a modern analogue of western united states summer palaeoclimate at 6000 years before present a modern summer climate analogue of western united states 6 ka palaeoclimate based on broad scale patterns from proxy data and general circulation model simulations provides details concerning its sum mer synoptic climatology august 1955 serves as a close analogue featuring an amplified subtropical ridge aloft and anticyclonic flow near the surface over the central united states and stronger westerly flow along the canadian northwest united states border the august 1955 summer temperature pattern similar to those from general circulation model simulations exhibits positive anomalies throughout most of the continental interior the august 1955 precipitation pattern corresponds very closely with patterns of effective moisture from proxy data showing widespread aridity and some stronger monsoonal activity in the southwest the analogue suggests that the amplified subtropical ridge is a very important feature of 6 ka palaeoclimate with the circulation pattern differing from that in the early holocene implying interactions between both milankov itch and sub milankovitch forcing factors

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

three dimensional gravity and magnetic modeling of crustal indentation and wedging in the western pyrenees cantabrian mountains 1 the complex crustal structure of the boundary zone between the iberian and european plates from the western pyrenees to the cantabrian mountains is probed by three dimensional 3 d gravity and magnetic modeling constrained by deep seismic profiles the 3 d modeling results support the presence of a continuous iberian crustal root and suggest that the geometry of the orogenic belt that formed in latest cretaceous tertiary times along the northern margin of iberia was conditioned by oblique structures separating areas characterized by different tectonic styles in the western pyrenees and the basque cantabrian basin a thick mesozoic basin presently incorporated to the pyrenean cantabrian belt the relatively narrow thinning of the crust inherited from the mesozoic rifting stage conditioned a structural style in which portions of the southward indenting european lower crust are interpreted to be back thrusted toward the north and uplifted to shallow depths promoting the appearance of significant potential field anomalies in the basque cantabrian basin the strongest aeromagnetic anomaly of the whole iberian mainland is superimposed on the eastern part of a well defined positive gravity anomaly similar in amplitude and wavelength to those located along the north pyrenean zone these observations suggest that the eastern part of the dense lower crustal causative body is strongly magnetized and may correspond to a gabbroic cumulate originated in the axis of the ancient mesozoic rift to the west tertiary compression affected the north iberian cantabrian passive continental margin whose geometry and inherited structures conditioned the formation of a double crustal delamination and the uplift of the cantabrian mountains

the ainsa fold and thrust oblique zone of the central pyrenees kinematics of a curved contractional system from paleomagnetic and structural data this work was funded by the project intectosal cgl2010 21968 c02 01 and by the grup de recerca de geodinamica i analisi de conques 2009sgr1198 secretaria d universitat i recerca del departament d economia i coneixement de la generalitat de catalunya josep poblet acknowledges financial support by research projects geociencias en iberia estudios integrados de topografia y evolucion 4d topo iberia csd2006 041 and desarrollo de fracturas y venas asociadas al plegamiento fraveple cgl2011 23628 funded by spanish ministries

inversion of small basins effects on structural variations at the leading edge of the axial zone antiformal stack southern pyrenees spain the southern axial zone of the pyrenees consists of a foreland dipping system of thrusts and backthrusts forming the leading edge of a basement involved antiformal stack several small stephanian permian extensional basins were strongly inverted during the pyrenean cainozoic orogeny and controlled thrust geometry and kinematics causing lateral structural variation palinspastic restoration reveals three main grabens erill castell estac and gramos basins capped by undeformed triassic strata in the lower orri and erta thrust sheets the overlying nogueres thrust sheet partitioned into several slices contains smaller basins northern basins the erill castell half graben with a steeply dipping sedimentary infill was inverted along its border fault and extruded by bedding parallel backthrusts the narrow thick estac basin which was less deformed internally was a heterogeneity causing major lateral variation in the system the gramos basin originally wide and covered by a gentle unconformity angle was also inverted along its border fault but caused less lateral variation in the thrust system the northern basins appear only in the western part of the present day nogueres thrust sheet where minor units are strongly stacked and rotated in the east thrust slices are large and shallow dipping the study case is a good example of complex 3d thrust development influenced by the initial geometry of a heterogeneous basin system

