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

seismic tomography of the colorado rocky mountains upper mantle from crest lithosphere asthenosphere interactions and mantle support of topography abstract the crest experiment colorado rocky mountains experiment and seismic transects integrated the earthscope usarray transportable array with portable and permanent stations to provide detailed seismic imaging of crust and mantle properties beneath the highest topography region of the rocky mountains inverting approximately 14 600 p and 3600 s wave arrival times recorded at 160 stations for upper mantle v p and v s structure we find that large v p perturbations relative to ak135 of 7 and v s variations of 8 take place over very short approaching tens of kilometers lateral distances highest heterogeneity is observed in the upper 300 km of the mantle but well resolved low velocity features extend to the top of the transition zone in portions of these images the previously noted low velocity upper mantle aspen anomaly is resolved into multiple features the lowest v p and v s velocities in the region are found beneath the san juan mountains which is clearly distinguished from other low velocity features of the northern rio grande rift taos latir region aspen region and below the never summer mountains we suggest that the san juan anomaly and a similar feature below the taos latir region of northern new mexico are related to delamination and remnant heat and melt beneath these sites of extraordinarily voluminous middle cenozoic volcanism we interpret a northeast southwest grain in velocity structure that parallels the colorado mineral belt to depths near 150 km as being reflective of control by uppermost mantle proterozoic accretionary lithospheric architecture further to the north and west the wyoming province and northern colorado plateau show high velocity features indicative of thick 150 km preserved archean and proterozoic lithosphere respectively overall we interpret the highly heterogeneous uppermost mantle velocity structure beneath the southern rocky mountains as reflecting interfingered chemical proterozoic lithosphere that has been is currently being replaced and modified by upwelling asthenosphere low velocity features resolved here indicate that this process may be sourced as deeply as the top of the mantle transition zone at 410 km one driving mechanism for this is upper mantle interaction between upwelling hydration induced partial melt and destabilized downwelling lithosphere in the deeper upper mantle tomographic imaging of mantle seismic velocity and crustal thickness results and modeling from the crest experiment indicate that the highest elevations of the colorado rocky mountains are substantially supported by the mantle and strong correlations between low velocity mantle and thin crust high topography are noted across the region this along with rich upper mantle seismic heterogeneity suggests that mantle buoyancy and dynamics are central to present day topographic support and recent geomorphic evolution of the region

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

constraints on the lateral strength of slabs from three dimensional dynamic flow models if the viscosity of subducted lithosphere is described purely by temperature and pressure it should remain considerably more viscous than the surrounding material as it descends through the upper mantle many dynamic models of the long wavelength geoid associated with slabs require an increase in viscosity from the upper to the lower mantle but assume the perturbation to the lateral viscosity structure arising from the slabs themselves can be ignored previous studies indicate that strong and localized viscosity variations should have a dramatic influence on the geoid we present 3d finite element models of the regional geoid of the western pacific subduction zones slab buoyancies and viscosities are defined using the distribution of seismicity the geoid is very sensitive to the lateral strength of the slab very viscous slabs penetrating a low viscosity mantle generate significant 10 50 m long wavelength geoid lows opposite to the geoid high which is observed over slabs to obtain a geoid high comparable to that observed the lower mantle viscosity must be 60 200 times greater than the upper mantle viscosity and the slab must be in contact with the lower mantle these strict requirements suggest that slabs have been weakened and cannot act as stress guides from the deep mantle to the surface

seismic discontinuities beneath the southwestern united states from s receiver functions abstract s receiver functions along the colorado plateau rio grande rift great plains transect known as la ristra in the southwestern united states have been utilized to map seismic discontinuities beneath this tectonically active region individual receiver functions were stacked according to ray piercing points with moveout corrections in order to improve the signal to noise ratio of the converted s to p phases a mantle discontinuity which is interpreted as the lithosphere asthenosphere boundary lab is observed along the profile with depth ranging from 80 km beneath the rio grande rift rgr to 100 km beneath the great plains gp and 120 180 km beneath the colorado plateau cp the shallow lab beneath the rio grande rift is indicative of lithosphere extension and asthenosphere upwarp the lab deepens sharply at the rgr cp and rgr gp boundaries providing evidence for edge driven small scale mantle convection beneath la ristra two local discontinuities beneath the southeastern colorado plateau are imaged at 250 km and 300 km and could be the top and base of the eroded lithosphere respectively the s receiver function images suggest that edge driven small scale convection is probably the mantle source for recent extension and uplift in the rio grande rift and the colorado plateau

