seismic reflection borehole and outcrop geometry of late wisconsin tills at a proposed landfill near toronto ontario the search for new landfill sites in the greater toronto area of southern ontario canada is producing a wealth of data regarding the subsurface stratigraphy and geometry of late wisconsin 25 ka

the miocene great basin of western north america as an extending back arc region abstract a narrow continental volcanic arc has lain near the western margin of the north american continent since mid miocene time its south end began retreating northward 18 m y ago the active remnant of this andesitic arc today is the cascade range of oregon washington and british columbia a large part of the great basin and region to its north was situated throughout this period first wholly then partially in a back arc position relative to the arc this region had and still has most of the characteristics of back arc regions elsewhere in the world a thinned faulted and extended crust and lithosphere limited development of a geophysical bilateral symmetry a mid to late miocene symmetry axis is marked by narrow swarms of basaltic dikes with associated positive linear magnetic anomalies that are parallel to the contemporary trench high heat flow low bouguer gravity and seismic wave velocity values notable seismic attenuation in the mantle and a fault plane solution record dominated by normal fault mechanisms this back arc region differs somewhat from most others those composed of oceanic crust because of its continental setting and recent geologic history despite its name the great basin stands high nearly 2 km above sea level this high elevation owes in part to the fact that extension and heating of the continental lithosphere are still underway though not as a back arc phenomenon the present lithospheric stretching results from oblique extension near the transform boundary of the north american plate the san andreas fault system the latest stretching has probably maintained the physical state of the lithosphere first developed during back arc spreading

evolution of volcanic and tectonic features in caldera settings and their importance in localization of ore deposits many calderas are located along regionally important fault zones that are intermittently active before and after the caldera cycle in mineralized calderas the ore deposits are controlled by structures developed during caldera formation and by regional faults which intersect and reactivate the caldera related structures in the earliest phase of the caldera cycle tumescence occurs over the shallowly emplaced magma chamber and normal faults develop within the structural dome above the magma chamber this early phase of faulting is disrupted by caldera formation but some of the faults are preserved outside the caldera margin and may become important ore controlling structures such as the ruja mercury deposit in the mcdermitt caldera complex nevada and the precious metal deposits in the atlanta district nevada outside the indian peak caldera caldera related structures such as ring fracture faults are commonly the most important in controlling the emplacement of resurgent magma as well as postcollapse intrusions and associated ore deposits during the resurgence phase of the caldera faults related to an apical graben and radial and ring faults and fractures form and may extend outside the collapse structure radial and ring faults and fractures provided the structural control for gold alunite type deposits in the rodalquilar caldera complex spain at the end of the caldera cycle the area over the batholith related to the caldera forming magma chambers is uplifted as isostatic compensation occurs or subsides if the volcanic field is developed within a rapidly subsiding graben volume change resulting from crystallization will also contribute to subsidence regional subsidence of the lake owyhee volcanic field in southeastern oregon at the end of the caldera cycle is reflected by arkosic sedimentation in the caldera basins and throughout the eastern part of the volcanic field crystallization of the underlying batholith permits the reestablishment of brittle deformation along the regional faults which initially may have controlled magma emplacement this tectonic activity may reactivate caldera structures and these typically become important for localization of hydrothermal activity hot spring type gold deposits in the lake owyhee volcanic field formed when a regional northwest trending structure reactivated the caldera ring fracture fault zone of the three fingers caldera the formation of hydrothermal ore deposits at the end of the caldera cycle is often related to tectonic reactivation of caldera structures by regional faults and emplacement of small volumes of magma as stocks and vent complexes along these structures

delineation of fluvial sediment architecture of subalpine riverine systems using noninvasive hydrogeophysical methods river management and restoration measures are of increasing importance for integrated water resources management iwrm as well as for ecosystem services however often river management mainly considers engineering and construction aspects only and the hydrogeological settings as the properties and functions of ancient fluvial systems are neglected which often do not lead to the desired outcome knowledge of the distribution of sediment units could contribute to a more efficient restoration in this study we present two noninvasive approaches for delineation of fluvial sediment architecture that can form a basis for the restoration particularly in areas where site disturbance is not permitted we investigate the floodplain of a heavily modified low mountain river in switzerland using different hydrogeophysical methods in the first approach we use data from electromagnetic induction emi with four different integral depths 0 75 6 m and gamma spectrometry as well as the elevation data as input for a k means cluster algorithm the generated cluster map of the surface combines the main characteristics from multilayered input data and delineates areas of varying soil properties the resulting map provides an indication of areas with different sedimentary units in the second approach we develop a new iterative method for the generation of a geological structure model gsm by means of various emi forward models we vary the geological input parameters based on the measured data until the predicted emi maps match the measured emi values subsequently we use the best matched input data for the gsm generation the derived gsm provides a 3d delineation of possible ancient stream courses a comparison with an independent ground penetrating radar gpr profile confirmed the delineations on the cluster map as well as the vertical changes of the gsm qualitatively thus each of the approaches had the capacity for detecting sedimentary units with distinct hydraulic properties as an indication of former stream courses the developed methodology presents a promising tool for the characterization of test sites with no additional subsurface information

