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

age of the peach springs tuff southeastern california and western arizona sanidine separates from pumice of the early miocene peach springs tuff are concordantly dated at 18 5 0 2 ma by two isotopic techniques the peach springs tuff is the only known unit that can be correlated between isolated outcrops of miocene strata from the central mojave desert of southeastern california to the western colorado plateau in arizona across five structural provinces a distance of 350 km thus the age of the peach springs tuff is important to structural and paleogeographic reconstructions of a large region biotite and sanidine separates from bulk samples of the peach springs tuff from zones of welding and vapor phase alteration have not produced consistent ages by the k ar method published ages of mineral separates from 17 localities ranged from 16 2 to 20 5 ma discordant 40ar 39ar incremental release spectra were obtained for one biotite and two of the sanidine separates ages that correspond to the last gas increments are as old as 27 ma the 40ar 39ar incremental release determinations on sanidine separated from blocks of peach springs tuff pumice yield ages of 18 3 0 3 and 18 6 0 4 ma laser fusion measurements yield a mean age of 18 51 0 10 the results suggest that sanidine and biotite k ar ages older than about 18 5 ma are due to inherited ar from pre tertiary contaminants which likely were incorporated into the tuff during deposition sanidine k ar ages younger than 18 ma probably indicate incomplete extraction of radiogenic 40ar whereas laser fusion dates of biotite and hornblende younger than 18 ma likely are due to postdepositional alteration laser fusion ages as high as 19 01 ma on biotite grains from pumice suggest that minerals from pre tertiary country rocks also were incorporated in the magma chamber

arc rift transition volcanism in the puertecitos volcanic province northeastern baja california mexico the neogene puertecitos volcanic province of northeastern baja california records a transition from arc related volcanic activity to rift volcanism associated with opening of the gulf of california the eastern puertecitos volcanic province is divided into three volcanic sequences based on mapping petrology and 40 ar 39 ar geochronology the lowest sequence comprises early to middle miocene 20 16 ma arc related andesitic lava flows volcanic necks and proximal pyroclastic and epiclastic deposits up to 400 m in thickness with minor basaltic lava flows following the initiation of crustal extension in the region 11 6 ma synrift volcanism produced two rhyolitic sequences that discordantly overlie the arc related rocks the older synrift sequence 6 4 5 8 ma is composed of rhyolite domes and a series of pyroclastic flows up to 300 m thick the upper sequence 3 2 2 7 ma consists of ash flow tuffs and pumice lapilli pyroclastic flows collectively up to 200 m thick minor andesite eruptions followed each episode of silicic synrift volcanism synvolcanic faults produced topographic relief that controlled deposition of the pyroclastic flows and caused gentler dips upsection rhyolite domes are aligned parallel to the predominant north northwest to north northeast fault pattern all three volcanic sequences are calc alkaline however the synrift andesite is characterized by lower k 2 o lower incompatible element concentrations and less fractionation of light rare earth elements than the arc related basalt and andesite this suggests that the primary melts were more primitive for synrift andesite than for the arc related rocks

lowermost mantle anisotropy and deformation along the boundary of the african llsvp shear wave splitting of sk k s phases is often used to examine upper mantle anisotropy in specific cases however splitting of these phases may reflect anisotropy in the lowermost mantle here we present sks and skks splitting measurements for 233 event station pairs at 34 seismic stations that sample d beneath africa of these 36 pairs show significantly different splitting between the two phases which likely reflects a contribution from lowermost mantle anisotropy the vast majority of discrepant pairs sample the boundary of the african large low shear velocity province llsvp which dominates the lower mantle structure beneath this region in general we observe little or no splitting of phases that have passed through the llsvp itself and significant splitting for phases that have sampled the boundary of the llsvp we infer that the d region just outside the llsvp boundary is strongly deformed while its interior remains undeformed or weakly deformed

