Abstract
This paper shows present the present state of the geologic and petrographic studies of the silicified tectonic breccia in the Cabo Frio area, State of Rio de Janeiro, Brazil. Based on the results, the authors propose a model for the stick-slip fault displacement. The tectonic breccia zones are 50 cm to 20 m wide and of NE-SW general strike. They are cut by early Cretaceous mafic dykes. The clasts are angular and characterized by auto-brecciation texture. The matrix is firmly consolidated by the silicification with the dissemination of hematite, carbonates, and sericite. The breccia zones are distributed randomly in a wade area of the State of Rio de Janeiro. This observation affirms that the silicification cannot be related to the local magmatism of Cretaceous to early Cenozoic alkaline intrusions. An alternative idea is the hydrothermalism originated form geothermal gradient. According to the normal continental geothermal gradient, of 25 to 30ºC/km, the temperature attributed to the depth from 6 to 8 km is 180 to 220ºC. In this condition, the H2O present there behaves as hydrothermal liquid. This type of hydrothermalism, without participation of magmas, is called amagmatic hydrothermalism. When a earthquake occurs, a cataclastic zone is formed. The surface water migrated down through it and arrived at the depth of 6 to 8 km transforms into amagmatic hydrothermal liquid. According to the stress adjustment and consequent accommodation of the crustal rock blocks, the hydrothermal liquid is compressed and forced to rise up along the cataclastic zone. During the ascension and cooling of the hydrothermal liquid, the cataclastic zone is silicified and consolidated. The silicified tectonic breccia observed in the studied area can represent this phenomenon which took place in the geologic time. By means of the silicification and consolidation, the fault plane transforms form a weak zone into a mechanically firm zone. This transformation enables the accumulation of tectonic energy in form of elastic deformation. That is, during and just after the earthquake, the fault plane is a weak zone. However, in the period between earthquakes it is a firm zone. The clasts of the fault breccia are composed of silicified cataclastic rock. This observation indicates that the fault plane rupture and its hydrothermal consolidation took place repeatedly, which implies repeated earthquakes along the same fault. The authors propose this process as an additional mechanism for stick-slip movement of the faults. The amagmatic hydrothermalism can be a geologic representation of asperity. It is a seismologic conception that corresponds to the specific locality on the fault plane in which the fault movement is stagnated and tectonic energy is accumulated in form of elastic deformation. A new earthquake occurs by means of asperity rupture.
Highlights
Entretanto, de fato, as falhas com deslizamento contínuo sem grandes terremotos são apenas casos excepcionais (e.g. Scholz et al, 1969; Lienkaemper et al, 2001)
Falha é a fratura planar com deslocamento, ou seja, o plano de descontinuidade mecânica com movimento, que estão presentes na crosta ou no manto (e.g. Bates & Jackson, 1987; Hobbs et al, 1976)
As observações microscópicas confirmam em detalhes as características texturais de cataclasito
Summary
Falha é a fratura planar com deslocamento, ou seja, o plano de descontinuidade mecânica com movimento, que estão presentes na crosta ou no manto (e.g. Bates & Jackson, 1987; Hobbs et al, 1976). As falhas de grande rejeito de deslocamento são formadas por esforços tectônicos. Quando o esforço tectônico supera o limite da resistência mecânica da crosta (yield stress), cria-se a fratura planar com deslocamento no corpo rochoso intacto, que corresponde ao surgimento de uma nova falha. Os movimentos repetitivos da falha geram uma zona de cataclase (fault fracture zone; cataclastic zone), que é caracterizada por brecha tectônica (fault breccia) e argila de falha (fault gouge). Byerlee, 1970; Byerlee et al, 1978), não se esclareceu suficientemente por que o movimento intermitente com terremoto é a maneira principal. Interpretando essa como um “fóssil” de terremoto que ocorreu repetitivamente no tempo geológico, os autores propõem um modelo para explicar o mecanismo de deslocamentos intermitentes de falhas, ou seja, terremotos, através de repetidos processos de consolidação hidrotermal amagmático e ruptura mecânica
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