evidence for an underground runoff and soil permeability at the ouled fayet algiers algeria subsurface landfill pilot project from geophysical investigations results from geophysical investigations electrical resistivity electromagnetic mapping and seismic refraction on an excavated cell of the ouled fayet algiers algeria pilot landfill indicated the presence of an underground runoff and permeable soil underneath the cell these results contradict those obtained by a feasibility study based however only on the analysis of seventy six 10 m drilling cores the 1d boreholes information has been proven to be insufficient and to give biased results the presence of water at depth is evidenced by lower resistivity high conductivity anomalies and increase of p wave velocity thus to the contrary of what is claimed in the feasibility study a threat of leachate pollution is real this study shows that landfill construction studies cannot give trustful results without geophysical investigations more specifically in algeria it is imperative to elaborate a landfill construction code which should include mandatory geophysical prospecting and deeper drilling cores

investigation of waste disposal areas using electrical methods a case study from chania crete greece a geophysical investigation of a landfill area was conducted as inextricable phase of a preliminary geotechnical geological and hydrogeological study of the area under investigation electrical resistivity tomography ert one of the most promising prospecting techniques mainly concerning its effective contribution to resolve several environmental problems was applied for the geophysical modeling ert is a robust imaging method the theory and implementation of which are well documented in geophysical research literature the survey site is located in akrotirion peninsula about 7 km east of chania city in crete i the geological setting comprises trypalion limestones marly limestones and a clayed weathered layer cavities are also identified at various scales which are recent tectono karstic voids due to the above mentioned geological singularity and in the light of the requirement for an environmentally safe construction of the landfill an ert survey was carried out specifically seven geoelectrical tomographies were conducted reaching the prospecting depth of 36 m the resulted images conduced to overcome geotechnical problems since the spatial distribution of karstic voids was determined to plan the future sites for waste disposal as the geological conditions were studied in detail and to reliably estimate the thickness of the already deposited wastes the resulted images were confirmed using available borehole logs and ambient noise measurements

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

epithermal gold mineralization in a fossil hot spring system red butte oregon the red butte gold prospect is a well preserved sediment hosted fossil hot spring system located within the lake owyhee volcanic field of eastern oregon 10 km west of the owyhee reservoir its features include siliceous hot spring sinter a blanket of intense argillic alteration produced by surficial steam heated waters and a 50 m wide hydrothermal eruption crater filled with bedded breccia deposits mineralization is hosted by north and northwest striking faults in volcaniclastic sediments of the miocene deer butte formation red butte is capped by 10 m of syngenetic fluvial sedimentary rocks consisting of a basal silicified mudstone overlain by arkosic sandstones and conglomerates interbedded with siliceous hot spring sinter the sediments at this paleosurface horizon have been cemented by hydrothermal quartz and adularia and form the resistant cap of the butte a blanketlike zone of pervasive argillic alteration occurs at the base of the fluvial sediments numerous hydrothermal veins and breccias located within the lacustrine sedimentary rocks beneath the argillic blanket contain a telescoped mineral assemblage related spatially to the paleosurface quartz and adularia are the most abundant hydrothermal minerals in this zone along with minor pyrite marcasite electrum bladed calcite stellerite ca 4 al 8 si 28 o 72 28h 2 o illite smectite and smectite clays the veins and argillic blanket are cut by a 50 m wide hydrothermal eruption crater filled with bedded breccia deposits the delta 18 o smow values for hydrothermal quartz and adularia range from 5 3 to 8 3 and 0 1 to 2 4 per mil respectively the delta 18 o smow and delta 13 c pdb values for calcite range from 3 6 to 8 9 and 8 9 to 5 2 per mil respectively these values are consistent with those of minerals from meteoric hydrothermal systems a plot of delta 18 o versus delta 13 c values for calcite shows that hco super 3 was the dominant aqueous carbon species inaa analyses of altered sedimentary rocks and hydrothermal vein and breccia samples reveal the presence of anomalous concentrations of au ag as sb hg and mo q mode factor analysis indicates that much of the variation in the geochemical data set can be explained in terms of five end member components factors which reflect the composition and evolution of the hydrothermal fluid and the initial composition of the host rock elemental associations for these factors are 1 au k 2 o ag feo hg 2 cao mo hg as 3 feo na 2 o hg sb 4 k 2 o zn na 2 o as and 5 as mo sb ag the abundance of adularia and stability of stellerite at red butte suggests that fluids reaching the surface had experienced a high degree of vapor separation due to boiling au mineralization occurred at the peak of hydrothermal activity in response both to boiling and mixing of the boiling hydrothermal fluid with surficial steam heated waters

