Análise da tectônica de colocação de diques cretácicos na região de São Sebastião, SP

• Autor(a) principal: Gomes, Leonardo Corrêa
• Data de Publicação: 2012
• Tipo de documento: Dissertação
• Idioma: por
• Título da fonte: Biblioteca Digital de Teses e Dissertações da UERJ
• Texto Completo: http://www.bdtd.uerj.br/handle/1/7071
• topic: Structural geology; Dykes; Tectonics; Magmatism; Cretaceous; Southeastern Brazil; Geologia estrutural; Diques; Tectônica; Magmatismo; Cretáceo; Sudeste do Brasil; CNPQ::CIENCIAS EXATAS E DA TERRA::GEOCIENCIAS::GEOLOGIA

The study was developed at the northern coast of São Paulo state, southeastern Brazil, where there are good exposures of intrusive rocks of the southern portion of the Early Cretaceous Serra do Mar Dyke Swarm. The main purpose is to define the tectonic regimes related to the emplacement and deformation of mafic dykes in the area of São Sebastião (SP) and their spatial and relative temporal distribution, based on interpretations of remote sensing images, field analysis of structural data, and petrographic description of igneous rocks. The area is quite complex in terms of Cretaceous magmatim, since there are dolerite dykes (tholeiitic and alkaline), lamprophyric dykes and felsic alkaline rocks (mainly phonolites, trachytes and syenites) which occur as dykes, sills and plugs. The tholeiitic dolerites yield ages around 134 Ma, related to the early South Atlantic rifting, while the alkaline rocks are dated as 86 Ma and are related to a subsequent intraplate magmatism. The structural lineaments are oriented mostly ENE-WSW, parallel to the metamorphic foliation and shear zones observed in the field and described in the literature, as part of the Costeiro Complex of the Ribeira Belt. The dykes are oriented NE-SW, with similar azimuth but different dip angles compared to the foliations, which are gently dipping in many areas. A second set of lineaments oriented NW-SE occurs as a major set of brittle fractures which cut both the Proterozoic rocks and the Late Cretaceous alkaline rocks. Dykes with this orientation are scarce. A third set oriented NNE-SSW occurs in the western area associated with some dolerite dykes which sometimes show evidence of sinistral component during emplacement. The kinematic analysis of the dykes shows a predominance of pure extension during emplacement, with an extension direction oriented NW-SE, orthogonal to the main dyke trend. Directional components, sometimes ambiguous, are commonly observed, with a slight predominance of sinistral components. The same kinematic pattern is observed for both tholeiitic and alkaline dykes, so that the local stress field orientation remained unaltered during the Cretaceous. Although the basement has not been directly reactivated during dyke emplacement, their anisotropies can account for some control on the orientation of the local stress field during the Cretaceous. The available geophysical maps of the Santos Basin suggest certain parallelism among the structures observed in the study area and those interpreted in the basin. The NNE-SSW trending structures are parallel to the trend of sub-basins and to the Merluza graben, while the NW-SE structures are parallel to transfer zones described in the literature.