Analogue model of rift linkage and inversion with application to the Western Alps

Along-strike segmentation of orogens raises questions because its causes may predate orogeny in relation to structural inheritance. Here we focus on rift/margin linkage domains and their inversion by using analogue models with image analysis to extract the 3D strain field. Extensional models document, depending on the strike-perpendicular offset and the brittle-crust thickness, three types of rift linkage modes: (1) oblique linkage with early T-fault, (2) strongly-oblique linkage with R-fault network and, (3) transfer-linkage with late Y-strike-slip fault. Analogue model of inverted rift basins is used to analyse the misunderstood tectonic evolution of transition zones in the segmented Western Alpine belt

Granite intrusion in a metamorphic core complex: The example of the Mykonos laccolith (Cyclades, Greece)

International audienceThe Aegean domain is a well-suited place to study the formation of metamorphic core complex (MCC) and to investigate the role of syn-tectonic granites on their development. In the northern Cyclades, the Mykonos-Delos-Rhenia MCC is characterised by the intrusion of a kilometer-scale Late Miocene pluton of I-type granitoids within a migmatitic gneiss dome. New combined AMS (Anisotropy of Magnetic Susceptibility) and microstructural studies on the Mykonos granitoids together with recently published thermochronological data allow us to use the granitoids as strain markers. The Mykonos granitoids form a laccolith-like intrusion with a N70°E long axis. The laccolith is strongly asymmetric with an outlying root zone to the SW and a major body mainly developed to the NE. The laccolith construction is due to successive pulses of more or less differentiated magma that intruded the Cycladic Blueschist Unit. The attitude of stretching markers suggests an important (about 60°) vertical-axis local rotation phenomenon in the cycladic upper crust during the exhumation of the Mykonos MCC. Structural data suggest a four-stage evolution of the Mykonos MCC: (i) a first stage characterized by flat shearing toward the N-NE and by the formation of a domal structure in migmatitic paragneisses with multi-scale generation of folds with axes either perpendicular or parallel to the regional stretching, as a result of the interplay between regional N20E°-directed extension and EW shortening; (ii) a second stage marked by the emplacement of the Mykonos laccolith at 13.5 ± 0.3 Ma at the top of the migmatitic paragneisses; (iii) the third stage corresponding to the development of protomylonitic foliations and lineations in the whole laccolith in high to medium temperature conditions; (iv) the late stage marked by an acceleration of the exhumation of the Mykonos MCC. This exhumation was accommodated by important rotations of upper crustal blocks. During the end of the exhumation processes, around 10 Ma, deformation localized at the top of the laccolith in semi-ductile conditions and then in brittle conditions in the major detachment plane. Our study shows that the Cycladic plutonism event had no role on the initiation of the MCC. However, the geometry of the Mykonos intrusion supports that the magmas are "sucked" into the direction of regional extension and that the intrusion of magmas has caused an acceleration of the last stages of the MCC development. This acceleration was marked by a very fast exhumation of the laccolith after its emplacement

The North Cycladic Detachment System

International audienceLow-angle normal faults accommodate a large part of continental post-orogenic extension. Besides the intrinsic rheological characteristics of the continental crust that may lead to the formation of shallow-dipping shear zones at the brittle–ductile transition, the role of pre-existing low-angle structures such as large thrusts has been proposed by several authors. We explore this question with the example of the North Cycladic Detachment System (NCDS) that is composed of a series of distinct detachments cropping out on the islands of Andros, Tinos and Mykonos, separating the Cycladic Blueschists in the footwall from the Upper Cycladic Nappe in the hanging wall. We show that these extensional structures are part of a single large-scale structure (more than 200 km along strike) that reactivates the Vardar suture zone. It extends eastward on Ikaria and westward offshore Evia and Thessalia where it probably connects to recent shallow-dipping normal faults evidenced on published seismic reflection profiles. The NCDS started its activity in the Oligocene concommitantly with the Aegean extension, and was still active in the Late Miocene. It has exhumed a series of metamorphic domes from southern Evia to Mykonos below low-angle detachment systems, made of low-angle normal faults and low-angle ductile shear zones. The ductile shear zones and the faults were created with a low dip and they kept the same attitude throughout their exhumation. We identify three main detachments that are part of a continuum of extension on the NCDS : Tinos detachment, Livada detachment and Mykonos detachment. A fourth detachment (Vari detachment) is the reactivation of an Eocene exhumation-related structure. Deformation in the footwall is characterized by intense stretching and flattening. Using the spatial evolution observed along strike from Andros to Mykonos we construct a history of formation of the NCDS starting with the reactivation of former thrusts leading to the exhumation of high-temperature metamorphic domes. The Aegean example shows that reactivation of earlier shallow-dipping discontinuities can play a fundamental role in continental post-orogenic extension

