Melt-poor spreading in the Shikoku back-arc basin: geophysical investigation of the initial spreading stage

Abstract Back-arc basins play a crucial role in the formation of oceanic crust, similar to mid-ocean ridges. While magmatic and tectonic processes in the mature spreading stage resemble those in mid-ocean ridges, the initiation and cessation of back-arc spreading remain unclear. The Shikoku Basin and its associated remnant arc in the Philippine Sea are among the most studied back-arc systems, providing an excellent opportunity to understand the detailed processes of oceanic arc rifting and the initial stages of back-arc opening. We have recently collected bathymetry, magnetic and gravity data from the southwestern part of the Shikoku Basin. This area is characterized by a deeper basin floor and numerous oceanic core complexes with high gravity anomalies, indicating that magma-poor spreading was predominant in the initial stage. The magnetic lineation pattern is only discernible in a limited abyssal hill area, but the identified geochrons align with previous studies in the northern part of the basin and new zircon U–Pb age data from a couple of minor gabbroic samples. The morphology of closely located oceanic core complexes suggests a ridge jump. The structural features of the basin floor correlate with the morphology of the remnant arc, the Kyushu-Palau Ridge, implying that the preexisting arc crustal structure influences the style of rifting/spreading and may affect the development of ridge segmentation.