variations in magmatic processes along the east greenland volcanic margin summary seismic velocities and the associated thicknesses of rifted and igneous crust provide key constraints on the rifting history the differentiation between non volcanic and volcanic rifted margins the driving force of magmatism at volcanic margins that is active or passive upwelling and the temperature anomaly in the lithosphere this paper presents two new wide angle seismic transects of the east greenland margin and combines the velocity models with a compilation of 30 wide angle seismic velocity models from several publications along the entire east greenland margin compiled maps show the depth to basement depth to moho crustal thickness and thickness of high velocity lower crust hvlc with velocities above 7 0 km s 1 first we present two new wide angle seismic transects which contribute to the compilation at the northeast greenland margin and over the oceanic crust between shannon island and the greenland fracture zone velocity models produced by ray tracing result in total traveltime rms misfits of 100 120 milliseconds and 2 values of 3 7 and 2 3 for the northern and southern profiles with respect to the data quality and structural complexity 2 d gravity modelling is used to verify the structural and lithologic constraints the northernmost profile awi 20030200 reveals a magma starved break up and a rapidly thinning oceanic crust until magnetic anomaly c21 47 1 ma the southern seismic transect awi 20030300 exhibits a positive velocity anomaly associated with the shannon high and a basin of up to 15 km depth beneath flood basalts between shannon island and the continent ocean boundary break up is associated with minor crustal thickening and a rapidly decreasing thickness of oceanic crust out to anomaly c21 the continental region is proposed to be only sparsely penetrated by volcanism and not underplated by magmatic material at all compared to the vast amount of magmatism further south break up is proposed to have occurred at the seaward boundaries of the continent ocean transition zones at between 50 and 54 ma propagating from north to south based on a joint analysis incorporating transects from the kejser franz joseph fjord and godthab gulf secondly the variation of the hvlc along the east greenland margin from 60 to 77 n and from transects of its conjugate margin shows inverted emplacement of prominent landward and seaward hvlc thickness portions from north to south in a distribution chart the differences in the hvlc distribution are attributed to one or more of the following three models in the first model it is inferred that a transfer zone detachment acts as a barrier to northward magma flow in the second model underplating results in thicker and highly intruded lower crust with several small scale feeder dykes that locally increase the lower crustal velocities in the third model a second magmatic event associated with the separation of the jan mayen microcontinent is considered lithospheric scale inhomogeneities might be responsible for the heterogeneous melt generation the inversion of the hvlc distribution in continental and oceanic domains and differences in its velocities

the computation of body wave synthetic seismograms in laterally homogeneous media synthetic seismograms computed for realistic horizontally stratified media are now routinely used as an aid to seismic interpretation this paper reviews the theoretical background to these methods and presents comparisons of the two popular algorithms reflectivity and wkbj seismograms for a variety of earth models the transformed wave equations are developed from the equations for a spherical gravitating medium in a symmetric form suitable for body wave calculations four methods of solving these equations in general inhomogeneous layers are described the wkbj and langer asymptotic expansions and the wkbj and langer iterative solutions together with the earth flattening transformation and the ray expansion transformed solutions for body waves can then be obtained for realistic layered media four methods of inverting the frequency and wave number transformations are also described the real and complex spectral and slowness methods although realistic seismic models are normally sufficiently complicated that numerical calculations are essential before proceeding with numerical comparisons we have included a review of the canonical signals included in body wave seismograms these analytic results for direct rays partial and total reflections turning rays on forward and reversed branches head waves interface waves airy caustics and fresnel and interface shadows are useful to anticipate and understand numerical problems and results finally a comparison of green s functions for crustal mantle and whole earth models calculated using the wkbj and reflectivity algorithms is included

propagating rifts and continental breakup a simple quantitative model is proposed whereby continental breakup starts with a phase of continental rifting and continues with the propagation of rifts toward locked zones these zones are deformed in the rifting process while oceanic crust is generated in the wake of the rift tip the deformed zones will cause overlaps when a continental reassembly is attempted the model accounts for the shape and distribution of magnetic quiet zones at ocean continent boundaries and for the detailed geometry of some v shaped zones of oceanic magnetic anomalies the former provide an estimate of the scale at which the hypothesis of rigid plates fails and the latter are used to measure the velocity of rift propagation illustrations are provided for present rifting in the red sea and gulf of aden and for early rifting of the south atlantic and breakup of pangea

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

crustal structure of the french guiana margin west equatorial atlantic summary geophysical data from the amazon cone experiment are used to determine the structure and evolution of the french guiana and northeast brazil continental margin and to better understand the origin and development of along margin segmentation a 427 km long combined multichannel reflection and wide angle refraction seismic profile acquired across the southern french guiana margin is interpreted where plate reconstructions suggest a rift type setting the resulting model shows a crustal structure in which 35 37 km thick pre rift continental crust is thinned by a factor of 6 4 over a distance of 70 km associated with continental breakup and the initiation and establishment of seafloor spreading the ocean continent boundary is a transition zone up to 45 km in width in which the two layered oceanic type crustal structure develops although relatively thin at 3 5 5 0 km such thin oceanic crust appears characteristic of the margin as a whole there is no evidence of rift related magmatism either as seaward dipping sequences in the reflection data or as a high velocity region in the lower crust in the p wave velocity model and as a such the margin is identified as non volcanic in type however there is also no evidence of the rotated fault block and graben structures characteristic of rifted margins consequently the thin oceanic crust the rapidity of continental crustal thinning and the absence of characteristic riftrelated structures leads to the conclusion that the southern french guiana margin has instead developed in an oblique rift setting in which transform motion also played a significant role in the evolution of the resulting crustal structure and along margin segmentation in structural style

crustal structure of the northwestern basin and range province and its transition to unextended volcanic plateaus 1 the northwestern margin of the basin and range province is characterized by a transition from low magnitude 20 extension in northwestern nevada to relatively unextended volcanic plateaus in northeastern california seismic velocity and potential field modeling provides new control on the mesozoic to present tectonic evolution of this poorly understood portion of the u s cordillera we document 20 crustal thinning associated with basin and range extension from a crustal thickness of 37 km under northeastern california to 31 km under northwestern nevada that is consistent with the amount of extension recorded in the upper crust in northwestern nevada suggesting the crustal response to extension was relatively homogeneous over the entire crustal column our modeling also shows a well defined 80 km wide zone of unusually low upper crustal velocities 5 9 6 1 km s that coincide with the surface location of sparse cretaceous granites locating the elusive northern extension of the sierra nevada batholith through northwestern nevada for the first time in the subsurface combining geological and geophysical data we reconstruct the late cretaceous to present crustal evolution of this region documenting an interplay between magmatic addition to the crust erosional exhumation sedimentation and extension that has reversed the direction of crustal thinning from a west facing continental margin to an east facing interior basin margin over this time interval finally we find no evidence in northwestern nevada for unusually thick crust 40 km prior to basin and range extension

deep structure of the southern kerguelen plateau southern indian ocean from ocean bottom seismometer wide angle seismic data wide angle seismic data collected during the kerguelen ocean bottom seismometer experiment provide the first images of the deep structure of the southern kerguelen plateau and support a new interpretation of the origin of the plateau velocity models based on travel time inversions and reflectivity synthetic seismograms show a 22 km thick crust composed of 1 6 km of sedimentary cover 5 3 km of upper crust 11 0 km of lower crust and a 4 to 6 km thick reflective zone immediately above moho velocities in the upper crust from 3 8 4 5 km s at top to 6 0 6 5 km s at bottom are consistent with the basaltic nature of this layer the top of which was sampled during the ocean drilling program velocities in the lower crust increase continuously from 6 60 km s at the top to 6 90 km s at 19 5 km depth the reflective zone at the base of the crust identified by wide angle reflections is observed only along the nnw sse direction it consists of alternating high and low velocity layers with an average velocity of 6 70 km s in the nnw sse direction and 6 90 km s in the perpendicular direction strong azimuthal anisotropy is also observed in the upper mantle with velocities of 8 60 and 8 00 km s in the nnw sse and e w directions respectively the absence of high velocities at the base of the crust that characterizes many large volume mafic provinces the reflective lower crust and anisotropy in upper mantle suggest that the southern kerguelen plateau represents a stretched continental fragment overlain by basaltic flows isolated from the antarctic margin during the early opening of the indian ocean

