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[EN]: 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., [FR]: 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.

In the South-Central Andes, the crustal structures driving the tectonic evolution of the Andean Cordillera remain unresolved. So far, most seismological studies focused on the subduction interface, leaving crustal seismicity and its relationship with crustal deformation and Andean volcanism mostly unconstrained. However, because of their large number compared to higher magnitude events, the characterization of small-magnitude crustal earthquakes is key to identify active structures and better constrain the tectonic models. In this work, we exploit 53 months of continuously recorded, three-component waveforms from the permanent seismic network in central Chile using a deep-learning approach to improve the detection of small-magnitude earthquakes. To increase station coverage, we also use the seismic phases obtained from a previous temporary seismic deployment. We use the obtained seismicity catalog to refine tomographic models of that region, revealing a more detailed architecture of the Chilean forearc. Travel times calculated in the new 3-D velocity model allowed us to locate ∼14,000 earthquakes. Refined double-difference relocations of ∼4,900 events located beneath the West Andean Thrust suggest a large-scale, west-dipping structure which, together with the west-verging tectonic front, likely contributed to the uplift and crustal deformation during the past ∼20 Myr

The Pyrenean domain records the development of a hyperextended system during the Early Cretaceous at Iberia/Eurasia plate-boundary. This rifting stage is controlled by the coeval development of N120° longitudinal and N20° transverse tectonic features. In the west-Pyrenean Mauléon basin, preserved in the heart of a N120° lithospheric pop-up, the Iholdy, Saison and Barlanès transverse structures are known to play a significant role during the Cretaceous hyperextension. Using a multidisciplinary approach combining Raman thermometry, paleostress reconstructions, seismic interpretations, 3D implicit geological modeling and passive seismic interpretation, we define these three N20° structures as syn-collisional transfer zones rooting at depth in the upper lithospheric mantle. These tectonic features significantly control the 3D structural architecture of the Mauléon basin pop-up. Indeed, the N120°-oriented thrust systems, defining the edges of the Mauléon basin pop-up, branch into these transfer zones and define corridors with differing amounts of shortening. This overall structural pattern defines drawer-like structures allowing the closure, by stages, of the former rift domain. Thus, this study clarifies the role of inherited lithospheric transfer zones in the reactivation of a hyperextended rift basin and bears upon the origin of the non-cylindrical shape of the West-Pyrenean belt

This contribution reviews the challenges of imaging collisional orogens, focusing on the example of the Pyrenean domain. Indeed, important progresses have been accomplished regarding our understanding of the architecture of this mountain range over the last decades, thanks to the development of innovative passive imaging techniques, relying on a more thorough exploitation of the information in seismic signals, as well as new seismic acquisitions. New tomographic images provide evidence for continental subduction of Iberian crust beneath the western and central Pyrénées, but not beneath the eastern Pyrénées. Relics of a Cretaceous hyper-extended and segmented rift are found within the North Pyrenean Zone, where the imaged crust is thinner (10–25 km). This zone of thinned crust coincides with a band of positive Bouguer anomalies that is absent in the Eastern Pyrénées. Overall, the new tomographic images provide further support to the idea that the Pyrénées result from the inversion of hyperextended segmented rift systems., Cette contribution passe en revue les enjeux de l’imagerie des orogènes collisionnels, en se focalisant sur l’exemple du domaine pyrénéen. Au cours des dernières décennies, notre compréhension de l’architecture de cette chaîne de montagnes a connu des avancées importantes grâce au développement de techniques innovantes d’imagerie passive, s’appuyant sur une exploitation plus poussée de l’information dans les signaux sismiques, ainsi que de nouvelles acquisitions sismiques. Les nouvelles images tomographiques mettent en évidence la subduction continentale de la croûte ibérique sous les Pyrénées occidentales et centrales, mais pas sous les Pyrénées orientales. Les reliques d’un rift Crétacé hyper-étendu et segmenté se trouvent dans la zone nord-pyrénéenne, où la croûte imagée est plus mince (10–25 km). Cette zone de croûte amincie coïncide avec une bande d’anomalies de Bouguer positives qui est absente dans les Pyrénées orientales. Les nouvelles images tomographiques viennent globalement renforcer l’idée que les Pyrénées résultent de l’inversion de systèmes de rifts segmentés et hyper-étirés.

