seismic structure of the crust and uppermost mantle of north america and adjacent oceanic basins a synthesis we present a new set of contour maps of the seismic structure of north america and the surrounding ocean basins these maps include the crustal thickness whole crustal average p wave and s wave velocity and seismic velocity of the up permost mantle that is pn and sn we found the following 1 the average thickness of the crust under north america is 36 7 km standard deviation s d 8 4 km which is 2 5 km thinner than the world average of 39 2 km s d 8 5 for continental crust 2 histograms of whole crustal p and s wave velocities for the north amer ican crust are bimodal with the lower peak occurring for crust without a high velocity 6 9 7 3 km sec lower crustal layer 3 regions with anomalously high average crustal p wave velocities correlate with precambrian and paleozoic orogens low average crustal velocities are correlated with modern extensional regimes 4 the average pn velocity beneath north america is 8 03 km sec s d 0 19 km sec 5 the well known thin crust beneath the western united states extends into north west canada 6 the average p wave velocity of layer 3 of oceanic crust is 6 61 km sec s d 0 47 km sec however the average crustal p wave velocity under the eastern pacific seafloor is higher than the western atlantic seafloor due to the thicker sediment layer on the older atlantic seafloor

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

a hypothesis on the heat flow similarity between oceans and continents abstract a simple physical model the thickening plate model gives a possible explanation of the fact that the averaged terrestrial heat flow over oceans and continents is roughly equal in the model latent heat released by solidification of asthenospheric material plays an important role faster development thickening of oceanic lithosphere releases a larger amount of latent heat than does continental lithosphere and the heat production becomes comparable with radiogenic heat production within continental crust the model also suggests that the rough equality of the heat flows is not an essential feature of the earth but merely coincidental under the conditions of the present plate motion a more important feature may be the similarity of the terrestrial heat flow over the oldest regions of both continents and oceans terrestrial heat flow over an oceanic area may be a function of the spreading rate of the lithosphere if the spreading rate of the organic lithosphere is slower than the present rate the averaged terrestrial heat flow over oceans should be lower

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

physical properties of the lithosphere asthenosphere system in europe abstract all the geophysical fields considered evidence a clear structural difference between the adriatic sea ionian sea basin on one side and the tyrrhenian sea western mediterranean basin on the other the former displays continental type characteristics while the latter shows features definitely related to young ocean basins lateral variations in the thickness of the high velocity sub crustal layer the lid are clearly evidenced in the whole european area the lateral changes in the shear wave velocities of the lithosphere asthenosphere system are quite pronounced gravity data imply a positive density contrast 0 03 0 07 g cm 3 in the upper mantle related to almost aseismic high velocity lithospheric roots which seem to characterize many orogenic belts the analysis of a transect along the longitudinal axis of the alps seems to indicate that relevant lateral variations in density are present in the lithospheric part of the mantle

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

crustal structure of northern italy from the ellipticity of rayleigh waves abstract northern italy is a diverse geological region including the wide and thick po plain sedimentary basin which is bounded by the alps and the apennines the seismically slow shallow structure of the po plain is difficult to retrieve with classical seismic measurements such as surface wave dispersion yet the detailed structure of the region greatly affects seismic wave propagation and hence seismic ground shaking here we invert rayleigh wave ellipticity measurements in the period range 10 60 s for 95 stations in northern italy using a fully non linear approach to constrain vertical v s v p and density profiles of the crust beneath each station the ellipticity of rayleigh wave ground motion is primarily sensitive to shear wave velocity beneath the recording station which reduces along path contamination effects we use the 3d layering structure in mambo a previous model based on a compilation of geological and geophysical information for the po plain and surrounding regions of northern italy and employ ellipticity data to constrain v s v p and density within its layers we show that ellipticity data from ballistic teleseismic wave trains alone constrain the crustal structure well this leads to mambo e an updated seismic model of the region s crust that inherits information available from previous seismic prospection and geological studies while fitting new seismic data well mambo e brings new insights into lateral heterogeneity in the region s subsurface compared to mambo it shows overall faster seismic anomalies in the region s quaternary pliocene and oligo miocene layers and better delineates the seismic structures of the po plain at depth two low velocity regions are mapped in the mesozoic layer in the western and eastern parts of the plain which seem to correspond to the monferrato sedimentary basin and to the ferrara romagna thrust system respectively

crustal structure beneath portugal from teleseismic rayleigh wave ellipticity up until now portugal lacked a countrywide shear velocity model sampling short length scale crustal structure which limits interpretations of seismicity and tectonics and predictions of strong ground motion in turn such interpretations and predictions are important to help mitigate risk of destruction from future large on and offshore earthquakes similar to those that portugal has experienced in the past e g the mw 8 5 8 7 tsunamigenic event in 1755 in this study we measured teleseismic rayleigh wave ellipticity rwe from 33 permanent and temporary seismic stations in portugal with wave periods between 15 s and 60 s and inverted it for 1 d models of shear wave velocity vs structure beneath each station using a fully non linear monte carlo method because rwe is strongly sensitive to the uppermost few kilometres of the crust both rwe measurements and vs models are spatially correlated with surface geology in portugal for instance we find that sedimentary basins produced by rifting that had begun in the mesozoic such as the lusitanian basin lb and the lower tagus sado basin ltsb are characterised by higher rwe lower vs interestingly we observe similar rwe and vs values in the interior of the central iberian zone ciz which is a metamorphic belt of paleozoic age together with reduced crustal thickness previously estimated for the same parts of the ciz this suggests that the ciz might have experienced an episode of extension possibly simultaneous to mesozoic rifting the galicia tras os montes zone gtmz that has undergone polyphased deformation since the paleozoic is characterised by the lowest rwe highest vs in portugal ossa morena zone and the south portuguese zone exhibit intermediate vs values when compared to that of basins and the gtmz our crustal vs model can be used to provide new insights into the tectonics seismicity and strong ground motion in portugal

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

evidence for an upper mantle low velocity zone beneath the southern basin and range colorado plateau transition zone a 370 km long seismic refraction wide angle reflection profile recorded during the pacific to arizona crustal experiment pace detected an upper mantle p wave low velocity zone lvz in the depth range 40 to 55 km beneath the basin and range in southern arizona interpretation of seismic data places constraints on the sub crustal lithosphere of the southern basin and range province which is important in light of the active tectonics of the region and the unknown role of the sub crustal lithosphere in the development of the western united states forward travel time and synthetic seismogram techniques are used to model this shallow upper mantle lvz modeling results show that the lvz is defined by a 5 velocity decrease relative to a pn velocity of 7 95 km s 1 suggesting either a 3 5 mafic partial melt or high temperature sub solidus peridotite

crustal structure of the colorado plateau arizona application of new long offset seismic data analysis techniques the colorado plateau is a large crustal block in the southwestern united states that has been raised intact nearly 2 km above sea level since cretaceous marine sediments were deposited on its surface controversy exists concerning the thickness of the plateau crust and the source of its buoyancy interpretations of seismic data collected on the plateau vary as to whether the crust is closer to 40 or 50 km thick a thick crust could support the observed topography of the colorado plateau isostatically while a thinner crust would indicate the presence of an underlying low density mantle this paper reports results on long offset seismic data collected during the 1989 segment of the u s geological survey pacific to arizona crustal experiment that extended from the transition zone into the colorado plateau in northwest arizona we apply two new methods to analyze long offset data that employ finite difference travel time calculations 1 a first arrival time inverter to find upper crustal velocity structure and 2 a forward modeling technique that allows the direct use of the inverted upper crustal solution in modeling secondary reflected arrivals we find that the crustal thickness increases from 30 km beneath the metamorphic core complexes in the southern basin and range province to about 42 km beneath the northern transition zone and southern colorado plateau margin we observe some crustal thinning to 37 km thick and slightly higher lower crustal velocities farther inboard beneath the kaibab uplift on the north rim of the grand canyon the crust thickens to a maximum of 48 km we observe a nonuniform crustal thickness beneath the colorado plateau that varies by 15 and corresponds approximately to variations in topography with the thickest crust underlying the highest elevations crustal compositions as inferred from seismic velocities appear to be the same beneath the colorado plateau as those in the basin and range province to the southwest implying that the plateau crust represents an unextended version of the basin and range some of the variability in crustal structure appears to correspond to preserved lithospheric discontinuities that date back to the proterozoic era

