the metagabbros orthogneisses and paragneisses of the connemara complex western ireland although now largely disappeared within the 400 ma old galway granite the syntectonic connemara metagabbro gneiss complex which crystallized at 490 ma arenig is the largest intrusion into the dalradian rocks it is part of widespread cambro ordovician volcanism and plutonic injection into continental crust in the appalachian caledonide orogenic belt being synchronous in crystallization with the widely obducted 490 ma ophiolites within the orogeny the peridotites and hornblende gabbros were injected broken up and shouldered aside by quartz diorite granodiorite granite and trondhjemite gneisses and it is shown that these orthogneisses are not the in situ differentiation products of the gabbro magma the hornblende in the metagabbros is mainly igneous but is partly metamorphic and replaces olivine fo 75 92 orthopyroxene en 65 92 clinopyroxene wo 48 5 en 50 39 fs 5 14 and plagioclase typically an 80 92 range an 50 96 an aureole at least 10 km wide in the sillimanite zone was imposed and cordierite garnet sillimanite dalradian hornfelses were partially melted at temperatures up to 1000 o c near to the gabbros at 4 5 1 kbar corresponding to c 18 km depth this suggests that the complex forms part of the root of a now eroded batholith of calc alkaline affinities the gabbros corresponding to the transition zone to a basic bottom which is often supposed to underlie many batholiths within the orthogneisses are sheets of paragneiss formed from dalradian metasediments which have been folded up from the floor and down into sides of the magma chamber which process could be illustrative of the means by which basic magma can intimately interact with crustal material the 18 o 16 o d h nd and sr isotope evidence and the existence of inherited zircons in the gneisses indicate that the gabbro and gneiss magmas were significantly contaminated with crustal possibly dalradian material both before and after intrusion with the lowest initial 87 sr 86 sr at 490 ma being 0 708 even in samples far removed from metasediment the metagabbros are tholeiitic to high alumina basalt in character but the gneisses have distinct calc alkaline geochemistry the chemistry igneous hornblende and highly calcic plagioclase match many arc plutons in the western usa the syntectonic nature and the ubiquitous gneissic fabric in the quartz diorites trondhjemites and granites is a consequence of early movements which were probably in part connected with the separation of the connemara massif and which culminated in slicing off the largely inverted complex and carrying it southwards on the mannin thrust which underlies the whole of connemara the complex is thus a fragment of a now disrupted cambro ordovician continental magmatic arc or continent island arc environment which can be traced along the se edge of the laurentian continent

a crustal seismic velocity model for the uk ireland and surrounding seas summary a regional model of the 3 d variation in seismic p wave velocity structure in the crust of nw europe has been compiled from wide angle reflection refraction profiles along each 2 d profile a velocity depth function has been digitised at 5 km intervals these 1 d velocity functions were mapped into three dimensions using ordinary kriging with weights determined to minimise the difference between digitised and interpolated values an analysis of variograms of the digitised data suggested a radial isotropic weighting scheme was most appropriate horizontal dimensions of the model cells are optimised at 40 40 km and the vertical dimension at 1 km the resulting model provides a higher resolution image of the 3 d variation in seismic velocity structure of the uk ireland and surrounding areas than existing models the construction of the model through kriging allows the uncertainty in the velocity structure to be assessed this uncertainty indicates the high density of data required to confidently interpolate the crustal velocity structure and shows that for this region the velocity is poorly constrained for large areas away from the input data

nd and sr isotope constraints on the origin of the cornubian batholith sw england combined nd and sr isotope data for the hercynian granites of sw england provide important new evidence for the origin of the cornubian batholith the nd isotope systematics are shown to be robust even in areas of extensive hydrothermal alteration e nd values for the main plutons range from 4 7 to 7 1 whereas values for the palaeozoic country rocks are significantly lower typically 8 to 11 depleted mantle nd model ages 1 3 1 8 ga indicate a heterogeneous source region for the granite magmas and imply that the age of the basement beneath southern england is significantly older than previously estimated calculated e nd values for the palaeozoic country rocks at the time of granite intrusion suggest that they were unlikely to have been the main source material for the magmas e nd e sr data for the granites do not define a simple binary mixing array between a mafic depleted mantle melt and proterozoic crust the best fit model is consistent with partial melting of a composite lower crustal source comprising immature metasedimentary and mafic meta volcanic rocks and mixing with a relatively minor contribution of basaltic magma extracted from a slightly enriched mantle source such hybrid magmas are thought to have been generated during an ivrea type underplating of the lower crust during continental collision their isotope signatures being determined by the relative proportions of crust and mantle material

