Geodynamics of the eastern sector of the Arrábida chain (W Portugal)

N.º4, p.263-267The Arrábida chain - located in the southern segment of the Lusitanian basin - is a S-wards directed fold and thrust belt of Miocene age of deformation. ENE-WSW striking frontal ramps and associated folds are connected to sinistral NNE-SSW and N-S striking lateral ramps, that show left-lateral and SE-wards reverse movements. Both frontal and oblique ramps form imbricate duplexes. Constriction increases SE-wards due to the presence of a basin boundary fault (SetúbaI- Pinhal Novo fault) that acts as a buttress to the oblique thrust movement. Kilometre scale transpressive structures can be observed along the basin boundary fault. A map scale sheath fold like structure can be observed at the Viso anticline, which is a consequence of a strong anisotropy contrast of the Mesocenozoic strata.The Arrábida chain thrusts migrated backward, i.e. to the north, according to an overstep model. Detailed geometrical reconstruction and kinematic analysis of the structures of the eastern region of Arrábida (Formosinho, Vise and Serra de S. Luis anticlines) show that a thin-skinned tectonic model (Ribeiro, A. et aI., 1990) fits very well with the geometry of the structures of the Arrábida chain

Tectónica da região de Sintra

Photo-interpretation of aerial stereopairs of the Sintra region on the approx. 1/32 000 scale together with field work allowed the production of the present Tectonic Map of the Sintra region. It is now possible to separate structures which resulted from two different tectonic events: one, corresponding to the intrusion of the Late Cretaceous Sintra igneous diapir, and the other the Miocene compressive event, the most important tectonic inversion phase of the Lusitanian Basin. The former are present to the south, southeast and east of the intrusion and within the intrusion itself, affecting the peripheral granites and their contacts with the gabbro-syenite core. These structures comprehend: i) faults and conical fractures striking parallel to the massif boundary, which were intruded by dykes, ii) vertical faults and fractures of two conjugate sets, dextral NNW-SSE and sinistral NNE-SSW. These faults are certainly associated with the E-W striking massif's northwards directed thrust and indicate a N-S oriented horizontal maximum compressive stress. The Miocene compressive event reactivated most of the inherited structures as follows. The NNWSSE faults located on the Sintra southern platform were reactivated as dextral strike slip faults and the E-W thrust along the northern boundary of the massif was also reactivated. This thrust propagated to the east. It also enhanced the asymmetry of the rim-syncline, uplifted the massif and reactivated the NNE-SSW faults as sinistral lateral ramps, which also accommodated vertical throw. The present Tectonic Map of Sintra together with the available geophysical data (MOREIRA, 1984, KULLBERG et al., 1991, SILVA & MIRANDA, 1994) allowed reassessment of the models proposed for the emplacement of the Sintra, Sines and Monchique igneous massifs, which intruded during Late Cretaceous times along the deep dextral NNW-SSE oriented strike slip fault (RIBEIRO et al., 1979; TERRINHA, 1998; TERRINHA & KULLBERG, 1998)

Interpretação tectónica de um conjunto de lineamentos WNW-ESE observados em imagens landsat da margem oeste ibérica

The existence of satellite images ofthe West Iberian Margin allowed comparative study of images as a tool applied to structural geology. Interpretation of LANDSAT images of the Lusitanian Basin domain showed the existence of a not previously described WNW-ESE trending set oflineaments. These lineaments are persistent and only observable on small scale images (e.g. approx. 11200000 and 11500 000) with various radiometric characteristics. They are approximately 20 km long, trend l200±15° and cross cut any other families oflineaments. The fact that these lineaments are perpendicular to the Quaternary thrusts of the Lower Tagus Valley and also because they show no off-set across them, suggests that they resulted from intersection oflarge tensile fractures on the earth's surface. It is proposed in this work that these lineaments formed on a crustal flexure of tens ofkm long, associated with the Quaternary WNW-ESE oriented maximum compressive stress on the West Iberian Margin. The maximum compressive stress rotated anticlockwise from a NW -SE orientation to approximately WNW-ESE, from Late Miocene to Quaternary times (RIBEIRO et aI., 1996). Field inspection of the lineaments revealed zones of norm~1.J. faulting and cataclasis, which are coincident with the lineaments and affect sediments of upper Miocene up to Quaternary age. These deformation structures show localized extension perpendicular to the lineaments, i.e. perpendicular to the maximum compressive direction, after recent stress data along the West Portuguese Margin (CABRAL & RIBEIRO, 1989; RIBEIRO et at., 1996). Also, on a first approach, the geographical distribution of these lineaments correlates well with earthquake epicenters and areas of largest Quaternary Vertical Movements within the inverted Lusitanian Basin (CABRAL, 1995)

