The magnetic structure of Axial Seamount, Juan de Fuca Ridge

Axial Seamount is a large, volcanically active seamount located in the eastern Pacific on the central Juan de Fuca (JDF) ridge at 46°N, 130′W. Sea surface magnetic anomaly data show that Axial lies completely within crust formed during the Brunhes normal polarity epoch. The edifice, however, does not produce a simple positive magnetic anomaly related to the excess mass above mean seafloor. Several pronounced negative magnetic anomalies are associated with the seamount, including both a circular‐shaped low located west of the summit and an elongate low (trending NW) on the southeast flank of the volcano. These two anomalies form part of a NW‐SE magnetic trough which extends across the seamount edifice oblique to the general JDF ridge trend of N20°E. A third negative anomaly is observed on the northern flank of the seamount associated with North Helium Basin, a deep elongate basin which indents the northeast flank of the seamount. Both inversion of sea surface magnetics data and forward modelling of topography, constrained by magnetization values of dredged samples, show that bathymetric effects can explain the Helium Basin anomaly, but topography alone cannot produce the low magnetization values which extend across the seamount summit. Magnetization of crustal rocks does not vary spatially over the seamount, so that negative anomalies at the summit can be explained by variations in the effective thickness of the magnetic source layer. A simple forward model of uniform magnetization shows that the anomaly source layer would have to thin by 50% (to less than 700 m thick) in order to generate the observed magnetic response. A thinned source layer is consistent with the presence of either a substantial magma chamber at depth beneath the summit of Axial or a zone of substantial crustal alteration. The shape of the anomaly implies the trend of either of these sources must be oriented obliquely to the general ridge trend but parallel to the direction of the absolute plate motion.