seismic anisotropy tracing plate dynamics in the mantle elastic anisotropy is present where the speed of a seismic wave depends on its direction in earth9s mantle elastic anisotropy is induced by minerals that are preferentially oriented in a directional flow or deformation earthquakes generate two seismic wave types compressional p and shear s waves whose coupling in anisotropic rocks leads to scattering birefringence and waves with hybrid polarizations this varied behavior is helping geophysicists explore rock textures within earth9s mantle and crust map present day upper mantle convection and study the formation of lithospheric plates and the accretion of continents in earth history

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

evidence for ancient lithospheric deformation in the east european craton based on mantle seismic anisotropy and crustal magnetics abstract we present new shear wave splitting measurements performed at 16 stations on the east european craton and discuss their implications in terms of upper mantle anisotropy for this geophysically poorly known region previous investigations of mantle anisotropy in central europe have shown fast directions aligning smoothly with the craton s margin and various suggestions have been proposed to explain their origin such as asthenospheric flow or lithospheric frozen in deformation here we aim at investigating the continuation of this shear wave splitting pattern further to the east into the east european craton for the craton the interpretation appears to be less ambiguous than for central europe since several arguments support lithospheric anisotropy in this region 1 the large scale coherence within either of the four constituting blocks and the significant variations between the blocks on a small scale 2 the weak correlation with absolute plate motion vectors and 3 the good correlation between anisotropy and crustal features for which we use magnetic field alignments as a proxy rather good correlation of these magnetic features with seismic fast orientations strongly supports the idea of vertically coherent deformation throughout upper mantle and crust the observed splitting orientations thus reflect the last tectonic events of each block frozen in into the lithosphere for hundreds of millions of years

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

anisotropic zonation in the lithosphere of central north america influence of a strong cratonic lithosphere on the mid continent rift we present shear wave splitting analyses of sks and skks waves recorded at sixteen superior province rifting earthscope experiment spree seismic stations on the north shore of lake superior as well as fifteen selected earthscope transportable array instruments south of the lake these instruments bracket the mid continent rift mcr and sample the superior penokean yavapai and mazatzal tectonic provinces the data set can be explained by a single layer of anisotropic fabric which we interpret to be dominated by a lithospheric contribution the fast s polarization directions are consistently ene wsw but the split time varies greatly across the study area showing strong anisotropy up to 1 48 s in the western superior moderate anisotropy in the eastern superior and moderate to low anisotropy in the terranes south of lake superior we locate two localized zones of very low split time 0 6 s adjacent to the mcr one in the nipigon embayment an mcr related magmatic feature immediately north of lake superior and the other adjacent to the eastern end of the lake at the southern end of the kapuskasing structural zone ksz both low splitting zones are adjacent to sharp bends in the mcr axis we interpret these two zones along with a low velocity linear feature imaged by a previous tomographic study beneath minnesota and the dakotas as failed lithospheric branches of the mcr given that all three of these branches failed to propagate into the superior province lithosphere we propose that the sharp bend of the mcr through lake superior is a consequence of the high mechanical strength of the superior lithosphere ca 1 1 ga

constraints on mantle anisotropy beneath precambrian north america from a transportable teleseismic experiment data from the apt89 transportable teleseismic experiment was examined to characterize the variations in shear wave splitting along a 1500 km traverse in north america see silver et al 1993 this issue three geologic terranes were sampled the western superior province of the canadian shield the trans hudson orogen and the wyoming craton the primary goal was to determine the extent to which the anisotropy was controlled by archean and proterozoic tectonic episodes of the large suite of teleseismic events recorded 5 were suitable for determining the fast polarization direction and delay time t from the phases sks and skks the resulting observations reveal two important features first the values of display good geologic coherence stations along the northern part of the array in the exposed shield reveal a consistent ene direction for that is generally parallel to exposed geologic structures within the part of the superior province buried beneath phanerozoic cover rotates to a ne sw azimuth as do geophysical indicators of geologic fabric moving onto the trans hudson changes abruptly to a more ew azimuth the stations within the wyoming craton suggest values with a more northerly direction although the waveforms from these stations are more complex second t which ranges between 0 40s and 1 75s shows systematic variations across the traverse the largest delay times approaching 2 s are found in a 250 km wide band from red lake ontario to the u s canada border t is reduced to about 1 s or less both north and south of this region for the stations with detectable splitting we find an inverse relationship between s isotropic station delays derived from the portable data set and t which is most easily explained by the anisotropy being primarily localized in the fast presumably lithospheric mantle the splitting observations overall suggest that mantle anisotropy along this traverse is dominated by precambrian fossil anisotropy that is preserved in the subcontinental lithosphere

laboratory and seismological observations of lower mantle isotropy we have carried out seismological and mineral physics investigations to identify the extent and origin of elastic anisotropy in the lower mantle based on observations of shear wave splitting we conclude that the lower mantle is effectively isotropic this result is surprising since the lower mantle is composed of elastically anisotropic minerals silicate perovskite mgo sio 2 that have been extensively strained by geologic processes to reconcile the seismic observations we have measured texture development during deformation and high temperature recrystallization of mantle silicates we show that these experiments provide a direct explanation for the shear wave splitting measurements the lower mantle appears isotropic because silicate perovskite maintains an isotropic texture during deformation and recrystallization

influences of storm erosion and deposition on rhythmites of the upper wenchang formation upper ordovician around tonglu zhejiang province china abstract tonglu rhythmites upper ordovician in zhejiang province east central china display three orders of cyclicity in sandstone and mudstone layer thickness millimeter thick alternations of sandstone and mudstone laminae are ascribed to single tidal cycles centimeter thick alternations of sand dominated layers sdls and mud dominated layers mdls are interpreted to be related to alternation of storm and calm weather conditions with a periodicity longer than that forced by neap spring tidal cyclicity the sdls are interpreted as storm deposits on the basis of presence of scour structures abundant intraformational mud pebbles oscillation ripples and thinning upward trends in the sandstone laminae a third meter thick cycle of variations in sandstone lamina thickness is interpreted as a reflection of cross shore changes in coastal dynamics and water depth in the subtidal intertidal environment storm waves usually considered to be random destructive factors to normal cyclic deposits are here highlighted as effective agents of sediment transport and deposition of the sand dominated layers this study aims at improving our facies level understanding of the genesis and preservation of storm related tidal flat rhythmites on open coasts and highlights the fact that storm related facies can mimic the cyclicity that is commonly ascribed to neap spring tidal variation

quantifying the oldest tidal record the 3 2 ga moodies group barberton greenstone belt south africa the 3 2 ga moodies group in the barberton greenstone belt south africa contains the oldest preserved record of tides the tidal record is preserved in a tidal sand wave deposit in the lower moodies group as bundles of sandstone foresets separated by mudstone drapes detailed analysis of rhythmic foreset bundles permits quantification of the tidal record and reveals a hierarchy of diurnal fortnightly and monthly tidal periodicities thick thin pairs of foreset bundles reflect deposition from semidiurnal dominant and subordinate flood tidal currents respectively cyclic variations in foreset bundle thicknesses record longer period changes in strength of the dominant semidiurnal tidal currents consistent with neap spring neap tidal cyclicity alternating thicker and thinner neap spring neap cycles are comparable to anomalistic perigean apogean tidal signatures this quantitative record of tides in the middle archean moodies group represents by 2 2 b y the oldest such documentation tidal cyclicity recognized in the moodies sand wave deposit is comparable to that recorded in modern tidal settings and identified in the carboniferous rock record and is most compatible with a lunar orbital shape similar to that existing today

hummocky cross stratification tropical hurricanes and intense winter storms most previous workers have inferred a storm origin for hummocky cross stratification which typically occurs in shallow marine deposits on the modern earth the only storms capable of profoundly affecting shallow marine depositional environments are severe tropical cyclones hurricanes and mid latitude winter wave cyclones intense winter storms this paper examines the palaeogeographic distribution including palaeolatitude and palaeogeographic setting of 107 occurrences of hummocky cross stratification ranging in age from the proterozoic to recent in each of these stratigraphic units both palaeolatitude and palaeogeography are consistent with a direct storm influence associated with the passage of hurricanes or winter storms directly over the site of deposition this palaeogeographic evidence lends support to the inferred storm origin for hummocky cross stratification further the distribution of the structure suggests that most occurrences 73 were generated by tropical hurricanes the remaining 27 being generated by intense mid latitude winter storms the preferential generation of hummocky cross stratification by hurricanes is consistent with 1 the known differences in the nature of the bottom flows generated by the two major storm types and 2 the inferred nature of the flows which form hummocky cross stratification hurricanes couple less effectively with the water column than do intense winter storms due to this ineffective coupling hurricane generated bottom flows tend to be oscillatory or multidirectional dominant with only minor unidirectional components of motion in contrast intense winter storms generally do couple effectively with the water column generating bottom flows which possess a dominant or significant unidirectional component most previous workers have suggested that hummocky cross stratification forms under oscillatory or multidirectional dominant flow thus it is conceptually reasonable that the vast majority of ancient occurrences of hummocky cross stratification were probably hurricane generated as suggested by the aforementioned palaeogeographic distribution the proterozoic palaeozoic neogene and quaternary were times when global climate was similar to that of today the distribution of hummocky cross stratification deposited during these times suggests that both hurricanes and intense winter storms occupied latitudinal belts during these times which were essentially identical to those occupied by their modern counterparts the mesozoic and palaeogene were non glacial times when global climate was much warmer than that of today the distribution of hummocky cross stratification deposited during this interval suggests that hurricanes occurred more frequently at higher latitudes during non glacial times than they do at present the possibility of a broadened hurricane belt during the mesozoic and palaeogene is consistent with climatic considerations a limited number of mesozoic and palaeogene rock units containing hummocky cross stratification were deposited in palaeogeographic settings that preclude a direct hurricane influence these examples were deposited in the middle latitudes suggesting that intense winter storms continued to form hummocky cross stratification in the middle latitudes during these much warmer times some previous workers have suggested that tsunamis may be capable of generating hummocky cross stratification the palaeogeographic distribution of the structure does not support an origin due to tsunamis lacustrine examples of hummocky cross stratification reported herein are the first known non marine occurrences they suggest that storm effects strongly influence the sedimentary record of some lakes