Drawing and use of auxiliary projection nets (the program STEGRAPH)

The development of an algorithm for the construction of auxiliary projection nets (conform, equivalent and orthographic), in the equatorial and polar versions, is presented. The algorithm for the drawing of the "IGAREA 220" counting net (ALYES & MENDES, 1972), is also presented. Those algorithms are the base of STEGRAPH program (vers. 2.0), for MS-DOS computers, which has other applications

Tectónica da cadeia da Arrábida

Geociências, Museu Nac. Hist. Nat. Univ. Lisboa, nº 2, 35-84A tectonic map of the Arrábida fold and thrust belt ( 30 km x 6 km) based on aerial-photo interpretation and field work is presented together with geological cross-sections constrained by seismic and gravimetric data, one restored section and kinematic models for the evolution of the Sesimbra salt-wall and the Palmela gravitational slide. Two rifting episodes of Lias-Dogger, and Malm ages, were recognized in the Arrábida rotated fault-block that were accommodated by two normal fault systems striking approximately N-S and E-W. Two episodes of tectonic inversion occurred during Miocene times (Burdigalian and Late Tortonian) during which the N-S faults were reactivated as lateral ramps and the E-W faults as thrusts that formed on a backward propagation sequence. The structure and style of deformation of the Arrábida belt were controlled by the inherited Mesozoic structure and the non-stretched basement that bounds the Arrábida block to the south and east. Although most of the inversion structure appears to be thin-skinned gravimetric modelling (SILVA, 1992) indicates some involvement of the basement in the Viso duplex. The existence of only one detachment, the Hettangian evaporite complex, whose depth to is estimated between 3.5 km and 2.2 km, as constrained by gravimetric and geometric models, is the reason for the simple and elegant geometry of the Arrábida fold and thrust belt. Shortening across the belt was estimated in e = 35% (RAMSAY, 1967) using a 10 km section drawn between Quinta do Anjo (loose line) and Albarquel (pin point) in the eastern sector of the Arrábida belt, across the Serra de São Luís and Viso structures.Apositive strain gradient from north to south and west to east is observed at cartographic scale and was corroborated by independent strain analysis ( P. RIBEIRO, et al., 1996; A. RIBEIRO, et al., 1996

Desk top calculation strategies for reactor analysis using Mathematica

Mathematics is one of several recently developed equations analysis programs that is particularly well suited for solving a broad range of intermediate engineering problems. The objective of this paper is to demonstrate, using a couple of reactor-related examples, how Mathematica can be exploited as a user-friendly analysis tool to symbolically and numerically handle systems of algebraic and differential equations. 7 refs., 3 figs

