boron isotopic variations in nw usa rhyolites yellowstone snake river plain eastern oregon the geochemistry of nw usa rhyolites correlates strongly with geography and the nature of the underlying basement terranes rhyolites from the snake river plain yellowstone srpy province have higher 87 sr 86 sr 207 pb 206 pb and lower 143 nd 144 nd than those from the oregon high lava plains hlp province reflecting a dominant influence of precambrian cratonic crust east of the western idaho suture zone versus accreted oceanic terranes of phanerozoic age to the west rhyolites from the cratonic domain show significant enrichments of th u and lree hree whereas b concentration and especially b nb and b rb are systematically higher west of the tectonic boundary this decoupling of b from the other incompatible elements is best explained in terms of distinctive magmatic sources east and west of the suture zone b isotopic composition 11 b was measured for natural and synthetic glasses via multiple multiplier laser ablation icp ms 11 b values are systematically lighter in srpy rhyolites 5 6 to 8 9 compared to those from the hlp 0 8 to 3 1 these data are consistent with strongly fluid depleted and or metamorphosed sources for srpy rhyolites whereas hlp sources resemble those of typical oceanic basalts and could reflect melting of juvenile basalt derived protoliths in the crust b isotope ratios of low 18 o rhyolites are indistinct from those with normal 8 o suggesting that 11 b values are not strongly affected by hydrothermal processes that alter source materials with meteoric water considering all data it is likely that b compositions of the rhyolites are inherited from their sources in the crust although low 11 b 0 is also observed in many mantle derived basalts i e oib morb in srpy rhyolites it is associated with enrichments of elements e g u th rb lree typically concentrated in continental crust and thus old high grade metamorphic continental crust is inferred to be b depelted if rhyolite protoliths were originally metasediments it is likely that bulk b and 11 b were selectively removed by metamorphic dehydration reactions and transported to the surface via b enriched fluids

drained and undrained anisotropic elastic stiffness parameters equations that map drained to undrained elastic stiffness parameters in a cross anisotropic material are presented graphs of various anisotropic stiffness ratios are used to illustrate the important trends a new 2 d graph showing the elastic bound enables measured elastic parameters to be plotted which all lie well within it the 2 d graph enables uncoupled and incompressible materials to be shown plus materials with particular initial effective stress path slopes in an undrained triaxial test bounds more restrictive than the elastic one are also discussed and the likely ranges of various anisotropic stiffness ratios are presented in general undrained anisotropy is much less than drained that is closer to isotropic in undrained plane strain conditions vertical and horizontal moduli are found to be the same in drained plane strain conditions the ratios of the moduli are almost the same as the young s moduli

columbia river flood basalts from a centralized crustal magmatic system the columbia river basalt group in the northwestern united states derived from flood basalt eruptions that occurred 16 million years ago exhibits variability in geography and trace element geochemistry that has led to a number of proposed magma origins however the geochemical variability can be explained by a relatively simple model in which magma is derived from a mantle plume that assimilated continental crust in a centralized magma system the columbia river basalt group in the northwestern united states1 comprising about 230 000 cubic kilometres of rock exhibits unusual patterns in lava distribution geochemistry and its apparent relationship to regional tectonics consequently there is little consensus on the origin of its magmas2 3 4 5 6 7 8 9 10 11 12 here we examine the isotopic ratios of sr nd pb and os and trace element abundances in columbia river basalts the results suggest that most of the lava was produced when magma derived from a mantle plume assimilated continental crust in a central magma chamber system located at the boundary between the north american craton and the accreted terranes of idaho and oregon other related basalts are the product of mixing between the mantle plume and different types of regional upper mantle magma was then transported over a wide region by an extensive network of dykes a process that has been identified in other flood basalt provinces as well13 interactions of the plume with surrounding upper mantle and of mantle derived magmas with regional crust provide a relatively simple6 7 9 14 15 model to explain the more unusual features of the main phase columbia river basalts

mid miocene propagation of the yellowstone mantle plume head beneath the columbia river basalt source region columbia river flood basalt volcanism is the product of mid miocene distortion of the yellowstone mantle plume head against the precambrian margin of north america the overall migration pattern of columbia river basalt vents reflects northward propagation of the plume head beneath the west southwest migrating north american plate thermal uplift mimics the vent migration over the 1 5 m y of grande ronde basalt extrusion the projected crest of the plume head migrated 105 km 7 cm yr from the chief joseph dike swarm east oortheast into the clearwater embayment where uplift occurred over the same period at a rate of 0 67 mm yr chemical variations reflect progressive melting of the plume and mantle lithosphere followed by the mixing of these components with an enriched source from the overriding craton rapid thickening of the plume head against the cratonic margin led to decompressional melting and magma rise beneath a thin oceanic lithosphere of accreted terranes

stress strain responses of block samples of compressible chicago glacial clays this paper presents the results and analysis of a laboratory investigation of the behavior of lightly overconsolidated compressible chicago glacial clays over a wide strain range each specimen was trimmed from high quality block samples taken from an excavation in evanston illinois specimens were instrumented with three sets of bender elements and local lvdts after k0 consolidation to the in situ vertical effective stress of the block drained stress probe tests were conducted results of bender elements tests obtained prior to stress probing show that compressible chicago glacial clay initially is cross anisotropic propagation velocities measured by bender elements in axial direction after k0 reconsolidation and drained creep agrees well with the in situ shear wave velocity measured by seismic cone penetration tests results of drained stress probe tests are analyzed in terms of shear volumetric and coupled stiffness stiffness degradation and direction of loading the significant variability of s

undrained stiffness anisotropy from hollow cylinder experiments on four eocene to jurassic uk stiff clays the paper describes the anisotropic undrained stiffness behaviour of four medium plasticity heavily overconsolidated uk stiff marine clays as revealed through hollow cylinder testing the experimen

helium and neon isotopes in the imnaha basalt columbia river basalt group evidence for a yellowstone plume source abstract helium neon and argon isotopic compositions were measured in two flows of the columbia river flood basalt the imnaha basalt has a 3 he 4 he ratio of 11 4 times atmospheric and 20 ne 22 ne and 21 ne 22 ne ratios characteristic of a plume component the measured 3 he 4 he is a lower limit due to possible preferential 3 he loss and or addition of radiogenic 4 he a wanapum basalt flow erupted approximately 2 ma later in the waning stages of volcanism has more morb like noble gases the he nd and sr isotopic compositions of these lavas suggest that the columbia river basalts were derived from the yellowstone plume head which contained both high helium plume material and entrained depleted mantle as the eruptions progressed the plume component in the melting region was gradually diluted or replaced

continental basalts of the boise river group near smith prairie idaho the boise river group of late cenozoic age consists of basaltic lavas and intercalated fluvial and lacustrine sediments deposited in drainages of the boise river and boise river south fork from 1 8 to 0 2 m y basalts of the boise river group can be divided into two groups based on major and trace element chemistry boise river group 1 basalts brg 1 which comprise the three oldest flows in the smith prairie area are silica saturated olivine tholeiites characterized by low alkalis mg fe and ni high concentrations of high field strength elements hfse and enrichment in the heavy isotopes of sr nd and pb they are chemically similar to basalts of the snake river plain and both units probably derive from a similar enriched mantle lithosphere source some brg 1 flows long gulch rock creek record the affects of crustal assimilation and fractional crystallization boise river group 2 basalts brg 2 are all younger than brg 1 0 7 m y and are transitional between olivine tholeiites and alkali olivine basalts brg 2 basalts are characterized by high alkalis mg fe and ni lower hfse concentrations and isotopic compositions of sr and nd near bulk earth these lavas have distinct geochemical characteristics and are not cogenetic with the older brg 1 flows or with coeval lavas of the snake river plain chemical variations within individual flows are consistent with low pressure crystal fractionation of the observed phenocryst phases olivine plagioclase chemical variations between different flows within the same group cannot result from low pressure fractionation but can be modeled by combined high pressure pyroxene fractionation and low pressure olivine plagioclase fractionation the high mg numbers and ni of the younger brg 2 basalts are not consistent with enrichment of the light field strength elements by crustal assimilation this enrichment must reflect a mantle source region characteristic the trace element and isotopic systematics of this source region are similar but not identical to the asthenospheric source inferred for ocean island basalts in contrast chemical and isotopic systematics of the brg 1 basalts imply derivation from ancient subcontinental lithosphere which has been isolated from the asthenosphere for at least 1 5 eons the subcontinental lithosphere was affected by an early enrichment in rb sr which supported growth of high 87sr 86sr a second enrichment event resulting in fe ti metasomatism of the lithosphere occurred later in response to intrusion of partial melts from a mantle plume rooted in the underlying asthenosphere this is second enrichment event is required by the high nb rb ratios of the brg 1 lavas and of similar lavas of the snake river plain the transition from saturated olivine tholeiites of boise river group 1 to transitional alkalic lavas of brg 2 in the smith prairie region implies a time dependent change in mantle source region with early magmas derived from a shallow lithospheric source giving way to younger magmas derived from a deeper asthenospheric source the eruption of these younger lavas along the flanks of the snake river plain coeval with eruption of saturated tholeiites of the snake river group to the south further implies an axial zonation to magmatism in the snake river province the nature of this axial zonation suggests that the deeper asthenospheric melts were preferentially tapped along the margins of the snake river plain but were blocked from reaching the surface in the axial zone these time space relationships are consistent with a model in which thinned lithosphere near the rift axis undergoes extensive partial melting which overwhelms any asthenospheric melts that may attempt passage the lithosphere is too cool to melt along the rift margins where pressure release by extensional thinning is minor subsequent small volumes of asthenospheric melt are able to traverse this region more easily and erupt with little or no lithospheric contamination

