crustal velocity structure of the british isles a comparison of receiver functions and wide angle seismic data summary magmatic underplating at the base of the continental crust is thought to be an important consequence of mantle plume activity geochemical evidence supports the hypothesis of underplating beneath the british tertiary igneous province during palaeocene times a process that has been invoked to explain at least part of the observed pattern of cenozoic denudation and consequent offshore deposition this study uses receiver functions to independently test the results of modelling of four wide angle seismic lines across the british isles several of which indicate the presence of a lens of high velocity material at the base of the crust this lens has been interpreted as evidence of palaeogene magmatic underplating receiver functions were calculated for three permanent and two temporary three component broad band seismometers in order to detect p to s conversions from intracrustal velocity discontinuities high frequencies were preserved in the calculation of the receiver functions receiver functions were modelled using the delay times of the p to s converted arrivals only with constraints on the vp and vp vs structure imposed from controlled source data this pragmatic method of interpreting receiver function data agrees well with the results of applying an established inversion technique resulting crustal velocity models are broadly consistent with the velocity models from the wide angle lines showing evidence for a four layer crust with laterally varying moho depths of between 33 2 5 km beneath southwest ireland 32 2 5 km beneath northern and eastern ireland 31 2 km beneath the isle of man and 35 3 km beneath northern england while these velocity models are consistent with the presence of a lower crustal layer receiver functions are insensitive to the p wave velocity structure and so unfortunately they cannot be used to independently constrain the p wave velocity of the lower crust which is an important diagnostic of the presence of magmatic underplating

seismic imaging of a hot upwelling beneath the british isles the iceland plume has had an important influence on vertical motions in the north atlantic the convecting mantle in this region contains a large scale low velocity seismic anomaly which correlates with a long wavelength gravity high and bathymetric feature this suggests that an arm of plume material has extended or is extending from iceland in a direction perpendicular to the mid atlantic ridge here we present the results of a detailed teleseismic traveltime study that reveals the high resolution morphology of this low velocity anomaly beneath the british isles our images provide insights into the nature of plume lithosphere interactions the low velocity anomaly imaged in this study correlates geographically with a region of high gravity anomalies and high topography that was associated with paleogene magmatism and phases of epeirogenic uplift during the cenozoic era there is evidence that the distribution of british earthquakes is also related to the low velocity anomaly the low velocity anomaly is interpreted to represent hot material from the original iceland plume head that became trapped beneath thinned regions of lithosphere ca 60 ma

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

control of regional sea level by surface uplift and subsidence caused by magmatic underplating of earth s crust magmatic underplating of the crust is a common feature of major basalt provinces the emplacement of magma within the lithosphere leads to surface uplift with a magnitude of the order of 10 of the thickness of the underplated material the composition of associated igneous rock samples suggests that much of the underplated material is made of gabbro this gabbro is denser than the magma from which it crystallizes so uplift of earth s surface caused by the emplacement of magma at the crust mantle boundary must be followed by subsidence as the magma solidifies this subsidence is equal to approximately half the original uplift and takes place within 0 1 m y of injection of the magma the association of magmatic underplating with surface uplift followed by subsidence provides a mechanism for changes of sea level including previously unexplained episodic highstands of sea level

sedimentary basin inversion caused by igneous underplating northwest european continental shelf a considerable body of evidence indicates that many of the extensional sedimentary basins in the vicinity of the british isles underwent permanent exhumation during the tertiary the most dramatic indicator of this process is the present day absence of as much as 4 km of anticipated postrift thermal subsidence in basins just north and west of scotland any explanation of this observation must take into account the fact that the entire region has very small long wavelength free air gravity anomalies this important constraint implies either that the crust has been thickened or that low density material has been added to or formed from the lithosphere and rules out models that invoke flexural effects arising from the opening of the north atlantic tertiary epeirogeny is often attributed to compression that is assumed to be related in a general sense to alpine mountain building however to remove 3 km of sedi mentary rock from a basin 100 km wide requires 15 km of shortening minor tertiary compression is observed all over the continental shelf but nowhere is it sufficient to account for the required amount of uplift and erosion in addition exhumation dramatically increases from south to north whereas the observed compression decreases markedly in the same direction at the beginning of the tertiary rifting associated with the initiation of the iceland plume generated substantial volumes of melt inversion of rare earth element concentrations of mgorich igneous rocks suggests that a minimum of 5 km of melt was produced beneath at least part of the continental shelf we infer that much of this melt remains trapped within the lithosphere presumably close to the moho which acted as a density filter such underplating will generate rapid uplift

