upper mantle tomography beneath the pacific northwest interior abstract the pacific northwest has undergone extensive tectonic and magmatic construction in the cenozoic and many clues to its history are in a well resolved upper mantle structure we present new p s and v p v s models that benefit from improved algorithms and additional data from recently deployed flexible array experiments in the region of the columbia river basalts western snake river plain and idaho batholith we refine the geometries of previously imaged structures and identify new anomalies beneath the pacific northwest a clear separation is resolved between the wallowa and idaho structures that extends from the base of the crust to the depth extent of these structures the separation follows the precambrian margin of north america possibly indicating separate tectonic histories for the two structures a shallow fast anomaly beneath idaho batholith may be responsible for the observed curvature of the yellowstone track around this batholith and a strong and unusually deep 180 250 km slow anomaly is present east of the apparent juan de fuca slab gap resolving these structures well is important in understanding the origin of the lithospheric fragments in the mantle and key to any reconstruction of the evolution of the pacific northwest farallon kula juan de fuca tectonic system of most relevance understanding the origin of the imaged high velocity mantle structures provides strong constraint on their density that otherwise would be largely unknown which controls the evolution of vertical and horizontal stresses acting at the base of the lithosphere and hence the evolution of topography also resolving the volumes of partially molten upper mantle shapes our understanding on the origin of the abundant volcanism in the pacific northwest interior

determination of rapid deccan eruptions across the cretaceous tertiary boundary using paleomagnetic secular variation 2 constraints from analysis of eight new sections and synthesis for a 3500 m thick composite section 1 the present paper completes a restudy of the main lava pile in the deccan flood basalt province trap of india chenet et al 2008 reported results from the upper third and this paper reports the lower two thirds of the 3500 m thick composite section the methods employed are the same i e combined use of petrology volcanology chemostratigraphy morphology k ar absolute dating study of sedimentary alteration horizons and as the main correlation tool analysis of detailed paleomagnetic remanence directions the thickness and volume of the flood basalt province studied in this way are therefore tripled a total of 169 sites from eight new sections are reported in this paper together with the results of chenet et al 2008 these data represent in total 70 of the 3500 m combined section of the main deccan traps province this lava pile was erupted in some 30 major eruptive periods or single eruptive events see each with volumes ranging from 1000 to 20 000 km3 and 41 individual lava units with a typical volume of 1300 km3 paleomagnetic analysis shows that some sees with thicknesses attaining 200 m were emplaced over distances in excess of 100 km both likely underestimates due to outcrop conditions and up to 800 km the total time of emission of all combined sees could have been much less than 10 ka with most of the time recorded in a very small number of intervening alteration levels marking periods of volcanic quiescence so called big red boles the number of boles thickness of the pulses and morphology of the traps suggest that eruptive fluxes and volumes were larger in the older formations and slowed down with more and longer quiescence periods in the end on the basis of geochronologic results published by chenet et al 2007 and paleontological results from keller et al 2008 we propose that volcanism occurred in three rather short discrete phases or megapulses an early one at 67 5 1 ma near the c30r c30n transition and the two largest around 65 1 ma one entirely within c29r just before the k t boundary the other shortly afterward spanning the c29r c29n reversal we next estimate sulfur dioxide likely a major agent of environmental stress amounts and fluxes released by sees they would have ranged from 5 to 100 gt and 0 1 to 1 gt a respectively over durations possibly as short as 100 years for each see the chemical input of the chicxulub impact would have been on the same order as that of a very large single pulse the impact therefore appears as important but incremental neither the sole nor main cause of the cretaceous tertiary mass extinctions

parana flood basalt volcanism primarily limited to 1 myr beginning at 135 ma new 40 ar 39 ar ages for rocks from rio grande do sul and critical evaluation of published radiometric data abstract 40 ar 39 ar step heating analyses were carried out on seven rocks five basalts an andesite and a rhyolite from the southern parana province 28 s 30 s they yield plateau isochron ages of 135 134 ma in good agreement with published step heating data on rocks from the same area critical review of laser spot isochron ages for rocks from the province ranging from 140 to 130 ma are shown to be unreliable estimates of crystallization ages as the rocks were substantially altered step heating results on three of these rocks thought to yield good plateau ages are shown to be incorrect as a result of a technicality in dating procedures followed u pb ages on zircon and baddeleyite separated from a variety of rock types 30 s 23 s fall in the range 135 to 134 ma all reliable 40 ar 39 ar and u pb ages indicate volcanism was sharply focused initiated at 135 ma and 1 myr in duration no variation of age with either latitude or longitude is noted scrutiny of published 40 ar 39 ar ages on the florianopolis dykes shows they cannot be used as reliable crystallization ages u pb work shows that this dyke swarm was formed coevally with the main part of the parana province most of the published 40 ar 39 ar ages on the ponta grossa dyke swarm are unreliable a few ages appear reliable and suggest the magmatic event in this area may have postdated the main parana pulse by 1 2 myr a single 40 ar 39 ar age from a high nb basalt in the southernmost part 34 s of the parana at 135 ma highlights the need for further radiometric work on other areas of this flood basalt province the parana province postdates the time of the jurassic cretaceous bound ary by 10 myr