[EN] A long-standing challenge in tectonics is to evaluate the role of inheritance and define the initial conditions of a geodynamic system, which are prerequisites to understand and model its evolution with some accuracy. Here we revisit the concept of “inheritance” by distinguishing “interface shape inheritance”, which includes the transient thermal state and gravitational potential energy, and “persisting inheritance”, which encompasses long-lasting structural and compositional inheritance. This new approach allows us to investigate, at each stage of a Wilson Cycle, the interplay between inheritance (innate/“genetic code”) and the physical processes at play (extension/compression, magmatism etc.). The aim of this paper is to provide a conceptual framework that integrates the role of inheritance in the study of rifts, rifted margins and collisional orogens based on the work done in the OROGEN project, which focuses on the Biscay-Pyrenean system. The Biscay-Pyrenean rift system resulted from a multistage rift evolution that developed over a complex lithosphere pre-structured by the Variscan orogenic cycle. There is a general agreement that the Pyrenean-Cantabrian orogen resulted from the reactivation of an increasingly mature rift system along-strike, ranging from mature rifted margins in the west to an immature and segmented hyperextended rift in the east. However, different models have been proposed to explain the preceding rifting and its influence on the subsequent reactivation. Results from the OROGEN project highlight the sequential reactivation of rift-inherited decoupling horizons and identify the specific role of exhumed mantle, hyperextended and necking domains during compressional reactivation. They also highlight the contrasting fate of rift segment centres versus segment boundaries during convergence, explaining the non-cylindricity of internal parts of collisional orogens. Results from the OROGEN project also suggest that the role of inheritance is mo, [FR] Un défi de longue date en tectonique consiste à évaluer le rôle de l’héritage et à définir les conditions initiales d’un système géodynamique. Ceux-ci sont en effet des prérequis pour comprendre et modéliser l’évolution d’un tel système avec une certaine précision. Nous revisitons ici le concept d’« héritage » en distinguant « l’héritage (transitoire) d’interfaces », qui comprend d’un côté l’état thermique et de l’autre l’énergie potentielle gravitationnelle, et « l’héritage persistant » qui englobe les hétérogénéités structurales et compositionnelles. Cette nouvelle approche permet d’étudier, à chaque étape du Cycle de Wilson, l’interaction entre l’héritage (inné/« code génétique » du système) et les processus physiques en jeu (extension/compression, magmatisme, etc.). Le but de cet article est de fournir un cadre conceptuel qui intègre le rôle de l’héritage dans l’étude des rifts, des marges riftées et des orogènes de collision, à partir des travaux réalisés dans le projet OROGEN dans le système Gascogne-Pyrénées. Le système de rift Gascogne-Pyrénées résulte de plusieurs épisodes extensifs qui ont successivement affecté une lithosphère déjà complexe car pré-structurée par le cycle orogénique Varisque. Il est généralement accepté que l’orogénèse pyrénéo-cantabrique a résulté de la réactivation d’un système de rift dont la maturité varie d’est en ouest, allant de marges conjuguées matures à l’ouest à un rift hyper-étiré, immature et segmenté à l’est. Cependant, différents modèles ont été proposés pour expliquer l’évolution précédant le rifting et son influence sur la réactivation ultérieure. Les résultats du projet OROGEN montrent une réactivation séquentielle des horizons de découplage hérités du rift et identifient le rôle spécifique des domaines de manteau exhumé, d’hyperextension et d’étranglement lors de la réactivation. Ils mettent également en évidence le sort contrasté des centres de segments de rifts par rapport à leurs bordures lors de l’inversion, ex

