3-DIMENSIONAL GRAVITY STUDY OF THE MID-ATLANTIC RIDGE - EVOLUTION OF THE SEGMENTATION BETWEEN 28-DEGREES-N AND 29-DEGREES-N DURING THE LAST10-MY

During the high-resolution survey SARA (Segmentation Ancienne de la Ri de Atlantique), Sea Beam bathymetry, magnetic, gravity, and seismic re flection data were collected on the flanks of the Mid-Atlantic Ridge, between 28-degrees and 29-degrees-N. This survey was designed to provi de off-axis information (up to approximately 10 m.y.) and to complemen t a detailed on-axis survey carried out in 1988 and 1989 (Sempere et a l., 1990) between the Atlantis and Kane fracture zones. A previous gra vity study had revealed the existence of ''bulls-eye'' shaped gravity lows centered on the axial segments and gravity highs centered on the non-transform discontinuities (Lin et al., 1990). We carried out a thr ee-dimensional calculation of the mantle Bouguer anomaly in order to i nvestigate if the axial pattern of circular anomaly lows can be follow ed on the flanks of the spreading center. The off-axis gravity anomali es are characterized by anomaly lows over the centers of the segments and anomaly highs over the discontinuities. After correcting for the g ravity effect of lithospheric cooling away from the ridge, the segment ation configuration determined from gravity data appears to be very si milar to that deduced from bathymetry. Off-axis bathymetry is characte rized by southward pointing, V-shaped basins which indicate that the t races of the nontransform discontinuities do not follow plate motion f low lines and that each ridge segment advances and retreats continuous ly. However, the gravity trace of the discontinuities is always slight ly offset with respect to the bathymetric lows: northward on the weste rn flank and southward on the eastern flank. The sense of offset betwe en the gravity and bathymetric traces appears to be related to the rig ht-stepping nature of the axial discontinuities.