seismic tomography a window into deep earth abstract the goal of this paper is to provide an overview of the current state of the art in seismic tomography and trace its origins from pioneering work in the early 1970s to its present status as the pre eminent tool for imaging the earth s interior at a variety of scales due to length limitations we cannot hope to cover every aspect of this diverse topic or include mathematical derivations of the underlying principles rather we will provide a largely descriptive coverage of the methodology that is targeted at readers not intimately familiar with the topic the relative merits of local versus global parameterization ray tracing versus wavefront tracking backprojection versus gradient based inversion and synthetic testing versus model covariance are explored a variety of key application areas are also discussed including body wave traveltime tomography surface wave tomography attenuation tomography and ambient noise tomography established and emerging trends many of which are driven by the ongoing rapid increases in available computing power will also be examined including finite frequency tomography full waveform tomography and joint tomography using multiple datasets several practical applications of seismic tomography including body wave traveltime attenuation and surface waveform are presented in order to reinforce prior discussion of theory

the 2013 wyoming upper mantle earthquakes tomography and tectonic implications on 21 september 2013 two earthquakes m 4 8 and 3 0 focal depths 70 km occurred in the lithospheric mantle beneath central wyoming which provide a rare opportunity to investigate the lithosphere rheology and dynamics we determined a detailed 3 d p wave tomography in the source area of the wyoming earthquakes using a large number of high quality travel time data of local earthquakes and teleseismic events our results show that the 2013 wyoming earthquakes occurred in an isolated high velocity high v anomaly which extends down to a depth of 160 km we propose two possible scenarios about the cause of the 2013 wyoming earthquakes one is that the high v anomaly reflects a remnant of the subducted oceanic slab which fractured under long term negative buoyancy and heating by the infilling asthenosphere the other is removing of a dense mantle lithospheric root the present results shed new light on the rheology and seismotectonics of the continental lithosphere

the 3d numerical modeling of subduction dynamics plate stagnation and segmentation and crustal advection in the wet mantle transition zone 1 water content in the mantle transition zone mtz has been broadly debated in the earth science community as a key issue for plate dynamics in this study a systematic series of three dimensional 3d numerical simulation are performed in an attempt to verify a hypothesis regarding the behavior of subducted oceanic plate and the role of crust in the mtz under wet conditions this hypothesis is based on the seismic observations of structural variations associated with large scale flattened high velocity anomalies i e stagnant slabs in the mtz in our model weak low viscosity fault zones wfzs which presumably correspond to the fault boundaries of large subduction earthquakes are imposed on the top part of subducting plates the phase transitions of olivine to wadsleyite and ringwoodite to perovskite magnesiowustite under both dry and wet conditions are considered based on recent high pressure experiments the effect of viscosity reduction of wet garnet is also considered for oceanic crust in the mtz results show that there is a substantial difference in the behaviors of subducting plate and the trace of crustal materials between the models under dry and wet conditions under wet conditions the subducting plate tends to stagnate with a maximum lateral extent of over 1000 km the weaker and denser crustal material garnet rich is fed into the wfzs and is advected faster than the plate main body of peridotite olivine rich thus the viscosity and density contrast between the crustal materials and the peridotite permits the plate segmentation under a wet mtz condition

upper mantle structure of the northern cascadia subduction zone previous knowledge of the structure of the cascadia subduction zone north of the canada united states border has been derived from a variety of geophysical studies that accurately delineated the downgoing juan de fuca plate from the offshore deformation front to depths of 50 60 km beneath south central vancouver island and the georgia strait little is known however of the structure of the cascadia subduction zone farther westward and to greater depths in the upper mantle we have assembled a set of some 1100 teleseismic traveltimes from events recorded on the western canadian telemetered network to augment a previously existing data set recorded on the washington regional seismograph network the composite data set is inverted for upper mantle structure below washington oregon and southwestern british columbia we analyze the new northern portion of the model between 48 5 50 n and 118 127 w which provides the first images of the deep slab structure in this region the model is parameterized using s

mesozoic plate motion history below the northeast pacific ocean from seismic images of the subducted farallon slab the high resolution seismic imaging of subducted oceanic slabs1 2 has become a powerful tool for reconstructing palaeogeography3 the images can now be interpreted quantitatively by comparison with models of the general circulation of the earth s mantle4 here we use a three dimensional spherical computer model of mantle convection5 6 to show that seismic images of the subducted farallon plate provide strong evidence for a mesozoic period of low angle subduction under north america such a period of low angle subduction has been invoked independently to explain rocky mountain uplift far inland from the plate boundary during the laramide orogeny7 the computer simulations also allow us to locate the largely unknown kula farallon spreading plate boundary the location of which is important for inferring the trajectories of suspect terrain across the pacific basin8

crust and upper mantle structure beneath the pacific northwest from joint inversions of ambient noise and earthquake data 1 we perform a joint inversion of phase velocities from both earthquake and ambient noise induced rayleigh waves to determine shear wave velocity structure in the crust and upper mantle beneath the pacific northwest we focus particularly on the areas affected by mid miocene to present volcanic activity the joint inversion combined with the high density seismic network of the high lava plains seismic experiment and data from the earthscope transportable array provides outstanding resolution for this area in oregon we find that the pattern of low velocities in the crust and uppermost mantle varies between the high lava plains physiographic province and the adjacent northwestern basin and range these patterns may be due to the presence of the brothers fault zone which separates the clockwise rotating northwest basin and range from the relatively undeformed areas further north further to the east the owyhee plateau snake river plain srp and northeastern basin and range are characterized by high crustal velocities though the depth extent of these fast wave speeds varies by province of particular interest is the mid crustal high velocity sill previously only identified within the srp we show this anomaly extends significantly further south into utah and nevada we suggest that one possible explanation is lateral crustal extrusion due to the emplacement of the high density mafic mid crustal sill structures within the srp

