imaging granitic plutons along the iberseis profile abstract the parameters used for the acquisition of the iberseis deep seismic reflection profile in the sw iberian peninsula provide seismic images of the deep crust as well as a high resolution section of the shallow subsurface a very dense array of sources and receivers allowed high resolution tomographic studies in zones of special interest granitic plutons the three dimensional tomographic inversion produced velocity models along a 500 m wide and 1000 m deep strip along the iberseis transect in the areas of la bazana la dehesilla feria and villafranca in these high resolution velocity models sampled by 50 50 50 m cells high velocity anomalies indicate the geology and extension of the granitic plutons at depth this directly correlates with the surface outcrops moreover tomographic models provided valuable information for the geometry and characterization of fractured and fresh domains in a rock volume furthermore a piggy back seismic acquisition experiment using additional seismic instrumentation from the university of paris sud 40 channel dmt provided perpendicular offline recordings of the vibroseis sources this additional recording system was deployed perpendicularly to the main iberseis seismic reflection line and provided additional 3d control

geodynamic evolution of the sw europe variscides 1 the early evolution of sw europe variscides started by opening of the rheic ocean at 500 ma splitting avalonia from armorica iberia subduction on the se side of rheic generated the paleotethys back arc basin 430 390 ma splitting armorica from iberia with development of porto tomar ferreira do alentejo ptfa dextral transform defining the boundary between continental armorica and finisterra microplate to the w obduction of paleotethys was followed by armorica iberia collision and emplacement of nw iberian allochthonous units at 390 370 ma whereas toward the west of ptfa there was antithetic ophiolite obduction beja acebuches and rheic ophiolites plus finisterra continental slices on top of ossa morena zone with simultaneous development of eclogites and orogenic magmatism under a flake double wedge tectonic regime continued convergence 370 ma proceeded by intracontinental deformation with progressive tightening of the ibero armorican arc through dextral transpression on the cantabrian indentor from iberia to armorica the proposed model is discussed at the light of the driving mechanism of soft plate tectonics

kirchhoff migration using eikonal equation traveltimes the use of ray shooting followed by interpolation of traveltimes onto a regular grid is a popular and robust method for computing diffraction curves for kirchhoff migration an alternative to this method is to compute the traveltimes by directly solving the eikonal equation on a regular grid without computing raypaths solving the eikonal equation on such a grid simplifies the problem of interpolating times onto the migration grid but this method is not well defined at points where two different branches of the traveltime field meet also computational and data storage issues that are relatively unimportant for performance in two dimensions limit the applicability of both schemes in three dimensions a new implementation of a gridded eikonal equation solver has been designed to address these problems a 2 d version of this algorithm is tested by using it to generate traveltimes to migrate the marmousi synthetic data set using the exact velocity model the results are compared with three other images an f x migration a standard for comparison a kirchhoff migration using ray tracing and a kirchhoff migration using traveltimes generated by a commonly used eikonal equation solver the f x migrated image shows the imaging objective more clearly than any of themore kirchhoff migrations and the authors advance a heuristic reason to explain this fact of the kirchhoff migrations the one using ray tracing produces the best image and the other two are of comparable quality less

reverse time migration from irregular surface by flattening surface topography abstract prestack reverse time migration rtm is a powerful tool to provide high resolution structure images of crust and upper mantle in wide angle seismic experiments as a standard migration algorithm assumes that data are recorded on a flat surface rtm meets severe challenge on the issue of complicated topography where many seismic surveys are carried out up till now there are two popular techniques for handling elevation changes i e elevation static correction or time shift and wave equation datuming numerous studies demonstrate that the elevation static correction is inaccurate for non vertically traveling energy and subsequently may migrate reflectors to incorrect positions wave equation datuming is more accurate but more sophisticated and too computationally demanding for routine use here we present an alternative scheme to deal with topography in prestack rtm that can migrate seismic data acquired in areas with irregular topography to the reflector positions directly without datuming or elevation static corrections in this scheme surface topography is flattened by a transformation of coordinates from cartesian to curvilinear such a transformation is equivalent to map a rectangular grid onto a curved one and is implemented by the introduction of boundary conforming grid then we use an explicit finite difference scheme with second order accuracy to discretize the elastic wave equations and perform the rtm in the curvilinear coordinate system moreover a flux corrected transport fct algorithm adapted from hydrodynamics is incorporated in the rtm implementation which can not only overcome problems of numerical dispersion that arise in finite difference algorithms but also offer the opportunity to use a coarse grid to obtain the same accuracy as conventional finite difference methods on a dense grid the fctrtm method presented here has no limitations on surface irregularity complexity of the subsurface structure or velocity function extensive numerical examples demonstrate the feasibility and effectiveness of our method

application of the topographic datuming operator to a data setfrom the eastern arabian peninsula we apply a redatuming methodology designed to handle rugged topography and the presence of high velocity layers near the acquisition surface to a 2d land seismic data set acquired in saudi arabia this methodology is based on a recently developed prestack operator which we call the topographic datuming operator tdo the tdo unlike static corrections allows for the movement of reflections laterally with respect to their true locations corresponding to the new datum level thus it mitigates mispositioning of events and velocity bias introduced by the assumption of surface consistency and the time invariant time shifts brought about by static corrections using the shallow velocities estimated from refracted events the tdo provides a superior continuity of reflections and better focusing than that obtained from conventional static corrections in most parts of the processed 2d line the computational cost of applying the tdo is only slightly higher than static corrections the marginal additional computational cost and the possibility of estimating after tdo redatuming stacking velocities that are not affected by a spurious positive bias as in the case of static corrections are further advantages of the proposed methodology the likelihood of strong heterogeneities in the most complex part of the line limits the applicability of any approach based upon geometrical optics however the tdo produces results that are slightly better than those obtained from static corrections because of its ability to partially collapse diffractions generated in the near surface

a bump on the upper surface of the philippine sea plate beneath the boso peninsula japan inferred from seismic reflection surveys a possible asperity of the 1703 genroku earthquake abstract to reveal a subsurface structure beneath the southern part of the boso peninsula japan where the philippine sea plate is subducting and great interplate earthquakes associated with the subduction occur repeatedly we conducted a new seismic reflection survey from march to april 2005 boso05 we also reanalyzed old multi channel seismic mcs survey data that had been collected off the boso peninsula in 1978 sk78 we found clear strong reflectors beneath the southern coast of the boso peninsula since common mid points cmps were distributed widely beneath the study area owing to the design of the receiver and shot lines of boso05 we selected appropriate directions of stacking lines to get the best image of the dipping reflectors by optimum azimuth search oas processing we carefully checked the seismic profiles at the intersections of the survey lines to confirm the nne dipping configuration of the strong reflectors these strong reflectors were interpreted as the upper surface of the subducting phs plate from their locations and the estimated velocities beneath the reflectors furthermore these reflectors revealed a topographic high bump beneath the southern coast of the boso peninsula where the source fault of the genroku earthquake of 1703 is thought to be located

crustal geometry of the abitibi subprovince in light of three dimensional seismic reflector orientations we provide precise estimates of reflector orientations beneath the archean abitibi subprovince using two distinct approaches based on lithoprobe seismic reflection data in the first we use the dip of reflections observed on intersecting profiles to establish the three dimensional orientation of reflective structures in the second the strikes and dips of reflectors are estimated in the crooked parts of seismic reflection profiles by calculating a measure of coherency along the traveltime trajectories defined by a particular azimuth dip depth and medium velocity mid crustal reflectors define two areas with distinctive geometry reflectors beneath the southern abitibi belt are oppositely dipping and convergent at depth providing a v shape aspect to the greenstone rocks other reflectors beneath the northern abitibi belt are in general subparallel dipping at an average of 30 toward the north these north dipping reflectors are partly disrupted by a low reflectivity zone which is attributed to

crooked line 2d seismic reflection imaging in crystalline terrains part 2 migration seismic reflection surveys are frequently conducted over very complicated geological structure but surveying often must be confined to existing crooked roads or tracks typically data from such 2d crooked line surveys are processed using standard common midpoint cmp stacking techniques to obtain a 2d time section which is then 2d migrated in part i we show that a reflector dip component across the processing line can cause serious problems for standard cmp stacking we also propose a supplementary processing step in which cross dip is determined locally and cross dip moveout cdmo is removed from data to form an optimum cross dip stack however a crooked line survey is really a swath 3d survey and ideally we would like to obtain a 3d image of reflectivity surrounding the profile here we investigate the potential of 3d prestack kirchhoff migration to directly image all observed reflections i e we attempt to construct a 3d image volume of all reflectors viewed by the survey because reflectors t

correlation length and fractal dimension interpretation from seismic data using variograms and power spectra recent modeling techniques for fluid flow in a reservoir or an aquifer require characterizing the statistics of the spatial variability of physical properties such as porosity or reflectivity the major interest of such modeling is in horizontal variations we acquired a nontraditional seismic survey and analyzed it for the statistical parameters of correlation length and fractal dimension variograms and power spectra can extract these parameters from migrated sections statistical parameters are valuable for newly established hydrogeologic modeling techniques with their definition greatly increasing the understanding of the aquifer under study applying similar techniques may also prove valuable for reservoir exploitation in the petroleum industry we acquired our survey on top of a bench of an open pit diatomite quarry and extracted the statistical parameters for an exposed vertical face the imaged zone of interest is therefore physically accessible to future permeability studies that could validate our interpreted parameters we can interpret horizontal but not vertical correlation lengths and fractal dimensions from either variograms or power spectra of our migrated seismic images the horizontal correlation length and fractal dimension extracted from the seismic variograms match those extracted from photographs taken of the quarry face