a north south seismic profile across the caledonian suture zone in ireland abstract coole i caledonian onshore offshore lithospheric experiment line i is a 284 km seismic refraction profile extending from bundoran n to ardmore s in ireland fifty nine recording stations were deployed along this profile and it was served by a total of six shotpoints the profile crosses the predominant ne sw caledonian strike at a steep angle using both one dimensional and two dimensional seismic inversion techniques a travel time and relative amplitude study of the observed p wave data was undertaken gravity data along the profile was modelled to test the plausability of the derived velocity depth section the upper crustal structure along the profile is highly variable but some features stand out a large low velocity body towards the south of the traverse correlates well with a bouguer anomaly low and is interpreted as the southwest extension of the buried leinster batholith the mid crust is approximately 10 km thick and is bound at its top and bottom by two laterally continuous reflecting horizons both reflectors shallow northwards by about 5 km over a horizontal distance of some 60 km this northwards shallowing is a feature also displayed by a number of isolines in the upper crust and it marks a broad region of crustal change along the traverse in the zone of proposed caledonian suturing crustal thickness beneath coole i is approximately 30 km and the moho is a transition zone about 2 km thick which shallows slightly towards the coasts

crustal structure of edoras bank continental margin and mantle thermal anomalies beneath the north atlantic we discuss the structure of the continent ocean transition of the edoras bank area of the ne atlantic margin and the influence of the iceland mantle plume during continental breakup edoras bank lies close to the present day limit of influence of the iceland plume sequences of seaward dipping reflectors imaged on multichannel seismic profiles are well developed over much of the margin they form aerially extensive dipping sheets with individual reflectors imaged on margin parallel profiles as laterally continuous and subhorizontal features we interpret them as basalt flows extruded from linear fissure vents close to sea level at the time of continental breakup a crustal velocity model for the margin was developed by travel time modeling of wide angle refractions and reflections recorded on digital ocean bottom hydrophones and the structure confirmed by gravity modeling oceanic crust formed immediately after breakup is 10 km thick and exhibits classic layer 2 and layer 3 seismic velocities the crustal thickness increases to the se reaching 17 km beneath the continental fragment of edoras bank the continent ocean transition is dominated in the lower crust by a lens up to 8 km thick of high velocity material 7 2 7 5 km s 1 which exhibits a very low velocity gradient it extends over a 60 km wide region beneath the seaward dipping reflectors the high velocities and densities of the lower crust are probably caused by igneous rock with a high mgo content that underplated the margin or heavily intruded pre existing crustal material during continental breakup both the addition of large quantities of melt to the margin and the subsidence curves inferred from postrift sediments sampled by deep sea drilling project dsdp boreholes on the margin indicate the presence of abnormally hot mantle beneath the area at the time of continental breakup comparison of the structure and subsidence history of the edoras bank margin with other areas of the nw european margin allows us to deduce the spatial extent and temporal variation of the thermal anomalies caused by the iceland mantle plume during and following continental breakup we find that at the time of breakup a thin layer of anomalously hot asthenospheric mantle 150 c hotter than normal lay beneath the 2000 km long rift extending from the lofoten margin off norway in the north to the edoras bank margin in the south over a period of 5 10 m y following the onset of rifting the mantle thermal anomaly decayed rapidly to only 50 c or so above normal as the thin layer of abnormally hot mantle decompressed beneath the rift and produced a large pulse of magmatism

crustal trace of a hot convective sheet the iceland plume has played an influential role in the evolution of the north atlantic ocean and margins over the past 60 m y it is believed that this plume formed at the conjunction of a tetrad of hot subvertical convective sheets the impingement of these hot sheets at the base of the lithospheric lid caused decompressional melting generating substantial quantities of high temperature magma that were injected into the cold overlying lid over the next 10 m y these sheets partly coalesced to form a crudely axisymmetric plume head here we analyze the lithospheric fingerprint of one of these hot convective sheets by forward and inverse modeling of densely sampled wide angle seismic data in conjunction with gravity observations we determined the three dimensional shape of magmatic underplating trapped within the lithosphere the injection of this melt into the lithosphere generated substantial permanent and minor transient uplift of earth s surface predicted and measured amounts of consequent denudation and sedimentation agree within error temporal variations in the patterns of deposition and oceanic circulation adjacent to the convective sheet show its evolution through time and space our results suggest that this linear sheet has probably been directly and indirectly responsible for cyclical events over 60 m y these events have 0 5 1 and 4 6 m y periodicities the existence of which may help to elucidate the dynamic behavior of convective sheets during and after impingement thus in particular circumstances surficial geological processes yield an indirect record of mantle convection and melt generation processes