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

a comparative analysis of pre silurian crustal building blocks of the northern and the southern appalachian orogen the new york promontory serves as the divide between the northern and southern segments of the appalachian orogen antiquated subdivisions distinct for each segment implied that they had lithotectonic histories that were independent of each other using new lithotectonic subdivisions we compare first order features of the pre silurian orogenic building blocks in order to test the validity of the implication of independent lithotectonic histories for the two segments three lithotectonic divisions termed here the laurentian iapetan and the peri gondwanan realms characterize the entire orogen the laurentian realm composed of native north american rocks is remarkably uniform for the length of the orogen it records the multistage neoproterozoic early paleozoic rift drift history of the appalachian passive margin formation of a taconic seaway and the ultimate demise of both in the middle ordovician the iapetan realm encompasses mainly oceanic and magmatic arc tracts that once lay within the iapetus ocean between laurentia and gondwana in the northern segment the realm is divisible on the basis of stratigraphy and faunal provinciality into peri laurentian and peri gondwanan tracts that were amalgamated in the late ordovician south of new york stratigraphic and faunal controls decrease markedly rock associations are not inconsistent with those of the northern appalachians although second order differences exist exposed exotic crustal blocks of the peri gondwanan realm include ganderia avalonia and meguma in the north and carolinia in the south carolinia most closely resembles ganderia both in early evolution and late ordovician silurian docking to laurentia our comparison indicates that to a first order the pre silurian appalachian orogen developed uniformly starting with complex rifting and a subsequent drift phase to form the appalachian margin followed by the consolidation of iapetan components and ending with accretion of the peri gonwanan ganderia and carolinia this deduction implies that any first order differences between northern and southern segments post date late ordovician consolidation of a large portion of the orogen

large magnitude miocene extension in the central mojave desert implications for paleozoic to tertiary paleogeography and tectonics this is the published version copyright 1990 american geophysical union all rights reserved

archean continental assembly in the southeastern superior province of canada between 1988 and 1993 seismic reflection and refraction surveys were acquired across the medium to high grade opatica plutonic gneiss belt the low grade abitibi greenstone belt and the pontiac metasedimentary belt all of which form part of the late archean superior province shallowly north dipping reflections define a structural style consistent with the northward underthrusting and accretion over about 30 ma of various exotic terranes against a backstop provided by the opatica belt this rapid southward growth of the archean protocraton was driven by at least one north dipping subduction zone as revealed by north dipping reflections that extend to 65 km depth in the upper mantle below the opatica belt in contrast to the mainly orthogneissic opatica and pontiac belts the midcrust of the abitibi belt comprises metasedimentary and igneous rocks plus imbricated units of unknown affinity relict midcrustal accretionary complexes of substantial size whieh are indicative of primary suture zones are interpreted near the northern and southern limits of the abitibi belt an interpreted basal decollement and significantly older ages in the north suggest that the upper crustal greenstone rocks are allochthonous evidence of large scale extension appears to be confined to the southern volcanic zone of the abitibi which developed into a half graben as the original suture zone was reactivated in extension unusually high seismic p wave velocities 7 5 8 2 kms 1 are present in the lower 8 km of the abitibi crust and they correlate well with a downward reduction in seismic reflectivity attributable to late modification of the deepest part of the crust crustal xenolith studies suggest that this process may be linked to early proterozoic magmatism

shear wave splitting in the appalachians and the urals a case for multilayered anisotropy observations of shear wave splitting in the northeastern u s appalachians and in the foredeep of the urals vary significantly with the back azimuth and incidence angle of the incoming phase these variations suggest two or more layers within the upper mantle with different anisotropic properties synthetic seismograms for simple multilayered anisotropic structures show that shear wave splitting parameters tend to vary substantially with the direction of approach relying on a subset of back azimuth and incidence angle may strongly bias the model inferred especially if the observations are averaged on the other hand the azimuthal splitting pattern provides additional constraints on vertical or lateral variation of anisotropic properties in the earth using a new error estimation technique for splitting we find that individual measurements from broadband data have errors of the order of 3 7 for the fast direction and 0 1 0 2 s for the delay of split shear waves the azimuthal variation of splitting parameters is broadly consistent throughout the appalachian terranes in the northeast united states especially for two long running stations in the northeast united states hrv harvard massachusetts and pal palisades new york observations can be separated into two distinct populations with mean fast axis azimuths of n60 e 4 and n119 e 2 delay values within each population range from near zero to 1 s azimuthal splitting variation for aru arti russia in the foredeep of uralian mountains is characterized by sharp transitions between different groups of observations using synthetic seismograms in simple structures we develop one dimensional anisotropic models under stations hrv and aru the model for hrv includes two layers of anisotropic material under an isotropic crust with fast axis azimuths n53 e and n115 e for the bottom and the top layers respectively the model for the upper mantle under aru includes a layer with a fast axis at n50 e atop a layer with fast axis azimuth n90 e our modeling confirms the need for a layer of strong anisotropy with a slow axis of symmetry in the lower crust under aru reported by levin and park 1997a our results suggest that both urals and appalachians possess a relict anisotropy in the tectosphere associated with past continental collision and accretion underlain by anisotropy with orientation similar to the local absolute plate motion suggesting an asthenospheric component to the signal