crustal structure of the eastern borborema province ne brazil from the joint inversion of receiver functions and surface wave dispersion implications for plateau uplift we investigate the crustal structure of the borborema province of ne brazil by developing 44 s wave velocity depth profiles from the joint inversion of receiver functions and fundamental mode rayleigh wave group velocities the borborema province is located in the northeasternmost corner of the south american continent and represents a portion of a larger neoproterozoic mobile belt that formed during the brasiliano pan african orogeny extensional processes in the mesozoic eventually leading to the separation of africa and south america left a number of aborted rift basins in the continental interiors and episodes of diffuse intraplate volcanism and uplift marked the evolution of the province after continental breakup our velocity depth profiles reveal the existence of two crustal types in the province i the thin crustal type which consists of 30 32 5 km thick crust with an upper layer of 3 4 3 6 km s overlying a lower layer of 3 7 3 8 km s and ii the thick crustal type which consists of a 35 37 5 km thick crust with velocities between 3 5 and 3 9 km s down to 30 km depth and a gradational increase in velocity vs 4 0 km s down to upper mantle depths the crustal types correlate well with topography with the thick crustal type being mainly found in the high standing southern borborema plateau and the thin crustal type being mostly found in the low lying sertaneja depression and coastal cuestas interestingly the thin crustal type is also observed under the elevated topography of the northern plateau we argue that the thick crustal type is rheologically strong and not necessarily related to postbreakup mantle processes as it is commonly believed we propose that extensional processes in the mesozoic stretched portions of the brasiliano crust and formed the thin crustal type that is now observed in the regions of low lying topography leaving the rheologically strong thick crust of the southern plateau at higher elevations the crust making the northern plateau would have thinned and subsided during mesozoic extension as part of a greater sertaneja depression to then experience uplift in the cenozoic and achieve its present elevation

new evidence for dislocation creep from 3 d geodynamic modeling of the pacific upper mantle structure laboratory studies on deformation of olivine in response to applied stress suggest two distinct deformation mechanisms in the earth s upper mantle diffusion creep through diffusion of atoms along grain boundaries and dislocation creep by slipping along crystallographic glide planes each mechanism has very different and important consequences on the dynamical evolution of the mantle and the development of mantle fabric due to the lack of in situ observations it is unclear which deformation mechanism dominates in the upper mantle although observed seismic anisotropy in the upper mantle suggests the presence of dislocation creep we examined the thermo mechanical erosion of the lithosphere by thermal boundary layer instabilities in 3 d dynamical models this study demonstrates that the seismically derived thermal structure of the pacific lithosphere and upper mantle imposes an important constraint on the upper mantle deformation mechanism the predominant deformation mechanism in the upper mantle is dislocation creep consistent with observed seismic anisotropy the acceptable activation energy range of 360 540 kj mol is consistent with although at the lower end of those determined from laboratory studies

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

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

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

resolving time space histories of late cenozoic bedrock incision along the upper colorado river usa abstract the upper colorado river basin drains the western slope of the rocky mountains province of north america and hosts a rich record of fluvial terraces and volcanic deposits that provide an archive of bedrock river incision since the late miocene here we present new geochronology from fluvial deposits and associated volcanic units using multiple methods luminescence cosmogenic burial 40ar 39ar basalt and detrital sanidine and u pb detrital zircon to reconstruct a detailed record of bedrock incision in the upper colorado river over timescales ranging from miocene to late pleistocene late miocene ca 11 8 ma basalt flows perched 800 1700 m above the colorado river and the oldest known ancestral colorado river deposits preserved beneath basalt flows on grand mesa provide the basis for average long term 107 yr incision rates rates range from 110 160 m ma between grand junction downstream and glenwood canyon upstream with rates decreasing upvalley intermediate scale 105 106 yr incision rates are 200 320 m m y over the past 1 3 ma and these rates are faster than the average over the past 8 11 ma short term

