widespread volcanism in the greenland north atlantic region explained by the iceland plume in the classical concept a hotspot track is a line of volcanics formed as a plate moves over a stationary mantle plume defying this concept intraplate volcanism in greenland and the north atlantic region occurred simultaneously over a wide area particularly around 60 million years ago showing no resemblance to a hotspot track here we show that most of this volcanism can nonetheless be explained solely by the iceland plume interacting with seafloor spreading ridges global mantle flow and a lithosphere the outermost rigid layer of the earth with strongly variable thickness an east west corridor of thinned lithosphere across central greenland as inferred from new highly resolved tomographic images could have formed as greenland moved westward over the iceland plume between 90 and 60 million years ago our numerical geodynamic model demonstrates how plume material may have accumulated in this corridor and in areas east and west of greenland simultaneous plume related volcanic activities starting about 62 million years ago on either side of greenland could occur where and when the lithosphere was thin enough due to continental rifting and seafloor spreading possibly long after the plume reached the base of the lithosphere volcanism across the north atlantic region 62 million years ago is consistent with an iceland plume source despite the absence of a classic hotspot track suggest tomographic images and geodynamic models

melting at the base of the greenland ice sheet explained by iceland hotspot history basal melting is widespread in the north central greenland ice sheet geophysical data and numerical modelling suggest a geothermal anomaly in this region resulting from the earlier passage of greenland over the iceland mantle plume

the contact aureole of the south mountain batholith in halifax nova scotia geology mineral assemblages and isograds the contact aureole at the eastern margin of the south mountain batholith halifax pluton underlies most of the city of halifax nova scotia the halifax group in the study area includes two lithological units the cunard and bluestone formations before intrusion both had been affected by greenschist facies regional metamorphism and deformed into northeast southwest trending regional upright folds associated with a strong slaty cleavage contact metamorphic isograds trend obliquely across the halifax peninsula at a high angle to regional structural trends at 2 5 3 km from the intrusive contact sparse cordierite spots mark the outer limit of the contact aureole the biotite in isograd is marked by the development of biotite within chlorite muscovite stacks inherited from regional metamorphism pyrrhotite is the dominant sulphide mineral throughout the contact aureole with increasing metamorphic grade assemblages in both units are marked by increasing modal abundance of cordierite and biotite with k feldspar variably developed within ca 600 m of the contact however there is a marked difference in the distribution and appearance of andalusite between the two units in aluminous pelites of the cunard formation idioblastic chiastolite appears before biotite more than 1500 m from the contact in the less aluminous bluestone formation andalusite is present only within ca 500 m of the contact where it forms xenoblastic spongy crystals in both units the assemblage andalusite biotite k feldspar cordierite is developed near the contact with local fibrolite and evidence of incipient partial melting petrographic constraints suggest pressure temperature conditions at the contact of ca 2 5 3 0 kbar 1 kbar 100 mpa at ca 650 c

causes and consequences of asymmetric lateral plume flow during south atlantic rifting volcanic rifted margins are typically associated with a thick magmatic layer of seaward dipping reflectors and anomalous regional uplift this is conventionally interpreted as due to melting of an arriving mantle plume head at the onset of rifting however seaward dipping reflectors and uplift are sometimes asymmetrically distributed with respect to the subsequent plume track here we investigate if these asymmetries are induced by preexisting lateral variations in the thickness of continental lithosphere and or lithospheric stretching rates variations that promote lateral sublithospheric flow of plume material below only one arm of the extending rift using three dimensional numerical experiments we find that south atlantic rifting is predicted to develop a strong southward asymmetry in its distribution of seaward dipping reflectors and associated anomalous relief with respect to the tristan plume that drove this volcanic rifted margin and that the region where plume material drains into the rift should experience long lived uplift during rifting both as observed we conclude that a mantle plume is still needed to source the anomalously hot sublithospheric material that generates a volcanic rifted margin but lateral along rift flow from this plume not a broad starting plume head is what controls when and where a volcanic rifted margin will form

major influence of plume ridge interaction lithosphere thickness variations and global mantle flow on hotspot volcanism the example of tristan hotspot tracks are thought to originate when mantle plumes impinge moving plates however many observed cases close to mid ocean ridges do not form a single age progressive line but vary in width are separated into several volcanic chains or are distributed over different plates here we study plume ridge interaction at the example of the tristan plume which features all of these complexities additionally the south atlantic formed close to where plume volcanism began opening from the south and progressing northward with a notable decrease in magmatism across the florianopolis fracture zone we study the full evolution of the tristan plume in a series of three dimensional regional models created with the convection code aspect we then compute crustal thickness maps and compare them to seismic profiles and the topography of the south atlantic we find that the separation of volcanism into the tristan and gough chain can be explained by the position of the plume relative to the ridge and the influence of the global flow field plume material below the off ridge track can flow toward the ridge and regions of thinner lithosphere where decompression melting leads to the development of a second volcanic chain resembling the tristan and gough hotspot tracks agreement with the observations is best for a small plume buoyancy flux of 500 kg s or a low excess temperature of 150 k the model explains the distribution of syn rift magmatism by hot plume material that flows into the rift and increases melt generation