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

application of integrated methods in mapping waste disposal areas an integrated suite of environmental methods was used to characterize the hydrogeological geological and tectonic regime of the largest waste disposal landfill of crete island the fodele municipal solid waste site msw to determine the geometry of the landfill depth and spatial extent of electrically conductive anomalies to define the anisotropy caused by bedrock fabric fractures and to locate potential zones of electrically conductive contamination a combination of geophysical methods and chemical analysis was implemented for the characterization and management of the landfill five different types of geophysical surveys were performed 1 2d electrical resistance tomography ert 2 electromagnetic measurements using very low frequencies vlf 3 electromagnetic conductivity em31 4 seismic refraction measurements sr and 5 ambient noise measurements hvsr the above geophysical methods were used with the aim of studying the subsurface properties of the landfill and to define the exact geometrical characteristics of the site under investigation

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

environmental geological and geotechnical investigations related to the potential use of ankara clay as a compacted landfill liner material turkey clay bearing upper pliocene red clastics and quaternary alluvial deposits occupy the ankara basin the clayey levels of the upper pliocene deposits referred to as ankara clay is considered as a source for compacted clay liners due to their low coefficients of permeability and widespread distributions throughout ankara this study investigates the geological geotechnical and mineralogical properties of the founding clayey soils at two sites of the ankara region the geotechnical index properties along with the hydraulic conductivities of the clayey soil samples collected from these sites were determined a mathematical relation between the clay mineralogical content and hydraulic conductivity was established the results of this investigation show that from a geotechnical point of view ankara clay may be regarded as a highly suitable material for a compacted clay landfill liner given that its mineralogical compatibility with leachate is confirmed

direct current and self potential monitoring of an evolving plume in partially saturated fractured rock abstract hole surface and hole hole mise a la masse malm measurements and surface self potential sp measurements were taken over a 173 day period during an 8 well radial injection tracer experiment in partially saturated fractured columbia river basalt a potassium chloride tracer stream was injected into the fractured basalt and was energized directly for the malm experiments the constant concentration tracer solution was injected for 76 days above a perched water table at an average rate of 10 07 l day tracer injection was followed by a 34 day period of tap water injection and a 63 day drainage period the associated changes in potential distributions and sp s over time indicate that an asymmetrical vadose zone plume evolved in the fractured basalt hole surface and sp measurements were used to track the lateral evolution of the tracer plume however there was some noise and variability in the sp data the hole hole measurements helped delineate the depth of preferential tracer migration in the fractured basalt

geomorphology of the burnt river eastern oregon usa topographic adjustments to tectonic and dynamic deformation abstract eastern oregon contains the deepest gorge in north america where the snake river cuts vertically down 2300 m this deep gorge is known as hells canyon a landscape containing such a topographic feature is likely undergoing relatively recent deformation study of the burnt river a tributary to the snake river at the upstream end of hells canyon yields data on active river incision in eastern oregon indicating that quaternary faults are a first order control on regional landscape development through 1 24 000 scale geologic mapping a 500 000 year record of fluvial incision along the burnt river was constructed and is chronologically anchored by optically stimulated luminescence dating and tephrochronology analyses a conceptual model of fluvial terrace formation was developed using these ages and likely applies to other non glaciated catchments in eastern oregon mapped terraces inferred to have formed during glacial interglacial cycles provide constraints on rates of incision of the burnt river incision through these terraces indicates that the burnt river is down cutting at 0 15 to 0 57 m kyr 1 this incision appears to reflect a combination of local base level adjustments tied to movement along the newly mapped durkee fault and regional base level control imposed by the downcutting of the snake river deformation of terraces as young as 38 7 5 1 ka indicates quaternary activity along the durkee fault and when combined with topographic metrics slope relief hypsometry and stream steepness reveals a landscape in disequilibrium longer wavelength lithospheric dynamics delamination and crustal foundering that initiated in the miocene may also be responsible for continued regional deformation of the earth s surface

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

dc resistivity mapping of internal landfill structures two pre excavation surveys geophysical investigations using 2 d dc resistivity were carried out on old parts of two similar landfills with waste of different ages the data sets which included high data density in both vertical and horizontal directions were interpreted with 2 d smoothness constrained inversion the landfills were excavated after the surveying the objective was to test the capability of the resistivity method as a pre characterization technique the objectives were only partially fulfilled first the moisture content was the parameter that appeared to exert the dominant control over the resistivity distribution of the landfill the most important potential information that can be recovered is therefore an indication of the waste piles hydraulics second it was neither possible to estimate the amount of recoverable soils nor to correlate the type of waste with the resistivity models however discrete anomalies were identified and if specific materials are searched for the resistivity models indicate possible places to search

