Fine‐scale crustal magnetization variations and segmentation of the East Pacific Rise, 9°10′–9°50′N

Abstract

Sea‐surface scalar magnetic field measurements collected over the East Pacific Rise near 9°30′N and within the Brunhes chron have been inverted for magnetization of the upper crust. A close track spacing (1–3 km) and careful corrections for sources of background noise make this data set particularly suitable for investigating fine‐scale crustal magnetization. A well‐defined axial magnetization high can be traced throughout the survey area but varies considerably along axis. Marked changes in the amplitude of axial magnetization are often accompanied by offsets in its along‐axis linearity (e.g., at 9°25′, 9°37′ and 9°46′N). On the basis of comparisons with seismic reflection data and geochemical measurements on basalts dredged from this rise area, we argue that the along‐axis variation in the axial magnetic high is the combined result of variations in the thickness of the magnetic source layer and in the magnetization of the layer caused by variations in the iron and titanium abundances in the extrusive upper crust. These two explanations are probably coupled, in that a thicker source layer arises from a deeper axial magma chamber, which is more prone to the production of evolved magmas rich in iron and titanium. Because crustal magnetization represents a time‐integrated signal averaged over a substantial volume of rock, the observation of offsets in the linearity of the axial magnetization high implies that such features, which we suggest mark boundaries between distinct magmatic systems within the axial crust, are stable on timescales of at least several tens of thousands of years. A distinct pattern of short‐wavelength (∼5 km) magnetization anomalies parallel to the rise axis to the north of 9°25′N correlates well with short reversed subchrons within the Brunhes chron. Although independent observations are needed to confirm this explanation, the correlation offers the potential that the determination of fine‐scale magnetization anomalies may prove useful for investigating temporal details of crustal evolution at fast spreading ridges.