Gráben Ribeirão das Lajes, RJ: evolução estrutural cenozoica e morfotectônica
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2019-09-06
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Real, Sílvia
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Universidade Estadual Paulista (Unesp)
Resumo
O Gráben Ribeirão das Lajes (GRL) foi mapeado no limite entre as escarpas da Serra do Mar e o Gráben da Guanabara, no estado do Rio de Janeiro, Brasil. O presente estudo buscou caracterizar a configuração e evolução tectônica cenozoica do GRL a partir de métodos que integram procedimentos em laboratório e trabalhos de campo, envolvendo Geologia Estrutural e Geomorfologia Tectônica. Esta bacia tipo gráben encontra-se encaixada na estrutura pretérita NE-SW do embasamento pré-cambriano, composto de gnaisse bandado milonítico da Unidade Arcádia-Areal, juntamente com ortognaisses e granitoides do Arco Magmático Rio Negro. Essas rochas estão intrudidas por diques de diabásio cretáceos de direção NE-SW. O registro sedimentar está associado, principalmente, aos depósitos aluvionares e coluvionares, que se estendem por 20 km de comprimento por 9 km de largura, expostos de maneira irregular e separados por altos intrabacinais. As características da rede de drenagem e do relevo da área, somados aos inúmeros e persistentes lineamentos, mostram, em conjunto com os dados levantados em campo, que os traços principais se associam às fraturas. O GRL é uma estrutura alongada e estreita delimitada por falhas NE-SW, em ambas as bordas, e compartimentada fundamentalmente por fraturas de direção NW-SE e E-W. O primeiro evento de formação do GRL se relaciona com as falhas normais NE-SW do embasamento, equivalente ao processo de geração das bacias do Sistema de Riftes Cenozoicos do Sudeste do Brasil no Paleógeno. As estruturas NE-SW são expressivas no sistema fluvial, nas feições morfotectônicas e nas zonas preferenciais de erosão e sedimentação. Na escala de trabalho local, as fraturas NW-SE de alto ângulo, reconhecidas em campo, têm papel significativo na dissecação da paisagem, definindo marcantes rupturas de relevo, a partir do condicionamento de tributários e feições morfotectônicas. As novas descontinuidades, representadas por fraturas E-W e N-S, controlam pequenos canais, cristas e facetas. Destaca-se a influência do feixe E-W na dissecação e remodelamento da paisagem local, com expressividade nos dados de juntas e falhas que afetam embasamento, diques e cobertura sedimentar. O condicionamento tectônico do relevo e, principalmente, da rede de drenagem, que responde a movimentações mais jovens, indica atividades tectônicas no Cenozoico. O regime atual de tensão é definido na literatura por uma transcorrência dextral, caracterizado por estruturas extensionais NW-SE, correspondente às estruturas encontradas em campo, que afetam tanto a cobertura sedimentar quanto o embasamento intemperizado, com planos estriados preenchidos com óxidos. Os estudos realizados expõem os reflexos da atuação tectônica rúptil cenozoica que modificaram a paisagem e levaram à erosão da bacia do GRL.
Ribeirão das Lajes Graben (RLG) was mapped at the boundary between Serra do Mar escarpments and Guanabara Graben, in the state of Rio de Janeiro, Brazil. The present study aimed to characterize the Cenozoic tectonic evolution and shape of RLG from methods which incorporate procedures in laboratory and field work, involving structural geology and tectonics geomorphology. This basin was developed on thrust-sheared precambrian NE-SW substrate, consisting of banded milonytic gneisses from the Arcadia-Areal Unit, along with orthognaisses and granitoids from the Rio Negro Magmatic Arc. These rocks are intruded by cretaceous diabase dikes with NE-SW direction. The sedimentary record is mainly associated with alluvial and colluvial deposits, extending 20 km long and 9 km wide, irregularly exposed and separated by intrabacinal highs. The drainage network and relief characteristics, added to the numerous and persistent lineaments, even along with the data collected in the field, demonstrate that the main features are associated to fractures. This elongated and narrow graben is delimited by NE-SW faults at both edges, in addition to NW-SE and E-W fractures which compartmentalize it. The RLG was formed due to recognized NE-SW normal faults, event related to Cenozoic Rift System of Southeastern Brazil generation process. NE-SW structures are expressive in the fluvial system, morphotectonic pattern and preferential erosion and sedimentation zones. In a local scale, field-recognized high-angle NW-SE fractures perform a significant role in the dissection of graben landscape, defining remarkable relief ruptures from the control of tributaries and morphotectonic features. The new discontinuities, represented by E-W and N-S fractures, control small drainage channels, ridges and facets. Moreover, there is the great influence of E-W fractures on the remodeling and dissection of the local landscape, with expressiveness in the joints and faults data that affect basement, dikes and sedimentary cover. The tectonic conditioning of the relief and, especially the drainage network, which responds to younger movements, indicates tectonic activities from Cenozoic Era. The current tectonic regime is defined in the literature by a dextral movement, characterized by NW-SE extensional structures, identical to the structures found at the field, affecting both sedimentary cover and altered basement, with fault planes characterized by striations filled with oxides. The studies performed expose the effects of the Cenozoic brittle tectonic, which modified the landscape and led to the erosion of the GRL basin.
Ribeirão das Lajes Graben (RLG) was mapped at the boundary between Serra do Mar escarpments and Guanabara Graben, in the state of Rio de Janeiro, Brazil. The present study aimed to characterize the Cenozoic tectonic evolution and shape of RLG from methods which incorporate procedures in laboratory and field work, involving structural geology and tectonics geomorphology. This basin was developed on thrust-sheared precambrian NE-SW substrate, consisting of banded milonytic gneisses from the Arcadia-Areal Unit, along with orthognaisses and granitoids from the Rio Negro Magmatic Arc. These rocks are intruded by cretaceous diabase dikes with NE-SW direction. The sedimentary record is mainly associated with alluvial and colluvial deposits, extending 20 km long and 9 km wide, irregularly exposed and separated by intrabacinal highs. The drainage network and relief characteristics, added to the numerous and persistent lineaments, even along with the data collected in the field, demonstrate that the main features are associated to fractures. This elongated and narrow graben is delimited by NE-SW faults at both edges, in addition to NW-SE and E-W fractures which compartmentalize it. The RLG was formed due to recognized NE-SW normal faults, event related to Cenozoic Rift System of Southeastern Brazil generation process. NE-SW structures are expressive in the fluvial system, morphotectonic pattern and preferential erosion and sedimentation zones. In a local scale, field-recognized high-angle NW-SE fractures perform a significant role in the dissection of graben landscape, defining remarkable relief ruptures from the control of tributaries and morphotectonic features. The new discontinuities, represented by E-W and N-S fractures, control small drainage channels, ridges and facets. Moreover, there is the great influence of E-W fractures on the remodeling and dissection of the local landscape, with expressiveness in the joints and faults data that affect basement, dikes and sedimentary cover. The tectonic conditioning of the relief and, especially the drainage network, which responds to younger movements, indicates tectonic activities from Cenozoic Era. The current tectonic regime is defined in the literature by a dextral movement, characterized by NW-SE extensional structures, identical to the structures found at the field, affecting both sedimentary cover and altered basement, with fault planes characterized by striations filled with oxides. The studies performed expose the effects of the Cenozoic brittle tectonic, which modified the landscape and led to the erosion of the GRL basin.
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Geociências, Geologia estrutural, Geomorfologia, Ribeirão das Lajes, Neotectônica, Morfotectônica, Tectônica rúptil, Neotectonics, Morphotectonics, Brittle tectonics