stress history and tectonic development of the rio grande rift new mexico paleostress determinations from radiometrically dated dikes indicate ene wsw regional extension began by about 32 ma in new mexico and perhaps earlier in the southern part of the state the extension direction maintained this orientation until sometime after 23 ma when it rotated clockwise to an e w or wnw ese direction which is the present orientation the spreading direction in the rio grande rift however has been e w since the beginning of rifting about 31 ma and reflects control of spreading by preexisting structural grain ne sw extension occurred in the grants pie town area of the colorado plateau and in the roofs of batholiths in the socorro and questa areas concurrently with e w spreading in the rio grande rift nearly orthogonal orientations of late eocene oligocene 38 32 ma paleostress indicators in southern new mexico indicate a nearly isotropic middle tertiary stress field both the western and eastern boundaries of the zone of e w spreading along the rio grande rift in northern new mexico shifted 60 200 km eastward sometime between 23 and 16 ma perhaps coeval with rotation of the direction of regional extension this eastward shift also coincided approximately with the renewal of volcanism following an early miocene 22 17 ma lull during the late miocene 9 5 ma the eastern boundary of e w spreading shifted back to the present eastern margin of the rio grande rift the n s to nne ssw orientation of the least principal horizontal stress in the southern great plains province has persisted since at least 28 ma stress province boundaries can be remarkably sharp with stress orientations changing by 90 within 30 km or less the ne trending jemez lineament transects the colorado plateau rio grande rift basin and range and southern great plains stress provinces but except for a transition zone along the present southeastern boundary of the colorado plateau both paleostress and contemporary stress reflect the respective stress provinces not the lineament chemical analyses of dated dikes reveal highly variable compositions but indicate that true basalts are more common among early rift 20 ma volcanic rocks than is generally recognized a histogram constructed from nearly 200 radiometric ages of igneous rocks in new mexico between 104 w and 108 w longitude indicates that volcanism along the rio grande rift has been episodic with peaks at 37 34 31 27 26 22 17 13 12 6 and 5 0 ma

high resolution receiver function imaging reveals colorado plateau lithospheric architecture and mantle supported topography 1 after maintaining elevations near sea level for over 500 million years the colorado plateau cp has a present average elevation of 2 km we compute new receiver function images from the first dense seismic transect to cross the plateau that reveal a central cp crustal thickness of 42 50 km thinning to 30 35 km at the cp margins isostatic calculations show that only approximately 20 of central cp elevations can be explained by thickened crust alone with the cp edges requiring nearly total mantle compensation we calculate an uplift budget showing that cp buoyancy arises from a combination of crustal thickening heating and alteration of the lithospheric root dynamic support from mantle upwelling and significant buoyant edge effects produced by small scale convecting asthenosphere at its margins

insights into the kinematic cenozoic evolution of the basin and range colorado plateau transition from coincident seismic refraction and reflection data estimates of surface extension in the southern basin and range province and transition into the colorado plateau range from a few percent to several hundred percent locally yet the crustal thickness varies perhaps only 10 15 km across these provinces within the southern basin and range and the metamorphic core complex belt extremely extended crust is directly juxtaposed against equally thick or thinner crust that underwent far milder extension unless preextension crustal thickness varied dramatically over a short distance the crust must have maintained its thickness during extension through mechanisms that involve crustal flow and magmatism we employ a 300 km long profile of seismic refraction and coincident vertical incidence reflection data to investigate the geophysical signature of these processes from the extended southern basin and range province to the unextended colorado plateau by integrating the seismic velocity with the pattern of reflectivity along the profile we estimate the amounts of tertiary magmatism and flow that have occurred we estimate an upper bound of 8 km of mafic material intruded beneath the metamorphic core complex belt and 4 and 5 km of intruded material beneath the transition zone and southern basin and range province respectively we emphasize that this 8 km estimate is strictly an upper bound and that the actual amount of magmatism was probably less 3 to 4 km we further speculate that several kilo meters of silicic rock was added to the metamorphic core complex belt via ductile flow as suggested by numerous numerical models of crustal extension we conclude that a mobile felsic midcrustal layer accommodated most of this crustal flow this ductile midcrustal layer appears to be thickest beneath the most extended terranes and thinnest beneath the less extended transition zone and colorado plateau in contrast the lowermost crust appears to have thinned passively in an amount that corresponds more directly to the regional surface extension

the yellowstone magmatic system from the mantle plume to the upper crust the yellowstone supervolcano is one of the largest active continental silicic volcanic fields in the world an understanding of its properties is key to enhancing our knowledge of volcanic mechanisms and corresponding risk using a joint local and teleseismic earthquake p wave seismic inversion we revealed a basaltic lower crustal magma body that provides a magmatic link between the yellowstone mantle plume and the previously imaged upper crustal magma reservoir this lower crustal magma body has a volume of 46 000 cubic kilometers 4 5 times that of the upper crustal magma reservoir and contains a melt fraction of 2 these estimates are critical to understanding the evolution of bimodal basaltic rhyolitic volcanism explaining the magnitude of co2 discharge and constraining dynamic models of the magmatic system for volcanic hazard assessment

cenozoic thermal mechanical and tectonic evolution of the rio grande rift careful documentation of the cenozoic geologic history of the rio grande rift in new mexico reveals a complex sequence of events at least two phases of extension have been identified an early phase of extension began in the mid oligocene about 30 ma and may have continued to the early miocene about 18 ma this phase of extension was characterized by local high strain extension events locally 50 100 regionally 30 50 low angle faulting and the development of broad relatively shallow basins all indicating an approximately ne sw 25 extension direction consistent with the regional stress field at that time extension events were not synchronous during early phase extension and were often temporally and spatially associated with major magmatism a late phase of extension occurred primarily in the late miocene 10 5 ma with minor extension continuing to the present it was characterized by apparently synchronous high angle faulting giving large vertical strains with relatively minor lateral strain 5 20 which produced the modern rio grande rift morphology extension direction was approximately e w consistent with the contemporary regional stress field late phase graben or half graben basins cut and often obscure early phase broad basins early phase extensional style and basin formation indicate a ductile lithosphere and this extension occurred during the climax of paleogene magmatic activity in this zone late phase extensional style indicates a more brittle lithosphere and this extension followed a middle miocene lull in volcanism regional uplift of about 1 km appears to have accompanied late phase extension and relatively minor volcanism has continued to the present we have estimated geotherms and calculated lithospheric strength curves for the two phases of extension using geologic data to constrain earlier events and geophysical data to constrain the modern geotherm and crustal structure a high geotherm was deduced for early phase extension resulting in a shallow crustal brittle ductile transition and negligible mantle strength the lithosphere cooled after early phase extension resulting in a deeper crustal brittle ductile transition and perhaps more significantly a considerable zone of mantle strength immediately beneath the moho these results indicate that early phase extensional style was controlled by a crustal decollement near the brittle ductile transition which was prevented during late phase extension by significant strength in the uppermost mantle late cenozoic uplift of the rift zone cannot be explained by crustal thinning during extension and geotherm evolution predicted from simple cooling however this uplift does not appear to be restricted to the rift zone and pliocene to recent volcanism and heat flow data suggest that uplift may be caused by magmatic thickening of the crust perhaps unrelated to rifting the complex interrelationship among regional and local prerifting synrifting and postrifting events in the rio grande rift suggests that rifting at least in this region should not be considered in isolation of other geologic events

xenolith constraints on the thermal history of the mantle below the colorado plateau analyses of minerals in xenoliths from the grand canyon volcanic field 1 ma and published results from the navajo field 20 30 ma constrain the history of the mantle below the colorado plateau the navajo xenoliths establish that the mantle was cool to a depth of at least 140 km until heated by magma just before eruption and so no frictional heating was recorded from shallow subduction below the plateau in cenozoic time the temperature record is consistent with a low elevation of the plateau until uplift consequent to igneous events at 25 ma rims of minerals of grand canyon spinel peridotite xenoliths record mantle temperatures in the range 800 1000 c at 1 ma orthopyroxene grains in two rocks however contain interior domains with ca like that of grain rims but with much lower al and cr the zonation of orthopyroxene records heating and annealing over a period of at least several million years and this history is consistent with the isotopically unusual grand canyon xenoliths being from mantle lithosphere that persisted through a period of shallow subduction temperatures recorded by rims of minerals typically are cooler by about 100 c than those of xenoliths from similar localities in the basin and range province consistent with seismic studies that indicate relatively faster velocities for the uppermost mantle of the plateau

