Takaaki Moriguchi

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.

Mylonitic mantle peridotites exposed at the Tosa Megamullion in the Shikoku Basin, Philippine Sea, provide direct evidence for amagmatic ductile shear deformation of the upper mantle beneath a back-arc spreading center. Oceanic core complexes (OCCs), or megamullions, are dome-shaped structures formed by detachment faulting and occur locally along slow-spreading mid-ocean ridges and back-arc basins, where they expose fault rocks derived from ductile shear zones in the lower crust and upper mantle. The Shikoku Basin hosts several OCCs, including the Tosa Megamullion, which formed during the early stage of back-arc spreading. In this study, nine ultramafic rocks were collected from the Tosa Megamullion using the submersible Shinkai6500 during cruise YK23-05S. Although all samples were highly serpentinized, several preserved primary peridotitic textures were composed mainly of olivine, orthopyroxene, with subordinate clinopyroxene, plagioclase, and spinel. Seven samples exhibit well-developed foliation and porphyroclastic textures dominated by orthopyroxene porphyroclasts, ranging from rounded to strongly elongated forms, commonly showing microkinks and undulose extinction. Crystallographic preferred orientations (CPOs) of three representative samples, analyzed using SEM-EBSD, reveal E-type-dominant olivine fabrics characterized by the (001)[100] slip system, with a subordinate contribution from C-type (100)[001] slip. These CPOs suggest deformation under non-dry conditions involving moderate hydration and/or elevated differential stress. These results indicate that the ultramafic rocks from the Tosa Megamullion represent mantle-derived mylonitic peridotites formed by ductile shear beneath the spreading axis and subsequently exhumed under strongly magma-poor, amagmatic conditions. The Tosa Megamullion thus represents an amagmatic end-member of the OCC formation in back-arc basins, dominated by tectonic strain localization rather than by magmatic accretion.