lithospheric dynamics and tectonic stratigraphic evolution of the ebro basin we present analysis of two dimensional modeling of the tectonic subsidence in the ebro basin along three profiles crossing the pyrenees the ebro basin and the catalan coastal range to the east and the iberic cordillera to the west the deformation of the foreland basin during tertiary times is simulated using a lithospheric flexure model incorporating laterally varying elastic thicknesses the deflection of the plate is the combined result of pyrenean loading and the loads exerted by the iberic cordillera and catalan coastal range in the eastern part of the ebro basin no extra driving force on the iberian plate is required the flexural modeling supports the hypothesis that underplating of the iberian plate under the european plate is not associated with subduction of old oceanic lithosphere a good fit with the tertiary basin subsidence data and bouguer anomalies is obtained for an elastic plate with local weakness zones under the ebro basin the inferred lateral variation in the elastic rigidity is consistent with heat flow measurements in northern spain and with major mesozoic extension in the iberic cordillera and valencia trough

neotectonics and intraplate continental topography of the northern alpine foreland research on neotectonics and related seismicity has hitherto been mostly focused on active plate boundaries that are characterized by generally high levels of earthquake activity current seismic hazard estimates for intraplate domains are mainly based on probabilistic analyses of historical and instrumental earthquake catalogues the accuracy of such hazard estimates is limited by the fact that available catalogues are restricted to a few hundred years which on geological time scales is insignificant and not suitable for the assessment of tectonic processes controlling the observed earthquake activity more reliable hazard prediction requires access to high quality data sets covering a geologically significant time span in order to obtain a better understanding of processes controlling on going intraplate deformation the alpine orogen and the intraplate sedimentary basins and rifts in its northern foreland are associated with a much higher level of neotectonic activity than hitherto assumed seismicity and stress indicator data combined with geodetic and geomorphologic observations demonstrate that deformation of the northern alpine foreland is still on going and will continue in the future this has major implications for the assessment of natural hazards and the environmental degradation potential of this densely populated area we examine relationships between deeper lithospheric processes neotectonics and surface processes in the northern alpine foreland and their implications for tectonically induced topography for the environmental tectonics project entec the upper and lower rhine graben urg and lrg and the vienna basin vb were selected as natural laboratories the vienna basin developed during the middle miocene as a sinistral pull apart structure on top of the east alpine nappe stack whereas the upper and lower rhine grabens are typical intracontinental rifts the upper rhine graben opened during its late eocene and oligocene initial rifting phase by nearly orthogonal crustal extension whereas its neogene evolution was controlled by oblique extension seismic tomography suggests that during extension the mantle lithosphere was partially decoupled from the upper crust at the level of the lower crust however whole lithospheric folding controlled the mid miocene to pliocene uplift of the vosges black forest arch whereas thermal thinning of the mantle lithosphere above a mantle plume contributed substantially to the past and present uplift of the rhenish massif by contrast oblique crustal extension controlling the late oligocene initial subsidence stage of the lower rhine graben gave way to orthogonal extension at the transition to the neogene the entec project integrated geological geophysical geomorphologic geodetic and seismological data and developed dynamic models to quantify the societal impact of neotectonics in areas hosting major urban and industrial activity concentrations the response of europe s intraplate lithosphere to late neogene compressional stresses depends largely on its thermo mechanical structure which in turn controls vertical motions topography evolution and related surface processes

on the linkage between labrador sea water volume and overturning circulation in the labrador sea a case study on proxies abstractalthough proxies have generally been used to study deep ocean convection and overturning circulation in the labrador sea their efficacy has not been explicitly evaluated because observations that directly measure those variables are scarce in this study the volume of newly formed labrador sea water lsw and the overturning circulation in the labrador sea are estimated using observational data and output from a high resolution ocean model and then compared to proxies used to represent those variables the comparisons reveal the limitations of proxies highlighting the desirability of robust estimates derived from direct monitoring in the region i e from argo and overturning in the subpolar north atlantic program osnap a linkage among lsw formation overturning circulation in the labrador sea and the export of lsw from the basin on interannual time scales is found in the model