heterogeneous modification and reactivation of a craton margin beneath the korean peninsula from teleseismic travel time tomography abstract margins of old continental lithosphere are likely prone to ongoing modification processes therefore constraining detailed structures beneath the margin can be essential in understanding the evolution of the continental lithosphere the eastern margin of the eurasian plate is a natural laboratory that allows us to study the strong effects from multiple episodes of continental collision and subduction of different oceanic plates since their formation to reveal the detailed evolution of cratons at their margins we describe for the first time the upper mantle structures beneath the southern korean peninsula skp based strictly on teleseismic relative arrival time data from densely deployed local seismic arrays which allows us to constrain the details of the lithospheric structures beneath the archean proterozoic basement we imaged a thick 150 km high velocity anomaly mainly beneath the proterozoic yeongnam massif with large velocity contrasts dlnvp 4 0 and dlnvs 6 0 at its boundaries suggesting the presence of a long lasting cratonic root in the southwestern skp on the other hand low velocity anomalies were found beneath the proterozoic gyeonggi massif gyeongsang arc back arc basin and along the eastern margin of the skp indicating significantly modified regions the possible existence of a remnant cratonic root beneath the skp and contrasting lithospheric structures across the different precambrian massifs suggests the highly heterogeneous modification of cratonic lithosphere at the eastern eurasian plate margin strong velocity reductions which indicate a thermally elevated upper mantle potentially with partial melts correspond to areas of cenozoic basalts high surface heat flow and high topography along the eastern kp margin we interpret this coincidence as a result of recent reactivation of a craton margin which is controlled by intense interaction between the convective upper mantle and heterogeneous continental lithosphere

a hotspot model for iceland and the voring plateau the proximity of a hotspot to a spreading center may result in the channeling of atmosphere to the section of rise crest closest to the hotspot this produces more basalt and thicker crust at these locations thus forming a plateau over time the geometric constraints of such a model predict a unique orientation location and age progression for a plateau formed by this mechanism the hotspot will channel material to the closest part of the rise therefore the orientation of the plateau will differ from that of a hotspot track by the component of absolute motion perpendicular to the rise axis the plateau will be symmetric with respect to the location of the rise axis at the time of formation also the age progression of the plateau will be contemporaneous with the age of formation of the seafloor on either side because the plateau is seafloor just with thicker crust a set of reconstructions based upon magnetic isochrones and a fixed hotspot reference frame is presented for the norwegian greenland sea as a means of evaluating the model s predictions by locating the iceland hotspot reconstructing the relative positions of the greenland and european plates and then assuming material would be channeled from the hotspot to the closest section of the rise crest we can trace the tectonic evolution of the greenland faeroe and voring plateaus the model is able to locate the plateaus explain their orientations and predict an age progression that satisfies observed age determinations the analysis demonstrates that both plateaus could have been formed by the iceland hotspot with the greenland faeroe plateau being in effect a continuation of the voring plateau which was cut off due to transform motion between the northern and southern spreading centers

the key role of global solid earth processes in preconditioning greenland s glaciation since the pliocene after 500 ma of absence major northern hemisphere glaciations appeared during the plio pleistocene with greenland leading other northern areas here we propose that three major solid earth processes underpinned build up of the greenland ice sheet first a mantle plume pulse responsible for the north atlantic igneous province at 60 ma regionally thinned the lithosphere younger plume pulses led to uplift which accelerated at 5 ma lifting the parts of the east greenland margin closest to iceland to elevations of more than 3 km above sea level second plate tectonic reconstruction shows a 6 northward component of greenland motion relative to the mantle since 60 ma third a concurrent northward rotation of the entire mantle and crust towards the pole dubbed true polar wander tpw contributed an additional 12 change in latitude these global geodynamic processes preconditioned greenland to sustain long term glaciation emphasizing the role of solid earth processes in driving long term global climatic transitions

