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

the localized role of base level lowering on channel adjustment of tributary streams in the trinity river basin downstream of livingston dam texas usa the purpose of this study is to describe and explain river channel cross sectional change in tributary streams within the 60 km scour zone of the trinity river texas downstream of livingston dam this study investigates changes within the tributary systems to base level changes triggered by incision associated with scour and channel migration within the dammed trunk stream the dam represents a precise moment and place of a system perturbation and therefore represents an opportunistic geomorphic experiment geomorphological effects of the lower trinity river tributaries were investigated through three different types of data resurveys of bridge cross sections planform change as measured from aerial photographs and field mapping of changes in channel geometry using fallen trees undercut banks and other geomorphic indicators of change while field evidence of channel change was found at sites well upstream of tributary confluences with the trinity these were not consistent with responses to lower base level downcutting and lateral changes in response to lowered base level were confined to lower tributary reaches this suggests distance decay and lag times in transmission of responses upstream and or masking of dam induced effects by other processes and controls the most dynamic locations of geomorphic change are near the confluences of the tributaries with the trinity river where the response to dam induced incision is strongly influenced by local trunk stream morphology indicating a high degree of historical and geographical contingency

late quaternary alluvial stratigraphy of a low order tributary in central texas usa and its response to climate and sediment supply abstract this paper presents the first comprehensive late quaternary alluvial stratigraphic study of a low order tributary in central texas using cowhouse creek as a case study the late pleistocene jackson ja alluvium forms the elevated t2 terrace the entrenched holocene valley is filled with the buried georgetown gt alluvium approximately 11 000 to 8000 14 c yr b p and associated royalty paleosol and the surficially exposed fort hood fh alluvium approximately 7000 to 5000 14 c yr b p and west range wr alluvium approximately 4200 to 600 14 c yr b p forming the broad t1 terrace the ford fo alluvium 14 c yr b p forms the modern t0 floodplain entrenched into t1 conditioned by cooler and wetter climates cowhouse creek was characterized by relatively high base flow and low sediment supply during deposition of the ja and gt alluvium appreciable upland soil erosion ensued during the middle holocene in response to warmer climate conditions resulting in widespread valley filling by the fh alluvium deposition of the late holocene wr and f0 alluvium was characterized by diminished sediment storage during relatively stable climate conditions the temporal alluvial stratigraphic framework of the bedrock confined cowhouse creek valley is out of phase with the alluvial sequence in the larger brazos river valley

tributary trunk stream relations in a cut and fill landscape a case study from wolumla catchment new south wales australia at the time of european settlement of the south coast of new south wales australia in the mid nineteenth century river courses were discontinuous throughout wolumla catchment within a few decades of settlement intact valley fills had been transformed into incised channels along trunk streams this induced associated patterns of secondary incision into tributary valley fills three of the primary subcatchments in wolumla catchment became fully incised while an upland swamp and a floodout i e sand sheet deposits atop an intact valley floor remain in mid catchment of the fourth subcatchment of the 70 lower order tributaries which join the trunk streams in the four subcatchments 27 have incised 12 of which are discontinuously incised incision of tributary streams in wolumla catchment cannot be explained by subcatchment area and slope relations the position of the trunk stream channel within the valley floor trough is the key determinant of whether or not tributary streams have incised changes to river morphology since european settlement have altered the linkage of tributary streams to the trunk stream impacting directly on the within catchment transfer of water and sediment

some effects of human modifications on sand bed channels in southwestern mississippi u s a this study examines the effects of land use changes and channel foreshortening on the sediment regime and channel morphology of two sand bed rivers in southwestern mississippi historical data including sequences of channel surveys at various bridge sites document the impact of these human modifications land use changes caused an increase in point bar size but this trend was found to be reversed as the result of conservation and reforestation practices channel foreshortening has produced a knickpoint that has migrated 50 km upriver river response to this disturbance included 5 m of channel incision a 4 5 fold increase in bankfull width and a fourfold increase in point bar size

