contemporary tectonic motion of the eastern snake river plain a campaign global positioning system study a comparison of precision campaign gps data from 1995 and 2004 from ten benchmarks on the eastern snake river plain esrp has revealed that the province moved 2 8 0 3 mm yr to the sw 232 4 6 3 relative to a fixed north american reference frame the benchmarks had no measurable displacement relative to one another at the resolution of the gps during the nine year study evidence that the province moves as a rigid non extending block this scenario is supported by the aseismic nature of the province and the lack of measurable horizontal stress in boreholes however an additional small component of intra plain extension must also be invoked to account for the observed nw trending volcanic rift zones that transect the esrp we suggest that intra plain extension is too slow 1 mm yr to measure using our campaign gps methods but may be sufficient over millennial time scales to accommodate rift zone formation slower velocities measured on three benchmarks within the neighboring basin and range seismic parabola are consistent with this region serving as a zone of detachment between the north american craton and the faster moving esrp

sedimentology stratigraphy and paleoclimate at the late miocene coffee ranch fossil site in the texas panhandle abstract the late miocene coffee ranch fossil assemblage contains some of the earliest evidence of c4 herbivory in north america however little is known regarding the environmental setting associated with the fauna and a general lack of detailed paleoclimate data exists for the late miocene from the north american continental interior in this study the sedimentary environments stratigraphy geochronology and paleoclimate are interpreted for a series of outcrops of the late miocene ogallala formation in the texas panhandle that includes the coffee ranch locality updated magnetostratigraphy coupled with previously published geochronology indicates that all exposed strata were deposited over a 277 kyr interval within chron c3an 2n from 6 42 6 70 ma depositional environments include fluvial channels floodplain ponds floodplain paleosols eolian paleosols riverine tufa and reworked volcanic ash trunk and tributary fluvial channels are differentiated using channel dimensions sedimentary structures and bounding surface architecture and indicate seasonally variable discharge paleosols from nine pedofacies are described and preserve a spectrum of weakly developed entisols and weakly to moderately mature inceptisols and vertisols constitutive mass balance calculations reveal that mature paleosols formed along distinct pedogenic pathways calcic vertisols accumulated pedogenic carbonate and exhibited either net volumetric dilation or collapse as a result of mineral weathering in contrast non calcic vertisols show patterns of decalcification and variable degrees of volumetric dilation and collapse the presence of the paleosols described in this study requires paleoprecipitation between of 900 1150 mm yr approximately double modern values and a westward shift of the continental udic ustic soil moisture boundary by 400 km during the late miocene this finding suggests that climates in the southern midcontinent were not sufficiently arid to select for c4 photosynthetic pathways over c3 photosynthesis in the late miocene

rheomorphic ignimbrites of the rogerson formation central snake river plain usa record of mid miocene rhyolitic explosive eruptions and associated crustal subsidence along the yellowstone hotspot track rogerson graben usa is critically placed at the intersection between the yellowstone hotspot track and the southern projection of the west snake river rift eleven rhyolitic members of the re defined 420 m thick rogerson formation record voluminous high temperature explosive eruptions emplacing extensive ashfall and rheomorphic ignimbrite sheets yet each member has subtly distinct field chemical and palaeomagnetic characteristics new regional correlations reveal that the brown s view ignimbrite covers 3300 km2 and the wooden shoe ignimbrite covers 4400 km2 and extends into nevada between 11 9 and 8 ma the average frequency of large explosive eruptions in this region was 1 per 354 ky about twice that at yellowstone the chemistry and mineralogy of the early rhyolites show increasing maturity with time possibly by progressive fractional crystallisation this was followed by a trend towards less evolved rhyolites that may record melting and hybridisation of a mid crustal source region contemporaneous magmatism induced crustal subsidence of the central snake river basin is recorded by successive ignimbrites offlapping and thinning up the n facing limb of a regional basin margin monocline which developed between 10 59 and 8 ma the syn volcanic basin topography contrasted significantly with the present day elevated yellowstone hotspot plateau concurrent basin and range extension produced the n trending rogerson graben early uplift of the shoshone hills 10 34 ma was followed by initiation of the shoshone fault and an e sloping half graben 10 3 10 1 ma the graben asymmetry then reversed with initiation of the brown s bench fault 8 ma which remained intermittently active until the pliocene