the carbon and sulfur cycles and atmospheric oxygen from middle permian to middle triassic the results of a theoretical isotope mass balance model are presented for the time dependence of burial and weathering plus degassing fluxes within the combined long term carbon and sulfur cycles averaged data for oceanic 13c and 34s were entered for every million years from 270 to 240 ma middle permian to middle triassic to study general trends across the permian triassic boundary results show a drop in the rate of global organic matter burial during the late permian and a predominance of low values during the early to middle triassic this overall decrease with time is ascribed mainly to epochs of conversion of high biomass forests to low biomass herbaceous vegetation resulting in a decrease in the production of terrestrially derived organic debris additional contributions to lessened terrestrial carbon burial were increased aridity and a drop in sea level during the late permian which led to smaller areas of low lying coastal wetlands suitable for coal and peat deposition mirroring the drop in organic matter deposition was an increase in the burial of sedimentary pyrite and a dramatic increase in the calculated global mean ratio of pyrite s to organic c high s c values resulted from an increase of deposition in marine euxinic basins combined with a decrease in the burial of low pyrite associated terrestrial organic matter the prediction of increased oceanic anoxia during the late permian and early triassic agrees with independent studies of the composition of sedimentary rocks weathering plus burial fluxes for organic carbon and pyrite sulfur were used to calculate changes in atmospheric oxygen the striking result is a continuous drop in o2 concentration from 30 to 13 over a twenty million year period this drop was brought about mainly by a decrease in the burial of terrestrially derived organic matter but with a possible contribution from the weathering of older organic matter on land it must have exerted a considerable influence on animal evolution because of the role of o2 in respiration some examples are the extinction of many vertebrates loss of giant insects and amphibians and the restriction of animals to low elevations it is concluded that the extinction of plants may have contributed to the extinction of animals

the geochemical and mineralogical fingerprint of west antarctica s weak underbelly pine island and thwaites glaciers abstract the marine based west antarctic ice sheet wais is considered the most unstable part of the antarctic ice sheet with particular vulnerability in the amundsen sea sector where glaciers are melting at an alarming rate far field sea level data and ice sheet models have pointed towards at least one major wais disintegration during the late quaternary but direct evidence for past collapse s from ice proximal geological archives remains elusive in order to facilitate geochemical and mineralogical tracing of the two most important glaciers draining into the amundsen sea i e pine island glacier pig and thwaites glacier tg we here provide the first multi proxy provenance analysis of 26 seafloor surface sediment samples from pine island bay our data show that the fingerprints of detritus delivered by pig and tg are clearly distinct near the ice shelf fronts of both ice stream systems for all grain sizes and proxies investigated glacial detritus delivered by pig is characterised by low end values 9 high 87sr 86sr ratios 0 728 low smectite content the geochemical and mineralogical fingerprints for pig and tg reported here provide novel insights into sub ice geology and allow us to trace both drainage systems in the geological past under environmental conditions more similar to those envisioned in the next 50 to 100 years

seismic velocities and anisotropy of the lower continental crust a review seismic anisotropy is often neglected in seismic studies of the earth s crust since anisotropy is a common property of many typically deep crustal rocks its potential contribution to solving questions of the deep crust is evaluated the anisotropic seismic velocities obtained from laboratory measurements can be verified by computations based on the elastic constants and on numerical data pertaining to the texture of rock forming minerals for typical lower crustal rocks the influence of layering is significantly less important than the influence of rock texture surprisingly most natural lower crustal rocks show a hexagonal type of anisotropy maximum anisotropy is observed for rocks with a high content of aligned mica it seems possible to distinguish between layered intrusives and metasediments on the basis of in situ measurements of anisotropy which can thus be used to validate different scenarios of crustal evolution

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

anisotropy in multi offset deep crustal seismic experiments modelling of deep seismic wide angle data commonly assumes that the earth is heterogeneous and isotropic it is important to know the magnitudes of errors that may be introduced by isotropic based wide angle models when the earth is anisotropic it is equally important to find ways of detecting anisotropy and determining its properties this paper explores the errors introduced by interpreting anisotropic seismic data with isotropic models errors in p wave reflector depths are dependent on the magnitude of the velocity anisotropy and the direction of the fast axis the interpreted isotropic model velocity function is found to correspond closely to the horizontal velocity of the anisotropic medium an additional observed parameter is the time mismatch which we define to be the difference between the vertical two way traveltime to a reflector and the time converted wide angle position of the reflector the magnitude of the time mismatch is typically 1 0 s when the whole crust is anisotropic and is found to be closely related to the magnitude and sign of the anisotropic anellipticity the relationships are extendible to more complicated models including those with vertical velocity gradients crustal zonation and lower symmetry orders a time mismatch may be symptomatic of the presence of anisotropy we illustrate the observation of a time mismatch for a real multi offset seismic data set collected north of scotland and discuss the implications for crustal anisotropy in that region