radiative heat transfer in lithospheric geodynamics implications for asthenospheric partial melting and basin modelling concepts abstract in geodynamic models describing basin formation by stretching the base of the lithosphere is commonly defined by a fixed temperature an alternative definition is proposed where the base of the lithosphere is determined by the solidus of mantle rocks some consequences of this definition are illustrated in a one dimensional model the model applies the finite difference method and approximates the temperature regime in the upper 400 km of the earth the model uses temperature dependent thermal conductivity and includes effects from heat storage by partial melting in the mantle low velocity layer mlv radiative heat transfer contributes significantly at elevated temperatures giving rise to a steepening of the thermal gradient below the lithosphere this change in gradient allows the perturbed geotherm to exceed the mantle solidus in a limited depth interval which accounts for the mlv and thus defines the base of the lithosphere in the model variations in thickness of and degree of partial melting in the mlv results from thinning of the overlying lithospheric plate thus the thickness of the mlv is roughly inversely proportional to the thickness of the lithosphere whereas the base of the mlv is almost flat temperature dependent variations in the carrying capacity of the mlv where the main isostatic compensation occurs is proposed as an epeirogenetic agent this effect may be significant and is expressed as an enhancement of the initial subsidence if no preexisting mlv is available a secondary effect occurs as solidification of the mlv ceases a long time 300 ma after stretching and causes slow uplift

mantle structure beneath the western edge of the colorado plateau 1 teleseismic traveltime data are inverted for mantle vp and vs variations beneath a 1400 km long line of broadband seismometers extending from eastern new mexico to western utah the model spans 600 km beneath the moho with resolution of 50 km inversions show a sharp large magnitude velocity contrast across the colorado plateau great basin transition extending 200 km below the crust also imaged is a fast anomaly 300 to 600 km beneath the nw portion of the array very slow velocities beneath the great basin imply partial melting and or anomalously wet mantle we propose that the sharp contrast in mantle velocities across the western edge of the plateau corresponds to differential lithospheric modification during and following farallon subduction across a boundary defining the western extent of unmodified proterozoic mantle lithosphere the deep fast anomaly corresponds to thickened farallon plate or detached continental lithosphere at transition zone depths

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

anisotropy and vp vs in the uppermost mantle beneath the western united states from joint analysis of pn and sn phases pn and sn phases are valuable for resolving velocity structure in the mantle lid as they propagate horizontally right below the moho relatively few sn tomography attempts have been made compared to pn because sn is often highly attenuated or buried in p wave coda usarray has greatly increased data coverage for regional phases and both pn and sn are routinely picked by network analysts here we jointly invert pn and sn arrival time residuals with a modified time term analysis and a regularized tomography method and present new maps of crustal thickness uppermost mantle p velocity perturbations vp vs ratios and azimuthal anisotropy strength and orientation beneath the western united states the results indicate partially molten mantle below the snake river plain and the colorado plateau the seismic structure of the top 40 km of the mantle below the colorado plateau differs from that seen at greater depths in other studies such as surface wave or teleseismic body wave tomography whereas the snake river plain anomaly just below the moho is comparable to structures seen at about 200 km depth pn fast axes provide complementary information to sks shear wave splitting observations and our analysis indicates that in several regions in the western united states the orientation of azimuthal anisotropy changes with depth in the upper mantle however we have so far been unable to resolve shear wave splitting directly in sn waveforms which seem to be dominated by sn sv energy

upper mantle convection beneath the central rio grande rift imaged by p and s wave tomography 1 we present models for upper mantle p and s velocity structure beneath a southwestern united states transect extending from near the center of the colorado plateau across the rio grande rift to the great plains the models were derived from travel times of compressional and shear seismic phases recorded by the la ristra passive seismic array deployed from july 1999 to may 2001 large variations in seismic velocity up to 8 in s and 5 in p are seen across the transect in the upper 200 km of the mantle seismically slow mantle underlies the rio grande rift and jemez lineament and relatively slow mantle is seen beneath the navajo volcanic field within the colorado plateau the relative variations of p and s velocity imply that high temperatures are the primary cause of the slow mantle although a small amount of partial melting or hydration cannot be ruled out sharp boundaries in mantle seismic velocity are coincident with boundaries of proterozoic structural trends implying that ancient lithospheric structure exerts a control on the tectonic and magmatic activity in the region weaker seismic variations are imaged below 200 km depth with three southeastward dipping structures in our images two of the structures have fast seismic anomalies beneath the central colorado plateau and the great plains respectively and the third has anomalously slow seismic wave speed we interpret the western deep seismic anomaly to be foundering farallon slab and the slow anomaly just to the east as upwelling mantle possibly associated with volatile release from the sinking farallon slab beneath the great plains there is also downwelling in the upper mantle the combination of upwelling in the west and downwelling in the east are likely part of an upper mantle convection cell that may include entrained lithosphere from beneath the rift index terms 7218 seismology lithosphere and upper mantle 8121 tectonophysics dynamics convection currents and mantle plumes 8180 tectonophysics evolution of the earth tomography keywords convection rio grande rift colorado plateau

numerical modeling for analyzing thermal surface anomalies induced by underground coal fires abstract coal seams burning underneath the surface are recognized all over the world and have drawn increasing public attention in the past years frequently such fires are analyzed by detecting anomalies like increased exhaust gas concentrations and soil temperatures at the surface a proper analysis presumes the understanding of involved processes which determine the spatial distribution and dynamic behavior of the anomalies in this paper we explain the relevance of mechanical and energy transport processes with respect to the occurrence of temperature anomalies at the surface two approaches are presented aiming to obtain insight into the underground coal fire situation in situ temperature mapping and numerical simulation in 2000 to 2005 annual temperature mapping in the wuda inner mongolia pr china coal fire area showed that most thermal anomalies on the surface are closely related to fractures where hot exhaust gases from the coal fire are released those fractures develop due to rock mechanical failure after volume reduction in the seams the measured signals at the surface are therefore strongly affected by mechanical processes more insight into causes and effects of involved energy transport processes is obtained by numerical simulation of the dynamic behavior of coal fires simulations show the inter relation between release and transport of thermal energy in and around underground coal fires our simulation results show a time delay between the coal fire propagation and the observed appearance of the surface temperature signal additionally the overall energy flux away from the burning coal seam into the surrounding bedrock is about 30 times higher than the flux through the surface this is of particular importance for an estimation of the energy released based on surface temperature measurements finally the simulation results also prove that a fire propagation rate estimated from the interpretation of surface anomalies can differ from the actual rate in the seam

shear velocity structure and azimuthal anisotropy beneath eastern north america from rayleigh wave inversion 1 we have obtained shear velocity structure beneath the northeastern united states and southeastern canada using rayleigh wave phases and amplitudes thin crust 36 42 km is observed along the atlantic coast and in the eastern appalachian orogen and thick crust 42 46 km is imaged in the western appalachians and in the western new york portion of the grenville province the variation of crustal thickness correlates well with the observed bouguer gravity anomalies in the upper mantle the high velocity continental keel of cratonic north america is present in the western part of the study area while a broad low velocity region is imaged in new england from the hudson river valley to the white mountains this low velocity anomaly is probably the consequence of past heating of the lithospheric mantle associated with the monteregian hotspot and may represent intrusion of asthenosphere into the edge of the keel in addition to lateral variations in velocity we estimate the azimuthal dependence of phase velocity strong and relatively uniform shear wave splitting is observed in the study region but at periods of 100 s or less the average azimuthal anisotropy of rayleigh waves is less than 1 and is not significantly different from zero at any individual period this small degree of azimuthal anisotropy is not consistent with a substantial contribution to shear wave splitting from fossil anisotropy in the lithosphere much of the source of the shear wave splitting must lie deeper than 200 km

