Brittle reactivation of mylonitic fabric and the origin of the Cenozoic Rio Santana Graben,southeastern Brazil |
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| Authors: | Ambrosina Gontijo-Pascutti Francisco HR Bezerra Emanuele La Terra Julio CH Almeida |
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| Institution: | 1. Universidade Federal do Rio Grande do Norte, Brazil;2. INCT-ET, Instituto Nacional em Ciência e Tecnologia em Estudos Tectônicos, CNPq, Brazil;3. Instituto Nacional de Pesquisas Espaciais—INPE, Brazil;4. Serviço Geológico do Brasil (CPRM), Brazil;5. INCT-GP, Instituto Nacional em Ciência e Tecnologia em Geofísica do Petróleo (CNPq), Brazil;6. Instituto de Geociências, Universidade de São Paulo, Brazil;7. University of Parma, Italy;8. Universidade Federal de Campina Grande, Brazil;9. Universidade de Brasília, Brazil;10. Universidade Estadual do Rio Grande do Sul, Brazil |
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| Abstract: | In the Ribeira belt, southeastern Brazil, the Precambrian mylonitic fabric mainly formed during the Brasiliano/Pan-African orogeny (640–480 Ma) and was reactivated as fault zones in the Cretaceous and Cenozoic. The reactivation process led to the development of the System of Continental Rifts of southeastern Brazil, from the Paleogene to the Quaternary. We investigated the brittle reactivation of a mylonitic zone, which is part of a major mylonitic belt, Arcádia-Areal. We used geological and geomorphological mapping, resistivity survey, controlled source audiomagnetotelluric survey, and luminescence dating. Our results indicate that this shear zone was reactivated and formed a 15 km long and 2 km wide sedimentary-filled trough, the Rio Santana Graben. It is located on the northwest border of a major structure, the Guanabara Graben, in the State of Rio de Janeiro. The Rio Santana Graben forms an almost entirely fault-bounded, NE-elongated depression that was accommodated entirely within the Arcádia-Areal shear zone. The graben consists of two main depocenters separated by a relay ramp. The graben formed by means of multistage activity of several faults during at least two main periods. The first period formed silicified fault breccia and occurred during alkaline magmatism in the Paleogene. The second formed fault breccia and gouge in shallow conditions and occurred at least until the Quaternary. The NE-trending and NW-dipping Precambrian fabric was reactivated as dip-slip and strike-slip faults. These faults triggered clastic-sediment deposition at least 300 m thick. The upper part of the graben consists of Quaternary alluvial and colluvial sediment fill, which yielded maximum luminescence deposition ages from 49 to 13 ka in the center of the trough. An organic layer at the top of the Quaternary alluvial deposits yielded 14C ages at ～6000 years BP. The lower part of the graben may be composed of Paleogene to Neogene sedimentary deposits, which occur in other basins of the System of Continental Rifts of southeastern Brazil. We conclude that the Rio Santana Graben is an example of the direct control of a preexisting continental-scale rheological boundary on the geometry and location of fault systems and sediment deposition. Quaternary fault reactivation of the preexisting fabrics represents only the latest movement of a major structure. |
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