paleomagnetic secular variation at the azores during the last 3 ka 1 we report on 33 new paleomagnetic directions obtained from 16 lava flows emplaced in the last 3 ka on sao miguel the largest island of the azores the data provide 27 well dated directions from historical or 14c dated flows which together with 6 directions previously gathered from the same flows by johnson et al 1998 yield the first paleomagnetic directional record of the last 3 ka from the atlantic ocean within flow directions are consistent suggesting that inclination swings from 60 to 25 and declination changes between 10 to 20 reflect variations in the geomagnetic field over the last 3 ka to a first approximation the declination record is consistent with predictions from cals3k 4 and gufm1 global field models conversely inclination values are lower than model predictions at two different ages 1 four sites from the 1652 ad flow yield i 48 instead of i 63 predicted by gufm1 2 data from several flows nicely mimic the inclination minimum of 800 1400 ad but inclination values are lower by 10 than cals3k 4 model predictions by interpolating a cubic spline fit on declination inclination versus age data we tentatively infer the directional evolution of the geomagnetic field at the azores from 1000 bc to 1600 ad the obtained curve shows three tracks in virtual overlap during the 1000 800 bc 800 500 bc and 400 700 ad time spans

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

geomagnetic field for 0 3 ka 2 a new series of time varying global models 1 steadily increasing numbers of archeomagnetic and paleomagnetic data for the holocene have allowed development of temporally continuous global spherical harmonic models of the geomagnetic field extending present and historical global descriptions of magnetic field evolution the current work uses various subsets of improved data compilations details of which are given in a companion paper by donadini et al 2009 and minor modifications of standard modeling strategies using temporally and spatially regularized inversion of the data and cubic spline parameterizations for temporal variations to produce five models with enhanced spatial and temporal resolution for 0 3 ka spurious end effects present in earlier models are eliminated by enforcing large scale agreement with the gufm1 historical model for 1650 1990 a d and by extending the model range to accommodate data older than 3 ka age errors are not considered as a contribution to data uncertainties but are included along with data uncertainties in an investigation of statistical uncertainty estimates for the models using parametric bootstrap resampling techniques we find common features but also significant differences among the various models indicating intrinsic uncertainties in global models based on the currently available holocene data model cals3k 3 based on all available archeomagnetic and sediment data without a priori quality selection currently constitutes the best global representation of the past field the new models have slightly higher dipole moments than our previous models virtual axial dipole moments vadms calculated directly from the data are in good agreement with all corresponding model predictions of vadms these are always higher than the spherical harmonic dipole moment indicating the limitations of using vadms as a measure of geomagnetic dipole moments

first archaeomagnetic secular variation curve for the iberian peninsula comparison with other data from western europe and with global geomagnetic field models 1 a first secular variation sv curve for the iberian peninsula was computed by hierarchical bayesian method using a total of 134 archaeomagnetic directions with ages ranging from 775 to 1959 a d a general agreement is observed between the iberian curve and the french and german sv curves although some interesting differences were found such as the occurrence of lower inclinations between the 11th and 14th centuries in the iberian curve the analysis of these three reference curves indicates that sv in western europe is characterized by three major directional changes at 125 200 and 1350 a d it is suggested that these cusps are regional features of the geomagnetic field the iberian curve has been compared with the predictions of the jackson calsk7k 2 and hongre global models despite large differences recognized between these models even for the dipolar terms they predict reasonably well the iberian archaeomagnetic sv

pseudopaleosecular variation due to remanence anisotropy in a pyroclastic flow succession 1 in order to check the reliability of a paleosecular variation record in a sequence of pyroclastic rocks miocene sardinia we measured various types of anisotropy magnetic susceptibility ams anhysteretic remanent magnetization aarm and thermoremanent magnetization atrm the main magnetic carrier is pseudo single domain titanomagnetite no general relation appears between the degrees of ams and aarm while the degrees of aarm and atrm are almost identical measuring the aarm thus provides a fast and reliable method to correct paleomagnetic deviation in volcanic rocks once corrected for anisotropy deviation the apparent scatter of the paleomagnetic directions mostly on inclination is drastically reduced

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

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

steep transient density gradients in the martian ionosphere similar to the ionopause at venus 1 with the mars advanced radar for subsurface and ionospheric sounding marsis on the mars express mex spacecraft the electron density can be measured by two methods from the excitation of local plasma oscillations and from remote sounding a study of the local electron density versus time for 1664 orbits revealed that in 132 orbits very sharp gradients in the electron density occurred that are similar to the ionopause boundary commonly observed at venus in 40 of these cases remote sounding data have also confirmed identical locations of steep ionopause like density gradients measurements from the analyzer of space plasma and energetic atoms aspera 3 electron spectrometer and ion mass analyzer instruments also on mars express verify that these sharp decreases in the electron density occur somewhere between the end of the region where ionospheric photoelectrons are dominant and the magnetosheath combined studies of the two experiments reveal that the steep density gradients define a boundary where the magnetic fields change from open to closed this study shows that although the individual cases are from a wide range of altitudes the average altitude of the boundary as a function of solar zenith angle is almost constant the average altitude is approximately 500 km up to solar zenith angles of 60 after which it shows a slight increase the average thickness of the boundary is about 22 km according to remote sounding measurements the altitude of the steep gradients shows an increase at locations with strong crustal magnetic fields

