Volume change in contractional kink bands; examples from the NNE-SSW Late Variscan kink bands of Portugal

Kink bands are usually associated to the late deformation stages in polydeformed areas, being developed in rocks that present a strong planar mechanical anisotropy (e.g. layering or cleavage). In Portugal, the Late Variscan deformation episode generates in Almograve and Abrantes regions (SW Iberian Massif), brittle to brittle-ductile NNE-SSW sinistral kink bands, which deflects and deforms the NNW-SSE regional trend related to the earlier tectonic events. These defection induces a counterclockwise rotation of subvertical primary layering/foliation in inner domains of the kink bands. The rotation inevitably distort previous structures, generating a complex structural pattern in response to kink folding mechanism, but with particular features in each case. In Almograve, within metric to decametric sinistral kink bands, a wide spectrum of structures were developed, including distinct fold patterns (among which flexural slip folds) and brittle to brittle-ductile shear zones. In the inner domains of the Abrantes metric kink band, a high strain pattern composed by 2nd order conjugate kink bands, acting as “strain slip cleavage” is found. The developed mesoscopic structures allows to calculate internal and external stress fields, showing a variation from the outer to inner domains of kink band. The stress field pattern also shows a rotation which is also similar to the exhibit rotation of structures within inner domains. This stress field variation should be related with the deformation mechanisms induced by rotation of primary layering. Indeed, the generated internal structures are clearly controlled by layer parallel shortening and layer parallel slip, which were the two main mechanisms responsible by accommodation of volume distortion within kink bands. This deformation was also conditioned by the rheology and heterogeneities of deformed sequence. As mentioned, during the development of kink bands, the strain is localized within kink band due the internal rotation of primary layering and/or simple shear parallel to kink boundaries. There are no evidences of kink band boundary migration, allowing to classify the kink bands as fixed hinge type III kink bands. In order to estimate the internal shortening parallel to layering during the rotation, it is proposed a new theoretical graphical method for type III kink bands, which allows an expedite shortening measurement based on angular parameters of kink bands. The proposal graphic shows that kink bands presents variable shortening ratio ranging between 5 to 25%. Some care should be used with such shortening values, because small variations in kink band angular values could occur, being induced either by slip in kink band boundaries or the layer parallel slip. However, the new method presents itself as an excellent approach method, as proved by the similar shortening calculated for real structures. The correct geometric analysis also allows to determine that shortening component is dissipated in the other two orthogonal directions, i.e. perpendicular do layering and in vertical direction

Domino structures as a local accommodation process in heterogeneous shear zones

Usually, deformation in rocks is heterogeneously distributed, concentrating on planar zones located between rigid blocks, named shear zones. The geometry and kinematic criteria analysis becomes essential to understand the shear zones dynamics; the dominoes are one of the structures which can be used as a shear kinematic criteria. However, for the application of these structures as kinematic criteria, it is necessary a careful analysis of the tectonic environment associated with these structures. These structures are developed under ductile-brittle to brittle regimes associated to a non-coaxial deformation and obeying Coulomb criterion for failure (Jaeger & Cook, 1981). Dominoes are characterized by the clear predominance of one shear family, that induces the block rotation during the deformation process. These structures are commonly associated to extensive regimes and in strike-slip environments their development is poorly known. In such non coaxial wrench domains, these structures are frequently interpreted as asymmetric boudins at mesoscale. Current work, in Abrantes region (Central Portugal), emphasizes the presence of two major Variscan deformation phases. The second one (D2) is associated with a NW-SE right-lateral non-coaxial shear component; This deformation increase to West, when approaching the Porto-Tomar-Ferreira do Alentejo shear zone, which leads to consider that this dextral first order shear was responsible by D2. The D2 structures are mostly right-lateral shear zones, developed at all scales, affecting clearly the first deformation phase structures. The D2 structures present a heterogeneous development with simple shear dominated transpression domains, alternating with domains that exposing pure shear dominated transpression. Locally, in simple shear dominated transpression domains, it is possible to see a complex Sfabric, characterized by the presence of several families of planar strucures, which accommodates the internal shear zone stress. This fabric is constrained to decimetric layer with well-defined borders and extensively laminated. It’s possible to separate four families of planar structures given our geometry and kinematics: (1) main shear represents NNW-SSE right-lateral kinematics, parallel to the previous anisotropy (S1). This shear acts like a rigid barrier and the other families don’t cut this shear. (2) Shear family 1 are conjugate NNE-SSW to N-S left-lateral shears. Near the main shear the shear orientation is variable. (3) Shear family 2 are NE-SW to E-W left lateral shears, delimiting centimetric to milimetric blocks. This blocks exhibits rotation. It is possible to show more than one generation of shears. (4) Shear family 3 are NNE−SSW shears, with main shear synthetic kinematics. These family behave as c’-bands and have a punctual development. Associated with this S-tectonite, appears cohesive fault rocks (cataclasite; e.g. Sibson, 1977). Cataclasite is characterized by distributed fracture and grain size reduction throughout shears. Therefore the blocks have rigid rotation in a plastic matrix (cataclasite); this heterogeneous flow accommodates the overlaps and gaps creation due the shear zones activity. The cataclastic flow is located near the main shear and shear family 1, yielding a crush zone. The field data shows that the domino spinning is result from dual acting between main shear and family 1 shear

