upper mantle tomography beneath the pacific northwest interior abstract the pacific northwest has undergone extensive tectonic and magmatic construction in the cenozoic and many clues to its history are in a well resolved upper mantle structure we present new p s and v p v s models that benefit from improved algorithms and additional data from recently deployed flexible array experiments in the region of the columbia river basalts western snake river plain and idaho batholith we refine the geometries of previously imaged structures and identify new anomalies beneath the pacific northwest a clear separation is resolved between the wallowa and idaho structures that extends from the base of the crust to the depth extent of these structures the separation follows the precambrian margin of north america possibly indicating separate tectonic histories for the two structures a shallow fast anomaly beneath idaho batholith may be responsible for the observed curvature of the yellowstone track around this batholith and a strong and unusually deep 180 250 km slow anomaly is present east of the apparent juan de fuca slab gap resolving these structures well is important in understanding the origin of the lithospheric fragments in the mantle and key to any reconstruction of the evolution of the pacific northwest farallon kula juan de fuca tectonic system of most relevance understanding the origin of the imaged high velocity mantle structures provides strong constraint on their density that otherwise would be largely unknown which controls the evolution of vertical and horizontal stresses acting at the base of the lithosphere and hence the evolution of topography also resolving the volumes of partially molten upper mantle shapes our understanding on the origin of the abundant volcanism in the pacific northwest interior

complex upper mantle anisotropy in the pacific northwest evidence from sks splitting abstract we use a dense network of observations and an automated method of analysis to investigate complex patterns of seismic anisotropy in the pacific northwest we present sks splitting results for approximately 220 broadband seismic stations including 33 stations from the new wallowa2 array deployed between 2016 and 2018 in northeast oregon our data set contains approximately 3300 splitting measurements our anisotropy measurements indicate that anisotropy in the pacific northwest can be divided into two domains near the subduction zone anisotropy is perpendicular to the slab and inboard of the subduction zone anisotropy tends to parallel the velocity structure and is roughly oriented with absolute plate motion however splitting analysis performs poorly in northeast oregon and results are not consistent with uniform or layered anisotropy we argue that the complex splitting behavior in northern oregon is a result of spatially variable upper mantle anisotropy related to the complex upper mantle seismic structure interpreting the mantle anisotropy in this region requires methods not limited to standard ray theoretical shear wave splitting analysis

refrigeration of the western cordilleran lithosphere during laramide shallow angle subduction the laramide orogeny has generally been attributed to a shift from normal angle to shallow angle subduction beneath the western margin of the north american plate in addition to important mechanical effects this shift may have had important thermal effects on the lithosphere in the western cordillera before the laramide geothermal gradients in the western cordillera were probably normal to high because of the presence below of a hot asthenospheric wedge with the shift to shallow subduction the wedge was probably expelled and replaced by a cold subducting slab that extended just below the cordilleran lithosphere this would shift the western cordillera into a cold forearc like thermal setting dominated by the refrigeration effects of the subducting slab thermochronologic data from the sierra nevada great basin mojave desert western arizona and perhaps the colorado plateau record latest cretaceous early tertiary cooling that may be evidence of such refrigeration if regional refrigeration occurred it would have several important implications 1 metamorphism in the western cordillera would have waned with the start of the laramide orogeny 2 the crust in the western cordillera would have strengthened and become much more resistant to deformation e g gravitational collapse and extension of thickened crust in the sevier hinterland would have been impeded and 3 latest cretaceous early tertiary isotopic cooling ages may not be valid indicators of a period of major regional uplift and unroofng

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

helium and neon isotopes in the imnaha basalt columbia river basalt group evidence for a yellowstone plume source abstract helium neon and argon isotopic compositions were measured in two flows of the columbia river flood basalt the imnaha basalt has a 3 he 4 he ratio of 11 4 times atmospheric and 20 ne 22 ne and 21 ne 22 ne ratios characteristic of a plume component the measured 3 he 4 he is a lower limit due to possible preferential 3 he loss and or addition of radiogenic 4 he a wanapum basalt flow erupted approximately 2 ma later in the waning stages of volcanism has more morb like noble gases the he nd and sr isotopic compositions of these lavas suggest that the columbia river basalts were derived from the yellowstone plume head which contained both high helium plume material and entrained depleted mantle as the eruptions progressed the plume component in the melting region was gradually diluted or replaced

plume modified orogeny an example from the western united states plate tectonic theory implies that orogeny at convergent margins results from several processes including subduction of oceanic crust subduction of aseismic ridges accretion of terranes and continental collision each of these processes involves the consumption of an oceanic tract and each has its own characteristic style of tectonothermal activity an additional potentially important orogenic process has been largely overlooked oceanic regions contain chains of volcanic islands formed at hotspots which are generally considered to reflect the sites of rising plumes in a hotspot reference frame where active continental margins advance and override a plume the plume9s buoyant swell may profoundly change the tectonothermal expression of subduction and hence orogenic processes at the continental margin the late cretaceous tertiary evolution of the western united states may be an example of this process which has been overlooked in the development of orogenic paradigms

historical reconstruction of the atlantic meridional overturning circulation from the ecmwf operational ocean reanalysis a reconstruction of the atlantic meridional overturning circulation moc for theperiod 1959 2006 has been derived from the ecmwf operational ocean reanalysis the reconstruction shows a wide range of time variability including a downward trend at 26n both the moc intensity and changes in its vertical structure are in good agreement with previous estimates based on trans atlantic surveys at 50n the moc and strength of the subpolar gyre are correlated at interannual time scales but show opposite secular trends heat transport variability is highly correlated with the moc but shows a smaller trend due to the warming of the upper ocean which partially compensates for the weakening of the circulation results from sensitivity experiments show that although the time varying upper boundary forcing provides useful moc information the sequential assimilation of ocean data further improves the moc estimation by increasing both the mean and the time variability

geochemical confirmation of the kula farallon slab window beneath the pacific northwest in eocene time plate tectonic models indicate subduction of the kula farallon spreading ridge and thus imply formation of the kula farallon slab window beneath western north america from late cretaceous to middle eocene time seafloor magnetic anomalies however pro vide few constraints on the configuration of the subducted ridge and the location of the slab window we use geochemical data from eocene igneous rocks to provide an estimate of the slab window s position at 50 ma contouring of the trace element ratios k 2o sio2 rb zr and ta ce for lavas of the eocene challis kamloops volcanic belt demonstrates a southward trend toward enriched geochemical character this trend is consistent with a pacific northwest position for the slab window this is supported by the presence of adak ites slab melts near the canada united states border and by ca 51 ma forearc intru sions and volcanic rocks on vancouver island that are regarded as products of ridge subduction a new reconstruction of the slab window s geometry over time indicates that its chemically defined position is kinematically viable