source and partitioning of b and sn in the cornubian batholith of southwest england abstract emplacement of the variscan s type cornubian batholith initiated the formation of the historical world class sn and cu deposits and extensive tourmalinisation of southwest england the hypothesis is tested that the sn and tourmaline mineralisation were wholly formed by magmatically derived hydrothermal fluids with no requirement for a contribution from the stripping of country rocks by circulating hydrothermal fluids from simple geochemical modelling sufficient sn concentrations to match those in the main 90 of granite outcrop biotite granites average of 23 mg kg and to produce the large sn deposits can be derived by partial melting of pre granite southwest england crust at geologically realistic levels of partial melting 30 fractionation 10 to 50 and melt aqueous fluid partitioning assumed to be 6 wt of the mass of the granite batholith in contrast insufficient b 700 mg kg or b concentrations found in many of the granites we conclude that tourmaline in the biotite granites was formed from magmatically derived hydrothermal fluids which emanated as a result of first and possibly second boiling an additional source of b for the hydrothermal fluids may have been the devolatilisation of stoped blocks of country rock mudstones these findings provide fundamental constraints on the evolution of the cornubian batholith which is used as a model for a large number of sn provinces worldwide

the structure and evolution of the wessex basin southern england an example of inversion tectonics abstract the wessex basin of southern england consists of four main sub basins each sub basin is thought to be fundamentally controlled by the normal reactivation of hercynian basement thrust and wrench or transfer faults post hercynian polyphase reactivation of both east west thrust and northwest southeast transfer faults tended to compartmentalise the basement thereby leading to the initiation of discrete permian cretaceous depocentres while rifting was confined to the western portions of the area during the permo triassic easterly migration in the mesozoic allowed the initiation of the weald and channel basins the sub basins represent therefore simple sinistral pull apart basins opening along northwest southeast faults this same fault direction is postulated to have also played a major role in controlling and or modifying the development of european basins in general major pre aptian uplift and erosion is largely attributed to the thermal effects and hence the isostatic uplift of the basin associated with continental rifting of the bay of biscay and the north atlantic margins major inversion and hence basin destruction began in the late cretaceous early tertiary with the uplift of the channel weald and vale of pewsey basins and culminated in the oligocene miocene synchronous with the helvetic phase of alpine deformation wessex basin inversion was facilitated by the thrust reactivation of variscan faults above an intracrustal detachment surface the same surface which allowed upper crustal extension and hence basin initiation in the permo triassic the structural development of the wessex basin both during extension basin formation phase and compression inversion phase was ultimately controlled by the plate motion of africa relative to europe such plate motions successfully predict the observed nw se sinistral motion from the stephanian to the aptian e w sinistral motion during the aptian cenomanian and largely nw se dextral motion from the cenomanian to the present day

evolution of the rheic ocean the rheic ocean which separated laurussia from gondwana following the closure of iapetus is arguably the most important ocean of the palaeozoic its suture extends from mexico to turkey and its closure produced the climactic variscan alleghanian ouachita orogeny that assembled the supercontinent pangaea following protracted cambrian rifting that represented a continuum from neoproterozoic orogenic processes the rheic ocean opened in the early ordovician with the separation of several neoproterozoic arc terranes from the continental margin of northern gondwana separation occurred along the line of a former neoproterozoic suture following the onset of subduction in the outboard iapetus ocean the timing of rift drift transition and drive for subsequent spreading was likely governed by slab pull accounting for the rapid rate 8 10 cm yr at which the rheic ocean widened during the ordovician the ocean broadened at the expense of iapetus and attained its greatest width 4000 km in the silurian by which time baltica had sutured to laurentia and the neoproterozoic arc terranes had accreted to laurussia closing iapetus in the process closure of the rheic ocean began in the devonian and was facilitated by northward subduction beneath southern baltica and southward subduction beneath northwest gondwana closure was largely complete by the mississippian as gondwana and laurussia sutured to build pangaea north africa colliding with southern europe to create the variscan orogen in the devonian carboniferous and west africa and south america suturing to north america to form the alleghanian and ouachita orogens respectively during the carboniferous permian the rheic ocean consequently plays a dominant role in the basement geology of southern europe in the appalachian ouachita orogeny of north america and in the palaeozoic sedimentary structural and tectonothermal record from middle america to the middle east with its closure the ocean brought about the assembly of pangaea and brought the palaeozoic era to an end