dam to delta sediment inputs and storage in the lower trinity river texas livingston dam on the trinity river in se texas usa disrupts the transport of sediment to the lower trinity river and the trinity bay galveston bay estuary however a sediment budget of the lower basin shows that the effects of this disruption are undetectable in the lower river sediment trapped in lake livingston is partly offset by channel erosion downstream of the dam and by inputs from the lower basin most importantly however the lower coastal plain reaches of the trinity are characterized by extensive alluvial storage and are a bottleneck that buffers the bay from effects of upstream changes in sediment flux storage is so extensive that the upper trinity basin and the lowermost river reaches were essentially decoupled in the sense that very little upper basin sediment reached the lower river long before the dam was constructed whereas sediment storage in lake livingston is extensive alluvial storage on the trinity flood plain is even more extensive dam related sediment starvation effects are noted for about 52 km downstream and the sediment budget suggests that a majority of the sediment in this reach is likely derived from channel scour and bank erosion the capacious alluvial storage in the lower trinity not only limits flux to the bay but the large amount of remobilizable alluvium also allows the system to adjust to localized sediment shortages as illustrated in the dam to romayor reach internal adjustments within the lower trinity river valley thus buffer the bay from changes in sediment supply upstream

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

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

seismic imaging of the crust and upper mantle using regularized joint receiver functions frequency wave number filtering and multimode kirchhoff migration 1 receiver functions provide an indispensable tool for producing discontinuity images of the crust and upper mantle from teleseismic earthquake arrivals however image quality can be significantly compromised by instabilities inherent in the deconvolution process and by irregularities in data coverage driven by source and or receiver geometries we present receiver function estimation and prestack migration techniques which both reduce receiver function deconvolution instability and produce regularized multimode receiver function images the receiver function estimation technique exploits frequency wave number filtering methods using frequency pseudowave number domain filtering of gathers to enhance arrivals with consistent moveout characteristics synthetic tests confirm robust recovery of features in the presence of high noise levels to process regularized receiver functions into accurate images of the subsurface seismic scattering potential we employ a regularized kirchhoff migration that migrates both direct and reverberated p to s converted modes to their correct subsurface locations the subsequent images can be stacked to mute mismigrated modes synthetic studies and model resolution tests demonstrate that this methodology is well suited to irregularly spaced stations and uneven data coverage and does not require the recording of a single earthquake at multiple stations

sedimentation in bottomland hardwoods downstream of an east texas dam dams and reservoirs are often efficient sediment traps and conventional wisdom holds that fluvial sediment supplies are reduced well downstream however there are reasons to question the extent to which fluvial and alluvial sediment supplies are reduced more than a few kilometers downstream of dams sedimentation in bottomlands of loco bayou east texas was investigated at a site less than 16 km downstream of loco dam and lake nacogdoches which controls 86 of the 265 km2 drainage area turbidity levels are generally as high or higher than those on loco bayou upstream of the lake sedimentation rates on the lower floodplain since the dam was completed are 11 mm year 1 or more this rate is high enough to suggest that the dam has no effect on sediment supplies 16 km downstream the spatial pattern of sedimentation and the vegetation distribution suggest that the elevation and frequency of flooding not fluvial sediment availability are the critical factors in determining sediment supplies to these floodplains

tectonic implications of the age composition and orientation of lamprophyre dikes navajo volcanic field arizona abstract fifty one lamprophyre dikes of the navajo volcanic field are randomly oriented in contrast to dikes of similar age from adjacent tectonic provinces k ar ages obtained on nine of these dikes indicate that they were emplaced between 28 and 19 m y ago initiation of dike emplacement was approximately coeval with the initial demarcation of the colorado plateau in the mid tertiary chemical analyses obtained on nine dike samples show a wide range in composition silica contents range from 33 6 in a phlogopite katungite to 55 9 in a felsic minette two of the samples have na 2 o k 2 o indicating that sodic volcanism was occurring concurrently with the dominant potassic volcanism published results of high pressure experimental studies of katungite suggest that this magma was derived from k metasomatised mantle because this potassium metasomatism produces a positive volume change we propose a model that relates uplift of the colorado plateau to metasomatism magma genesis and dike emplacement

crustal structure and evolution beneath the colorado plateau and the southern basin and range province results from receiver function and gravity studies over the past several decades contrasting models have been proposed for the physical and chemical processes responsible for the uplift and long term stability of the colorado plateau cp and crustal thinning beneath the basin and range province brp in the southwestern united states here we provide new constraints on the models by modeling gravity anomalies and by systematically analyzing over 15 500 p to s receiver functions recorded at 72 usarray and other broadband seismic stations on the southwestern cp and the southern brp our results reveal that the brp is characterized by a thin crust 28 2 0 5 km a mean vp vs of 1 761 0 014 and a mean amplitude r of p to s converted wave relative to that of the direct p wave of 0 181 0 014 that are similar to a typical continental crust consistent with the model that the thin crust was the consequence of lithospheric stretching during the cenozoic the cp is characterized by the thickest crust 42 3 0 8 km largest vp vs 1 825 0 009 and smallest r 0 105 0 007 values in the study area in addition many stations on the cp exhibit a clear arrival before the p to s converted phase from the moho corresponding to a lower crustal layer of about 12 km thick with a mafic composition we hypothesize that the lower crustal layer which has an anomalously large density as revealed by gravity modeling and high velocities in seismic refraction lines contributed to the long term stability and preuplift low elevation of the colorado plateau

