Three‐dimensional gravity study of the Mid‐Atlantic Ridge: Evolution of the segmentation between 28° and 29°N during the last 10 m.y.

During the high‐resolution survey SARA (Segmentation Ancienne de la Ride Atlantique), Sea Beam bathymetry, magnetic, gravity, and seismic reflection data were collected on the flanks of the Mid‐Atlantic Ridge, between 28° and 29°N. This survey was designed to provide off‐axis information (up to approximately 10 m.y.) and to complement a detailed on‐axis survey carried out in 1988 and 1989 (Sempéré et al., 1990) between the Atlantis and Kane fracture zones. A previous gravity 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 three‐dimensional calculation of the mantle Bouguer anomaly in order to investigate if the axial pattern of circular anomaly lows can be followed on the flanks of the spreading center. The off‐axis gravity anomalies are characterized by anomaly lows over the centers of the segments and anomaly highs over the discontinuities. After correcting for the gravity effect of lithospheric cooling away from the ridge, the segmentation configuration determined from gravity data appears to be very similar to that deduced from bathymetry. Off‐axis bathymetry is characterized by southward pointing, V‐shaped basins which indicate that the traces of the nontransform discontinuities do not follow plate motion flow lines and that each ridge segment advances and retreats continuously. However, the gravity trace of the discontinuities is always slightly offset with respect to the bathymetric lows: northward on the western flank and southward on the eastern flank. The sense of offset between the gravity and bathymetric traces appears to be related to the right‐stepping nature of the axial discontinuities.