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

a sharp lithosphere asthenosphere boundary imaged beneath eastern north america although a rigid lithosphere about 100 km of crust and uppermost mantle moving over a weaker asthenosphere a few hundred kilometres of more deformable rock is a widely accepted aspect of plate tectonics the properties of the boundary between them are poorly understood when compressional waves p waves from an earthquake encounter an interface at an angle some of the energy is converted to shear waves or s waves these converted waves can be used to construct an image of the interface based on converted waves recorded in eastern north america the lithosphere asthenosphere boundary has been resolved as a sharp gradient that cannot be explained by thermal gradients alone indicating that the boundary is controlled by melt or high water content in the asthenosphere plate tectonic theory hinges on the concept of a relatively rigid lithosphere moving over a weaker asthenosphere yet the nature of the lithosphere asthenosphere boundary remains poorly understood the gradient in seismic velocity that occurs at this boundary is central to constraining the physical and chemical properties that create differences in mechanical strength between the two layers for example if the lithosphere is simply a thermal boundary layer that is more rigid owing to colder temperatures mantle flow models1 2 indicate that the velocity gradient at its base would occur over tens of kilometres in contrast if the asthenosphere is weak owing to volatile enrichment3 4 5 6 or the presence of partial melt7 the lithosphere asthenosphere boundary could occur over a much smaller depth range here we use converted seismic phases in eastern north america to image a very sharp seismic velocity gradient at the base of the lithosphere a 3 11 per cent drop in shear wave velocity over a depth range of 11 km or less at 90 110 km depth such a strong sharp boundary cannot be reconciled with a purely thermal gradient but could be explained by an asthenosphere that contains a few per cent partial melt7 or that is enriched in volatiles relative to the lithosphere3 4 5 6

the origin of mount st helens andesites abstract mount st helens volcano has intermittently produced mainly dacitic products but occasionally erupted a more diverse suite of lavas including basalts and andesites petrogenetic relations between these magmas provide insight into the dynamics of the subjacent magma system mineralogical and geochemical features of representative lavas erupted during the past 2200 years can distinguish three basaltic and three andesitic variants the mafic lavas include a transitional olivine plagioclase basalts with low k 2 o and incompatible trace element abundances b calc alkaline olivine plagioclase clinopyroxene basalts enriched in k 2 o tio 2 and incompatible trace elements and c calc alkaline olivine plagioclase basaltic andesites with incompatible trace element contents transitional between the two basalt types intermediate lavas include a tholeiitic two pyroxene andesites b calc alkaline plagioclase two pyroxenes olivine amphibole mafic andesites 56 59 wt sio 2 and c calc alkaline plagioclase two pyroxenes amphibole high silica andesites 61 62 wt sio 2 eruption of these magmatic variants within the same eruptive phase implies the existence of different petrogenetic lineages and that the plumbing system is sufficiently complex to simultaneously isolate and preserve numerous magma batches it is unlikely that any of the andesites or dacites are derived by fractional crystallization of the recognized basaltic variants formation of the andesites simply by contamination or assimilation fractional crystallization of basaltic magma is also improbable more plausibly the andesites represent mixing between basaltic and dacitic end member magmas each of which may be somewhat heterogeneous or vary in composition with time in this model efficient mixing must occur in some parts of the magma plumbing system while some conduits or storage reservoirs must be effectively isolated

lithospheric deformation inferred from teleseismic shear wave splitting observations in the united kingdom observations of splitting in teleseismic shear waves recorded at stations in the united kingdom uk seismic network operated by the british geological survey are related to the tectonic structures and history in the uk the combination of network density and tectonic variability in the uk permits an assessment to be made of the relationship between splitting and geology fast polarization directions are most commonly parallel to geologic features resulting from compressional or strike slip events and splitting delay times mirror the increasing involvement of basement in deformation as observed through field investigation different orogenic episodes may be distinguished in the splitting orientations a peak in delay times is suggested in the observations at the iapetus suture a site of paleozoic subduction now in the uk interior these observations are consistent with the idea that continental anisotropy arises through coherent deformation of the lithosphere and is preserved long after orogenic deformation ceases