crustal structure of the lofoten continental margin off northern norway from ocean bottom seismographic studies summary in 1988 an extensive geophysical experiment was performed on the lofoten passive continental margin which was formed by the continental break up between greenland and northern norway at 58 ma the geological units of the outer lofoten margin are characterized by seaward dipping reflectors sdrs and landward flood basalt which extends up to 100 km landwards of sdrs in this study we obtain the p wave velocity structure beneath the lofoten basin the sdrs and the landward flood basalt by use of ocean bottom seismograph refraction profiling and we also discuss the formation of the northern norwegian passive continental margin in the lofoten basin the crust is of oceanic type consisting of sedimentary layers oceanic laver 2 4 9 5 5 km s 1 layer 3a 6 3 6 8 km s 1 and layer 3b 7 0 7 1 km s 1 beneath the sdrs the crustal layers are identical to those of the lofoten basin but the thickness of the lower crust which represents the same velocity as layer 3b increases to 5 km towards the continent side and a high velocity lower crustal layer 7 3 km s 1 is formed at the base of the crust the ocean continent transition zone is situated between the landward side of the sdrs and the northward continuation of the voring plateau escarpment in this region the velocity of the lower crust gradually decreases and approaches the lower crustal velocity beneath the lofoten islands 6 8 km s 1 the model also indicates that the high velocity layer disappears in this region comparing our model with the crustal structure on the voring margin it is clear that the lower crustal body 7 km s 1 thickens southwards along the northern norwegian continental margin recent results from petrological and geophysical studies of the generation of the oceanic crust have shown that increasing the temperature of the upwelling asthenospheric material increases the thickness of the oceanic crust we interpret this as that the oceanic crust in the southern area in the voring lofoten margin was generated by hotter material than that of the northern area

imaging beneath an opaque basaltic layer using densely sampled wide angle obs data a combined reflection refraction wide angle seismic survey was conducted on the continental shelf north west of britain using a conventional streamer with an air gun source and static ocean bottom seismometers obs to record wide angle energy the shallow structure down to a basaltic layer was reasonably well imaged on the stacked reflection section the basalts however proved to be opaque to the conventional reflection method and prevented the imaging of deeper horizons where an important velocity inversion was anticipated this paper reports on the processing modeling and interpretation of the densely sampled wide angle obs data that were coincident with the reflection profile eleven obs instruments were deployed along a 75 km line and recorded signal from a powerful 149 liter 9100 in sup 3 air gun array fired every 50 m data processing was performed using a standard industrial reflection seismic software package prior to first arrival picking processing steps included geometry definition trace summation and display of the data using various scaling algorithms an initial model was constructed from 1d velocity time profiles digitized every 4 km along the stacked section first arrival travel time modeling rapidly converged to a detailed model of the structure of the top 5 km of the crust modeling revealedmore the existence of a buried low velocity mesozoic sedimentary basin of a prominent basement horst and of a normal fault penetrating to the basement less

the spacing of rift axis highs dependence upon diapiric processes in the underlying asthenosphere abstract detailed bathymetric mapping has revealed a near harmonic distribution of intermediate wavelength morphologic highs on slow to fast spreading centers one model suggests that the highs may result from the thermal instabilities generated by the interaction of cold fracture zone walls with the axial asthenosphere this may force a deepening of the isotherms near the fracture zone and an uplift of the isotherms midway between the fracture zones a preferred model based on newly accumulated data suggests that the highs may be a result of upwelling within the asthenosphere underlying the ridge axis where a linear region of instability develops at the base of a partial melting zone less dense material at depths of 40 80 km may penetrate the overlying material at discrete and ideally evenly spaced points harmonically spaced upwellings should generate excess magma at discrete points on the ridge axis creating volcanic highs the spacing between highs may depend upon the continuity and thickness of the deep axial instability layer the temperature and viscosity ratios within the magma and the distance between major transform faults instability of the diapirs may stimulate the migration of the highs through time

the east pacific rise near 21 n 13 n and 20 s inferences for along strike variability of axial processes of the mid ocean ridge abstract data obtained along various segments of the mid ocean ridge mor are used to construct an idealized model for crustal accretion the model seeks to predict the topographic volcanic tectonic and hydrothermal characteristics of any given spreading segment of the mor as a function of distance away from the bounding transform faults this model is based on a series of detailed mapping efforts carried out on segments of the mor having a broad range of spreading rates between 2 and 16 cm yr this paper includes the results of the french searise program carried out during the summer of 1980 aboard the n o jean charcot and two american cruises conducted in 1981 aboard the r v melville

crustal structure of a nonvolcanic rifted margin east of newfoundland a detailed seismic refraction study was undertaken of the rifted continental margin southeast of the grand banks of newfoundland along a preexisting deep multichannel seismic reflection profile the primary objectives were to determine the topography of moho across the margin to detect any evidence for crustal underplating and to determine the position of the continent ocean boundary data were obtained from nine ocean bottom seismometers deployed along the profile with an array of six 1000 inch3 air guns as the source two additional refraction lines were shot parallel to structure over the continental slope the resultant velocity model is well constrained and consistent with the observed gravity the crust thins abruptly beneath the continental slope to a major structural transition identified as the continent ocean boundary there is no significant magmatic underplating of the thinned continental crust seaward of the transition the seismic velocity structure is characterized by a 50 km wide zone of intermediate velocity 7 2 7 6 km s beyond this is a thin 3 5 km crust with low velocity 4 5 5 km s and high gradient over mantle with a velocity of 7 7 km s increasing to 8 0 km s the zone of intermediate velocity is interpreted to be highly serpentinized mantle material beneath a very thin crust suggesting little or no magmatic activity during and immediately after rifting this interpretation is consistent with results from adjacent and conjugate margins

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

lead isotope evidence for the structure of the rockall dipping reflector passive margin the passive continental margin south west of rockall plateau is characterized by a thick sequence of oceanward dipping seismic reflectors during leg 81 of the deep sea drilling project these reflectors were sampled at site 553 and proved to consist almost exclusively of basalt here we present lead isotope data which indicate that these basalts may have been contaminated by ancient uranium depleted continental crust or alternatively derived from a sub continental lithospheric mantle source in either case the implications are that the basalts of the south west rockall plateau formed by eruption through and onto continental basement not by subaerial seafloor spreading this conclusion is in accord with gravity models of the area which predict stretched continental crust beneath the dipping reflector sequence

crustal structure of the madeira tore rise eastern north atlantic results of a dobs wide angle and normal incidence seismic experiment in the josephine seamount region summary during november december 1988 an extensive geophysical data set was collected over the josephine seamount which is located at the northeasterly end of the madeira tore rise in the eastern north atlantic the josephine seamount lies at the intersection of the madeira tore rise and azores gibraltar fracture zone the latter representing the africa eurasia plate boundary in this part of the eastern north atlantic from this data set a 275 km long explosive refraction line has been modelled together with wide angle airgun profiles seismic reflection and gravity data the velocity depth model shows that the crust either side of the seamount is typically oceanic in character however beneath the seamount there exists a region of anomalously high velocity and crustal thickening to a depth of about 17 18 km gravity modelling also suggests that the josephine seamount is compensated by a crustal root and that the josephine seamount madeira tore rise system is in local isostatic equilibrium calculations of the flexural rigidity and effective elastic thickness of the lithosphere in this region suggest that the madeira tore rise formed contemporaneously with the lithosphere on which it lies this age of crustal loading is consistent with the proposal that the madeira tore rise is an aseismic ridge which formed at or near the mid atlantic ridge

from orogenic collapse to rifting a case study of the northern porcupine basin offshore ireland abstract offshore ireland the north atlantic opening is generally interpreted as successive 10 to 15 myr long rifting events during the mesozoic however their interaction is poorly documented in terms of structural inheritance and fault reactivation from extensive seismic and well data in the northern porcupine basin we show that extension actually evolved over an abnormal period of ca 220 myrs with overlapping tectonic stages that all together reflect the irish atlantic margin construction from the palaeozoic during the carboniferous extension initiated throughout the variscan and caledonian thickened crusts and controlled the deposition of generally slightly tilted continental clastics two subsequent rifting events during the triassic lower jurassic and upper jurassic lower cretaceous controlled shallow to deep marine sedimentation in separated sub basins that progressively widen and connect once extension localised along bounding faults we propose that extensional tectonics first controlled by a general orogenic collapse reflecting an early stage of stretching is then followed by several rifting periods that ultimately evolved toward the hyper thinning of the margin and shaped an aborted rift propagator this evolution is due to pre existing orogenic related structures and boundary conditions variations such a continuum of deformation although pulsed implies varying structural interactions of crustal scale that are often recorded in syn tectonic sediments