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

rayleigh wave phase velocities small scale convection and azimuthal anisotropy beneath southern california 1 we use rayleigh waves to invert for shear velocities in the upper mantle beneath southern california a one dimensional shear velocity model reveals a pronounced low velocity zone lvz from 90 to 210 km the pattern of velocity anomalies indicates that there is active small scale convection in the asthenosphere and that the dominant form of convection is three dimensional 3 d lithospheric drips and asthenospheric upwellings rather than 2 d sheets or slabs several of the features that we observe have been previously detected by body wave tomography these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern sierra nevada and walker lane region sinking lithosphere beneath the southern central valley upwelling beneath the salton trough and downwelling beneath the transverse ranges our new observations provide better constraints on the lateral and vertical extent of these anomalies in addition we detect two previously undetected features a high velocity anomaly beneath the northern peninsular range and a low velocity anomaly beneath the northeastern mojave block we also estimate the azimuthal anisotropy from rayleigh wave data the strength is 1 7 at periods shorter than 100 s and decreases to below 1 at longer periods the fast direction is nearly e w the anisotropic layer is more than 300 km thick the e w fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms pure shear in the lithosphere due to n s tectonic shortening and simple shear in sublithosphere mantle due to mantle flow

crust and mantle structure across the basin and range colorado plateau boundary at 37 n latitude and implications for cenozoic extensional mechanism we present new evidence on the seismic velocity and density of the crust and upper mantle along a 200 km long transect across the eastern basin and range and western colorado plateau at 37 n latitude receiver functions computed from the p waveforms recorded with 10 portable broadband stations deployed along the transect were used to estimate crustal thickness variations the crust is 30 35 km thick within the eastern basin and range and increases over a distance of 100 km at the western edge of the colorado plateau reaching a maximum of approximately 45 km east of the hurricane fault the timing of crustal multiples within the reciever functions were used to estimate the v p v s of the crust along the profile and we found that the western colorado plateau crust is characterized by a high poisson s ratio 0 28 0 29 indicative of a crust with an average mafic composition we estimated the upper mantle lid thickness along our profile based on teleseismic p wave travel times and constraints provided by gravity data our data and available geophysical constraints are most consistent with a lithosphere that thickens from an average thickness of 60 km beneath the basin and range to 100 km beneath the western colorado plateau although the basin and range lithosphere may have significant thickness variations the thick strong mafic crust and thicker mantle lid under the colorado plateau can account for the relative geologic stability and subdued magmatism of the plateau during laramide compression and cenozoic extension compared to surrounding regions the crustal and lithospheric thinning across the tectonic boundary occurs over a short distance 100 km suggesting it is a geologically young feature produced by a predominantly mechanical response to late cenozoic extension our new lithosphere model at 37 n latitude is consistent with the existence in early cenozoic time of a flat subducted slab at 100 km depth and a relict sevier laramide 50 60 km thick crustal welt and 60 100 pure shear extension values of 1 6 2 0 during the late cenozoic

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

preservation of proterozoic terrane boundaries within the colorado plateau and implications for its tectonic evolution abstract during the proterozoic the north american continent grew by the accretion of arcs onto the archean craton subsequent tectonic events affected much of the lithosphere of western north american resulting in the destruction of many of the features that reflect continental growth the colorado plateau which is centrally located within the tectonically active western portion of north america experienced only minor amounts of volcanism during subsequent magmatic events and has undergone comparatively little internal deformation during the remainder of the precambrian and phanerozic despite the occurrence of some significant tectonic events because the lithosphere of the plateau has not been significantly reworked by tectonic events it may have retained structures since accretion that give insight into processes of continental growth the crust of the southeastern portion of the colorado plateau revealed here through seismic data recorded by the temporary spe and la ristra arrays is greater than 40 km thick and possesses distinct changes in intercrustal layering and the character of the moho the moho appears quite dim towards the center of the plateau and much brighter along its southeastern margins connecting changes in crustal features that are common between the two arrays indicates a northeastern structural trend that matches the pattern of gravity anomalies in the region differences in crustal structures between terranes extend across much of the crust suggesting that entire distinct crustal columns align with terrane boundaries it is unlikely that extensive crustal flow as required by some models of plateau evolution could have traversed across the entire colorado plateau without producing features that cross cut these boundaries

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

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

the anatomy of uppermost mantle shear wave speed anomalies in the western u s from surface wave amplification abstract we build swus amp a three dimensional shear wave speed model of the uppermost mantle of the western u s using rayleigh wave amplification measurements in the period range of 35 125 s from teleseismic earthquakes this represents the first ever attempt to invert for velocity structures using rayleigh wave amplification data alone we use over 350 000 rayleigh wave amplitude measurements which are inverted using a monte carlo technique including uncertainty quantification being a local seismic observable rayleigh wave amplification is little affected by path averaged effects and in principle has stronger depth resolution than classical seismic observables such as surface wave dispersion data swus amp confirms shallow mantle heterogeneities found in previous models in the top 100 km of the mantle we observe low velocity anomalies associated with yellowstone and the basin range province as well as a fast velocity anomaly underneath the colorado plateau where a strong velocity gradient at its edges shows a drastic contrast with its surroundings swus amp also gives additional insights into the current state of the uppermost mantle in the region we image a high velocity anomaly beneath the high topography wyoming province with a maximum depth extent of about 150 170 km which is shallower than in previous tomographic models and resolves previous inconsistencies with geological information beneath the snake river plain a finger like low velocity anomaly dips to the west suggesting lateral flow in the region below about 150 km depth swus amp shows a north south dichotomy in shear wave speed structure with the northern region showing mostly high velocity anomalies whereas the southern region shows low velocity anomalies this is consistent with the continuous subduction history of the western u s and with the recent extension and uplift of the southern region

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

melting of forsterite mg2sio4 up to 15 gpa the melting curve of forsterite has been studied by static experiment up to a pressure of 15 gpa forsterite melts congruently at least up to 12 7 gpa the congruent melting temperature is expressed by the kraut kennedy equation in the following form tm k 2163 1 3 0 v0 v v0 where the volume change with pressure was calculated by the birch managhan equation of state with the isothermal bulk modulus k0 125 4 gpa and its pressure derivative k 5 33 the triple point of forsterite mg2sio4 liquid will be located at about 2600 c and 20 gpa assuming that congruent melting persists up to the limit of the stability field of forsterite the extrapolation of the previous melting data on enstatite and periclase indicates that the eutectic composition of the forsterite enstatite system should shift toward the forsterite component with increasing pressure and there is a possibility of incongruent melting of forsterite into periclase and liquid at higher pressure although no evidence on incongruent melting has been obtained in the present experiment

pn tomography of the western united states using usarray 1 usarray has now provided several years of high quality seismic data and improved ray coverage for much of the western united states which will enable increased resolution for studies of the lithospheric and deeper structure of the north american continent here we analyze pn arrival times from the transportable stations of usarray to resolve crustal thickness and uppermost mantle structure we use 123 008 pn picks from april 2004 to october 2009 as measured by the array network facility at epicentral distances from 180 to 1450 km these picks are derived from 778 stations at 70 km spacing and 7903 earthquakes and quarry blasts applying the classic time term method we use a regularized least squares inversion to estimate crustal thickness variations and image velocity perturbations in the uppermost mantle just below the moho we also consider upper mantle anisotropy and describe the velocity perturbations with a cos 2 azimuthal variation our crustal thickness map generally agrees with receiver function results from other researchers but differs in some details we obtain an average upper mantle velocity of 7 93 km s with higher velocities beneath eastern washington and northern idaho and lower velocities near the california mexico border the sierra nevada the northern coastal california region and the greater yellowstone area we observe large anisotropic anomalies in southern california as well as in the snake river plain area these results should complement other seismic studies e g body and surface wave tomography and shear wave splitting to provide information about composition temperature and tectonic processes in the western united states

western u s seismic anisotropy revealing complex mantle dynamics abstract the origin of the complex pattern of sks splitting over the western united states u s remains a long lasting debate where a model that simultaneously matches the various sks features is still lacking here we present a series of quantitative geodynamic models with data assimilation that systematically evaluate the influence of different lithospheric and mantle structures on mantle flow and seismic anisotropy these tests reveal a configuration of mantle deformation more complex than ever envisioned before in particular we find that both lithospheric thickness variations and toroidal flows around the juan de fuca slab modulate flow locally but their co existence enhances large scale mantle deformation below the western u s the ancient farallon slab below the east coast pulls the western u s upper mantle eastward spanning the regionally extensive circular pattern of sks splitting the prominent e w oriented anisotropy pattern within the pacific northwest reflects the existence of sustaining eastward intrusion of the hot pacific oceanic mantle to beneath the continental interior from within slab tears below oregon to under the snake river plain and the yellowstone caldera this work provides an independent support to the formation of intra plate volcanism due to intruding shallow hot mantle instead of a rising mantle plume

tomography reveals buoyant asthenosphere accumulating beneath the juan de fuca plate the boundary between earth s strong lithospheric plates and the underlying mantle asthenosphere corresponds to an abrupt seismic velocity decrease and electrical conductivity increase with depth perhaps indicating a thin weak layer that may strongly influence plate motion dynamics the behavior of such a layer at subduction zones remains unexplored we present a tomographic model derived from on and offshore seismic experiments that reveals a strong low velocity feature beneath the subducting juan de fuca slab along the entire cascadia subduction zone through simple geodynamic arguments we propose that this low velocity feature is the accumulation of material from a thin weak buoyant layer present beneath the entire oceanic lithosphere the presence of this feature could have major implications for our understanding of the asthenosphere and subduction zone dynamics

