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- The emergence of volcanic oceanic islands on a slow-moving plate: the example of Madeira Island, NE AtlanticPublication . Ramalho, Ricardo S.; Silveira, António Brum da; Fonseca, Paulo E.; Madeira, José; Cosca, Michael; Cachão, Mário; Fonseca, Maria M.; Prada, Susana N.; PRADA, SUSANAAbstractThe transition from seamount to oceanic island typically involves surtseyan volcanism. However, the geological record at many islands in the NE Atlantic—all located within the slow‐moving Nubian plate—does not exhibit evidence for an emergent surtseyan phase but rather an erosive unconformity between the submarine basement and the overlying subaerial shield sequences. This suggests that the transition between seamount and island may frequently occur by a relative fall of sea level through uplift, eustatic changes, or a combination of both, and may not involve summit volcanism. In this study, we explore the consequences for island evolutionary models using Madeira Island (Portugal) as a case study. We have examined the geologic record at Madeira using a combination of detailed fieldwork, biostratigraphy, and geochronology in order to document the mode, timing, and duration of edifice emergence above sea level. Our study confirms that Madeira's subaerial shield volcano was built upon the eroded remains of an uplifted seamount, with shallow marine sediments found between the two eruptive sequences and presently located at 320–430 m above sea level. This study reveals that Madeira emerged around 7.0–5.6 Ma essentially through an uplift process and before volcanic activity resumed to form the subaerial shield volcano. Basal intrusions are a likely uplift mechanism, and their emplacement is possibly enhanced by the slow motion of the Nubian plate relative to the source of partial melting. Alternating uplift and subsidence episodes suggest that island edifice growth may be governed by competing dominantly volcanic and dominantly intrusive processes.
- Tectónica da Ponta de São Lourenço, Ilha da MadeiraPublication . PRADA, SUSANA; Madeira, J.; Fonseca, P. E.; Silveira, A.; Prada, S.; Rodrigues, C. F.The tectonics of Madeira Island is, in its generality, still quite unknown. A structural characterization togeth er with the formulation of hypothesis contributing to this problematic is, in our opinion, of extreme impor tance to relate this context with the regional and global tectonic models. The Ponta de São Lourenço, in Madeira Island, exhibited an unusual situation for studies of this nature, due to the diversity of lithologies found there, as well as the presence of excellent natural cross-sections originated by marine erosion. In terms of tectonics, the studied area is dominated by accidents of distensive nature, organized in two out standing families: E-W and NW-SE. From these structures the most important are: a dyke swarm, very dense and with an approximate WNW-ESE direction; and a fracture pattern where we highlight the Seixal Machico (N50ºW, subvertical), Pedra Furada (E-W, 70ºN) and Estreito (N50ºW, 70ºN) faults. The analysis of those structures suggests a distensive tension field with minimum compression axis (s3) oriented with NNE-SSW direction, and maximum compression axis (s1) subvertical, related to the weight of the lytho static column (gravitical origin). This tension field suggests a hotspot rift model, similar to the one proposed by WALKER (1987), for shield volcanoes of the Hawaiian kind. This tension field seems to have a constant space-temporal orientation, with probable neotectonic activity. It is also proposed a tectonic control of vol canism, confirmed by the presence of aligned volcanic structures with the same directions previously described. It is equally proposed the existence of a structural control of the coast line and the landforms in the area.
- Stratigraphy and evolutionary model of Madeira IslandPublication . Prada, S. Nascimento; PRADA, SUSANA; Serralheiro, A.
- O inventário nacional do património geológico: abordagem metodológica e resultadosPublication . Prada, S.; PRADA, SUSANAA existência de um inventário nacional de património geológico é fundamental para se poderem implementar estratégias de geoconservação. Este trabalho apresenta a metodologia usada no desenvolvimento do mais completo inventário de geossítios, realizado até ao momento em Portugal, assim como os principais resultados obtidos. O inventário vai integrar o Sistema de Informação do Património Natural e o Cadastro Nacional dos Valores Naturais Classificados, ambos geridos pelo Instituto de Conservação da Natureza e da Biodiversidade.