structural and tectonic setting of the permo triassic basins of northwest europe abstract the permo triassic basins of nw europe formed by i late orogenic spreading of the variscan mountain belt eventually leading to ocean floor formation in the mid atlantic ii continental rifting propagating south from the arctic and iii thermal subsidence following carboniferous volcanic activity in the north sea spreading values in the northernmost north sea west of shetland and west and southwest of britain are relatively high but the sediments are dominantly continental the permo triassic rifting affected lithosphere thickened during caledonian and variscan orogenesis however subsidence associated with the arctic rifting together with global sea level changes allowed marine incursions into the north sea producing the large zechstein salt basins interference of the different basin formation mechanisms led to a complex three dimensional fault pattern and multidirectional extension across britain and the north sea this fault pattern formed the basic framework for subsequent jurassic and cretaceous basin formation

global changes in intensity of the earth s magnetic field during the past 800kyr recent advances in palaeomagnetic and dating techniques have led to increasingly precise records of the relative intensity of the earth s past magnetic field at numerous field sites the compilation and analysis of these records can provide important constraints on changes in global magnetic field intensity and therefore on the earth s geodynamo itself a previous compilation for the past 200 kyr integrated 17 marine records into a composite curve1 with the geomagnetic origin of the signal supported by an independent analysis of 10be production made on different cores2 the persistence of long term features in the earth s magnetic intensity or the existence of long term periodic changes cannot however be resolved in this relatively short time span here we present the integration of 33 records of relative palaeointensity3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 into a composite curve spanning the past 800 kyr we find that the intensity of the earth s dipole field has experienced large amplitude variations over this time period with pronounced intensity minima coinciding with known excursions in field direction reflecting the emergence of non dipole components no stable periodicity was found in our composite record and therefore our data set does not support the hypothesis that the earth s orbital parameters have a direct and strong influence on the geodynamo

tectonics of the bude formation north cornwall the recognition of northerly directed decollement the upper carboniferous bude formation of north cornwall and devon underwent deformation beginning shortly after its deposition which resulted in a complex geometry of folding and thrusting the deformation initially exploited slumped beds as decollement horizons and resulted in a significant northerly translation of thrust sheets the development of chevron folds for which the area is famous commenced at this stage but continued causing folding of thrust planes and cleavages during the deformation the dominant sense of thrust movement switched from a northerly to a southerly direction and this is interpreted as the effect of backthrusting associated with the northwards migration of the deformation front

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

helium radon and hydrothermal circulation associated with the carnmenellis radiothermal granite of southwest england the carnmenellis granite of southwest england has higher than average radioelement concentrations and a heat flow greater than 100 mw m 2 more than twice the united kingdom average measurements were made of 4he concentrations and 222rn activities in surface waters over the granite and its immediate host rocks high values greater than 48 ppb 4he 160 c min 222rn for either or both gases are interpreted as indicating discharge of groundwater from the upwelling limbs of hydrothermal convection cells 4he has a spatial distribution which agrees well with the observed variations of heat flow within the pluton but 222rn is also influenced by lithology and depth of groundwater flow although the distribution of convection cells is partly determined by the presence of nw se faults there is close similarity with theoretical models of groundwater circulation it is concluded that 4he and 222rn surveys provide a simple but effective method for mapping areas of modern hydrothermal activity