mantle variation within the canadian shield travel times from the portable broadband archean proterozoic transect 1989 we report travel times from the archean proterozoic transect 1989 this type of data set recorded by a transect of portable broadband instruments allows us to make inferences about mantle structure in the region between the wyoming craton and the superior province of the canadian shield with station separations of 50 to 100 km and frequencies up to 5 hz the resolution of lateral changes is increased by nearly an order of magnitude over previous studies to a scale that allows us to study the relation between velocity variation in the continental upper mantle surface geology tectonic features and age provinces travel times of direct and later phases are obtained from waveform matching the values are corrected for crustal contribution and inverted for the vertical path upper mantle delay tum under each station as well as the azimuthal dependence of this quantity the prominent feature in the upper mantle delays tum is the variation by at least 1 5 s for s much of which occurs over a narrow zone of just a few hundred kilometers width this suggests a major lateral upper mantle transition which does not coincide with the surface geological edge of the canadian shield but is located within the shield this same transition is also observed in shear wave splitting delay times surprisingly however the p delays do not exhibit a corresponding variation we address this apparent contradiction and show how it may be explained in conjunction with anisotropy in the subcontinental lithosphere a simplified thermal model of the lithospheric transition zone in which temperature controls the degree of crystallographic alignment and thus seismic anisotropy predicts this phenomenon

tidal bedding and tidal cyclicities within the intertidal sediments of a microtidal estuary dyfi river estuary west wales u k abstract in the microtidal dyfi river estuary west wales the characteristic sedimentary structure within the intertidal sediments is tidal bedding this consists of sand mud couplets by means of thickness measurements and detailed observations the following conclusions can be drawn 1 the thicknesses of and structures in both mud layer and sand layer of each couplet vary systematically downward in the studied cores and trenching profiles variations are mainly due to differences in the current speed and tidal range between tides to difference of individual neap spring tides and to the possible effect of filtering of currents due to being in the intertidal zone 2 the patterns of sand mud couplet thickness variations reflect two main types of tidal cyclicities diurnal inequality and the neap spring neap tidal cycle which is in turn related to the lunar cycle 3 the mud layer of each couplet represents deposition from suspension at times of tidal slack no ebb slack water period during low tide in the intertidal zone thus no ebb mud deposition during low water whilst the sand layer is deposited by asymmetrical bidirectional tidal currents in either the ebb or flood half of a tidal cycle 4 the tidal regime derived from these variation patterns is characterized by a mixed predominantly semi diurnal form which is comparable with the present tidal regime 5 finally it can be concluded that tidal bedding patterns are the structural expressions of tidal cyclicity

granulometric characterization and evaluation of annually banded mid holocene estuarine silts welsh severn estuary uk coastal change sea level and climate abstract holocene silts salt marshes and highest intertidal supratidal peats are superbly exposed on a 15 km coastal transect which reveals two laterally extensive units of annually banded silts beds 3 7 associated with three transgressive regressive silt peat cycles early sixth early fourth millennium bc bed 3 in places is concordantly and gradationally related to peats above and below but in others transgresses older strata bed 7 also grades up into peat but everywhere overlies a discordance the banding in bed 3 at three main and two minor sites was resolved and characterized texturally at high resolution 2 5 5 mm contiguous slices using laser granulometry ls230 with pids and a comprehensive scheme of data assessment most of bed 3 formed very rapidly at peak values of several tens of millimetres annually in accordance with modelled effects of sea level fluctuations on mature marshes bed concordant and gradational and on marshes growing up after coastal erosion and retreat bed with discordant base using data from the modern severn estuary the textural contrast within bands and its variation between bands points to a variable but overall milder mid holocene climate than today the inter annual variability affected marsh dynamics as shown by the behaviour of the finely divided plant tissues present given local calibration the methodology is applicable to other tidal systems with banded silts in britain and mainland northwest europe

rayleigh wave phase velocity heterogeneity and multilayered azimuthal anisotropy of the superior craton ontario summary 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 phasevelocity 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

evidence of deposition from individual tides and of tidal cycles from the francis creek shale host rock to the mazon creek biota westphalian d pennsylvanian northeastern illinois abstract a silt clayrock sequence in the francis creek shale pennsylvanian westphalian d of northeastern illinois is characterized by 1 the presence of numerous clayrock paired bands and 2 cyclic variations in thickness of silt between and within these clayrock pairs the sequence is interpreted to result from tidal influence on inner estuarine sediments the clayrocks representing deposition from suspension at times of tidal slack the interbedded silts and fine sands representing deposition from bottom traction during ebb and flood tidal flow the cyclic variations in thickness is interpreted to represent lunar neap spring forcing of tidal range in a dominantly diurnal tidal system the association of tidal characteristics provides a means whereby the rate of sedimentation can be quantified at about 1 0 m yr of compacted sediment during times of active deposition such a high rate of sedimentation would account for the rapid burial and preservation of soft bodied organisms as indicated by the associated mazon creek biota similar rates in similar settings i e tidal estuaries have recently been reported although the depositional setting of many westphalian sequences was favorable to the preservation of tidal features few examples have been recorded it is suggested that evidence of westphalian tides may have been misinterpreted or overlooked in field studies elsewhere

seismic studies of the upper mantle and transition zone in the last few years seismic studies of the upper mantle and transition zone have achieved significant new results in several areas progess has been driven largely by the increasing size quality and accessibility of digital seismic datasets from global and regional networks and from experiments using portable instruments this paper focuses on a few of the most interesting and important developments these include 1 measurements of upper mantle anisotropy obtained using a variety of techniques which can reveal details of the flow regimes and tectonic history of the upper mantle 2 improvements in models of upper mantle velocity and attenuation structure with some controversy arising regarding how deep slow velocity anomalies extend beneath oceanic ridges 3 direct seismic observations of topography on the 410 and 660 km discontinuities in the upper mantle providing constraints on mantle temperatures and dynamics and 4 observations suggesting secondary seismic discontinuities in some regions

surficial glaciology of jakobshavns isbrae west greenland part i surface morphology jakobshavns isbrae is a large fast moving ice stream outlet glacier in west greenland which ends at a floating calving front it drains about 6 5 of the area of the greenland ice sheet studies of its surface morphology are described in this paper the surface is relatively steep 0 01 0 03 and the thickness is large up to 2600 m along the center line clarke and echelmeyer 1989 indicating very high driving stresses 200 300 kpa the ice stream is about 6 km wide and 85 90 km long all of which is in an area of surface melting the base of the ice stream and of much of the drainage area is below sea level marginal crevasse zones have a width on the order of the width of the ice stream itself unique surficial features are ice blisters and lakes the latter have a sequence of ogive like features on their floating ice cover which can be used to determine velocity there is a pinning point near the terminus which may act as a stabilizing influence possibly playing a role in halting at least temporarily a recent retreat of the terminus ice thickness estimates at the terminus lead to a flux which is less than previously assumed by others e g bindschadler 1984 pelto and others 1989 when estimating jakobshavns isbrae s drainage basin to be nearly in balance the driving stresses on jakobshavns isbrae are an order of magnitude higher than those of the ice streams of west antarctica its crevasse patterns are much less localized its relatively unconfined terminus is more comparable to that of relatively unbuttressed ice streams such as pine island and thwaites glaciers than it is to other west antarctic ice streams which terminate in large confined ice shelves

