nd sr isotopic composition of granulites and constraints on the evolution of the lower continental crust in a statistical study of granulite facies rocks we have analysed 32 samples with ages ranging from 2 900 to 200 myr for sm nd rb and sr and 143nd 144nd and 87sr 86sr ratios the sm nd ratio is found to be constant compared with the highly variable rb sr ratio using the model age formalism we show that in many cases internal differentiation of continental crust follows external differentiation with a large time interval

deep crustal reflections from a vibroseis survey in northern switzerland abstract in 1982 a vibroseis survey comprising 180 km of reflection profiles was run in northern switzerland in order to investigate the suitability of the crystalline basement for the deposition of highly radioactive waste a configuration was chosen with 144 channels 25 m of geophone spacing 20 s sweeps ranging from 11 to 61 hz and stacking of 4 or 8 sweeps of 3 simultaneous vibrators at twice the geophone spacing the listening time was generally 4 s and at 4 sites it was extended to 11s for the detection of deeper crustal reflectors this survey unravelled the complicated fault and thrust system beneath the swiss folded jura mountains the stack from 4 s to 11 s reveals clearly a strong sloping reflector between 3 0 and 3 5 s which is strong evidence for a pronounced differentiation in the upper crust a series of reflections is observed between 5 8 and 7 2 s the top of which can be correlated with the conrad discontinuity a strong layered signal between 9 0 and 9 5 s is interpreted as reflections from the m discontinuity the main features are compatible with results from nearby refraction surveys in the southern rhinegraben rift system which show a distinct velocity increase of about 0 5 km s in the lower crust at a depth ranging from 15 to 20 km followed by an inversion zone or a laminated structure before reaching the moho at about 27 km depth the correlation of the field recordings with the first 10 s of the up sweep only shows some loss of resolution in the uppermost 3 s because of the lower frequency content of the signal however the lower parts of the sections are nearly identical the fact that the deeper reflectors in the sections can consistently be traced laterally is a strong argument for using this processing technique thus high coverage vibroseis surveys utilizing up sweep can be processed for deep crustal reflections even if the recording time is restricted to the standard 4 s provided the surface static corrections are carried out with high precision

contribution of deep seismic profiling to the knowledge of the lower crust in france and neighbouring areas abstract in france and neighbouring areas the crust is essentially formed of segments of the variscan orogen 380 300 ma and older formations cratonic basins began to form in the permian but extensive subsidence only occurred from the lias in basins such as the paris and aquitaine basins and also along the margin of the alpine tethys ocean the pyrenees and alps range eventually formed during the tertiary as a result of the collision between the european and african continental plates large grabens were also rifted in the rhine and rhone valleys deep seismic profiles show remarkable features common to the crust of the whole area although it underwent such a complicated geological history the most striking feature is the occurrence of conspicuous flat reflections densely distributed in the lower part of the crust below a comparatively transparent upper crust these reflections hereafter called laminations have been found to occur on most profiles within a large area covering western europe although they vary in depth thickness and energy dipping reflections have frequently been observed in the upper crust some of them extend across the lower crust and even penetrate into the upper mantle they may be reasonably interpreted as variscan compressional features by correlation with surface geology and comparison with similar features found in deep seismic profiles from other regions the origin of the flat laminations which generally lie in the lower part of the crust is more controversial this paper attempts to use the well known geological history of the region to place some constraints on the hypotheses which may be advanced to explain the formation of these laminations these features may be explained by the following tectonothermal events 1 1 compression which prevailed during the variscan orogeny may have formed the laminations as a result of delamination and shearing near the bottom of the crust associated with melting intrusion and metamorphism during the different stages of the crustal evolution 2 2 the thermal equilibration of the upper mantle which occurred in the permian at the end of the variscan orogeny may have caused metamorphism and magmatism in the overlying crust 3 3 a regional thermal event which might be associated with subordinate extension occurred during the rifting of the alpine tethys and the initiation of mesozoic cratonic basins in late triassic middle jurassic times it may also have caused metamorphism and magmatism in the lower crust while the first model implies conservation of the crust material the other two suggest that the crust mantle transition has been somewhat uplifted since the end of the variscan orogeny the latter hypothesis is one way to explain the undeniable occurrence of seismic events below the present moho reflection the above three models are not mutually exclusive and all these phenomena may well have affected the crust to some extent however the data at hand are still insufficient to determine what parts tectonic and nontectonic processes have played in the creation of the laminations in the lower crust and the moho further experiments are needed to check these hypotheses

u pb zircon and monazite dating of a mafic ultramafic complex and its country rocks u pb data on zircons from the largest mafic ultramafic body 6 2 km of the french central massif sauviat sur vige yield the following age results primary magmatic crystallization of the gabbroic and peridotitic protoliths took place in the cambro ordovician 496 25 17 m y variable transformation under eclogite facies conditions was hercynian 320 29 36 m y the same age pattern derived by u pb monazite analyses was found also for the immediate country rocks i e kyanite bearing coarse grained metagranites occurring to the w and n of the sauviat massif due to the fact that there is no regional hercynian high grade metamorphism in this part of the french central massif e g duthou 1977 bernard griffiths 1975 both mafic ultramafic complex as well as immediate felsic country rocks must have been emplaced tectonically into pre hercynian acadian caledonian crustal rocks the cause for such a hercynian tectonism is thought to be due to continent continent collision of the spanish with the armorican plate

precambrian u pb zircon ages in eclogites and garnet pyroxenites from south brittany france an old oceanic crust in the west european hercynian belt abstract u pb zircon ages have been determined for two eclogites from the vendee and for two garnet pyroxenites from the baie d audierne in an episodic pb loss model the two discordia would give upper intercept ages around 1300 1250 ma and lower intercepts ages of 436 384 ma two interpretations are proposed 1 the 1250 1300 ma ages may reflect an unspecified upper mantle event or process the paleozoic ages correspond to the tectonic emplacement of an eclogitic mantle fragment into the continental crust 2 the protolith may have been extracted from the upper mantle 1250 1300 ma ago and stored in a crustal environment until it was metamorphosed under high pressure conditions around 400 ma ago this latter model is favoured by available geologic and isotopic data consequently we propose that a 1300 ma old oceanic crust was tectonicly incorporated into a sialic basement during the proterozoic this mixture was subsequently subducted during the paleozoic