dating the oldest terrestrial rocks fact and fiction abstract this paper is a review of well documented geochronological and related isotopic evidence for the age and total crustal residence time of the oldest known terrestrial rocks mainly using rb 1bsr sm 1bnd and u 1bpb decay schemes the oldest known stable sialic crust of true continental character predominantly composed of calc alkaline orthogneisses was produced during major mantle differentiation some 3 7 3 5 ga ago and is represented on several continents enclaves of older supracrustal rocks as well as a major remnant of a volcano sedimentary sequence of greenstone belt affinity dated at 3 8 ga are preserved within the surrounding gneisses in southern west greenland recent ionmicroprobe evidence for 4 1 ga old detrital zircon grains in a younger early archaean metasedimentary sequence in western australia is also discussed several cases are reviewed where claims for early archaean 3 4 ga dates are believed to be erroneous and to result from oversimplistic and incorrect interpretation of isotopic data some of the geological and geochemical mechanisms which have given rise to claims for spuriously old sm 1bnd u 1bpb and pb pb dates are critically discussed to supplement published data we present several new sets of whole rock sm 1bnd and pb pb isotopic data from west greenland labrador zimbabwe and eastern india relevant to the overall topic of this review

paleomagnetic evidence for tectonic rotations in the variscan mountain belt a critical review of devonian and carboniferous paleomagnetic results from the hercynian basement of western and central europe reveals large rotations their geographic distribution suggests a tectonic origin for large scale curved structures such as the ibero armorican arc or the variscan v together with a coherence of the hercynian domain with time geodynamic models which can have produced the observed rotation pattern are discussed it appears that lateral compression due to the accommodation of continental collision as in the india asia convergence is a plausible mechanism although variscan and himalayan mountain belts have different characteristics

a new sulphur isotopic study of some iberian pyrite belt deposits evidence of a textural control on sulphur isotope composition the sulphide deposits of the iberian pyrite belt ipb represent an ore province of global importance our study presents 113 new sulphur isotope analyses from deposits selected to represent the textural spectrum of ores measured 34s values range from 26 to 10 mostly for massive and stockwork ores in agreement with data previously published in situ laser 34s analyses reveals a close correlation of 34s with texture primary diagenetic textures are dominated by relatively low 34s 8 to 2 whereas stockwork feeder textures are dominated by higher 34s 3 to 5 intermediate textures mainly coarse textures in stratiform zones have intermediate 34s although they are mostly dominated by the high 34s component rare barite has a homogeneous 34s around 18 which is consistent with direct derivation from lower carboniferous seawater sulphate a dual source of sulphide sulphur in the ipb deposits has been considered a hydrothermal source derived from reduction of coeval seawater sulphate in the convective systems is represented by sulphide in the feeder zones here variations in 34s are caused by variations in the extent of the sulphate reduction which governs the so4 h2s ratio the second end member was derived from the bacterial reduction of coeval seawater sulphate at or near the surface as reflected in the primary textures a distinct geographical variation in 34s and texture from sw more bacteriogenic and primary textures to ne more hydrothermal textures and 34s which reflects a variation in the relative input of each source was likely controlled by local geological environments given that the sulphur isotope characteristics of the ipb deposits are unlike most vms and kuroko deposits and noting the dominance of a mixed reduced sedimentary and volcanic environment we suggest that the ipb could represent an ore style which is intermediate between volcanic and sedimentary hosted massive sulphide types

flooding the topography wave equation datuming of land data with rugged acquisition topography wave equation datuming overcomes some of the problems that seismic data recorded on rugged surface topography present in routine image processing the main problems are that 1 standard optimized migration and processing algorithms assume data are recorded on a flat surface and that 2 the static correction applied routinely to compensate for topography is inaccurate for waves that do not propagate vertically wave based processes such as stacking dip moveout correction normal moveout correction velocity analysis and migration after static shift can be severely affected by the nonhyperbolic character of the reflections to alleviate these problems i apply wave equation datuming early in the processing flow to upward continue the data to a flat datum above the highest topography this is what i refer to as flooding the topography this approach does not require detailed a priori knowledge of the near surface velocity and it streamlines subsequent processing because the data are regridded onto a regularly sampled datum wave equation datuming unravels the distortions caused by rugged topography and unlike the static shift method it does not adversely effect subsequent wave based processing the image obtained after wave equation datuming exhibits better reflector continuity and more accurately represents the true structural image than the image obtained after static shift

seismic imaging of the crust and upper mantle using regularized joint receiver functions frequency wave number filtering and multimode kirchhoff migration 1 receiver functions provide an indispensable tool for producing discontinuity images of the crust and upper mantle from teleseismic earthquake arrivals however image quality can be significantly compromised by instabilities inherent in the deconvolution process and by irregularities in data coverage driven by source and or receiver geometries we present receiver function estimation and prestack migration techniques which both reduce receiver function deconvolution instability and produce regularized multimode receiver function images the receiver function estimation technique exploits frequency wave number filtering methods using frequency pseudowave number domain filtering of gathers to enhance arrivals with consistent moveout characteristics synthetic tests confirm robust recovery of features in the presence of high noise levels to process regularized receiver functions into accurate images of the subsurface seismic scattering potential we employ a regularized kirchhoff migration that migrates both direct and reverberated p to s converted modes to their correct subsurface locations the subsequent images can be stacked to mute mismigrated modes synthetic studies and model resolution tests demonstrate that this methodology is well suited to irregularly spaced stations and uneven data coverage and does not require the recording of a single earthquake at multiple stations

multifocusing imaging over an irregular topography if seismic data are acquired over an irregular topography standard elevation statics methods may be inaccurate because the assumption of vertical raypaths will no longer be valid an effective solution to the problem of irregular topography can be found through the use of the multifocusing method in which large supergathers of seismic traces are stacked each of which can span many common midpoint cmp gathers this can be done by extending the multifocusing moveout formula to explicitly account for nonzero elevations of the source and receiver as well as their horizontal coordinates implementation of this formula into the multifocusing algorithm is straightforward because estimating the necessary raypath information i e emergence angles is an integral part of the algorithm the extended multifocusing moveout correction can be applied directly to the data acquired in areas of irregular topography without the need for prior elevation static corrections synthetic tests on such data show that the proposed technique results in a better alignment of reflection events

potential pitfalls of crooked line seismic reflection surveys during the last few years the geological survey of canada has pioneered the application of seismic reflection profiling to mineral exploration in close collaboration with canadian mining companies and with the lithoprobe project e g spencer et al 1993 milkereit et al 1994 because of the rugged terrain in crystalline rock environments dahle et al 1985 spencer et al 1993 vibroseis seismic surveys are frequently conducted along existing roads resulting in extremely crooked survey profiles crooked profiling geometry coupled with the complex nature of the geological targets pose special challenges for seismic data processing and interpretation many common midpoint seismic processing techniques are based on an implicit assumption of a straight line survey and are most effective with uniform fold and even offset distribution within common midpoint cmp gathers however with crooked line acquisition the cmp gathers are characterized by variable fold and uneven offset distribution based

migration from 3d irregular surfaces a prestack time migration approach abstractwe have developed a modified 3d prestack time migration pstm scheme that can handle rugged topography as well as high near surface velocities in land seismic imaging the proposed topography pstm can be applied to seismic data recorded on a 3d irregular surface without static corrections two effective velocity parameters were found to describe wave propagation through inhomogeneous media above and below a chosen datum as a result wave propagation phenomena in the complex near surface such as near vertical incidences through a weathering layer and raypaths bending away from vertical in the presence of high near surface velocities are correctly considered the two effective velocity parameters can be estimated by flattening events in imaging gathers hence it is not necessary to have detailed knowledge of the near surface velocity model and velocity field below the datum when applying topography pstm we integrated residual static corrections into topography pstm this eliminated the distort

interpretation of 2d seismic profiles in complex geological terrains examples from the flin flon mining camp canada abstracta methodology was demonstrated for the 3d interpretation of networks of 2d seismic profiles in conjunction with other 3d geological constraints the methodology employs 3d migration of 2d seismic data as a means of directly correlating reflections with out of plane geology followed by ray trace modeling of interpreted 3d geological surfaces the proposed interpretation workflow was demonstrated with examples taken from 2d seismic profiles that were recently acquired for vms ore exploration within the flin flon mining camp canada in each example the utility of the method was demonstrated and the resulting inferences were validated by comparison with a true 3d seismic survey acquired over a subset of the same area

true amplitude seismic migration a comparison of three approaches knowledge of elastic parameter compressional and shear velocities and density contrasts within the earth can yield knowledge of lithology changes elastic parameter contrasts manifest themselves on seismic records as angle dependent reflectivity interpretation of angle dependent reflectivity or amplitude variation with offset avo on unmigrated records is often hindered by the effects of common depth point smear incorrectly specified geometrical spreading loss source receiver directivity as well as other factors it is possible to correct some of these problems by analyzing common reflection point gathers after prestack migration provided that the migration is capable of undoing all the amplitude distortions of wave propagation between the sources and the receivers a migration method capable of undoing such distortions and thus producing angle dependent reflection coefficients at analysis points in a lossless isotropic elastic earth is called a true amplitude migration the principles of tr

true amplitude migration of 2d synthetic data true amplitude ta migration which is a kirchhoff type modified weighted diffraction stack recovers possibly complex angle dependent reflection coefficients which are important for amplitude versus offset avo inversion the method can be implemented using existing prestack or post stack kirchhoff migration and fast green s function computation programs here it is applied to synthetic single shot and constant offset seismic data that include post critical reflections complex reflection coefficients and caustics comparisons of the amplitudes of the ta migration image with theoretical reflection coefficients show that the possibly complex angle dependent reflection coefficients are correctly estimated