a model for the tectonic evolution of south cornwall a stratigraphy for south cornwall is combined with an analysis of the chronology of thrust nappe emplacement the resulting model suggests that northward overthrusting of nappes of flysch and olistostrome ophiolite and continental basement is related to the closure of an oceanic basin in south cornwall involving southward subduction throughout the devonian the subsequent continental collision between the normannian high and part of laurentia occurred in the upper devonian with deformatian continuing until late carboniferous times

the sticklepath lustleigh fault zone tertiary sinistral reactivation of a variscan dextral strike slip fault dextral strike slip movement on the sticklepath lustleigh fault zone slfz is indicated by displacements of permian and older rocks previous authors have inferred that the main dextral movement which caused these displacements was post permian and noting the presence of small tertiary pull apart basins along the fault zone probably of tertiary age however the geometry of these early tertiary pull apart basins indicates sinistral rather than dextral strike slip movement we present an alternative model for the history of the sticklepath lustleigh fault zone summarized below 1 late variscan strike slip movement with a total displacement of up to 10 km produced the slfz and offset dextrally an earlier variscan thrust 2 extensional reactivation of this thrust led to rapid permo triassic subsidence in the crediton trough and a dextrally offset neighbour the hatherleigh outlier 3 approximately 6 km of early tertiary sinistral movement on the slfz produced small pull apart sedimentary basins and reduced the net dextral offset across the fault 4 in mid tertiary times minor dextral movements on the slfz may have produced reverse faulting on the margins of the tertiary basins

superimposed variscan caledonian and proterozoic features inferred from deep seismic profiles recorded between southern ireland southwestern britain and western france abstract from southern ireland to the northern bay of biscay subduction was active from the proterozoic to the late palaeozoic they led to the cadomian caledonian and variscan collisions features characterizing these three orogenies have been crossed by the winch swat wam and bay of biscay deep seismic profiles recorded by the birps and ecors groups in the celtic sea the variscan front is characterized by a s dipping ramp rising directly from the middle crust with much less northward flattening than shown by the front in the northern france and west germany profiles south of the variscan front other s dipping reflections occur above a possible deep detachment the lizard ophiolitic suture marks s dipping events traversing the entire crust and the present moho in the english channel several sets of prominent n dipping reflections occurring in the celtic sea and south irish sea are interpreted as caledonian features running parallel to the caledonian iapetus suture and the pattern of folds and faults recognized in wales and southeastern ireland proterozoic s dipping subduction is recorded by geological data in northern wales and the english channel related crustal structures correspond to deep seismic events cutting across the lower crust and the moho and show the same apparent dip as the variscan features parallelism between dips and or strikes of structural features with different ages suggests a tectonic heritage the crust thus appears to be characterized by n and s dipping features which cut across large parts of it and may be assigned to variscan caledonian or candomian thrusts or shear zones the lower part of the crust shows a band of high amplitude subhorizontal reflections and a flat moho reflection which seem to have overprinted the above mentioned dipping features in a later period the lack of evidence of any shallow detachment in the caledonian and variscan belts together with the presence of dipping crustal thrust like features suggest that the region may have reached the very latest collisional stage the end of both of these orogenies

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

characterization of a proposed palaeoproterozoic suture in the crust beneath the curnamona province australia abstract we use constrained forward and inverse modeling of bouguer gravity data to investigate a regional positive gravity anomaly that defines but is not confined to the eastern margin of the curnamona province best fit results imply that the positive gravity values are sourced from a relatively dense terrane 0 04 mg m 3 with respect to surrounding crust located at middle crustal levels 16 27 km depth which is likely dismembered and offset into discrete slivers by later tectonism consideration of pre 1500 ma plate reconstructions of the australian continent correlates this terrane with an interpreted buried suture in the mount isa inlier of northern australia where allochthonous crust was amalgamated to the australian margin at ca 2 20 1 85 ga bouguer gravity forward and inverse modeling also delineates a shallow west dipping crustal scale structure that possibly represents a primary boundary between precambrian basement of australia and the eastern fold belts accreted to the precambrian margin during the palaeozoic