crust and mantle lithospheric structure of the iberian peninsula deduced from potential field modeling and thermal analysis abstract we investigate the lithospheric structure of the iberian peninsula and lateral crustal density variations using a three step approach first the crustal and mantle lithosphere thicknesses are calculated from joint geoid and elevation modeling combined with thermal analysis further constrained by seismic data we then compute the 3d gravity effect of the resulting lithospheric structure to separate the measured bouguer anomaly into its regional and local components finally we invert the residual gravity anomalies to highlight lateral average crustal density variations and discuss them in terms of crustal structures our results show that for the majority of the study area the crustal thickness does correlate with the regional topography pattern the highest topography above 1500 m shows thicknesses above 44 km with local values up to 48 km crustal thicknesses in the range of 36 40 km are obtained in the uplifted alpine areas while a thinner crust is observed in sedimentary basins and in the iberian massif 30 to 35 km with the exception of sw iberia region where the crust thins from 30 to 28 km thick lithosphere above 140 km is found along the pyrenees the cantabrian mountains the iberian chain and in the betics while the thinnest lithosphere is found in sw iberia 90 km 3d inversion of residual anomalies show that for the majority of the area the average density of the crust is in the range of 2810 10 kg m 3 the denser crust is found in the nw and sw regions of the iberian massif 30 kg m 3 on average and locally in the pyrenees above 70 kg m 3 nw of the iberian chain 15 kg m 3 on average and in the southern internal betics 70 kg m 3 the least dense crust is found in the central and western betic chain 30 kg m 3 on average and in sedimentary basin depocenters

thermochronology constraints for the propagation sequence of the south pyrenean basement thrust system france spain 1 in this work we combined apatite fission track and biotite k feldspar 40ar 39ar ages with tectonic data in the west central part of the axial zone of the pyrenees we discuss the exhumation ages and rates of the neouvielle bordere louron and bielsa variscan granites and their relationships with the timing and sequence of south vergent basement thrusting within the pyrenean orogenic prism the 40ar 39ar ages on k feldspars from the neouvielle massif sample nv7 seem to indicate tectonic movements on the eaux chaudes thrust during the early middle eocene fission track results suggest that the exhumation of the neouvielle massif occurred around 35 ma and exhumation of the bordere louron massif around 32 ma in relation to thrusting on the gavarnie thrust the bielsa massif was exhumed from around 19 ma by out of sequence movements on the bielsa thrust we thus show that whereas most of the pyrenean basement thrust faults here the eaux chaudes gavarnie and guarga thrusts were active in sequence toward the southern foreland from the early eocene to the earliest miocene some of them here the bielsa thrust were activated out of sequence in the hinterland later than the generally accepted aquitanian age for the end of the pyrenean compression finally the apatite fission track modeling indicate a last cooling episode starting around 5 ma which is most certainly related to the pliocene reexcavation of the southern and northern flanks of the pyrenees

the role of the bay of biscay mesozoic extensional structure in the configuration of the pyrenean orogen constraints from the marconi deep seismic reflection survey 1 seismic interpretation of the marconi deep seismic survey enables recognition of the upper crustal structure of the eastern part of the bay of biscay and the main features of its alpine geodynamic evolution the new data denotes that two domains with different pyrenean and north foreland structures exist in the bay of biscay in the eastern or basque parentis domain the north pyrenean front is located close to the spanish coast and the northern foreland of the pyrenees is constituted by a continental crust thinned by a north dipping fault that induced the formation of the early cretaceous parentis basin in the western or cantabrian domain the north pyrenean front is shifted to the north and deforms a narrower and deeper foreland basin which lies on the top of a transitional crust formed from the exhumation of lithospheric mantle along a south dipping extensional low angle fault during the early cretaceous the transition between these two domains corresponds to a soft transfer zone linking the shifted north pyrenean fronts and a north to wnw directed thrust that places the continental crust of the landes plateau over the transitional crust of the bay of biscay abyssal plain comparison between this structure and regional data enables characterization of the extensional rift system developed between iberia and eurasia during the late jurassic and cretaceous and recognizes that this rift system controlled not only the location and features of the pyrenean thrust sheets but also the overall structure of this orogen

