Spiess Ridge: An axial high on the slow spreading Southwest Indian Ridge

We report recent mapping of Spiess Ridge with the Hawaii-MR1 sidescan sonar. Spiess Ridge is an unusual elongate 90 by 50 km volcanic feature on the slow spreading Southwest Indian Ridge near the Bouvet hotspot. The northwest half of the ridge has a narrow ∼7-km neovolcanic zone in MR1 sonar images and a simple magnetic anomaly sequence including possible anomalies C1n and C2n. In contrast, the southeast half shows extrusive volcanism over >40 km with distributed eruption sites, a broadened central magnetic anomaly with no anomaly C2n, and a volcanic ridge radiating from the summit of Spiess Ridge, oblique to the spreading orthogonal trend. The images show no evidence for large-offset normal faults or an axial rift valley typical of slow spreading ridges. Overall, Spiess Ridge has an appearance very unlike that of either fast or slow spreading ridges and more transitional between a spreading ridge and a seamount. We compare the morphology of Spiess Ridge to other large volcanic structures in oceanic rifts. Spiess Ridge and Bouvet Island represent localized zones of excess melting along the Southwest Indian Ridge and we discuss their origin in the light of current ideas on ridge-hotspot interaction.