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

migration of contaminants in groundwater at a landfill a case study 7 dc vlf and inductive resistivity surveys abstract we report here the results of direct current dc very low frequency vlf and inductive resistivity surveys of the abandoned landfill at canadian forces base borden these surveys were undertaken initially to assist in mapping the migration of contaminants and continued to assess the relative usefulness of the three techniques for the task all three techniques were successful in outlining the regions of highest landfill derived dissolved constituents in the sand aquifer but the electromagnetic methods have a significant advantage over dc resistivity in both resolution and cost this conclusion is very site specific however we emphasize the importance of evaluating geophysical methods through preliminary modelling before undertaking field work at a new site where the contaminant levels at borden are less than about twice background mapping their distribution with surface geophysics became unreliable as has been noted in previous case histories of this type surface surveys are ultimately limited in their usefulness by natural scatter in the apparent resistivity variations which are unrelated to the variations in contaminant level this lithological and topographical noise can easily mask the detail of a contaminant pattern and is generally inseparable from contaminant related variations if geophysical monitoring is to be useful it is essential that resistivity surveys be undertaken as part of the pre landfill hydrogeological studies in order to separate the natural scatter from post landfill surveys

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

migration of contaminants in groundwater at a landfill a case study 4 a natural gradient dispersion test abstract a natural gradient tracer test using a chloride solution with an initial injection volume of 0 7 m 3 was performed in the sandy aquifer at the borden site the solution was injected into five well points set 1 m below the water table in an uncontaminated zone situated above the contaminant plume at a location 450 m downflow from the landfill under conditions of natural groundwater flow the tracer slug was monitored for a period of 4 months by withdrawing small volume samples from points in a three dimensional array of bundle type multilevel samplers measurements of hydraulic head were obtained from a network of miniature piezometers soon after injection the tracer slug gradually split into two halves one half moving horizontally at an average velocity of 2 9 10 6 ms 1 and the other horizontally at 8 2 10 7 ms 1 although the split has been attributed to local lateral heterogeneity the nature of the heterogeneity and its influence on the hydraulic head distribution were not clearly distinguishable in the field data obtained before during or after the test the chloride patterns for each of the two halves of the tracer slug evolved into gaussian forms although the patterns demonstrated some irregularity at early time the relatively smooth gaussian forms were unexpected because the aquifer has numerous small scale heterogeneities observed in vertical cores obtained from the tracer zone and because the glaciofluvial origin of the aquifer suggests that heterogeneities are not laterally continuous simulated chloride distributions from a three dimensional analytical solution to the advection dispersion equation were fitted to the field data to obtain best fit estimates of the values of longitudinal transverse horizontal and transverse vertical dispersivity at various travel distances for each of the two halves of the tracer zone this is the first known field test that has permitted the estimation of three principal dispersion coefficients in layered media the longitudinal dispersivity was found to increase from 0 01 m at a distance of 0 75 m from the source to 0 08 m at 11 0 m the transverse horizontal dispersivity increased also to a value of 0 03 m at 11 0 m transverse vertical dispersion was very weak and was accounted for by molecular diffusion the relative lack of vertical dispersion is consistent with the shape of the plume of leachate contamination from the landfill it was concluded that the observed increase in dispersivity along the path of migration is likely caused by heterogeneities information on the dispersion controlling heterogeneities is not yet available as practical field methodologies for their identification and description have not yet been developed until such information is incorporated into mass transport models a realistic solution of the dispersion problem in heterogeneous media will remain elusive

smooth bumps in h v curves over a broad area from single station ambient noise recordings are meaningful and reveal the importance of q in array processing the boumerdes algeria case 1 single station h v curves from ambient noise recordings in boumerdes algeria show smooth bumps around 1 and 3 hz a complementary microtremor study based on two 34 and 134 meter aperture arrays evidences that these bumps are indeed real peaks produced by two strong vs contrasts at 37 and 118 meters depth strongly smoothed by very high s wave attenuation in the two sedimentary layers these two h v bumps observed over a broad area are meaningful and reveal the importance of q in s wave velocity modeling from microtremor array data processing it also appears that tertiary rocks should be at least in some cases taken into account together with the quaternary sediments to explain single station h v frequency peaks and therefore that considering only the first 30 m of soil for vs amplification evaluation as usually recommended sometimes leads to flaky results by artificially eliminating non explained low frequency peaks from the analysis