anomalous mantle transition zone beneath the yellowstone hotspot track the origin of the yellowstone and snake river plain volcanism has been strongly debated the mantle plume model successfully explains the age progressive volcanic track but a deep plume structure has been absent in seismic imaging here i apply diffractional tomography to receiver functions recorded at usarray stations to map high resolution topography of mantle transition zone discontinuities the images reveal a trail of anomalies that closely follow the surface hotspot track and correlate well with a seismic wave speed gap in the subducting farallon slab this observation contradicts the plume model which requires anomalies in the mid mantle to be confined in a narrow region directly beneath the present day yellowstone caldera i propose an alternative interpretation of the yellowstone volcanism about 16 million years ago a section of young slab that had broken off from a subducted spreading centre in the mantle first penetrated the 660 km discontinuity beneath oregon and idaho and pulled down older stagnant slab slab tearing occurred along pre existing fracture zones and propagated northeastward this reversed polarity subduction generated passive upwellings from the lower mantle which ascended through a water rich mantle transition zone to produce melting and age progressive volcanism the mantle transition zone in the western united states is perturbed along a path that mirrors the line of the yellowstone hotspot track at the surface according to analysis of tomographic data

location location location the variable lifespan of the laramide orogeny the laramide orogeny had a spatially variable lifespan which we explain using a geodynamic model that incorporates onset and demise of at slab subduction laramide shortening and attendant uplift began in southeast california usa at ca 90 ma swept to the northeast to arrive in the black hills of south dakota usa at ca 60 ma and concluded in south dakotawithin 10 m y during subsequent slab rollback the areal extent of laramide deformation decreased as the eastern edge of active deformation retreated to the southwest rapidly from ca 55 to 45 ma and more slowly from ca 45 to 40 ma with deformation ultimately ceasing in the southwestern part of the orogen at ca 30 ma geodynamic modeling of this process suggests that changes in the strength of the north america plate and densifcation of the farallon plate played important roles in controlling the areal extent of the laramide orogen and hence the lifespan of the orogenic event at any particular location in western north america

trench migration and subduction zone geometry seismic tomography results show a large variety in upper mantle structure along convergent plate boundaries we numerically investigate the effect of trench migration on the evolution of a slab with temperature dependent viscosity encountering a viscosity interface we find that subduction zone geometry is sensitive to even small rates 1 cm yr of retrograde motion increase in trench migration rate decreases the dip angle of the slab and its ability to penetrate the lower mantle upon including a background mantle flow it turns out that trench migration relative to the upper mantle flow is more decisive than the absolute plate velocities subduction zone geometry appears to be strongly time dependent we conclude that individual tectonic setting and time dependent slab behaviour can account for many different types of observable subduction zone anomalies our model temperature can adequately account for magnitudes and patterns of seismic anomalies as obtained from seismic tomography

shear wave splitting and the pattern of mantle flow beneath eastern oregon a r t i c l e i n f o article history the tectonic and geologic setting of eastern oregon includes the volcanically active high lava plains hlp province and the accreted terrains of the blue and wallowa mountains and is bounded by the columbia river ood basalts to the north basin and range extension to the south the cascade arc to the west and stable north america to the east several models have been proposed to explain the tectonic evolution of eastern oregon and in particular the voluminous volcanic activity in the hlp but a consensus on which model fully describes the complex range of processes remains elusive measurements of the seismic anisotropy that results from active mantle ow beneath the region can provide a crucial test of such models to constrain this anisotropy here we present new sks splitting results obtained at approximately 200 broadband seismic stations in eastern oregon and the surrounding region data come from the usarray transportable array ta and two temporary experiments carried out in the hlp and in the wallowa mountains our splitting data set includes 2900 individual splitting measurements from sks phases recorded between 2006 and 2008 stations in eastern oregon exhibit signi cant shear wave splitting with average delay times at individual stations between 0 8 s and 2 7 s in the hlp nearly all observed fast directions are approximately e w while to the north in the blue and wallowa mountains there is more variability in the splitting patterns the average delay time observed at stations located in the heart of the hlp province is 2 s well above the global average of 1 s for continental regions we infer from the large split times and homogeneous fast directions that there must be signi cant active ow in a roughly e w direction in the asthenosphere beneath the hlp this inferred ow eld places a strong constraint on models that seek to explain the young tectonomagmatic activity in the region in the wallowa region the anisotropic signature is more complicated and there may be a signi cant contribution from fossil fabrics in the crust or mantle lithosphere

slab mantle flow interaction influence on subduction dynamics and duration we investigate the influence of mantle flow relative to the lithosphere on subduction dynamics we use 2d thermo mechanical models assuming incompressible non newtonian fluid rheology different mantle flow velocities consistent with absolute plate motion models are tested as well as both directions of flow either sustaining or opposing slab dip the effects of different inflow outflow velocity profiles slab strengths and upper lower mantle viscosity contrasts are also evaluated slab dip deviations between models with opposite mantle flow directions range from 37 for relatively strong slabs max 1025 pa s to 50 for weaker slabs max 1024 pa s accounting for a significant amount of natural slab dip variability for imposed mantle flow supporting the slab the initial stage of slab steepening is followed by a stage of continuous slab dip decrease this slab shallowing eventually leads to mantle wedge closure subduction cessation and slab break off possibly driving subduction flips

