Pyrenean Compression Research Articles

The Mid-Albian unconformity, a key to understand the geodynamics of the North Pyrenean Trough

The Pyrenees are the result of the structural inversion of a rift of general direction N110E, established at the articulation of the European and Iberian plates. This rift was created by left-lateral transtension with severe crustal thinning during the latest Early Cretaceous and Mid-Cretaceous. Its inversion, which led to the uplifting of the Pyrenean chain, took place gradually, by transpression from east to west, during the Late Cretaceous followed by an Eocene collision. From historical fieldwork, this note mainly focuses on the rifting period at the scale of the Pyrenean domain. It clearly shows the fundamental role of the stratigraphic approach in structural interpretation. Although synchronous at the scale of the Pyrenees, the Cretaceous extension is more important in the western part of the chain, near the Bay of Biscay. It is therefore at this location, within the Tardets-Mauléon Basin, that the various stages of creation and evolution of the rift could be clearly recognized with, successively: (1) an epirogenic pre-rifting stage, during which the facies lines, submeridian during the Jurassic, gradually change into N110E during the Neocomian before giving way, in the Early Cretaceous (Barremian-Gargasian), to a moderate acceleration of subsidence and, consequently, to the gentle uplift of a southern and western margin, future High Primary Belt (also often called Axial Zone) and Landes Plateau, initiating the next stage; (2) a major, brittle, newly identified rifting stage in the Latest Aptian (Clansayesian) and Early Albian. It is characterized by the installation of elongated lows on the downthrown side of transverse N20 normal faults, active diapirism of the Triassic evaporites and central detachment of the post-Triassic cover. During the Mid-Albian, a major surface of stratigraphic unconformity, often associated to uplift, erosion, and local centripetal gliding of the basin borders, marks a new structural organization, a rapid mantle ascent and the end of this episode. Although obscured by the later Pyrenean compression, the reconstructed basin geometry is interpreted as a “pull-apart” basin associated to a modest left-lateral motion; (3) a late rifting stage characterized by the collapse of the internal zones by rapid mantle cooling. The erosion of the coeval uplift of the southern High Primary Belt and the Landes Plateau feeds the accumulation of the thick, deep and transgressive unconformable formation of the Black Flysch of Mid-Albian to Early Cenomanian age in a larger coalescing basin. Faulting associated with a simple NS distension is only active on the last major border faults; (4) finally, a post-rift stage of Late Cretaceous age illustrated by the establishment of a passively subsiding basin, devoid of major deformations and whose heterogeneity of the previous depocenters is gradually erased. The Pyrenean transpression, later and less marked than in the eastern and central Pyrenees, allowed the conservation of most of these Cretaceous transtensive structures. Within the Tardets-Mauléon Basin, the Pernes and Col d’Urdach sector, the only Pyrenean location where the Mid-cretaceous transtension led to a proven underwater mantle exhumation, illustrates from outcrops the various stages that have marked the history of the Pyrenean Trough. Towards the east, along the Internal Metamorphic Zone, the Lourdes-Bagnères, Baronnies, Ballongue, Aulus and Saint-Paul de Fenouillet-Boucheville Basins, or those, further north like the Camarade and Quillan Basins, successively examined, testify to the same sedimentary and structural evolution. As in the case of the Tardets-Mauléon Basin, we deal here again with “pull-apart” type basins established on a thinned crust, arranged in transtensive or locally transpressive relays and separated by zones of thicker crust and shallower facies successions. The Mid-Albian unconformity is always present, heralding a general drowning of the North Pyrenean Trough and cessation of the extensional activity. The Pyrenean deformation, however, constitutes an obstacle for a precise geodynamic reconstruction, in particular in the eastern and central Pyrénées. Towards the west and north-west, in the South Aquitaine, Parentis and Basque-Cantabrian Basins, where Pyrenean deformation has far less obscured the rifting events, a very comparable Cretaceous geodynamic evolution is observed thus strengthening the present interpretation.

U–Pb dating of middle Eocene–Pliocene multiple tectonic pulses in the Alpine foreland

