Viscous remanent magnetization model for the Broken Ridge Satellite Magnetic Anomaly

Abstract

A range of crustal models for the interpretation of the Broken Ridge satellite magnetic anomaly have been constructed from bathymetric data. The source magnetizations were assumed to be constant in direction throughout the source region and parallel to the present earth's field at the center of the source region. The model calculations were made at the observation points of a selected set of Magsat Investigator B passes, with a variable magnetic field direction calculated from the MGST (4/81) field model. The horizontal perimeter of all the models was defined by the departure of the Broken Ridge bathymetry from an average oceanic depth of 4.5 km. A topographic model, with an upper surface defined by the bathymetry of the ridge and a lower surface defined by the average oceanic depth, requires a magnetization of 42 A/m to account for the observed anomaly amplitude. An Airy‐type isostatic model, with an upper surface defined by the bathymetry of the ridge and a lower surface predicted from the bathymetry, requires a magnetization of 6 A/m to account for the observed anomaly amplitude. A midcrust model, with an upper surface at the average oceanic depth and a lower surface at the average oceanic Moho, requires a magnetization of 13 A/m to account for the observed anomaly amplitude. A comparison of the locations of the model and observed anomaly peaks shows that the Broken Ridge anomaly can be modeled assuming present field directions. There is not sufficient sensitivity in the data to distinguish between the models on the basis of anomaly location. The model magnetizations vary from 6 A/m, where the whole crust is uniformly magnetized, to 42 A/m, where the topographic expression of the ridge is the source of the anomaly. The strength of the required magnetization contrasts cannot be accounted for by induced magnetization if normal oceanic crustal materials are assumed to be the source for the observed anomaly. Measured and modeled remanent magnetizations of seafloor rocks are typically within the lower part of the range of required values, but the higher magnetizations are associated with very young rocks and dredge samples. It is concluded that the source magnetizations have been enhanced by the development of a viscous remanence effect since the last reversal of the earth's field.