Conjugate Divergent Margins

The main focus of the book is the geological and geophysical interpretation of sedimentary basins along the South, Central and North Atlantic conjugate margins, but concepts derived from physical models, outcrop analogues and present-day margins are also discussed in some chapters. There is an encompassing description of several conjugate margins worldwide, based on recent geophysical and geological datasets. An overview of important aspects related to the geodynamic development and petroleum geology of Atlantic-type sedimentary basins is also included. Several chapters analyse genetic mechanisms and break-up processes associated with rift-phase structures and salt tectonics, providing a full description of conjugate margin basins based on deep seismic profiles and potential field methods.
Geochronology (Ar/Ar and K–Ar) of the South Atlantic post-break-up magmatism Available to Purchase
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Published:January 01, 2013
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CiteCitation
M. C. Geraldes, A. Motoki, A. Costa, C. E. Mota, W. U. Mohriak, 2013. "Geochronology (Ar/Ar and K–Ar) of the South Atlantic post-break-up magmatism", Conjugate Divergent Margins, W. U. Mohriak, A. Danforth, P. J. Post, D. E. Brown, G. C. Tari, M. Nemčok, S. T. Sinha
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Abstract
This work integrates the available geological information and geochronology data for the Cretaceous–Recent magmatism in the South Atlantic, represented by onshore and offshore magmatic events, including the oceanic islands along the transform faults and near the mid-ocean ridge. The analysis of the igneous rocks and their tectonic settings allows new insights into the evolution of the African and Brazilian continental margins during the South Atlantic opening. Following the abundant volcanism in the Early Cretaceous, the magmatic quiescence during the Aptian–Albian times is a common characteristic of almost all Brazilian and West African marginal basins. However, rocks ascribed to the Cabo Granite (104 Ma) are observed in NE Brazil. In West Africa, sparse Aptian–Albian ages are observed in a few coastal igneous centres. In the SE Brazilian margin, an east–west alkaline magmatic trend is observed from Poços de Caldas to Cabo Frio, comprising igneous intrusions dated from 87 to 64 Ma. Mafic dyke swarms trending NW also occur in the region extending from the Cabo Frio Province towards the Central Brazilian Craton. On the West African side, Early Cretaceous–Recent volcanism is observed in the Walvis Ridge (139 Ma), the St Helena Ridge (81 Ma) and the Cameroon Volcanic Line (Early Tertiary–Recent). Volcanic islands such as Ascencion (1.0–0.65 Ma), Tristão da Cunha (2.5–0.13 Ma) and the St Helena islands (12 Ma) most probably correspond to mantle plumes or hot spots presently located near the mid-Atlantic spreading centre. Within the South America platform and deep oceanic regions, the following volcanic islands are observed: the Rio Grande Rise (88–86 Ma), Abrolhos (54–44 Ma), the Vitória–Trindade Chain (no age), Trindade (2.8–1.2 Ma) and Fernando de Noronha (12–1.5 Ma). There are several volcanic features along the NW–SE-trending Cruzeiro do Sul Lineament from Cabo Frio to the Rio Grande Rise, but they have not been dated. The only known occurrence of serpentinized mantle rocks in the South Atlantic margin is associated with the Saint Peter and Saint Paul Rocks located along the São Paulo Fracture Zone. The Cameroon Volcanic Line in NW Africa is related to the magmatism that started in the Late Cretaceous and shows local manifestations up to the Present. The compilation of all available magmatic ages suggests an asymmetrical evolution between the African and South America platforms with more pre-break-up and post-break-up magmatism observed in the Brazilian margin. This is most likely to have resulted from the different geological processes operating during the South Atlantic Ocean opening, shifts in the spreading centre, and, possibly, the rising and waning of mantle plumes.
A complete table with radiometric dates that have been obtained by universities, government agencies and research groups is available at: www.geolsoc.org.uk/SUP18596
- absolute age
- Africa
- Albian
- alkalic composition
- Aptian
- Ar/Ar
- Atlantic Ocean
- Brazil
- Brazil Basin
- Brazilian Shield
- Cameroon Line
- Cenozoic
- continental margin
- Cretaceous
- dates
- faults
- geochronology
- igneous rocks
- K/Ar
- Lower Cretaceous
- magmatism
- Mesozoic
- mid-ocean ridges
- Minas Gerais Brazil
- ocean floors
- plate tectonics
- Pocos de Caldas Brazil
- rifting
- Rio Grande Rise
- Saint Helena
- South America
- South Atlantic
- strike-slip faults
- transform faults
- volcanic rocks
- volcanism
- Walvis Ridge
- West Africa
- Cabo Frio
- Fernando de Noronha
- Cruzeiro do Sul Lineament