Abstract. Foreland fold-and-thrust belts (FTBs) record long-lived tectono-sedimentary activity, from passive margin sedimentation, flexuring, and further evolution into wedge accretion ahead of an advancing orogen. Therefore, dating fault activity is fundamental for plate movement reconstruction, resource exploration, and earthquake hazard assessment. Here, we report U–Pb ages of syn-tectonic calcite mineralizations from four thrusts and three tear faults sampled at the regional scale across the Jura fold-and-thrust belt in the northwestern Alpine foreland (eastern France). Three regional tectonic phases are recognized in the middle Eocene–Pliocene interval: (1) pre-orogenic faulting at 48.4±1.5 and 44.7±2.6 Ma associated with the far-field effect of the Alpine or Pyrenean compression, (2) syn-orogenic thrusting at 11.4±1.1, 10.6±0.5, 9.7±1.4, 9.6±0.3, and 7.5±1.1 Ma associated with the formation of the Jura fold-and-thrust belt with possible in-sequence thrust propagation, and (3) syn-orogenic tear faulting at 10.5±0.4, 9.1±6.5, 5.7±4.7, and at 4.8±1.7 Ma including the reactivation of a pre-orogenic fault at 3.9±2.9 Ma. Previously unknown faulting events at 48.4±1.5 and 44.7±2.6 Ma predate the reported late Eocene age for tectonic activity onset in the Alpine foreland by ∼10 Myr. In addition, we date the previously inferred reactivation of pre-orogenic strike-slip faults as tear faults during Jura imbrication. The U–Pb ages document a minimal time frame for the evolution of the Jura FTB wedge by possible in-sequence thrust imbrication above the low-friction basal decollement consisting of evaporites.

Contractional salt tectonics and role of pre-existing diapiric structures in the Southern Pyrenean foreland fold–thrust belt (Montsec and Serres Marginals)

Triassic Keuper evaporites have long been recognized as the main detachment level for thrusting in the Pyrenean fold–thrust belts. The deformed Late Cretaceous–Eocene foreland basin of the Southern Pyrenees has structures and stratal geometries that can be interpreted as related to salt tectonics (e.g. unconformities, rapid thickness variations, long-lived growth fans and overturned flaps), although they have been overprinted by shortening and thrusting. Based on field observations and published maps, we build new structural cross-sections reinterpreting two classic transects of the Southern Pyrenees (Noguera Ribagorçana and Noguera Pallaresa river transects). The sequential restoration of the sections explores the variations in structural style, addressing the role of halokinesis in the tectonic and sedimentary development. In the Serres Marginals area, we propose that salt pillows and diapirs started developing locally during the Mesozoic pre-orogenic episode, evolving into a system of salt ridges and intervening synclines filled with early synorogenic sediments. Rapid amplification of folds recorded by widespread latest Cretaceous–Paleocene growth strata is taken as marking the onset of contractional folding in the area. During Pyrenean compression, folding mechanisms transitioned from dominantly halokinetic to a combination of buckling and differential sedimentary loading. Squeezing of salt diapirs and thrust welding occurred as salt ridges were unroofed. We provide new field observations that lead to a reinterpretation of the regional structural development and contribute to the debate about the role of salt tectonics in the Pyrenees. Supplementary material: Table S1, giving the thickness of the main stratigraphic units, is available at https://doi.org/10.6084/m9.figshare.c.5287737

Polyphased brittle deformation around a crustal fault: A multi-scale approach based on remote sensing and field data on the mountains surrounding the Têt hydrothermal system (Eastern Pyrénées, France)

Studying hydrothermal systems in basement environments requires knowledge of fault and fracture network distributions. This study addresses this through multi-scale structural analysis of the Têt fault and its surrounding fracture systems (Eastern Pyrénées) using remote sensing and field data. This study aims to achieve this through: 1) precisely mapping and describing the brittle fault network, 2) analysing the distribution of lineaments and outcrop-scale fractures related to these faults, 3) making comparisons to fault-kinematic evidence combined in a new regional review, 4) examining the relations between fractures features and lithology, 5) applying statistical analysis to highlight relations between different scales of deformation. The complex fault network is inherited from consecutive tectonic stages (Hercynian, Pyrenean compression, Neogene extension) and has been reactivated since the middle-Miocene. NE–SW secondary faults are abundant at the regional scale, even away from the Têt fault. Major NW–SE faults are constituted by 10s-m wide brittle core zones, and NW–SE secondary faults are concentrated around the Têt fault, attesting that they had formed at shallow crust levels after the Oligo-Miocene extension. N–S fractures, formed during Pyrenean compression, are part of the background fracturing and are scattered throughout the study area. Intersections of fault and fracture networks provide efficient permeable pathways for meteoric and hydrothermal fluids. Finally, a dislocation model reveals a lithological control on fracture apertures in crystalline rocks, which appears more preserved at depth than in metasediments. All of these elements are integrated in a global model of the hydrothermal system establishment in accordance with the faulting sequence, with the damage distribution and with the lithology. This distributed fault system could represent the surface expression of the crustal thinning revealed by recent geophysical data. The realized identification of the lithological and structural characteristics of the surrounding mountains, allowing hydrothermal circulation to establish itself, provides a better understanding of the orogenic-belt related hydrothermal systems necessary to the geothermal exploration.