origin of the columbia plateau and snake river plain deflection of the yellowstone plume previous models relating the snake river plain and columbia river basalt northwest united states to the yellowstone plume have not taken into account the effects of the subducted farallon and juan de fuca plates not only did subduction shield the surface from mantle plume activity but it deflected the plume in the direction of plate motion a variety of geophysical and geologic evidence suggests that the plume penetrated the subducted slab at 17 5 ma causing eruption of the columbia river basalt the plate boundary then migrated over the plume and the yellowstone plume has recovered to a more nearly vertical conduit forming the snake river plain and yellowstone plateau

mantle structure beneath the western united states and its implications for convection processes 1 we present tomographic images of the mantle structure beneath the western united states our dynamic north america models of p and s velocity structure dna07 p and dna07 s use teleseismic body waves recorded at 600 seismic stations provided by the earthscope transportable array and regional networks dna07 p and s benefit from the unprecedented aperture of the network while maintaining a dense station distribution providing high resolution body wave imaging of features through the transition zone and into the lower mantle the main features imaged include 1 the juan de fuca subduction system that bottoms at 400 km beneath oregon implying interaction with the yellowstone anomaly 2 a low velocity conduit beneath yellowstone national park that bottoms at 500 km and dips toward the northwest 3 shallow low velocity anomalies upper 200 km beneath the eastern snake river plain esrp and the high lava plains and a deep low velocity anomaly 600 km beneath the esrp but not newberry 4 a low velocity slab gap to 400 km depth immediately south of the mendocino triple junction and south of the gorda slab and 5 high velocity drips beneath the transverse ranges the southern central valley sierra nevada and central nevada these observations reveal extremely heterogeneous mantle structure for the western united states and suggest that we are only just beginning to image the complex interactions between geologic objects the transportable array allows for analysis of the relationships between these anomalies in an internally consistent single tomographic model the dna velocity models are available for download and slicing at http dna berkeley edu

stiffness and strength anisotropy of overconsolidated bootlegger cove clays this paper presents the evaluation of the stiffness and strength anisotropy of overconsolidated oc bootlegger cove formation bcf clays at the port of alaska formerly known as the port of ancho

crustal structure beneath the high lava plains of eastern oregon and surrounding regions from receiver function analysis 1 we analyze teleseismic p to s receiver functions to image crustal structure beneath the high lava plains hlp of eastern oregon and surrounding regions coverage from 206 broadband seismic stations provides the first opportunity to resolve variations in crustal composition thickness and heterogeneity on scales of a few km in depth and tens of km laterally across the hlp region we utilize both h stacking and a new gaussian weighted common conversion point stacking technique we find crust that is 40 km thick beneath the cascades idaho batholith and owyhee plateau and thinner 31 km crust beneath the hlp and northern great basin low poisson s ratios of 0 240 characterize the granitic crust beneath the idaho batholith while the owyhee plateau exhibits values of 0 270 typical of average continental crust the owyhee plateau is a thick simple crustal block with distinct edges at depth the western hlp exhibits high average values of 0 304 typical for regions of widespread basaltic volcanism combined with other geological and geophysical observations the areas of abnormally high poisson s ratios 0 320 and low velocity zones in the crust beneath north central and southern oregon are consistent with the presence of partial melt on either side of the hlp trend suggesting a central zone where crustal melts have drained to the surface perhaps enabled by the brothers fault zone thicker crust and an anomalous n s band of low poisson s ratios 0 252 skirting the steens mountain escarpment is consistent with residuum from a midcrustal magma source of the massive flood basalts supporting the view of extensive mafic underplating and intraplating of the crust from cenozoic volcanism

modelling of long term ground response to tunnelling under st james s park london following a tunnel excavation in low permeability soil it is commonly observed that the ground surface continues to settle and ground loading on the tunnel lining changes as the pore pressures in the ground approach a new equilibrium condition the monitored ground response following the tunnelling under st james s park london shows that the mechanism of subsurface deformation is composed of three different zones swelling consolidation and rigid body movement the swelling took place in a confined zone above the tunnel crown extending vertically to approximately 5 m above it on the sides of the tunnel the consolidation of the soil occurred in the zone primarily within the tunnel horizon from the shoulder to just beneath the invert and extending laterally to a large offset from the tunnel centreline above these swelling and consolidation zones the soil moved downward as a rigid body in this study soil fluid coupled three dimensional finite element analyses were performed to simulate the mechanism of long term ground response monitored at st james s park an advanced critical state soil model which can simulate the behaviour of london clay in both drained and undrained conditions was adopted for the analyses the analysis results are discussed and compared with the field monitoring data it is found that the observed mechanism of long term subsurface ground and tunnel lining response at st james s park can be simulated accurately only when stiffness anisotropy the variation of permeability between different units within the london clay and non uniform drainage conditions for the tunnel lining are considered this has important implications for future prediction of the long term behaviour of tunnels in clays

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

large volume low δ18o rhyolites of the central snake river plain idaho usa the miocene bruneau jarbidge and adjacent volcanic fields of the central snake river plain southwest idaho are dominated by high temperature rhyolitic tuffs and lavas having an aggregate volume estimated as 7000 km3 samples from units representing at least 50 of this volume are strongly depleted in 18o with magmatic feldspar 18ovsmow vienna standard mean ocean water values between 1 4 and 3 8 the magnitude of the 18o depletion and the complete lack of any rhyolites with normal values 7 10 combine to suggest that assimilation or melting of a caldera block altered by near contemporaneous hydrothermal activity is unlikely instead we envisage generation of the high temperature rhyolites by shallow melting of idaho batholith rocks under the influence of the yellowstone hotspot affected by eocene meteoric hydrothermal events the seeming worldwide scarcity of strongly 18o depleted rhyolites may simply reflect a similar scarcity of suitable crustal protoliths

tectonic and magmatic evolution of the northwestern basin and range and its transition to unextended volcanic plateaus black rock range nevada the seismically active eastern and western margins of the northern basin and range have been extensively studied yet the northwestern margin of the province remains incompletely understood the black rock range of northwestern nevada straddles the transition from the basin and range province to the south and east and flat lying volcanic plateaus to the west this poorly understood range preserves a remarkably complete record of cenozoic magmatism and provides an important window into the pre miocene history of the unextended volcanic plateaus of northeastern california and southern oregon geologic mapping and 40ar 39ar geochronology from the northern black rock range document three significant episodes of eocene to middle miocene volcanism eocene 35 ma basalts directly overlie mesozoic granites and arc related volcanic and sedimentary rocks locally erupted oligocene to early miocene 27 21 ma bimodal volcanic rocks comprise the bulk of the cenozoic section and conformably overlie the eocene basalt flows these bimodal units include rhyolitic lavas variably welded rhyolitic ash flows unwelded ash fall deposits and thin basalt flows in the neighboring pine forest range 20 km to the north similar oligocene to early miocene units are overlain by more than 500 m of ca 16 4 ma steens equivalent basalt flows and are capped by ca 16 ma rhyolitic ash flow tuffs in the northern black rock range the ca 16 4 ma middle miocene basalts are absent from the section and a 16 2 ma rhyolitic ash flow tuff directly overlies the early miocene flows basaltic and rhyolitic volcanic products in the northern black rock range span 35 16 ma with many of the oligocene volcanic units derived from local vents and dikes despite the map scale complexities of locally derived lava flows the cenozoic section is broadly conformable and dips gently 5 10 to the northwest the region experi enced no significant tilting between 35 and 16 ma with moderate tilting 5 10 and concomitant uplift occurring after 16 ma this tectonic history is consistent with that of the nearby pine forest and santa rosa ranges where low temperature thermochronology documents footwall exhumation along the range bounding normal faults after 12 ma the velocity structure of the crust beneath the northern black rock range is constrained by a recent geophysical survey seismic reflection refraction and gravity and contains gradients that correspond to basin depths predicted by our geologic mapping together with recently completed geological and geophysical studies from the surrounding region our results suggest that the evolution of the northwestern margin of the basin and range was characterized by long lived and voluminous volcanism without significant tectonism followed by low magnitude 20 extension along high angle normal faults

strontium and oxygen isotopic variations in mesozoic and tertiary plutons of central idaho regional variations in initial 87sr 86sr ratios r i of mesozoic plutons in central idaho locate the edge of precambrian continental crust at the boundary between the late paleozoic mesozoic accreted terranes and precambrian sialic crust in western idaho the r i values increase abruptly but continuously from less than 0 704 in the accreted terranes to greater than 0 708 across a narrow 5 to 15 km zone characterized by elongate lens shaped highly deformed plutons and schistose metasedimentary and metavolcanic units the chemical and petrologic character of the plutons changes concomitantly from ocean arc type diorite tonalite trondhjemite units to a weakly peraluminous calcic to calcalkalic tonalite granodiorite granite suite the idaho batholith plutons in both suites yield late cretaceous ages but permian through early cretaceous bodies are confined to the accreted terranes and early tertiary intrusions are restricted to areas underlain by precambrian crust the two major terranes were juxtaposed between 75 and 130 m y ago probably between 80 and 95 m y oxygen and strontium isotopic ratios and rb and sr concentrations of the plutonic rocks document a significant upper crustal contribution to the magmas that intrude precambrian crust magmas intruding the arc terranes were derived from the upper mantle subducted oceanic lithosphere and may have been modified by anatexis of earlier island arc volcanic and sedimentary units plutons near the edge of precambrian sialic crust represent simple mixtures of the precambrian wall rocks with melts derived from the upper mantle or subducted oceanic lithosphere with r i of 0 7035 rb sr varies linearly with r i producing pseudoisochrons with apparent ages close to the age of the wall rocks measured 18o values of the wall rocks are less than those required for the assimilated end member by sr o covariation in the plutons however indicating that wall rock 18o was reduced significantly by exchange with circulating fluids metasedimentary rocks of the belt supergroup are similarly affected near the batholith documenting a systematic depletion in 18o as much as 50 km from the margin of the batholith plutons of the bitterroot lobe of the idaho batholith are remote from the accreted terranes and represent mixtures of precambrian wall rocks with melts dominated by continental lower crust r i 0 708 rather than mantle pseudoisochrons resulting from these data are actually mixing lines that yield apparent ages less than the true age of the wall rocks and meaningless ri assimilation fractional crystallization models permit only insignificant amounts of crystal fractionation during anatexis and mixing for the majority of plutons of the region

