Lithospheric magnetic property contrasts within the South American plate derived from damped least-squares inversion of satellite magnetic data

Abstract

A ridge regression (or damped least-squares) method has been used in a large-scale equivalent source inversion of South American MAGSAT data. The optimum damping parameter is selected based on percent increase in the sum of squared residual of the observed and the calculated anomaly and on the variance and the range of the solution for a series of damping parameters. Solutions obtained by the damping parameter selected in this manner are numerically stable and geologically meaningful. Consequently, these solutions are used in various applications such as transformation into effective magnetic susceptibility variation, altitude normalization, differential reduction-to-pole, and downward continuation of the data set. The spacing of equivalent sources and the source-observation distance are shown to introduce significant errors in the estimation of the physical property variation from equivalent source solutions for the large source spacing (> 1°) commonly in use. The correction for these geometric factors is essential in estimating the physical property variation from the equivalent source solutions. A continental and an oceanic profile have been analyzed from the South American maps derived from the above techniques. The continental profile transects a variety of geologic/tectonic features, including shields, aulacogens, basins and fold belts, near the north coast of South America. Along the profile, there is a good correlation between the radially polarized MAGSAT magnetic anomalies and other available geologic and geophysical information. In addition, the variation in effective susceptibility obtained from the damped equivalent source inversion is consistent with our present knowledge of continental lithospheric susceptibilities expected from the geologic/tectonic provinces along the profile. The oceanic profile extends from the trailing margin of South America through the Cretaceous normal polarity rocks of the South Atlantic Ocean. The variations in the magnetic properties along this profile can be broadly correlated with the depth anomaly variation, indicating a possible genetic relationship. Two conceptual models requiring undepleted mantle intrusions under the oceanic uplifts appear possible. In the first model, the high magnetization can be accounted for by a (weakly) magnetic upper mantle below the uplifts; in the second model, very highly magnetic rocks are required in the crust of these uplifted regions to account for the observed anomaly values.