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

the geology and gravity field in the central core of the vredefort structure the centre of the vredefort structure in south africa comprises an uplifted granitic basement with an associated 300 g u gravity high ringed by upturned sedimentary strata detailed geological mapping of the basement core has delineated a three fold concentric zoning of outer granite gneisses ogg steynskraal formation sf and inlandsee leucogranofels ilg an increase in metamorphic grade occurs toward the centre of the structure from greenschist relics on the basement perimeter to upper amphibolite and granulite facies in the sf and granulite facies in the ilg a marked lithological change occurs within the sf in which perimeter greenschists and inner amphibolites give way to paragneisses in the ilg paragneiss and mafic granulite relics predominate these divisions are consistent with the standing on edge model for the basement 1 three regional metamorphic episodes and two thermal events have been recognised m1 and m3 both attained granulite facies at low medium load pressures in the ilg m1 granulite facies assemblages extended into the ogg but the m3 facies was restricted to the ilg m2 was a k metasomatism event limited to the ogg an extensive thermal episode m4a is provisionally correlated with the collar metamorphism pyroxene hornfels facies t 800 c was attained in the centre as opposed to conditions of p 4 5 kbar and t 650 c for alkali granite stocks in the sedimentary collar 2 this event was thus pre or syn uplift but not post uplift a high level high temperature p 3 kbar and t 800 900 c event m4b occurred at a late stage in the overall sequence a suite of amphibole orthopyroxene rocks of basaltic komatiite type cao al2o3 1 are chemically related to younger high mg tholeiites these tholeiites are correlated with similar rocks in the sedimentary collar and were intruded syn to post m4a in several episodes of decreasing basicity shock metamorphic features granitic breccias and pseudotachylite are linked to the late stage history of the structure and a case is presented for a dynamic metamorphic front advancing outward and upward from the centre in response to unroofing of the basement the central gravity high3 is delineated by 1239 new gravity stations at 1 km and 0 5 km intervals a low frequency oval shaped anomaly with a long axis trending nw se has eccentrically positioned on it a plateau extending southwards as a spur a dyke is responsible for the spur but the source for the plateau is from a recent borehole thought to be largely due to sheared altered harzburgite modelling of the low frequency gravity field component was based on the mapped divisions of the ogg sf and ilg and a density contrast of 200 kg m3 was determined between the ogg and the sf and ilg lithologies a purely crustal solution for the excess mass distribution in the basement is conclusively demonstrated

geophysical evidence of a mantle derived intrusion in sw iberia 1 deep seismic reflection data acquired as part of the sw iberia europrobe project across the transpressional variscan orogen sample three tectonic terranes the south portuguese zone the ossa morena zone and the central iberian zone the seismic data reveal the existence of a mid crustal reflective body 140 km long and of variable thickness up to 5 km the iberian reflective body the conductivity image provided by coincident mt soundings the amplitude characteristics of the seismics mineralization studies related to magmatic ore deposits and the surface geology suggest that the irb is a mantle derived mafic intrusion the geophysical geological and petrological data suggest that the irb is most probably an early carboniferous approximately at 350 340 ma mantle derived intrusion possibly linked to plume activity that took place in europe in the carboniferous and permian

a seismic geotraverse across the iberian variscides orogenic shortening collisional magmatism and orocline development 1 a crustal geotraverse through the iberian variscides is presented by integrating the available geological and seismic profiling data different modes of orogenic shortening are identified with varying degrees of coupling between upper and lower crust in northern and southern regions of the geotraverse a decoupling in the middle crust permits the lower crust to subduct underthrust thus compensating for strong upper crustal shortening this behavior does not result in great crustal thickening except in sectors of crustal underthrusting in southern central iberia moderate upper crustal shortening is due to a mechanically strong lower crust impeded to subduct underthrust in this region shortening is partitioned between upper and lower crust deformation being distributed in the upper crust while localized at major fault zones in the lower crust finally the central region of the geotraverse northern central iberia shows a coupled crustal deformation having given way to the largest orogenic thickening in the iberian variscides the thermal maturity of this much thickened crust originated voluminous crustal melting and concomitant normal fault detachments developed while shortening dominated in other regions theoretical models suggest that compressive stresses may prevail in the lower crust beneath the extending upper crust thus explaining the efficient syncollisional exhumation in this part of the orogen a particular feature of the iberian variscan geotraverse is the great importance of out of section mass movements mainly left lateral shear zones concentrated in two suture boundaries which displaced to the nw present coordinates central and northern iberia with respect to southern iberia

accretionary history and crustal evolution of the variscan belt in western europe abstract the variscan belt of western europe is part of a large intra paleozoic belt extending on both sides of the atlantic from the ouachitas in the us and the mauritanides in west africa to the bohemian massif in czechoslovakia and poland the belt was constructed between 500 and 250 ma as a result of convergence and collision between two main continents laurentia baltica and africa after the closure of various oceanic basins iapetus rheic and galicia massif central now recorded as discrete in part cryptic sutures which form the roots of large crystalline nappes in which are found dismembered remnants of oceanic crust and mantle the accretionary history of the variscan belt in western europe is related to the progressive closure of probably two oceanic basins the rheic in the north and another the galicia massif central in the south by respectively southward and northward intraoceanic subduction followed by obduction and intracontinental lithospheric subduction the result is a broad 700 800 km fan like divergent belt which must be considered as a classical obduction collision orogen with a long intracontinental history of about 100 150 ma its arcuate shape is probably due to the northwestward impingement of a promontory of the african continent into laurentia baltica

crustal structure of the transpressional variscan orogen of sw iberia sw iberia deep seismic reflection profile iberseis 1 iberseis a 303 km long 20 s deep seismic reflection profile was acquired across the variscan belt in sw iberian peninsula the acquisition parameters were designed to obtain a high resolution crustal scale image of this orogen the seismic profile samples three major tectonic terranes the south portuguese zone the ossa morena zone and the central iberian zone which were accreted in late paleozoic times these terranes show a distinctive seismic signature as do the sutures separating them late strike slip movements through crustal wedges are apparent in the seismic image and have strongly modified the geometry of sutures the upper crust appears to be decoupled from the lower crust all along the seismic line but some deformation has been accommodated at deeper levels a sill like structure is imaged in the middle crust as a 1 2 s thick and 175 km long high amplitude conspicuous reflective band it is interpreted as a great intrusion of mafic magma in a midcrustal decollement taking into account surface geological data and the revealed crustal architecture a tectonic evolution is proposed for sw iberia which includes transpressional collision interacting during early carboniferous with a mantle plume the moho can be identified along the entire transect as subhorizontal and located at 10 to 11 s indicating a 30 35 km average crustal thickness its seismic signature changes laterally being very reflective beneath the south portuguese zone and the central iberian zone but discontinuous and diffuse below the ossa morena zone

tectonic evolution of the boundary between the central iberian and ossa morena zones variscan belt southwest spain three tectonometamorphic units can be differentiated in the boundary between the central iberian and ossa morena zones of the iberian massif variscan belt the three units have been named the northern unit central unit and southern unit the northern unit corresponds to the border of the central iberian zone it evolved at low temperature and intermediate or low pressure metamorphic conditions and was affected by top to the se ductile shearing the central unit placed under the northern unit preserves high pressure silurian eclogitic assemblages retrograded by high to medium temperature and finally low temperature ductile shearing with top to the nw sense of movement oblique left lateral the central unit is superposed on the southern unit the latter corresponds to the border of the ossa morena zone and underwent low pressure intermediate temperature metamorphism synchronous with right lateral top to the se ductile shearing the envisaged tectonic evolution is as follows after a stage of lower paleozoic rifting subduction of the central unit under the northern unit took place in silurian times as a result of the crustal thickening a gravitational instability developed giving way to left lateral extensional shearing that affected the entire central unit the combined action of oblique thrusting at the front of the wedge and oblique extensional shearing at the rear caused the exhumation of the high pressure rocks of the central unit this tectonic evolution reveals that the boundary between the central iberian and ossa morena zones is a suture of the variscan belt

topographic elastic least squares reverse time migration based on vector p and s wave equations in the curvilinear coordinates elastic least squares reverse time migration has been applied to multi component seismic data to obtain high quality images however the final images may suffer from artefacts caused by p and s wave crosstalk and severe spurious diffractions caused by complex topographic surface conditions to suppress these crosstalk artefacts and spurious diffractions we have developed a topographic separated wavefield elastic least squares reverse time migration algorithm in this method we apply p and s wave separated elastic velocity stress wave equations in the curvilinear coordinates to derive demigration equations and gradient formulas with respect to p and s velocity for the implementation of topographic separated wavefield elastic least squares reverse time migration the wavefields gradient directions and step lengths are all calculated in the curvilinear coordinates numerical experiments conducted with the two component data synthetized by a three topographic layer with anomalies model and the canadian foothills model are considered to verify our method the results reveal that compared with the conventional method our method promises imaging results with higher resolution and has a faster residual convergence speed finally we carry out numerical examples on noisy data imperfect migration velocity and inaccurate surface elevation to analyse its sensitivity to noise migration velocity and surface elevation error the results prove that our method is less sensitive to noise compared with the conventional elastic least squares reverse time migration and needs good migration velocities as other least squares reverse time migration methods in addition when implementing the proposed method an accurate surface elevation should be obtained by global positioning system to yield high quality images