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

confined deep water system development on the accretionary wedge miocene kahramanmaras foreland basin s turkey abstract according to theoretical studies the foreland basin consists of accretionary wedge including wedge top or piggyback basin foredeep forebulge and backbulge depozones all of them are parallel to the orogenic belts of the overlying and underlying plates the closure of the southern branch of the neotethys during the late cretaceous led to an oblique collision of the arabian plate and the anatolide taurides platform leading to the development of the miocene kahramanmaras foreland basin kfb thus the promontory shape of the arabian plate prevented the development of an accretionary wedge parallel to the orogenic belt the accretionary wedge of the kfb includes blocks of various sizes and age mainly mesozoic limestone scattered within an early tertiary matrix mass wasting deposits and shallow to deep marine sediments at the beginning of the miocene transtensional tectonism led to the development of half graben basins on top of the accretionary wedge these basins namely the tekir and cukurhisar also cut the foredeep of the kfb obliquely in contrast with the theoretical study this paper focuses on the evolution and fillings of those basins initially claystone and basin margin reef deposits filled the half graben basins as a consequence of the lower miocene sea invasion then long and narrow conglomeratic channels starting from the northern edge of the basins fan delta progressed southwards passing into sandy lobes then into claystones an activation of the boundary faults of the wedge top basin stopped the progression of the lower middle miocene sediments and led to their deformation then the sedimentation of the kfb shifted towards the basin centre during the middle miocene

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

complex tectonic and tectonostratigraphic evolution of an alpine foreland basin the western duero basin and the related tertiary depressions of the nw iberian peninsula abstract the tectonic and tectonostratigaphic evolution of foreland basins and related tertiary depressions are the key to investigate deformation history and the uplifting of the continental lithosphere of the alpine pyrenean orogeny the northern part of the duero basin is the foreland basin of the cantabrian mountains which are in turn the western part of the pyrenean orogen we have studied the western sharp end of the duero foreland basin and its relation to the tertiary deposits of the nw iberian peninsula and the topography evolution in order to propose a coherent tectonic and tectonosedimentary model that could explain all tertiary deposits we have analysed the depositional environment stratigraphic sequences paleocurrents and established a correlation of the main outcrops besides a detailed structural mapping of the alpine structures that limit and affect the main tertiary outcrops has been carried out the tertiary deposits of the nw iberian peninsula depressions are affected and fragmented by alpine structures that limit their extensions and locations the stratigraphic succession is similar in the nw tertiary outcrops they are mainly terrigenous and carbonated continental deposits formed by assemblage of alluvial fans developed at the mountains front in arid or semiarid conditions three formations can be identified in the main depressions toral fm santalla fm and medulas fm the nw tertiary outcrops were the western deposits of the duero foreland basin that surrounded the lateral termination of the pyrenean orogen these deposits were fragmented and eroded by the subsequent uplift of the galaico leoneses mountains and the ne sw strike slip faults activity broken foreland basin only the latest stages of some of these outcrops can be considered as intramontane basins as traditionally have been interpreted the sedimentation started in the northeast oviedo infiesto during the eocene and migrated to the west as pontes during the late oligocene early miocene times meanwhile southern areas were far away from the tectonic activity el bierzo o barco monforte sarria depressions