the north atlantic igneous province reconstructed and its relation to the plume generation zone the antrim lava group revisited summary large igneous provinces lips have recently been suggested to originate at the edges of low velocity zones on the core mantle boundary plume generation zones if true lips can potentially be used to constrain paleolongitude in plate tectonic reconstructions to validate the hypothesis it is essential to study lips of which the paleolongitude can be constrained by other methods such as hotspot reference frames an ideal candidate to this end is the early cenozoic north atlantic igneous province naip despite being the largest volcanic unit of the british tertiary igneous province btip part of the naip the age and paleoposition of the antrim lava group alg in northern ireland which is key to the naip as a whole was hitherto poorly constrained in this paper we therefore present an integrated high resolution paleomagnetic and geochronological study the alg is divided into three formations the lower basalt formation lbf interbasaltic formation ibf and the upper basalt formation ubf the ibf is mostly lateritic and encloses the tardree rhyolite we offer new age constraints from all three formations using the 40 ar 39 ar method and propose that 62 6 0 3 61 3 0 3 and 59 6 0 3 ma 1 internal uncertainties are sound estimates of the age of emplacement of the lbf tardree rhyolite ibf and ubf respectively this constrains the nominal duration of emplacement of the alg to 3 0 6 ma 1 this reevaluation of the magnetic signature in the alg revealed reverse polarity remanence in all three formations and an overall paleomagnetic north pole at latitude 78 9 n longitude 167 e a95 6 3 age 61 ma in the european reference system this appears consistent with paleomagnetic poles from the rest of the naip both in europe and greenland as well as predictions from modern apparent polar wander paths the new radiometric ages span magnetochron c26r c27n and c27r the normal polarity chron c27n most probably occurred during the ibf hiatus explaining why no normal polarity remanence was detected in the paleomagnetic investigation emplacement of the lbf falls in magnetochron c27r making this one of the oldest lava sequence in the naip older than the c27n lava pile in western greenland the 60 ma position of the naip in a paleomagnetic reference frame puts it close to the northern edge of the african large low shear wave velocity anomaly at the core mantle boundary and therefore in the line with the plume generation zone hypothesis however the back projected icelandic hotspot normally considered to have formed the naip is located 1500 km north of the latitude at which the naip erupted the northward motion of the north atlantic lithosphere since the late cretaceous challenging the existing correlation of the naip to the icelandic hotspot normally used to explain the observed pre and syn breakup north atlantic magmatism 63 55 ma and either an additional plume located further south in the north atlantic may be invoked to create the naip or the icelandic hotspot must have

elastic anisotropy in textured hcp iron to 112 gpa from sound wave propagation measurements we report the first experimental determination of elastic anisotropy in highly textured polycrystalline hexagonal closed packed hcp iron obtained at room temperature and 112 gpa by inelastic x ray scattering ixs in a diamond anvil cell dac the compressional sound velocity vp at 50 and 90 from the c axis was determined to be vp 50 9900 200 m s and vp 90 9450 150 m s respectively the difference of 4 5 between the two velocities is of the same order of the seismic observations of the anisotropy in earth s inner core combining our new results with the equation of state of hcp iron we derive the pressure dependence of the shear velocity vs the vs values extrapolated to inner core densities are 30 higher than the prem results the extrapolated vp values are instead in fair agreement with the prem seismic model though differences in the slope remain furthermore assuming a strong texture characterized by cylindrical symmetry we provide an estimate for the single crystal elastic modulus c11 as a function of pressure while our results are in general agreement with the predominant existence of hcp iron in earth s inner core the remaining differences might be assigned to the presence of another solid iron phase light elements or liquid inclusions as well as anharmonic high temperature effects

seismic structure of the lithosphere beneath nw namibia impact of the tristan da cunha mantle plume northwestern namibia at the landfall of the walvis ridge was affected by the tristan da cunha mantle plume during continental rupture between africa and south america as evidenced by the presence of the etendeka continental flood basalts here we use data from a passive source seismological network to investigate the upper mantle structure and to elucidate the cretaceous mantle plume lithosphere interaction receiver functions reveal an interface associated with a negative velocity contrast within the lithosphere at an average depth of 80 km we interpret this interface as the relic of the lithosphereasthenosphere boundary lab formed during the mesozoic by interaction of the tristan da cunha plume head with the pre existing lithosphere the velocity contrast might be explained by stagnated and frozen melts beneath an intensively depleted and dehydrated peridotitic mantle the present day lab is poorly visible with converted waves indicating a gradual impedance contrast beneath much of the study area converted phases of the 410 and 660 km mantle transition zone discontinuities arrive 1 5 s earlier than in the landward plume unaffected continental interior suggesting high velocities in the upper mantle caused by a thick lithosphere this indicates that after lithospheric thinning during continental breakup the lithosphere has increased in thickness during the last 132 myr thermal cooling of the continental lithosphere alone cannot produce the lithospheric thickness required here we propose that the remnant plume material which has a higher seismic velocity than the ambient mantle due to melt depletion and dehydration significantly contributed to the thickening of the mantle lithosphere

lateral flow and ponding of starting plume material many mantle plumes started with voluminous heads of hot material that ascended as diapirs through the mantle flood basalts radial dike swarms and frequently continental breakup are associated with the impingement of starting plume heads on the base of the lithosphere the buoyant plume head material ponds at the base of the lithosphere and then flows laterally relief on the base of the lithosphere acts as an upside down drainage pattern with enclosed catchments for this flow that is plume material preferentially collects beneath regions where the lithosphere is locally thin such as thermally subsiding sedimentary basins an example is uplift associated with the iceland starting plume which inverted the mesozoic irish sea basin additional relief at the base of the lithosphere is associated with rifting and seafloor spreading in the case of the northern north sea material from the iceland starting plume ponded beneath a basin with locally thin lithosphere rifting breached the enclosed catchment allowing the ponded plume material to drain across the new passive margin toward very thin lithosphere produced by seafloor spreading rifting also creates a conduit for the flow of plume material away from the region underlain by the starting plume head an example is the volcanic passive margin off nova scotia which is 2000 km from the center of the bahama starting plume head numerical and analytical calculations indicate that lateral flow of plume material consistent with this observation is expected if its viscosity is around 1018 pa s