influence of the iceland mantle plume on oceanic crust generation in the north atlantic when a mantle plume with elevated temperature underlies an oceanic spreading centre it affects the generation of oceanic crust by creating thicker crust we map the variation in crustal thickness and seismic velocity along three long offset seismic profiles acquired over oceanic crust generated shortly after continental breakup in the north atlantic a 212 kmlong flowline from the faroes rifted continental margin across crust of 51 42 ma age where oceanic spreading developed close to the inferred centre of the iceland mantle plume a 256 km flowline extending from the hatton rifted continental margin across crust of 52 40 ma age about 800 km south of the presumed centre of the mantle plume and a 99 km strike line over oceanic crust formed at 43 ma in the iceland basin off the hatton continental margin the crustal velocity structure along each profile is constrained by multichannel seismic reflection data which is used primarily to map the sediments and by densely spaced ocean bottom seismometers which recorded wide angle reflections and refractions to offsets of more than 100 km over 56 000 crustal diving wave and moho wide angle reflection arrivals were used in joint crustal refraction and reflection tomographic inversions quantitative error analysis shows that the seismic velocity of the crust is mostly constrained to within 0 1 km s 1 and the depth of the moho to within 250 m we interpret the crustal thickness and velocity changes along the profiles as caused primarily by changes in the mantle temperature at the time of crustal formation if all the oceanic crustal thickness variations are ascribed to mantle temperature changes we infer that as mature seafloor spreading developed following continental breakup the mantle cooled by ca 75 c over a 10 myr period although it still remained hotter than the global average of normal oceanic crust the crust formed close to iceland is at all times thicker than that formed further away which we interpret as reflecting higher temperatures close to the centre of the thermal anomaly created by the mantle plume currently at the reykjanes ridge south of iceland we interpret thicker than normal oceanic crust as being caused by the presence of hotter mantle modulated by thickness variations of 1 5 2 0 km which are attributed to temporal variations in the mantle plume temperature of about 25 c on a 3 6 myr timescale a 1 5 km increase in thickness of oceanic crust generated between 48 and 45 ma on the faroes line is similar in magnitude and duration to those occurring on the present day reykjanes ridge which we suggest is due to a temperature pulse of 25 c gravity lineations in the northern north atlantic suggest that the oceanic crust has exhibited small thickness fluctuations of similar size throughout its history interpreted as due to small fluctuations in the temperature of the iceland mantle plume

basin structure on the norwegian margin from analysis of digitally recorded sonobuoys during r v robert d conrad cruise 21 a large number of sonobuoys were recorded in the voring more and adjacent basins at the margin off norway the digital sonobuoy data have been transformed into the domain of intercept time tau and ray parameter p and inverted by the tau sum method to obtain better definition of the near seafloor velocities using also the precritical energy a move out correction has been applied to the tau p data conventional analyses of the x t data were also performed in general the various types of analyses complement each other and should be used interactively together with forward modeling to obtain the optimal velocity depth function the sonobuoy results have outlined in a regional sense the pre cenozoic sedimentary basin at the margin this basin attains a maximum sediment thickness of 8 10 km of which only the upper half is defined by seismic reflection profiling at the outer margin the marginal escarpments form major discontinuities in the lateral velocity distribution the survey has proven that sonobuoy profiles recorded during standard seismic reflection profiling in major marginal basins may provide important new information particularly about the deeper parts of the basins that is not directly available from the reflection profiles

frequent shifts of the volcanic zone in iceland the heterogeneous and locally anomalously thick icelandic upper crust cannot be fully explained with current models of crustal construction a model that invokes frequent shifts of the volcanic zone accreting plate boundary is proposed as an alternative three important parameters which result from a complex history of shifting determine the upper crustal structure of iceland 1 the time interval between two successive shifts 2 the scale of lateral displacements of the volcanic zone and 3 the intensity of volcanism at each volcanic zone

continent ocean transition and voluminous magmatic underplating derived from p wave velocity modelling of the east greenland continental margin summary deep seismic refraction data were gathered across the entire east greenland rifted margin north of the jan mayen fracture zone between 72 n and 75 n in 2003 investigations of the deep crustal structure of this continental margin provide constraints on the formation of the margin and its structural evolution during and after late cretaceous early tertiary rifting and continental break up we present here the results along two profiles located in the prolongation of the godthab gulf and the kejser franz joseph fjord regional p wave velocity models were derived from forward traveltime modelling of land stations and ocean bottom hydrophone obh recordings for the first time long deep seismic sounding transects off east greenland provide a full insight into the crustal architecture of the transition from continental to oceanic crust a mean result is the identification of voluminous magmatic underplating which is wider and thicker than previously thought p wave velocities of the underplated material range between 7 1 and 7 4 km s 1 and the horizontal extents on the profiles are 225 and 190 km the maximum thickness of the underplated material is 15 16 km furthermore the p wave velocity models reveal a 120 130 km wide continent ocean transition zone cot based on an interpretation of the extent of cretaceous syn rift sediments mixed with basaltic intrusions and the lateral increase of velocities in the crustal layers excess magmatism must have been present during a long term rifting process accompanying the extension of the continental crust and giving rise to the voluminous magmatic underplating a consequence of our interpretation of the seismic refraction data is a likely rift propagation in the greenland sea from north to south additionally a comparison of p wave velocity models of the east greenland margin and voring margin reveals significantly asymmetric crustal architectures the voluminous magmatic underplating and asymmetrical conjugate margins formations are considered as a mirror of complex pre and syn rift processes

segmentation of mid ocean ridges studies of mid ocean ridges in the pacific and atlantic oceans show that the volcanism that forms the oceanic crust along the spreading plate boundaries is concentrated at regular intervals related to spreading rate this observation and a new calculation for a rayleigh taylor type of gravitational instability of a partially molten mantle region growing under spreading centres yield reasonable estimates of upper mantle viscosities

aeromagnetic study of the continental crust of northeast greenland a reconnaissance type aeromagnetic survey of northeast greenland was undertaken by the alfred wegener institute for polar and marine research between 1993 and 1996 these data were compiled in a regional magnetic anomaly map at 3700 m altitude covering the caledonian orogen of east greenland and parts of the precambrian shield of north greenland the survey area was divided into regions with distinct magnetic anomalies the interpretation of the magnetic anomalies is based on comparison with exposed geological structures and is locally supported by in situ measurements of magnetic susceptibilities middle proterozoic magmatic rocks determine the magnetic character of the precambrian shield of north greenland rifting and downfaulting of the precambrian basement at its northern and eastern margin are reflected by gradients in the magnetic field the caledonian fold belt of east greenland could be subdivided into a northern only weakly magnetic part where pervasive caledonian reworking probably demagnetized the crust and a southern part where isolated positive anomalies occur magnetite survived caledonian metamorphism in these areas the sensitivity of magnetite to caledonian metamorphism was recognized in several places in the caledonides of east greenland this observation might be useful in determining the much debated degree of caledonian reworking of the crystalline complexes of east greenland

volcanism on the rockall continental margin continental break up in the northern north atlantic was accompanied by massive volcanism this produced sequences of basalt flows erupted close to sea level which are imaged as seaward dipping reflectors on seismic reflection profiles they comprise the upper 4 5 km of transitional crust in both the edoras bank and hatton bank areas of the rockall continental margin detailed surveys show that the reflectors are convex upward on dip lines but are sub horizontal on strike lines they are probably produced by aerially extensive basalt flows extruded from linear fissure vents aligned parallel to the rift axis along the rockall margin two sequences of seaward dipping reflectors occur separated by an 9outer high9 near the foot of the continental margin the landward set of basalt flows are emplaced above stretched and intruded continental crust while the seaward set span the region from highly distended continental crust to fully oceanic crust the outer high is capped by basalt flows but exhibits little coherent internal reflectivity we discuss possible origins of the outer high as a residual block of thinned and intruded continental crust or as a central volcano cut by fissure vents similar to those found in the neovolcanic zones of iceland

the oceanographer fracture zone i crustal structure from seismic refraction studies summary this paper presents the results of four lookm long seismic refraction lines carried out in and around the active transform of the oceanographer fracture zone which offsets the mid atlantic ridge rightlaterally by 130 km near 35 n the seismic data are interpreted using standard travel time inversions the time term method and two dimensional ray tracing techniques the results show that beneath the oceanographer fracture zone the crust is typically only 4 5 km thick it consists of a thin upper crustal layer in which the p wave velocity increases rapidly with depth and a main crustal layer with velocity increasing from 6 4 km s l at the top to 6 8 km s at the base of the crust some degree of crustal thinning is found to extend for distances of 30 40 km on either side of the fracture zone axis in addition to this a small area 10 15 km long of crust less than 2 5 km thick is found beneath the centre of the transform valley crust less than 4 km thick is also observed on a smaller offset fracture zone 85 km to the south of the oceanographer but no evidence is found for significantly thin crust beneath another small offset fracture zone 65 km to the north of it we conclude that the moderate thnning of the crust at the fracture zone is due to a progressive reduction in the crustal production rates of the oceanic spreading centres as they approach the transform fault however we attribute the occurrence locally of extremely thin crust to a different process in which a thin lid of mafic and ultramafic rocks overlying unaltered upper mantle material is accreted at the junction of a spreading centre and a transform fault this may happen if the accretion rate at the end of the spreading centre becomes so slow that the usual processes which give rise to normal oceanic crust can no longer operate it therefore seems that there are two distinct types of crust either of which may be found at different points along a fracture zone this might account for the extreme variability in crustal thickness found in different fracture zones by previous studies