loose boundary hydraulics and fluid mechanics selected advances since 1961 s u m m a r y a maj or and acknowledged role in the advance o f sedimentological knowledge is played by the methods of physics as expressed through fluid mechanics and loose boundary hydraulics turbulence is increasingly being seen as involving orderly flow structures and these are significant for the origin of several sedimentary structures and for suspension transport recent advances in sediment transport theory are rooted in the concept of the sediment load as a downward acting force and in the notion of the flow as a transporting machine laboratory experiments are increasing our understanding of bedforms due to rivers and waves but there is a need for well framed field studies to clarify the influence of change in natural environments physical explanations are being proposed for tidal bedforms field studies in both modern environments and the stratigraphic record suggest that tidal patterns and strengths are recognizable from the internal features of cross bedded units formed by sand waves tidal patterns are complex however and a wider range of situations needs to be explored than has hitherto been the case mathematical modelling could help to define the expectable bedding patterns although successful mathematical models have been constructed of flow and sedimentation in channel bends secondary flow in real bends and the evolution of those bends are not yet well understood turbidity currents on a geologically significant scale have not been observed directly but valuable insights into their character and processes have come from laboratory experiments recent laboratory work points to the limitations of lock exchange experiments and emphasizes the various ways in which turbidity currents mix with the ambient medium these studies seem to have implications yet to be worked out for the internal features and some sole marks of turbidites

measurement and modelling of an artificial coastal lagoon breach a field data set of the artificial breaching of a coastal lagoon berm is presented and includes a detailed analysis of the breach evolution in plan and elevation together with water levels and flow velocities a semi coupled two dimensional depth averaged numerical model describing both the shallow water hydrodynamics and morphodynamics is developed and tested against the field data key hydrodynamic and morphodynamic processes are discussed and strategies to model these processes are presented and evaluated such as accounting for modified roughness under transcritical flows and testing an improved algorithm for widening of the breach channel through side wall erosion while further research is warranted the processes relating to the erosion of the breach side walls and sediment transport under transcritical flow regimes were found to be essential to developing a realistic model of the overall breach process a new channel bank erosion model is developed and implemented which shows improved performance

response to baksi a 2012 new 40ar 39ar dating of the grande ronde lavas columbia river basalts usa implications for duration of flood basalt eruption episodes by barry et al 2010 discussion abstract baksi 2012 claims that a number of radiometric ages presented in barry et al 2010 are statistically invalid and that many of the samples were altered thus not able to date the eruptions and that there is no compelling evidence for a limited duration for the eruption of the grande ronde basalts gr basalts usa however baksi 2012 has simply reiterated some of the assessment of the data presented in the paper and has used an alteration index in order to assess the data the alteration index commonly shows that basaltic groundmass is altered and from this baksi 2012 deduces that the ages are incorrect in contrast much work on young volcanics has shown that phenocrysts are often sources of incorrect ages in any assessment of data we maintain that whilst alteration is an important consideration the best approach should include not just alteration but also experimental reproducibility stratigraphic order comparison with other analyses and analytical techniques and that the application of an alteration index is not a straightforward guide to the validity of an age

new 40ar 39ar dating of the grande ronde lavas columbia river basalts usa implications for duration of flood basalt eruption episodes by discussion abstract barry et al 2010 presented 40 ar 39 ar ages to argue for a very limited duration 0 4 m y for the eruption of the grande ronde lavas critical examination of their data sets shows 1 a number of their plateau ages are statistically invalid and 2 many of their groundmass matrix samples were altered and did not yield valid estimates of the time of crystallization in general their ages err on the young side reflecting partial 40 ar loss due to alteration and there is no compelling evidence therein to argue for very limited duration for the grande ronde basalt the imnaha grande ronde and wanapum basalt erupted within 0 7 m y starting at 16 4 ma by contrast the ages of barry et al 2010 span the interval 16 6 to 14 7 ma and in some instances are out of stratigraphic sequence

broadband seismic observation at the sakurajima volcano japan we installed a portable broadband seismometer streckeisen sts 2 at the sakurajima volcano which has been very active in the recent years the recorded seismograms show a wide variety both in temporal and spectral contents of seismic events from explosions to tremors and exhibit the importance of such broadband seismometry at volcanos we present examples of seismograms to show the potential of broadband seismic observation in monitoring volcanic activities

highlights from a seismic broadband array on stromboli volcano an array of nine three component broadband seismometers was deployed in two different configurations on stromboli volcano the analysis of the seismic wavefield related to volcanic explosions revealed some observations which offer a completely new insight into the internal dynamics of a volcano these new observations are restricted to the low frequency range below 1 hz and underline therefore the superiority of broadband recordings over conventional short period observations surprisingly simple wavelets indicate an initially contracting source mechanism gas jets that could not be seen in a short period seismic record at all generate a clear dilatational wavelet in a broadband recording suggesting the same contracting source mechanism the analysis of particle motion and seismic array techniques permits a location of the seismic source we find low frequency signals of 3s and 6s period that are not related to eruptions and do not share a common source with the eruption related events a video recording of visible volcanic activity at the crater region allows one to correlate precisely eruptive features with seismic signals