plugs or flood makers the unstable landslide dams of eastern oregon abstract landslides into valley bottoms can affect longitudinal profiles of rivers thereby influencing landscape evolution through base level changes large landslides can hinder river incision by temporarily damming rivers but catastrophic failure of landslide dams may generate large floods that could promote incision dam stability therefore strongly modulates the effects of landslide dams and might be expected to vary among geologic settings here we investigate the morphometry stability and effects on adjacent channel profiles of 17 former and current landslide dams in eastern oregon data on landslide dam dimensions former impoundment size and longitudinal profile form were obtained from digital elevation data constrained by field observations and aerial imagery while evidence for catastrophic dam breaching was assessed in the field the dry primarily extensional terrain of low gradient volcanic tablelands and basins contrasts with the tectonically active mountainous landscapes more commonly associated with large landslides all but one of the eastern oregon landslide dams are ancient likely of order 10 3 to 10 4 years old and all but one has been breached the portions of the oregon landslide dams blocking channels are small relative to the area of their source landslide complexes 0 4 33 6 km 2 the multipronged landslides in eastern oregon produce marginally smaller volume dams but affect much larger channels and impound more water than do landslide dams in mountainous settings as a result at least 14 of the 17 82 large landslide dams in our study area appear to have failed cataclysmically producing large downstream floods now marked by boulder outwash compared to a 40 70 failure rate for landslide dams in steep mountain environments morphometric indices of landslide dam stability calibrated in other environments were applied to the oregon dams threshold values of the blockage and dimensionless blockage indices calibrated to worldwide data sets successfully separate dam sites in eastern oregon that failed catastrophically from those that did not accumulated sediments upstream of about 50 of the dam sites indicate at least short term persistence of landslide dams prior to eventual failure nevertheless only three landslide dam remnants and one extant dam significantly elevate the modern river profile we conclude that eastern oregon s landslide dams are indeed floodmakers but we lack clear evidence that they form lasting plugs

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

geology of the deer butte formation malheur county oregon faulting sedimentation and volcanism in a post caldera setting abstract the deer butte formation accumulated during the middle miocene in fault controlled basins in an extensional setting the basins developed as regional faults asserted influence after eruption of ash flow sheets and collapse of calderas of the lake owyhee volcanic field the sequences of hurley flat dry creek and oxbow basin contain a lower basalt tephradominated unit formed by basalt hydrovolcanism overlain by fine grained fluvial and lacustrine volcaniclastic sedimentary units the sequence of freezeout creek was deposited in an erosional valley that was incised into older units and cut across the concurrently active wall rock ridge fault zone the sequence of hurley flat and dry creek contain alkaline tholeiitic basalt flows and tephra deposits whereas the sequences of freezeout creek and oxbow basin contain subalkaline calcalkaline basaltic andesite the compositional change occurred after local uplift due to faulting along the wall rock ridge fault zone the youngest unit well sorted medium grained muscovite bearing arkose of the arkose of dry creek buttes was deposited in a large river that drained westward from source areas in western idaho the deer butte formation was deposited between approximately 15 and 12 6 ma while basin and range type faulting dominated regional structural patterns

assessing a fractured landfill cover using electrical resistivity and seismic refraction techniques fracturing or erosion of landfill cover materials may lead to escape of landfill gases and creation of leachate through infiltration of surface water periodic geophysical surveys could be used to assess landfill covers noninvasively and identify fractured or thinned areas models derived from electrical resistivity and seismic refraction measurements were evaluated over various portions of a closed municipal landfill in west surburban chicago resistivity soundings could not delineate cover thickness over areas of fractured or new cover where resistivity contrasts with the underlying refuse were minor cover thickness estimates accurate to within 0 7 m however were obtained over unfractured mature cover where resistivities were 10 20 ohm m higher than in the refuse azimuthal resistivity variations as large as 16 ohm m indicate opening of cover fractures during dry weather these same areas showed little or no azimuthal resistivity variation under moist conditions strong absorption of seismic energy a

effects of the hanging wall and footwall on ground motions recorded during the northridge earthquake abstract systematic differences in ground motion on the hanging wall and footwall during the northridge earthquake are evaluated using empirical data an empirical model for the hanging wall effect is developed for the northridge earthquake this empirical model results in up to a 50 increase in peak horizontal accelerations on the hanging wall over the distance range of 10 to 20 km relative to the median attenuation for the northridge earthquake in contrast the peak accelerations on the footwall are not significantly different from the median attenuation over this distance range recordings from other reverse events show a similar trend of an increase in the peak accelerations on the hanging wall indicating that this systematic difference in hanging wall peak accelerations is likely to be observed in future reverse events