digital image processing of seabeam bathymetric data for structural studies of seamounts near the east pacific rise digital marine bathymetric data are rapidly increasing in both volume and variety these data provide new perspectives on the morphology of the seafloor they also present a number of problems with regard to developing display methods that convey the large amount of information contained in these data we have used digital image processing techniques to reduce and display seabeam multibeam data for three young less than 5 ma seamounts on the eastern flank of the east pacific rise navigational errors in location were corrected by identifying offsets between features observed in crossing tracks and then translating the tracks relative to a control track images of the seamounts were then generated using spatial filtering techniques the images reveal more information than contour maps because 1 more depth intervals can be displayed and 2 the continuous tone nature of the images shown in shaded relief or color coded form more readily conveys morphologic information the images suggest a significant degree of structural control on seamount shape more so than can be delineated by examination of contour maps alone in one case a chain of seamounts strikes at an angle perpendicular to the abyssal hill direction although each seamount within the chain is elongate in a direction parallel to the hills in another case a polygonally shaped flat topped seamount overlies a region of the crust where the abyssal hills display two strike directions images of the third seamount demonstrate a complicated structural control on morphology including a control on the distribution of satellite constructs finally the image presentations clearly depict regions where cross tracks could not be registered because of 1 residual navigation errors and 2 the presence of steep slopes where the depth and location estimates can vary depending on the angular relationship between the ship track azimuth and the strike of the slope

on the evolution of rifted continental margins comparison of models and observations for the nova scotian margin summary a model for the formation and evolution of rifted continental margins based on lithospheric extension during rifting and its thermal and mechanical consequences is proposed model predictions are then compared with geological and geophysical observations from a transect of the 185 ma old rifted margin off nova scotia through the scotian basin three kinematic models of the rifting process are discussed these are 1 the uniform extension model in which the amount of extension is uniform with depth but varies with position across the margin 2 the uniform extension and melt segregation model which has similar properties but also provides an explanation for the properties of the extended continental crust and its transition to oceanic crust by postulating that basaltic melt segregates from the asthenosphere and migrates to the crust and 3 the depth dependent extension model in which the first order consequences of rapidly changing rheological properties with depth in the lithosphere are included by decoupling the lithosphere into two zones with depth each of which undergoes differing amounts of extension these rift models predict the form of crustal and lithospheric thinning subsidence and temperature change once the amount of extension has been determined this is estimated from seismic measurements of present crustal thickness on the assumption that the crust had a uniform thickness equal to that currently measured in the adjacent continental region before rifting occurred rifting is also considered to occur instantaneously a time stepping thermo mechanical model is used to predict the cooling of rifted margin additional thermal contraction subsidence and its amplification by water and sediment loading thermal aspects are calculated using a finite difference model of time dependent conductive heat transport whereas regional isostatic response to loading is calculated by a finite element model the models are coupled because the temperature distribution is used to define a rheological lithosphere an elastic region with thickness that varies in time and space as the model evolves for flexural calculations in the mechanical model secondary coupling occurs through perturbations to the temperature field by sedimentary thermal blanketing and advection of heat during isostatic adjustment the model predictions of 1 sedimentary basin stratigraphy 2 moho position 3 free air gravity anomaly 4 age depth relations for deep exploratory wells and 5 subsidence and temperature histories agree well with observations from the scotian basin additional effects due to sedimentary and crustal radiogenic heat production lateral heat transport and the possible existence of a near surface brittle listric faulted region created during rifting are also considered it is concluded that a model in which rifting occurred by depth dependent extension and which includes a finite thickness for the rheological lithosphere radiogenic heat production in the sediments and crust and a brittle listric faulted crustal layer affords an accurate description of the first order processes that occur during rifting and evolution of this margin

the continent ocean boundary at the rifted margin off eastern canada new results from deep seismic reflection studies seismic reflection data were collected and processed to 20 s two way travel time along four lines which cross the rifted continent ocean boundary off the grand banks region of eastern canada specifically to examine the origin age and nature of this fundamental boundary this represents the first regional study of its kind the most important result is the presence of landward dipping reflectors near the foot of the continental slope these occur where oceanic crust appears to terminate against the continent we suggest that the dipping reflectors mark the continent ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary sedimentary basins lie just landward of the continent ocean boundary their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting and this may be an important stage leading to continental breakup rift basins formed further landward on the grand banks do not exhibit this thinning other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent ocean boundary also the oceanic crust exhibits a complexity of reflections some of which may be due to compositional zonation during magmatic crystallization finally our results may have important consequences for continental predrift reconstruction as oceanic crust appears to extend farther landward in the newfoundland basin than some recent studies have indicated

the ocean continent boundary off the western continental margin of iberia ii crustal structure in the tagus abyssal plain summary an 80 km long reversed seismic refraction line line 5 was shot over the tagus abyssal plain off portugal the main p wave reflected and refracted phases were modelled both for traveltime and amplitude the resulting p wave velocity depth model has the following features a an extremely thin crust of about 2 km b the absence of oceanic layer 3 and c very low upper mantle velocities between 7 6 and 7 9 km s this very unusual seismic velocity crustal structure is quite unlike thinned continental crust but is remarkably similar to the seismic crustal structures found at atlantic fracture zones and in particular to the structures found in profiles shot along the transform valley and near ridge transform intersections a magnetic anomaly chart seems to allow the possibility of several fracture zones one of which could intersect the centre of line 5 as an alternative to the fracture zone hypothesis we show that if the oceancontinent transition in the tagus abyssal plain is located at about 11 30 w in a symmetric position with respect to the ocean continent transition in the conjugate south newfoundland basin then magnetic anomalies can be modelled simply by assuming sea floor spreading west of 11 45 w at 10 mm yr beginning at m11 time 133 myr bp and blocks of rifted continental crust to the east the location of the proposed ocean continent transition in the tagus abyssal plain is marked by a well defined n s linear magnetic anomaly which is adjacent to the oldest sea floor spreading block east of the proposed ocean continent transition there is an increase in the depth to basement similar to that found east of the ocean continent transition in the iberia abyssal plain and elsewhere this model also allows us to explain why purdy s 1975 seismic refraction line a ar in the tagus abyssal plain cannot be interpreted as a conventional reversed pair because most of line a was shot over the ocean continent transition zone and most of line ar over thinned continental crust remarkably similar velocity depth structures to that under line 5 are found close to the ocean continent transition zone off the whole of western iberia in areas which show no clear evidence of fracture zones therefore it appears more likely that the seismic structure of line 5 is due to its proximity to the ocean continent transition than to a local association with a fracture zone and further that its structure is typical of this transition off the western margin of iberia we also suspect that the low upper mantle velocities associated with the ocean continent transition indicate the widespread occurrence of serpentinized peridotite

coincident seismic reflection refraction studies of the continental lithosphere a global review nearly 50 coincident seismic reflection refraction studies to depths of at least the moho provide an improved understanding of the continental lithosphere some conclusions include the following 1 a transparent upper crust a common observation on vertical reflection profiles cannot generally be correlated with velocity gradients or low velocity zones rather a commonly transparent upper crust may be explained by short wavelength steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source generated noise 2 the reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity 6 6 7 3 km s and to consist of laminated high and low velocity layers with typical thicknesses of 100 200 m 3 landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high and low velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin 4 the crust mantle boundary may differ sufficiently when imaged with vertical incidence and wide angle data to justify the retention for the present of the concept of separate reflection and refraction mohos while there is good evidence that these features are coincident within measurement uncertainties in most regions recently recorded data from the basin and range admit the possibility for noncoincidence in that area 5 upper mantle reflections which cannot be migrated into the lower crust remain rare despite isolated unequivocal examples thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust the wide angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come 6 there appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes 7 no truly three dimensional coincident experiment i e including three dimensional migration has been conducted but some three dimensional data have been collected using both methods measurements of attenuation poisson s ratio and anisotropy within the crust using coincident data sets remain frontiers

crustal structure interpreted from magnetic anomalies this review discusses publications during the last quadrennium 1987 1990 that used aeromagnetic data marine magnetic data satellite magnetic data and rock magnetic and petrologic data to provide information on the sources of magnetic anomalies the publications reviewed reflect increased integration of rock magnetic property and petrologic studies with magnetic anomaly interpretation studies particularly in deep crustal magnetization exploration for hydrocarbons and inversion of marine magnetic anomalies interpretations of aeromagnetic data featuring image display techniques and using the horizontal gradient method for locating magnetization boundaries became standard

