depth of the moho discontinuity beneath the japanese islands estimated by traveltime analysis 1 the crustal structure beneath the japanese islands including depth distributions of the conrad and moho discontinuities was estimated using a tomographic inversion of regional body wave arrival times depth distributions of the bottom of the surface layer the conrad and the moho were modeled with two dimensional b spline functions while velocity distributions in layers were expressed by three dimensional b spline functions the depth of the discontinuities and the velocity in the layers were estimated simultaneously by the least squares method the velocity structure was sequentially estimated from shallower parts to deeper parts to avoid correlation between them this sequential analysis provided improved depth resolution the deepest region of the moho discontinuity was located in central honshu reaching about 40 km the moho discontinuity was generally deep in the central part of the islands whereas it was relatively shallow in the kanto southwestern chubu and chugoku districts in honshu and in northern kyushu some of the shallow moho regions would be related to graben formation due to tensile tectonic stress since the miocene the results were compared with those of seismic refraction surveys and receiver function analyses and it was found that the obtained model was consistent with many of these studies

topographic expressions of mantle dynamics in the mediterranean abstract the surface of the earth is the ever changing expression of the dynamic processes occurring at depth and at or above its surface however our ability to read landscapes in terms of their underlying tectonic or climatic forcing remains rudimentary during the last decade particular attention has been drawn to the deep dynamic contributions to topography related to the stresses that are produced at the base of the lithosphere by mantle convection and their relevance compared to the iso static component of topography here we use examples from the mediterranean and estimate residual and dynamic topography we then compare those with surface uplift from geology for specific regions to disentangle the dynamic from the static components considering the different topographic signatures of tectonic processes e g actual residual and dynamic topography as well as uplift rates jointly provides a powerful way to distinguish between the contributions of mantle crustal and surface processes such an approach might bring us closer to reading topographic expressions in terms of their geological cause

deep mantle forces and the uplift of the colorado plateau 1 we introduce a quantitative model of global mantle convection that reconstructs the detailed motion of a warm mantle upwelling over the last 30 ma towards the interior of the southwestern usa from observed present day mantle heterogeneity the onset and evolution of uplift in the central basin and range province and colorado plateau during this time is determined by tracking the topographic swell due to this mantle upwelling we show that 1 the extension and basaltic volcanism post 25 ma in the central basin and range coincides with the arrival and eastward progression of this upwelling and 2 dynamic uplift of the southern colorado plateau totaling about 1 km transpired in the last 20 ma since 10 ma the center of uplift continued northeastward from the southwestern rim of the plateau consistent with a young grand canyon model and eastward sweep of magmatism in the western colorado plateau

on recent seismic tomography for the western united states 1 in recent years a number of relatively high resolution seismic tomography models for the uppermost mantle underneath the western united states have been published and vigorous debate has ensued about their tectonic interpretation i present a straightforward yet quantitative comparison between models in order to help establish a framework for geodynamic interpretation and to help judge the role of tomographic theory vs data selection mapped s and p wave anomalies are found to be remarkably consistent between models which implies that seismologists are beginning to narrow down the structure underneath active continental margins to scales of 200 km large discrepancies between published anomaly amplitudes exist however and the models on the high end of the spectrum raise questions as to how they are to be interpreted in terms of temperature composition and melting

deep low frequency microearthquakes in or around seismic low velocity zones beneath active volcanoes in northeastern japan abstract precise hypocenter relocations have been made for shallow intraplate microearthquakes beneath northeastern japan by applying source region station corrections the relocated hypocenter distribution shows that most of the events are confined to the upper 15 km of the crust which forms the brittle seismogenic zone in this volcanic arc it also shows that exceptionally deep microearthquakes well below the base of the seismic aseismic boundary occur at twelve locations of northeastern japan they occur in the lowermost crust or in the uppermost mantle where rheological properties of rocks are presumed to be in the ductile regime all the 153 events presently detected in the lowermost crust or in the uppermost mantle have anomalously low predominant frequencies 1 5 5 hz both for p and s waves in contrast to those 8 20 hz of normal shallow events in the brittle seismogenic zone the magnitudes of these events are small m 2 2 moreover all of these events are located beneath active volcanoes and or near p wave low velocity zones in the uppermost mantle the proximity of these events to the active volcanoes and other anomalous features suggest that these deep low frequency events are generated by magmatic activity of mantle diapirs in the uppermost mantle

lateral variations in mantle rheology implications for convection related surface observables and inferred viscosity models summary over the past decade numerous analyses of convection related observables such as horizontal surface divergence geoid or gravity anomalies and dynamic surface topography have been carried out in the context of tomography based mantle flow models one of the major objectives of this modelling has been the inference of the rheological structure of the mantle with few exceptions these studies have been conducted in the framework of a viscous flow theory which assumes that the mantle rheology may be represented in terms of an effective viscosity which varies with depth only here we present a detailed assessment of the impact of lateral variations in viscosity on global convection related observables using forward modelling of buoyancy induced flow in a 3 d spherical shell we find that the resulting dynamic topography at the surface and the core mantle boundary as well as the gravitational response of the earth are affected relatively little by the inclusion of lateral viscosity variations lvv when compared with results for a purely 1 d radial viscosity model in particular we found that the effect of lvv on the global observables is significantly smaller than the variability due to uncertainties in the current seismic tomography models we also quantify the effect of lvv in the context of the viscosity inverse problem using two synthetic data sets generated with a 1 d viscosity profile and with a fully 3 d viscosity model in which the lvv span across three orders of magnitude we compared the 1 d viscosity profiles recovered from the inversions and found that lvv have virtually no effect on our inversion results the synthetic viscosity inversion further revealed that the effect of lvv is small in comparison to the uncertainties arising from the seismic tomography models the inversions also suggest that the 1 d viscosity profiles derived from actual surface data represent the depth variation of the horizontally averaged logarithm of the 3 d viscosity distribution in the mantle intr o duction constraining the rheological properties of the mantle using surface geophysical data has traditionally involved separate treatments of short seismic intermediate solid earth tides long postglacial rebound and very long mantle convection timescale processes in each case the initial focus historically speaking has been on the inversion of the data to determine a one dimensional 1 d purely radial variation of the relevant rheological parameter e g elastic moduli intrinsic attenuation and viscosity which provides the best fit to the data it has long been recognized particularly in the context of the seismic inverse problem that the search for a 1 d earth model is subject to possible bias due to the presence of lateral variations in structure which may be as significant as the radial variations especially at shallow depths this paper is concerned with the mantle viscosity field the earliest inferences of the radial i e horizontally averaged profile of viscosity based on glacial isostatic adjustment gia data e g mcconnell 1968 parsons 1972 were characterized by a pronounced increase of viscosity with depth in the top half of the mantle however these were followed by two decades of gia research that argued for an essentially isoviscous

long term sea level fluctuations driven by ocean basin dynamics earth9s long term sea level history is characterized by widespread continental flooding in the cretaceous period 145 to 65 million years ago followed by gradual regression of inland seas however published estimates of the late cretaceous sea level high differ by half an order of magnitude from 40 to 250 meters above the present level the low estimate is based on the stratigraphy of the new jersey margin by assimilating marine geophysical data into reconstructions of ancient ocean basins we model a late cretaceous sea level that is 170 85 to 270 meters higher than it is today we use a mantle convection model to suggest that new jersey subsided by 105 to 180 meters in the past 70 million years because of north america9s westward passage over the subducted farallon plate this mechanism reconciles new jersey margin based sea level estimates with ocean basin reconstructions

topography the geoid and compensation mechanisms for the southern rocky mountains the southern rockies of colorado are anomalously high elevations greater than 2800 m topographically rough implying active uplift and underlain by significant low velocity anomalies in the upper mantle that suggest an intimate relationship between mantle geodynamic processes and the surface topography the region is in isostatic equilibrium i e near zero free air gravity anomaly however the poor correlation between the high topography and crustal thickness makes the application of simple compensation models e g pure heiskanen or pratt hayford problematic knowledge of how the current topography of the rockies is isostatically compensated could provide constraints on the relative role of sublithospheric buoyancy versus lithospheric support here we evaluate the geoid and its relationship to the topography using the geoid to elevation ratio gtr in the spatial domain and the admittance in the frequency domain to constrain the mechanism of compensation we separate the upper mantle geoid anomalies from those with deeper sources through the use of spherical harmonic filtering of the egm2008 geoid we exploit the fact that at wavelengths greater than the flexural wavelength where features are isostatically compensated the geoid topography ratio can be used to estimate the depth of compensation and the elastic thickness of the lithosphere the results presented below indicate that the main tectonic provinces of the western united states have moderate geoid topography ratios between 3 5 and 5 5 m km 3 9 for the southern rockies 4 25 for the colorado plateau and 5 2 for the northern basin and range suggesting shallow levels of isostatic compensation in terms of the elastic thickness of the lithosphere our results indicate an elastic thickness of less than 20 km these value support the notion that a major portion of the buoyancy that has driven uplift resides at depths less than 100 km and that upper mantle processes such as small scale convection may play a significant role in the buoyant uplift of the southern rockies as well as other actively uplifting areas of the western united states further support for this hypothesis is provided by high coherence for the geoid topography relationship for nearly all wavelengths between 50 and 1000 km

