Kinematic interpretation of shearband boudins : new parameters and ratios useful in HT simple shear zones

Shearband boudins (asymmetric boudins showing slip along the inter-boudin surface, which is synthetic with respect to the bulk shear sense) are ubiquitous and well-exposed in HT simple shear zones. The present work aims to extend the methodology of analysis of shearband boudins developed by Goscombe and Passchier (2003). Such shearband boudins represent complex objects that require an adequate methodology for unambiguous kinematic interpretation. We propose new geometric parameters (Bbs, B-t, c’, D’, d, ψ’) in order to describe and identify, with confidence, the kinematics of boudinage in this particular geological framework. The key-observation for kinematic interpretation in simple shear regimes is the boudin axis (Lb) that is commonly present, excluding the necessity to identify the regional stretching lineation (Lx) in the metasedimentary matrix. In monoclinic HT simple shear zones, this approach involves the use of a local displacement plane (Sx), which is always normal to the boudin axis (Lb).Partial funding to JP was provided by the Centro de Investigacao GeolUgica, Ordenamento e Valorizacao de Recursos that is supported by the Pluriannual program of the Fundacao para a Cilncia e a Tecnologia, funded by the European Union (FEDER program) and the national budget of the Portuguese Republic. Very insightful and helpful reviews were done by C. Fernandez (University of Huelva) and two reviewers, for which we are grateful. We want to acknowledge to T. Valente (University of Minho) for her suggestions that helped to simplify the manuscript structure and to A. McCaig (University of Leeds) for his final comments and English revision that significantly improved this manuscript

Fold boudins : what is that?

The hypothesis proposed in this work is related with the genesis and evolution, in HT simple shear zones, of a new conceptual structure designed by fold boudin. The field evidences, achieved in the scope of a methodology to analyze the kinematics of shearband boudins (Pamplona & Rodrigues, 2011), are the basis to show how folds evolute until reach a shearband boudin final state. The critical mass factor (Mc) is the responsible for that mechanism. This parameter, which is now being introduced, is controlled by the hickness (t). The systematic analyses of shearband oudins geometry point to the hypothesis that the development of this structure is precluded by a folding phase, according to the following path evolution in HT shear zones shearband boudins: fold => fold boudin => shearband boudin.Fundação para a Ciência e a Tecnologia (FCT

Folding as a precursor of asymmetric boudinage in shear zones affecting migmatitic terranes

The folding laws control the process responsible for the genesis and the initial morphological evolution of shearband boudins. In this model, the generation of boudins begins with folding of tabular competent veins of different thickness immersed in a less competent matrix. Veins thicker than a given critical value form initial asymmetric open folds that evolve by asymmetric antithetical rotation and later by an internal mass redistribution with translation along the interface vein-matrix. On the other side, the veins thinner than the critical value follow the folding process until it culminates with the coalescence and stacking of the folds. By this mechanism the critical thickness is reached and these bodies are prone to suffer a new phase of boudinage similar to that followed by the originally thicker veins. Therefore, all deformed tabular veins, embedded in a ductile matrix, converge to classical shearband boudin morphology, independently from their original thickness.La génesis y evolución morfológica inicial de los "boudins” de tipo “shearband" están controlados por mecanismos de plegamiento. En este modelo, el "boudinage" comienza con el plegamiento de cuerpos tabulares (venas) de diferentes espesores, inmersos en una matriz menos competente. Las venas más gruesas exceden un espesor crítico que determina la formación inicial de un pliegue abierto. Posteriormente evoluciona por rotación antitética en relación con la cinemática de la zona de cizalla y por traslación de masa a lo largo de la interfase vena-matriz,conduciendo a la morfología clásica de un "shearband boudin”. Por su parte, los cuerpos comparativamente más delgados continúan plegándose hasta que se produce la unión y apilamiento de los flancos de los pliegues (“stacked-folds”). Mediante este proceso se alcanza el espesor crítico necesario para que se desarrolle una nueva fase de "boudinage" similar a la seguida por las venas gruesas

Boudins: an useful tool in structural geology teaching

The boudins are generally finite objects of easy identification in the field. They are excellent structures for learning the measurement of linear and planar structures. Geometric and morphological parameters of the boudins allow drawing conclusions about several structural parameters, such as, kinematics. vorticity, strain rate and shear strain.The author J. Pamplona was co-funded by the EU, based on COMPETE 2020, project ICT (UID/GEO/04683/2013), reference POCI-01-0145-FEDER-007690, national funds provided by FCT

Isolated boudins or isolated look-like boudins structures?