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

seismic reflection signature of cretaceous continental breakup on the wilkes land margin antarctica the passive rifted continental margin of wilkes land antarctica is characterized on seismic reflection records by i in the south a block faulted sequence of highly stratified continental beds overlain by two distinct unconformities ii a transitional greatly thinned continental crust overlain by material interpreted to be flood basalt and iii in the north oceanic crust with a boundary ridge at its edge the mohorovicic discontinuity can be followed across the continent ocean boundary and shows a progressive thinning of continental crust to a minimum of 2 5 kilometers at its northern edge

crustal structure of the northern part of the voring basin mid norway margin from wide angle seismic and gravity data abstract three regional ocean bottom seismograph obs profiles with a total of fifteen recovered obss were acquired in the northern part of the voring basin mid norway margin in 1996 the data have been modelled by inversion and forward modelling of the obs vertical components p waves the velocity is found to increase downwards within the sedimentary layers due to increasing depth of burial confining pressure within the two deepest sedimentary layers there seems to be an increase of 0 5 1 km s in velocity northwestwards this increase is most likely caused by high velocity sill intrusions in the sedimentary rocks emplaced during the rift episode leading to early eocene opening of the ne atlantic the presence of significant amounts of sills seems to terminate close to the nyk high along profile 5 dip profile and close to a dome northwest of the shelf edge along profile 6 dip profile the velocity in the top of this dome is anomalously high 3 4 km s at only 1 km depth beneath the seafloor suggesting that the high consists of volcanic rocks or heavily intruded sedimentary rocks the relatively low velocities derived from within the upper crystalline crust 5 9 6 2 km s confirms that the crust is of continental origin within the lower crust there seems to be a clear increase in velocity northwestwards from about 7 0 km s to about 7 4 km s suggesting that the amount of high velocity intrusions underplating in the lower crust decreases landwards the modelling of the strike profile profile 7 suggests that the transition from a lower crust dominated by magmatic intrusions to a more undisturbed continental lower crust is located approximately beneath this profile the model for profile 7 ties closely southwestwards to the models derived from obs data acquired in 1992 and northeastwards to lofoten obs data acquired in 1988 the models indicate that the thick magmatic intrusions in the sedimentary rocks and lower crust extend further landwards in the area of the 1992 survey and that the transition zone is related to the surt lineament the distribution of magmatic rocks thus seems to correlate strongly with pre breakup structures in addition to the p wave modelling gravity modelling has been performed along all profiles the gravity modelling confirmed all main aspects of the obs models and provided constraints on the crustal structures towards the ends of the profiles

three dimensional model of an ultramafic feeder system to the nikolai greenstone mafic large igneous province central alaska range the amphitheater mountains and southern central alaska range expose a thick sequence of triassic nikolai basalts that is underlain by several mafic ultramafic complexes the largest and best exposed being the fish lake and tangle fl t mafic ultramafic sills that flank the amphitheater mountains synform three dimensional 3 d modeling of gravity and magnetic data reveals details of the structure of the amphitheater mountains such as the orientation and thickness of nikolai basalts and the geometry of the fl t intrusions the 3 d model 50 70 km includes the full geographic extent of the fl t complexes and consists of 11 layers layer surfaces and properties density and magnetic susceptibility were modified by forward and inverse methods to reduce differences between the observed and calculated gravity and magnetic grids the model suggests that the outcropping fl t sills are apparently connected and traceable at depth and reveals variations in thickness shape and orientation of the ultramafic bodies that may identify paths of magma flow the model shows that a significant volume 2000 km3 of ultramafic material occurs in the subsurface gradually thickening and plunging westward to depths exceeding 4 km this deep ultramafic material is interpreted as the top of a keel or root system that supplied magma to the nikolai lavas and controlled emplacement of related magmatic intrusions the presence of this deep keel like structure and asymmetry of the synform supports a sag basin model for development of the amphitheater mountains structure and reveals that the feeders to the nikolai are much more extensive than previously known

eocene post rift tectonostratigraphy of the rockall plateau atlantic margin of nw britain linking early spreading tectonics and passive margin response a regional study of the eocene succession in the uk sector of the rockall plateau has yielded new insights into the early opening history of the ne atlantic continental margin data acquired from british geological survey borehole 94 3 on the rockall high provides a high resolution record of post rift early to mid eocene subaqueous fan delta development and sporadic volcanic activity represented by pillow lavas tuffs and subaerial lavas this sequence correlates with the east rockall wedge which is one of several prograding sediment wedges identified across the rockall plateau whose development was largely terminated in the mid lutetian linking the biostratigraphical data with the magnetic anomaly pattern in the adjacent ocean basin indicates that this switch off in fan delta sedimentation and volcanism was coincident with the change from a segmented transform margin to a continuously spreading margin during chron c21 however late stage easterly prograding sediment wedges developed on the hatton high during late mid to late eocene times these can only have been sourced from the hatton high which was developing as an anticline during this interval this deformation occurred in response to mid to late eocene compression along the ocean margin possibly associated with the reorganisation to oblique spreading in the iceland basin which culminated at the end of the eocene with the formation of the north hatton anticline and the deformation tilting of these wedges a series of intra eocene unconformities of which the mid lutetian unconformity is the best example has been traced from the rockall plateau to the faroe shetland region and onto the greenland conjugate margin bordering the early ocean basin whilst there appears to be some correlation with 3rd order changes in eustatic sea level it is clear from this study that tectonomagmatic processes related to changes in spreading directions between greenland and eurasia and or mantle thermal perturbations cannot be discounted

evidence for a thick oceanic crust adjacent to the norwegian margin the oceanic crust created during this first few million years of accretion in the norwegian greenland sea lies at an unusually shallow depth for its age has a smooth upper surface and in many places the results of multichannel seismic reflection profiling reveal that its upper layers comprise a remarkable sequence of arcuate seaward dipping reflectors these have been attributed to lava flows generated during a brief period of subaerial seafloor spreading we describe the results of inversions of digitally recorded sonobuoy measurements and two ship expanded spread profiles collected over the oceanic crust adjacent to the norwegian passive margin we find that the crust of the deep lofoten basin is indistinguishable from normal oceanic crust in thickness and structure closer to the margin we observe up to a four times expansion in thickness of layers with velocities equal to those of oceanic layer 2 while the layer 3 region retains approximately the same thickness the area over which the seaward dipping reflectors can be observed on reflection profiles corresponds to the region of greatest expansion in layer 2 thickness in the very oldest crust immediately adjacent to an escarpment that probably marks the continent ocean boundary we see evidence for a low velocity zone overlying an indistinct reflector that may mark the dyke lava interface in the thick crust comparing the structure of the thick crust to that of eastern iceland we find a strong resemblance especially in the expansion in thickness of material with layer 2 velocities these results support the suggestion that during the earliest stages of spreading extrusive volcanism at the ridge crest was unusually voluminous building a thick pile of lavas erupted from a subaerial spreading center

the structure and subsidence of rockall trough from two ship seismic experiments coincident multichannel seismic reflection and refraction profiles acquired northwest of britain during a two ship seismic experiment in the southern rockall trough provide evidence for a thin 6 km syn rift crust below rockall trough the variation of seismic velocity with depth is consistent with either oceanic crust similar to that found in other parts of the north atlantic or with thinned continental crust heavily intruded by syn rift igneous rocks the syn rift crust beneath rockall trough is now buried by nearly 5 km of sediments that are intruded by tertiary volcanics varying in extent from isolated sills to a large sill complex extending 100 km laterally the basement imaged on seismic profiles exhibits a band of subhorizontal reflectors that extend laterally over distances of up to 40 km which they interpret as sediments intercalated with submarine lava flow generated during the opening of the trough estimates of stretching from crustal thinning and from subsidence indicate that rockall trough has undergone extensive rifting beta 6 upwelling asthenosphere beneath the thinned lithosphere generated at least 1 3 km thickness of melt as it decompressed the molten rock rose upward from the mantle until it was in part extruded as lava flows and in partmore intruded into the crust less

a geophysical study of the hatton bank volcanic margin a summary of the results from a combined seismic gravity and magnetic experiment abstract this paper summarizes the results of a geophysical study of the hatton bank volcanic margin in the north atlantic normal incidence and synthetic aperture profiles saps show seaward dipping reflectors interpreted as subaerial basalt flows on the continental slope and in the adjacent ocean basin velocity depth profiles derived from expanding spread profiles esps shot along strike show elevated velocities in the lower crust on the continental slope the high velocities are interpreted as evidence for igneous underplating or intrusion gravity modelling shows high densities in the lower crust supporting a model of igneous accretion an ocean bottom seismometer obs and multichannel refraction line shot perpendicularly to strike through the midpoints of the esps further constrained the velocity structure across the margin the velocity model was refined as each dataset was processed clearly showing that multiple datasets were required in order to constrain the velocity structure tightly in particular esps were found to produce misleading results in places where the assumption that the sub surface structure is one dimensional inherent in the esp technique was not valid the final velocity model is used to infer the thickness of igneous material accreted across the hatton margin which is estimated to increase westwards from zero at the top of the bank to 18 km at the bottom of the continental slope and then thin rapidly to 11 km in the adjacent iceland basin as the oceanic crust thins its seismic velocity decreases suggesting a decrease in percentage partial melting these observations strongly support a model in which volcanic margins are produced by elevated asthenospheric temperatures

