Stress states during the emplacement of the eastern Rio Ceará-Mirim dike swarm, Borborema Province, northeastern Brazil

Abstract

The Rio Ceará-Mirim dike swarm (RCMDS) of the Borborema Province is a suite of subvertical intrusions classically described in the state of Rio Grande do Norte as E-W-trending dikes, up to southern Ceará, progressively deflecting to NE-SW. The rifting processes involving the Atlantic Ocean opening in the Early Cretaceous is responsible for the Northeast Brazilian Rift System (NBRS) and RCMDS development. This paper investigates the morphological styles and stress states in eastern RCMDS, focusing on well-preserved dikes at the localities of Rio Salgado and Lajes (state of Rio Grande do Norte). Remote sensing techniques, fieldwork, and numerical models aimed to obtain data to propose correlations between the eastern RCMDS emplacement and the NBRS tectonic settling in the Early Cretaceous. The studied dikes are predominantly tholeiitic basalt-diabase ranging from a few centimeters to 150 meters in thickness, locally achieving tens of kilometers in length. Their morphological styles vary from symmetrical to asymmetrical, from sharp and straight to anastomosing/braided dikes, showing diverse en echelon patterns, steps, horns, bridges, and bridge xenoliths. Fractal analysis of the dikes framework indicate syn-magmatic strike-slip components. The majority of morphological markers and the average orientation of the Rio Salgado dikes indicate a NNS-SSW (010 Az) orientation for the least compressive axis and dextral displacements. Based on some occurrences of en echelon dikes showing syn-emplacement stages in the extension direction, we propose three main stages for the least compressive axis -initially oriented to NW-SE, changing to N-S, and finally to NNE-SSW. Mechanical models indicate that the eastern RCMDS were emplaced in deviatoric stress with low/intermediate fluid pressure (Pf < σ2), which is in agreement with the observed morphological patterns.Stress states during the emplacement of the eastern Rio Ceará-Mirim dike swarm, Borborema Province, northeastern Brazil