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

terrestrial maria the origins of large basalt plateaus hotspot tracks and spreading ridges large lava plateaus which form abruptly within plates without apparent tectonic cause appear to be the terrestrial equivalents of lunar maria deep erosion of lava plateaus reveals that their magma chambers are gabbro and granophyre complexes the classic lopoliths the close association of the deccan plateau with the timing of the terminal cretaceous boundary clay suggests that it is an impact crater large enough to cause pressure relief melting in the asthenosphere basalt then flooded the crater to form a lava lake the terrestrial equivalent of a lunar mare part of the crater rim survives in the amirante ridge the carlsberg oceanic ridge and chagos laccadive hotspot track formed simultaneously with the deccan plateau several other mesozoic and cenozoic lava plateaus are parts of similar arrays of simultaneous features one of those is the southern oregon part of the columbia plateau with its associated continental rifting in the northern basin and range and the yellowstone hotspot track along t

areal distribution and age of low k high alumina olivine tholeiite magmatism in the northwestern great basin field petrographic chemical and geochronologic information has led to the identification and characterization of a widespread low k high alumina olivine tholeiite haot magma type in the northwestern great basin this basalt covers at least 22 000 km 2 and is estimated to represent a total volume of at least 650 km 3 the time period over which haot lavas were erupted extends from late miocene to holocene 10 5 0 m y b p this interval overlaps with the timing of snake river cascade and northwestern basin and range volcanism but distinguishes haot from the main pulse of columbia river volcanism 15 m y b p furthermore three major pulses of haot magmatism are suggested from the geochronology of this study 0 to 2 5 m y b p 3 5 to 6 m y b p and 7 to 10 m y b p the distinctive holocrystalline nonporphyritic and diktytaxitic texture the low incompatible element concentrations and the high mgo feo of haot serve to distinguish this basalt from other basalts of the northwestern united states the low incompatible element signature accentuates the similarities between haot mid ocean ridge basalts and back arc basin basalts these similarities combined with the haot chronology support the idea that the processes giving rise to extensional tectonism and haot magmatism in the northwestern great basin are similar to those acting in active back arc spreading regions

migration of hydrocarbons in carbonate source rocks of the stasfurt member ca2 of the permian zechstein borehole aue 1 germany the role of solution seams abstract the effects of petroleum migration from carbonate source rocks through preferential expulsion of hydrocarbons from stylolites is investigated this type of expulsion is associated with compositional fractionation saturated hydrocarbons and nso compounds are expelled preferentially pristane and phytane show higher relative expulsion efficiencies than n c 17 and n c 18 alkanes aromatic hydrocarbons especially trimethylnaphthalenes are fractionated by the expulsion processes triterpanes and steranes are not fractionated by expulsion from solution seams and could be valuable tools for oil source rock correlation in carbonate rocks

geochronology structure and basin range tectonism of the warner range northeastern california the warner range of northeastern california is a north trending horst at the northwest margin of the great basin the 3 000 m thick bedrock section exposed in the southern part of the range is tertiary in age and it dips homoclinally 20 to 25 west southwest much of the section consists of volcanic rocks whose k ar ages range from about 32 m y near the base of the section to about 14 m y at the top rocks older than about 26 m y are principally andesitic lahars those between 26 and 17 m y are andesitic lava flows and rhyolitic welded ash flow tuffs and those younger than about 17 m y are principally basaltic and andesitic lava flows the homoclinal character of the warner section indicates that block faulting responsible for uplift and tilting of the range began after emplacement of the youngest exposed rocks drilling in the deeply alluviated graben adjacent to the eastern face of the range indicates that aggregate vertical offset across the range front fault is at least 3 600 m the difference in elevation between alluvium in the bottom of the deepest bore hole and the crest of the nearby part of the range thus vertical displacement has occurred at a minimum average rate of 26 cm 1 000 yr during the past 14 m y offset of alluvium along the eastern base of the range records the continuation of block faulting into holocene time the onset of block faulting somewhat less than 14 m y ago and the change in character of volcanic rocks from principally andesitic to principally basaltic in mid miocene time are similar to volcano tectonic events of a much broader region proposed by previous workers the warner range is remarkable for the nearly complete record of regional late cenozoic volcanic and tectonic events found within a single horst

application of electrical resistivity imaging to the development of a geologic model for a proposed edmonton landfill site electrical resistivity imaging eri was used to further characterize the geologic setting at the proposed aurum solid waste landfill site near edmonton alberta two bedrock channel aquifers the east and south channels exist in or near the site previous studies at the proposed site used borehole and pumping test data to determine that the two aquifers are separate and hydraulically disconnected by a sheet of ice thrust bedrock the three objectives of the eri were to resolve the sand channels and terrace sands resolve the top of the thrust bedrock and resolve the sand channels beneath the thrust bedrock the eri survey in combination with the borehole data presented a more detailed representation of the site s complex geology than borehole data alone the south channel has a long and even bottom steeply sloping sides and two levels the thrust bedrock occurs as irregular massive blocks throughout the site and the aquifers are hydraulically confined by the glacial till the bedrock surface is hi