Evidence of decoupled deformation during Jurassic rifting and Cenozoic inversion phases in the salt-rich Corbières-Languedoc Transfer Zone (Pyreneo-Provençal orogen, France)

A detailed field study of Jurassic tectono-stratigraphic architecture of the southwestern part of the Corbières-Languedoc Transfer Zone (CLTZ, NE-SW oriented), between the Pyrenean and Provençal orogenic segments (N110 oriented) in SE France, document for the first time variations in thickness and stratigraphic geometries in relation to oblique extensional cover structures (NE-SW and N110 oriented). These structures (low-dip normal faults, salt rollers, rollovers, forced folds) formed during a Jurassic extension phase with strong decoupling between basement and cover on the Keuper pre-rift salt (Carnian-Norian evaporites). Some of these structures such as the Treilles Fault, the Valdria and the Terres Noires fold pairs, were previously interpreted as compressional and Pyrenean in origin (Late Santonian-Bartonian). Our study instead shows that these are Jurassic extensional and salt related structures, which were later affected by Pyrenean compression and Oligo-Miocene extension. Evidence of Jurassic extension is still observable in the field despite later rectivations, making these good field analogs for gently inverted extensional salt structures. During the Jurassic the interference between oblique structures above Keuper, leads to the formation of three dimensional growth strata observable at kilometric scale. Despite the mechanical decoupling effect of Keuper, we infer that their formation was linked to a strong interaction between oblique basement structures (NE-SW and N110) as the area lies during the Jurassic at the intersection between the eastern part of the Pyrenean E-W trending rift system and the NE-SW trending European margin of the Alpine Tethys rift. The decoupled deformation and the strong segmentation above and below Keuper detachement are here identified as Jurassic structural inheritence for later tectonic events, notably for the Nappe des Corbières Orientales emplaced at the end of Pyrenean orogenesis. Associated to previous works, this study highlights that the CLTZ is a key area to better understand Pyreneo-Provençal system evolution along its whole Wilson cycle and to better understand the processes that govern the formation of a salt-rich transfer zone in a strongly pre-structured crust, its multiple reactivations and the decoupling role of salt.

Salt tectonics and controls on halokinetic-sequence development of an exposed deepwater diapir: The Bakio Diapir, Basque-Cantabrian Basin, Pyrenees

This work evaluates growth strata adjacent to the Bakio Diapir in the Basque Pyrenees, aiming to discuss the application of halokinetic-sequence concepts, mainly developed in shallow-water to subaerial environments, to deepwater depositional settings. This is one of the few exposed passive diapirs developed in deepwater environments, with both synkinematic carbonate and siliciclastic strata. Thus, considering that large hydrocarbon producing areas are located in salt provinces developed in deepwater environments, this study is of interest not only to researchers in salt tectonics, but also to industry geoscientists focusing on hydrocarbon exploration and production. We present a 3D analysis of this outstanding salt structure by integrating detailed geological cartography, high-resolution bathymetry, seismic, and well data. The resulting reconstruction enables us to trace the extent of the diapir offshore and decipher its evolution from its formation as a salt wall developed above the overlap of two basement-involved faults until its squeezing during the Pyrenean compression. But more significantly, it allows us to discern the roles played by bathymetry, subsidence, and sedimentation type in the configuration of halokinetic sequences developed in deepwater environments. Thus, our study shows that: 1) the geometry of halokinetic sequences is defined by the thickness of the roof edges, the dip of the salt-sediment interface in the limbs of the active drape fold, and the onlap angle of the synkinematic sediments over the salt; 2) the roof thickness is controlled not only by the ratio between salt-rise and regional sediment-accumulation rate, but also by the water depth of the diapir roof and the depositional environment that can promote vertical aggradation of a supra-diapir carbonate buildup; and 3) high surface slopes and consequent debrites are not exclusive to, and characteristic of, hook halokinetic sequences and tabular composite halokinetic sequences.

Past hot fluid flows in limestones detected by Δ47–(U-Pb) and not recorded by other geothermometers

Abstract Geothermometers are commonly used to reconstruct the diagenetic and thermal history of rocks. However, characterizing the timing, origin, and temperature of paleofluid flow remains challenging because it must be assessed indirectly through the analysis of microscopic cements that precipitate and fill intergranular spaces during fluid circulation. Here, we measure both the clumped isotope (Δ47) temperature and in situ U-Pb age of individual diagenetic calcite cements within a sedimentary section of the Paris Basin (France), whose thermal history has been previously inferred to be <60 °C. We show that cementation occurred during two stages associated with major events at the western European lithospheric scale: (1) the Bay of Biscay rifting (Late Jurassic–Early Cretaceous), and (2) north-south Pyrenean compression (Eocene) followed by east-west extension during the European Cenozoic rift system event (Oligocene). Related to both events, we report unexpectedly hot fluids, up to 110 °C, contrasting with the lower temperatures inferred from other geothermometers (e.g., fluid inclusions, clay minerals, apatite fission tracks, maturity of organic matter by Rock-Eval pyrolysis, or vitrinite reflectance). These high temperatures (>70 °C) have been measured for calcite cements containing single-phase aqueous fluid inclusions, challenging the commonly accepted assertion that the absence of nucleation of a vapor phase indicates crystallization at low temperature (∼<70 °C). We suggest that the kinetics of mineralization events prevented the recording of short-lived hot fluid flows by other geothermometers.