lithospheric topography tilted plumes and the track of the snake river yellowstone hot spot 1 the trace of the snake river yellowstone hot spot is the world s best example of a mantle plume that has been overridden by continental lithosphere the standard model calls for the plume head to rise under northern nevada and be forced northward to form basalts of the columbia plateau subsequent movement of north america to the southwest over the plume tail created a hot spot trace on the surface we present a new conceptual model for the origin of this feature that resolves inconsistencies in the current standard model and explains the recent documentation of a thermal anomaly in the mantle below yellowstone today that plunges 65 wnw our model implies that the plume tail was forced beneath thinned cratonic lithosphere to the se along with part of the plume head and has remained in this orientation for the last 12 ma we infer that almost all of the volcanism in se oregon and sw idaho prior to 12 ma results from overriding the southern extension of the plume head not the plume tail and that a distinct plume tail hot spot track was not established until formation of the bruneau jarbidge eruptive center around 12 ma the plume tail track may also be controlled by a preexisting structural boundary in lithosphere that is thinner than adjacent lithosphere this model demonstrates the potential importance of lithospheric topography on controlling the surface manifestation of plume volcanism and the complexity that may arise when lithospheric thickness is nonuniform

distribution and geochronology of oregon plateau u s a flood basalt volcanism the steens basalt revisited the timing and petrogenesis of mid miocene flood basalt volcanism in the northwest united states has been extensively addressed yet the chemical characteristics and temporal details of the steens basalt exposed on the oregon plateau are poorly defined steens basalt volcanism has generally been accepted to have occurred at 16 6 ma coeval and or just prior to the onset of columbia river basalt group volcanism to the north new major and trace element analyses and nine 40 ar 39 ar ages ranging from 15 51 0 28 to 16 58 0 18 ma were obtained on oregon plateau flood basalt lava flows from stratigraphic sections in close proximity to steens mountain additionally new 40 ar 39 ar ages were obtained on the uppermost and thirty first lava flow down from the top of the 1 km section of steens basalt exposed at steens mountain and yield eruption ages of 16 59 0 10 and 16 55 0 10 ma respectively field relations between these basalt sections suggest that multiple eruptive centers were present in the vicinity of steens mountain the chemical and chronologic data presented in this study illustrate that flood basalts with the steens basalt chemical signature erupted across the southern oregon plateau over a much greater timespan than what is typically quoted for the steens mountain type section these data suggest that the main volume of steens basalt volcanism erupted over at least an 1 m y duration from eruptive loci in the vicinity of steens mountain while likely much less voluminous volcanism and lithospheric input of mafic magma appears to have occurred for n2 ma across the oregon plateau these new geochemical and geochronological constraints verify a common temporal link between steens malheur gorge region and columbia river basalt group crbg volcanism this direct temporal link requires that petrogenetic and tectonic models of mid miocene northwestern u s flood basalt volcanism recognize that the northern columbia plateau and southern oregon plateau portions of this mid miocene basalt province were erupting simultaneously during portions of the regional event 2007 elsevier b v all rights reserved

geology of the mid miocene rooster comb caldera and lake owyhee volcanic field eastern oregon silicic volcanism associated with grande ronde flood basalt abstract the lake owyhee volcanic field lovf of eastern oregon consists of rhyolitic caldera centers and lava fields contemporaneous with and spatially related to mid miocene columbia river flood basalt volcanism previous studies delineated two calderas in the southeastern part of lovf near owyhee reservoir the result of eruptions of two ignimbrites the tuff of leslie gulch and the tuff of spring creek our new interpretation is that these two map units are differentially altered parts of a single ignimbrite produced in a major phreatomagmatic eruption at 15 8 ma areas previously mapped as tuff of spring creek are locations where the ignimbrite contains abundant clinoptilolite mordenite which made it susceptible to erosion the resistant intracaldera tuff of leslie gulch has an alteration assemblage of albite quartz indicative of low temperature hydrothermal alteration our new mapping of caldera lake sediments and pre and post caldera rhyolitic lavas and intrusions that are chemically similar to intracaldera tuff of leslie gulch point to a single 20 25 km caldera which we name the rooster comb caldera erosion of the resurgently uplifted southern half of the caldera created dramatic exposures of intracaldera tuff of leslie gulch cut by post caldera rhyolite dikes and intrusions that are the deeper level equivalents of lava domes and flows that erupted into the caldera lake preserved in exposures to the northeast the rooster comb caldera has features in common with more southerly mid miocene calderas of the mcdermitt volcanic field and high rock caldera complex including formation in a basinal setting shortly after flood basalt eruptions ceased in the region and forming on eruption of peralkaline ignimbrite the volcanism at rooster comb caldera postdates the main activity at mcdermitt and high rock but like it begins 300 ky after flood basalt volcanism begins in the area and while flood basalts don t erupt through the silicic focus are contemporaneous with the latest stages of eruptions nearby high rock and mcdermitt rhyolites are associated with propagation of steens basalt dikes to the south and lovf rhyolites with later propagation of grande ronde basalt dikes to the north and north northwest

calibration of ga1550 biotite standard for k ar and 40ar 39ar dating abstract the mineral separate ga1550 biotite has become an international standard for k ar and 40 ar 39 ar dating studies although it was prepared as an intralaboratory standard at anu to monitor tracer depletion from a gas pipette it is one of a small number of samples that has been calibrated against 38 ar tracers some of which had been mixed with known amounts of atmospheric argon so that a so called primary calibration has been performed by measuring ga1550 biotite against additional tracers from the same batch we have determined the radiogenic argon content of this sample as 1 342 0 007 10 9 mol g and together with the measured k content of 7 645 0 050 weight percent we derive a best estimate for the k ar age as 98 5 0 5 ma where the error is derived from averaging the ages determined relative to the 38 ar tracer

lithosphere scale thrusting in the western u s cordillera as constrained by sr and nd isotopic transitions in neogene volcanic rocks the western idaho suture zone juxtaposes continental and oceanic terranes with distinctive sr and nd isotopic signatures across a sharp isotopic transition coincident with the mesozoic sup 87 sr sup 86 sr 0 706 line msl late cretaceous to eocene granitic plutons that define the msl are allochthonous and were transported eastward by approximately 150 km during late mesozoic to tertiary ca 100 55 ma thrusting sr and nd isotopic compositions of neogene rhyolites and basalts from southwestern idaho southeastern oregon and northern nevada define isotopic transitions coincident with the msl there are continental affinities east of long 116 degree 30minutesw and oceanic affinities west of 119w between these longitudes intermediate isotopic compositions imply derivation of magmas from both continental and oceanic mantle basalts and crustal rhyolites sources it is inferred that the basal decollement of the mesozoic and tertiary contractional orogen dips westward below depths of segregation of basaltic magmas and that oceanic lithospheric mantle was thrust eastward over a shelf of old isotopically evolved continental lithospheric mantle that now protrudes approximately 150 km west of the msl

episodic north america and pacific plate motions the north america plate motion for the period 20 0 m y is modeled by two absolute angular velocity vectors with a change occurring at about 9 m y the model incorporates new geologic constraints based on 1 the recognition of a discontinuity in north america plate motion at 9 m y 2 a reexamination of the iceland and yellowstone hotspot traces 3 improved delineation of the present north america pacific relative motion and 4 a revised history of pacific absolute motion the two stage model is consistent with several features of the late cenozoic geologic history of north america including geological observations relating to tectonism along a large portion of both the eastern and western boundaries of north america this plate model combined with a previously established pacific plate model makes specific predictions concerning the 15 0 m y history of the north america pacific plate boundary which compare favorably with the known history these models together resolve most of the discrepancies previously encountered between absolute and relative motion models of the two plates the implications of the north america plate model are the greatest for those events related to the 9 m y change in its motion among the geologic consequences of this change are rifting in the western united states intense compressional deformation along the northern caribbean plate boundary a hiatus in aleutian arc volcanic activity and a change in the orientation of the lesser antilles arc this picture reinforces the notion that the major plates behave rigidly and transmit stresses over great distances the predictive power of the model may help in interpreting the history of the north america plate boundary in those areas where clear observations are lacking an extra force needs to be applied to a plate to cause a change in plate motion by treating the plate as a rigid spherical shell overlying a uniformly viscous fluid we derive a simple class of forces sufficient to cause a given change in plate motion such force balance considerations indicate that an additional force applied in the northwest pacific region toward the northeast would result in the observed change in north america plate motion this force is surprisingly similar to that which apparently caused a large change in pacific plate motion about 5 m y later we propose that a common mantle process is responsible for both changes and is related to the ability of the northwest pacific subducted lithosphere to enter into the lower mantle

the steens mountain oregon geomagnetic polarity transition 3 its regional significance study of the variations of direction and intensity of the geomagnetic field as recorded by the miocene lava flows on steens mountain southeastern oregon has resulted in a detailed description of total field behavior during a reversal in polarity in addition to information about the polarity reversal itself the detailed paleomagnetic record includes several thousand years of geomagnetic history preceding and following the polarity transition at 15 5 ma in order to test the feasibility of using this record as a means of correlation in this part of the western united states comparisons are made of reconnaissance and previously published paleomagnetic records obtained from what has been thought to be the steens basalt or rocks of equivalent age despite the fact that many of these earlier studies were not done in detail and were not intended for correlation purposes convincing similarities among some of the records are evident the steens basalt paleomagnetic record does indeed have potential as a correlation tool during this time of widespread basaltic volcanism additionally paleomagnetic data from flows that were sampled in detail yield a middle miocene paleomagnetic pole at 88 3 n 209 0 95 6 3 for the high lava plains of oregon this pole position is statistically indistinguishable from the earth s rotational axis and implies that no tectonic rotation of this region has occurred since these lava flows were erupted data from selected sites within the coeval part of the columbia river basalt group yield a paleomagnetic pole at 88 7 n 171 6 e 95 4 0 the columbia river basalt group pole is statistically indistinguishable from either the rotational axis or from the high lava plains pole these findings indicate no post 20 ma differential rotation between south eastern washington and south central oregon in contrast to previous interpretations

