Desenvolvimento morfo-estrutural do Terraço Ceará: o Alto Marginal Oeste da Zona de Fratura Romanche

Abstract

A Marginal Ridge is a prominent morph-structural feature associated with an extremity of a Fracture Zone. The evolution of rifted and sheared margin segments might control the formation of marginal ridges. The Ceará Terrace (CT) is a marginal ridge at the western limit of the Romanche Fracture Zone (RFZ) in the Brazilian Equatorial Margin. The corresponding opposite end of the RFZ is the Ivory-Coast Ghana ridge, in the eastern African continental margin. We investigate the tectonic influences of continental and oceanic structures on the CT formation and the differences between CT and Ivory-Coast Ghana ridges. Our data consist of 1000 km of 2D seismic lines crossing (parallel and transverse to the continental margin) the CT and the RFZ, and 4 exploratory wells located in deep waters of Ceará Basin. The CT morphology on surface displays an asymmetric ridge; the north slope is aligned to the RFZ, which is related to a bathymetric step at ~850 m. Morphology on subsurface is represented by a fossil ridge similar to the actual relief of CT compound by Rift-Sequence. The fossil ridge is bounded at southeast by two half-grabens associated to reactivation of preexisting tectonic weakness related to the Transbrasiliano Lineament. This structure was buried by Drift Sequence which was divided in three sedimentary units: U1 (shale), U2 (carbonate interfingered with shale and sandstone) and U3 (carbonate and shale). The fossil ridge locates near to a seamount associated to important magmatic events in Oligocene. Both CT ridges associated to RFZ resulted of the Late Albian to Cenomanian transpression, flexural uplift due erosion and thermal exchanges with oceanic spread center.