evolution of supra glacial lakes across the greenland ice sheet abstract we used 268 cloud free moderate resolution imaging spectroradiometer modis images from 2003 and 2005 2007 to study the seasonal evolution of supra glacial lakes in three different regions of the greenland ice sheet lake area estimates were obtained by developing an automated classification method for their identification based on 250 m resolution modis surface reflectance observations widespread supra glacial lake formation and drainage is observed across the ice sheet with a 2 3 week delay in the evolution of total supra glacial lake area in the northern areas compared to the south west the onset of lake growth varies by up to one month inter annually and lakes form and drain at progressively higher altitudes during the melt season a positive correlation was found between the annual peak in total lake area and modelled annual runoff high runoff and lake extent years are generally characterised by low accumulation and high melt season temperatures and vice versa our results indicate that in a future warmer climate meehl g a stocker t f collins w d friedlingstein p gaye a t gregory j m kitoh a knutti r murphy j m noda a raper s c b watterson i g weaver a j zhao z c 2007 global climate projections in climate change 2007 the physical science basis contribution of working group i to the fourth assessment report of the intergovernmental panel on climate change solomon s d qin m manning z chen m marquis k b averyt m tignor h l miller eds cambridge university press cambridge united kingdom and new york ny usa greenland supra glacial lakes can be expected to form at higher altitudes and over a longer time period than is presently the case expanding the area and time period over which connections between the ice sheet surface and base may be established das s joughin m behn m howat i king m lizarralde d bhatia m 2008 fracture propagation to the base of the greenland ice sheet during supra glacial lake drainage science 5877 778 781 with potential consequences for ice sheet discharge zwally h j abdalati w herring t larson k saba j steffen k 2002 surface melt induced acceleration of greenland ice sheet flow science 297 218 221

lehmann discontinuity as the base of an anisotropic layer beneath continents long period surface wave r 1 g 1 body wave s ss sss and scs reverberation data have been inverted to obtain anisotropic structures along seismic corridors that sample australia and the western pacific these models support the proposal that the lehmann discontinuity beneath stable continents represents a transition from an anisotropic lithosphere to a more isotropic material in the lower part of the continental tectosphere

fourier and autocorrelation analysis of estuarine tidal rhythmites lower breathitt formation pennsylvanian eastern kentucky usa abstract outcrops of the pennsylvanian breathitt formation in eastern kentucky reveal a rhythmic pattern of siliciclastic sedimentation in a marginal marine coastal setting a 15 23 m thick stratigraphic interval of thinly interbedded fine sandstone and shale displays tidally generated features such as flaser and wavy current ripple bedding bipolar paleocurrents and cyclic thickening and thinning of mud draped sandstone layers bioturbation is common and increases toward the top of the section ichnogenera include asterosoma arenicolites olivellites conostichus scalarituba planolites zoophycos teichichnus curvolithus chondrites neonereites and rosselia a statistical analysis of sand layer thickness was carried out using shale partings as bounding surfaces for the individual sand units fourier and autocorrelation analyses were performed on two vertical sequences containing a total of over 2100 layers the results reveal the presence of four cycles of thickness variation first order cycles consist of alternating thick thin sand layers these daily couplets may reflect unequal flood and ebb currents during a single tidal cycle or dominant and subordinate tidal deposits in an ebb or flood dominated semidiurnal or mixed system second order cycles typically consist of 11 14 sand layers and reflect spring neap variations in tidal range and current velocities third order cycles are usually composed of 24 35 layers and are formed in resp nse to monthly variations in tidal range resulting from the ellipticity of the moon s orbit fourth order cycles generally contain about 150 layers range 100 166 and were caused by seasonal maxima in tidal range associated with the solstice winter summer and seasonal minima associated with the equinox spring fall

intermediate scale observations of the earth s deep interior from the apt89 transportable teleseismic experiment the archean proterozoic transition seismic experiment of 1989 apt89 represents one of the first of a new class of teleseismic experiments aimed at examining deep earth structure using portable instruments we show in the following three articles that such arrays can be utilized to examine properties of the earth in great detail from the surface of the earth to the core

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

evidence for a deep asthenosphere beneath north america from western united states sks splits the uniform southwesterly trend of fast traveling split sks waves that traverse the upper mantle beneath the yellowstone swell provides good evidence for both southwesterly motion of north america over a relatively stable deep earth interior and young strain accommodation within the yellowstone swell mantle via dislocation creep of olivine these results contrast with the many sks splits recorded across the western united states the splitting behavior of which often is very complex at individual sites and the orientations of which as a set cannot be attributed simply to plate motion or to small scale convection beneath north america lithosphere these data suggest that away from yellowstone 1 mantle fabric is complex and 2 north america motion is accommodated by strain at depths greater than those typically associated with asthenosphere

interpretation of sks waves using samples from the subcontinental lithosphere abstract the seismic properties of five south african kimberlite nodules have been calculated from petrofabric measurements these garnet lherzolite and harzburgite composition nodules 56 80 olivine are known from previous studies to have originated at depths of 120 170 km in the subcontinental lithosphere their in situ seismic properties have been calculated by extrapolating the elastic constants to the appropriate temperature 900 1050 c and pressure 3 0 3 5 gpa conditions the average of the five samples has a maximum s wave anisotropy v s of 3 7 which is sufficiently high to explain previously reported teleseismic sks delay times t in continental shield areas of up to 1 7 s the biggest delay time is observed when the foliation is vertical the lineation horizontal and the fastest s wave polarized parallel to the lineation i e an orientation for transcurrent motion delay times of over 1 s can only be generated by this orientation a strong mantle crust mechanical coupling is suggested for such situations the potential use of seismic anisotropy as an indicator of strain in the lithosphere has been investigated using experimental and simulated fabric data for olivine the most abundant phase 70 in the upper mantle the v p and v s seismic anisotropy increases with fabric strength and finite strain recrystallization tends to reduce fabric strength and seismic anisotropy resulting in saturation values for experimental and simulated fabrics which correspond to approximately 50 60 strain or 8 9 v s for pure olivine aggregates a survey of 25 naturally deformed peridotites of oceanic origin suggests an average maximum s wave anisotropy for the olivine component of 9 or about 8 for a lherzolite or harzburgite rock composition when the orthopyroxene component is taken into account the ophiolite samples are twice as anisotropic as the kimberlite nodules if an average anisotropy value is representative of the lithosphere then the sks delay times represent variations in anisotropic layer thickness for delay times over 1 s as the orientation is constrained to be foliation vertical and lineation horizontal the magnitude of s wave anisotropy is less sensitive than v p to variations in olivine volume fraction in range 50 100 and to deformation or fabric intensity further suggesting that s wave anisotropy is particularly apt for the determination of the anisotropic lithosphere thickness for smaller delay times there is some trade off between structural orientation and thickness

tidal deposits associated with low sulfur coals brazil fm lower pennsylvanian indiana abstract the interval between the lower and upper block coal members of the brazil formation lower pennsylvanian indiana records a transgression of brackish water tidal deposits over a coastal domed peat field followed by a regression and then re establishment of the peat forming environment within the study area the interval averages 8 m in thickness and consists of a basal sequence of laminated mudstone that coarsens upwards to thinly interlayered sandstone and mudstone lenticular wavy and flaser bedding this is capped by an upward fining sequence into a rooted mudstone sedimentation rates can be estimated by derailed thickness measurements of vertically stacked mudstone laminae and sandstone layers that thicken and thin rhythmically this rhythmic thickness variation can be elated to neap spring tidal current cyclicity and indicates that sedimentation varied from 1 cm yr to 1 m yr locally as much as 8 m of accommodation space for the sediments appears to have been generated and mostly filled in a matter of decades to a few centuries such rapid rates of filling indicate the possibility that the accommodation space was generated by compaction of the transgressed peat and mud rich sediments

observations of laterally inhomogeneous anisotropy in the continental lithosphere elastic anisotropy may be an important factor when seismic data are used to determine the structure composition and dynamics of the earth s interior1 2 azimuthal anisotropy has been found in the oceanic and although less definitely in the continental lithosphere from field experiments where the pn wave velocity was observed to vary with direction3 5 these studies suggest a 180 periodicity in travel time variations as the only criterion of the presence of azimuthal anisotropy this criterion however may fail to distinguish between anisotropy and lateral inhomogeneity of the isotropic medium a new method for measuring azimuthal anisotropy has been described recently6 based on the phenomenon of shear wave splitting in anisotropic media which suggests three criteria instead of one to identify azimuthal anisotropy we have now applied this method to the records of permanent seismograph stations in southern germany our results provide strong evidence for the presence of azimuthal anisotropy in the lithosphere of the region and reveal pronounced lateral variations in the parameters of anisotropy on a scale of about 200 km

