Three‐dimensional modeling of a magnetic reversal boundary from inversion of deep‐tow measurements

Abstract

A near‐bottom magnetic survey was conducted over the Brunhes/Matuyama reversal boundary near the East Pacific Rise crest at 21° N. Magnetic measurements were made on a level plane approximately 200 m above the sea floor using the Marine Physical Laboratory's deep‐tow vehicle, with precise transponder navigation. Track density was high both parallel and perpendicular to the magnetic lineations in order to study fine scale deviations from two‐dimensionality. The magnetic field on a gridded map was inverted to obtain the crustal magnetization distribution (including the effects of topography) by extension of the Fourier technique of Parker and Huestis [1974]. Linearity of sources parallel to the spreading center was not assumed, nor was upward continuation necessary in this treatment. We found that the polarity transition boundary is extremely straight and sharp and is very close to two‐dimensional even on a scale of hundreds of meters. Deviations from two‐dimensionality, however, occur within the magnetized blocks away from the transition zone. The polarity transition width is narrow, only 1000 m to 1400 m throughout the study area. This suggests a zone of crustal emplacement which is only 600–1000 m wide at the spreading center, which is in excellent agreement with geologic observations in the area. Comparisons are made with a two‐dimensional treatment of the same data from profiles (i.e., assuming linearity of sources). These studies also document a long‐wavelength (≈ 60 km) sinuosity in the trend of the magnetic anomalies. This sinuosity is the result of offsets of the spreading center which are not transform faults but which involve a component of strike slip motion subparallel to the spreading center.