THE MID-ATLANTIC RIDGE BETWEEN 29-DEGREES-N AND 31-DEGREES-30'N IN THE LAST 10 MA

The segmentation of the Mid-Atlantic Ridge between 29 degrees N and 31 degrees 30' N during the last 10 Ma was studied. Within our survey ar ea the spreading center is segmented at a scale of 25-100 km by non-tr ansform discontinuities and by the 70 km offset Atlantis Transform. Th e morphology of the spreading center differs north and south of the At lantis Transform. The spreading axis between 30 degrees 30'N and 31 de grees 30'N consists of en echelon volcanic ridges, located within a ri ft valley with a regional trend of similar to 040 degrees. South of th e transform, the spreading center is associated with a well-defined ri ft valley trending similar to 015 degrees. Magnetic anomalies and the bathymetric traces left by non-transform discontinuities on the flanks of the Mid-Atlantic Ridge provide a record of the evolution of this s low-spreading center over the last 10 Ma. Migration of non-transform o ffsets was predominantly to the south, except perhaps in the last 2 Ma . The discontinuity traces and the pattern of crustal thickness variat ions calculated from gravity data suggest that focused mantle upwellin g has been maintained for at least 10 Ma south of 30 degrees 30' N. In contrast, north of 30 degrees 30'N, the present segmentation configur ation and the mantle upwelling centers inferred from gravity data appe ar to have been established more recently. The orientation of the bath ymetric traces suggests that the migration of non-transform offsets is not controlled by the motion of the ridge axis with respect to the ma ntle. The evolution of the spreading center and the pattern of segment ation is influenced by relative plate motion changes, and by local pro cesses, perhaps related to the amount of melt delivered to spreading s egments. Relative plate motion changes over the last 10 Ma in our surv ey area have included a decrease in spreading rate from similar to 32 mm a(-1) to similar to 24 mm a(-1), as well as a clockwise change in s preading direction of 13 degrees between anomalies 5 and 4, followed b y a counterclockwise change of 4 degrees between anomaly 4 and the pre sent. Interpretation of magnetic anomalies indicates that there are si gnificant variations in spreading asymmetry and rate within and betwee n segments for a given anomaly time. These differences, as well as var iations in crustal thickness inferred from gravity data on the flanks of spreading segments, indicate that magmatic and tectonic activity ar e, in general, not coordinated between adjacent spreading segments.