Accommodation structures during kink bands evolution; quantitative methods applied to Late Variscan deformation of Portugal

Kink bands are common in rocks with a strong planar mechanical anisotropy, being generally developed in polydeformed areas during the late stages of deformation, in brittle to brittle-ductile conditions. In the Late Variscan deformation events in the Iberian Massif, several kink band structures were developed in association to the NNE-SSW sinistral strike-slip faults, which are induced by the Laurentia and Gondwana E-W dextral collision on Carboniferous times. Within two metric-scale kink bands, several 2nd order structures are generated in response to internal shortening during layering rotation. The present paper describes the internal geometry and kinematics of these type III fixed hinges kinks bands, discussing their deformation mechanisms. In order to quantify the internal shortening within kink bands, a new graphical method, using simple angular parameters, is proposed. This graphical approach applied to the studied kink bands allows their internal shortening to be quantified; it ranges between 12 to 18%. The genesis of 2nd order structures within kink bands results from the accommodation of the internal layer distortion during kink bands progressive deformation and is controlled by two main deformation processes: layer parallel shortening and layer parallel slip

Guia de Campo – Recursos Geológicos no Sector de Estremoz-Barrancos

Guia de Campo para a região de Estremoz preparado no âmbito das acções de divulgação do Projecto Modelos Metalogénicos 3D da Zona de Ossa Morena

Devonian Sedimentation in the SW boundary of the Ossa-Morena Zone: State of Art and Paleogeography

The Devonian sedimentary record of the Ossa-Morena Zone is scarce. In its southwestern domains, the Odivelas Limestone and correlatable units represent the single evidence of latest Early Devonian to Middle Devonia n (possibly into Frasnian) sedimentation in this area. Reefal and perireefa l sediments are frequently, but not always, associated to coeval volcanic rocks that probably supported atoll-like systems on the top of volcanic edifices. The northernmost occurrences are spatially associated with mississippian volcani c-sedimentary complexes. Their geometrical relation is still unclear, the limestones are possibly embedded as olitoliths, but can also be tectonically imbricated or simply reflecting draping of younger sediments around and above older (Devonian) sea-bed topography

The central Alentejo plateaus: a review of the regional relief units

Since the 1990s, detailed studies on the relief units of the central and upper Alentejo region have been lacking. Previously, tectonics were used to explain most of the relief units, even those in which lithological differentiation is remarkable, such as the dolomitic plateaus of Elvas and Estremoz. The morphotectonic reliefs like Serra d’Ossa and Serra de S. Mamede were previously explained by vertical tectonics, in a horst-graben system, difficult to understand under the Cenozoic compressive tectonic regime affecting the Western Iberian margin. The superposition of geological maps with digital terrain models suggests a more complex genesis in the formation of the morphotectonic regional reliefs. The Serra d’Ossa (652 m) develops in a WNW-ESE general trend (N80ºW), slightly asymmetrical with a 200 m high north-facing escarpment, much steeper than the south-facing slope. The north-facing escarpment (Ossa Fault – OF) is transversal to the NW-SE variscan structures (N40ºW). Thus, this escarpment cannot be explained by differential erosion. Indeed, Feio (1983) already hypothesized a tectonic origin to the Serra d'Ossa, although without presenting a tectonic model. The western termination of the Serra d‘Ossa small scarps, with NE-SW orientation, coincide with the horse tail terminations of the NNE-SSW left strike-slip Graça do Divor fault (GDF). If both GDF and OF are connected, the Serra d’Ossa can be interpreted as a push up deformation of the South Portuguese Planation Surface (SPPS). In this work, a greater relevance of differential erosion is highlighted in the individualisation of Elvas and Estremoz plateaus, as well as the Serra de Monfurado. Towards the north of the town of Évora, the landscape is formed by broad-bottomed valleys at 240 m, with gentle slope and flat uplands at ca. 320-340 m. Looked in Davisian terms, the valleys looks like a mid-cycle maturity landscape. The flat upland level corresponds to the SPPS, well developed in the upper Alentejo (Nisa and Alpalhão), while the bottom of the valleys correspond to a younger level, embedded ca. 80–100 m in the SPPS. The valley bottoms widens to downstream forming a flattening surface (named N1 fluvial surface) related with the beginning of the incision of the drainage network in the SPPS and with the first (older) terraces of the Tejo and Guadiana rivers. The prominence in the landscape of the Serra de Monfurado should be understood as a resistant ridge, whose summits were not completely flattened due to the lithological diversity and to the geographic location in the watershed limit of the Tejo, Guadiana and Sado rivers, where the flattening of the SPPS was difficult to achieve. Differential erosion during the formation of the N1 fluvial surface is thought to be the main responsible for the prominence of this ridge in the central Alentejo landscape, as well as other resistant reliefs as the Monsaraz inselberg

