volcanism in the afar rift sustained by decompression melting with minimal plume influence continental breakup is caused by some combination of heating and stretching1 2 the afar rift system in africa is an example of active continental rifting where a mantle plume probably weakened the lithosphere through thermal erosion and magma infiltration however the location and degree of plume influence today are debated2 3 here we use seismic s to p receiver functions to image the mantle structure beneath afar we identify the transition between the lithosphere and underlying asthenosphere at about 75 km depth beneath the flanks of the continental rift however this boundary is absent beneath the rift itself and we instead observe a strong increase in seismic velocities with depth at about 75 km we use geodynamic modelling to show that the velocity increase at this depth is best explained by decompression melting of the mantle in the absence of a strong thermal plume so although the absence of mantle lithosphere beneath the rift implies a plume may have once been active we conclude that the influence of a thermal plume directly beneath afar today is minimal

an experimental method for gold partitioning between two immiscible fluids brine and n dodecane abstract organic matter can be found in many different types of ore deposits but its role in ore forming processes is not yet fully understood here we present an experimental method that can be used to determine the partition coefficient dauorg aq partition coefficient of au between an hydrocarbon and an aqueous fluid of gold between two immiscible liquids and thus whether liquid hydrocarbon fluids such as petroleum can act as ore fluids and transport gold or other metals of interest to investigate liquid hydrocarbons in the presence of an aqueous liquid doped with gold we modified the hfs 340z hydrothermal flow system coretest systems inc to enable sampling at hydrothermal p t conditions 150 c and 5 bar of each of two density stratified immiscible liquids a saline aqueous solution 10 wt nacl was doped with gold and heated with n dodecane ch3 ch2 10ch3 to 105 c and 150 c each brine sample was directly followed by an organic sample until three samples of each liquid were taken aqua regia was added to the brine samples to stabilize the gold before icp ms analyses each organic sample was digested chemically with a mixture of ultra pure nitric acid and hydrogen peroxide to generate carbon free solutions prior to icp ms analysis this procedure generates reproducible partition coefficients for gold or presumably any other metal between hydrocarbon and aqueous liquids if passivation procedures of the hfs 340z hydrothermal flow system are strictly followed and error sources are monitored rigorously the preferred dauorg aq between n dodecane and the brine is 0 05 0 04

a low velocity zone within the layer 3 region of 118 myr old oceanic crust in the western north atlantic summary during the north atlantic transect nat experiment 11 expanding spread profiles esps were acquired using a 3300 cubic inch 30 element tuned airgun array s v prospekta and a 48 channel seismic streamer r v moore extremely high density seismograms were obtained that have allowed us to transform the observed data to the domain of intercept time and ray parameter tau p with high precision velocity structures have been obtained by tau sum inversion ray tracing and synthetic wkbj and reflectivity seismogram methods both in tau p and x t domains a detailed analysis of esp 5 suggests the presence of a low velocity zone within the lower oceanic crust at the location of magnetic anomaly mo 118 ma it is apparently 2 86km thick has a p wave velocity of 6 5 km s 1 s wave velocity of 3 6 km s 1 and occupies much of the depth interval normally associated with oceanic layer 3 converted shear wave arrivals have been used to obtain the shear velocity structure we find that the vp vs ratio in the uppermost part of the crust is about 1 9 and much of the crustal section has a vp vsratio greater than 3 attenuation of p and s waves has been studied by amplitude analysis and we find that in most of the lower crust qs is equal to qp comparison with solutions from esps in younger and older parts of the basin suggests that esp 5 is unique in displaying a low velocity zone despite this the total thickness at esp 5 is not significantly different than at other locations we suggest that this provides strong evidence that the low velocity zone occurs within the lower crust and does not result from serpentinization of the upper mantle beneath thin oceanic crust

modification of continental lithosphere by tectonic processes a tomographic image of central north america the assembly of laurentia by precambrian accretion is also believed to have formed the underlying lithosphere accretionary signatures are detectable by seismic observations but subject to modification by later processes e g orogeny rifting and plumes we examine the archean superior province sp and environs using a set of teleseismic p wave arrivals from canadian and american instruments the resulting tomographic model has high resolution beneath the dakotas and minnesota provides a first look at the lithosphere beneath manitoba and sharpens previously documented features in ontario from the model and previous anisotropy observations we detect i a large high velocity feature beneath the western sp associated with elevated lithospheric anisotropy the high velocity feature does not match crustal boundaries notably its western edge lies approximately 200 km east of the contact with the proterozoic trans hudson orogen tho ii a low velocity channel shaped feature strikes northwest through minnesota and the dakotas associated with weakening anisotropy iii high velocities southwest of ii beneath the minnesota river valley terrane mrvt associated with low anisotropy we interpret i to be accretionary and contemporaneous with superior assembly similar velocity but weaker anisotropy of the mrvt is consistent with vertical tectonic mechanisms the inboard location of the tho contact may indicate modification of the superior root the low velocity channel has no obvious crustal expression but connects to an offset in the proterozoic mid continent rift mcr and may be rift related the mcr is not well imaged but will be examined via the temporary superior province rifting earthscope experiment in progress

imaging the inner core under africa and europe abstract the inner core under africa is thought to be a region where the nature of inner core texture changes from the strongly anisotropic western part of the inner core to the weakly anisotropic or isotropic eastern part of the inner core additionally observations of a difference in isotropic velocity between the two hemispheres have been made a very large new dataset of simultaneous pkpdf and pkpbc observations on which differential travel times have been measured is used to examine the upper 360 km of the inner core under europe africa and the surrounding oceans inversion of the differential travel time data for laterally varying inner core anisotropy reveals that inner core anisotropy is stronger under central africa and the atlantic ocean than under the western indian ocean no hemispherical pattern is present in voigt isotropic velocities indicating that the variation in anisotropy is due to differing degrees of crystal alignment in the inner core not material differences when anisotropy is permitted to change with depth the upper east most part of the study region shows weaker anisotropy than the central and western regions when depth dependence in the inner core is neglected the hemisphere boundary is better represented as a line at 40 e than one at 10 e however it is apparent that the variation of anisotropy as a function of depth means that one line of longitude cannot truly separate the more and less anisotropic regions of the inner core the anisotropy observed in the part of the inner core under africa which lies in the western hemisphere is much weaker than that under central america showing that the western hemisphere is not uniformly anisotropic as the region of low anisotropy spans a significant depth extent it is likely that heterogeneous heat fluxes in the core which may cause variations in inner core anisotropy have persisted for several hundred million years

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

a reconnaissance teleseismic study of the upper mantle and transition zone beneath the archean slave craton in nw canada abstract the objective of this study is to investigate upper mantle structure below the archean slave craton site of the oldest known rocks on earth and numerous diamondiferous kimberlites and thence to gain an understanding of early craton formation and kimberlite genesis to this aim a temporary array consisting of 13 sites equipped with broadband seismometers recorded teleseisms between november 1996 and may 1998 this data set has been augmented with recordings from the yellowknife seismic array our three most robust observations and their interpretation are 1 p wave travel time tomography reveals the oldest part of the craton the central slave basement complex to be underlain by the fastest seismic velocities indicating that this block remains distinct well into mantle depths 2 receiver function analysis requires only the moho as major s wave velocity discontinuity and points to a fairly constant crustal thickness throughout the slave province and 3 sks splitting analysis shows little variation in delay times and fast polarization directions across the array leading us to conclude that the present day plate motion of north america is the primary cause for mantle fabric beneath the entire array furthermore the data let us rule out the possibility that the mackenzie plume had any seismologically detectable effect on the slave lithosphere more speculative results of our investigation namely a possible genetic link between a low seismic velocity anomaly at depth with the overlying lac de gras kimberlite field and a possible archean origin of two shallow low velocity anomalies will require further investigation

crustal structure from an obs survey of the nootka fault zone off western canada summary the nootka fault zone is the boundary between the small explorer and juan de fuca plates which are situated between the america and pacific plates off western canada to investigate the crustal structure in the region three explosive large airgun refraction lines were shot into three ocean bottom seismometers obss with three component geophone assemblies in this phase of the study p wave velocity depth models are interpreted by comparison of the travel time and amplitude characteristics of the observed data with theoretical seismograms computed using a wkbj algorithm the interpretation gives relatively consistent results for the upper crust however the structure of the lower crust is significantly different among the various profiles upper mantle velocities range from 7 5 to 8 3 kms 1 and the sub bottom crustal thickness vanes from 6 4 to 11 km nevertheless these seismic models are consistent in general terms with oceanic crustal models represented by ophiolite complexes some aspects of the differences among profiles can be explained by consideration of a recent tectonic model for the development of the fault zone this requires within a 1 myr time interval variations in the process of crustal formation at the ridge crustal maturing or both the abnormally thick crust near a spreading centre may result in part from the complex interaction of the juan de fuca and explorer plates with the larger and older america and pacific plates upper mantle velocity variations are consistent with the concept of velocity anisotropy the different record sections show that seismic energy is attenuated for ray paths traversing the nootka fault zone

compressional and shear wave anisotropy in the oceanic lithosphere the ngendei seismic refraction experiment summary synthetic seismogram modelling of seismic refraction data from the 1983 ngendei expedition to the south pacific indicate seismic anisotropy both within the top kilometre of oceanic crust and in the uppermost mantle we calculate average p and s wave velocity versus depth functions for two orthogonal azimuths n3ooe approximately aligned with the fast mantle direction and n120 e close to the fast crustal direction probable lateral heterogeneities at the ngendei site prevent perfect matching of data and synthetic waveforms crustal anisotropy is indicated by a 28 pattern of both p and s wave travel times as a function of azimuth and is consistent with an approximate 0 2 km s difference in p wave velocities and 0 1 km s difference in s wave velocities upper mantle anisotropy is characterized by pn velocity differences of 7 95 8 4 km s l and a nearly uniform s velocity of 4 65 km s we use these velocity profiles and traveltime data to calculate bounds on the properties of the elastic constants of the crust and upper mantle the crustal anisotropy can be explained by a model involving aligned cracks parallel to the original spreading ridge resulting in a fast direction perpendicular to the fossil spreading direction the upper mantle anisotropy is consistent with aligned olivine crystal models in which the fast direction is parallel to the fossil spreading direction for both of these models we use bounds from the ngendei data to place constraints on the physical properties of the lithosphere

