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Evolução tectono-estratigráfica cenozóica do litoral alentejano (sector Melides- Odemira) e enquadramento no regime geodinâmico actual
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Abstract(s)
O presente estudo foca-se nos processos de sedimentação e deformação tectónica no sector litoral do Alentejo que ocorreram ao longo do Cenozóico, num contexto de tectónica compressiva relacionada com a orogenia alpina. A região considerada é caracterizada por três domínios paleogeográficos com sedimentação associada, que rodeiam os relevos litorais de Grândola e Cercal: a planície litoral, a bacia de Alvalade e a zona de transição entre as bacias de Alvalade e do Baixo Tejo. A diferenciação entre a planície litoral e a bacia de Alvalade iniciou-se durante o Paleogénico associada a uma fase tectónica que foi responsável pela movimentação em falhas e consequente criação de relevos e áreas deprimidas adjacentes, onde ocorreu a sedimentação. O registo sedimentar cenozóico apresenta-se relativamente completo, tendo-se identificado seis etapas de sedimentação principais relacionadas com factores tectónicos, eustáticos e climáticos: 1) no Eocénico-Aquitaniano, com deposição em ambiente de leque aluvial; 2) no Serravaliano superior-Tortoniano inferior, representada por sedimentação marinha; 3) no Messiniano, com sedimentação marinha e aluvial; 4) no Zancleano-Placenciano, com ocupação marinha na área litoral e desenvolvimento de importantes sistemas fluviais; 5) no Gelasiano, com deposição em ambiente fluvial e de leque aluvial; 6) no Plistocénico Médio Superior, com geração de terraços fluviais e marinhos. Intercalados e contemporaneamente a estas fases sedimentares ocorreram episódios de deformação tectónica durante o Eocénico, o Burdigaliano, o Tortoniano, e ao longo de todo o Pliocénico e Plistocénico. Esta deformação deu-se principalmente em regime transpressivo. As deformações verificadas durante o período neotectónico considerado (aproximadamente os últimos 5 Ma) estão expressas por deslocamentos associados a falhas e por movimentos verticais positivos da crusta. As falhas identificadas apresentam capacidade de gerar sismos de magnitude significativa (Mw ≥ 5,5), embora as taxas de actividade estimadas sejam baixas (0,007-0,038 mm/ano) e os intervalos de recorrência muito longos (9700-117000 anos). As taxas de levantamento tectónico são também baixas (0,005-0,056 mm/ano), inferiores às anteriormente admitidas. De forma geral, verifica-se uma tendência de aumento da amplitude dos movimentos verticais de N para S. A partir do Plistocénico Médio o levantamento poderá anular-se ou mesmo ocorrer subsidência na área de Porto Covo-Sines.
This study is focused on the sedimentary and tectonic processes that occurred on the Alentejo coastal sector throughout the Cenozoic, in a context of compressive tectonics related to the alpine orogeny. The study area comprises three paleogeographical domains with associated sedimentation, surrounding the Grândola and Cercal coastal reliefs: the littoral plain, the Alvalade basin and the transition area between the Alvalade and the Lower Tagus basins. The differentiation between the littoral plain and the Alvalade basin was initiated during the Paleogene in relation to a tectonic phase that was responsible for movements along faults, generating relief and adjacent lowered areas were sedimentation took place. The Cenozoic sedimentary record is fairly complete and six major sedimentation phases were identified, related to tectonic, eustatic and climatic processes, which occurred in the following time periods: 1) Eocene-Aquitanian, with alluvial fan deposition; 2) upper Serravalian-lower Tortonian, represented by marine sedimentation; 3) Messinian, with marine and alluvial sedimentation; 4) Zanclean-Piacenzian, with a marine occupation of the littoral area and development of important fluvial systems; 5) Gelasian, with alluvial fan and fluvial sedimentation; 6) Middle-Upper Pleistocene, characterized by the generation of fluvial and marine terraces. Tectonic deformation episodes occurred intercalated and coeval to these sedimentation phases during the Eocene, Burdigalian, Tortonian, and along the Pliocene and Pleistocene. This deformation occurred mainly in a transpressive context. Neotectonic deformation (approximately in the last 5 Ma) is represented by fault related displacements and positive vertical crustal movements. The identified faults are able to generate seismic events of considerable magnitude (Mw ≥ 5.5), although the estimated activity rates are low (0.007 0.038 mm/yr) and recurrence intervals are very long (9700-117000 years). Uplift rates are also low (0.005 0.056 mm/yr), lower than previously thought. There is a general trend of increase of uplift amplitude from N to S. Evidences show that in the Porto Covo-Sines area the uplift might be nulled since Middle Pleistocene or even there is subsidence.
This study is focused on the sedimentary and tectonic processes that occurred on the Alentejo coastal sector throughout the Cenozoic, in a context of compressive tectonics related to the alpine orogeny. The study area comprises three paleogeographical domains with associated sedimentation, surrounding the Grândola and Cercal coastal reliefs: the littoral plain, the Alvalade basin and the transition area between the Alvalade and the Lower Tagus basins. The differentiation between the littoral plain and the Alvalade basin was initiated during the Paleogene in relation to a tectonic phase that was responsible for movements along faults, generating relief and adjacent lowered areas were sedimentation took place. The Cenozoic sedimentary record is fairly complete and six major sedimentation phases were identified, related to tectonic, eustatic and climatic processes, which occurred in the following time periods: 1) Eocene-Aquitanian, with alluvial fan deposition; 2) upper Serravalian-lower Tortonian, represented by marine sedimentation; 3) Messinian, with marine and alluvial sedimentation; 4) Zanclean-Piacenzian, with a marine occupation of the littoral area and development of important fluvial systems; 5) Gelasian, with alluvial fan and fluvial sedimentation; 6) Middle-Upper Pleistocene, characterized by the generation of fluvial and marine terraces. Tectonic deformation episodes occurred intercalated and coeval to these sedimentation phases during the Eocene, Burdigalian, Tortonian, and along the Pliocene and Pleistocene. This deformation occurred mainly in a transpressive context. Neotectonic deformation (approximately in the last 5 Ma) is represented by fault related displacements and positive vertical crustal movements. The identified faults are able to generate seismic events of considerable magnitude (Mw ≥ 5.5), although the estimated activity rates are low (0.007 0.038 mm/yr) and recurrence intervals are very long (9700-117000 years). Uplift rates are also low (0.005 0.056 mm/yr), lower than previously thought. There is a general trend of increase of uplift amplitude from N to S. Evidences show that in the Porto Covo-Sines area the uplift might be nulled since Middle Pleistocene or even there is subsidence.
Description
Tese de doutoramento, Geologia (Geodinâmica Interna), Universidade de Lisboa, Faculdade de Ciências, 2018
Keywords
Teses de doutoramento - 2018