devonian palaeontological data and the armorica problem abstract various palaeogeographic ideas about the relation between gondwana and euramerica during the devonian period are formalised into four competing hypotheses involving three areas euramerica gondwana armorica separated by two marine faunal barriers palaeontological evidence from devonian vertebrates is summarised to show support for the hypothesis that gondwana and euramerica were contiguous by the middle devonian and that the intervening ocean had been occluded by the late devonian palaeontological data provide no support for the existence of a wide devonian ocean separating euramerica from gondwana nor for a discrete continental block armorica which may have traversed this ocean during the devonian the armorican region of southern europe was apparently part of gondwana during the early devonian biostratigraphic evidence of discrepancies in the first appearance of various invertebrate taxa between southern europe north africa and north america are consistent with a biotic dispersal model involving shallow marine invertebrates and vertebrates during the early middle devonian and continental faunas and floras in the late devonian the latter event may have occurred at or near the frasnian famennian boundary

images of crustal variations in the intermountain west 1 we develop a map of crustal thickness variations across the great basin colorado plateau rocky mountain and great plains provinces of the western united states using common conversion point stacking of teleseismic receiver functions below the rocky mountains and high plains in colorado we find the thickest crust in the region at 45 50 km thick beneath the basin and range thinner between 30 and 40 km crust is found thin 30 km thick crust is present in the northern portion of nevada and utah despite elevations similar to those farther south crustal thickness across the colorado plateau can be characterized as a broad transitional region between the thin crust of basin and range to the thicker crust of the rocky mountains the impedance contrast across the mohorovicic discontinuity decreases below the colorado plateau as converted arrivals recorded in this region appear weak compared to surrounding areas variations in vp vs across the region indicate higher values along the western boundary of the basin and range in the rocky mountains and in the western great plains we are not able to characterize vp vs in the colorado plateau we find that crustal thickness does not closely correlate with surface topography within each region or across the region as a whole differences in crustal thickness in each tectonic province indicate the need for a mantle component to support the high elevations across the western united states

imaging crustal and upper mantle structure beneath the colorado plateau using finite frequency rayleigh wave tomography a new 3 d shear velocity model of the crust and upper mantle beneath the colorado plateau and surrounding regions of the southwestern united states was made with finite frequency rayleigh wave tomography using earthscope usarray data the goal of our study is to examine the colorado plateau lithospheric modification that has resulted from cenozoic tectonism and magmatism we have inverted for the isotropic vs model from a grid of rayleigh wave dispersion curves obtained by a modified two plane wave method for periods from 20 to 167 s we map the lithosphere asthenosphere boundary under the colorado plateau by identifying the middle of the shallowest upper mantle negative vs gradient the depths of the lithosphere asthenosphere boundary inferred here agree well with receiver function estimates made independently the strong lateral heterogeneity of shear velocity can be mainly attributed to 200 400 k variations in temperature together with 1 partial melt fraction in the shallow upper mantle the resulting vs structures clearly image the upper mantle low velocity zones under the colorado plateau margins that are associated with magmatic encroachment these upper mantle low velocity zones resulted from the convective removal of the colorado plateau lithosphere that had been rehydrated by subduction released water refertilizing and destabilizing it this convective erosion by the asthenosphere at the low viscosity part of the lithosphere is driven by the large step in lithospheric thickness and the thermal gradient across the boundary between the plateau and the extended basin and range since the mid cenozoic at a rate similar to that of magmatic migration into the plateau from the southeast south and northwest moreover the rayleigh wave tomography model images parts of a high velocity drip in the western colorado plateau and thus provides additional seismic evidence for ongoing convective downwelling of the lithosphere that was initially suggested by receiver functions and body wave tomography the widespread edge convective erosion which the regional delamination style downwelling processes are a 3 d manifestation of could provide additional buoyancy sources to support the excess uplift at the margins of the plateau

the lithosphere asthenosphere boundary beneath the western united states summary s receiver functions from 67 broad band seismic stations in the western united states clearly reveal the existence of a mantle discontinuity with velocity reduction downward which we interpret as the lithosphere asthenosphere boundary lab the average depth of the lab is 70 km the boundary is relatively sharp with an overall sharpness of less than 20 km the boundary is more prominent south of the mendocino triple junction where the farallon plate has completely subducted this may indicate partial melts at the base of the lithosphere caused by the upwelling of the asthenospheric flow through the slab window a double low velocity zone is observed at base of the lithosphere beneath southern sierra nevada implying a second melting zone at a depth of 100 km well correlated with previous studies of lithospheric delamination in the area

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

melting of garnet peridotite to 13 gpa and the early history of the upper mantle by analogy with the evolution of the moon it has been suggested that the earth may have had a magma ocean or liquid outer layer before 3 800 myr ago 1 its presence would have had profound implications for the primary stratification of the mantle into upper mantle transition zone and lower mantle2 3 an essential constraint on this hypothesis is a knowledge of the high pressure melting characteristics of mantle material generally assumed to be peridotite in the upper mantle we recently described4 the first melting experiments on mantle peridotite to 14 gpa using spinel iherzolite klb 1 from kilbourne hole new mexico we showed that partial melts close to the solidus at 5 7 gpa are komatiitic in composition and that the solidus and liquidus converge at high pressures here we report further melting experiments on garnet iherzolite phn1611 from thaba putsoa lesotho we confirm the convergence of the solidus and liquidus and we show a negative dt dp slope for the liquidus above 7 gpa these observations are briefly discussed in terms of their importance to the petrological evolution of the upper mantle and the presence of a magma ocean in the early archaean

western u s upper mantle structure regional three dimensional inversions of upper mande p wave velocity structure are created from teleseismic p wave travel time residuals recorded by many of the high density high frequency regional arrays operated within the western united states these inversions are adjusted to a global international seismic centre reference and merged to obtain an image of the upper mantle beneath western united states the p wave velocities in the upper mantle are slow on average and the structure is very heterogeneous where resolution is good coherent upper mantle structures are imaged that extend as deep as 200 km the juan de fuca and gorda slabs which penetrate to greater depths are exceptions and deviate from the average velocity at a given depth by as much as 4 lateral resolution of these structures usually is very good although the magnitude of the actual seismic variations is probably greater than that imaged the long wavelength part of the imaged mantle structure defines coherent elongate features with wavelengths of 200 500 km within 250 km of the pacific coast these structures have a wavelength of 250 km and trend parallel to the surface physiography and young tectonic structures beneath the continental interior where use is made of additional seismic studies to infer average structure in regions of poor teleseismic data coverage structures have a wavelength of 500 km and trend northeasterly this northeast orientation is discordant with young tectonic structures but aligns with young volcanic activity

rayleigh and s wave tomography constraints on subduction termination and lithospheric foundering in central california the crust and upper mantle structure of central california have been modified by subduction termination growth of the san andreas plate boundary fault system and small scale upper mantle convection since the early miocene here we investigate the contributions of these processes to the creation of the isabella anomaly which is a high seismic velocity volume in the upper mantle there are two types of hypotheses for its origin one is that it is the foundered mafic lower crust and mantle lithosphere of the southern sierra nevada batholith the alternative suggests that it is a fossil slab connected to the monterey microplate a dense broadband seismic transect was deployed from the coast to the western sierra nevada to fill in the least sampled areas above the isabella anomaly and regional scale rayleigh and s wave tomography are used to evaluate the two hypotheses new shear velocity vs tomography images a high velocity anomaly beneath coastal california that is sub horizontal at depths of 40 80 km east of the san andreas fault a continuous extension of the high velocity anomaly dips east and is located beneath the sierra nevada at 150 200 km depth the western position of the isabella anomaly in the uppermost mantle is inconsistent with earlier interpretations that the isabella anomaly is connected to actively foundering foothills lower crust based on the new vs images we interpret that the isabella anomaly is not the dense destabilized root of the sierra nevada but rather a remnant of miocene subduction termination that is translating north beneath the central san andreas fault our results support the occurrence of localized lithospheric foundering beneath the high elevation eastern sierra nevada where we find a lower crustal low vs layer consistent with a small amount of partial melt the high elevations relative to crust thickness and lower crustal low vs zone are consistent with geological inferences that lithospheric foundering drove uplift and a 3 4 ma pulse of basaltic magmatism