Evolution of salt structures of the Pyrenean rift (Chaînons Béarnais, France): From hyper-extension to tectonic inversion

The Chaînons Béarnais is a salt-detached fold belt in the northern Pyrenees that formerly occupied the axis of the Cretaceous Pyrenean rift. Geological map revision and cross-section construction from surface geology and industrial well and seismic reflection data emphasize the role of salt diapirism in the folding of the belt during the Cretaceous extension and the subsequent Pyrenean orogeny. Pre-rift Triassic evaporites played a fundamental role during rifting, allowing the sedimentary basin lying above to detach and slide down the hyper-extended margins onto a central exhumed mantle tract. Since the Early Cretaceous (and locally probably since the Jurassic) a system of low-amplitude salt walls evolved in shallow marine environments punctuated by episodic emersion. During the main stage of crustal extension, in late Aptian to early Cenomanian times, carbonate shelves rapidly drowned giving rise to deeper marine sedimentation. This was a period of major rise of salt walls, progressively detached from their substratum. These salt walls enclosed minibasins that accumulated thick flysch deposits arranged in growth stratal patterns. Depocenter migration and foundering of previous diapiric highs controlled further flysch deposition during the Late Cretaceous, while moderate extension probably persisted until the onset of the Pyrenean compression. During the late Santonian to Paleogene Pyrenean orogeny, the sedimentary lid of the Chaînons Béarnais basin climbed back along the Triassic detachment onto the colliding continental margins, leading to salt wall squeezing and further rising. Based on the Cretaceous timing and style of growth folding, we suggest that salt wall squeezing was not solely related to the Pyrenean compression, but shortening affected the diapiric ridges during the syn-rift sliding by gravity and crowding in the basin center, as the rifted margins were pulled apart from beneath. This makes the Chaînons Béarnais belt a unique field analog for contractional salt-tectonic systems in distal continental margins.

RESERVOIR PROPERTIES OF BARREMIAN – APTIAN URGONIAN LIMESTONES, SE FRANCE, PART 1: INFLUENCE OF STRUCTURAL HISTORY ON POROSITY‐PERMEABILITY VARIATIONS

Upper Barremian – Lower Aptian inner platform “Urgonian” limestones in the Mont de Vaucluse region, SE France, consist of alternating metre‐scale microporous and tight intervals. This paper focuses on the influence of structural deformation on the reservoir properties of the Urgonian limestone succession in a study area near the town of Rustrel. Petrographic, petrophysical and structural data were recovered from five fully‐cored boreholes, from the walls of a 100 m long underground tunnel, and from a 50 m long transect at a nearby outcrop. The data allowed reservoir property variations in the Urgonian limestones to be studied from core to reservoir scale.Eleven Reservoir Rock Types (RRTs) were identified based on petrographic features (texture, grain size), reservoir properties (porosity, permeability), and the frequency of structural discontinuities such as fractures, faults and stylolites. Tight and microporous reservoir rock types were distinguished. Tight reservoir rock types were characterised by early cementation of intergranular pore spaces and by the presence of frequent structural discontinuities. By contrast microporous reservoir rock types contained preserved intragranular microporosity and matrix permeability, but had very few structural discontinuities. Observed vertical alternations of microporous and tight rock types are interpreted to have been controlled by the early diagenesis of the Urgonian carbonates.Deformation associated with regional‐scale tectonic phases, including Albian – Cenomanian “Durancian” uplift (∼105 to 96 Ma) and Pyrenean compression (∼55 to 25 Ma), resulted in the modification of the initial petrophysical properties of the Urgonian limestones. An early diagenetic imprint conditioned both the intensity of structural deformations and the associated circulations of diagenetic and meteoric fluids. Evolution of the Reservoir Rock Types is therefore linked both to the depositional conditions and to subsequent phases of structural deformation.

