Thermomagnetic features of Pirapora region, central Brazil

Abstract

We report results of a detailed analysis of aeromagnetic survey data of Pirapora region, situated in the São Francisco Craton region, central Brazil. The main residual anomaly is wide, spanning over an area of about 9000 square kilometers, and has a maximum intensity of ± 300 nT. The analytic signal of the anomaly is located between 44.5^°and 45.5° W and between 16.5° and 18.5 S and has a maximum value of about 0.028 nTm^-1. Results of spectral analysis, based on matched bandpass filtering method, indicate that the anomaly is composed of signals arising from three different layers. The top layer is at depth shallower than 5 km while the intermediate one extends from 5 to 15 km depth. The bottom of deepest magnetic layer extends over depths varying from 20 to 55 km. Along an east-west belt, south of Pirapora anomaly, the depth to bottom of magnetized crust is less than 35 km and comparable to the thickness of the local crust obtained in seismic surveys. However, the thickness of the magnetized crust can exceed 35 km to the north and to the south of the Pirapora anomaly. This implies that the top layer of the mantle itself is magnetized. Results of geothermal model studies indicate Curie isotherm of 580^oC may lie at depths greater than 35 km, along wide sections of the Pirapora region, within the São Francisco Craton.