Evolução tectónica mesozóica da Bacia Lusitaniana

The evolution of the Lusitanian Basin, localized on the western Iberian margin, is closely associated with the first opening phases of the North Atlantic. It persisted from the Late Triassic to the Early Cretaceous, more precisely until the end of the Early Aptian, and its evolution was conditioned by inherited structures from the variscan basement. The part played by the faults that establish its boundaries, as regards the geometric and kinematic evolution and the organization of the sedimentary bodies, is discussed here, as well as with respect to important faults transversal to the Basin. A basin evolution model is proposed consisting of four rifting episodes which show: i) periods of symmetrical (horst and graben organization) and asymmetrical (half graben organization) geometric evolution; ii) diachronous fracturing; iii) rotation of the main extensional direction; iv) rooting in the variscan basement of the main faults of the basin (predominantly thick skinned style). The analysis and regional comparison, particularly with the Algarve Basin, of the time intervals represented by important basin scale hiatuses near to the renovation of the rifting episodes, have led to assume the occurrence of early tectonic inversions (Callovian–Oxfordian and Tithonian–Berriasian). The latter, however, had a subsequent evolution distinct from the first: there is no subsidence renovation, which is discussed here, and it is related to a magmatic event. Although the Lusitanian Basin is located on a rift margin which is considered non-volcanic, the three magmatic cycles as defined by many authors, particularly the second (approx. 130 to 110 My ?), performed a fundamental part in the mobilization of the Hettangian evaporites, resulting in the main diapiric events of the Lusitanian Basin. The manner and time in which the basin definitely ends its evolution (Early Aptian) is discussed here. Comparisons are established with other west Iberian margin basins and with Newfoundland basins. A model of oceanization of this area of the North Atlantic is also presented, consisting of two events separated by approximately 10 My, and of distinct areas separated by the Nazaré fault. The elaboration of this synthesis was based on: - information contained in previously published papers (1990 – 2000); - field-work carried out over the last years, the results of which have not yet been published; - information gathered from the reinterpretation of geological mapping and geophysical (seismic and well logs) elements, and from generic literature concerning the Mesozoic of the west iberian margin

Evolução tectónica mesozóica da bacia lusitaniana

A evolução da Bacia Lusitaniana, localizada na margem ocidental ibérica, está intimamente associada às primeiras fases de abertura do Atlântico Norte. Perdurou desde o Triásico superior até o Cretácico inferior, mais exactamente até o topo do Aptiano inferior, e desenvolveu-se condicionada por estruturas herdadas do soco varisco. É discutido o papel desempenhado pelas falhas que constituem os seus limites, no que respeita a evolução geométrica e cinemática e a organização dos corpos sedimentares. O mesmo é efectuado relativamente a importantes falhas transversais à bacia. É proposto modelo de evolução da bacia ao longo de quatro episódios de rifting que mostram: i) períodos de simetria (organização em horsts e grabens) e assimetria (organização em half graben) na sua evolução geométrica; ii) diacronismo na fracturação; iii) rotação da direcção de extensão principal; iv) enraizamento no soco varisco das principais falhas da bacia (estilo predominantemente thick skinned). A análise e comparação regional, nomeadamente com a bacia do Algarve, de intervalos temporais representados por importantes hiatos à escala da bacia, próximos da renovação dos episódios de rifting, permitiram concluir sobre a ocorrência de inversões tectónicas precoces (Caloviano-Oxfordiano e Titoniano-Berriasiano). A última, no entanto, teve evolução subsequente diferente da primeira: não se verifica renovação da subsidência, que se discute, e relaciona-se com evento magmático. Embora a Bacia Lusitaniana se encontre numa margem de rift que se considera como não-vulcânica, os três ciclosmagmáticos definidos por vários autores, em especial o segundo (apr. 130 a 110 M.a. ?), desempenhou papel fundamental na mobilização dos evaporitos do Hetangiano, que resultou no intervalo principal de diapirismo na Bacia Lusitaniana. É discutida a forma e o momento em que a bacia aborta definitivamente (Aptiano inferior). São estabelecidas comparações com outras bacias da margem ocidental ibérica e da Terra Nova e proposto modelo de oceanização deste troço do Atlântico Norte, em dois momentos, separados por intervalo de cerca de 10 M.a. e em áreas distintas, separadas pela falha da Nazaré. Esta síntese foi elaborada com base: - na informação dada por um conjunto de trabalhos já publicados (1990-2000), - nos trabalhos de campo efectuados nos últimos anos e cujos resultados não foram ainda publicados, - na reunião de informação proveniente da reinterpretação de elementos de cartografia geológica e de geofísica (sísmica e sondagens) e de outros de bibliografia geral sobre o Mesozóico da margem ocidental ibérica

Paleolítico médio em Galapos (Arrábida)

Descrevem-se artefactos líticos de quartzo (e sílex) recolhidos em Galapos (Creiro), na vertente meridional da Serra da Arrábida. São característicos do Paleolítico médio (Mustierense) e reforçam a evidência da importante ocupação humana da região, já salientada pelo estudo da gruta da Figueira Brava.info:eu-repo/semantics/publishedVersio

Toll-like receptors and the host defense against microbial pathogens: bringing specificity to the innate-immune system

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