the continental margin of the mesozoic tethys in the western alps abstract most of the structural units of the western alps were derived from the european continental margin of the ligurian ocean a segment of the mesozoic tethys ocean their mesozoic palaeotectonic and stratigraphic evolution bears witness of the following main stages 1 deposition of the triassic platform carbonates essentially prerift but nevertheless bearing the imprint of some extensional movements 2 the liassic middle jurassic rifting corresponds to the creation of a horst and graben system with especially tilted blocks and hence a shoal and basin palaeogeography but without a preliminary doming event 3 the late jurassic early cretaceous opening and spreading of the ligurian ocean began with a general collapse of the continental margin thermal subsidence latest middle jurassic and earliest late jurassic in this paper emphasis is given to the refting derived structures more especially to the major tilted crustal blocks a few tens of kilometres wide which can be either reconstructed or directly observed the rifting stage lasted roughly 40 m y with the alternation of extensional tectonic phases and of relatively quiet periods during the extensional phases movements along fault planes and related episodes of tilting were followed by sudden and rapid episodes of subsidence the latter may be interpreted as resulting from the isostatic and thermal readjustment that follows a phase of stretching of the crust and of the lithosphere the tectonic evolution of the margin continued but decreased during the late jurassic early cretaceous oceanspreading stage tectonic activity resumed in the late cretaceous this probably resulting from the beginning of contraction of both the ocean and the continental margin leading progressively to the continental collision in the tertiary

major geodynamic processes studied from the ecors deep seismic profiles in france and adjacent areas abstract for more than five years the french ecors project has obtained deep seismic profiles across major structural features that are related to the variscan alpine and pyrenean belts as well as the permo mesozoic subsidence in the celtic sea the english channel and the bay of biscay profiles across the orogenic belts show striking similarities such as the flexure of the down going plate and the geometry of the frontal detachment and related thrusts and nappes whatever the age of the belt however straight dipping reflections cutting through large sections of the crust seem to be common in the variscan belt and are interpreted as late features in its tectonic history the tertiary alpine and pyrenean belts show extensive crustal thickening and crust mantle imbrication while the layered lower crust and its bottom are flat beneath the late palaeozoic variscan belt this is interpreted as a late equilibration of the crust mantle boundary which involved deep metamorphism in the crust and occurred between the permian and the middle jurassic the permo jurassic basins reveal an unattenuated lower crust while both the upper and lower crusts are thinned beneath the cretaceous tertiary ones observation does not support the current models of basin formation and nowhere does extension appear to have played a major role in subsidence the lower crust of the permo jurassic basins may have been reworked and equilibrated after their rifting

early palaeozoic palaeobiogeography of the variscan regions abstract the palaeobiogeographic method appears to be an attractive alternative for reconstruction of the palaeogeography of the variscan regions during the palaeozoic the evolution of faunal assemblages and climatic characteristics from the cambrian up to the end of the devonian demonstrates that baltica and north gondwana represent two major and independent palaeogeographic entities separated by a mid european rheic ocean which opened as early as the cambrian and closed progressively from the devonian the south armorican ocean whose closure generated the so called ligerian orogen is regarded as a branch of the rheic ocean both a tornquist s sea separating baltica from its western extension avalonia during the ordovician and a proto tethys ocean extending between southwestern europe and north africa during the devonian are excluded from our reconstructions

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

crustal evolution of the rhinegraben area 1 exploring the lower crust in the rhinegraben rift by unified geophysical experiments abstract unified geophysical investigations of the lithosphere in the rhinegraben rift system have revealed new details of the lower crust and its role in the rifting process the new findings allow an assessment of the compatibility of four different geophysical notions and properties of the lower crust 1 1 the lower crust as the layer beneath the conrad discontinuity with a p wave velocity of about 6 5 km s or greater refraction seismics 2 2 the laminated band of reflections as seen in the near vertical reflection seismic experiments in many parts of the continents 3 3 the ductile part of the crust below the brittle ductile transition devoid of earthquakes in seismically active regions 4 4 the electrical conductivity of the lower crust indicative of dry or wet conditions or still unknown conduction phenomena in the rhinegraben area the lamination of the lower crust serves as an outstanding marker of deep tectonic activity during the rifting process in which the crust of the rhinegraben rift system has been subjected to three different natural dynamic processes 1 uplift by 2 to 3 km with subsequent erosion of the rhinegraben shoulders black forest and vosges mountains caused decompression possibly leading to the formation of a low velocity high electrical conductivity zone right on top of the laminated lower crust beneath the elevated shoulders of the black forest 2 the brittle crystalline wedge of the graben proper subsided nearly undeformed into the lower crust which became about 5 to 7 km thinner below the graben than below the shoulders 3 the deepest hypocentres in the black forest dinkelberg area if projected onto the neighbouring reflection profile would be located 7 to 8 km within the laminated lower crust beneath the southern black forest indicating a discrepancy between the top of the lower crust as defined by the brittle ductile transition as seen by the deepest earthquakes and by the top of the laminated reflection band the rhinegraben rift system reveals the properties and behaviour of the lower crust under a wide variety of tectonic situations

p wave velocity features of the lithosphere under the eromanga basin eastern australia including a prominent mid crustal conrad discontinuity abstract the crustal structure of the central eromanga basin in the northern part of the australian tasman geosyncline revealed by coincident seismic reflection and refraction shooting contrasts with some neighbouring regions of the continent the depth to the crust mantle boundary moho of 36 41 km is much less than that under the north australian craton to the northwest 50 55 km and the lachlan fold belt to the southeast 43 51 km but is similar to that under the drummond and bowen basins to the east the seismic velocity boundaries within the crust are sharp compared with the transitional nature of the boundaries under the north australian and lachlan provinces in particular there is a sharp velocity increase at mid crustal depths 21 24 km which has not been observed with such clarity elsewhere in australia the conrad discontinuity in the lower crust the many discontinuous sub horizontal reflections are in marked contrast to lack of reflecting horizons in the upper crust further emphasising the differences between the upper and lower crust the crust mantle boundary moho is characterised by an increase in velocity from 7 1 7 7 km s to a value of 8 15 0 04 km s the depth to the moho under the canaway ridge a prominent basement high is shallower by about 5 km than the regional moho depth there is also no mid crustal horizon under the canaway ridge but there is a very sharp velocity increase at the moho depth of 34 km the ridge could be interpreted as a horst structure extending to at least moho depths but it could also have a different intra crustal structure from the surrounding area the sub crustal lithosphere has features which have been interpreted from limited data as being caused by a velocity gradient at 56 57 km depth with a low velocity zone above it because of the contrasting crustal thicknesses and velocity gradients the lithosphere of the central eromanga basin cannot be considered as an extension of the exposed lachlan fold belt or the north australian craton the lack of seismic reflections from the upper crust indicates no coherent accoustic impedance pattern at wavelengths greater than 100 m consistent with an upper crustal basement of tightly folded meta sedimentary and meta volcanic rocks the crustal structure is consistent with a pericratonic or arc back arc basin being cratonised in an episode of convergent tectonics in the early palaeozoic the seismic reflections from the lower crust indicate that it could have developed in a different tectonic environment