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

ages of the steens and columbia river flood basalts and their relationship to extension related calc alkalic volcanism in eastern oregon stratigraphic and chemical correlations of tertiary volcanic units in eastern oregon confirm that the steens basalt represents the earliest eruptions of the columbia river flood basalt province field correlations are supported by major and trace element analyses and confirmed by 4 0 ar 3 9 ar dates within the basalt of malheur gorge situated between steens mountain and the southernmost extent of the previously mapped columbia river basalt group the lowest unit correlates with the steens basalt and the conformably overlying middle and upper units correlate with the imnaha and grande ronde basalt formations of the columbia river basalt group new dates indicate that imnaha and grande ronde basalt formations on the columbia plateau 90 of the columbia river basalt group erupted between 16 1 and 15 0 ma these were immediately preceded by the steens basalt a plagioclase phyric tholeiite that erupted above the calculated position of the yellowstone hotspot at 16 6 ma in eastern oregon the flood basalt tholeiites of steens mountain and malheur gorge form a voluminous but brief interlude 16 6 15 3 ma superimposed on the low volume calc alkalic to mildly alkalic volcanism associated with continuing eocene to present east west extension

analysis of long period seismic waves excited by the november 1987 eruption of izu oshima volcano long period seismic waves which were excited by two eruptions of izu oshima volcano in november 1987 and recorded at an abbreviated seismic research observatory station and at several volumetric strain meter stations are analyzed to determine kinematic seismic source models associated with the eruptions the seismic waves have unusual features in the radiation pattern and waveforms of rayleigh waves which cannot be explained in terms of a shear dislocation model waveform inversion is applied to obtain a kinematic source model which is parameterized by a single force and a seismic moment tensor many geophysical and field observations make clear that the two eruptions occurred in a lava drain back stage of the volcanic activity the source model of the seismic event associated with the eruption on november 16 1987 is mainly composed of a vertical single force with a magnitude of 4 1011 n and suggests a rapid collapse of overloading lava into the deeper vent the long period seismic waves excited by the eruption on november 17 1987 can be explained by a force system composed of two vector dipoles with nearly vertical orientation which are separated in time by 81 s the sum of the two dipoles is 1 7 1017 n m this source model suggests that tension crack openings or volume expansions of horizontally extended magma reservoirs or of aquifers occurred because of lava drain back

evidence from lava flows for complex polarity transitions the new composite steens mountain reversal record summary geomagnetic polarity transitions may be significantly more complex than are currently depicted in many sedimentary and lava flow records by splicing together paleomagnetic results from earlier studies at steens mountain with those from three newly studied sections of oregon plateau flood basalts at catlow peak and poker jim ridge 70 90 km to the southeast and west respectively we provide support for this interpretation with the most detailed account of a magnetic field reversal yet observed in volcanic rocks forty five new distinguishable transitional t directions together with 30 earlier ones reveal a much more complex and detailed record of the 16 7 ma reversed r to normal n polarity transition that marks the end of chron c5cr compared to the earlier r t n t n reversal record the new record can be described as r t n t n t r t n the composite record confirms earlier features adds new west and up directions and an entire large n t r t segment to the path and fills in directions on the path between earlier directional jumps persistent virtual geomagnetic pole vgp clusters and separate vgps have a preference for previously described longitudinal bands from transition study compilations which suggests the presence of features at the core mantle boundary that influence the flow of core fluid and distribution of magnetic flux overall the record is consistent with the generalization that vgp paths vary greatly from reversal to reversal and depend on the location of the observer rates of secular variation confirm that the flows comprising these sections were erupted rapidly with maximum rates estimated to be 85 120 m ka 1 at catlow and 130 195 m ka 1 at poker jim south paleomagnetic poles from other studies are combined with 32 non transitional poles found here to give a clockwise rotation of the oregon plateau of 11 4 5 6 with respect to the younger columbia river basalt group flows to the north and 14 5 4 6 with respect to cratonic north america 95 per cent confidence interval

