Evolution of a low convergence collisional orogen: a review of Pyrenean orogenesis

The Pyrenees is a collisional orogen built by inversion of an immature rift system during convergence of the Iberian and European plates from Late Cretaceous to late Cenozoic. The full mountain belt consists of the pro-foreland southern Pyrenees and the retro-foreland northern Pyrenees, where the inverted lower Cretaceous rift system is mainly preserved. Due to low overall convergence and absence of oceanic subduction, this orogen preserves one of the best geological records of early orogenesis, the transition from early convergence to main collision and the transition from collision to post-convergence. During these transitional periods major changes in orogen behavior reflect evolving lithospheric processes and tectonic drivers. Contributions by the OROGEN project have shed new light on these critical periods, on the evolution of the orogen as a whole, and in particular on the early convergence stage. By integrating results of OROGEN with those of other recent collaborative projects in the Pyrenean domain (e.g., PYRAMID, PYROPE, RGF-Pyrénées), this paper offers a synthesis of current knowledge and debate on the evolution of this immature orogen as recorded in the synorogenic basins and fold and thrust belts of both the upper European and lower Iberian plates. Expanding insight on the role of salt tectonics at local to regional scales is summarised and discussed. Uncertainties involved in data compilation across a whole orogen using different datasets are discussed, for example for deriving shortening values and distribution

Du système de rift à l'orogène à double vergence : un modèle évolutif basé sur l'étude de cas des Pyrénées Orientales et une étude des facteurs de contrôle à partir des modèles numériques