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

sedimentary records of relative paleointensity of the geomagnetic field theory and practice sediments have proved irresistible targets for attempts at determining the relative variations in the earth s magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution the assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short no dependence on magnetic field was expected nonetheless a number of experiments showed that sedimentary magnetizations varied in accordance with the field albeit not always in a simple linear fashion experiments in which the sediments were stirred in the presence of a field to simulate bioturbation showed a reasonably linear relationship with the applied field and these results spurred the hope that variations in the earth s magnetic field might indeed be recoverable from appropriate sedimentary sequences examination of existing paleointensity data sets allows a few general conclusions to be drawn it appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the earth s magnetic field the dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences moreover when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences there is considerable agreement among the various records when viewed over timescales of hundreds to thousands of years relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another suggesting that they are dominated by nondipole field behavior when viewed over timescales of a few tens of thousands to hundreds of thousands of years however the records show coherence over large distances at least thousands of kilometers and may reflect changes in the dipole field on the basis of a sequence spanning the brunhes and matuyama chrons the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years but these oscillations are not clear in the record prior to about 300 ka thus they are probably not an inherent feature in the geomagnetic field and the correspondence of the period of oscillation to that of obliquity is probably coincidence

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

paleomagnetic evidence for rotation in opposite senses of adjacent blocks in northeastern aegea and western anatolia paleomagnetic data from western anatolia and from the island of lesbos demonstrate significant counterclockwise clockwise or null rotations of coherent blocks which occurred during the neotectonic period these results suggest that the brittle carapace of the lithosphere may not everywhere follow the motion of the ductile lower part but that its movement arises from adjustment to the extensional tectonic regime

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

secular variation of the earth s magnetic field in the balkan region during the last eight millennia based on archaeomagnetic data summary the first archaeomagnetic secular variation sv curves for the whole southern balkan peninsula are presented these are based on all data within a 700 km circle centred at thessaloniki 40 60on 23 00oe this data set consists of 325 directional and 625 intensity data mainly from greece bulgaria serbia and southern hungary some data from southern italy are also included the sliding moving window technique was used to calculate a continuous sv curve for intensity while the directional sv curves were calculated using the bivariate extension of the fisher statistics these curves are well constrained and clearly show the main features of the geomagnetic field variation in this region during the last eight millennia comparisons with the predictions of the scha dif 3k and scha dif 8k regional and the cals7k 2 and arch3k 1 global geomagnetic field models show a good agreement for the last 3000 years but differences for older times the balkan sv curves identify several rapid changes of the geomagnetic field in eastern europe and can be used as reference curves for archaeomagnetic dating in the balkan peninsula

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

retention of an atmosphere on early mars the presence of valley networks and indications of high erosion rates in ancient terrains on mars suggest that mars was warm and wet during heavy bombardment various processes that could occur on early mars were integrated into a self consistent model to determine what circumstances might lead to warm temperatures during and at the end of heavy bombardment included were weathering and burial of co2 as carbonates impact erosion sputtering and recycling of co2 back into the atmosphere by burial and heating the models suggest that despite losses from the atmosphere by weathering and impact erosion mars could retain a 0 5 to 1 bar atmosphere at the end of heavy bombardment partly because weathering temporarily sequesters co2 in the ground and protects it from impact erosion while the impact rate is declining and impact erosion is becoming less effective because of the low output of the early sun surface temperatures can be above freezing only for a very efficient greenhouse such as that suggested by forget and pierrehumbert 1997 with weak greenhouse models temperatures are below freezing throughout heavy bombardment and such a large amount of co2 is left in the atmosphere at the end of heavy bombardment that it is difficult to eliminate subsequently to arrive at the present surface inventory with strong greenhouse models temperatures are well above freezing during heavy bombardment and drop to close to freezing at the end of heavy bombardment at which time the atmosphere contains 0 5 to 1 bar of co2 this can be largely eliminated subsequently by sputtering and low temperature weathering such a model is consistent with the change in erosion rate and the declining rate of valley formation at the end of heavy bombardment conditions that favor warm temperatures at the end of heavy bombardment are an efficient greenhouse low weathering rates low impact erosion rates and a smaller fraction of heat lost by conduction as opposed to transport of lava to the surface