crustal thickness and vp vs variations in the grenville orogen ontario canada from analysis of teleseismic receiver functions abstract we have developed a simple semblance weighted stacking technique to estimate crustal thickness and average vp vs ratio using teleseismic receiver functions we have applied our method to data from 32 broadband seismograph stations that cover a 700 400 km2 region of the grenville orogen a 1 2 0 98 ga himalayan scale collisional belt in eastern north america our seismograph network partly overlaps with l ithoprobe and other crustal refraction surveys in 8 out of 9 cases where a crustal refraction profile passes within 30 km of a seismograph station the two independent crustal thickness estimates agree to within 7 our regional crustal thickness model constructed using both teleseismic and refraction observations ranges between 34 0 and 52 4 km crustal thickness trends show a strong correlation with geological belts but do not correlate with surface topography and are far in excess of relief required to maintain local isostatic equilibrium the thickest crust 52 4 1 7 km was found at a station located within the 1 1 ga mid continent failed rift the central gneiss belt which contains rocks exhumed from deep levels of the crust is characterized by vp vs ranging from 1 78 to 1 85 in other parts of the grenville orogen vp vs is found to be generally less than 1 80 the thinnest crust 34 5 37 0 km occurs northeast of the 0 7 ga ottawa bonnechere graben and correlates with areas of high intraplate seismicity

reflection waveform inversion using local descent methods estimating attenuation and velocity over a gas sand deposit prestack seismic reflection data contain amplitudes traveltimes and moveout information waveform inversion of such data has the potential to estimate attenuation levels reflector depths and geometry and background velocities however when inverting reflection data strong nonlinearities can cause reflectors to be incorrectly imaged and can prevent background velocities from being updated to successfully recover background velocities previous authors have resorted to nonlinear global search inversion techniques we propose a two step inversion procedure using local descent methods in which we perform alternate inversions for the reflectors and the background velocities for our reflector inversion we exploit the efficiency of the back propagation method when inverting for a large parameter set for our background velocity inversion we use newton inverse methods during the background velocity inversions it is crucial to adaptively depth stretch the model to preserve the vertical traveltimes this reduces nonlinearity by largely decoupling the effects of the background velocities and reflectors on the data nonlinearity is further reduced by choosing to invert for slownesses and by inverting for a sparse parameter set which is partially defined using geological reflector picks applying our approach to shallow seismic data from the north sea collected over a gas sand deposit we demonstrate that the proposed method is able to estimate both the geometry and internal velocity of a significant velocity structure not present in the initial model over successive iterations the use of adaptive depth stretching corrects the pull down of the base of the gas sand vertical background velocity gradients are also resolved for an insignificant extra cost the acoustic attenuation parameter q is included in the inversion scheme the final attenuation tomogram contains realistic values of q for the expected lithologies and for the effect of partial fluid saturation associated with a shallow bright spot the attenuation image may also indicate the presence of fracturing

new insights into archean granite greenstone architecture through joint gravity and magnetic inversion archean tectonic evolution and metallogenic endowment in cratonic areas are intrinsically linked to lithological diversity and complex architectures unfortunately reliable models of the deep structure of such areas are scarce due to poor surface exposure and limited availability of drill holes and high resolution geophysical information by combining multiple geophysical data sets the strategy of cross gradient joint geophysical inversion permits improved subsurface imaging of geological heterogeneity and structure in this work we showcase the application of cross gradient joint inversion of gravity and magnetic data to characterize the heterogeneity of the leonora district in the eastern yilgarn craton western australia our results harmonize both geophysical data types into integrated images of the top 15 km of the crust and successfully predict observations made by a seismic experiment and surface geological mapping

full wave equation downward continuation of seismic reflection data a new method for suppressing multiple reflections in seismograms is developed it is based on a downward continuation procedure which uses the full acoustic wave equation hyperbolic form as a downward continuation operator we demonstrate that the downward continuation of the recorded wave field maps a reflectivity function without multiply reflected events the method is applied successfully to individual traces of plane wave decomposed slant stacked synthetic and field data

fluid density control on vapor liquid partitioning of metals in hydrothermal systems hot aqueous fluids both vapor and saline liquid are primary transporting media for metals in hydrothermal magmatic systems despite the growing geological evidence that the vapor phase formed through boiling of magmatic ore bearing fluids can selectively concentrate and transport metals the physical chemical mechanisms that control the metal vapor liquid fractionation remain poorly understood we performed systematic experiments to investigate the metal vapor liquid partitioning in model water salt gas systems h2o nacl kcl hcl at hydrothermal conditions measurements show that equilibrium vapor liquid fractionation patterns of many metals are directly related to the densities of the coexisting vapor and liquid phases despite differences in the vapor phase chemistry of various metals that form hydroxide chloride or sulfide gaseous molecules of contrasting volatile properties water solute interaction is a key factor that controls the metal transfer by vapor like fluids in earth s crust these findings allow quantitative prediction of the vapor liquid distribution patterns and vapor phase metal transport in a wide range of conditions our density model accounts well for the vapor brine distribution patterns of na si fe zn as sb and ag observed in fluid inclusions from magmatic hydrothermal deposits for au and cu the partitioning in favor of the liquid phase predicted in a sulfur free system contrasts with the copper and gold enrichment observed in natural vapor like inclusions the formation of stable complexes of cu and au with reduced sulfur may allow for their enhanced transport by sulfur enriched magmatic hydrothermal vapors

seismic imaging of the crust and upper mantle using regularized joint receiver functions frequency wave number filtering and multimode kirchhoff migration 1 receiver functions provide an indispensable tool for producing discontinuity images of the crust and upper mantle from teleseismic earthquake arrivals however image quality can be significantly compromised by instabilities inherent in the deconvolution process and by irregularities in data coverage driven by source and or receiver geometries we present receiver function estimation and prestack migration techniques which both reduce receiver function deconvolution instability and produce regularized multimode receiver function images the receiver function estimation technique exploits frequency wave number filtering methods using frequency pseudowave number domain filtering of gathers to enhance arrivals with consistent moveout characteristics synthetic tests confirm robust recovery of features in the presence of high noise levels to process regularized receiver functions into accurate images of the subsurface seismic scattering potential we employ a regularized kirchhoff migration that migrates both direct and reverberated p to s converted modes to their correct subsurface locations the subsequent images can be stacked to mute mismigrated modes synthetic studies and model resolution tests demonstrate that this methodology is well suited to irregularly spaced stations and uneven data coverage and does not require the recording of a single earthquake at multiple stations

constraints on the interpretation of s to p receiver functions summary we present results from forward modelling to study the feasibility of using s to p converted phases to image the seismic discontinuity structure of the crust and upper mantle we show that a significant level of p wave energy arriving before the direct s wave arrival can interfere with the s to p converted phases of interest and may result in sp receiver function phases that do not represent true earth structure the source of this p wave energy is attributable to a number of phases including those that have undergone multiple reflections off the earth s surface for deep focus earthquakes 300 600 km deep a significant amount of p wave energy is observed from pppp ppppp and spppp phases and arrives within the same time window as predicted for s to p converted phases from the direct s phase arrival furthermore for earthquakes at all depths interfering p wave energy arrives within the same time window as predicted for s to p converted phases from the sks phase arrival limiting the usefulness of sksp receiver functions for upper mantle imaging to isolate true sp receiver function phases from contamination due to other p wave phases we find it necessary to stack receiver functions from a range of epicentral distances and depths in order to aid the suppression of noise and other unwanted phases we provide constraints on the noise levels to be expected as a function of epicentral distance and earthquake depth we find that the lowest noise levels are achievable by restricting epicentral distance to less than 75 degrees and the depth of earthquakes used to less than 300 km

imaging crust and upper mantle seismic structure in the southwestern united states using teleseismic receiver functions rio grande rift seismic transect ristra is designed to image and interpret crust and mantle structures beneath the southwestern united states including the nature of the colorado plateau and rio grande rift figure 1 key questions include the nature of crustal and mantle structures and thermal conditions beneath one of the more dramatic continental rifts on earth and clues to understanding the anomalously unextended colorado plateau which is currently surrounded by the predominantly extensional deformation of the western united states figure 1 regional tectonic map with ristra station locations stars indicate the 54 main transect stations the diamond locates global seismic network borehole station anmo stations tx01 tx06 lie in texas nm07 nm44 in new mexico az45 az50 in arizona and ut51 ut54 in utah the data collecting stage of ristra provided an exceptional 18 months 1999 2001 of broadband streckheisen sts 2 120 s seismometers teleseismic iris passcal data along a 950 km great circle transect of 54 sites with 18 3 6 km station spacing with endpoints near lake powell utah and pecos texas the northwest southeast trending transect was oriented parallel to the azimuth of the very active western pacific seismogenic zones analysis applied to this seismic data set includes surface wave dispersion inversion for crustal and mantle structure teleseismic body wave tomography and teleseismic mantle anisotropy here we discuss the use of ristra data to examine lithospheric structure using p to s mantle and crustal forward scattering observed in the s phases immediately following several tens of seconds the teleseismic p arrival figure 2 we migrate these signals to produce p to s converted mode images of the structure along the ristra array figure 2 global distribution of the 285 earthquakes red points with body wave magnitude 5 6 recorded during the ristra network deployment which have suitable offsets for calculating the 8361 receiver functions used in this study we use teleseisms from epicentral distances of

a two dimensional p sv hybrid method and its application to modeling localized structures near the core mantle boundary a p sv hybrid method is developed for calculating synthetic seismograms involving two dimensional localized heterogeneous structures the finite difference technique is applied in the heterogeneous region and generalized ray theory solutions from a seismic source are used in the finite difference initiation process the seismic motions after interacting with the heterogeneous structures are propagated back to the earth s surface analytically with the aid of the kirchhoff method anomalous long period sks spdks observations sampling a region near the core mantle boundary beneath the southwest pacific are modeled with the hybrid method localized structures just above the core mantle boundary with lateral dimensions of 250 to 400 km can explain even the most anomalous data observed to date if s velocity drops up to 30 are allowed for a p velocity drop of 10 structural shapes and seismic properties of those anomalies are constrained from the data since synthetic waveforms are sensitive to the location and lateral dimension of seismic anomalies near the core mantle boundary some important issues such as the density change and roughness of the structures and the sharpness of the transition from the structures to the surrounding mantle however remain unresolved due to the nature of the data

theoretical body wave interactions with upper mantle structure long period seismograms are generated by a spectral method for body waves interacting with the upper mantly discontinuities that define the low velocity and transition zones of the spherically symmetric but radially inhomogeneous earth models pem c and t7 a complex velocity profile incorporates the sl8 q model models pem c and t7 predict distinct differences in displacement waveforms for p waves that interact with the 400 and 670 km discontinuities the waveforms demonstrate that later arrivals such as interference head waves can be important constraints upon earth models theoretical seismograms were computed by a spectral method that modeled the velocity profile with arbitrarily thick inhomogeneous layer full wave theory the seismograms agreed well with those generated by methods that approximate the velocity profile with thin homogeneous layers the cagniard and reflectivity methods provided the displacements were convolved with source time functions and a long period instrument response minor differences among the results of the methods do not affect the interpretation of observed waveform data but do help define the limitations of the methods for example the thin layer methods suffer from numerical limitations on layer thickness and the umber of layers needed to approximate the earth model the full wave theory may occasionally also require the usemore of thin layers when the zero order asymptotic theory it uses fails to describe wave propagation properly in layers having rapid velocity variation with depth less