a tectonic model for the emplacement of the main donegal granite nw ireland the main donegal granite is shown to lie in a major late stage ne sw trending transcurrent sinistral shear zone in the caledonides of nw ireland the zone which is bent both laterally and upwards around the pluton has displacements which diminish sw along its length from a maximum of approximately 20 km at the ne end of the pluton the granite has also been deformed by this sinistral shear but it has a strain component of its own the nature and development of which indicate that the pluton was accommodated rather than forcefully emplaced in the shear zone strain variations between granite and wall rock and an early plagioclase fabric associated with the petrographic banding of the granite suggest that intrusion and deformation were more or less synchronous at around 400 ma it is suggested that unconsolidated granite in the shear zone locally created high strain rates and instability which caused the zone to bend split lengthwise and progressively create an internal low pressure zone into which the main granite was emplaced the wide variety of emplacement styles of the other radiometrically synchronous plutons of the batholith can be attributed to the progressive development of the shear zone thorr and fanad in tension between the zone and a complimentary structure to the n ardara and toories in local compression at the shear zone termination rosses in tension on the outer arc of the bending zone and trawenagh bay as displaced main granite magma in a similar tensional setting during the final stages of deformation

a wide angle seismic traverse through the variscan of southwest ireland summary a wide angle seismic profile across the western peninsulas of sw ireland was performed this region corresponds to the northernmost variscan thrust and fold deformation the dense set of 13 shots and 109 stations along the 120 km long profile provides a detailed velocity model of the crust the seismic velocity model obtained by forward and inverse modelling defines a five layer crust a sedimentary layer 5 8 km thick is underlain by an upper crustal layer of variable thickness with a base generally at a depth of 10 12 km two midcrustal layers are defined and a lower crustal layer below 22 km the moho lies at a depth of 30 32 km a low velocity zone which coincides with a well defined gravity low is observed in the central part of the region and is modelled as a caledonian granite which intruded upper crustal basement the granite may have acted as a buver to northward directed variscan thrusting the dingle dungarvan line ddl marks a major change in sedimentary and crustal velocity and structure it lies immediately to the north of the velocity and gravity low and shows thickness and velocity diverences in many of the underlying crustal layers and even in the moho this suggests a deep pre variscan control of the structural development of this area the model is compatible with thin skinned tectonics which terminated at the ddl and which incorporated thrusts involving the sedimentary and upper crustal layers

lithospheric structure north of scotland ii poisson s ratios and waveform modelling summary a 200 by 50 km grid of near normal incidence seismic reflection data and a 350 km wide angle seismic line have been acquired across the flannan and w mantle reflectors north of scotland a number of geological models have been published to explain the origin of these reflections we examine these models by considering the physical properties of the lithosphere in this region we present p and s wave velocity models for the lithosphere obtained using ray based inversion schemes we show full wavefield synthetic p wave data generated using a 2 d finite difference code to confirm that our velocity model can reproduce the amplitude behaviour of the observed mantle arrivals p and s wave velocities beneath the moho give a poisson s ratio of 0 25t0 01 which together with evidence from local mantle xenoliths suggests a normal peridotitic upper mantle we observe both normal incidence and wide angle reflections from the mantle reflectors modelling and inversion of these reflection events show that the flannan and w reflectors represent at least a 20 per cent increase in acoustic impedance within the mantle and that the region of elevated acoustic impedance has a thickness of at least 3 km a mafic eclogite is the only common mantle rock type that can have an acoustic impedance significantly higher than peridotite the physical properties of the upper mantle north of scotland thus suggest that the flannan and w reflectors represent reflection events from the tops of slabs of eclogite a weak event is interpreted as a wideangle reflection from the base of one of these slabs a slab thickness of 6 0t1 0 km with a poisson s ratio of 0 29t0 02 was used to model this phase the high poisson s ratio within the slab gives further support to the interpretation that these slabs are eclogitic the lateral extent of these mantle reflection events and the thickness of the slab are most readily explained as oceanic crust that was metamorphosed to eclogite when subducted beneath the existing continent we conclude that the eclogitic slabs are relicts of oceanic crust that remained within the lithosphere when subduction halted

evidence for kilometre scale neogene exhumation driven by compressional deformation in the irish sea basin system simon p holford paul f green jonathan p turner gareth a williams richard r hillis david r tappin ian r duddy