mapping p wave azimuthal anisotropy in the crust and upper mantle beneath the united states abstract much progress has been made on revealing seismic structure and mantle dynamics beneath the united states us with the earthscope usarray project seismic anisotropy revealed by shear wave splitting studies provides important constraints on constructing geodynamic models with regard to the seismic images but the shear wave splitting observations have poor vertical resolution and so their interpretations are often not unique in this work we used a large number of arrival time data from local and distant earthquakes recorded by the usarray to determine the first p wave azimuthal anisotropy tomography of the crust and upper mantle beneath the us our results show that fast velocity directions fvds in the lithosphere under the tectonically active areas correlate well with the surface tectonic features suggesting that the p wave anisotropy mainly reflects the present deformation a circular pattern of the fvds centered in the great basin is revealed which is well consistent with the specific circular shear wave splitting observations there suggesting that the anisotropy occurs in the crust and uppermost mantle in contrast beneath the stable cratonic region the fvds revealed by this study differ from the shear wave splitting observations but consistent with the features of gravity and magnetic anomalies indicating that the p wave fvds mainly reflect the fossil anisotropy in the lithosphere whereas the s wave splitting observations mainly reflect the significant anisotropy in the asthenosphere the present results shed new light on the seismic anisotropy in the crust and upper mantle and provide new constraints on constructing geodynamic models beneath the us

variability and origin of seismic anisotropy across eastern canada evidence from shear wave splitting measurements measurements of seismic anisotropy in continental regions are frequently interpreted with respect to past tectonic processes preserved in the lithosphere as fossil fabrics models of the present day sublithospheric flow often using absolute plate motion as a proxy are also used to explain the observations discriminating between these different sources of seismic anisotropy is particularly challenging beneath shields whose thick 200 km lithospheric roots may record a protracted history of deformation and strongly influence underlying mantle flow eastern canada where the geological record spans 3 ga of earth history is an ideal region to address this issue we use shear wave splitting measurements of core phases such as sks to define upper mantle anisotropy using the orientation of the fast polarization direction and delay time t between fast and slow shear wave arrivals comparison with structural trends in surface geology and aeromagnetic data helps to determine the contribution of fossil lithospheric fabrics to the anisotropy we also assess the influence of sublithospheric mantle flow via flow directions derived from global geodynamic models fast polarization orientations are generally ene wsw to ese wnw across the region but significant lateral variability in splitting parameters on a 100 km scale implies a lithospheric contribution to the results correlations with structural geologic and magnetic trends are not ubiquitous however nor are correlations with geodynamically predicted mantle flow directions we therefore consider that the splitting parameters likely record a combination of the present day mantle flow and older lithospheric fabrics consideration of both sources of anisotropy is critical in shield regions when interpreting splitting observations

hummocky cross stratification like structures and combined flow ripples in the punta negra formation lower middle devonian argentine precordillera a turbiditic deep water or storm dominated prodelta inner shelf system abstract turbidity current and storm induced deposits may exhibit similarities in particularly when the latter is laid down by a combination of oscillatory and unidirectional flows recent progress in facies analysis helps to discriminate the sedimentary effects of oscillatory from unidirectional components of the flow on the basis of detailed analysis of sedimentary facies strata geometry and palaeocurrent data the present study reinterprets the punta negra formation pnf lower middle devonian argentine precordillera previously considered as a depositional system of deep water as a storm dominated prodeltaic shelf depositional system in the sandstone beds of the pnf planar low angle and undulating laminations with weakly asymmetric hummocky and swaley bedforms combined flow ripples accretionary hummocky cross stratification like hcs like and anisotropic hcs like suggest the action of oscillatory currents combined with unidirectional currents in forming the deposits different hypotheses on the origin of the oscillatory currents have been examined the most convincing interpretation is that the oscillatory component of the velocity is attributed to storm induced waves the palaeocurrent data indicate offshore current directions suggesting that the unidirectional flow was a gravity induced bottom current inverse grading at the base and overlying normally graded divisions of the sandstone beds testify to waxing waning behaviour of the depositional flows interbedding of sedimentary structures undulating laminations low angle and parallel laminations and combined flow ripples in the lower and intermediate divisions of the beds indicate fluctuations of flow velocity this organisation of the sedimentary structures permits association of the unidirectional component with hyperpycnal bottom currents the terrestrial origin of the hyperpycnal flows is suggested by the abundance of terrestrial plant remains the mineralogical and textural immaturity of the sandstone composition and the relative scarcity of bioturbation which was likely controlled by fresh water input and a high rate of sedimentation storm influenced hyperpycnal flows generated subaqueous channelised forms at the mouth of the river deltas which later filled with sand at the distal end of the channelised forms lobe shaped sandstone beds were deposited evolving distally into thin sandstone beds alternating with sandy mudstone

seismic structural and petrological models of the subcrustal lithosphere in southern germany a quantitative revaluation anisotropy in the subcontinental lithosphere becomes increasingly important because it is observed in many seismic studies especially for p n waves typical rocks of the uppermost mantle are peridotites which predominantly exhibit a pronounced elastic anisotropy this anisotropy is mainly caused by the anisotropic elastic properties and the lattice preferred orientation here referred to as texture of olivine to evaluate the elastic anisotropy of peridotites from the subcontinental lithosphere specimens of the northern hessian depression germany and the balmuccia ultramafic massif northern italy have been used they comprise four olivine texture types which are characteristic for olivine textures observed worldwide the bulk rock elastic properties have been calculated using olivine and orthopyroxene textures their single crystal elastic constants at ambient pressure temperature conditions and their volume fraction clinopyroxene and spinel are assumed to be randomly distributed the effect of four different orientations of the foliation within the uppermost mantle has been evaluated since this orientation is usually unknown two of the olivine textures have a pronounced azimuthal dependence of compressional waves when a horizontal foliation within the uppermost mantle is presumed these variations cause significant azimuthal variations of the p wave reflections coefficients at the moho primarily we predict a significant azimuthal dependence of the critical points where the reflected amplitude increases from approximately 15 to 95 possibly these azimuthal variations can be detected by seismic reflection measurements carried out at earth surface the remaining two texture types only manifest a small directional dependence when anisotropy of compressional waves is observed in seismic studies these latter types can only be of subordinate importance however all of the peridotites investigated are able to explain the seismically observed azimuthal variations of compressional waves when a vertical foliation is proposed this ambiguity can be substantially reduced when shear waves s waves are considered the directional distribution of s wave velocities and of the s wave splitting exhibits characteristic patterns for the different olivine texture types this could be used to discriminate between different texture types and orientations of the foliation within the uppermost mantle a fundamental requirement for a more comprehensive interpretation is the availability of detailed s wave observations the maximum s wave splitting in the peridotites investigated coincides with the maximum of the faster leading s wave this may be of importance to detect s wave splitting in future seismic studies

a temporal and spatial study of mudflat erosion and deposition sediment elevations were measured on an intertidal mudflat in maine to determine the temporal and spatial changes in sediment volume the winter and spring months were times of greatest sediment change with erosion in winter months followed by rapid accretion in the spring in some months erosion and deposition took place during the same time interval but on different parts of the mudflat the data suggest that areas of erosion quickly become sites of accretion indicating that an equilibrium surface level is being maintained mudflats may strongly influence the adjacent estuary by acting as sediment banks that can periodically lend sediments to the particulate pool offshore we conclude that mudflats should not be thought of as steady accretionary environments but rather as more variable environments that respond to seasonal changes

seismic studies on the trans hudson orogen of western canada abstract the discovery of diamondiferous kimberlites in the late 1980 s within the central saskatchewan segment of the trans hudson orogen prompted an aggressive diamond exploration program in the region the presence of this apparently uncharacteristic mineralization within the proterozoic orogen was initially a puzzle recent seismic reflection and geochronology evidence however suggests that the glennie domain of the orogen may be cored by a microcontinental archean block while geochemical results imply that segments of the precambrian basement of southern saskatchewan mainly comprise archean crust which has been reworked by thermotectonism we present the initial results of two seismic programs designed to elucidate the lithospheric structure associated with the economically targeted segment of the orogen preliminary interpretation of wide angle reflection data along a 730 km north south profile in north central saskatchewan reveals a very complex structure with the crustal thickness varying from a maximum of 50 km to a minimum of 37 km over a distance of 200 km beneath the glennie domain a teleseismic feasibility study with an 8 station array with individual seismographs operating for 4 to 6 months supports and extends this image of complexity relative travel time differentials across a 500 km array are as large as 700 ms while key features of these delay times correlate with the crustal structure a significant portion of the variations must be assigned to deeper levels of the lithosphere receiver function analysis for crustal structure is shown to be feasible in this environment where the phanerozoic sediment thickness varies from 0 to 2 1 km and provides evidence for large variations 7 km in crustal thickness in the southwestern area of the province sks analysis at a single station also adds to the evidence which shows rapid variations in anisotropy within the orogen a teleseismic program with seventeen 3 component stations is now underway to clarify the nature of this complex crustal and upper mantle structure