mapping of crustal scale tectonic boundaries in the ossa morena zone using reprocessed iberseis reflection seismic data the iberseis deep seismic reflection profile imaged crustal scale structures in the sw iberian variscan belt crossing the south portuguese zone the ossa morena zone and the central iberian zone in spain two subsets of the profile corresponding to the south portuguese zone ossa morena zone and the ossa morena zone central iberian zone tectonic contacts have been reprocessed with the aim of investigating the influence of cross dip and to better image steeply dipping features alternative strategies for binning midpoints into common depth point cdp bins using different azimuths were examined for synthetic data we show that the choice of the cdp processing line and the bin azimuth orientation has a significant impact on the normal moveout and dip moveout velocities and is crucial to optimizing the quality of the stacked seismic image along the crooked profile multi azimuth binning normal moveout dip moveout and migration velocity analysis on synthetic and real data show the presence of clear sub vertical upper crustal structures near the south portuguese zone ossa morena zone suture the aroche fault this sub vertical reflectivity that was not imaged earlier projects into a location in the lower crust with low reflectivity

high resolution reflection seismic imaging of the ullared deformation zone southern sweden the ullared deformation zone udz is one of a few structures worldwide known to contain decompressed eclogite facies rocks of precambrian age given the unique nature of the ullared eclogites a 1

4 transpressional tectonics lower crust decoupling and intrusion of deep mafic sills a model for the unusual metallogenesis of sw iberia abstract sw iberia is interpreted as an accretionary magmatic belt resulting from the collision between the south portuguese zone and the autochthonous iberian terrane in variscan times 350 to 330 ma in the south portuguese zone pull apart basins were filled with a thick sequence of siliciclastic sediments and bimodal volcanic rocks that host the giant massive sulphides of the iberian pyrite belt massive sulphides precipitated in highly efficient geochemical traps where metal rich but sulphur depleted fluids of dominant basinal derivation mixed with sulphide rich modified seawater massive sulphides formed either in porous reactive volcanic rocks by sub seafloor replacement or in dark shale by replacement of mud or by exhalation within confined basins with high biogenic activity crustal thinning and magma intrusion were responsible for thermal maturation and dehydration of sedimentary rocks while magmatic fluids probably had a minor influence on the observed geochemical signatures the ossa morena zone was a coeval calc alkaline magmatic arc it was the site for unusual mineralization particularly magmatic ni cu and hydrothermal fe oxide cu au ores iocg most magmatism and mineralization took place at local extensional zones along first order strike slip faults and thrusts the source of magmas and iocg and ni cu deposits probably lay in a large mafic ultramafic layered complex intruded along a detachment at the boundary between the upper and lower crust here juvenile melts extensively interacted with low grade metamorphic rocks inducing widespread anatexis magma contamination and further exsolution of hydrothermal fluids hypersaline fluids 18 o fluid 5 4 to 12 were focused upward into thrusts and faults leading to early magnetite mineralization associated with a high temperature 500 c albite actinolite salite alteration and subsequent copper gold bearing vein mineralization at somewhat lower temperatures assimilation of sediments by magmas led in turn to the formation of immiscible sulphide and silicate melts that accumulated in the footwall of the layered igneous complex further injection of both basic and sulphide rich magmas into the upper crust led to the formation of ni cu rich breccia pipes younger 330 to 280 ma peraluminous granitoids probably reflect the slow ascent of relatively dry and viscous magmas formed by contact anatexis these granitoids have w sn and pb zn related mineralization that also shows geochemical evidence of major mantle crust interaction late epithermal hg cu sb and pb zn ag mineralization was driven by convective hydrothermal cells resulting from the high geothermal gradients that were set up in the zone by intrusion of the layered igneous complex in all cases most of the sulphur seems to have been derived from leaching of the host sedimentary rocks 34 s 7 to 20 with only limited mixing with sulphur of magmatic derivation the metallogenic characteristics of the two terranes are quite different in the ossa morena zone juvenile magmatism played a major role as the source of metals and controlled the styles of mineralization in the south portuguese zone magmas only acted as heat sources but seem to have had no major influence as sources of metals and fluids which are dominated by crustal signatures most of the magmatic and tectonic features related to the variscan subduction and collision seem to be masked by those resulting from transpressional deformation and deep mafic intrusion which led to the development of a metallogenic belt with little resemblance to other accretionary magmatic arcs

radioelement concentrations in the deep profile through precambrian basement of the vredefort structure the vredefort basement granite in south africa forms the core of an undomed and overturned sequence of precambrian strata slawson 1976 proposed that the granite core has also been overturned exposing to view a thick section through the archean crust incrasing in depth toward the center of the dome profiles across the vredefort basement provide an opportunity for studying the distribution of u th and k with depth in the upper and middle crust detailed radioelement concentration profiles across the granitic basement showed that 1 the central part of the basement core is depleted in th and u relative to the perimeter and the concentration of th and u falls off regularly from the basement margin inward 2 the distribution of the radioelements and the distribution of heat production with depth over the outer 8 km are consistent with an exponential depth function 3 the felsic rocks of the central basement are characterized by high k u and th u ratios and the ratios k u k th and th u increase from the margin inward and 4 mafic granulites which occur in the central regions of the dome have normal k u k th and th u ratios high grade metamorphic environment resulted in the depletion of bothmore u and th relative to k in the greater proportion of the felsic rocks from the central regions of the vredefort basement depletion of u was far greater than that of th the normal th u k u and k th ratios in mafic rocks in close proximity to the felsic rocks suggest that mineral assemblages have a strong control on the behavior of the radioelements during regional metamorphism in the vredefort basement less

the zero velocity layer migration from irregular surfaces seismic data acquired in areas with irregular topography are usually corrected to a flat datum before migration a time honored technique for handling elevation changes is to time shift the data before application of migration this simple time shift or elevation static correction cannot properly represent wide angle or dipping reflections as they would have been recorded at the datum as a result when elevation varies significantly accuracy in event positioning may be compromised for migration and other wave equation processes such as dip moveout processing dmo traditionally such over and under migration artifacts have been dealt with by increasing or decreasing the migration velocity however simple adjustment of the migration velocity cannot undo the wave field distortions induced in seismic data acquired over varying elevations more sophisticated and accurate solutions such as wave equation datuming are too computationally demanding for routine use here we propose an efficient and accura

application of an integrated wave equation datuming scheme to overthrust data a case history from the chinese foothills an integrated wave equation datuming scheme improves the imaging quality of seismic data from overthrust areas it can be regarded as integrated because upward layer replacement is included in this scheme data are downward continued to a nonplanar datum such as the base of the weathering layer followed by upward continuation from the nonplanar datum to a final planar datum using a one way extrapolator when compared with a kirchhoff integral this method can deal better with the strong lateral velocity variation within the near surface after a test on synthetic data the scheme is applied successfully to real 2d overthrust data acquired in the qi lian foothills western china compared with results using static corrections integrated wave equation datuming results lead to better reconstruction of the diffractions and reflections more reliable migration velocity analyses and stronger stack and final depth images

tectonics of the beja acebuches ophiolite a major suture in the iberian variscan foldbelt the beja acebuches ophiolitic complex baoc south portugal spain corresponds to a high grade metamorphic belt along the boundary between ossa morena and the south portuguese zones and comprises a lithostratigraphic sequence including from top to bottom metabasalts metamorphosed multiple dyke intrusions in gabbro flasergabbros and metaserpentinites it is affected by three deformation phases d1 affects the ophiolite lower stratigraphic units and is represented by a mylonitic cleavage with a stretching lineation where shear criteria indicates the sense of shear to be towards the north north east this deformation event can be related to the ophiolite emplacement above the crystalline footwall of the serpa antiform affecting precambrian basement and cambrian cover rocks the obduction polarity ist north eastwards similar to the subduction polarity that generates the beja gabbroic complex bgc implying a flake geometry the second deformation phase d2 is represented by an intense wnw ese sinistral shear event which is responsible for the shattered appearance of the suture d2 is reactivated later by a more brittle d3 event involving thrusting to the south west again with a sinistral component 40ar 39ar isotopic ages were obtained for metamorphosed multiple dyke intrusions in the baoc s gabbro 342 6 1 4 ma for metagabbroic cumulates 340 7 1 9 ma and for the undeformed unmetamorphosed bgc 341 1 1 3 ma occurring to the north of the ophiolitic suture these ages reflect a last regional cooling event in the area which post dates the ophiolite emplacement and the intrusion of the bgc through this oceanic sequence