break up and seafloor spreading domains in the ne atlantic an updated magnetic anomaly grid of the ne atlantic and an improved database of magnetic anomaly and fracture zone identifications allow the kinematic history of this region to be revisited at break up time continental rupture occurred parallel to the mesozoic rift axes in the south but obliquely to the previous rifting trend in the north probably due to the proximity of the iceland plume at 57 54 ma the new oceanic lithosphere age grid is based on 30 isochrons c from c24n old 53 93 ma to c1n old 0 78 ma and documents ridge reorganizations in the se lofoten basin the jan mayen fracture zone region in iceland and offshore faroe islands updated continent ocean boundaries including the jan mayen microcontinent and detailed kinematics of the eocene present greenland eurasia relative motions are included in this model variations in the subduction regime in the ne pacific could have caused the sudden northwards motion of greenland and subsequent eurekan deformation these events caused seafloor spreading changes in the neighbouring labrador sea and a decrease in spreading rates in the ne atlantic boundaries between major oceanic crustal domains were formed when the european plate changed its absolute motion direction probably caused by successive adjustments along its southern boundary supplementary material figures showing the long wavelength of the nag tec magnetic anomaly grid detailed magnetic anomalies and isochrons and a table documenting aeromagnetic surveys for nag tec magnetic compilation are available at https doi org 10 6084 m9 figshare c 3661925

yellowstone debate erupts again the heat driving yellowstone s volcanism originates from a depth of at least 700 km according to images of the mantle created using novel seismic methods

the fate of the juan de fuca plate implications for a yellowstone plume head beneath the pacific northwest the juan de fuca plate a remnant of the farallon plate continues subducting beneath the north american continent to the east of the cascadia subduction zone lies the yellowstone hotspot track the origins of this track can be traced back to the voluminous basaltic outpourings in the columbia plateau around 17 ma if these basalts are the result of a large melting anomaly rising through the mantle to the base of the north american continent such as a mantle plume head the anomaly would need to punch through the subducting juan de fuca slab here we use teleseismic body wave travel time tomography to investigate the fate of the subducted slab and its possible interaction with a plume head our dataset is derived from the oregon array for teleseismic study oats deployment in oregon and all other available seismic data in this region during the same period in our jdf07 models we image the subducted juan de fuca plate in the mantle east of the cascades beneath oregon where the slab has not been imaged before to a depth of 400 km but no deeper the slab dips similar to 50 degrees e and has a thickness of similar to 75 km immediately beneath the slab we image a low velocity layer with a similar geometry to the slab and extending down to at least similar to 575 km depth in the v model the total length of the high velocity slab is similar to 660 km about 180 km longer than the estimated length of slab subducted since 17 ma assuming similar slab geometry to today this 180 km length of slab would reach similar to 60 km depth comparable to the thickness of continental lithosphere we propose that the absence of the slab below 400 km today is due to the arrival of the yellowstone plume head similar to 17 ma which destroyed the juan de fuca slab at depths greater than the thickness of the continental lithosphere given this scenario the low velocity anomaly beneath the slab is likely the remnant plume head material which has been pulled down by traction with the subducting plate the amplitude of the observed low velocity anomaly is comparable with that expected for plume head material 100 300 degrees c hotter than the surrounding asthenosphere c 2007 elsevier b v all rights reserved

a data adaptive multiscale approach of finite frequency traveltime tomography with special reference to p and s wave data from central tibet 1 we discuss an innovation in traveltime tomography that combines wavelet based multiscale parameterization and finite frequency theory to solve two outstanding issues that inevitably arise from uneven source station distributions and from the three dimensional 3 d nature of wavefront healing how to objectively address the intrinsically multiscale nature of data coverage while simultaneously maintain model resolution at each scale level we apply the new integrated methodology to investigate 3 d variations of p and s wave speeds lnvp and lnvs beneath the himalayan tibetan orogen in particular we are able to constrain variations in the poisson s ratio via ln vp vs the formulation is naturally data adaptive resolving features at each scale only if the required data converge is available the very first long wavelength feature that emerges is a clear anomaly of high lnv that extends over more than 500 km beyond the northern edge of the lhasa terrane at places farther northward a strong negative anomaly underlies the region where recent volcanism occurs in northern tibet regions of negative ln vp vs delineate a slab like subhorizontal feature concentrated between depths of 100 250 km such characteristics are consistent with the notion that chemically refractory and therefore buoyant mantle lithosphere of the indian shield greater india has advanced subhorizontally northward far beyond the surficial bangong nujiang suture in the crust two isolated regions of low lnv each extending to depths near 100 km occur along the lunggar and the yadong gulu active rifts in southern tibet deep penetrating rifts imply that only a limited amount of horizontal displacement is being accommodated on subvertical structures

viscosity undulations in the lower mantle the dynamical role of iron spin transition a b s t r a c t a proper determination of the lower mantle viscosity profile is fundamental to understanding earth geodynamics based on results coming from different sources several models have been proposed to constrain the variations of viscosity as a function of pressure stress and temperature while some models have proposed a relatively modest viscosity variation across the lower mantle others have proposed variations of several orders of magnitude here we have determined the viscosity of ferropericlase a major mantle mineral and explored the role of the iron high to low spin transition viscosity was described within the elastic strain energy model in which the activation parameters are obtained from the bulk and shear wave velocities those velocities were computed combining first principles total energy calculations and the quasi harmonic approximation as ar esult of a strong elasticity softening across the spin transition there is a large reduction in the activation free energies of the materials creep properties leading to viscosity undulations these results suggest that the variations of the viscosity across the lower mantle resulting from geoid inversion and postglacial rebound studies may be caused by the iron spin transition in mantle minerals implications of the undulated lower mantle viscosity profile exist for both down and up wellings in the mantle we find that a viscosity profile characterized by an activation free energy of g z0 300 400 kj mol based on diffusion creep and dilation factor 0 5b etter fits the observed high velocity layer at mid mantle depths which can be explained by the stagnation and mixing of mantle material our model also accounts for the growth of mantle plume heads up to the size necessary to explain the large igneous provinces that characterize the start of most plume tracks