The doubly vergent nature of some natural orogens is classically understood as two opposing thrust wedges (pro and retro) that comply with critical taper theory. The evidence that retro-wedges and their associated basins behave differently from their pro-wedge counterparts has been steadily increasing over the past few decades. However, what causes an orogen to become doubly vergent is currently not well understood. Nor is the relationship between the pro- and retro-wedge during the evolution of a doubly vergent orogen. It is the aim of this work to improve our understanding of: 1) how the pro- and retro-wedges relate to each other during the orogenic process, 2) what factors control the evolution of a doubly vergent orogen and 3) a possible link between the pro- and retro-wedge. Answering these questions requires an improved knowledge of the evolution of a doubly vergent orogen. We focussed on the Eastern Pyrenees as a type example of a doubly vergent orogen, due to the large amount of available data. We performed a detailed tectonostratigraphic study of the retro-foreland of the Eastern Pyrenees (European plate), updating the interpretation based on recent insights into its hyperextended rift origins. We link the evolution of the retro-foreland to that of the pro-foreland (Iberian plate) in order to derive insight into the crustal scale dynamics. Based on cross section restoration, reconstructed shortening rates and subsidence analysis, we subdivide the East Pyrenean evolution into four phases. The first (Late Cretaceous) phase is characterised by closure of an exhumed mantle domain between the European and Iberian rifted margins, and simultaneous inversion of a salt-rich, thermally unequilibrated rift system. Shortening was distributed roughly equally between both margins during this early inversion phase. Following inversion, a quiescent phase (Paleocene) was apparently restricted to the retro-foreland. This phase may record the period of transition between inversion and full collision in the Eastern Pyrenees. The main collision phase (Eocene) records the highest shortening rates, which was predominantly accommodated in the pro-wedge. Retro-wedge shortening rates were lower than during the rift inversion phase. During the final phase (Oligocene) the retro-wedge was apparently inactive and shortening of the pro-wedge slowed. This demonstrates that the relationship between the pro- and retro-wedges changes through time. We used lithosphere-scale thermo-mechanical numerical models to simulate the evolution of a doubly vergent orogen. Our results show a similar evolutionary pattern as observed in the Pyrenees: A roughly symmetrical rift inversion phase is followed by an asymmetric collision phase. Rift inheritance was found to be essential for enabling double vergence. Other factors, such as surface processes and thin-skinned deformation, were found to have a significant effect on the crustal structure and strain partitioning between both wedges. A salt décollement layer in the sedimentary cover promotes the formation of a crustal antiformal stack such as observed in the Pyrenees and Alps by forming a wide and low-taper thin-skinned fold-and-thrust belt that forces crustal deformation to focus in the hinterland. Finally, we show that the evolution of the pro- and retro-wedges is inextricably linked: events or conditions on one side of the doubly vergent orogen have an immediate effect on the other side of the orogen. This is clearly demonstrated in our models by constant variations in shortening rates of the pro- and retro-wedge in response to accretion of new pro-wedge thrust sheets. The High Atlas (Morocco) and Pyrenees can be seen as examples of symmetric rift inversion and later asymmetric collision phases, respectivelyLes orogènes à double vergence sont classiquement définis comme deux prismes critiques opposés (pro et retro) qui évoluent ensemble. Les études récentes montrent que les rétro-prismes et leurs bassins d’avant-pays associés se comportent différemment des pro-prismes. Cependant, ni les facteurs qui mènent un orogène à devenir doublement vergent, ni la relation entre le pro- et rétro-prisme ne sont bien compris. Le but de cette étude est d'améliorer notre connaissance 1) de la relation entre le pro- et le rétro-prisme pendant l'orogénèse, 2) des facteurs contrôlant l'évolution d'un orogène à double vergence, et 3) d’un lien dynamique possible entre le pro- et le rétro-prisme. Répondre à ces questions nécessite une connaissance améliorée de l'évolution d'un orogène à double vergence. Nous nous sommes concentrés sur les Pyrénées Orientales, en raison de la grande quantité de données disponibles. Nous avons effectué une étude de terrain tectono-stratigraphique détaillée à l’est du Massif de Saint Barthelemy et dans l’avant-pays autour de Lavelanet (plaque Européenne). Notre interprétation d’une coupe restaurée intègre une configuration crustale pré-orogenique en tant qu'une marge hyper-amincie. Nous relions l'évolution détaillée du rétro-prisme à celle du pro-prisme (plaque Ibérique), afin de mieux contraindre la dynamique à l'échelle crustale. Nous subdivisons l'évolution des Pyrénées Orientales en quatre phases. La première phase (Crétacé Supérieur) est caractérisée par la fermeture d'un domaine de manteau exhumé entre les plaques et l'inversion synchrone d'un système de rift riche en sel et thermiquement déséquilibré. Le raccourcissement était distribué de façon égale entre les deux marges pendant cette première phase d’inversion. Une phase de quiescence (Paléocène), limitée au rétro-prisme, enregistre la transition entre l'inversion et la phase de collision. La phase de collision principale (Éocène) enregistre le taux de raccourcissement le plus élevé, et était principalement accommodé dans le pro-prisme. Pendant la phase finale (Oligocène) le rétro-prisme était largement inactif et le raccourcissement du pro-prisme a ralenti. Cela démontre que la relation entre le pro- et rétro-prisme change avec le temps. Nous avons utilisé des modèles numériques 2D thermomécaniques à l'échelle lithosphérique pour simuler l'évolution d'un orogène à double vergence s'initie après avec un rift. Nos résultats montrent un modèle évolutif similaire à celui observé dans les Pyrénées Orientales avec une phase d'inversion du rift approximativement symétrique suivie d'une phase de collision asymétrique. L'héritage du rift est essentiel pour permettre le développement d’un orogène à double vergence. Des autres facteurs, comme les processus de surface et la déformation de la couverture, ont un effet significatif sur la structure crustale et la répartition du raccourcissement entre les deux prismes. Un niveau de décollement (sel) à la base de la couverture favorise la formation d'un empilement antiformal d’écailles crustales, similaire à la géométrie observée dans la Zone Axiale des Pyrénées, en formant un prisme à faible pente qui force la déformation crustale à se concentrer dans l'arrière-pays. Enfin, nous montrons que l'évolution des pro- et rétro-prismes est inextricablement liée : des événements ou des conditions d'un côté de l'orogène ont un effet direct sur l'autre côté de l'orogène. Ceci est clairement démontré dans nos modèles par des variations constantes des taux de raccourcissement du pro- et rétro-prisme en réponse à l'accrétion dans le pro-prisme. Le Haut Atlas (Maroc) et Pyrénées peuvent être respectivement considérés comme des exemples d'inversion de rift symétrique et de phases de collision asymétrique ultérieure

Salt decollement and rift inheritance controls on crustal deformation in orogens

We investigate the factors that control the shortening distribution and its evolution through time in orogenic belts using numerical models. We present self‐consistent high‐resolution numerical models that simulate the inversion of a rift to generate an upper crustal antiformal stack, a wide outer pro‐wedge fold‐and‐thrust belt, characterised by a two‐phase evolution with early symmetric inversion followed by formation of an asymmetric doubly‐vergent orogen. We show that a weak viscous salt décollement promotes gravitational collapse of the cover. When combined with efficient erosion of the orogenic core and sedimentation in adjacent forelands, it ensures the thick‐skinned pro‐wedge taper remains subcritical, promoting formation of an upper crustal antiformal stack. Rift inheritance promotes a two‐phase shortening distribution evolution regardless of the shallow structure and other factors. Comparison to the Pyrenees strongly suggests that this combination of factors led to a very similar evolution and structural style.publishedVersio

