Focused Magmatism versus Amagmatic spreading along the ultraslow-spreading Southwest Indian Ridge. Evidence from TOBI side scan sonar imagery

International audienceThe analysis of the Towed Ocean Bottom Instrument (TOBI) side scan sonar images along theSouthwest Indian Ridge between 63400E and 65400E reveals strong focusing of magmatic activityand long amagmatic accretionary ridge segments. Fresh-looking volcanic terrains are observed atdistinct locations along the axis separated by highly tectonized and sedimented terrains of an alongaxisextent as much as 82 km. The largest tectonized section corresponds to a dramatically thin crustarea with moderate magnetization anomalies. We suggest that seafloor spreading is mainly amagmaticin this tectonized section of the Southwest Indian Ridge with upper mantle rocks exposed at theseafloor. Amagmatic accretionary ridge segments of such dimensions are quite distinct from what isobserved at the Mid-Atlantic Ridge but are also recognized at the Gakkel Ridge and may thus becharacteristic of ultra-slow spreading ridges. The correlation between the distribution of fresh-lookingvolcanic terrains, the occurrence of shallow areas crowned by axial volcanic ridges, and highmagnetization values suggests a shallow segmentation of the ridge mainly related to variation in thethickness and/or the intrinsic magnetization of the basaltic source layer. By contrast, strong along-axisvariations of the gravity-derived crustal thickness are shrunken in length relative to this shallowsegmentation of the ridge and occur only beneath the elevated segments. Adjacent to these elevatedsegments, small bathymetric swells with fresh-looking volcanic constructions do not correspond tothicker crust areas. This suggests a highly focused melt supply beneath the elevated segments whichmay feed volcanic constructions up to 60 km from the center of these segments by shallow lateral meltmigration in the crust, probably through large dikes. Neither the ultra-slow spreading rate nor the ridgeobliquity explains the variation of the magmatic vigor along the ridge. Mantle source heterogeneitiestogether with lower mantle temperatures beneath the easternmost Southwest Indian Ridge could partlycontrol its segmentation