subduction history reveals cretaceous slab superflux as a possible cause for the mid cretaceous plume pulse and superswell events abstract subduction is a fundamental mechanism of material exchange between the planetary interior and the surface despite its significance our current understanding of fluctuating subducting plate area and slab volume flux has been limited to a range of proxy estimates here we present a new detailed quantification of subduction zone parameters from the late triassic to present day 230 0 ma we use a community plate motion model with evolving plate topologies to extract trench normal convergence rates through time to compute subducting plate areas and we use seafloor paleo age grids to estimate the thickness of subducting lithosphere to derive the slab flux through time our results imply that slab flux doubled to values greater than 500 km3 yr from 180 ma in the jurassic to 130 ma in the mid cretaceous subsequently halving again towards the cretaceous paleogene boundary largely driven by subduction zones rimming the pacific ocean basin the 130 ma spike can be attributed to a two fold increase in mid ocean ridge lengths following the break up of pangea and a coincident increase in convergence rates with average speeds exceeding 10 cm yr with one third of the total 230 0 ma subducted volume entering the mantle during this short 50 myr period we suggest this slab superflux drove a surge in slab penetration into the lower mantle and an associated increase in the vigour of mantle return flow this mid cretaceous event may have triggered or at least contributed to the formation of the darwin rise mantle superswell dynamic uplift of the south african plateau and the plume pulse that produced the ontong java hikurangi manihiki and kerguelen plateaus among others the models presented here contribute to an improved understanding of the time evolving flux of material consumed by subduction and suggest that slab superflux may be a general feature of continental dispersal following supercontinent breakup these insights may be useful for better understanding how supercontinent cycles are related to transient episodes of large igneous province and superswell formation and the associated deep cycling of minerals and volatiles as well as leading to a better understanding of tectonic drivers of long term climate and icehouse to greenhouse transitions

paleovalley morphology and fluvial system evolution of eocene oligocene sediments auriferous gravels northern sierra nevada california implications for climate tectonics and topography in the northern sierra nevada california eocene oligocene fluvial sediments the auriferous gravels rest unconformably in well defined basement incised paleovalleys providing a record of paleolandscape geomorphology and drainage system evolution paleovalley surface mapping shows that local topography and bedrock lithology controlled the location of broad terraced paleovalley segments which formed in the central and eastern metamorphic bedrock belts and subsequently became areas of significant fluvial deposition high gradient high energy deposits are present near the edges of these areas suggesting that tributaries and outlets resided in steeper narrow paleovalleys multiple strath terraces are common in broad paleovalleys indicating that incision events occurred both before deposition and during paleovalley widening followed by periods of aggradation broad paleovalley deposits contain predominantly braided stream facies deposited in an energetically variable system multiple upward fining cycles bounded by erosional surfaces are composed of alternating coarse grained channel belt and fine grained floodplain packages an overall upward fining succession suggests that the locus of fluvial deposition shifted and depositional energy waned over time as the system backfilled successive broad paleovalleys paleochannel slope estimates of 0 004 0 055 were obtained based on average grain sizes of coarse channel fills anomalously thick up to 11 m laterally continuous 1 3 km floodplain units containing kaolinite rich clay and oxisol intervals record high fine grained sediment supply driven by high rates of chemical weathering in the warm eocene climate based on sedimentary characteristics combined with floral and detrital zircon data braided stream deposition within the basin occurred from the late early eocene through late eocene and aggradation primarily occurred in broad lower gradient reaches behind a major bedrock knickpoint created in the late mesozoic early cenozoic possibly due to asymmetric batholith unroofing the diachronous localized nature of paleovalley incision and braided stream aggradation within the northern sierra nevada invalidates previous estimates of the timing and amount of range uplift based on paleochannel reconstructions and gradients both internal and external drivers influenced the evolution of the fluvial system through 1 the pattern of basin filling as the locus of deposition migrated east to west 2 the warm eocene climate which increased chemical weathering and contributed a high sediment supply and 3 changes in base level due to eustasy and or laramide tectonism this record of laramide tectonics in the sierra nevada and basin and range is important for the timing of sierra nevada uplift for tectonic reconstructions of western north america and for studies of landscape evolution in tectonically active orogens

the role of the overriding plate thermal state on slab dip variability and on the occurrence of flat subduction 1 slab dip varies significantly both between different and along single subduction zones provided that old subducting plates are colder and denser than young plates variations in the slab dip should correlate with slab age however recent statistical analyses do not show this expected correlation we present the results of non newtonian numerical dynamic models where subduction is driven by means of a kinematic boundary condition we systematically vary the age of both the overriding and subducting plates in order to test these effects on the slab dip at different depth ranges we find that colder overriding plates result in shallower slab dips and episodes of flat slab subduction as a result of the increased suction force in the mantle wedge the influence of the thermal state of the overriding plate on slab dip is shown here to be more important than that of the age of subducting lithosphere modeling results are qualitatively compared to the large dip variability of the cocos slab including a flat slab segment we suggest that this variability is likely related to the change of the thermal state of the overriding plates with flat subduction occurring under cold lithosphere in southwestern mexico and steep subduction under the warmer lithosphere of the northwestern caribbean plate