crustal profile of mountain belt cocorp deep seismic reflection profiling in new england appalachians and implications for architecture of convergent mountain chains relatively shallow reflections from beneath the taconic allochthon and green mountain anticlinorium are suggested to mark a zone of detachment that carries precambrian grenville basement of the green mountains in its upper plate the data suggest that tectonic thickening of the shelf sedimentary and synorogenic flysch sequence beneath the taconic allochthon has occurred and that major thrust fault systems champlain thrust of western vermont which imbricate shelf lithologies project downplunge into the seismic profile eastward dipping reflections beneath the east flank of the green mountains appear to project across the connecticut river valley into new hampshire because some of these reflections can be projected into an exposed zone of probable thrust imbricated precambrian grenville basement and younger metasediments there is a strong implication that similar rock types exist in the deeper crust beneath new hampshire the gross geometry of the eastward dipping zone is a large thrust ramp that facilitated faulting of continental basement in the green mountains to higher structural levels thus suggesting the anticlinorium is a large hanging wall structure or ramp anticlinorium highly deformed satellitic massifs of grenville basement such as the chester dome and sub silurian and silurian devonian rocks of the bronson hill anticlinorium and merrimack synclinorium are probably transported along intracrustal detachments as well data suggest the possiblity of a tectonically buried continental margin transition zone much like that interpreted for the southern appalachians this zone would have been preserved from prior rifting of the proto atlantic ocean and its inherited geometry could have controlled subsequent convergent deformation during appalachian orogenesis although regional deep seismic reflection profiling has not been carried out across the alps existing data imply lateral changes in crustal reflection character similar to those observed in the appalachians deep seismic reflection data from convergent mountain belts raises the possibility of such preserved transition zones beneath transported rocks and provides an alternative interpretation to lithosphere penetrating sutures in explaining lateral geologic and geophysical contrasts in the crust modified journal abstract

crustal laminations in deep seismic profiles in france and neighbouring areas summary remarkable crustal features appear on the ecors profiles carried out in northern france and the bay of biscay as well as on the swat profiles shot in the western channel and the celtic sea the most striking one is the occurrence of flat laminations in the lower crust dipping events and laminations are also present in the upper and lower crust especially in the swat profiles they can readily be related to tectonic events variscan in age some of them identified in the field the flat laminations in the lower crust are at first interpreted as resulting from delamination shearing magmatism and metamorphism at the crust mantle transition during the variscan orogeny this interpretation raises some difficulty concerning the space and time correlation of the laminations with the variscan orogeny they seem to have been emplaced after the permian triassic infilling of the plymouth bay basin and before the early cretaceous opening of the bay of biscay an early to middle jurassic age is suggested a period when large cratonic basins were formed without noticeable extension heat flow increase and magmatism are proposed as a second hypothesis for the formation of the lower crust laminations choosing between orogenic and non orogenic causes of these laminations will require further deep seismic profiles together with good velocity determination

cocorp and the continental crust this is a speculative paper on the continental crust a major frontier of modern earth science seismic reflection profiling data from the cocorp project coupled with other kinds of geophysical and geological observations suggest some fresh perspectives on the crust for example there is evidence for great differences within the basement at a single site and from one cocorp site to another a simple layered model is surely not an adequate representation of the crust in most areas there is however the puzzling and rather common occurrence of limited zones of parallel though not always flat layered features at various depths in certain parts of the crust are the corresponding rocks metasedimentary or igneous or both there are transparent zones from which little or no coherent reflected seismic energy is detected are the corresponding rocks intrusions or highly deformed metasediments can degree of deformation suggest whether rocks were deformed in the hanging wall or footwall of a thrust there is evidence for large scale thrusting and related features that emphasizes the great importance of the wilson cycle to continental tectonics and that raises the possibility that water and rocks of sedimentary origin are trapped deep in the crust by the thrusting process and remain there for long periods with a variety of possible rheological and petrological consequences in some places the modern moho and adjacent rocks may correspond to parts of former oceanic or continental margin sections that have been emplaced at depth as a result of thrusting the widely held concept of the moho as a consistent boundary that is everywhere similar and that always separates the same kinds of rocks may be a major barrier to further progress in understanding the continental crust

deep seismic reflection survey across the variscan front of southern england in 1979 the institute of geological sciences commissioned two lines of seismic reflection profile a technique which has proved successful1 for studying crystalline basement the survey was sited to cross the concealed variscan front in southern england with recording time extended to 12 s two way time twt similar profiles in adjacent parts of europe have suggested that the variscan front is characterized by major thrust zones extending to considerable depths2 3 our profiles reported here show good primary reflections to 6 s twt and give new detail on the nature of variscan deformation in southern england zones of differing reflection character are evident a northern zone of low northerly apparent dips a central zone of poor reflector quality and a southern zone of steeper dips to the sse the southern zone is presumed to represent deformed variscan basement including several minor thrust planes and a proposed major thrust zone analogous to similar structures in europe impersistent reflections between 9 and 10 s twt could possibly relate to the moho

crustal structure of the appalachian highlands in tennessee abstract crustal structure of the southern appalachians and adjacent interior low plateaus in tennessee is derived from seismic refraction measurements observed by the u s geological survey in 1965 along reversed lines normal nw se and parallel ne sw to the structure of the appalachian highlands major geologic divisions its easternmost part is located approximately 80 km southwest of the westernmost part of the cocorp seismic reflection traverse within the blue ridge province the velocity depth models derived for both observational directions consist of three crustal layers with surprisingly high velocities being about 6 1 6 2 km s in the upper crust down to 7 10 km depth 6 7 6 8 km s for the middle crust between about 17 and 34 km and varying from 7 1 to 7 4 km s for the lower crust at about 40 47 km depth the boundaries between the three crustal layers as well as the crust mantle boundary are transition zones of up to 11 km thickness similar to old orogens in other parts of the earth the main result is a thick crust at places in excess of 50 km with high average velocity and a broad crust mantle transition zone

