The southern Indian Ocean has several prominent aseismic ridges recognized as oceanic large igneous provinces (i.e., the Madagascar Ridge, Del Cano Rise, Crozet Plateau, and Conrad Rise) in the off-axis areas of the Southwest Indian Ridge (SWIR). However, previously obtained magnetic survey lines are sparse and not correctly aligned with the seafloor spreading direction; thus, the detailed spreading history, including the formation of these aseismic ridges, remains an open question. We reconstructed the tectonic history of two segments between the Discovery II and Gallieni fracture zones in the SWIR using newly obtained magnetic data (total and vector magnetic field) and an open-source magnetic dataset. We revealed that the southern Madagascar Ridge and the Del Cano Rise once formed a single bathymetric high and separated by at least Chron 30y, which is quite different from the global age model. In addition, the rises may have formed before Chron 34y, assuming an extinct ridge south of the Del Cano Rise. The two rises have been recognized as having formed by Marion hotspot plume-induced excess volcanism around the active spreading ridge of the SWIR, which can explain locally isostatically compensated thicker-than-normal crust. However, linear magnetic anomalies have not been observed across the main part of these rises, suggesting that magmatic activity controlled by seafloor spreading is unlikely. Like other aseismic ridges in the southern Indian Ocean, these two rises may possibly have been formed partly by continental fragments rather than plume-induced excess volcanism.

Plain Language Summary

The southern Indian Ocean contains several bathymetric high regions previously interpreted as oceanic large igneous provinces. Hotspot-ridge interaction is considered the key factor in the formation of such large volcanic structures. However, the detailed seafloor spreading history and origin of these large volcanic structures are not understood as well as those in other oceans because of the vast and remote Indian Ocean. We conducted magnetic surveys and reconstructed a new tectonic history of two segments along the Southwest Indian Ridge (SWIR). We found that the southern Madagascar Ridge and the Del Cano Rise once formed a single bathymetric high and that they separated by at least 66 million years ago. Moreover, the rises may have formed before 84 million years ago, assuming an extinct ridge south of the Del Cano Rise. The two rises might have formed by interaction between the Marion hotspot and SWIR; however, linear magnetic anomalies implying seafloor spreading are not observed in the main parts of these rises. This absence suggests that hotspot-ridge interaction cannot fully explain the origin of these rises. We cannot exclude the possibility that the two rises are partly continental fragments rather than products of hotspot-induced excess volcanism.