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seismic anisotropy of oceanic upper mantle shear wave splitting methodologies and observations we develop methodologies to obtain accurate measurements of shear wave splitting and apply these techniques to examine the pattern of oceanic upper mantle anisotropy to obtain high quality estimates of receiver splitting at island stations we devise a stacking method that finds the optimum splitting parameters and 95 error region for teleseismic shear wave phases from a suite of earthquake events to obtain additional measurements in oceanic regions we develop techniques to measure splitting parameters and errors for ss phases that sample upper mantle anisotropy at their bounce points however we find the data are often of low resolution and anomalous characteristics are sometimes found that make splitting difficult to interpret ten second splitting of ss is observed across the banjo seismic array but we cannot unambiguously attribute this signal to mantle anisotropy at the bounce the receiver splitting methods are used to assess the adequacy of a two anisotropic layer model for the pacific region with fast polarization azimuth in the lithosphere oriented in the fossil spreading direction and in the asthenosphere oriented in the absolute plate motion direction this model has been proposed to explain surface wave data in the pacific ocean but our splitting results demonstrate that oceanic anisotropy patterns are more heterogeneous than would be predicted while the island splitting measurements could reflect the influence of individual hotspot upwellings hotspot effects do not appear to be universally dominant we propose that splitting observations alternatively indicate broad scale differences in the underlying character of oceanic upper mantle anisotropy associated with coherant patters of lithospheric structure and asthenospheric flow in particular splitting surface wave models and regional studies all support a model where lithospheric anisotropy throughout the south pacific has been erased or reoriented toward the absolute plate motion direction whereas more limited observations in the north pacific indicate that the fossil lithospheric signature appears to be preserved

a depositional model for storm and tide influenced prograding siliciclastic shorelines from the middle devonian of the central appalachian foreland basin usa a stratigraphic motif observed in many foreland basins is the development of basinward tapering siliciclastic wedges characterized by various scales of depositional cycles the middle devonian givetian mahantango formation in the central appalachian foreland basin is such an example it consists of both small and large scale thickening and coarsening upward cycles the small scale cycles are typically less than 10 m thick whereas larger scale cycles are generally a few tens of metres thick and commonly contain several of the smaller scale cycles outcrop based facies analyses indicate that the depositional cyclicity resulted from episodic progradation of a regionally straight tide dominated shoreline onto a storm dominated shallow marine shelf the depositional model for this ancient shallow marine system consists of a vertical facies succession in which storm dominated offshore marine mudstone and fine sandstone pass gradationally upward into storm dominated nearshore marine shelf and shoreface sandstone overlain by in proximal sections tide dominated shoreline sandstone pebbly sandstone and mudstone transgressively reworked lag deposits cap most of the thickening and coarsening upward packets in this model coarse grained rocks rather than implying basinward shifts of facies are a consanguineous part of the stacked shoaling cycles lateral facies relationships show that the dominance of storm vs tide generated sedimentary features is simply a function of palaeogeographical position within the basin proximal sections contain tidally influenced sedimentary features whereas more distal sections only display evidence for storm influenced deposition these results suggest caution when inferring palaeoceanographic conditions from sedimentological datasets that do not contain preserved examples of palaeoshorelines

two distinct shear wave splitting directions in the northwestern deccan volcanic province 1 the northwestern deccan volcanic province in india has witnessed several tectonic episodes resulting in the formation of rift zones wide spread magmatism and deep seated faults that are host to some deadly intraplate earthquakes in this study we attempt to decipher the mantle deformation beneath the region using the sks splitting technique applied to high quality data from a regional network comprising 44 broadband seismic stations sited on diverse tectonic environments the first measurements of 461 312 sks and 149 skks splitting parameters from 117 earthquake sources reveal two major trends one coinciding with the absolute plate motion apm of the indian plate and the other roughly oriented in the e w direction forward modeling of the dependence of the splitting parameters on incoming polarization assuming horizontal axis of symmetry brings out two layers of anisotropy beneath the study region with the fast axis azimuths oriented n22 e in the bottom layer and n77 e in the top layer while stronger anisotropy t 1 2 s in the bottom layer is related to asthenospheric flow a weaker anisotropy t 0 5 s in the upper layer could represent anisotropy frozen in the lithosphere due to deformation related to widespread magmatism in this plume affected region

seismic constraints on dynamics of the mantle of the kaapvaal craton abstract shear wave splitting in the upper mantle of the kaapvaal craton in southern africa is measured by applying the sks technique to the records of a portable array of digital broad band seismographs the data are interpreted in terms of lattice preferred orientation in olivine fast wave polarization directions at all stations of the array are close to the absolute plate motion apm direction of southern africa since the end of the jurassic this alignment is similar to that previously reported for the north american craton and suggests that the large scale component of mantle anisotropy on both sides of the atlantic can be related to resistive drag at the base of the plates a likely depth range of the corresponding deformation is 150 400 km analysis of the mantle converted phases suggests that the high velocity mantle root of the kaapvaal craton which translates coherently with the plate resides in the same depth range the bottom of the root is found at a depth close to 380 km we conclude that the root is deformed by the recent plate motion but the deformations are not strong enough to be seen in the available tomographic models

comparing crustal and mantle fabric from the north american craton using magnetics and seismic anisotropy a central target in earth sciences is the study of deformation at various depth levels within the earth seismology has offered a remarkable tool for doing this via seismic anisotropy it is however not always clear how to interpret those observations a question of interest is to understand the relation between the deformation of the mantle and the crust and in studying the relation between the two mantle deformation is expressed in seismic anisotropy in this paper we seek an objective way of extracting information about crustal fabric as well to be able to compare with seismic anisotropy the magnetization of crustal rocks offers an attractive possibility for doing this we thus explore the use of magnetic data and we compare magnetic crustal fabric orientation with mantle fabric observations from seismic anisotropy we apply our technique to the north american craton for which we have an excellent magnetic dataset and we show that there is a clear relation between crustal and mantle fabric for the cratonic region this has important implications for crustal formation and for interpreting seismic anisotropy observations

an anomaly in the amplitude ratio of skks sks in the range 100 108 from portable teleseismic data we have examined the amplitude ratio skks sks from two deep focus events recorded by the apt89 portable seismic experiment for one of the events beneath the solomon islands we obtain a profile of 18 stations in the distance range 99 108 we observe large systematic variations in this ratio as a function of distance from 99 to 106 the ratio is fairly constant and in the range 0 1 to 0 5 but it then rapidly increases to the range 0 5 3 5 between 106 and 108 such variations are in fact predicted by standard earth models such as prem although this feature of the wavefield has not previously been observed it is due to the momentary drop in sks amplitude at the ray parameter where the reflected phase scp is critically reflected because ray theory is not valid in this distance range we use full wave theory to model the amplitudes we have been able to match the distance pattern reasonably well if we use a model with the p velocity at the base of the mantle reduced by 1 5 compared to prem although fewer stations are available the second event shows the same basic distance pattern except that the abrupt increase in the ratio occurs at a closer distance this suggests an even lower p velocity for this region about 3 5 slower than prem and is consistent with lateral heterogeneity at the base of the mantle of a few percent

archaean cratonic roots mantle shear zones and deep electrical anisotropy the extent to which the mantle participated in the growth and stabilization of ancient cratons is central to our understanding of the evolution of the continents1 the detection of seismic anisotropy beneath precambrian north america for example has been interpreted as showing that strain induced orientation of mantle minerals in subcontinental lithospheric mantle can preserve a record of ancient episodes of deformation2 here we present mag netotelluric measurements from the superior province of the canadian shield which reveal pronounced electrical anisotropy in the upper 100 km of the underlying mantle we argue that this anisotropy is best explained by conducting graphite films oriented within fractures or on grain boundaries and associated with metasomatism of the mantle roots of major archaean shear zones which transect the entire superior province the uppermost mantle beneath the canadian shield has therefore remained fixed to the crust and isolated from significant tectonic reworking since the late archaean

anisotropy of the yellowstone hot spot wake eastern snake river plain idaho over the last 10 million years the yellowstone hot spot has passed beneath the eastern snake river plain both magmatically modifying the snake river plain crust and creating a wider wake like tectonic parabola of seismicity and uplift analysis of sks arrivals to a line array of 55 mostly broadband stations distribution across the tectonic parabola reveals a nearly uniform orientation of anisotropy with an average fast axis orientation of n64e the back azimuth of null splitting events is parallel to the measured fast axis suggesting that anisotropic material consists of a single layer splitting parameters are independent of backazimuth suggesting that anisotropy is constant beneath each station thus station averaged split parameters are representative of the anisotropy beneath the station station averaged split times range from 0 6 1 5 s and define a pronounced depression in split times centered about 80 km southeast of the axis of the snake river plain

seismic anisotropy and mantle flow driven by the cocos slab under southern mexico shear wave splitting measurements were made using sks and skks waves recorded by the meso american subduction experiment which was deployed in southern mexico starting at the coast of the pacific ocean and running north toward the gulf of mexico in this segment of the middle america trench the oceanic cocos plate subducts under the continental north american plate the active volcanic arc is located at the southern end of the trans mexican volcanic belt unlike most subduction zones however the volcanic arc is not subparallel to the trench in the fore arc between the trench and the trans mexican volcanic belt the cocos slab subducts subhorizontally beneath the volcanic belt however the slab dives steeply into the mantle a marked difference in the orientation of the fast polarization directions is observed between the fore arc and the back arc in the fore arc the fast axes determined using sks phases are oriented ne sw in the same direction as the relative motion between the cocos and north american plates and are approximately perpendicular to the trench physical conditions in the subslab mantle are consistent with the existence of a type olivine and consequently entrained mantle flow is inferred strong coupling between the slab and the surrounding mantle is observed in the back arc sks fast polarization directions are oriented n s and are perpendicular to the strike of the slab given the high temperatures in the mantle wedge tip the development of a type or similar olivine fabric throughout the mantle wedge is expected the orientation of the fast axes is consistent with corner flow in the mantle wedge