crustal structure of the british isles and its epeirogenic consequences crustal receiver functions have been calculated for a network of 51 three component broadband seismometers distributed across the british isles and nw europe over 3200 receiver functions were assembled for 1055 events for each station preliminary estimates of crustal thickness and vp vs ratio were obtained from h plots stacked receiver functions were then inverted to determine shear wave velocity as a function of depth each result was checked by guided forward modelling and by monte carlo error analysis in this way the robustness of our final calculated velocity profiles was carefully tested a set of depth migrated profiles was also constructed using an average of 50 events for each station over a range of backazimuths these profiles agree well with legacy wide angle crustal models our results show that crustal thickness varies between 24 and 36 km across the british isles thicker crust is found beneath north wales and beneath central scotland thinner crust occurs beneath northwest scotland and northwest ireland by combining our database with the results of controlled source wide angle experiments and with depth converted reflection profiles we have produced a detailed crustal thickness map for a region encompassing the british isles our synthesis of crustal thickness and structure has important implications for the tectonic and magmatic histories of this region complex moho structure with lower crustal p wave velocities of 7 km s 1 occurs beneath regions of cenozoic magmatism which may be consistent with magmatic underplating thin crust beneath northern britain suggests that present day long wavelength topography is maintained by regional dynamic support originating beneath the lithospheric plate

seismic velocities beneath central and southern britain from broad band surface waves abstract fundamental mode rayleigh waves recorded across the uk broad band network have been analysed to produce phase velocity curves in the period range of 12 to 85 s an inter station method has been used for two paths across southern england the resulting mean phase velocity curve has been compared with a mean curve derived using three stations in central britain lateral refraction characteristics of tripartite phase velocities have been analysed and shown to be less than 5 at periods of greater than 15 s forward modelling to s wave velocity depth profiles using a hedgehog search procedure suggests that central britain is underlain by a thicker crust than southern england with a steady velocity increase down to the moho for southern england low velocities are apparent either at the base of the crust or in the uppermost mantle for both regions sub crustal lithosphere velocity is contrained to be between 4 6 and 4 7 km s 1 with a layer thickness of less than 140 km for southern england and between 80 and 160 km for central britain both regions appear to exhibit low velocity zone lvz development however for central britain more data are required to place better constraints on the magnitude of the lvz channel velocities in the lower crust have been used to place constraints on poisson s ratio which appears to decrease northwards from greater than 0 27 beneath southern england to between 0 25 and 0 27 beneath central britain the distributions of p wave velocities s wave velocities and poisson s ratio have been interpreted of compositional variation and or fluid content within the lower crust

thrust tectonics of s devon the tectonics of the area from start point to chudleigh sw england are shown to be dominated by northward verging thrust nappes two major thrusts are postulated the dartmouth antiform is considered to be a major thrust structure not a fold structure as previously interpreted the torquay limestones are allochthonous and part of a klippe of a major thrust nappe with a minimum displacement of 13 km northwards a progressive and diachronous sequence of deformation has been established with deformation starting in the s and moving n many of the folds and cleavages are probably related to the strains developed locally in thrust tips and may have no regional correlation more intense and complex deformation is found in the s compared to the n the diachroneity is also reflected in the k ar mineral ages in the s at start point southward verging backfolding is the last significant hercynian deformation phase folding the thrust nappes

seismic refraction investigation of the structural setting of the lizard and start complexes sw england a long refraction line was established between the lizard and start peninsulas during a large scale seismic experiment carried out in sw england during 1979 interpretation of first arrival data indicates a shallow crustal structure consisting of two layers of regional extent the upper layer of average velocity 5 15 km s is interpreted as devonian metasediments the underlying layer with a velocity of about 6 10 km s lies at a depth of several kilometres beneath the lizard complex but rises eastwards to occur at or near surface in the eddystone and start areas the lower layer is believed to represent a pre devonian crystalline basement of which the metamorphic rocks of start and eddystone are constituent parts by contrast the lizard complex is interpreted as a thin surface sheet overlying devonian metasediments