the transition from thick skinned to thin skinned tectonics in the basque cantabrian pyrenees the burgalesa platform and surroundings interpretation of seismic data in the margins of the burgalesa platform in the basque cantabrian pyrenees has allowed proposition of a new structural model that combines different modes of deformation during oblique tectonic inversion conditioned by the distribution of triassic salts deformation was decoupled by the presence of the salt horizon between basement involved thrusts inverting formerly triassic and late jurassic early cretaceous extensional faults and a detached thrust system involving the upper triassic to neogene sedimentary package structural units experiencing different styles of deformation are not only stacked vertically above and below the salt but most importantly they change from one to the other along strike across the transversal edges of the triassic salts the burgalesa platform detached thrust system was confined between the basement involved structures of the cantabrian mountains westward and the nw tip of the iberian basement involved structures san pedro southward this together with the obliquity between the pyrenean shortening direction and the strike of the previous extensional faults mostly during the late stages of deformation determined the strike slip reactivation of the basement involved inverted faults and the lateral extrusion of the burgalesa platform detached mesozoic successions above the salt towards the se to form a prominent thrust salient oblique to the main pyrenean trend the proposed model combines thick skinned with thin skinned structural styles during oblique tectonic inversion and is consistent with the surface data including the fracture system the available subsurface data and the mechanical stratigraphy

seismic image of the cantabrian mountains in the western extension of the pyrenees from integrated escin reflection and refraction data abstract integrated analysis of normal incidence and large aperture seismic reflection data collected in 1992 and 1993 within the spanish escin and complementary projects provide a first complete n s crustal transect across the northern iberian peninsula and continental margin images of the crustal structure of the cantabrian mountains and their transition to the duero basin and to the cantabrian margin are obtained from a a 65 km long vertical reflection profile escin 2 on land b a 200 km long reversed refraction profile and c wide angle recordings of the marine escin 4 profile consistent results between reflectivity pattern and velocity depth distribution reveal important lateral variations in the deep structure the reflective crust imaged in the escin 2 profile changes its attitude from sub horizontal beneath the duero basin to north dipping beyond the mountain front basement thrusts are observed in the upper crust merging into a detachment at 6 s twt and may have triggered the alpine uplift of the range the moho is identified at the bottom of the reflective lower crust and deepens from 12 to 15 s at the northern end of the profile about 35 km inland modelling of the refraction data laterally extends the seismic image and provides evidence for variscan crustal features beneath the duero basin northwards the velocity in the lower crust decreases and the moho constrained by the wide angle data from profile escin 2 deepens to about 60 km ending abruptly at the shoreline the velocity depth model is constrained along the asturian platform up to the continental slope where the crust mantle boundary is located at 24 km depth this margin moho shows a progressive deepening southwards and extends to the coast where it is found at 30 km depth the present seismic data support an important alpine reworking and thickening of the crust under the cantabrian mountains the onshore offshore transition is marked by an imbrication of two crusts of very different thicknesses this signature offers a strong parallelism with the one previously observed further east across the pyrenees in the ecors seismic profile

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

large scale distributed deformation controlled topography along the western africa eurasia limit tectonic constraints in the interior of the iberian peninsula the main geomorphic features mountain ranges and basins seems to be arranged in several directions whose origin can be related to the n s plate convergence which occurred along the cantabro pyrenean border during the eocene lower miocene time span the iberian variscan basement accommodated part of this plate convergence in three e w trending crustal folds as well as in the reactivation of two left lateral nne ssw strike slip belts the rest of the convergence was assumed through the inversion of the iberian mesozoic rift to form the iberian chain this inversion gave rise to a process of oblique crustal shortening involving the development of two right lateral nw se shear zones crustal folds strike slip corridors and one inverted rift compose a tectonic mechanism of pure shear in which the shortening is solved vertically by the development of mountain ranges and related sedimentary basins this model can be expanded to nw africa up to the atlasic system where n s plate convergence seems also to be accommodated in several basement uplifts anti atlas and meseta and through the inversion of two mesozoic rifts high and middle atlas in this tectonic situation the microcontinent iberia used to be firmly attached to africa during most part of the tertiary in such a way that n s compressive stresses could be transmitted from the collision of the pyrenean boundary this tectonic scenario implies that most part of the tertiary eurasia africa convergence was not accommodated along the iberia africa interface but in the pyrenean plateboundary a broad zone of distributed deformation resulted from the transmission of compressive stresses from the collision at the pyrenean border this distributed intraplate deformation can be easily related to the topographic pattern of the africa eurasia interface at the longitude of the iberian peninsula shortening in the rif betics external zones and their related topographic features must be conversely related to more local driven mechanisms the westward displacement of the exotic alboran domain other than n s convergence the remaining nnw sse to nw se latest miocene up to present convergence is also being accommodated in this zone straddling iberia and morocco at the same time as a new ill defined plate boundary that is being developed between europe and africa

