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

Abstract

The Rio Ceará-Mirim Dikes Swarm (RCMDS) is a suite of subvertical E-W dikes classically described in the State of Rio Grande do Norte, 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 from the eastern RCMDS, focusing on well-preserved dikes at Rio Salgado and across Lajes-RN. Remote sensing approaches, 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 majorly tholeiitic basalt-diabase ranging from a few centimeters to 150 meters thick, in some cases, achieving tens of kilometers in length. Their morphological styles vary from symmetrical to asymmetrical, from sharp and straight to anastomosing/braided dikes, showing diverse shapes of en echelon patterns, steps, horns, bridges, and bridge xenoliths. Fractal analyses from the dikes’ framework indicate synmagmatic variations in the least compressive axis. The majority of morphological markers and the main orientation in the Rio Salgado dikes indicate an NNS-SSW (010Az) least compressive axis with a dextral strike-slip rate. Based on a punctual occurrence of en echelon dikes showing three syn-emplacement stages in the extension direction, we propose three main stages for the least compressive axis – initially, NW-SE oriented, changing to N-S, and afterward to NNE-SSW. Mechanical models indicate that the Rio Salgado dikes were emplaced in deviatoric stress with low/intermediate fluid pressure (Pf < σ2), which agrees with the observed morphological patterns.

Keywords

• Basalt-diabases dikes
• Structural control
• Morphology
• Propagation mode