identification of a 15 m wisconsin shoreline on the texas inner continental shelf abstract the maximum sea level position during the middle wisconsin oxygen isotope stage 3 is controversial it being interpreted as anywhere from at or above present day levels to 85 m high resolution seismic lithologic paleontologic and radiocarbon data from the east texas continental shelf were used to map a stage 3 shoreline the shoreline was placed in a regional context with a study of the texas shelf based on seismic profiles core and platform boring descriptions that identifies the stage 3 maximum flooding surface and the stage 2 sequence boundary these surfaces were then traced up dip on the east texas shelf where a shore parallel escarpment was recognized landward of the escarpment the stage 3 maximum flooding surface is amalgamated with the stage 2 sequence boundary therefore the location of the escarpment marks the up dip limit of stage 3 deposits and a stage 3 shoreline lagoonal sediments with in situ brackish water fossils on lap the escarpment seaward of the escarpment remnants of barrier island facies have been preserved as a bank freeport rocks bathymetric high located in 18 m of water the shoreline adjusted for subsidence is located at 15 2 m which is shallower than what oxygen isotope and coral records indicate

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

xenolithic evidence for proterozoic crustal evolution beneath the colorado plateau tertiary diatremes of the navajo volcanic field brought a wide variety of proterozoic xenoliths to the surface of the colorado plateau examination of crustal xenoliths from the navajo volcanic field diatremes permits reconstruction of proterozoic pressure temperature p t histories beneath the colorado plateau diatremes from the northwest part of the navajo volcanic field carry the greatest variety of xenoliths including metasedimentary rocks amphibolites felsic gneisses mafic granulites and crustally derived eclogites these rock types show variable degrees of hydrous alteration and evidence for complex reaction histories in contrast diatremes from the southeast part of the navajo volcanic field contain primarily mafic and felsic granulites that show fewer reaction textures and less alteration metasedimentary and eclogitic xenoliths are absent in these diatremes the p t paths from the northwest xenoliths are counterclockwise and reach temperatures as high as 850 c and pressures equal to or greater than 10 kbar later hydrous alteration occurred around 500 c and 8 12 kbar xenoliths from the southeast diatremes preserve little evidence of their p t evolution but paths involve heating and or decompression the differing rock types p t paths and alteration histories of the northwest and southeast populations suggest that proterozoic tectonism juxtaposed two distinct crustal blocks beneath the colorado plateau previous workers have postulated that the boundary between the proterozoic yavapai and mazatzal provinces occurs in the region our data support the existence of a northeast trending boundary beneath the four corners area if the eclogitic metamorphism and hydrous alteration are proterozoic in age their restricted occurrence beneath the northwest part of the plateau suggests that subduction was northwest dipping during proterozoic accretion of the mazatzal province onto north america

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

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

quantifying downstream impacts of impoundment on flow regime and channel planform lower trinity river texas abstract as human population worldwide has grown so has interest in harnessing and manipulating the flow of water for the benefit of humans the trinity river of eastern texas is one such watershed greatly impacted by engineering and urbanization draining the dallas fort worth metroplex just under 30 reservoirs are in operation in the basin regulating flow while containing public supplies supporting recreation and providing flood control lake livingston is the lowest as well as largest reservoir in the basin a mere 95 km above the trinity s outlet near galveston bay this study seeks to describe and quantify channel activity and flow regime identifying effects of the 1968 closure of livingston dam using historic daily and peak discharge data from usgs gauging stations flow duration curves are constructed identifying pre and post dam flow conditions a digital historic photo archive was also constructed using six sets of aerial photographs spanning from 1938 to 1995 and three measures of channel activity applied using a gis results show no changes in high flow conditions following impoundment while low flows are elevated however the entire post dam period is characterized by significantly higher rainfall which may be obscuring the full impact of flow regulation channel activity rates do not indicate a more stabilized planform following dam closure rather they suggest that the trinity river is adjusting itself to the stress of livingston dam in a slow gradual process that may not be apparent in a modern time scale

