The post-intrusion fault reactivation of the Rio Ceará Mirim dike swarm (Mesozoic): its implications for Potiguar Basin tectonic evolution

Abstract

It was found that in the geological context where the Rio Céará Mirim Dike Swarm (EDCM, northeastem Brazil) cross-cuts the Precambrian shear zones, an intense deformation of the magmatic bodies is present. Since the age (150 - 120 m.y.) and the Iithotectonic evolution of this Dike Swarm are well known, it was possible to infer on the physico-chemical conditions of post-intrusion fault reactivation of the preexistent anisotropies to which the Precambrian framework were submitted. The process of fault reactivation developed mainly hybrid fractures, cataclastic rocks, and gouge zones. Through the dynamic analysis of the structural elements with kinematic behavior, a NE-SW compression was verified. Within the hybrid fractures minerais such as serpentine and calcite were developed. Concentrates of the mineraIs developed within the hybrid fractures were analyzed for its major elements. The results arising out with the analyses of these concentrates presented a strong influence of fluid pressure during the development of those mineral phases. The cataclastic rocks are represented by breccias and ultracataclasites The mineral assemblage (cataclasites and hybrid fractures) chlorite + serpentine + calcite + dolomite + white mica + opaques derives from alteration, mainly from clinopyroxene, plagioclase and matrix, that took place during the cataclastic processo From information collected when studying the petrogenesis of these minerais, it was possible to postulate the physico-chemical conditions during the catac1astic process ranging from 200 to 350°C, and from 0.5 to 7.0 kbar, and deformation occurring at depths of 0.5 to 7.5 km. These data are consistent with low grade metamorphism, where the frictional regime was effective. On the other hand, the gouge zones represent the last pulses of fault reactivation processes at very shallow crustallevel.