seismic crustal structure of the proterozoic north australian craton between tennant creek and mount isa seismic recording along a 600 km traverse across the southern part of the north australian craton indicates that lateral velocity variations exist in the middle lower crust taken together with other data from the craton there is evidence for midcrustal velocities increasing from east to west and from south to north these trends could result from a greater proportion of high velocity granulites being emplaced at higher levels in the crust between tennant creek and mount isa the velocity structures near the surface are compatible with an assemblage of younger and older metamorphic rocks intruded by granites near surface 0 5 km p wave velocities on the tennant creek inlier increase from 5 47 km s at the surface to about 6 2 km s at 6 km depth and correspond with low grade metamorphics of the warramunga group interspersed with weathered granites overlying amphibolite facies rocks of older metamorphic domains in the mount isa geosyncline velocities of 6 03 6 15 km s correspond to the leichhart metamorphics interspersed with granites in the middle crust p wave velocities of 6 85 km s occur at a depth of 26 km near tennant creek whereas such velocities are not evident until depths of about 37 km near mount isa there is therefore a trend for midcrust velocities to increase from east to west the lower crust is characterized by velocities of 7 3 7 5 km s and the upper mantle velocities of 8 16 8 20 km s are reached at depths of 51 54 km exposed granulite facies rocks in central australia are low in heat producing elements and are compatible with the low surface heat flow generally measured in precambrian australia these data taken together with laboratory velocity measurements on worldwide samples reported elsewhere indicate that middle to lower crustal rocks could be an assemblage of granulite facies rocks varying from pyroxene granulite in the middle crust through hornblende granulite to garnet granulite in the lower crust

thin skinned tectonics in the northern rhenish massif germany a network of profiles in the hercynian mountain system geo traverse rhenoherzynikum is needed to determine details of the geological structure and find the reasons for tectonic movements outstanding features within the mountain system such as the large fault zone along the southern boundary of the hunsruck1 and the left flank of the rhinegraben have already been investigated by reflection seismology2 3 we describe here a new reflection refraction experiment carried out in 1978 in the most northern part of the rhenish massif across the stavelot venn massif near aachen fig 1 while layers in the lower crust dip to the nnw a strong upper reflector at 3 4 km depth shows a slight dip to the sse and indicates a prominent fault zone near aachen known in belgium as the faille du midi here a comparable and apparently identical reflector was found some 100 and 175 km to the west of our profile we interpreted this reflector as a prominent and well lubricated thrust fault along which a huge horizontal nappe displacement took place during the last stages of the variscan orogeny

the large scale structure of the earth s crust beneath southern britain recently acquired deep seismic reflection profiles recorded to 12 s two way travel time in southern britain suggest that the crust beneath this area is commonly divisible vertically into three zones these zones based upon seismic character can be related to structural or tectonic units they comprise an upper zone of cover rocks of various ages a middle zone of folded and thrust rocks and a lower zone tentatively identified as metamorphic or crystalline basement a sketch crustal profile from dorset to yorkshire summarizes the data and illustrates the distribution of the zones

datation u pb sur zircons de l eclogite de la borie haut allier france et consequences sur l evolution ante hercynienne de l europe occidentale a crystallization age of 432 10 20 ma has been determined for the la borie eclogite haut allier france by u pb dating on zircons this age is yielded by the upper intercept with the concordia curve in agreement with recent dates obtained in the southern part of the massif central in brittany and in vendee these data indicate that the high pressure high temperature metamorphic phase representative of the pre variscan evolution of the western europe took place at the silurian ordovician boundary this high pressure high temperature metamorphism could be significant of subduction processes more or less synchronous preceding the main continental collision responsible of the variscan belt s s

the seismic reflection character of the continental mohorovicic discontinuity the continental mohorovicic discontinuity is most often interpreted as a step function velocity boundary however on deep crustal seismic reflection profiles reflections at depths where refraction data places the moho have laminated character and are laterally discontinuous these observations on continental reflection data point to a model of the moho that has thin layered rather than block structure previous workers who have synthetically generated seismic responses from various crust mantle boundary models have shown that thin layered models of alternating high and low velocity generate responses that best emulate observed moho arrivals geologic interpretations of the thin layering include relatively undeformed metasediments cumulate layering tectonic banding and lenses of partial melt to obtain more direct geologic evidence a synthetic seismogram is generated from geologic cross sections of the ivrea verbano zone an exposed section through the lower crust and upper mantle the synthetic shows laminations similar to those observed on reflection data without more direct evidence no single geologic interpretation of the moho is reasonable indeed the moho may be a laterally variable boundary its composition and structure dependent upon the geologic history of the overlying crust

a deep seismic reflection profile across the northern north sea the viking graben is a major mesozoic rift basin in the northern north sea1 seismic refraction data2 gravity surveys3 and theoretical modelling4 have led to the general acceptance of an extensional tectonic origin for the viking graben involving an early mesozoic phase of extension and fault controlled subsidence followed by a more regional thermal contraction subsidence the early success of the birps deep seismic reflection profiling5 and the results from cocorp6 led to the planning and acquisition during 1983 of a deep seismic line recorded to 15 s twt two way time across the north viking graben the aims of the present study were to provide constraints on the geometry of the viking graben and its extensional faults to provide data on the geometry of crustal thinning beneath the graben and to provide an insight into the relation between the shallow extensional features of the sedimentary basin and the deep crustal thinning here we present a specific interpretation of this seismic profile further details will be given elsewhere7

deep crustal features of the celtic sea from complementary processing on the swat data complementary processing develop by bano 1989 including automatic extraction of reflections and migration of deep seismic data are applied to the swat south west approaches traverse deep seismic profiles in the celtic sea the automatic extraction of reflection gives a more reliable definition of the basement under the deep celtic sea basins using migration we derive a more accurate geometry of these basins

seismic reflection coefficients from mantle fault zones summary several bright reflections from structures within the mantle can be seen on birps deep seismic reflection profiles we have calculated apparent reflection coefficients for the brightest of these events and obtain values around 0 1 it is not possible to produce such large reflections by either compositional layering or seismic anisotropy if olivine and pyroxene are the only significant minerals in the mantle these large reflections can be produced by a mafic layer or a partially hydrated layer within normal peridotite the brightest reflections seem to be best explained as major faults or shear zones within the mantle

