crustal structure and orogenic material budget in the west central pyrenees surface and subsurface data are combined to construct a crustal scale cross section of the western central pyrenees france and spain at the boundary between the european and iberian plates the position of moho reflections in the ecors arzacq reflection profile suggests a tectonic wedge of european crust and upper mantle had indented the iberian plate at lower crustal levels the european wedge is overlain by an upper iberian wedge thus constituting a double stacked wedge geometry the upper wedge was delaminated and deformed giving rise to the pyrenean orogenic prism manifested as a bivergent fan in upper crustal levels the underthrust lower iberian plate has been imaged to depths of 55 60 km but crustal budget considerations based on a palinspastic reconstruction require that this continental root subducted to depths up to 90 km total orogenic contraction calculated from surface structures is about 75 80 km which was accomplished between the latest cretaceous and the early miocene at an averaged rate of 1 2 mm yr consistent with these moderate values exhumation of the orogen is much less than in the more shortened eastern parts of the range paleozoic basement and preorogenic mesozoic rocks are little eroded and much of the exhumation involved cannibalization of early foreland basins which once covered the entire poorly emergent orogen at this transect this resulted in a continuous process of sediment recycling and coupled with a considerable lateral arrival of material a bulk negative erosion sedimentation budget

formation and deformation of hyperextended rift systems insights from rift domain mapping in the bay of biscay pyrenees the bay of biscay and the pyrenees correspond to a lower cretaceous rift system including both oceanic and hyperextended rift domains the transition from preserved oceanic and rift domains in the west to their complete inversion in the east enables us to study the progressive reactivation of a hyperextended rift system we use seismic interpretation gravity inversion and field mapping to identify and map former rift domains and their subsequent reactivation we propose a new map and sections across the system illustrating the progressive integration of the rift domains into the orogen this study aims to provide insights on the formation of hyperextended rift systems and discuss their role during reactivation two spatially and temporally distinct rift systems can be distinguished the bay of biscay parentis and the pyrenean basque cantabrian rifts while the offshore bay of biscay represent a former mature oceanic domain the fossil remnants of hyperextended domains preserved onshore in the pyrenean cantabrian orogen record distributed extensional deformation partitioned between strongly segmented rift basins reactivation initiated in the exhumed mantle domain before it affected the hyperthinned domain both domains accommodated most of the shortening the final architecture of the orogen is acquired once the conjugate necking domains became involved in collisional processes the complex 3 d architecture of the initial rift system may partly explain the heterogeneous reactivation of the overall system these results have important implications for the formation and reactivation of hyperextended rift systems and for the restoration of the bay of biscay and pyrenean domains

anisotropy beneath the pyrenees range from teleseismic shear wave splitting results from a test experiment we have investigated the seismic anisotropy from sks and skks wave splitting along a n s profile perpendicular to the pyrenean mountain belt the line crossed the north pyrenean fault npf which probably represents the plate boundary between the european and the iberian plates we observed large delay times t 1 3 to 1 5 s and uniform fast polarization directions n100e at three stations located on the iberian plate on the other hand t are smaller 0 5 1 0 s and the scattered at stations located north of the npf the small scale variability the relation with surface geology and the correlation of the t with lithospheric thickness suggest that the anisotropy could be predominantly of lithospheric origin however an asthenospheric contribution cannot be ruled out in the north pyrenean zone an important thermal event before the pyrenean compression has likely erased the former mantle fabric the observed anisotropy is then likely related to the pyrenean orogeny on the iberian plate our splitting parameters are similar to those obtained at two stations located on hercynian structures in central spain this suggests that two contributions are likely superposed to that of the pyrenean built up one related to the hercynian orogeny and the other one related to the rotation of the iberian plate

a detailed analysis of the february 1996 aftershock sequence in the eastern pyrenees france summary following the 1996 february 18 m l 5 2 earthquake in the agly massif in the eastern french pyrenees we installed a temporary network of seismometers around the epicentre in this paper we analyse 336 well located aftershocks recorded from february 19 to february 23 by 18 temporary stations and two permanent stations located less than 35 km from the epicentre most aftershocks have been located with an accuracy better than 1 5 km in both horizontal and vertical positions their spatial distribution suggests the reactivation of a known fault system we determined 39 faultplane solutions using p wave first motions despite their diversity the focal mechanisms yield an e w subhorizontal t axis we also determined fault plane solutions and principal stress axes using the method developed by rivera cisternas 1990 for the 15 best recorded events we obtain a pure shear rupture tectonic regime under n s subhorizontal compression and e w subhorizontal extension these principal stress axes which explain the focal mechanisms for at least 75 per cent of the 39 aftershocks are diverent from the axes deduced from the main shock the post earthquake stress field caused by the main shock rupture modelled as sinistral strike slip on three vertical fault segments is computed for various orientations and magnitudes of the regional stress field assumed to be horizontal the aftershock distribution is best explained for a compressive stress field oriented n30 e most aftershocks concentrate where the coulomb failure stress change increases by more than 0 2 mpa the diversity of aftershock focal mechanisms poorly explained by this model may reflect the great diversity in the orientations of pre existing fractures in the agly massif

thermal imprint of rift related processes in orogens as recorded in the pyrenees 19 the extent to which heat recorded in orogens reflects thermal conditions inherited from 20 previous rift related processes is still debated and poorly documented as a case study we 21 examine the mauleon basin in the north western pyrenees that experienced both extreme 22 crustal thinning and tectonic inversion within a period of 30 myrs to constrain the time 23 temperature history of the basin in such a scenario we provide new detrital zircon fission 24 track and u th sm he thermochronology data the role of rift related processes in 25 subsequent collision is captured by inverse modeling of our thermochronological data using 26 relationships between zircon u th sm he ages and uranium content combined with 27 thermo kinematic models of a rift orogen cycle we show that the basin recorded significant 28 heating at about 100 ma characterized by high geothermal gradients 80 c km our 29 thermo kinematic modeling and geological constraints support the view that subcontinental 30 lithospheric mantle was exhumed at that time below the mauleon basin such a high 31 geothermal gradient lasted 30 myr after onset of convergence at 83 ma and was relaxed 32 during the collision phase from 50 ma this study suggests that heat needed for ductile 33 shortening during convergence is primarily inherited from extension rather than being only 34 related to tectonic and or sedimentary burial this should have strong implications on tectonic 35 reconstructions in many collision belts that resulted from inversion of hyper extended rift 36 basins

interaction between prerift salt and detachment faulting in hyperextended rift systems the example of the parentis and mauleon basins bay of biscay and western pyrenees prerift salt layers associated with extensional detachment faults exhuming mantle and deeper crustal rocks at the sea floor are observed in the parentis and arzacq mauleon basins located at the eastern termination of the bay of biscay how detachment faults interact with salt in hyperextended rift systems is yet little understood based on field observations and drill hole and seismic data we propose a new model to explain the interaction between salt tectonics and extensional detachment systems we demonstrate that the presence of a thick prerift salt layer in an area undergoing extreme crustal thinning can control the geometry and evolution of rift systems and obscure the rift related structures in the underlying basement during an initial stage of rifting prerift salt layers act as a decoupling horizon between sub and suprasalt units and hinder crustal detachment faults to cut through the salt layers and form breakaways at the sea floor as a consequence they sole out along the ductile salt layer and no subsalt material can be exhumed to the sea floor thus sub and suprasalt layers deform by different deformation modes which makes that detachment fault difficult to identify on seismic images in a later stage when salt has migrated and thinned out sub and suprasalt layers can locally couple and detachment faults can be exposed at the surface resulting in windows of exhumed basement surrounded by extensional allochthons formed by suprasalt sedimentary units

