Morphotectonic development of the Ceará Terrace: a marginal ridge on the western side of the Romanche Fracture Zone in the Brazilian Equatorial Margin

Abstract

The evolution of a rifted and sheared continental margin may control the formation of marginal ridges, which are prominent morphostructural features located at the extremities of fracture zones. We investigate the Ceara Terrace, a marginal ridge located at the western limit of the Romanche Fracture Zone in the Brazilian Equatorial Margin. Our data consist of 2000 km of 2D seismic lines parallel and orthogonal to the continental equatorial margin, as well as four exploratory wells. The Ceara Terrace has an evolution similar to that of the Ivory Coast-Ghana Ridge, the corresponding conjugate end of the Romanche Fracture Zone in the western African continental margin. The Ceara Terrace surface morphology displays an asymmetric ridge; the north slope is aligned to the Romanche Fracture Zone, related to a bathymetric step at ~ 850 m. The paleo relief represents a ridge consisting of a rift sequence (Lower Cretaceous) that is similar to the Ivory Coast-Ghana Ridge. This ridge is bounded by two half-grabens to the southeast associated with the reactivation of preexisting tectonic weakness zones related to the Precambrian Transbrasiliano Lineament. This structure was buried by a drift sequence, which comprises three sedimentary units: Unit 1, shale; unit 2, limestone that interfingers with shale and sandstone; and unit 3, limestone and shale. The fossil ridge is located near a seamount associated with important Oligocene volcanic units. Both conjugate marginal ridges were formed by a Late Albian to Cenomanian transpressional event (marked by folds and tectonic uplift) and flexural uplift due to erosion and thermal exchanges with oceanic spreading centers. Differences in the amplitude of uplift are related to the thermal influences of the distinct oceanic spreading centers. Furthermore, the results indicate that the studied area is still tectonically active.