electric lithosphere of the slave craton the archean slave craton in northwestern canada is an ideal natural laboratory for investigating lithosphere formation and evolution and has become an international focus of broad geoscientific investigation following the discovery of economic diamondiferous kimberlite pipes three deep probing magnetotelluric surveys have recently been carried out on the craton using novel acquisition procedures the magnetotelluric responses reveal an unexpected and remarkable anomaly in electrical conductivity collocated with the kimberlite field that is modeled as a spatially confined upper mantle region of low resistivity 30 m at depths of 80 100 km and is interpreted to be due to dissolved hydrogen or carbon in graphite form this geophysically anomalous upper mantle region is also spatially coincident with a geochemically defined ultradepleted harzburgitic layer the tectonic processes that emplaced this structure are possibly related to the lithospheric subduction and trapping of overlying oceanic mantle at 2630 2620 ma

mantle discontinuity structure beneath the colorado rocky mountains and high plains analysis of mantle discontinuity structure using converted p to s pas phases beneath colorado from the program for array seismic studies of the continental lithosphere passcal rocky mountain front rmf experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations a stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km respectively imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the pds conversion points and then stacking the receiver functions in each bin to form spatial images analogous to common depth point stacking corrections for lateral velocity heterogeneity are calculated using the local s wave tomographic model of lee and grand 1996 and a constant lnvs lnvp scaling of 1 3 this scaling value is determined from the relative scaling between teleseismic p and s wave travel time residuals measured from the rocky mountain front deployment mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography 30 km of 660 km discontinuity topography and up to 40 km of transition zone thickness variation features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western colorado and a gradual 10 km dip of the 410 km discontinuity beneath the high plains the thickening of the transition zone beneath southwest colorado is consistent with the presence of the subducted farallon slab in this region as imaged by van der lee and nolet 1997 in general our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies

gold partitioning between 1 dodecanethiol and brine at elevated temperatures implications of au transport in hydrocarbons for oil brine ore systems abstract gold can be associated with hydrocarbons in hydrothermal gold deposits but the near absence of experimental data on gold hydrocarbon interactions at hydrothermal conditions prevents a quantitative interpretation of this relationship in this study we investigate the ability of liquid hydrocarbons to act as gold transporting ore fluids using experiments on the partitioning of gold between brine 10 wt nacl and 1 dodecanethiol ddt experiments were conducted at 105 c and 150 c and

the lithosphere asthenosphere boundary in the tien shan karakoram region from s receiver functions evidence for continental subduction 1 significant s to p converted wave energy from the crust mantle and lithosphere asthenosphere boundaries the moho and the lab respectively has been observed in the tien shan and karakoram orogenic belts and used for the determination of the thickness of the crust and the mantle lithosphere the moho lies at depths of 45 65 km in the tien shan and adjacent regions and deepens to 70 km depth in pamir and karakoram the thickness of the lithosphere varies between 90 and 120 km underneath the tien shan and increases to a value of 160 km beneath the tarim basin to the south of the tarim basin the lower boundary of the asian lithosphere can be clearly followed to a depth of 270 km beneath central pamir and karakoram while the indian lithosphere is observed beneath the karakoram dipping from 130 km to 170 km towards the north the observations indicate a scenario of continental collision and subduction consistent with seismicity and the surface wave tomography results

prestack waveform inversion of three dimensional seismic data an example from the rock springs uplift wyoming usa abstractapplying seismic inversion to estimate subsurface elastic earth properties for reservoir characterization is a challenge in exploration seismology in recent years waveform based seismic inversions have gained popularity but due to high computational costs their applications are limited and amplitude variation with offset angle inversion is still the current state of the art we have developed a genetic algorithm based prestack seismic waveform inversion methodology by parallelizing at multiple levels and assuming a locally 1d structure such that forward computation of wave equation synthetics is computationally efficient this method is capable of inverting 3d prestack seismic data on parallel computers applying this inversion to a real prestack seismic data volume from the rock springs uplift rsu located in wyoming usa we determined that our method is capable of inverting the data in a reasonable runtime and producing much higher quality results than amplitude variation with offset ang

seismic evidence for preservation of the archean uchi granite greenstone belt by crustal scale extension abstract in the westernmost superior province of canada the east west alignment of granite greenstone belts and the adjacent highly deformed gneiss belts led to the first proposals that plate tectonics existed before 2 5 ga ago with the belts thrust against one another by east west oriented subduction zones here we present seismic reflection data which demonstrate that in this region the present juxtaposition of the uchi granite greenstone belt and the north caribou gneiss terrane occurred along a late southeast dipping extensional shear zone that extends from the surface into the lower crust the preservation of the uchi belt and probably the english river metasedimentary belt is directly related to their dropping along extensional shear zones which limited subsequent erosion the relative lateral transport of these greenstone rocks implies that they were neither derived from the immediately underlying crust nor preserved by vertical crustal movements as might occur in the absence of plate tectonics extension may have been associated with the emplacement of mantle derived magmas at 2700 ma which has been linked to slab break off or lithospheric delamination making the extension approximately coeval with local gold mineralisation since crustal scale faults can facilitate the circulation of gold bearing fluids we suggest that greenstone rocks preserved in the hanging walls of syn to post accretion extensional shear zones may preferentially host archean lode gold deposits in the westernmost superior province our seismic observations imply that some of the late structures in the well developed belts defined by surface mapping arose through the collapse of a collage of laterally accreted terranes

revisiting hydrocarbon phase mobilization of au in the au hg mclaughlin mine geysers clear lake area california abstract carbonaceous material cm enriched silica is co located with gold au mineralization at the au hg mclaughlin deposit geysers clear lake area california u s a the co location suggests that hydrocarbons hc may be involved in mineralization and metal concentration processes but little is known about the role of hc in the formation of ore deposits previous studies noted liquid oil inclusions in samples from the mclaughlin deposit and proposed that the hc were liquid at the time of ore deposition hydrocarbon materials in the mclaughlin deposit occur as solid and liquid textural evidence suggests that hydrocarbon rich and aqueous silica rich fluids were present simultaneously as well as separately in alternating pulses synchrotron x ray fluorescence microscopy of microscopic silica free carbonaceous material reveals that the cm contains abundant ore metals e g au ag hg and pb the cm could have become metal enriched by scavenging metals from other ore fluids or it could have transported metals when the cm was still liquid with subsequent in situ degradation due to hydrothermal heat gold concentrations of up to 18 ppm were measured via acid digestion of solid and liquid hc material and subsequent inductively coupled plasma mass spectrometry icp ms analyses hydrocarbon material with liquid to medium viscous properties bearing 10 8 ppm au provides evidence that au in liquid hc in the mclaughlin au hg deposit is still mobile and that remobilization and or transport of metals to the deposit by hc liquids cannot be ruled out

stratigraphy of the archean western superior province from p and s wave receiver functions further evidence for tectonic accretion the archean western superior province in canada represents the nucleus of the north american continent whose origin has been speculated to be the result of widespread crustal accretion some 2 7 ga ago in this paper crustal and upper mantle seismic discontinuities beneath the western superior province of the canadian shield are imaged with teleseismic p to s and s to p converted phases using the receiver function method three crustal discontinuities are observed the moho ranging in depth between 38 and 47 km and dipping to the south and two intra crustal discontinuities having depths of approximately 15 and 30 km the crustal discontinuities undulate laterally and often lose continuity possibly indicating an imbricated structure and or regions of velocity gradients in the shallow lithosphere a positive discontinuity is imaged at approximately 65 km depth and is consistent with earlier refraction and wide angle reflection results additionally two zones of negative receiver function amplitudes at 55 km depth are observed and are coincident with a region of anomalous tomographic low p and s wave velocities as well as a zone of high electrical conductivity the images for the crust and shallow upper mantle when integrated with previous geophysical studies are consistent with ideas of continental root formation due to imbrication of archean subducted material and accretion of island arcs observed in surface geology

the application of finite element analysis to body wave propagation problems summary the finite element method is shown to be a powerful tool for the numerical modelling of seismic body wave propagation problems applications extend to both problems on a scale of interest to engineers and also to large scale seismological problems solutions are sought in the time domain efficient programs have been written to accomplish this the scope of numerical solutions has been greatly enhanced by the use of a previously reported scheme for exactly cancelling reflections at the boundaries of the model the finite difference results of boore and the analytical results of trifunac for the amplification due to a mountain and an alluvial valley respectively are compared with new finite element results the new results agree well although there are some difficulties with resonance in the alluvial valley problem boore s sh results have been extended to vertical p and sv incidence a deep earthquake zone has been modelled realistically in two dimensions and earthquakes simulated at depth it is suggested that the variation in observed amplitude across the top of the zone due to refraction away from the slab may be used to provide an estimate of the thickness of the slab from long period observations of local earthquakes

seismic anisotropy of the slave craton nw canada from joint interpretation of sks and rayleigh waves summary teleseismic events recorded at a 25 element array in nw canada between 2001 and 2006 provided sufficient distribution in back azimuth to demonstrate birefringence in sks and skks waves as well as directional dependence of rayleigh wave phase velocities typical delays between orthogonally polarized sks waves are 0 8 1 2 s and modelling of azimuthal dependence indicates two nearly horizontal layers of anisotropy within the mantle anisotropy of rayleigh waves is generally consistent with models of layered vs anisotropies that increase with depth from 1 per cent at the moho to 9 per cent at 200 km but vary between subarrays consistency between the sks and rayleigh wave anisotropies in one subarray suggests that the assumption of symmetry about a horizontal axis is valid there but is not fully valid in other parts of the craton the upper layer of anisotropy occupies approximately the uppermost 120 km in which the fast polarization direction strikes generally north south coinciding with regional scale fold axes mapped at the surface the fast polarization direction of the deeper layer aligns with current north america plate motion but its correlation with trends of coeval kimberlite eruptions within the lac de gras field suggests it can be at least partly attributed to structural preferred orientation of vertical dykes inferred to exist to depths of 200 km

structure of the earth oceanic crust and uppermost mantle the past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere the application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere borehole seismic instrumentation has become more widely employed ocean bottom seismographs while generally decreasing in number have become significantly more reliable and useful and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes the techniques available for the analysis of seismic data have become more sophisticated waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models and travel time data are regularly inverted directly for structure trial and error modeling has become increasingly unimportant marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere this has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves

metal distribution in pyrobitumen of the carbon leader reef witwatersrand supergroup south africa evidence for liquid hydrocarbon ore fluids abstract high grade gold and uranium mineralization in the witwatersrand basin is intimately associated with pyrobitumen implying that hydrocarbons may have played a role in the genesis of these ores the nature of the pyrobitumen in the carbon leader reef was therefore investigated to determine if hydrocarbons were essential to the ore forming process the relative proportions of trace elements in pyrobitumen seams and nodules are in large part similar to those of organic rich shales located within the sedimentary succession this indicates a strong genetic relationship between the pyrobitumen and the shales a network of veins and veinlets filled with pyrobitumen and the occurrence of oil inclusions in detrital and authigenic quartz grains provide compelling evidence that the pyrobitumen was derived from a mobile liquid hydrocarbon precursor that circulated in the carbon leader reef uranium au sb pb ag te th bi se and w are enriched relative to their concentrations in the shales this is particularly the case for u and au the pervasive and relatively homogeneous distribution of the uranium is consistent with the presence of ubiquitous submicroscopic uraninite in the pyrobitumen documented in an earlier study in contrast the distribution of the gold is heterogeneous indicating the presence of irregularly distributed gold micrograins these grains are concentrated mainly in the interstices between pyrobitumen nodules secondary minerals such as brannerite monazite ce and galena are also concentrated in these interstices on the basis of this study we propose that intrabasinal shale units of the witwatersrand supergroup were the source for the pyrobitumen in the carbon leader reef these shales produced and released liquid hydrocarbons during the burial of the sedimentary succession phyllosilicates enriched in trace metals were transferred from the shales into the newly produced liquid hydrocarbons and were transported stratigraphically upward by the latter into the carbon leader reef during migration the liquid hydrocarbons dissolved detrital uraninite present in the basin adding to the uranium budget the liquid hydrocarbons were converted into pyrobitumen in response to further burial and precipitated uraninite nanocrystals post depositional hydrothermal fluids interacted with the hydrocarbons and deposited gold by reduction on hydrocarbon surfaces the interaction of these fluids with the hydrocarbons may have released u th ree pb and ti from the hydrocarbons and triggered the formation of secondary brannerite monazite ce and galena the results of this study provide convincing evidence that the hydrocarbon liquids that formed the pyrobitumen seams in the carbon leader reef played an essential role in the remobilization and redeposition of uranium and gold respectively in the witwatersrand supergroup

organic matter and gold ore association in a hydrothermal deposit france abstract the aim of this study was to define the part played by organic matter in au ore genesis the possible associations and relations between carbonaceous matter and au were studied at various observation scales drill hole rock sample and microanalysis in an early stage au shear zone type deposit the west viges prospect creuse department france prior work had shown that in this type of deposit au mainly occurs within the structure of arsenopyrite the study of three cores from drill holes revealed a relation between the distribution of the mineralization and the presence of moderately brecciated anthracitic layers within the host volcanosedimentary tuffs chemical assays for au performed on the rock samples after destruction of the carbonaceous material by calcination showed that the au content is nearly always higher after calcination this may indicate that part of the au initially contained in the rock is trapped in the organic phase x ray diffraction spectra on carbonaceous phases isolated from some samples indicated the presence of elemental au these results indicate that like arsenopyrite organic matter is the bearer of au and retains it during conventional acid treatment of the ore

lithospheric assembly and modification of the se canadian shield abitibi grenville teleseismic experiment this paper presents the results of a joint lithoprobe incorporated research institutions for seismology iris program for array seismic studies of the continental lithosphere passcal teleseismic experiment that investigates portions of the grenville and superior provinces of the canadian shield along the quebec ontario border data from a 600 km long n s array of 28 broadband seismographs deployed between may and october 1996 have been supplemented with additional recordings from an earlier 1994 deployment and from stations of the canadian national seismograph network and the southern ontario seismic network relative delay times of p and s waves from 123 and 40 teleseismic events respectively have been inverted for velocity perturbations in the upper mantle and reveal a low velocity nw se striking corridor that crosses the southern portion of the line at latitude 46 n and lies between 50 and 300 km depth multievent s k s splitting results yield an average delay time of 0 57 0 22 s and a direction of fast polarization of n93 e 18 which is consistent with an earlier interpretation as being due to fossil strain fields related to the last major regional tectonic event subtle variations in splitting parameters over the low velocity corridor may suggest an associated disruption in mantle fabric profiling of radial receiver functions reveals large and abrupt variations in moho topography specifically a gradual thickening in crust from 40 to 45 km between latitudes 45 n and 46 n which is followed by an abrupt thinning to 35 km at 46 6 n some 65 km southeast of the grenville front this structure is interpreted as a subduction suture extending the full length of the front and punctuating a major pre grenvillian archean proterozoic episode of lithospheric assembly in the southeast canadian shield the low velocity mantle corridor by contrast is better explained as the extension of the monteregian white mountain new england seamount hotspot track below the craton and is here postulated to represent interaction of the great meteor plume with zones of weakness within the craton developed during earlier rifting episodes

on the relations between cratonic lithosphere thickness plate motions and basal drag an overview of seismic thermal and petrological evidence on the structure of precambrian lithosphere suggests that its local maximum thickness is highly variable 140 350 km with a bimodal distribution for archean cratons 200 220 km and 300 350 km we discuss the origin of such large differences in lithospheric thickness and propose that the lithospheric base can have large depth variations over short distances the topography of bryce canyon western usa is proposed as an inverted analog of the base of the lithosphere the horizontal and vertical dimensions of archean cratons are strongly correlated larger cratons have thicker lithosphere analysis of the bimodal distribution of lithospheric thickness in archean cratons shows that the critical surface area for cratons to have thick 300 km keels is 6 8 10 6 km 2 extrapolation of the linear trend between archean lithospheric thickness and cratonic area to zero area yields a thickness of 180 km this implies that the reworking of archean crust should be accompanied by thinning and reworking of the entire lithospheric column to a thickness of 180 km in accord with thickness estimates for proterozoic lithosphere likewise extrapolation of the same trend to the size equal to the total area of all archean cratons implies that the lithospheric thickness of a hypothesized early archean supercontinent could have been 350 450 km decreasing to 280 400 km for gondwanaland we evaluate the basal drag model as a possible mechanism that may thin the cratonic lithosphere inverse correlations are found between lithospheric thickness and a fractional subduction length and b the effective ridge length in agreement with theoretical predictions lithospheric thickness of archean keels is proportional to the square root of the ratio of the craton length along the direction of plate motion to the plate velocity large cratons with thick keels and low plate velocities are less eroded by basal drag than small fast moving cratons basal drag may have varied in magnitude over the past 4 ga higher mantle temperatures in the archean would have resulted in lower mantle viscosity this in turn would have reduced basal drag and basal erosion and promoted the preservation of thick 300 km archean keels even if plate velocities were high during the archean d 2002 elsevier science b v all rights reserved

the growth and concentration of uranium and titanium minerals in hydrocarbons of the carbon leader reef witwatersrand supergroup south africa abstract uranium and gold bearing pyrobitumen from the carbon leader reef in the witwatersrand basin south africa was investigated by high resolution transmission electron microscopy this study provides evidence for the in situ growth of uraninite and anatase nanocrystals in the pyrobitumen implying mobilization and concentration of uranium and titanium by formerly mobile liquid hydrocarbons individual nanocrystals of uraninite and anatase are pervasively distributed and locally isolated within the pyrobitumen matrix crystallization of uraninite and anatase led to the formation of complex nanocrystal aggregates by oriented attachment in which anatase generally provided nuclei for the growth of uraninite single nanocrystals of curite occur locally in channel ways within masses of uraninite nanocrystals consistent with later auto oxidation of uraninite and limited release of water during hydrocarbon maturation on the basis of evidence for the migration of liquid hydrocarbons in the witwatersrand basin and the presence of abundant uraninite and anatase nanoparticles in pyrobitumen a new model is proposed for the transport and concentration of uranium and titanium in the carbon leader reef according to this model liquid hydrocarbons that were circulating in the witwatersrand basin dissolved detrital u ti bearing minerals and transported the uranium and titanium until thermal degradation immobilized the hydrocarbons by solidifying them as pyrobitumen the latter process involved the release of volatiles and the destruction of bonds that may have held the uranium and titanium in solution thereby inducing the growth of individual uraninite and anatase nanocrystals and the formation of complex nanocrystal aggregates within the pyrobitumen

geological constraints on the origin of the mantle root beneath the canadian shield cratonic north america is composed of a cluster of archaean microcontinents centred on the canadian shield and juvenile proterozoic crust that lies mainly buried beneath the sedimentary cover of the western and southern interior platforms the shield is underlain by an anomalous low temperature mantle root that is absent beneath the platform as there appears to be no systematic difference in crustal thickness or density between the shield and the platform the long lived arching of the shield implies an intrinsic buoyancy imparted by the mantle root that more than offsets its colder temperature isotopic and seismic anisotropy data indicate an archaean age for the mantle root close to the time of formation of the overlying crust the preferential development of the mantle root beneath archaean crust is consistent with an origin by imbrication of partly subducted slabs of highly depleted oceanic lithosphere assuming that buoyant subduction was more common in the archaean formation of the mantle root was not dependent on collisional orogenesis as has been suggested but the archaean cratonic mantle was sufficiently buoyant and refractory to survive later tectonic thickening the mantle root persists beneath archaean crust that was transected by mafic dyke swarms and subjected to short lived episodes of post orogenic crustal melting but the root is reduced at mantle plume initiation sites the partitioning of archaean and proterozic crust between the shield and the platform respectively causes the shield to misrepresent precambrian crust as a whole studies of the shield falsely conclude that a high percentage of precambrian crust formed in the archaean and that the proterozoic was characterized by epicontinental volcanism and sedimentation and crustal reworking furthermore the isotopic ratios of detritus eroded from the craton may tend to overestimate the mean age of continental crust

the accuracy of models derived by wkbj waveform inversion summary in this paper the accuracy of velocity depth profiles derived by matching wkbj seismograms to observations is quantitatively evaluated seismograms computed with the wkbj method are generally quite reliable but possess predictable systematic inaccuracies in the presence of strong velocity gradients the effects of these inaccuracies on models derived through wkbj waveform inversion are studied using reflectivity seismograms as data the velocity structure used is an oceanic lithosphere model that contains several transition regions separated by relatively homogeneous layers producing partially reflected reverberations in the reflectivity synthetics that are absent from the wkbj seismograms the inversion incorporates the jumping strategy to solve for the smoothest models consistent with the data we find these solutions to be independent of the starting model and to have a stable basic structure that agrees well with the correct model the differences everywhere less than a seismic wavelength depend on the frequency content of the seismograms reverberations in the reflectivity seismograms that are well separated from wkbj arrivals are treated as noise in the inversion