microsedimentological characterization using image analysis and μ xrf as indicators of sedimentary processes and climate changes during lateglacial at laguna potrok aike santa cruz argentina palaeoclimatic and paleoenvironmental high latitude records in the southern hemisphere are scarce compared to the northern counterpart however understanding global evolution of environmental systems during sudden climate changes is inseparable from an equivalent knowledge of both hemispheres in this context a high resolution study of lacustrine sediments from laguna potrok aike santa cruz province patagonia argentina was conducted for the lateglacial period using concurrent x ray fluorescence xrf and scanning electron microscope analyses peaks of ca si and mn and occurrences of the green alga phacotus lenticularis have been interpreted as variations in ventilation of the water column from 13 6 to 11 1 ka cal bp during this interval mild climate conditions during the younger dryas are characterized by relatively weak westerlies favouring the formation of a stratified water body as indicated by preserved manganese and ca si peaks and high total organic carbon toc values in this environment water in the epilimnion can reach sufficiently high temperature to allow p lenticularis to grow colder conditions are marked by peaks in ca without p lenticularis and occur during the antarctic cold reversal acr in this lateglacial interval micropumices were also detected in large amount image analysis of thin sections allowed the counting and size measurement of detrital particles and micropumices separately micropumices significantly influence the iron and titanium content hence preventing to use them as proxies of detrital input in this interval

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

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

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

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

colorado river chronostratigraphy at lee s ferry arizona and the colorado plateau bull s eye of incision abstractlee s ferry arizona united states lies at an important geo logic transition between the grand canyon margin and the canyon lands center of the colorado plateau it marks a knickpoint along the colorado river at the top of the steep grand canyon and it is central to debate about the patterns of erosion and sources of uplift in this famous landscape new chronostratigraphic data from the suite of ll terraces here indicate a strong uvial response to climate driv ers superimposed upon an integrated mid to late pleistocene incision rate of 350 m m y a regional compilation of well constrained results over the same timescale reveals that this is intermediate between slower rates downstream in grand canyon and even faster rates in the central colorado plateau which taper off again farther upstream near the plateau s eastern edge this bull s eye pattern of rapid inci sion in the central colorado plateau does not match proposed sources of uplift from mantle dynamics at the south and west ank of the pla teau nor patterns of river steepness and energy instead we suggest that this incision pattern is primarily driven by transient response to drainage integration and isostatic feedback from the deep exhuma tion of weak rocks in the central plateau introduction

the deep structure of the scandes and its relation to tectonic history and present day topography abstract we review the results of the toposcandiadeep project a component of the topoeurope project in which we have studied the crustal and upper mantle structures of southern norway in relation to its high topography the scandinavian mountain chain the scandes is an intracontinental mountain chain at the western edge of the baltic shield and has its southern part located in southern norway the timing as well as the processes causing the formation of the scandes are disputed we bring new geophysical constraints to this issue by providing crustal and mantle seismic models for the area and by integrated modeling of the lithosphere and its potential deformation new maps of moho depth and crustal seismic velocities have been compiled using data from refraction lines p receiver functions and noise cross correlation these results show a thickening of the crust from southwest to northeast and a small crustal root not directly located below the topographic high p s and surface wave tomography infer seismic mantle velocities lower than in normal shield structure with a possible sharp boundary close to the oslo graben these low velocities are imaged in the lithosphere and in the underlying mantle down to the 410 km discontinuity integrated modeling of seismic models and gravity data shows that the low velocities below southern norway are compatible with a change in lithosphere thickness from c 100 km under southern norway to nearly 200 km under southern sweden with possible additional differences in composition the study also indicates that the topography can be isostatically sustained by the density distribution in the crust and lithospheric mantle we argue that the lithospheric lateral variation has been present for at least 300 my and has had a significant influence on the localization of the topography independently of the mechanism for uplift