[EN] We present a 3-D shear wave velocity model of the Mauléon and Arzacq Basins from the surface down to 10 km depth, inverted from phase velocity maps at periods between 2 and 9 s. These phase velocity maps were obtained by analyzing coherent surface wave fronts extracted from ambient seismic noise recorded by the large-N Maupasacq seismic array with a matched filtering approach. This new model is in good agreement with a local earthquake tomography study performed on the same acquisition dataset. Our passive imaging models reveal the upper crustal architecture of the Mauléon and Arzacq Basins, with new details on the basement and its relationship with the overlying sedimentary cover. Combining these new tomographic images with surface and subsurface geological information allows us to trace major orogenic structures from the surface down to the basement. In the basin, the models image the first-order basin architecture with a kilometric resolution. At depth, high velocity anomalies suggest the presence of dense deep crustal and mantle rocks in the hanging wall of north-vergent Pyrenean Thrusts. These high velocity anomalies spatially coincide with a positive gravity anomaly in the western Mauléon Basin. In addition, our models reveal major changes from the Chaînons Béarnais to the western Mauléon Basin across a set of orogen-perpendicular structures, the Saison and the Barlanès transfer zones. These changes reflect the along-strike variation of the orogenic evolution that led to the preservation of the former rifted domain and its underlying mantle in the orogenic wedge of the Western Pyrenees. We discuss the implications of these results for the 3-D architecture of the Mauléon Basin and its underlying basement, [FR] Nous présentons un modèle 3-D de vitesse des ondes de cisaillement des bassins de Mauléon et d’Arzacq de la surface jusqu’à 10 km de profondeur inversé à partir de cartes de vitesse de phase pour des périodes entre 2 et 9 s. Ces cartes ont été obtenues à partir de l’analyse de fronts d’onde de surface cohérents extraits du bruit sismique ambiant enregistré par le réseau Maupasacq par filtrage adaptatif. Ce nouveau modèle est en bon accord avec la tomographie locale réalisée sur ce même jeu de données. Nos nouvelles images tomographiques révèlent l’architecture supra-crustale des bassins de Mauléon et d’Arzacq, avec des informations nouvelles sur la nature du socle et sa relation à la couverture sédimentaire. En combinant ces nouvelles images tomographiques aux informations géologiques, il est possible de tracer les principales structures orogéniques de la surface jusqu’au socle des bassins. Dans le bassin, les modèles nous fournissent une image de premier ordre des plis et chevauchements à l’échelle kilométrique. En profondeur, les anomalies rapides suggèrent la présence de roches de la croûte inférieure et du manteau dans le toit des chevauchements pyrénéens de pendage nord. Ces anomalies rapides coïncident spatialement avec l’anomalie gravimétrique positive dans la partie ouest du bassin de Mauléon. Nos modèles tomographiques documentent également des changements de structures majeurs entre les Chaînons Béarnais et la partie ouest du bassin de Mauléon à travers des structures perpendiculaires à l’axe de la chaîne, représentées par les structures transverses du Saison et du Barlanès. Ce changement structural reflète les variations latérales de l’évolution orogénique qui a conduit à la préservation des domaines de rift hyper-étirés et du manteau sous-jacent dans le prisme orogénique. Nous discutons les implications de ces résultats concernant l’architecture 3-D du bassin de Mauléon et du socle sous-jacent

Horizontal-to-Vertical Spectral Ratios (HVSR) and Rayleigh group velocity dispersion curves (DC) can be used to estimate the shallow S-wave velocity (Vs) structure. Knowing the shallow Vs structure is important for geophysical data interpretation either in order to better constrain data inversions for P-wave velocity (Vp) structures such as travel time tomography or full waveform inversions, or to directly study the Vs structure for geo-engineering purposes (e.g. ground motion prediction). The purpose of this study is to appraise in particular how much information HVSR can add in a large N experiment and how different instrumentation types affect this. During the Maupasacq large-scale experiment, 197 three-component short-period stations, 190 geophone nodes and 54 broadband seismometers were continuously operated in Southern France for 6 months (April to October 2017) covering an area of approximately 1500 km2 with a site spacing of approximately 1 to 3 km. On the obtained HVSR and DC data, a statistical Joint inversion is performed for the shallow Vs structure. The results indicate that the addition of HVSR data to the DC inversion reduces the variance of the recovered shallow Vs model and improves the convergence to a smaller data misfit. While broadband and short period instruments delivered similar results, geophone nodes performed significantly worse due to their much higher cut off frequency.