three dimensionality of slab detachment due to ridge trench collision laterally simultaneous boudinage versus tear propagation the detachment of subducted tectonic plates is a process that has been increasingly associated with collisional scenarios and the end of subduction in various locations worldwide in particular the propagation of slab detachment tearing of a subducting plate has been described in conceptual models as a cause for spatially and temporally progressing surface effects such as slab gap volcanism and uplift however there is little understanding of the causes and dynamics associated with three dimensional 3 d slab tearing especially in the case of ridge trench collision here we show using fully dynamic 3 d numerical models that the process of detachment due to ridge trench collision depends on the geometry of the ridge segments approaching the trench for a finite laterally symmetric slab the 3 d detachment process occurs nearly simultaneously along strike by way of boudinage type necking and opening of holes central to the slab for a case involving the approach of two offset ridge segments to the trench slab tearing occurs in the form of 1 a vertical propagating separation along the age offset boundary within the slab that was previously weakened by a transform weak zone and 2 horizontal propagating detachment controlled by lateral transfer of slab pull to adjacent surface plate segments however lateral decoupling between offset adjacent plate segments and the propagating nature of the vertical and horizontal tearing are dependent upon fracture zones remaining weak through the subduction zone whether detachment occurs simultaneously along strike or propagates laterally the process is controlled by plastic yielding of the slab interior when young lithosphere entering the trench can no longer support slab pull

intraplate volcanism due to convective instability of stagnant slabs in the mantle transition zone the study of volcanism can further our understanding of earth s mantle processes and composition continental intraplate volcanism commonly occurs above subducted slabs that stagnate in the mantle transition zone mtz such as in europe eastern china and western north america here we use two dimensional numerical models to explore the evolution of stagnant slabs in the mtz and their potential to sustain mantle upwellings that can support volcanism we find that weak slabs may go convectively unstable within tens of million years upwellings rise out of the relatively warm underbelly of the slab are entrained by ambient mantle flow and reach the base of the lithosphere the first and most vigorous upwellings rise adjacent to lateral heterogeneity within the slab ultimately convective instability also acts to separate the compositional components of the slab harzburgite and eclogite from each other with harzburgite rising into the upper mantle and eclogite sinking into the lower mantle such a physical filtering process may sustain a long term compositional gradient across the mtz

palinspastic restoration of navdat and implications for the origin of magmatism in southwestern north america 1 simultaneous palinspastic restoration of deformation and volcanism illuminates relationships between magmatism and tectonics in western north america using arcgis we retrodeformed the navdat north american volcanic database navdat geongrid org using the western north america reconstruction of mcquarrie and wernicke 2005 from these data sets we quantitatively compare rates of magmatism and deformation and evaluate the age composition and migration of cenozoic volcanism from 36 ma to present these relationships are shown in a series of palinspastic maps as well as animations that highlight migrating extension and volcanism with time western north america is grouped into eight different regions with distinct relationships between strain and volcanism to evaluate competing hypotheses regarding the relationship of extension to continental magmatism a first order observation from this study is that magmatism throughout the basin and range appears to be primarily driven by plate boundary effects notably subducting and foundering slabs as well as slab windows exceptions include the yellowstone hotspot system along the northern border of our study area and late stage 8 ma passive extension related asthenospheric upwelling along the eastern and western margins of the basin and range the palinspastic reconstructions presented here highlight that the classic high angle basin and range faulting that comprises most of the physiographic basin and range province commenced during a magmatic lull more broadly with the exception of the rio grande rift we find that pulses of magmatism lag the onset of extension these observations largely contradict the active rifting model where magmatism triggers basin and range extension

tearing of stagnant slab subducted slabs of oceanic lithosphere below the western pacific tend to be stagnant in the transition zone with poorly known mechanical properties typical examples are the izu bonin and japan slabs that meet each other to form a cusplike junction beneath southwest japan here we show that these two slabs are torn apart at their junction when they bend to flatten over the 660 kilometer discontinuity as is expected from a simple geometric argument we present three lines of evidence for this ongoing slab tear

assembling north china craton within the columbia supercontinent the role of double sided subduction abstract an evaluation of recent s wave receiver functions s wave velocities and two versions of p wave tomographic images along various transects in the north china craton provides some clues on the subduction collision history of the different crustal blocks and their final amalgamation within the paleoproterozoic columbia supercontinent interpretation of a n s seismic section of the craton suggests thick slab debris sinking to various depths in the mantle the w e seismic corridors show the preservation of a thick 200 km lithospheric root tectosphere beneath the ordos block and its variable and extensive erosion towards the yanliao block eastern block this zone is characterized by layers with marked velocity contrast and suggests repeated stacking of the remnants of underplated and accreted paleoproterozoic oceanic lithosphere the present day lithosphere asthenosphere boundary beneath this region probably marks the erosional plane along which decratonization occurred through subduction erosion from the east and thermal and material erosion by upwelling asthenosphere from below resulting in the partial destruction of the tectosphere and its thinning towards the east within the asthenosphere below the yanliao block younger and thinner slabs predominate in the absence of any prominent thick high velocity layers these younger slabs define a westward polarity and constitute a mega scale duplex formed by underplating through phanerozoic subduction process particularly the pacific plate subduction from the east the lithologic associations within the inner mongolia suture zone dividing the yinshan block to the north and ordos block to the south correspond to an accreted ocean plate stratigraphic sequence with the tonalite trondhjemite granodiorite ttg gneisses charnockites and calc alkaline granites representing a continental arc built up through subduction from the north the seismic transects bring out a contrasting polarity in the subduction regime with an oblique east to southward subduction of the yinshan block and a westward subduction of the yanliao block here i propose a double sided subduction history for the ncc similar to the ongoing subduction process in the western pacific such double sided subduction is considered to promote rapid amalgamation of continental fragments within supercontinents and the subduction polarities and mantle dynamics of ncc are therefore considered to be critical in evaluating the final assembly of the paleoproterozoic supercontinent columbia