crustal structure of the celtic sea and western approaches from gravity data and deep seismic profiles constraints on the formation of continental basins a free air anomaly map of the celtic sea and the western approaches is presented including all available marine gravimetric data and bouguer anomalies on land n60 features are predominant in the area they correspond to the n60 so called caledonian grain which played a major role in the variscan orogeny and in the formation of the permo triassic sedimentary basins the fundamental new point which comes out from this gravimetric map is the existence of a closely spaced pattern of n105 features which we regard as late carboniferous transpressional structures in the eastern pan of the celtic sea thrusts are in the same n105 direction west of 6 w longitude however folds and thrusts trend preferentially in a n60 direction following the older grain there the n105 lineaments may be regarded as dextral shear structures with a possible compressive component the hercynian shortening of the celtic sea domain located between the variscan front and the lizard suture resulted from the displacement along the n160 direction of the two adjacent rigid domains north of the variscan front the n105 and n135 features were possibly active in more recent times then both the n60 and n105 trends controlled the formation of permo triassic basins the n105 directions being transfer faults two dimensional gravity modelling of the swat profiles in the celtic sea shows that the base of the reflective lower crust may be associated with the moho discontinuity everywhere in the celtic sea except below the saint george s channel basin the lower crust does not change significantly in depth and thickness beneath the permo triassic basins none of the available models simply satisfies such conditions they all require at least a modification of the geometry of the lower crust after the formation of the celtic sea basins nevertheless if the simple shear model can be ruled out mainly from structural arguments the uniform stretching and to a lesser degree the thermomechanical models seem to be more relevant models however two additional conditions are necessary 1 the preservation of crustal seismic features formed before the creation of basins must be explained and 2 a significant portion of the volume of the stretched lower crust about 40 of this volume for the north celtic sea basin must be added below or perhaps partly above the lower crust by lateral flow of the crust or at the expense of the upper mantle

some unsolved birps problems summary birps has 8000 km of records around britain that are relevant to the rheology of continents and to faulting of the lithosphere problems include the need to measure seismic velocities with better resolution and to eliminate multiples in deep water the question are blank bits of records due to noise increase or to geology and the need to know the physics of the origin of deep reflections in order to answer geologists questions

deep seismic reflection and refraction profiling along the aquitaine shelf bay of biscay summary the 300 km ecors bay of biscay profile was carried out along the aquitaine shelf and comprised a complete set of experiments including zero offset and 7 5 km constant offset vertical seismic reflection and six expanding spread profiles large offset recordings were fundamental for the definition of the layered lower crust and the moho while esps provided decisive complementary information for the geological interpretation these data show a strong variation in crustal thickness from about 20 km beneath the rifted parentis basin a failed arm of the oceanic bay of biscay up to 35 km to the north below the armorican shelf in the hercynian domain and to the south below the cantabria shelf in the vicinity of the pyrenean deformation front the results have important implications for the behaviour of the crust during the formation of rifted sedimentary basins and during continental collision

reflectivity of the crystalline crust hypotheses and tests summary the nature of reflectors within the crystalline basement remains the subject of inference except where reflections have been traced directly to outcrop geological models of basement reflectors need to be developed which incorporate geophysical constraints obtained from measurements on seismograms but most geological information still comes from speculative interpretations of seismic experiments run in different regimes pronounced lower crustal reflectivity detected worldwide is ascribed in various geological hypotheses to primary lithologic layering to ductile strain banding or to trapped fluids a birps deep crustal profile across the atlantic continental margin suggests that the observed reflectivity is not related in any simple way to the amount of extensional strain undergone study of worldwide crustal profiles shows that exposed high pressure terranes are not as reflective as in situ lower crust at high pressure suggesting either that these granulite terranes are not representative of the lower crust or that physical properties possibly the presence of fluids or thermally controlled ductile strain banding are more likely responsible for observed reflectivity than are simple lithologic boundaries the argument for the importance of physical properties in causing observed lower crustal reflectivity is strengthened by an observed negative correlation between depth to the reflective lower crust and regional surface heat flow 1 geophysical constraints the notion that geometric effects thin layer interference and focussing in the absence of otherwise significant impedance contrasts are responsible for much observed reflectivity is common in the literature because many reflections are observed on deep crustal profiles velocity and density cannot increase monotonically with depth while remaining within bounds set by refraction experiments implying the existence of a funtion of velocity with depth that is oscillatory while the concept thus developed of the existence of many thin layers in the lower crust is justly important in geologic models of the lower crust seismic interference from such laminae cannot be the only explanation of observed crustal reflectivity for normal incidence no structure embedded in a constant impedance medium gives a stronger reflection than a quarter wavelength layer which produces an amplitude twice that due to a simple interface complex structures can increase the total energy reflected but the energy is spread over longer times and the peak reflected amplitude is not increased observed reflection amplitudes from the lower crust vary by factors u2 so that the presence or absence of thin layer interference cannot be the only effect causing lower crustal

structure of the lower crust beneath the gulf of maine summary the variability of deep crustal reflections in usgs line 1a across the offshore new england appalachians shows the differing influence of paleozoic and mesozoic tectonic events mesozoic extension has not significantly modified paleozoic thrust faults penetrating the lower crust in the northern gulf of maine mesozoic extension or paleozoic crustal melting could explain the lower crustal character in the central gulf

undercrusting by serpentinite beneath rifted margins the west galicia passive margin spain is bounded by a continuous belt of serpentinized peridotite here we present the results of petrological studies of the peridotite 39ar 40ar dating of high temperature ductile shearing of a dioritic dyke crossing the peridotite and regional seismic stratigraphy which demonstrate that the uplift ductile shearing and pervasive serpentinization of mantle rocks occurred at early cretaceous time during the rifting stage of the margin the serpentinite layer resting on fresh peridotite is probably several kilometres thick at present seismic data and gravity data cannot distinguish serpentinite from crustal material but our results from the galicia margin indicate that the seismic crust beneath rifted margin may include mantle material transformed into serpentinite by syn rift hydrothermal activity in some cases this undercrusting process might explain the young age and mobility of the moho beneath stretched continental basement