Detrital zircon U–Pb insights into the timing and provenance of the South Pyrenean Jaca basin

We investigate how sedimentary systems respond to tectonic uplift during the last stages of infilling of a foreland basin. Massive conglomeratic deposits characterize the last stage of basin fill in the Jaca basin, the western portion of the South Pyrenean foreland. We conduct detrital zircon U–Pb geochronology in a 3000 m thick Cenozoic section containing the transition from deltaic to alluvial environments with the aim of reconstructing changes of provenance in this key stage of evolution of the basin. The last conglomeratic deposits of the basin belong to the San Juan de la Peña alluvial fan, which has previously been assigned an early Oligocene age. Our results show the presence of earliest Miocene detrital zircons at the base of the fan. Zircon double-dating reveals identical U–Pb and (U–Th)/He ages, which is indicative of a volcanic origin for these zircons. We therefore conclude that this fan was deposited during early Miocene times. Our data establish a new chronological framework for deformation and sedimentation in the South Pyrenean foreland, with strong implications for the dynamics and palaeogeography governing the last episodes of basin fill. Our results support synchronous sedimentation in the Jaca piggy-back basin and in the autochthonous Ebro basin during the last stages of Pyrenean compression in the Miocene. This work emphasizes the tectonic controls on the configuration of the drainage basin and gives insights into the evolution of the drainage network in internally drained basins. Supplementary material: S1: Extended Zircon U-Pb LA-ICP-MS methodology. S2: Reduced detrital zircon U-Pb dataset for all analysis. S3: All age spectra. S4: zHe data. S5: Sample location for the analyzed samples are available at: https://doi.org/10.6084/m9.figshare.c.4507073

Diagenesis in Mesozoic carbonate rocks in the North Pyrénées (France) from mineralogy and fluid inclusion analysis: Example of Rousse reservoir and caprock

Fluid-rock interaction is of key interest for deciphering the geodynamic history of complex geological environments. In this paper, we describe the mineralogy and petrography of the Rousse reservoir (the Upper Jurassic “Dolomie de Mano” formation) as well as the reconstruction of its thermal and pressure history. We also investigate the base of the reservoir caprock (the Campanian breccia). A satellite of the giant gas field of Lacq in the south-west of France, Rousse was a gas producer until it moves to a CO2 storage site in 2010. Rousse is located within a deep, isolated, faulted, carbonate Jurassic horst overlain by a 4500 m thick overburden that is composed of a series of turbiditic flysch deposits of Upper Cretaceous (Cenomanian) to Tertiary (Eocene) age. Cores were sampled from two wells, Rousse 1 and 2, and were analyzed by ICP-MS, XRD, SEM, EPMA, TEM, cathodoluminescence and fluid inclusion techniques. Pressure-Temperature (P-T) conditions were derived by modeling data from fluid inclusions trapped in the fracture minerals of the “Dolomie de Mano” formation and the Campanian breccia. In the “Dolomie de Mano”, diagenesis was dominated by three dolomite episodes followed by a calcite stage in fractures. The second and third dolomitizations were associated with fracturing episodes. These dolomites precipitated from low-saline (<1 mol·kg−1 eq. NaCl) hydrothermal fluids at temperatures around 150 °C. These two characteristics exclude dolomitization during a synsedimentary slope brecciation event and also the role of hypothetical secondary brines. In the Campanian breccia, dolomitization is marginal and the main precipitation is calcite. In association, two types of fluid inclusion (liquid water-vapor and vapor) can be distinguished in the two formations and are marked by predominant CH4 and variable CO2 contents. The presence of gas inclusions in the reservoir as in the caprock is indicative of several gas invasions. A P-T modeling of the geologic evolution of the area reveals two periods of fluid circulation: a first stage (150–170 °C, 20 MPa) during the Late Jurassic to Early Cretaceous for the reservoir rock, corresponding to the rifting period of the North-Pyrénées western zone; and a second stage (170 °C, 41 MPa) during the late Eocene period in breccias, corresponding to the Pyrenean compression period. The petrophysical properties of the reservoir, marked by an intense fracture network, were acquired early during an extensional period, whereas the impact of fluids associated with a period of compression appears to have been limited in space to drainage of tectonic origin. Finally, we note significant similarities between the petrographic and fluid signatures of the Chaînons béarnais, in the folded orogenic wedge located south of the North Pyrenean Frontal Thrust, and Rousse, located in the foreland basin to the North. Nevertheless, the temperature recorded in the Chaînons béarnais reached 300–350 °C, much higher than in Rousse due to its position in the vicinity of an over-thinned palaeomargin during the early Cretaceous.