Salt decollement and rift inheritance controls on crustal deformation in orogens

We investigate the factors that control the shortening distribution and its evolution through time in orogenic belts using numerical models. We present self‐consistent high‐resolution numerical models that simulate the inversion of a rift to generate an upper crustal antiformal stack, a wide outer pro‐wedge fold‐and‐thrust belt, characterised by a two‐phase evolution with early symmetric inversion followed by formation of an asymmetric doubly‐vergent orogen. We show that a weak viscous salt décollement promotes gravitational collapse of the cover. When combined with efficient erosion of the orogenic core and sedimentation in adjacent forelands, it ensures the thick‐skinned pro‐wedge taper remains subcritical, promoting formation of an upper crustal antiformal stack. Rift inheritance promotes a two‐phase shortening distribution evolution regardless of the shallow structure and other factors. Comparison to the Pyrenees strongly suggests that this combination of factors led to a very similar evolution and structural style

Evolution of a low convergence collisional orogen: a review of Pyrenean orogenesis

International audienceThe Pyrenees is a collisional orogen built by inversion of an immature rift system during convergence of the Iberian and European plates from Late Cretaceous to late Cenozoic. The full mountain belt consists of the pro-foreland southern Pyrenees and the retro-foreland northern Pyrenees, where the inverted lower Cretaceous rift system is mainly preserved. Due to low overall convergence and absence of oceanic subduction, this orogen preserves one of the best geological records of early orogenesis, the transition from early convergence to main collision and the transition from collision to post-convergence. During these transitional periods major changes in orogen behavior reflect evolving lithospheric processes and tectonic drivers. Contributions by the OROGEN project have shed new light on these critical periods, on the evolution of the orogen as a whole, and in particular on the early convergence stage. By integrating results of OROGEN with those of other recent collaborative projects in the Pyrenean domain (e.g., PYRAMID, PYROPE, RGF-Pyrénées), this paper offers a synthesis of current knowledge and debate on the evolution of this immature orogen as recorded in the synorogenic basins and fold and thrust belts of both the upper European and lower Iberian plates. Expanding insight on the role of salt tectonics at local to regional scales is summarised and discussed. Uncertainties involved in data compilation across a whole orogen using different datasets are discussed, for example for deriving shortening values and distribution.Les Pyrénées sont un petit orogène de collision à faible convergence construit par inversion d’un système de rift immature au cours de la convergence des plaques ibérique et européenne du Crétacé supérieur au Cénozoïque. La ceinture montagneuse comprend les Pyrénées méridionales (pro-avant-pays) et les Pyrénées septentrionales (rétro-avant-pays), où le système de rift hérité du Crétacé inférieur est principalement préservé. En raison de la faible convergence globale et de l’absence de subduction océanique, l’orogène pyrénéen conserve l’un des meilleurs enregistrements géologiques de l’orogenèse précoce, de la transition de la convergence précoce à la collision principale et de la transition de la collision à la post-convergence. Ces périodes de transition enregistrent des changements majeurs dans le comportement de l’orogène, reflétant l’évolution des processus lithosphériques et des moteurs tectoniques. Les contributions du projet OROGEN ont apporté un nouvel éclairage sur ces périodes critiques, sur l’évolution de l’orogène dans son ensemble, et en particulier sur la phase de convergence précoce. En intégrant les résultats d’OROGEN aux résultats d’autres projets de recherche collaboratifs récents sur le domaine pyrénéen (PYRAMID, PYROPE, RGF-Pyrénées), cet article propose une synthèse des connaissances actuelles et des débats sur l’évolution de cet orogène immature tel qu’enregistré en particulier dans les bassins synorogéniques et les chaînes plissées des plaques européennes et ibériques. L’élargissement des connaissances sur le rôle de la tectonique salifère aux échelles locales et régionales est résumé et discuté. Les incertitudes impliquées dans la compilation des données sur l’ensemble d’un orogène à l’aide de différents ensembles de données sont discutées, par exemple pour estimer les valeurs de raccourcissement et sa distribution

Pyrenean hyper-extension : breaking, thinning, or stretching of the crust ?A view from the central north-Pyrenean zone