deep crustal reflection results from the central eromanga basin australia abstract from 1980 to 1982 deep seismic reflection profiles were recorded across the central eromanga basin in eastern australia to study the regional structure stratigraphy and geological history of the eromanga basin and infra basins the reflection data were recorded to 20 s to obtain additional information on the nature and structure of the crust below the sediments and their relationship to the development of the basins the seismic sections show good quality reflections from the deep crust as well as from the sedimentary layers based on the character strength coherence continuity and spatial distribution of the reflections the sections can be divided into four zones the tope zone between 0 and 2 5 s shows fairly uniform coherent and continuous events which correlate with the mesozoic and late palaeozoic sediments the zone from 2 5 to 8 s 4 to 22 km does not show any primary reflections and is interpreted as the highly deformed metasedimentary and metavolcanic rocks of the early palaeozoic thomson orogen underlying the sediments without any recognisable reflection or diffraction patterns in this zone it is difficult to say whether the faulting and folding observed in the sediments extend into the upper crustal basement the deeper zone of numerous reflection segments between 8 and 12 5 s 22 to 36 km is interpreted as thin laminae of alternating low and high velocity intermediate and basic rocks and correlates with the lower crust bounded by refraction velocity discontinuities the lowest zone of no reflections below 12 5 s corresponds with the upper mantle the reflection character and thickness as well as the refraction velocity structure of the crust under the central eromanga basin area are significantly different from those of the precambrian crust under the georgina basin to the northwest it is proposed that the crust under the eromanga basin is extensionally attenuated crust which had been intruded by sills of basaltic melt from the underlying asthenosphere

the moho in the northern basin and range province nevada along the cocorp 40 n seismic reflection transect cocorp seismic reflection profiles across nevada at about 40 n image a prominent essentially continuous band of reflectors at a two way traveltime of 9 to 11 s the approximate correspondence of this reflection time with estimates of the two way traveltime to the moho in this area provided by seismic refraction data suggests that the prominent reflections are from the moho the relief on these reflectors the reflection moho beneath nevada across a latitudinal transect of

evolution of the lower continental crust granulite facies xenoliths from the eifel west germany the lower continental crust is a key region to understanding processes of crust mantle differentiation and subsequent evolution of the crust the region is metamorphosed into granulite or amphibolite facies1 2 whereas felsic to mafic granulites with an average intermediate major element composition are typically observed in old exposed lower crustal terranes mafic compositions are preponderant among granulite facies xenoliths transported by young alkalic basaltic volcanics and kimberlites1 radiometrie ages for granulite bulk rocks using the rb sr or u pb methods are usually interpreted as metamorphic ages in contrast sm nd ages obtained on such metamorphics are interpreted to reflect pre metamorphic events3 7 although a case of severe disturbance of the sm nd isotope system during granulite facies metamorphism has also been reported8 we present here sm nd and rb sr isotope data for a suite of mafic lower crustal xenoliths from the hercynian belt our data suggest that the igneous precursors of these granulites differentiated from depleted mantle 1 5 gyr ago in most of these granulites this evidence for a mid proterozoic age has been obliterated by a young 172 myr metasomatic overprint by mantle derived fluids or melts

crustal reflection seismics the contributions of oblique low frequency and shear wave illuminations summary continuous vertical seismic reflection profiling the use of which has been extended by cocorp birps ecors dekorp and other programmes barazangi brown 1986 from stratified sedimentary basins to the entire crust provides a high resolution cross section of reflectivity a similar extension of oblique or variable offset sounding would be expected only to provide complementary velocity information however combined use of the two methods at crustal scale is new and we show that refractions and wide angle reflections do contribute original and relevant information but generally in areas other than velocities the reason being that interfaces and layers may have a different nature in the crystalline crust from that in sediments

the mid crustal horizon under the eromanga basin eastern australia abstract long line explosion seismic recordings indicate that a major mid crustal p wave velocity increase exists under the eromanga basin which contrasts the crust of this region from its neighbouring tectonic provinces the velocity increase is shown to be a regional feature and to correspond to the upper boundary of a zone of discontinuous seismic reflections characteristic of the lower crust the velocity increase of 0 35 0 6 km s occurs over a small depth range at an average depth of 24 km this velocity gradient produces conspicuous wide angle reflections at distances greater than 60 km from the shot point the seismic data present the clearest evidence to date of the existence of a major mid crustal horizon in continental australia a feature not prominent in other cratonic provinces which have been investigated in detail the velocity in the lower crust below the horizon is 6 7 7 0 km s the horizon represents a boundary between the upper and lower crust which could imply a different tectonic history for these two zones

long listening multichannel seismic profiles in the valencia trough valsis 2 and the gulf of lions ecors a comparison abstract the long listening multichannel seismic mcs profiles recorded during the valsis and ecors experiments were analysed in order to understand the rifting processes of a young passive margin the seismic lines were interpreted within a geological framework which is considerably improved by the results from industrial wells the disturbed upper crust off catalonia is tentatively connected with a variscan front the origin of lamellae in the lower crust beneath the valencia shelf is discussed a variscan origin is rejected and a relation to a jurassic extensional event is proposed a companion basin to the onshore maestrazgo basin is located off shore of valencia and these two basins are probably connected by the left lateral strike slip castellon fault this fault is parallel to the north pyrenean fault which is the trace of the rotation of spain relative to europe the castellon fault may be connected with the asturian fault which is located on the atlantic side of spain a contour map down to the moho has revealed some new deep features and the results are compared with the previous refraction results the sedimentary column of the deep north balearic provencal basin is much thicker than that of the eastern part of the valencia trough and this diversity in the sedimentary infilling is probably related to a difference in the subsidence processes a comparison of the valencia trough to the gulf of lions shows that the thinning of the continental crust looks similar in both regions but the crust is thinner beneath the valencia continental shelf than that below the gulf of lions in both regions the moho rises possibly at the expense of the continental crust by a process as yet unexplained petrological or and thermal variation

exposed cross sections through the continental crust implications for crustal structure petrology and evolution abstract the continental crust is exposed in cross section at numerous sites on the earth s surface these exposures which appear to have formed by obduction along great faults during continental collision may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle from an examination of five complexes which meet these criteria it is concluded that the most prominent layering in the crust is not compositional but metamorphic the lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks respectively post metamorphic granitic intrusions are common at intermediate to shallow levels position of discontinuities in refraction velocity where present commonly correspond to changes in composition or metamorphic grade with depth the continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by cocorp field observations coupled with geochemical data indicate a complex evolution of the lower crust which can include anatexis multiple deformation polymetamorphism and reworking of older crustal material the complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments

residence time of diffuse waves in the crust as a physical interpretation of coda q application to seismograms recorded in mexico summary we consider a simple model for elastic wave propagation in the earth s litho sphere consisting of a heterogeneous and scattering crust overlying a homogeneous mantle that is we neglect backscattering from the upper mantle this rough condition is not strictly valid everywhere but this model allows us to investigate the effects of variations of diffracting properties between crust and mantle on the coda decay the free parameters describing the wave propagation are the mean free path of the waves and the intrinsic absorption in the crust in the framework of the radiative transfer theory we treat accurately jumps in s wave speed and the mean free path at the moho without intrinsic absorption this configuration yields a synthetic coda decay 1 texp 2 ft qc a form similar to that proposed by aki chouet 1975 to parametrize the observations the exponential decay in our purely elastic model is due to the partial leakage of diffuse energy into the mantle in order to test the applicability of our model we have determined the coda q parameter qc in the 1 15 hz frequency band from data recorded at four stations in mexico in the low frequency regime around 1 hz we suggest that the energy leakage significantly affects coda q whilst at higher frequencies around 10 hz the anelastic attenuation dominates a weak intrinsic absorption expressed by a frequency independent parameter qi of about 1000 accounts satisfactorily for the observed qc values in the whole frequency range our very simple model has only two free parameters and offers an attractive alternative physical interpretation of coda q in mexico this model is sufficient to interpret the coda observations according to our model in regions where the mean free path is much larger than the crustal thickness weakly heterogeneous crust the leakage effect should be small and the observed qc will mainly be due to the intrinsic absorption on the other hand when the mean free path is of the order of the crustal thickness strongly heterogeneous crust we predict a strong sensitivity of the decay rate of the coda to the crustal thickness this dependence may be important for the interpretation of the regional variations of coda q we propose introducing a new parameter d the residence time of diffuse energy in the crust which is physically more meaningful than a quality factor to quantify the leakage effect

the variscan belt in central europe main structures geodynamic implications open questions abstract the variscan belt of europe originated from the confrontation of northern europe and gondwana with intervening pre variscan blocks cadomian and older armorica tepla barrandean moravo silesian though a strike slip component cannot be excluded geological evidence suggests a subduction collision process newly discovered features include basaltic rocks with morb affinities high pressure metamorphic rocks trench deposits and large scale allochthonism which also affects the pre variscan basement and has often led to an inversion of the metamorphic isogrades large scale thrusting is evident at all crustal levels and is also observable by geophysical methods the structure of the variscan crust is best described as horizontal tectonic layering late ordovician through carboniferous convergence closed one rhenohercynian and one saxo thuringian basin to the north and at least one mediterranean basin to the south of the pre variscan blocks subduction occurred from the north as well as from the south down under the dorsal core region the exact timing and the driving mechanism of this bilateral activity remains a problem likewise uncertain are the exact limination of the pre variscan blocks and the existence and width of oceanic areas in the saxothuringian zone and in the south facing part of the moldanubian zone there is good evidence of early palaeozoic rifting and of late ordovician through carboniferous plate convergence palaeomagnetic and palaeoclimatic constraints however suggest only a limited extent of oceanic basins in these areas while they appear to allow a wider ocean at the rhenohercynian saxothuringian boundary which is relatively unobtrusive from the geological point of view research during the past decade has produced a wealth of information on the variscan fold belt in europe the resulting geodynamic models however are extremely diverging evidently it is not the time yet for definitive statements the present paper intends to review the state of knowledge to deduce such basic items of a geodynamic model as appear reasonably well documented and to point out the most urgent open questions stress will be laid on central europe which offers a complete cross section through the northern flank of the variscan belt and where we have carried out most of our studies during the last years

a deep seismic reflection survey across the betic chain southern spain first results abstract two land seismic reflection profiles across the betic chain have imaged the deep structure of the crust belonging to two different crustal domains to the north one profile samples the crust of the variscan iberian massif that underlies the sedimentary cover of both the guadalquivir foreland basin and the south iberian crustal domain the upper crust is non reflective along the profile but the lower crust is reflective in the northern half of the profile between 7 and 12 s two way travel time in this segment of the profile the mono is also well imaged the quality of the data diminishes significantly as the profile enters the neogene guadix baza basin a second profile crossing the alpine metamorphic complexes of the betics reveals a reflective lower crust and a conspicuous reflection in the upper crust that is continuous for about 10 km at 11 s a distinct reflection has been interpreted as the reflection moho deeper reflections are also seen in the central part of the profile in a segment about 10 km long comparison with available refraction and wide angle reflection data shows some differences in both the crustal configuration and in the depth to the moho

deep seismic reflection profiles across the western barents sea summary the continental crust beneath the western barents sea has been acoustically imaged down to moho depths in a large scale deep seismic reflection experiment a first order pattern of crustal reflectivity has been established and the thickness of the crust determined a number of features with important implications for the tectonics of the area have been discovered the results are presented in the form of two transects

new cocorp profiling in the southeastern united states part i late paleozoic suture and mesozoic rift basin new cocorp profiling in the southeastern united states has revealed a broad zone of dipping reflections that extends downward through the crust beneath the coastal plain in western georgia the zone is over 50 km wide and most of the reflections dip moderately steeply toward the south regional considerations suggest that this feature marks the late paleozoic suture between north america and africa where crossed by the cocorp survey the suture occurs beneath the north flank of the triassic early jurassic south georgia basin the main depocenter of the south georgia basin occurs about 90 km to the south and is formed by a large half graben containing more than 5 km of rift basin fill farther south the paleozoic suwannee basin sequence beneath northern florida is poorly imaged on the cocorp profiles however weak reflections suggest that these strata including basal felsic volcanics may have an aggregate thickness of about 6 km in north central florida at the northwest end of the cocorp traverse a prominent horizon imaged in the upper crust beneath the inner piedmont probably marks the southern appalachian detachment the detachment appears to be cut off by the towaliga fault implying that the towaliga fault is in part a down to the north normal fault intermittent moho reflections occur at 11 12 s two way time along the length of the cocorp survey indicating that the crust in the region has a roughly uniform thickness of about 33 36 km assuming an average crustal velocity of 6 km s