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

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

thermal history of colorado plateau lithosphere from sm nd mineral geochronology of xenoliths the thermal history of the lower crust and upper mantle of the colorado plateau region is reconstructed on the basis of nd and sr isotopes in minerals and whole rock xenoliths hosted by tertiary minette and kimberlite the mineral data garnet and clinopyroxene indicate that lower crustal granulite and amphibolite equilibration depth 25 km equilibration temperature 700 c were last equilibrated on a mineral scale at 1345 10 ma a sm nd garnet whole rock age for a granitoid xenolith is also 1345 10 ma whole rock data indicate that the crustal rocks were extracted from the mantle at ca 1900 ma the mineral ages which are 30 100 m y younger than crystallization ages of proterozoic anorogenic granitoids from regions bordering the colorado plateau are interpreted as cooling ages set following the crustal thermal maximum at 1380 1440 ma the granulites and amphibolites have remained at temperatures below 450 c since 1350 ma two eclogite xenoliths equilibration depth 45 60 km equilibration temperature 600 c which are inferred to be precambrian as well yield sm nd garnet clinopyroxene ages of 21 6 1 2 and 21 0 0 8 ma the eclogite mineral ages are probably the ages of the host garnet ridge and moses rock diatremes and require that nd isotopes were maintained in equilibrium right up to the time of entrainment the isotopic data and the mineral textures suggest that the eclogites were undergoing active recrystallization at 21 ma the contrast in mineral ages between granulite and eclogite xenoliths indicates that the equilibration temperatures of the two rock types reflect different times of equilibration and therefore cannot be considered as evidence for a negative thermal gradient at depth the rb sr mineral data from the xenoliths give variable early paleozoic and proterozoic ages that cannot easily be assigned to geologic events

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

time transgressive and extension related basaltic volcanism in southwest utah and vicinity late cenozoic 15 ma basaltic rocks define two temporally and spatially distinct magmatic belts in southwest utah that are located in the basin and range province the transition zone and into the colorado plateau interior the belts share certain time space composition characteristics and are dissimilar in others the north south black rock grand canyon belt generally 2 5 0 ma exhibits no province wide time space patterns but shows distinct space composition variations involving both lithospheric and asthenospheric magma sources magmatism is inferred to have occurred in response to east west extension and the composition of the basalts is compatible with maximum thinning and extension by pure shear of the mantle lithosphere occurs near the transition zone in contrast the pahranagat san rafael belt 15 3 5 ma exhibits a northeastward time space progression of magmatism the magmatic migration vector is similar to the yellowstone trend suggesting the ultimate influence of a fixed heat or magma source in the asthenosphere although the basalts record apparent lithospheric signatures basalts from this belt are progressively more isotopically primitive and more alkaline into the colorado plateau these chemical variations can be interpreted as a change in the mantle source depth or degree of partial melting or decreasing crustal contamination to the northeast

the crustal structure of the arizona transition zone and southern colorado plateau from multiobservable probabilistic inversion the arizona transition zone is a narrow band that separates two of the main and most contrasting tectonic provinces in western us namely the southern colorado plateau and the southern basin and range provinces as such the internal crustal structure and physical state of this transitional zone hold clues for understanding i the amalgamation of these provinces ii the partitioning of deformation due to both past and present day stress fields and iii the role of thermal versus compositional effects in controlling surface observables here we employ and expand a novel multiobservable probabilistic inversion method and jointly invert fundamental mode rayleigh phase velocities receiver functions surface heat flow geoid height and absolute elevation to obtain an internally consistent 3 d model of the temperature density vs and vp of the arizona transition zone and the southern portions of the colorado plateau and basin and range our results confirm a significant crustal thickening from 28 km in the sw of the arizona transition zone and southern basin and range to 48 km beneath the southern colorado plateau inverted temperatures agree well with the location of recent volcanism and indicate that the lithosphere asthenosphere boundary is not deeper than 70 km in most of the region we find that major pre cambrian surface structures and or shear zones separate crustal domains with distinct bulk properties suggesting that the juxtaposed crustal blocks still retain at least in part their original characteristics however widespread intrusions of significant volumes of mafic magmas have affected these blocks at different depths locally overprinting their original compositions and creating highly heterogeneous crustal sections a dominant and large scale internal crustal pattern of sw dipping planes structures is evident in our models coinciding with the orientation of deep faults previously inferred from earthquake focal mechanisms while we cannot categorically corroborate the presence of melt or aqueous fluids within the crust our results are compatible with these scenarios beneath some parts of the basin and range the mogollon datil and springerville volcanic fields

