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- High-resolution P- and S-wave reflection studies of an intraplate structure: The Azambuja fault, PortugalPublication . Ghose, Ranajit; Carvalho, JoĆ£o; Alves, Daniela; Santos, Luiz Alberto; RessurreiĆ§Ć£o, Ricardo; Alves, Paulo Henrique Bastos; Leote, JaimeABSTRACT: The Azambuja fault is a NNE trending structure located 50 km north of Lisbon, the capital and most populous city of Portugal. The fault has been considered as a possible source for the historical, large earthquakes. Under-standing this fault is a priority in seismic hazard evaluation of this region. The fault has a clear morphological signature. Miocene and Pliocene sediments are tilted eastward and cut by steeply dipping mesoscale fault seg-ments, presenting reverse and normal offsets with a net downthrow to the east. Neotectonic studies indicate a Quaternary slip on the fault of 0.05-0.06 mm/year. However, no direct evidence of the Azambuja fault affecting the Pleistocene or Holocene sediments was found so far. Here, we present the findings from high-resolution seismic reflection studies using both P-and S-waves over the Holocene deposits. The detection of small-throw faulting in ductile sediments is a challenging task. We show that multiple signatures, like perturbations in the reflection hyperbolae visible in shot and CMP gathers, interruptions of reflectors in stacked sections, lateral seismic velocity variations obtained by horizon velocity analysis, all at coincident locations, strongly suggest that the activity of the Azambuja fault has affected the Holocene sediments in the study area. The lateral velocity variations are corroborated by wavepath eikonal traveltime tomography and velocity analysis supported by seismic modeling. By means of 2D viscoelastic modeling, we explain the absence of fault-related diffractions and negligible back-scattered energy from the fault. Using data from nearby boreholes, we find that the 15 ka old alluvium cover has indeed been disturbed by the presence of shallow fault strands. Considering the estimated vertical throws and the empirical relationships between fault length, co-seismic rupture and magnitude, a slip rate of 0.07 mm/y, slightly larger than previously thought, is expected for this fault.
- SismotectĆ³nica do AlgarvePublication . Dias, Ruben Pereira; Terrinha, Pedro; Carvalho, JoĆ£o; Cunha, Tiago A.; RessurreiĆ§Ć£o, Ricardo; GonƧalves, JoĆ£o Noiva; Silva, S.; Ramalho, Elsa; Pinto, Carlos C.; Roque, Ana Cristina; Duarte, JoĆ£o C.; Valadares, Vasco; Carrara, Gabriela; Duarte, Henrique; Batista, LuĆs Filipe; Rego, J.; Gomes, F.; Caneiras, F.; Marquilhas, J.; Catrapona, JoĆ£o; Matias, L.; Vicente, J.
- DeformaĆ§Ć£o tectĆ³nica no bordo ocidental da bacia de Rio Maior : a falha do CidralPublication . RessurreiĆ§Ć£o, Ricardo; Dias, Ruben Pereira; Cabral, JoĆ£o; Charifo, G.SUMMARY: The Rio Maior basin is a NNW-SSE elongated tectonic basin, controlled by main faults with the same orientation. The basin was filled with sediments during the Pliocene, over Miocene deposits. Borehole data show a depocenter located near its eastern margin with the Pliocene sediments forming a dissymmetrical synform controlled by the NNW-SSE Cidral border fault. The sedimentation culminates in Pleistocene conglomeratic sandstones, apparently not deformed. During recent field work in a sand exploration quarry located in the SE extremity of the basin, a N-S fault affecting the Miocene and Pliocene was identified along ca. 400 m. Striated slickensides in the Miocene deposits indicate a predominant normal dip-slip component. This was interpreted as a prolongation or major ramification of the Cidral fault. Considering the proximity of extruded Jurassic evaporitic sediments at Fonte da Bica diapir, the extensional deformation in the Rio Maior area during the Pliocene is probably related with tectonic diapirism.