two stage subduction history under north america inferred from multiple frequency tomography eastward subduction of oceanic tectonic plates has shaped the geologic history of western north america over the past 150 million years the mountain building and volcanism that brought forth the spectacular landscapes of the west are credited to the vast ancient farallon plate which interacted mechanically and chemically with the overlying continent as it plunged back into the mantle here we use finite frequency travel time and amplitude measurements of teleseismic p waves in seven frequency bands to obtain a high resolution tomographic image to approx1 800 km depth we discover several large previously unknown pieces of the plate which show that two distinct stages of whole mantle subduction are present under north america the currently active one descends from the pacific northwest coast to 1 500 km depth beneath the great plains whereas its stalled predecessor occupies the transition zone and lower mantle beneath the eastern half of the continent we argue that the separation between them is linked to the laramide era 70 50 myr ago a time of unusual volcanism and mountain building far inland generally explained by an episode of extremely flat subduction

depth dependent azimuthal anisotropy in the western us upper mantle abstract we present the results of a joint inversion of long period seismic waveforms and sks splitting measurements for 3d lateral variations of anisotropy in the upper mantle beneath the western us incorporating recent datasets generated by the usarray deployment as well as other permanent and temporary stations in the region we find that shallow azimuthal anisotropy closely reflects plate motion generated shear in the asthenosphere in the shallow upper mantle 70 150 km depth whereas at depths greater than 150 km it is dominated by northward and upward flow associated with the extension of the east pacific rise under the continent constrained to the east by the western edge of the north american craton and to the north by the presence of the east west trending subduction zone in particular the depth integrated effects of this anisotropy explain the apparent circular pattern of sks splitting measurements observed in nevada without the need to invoke any local anomalous structures

segmentation of the farallon slab abstract recent tomography images reveal a complex 3d mantle structure beneath western united states with feature morphology varying rapidly with depth by assimilating plate motion history paleo age of sea floor and paleo geography of plate boundaries in a 3 d numerical model we simulate the farallon juan de fuca subduction during the past 40 ma we find that the highly segmented upper mantle structure of western u s is a direct result of the farallon subduction we show that the tilted horseshoe shaped fast seismic anomaly beneath nevada and utah at 300 600 km depth range is in fact a segment of curled slab subducted since 15 ma and the shallower linear slab beneath the cascades is younger than 5 ma the distinct morphology between these two parts of the subduction system indicates the strong influence of the fast trench rollback since 20 ma the northward migrating jf pa na triple junction and the toroidal flow around slab edges the observed mantle structures are used to constrain the rheology of the upper mantle through matching the shape depth and location of modeled subducted slab segments the inferred viscosity for the asthenosphere is 5 10 19 pa s and those for the transition zone and lower mantle are 1 5 10 21 pa s and 2 10 22 pa s respectively the slab is found to be about 2 orders of magnitude stronger than the ambient mantle above 410 km depth but of similar order of magnitude viscosity in the transition zone

depth constraints on azimuthal anisotropy in the great basin from rayleigh wave phase velocity maps abstract we present fundamental mode rayleigh wave azimuthally anisotropic phase velocity maps obtained for the great basin region at periods between 16 s and 102 s these maps offer the first depth constraints on the origin of the semi circular shear wave splitting pattern observed in central nevada around a weak azimuthal anisotropy zone a variety of explanations have been proposed to explain this signal including an upwelling toroidal mantle flow around a slab lithospheric drip and a megadetachment but no consensus has been reached our phase velocity study helps constrain the three dimensional anisotropic structure of the upper mantle in this region and contributes to a better understanding of the deformation mechanisms taking place beneath the western united states the dispersion measurements were made using data from the usarray transportable array at periods of 16 s and 18 s which mostly sample the crust we find a region of low anisotropy in central nevada coinciding with locally reduced phase velocities and surrounded by a semi circular pattern of fast seismic directions away from central nevada the fast directions are n s in the eastern great basin nw se in the walker lane region and they transition from e w to n s in the northwestern great basin our short period phase velocity maps combined with recent crustal receiver function results are consistent with the presence of a semi circular anisotropy signal in the lithosphere in the vicinity of a locally thick crust at longer periods 28 102 s which sample the uppermost mantle isotropic phase velocities are significantly reduced across the study region and fast directions are more uniform with an e w fast axis the transition in phase velocities and anisotropy can be attributed to the lithosphere asthenosphere boundary at depths of 60 km we interpret the fast seismic directions observed at longer periods in terms of present day asthenospheric flow driven deformation possibly related to a combination of juan de fuca slab rollback and eastward driven mantle flow from the pacific asthenosphere our results also provide context to regional sks splitting observations we find that our short period phase velocity anisotropy can only explain 30 of the sks splitting times despite similar patterns in fast directions this implies that the origin of the regional shear wave splitting signal is complex and must also have a significant sublithospheric component

multiple transition zone seismic discontinuities and low velocity layers below western united states with p to s converted waves recorded at seismic stations of the us transportable array we image the fine structure of upper mantle and transition zone tz beneath the western us we map the topographies of seismic discontinuities by stacking data by common conversion points along profiles systematic depth and amplitude measurements are performed not only for the well known 410 and 660 interfaces but also for minor seismic discontinuities identified around 350 590 and 630 km depths the amplitude of conversion suggests shear wave velocity vs increase by 4 at the 410 and the 660 the observed 660 velocity contrast is smaller than expected from the 6 in iasp91 but consistent with a pyrolitic model of mantle composition the gorda plate subducted under northern california is tracked to the tz where it seems to flatten and induce uplift of the 410 under northern nevada maps of 410 660 amplitude topography reveal that the tz is anomalous beneath the geographical borders of washington oregon and idaho with i a thickened tz ii a sharp change in depth of the 660 iii a reduced 410 conversion amplitude in the north and iv a positive 630 discontinuity such anomalous structure might be inherited from the past history of plate subduction accretion a thinned tz under the yellowstone suggests higher than average temperatures perhaps due to a deep thermal plume both the 350 and the 590 negative discontinuities extend over very large areas they might be related either to an increased water content in the tz a significant amount of oceanic material accumulated through the past 100 my or both