biogenic sedimentation in the equatorial pacific carbon cycling and paleoproduction 12 24 ma 1 the equatorial pacific is an important part of the global carbon cycle and has been affected by climate change through the cenozoic 65 ma to present we present a miocene 12 24 ma biogenic sediment record from deep sea drilling project dsdp site 574 and show that a caco3 minimum at 17 ma was caused by elevated caco3 dissolution when pacific plate motion carried site 574 under the equator at about 16 2 ma there is a minor increase in biogenic deposition associated with passing under the equatorial upwelling zone the burial rates of the primary productivity proxies biogenic silica bio sio2 and biogenic barium bio ba increase but biogenic caco3 decreases the carbonate minimum is at 17 ma coincident with the beginning of the miocene climate optimum the transient lasts from 18 to 15 ma bio sio2 and bio ba are positively correlated and increase as the equator was approached corg is poorly preserved and is strongly affected by changing carbonate burial terrestrial 232th deposition a proxy for aeolian dust increases only after the site 574 equator crossing since surface production of bio sio2 bio ba and caco3 correlate in the modern equatorial pacific the decreased caco3 burial rate during the site 574 equator crossing is driven by elevated caco3 dissolution representing elevated ocean carbon storage and elevated atmospheric co2 the length of the 17 ma caco3 dissolution transient requires interaction with a slow part of the carbon cycle perhaps elevated mantle degassing associated with the early stages of columbia river basalt emplacement

emplacement of columbia river flood basalt evidence is examined for the emplacement of the umatilla wilbur creek and the asotin members of columbia river basalt group these flows erupted in the eastern part of the columbia plateau during the waning phases of volcanism the umatilla member consists of two flows in the lewiston basin area and southwestern columbia plateau these flows mixed to form one flow in the central columbia plateau the composition of the younger flow is preserved in the center and the composition of the older flow is at the top and bottom there is a complete gradation between the two flows of the wilbur creek and asotin members erupted individually in the eastern columbia plateau and also mixed together in the central columbia plateau comparison of the emplacement patterns to intraflow structures and textures of the flows suggests that very little time elapsed between eruptions in addition the amount of crust that formed on the earlier flows prior to mixing also suggests rapid emplacement calculations of volumetric flow rates through constrictions in channels suggest emplacement times of weeks to months under fast laminar flow for all three members a new model for the emplacement of columbia river basalt group flows is proposed that suggests rapid eruption and emplacement for the main part of the flow and slower emplacement along the margins as the of the flow margin expands

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

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

transitional geomagnetic impulse hypothesis geomagnetic fact or rock magnetic artifact a striking feature of the steens mountain oregon geomagnetic polarity reversal is the two maybe three extremely rapid field directional changes 6 degrees per day proposed to account for unusual behavior in direction of remanent magnetization in a single lava flow each of these very fast field changes or impulses is associated with a large directional gap some 90 in the record in order to check the spatial reproducibility of the paleomagnetic signal over distances up to several kilometers we have carried out a paleomagnetic investigation of two new sections b and f in the steens summit region which cover the second and the third directional gap the main result is the description of two new directions which are located between the pre second and post second impulse directions these findings weigh against the hypothesis that the geomagnetic field cause the unusual intraflow fluctuations which now appears to be more ad hoc as an explanation of the paleomagnetic data however the alternative baking hypothesis remains also ad hoc since we have to assume variable rock magnetic properties that we have not yet been able to detect within the flows at the original section steens a and d 1 5 km to the north in addition new results for 22 transitional and normal lava flows in section b are presented that correlate well with earlier results from section a

eocene to miocene magnetostratigraphy biostratigraphy and chemostratigraphy at odp site 1090 sub antarctic south atlantic at ocean drilling program odp site 1090 lat 42854 89s long 8854 09e located in a water depth of 3702 m on the agulhas ridge in the sub antarctic south atlantic 300 m of middle eocene to middle miocene sediments were recovered with the advanced piston corer apc and the extended core barrel xcb u channel samples from the 70 230 meters composite depth mcd interval provide a magnetic polarity stratigraphy that is extended to 380 mcd by shipboard whole core and discrete sample data the magnetostratigraphy can be interpreted by the fit of the polarity zone pattern to the geomagnetic polarity time scale gpts augmented by isotope data and bioevents with documented correlation to the gpts three normal polarity subchrons c5dr 1n c7ar 1n and c13r 1n not included in the standard gpts are recorded at site 1090 the base of the sampled section is correlated to c19n middle eocene although the interpretation is unclear beyond c17r the top of the sampled section is correlated to c5cn late early miocene although in the uppermost 10 m

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

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

modelling of the construction of the rhone alluvial plain since 15 000 years bp a numerical model presented here develops a three dimensional image of alluvial media on an elementary scale significant for groundwater flow modelling the model was tested on the alluvial plain of the rhone river france on a scale of several kilometres and from geomorphological observations and dating reproduced the construction of this alluvial plain from 15 000 years bp to the present the history of the alluvial plain during the late glacial and holocene periods is summarized through most of this time the river rhone has maintained a braided pattern with the exception of two incising phases with a meander pattern the model does not use any physically based equations or water representation the main processes governing the construction of the plain are modelled by simple rules chosen according to geometrical or empirical laws taken from the literature or as modelling assumptions using multi agent concepts of distribution and interaction of elementary entities these sedimentary rules are applied to sediment entities or to conceptual erosion entities that simulate local deposition and erosion of sediments the sedimentation model reproduces the various climatic periods during which the sediments were deposited by simulating genetic periods and associated modelled processes for each period the model was constrained by quantitative field data such as altitude of ancient channels and deposits or thickness of sediments the general geometry of the alluvial deposits was satisfactorily reproduced during the simulation characteristic large scale features emerge despite the use of local rules the model results are discussed with reference to other approaches such as geostatistical or boolean models and the applicability of the model to other less documented alluvial plains is outlined