the many surface expressions of mantle dynamics plate tectonic theory suggests that present day topography can be explained by the repeated interactions between the tectonic plates moving along earth s surface however mounting evidence indicates that a significant proportion of earth s topography results from the viscous stresses created by flow within the underlying mantle rather than by the moving plates this dynamic topography is transient varying as mantle flow changes and is characterized by small amplitudes and long wavelengths it is therefore often hidden by or confused with the more obvious topographic anomalies resulting from horizontal tectonic movements however dynamic topography can influence surface processes and thus enter the geological record it has for example played a role in the establishment of amazon drainage patterns in turn surface processes such as the erosion of topographical anomalies could affect mantle flow this emerging view of dynamic topography suggests that the concept of plate tectonics as the driver of surface deformation needs to be extended to include the vertical coupling between the mantle and the surface unravelling this coupling back in time with the help of models and the geological record can potentially provide unprecedented insights into past mantle dynamics

reply to comment by i pineda velasco t t nguyen h kitagawa and e nakamura on diverse magmatic effects of subducting a hot slab in sw japan results from forward modeling

a 3 d model of the crust and uppermost mantle beneath the central and western us by joint inversion of receiver functions and surface wave dispersion 1 rayleigh wave phase velocity maps from ambient noise and earthquake data are inverted jointly with receiver functions observed at 828 stations from the usarray transportable array west of 100 w longitude for data recorded in the years 2005 through 2010 to produce a 3 d model of shear wave speeds beneath the central and western us to a depth of 150 km eikonal tomography is applied to ambient noise data to produce about 300 000 rayleigh wave phase speed curves and helmholtz tomography is applied to data following 1550 ms 5 0 earthquakes so that rayleigh wave dispersion maps are constructed from 8 to 80 s period with associated uncertainties across the region harmonic stripping generates back azimuth independent receiver functions with uncertainty estimates for each of the stations a nonlinear bayesian monte carlo method is used to estimate a distribution of shear wave speed vs models beneath each station by jointly interpreting surface wave dispersion and receiver functions and their uncertainties the assimilation of receiver functions improves the vertical resolution of the model by reducing the range of estimated moho depths improving the determination of the shear velocity jump across moho and improving the resolution of the depth of anomalies in the uppermost mantle a great variety of geological and tectonic features are revealed in the 3 d model that form the basis for future detailed local to regional scale analysis and interpretation

isostasy dynamic topography and the elevation of the apennines of italy abstract the elevation of an orogenic belt is commonly related to crustal lithosphere thickening here we discuss the apennines as an example to show that topography at a plate margin may be controlled not only by isostatic adjustment but also by dynamic mantle driven processes using recent structural constraints for the crust and mantle we find that the expected crustal isostatic component explains only a fraction of the topography of the belt indicating positive residual topography in the central apennines and negative residual topography in the northern apennines and calabria the trend of the residual topography matches the mantle flow induced dynamic topography estimated from regional tomography models we infer that a large fraction of the apennines topography is related to mantle dynamics producing relative upwellings in the central apennines and downwellings in the northern apennines and calabria where subduction is still ongoing comparison between geodetic and geological data on vertical motions indicates that this dynamic process started in the early pleistocene and the resulting uplift appears related to the formation and enlargement of a slab window below the central apennines the case of the apennines shows that at convergent margins the elevation of a mountain belt may be significantly different from that predicted solely by crustal isostasy and that a large fraction of the elevation and its rate of change are dynamically controlled by mantle convection

vs and density structure beneath the colorado plateau constrained by gravity anomalies and joint inversions of receiver function and phase velocity data 1 the colorado plateau is a physiographic province in the western us with an average elevation of 1 9 km where in contrast to neighboring provinces there is little evidence of large scale tectonic deformation or magmatism recent availability of earthscope usarray seismic data allow us to better examine the crust and upper mantle structure beneath the region and test proposed explanations for the plateau s uplift and relative stability using phase velocities for fundamental mode rayleigh waves and p receiver functions we perform over 800 joint inversions for 1 d shear wave velocity vs profiles sampling the plateau and surrounding regions down to 150 km depth we image a sharp change in crustal thickness at the western edge of the colorado plateau with a more gradual increase eastward moving into the rocky mountains a relatively thick 100 km lithosphere beneath the plateau extends into the rocky mountains to the north we use empirical scaling relations to estimate densities from our vs results and predict the associated gravity anomalies which are inconsistent with the observed distribution of the bouguer gravity anomalies we somewhat reconcile the prediction and observations by assuming that lateral density variations below 50 km can be ignored and the lithospheric root is therefore neutrally buoyant while there is some evidence for small scale convection and lithospheric removal at its edges the shape of the lithospheric mantle anomaly is consistent with a large scale uplift of the plateau by heating since removal of the farallon slab we conclude that the lithospheric root is key to the long term stability of the colorado plateau leading to a colder stronger crust

the ups and downs of north america evaluating the role of mantle dynamic topography since the mesozoic the driving force for transient vertical motions of earth s surface remains an outstanding question a main difficulty lies in the uncertain role of the underlying mantle especially during the geological past here i review previous studies on both observational constraints and physical mechanisms of north american topographic evolution since the mesozoic i first summarize the north american vertical motion history using proxies from structural geology geochronology sedimentary stratigraphy and geomorphology based on which i then discuss the published physical models overall there is a progressive consensus on the contribution of mantle dynamic topography due to buoyancy structures associated with the past subduction at the continental scale a largely west to east migrating deformation pattern suggests an eastward translation of mantle dynamic effects consistent with models involving an eastward subduction and sinking of former farallon slabs since the cretaceous among the existing models the inverse model based on an adjoint algorithm and time dependent data constraints provides the most extensive explanations for the temporal changes of north american topography since the mesozoic at regional scales debates still exist on the predicted surface subsidence and uplift within both the western and eastern united states where discrepancies are likely due to differences in model setup e g mantle dynamic properties and boundary conditions and the amount of time dependent observational constraints toward the development of the next generation predictive geodynamic models new research directions may include 1 development of enhanced data assimilation capabilities 2 exploration of multiscale and multiphysics processes and 3 cross disciplinary code coupling

along strike translation of a fossil slab abstract the isabella anomaly is a high seismic velocity anomaly beneath the southern central valley of california that has previously been interpreted to represent a lithospheric drip or delaminated sierra nevada root however recent work suggests that the anomaly is a remnant slab left over from cenozoic subduction attached to the pacific plate underneath the edge of the north american plate this hypothesis requires the slab to translate hundreds of kilometers along strike while remaining intact and attached to the pacific plate despite drag from the surrounding asthenosphere and overriding lithosphere we use 3 d finite element models to simulate this scenario and calculate the viscosity ratio between the slab and the surrounding asthenosphere required for the slab to translate undeformed the required viscosity ratio increases with increasing downdip extent of the slab and decreases with increasing slab dip for geometries approximating that of our proposed slab the required ratio ranges from 10 2 to 10 4 given the thermal and petrologic history of the slab mantle flow laws predict viscosity contrasts greater than or equal to these requirements as such we conclude that along strike translation of a remnant slab is feasible and serves as a possible explanation of the isabella anomaly the significance of this finding extends beyond our general understanding of subduction dynamics in that the presence of such a slab could have implications for the water budget of the san andreas fault and its role in aseismic slip

influence of fluids and magma on earthquakes seismological evidence abstract in this paper we present seismological evidence for the influence of fluids and magma on the generation of large earthquakes in the crust and the subducting oceanic slabs under the japan islands the relationship between seismic tomography and large crustal earthquakes m 5 7 8 0 in japan during a period of 116 years from 1885 to 2000 is investigated and it is found that most of the large crustal earthquakes occurred in or around the areas of low seismic velocity the low velocity zones represent weak sections of the seismogenic crust the crustal weakening is closely related to the subduction process in this region along the volcanic front and in back arc areas the crustal weakening is caused by active volcanoes and arc magma resulting from the convective circulation process in the mantle wedge and dehydration reactions in the subducting slab in the forearc region of southwest japan fluids are suggested in the 1995 kobe earthquake source zone which have contributed to the rupture nucleation the fluids originate from the dehydration of the subducting philippine sea slab the recent 2001 geiyo earthquake m 6 8 occurred at 50 km depth within the subducting philippine sea slab and it was also related to the slab dehydration process a detailed 3d velocity structure is determined for the northeast japan forearc region using data from 598 earthquakes that occurred under the pacific ocean with hypocenters well located with sp depth phases the results show that strong lateral heterogeneities exist along the slab boundary which represent asperities and results of slab dehydration and affect the degree and extent of the interplate seismic coupling these results indicate that large earthquakes do not strike anywhere but only anomalous areas which can be detected with geophysical methods the generation of a large earthquake is not a pure mechanical process but is closely related to physical and chemical properties of materials in the crust and upper mantle such as magma fluids etc