Distinguishing the effect of diapir growth on magnetic fabrics of syn‐diapiric overburden rocks: Basque–Cantabrian basin, Northern Spain

AbstractAn analysis of Anisotropy of Magnetic Susceptibility was done on Aptian–Albian sediments from the Basque–Cantabrian basin. Samples were collected from 39 sites in the halokinetic sequences of the Bakio, Bermeo, Guernica and Mungia diapirs; 28 sites were sampled close to diapirs, and 11 sites were far from the diapir edges. The magnetic foliation is parallel to bedding, suggesting it reflects depositional and compaction processes, whereas the orientation of magnetic lineation varies. Far from the diapir edges, the magnetic lineation is interpreted as being related to the regional Pyrenean compression. Close to diapir edges, the observed behaviour shows that diapirs, predominantly formed by rigid ophites, have acted as buttresses, with shadow areas at their northern faces being protected from the Pyrenean compression. The high sensitivity of AMS makes it a very useful tool to distinguish deformation in halokinetic sequences related to diapir growth from that related to subsequent compression.

Interactions between tectonics and fluid circulations in an inverted hyper-extended basin: Example of mesozoic carbonate rocks of the western North Pyrenean Zone (Chaînons Béarnais, France)

Interactions between fracturing, fluid circulations and fluid chemistry in hyper-extended margins are still poorly described as most of them are located offshore, buried underneath post-rift sediments. The southern Aquitaine basin and the northern Pyrenees constitute an appropriate case study to investigate these interactions since a model of hyper extended margin with mantle exhumation during the Lower Cretaceous subsequently inverted was recently proposed. From a field study, we here describe three main sets of fractures (set 1 to set 3). They are correlated with main stages of the geodynamic evolution of the basin corresponding to the Liassic rifting, the Aptian-Cenomanian hyper-extension, and the Pyrenean compression. Petrographic observations, Raman and micro-thermometry analysis on fluid inclusions, ICP-MS, and isotope analysis permitted to determine chemistries, temperatures, redox conditions, gas compositions, oxygen and carbon isotopic signatures, and REE contents of parent fluids for cements precipitated during each episode. In particular saddle dolomite and chlorite precipitated in set 2 fractures during the hyper-extension corresponding to the thermal peak at temperatures higher than 300 °C. The isotopic signature, the high CO2 content, the occurrence of H2S and the high salinity of parent fluids suggest ascending magmatic fluids percolating across Triassic evaporites. The late and post hyper-extensional phase is characterized by hydraulic brecciation in dolomitic formations, a decrease in temperature and salinity, a decrease in magmatic contribution in parent fluids, a closing of the diagenetic system during burial and a switch to reducing conditions during the precipitation of quartz, pyrite and calcite. The Pyrenean compressive phase associated with the third fracturing stage induced a reopening of the diagenetic system and favored a return to oxidizing conditions and infiltrations of meteoric fluids.

Multi-episodic remagnetization related to deformation in the Pyrenean Internal Sierras

The Internal Sierras (IS) in the southern margin of the Western and Central Axial Zone (Southern Pyrenees) are affected by a syn-orogenic remagnetization that provides information to reconstruct deformation geometries at the time of acquisition of magnetization. Furthermore, the IS structure changes strike along its structural trend, from ∼N120 to 130°E in the western and eastern margins to ∼N070–090°E in the central part. Palaeomagnetic techniques have been used to (i) accurately define the timing of remagnetization with regard to deformation and (ii) determine if the along-strike trend variation in the IS was induced by deformation and thrust emplacement during the Pyrenean compression or, on the contrary, was the result of a primary orientation controlled by structures inherited from pre-orogenic times. From 23 new palaeomagnetic sites, collected in Upper Cretaceous marls and marly limestones, two meaningful and stable palaeomagnetic components were resolved, principally carried by magnetite: (1) a lower-temperature component (B) that unblocks between 200 °C and 325–400 °C and (2) a higher-temperature component (C) that has been successfully isolated by means of combined thermal (up to 400 °C) and AF demagnetization (generally up to 50–100 mT). The B component is a late remagnetization that post-dates folding and emplacement of basement thrust sheets in the IS (mainly the Gavarnie thrust). It supports small but statistically significant clockwise rotations in the western part of the IS (from +18 to +26°). These rotations can be attributed to the westwards shortening decrease in the thrust system below the Gavarnie unit that results from its along-strike structural change, with a higher number of basement thrusts to the east. The C component has been interpreted as an early remagnetization, based on the results of conglomerate and fold tests. This component predates basement thrusting and is diachronous across the study area: reverse and normal polarities dominate in the eastern and western margins of the IS, respectively. New and previous palaeomagnetic data point out that curvature in the IS is probably a primary feature and the along-strike change in their trend could be interpreted as the result of basement geometrical features inherited from Variscan, Late Variscan or Mesozoic times. A complex, multi-episodic remagnetization probably related to burial and deformation processes occurred during Eocene times.

Fluid emissions at the Aquitaine Shelf (Bay of Biscay, France): A biogenic origin or the expression of hydrocarbon leakage?