seismic crustal structure of the north china craton and surrounding area synthesis and analysis we present a new digital model nccrust of the seismic crustal structure of the neoarchean north china craton ncc and its surrounding paleozoic mesozoic orogenic belts 30 45 n 100 130 e all available seismic profiles complemented by receiver function interpretations of crustal thickness are used to constrain a new comprehensive crustal model nccrust the model presented on a 0 25 0 25 grid includes the moho depth and the internal structure thickness and velocity of the crust specified for four layers the sedimentary cover upper middle and lower crust and the pn velocity in the uppermost mantle the crust is thin 30 32 km in the east while the moho depth in the western part of the ncc is 38 44 km the moho depth of the sulu dabie qinling qilian orogenic belt ranges from 31 km to 51 km with a general westward increase in crustal thickness the sedimentary cover is 2 5 km thick in most of the region and typical thicknesses of the upper crust middle crust and lower crust are 16 24 km 6 24 km and 0 6 km respectively we document a general trend of westward increase in the thickness of all crustal layers of the crystalline basement and as a consequence the depth of the moho there is no systematic regional pattern in the average crustal vp velocity and the pn velocity we examine correlation between the moho depth and topography for seven tectonic provinces in the north china craton and speculate on mechanisms of isostatic compensation

imaging the moho and vp vs ratio in the western superior archean craton with wide angle reflections a refraction wide angle reflection survey in northwestern ontario investigated the archean superior province of the canadian shield images of apparent moho topography were produced from both the pmp and sms moho wide angle reflections the average moho depth is 41 km and average p and s wave velocities 6 6 and 3 8 0 05 km s respectively the moho dips southward from 39 5 to 42 km depth over roughly 400 km from ratios of s and p travel times to the moho profiles of vp vs were constructed the average vp vs ratios are 1 74 and 1 76 for on the e w and n s lines poisson s ratio 0 253 and 0 262 respectively vp vs profiles show a sharp increase from west to east and a gradual increase from north to south correlating with the distribution of more felsic continental crust and composite crust in the western superior

integrated potential field and seismic constraints on the structure of the archean metasedimentary english river belt western superior craton canada the english river subprovince is a prominent belt of metasedimentary rocks in the archean western superior province the structure of its western half was investigated by using techniques of enhancement and automatic interpretation of magnetic data and integration of magnetic derived information with seismic and gravity data the results indicate that a suite of exposed felsic plutons that intruded the belt at ca 2698 ma extends under most of the metasedimentary rocks that are exposed at the surface the thickness of the metasedimentary rocks is interpreted to be less than 1 km in areas where it is underlain by the members of this intrusive suite in other areas the metasedimentary rocks attain thicknesses of 3 4 km and appear to be underlain by rocks similar to the gneissic rocks that are exposed in the adjacent metaplutonic winnipeg river subprovince the integration of enhanced magnetic data with gravity data indicates that the large gravity anomaly that extends along the english river belt correlates well spatially and morphologically with the extensive suite of felsic intrusions that underlies the belt suggesting that the crustal component of the gravity anomaly is related to this suite of intrusions we interpret the source of the gravity anomaly as a dense unit comprising anhydrous mineral assemblages that formed within these felsic intrusions in response to low pressure high temperature metamorphism that affected the belt at ca 2691 ma on the basis of geochronological geological and geophysical constraints we propose that this metamorphic episode is linked to the continuation of magmatism at depth after the emplacement of the ca 2698 ma felsic plutons being ultimately related to the advection of mantle heat into the crust during a period of regional extension 2005 elsevier b v all rights reserved

variations in oceanic layer 2 elastic velocities near hawaii and their correlation to lithospheric flexure analysis of the travel times and amplitude range distributions of both compressional and shear wave arrivals on 15 closely spaced refraction profiles reveals a significant systematic and symmetric dependence of average layer 2 velocities on their distance to the north and south of the hawaiian ridge beyond the flexural arch surrounding the ridge the velocity depth solutions indicate a normal layer 2 within the arch approximately 155 km from the ridge the average elastic velocities in layer 2 are lowered by 0 8 0 9 0 2 km s within 75 km of the ridge the average velocities are again normal elastic and elastic plastic flexural models for the regional compensation of the hawaiian islands predict tensional stress drops of 0 8 kbar in the upper lithosphere for the region having lowered velocities in layer 2 which are similar in magnitude with laboratory measurements of the confining pressure drop necessary to reduce velocities in porous basalts by 0 5 km s a significant inverse relationship correlation coefficient of 0 818 exists between the average elastic velocities in the upper 1 0 km of the igneous crust and the strain in the upper crust calculated from these flexural models the correlation between lowered average velocities in layer 2 and increased tensional stresses and strains suggest that crack opening in the upper crust accompanies the flexure these observations are the first reported for a midplate load and corroborate previous suggestions from seismic and flexural data at subduction zones

full waveform inversion of reflection data the theory of a direct inversion method to obtain the p and s wave seismic velocities and the density of a horizontally finely layered elastic medium from vertical and horizontal component plane wave seismograms in the tau p slant stacked domain has been developed the method is based on the downward continuation method of bube and burridge 1983 but plane wave seismograms with a range of ray parameters are used simultaneously to obtain the velocity and density profiles a corrected acoustic approximation of the elastic case has also been developed with which one may obtain a model of the p wave velocity and density from vertical component seismic recordings however it requires the s wave velocity to be estimated a priori for instance as a fixed fraction of the p wave velocity many direct inverse methods including the bube and burridge procedure followed here may lack stability when implemented numerically specially when they are applied to real data which necessarily are band limited at both low and high frequencies the possibility of a stable implementation of our methods has been investigated firstly by using synthetic data for layered acoustic media containing a full range of low frequencies in this case the inverted velocity and density profiles are in good agreement with the test models secondly the indeterminacy associated with the lack in practice of low frequencies has been studied and a method has been devised to estimate the missing low frequency data from travel time information finally the inversion algorithm has then been applied to high quality marine seismic reflection data obtained by the north atlantic transect nat group in the deep ocean using an expanding spread profile esp in this test only the p wave velocity profile was estimated independently because the move out data cannot provide low frequency information about density also the source wavelet of the seismic system had to be estimated from the seismic recordings a stable and plausible estimate of the velocity variation was obtained unfortunately no independent measurement of this velocity structure is available with which to verify the method s accuracy

frequency dependence of q in the mantle underlying the shield areas of eurasia part i analyses of short and intermediate period data summary the results ot short and intermediate period data analyses for the determination of i frequency dependent l model of the niantlc under the shield areas of eurasia are presented the spectra of short period p waves from nuclear explosions in the 1 8 hz frequency range give 0 15 0 2 s using recordings of soviet nuclear explosions at norsar p wave profiles were assembled to study q in the upper mantle under the russian fennoscandian shield analyses of the relative amplitudes and frequency contents of the various upper mantle arrivals support the existence of an upper mantle low q layer under this region but the qp in this layer is high around 700 compared to well studied tectonic regions of the western united states a variety of measurements were used in the 0 3 1 hz intermediate frequency band including s wave periods p and s rise times scs scp periods and s ss periods giving 2 to 3 s or 0 5 to 0 75 s

lower crustal reflectivity from waveform inversion summary deep seismic reflection profiles show that the lower part of the continental crust often contains many strong subhorizontal reflections based on forward modelling these reflections are interpreted as a sequence of alternating high and low velocity layers a waveform inversion provides an optimum model by minimizing the misfit between observed data and synthetic seismograms however it is very difficult to invert for the whole data set partly because of the large computation time needed in modelling and partly because the high amplitude reflected energy in the upper crust dominates the misfit function to overcome these difficulties we propose a three step strategy for inverting the deeper part of seismic reflection data over a narrow aperture in which the velocity structure can be assumed to be 1 d first both source wavelet and recorded wavefield are propagated in the plane wave domain downwards to a particular depth for a given 1 d medium because the source receiver distances are short for deep reflections it is assumed that these reflections are at normal incidence it is also assumed that the near surface velocity is known or has been estimated using other inversion schemes the downward propagation algorithm takes account of nearsurface effects such as multiple reflections and attenuation second starting from a known background velocity model a non linear iterative waveform inversion is applied to the propagated data set the method is based on minimizing the difference sample by sample between observed and calculated wavefields in a least squares sense the forward modelling which consists of calculating the data from the model uses a reflectivity method to compute the synthetic seismogram in the plane wave domain the model perturbation is calculated with the help of the conjugate gradients method the non linear inversion gives the short wavelengths of impedance contrast which is a function of density and velocity in the lower crust third an interpretational step is tried in order to recover the medium wavelengths of velocity the algorithm is tested on synthetic data and then applied to a part of the wam data set south west of england a good fit to the real data with the optimum model predicts that the velocity contrast of the layers in the lower crust varies from 0 2 km s to 0 5 km s and the thickness from at least 0 075 km to 1 km

seismic imaging of the north american midcontinent rift using s to p receiver functions north america s 1 1 ga failed midcontinent rift mcr is a striking feature of gravity and magnetic anomaly maps across the continent however how the rift affected the underlying lithosphere is not well understood with data from the superior province rifting earthscope experiment and the usarray transportable array we constrain three dimensional seismic velocity discontinuity structure in the lithosphere beneath the southwestward arm of the mcr using s to p receiver functions we image a velocity increase with depth associated with the moho at depths of 33 40 4 km generally deepening toward the east the moho amplitude decreases beneath the rift axis in minnesota and wisconsin where the velocity gradient is more gradual possibly due to crustal underplating we see hints of a deeper velocity increase at 61 4 km depth that may be the base of underplating beneath the rift axis further south in iowa we image two distinct positive phases at 34 39 4 and 62 65 4 km likely related to an altered moho and an underplated layer we image velocity decreases with depth at depths of 90 190 7 km in some locations that do not geographically correlate with the rift these include a discontinuity at depths of 90 120 7 km with a northerly dip in the south that abruptly deepens to 150 190 7 km across the spirit lake tectonic zone provincial suture the negative phases may represent a patchy frozen in midlithosphere discontinuity feature that likely predates the mcr and or be related to lithospheric thickness

the s receiver functions synthetics and data example summary recently the s receiver function method has been successfully developed to identify upper mantle interfaces s receiver functions have the advantage of being free of s wave multiple reflections and can be more suitable than p receiver functions for studying mantle lithosphere however because of specific ray geometry and interference of diverse phases the s receiver function method has some technical difficulties and limitations we use synthetic seismograms to demonstrate the feasibility and limitations of s receiver functions for studying mantle structures full wavefield seismograms were calculated using the reflectivity method and processed to generate synthetic s receiver functions for s sks and scs waves results show that s receiver functions can be obtained from waveforms of s sks and scs waves the synthetic s receiver functions for these incident waves show s to p converted phases at all discontinuities in the crust and upper mantle useful ranges of epicentral distances for calculation of s receiver functions are 55 85 for s 85 for sks and 50 75 for scs waves we apply both the s and p receiver function methods to data recorded at broadband station ykw3 in northwest canada the study shows that there is significant agreement among different receiver function methods and demonstrates the usefulness of s receiver functions for imaging the mantle lithosphere

