Tectonic evolution of Brazilian equatorial continental margin basins
The structural style and stratigraphic relationships of sedimentary basins along the Brazilian Equatorial Atlantic Continental Margin were used to construct an empirical tectonic model for the development of ancient transform margins. The model is constrained by detailed structural and subsidence analyses of several basins along the margin. The structural framework of the basins was defined at shallow and deep levels by the integration of many geophysical and geological data sets. Basin-forming mechanisms and their thermal and mechanical effects on the initiation and development of the basins were then evaluated. The results of these analyses, together with a kinematic framework of the Atlantic opening, helped to describe and constrain the tectonic model for the equatorial margin of Brazil. A comprehensive review of the continental and oceanic geology of the Equatorial Atlantic Ocean and its continental margins shows that fundamental discontinuities of the continental Hthosphere of the South American and African plates are correlated across the Atlantic and are linked to major E-W oriented oceanic fracture zones. The Romanche Fracture Zone, in particular, and its continental margin extensions, are linked with the Kandi/Sobral-Transbrasiliano Shear Zones in the continental crust of West Africa and North Brazil respectively. The extension of the oceanic fracture zones to both continents are marked by Precambrian age shear zones which show histories of multiple strikeslip reactivation during the Phanerozoic. The original assumption of strong rotational rigidity used in earlier kinematic plate-tectonic models is challenged based on mounting evidence of intraplate deformation of the South American plate during the Early Cretaceous opening of the South Atlantic Ocean. The Equatorial Atlantic Ocean, defined as an oceanic basin that developed between the Ascencion and the Bahamas Oceanic Fracture Zones, was initially formed by fragmentation and breakup of the northwestern Gondwana during the Aptian-Cenomanian interval. A transtensional shear corridor with dextral sense of displacement was developed at the site of the present-day northern continental margin of Brazil which, in the segment studied in detail in this thesis. Abstract Page VI Tectonic Evolution of Brazilian Equatorial Continental Margin Basins formed the Barreirinhas and Para-Maranhao marginal basins. The Barreirinhas and Para-Maranhao Basins were divided in three tectonic domains: the Tutoia, Caete and Tromai Sub-basins. The Caete area is characterized by NW-SE striking and northeast-dipping normal faults. A pure shear mechanism of basin formation is suggested for its development. The structure of the Tutoia and Tromaf Sub-basins are more complex and indicative of a major strike-slip component with dextral sense of displacement, during early stages of basin evolution. These two later sub-basins were developed on a lithosphere characterized by an abrupt transition (<50 km wide) from an unstretched continent to an oceanic lithosphere. The transitional lithosphere is marked by fracture zones in which horizontal shearing along vertical zones was the dominant process. The subsidence history of these basins do not comply with the classical models developed for passive margins or continental rifting. The Gurupi Graben System is an onshore chain of asymmetrical NW-SE striking grabens developed simultaneously with the offshore basins. The system was formed by minor upper crustal extension (9-16%) which is suggested to have occurred as a result of NE-SW extension during the Aptian-Early Albian. A simple shear mechanism comprising a low-angle detachment linking the crustal deformation of the graben system to that of the offshore basins is thought to explain the geometrical relationship with the transtensional shear corridor. The grabens are interpreted as being produced by the extensional reactivation of Precambrian age shear zones. Basement anisotropy played a dominant role in the external geometry and internal organization of these basins. The thermo-mechanical model proposed for the Brazilian Equatorial Margin includes heterogeneous stretching combined with shearing at the plate margin. The tectonic history comprises: (1) Triassic-Jurassic limited extension associated with the Central Atlantic evolution (Marajo Rift System); (2) Neocomian intraplate deformation consisting of strike-slip reactivation of pre-existing shear zones and development of the Potiguar Graben; (3) Aptian-Cenomanian twophase period of dextral shearing documented in the Pard-Maranhao/ Barreirinhas Basin System; and (4) Late Cretaceous-Cenozoic sea-floor spreading.