integrated geological and geophysical studies in the sg4 borehole area tagil volcanic arc middle urals location of seismic reflectors and source of the reflectivity near vertical incidence reflection seismic data acquired in the tagil volcanic arc middle urals show the upper crust to be highly reflective two intersecting seismic lines located near the ongoing 5400 m deep sg4 borehole show that the main reflectivity strikes approximately n s and dips 35 55 to the east prominent reflections intercept the borehole at 1000 1500 2800 2900 3400 and between 4000 and 5400 m which correspond to intervals of low velocity low density low resistivity the surface projections of these reflections lie parallel to the strike of magnetic anomaly trends multioffset vertical seismic profile vsp data acquired in the sg4 borehole show a seismic response dominated by p to s reflected converted waves from the moderately east dipping reflectivity and from a set of very steep east dipping reflectors not imaged by the surface data modeling of the vsp data constrains the depth at which reflectors intercept the borehole and suggests that the p to s conversions are best explained by low velocity porous intervals rather than higher velocity mafic material the most prominent east dipping reflection on the surface seismic data is only imaged on vsp shots that sample the crust closer to the e w seismic line this discrepancy between the vsp and the surface seismic data is attributed to rapid lateral changes in the physical properties of the reflector surface and borehole data suggest that the low velocity low density low resistivity intervals are the most important source of reflectivity in the sg4 borehole area although lithological contrasts may also play a role drill cores from the these zones contain hydrothermal alteration minerals indicating interaction with fluids tectonic criteria suggest that they might represent imbricated fracture zones often bounding different lithologies and or intrusions some of them might also represent high porosity lava flows or pyroclastic units common in island arc environments

variscan intracontinental deformation the coimbra cordoba shear zone sw iberian peninsula the coimbra cordoba shear zone is a major lineament roughly parallel to the nw se variscan structures of the southern iberian peninsula inside the shear zone evidence for a non coaxial strain path exists from structural microstructural and lattice preferred orientation data on quartz plane strain is suggested by the study of microstructures in the xy and yz sections in all formations occurring in the shear zone with increasing strain a mylonitic series develops the deformation contemporaneous with epizonal to mesozonal metamorphism is responsible for 1 1 the regional subvertical foliation oblique to the shear zone 2 2 the well defined subhorizontal lineation striking nw se 3 3 the discrete planes of shear roughly parallel to the shear zone the data are interpreted as resulting from a left lateral slip motion with a displacement of at least 72 km in the light of plate tectonics it is inferred that the coimbra cordoba lineament could have been a suture zone which evolved into an intracontinental left lateral shear zone during the variscan orogeny of the ibero armorican arc

constraints on the geometry of the suasselka post glacial fault northern finland based on reflection seismic imaging abstract unloading of the ice during the last glacial period in northern fennoscandia is believed to have generated major faulting these faults often referred to as post glacial faults typically have clear surface exposures but their geometry at depth is poorly known in order to better understand the geometry at depth of the suasselka post glacial fault in finland three high resolution 2d reflection seismic profiles over the fault were reprocessed their total profile length is about 60 km and they were acquired as part of a major effort in finland to map the uppermost crust in mining areas the reprocessing led to significantly improved images that could be used to map the fault at depth two approximately n s striking profiles and one e w striking profile were reprocessed the different azimuths and the crooked nature of the profiles allowed the fault geometry to be relatively well constrained clear reflections from the fault dipping towards the se can be traced from the shallow subsurface down to about 3 km the strike and dip of two sets of dipping reflections in the stacked data along with geometrical constraints and cross dip analysis give a consistent dip of about 35 45 towards the se for the fault the strike and dip vary from n55e with a dip of 35 in the east to a strike of n48e with a dip of 45 in the west existence of the two sets of reflections indicates that the fault surface is non planar aside from allowing the geometry of the fault to be determined the seismic data show a complex reflectivity pattern in the area and indications of both reverse and normal movement along fault planes with similar orientation to the suasselka post glacial fault these images can be used as a basis for better characterizing the 3d geology of the area

magnetotelluric survey of the electrical conductivity of the crust across the ossa morena zone and south portuguese zone suture abstract the transition between the south portuguese zone spz and the ossa morena zone omz is made up of a major geosuture which is indicated by the beja acebuches ophiolite several geological data suggest that this suture is the result of an oblique collision with a northward propagation between the spz and omz from the structural point of view the omz is much more complex than the adjacent central iberian zone and the spz the frequently complicated internal structure is the result of superposition of various structural elements developed during different tectonic events with the aim of providing new constraints to this complex deep structure a magnetotelluric mt survey was designed by the universities of lisbon and barcelona in the scope of the bilateral cooperation and the europrobe programs a first nne ssw profile across the spz and omz was carried out in september 1997 the profile is 40 km long and consists of nine deep mt soundings with periods ranging from 0 0039 to 4000 s the time series were processed using a robust algorithm after visual inspection the study of the dimensionality and directionality of the regional electrical structures is based on the analysis of the induction arrows and the groom bailey decomposition a strike of n125e was determined in accordance with regional geological features 2 d inversion was undertaken on both te and tm modes using the rri method the main feature of the model is a large conductive body

geochronology of the deep profile through archean basement at vredefort with implications for early crustal evolution the presence of archean granite forming the core of an updomed and overturned sequence of strata at vredefort south africa has been known for over 70 years recent geophysical geochemical and geological evidence has given rise to the proposal that the basement granite core has also been overturned presenting a section of the earth s granitic crust to view and that radial traverses in the vredefort basement penetrate through a 15 km thick section of the archean crust when moving from the contact of collar strata to the center of the dome the upper levels of the vredefort crust are dominated by a relatively homogeneous outer granite gneiss ogg displaying marked gradients in large ion lithophile elements these rocks grade inward into the deeper levels which occur in granulite facies metamorphism the deeper levels of the vredefort crust exposed in the central parts of the dome consist predominantly of felsic inlandsee leucogranofels ilg which occurs interleaved with inclusions of mafic metavolcanic and metasedimentary rocks termed steynskraal metamorphic zone smz a comprehensive isotopic study using four decay schemes 87rb 232th 238u and 235u on this crustal section is now reported the results permit unique and direct tests on the theories of archean crust forming processes in the upper vredefort crust whole rock isochron ages of 3050 m y are obtained from the four decay schemes and almost all evidence of an extended crustal history prior to 3050 m y has been obliterated establishment of strong vertical gradients in rb sr ratio shortly before setting of rb sr whole rock systems has wide implications for the interpretation of sr initial ratios although the vredefort upper levels of the crust has a low 87sr 86sr initial ratio of 0 7019 0 0002 it probably participated in a crustal prehistory for at least 500 m y in the deeper levels of the vredefort basement the geochronology is much more complex rb sr and th pb isochrons of 3500 m y are recorded in the mafic granulite relicts a companion paper welke and nicolaysen this issue provides evidence for an early crust forming event in this sector 3800 m y ago from 3500 m y onward these deeper crustal levels did not undergo addition of new archean crust forming material on a major scale nevertheless a most important younger archean event affected these levels 2800 m y ago cessation of high grade metamorphism is recorded by setting of the rb sr isotopic system in the felsic rocks and zircon u pb ages an episode of strong uranium depletion affected both the felsic and mafic rocks of this sector 2800 m y ago the inlandsee leucogranofels is evidently a rock unit with a long and complex history beginning with the emplacement as a crustal unit at least 3800 m y ago since then it has been subject to reworking and resetting of isotopic systems which terminated 2800 m y ago in the deeper crust there is scant evidence of a 3050 m y event similarly there is no record of a 2800 m y old imprint in the upper vredefort crust thus the last major events that affected the geochemical constitution of the vredefort upper and deeper crustal sectors differed in time and nature nevertheless they brought about isotopic setting in a downward moving sequence while the overlying crust remained cooler rigid and unaffected these events probably reflect significant discontinuities in the cooling of the archean crust

an integrated method for resolving the seismic complex near surface problem we describe an integrated method for solving the complex near surface problem in land seismic imaging this solution is based on an imaging approach and is obtained without deriving a complex near surface velocity model we start by obtaining from the data the kinematics of the one way focusing operators i e time reversed green s functions that describe propagation between the acquisition surface and a chosen datum reflector using the common focus point technology the conventional statics solutions obtained from prior information about the near surface are integrated in the initial estimates of the focusing operators the focusing operators are updated iteratively until the imaging principle of equal traveltime is fulfilled for each subsurface gridpoint of the datum reflector therefore the seismic data is left intact without any application of time shifts which makes this method an uncommitted statics solution the focusing operators can be used directly for wave equation redatuming to the respective reflector or for prestack imaging if determined for multiple reflecting boundaries the underlying velocity model is determined by tomographic inversion of the focusing operators while also integrating any hard prior information e g well information this velocity model can be used to perform prestack depth imaging or to calculate the depth of the new datum level we demonstrate this approach on 2d seismic data acquired in saudi arabia in an area characterized by rugged topography and complex near surface geology

basement components of the xiangshan yuhuashan area south china defining the boundary between the yangtze and cathaysia blocks abstract the exposed metamorphic rocks and xenolithic metamorphic rocks in the xiangshan yuhuashan xsyhs area in central jiangxi province south china were studied in order to reveal the compositions of the basement rocks and their tectonic implications bulk geochemistry and zircon u pb hf isotopic compositions suggest that the basement metamorphic rocks in the xsyhs area are mainly composed of neoproterozoic after 822 683 ma meta sedimentary rocks the exposed meta sedimentary rocks show higher k 2 o na 2 o and al 2 o 3 na 2 o cao and lower eu eu than those occurring as xenoliths in the yuhuashan volcanic intrusive complex indicating relatively strong weathering and leaching in the provenance area geochemical data suggest that the detritus of the exposed metasedimentary rocks was derived mainly from felsic to intermediate igneous rocks similar to neoproterozoic sedimentary rocks in the southern yangtze block whereas the xenolithic metasedimentary rocks contain more recycled material similar to those in the cathaysia block these xenoliths from the mid lower crust also have significantly high u contents consequently they probably are an important source for uranium in the xsyhs volcanic rocks u pb ages of detrital zircons show that the protoliths of the xenoliths are mainly composed of neoproterozoic neoarchean and paleoproterozoic detritus similar to the components of the wuyi terrane in the eastern cathaysia block in contrast the protoliths of the exposed metasedimentary rocks consist mainly of neoproterozoic detritus with minor neoarchean and paleoproterozoic fragments similar to those in the southern yangtze block both geochemistry and zircon u pb hf isotope compositions suggest that the deep basement metamorphic rocks in the xsyhs area belong to the wuyi terrane whereas the surface metamorphic rocks are derived from the southern yangtze block therefore the real boundary between the yangtze and cathaysia blocks lies north of the study area the exposed yangtze related metamorphic rocks were overthrust southward from the southern yangtze block and onto the cathaysia basement during the indosinian to yanshanian orogeny