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

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

the magnitude symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonites summary seismic anisotropy within the upper mantle originates from the preferred orientation of highly anisotropic single crystals the symmetry and magnitude of anisotropy depend upon 1 the volume percentages of the minerals constituting the upper mantle 2 the degree and symmetry of preferred orientation of each mineral and 3 the alignment of the minerals crystallographic axes relative to one another the nature of upper mantle anisotropy can be examined by studying mineral orientations within ultramafic rocks which were once part of the mantle petrofabric data for olivine and pyroxene have been used to obtain velocity anisotropy patterns over large regions of ultramafic rocks from the samail ophiolite oman the troodos ophiolite cyprus the bay of islands ophiolife newfoundland the twin sisters ultramafic washington usa the dun mountain ophiolite new zealand the red hills ophiolite new zealand and the red mountain ophiolite new zealand the compressional wave anisotropy calculated for these massifs ranges from 3 to 8 per cent in excellent agreement with observed seismic anisotropy in the upper continental and oceanic mantle the symmetry varies from orthorhombic to axial with the axial symmetry axis corresponding to the olivine a axes maxima and subparallel to spreading directions in oceanic upper mantle pyroxene a b and c axes maxima generally parallel olivine b c and a axes respectively and anisotropy decreases with increasing pyroxene content shear wave splitting is predicted for all propagation directions within the upper mantle symmetry is also orthorhombic or axial with the minimum difference in velocity between the two shear waves parallel to the maximum compressional wave velocity

shear wave birefringence in wedge shaped anisotropic regions summary a popular method for studies of seismic wave speed anisotropy within the earth is based on observations of birefringence in subvertically propagating shear waves a well recognized shortcoming of the method is that individual birefringence measurements commonly interpreted as a path average of anisotropy do not reveal the potential complexity of anisotropic structure consequently there is a necessary ambiguity in the interpretation of observed birefringence parameters fast polarization directions and delays between components in this paper we explore the influence of observational geometry and the configuration of an anisotropic region on the birefringence parameters we use full wavefield simulations in 3 d models to access these influences in a subduction zone setting our primary findings are that a modest wavefield complications associated with the non planar geometry of a subduction zone can significantly bias the determination of the fast polarization and time delay associated with shear wave birefringence b shear wave splitting associated with flow in the mantle wedge near the trench e g the forearc is likely to be weak owing to short path lengths through an anisotropic region and c popular splitting estimators are vulnerable to waveform distortions caused by s wave interaction with the dipping slab interface we show that simultaneous interpretation of numerous birefringence observations from a variety of propagation directions is a more reliable way to constrain parameters of anisotropy at depth

3 d shear wave radially and azimuthally anisotropic velocity model of the north american upper mantle summary using a combination of long period seismic waveforms and sks splitting measurements we have developed a 3 d upper mantle model sawum na2 of north america that includes isotropic shear velocity with a lateral resolution of 250 km as well as radial and azimuthal anisotropy with a lateral resolution of 500 km combining these results we infer several key features of lithosphere and asthenosphere structure a rapid change from thin 70 80 km lithosphere in the western united states wus to thick lithosphere 200 km in the central cratonic part of the continent closely follows the rocky mountain front rmf changes with depth of the fast axis direction of azimuthal anisotropy reveal the presence of two layers in the cratonic lithosphere corresponding to the fast to slow discontinuity found in receiver functions below the lithosphere azimuthal anisotropy manifests a maximum stronger in the wus than under the craton and the fast axis of anisotropy aligns with the absolute plate motion as described in the hotspot reference frame hs3 nuvel 1a in the wus this zone is confined between 70 and 150 km decreasing in strengthwithdepthfromthetop fromthermftothesanandreasfaultsystemandthejuande fuca gorda ridges this result suggests that shear associated with lithosphere asthenosphere couplingdominatesmantledeformationdowntothisdepthinthewesternpartofthecontinent the depth extent of the zone of increased azimuthal anisotropy below the cratonic lithosphere is not well resolved in our study although it is peaked around 270 km a robust result radial anisotropy is such that predominantly 1 where vsh vsv 2 under the continent and its borders down to 200 km with stronger in the bordering oceanic regions across the continent and below 200 km alternating zones of weaker and stronger radial anisotropy with predominantly 1 correlate with zones of small lateral changes in the fast axis direction of anisotropy and faster than average vs below the lab suggesting the presence of small scale convection with a wavelength of 2000 km finally in the western united states complex 3 d patterns of isotropic velocity and anisotropy reflect mantle dynamics associated with the rich tectonic history of the region