a balanced section across the pyrenean orogenic belt evolutionary models that have been proposed for the pyrenean orogenic belt involve gravitational gliding of major thrust sheets to both the north and south of an uplifted central axial zone it has been suggested by previous workers that uplift is a result of alpine compression or long lived strike slip movement on the north pyrenean fault we critically assess these models and propose a new model based on the generation of a thin skinned linked thrust system due to alpine collision of iberia and europe pyrenean alpine tectonics is dominated by southward thrusting on a major sole fault dipping north at approximately 6 within hercynian basement northerly directed thrusts of the north pyrenean zone and northern folded foreland are backthrusts from this sole fault a total orogenic shortening of 106 km in the central pyrenees was achieved between the palaeocene and oligocene at a time averaged rate of 0 62 cm yr 1 a thin skinned model requires that the north pyrenean fault is truncated by both northward and southward moving thrusts and occurs in the footwall of the sole fault some 60km north of its present surface trace

tectonosedimentary evolution related to extreme crustal thinning ahead of a propagating ocean example of the western pyrenees 1 in this paper we describe the tectonosedimentary evolution and its subsequent inversion of a basin that underwent extreme crustal thinning in a transtensional setting ahead of a propagating ocean in the western pyrenees the labourd mauleon area situated in the western pyrenees at the termination of the v shaped bay of biscay is an ideal natural laboratory to study how such complex basins evolve in time and space because of a mild inversion of the basin during pyrenean compression the rift structures and their relations to basement rocks and sediments are exposed and can be directly studied in the field the basin shows a complex polyphase evolution that starts with left lateral dominated transtension in latest jurassic early aptian time this event is overprinted by a late aptian early albian extension that is related to the counterclockwise rotation of iberia away from europe leading to the opening of the bay of biscay during this stage the late triassic to jurassic carbonate platform was stretched salt migrated and detachment faults exhumed upper and lower crustal and mantle rocks to the seafloor the final structure of the basin resembles a sag basin floored by exhumed rocks overlain by extensional allochthons and compartmentalized by n40 to n60 transfer faults the sedimentary architecture is characterized by late aptian synrift sediments e g urgonian limestones that were deposited in fault bounded basins and are overlain by thick latest aptian to albo cenomanian sediments e g flysch noir that define a sag sequence the complex tectonosedimentary evolution of the basin is associated with salt tectonics and overprinted by a major magmatic thermal event that postdates mantle exhumation

seismicity induced by gas production ii lithology correlated events induced stresses and deformation a 3d relocation technique permits precise locations of induced earthquakes geostatistical processing using the data of 87 boreholes provides the basis of a precise 3d structure with a dome geometry conventional laboratory mechanical tests performed on deep rock samples 1000 m to 5000 m define the rock properties at depths similar to those of the seismic events 1 ml 4 2 that range from 1 to 7 km

absolute earthquake locations using 3 d versus 1 d velocity models below a local seismic network example from the pyrenees this study was initiated during an ater attach e temporaire d enseignement et de recherche position funded by the university paul sabatier iii toulouse france at the institut de recherche en astrophysique et plan etologie irap umr5277 part of the observatoire midi pyr en ees omp this work was funded by edf electricit e de france under the sigma seismic ground motion assessment project this work was included in the work package 1 wp1 the seismic source in hazard models we acknowledge kevin manchuel leader of wp1 we also acknowledge people involved in the topo iberia and pyrope projects for providing us with the high quality data from these temporary seismic deployments we want also to acknowledge the institutions responsible for the maintenance of the different permanent seismic networks contributing data to this study instituto geogr afico nacional of spain ign catalonian geological institute igc commissariat a l energie atomique cea french broad band network rlbp and rap french strong motion network provided by resif r eseau sismologique et g eod esique franc ais seismic network doi 10 15778 resif fr resif is a national research infrastructure recognised as such by the french ministry of higher education and research resif is managed by the resif consortium composed of 18 research institutions and universities in france resif additionally supported by a public grant overseen by the french national research agency anr as part of the investissements d avenir program reference anr 11 eqpx 0040 and the french ministry of ecology sustainable development and energy we acknowledge thomas bardainne cgg microseismic and mario ruiz fernandez institute of earth sciences jaume almera of the spanish scientific research council ictja csic for providing us access to traveltime picks and velocity models from temporary seismic experiments we acknowledge robert sarda imerys for giving information about blasts at the luzenac quarry thanks to leo zijerveld for his suggestions and help with the english authors thank prof egill hauksson editor and two anonymous reviewers for their constructive reviews and careful handling of the manuscript

seismostructural studies in the pyrenees evolution and recent results seismic studies of the last ten years in the pyrenees deep seismic profiles fan profiles at critical distance teleseismic travel time residuals seismicity from temporary networks and their most significant results concerning crustal thickness in the different structural units of the range sharpness of the transition between these units at depth and east west lateral evolution are reviewed in this paper focal mechanisms for three recent earthquakes ofm 4 are given and connections of local seismicity with major tectonic structures such as the north pyrenean fault and its western prolongation at depth are described

seismicity induced by gas production i correlation of focal mechanisms and dome structure from august 1976 to june 1979 more than 220 seismic events were recorded by a 7 station network operating over an active gas field in france 120 earthquakes with magnitudes greater than 1 0 were selected for a detailed study in order to understand the seismic response to a decrease in the gas reservoir pressure of 50 mpa composite focal mechanisms were determined for these events we found that where preexisting fractures were known from well logs faults with the most favorable orientation were reactivated in the part of the gas field where no preexisting faults were known orientation of the fracture planes was close to the orientations of stratigraphic interfaces our kinematic model constrained by the distribution of the earthquakes and their focal mechanisms consists of the downward dropping of a block located close to the top of the gas field the main faults as defined by the spatial density of the seismic activity define the limits of this block the displacements observed along these faults can be either normal or reverse mechanism dependent on the preexisting orientation of each fault another important result is the evidence that the depletion of the gas reservoir exerts the major influence on local seismic displacements obscuring the effect of regional setting

the pyrenean architecture as revealed by teleseismic p to s converted waves recorded along two dense transects 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 our migration 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

recent activity and seismotectonics of the eastern pyrenees results from a recent earthquake in the eastern pyrenees are presented and the seismotectonics of the region is analyzed from the presently available data on 26 september 1984 an earthquake ml 4 4 took place in the area of the historical destructive earthquake of 1428 several portable stations installed in the epicentral area to record aftershocks permitted of defining a precise location at 42 19 2 n 2 10 2 e and 5 km depth a maximum felt intensity of v msk is obtained from macroseismic data the epicentral location lies within a block bounded by e w trending structures and the focal solution shows right lateral shearing with a nw se pressure axis the seismicity in the eastern pyrenees shows a complex pattern which can be associated with both e w fractures and ne sw fault systems focal solutions of another two recent earthquakes of ml 4 with differences in horizontal pressure axis are also discussed