- Estudos de neotectĆ³nica na falha de Carcavai, Algarve OrientalPublication . RessurreiĆ§Ć£o, Ricardo; Cabral, JoĆ£o; Dias, Ruben Pereira; Carvalho, JoĆ£o P. F.; Pinto, Carlos C.Estudos levados a cabo na zona da falha de Carcavai revelaram a ocorrĆŖncia de deformaƧƵes (fracturaĆ§Ć£o e filƵes detrĆticos) em sedimentos plio-quaternĆ”rios, indicando actividade neotectĆ³nica. A generalidade das fracturas aparenta corresponder Ć expressĆ£o superficial secundĆ”ria da actividade naquela zona de falha. Os dados adquiridos apontampara uma zona de falha complexa de desligamento esquerdo, com componente vertical inversa, desde o final do MesozĆ³ico ou inĆcio do CenozĆ³ico. Os filƵes detrĆticos foram interpretados como estruturas resultantes de liquefacĆ§Ć£o induzida sismicamente.
- Neotectonic activity on the Carcavai fault zone (Algarve, Portugal)Publication . RessurreiĆ§Ć£o, Ricardo; Cabral, JoĆ£o; Dias, Ruben Pereira; Carvalho, JoĆ£o; Pinto, Carlos C.Studies performed on the Carcavai fault zone, Eastern Algarve, revealed several deformational features in Plio-Quaternary sediments with potential neotectonic implications. These features include fracturing (faults and joints) and clastic dikes, located along the Carcavai fault. Most of the fractures appear to be the expression of the fault activity affecting the Plio-Quaternary sediments. This is suggested by a consistent strike, subparallel to the major tectonic structure, and by the location pattern. Evidences of the fault activity were also identified in the Mesozoic and Paleozoic rocks, though the age of that deformation is poorly constrained. It consists in the presence of zones of intensely brecciated rock and of adjoining coarse sedimentary deposits probably related to the dismantling of a fault scarp. The acquired data point to a late hercynian structure, reactivated as a left-lateral strike-slip fault with a thrust component during the Plio-Quaternary.
- Contributo do acompanhamento das grandes obras de engenharia para a cartografia geolĆ³gica : Caso de estudo do Corredor Internacional SulPublication . Morais, Igor; Albardeiro, LuĆs; GonƧalves, Pedro; Soares, S.; SolĆ”, A. Rita; Dias, Ruben Pereira; RessurreiĆ§Ć£o, Ricardo; Castro-Pina, M.; Serra, B.; Pereira, ZĆ©lia; Mendes, MĆ”rciaSUMMARY: The follow up of new railway works, which will be part of the International South Corridor (Corredor Internacional Sulā, allowing the privileged access to new Ossa-Morena Zone outcrops, contributes to refine detailed geological mapping, as part of LNEG's mission. Along the different sections of the railway line, systematic geological sampling was carried out for petrographic, geochronological and paleontological studies. In addition, photographic record and three-dimensional modeling of the main slopes were performed using a drone. This methodology allows the construction of an information repository that can be consulted long after the works have been concluded and the outcrops become inaccessible. Besides the accuracy improvement of the geological contacts of the Ossa-Morena Zone rocks, the preliminary results confirmed the extent of Miocene debris flow sedimentation (conglomerates) and subsequent calcretization, and has identified new mineral occurrences.
- Neotectonic structures affecting Plio-Pleistocene sediments in Boliqueime and Quarteira areasPublication . Cabral, JoĆ£o; Dias, Ruben Pereira; RessurreiĆ§Ć£o, Ricardo
- Cartografia geolĆ³gica sistemĆ”tica para a ediĆ§Ć£o da Folha 43-A, Cuba (Carta GeolĆ³gica de Portugal, escala 1:50 000) - Ponto da situaĆ§Ć£oPublication . Ferreira, Pedro Tavares; Caldeira, Rita; PiƧarra, JosĆ© Manuel; Dias, Ruben Pereira; Calvo, RogĆ©rio; Cunha, Teresa Arriaga; Pestana, Ana Margarida; Pais, JoĆ£o; RessurreiĆ§Ć£o, RicardoA cartografia geolĆ³gica sistemĆ”tica para a ediĆ§Ć£o da Folha 43-A, Cuba, tem vindo a desenvolver-se desde 2009, com base em levantamentos de campo realizados Ć escala 1:25000. As unidades cartografadas atĆ© Ć actualidade sĆ£o incluĆdas (1) no MaciƧo de Beja - PĆ³rfiros de BaleizĆ£o, Dioritos de Monte Novo e Complexo Gabro DiorĆtico de Cuba, (2) no sector de Montemor-Ficalho - formaƧƵes meta-sedimentares e vulcano-sedimentares do Neo-ProterozĆ³ico ao CarbĆ³nico e (3) nos depĆ³sitos do CenozĆ³ico - constituĆdos por alternĆ¢ncias de sedimentos finos e grosseiros de idades que variam entre o PaleogĆ©nico e o QuaternĆ”rio. Com este trabalho pretende-se apresentar a cartografia geolĆ³gica desenvolvida atĆ© Ć actualidade na regiĆ£o de Cuba, Alentejo.