lithospheric expression of geological units in central and eastern north america from full waveform tomography abstract the earthscope ta deployment has provided dense array coverage throughout the continental us and with it the opportunity for high resolution 3d seismic velocity imaging of both lithosphere and asthenosphere in the continent building upon our previous long period waveform tomographic modeling in north america we present a higher resolution 3d isotropic and radially anisotropic shear wave velocity model of the north american lithospheric mantle constructed tomographically using the spectral element method for wavefield computations and waveform data down to 40 s period the new model exhibits pronounced spatial correlation between lateral variations in seismic velocity and anisotropy and major tectonic units as defined from surface geology in the center of the continent the north american craton exhibits uniformly thick lithosphere down to 200 250 km while major tectonic sutures of proterozoic age visible in the surface geology extend down to 100 150 km as relatively narrow zones of distinct radial anisotropy with vsv vsh notably the upper mantle low velocity zone is present everywhere under the craton between 200 and 300 km depth east of the continental rift margin the lithosphere is broken up into a series of large somewhat thinner 150 km high velocity blocks which extend laterally 200 300 km offshore into the atlantic ocean between the craton and these deep rooted blocks we find a prominent narrow band of low velocities that roughly follows the southern and eastern laurentia rift margin and extends into new england we suggest that the lithosphere along this band of low velocities may be thinned due to the combined effects of repeated rifting processes and northward extension of the hotspot related bermuda low velocity channel across the new england region we propose that the deep rooted high velocity blocks east of the laurentia margin represent the proterozoic gondwanian terranes of pan african affinity which were captured during the rodinia formation but left behind after the opening of the atlantic ocean our results suggest that recurring episodes of tectonic events that are well exposed at the surface also leave persistent scars in the continental lithosphere mantle marked by isotropic and radially anisotropic velocity anomalies that reach as deep as 100 150 km

cenozoic evolution of the mojave block of southern california the recorded cenozoic history of the mojave desert region of southern california began in the latest oligocene when intense volcanism and tectonism interrupted a long early tertiary silence volcanism commenced across the region in an east west band ca 24 22 ma northwest of barstow volcanism was accompanied by intense crustal extension and development of a metamorphic core complex outside of this relatively restricted area extension was minor or absent after extension ceased ca 18 ma volcanism shifted to small volume eruptions of basalt post extensional deformation has largely been by strike slip faulting along northwest striking dextral faults and west striking sinistral faults and total dextral slip across the mojave desert region since the early miocene is 45 60 km strike slip deformation has been overprinted locally by intense north south contraction that is the dominant style of deformation in the western mojave block paleomagnetic data indicate that parts of the mojave block were rotated clockwise although the magnitude and timing of this rotation are poorly determined the best evidence for large 45 rotation comes from the area east of barstow where large clockwise declination anomalies and mesozoic and cenozoic dikes with anomalous strikes may reflect early miocene clockwise deflection along the mojave river fault volcanism and tectonism in the mojave block resulted from interactions among the north american pacific and various oceanic plates patterns of volcanism and tectonism do not correlate with growth of slab windows beneath the continent but do correlate with the position of the subducted mendocino fracture zone plate circuit reconstructions suggest that the driving force for extension was divergence between the pacific and north american plates along the transform margin that separated the two this hypothesis accounts for the direction magnitude and rate of extension in the mojave block

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

alkalic ocean island basalt type and calc alkalic volcanism in the mexican volcanic belt a case for plume related magmatism and propagating rifting at an active margin the mexican volcanic belt has been traditionally regarded as a classic case of subduction related calc alkalic volcanism however a series of geologic geophysical and petrological arguments makes this simple relationship doubtful a seismic gap beneath the belt a large scale mantle anomaly a graben triple junction domain and the presence of volumetrically important oceanic island basalt oib volcanism throughout the belt suggest a more complex tectonic scenario involving plume and subduction related processes we here propose a model involving the development of a propagating rift opening from west to east in response to plume activity the process started in miocene time within the western sector of the belt guadalajara and gave rise to a graben triple junction and oib type and calc alkalic volcanism extension and volcanism proceeded to the east giving rise to progressively younger ages for the initiation of oib type volcanism 1 miocene in the west e g guadalajara 2 pliocene in the central zone e g michoacan guanajuato and 3 quaternary farther east e g chichinautzin geochemical evidence suggests that part of the modern calc alkalic volcanism e g chichinautzin may be derived from magma mixing between the oib mafic magmas and silicic crust derived magmas however we do not preclude some influence of the subducting slab in the generation of other e g jorullo calc alkalic volcanic rocks our model suggests a currently unrooted upper plume attached to the subcontinental lithosphere which defines a hot zone beneath the mexican volcanic belt

deep mantle contributions to the surface dynamics of the north american continent abstract the regional and continental scale dynamics of north america and vicinity are explored using a high resolution model of mantle flow the model is constrained by simultaneously inverting global seismic and mantle convection data sets and it includes an explicit treatment of the positive chemical buoyancy of the continental tectosphere moreover it adopts a depth dependent mantle viscosity structure which reconciles both glacial isostatic adjustment gia and convection data the flow model successfully reproduces plate velocities and observations of surface gravity and topography including the continent scale quasi linear depression after corrections for gia and crustal heterogeneity extending from northern alaska to venezuela the predictions also match lithospheric flow and stress fields inferred from local and regional measurements of seismic anisotropy and surface deformation we demonstrate that these signals are largely driven by viscous flow coupled to density anomalies in the deep portions of the upper mantle and within the lower mantle where the latter may be associated with the descent of the ancient kula farallon plate system and an active mantle upwelling below the pacific margin of the north american plate more importantly the flow calculations elucidate how these large scale heterogeneities give rise to regional scale flow and stress patterns below the southwestern u s and below the central u s