the orogenic superstructure infrastructure concept revisited quantified and revived the historical superstructure infrastructure concept s i expressed contrasts in structural style and metamorphic grade between shallow and deep orogenic levels two dimensional thermal mechanical models provide a quantitative explanation in terms of progressive crustal shortening and thickening phase 1 thermal relaxation and rheological weakening phase 2 and ductile flow at depth phase 3 results predict an upper crustal superstructure dominated by early steep structures separated across a subhorizontal high strain zone from a ductile infrastructure with late gently dipping structures this is consistent with observations from the western superior province these models can account for contrasts in structural style metamorphic grade seismic reflectivity and age between upper and lower crustal levels in contrast to conventional thrust tectonics models the revived s i model shows how young structures can form beneath older ones during progressive convergence thereby encouraging reassessment of standard seismic reflection interpretations

anisotropic upper mantle stratigraphy and architecture of the slave craton the earth s physical properties show a dominantly radial structure which is the result of compositional differentiation isochemical phase changes1 and rheological layering2 rheological layering is perhaps the most difficult to investigate using conventional seismological techniques because the seismic manifestation of this property elastic anisotropy may closely mimic the effects of isotropic heterogeneity3 nonetheless an improved characterization of earth s rheological structure promises important insights into such processes as plate dynamics and continental evolution here i present a methodology for effectively characterizing sharp transitions in anisotropic upper mantle structure using the coda of teleseismic p waves application to seismograms from the slave craton reveals a well developed stratigraphy at least in part anisotropic with major boundaries occurring at nominal depths of 75 135 and 195 km the geometry and sharpness of these discontinuities suggest a structural origin perhaps involving shallow subduction

receiver function imaging of lithospheric structure and the onset of melting beneath the galapagos archipelago abstract the galapagos archipelago represents an opportunity to investigate the properties of young oceanic lithosphere the effects of a hotspot anomaly on lithospheric thickness and melting dynamics in a hotspot ridge interaction here we use data recorded by the signet array and permanent station payg on the islands santa cruz and isabela respectively we used p to s ps and s to p sp receiver functions to constrain crust and mantle structure a simultaneous deconvolution method was used to constrain 1 d structure and also for the modeling of robust features a migrated extended multitaper method was used to investigate 3 d structural variations ps images a velocity increase with depth at 11 7 km probably the base of the pre plume crust or old moho sp imaging and modeling images a second deeper velocity increase at 37 7 km depth a velocity decrease with depth is imaged on average at 75 12 km likely associated with the lithosphere asthenosphere boundary this discontinuity is imaged deeper 82 km in the southwest and shallower 66 km in the northeast near the spreading ridge although the trend is consistent with lithospheric thickening with age the thickness is much larger than predicted by conductive cooling models of 0 10 my oceanic lithosphere we infer a compositional contribution to velocity variations finally a velocity increase with depth is imaged at 125 to 145 15 km depth that is likely associated with the onset of melting the discontinuity is imaged deeper in 3 sectors of the galapagos platform ridge region all coincident with the slowest surface wave shear velocity anomalies in the upper 100 km one is located in the southwest in a hypothesized plume location the other two are to the northwest and northeast possibly illuminating multiple plume diversions related to complex plume ridge interactions

full wavefield inversion of wide aperture sh and love wave data summary reverse time linearized inversion is implemented for synthetic wide aperture sh and love wave data including multiple reflections to estimate the s wave velocity in a two dimensionally inhomogeneous medium complete wide aperture wavefields in which triplications pre and post critical reflections and surface waves love waves are present can be imaged by reverse time inversion to estimate the sh velocity distribution the algorithm operates on common source gathers and involves cross correlation of the source and recorded wavefields to define the steepest descent direction to update the velocity at each point on a finite difference grid effects of noise can be reduced by stacking information from different sources

new insights into the lithosphere beneath the superior province from rayleigh wave dispersion and receiver function analysis we study the azimuthally anisotropic upper mantle structure of the superior craton and grenville province in ontario canada using rayleigh wave phase velocity data in the period range 40 160 s 152 two station dispersion measurements are combined in a tomographic inversion that solves simultaneously for isotropic and anisotropic terms using a least squares technique we perform a series of tests to derive optimal regularization smoothing and damping and to assess the resolution of and trade offs between isotropic and anisotropic anomalies the tomographic inversion is able to resolve isotropic phase velocity anomalies on a scale of 200 300 km and to distinguish between different anisotropic regimes on a 500 km scale across the study region isotropic phase velocity anomalies in the tomographic model span a range of up to 2 per cent around a regional average which is similar to the canadian shield dispersion curve of brune dorman 1963 with phase velocities up to 3 per cent above global reference models the amplitude of azimuthal phase velocity anisotropy reaches a maximum of 1 2 per cent a clear east west division of the study area based on both isotropic phase velocity anomalies and azimuthal anisotropy is apparent in the western superior isotropic phase velocities are generally higher than the regional average anisotropy is observed at all periods with ene wsw to ne sw fast propagation directions at periods 120 s the anisotropy likely results from frozen lithospheric fabric aligned with tectonic boundaries whereas the anisotropy at longer periods is interpreted to arise from present day sublithospheric flow the fast directions from published sks measurements are close to the fast rayleigh wave propagation directions throughout the period range sampled and the large sks splitting times may be accounted for by this near coincidence of fast propagation directions across most of eastern ontario phase velocities are lower than the regional average fast propagation directions rotate from nw se at 40 130 s period to wnw ese at periods 140 160 s the results suggest a difference in fast propagation directions between the anisotropic fabric frozen into the lithosphere and the fabric due to current and recent sublithospheric flow the superior craton and grenville province are characterized by large scale structural variations that reflect the complex tectonic history of the region this study highlights differences between the characteristics of eastern and western ontario and indicates the occurrence of multiple layers of anisotropy in the subcratonic upper mantle

constructing synthetics from deep earth tomographic models recent studies of deep mantle structure indicate strong heterogeneity to conduct high resolution waveform modelling of these structures we have developed a new method to construct 2 d synthetics directly from block style tomographic models unlike the wkbj approximation which utilizes rays overshooting and undershooting receivers our method wkm approximation uses rays that arrive at the receiver first the ray paths from the 1 d layered reference model are used to localize each ray segment where the anomalous velocities are applied by overlay as in tomography next new p i t i p i ray parameter ti traveltime are computed to satisfy snell s law along with their numerical derivative p t which is used to construct a synthetic seismogram similar to the wkbj method as a demonstration of the usefulness of this method we generated wkm synthetics for the d region of high velocities beneath central america based on grand s tomography model reasonable fits to broad band data are obtained by condensing his distributed anomalies into his lowermost mantle layer such a 2 d model predicts synthetics containing a laterally varying s cd triplication similar to observations

the nature of the crust beneath the afar triple junction evidence from receiver functions the afar depression is an ideal locale to study the role of extension and magmatism as rifting progresses to seafloor spreading here we present receiver function results from new and legacy experiments crustal thickness ranges from 45 km beneath the highlands to 16 km beneath an incipient oceanic spreading center in northern afar the crust beneath afar has a thickness of 20 26 km outside the currently active rift segments and thins northward it is bounded by thick crust beneath the highlands of the western plateau 40 km and southeastern plateau 35 km the western plateau shows vp vs ranging between 1 7 1 9 suggesting a mafic altered crust likely associated with cenozoic flood basalts or current magmatism the southeastern plateau shows vp vs more typical of silicic continental crust 1 78 for crustal thicknesses 2 0 can only be explained by significant amounts of magmatic intrusions in the lower crust this suggests that melt emplacement plays an important role in late stage rifting and melt in the lower crust likely feeds magmatic activity the crust between the location of the miocene red sea rift axis and the current rift axis is thinner 2 0 than beneath the eastern part of afar 26 km vp vs 1 9 this suggests that the eastern region contains less partial melt has undergone less stretching extension and has preserved a more continental crustal signature than west of the current rift axis the red sea rift axis appears to have migrated eastward through time to accommodate the migration of the afar triple junction

testing the limits of virtual deep seismic sounding via new crustal thickness estimates of the australian continent we apply virtual deep seismic sounding vdss to data collected from both permanent and temporary seismic stations in australia with the goal of examining i the resilience of the method to the presence of complex lithospheric structure and ii the effectiveness of different approaches for estimating bulk crustal properties namely thickness and vp data from the permanent wrab in the northern territory station is ideal for benchmarking vdss large number and favourable distribution of recorded earthquakes with the results from several approaches agreeing on a thickness of 40 42 km application of vdss to data from the temporary bilby array a linear distribution of broadband stations that traverses central australia shows that strong moho reflections can be retrieved with as few as two earthquakes even at the transition between crustal blocks of different character and in the presence of thick sedimentary basins crustal thickness varies between 36 54 km and compare well with the reflectivity character of nearby deep seismic reflection lines furthermore we find that off line estimates of crustal thickness calculated by binning the source regions according to back azimuth produce estimates of crustal thickness that are consistent with the regional geology overall we find that vdss is a powerful technique for estimating crustal thickness and velocity due to its insensitivity to complex short wavelength structure and requirement of a small number earthquakes to produce a stable result however not all schemes tested for extracting bulk crustal properties appear to be robust and stringent data quality checking is still required during implementation

a global study of s to p and p to s conversions from the upper mantle transition zone summary long period data of the global digital seismograph network gdsn recorded over the three year period from 1984 to 1986 were studied for the occurrence of s p and p s conversions from the upper mantle transition zone that appear as precursors to teleseismic s arrivals conversions of this type were identified on a large number of single station records simple stacking of many records enhanced the appearance of converted phases and demonstrated that no major lateral variations in the nature of the transition zone exist between various tectonic regions s p and p s conversions from the 400 km discontinuity were best observed at distances between 70 and 85 while conversions from the 670 km discontinuity showed up best at distances beyond 87 the analysis of published source mechanisms and comparison with synthetic seismograms suggests that the appearance of converted phases is primarily governed by the earthquake radiation pattern phases that have undergone s p conversions beneath the receiver are best observed from dip slip events that radiate strong sv and weak p waves towards the station p s conversions beneath the source area on the other hand are frequently observed from events that radiate strong p and little sv energy towards the station and also from some strike slip events comparison of observed with synthetic seismograms suggests that the prem model of dziewonski anderson 1981 explains most of the observations observed s p and p s conversions from the 670 km discontinuity however often have larger amplitudes than in the synthetics constructive interference of converted waves with the p wave coda source radiation effects and a velocity contrast across the 670 km discontinuity which is higher than in prem may all contribute to the discrepancy