origin of the columbia plateau and snake river plain deflection of the yellowstone plume previous models relating the snake river plain and columbia river basalt northwest united states to the yellowstone plume have not taken into account the effects of the subducted farallon and juan de fuca plates not only did subduction shield the surface from mantle plume activity but it deflected the plume in the direction of plate motion a variety of geophysical and geologic evidence suggests that the plume penetrated the subducted slab at 17 5 ma causing eruption of the columbia river basalt the plate boundary then migrated over the plume and the yellowstone plume has recovered to a more nearly vertical conduit forming the snake river plain and yellowstone plateau

mantle structure beneath the western united states and its implications for convection processes 1 we present tomographic images of the mantle structure beneath the western united states our dynamic north america models of p and s velocity structure dna07 p and dna07 s use teleseismic body waves recorded at 600 seismic stations provided by the earthscope transportable array and regional networks dna07 p and s benefit from the unprecedented aperture of the network while maintaining a dense station distribution providing high resolution body wave imaging of features through the transition zone and into the lower mantle the main features imaged include 1 the juan de fuca subduction system that bottoms at 400 km beneath oregon implying interaction with the yellowstone anomaly 2 a low velocity conduit beneath yellowstone national park that bottoms at 500 km and dips toward the northwest 3 shallow low velocity anomalies upper 200 km beneath the eastern snake river plain esrp and the high lava plains and a deep low velocity anomaly 600 km beneath the esrp but not newberry 4 a low velocity slab gap to 400 km depth immediately south of the mendocino triple junction and south of the gorda slab and 5 high velocity drips beneath the transverse ranges the southern central valley sierra nevada and central nevada these observations reveal extremely heterogeneous mantle structure for the western united states and suggest that we are only just beginning to image the complex interactions between geologic objects the transportable array allows for analysis of the relationships between these anomalies in an internally consistent single tomographic model the dna velocity models are available for download and slicing at http dna berkeley edu

seismically imaged relict slab from the 55 ma siletzia accretion to the northwest united states teleseismic tomography images a large high velocity curtain extending vertically beneath the ca 50 ma challis magmatic trend to maximum depths of 230 600 km we interpret this structure as subducted farallon ocean lithosphere that stalled with the ca 55 ma accretion of the siletzia microplate to north america within the columbia embayment and we consider the regional tectonic implications the abrupt switch ca 53 ma from laramide thrusting and magmatic quiescence to extension and vigorous magmatism in the northwestern united states is evidence for foundering of the fl at subducting farallon slab to account for the imaged curtain foundering apparently occurred by rollback after siletzia accretion terminated subduction within the columbia embayment the magnitude of the high seismic velocity curtain is approximately that expected for 20 50 m y old ocean lithosphere that stalled in the upper mantle ca 50 ma we suggest that the stalled slab became nearly neutrally buoyant as a result of removal of its basaltic crust the melting of which likely contributed to the short lived vigor of challis magmatism after siletzia accretion normal dip cascadia subduction initiated west of siletzia evidenced by arc volcanism in oregon and washington beginning ca 45 40 ma while continued quiescence of the sierra nevada arc suggests persistence of flsubduction to the south kinematically this requires a tear in the subducted farallon slab near the latitude of southern oregon we propose that north to south propagation of this torn slab edge propagated the ignimbrite fl are up across what now is the northern basin and range and ended the laramide orogeny

missing idaho arc transpressional modification of the 87sr 86sr transition on the western edge of the idaho batholith the north american cordillera is characterized by subduction related mesozoic batholiths with voluminous middle cretaceous plutonism in the cordillera the idaho batholith is anomalous containing only a narrow 10 km band of middle cretaceous plutons on its extreme western margin these plutons are spatially coincident with the late cretaceous crustal scale dextral transpressional western idaho shear zone finite strain analysis indicates that there is significant east west shortening in the western idaho shear zone a reconstruction using isotopic variations as strain markers and comparing them to the undeformed middle cretaceous sierra nevada batholith results in a middle cretaceous magmatic arc in western idaho similar in width 85 100 km to other sections of the cordilleran magmatic arc the tectonic implications include 1 a full magmatic arc existed in idaho in the middle cretaceous prior to late cretaceous deformation 2 the currently sharp 87sr 86sr transition in western idaho represents structural modification of the batholith after emplacement i e the steep isotopic gradient results from deformation and 3 as a result of late cretaceous shortening in the western idaho shear zone the main part of the idaho batholith i e post ca 90 ma intruded farther eastward into thick north american lithosphere consequently the main part of the idaho batholith has a stronger continental signature than the other coastal batholiths

seismic structure of the upper mantle beneath the northern canadian cordillera from teleseismic travel time inversion abstract teleseismic tomography is used to image upper mantle structure beneath the northern canadian cordillera with the objective of determining the physical state of the upper mantle along the western half of the snorcle lithoprobe transect the resulting three dimensional p wave velocity model resolves structure beneath the southern yukon and northernmost british columbia between 100 and 600 km depth two significant anomalies are identified the first a relatively shallow high velocity feature located at the western edge of the model is interpreted as being the edge of the pacific slab from the southern alaska subduction zone the second is a large tabular low velocity anomaly centred at 60 n by 136 w elongate northwest southeast dipping southwest and reaching a depth of 450 500 km this low velocity anomaly is judged to reflect a thermal anomaly of 100 200 c with a possible compositional component its northeastern boundary is particularly sharp and is interpreted to represent the boundary between thin cordilleran lithosphere and a colder cratonic mantle root our preferred explanation for the low velocity feature is a thermal anomaly resulting from the advective upflow produced by the opening of a slab window beneath the northern cordillera a possible alternative is dense downward percolating partial melt generated by sub orogenic heating as past work e g stolper et al j geophys res 86 1981 6261 6271 suggests that basic partial melts may be denser than peridotite below 200 km depth

