contemporary tectonic stress in northeastern iberia new results from borehole breakout analysis abstract in situ data on the orientation of tectonic stress in northern iberia are scarce fault slip and focal mechanism data show a variety of stress orientations difficult to interpret in terms of a general stress pattern breakout analysis performed on oriented caliper logs from 17 hydrocarbon exploration wells has provided new data on the orientation of tectonic stress in northeastern iberia the southern pyrenees the ebro basin and the northwestern valencia trough the majority of breakout data indicate a rather consistent ne sw to ene wsw directional signature of the larger horizontal principal stress orientation this orientation differs significantly from the nw western european mean stress orientation it is consistent with depth within the first 4 7 km of both the sedimentary fill and the sedimentary basins substrate and also within allochthonous thrust sheets in the southern pyrenees and in the underlying autochthonous and parautochthonous foreland basin sediments

fault reactivation in brittle viscous wrench systems dynamically scaled analogue models and application to the rhine bresse transfer zone abstract analogue experiments on oblique rifting and subsequent transpressional reactivation were performed with two layer slabs of sand and silicone in this brittle viscous system transtensional and transpressional wrench faulting was induced by movements of a basal rigid plate the dynamically scaled analogue models are confronted with the structural evolution of the rhine bresse transfer zone rbtz that linked palaeogene rifting in the upper rhine and bresse grabens and that appears to have been transpressively reactivated in neogene to recent times fault patterns produced in the sand layer above the basal silicone layer are compared with structural elements in the sedimentary cover separated from the basement by an evaporitic decollement layer in order to investigate strain rate dependence of fault reactivation and graben inversion transpressional shortening was performed under different displacement rates experimental results suggest that the reactivation of pre existing structures in a brittle cover above a viscous decollement is strongly dependent on the strain rate within the viscous layer under low to intermediate displacement rates 2 6 cm h 1 deformation concentrates within the basal viscous layer and former normal faults within the cover are not reactivated the reactivation at higher displacement rates 5 cm h 1 results in a complete inversion of graben structures within the cover ongoing shortening produces lobed thrust fronts which crosscut pre existing normal faults late pliocene to recent en echelon aligned folds and isolated thrust faults in the cover of the rbtz are attributed to thick skinned reactivation of basement faults a comparison of natural and experimentally obtained structures suggests that fault reactivation occurred under low displacement rates

extensional forced folding and decollement of the pre rift series along the rhine graben and their influence on the geometry of the syn rift sequences

shear fracture pattern and microstructural evolution in transpressional fault zones from field and laboratory studies abstract zones of transpressional shear deformation accommodate strike slip and oblique slip displacements field work in a transpressive shear zone and transpressional analogue clay box modelling show that a p oriented foliation and associated p shears are preferentially developed over the more common r 1 riedel shears the carboneras fault system cfs in se spain is a left lateral transpressional shear zone with an internal geometry characterized by first order y oriented faults and widespread p oriented second order faults the mesoscopic to microscopic gouge fabric reflects the regional architecture of the shear zone being dominated by a pervasive poriented foliation and discrete y and p shears friction experiments carried out to investigate the textural evolution of gouge fabrics showed four textural stages of fabric development from foliation formation to extreme shear localization resulting in cross gouge failure transpression clay box models favoured the formation of secondary p oriented shear fractures and p oriented shear lenses further deformation caused differential shear lens rotation and shear lens orientations closer to the mean displacement direction our field studies and laboratory analogue experiments indicate that shear zones dominated by p shears are diagnostic of a transpressional deformation regime

analysis of the 2002 may earthquake sequence in the central pyrenees consequences for the evaluation of the seismic risk at lourdes france summary three earthquakes of magnitudes 4 6 4 3 and 3 7 occurred in 2002 may at two locations 20 km from the pilgrimage city of lourdes in the french pyrenees they were well recorded by the permanent pyrenean seismic networks by a temporary local network as well as by accelerometric stations in order to understand their tectonic contexts and to come to a better evaluation of the seismic risk at lourdes a detailed analysis of these events is performed the first two events are located south of lourdes in an area where only a few earthquakes have occurred up to now their focal solutions derived from first motion polarities indicate reverse faulting with a n110 e strike consistent with the geological structures 10 aftershocks were recorded and relocated with respect to the main events benefiting from the waveform similarity of the various events this analysis reveals that the two main events concern probably the same fault the second rupture being in the prolongation of the first one whereas the other small aftershocks are located on fault segments in the vicinity of the hypocentre of the second event the third large event located to the se of lourdes involves a normal mechanism with a n120 e plane parallel to the main geological structures it occurred in a region of intense activity including in particular an event of maximum macroseismic intensity ix in 1660 the first two events are at the boundary of a large quiet zone in order to understand the related structural context a new crustal tomographic model has been computed it reveals that this quiet zone coincides with a block of high p velocity in contrast the seismicity appears to be stronger at the northern and eastern boundaries of this block the accelerometric data of the three main events recorded at lourdes have been used to estimate the maximum peak ground accelerations in this city if a large event occurred similar to those which damaged the city in the seventeenth and eighteenth centuries horizontal accelerations of 0 25 0 07 g are predicted in the frequency domain 1 5 hz at the location of the sanctuary for a magnitude 6 event occurring 10 km away from the city taking into account the error bars these values could in some cases exceed those specified by the building codes in this region

