Magnetic Anomalies Of The Upper Continental Margin Of Brazil
Authors: Fainstein Roberto 
Published: October 2008

Magnetic anomaly trends of the upper continental margin of Brazil have been defined from the study of 42,000 km of oceanographic magnetic information. Off the Amazon, a series of long-wavelength anomalies strike perpendicular to the coast line. Magnetic anomalies in northeastern/eastern Brazil correlate with basement highs, seamounts, and marginal fracture zones. On the southern shelf most anomalies are caused by downfaulted blocks and marginal ridges.

INTRODUCTION

This report describes the general characteristics of magnetic anomaly profiles along the upper continental margin of Brazil. The magnetic data was obtained as part of a broad geophysical reconnaissance survey of the Brazilian Continental Margin carried out by Projeto REMAC*. This survey consisted of two oceanographic cruises:The 1972/ 73/74 R/V Besnard cruise collected about 35,000 km of magnetic profiles from Chui (southernmost Brazil) to Cape Orange (northernmost Brazil);the R/V Chain cruise in 1974 was restricted to the northern/ northeastern margins and collected about 7,000 km of magnetic information.

Previous magnetic investigations of the upper continental margin of Brazil have been sparse with the exception of an aeromagnetic survey carried out by Prakla/Petrobras whose results remain largely unpublished. Former studies included magnetic trend analyses derived from contour mapsl,2 and description of aeromagnetic profiles obtained by Project MAGNET3.

The Brazilian continental margin encompasses mostly low magnetic latitudes and therefore the orientation of the magnetic profiles may influence the shape of magnetic contour maps. Also, as spacing between lines in certain areas were not fully adequate for contouring, the analysis of individual profiles presented here yields a better correlation between magnetic anomalies and the existing geological features.

MAGNETIC DATA COLLECTION/PROCESSING

Total intensities of the magnetic field vector were recorded with a proton-precession magnetometer. The analog records were digitized at five minute intervals (approximately every 1.2 km) and compared with regional estimates from the IGRF 1965.0 - International Geomagnetic Reference Field 1965.0 4,5. The resulting amplitude represented the magnetic anomaly at any particular point under consideration. Some bias was present near the equatorial region, probably due to the lack of sufficient information in computing the IGRF. Precise positioning for both cruises were provided by satellite navigation.

MAGNETIC ANOMALIES - Northern Margin

Off the Amazon River, a series of long wavelength anomalies strike approximately N45° E (Fig. 1). Wavelengths are of the order of 60-100 km with amplitudes between 200 and 300 gammas. Some of these trends correlate with a system of major grabens and horsts that incise the Amazon shelf6, 7. It is interesting to note that long-wavelength anomalies have been reported in earlier surveys over Guyana-Surinam and Sierra Leone-Ivory Coast-Ghana3 and over the Liberian coast8, 9.

North of the Amazon shelf between Cape Orange and Cape Norte (Fig. 1) magnetic anomalies are short wavelength/ high-amplitude near the coast, but the wavelengths become progressively longer seaward.