global distributions of glacier properties a stochastic scaling paradigm many problems in global climate and earth systems science require knowledge of regional or global scale distributions of glacier properties which includes mass balance ice velocity flux thickness volume and surface area among others with roughly 160 000 glaciers worldwide obtaining information on the global probability distributions of most ice properties is expensive and often infeasible only surface area distributions are relatively easy to measure either by direct observation or remote sensing while other properties are difficult to observe this work shows that scaling relationships from the continuum dynamics of ice can link the distribution of surface areas to the global and regional distributions of any other continuum property some data inventories already exist for constructing reasonable distributions of glacier sizes and this analysis presents theoretical arguments based on glacier network topologies similar in concept to river networks to suggest a power law times an exponential distribution of surface areas in agreement with the data therefore by using existing data or the theoretical distributions of surface areas specific distributions for other glacier properties in any region of the world can be constructed as an example predictions up to a scaling constant are made for the distribution of glacier volumes characteristic thicknesses characteristic velocities and characteristic response times in the alps

evolutionary aspects of lithosphere discontinuity structure in the western u s 1 we have produced common conversion point ccp stacked ps and sp receiver function image volumes of the moho and lithosphere asthenosphere boundary lab beneath the western united states using transportable array data the large image volumes and the diversity of tectonic environments they encompass allow us to investigate evolution of these structural discontinuities the moho is a nearly continuous topographic surface whereas the lab is not and the seismic images show a more complex expression the first order change in lab depth in the western u s occurs along the cordilleran hingeline the former laurasian passive margin along the southwestern precambrian north american terranes the lab is about 50 deeper to the east of the hingeline than to the west with most of the increase in lab thickness being in the mantle lithosphere we infer that the moho and the lab are late mesozoic or cenozoic everywhere west of the hingeline modified during farallon subduction and its aftermath between the hingeline and the rocky mountain front the lab and to a lesser extent the moho have been partially reset during the cenozoic by processes that continue today seismicity and recent volcanism in the interior of the western u s are concentrated along gradients in crustal and or lithospheric thickness for example the hingeline and the eastern edge of the coastal volcanic magmatic terranes to us this suggests that lateral gradients in integrated lithospheric strength focus deformation similarly areas conjectured to be the sites of convective downwellings and associated volcanism are located along gradients in regional lithosphere thickness

topographic response to mantle lithosphere removal in the southern sierra nevada region california geological studies of mantle xenoliths entrained in late neogene quaternary lavas from the southern sierra nevada region and regional geophysical studies suggest that the highdensity mantle lithosphere that formed beneath the sierra nevada batholith in conjunction with arc magmatism is being convectively removed as a drip structure this structure as imaged seismically is roughly cylindrical in shape with a diameter of 100 km and extends to 225 km depth centered above this structure is a region 120 km in diameter that is undergoing active subsidence relative to adjacent regions such subsidence is seen in the active fluvial alluvial sediment flooding of mountainous topography of the southwestern sierra and in the development of the adjacent tulare lake basin of the san joaquin valley dynamic modeling of such upper mantle drip structures predicts a phase of overlying surface subsidence during the most vigorous phase of drip formation the southern sierra upper mantle drip and the overlying crust appear to be in this phase of their dynamically coupled evolution

three dimensional seismic velocity structure of the northwestern united states 1 we present a high resolution three dimensional p wave tomography model for the northwestern united states major features of the model include clear well resolved images of the juan de fuca gorda slab to depths of at least 500 km reduced velocities beneath newberry volcano and north central oregon increased velocities beneath the idaho batholith and a deep slab like eastward dipping feature of increased velocities beneath nevada we find no evidence for a mantle plume in this region and demonstrate that suggested disruption of the juan de fuca slab beneath oregon in the vicinity of newberry volcano is an inversion artifact we interpret the primary features of this new model within the framework of post laramide tectonomagmatism and the continuing tectonic evolution of the cascadia subduction zone a model explaining the regional tectonomagmatic history consistent with the imaged seismic velocity structure is one driven by mantle flow responding to the evolving subduction system

juan de fuca subduction zone from a mixture of tomography and waveform modeling seismic tomography images of the upper mantle structures beneath the pacific northwestern united states display a maze of high velocity anomalies many of which produce distorted waveforms evident in the usarray observations indicative of the juan de fuca jdf slab the inferred location of the slab agrees quite well with existing contour lines defining the slab s upper interface synthetic waveforms generated from a recent tomography image fit teleseismic travel times quite well and also some of the waveform distortions regional earthquake data however require substantial changes to the tomographic velocities by modeling regional waveforms of the 2008 nevada earthquake we find that the uppermost mantle of the 1d reference model ak135 the reference velocity model used for most tomographic studies is too fast for the western united states here we replace ak135 with mt7 a modification of an older basin and range model t7 we present two hybrid velocity structures satisfying the waveform data based on modified tomographic images and conventional slab wisdom we derive p and sh velocity structures down to 660 km along two cross sections through the jdf slab our results indicate that the jdf slab is subducted to a depth of 250 km beneath the seattle region and terminates at a shallower depth beneath portland region of oregon to the south the slab is about 60 km thick and has a p velocity increase of 5 with respect to mt7 in order to fit waveform complexities of teleseismic gulf of mexico and south american events a slab like high velocity anomaly with velocity increases of 3 for p and 7 for sh is inferred just above the 660 discontinuity beneath nevada

multiple frequency sh wave tomography of the western us upper mantle summary we estimate the sh wave velocity and attenuation structures of the western us upper mantle using the dense network of the usarray and new techniques we observe a multiple frequency data set of both traveltime and amplitude anomalies and interpret these with full 3 d finitefrequency sensitivity kernels amplitudes show stronger frequency dependence than traveltimes we perform a joint inversion on the measured traveltime and amplitude anomalies interpreting them in terms of velocity and attenuation heterogeneities aside from the expected clear division between the slow tectonically active region in the west and the fast craton in the east several interesting smaller velocity anomalies are observed the subduction along the cascades at 100 300 km depths shows lateral discontinuity with a slab hole absence of fast anomalies observed around 45 n the delaminated sierra nevada mountains root is observed to have sunk to 200 km depth the yellowstone plume seems to have an origin weak slow velocity anomalies near 1000 km depth but the plume conduit seems to be interrupted by a fast anomaly which is identified as a fragment of the farallon slab the s velocity model shows a trench perpendicular slab gap absence of fast anomalies at almost the same location as in the p model recently published by sigloch et al 2008 the methodological improvements described in the first paragraph have several benefits amplitude data help to sharpen the edges of narrow velocity heterogeneities in the shallow upper mantle the focusing effect from velocity heterogeneity dominates over that of attenuation and must be considered when interpreting amplitude anomalies in general velocity and attenuation heterogeneities correlate positively suggesting that temperature plays a major role in forming the anomalies