microseismicity of the bearn range reactivation of inversion and collision structures at the northern edge of the iberian plate the bearn range located to the north of the axial zone in the western pyrenees is affected by numerous small magnitude seismic events these events overlap an area characterised by specific geological structures which are interpreted to have resulted from multistage extensional and compressional deformation an analysis of surface geology draped over dem together with field investigations allow identification of two main shortening episodes with differing direction of contraction d1 represents the inversion of the north pyrenean basin whose mesozoic infill was detached from highly extended crust and transported southwards over the necking zone and the northern margin of the iberian plate d2 corresponds to the collision stage and is characterised in the study area by backfolding and backthrusting deformation coeval with uplift in the axial part of the chain due to thickening of the iberian plate the microseismicity appears to concentrate along the basal part of the inverted basin units d1 where this initially low angle thrust has been tilted and steepened during collision d2 we propose that local steepening of this ancient inversion structure which should not be named north pyrenean fault provided the suitable dip for extensional solicitation in association with the present uplift of the axial zone whatever the driving mechanism of this uplift could be

a tomographic study of the lithosphere beneath the pyrenees from local and teleseismic data a tomographic image of the pyrenean lithosphere from the surface down to 200 km is deduced from the propagation of the seismic waves recorded at the regional pyrenean seismograph network crustal and subcrustal models are derived independently from the inversion of two different data sets p and s velocity models for the crust are obtained from the propagation of local earthquakes the upper crust appears very heterogeneous and exhibits sharp velocity contrasts in the uppermost 7 km an east west trending zone with velocities up to 6 higher than the surrounding material is observed all along the pyrenees this zone coincides in most places with the exposure of the paleozoic rocks of the axial zone low velocities are observed on both sides of this high velocity zone at the surface they coincide with well known sedimentary basins but dip down to about 11 km depth toward the axis of the range in the central and eastern pyrenees two large high velocity bodies stand out in the north pyrenean zone and extend down to the base of the upper crust they coincide with large positive gravity anomalies and are interpreted as lower crust or upper mantle blocks uplifted into the upper crust the lower crust is poorly resolved but appears to be less heterogeneous than the upper crust moreover the lower crustal structure is almost uncorrelated with the upper crustal structure the major feature of the lower crust is a low velocity anomaly which stretches along the north pyrenean fault in the western pyrenees and beneath the axial zone in the central pyrenees its location nearly coincides with the distribution of the seismicity in the upper mantle p models are derived from the travel times of the remote events recorded at the pyrenean stations the most important feature is the presence in the eastern and central pyrenees of a vertical low velocity heterogeneity south of the north pyrenean fault down to 80 100 km this structure exhibits a 2 velocity contrast with respect to the surrounding structures and is interpreted as lower crustal material subducted during the pyrenean convergence this heterogeneity is not observed in the western pyrenees suggesting that the crustal shortening has been completely accommodated by several thrusts in the thinned aquitaine crust

cave levels and cave development in the mitchell plain following base level lowering multiple levels evolved in buddha christian s cave and in donaldson s bronson s twin cave in a subtly different manner to that normally associated with the development of multilevel caves adjacent to entrenched river valleys it is often assumed that lower cave levels are generated by successive phases of valley entrenchment however multiple levels may also be induced by progressive adjustments in groundwater flow paths such adjustments occur as the effects of a change in base level propagate up basin in the mitchell plain much of the late pleistocene drop in the elevation of the principal drainage the east fork of the white river was accommodated by entrenchment along the lower reaches of tributary valleys caves initially developed in the mid sections of tributary basins at relatively shallow depths in the vicinity of the cave exit the piezometric surface gradually steepened and was eventually lowered through a combination of valley headwall retreat and the incremental concentration of flow through master conduits upper level passages were abandoned once the original high level flow paths were replaced by lower level routes