gravity interpretation of a unified 2 d acoustic image of the central alpine collision zone summary four individual deep seismic reflection profiles across the eastern and southern swiss alps have been combined along the trend of the bouguer gravity anomalies onto the european geotraverse the profiles were migrated ray theoretically with the migration velocity model derived from a smoothed reinterpretation of the seismic wide angle profiles running parallel to the strike of the alpine arc the contours of this velocity model were ray theoretically depth migrated together with the combined digitized line drawings of the seismic reflection profiles the resulting acoustic image depicts the subduction of parts of the lower crust of the european plate beneath the adriatic promontory of the african plate at a low angle of 15 based on the interpretation of the seismic data orogenic crustal thickening is attributed to the stacking of crystalline nappes onto the upper crust of the european plate and wedging of the european and adriatic middle and lower crusts the latter being ill constrained by the seismic evidence alone the south vergent thrusting of the southern alps can be accounted for by the observed downbending of the adriatic moho and the lower crust in conjunction with the inferred wedging at mid crustal levels using the geometric constraints provided by seismic data gravity modelling of the alpine lithosphere asthenosphere system relative to stable central european platform clearly favours a gently inclined subduction zone reaching down to at least 200 km depth such a gently dipping subduction zone is at odds with prominent models on alpine geodynamics which favour a near vertical orientation of the subducting lithosphere verschluckung in agreement with the wedging hypothesis indirectly inferred from the seismic data the short wavelength part of the alpine gravity anomaly requires a middle crust of anomalously high density in the axial zone of the orogen both seismic and gravimetric evidence therefore suggest that late orogenic lithospheric shortening and crustal thickening was governed by the mechanical decoupling of the upper middle and lower parts of the crust the amount of subducted lower crustal material cannot be constrained by gravity modelling since its gravity effect cannot be separated from the effect of the subducting slab

seismic images and evolution of the iberian crust in the pyrenees abstract crustal images of the pyrenean range based on coincident wide angle and vertical deep seismic profiles are presented in this study wide angle experiments in the thickened iberian crust along strike lines have provided no evidence for reflections from interfaces between layers of different velocities in the middle or lower crust in contrast reversed sounding on a line across the pyrenean range shows wide angle reflectivity its unusual reflectivity pattern points to widespread impedance contrasts of localized continuity embedded in the crust this observation is consistent with a new image of vertical incidence reflectivity obtained by coherency weighted migration of the coincident ecors traverse this profile shows numerous discontinuous and dipping reflections from the middle to the base of the crust the reflections dip towards the axis of the range the upper limit and the attitude of the reflective zones are so strongly variable that the crust can hardly be envisaged as the simple continuous three layered model previously used for geodynamical reconstructions the reflective zones cannot be brought back to the horizontal image of a layered lower crust of proposed extensional origin by plate unbending to the pre alpine stage the dipping reflections may result from compressional deformation as the south pyrenean deep crustal domain has undergone both hercynian and alpine orogenic deformation if part of the present reflectivity pattern was due to alpine remobilization the corresponding lower crustal deformation would have spread out thereby accomodating convergence by thickening even outside the axial zone where the upper crustal deformation appears to be concentrated

towards a new concept of the tectogenesis of the pyrenees abstract crustal structures of the central and eastern pyrenees are compared with those of the alps and carpathians by analogy northward subduction underthrusting in the central and eastern pyrenees is proposed southward subduction underthrusting is found along the north iberian margin and western pyrenees hence a flip a change in the direction of subduction underthrusting is postulated between the central and western pyrenees

gravity constraints on the deep structure of the pyrenean belt along the ecors profile abstract in order to furnish further constraints to modelling the ecors pyrenees deep seismic profile gravity data were collected along a stripe 20 km wide and 220 km long containing the seismic line a total of 4020 stations have been used to derive a bouguer anomaly map of the area a large and roughly symmetrical negative anomaly 100 mgal is centered on the axial zone a local positive anomaly over the north pyrenean zone disturbs the negative regional trend somewhat detailed densities were assigned to the sedimentary cover on the basis of borehole samples refraction data and a velocity density relationship were used to constrain the densities adopted for the deeper layers the results of the two dimensional gravity modelling confirm the asymmetrical shape of the crust mantle boundary beneath the pyrenean belt the downward deflection of the iberian moho from the ebro basin leads to a maximum depth of 65 km under the paleozoic axial zone whereas a depth of 35 km is found for the european moho under the north pyrenean zone two extreme models are proposed for the northern limit of the iberian crust a vertical contact in the downward continuity of the north pyrenean fault or a 70 northward sloping interface the positive anomalies beneath the north pyrenean zone may be explained by dense intracrustal mantle slices or lower crustal sheets the outcrop of granulitic basic gneiss within the labourd massif suggests that the anomaly results from lower crustal sheets

integrated lithospheric modeling combining thermal gravity and local isostasy analysis application to the ne spanish geotransect a two dimensional algorithm to determine the steady state thermal structure of the lithosphere that integrates thermal gravity and local isostasy analyses is presented gravity analyses together with seismic data are used to constrain spatial variations in density and crustal structure while absolute elevation is used to determine the lithospheric mantle thickness the calculation is performed using a finite element technique that links the different physical equations the program optionally calculates the temperature at any material boundary and with given rheological parameters the strength distribution and the total lithospheric strength in selected columns we apply the algorithm to the northeastern spanish geotransect which extends from the pyrenees to the balearic promontory and along which a strong variation in crustal and lithospheric thickness is evident we assess the use of two different inferred density models for the lithospheric mantle the first assumes a linear decrease in density with increasing temperature using the asthenospheric density as a reference the second model assumes a constant density for the whole lithospheric mantle although conceptually the two hypotheses differ substantially the results obtained do not show significant differences lithospheric thicknesses of 120 130 km below the pyrenees 60 65 km in the valencia trough and 65 75 km below the balearic promontory are deduced in all cases the mean lithospheric mantle density has to be 40 60 kg m 3 higher than the asthenospheric density the algorithm is shown to be a powerful tool in lithospheric thermal modeling especially in areas where surface heat flow is poorly constrained because of the temperature density elevation relationship

crustal parameters in the iberian peninsula abstract the structure of the crust in the iberian peninsula has been investigated for the last 15 years by spanish and portuguese groups in close collaboration with other european institutions the first experiments were carried out in portugal mueller et al 1973 with the aim of investigating the crustal structure of the hercynian belt in the southwest corner of the iberian peninsula other experiments have been subsequently realized to study different aspects of the crust in various regions of portugal in spain the main effort has been focused in alpine areas with the first experiments in the alboran sea and the betic cordilleras working group for deep seismic sounding in spain 1974 1975 1977 working group for deep seismic sounding in the alboran sea 1974 1975 1978 follow up experiments until 1981 completed the work in the betic cordillera extensive experiments were carried out in the pyrenees in 1978 further surveys covered the balearic islands in 1976 the valencia trough in 1976 and 1983 and the celtiberian chain or iberic system in 1981 the hercynian belt has only been studied in detail in the northwest corner of spain in 1982 with smaller studies in the central iberian massif in 1976 and 1986 mostaanpour 1984 has compiled some crustal parameters crustal thickness average crustal velocity and pn velocity for western europe meanwhile more complete data are available for the iberian peninsula the results presented here were derived from a large number of seismic refraction experiments which have been carried out mostly along or close to coastal areas of the iberian peninsula offshore explosions of various sizes were used as the energy source in most cases in addition to some quarry blasts unfortunately this leaves most of the inner part of the iberian peninsula unsurveyed our purpose is to summarize some of the crustal parameters obtained so far and to detail the appropriate literature for the interested reader