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

deflection of paleomagnetic directions due to magnetization of the underlying terrain in the summers of 1989 and 1991 we made 344 near ground level measurements of the ambient geomagnetic field above recent basalts on the island of hawaii using a three component fluxgate magnetometer we studied 12 surface features including a lava pond lava channels long tilted blocks smooth sloping surfaces two fissures and a deep u shaped road cut we observed substantial differences up to 20 between the observed and expected international geomagnetic reference field igrf magnetic field directions over these features except those composed of shelly pahoehoe and a flat horizontally thin lava pond we also observed inclinations that were systematically shallower than the igrf field by up to 5 we show that these shallower inclinations can be explained by the magnetization of the regionally sloping surface of the southern side of the island we found that all of the observed inclination deflections can be explained by simple two dimensional models which assume uniform induced and remanent magnetization parameters in the local terrain our observations imply that the inclination deflections cannot be corrected without a complete knowledge of the preexisting terrain and the remanence in the underlying flows upon which the lavas cooled since this information is rarely available it is difficult or impossible to discriminate between dispersion of paleomagnetic directions caused by the magnetic terrain effect and dispersion due to other factors such as paleosecular variation psv we therefore conclude that psv dispersion parameters cannot be accurately determined from paleomagnetic measurements on highly magnetic volcanic flows we also suggest that some of the geomagnetic excursions inferred from measurements on volcanic rocks may be at least in part due to the magnetic terrain effect it is unnecessary to invoke ad hoc mechanisms such as clastic block or crustal rotations distortion of the top crust or flow deformation to explain the large between site dispersions or inclination anomalies observed in many of the paleomagnetic data from volcanic rocks our observations also bring into question the general reliability of paleomagnetic pole positions inferred from volcanic rocks as a systematic inclination deflection due to local and regional slopes and irregular terrain such as those we observed would lead to a corresponding error in the inferred paleolatitude the magnetic terrain effects also offer alternative explanations for anomalous paleomagnetically inferred plate motions

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

characterization of low altitude nightside martian magnetic topology using electron pitch angle distributions magnetic field lines at mars act as direct pathways for both energy inflow and ion escape local variations in magnetic field topology can therefore directly impact the interaction between the solar wind and the martian ionosphere one method of analyzing magnetic topology is through the use of electron pitch angle distributions pads previous pad investigations have characterized magnetic topology in the martian system using data from the mars global surveyor spacecraft but these studies were orbitally constrained to 400 km altitude and 2 a m 2 p m local time with the mars atmosphere and volatile evolution maven mission we are now able to extend this analysis to a larger range of altitudes and local times here we use electron pads measured using the solar wind electrostatic analyzer and magnetometer instruments on maven to analyze the magnetic topology of the nightside martian environment we use several characteristic pad shapes to determine where martian magnetic field lines are open or closed to the solar wind and present frequency maps of how these pad shapes vary both geographically and with altitude finally we present an initial analysis of the variation of the pad shapes with local time finding that trapped electron distributions become increasingly frequent as crustal fields rotate from dusk to dawn across the nightside of mars

shallow paleomagnetic directions from historic lava flows hawaii a study designed to examine the basic theory of thermoremanent magnetization acquisition parallel to the earth s ambient magnetic field upon cooling and to evaluate intra flow variations of very young basalt flows has yielded some startling initial results two extensively sampled flows the 1950 flow of mauna loa and the 1972 flow from mauna ulu kilauea have been found to have paleomagnetic directions statistically different from the present geomagnetic field direction pfd in hawaii the 1950 flow shows very consistent but shallow directions throughout the flow with a mean inclination 6 shallower than the pfd directions from the 1972 flow vary from site to site both along the length of the unit and within vertical sections relative between site consistency is low with all sites but one having paleomagnetic directions distinct from the pfd however the mean inclination for this flow 33 4 is only about 3 shallower than expected the source of such anomalous behavior in the recording of ambient field directions is yet unknown although several possible causes are examined

new archaeomagnetic directions from portugal and evolution of the geomagnetic field in iberia from late bronze age to roman times abstract this study presents new archaeomagnetic results from 33 combustion structures kilns and hearths from the archaeological sites of castelinho crestelos olival poco da barca and fonte do milho in ne portugal the age of the investigated structures ranges from 1210 bc to 200 ad according to calibrated radiocarbon dating thermoluminescence dating and archaeological constraints stepwise thermal and alternating field demagnetization isolate a single stable characteristic remanence component with very well defined directions rock magnetic analyses suggest low ti titanomagnetite maghemite as the main magnetic carrier of the remanence mean directions are well grouped in most structures the effect of thermoremanent anisotropy on mean directions has been evaluated and was found to be important inclination increases of between 2 and 13 after applying the anisotropy correction at specimen level this highlights the requirement of evaluating this effect on the directions of small and flattened thin kilns and hearths the 31 new directional data improve both the temporal and spatial distribution of the iberian archaeomagnetic dataset from late bronze age to roman times finally a new directional palaeosecular variation curve for iberia for the last twelve centuries bc is proposed the curve has been computed using the bootstrap method and includes data coming from sites within 900 km of madrid the new palaeodirectional secular variation curve for iberia is consistent with the western european palaeosecular variation curve and with the prediction of regional european models