magnetotelluric imaging of lower crustal melt and lithospheric hydration in the rocky mountain front transition zone colorado usa we present an electrical resistivity model of the crust and upper mantle from two dimensional 2d anisotropic inversion of magnetotelluric data collected along a 450 km transect of the rio grande rift southern rocky mountains and high plains in colorado usa our model provides a window into the modern day lithosphere beneath the rocky mountain front to depths in excess of 150 km two key features of the 2d resistivity model are 1 a broad zone 200 km wide of enhanced electrical conductivity 20 m in the mid to lower crust that is centered beneath the highest elevations of the southern rocky mountains and 2 hydrated lithospheric mantle beneath the great plains with water content in excess of 100 ppm we interpret the high conductivity region of the lower crust as a zone of partially molten basalt and associated deep crustal fluids that is the result of recent less than 10 ma tectonic activity in the region the recent supply of volatiles and or heat to the base of the crust in the late cenozoic implies that modern day tectonic activity in the western united states extends to at least the western margin of the great plains the transition from conductive to resistive upper mantle is caused by a gradient in lithospheric modification likely including hydration of nominally anhydrous minerals with maximum hydration occurring beneath the rocky mountain front this lithospheric hydration front has implications for the tectonic evolution of the continental interior and the mechanisms by which water infiltrates the lithosphere

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

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

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

a geophysical model for the origin of volcano vent clusters in a colorado plateau volcanic field variation in spatial density of quaternary volcanic vents and the occurrence of vent clusters correlate with boundaries in proterozoic crust in the springerville volcanic field svf arizona usa inverse modeling using 538 gravity measurements shows that vent clusters correlate with gradients in the gravity field due to lateral variation in crustal density these lateral discontinuities in the crustal density can be explained by boundaries in the north american crust formed during proterozoic accretion spatial density of volcanic vents is low in regions of high density proterozoic crust high in areas of relatively low density proterozoic crust and is greatest adjacent to crustal boundaries vent alignments parallel these boundaries we have developed 2d and 3d numerical models of magma ascent through the crust to simulate long term average magma migration that led to the development of vent clusters in the svf assuming that a viscous fluid flow through a porous media is statistically equivalent to magma migration averaged over geological time in the full field scale the location and flux from the uniform magma source region are boundary conditions of the model changes in model diffusivity associated with changes in the bulk properties of the lithosphere can simulate preferential magma migration paths and alter estimated magma flux at the surface implying that large scale crustal structures such as inherited tectonic block boundaries influence magma ascent and clustering of volcanic vents probabilistic models of volcanic hazard for distributed volcanic fields can be improved by identifying crustal structures and assessing their impact on volcano distribution with the use of numerical models

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

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

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

age of a eucrite clast from the bholghati howardite 39ar 40ar rb sr and sm nd investigations of the large eucrite clast 1 studied by the bholghati consortium show that it crystallized from a melt 4 51 ga ago the crystallization age is given most precisely by a sm nd internal isochron of 4 510 0 034 ga 2 error limits for 147sm 0 00654 ga 1 142nd144nd measurements show that the clast contained live 146sm at the time of crystallization a solar system initial 146sm144sm0 0 00431 0 0039 0 0026 at 4 555 ga ago can be calculated from a 146sm 142nd isochron and the age of the clast alternatively using 146sm144sm 0 0 0045 as previously reported in the literature a 146sm142nd formation interval of 0 05 0 12 0 08 ga can be calculated which is concordant with the 147sm 143nd isochron age and in agreement with pu u xe and pu nd xe formation intervals reported in a companion consortium paper the initial 143nd144nd ind of the clast was ind 0 505958 50 2 error limits normalized to 146nd 144nd 0 724140 and is consistent with the chur reference value for evolution in a source having chondritic relative ree abundances prior to magma generation however when interlaboratory bias is considered the data are also permissive of a lree depleted source as suggested in a companion consortium paper a primary rb sr isochron yields an age of 4 54 0 06 ga 2 error limits for 87rb 0 01402 ga 1 in agreement with the more precise sm nd age the initial 87sr86sr isr of the clast was isr 0 699006 21 and was the same within error limits as isr previously determined for a pristine eucrite clast from polymict eucrite y75011 compatible with a common parent body the 39ar 40ar data and some of the rb sr data show that bholghati was affected by a major thermal event 2 3 ga ago and also provide weak evidence for a minor thermal event 1 ga ago the age and isotopic systematics of the clast are consistent with those of other eucrites and support the widely held assumption of a common parent body for howardites and eucrites the isotopic investigations of polymict eucrites and howardites point to a prolonged cratering record on the howardite eucrite diogenite hed parent body and suggest that it survived the cataclysm affecting the moon 3 8 4 0 ga ago

