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.

Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments

“…The Parker and Huestis [1974] FFT inversion method was used as adapted by Macdonald et al, [1980] for three-dimensional problems. Two multibeam bathymetry grids were combined, in the same way as the Carbotte and Macdonald [1992] and near-bottom anomaly grids, to provide bathymetric data over the same spatial area as the residual magnetic anomaly grid.…”

“…The residual anomaly data points were interpolated onto an even grid with 0.5 km grid spacing using the minimum curvature algorithm from Generic Mapping Tool (GMT) software [Wessel and Smith, 1991] . The grid was then inverted for crustal magnetization using the fast Fourier transform inversion method of Parker and Huestis [1974], as adapted by Macdonald et al [1980] for three-dimensional problems. The inversion assumes a source layer thickness of 1 km and uniform geocentric axial dipole magnetization direction (declination of 0' and inclination of 400).…”

Tectonic Background of Four Hydrothermal Fields Along the Central Indian Ridge

“…Total geomagnetic field intensity and shipboard gravity data were also obtained along most of the survey lines. We calculated the magnetic anomaly by subtracting the appropriate International Geomagnetic Reference Field models, and then estimated the equivalent magnetization using the three-dimensional inversion method of (Parker and Huestis 1974;Macdonald et al 1980), assuming a 500-m-thick magnetized layer. We also calculated the free-air gravity anomaly, and then estimated the Mantle Bouguer anomaly by subtracting the predicted gravity effects of the seafloor relief and a 6-kmthick crust from the free-air gravity anomaly (Kuo and Forsyth 1988).…”

The influence of magma supply and eruptive processes on axial morphology, crustal construction and magma chambers

“…Such a narrow dike intrusion width is supported by independent statistical observations of the proportion of dikes in ophiolites that exhibit one-sided cooling, due to having been split by a subsequent dike [1,17,18]. Studies of marine magnetic anomaly reversal widths and variability [4][5][6][7][8] suggest a relatively wide zone of lava emplacement, 1-3 km wide. However, these data can not resolve the relative contributions of the width of the intrusion zone and lava flow distribution to the lava emplacement process.…”

“…These two magmatic processes generate an upper crustal structure composed of lavas piled on top of one another, underlain by a sheeted dike complex. Constraints on the processes of upper crustal construction have come from ophiolite studies [1 ,2], observations in active rifts (e.g., Iceland [3D, modeling of marine magnetic anomaly shapes and transition thicknesses [4][5][6][7][8], mapping [9][10][11][12][13][14], dating [15], and drilling (e.g. DSDP Hole 504B [I6D of young ocean crust.…”