variations in shoreface progradation and ravinement along the texas coast gulf of mexico shoreface architecture evolution mid holocene to present and depths of transgressive ravinement were examined from sabine pass at the texas louisiana border to south padre island near the texas mexico border using 30 shoreface transects shoreface transects extend out to 16 m water depth each created from an echo sounding profile and on average seven sediment cores the shoreface is composed of three broad sedimentological facies the upper shoreface composed almost entirely of sand the proximal lower shoreface composed of sand with thickly to medium bedded 50 10 cm mud and the distal lower shoreface composed dominantly of mud with medium to thinly bedded 20 3 cm sand shoreface architecture and evolution is extremely variable along the texas coast shoreface gradients increase from 2 25 m km 1 in east texas to 3 50 m km 1 in south texas shoreface sands coarsen towards south texas east and south texas shoreface deposits are thin and retrograding whereas central texas shoreface deposits are thicker and prograding central texas is characterized by stacked shoreface successions whereas in east texas lower shoreface sands are preserved only in offshore banks preservation of shoreface deposits is low in south texas although eustatic fluctuations and accommodation space have a strong impact on overall mid holocene to present shoreface evolution and preservation potential along strike variations in sediment supply and wave energy are the main factors controlling shoreface architecture the transgressive ravinement surface varies from 6 to 15 m along the texas coast

dams and geomorphology research progress and future directions dams impose changes of flow and sediment transfer that drive changes of channel form along the downstream regulated river these changes have been described for more than 50 years but process form relationships have only been advanced with the establishment of a conceptual framework during the 1970s and then the extension of monitoring data and advancement of remote sensing technologies particularly over the past 20 years this paper reviews these developments and identifies three influential themes i channel dynamics ii the role of riparian vegetation and iii channel change as the driver of ecological change changes can be rapid in semi arid regions but elsewhere relaxation periods may extend to millenia in these latter cases regime or steady state models should be replaced by models of transient states applied to the reach scale in order to respond to the needs of river managers over decadal timescales

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

underlying gravel layers in a large sand bed river and their influence on downstream dam channel adjustment due to the changes in environmental factors during a river s historical development underlying gravel layers are found in many large plain rivers when the buried depth of this gravel layer is within the reach of down cutting by clear water scour after reservoir construction it may be exposed and exert a far reaching influence on channel adjustment in hanjiang river the longest tributary of the yangtze river in china down cutting has been greatly reduced and even stopped due to the total change of bed material composition the exposure of a gravel layer results in a dramatic increase in the bed s hydraulic roughness so the channel slope may increase after a decline during the period prior to the exposure of the gravel layer moreover the exposed gravel layer increases the erosion resistance of bed material making a lower relative erosion resistance of bank to bed material where the bank s erosional resistance is weak this may lead to a tendency towards channel widening

evolution of navajo eclogites and hydration of the mantle wedge below the colorado plateau southwestern united states 1 eclogite and pyroxenite xenoliths from ultramafic diatremes of the navajo province on the colorado plateau have been analyzed to investigate hydration of continental mantle and effects of low angle subduction on the mantle wedge xenoliths have been characterized by petrographic and electron probe analysis and by sm nd rb sr k ar and o isotopic analysis of mineral separates from one eclogite and by u pb isotopic analysis of zircons from three samples k ar analysis of phengite establishes eruption of a garnet ridge arizona diatreme at 30 ma sm nd and rb sr analyses of clinopyroxene and garnet from that eclogite document recrystallization shortly preceding eruption three zircon fractions have been analyzed from that eclogite and from two others representing the nearby moses rock and mule ear diatremes seven of nine small multigrain fractions scatter about a poorly fit discordia between ca 35 ma and 1515 ma fractions range from overlapping concordia at the lower intercept to a 207pb 206pb age of ca 1220 ma the discordant fractions establish a mid proterozoic zircon component in each eclogite inconsistent with an origin from basalt of the farallon plate the pressure recorded by one of these eclogites 3 3 gpa exceeds that of an eclogite previously attributed to the farallon plate nonetheless each of the eclogites contains a fraction of nearly concordant zircons with ages in the range 35 to 41 ma and one rock also contains a fraction that is nearly concordant at 70 ma these concordant ages are interpreted to record episodic zircon growth during recrystallization of proterozoic mantle the concordant zircon ages are consistent with published data that establish recrystallization of navajo eclogites from 81 to 33 ma a time interval similar to that of the laramide orogeny the eclogite facies recrystallization and growth of new zircon are attributed to the catalytic effects of water introduced into the mantle from the farallon slab water penetrated fracture zones extending for at least tens of kilometers into the mantle wedge above the farallon slab during low angle subduction magmatism in the san juan volcanic field to the northeast of the diatremes may be related to similar hydration