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

imaging the crustal structure of the central iberian zone variscan belt the alcudia deep seismic reflection transect alcudia is a 230 km long vertical incidence deep seismic reflection transect acquired in spring 2007 across the southern central iberian zone part of the pre mesozoic gondwana paleocontinent of the variscan orogen of spain the carefully designed acquisition parameters resulted in a 20 s twtt deep 60 90 fold high resolution seismic reflection transect the processed image shows a weakly reflective upper crust the scarce reflectivity matching structures identified at surface a thick highly reflective and laminated lower crust and a flat moho located at 10 s twtt 30 km depth the transect can be divided into three segments with different structural styles in the lower crust in the central segment the lower crust is imaged by regular horizontal and parallel reflectors whereas in the northern and southern segments it displays oblique reflectors interpreted as an important thrust north and tectonic wedging involving the mantle south the alcudia seismic image shows that in an intracontinental orogenic crust far from the suture zones the upper and lower crust may react differently to shortening in different sectors which is taken as evidence for decoupling the interpreted structures as deduced from surface geology and the seismic image show that deformation was distributed homogeneously in the upper crust whereas it was concentrated in wedge thrust structures at specific sectors in the lower crust the seismic image also shows the location of late variscan faults in spatial association with the lower crustal thickened areas

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

cretaceous tertiary tectonic inversion of the cotiella basin southern pyrenees spain the cotiella nappe includes one of the most important mesozoic basins of the southern pyrenees which was subsequently inverted during the tertiary compression the late cretaceous cotiella basin is here interpreted as the western sector of the cretaceous cotiella boixols basin 100 20 km wide located in the central part of the southern pyrenees the present day complex structure of the cotiella nappe is the result of the inversion process linked to the emplacement of basement thrust sheets of the axial zone in its western sector the cotiella nappe consists of several superimposed thrust sheets with complex geometry becoming simpler towards the east with a single thrust surface and smaller displacements the cotiella boixols basin underwent strong subsidence during the early cretaceous at its eastern sector and its depocentre migrated westward during the late cretaceous the reconstruction of the sedimentary basin to the pre compressional stage shows that during the mesozoic the cotiella boixols basin was located to the south of a basement high which later became the pyrenean axial zone from a balanced cross section it can be inferred that the cotiella north verging extensional system was connected with the north pyrenean rift by means of a 10 km deep horizontal detachment the compressional tertiary detachment within the upper crust was shallower than the extensional detachment and individualised four basement thrust sheets which form the axial zone antiform

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

the structural and sedimentary evolution of the cretaceous north pyrenean basin southern france during the cretaceous the north pyrenees of southern france suffered complex deformation the relatively deep marine north pyrenean basin formed a narrow band of rock adjacent to the north pyrenean fault was metamorphosed iherzolite was emplaced and the bay of biscay opened compression during the late cretaceous and tertiary pyrenean orogeny overprinted and partially masked this earlier history in order to better understand the regional tectonics off the cretaceous northern pyrenees and specifically the early history of the north pyrenean basin we compiled and then palinspastically restored a geologic map of the western north pyrenees lithofacies paleotransport and isopach maps for the early deposits of the basin and a pre albian palinspastic subcrop map show that the basin was an east west elongate fault bounded trough at least 300 km long and from 40 to 60 km wide containing a siliciclastic composite sedimentary package as much as 4 700 m thick detailed structural and stratigraphic studies of the massif d9igountze mendibelza which is located along the southern limit of the north pyrenean basin in the western pyrenees suggest that its cretaceous cover was deposited against an active relatively low angle listric normal fault this fault is on strike with the north pyrenean fault zone a major long lived zone of deformation in the central and eastern north pyrenees