midmantle deformation between the australian continent and the fiji tonga subduction zone 1 shear wave splitting measurements are performed at temporary stations deployed across the australian continent for direct s waves from events occurring in the neighboring subduction zones especially fiji tonga unlike core refracted shear waves direct s waves are not polarized at the core mantle boundary and are therefore subject to influences on the source side on the receiver side and along the ray path in between source side contamination due to the development of lattice preferred orientation of olivine can be minimized by considering deep events with an epicentral location below 410 km depth the olivine spinel phase transition depth anisotropic contributions from the transition zone or lower mantle can only be reliably inferred in the case of high delays that cannot be generated solely by the upper mantle structure underneath a station the analysis of a comprehensive data set for direct s waves from deep events leads to an average delay time of 0 9 s at stations deployed across the continent despite the limitations of such an arithmetic average the result highlights the lack of evidence for midmantle deformation between the fiji tonga subduction zone and the australian continent unlike previously suggested in studies dealing with the permanent seismological stations only a striking consistency between measurements performed across several stations belonging to an array in western australia suggests a very coherent lithospheric structure underneath the yilgarn craton

neap spring tidal cyclicity in laminated carbonate channel fill deposits and its implications salem limestone mississippian south central indiana u s a abstract limestone beds containing rhythmic parallel laminations occur within a channel fill sequence exposed locally in the salem limestone mississippian of indiana the laminations consist of carbonate grains and micrite in layers up to 2 mm thick capped by thin 0 1 0 3 mm drapes of organic matter each couplet was generated by mixed or semidiurnal tidal events during which carbonate sediment was mobilized and deposited by the dominant tide and organic material settled from suspension during slackwater conditions laminae thicken and thin systematically in a vertical sequence as a result of neap spring tidal current fluctuations that occur during a lunar month laminations are well preserved in certain beds ranging from 20 to 40 cm thick through much of the channel fill sequence other beds are extensively bioturbated and contain only a partial record of the laminations the sequence of bioturbated and unbioturbated units defines a rhythmic pattern suggesting periods of seasonal environmental restriction that precluded burrowing organisms estimation of sedimentation rates based on the neap spring tidal cycles indicates that deposition within the channel occurred very rapidly averaging 35 cm year a rate four orders of magnitude faster than calculated long term depositional rates

mass balance of the greenland ice sheet at dye 3 the mass balance of the greenland ice sheet a t dye 3 is estimated on the basis of 0 bserva tions of ice thickness accumulation rate surface velocities and surface strain rates the calculations indicate a rate of increase of surface elevation of 3 cm year with 95 confidence limits of 3 cm year and 9 cm year previous estimates of the mass balance of the greenland ice sheet by the same method reported large imbalances these are most probably due to lack of precise data and the use of quantities measured at the surface as representative of depth averaged quantities the most reliable observations indicate that the interior regions of the greenland ice sheet are at present thickening at a rate of a few centimetres per year a contributing cause for this may be the slow thinning of a bottom layer of relatively soft wisconsin ice resume bilan de masse de l lndlandsis groenlandais a dye 3 le bilan de masse est estime it partir des mesures d epaisseur taux d accumulation vitesses et vitesses de deformation superficielles le calcul don ne un epalss lssement de 3 cm an 1 avec un intervalle de confiance au seuil de 95 allant de 3 cm an 1 it 9 cm an i les estimations anterieures du bilan de l lndlandsis obtenues par la meme methode sont examinees on en deduit que les valeurs de bilan tres eloignees de l equilibre sont certainement dues au manque de donnees preclses et ou a i utilisation de donnees superficielles comme valeurs moyennes sur une

shear wave splitting around the northern atlantic frozen pangaean lithospheric anisotropy abstract although the number of teleseismic shear wave splitting measurements has considerably increased world wide during the last decade there is no consensus on a unique and universal interpretation of these data splitting might primarily originate in the lithospheric and or asthenospheric mantle but the technique has no vertical resolution and therefore leaves this question of anisotropy location unresolved generally the consequences of the two possibilities are used to infer which is more likely if splitting occurs in the lithosphere the splitting parameters the orientation of the fast split shear wave o and the delay time t should be related to frozen or active deformation on the other hand if the splitting occurs in the asthenosphere the observed splitting parameters should be related to the present day flow induced by the differential motion between the tectonic plates and the underlying mantle the aim of this paper is to test these hypotheses against shear wave splitting observations from western europe and the eastern u s indeed except in a few places splitting on either side of the atlantic on hercynian caledonian or grenvillian structures do not appear to have been deeply and pervasively affected by the opening of this ocean this provides us the opportunity to place these results in the pangaean reference frame of the hercynian caledonian belt before the opening of the atlantic in many places the close parallelism between lithospheric structure and the absolute plate motion apm impedes a reliable identification of the origin of the anisotropy especially an asthenospheric flow channelled around the deeply rooted north american craton may be compatible with the large scale anisotropy pattern in the eastern u s despite this ambiguity fast polarization directions o observed on or near some major lithospheric structures correlate better with the trend of the local tectonic structures than with the expected asthenospheric flow therefore we suggest that these lithospheric structures developed before the opening of the atlantic remain frozen since hercynian caledonian or grenvillian times and are responsible for the observed splitting at these places

seismic anisotropy beneath stable continental interiors a robust knowledge of seismic anisotropy beneath the continents is essential to our understanding of plate tectonic theory as anisotropy provides a unique constraint on the character of past and present deformation in the lithosphere and sublithospheric mantle this review paper summarizes the range of techniques currently available to image seismic anisotropy with passive source seismic data and addresses current issues surrounding the observation and interpretation of continental seismic anisotropy to this end we present case studies for four regions where seismic anisotropy has been extensively investigated in recent years eastern north america the canadian shield australia and southern africa based on this full suite of results we infer that stable continental regions generally contain seismic anisotropy that is clearly located within both the lithosphere and the sublithospheric mantle usually to depths of at least 200 km and perhaps more an implication of these results is that tectonic plates are at most only partially coupled to the underlying mantle the results from these case studies also demonstrate that while remarkable progress in seismic anisotropy imaging has been achieved in recent years it is clear that much more work will be required to adequately understand the origin of continental seismic anisotropy we suggest that a more robust characterization of anisotropic parameters can only be achieved by integrating complementary seismic datasets and by incorporating constraints from key related datasets from mineral physics magnetotellurics gravity and geodesy

seismic anisotropy and mantle flow beneath the northern great plains of north america a diverse set of tectonic features and the recent availability of high quality broadband seismic data from the usarray and other stations on the northern great plains of north america provide a distinct opportunity to test different anisotropy forming mechanisms a total of 4138 pairs of well defined splitting parameters observed at 445 stations show systematic spatial variations of anisotropic characteristics azimuthally invariant fast orientations subparallel to the absolute plate motion apm direction are observed at most of the stations on the superior craton and the southern yavapai province indicating that a single layer of anisotropy with a horizontal axis of symmetry is sufficient to explain the anisotropic structure for areas with simple anisotropy the application of a procedure for estimating the depth of anisotropy using spatial coherency of splitting parameters results in a depth of 200 250 km suggesting that the observed anisotropy mostly resides in the upper asthenosphere in the vicinity of the northern boundary of the yavapai province and the wyoming craton the splitting parameters can be adequately explained by a two horizontal layer model the lower layer has an apm parallel fast orientation and the upper layer has a fast orientation that is mostly consistent with the regional strike of the boundary based on the splitting measurements and previous results from seismic tomography and geodynamic modeling we propose a model involving deflecting of asthenosphere flow by the bottom of the lithosphere and channeling of flow by a zone of thinned lithosphere approximately along the northern boundary of the yavapai province

sks splitting beneath continental rift zones we present measurements of sks splitting at 28 digital seismic stations and 35 analog stations in the baikal rift zone siberia and adjacent areas and at 17 stations in the east african rift in kenya and compare them with previous measurements from the rio grande rift of north america fast directions in the inner region of the baikal rift zone are distributed in two orthogonal directions ne and nw approximately parallel and perpendicular to the ne strike of the rift in the adjacent siberian platform and northern mongolian fold belt only the rift orthogonal fast direction is observed in southcentral mongolia the dominant fast direction changes to rift parallel again although a small number of measurements are still rift orthogonal for the axial zones of the east african and rio grande rifts fast directions are oriented on average nne that is rotated clockwise from the n s trending rift all three rifts are underlain by low velocity upper mantle as determined from teleseismic tomography rift related mantle flow provides a plausible interpretation for the rift orthogonal fast directions the rift parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small scale mantle convection with rift parallel flow the agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift parallel fast directions are caused by oriented magmatic cracks

