Fault Characteristics and Strain Accommodation at the Ultra Slow Spreading South West Indian Ridge (SWIR).

Spreading rate is one of the fundamental physical processes controlling mid-ocean ridge structure. The ultra slow spreading (16mm/yr full rate) and expected low magma budget of the SWIR suggest that faulting characteristics may be particulary distinctive. Brittle failure extending through the crust into the upper mantle and extension along these faults forming low angle detachment surfaces along with normal faulting may produce fault patterns and parameters very different to those observed at faster spreading ridges. Data collected from the FUJI and INDOYO cruises of 1997 and 1998 have been used to map the faults at 64 deg. East, Segment 11 of the SWIR. TOBI sidescan sonar images and shipboard bathymetry display faults to a resolution of 100m. TOBI phase bathymetry supplements the datasets with a resolution of 20m. Fault parameters have been measured and strain calculations performed to make a direct comparison with 29 deg North of the slow spreading (25mm/yr) Mid-Atlantic Ridge (MAR). (JGR B, 104, 10,421. 1999) Total tectonic strain at the SWIR is shown to be 9% compared to 11.5% at the MAR. The strain measurements at the outside corner flank of each segment are more comparable with 6.67% and 6.8% at the SWIR and MAR respectively. Strain results for the inside corner and strain accommodation through outward facing faults at each ridge will also be presented. At the MAR tectonic strain remains constant along the segment axis suggesting that variations in fault development may be reflecting variations in the rheology of the lithosphere and not variations in tectonic strain or magmatic supply. The situation at the SWIR will be discussed. Gravity measurements are expected to show a Mantle Bouguer Anomaly over the FUJI Dome (inferred detachment surface) that will be modelled to determine relative crustal thickness. Sub-seafloor density measurements will discriminate crustal from mantle rocks and provide information about the origin of the FUJI Dome. Magnetic signatures from the detachment feature and the conjugate area to the north of the axis will be presented and examination of the TOBI sidescan image to the north should determine whether the detachment initiated during an amagmatic period at the ridge axis.