global assessment of benthic nepheloid layers and linkage with upper ocean dynamics abstract global maps of the maximum bottom concentration thickness and integrated particle mass in benthic nepheloid layers are published here to support collaborations to understand deep ocean sediment dynamics linkage with upper ocean dynamics and assessing the potential for scavenging of adsorption prone elements near the deep ocean seafloor mapping the intensity of benthic particle concentrations from natural oceanic processes also provides a baseline that will aid in quantifying the industrial impact of current and future deep sea mining benthic nepheloid layers have been mapped using 6 392 full depth profiles made during 64 cruises using our transmissometers mounted on ctds in multiple national international programs including woce save jgofs clivar repeat hydrography and go ship during the last four decades intense benthic nepheloid layers are found in areas where eddy kinetic energy in overlying waters mean kinetic energy 50 m above bottom mab and energy dissipation in the bottom boundary layer are near the highest values in the ocean areas of intense benthic nepheloid layers include the western north atlantic argentine basin in the south atlantic parts of the southern ocean and areas around south africa benthic nepheloid layers are weak or absent in most of the pacific indian and atlantic basins away from continental margins high surface eddy kinetic energy is associated with the kuroshio current east of japan data south of the kuroshio show weak nepheloid layers but no transmissometer data exist beneath the kuroshio a deficiency that should be remedied to increase understanding of eddy dynamics in un sampled and under sampled oceanic areas

the naval diapir southern pyrenees geometry of a salt wall associated with thrusting at an oblique ramp abstract the southern pyrenees provide several examples of diapiric structures linked to migration of triassic evaporites associated with the movement of thrust sheets among them the naval diapir the westernmost diapiric body in the sierras marginales unit is key to understanding the mechanisms controlling salt tectonics in this sector of the pyrenees in this paper we use geological mapping field data cross sections seismic reflection profiles and detailed gravity data to characterize the geometry of the naval diapir a body of middle and upper triassic evaporites and shales and its relationship with the sedimentary cover and compressional structures the information obtained from these methods was used to construct 2 5d and 3d models these models indicate the significant role of compressional oblique structures and crestal weakening in the origin of the n s salt wall geometry during the translation rotation of the western end of the sierras marginales thrust sheet in the late eocene oligocene times the uplift of low density materials outlived compressional activity in the area and continued during the oligocene early miocene

extensional salt tectonics in the partially inverted cotiella post rift basin south central pyrenees structure and evolution the cotiella massif in the south central pyrenees hosts upper cretaceous gravity driven extensional faults which were developed in the bay of biscay pyrenean paleorift margin of the atlantic ocean they accommodate up to 6 km of post rift carbonates above relict upper triassic salt subsequent pyrenean contractional deformation preserved the main extensional features but most of the upper triassic salt was expulsed and then dissolved leaving little indications of the original salt volume nonetheless several distinctive salt related features are still recognizable both at outcrop and at basin scale providing an exposed analogue for salt floored extensional basins developed on passive margins based on field research we re interpret the tectonic evolution of the area and suggest that passive diapirs were coeval with gravity driven extension during the development of the cotiella basin the given interpretations are supported with detailed geological maps original structural data cross sections and outcrop photographs the discovery of previously unknown post rift salt structures in the cotiella massif is an extra element to consider in the paleogeographic reconstructions of the upper cretaceous passive margin of the bay of biscay pyrenean realm and consequently helps in our understanding of the evolution of current atlantic type margins