the thermal and mechanical development of the wessex basin southern england summary the wessex basin of southern england is an e w trending late palaeozoic to tertiary extensional basin formed on early palaeozoic variscan crust extension occurred over a protracted period from late carboniferous to cretaceous times resulting in the evolution of four main depocentres or sub basins the channel basin winterborne kingston trough vale of pewsey and weald basins each sub basin was controlled fundamentally by the normal reactivation of basement thrust faults and wrench or transfer faults the basement faults represent primary structures associated with the fold and thrust terrain of the devonian carboniferous variscan orogeny subsequent but synchronous reactivation of both thrust and transfer faults tended to compartmentalize the basement leading to discrete depocentres and the production of general rhomboidal shaped sub basins characteristic of pull apart basins in strike slip terrains to investigate the thermo mechanical properties of the lithosphere during basin formation 50 boreholes within the wessex basin were back stripped to isolate the driving tectonic subsidence responsible for basin formation the resultant driving subsidence is characterized by a gentle sometimes negative exponential form with superimposed rapid discrete subsidence events as the geological development of the wessex basin was characterized by polyphase extension and the utilization of crustal detachments the lithospheric model used to determine crustal extension above the detachment and stretching below the detachment included the effects of depth dependent stretching and finite re rifting from the back stripping analysis the basin wide averaged upper crustal extension and lower crustal sub crustal lithospheric stretching were 11 and 5 respectively the distribution of is strongly influenced by the location of growth faults and hence basement thrust fault reactivation suggesting that is reflecting the brittle failure and collapse of hanging wall blocks repeated slip and block collapse resulted in the polyphase extension responsible for renewed basin subsidence in contrast is uniformly distributed across the basin suggestive of a ductile failure of the lower crust and responsible for the low magnitude thermal subsidence strain compatibility between the upper crust and lower crust sub crustal lithosphere is maintained partly within the basin 5 and partly external to the basin as the variscan basal thrust was the fundamental relay in the sense of wernicke responsible for the wessex basin formation we postulate that strain balancing induced a cumulative essentially thermal isostatic uplift of 60 m under the northern margin of the armorican massif

major crustal reflectors beneath sw england strong wide angle reflection events were detected during a crustal seismic experiment carried out in sw britain in 1979 three crustal reflectors have been identified referred to as rl r2 and r3 rl lies at a depth of about 8 km and appears to be restricted to the interior of the granite batholith underlying sw england the nature of this reflector is unknown r2 is a mid crustal reflector lying at a depth of 10 15 km and interpreted as rising gradually northwards it coincides locally with the base of the granite as predicted by gravity modeling but extends well beyond the northern limit of the granite body it is suggested that r2 may represent a major thrust of late variscan age r3 is the deepest reflector identified and lies at a depth of 27 30 km r3 is interpreted as the base of the crust and its computed depth is in close agreement with the local depth to the moho derived from an earlier seismic experiment in sw britain

variscan structure of the inner bristol channel uk major variscan structures under the bristol channel are concealed beneath a mesozoic and quaternary cover mapping of reflectors on offshore seismic sections reveals two major wnw ese striking thrusts within the underlying palaeozoic sequence the more southerly with a moderate ssw dip has previously been identified as the bristol channel thrust another thrust the previously undescribed gravel margin thrust has been identified in the eastern part of the inner bristol channel and lies in the footwall of the bristol channel thrust it has a similar strike but steeper dip and extends to anticipated precambrian basement depths the bristol channel and gravel margin thrusts together juxtaposed the contrasting palaeozoic successions of south wales and sw england before mesozoic reactivation reflection packages identified on the offshore seismic sections are tentatively correlated with specific onshore palaeozoic sequences interpretation of the offshore seismic sections in conjunction with geological mapping along the north devon coast shows that the bristol channel and gravel margin thrusts have orientations and geometries typical of several mesoscale thrusts observed and measured at foreland point north devon along strike structural changes beneath the inner bristol channel seen on seismic sections provide evidence additional to earlier refraction and structural studies for the offshore continuation of the nw se trending watchet cothelstone hatch fault and its linkage with the previously described central bristol channel fault zone the bristol channel and gravel margin thrusts are interpreted to be offset from the cannington park thrust in north somerset by a 14 16 km variscan dextral strike slip displacement across the watchet cothelstone hatch fault the bristol channel gravel margin thrust and the cannington park thrust are interpreted as syngenetic structures linked by their lateral ramp the watchet cothelstone hatch fault using all the above information the overall structure of the variscides of the inner bristol channel is re evaluated and a new tectonic model is presented the structures discussed in this paper at a scale portrayed by commercial seismic data and published geological maps may represent merely the tips of linked thrusts that have previously been mapped down to mid crustal depths on birps seismic sections from the south west approaches traverse we show that enough data now exist to resolve outstanding problems of the concealed variscan structure in the inner bristol channel providing local structural detail that can be incorporated into the wider tectonic framework of nw europe