mapping the moho with seismic surface waves a review resolution analysis and recommended inversion strategies abstract the strong sensitivity of seismic surface waves to the moho is evident from a mere visual inspection of their dispersion curves or waveforms rayleigh and love waves have been used to study the earth s crust since the early days of modern seismology yet strong trade offs between the moho depth and crustal and mantle structure in surface wave inversions prompted doubts regarding their capacity to resolve the moho here we review surface wave studies of the moho with a focus on early work and then use model space mapping to establish the waves sensitivity to the moho depth and the resolution of their inversion for it if seismic wavespeeds within the crust and upper mantle are known then moho depth variations of a few kilometres produce large 1 perturbations in phase velocities however in inversions of surface wave data with no a priori information wavespeeds not known strong moho depth shear speed trade offs will mask 90 of the moho depth signal with remaining phase velocity perturbations 0 1 only in order to resolve the moho with surface waves alone errors in the data must thus be small up to 0 2 for resolving continental moho with larger errors moho depth resolution is not warranted and depends on error distribution with period an effective strategy for the inversion of surface wave data alone for the moho depth is to first constrain the crustal and upper mantle structure by inversion in a broad period range and then determine the moho depth in inversion in a narrow period range most sensitive to it with the first step results used as reference prior information on crustal and mantle structure reduces the trade offs and thus enables resolving the moho depth with noisier data such information should be used whenever available joint analysis or inversion of surface wave and other data receiver functions topography gravity can reduce uncertainties further and facilitate moho mapping

hotspot tracks and the early rifting of the atlantic abstract many hotspot tracks appear to become the locus of later rifting as though the heat of the hotspot weakens the lithosphere and tens of millions of years later the continents are split along these weakened lines examples are the west coast of greenland east coast of labrador madeira hotspot the south coast of mexico north coast of honduras guyana hotspot and the south coast of west africa north coast of brazil st helena hotspot a modern day analog of a possible future rift is the snake river plain where the north american continent is being pre weakened by the yellowstone hotspot track this conclusion is based on reconstructions of the motions of the continents over hotspots for the past 200 million years the relative motions of the plates are determined from magnetic anomaly isochrons in the oceans and the motion of one plate is chosen ad hoc to best fit the motions of the plates over the hotspots however once the motion of this one plate is chosen the motions of all the other plates are prescribed by the relative motion constraints in addition to the correlation between the predicted tracks and sites of later continental breakup exposed continental shields correlate with the tracks their exposure may be the result of hotspot induced uplift which has led to erosion of their former platform sediment cover

crust generated above the iceland mantle plume from continental rift to oceanic spreading center since the north atlantic continental breakup in the early tertiary the process of rifting above a mantle plume has produced large thicknesses of igneous crust we report results of an integrated offshore onshore seismic study of the crust and upper mantle along a transect of the aseismic faroe iceland ridge between the continental fragment beneath the faroe islands and the present day spreading center in northeast iceland normal incidence seismic data provide an image of the uppermost crust which is complemented by a velocity model from streamer refraction analyses these data together image four sedimentary basins up to 400 m deep along the crest of the ridge the streamer refraction velocity model in turn forms the uppermost section of a full crustal velocity model derived from travel time modeling of air gun and explosive data the compressional to shear wave velocity ratio in the crust of the faroe iceland ridge is 1 77 0 02 similar to that of eastern iceland the moho beneath the faroe iceland ridge lies at a depth of 25 30 km crust of 25 30 km thickness requires an upper mantle potential temperature elevated 200 250 c above normal if formed by passive adiabatic decompression melting of the mantle or a somewhat less elevated temperature if as is likely there is a component of active convection in the underlying mantle plume core

the ocean continent transition in the mid norwegian margin insight from seismic data and an onshore caledonian field analogue understanding the structure of the ocean continent transition oct in passive margins is greatly enhanced by comparison with onshore analogues the north atlantic margins and the fossil system in the scandinavian caledonides show variations along strike between magma rich and magma poor margins but are different in terms of exposure and degree of maturity they both display the early stages of the wilson cycle seismic reflection data from the mid norwegian margin combined with results from ocean drilling program leg 104 drill core 642e allow for improved subbasalt imaging of the oct below the seaward dipping reflector sdr sequences vertical and inclined reflections are interpreted as dike feeder systems high amplitude reflections with abrupt termination and saucer shaped geometries are interpreted as sill intrusions implying the presence of sediments in the transition zone beneath the volcanic sequences the transitional crust located below the sdr of the mid norwegian margin has a well exposed analogue in the seve nappe complex snc at sarek sweden hornfelsed sediments are truncated by mafic dike swarms with densities of 70 80 or more the magmatic domain extends for at least 800 km along the caledonides and probably reached the size of a large igneous province it developed at ca 600 ma on the margin of the iapetus ocean and was probably linked to the magma poor hyperextended segment in the southern scandinavian caledonides these parts of the snc represent an onshore analogue to the deeper level of the mid norwegian margin permitting direct observation and sampling and providing an improved understanding particularly of the deeper levels of present day magma rich margins

