New Views of Seafloor Spreading Processes on the Very Slow Spreading Southwest Indian Ridge

We have studied segment 11 of the Southwest Indian Ridge near 64°E using bathymetry, gravity, magnetic field and deep-towed sidescan sonar. These have been combined to yield new information on the recent (<12Ma) spreading history of the ridge. We used seafloor gradient to help investigate faulting, gravity anomalies to estimate crustal thicknesses, magnetic field inverted with bathymetry to yield crustal magnetisation and infer crustal age, and sidescan draped on bathymetry to show details of volcanism and tectonism. The plate separation rate was 13.9±1 km/Ma, but spreading was highly asymmetric with each plate successively spreading ~50% faster with ~6Ma episodicity. The average crustal thickness of the African plate is confirmed to be ~50% greater than that of the Antarctic plate but is not closely correlated with spreading rate. Seafloor morphology is a mixture of lineated fault-bounded volcanic ridges combined with more equant massifs of probably predominantly volcanic origin. The African flank tends to have more regular, lineated topography like the Mid-Atlantic Ridge. We reconstructed maps of combined bathymetry and crustal thickness at various chrons. They show symmetric crustal thickening focused on the segment centre for the last 1-2 Ma reflecting the growth of highly focused magmatism at Mt. Jourdanne. Massif h8/m8 (Cannat et al. 2003) grew between 6 and 4 Ma over mainly symmetrically thickened crust and was rifted somewhat asymmetrically at 3 Ma. At other times, crustal thickness varied along the segment and was often different on the two plates. Sidescan sonar draped on bathymetry shows predominantly extrusive volcanism on the surface of the African plate, while the Antarctic plate is more disrupted by tectonism. The development of Mt. Jourdanne is accompanied by propagation of extrusive volcanism from the segment centre to its end. The FUJI Dome core complex is seen to be part of a continuous tectonic block whose exposed surface appears to grade continuously from extrusive in the segment centre to basement at the segment end.