Caracterização Crustal da Margem Sudeste Brasileira através de Magnetometria e suas Implicações Tectônicas

Abstract

The Southeastern Brazilian margin was investigated focusing on its basement structure and its relations with the adjacent oceanic features. This study was carried out based mainly upon a magnetic dataset. Two scales of analysis were applied: a regional analysis, through the recently released World Digital Magnetic Anomaly Map (WDMAM) compilation, and a detailed analysis, using high resolution aeromagnetic surveys covering the area between 21o and 24o S. The aim of this study is to characterize the crustal structure of this portion of the margin, correlating the magnetic anomalies with gravity and known geology. Ultimately, we investigated the existing correlations between structural, lithological and/or rheological variations, in order to understand the tectonic evolution of the margin during rifting. On a regional scale, the continental region displays smoother and lower intensity magnetic anomalies (< 50 nT) when compared to the oceanic area. The offshore area is characterized by linear magnetic trends (100-200 nT), following a main NE-SW direction, well correlated with the adjacent continental basement structural orientation. An exception is made between 23o and 24oS, where the offshore lineaments assume an E-W direction, in agreement with Mesozoic structural orientation, formed during rifting. Such pattern seems to represent the structural framework of the margin, marked by inherited NE-SW directions and newly formed E-W features related to a main WNWESE axis of extension. Two prominent lineaments were identified (MM1 and MM2), which represent an important change on the crustal characteristics, probably associated with igneous intrusions and deep faults. The most distal MM2 lineament may be correlated to the G Anomaly, interpreted as the continent-ocean transition zone. The large-scale magnetic anomalies brought to evidence the tectonic segmentation of the margin, where E-W negative lineaments offset the positive NE-SW trends, presenting, in general, a dextral movement. Such pattern is potentially associated with transfer faults, like the Rio de Janeiro Transfer Zone, as well the early activity of fracture zones, like Rio de Janeiro and Martins Vaz Fracture Zones. The aeromagnetic analysis for the continental basement between 21o and 24o S displays N45E-S45W e N30E-S30W magnetic lineaments, related to the main Ribeira Belt structural orientation. Strong gradient and circular-shaped anomalies are associated with late-tectonic granitic bodies and some alkaline rocks from Poços de Caldas-Cabo Frio Alignment, while curved-shaped lineaments can be related to ductile structures, like Precambrian shear zones. High amplitude and short-wavelength anomalies (100- 150 nT) are interpreted as Mesozoic maffic dyke swarms. Their association with Ribeira Belt fault systems suggests a tectonic reactivation of the NE-SW direction during rifting. Nevertheless, the Cabo Frio Tectonic Domain (CFTD) exhibits the same magnetic pattern despite its NW-SE basement orientation, implying that the Mesozoic rifting was characterized by widespread tectonic deformation regardless preexisting basement orientation. The rifting structures formation seems to have been controlled mainly by the stress tensors during the Mesozoic and the rheology of the lithosphere. The Campos Basin exhibits strong, positive magnetic lineaments, N30E-S30W oriented, which characterize an important region identified here as the Campos Magnetic High. A magnetic modeling on this area showed that a deep magmatic source x (4 km depth and 21 km wide), with high susceptibility values close to ultramaffic rocks (> 0,078 SI) would be necessary to account for the observed anomalies, in association with subvertical structures like dykes conduits, with high magnetization (> 0,04 SI) possibly extending close to the surface. We suggest a high extension rate as responsible for the formation of the Campos Magnetic High, and a possible mantle exhumation could be considered to explain this anomalous feature. The magnetic basement (up to 5 km). depth) between 21o and 24o S is characterized by positive and negative elongated zones, N45E-S45W oriented, which seems to be related to the Ribeira Belt terranes. The main tectonic boundaries are represented by long-wavelength, linear anomalies, which extend deep in the crust and represent regional suture zones. The Cabo Frio Tectonic Domain exhibits a distinct magnetic basement, showing no linearity, coherent with its late tectonic amalgamation during the Silurian.