extension of the yellowstone plateau eastern snake river plain and owyhee plateau formation of the late cenozoic volcanic province comprising the owyhee plateau eastern snake river plain and yellowstone plateau has been accompanied by east northeast directed crustal extension a new vector of 45 mm yr n56 degree e for the migration of silicic volcanism across the volcanic province is calculated if migration of volcanism reflects west southwest continental drift over a mantle plume a zone of crustal extension must separate the volcanic province from the more slowly moving north american craton space time relations of basin fill in the adjacent basin and range province provide evidence for a zone of extension about 125 km wide coincident with and east of coeval silicic volcanism since 16 ma the zone of extension has migrated along with silicic volcanism maintaining its position between the province and the unextended craton

cross anisotropic deformation characteristics of natural sedimentary clays the cross anisotropic elasticity parameters were experimentally determined for six sedimentary clays of different ages and origins at both natural and reconstituted states by using high precision triaxial apparatus all the parameters and their dependency on the effective stress are satisfactorily described by a simple conventional power law model the patterns of the inherent anisotropy exhibited close agreement between the natural and reconstituted states suggesting that the development of natural microstructures due to ageing and diagenesis does not play a primary role in determining the anisotropy patterns in clays despite different degrees of stiffness anisotropy observed in the six clays relatively well defined correlations encompassing all the tested clays were found between certain stiffness parameters while fully characterising the drained elasticity moduli is still a challenging laboratory task these correlations will help estimate some of them based on more readily measurable undrained mo

the age of the steens reversal and the columbia river basalt group abstract the columbia river basalt group crbg eruptions have a well defined relative magnetostratigraphy but have not been definitively correlated to the geomagnetic polarity time scale 40 ar 39 ar ages are presented from lavas erupted in the r 0 through n 1 magnetozones of the crbg and in the transition between r 0 and n 0 four ages from transitionally magnetized lava flows at steens mountain catlow peak and poker jim ridge with a weighted mean age 16 58 0 10 ma 1 and the more precise age 16 654 0 025 ma of the normally magnetized oregon canyon tuff at the top of the catlow peak section show that the oldest crbg magnetozone r 0 correlates with the c5cr chron bayesian statistical analysis applied to data from four flows at catlow peak using the mean age of the steens reversal gives a best and preferred age of the steens reversal of 16 73 0 13 0 08 ma 95 confidence depending on the geomagnetic polarity time scale model the eruption rate from n 0 through r 2 0 34 0 45 ma in the middle and the bulk of the crbg emplacement averaged 0 30 0 41 km 3 a and peaked at a rate 1 1 2 to 4 1 2 times higher during r 2

p and s wave velocity and vp vs in the wake of the yellowstone hot spot 1 seismic vp vs and vp vs structure is imaged across the yellowstone hot spot swell including the hot spot track where magmatism occurred at the eastern snake river plain 6 10 m y b p data are teleseismic p and s travel time delays that have been corrected for the well understood upper mantle anisotropy and crustal structure amplitude variations in the imaged structures are 6 2 11 2 and 8 for vp vs and vp vs respectively the dominant structure is a zone which extends beneath the snake river plain to a depth of 190 km that is high in vp vs and low in vp and vs the physical state of the upper mantle is inferred by assuming isostasy using the volume of melt segregated from the mantle that is inferred from estimates of magma addition to the crust and using relations that scale changes in temperature partial melt fraction and composition to density specifically we infer that the low velocity mantle beneath the snake river plain is partially molten up to 1 0 and the high velocity yellowstone swell mantle away from the snake river plain is 80 k cooler and 5 depleted in basaltic component the imaged large seismic velocity variations occur under near isothermal conditions

initial impingement of the yellowstone plume located by widespread silicic volcanism contemporaneous with columbia river flood basalts during the main phase of steens and columbia river united states flood basalt eruptions between 16 7 and 15 0 ma 3900 km 3 of silicic magma erupted from centers dispersed across 25 000 km 2 the largest and oldest silicic centers high rock mcdermitt and lake owyhee caldera complexes lie along a narrow nne trend at the transition between the craton and accreted oceanic crust suggesting that silicic magma generation was greatest where mafic magma intruded more felsic crust less voluminous and younger silicic centers track the spread of mafic dikes to the north the high rock caldera complex forms the western end of the snake river plain yellowstone trend of eastward younging silicic centers we attribute a regional lull in volcanism between ca 23 and 7 ma to lifting of the juan de fuca slab by a mantle plume beginning at ca 25 ma and the distribution of main stage flood basalts and coeval rhyolites to breakthrough of plume material at ca 17 ma to form a smaller plume head centered beneath steens mountain

anisotropy of small strain stiffness in ticino sand comprehensive tests performed in a large calibration chamber and consisting of the propagation of seismic body waves in dry ticino silica sand are presented by virtue of the fact that both shear and constrained compression waves have been propagated in vertical horizontal and oblique directions five independent constants of the cross anisotropic elasticity model of the test have been established the tests have also made it possible to evaluate the influence of the structural inherent and stress induced anisotropy on the velocity of the propagated seismic body waves the influence of the principal effective stresses on the velocity of these waves is also described de nombreux essais realises dans une grande chambre d etalonnage et consistant en la propagation d ondes sismiques dans des sables siliceux et sec de ticino sont presentes les ondes de cisaillement et de compression ayant w emises dans des directions verticales horizontales et obliques les 5 constantes independantes du modele eastique

tectonic controls on magma genesis and evolution in the northwestern united states abstract field chronologic chemical and isotopic data for late cenozoic basaltic rocks from the northwestern united states illustrate the relationship between crustal structure and tectonic forces in controlling the genesis and evolution of continental volcanism in the northwestern u s the first major episode of basaltic volcanism was triggered by crustal rifting in a back arc environment east of the westward migrating volcanic arc created by the subduction of the juan de fuca plate beneath the north american plate rifting and volcanism were concentrated by pre existing zones of crustal weakness associated with boundaries between the old archean core of the continent and newly accreted terranes basalts erupted during this time columbia river steens mountain show evidence of significant fractionation histories including contamination by crust of varying age depending on the crustal structure at the eruption site presumably this reflects ponding and stagnation of primary magmas in the crust or at the crust mantle interface due to their encounter with thick crust not yet extended and still containing its low density easily fusible component continued rifting of this crust and modification of its composition through extraction of rhyolitic partial melts and deposition of the fractionation products from primary basaltic melts coupled with a shift in stress orientation roughly 10 5 ma ago allowed relatively unfractionated and uncontaminated magmas to begin reaching the surface in the western part of the region oregon plateau these magmas tapped a mantle source similar to that which produced most of the ocean island basalts of the northern hemisphere to the east snake river plain however the mantle sampled by basaltic volcanism has isotopic characteristics suggesting it has preserved a record of incompatible element enrichment processes associated with the formation of the overlying archean crustal section some 2 6 ga ago

morphological and textural diversity of the steens basalt lava flows southeastern oregon usa implications for emplacement style and nature of eruptive episodes this study focuses on middle miocene tholeiitic flood basalt lava flows from the oregon plateau northwestern usa steens basalt and is the first to comprehensively document and evaluate their morphology field observations of flows from several sections within and proximal to the main exposures at steens mountain have been supplemented with textural and geochemical data and are used to offer preliminary insights into their emplacement compound pahoehoe flows of variable thickness appear to be common throughout the study area laterally and vertically these tend to be plagioclase phyric and the morphology and disposition of constituent flow lobes are quite similar to those from other provinces such as hawaii and the snake river plain classic a a flows with brecciated upper and basal crusts are not abundant but by no means uncommon flows with characters different from typical pahoehoe and a a are also common such flows display a range in morphology flows with preserved upper crusts but brecciated basal crusts as well as those displaying well developed flow top breccias and preserved basal crusts rubbly pahoehoe are observed the steens basalt appears to display greater morphological and textural diversity at the outcrop scale than that described for some other flood basalt provinces the abundant compound pahoehoe flows often rich in plagioclase phenocrysts were likely emplaced during slow but sustained eruptive episodes their constituent lobes show clear evidence for endogenous growth the relatively aphyric flows with brecciated surfaces including a a hint at higher strain rates and or higher viscosity probably caused by higher effusion rates a couple of sections are characterized by compositionally similar but morphologically different flows that were possibly part of the same eruption while differences in pre eruptive topography could explain this it is also possible that certain physical parameters changed substantially and abruptly during eruption and that such changes were accompanied by differentiation processes within the plumbing system it is possible that such observations indicate temporal fluctuations within complex magmatic and eruptive systems and deserve closer scrutiny

measurements and numerical simulations of inherent stiffness anisotropy in soft taipei clay abstractsoils are generally known not to be isotropic materials the inherent anisotropy of clays results from the deposition process which tends to induce a horizontal bedding plane in the soil layer in this paper the anisotropy of clays was studied to obtain more accurate analytical results for geotechnical problems especially for deep excavations in soft clay with nearby structures a series of k0 consolidated undrained triaxial compression ck0uc tests thus was conducted on tube samples of natural taipei silty clay with multidirectional bender elements a newly designed triaxial testing system equipped with a high precision servo motor and local strain sensors was developed a new soil suction control system was also developed to perform soil saturation for triaxial tests the suction control system was used to reduce the change in the void ratio during saturation in the triaxial tests soil samples were retrieved from a site near a well documented excavation case in taipei the anisotropy ratios