Fishery acoustic surveys conducted in the Bay of Biscay and dedicated to monitoring and predicting pelagic ecosystem evolution reveal numerous active seeps on the Aquitaine Shelf, east of the shelf break, at water depths ranging from 140 to 185m. Some acoustic anomalies recorded in the water column with hull-mounted single and multibeam echosounders are clearly caused by fluid escape at the seabed, most likely gases. These fluid emissions are associated at the seafloor with high backscatter subcircular small-scale mounds, on average less than 2m high and a few metres in diameter. Based on near-bottom video and acoustic surveys, these mounds are interpreted to be by-products of gas seepage, possibly methane-derived authigenic carbonates. The spatial distribution of the seeps and related structures, based on water column acoustic gas flares and high backscatter seabed patches, appears to be relatively broad, with a North–South extension of ~65km across the Parentis Basin and the Landes High, and a West–East extension along a few kilometres wide on the shelf. The seepage activity seems persistent through time at the annual scale, with acoustic evidence dating back to 1998. The spatial distribution of the fluid emissions at the Aquitaine Shelf may suggest possible sedimentary and tectonic controls in relation with the Pyrenean compression phase. The nature and the origin of the emitted fluids and seafloor mounds are unknown. The gases may correspond to biogenic methane from Late Pleistocene deposits or to thermogenic gases originating from deeper, Jurassic–Cretaceous levels. The oil province of the Parentis Basin raises questions regarding possible genetic links to the petroleum system.

Style of Alpine tectonic deformation in the Castellane fold-and-thrust belt (SW Alps, France): Insights from balanced cross-sections

This study proposes a reappraisal of the role of the basement tectonics in the structuration of the Alpine foreland, across the Castellane fold-and-thrust belt located in southwestern Alps. We construct three 30km length N–S balanced cross-sections across the entire fold-and-thrust belt, in order to quantify the amount of horizontal shortening due to the Pyrenean and Alpine deformations. We then assess the role of the basement inherited structures during the compressional phases which resulted in the exhumation of the Argentera–Mercantour External Crystalline Massif and the Barrot Dome. The construction of these balanced cross-sections suggests a dominant thick-skinned deformation style, which includes the reactivation of inherited Permian and Early Cretaceous basement normal faults. We reconstructed three restoration stages: the oldest one highlights the geometry of Permian and Early Cretaceous extensional structures; the intermediate one after the Late Cretaceous evidences a first compressional episode linked to the Pyrenean compression; and the latest one in the Oligocene shows the first Alpine folds resulting from the southward thrusting of the internal units over the external ones along of the Penninic Frontal Thrust. Balanced cross-sections suggest very moderate crystalline basement–sedimentary cover decoupling. On the western and central cross-sections, the estimated amount of shortening ranges from 9.5 to 10km (21%) whereas on the easternmost one shows ~5km of shortening (9%). These shortening values are consistent with previously published estimates in the surrounding foreland subalpine chains. They highlight a decreasing value of Pyrenean shortening toward the east, while the Alpine shortening dominates and amplifies this first phase in a similar direction. We interpret this dominantly thick-skinned structural style as a possible consequence of the Neogene thermal weakening in the European passive margin above the Ligurian slab rollback.

Tectonic versus diagenetic origin of fractures in a naturally fractured carbonate reservoir analog (Nerthe anticline, southeastern France)

Field analogs allow a better characterization of fracture networks to constrain naturally fractured reservoir models. In analogs, the origin, nature, geometry, and other attributes of fracture networks can be determined and can be related to the reservoir through the geodynamic history. In this article, we aim to determine the sedimentary and diagenetic controls on fracture patterns and the genetic correlation of fracture and diagenesis with tectonic and burial history. We targeted two outcrops of Barremian carbonates located on both limbs of the Nerthe anticline (southeastern France). We analyzed fracture patterns and rock facies as well as the tectonic, diagenetic, and burial history of both sites. Fracture patterns are determined from geometric, kinematic, and diagenetic criteria based on field and lab measurements. Fracture sequences are defined based on crosscutting and abutting relationships and compared with geodynamic history and subsidence curves. This analysis shows that fractures are organized in two close-to-perpendicular joint sets (i.e., mode I). Fracture average spacing is 50 cm (20 in.). Fracture size neither depends on fracture orientation nor is controlled by bed thickness. Neither mechanical stratigraphy nor fracture stratigraphy is observed at outcrop scale. Comparing fracture sequences and subsidence curves shows that fractures existed prior to folding and formed during early burial. Consequently, the Nerthe fold induced by the Pyrenean compression did not result in any new fracture initiation on the limbs of this fold. We assume that the studied Urgonian carbonates underwent early diagenesis, which conferred early brittle properties to the host rock.

