Abstract
There are a lot of marginal basins around Circum-Pacific Zone from the north to the south. One of large marginal basins is the South China Sea (SCS) . Understanding the development from syn-rift to spreading stage of the SCS is important in elucidating the western Pacific s tectonic evolution because the SCS is a major tectonic constituent among many marginal seas in this region.
This paper presents researches examining the transition from rifting to spreading along the northern margin of the SCS, made possible by the amalgamation of newly acquired and existing geophysical data installed in Frontier Database. The northernmost SCS was surveyed as a part of a joint Japan-China cooperative project (JCCP) in two phases in 1993 and 1994.The purpose of the investigation was to reveal seismic and magnetic characteristics of the transitional zone between continental crust and the abyssal basin. Compilation of marine gravity and magnetic data of the South China Sea clarified structural characteristics of its rifted continental and convergent margins, both past and present. Total and three component magnetic data clearly show the magnetic lineations in the oceanic basin and the magnetic characteristics of its historical development.
The analyses of magnetic, gravity and seismic data and other geophysical and geological information of the SCS using Frontier Database system lead the following results:
(1) N-Sdirection seafloor spreading continued from 32 to 17Ma. There were at least four separate evolutional stages. Directions and rates of the spreading are fluctuating and unstable.
(2) The apparent difference in the present tectonism between the eastern and western parts of Continent Ocean Boundary (COB) implies that the eastern side of the continental breakup is governed by a strike slip faulting.
(3) Seismic high velocity layer seems to be underplated beneath the stretched continental crust.
(4) Magnetic quiet zone anomaly (MQZ) in the continental margin area coincides with COB.
(5) The non-magnetic or very weakly magnetized layer is probably responsible for MQZ.
We are currently developing a synthetic database system containing datasets of seismicity, potential field data, crustal and thermal structures, and other geophysical data to facilitate the study of past, contemporary and future changes in the deep sea environment around Japan; i.e. trench, trough, subduction zones, marginal basins and island arcs. Several special characteristics are an obj ect-oriented approach to the collection and multi-faceted studies of global data from a variety of sources.