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

plate tectonics and net lithosphere rotation over the past 150 my we have developed an improved model of global digital palaeo plate boundaries and plate motion to describe the distribution and history of plates since the late jurassic from this history we computed net lithospheric rotation nr through time confirming the so called westward drift but only for the past 30 myr the nr has significantly smaller magnitudes 0 13 my past 5 my than for some other plate models it averages to 0 11 0 03 my for the past 50 my with a small but systematic increase toward the present the westward drift seen only for the past 30 my is attributed to the increased dominance of a steadily growing and accelerating pacific plate nr shows peaks with time but only an early tertiary peak of 0 33 my when the indian plate was undergoing the largest known acceleration deceleration can be interpreted with some confidence we find a linear decreasing trend in net rotation over the past 150 my but attribute this trend to increasing reconstruction uncertainties back in time as subduction consumed more than half of the oceanic crust since the jurassic after removing a linear time trend we find a nr average of about 0 12 my for the past 150 my

radial viscous fingering of hot asthenosphere within the icelandic plume beneath the north atlantic ocean abstract the icelandic mantle plume has had a significant influence on the geologic and oceanographic evolution of the north atlantic ocean during cenozoic times full waveform tomographic imaging of this region shows that the planform of this plume has a complex irregular shape with significant shear wave velocity anomalies lying beneath the lithospheric plates at a depth of 100 200 km the distribution of these anomalies suggests that about five horizontal fingers extend radially beneath the fringing continental margins the best imaged fingers lie beneath the british isles and beneath western norway where significant departures from crustal isostatic equilibrium have been measured here we propose that these radial fingers are generated by a phenomenon known as the saffman taylor instability experimental and theoretical analyses show that fingering occurs when a less viscous fluid is injected into a more viscous fluid in radial miscible fingering the wavelength and number of fingers are controlled by the mobility ratio i e the ratio of viscosities by the peclet number i e the ratio of advective and diffusive transport rates and by the thickness of the horizontal layer into which fluid is injected we combine shear wave velocity estimates with residual depth measurements around the atlantic margins to estimate the planform distribution of temperature and viscosity within a horizontal asthenospheric layer beneath the lithospheric plate our estimates suggest that the mobility ratio is at least 20 50 that the peclet number is o 10 4 and that the asthenospheric channel is 100 20 km thick the existence and planform of fingering is consistent with experimental observations and with theoretical arguments a useful rule of thumb is that the wavelength of fingering is 5 1 times the thickness of the horizontal layer our proposal has been further tested by examining plumes of different vigor and planform e g hawaii cape verde yellowstone our results support the notion that dynamic topography of the earth s surface can be influenced by fast irregular horizontal flow within thin but rapidly evolving asthenospheric fingers

shear wave splitting across the iceland hot spot results from the icemelt experiment 1 we report on observations of upper mantle anisotropy from the splitting of teleseismic shear waves sks skks and pks recorded by the icemelt broadband seismometer network in iceland in a ridge centered hot spot locale mantle anisotropy may be generated by flow induced lattice preferred orientation of olivine grains or the anisotropic distribution of magma splitting measurements of teleseismic shear waves may thus provide diagnostic information on upper mantle flow and or the distribution of retained melt associated with the iceland mantle plume in eastern iceland fast polarization directions lie between n10 w and n45 w and average n24 w delay times between the fast and slow shear waves are generally 0 7 1 35 s in western iceland in contrast the fast polarization directions while less well constrained yield an average value of n23 e and delay times are smaller 0 2 0 95 s we propose that splitting in eastern iceland is caused by a 100 to 200 km thick anisotropic layer in the upper mantle the observed fast directions in eastern iceland however do not correspond either to the plate spreading direction or to a pattern of radial mantle flow from the center of the iceland hot spot we suggest that the relatively uniform direction and magnitude of splitting in eastern iceland situated on the eurasian plate may therefore reflect the large scale flow field of the north atlantic upper mantle we hypothesize that the different pattern of anisotropy beneath western iceland part of the north american plate is due to the different absolute motions of the two plates by this view splitting in eastern and western iceland is the consequence of shear by north american and eurasian plate motion relative to the background mantle flow from absolute plate motion models in which the eurasian plate is approximately stationary and the north american plate is moving approximately westward the splitting observations in both eastern and western iceland can be satisfied by a background upper mantle flow in the direction n34 w and a velocity of 3 cm yr in a hot spot reference frame this inference can be used to test mantle flow models in particular it is inconsistent with kinematic flow models which predict southward flow or models where flow is dominated by subduction related sources of mantle buoyancy which predict westward flow our observations are more compatible with the flow field predicted from global seismic tomography models which in particular include the influence of the large scale lower mantle upwelling beneath southern africa while the hypothesized association between our observations and this upwelling is presently speculative it makes a very specific and testable prediction about the flow field and hence anisotropy beneath the rest of the atlantic basin index terms 7218 seismology lithosphere and upper mantle 8120 tectonophysics dynamics of lithosphere and mantle general 8155 tectonophysics plate motions general keywords iceland anisotropy hot spot plate motion shear wave splitting mantle flow

