Evolução estrutural da borda leste do Rifte Potiguar emerso, Bacia Potiguar (NE do Brasil)

Abstract

The Potiguar Basin is located at the intersection of the east and equatorial Brazilian margins, possessing a sedimentary fill that includes rocks deposited from the Neocomian to the Quaternary, which record its different evolutionary phases: rift, post-rift and drift. The rift phase, focus of this study, is divided into two distinct events thar occurred along the Lower Cretaceous: an earlier one, called Rift I, that occurred in response to distensive lithosphere stress associated with the opening of the Brazilian east margin, culminating in an onshore aborted rift; and a later one, called Rift II, that developed in response to transtrative stress during the opening of the equatorial Brazilian margin, resulting in a offshore rift that evolved until the lithospheric rupture. The final product of this phase is a complex structural framework that still lacks explanation of how the origin and evolution of the structures occurred. In this sense, the objective of this work was to investigate the eastern margin of the onshore Potiguar Rifte, a region that encompasses the main fault systems (Carnaubais and Baixa Grande) and depocenter (Umbuzeiro half-graben) of the rift. To achieve that, we analyzed 2D and 3D seismic data available along the margin in conjunction with geophysical logs of selected wells. The results of this analysis revealed a probable evolution of the main fault systems from the diachronic nucleation of several smaller segments, which grew, interacted with one another trhough relay ramps and, at a more advanced stage, were mechanically connected by connecting faults. The diachronism between individual fault segments that will constitute major fault systems is marked in the geological record by intra-rift angular unconformities. The connecting faults, responsible for connecting fault segments originally distinct, form, in subsurface, a sector with flat geometry, giving rise to faults with ramp-flat-ramp geometry. The smoother dip of the connecting faults was attributed to a possible interaction between the stress field generated in regions of contractional jogs between normal faults and zones of weakness of the lower dip. Physical structural models were made, corroborating and illustrating the interpretations. Finally, the results obtained with this study provide a proposal for the evolution of the eastern margin of the onshore Potiguar Rift, indicating áreas with a higher propensity to the occurrence of good reservoir facies together with areas of structural traps, as well as possible implications in the opening kinematics of the basin, that according to the results, would have a transtrative behavior.