experiments on simultaneous faulting and folding above a basement wrench fault abstract wrench zones commonly show en echelon patterns of folds in the stratified cover above a basement fault examples are the darien basin in panama and the inglewood fault trend in california folds are often associated with synthetic and antithetic wrench faults but the time relationships between folds and faults may not be obvious we have done experiments where simultaneous faulting and folding occur above a basement fault four experiments were performed using a simple apparatus capable of imposing localised strike slip motions this apparatus consists of two plastic plates one can be moved laterally past the other by a piston thus modelling lateral motion on a single basement fault models were properly scaled to account for gravitational forces brittle behaviour of sandstone and ductile behaviour of clay or shales in the upper crust each model is a vertical succession of silicone layers and sand layers each layer is 2 mm thick except for the basal silicone layer which is 2 4 6 or 8 mm thick in experiments 1 2 3 or 4 respectively all models were deformed at the same slip rate on the basal fault 1 cm h 1 experiments 1 2 and 3 with 8 cm total horizontal sinistral offset were designed to investigate the influence of basal silicone layer thickness on the deformation of a multilayered model experiment 4 with 20 cm of total offset was designed to investigate the influence of an increasing left lateral offset on folding and faulting with a 2 mm thick basal silicone layer experiment 1 synthetic strike slip faults appeared en echelon above the basement fault no folds were visible at the surface of the experiment but the interfaces between sand and silicone layers showed millimetric folds en echelon above the basement fault with a 4 mm thick basal silicone layer experiment 2 en echelon synthetic strike slip faults and folds appeared above the basement fault faults and folds rotated progressively anticlockwise and synclines were activated as dextral antithetic shear zones with a 6 mm thick basal silicone layer experiment 3 synthetic faults and en echelon folds appeared as before but the fold amplitude was larger than in experiment 2 an increasing left lateral offset produced anticlockwise rotation of faults and folds as before and synclines were activated as dextral antithetic shear zones in experiment 4 after 16 cm of left lateral offset the anticlockwise rotation of the folds was such that antithetic dextral shear zones became almost parallel to the basement fault and were then reactivated as synthetic sinistral shear zones

a structural model for the seismicity of the arudy 1980 epicentral area western pyrenees france summary the western pyrenees presents a diffuse and moderate m 5 7 instrumental seismicity it nevertheless historically suffered from strong earthquakes i ix msk the seismic sources of these events are not yet clearly identified we focus on the arudy 1980 epicentral area m 5 1 and propose here the reactivation of early cretaceous normal faults of the iberian margin as a potential source the late cretaceous inversion of this basin first in a left lateral strike slip mode and then in a more frontal convergence resulted in a pop up geometry this flower structure attests of the presence of a deep crustal discontinuity the present day geodynamic arrangement suggests that this accident is reactivated in a right lateral mode this reactivation leads to a strain partitioning between the deep discontinuity that accommodates the lateral component of the motion and shallow thrusts rooted on this discontinuity these thrusts accommodate the shortening component of the strain the distribution of the instrumental seismicity fits well the structural model of the arudy basin whatever the compressive regional context the structural behaviour of the system explains too the extensive stress tensor determined for the arudy crisis if we interpret it in terms of strain ellipsoid indeed numerical modelling has shown that this concomitant activity of strike slip and thrust faulting results in an extensive component that can rise 50 per cent of the finite strain we identify too a 25 30 km long potential seismic source for the arudy area the size of the structure and its potential reactivation in a strike slip mode suggest that a maximum earthquake magnitude of 6 5 could be expected the extrapolation of this model at the scale of the western pyrenees allows to propose other potential sources for major regional historical earthquakes

late pleistocene folding above the mail arrouy thrust north western pyrenees france abstract the western pyrenees have experienced several major earthquakes in the last 400 years herein we investigate the ongoing tectonic activity of the arudy area affected by one of the largest earthquakes m 5 1 1980 registered in the instrumental seismicity catalogue in the western pyrenees folding of alluvial terraces younger than 17 3 ky above the mail arrouy thrust mat attest from a late pleistocene tectonic activity this deformation resulted in a shallow fold with a wavelength of 2800 m and amplitude of 7 to 8 m the mat is rooted above a former normal fault of the cretaceous iberian margin which is a potential seismogenic source such observations suggest that the northern flank of the western pyrenees is still subjected to shortening in response to a compressional stress regime this compression is consistent with the african eurasian plate kinematics and the arudy 1980 earthquake nonetheless recent seismological data suggest the presence of an extensional stress regime in the range this extension is indicated by some seismic events mostly observed in the pyrenean high chain our hypothesis is that these extension driven events could be due to a local stress field induced by the elevation of the range on the contrary the compression associated with the regional stress regime could prevail in the outermost domain of the range