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

ensembles of low degree archeomagnetic field models for the past three millennia abstract we introduce ensembles of time varying archeomagnetic field models consisting of a reference model a mean model and a thousand individual models we present a set of three such ensembles built from archeomagnetic volcanic and sedimentary data sets that cover the past three millennia these ensembles can be used to describe the field at any location from the core surface to the magnetosphere and assess the way uncertainties due to the limited distribution and quality of the data affect any of its component or parameter such as individual gauss coefficients they provide alternative and we argue more complete descriptions of the archeomagnetic field to those provided by previously published archeomagnetic field models being better suited to existing and emerging needs such as those of geomagnetic data assimilation we present the data sets we rely on essentially the same as those used by other recent archeomagnetic field models and describe how errors affecting the data and errors due to non modelled small spatial scales of the field are taken into account we next explain our modeling strategy and motivation for building low degree spherical harmonic degree ensembles of models we carry on a number of end to end simulations to both illustrate the usefulness of such ensembles and point at the type of errors one should expect practical illustrations of what can be done with these three ensembles of models with examples of geomagnetic inferences are also described northern high latitude flux patches for instance appear to be the most robust features of all these patches tend to fluctuate but clearly have some favored locations resulting in the same clear signature with three tongues over northern america europe and asia in the time averaged field at the core mantle boundary similar to what had been found in earlier models inferences about the field behavior in the southern hemisphere are more difficult to draw still some suggestions that the well known present south atlantic reversed patch could have arisen as early as in 1500 a d are found in some of the ensembles we otherwise confirm that most of the current archeomagnetic field model limitations are related to a number of sediment cores identified as producing frequent outliers in the modeling process we provide evidence that such cores are likely affected by timing errors and timing delays between magnetization lock in and sediment deposition that future more advanced treatment should be able to handle all source files for the three ensembles of models together with appropriate matlab applications can be downloaded from http geomag ipgp fr download archeo fm zip

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

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

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

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

palaeomagnetic distortion modelling and possible recovery by inversion abstract a robust finite element technique is presented for computation of both the internal demagnetization effects and magnetic terrain effects in bodies with arbitrary shape and arbitrary susceptibility distribution this method facilitates a flexible analysis of the palaeomagnetic deflection problem tests on geologically realistic settings of highly magnetic rocks demonstrate that deflections of several degrees may occur even for relatively simple two dimensional models similarly the magnetic intensity may well be biased by 5 15 by demagnetization effects the present paper focuses on deflections and intensity variations inside the magnetized body where we find a systematic shallowing of inclination for bodies with a horizontal elongation because the bodies sampled at a typical palaeomagnetic site will have a dominant direction of elongation the magnetic deflection effect will tend to impose a systematic bias which doesn t average out an inversion based procedure for elimination of the deflection effect is presented it requires that the magnetic body is quite homogeneous and that its surface geometry is known as may be the case for historical lava flows tests demonstrate that in order to recover both ambient palaeofield direction and the effective susceptibility at blocking temperature it is necessary to sample near strong topographic elements in the magnetic body since the surface geometry rarely is known it is proposed as an alternative to inversion that an effective susceptibility is assessed and a horizontal slab correction is applied for samples taken far from topographical features when shape geometry is unknown and no correction applied palaeomagnetic conclusions must take into account the possible bias from internal demagnetization and magnetic terrain effects

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

reconstructing the holocene geomagnetic field abstract knowledge of the holocene evolution of earth s magnetic field is important for understanding geodynamo processes in the core is necessary for studying long term solar terrestrial relationships and can provide useful age constraints for archeologicaland stratigraphic applications continuous time varying global field models based on archeo and paleomagnetic data are useful tools in this regard we use a comprehensive data compilation and recently refined modelling strategies to produce cals10k 1b the first time varying spherical harmonic geomagnetic field model spanning 10 ky the model is an average obtained from bootstrap sampling to take account of uncertainties in magnetic components and ages in the data and hence has version number 1b instead of 1 this model shows less spatial and temporal resolution than earlier versions for 0 3 ka and particularly aims to provide a robust representation of the large scale field at the core mantle boundary cmb we discuss the geomagnetic dipole evolution and changes in holocene magnetic field morphology at the cmb as shown by the new reconstruction the results are compatible with earlier models cals3k 3 and cals3k 4 for 0 3 ka but reveal some clear deficiencies in the 0 7 ka cals7k 2 model prior to 3 ka cals10k 1b is able to resolve mobile and structurally evolving high latitude radial field flux lobes at the cmb in both hemispheres as well as persistent non zonal structure in the 10 ky average contributions to the average field from time varying structures in the equatorial indonesian australian region are particularly striking