Anisotropy of magnetic susceptibility of the Pyrenean granites

In this paper, we report on a compilation of more than 2200 sites (more than 10,000 individual measurements) where anisotropy of magnetic susceptibility (AMS) was studied in granites from the Variscan Pyrenees. The standardization and homogenization of this information has allowed us to produce three Main Maps that synthesize all the information related with the AMS of the Pyrenean granites. We also describe the problems found during the construction of the database (variable geo-positioning, different published information, etc.). The information derived from 21 granite bodies, the database, and the synthesis maps (magnetic susceptibility, Km, and the orientation of the magnetic foliation, plane perpendicular to k3, and of the magnetic lineation, k1) allow us to see for the first time a complete image of this important kinematic and petrographic indicator

Tectonic evolution of the Rehamna metamorphic dome (Morocco) in the context of the Alleghanian-Variscan orogeny

Structural and 40Ar/39Ar geochronological investigations of the Rehamna Massif (Meseta, Moroccan Variscan belt) provide new constraints on the tectonic evolution of the Alleghanian-Variscan orogen during the Upper Paleozoic. Three main tectonic events have been recognized: (1) Southward thrusting of an Ordovician sequence over the Proterozoic basement, its Cambrian sedimentary cover, and the overlying Devono-Carboniferous basin. This event caused subhorizontal shearing and prograde Barrovian metamorphism of the buried rocks. (2) Continuous shortening resulting in the development of a synconvergent extrusion of metamorphosed units to form a dome elongated E-W. This was responsible for synconvergent detachment of the Ordovician upper crustal sequence. The timing of these two episodes is constrained to 310–295 Ma by cooling and metamorphic amphibole and mica ages (3) A NW-WNW convergence in a direction orthogonal to the previous one and characterized by the accretion of the Rehamna dome to the continental basement in the east. Based on 40Ar/39Ar cooling ages from a syntectonic granitoid and its host rocks and metamorphic 40Ar/39Ar ages from greenschist facies mylonite, the timing of this event falls between 295 and 280 Ma. The end of the Variscan orogeny in the Moroccan Meseta is constrained by the 40Ar/39Ar cooling age of a posttectonic pluton dated at ~275 Ma. The tectonic events highlighted in Morocco coincide with the late Variscan-Alleghanian tectonic evolution of southern Europe and North America and can be correlated with the global reorganization of plates that accompanied suturing of Pangaea at around 295 Ma.Peer reviewe

Une sainte reine et franciscaine : les images de sainte Élisabeth de Hongrie au Trecento, en Italie