the crustal structure of the madagascar ridge summary the madagascar ridge is an elongated aseismic plateau in the southwest indian ocean which separates two ocean basins of mid to late cretaceous age from interpretations of a reversed refraction line on the ridge and a continuous gravity profile across it we conclude that it formed during the cretaceous as thickened oceanic crust at a mid ocean ridge which possibly overlay a mantle hot spot the seismic data consist of record sections from five recording sonobuoys forty one shots were fired along a 210 km long nne ssw line across the central part of the ridge simple horizontal layer analysis of first arrivals reveals a lower crustal refractor with vp 7 kms 1 at a depth of 8 8 5 km and the moho at an apparent depth of 18 km but fails to characterize the crustal structure as either oceanic or continental we obtain a definitively oceanic structure by using synthetic seismogram techniques together with tau p analysis and second arrival correlations the model which best fits the data consists of 2 2 km of water and 1 km of sediments overlying a velocity gradient from 4 to 7 kms 1 beneath this at a depth of 7 5 km is a 9 5 km thick layer with vp 6 9 7 1 kms 1 a 7 5 kms 1 layer not observed in the first arrival analysis is found at the base of the crust the moho is a transition zone 1 km thick and occurs at 25 km depth the gravity data consist of a continuous ne sw profile across the ridge and on to the flanking basins two dimensional gravity modelling allows a structure consistent with the synthetic seismogram results and supports the existence of a high density basal layer in the crust we use in addition a time series analysis approach to estimate the depth of compensation assuming an airy model and lithospheric thickness assuming an elastic plate model convolution of bathymetry along the profile with a range of model filters gives minimum rms gravity residuals for a crustal compensation depth of 23 5 km and a plate thickness of less than 2 5 km

effects of canary hotspot volcanism on structure of oceanic crust off morocco analysis of over 6400 km of multichannel seismics mcs and 50 sonobuoy reflection and refraction experiments reduced both in the domain of x t and tau p shows that a region within the jurassic quiet zone off morocco underwent dramatic changes as a result of the passage of the lithosphere over the canary hotspot commencing approximately 60 ma a seismic unit ucf interpreted as volcanic in origin is observed within the sediments in a region characterized by a broad bathymetric swell it shows diffractions from its upper surface and an internally chaotic seismic facies and pinches out between sedimentary units of continuous subparallel facies a velocity inversion is noted between the ucf 4 7km s and the underlying sediment 3 1 km s the ucf is time transgressive it lies near the cretaceous s tertiary boundary in the northern portion of the study area and is younger to the south kinematic studies of the movement of the canary hotspot relative to africa show that the hotspot first appeared off nw africa about 60 ma and was located beneath oceanic crust in the region where the ucf is observed depth to basement measurements in areas not effected by the hotspot show a consistent linear trend of increased depth with age in areas effected by the hotspot the thermal rejuvenation is evident as basement depths shoal with increased proximity to the present hotspot the reheating of the crust resets the thermal age of the lithosphere with many of the properties of crust of a younger age subsidence curves of the reheated crust off morocco show good correlation to subsidence curves of other reheated crust on a global basis a zone characterized by high crustal velocities 7 1 7 4 km s and greater crustal thicknesses by 1 2 km is observed in an area that corresponds to the bathymetric swell the region of the ucf and the reelevated basement the high velocities and increased crustal thickness are interpreted to be the result of underplating and assimilation of existing oceanic crust caused by the canary thermal anomaly the presence of high crustal velocities coupled with a thickened crustal section has been noted on various passive margins of the world they have generally been attributed to the thermal processes associated with continental rifting off morocco we believe that similar thermally induced phenomena have occurred but that here the heat anomaly was the midplate volcanism associated with the canary hotspot

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

crustal structure of the mid atlantic ridge at 23 n from seismic refraction studies an explosive seismic refraction experiment using four ocean bottom hydrophone receivers was carried out along a 120 km long section of the mid atlantic ridge mar median valley at latitude 23 n immediately south of its intersection with the kane fracture zone the data are interpreted in terms of laterally homogeneous horizontally layered structures using travel time inversion and synthetic seismogram modeling techniques these solutions are combined using two dimensional ray tracing to produce a single model defining the major along axis structural changes occurring beneath this section of the median valley with the exception of slightly low layer 3 velocities the crust beneath several tens of kilometers of the mar median valley has all the characteristics of simple mature ocean crust including a well defined moho transition zone 8 km s upper mantle velocities and a total crustal thickness of 6 7 km no evidence is found for the presence of a steady state axial magma chamber in the crust or upper mantle and the excellent propagation characteristics and orderly amplitude versus range relationships imply the existence of a considerable degree of lateral homogeneity beneath the median valley over distances of several tens of kilometers along axis we infer from these observations that the basic seismic characteristics of the ocean crust are frozen in in a time period less than that separating the major volcanic injection events 10 000 50 000 years by hydrothermal circulation penetrating to the base of the crust along the 100 km long ridge segment studied here two major along axis structural changes occur the first is a 10 to 15 km wide along axis zone of lowered velocities in the lower crust centered beneath a major along axis topographic high this is interpreted to be the remnants of the most recent phase of injection that has temporarily left behind it a region of elevated temperatures and pervasive cracking and thus reduced velocities the second occurs abruptly at latitude 23 15 n and is accompanied by both a major change in rift valley and crestal mountain morphology and the apparent noncoincidence of the median valley and central magnetic anomaly north of this boundary the 30 km long ridge segment that abuts the kane fracture zone has a crustal thickness of 4 5 km and no distinctive layering to produce the characteristic amplitude patterns that typify mature oceanic crust interpretation of the cause of this structural change is uncertain but may be related to local tectonic events perhaps a recent 10 to 20 km ridge jump to the east

the origin and microfabric of lake superior red clay abstract two red clay units the hanson creek and douglas members of the miller creek formation occur in wisconsin along the bluffs of lake superior the red clay is nonlaminated consists of 60 to 90 percent clay contains few pebbles and occurs dominantly basinward of the shorelines of ancient glacial lakes these characteristics have led previous workers to interpret the red clay as either lake sediment or subglacial till glacial striations flutes macrofabric an areal distribution characteristic of till grain size trends shear planes and outcrop characteristics indicate that the red clay is subglacial till and not lake sediment microfabric analysis of the red clay reveals a preferred orientation of elongate sand grains this preferred orientation is related to ice flow direction but in a way different from that shown by most analyses of till microfabric nearly half of the 25 samples analyzed have the major mode of sand grain trends oriented transverse to regional ice flow direction as indicated by flutes striations and macrofabric in the remaining samples the sand grain trends are parallel to ice flow the development of a transverse or parallel orientation may be influenced by clast content and subglacial shearing linear streaks of silt and clay referred to as microfoliation occur in the matrix of the red clay the microfoliation bands are planes that strike transverse to ice flow and dip in the up ice direction the structure of the microfoliation and its orientation relative to ice flow suggest that they may be shear planes caused by overriding ice the microfoliation shows a more consistent relationship to ice flow than the sand grain trends

the evolution of craters and calderas on young seamounts insights from sea marc i and sea beam sonar surveys of a small seamount group near the axis of the east pacific rise at 10 n a northwest trending group of seamounts located just west of the east pacific rise epr axis has been mapped using high resolution sea marc i side scan sonar and sea beam multibeam bathymetry these seamounts have 9 13 km basal diameters and the shapes of the volcanos change from conical domes near the epr axis to truncated cones and larger elliptical truncated cones farther away from the rise axis the detailed resolution of constructional volcanic features on these seamounts and the progressive development of summit depressions craters to large complex calderas and summit plateaus suggest to us a succession of evolutionary stages through which seamounts grow our evolutionary scheme for constructing these volcanos calls for the development of satellite extrusive centers around the base of a growing seamount which serve to drain off magmas and precipitate summit collapse the creation of a summit depression on a seamount will structurally constrain the style of eruptive volcanism to be principally along ring fractures which localize vents around the margin of a crater or caldera the concentration and channelization of volcanic eruptions around the top of the volcano lead to a progradation of the summit area and the establishment of a summit plateau the steep sides of a seamount reflect growth of the summit area out over the upper flanks of the volcano caused by continued eruption of lavas around the margin of the caldera the steep constructional slopes are sites for vigorous mass wasting that shapes and modifies flank terrain as a seamount evolves the crestal topography is the product of many volcanic cycles characterized by filling and subsequent draining of the caldera and expansion of caldera area through progressive collapse and migration of primary magmatic conduits the final phase of volcanism may be a calderafilling episode that builds a rough conical mount over the summit plateau