snake river sr type volcanism at the yellowstone hotspot track distinctive products from unusual high temperature silicic super eruptions a new category of large scale volcanism here termed snake river sr type volcanism is defined with reference to a distinctive volcanic facies association displayed by miocene rocks in the central snake river plain area of southern idaho and northern nevada usa the facies association contrasts with those typical of silicic volcanism elsewhere and records unusual voluminous and particularly environmentally devastating styles of eruption that remain poorly understood it includes 1 large volume lithic poor rhyolitic ignimbrites with scarce pumice lapilli 2 extensive parallel laminated medium to coarse grained ashfall deposits with large cuspate shards crystals and a paucity of pumice lapilli many are fused to black vitrophyre 3 unusually extensive large volume rhyolite lavas 4 unusually intense welding rheomorphism and widespread development of lava like facies in the ignimbrites 5 extensive fines rich ash deposits with abundant ash aggregates pellets and accretionary lapilli 6 the ashfall layers and ignimbrites contain abundant clasts of dense obsidian and vitrophyre 7 a bimodal association between the rhyolitic rocks and numerous coalescing low profile basalt lava shields and 8 widespread evidence of emplacement in lacustrine alluvial environments as revealed by intercalated lake sediments ignimbrite peperites rhyolitic and basaltic hyaloclastites basalt pillow lava deltas rhyolitic and basaltic phreatomagmatic tuffs alluvial sands and palaeosols many rhyolitic eruptions were high mass flux large volume and explosive vei 6 8 and involved h2o poor low 18o metaluminous rhyolite magmas with unusually low viscosities partly due to high magmatic temperatures 900 1 050 c sr type volcanism contrasts with silicic volcanism at many other volcanic fields where the fall deposits are typically plinian with pumice lapilli the ignimbrites are low to medium grade non welded to eutaxitic with abundant pumice lapilli or fiamme and the rhyolite extrusions are small volume silicic domes and coulees sr type volcanism seems to have occurred at numerous times in earth history because elements of the facies association occur within some other volcanic fields including trans pecos texas etendeka parana lebombo the english lake district the proterozoic keewanawan volcanics of minnesota and the yardea dacite of australia

new evidence for cretaceous strike slip faulting in the united states cordillera and implications for terrane displacement deformation patterns and plutonism early to mid cretaceous dextral strike slip faulting involving up to 400 km displacement has been documented in the southern sierra nevada of california in western idaho the salmon river suture juxtaposes unrelated oceanic and continen tal rocks along a major near vertical shear zone that was active in the early to mid cretaceous these two structural zones have been treated as separate entities however new data indicate that they likely connect via a structural discontinuity in northwest nevada and southeast oregon herein called the western nevada shear zone wns current structures within the wns record dip slip contractional deformation however regional relations structural and stratigraphic indicate a mismatch in the early mesozoic geology of rocks across the wns that is best explained by dextral strike slip displacement of several 100 km these relations argue that an early strike slip structure along the wns was reactivated and obscured during later regional shortening deformation most likely related to the cretaceous sevier orogeny the wns is along strike and broadly coeval with the salmon river suture zone and strike slip boundaries in the sierra nevada we hypothesize that all these features are related elements of a major structural boundary in the western united states cordil lera this boundary is interpreted to have accommodated significant dextral strike slip displacement of outboard arc terranes of the cordillera in the early cretaceous followed by reactivation as a localized contractional zone in the mid cretaceous during the sevier orogeny cretaceous batholiths of the united states cordillera closely parallel the boundary which we suggest formed a major crustal flaw that localized and facilitated emplacement of arc magmas

the stiffness of natural london clay an investigation of natural london clay is reported involving advanced triaxial hollow cylinder apparatus hca and dynamic testing techniques significant anisotropy was revealed at all scales of deformation and the framework of cross anisotropic elasticity was found to apply broadly to the initial elastic behaviour the stiffness parameters obtained by independent techniques generally exhibited good agreement with the greatest deviation being seen in the poisson s ratios which fell far from the values usually assumed in conventional foundation analysis probing tests established the limits to the elastic domain over a range of depths showing that these scaled in proportion to the mean effective stress level as did those of a second kinematic surface that surrounded the elastic domain once engaged this second surface signified a new pattern of strain increment directions faster elastic plastic stiffness decay with strain and also a greater dependence of behaviour on recent stress history howev

estimating the area fraction of geophysical fields from measurements along a transect methods are presented for estimating the fractional area coverage of geophysical phenomena from measurements taken along a transect these methods are most useful for assessing potential errors in sampling strategies but may also be used for the analysis of data the procedure provides a means to compute confidence interval estimates of the true area fraction when the autocovariance function for the geophysical field is known or assumed another approach that does not require a priori knowledge of the underlying autocovariance function is described for the special case of linear features modeled as a poisson line process

large scale fractures related to inception of the yellowstone hotspot during middle miocene time western north america was subject to flood basalt volcanism dike swarm injection and broad scale fracturing and folding of the crust we propose a simple model to account for these events and for a regional pattern of geologic and geophysical features aeromagnetic maps reveal some of the most important elements of this pattern which are several narrow arcuate anomalies here referred to as the northern nevada rifts these rifts extend hundreds of kilometers across nevada and are likely caused by highly magnetic middle miocene mafic dikes with the aid of filtering techniques the anomalies can be traced into oregon together with other geologic features such as fold axes dike swarms and faults they produce a spoke like pattern fanning over 2208 of arc that converges toward a point near the oregon idaho border lat 448n a possible cause for this pattern is a point source of stress at the base of the crust related to the formation of the yellowstone hotspot the spoke like pattern however does not persist at large distances from the emerging hotspot several hundred kilometers to the south the northern nevada rifts deviate significantly 308 from a radial trend we show that a simple model imposing a point source of stress at the base of the crust and a regional stress field aligned with the presumed middle miocene stress direction fits the observed fracture pattern it thus accounts for both the radial pattern present near the nascent hotspot and the far field pattern due to regional stresses

large volume rhyolite genesis in caldera complexes of the snake river plain insights from the kilgore tuff of the heise volcanic field idaho with comparison to yellowstone and bruneau jarbidge rhyolites generation of large volume rhyolites in the shallow crust is an important yet enigmatic process in the snake river plain and worldwide here we present data for voluminous rhyolites from the 6 6 4 5 ma heise volcanic field in eastern idaho heise is arguably the best site to evaluate shallow rhyolite genesis in the snake river plain it is the youngest complete record of caldera cluster volcanism along the yellowstone hotspot track and it culminated with the eruption of the most voluminous low 18 o rhyolite known on earth the 1800 km 3 kilgore tuff 18 o 3 4 such low 18 o values fingerprint meteoric waters and thus the shallow crust new oxygen isotope data for phenocrysts obtained by laser fluorination correspond to a low 18 o magma value of 3 4 0 1 2 standard error for kilgore tuff samples erupted 100 km apart however ion microprobe data for single zircon crystals show significant diversity with 18 o values that range from 1 3 to 6 1 u pb zircon ages mineral chemistry whole rock major and trace element geochemistry sr and nd isotope data and magmatic liquidus temperatures are similar and or overlapping for all studied samples of the kilgore tuff normal 18 o heise tuff units that preceded the kilgore tuff define a temporal compositional trend in trace element concentrations trace element ratios and sr and nd isotope ratios that is consistent with fractional crystallization from a common reservoir whereas low 18 o kilgore cycle units have compositions that define a sharp reversal in the temporal trend back towards the composition of the first normal 18 o heise tuff 6 62 ma blacktail creek tuff the data support derivation of the voluminous low 18 o kilgore tuff from remelting of hydrothermally altered 18 o depleted intracaldera and subvolcanic portions of the blacktail creek tuff single pockets of melt with variable low 18 o values were assembled and homogenized on a caldera wide scale prior to the climactic kilgore tuff eruption and the best record of this process is provided by the 18 o diversity in kilgore tuff zircons temporal trends of oxygen isotopic depletion and recovery in rhyolite eruptions of the heise volcanic field are clearly linked to caldera collapse events and remarkably consistent with trends in the yellowstone plateau volcanic field at heise and yellowstone magmatic 18 o values can be predicted on the basis of cumulative eruptive volumes with a decrease in 18 o by 1 for every 1000 km 3 of erupted rhyolite the kilgore tuff of the heise volcanic field has the same timing magnitude of 18 o depletion and cumulative eruptive volume as the youngest phase of voluminous rhyolitic eruptions in the yellowstone plateau volcanic field indicating that the kilgore tuff may serve as a useful analog for these and perhaps other large volume low 18 o rhyolites on earth

geology and 40ar 39ar geochronology of the middle miocene mcdermitt volcanic field oregon and nevada silicic volcanism associated with propagating flood basalt dikes at initiation of the yellowstone hotspot