In the framework of the MAUPASACQ project, a dense seismic network of 440 three-component stations was deployed throughout the Mauleon Basin, in the northern foothills of the Pyrenees, SW France. The network consisted of three different types of sensors, namely 190 SG-10 SERCEL geophone nodes, 197 5 Hz SEISMOTECH short-period sensors and 53 broadband sensors (Guralp CMG-40, Trillium Compact, Trillium 120), and operated continuously for 6 months, from April 1 until September 30, 2017. Numerous geological and geophysical observations provide evidence that the Mauleon Basin was created by the tectonic inversion of a precollisional rift system that was formed between the Iberian and European plates, involving processes of rift-related mantle exhumation. These observations, among others, characterize the area under investigation as an ideal locality to study and understand the structural and geodynamic evolution of the Pyrenean mountain range. The main scientific objective of the MAUPASACQ experiment is to provide new constraints on this topic, by applying different types of classical imaging techniques that incorporate a number of Passive Seismic methodologies (i.e. travel time tomography, ambient noise tomography, receiver functions, etc.). The scope of this work is to illustrate some preliminary observations deriving from the analysis of the ambient seismic noise field that was recorded in the Mauleon Basin. This was achieved by beamforming and by using the Rayleigh wave empirical Green’s functions that were estimated by seismic interferometry. Beamforming analysis provided valuable information about the spatial distribution and the directionality of the ambient noise sources. The main source region for the primary microseisms (12-20 s) is located to the northwest (N315o), being present over a larger backazimuth range across the northwest quadrant, while the secondary microseisms (5-10 s) seem to present two maxima, one in the west (N270o) and a second one in the nort

The presence of a crustal root beneath the central part of the Pyrenees has been extensivelly documented by seismic and gravity data and is related to the building of the Pyrenees Chain during the Alpine orogeny. The Eastern termination of the chain is affected by a second major orogenic process, the Neogene extension associated to the rotation of the Sardinia-Corsica block and the opening of the Valencia Trough. This extensional feature has resulted in a rather abrupt thinning of the crust, from 40-45 km about 80 km to the west of the Mediterranean coastline to less than 25 km beneath the eastern termination of the chain. The details of the transition between the central and eastern Pyrenees domains needs still to be fixed into detail. With this objective in mind, two passive seismic profiles have been acquired from mid 2015 to late 2016 within the OROGEN and Pyrope projects. Up to 38 broad-band stations were deployed along two orthogonal lines, with an interstation spacing close to 10 km. The NNE-SSW profile, oriented orthogonally to the Pyrenees trend, shows a well defined Moho beneath Iberia, slightly deepening from 32 to 35 km northwards. Beneath the Axial zone the Moho is located between 30 and 35 km and appears to be segmented in a couple of convertors. Further North, underneath the North Pyrenean Front Thrust , the Moho appears again as a clear and continuous convertor located at 28-30 km. This image clearly differs from the sections obtained in Central and Western Pyrenees, where the imbrication between the Iberian and Eurasian crusts is more conspicuous. Results of receiver function migration on the E-W profile suggest a smooth Moho thinning from a 40 km depth beneath the western termination of the line to 23 km close to the coastline, confirming the picture retrieved from previous, lower resolution experiments and evidencing the crustal thinning due to the Neogene extensional processes. Moderate magnitude earthquakes with epicenters located in the Gulf of