geomorphology of the burnt river eastern oregon usa topographic adjustments to tectonic and dynamic deformation abstract eastern oregon contains the deepest gorge in north america where the snake river cuts vertically down 2300 m this deep gorge is known as hells canyon a landscape containing such a topographic feature is likely undergoing relatively recent deformation study of the burnt river a tributary to the snake river at the upstream end of hells canyon yields data on active river incision in eastern oregon indicating that quaternary faults are a first order control on regional landscape development through 1 24 000 scale geologic mapping a 500 000 year record of fluvial incision along the burnt river was constructed and is chronologically anchored by optically stimulated luminescence dating and tephrochronology analyses a conceptual model of fluvial terrace formation was developed using these ages and likely applies to other non glaciated catchments in eastern oregon mapped terraces inferred to have formed during glacial interglacial cycles provide constraints on rates of incision of the burnt river incision through these terraces indicates that the burnt river is down cutting at 0 15 to 0 57 m kyr 1 this incision appears to reflect a combination of local base level adjustments tied to movement along the newly mapped durkee fault and regional base level control imposed by the downcutting of the snake river deformation of terraces as young as 38 7 5 1 ka indicates quaternary activity along the durkee fault and when combined with topographic metrics slope relief hypsometry and stream steepness reveals a landscape in disequilibrium longer wavelength lithospheric dynamics delamination and crustal foundering that initiated in the miocene may also be responsible for continued regional deformation of the earth s surface

constraints on north american plate velocity from the yellowstone hotspot deformation field oceanic hotspot tracks generally indicate that the mantle plumes that give rise to them are fixed in position relative to a lower mantle reference frame for timespans of 100 myr1 3 it has not been clear whether continental hotspots are equally stable for example there is evidence that the velocity of the yellowstone hotspot has been quite irregular with respect to the north ameri can plate over the past 16 myr4 7 but measurements of its velocity based on caldera locations have been too uncertain and irregular to be definitive here i describe a new method of obtaining the plume velocity which uses the broader lithospheric effects of the hotspot rather than the locations of individual calderas for the past 10 myr the results indicate a constant north american plate velocity of 2 2 cm yr 1 which agrees with independent estimates from global plate motion inversions8 9 this velocity is considerably lower than previous estimates obtained from caldera locations and clearly differs from the much higher but less well constrained velocities of the previous 6 myr refs 5 7

mantle provinces under north america from multifrequency p wave tomography this is a survey of mantle provinces large scale seismic anomalies under north america from the surface down to 1500 1800 km depth the underlying p velocity model was obtained by multifrequency tomography a waveform based method that systematically measures and models the frequency dependence of teleseismic body waves a novel kind of three dimensional rendering technique is used to make the considerable structural complexities under north america accessible in the transition zone and below the north american mantle is dominated by seismically fast provinces which represent distinct subduction episodes of the farallon plate i attempt to date and interpret the various slab fragments by reconciling their present positions with paleotrench locations from plate tectonic reconstructions and with major geologic surface episodes differences in vertical sinking velocity have led to large vertical offsets across adjacent coeval slabs some of the mantle provinces have not been discussed much previously including 1 a seismically slow blanket overlying the oldest farallon subduction along the eastern continental margin 2 a transition zone slab coeval with the laramide orogeny ca 80 60 myr which i discuss in analogy to the stagnant slab subduction style commonly found in the western pacific today 3 the lower mantle root of present day cascadia subduction which may have started out as intraoceanic subduction 4 a lower mantle slab under arizona and new mexico the last material to subduct before strike slip motion developed along the san andreas boundary and 5 two narrow plate tears thousands of kilometers long one of which is the subducted conjugate of the mendocino fracture zone

teleseismic p wave tomogram of the yellowstone plume 1 inversion of a new data set of teleseismic p wave travel times from three passcal seismic deployments around the yellowstone hotspot reveals a 100 km diameter upper mantle plume that extends from the yellowstone volcanic caldera to 500 km depth and dips 20 to the northwest a monotonic decrease in the velocity perturbation of the plume from 3 2 at 100 km to 0 9 at 450 km is consistent with a uniform thermal anomaly of 180 c where the plume crosses the 410 km discontinuity previous research shows a depression in the 410 km discontinuity consistent with a warm plume fee and dueker 2004 additionally a region of high velocities extends to 250 km beneath the wind river basin in nw wyoming that may represent a convective downwelling of the lithosphere

petrogenesis of the late archean 2 5 ga na and k rich granitoids in the zhongtiao wangwu region and its tectonic significance for the crustal evolution of the north china craton abstract petrogenesis of coexisting late archean 2 5 ga na and k rich granitoids in the zhongtiao wangwu region is significant for understanding the crustal evolution of the north china craton ncc in this paper we present new zircon u pb age and whole rock geochemical and isotope data on a 2 5 ga na and k rich granitoid suite from the zhongtiao wangwu region southern ncc the na rich granitoids ttg and k rich granitoids a type granites are zircon u pb dated to be 2 45 ga and 2 49 ga respectively integrated geochemical data indicate that the ttg were formed by partial melting of the mafic lower crust while the a type granites were formed by partial melting of a ttg like source in the shallower crust the petrogenesis likely occurred under lithospheric delamination and the subsequent asthenospheric upwelling and mafic magma underplating we proposed that the coeval occurrence of na and k rich granitoids in the southern ncc is probably ascribed to the partial melting of continental crust at different depths which could be an important way to induce the archean crustal reworking and compositional changes at 2 5 ga

