Herança tectónica e o ciclo dos supercontinentes

ResearchBlogging.org

Provavelmente a maior revolução na Geologia foi a constatação da mobilidade dos continentes, ou mais correctamente, de placas rígidas (placas litosféricas, que podem ser de natureza continental ou oceânica) sobre rocha mais fluída subjacente (astenosfera). A teoria da tectónica de placas é hoje um elemento central nesta disciplina científica, dado o seu poder para explicar uma grande variedade de fenómenos.

O estudo de formações rochosas a nível mundial à luz da tectónica de placas permitiu chegar não só à conclusão que os continentes não estiveram sempre nas posições actuais, mas também que em tempos a maioria deles esteve reunido em grandes massas continentais, geralmente designadas “supercontinentes”. Ao longo do tempo estes foram-se formando e desagregando ciclicamente, fenómeno explicado pelo chamado “Ciclo dos Supercontinentes” (complementar ao “Ciclo de Wilson”, que explica a abertura e fecho cíclicos dos oceanos). O supercontinente mais recente, e o mais conhecido, foi a Pangea, que se começou a separar durante o Jurássico, há cerca de 180 milhões de anos (Ma), mas vários supercontinentes existiram em tempos anteriores.

As placas continentais são geralmente estáveis, espessas e com rochas velhas e pouco alteradas no centro (regiões cratónicas), e bastante deformadas, pouco espessas e com rochas jovens na periferia – estas estão constantemente a ser envolvidas nos ciclos tectónicos, “poupando” os velhos cratões que se localizam no centro das placas continentais.

O mote foi dado pelo geólogo canadiano John Tuzo Wilson, pioneiro da teoria da tectónica de placas, no seu influente artigo de 1966 “Did the Atlantic close and then re-open?” (“Será que o Atlântico fechou e depois reabriu?”). Os estudos subsequentes da história geológica das margens continentais, e em particular das margens atlânticas, revela que nestas, para além das marcas da abertura do oceano, estão também as marcas da abertura e fecho de oceanos passados, marcas essas geralmente ausentes do interior continental.

 

Posição actual dos continentes - Fonte: http://www.scotese.com/

Também em Portugal temos evidências dessa reutilização de margens continentais nos ciclos tectónicos. Partindo do presente para o passado, fazendo o percurso contrário ao do tempo, começamos com a nossa margem continental atlântica, em expansão desde o Jurássico Superior. Isto é nos revelado pela datação das rochas sedimentares do Oeste e Sul portugueses e pela idade das rochas do fundo oceânico – estas são sucessivamente mais velhas quanto mais nos afastamos da Crista Médio-Atlântica, o rifte cuja actividade está na génese do Atlântico.

 

Posição dos continentes no Jurássico Superior - Fonte: http://www.scotese.com

Se estas evidências nos demonstram um oceano em expansão, teve que haver um período antes do oceano se expandir, neste caso um tempo em que ele esteve fechado, e os continentes em ambos os lados unidos. Este foi o tempo da Pangea, o último supercontinente, cuja formação se concluiu no Pérmico, há cerca de 280 Ma.

 

Posição dos continentes no final do Pérmico - Fonte: http://www.scotese.com

A geologia de Portugal está repleta de marcas (dobras, falhas, rochas metamorfizadas, magmatismo…) dos eventos de formação deste famoso supercontinente – as rochas que formam a “espinha dorsal” da geologia ibérica (o chamado Maciço Hespérico) foram formadas na margem de um grande continente do hemifério Sul, a Gondwana, à medida que os continentes se iam aproximando uns dos outros durante o Paleozóico Superior. Estas enormes forças de compressão levaram ao fecho do Oceano Reico, que separava a Gondwana do continente Laurussia. Na nossa pequena porção na margem da Gondwana, a colisão deu-se principalmente pelo fecho da parte ocidental de um outro oceano, o Paleo-Tétis, que originou a colisão da margem da Gondwana com o pequeno continente Armórica.

 

Posição dos continentes durante o Devónico – Fonte: http://www.scotese.com

A formação do Paleo-Tétis deu-se como consequência da abertura do Reico no início do Paleozóico, após os processos de desagregação do supercontinente anterior, Panótia (este existiu durante um curto espaço de tempo no final do Proterozóico, tendo sido precedido pelo supercontinente Rodínia). A formação deste oceano teve como consequência a individualização da Armórica a partir da margem da Gondwana.

Tudo isto ajuda-nos a perceber a ciclicidade da formação e desagregação dos supercontinentes, e em particular a forma como a margem continental do Atlântico mostra evidências de ter sido palco noutras ocasiões de grandes eventos tectónicos.

 

Referências

– Kearey, P., Klepeis, K.A. & Vine, F. (2009) Global Tectonics, third edition, Wiley-Blackwell, John Wiley & Sons, West Sussex, UK

– Pita, P. et al – Plano Regional de Ordenamento (PROT) para a Região do Norte – Recursos Geológicos e Hidrogeológicos

– Ribeiro, A., Munhá, J., Dias, R., Mateus, A., Pereira, E., Ribeiro, L., Fonseca, P., Araújo, A., Oliveira, T., Romão, J., Chaminé, H., Coke, C., & Pedro, J. (2007). Geodynamic evolution of the SW Europe Variscides Tectonics, 26 (6) DOI: 10.1029/2006TC002058

– Thomas, W. A. Tectonic inheritance at a continental margin. (2006) GSA Today 16,411

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