geology geochemistry and origin of volcanic rock hosted uranium deposits in northwestern nevada and southeastern oregon usa abstract northwestern nevada and southeastern oregon have the largest uranium u deposits in tertiary volcanic rocks in the us most deposits are in or adjacent to calderas or rhyolite lava dome fields and are hydrothermal almost all are associated with rhyolites that have high primary u concentrations 9 to 20 ppm but the rhyolites range from peralkaline to peraluminous caldera related occurrences are hosted by peralkaline rocks in the mcdermitt and virgin valley calderas at mcdermitt major deposits are along or just inboard of the southwestern kings river and northern ring fractures bretz opalite of the caldera complex the kings river deposits are strongly enriched in trace elements associated with epithermal precious metal deposits e g as au ag mo and sb much of the u occurs as hydrothermal uraniferous zircon and whole rock samples contain as much as 5 zr gangue minerals include quartz adularia fluorite and numerous sulfides 40 ar 39 ar dating of adjacent volcanic rocks and adularia indicates that mineralization was contemporaneous with igneous activity the bretz opalite deposits which include the large aurora resource 7 5 106 kg u3o8 are less enriched in epithermal trace elements and contain uraninite coffinite chalcedony opal and numerous sulfides at virgin valley deposits are mostly in tuffaceous sediments adjacent to ring fracture rhyolites trace element enrichment is minor u is hexavalent in all identified u minerals and associated phases include opal organic material and pyrite deposits in the lakeview u district oregon are hosted by weakly peraluminous rhyolite domes and tuffs and are highly enriched in epithermal trace elements the mineral assemblage includes coffinite opal and numerous sulfides minor u occurrences at buff peak nevada the newly recognized westernmost topaz peraluminous rhyolite in the us are also trace element enriched gangue minerals include quartz adularia and sulfides on the basis of their geologic setting geochemistry age and mineralogy the mcdermitt lakeview and buff peak deposits are hydrothermal sparse published fluid inclusion data indicate temperatures between 200 c and 330 c virgin valley deposits most likely are hydrothermal minor deposits in outflow ash flow tuffs are distant from any intrusive heat source and probably formed from low temperature groundwater

timing and development of the heise volcanic field snake river plain idaho western usa the snake river plain srp developed over the last 16 ma as a bimodal volcanic province in response to the southwest movement of the north american plate over a fixed melting anomaly volcanism along the srp is dominatedby eruptions of explosive high silica rhyolites and represents some of the largest eruptions known basaltic eruptions represent the final stages of volcanism forming a thin cap above voluminous rhyolitic deposits volcanism progressed generally from west to east along the plain episodically in successive volcanic fields comprised of nested caldera complexes with major caldera forming eruptions within a particular field separated by ca 0 5 1 ma similar to and in continuation with the present day yellowstone plateau volcanic field passage of the north american plate over the melting anomaly at a particular point in time and space was accompanied by uplift regional tectonism massive explosive eruptions and caldera subsidence and followed by basaltic volcanism and general subsidence the heise volcanic field in the eastern srp idaho represents an adjacent and slightly older field immediately to the southwest of the yellowstone plateau volcanic field five large volume 0 5 km 3 rhyolitic ignimbrites constitute a time stratigraphic framework of late miocene to early pliocene volcanism for the study region field relations and high precision 4 0 ar 3 9 ar age determinations establish that four of these regional ignimbrites were erupted from the heise volcanic field and form the framework of the heise group these are the blacktail creek tuff 6 62 0 03 ma walcott tuff 6 27 0 04 ma conant creek tuff 5 51 0 13 ma and kilgore tuff 4 45 0 05 ma all errors reported at 2a the fifth widespread ignimbrite in the region is the arbon valley tuff member of the star light formation 10 21 0 03 ma which erupted from a caldera source outside of the heise volcanic field these results establish the conant creek tuff as a distinct and widespread ignimbrite in the heise volcanic field eliminating former confusion resulting from previous discordant k ar and fission track dates new 4 0 ar 3 9 ar determinations when combined with geochemical lithologic geophysical and field data define the volcanic and tectonic history of the heise volcanic field and surrounding areas volcanic units erupted from the heise volcanic field also provide temporal control for tectonic events associated with late cenozoic extension in the snake range and with uplift of the teton range wyoming in the snake range movement of large 0 10 km 3 slide blocks of mississippian limestone exposed 50 km to the east of the heise field occurred between 6 3 and 5 5 ma and may have been catastrophically triggered by the caldera eruption of the 5 51 0 13 ma conant creek tuff this slide block movement of 300 vertical meters indicates that the snake range had significant relief by at least 5 5 ma in jackson hole the distribution of outflow facies of the 4 45 0 05 ma kilgore tuff related to eruption from the kilgore caldera in the heise volcanic field on the eastern srp indicates that the northern teton range was not a significant topographic feature at this time

the yellowstone hotspot in space and time nd and hf isotopes in silicic magmas abstract over the course of its 16 m y history the yellowstone hotspot has produced silicic magmas exhibiting systematic and often sympathetic variations in isotopic and chemical composition temperature and frequency of eruption nd and hf isotopic ratios vary systematically from initial eruptions at 16 ma contemporaneous with basaltic volcanism in eastern oregon and washington to the present day yellowstone volcanic plateau nd and hf isotopic ratios co vary and span the range of most terrestrial samples reflecting mixing of mantle and crustal sources earliest erupted silicic magmas were hot in excess of 1050 c relatively less evolved and have isotopic ratios within the range of contemporaneous columbia river flood basalts the transit of the hotspot across the lithospheric boundary between the western accreted oceanic terrain and the precambrian craton at 15 ma is marked by shifts in e nd from 4 to 11 and in e hf from 10 to 10 the duration of the transit yields a crustal magma source diameter of 70 km in the interval from 14 to 9 ma e nd systematically increases from 11 to 7 recording a minimum increase in the mantle component from 5 to 30 the mantle component could be twice as great depending upon the isotopic composition of crust and mantle reservoirs in this same interval peak temperatures of 1000 c occurred at 9 ma the last 8 m y are characterized by less frequent eruption of lower temperature 830 900 c and more compositionally evolved magmas

effect of cementation on the small strain parameters of sands natural cementation affects the properties of soils the interpretation of in situ and laboratory test results and the selection of criteria for geotechnical design in this paper published exper

crustal structure beneath the blue mountains terranes and cratonic north america eastern oregon and idaho from teleseismic receiver functions we present new images of lithospheric structure obtained from p to s conversions defined by receiver functions at the 85 broadband seismic stations of the earthscope idaho oregon experiment we resolve the crustal thickness beneath the blue mountains province and the former western margin of cratonic north america the geometry of the western idaho shear zone wisz and the boundary between the grouse creek and farmington provinces we calculated p to s receiver functions using the iterative time domain deconvolution method and we used the h k grid search method and common conversion point stacking to image the lithospheric structure moho depths beneath the blue mountains terranes range from 24 to 34 km whereas the crust is 32 40 km thick beneath the idaho batholith and the regions of extended crust of east central idaho the blue mountains group olds ferry terrane is characterized by the thinnest crust in the study area 24 km thick there is a clear break in the continuity of the moho across the wisz with depths increasing from 28 km west of the shear zone to 36 km just east of its surface expression the presence of a strong midcrustal converting interface at 18 km depth beneath the idaho batholith extending 20 km east of the wisz indicates tectonic wedging in this region a north striking 7 km offset in moho depth thinning to the east is present beneath the lost river range and pahsimeroi valley we identify this sharp offset as the boundary that juxtaposes the archean grouse creek block with the paleoproterozoic farmington zone

geodynamics of the yellowstone hotspot and mantle plume seismic and gps imaging kinematics and mantle flow abstract integration of geophysical and geological data show that the yellowstone hotspot resulted from a mantle plume interacting with the overriding north america plate a process that has highly modified continental lithosphere by magmatic and tectonic processes and produced the 16 17 ma 700 km long yellowstone snake river plain ysrp silicic volcanic system accessibility of the ysrp allowed large scale geophysical projects to seismically image the hotspot and evaluate its kinematic properties using geodetic measurements seismic tomography reveals a crustal magma reservoir of 8 to 15 melt 6 km to 16 km deep beneath the yellowstone caldera an upper mantle low p wave velocity body extends vertically from 80 km to 250 km beneath yellowstone but the anomalous body tilts 60 wnw and extends to 660 km depth into the mantle transition zone we interpret this conduit shaped low velocity body as a plume of up to 3 5 vp and 5 5 vs perturbation that corresponds to a 1 2 partial melt models of whole mantle convection reveal eastward upper mantle flow beneath yellowstone at relatively high rates of 5 cm yr that deflects the ascending plume into its west tilted geometry a geodynamic model of the yellowstone plume constrained by vp and vs velocities and attenuation parameters suggests low excess temperatures of up to 120 k corresponding to a maximum 2 5 melt and a small buoyancy flux of 0 25 mg s i e properties of a cool weak plume the buoyancy flux is many times smaller than for oceanic plumes nonetheless plume buoyancy has produced a 400 km wide 500 m high topographic swell centered on the yellowstone plateau contemporary deformation derived from gps measurements reveals sw extension of 2 3 mm yr across the yellowstone plateau one fourth of the total basin range opening rate which we consider to be part of basin range intraplate extension locally decadal episodes of subsidence and uplift averaging 2 cm yr characterize the 80 year yellowstone caldera monitored history and are modeled as hydrothermal magmatic sources moreover a recent episode 2004 2009 of accelerated uplift of the yellowstone caldera at rates up to 7 cm yr has been modeled as resulting from magmatic recharge of a 10 km deep sill at the top of the crustal magma reservoir regionally gravitational potential energy of the yellowstone swell drives the lithosphere southwest and downhill from the yellowstone plateau 400 km where it coalesces with basin range province wide westward extension based on the geometry and its assumed 660 km depth we extrapolate the plume source southwest to its original location at 17 ma and 600 km southwest and 200 km north of the ysrp importantly this location is beneath the southern part of the columbia plateau flood basalt field of the same age and implies that the yellowstone mantle plume may be the common source for both of these large volcanic fields our time progression model suggests that the original plume head rose vertically behind the juan de fuca plate but at 12 ma it lost the protection of the subducting plate from eastward mantle flow and encountered cooler thicker continental lithosphere becoming entrained in eastward upper mantle flow these results reveal that yellowstone plume plate processes have had a profound effect on late cenozoic geologic evolution and topography of a large part of the western u s

