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