International audienceThe geology of the North Pyrenean Zone in the central Pyrenees allows for the observation in the field of theentire section of the Pyrenean rift, from the mantle to the crust and the Mesozoic cover (pre, syn and post rift).The good knowledge we have of the pre-Alpine history of the Pyrenees allows us to properly constrain the Alpinegeological evolution of the pre-Triassic rocks which record both Variscan and Alpine orogenic cycles.The mantle outcrop as kilometric to centimetric fragments of peridotite dispersed within a carbonate metamorphicbreccia. The study of peridotite serpentinisation shows several events of low-temperature serpentinisation, incontact with seawater. In some locallities, we can observe a mixture of fragments of variously serpentinizedperidotites. This suggests a tectonic context where fragments of peridotites from different structural levels weresampled more or less synchronously.The granulitic basement is characterized by a Variscan syndeformational HT event (300-280 Ma). So farwe have not found any trace of a Cretaceous HT event (> 500C). On the other hand, the basement is affectedby a regional metasomatism that began during the Jurassic and became more spatially focused with time until itwas restricted to the Pyrenean rift during the Aptien, Albian and Cenomanian. The talc-chlorite metasomatism(120-95 Ma) shows an evolution from a static toward a syn-deformation hydrothermal event, under a more or lessnormal geothermal gradient. Extensional deformation is recorded by the reworking of several inherited low-angleVariscan tectonic contacts, but also by dispersed high-angle extensional shear zones formed under greenshistconditions.The metamorphic Mesozoic cover of the basement massifs, which constitute the so-called Internal MetamorphicZone, is an allochtonous unit made of lenses of Mesozoic rocks enclosed into the breccia, whichlocally contains peridotite and basement clasts. The Mesozoic metamorphic carbonates show a first phase ofsyn-metamorphic (450-600C, P < 2 kb) ductile deformation, and subsequent phases of folding and fracturing.The datation of neoformed minerals give a 108-85 Ma time span for the metamorphism. We interpret this brecciaas an abandonment breccia which marks the emergence of the main detachment. The basal contact of the Mesozoiccover has a complex 3D geometry traced by Triassic evaporites. It corresponds to a major pre- and synorogenicpolyphased tectonic contact.All these data show a geometrically complex hyper-extended rift where the crust was not stretched under ahigh geothermal gradient but thinned by the tectonic extraction of relatively thin lenses and perhaps cut by highangle low-grade shear zones. The 3D geometry, as well as the strain records and the breccia lithologies stronglysuggest a non-cylindricity for the exhumation process, probably within a transtentional syste

Insights Into the Crustal-Scale Dynamics of a Doubly Vergent Orogen From a Quantitative Analysis of Its Forelands: A Case Study of the Eastern Pyrenees

In natural doubly vergent orogens, the relationship between the pro- and retro-wedges is, as yet, poorly constrained. We present a detailed tectonostratigraphic study of the retro-wedge of the Eastern Pyrenees (Europe) and link its evolution to that of the pro-wedge (Iberia) in order to derive insight into the crustal-scale dynamics of doubly vergent orogens. Based on cross-section restoration and subsidence analyses, we divide the East Pyrenean evolution into four phases. The first phase (Late Cretaceous) is characterized by closure of an exhumed mantle domain between the Iberian and European plates and inversion of a salt-rich, thermally unequilibrated rift system. Overall shortening (~1 mm/yr) was distributed roughly equally between both margins over some 20 Myr. A quiescent phase (Paleocene) was apparently restricted to the retro-wedge with slow, continuous deformation in the pro-wedge (~0.4 mm/yr). This phase occurred between closure of the exhumed mantle domain and onset of main collision. The main collision phase (Eocene) records the highest shortening rate (~3.1 mm/yr), which was predominantly accommodated in the pro-wedge. During the final phase (Oligocene), the retro-wedge was apparently inactive, and shortening of the pro-wedge slowed (~2.2 mm/yr). Minimum total shortening of the Eastern Pyrenees is ~111 km, excluding closure of the exhumed mantle domain. The retro-wedge accommodated ~20 km of shortening. The shortening distribution between the pro- and retro-wedges evolved from roughly equal during rift inversion to pro-dominant during main collision. This change in shortening distribution may be intrinsic to all inverted rift systems. ©2018. American Geophysical Union.This study was funded by the ANR (France) PYRAMID research project. The French-Norwegian Foundation (13-06 PYR-FFTP; sedimentary basin and North Pyrenean foreland fold and thrust belt formation) supported study visits to the University of Bergen, Norway. Collaboration with CSIC Barcelona, Spain was funded by the project ALPIMED (PIE-CSIC-201530E082).Peer reviewe