Two passive seismic profiles have been acquired in the eastern Pyrenees as part of the Pyrope and Orogen projects to investigate the crustal structure differences between this area and the central and western Pyrenees. Up to 28 broad-band stations were deployed along two orthogonal lines, with an interstation spacing close to 10 km. High frequency receiver functions allowed us to obtain the main lithospheric interfaces along those lines. The NNE-SSW profile shows a well-defined Moho beneath Iberia, slightly deepening northwards. Beneath the Axial zone the Moho appears to be segmented but does not show evidence of crustal imbrication. Further North, the Moho appears again as a continuous interface located around 30 km depth. This image clearly differs from the conspicuous imbrication between the Iberian and Eurasian crusts observed westward. The E-W profile shows a smooth Moho thinning from a 40 km depth to the west of the profile to 23 km close to the coastline, evidencing the crustal thinning related to the Neogene extensional processes. Additional constraints on the geometry of the crust/mantle boundary in the Eastern Pyrenees are obtained from local moderate magnitude earthquakes recorded along the seismic lines during the experiment. In particular, a fan profile built from an event located near the Mediterranean coast suggests that crustal thickness differences between Iberian and Eurasian crusts can still be recognized 30 km westward of the Mediterranean coast, in an area that seems to mark the limit between regions dominated by compressive and extensive processes. We propose a model in which the observed dissimilar Moho structure beneath the Eastern Pyrenees is interpreted as the result of a different Mesozoic pre-shortening margin configuration. Seismic results are consistent with recently proposed tectonic models including an intermediate continental block separating Iberia and Europe by two basins with extremely thin crust or exhumed mantle.

We exploit the data from five seismic transects deployed across the Pyrenees to characterize the deep architecture of this collisional orogen. We map the main seismic interfaces beneath each transect by depth migration of P-to-S converted phases. The migrated sections, combined with the results of recent tomographic studies and with maps of Bouguer and isostatic anomalies, provide a coherent crustal-scale picture of the belt. In the Western Pyrenees, beneath the North Pyrenean Zone, a continuous band of high density/velocity material is found at a very shallow level (~10 km) beneath the Mauleon basin and near Saint-Gaudens. In the Western Pyrenees, we also find evidence for northward continental subduction of Iberian crust, down to 50–70 km depth. In the Eastern Pyrenees, these main structural features are not observed. The boundary between these two domains is near longitude 1.3 °E, where geological field studies document a major change in the structure of the Cretaceous rift system, and possibly a shift of its polarity, suggesting that the deep orogenic architecture of the Pyrenees is largely controlled by structural inheritance.

Local seismic networks are usually designed so that earthquakes are located inside them (primary azimuthal gap < < 180 degrees) and close to the seismic stations (0-100 km). With these local or near-regional networks (0 degrees-5 degrees), many seismological observatories still routinely locate earthquakes using 1-D velocity models. Moving towards 3-D location algorithms requires robust 3-D velocity models. This work takes advantage of seismic monitoring spanning more than 30 yr in the Pyrenean region. We investigate the influence of a well-designed 3-D model with station corrections including basins structure and the geometry of the Mohorovicic discontinuity on earthquake locations. In the most favourable cases (GAP < 180 degrees and distance to the first station lower than 15 km), results using 1-D velocity models are very similar to 3-D results. The horizontal accuracy in the 1-D case can be higher than in the 3-D case if lateral variations in the structure are not properly resolved. Depth is systematically better resolved in the 3-D model even on the boundaries of the seismic network (GAP > 180 degrees and distance to the first station higher than 15 km). Errors on velocity models and accuracy of absolute earthquake locations are assessed based on a reference data set made of active seismic, quarry blasts and passive temporary experiments. Solutions and uncertainties are estimated using the probabilistic approach of the NonLinLoc (NLLoc) software based on Equal Differential Time. Some updates have been added to NLLoc to better focus on the final solution (outlier exclusion, multiscale grid search, S-phases weighting). Errors in the probabilistic approach are defined to take into account errors on velocity models and on arrival times. The seismicity in the final 3-D catalogue is located with a horizontal uncertainty of about 2.0 +/- 1.9 km and a vertical uncertainty of about 3.0 +/- 2.0 km.