complex subduction and small scale convection revealed by body wave tomography of the western united states upper mantle abstract new high resolution p and s wave tomography of the united states upper mantle from the pacific coast to the great plains reveals strong multi scale heterogeneity closely correlated with tectonic and magmatic activity we invert teleseismic travel time residuals from the earthscope transportable array and more than 1700 additional temporary and permanent stations for 3 d velocity perturbations to a depth of 1000 km the inversion uses recent advances in western u s crust models to better isolate the mantle component of travel time residuals and frequency dependent 3 d sensitivity kernels to map travel time residuals measured in multiple frequency bands into velocity structure in addition to separate v p and v s models we jointly invert the two datasets for v p v s perturbations by imposing a smoothness constraint on the ln v s ln v p field the joint inversion helps us identify regions where partial melt is probable the amplitude of v p v s and v p v s variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small scale convection partially molten mantle is inferred beneath yellowstone and the eastern snake river plain srp the salton trough and the clear lake volcanic field the inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern srp and yellowstone despite continuous subduction since the cretaceous the distribution of sub lithospheric high velocity anomalies is dissected similar to other recent studies based on our new tomography models western u s geologic history and plate tectonic reconstructions we infer patchy and incomplete removal of the flat subducting laramide slab and slab tearing associated with eocene accretion in the northwestern u s

the age of the steens reversal and the columbia river basalt group abstract the columbia river basalt group crbg eruptions have a well defined relative magnetostratigraphy but have not been definitively correlated to the geomagnetic polarity time scale 40 ar 39 ar ages are presented from lavas erupted in the r 0 through n 1 magnetozones of the crbg and in the transition between r 0 and n 0 four ages from transitionally magnetized lava flows at steens mountain catlow peak and poker jim ridge with a weighted mean age 16 58 0 10 ma 1 and the more precise age 16 654 0 025 ma of the normally magnetized oregon canyon tuff at the top of the catlow peak section show that the oldest crbg magnetozone r 0 correlates with the c5cr chron bayesian statistical analysis applied to data from four flows at catlow peak using the mean age of the steens reversal gives a best and preferred age of the steens reversal of 16 73 0 13 0 08 ma 95 confidence depending on the geomagnetic polarity time scale model the eruption rate from n 0 through r 2 0 34 0 45 ma in the middle and the bulk of the crbg emplacement averaged 0 30 0 41 km 3 a and peaked at a rate 1 1 2 to 4 1 2 times higher during r 2

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

petrology geochemistry and genesis of high al tonalite and trondhjemites of the cornucopia stock blue mountains northeastern oregon tonalitic and trondhjemitic compositions span a narrow range of trondhjemite sio2 content 65 74 wt and exhibit characteristics of a highal tonalite trondhjemite granitoid ttg suite including light rare earth element lree enrichment low y nb and rb sr and high al2o3 and sr these compositions are consistent with an

new 40ar 39ar dating of the grande ronde lavas columbia river basalts usa implications for duration of flood basalt eruption episodes abstract grande ronde basalt grb lavas represent the most voluminous eruptive pulse of the columbia river snake river yellowstone hotspot volcanism with an estimated eruptive volume of 150 000 km 3 grb lavas form at least 66 of the total volume of the columbia river basalt group new 40 ar 39 ar dates for grb lavas reveal they were emplaced within a maximum period of 0 42 0 18 my a well documented stratigraphy indicates at least 110 grb flow fields or individual eruptions and on this basis suggests an average inter eruption hiatus of less than 4000 years isotopic age dating cannot resolve time gaps between grb eruptions and it is difficult to otherwise form a picture of the durations of eruptions because of non uniform weathering in the top of flow fields and a general paucity of sediments between gr lavas where sediment has formed on top of grb lavas it varies in thickness from zero to 20 30 cm of silty to fine sandy material with occasional diatomaceous sediment individual grb eruptions varied considerably in volume but many were greater than 1000 km 3 in size most probably eruptive events were not equally spaced in time some eruptions may have followed short periods of volcanic repose perhaps 10 2 to 10 3 of years whilst others could have been considerably longer many 1000 s to 10 4 years recent improvements in age dating for other continental flood basalt cfb lava sequences have yielded estimates of total eruptive durations of less than 1 my for high volume pulses of lava production the grb appears to be a similar example where the main pulse occupied a brief period even allowing for moderate to long duration pahoehoe flow field production the amount of time the system spends in active lava producing mode is small less than c 2 6 based on eruption durations of approximately 10 000 years compared to the duration of the entire eruptive pulse of c 420 000 years a review of available 40 ar 39 ar data for the major voluminous phases of the columbia river basalt group suggests that activity of the steens basalt imnaha basalt grb may have at times been simultaneous with obvious implications for climatic effects resolving intervals between successive eruptions during cfb province construction and durations of main eruptive pulses remains vital to determining the environmental impact of these huge eruptions

geology and 40ar 39ar geochronology of the middle miocene mcdermitt volcanic field oregon and nevada silicic volcanism associated with propagating flood basalt dikes at initiation of the yellowstone hotspot

climatic effects on glacier distribution across the southern coast mountains b c canada over a distance of 150 km inland across british columbia glaciation level altitudinal threshold for glacier generation rises north eastward from 1800 m near jervis inlet to 2800m near lillooet on fraser river the methods of 0strem 1966b are revised for more detailed work from air photographs inclusion of small but active glaciers gives a glaciation level some 200 m below 0strem s from i 250 000 maps or 110 m below that from 1 50000 maps mountains which rise slightly above the altitudinal threshold invariably support glaciers only on their northerly slopes mountain crests need to be some 300 m higher before they support glaciers on most slopes this defines an all sided glaciation level since summit heights vary from 2500 to 2900 m through most of the transect considered here east and west facing glacier sources are found mainly in the coastal half south facing sources are rare outside icefields for particular mountain ranges bounded by valleys and low passes the aspects azimuths of glacier sources can be summarised by vector analysis the higher the range rises above glaciation level the more widespread the aspects represented and the weaker the resultant vector strength strengths as high as 90 are observed on landward ranges with a relatively sunny continental climate while 20 60 is found for coastal ranges where cloudiness reduces slope variations in incident solar radiation vector mean aspects throughout are close to north perennial lee drifts on icefields show that wind directions during and immediately after snowfalls are from the south or from south south west together with oro graphic details of the glaciation level this suggests that snow bearing winds come from the south rather than the west these winds reinforce radiation contrasts giving north outh asymmetry of glacier aspect even near the coast