basal continental mantle lithosphere displaced by flat slab subduction during the subhorizontal flat subduction of an ocean lithosphere plate the overlying continental plate is deformed far inland of the plate boundary in addition arc magmatism which is caused by melting in the asthenospheric wedge during steep subduction wanes or ends when the subduction is subhorizontal the observed upper plate deformation patterns have been explained by an end load exerted at the plate boundary or by stress transmitted into the overlying plate along the top of the flat slab here we present numerical thermal mechanical models of flat slab subduction that show the flattening of the slab results in a compression of the continental plate through end loading the advancing flat slab scrapes off the lowermost 20 50 km of continental mantle lithosphere the displaced continental mantle lithosphere fills the asthenospheric wedge ending arc type melting if the displaced material is buoyant it accumulates in a growing keel that migrates ahead of the slab if it is dense the displaced material sinks with the slab flat slab removal renews the asthenospheric wedge and arc magmatism and leaves a step in the lithosphere asthenosphere boundary and or a keel that consists of displaced continental mantle lithosphere a fossil keel and a fossil step formed during laramide flat subduction are preserved below the western united states continental mantle lithosphere is scraped from the base of the overriding plate by the underlying oceanic slab during flat subduction according to numerical thermal mechanical models

timing of basaltic volcanism along the mesa butte fault in the san francisco volcanic field arizona from 40ar 39ar dates implications for longevity of cinder cone alignments using high precision stepwise heating 40ar 39ar methods we dated volcanic products of at least 12 vents cropping out along the northern segment of the mesa butte fault in the san francisco volcanic field arizona where the relationship between fault and vent alignment is unambiguous the 40ar 39ar isochron dates indicate that the mesa butte fault hosted five episodes of volcanism over a 1 million year interval from 1400 to 300 ka the first episode occurred 1 km northwest of the trace of the mesa butte fault whereas the next two episodes were centered near the intersection of the mesa butte and cedar ranch faults the final two episodes produced red mountain 750 100 ka and shadow mountain 300 100 ka respectively ages of vents representing the third mesa butte episode of volcanism 760 50 ka to 900 80 ka indicate a systematic southwestward shift in the locus of volcanism along the mesa butte fault that contrasts sharply with overall eastward younging of volcanism within the field these data demonstrate that structures in basaltic volcanic fields are capable of hosting multiple episodes of volcanism over periods of time comparable to duration periods of composite volcanoes 106 years but at significantly lower recurrence intervals these results suggest that probability models of spatiotemporal recurrence of volcanism within basaltic volcanic fields should incorporate mapped structures particularly where these structures have either hosted previous volcanic activity or are proximal to population centers or sensitive installations such as nuclear power plants or waste repositories

overview tectonic implications of cenozoic volcanism in coastal california cenozoic volcanic rocks of coastal california were erupted west of the magmatic arc trend related to subduction along the continental margin two assemblages representing discrete pulses of mid tertiary oligocene miocene and mid miocene volcanism occupied relatively compact tracts as restored palinspastically prior to disruption and dispersal by tectonic displacements within the neogene san andreas transform system a third assemblage records migratory post mid miocene volcanism at centers located progressively farther north the distribution and petrologic character of all three assemblages reflect slab window magmatism triggered by decompression melting of upwelling mantle paleogeographic reconstructions based on analysis of magnetic anomaly patterns offshore and restoration of on land features prior to san andreas transform slip and associated transrotational deformation indicate the paleotectonic positions of the cenozoic volcanic fields before structural disruption the pulses of mid tertiary and mid miocene volcanism were related to transient episodes of mantle upwelling generated successively by rise trench encounters associated with subduction of the vancouver farallon and monterey arguello plates respectively along the continental margin off southern california migratory post mid miocene volcanism in central california accompanied the incremental expansion of a growing slab window as the mendocino triple junction migrated northward along the continental margin mid miocene columbia river basalt volcanism in the pacific northwest was coeval with the mid miocene pulse of coastal volcanism and may have reflected tectonism induced by the final demise of offshore microplates to allow initial full integration of the san andreas transform as a coherent plate boundary columbia river volcanism may have stemmed from mantle perturbation caused by torsional deformation of the continental block in response to shear imposed by the pacific plate