foredeep geometries at the front of the apennines in the ionian sea central mediterranean abstract a new regional seismic section in the ionian sea across the apennines belt and related foreland shows how the present foredeep geometry may be an example for interpreting discontinuous tilted and deformed earlier basins now incorporated in the internal parts of the accretionary wedge onlap stratal terminations of the foredeep sediments on the foreland monocline may simulate downlap geometries once involved and tilted by back thrusting the geometry of the ionian foredeep is controlled by the dip of the regional monocline and internally by the variable dip and length of the limb of the external fold which may be either foreland verging or hinterland verging the generation of a new fold within the foredeep splits the basin into a new foredeep toward the foreland and a thrust top basin toward the hinterland the thrust top basin dimension is primarily controlled by the distance between the two folds and related thrusts at its margins the foredeep in its overall history is composed by a series of concave heterogeneous lenses progressively displaced and piled up toward the foreland to the east the formation of each sedimentary lens is controlled by the development of a new fold and the contemporaneous retreat of the regional monocline which creates new accommodation space the complex 3d geometry of the apennines foredeep mainly results from lateral variations of the latter parameters as well as variations in sediment supply and eustasy

an example of intraplate neotectonic activity vilarica basin northeast portugal the vilarica tectonic basin nw iberian peninsula is presented as an example of intraplate neotectonic activity the regional geological setting is first introduced then followed by a brief view on the neotectonic activity in portugal the basin consists of an elongated depression approximately 20 km long and 2 to 3 km wide trending nne ssw it is located astride a late variscan strike slip fault the manteigas braganca fault suggesting that it might correspond to a structural depression related to reactivation movements on that fault mapping of neogene and quaternary cover deposits preserved in the basin and its vicinity was accomplished and numerous evidence of tectonic deformation tilting and faulting affecting the sediments were found other geomorphological criteria are discussed showing the structural origin of the vilarica basin and its neotectonic activity attending to its location on the manteigas braganca fault the vilarica depression is interpreted as a pull apart basin resulting from left lateral strike slip displacement on that fault in the quaternary average fault slip rates are estimated with values comprised between 0 2 mm yr and 0 5 mm yr these slip rates are typical of moderate to moderately high degrees of fault activity and are compatible with the regional faulting geomorphic expression and the relatively important historical and instrumental seismicity affecting the vilarica region finally a geodynamical model is proposed and the studied area is presented in the geodynamic framework

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

assessment of recent tectonic activity on the nw iberian atlantic margin by means of geomorphic indices and field studies of the lower mino river terraces abstract until recently nw iberia was assumed to be tectonically quiescent stable however a combination of tectono morphological analyses demonstrates that neotectonic movements do occur which are in agreement with recent findings by other workers we use a dem based tectono geomorphic approach in combination with field work to assess the presence and nature of neotectonics in the lower mino catchment in nw iberia the area is characterized by clearly developed lineaments coinciding with paleozoic and mesozoic faults and fractures and remnants of a peneplain stepping down towards the coast longitudinal river profiles show knick points where lithological control can be excluded valley shapes show that some valleys have recently been rejuvenated systematic asymmetry tendencies of river catchments indicate tilting of fault bounded blocks finally at several locations faulted terrace deposits of the mino river have been found the types of motions that can clearly be evidenced by our analyses are normal faulting and tilting of fault bounded blocks little clear evidence exists for strike slip faulting whereas reverse faulting has not been found in this study a striking morphological feature of the study area is the presence of small fault bounded basins along the course of the river mino the tectonic movements take place along inherited paleozoic and mesozoic faults which apparently act as weakness zones the reactivation of these faults under the present day stress conditions is probably the reason for the complicated pattern and style of present day tectonic motions that we find the small basins are particularly difficult to explain they could result from the interaction of partially obliquely reactivated strike slip faults step over and normal faults