210pb in the pacific the geosecs measurements of particulate and dissolved concentrations abstract we report here on particulate and dissolved 210 pb profiles at 16 stations and on total 210 pb profiles at 3 stations all occupied during the pacific geosecs expedition comparison with measurements at yale on geosecs library samples indicates that during separation of particulate lead from dissolved lead our filtered water samples suffered some loss of 210 pb in the filtration system this effect appears to have reduced the dissolved 210 pb activities by 20 in stations where the water was filtered however for these first pacific data on the 210 pb distribution between the two phases this effect does not significantly interfere with our recognition of the major features of both particulate and dissolved 210 pb distributions the dissolved 210 pb profiles in general vary geographically following the 226 ra profiles in deep water 226 ra increases northward and eastward from the southwest pacific from 22dpm 100kg to over 40 dpm 100 kg in the northeast pacific our dissolved 210 pb profiles show a similar increase in deep water varying from about 10 to 20 dpm 100 kg along this line and are commonly characterized by a mid depth maximum this 210 pb maximum reflects the mid depth 226 ra maximum of the pacific deep water observed along the western boundary current in surface water at low latitudes there is a significant 210 pb flux from the atmosphere which produces a 210 pb 226 ra activity ratio generally greater than unity this flux penetrates as deep as 600 m as indicated by an induced 210 pb minimum caused by the surface maximum the surface water 210 pb excess decreases toward high southern latitudes and vanishes in the circumpolar region the particulate 210 pb profiles show a general increase with depth from 0 3dpm 100kg in subsurface water to 1 5dpm 100kg in bottom water with or without a mid depth maximum that reflects the 226 ra or dissolved 210 pb maximum the particulate 210 pb normally comprises about 2 of the total 210 pb in subsurface water and this fraction increases to about 10 near the bottom as the filtration loss is not taken into account the fraction of particulate 210 pb quoted here is an upper limit since the particulate matter concentrations are quite uniform in the water column below a few hundred meters the 210 pb activity of the particulate matter also increases with depth the particulate matter has a 210 pb concentration of 100dpm g in subsurface water but the concentration increases to 500dpm g or more toward the bottom this indicates that there is a cumulative adsorption of pb onto the suspended particles as they are sinking through the water column

azimuthal anisotropy in the earth from observations of sks at geoscope and nars broadband stations abstract we have analyzed observations of low frequency sks waves recorded on horizontal components at several broadband stations of the geoscope and nars arrays splitting of sks is consistently observed and in good agreement with a hypothesis of azimuthal anisotropy due to transverse anisotropy with a horizontal axis in the upper mantle beneath the receivers we have designed an inversion method which takes advantage of observations made simultaneously at different azimuths and which permits us to obtain estimates of the direction of fast axis of anisotropy as well as the time delay between the orthogonal quasi shear waves in general the directions of fast velocity obtained both for stations around the atlantic and around the pacific appear to be related to directions of past and present plate motions

seismic anisotropy and mantle flow seismic anisotropy in the mantle is primarily due to deformation induced lattice preferred orientation lpo of olivine crystals the aim of this paper is to better understand how such lpo is produced by flow and to learn whether observations of seismic anisotropy can be used to infer the local direction of flow within the mantle the basis of the work is a two dimensional theory which describes how lpo evolves during an arbitrary time dependent deformation ribe 1989 i investigate the evolution of lpo beneath ocean ridges above subducted slabs and in thermal convective flows the ocean ridge model predicts that the seismic anisotropy produced at a fast spreading ridge should be greater in magnitude than that produced at a slow spreading ridge and that the fast axis should be more nearly horizontal the subduction zone model predicts that strong seismic anisotropy should be produced above subducted slabs with the fast axis aligned with the slab regardless of the subduction angle the evolution of lpo in typical convective flows is extremely complex and depends strongly on such factors as the mode of heating and the surface boundary conditions in general there is no simple relation between the lpo and the local flow direction in any of the models considered the attempt to use seismic anisotropy to map flow patterns on a global scale is therefore probably unwarranted however the lpo does become aligned with the flow direction when the flow is a progressive simple shear such a flow occurs in limited portions of the mantle beneath the lithosphere at a fast spreading ridge above subducted slabs and in boundary layers beneath rigid surfaces such as continents observations of seismic anisotropy may therefore indicate the sense of shear at shallow depths in the mantle

