A tale of three methods: Volcanics in the Abrolhos Banks, Brazil

Abstract

Near-surface flood volcanics can create havoc when attempting geophysical imaging and geological understanding of the sub-surface. Abrolhos Bank, offshore Brazil, is one such area, and recent licensing of offshore blocks nearby or on the Banks has increased the economic stake in this problem. Extensive 3 dimensional modeling has been conducted using recently acquired marine seismic, gravity and magnetics data. Integrated geophysical analysis of these complimentary datasets has greatly improved our understanding of the extent of the flood basalts, as well as the thickness of the sub-volcanic sediments and the basement structure. No single one of these geophysical methods would have given us all this information in isolation: only the joint use of all three methods allowed the full range of interpretation. We interpreted the seismic data together with magnetic depth estimation techniques to constrain the depth to basement. We inverted on the gravity data in 3 dimensions to produce an isopach of the basalt, which was in turn constrained with the seismic and magnetic data interpretation. We have considered the important question of the source of the flood basalts: we found no evidence of basaltic feeder dikes in any of the magnetic, gravity or seismic data in the area, so a distal source seems most likely.