using 40 ar 39 ar ages of intercalated silicic tuffs to date flood basalts precise ages for steens basalt member of the columbia river basalt group abstract to establish causality between flood basalt eruptions and extinction events and global environmental effects recorded by isotopic excursions in marine sediments highly accurate and precise ages for the flood basalts are required but flood basalts are intrinsically difficult to date we illustrate how 40ar 39ar feldspar ages for silicic tuffs intercalated with and overlying sections of steens basalt the earliest lavas of the middle miocene columbia river basalt group in the northwestern united states provide high precision ages that for the first time make it possible to resolve age differences with stratigraphic position within a section of these flood lavas the stratigraphically lowest rhyolitic tuff a fall deposit yielded an age of 16 592 0 028 ma fcs 28 02 ma and the uppermost the alkali rhyolite ignimbrite tuff of oregon canyon is 16 468 0 014 ma the argon and stratigraphic data indicate that steens basalt eruptions occurred from 16 64 to 16 43 ma in the southern end of its distribution we estimate that the steens mountain geomagnetic reversal occurred at 16 496 0 028 ma 0 18 ma total error our estimates of the timing for initiation of volcanism and volumetric eruptive rates do not seem to support volcanic forcing by the initial stages of columbia river basalt group eruptions as an explanation for the abrupt warming and carbonate dissolution at the beginning of the miocene climatic optimum

uplift rupture and rollback of the farallon slab reflected in volcanic perturbations along the yellowstone adakite hot spot track field geochemical and geochronological data show that the southern segment of the ancestral cascades arc advanced into the oregon back arc region from 30 to 20 ma we attribute this event to thermal uplift of the farallon slab by the yellowstone mantle plume with heat diffusion decompression and the release of volatiles promoting high k calc alkaline volcanism throughout the back arc region the greatest degree of heating is expressed at the surface by a broad ene trending zone of adakites and related rocks generated by melting of oceanic crust from the farallon slab a hiatus in eruptive activity began at ca 22 20 ma but ended abruptly at 16 7 ma with renewed volcanism from slab rupture occurring in two separate regions the eastern rupture resulted in the extrusion of steens basalt during the ascent and melting of a dry mantle plume source contaminated with depleted mantle the contemporaneous western rupture resulted in renewed subduction melting of a wet mantle source and the rejuvenation of high k calc alkaline volcanism near the nevada california border at 16 7 ma here the initiation of slab rollback is evident in the westward migration of arc volcanism at 7 8 km ma today the uplifted slab is largely missing beneath the oregon back arc region replaced instead by a seismic hole which is bound on the south by the adakite hotspot track we attribute slab destruction to thermal uplift and mechanical dislocation that culminated in rapid tearing of the slab from 17 15 ma and possible foundering and sinking of slab segments from 16 10 ma

crustal velocity structure across the eastern snake river plain and the yellowstone swell teleseismic receiver functions are used to estimate the crustal structure beneath a 36 station 500 km long nw oriented linear array centered on the eastern snake river plain and crossing the yellowstone hotspot swell 250 km sw of yellowstone structure derived previously for this region from wide angle reflection data is used as an initial model and this structure explains most features observed in our receiver functions based on a combination of forward and inverse modeling our data require several modifications to the initial structure 1 moho depth is 42 km beneath most of the snake river plain shallows to 37 km to either side and thickens abruptly to 47 km beneath sw wyoming 2 a midcrustal layer interpreted previously as a 9 km thick gabbroic sill is flat topped across the entire 90 km width of the snake river plain and 3 a low velocity layer is found beneath and southeast of the snake river plain which probably is partially molten lower most crust using the seismic structure of the crust to estimate the crustal load upon the mantle and assuming local isostasy we calculate that mantle beneath the yellowstone swell is approximately uniformly as buoyant as 12 million year old ocean mantle and more buoyant than the adjacent wyoming mantle by an amount equivalent of 1 5 km of elevation the transition between these regions of greatly different mantle occurs across a major paleozoic boundary that now separates the basin and range from the rocky mountains

lateral flow of hot plume material ponded at sublithospheric depths local buoyancy forces drive the flow of ponded plume material along the base of the lithosphere this processes complicates the geological effects of plumes beyond those of simple passage of plates over point heat sources variations in the depth to the base of the lithosphere significantly affect lateral flow the flow rate is modified to a moderate extent by locally thin lithosphere beneath midplate hotspots such as hawaii the effect is relevant when numerical models are rescaled to better fit the observed uplift and the observed distance from the hotspot to the nose of the swell variations in the depth to the base of the lithosphere have a much stronger effect on flow near ridge axes effectively forming an upside down drainage pattern plume material flows upslope toward ridge axes from off axis plumes and along ridge axes from on axis plumes at sufficiently low spreading rates thinner lithosphere exists above plume material than above normal asthenosphere the edges of normal lithosphere adjacent to plume material act to some extent as levees which limit the lateral spreading of plume material pressure release melt is associated with the upslope flow of plume material it is expected to cause volcanism away from the plume and to sometimes supply melt depleted plume material to ridge axes that are upslope from the region of voluminous melting

quickly erupted volcanic sections of the steens basalt columbia river basalt group secular variation tectonic rotation and the steens mountain reversal 1 the steens basalt now considered part of the columbia river basalt group crbg contains the earliest eruptions of this magmatic episode lava flows of the steens basalt cover about 50 000 km2 of the oregon plateau in sections up to 1000 m thick the large number of continuously exposed quickly erupted lava flows some sections contain over 200 flows allows for small loops in the magnetic field direction paths to be detected for volcanic rocks this detail and fidelity are rarely found outside of the holocene and yield estimates of eruption durations at our four sections of 2 5 ka for 260 m at pueblo mountains 0 5 to 1 5 ka for 190 m at summit springs 1 3 ka for 170 m at north mickey and 3 ka for 160 m at guano rim that only one reversal of the geomagnetic field occurred during the eruption of the steens basalt the steens reversal at approximately 16 6 ma is supported by comparing 40ar 39ar ages and magnetic polarities to the geomagnetic polarity timescale at summit springs two 40ar 39ar ages from normal polarity flows 16 72 0 29 ma 16 61 and 16 92 0 52 ma 16 82 equals 2 error place their eruptions after the steens reversal while at pueblo mountains an 40ar 39ar age of 16 72 0 21 ma 16 61 from a reverse polarity flow places its eruption before the steens reversal paleomagnetic field directions yielded 50 nontransitional directional group poles which combined with 26 from steens mountain provide a paleomagnetic pole for the oregon plateau of 85 7 n 318 4 e k 15 1 a95 4 3 comparison of this new pole with a reference pole derived from crbg flows from eastern washington and a synthetic reference pole for north america derived from global data implies relative clockwise rotation of the oregon plateau of 7 4 5 0 or 14 5 5 4 respectively probably due to northward decreasing extension of the basin and range

quality of samples retrieved from great depth and its influence on consolidation properties pleistocene sediments deposited in osaka bay have been extensively investigated to depths of up to 400 m consolidation properties especially preconsolidation pressure are somewhat scattered with depth the quality of samples retrieved from great depths has been evaluated based on the indices calculated from the volumetric strain or the void ratio changes in the recompression process the study reveals that these indices do not simply increase with an increase in sampling depth the potential for sample disturbance during sample setting in the constant rate of strain crs oedometer was also investigated these tests have revealed that the preconsolidation pressure as measured in the laboratory is not influenced significantly by swelling in addition preconsolidation pressures indicate that the sediments are slightly overconsolidated with a departure from normally consolidated behaviour that could be explained by cementation key words sampling preconsolidation pressure sample disturbance oedomet

chemical discrimination of columbia river basalt flows 1 the homogeneity and distinctive chemical composition of individual columbia river basalt flows are illustrated in a series of chemical plots and the procedure for flow identification is outlined the unique characterization of many individual eruptions permits units to be mapped from feeder dike to flow margins across the columbia plateau for hundreds of kilometers and so provides critical constraints on the physical and chemical evolution of this continental flood basalt province

response to the comment by w h schwarz et al on joint determination of 40k decay constants and 40ar 40k for the fish canyon sanidine standard and improved accuracy for 40ar 39ar geochronology by p r renne et al 2010

genesis of flood basalts and basin and range volcanic rocks from steens mountain to the malheur river gorge oregon the middle and south forks of the malheur river provide a unique mapping corridor connecting two flood basalt successions steens basalt to the south and the basalt of malheur gorge to the north each contains chemically defined subtypes which merge stratigraphically across the north south length of the map area the lowermost flows of steens basalt are stratigraphically equivalent to the lowermost flows of the basalt of malheur gorge the uppermost flows of steens basalt pinch out in the map area but are partly interbedded with the middle and uppermost flows of the basalt of malheur gorge which continue to thicken northward the upper part of the tholeiitic succession is interbedded with a group of previously unrecognized lavas the venator ranch basalt flows tholeiitic volcanism ceased at ca 15 3 ma the last tholeiitic unit erupted is the hunter creek basalt which also thickens northward subsequent younger than 15 3 ma more localized eruptions were dominated by calc alkaline to mildly alkaline lavas associated with basin and range extension local uplift generated deep canyons which were filled by andesitic lavas of the keeney sequence ca 13 10 ma the final eruptive products include the devine canyon tuff ca 9 7 ma the drinkwater basalt ca 6 9 ma and the voltage flow older than 32 000 yr b p major and trace element analyses demonstrate that 1 crystal fractionation was a universal process in the derivation of the

diverse mid miocene silicic volcanism associated with the yellowstone newberry thermal anomaly the santa rosa calico volcanic field sc of northern nevada is a complex multi vent mid miocene eruptive complex that formed in response to regional lithospheric extension and flood basalt volcanism santa rosa calico volcanism initiated at 16 7 ma concurrent with regional steens columbia river flood basalt activity and is characterized by a complete compositional spectrum of basalt through high silica rhyolite to better understand the relationships between upwelling mafic magmatism coeval extension and magmatic system development on the oregon plateau we have conducted the first compre hensive study of santa rosa calico silicic volcanism detailed stratigraphic based field sampling and mapping illustrate that silicic activity in this volcanic field was primarily focused along its eastern and western margins at least five texturally distinct silicic units are found in the western santa rosa calico volcanic field including abun dant lava flows near vent deposits and shallow intrusive bodies similar physical features are found in the eastern portion of the volcanic field where four physically distinct units are present the western and eastern santa rosa calico units are characterized by abundant macro and microscopic disequilibrium textures reflecting a complex petrogenetic history additionally unlike other mid mio cene oregon plateau volcanic fields e g mcdermitt the santa rosa calico volcanic field is characterized by a paucity of caldera forming volcanism only the cold springs tuff which crops out across the central portion of the volcanic field was caldera derived major and trace element geochemical variations are present within and between eastern and western santa rosa calico silicic units and these chemical differences coupled with the observed disequilibrium textures illustrate the action of open system petrogenetic processes and melt derivation from heterogeneous source materials the processes and styles of santa rosa calico silicic magmatism are linked to three primary factors local focusing of and thermal and material contributions from the regional flood basalt event lithospheric extension within the northern portion of the northern nevada rift and interaction of mid miocene silicic magmas with pre santa rosa calico lithosphere similar processes and styles of mid miocene silicic volcanism likely occurred across the oregon plateau in regions characterized by both focused lithospheric exten sion and localized mafic magmatism