simultaneous batholith emplacement terrane continent collision and oroclinal bending in the blue mountains province north american cordillera the north american cordillera is a classic example of accretionary orogen consisting of multiple oceanic terranes attached to the western margin of laurentia during the mesozoic times although the cordillera is linear for most parts terrane boundaries are at a high angle to the overall structural grain in several segments of the orogen which has been a matter of longstanding controversy as to how and when these orogenic curvatures formed this paper discusses mechanisms kinematics and timing of initiation of one of these major curvatures the blue mountains province in northeastern oregon here magmatic fabric patterns and anisotropy of magnetic susceptibility in the wallowa batholith record three phases of progressive deformation of the host wallowa terrane during early cretaceous first is terrane oblique ne sw shortening interpreted as recording attachment of the amalgamated oceanic and fringing terranes to the continental margin during dextral convergence at 140 ma deformation subsequently switched to pure shear dominated nne ssw shortening associated with crustal thickening caused by continued impingement of the amalgamated blue mountains superterrane into a presumed westward concave reentrant in the continental margin at 135 128 ma upon impingement at 126 ma the northern portion of the superterrane became locked leading to reorientation of the principal shortening direction to nnw sse while its still deformable southern portion rotated clockwise about a vertical axis we thus propose oblique bending as the main mechanism of the orocline formation whereby horizontal compressive forces resulting from plate convergence acted at an angle to the terrane boundaries

geodynamics of the yellowstone hotspot and mantle plume seismic and gps imaging kinematics and mantle flow abstract integration of geophysical and geological data show that the yellowstone hotspot resulted from a mantle plume interacting with the overriding north america plate a process that has highly modified continental lithosphere by magmatic and tectonic processes and produced the 16 17 ma 700 km long yellowstone snake river plain ysrp silicic volcanic system accessibility of the ysrp allowed large scale geophysical projects to seismically image the hotspot and evaluate its kinematic properties using geodetic measurements seismic tomography reveals a crustal magma reservoir of 8 to 15 melt 6 km to 16 km deep beneath the yellowstone caldera an upper mantle low p wave velocity body extends vertically from 80 km to 250 km beneath yellowstone but the anomalous body tilts 60 wnw and extends to 660 km depth into the mantle transition zone we interpret this conduit shaped low velocity body as a plume of up to 3 5 vp and 5 5 vs perturbation that corresponds to a 1 2 partial melt models of whole mantle convection reveal eastward upper mantle flow beneath yellowstone at relatively high rates of 5 cm yr that deflects the ascending plume into its west tilted geometry a geodynamic model of the yellowstone plume constrained by vp and vs velocities and attenuation parameters suggests low excess temperatures of up to 120 k corresponding to a maximum 2 5 melt and a small buoyancy flux of 0 25 mg s i e properties of a cool weak plume the buoyancy flux is many times smaller than for oceanic plumes nonetheless plume buoyancy has produced a 400 km wide 500 m high topographic swell centered on the yellowstone plateau contemporary deformation derived from gps measurements reveals sw extension of 2 3 mm yr across the yellowstone plateau one fourth of the total basin range opening rate which we consider to be part of basin range intraplate extension locally decadal episodes of subsidence and uplift averaging 2 cm yr characterize the 80 year yellowstone caldera monitored history and are modeled as hydrothermal magmatic sources moreover a recent episode 2004 2009 of accelerated uplift of the yellowstone caldera at rates up to 7 cm yr has been modeled as resulting from magmatic recharge of a 10 km deep sill at the top of the crustal magma reservoir regionally gravitational potential energy of the yellowstone swell drives the lithosphere southwest and downhill from the yellowstone plateau 400 km where it coalesces with basin range province wide westward extension based on the geometry and its assumed 660 km depth we extrapolate the plume source southwest to its original location at 17 ma and 600 km southwest and 200 km north of the ysrp importantly this location is beneath the southern part of the columbia plateau flood basalt field of the same age and implies that the yellowstone mantle plume may be the common source for both of these large volcanic fields our time progression model suggests that the original plume head rose vertically behind the juan de fuca plate but at 12 ma it lost the protection of the subducting plate from eastward mantle flow and encountered cooler thicker continental lithosphere becoming entrained in eastward upper mantle flow these results reveal that yellowstone plume plate processes have had a profound effect on late cenozoic geologic evolution and topography of a large part of the western u s

geochronology of age progressive volcanism of the oregon high lava plains implications for the plume interpretation of yellowstone 1 the high lava plains province hlp is a late cenozoic bimodal volcanic field at the northern margin of the basin and range province in southeastern oregon that hosts a westward younging trend of silicic volcanism that crudely mirrors northeastward migration of silicic volcanism along the yellowstone snake river plain ysrp trend we present 40ar 39ar ages for 19 rhyolite domes 5 rhyolite ash flow tuffs and 34 basaltic lavas from the hlp the previously identified trend of westward migration of hlp rhyolites is confirmed the rate of propagation is 33 km m y from 10 to 5 ma slowing to 13 km m y after 5 ma the duration of silicic volcanism at any locus is 2 m y three older hlp dacite domes yielded ages of 15 5 ma basalts are not age progressive we identify several episodes of increased basaltic activity at 7 5 7 8 5 3 5 9 and 2 3 ma with the younger episode likely continuing into the recent the hlp and ysrp trends emerged from the axis of middle miocene basaltic volcanism of the columbia river and steens basalts we propose a model in which 1 miocene flood basalts and widespread silicic rocks are the result of emplacement of a plume head near the craton margin enhanced by flow up a topographic gradient along the base of the lithosphere at the craton margin 2 the hlp trend is the result of westward flow originating at the craton margin and 3 the ysrp trend is the trace of the motion of the north american plate over the tail of the plume