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

main results of the ecors pyrenees profile abstract the 250 km long deep seismic reflection survey across the pyrenees from one foreland ebro basin to the other aquitaine basin was designed to study the structure of the deformed mesozoic boundary between the iberian and european plates the results are 1 1 the entire crust shows well defined reflectors with a general fan shape geometry 2 2 a deep layered domain is evident above the moho discontinuity 3 3 near the surface in the external domains reflectors accurately define the geometry of major thrusts and structures affecting the mesozoic and cenozoic cover of the pyrenees 4 4 the middle crust shows two major sets of seismic events interpreted as hercynian structures using these results a balanced structural section of the pyrenean belt is proposed the mode of thickening of the crust during the alpine orogeny is briefly discussed only the iberian crust was significantly thickened by a southward stacking of slabs

crustal structure and evolution of the pyrenean cantabrian belt a review and new interpretations from recent concepts and data this paper provides a synthesis of current data and interpretations on the crustal structure of the pyrenean cantabrian orogenic belt and presents new tectonic models for representative transects the pyrenean orogeny lasted from santonian 84 ma to early miocene times 20 ma and consisted of a spatial and temporal succession of oceanic crust exhumed mantle subduction rift inversion and continental collision processes at the iberia eurasia plate boundary a good coverage by active source vertical incidence and wide angle reflection and passive source receiver functions seismic studies coupled with surface data have led to a reasonable knowledge of the present day crustal architecture of the pyrenean cantabrian belt although questions remain seismic imaging reveals a persistent structure from the central pyrenees to the central cantabrian mountains consisting of a wedge of eurasian lithosphere indented into the thicker iberian plate whose lower crust is detached and plunges northwards into the mantle for the pyrenees a new scheme of relationships between the southern upper crustal thrust sheets and the axial zone is here proposed for the cantabrian belt the depth reached by the n dipping iberian crust and the structure of the margin are also revised the common occurrence of lherzolite bodies in the northern pyrenees and the seismic velocity and potential field record of the bay of biscay indicate that the precursor of the pyrenees was a hyperextended and strongly segmented rift system where narrow domains of exhumed mantle separated the thinned iberian and eurasian continental margins since the albian cenomanian the exhumed mantle in the pyrenean rift was largely covered by a mesozoic sedimentary lid that had locally glided along detachments in triassic evaporites continental margin collision in the pyrenees was preceded by subduction of the exhumed mantle accompanied by the pop up thrust expulsion of the off scraped sedimentary lid above to the west oceanic subduction of the bay of biscay under the north iberian margin is supported by an upper plate thrust wedge gravity and magnetic anomalies and 3d inclined sub crustal reflections however discrepancies remain for the location of continent ocean transitions in the bay of biscay and for the extent of oceanic subduction the plate kinematic evolution during the mesozoic which involves issues as the timing and total amount of opening as well as the role of strike slip drift is also under debate discrepancies arising from first order interpretations of the adjacent oceanic magnetic anomaly record

extreme crustal thinning in the bay of biscay and the western pyrenees from observations to modeling recent observations and models of continental rifting in magma poor environments have led to the concept of multiphase stages of lithospheric extension in these concepts it is shown that extreme crustal thinning of the crust predates exhumation of lower crustal and subcontinental mantle rocks during final rifting the bay of biscay is a v shaped ocean basin that opened in aptian albian time in front of this propagating ocean several rift basins formed that show evidence for extreme crustal thinning and locally also mantle exhumation the parentis arzacq mauleon and cantabrian basins in this paper we propose based on geological and geophysical observations and using numerical modeling a model that can explain the extreme crustal thinning observed in the arzacq mauleon and parentis basins our results show that rifting in the bay of biscay was initiated by distributed oblique stretching latest jurassic to early aptian before it underwent an more orthogonal asymmetric thinning and exhumation phase from late aptian to albian time these last two stages of deformation are similar to those observed in orthogonal rift systems we show that thinning is accomplished by the formation of a semibrittle shear zone that allows for the transfer of middle to lower crustal material from the side of the rift collocated with the hanging wall to the side of the rift collocated with the footwall of the detachment system the main difference with an orthogonal rift system appears to be generated by the formation of flower structures during the distributed oblique phase and the capacity of localizing the deformation in the subsequent stages these oblique slip faults form very steep normal faults that induce the development of strongly localized compartmentalized and asymmetric rift basins in the case of the parentis and arzacq mauleon basins these strike slip faults separate upper plate sag basins to the north from lower plate sag basins to the south while the northern sag basins do not show any evidence for exhumation the southern ones are more complex and floored by detachment faults as indicated by the occurrence of syntectonic and posttectonic sediments onlapping directly onto exhumed lower crustal and mantle rocks

inflation predictable behavior and co eruption deformation at axial seamount deformation of the ground surface at active volcanoes provides information about magma movements at depth improved seafloor deformation measurements between 2011 and 2015 documented a fourfold increase in magma supply and confirmed that axial seamount s eruptive behavior is inflation predictable probably triggered by a critical level of magmatic pressure a 2015 eruption was successfully forecast on the basis of this deformation pattern and marked the first time that deflation and tilt were captured in real time by a new seafloor cabled observatory revealing the timing location and volume of eruption related magma movements improved modeling of the deformation suggests a steeply dipping prolate spheroid pressure source beneath the eastern caldera that is consistent with the location of the zone of highest melt within the subcaldera magma reservoir determined from multichannel seismic results

crust and upper mantle structure of the central iberian meseta spain summary quarry blasts recorded along three lines on the central iberian meseta are used in an attempt to interpret the crustal structure the results of the interpretation of the data together with published surface wave and earthquake data suggest a layered structure of the crust having the following features the basement in some areas covered by up to 4 km of sediments has a p velocity of 6 1 km s 1 a low velocity layer between 7 and 11 km depth seems to exist on the basis of both p and s interpretation of seismic data a thick middle crust of 12 km has a p velocity of 6 4 km s 1 and overlies a lower crust with a mean p velocity of 6 9 km s 1 and a possible slight negative gradient the mean vp vs ratio for the crust is about 1 75 the moho is reached at about 31 km depth and consists of a transition zone at least 1 5 km thick the p velocity of the upper mantle is close to 8 1 km s 1 and the s velocity about 4 5 km s 1 which gives a vp vs ratio of 1 8 for the uppermost mantle a tentative petrological interpretation of the velocities and composition of the layers is given

initiation of an active margin at the north iberian continent ocean transition 1 the north iberian or cantabrian margin located at the southern flank of the bay of biscay underwent successive tectonic regimes of rifting passive margin and compression from mesozoic to tertiary times a complete crustal cross section of the north iberian margin from the abyssal plain to the shoreline and a reconstruction of its undeformed upper cretaceous structure are presented here it is constrained after the compilation and interpretation of different geophysical data sets recently available in the area the reflective pattern is provided by deep seismic reflection profile escin 4 complemented by commercial profiles and well information from the north iberian platform and the velocity depth control comes from an onshore offshore wide angle seismic profile tested also with gravity modeling the detailed cross section of the meso tertiary basins that fill the platform and abyssal plain shows structures from the three main tectonic events 1 normal faults and asymmetric basins from the permian to lower cretaceous extensional stage 2 upper cretaceous sediments deposited under stable conditions during the passive margin period and 3 inverted faults thrusts and folds related to the tertiary compression the deep structure of the crust beneath the margin is poorly constrained from escin 4 profile that shows reflectivity in the lower crust only at a small area under the platform however the corresponding wide angle experiment provides a conspicuous image of the moho that shows a continued deepening toward the continent on the basis of the geophysical results we propose a new model for this margin where the lower crust is detached and underthrusted to the south as a result of the partial closure of the bay of biscay an interpreted mesozoic crustal scale extensional detachment could be the weakness zone that allowed the lower crust to slide southward under the upper crust resulting in the indentation of the cantabrian margin lower crust into the iberian crust plate convergence stopped at an early stage making this a unique area to study the initial stages of deformation of a passive margin