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

support of high elevation in the southern basin and range based on the composition and architecture of the crust in the basin and range and colorado plateau toexplorethenatureofhowthestructureandphysicalpropertiesofthecrustvaryfromextendedtorelativelyunextendeddomains we present teleseismic receiver functions which measure crustal thickness shear wavespeed structure and the vp vs ratio at 12 seismic stations in eastern arizona the crustal thickness is 28 km increases slightly eastward and remains nearly uniform beneath the varying elevations in the southern basin and range the observed vp vs ratio in the basin and range 1 78 exceeds the global average the southern colorado plateau exhibits thicker crust 40 km and a slightly greater observed vp vs 1 81 a discrete region in the colorado plateau generates an unusually high vp vs ratio 1 90 and contains low wavespeed zones which serve as evidence of partial melt related to quaternary volcanism the metamorphic core complexes in the southern basin and range likewise exhibit anomalously high vp vs values 1 79 1 87 and lack locally compensating crustal roots density models show that 85 kg m 3 lighter crust or 35 kg m 3 lighter mantle than that of the surrounding basin and range helps these metamorphic core complexes maintain their high elevation compositional modeling of intrusive bodies exposed throughout the catalina rincon metamorphic core complex indicates that the observed high vp vs ratio and modeled low density could result from substantial amounts of a plagioclase rich quartz poor rock these vp vs data are evidence that significant compositional heterogeneity of the crust can occur over a short distance and provide a clue as to how these areas that underwent significant tertiary extension may have been preconditioned for orogenic collapse

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

deep water formation in the western mediterranean wintertime deep convection was studied in the gulf of lions northwestern mediterranean using the chlorofiuoromethane cfm distributions components f11 and f12 from december 1991 and february march 1992 convection occurred on small spatial and temporal scales and it was not complete only convection to 1900 m depth was observed the vertical mixing of the water column above the convection depth turned out to be of major influence regarding the characteristic of newly formed deep water and regarding the boundary conditions for cfms in modeling deep water formation air sea gas exchange and entrainment were found to be negligible these findings are important in estimating deep water formation rates for all water masses renewed by convection e g greenland sea deep water labrador sea water and in evaluating the spreading of deep water masses along the western boundary current a box model is presented which uses cfm and tritium distributions to estimate formation of western mediterranean deep water below 1000 m the model starts with zero tracer concentrations in 1945 the integration step is 1 year and the deep water formation rate is assumed to be constant from 1945 to 1992 the best fit between cfm model concentrations and measurements were obtained at a mean deep water renewal time of 5 7 years equivalent to a conversion of 0 46 sv modified atlantic water 0 to 150 m depth and 0 76 sv levantine intermediate water 150 to 400 m depth into deep water however the tritium measurements are only partly matched by the model by assuming that convection stopped in the years 1964 1968

dynamic topography gravity and the role of lateral viscosity variations from inversion of global mantle flow lateral viscosity variations lvvs in the mantle influence geodynamic processes and their surface expressions with the observed long wavelength geoid free air anomaly gravity gradient in three directions and discrete high accuracy residual topography we invert for depth and temperature dependent and tectonically regionalized mantle viscosity with a mantle flow model the inversions suggest that long wavelength gravitational and topographic signals are mainly controlled by the radial viscosity profile the pre cambrian lithosphere viscosity is slightly one order of magnitude higher than that of oceanic and phanerozoic lithosphere plate margins are substantially weaker than plate interiors and viscosity has only a weak apparent dependence on temperature suggesting either a balancing between factors or a smoothing of actual higher amplitude but short wavelength lvvs the predicted large scale lithospheric stress regime compression or extension is consistent with the world stress map thrust or normal faulting both recent compiled high accuracy residual topography and the predicted dynamic topography yield 1 km amplitude long wavelength dynamic topography inconsistent with recent studies suggesting amplitudes of 100 to 500 m such studies use a constant positive admittance transfer function between topography and gravity in contrast to the evidence which shows that the earth has a spatially and wavelength dependent admittance with large negative admittances between 4000 and 10 4 km wavelengths

unraveling the geometry of the farallon plate synthesis of three dimensional imaging results from usarray abstract we compare 12 recent three dimensional 3d seismic imaging results that made extensive use of data from the earthscope transportable array ta our goal is to sort out what can be said about the geometry of the farallon plate our main approach is 3d visualization using a kinematic plate motion model as a framework comparison of results from all 12 image volumes indicates that the results are most consistent with a single coherent farallon slab overridden by north american the farallon can be tracked from the trench in the pacific northwest to its remnants in the lower mantle under eastern north america from the trench the lithosphere has a low dip to the volcanic arc immediately east of the arc the slab steepens sharply before undergoing a decrease in dip above the 410 km discontinuity the gently dipping section varies along strike under washington the deflection is minor but to the south the slab flattens to become nearly horizontal beneath southern idaho there is a strong agreement that the high velocity anomaly associated with the slab vanishes under eastern oregon scattered wave imaging results however suggest the top of the anomaly is continuous these can be reconciled if one assumes the wavespeed anomaly has been neutralized by processes linked to the yellowstone system we find that all results are consistent with a 4d kinematic model of the mendocino slab window under nevada and utah in the eastern us the larger scale models all show a lower mantle anomaly related to the older history of farallon subduction the link between the lower mantle and new results in the u s cordillera lies under the high plains where the required usarray coverage is not yet complete image volumes in a unified format are supplied in an electronic supplement

shaping mobile belts by small scale convection claudio faccenna and thorsten becker compute global mantle flow based on a recent high resolution seismic tomography model to investigate the role of buoyancy driven and plate motion induced mantle circulation for the mediterranean they show that mantle flow provides an explanation for much of the observed dynamic topography and microplate motion in the region and conclude that vigorous small scale convection in the uppermost mantle may also hold the key for understanding other complex mobile belts such as those in the north american cordillera and the himalayan tibetan collision zone here global mantle flow is computed on the basis of recent high resolution seismic tomography to investigate the role of buoyancy driven and plate motion induced mantle circulation in the mediterranean region the findings show that mantle flow explains much of the observed dynamic topography and microplate motion in the region small scale convection in the uppermost mantle may also hold the key to understanding complex mobile belts elsewhere mobile belts are long lived deformation zones composed of an ensemble of crustal fragments distributed over hundreds of kilometres inside continental convergent margins1 2 the mediterranean represents a remarkable example of this tectonic setting3 the region hosts a diffuse boundary between the nubia and eurasia plates comprised of a mosaic of microplates that move and deform independently from the overall plate convergence4 surface expressions of mediterranean tectonics include deep subsiding backarc basins intraplate plateaux and uplifting orogenic belts although the kinematics of the area are now fairly well defined the dynamical origins of many of these active features are controversial and usually attributed to crustal and lithospheric interactions however the effects of mantle convection well established for continental interiors5 6 7 should be particularly relevant in a mobile belt and modelling may constrain important parameters such as slab coherence and lithospheric strength here we compute global mantle flow on the basis of recent high resolution seismic tomography to investigate the role of buoyancy driven and plate motion induced mantle circulation for the mediterranean we show that mantle flow provides an explanation for much of the observed dynamic topography and microplate motion in the region more generally vigorous small scale convection in the uppermost mantle may also underpin other complex mobile belts such as the north american cordillera or the himalayan tibetan collision zone

tectonic volcanic and semi volcanic deep low frequency earthquakes in western japan abstract in western japan relatively small m the present paper characterizes the seismicity of the three types of lfes in five regions based on a more complete catalog thanks to automated event detection and precise relocation using waveform correlation the results show that isolated intraplate lfes are more similar to volcanic lfes than to tectonic lfes the volcanic and isolated intraplate lfes are insensitive to tidal stress are characterized by power law magnitude frequency statistics such as the gutenberg richter relation and are distributed mainly in small vertical clusters while the tectonic lfes are modulated by tidal stress are limited in size and are distributed along the plate boundary hence we propose that isolated intraplate lfes should be named semi volcanic lfes since tectonic lfes and volcanic lfes are thought to be related to the fluid existence and fluid movements respectively isolated intraplate semi volcanic lfes may be related to crustal fluid movements in the source area