Passive Seismic is a broad term, incorporating various techniques and methodologies, which all exploit some part of the seismic signal that naturally exists or occurs in the Earth’s subsurface. This signal may differ significantly in the form and/or the provenance (e.g. earthquakes, ambient seismic noise, etc.), as well as the frequency content and, subsequently, the part of the subspace on which it may carry useful information. People involved in Passive Seismic often encounter the question: ‘What type of instrument is suitable for a passive seismic survey?’. Passive Seismic instrumentation usually consists of three-component seismic sensors, which mainly differ in the frequency range they are able to record (broad-band, short-period or geophone nodes). Having its roots in seismology, where traditionally broadband stations have been used for decades, but heading towards exploration, where instrumentation has to be cost-efficient and easy to handle in order to permit the adaptation at a reservoir scale, Passive Seismic instrumentation still tries to strike a balance between cost and bandwidth. Having in mind the variability of Passive Seismic methodologies and instruments, the Maupasacq experiment, a large passive seismic survey, has been launched in the Mauleon basin, SW France. The scope of the experiment was to image the area of interest by jointly applying a number of passive seismic methodologies, each one contributing to the final image with a different piece of useful information. The area of interest was carefully selected as, on one hand, the Mauleon basin consists of a former Cretaceous hyper-extended rift, inverted during pyrenean orogeny and, on the other hand, it provides a means of evaluating the results acquired, as an abundance of geological and geophysical data already exist in the area. In this context, a dense seismic network of 417 three-component (3C) sensors was deployed in an area of approximately 1500 km2. In addition to those, 24 peripheral

New tomographic images of the upper mantle beneath the westernmost Mediterranean suggest that the evolution of the region experienced two subduction-related episodes. First subduction of oceanic and/or extended continental lithosphere, now located mainly beneath the Betics at depths greater than 400 km, took place on a NW-SE oriented subduction zone. This was followed by a slab-tear process that initiated in the east and propagated to the west, leading to westward slab rollback and possibly lower crustal delamination. The current position of the slab tear is located approximately at 4°W, and to the west of this location the subducted lithosphere is still attached to the surface along the Gibraltar Arc. Our new P-wave velocity model is able to image the attached subducted lithosphere as a narrow high-velocity body extending to shallow depths, coinciding with the region of maximum curvature of the Gibraltar Arc, the occurrence of intermediate-depth earthquakes, and anomalously thick crust. This thick crust has a large influence in the measured teleseismic travel time residuals and therefore in the obtained P-wave tomographic model. We show that removing the effects of the thick crust significantly improves the shallow images of the slab and therefore the interpretations based on the seismic structure

© The Authors 2014. Between 2011 and 2013, two dense transects were deployed across the central and western Pyrenees to get better constraints on the deep lithospheric architecture and discriminate the competing models of the structure and formation of the Pyrenees. Each transect recorded the regional and global seismicity during a period of approximately 1 yr. Here, we exploit the records of teleseismic compressional waves and of their conversions to shear waves on internal discontinuities in order to map lithospheric interfaces beneath the two transects. The migrated sections, obtained by performing common conversion point stacks, are in remarkable agreement with the results of the ECORS-Pyrenees and ECORS-Arzacq deep seismic surveys. However, the migrations of converted waves reveal new details of the deep lithospheric architecture that could not be seen with the active source experiments. The new images provide clear and definite evidence for the subduction of a thinned Iberian crust down to at least ~70 km depth, a result that has important implications for the formation of the Pyrenees. The subduction of the Iberian lithosphere leads to reconsider the amount of convergence between Iberia and Europe during the Cenozoic. A recent regional P-wave tomography, relying on the data of the PYROPE and IBERARRAY temporary experiments, revealed the segmentation of lithospheric structures by inherited Hercynian NE-SW transfer faults that were reactivated during the Albian rifting. Ourmigration images are consistent with this model, and give further support to the idea that the Pyrenees were produced by the tectonic inversion of a segmented hyperextended rift that was buried by subduction beneath the European Plate.