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

physical experiments of transpressional folding abstract in order to understand the process of folding in obliquely convergent settings we formed folds within a shear box capable of creating homogeneous transpressional deformations folds were created in a single layer of stiff mixed plasticine and silicone that overlay a newtonian silicone for a variety of plate convergence angles as small amplitude folds became visible they were parallel to the long axis of the horizontal finite strain ellipse with increasing deformation the fold hinges rotated parallel with the long axis of the horizontal finite strain ellipse for all angles of convergence this parallelism indicates that fold hinges once formed rotate with the horizontal strain ellipse rather than as material lines the experiments highlight several interesting effects of transpression dynamics the fold hinges initiate parallel to either 1 or 2 and are parallel to either s 1 or s 2 with increasing deformation neither infinitesimal strain stress nor finite strain is resolvable solely from fold geometry further the net amount of contraction determined by folding across the zone was overestimated in all cases except pure contraction this effect is obvious for the case of wrenching where folding implies that the zone contracts if elongation parallel to the fold hinge is not considered therefore attempts to balance cross sections in transpressional zones will tend to overestimate contraction unless the wrench component of deformation is addressed this result is validated by applying the modeling results in folding in central california adjacent to the san andreas fault where cross section balancing indicates higher amounts of contraction than predicted by plate motion

geodetic measurements of crustal deformation in the western mediterranean and europe geodetic measurements of crustal deformation over large areas deforming at slow rates 5 mm yr over more than 1000 km such as the western mediterranean and western europe are still a challenge because 1 these rates are close to the current resolution of the geodetic techniques 2 inaccuracies in the reference frame implementation may be on the same order as the tectonic velocities we present a new velocity field for western europe and the western mediterranean derived from a rigorous combination of 1 a selection of sites from the itrf2000 solution 2 a subset of sites from the european permanent gps network solution 3 a solution of the french national geodetic permanent gps network rgp and 4 a solution of a permanent gps network in the western alps regal the resulting velocity field describes horizontal crustal motion at 64 sites in western europe with an accuracy on the order of 1 mm yr or better its analysis shows that central europe behaves rigidly at a 0 4 mm yr level and can therefore be used to define a stable europe reference frame in that reference frame we find that most of europe including areas west of the rhine graben the iberian peninsula the ligurian basin and the corsica sardinian block behaves rigidly at a 0 5 mm yr level in a second step we map recently published geodetic results in the reference frame previously defined geodetic data confirm a counterclockwise rotation of the adriatic microplate with respect to stable europe that appears to control the strain pattern along its boundaries active deformation in the alps apennines and dinarides is probably driven by the independent motion of the adriatic plate rather than by the africa eurasia convergence the analysis of a global gps solution and recently published new estimates for the african plate kinematics indicate that the africa eurasia plate motion may be significantly different from the nuvel1a values in particular geodetic solutions show that the convergence rate between africa and stable europe may be 30 60 slower than the nuvelia prediction and rotated 10 30 counterclockwise in the mediterranean

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

analogue modelling of fault reactivation tectonic inversion and oblique remobilisation of grabens abstract analogue models of sand above a silicone layer were examined to determine the effects of normal fault reactivation models were first subjected to extension which lead to the formation of two linear grabens each model was then subjected to a second phase deformation either parallel or oblique to the previous initial extension and either extensional or contractional the influence of sedimentation has been evaluated using experiments with and without sedimentation in cases of oblique secondary deformation all newly formed faults were parallel to the older grabens thus they were oblique to the direction of the principal stress directions during the second deformation phase in experiments without sedimentation all older faults were reactivated whereas in experiments with sedimentation some of the older faults were not reactivated in the case of an oblique compressional secondary deformation phase with post rift sedimentation strain partitioning occurred between reactivated older normal faults and new reverse faults 2 was vertical on reactivated normal fault planes whereas 3 was vertical on reverse fault planes in the case of oblique compressional secondary deformation phase without post rift sedimentation no strain partitioning was observed in this model 3 was vertical on every fault plane it is therefore concluded that sedimentation within grabens induces a variation of stress orientation and strain partitioning

transpression along the southern san andreas fault durmid hill california transpression is expressed at durmid hill in the development and subsequent rotation of en echelon folds and ongoing vertical uplift the transpression related strain field can be quantified by an analysis of 1 orientation of rotated fold axes which display an angle of about 20 between the san andreas fault and the fold axial traces 2 shortening perpendicular to the fold axes 40 60 3 extension parallel to the fold axes 58 68 measured using a boudinaged ash bed and 4 uplift determined from an antecedent stream 1 1 1 7 mm yr the topographic expression of durmid hill and the exposed stratigraphy the observed deformation may be approximated by a homogeneous transpressional deformation this model suggests that an estimated 9 14 of the total displacement predicted by plate motion models across the san andreas fault has been accommodated by the durmid hill deformation zone adjacent to the fault this off fault deformation is not directly coupled to coseismic or interseismic movement on the fault itself