geophysical methods applied to fault characterization and earthquake potential assessment in the lower tagus valley portugal abstract the study region is located in the lower tagus valley central portugal and includes a large portion of the densely populated area of lisbon it is characterized by a moderate seismicity with a diffuse pattern with historical earthquakes causing many casualties serious damage and economic losses occurrence of earthquakes in the area indicates the presence of seismogenic structures at depth that are deficiently known due to a thick cenozoic sedimentary cover the hidden character of many of the faults in the lower tagus valley requires the use of indirect methodologies for their study this paper focuses on the application of high resolution seismic reflection method for the detection of near surface faulting on two major tectonic structures that are hidden under the recent alluvial cover of the tagus valley and that have been recognized on deep oil industry seismic reflection profiles and or inferred from the surface geology these are a wnw ese trending fault zone located within the lower tagus cenozoic basin across the tagus river estuary porto alto fault and a nne ssw trending reverse fault zone that borders the cenozoic basin at the w vila franca de xira lisbon fault vertical electrical soundings were also acquired over the seismic profiles and the refraction interpretation of the reflection data was carried out according to the interpretation of the collected data a complex fault pattern disrupts the near surface first 400 m at porto alto affecting the upper neogene and at least for one fault the quaternary with a normal offset component the consistency with the previous oil industry profiles interpretation supports the location and geometry of this fault zone concerning the second structure two major faults were detected north of vila franca de xira supporting the extension of the vila franca de xira lisbon fault zone northwards one of these faults presents a reverse geometry apparently displacing holocene alluvium vertical offsets of the holocene sediments detected in the studied geophysical data of porto alto and vila franca de xira lisbon faults imply minimum slip rates of 0 15 0 30 mm year three times larger than previously inferred for active faults in the lower tagus valley and maximum estimates of average return periods of 2000 5000 years for m 6 5 7 co seismic ruptures

cenozoic tectonic activity in a variscan basement evidence from geomorphological markers and structural mapping nw iberian massif abstract the nw iberian massif is part of an ancient basement that has been considered a seismically stable area with no outstanding cenozoic tectonics however recent seismic activity revealed the need for better knowledge of the cenozoic structures in the area because of the lack of mesozoic deposits and the scarcity of cenozoic sediments as well as the intense deformation of the pre mesozoic variscan basement it is difficult to study the cenozoic tectonic structures in this work the combination of detailed structural mapping and study of geomorphological markers in the variscan basement has allowed recognition of cenozoic tectonic structures kinematics and processes that otherwise would not have been identified the identified structures have been gathered into three groups a ne sw trending strike slip faults mainly sinistral b nnw vergent thrusts that uplift the caurel mountains and galaico leoneses mountains and c e w and ene wsw thrusts that uplift the ancares mountains in a pop up structure the structures cut the pre cenozoic erosion surface and affect the drainage network that shows patterns characteristic of tectonic activity the three groups of structures define sectors with different relief showing a strong link between geomorphological elements and tectonic structures the intense drainage reorganisations observed in the area and the deformation of miocene pliocene deposits point out to a significant late miocene tectonic activity in the region thus the cenozoic tectonic activity in the northwest of the iberian peninsula takes place during an extensive period of time which started with the episodes of compression in the cantabrian margin and it is identified nowadays by the recent seismic activity recorded in the north western iberian peninsula 1995 and 1997 lugo events the seismicity is related to the cenozoic structures identified in the area which move under the present se nw horizontal maximum compression and coincide with the proposed seismogenic faults

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