the thickness of tectonic plates a fundamental premise of plate tectonics on earth is that rigid lithospheric plates formed at mid ocean ridges float above a more deformable substratum the asthenosphere 1 the precise nature of the asthenosphere is still debated mechanical models predict a well defined sharp lithosphere asthenosphere boundary lab but evidence for such a boundary from conventional seismic measurements is ambiguous on pages 499 1 and 495 2 of this issue kawakatsu et al 2 and rychert and shearer 3 present analyses of more sophisticated seismic studies that help refine the lab and hence the thickness of the lithosphere and tectonic plates although challenges still remain in picking out this boundary versus other structures within the lithosphere 1 lookup doi 10 1126 science 1169499 2 lookup doi 10 1126 science 1169754

shear wave splitting observations in the lower great lakes region evidence for regional anisotropic domains and keel modified asthenospheric flow 1 fast axis directions obtained from shear wave splitting analysis of core refracted shear waves reveal several distinct anisotropic domains in the lower great lakes region we used data from 27 broadband seismograph stations extending across a low velocity anomaly in the lithospheric mantle observed splitting times vary from 0 4 to 1 4 s with a mean of 0 7 s a subset of the splitting vectors across a failed rift are oriented parallel to one arm of the rift zone most others show a coherent pattern of fast directions close to the direction of plate motion but oblique to surface tectonic belts within the area of the low velocity anomaly our observations are most simply explained by single layer anisotropy induced by asthenospheric flow spatial variability in the direction and magnitude of splitting are consistent with a flow regime influenced by basal topology of the lithospheric keel

geological analysis of the silver lake marsokhod field test from ground truth sampling and mapping we compare the results of the observations and conclusions obtained from a remote science team and field geologists during the 1999 marsokhod silver lake field experiment this comparison is used to determine methods that enhance the ability of a remote team using a rover to determine the geology and paleoclimate of a landing site analogous to sites that may be encountered during the mars surveyor program the mobility of the field geologists and their ability to rapidly gain visual and physical data from the field site give an immediate confidence to their interpretation of the geology the finite time of teleoperation and the initial decisions and mechanical problems that caused the rover to linger and collect data in a small area of the test site for the first 5 days of the test limited the ability of the remote science team to gain a similar confidence nevertheless by appropriate use of orbital and descent images as well as the rover s science payload the remote science team interpreted the present and former climates the geology and geological history and present day biology relatively accurately the few erroneous interpretations by the remote team resulted in part from biases brought to the test about the nature of the geology that was likely to be encountered data quality and variety also impacted the quality of the geological interpretations significant misinterpretations in the remotely determined geology resulted from the lack of detail in the optical imagery a less than optimal understanding of the site s topography and misinterpretation of some infrared data all in all this field test showed that the geology paleoclimate and biology of a site can be assessed to a reasonable level of confidence using a remotely controlled vehicle it also showed that it is both feasible and desirable for a distributed science team to participate in long science missions from home institutions as long as data flows efficiently to and from these institutions

mantle dynamics and seismic anisotropy abstract observations of seismic anisotropy yield some of the most direct constraints available on both past and present day deformation in the earth s mantle insight into the character of mantle flow can also be gained from the geodynamical modeling of mantle processes on both global and regional scales we highlight recent progress toward understanding mantle flow from both observations and modeling and discuss outstanding problems and avenues for progress particularly in the integration of seismological and geodynamical constraints to understand seismic anisotropy and the deformation that produces it to first order the predictions of upper mantle anisotropy made by global mantle circulation models match seismological observations well beneath the ocean basins but the fit is poorer in regions of greater tectonic complexity such as beneath continental interiors and within subduction systems in many regions of the upper mantle models of anisotropy derived from surface waves are seemingly inconsistent with shear wave splitting observations which suggests that our understanding of complex anisotropic regions remains incomplete observations of anisotropy in the d layer hold promise for improving our understanding of dynamic processes in the deep earth but much progress remains to be made in characterizing anisotropic structure and relating it to the geometry of flow geochemical heterogeneity or phase transitions major outstanding problems related to understanding mantle anisotropy remain particularly regarding the deformation and evolution of continents the nature of the asthenosphere subduction zone geodynamics and the thermo chemical state of the lowermost mantle however we expect that new seismological deployments and closer integration of observations with geodynamical models will yield rapid progress in these areas

descent of the ancient farallon slab drives localized mantle flow below the new madrid seismic zone 1 the great earthquake sequence that occurred in the central mississippi river valley during the winter of 1811 1812 is unprecedented in the historical record of seismicity within stable continental plate interiors we show using viscous flow models based on high resolution seismic tomography that the descent of the ancient farallon slab into the deep mantle beneath central north america induces a highly localized flow directly below the new madrid seismic zone nmsz this localization arises because of structural variability in the farallon slab and the low viscosity of the sub lithospheric upper mantle and it represents a heretofore unrecognized and possibly significant driving mechanism for the enigmatic intraplate seismicity