evolution of estuarine facies in a tidal channel environment sw spain evidence for a change from tide to wave domination abstract the transition from tide to wave dominated environments is documented in holocene estuaries along the mesotidal coast of huelva southwestern spain this transition is a consequence of the decreasing tidal prism caused by estuarine siltation a process favoured by the inherited morphology of the prior fluvial valleys the domingo rubio tidal channel is a small estuary adjacent to the mouth of two wave dominated estuaries along the coast of huelva coring and vibracoring of both surficial and subsurface sediments allow the distinction of six lithofacies that form five depositional environments 1 salt marsh 2 active channel margin 3 washover fan 4 tidal channel and 5 estuarine channel the facies architecture reveals the infilling process of the tidal channel due to the interaction of both waves and tides wave activity is registered in the elongation of the spit whereas tidal action produces facies associations characteristic of tide dominated estuaries the recent history of this estuary records three successive stages 1 an initial tide dominated open marine environment beginning at 3090 60 years b p with strong tidal currents in very energetic channels displaying a straight meandering straight morphology 2 a wave dominated period during which waves transport sand into the inner estuary and also along the mouth spit at about 2500 years b p and 3 a second tide dominated stage from 300 80 years b p onward during which weaker tidal currents distribute the sediment four factors controlled the transition from tide to wave domination 1 the relief formed during deposition 2 the decreasing tidal prism 3 the inner facies distribution versus those of the outlet and 4 the seaward spit elongation of adjacent estuaries

downstream trends in discharge slope and stream power in a lower coastal plain river conceptual models of river estuary interaction are typically based on a notion of systematic downstream change in the intensity of fluvial processes low slopes backwater effects and effects of antecedent topography and landforms may complicate downstream tends in water and sediment flux in coastal plain rivers an analysis of the lower trinity river texas shows no consistent downstream pattern of increases or decreases in the discharge stream power or water surface slope flows may decrease downstream due to coastal backwater effects in the lowermost reaches and due to diversion of flow into valley bottom depressions during high flows in both the fluvial and fluvial estuarine transition reaches in general however stream power and slope decrease in the lower reaches consistent with earlier findings of limited fluvial sediment delivery to the coastal zone some tributaries may become distributaries at high but sub bankfull flows as backwater effects reverse flows into depressions associated with paleomeanders the paleomeanders and possibly the locations of these reversible channels are related to antecedent topography associated with aggradation degradation cycles over the past 100 ka low gradient coastal plain rivers may not function as simple conduits from land to sea further the transition from fluvial to coastal dominance may be variable along the river with the variability controlled not just by the relative magnitude of river and tidal or backwater forcing but also by valley topography controlled in part by antecedent landforms

a comparison of factors controlling sedimentation rates and wetland loss in fluvial deltaic systems texas gulf coast abstract submergence of coastal marshes in areas where rates of relative sea level rise exceed rates of marsh sedimentation or vertical accretion is a global problem that requires detailed examination of the principal processes that establish maintain and degrade these biologically productive environments using a simple 210 pb dating model we measured sedimentation rates in cores from the trinity lavaca navidad and nueces bayhead fluvial deltaic systems in texas where more than 2000 ha of wetlands have been lost since the 1950s long term average rates of fluvial deltaic aggradation decrease southwestward from 0 514 0 008 cm year 1 in the trinity 0 328 0 022 cm year 1 in the lavaca navidad to 0 262 0 034 cm year 1 in the nueces the relative magnitudes of sedimentation and wetland loss correlate with several parameters that define the differing fluvial deltaic settings including size of coastal drainage basin average annual rainfall suspended sediment load thickness of holocene mud in the valley fill and rates of relative sea level rise there is some evidence that upstream reservoirs have reduced wetland sedimentation rates which are now about one half the local rates of relative sea level rise the extant conditions indicate that fluvial deltaic marshes in these valleys will continue to be lost as a result of submergence and erosion

