Evolução tectônica do segmento extremo leste da Margem Equatorial brasileira: contribuição do mapeamento estrutural (onshore) e da modelagem física

Abstract

The discovery in recent decades of hydrocarbon provinces associated with the Equatorial Transform Margin (ETM), has generated a growing interest in better understanding the evolution of this margin, being considered a new exploratory frontier. In the Brazilian portion, this margin comprises intracontinental rifts and passive margins. In the present work, we investigated the evolution of the ETM, from the application of the structural physical modeling tool and a multiscale analysis of the brittle deformation record in the crystalline basement of the Potiguar and Ceará basins, the easternmost portion of the Brazilian Equatorial Margin (BEM). The modeling data show that during the ETM rifting process occurred a dominantly oblique kinematic regime. This margin comprised during its initial stage, on a plate scale, an important oblique transfer zone (α = 20º) developed between the South and Central Atlantic. In the mature stage, this zone evolved into diachronic oblique rifts, whose margins are composed of NW-SE oblique (dextral-normal) faults and E-W to WNW-ESE transcurrent faults, that together define a zigzag geometry. Two distinct tectonic domains occur in the ETM: (i) the extremity regions characterized by oblique deformation with intense rotation and tilting of the substrate; and (ii) the central region dominated by transcurrent (and oblique) deformation, with the presence of oblique and pull-apart basins, in addition to marginal plateaus. In the easternmost segment of the BEM has shown evidence of the superimposed deformational events related to pre- and syn-structuring of this margin. The oldest event, D1 (Late-Brasiliano) is expressed by the NE-SW dextral shear fractures staggered and extensional fractures fill and unfilled with quartz and chalcedony (NE-SW to E-W). This event is related to an intraplate NESW transpression associated with the progressive exhumation of the Brazilian-Pan African orogenic chain. The D2 event (Neo-Jurassic to Aptian) is characterized by NESW extensional fractures and normal faults, NNE-SSW sinistral shear fractures staggered, and NW-SE and E-W basic dykes (Rio Ceará-Mirim magmatism); whose formation indicates a NW-SE extensional paleostress field compatible with the active one that occurred during the formation of the Brazilian East Margin and development of the interior basins of the Northeast. The D3 (Aptian) event, beginning of the installation of the equatorial proto-margin, is represented by the several hybrids (E-W, NNE-SSW and NNW-SSE), extensional (NW-SE) and shear (WNW-ESE and ENE-WSW dextral; NNE-SSW and NNW-SSE sinistral) fractures. These fractures show unfilled or filled (chalcedony, iron oxide and hydroxide, and calcite), which together express an E-W dextral transtensional kinematics. Extensional jogs are characteristic of this event, expressing a fractal relationship with the Messejana and Jacaúna grabens from the Potiguar Basin. N-S normal faults and extensional fractures, NNW-SSE hybrid fractures unfilled and filled (carbonate and iron oxide-hydroxide) express the youngest event (D4). This event exhibits a N-S compressional and an E-W extensional paleostress field, interpreted as related to rotation and drift between post-Aptian tectonic plates. Previous anisotropies, such as the Transbrasiliano Lineament (TBL), can control the nucleation and structuring of sedimentary basins. The role played by the TBL in the structuring of the BEM, however, is still poorly understood. Thus, the physical modeling data indicate that the TBL was reactivated during the development of the BEM, controlling the structuring, configuration and deformation of the sub-basins that compose the Ceará Basin. NE-SW normal-dextral faults comprised the main expressions of the TBL reactivation. Wide synform and antiform folds (generated by the bending mechanism) accommodate the deformation between fault sets. To the east of this lineament, the deformation exhibits a distributed character associated with a transtensional/extensional kinematics with the generation of oblique depocenters and pull part. West of TBL, the deformation exhibits more localized, dominantly transcurrent to compressional resulting in the formation of reverse and transcurrent/transtensional faults.