extensive soft sediment deformation and peperite formation at the base of a rhyolite lava owyhee mountains sw idaho usa in the northern owyhee mountains sw idaho a 200 m thick flow of the miocene jump creek rhyolite was erupted on to a sequence of tuffs lapilli tuffs breccias and lacustrine siltstones of the sucker creek formation the rhyolite lava flowed over steep palaeotopography resulting in the forceful emplacement of lava into poorly consolidated sediments the lava invaded this sequence liquefying and mobilising the sediment propagating sediment subvertically in large metre scale fluidal diapirs and sediment injectites the heat and the overlying pressure of the thick jump creek rhyolite extensively liquefied and mobilised the sediment resulting in the homogenization of the sucker creek formation units and the formation of metre scale loading structures simple and pendulous load casts detached pseudonodules density contrasts between the semi molten rhyolite and liquefied sediment produced highly fluidal rayleigh taylor structures local fluidisation formed peperite at the margins of the lava and elutriation structures in the disrupted sediment the result is a 30 40 m zone beneath the rhyolite lava of extremely deformed stratigraphy brittle failure and folding is recorded in more consolidated sediments indicating a differential response to loading due to the consolidation state of the sediments the lava sediment interaction is interpreted as being a function of 1 the poorly consolidated nature of the sediments 2 the thickness and heat retention of the rhyolite lava 3 the density contrast between the lava and the sediment and 4 the forceful emplacement of the lava this study demonstrates how large lava bodies have the potential to extensively disrupt sediments and form significant lateral and vertical discontinuities that complicate volcanic facies architecture

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

crustal genesis on the oregon plateau the areal and temporal distribution and the chemical and isotopic characteristics of cenozoic volcanic rocks from the oregon plateau reflect the changing tectonic conditions affecting this area over the last 20 m y following subduction zone related calc alkaline volcanism flood basalt eruptions occurred in the time interval 18 14 ma this primarily basaltic volcanism is interpreted to be the result of a major shift in spreading orientation of the juan de fuca ridge that resulted in changes in the geometry of subduction against the pacific northwest the compositions of the mid miocene flood basalts show signs of extensive crystal fractionation in some cases accompanied by crustal contamination roughly 11 m y ago basalt eruptions showed an increasing tendency toward smaller volumes and less evolved compositions this implies that as extension in the oregon plateau proceeded the crust became less of an impediment to the passage of relatively unfractionated mantle melts initial sr nd and pb isotopic compositions of all the basalts show more correlation with the geographic position of eruption than with the composition of the lava primary control over the isotope systematics appears to be due to changes in the age and chemical characteristics of the crust and mantle throughout this area these observations suggest that an area of some 130 000 km2 of crust and basalt depleted subcontinental mantle has been added to northwestern north america over the last 20 m y the intensity of basalt volcanism as manifest in the columbia river group and in the oregon plateau does not appear to reflect the presence of a hot spot beneath the area both the areal and temporal distribution of the lavas and their geochemical characteristics are more easily reconciled with the suggestion that this crustal section grew by back arc spreading in a manner analogous to the formation of the marginal basins of the western pacific this spreading initiated along the western border of the archean wyoming craton with the location of mantle ascent melt production and extraction controlled by the changing geometry of subduction the existence of a zone of lithospheric weakness and the presence of a thick low density subcontinental mantle keel existing beneath the archean craton in this area

a miocene subcontinental plume in the pacific northwest geochemical evidence most of the recent discussions on the number and kinds of components which can be distinguished in the columbia river basalt group crbg magmas have used arguments developed from isotopic evidence in this paper we consider relative contents of excluded trace elements interpreted by means of spidergrams and abundance ratios and compare data from the crbg and related lavas with those of selected oceanic basalts although many recent models for crbg petrogeneses do not include a plume component spidergrams for the american bar flows of the imnaha basalt of the crbg have broad humps at ba th and nb ta this feature suggests that an enriched mantle plume component is present in the american bar magmas an inference which is weakly confirmed by 87sr 86sr data this plume may have been identical to that presently beneath yellowstone the inferred hot spot track lies almost directly east west and requires a vector of motion of the pacific northwest part of the north american plate significantly different from that inferred by minster and jordan 1978 31 our proposed vector for the north american plate agrees however with that suggested by the anahim belt a volcanic lineament in southern british columbia that has been proposed as a hot spot track this proposed vector requires alterations in current models for pacific and north american plate movement during the cenozoic

