Shikoku Basin and Its Margins

Abstract

The Shikoku Basin is an active backarc basin located south of the southwest Japan arc in the northwestern Pacific margin. Its characteristic features are summarized on the basis of updated geophysical and geological data, including swath bathymetry, gravity, magnetic, and seismic reflection profiling records as well as results from the DSDP/ODP drilling holes. It has been proposed that the Shikoku Basin was born as a rift separating the N-S trending paleo-Kyushu-Palau Ridge at its northern end. The rifting rapidly propagated southward. It was succeeded by seafloor spreading that formed a narrow triangular trough bounded by steep scarps on both east and west margins. Magnetic and bathymetric data indicate that the spreading center has changed its trend at least twice, first at 23 Ma and then at 19 Ma. Widespread off-ridge volcanism occurred after extinction of spreading at 15 Ma. The Kinan seamount chain was formed at this stage. Most of the rocks constituting the basin are tholeiite similar to MORB, whereas some of the off-ridge magmas are alkali basalt. Igneous basement is overlain mostly by hemipelagic sediments containing dispersed detrital clays and interbedded tephra layers. The Shikoku Basin lithosphere is subducting at the Nankai Trough beneath southwest Japan in a NNW direction. Deep-focus earthquakes are not observed at depth greater than 80 km, implying that the subducted young lithosphere loses its rigidity below such depths. It seems plausible to presume that the Shikoku Basin was subducting at the Nankai Trough at 15-12 Ma while the basin floor underwent extensive volcanic intrusions. Emplacement of intermediate to felsic rocks at the outer zone and Setouchi Province of southwest Japan probably originated from this unusual circumstance.