geomagnetic field inclinations for the past 400 kyr from the 1 km core of the hawaii scientific drilling project a volcanic record of geomagnetic field inclination for the past 400 kyr at hilo hawaii has been obtained from the 941 5 m of core recovered by the hawaii scientific drilling project the analysis of 195 lava flows reveals six instances of near zero inclination and two instances of fully negative inclination reverse polarity within an otherwise normal polarity core in particular flow unit 23 178 m depth records a horizontal inclination and may be associated with the laschamp event flow units 40 and 42 260 m depth record negative inclinations and are close in age to the blake event and flow unit 55 320 m depth records a negative inclination with a relative declination change of 75 with respect to the overlying flow and is probably the jamaica biwa i pringle falls event the five instances of shallow inclination found below 400 m depth appear to have resulted from long term secular variation as they are part of inclination swings between 0 and 60 with a periodicity of 10 50 kyr in contrast the inclination shifts at 178 m and 320 m depths significantly deviate from long term trends suggesting the existence of at least two independent processes producing time variations of the geomagnetic field the secular variation has a mean of 30 9 95 2 27 which is significantly shallower than the expected dipole mean of 36 the dispersion 12 5 agrees with global paleosecular variation data for 0 5 ma and secular variation models

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

magnetic anomalies of lava fields in the canary islands possible consequences for paleomagnetic records abstract measurements of the total magnetic field have been performed above the surface of 12 lava flows in the canary islands of la palma and tenerife directions can differ by up to 15 and intensity changes reach 20 the mean flow directions do not coincide and the mean flow virtual dipole moments vdms differ by more than 10 the deviations are linked to topographic features at the surface of the flows and should persist during emplacement of the next overlying flow such effects can thus limit the resolution of paleomagnetic records of detailed geomagnetic features and the field intensity variations above the flows and between the mean flow values could explain a significant part of the scatter inherent to many paleointensity studies historical field variations have been recorded from the same flows and are in agreement with the european curve for the same period however there is also some correlation with the present field which suggest that the underlying magnetized bodies could have affected the paleomagnetic records biases induced by the crustal field would increase considerably during periods of reduced field intensity like during reversals and induce erroneous records samplings performed over large distances within each lava flow would provide the best way to average out the contribution of the anomalies in any case such effects should not be neglected in paleomagnetic studies

archeomagnetic dating in western north america an updated reference curve based on paleomagnetic and archeomagnetic data sets 1 a robust database of paleomagnetic pm and archeomagnetic am data for western north america covering the past 4 millennia is assembled from three data sets 1 published pm data for dated lava flows 2 recently published am data generated by r l dubois and 3 published am data generated by j l eighmy chronologic information for the am data sets has been reassessed and the data points have been assigned to age ranges depending on their archeological contexts we have selected the computer program archeocourbe available from m le goff http www ipgp fr legoff or the auxiliary material see also http earthref org to construct am reference curves and to quantitatively estimate am dates for archeological and geological samples the updated curve provides a more detailed record of prehistoric secular variation for western north america than was previously available and shows a short interval of rapid secular variation sv in the 10th or 11th centuries at near excursional rates similar and coeval features in the hawaiian and to a lesser extent arkansan sv data sets possibly indicate a rapid amplitudinal shift in the regional nondipole field at that time

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

a regional archeomagnetic model for europe for the last 3000 years scha dif 3k applications to archeomagnetic dating 1 the available european database of archeomagnetic field values and instrumental data has been used to produce a regional model for the geomagnetic field in europe for the last 3000 years from 1000 b c to 1900 a d connecting with the epoch covered by the igrf models this new model scha dif 3k constitutes an improvement with respect to the previous regional archeomagnetic model scha di 00 f which used relocated values and was only valid for the last 2000 years the new model has been obtained by least sums of absolute deviation inversion of paleomagnetic data using spherical cap harmonics for the spatial representation of the field and sliding windows in time an algorithm has been developed to jointly model the three archeomagnetic elements declination inclination and intensity the resulting model provides the direction and intensity of the earth s magnetic field over the european continent northern africa and western asia for the last 3000 years the fit to the european archeomagnetic database is more accurate than that provided by global archeomagnetic models in addition this model represents a step forward in archeomagnetic dating studies since the relocation error is avoided and can also be used to study the rapid changes of the geomagnetic field archeomagnetic jerks that have been recently proposed

