RIDGE SEGMENTATION, FAULTING AND CRUSTAL THICKNESS IN THE ATLANTIC-OCEAN

THE Mid-Atlantic Ridge (MAR) between the Kane and Atlantis fracture zones consists of segments 20-85 km in length1; bull's-eve patterns in the mantle Bouguer gravity anomaly field2 centred on several segments associated with narrow rift valleys3 have been interpreted as centres of strong mantle upwelling and thick crust. Here I present a map of normal faults inferred from approximately 5 x 10(4) km2 of Sea Beam bathymetry4 along the MAR between 28-degrees-N and the Atlantis fracture zone. Faults are mapped both on- and off-ridge using a criterion that distinguishes them from volcanic topography. Abyssal hill lineations seem to form at the rift valley walls through the growth of the normal faults; towards ridge segment ends these faults are more widely spaced, with larger throws than those at segment centres, and often define the boundaries of the gravity bull's-eyes. These two different faulting styles, which probably reflect changes in lithospheric strength, are preserved into the rift mountains. Large-throw faults are highly correlated with the mantle Bouguer highs, suggesting that amagmatic extension on the large faults contributes to the crustal thinning inferred towards the segment ends.