geophysical constraints on the deep structure of a limited ocean continent subduction zone at the north iberian margin 1 late cretaceous to cenozoic convergence between iberia and europe led to the partial closure of the bay of biscay with limited southward subduction of oceanic crust below the north iberian margin inclined sub moho reflections and diffractions observed in deep seismic reflection profiles shot across the margin are especially well represented in two reflection profiles escin 3 2 and escin 3 3 these two profiles have been chosen to test if the sub moho reflections correspond to true primary deep events and provided that they are reflecting off the subduction zone to investigate its deep structure spectral analysis together with travel time estimation and migration allow us to characterize a number of these sub moho events as deep source low frequency 19 hz reflections and diffractions synthetic seismograms were generated by three dimensional seismic modeling of a sub moho southward dipping surface interpreted to correspond to the top of subducted oceanic crust comparison between the real and synthetic data show that inclined low frequency sub moho reflections in both escin 3 2 and escin 3 3 profiles may correspond to reflections from southward subducted bay of biscay oceanic crust geoid free air gravity and absolute topography modeling provides additional constraints on the lithospheric scale structure of this limited ocean continent subduction zone beneath the north iberian margin

crustal thinning on the aquitaine shelf bay of biscay from deep seismic data in recent years an increasing amount of new scientific data on the structure and nature of the whole continental crust has been acquired from deep seismic profiling the bay of biscay profile was recorded in 1984 at sea along the aquitaine platform for the french ecors group in association with hispanoil it collected a unique set of two ship multi channel seismic data with constant offset and expanded spread profiles in a complex geological area the data are of very good quality and contain clear moho and lower crust reflections as seen under most parts of western europe where similar experiments have been conducted1 4 this profile provided for the first time the most convincing evidence for an anomalously thin crust below a rifted basin the parentis basin which is directly connected to the oceanic area of the bay of biscay it gives new constraints for understanding the formation of extensional sedimentary basins

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

asymmetric growth of the pyrenees revealed through measurement and modeling of orogenic fluxes orogenic asymmetry is ubiquitous in terms of topography structure and metamorphism but the relative influences of climate and tectonics in determining such asymmetry remain contentious this study demonstrates that for the case of the european pyrenees orogenic asymmetry is not defined by inherited structural asymmetry nor by orographic asymmetry of precipitation but is the simple consequence of the inherent tectonic growth of a doubly vergent orogen the pyrenees record approximately 60 myr of upper crustal accretion erosion and sedimentation that developed from late cretaceous to early miocene times approximately 165 km of plate convergence was accommodated by four phases of orogenic growth that can be summarized in terms of their generic orogenic development as follows 1 early inversion of passive margin extensional faults initiating the pro wedge 2 growth by a combination of frontal accretion and underplating to the pro wedge symmetrical and steady record of erosion and foreland basin sedimentation 3 cessation of pro wedge frontal accretion with propagation of the retro wedge deformation front 4 accelerated growth of pro wedge antiformal stack induced by underplating accelerated erosion over antiformal stack and increase in sediment discharge to pro foreland basin stabilization of outer pro wedge by sediment blanketing discrete element model experiments are used to explore the natural tendency of the system towards asymmetry model output demonstrates that during active frontal accretion to the pro wedge of an orogen the maxima in erosional denudation is offset towards the pro wedge relative to the maximum depths of exhumation this asymmetry is fundamentally driven by the progressive reduction in advection velocities of material within the orogen from the pro wedge into the retro wedge hence for an efficient erosional regime a dynamic coupling between erosion and structural uplift of the pro wedge will reduce activity of the retro wedge and hence drive asymmetry in terms of low temperature thermochronometry erosion and sedimentation the models also demonstrate the tendency for pro wedge growth through frontal accretion to slow however the cessation of frontal accretion as recorded by the south pyrenean thrust belt phases 3 and 4 requires additional forcing it is proposed that the thick wedge top sedimentation recorded at this time increased taper angles and hence stabilized the wedge accretionary influx into the orogen at this time was accomplished through underplating these processes governing asymmetry do not require a spatially variable climate or a laterally heterogeneous underthrust plate