Intramontane basin development related to contractional and extensional structure interaction at the termination of a major sinistral fault: The Huércal-Overa Basin (Eastern Betic Cordillera)

Among the classical minor structural associations on the termination of transcurrent faults are horsetail splays formed by reverse, normal or strike-slip faults developing duplexes. However, temporal and spatial coexistence of contractional and extensional structures is very rarely documented. We discuss the relationships of contractional and extensional structures and associated sedimentary depocenters at the termination of a major strike-slip fault in the Eastern Betic Cordillera. Field mapping, kinematic fault analysis, paleostress determination and gravity prospecting in the Huércal-Overa Basin, at the southern termination of the NE–SW Alhama de Murcia transcurrent fault (AMF), are used to establish the relationships of tectonic structures and associated sedimentary depocenters. Here, ENE–WSW and WNW–ESE folds interact with two sets of normal faults having the same orientation as well as ENE–WSW reverse faults. Progressive unconformities associated with folds reveal that the beginning of the AMF activity occurred in the Tortonian. The folds progressively grew and rotated from ENE–WSW up to WNW–ESE close to the transcurrent fault. We propose that the development of the normal faults developed during short-term episodes characterized by vertical major stress axis and are, in turn, related to gravitational instability linked to the thickening of a crust relatively hot at depth. This setting may have become predominant in between the main activity, compressive pulses along transcurrent faults.     

A Multistep Methodology for Building a Stochastic Model of Gold Grades in the Disseminated and Complex Deposit of Casas Novas in Alentejo,Southern Portugal

This paper proposes a multistep approach for creating a 3D stochastic model of gold grades in a complex disseminated auriferous deposit located in the Alentejo region, southern Portugal. The approach involves the following steps: (i) the creation of a 3D low‐resolution vector object model of two geological domains that better discriminate gold grades; (ii) the definition of three intervals for gold grades (low, intermediate, and high values); (iii) the estimation of the probability of each grid block belonging to each interval constrained by the geological domains; (iv) the calculation of local conditional cumulative distribution functions (CCDFs) of gold grades, for each block, taking into account the geological domains and the borehole data; and finally (v) the simulation of images of gold grades. The results demonstrate the usefulness and accuracy of the proposed procedure, as they are in compliance with the conceptual model, the distribution laws are conditioned by the geological domains, and the transition of grades between domains is continuous.     

Magnetic Environments of Young Stellar Objects

We report on early results of magnetic field measurements towards a sample of young stellar objects. The results show a variety of field configurations, some of which can be explained by conventional models, while others cannot. We find that the field in some cases is curved over large scales and influenced by the gas kinematics in the local environment. This implies that, at these scales at least, the magnetic field plays a passive rôle in the star formation process.     

Geosynclinal evolution of the Epi-Baykalian platform of Central Brazil

Evidences of geosynclinal polarity (orogenic and sedimentary) in foldbelts of Central Brazil led the authors to the recognition of seven tectonic isopic zones as parts of a wide geosynclinal system policyclically developed during a long Baykalian Cycle of 1,500-500 m. y. ago. Tectonic and stratigraphic evolutions are reinterpreted on the light of the new concepts.The orogenic polarity of contiguous miogeosynclinal and eugeosynclinal foldbelts provided the basis for enlarging the Brasiliane Cycle of short duration (850–550 m. y.) to a Brasiliane Cycle of 1,500-550 m.y. (= Baykalian Cycle). The Minas Cycle is viewed as a phase within a distinct platform-making process.

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Zusammenfassung Die PolaritÄt orogener und sedimentÄrer Erscheinungen in Geosynklinalen der Faltengürtel Zentralbrasiliens veranla\te die Autoren, sieben tektonische Zonen als Teile eines gro\rÄumigen geosynklinalen Systems, das wÄhrend eines langen Baykalischen Zyklusses vor 1500 bis 500 Millionen Jahren entstand, zu unterscheiden. Tektonische und stratigraphische Entwicklungen werden im Lichte dieser neuen Auffassung neu interpretiert.Die orogene PolaritÄt von aneinandergrenzenden miogeosynklinalen und eugeosynklinalen Faltengürteln lieferte die Grundlage für die Erweiterung des Brasilianischen Zyklusses von kurzer Dauer (850-550 Millionen Jahre) zu einem Brasilianischen Zyklus von 1500-500 Millionen Jahren (=Baykalischer Zyklus). Der Minas-Zyklus wird als eine Phase innerhalb eines Plattform bildenden Prozesses angesehen.