geodynamic evolution of northernmost sectors of Ossa-Morena zone in iberian variscides context

Os estudos levados a cabo na região Abrantes-Tomar permitiram reconhecer dois domínios distintos: um domínio Este com claras afinidades litoestratigráficas, estruturais e geoquímicas com a Zona de Ossa-Morena (ZOM) e um domínio a Oeste com características tectonoestratigráficas, metamórficas e magmáticas próprias que permitem a sua distinção do restante Terreno Varisco Ibérico. O domínio Este apresenta uma sucessão litoestratigráfica com afinidades à transição Neoproterozóico-Câmbrico da ZOM. Aqui distinguiram-se duas sequências distintas, que colocam em evidência a presença de uma evolução policíclica. Com efeito, a sequência Neoproterozóica mostra o desenvolvimento de um arco vulcânico, ao qual se associa a génese de uma bacia de back-arc. Esta bacia poderá apresentar oceanização incipiente, encontrando-se materializada na Zona de Cisalhamento Tomar-Badajoz-Córdoba. Estes dados são compatíveis com os modelos propostos para a ZOM durante o Ciclo Cadomiano, tendo esta zona de cisalhamento sido reactivada como um importante cisalhamento intraplaca durante o Ciclo Varisco. A sequência Câmbrica apresenta claras afinidades litoestratigráficas e geoquímicas com as sucessões sin-rift intra-continental que caracterizam o início do Ciclo Varisco na ZOM. O domínio Oeste é caracterizado pela presença de unidades tectono-estratigráficas com características metamórficas e magmáticas distintas no contexto do Maciço Ibérico, sendo a sua evolução geodinâmica condicionada pela Zona de Cisalhamento Porto-Tomar-Ferreira do Alentejo, que delimita o bordo Este deste domínio. A comparação com os sectores de Coimbra, Porto-Albergaria e Berlengas permitiu a caracterização de um terreno com características tectono-estratigráficas próprias (Terreno Finisterra), que apresenta características análogas ao Bloco de Léon (Maciço Armoricano) e ao Mid-German Crystalline Rise, permitindo a sua correlação à escala do Varisco Europeu. A integração dos dados do sector de Abrantes-Tomar nos modelos geodinâmicos propostos para o Maciço Ibérico, implicou a realização de estudos pontuais noutras regiões, como sejam Almograve, Ponta Ruiva e Vila Boim; Geodynamic Evolution of northernmost sectors of Ossa-Morena Zone in Iberian Variscides context Abstract: The studies in the Abrantes-Tomar region allowed to recognize two distinct domains: an eastern one with clear lithostratigraphic, structural and geochemical affinities with the Ossa-Morena Zone (OMZ) and a western domain which has its own tectonostratigraphic, metamorphic and magmatic features, allowing its individualization from the Iberian Variscan Terrane. The eastern domain presents a lithostratigraphic succession with affinities to the Neoproterozoic-Cambrian of the OMZ. Two sequences were distinguished, highlighting the presence of a polycyclic geodynamic evolution. Indeed, the Neoproterozoic sequence shows the development of a volcanic arc and a coeval back-arc basin. This basin may present incipient oceanization, being materialized in the Tomar-Badajoz-Córdoba Shear Zone. These data are compatible with the models proposed for the OMZ during the Cadomian Cycle. This shear zone was reactivated as an intraplate shear zone during the Variscan Cycle. The Cambrian sequence presents lithostratigraphic and geochemical affinities with the intra-continental sin-rift successions that typify the beginning of the Variscan Cycle in the OMZ. The western domain is characterized by the presence of tectonostratigraphic units with particular tectonostratigraphic, metamorphic and magmatic features in the context of the Iberian Massif. Its geodynamic evolution was controlled by the Porto-Tomar-Ferreira do Alentejo Shear Zone, which delimits its eastern boundary. The comparison with the sectors of Coimbra, Porto-Albergaria and Berlengas sectors allowed the characterization of the Finisterra Terrane. The geological features of this Terrane are similar to those exhibited in the Léon Block (Armorican Massif) and Mid-German Crystalline Rise ones. Such behaviour allows their correlation in the European Variscan Belt context. The obtained data in Abrantes-Tomar region was integrated into the geodynamic models proposed for the Iberian Massif. In order to strengthen the proposed geodynamic models, new data were obtained in other regions, as in the Almograve, Ponta Ruiva and Vila Boim regions