directional change during a miocene r n geomagnetic polarity reversal recorded by mafic lava flows sheep creek range north central nevada usa recurring transitional field directions during three miocene geomagnetic reversals provide evidence that lateral inhomogeneity of the lower mantle affects flow in the outer core we compare new paleomagnetic results from a composite sequence of 15 2 ma lava flows in north central nevada sheep creek range 40 7n 243 2e erupted during a polarity reversal to published data from steens mountain 250 km to the northwest in oregon and the newberry mountains 650 km to the south in california that document reversals occurring millions of years and many polarity switches earlier alternating field demagnetization followed by thermal demagnetization in half the samples clearly isolated the primary thermoremanent magnetization of sheep creek range flows we correlated results from our three sampled sections to produce a composite record that begins with a single virtual geomagnetic pole vgp at low latitude in the atlantic followed by two vgps situated near latitude 30n in ne africa after jumping to 83n one vgp the pole moves to equatorial south america one vgp back to ne africa three vgps to high southern latitudes two vgps back to equatorial south america three vgps and finally to high northern latitudes nine vgps the repeated visits of the transitional vgp to positions in south america and near ne africa as well as the similar behavior recorded at steens mountain and the newberry mountains suggest that lower mantle or core mantle boundary features localize core flow structures thereby imparting a discernible regional structure on the transitional geomagnetic field that persists for millions of years

evidence for errors in the geomagnetic polarity time scale at 17 15 ma 40ar 39ar dating of basalts from the pacific northwest usa 40ar 39ar incremental heating studies have been carried out on whole rock basalt samples from the columbia river basalt crb and the steens basalt to obtain crystallization ages with a standard error 1 these high precision accurate ages are utilized with the known magnetostratigraphy of the crb to test the accuracy of the early to middle miocene section of the geomagnetic polarity time scales currently in use we conclude that these time scales underestimate the ages of various chrons within this time range by 0 9 m y

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

the relationship between flow shear stress and stream pattern based on simple hydraulic reasoning a discharge q slope s diagram for streams is given a physical interpretation permitting the assignment of a relative shear stress value to each stream a sample of 359 streams is used ranging in mean annual discharge from 900 000 to 0 05 ft 3s 1 varying in pattern from highly sinuous p 3 1 to straight and braided sorted according to their relative shear stress values 0 avg streams of high sinuosity p 1 5 are characterized by low relative shear stress median 0 56 which is significantly lower than that characterizing braided streams median 1 70 it is concluded that for natural stream populations the passage from one pattern to another is gradual and the geomorphic thresholds defined by relative shear stress are fuzzy ones

statistical analysis of lateral migration of the rio grande new mexico abstract the lateral migration rates of alluvial rivers are affected by changes in water and sediment regimes the rio grande downstream from cochiti dam exhibits spatial and temporal variability in lateral movement rates documented since 1918 a tremendous database exists that documents planform bed material size channel geometry and water and sediment regimes a statistical analysis reveals that migration rates primarily decreased with decreasing flow energy r2 0 50 p 60 the findings show that lateral movement increases with increasing flow energy and with degree of braiding

incision and morphologic evolution of an alluvial channel recovering from hydraulic mining sediment hydraulic gold mining in the sierra nevada california produced such a large volume of sediment from 1853 to 1884 that channels could not carry a substantial proportion of the delivered sediment the lower bear river aggraded as much as 5 m and underwent a major avulsion during the 1870s channel morphology continues to respond to these changes more than 100 years after the cessation of most hydraulic mining textures of mine tailings and older alluvium are contrasted and channel incision and channel shape changes from 1930 to 1985 are documented unlike the yuba and sacramento rivers the lower bear river had not returned to pre mining base levels by 1950 as sediment loads decreased nor were incision rates or morphological change governed by reduced sediment loads below dams built in 1928 and in the mid 1960s channel stability was due to resistance of a cohesive stratum on the channel bed an episode of channel down cutting was instigated by a moderately large flood in december 1955 that pierced the cohesive layer and was sustained through the 1970s by moderate magnitude floods the 1955 flood did not directly erode large volumes of sediment but it destabilized the channel allowing smaller floods to erode the channel at an accelerated rate episodic channel bed incision is contrasted to progressive morphologic changes cross section shape narrowed and deepened steadily from 1930 through the mid 1970s as the channel was superimposed onto cohesive older alluvium progressive deepening set up a positive feedback that ensured the ultimate penetration of the paved bed channel morphologic readjustment from the avulsion thus was both progressive and episodic