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Résumé Certaines évidences de polarité géosynclinale (orogéniques et sedimentaires) dans les régions plissées du Brésil Central ont conduit les auteurs a établir sept zones tectoniques rassemblées dans un seul système géosynclinal, qui s'est développé policycliquement au cours d'un Cycle Baykalien de 1500 à 500 millions d'années. Les évolutions stratigraphiques et tectoniques sont réinterprétées à la lumière de nouveaux concepts.La polarité orogénique du couple eugéosynclinal-miogéosynclinal de ces zones plissées a fourni des données suffisantes pour étendre les limites du Cycle Brésilien (= Baykalien) de 850–550 millions d'années à 1500–550 millions d'années: le Cycle Minas est devenu simplement une phase dans ce processus de formation de plateformes.

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7 , 1500-500 . . 850–500 1500-500 ( ). , .

     

Perturbazione indotta nel campo di un elettrodo rettilineo indefinito da un piano perfettamente conduttore parallelo all'elettrodo

Riassunto Si studiano il campo ed i potenziali generati da un elettrodo rettilineo indefinito posto alla superficie di un suolo omogeneo, contenente un piano verticale perfettamente conduttivo e posto a data profondità (che potrebbe, in pratica, schematizare un filone metallifero di spessore piccolo rispetto alle altre dimensioni). Si mette in evidenza il notevole mutamento dell'andamento del potenziale rispetto al caso dello stesso elettrodo agente su un suolo omogeneo.

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Summary This paper deals with the electrical field and potential, generated by a rectilinear indefinite electrode placed at the surface of a homogeneous soil, embedding, at given depth, a perfecly conducting plane (which, in practice, may be the scheme of a metalliferous vein of small thickness with regard to other dimensions). Furthermore it is emphasized the remarkable change in potential shape in comparison to the case of the same electrode acting on a homogeneous soil.Zusammenfassung Das, von einem geradlinigen unbegrenzten Elektrode, an die Oberfläche eines homogenen Bodens liegend, das eine, mit gewissener Tiefe, leitfähige in vollendeter Weise, senkrechte Fläche umfasst (die, in der Praxis, als Schema einer Metallführenden Ader, geringer Dicke im Vergleich anderen Dimensionen, betrachtet sein kann), erzeugte Feld und Potential wird erforscht. Die des Potential Verlaufes bedeutende Veränderung, in Betreff des Falles von derselbe, auf homogenen Boden, wirksame Elektrode, wird deutlich erklärt.

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Relazione presentata il 22 Aprile 1960 alla Ottava Assemblea Generale della «Società Italiana di Geofisica e Meteorologia» (Genova: 22–24 Aprile 1960).     

Uncertainty in the application of the Parabolic Bay Shape Equation: Part 1

This paper aims to quantify the uncertainty in the application of the Parabolic Bay Shape Equation (PBSE) for a bay in static equilibrium. By means of expert elicitation, a database was generated consisting of the position of the control points that determine the Static Equilibrium Planform (SEP). The elicitation consisted of two parts. In Part 1, 22 expert volunteers were asked to place the three control points that determine the SEP on a vertical aerial photograph of Taquaras–Taquarinhas Bay, a morphologically stable bay in the south of Brazil, approximately 1800 m in width and with an indentation of 750 m. The distribution of the location of the SEP along four evenly spaced (200 m) profiles in the southern part of the bay was determined. The overall bias of the location of the SEP calculated over the four profiles was in the order of 41 m (landward) and the average standard deviation was 116 m. These parameters increased when moving alongshore toward the curved section of the bay. This means that the uncertainty in the application of the PBSE is dependent on the particular point of interest along the bay. In Part 2 of the elicitation, 30 volunteers participated. This time the consequence of the placement of the control points (the corresponding SEP) was visible. Comparing the results from Parts 1 and 2, it was observed that when volunteers were directly confronted with the result of the placement of the control points (a plotted SEP) a much smaller variation in the position of the SEP occurred. This implies that when feedback on the result is provided the PBSE is a more robust method.