aeromagnetic signature of porphyry copper systems in northern chile and its geologic implications the compilation of a large aeromagnetic high resolution dataset acquired for base metals exploration in northern chile between lat 19 45 s and 27 15 s revealed a pattern of transorogenic regional magnetic anomalies these anomalies occur as generally east west stripes of negative residual magnetic intensity of more than 100 nanotesla nt they have an east west extension of some tens of kilometers and a north south width of some 5 to 10 km some of them can be observed continuously from the coast to the volcanic belt their residual intensity is typically more than 100 nt they also show a loose north south spacing it is observed that all known porphyry copper deposits of the investigated segment are spatially related to these transverse magnetic anomalies we propose that the transverse magnetic anomalies are the magnetic response to the loci of emplacement of intrusive bodies of batholithic size along paths of the advancing magmatic front of the active continental margin the occurrence of several important porphyry copper deposits grouped in clusters within a district is explained as being related to a common parental intrusive complex which is geophysically signaled by a corresponding transverse magnetic anomaly a possible implication of the observed relationship between porphyry copper deposits and transverse magnetic anomalies is the quasideterministic restriction of porphyry copper deposits to certain orogen transverse loci necessary condition and the corresponding consequences for future exploration strategies

origin of seaward dipping reflectors in oceanic crust off the norwegian margin by subaerial sea floor spreading a remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the norwegian continental margin it comprises a seaward dipping complex of reflectors in the form of a wedge we suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean basin genesis

the dynamics of rifting deformation of the lithosphere by active and passive driving forces summary the thermal and mechanical behaviour of the lithosphere during rifting is investigated using a two layer model for its rheology and different driving forces for the rift process the uppermost crustal layer is assumed to deform by brittle failure while the lower lithosphere including the lower crust is assumed to behave as a newtonian viscous fluid results of numerical modelling of the deformation flow and heat transport in the viscous layer are presented in which viscosity is a strong function of depth and temperature deformation of the upper brittle layer is complex and difficult to quantify thus deformation of this upper layer was approximated by a simple kinematic model in which the layer is thinned by an amount proportional to the amount of extension it experiences two fundamentally different driving forces were investigated they correspond to active rifting in which the litho sphere is thinned by an upwelling limb of a convection cell or mantle plume and to passive rifting or stretching of the lithosphere the results show that for zones of stretching which are wider than the thickness of the viscous layer thinning at all levels in the lithosphere can be described by a single parameter as in earlier stretching models this is not always true when the rift zone is narrow or near the edges of the rift zone initial subsidence due to isostatic and thermal changes during stretching slows as stretching progresses giving an exponential character to the rift stage subsidence active rifting is associated with thermal uplift in this case lithospheric deformation is confined to a narrow zone near the base of the lithosphere and thinning of the crust does not occur until the base of the lithosphere approaches that level the rate of lithospheric thinning and width of the rift zone depend on the convection velocities in the underlying asthenosphere and on the viscosity depth gradient near the lithosphere asthenosphere boundary also contributing to the elevation changes during rifting are dynamic stresses which result from the flow these dynamic affects may significantly modify the uplift or subsidence of the rift zone another important process which results from viscous flow is the generation of secondary convection in the upwelling asthenosphere ths will further thin the lithosphere and can cause depth dependent stretching in which the lower lithosphere is thinned more than the upper lithosphere both of these properties of the flow dynamic stresses and secondary convection critically depend on absolute values of viscosity and so are not well constrained at present the model results presented here exhibit the degree of complexity which results from viscous flow in the lower lithosphere and which is matched by the wide variety of observations on rifting

sea floor spreading in the presence of the iceland plume the structure of the reykjanes ridge at 61 40 n abstract we present results from a combined normal incidence and wide angle seismic survey of the reykjanes ridge at 61 40 n in the north atlantic the seismic data show that the crust is 8 4 10 0 km thick on the ridge axis we attribute this thicker than normal oceanic crust to a high degree of melting caused by hot mantle from the iceland plume beneath the speading centre layer 2a of the oceanic crust is significantly thicker at 0 ma than in crust of age 4 ma our results combined with other seismic data from the north atlantic show that the crustal thickness decreases away from the plume centre to normal oceanic crustal thickness at a distance of approximately 1000 km

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

a refraction seismic transect from the faroe islands to the hatton rockall basin 1 the crustal structure of the faroe rockall plateau was studied by a 790 km long refraction seismic transect consisting of two intersecting lines the air gun shot spacing was 200 m and the signals were recorded by 77 ocean bottom seismometers a p wave velocity model was developed from forward and inverse modeling of the wide angle seismic data and incorporation of coincident multichannel reflection seismic data continental crust with velocities ranging from 5 6 to 6 8 km s can be traced from the faroe islands across the banks to the sw of the faroes and into the hatton rockall basin the thickness of the subvolcanic crust is up to 25 km on the banks but is as little as 8 km in the channels between the banks the thinning in the channels may be related to nw trending shear zones extending from major lineaments in ne rockall trough basalt layers are found along the entire transect with a total thickness of up to 4 km two layers with velocities of 4 9 5 2 and 5 3 5 6 km s are thought to represent paleogene flood basalts that can be correlated from the faroe islands to george bligh bank close to george bligh bank an 80 km wide and up to 9 km thick body with velocities of 6 5 km s is interpreted as intrusion a 5 km thick high velocity lower crustal layer 7 3 km s extends from the area of the intrusion into the northern hatton rockall basin at the northern flank of lousy bank the transition zone to oceanic crust was encountered

spatial distribution of hotspot material added to the lithosphere under la reunion from wide angle seismic data wide angle seismic lines recorded by ocean bottom and land seismometers provide a pseudo three dimensional investigation of the crust and upper mantle structure around the volcanically active hotspot island of la reunion the submarine part of the edifice has fairly low seismic velocities without evidence for intrusives an upper unit with a velocity depth gradient is interpreted as made of material erupted subaerially then transported and compacted downslope between this unit and the top of the oceanic plate imaged by normal incidence seismic reflection a more homogeneous unit indicated by shadow zones on several wide angle sections may correspond to lavas of a different nature extruded underwater in the earlier phase of volcanism coincident wide angle and normal incidence reflections document that the oceanic plate is not generally downwarping toward the island but doming instead toward its southeastern part with limited evidence for some intracrustal intrusion deeper in the lithosphere the presence of a layer of intermediate velocity between the crust and mantle is firmly established it is interpreted as resulting from the advection of hotspot magmatic products possibly partially molten and of a composition for which the crust is a density barrier the extensive wide angle coverage constrains the extent of this body it does not show the elongated shape expected from plate drift above a steady hotspot supply alternative propositions can hence be considered for example that la reunion is caused by a solitary wave of hotspot material or by a young hotspot the size of the underplate 140 km wide and up to 3 km thick corresponds to less than half the volume of the edifice on top of the plate

regional anomalies of sediment thickness basement depth and isostatic crustal thickness in the north atlantic ocean abstract we calculate the anomalous basement topography for the north atlantic ocean from 30 to 70 n latitude and from 0 to 70 w longitude at a resolution of roughly 6 6 km using grids of total sediment thickness and observed and predicted sea floor bathymetry to correct for the effects of isostatic sediment loading and lithospheric age plotting this residual topography for various plate reconstructions during opening of the north atlantic we delineate consistent patterns of basement highs related to variations in hotspot related volcanism in addition to iceland and the azores we recognize three centers of excess volcanism at the mid atlantic ridge the milne seamounts and azores biscay rise 75 40 ma the southeast newfoundland ridge and madeira tore rise 130 110 ma and the east and west thulean rises 60 50 ma the duration of volcanic activity ranges from 8 to 10 m y thulean rises to 60 m y iceland and thus it appears that both long and short lived hotspots coexist even in relatively close proximity in contrast during the period 110 60 ma we observe little excess volcanism during either continental breakup or seafloor spreading we estimate isostatic crustal thickness from the anomalous basement depths after first removing dynamic effects created by mantle flow maximum thicknesses of volcanic features from 30 km beneath the greenland iceland faeroe ridge to 15 km beneath the azores biscay rise are broadly consistent with seismic data and predictions of decompression melting widths of volcanic features indicate that thickening primarily occurs within 100 200 km of hotspots except along continental margins that rifted at the time of the hotspot activity i e east greenland and the hatton rockall bank we observe conjugate structures south of greenland and edoras bank where excess volcanism appears to have extended beyond the margin proper and into oceanic crust similar conjugate features appear in the labrador sea south of davis strait finally we identify anomalous oceanic regions adjacent to some continental margins where unusually low values of predicted crustal thickness suggest either additional variations in plate properties or non isostatic effects within the mantle