moist and the continuity of crustal reflector geometry along the caledonian appalachian orogen new aspects of the deep structure of the scottish caledonides are revealed by the moine and outer isles deep seismic reflection traverse moist the caledonian foreland is underlain by an easterly dipping strongly reflecting surface cutting through the moho and traceable to more than 45 km depth thrusts within the foreland basement and caledonian orogen have been reactivated as normal faults bounding half grabens filled with sediments of late palaeozoic to mesozoic age the moine thrust which carries rocks of the orogen over lewisian foreland dips at 20 25 to the e on moist and is either a the westernmost of a series of easterly dipping reflections thrusts which flatten or terminate at 17 20 km depth or b a thrust lying further e which structurally overlies these easterly dipping reflectors by comparison with easterly dipping reflections on cocorp lines in the northern and southern appalachians the latter interpretation is preferred implying that many of the easterly dipping reflections on moist correspond to off shelf metasedimentary rocks with slivers of basement stacked and imbricated against the lewisian basement edge regional seismic refraction and conductivity data from northern scotland suggest that regions of the caledonian orogen have lower mid crustal velocity gradients and higher conductivities than the foreland which might be explained by this interpretation of the easterly dipping reflections despite similarities in crustal reflector geometries there are important differences between the moist and cocorp lines including 1 a remarkably continuous relatively horizontal moho seen on moist data at about 25 km depth and 2 the sedimentary basins offshore from northern scotland which have formed by reactivation under crustal extension of the easterly dipping thrusts furthermore it is probable that the easterly dipping reflectors in these areas of the moist and cocorp lines were formed at different times in the early palaeozoic

structural comparison of archetypal atlantic rifted margins a review of observations and concepts abstract in this study we compare three pairs of conjugate rifted margins that are often referred to as archetypes of rift systems despite numerous differences a remarkable first order structural similarity appears between the magma poor iberia newfoundland the magma rich mid norway east greenland and the sediment rich angola brazil rifted margins typical is the seaward arrangement of distinct domains proximal necking distal and outer that each present specific comparable structural characteristics our study also suggests that magma rich systems may go through a stage of hyper extension indicating that extreme crustal thinning does not preclude a magmatic breakup in this contribution we clarify the definition of a number of terms introduced recently in rifted margin studies we review the major features constitutive of these key referenced systems discuss their similarities and differences and examine how the related deformation modes develop in the life cycle of a rift that goes to seafloor spreading we conclude that the distinct domains observed in the margin architecture represent distinct stages in the evolution of rifted margin independently of their later evolution into magma poor or magma rich environments

nontypical birps on the margin of the northern north sea the shet survey summary striking similarities in the reflectivity of the crust and upper mantle on birps profiles has led to the development of the typical birp a model seismic section for the british continental lithosphere the shet survey collected in the region of the shetland islands and the northern north sea fits the general pattern to a certain extent caledonian structures and devonian or younger basins are imaged in the otherwise acoustically transparent upper crust an unexpected and exciting feature imaged on shet is a short wavelength structure on the moho or abrupt moho offset beneath the strike slip walls boundary fault shet differs markedly from the swat typical birp however by showing a poorly reflective lower crust only a narrow zone 1 s at the base of the crust contains high amplitude reflections the shet survey therefore highlights the wide variation in lower crustal reflectivity within the total birps data set rather than the similarities

a deep seismic reflection profile over a caledonian granite in central england in 1980 a short deep reflection profile undertaken jointly by the university of leicester and the institute of geological sciences crossed a buried caledonian granite to the south west of melton mowbray in leicestershire little structure is seen in the section between the top of the granite at 0 25 s two way time stwt and a zone of very strong sub horizontal reflections at 6 s twt a further reflection zone is identified at 7 2s twt below which there are a series of strong but impersistent reflections to the full reording time of 12 s twt the data indicate that the granite must be relatively homogeneous and underlain by several marked sub horizontal boundaries at mid to lower crustal levels this suggests that during emplacement either the granite must have had little effect on the lower crust sampled by the profile or the lower crust was mobile enough to have developed layering since the granite was emplaced during the caledonian orogeny

operations and main results of the ecors project in france summary the ecors project was launched in 1983 with the aim of studying fundamental mechanisms of geodynamics in france the ecors deep seismic profiles have concentrated on a few structures of major significance outer zone of the variscan orogen in northern france the basin formed during the bay of biscay s opening and transects of recent orogenic ranges pyrenees and alps the seismic profiles have been carried out with all the available modern techniques of industry and completed wherever possible by additional geophysical surveys magnetism gravity mt wide angle and refraction seismics and geological surveys the first results already shed new light on major geodynamic phenomena such as variations in the frontal variscan detachment lower crust formation crustal behaviour during orogenesis and variations in the formation of cratonic basins

lower crustal petrology from wide angle p and s wave measurements in the black forest abstract the lower crust of the black forest in southwest germany is strongly reflective for compressional waves but almost non reflective for shear waves if the reflectivity for p waves is caused by alternating high and low velocity rocks the s wave observations require that the low velocity rocks have a low poisson s ratio and vice versa a purely compositional petrological model for lower crustal rocks can fit the observed p and s wave velocity data provided a rather high quartz content 50 is acceptable for the lower crust

united states crustal thickness abstract the thickness of the crust the thickness of the basal intermediate or lower crustal layer and the average velocity at the top of the mantle p n have been mapped using all available deep penetrating seismic refraction profiles in the conterminous united states and surrounding border areas these profiles are indexed to their literature data sources the more significant long wavelength anomalies on the three maps are briefly discussed and analyzed an attempt to use bouguer gravity to validate mantle structure was inconclusive

large variscan overthrusts beneath the paris basin the ecors project is devoted to the deep geophysical investigation of the continental and oceanic crust in france through the study of fundamental problems such as the formation of basins mountain belts and continental margins the first of the profiles completed is the nord de la france which is mainly devoted to the survey of the major units of the northern hercynian belt and of the deep crust below the mesozoic paris basin the deep geophysical investigations undertaken on this profile have led tothe acquisition of diverse information such as vibroseismic wide angle reflection refraction and magnetotelluric data the main results presented here show evidence of large overthrusts in the variscan chain which is currently buried beneath the thin mesozoic cover of the paris basin they show a differentiation of structural units in the upper crust their link with the lower crust and the depth and geometry of the moho they also give rise to some speculations regarding the nature of various gravimetric and magnetic anomalies such as the large anomalie magnetique du bassin de paris ambp