joint inversion for global isotropic and radially anisotropic mantle structure including crustal thickness perturbations we present a new global whole mantle model of isotropic and radially anisotropic s velocity structure sglobe rani based on 43 000 000 surface wave and 420 000 body wave travel time measurements which is expanded in spherical harmonic basis functions up to degree 35 we incorporate crustal thickness perturbations as model parameters in the inversions to properly consider crustal effects and suppress the leakage of crustal structure into mantle structure this is possible since we utilize short period group velocity data with a period range down to 16 s which are strongly sensitive to the crust the isotropic s velocity model shares common features with previous global s velocity models and shows excellent consistency with several high resolution upper mantle models our anisotropic model also agrees well with previous regional studies anomalous features in our anisotropic model are faster sv velocity anomalies along subduction zones at transition zone depths and faster sh velocity beneath slabs in the lower mantle the derived crustal thickness perturbations also bring potentially important information about the crustal thickness beneath oceanic crusts which has been difficult to constrain due to poor access compared with continental crusts

three dimensional models of mantle flow across a low viscosity zone implications for hotspot dynamics the influence of a shallow low viscosity zone on the planform of infinite prandtl number rayleigh benard convection has been investigated at rayleigh numbers ra ranging from 1 5 103 to 3 105 a viscosity contrast of130 has been set between the low and high viscosity layers for ra exceeding about 3 104 and no slip conditions at the top and bottom boundaries a hexagonal convective planform is systematically observed free slip boundary conditions lead to the development of a square convective planform the flow is ascending along the center of the cells and is strongly narrowed in the low viscosity layer a moving top boundary does not destabilize significantly these plumes as long as the drift velocity is lower than the upwelling velocity in the low viscosity layer a higher drift velocity of the upper boundary rapidly induces the formation of convective rolls at ra of 3 105 unsteady flow occurs along the descending currents when the convective cells have the horizontal size predicted by the linear theory this time dependence is prevented when the cells are smaller suggesting that there is a competition between the aspect ratio and the steadiness of the cells such a reduction of the horizontal dimensions of the cells with increasing ra has been observed in large experiments these results show that a moderate viscosity drop at shallow depth in the mantle compatible with the geoid and bathymetric data at hotspot swells induces a marked asymmetry between the upper and lower convective boundary layers this promotes the development of ascending plumes large at depth and very narrow a few tens of kilometers in section below the lithosphere these plumes are very stable in time and space it is inferred that the channelling of the ascending plumes induced by a viscosity drop precludes the development of secondary convection in a top low viscosity layer

convective partial melting 1 a model for the formation of thick basaltic sequences during the initiation of spreading stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure temperature environment where partial melting occurs the amount of melt liberated is however insufficient to account for the thickness of igneous crust emplaced near the continent ocean boundary at some passive margins these igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide seismic studies also suggest that the transition from continental to oceanic crust on these volcanic margins is relatively abrupt their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading to account for these observations we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle deep hot mantle material is transported upward by the convection and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling numerical calculations show that convective partial melting can provide substantial additions to melting due to passive upwelling alone using this model we propose an evolutionary scheme for volcanic passive margins emphasize the contrast with nonvolcanic margins and describe conditions which might lead to the development of both types

local amplification of rayleigh waves in the continental united states observed on the usarray abstract we develop a method based on ratios of amplitudes measured at adjacent stations to determine local amplification of surface waves across an array of seismic stations we isolate the effects of local structure from those of the earthquake and propagation by systematic averaging of ratios corresponding to many sources we apply the method to data recorded on the usarray for the years 2006 2011 and determine amplification factors at each station of the array for rayleigh waves at periods between 35 s and 125 s local amplification factors are spatially coherent and display variations of 10 at a period of 125 s and greater variations at shorter periods maps of local amplification exhibit spatial correlation with topography and geologic structures in the western and central united states at long periods the observed amplification factors correlate well with predictions from a regional crust and mantle model of north america at short periods correlations are weaker suggesting that the local amplification factors can be useful for constraining shallow structure better

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

seismic evidence for partial lithospheric delamination model of colorado plateau uplifts the lithospheric structure beneath the colorado plateau and great basin of the western united states is investigated using pn and surface wave data from the colorado plateau great basin broadband seismic experiment average pn velocities of 8 0 km s in the colorado plateau and 7 8 km s in the eastern great basin are found teleseismic rayleigh waves are also measured and eastern great basin phase velocities are found to be 3 4 higher than colorado plateau phase velocities in the 20 35 s range but indistinguishable at periods greater than 40 s the apparent pn surface wave discrepancy is resolved with models that include a thicker crust and faster mantle lid for the colorado plateau than the eastern great basin models for both regions require a mantle low velocity zone the slow mantle deep beneath the colorado plateau may suggest a thermal origin of plateau uplift which has not yet penetrated the mantle lid

upper mantle s velocity structure of north america we have inverted fundamental and higher mode rayleigh waveforms from 685 vertical component broadband seismograms with wave paths over north america to obtain an image of the upper mantle s velocity structure down to 660 km among the well resolved features of the new model are 1 a high velocity root beneath the north american craton which extends no deeper than 250 km except near the archean core of the craton where depths of 350 km are reached 2 a weak band of low velocity along the eastern margin of the north american craton which reaches into the transition zone 3 a low velocity slab window beneath the western united states down to a depth of 300 km 4 areas of low uppermost mantle velocity beneath the cascade volcanoes the yellowstone hotspot track the colorado plateau the sierra madre occidental and the grabens bordering the jalisco block and 5 a pronounced band of high velocities in the transition zone coinciding with the expected location of the subducted trailing fragments of the farallon plate we introduce several improvements to the method of partitioned waveform inversion which was used to compute the new model rather than to correct for an estimated depth to the mohorovicic discontinuity we leave the crustal thickness as a free though yet poorly resolved parameter in the inversion we also improve the windowing and filtering operator used to select uncontaminated waveforms

anatomy of a metamorphic core complex seismic refraction wide angle reflection profiling in southeastern california and western arizona the metamorphic core complex belt in southeastern california and western arizona is a nw se trending zone of unusually large tertiary extension and uplift midcrustal rocks exposed in this belt raise questions about the crustal thickness crustal structure and the tectonic evolution of the region three seismic refraction wide angle reflection profiles acquired and analyzed as a part of the u s geological survey s pacific to arizona crustal experiment were collected to address these issues the results presented here which focus on the whipple and buckskin rawhide mountains yield a consistent three dimensional image of this part of the metamorphic core complex belt the seismic refraction wide angle reflection data are of excellent quality and are characterized by six principal phases that can be observed on all three profiles these phases include refractions from the near surface and crystalline basement reflections from boundaries in the middle and lower crust and reflections and refractions from the upper mantle the final model consists of a thin veneer 2 km of upper plate and fractured lower plate rocks 1 5 5 5 kms 1 overlying a fairly homogeneous basement 6 0 km s 1 and a localized high velocity 6 4 km s 1 body situated beneath the western whipple mountains a prominent midcrustal reflection is identified beneath the whipple and buckskin rawhide mountains between 10 and 20 km depth this reflector has an arch like shape and is centered beneath or just west of the metamorphic core complex belt this event is underlain by a weaker approximately subhorizontal reflection at 24 km depth together these two discontinuities define a lens shaped midcrustal layer with a velocity of 6 35 6 5 km s 1 the apex of this midcrustal layer corresponds roughly to a region of major tectonic denudation and uplift 10 km defined by surface geologic mapping and petrologic barometry studies the layer thins to the northeast and is absent in the transition zone the 6 35 6 5 km s 1 velocities are compatible with a diorite composition or a mixture of mafic and silicic rocks this midcrustal layer is underlain by a higher velocity lower crustal layer that is modeled as only 3 6 km thick beneath the metamorphic core complex belt and regions to the southwest to the northeast however this layer thickens to 8 10 km as the midcrustal layer pinches out above it the velocity of the lower crust is constrained by traveltime modeling and is 6 6 0 15 kms 1 beneath the western transition zone and the metamorphic core complex belt higher velocities may be present farther to the southwest where the layer is thin the velocity of the lower crust is too low to accommodate significant amount of mafic underplating at the base of the crust instead we interpret the velocities to indicate that the lower crust is passively thinned beneath these regions without significant addition of mafic mantle derived intrusions the crust mantle boundary does not dome up beneath the core complexes but remains approximately subhorizontal at a depth of 26 28 km or in the case of the whipple mountains actually deepens a 3 km crustal root is modeled this lack of upward doming of the moho together with the vertical alignment of the metamorphic core complex belt over what are believed to be extension related structures in the middle and lower crust suggest that there is no lateral offset of upper crustal deformation from deeper zones of extension as one would expect if extension occurred along crust penetrating shear zones wernicke 1981 wernicke et al 1985 instead domed and inflated middle crust and thinned lower crust directly underlie the region of greatest thinning of the upper crust

