area in 1984. The purpose of this survey is to collect the basic information of Japanese continental shelves and to publish the results in bathymetric chart, geological structure, geomagnetic and gravitational anomaly maps. The surveyed area in this report includes southwestern region of Tunghai Shelf, Okinawa Trough and Nansei Syoto (Ryukyu Island Arc). Okinawa Trough is a back-arc basin behind Ryukyu Island Arc. Whole survey area is included in Eastern China Sea as a Marginal Sea between Asian Continent and Ryukyu Island Arc. It was reported that the Okinawa Trough probably represents a feature of the primary stage of back arc spreading related to subduction of Philippine Sea plate along the Nansei Syoto Trench. High heat flow values, fresh igneous rock intrusions and remarkable graben structures have been observed at the center of the Trough. These evidences have been considered as active back arc spreading occurring in this place. We confirmed that the graben formed en-echelon pattern in the trough axis. We also found a conspicuous small knoll (tentative name is Yaeyama central knoll) at the central graben in this survey area. That knoll have been consisted of two pyroxene andesite. According to the bottom photograph, there is biological colony by Glassy Sponges, Sea Anemones and other benthos at this Yaeyama central knoll. Warm water probably exhales here because of existence of this colony and photographic flickering phenomena. It is supported that the crust of southwestern Okinawa Trough has continental character, because fresh andesitic rocks formed central part of this trough ares. Gravitational data and previous seismic refraction data support this assumption that thin continental crust exists under this surveyed area. The results from interpretation of seismic profiles, sedimentary pattern showing at the central graben formed by rapid subsidence of basement at the axis zone of trough. We could not determined in the present-day’s actual spreading of Okinawa Trough in our data.