Evolution of salt structures during extension and inversion of the Offshore Parentis Basin (Eastern Bay of Biscay)

Abstract The Late Jurassic–Cretaceous Parentis Basin (Eastern Bay of Biscay) illustrates a complex geological interplay between crustal tectonics and salt tectonics. Salt structures are mainly near the edges of the basin, where Jurassic–Lower Cretaceous overburden is thinner than in the basin centre and allowed salt anticlines and diapirs to form. Salt diapirs and walls began to rise reactively during the Late Jurassic as the North Atlantic Ocean and the Bay of Biscay opened. Some salt-cored drape folds formed above basement faults from the Upper Jurassic to Albian. During Albian–Late Cretaceous times, passive salt diapirs rose in chains of massive salt walls. Many salt diapirs stopped growing in the Mid-Cretaceous when their source layer depleted. During the Pyrenean orogeny (Late Cretaceous–Cenozoic), the basin was mildly shortened. Salt structures absorbed almost all the shortening and were rejuvenated to form squeezed diapirs, salt glaciers and probably subvertical welds, some of which were later reactivated as reverse faults. No new diapirs formed during the Pyrenean compression, and salt tectonics ended with the close of the Pyrenean orogeny in the Middle Miocene. Using reprocessed industrial seismic surveys, we document how salt tectonics affected the structural evolution of this offshore basin largely unknown to the international audience.

Anisotropic magnetic susceptibility record of the kinematics of the Boltaña Anticline (Southern Pyrenees)

AbstractAnisotropic magnetic susceptibility (AMS) analysis of 1454 samples evenly distributed along an 1800 m section in the South Pyrenean Zone is presented. The sampled rocks are pre‐ and syn‐tectonic limestones, marls and sandstones of Ypresian‐Lutetian age. They are weakly deformed rocks sampled from both limbs of the N–S trending Boltaña Anticline (oblique to the WNW–ESE main Pyrenean trend). Rock magnetism analyses, including low temperature and high field magnetic susceptibility measurements confirm a dominant paramagnetic signal and hence allow the interpretation of AMS in terms of preferred rock fabric or finite strain orientation. Therefore, this study was focused on obtaining an independent kinematic information from a vast set of AMS data from particular structural positions. The three directional maxima for the maximum axis of the magnetic ellipsoid (KMAX, magnetic lineation) can be tentatively related to major tectonic events in the Central Pyrenees. A first stage (recording a N–S magnetic lineation) is related to an oblique ramp of the Cotiella Thrust, emplaced during Ypresian times. A pseudo‐progressive variation turns into a second stage (linked to a NW–SE KMAX orientation maximum) that can be related to Pyrenean compression. Both stages witness a far‐field imprint of active structures on the foreland sediments. Once the Boltaña Anticline was nucleated (during the Lutetian), a predominant, more constant NW–SE trend is observed, this near‐field magnetic fabric being the only lineation imprinted in the rocks. A final stage arrived in the course of vertical axis rotation during the Late Lutetian‐Bartonian. This latest event was responsible for the present configuration of the Boltaña Anticline (N–S). Copyright © 2010 John Wiley &amp; Sons, Ltd.

Timing, controls and consequences of compression in the Rockall-Faroe area of the NE Atlantic Margin

Abstract The simplest models of passive margins would suggest that they are characterized by tectonic quiescence as they experienced gentle thermal subsidence following the extensional events that originally formed them. Analysis of newly acquired and pre-existing 2D seismic data from the Rockall Plateau to the Faroe Shelf, however, has confirmed that the NE Atlantic Margin was the site of significant active deformation. Seismic data have revealed the presence of numerous compression-related Cenozoic folds, such as the Hatton Bank, Alpin, Ymir Ridge and Wyville–Thomson Ridge Anticlines. The distribution, timing of formation and nature of these structures have provided new insights into the controls and effects of contractional deformation in the region. Growth of these compressional features occurred in five main phases: Thanetian, late Ypresian, late Lutetian, Late Eocene (C30) and Early Oligocene. Compression has been linked to hotspot-influenced ridge push, far-field Alpine and Pyrenean compression, asthenospheric upwelling and associated depth-dependent stretching. Regional studies make it clear that compression can have a profound effect on seabed bathymetry and consequent bottom-water current activity. Bottom-water currents have directly formed the early Late Oligocene, late Early Miocene (C20), Late Miocene–Early Pliocene, and late Early Pliocene (C10) unconformities. The present-day Norwegian Sea Overflow (NSO) from the Faroe–Shetland Channel into the Rockall Trough is restricted by the Wyville–Ymir Ridge Complex, and takes place via the syncline (Auðhumla Basin) between the two ridges. The Auðhumla Basin Syncline is now thought to have controlled the path of the NSO into the Rockall Trough and the resulting unconformity formation and sedimentation therein, no later than the Mid Miocene.