RIDGE SEGMENTATION AND THE MAGNETIC-STRUCTURE OF THE SOUTHERN MID-ATLANTIC RIDGE 26-DEGREES-S AND 31-DEGREES-35-DEGREES-S - IMPLICATIONS FOR MAGMATIC PROCESSES AT SLOW-SPREADING CENTERS

Along-axis profiles of three-dimensional magnetic inversions for the M id-Atlantic Ridge (MAR) 31 degrees-35 degrees S show low magnetization near the middle of ridge segments and high magnetization at the segme nt tips for three adjacent spreading segments; thus there is an invers e relation between axial magnetization and axial topography. The ridge segment at 26 degrees S on the MAR has the same inverse relationship between magnetization and topography. The common occurrence of this re lationship suggests that it reflects a fundamental process of crustal accretion at the MAR. We analyze the rock magnetic properties from 42 locations within the four ridge segments in the South Atlantic to cons train the inherent trade-off between source intensity and source thick ness in the magnetization model. The natural remanent magnetization (N RM) intensities from the four ridge segments, averaged together, corre late with the magnetic inversion profiles. This finding implies that c hanges in the magnetization of the extrusives may account for much of the observed magnetic anomaly amplitude variation. A direct correlatio n of FeO content and magnetization suggests that magnetic anomaly ampl itudes may bean indicator of FeTi-rich basalts at the slow spreading M AR, even though the iron content of the basalts from high magnetizatio n areas is not as high as observed at Pacific spreading centers. Despi te the different magma plumbing systematics of the Pacific spreading c enters and the MAR, it appears that the segment-scale magma system of the MAR also results in segment-scale crustal magnetization variations . Further evidence that the axial magnetic variations result from sour ce intensity variations is that older isochrons have higher intensitie s near the ridge-discontinuities, similar to the behavior on-axis. Bet ween 0 and 5 Ma the decay in magnetization is similar to 50% independe nt of location within a spreading segment.