- 87Sr/86Sr dating of the AlcĆ”cer do Sal Formation (upper Miocene, mainland Portugal)Publication . RessurreiĆ§Ć£o, Ricardo; Legoinha, Paulo; Dias, Ruben Pereira; Santos, JosĆ© Francisco; Ribeiro, Sara; Patinha, Carla; Miranda, RuiThe Lower Tagus Basin was subjected to several marine transgressions along the Neogene, related to positive eustatic oscillations and also controlled by the regional tectonic activity. In the AlcĆ”cer do SalSines region, previous studies identified an episode of marine sedimentation interpreted as having occurred during the late Serravalian to early Tortonian interval, based on biostratigraphic data. This episode is represented in the AlcĆ”cer do Sal Formation. The study of the SMS-12- 01B borehole, drilled for mineral prospection purposes, located about 10 km NNE of Melides, allowed the identification of a 3 m thick layer of sediments containing marine fossils. 87Sr/86Sr determinations of oyster shells point to deposition about 11.5 Ma ago, validating previous proposals for the age of the formation. These data correspond to the first numerical ages obtained for the AlcĆ”cer do Sal Formation, allowing a solid correlation with other known marine units in the Lower Tagus and Algarve basins related to the same transgression episode
- Programa de cartografia transfronteiriƧa do LNEG-IGME na Zona Sul Portuguesa nas escalas 1/200 000 e 1/400 000. Um contributo para a estratigrafia do SW IbĆ©ricoPublication . Matos, JoĆ£o Xavier; DĆez-Montes, Alejandro; Dias, Ruben Pereira; Pereira, ZĆ©lia; Machado, Susana; SolĆ”, A. Rita; Carmona, J.J. Huerta; RessurreiĆ§Ć£o, Ricardo; Albardeiro, LuĆs; Mendes, MĆ”rcia; Morais, Igor; GonzĆ”lez-Clavijo, Emilio J.; GonƧalves, Pedro; Santos, Sara; de Oliveira, Daniel Pipa Soares; Salgueiro, Rute; Henriques, S.B.A.; Oliveira, JosĆ© TomĆ”s; Fernandes, Paulo; Inverno, Carlos; Lopez, M. T.SUMMARY: The South Portuguese Zone (SPZ) 1/200 000 and 1/400 000 scale maps and stratigraphic program in SW Iberia is presented by a LNEG-IGME consortium and funded by GEO_FPI/Interreg-POCTEP. The 1/200 000 mapped area corresponds to the border area and is based on previous work (Folha 8 and Sevilla-Puebla de GuzmĆ”n maps). The 1/400 000 map covered a larger area, from Lisbon to Seville. The following domains of SPZ were considered: Pulo do Lobo (early Frasnian-late Famennian), Iberian Pyrite Belt (late Famennian-late Visean) and Baixo Alentejo Flysch Group (late Visean-late Moscovian). The maps include the Mesozoic Algarve basin and the Cenozoic Guadalquivir/Algarve basins. Field surveys were done using standard stratigraphic and GIS database methodologies. Palynological studies of Paleozoic metasedimentary rocks and U/Pb zircon geochronology of plutonic and volcanic rocks were essential to refine and update the Paleozoic stratigraphy. The new data allowed new knowledge and the validation of the geological models.