Crustal-Scale Geometries and Architecture Along the Northeastern Brazilian Margin Based on Integrated Analysis of Seismic Reflection and Potential Field Data and Modelling

Blaich, O.A., Tsikalas, F., and Faleide, J.I.
Department of Geosciences, University of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo, Norway

The Brazilian margin has been for the last years the locus of extensive hydrocarbon exploration. However, remaining questions and uncertainties still exist and concern the crustal-scale evolution, the architecture of onshore and offshore basins, and the continent-ocean boundary (COB) location and character. In this study, we concentrate on the northeastern Brazilian margin, although considerations are depicted on a conjugate (Brazil-West Africa) margin framework, and primarily investigate the offshore Jacuípe and Sergipe-Alagoas basins, and the onshore Recôncavo, Tucano and Jatobá basins. We compile and re-interpret available conventional and deep multichannel seismic reflection profiles. Complemented by crustal scale gravity modelling we illustrate the crustal structure of the province by representative transects. We emphasize on the crustal type, tectonic development, the character of the continent-ocean boundary/transition expressed in potential field data, and the relationship of crustal structure to regional variations of potential field anomalies.

The analysis shows that the onshore Tucano Basin has approximately the same depth as the Sergipe-Alagoas Basin implying that the amount of crustal extension and deformation was probably similar for both basins. However, the absence of post-rift sediments in the onshore Recôncavo, Tucano and Jatobá basins and the prominent gravity anomaly amplitude difference observed across the Tucano and Sergipe-Alagoas basins indicates that both crustal and lithospheric mantle extension was dissimilar for the offshore and onshore basins. Furthermore, the study reveals that Moho shallows significantly more across the offshore Sergipe-Alagoas/Jacuípe basins than across the onshore Tucano and Recôncavo basins indicating that lithospheric mantle extension was considerable larger across the offshore basins in comparison to the onshore ones. These observations clearly indicate that the deep crustal structure along the Northeastern Brazilian margin exhibits a different degree of extension between the crust and the lithospheric mantle, reflecting a depth-dependent stretching pattern. Therefore the existence of an intracrustal detachment zone is invoked and placed at the mapped physical boundary that separates the heterogeneous crust into zones with different deformation characteristics. The crustal heterogeneities are controlled by the complex network of shear zones and by two different kinds of terrains, namely Proterozoic granulites and Proterozoic metasediments. Finally, the dip of the detachment was interpreted based on the geometry of the basins, i.e. the area of greatest subsidence coinciding with the depocenter location at the onset of the detachment. The analysis also reveals discernible along-margin segmentation due to a number of transfer systems which evolved with different detachment dips and different rift geometries. The observations on the different segments of the Northeastern Brazilian margin are evaluated and integrated within the framework of simple shear, pure shear and combined-shear hybrid crustal extension models.

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands

 

AAPG Search and Discover Article #90066©2007 AAPG Hedberg Conference, The Hague, The Netherlands