mapping high pn velocity beneath the colorado plateau constrains uplift models the massive international seismological centre data set of the past 20 years and the two station method are used to determine pn velocities in the mantle lid beneath the colorado plateau in this method the event is located at distances where pn is the first arrival 2 16 and the path is in or very near the azimuth of the two station pair and crosses the plateau this technique to a large extent minimizes the hypocenter mislocation effect and possible errors due to variations in the crustal structure near the source since only the difference in travel times at the two stations is used however this technique has a few underlying assumptions and possible sources of errors such as the quality of the pn data base and station delays caused by varying crustal structure that require an extremely careful application of the method a detailed study of the source of errors and a methodology of selection of the data are presented application of this method to the colorado plateau using all possible two station pairs from 53 stations located within or along the margin of the plateau yields an average high pn velocity of 8 12 0 09 km s this value is considerably larger than the average value of 7 83 km s based on available but very limited seismic refraction profiles but is remarkably similar to the average value of 8 1 km s for the relatively stable midcontinent region our new pn velocity for the colorado plateau eliminates the paradox in the literature that emphasizes the rather close similarity between average pn velocities beneath the colorado plateau and the basin and range province while their tectonic and magmatic cenozoic history is dramatically different previous models for the structure and evolution of the plateau have used the low pn velocity as an important constraint on density and thermal state of the lithosphere hence such models should be reexamined on the basis of this new uppermost mantle pn velocity determination there are two main models that have been proposed to explain the 2 km uplift of the colorado plateau one is based on a combination of thermal thinning of the lithosphere and crustal thickening and the other involves a combination of the delamination of the subducted subhorizontal farallon oceanic plate from the overriding north american plate and crustal thickening we show that the delamination model is more readily consistent not only with our velocity determination and the elevation of the plateau but also with varied geological observations reported in the literature that concern the cenozoic evolution of western north america

laramide crustal thickening event in the rocky mountain foreland and great plains the rocky mountain foreland and great plains of the western united states were formerly part of a continental platform adjusted by erosion and deposition in cambrian through jurassic time to near mean sea level and therefore to a near uniform crustal thickness of approximately 33 km today the region stands at regional elevations up to 2 km isostatically supported by a crust exceeding 50 km in thickness reasonable estimates of tertiary sedimentation and laramide strain do not account for more than 15 of the implied thickening however from approximately 70 40 m y b p this region was underlain by a horizontally subducting slab of farallon plate lithosphere moving northeast this slab could have been the cause of the thickening finite difference thermal models with specified kinematics show only a temporary cooling of the base of the north american lithosphere by this slab the excess weight of the slab would have depressed the region plate bending calculations show a quantitative agreement of predicted depression with upper cretaceous isopachs since depression by this slab lasted until the eocene at least the latest cretaceous regression was probably caused by a laramide crustal thickening event the farallon slab might have caused crustal thickening in two ways its excess weight would have drawn in ductile lower crust from surrounding regions however calculations show that this effect is too slow too local and too reversible to explain most of the crustal thickening therefore it seems likely that ductile lower crust was transported from sw to ne by shear stresses which the farallon plate exerted on the base of the north american lithosphere a preliminary finite element calculation based on this hypothesis shows the correct general pattern of crustal thickening an unexpected but encouraging result is that predicted principal compression directions are orthogonal to many laramide basement uplifts

bulk crustal properties of the borborema province ne brazil from p wave receiver functions implications for models of intraplate cenozoic uplift abstract we investigate variations in crustal thickness and bulk vp vs ratio across the borborema province of ne brazil by analyzing teleseismic p to s conversions recorded at 52 seismic stations in the province the borborema province represents the western portion of a larger neoproterozoic mobile belt that resulted from the assembly of gondwanaland and that split from the african continent during mesozoic times the evolution of the province after continental breakup was marked by episodes of diffuse intraplate magmatism perhaps leading to uplift of the borborema plateau in the cenozoic a number of geodynamic models have been proposed to explain coeval cenozoic magmatism and uplift in the province which invoke either thermal anomalies under the plateau and related mantle upwellings channeling along lithospheric thin spots from a distant mantle plume and small scale convection at the continental edge alternatively plateau uplift might have resulted from thickening of the crust after depth dependent stretching of the continental lithosphere in the mesozoic most of the models imply mafic underplating of the plateau s crust in order to fully explain its elevated topography but the volume of such mafic underplate varies among them our results show that i the crust is 32 40 km thick under the borborema plateau ii the crust is generally thinner about 30 33 km under the lower topographies surrounding the plateau and iii vp vs ratios are in the 1 68 1 80 range for both regions of higher and lower topography no apparent correlation is observed between vp vs ratio and crustal thickness our results suggest that compositional differences between thick and thin crust across the borborema province are minimal and that models of plateau uplift involving a small volume of mafic underplate provide a more plausible explanation for the observed topography of the borborema plateau

