The Southwest Indian Ridge between 49 degrees 15 ' E and 57 degrees E: focused accretion and magma redistribution

Bathymetric, gravity, magnetic and backscattering strength data have been u sed to characterise the segmentation of an 800 km long portion of the ultra slow-spreading Southwest Indian Ridge (SWIR, full rate 14 mm/yr) between 49 *15'E and 57E. This analysis reveals that the segmentation defined by along -axis variations of depth and by occurrence of axial offsets does not syste matically correspond to the segmentation determined by the along-axis varia tions of backscattering strength, mantle Bouguer anomaly (MBA) and amplitud e of the central magnetic anomalies (CMA). At axial discontinuities with of fsets larger than 15 kin, thin crust and reduced volcanic production are su ggested by the occurrence of MBA highs, almost non-existent CMA and 50% low er backscattering strength relative to the segment centres. By contrast, sm aller non-transform discontinuities, with offsets smaller than 15 km, corre spond to very weak variations or to no variation of the MBA, the CMA or the reflectivity of the seafloor, suggesting that there is little or no variat ion of volcanic production and crustal thickness associated with those smal l discontinuities. These small axial discontinuities bound low-relief bathy metric segments (500-700 m), corresponding to weak or no MBA lows (amplitud e < 11 mGal), and robust high-relief segments ( > 1000 m), corresponding to large MBA lows (amplitude > 30 mGal). We suggest that the magma supply to these low-relief segments is controlled by near-surface processes such as m elt migration and/or crustal plumbing from adjacent high-relief segments. P ronounced MBA lows at high-relief segments are thought to correspond to spr eading cells where magma supply is focused in the mantle. These spreading c ells are spaced by about 100 kin along the SWIR axis. We suggest that the s pacing of spreading cells on slow-spreading ridges is primarily controlled by the spreading rate with larger spacing between spreading cells on ultras low-spreading ridges than on slow-spreading ridges. (C) 2001 Elsevier Scien ce BN. All rights reserved.