the vredefort structure an introduction and a guide to recent literature abstract the 50 km wide vredefort structure appears to present the eroded roots of an area affected by rapid uplift and violent rock deformation widely regarded as shock deformation on an immense scale the origin of the structure is disputed external impact and internal agencies have been suggested this note introduces the vredefort structure and its geological environment emphasizing information published in the decade prior to the 1987 international workshop on cryptoexplosions and catastrophes in the geological record with a special focus on the vredefort structure this typical cryptoexplosion structure has a central and polygonal basement uplift containing violently deformed and brecciated rocks with the northern half surrounded by steeply dipping and heavily faulted collar strata and then by a rim synclinorium of less deformed strata the gains made in research on all three sectors are noted traverses through the basement from collar to centre effectively penetrate through a 15 km thick section of crust the archaean history of the crustal section has been documented the thermal and fluid inclusion evolution near the time of uplift has been studied and a deep seated co 2 rich gas explosion has been proposed as an alternative to the impact hypothesis for the shock deformation meticulous study of the effects of thermal metamorphism on the shock deformations in the collar strata led to a bold proposal more than one shock event in the evolution of vredefort analysis of the character of the faulting of the collar supports an earlier proposal regarding inward movement of blocks of collar strata renewed research on shatter cone orientations in the collar strata and two detailed studies of the structural history within the rim synclinorium are noted through the intensive mapping geophysical exploration and drilling which have been completed within the witwatersrand and transvaal basins close spatial and temporal connections between vredefort and the bushveld complex have been documented in both cases basin centres were intruded by horizontal mafic eruptives and basin subsidence took place after these eruptives had consolidated and cooled the vredefort structure has long been discussed as a possible deeply eroded remnant of a major cratering event or events the connections of this structure with the bushveld complex carry important implications for such discussion

prestack and poststack migration of crooked line seismic reflection data a case study from the south portuguese zone fold belt southwestern iberia crooked line 2d seismic reflection survey geometries violate underlying assumptions of 2d imaging routines affecting our ability to resolve the subsurface reliably we compare three crooked line

shallow subsurface structure of the wasatch fault provo segment utah from integrated compressional and shear wave seismic reflection profiles with implications for fault structure and development integrated vibroseis compressional and experimental hammer source shear wave seismic reflection profiles across the provo segment of the wasatch fault zone in utah reveal near surface and shallow bedrock structures caused by geologically recent deformation combining information from the seismic surveys geologic mapping terrain analysis and previous seismic first arrival modeling provides a well constrained cross section of the upper 500 m of the subsurface faults are mapped from the surface through shallow poorly consolidated deltaic sediments and cutting through a rigid bedrock surface the new seismic data are used to test hypotheses on changing fault orientation with depth the number of subsidiary faults within the fault zone and the width of the fault zone and the utility of integrating separate elastic methods to provide information on a complex structural zone although previous surface mapping has indicated only a few faults the seismic section shows a wider and more complex deformation zone with both synthetic and antithetic normal faults our study demonstrates the usefulness of a combined shallow and deeper penetrating geophysical survey integrated with detailed geologic mapping to constrain subsurface fault structure due to the complexity of the fault zone accurate seismic velocity information is essential and was obtained from a first break tomography model the new constraints on fault geometry can be used to refine estimates of vertical versus lateral tectonic movements and to improve seismic hazard assessment along the wasatch fault through an urban area we suggest that earthquake hazard assessments made without seismic reflection imaging may be biased by the previous mapping of too few faults

the issues of prestack reverse time migration and solutions with graphic processing unit implementation prestack reverse time migration rtm is a very useful tool for seismic imaging but has mainly three bottlenecks highly intensive computation cost low frequency band imaging noise and massive memory demand traditionally pc clusters with thousands of computation nodes are used to perform rtm but it is too expensive for small companies and oilfields in this article we use graphic processing unit gpu architecture which is cheaper and faster to implement rtm and we obtain an order of magnitude higher speedup ratio to solve the problem of intensive computation cost aiming at the massive memory demand we adopt the pseudo random boundary condition that sacrifices the computation cost but reduces the memory demand for rugged topography rtm it is difficult to deal with the rugged free boundary condition with the finite difference method we employ a simplified boundary condition that avoids the abundant logical judgment to make the gpu implementation possible and does not induce any sacrifice on efficiency besides we have also done some tests on multi gpu implementation for wide azimuth geometries using the latest gpu cards and drivers finally we discuss the challenges of anisotropy rtm and gpu solutions all the jobs stated above are based on gpu and the synthetic data examples will show the efficiency of the algorithm and solutions

where is the zero velocity layer the zero velocity layer was introduced in higginbotham et al 1985 to increase the maximum dip imaging capability of finite difference depth migration beasley and lynn 1992 adapted the idea to improve the imaging again using finite difference depth migration of seismic data acquired in areas of irregular topography beasley and lynn s application improves upon the conventional method of processing which is to time shift the data from the acquisition surface to a horizontal datum and then migrate using the near surface velocity above the surface and the best estimate of seismic velocity below the surface the conventional procedure typically produces artifacts in the shallow part of the section that are characteristic of overmigration to reduce these artifacts velocities are often reduced for the migration step the use of the zero velocity layer overcomes the need to adjust the migration velocities here a component of the migration velocity is set to zero in the layer between the datum and th

common shot fresnel beam migration based on wave field approximation in effective vicinity under complex topographic conditions gaussian beam migration is a versatile imaging method for geologically complex land areas which overcomes the limitation of kirchhoff migration in imaging multiple arrivals and has no steep dip limits of one way wave equation migration however its imaging accuracy depends on the geometry of gaussian beam that is determined by the initial parameter of dynamic ray tracing as a result its applications in exploration areaswith strong variations in topography and near surface velocity are limited combined with the concept of fresnel zone and the theory of wave field approximation in effective vicinity we present a more robust common shot fresnel beam imaging method for complex topographic land areas in this paper compared with the conventional gaussian beam migration for irregular topography our method improves the beam geometry by limiting its effective half width with fresnel zone radius moreover through a quadratic travel time correction and an amplitude correction that is based on the wave field approximation in effective vicinity it gives an accurate method for plane wave decomposition at complex topography which produces good imaging results in both shallow and deep zones trials of two typical models and its application in field data demonstrated the validity and robustness of our method

geological constraints on the paleozoic tectonic evolution of tectonostratigraphic terranes in the iberian massif abstract the presence of oceanic paleozoic sequences in the iberian variscan fold belt is used to identify and separate suspected exotic terranes from a reference iberian autochthon terrane which constitutes the greater part of it in this paper the relevant data characterizing the pre accretionary evolution of the paleozoic iberian terranes are presented as well as the available evidence concerning the accretionary process itself emphasis is placed on the analysis of the sedimentary igneous and structural expression of the major events which affected the iberian terranes before during and after their amalgamation the main conclusions of such an analysis may be summarized as follows 1 a recognition of rift and passive margin stages in all the iberian massif terranes during the early paleozoic 2 b late paleozoic accretion of the exotic terranes to the iberian autochthon via a long and complex process variscan orogeny which involved initial subduction underneath the latter and eventual obduction followed by oblique collision of a promontory the iberian autochthon northwestern margin with a northwestern continental block this latter process could account for the large scale crustal imbrication shown by the northern branch of the orogen coeval with predominant strike slip faulting in the southern branch as well as with the formation of the typical arcuate shape of this orogenic belt major present uncertainties relate to the palinspastic provenance of continental suspect terranes and their correlation with similar units in other circum atlantic paleozoic orogens a discussion and speculations on this subject are dealt with in the final section of this paper

geochemistry across an exposed section of archaean crust at vredefort south africa with implications for mid crustal discontinuities abstract the central region of the vredefort structure consists of a semi circular multi layered sequence of crystalline rocks which are nearly vertical in attitude and which increase in metamorphic grade towards the core of the structure together with the overlying precambrian strata this sequence provides a cross section through almost the entire crustal section of the kaapvaal craton 36 km the upper part of the vredefort crystalline crust consists of a 3 0 ga sequence of differentiated felsic rocks in amphibolite facies the lower crust consists of a complex and heterogeneous both chemically and isotopically high grade metamorphic terrain of charnockites granulites mafic and felsic and supracrustal rocks the upper crust is separated from the lower crust by the vredefort discontinuity a brittle ductile shear zone characterised by a high concentration of pseudotachylite and brecciated rock petrographic chemical and isotopic evidence suggest that the upper and lower crust have undergone very different styles of evolution this is indicative of different geological environments prior to their present juxtaposition we speculate that the upper and the lower parts of the vredefort crystalline crust were juxtaposed during intracratonic thrusting 2 8 ga ago and that the entire 36 km section of crust including the vredefort discontinuity were rotated and uplifted into their present vertical position during the 2 0 ga vredefort catastrophe