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

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

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

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

gravity modelling of the lherzolite body at lers french pyrenees some regional implications lherzolites outcrop throughout the north pyrenean zone of the pyrenees and are everywhere associated with metamorphosed carbonates it has been suggested that heat from the cooling of the lherzolites was responsible for the high temperature metamorphism of the carbonates a gravity survey reported here shows that the volume of the lherzolite body at lers is approximately 0 8 cubic km the maximum volume of carbonates that such a body could metamorphose is 3 2 cubic km this latter value is so much less than the volume of carbonates inferred from field mapping that the lherzolite body cannot have been the sole source of heat for metamorphism of the carbonates it has been suggested from seismic data that there is a step in the moho beneath the north pyrenean fault in the central pyrenees gravity anomalies reported here show that either the step is less than 10 km high or that the density contrast is very low at the base of the crust in the pyrenees

aegean paleomagnetic inclination anomalies is there a tectonic explanation abstract more than a decade of paleomagnetic research in the aegean area mainly by french investigators c laj c kissel and their colleagues has provided a valuable data base for the evaluation of cenozoic aegean tectonics systematic rotations are documented and have been interpreted by the original investigators in the light of the evolution of the hellenic arc however less attention has been paid to the significance of the paleomagnetic inclinations which also are anomalous twenty one out of 22 paleomagnetic studies performed to date within the general aegean region have yielded paleolatitudes that are too shallow when compared with reference curves for either africa or eurasia suggesting northward relative transport with respect to both continents sedimentary volcanic and plutonic units are involved ages range from oligocene to pliocene some of these apparent paleolatitude anomalies may merely be the result of inclination flattening in certain kinds of clastic sedimentary rocks however shallow inclinations in igneous rocks cannot be so easily dismissed the explanation for these anomalously shallow inclinations does not appear to lie with the reference curves because several other reference curves calculated in several different ways yield essentially the same result we suggest that the paleomagnetic pattern may reflect real northward displacement of crustal blocks within the aegean region measured with respect to both europe and africa the amount of displacement appears to increase systematically to the west the pattern of local block rotations noticed by earlier investigators argues that the displacement did not involve a single coherent aegean block but rather a collection of smaller blocks moving quasi independently geological consequences of northward relative displacement of the aegean block ought to include mid tertiary and or later compressional structures in the north e g rhodope and extensional structures in the leading edge of the african plate

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

a method for correcting geographically separated remanence directions for the purpose of archaeomagnetic dating summary spatial variations in the geomagnetic field must be taken into account if secular variation master curves and directional magnetic dates are to be optimized two methods for relocating remanence vectors have been proposed and in this paper their relative accuracies are compared using a numerical model based on the present day field a method which converts archaeomagnetic directions via a virtual geomagnetic pole is shown to be the more efficient transformation for an archaeomagnetic region the size of the british isles 900 km radius the maximum error in relocating vectors to a central location is predicted to be of the order of 1 2

paleomagnetic secular variation at vulcano aeolian islands during the last 135 kyr abstract paleosecular variation psv of the earth s magnetic field during the last 135 kyr has been investigated in lavas scoriae and pyroclastic rocks of vulcano aeolian islands about 1000 samples have been collected at 77 sites from 25 distinct volcanic units whose age is either known from published isotopical data or constrained on the grounds of statigraphical relationships magnetic mineralogy investigation shows that ti magnetite is the main ferromagnetic mineral at most sites secondary magnetization components are either absent or easily removed by stepwise thermal or alternating field demagnetization the mean site direction of the characteristic remanent magnetization is usually well defined since the semi angle of confidence is greater than 5 at only four sites the mean paleomagnetic direction over the last 135 kyr d 9 4 i 53 2 95 3 5 differs from the geocentric axial dipole gad at vulcano d 0 i 57 8 and might be interpreted as the effect of a long term non axial dipolar component the psv record from vulcano agrees well with those from the lacustrine sediments of lago grande di monticchio 100 kyr bp and lago di mezzano 30 kyr bp located in the italian peninsula brandt et al quat sci rev 18 1999 961 976 the inclination anomaly i found at vulcano corresponds to about half of the shallowing observed in the sediments of the two lakes and the declination anomaly d may be used to tie the declination values derived from azimuthally unoriented cores to the geographical reference system in order to find the optimum site to be used as reference for psv studies in italy the angular values of the earth s magnetic field measured at the 113 repeat stations of the italian geomagnetic network coticchia et al boll geod sci aff 40 2001 277 291 have been analyzed with the relocation via pole method noel and batt geophys j int 102 1990 753 756 the viterbo station lat 42 27 n long 12 02 e proved the best since the mean error is 0 3 for both declination and inclination wherever the original psv site is located in italy a preliminary composite psv curve for the last 30 kyr bp is thus proposed merging and relocating to viterbo the data from vulcano and the curve from lago di mezzano corrected for the gad deviation found at vulcano

unbiased analysis of geomagnetic data sets and comparison of historical data with paleomagnetic and archeomagnetic records reconstructions of the past geomagnetic field provide fundamental constraints for understanding the dynamics of the earth s interior as well as serving as basis for magnetostratigraphic and archeomagnetic dating tools such reconstructions when extending over epochs that precede the advent of instrumental measurements rely exclusively on magnetic records from archeological artifacts and farther in the past from rocks and sediments the most critical component of such indirect records is field intensity because of possible biases introduced by material properties and by laboratory protocols which do not reproduce exactly the original field recording conditions large biases are usually avoided by the use of appropriate checking procedures however smaller ones can remain undetected in individual studies and might significantly affect field reconstructions we introduce a new general approach for analyzing geomagnetic databases in order to investigate the reliability of indirect records this approach is based on the comparison of historical records with archeomagnetic and volcanic data considering temporal and spatial mismatches with adequate weighting functions and error estimation a good overall agreement is found between indirect records and historical measurements while for several subsets systematic bias is detected e g inclination shallowing of lava records we also demonstrate that simple approaches to analyzing highly inhomogeneous and internally correlated paleomagnetic datasets can lead to incorrect conclusions about the efficiency of quality checks and corrections consistent criteria for selecting and weighting data are presented in this review and can be used to improve current geomagnetic field modeling techniques