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

coupled variations in helium isotopes and fluid chemistry shoshone geyser basin yellowstone national park abstract early studies of 3he 4he variations in geothermal systems have generally attributed these fluctuations to either differences in the source of the magmatic 3he rich helium or to local differences in the deep flux of magmatic 3he rich helium kennedy et al 1987 however show that near surface processes such as boiling and dilution may also drastically affect 3 he 4 he ratios of geothermal vapors helium isotope ratios were determined for several hot springs at shoshone geyser basin of yellowstone national park for this study along with other noble gas data stable isotope data and water and gas chemistry data for each spring were also compiled the water chemistry indicates that there is one deep hot thermal water in the area which is mixing with dilute meteoric water that has entered the system at depth spring hco3 concentrations correlate with 3 he 4 he values as in nearby lower geyser basin this correlation is attributed to variable amounts of deep dilution of thermal waters with a relatively cool water that inhibits boiling at depth thus preventing the loss of co2 and therefore hco3 and magmatic he in the most diluted samples oxygen and hydrogen isotope data also support a boiling and dilution model but to produce the observed fractionations the boiling event would have to be extensive with steam loss at the surface whereas the boiling that affected the helium isotope ratios was probably a small scale event with steam loss at depth it is possible that deep boiling occurred in the basin and that small amounts of steam escaped along fractures at about 500 m below the surface while all subsequently produced steam was lost near or at the surface

distinctive upper mantle anisotropy beneath the high lava plains and eastern snake river plain pacific northwest usa 1 the pacific northwest pnw has experienced voluminous intraplate volcanism over the past 17 ma beginning with the steens columbia river flood basalts and continuing with the still ongoing volcanism in the high lava plains hlp and eastern snake river plain srp here we present two complementary datasets sks splitting and rayleigh wave phase velocity anisotropy that place constraints on the anisotropic structure of the upper mantle beneath the hlp and srp regions beneath the hlp sks phases reveal dominantly e w fast splitting directions and large up to 2 7 s delay times with pronounced lateral variations in t lateral and depth variability in the strength of anisotropy beneath the hlp is also evident from rayleigh wave dispersion beneath the srp sks splitting delay times are much smaller 0 5 s and surface wave observations suggest a region of upper mantle anisotropy 50 150 km depth with a geometry that deviates significantly from the generally plate motion parallel fast directions observed just outside of the srp beneath the hlp the geometry of the anomalously strong anisotropy is similar to the anisotropy in the deeper parts of the upper mantle resulting in constructive interference and large sks splitting delay times beneath the srp the geometry of the anomalous anisotropic region in the shallow mantle is different resulting in destructive interference and reduced sks splitting delay times we discuss several possible explanations for these observations including variations in olivine lattice preferred orientation lpo strength transitions in olivine fabric type and a contribution from aligned partial melt

structure and chronology of the oval peak batholith and adjacent rocks implications for the ross lake fault zone north cascades washington the 200 km long ross lake fault zone rlfz forms the northeastern border of the crystalline core of the north cascades this late cretaceous and paleogene structure is an 15 km wide system of faults and fault zones several of which deform the paleocene 65 ma u pb zircon oval peak batholith the southern segment of the gabriel peak tectonic belt of the rlfz is defined by strongly deformed rocks of the foliated southwestern margin of the oval peak batholith structurally underlying pre late cretaceous twisp valley schist and structurally highest part of the subjacent skagit gneiss which here consists of late cretaceous 87 ma and paleocene 60 ma orthogneiss kinematic indicators in moderately steeply northeast dipping mylonitic orthogneiss record reverse slip the strong solid state flattening fabric in the foliated margin of the oval peak batholith formed during or shortly after emplacement and overprinted a magmatic fabric observed in the core of the batholith foliation and associated stretching lineation within the gabriel peak tectonic belt are oblique to the regional structural grain but concordant with the contacts of the batholith these patterns in part record emplacement of the batholith as an expanding diapir into the tectonic belt reverse slip in the tectonic belt occurred before during and shortly after emplacement but ended by 55 58 ma the core of the oval peak batholith intruded a large tongue of twisp valley schist on the north porphyroblast microstructures and ar ar and k ar geochronology indicate that amphibolite facies dynamothermal metamorphism of the schist occurred during intrusion the foggy dew and twisp river fault zones separate the batholith and its wall rocks on the northeast from weakly metamorphosed strata of the methow basin the foggy dew fault zone contains mylonitic gneiss derived from the oval peak batholith skagit gneiss and tonalitic rock that displays u pb zircon systematics suggestive of an eocene age sillimanite schist and amphibolite are also present mylonites record dextral strike slip with a component of normal slip down to the northeast movement in the fault zone continued subsequent to deformation in the gabriel peak tectonic belt and ended by 48 ma mylonites in the twisp river fault zone also reveal dextral strike slip but apparently with less normal slip the rlfz experienced a diachronous history from at least 65 to 45 ma this history was dominated by dextral strike slip but there were significant components of normal slip and reverse slip on some structures during the latter part of this interval 57 45 ma the rlfz probably recorded a transtensional tectonic regime transitional between extension to the east in omineca metamorphic core complexes and dextral strike slip to the west related to oblique convergence along the continental margin early cenozoic plutonism metamorphism and ductile deformation have strongly overprinted any of the major cretaceous deformation postulated in previous tectonic models for the rlfz

a miocene subcontinental plume in the pacific northwest geochemical evidence most of the recent discussions on the number and kinds of components which can be distinguished in the columbia river basalt group crbg magmas have used arguments developed from isotopic evidence in this paper we consider relative contents of excluded trace elements interpreted by means of spidergrams and abundance ratios and compare data from the crbg and related lavas with those of selected oceanic basalts although many recent models for crbg petrogeneses do not include a plume component spidergrams for the american bar flows of the imnaha basalt of the crbg have broad humps at ba th and nb ta this feature suggests that an enriched mantle plume component is present in the american bar magmas an inference which is weakly confirmed by 87sr 86sr data this plume may have been identical to that presently beneath yellowstone the inferred hot spot track lies almost directly east west and requires a vector of motion of the pacific northwest part of the north american plate significantly different from that inferred by minster and jordan 1978 31 our proposed vector for the north american plate agrees however with that suggested by the anahim belt a volcanic lineament in southern british columbia that has been proposed as a hot spot track this proposed vector requires alterations in current models for pacific and north american plate movement during the cenozoic

time corrections to teleseismic p delays derived from sks splitting parameters and implications for western u s p wave tomography 1 upper mantle anisotropy will affect teleseismic p wave arrival times and create artifacts in isotropic travel time tomography compiled sks splitting results indicate variations in the strength and orientation of azimuthal anisotropy beneath the western u s we use sks splitting parameters and a hexagonally anisotropic elastic tensor fast symmetry axis to estimate azimuthal anisotropy contributions to p wave delay times and evaluate the effects on isotropic p wave tomography estimated anisotropy correction times have a root mean square rms value of 0 16 s which is 37 of p wave delay time rms the magnitude of azimuthal anisotropy rather than its azimuth has the strongest effect on p delay times however if the anisotropy symmetry axis has a plunge p wave travel times may be strongly affected by both magnitude and azimuth applying azimuthal anisotropy corrections significantly increases tomographic p wave velocity beneath the high lava plains and central california coast and decreases velocity beneath the great basin