mantle mixing and continental breakup magmatism abstract the frequent formation of large igneous provinces during the opening of the atlantic ocean is a surface manifestation of the thermal and chemical state of convecting mantle beneath the supercontinent pangea recent geochemical and geophysical findings from the north atlantic igneous province all point to the significant role of incomplete mantle mixing in igneous petrogenesis on the basis of a whole mantle convection model with chemical tracers i demonstrate that sublithospheric convection driven by surface cooling can bring up dense fertile mantle without a thermal anomaly when small scale convection in the upper mantle breaks down into the lower mantle strong counter upwelling takes place entraining a large amount of dense crustal fragments accumulated at the base of the mantle transition zone this multi scale mantle mixing could potentially explain a variety of hotspot phenomenology as well as the formation of both volcanic and non volcanic rifted margins with a spatially and temporally varying distribution of fertile mantle

hawaiian lava flows in the third dimension identification and interpretation of pahoehoe and a a distribution in the kp 1 and soh 4 cores 1 hawaiian lava flows are classified as pahoehoe or a a by their surface morphology as surface morphology reflects flow emplacement conditions the surface distribution of morphologic flow types has been used to study the evolution and eruptive history of basaltic volcanoes we extend this analysis to the third dimension by determining the distribution of flow types in two deep drill cores the scientific observation hole 4 soh 4 core drilled near kilauea s east rift zone erz and the pilot hole kahi puka 1 kp 1 for the hawaiian scientific drilling project hsdp drilled through distal flows from mauna loa and mauna kea flows are classified using both internal structures and groundmass textures with the latter useful when identification based on mesoscopic flow features e g surface morphology and vesicle content and distribution is ambiguous we then examine the temporal distribution of pahoehoe and a a flows in proximal soh 4 and distal kp 1 settings sequence analysis shows that the two flow types are not randomly distributed in either core but instead are strongly clustered the proximal soh 4 core is dominated by thin pahoehoe flows 60 by volume consistent with the common occurrence of surface fed pahoehoe flows in near vent settings the distal kp 1 core has a high proportion of a a 58 by volume although the proportion of pahoehoe and a a varies dramatically throughout the mauna kea sequence thick inflated pahoehoe flows dominate when the drill site was near sea level consistent with the numerous inflated pahoehoe fields on the current coastal plains of kilauea and mauna loa a a flows are abundant when the site was far above sea level as slope increases from the coastal plains to mauna kea s flank this correlation may reflect the combined effect of long transport distances and increased slopes on flow emplacement these results demonstrate that flow type and thickness variations in cores provide valuable information about both vent location and local site environment observed variations in flow type within the kp 1 core raise interesting questions about feedback between volcano evolution and flow morphology and suggest that flow type is an important variable in models of volcano growth and related models for lava flow hazard assessment

timing and duration of volcanism in the north atlantic igneous province implications for geodynamics and links to the iceland hotspot abstract we combine new and published 40 ar 39 ar age determinations from incremental heating experiments on whole rocks and mineral separates to assess the timing duration and distribution of volcanic activity during construction of the north atlantic igneous province we use these ages together with volume estimates of erupted magmas and their cumulates to calculate melt production rates for the early tertiary flood basalts of east greenland and the faeroes islands the lavas lie at opposite ends of the greenland iceland faeroes ridge the postulated iceland hotspot track and record volcanic activity leading up to during and following continental breakup between greenland and europe dominantly basaltic but also alkalic and picritic magmas were erupted from beneath thick continental lithosphere simultaneously at 61 ma from baffin island to the western and eastern margins of greenland to the faeroe islands and the western british isles a roughly circular area 2000 km in diameter volcanic activity was increasingly intermittent by 57 56 ma but at 56 1 0 5 ma the average melt production rate increased by more than an order of magnitude over previous levels from 3000 km 3 km of rift my coinciding with continental rifting and shallow decompressional melting of the mantle that fed subaerial volcanic activity along most of the eastern margin of greenland and its complement on the northwestern european margin offshore abnormally thick oceanic crust was produced along the line of plate separation the upper part of this crust comprises seismically imaged seaward dipping subaerially erupted lavas by 50 ma eruption rates had diminished drastically and volcanic activity had narrowed to a much restricted portion of the east greenland margin at the western end of the greenland iceland ridge that subsequently connected in an age progressive trend to present hotspot activity in southeastern iceland the startup mantle plume head and tail model with moderate excess temperature t 100 c and active upwelling best explains the time space and compositional aspects of volcanism in this province

the effect of the large scale mantle flow field on the iceland hotspot track abstract fluid dynamical simulations were carried out in order to investigate the effect of the large scale mantle flow field and the depth of the plume source on the structure of the iceland plume through time the time dependent location and shape of the plume in the earth s mantle was calculated in a global model and it was refined in the upper mantle using a 3d cartesian model box global flow was computed based on density heterogeneities derived from seismic tomography plate motion history served as a velocity boundary condition in both models hotspot tracks of the plume conduits and the plume head were calculated and compared to actual bathymetry of the north atlantic if a plume source in the lowermost mantle is assumed the calculated surface position of the plume conduit has a southward component of motion due to southward flow in the lower mantle depending on tomography model assumed plume age and buoyancy the southward component is more or less dominating plume models having a source at the 660 km discontinuity are only influenced by flow in the upper mantle and transition zone and hence rather yield westward hotspot motion many whole mantle plume models result in a v shaped track which does not match the straight greenland iceland faroe ridge models without strong southward motion such as for a plume source at 660 km depth match actual bathymetry better plume tracks were calculated from both plume conduits and plume heads a plume head of 120 k anomalous temperature gives the best match between plume head track and bathymetry