lithospheric anisotropy beneath the pyrenees from shear wave splitting we investigate upper mantle anisotropy beneath the pyrenean range along three n s profiles across the mountain belt the results of a first profile that operated in 1993 in the central part of the belt have been presented elsewhere we present the results of two other profiles that ran in 1995 1996 and 1996 1997 in the eastern and western part of the belt respectively and propose an interpretation of the whole results teleseismic shear waves sks skks and pks are used to determine splitting parameters the fast polarization direction and the delay time t teleseismic shear wave splitting in the eastern pyrenees displays homogeneous values trending n100 e and t values in the range 1 1 to 1 5 s a station located in the southern massif central 100 km north of the range is characterized by different splitting parameters n70 e t 0 7 s in the western part of the belt anisotropy parameters are similar across the whole belt n110 e and t 1 3 to 1 5 s most of the measured delay times including those obtained in the central part of the range are above the global average of the sks splitting around 1 s at the belt scale is generally poorly correlated with recent estimations of the absolute plate motion which predicts a fast direction ranging between n50 e and n80 e instead the orientation of n100 e is parallel to the trend of the pyrenean belt but also to hercynian preexisting structures this parallelism supports an anisotropy primarily related to frozen or active lithospheric structures we show that a signature related to the pyrenean orogeny is likely for the stations located in the internal domains of the belt by contrast the anisotropy measured at the stations located on the external parts of the belt could reflect a pre pyrenean hercynian deformation we suggest that a late hercynian strike slip deformation is responsible for this frozen upper mantle anisotropy and that the pyrenean tectonic fabric developped parallel to this preexisting fabric finally no particularly strong splitting is related to the north pyrenean fault commonly believed to represent the plate boundary between iberia and eurasia

the variscan geology of the pyrenees abstract the main outlines of the geology of the variscan part of the pyrenees are discussed rocks involved in this cycle are high grade basement gneisses palaeozoic sediments and their metamorphic equivalents late intrusive granodiorites and early pre variscan granites the main features of the stratigraphy of the palaeozoic are given structures fall into two domains a low grade suprastructure essentially with steep folds and cleavages and a high grade infrastructure with dominantly low dipping foliations an important phase of early pre cleavage folding occurs in low grade rocks mainly along the southern border of the axial zone in high grade rocks most structures and the metamorphism postdate the main cleavage phase in low grade rocks the influence of the alpine orogeny on the variscan structures consists mainly of faults steep reverse faults in the northern and south directed thrusts in the southern part of the pyrenees metamorphism took place under high geothermal gradients and low pressures as indicated by the abundant occurrence of andalusite and cordierite

structural evolution of the pyrenees tectonic inheritance and flexural behaviour in the continental crust abstract the ecors deep seismic reflection profile and additional geological data are used to understand deformation processes in the pyrenees the middle crust shows two major sets of seismic reflectors interpreted as hercynian structures during permian and the late jurassic early cretaceous rifting events these structures were extensionally reactivated mainly within the european plate during the pyrenean orogeny tectonic inversion of the early cretaceous basins took place located mainly on the european plate and hercynian southward directed verging thrusts were also reactivated in the iberian foreland a regional model is proposed which takes into account the lithospheric flexure and the evolution of the sedimentary load this study shows the different rigidities of the european and iberian plates difference related to differential thinning events as well as an east west diachronism of the flexure for the european plate and unflexing behaviour after the orogeny

ten years of seismic monitoring over a gas field abstract ten years of seismic recording allows us to locate more than 800 tremors with magnitudes ranging from 1 0 to 4 2 in a small 10 by 10 by 5 km volume near a major gas field lacq france the seismic activity began 10 yr after gas extraction began nearly all of the epicenters are located within the lateral extent of the gas field the only perturbation of the stress or strain fields which has the same spatial and temporal scale as the seismicity is the 500 bar drop in gas pressure due to 20 yr of gas exploitation demonstrating that these events are induced the network established to monitor this activity consists of eight telemetered stations including a well bottom geophone located at 3200 m depth using three dimensional relocation techniques we computed new hypocenter positions for more than 280 events these better located hypocenters form a diffuse zone that mimics the local dome structure taking the regional tectonic stresses into account we suggest that these ruptures may be the result of unfolding of the dome structure

an approach to the thermal field in northeastern spain temperature data from oil wells thermal springs and recent water wells have been compiled and used to construct a temperature gradient map for the northeastern part of the iberian peninsula this map shows a gentle increase in thermal gradient from north to south and west to east with individual areas displaying both cold and warm temperature data a very cold area near the ebro mouth and a nearby anomalously warm area offshore are the most important features of the gradient map these data are interpreted in terms of normal 30 35 mk m 1 geothermal gradient with conspicuous anomalies caused by groundwater circulation this interpretation differs from previously published models of the thermal regime in northeastern spain

thick and thin skinned tectonics in the pyrenees abstract a comparison is made between the gavarnie thrust and the merens fault in the axial zone of the pyrenees the former has a gentle dip and quite a large displacement at least 12 km but does not cut through either hercynian or alpine isograds the latter has a smaller displacement 5 km but dips steeply and cuts through both hercynian and alpine isograds at a high angle on this basis and on the basis of shear zone geometries immediately north of it it is proposed that the merens fault nucleated as a steeply 65 80 dipping structure while the gavarnie thrust nucleated with a shallow attitude the merens fault is not a backward rotated thrust fault nor is it the root zone for any major nappe structure similar steep ductile structures occur within the gavarnie nappe and may reflect considerable internal strain in basement lithologies the relationship between steep and shallow structures is not yet clear the shear zones may pre date the thrusting in which case they may be thick skinned structures affecting the whole lithosphere or they may be contemporary with thrusting reflecting only local thickening above a decollement rheological models can be used to test proposed geometrical and kinematic models for the lithosphere scale evolution of the pyrenees suggested models are dominated by a cool rigid high level mantle wedge beneath the north pyrenean zone which probably controlled the location of north dipping thrust faults thick skinned shortening is possible in thick crust in the axial zone but is very unlikely in the north pyrenean zone where steeply rooted structures would have to cut through the strongest part of the lithosphere

unique large diamonds in a ureilite from almahata sitta 2008 tc3 asteroid the almahata sitta ms 170 ureilite a piece of a breccia originating from the asteroid 2008 tc3 consists mainly of olivine with many diamond and graphite grains existing between the olivine grains the occurrences of the diamonds are unique i e i some diamonds exhibit sub euhedral habits and ii some diamonds have large grain size up to about 40 mu m several diamonds are segmented into many fragments by fractures individual fragments have similar crystallographic orientation which implies that the adjacent diamond segments were originally a single crystal large diamond assemblages occur besides such individual diamond grains in one of the largest assemblages almost about 100 mu m in size has also the same crystallographic orientation they can be regarded as the pieces of a previously unique single diamond which provides evidence for large single crystals diamond in meteorites almahata sitta ms 170 is a meteorite fragment from the 2008 tc3 asteroid that underwent less shock than other ureilitic meteorites it is unlikely that such large diamonds were formed from graphite through a shock induced phase transformation during planetesimal collision despite this idea being now widely accepted as the diamond formation mechanism of ureilites fine scale heterogeneous distribution of impurities hydrogen nitrogen and oxygen exists in single crystal diamonds indicative of sluggish growth this distribution is reminiscent of sector zoning growth its grain size the shock features of ms 170 and the c and n isotopic composition signatures allow us to revive classical and but not widely accepted models for diamond formation in ureilites i e a diamond formed from partially melted magma or a c o h fluid in the deep interior of the ureilite parent body or alternatively through a chemical vapor deposition cvd process in the solar nebula considering present mineralogical and isotopic features the former scenario is more favorable c 2015 elsevier ltd all rights reserved