slabs do not go gently in geology textbooks the fate of the oceanic crust seems straightforward the ocean floor is created by upwelling of lighter magma at spreading ridges the magma cools as it moves away from the ridge forming a stiff layer or plate called the oceanic lithosphere having increased in density it then descends back into the mantle in trench regions precise seismic tomography studies have revealed that many descending slabs have a more complex evolution and have developed tears detached from the surface plate or even broken up into fragments on page 1173 1 of this issue obayashi et al 1 not only show clear tomographic evidence for the development of a vertical tear under southwest japan but have also found evidence for ongoing plate rupturing the authors correlated the images directly with measurements of stress revealed by active seismic sources 1 lookup doi 10 1126 science 1172496

subduction triggered magmatic pulses a new class of plumes article i nfo a variety of atypical plume like structures and focused upwellings that are not rooted in the lower mantle have recently been discussed and seismological imaging has shown ubiquitous small scale convection in the uppermost mantle in regions such as the mediterranean region the western us and around the western pacific we argue that the three dimensional return flow and slab fragmentation associated with complex oceanic subduction trajectories within the upper mantle can generate focused upwellings and that these may play a significant role in regional tectonics the testable surface expressions of this process are the outside arc alkaline volcanism topographic swell and low velocity seismic anomalies associated with partial melt using three dimensional simplified numerical subduction models we show that focused upwellings can be generated both ahead of the slab in the back arc region though five times further inward from the trench than arc volcanism and around the lateral edges of the slab in the order of 100 km away from slab edges vertical mass transport and by inference the associated decompression melting in these regions appears strongly correlated with the interplay between relative trench motion and subduction velocities the upward flux of material from the depths is expected to be most pronounced during the first phase of slab descent into the upper mantle or during slab fragmentation we discuss representative case histories from the pacific and the mediterranean where we find possible evidence for such slab related volcanism

uplift rupture and rollback of the farallon slab reflected in volcanic perturbations along the yellowstone adakite hot spot track field geochemical and geochronological data show that the southern segment of the ancestral cascades arc advanced into the oregon back arc region from 30 to 20 ma we attribute this event to thermal uplift of the farallon slab by the yellowstone mantle plume with heat diffusion decompression and the release of volatiles promoting high k calc alkaline volcanism throughout the back arc region the greatest degree of heating is expressed at the surface by a broad ene trending zone of adakites and related rocks generated by melting of oceanic crust from the farallon slab a hiatus in eruptive activity began at ca 22 20 ma but ended abruptly at 16 7 ma with renewed volcanism from slab rupture occurring in two separate regions the eastern rupture resulted in the extrusion of steens basalt during the ascent and melting of a dry mantle plume source contaminated with depleted mantle the contemporaneous western rupture resulted in renewed subduction melting of a wet mantle source and the rejuvenation of high k calc alkaline volcanism near the nevada california border at 16 7 ma here the initiation of slab rollback is evident in the westward migration of arc volcanism at 7 8 km ma today the uplifted slab is largely missing beneath the oregon back arc region replaced instead by a seismic hole which is bound on the south by the adakite hotspot track we attribute slab destruction to thermal uplift and mechanical dislocation that culminated in rapid tearing of the slab from 17 15 ma and possible foundering and sinking of slab segments from 16 10 ma

the influence of trench migration on slab penetration into the lower mantle abstract a two dimensional numerical convection model in cartesian geometry is used to study the influence of trench migration on the ability of subducted slabs to penetrate an endothermic phase boundary at 660 km depth the transient subduction history of an oceanic plate is modelled by imposing plate and trench motion at the surface the viscosity depends on temperature and depth a variety of styles of slab behaviour is found depending predominantly on the trench velocity when trench retreat is faster than 2 4 cm a the descending slab flattens above the phase boundary at slower rates it penetrates straight into the lower mantle although flattening in the transition zone may occur later leading to a complex slab morphology the slab can buckle independent of whether it penetrates or not especially when there is a localised increase in viscosity at the phase boundary flattened slabs are only temporarily arrested in the transition zone and sink ultimately into the lower mantle the results offer a framework for understanding the variety in slab geometry revealed by seismic tomography

assessment of tomographic mantle models using spectral element seismograms summary we investigated the agreement between real seismograms and those predicted by long wavelength mantle models by looking at phase and amplitude differences we computed full synthetic seismograms using a spectral element method together with 3 d mantle models and the appropriate crustal model on top we selected differently damped mantle models to see the effect of regularization on the computed seismograms to check the phase agreement we measured time shifts between the real and synthetic surface waves and body wave phases such as p s and ss using a cross correlation technique we also compared the amplitudes of the real and synthetic seismograms to understand how well the models explain not only the phases but the whole waveforms 3 d mantle models improve the phase agreement of surface waves in particular the remaining misfit however is still so large that we cannot distinguish between different tomographic models we suggest that this is mainly due to an imperfect modelling of the crust and or source location if body waves are included which have to be addressed in future inversions amplitude mismatches are large regardless which 3 d mantle model is used we observe that 3 d scattering or focusing defocusing effects can only explain half of the surface wave amplitudes whereas body wave amplitudes are dominated by scattering effects 1 d q models particularly in the crust and upper mantle strongly affect surface wave amplitudes and have to be modelled properly

