Mapping a Gondwana suture zone integrating magnetic methods and geology at the SE Brazilian continental margin

Abstract

According to previous reconstructions, the South Atlantic Ocean is the biggest gap ofGondwana, with important crustal limits that controls convergent and divergent tectonicevents since the Paleoproterozoic. In the SE Brazilian continental margin, a Cambrian NE-SWsuture zone is marked by an expressive magnetic anomaly onshore that extends offshore tothe proximal Santos basin (Rio de Janeiro). This suture separates two contrasting geologicalterranes: a Neoproterozoic magmatic domain (Oriental Terrane), to the west, and aPaleoproterozoic gneissic terrane, to the east (Cabo Frio Tectonic Domain). The focus is todetermine how this suture extends offshore, hence determining the nature of the basementthat constitutes the continental margin, and how it continues in depth, determining theorientation of the Ediacaran paleo subduction zone. We made a geological and geophysicalintegration aiming to characterize the magnetic pattern of terranes and structures in order todeterminate the geometry and dip direction of the suture in depth. The methodology includedthe generation and interpretation of aeromagnetic images combined with geological mapping.Also, two magnetic sections were made on field along the suture zone and modeled on GMSYSadopting magnetic susceptibility values obtained both on field and laboratory. The studiedarea comprises mainly two geological units separated by a thrust fault (suture zone): dioritic togranitic Paleoproterozoic rocks with metabasite layers (Região dos Lagos Complex - RLC) andEdiacaran aluminous paragneisses with calcsilicate layers (Palmital Succession). Based on theamplitude (intensity), geometry, magnetic signal’s texture and lineament pattern, in map view,two domains were defined: A1 and A2. The A1 is correlated with the Paleoproterozoicgneisses, with high amplitude, long wavelength (55 km) with numerous magnetic rectilinearand curvilinear lineaments. Amplitude ranges between -100 and 452 nT, mostly higher than150 nT. A2 coincides with the Palmital paragneisses, defined by a low frequency, longwavelength (55 km) magnetic domain, with amplitudes <0 nT, resulting in a lateral contrast ofmore than 500 nT with A1, where the magnetic gradient decreases to NW. Magneticlineaments display a preferential NE-SW direction, but in A1 a deep NW-SE fabric occurs, thatmight be related with the Paleoproterozoic tectonic fabric. The suture is represented by a highpositive curvilinear lineament, extending offshore, crossing the coastline to SW beforeinflecting to the east. Modeling and geological/magnetic maps correlation suggest that thesuture dips to NW.