crustal structure of the north iberian continental margin from seismic refraction wide angle reflection profiles abstract the structure and geodynamics of the southern margin of the bay of biscay have been investigated from a set of 11 multichannel seismic reflection profiles recorded also at wide angle offsets in an onshore offshore network of 24 obs obh and 46 land sites this contribution focuses on the analysis of the wide angle reflection refraction data along representative profiles the results document strong lateral variations of the crustal structure along the margin and provide an extensive test of the crustal models previously proposed for the northern part of the iberian peninsula offshore the crust has a typical continental structure in the eastern tip of the bay which disappears smoothly towards the nw to reach crustal thickness close to 10 km at the edge of the studied area 45 n 6 w the analysis of the velocity depth profiles altogether with additional information provided by the multichannel seismic data and magnetic surveys led to the conclusion that the crust in this part of the bay should be interpreted as transitional from continental to oceanic typical oceanic crust has not been imaged in the investigated area onshore the new results are in good agreement with previous results and document the indentation of the bay of biscay crust into the iberian crust forcing its subduction to the north the interpreted profiles show that the extent of the southward indentation is not uniform with an alpine root less developed in the central and western sector of the basque cantabrian basin n s to ne sw transfer structures seem to control those variations in the indentation degree

the influence of inherited extensional structures on the tectonic evolution of an intracratonic chain the example of the western pyrenees abstract a geotraverse constructed on the basis of surface and subsurface data across the basque bearnais portion of the pyrenees east of the pamplona fault reveals from north to south the existence of three major tectono sedimentary units the north pyrenean zone npz the central zone cz and the axial zone az these zones are characterized both by the type and age of their sediments and volcanics and by the style of their cretaceous synsedimentary structures paleogeographically they correspond to a cretaceous platform az margin cz and basin npz separated by zones of normal faults during alpine convergence the cretaceous fault system was to varying degrees reactivated in the reverse sense the npz grades into the cz without any significant hiatus by contrast the cz is separated from the az by a major southward overthrust the orly lakhoura overthrust which shows up as the most important fault in the cross section its subsurface continuation is constrained by regional microseismicity and other seismic data all these data lead us to consider the orhy lakhoura overthrust as the surface evidence of the alpine crustal underthrust of iberia below the european continental domain restoration of the cross section suggests that this alpine underthrust follows a previous crustal discontinuity linked to the cretaceous extensional episode

geophysical constraints on the deep structure along the ecors pyrenees line the structure of the central part of the pyrenean range traversed by the ecors vertical reflection seismic line is further constrained by a joint interpretation of complementary and reprocessed data wide angle low frequency profiling of moho topography indicates an abrupt termination of the northern european moho at the north pyrenean fault and a range of possible geometries of a deeper iberian moho which does not seem to extend significantly to the north of this fault when reprocessing vertical reflection data to 25 s two way travel time strong clear straight reflections are discovered in the part of the section under the northern foreland long after the moho reflection their strong northward dip indicates that they originate in fact at reflectors at depth beneath the north pyrenean fault which may correspond to the connection between the two levels of moho the reflections then being from the northward inclined limit between the wedge of iberian crust underlying the southern edge of the european mantle a range of possible shapes is allowed by the conjunction of these seismic data which is compatible with the results of a detailed gravity profile this would indicate that the imbrication of european into iberian lithosphere has been of limited extent with respect to the surface trace of the north pyrenean fault provided no strong density contrast in the mantle part of the section contributes significantly to the gravity anomaly

a three dimensional model of the pyrenean deep structure based on gravity modelling seismic images and petrological constraints summary the present study aims at building a 3 d density model of a whole collisional mountain belt at the lithospheric scale the geometry of internal structures is constrained by seismic studies while density values are imposed by petrological considerations the model must fit both the bouguer and geoid anomalies this approach is applied to the pyrenees for which numerous geophysical data are available the crustal structure is constrained from the results of seismic profiles the most important crustal feature is the large moho jump which coincides with the surface expression of the north pyrenean fault considered as the suture between the iberian and eurasian plates the eurasian crust is about 30 km thick whereas the iberian crust is 50 km thick in the central part of the range thick sedimentary basins at the piedmont and two large heavy intracrustal bodies located in the western and central parts of the range are also important features of the crustal model at the lithospheric scale a low velocity strip under the north pyrenean fault down to 80 100 km depth interpreted as the subduction of the iberian lower crust has been revealed by tomographic models the density of this structure has been constrained using petrological arguments which assume that the crustal material is transformed into eclogite at about 12 15 kbar 40 50 km depth an eclogite type density value much higher than the mantle density is thus imposed from 50 to 100 km depth theoretical bouguer and geoid anomalies are computed for this 3 d density model using a summation of density block anomalies a good agreement is found with observations giving strong support to the proposed model the crustal structure as seen by seismic profiles explains most of the observed long wavelength negative bouguer anomaly as well the local anomalies related to the intracrustal bodies a possible mantle origin of these bodies is ruled out on the other hand the transformation of the subducted lower crust into dense eclogite provides a successful fit to the long wavelength positive geoid anomalies in addition the water released during eclogite formation may help to explain the high conductivity zone detected down to 80 km south of the north pyrenean fault the 3 d model proposed here thus reconciles petrological considerations and geophysical data

high resolution imaging of the pyrenees and massif central from the data of the pyrope and iberarray portable array deployments 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

implications of the seismic structure for the orogenic evolution of the pyrenean range abstract the dynamic evolution of the pyrenees is discussed in the light of geophysical data recent deep seismic sounding have revealed the crustal structure of the pyrenees which is used to test the different evolutionary models proposed until now the crustal thickness of the paleozoic axial zone paz and the north pyrenean zone npz differ by more than 10 km ranging from about 30 km in the npz to 40 50 km in the paz the transition from paz to the npz identified at the surface as the north pyrenean fault npf is sharp at depth and marked by a vertical step at least in the eastern half of the range the npz is characterized by additional throws and dips of the moho in the east whereas in the west a heterogeneous middle to lower crust is encountered with high velocity anomalies the seismic results suggest that the paz and the npz belong to different plates the npf being the plate boundary these results are inconsistent with evolutionary models involving lithospheric subduction or crustal doubling and intracratonic rifting with the main tectonic lineations following nne ssw directions they rather suggest that after a period of extension two main orogenic events took place a phase involving shearing and thinning which affected mainly the present day npz and a later compressive phase which explains the building up of the chain the thickening of the crust and the enhancement of a pre existing difference in crustal thickness between the european and iberian plates

transverse fold evolution in the external sierra southern pyrenees spain fault slip data are used to reconstruct varying tectonic regimes associated with transverse fold development along the eastern and southern margins of the jaca basin southern pyrenees spain the spanish pyrenean foreland consists of thrust sheets and leading edge decollement folds which developed within piggyback basins guara formation limestones on the margins of the jaca basin were deposited synchronously with deformation and are exposed in the external sierra within the transverse folds principal shortening axes determined from p and t dihedra plots of fault slip data show a shift from steep shortening in stratigraphically older beds to nne ssw horizontal shortening in younger beds older strata are characterized by extensional faults interpreted to result from halotectonic salt tectonics deformation whereas younger strata are characterized by contraction and strike slip faults interpreted to result from thrust sheet emplacement the interpretation of the timing for the shortening axes in the younger strata is supported by the observation that these axes are parallel to shortening axes determined from finite strain analysis calcite twins and regional thrusting directions determined from fault related folds and slickenlines this study shows that fault population analysis in syntectonic strata provides an opportunity to constrain kinematic evolution during orogeny