archaean subduction inferred from seismic images of a mantle suture in the superior province plate tectonics provides the basis for the interpretation of most current terrestrial tectonic activity and is widely accepted as having been active over much of the earth s history1 yet the timing of initiation of this process is subject to debate2 9 so far the earliest seismic evidence for plate tectonics has come from a fossil mantle suture in the svecofennian orogen 1 89 gyr ago 10 and from inferred plate convergence subduction and accretion in the trans hudson orogen 1 91 1 79 gyr ago 11 as yet seismic data from archaean areas have been able to demonstrate only the importance of compression in the construction of the continental crust12 15 here we present seismic data from a collision zone in the superior province of canada involving the abitibi granite greenstone subprovince and the plutonic arc related opatica belt we interpret dipping seismic reflections that extend 30km into the mantle as representing a relict 2 69 gyr old suture associated with subduction although crustal structure lithospheric thicknesses and convergence rates may have differed from those seen today these seismic data provide direct evidence that plate tectonics was active in late archaean times 800 myr earlier than indicated by previous seismic reflection surveys

upper mantle s velocity structure of north america we have inverted fundamental and higher mode rayleigh waveforms from 685 vertical component broadband seismograms with wave paths over north america to obtain an image of the upper mantle s velocity structure down to 660 km among the well resolved features of the new model are 1 a high velocity root beneath the north american craton which extends no deeper than 250 km except near the archean core of the craton where depths of 350 km are reached 2 a weak band of low velocity along the eastern margin of the north american craton which reaches into the transition zone 3 a low velocity slab window beneath the western united states down to a depth of 300 km 4 areas of low uppermost mantle velocity beneath the cascade volcanoes the yellowstone hotspot track the colorado plateau the sierra madre occidental and the grabens bordering the jalisco block and 5 a pronounced band of high velocities in the transition zone coinciding with the expected location of the subducted trailing fragments of the farallon plate we introduce several improvements to the method of partitioned waveform inversion which was used to compute the new model rather than to correct for an estimated depth to the mohorovicic discontinuity we leave the crustal thickness as a free though yet poorly resolved parameter in the inversion we also improve the windowing and filtering operator used to select uncontaminated waveforms

2 5d full wavefield viscoacoustic invercion1 full wavefield inversion for distributions of acoustic velocity density and q on a vertical slice through a25d model is implemented for common source gathers in a cross hole geometry the wavefield extrapolation used is 3d so all geometrical spreading scattering reflection and transmission effects are correctly and automatically compensated for in order to keep the number of unknowns tractable application was limited to 2 5d models of known geometry the latter assurnes a prior step such as tomography to fix the layer geometries with the model geometry fixed reliable solutions are obtained using synthetic data from only two independent source locations solutions from data with noisy and missing traces are comparable to those from noise free data but with higher residuals when the source locations are spatially widely separated conunon source gathers may be summed and treated as a single wavefield to yield the same model estimates as when the individual source wavefields are treated separately at substantially reduced cost inversions for full 3d parameter distributions can be handled with the same software requiring only solution for more unknowns

imaging the upper part of the red lake greenstone belt northwestern ontario with 3 d traveltime tomography seismic reflection line 2b was shot across the archean red lake greenstone belt and sydney lake fault zone that marks the northern boundary of the english river metasedimentary belt as part of the western superior lithoprobe transect three dimensional tomographic inversion of first arrival traveltimes recorded in this survey delineate the subsurface to depths as great as 1 5 km around this crooked two dimensional seismic line within the red lake greenstone belt p wave velocities of 6 2 7 0 km s 1 occur at 500 m depth in the mesoarchean balmer assemblage clearly distinguishable from the lower velocities of 5 1 6 1 km s 1 of the neoarchean confederation assemblage although the overall range of velocities in the metasedimentary rocks of the english river subprovince is similar to that found in the confederation assemblage lower velocities of 5 1 5 4 km s 1 are found in the upper 300 m of the metasedimentary rocks in particular two 2 3 km wide east northeast striking zones of low velocity are associ

derivation of layer parameters of an elastic medium from reflection coefficient matrices in this paper we develop a recursive algorithm to obtain the layer parameters of an elastic medium density p wave velocity s wave velocity from reflection coefficient matrices in terms of energy flux ratios for a non normal incidence case we define a layer impedance matrix analogous to the impedance of an acoustic medium next we derive a matrix relationship between the layer impedance matrix of the n 1st layer and the reflection coefficient and parameter matrices of the n th layer this relationship leads to recursively computing the parameters of the subsurface we show that the elastic case unlike the acoustic case allows one to recover the layer parameters from the impedance matrix for non normal incidence the results of this work play a key role in the solution of the inverse problem with non normal incidence plane wave seismic data when using a downward continuation technique

geoelectric response of archean lithosphere in the western superior province central canada abstract magnetotelluric mt soundings were made at more than 230 sites in the western superior province in central canada as part of the lithoprobe western superior transect investigations the superior province forms the nucleus of the north american continent its western part contains a regional pattern of east west trending 100 200 km wide granite greenstone and metasedimentary belts or subprovinces the objectives of the mt study are to define the electrical resistivity structure of the crust of the subprovinces and their boundaries and to determine if there exists a signature in the upper mantle related to the formation of the craton the exceptionally large mt database for the western superior province provides a three dimensional sampling of the resistivity structure in this paper the primary mt responses from the data set including geoelectric strike apparent resistivity and phase are used to define first order geoelectrical structures in the region apparent resistivity responses indicate the ca 3 0 ga north caribou terrane contains extremely resistive crust crust to the south accreted at ca 2 7 ga is significantly more conductive with resistivities similar to in the southeast superior province the results suggest that the supracrustal component of the north caribou terrane provides minimal enhancement of the crustal conductivity at the southern margin of the former north caribou superterrane induction arrows define a conductivity anomaly that extends in an east west direction for at least 600 km and causes a reversal of arrows in a region 100 200 km wide this anomaly is interpreted to be caused by a conductive component of the 2 7 ga metasedimentary rocks of the english river subprovince the observations demonstrate the ability of electromagnetic methods to map some neoarchean continental margins in the southwest superior province geoelectric strike azimuths for periods corresponding to signal penetration into the upper mantle are aligned with the trend of subprovince boundaries and are interpreted to be related to fossil structure in the mantle lithosphere this result provides support to the interpretation that the lithospheric mantle of the western superior province has remained intact since the 2 7 ga accretion of the subprovinces the mt responses from the western superior province have thus defined a number of large scale resistivity structures providing information on tectonic structure and history and confirming the value of electromagnetic imaging of archean lithosphere

tau p seismic data for viscoelastic media part 2 linearized inversion1 an approach to extraction of viscoelastic parameters from seismic data is implemented and succesfully tested viscoelastic inversion is performed using adaptive damping factors to control the sensitivity of the viscoelastic parameters in relation to the p seismic data a priori information is incorporated through the damping factors as standard deviations of the data and of the viscoelastic model parameters the stability of the inversion process is controlled by the variation of the damping factors as a function of the residual errors and parameter updates at each iteration tests on synthetic and real data show that p and s wave quality factors qp and qs in addition to p and s wave velocities and density cp cs and p can be extracted from p seismic information singular value decomposition analysis demonstrates that estimated qp and qs values are more affected by the presence of data inaccuracies and noise than are those of cp and p cs and qs are not uniquely recovered due to the limited contribution of p s converted waves knowledge of the viscoelastic parameters is of particular importance in accurately describing petrophysical properties of rocks and pore fluids existing in the subsurface this is demonstrated with real data from the gulf of mexico

shear wave splitting observations in the archean craton of western superior shear wave splitting observations in the archean superior province of the canadian shield show moderate to large delay times 1 1 2 1 s with azimuths suggesting upper mantle anisotropy is subparallel to the structural grain of the craton two regions with uniform anisotropy azimuth 62 7 and 87 3 are separated by a transitional zone showing strong dependence of observed azimuth on source direction suggesting that lateral structure is being observed no splitting was observed at stations near the coast of hudson bay continuing a pattern of weak splitting in surrounding proterozoic orogenic belts current absolute plate motion is consistent with the direction of anisotropy but would not explain the regional contrasts instead the anisotropy appears to be related to archean structure and tectonic history

seismic anisotropy the state of the art ii summary the theory causes observations and possible applications of seismic anisotropy in the earth have developed considerably since the previous state of the art paper was published in 1977 the behaviour of waves in layered anisotropic media is now much better understood and the evidence for seismic anisotropy indicates that anisotropy is likely to be present throughout much of the crust and upper mantle the top few hundred kilometres of the mantle appears to be anisotropic with the orientations aligned by the present or palaeo stress field the upper part of the crust is frequently anisotropic probably due to cracks differentially aligned by the non lithostatic stresses the possibility of being able to monitor crack geometry by seismic techniques opens a wide range of applications in currently important activities

present seismic evidence for a boundary layer at the base of the mantle to improve the global coverage needed in investigating the observational consequences of boundary layer models the existing data set has been extended with long period diffracted p and sh from the sro network and short period diffracted p from the norsar array it was found that on average the amplitude decay of diffracted sh is relatively small in accordance with the majority of previously published wwssn data and consistent with the observed weakness in dispersion of this phase these results in combination with the apparent absence of anomalously high dt d can be explained by a relatively thin 100 km low velocity zone at the base of the mantle long period diffracted p wave data probably lack the required resolution for detecting a relatively thin transition zone such a zone with relatively low velocity is allowed by an average of travel time and amplitude data across the core shadow boundary and appears to be required by the norsar observations of short period diffraction far into the shadow however recent results of individual short period p wave studies pertaining to geographically limited regions only including also the norsar observations are partly conflicting unless lateral variations exist on balance current thermal models appear to be consistent with but not required by the present seismic evidence

images of a lower crustal oceanic slab direct evidence for tectonic accretion in the archean western superior province the archean western superior province in canada is the type area for proposed archean plate tectonics seismic images from this region provide direct evidence for assembly of the craton by terrane accretion and for a large slab of remnant oceanic crust preserved at the base of the crust this slab with inferred garnet amphibolite composition adds a critical piece of evidence to previous suggestions that archean subduction was at a shallow angle and that some neoarchean tonalite trondhjemite granodiorite suites distinct from most modern day suprasubduction magmas are melts primarily derived directly from subducted slabs