40ar 39ar ages of flood basalt provinces in russia and china and their possible link to global faunal extinction events a cautionary tale regarding alteration and loss of 40ar abstract the hypothesis that the permo triassic boundary ptb mass extinctions were caused by flood basalt volcanism in russia siberian traps and or china the emeishan traps is investigated from the point of view of time of occurrence 40 ar 39 ar ages numerous published ages in the literature are rejected as good estimates of the time of crystallization the filters applied in this respect are a statistical reliability of plateau isochron sections of stepheating data and b the alteration state of the material that was dated alteration appears to be ubiquitous unsurprising since most of the material dated was used without acid leaching a procedure that is effective in yielding fresh er samples of 70 ages in the literature for the main pulse of siberian trap volcanism less than ten prove to be reliable ages similar techniques applied to 40 ar 39 ar for the emeishan traps leaves only a single reliable age for the magmatic episode these ages are compared to both published and new 40 ar 39 ar ages for the ptb as based on analysis of minerals from critical ash beds in china there is good overlap in the ages ptb 250 0 0 1 ma siberian trap lavas 250 1 0 4 ma lending credence to a genetic link between the formation of the siberian traps and the faunal extinction event at the ptb a similar link for the formation of the viluy traps russia and the late devonian extinction event is investigated only a single reliable 40 ar 39 ar age is available for the viluy traps and falls close to the interpolated age for the frasnian fammenian boundary the use of the unspiked k ar technique to yield accurate ages for such altered samples is questioned a review of u pb data pertinent to these problems suggests a close temporal link between the formation of the siberian traps and the ptb comparison of u pb and 40 ar 39 ar ages for the ptb raises questions about the accuracy of high precision sanidine ages possibly resulting from very slow leakage of 40 ar from this mineral

the source of explosive eruptions of sakurajima volcano japan to understand the nature of explosive eruptions in particular the role played by exsolution of magmatic gases we studied the seismic waveforms produced by two such eruptions of sakurajima volcano on october 30 and november 14 1986 an explosion begins when a mixture of magmatic gases and fragmented magma flows upward through a narrow vertical channel to the surface the source region is a reservoir of gas rich magma where the pressure drops as gases flow out the form of low frequency seismic waves can be explained by a source that consists of two components one component is a single force directed downward that represents the reaction to ejection of gas and magma the other more important component is a moment tensor that represents expansion or contraction of rock around a source the first motion of seismic waves produced by an explosion is directed away from the volcano tilt and strain measurements show that the initial motion directed outward is soon followed by subsidence around the volcano the eigenvectors of the seismic moment tensor are either nearly horizontal or vertical the ratio of horizontal to vertical eigenvalues was 1 8 1 for the october 30 explosion this suggests the geometry of the explosive source can be approximated as a buried vertical cylinder the volume of contraction 2 l03 m3 can be produced by a pressure change of 0 3 mpa over a cylinder with a radius of 160 m and a height of 300 m we propose that an explosive eruption occurs after a small shallow source of high pressure forms within a vertical conduit possibly exsolution of magmatic gases eventually the small source bursts and pushes open a pathway to the surface providing an upward escape for trapped gases it is this burst that causes the initial outwardly directed motion at the surface a deeper reservoir of gas rich magma is then subjected to a lower confining pressure and also bursts causing the explosion

back arc convection model for columbia river basalt genesis abstract genesis of the columbia river basalt group crbg may be accounted for by the progressive thermal erosion of an attenuated heterogenous subcontinental lithosphere scl by back arc convection behind the cascade arc assembly of the source components of the crbg began during the early mesozoic when slices of former oceanic lithosphere were isolated below terranes accreted to the continental margin subsequent lithospheric shearing accompanying northward translation of the terranes allowed the injection of small degree partial melts from the asthenosphere which solidified in the scl as pyroxenite veins the scl source of the crbg underwent a further episode of shearing accompanied by attenuation during the eocene and oligocene when it was caught between contrasting right lateral terrane translation and e w extension regimes in the northern cordillera and basin and range province respectively the attenuation resulted in the raising of the geotherm close to that of the pyroxenite and iherzolite solidi melting was triggered by back arc convection behind the cascade arc the pyroxenite was the first component mobilised constituting the principal source of the imnaha formation continued back arc convection resulted in the failure of the scl and the generation of the voluminous grande ronde formation from pyroxenite and the surrounding scl the change to wanapum volcanism marks the stage of exhaustion of in situ sources and their replacement with a combination of proterozoic scl from the east and asthenospheric material by back arc convection the saddle mountains formation was derived entirely from proterozoic scl small magma volumes resulted in a susceptibility to local contamination such that the distinct isotopic arrays of the saddle mountains formation comprise three mixing lines and do not have any age significance

