La faille normale ductile du massif du Saint Barthélémy. Age et signification de l'extension crustale dans le zone Nord-pyrénéenne

The first part of this work is composed of a synthesis about Alpine and Variscan orogenies in the Pyrenees. Recent works (ECORS profile), are presented and discussed. The second part is concerning with the Saint Barthelemy massif. The results of structural and petrological studies are shawn. The structure of the massif is interpreted to be the result of a single progressive deformation event. Structural analysis and thermobarometrical data confirm the existence of extensional tectonics. The singularity of the SBM, with the occurence of a major low angle shear zone, is shawn. In the third part, we proposed a thermotectonical madel which is integrated in the regional geological context.La première partie de ce travail correspond à une synthèse bibliographique sur les chaînes alpines et hercyniennes des Pyrénées. Les résultats récents (profil ECORS) y sont présentés et discutés. La deuxième partie concerne le Massif Nord Pyrénéen du Saint Barthélémy. Elle présente les résultats d'études structurales et pétrographiques combinées qui permettent de caractériser différents stades d'évolution. Ceux-ci s'inscrivent dans un continuum de déformation progressive qui s'effectue sous des conditions rétromorphiques. L'analyse structurale et les données thermobarométriques confirment l'existence d'une tectonique extensive. La singularité du massif du Saint Barthélémy apparaît dans la présence d'une zone de cisaillement d'ampleur régionale, la Bande Mylonitique Majeure, que l'on ne retrouve pas dans les autres MNP. Dans la troisième partie, l'ensemble des données est utilisé pour élaborer un modèle d'évolution thermotectonique, qui est ensuite confronté à l'évolution géologique régionale. Les données géochronologiques sont présentées et discutées. Différents modèles d'évolution tectonique sont proposés

Spatial and temporal evolution of Liassic to Paleocene arc activity in southern Peru unraveled by zircon U-Pb and Hf in-situ data on plutonic rocks

International audienceCordilleran-type batholiths are built by prolonged arc activity along active continental margins and provide detailed magmatic records of the subduction system evolution. They complement the stratigraphic record from the associated forearcs and backarcs. We performed in-situ U-Pb geochronology and Hf isotope measurements on zircon grains from a large set of plutonic rocks from the Coastal Batholith in southern Peru. This batholith emplaced into the Precambrian basement and the Mesozoic sedimentary cover. We identify two major periods of voluminous arc activity, during the Jurassic (200-175 Ma) and the Late Cretaceous-Paleocene (90-60 Ma). Jurassic arc magmatism mainly resulted in the emplacement of a dominantly mafic suite with εHf values ranging from − 9.5 to + 0.1. Published ages south of the Arequipa area suggest that the arc migrated southwestward out of the study area during the Middle Jurassic. After a magmatic gap of 85 Ma, arc activity abruptly resumed 90 Ma ago in Arequipa. Intrusive bodies emplaced into both basement and older Jurassic intrusions and strata. This activity culminated between 70 and 60 Ma with the emplacement of very large volumes of dominantly quartz-dioritic magmas. This last episode may be considered as a flare-up event, characterized by intense magmatic transfers into the crust and rapid relief creation. The Late Cretaceous-Paleocene initial εHf are shifted toward positive values (up to + 3.3 and + 2.6) compared to the Jurassic ones, indicating either a larger input of juvenile magmas, a lesser interaction with the ancient crust, or an increase of re-melting of young mantle-derived mafic lower crust. These magmatic fluxes with juvenile component are coeval with the onset of the crustal thickening at 90 Ma and represent a significant contribution to the formation of the continental crust in this area

High-resolution imaging of the Pyrenees and Massif Central from the data of the PYROPE and IBERARRAY portable array deployments