pluton shapes in the cornubian batholith new perspectives from gravity modelling gravity modelling of the dartmoor bodmin st austell and carnmenellis plutons of the cornubian batholith of sw england strongly supports a tabular form for the exposed granites modelled thicknesses for the individual bodies with the exception of dartmoor are consistent with those derived from empirical relationships for plutons and laccoliths in the case of the dartmoor bodmin and st austell plutons the deepest parts are located near their southern margins suggesting a steep conduit with northerly and outward direct flows modelling of the carnmenellis granite suggests a more centrally located feeder in the case of dartmoor at least the granite appears to have exploited the devonian carboniferous interface during its emplacement

a deep crustal model to explain regional aeromagnetic and gravity anomalies in southwest england abstract regional aeromagnetic anomaly maps of southwest england produced by upward continuation trend surface analysis and low pass wavenumber filtering of the published aeromagnetic map show a pronounced gradient zone of an east west trend crossing central devon across a 60 km wide zone regional field values reduce southwards by about 150 nt this gradient zone can be modelled as the edge effect of a 6 km thick permanently magnetised layer at the base of the crust a dense layer of similar geometry can explain the high regional gravity field that characterises southwest england

khatyrka a new cv3 find from the koryak mountains eastern russia a new meteorite find named khatyrka was recovered from eastern siberia as a result of a search for naturally occurring quasicrystals the meteorite occurs as clastic grains within postglacial clay rich layers along the banks of a small stream in the koryak mountains chukotka autonomous okrug of far eastern russia some of the grains are clearly chondritic and contain type ia porphyritic olivine chondrules enclosed in matrices that have the characteristic platy olivine texture matrix olivine composition and mineralogy olivine pentlandite nickel rich iron nickel metal nepheline and calcic pyroxene diopside hedenbergite solid solution of oxidized subgroup cv3 chondrites a few grains are fine grained spinel rich calcium aluminum rich inclusions with mineral oxygen isotopic compositions again typical of such objects in cv3 chondrites the chondritic and cai grains contain small fragments of metallic copper aluminum iron alloys that include the quasicrystalline phase icosahedrite one grain is an achondritic intergrowth of cu al metal alloys and forsteritic olivine diopsidic pyroxene both of which have meteoritic cv3 like oxygen isotopic compositions finally some grains consist almost entirely of metallic alloys of aluminum copper iron the cu al fe metal alloys and the alloy bearing achondrite clast are interpreted to be an accretionary component of what otherwise is a fairly normal cv3 oxidized chondrite this association of cv3 chondritic grains with metallic copper aluminum alloys makes khatyrka a unique meteorite perhaps best described as a complex cv3 ox breccia

seismic attenuation in the continental crust sw of england summary significant variability in crustal attenuation exists off england s sw coast as shown by birps deep seismic reflection lines the presence of the thick cornubian batholith with no sedimentary cover acting as a low attenuating region compared to normal upper crust with about 300 m of sedimentary cover can account for the observed differences in the strength of signal returning from the lower crust application of the spectral amplitude ratio method to determine the effective quality factor q of the lower crust beneath the granite yields a value of 400 200 a modification of the spectral amplitude ratio method to measure relative q s at different locations indicates that the average q of the upper and middle crust containing the granite is twice that elsewhere although the hypothesis that the lower crustal composition differs significantly beneath the granite cannot be ruled out geological and seismic observations suggest that differential attenuation is the main factor causing variation in the observed reflectivity thus thick granites act as low attenuating windows to the lower crust