Abstract: A Unique Transform Margin: The Equatorial Atlantic

Matos, Renato M.D. and Ricardo N. Waick - Petrobras/E&P

Conventional extensional processes can not truly explain the kinematics and rift geometry of the Equatorial Atlantic basins. The commonly accepted causal processes for rifting, such as passive/active or diffuse/discrete rifting can not accommodate the equatorial data set. Even pure-shear or simple-shear rift mechanisms can not be promptly used when modeling basins generated as a response of major transform motions of a continental scale plate boundary.

During the Aptian, the entire region started to break apart, when transtensional conditions created a series of NW-SE trending en-echelon basins (regarding South America), throughout the Equatorial domain. Even though shearing signatures and pull apart features are easily recognized throughout the margin, their magnitude and rift architecture varies significantly accordingly with the distance from the main transform faults, generating significant differences in the thermal evolution, basin subsidence and geometry, facies distribution and uplift history.

The well known rift architecture, defined by tilted half-grabens, controlled by planar/listric faults (Fig. 1.c), filled with a divergent wedge of siliciclastics sediments with fanglomerades adjacent to the main border fault are not promptly recognized in the Equatorial domain. In many cases it is difficult to see a divergent seismic pattern as illustrated in the Potiguar and Ceará basins in Brazil (Fig. 1 a,b), where even coarse grained alluvial sediments may be missing adjacent to main border faults. Another major difference includes the lack of a pre-rift section, as well as a transitional section between the rift and drift stages. Local tectonics and magmatism had a keyhole in the post-break-up subsidence.

Abrupt variations in the temporal and spatial location of magmatism as well as the spatial distribution of releasing and restraining bends through the region led to a quite different post-rift evolution from basin to basin. Another key factor was deformation rate. Although this multi-stage rifting process evolved throughout the Aptian-Cenomanian interval, an almost instantaneous Aptian rift stage was responsible for a widespread fracturing of the Equatorial Atlantic, while the South Atlantic passive margin had a longer rift stage (Neocomian-Barremian). The tectonic evolution of the Equatorial Atlantic is better understood if considering three stages: pre-, syn-, and post-transform movements.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil