gravitational potential energy and regional stress and strain rate fields for continental plateaus examples from the central andes and colorado plateau abstract the commonly observed extension in areas of elevated and thickened crust is an expected consequence of having excess gravitational potential energy gpe compared to the low gpe of the surrounding crust while this conceptual model is well founded it is less clear how well gpe related stress orientations compare quantitatively to observed stress and strain rate orientations and what any inconsistency tells us about the presence of other competing forces we estimate the gpe distribution for the central andes and the greater colorado plateau area using topography and crustal thickness variations respectively and compare the related stress fields with the world stress map as well as with a geodetic strain rate field for the colorado plateau only in both areas deviatoric stresses associated with gpe variations alone cannot fully account for the observed deformation rate field for the central andes only a combination of deviatoric stresses associated with gpe and relative plate motions can account for the near n s tensional stress observed in the peruvian andes and the margin normal compressional stress along the eastern cordillera and sub andean fold and thrust belt the observed deformation field around the colorado plateau shows e w extension largely inconsistent with the deviatoric stresses associated with gpe variations except for the area east of the rio grande rift the ne sw oriented stress observed on the southwestern colorado plateau is consistent with the orientations of tensional deviatoric stresses associated with gpe variations we argue that this consistency could be haphazard stress observations may not reflect the current state of stress due to inherited structure or could result from the relative high strength of colorado plateau that allows for regional gpe variations and possibly basal shear to be more significant forces than far field plate interactions for the central andes and colorado plateau stresses associated with gpe variations have a strong influence on the total stress field and can thus be used to calibrate the overall level of deviatoric stress acting within the lithosphere

the crustal structure of the northwestern basin and range province nevada from wide angle seismic data i present a model of lithospheric p wave velocity structure in the northwestern basin and range province nevada based on wide angle seismic data collected in 1986 under the auspices of the program for array seismic studies of the continental lithosphere passcal the lithosphere in the basin and range consists of 1 2 to 3 km thick low velocity 2 5 4 0 km s sedimentary basins and high velocity 4 5 km s ranges overlying a basement of velocity 5 5 6 1 km s 2 a mid crust 6 1 6 2 km s which consists of rocks of probable granitic to granodioritic composition in greenschist to amphibolite facies and which includes two local low velocity zones 5 5 5 7 km s 3 a laterally varying lower crust which consists of rocks of probable gabbroic composition in upper amphibolite to granulite fades and which comprises layers of 6 6 and 7 0 7 2 km s average velocity 4 a complex crust mantle transition best modeled as a velocity gradient zone 1 2 km thick and 5 an upper mantle with velocities of 7 7 7 9 km s which includes a local low velocity zone 7 5 km s crustal thickness in the region varies laterally from 28 to 37 km significantly thicker than several previous estimates the upper mantle velocities and velocity gradients differ slightly on the two perpendicular profiles suggesting possible azimuthal anisotropy a wedge shaped eastward thickening lower crust of high seismic velocity 7 0 7 2 km s on the eastern pan of the survey may represent the westward edge of rift stage precambrien continental crust or alternatively mafic material underplated and intruded into the crust during cenozoic extension comparison of the velocity model with coincident seismic reflection data shows that the reflection and refraction mohos are coincident across the entire profile thus indicating that deep reflections 4 10 sec two way travel time seen on cocorp and passcal piggyback reflection profiles originate entirely within the crust the top of the reflective zone however lies consistently above the mid crustal increase in average velocity indicating that the reflectivity is not restricted to the mafic part of the crust on passcal piggyback profiles in fact the middle silicic crust appears more reflective than the lower mafic crust this can be explained as the result of either mafic sill like intrusions or ductile strain fabrics both of these processes are likely to have accompanied cenozoic extension in the region

basin and range crustal and upper mantle structure northwest to central nevada we present an interpretation of the crustal and uppermost mantle structure of the basin and range of northwestern nevada based on seismic refraction wide angle reflection near vertical reflection and gravity data in comparison to most previous estimates we find that the crust is somewhat thicker 32 36 km versus 22 30 km and the uppermost mantle velocity is somewhat higher 8 0 km s versus 7 3 7 9 km s along our transects the crust is thinnest 32 km in the carson sink buena vista valley region and increases by 2 4 km to the west and east respectively there is considerable complexity throughout the crust where velocities range from of 2 5 km s at the surface to 7 4 km s in the lowermost crust variations in velocity and structure of the upper crustal layers reveal apparent basement velocity depressions areas of lower velocities extending up to 10 km in depth that underlie some surface ranges as well as the basins the middle crust rises from about 20 km beneath central nevada to within 12 km of the surface beneath the area of thinnest crust and is characterized by a modest 0 1 km s change in velocity and low velocity gradients these midcrustal layers mark the onset of high crustal reflectivity and the apparent limiting depth to which basin and range faults can be traced in near vertical reflection profiles suggesting that these midcrustal layers represent the transition between the brittle and ductile zones of the crust the lower crust is more structurally complex with layers thickening and thinning in a systematic manner with the upper crustal layers generally where there are velocity depressions in the upper crust the lower crust is thickest and shallowest the geometry of these lower crustal layers derived from refraction modeling coincides with changes in the crustal reflectivity determined from the consortium of continental reflection profiling reflection data the lower crustal layer is unusually high in velocity 7 4 km s and is likely the layer identified as mantle in some previous studies we do not identify the 7 4 km s layer as mantle because 1 there is an underlying layer with a velocity 8 0 km s that is more consistent with the worldwide average velocity for the upper mantle and 2 the 7 4 km s layer does not correspond to the reflection moho gravity modeling and comparison to existing seismic models show a general consensus in many aspects with respect to crustal structure this new model forms the basis for speculation on some of the processes associated with rifting of the basin and range province one such process lithospheric magmatism is inferred from the strong attenuation of transmitted seismic waves which occurs at the same interface at which high amplitude bright spot reflections originate unlike previous models the overall structure and velocity of the crust and uppermost mantle of our new model are similar to other regions worldwide which have undergone high degrees of extension

methane at the sediment water transition in black sea sediments abstract we present high resolution profiles for the methane concentration and the carbon isotope composition of methane from surface sediments and from the sediment water transition in the black sea at shallow water sites methane migrates from the sediment into the water column and the magnitude of this upward migrating flux depends on the depth of the sulfate methane transition smt in the sediment the isotope data reveal that the sediments at shallow water sites are a source for methane depleted in 13 c relative to the isotope composition of methane in the water column at deep water sites the methane concentration first decreases with depth in the sediment to reach lowest values at the unit i to unit ii transition below this transition the concentration increases again numerical modeling of methane concentration and isotope data shows that high methane oxidation rates occur in the surface sediment layer indicating that the removal of methane in the surface sediments is not related to the anaerobic oxidation of methane coupled to sulfate reduction that occurs a few meters deep in the sediment at the smt instead near surface methane consumption in the euxinic black sea sediments appears to be related to lithological stratification furthermore a map of the diffusive methane fluxes in the black sea surface sediments indicates that approximately half of the black sea seafloor acts as a sink for methane and thus limits the flux of methane to the atmosphere

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

small scale heterogeneity and large scale velocity structure of the continental crust since conrad 1925 first identified a midcrustal refraction event in central europe seismologists have developed layered models of the continental crust with velocity steps at various levels modern analysis of crustal refraction data including automated inversion relies on ray theoretical techniques or wave theory for plane layered media which generally parameterize the earth as a series of grossly horizontal layers the bias toward layered models is in part based on physical and geologic intuition but it is to a large measure a result of available interpretation methods for example layer based reflectivity and ray tracing methods in contrast reflection seismology has produced a picture of a highly heterogeneous crust with wavelength scale variations in velocity existing at different levels of the crust in different tectonic regimes finite difference modeling shows that zones of small amplitude random velocity fluctuations not only produce short discontinuous reflection segments as observed on deep seismic reflection data but also produce strong wide angle reflections some of the events identified as crustal wide angle reflections in field data may in fact be the result of scattering from wavelength scale heterogeneities and need not result from reflection from a first or second order velocity discontinuity significant changes in the large scale velocity structure associated with zones of random velocity fluctuations produce relatively small changes in the dynamic properties of the backscattered wave field this observation is in disagreement with the common credo that the seismic refraction method is primarily sensitive to changes in the large scale velocity structure as a consequence the ray theoretical interpretation of such wide angle reflections as first order discontinuities leads to crustal velocity structures that are nonunique and may be erroneous in detail while correctly predicting average crustal velocity and thickness we are not suggesting that crustal velocity does not change with depth on the large scale rather we are proposing that crustal seismic events observed at intermediate and large offsets may be the result of scattering from wavelength scale velocity fabric rather than specular reflection or refraction from first order discontinuities

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

raising the colorado plateau shallow marine rocks exposed on the 2 km high 45 km thick colorado plateau in the western united states indicate that it was near sea level during much of the phanerozoic isostatic calculations however illuminate the difficulty in maintaining a 45 km thick crust at or near sea level we propose that an isostatically balanced 30 km thick proto colorado plateau crust was thickened during the late cretaceous to early tertiary by intracrustal flow out of an overthickened sevier orogenic hinterland this plateau would have been supported by a thick 70 km crustal root which is proposed to have been the source region for hot and weak mid crustal material that flowed eastward from the plateau toward the low elevation proto colorado plateau

a stochastic view of lower crustal fabric based on evidence from the ivrea zone despite its complicated history the ivrea zone is considered to be a representative surface exposure of extended continental crust we have digitized two standard 1 25 000 geological maps from this area and evaluated their structural statistics because of the subvertical orientation of the ivrea zone these maps can be considered as small scale cross sections through the lower continental crust the autocorrelation functions of the digitized lithologies measured from these maps show a clear self similar or fractal rather than a gaussian or deterministic trend we found that an anisotropic von karman correlation function with an aspect ratio around 4 and a hurst number of 0 3 corresponding to a fractal dimension of 2 7 matches the observed data our results represent an explicit confirmation of previous indirect evidence for the fractal nature of lithospheric heterogeneities and provide the means to construct realistic crustal scale seismic models of ivrea type lower continental crust

the dynamics of western north america stress magnitudes and the relative role of gravitational potential energy plate interaction at the boundary and basal tractions summary we investigate the forces involved in driving long term large scale continental deformation in western north america and quantify the vertically averaged deviatoric stress field arising from internal buoyancy forces and the accommodation of relative plate motions in addition we investigate the ability of regional models to resolve the level of tractions acting at the base of the lithosphere we directly solve force balance equations for vertically averaged deviatoric stresses associated with differences in values of 1 lithospheric thickness times the gravitational potential energy per unit area gpe the gpe values are inferred using both the etopo5 topographic data set and the crust2 0 crustal thickness model deviatoric stresses associated with basal tractions are calculated globally with inputs determined from an isoviscous upper mantle 10 21 pa s 3 d large scale convection model in which mantle density variations were inferred from tomographic data and the history of subduction in a 211parameter iterative inversion we then solve for a stress field boundary condition by fitting stress field indicators i e the directions and relative magnitudes of the principal axes of kinematic strain rates magnitudes of the total vertically averaged deviatoric stress field sum of gpe solution with the boundary condition solution range from 5 to 10 mpa within a 100 km thick lithosphere these magnitudes are calibrated by the gpe differences along with the spatial variation in deformation style there is a trade off between the scaling of the basal traction deviatoric stress field and the boundary condition solution however the combined boundary conditions plus basal traction solution is robust in both magnitude and style and when added to the contribution from gpe differences provides a global minimum of misfit between the total deviatoric stress solution and the stress field indicators gpe variations account for 50 per cent of the deviatoric stress magnitudes driving deformation while boundary condition stresses account for the remaining 50 per cent of deviatoric stress magnitude by comparing possible end member strength profiles with our vertically averaged deviatoric stresses we infer that the bulk of the strength within the lithosphere in western north america lies within the brittle seismogenic layer

attenuation of lg in the western us using the usarray 1 the unprecedented breadth and density of the usarray allows us to resolve lateral variations of 1 hz lg attenuation to 0 5 degrees over much of the western united states we collected over 31 000 lg amplitudes from 547 events and 408 stations of the initial usarray footprint two dimensional tomographic inversion yielded q ranging from 60 to 550 with a variance reduction of 47 relative to the best fit constant q model q 155 q is low over most of the region with lowest values associated with recent volcanic activity and active shear zones high q is found in older stable crust notably the colorado and columbia plateaus while smaller islands of high q are associated with batholiths including the sierra nevada peninsular and bitterroot ranges the western mojave and okanogan highlands as well as other areas of competent crust such as the harcuvar range and two topographically bounded blocks in the basin and range

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

origins of topography in the western u s mapping crustal and upper mantle density variations using a uniform seismic velocity model to investigate the physical basis for support of topography in the western u s we construct a subcontinent scale 3 d density model using 1000 estimated crustal thicknesses and s velocity profiles to 150 km depth at each of 947 seismic stations crustal temperature and composition are considered but we assume that mantle velocity variations are thermal in origin from these densities we calculate crustal and mantle topographic contributions typical 2 uncertainty of topography is 500 m and elevations in 84 of the region are reproduced within error remaining deviations from observed elevations are attributed to melt variations in crustal quartz content and dynamic topography compositional variations in the mantle while plausible are not necessary to reproduce topography support for western u s topography is heterogeneous with each province having a unique combination of mechanisms topography due to mantle buoyancy is nearly constant within 250 m across the cordillera relief there 2 km results from variations in crustal chemistry and thickness cold mantle provides 1 5 km of ballast to the thick crust of the great plains and wyoming craton crustal temperature variations and dynamic pressures have smaller magnitude and or more localized impacts positive gravitational potential energy gpe anomalies 2 1012n m calculated from our model promote extension in the northern basin and range and near the sierra nevada negative gpe anomalies 3 1012n m along the western north american margin and yakima fold and thrust belt add compressive stresses stresses derived from lithospheric density variations may strongly modulate tectonic stresses in the western u s continental interior

a tectonic redefinition of the southern rocky mountains abstract the southern rocky mountains of the western united states physiographically defined and described by n m fenneman nearly 60 yrs ago are tectonically redefined and extended they are shown to constitute the crestal range of a mammoth continental arch like feature here named the alvarado ridge its axis trends south from casper wyoming at least as far south as el paso texas beyond which the ridge begins to lose morphological identity maximum elevations along the crest of the ridge exceed 4 2 km the summit ranges are bordered on either side by gently sloping rises that extend outward for at least 1300 km falling to elevations of less than 400 m modest rift structures along the ridge axis continue beyond the southern terminus of the mountains before playing out a major sediment filled axial graben exists over the southern two thirds of the ridge but equivalent parts of it farther north were stripped of their neogene fill by erosion in the headwaters areas of the colorado and north platte rivers the maximum elevation of earlier laramide mountains in this area has been estimated to have been no more than 2 km in colorado half that of the present range related topography was nearly obliterated by erosion prior to late eocene time lateral stream planation produced a southeast sloping major late eocene erosion surface across the region that had what was probably an isostatically adjusted average maximum elevation of less than 900 m present day elevations and relief on the ridge crest are the result of steep crestal normal faulting pronounced block uplift and regional arching with extensional strain limited to an axial corridor less than 200 km wide this episode of mountain building began in middle miocene time 17 to 12 ma culminating in latest miocene and early pliocene time between 7 and 4 ma ago debris from the newly elevated range the southern rocky mountains sensu lato was shed along the full length of the neogene alvarado ridge down parts of both rises as well as into its medial graben the uplifted and tilted east rise the great plains ramp has an extensive aggradational cover that spread eastward to distances of 700 km and more in middle miocene to earliest pliocene time its fluviatile lacustrine and eolian sediments are undergoing dissection today but a sufficient portion remains to define the unmodified profile of the east rise as well as the original length of the ridge it is coincidentally similar in configuration and scale to the west rise of the mid atlantic ridge the topography of the crestal mountain ranges on both structures is also remarkably similar the southern rocky mountains are as high as the alps but are not the product of lithospheric compression and crustal thickening rather they reflect profound epeirogenic uplift driven by lithospheric thinning and accompanied by extensional strain and differential vertical jostling along the crest of the ridge

upper mantle seismic wave velocity effects of realistic partial melt geometries we investigate seismic wave velocity reduction resulting from the presence of partial melt in the upper mantle the amount of shear and bulk modulus reduction produced by the presence of a connected network of realistically shaped and naturally organized melt inclusions is found using finite element calculations the geometries of the inclusions are taken directly from laboratory experiments of mantle melting with finite element meshes constructed to conform to these shapes the shear and bulk moduli of the composite material are found for both the unrelaxed isolated inclusions and relaxed pressure equalized inclusions cases by assigning appropriate material properties to the fluid modulus reduction from deformation simulations of a solid containing realistically shaped and ellipse shaped melt inclusions quantify the effect of melt pocket cuspateness and melt pocket organization on seismic velocity reduction the three dimensional response is estimated from two dimensional distributions of the melt phase by determining the mode ii and mode iii components of elastic modulus reduction separately and summing their effects in general cuspate and naturally organized melt inclusions cause greater velocity reduction it is shown that v and vs reduction per percent partial melt are at least 3 6 and 7 9 respectively even higher values for velocity reduction are possible above 1 melt fraction if melt exists only in tubules below 1 melt fraction the lower more conservative values of velocity reduction are 070 greater for vp and 84 greater for vs than the analytically determined values for ellipsoidal inclusions somewhat greater effects are possible if nonrandom organization of melt occurs on scales greater than our model

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

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

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

intra oceanic subduction shaped the assembly of cordilleran north america the western quarter of north america consists of accreted terranes crustal blocks added over the past 200 million years but the reason for this is unclear the widely accepted explanation posits that the oceanic farallon plate acted as a conveyor belt sweeping terranes into the continental margin while subducting under it here we show that this hypothesis which fails to explain many terrane complexities is also inconsistent with new tomographic images of lower mantle slabs and with their locations relative to plate reconstructions we offer a reinterpretation of north american palaeogeography and test it quantitatively collision events are clearly recorded by slab geometry and can be time calibrated and reconciled with plate reconstructions and surface geology the seas west of cretaceous north america must have resembled today s western pacific strung with island arcs all proto pacific plates initially subducted into almost stationary intra oceanic trenches and accumulated below as massive vertical slab walls above the slabs long lived volcanic archipelagos and subduction complexes grew crustal accretion occurred when north america overrode the archipelagos causing major episodes of cordilleran mountain building a new explanation for the origin of the accreted terranes that form the mountainous cordillera of western north america is proposed and tested stationary intra oceanic subduction deposited massive slab walls in the mantle and grew volcanic archipelagos at the surface which were overridden by and accreted to north america during cretaceous times the generally accepted account of how the geology of western north america developed involves the oceanic farallon plate acting as a conveyor belt sweeping crustal terranes onto the continental margin as it moves beneath the north american plate this model fails to explain many aspects of this accreted terrane and here karin sigloch and mitchell mihalynuk present an alternative model based on new tomographic images of lower mantle slabs and modelling of deep mantle structure in the region they propose that during the cretaceous period there were a series of island arcs west of north america beneath which proto pacific plates subducted into almost stationary trenches long lived volcanic archipelagos and subduction complexes grew above the slabs and then accreted onto north america as it overrode the archipelagos causing episodes of cordilleran mountain building

through thick and thin a new model for the colorado plateau from seismic refraction data from pacific to arizona crustal experiment we have constructed a model for the crust and uppermost mantle beneath the colorado plateau by combining newly acquired seismic refraction wide angle reflection data from the 1989 pacific to arizona crustal experiment pace cross profile with data collected by roller 1965 a combination of forward modeling methods was used to model travel times and relative amplitudes of crustal and upper mantle phases the salient features of the model for the colorado plateau hereafter referred to as the plateau are 1 an upper crust having an average velocity of 6 1 km s with a low vertical velocity gradient 2 midcrustal discontinuities at depths of 30 to 38 km beneath the plateau 3 a lower crust that varies in thickness 5 to 16 km and has an average velocity of 6 8 km s 4 a transitional lower crust upper mantle boundary and 5 a crustal thickness of 45 3 km a comparison of our new model for the colorado plateau with recent models for the basin and range shows that the upper crusts of the two provinces are similar but that the lower crust of the plateau is thicker particularly in the plateau s central region these observations are consistent with the idea that the basin and range could have formed from crust like that of the colorado plateau

western united states extension how the west was widened abstract cenozoic extension in the western united states presents a complex interrelation of extension volcanism and plate boundary tectonics that defeats simple notions of active or passive rifting forces driving extension can originate at plate boundaries through basal traction basal normal forces or from buoyancy forces internal to the crust and lithospheric mantle the latter two are most responsible for driving extension where it is observed in the basin and range the complex evolution of the northern basin and range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust in contrast crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 ma to drive extension in the southern basin and range the central basin and range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western united states

three dimensional seismic model of the sierra nevada arc california and its implications for crustal and upper mantle composition a three dimensional p wave velocity model of south central california from the coast ranges to the sierra nevada shows that the crust under most of the southern sierra nevada batholith has seismic velocities 5 9 6 3 km s below the continental average the crust is not much thicker on average about 35 km than in the adjacent great valley and basin and range province apart from a small northward thickening crustal root under the western sierra nevada that reaches a depth of 42 km crustal velocities above the continental average are observed beneath much of the great valley due to a high velocity body underlying the sedimentary basin and the foothills metamorphic belt 6 4 7 0 km s upper mantle velocities are generally low 7 8 km s but span a wide range 7 4 8 2 km s we display the velocity model in several cross sections and maps of moho depth and average crustal velocity the measured velocities in the upper and mid crust of the sierra nevada batholith are in good agreement with laboratory measurements on sierra nevada tonalites after corrections for density and temperature peridotite xenoliths from the eastern sierra nevada suggest strong upper mantle anisotropy which could explain some of the velocity heterogeneity in the sierra nevada mantle by the time cretaceous subduction related magmatism ceased the sierra nevada arc must have had a thick mafic lower crust yet a principal result of our work is that today the batholith has a crust of mainly felsic composition throughout a subcrustal layer with velocities below normal pn velocities 7 6 km s may indicate the presence of lower crustal material in eclogite facies

thermal evolution of the sierra nevada tectonic implications of new heat flow data eight new heat flow measurements in the southern sierra nevada constrain models of thermal evolution and lithospheric structure low reduced heat flows 18 to 21 mw m in the southwest sierra nevada are consistent with previous results from the northwestern and central sierra nevada and extend the known region of linear heat flow heat production correlation an additional 150 km to the south isostatic residual gravity and measured rock densities are also linearly correlated in the central sierra nevada suggesting a general association between the upper crustal distribution of density and heat production the linear residual gravity density relation implies that isostatic residual gravity anomalies have upper crustal sources and therefore is evidence against a flexural model of cenozoic sierra nevada uplift new reduced heat flows measured in the southeast sierra nevada are relatively high 32 to 57 mw m and correlate spatially with a region of high seismicity that includes extensional earthquake swarms this correlation supports the view that basin and range extensional tectonics and associated magmatic processes are encroaching on the eastern sierra nevada in the stable southwest sierra nevada as in the central and northwestern sierra nevada the persistence of low reduced heat flow at the surface is consistent with a thermal origin for the cenozoic uplift of the sierra nevada provided the conductive lithosphere is at least 60 to 90 km thick simple order of magnitude calculations show that thermal uplift models combining mechanisms such as advective warming and thinning of the mantle lithosphere advection of basaltic magma and heat into the crust simple thermal expansion and eclogite to basalt phase conversion can account for the timing and amount of cenozoic sierra nevada uplift thermal models do not need the assumptions of unusual lithospheric strength and crustal buoyancy required by mechanical models for cenozoic uplift driven by a mesozoic crustal root

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

mantle avalanches and the dynamic topography of continents abstract the dynamic topography of continents associated with mantle avalanche events are discussed our flow simulations involve two dimensional axisymmetric multi phase mantle models which incorporate compressibility and a realistic factor of 40 increase in viscosity with depth the surface kinematic boundary conditions involve a mix of free and rigid i e no slip constraints with the latter intended to mimic the influence of a spherical continental cap we find that avalanche events support regional vertical deflections of up to 2 5 km the horizontal length scale of the dynamic topography is 2500 km and non negligible deflections can persist on the order of 100 200 ma we also find that convergence rates in regions surrounding the continents are variable but can reach rates of the order of 3 cm a finally we speculate that an avalanche event may have been responsible for the development of the large scale intracontinental basin system in the western interior of north america from the early devonian to late carboniferous 1

the thermal blanketing effect of sediments on the rate and amount of subsidence in sedimentary basins formed by extension abstract the thermal blanketing effect of sediments with finite thermal conductivity on the rate and amount of subsidence in rift basins is presented in contrast with mckenzie s 1978 model which assumed infinite thermal conductivity of the sedimentary covers the post rift cooling of the lithosphere is shown to be very slow owing to the blanketing effect even 200 m y after extension ceases there is still an enormous amount of residual heat in the lithosphere with moderate thermal conductivity of the sediments k s 2 0 w mk the maximum discrepancy in subsidence predicted by the two models i e the present model and that of mckenzie are 2 km 2 6 km and 3 2 km for thinning factors of 1 5 2 and 3 respectively subsidence is moderately sensitive to k s when k s is greater than 2 0 w mk becoming more sensitive to k s when k s is less than 2 0 w mk the presence of mudstone would further slow down the cooling the thermal conductivity of the sediment is shown to govern the temperature gradient of rift basins although it has little influence on heat flow the heat production of the upper crust and sediment comprises an important component of heat flow unless heat absorption in rift basins is unusually high subsidence is insensitive to the heat production of sediments when the thinning factor is less than 2 the mean temperature gradient of rift basins is shown to decrease quickly over a few million years and as a result such basins are not favourable as regards hydrocarbon maturity the central graben of the north sea is chosen to test the model the model satisfactorily predicts the subsidence history and thermal evolution with the base zechstein as the basement and the cenozoic subsidence as the thermal subsidence the best fit value of the mean thermal conductivity of the sediments in the graben is 1 67 w mk and the thinning factor is about 2

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

seismic evidence for widespread western us deep crustal deformation caused by extension morgan moschetti and colleagues use ambient noise tomography data from the western united states to show that strong radial seismic anisotropy exists in the deep crust confined mainly to the geological provinces that have undergone significant extension during the cenozoic era since about 65 million years ago the coincidence of crustal radial anisotropy with these extensional provinces supports the hypothesis that the deep crust within these regions has undergone widespread and relatively uniform strain in response to crustal thinning and extension here a method of seismic wave imaging known as ambient noise tomography is used to generate high resolution images of seismic wave speeds in the crust and uppermost mantle the observations reveal strong and uniform anisotropy where waves travel through rock at different speeds depending on direction in the deep crust in areas of the western united states that have undergone significant extension during the past 65 million years laboratory experiments have established that many of the materials comprising the earth are strongly anisotropic in terms of seismic wave speeds1 observations of azimuthal2 3 and radial4 5 anisotropy in the upper mantle are attributed to the lattice preferred orientation of olivine caused by the shear strains associated with deformation and provide some of the most direct evidence for deformation and flow within the earth s interior although observations of crustal radial anisotropy would improve our understanding of crustal deformation and flow patterns resulting from tectonic processes large scale observations have been limited to regions of particularly thick crust6 here we show that observations from ambient noise tomography in the western united states reveal strong deep middle to lower crustal radial anisotropy that is confined mainly to the geological provinces that have undergone significant extension during the cenozoic era since 65 myr ago 7 8 the coincidence of crustal radial anisotropy with the extensional provinces of the western united states suggests that the radial anisotropy results from the lattice preferred orientation of anisotropic crustal minerals caused by extensional deformation these observations also provide support for the hypothesis that the deep crust within these regions has undergone widespread and relatively uniform strain in response to crustal thinning and extension9 10 11

dynamics of the pacific north american plate boundary in the western united states the vertically averaged deviatoric stress tensor field within the western united states was determined with topographic data geoid data recent global positioning system observations and strain rate magnitudes and styles from quaternary faults gravitational potential energy differences control the large fault normal compression on the california coast deformation in the basin and range is driven in part by gravitational potential energy differences but extension directions there are modified by plate interaction stresses the california shear zone has relatively low vertically averaged viscosity of about 1021 pascal seconds whereas the basin and range has a higher vertically averaged viscosity of 1022pascal seconds

isotopic evidence for preservation of cordilleran lithospheric mantle during the sevier laramide orogeny western united states it has been inferred that sevier laramide flat subduction imparted a large basal shear traction force to the overriding north american plate resulting in laramide rocky mountain foreland deformation and significant thinning of cordilleran lithosphere additional inferred consequences include regional refrigeration of cordilleran crust and suppression of synorogenic extensional collapse nd isotopic data from cenozoic mafic volcanic rocks indicate that normal thickness lithospheric mantle was preserved beneath the cordilleran craton from precambrian to late cenozoic time therefore cordilleran lithosphere was not thinned significantly and extensional collapse of orogenically thickened crust was not suppressed by refrigeration analogy with the south american cordillera suggests that flat subduction does not apply significant shear stress to the base of overriding plate lithosphere beyond the fore arc region and therefore does not mechanically thin overriding plate lithosphere except in the fore arc region the principal result of flat subduction was enhanced mechanical coupling between the underriding and overriding plates along the fore arc region this increased the horizontal end load due to relative plate convergence placed on the north american plate horizontal end load stress was transmitted laterally across both 1 a hinterland that had attained maximum crustal thickness and 2 the rigid colorado plateau into the laramide rocky mountain foreland early tertiary waning of plate convergence rates then allowed radial extensional collapse of the southern cordilleran region to drive the colorado plateau block northward resulting in late laramide rocky mountain foreland deformation

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

diffuse pacific north american plate boundary 1000 km of dextral shear inferred from modeling geodetic data geodetic measurements tell us that the eastern part of the basin and range province expands in an east west direction relative to stable north america whereas the western part of the province moves to the northwest we develop three dimensional finite element representations of the western united states lithosphere in an effort to understand the global positioning system gps signal the models are constrained by known bounding block velocities and topography and basin and range province deformation is represented by simple plastic thermal creep rheology we show that active basin and range spreading by gravity collapse is expected to have a strong southward component that does not match the gps signal we can reconcile the gravitational component of displacement with observed velocity vectors if the pacific plate applies northwest directed shear stress to the basin and range via the sierra nevada block this effect reaches at least 1000 km east of the san andreas fault in our models

relation of flat subduction to magmatism and deformation in the western united states flat subduction of the farallon plate beneath the western united states during the laramide orogeny was caused by the combined effects of oceanic plateau subduction and unusually great suction in the mantle wedge the latter of which was caused by the shallowing slab approaching the north american craton root once in contact with basal north america the slab cooled and hydrated the lithosphere upon removal asthenospheric contact with lithosphere resulted in magma production that was especially intense where the basal lithosphere was fertile in what now is the basin and range province and this heating weakened the lithosphere and made it convectively unstable small scale convection has since affected many areas with slab sinking and unloading the western united states elevated into a broad plateau and the weak portion gravitationally collapsed with development of a transform plate boundary the western part of the weak zone became entrained with the pacifi c plate and deformation there is dominated by shear 2 humphreys mwr204 04 1st pages e surface heat flow p wave velocity d seismic velocity at 100 km background s wave velocity 2 100 175 km c s wave velocity

formation of the rocky mountains western united states a continuum computer model one hypothesis for the information of the rocky mountain structures in late cretaceous through eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the north american lithosphere the horizontal components of the motion of this plate are known from paleomagnetism and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism new techniques of finite element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the north american plate a model that has a realistic temperature dependent rheology and a simple plane layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present successful prediction of this model include i the location amount and direction of horizontal shortening that has been inferred from laramide structures ii massive transport of lower crust from southwest to northeast iii the location and timing of the subsequent extension in metamorphic core complexes and the rio grande rift and iv the total area eventually involved in basin and range style extension in a broad sense this model has predicted the belt of laramide structures the transport of crust from the coastal region to the continental interior the subsequent extension in metamorphic core complexes and the rio grande rift and the geographic region of late tertiary basin and range extension its principal defects are that i many events are predicted about 5 million to 10 million years too late and ii the wave of crustal thickening does not travel far enough to the east reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects the correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the laramide orogeny is probably correct the rocky mountain thrust and reverse faults formed in an environment of east west to northeast southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the north american crust nonuniform crustal thickening by simple shear transport also caused relative uplifts therefore this model is consistent with both of the range forming mechanisms that have been inferred 1 a new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional basin and range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away

why is it downhill from tonopah to las vegas a case for mantle plume support of the high northern basin and range a major abrupt southward decline in elevation 800 m with an accompanying increase in bouguer gravity anomaly 80 mgal crosses a seismically active region of southern nevada at about 37 n the steepness of the gravity gradient requires significant crustal density contrast some of which is probably caused by the plutonic roots of voluminous tertiary volcanism to the north but the amplitude of the anomaly requires additional contrast within the mantle the topographic step probably developed in mid miocene time coincident with the arrival of the starting head of the yellowstone thermal plume at the base of the lithosphere a plausible combination of crustal and deep buoyancy sources related to the heat and melt input of an anomalously hot asthenospheric source is consistent with gravity seismic heat flow and isotopic observations and explains the origin of the topographic step