crust and upper mantle structure beneath the pacific northwest from joint inversions of ambient noise and earthquake data 1 we perform a joint inversion of phase velocities from both earthquake and ambient noise induced rayleigh waves to determine shear wave velocity structure in the crust and upper mantle beneath the pacific northwest we focus particularly on the areas affected by mid miocene to present volcanic activity the joint inversion combined with the high density seismic network of the high lava plains seismic experiment and data from the earthscope transportable array provides outstanding resolution for this area in oregon we find that the pattern of low velocities in the crust and uppermost mantle varies between the high lava plains physiographic province and the adjacent northwestern basin and range these patterns may be due to the presence of the brothers fault zone which separates the clockwise rotating northwest basin and range from the relatively undeformed areas further north further to the east the owyhee plateau snake river plain srp and northeastern basin and range are characterized by high crustal velocities though the depth extent of these fast wave speeds varies by province of particular interest is the mid crustal high velocity sill previously only identified within the srp we show this anomaly extends significantly further south into utah and nevada we suggest that one possible explanation is lateral crustal extrusion due to the emplacement of the high density mafic mid crustal sill structures within the srp

stable isotope compositions of herbivore teeth indicate climatic stability leading into the mid miocene climatic optimum in idaho u s a abstract the effects of global climate change are manifested at the regional level consequently evaluation of links between palaeoenvironmental change and turnover of past biotas must use regional scale climate data here we test whether climate change influenced faunal and floral patterns leading up to and during the mid miocene climatic optimum mmco based on datasets collected in the railroad canyon section rcs central eastern idaho u s a specifically we use isotope compositions of herbivore tooth enamel to investigate how mean annual precipitation map mean annual temperature mat cold month mean temperature cmmt and temperature seasonality varied through the rcs and how this local record compares to global climate reconstructions isotope compositions of teeth from the fossil equid merychippus and rhinocerotid diceratherium were compared across five time bins from c 23 to c 15 ma bulk 18o values of both taxa indicate that meteoric water compositions were unchanged through the study interval consistently c 15 6 but were c 1 9 higher than current regional meteoric water this difference points to warmer climates in central eastern idaho during the miocene post middle miocene topographic uplift in western north america and or shifts to air mass trajectories serial sampling of enamel shows seasonal fluctuations in water compositions and temperature bulk 13c values indicate an average map of 190 range 10 510 mm yr 1 with no significant change through the study interval comparable to today s map 236 mm yr 1 the reconstructed semi arid seasonal warm climate for the rcs agrees with phytolith assemblage data from the same strata indicating dominantly open grassy habitats however it is inconsistent with previously held ideas of warm wet mid miocene climates in the northern rocky mountains nrm based on paleosol analyses our inferred stable climate conditions for the mmco of the nrm contrast with coeval records from europe and asia indicating sustained or enhanced warm and wet climates these differences point to a decoupling of regional climate from global trends and highlight the necessity of studying regional variation to understand the biotic impacts of global climate change

evidence for a long lived accommodation transfer zone beneath the snake river plain a possible influence on neogene magmatism geochronologic data compiled from 12 metamorphic core complexes and their flanking regions outline important differences in tectonic and magmatic histories north and south of the snake river plain yellowstone province srp y magmatism crustal flow metamorphism and extensional exhumation of core complexes north of the srp occurred mostly between 55 and 42 ma as compared to 42 25 ma south of the srp with final exhumation of the southern complexes occurring only during younger miocene 20 0 ma basin and range faulting these significant differences in the timing of events suggest that the now lava covered srp which is at a high angle to cordilleran trends may have at times operated as a steep shear or transfer zone accommodating difference in strain to the north and south following previous suggestions we infer that this proposed accommodation or transfer zone developed above an important lithospheric boundary localized above a tear in the subducting slab shallower slab angle to the south used to explain both the locus of late cretaceous paleocene magmatism and the different ages and mechanisms of slab reconfiguration and removal north and south of the srp during the cenozoic the details of these different histories help outline the complex evolution of this zone and also suggest that this zone of lithospheric weakness may have subsequently focused miocene srp y hot spot magmatism

