Magnetic anomalies and tectonic evolution of the shikoku inter-arc basin

Detailed analysis of magnetic anomalies has revealed a clear pattern of symmetric lineations in the Shikoku Inter-arc Basin, northern Philippine Sea. Amplitudes of anomalies are in general a few hundred nannotesla (gammas, peak to peak), which are moderate compared to those of the normal ocean basins accreted from the mid-oceanic ridges and are relatively larger than those of some other inter-arc basins such as the Parece Vela Basin, Mariana Trough and West Philippine Basin. Correlation of anomalies is usually so good that age identification can be convincingly performed except for the axial irregular zone. Mode of opening derived from the distribution of magnetic anomalies as well as the topographic features provides the evolutionary history of the Shikoku Basin in the following manner: 1) The Kyushu-Palau and Shichito-Iwojima Ridges began rifting at their northern end at about 30 mybp. The rifting propagated towards south at a speed of about 10 cm/ year. 2) After the whole basin was rifted at about 25 mybp, it continued to open symmetrically from the central spreading axis at a half rate of nearly 4 cm/year until about 22 mybp. 3) In the latest stage of opening the spreading became slower and even irregular. The spreading axis jumped in some parts of the basin. A chain of seamounts was formed and widespread Off-ridge intrusions occurred in the eastern portion of the basin.