constraints on crustal structure and complex moho topography beneath pinon flat california from teleseismic receiver functions abstract we use teleseismic p waves recorded at pinon flat observatory pfo to constrain the three dimensional crustal and upper mantle velocity structure beneath the station by forward modeling radial receiver function waveforms we construct a one dimensional crustal model that includes a significant shear velocity inversion at 9 km in depth arrivals on the tangential components indicate dip of at least the uppermost discontinuity complicated moho topography deepening to the northwest of pfo is suggested by azimuthal dependence of travel times and amplitudes of the receiver functions and travel times of crustal p wave reverberations although fine details cannot be resolved each of those sets of observations plus mislocation vectors provide strong indications of abrupt moho topography possibly including step offsets of several kilometers this is not only consistent with gravity data in implying airy isostasy with compensation at moho depth but extends that model to a much finer length scale than had been resolved

the hatton bank continental margin ii deep structure from two ship expanding spread seismic profiles summary the continent ocean transition adjacent to hatton bank was studied using a dense grid of single ship and two ship multichannel seismic profiles the interpretation of the explosive expanding spread profiles esps which were shot as part of this survey are discussed here in detail extensive seaward dipping reflectors are developed in the upper crust across the entire margin these seaward dipping reflectors continue northwards on the faeroes and voring margins where they have been shown to be caused by basaltic lavas as well as on the conjugate margin of east greenland the dipping reflectors are an important feature of the rifting history of the margin and show that extensive volcanism was associated with the extension the esps show clear seismic arrivals out to ranges of 100 km wide angle moho reflections can be seen on all the lines as well as good mid and lower crustal arrivals the determination of seismic velocity structure was constrained by ray tracing and by amplitude modelling using reflectivity synthetic seismograms the results from the esps show that there is a thick region of lower crustal material beneath the margin with an unusually high crustal velocity of 7 3 7 4 km s this lower crustal material reaches a maximum thickness of 14 km beneath the central part of the margin and is terminated at depth by the moho the lower crustal lens of high velocity material is interpreted as underplated or intruded igneous rocks associated with the large volumes of extrusive basaltic lavas now seen as dipping reflectors on the margin

uk crustal structure close to the iapetus suture a receiver function perspective summary teleseismic receiver functions have been calculated for data from 10 short period three component seismic recording stations across northern britain to investigate variations in crustal and upper mantle velocity structure the stations straddle the iapetus suture zone the inferred boundary between two of the continents fused together during the caledonian orogeny the receiver function data shows that there is considerable azimuthal variation in both crustal and upper mantle structure beneath several stations the data are projected on to a 2 d profile showing laterally continuous moho ps conversions at delay times between 3 and 4 s synthetic receiver functions generated using the velocity model from a previous deep seismic reflection refraction survey show ps and pppms phases comparable to the observed data 1 d forward modelling of the data gives crustal thicknesses of 30 km there is a significant velocity depth trade off in the receiver function method and the crustal thicknesses have been constrained by a priori velocity information investigation of the data from station gim close to the iapetus suture shows a sub moho phase which is found only on data from northerly backazimuths phase modelling is consistent with the presence of a gradational low velocity zone with a minimum vp of 6 5 km s 1 at a depth of 43 km this feature has similar characteristics to the wide angle seismic reflection and refraction velocity model of the w reflector in northern scotland the large scale heterogeneity at gim is attributed to structures associated with the iapetus suture however modelling has been performed using 1 d approximations while the phase signature could result from complex 3 d morphology we therefore conclude only that the results provide evidence of significant lateral variation in subcrustal structure

lispb revisited a new look under the caledonides of northern britain summary the lithospheric seismic profile in britain lispb shot in 1974 included a 700 km long north south profile of the crust and upper mantle of northern britain the wide angle seismic data from lispb was of exceptional quality and coverage making a reinterpretation using current modelling programs worthwhile this reinterpretation shows that the original crustal model published by bamford et al 1978 satisfies the observed first arrival traveltimes but that further detail of crustal velocity structure may be obtained by model shows strong variations in crustal velocity structure that appear to be correlated with geological and tectonic structures mapped at the surface the new lithospheric model is compatible with the observed gravity field when converted to appropriate densities and the variations in loading along the base of the density model show that the lithosphere is close to local isostatic equilibrium profiles of the variation of velocity and density with depth from the new model are compared with measurements of lewisian rocks made at confining pressures up to 8 kbar allowing the seismic model to be interpreted geologically a synthetic normal incidence profile of the new model is superimposed on the nearby nec deep seismic reflection profile and the features imaged by the two techniques are compared