Ridge segmentation and the magnetic structure of the Southern Mid‐Atlantic Ridge 26°S and 31°–35°S: Implications for magmatic processes at slow spreading centers

Abstract

Along‐axis profiles of three‐dimensional magnetic inversions for the Mid‐Atlantic Ridge (MAR) 31°–35°S show low magnetization near the middle of ridge segments and high magnetization at the segment tips for three adjacent spreading segments; thus there is an inverse relation between axial magnetization and axial topography. The ridge segment at 26°S on the MAR has the same inverse relationship between magnetization and topography. The common occurrence of this relationship 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 constrain the inherent trade‐off between source intensity and source thickness in the magnetization model. The natural remanent magnetization (NRM) intensities from the four ridge segments, averaged together, correlate with the magnetic inversion profiles. This finding implies that changes in the magnetization of the extrusives may account for much of the observed magnetic anomaly amplitude variation. A direct correlation of FeO content and magnetization suggests that magnetic anomaly amplitudes may be an indicator of FeTi‐rich basalts at the slow spreading MAR, even though the iron content of the basalts from high magnetization areas is not as high as observed at Pacific spreading centers. Despite the different magma plumbing systematics of the Pacific spreading centers 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 source intensity variations is that older isochrons have higher intensities near the ridge‐discontinuities, similar to the behavior on‐axis. Between 0 and 5 Ma the decay in magnetization is ∼50% independent of location within a spreading segment.