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

mafic enclaves in the wilson ridge pluton northwestern arizona implications for the generation of a calc alkaline intermediate pluton in an extensional environment the wilson ridge pluton is an epizonal calc alkaline pluton that formed about 13 5 ma during a period of mid miocene extension faulting and erosional dissection provide a cross section of the pluton the apex of the pluton in the boulder wash area nevada is composed of hypabyssal quartz monzonite and dacite the base of the pluton is 20 km to the south where quartz monzodiorite monzodiorite and diorite are in low angle intrusive contact with precambrian basement the pluton was separated from cogenetic volcanic rocks in the river mountains by movement along the saddle island detachment fault at about 13 4 ma the river mountains now lie 20 km to the west of the pluton the wilson ridge pluton is composed of the teakettle pass suite consisting of foliated monzodiorite and quartz monzodiorite and unfoliated quartz monzonite and the older horsethief canyon diorite rocks of the pluton contain 2 4 modal percent sphene intermediate rocks of the teakettle pass suite contain abundant basaltic and diorite enclaves basaltic enclaves are lensoidal and pillow like and commonly have crenulate and fine grained margins enclaves are chemically similar to mafic dikes of the wilson ridge pluton and to cogenetic alkali basalt flows in the river mountains they probably represent blobs of mafic liquid that commingled and mechanically mixed with felsic magma to produce the intermediate rocks of the pluton basaltic enclaves commonly occur as inclusion rich zones that represent synplutonic mafic dikes that were injected into a quartz monzonite host mafic magma was entrained and mechanically broken down by magmatic flow shear a continuum in shape exists from enclaves that are bulbous and ellipsoidal to those that are thin tabular mafic selvages and schlieren and ultimately to the mafic component in foliated quartz monzodiorite and monzodiorite diorite enclaves have angular contacts with host rocks and are interpreted as xenoliths field evidence and major and trace element models suggest that the intermediate rocks of the pluton were produced by the commingling of a large volume of mafic magma with a smaller volume of felsic magma a mafic felsic ratio of about 70 30 as well as fractional crystallization similar open system processes may be responsible for the production of calc alkaline intermediate rocks in other parts of the great basin

petrology of the lower tertiary clarno formation in north central oregon the importance of magma mixing predominantly eocene rocks of the clarno formation in north central oregon consist of lava flows volcaniclastic rocks and tuffaceous sedimentary rocks lying between cretaceous shallow marine deltaic sedimentary rocks and younger volcanic rocks whole rock rb and sr isotopic data yield a poor isochron with an age of 47 5 ma clarno rocks are broadly calc alkaline range from basalt to rhyolite and show complex textural and compositional properties plagioclase and clinopyroxene phenocrysts are present in most lavas with olivine in mafic rocks and orthopyroxene quartz and subordinate biotite and hornblende in intermediate rocks spongy sieve textured plagioclase and glass bearing plagioclase are both common many quartz grains are resorbed and some pyroxene grains are spongy some samples contain quartz in which plagioclase inclusions have undergone partial melting along the plagioclase quartz boundaries plagioclase inclusions in these rocks are more calcic an54 66 than the cores of their companion xenocrysts an41 suggesting digestion of earlier mafic rocks mafic cumulates have high mgo cr and ni values some oxide and element sio2 diagrams show linear trends but scatter from the trends is large plots of sio2 versus al2o3 sr y and possibly other components show inflections in trends in the range of 55 60 sio2 the abundance of disequilibrium textures shows that the dominant process in the evolution of the clarno suite was mixing of magmas with liquid and partly solid sources probably including earlier crystallized clarno magmatic rocks cumulate textures and compositions however indicate that clarno magmas were modified by crystal fractionation which also may have contributed to the scatter from linear trends on composition diagrams initial 87sr 86sr ratios corrected for a model age of 50 ma range from 0 70350 to 0 70397 consistent with mantle source s or with contamination by young mafic material but not with interaction with old upper continental crust