temporal distribution of diastems in deposits of an open coast tidal flat with high suspended sediment concentrations abstract based on field observations of the preservation potential and genesis of couplets and small sequences diastems occur within tidal flat deposits at scales of semi tidal daily monthly and yearly intervals thus tidal flat deposition is riddled with more diastems of different scales than previous assumptions units that are apparently complete when examined at longer time scales actually contain many diastems when examined at a shorter time scale diastems within tidal flat deposits are generated not only by high magnitude storm waves which occur during typhoon seasons but also by daily erosion produced by tides and weak waves during calm weather seasons the missing intervals in diastems generated by storm waves can be several days to several years preservation potential of storm generated small sequences is of the order of one third diastems generated by erosion by tides and weak waves can have spans from a semidiurnal cycle to several days they are easily overlooked because of the low magnitude destruction and lack of distinctive sedimentary features net erosion by tides and weak waves is unexpectedly increased by day to day processes alternations of sandy and muddy laminae are related to tidal rhythm but individual laminae can be separated by a significant diastem documentation of erosion by tides and weak waves under normal non typhoon conditions is the principal focus of this report

lower cretaceous tides revealed by cross bedding with mud drapes the sand dominated folkestone beds widely exposed around the weald of south east england are of upper aptian to lower albian lower cretaceous age and range in thickness up to 84 m refs 1 3 their content of ammonites bivalves and sponges indicates a marine origin while their relative coarseness and the incorporation of drifted timber point to a shallow water deposition near land one striking sedimentary facies is formed by medium to coarse grained cross bedded sands in which several foreset and or bottomset surfaces support mud drapes although it was suggested4 that this facies may have been formed by the storm induced movement of sand waves i now consider that the facies is best explained by the migration of sand waves under the influence of strongly asymmetrical tides and that the mud drapes are formed at slack water a model of this process has been described elsewhere5 and i now describe measurements of a section which supports my earlier conjecture6that the separation of the mud drapes is systematically related to the phase of the spring neap tidal cycle at the time of deposition although criteria for recognizing tidal deposits have long been available it has not previously been possible to analyse a series of ancient sediments with a view to characterizing the tidal regime

late mississippian prodeltaic rhythmites in the appalachian basin a hierarchical record of tidal and climatic periodicities abstract the pride shale is a 60 m thick succession of thinly interlaminated dark shales siltstones and fine grained sandstones within the bluestone formation upper mississippian of southern west virginia a hierarchy of submillimeter to meter scale cycles in this succession preserves a spectrum of tidal and climatic periodicities submillimeter thick fine grained sandstone shale or siltstone shale couplets are interpreted as the product of suspension settling associated with individual ebb tidal flows up to 17 couplets systematically thicken and thin within millimeter to centimeter scale bundles interpreted as neap spring tidal cycles successive neap spring cycles display a thick thin relationship interpreted to reflect unequal perigean and apogean tides the fine scale and abbreviat d character of these microlaminated rhythmites is suggestive of a distal subtidal setting wherein subordinate daily and neap and apogean ebb flows were generally of insufficient strength to transport sand and silt decimeter scale cycles in the pride shale are manifested by the progressive upward thickening and thinning of up to 18 millimeter to centimeter thick neap spring cycles this bundling is interpreted to reflect a climatic monsoonal signal which defines that part of the year when adequate fluvial tidal discharge forced suspended sediment into the basin lastly a multi year cyclicity is recognized in thick exposures where 17 22 annual beds display a crude upward thickening and thinning within meter scale bundles this bundling is interpreted to reflect the 18 6 year lunar odal cycle with the thick annual beds representing years during which the inclination of the lunar orbital plane favored increased tidal amplitudes decompacted thicknesses of cycles indicate that accumulation rates for the pride shale typically ranged between 3 and 20 cm per year but reached over 60 cm per year where sandy rhythmites developed as the marginal infill of large slump scars the stratigraphic relationships and sedimentation rates for the pride shale are consistent with paleontological evidence of rapid sedimentation in turbid waters seaward of multiple points of fluvial discharge the pride shale is interpreted as an extensive prodeltaic deposit which in places records hundreds of years of nearly continuous tidal sedimentation along the eastern margin of the appalachian basin

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

seismic anisotropy in the eastern united states deep structure of a complex continental plate we have analyzed shear wave splitting recorded by portable and permanent broadband and long period stations located in the eastern united states teleseismic shear waves sks skks and pks were used to retrieve the splitting parameters the orientation of the fast wave polarization plane and the delay time t in total 120 seismic events were processed allowing for more than 600 splitting measurements within the appalachians stations located in the western external part are characterized by t 1s and trending n50 70 e in the south and central regions and n30 40 e in the north closely following the trend of the orogenic belt in these areas the transition region between north and central is characterized by t 1 1 3 s and by e w trending that are at a high angle to the regional geologic trend measurements at two stations located in the eastern internal part of the belt indicate very weak anisotropy the large scale pattern of anisotropy is not consistent with that predicted for simple asthenospheric flow beneath the plate splitting along the southern and eastern margins of the continent is consistent with that expected for grenvillian deformation an alternative model of asthenospheric flow around the cratonic keel cannot be ruled out within the cratonic core the correlation between t and lithospheric thickness suggests a lithospheric anisotropy smaller length scale variations also argue for a significant contribution of lithospheric structures the fabric responsible for shear wave splitting may have formed during tectonic episodes that affected the eastern united states i e the grenville and appalachian orogenies and the subsequent rifting of the north atlantic ocean our observations in the western appalachians suggest that the anisotropy may be preserved since the grenvillian orogeny the absence of detectable splitting in the two stations in the eastern appalachians is attributed to the igneous intrusions related to the atlantic rifting the measurements in the transition between the northern and central southern appalachians constitute an intriguing anomaly whose e w have little obvious relation to the regional surface geology we suggest two possible causes 1 the local dominance of asthenospheric flow motivated by the proximity of a pervasive low velocity anomaly and 2 lithospheric deformation in a transcontinental strike slip fault zone active during the appalachian collision

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

seismic and electrical anisotropies in the lithosphere across the grenville front canada the grenville front is a prominent tectonic feature of the canadian shield which has been the focus of a lithoprobe transect a strong electrical anisotropy has been detected in a frequency range of 0 1 0 01 hz 50 to 150 km the most conductive direction being oriented n80e in order to compare electrical and seismic anisotropies ten seismometers have been deployed across the grenville front to record teleseismic events s wave splitting measurements show an average delay of 1 54 0 21 s between the fast and the slow components the fast axis direction is approximately e w at all stations showing a strong coherence between the stations along the complete transect this azimuth correlates well with that of the electrical anisotropy the electrical anisotropy originates in the subcrustal lithosphere between 50 and 150 km the similarity of the magnetotelluric and seismic anisotropy directions across the grenville front is an argument in favor of an origin of the seismic anisotropy in the subcrustal lithosphere as advocated by silver and kaneshima 1993

anisotropic measurements in the rhinegraben area and the french massif central geodynamic implications as part of an integrated seismic study polarization of shear waves has been analyzed for teleseismic events recorded at a set of permanent broadband semi permanent long and short period and temporary short period seismological stations located in two geodynamically important areas in western europe namely the rhinegraben urach area and the french massif central volcanic field

calibration of shear wave splitting in the subcontinental upper mantle beneath active orogenic belts using ultramafic xenoliths from the canadian cordillera and alaska abstract despite the abundance of measurements of shear wave splitting around the world some fundamental questions about its geological interpretation have not yet been answered in order to constrain 1 i the orientation and magnitude of s wave anisotropy 2 ii the thickness of the anisotropic layer and 3 iii the possible variation of the anisotropy with depth in the subcontinental upper mantle beneath active orogenic belts we have carried out a systematic investigation on shear wave properties of mantle xenoliths from recent alkaline basalts of the canadian cordillera and alaska it is found that the polarization direction of the fast s wave as well as the time delay t between the two arrivals are strongly dependent on the propagation direction with respect to the structural frame at a single station the variations of measured and t values for sks phases from different events can result from this dependence because the average grain size of olivine in upper mantle samples is commonly larger than 0 5 mm dislocation creep prevails over a thickness of at least 250 km and results in olivine lpo and hence anisotropy and shear wave splitting a single thick anisotropic layer in mountain parallel subvertical shear zones may cause high values of tgd while multiple subhorizontal layers will produce multiple splitting making the measurement and interpretation of s wave splitting difficult our results also suggest that the s wave splitting observed in cold old stable cratons such as the canadian shield is likely dominated by fossil anisotropy whereas that observed in hot active orogenic regions such as the northwest american cordillera and the tibetan plateau may be dominated by modern tectonic deformation in the lithosphere and flow in the asthenosphere

the lunar orbit in the late precambrian and the elatina sandstone laminae the discovery of layered sediments of the elatina formation of south australia has attracted intense speculation regarding their origin1 2 the formation 10 m layer of graded sandstone consists of a pattern of light bands of periodically varying thickness3 a sequence of dark bands separates on average 11 6 light bands the spacing of these dark bands exhibits a rich and complex spectrum previously the periodicity has been attributed to the sunspot cycle and unassigned but putative solar periods prompted by a letter from g e williams summarizing new field data from the adelaide region south australia which imply that the elatina varves may be tidal we re examined the dark band spectrum and propose a luni solar tidal interaction model as the source of the laminae