cretaceous tertiary geodynamics a north atlantic exercise summary new reconstructions are presented for the cretaceous early tertiary north atlantic using a combination of palaeomagnetic hotspot and magnetic anomaly data we utilize these reconstructions in an analysis of previously described misfits between the north atlantic plate elements at successive intervals during this time period we are able to achieve reasonable overlap between the hotspot and palaeomagnetic reconstructions between 40 and 95 ma and thus are able to support the idea that the indo atlantic hotspots are relatively stationary small but systematic discrepancies for this time interval can readily be modelled with a long term octopole non dipole field contribution g3 g30 g10 0 08 however hotspot and palaeomagnetic reconstructions for the early cretaceous north atlantic show substantial differences that cannot be explained by constant non dipole fields and we favour an explanation for these discrepancies in terms of true polar wander tpw triggered by mantle instabilities between 125 and 95 ma this constitutes the only identifiable event of significant tpw since the early cretaceous taken in the context of available geochronological and geological data and seismic tomography from the region the 95 40 ma reconstructions and their time consequent geological products are interpreted in terms of specific conditions of mantle crust coupling and global plate motions tectonic activity highlights from these reconstructions show uniform ne movement of the coupled north american greenland and eurasian plates from 95 to 80 ma a marked cusp in the paths for all three elements at 80 ma where the three plates simultaneously change direction and follow a uniform nw directed motion until c 20 ma when eurasia diverges ne away from the still nw moving greenland and north american elements positioning of the iceland plume beneath the spreading ridge at 20 ma may have increased upwelling below the ridge increased the ridge push and caused a ne shift in the absolute direction of eurasia

global upper mantle tomography with the automated multimode inversion of surface and s wave forms summary we apply the automated multimode inversion of surface and s wave forms to a large global data set verify the accuracy of the method and assumptions behind it and compute an s vvelocity model of the upper mantle crust 660 km the model is constrained with 51 000 seismograms recorded at 368 permanent and temporary broadband seismic stations structure of the mantle and crust is constrained by waveform information both from the fundamentalmode rayleigh waves periods from 20 to 400 s and from s and multiple s waves higher modes in order to enhance the validity of the path average approximation we implement the automated inversion of surface and s wave forms with a three dimensional 3 d reference model linear equations obtained from the processing of all the seismograms of the data set are inverted for seismic velocity variations also relative to a 3 d reference in this study composed of a 3 d model of the crust and a one dimensional 1 d global average depth profile in the mantle below waveform information is related to shear and compressional velocity structure within approximate waveform sensitivity areas we use two global triangular grids of knots with approximately equal interknot spacing within each a finely spaced grid for integration over sensitivity areas and a rougher spaced one for the model parametrization for the tomographic inversion we use lsqr with horizontal and vertical smoothing and norm damping we invert for isotropic variations in s and p wave velocities but also allow for s wave azimuthal anisotropy in order to minimize errors due to possible mapping of anisotropy into isotropic heterogeneity the lateral resolution of the resulting isotropic upper mantle images is a few hundred kilometres varying with data sampling we validate the imaging technique with a spectral element resolution test inverting a published global synthetic data set computed with the spectral element method using a laterally heterogeneous mantle model we are able to reconstruct the synthetic model accurately this test confirms both the accuracy of the implementation of the method and the validity of the jwkb and path average approximations as applied in it reviewing the tomographic model we observe that low s v velocity anomalies beneath mid ocean ridges and backarc basins extend down to 100 km depth only shallower than according to some previous tomographic models this presents a close match to published estimates of primary melt production depth ranges there in the seismic lithosphere beneath cratons unambiguous high velocity anomalies extend to 200 km pronounced low velocity zones beneath cratonic lithosphere are rare where present south america tanzania they are neighboured by volcanic areas near cratonic boundaries the images of these low velocity zones may indicate hot material possibly of mantle plume origin trapped or spreading beneath the thick cratonic lithosphere

numerical modeling of mantle flow beneath madagascar to constrain upper mantle rheology beneath continental regions over the past few decades azimuthal seismic anisotropy measurements have been widely used proxy to study past and present day deformation of the lithosphere and to characterize convection in the mantle beneath continental regions distinguishing between shallow and deep sources of anisotropy remains difficult due to poor depth constraints of measurements and a lack of regional scale geodynamic modeling here we constrain the sources of seismic anisotropy beneath madagascar where a complex pattern cannot be explained by a single process such as absolute plate motion global mantle flow or geology we test the hypotheses that either edge driven convection edc or mantle flow derived from mantle wind interactions with lithospheric topography is the dominant source of anisotropy beneath madagascar we therefore simulate two sets of mantle convection models using regional scale 3 d computational modeling we then calculate lattice preferred orientation that develops along pathlines of the mantle flow models and use them to calculate synthetic splitting parameters comparison of predicted with observed seismic anisotropy shows a good fit in northern and southern madagascar for the edc model but the mantle wind case only fits well in northern madagascar this result suggests the dominant control of the measured anisotropy may be from edc but the role of localized fossil anisotropy in narrow shear zones cannot be ruled out in southern madagascar our results suggest that the asthenosphere beneath northern and southern madagascar is dominated by dislocation creep dislocation creep rheology may be dominant in the upper asthenosphere beneath other regions of continental lithosphere