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

upper mantle seismic structure beneath the pacific northwest a plume triggered delamination origin for the columbia river flood basalt eruptions abstract we invert teleseismic p and s body waves constrained by an ambient noise surface wave model and moho depth inferred from receiver function analysis gao et al 2011 to image mantle structures continuously from the surface to the base of the upper mantle the major structures coincide with prominent geological features we focus on a ne oregon structure termed here the wallowa anomaly which coincides with the source area for the 16 ma columbia river flood basalt eruptions and a circular area of topographic relief created during and after these eruptions resolution tests indicate that the curtain like structure previously interpreted as farallon lithosphere connects with the wallowa anomaly above 150 km along the northeast margin of the wallowa anomaly this connection along with the pre flood basalt magmatic and tectonic history of the pacific northwest lead us to conclude that arrival of the yellowstone plume to south central oregon initiated delamination of remnant farallon lithosphere from the base of ne oregon exposing ocean crust to yellowstone asthenosphere this hypothesis accounts for the propagation of flood basalt volcanism far north of the yellowstone hotspot track and for the high silica composition of most of the flood basalt magmas

termination of the northwestern basin and range province into a clockwise rotating region of transtension and volcanism southeast oregon abstract new data indicate that northeast directed extensional faulting characterizes slip across the brothers fault zone bfz which marks the northern limit of the northwestern basin and range nwbr extensional province in southeastern oregon structural separation across individual north northeast striking nwbr faults decreases to zero south of the bfz field relationships and cross sections demonstrate limited kinematic linkage and independent evolution of the two fault systems since 7 ma west directed extension accumulated on nwbr faults at 0 01 mm yr and lengthened northward after 7 05 ma bfz faults accumulated northeast directed extension at rates of 0 01 mm yr since 5 68 ma deformation coincides with periods of heightened basaltic magmatism in the high lava plains implying that volcanism weakened the crust and promoted extension in the bfz in a new model we reconcile the observed northward diminishing rate and clockwise motion of the modern nwbr deformation field with regional geology the bfz defines a small circle about the pole of rotation and separates a stable block to the ne from the extending region to the south faults to the south are growing northward consistent with the northward decrease in rate and magnitude of extension in the nwbr

stratigraphic and structural evolution of the middle miocene synvolcanic oregon idaho graben the oregon idaho graben is a newly identified north south trending synvolcanic graben in southeastern oregon and southwestern idaho within the middle miocene backarc rift system that extends 1100 km from southern nevada to southeastern washington the graben formed along the western margin of the north american craton shortly after the largest volumes of tholeiitic flood basalt erupted columbia river basalt group steens pueblo basalt basalt of malheur gorge basalt and latite unit of ekren et al 1981 1 rhyolite flows and ash flow tuffs 16 1 14 0 ma erupted from northeastern oregon dooley volcanics to northern nevada mcdermitt volcanic field shortly after the flood basalt was emplaced subsidence of the oregon idaho graben 15 5 15 3 ma coincides with eruption of rhyolite flows and caldera related ash flow tuffs from vents along the margins and within the graben mafic and silicic intragraben volcanism accompanied sedimentation from about 15 3 to 10 5 ma sedimentary and volcanic rocks from extrabasinal sources especially southwestern idaho were introduced periodically after initial subsidence the evolution of the oregon idaho graben is divided into three stages stage 1 15 3 14 3 ma followed intragraben caldera collapse and was marked by deposition of fluvial and lacustrine sediment across the graben stage 2 14 3 12 6 ma movement on intragraben fault zones divided the graben into distinct subbasins and marked the onset of calc alkalic volcanism fine grained tuffaceous sediment derived from glassy rhyolite and pyroclastic deposits and basalt tuff cones interbedded with rhyolite ash and lapilli fall deposits and locally erupted basalt hydrovolcanic deposits predominated during synvolcanic subsidence synsedimentary hot spring alteration and precious metals mineralization of graben fill were controlled by the same intragraben fault zones that served as magmatic conduits during stage 3 12 6 10 5 ma the subbasins were filled and graben wide fluviatile and lacustrine sedimentation resumed at about the same time renewed rhyolitic volcanism occurred on both flanks and tholeiitic volcanism resumed within the oregon idaho graben subsidence in the oregon idaho graben ceased as west northwest striking faults related to the formation of the western snake river plain became active 1 ref 28

local lithospheric relief associated with fracture zones and ponded plume material 1 buoyant plume material ponds at the base of oceanic lithosphere it is a natural probe for the relief on the base of the lithosphere owing to plate age because it flows laterally along an upside down drainage pattern geomorphic and geochemical studies of near ridge and on ridge hot spots indicate as expected that the oceanic lithosphere thickens as it ages corridors of thin lithosphere channel plume material along the axis while the thick lithosphere at the end of ridge segments dams this along axis flow channel hot spots extend from plumes toward ridge axes however a good correlation between local lithospheric thickness and local age is not evident at fracture zones within the old ocean basins such a correlation would be expected if the lithosphere cooled only by conduction then pressure release melting and volcanoes would occur preferentially near fracture zones where plume material cascades from thick old to thin young lithosphere this concentration of volcanic edifices does not occur this lack of correlation between lithosphere thickness and plate age is an expected consequence of small scale convection at the base of the lithosphere scaling relationships and numerical models indicate that modestly vigorous convection of several milliwatt per meter squared is sufficient to perturb local lithospheric relief beneath fracture zones without changing the gross correlation of lithospheric thickness with plate age typically the step in lithospheric thickness moves toward the old side of the fracture zone

timing of cenozoic volcanism and basin and range extension in northwestern nevada new constraints from the northern pine forest range eocene middle miocene volcanic rocks in the northern pine forest range nevada are ideally situated for reconstructing the timing and style of volcanism and extensional faulting in the northwesternmost part of the basin and range province a conformable sequence of cenozoic volcanic and sedimentary strata in the northern pine forest range dips 30 w and 11 new 40 ar 39 ar ages from this sequence defi ne 3 major episodes of volcanic activity pre tertiary basement and older ca 38 ma tertiary intrusive rocks are overlain unconformably by oligocene ca 30 23 ma basalt fl ows and dacitic to rhyolitic ash fl ow tuffs interbedded with fi ne grained tuffaceous sedimentary rocks oligocene rocks are overlain by 550 m of ca 17 16 ma basalt fl ows equivalent to the steens basalt in southern oregon and basalt fl ows are capped by a thin 16 3 ma ignimbrite that likely is correlative with either the idaho canyon tuff or the tuff of oregon canyon the northern pine forest range is bounded to the east by a major down to the east normal fault that dips 40 e with well developed fault striations indicative of dip slip motion this fault initiated at an angle of 70 and was rotated 30 during uplift of the range a suite of 17 apatite fi ssion track ages from the pine forest footwall block demonstrates that exhumation uplift and slip on the range bounding fault began ca 12 11 ma and continued until at least 7 ma with moderate slip since then the pine forest range did not undergo signifi cant extension before or during peak oligocene and miocene volcanism and similar geologic relationships in nearby ranges suggest that a larger region of northwestern nevada was also little extended during this interval basin and range faulting in northwestern nevada appears to have begun no earlier than 12 ma making it distinctly younger than deformation in much of central and southern nevada where peak extension occurred in the middle miocene or earlier

the northern nevada rift regional tectono magmatic relations and middle miocene stress direction as defined by the most recent aeromagnetic surveys the north northwest trending northern nevada rift zone extends for at least 500 km from southern nevada to the oregon nevada border at several places along the rift the magnetic anomaly is clearly related to north northwest trending dikes and flows that based on new radiometric dating erupted between 17 and 14 ma and probably during an even shorter time interval the tectonic significance of the rift is dramatized by its length its coincidence in time and space at its northern terminus with the oldest silicic caldera complex along the yellowstone hot spot trend and its parallelism with the subduction zone along the north american coast prior to the establishment of the san andreas fault the northern nevada rift is also equivalent in age trend and composition to feeder dikes that fed the main eruptive pulse 95 volumetrically off the columbia river flood basalts in northern oregon 15 5 16 5 ma because of these similarities both regions are considered to be part of an enormous lithospheric rift that propagated rapidly south southeast and north northwest respectively from a central mantle plume the site of the initial breaching of the north america plate by this plume is probably the mcdermitt volcanic center at the north end off the rift near the oregon nevada border the present north northwest trend of the rift and its internal elements such as dikes and lava filled grabens record the orientation of the arc normal extensional stress in this back arc region at the time of emplacement paleomagnetic evidence presented by others and interpreted to indicate block rotations at three sample localities is not consistent with either a rotation of dikes within the rift or with a regional rotation of the entire rift the present north northwest trend of the rift reflects the state of stress in the basin and range during middle miocene time and is consistent with stress indicators of similar age throughout the basin and range and rio grande rift provinces