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

a possible mechanism for epeirogenic uplift continental geology is dominated by vertical movements some of these are the result of crustal shortening and extension associated with large horizontal displacements involving processes which are now understood at least in outline however both uplift and subsidence can occur without associated shortening or extension of the continental crust most investigations of such epeirogenic movements have been concerned with subsidence partly because of the economic importance of the resulting sedimentary basins and partly because of the existence of a simple model for the process1 which can account for the development of several basins in some detail2 4 however uplift has been relatively neglected and cannot be produced by the same mechanism the purpose of this paper is to examine whether epeirogenic uplift can result from the intrusion of large thicknesses of basic magma into the lower part of continental crust this suggestion was considered as a possible mechanism for the regional uplift of the colorado plateau by gilbert5 more than 100 years ago and also by holmes6 though both rejected the idea however a recent discussion of no less than 14 different mechanisms for generating uplift7 did not include this process even though it seems able to produce the observed vertical motions and also to account for some of the features of the evolution of sedimentary basins that cannot be explained by the uniform stretching model3 8 9 although the suggestion is not new the argument used here to support it is and depends on an understanding of the relationship between ocean island volcanoes and mantle circulation

the hatton bank continental margin iii structure from wide angle obs and multichannel seismic refraction profiles summary in two recent papers normal incidence and expanding spread profiles esps shot across the hatton bank passive continental margin showed seaward dipping reflectors in the upper crust and elevated velocities in the lower crust the reflectors were interpreted as subaerial basalt flows and the high velocities as underplated or intruded igneous material in the lower crust a recent gravity model shows high densities in the lower crust supporting the velocity model in this study we present the forward modelling of two wide angle seismic refraction datasets shot perpendicular to strike and through the esp midpoints the final velocity model across the margin is consistent with all the seismic datasets but differs in some important respects from the esp based model a high velocity volcanic pile is outlined mid way up the continental slope and the inferred velocities in the lower cnst are slightly lower the differences between the two models can be explained by the three dimensional nature of the margin the esps were modelled assuming that the sub sediment structure was invariant along strike the final velocity model is used to obtain an estimate of the total thickness of melt accreted across the margin which is calculated to thicken from zero at hatton bank to about 12 km mid way down the continental slope at the bottom of the continental slope the adjacent igneous section forming the oceanic crust is about 18 km thick and thins rapidly northwestwards to 11 km as the oceanic crust thins its seismic velocity decreases this suggests that it is less mafic and that there is a post rift decrease in the percentage of mantle melting this decrease may be explained by a decrease in asthenospheric temperature and or by a reduction in volatile content

the hatton basin and continental margin crustal structure from wide angle seismic and gravity data results from a wide angle seismic and gravity study between the rockall bank and the iceland basin in the north atlantic are presented crustal and sedimentary structures are resolved in the hatton basin and across the hatton continental margin hcm east of magnetic anomaly 24 the structure of the oceanic crust west of the anomaly is also determined gravity data support the seismic model in areas of good seismic coverage and are used to control the model where the wide angle seismic data are poor a two layer sedimentary sequence is present both in the hatton basin and across the continental margin the lower layer with p wave velocity of about 4 km s is interpreted as pre eocene synrift sediments and is up to 3 5 km thick a younger and thinner 1 2 5 km postrift sequence with a velocity of about 2 km s defines a strong velocity contrast which suggests an erosional unconformity surface the sedimentary structure is distinctly different from that in the rockall trough where a third intermediate layer vp 3 km s occurs the three layer crust characterized by two intracrustal reflections pip1 and pip2 varies from 30 km thick under the rockall bank to about 15 km below the hatton basin where it is stretched by a factor of 2 relative to onshore ireland the crust is thinnest below the hatton bank where the presence of a single intracrustal reflection indicates that the lower crustal layer thins to below the seismic resolution limit below the hcm a region of thick lower crust with anomalously high velocity vp 7 2 km s is resolved by the seismic and gravity data it is connected west of anomaly 24 to a region of oceanic crust which is thicker than in the iceland basin these relationships between the thick lower crust below the hcm and the oceanic crust in the iceland basin are interpreted as evidence for magmatic underplating consistent with previous models for the hcm the inferred unconformity surface between the synrift and postrift layers may be due to regional uplift driven by upwelling of hot asthenosphere before anomaly 24 early eocene time

wavelet and multitaper coherence methods for assessing the elastic thickness of the irish atlantic margin summary there have been some inconsistencies in estimates of the effective elastic thickness of the continental lithosphere te based upon admittance or coherence relationships between gravity and topography this paper compares multitaper and wavelet methods to analyse the coherence between bouguer gravity and bathymetric data over the irish atlantic margin the analyses show that similar lateral te variations can be recovered from the data but demonstrate that the size of the data window can give rise to a significant downward bias in te estimates a seismically constrained 3 d gravity inversion over the rockall basin shows the presence of surface and subsurface loads whose ratio is loosely correlated with load ratio variations generated from the wavelet coherence method the te and load ratio f variations can be plausibly related to major geological structures on the margin if the load ratio variations can be interpreted geologically it implies that spectral based methods to estimate effective elastic thickness must incorporate subsurface loads within the underlying theoretical model on the irish atlantic margin te is generally low 6 18 km and is associated with a ne sw caledonian trend the weakest lithosphere is in the southern rockall basin porcupine bank and porcupine basin and the strongest lithosphere is along the rockall hatton region the low te values are consistent with results from other passive margins the reasons for such low te values on the irish atlantic margin remain unclear but may be the consequence of te being frozen into the lithosphere when loads were emplaced during continental breakup and temperature gradients were high the process of sedimentation and the presence of fluids may be contributory factors there is an indication of a geological and rheological divide between the rockall hatton region and the rockall basin possibly associated with the caledonian orogenic front

comparison between a regional and semi regional crustal obs model in the voring basin mid norway margin semi regional ocean bottom seismograph obs data acquired in the central and northern part of the voring basin mid norway margin have been modeled by use of 2 d ray tracing the semi regional model derived from the study of twenty five obss deployed along a 120 km long profile is compared with a regional model consisting of five obss from the same profile the semi regional model is somewhat more detailed than the regional model due to the considerably closer receiver spacing the overall geometry and velocity distribution of the two models are remarkably similar however proving that the regional procedure with large obs spacing provides a reliable regional model intrusions of sills related to early tertiary continental rifting and break up are important at intermediate and deep sedimentary levels 2 10 km below sea floor in most parts of the area the semi regional modeling suggests that one of the deepest sills extends much further east and is substantially thicker locally more than 500 m than indicated in the regional model another important difference is a high velocity body within the upper crystalline crust at 11 12 km depth in the nw part of the area indicating that the closer obs spacing in the semi regional modeling allows detection of local intra crustal intrusions local differences are also inferred in the lower crust at about 20 km depth a structure is inferred within the lower crust from wide angle reflections this might suggest that the high velocity lower crustal layer interpreted as magmatic underplating consists of a mixture of underplated intruded magmatic material and blocks of continental lower crust

mantle thermal structure and active upwelling during continental breakup in the north atlantic seismic reflection and refraction data acquired on four transects spanning the southeast greenland rifted margin and greenland iceland ridge gir provide new constraints on mantle thermal structure and melting processes during continental breakup in the north atlantic maximum igneous crustal thickness varies along the margin from s 30 km in the near hotspot zone 6 500 km from the hotspot track to v18 km in the distal zone 500 1100 km magmatic productivity on summed conjugate margins of the north atlantic decreases through time from 1800 300 to 600 50 km 3 km ma in the near hotspot zone and from 700 200 to 300 50 km 3 km ma in the distal zone comparison of our data with the british faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist joint consideration of crustal thickness and mean crustal seismic velocity suggests that along margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature the thermal anomaly vt at breakup was modest v100 125 c varied little along the margin and transient data along the gir indicate that the potential temperature anomaly 125 50 c and upwelling ratio v4 times passive of the iceland hotspot have remained roughly constant since 56 ma our results are consistent with a plume impact model in which 1 a plume of radius v300 km and vt of v125 c impacted the margin around 61 ma and delivered warm material to distal portions of the margin 2 at breakup 56 ma the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin and 3 by 45 ma both the remaining plume head and the distal warm layer were exhausted with excess magmatism thereafter largely confined to a narrow 6 200 km radius zone immediately above the iceland plume stem alternatively the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre existing thermal anomaly unrelated to the plume fl 2001 elsevier science b v all rights reserved