active faulting in the western pyrenees france paleoseismic evidence for late holocene ruptures abstract we have identified a 50 km long active fault scarp called herewith the lourdes fault between the city of lourdes and arette village in the french pyrenees this region was affected by large and moderate earthquakes in 1660 io viii ix msk 64 in 1750 io viii msk 64 and in 1967 md 5 3 io viii msk 64 most earthquakes in this area are shallow and the few available focal mechanism solutions do not indicate a consistent pattern of active deformation field investigations in active tectonics indicate an east west trending and up to 50 m high fault scarp in average made of 3 contiguous linear fault sub segments to the north the fault controls quaternary basins and shows uplifted and tilted alluvial terraces deviated and abandoned stream channels of the southern block are likely due to the successive uplift of the northern block of the fault paleoseismic investigations coupled with geomorphic studies georadar prospecting and trenching along the fault scarp illustrate the cumulative fault movements during the late holocene trenches exhibit shear contacts with flexural slip faulting and thrust ruptures showing deformed alluvial units in buried channels 14c dating of alluvial and colluvial units indicates a consistent age bracket from two different trenches and shows that the most recent fault movements occurred between 4221 bc and 2918 bc fault parameters and paleoseismic results imply that the lourdes fault and related sub segments may produce a mw 6 5 to 7 1 earthquake fault parameters imply that the lourdes fault segment corresponds to a major seismic source in the western pyrenees that may generate earthquakes possibly larger than the 1660 historical event

tertiary compressional overprint on aptian albian extensional magnetic fabrics north pyrenean zone abstract the mauleon basin constitutes part of the north pyrenean mesozoic extensional basins inverted during pyrenean late cretaceous tertiary orogeny a structural and magnetic fabric study of the aptian albian black marls of the mauleon basin north pyrenean zone provides new data indicating that when there is an intense deformation associated with the compressional stage formation of a pervasive foliation the magnetic lineation is strongly controlled by the latter tectonic deformation recorded in the basin however some areas of the sedimentary basin can still preserve the primary extensional event depending on its position with respect to basin margin faults and heterogeneous deformation areas structural and ams data define a fabric transition from extensional fabrics indicating an approximate n s extension in the central inner domain of the basin where compressional deformation was moderate to ne sw compressional fabrics in boundary domains close to the inverted faults where compressional deformation was more important k max becomes nearly parallel to the main pyrenean direction nw se consequently fabrics in the mauleon basin can be interpreted as the result of the overprint of compressional deformation onto a primary extensional fabric which in turn endures in the central domain of the basin in spite of its complicated pattern ams is revealed useful when considering the long term history of sedimentary basins undergoing several deformation events

hinge migration as a mechanism of superimposed folding abstract paraffin wax analogue modelling of superimposed folding shows how early fold shape and orientation determine fold axis orientation type of interference pattern and mechanism of superposed folding during two successive deformations the behaviour depends on the angle between the first and second compression directions i when they are perpendicular to each other classic superposed folding with either domes and basins or folds with vertical axes develop depending on whether the earlier folds were open or closed ii when the two compression directions are oblique the earlier folds rotate the degree of rotation depends on the angle between the first folds and the later compression direction and is proportional to the amount of strain the orientations of the new folds are close to those of the earlier open folds the final deformation pattern consists of domains where the orientations of the axes depend on whether the early folds were previously either open or closed many early folds are reused to become later folds by the mechanism of hinge migration these results are consistent with the pattern of superimposed folding observed in the devoluy structure of the french alps they show that simply by studying the directions of fold axes in areas of superposed folding it is not possible to define compression directions moreover there is the possibility that superposed folding may go unnoticed if hinge migration has occurred

intraplate seismicity new seismotectonic data in western europe abstract fault plane solutions for 128 earthquakes which occurred in france and neighbouring areas have been determined stress field information given by these data are discussed in relation to previous tectonic knowledge the western alps are characterized by a compressional regime with a fan shaped distribution of p axes which might be related to the interaction mechanism between europe and the adriatic plate the pyrenean region is mainly submitted to a south north compression tectonic activity in this area seems to be determined by the presence of two subduction zones with reverse dips in the south bay of biscay and in the central pyrenees the transition zone between these two active belts is characterized by significant seismicity with a transpression regime