Caracterização estrutural da zona de cisalhamento Tomar-Badajoz- Córdova no sector de Abrantes

As zonas de cisalhamento Tomar – Badajoz – Córdova (TBCSZ) e Porto – Tomar – Ferreira do Alentejo (PTASZ) são duas das principais estruturas do Varisco Ibérico, cuja caracterização é fundamental para a compreensão da sua evolução geodinâmica. A região de Abrantes surge então como um sector chave para a caracterização destas estruturas, visto que corresponde à zona de interferência ambas. Os trabalhos em curso sugerem a ocorrência de uma sucessão característica da transição Neoproterozóico-Câmbrico, por correlação com outros sectores da Zona de Ossa-Morena. Foi possível evidenciar a presença de duas fases de deformação dúcteis principais. A primeira fase é responsável pelo desenvolvimento de uma xistosidade mostrando transporte tangencial para o quadrante NW, relacionada com possível formação de uma dobra em bainha quilométrica associada à terminação NW da TBCSZ. A segunda fase de deformação afecta as estruturas anteriores, estado associada a um regime de deformação não-coaxial direito, correlacionável com a PTASZ; ABSTRACT: The Tomar-Badajoz-Cordoba (TBCSZ) and the Porto-Tomar-Ferreira do Alentejo shear zones (PTASZ) are two of the major variscan structures in the Iberian Massif. Their characterization is thus crucial to understand the geodynamic evolution of the Variscan Chain. These two major shear zones interfere in the Abrantes region, making this a key sector for the understanding the Iberian Variscan geodynamics. Ongoing work suggests the occurrence of a characteristic succession of Neoproterozoic-Cambrian transition that could be correlated with other sectors of Ossa-Morena Zone. It was possible to demonstrate the presence of two main phases of ductile deformation. The first phase generates a schistosity showing tangential transport to NW, which could be related with a kilometric sheath fold associated to TBCSZ NW termination. The second deformation phase affects all previous structures, being associated with a non-coaxial deformation regime, induced by PTASZ kinematics

Geodynamic evolution of Ossa-Morena Zone in a SW Iberian context during the Variscan cycle

Ossa-Morena Zone (OMZ) is crucial to understanding the geodynamic evolution of the Variscan cycle in SW Iberia. We review previous data, from Early to Late Paleozoic. The early Cambrian (conglomeratic and felsic metavolcanic units) lies unconformably upon Neoproterozoic formations and shows a carbonate sequence with bimodal volcanic rocks, accompanied by intrusion of plutonic bodies (535–520 Ma). This could be interpreted as result of rifting process (Sánchez-García et al., 2010). The middle Cambrian is marked by a significant crustal stretching episode: siliciclastic sedimentation is accompanied by bimodal volcanism, with transitional alkaline geochemical features, symptomatic of an intra-plate environment. The Cambrian–Ordovician transition is marked by the absence of sedimentation and/or an erosional episode. This period is concomitant with large plutonic intrusions (~ 510–485 Ma). This is related to opening of the Rheic Ocean: geochemical (N- and T-MORB signatures) and geochronological data support the existence of anorogenic oceanic magmatic activity during this period (~ 485–480 Ma). From the Mid Ordovician until the end of Silurian, magmatic features are related to passive margin evolution and tectonic stability

Do rifting continental à abertura do Oceano Rheic; evidências de cariz multidisciplinar na Zona de Ossa-Morena

A evolução geodinâmica da Zona de Ossa-Morena é essencial na compreensão do Ciclo Varisco no Terreno Autóctone Ibérico. Vários estudos têm sido efectuados nesta zona tectono-estratigráfica. Contudo, todos os dados devem ser enquadrados de forma integrada, espacial e temporalmente, na evolução geodinâmica desta zona. O trabalho em causa apresenta-se como uma síntese multidisciplinar crítica desta zona, durante as fases iniciais do Ciclo de Wilson Varisco (Paleozóico inferior). A (re)interpretação dos dados bibliográficos existentes para esta zona permitiu colocar em evidência a presença de vários impulsos de extensão crustal durante as fases iniciais do ciclo (entre o Câmbrico e o Ordovícico inferior), que culminam com a abertura do oceano Rheic no Ordovícico. Os dados de cariz estratigráfico parecem mostrar que o limite de placas se mantém como um limite passivo pelo menos até ao topo do Silúrico