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

three dimensional radial anisotropic structure of the north american upper mantle from inversion of surface waveform data summary seismic anisotropy provides insight into palaeo and recent deformation processes and there fore mantle dynamics in a first step towards a model for the north american upper mantle with anisotropy characterized by a symmetry axis of arbitrary orientation aimed at filling the gap between global tomography and sks splitting studies we inverted long period waveform data simultaneously for perturbations in the isotropic s velocity structure and the anisotropic parameter v 2 sh v2 in the framework of normal mode asymptotic coupling theory nact the resulting 2 d broad band sensitivity kernels allow us to exploit the information contained in long period seismograms for fundamental and higher mode surface waves at the same time to ensure high quality of the retrieved regional upper mantle structure accurate crustal cor rections are essential here we follow an approach which goes beyond the linear perturbation approximation and split the correction into a linear and non linear part the inverted data set consists of more than 40 000 high quality three component fundamental and overtone sur face waveforms recorded at broad band seismic stations in north america from teleseismic events and provides a fairly homogeneous path and azimuthal coverage the isotropic part of our tomographic model shares the large scale features of previous regional studies for north america we confirm the pronounced difference in the isotropic velocity structure between the western active tectonic region and the central eastern stable shield as well as the presence of subducted material juan de fuca and farallon plate at transition zone depths the new regional 3 d radial anisotropic model indicates the presence of two distinct anisotropic layers beneath the cratonic part of the north american continent a deep asthenospheric layer con sistent with present day mantle flow and a shallower lithospheric layer possibly a record of ancient tectonic events

precambrian basement geology of north and south dakota combined analysis of drill hole gravity and magnetic data indicates that the buried precambrian basement rocks of the dakotas can be divided into several lithotectonic terranes eastern north dakota and northeastern south dakota are underlain by archean gneiss except for the black hills region of south dakota where archean rocks are also exposed the western third of both dakotas is underlain mainly by early proterozoic gneiss and metasedimentary rocks part of this region is underlain by archean crust with an early proterozoic tectonic overprint a broad transition zone of strongly overprinted archean crust occurs between the proterozoic rocks to the west and the archean rocks to the east south central south dakota is underlain by an early proterozoic batholith early proterozoic felsic volcanic rocks occur in southeast south dakota the bootheel portion of south dakota contains a diverse assemblage of basement rocks that are partly archean in age churchill province rocks of the trans hudson foldbel

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

precambrian tidal and glacial clastic deposits implications for precambrian earth moon dynamics and palaeoclimate abstract over the past decade the study of precambrian clastic tidal rhythmites stacked laminae of sandstone siltstone and mudstone that display periodic variations in thickness reflecting a strong tidal influence on sedimentation has provided accurate palaeotidal and palaeorotational data palaeotidal records obtained from tidal rhythmites may be systematically abbreviated however and derived periods and frequencies can be misleading the validity of such values including past length of day can be assessed by testing for internal self consistency through application of the laws of celestial mechanics such a test supports the estimated length of day of 21 9 0 4 h derived from the late neoproterozoic 620 ma elatina reynella rhythmites in south australia and the indicated mean rate of lunar retreat of 2 16 0 31 cm year since 620 ma the validity of estimated lengths of day obtained from other precambrian tidal rhythmites remain unverified because the data sets contain only one primary value directly determined from the rhythmites the elatina reynella data militate against significant earth expansion at least since 620 ma and suggest that the free nutation or tipping of the earth s fluid core has undergone a resonance with the earth s annual forced nutation since the neoproterozoic glaciogenic deposits are readily distinguishable from ejecta resulting from impacts with earth crossing bodies palaeomagnetic data based on the geocentric axial dipole model for the geomagnetic field indicate that neoproterozoic and palaeoproterozoic glaciation and cold climate near sea level occurred in low palaeolatitudes this enigmatic finding may imply global glaciation or an increased obliquity of the ecliptic and is relevant to modelling the effect of ice sheet formation on the earth s obliquity history by obliquity oblateness feedback mechanisms through multidisciplinary studies clastic sedimentology and geophysics together can make substantial contributions to understanding precambrian earth moon dynamics and global palaeoenvironments

modelling of cyclic tidal rhythmites carboniferous of indiana and kansas precambrian of utah usa as a basis for reconstruction of intertidal positioning and palaeotidal regimes analyses are presented for a variety of ancient cyclic tidal rhythmites which exhibit well developed neap spring tidal periods many such rhythmites were formed within the upper intertidal zone and exhibit truncated cycles that contain relatively few discrete lamina per neap spring cycle in such cases it can be difficult to determine if the originated palaeotidal system was diurnal or semidiurnal based on sedimentological controls observed in modern analogues the development of cyclic tidal rhythmites can be modelled by use of predicted tidal height information the modelling used a 19 year long series of tidal heights which were subsequently used to approximate tidal velocities and rhythmite sedimentation modelling was based on a range of diurnal to semidiurnal regimes and comparisons were performed at a number of levels within the intertidal zone this modelling produced a long series of simulated laminae thickness series which could be cross correlated with ancient laminae thickness series measured directly from ancient rhythmites this approach involved 1 6 106 comparisons of each ancient rhythmite series to the series simulated from the predictive tidal height information the high correlations derived for such comparisons indicate that reasonable approximations to the palaeotidal systems can be made in some cases it is possible to determine the diurnal or semidiurnal nature of the originating palaeotidal system

p wave teleseismic traveltime tomography of the north american midcontinent the remains of the 1 1 ga midcontinent rift mcr lie in the middle of the tectonically stable portion of north america previous and ongoing studies have imaged strong heterogeneity associated with the mcr in the crust but have not imaged such within the mantle it is unclear whether this is due to the absence of rift related mantle structures or these studies had insufficient resolution to image them to address this issue we measured 46 374 teleseismic p wave delay times from seismograms recorded by the usarray transportable array superior province rifting earthscope experiment and surrounding permanent stations we included these and 54 866 delay times from prior studies in our tomographic inversion we find that high velocity anomalies are widespread in our study area but there are also prominent low velocity anomalies two of these are coincident with high bouguer gravity anomalies associated with the mcr in iowa and the minnesota wisconsin border at 50 to 150 km depth extensive resolution testing shows that these anomalies could be the result of downward vertical smearing of relatively low velocities from rift related material that underplated the crust although we cannot exclude that the subcrustal mantle lithosphere beneath the mcr is anomalously enriched hydrated or warm other anomalies occur at syntaxes of the penokean orogen one with the superior province and marshfield terrane in southern minnesota and another with the yavapai and mazatzal terranes both at 100 to 250 km depth in the midmantle we image two linear high velocity anomalies interpreted as subducted fragments of the farallon and kula plates

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

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

rapid resetting of an estuarine recorder of the 1964 alaska earthquake tides and plants have already restored much of a landscape that the 1964 alaska earthquake destroyed at the head of a macrotidal estuary near anchorage in the vicinity of portage subsidence during the earthquake changed meadows thickets and spruce groves into barren tidal flats tidal flat silt and sand soon buried the pre earthquake landscape while filling intertidal space that the subsidence had made the flats supported new meadows and thickets by 1973 and new spruce by 1980 three new findings confirm that the flats aggraded rapidly and that their vegetation is maturing 1 most of the postearthquake deposits at portage date from the first decade after the 1964 earthquake their thickness of 23 sites in a 0 5 km 2 area was 1 4 0 2 m in 1973 1 6 0 2 m in 1991 and 1 6 0 3 m in 1998 2 many of the deposits probably date from the first months after the earthquake the deposits contain sedimentary couplets in which coarse silt or very fine sand is capped by fine or medium silt about 100 such couplets make up the lowest 0 5 m or more of the postearthquake deposits in two outcrops these couplets thicken and thin rhythmically both as groups of 5 20 couplets and as pairs of successive couplets probably the groups of thick couplets represent the highest tides the groups of thin couplets represent some of the lesser high tides and the pairs record inequality between twice daily high tides 3 in the 1980s and 1990s thickets expanded and spruce multiplied the vegetation now resembles the fossil assemblage rooted in the buried landscape from 1964 had the 1964 alaska earthquake been repeated a decade later the two earthquakes would now be recorded by two superposed buried landscapes near portage much more than a decade is probably needed to reset similar recorders at mesotidal estuaries of the cascadia subduction zone