kinematics of compressional fold development in convergent wrench terranes abstract kinematic models are presented for compressional fold development in wrench and convergent wrench terranes that relate fold shortening axial rotation and axial extension fold shortening may be derived from final fold geometry existing fold geometry and axial orientation two readily measurable quantities provide the data needed to determine the relative components of shearing and convergence within the fold system analyses utilizing these kinematic models indicate that folds developed in sedimentary rocks in the wrench borderlands of both the rineonada and san andreas wrench faults in central california are the product of strongly convergent wrenching the axes of these folds have been rotated no more than a few degrees during the course of their development in contrast folds developed in the alpine schists along the alpine fault in new zealand and in pleistocene sediments along the southern limit of the san andreas fault suggest an almost pure wrench setting and large 25 axial rotations significant axial extension is inherent in wrench related compressional folds this axial extension is commonly manifest in the form of normal and strike slip faults that are internal to the folds and trend at high angles to the fold axes the relative amount of axial extension diminishes as the degree of convergence increases this axial extension and the associated extensional features can be a diagnostic indication of the influence of wrenching

initiation of folding and boudinage in wrench shear and transpression abstract a linear stability analysis is performed on a deforming layered three dimensional linear viscous system the system consists of a single layer of viscosity embedded in a medium of viscosity the layer is oriented normal to the z axis and both the layer and medium are subjected to uniform simple shearing parallel to the y axis wrench shear with superposed uniform shortening parallel to x and extension parallel to z transpression the stability of cylindrical perturbations of the form a t cos ax y is examined it is found that fold type perturbations are unstable and pinch and swell disturbances are unstable in some cases for the case of wrench shear alone the fastest growing buckling disturbances are oriented at 45 to the positive y axis while the fastest growing pinch and swell disturbances have positive growth rates and are oriented 90 to the fold axes additional shortening parallel to the x axis transpression causes the fold axes to initiate at lower angles to the y axis pinch and swell disturbances may or may not be unstable in transpression depending on the magnitude of stretching parallel to the z axis

triassic paleomagnetism from the western pyrenees revisited implications for the iberian eurasian mesozoic plate boundary since the pioneering studies of van der voo tectonophysics 7 1969 5 and van der voo and boessenkool j geophys res 78 1973 5118 paleomagnetism of permo triassic redbeds and volcanics from the western pyrenees has furnished important contributions for delineating the mesozoic boundary between the iberian and eurasian plates in this paper we present a new paleomagnetic study focussed on triassic red beds 23 sites of the paleozoic basque massifs pbm the aim of this study is to complement previous studies done in those massifs to better constrain the complex kinematics of the western pyrenees two stable magnetic components have been isolated 1 a dual polarity pre folding magnetisation carried by specular hematite and 2 a secondary normal polarity component also carried by hematite our data confirm both the origin and the rotation pattern of the primary remanence described in previous works nevertheless field tests performed on the secondary component do not confirm the earlier interpretations by schott and peres tectonophysics 156 1988 75 as they indicate a synfolding nature of the remagnetisation instead of a post folding origin we consider that the secondary component is better explained if a cretaceous age is considered the presence of such remagnetisation in the western pyrenees strengthens the widespread occurrence of similar remagnetisation events reported in northern iberia in connection with the extensional tectonic events that occurred during cretaceous times a comparison of the rotations recorded by the triassic component and by the remagnetisation indicate that the paleozoic units underwent variable tectonic rotations before the remagnetisation was acquired most likely in connection with the counterclockwise rotation of iberia with respect to eurasia these results favour that the mesozoic plate boundary between the iberian and eurasian plates was a wide domain of distributed deformation and therefore contradict previous interpretations claiming for a discrete plate boundary

paleomagnetism of permo triassic red beds in the western pyrenees evidence for strong clockwise rotations of the paleozoic units abstract the paleomagnetic study of permo triassic red bed outcrops located in several basement units and slices of the western pyrenees is presented despite strong overprintings attributed to remagnetization processes connected with the early tertiary compressional phase we were able to identify the original magnetization in all sites the directions yielded reveal the occurrence in the whole area of clockwise rotations varying with respect to the european reference direction from nearly 0 to 102 the secondary directions are interpreted as more or less contemporary with the inferred rotations the paleomagnetic data are discussed in the light of two kinematic models for the iberian plate the examination of the models involving a left lateral displacement of iberia during the mid cretaceous is most conclusive and leads us to suggest that at that time the boundary between europe and iberia was located south of the cinco villas and aldudes massifs the clockwise rotations were induced by the early tertiary convergence of the plates which markedly disturbed the rather simple pattern inferred for the mid cretaceous period

transected folds and transpression how are they associated abstract folds transected by cleavage are a distinctive structural feature in the caledonian appalachian orogenic belt with the advent of a tectonic model of transpression for this belt there has been an assumption that transected folds are a symptom of transpressive deformation in particular that clockwise transection signifies sinistral transpression we examine this tenet and conclude that there is no unique explanation for folds transected by a synchronous cleavage explanations can be found for either clockwise or anticlockwise transection for sinistral or dextral transpression each requiring assumptions about the inclination of bedding to the transpression vector or requiring that cleavage does not track the finite xy plane a further explanation which may fit the regional data best requires a non constant transpressive deformation with initial pure shear dominated deformation giving way to increasing transcurrent movement in time none of these models can be assumed to be right without supporting data on the incremental and finite strain histories of such regions