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

dependence of the duration of geomagnetic polarity reversals on site latitude an important constraint on the processes governing the geodynamo the flow in the outer core responsible for generating earth s magnetic field is the duration of geomagnetic polarity reversals that is how long it takes for earth s magnetic field to reverse1 it is generally accepted that earth s magnetic field strength drops to low levels during polarity reversals and the field direction progresses through a 180 change while the field is weak1 the time it takes for this process to happen however remains uncertain with estimates ranging from a few thousand up to 28 000 years here i present an analysis of the available sediment records of the four most recent polarity reversals these records yield an average estimate of about 7 000 years for the time it takes for the directional change to occur the variation about this mean duration is not random but instead varies with site latitude with shorter durations observed at low latitude sites and longer durations observed at mid to high latitude sites such variation of duration with site latitude is predicted by simple geometrical reversal models in which non dipole fields are allowed to persist while the axial dipole decays through zero and then builds in the opposite direction and provides a constraint on numerical dynamo models

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

very rapid geomagnetic field change recorded by the partial remagnetization of a lava flow 1 a new paleomagnetic result from a lava flow with a distinctive two part remanence reinforces the controversial hypothesis that geomagnetic change during a polarity reversal can be much faster than normal the 3 9 m thick lava flow 20 is exposed in the sheep creek range north central nevada and was erupted during a reverse to normal r n geomagnetic polarity switch at 15 6 ma flow 20 began to acquire a primary thermoremanence while the field was pointing east and down but was soon buried reheated and partially remagnetized in a north down direction by the 8 2 m thick flow that succeeded it a simple conductive cooling calculation shows that the observed remagnetization could not have occurred unless flow 20 was still warm about 150 c near its base when buried and that the 53 change from east down to north down field occurred at an average rate of approximately 1 week several orders of magnitude faster than typical of secular variation

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

the geomorphic effectiveness of a large flood on the rio grande in the big bend region insights on geomorphic controls and post flood geomorphic response abstract since the 1940s the rio grande in the big bend region has undergone long periods of channel narrowing which have been occasionally interrupted by rare large floods that widen the channel termed a channel reset the most recent channel reset occurred in 2008 following a 17 year period of extremely low stream flow and rapid channel narrowing flooding was caused by precipitation associated with the remnants of tropical depression lowell in the rio conchos watershed the largest tributary to the rio grande floodwaters approached 1500 m 3 s between a 13 and 15 year recurrence interval and breached levees inundated communities and flooded the alluvial valley of the rio grande the wetted width exceeding 2 5 km in some locations the 2008 flood had the 7th largest magnitude of record however conveyed the largest volume of water than any other flood because of the narrow pre flood channel conditions record flood stages occurred we used pre and post flood aerial photographs channel and floodplain surveys and 1 dimensional hydraulic models to quantify the magnitude of channel change investigate the controls of flood induced geomorphic changes and measure the post flood response of the widened channel these analyses show that geomorphic changes included channel widening meander migration avulsions extensive bar formation and vertical floodplain accretion reach averaged channel widening between 26 and 52 occurred but in some localities exceeded 500 the degree and style of channel response was related but not limited to three factors 1 bed load supply and transport 2 pre flood channel plan form and 3 rapid declines in specific stream power downstream of constrictions and areas of high channel bed slope the post flood channel response has consisted of channel contraction through the aggradation of the channel bed and the formation of fine grained benches inset within the widened channel margins the most significant post flood geomorphic changes have occurred at and downstream from ephemeral tributaries that contribute large volumes of sediment

volcanic tremors at etna a model for hydraulic origin spectral analysis of volcanic tremors at etna reveals 2 characteristic features 1 the escaping vapor and gas streams drive the magma in random turbulent motion generating a seismic wave emission according toa f n exp m f k wherea amplitude f frequency n m k parameters 2 sudden transients in the boundary conditions of the piping system generate hydraulic pressure pulses which in the seismogram appear as wave packages their monochromatic frequency content suggests that their origin is associated with strong resonance phenomena it is conclusive that volcanic eruptions may be distinguished and classified according to different eigenfrequencies of the fissure and pipe network involved in the volcanic activity furthermore positions of active dykes may be traced following the amplitude distributions associated with pronounced spectral lines in the tremor spectra

10s period volcanic tremors observed over a wide area in southwestern japan unusual seismic oscillations with a period of 10 seconds were observed over a wide area of southwestern japan several lines of evidence indicate that the origin of these 10s waves must be around the aso volcano located in the central part of kyushu island japan although a 10s period is unusually long for a volcanic tremor and no corresponding surface activity at the crater was reported by the local observatory the volcanic origin of the tremor is indisputable a further astonishing aspects of the observation is that there exists a strong correlation between the onset of 10s waves observed remotely and cessation of high frequency 1hz volcanic tremors at the aso volcano the aso volcano appears to be trembling with a 10s period without this having been noticed since seismic monitoring of volcanic activity is limited to high frequencies above 1 hz at the most of the volcanoes in the world long period tremors of the type observed here might be a widespread phenomenon that has been overlooked

the measurement of river bank erosion and lateral channel change a review a detailed review and chronological survey is presented of the various techniques which have been used for the measurement of river bank erosion and channel change the techniques are classified according to the time scales involved long intermediate and short and each is discussed with respect to accuracy and repeatability the methods covered include sedimentological evidence botanical evidence historical sources planimetric resurvey repeated cross profiling erosion pins and terrestrial photogrammetry prospects for future developments are also discussed