international geomagnetic reference field the eleventh generation the eleventh generation of the international geomagnetic reference field igrf was adopted in december 2009 by the international association of geomagnetism and aeronomy working group v mod it updates the previous igrf generation with a definitive main field model for epoch 2005 0 a main field model for epoch 2010 0 and a linear predictive secular variation model for 2010 0 2015 0 in this note the equations defining the igrf model are provided along with the spherical harmonic coefficients for the eleventh generation maps of the magnetic declination inclination and total intensity for epoch 2010 0 and their predicted rates of change for 2010 0 2015 0 are presented the recent evolution of the south atlantic anomaly and magnetic pole positions are also examined

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

distribution analysis of errors due to relocation of geomagnetic data using the conversion via pole cvp method implications on archaeomagnetic data summary secular variation master curves are built up using geomagnetic historical observations or archaeomagnetic data from a limited area and their use is usually restricted to regions of around 1000 km radius relocation of data within this distance is a common practice to enable comparison of data although the errors due to such process are rarely taken into account a detailed analysis of the distribution of relocating geomagnetic data has been done using three popular sets of geomagnetic models igrf 9 gufm and cals7k 2 this study improves the error analysis of relocating geomagnetic directions made up to date and expands it to geomagnetic intensities maximum errors correlate with the non dipole to dipole field ratio archaeomagnetists could use this analysis to valuate the error introduced by reducing data

three millennia of directional variation of the earth s magnetic field in western europe as revealed by archeological artefacts directional secular variation of the geomagnetic field over the last 2000 years has been defined in western europe from numerous archeomagnetic studies however the number of archeomagnetic results for older periods is much more limited for this reason we present new data obtained from fired archeological structures found in two french sites loupiac and aspiran dated within the first millennium b c latest bronze earliest iron age transition 850 700 b c and iron age 525 475 b c respectively from a compilation of archeomagnetic results from western europe great britain italy and france and northern africa tunisia we propose a directional secular variation curve for western europe that covers the entire first millennium b c this curve exhibits a large clockwise motion with rapid changes during the first half of the millennium while the last four centuries b c are characterized by weak variations

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 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 geomagnetic field model for the holocene based on archaeomagnetic and lava flow data abstract we propose a new geomagnetic field model for the holocene period based on archaeomagnetic and lava flow data avoiding the use of lake sediment data the source of data comes from the geomagia50v2 database which has been updated with the new archaeomagnetic and volcanic studies published during the last 3 yr the model called sha dif 14k allows us to analyse the behaviour of the geomagnetic field for the last 14 000 yr from 12 000 bc to 1900 ad for the model construction we use the spherical harmonic analysis in space and the penalized cubic b splines in time both spatial and temporal regularization norms are used to constrain the inversion problem and applied at the core mantle boundary cmb to assure the convergence of the model for the last 3 ka the model predictions agree with those given by the global model arch3k 1 and the european model scha dif 3k for older epochs the new model presents a clear improvement in field resolution with respect to other current models of the geomagnetic field for the holocene for the last 9 ka the time evolution of the dipolar moment obtained from the dipole field shows a clear minimum between 5500 bc and 3000 bc and the well known continuous decreasing trend of the geomagnetic field strength for the last millennium and a half a general view of the time average evolution of the geomagnetic field flux lobes at the cmb for the northern hemisphere suggests a marked lobe of positive magnetic flux when the dipole moment was maximum this lobe vanishes when the dipolar field is decreasing the north polar wander paths of both north magnetic dip and geomagnetic poles were obtained showing an average rate of motion of 5 1 km yr and 3 7 km yr respectively the model shows that the geomagnetic field can be averaged as axial dipolar in 2000 yr within an error of 5 the typical uncertainty of the palaeomagnetic studies finally and following the recent definition of archaeomagnetic jerks we found 8 critical events in the time evolution of the geomagnetic field for the last 8 ka characterized by a maximum in the hemispheric asymmetry of the proposed model the model is available in the earth ref digital archive at http earthref org erda 1897

