A Geophysical Investigation and Geological Interpretation of a Super-Slow Spreading Segment at the South West Indian Ridge (SWIR).

The SWIR has an average full spreading rate of 16mm/year and inter- and intra-segment style and structure are expected to show more extreme characteristics than are observed at faster spreading ridges. Segment 11 at 64 deg. E is a magmatically robust segment, 60 Km in length with a large non-transform discontinuity (NTD) at its western end. Two recent cruises carried out geophyscial investigations both along and across the axis at Segment 11. The FUJI cruise of 1997 collected TOBI sidescan sonar and phase bathymetry, shipboard multibeam bathymetry and gravity and sea surface magnetics. It included a small across-axis box out to 2.8My. TOBI sidescan and shipboard bathymetry have been analysed jointly to interpret seafloor terrains and dominantly magmatic or tectonic regimes and their interaction. The results are compared to the 29 deg. N segment of the faster spreading Mid-Atlantic Ridge. A large detachment surface is seen south of the axis on sidescan images and topographic data. Sidescan, topographic and magnetic data at the conjugate point to the north is used to investigate conditions at the initiation of the detachment. The INDOYO cruise of 1998 provides extended coverage of Segment 11 out to magnetic anomaly 5. Both gravity and magnetic data have been used to further constrain the nature of the detachment surface. Modelling of the gravity anomalies has been used to constrain the relative crustal thickness and subsurface density along and across the segment. Combined bathymetry and gravity datasets also show evidence for potential older inactive detachment surfaces along the inside corner trace. Magnetic anomalies constrain the temporal spreading history over the last 10 million years, and are used to investigate variations in local spreading rate and asymmetry.