a new tomographic image of the pyrenean lithosphere from teleseismic data abstract a new tomographic model of the pyrenean lithosphere is determined down to 200 km depth from teleseismic p and pkp travel times with a lateral resolution of 0 25 compared to previous models two important improvements are 1 a larger number of stations with a more even distribution in particular to the west of the range and 2 the introduction before inversion of crustal corrections inferred from previous refraction and reflection experiments this last point is crucial because a strong moho jump up to 20 km is present at the north pyrenean fault the former boundary between eurasian and iberian plates the comparison of the models obtained with and without crustal corrections reveals the strong contamination of the models by the crust down to 100 km depth in the uncorrected model a large strip with negative p velocity anomalies previously interpreted as subduction of lower crust is observed it disappears in the corrected model moreover the introduction of crustal corrections allows us to reveal short wavelength heterogeneities which were hidden by the crustal signal an attempt is made to relate the heterogeneities revealed by the tomographic model with the tectonic history of the pyrenees in particular with the alpine orogeny the alpine phase includes an extensive episode with generation of the thin continental crust and possibly the opening of an oceanic sea floor and then a compressive stage in our model no signature of an oceanic subducted slab could be detected all along the range a result which rules out the opening of a large oceanic floor before the compressive stage a subduction of continental crust is possible but due to the transformation of lower crust into eclogite at depth it can not be detected by seismological methods whereas it was observed from electrical and gravity data to the east of the range large heterogeneities with low velocities are ascribable to the neogene extension related to the rotation of the corso sardo block and the opening of the gulf of lion a prominent high velocity anomaly extending down to 200 km in eastern central pyrenees could possibly be interpreted as a detached piece of the tethys slab in north of iberia outside the range deep down to 200 km low velocity structures oriented n130 e are probably related to hercynian orogeny

a three dimensional model of the pyrenees and their foreland basins from geological and gravimetric data abstract we construct a three dimensional geological model of the pyrenees and their foreland basins with the geomodeller this model which accounts for different sources of geological and geophysical informations covers the whole pyrenees from the atlantic ocean to the mediterranean sea and from the iberian range to the massif central down to 70 km depth we model the geological structure with a stratigraphic column composed of a superposition of layers representing the mantle lower middle and upper crusts the sedimentary basins are described by two layers which allow us to make the distinction between mesozoic and cenozoic sediments which are characterized by markedly different densities and seismic velocities since the pyrenees result from the convergence between the iberian and european plates we ascribe to each plate its own stratigraphic column in order to be able to model the imbrication of iberian and european crusts along this fossile plate boundary we also introduce two additional units which describe the orogenic prism and the water column in the bay of biscay and in the mediterranean sea the last ingredient is a unit that represents bodies of shallow exhumed and partly serpentinized lithospheric mantle which are assumed to produce the positive bouguer gravity anomalies in the north pyrenean zone a first 3d model is built using only the geological information coming from geological maps drill holes and seismic sections we use the potential field method implemented in geomodeller to interpolate these geological data this model is then refined in order to better explain the observed bouguer anomalies by adding new constraints on the main crustal interfaces the final model explains the observed bouguer anomalies with a standard deviation less than 3 4 mgal and reveals anomalous deep structures beneath the eastern pyrenees

a new kinematic model for iberia further palaeomagnetic and isotopic age evidence abstract palaeomagnetic results and k ar age data from the algarve igneous province have uncovered three ancient relative field components it is inferred that the principal magnetization of the monchique complex with a mean direction of d 42 i 181 95 4 5 was acquired at around the time of the pluton s emplacement some 72 2 ma ago a second magmatic pulse affected the region at 68 2 ma leaving behind a relative field component of d 42 i 341 95 1 7 furthermore an overprinted magnetization characterized by shallow inclination and somewhat smeared northwesterly southeasterly declination most likely originates from the lower tertiary the new palaeomagnetic data are seen in conjunction with previously published upper cretaceous results from the sintra and sines intrusive complexes the resulting database gives entirely new perspectives for evaluating the plate tectonic history of the peninsula in upper cretaceous lower tertiary time a two phase rotation of the iberian micro plate an anticlockwise one of 40 followed by a clockwise one of 70 is clearly in evidence the net clockwise rotation provided a major impetus for the compressive deformation in the pyrenees and the bay of biscay and led to extensive remagnetization of older rock formations in the northern regions of iberia previous palaeomagnetic conclusions in favour of a single anticlockwise rotation of 35 supposed to be directly connected with the formation of the bay of biscay appear to be artefacts which stem from an incorrectly adopted palaeomagnetic reference frame

deep structure of pyrenees range sw europe imaged by joint inversion of gravity and teleseismic delay time we present a new model of the lithosphere and asthenosphere structure down to 300 km depth beneath the pyrenees from the joint inversion of recent gravity and teleseismic data unlike previous studies crustal correction was not applied on teleseismic data in order i to preserve the consistency between gravity data which are mainly sensitive to the density structure of the crust lithosphere and traveltime data and ii to avoid the introduction of biases resulting from crustal reductions the density model down to 100 km depth is preferentially used here to discuss the lithospheric structure of the pyrenees whereas the asthenospheric structure from 100 to 300 km depth is discussed from our velocity model the absence of a high density anomaly in our model between 30 and 100 km depth except the labourd density anomaly in the northern part of the pyrenees seems to preclude eclogitization of the subducted iberian crust at the scale of the entire pyrenean range local eclogitization of the deep pyrenean crust beneath the western part of the axial zone west of andorra associated with the positive central density anomaly is proposed the pyrenean lithosphere in density and velocity models appears segmented from east to west no clear relation between the along strike segmentation and mapped major faults is visible in our models the pyrenees lithosphere segments are associated with different seismicity pattern in the pyrenees suggesting a possible relation between the deep structure of the pyrenees and its seismicity in the upper crust the concentration of earthquakes localized just straight up the central density anomaly can reflect the subsidence and or delamination of an eclogitized pyrenean deep root the velocity model in the asthenosphere is similar to previous studies the absence of a high velocity anomaly in the upper mantle and transition zone i e 125 to 225 km depth seems to preclude the presence of a detached oceanic lithosphere beneath the european lithosphere

on the consistency of proposed models for the pyrenees with the structure of the eastern parts of the belt abstract the structure of the eastern pyrenees consists mainly of south directed thrusts involving basement and cover rocks an antiformal stack developed by the piling up of basement thrust sheets which outcrop in the axial zone these structures account for a thin skinned thrust model rather than a vertical fault model in which the axial zone would be essentially autochthonous and the north pyrenean fault the axial plane of a fan thrust system new data from the eastern pyrenees and the thin skinned model suggest that 1 the structure east of the pedraforca nappe is similar to that of the central pyrenees 2 the cover rocks of the south pyrenean units and of the axial zone after restoration built up a northwards thickening prism consistent with the existence of a unique pyrenean sedimentary basin during mesozoic time 3 the axial zone is only a complex antiformal stack developed as a part of south pyrenean system related to the paleogene thrusting tectonics the axial zone palaeogeographic area had no special meaning during mesozoic time

gravity anomaly map of the pyrenean region and a comparison of the deep geological structure of the western and eastern pyrenees abstract a gravity anomaly map based on over 13 000 observations accurately delineates the major features of the pyrenean region an extensive gravity anomaly low in the south of the region appears to result from crustal thickening associated with the pyrenean mountain building episode several large positive gravity anomalies are apparent in the north pyrenean zone in the centre and west of the mountain chain but are absent in the east the interpretation of a series of gravity anomaly profiles along the pyrenees has been controlled by existing geophysical data the interpretations confirm the cause of the southern gravity anomaly low as thickened iberian crust and demonstrate as shown by previous seismic investigations that the transition to much thinner european crust is quite abrupt the sources of the positive anomalies in the north pyrenean zone are bodies in the upper crust with densities too high to be explained in terms of crustal materials these are interpreted as segments of upper mantle that originated in transtensional n s separation across the north pyrenean fault and were emplaced at shallow levels by thrusting during pyrenean orogenesis this is the first time that evidence has been presented for the presence of such mantle bodies in the pyrenees it is suggested that their absence in the east is caused by less extension between iberia and europe in this region during the transform motion between them prior to orogenesis allowing only limited ascent of deep material the greater extension in the west would have allowed the ascent of upper mantle into the space between iberia and europe which would have been further elevated during the thrusting associated with pyrenean orogenesis