seismic activity at the western pyrenean edge abstract the present day seismicity at the westernmost part of the pyrenean domain reported from permanent networks is of low to moderate magnitude however it is poorly constrained due to the scarce station coverage of the area we present new seismic data collected from a temporary network deployed there for 17 months that provides an enhanced image of the seismic activity and its tectonic implications our results delineate the westward continuity of the e w pyrenean band of seismicity through the variscan basque massifs along the leiza fault ending up at the hendaya fault this seismicity belt is distributed on a crustal scale dipping northward to almost 30 km depth other relevant seismic events located in the area can be related to the central segment of the pamplona fault and to different e w thrust structures

influence of syntectonic sedimentation on thrust geometry field examples from the iberian chain spain and analogue modelling abstract steep thrusts are usually interpreted as oblique slip thrusts or inverted normal faults however recent analogical and numerical models have emphasised the influence of surface mass transfer phenomena on the structural evolution of compressive systems this research points to sedimentation and erosion during deformation as an additional explanation for the origin of steeply dipping thrusts the present study uses both field observations and analogue modelling to attempt to isolate critical parameters of syntectonic sedimentation that might control the geometry of the upper part of thrust systems a field study of thrust systems bounding two compressive intermountain tertiary basins of the iberian chain is first briefly presented we describe the surface geometry of thrusts surrounding the montalban basin and the alto tajo syncline at the vicinity of deposits of oligocene early miocene alluvial fans at the footwall of faults field observations suggest that synthrusting sedimentation should influence the structure of thrusts indeed the faults are steeper and splitted at the edge of the syntectonic deposits effects of sedimentation rate on footwall of thrusts and of its change along fault strike are further investigated on two layer brittle ductile analogue models submitted to compression and syntectonic sediment supply two series of experiments were made corresponding to two end members of depositional geometries in the first series the sedimentation was homogeneously distributed on both sides of the relief developed above the thrust front in the second series deposits were localised on a particular area of the footwall of thrust front in all experiments the sedimentation rate controls the number and the dip of faults for low sedimentation rates a single low angle thrust develops whereas for high sedimentation rates a series of steeper dipping thrust is observed in experiments with changing sedimentation rate along fault strike the thrust geometry varies behind the areas with the thickest sediment pile

stress tensor and fault plane solutions for a population of earthquakes an algorithm for the simultaneous estimation of the orientation and shape of the stress tensor and the individual fault plane solutions for a population of earthquakes is studied it corresponds to a synthesis of the methods used by brillinger et al 1980 to obtain focal mechanisms and by armijo and cisternas 1978 for stress tensor analysis in microtectonics the input data are the polarities of the p arrival and take off angles for the set of source station pairs the method distinguishes in general which one of the nodal planes corresponds to the fault and gives the direction of the slip the application to the aftershock sequence of the 1980 arudy earthquake western pyrenees shows that the observations may be explained by a single stress tensor producing a n32 e extension with a likelihood of 95 per cent

rare earth element geochemistry of sulphide weathering in the sao domingos mine area iberian pyrite belt a proxy for fluid rock interaction and ancient mining pollution gossan disseminated orebody waste other mining wastes minesoils and acid mine drainage amd in the abandoned sao domingos mine area iberian pyrite belt ipb have been analyzed for rare earth elements ree the main aim is to understand ree mobility during sulphide weathering so that the lanthanide series can be used both as a record of the water rock interaction and as a tool for identifying impacts of amd on natural ecosystems north american shale composite nasc normalized ree patterns corresponding to the disseminated orebody waste are relatively flat emree 0 01 0 12 however nasc normalized ree distributions in amd from sulphide oxidation tend to be enriched in middle ree mree compared to light ree lree and heavy ree hree as a consequence gossan resulting from supergene alteration of massive sulphide presents an evident nasc normalized mree depleted signature thus the overall water sulphide interaction defines complementary convex emree 0 72 0 25 and concave emree 0 31 0 12 nasc normalized patterns in draining waters and oxidation products respectively solutions extracted from minesoils have also nasc normalized patterns with mree enriched signature emree 0 62 0 22 similar to amd the emree parameter is proposed to measure the curvature in the mree segment and its size is quantified as the normalized maximum vertical difference between the polynomial curve fitting of the mree region and its theoretical y axis position in the absence of enrichment or depletion emree 0 for mree enriched signatures 0 for mree depleted signatures and 0 for horizontal patterns the sao domingos stream although it has been highly affected by amd flows into the guadiana river that has an estuarine system where pollution is considerably attenuated due to the mixing according to the metal geoaccumulation indexes currently used in the literature however sediments of this estuary were also analyzed and reflect mree enriched signatures emree 0 25 0 03 which demonstrate that this apparently non polluted estuarine system is being certainly affected by historical mining activities from the ipb the emree index is more sensitive to recognize curved mree signatures than other normalized ratios such as la gd nasc validating the use of ree patterns as a proxy for environmental pollution by amd