upper mantle structure beneath the eastern colorado plateau and rio grande rift revealed by bouguer gravity seismic velocities and xenolith data 1 the lithospheric architecture of the colorado plateau rio grande rift transition zone reflects a narrow corridor of active tectonism and magmatism between the tectonically stable colorado plateau and the stable interior of north america in this paper we analyze bouguer gravity data crustal thickness variations xenolith data and seismic wave speed structure to argue that the rio grande rift and southeastern colorado plateau is underlain by a low density upper mantle province that does not trend along upper crustal tectonic boundaries but rather is correlated with regions of middle to late tertiary magmatism we model uppermost mantle density contrasts using bouguer gravity along five parallel profiles across the colorado plateau and validate our models by converting modeled density contrasts to seismic velocity variations for comparison with regional seismic data as well as comparing modeled densities to calculated densities from xenoliths we find that upper crustal features and crustal thickness variations do not explain long wavelength 200 km variations in bouguer gravity across the colorado plateau rio grande rift a large 200 440 km wide region of low uppermost mantle density is modeled beneath the eastern colorado plateau margin jemez lineament rio grande rift and parts of the westernmost great plains this region has a density contrast of 90 to 30 kg m3 relative to the adjacent colorado plateau and great plains provinces and coincides with previously imaged regions of low upper mantle seismic velocities in situ densities inferred from xenolith data in the colorado plateau support the modeled upper mantle density variations and support the idea that the eastern and western margins of the plateau have been significantly modified by metasomatism we suggest that the region of low upper mantle density likely reflects some combination of temperature variations partial melt contents and compositional variations related to tertiary magmatic activity and or thinned lithosphere the relationship of this zone of modified upper mantle to extension in the rio grande rift remains enigmatic however particularly given the lack of spatial correlation between the trends of upper mantle structures and surficial extensional structures

extensional collapse along the sevier desert reflection northern sevier desert basin western united states newly released and previously published seismic reflection data from the northern sevier desert basin provide a complete seismic transect between the tilted western margin of the basin and the eastern breakaway zone when tied to well and surface age data the transect delineates a continuum of extensional fault and basin fill geometries that developed between late oligocene and pleistocene time across the basin a minimum of 18 km of top to the west normal displacement is estimated across the sevier desert from only the most conspicuous growth geometries and offsets across listric normal faults that sole downward into the sevier desert reflection sdr the sdr clearly marks a normal fault zone beneath the entire basin where stratal truncations are imaged for 50 of the 39 km length of the reflection east of the cricket mountains block restoration of extensional displacement along this entire 39 km fault length is necessary to reconstruct the pre oligocene configuration and erosion level of sevier thrust sheets across the sevier desert area the sdr normal fault zone underlies the former topographic crest of the sevier orogenic belt where it accommodated extensional collapse after cessation of regional contractile tectonism

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

suspended sediment transport effectiveness of three large impounded rivers u s gulf coastal plain suspended sediment transport effectiveness was examined near the mouths of three large impounded rivers rio grande brazos and pearl rivers in differing precipitation regimes in the u s gulf coastal plain magnitude and frequency analysis of suspended sediment transport was performed by examining the effectiveness of both discharge and time in transporting suspended sediment bivariate plots of discharge with infrequent values of silt clay and sand provide an insight into the relative proportion of coarse versus fine grained sediment transport for the three rivers despite the aridity of the rio grande and brazos drainage basins which is often associated with infrequent or episodic transport of sediment the duration of the effective discharge is similar to humid basins described in the literature the majority of sediment transport occurs during the moderate discharge events having a duration of 2 4 1 5 and 4 4 for the rio grande brazos and pearl rivers respectively this may be due to the influence of scale or the influence of upstream dams and reservoirs on discharge and sediment transport findings from this research suggest that magnitude and frequency analysis of discharge and suspended sediment near the mouths of large rivers may provide a useful framework for understanding the timing and delivery of riverine sediments to the nearshore coastal environment from rivers draining a range of geologic and climatic settings

3 d structure of the rio grande rift from 1 d constrained joint inversion of receiver functions and surface wave dispersion the southern terminus of the rio grande rift region has been poorly defined in the geologic record with few seismic studies that provide information on the deeper rift structure in consequence important questions related to tectonic and lithospheric activity of the rio grande rift remain unresolved to address some of these geological questions we collect and analyze seismic data from 147 earthscope transportable array usarray and other seismic stations in the region to develop a 3 d crust and upper mantle velocity model we apply a constrained optimization approach for joint inversion of surface wave and receiver functions using seismic s wave velocities as a model parameter in particular we compute receiver functions stacks based on ray parameter and invert them jointly with collected surface wave group velocity dispersion observations the inversions estimate 1 d seismic s wave velocity profiles to 300 km depth which are then interpolated to a 3 d velocity model using a bayesian kriging scheme our 3 d models show a thin lower velocity crust anomaly along the southeastern rio grande rift a persistent low velocity anomaly underneath the colorado plateau and basin and range province and another one at depth beneath the jemez lineament and the southern rgr