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

global shear speed structure of the upper mantle and transition zone s u m m a r y the rapid expansion of broad band seismic networks over the last decade has paved the way for a new generation of global tomographic models significantly improved resolution of global upper mantle and crustal structure can now be achieved provided that structural information is extracted effectively from both surface and body waves and that the effects of errors in the data are controlled and minimized here we present a new global vertically polarized shear speed model that yields considerable improvements in resolution compared to previous ones for a variety of features in the upper mantle and crust the model sl2013sv is constrained by an unprecedentedly large set of waveform fits 3 4 of a million broad band seismograms computed in seismogram dependent frequency bands up to a maximum period range of 11 450 s automated multimode inversion of surface and s wave forms was used to extract a set of linear equations with uncorrelated uncertainties from each seismogram the equations described perturbations in elastic structure within approximate sensitivity volumes between sources and receivers going beyond ray theory we calculated the phase of every mode at every frequency and its derivative with respect to sand p velocity perturbations by integration over a sensitivity area in a 3 d reference model the normally small perturbations of the 3 d model required to fit the waveforms were then linearized using these accurate derivatives the equations yielded by the waveform inversion of all the seismograms were simultaneously inverted for a 3 d model of shear and compressional speeds and azimuthal anisotropy within the crust and upper mantle elaborate outlier analysis was used to control the propagation of errors in the data source parameters timing at the stations etc the selection of only the most mutually consistent equations exploited the data redundancy provided by our data set and strongly reduced the effect of the errors increasing the resolution of the imaging our new shear speed model is parametrized on a triangular grid with a 280 km spacing in well sampled continental domains lateral resolution approaches or exceeds that of regionalscale studies the close match of known surface expressions of deep structure with the distribution of anomalies in the model provides a useful benchmark in oceanic regions spreading ridges are very well resolved with narrow anomalies in the shallow mantle closely confined near the ridge axis and those deeper down to 100 120 km showing variability in their width and location with respect to the ridge major subduction zones worldwide are well captured extending from shallow depths down to the transition zone the large size of our waveform fit data set also provides a strong statistical foundation to re examine the validity field of the jwkb approximation and surface wave ray theory our analysis shows that the approximations are likely to be valid within certain time frequency portions of most seismograms with high signal to noise ratios and these portions can be identified using a set of consistent criteria that we apply in the course of waveform fitting

the effect of nickel on the properties of iron at the conditions of earth s inner core ab initio calculations of seismic wave velocities of fe ni alloys abstract we have performed athermal periodic plane wave density functional calculations within the generalised gradient approximation on the bcc fcc and hcp structures of fe 1 x ni x alloys x 0 0 0625 0 125 0 25 and 1 in order to obtain their relative stability and elastic properties at 360 gpa and 0 k for the hcp structure using ab initio molecular dynamics we have also calculated the elastic properties and wave velocities for x 0 0 0625 and 0 125 at 360 gpa and 5500 k with further calculations for x 0 and 0 125 at 360 gpa and 2000 k at 0 k the hcp structure is the most stable for x 0 0 0625 0 125 and 0 25 with the fcc structure becoming the most stable above x 0 45 the bcc structure is not the most stable phase for any composition at 0 k compressional and shear wave velocities are structure dependent in the case of fcc the velocities are very similar to pure fe but for the hcp structure the addition of ni strongly reduces v s ni also reduced velocities in fcc iron but to a lesser extent however at 5500 k and 360 gpa ni has little effect on the wave velocities of the hcp structure which remain similar to those of pure iron throughout the range of compositions studied and in the case of v s 30 greater than that from seismological models the effect of temperature on fe ni alloys is therefore very significant indicating that conclusions based on the extrapolation of results obtained at much lower temperatures must be treated with great caution the significance of temperature is confirmed by the additional simulation at 2000 k for x 0 and 0 125 which reveals a remarkably linear temperature dependence of the change in v s relative to that of pure iron at 0 k the maximum anisotropy in v p is found to be only very weakly dependent on nickel content but dependent on structure being 15 for fcc and 8 for hcp for the hcp structure at 2000 and 5500 k the maximum anisotropy in v p is also 8 and almost independent of the ni content we conclude that ni can safely be ignored when considering its effect on the seismic properties of hcp fe under core pressures and temperatures and that the negligible effect of nickel on the physical properties of iron in the core arises not because of the chemical similarities between iron and nickel but because of the high temperature of the system