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

a coupled sediment routing and lateral migration model for gravel bed rivers we describe additions made to a multi size sediment routing model enabling it to simulate width adjustment simultaneously alongside bed aggradation incision and fining coarsening the model is intended for use in single thread gravel bed rivers over annual to decadal timescales and for reach lengths of 1 10 km it uses a split channel approach with separate calculations of flow and sediment transport in the left and right sides of the channel bank erosion is treated as a function of excess shear stress with bank accretion occurring when shear stress falls below a second low threshold a curvature function redistributes shear stress to either side of the channel we illustrate the model through applications to a 5 6 km reach of the upper river wharfe in northern england the sediment routing component with default parameter values gives excellent agreement with field data on downstream fining and down reach reduction in bedload flux and the width adjustment components with approximate calibration to match maximum observed rates of bank shifting give plausible patterns of local change the approach may be useful for exploring interactions between sediment delivery river management and channel change in upland settings

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

stream curvature and bank erosion a model based on the momentum equation based on the momentum equation of flow the force per unit area which the flow exerts radially on the outer bank of a stream bend is expressed by an equation in which the force is explicitly related to the curvature ratio r w curve radius channel width a dimensionless curvature coefficient c alpha is defined which is dependent on r w and the coefficient of dynamic bedload friction tan alpha exploring the changes of c alpha with r w reveals a functional relationship which for the known values of tan alpha shows maximal values of c alpha for r w values between 2 and 4 the theoretical results agree with field measurements of the relationship between r w and the rate of channel migration

excess argon in mount st helens plagioclase as a recorder of magmatic processes excess argon in plagioclase crystals from young 4000 ybp tephra layers from mount st helens washington illustrates the importance of argon isotopes in understanding magmatic processes 40ar 39ar step heating identifies two distinct argon reservoirs in the plagioclase a high cl k low ca k reservoir with atmospheric 40ar 36ar and a low cl k higher ca k reservoir with variable 40ar 36ar the first is probably glass whereas the second is true plagioclase felsic dacite plagioclases have little or no excess argon but those from more mafic layers have significantly higher 40ar 36ar indicating a non atmospheric source these variations are seen within a layer that contains both mafic and felsic dacite and are inconsistent with either xenocrystic or restitic origin for plagioclase the magma chamber exhibits long term open system degassing behavior punctuated by short term fluctuations from influx of high 40ar 36ar basalt the relative timing between mixing and eruption can affect the amount of excess argon recorded in plagioclase

astronomic calibration of the late oligocene through early miocene geomagnetic polarity time scale at ocean drilling program odp site 1090 subantarctic south atlantic benthic foraminiferal stable isotope data from cibicidoides and oridorsalis span the late oligocene through early miocene f24 16 ma at a temporal resolution of f5 ky over the same interval a magnetic polarity stratigraphy can be unequivocally correlated to the geomagnetic polarity time scale gpts thereby providing direct correlation of the isotope record to the gpts in an initial age model we use the newly derived age of the oligocene miocene o m boundary of 23 0 ma of shackleton et al geology 28 2000 447 revised to the new astronomical calculation la2003 of laskar et al icarus in press to recalculate the spline ages of cande and kent j geophys res 100 1995 6093 we then tune the site 1090 y 18 o record to obliquity using la2003 in this manner we are able to refine the ages of polarity chrons c7n through c5cn 1n the new age model is consistent within one obliquity cycle with previously tuned ages for polarity chrons c7n through c6bn from shackleton et al geology 28 447 450 2000 when rescaled to la2003 the results from site 1090 provide independent evidence for the revised age of the oligocene miocene boundary of 23 0 ma for early miocene polarity chrons c6aar through c5cn our obliquity scale age model is the first to allow a direct calibration to the gpts the new ages are generally within one obliquity cycle of those obtained by rescaling the cande and kent j geophys res 100 1995 6093 interpolation using the new age of the o m boundary 23 0 ma and the

pyroclastic edifices record vigorous lava fountains during the emplacement of a flood basalt flow field roza member columbia river basalt province usa the 1300 km 3 tholeiitic lava flow field of the 14 98 ma roza member of the columbia river basalt province has the best preserved vent system of any known continental flood basalt detailed geological mapping and logging of the exposed pyroclastic rocks along the 180 km long vent system enable the reconstruction of pyroclastic edifices partial cones built around vents the pyroclastic cones differ from those constructed during typical basaltic effusive eruptions and are characterized by low to moderate slope angles 1 km fountains enhanced by fallout from the lower parts of convective plumes that rose above the fountains