International audienceThe lithospheric structures beneath the Pyrenees, which holds the key to settle long-standing controversies regarding the opening of the Bay of Biscay and the formation of the Pyrenees, are still poorly known. The temporary PYROPE and IBERARRAY experiments have recently filled a strong deficit of seismological stations in this part of western Europe, offering a new and unique opportunity to image crustal and mantle structures with unprecedented resolution. Here we report the results of the first tomographic study of the Pyrenees relying on this rich data set. The important aspects of our tomographic study are the precision of both absolute and relative traveltime measurements obtained by a nonlinear simulated annealing waveform fit and the detailed crustal model that has been constructed to compute accurate crustal corrections. Beneath the Massif Central, the most prominent feature is a widespread slow anomaly that reflects a strong thermal anomaly resulting from the thinning of the lithosphere and upwelling of the asthenosphere. Our tomographic images clearly exclude scenarios involving subduction of oceanic lithosphere beneath the Pyrenees. In contrast, they reveal the segmentation of lithospheric structures, mainly by two major lithospheric faults, the Toulouse fault in the central Pyrenees and the Pamplona fault in the western Pyrenees. These inherited Hercynian faults were reactivated during the Cretaceous rifting of the Aquitaine and Iberian margins and during the Cenozoic Alpine convergence. Therefore, the Pyrenees can be seen as resulting from the tectonic inversion of a segmented continental rift that was buried by subduction beneath the European plate

Structures Related to the Emplacement of Shallow-Level Intrusions

A systematic view of the vast nomenclature used to describe the structures of shallow-level intrusions is presented here. Structures are organised in four main groups, according to logical breaks in the timing of magma emplacement, independent of the scales of features: (1) Intrusion-related structures, formed as the magma is making space and then develops into its intrusion shape; (2) Magmatic flow-related structures, developed as magma moves with suspended crystals that are free to rotate; (3) Solid-state, flow-related structures that formed in portions of the intrusions affected by continuing flow of nearby magma, therefore considered to have a syn-magmatic, non-tectonic origin; (4) Thermal and fragmental structures, related to creation of space and impact on host materials. This scheme appears as a rational organisation, helpful in describing and interpreting the large variety of structures observed in shallow-level intrusions

Étude des brèches Méso-Cénozoïques duNord-Est des Pyrénées (Latour-de-France),origines et implication sur l’évolutiongéologique des Pyrénées

National audienceLes brèches de la zone nord pyrénéenne sont un des témoinsde l’histoire géodynamique des Pyrénées. Les affleurements debrèches sont nombreux dans cette zone et il existe une variété importanteen termes de faciès et de géométries. Les différents typesd’associations faciès-géométries sont dépendant du processus deformation (tels que des processus sédimentaires, tectoniques ouencore de fracturation hydraulique) et constituent ainsi des indicesdu contexte géodynamique local. Ainsi, il convient de définir quelssont les critères discriminants dans la caractérisation des différentsfaciès de brèches afin de déterminer quelles sont les origines de laformation des brèches pyrénéennes. Cette étude porte sur un desfaciès de brèches présent dans la région de Latour-de-France : lesbrèches dite Éocène (eBr). Ces brèches (eBr) se retrouvent sur lesbordures des synclinaux du Bas Agly et Saint Paul de Fenouillet,ainsi que coincées dans le Massif de l’Agly. Cette étude focaliséesur la carrière de Roc Pointu sur le flanc sud du synclinal du BasAgly entre Cases de Pène et Estagel a permis de confirmer que lesbrèches de type eBr, considérées comme sédimentaires et Eocènesdonc syn-compression alpine (Mattauer et Proust, 1962 ; Bergeret al., 1993), sont effectivement d’origine sédimentaire mais sesont mises en place antérieurement à l’Albo-Cénomanien. La fortereprésentativité des brèches de type eBr dans la zone d’étude indiqueraitdonc, qu’à l’Albo-Cénomanien, de fortes pentes jalonnaientles zones actuelles synclinales et que leur déstabilisationserait à l’origine de la mise en place des brèches sur les formationsante-albiennes. Dans la mesure où cette époque correspondà une phase d’extension (plus ou moins décrochante), l’origine deces escarpements est à relier à des failles normales (plus ou moinsdécrochantes) qui formaient les bordures des bassins