genesis of flood basalts and basin and range volcanic rocks from steens mountain to the malheur river gorge oregon the middle and south forks of the malheur river provide a unique mapping corridor connecting two flood basalt successions steens basalt to the south and the basalt of malheur gorge to the north each contains chemically defined subtypes which merge stratigraphically across the north south length of the map area the lowermost flows of steens basalt are stratigraphically equivalent to the lowermost flows of the basalt of malheur gorge the uppermost flows of steens basalt pinch out in the map area but are partly interbedded with the middle and uppermost flows of the basalt of malheur gorge which continue to thicken northward the upper part of the tholeiitic succession is interbedded with a group of previously unrecognized lavas the venator ranch basalt flows tholeiitic volcanism ceased at ca 15 3 ma the last tholeiitic unit erupted is the hunter creek basalt which also thickens northward subsequent younger than 15 3 ma more localized eruptions were dominated by calc alkaline to mildly alkaline lavas associated with basin and range extension local uplift generated deep canyons which were filled by andesitic lavas of the keeney sequence ca 13 10 ma the final eruptive products include the devine canyon tuff ca 9 7 ma the drinkwater basalt ca 6 9 ma and the voltage flow older than 32 000 yr b p major and trace element analyses demonstrate that 1 crystal fractionation was a universal process in the derivation of the

seismic heterogeneity and small scale convection in the southern california upper mantle we invert traveltime residuals of teleseismic p and s phases for 3 d perturbations in vp vs and vp vs structures in the southern california upper mantle the tomographic inversion uses frequency dependent 3 d sensitivity kernels to interpret traveltime residuals measured in multiple frequency bands and recent advances in regional crustal thickness and velocity models to better isolate the mantle component of traveltime residuals in addition to separate vp and vs tomography we jointly invert the p and s data sets for vp vs perturbations by imposing a smoothness constraint on the lnvs lnvp field the regional upper mantle is very heterogeneous with the greatest amplitude of velocity and vp vs perturbations in the upper 200 km and mantle structures that correlate spatially with the major physiographic provinces of southern california our imaging improves the resolution of the geometry and amplitude of these features in addition to the major structures imaged and discussed in previous papers we find a large high velocity anomaly at depths between 340 and 500 km beneath the borderland transverse ranges and northern peninsular ranges this anomaly is separate from the well known uppermost mantle transverse ranges anomaly which extends no deeper than 175 200 km the strongest low velocity high vp vs anomaly is found beneath and west of the salton trough and is attributed to relatively high partial melt in the asthenosphere as a result of lithospheric thinning the magnitudes of vp vs and vp vs perturbations and knowledge of regional 1 d velocity and attenuation lead us to conclude that the asthenosphere contains up to 1 partial melt extending to depths as great as 150 200 km

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

western us volcanism due to intruding oceanic mantle driven by ancient farallon slabs the origin of late cenozoic intraplate volcanism over the western united states is debated one important reason is the lack of a clear understanding of the mantle dynamics during this volcanic history here we reconstruct the mantle thermal states beneath north america since 20 million years ago using a hybrid inverse geodynamic model with data assimilation the model simultaneously satisfies the past subduction kinematics present mantle tomographic image and the volcanic history we find that volcanism in both the yellowstone volcanic province and the basin and range province corresponds to a similar eastward intruding mantle derived from beneath the pacific ocean and driven mostly by the sinking farallon slab below the central eastern united states the hot mantle that forms the columbia river flood basalt and subsequent yellowstone newberry hotspot tracks first enters the western united states through tears within the juan de fuca slab subsequent coexistence of the westward asthenospheric flow above the retreating juan de fuca slab and eastward propagating mantle beyond the back arc region reproduces the bifurcating hotspot chains a similar but weaker heat source intrudes below the basin and range around the southern edge of the slab and can explain the diffuse basaltic volcanism in this region according to our models the putative yellowstone plume contributes little to the formation of the yellowstone volcanic province volcanism in the western us may result from warm oceanic mantle beneath the pacific ocean being drawn eastwards by mantle flow induced by the sinking of farallon slabs according to numerical model simulations

upper mantle seismic structure beneath the pacific northwest a plume triggered delamination origin for the columbia river flood basalt eruptions abstract we invert teleseismic p and s body waves constrained by an ambient noise surface wave model and moho depth inferred from receiver function analysis gao et al 2011 to image mantle structures continuously from the surface to the base of the upper mantle the major structures coincide with prominent geological features we focus on a ne oregon structure termed here the wallowa anomaly which coincides with the source area for the 16 ma columbia river flood basalt eruptions and a circular area of topographic relief created during and after these eruptions resolution tests indicate that the curtain like structure previously interpreted as farallon lithosphere connects with the wallowa anomaly above 150 km along the northeast margin of the wallowa anomaly this connection along with the pre flood basalt magmatic and tectonic history of the pacific northwest lead us to conclude that arrival of the yellowstone plume to south central oregon initiated delamination of remnant farallon lithosphere from the base of ne oregon exposing ocean crust to yellowstone asthenosphere this hypothesis accounts for the propagation of flood basalt volcanism far north of the yellowstone hotspot track and for the high silica composition of most of the flood basalt magmas

global variations of local asymmetry in glacier altitude separation of north south and east west components north south and east west differences in firn line altitude equilibrium line altitude or middle altitudes of glaciers can be separated by regression on the cosine and sine of glacier aspect accumulation area azimuth allowing for regional trends in altitude the north south differences expected from radiation and shade effects can be reliably quantified from world glacier inventory wgi data the north south differences are greater in sunnier climates mid latitudes and steeper relief local altitude differences between north and south facing glaciers are commonly 70 320 m such asymmetry is near universal although weak in the arctic and tropics east west contrasts are less and found mainly in the tropics and areas most exposed to strong winds altitude latitude glacier gradient and height range calculable from most of the wgi data are potential controls on the degree of north south contrast as well as surrogates for climatic controls temperature precipitation radiation and cloudiness an asymmetric sine cosine power model is developed to describe the variation of north south contrast with latitude multiple regression over 51 regions shows altitude and latitude to be the strongest controls of this contrast aspect altitude analysis for former glaciers provides new evidence of cloudiness

crustal seismic structure beneath the source area of the columbia river flood basalt bifurcation of the moho driven by lithosphere delamination