geophysical constraints on the crustal structure of the southern rio grande rift abstract evidence of geologic activity still occurring in the rio grande rift rgr includes quaternary faulting seismicity and widening at a small rate we map the crustal thickness and seismic velocity ratio to generate constrained crustal model cross sections that highlight the regional extension of the southern rgr srgr specifically we computed receiver functions and receiver function stacks for 147 stations from earthscope usarray transportable array and previously collected data in the southwestern united states and interpolated the crustal and velocity results using a kriging interpolation scheme we include a new optimized gravity inversion approach using the receiver function results as a constraint for inverted density profiles we produce constrained crustal models that characterize the srgr as showing a shallower moho 30 km in the region of the srgr which is thicker than previously suggested the crust appears to be delaminated west of the rgr in the basin and range province and thicker east of the rgr toward the great plains with a denser lower crustal layer we also find that the delaware basin shows a significant geophysical anomaly likely due to thick saturated sediments given that there is no deep mantle anomaly determined from other studies in this region we suggest that rifting could be the result of small scale convection of the uppermost mantle and we conclude that the rgr is at a minimum the boundary between the basin and range province and the great plains perhaps reflecting the terminus of flat slab subduction

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

nd and sr isotope chronostratigraphy of colorado plateau lithosphere implications for magmatic and tectonic underplating of the continental crust nd sr isotopes and major and trace element abundances were measured in lower and upper crustal xenoliths carried in tertiary minettes and kimberlitic diatremes of the colorado plateau the objective was to establish the vertical distribution of mantle extraction ages for an undisturbed precambrian crustal province characterized by normal thickness and low cratonic heat flow and to thereby evaluate the importance of magmatic or tectonic underplating in crustal growth mid proterozoic age 1 7 2 0 ga material is found at all levels of the crust although mafic granulites and amphibolites have major and trace element compositions compatible with an origin as magmatically underplated material nd model ages do not indicate that there has been significant mantle contribution to the lower crust by igneous processes such as magmatic underplating since the main crust forming event ca 1 85 ga ago the meta igneous lower crustal xenoliths have relatively radiogenic nd isotopic compositions and unradiogenic sr compositions there is little variability in nd model ages in spite of large variability in sm nd high and variable sm nd ratios were therefore established early in the history of the protolith during igneous or metamorphic processes coincident with or shortly after extraction from the mantle metapelitic xenoliths tectonically emplaced into the lower crust have the same proterozoic model ages as meta igneous parts of the lower crust these paragneisses are probably arc associated sediments that were underthrust during terrane accretion in the proterozoic the isotopic data indicate that eclogite xenoliths are likely to represent proterozoic lithosphere rather than subducted mesozoic morb as previously proposed a number of the eclogite xenoliths have experienced a metasomatic event which introduced lree sio2 and na2o and shifted the nd and sr isotopic compositions the least altered eclogite xenoliths have major and trace element and radiogenic nd isotopic compositions similar to those of other proterozoic mafic lower crustal xenoliths a more plausible protolith for the eclogites is proterozoic lithosphere that was metamorphosed to eclogite facies conditions either during precambrian continental assembly or in more recent times

an uplift history of the colorado plateau and its surroundings from inverse modeling of longitudinal river profiles 1 it is generally agreed that a region encompassing the colorado plateau has been uplifted by sub crustal processes admittance calculations tomographic studies and receiver function analyses suggest that dynamic support is generated by some combination of convective upwelling and lithospheric thickness changes notwithstanding advances in our understanding of present day setting uplift rate histories are poorly constrained and debated an improved history will aid discrimination between proposed models here we show that a regional uplift rate history can be obtained by inverting longitudinal river profiles we assume that the shape of a river profile is controlled by uplift rate and moderated by erosion in our model uplift rate is allowed to vary smoothly as a function of space and time upstream drainage area is invariant with time simultaneous inversion of river profiles from the colorado rio grande columbia and mississippi catchments shows that three phases of regional uplift occurred the first phase occurred between 80 and 50 myrs when 1 km of uplift was generated at a rate of 0 03 mm yr a second phase occurred between 35 and 15 myrs when 1 5 km of uplift was generated at a faster rate of 0 06 mm yr a final phase of uplift commenced 5 myrs ago these distinct phases of late cretaceous and oligocene uplift are corroborated by stratigraphic considerations by thermochronometric data and by stratigraphic evidence of periodic clastic efflux delivered into the gulf of mexico an episodic uplift history is consistent with staged removal of thick lithospheric mantle beneath a large region which is currently centered on yellowstone