Cet article propose d’analyser la fortune de l’iconographie de sainte Elisabeth de Hongrie dans l’Italie de la fin du Moyen-âge. L’apparition des deux principaux types iconographiques au tournant du XIVe siècle –celui de la sainte princesse et celui de la sainte franciscaine – est liée à la présence angevine en Italie et à sa politique des images. Elisabeth, grand-tante de Marie de Hongrie, prend place au milieu du panthéon angevin et participe alors à la monstration de la sainteté de leur lignage : par la représentation de ces saints issus de leur dynastie, les Anjou apportent une caution religieuse à leur domination politique. Leurs partisans guelfes dans les villes centre-italiennes prennent le relais des commandes des images de la sainte chère aux souverains. L’affaiblissement puis la disparition du pouvoir angevin de la Péninsule à la fin du XIVe siècle marque la fin des images du premier type iconographique, celui de la sainte princesse, alors que le second jouit d’une seconde phase de rayonnement au XVe siècle grâce à la présence franciscaine.This article analyzes the fortune of the iconography of St. Elizabeth of Hungary in late Middle-Ages Italy. The emergence of the two main iconographic types at the turn of the fourteenth century - the holy princess and the Franciscan saint - is linked to the presence of the Anjou family in Italy and to their policy images. Elizabeth, great aunt of Mary of Hungary, is part of the angevin’s Pantheon and participates in ascertaining holiness of their family: the picturing of saints coming from their own family help Anjou add a religious pledge to their political domination. Their Guelph supporters in center Italy cities take over the indent of pictures of this saint dear to the sovereign. The weakening and disappearance of Angevin power in the Peninsula at the end of the fourteenth century marks the end of the first iconographic type, the holy princess one, while the second still continues during the fifteenth century thanks to the Franciscan presence

The Saint Michel Archangel iconography in the murals paintings and painted panels in Italy : (1200-1518)

Cette thèse est une enquête sur les peintures murales et sur panneaux représentant l’archange Michel en Italie, entre 1200 et 1518. Elle propose une large mise au point historiographique et un panorama du développement du culte et de l’iconographie michaélique des origines à 1200. À travers un corpus de plus de 500 images, les représentations de l’archange sont étudiées dans les moindres détails, et leur évolution générale est située dans un cadre spatio-temporel, propre à en faire ressortir les spécificités. Au niveau formel et iconographique, la figure de Michel est partagée entre évocation de la spiritualité de sa nature et monstration de sa force physique, à forme humaine, alors que son image se simplifie par une cristallisation autour du guerrier dès le milieu du Trecento. Cette étude considère en outre la peinture en tant qu’objet fabriqué, pensé, reçu et utilisé. Les évolutions iconographiques participent à une sanctification de l’archange et sont au cœur d’expériences visuelles mêlant images peintes, représentations et visions miraculeuses de l’archange. Symbole universel du bien contre le mal et de la justice divine, et acteur efficace de l’au-delà intermédiaire, organisé et géré par l’Église, l’iconographie michaélique est un outil de son système pénitentiel. Mais Michel est un être sans apparence réelle et sa représentation est donc un reflet de sa perception par les hommes. Les représentations du plus humain des anges et du plus céleste des saints, sont ainsi un moyen de penser l’homme, dans sa relation avec l’Église, avec Dieu, et surtout dans la perception de l’homme par lui-même, de son rôle et de sa responsabilité au moment même du salut.This work is an investigation into murals and panel paintings depicting Archangel Michael in Italy, between 1200 and 1518. It presents a broad historiographical update and an overview of the development of the michaelic cult and iconography from its origins to 1200. With a corpus of over 500 paintings, the images of the Archangel are scrutinized in their finest details and their general evolution is put back into a spatiotemporal framework, so as to bring out its specificities. On both formal and iconographical levels, Michael's figure is split between evoking his spiritual nature and showing his physical strength, in human form, and it crystallises around the image of the warrior in the middle of the fourteenth century. This study considers a painting to be a manufactured object, a thought-through object, a received object, an used object. The iconographic developments play a role in the archangel's sanctification and are at the heart of visual experiences using painted images, representations and miraculous visions of the archangel. Universal symbol of the fight of Good against Evil or divine justice, and efficient agent in the intermediate afterlife, organised and managed by the Church, the michaelic iconography is a tool of its penitential system. But Michael has no real figure, therefore his representation is a reflection of how men perceive him. The representations of the most human of angels and the most heavenly of saints, are no less than a way of thinking Man itself, in his relationship with the Church, with God, and especially in the way Man perceives himself, perceives his role and his responsibility when time of salvation arises