the pyrenean earthquake of february 29 1980 an example of complex faulting abstract an m b 5 1 earthquake occurred on february 29 1980 in the western pyrenees near arudy france the telemetred network of the i p g p operating in this area since 1978 allowed a good localization of this earthquake 43 4 21 n 0 24 59 w depth 4km during the two years preceding this earthquake the seismic activity exhibited a gradual decrease a foreshock of magnitude 1 6 was recorded three hours before the main shock a temporary network set up in the epicentral area a few days after the main shock permitted a precise study of the aftershock sequence fifty fault plane solutions for earthquakes ranging from magnitude 1 5 4 were obtained a complex pattern of faulting was revealed with both strike slip and normal faulting however a regional tectonic stress tensor can be proposed from a detailed investigation of the aftershock sequence this stress tensor is in agreement with previous results in this area

the recent upper miocene to quaternary and present tectonic stress distributions in the iberian peninsula a general synthesis of the recent and present stress situation and evolution in the iberian peninsula was obtained from microstructural and seismological analysis the stress evolution was deduced from 1 fault population analysis fpa from 409 sites distributed throughout the iberian peninsula 2 paleostress indicators given by 324 stations taken from the bibliography and 3 seismic data corresponding to 161 focal mechanisms evenly spread in the studied region the application of fpa together with the determination of stress tensors and focal mechanisms for the whole iberian microplate has provided two main results 1 the iberian peninsula is undergoing a nw se oriented compression except for the northeastern part pyrenees ebro basin and iberian chain where it is n s to ne sw and the gulf of cadiz where it seems to be e w and 2 the main trends of the stress field have remained almost constant since the upper miocene the analysis performed by zones suggests the presence of local heterogeneities in the stress field

graben inversion in nature and experiments the inversion of crustal scale basement grabens is studied here through laboratory experiments on small scale models and available oil industry seismic lines from the southern north sea two basic configurations are considered first both the basement and the sedimentary cover are brittle and inversion does not involve any potential decollement between them second the basement and sedimentary cover are separated by a weak ductile layer e g salt which can allow decollement of the cover during both extension and later compression and inversion the second configuration is more complicated and can lead to a large variety of geological structures laboratory experiments were carried out on brittle ductile models built with sand to represent brittle layers basement and sedimentary cover and silicone putty to simulate the decollement layer between basement and cover a mechanically based classification of inversion structures is proposed the effects of some crucial parameters are investigated including obliquity between the direction of shortening and normal faults as well as strength profiles and the presence or absence of salt diapirs the experimental investigation leads to the following conclusions 1 the inversion of the graben by reactivation of normal faults implies that the angle between the direction of compression and the graben is less than 45 2 if there is a superficial decollement e g basement cover interface inversion initiates low dipping thrust faults in the cover localized at graben borders 3 salt diapirs or salt walls localized along the graben borders in the cover are preferential sites for the development of thrust faults and 4 when the cover is decoupled from the basement by a decollement layer inversion induced deformation in the cover which is partitioned between thrust faults along the graben borders and strike slip faults within the graben trending oblique to the graben borders experimental results are compared with field examples in particular from the southern north sea

structural analysis of hypocentral distribution of an earthquake sequence using anisotropic wavelets method and application 1 spatial organization of earthquake sequences is investigated to localize active rupture planes and to reconstruct the geometry of the inferred rupture zone we have developed a new approach the normalized optimized anisotropic wavelet coefficient noawc method to extract from a set of hypocenters the active ruptures planes our approach permits the detection of organized structures within a plane regardless of its size location shape anisotropy and orientation it includes the determination of a system of three perpendicular sections minimizing the effects of projections of the hypocenters and intrinsically accounts for uncertainty in the location of the seismic events the accuracy and the effectiveness of the noawc procedure are illustrated on both synthetic and real data an application to the m 5 1 arudy french pyrenees aftershock sequence shows how a combination of the possible mathematically reconstructed geometries can be combined with the available fault plane solutions and geomorphological markers to determine the active rupture planes and to propose and validate a local tectonic model this new multitool approach lends itself to quantitative and computer assisted analyses of large data sets