p and s wave tomography of the mantle beneath the united states mantle seismic structure beneath the united states spanning from the active western plate margin to the passive eastern margin was imaged with teleseismic p and s wave traveltime tomography including usarray data up to may 2014 to mitigate artifacts from crustal structure 5 40 s rayleigh wave phase velocities were used to create a 3 d starting model major features of the final p and s models include two distinct low velocity anomalies at depths of 60 300 km beneath the central and northern appalachians and passive margin the central appalachian low velocity anomaly coincides with eocene basaltic magmatism and the northern anomaly is located along the cretaceous track of the great meteor hot spot at depths of 300 700 km beneath the central and eastern u s large high velocity anomalies are inferred to be remnants of the farallon slab that subducted prior to 40 ma during the laramide orogeny

a unified continental thickness from seismology and diamonds suggests a melt defined plate thick rigid continents move over the weaker underlying mantle although geophysical and geochemical constraints on the exact thickness and defining mechanism of the continental plates are widely discrepant xenoliths suggest a chemical continental lithosphere 175 kilometers thick whereas seismic tomography supports a much thicker root 250 kilometers and a gradual lithosphere asthenosphere transition consistent with a thermal definition we modeled ss precursor waveforms from continental interiors and found a 7 to 9 velocity drop at depths of 130 to 190 kilometers the discontinuity depth is well correlated with the origin depths of diamond bearing xenoliths and corresponds to the transition from coarse to deformed xenoliths at this depth the xenolith derived geotherm also intersects the carbonate silicate solidus suggesting that partial melt defines the plate boundaries beneath the continental interior

contrasting lithospheric structure between the colorado plateau and great basin initial results from colorado plateau great basin passcal experiment the colorado plateau great basin cpgb passcal experiment provides broadband seismic data that allow us to determine crustal and upper mantle structure in the colorado plateau and eastern great basin twelve portable broadband seismometers throughout utah and nevada are supplemented by permanent broadband stations in the region receiver functions indicate an average crustal thickness of 35 5 km in the great basin gb with a strong p to s ps conversion from the moho the crust in the transition between the great basin and colorado plateau cp produces a strong intracrustal conversion the cp has somewhat thicker crust 42 5 km and weaker moho conversion br surface wave phase velocities are 3 4 higher than cp phase velocities in the 20 35 s range consistent with a thinner crust under the gb the phase velocities at periods greater than 40 s are indistinguishable between the two provinces indicating that differences between upper mantle velocities are minor shear wave splitting yields fast polarizations that parallel the western and northern boundaries of the cp with delay times decreasing towards the center of the plateau

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

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

the global attenuation structure of the upper mantle 1 a large data set of fundamental mode rayleigh wave amplitudes is analyzed to derive a new global three dimensional model of shear wave attenuation in the upper mantle the amplitude observations span a range of periods between 50 and 250 s and are derived from earthquakes with mw 6 0 that occurred between 1993 and 2005 four separate factors may influence an amplitude anomaly intrinsic attenuation along the raypath elastic focusing effects along the raypath uncertainties in the strength of excitation and uncertainties in the response at the station in an earlier paper dalton and ekstrom 2006a dependence of the retrieved attenuation structure on these terms was shown to be significant and an approach was developed to invert the amplitudes simultaneously for each term the new three dimensional attenuation model qrfsi12 which is the subject of this paper is derived using this method the model contains large lateral variations in upper mantle attenuation 60 to 100 and exhibits strong agreement with surface tectonic features at depths shallower than 200 km at greater depth qrfsi12 is dominated by high attenuation in the southeastern pacific and eastern africa and low attenuation along many subduction zones in the western pacific resolution tests confirm that the change in pattern of attenuation above and below 200 km depth can be determined with confidence using the fundamental mode data set the new model is highly correlated with global models of shear wave velocity particularly in the uppermost mantle suggesting that the same factors may control both seismic attenuation and velocity in this depth range however forcing the lateral perturbations in attenuation to match those found in global velocity models decreases the data variance reduction which suggests that subtle differences between patterns of attenuation and velocity are robust

upper mantle origin of the yellowstone hotspot fundamental features of the geology and tectonic setting of the northeast propagating yellowstone hotspot are not explained by a simple deep mantle plume hypothesis and within that framework must be attributed to coincidence or be explained by auxiliary hypotheses these features include the persistence of basaltic magmatism along the hotspot track the origin of the hotspot during a regional middle miocene tectonic reorganization a similar and coeval zone of northwestward magmatic propagation the occurrence of both zones of magmatic propagation along a first order tectonic boundary and control of the hotspot track by preexisting structures seismic imaging provides no evidence for and several contraindications of a vertically extensive plume like structure beneath yellowstone or a broad trailing plume head beneath the eastern snake river plain the high helium isotope ratios observed at yellowstone and other hotspots are commonly assumed to arise from the lower mantle but upper mantle processes can explain the observations the available evidence thus renders an upper mantle origin for the yellowstone system the preferred model there is no evidence that the system extends deeper than 200 km and some evidence that it does not a model whereby the yellowstone system reflects feedback between upper mantle convection and regional lithospheric tectonics is able to explain the observations better than a deep mantle plume hypothesis

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

assumptions and observations in tectonic modelling of rift basins some implications of thermo isostasy stress and rheology for intrabasinal structure abstract there has been a multitude of studies showing to what extent alterations in fundamental tectonic assumptions and or modelling parameters can affect predictions on how extensional sedimentary basins evolve however that little is known about how mantle processes initiate lithospheric rifting extension and affect basin evolution limits the extent to which improvements in tectonic modelling assumptions and parameter resolution can lead to a better understanding of intrabasinal or reservoir development processes the tectonic heat budget appears to be constrained by empirical means with the aid of kinematic models without the necessity of a thorough understanding of the dynamic processes involved in basin formation nevertheless the thermal coupling of tectonic model with intrabasinal development is sensitive to the choice of sediment thermal conductivities an important but often neglected tectonic modelling parameter to resolve is when the state of stress changes through time as a result of the interaction of all the known and unknown tectonic processes actively affecting the basin lithosphere system the occurrence of geologically short lived episodes of rapid basin subsidence may be a response to a change in the tectonic setting regardless of the relative sense of change of the ambient stress field recent theoretical studies suggest the same may be true for the evolution of salt diapirs

the role of oceanic plateau subduction in the laramide orogeny the cause of the laramide phase of mountain building remains uncertain conceptual models implicate the subduction of either ocean ridges or conjugates of the buoyant hess or shatsky oceanic plateaux independent verification of these models has remained elusive because the putative ridges or plateaux are no longer at the earth s surface inverse convection models have identified two prominent seismic anomalies on the recovered farallon plate here we combine inverse convection models with reconstructions of plate motions to show that these seismic anomalies coincide palaeogeographically with the restored positions of the shatsky and hess conjugate plateaux as they subducted beneath north america specifically the distribution of laramide crustal shortening events tracked the passage of the shatsky conjugate beneath north america whereas the effects of the hess conjugate subduction were restricted to the northern mexico foreland belt we propose that continued subduction caused the oceanic crust to undergo the basalt eclogite phase transformation during which the shatsky conjugate lost its extra buoyancy and was effectively removed increases in slab density and coupling between the overriding and subducting plates initially dragged the surface downward followed by regional scale surface rebound we conclude that laramide uplift resulted from the removal rather than emplacement of the shatsky conjugate