seismic evidence for lithospheric modification beneath the mojave neovolcanic province southern california 1 the mojave neovolcanic province mnvp located in the mojave block of southern california comprises late miocene to quaternary small volume basaltic centers geochemistry indicates an asthenospheric source for the mnvp beginning in the late miocene but no physical evidence of missing mantle lithosphere has been presented we utilize receiver functions and ambient noise tomography to image the lithosphere beneath the mojave block regionally we find thin crust that thickens distal to sites of mnvp volcanism shear wave velocities between 40 and 75 km depth are consistent with the presence of mantle lithosphere in the southern mojave block and very thin or missing mantle lithosphere to the north with one exception mnvp volcanoes lie along this sharp boundary our observations together with the established geologic history and geochemistry of the mnvp can be explained by small scale edge driven convection producing ongoing lithospheric removal within the mojave block our results provide another example of lithospheric instability that occurs in response to rapid changes in mantle dynamics induced by major changes in tectonic plate geometry

strong lateral strength contrasts in the mantle lithosphere of continents a case study from the hot sw canadian cordillera abstract this study aims at quantifying the 3 d variability in lithosphere strength of the south eastern canadian cordillera and adjacent craton to the east strength is calculated in a forward manner starting from rheological laws of brittle and ductile deformation the work flow calculates a temperature model based on a multi layer compositional model and subsequently estimates the strength distribution from both the compositional and temperature models the temperature modeling involves numerical inversion and cubic spline algorithms which enable to impose boundary conditions that account better for strong lateral variations in lithosphere thickness and lateral heat flow this addresses the lithosphere structure between the canadian cordillera and craton the canadian cordillera is marked by a hot and thin lithosphere of 60 km thickness that stands in contrast to a cold and thick craton of 170 km thickness to the east which consequently results in pronounced changes in bulk lithosphere strength the high surface heat flow of the cordillera interior and its contrast with the foreland basin can be reproduced by temperature models that combine elevated mantle heat flow due to the thin lithosphere and higher crustal heat production from magmatic intrusions a series of rheological models which examines the role of different temperature input models composition and strain rate shows that the first order strength pattern is persistent for the hot canadian cordillera strength resides for 80 in the upper crust and with integrated lithosphere strength of 2 0 3 5 106 mpa m for the cold craton the upper mantle provides 75 of the integrated strength with values of 4 0 8 0 106 mpa m for the crust and 20 65 106 mpa m for the lithosphere effective elastic thickness is estimated between 5 and 15 km for the cordillera and 40 80 km for the craton this illustrates the cordillera to craton transition as a prominent rheological feature for upper mantle flow dynamics and spatial seismicity trends

colorado river long profile metrics knickzones and their meaning abstract the numerical analysis of river long profiles has a rich history and is experiencing a rejuvenation with the advent of improved terrain models and computational tools the highly variable profile and the unusual hydrology of the green colorado river system crossing the colorado plateau provide an example that is germane to debates about transient versus equilibrium knickzone features about the controls of baselevel change due to drainage integration or differential uplift and the role of adjustments to climate hydrology and changing bedrock in order to explore these controls in more detail we define objective geologic study reaches along the green colorado river calculate unit stream power and develop a discharge adjusted steepness index kqsn that avoids the scaling assumptions in substituting contributing area for discharge which are erroneous in this dryland setting determining the nature of knickzones is key and the colorado plateau is a prime setting for study because of the legacy of previous work and a burst of new research and debate the four canyon knickzones along the green colorado are characterized by energy expenditure an order of magnitude greater than in intervening reaches and our kqsn proxy tracks stream power well the strong coincidence of the grand canyon cataract canyon and eastern uinta knickzones with stronger bedrock and mass movement inputs suggests they are pinned equilibrium adjustments to greater bed resistance with the possible exception of desolation canyon as a transient feature marked by anomalous profile convexity along the trunk drainage the knickzone anomalies become greater downstream this and patterns of kqsn in headwater tributaries correspond to some proposed sources of broad mantle driven uplift but they do not match other proposed uplift patterns thus a poorly understood decoupling of topography and proposed uplift exists in some regions