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

the active southwest margin of the colorado plateau uplift of mantle origin during cenozoic time the colorado plateau was raised about 2 km above sea level the most recent and best documented uplift of the plateau 1 km has been concentratedatitssouthwestmarginbetween6 and 1 ma whereas the eastern colorado plateau may have been at high elevations sinceeocenetime tobetterunderstandthe recent tectonic activity at the southwest marginofthecoloradoplateau wecompile detailed crustal thickness and density informationfromseismicandgravitydatafor a region that includes northwest arizona and the southern tip of nevada this information is used to isolate the mantle contribution to uplift wefind that there is relatively low density mantle underlying the southernmarginoftheplateauinnorthwest arizona which could result from about 60 80 km of thinning of the dense mantle lithosphere combined with about 100 cof heating through a 100 km thick mantle layer the available estimates from earthquake source seismology in or near the studyareaarecompatiblewiththisestimate of lithospheric thinning we speculate that uplift may result from subduction related thinning of the continental lithosphere

slabs in the lower mantle results of dynamic modelling compared with tomographic images and the geoid abstract the recent increase in resolution of tomographic images of the earth s interior has enabled us to see slabs in the lower mantle on the other hand the distribution of slabs can be inferred from subduction history using a dynamical model of mantle flow comparison of tomographic images with model results can help to distinguish between alternative models of subduction history tomographic models and mantle flow models and thus improve our understanding of the earth here a simple dynamic model of subduction is used and some results are presented amounts and locations of subduction are inferred from published models of plate motion and plate boundaries for the past 120 ma the latter have been interpolated on 2 ma time intervals mantle flow driven by the density anomalies corresponding to subducted slabs is calculated with the method of hager and o connell for a viscosity structure that features an increase in viscosity from 4 10 20 pa below the lithosphere to 4 10 22 pa in the lower part of the lower mantle slabs are advected in the flow field in the model slabs sink on average about 1700 km through the mantle during 100 ma however the calculated average horizontal motion of slabs throughout most of the mantle is only about 600 700 km for slabs that have sunk to the lowermost mantle it is about 1000 km this is in accordance with the previously observed good correlation between subduction locations and lowermost mantle heterogeneities slabs sink fastest in regions with abundant subduction and little trench migration e g below east asia in the lowermost mantle predicted slab locations tend to lie in areas with high seismic velocities however there are large areas of fast seismic velocities where no slabs are predicted and which therefore presumably correspond to earlier subduction in some regions e g below japan our model approximately reproduces the observed shape of the slab in other regions where the agreement is poor the results can help to further constrain models of subduction history and associated mantle flow additionally the predicted geoid is computed and compared with the actual geoid predicted and observed geoid have similar magnitude and a number of features in common however an optimization of the fit is not attempted here since in order to do this also the hot upwellings from the lower boundary layer presumably in the form of plumes would have to be included

effects of lateral viscosity variations on the geoid 1 we investigate the effects of lateral viscosity variations lvvs on mantle circulation model predictions of the global geoid the present study is motivated in part by earlier findings that lvvs due to stiff slabs in the lower mantle have a strong influence on the geoid and that slabs in the lower mantle are perhaps no stronger than the ambient mantle however more recently it has been argued based on global seismic tomography models that lvvs are a minor effect on the geoid in the light of these seemingly contradictory results we re visit the problem of slab strength in the lower mantle we substantiate that the geoid calculated from tomography is hardly affected by the presence of lvvs whereas the geoid computed from global slab models yields a poor fit to the observed geoid when lvvs are considered however this degradation of fit only occurs for very long wavelengths of flow indicating inherent differences between the slab and tomography models we also investigate the effects on the geoid due to weak plate boundaries strong cratonic keels and a low viscosity region in the d layer due to post perovskite in addition to the geoid we attempt to fit plate motions with a circulation model that has prescribed weak zones at plate boundaries motions are matched well and taking into account lvvs the geoid with appropriate surface velocity boundary conditions agrees with the observed geoid as well as for free slip cases

east asia mantle tomography new insight into plate subduction and intraplate volcanism abstract we present a new p wave tomographic model beneath east asia by inverting 1 401 797 arrival time data recorded by the regional seismic networks in east asia and temporary seismic arrays deployed on the tibetan plateau our high resolution tomography shows that the subducted pacific slab is revealed clearly as a high velocity high v zone and it becomes stagnant in the mantle transition zone beneath eastern china prominent high v anomalies atop the 410 km discontinuity are detected beneath the eastern north china craton and northeast china suggesting that lithospheric delamination may have occurred and greatly affected the thermal state intraplate magmatism and surface topography of this region the indian lithosphere is characterized by a high v anomaly and it is subducting nearly horizontally beneath the entire or most parts of western tibet and with a small dipping angle to the southernmost part of eastern tibet the intraplate magmatism in different parts of east asia has different origins the active tengchong volcano is underlain by a prominent low velocity low v anomaly in the shallow mantle which may be caused by the subduction and dehydration of the burma microplate plate or indian plate the hainan volcano is underlain by a plume like low v anomaly that extends down to at least 1000 km depth and seems to be related to the deep subductions of the burma microplate or indian plate in the west and the philippine sea plate in the east prominent low v anomalies are detected beneath the changbai longgang and xianjindao volcanoes in northeast asia which may be related to the rollback of the subducting pacific slab as well as the convective circulation process in the big mantle wedge above the stagnant slab the origin of the wudalianchi volcano in northeast china seems associated with the upwelling of asthenospheric materials caused by the subduction induced lithospheric delamination

mantle conveyor beneath the tethyan collisional belt abstract collisional belts are generated by the arrival of continental lithosphere into a subduction zone the tethyan suture from the bitlis to the himalayas is a prime example where the arabian and indian plates collided with eurasia during the cenozoic while the kinematics of this process are well established its dynamics are more uncertain india and arabia intriguingly keep advancing in spite of large collisional resisting forces and in the absence of a substantial upper mantle slab driving force at present day we perform global mantle circulation computations to test the role of deep mantle flow as a driving force for the kinematics of the tethyan collisional belt evaluating different boundary conditions and mantle density distributions as inferred from seismic tomography or slab models our results show that mantle drag exerted on the base of the lithosphere by a large scale convective conveyor belt with an active upwelling component is likely the main cause for the ongoing indentation of the indian and arabian plates into eurasia

small scale convection at the edge of the colorado plateau implications for topography magmatism and evolution of proterozoic lithosphere the colorado plateau of the southwestern united states is characterized by a bowl shaped high elevation late neogene quaternary magmatism at its edge large gradients in seismic wave velocity across its margins and relatively low lithospheric seismic wave velocities we explain these observations by edge driven convection following rehydration of colorado plateau lithosphere a rapidly emplaced cenozoic step in lithosphere thickness between the colorado plateau and adjacent extended rio grande rift and basin and range province causes small scale convection in the asthenosphere a lithospheric drip below the plateau is removing lithosphere material from the edge that is heated and metasomatized resulting in magmatism edgedriven convection also drives margin uplift giving the plateau its characteristic bowl shape the edge driven convection model shows good consistency with features resolved by seismic tomography

on modeling the thermal evolution of the oceanic upper mantle an assessment of the cooling plate model the plate cooling model for the thermal evolution of the oceanic upper mantle has been widely accepted to explain observed variations of depth to oceanic basement and conductive heat flow with the age of the seafloor several estimates of best fitting plate model parameters derived from depth heat flow and age data have been proposed but the viability of the plate model itself has not been rigorously evaluated we have used published mean depths and depths to basement at deep sea drilling project ocean drilling program dsdp odp drilling sites to test the plate cooling model based on two criteria first viable plate models must have coefficients that are consistent with the slope of the corresponding root t line because the half space or root t subsidence of young seafloor is implicit in the plate model i e the slope of the root t line can be calculated directly from the coefficients of the plate subsidence model second any viable physical model must fit the data with an acceptable degree of systematic misfit large systematic misfits indicate that the model cannot explain the observations fits of half space root t models to depth versus age data for young 80 ma seafloor indicate basal temperatures in the range 1300 to 1370 c based on the age at which the depths deviate from the root t line the minimum plate thickness that is compatible with the best fitting half space models for young seafloor is 120 km in contrast all best fitting plate models yield systematically higher temperatures 1450 to 1470 c and thinner plates 102 118 km the plate model can explain the depth to basement at dsdp odp drill sites with a satisfactory degree of systematic misfit but we find that there is no plate cooling model that can explain the variation of the mean depths derived from the dbdb5 database with age over the entire range of ages 0 to 165 ma models that best fit the entire data set have unacceptably large systematic misfits over the entire range of seafloor age whereas models that minimize the systematic misfit for young seafloor 0 to 81 ma fail for older seafloor the plate model clearly fails to explain the observations the cooling history of old seafloor is not simply an extension of the cooling history of young seafloor according to a simple plate cooling model previous studies have suggested as an alternative to the plate model that observed variations of basement depth with age are best explained by the combined effects of a normal half space cooling process and the dynamic and or thermal effects of hot spots or mantle plumes our results are entirely consistent with the half space cooling model and we find that the best reference model for normal subsidence is d t 2600 20 m 345 3 m m y t we also find that the heat flow predicted by this half space model is consistent with the most reliable average heat flow values from the pacific

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

why is the north america cordillera high hot backarcs thermal isostasy and mountain belts global mountain belts are commonly concluded to be a consequence of crustal thickening resulting from continental collision with high elevations supported by crustal roots however accumulating seismic structure data indicate that many mountain belts have no crustal root most of the north american cordillera has a 30 35 km crust in contrast to 40 45 km for the lower elevation craton and other stable areas it has been shown previously that most such mountain belts are in present or recent backarcs that are uniformly hot from thermal constraints we predict a uniform 1600 m elevation support of the cordillera by thermal expansion compared to stable areas over most of the cordillera relative to stable areas the elevations after correction for variable crustal thickness and density are in excellent agreement when subduction and shallow backarc convection stop the lithosphere may cool and the elevations of mountain belts subside over 300 m y

models of isostatic and dynamic topography geoid anomalies and their uncertainties knowledge of the dynamic topography at earth s surface caused by sublithospheric density contrasts would provide crucial constraints on models of mantle dynamics we calculate global models of this dynamic topography by subtracting from the observed topography estimates of the topography caused by assumed isostatically compensated density variations in the crust the oceanic lithosphere and the continental tectosphere we also calculate the isostatic geoid anomaly that would result from these compensated near surface density anomalies because there is controversy about the thermal structure of old oceanic lithosphere we investigate two models of the cooling of the oceanic lithosphere half space and plate and calculate the effect of each on the inferred dynamic topography peak amplitudes of dynamic topography from sublithospheric sources expanded to spherical harmonic degree 6 are approximately 1 km we estimate the uncertainties in the calculated dynamic topography and geoid by combining the errors in the data and systematic errors to obtain the spectral coefficients of the fields we use weighted least squares inversions the calculated dynamic topography arising from sublithospheric sources residual geoid and errors associated with these fields can be used in studies of mantle dynamics

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

three dimensional seismic structure beneath southwest japan the subducting philippine sea plate abstract a three dimensional seismic structure beneath southwest japan has been investigated in detail by applying an inversion method this analysis clearly reveals the present and possibly the past subduction of the philippine sea plate beneath southwest japan beneath the izu peninsula the philippine sea plate does not appear to subduct but collides against the continental asian plate here and its thickness appears thinnest at most about 30 km from the tokai to the shikoku regions the high velocity zone corresponds well to the zone of subcrustal earthquakes down to a depth of about 60 70 km indicating that the subduction of the present philippine sea plate does not reach below this depth here however a small high velocity portion appears at depths between 100 km and 150 km beneath a part of the kinki region and probably extends to the chugoku region where no corresponding seismic activities exist these high velocities might be an indication of the remnants of the past subduction of the philippine sea plate in the kyushu region the high velocity zone continues down to a depth of about 150 km or even to 200 km in the southern part corresponding to a steeply inclined seismic zone the philippine sea plate in this region appears to be thicker than in regions east of the kyushu palau ridge it was also found that there exist narrow low velocity zones intervening within the philippine sea plate where two aseismic ridges the kyushu palau and shichito iwojima ridges extend northwards and the nankai trough shows warping these low velocity zones give support to the buoyant subduction hypothesis it also appears that lateral heterogeneities existing within and around the subducting plate are closely related to the mode of rupture of large earthquakes occurring around there

melting and seismic structure beneath the northeast japan arc transportation of h2o associated with subduction of the pacific plate beneath the northeast japan arc is modeled to predict distribution of aqueous solution and melt and the consequent p wave velocity structure the observed velocity structure coincides well with the model for equilibrium transport of h2o most of h2o subducted is brought down to 150 200 km depth along the slab then the aqueous fluid generated ascends to initiate melting which explains the low velocity regions 6 reduction observed beneath the backarc rather than beneath the volcanic front if equilibrium transport of h2o occurs initiation of melting beneath the backarc with deep subduction of h2o is likely to be the case also for other subduction zones with slabs older than several tens of m y cold enough to stabilize serpentine to a great depth

lithosphere asthenosphere interaction beneath the western united states from the joint inversion of body wave traveltimes and surface wave phase velocities summary the relation between the complex geological history of the western margin of the north american plate and the processes in the mantle is still not fully documented and understood several pre usarray local seismic studies showed how the characteristics of key geological features such as the colorado plateau and the yellowstone snake river plains are linked to their deep mantle structure recent body wave models based on the deployment of the high density large aperture usarray have provided far more details on the mantle structure while surface wave tomography ballistic waves and noise correlations informs us on the shallow structure here we combine constraints from these two data sets to image and study the link between the geology of the western united states the shallow structure of the earth and the convective processes in mantle our multiphase dna10 s model provides new constraints on the extent of the archean lithosphere imaged as a large deeply rooted fast body that encompasses the stable great plains and a large portion of the northern and central rocky mountains widespread slow anomalies are found in the lower crust and upper mantle suggesting that low density rocks isostatically sustain part of the high topography of the western united states the yellowstone anomaly is imaged as a large slow body rising from the lower mantle intruding the overlying lithosphere and controlling locally the seismicity and the topography the large e w extent of the usarray used in this study allows imaging the slab graveyard a sequence of farallon fragments aligned with the currently subducting juan de fuca slab north of the mendocino triple junction the lithospheric root of the colorado plateau has apparently been weakened and partly removed through dripping the distribution of the slower regions around the colorado plateau and other rigid blocks follows closely the trend of cenozoic volcanic fields and ancient lithospheric sutures suggesting that the later exert a control on the locus of magmato tectonic activity today the dna velocity models are available for download and slicing at http dna berkeley edu

colorado plateau magmatism and uplift by warming of heterogeneous lithosphere the forces that drove rock uplift of the low relief high elevation tectonically stable colorado plateau are the subject of long standing debate while the adjacent basin and range province and rio grande rift province underwent cenozoic shortening followed by extension the plateau experienced approximately 2 km of rock uplift without significant internal deformation here we propose that warming of the thicker more iron depleted colorado plateau lithosphere over 35 40 myr following mid cenozoic removal of the farallon plate from beneath north america is the primary mechanism driving rock uplift in our model conductive re equilibration not only explains the rock uplift of the plateau but also provides a robust geodynamic interpretation of observed contrasts between the colorado plateau margins and the plateau interior in particular the model matches the encroachment of cenozoic magmatism from the margins towards the plateau interior at rates of 3 6 km myr 1 and is consistent with lower seismic velocities and more negative bouguer gravity at the margins than in the plateau interior we suggest that warming of heterogeneous lithosphere is a powerful mechanism for driving epeirogenic rock uplift of the colorado plateau and may be of general importance in plate interior settings

effects of water and melt on seismic velocities and their application to characterization of seismic reflectors the effects of a silicate melt and water on seismic velocities are compared at relatively small fluid fraction less than 20 vol on the basis of a theoretical model of composite materials to show that the two fluids are clearly characterized by the velocity ratio vp vs for a silicate melt vp vs increases with increasing fluid fraction when a significant reflection is expected vp vs becomes much larger than 2 for water vp vs decreases as the fluid fraction increases to 10 vol then it increases but it remains similar to a solid state value about 1 8 unless the amount of water exceeds 15 vol this will be a good measure to distinguish between two candidates for seismic reflectors partially molten rocks and rocks containing free water if a reflector can be treated as a thin low velocity layer the velocity ratio in it can be estimated from the frequency dependence of reflection coefficient

aseismic deep subduction of the philippine sea plate and slab window abstract we have made great efforts to collect and combine a large number of high quality data from local earthquakes and teleseismic events recorded by the dense seismic networks in both south korea and west japan this is the first time that a large number of korean and japanese seismic data sets are analyzed jointly as a result a high resolution 3 d p wave velocity model down to 700 km depth is determined which clearly shows that the philippine sea phs plate has subducted aseismically down to 460 km depth under the japan sea tsushima strait and east china sea the aseismic phs slab is visible in two areas one is under the japan sea off western honshu and the other is under east china sea off western kyushu however the aseismic phs slab is not visible between the two areas where a slab window has formed the slab window is located beneath the center of the present study region where many teleseismic rays crisscross detailed synthetic tests were conducted which indicate that both the aseismic phs slab and the slab window are robust features using the teleseismic data recorded by the japanese stations alone the aseismic phs slab and the slab window were also revealed zhao et al 2012 though the ray paths in the japanese data set crisscross less well offshore the slab window may be caused by the subducted kyushu palau ridge and kinan seamount chain where the phs slab may be segmented hot mantle upwelling is revealed in the big mantle wedge above the pacific slab under the present study region which may have facilitated the formation of the phs slab window these novel findings may shed new light on the subduction history of the phs plate and the dynamic evolution of the japan subduction zone

mantle convection and the recent evolution of the colorado plateau and the rio grande rift valley the colorado plateau contains late cretaceous marine strata that are at a mean elevation of 2 km the timing and amount of uplift since the cretaceous has generated considerable debate with the exception of a few studies topography supported by vertical stresses generated by viscous fl ow in the mantle has not been explicitly considered to contribute to the elevation of this region herein we compute the viscous fl ow beneath north america that is driven by density anomalies inferred from joint seismic geodynamic modeling we fithat the colorado plateau overlies a strong mantle upwelling that is coupled to the sinking farallon slab currently beneath the eastern united states consequently the colorado plateau is currently a focused dynamic topography high within the western u s cordillera moreover this strong upwelling impacts the base of the lithosphere at an oblique angle east of the plateau directly below the rio grande rift we attribute this fl ow as being responsible for some of the recent magmatic activity along the jemez lineament as well as contributing to the recent rifting process in the rio grande rift valley

topography and geoid due to lithospheric mass anomalies summary a model of lithospheric thickness and a recent compilation of moho depths are used to compute the earth s isostatic surface topography and associated gravity anomalies the results are strongly influenced by the uncertainties in lithospheric depth and crustal density profiles the preferred models explain most of the observed topography and are highly correlated with observed gravity anomalies for harmonic degrees larger than 10 comparisons of our residual topography with geodynamical calculations of dynamic topography based on mantle circulation are rather poor

continental scale landscape evolution a history of north american topography the generation and evolution of continental topography are fundamental geologic and geomorphic concerns in particular the history of landscape development might contain useful information about the spatiotemporal evolution of deep earth processes such as mantle convection a significant challenge is to generate observations and theoretical predictions of sufficient fidelity to enable landscape evolution to be constrained at scales of interest here we combine substantial inventories of stratigraphic and geomorphic observations with inverse and forward modeling approaches to determine how the north american landscape evolved first stratigraphic markers are used to estimate postdepositional regional uplift present day elevations of these deposits demonstrate that 2 km of long wavelength surface uplift centered on the colorado rocky mountain plateaus occurred in cenozoic times second to bridge the gaps between these measurements an inverse modeling scheme is used to calculate the smoothest spatiotemporal pattern of rock uplift rate that yields the smallest misfit between 4 161 observed and calculated longitudinal river profiles our results suggest that cenozoic regional uplift occurred in a series of stages in agreement with independent stratigraphic observations finally a landscape evolution model driven by this calculated rock uplift history is used to determine drainage patterns denudation and sedimentary flux from late cretaceous times until the present day these patterns are broadly consistent with stratigraphic and thermochronologic observations we conclude that a calibrated inverse modeling strategy can be used to reliably extract the temporal and spatial evolution of the north american landscape at geodynamically useful scales

constraints on residual topography and crustal properties in the western united states from virtual deep seismic sounding we use virtual deep seismic sounding vdss and data from 1 000 broadband seismic stations to provide high resolution estimates of crustal structure in the western cordillera of the united states us the most robust result is the geographic distribution of residual topography that is the difference between observed elevation and that expected from crustal buoyancy alone and by implication thermal or petrologic anomalies in the mantle overall residual topography of the western us cordillera varies considerably with contrasts of up to about 3 km across distances of 200 km or less high residual topography indicating large mantle effects is evident along the periphery of the colorado plateau and the surroundings of the great basin in contrast the central colorado plateau and the wyoming basin show low residual topography close to what is expected of a geologically stable lithosphere overall in regions to the east of the wasatch hinge line the eastern limit of significant extension in the north american cratonic basement patterns of high residual topography and anomalies of low seismic wave speeds in the upper mantle are similar suggestive of a common thermal origin in contrast such a similarity is absent in regions to the west of the hinge line suggesting substantial effects of petrological heterogeneities in the mantle finally joint analyses of vdss and conventional receiver functions reveal a wide range of crustal p wave speeds locally as high as 6 7 km s perhaps indicating magmatic modification of the crust

mapping of a magma reservoir beneath nikko shirane volcano in northern kanto japan from travel time and seismogram shape anomalies since ray paths of direct p and s waves are largely distorted in a magma body a seismic shadow zone is created however scattered or diffracted waves with very small amplitudes can arrive in the shadow zone because their ray paths are different from those of the direct waves they become the first arrival in the shadow zone more than 100 seismologists from many institutions in japan joined together to perform an intensified seismic study in the nikko ashio area northern kanto japan it was suggested that there is a magma body beneath mount nikko shirane an active volcano that causes a seismic shadow zone because p waves recorded by the joint observation are anomalously attenuated in cases when they propagate through a zone beneath the eastern part of the volcano we developed a new method to estimate the shape of the anomalous attenuation zone by proposing the use of a new parameter the energy ratio between directp or s waves and their coda waves to discriminate whether or not each observation station is located inside or outside the shadow zone we estimated the shape of the anomalous attenuation zone by dividing the study area into blocks with dimensions of 2 km 2 km 1 km and using the energy ratio data of p waves for local events the obtained result shows the existence of an anomalous attenuation zone in the area east of nikko shirane volcano with a diameter of about 10 km at depths greater than 3 km it was also found from plots of deep event seismograms recorded by the same observation that travel times are delayed by about 0 7 s for ray paths crossing the anomalous attenuation zone obtained by the energy ratio data this value requires the p wave velocity in the anomalous zone to be less than that of the surrounding crustal material by at least about 30 suggesting the existence of a huge magma reservoir in this area

thermochemical structure and dynamics of the african superplume 1 we present a new three dimensional 3 d model of the thermochemical structure of the african superplume region obtained from simultaneous inversion of global seismic and convection related data convection related observations include the global free air gravity field tectonic plate motions dynamic surface topography and the excess ellipticity of the core mantle boundary a 3 d image of the chemically induced density perturbations provides direct evidence that intrinsically dense material is entrained within the superplume and concentrated into a rounded structure 1000 km above the core mantle boundary the thermally induced density perturbations are greater in magnitude than the chemically induced implying overall positive buoyancy throughout the superplume the observed morphology and density signatures are consistent with a thermochemical plume that has risen from a compositionally distinct pile at the base of the mantle and may be currently deforming under the influence of its intrinsic negative chemical buoyancy citation simmons n a a m forte and s p grand 2007 thermochemical structure and dynamics of the african superplume geophys res lett 34 l02301 doi 10 1029 2006gl028009

crustal heterogeneity in the 2000 western tottori earthquake region effect of fluids from slab dehydration abstract in order to understand the generating mechanism of the western tottori earthquake m 7 3 that occurred on 6 october 2000 in southwest japan we determined high resolution 3d p and s wave velocity and poisson s ratio structures in the epicentral area we used 17 542 p and 13 831 s wave high quality arrival times from 721 tottori aftershocks and other local microearthquakes recorded by the dense high sensitivity seismic network hi net installed recently on the japan islands significant variations of up to 4 for seismic velocity and 9 for poisson s ratio are revealed in the aftershock area areas with large coseismic slips and high aftershock activity are associated with high p wave velocity v p high poisson s ratio and high electrical conductivity which may represent strong and competent parts of the fault zone containing fluids the western tottori mainshock hypocenter is located in a boundary zone where both velocity and poisson s ratio change drastically both p and s wave velocities are slower in the lower crust under the mainshock hypocenter and low frequency microearthquakes were detected within the slow anomalies around the moho discontinuity before and after the occurrence of the 2000 western tottori earthquake these results and other related evidence suggest that the strong crustal heterogeneity in the western tottori source area is associated with fluids and arc magma under nearby the daisen volcano which may have influenced the nucleation and rupture process of the western tottori earthquake the arc magma and fluids are thought to result from the dehydration reactions of the subducting philippine sea slab in this region these results suggest that the nucleation of shallow earthquakes could be controlled by a deep structure and regime and so it is important to study the deeper crust and upper mantle structure and processes for clarifying the generating mechanism of large crustal earthquakes

tottori earthquakes and daisen volcano effects of fluids slab melting and hot mantle upwelling abstract we investigate the 3 d seismic structure of source areas of the 6 october 2000 western tottori earthquake m 7 3 and the 21 october 2016 central tottori earthquake m 6 6 which occurred near the daisen volcano in sw japan the two large events took place in a high velocity zone in the upper crust whereas low velocity low v and high poisson s ratio high anomalies are revealed in the lower crust and upper mantle low frequency micro earthquakes m 0 0 2 1 occur in or around the low v and high zones which reflect upward migration of magmatic fluids from the upper mantle to the crust under the daisen volcano the nucleation of the tottori earthquakes may be affected by the ascending fluids the flat subducting philippine sea phs slab has a younger lithosphere age and so a higher temperature beneath the daisen and tottori area facilitating the phs slab melting it is also possible that a phs slab window has formed along the extinct shikoku basin spreading ridge beneath sw japan and mantle materials below the phs slab may ascend to the shallow area through the slab window these results suggest that the daisen adakite magma was affected by the phs slab melting and upwelling flow in the upper mantle above the subducting pacific slab

mapping the crustal thickness in southwest japan using moho reflected waves in this work we have tried to detect and collect later phases associated with the moho discontinuity and use them to study the lateral variations of the crustal thickness in southwest sw japan we first compute synthetic seismograms for local earthquakes taking into account the focal mechanism solutions estimated from first motion polarity data then we compare the synthetics with the observed waveforms to detect the major later phases in seismograms of shallow crustal earthquakes in sw japan taking the advantage of the high quality and great quantity of hi net waveform data now available throughout the entire japan islands we have detected 1659 moho reflected phases pmp and sms from shallow events in sw japan we estimated the crustal thickness in the study area using travel time differences between these later phases and the first p and s arrivals our results show that the moho is deep in chubu district and becomes shallower along the coastlines of the japan sea and the pacific ocean a thinner crust appears in osaka bay and awajii island where the 1995 kobe earthquake m7 2 occurred

subsidence of a volcanic basin by flexure and lower crustal flow the eastern snake river plain idaho the eastern snake river plain esrp is a linear volcanic basin interpreted by many workers to reflect late cenozoic migration of north america over the yellowstone hotspot thermal subsidence of this volcanic province with respect to yellowstone has been documented by several workers but no one has characterized subsidence with respect to the adjacent basin and range province this paper documents crustal flexure along the northwest edge of the esrp uses flexure to model the dimensions of a dense load beneath the basin and presents evidence in support of density driven subsidence and lower crustal flow away from the basin crustal flexure adjacent to the esrp is reflected by the attitudes of mesozoic fold hinges and neogene volcanic rocks fold hinges formed with a subhorizontal plunge and a trend perpendicular to the esrp but now show a southward plunge near the esrp of as much as 20 25 we present a contour map of equal fold plunges proximal to the esrp that shows flexure is roughly parallel to and extends 10 20 km north of the average edge of the esrp flexural profiles indicate the minimum amount of esrp subsidence with respect to the basin and range subsidence ranges from 4 5 to 8 5 km the structural contour map and published seismic and gravity data were used to develop and constrain flexural subsidence models these models indicate the flexed crust is very weak flexural parameter of 4 10 km interpreted to be a result of the high heat flow of the esrp assuming subsidence was induced by emplacement of a dense crustal layer beneath the esrp a midcrustal sill identified in previous seismic surveys is too wide and probably too thin to produce the measured flexure new dimensions include a thickness of 17 25 km and a half width of 40 50 km which place the edge of the sill beneath the edge of the esrp the dimensions of the esrp sill are based on isostatic compensation in the lower crust because compensation in the asthenosphere requires an unreasonable sill thickness of 30 km and because esrp seismic gravity and heat flow data support lower crustal compensation density driven lower crustal flow away from the esrp is proposed to accommodate subsidence and maintain isostatic equilibrium timing of subsidence is constrained by esrp exploratory wells where 6 6 ma rhyolites at a depth of 1 5 km indicate most subsidence occurred prior to their emplacement and by strong spatial correlations between plunge contours and quaternary volcanic rift zones two processes interpreted to contribute to the load include an extensive midcrustal mafic load emplaced at 10 ma which provided the heat source for the initial rhyolitic volcanism on the esrp and continuing localized loads from dikes and sills associated with quaternary basalts widespread 10 ma magmatism and subsidence conflicts with simple time transgressive migration of the yellowstone hotspot indicating a need for revision of the hotspot paradigm

mantle plume influence on the neogene uplift and extension of the u s western cordillera espite its highly extended and thinned crust much of the western cordillera in the united states is elevated more than 1 km above sea level therefore this region cannot be thought of as thick crust floating isostatically in a uniform mantle rather the lithospheric mantle and or the upper asthenosphere must vary in thickness or density across the region utilizing crustal thickness and density constraints we modeled the residual mass deficit that must occur in the mantle lithosphere and asthenosphere beneath the western cordillera a major hot spot broke out during a complex series of cenozoic tectonic events that included lithospheric thickening backarc extension and transition from a subduction to a transform plate boundary we suggest that many of the characteristics that make the western cordillera unique among extensional provinces can be ttributed to the mantle plume that created the yellowstone hot spot

topographic evolution of the western united states since the early miocene abstract the origin of the high topography within the western united states has been attributed to either crustal lithospheric isostasy or dynamic topography but their relative contributions remain unconstrained here we investigate this problem using gravity residual topography and geodynamic modeling we first evaluate two end member scenarios of isostatic balance crustal isostasy and lithospheric isostasy both cases lead to prominent negative mantle residual gravity within the tectonically active western u s and unrealistic crustal lithospheric density structures requiring the presence of low density mantle underneath the negative mantle residual gravity is consistent with both the estimated positive residual topography and calculated dynamic uplift due to the presence of hot asthenospheric mantle underneath geodynamic modeling further reveals that this landward migrating dynamic uplift originates from the eastward intrusion of the hot pacific mantle through tears and edges of the juan de fuca slab since middle miocene the estimated paleotopography maps by combining dynamic topography and lithosphere isostasy over the western u s are consistent with several observational constraints including episodic uplifts of the sierra nevada post mid miocene uplift of the idaho batholith the sustaining subsidence within most of the b r and the largely stable topography of central colorado plateau since 20 ma

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

western us intermountain seismicity caused by changes in upper mantle flow understanding the causes of intraplate earthquakes is challenging as it requires extending plate tectonic theory to the dynamics of continental deformation seismicity in the western united states away from the plate boundary is clustered along a meandering north south trending intermountain belt this zone coincides with a transition from thin actively deforming to thicker less tectonically active crust and lithosphere although such structural gradients have been invoked to explain seismicity localization the underlying cause of seismicity remains unclear here we show results from improved mantle flow models that reveal a relationship between seismicity and the rate change of dynamic topography that is vertical normal stress from mantle flow the associated predictive skill is greater than that of any of the other forcings we examined we suggest that active mantle flow is a major contributor to seismogenic intraplate deformation while gravitational potential energy variations have a minor role seismicity localization should occur where convective changes in vertical normal stress are modulated by lithospheric strength heterogeneities our results on deformation processes appear consistent with findings from other mobile belts and imply that mantle flow plays a significant and quantifiable part in shaping topography tectonics and seismic hazard within intraplate settings

crustal signatures of the tectonic development of the north american midcontinent abstract the stable eastern portion of the north american continent offers an excellent environment to study the tectonic development of intra continental structures the midcontinent of north america formed by the accretion of proterozoic terranes and has since experienced episodes of deformation during the subsidence of the illinois basin and uplift of the ozark plateau rifting also initiated in eastern north america but extension did not continue and arms of failed rifts extend across the region the new madrid seismic zone situated within a portion of the failed reelfoot rift represents an active zone of intraplate seismicity analyzing the structure of the crust and upper mantle within the midcontinent will therefore provide insight into the factors that lead to intraplate deformation using data from over 180 transportable array seismic stations we calculate receiver functions to investigate the crust and upper mantle of the midcontinent at close to 40 km thick the crust of the new madrid seismic zone is thinner than in the surrounding areas outside of the reelfoot rift and rough creek graben the illinois basin cannot be characterized by a single crustal structure as crust near 50 km thick in the central portion of the basin thins to between 40 and 45 km thick towards the northern and southern portions of the basin discontinuities within the crust and upper mantle are prominent in and around the new madrid seismic zone and mark locations of crustal modification and underplating comparing changes in crustal structure to the distribution of bouguer gravity anomalies the presence of positive gravity anomalies suggests that higher density crust plays a role in maintaining low surface elevations within the reelfoot rift conversely a negative gravity anomaly in an area of thinner crust within the ozark plateau supports the need for low density crustal material to influence the uplift of the plateau

a major dissolution event at the close of mis 5e in the western equatorial atlantic measurements of the caco3 content and the caco3 size index on a core from the equatorial atlantic s ceara rise reveal a pronounced dissolution event associated with the transition between mis 5e and 5d we tentatively correlate this event with the dramatic drop in atmospheric co2 content that occurred at the end of mis 5e attempts to quantify the carbonate ion concentration drop associated with this event failed because the shells in this zone had been largely destroyed by dissolution by leaving out the sonification step in our cleaning procedure we were able to rescue enough shells to weigh but they showed no dramatic weight loss we attribute this to the survival of the hardiest shells

dynamic subsidence and uplift of the colorado plateau we use inverse models of mantle convection to explore the vertical evolution of the colorado plateau by satisfying multiple constraints seismic tomography stratigraphy in the western united states and great plains and other structural and volcanic data adjacent to the plateau the model provides predictions on the continuous history of colorado plateau vertical motion since 100 ma with the arrival of the flat lying farallon slab dynamic subsidence swept from west to east over the plateau and reached a maximum ca 86 ma two stages of uplift followed the removal of the farallon slab below the plateau one in the latest cretaceous and the other in the eocene with a cumulative uplift of 1 2 km both the descent of the slab and buoyant upwellings raised the plateau to its current elevation during the oligocene a locally thick plateau lithosphere enhances the coupling to the upper mantle so that the plateau has a higher topography with sharp edges the models predict that the plateau tilted downward to the northeast before the oligocene caused by northeast trending subduction of the farallon slab and that this northeast tilting diminished and reversed to the southwest during the miocene in response to buoyant upwellings

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

the role of gravitational potential energy in active deformation in the southwestern united states in orogenic belts intrinsic buoyancy and extrinsic boundary forces act on the continental lithosphere determining the relative importance of these forces is fundamental to understanding the dynamics of continental deformation in the western united states estimates of gravitational potential energy show that given available constraints on rheology buoyancy forces are sufficient to produce the wide variety of rates and styles of observed deformation including absence of deformation this underscores the importance of gravitational potential energy for deformation in orogenic belts

slab melting as an explanation of quaternary volcanism and aseismicity in southwest japan two volcanoes on the quaternary volcanic front in southwest japan have erupted medium k andesite and dacite which are chemically similar to adakites i e high al 2 o 3 sr y low y and steep rare earth element ree patterns with no negative eu anomalies unlike other subduction related volcanoes in japan earthquake data and the extrapolated position of the philippine sea plate from its near trench position indicate that the leading edge of the plate is 75 100 km deep beneath these volcanoes melting of the plate at this depth would leave a garnet eclogite residual accounting for the y and heavy ree depletion the lack of seismic data and active volcanism in southwest japan can be attributed to the attenuated seismic response of the plate due to melting and to the atypical nature of magma generation

seismic anisotropy and mantle flow from the great basin to the great plains western united states shear wave splitting and p sks and s travel time residuals are calculated for teleseismic arrivals recorded on the colorado plateau great basin program for array seismic studies of the continental lithosphere passcal portable broadband seismic deployment and for permanent stations in the western united states little shear wave splitting is observed for broadband recordings in the northern colorado plateau the rocky mountains or the central great basin the transition between the colorado plateau and the great basin is marked by moderate shear wave splitting 1 0 s and unusually late teleseismic phase arrivals this suggests material with a higher content of mantle melt or volatiles than regions to either side splitting in the transition between the colorado plateau and great basin is part of a pattern of fast polarizations that align in a semicircle surrounding a central great basin region of null no splitting measurements away from the california plate boundary splitting to the north and south of our study region aligns roughly parallel to the absolute plate motion of the north american plate no simple spatial relation of splitting with geological and geophysical features such as mountain ranges velocity anomalies gravity magnetics or heat flow is evident in most of the western united states however splitting in the great basin is compatible with asthenospheric flow the smallest shear wave splitting delay times coincide with the eureka low in heat flow also having low s velocity at 300 km depth we suggest that the circumferential pattern of fast polarization directions ringing a central region of nulls in the great basin is caused by mantle flow by the interaction of mantle up welling and the absolute motion of the north american plate

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

flat slab subduction topography and mantle dynamics in southwestern mexico topography above subduction zones arises from the isostatic contribution of crustal and lithospheric buoyancy as well as the dynamic contribution from slab driven mantle flow we evaluate those effects in southwestern mexico where a segment of the cocos slab subducts horizontally the eastern part of the volcanic arc the trans mexican volcanic belt stands at an average elevation of 2 3 km nearly 1 3 km above the fore arc lateral changes in bulk crustal density are relatively small and seismic imaging shows that there is little variation in crustal thickness between these two regions thus the elevation difference between the arc and the fore arc should arise from differences in mantle properties we present finite element models of flat slab subduction that provide a simultaneous match to topography plate velocities and stress state in the overriding plate we find that the dynamic effects are primarily controlled by the amount of coupling at the subduction interface and in the mantle wedge the lack of slab anchoring into the lower mantle and the absence of continental mantle lithosphere with a mantle wedge and a subduction interface that are respectively 2 and 4 orders of magnitude weaker than the asthenosphere the flat slab exerts a downward pull that can explain most of the elevation difference between the fore arc and the arc we infer that lateral viscosity variations play a significant role in shaping dynamic topography in complex tectonic settings and that sublithospheric dynamics can influence the topography at wavelengths that are significantly shorter than previously recognized

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

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

an introduction to the special issue of earth and planetary science letters on usarray science abstract the usarray observatory a component of the earthscope science initiative has provided a geophysical dataset that densely samples the continental us with unprecedented scale and resolution the major scientific target of the multidisciplinary earthscope project is an understanding of the structure dynamics and evolution of the north american continent with emphasis on imaging the continental crust and lithosphere as well as illuminating dynamic processes in the deep earth this special issue of earth and planetary science letters presents a collection of papers that leverage data from the usarray observatory to provide fundamental insights into earth s structure and dynamics here we present an overview of the papers in this issue on a range of topics including the nature of crustal and mantle heterogeneity across north america the dynamics of the subcontinental mantle the assembly and preservation of continental interiors and the physics of earthquakes and faulting

tracing extinct spreading center in sw japan by helium 3 emanation abstract in the kinki district of southwestern japan anomalously high 3 he 4 he ratios high ra are observed in the frontal arc region this globally unique phenomenon is called the kinki spot these high ra values are not attributable to dehydration of the young warm subducting philippine sea slab based on the magma aging effect a recent study reported that the source melt generated below the plate may penetrate a large slab tear fissure and arrive at the shallow crust by upwelling flow to verify this hypothesis we collected 25 gas and water samples from hot springs in the northern district and measured the 3 he 4 he and 4 he 20 ne ratios of these samples the geographical distribution of high ra values revealed not a spot but a region extending parallel to the extinct spreading center in the shikoku basin more precisely the distribution is shaped like a backwards letter l suggesting that the subducted spreading center may have been a mid ocean ridge with a transform fault this hypothesis is supported by seismic tomography results and is consistent with the geometry of the subducting slab deduced from seismic isodepth contours as well as that of a young magnetic anomaly in the region thus the high 3 he emanation may trace an extinct ridge transform ridge type spreading center in a subduction zone

variations of the lithospheric strength and elastic thickness in north america we evaluate the effect of temperature variations on strength and effective elastic thickness te of the lithosphere of the north american na continent to this purpose we use two thermal models that are corrected for compositional variations and anelasticity effects in the upper mantle these thermal models are obtained from a joint inversion of gravity data and two recent seismic tomography models na07 and sl2013sv the crustal rheology was defined using nacr14 the most recent na crustal model this model specifies seismic velocities and thickness for a three layer model of the crystalline crust strength in the lithosphere and in the crust has similar distributions indicating that local geotherms play a dominant role in determining strength rather than crustal composition a pronounced contrast is present in strength between cratonic and off cratonic regions lithospheric strength in the off cratonic regions is prevalently localized within the crust and te shows low values 150 km in contrast to previous results our models indicate that phanerozoic regions located close to the edge of the cratons as the appalachians are characterized by low strength we also find that locally weak zones exist within the cratons e g beneath the intracratonic illinois basin and midcontinent rift seismic tomography models na07 and sl2013sv differ mainly in some peripheral parts of the cratons as the proterozoic canadian platform the grenville and the western part of the yavapai mazatzal province where the integrated strength for the model na07 is 10 times larger than in model sl2013sv due to a temperature difference 200 c in the uppermost mantle the differences in te between the two models are less pronounced in both models proterozoic regions reactivated by meso cenozoic tectonics e g rocky mountains and the mississippi embayment are characterized by a weak lithosphere due to the absence of the mechanically strong part of the mantle lithospheric layer intraplate earthquakes are distributed along the edges of the cratons demonstrating that tectonic stress accumulates there while the cores of the cratons remain undeformed in both models intraplate earthquakes occur in weak lithosphere 0 5 1013 pa s te 15 km or near the edges of strong cratonic blocks characterized by pronounced contrasts of strength and te

static and dynamic support of western united states topography article history accepted 7 october 2013 available online 31 october 2013 editor p shearer isostatic and dynamic models of earth s surface topography can provide important insights into the driving processes of tectonic deformation we analyze these two estimates for the tectonically active western united states using refined structural models derived from earthscope usarray for the crust use of recent moho depth measurements and crustal density anomalies inferred from passive source seismology improve isostatic models however seismically determined lithospheric thickness variations from lithosphere asthenosphere boundary lab maps and lithospheric and mantle density anomalies derived from heat flow or uppermost mantle tomography do not improve isostatic models substantially perhaps this is a consequence of compositional heterogeneity a mismatch between thermal and seismological lab and structural complexity caused by smaller scale dynamics the remaining non isostatic dynamic component of topography is large topography anomalies include negative residuals likely due to active subduction of the juan de fuca plate and perhaps remnants of formerly active convergence further south along the margin our finding of broad scale positive residual topography in the basin and range substantiates previous results implying the presence of anomalous buoyancy there which we cannot fully explain the colorado plateau does not appear dynamically anomalous at present except at its edges many of the residual topography features are consistent with predictions from mantle flow computations this suggests a convective origin and important interactions between vigorous upper mantle convection and intraplate deformation

isostasy flexure and dynamic topography abstract a fundamental scientific question is what controls the earth s topography although the theoretical principles of isostasy flexure and dynamic topography are widely discussed the parameters needed to apply these principles are frequently not available isostatic factors controlling lithospheric buoyancy are frequently uncertain and non isostatic factors such as lithospheric bending towards subduction zones and dynamic topography are hard to distinguish the question discussed here is whether a set of simple rules that relate topography to lithospheric structure in various tectonic environments can be deduced in a way that missing parameters can be approximated or does each area behave differently making generalizations problematic we contribute to this issue analyzing the asia africa arabia europe domain following a top down strategy we compile a new crustal thickness map and remove the contribution of the crust from the observed elevation then the challenge is to interpret the residual topography in terms of mantle lithosphere buoyancy and dynamics based on systematic relationships between tectonic environments and factors controlling topography we argue that crustal buoyancy and mantle lithospheric density can be approximated from available geological data and that regions near mantle upwelling or downwelling are easily identified by their extreme residual topography yet even for other areas calculating lithospheric thickness from residual topography is problematic because distinguishing variations in mantle lithosphere thickness from sub lithospheric dynamics is difficult fortunately the area studied here provides an opportunity to examine this issue based on the conjunction between the afar plume and the mid ocean ridge in the nearby gulf of aden and southern red sea we constrain the maximal amplitude of dynamic topography to 1 km this estimate is based on a narrow definition of dynamic topography that only includes sub lithospheric processes and using mid ocean ridges as reference where mantle lithosphere buoyancy is zero