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

causes of river bed degradation river bed degradation can proceed downstream as well as upstream depending upon the cause of degradation the causes of downstream progressing degradation are primarily related to changes in independent river channel variables such as increase in water discharge decrease in size of bed material and decrease in bed material discharge the causes of upstream progressing degradation are all related to an imposed increase in river slope which can occur as a result of natural river behavior or by man made changes study of various case histories indicates that river slopes are increased by lowering a base level by decreasing the length of a river or by removal of a control point case histories also show that downstream and upstream progressing degradation can act in combination along a river system downstream progressing degradation along the main stream of a river system can initiate upstream progressing degradation on a tributary

reductions in fluvial sediment discharge by coastal dams in california and implications for beach sustainability abstract the long term sustainability of california s beaches depends on periodic deliveries of sand and gravel from coastal rivers and streams to assess the long term health of california s beaches this study characterized the current state of fluvial sediment delivery and quantified on a littoral cell basis the cumulative impacts of dams in decreasing annual discharge presently more than 500 dams impound more than 42 000 km2 or 38 of california s coastal watershed area flow modeling suggests that by diminishing flood hydrographs these dams have reduced the average annual sand and gravel flux to 20 major littoral cells by 2 8 million m3 yr or 25 in 70 of the streams considered in this study suspended sediment loads during equivalent discharge events have declined over the past three decades which indicates that dams have also significantly reduced downstream sediment supplies approximately 23 or 274 km of the 1193 km of beaches in california are downcoast from rivers that have had s

heat flow data in the four corners area suggest neogene crustal warming resulting from partial lithosphere replacement in the colorado plateau interior southwest usa the uplift of the colorado plateau is of great geologic interest and low noise terrestrial heat flow data can provide boundary conditions that should be met by proposed models of uplift the structural continuity of the plateau interior with respect to the neighboring basin and range and southern rocky mountains implies a distinctive neogene lithosphere crustal evolution the four corners area represents a unique location in the colorado plateau interior and likely the world where both heat flow data with little variability and mineralogy studies relating to pre neogene lithosphere temperatures are available together these studies allow one to calculate data derived heat flow values at two different times ca 25 ma and the present using a first order model for temperature depth profiles these two heat flow values imply 100 km of neogene lithosphere asthenosphere boundary lab shallowing in the study area if crustal radiogenic heat remains constant the calculated neogene increase in heat flow of 17 mw m 2 will be produced below the moho several mechanisms are considered that might cause the lab to shallow and the near surface heat flow to increase derived neogene lithosphere thinning of 100 km leads one to consider lithosphere displacement of the same order if the presented finite difference model approximates neogene thermal phenomena in the four corners then partial lithosphere replacement of the upper mantle lithosphere appears to be the most suitable cause of the heat flow rise in the area as well as the near uniform heat flow over large regions of the plateau interior identified by low noise heat flow data

residence times of alluvium in an east texas stream as indicated by sediment color abstract the relationships between sediment production storage and transport in fluvial systems are complex and variable key issues in addressing these relationships are the residence times of sediment delivered to the channel and the proportion derived from recent upland erosion as opposed to remobilized alluvium the systematic changes in iron geochemistry often experienced by sediments deposited in an anaerobic environment such as a stream channel or waterlogged floodplain are used here as an indicator of residence time over contemporary time scales in areas such as east texas where upland soils are high in iron oxide content these changes are reflected in soil color alluvium with red yellow or brown colors indicating ferric oxidized iron and sufficient organic matter for reduction to occur indicates a short 1 year sediments with the longest residence times in alluvial environments 1 year will not oxidize on exposure to the atmosphere due to the loss of iron while those with ferrous iron remaining will experience oxidation and color change on exposure in loco bayou texas these indicators of residence time are shown to be generally consistent with other field evidence of erosion and sedimentation further the color indicators correctly indicate the residence time in several cases where the latter is known from field observations

sedimentation within a regulated river channel change induced by tributary sediment injection to a regulated gravel bed river is described for the river rheidol u k eighteen years after dam closure morphological and sedimentological data are presented for an aggrading reach a primary lobate bar appears to have initiated flow separation and the subsequent construction of channel side berms hydraulic sorting has operated to produce both longitudinal and lateral particle size reduction and selective particle size dispersal appears to be an important process influencing channel change the channel is adjusting to a lower width depth ratio form of smaller capacity and with a steeper slope