the graveyard point intrusion an example of extreme differentiation of snake river plain basalt in a shallow crustal pluton the graveyard point intrusion is the only known example of a wellexposed differentiated mafic pluton associated with the late miocene pleistocene magmatism of the western snake river plain srp it is exposed in a 6 km by 4 km area adjacent to the oregon idaho border and exposures range in thickness from 20 to 160 m the thicker parts of the intrusion are strongly differentiated and contain a 25 60 m thick section of well laminated cumulus textured gabbros that grade upward into pegmatoidal ferrogabbro evolved liquids formed sheets of fe rich siliceous granophyre at least two injections of magma are indicated by abrupt discontinuities in the rock and mineral compositions and by the lack of mass balance between the bulk intrusion and its chilled borders the laminated gabbros are interpreted to have formed from a tongue of augite and plagioclase crystals that were carried in with the second pulse of magma following the final emplacement of the intrusion in situ differentiation proceeded through a two stage process the ferrogabbros are explained as interstitial liquids forced out of the crystal mush by compaction and the siliceous granophyres are interpreted to be residual liquids that migrated out of the partly crystallized ferrogabbros in response to the exsolution of volatiles because the geochemical trend inferred for the mafic to intermediate composition liquids in the graveyard point intrusion is similar to the trend for many western snake river plain lavas the pluton may be a good model for shallow sub volcanic magma chambers elsewhere in the srp however some western srp lavas contain anomalously high concentrations of p2o5 which are best explained by mixing within the active crustal mush column or with partial melts of previously formed differentiated mafic intrusions

evaluation of landfill disposal boundary by means of electrical resistivity imaging this paper deals with an employment of electrical resistivity imaging eri for survey of leachate content on the waste disposal site in northern israel the research consisted of conducting ten eri lines and drilling investigation wells data simulation used a 2d earthimager inversion program analysis of 2d eri interpretation results shows that determination of the boundary between the landfill body bottom intensively saturated with leachates and underlying layers of highly water saturated fat nonconsolidated clays presents a challenge however statistical analysis of eri data indicates that standard deviation and confidence interval of a set of resistivity data measured in the landfill body are significantly larger than those in underlying clays moreover maximum changes of these parameters are found on the boundary between landfill body and underlying soil thus reflecting natural differences in scattering of resistivity data measured in these two objects

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

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

post oligocene tectonic rotation of the oregon western cascade range and the klamath mountains paleomagnetic data indicate that the western cascade range from northern california to central washington has undergone about 27 of clockwise rotation since 25 m y b p all the available regional paleo magnetic data fit a two phase rotation model the first phase being an eocene rotation that accompanied the accretion of the oregon coast range to the continent and the second phase being a rotation associated with post early miocene extension in the basin and range province

controls on large landslide distribution and implications for the geomorphic evolution of the southern interior columbia river basin large landslides 0 1 km 2 are important agents of geomorphic change while most common in rugged mountain ranges large landslides can also be widespread in relatively low relief several 100 m terrain where their distribution has been relatively little studied a fuller understanding of the role of large landslides in landscape evolution requires addressing this gap since the distribution of large landslides may affect broad regions through interactions with channel processes and since the dominant controls on landslide distribution might be expected to vary with tectonic setting we documented 400 landslides between 0 1 and 40 km 2 across 140 000 km 2 of eastern oregon in the semiarid southern interior columbia river basin the mapped landslides cluster in a nw se trending band that is 50 100 km wide landslides predominantly occur where even modest local relief 100 m exists near key contacts between weak sedimentary or volcaniclastic rock and coherent cap rock fault density exerts no control on landslide distribution while 10 of mapped landslides cluster within 3 10 km of mapped fold axes landslide occurrence is curtailed to the ne by thick packages of coherent basalt and to the sw by limited local relief our results suggest that future mass movements will localize in areas stratigraphically preconditioned for landsliding by a geologic history of fluviolacustrine and volcaniclastic sedimentation and episodic capping by coherent lava flows in such areas episodic landsliding may persist for hundreds of thousands of years or more producing valley wall slopes of 7 13 and impacting local channels with an evolving array of mass movement styles

a new interpretation of deformation rates in the snake river plain and adjacent basin and range regions based on gps measurements we evaluate horizontal global positioning system gps velocities together with geologic volcanic and seismic data to interpret extension shear and contraction within the snake river plain and the northern basin and range province u s a we estimate horizontal surface velocities using gps data collected at 385 sites from 1994 to 2009 and present an updated velocity field within the stable north american reference frame snarf our results show an ene oriented extensional strain rate of 5 9 0 7 x 10 sup 9 yr sup 1 in the centennial tectonic belt and an e oriented extensional strain rate of 6 2 0 3 x 10 sup 9 yr sup 1 in the intermountain seismic belt combined with the northern great basin these extensional strain rates contrast with the regional north south contraction of 2 6 1 1 x 10 sup 9 yr sup 1 calculated in the snake river plain and owyhee oregon plateau over a 125 x 650 km region tests that include dike opening reveal that rapid extension by dike intrusion in volcanic rift zones does not occur in the snake river plain at present this slow internal deformation in the snake river plain is in contrast to the rapidly extending adjacent basin and range provinces and implies shear along boundaries of the snakemore river plain we estimate right lateral shear with slip rates of 0 5 1 5 mm yr along the northwestern boundary adjacent to the centennial tectonic belt and left lateral oblique extension with slip rates of 10 sup 6 yrs the low strain field in the snake river plain and owyhee oregon plateau would extend through the quaternary less

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