dynamics of extensional basin formation and inversion introduction this volume contains a cohection of papers on the dynamics of extensional basin formation and inversion presented at the 3rd annual workshop of the international lithosphere program task force origin of sedimentary basins in sundvollen norway 18 23 august 1992 previous workshops of the ilp task force had been in rueil malmaison france 1990 and in matrahaza hungary 19911 on the rim of the pannonian basin the task force also organized a number of sessions during the 1992 european geophysical society and the 1993 european union of geosciences conferences reports of these meetings have been published in a number of special volumes and papers cloetingh et al 1993a b stephenson 1993 cloetingh sassi and task force team 1994 the 4th task force meeting focusing on the interplay of extension and compression was held in benevento southern italy in october 1993 a selection of papers presented at that task force workshop will be published in a forthcoming special volume of tectonophysics the emphasis of the 5th workshop of the task force taking place in the dead sea rift israel is on rifting and strike slip tectonics the 1995 workshop will be organized in the southern pyrenean foreland basin with the aim of examining the relationships between tectonics and eustasy in foreland fold and thrust belts the emphasis of the sundvollen workshop was on modelling studies as well as observational approaches to the dynamics of basin extension and inversion important elements of the meeting were the discussion of extensional tectonics rifting in passive margin settings and volcanism and magmatism in rifts and basins following the task force strategy to link subsurface data to field analogues an important part of the meeting was devoted to a thematic excursion in the oslo rift field guides have been published separately larsen et al 1992a

thermal modelling of the laramide orogeny testing the flat slab subduction hypothesis abstract the laramide orogeny is the late cretaceous to palaeocene 80 55 ma orogenic event that gave rise to the rocky mountain fold and thrust belt in canada the laramide block uplifts in the usa and the sierra madre oriental fold and thrust belt in mexico the leading model for driving laramide orogenesis in the usa is flat slab subduction whereby stress coupling of a subhorizontal oceanic slab to the upper plate transmitted stresses eastwards producing basement cored block uplifts and arc magmatism in the foreland the thermal models presented here indicate that arc magma generation at significant distances inboard of the trench 600 km during flat slab subduction is problematic this conclusion is consistent with the coincidence of volcanic gaps and flat slab subduction at modern convergent margins lawsonite eclogite xenoliths erupted through the colorado plateau in oligocene time are inferred to originate from the subducted farallon slab and indicate that the laramide flat slab subduction zone was characterised by a cold thermal regime thermal modelling indicates that this regime can be produced by flat slab subduction of old 50 myr oceanic lithosphere at high convergence rates in the canadian and mexican portions of the laramide orogen the coeval development of a magmatic arc within 300 km of the trench refutes the existence of flat slab subduction in these regions it is proposed that subduction of an oceanic plateau aseismic ridge may have overcome the negative buoyancy inherent in old oceanic lithosphere and resulted in a spatially restricted zone of flat slab subduction in the usa these findings cast doubt on the flat slab model as a primary means of driving laramide orogenesis along its entire length and instead point to the need for an alternative mechanism for cordilleran wide laramide orogenesis

teleseismic surface wave tomography in the western u s using the transportable array component of usarray 1 fundamental mode rayleigh waves recorded by the transportable array component of earthscope usarray from january 2006 through april 2007 are used to generate phase velocity maps at periods from 25 to 100 sec across the western u s including washington oregon california nevada and western idaho at short periods 25 33 s low velocity anomalies are observed in western washington western and central oregon northern california the southern sierra nevada and the snake river plain at intermediate and long periods 50 100 s high velocities are seen in the cascades the southern central valley of california california s transverse range and the columbia river flood basalt province the phase velocity maps are consistent with those obtained from ambient noise tomography at comparable periods short period phase velocities from ambient noise tomography and the longer period phase velocities from teleseismic tomography therefore present natural data sets to invert jointly 3 d structure across the western u s

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

crust and upper mantle electrical conductivity beneath the yellowstone hotspot track combining long period magnetotelluric data from the spatially uniform earthscope usarray and higher resolution profiles we obtain a regional three dimensional electrical resistivity model in the snake river plain and yellowstone areas idaho and wyoming united states and provide new constraints on the large scale distribution of melt and fluids beneath the yellowstone hotspot track contrary to what would be expected from standard mantle plume models the electromagnetic data suggest that there is little or no melt in the lower crust and upper mantle directly beneath yellowstone caldera instead low mantle resistivities 10 m and below which we infer to result from 1 3 partial melt are found 40 80 km beneath the eastern snake river plain extending at least 200 km southwest of the caldera beneath the area of modern basaltic magmatism the reduced resistivities extend upward into the mid crust primarily around the edges of the snake river plain suggesting upward migration of melt and or fluid is concentrated in these areas the anomaly also shallows toward yellowstone where higher temperatures enhance permeability and allow melts to ascend into the crust the top of the conductive layer is at its shallowest in the upper crust directly beneath the modern yellowstone supervolcano

paleozoic lake faunas establishing aquatic life on land abstract lakes are important archives for continental records of paleoenvironmental as well as paleoclimatic change they also record a unique macroevolutionary pattern that occurred when faunas invaded the continental realm in order to document that pattern we compiled a database of over one hundred and six lake basins from the neoproterozoic to permian looking at this critical time in lacustrine evolution several patterns emerge for the neoproterozoic through silurian time there is a paucity of documented lake deposits and lake faunal records however the records that are available indicate that lakes were oligotrophic and their nutrient cycling regimes primitive it is not until the establishment of land plants in the silurian that lakes began to respond with higher diversities and more complex physical and chemical conditions during the devonian carboniferous periods diversity was on the rise as trophic structure became more complex most lakes of the devonian and carboniferous formed along continental margins or in tectonic basins with occasional connection to the marine realm the faunas from these types of lakes were commonly comprised of mixed marine and freshwater elements and were far more diverse than other more inland lakes this estuary effect created a gateway or filter through which faunas invaded the continental realm the early history of lake faunas is one of opportunity and amelioration the feedback loops created by the establishment of vascular plants altered the nutrient cycle on land and in lakes all trophic levels were established early but became increasingly complex throughout the paleozoic as roles changed and faunal elements became established groups invading the continents via the estuary effect did so numerous times before establishing themselves permanently this was linked with the episodic re establishment in marine freshwater connections along these continental margins

a kinematic model for the late cenozoic development of southern california crust and upper mantle a model is presented for the young and ongoing kinematic deformation of southern california crust and upper mantle that accounts for both the known late cenozoic geologic record and upper mantle structure in this model the high velocity upper mantle transverse ranges anomaly is created through the convergence and sinking of the entire thickness of subcrustal lithosphere using typical scaling relations we estimate this anomaly to be 500 c colder and 1 more dense than average southern california mantle of the same depth we infer that subcrustal lithosphere from both sides of the convergence zone participates nearly equally in the construction of the anomaly the low velocity upper mantle anomaly beneath the salton trough region is attributed to high temperatures and 1 4 partial melt related to adiabatic decompression during mantle upwelling our model has crustal motions in the central transverse ranges region differing from mantle motions while the subcrustal lithosphere south of the transverse ranges moves toward and sinks beneath the transverse ranges the corresponding crust largely avoids convergence by adopting a more westerly direction of motion the differing velocity fields necessitates of a decoupling zone immediately south of the transverse ranges near the moho we interpret the upper mantle anomalies as results of small scale convection beneath southern california to which we attribute much of the recent tectonic activity in the region

upper mantle structure beneath the caribbean south american plate boundary from surface wave tomography 1 we have measured shear wave velocity structure of the crust and upper mantle of the caribbean south american boundary region by analysis of fundamental mode rayleigh waves in the 20 to 100 s period band recorded at the bolivar geodinos stations from 2003 to 2005 the model shows lateral variations that primarily correspond to tectonic provinces and boundaries a clear linear velocity change parallels the plate bounding dextral strike slip fault system along the northern coast of venezuela illustrating the differences between the south american continental lithosphere the venezuelan archipelago and the caribbean oceanic lithosphere at depths up to 120 km beneath the venezuelan andes and the maracaibo block there is evidence of underthrusting of the caribbean plate but there is no other evidence of subduction of the caribbean plate beneath the south american plate in eastern venezuela linear crustal low velocities are associated with the fold and thrust belts whereas as higher crustal velocities are imaged in the guayana shield lithosphere the subducting oceanic part of the south american plate is imaged beneath the antilles arc the surface wave images combined with seismicity data suggest shear tearing of the oceanic lithosphere away from the buoyant continental south american plate offshore of northeastern venezuela the continental lithosphere south of the slab tear is bent down toward the plate boundary in response to the propagating tear in the lithosphere we interpret a nearly vertical low velocity column west of the tear centered beneath the cariaco basin with three dimensional asthenospheric flow around the southern edge of the subducting oceanic lithosphere with the asthenosphere escaping from beneath continental south america and rising into the plate boundary zone the complex plate boundary structure is best examined in three dimensions we discuss the new surface wave tomographic inversion in the context of results from other researchers including local seismicity teleseismic shear wave splits and interpretations from active source profiling