electromagnetic imaging of variscan crustal structures in sw iberia the role of interconnected graphite abstract the western part of the iberian peninsula iberian massif is the best exposed fragment of the variscan orogen in europe its southern half was generated by an oblique collision between three continental terranes belonging to the margins of laurassia avalonia the south portuguese zone spz and gondwana the ossa morena zone omz and the central iberian zone ciz the boundaries between them are considered to be suture zones a 200 km long magnetotelluric profile across the three variscan terranes was done in a nne direction approximately perpendicular to the main tectonic features the results of two dimensional inversion of the mt dataset reveal high conductivity zones coinciding with the transitions spz omz and omz ciz these conductive bodies related to the sutures at depth were interpreted as graphite enrichments along shear planes formed due to the overall transpressive regime a high conductivity layer extending along the whole omz was found at a depth of 15 25 km the top of which spatially correlates with a broad reflector detected by a recently acquired deep seismic reflection profile the high conductivity was interpreted as caused by the precambrian serie negra graphite rich rocks carbon and oxygen x ray mapping with electron microprobe on polished sections of serie negra samples from omz revealed the presence of interconnected graphite which supports the hypothesis that graphite is determinant for the high conductivity two graphite types which help to record the geological evolution were identified graphite accumulations in the schistosity surfaces produced by folding and metamorphism and metallic films of graphite developed along late faults the conductive layer shows blobs of higher conductivity suggesting macro anisotropy additional mylonitisation and shearing produced by thrusting at depth can be the origin of these zones of enhanced conductivity given that the detachment level is located within the serie negra several high resistivity features were found in the upper crust related to devonian and carboniferous successions and probably to some unexposed plutons in the spz and the palaeozoic series of omz plus some granitic intrusions in the ciz a high resistivity zone extending to the whole crust is correlated with extensive late variscan granite intrusions

p and sv velocity structure of the south portuguese zone fold and thrust belt sw iberia from traveltime tomography imaging the architecture of the shallow crust of the south portuguese zone fold and thrust belt is essential to extend surface mapped geological information to depth and to help in developing model

geodynamic evolution of the european variscan fold belt palaeomagnetic and geological constraints the variscan fold belt of europe resulted from the collision of africa baltica laurentia and the intervening microplates in early paleozoic times over the past few years many geological palaeobiogeographic and palaeomagnetic studies have led to significant improvements in our understanding of this orogenic belt whereas it is now fairly well established that avalonia drifted from the northern margin of gondwana in early ordovician times and collided with baltica in the late ordovician early silurian the nature of the gondwana derived armorican microplate is more enigmatic geological and new palaeomagnetic data suggest armorica comprises an assemblage of terranes or microblocks palaeobiogeographic data indicate that these terranes had similar drift histories and the rheic ocean separating avalonia from the armorican terrane assemblage closed in late silurian early devonian times an early to mid devonian phase of extensional tectonics along this suture zone resulted in formation of the relatively narrow rhenohercynian basin which closed progressively between the late devonian and early carboniferous in this contribution we review the constraints provided by palaeomagnetic data compare these with geological and palaeobiogeographic evidence and present a sequence of palaeogeographic reconstructions for these circum atlantic plates and microplates from ordovician through to devonian times

a new interpretive procedure for whole rock u pb systems applied to the vredefort crustal profile granulite grade precambrian gneisses have usually undergone at least one period of strong u depletion whole rock u pb isotope studies can determine the time s of the severe depletion and this work attempts to place such studies on a more rigorous footing two stage u pb systems can be described in terms of one major episodic differentiation into rocks with varying u pb ratios while three stage systems can be described by two such distinct episodes most of the precambrian granulites that have been isotopically analyzed have histories too complex to be described as two stage systems however it is demonstrated here that u pb data on whole rock suites can yield the complete u pb chemical history of a three stage system in terms of u pb ratios for a suite of granulites present day 207pb 204pb and 206pb 204pb ratios and element concentration data allow these ratios to be calculated at a number of specific past times and plotted as an array the degree of scatter in each of these past arrays is graphed as a function of time the point of least scatter denotes the age of the end of stage 2 in the history of the system the array slope and the dating of the end of stage 2 also permit the beginning of stage 2 to be calculated all other parameters in the system u and pb concentrations pb isotopic ratios can now be determined for each individual rock throughout its history the new interpretive method also distinguishes sensitively among various kinds of uranium fractionation which may have operated during the differentiation episodes it is applied here to uranium depleted granulites in the deeper part of the vredefort crustal profile the times of the two fractionating episodes are calculated at 3860 and 2760 m y respectively the vredefort 3070 m y event when geochemical systems in the upper half of the crystalline basement became permanently closed evidently had little significance for the lower half of the crystalline basement some fundamental features of archaean basement evolution whose detailed significance is still poorly understood were elucidated for the first time by this u pb isotope study on whole rock samples rocks which suffered little uranium depletion at the end of stage 1 suffered least at the end of stage 2 rocks which were most affected at the end of stage 1 suffered the strongest depletion at the end of stage 2 evidently repeated high grade metamorphism purges certain granulites of their u content but leaves others relatively unscathed

imaging of offset vsp data with an elastic iterative migration scheme vsp data are usually acquired in order to obtain high resolution images of complex structures in reservoirs and near boreholes the authors present an elastic iterative migration scheme which has few limitations regarding the complexity of the geology and where the macromodel for both p and s wave velocities is automatically improved and updated at each iteration they avoid wavefield separation up down and p s and the simplifying assumptions of small dips underlying most such methods the migration scheme is based on elastic inversion theory the wavefield extrapolation is based on a high order coarse grid finite difference solution to the elastic two way wave equation at each iteration the macromodel is updated using a gradient method in which the gradient is computed by correlation of forward modelled fields with back propagated residual fields the first iteration of the migration scheme is equivalent to elastic reverse time migration with an imaging condition similar to claerbout s principle both p and s wave reflections contribute to the images in numerical examples with both synthetic and real offset vsp data they find that increasing the number of iterations improves the image quality images based on both p and s wave energy give more near well information and higher spatial resolution than images on only acoustic energy more in the real data example the authors show that the iterative migration scheme can image a relatively complex geological structure a fault and a small graben intersecting the well can be identified less

the structure of a major suture zone in the sw iberian massif the ossa morena central iberian contact abstract we have investigated the stratigraphy structure and metamorphism of the boundary between the ossa morena zone omz and the central iberian zone ciz two significant continental portions of the variscan iberian massif the omz ciz contact is marked by a strongly deformed and metamorphosed nw se trending narrow band namely the central unit in which partially retrogressed eclogites are included during the middle late devonian the ciz overthrust the omz and in the footwall km scale recumbent folds and thrusts developed with decoupling and underplating of the lower crust at the same time in the hanging wall there took place intense though localized back folding and back shearing in the early carboniferous a transtensional tectonic regime sank the overthrust block resulting in the exhumation of eclogites these eclogites probably came from the underthrust omz lower crust and they are at present included in the suture zone central unit of this continental collision the extension is responsible for the origin of a basin and bimodal magmatism on the southern border of the ciz a late episode of folding and fracturing significantly contributed to the final complex picture of this suture boundary

seismic reflection imaging over the south portuguese zone fold and thrust belt sw iberia 1 we reprocessed an 35 km long part of the ibesreis seismic reflection profile which runs over the iberian pyrite belt section of the south portuguese zone sw iberia with the goal to image the upper crust 15 km depth the applied processing sequence enhanced numerous prominent reflections and diffraction patterns within the uppermost 5 s travel time relative to high amplitude source generated noise a complex subsurface characterized by conflicting dips and a survey following winding roads require a crooked line prestack migration scheme for coherent imaging to interpret sources of diffracted energy we additionally employed a diffraction imaging scheme which enhances diffractions at the expense of reflections the final seismic images show south vergent imbricate fold and thrust tectonics documenting the contractive deformation that the south portuguese zone experienced during the variscan orogeny based on surface geological information we correlate a low reflective unit with the shallow upper carboniferous flysch group a highly reflective unit ranging in depth from 2 to 4 km with the middle carboniferous volcano sedimentary complex group which hosts massive sulfide deposits and a moderately reflective unit with the upper devonian phyllite quartzite group below these units another low reflective facies is present which may represent older paleozoic metasediments in addition the seismic and diffraction images reveal bands of high reflectivity and distinct diffraction patterns that were interpreted as extensive layered mafic intrusions these proposed mafic bodies may be related to the same event that triggered a huge hydrothermal activity assumed in early carbonifereous times

topographic migration of georadar data implications for acquisition and processing application of conventional elevation static corrections and migration to wavefield data recorded on irregular surfaces may result in poor reconstructions of complex subsurface features particulary poor images may be obtained at locations where the depths to target structures are comparable to undulations in the surface topography for example topographic relief of only 1 2 m may be important for the processing of georadar data we describe an algorithm that allows georadar data to be migrated directly from gently to highly irregular acquisition surfaces when applied to a variety of complicated synthetic data sets topographically migrated images are observed to be markedly superior to those produced by two standard processing schemes extensive tests demonstrate that topographic migration should be considered in regions characterized by surface gradients 10 i e dips 6 for effective topographic migration lateral and vertical coordinates of the georadar antennas should be determined to better

