We conducted geophysical surveys, including bathymetry, gravity, and magnetism, within a first‐order segment of the Southwest Indian Ridge (SWIR) between the Prince Edward and Eric Simpson fracture zones (FZs) (latitude 35°–40°E, segment PE), in the vicinity of the Marion hotspot. Segment PE includes four orthogonally spreading second‐order segments (PE‐1, PE‐2, PE‐3, and PE‐4) and a long, oblique axial valley (NTD‐1). Segments PE‐1, PE‐2, and PE‐4 are magmatic, whereas segment PE‐3 and NTD‐1 are characterized by low magmatic activity. Segment PE‐3 is a nascent segment and NTD‐1 contains three tiny magmatic sections. Each low‐magmatic interval along the axis of segment PE lies between two magmatic segments. This segmentation pattern is similar to the SWIR between the Gallieni and Melville FZs; therefore, a strong melt‐focusing process can be expected. Different characteristics of second‐order magmatic segments suggest that the magmatic activity in each segment varies among each other as well as that of the other segments of SWIR. Continuous seafloor morphology and isochrons over off‐axis areas of segment PE‐1 and NTD‐1 suggest that PE‐1 shortened after the C2An chron. The V‐shaped bathymetric structure between segment PE‐1 and NTD‐1 suggests that the melt supply center has migrated westward. This westward melt migration would have reduced magmatic activity at NTD‐1 after C2An. Ridge obliquity may also have reduced magmatic activity. Geophysical characteristics of second‐order segments suggest that magmatic activity of segment PE is mainly controlled by a strong melt‐focusing process and a comparatively low contribution of melt supply from Marion hotspot.
Abstract Detachment faulting is one of the main styles of seafloor spreading at slow to intermediate mid-ocean ridges. However, we have limited insight into its role in back-arc basin formation. We surveyed a remnant back-arc spreading center in the Philippine Sea and determined the detailed features and formation processes of the Mado Megamullion (Mado MM) oceanic core complex (OCC). This was undertaken in the context of back-arc evolution, based on the shipborne bathymetry, magnetics, and gravity with radiometric age dating of the rock samples collected. The Mado MM OCC has a typical OCC morphology with prominent corrugations on the domed surface and positive gravity anomalies, suggesting that there has been an exposure of the lower crust and mantle materials by a detachment fault. The downdip side of the detachment continues to the relict axial rift valley, which has indicated that the Mado MM OCC was formed at the end of the back-arc basin opening. The spreading rate of the basin decreased once when the spreading direction changed after six million years of stable trench perpendicular spreading. The rate then further decreased immediately prior to the end of the spreading when the Mado MM OCC was formed. The existence of other OCC-like structures in the neighboring segment and the previously reported OCCs along the Parece Vela Rift have indicated that the melt-poor, tectonic-dominant spreading is a widespread phenomenon at the terminal phase of back-arc spreading. The decrease in spreading rate in the later stage is consistent with the previous numerical modeling because of the decrease in trench retreat. In the Izu–Bonin–Mariana arc trench system, the rotation of the spreading axis and the resultant axis segmentation have enhanced the lithosphere cooling and constrained mantle upwelling, which caused the tectonic-dominant spreading at the final phase of the basin evolution.