archaeointensity determinations from italy new data and the earth s magnetic field strength variation over the past three millennia summary three kilns and a collection of baked bricks from italian archaeological sites have been studied for archaeointensity determination using the thellier method as modified by coe all sites are dated based on archaeological information and their ages range from 500 to 800 ad and 1500 to 1700 ad rock magnetic studies identify magnetite ti magnetite and hematite as the main magnetic minerals and magnetic susceptibility versus temperature shows a good thermal stability of the samples the intensity results have been corrected for anisotropy of the thermoremanent magnetization and cooling rate effects the new data together with 136 previously published results are used to estimate the variation of the earth s magnetic field over the past three millennia the time distribution of the italian absolute intensity data is irregular with the majority of determinations concentrated during the last four centuries while older periods are very poorly covered most of the data come from volcanic rocks and show significant discrepancies all data have been compared with archaeointensity results from europe and regional and global models predictions this work indicates the need to obtain more high quality archaeointensity results in particular for the periods older than 200 bc and between 200 and 1000 ad in order to determine a robust italian intensity secular variation curve that in combination with directional data could be used for archaeomagnetic dating

geomagnetic secular variation in historical lavas from the canary islands summary the age of historical lava flows on the canary islands can be traced back at least to 1585 ad during the last 400 years eruptions occurred at fairly regular intervals the palaeomagnetic directions of these lavas record the ambient field direction with high precision so that an historical secular variation curve svc can be constructed for the canaries which closely resembles the curve registered at paris the svc can be used as a dating tool for historical eruptions of unknown age and can probably be extended into the last 4000 5000 yr of recent volcanic activity on the canaries

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

magnetization age from paleomagnetism of the copper harbor red beds northern michigan usa and its keweenawan geologic consequences the copper harbor formation on lake superior s keweenaw peninsula records the transition from volcanic to sedimentary infilling of north america s 1 1 ga keweenawan rift radiometric dating shows t

statistical analysis of palaeomagnetic data from the last four centuries evidence of systematic inclination shallowing in lava flow records the main objective of this work is to compare directional declination and inclination volcanic and archaeomagnetic data for the last four centuries 1600 1990 with the historical geomagnetic predictions given by the gufm1 model which spans from 1590 to 1990 the results show statistical agreement between archaeomagnetic data and directions given by the geomagnetic field model however when comparing the volcanic data with the model predictions marked inclination shallowing is observed this systematically lower inclination has already been observed in local palaeomagnetic studies italy mexico and hawaii for the 20th century by comparing recent lava flows with the international reference geomagnetic field igrf model here we show how this inclination shallowing is statistically present at worldwide scale for the last 400 years with mean inclination deviation around 3 lower than the historical geomagnetic field model predictions

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

martian magnetic storms the response of mars to the major space weather events called interplanetary coronal mass ejections icmes is of interest for both general planetary solar wind interaction studies and related speculations on their evolutionary consequences especially with respect to atmosphere escape various particle and field signatures of icmes have been observed on phobos 2 mars global surveyor mgs mars express mex and now mars atmosphere and volatile evolution maven of these maven s combined instrumentation and orbit geometry is particularly well suited to characterize both the event drivers and their consequences however maven has detected only moderate disturbances at mars due in large part to the general weakness of the present solar cycle nevertheless the strongest event observed by maven in march 2015 provides an example illustrating how further insights can be gained from available models here we first look more closely at what previously run bats r us mhd simulations of the combined maven observations tell us about the march 2015 event consequences we then use analogous models to infer those same responses including magnetic field topology changes and ionospheric consequences to a hypothetical extreme icme at mars based on stereo a measurements in july 2012 the results suggest how greatly enhanced yet realistic solar wind pressure magnetic field and convection electric field combine to produce strong magnetospheric coupling with important consequences for upper atmosphere and ionosphere energization

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

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

distortion of the geomagnetic field in volcanic terrains an experimental study of the mount etna stratovolcano abstract the inclination i declination d and total intensity f of the geomagnetic field were measured on mount etna in 1989 1991 at a dozen sites previously sampled for archeomagnetic studies the purpose of the work was to determine the variations of these parameters at 30 cm above ground level and how the distortion from the main field can affect the archeomagnetic record of volcanic rocks ten measurements were usually performed at each site with a three component flux gate magnetometer whose estimated precision is 0 2 on direction and 50 nt on intensity this was considered sufficient on volcanic areas with highly magnetized rocks and where the geomagnetic gradient may be in excess of 1000 nt m results averaged for each site generally show small variations in intensity 3 of the total field and direction 1 5 the averaged values of the 12 sites i 52 6 d 0 3 f 44010 nt are very close to those measured in sedimentary terrain away from the volcano i 52 9 d 0 35 f 44110 nt themselves consistent with the interpolated igrf in eastern sicily the largest deviations of the geomagnetic direction have been observed on four sites three of them located on the south flank between 1900 and 700 m elevation it is suggested that these anomalies are mainly related to dyke swarms which are common within the south rift zone of mount etna our findings show that reliable archeomagnetic results can be obtained from volcanic rocks provided that lavas of the same eruption are sampled on several sites distributed over the largest possible area