Investigação estrutural de uma região do Cerrito do Ouro, São Sepé – RS, e relações com anomalias magnéticas NW

Abstract

The Sul-rio-grandense Shield (ESrg) is composed of petrotectonic assemblies characterized and studied from the perspective of geochemistry, structural geology, stratigraphy and metalogenesis. However, few studies directly or indirectly address the relationship between the brittle tectonics of the ESrg and magnetic data on a scale of detail. A region of Cerrito do Ouro district, São Sepé – RS, is characterized by occurrences of fracture systems possibly related to the evolution of the Rio Grande Arch, a positive flexure of the crust, directed NW that lifts the units of the ESrg and the Paraná Basin. In this context, the present study aimed to investigate these fracture systems and verify their relationships with magnetic anomalies of Analytical Signal Amplitude (ASA) of NW direction, identified in the study area previously. The first part of the work consisted of the bibliographic review and integration of remote sensing data in order to recognize and catalogue geomorphological lineaments on the scales 1:250,000, 1:100,000, 1:50,000 and 1:25,000. Subsequently, fieldwork carried out in profiles approximately perpendicular to the magnetic anomalies investigated, using basic geological-structural mapping techniques. From remote sensing, it was possible to verify the predominance of NE-SW direction geomorphological lineaments, followed by those directed NW-SE. Direction lineaments E-W and N-S were more frequent than expected through the review bibliographical. With regard to field data, at magnetic anomaly sites, the clearly predominant fracturing system is NW-SE direction and, outside it, this fracture becomes less frequent. Cutting relationships make it possible to identify NE-SW fractures of shear older than the opening NW-SE, a transcurrent E-W fracture more recent than the previous ones, and, like the most recent structures, N-S fracture systems and reactivations NE-SW. Therefore, it claims that there is a correlation between magnetic and morphological alignments, and that the behaviour of the brittle structures seen in the field reflects in geophysics. The NW magnetic anomalies, in study area, represent areas of NW fissures, filled by dikes possibly feeders of Serra Geral volcanism, being displaced by more recent E-W fracture systems and probably related to the opening of the Atlantic Ocean. NE-SW fracture systems are associated with an older event to those responsible for generating the NW-SE structures, and reactivated later. The integration of field results, bibliography and remote sensing allowed obtaining detail on the brittle tectonics in the study area, as well as a greater understanding of the meaning of the NW magnetic anomalies in the ESrg and its relationship with the evolution of the Rio Grande Arch.