Abstract: Rift Dynamics in Northeastern Brazil

Milani, Edison J. - Petrobras/E&P and Peter Szatmari - Petrobras/Cenpes

Northeastern Brazil (Fig. 1) is a particularly favorable geologic domain for analyzing the dynamics of rifting. In this region, a series of sedimentary basin and related structural features developed during the opening of the South Atlantic, recording some of the key processes that characterized the Mesozoic breakup Gondwana.

The Recôncavo-Tucano-Jatobá basin is a 500 km-long, N-S striking interior rift system filled with a package of Early Cretaceous siliciclastic rocks in excess of 10 km-thick. Regional extensional stresses were oriented NW-SE and defined series of NE-SW major normal faults that nucleated the subsidence of each one the five individual grabens of the rift system. NW-SE transfer zones accommodated the lateral displacement between disrupted crustal blocks. A complex tectonic pattern characterizes the southernmost portion of this rift system, where it merges with the continental margin of Brazil.

In the marginal, SW-NE trending Sergipe-Alagoas basin, transtensional subsidence seems to have been an important mechanism. N-S oriented, en échélon Neocomian grabens provide plentiful geometric evidence for a wrench tectonic regime operating when African and South American plates started to individualize. The progressive movement of the SW-NE left-lateral shear couple led to the development of important transpressive deformation and uplift in the Alagoas Basin during the Aptian times, simultaneously with a strong, renewed phase of subsidence in the southernmost Sergipe Basin. The northern tips of Alagoas and Jatobá basins are linked by the Pernambuco fault, a regional tectonic feature inherited from Precambrian times, which displays a complex assemblage of extensional, transcurrent and compressional structures synchronous with the Cretaceous rifting.

The inland Recõncavo-Tucano-Jatobá rift and Pernambuco fault, together with the Sergipe-Alagoas marginal basin along the continental margin, define the three sides of the NE-Brazilian Microplate, a unique dynamic element in the rift system. Differential rotation of the NE-Brazilian Microplate relative to the bulk of the South American continent is well defined. The microplate rotated about 10° counterclockwise relatively to a pole which lies along the Pernambuco fault at about 8.5°S and 38.5°W. As a result of the rotation of the microplate, to the east of the pole an eastward widening wedge of granites is thrusted over the northern flank of the Pernambuco fault. Whereas to the west of the pole the Jatobá basin opens as a westward widening wedge-like graben.

Further north, in the northeastern corner of Brazil, the Potiguar Basin is a classical example of an active rift basin. There, magmatic activity along a mainly E-W trending dike swarm marked the initial stages of rift evolution, along a structural trend that continued eastward into the Benue trough of Nigeria. Neocomian rift subsidence in the Potiguar basin was controlled by major SW-NE border faults; tilted blocks dipping to the SE define half-grabens in this basin, whose subsequent evolution caused it to become part of the Equatorial margin of Brazil.

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