kirchhoff modeling inversion for reflectivity and subsurface illumination because of its computational efficiency prestack kirchhoff depth migration is currently one of the most popular algorithms used in 2 d and 3 d subsurface depth imaging nevertheless kirchhoff algorithms in their typical implementation produce less than ideal results in complex terranes where multipathing from the surface to a given image point may occur and beneath fast carbonates salt or volcanics through which ray theoretical energy cannot penetrate to illuminate underlying slower velocity sediments to evaluate the likely effectiveness of a proposed seismic acquisition program we could perform a forward modeling study but this can be expensive we show how kirchhoff modeling can be defined as the mathematical transpose of kirchhoff migration the resulting kirchhoff modeling algorithm has the same low computational cost as kirchhoff migration and unlike expensive full acoustic or elastic wave equation methods only models the events that kirchhoff migration can image kirchhoff modeling is also

rg waves from rockbursts in south africa abstract most of the recorded seismic activity in south africa is induced by mining operation seismograms of rockbursts show well developed rg waves with the fundamental mode propagating at 3 1 km s some records also display clear group velocity dispersion recognition of rg waves on the seismograms indicates that the event has a shallow focal depth 0 5 km and the large amplitudes and dispersion imply a heterogeneous crustal structure studies of the analytical attenuation of rg waves in south africa yield a specific quality factor q of 195 which is much lower than the value of 495 obtained using coda waves this discrepancy can be explained by the presence of a low velocity upper crustal layer and hence supports previous suggestions of a heterogeneous crust analysis of rg waves therefore gives information on local crustal structure and may also in some cases provide a discriminator between rockbursts and earthquakes

efficient migration through complex water bottom topography an efficient means of imaging structures beneath complex water bottom topography is obtained using a conventional time migration algorithm with a simple modification to the migration velocity field the process consists of two migration steps one with the migration velocity set to zero below the water bottom and the other with the migration velocity set to zero above the water bottom between the two steps the data are vertically time shifted to account for the lateral velocity variations between the water sediment interface the time shifts used are equivalent to the so called thin lens term used in depth migration algorithms efficiency is obtained by applying the thin lens term only once and by using computationally optimized time migration algorithms results obtained from this technique are nearly identical to more costly wave equation layer replacement and depth migration techniques

crooked line rough topography advancing towards the correct seismic image seismic exploration in mountainous areas imposes serious compromises on both acquisition and processing access restrictions usually result in profiles that are not straight and are not recorded along the true dip direction if there is a true dip direction processing constraints often result in very poor approximate corrections for elevations and for deviations from a straight line most fundamentally 2d acquisition and processing assumes that the earth is 2d this assumption is often seriously violated in mountainous areas while we cannot efficiently correct 2d seismic data for the effects of a fully 3d subsurface we can improve the data quality in thrust areas where the assumption of 2d subsurface variation is reasonable we do this in a series of small steps which improves the accuracy of several approximations made in processing 2d land data

multidisciplinary study of the hanging wall of the kiirunavaara iron ore deposit northern sweden potential weakness zones due to mining related fracture development under the town of kiruna sweden have been investigated by integration of seismic gravity and petrophysical data reflection s

mt and reflection seismics in northwestern skellefte ore district sweden a seismic reflection and mt survey was carried out along a 27 km long transect in northwestern skellefte district as part of a bigger 3d modeling project the main motivation for the data acquisit

velocity inversion by differential semblance optimization differential semblance optimization dso is an approach to inversion of reflection seismograms which avoids the severe convergence difficulties associated with nonlinear least squares inversion the method exploits both moveout and amplitude characteristics of reflections we have implemented a version appropriate to plane wave p tau seismograms and layered constant density acoustic earth models theoretical and numerical analyses of this version of dso indicate that stable and reasonably accurate estimates of both velocity trend and reflectivity can be derived to test dso further we applied it to a marine data set from the gulf of mexico where the method produced results which compare favorably to well log information the method can be extended to incorporate laterally heterogeneous velocity models

crooked line 2d seismic reflection imaging in crystalline terrains part 1 data processing for cost and access reasons most of the seismic reflection data collected in crystalline terrains have been acquired by 2d crooked line profiling when the survey geometry is significantly irregular and the geologic structures have cross profile dip several standard 2d imaging procedures severely underperform as a result reflection signal is poorly aligned across individual common midpoint cmp gathers and much is lost during the cmp stack to improve imaging either the methods used to align signal before stack need to be modified or more tolerant methods of combining trace signals than the standard cmp stack need to be applied because a high fold 2d crooked line profile is really a 3d survey of a swath of terrain around the processing line better signal alignment before cmp stacking may be achieved by revisiting the traveltime equation and including the cross dip terms into the moveout calculations therefore in addition to the correction of nmo and in line dip moveout dmo we also locally com

directional statics in common reflection surface stack for rugged surface topography seismic data acquired along rugged topographic surfaces present well known problems in seismic imaging in conventional seismic data processing datum statics are approximated by the surface consistence assumption which states that all seismic rays travel vertically in the top layer hence the datum static for each single trace is constant in case this assumption does not apply non constant statics are required the common reflection surface crs stack for rugged surface topography provides the capability to deal with this non vertical static issue it handles the surface elevation as a coordinate component and treats the elevation variation in the sense of directional datuming in this paper i apply the crs stack method to a synthetic data set that simulates the acquisition along an irregular surface topography after the crs stack by means of the wavefield attributes a simple algorithm for redatuming the crs stack section to an arbitrarily chosen planar surface is performed the redatumed section simulates a stack section whose acquisition surface is the chosen planar surface

least squares migration of incomplete reflection data a least squares migration algorithm is presented that reduces the migration artifacts i e recording footprint noise arising from incomplete data instead of migrating data with the adjoint of the forward modeling operator the normal equations are inverted by using a preconditioned linear conjugate gradient scheme that employs regularization the modeling operator is constructed from an asymptotic acoustic integral equation and its adjoint is the kirchhoff migration operator we tested the performance of the least squares migration on synthetic and field data in the cases of limited recording aperture coarse sampling and acquisition gaps in the data numerical results show that the least squares migrated sections are typically more focused than are the corresponding kirchhoff migrated sections and their reflectivity frequency distributions are closer to those of the true model frequency distribution regularization helps attenuate migration artifacts and provides a sharper better frequency distribution of estimated reflectivity the least squares migrated sections can be used to predict the missing data traces and interpolate and extrapolate them according to the governing modeling equations several field data examples are presented a ground penetrating radar data example demonstrates the suppression of the recording footprint noise due to a limited aperture a large gap and an undersampled receiver line in addition better fault resolution was achieved after applying least squares migration to a poststack marine data set and a reverse vertical seismic profiling example shows that the recording footprint noise due to a coarse receiver interval can be suppressed by least squares migration

pseudo 3d study using crooked line processing from the swiss alpine western profile line 2 val d anniviers valais abstract first results are presented of the processing of one of the seismic lines of the swiss national program to study the deep geological structure of the swiss alps located in the anniviers valley val d anniviers valais the rough topography of the study area gives rise to a crooked line geometry allowing analysis of lateral dips across the common mid point cmp swath corresponding to a pseudo 3d study the coherency of the seismic reflections has been improved by accounting for the lateral dips and applying a correction before performing the cmp stack the main e w dip found by the seismic pseudo 3d investigation confirms the surface geological survey preliminary geological interpretation suggests large scale recumbent folding at depths between 10 and 20 km

efficient amplitude encoding least squares reverse time migration using cosine basis least squares reverse time migration provides better imaging result than conventional reverse time migration by reducing the migration artefacts improving the resolution of the image and balancing the amplitudes of the reflectors however it is computationally intensive to reduce its computational cost we propose an efficient amplitude encoding least squares reverse time migration scheme in the time domain although the encoding scheme is effective in increasing the computational efficiency it also introduces the well known crosstalk noise in the gradient that degrades the quality of the imaging result we analyse the cause of the crosstalk noise using an encoding correlation matrix and then develop two numerical schemes to suppress the crosstalk noise during the inversion process we test the proposed method with synthetic and field data numerical examples show that the proposed scheme can provide better imaging result than reverse time migration and it also generates images comparable with those from common shot least squares reverse time migration but with less computational cost

structure and kinematics of a sinistral transpressive suture between the ossa morena and the south portuguese zones south iberian massif the ossa morena zone and the south portuguese zone represent the southernmost hercynian domains of the iberian massif lithostratigraphic petrographic and structural differences exist between both zones the boundary between the ossa morena and the south portuguese zones is marked by a narrow belt of amphibolites of tholeiitic affinity and related oceanic sequences the beja acebuches ophiolites the southernmost area of the ossa morena zone which bounds the acebuches amphibolites is composed of medium to high grade metamorphic rocks the ossa morena zone south portuguese zone suture preserves a complex story of middle to upper devonian to upper stephanian sinistral transpression the resulting structures were generated in a ductile to brittle regime transpressive shear zones affect the ophiolitic remnants and the high grade rocks of the southernmost area of the ossa morena zone the late hercynian faulting combines sinistral strike slip movements and some thrusting component which cut the suture to produce fish like megaboudins the transpressive regime can be explained by the oblique collision of the two plates now represented by the ossa morena zone and the south portuguese zone during the continental accretion on the scale of the hercynian belt this sinistral transpression belongs to a network of comparable structures explained by the impingement of the ibero armorican terranes