the eastern end of the pyrenees seismic features at the transition to the nw mediterranean the deep crustal features at the transition from the pyrenean range to the nw mediterranean basin have been investigated by onshore offshore recording of ligurian sardinian lisa seismic profiles the predominance in this area of the neogene extensional tectonics over the paleogene compressional one is revealed from the present day deep structure sampled by the new wide angle data the existence of a crustal root beneath the pyrenean axial zone and the outstanding differences in moho depths between the iberian and european domains well established from previous seismic studies on central and western pyrenees are not observed anymore at the easternmost part of the range this area is marked by a continued crustal thinning seawards the moho rises about 10 km along distances of 60 km that affects already the mainland further evidences on lateral variations and topography of the crust mantle boundary are indicative of the extent of the different tectonic episodes

the influence of the evolution of the pyrenees on adjacent basins abstract important thicknesses of syn and post orogenic sediments are observed in the southern part of the aquitaine basin and in the north of the ebro basin their deposition cannot be explained only as a result of the extensional history of the early cretaceous thus the influence of the compression associated with the pyrenees formation upon the subsidence of these basins is studied europe and iberia are considered to behave as two semi infinite elastic plates the evolution through time of the sedimentary thicknesses is explained by the deflections of these two plates induced not only by the topographic load of the pyrenees but also by additional forces 10 12 n m that may have their origin in the formation of a cold lithospheric root during the shortening a series of calculations is presented for an approximately north south profile across the aquitaine basin the pyrenees and the ebro basin a good fit for the shape of the basins and the gravity anomalies was obtained with flexural rigidities of 1 5 10 23 nm for the northern plate and the outer part of the southern plate and 1 4 10 21 nm for the inner part of the southern plate

thrust tectonics in the north pyrenees abstract balanced and restored cross sections through the central and eastern pyrenees constructed using both surface and borehole data demonstrate the presence of c 18km of shortening above a flat lying n directed alpine decollement surface hangingwall diagrams show how the north pyrenean satellite massifs are culminations within this thrust system pre thrusting structures such as subhorizontal stretching lineations in the north pyrenean fault zone became rotated above these culminations as the north pyrenean fault was cut by alpine thrusts stratigraphic evidence demonstrates that n directed thrust movements occurred between mid eocene and oligocene time and this is similar to the age of major s directed thrust movements on the south side of the axial zone the n directed thrust system probably originated as a series of backthrusts to the dominant s directed structures

seismic signature of variscan and alpine tectonics in nw iberia crustal structure of the cantabrian mountains and duero basin the seismic structure of the crust of the nw iberian peninsula along the cantabrian mountains and their southern foreland the duero basin is investigated from the first data set available from refraction and wide angle reflection profiles the velocity depth distributions obtained along e w and n s transects evidence lateral variations in crustal structure that can be associated to variscan and alpine tectonic episodes the western part of the range toward the variscan hinterland zones as well as the duero basin below the sediments show a crustal structure similar to that of the variscan belt elsewere in europe the total crustal thickness is 30 32 km and three main layers upper middle and lower crust with average velocities 6 0 6 25 and 6 8 km s are resolved some paleozoic structures such as the basal detachment of the cantabrian zone are expressed in the seismic models the crust appears to vary eastward in the external zones the most outstanding feature is a crustal root beneath the highest cantabrian summits where the moho is found at 47 km depth a prominent alpine reworking of the crust in the eastern part of the cantabrian zone is thus revealed and the n s seismic transects delineate a geometry similar to the one found across the central pyrenees in the etude continentale et oceanique par reflexion et refraction sismique ecors profile revealing the importance of the alpine deformations in the northern part of the iberian plate

palaeo elevation and effective elastic thickness evolution at mountain ranges inferences from flexural modelling in the eastern pyrenees and ebro basin abstract the results are presented of a two dimensional flexural modelling study of the lithosphere underlying the southern pyrenees and the ebro basin the modelling is based on a crustal scale balanced cross section along a profile through the north eastern part of the iberian plate two time slices of the structural evolutions of the mountain chain are modelled the present day configuration and the configuration at middle lutetian time 47 ma where important structural and sedimentological changes are observed the flexure model incorporates lateral variations in the effective elastic thickness eet of the lithosphere the present day deflection in the profile is simulated using boundary forces and a northward decreasing eet that varies from 30 to 11 km models for middle lutetian times indicate eet values of 26 18 km in the northern part of the profile assuming that the eet at the distal margin of the ebro basin has not significantly changed since middle lutetian times these higher values for the eet at middle lutetian times suggest that the effect of the cretaceous extensional phase on the present day flexural rigidity is small and therfore the inferred northward decreasing rigidity is predominantly related to the pyrenean collision flexural modelling provides also constraints for the palaeo elevation of the inner part of the chain including the assumption that the eet at the distal margin of the ebro basin has not significantly changed since middle lutetian times the model predicts a maximum palaeo elevation of 2000 m which is in agreement with geological observations concerning the relation between basin fill and palaeo elevation

thermomechanical evolution of a thinned continental lithosphere under compression implications for the pyrenees in order to demonstrate the interactions between plate boundary forces and preexisting zones of weakness and how those control the localization of continental deformation inside the lithosphere the thermomechanical evolution of an initially thinned and thermally perturbed continental lithosphere in compression was analyzed within the framework of the pyrenees while a concensus exists as to the shallow structures the extent and geometry of deformation at depth have been the subject of great controversy the thermomechanical implications of recent geological hypotheses on the initial state prior to the compression recently derived from balanced cross section reconstructions are here quantified and compared to the recent ecors deep seismic reflection profile across the pyrenees a thermomechanical model based on finite elastoplastic viscoplastic deformations has therefore been formulated using phenomenological constitutive equations derived from experimental studies the thermally coupled problem is solved by an appropriate vertical two dimensional plane strain finite element approximation in those models most of the shortening is accommodated inside a local zone of intense plastic shearing that creates a sharp wedge shaped root progressively sinking deep into the mantle the main thickening takes place inside the lower crust where it can reach a factor 2 this indentation and crustal imbrication process is controlled by a buckling instability occuring inside the uppermost part of the mantle the evolution of the topography can be related to these phenomena lateral conductive heat transfer overtakes the advective transfer in the vicinity of the inflow of colder crustal material inside a warmer mantle the deformation pattern can be explained by a three layer mechanical lithosphere with a soft ductile internal layer of potential decoupling between two more competent layers characterized by a significant elastic stored energy the overall response of this composite plate is not much influenced by smaller scale variations of crustal rheology but is very much dependent on the contrast in rates of viscous dissipation between the lower crust and the upper mantle and on the possibility of singling out a layer of significant elastic stored energy by the balance between rate of loading and rate of dissipation inside the uppermost mantle predicted deformation pattern is found to agree with the seismic reflection and gravity data even though extrapolation to other rates of shortening must be verified by appropriate numerical modelling these physical mechanisms may be regarded as a paradigm for other crustal imbrication phenomena found in the himalayas tibet or the alps