From rifting to mountain building: The Pyrenean Belt

International audienceThe Pyrenees are again under the spotlights! Today, anincreasing number of research groups are focusing on thisapparently simple double-verging orogeny. These recent(and ongoing) studies using a broad range of disciplinaryapproaches have greatly improved our understanding ofthe geological structure and tectonostratigraphic evolutionof the Pyrenees and sometimes also challenged wellestablishedmodels

Rates of magma transfer in the crust: Insights into magma reservoir recharge and pluton growth

International audiencePlutons have long been viewed as crystallized remnants of large magma reservoirs, a concept now challenged by high precision geochronological data coupled with thermal models. Similarly, the classical view of silicic eruptions fed by long-lived magma reservoirs that slowly differentiate between mafic recharges is being questioned by petrological and geophysical studies. In both cases, a key and yet unresolved issue is the rate of magma transfer in the crust. Here, we use thermal analysis of magma transport to calculate the minimum rate of magma transfer through dykes. We find that unless the crust is exceptionally hot the recharge of magma reservoirs requires a magma supply rate of at least ~0.01 km 3 /yr, much higher than the long-term growth rate of plutons, which demonstrates unequivocally that igneous bodies must grow incrementally. This analysis argues also that magma reservoirs are short-lived and erupt rapidly after being recharged by already differentiated magma. These findings have significant implications for the monitoring of dormant volcanic systems and our ability to interpret geodetic surface signals related to incipient eruptions

Emplacement of magma pulses and growth of magma bodies

International audienceMagmatism is responsible for the formation of continental and oceanic crusts. It is the main agent of mass and heat transfers from the mantle towards the crust, the hydrosphere (oceanic and continental hydrothermalism) and the atmosphere (emissions of volcanic gas and ashes). Its expressions are the crystallization of intrusive rocks and the eruption of volcanic products. However, with a ratio between the volumes of extrusive and intrusive magmatic rocks estimated to be of the order of 1:5 for most magmatic systems (White et al., 2006), one of the main characteristics of magmatism is that magmas rarely reach the Earth's surface. This suggests that the prevailing conditions in the crust are not favourable to the arrival of magma on the Earth's surface, but instead lean heavily towards the formation of intrusive bodies in the crust. These intrusive bodies, referred to here as plutons, constitute the elementary building brick of the continental crust. An increasing number of geophyscial and geochronological data as well as geological observations is currently modifying our understanding of pluton construction. When plutons were initially envisaged as quasi-spherical bodies growing slowly and essentially by an overall inflation, they are now recognized as growing incrementally by the accretion of successive and relatively small magma pulses, over variable periods of time, from hundred to millions of years, depending on geodynamic setting and source fertility. This brings new challenges and has far-reaching implications. According to this new model of plutonism, the evolution of magma bodies is related to the processes that control the timescale and the spatial distribution of the successive pulses. Depending on their emplacement rate and on their ability to amalgamate, repeated magma pulses can either rapidly solidify or ultimately build up an active magma chamber. Thus understanding how magma bodies grow has fundamental implications for the link between volcanism and plutonism as well as for magma differentiation and ultimately for our understanding of the growth and evolution of the Earth crusts. The concept of pluton incremental growth challenges our understanding as well as our field interpretations of the processes involved during pluton construction. Indeed, these processes and how they operate in governing the emplacement and growth of plutons, both in space and time, are still debated.A state-of-the-art session on these very issues was held at the 2008 General Assembly of the European Geosciences Union. As a follow-up of this session, this special volume Emplacement of magma pulses and growth of magma bodies brings together both theoretical models and field studies that cover most aspects of the emplacement and growth of plutons

Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies

International audienceThe Pyrenean peridotites (lherzolites) form numerous small bodies of subcontinental mantle, a few meters to 3 km across, exposed within the narrow north Pyrenean zone (NPZ) of Mesozoic sediments paralleling the north Pyrenean Fault. Recent studies have shown that mantle exhumation occurred along the future NPZ during the formation of the Albian-Cenomanian Pyrenean basins in relation with detachment tectonics. This paper reviews the geological setting of the Pyrenean lherzolite bodies and reports new detailed field data from key outcrops in the Bearn region. Only two types of geological settings have to be distinguished among the Pyrenean ultramafic bodies. In the first type (sedimented type or S type), the lherzolites occur as clasts of various sizes, ranging from millimetric grains to hectometric olistoliths, within monogenic or polymictic debris flow deposits of Cretaceous age, reworking Mesozoic sediments in dominant proportions as observed around the Lherz body. In the second type (tectonic type or T type), the mantle rocks form hectometric to kilometric slices associated with crustal tectonic lenses. Both crustal and mantle tectonic lenses are in turn systematically associated with large volumes of strongly deformed Triassic rocks and have fault contacts with units of deformed Jurassic and Lower Cretaceous sediments belonging to the cover of the NPZ. These deformed Mesozoic formations are not older that the Aptian-early Albian. They are unconformably overlain by the Albian-Cenomanian flysch formations and have experienced high temperature-low pressure mid-Cretaceous metamorphism at variable grades. Such a tectonic setting characterizes most of the lherzolite bodies exposed in the western Pyrenees. These geological data first provide evidence of detachment tectonics leading to manle exhumation and second emphasize the role of gravity sliding of the Mesozoic cover in the preorogenic evolution of the Pyrenean realm. In the light of such evidence, a simple model of basin development can be inferred, involving extreme thinning of the crust, and mantle uprising along a major detachment fault. We demonstrate coeval development of a crust-mantle detachment fault and generalized gravitational sliding of the Mesozoic cover along low-angle faults involving Triassic salt deposits as a tectonic sole. This model accounts for the basic characteristics of the precollisional rift evolution in the Pyrenean realm

Effects of magnetic interactions in anisotropy of magnetic susceptibility: Models, experiments and implications for igneous rock fabrics quantification

International audienceBesides granites of the ilmenite series, in which the anisotropy of magnetic susceptibility (AMS) is mainly controlled by paramagnetic minerals, the AMS of igneous rocks is commonly interpreted as the result of the shape-preferred orientation of unequant ferromagnetic grains. In a few instances, the anisotropy due to the distribution of ferromagnetic grains, irrespective of their shape, has also been proposed as an important AMS source. Former analytical models that consider infinite geometry of identical and uniformly magnetized and coaxial particles confirm that shape fabric may be overcome by dipolar contributions if neighboring grains are close enough to each other to magnetically interact. On these bases we present and experimentally validate a two-grain macroscopic numerical model in which each grain carries its own magnetic anisotropy, volume, orientation and location in space. Compared with analytical predictions and available experiments, our results allow to list and quantify the factors that affect the effects of magnetic interactions. In particular, we discuss the effects of (i) the infinite geometry used in the analytical models, (ii) the intrinsic shape anisotropy of the grains, (iii) the relative orientation in space of the grains, and (iv) the spatial distribution of grains with a particular focus on the inter-grain distance distribution. Using documented case studies, these findings are summarized and discussed in the framework of the generalized total AMS tensor recently introduced by Cañon-Tapia (Cañon-Tapia, E., 2001. Factors affecting the relative importance of shape and distribution anisotropy in rocks: theory and experiments. Tectonophysics, 340, 117–131.). The most important result of our work is that analytical models far overestimate the role of magnetic interaction in rock fabric quantification. Considering natural rocks as an assemblage of interacting and non-interacting grains, and that the effects of interaction are reduced by (i) the finite geometry of the interacting clusters, (ii) the relative orientation between interacting grains, (iii) their heterogeneity in orientation, shape and bulk susceptibility, and (iv) their inter-distance distribution, we reconcile analytical models and experiments with real case studies that minimize the role of magnetic interaction onto the measured AMS. Limitations of our results are discussed and guidelines are provided for the use of AMS in geological interpretation of igneous rock fabrics where magnetic interactions are likely to occur