The difficulties in the identification of boudinated bodies happen whenever isolated or isolated look-like boudins structures occur. Figure shows the criteria for identifying micro and meso structures that could be misunderstand with boudins.Co-funded by the European Union through the European Regional Development Fund, based on COMPETE 2020 (Programa Operacional da Competitividade e Internacionalização), project ICT (UID/GEO/04683/2013) with reference POCI-01-0145-FEDER-007690 and national funds provided by Fundação para a Ciência e a Tecnologi

Application of shearband boudins analysis to understand ductile shear zones

The methodology used for the kinematic analysis of H-T ductile shear zones based on geometric analysis of boudins, proved to be a easy application tool. The boudin analysis begans with the measurement of the Lb (boudin axis) orientation, as a fundamental element of the kinematic analysis in simple shear – Lb proved always to be perpendicular to a local displacement plane (Sx) that is defined as the plane that contains different lineations (e.g., stretching lineation in the host rock) and so, validates the orientation of the outcrop plane. The methodology also includes the measurement of several angular and dimensional parameters. The main objective of this study deals with the validation of the broad utility of this methodology in shear zones with different P-T conditions. It was studied a shearband boudin field on a Grt-St shear zone, which was compared with previous studies on Sil shear zone.Fundação para a Ciência e a Tecnologia (FCT

Characterization of ductile shear zones with different P-T conditions based on the geometric analyzes of shearband boudins

No presente trabalho analisou-se a relação entre a morfologia de corpos boudinados e as condições tectono-metamórficas em que estes se formaram e evoluíram. Com base neste pressuposto formula-se a hipótese de ser possível associar boudins, com características geométricas bem definidas, a ambientes metamórficos específicos. Foram estudados 345 boudins, distribuidos por quatro regiões com distintos graus de metamorfismo: Serra da Salgosa, Praia da Granja, Aracena e Acebuches. Relativamente a cada boudin foi analisada a relação entre os rácios de parâmetros geométricos e o ângulo θ (ângulo entre a superficie interna do boudin – Sib – e a superfície externa do boudin – Sb). Os dados obtidos permitiram concluir que a morfologia do boudin parece depender de fatores, tais como, a espessura e natureza do corpo boudinado, o contraste de viscosidade e a taxa de deformaçao, não sendo clara uma relação entre a morfologia do boudin e o ambiente metamórfico em que este se formou.In this study we pretend to analyze the relation between the boudins morphology and the tectono-metamorphic environment where they were formed and evolved. Therefore, it would be possible to associate boudins, with well-defined geometric parameters, to a specific metamorphic environment. During this study 345 boudins have been analyzed, distributed along four different areas with distinct metamorphism degrees: Serra da Salgosa, Praia da Granja, Aracena and Acebuches. For each boudin, the relation between ratios of different geometric parameters and the θ angle (angle between the boudin internal surface – Sib – and the boudin external surface – Sb) were analyzed. The obtain data allow us to conclude that the boudin morphology appears to depend on factors such as the layer thickness and nature, matrix/layer viscosity contrast and strain rate. The relation between the boudin morphology and the metamorphic environment where it was formed doesn´t seems clear.Fundação para a Ciência e a Tecnologia (FCT

Isolated anaemia as a manifestation of Rh isoimmunisation

Rh isoimmunisation leads to haemolytic anaemia and hyperbilirubinaemia in the first h of life. Isolated early onset neonatal anaemia has rarely been reported. The authors describe the case of a term infant, born to an 'A' negative, second gravida mother. On the second day of life, pallor was noticed. His haemoglobin (Hb) was 6.8 g/dl, he had reticulocytosis and a positive direct antiglobulin test. However, he did not have a high total serum bilirubin (TSB) (87.2 μmol/l). He was transfused with red blood cells and kept under phototherapy for 3 days. Three weeks later, he received another transfusion for severe anaemia (Hb 6 5 g/dl). During this period, he was never jaundiced and the maximum level of TSB was 122 μmol/l. On follow-up, his Hb stabilised and he had no further problems. This report highlights the possibility of early onset anaemia without jaundice as the sole manifestation of Rh isoimmunisation

Quantification of synmagmatic flow structures: a tool for Rock Quality Evaluation

The result of the analysis using methods based on fractal geometry provides information about magmatic flux directions and mineral equilibrium processes in the crystallizing magma chamber. Additionally, the used methods may also provide important information for the rock industry, because they allow a fast and automatically evaluation of rock quality.Fundação para a Ciência e a Tecnologia (FCT

Microstructural analysis of shearband boudin - preliminary results

The internal structure of a shearband boudin resulting from an original igneous, hydrothermal or metamorphic segregation tabular rigid body is a subject of scientific interest. It allows understanding the deformation mechanisms acting on homogeneous quartz aggregate activated during simple shear progressive deformation. This communication is focused on the characterization of two main structural aspects: (i) the existence of different internal domains in shearband boudins; (ii) the development of internal structures related with its genesis and evolution, namely, secondary shear planes c’ type-I and c’ type-II, parameter B-t (mass accumulation sector on blunt tip of shearband boudin) and S-t (sharp tip of shearband boudin), internal migration of mass (rotation and translation) and mechanisms of ductile deformation activated on the process.Fundação para a Ciência e a Tecnologia (FCT