the influence of hydrous phases on the microstructure and seismic properties of a hydrated mantle rock abstract to better understand the microstructural evolution of a serpentinized mantle rock and the influence of various hydrous phases on the seismic properties of the mantle wedge we have conducted the detailed microstructural analyses of a sample of tremolite chlorite antigorite schist collected from the moses rock dike central part of the colorado plateau we performed differential effective media dem modelling to study the effect of three hydrous phases forming two phase aggregates with olivine considering the crystallographic preferred orientation cpo of each phase and the shape ratio of the hydrous phases we have demonstrated that in a partially serpentinized peridotite the olivine cpo characteristic of 100 010 dislocation glide is still preserved and the high temperature asthenospheric flow is preserved with a foliation normal to that of antigorite schist the transformation of olivine into antigorite occurs predominantly 75 by the relationship 100 ol 001 atg with 001 ol 010 atg with the 010 ol 001 atg and 001 ol 010 atg relationship observed in areas of weak antigorite cpo chlorite results from the phase transformation of olivine in a relatively static environment as shown by the correlation between the olivine chlorite cpos with 100 ol 100 ch 010 ol 001 ch and 001 ol 010 ch the fluid percolation that caused the localized metasomatism and partial hydration of the mantle occurred possibly along trans lithospheric shear zones the presence of chlorite induces the most important drop on the p wave velocities and may help to explain some local low velocities in the fore arc mantle wedges but is unlikely to be of global importance due to its very high vp vs ratio 1 9 on the other hand antigorite is the only phase that causes important modification on the propagation directions of p and s waves and the only phase to explain the polarization of the fastest shear waves parallel to the subduction trench

post laramide removal of the farallon slab western united states i propose that post laramide removal of the subhorizontally subducting farallon slab occurred by buckling downward along an approximately east northeast trending axis this process was accomplished by a tearing or necking separation of the subducted slab near the northern and southern boundaries of the united states and propagation of the separated edges toward the central axis of downwelling accompanied by aesthenosphic upwelling behind the trailing edges initial buckling probably began near 50 ma and slab removal was complete by 20 ma this model is based primarily on the space time evolution of the ignimbrite flare up a major mid tertiary igneous event of mantle origin which involved two propagating fronts of initiation of volcanism that followed the proposed motions of the separated slab edges as they converged on central nevada from the north and southeast post laramide uplift extension establishment of the cascadia subduction zone and active magmatism may be consequences of lithosphere scale modifications caused by the laramide removal of the slab and the resulting asthenospheric upwelling

petrology and petrogenesis of the ministers island dike southwest new brunswick canada the ministers island dike in southwest new brunswick is a quartz tholeiitic member of the late triassic to early jurassic dike suite that outcrops along the east coast of north america the dike s phenocryst assemblage is orthopyroxene augite plagioclase the combination of petrographic chemical both phase and whole rock isotopic and experimental work on representative samples from the dike places important constraints on the petrogenesis of the ministers island dike and by analogy on the other members of the dike suite the petrographic analytical and experimental results show that the ministers island dike magma underwent high pressure 0 8 to 1 0 gpa fractionation of augite followed by augite orthopyroxene and finally augite plagioclase the absence of olivine as either a phenocryst or an experimentally observed high pressure liquidus phase indicates that the magma evolved away from the ol cpx opx plag pseudo invariant point while still at high pressure and there was sufficient time for any olivine to settle out of the magma prior to emplacement of the dike the sr and nd isotopic results support a metasomatised mantle source similar to emi but with slightly more radiogenic nd

automated large scale full seismic waveform inversion for north america and the north atlantic we present a new anisotropic seismic tomography model based on a multiscale full seismic waveform inversion for crustal and upper mantle structure from the western edge of north america across the north atlantic and into europe the gradient based inversion strategy utilizes the adjoint state method coupled with an l bfgs quasi newton optimization scheme to improve the handling of large data quantities in the context of full seismic waveform inversions we developed a workflow framework automating the procedure across all stages enabling us to confidently invert for waveforms from 72 events recorded at 7 737 unique stations resulting in a total of 144 693 raypaths most of them with three component recordings the final model after 20 iterations is able to explain complete waveforms including body as well as surface waves of earthquakes that were not used in the inversion down to periods of around 30s the model is complemented by a detailed resolution analysis in the form of 3 d distributions of direction dependent resolution lengths

finite frequency whole mantle p wave tomography improvement of subducted slab images received 23 july 2013 revised 14 october 2013 accepted 17 october 2013 1 we present a new whole mantle p wave tomographic model gap p4 we used two data groups short period data of more than 10 million picked up onset times and long period data of more than 20 thousand differential travel times measured by waveform cross correlation finite frequency kernels were calculated at the corresponding frequency bands for both long and short period data with respect to an earlier model gap p2 we find important improvements especially in the transition zone and uppermost lower mantle beneath the south china sea and the southern philippine sea owing to broadband ocean bottom seismometers bbobss deployed in the western pacific ocean where station coverage is poor this new model is different from a model in which the full data set is interpreted with classical ray theory bbobs observations should be more useful to sharpen images of subducted slabs than expected from simple raypath coverage arguments citation obayashi m j yoshimitsu g nolet y fukao h shiobara h sugioka h miyamachi and y gao 2013 finite frequency whole mantle p wave tomography improvement of subducted slab images geophys res lett 40 doi 10 1002 2013gl057401

chronostratigraphy of eocene volcanism central british columbia onset and termination of eocene felsic volcanism in the chilcotin plateau of central british columbia is constrained between 54 6 and 46 6 ma by 33 new u pb and 40ar 39ar isotopic age determinations dates were obtained from representative felsic coherent and fragmental volcanic rocks that comprise the ootsa lake group the resulting chronostratigraphy shows that magma compositions evolved from felsic to intermediate with no spatial migration of the volcanic activity rhyolitic compositions are oldest and are overlain by dacitic rocks with varied phenocrysts assemblages in many parts of the chilcotin plateau the eocene stratigraphy is capped by distinctive vitreous black dacite lavas which are contemporaneous with andesitic lavas of the endako group in the nechako plateau to the north crystallization ages from ootsa lake group rocks of the chilcotin plateau overlap age determinations from correlative rocks of the nechako plateau and southern bc collectively this geochronological dataset supports p