variations in axial magma lens properties along the east pacific rise 9 30 n 10 00 n from swath 3 d seismic imaging and 1 d waveform inversion we use three dimensional multistreamer seismic reflection data to investigate variations in axial magma lens aml physical properties along the east pacific rise between 9 30 n and 10 00 n using partial offset stacks of p and s converted waves reflecting off the top of the aml we image four 2 4 km long melt rich sections spaced 5 10 km from each other one dimensional waveform inversion indicates that the aml in a melt rich section is best modeled with a low vp 2 95 3 23 km s and vs 0 3 1 5 km s indicating 70 melt fraction in contrast the aml in a melt poor section requires higher vp 4 52 4 82 km s and vs 2 0 3 0 km s which indicates 40 melt fraction the thicknesses of the aml are constrained to be 8 32 m and 8 120 m at the melt rich and poor sites respectively based on the aml melt mush segmentation imaged in the area around the 2005 2006 eruption we infer that the main source of this eruption was a 5 km long section of the aml between 9 48 n and 51 n the eruption drained most of the melt in this section of the aml leaving behind a large fraction of connected crystals we estimate that during the 2005 2006 eruption a total magma volume of 9 83 106 m3 was extracted from the aml with a maximum of 71 106 m3 left unerupted in the crust as dikes from this we conclude that an eruption of similar dimensions to the 2005 2006 one would be needed with a frequency of years to decades in order to sustain the long term average seafloor spreading rate at this location

seismic tomographic images of the cratonic upper mantle beneath the western superior province of the canadian shield a remnant archean slab abstract knowledge of the velocity structure of the upper mantle beneath the western superior province wsp is key to understanding better the accretionary processes active during the archean to that end teleseismic p and s wave travel times recorded as part of the teleseismic western superior transect twst were inverted for their respective seismic velocities this experiment involved 17 portable broadband stations arrayed in northern ontario canada so as to cross cut the strike of many subprovinces as well as the boundary with the proterozoic trans hudson orogen to the north the 5 month deployment yielded 1423 p wave and 651 s wave high quality residuals for inversion the resulting tomographic images reveal three apparently robust velocity anomalies represented by i a dipping tabular high velocity anomaly ii a relatively shallow low velocity anomaly directly above the positive anomaly and iii a deep low velocity body the first anomaly may be interpreted in the western superior context as a 30 50 km thick eclogite dunite layer representing remnant subducted oceanic lithosphere the presence of such a body within the cratonic root would suggest its origin at around 2 7 ga and the apparent se nw strike is noticeably oblique to the main ew trend of the subprovince boundaries the low velocity anomalies may be related to processes that occurred at the edges of the descending slab or they may be expressions of later upwelling material the presence of a thick cratonic root 300 km may also be revealed by the tomographic images overall these travel time results are considered compatible with late archean structures at depth resembling those of modern subduction tectonics though other possibilities e g small scale convection cells exist

the velocity structure of the crust and upper mantle under china from broad band p and pp waveform analysis summary we model 45 p and pp waveforms at epicentral distances of 14 to 40 primarily sampling western and central china to determine regionalized p wave velocity variations for the crust and upper mantle in this region observed p and pp waveforms are matched by 1 d forward modelling using the reflectivity technique our approach is to first find a homogeneous average velocity model for china which can match the observed broad band waveforms filtered with a long period instrument response this model is then used as a starting model for the matching of the broad band waveforms to investigate the lateral velocity variations our preferred average model wch has a 50 km thick crust with a velocity of 6 4 km s a p velocity of 8 15 km s and a low velocity zone between 100 km and 175 km depth it has intermediate characteristics between an active tectonic region and a stable shield compatible with previous s wave modelling results modelling of the broad band waveforms indicates significant lateral deviations from model wch for the tibetan plateau we observe larger crustal thickness 55 70 km and p velocity 8 15 8 25 km sc in combination with a low velocity zone that varies in magnitude throughout the tibetan region lower velocities than our average model may be present below 200 km depth our modelling indicates a thin lid of about 30 45 km under tibet we infer from our results and previous work that crustal shortening and thickening is the most likely process responsible for the central and northern tibetan plateau s thick crust and high average elevation and that the indian plate has probably only underplated the southernmost part of the plateau mantle convection associated with the crustal shortening process has led to strong lateral heterogeneity in the upper mantle under tibet higher p velocities in western tibet might indicate that this region is presently undergoing crustal shortening paths traversing eastern china require a constant velocity lid of about 100 km thickness a p velocity of 8 12 8 14kms and a crustal thickness between 27 and 35 km a preliminary model for northern china suggests a crustal thickness between 43 and 50km a p velocity of about 8 0kms and a 100km thick lid with a positive velocity gradient for this region

further evidence for the lay discontinuity beneath northern siberia and the north atlantic from short period p waves recorded in france abstract we analyze data from the french laboratoire de detection geophysique ldg network of digital short period seismic stations the p waves from several earthquakes in the kuril okhotsk region display clear secondary arrivals in the 75 85 range which are not explained by smooth lower mantle models the move out of the arrivals across the network indicates a deep mantle origin we interpret them as waves reflected off or transmitted at the lay discontinuity about 300 km above the core mantle boundary the bounce points at this discontinuity are located beneath northern siberia slightly to the northwest of the region for which a similar structure has been reported we propose tools for investigating the lateral variations of the d structure on the scale 100 1000 km the method we propose is based on the deconvolution of the data or convolution of synthetics by a source time function derived from the data we show that the method helps identification of the waves that interact with the lay discontinuity and enables a more objective mapping of its lateral variations evidence for bumps a few kilometers high on the lateral scale of a few tens of kilometers is proposed we also probe a spot of the lower mantle beneath the north atlantic a detailed analysis of the waveforms from a chiapas earthquake suggests that the lay discontinuity is also present in that part of the world

shear wave velocity structure of the oceanic lithosphere from ocean bottom seismometer studies summary a comprehensive interpretation of two shear wave refraction profiles recorded on ocean bottom seismometers provides velocity depth models for the oceanic crust and upper mantle the data were obtained as part of an investigation of lithospheric structure carried out in the nootka fault zone area off the west coast of canada one dimensional s wave velocity depth models are derived from the data by trial and error comparisons with wkbj synthetic seismograms the results of this study confirm crustal characteristics interpreted previously from the compressional wave data layers 3a and 2b in the upper crust are characterized by rapid increases in seismic velocities shear wave velocity increases from 2 2 to 3 6 km s l over an interval of 3 5 km although the exact nature of the shearwave velocity gradient is uncertain in contrast the shear velocity in layer 3a remains fairly constant with depth at values near 3 6km s no velocity discontinuity is observed at the moho instead the transition from crust to upper mantle is marked by the presence of a velocity gradient zone layer 3b the shear wave velocities in the upper mantle are 4 5 and 4 6 km s l in directions approximately parallel to the spreading ridge and parallel to the direction of spreading respectively this low degree of velocity anisotropy contrasts sharply with the 10 per cent anisotropic effect observed for the pn velocities combining the p wave interpretation with this s wave study enables evaluation of poisson s ratio as a function of depth except for low values of poisson s ratio found in the upper crust the seismic properties of the oceanic crust and upper mantle interpreted in this study agree remarkably well with those of laboratory measurements on ophiolite samples

lithospheric structure of the arabian and eurasian collision zone in eastern turkey from s wave receiver functions summary crustal and upper mantle seismic discontinuities beneath eastern turkey are imaged using teleseismic s to p converted phases three crustal phases are observed the moho with depth ranging between 30 and 55 km indicating variable tectonic regimes within this continental collision zone an upper crustal discontinuity at approximately 10 km depth and various crustal low velocity zones possibly associated with recent quaternary volcanism imaging of the upper mantle is complicated by the 3 d geometry of the region in particular due to the bitlis zagros suture zone however several upper mantle s to p converted phase are identified as being the signature of the lithosphere asthenosphere boundary lab the inferred lab for the eastern anatolian accretionary complex indicates that eastern turkey has an anomalously thin between 60 and 80 km lithosphere which is consistent with an oceanic slab detachment model the observed lab phases for the arabian shield and iranian plateau indicate that lithospheric thickness for these stable regions is on the order of 100 to 125 km thick which is typical of continental margins

the structure of the lowermost mantle determined by short period p wave amplitudes summary the seismic velocities in the d region lowermost 200 km of the mantle are recognized to be anomalously low though the details of the velocity structure are not known the structure of d is important in particular whether a smooth velocity model is appropriate or not a smooth decrease in the seismic velocities would be consistent with a thermal boundary layer at the base of the mantle we have used the amplitudes of short period t 1s p and diffracted p waves to investigate the internal structure of d a short period amplitude data set is obtained by using underground nuclear events as sources and applying receiver corrections to the amplitudes receiver effects are largely responsible for the factor of 8 scatter in the amplitudes of the north american wwssn stations applying receiver corrections reduces the scatter to a factor of 2 thereby providing a quantitatively useful amplitude profile into the core shadow using soviet events and north american wwssn stations the d layer beneath the north polar region is well sampled the core shadow at t 1 s begins sharply at a distance of 95 5 and the slope of the amplitude decay is well defined also the amplitudes decrease slightly from 87 to 90 then increase to 95 synthetic seismograms are used to test various earth models with the important conclusion that the amplitudes from smooth d models with a small velocity gradient in d decay too slowly in the shadow this mismatch cannot be satisfactorily explained by random forward scattering or a thin low q layer within d anelastic calculations show that a thin low q layer in d decreases the amplitudes before the shadow with little effect on the decay slope in the shadow all of the features of the observed amplitude profile can be explained as the interference effects of a model that has a low velocity zone in the upper part of d followed by a normal velocity gradient in the lower part of d this type of model polar series also explains the scatter often observed in dt d beyond 90 the interference effects and required velocity changes in d are small and long period amplitudes will respond only to the averaged velocity gradient in d the polar models imply a compositional and or phase change at the top of d thus the preferred seismological model does not allow the d region to be interpreted as a single thermal boundary layer between the mantle and core

lithospheric variations across the superior province ontario canada evidence from tomography and shear wave splitting 1 the superior province of the canadian shield is the largest contiguous region of the archean crust a combination of data from multiple experiments is used to obtain shear wave splitting parameters and a three dimensional tomographic velocity model beneath a large portion of the superior corresponding approximately to the province of ontario shear wave splits are obtained at 24 sites across the superior displaying strong averaging 1 34 s ene wsw splitting at stations west of 86 w and weaker 0 67 s e w splitting in the east the fast direction is subparallel to both absolute plate motion and tectonic belt boundaries the recovered tomographic velocity model shows a major boundary oriented nnw sse separating high velocities in the western superior ws from low velocities in the east and coinciding with the divide between weak and strong shear wave splits other features include a 200 km thick low velocity anomaly corresponding to the nipigon embayment a 1 0 ga failed rift branch and a linear low velocity anomaly in the east attributed to the great meteor hot spot track the nipigon anomaly in the western portion of the model is probably in situ while the great meteor track is displaced from crustal features associated with the hot spot we interpret this displacement as evidence that the eastern lithosphere has been deformed by basal drag while the western lithosphere has remained stable and propose that the east west lithospheric boundary we have detected represents a change in mechanical properties between stronger higher velocity western material with consistent anisotropic fabric and weaker eastern material with more variable fabric