Ocean activity produces continuous and ubiquitous seismic energy mostly in the 2¿20 s period band, known as microseismic noise. Between 2 and 10 s period, secondary microseisms (SM) are generated by swell reflections close to the shores and/or by opposing swells in the deep ocean. However, unique conditions are required in order for surface waves generated by deep-ocean microseisms to be observed on land. By comparing short-duration power spectral densities at both Atlantic shoreline and inland seismic stations, we show that ocean tides strongly modulate the seismic energy in a wide period band except between 2.5 and 5 s. This tidal proxy reveals the existence of an ex situ short-period contribution of the SM peak. Comparison with swell spectra at surrounding buoys suggests that the largest part of this extra energy comes from deep ocean-generated microseisms. The energy modulation might be also used in numerical models of microseismic generation to constrain coastal reflection coefficients.

The lithospheric architecture of the Pyrenees is still uncertain and highly debated. Here, we provide new constraints from a high-resolution 3-D S-wave velocity model of the Pyrenees and the adjacent foreland basins. This model is obtained from ambient noise tomography on records of temporary and permanent seismic arrays installed in southwestern France and northern Spain. We first computed group velocity maps for Rayleigh waves in the 5 to 55 s period range using noise correlation stacks at 1500-8500 station pairs. As the crust is very heterogeneous, poor results were obtained using a single starting model in a linearized inversion of group velocity dispersion curves for the shear wave structure. We therefore built a starting model for each grid node by full exploration of the model space. The resulting 3-D shear wave velocity model is compared to data from previous geophysical studies as a validation test. Despite the poor sensitivity of surface waves to seismic discontinuities, the geometry of the top of the basement and the Moho depth are retrieved well, except along the Cantabrian coast. Major reflectors of the ECORS deep seismic sounding profiles in the central and western Pyrenees coincide with sharp velocity gradients in our velocity model.We retrieve the difference between the thicker Iberian crust and the thinner European crust, the presence of low-velocity material of the Iberian crust underthrust beneath the European crust in the central Pyrenees, and the structural dissymmetry between the South Pyrenean Zone and the North Pyrenean Zone at the shallow crustal level. In the Labourd-Mauléon-Arzacq region (western Pyrenees), there is a high S-wave velocity anomaly at 20-30 km in depth, which might explain the positive Bouguer anomaly of the Labourd Massif. This high-velocity lower crust, which is also detected beneath the Parentis area, might be an imprint of the Albian-Aptian rifting phase. The southeastern part of the Massif Central has an unusual velocit

The 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.

We present new tomographic images of the Iberian Peninsula and northern Morocco based on broadband seismic data from the IberArray experiment, complemented with other permanent stations from local and regional networks. The quality of this combined dataset allows to apply a number of imaging techniques, including local and teleseismic travel- time tomography, surface-wave tomography from regional and distant earthquakes, and surface-wave tomography from correlations of seismic ambient noise. Each of these methods has advantages and disadvantages, and the combination of all allows to obtain a detailed and consistent picture of the crustal and mantle structure beneath the Iberian Peninsula and Morocco. Using local earthquake and seismic ambient noise tomography we are able to image the shallow crustal structure, also in regions with low seismicity. In particular we have obtained constraints on the lateral and vertical extent of the major sedimentary basins (Gulf of Cadis, west Alboran basin, Valencia trough, and Ebro basin) and the areas of greater than average crustal thickness beneath the mountain ranges of the Rif, Betics, Iberian chain and Pyrenees. Teleseismic travel-time tomography is best suited to investigate the structure of the upper mantle. Using very precise travel-time measurements determined using waveform similarity we have obtained a well resolved image of a high velocity anomaly beneath the Betics and Alboran Sea that extends down to the mantle transition zone and probably corresponds to a subducted slab.

We present an extension of the previously developed algorithm for Simulated Annealing Waveform Inversion of Body waves (SAWIB) to resolve the interference between direct PKP seismic phases and their corresponding depth phases (pPKP and sPKP) which occurs