paleoseismology and seismic hazards evaluations in extensional volcanic terrains extensional structures in volcanic terrains are the surface expression of shallow dike intrusion and can be misinterpreted as structures associated with major tectonic faults dike induced structures can be distinguished from their tectonic counterparts by their association with cogenetic volcanic rocks and by several geometric relationships tensile fissures with little or no vertical displacement fissure swarms flexural monoclines and normal faults are commonly symmetrically distributed about a central eruptive fissure sometimes forming a graben above shallow dikes the structures typically occur within broad zones not narrow belts near a main fault zone reflecting their origin by repeated dike injection colluvial wedges containing records of single large earthquakes generally do not form instead fault scarps with several meter vertical displacements in volcanic bedrock may reflect the cumulative effects of many decimeter scale displacement events from several dike injection episodes the mechanism of dike intrusion and the nature of cointrusive seismicity have important implications for determination of the maximum magnitude and recurrence of earthquakes in extensional volcanic terrains observational seismicity from volcanic rift zones worldwide suggests the maximum magnitudes of dike induced earthquakes are 3 8 0 8 earthquakes are generally small to moderate because downdip extents of faults and fissures are controlled by the depth to the top of the associated dike usually 5 km permitting only small rupture areas also rupture and displacement on faults and fissures migrate incrementally at about the velocity of propagating dikes 0 5 m s as dike dilation stresses the zone above and ahead of the dike earthquake recurrence is tied to recurrence of volcanic cycles based on the geochronology of the associated volcanic materials based on these concepts innovative approaches have led to estimates of the maximum magnitude and recurrence for magma induced seismicity in eastern snake river plain esrp volcanic rift zones the most conservative approach uses surface length fault width downdip extent and rupture areas of normal faults and fissures produced by dike injection to estimate a maximum m5 5 earthquake for an episode of rift zone volcanism chronology of volcanic rocks suggests annual probabilities of 10 4 to 10 5 for volcanic rift zones near idaho national engineering laboratory facilities probabilistic assessments show that ground motion hazard due to volcanic rift zone seismicity is lower than the hazard from other earthquake sources this is because of the relatively low magnitudes and long recurrence intervals of cointrusive earthquakes in volcanic rift zones

how the development of magma chambers affects collapse calderas insights from an overview here we review the studies of pluton emplacement and caldera collapse proposing a model linking the two processes the shallow rise of magma occurs by dyking and its emplacement occurs along major anisotropies many plutons are emplaced at subhorizontal discontinuities forming sills these will eventually grow forming laccoliths the most common mechanism to store shallow magma calderas are a surface expression of these reservoirs and can be approximated by a piston cylinder which sinks due to 1 underpressure in the reservoir producing outward dipping reverse faults 2 overpressure within a silllike chamber undergoing doming and 3 overpressure within a laccolith generating apical tensile stresses in the latter two cases inward dipping normal faults form we suggest that collapse calderas are the surface expression of pressure variations within laccoliths or tabular intrusions their geometric relationships depend on the shape and aspect ratio of the intrusion and its pressure conditions tabular intrusions generating ring faults mainly at their tips form calderas with a similar width to the intrusion laccoliths may generate ring faults along the intrusion roof forming calderas narrower than the intrusion the outward or inward dip of the caldera faults results from the underpressure or overpressure conditions within the

kinematic analysis of fractures in the great rift idaho implications for subsurface dike geometry crustal extension and magma dynamics 1 extension across the southern great rift of the eastern snake river plain esrp idaho was measured to calculate the dimensions of underlying dikes and interpret magmatic and extensional processes cumulative rift perpendicular extension ranges from 0 64 to 4 50 m along the 14 km long kings bowl segment from 1 33 to 4 41 m along the 14 km long new butte segment and from 0 74 to 1 57 m along the 4 km long minidoka segment along strike of each segment extension increases toward coeval vents each rift segment is interpreted to be underlain by a subsurface dike whose dimensions are calculated using buoyancy equilibrium and boundary element models dikes are calculated to have tops that are 950 530 m deep bottoms that are 23 31 km deep and widths that taper to zero from a maximum of 2 21 m modeling suggests that the kings bowl dike has a maximum probable width of 8 m and a volume of 2 km3 about 400 times the volume of its coeval lava flow dike widths and ages at the southern great rift provide evidence for a holocene esrp strain rate of about 1 to 3 10 16 s 1 which is as much as an order of magnitude slower than strain rates in the adjacent seismically active basin and range province eruptive fissures are present where rift width is 1650 m this corresponds to a depth to dike top of 700 m which we propose was the depth where vesiculation initiated thus increasing magma pressure and inducing eruption

high temperature low h2o silicic magmas of the yellowstone hotspot an experimental study of rhyolite from the bruneau jarbidge eruptive center central snake river plain usa the phase relations have been investigated experimentally at 200 and 500 mpa as a function of water activity for one of the least evolved indian batt rhyolite and of a more evolved rhyolite composition cougar point tuff xv from the 12 8 8 1 ma bruneau jarbidge eruptive center of the yellowstone hotspot particular priority was given to accurate determination of the water content of the quenched glasses using infrared spectroscopic techniques comparison of the composition of natural and experimentally synthesized phases confirms that high temperatures 900 c and extremely low melt water contents 1 5 wt h o are required to reproduce the natural mineral assemblages in melts containing 0 5 1 5 wt h o the liquidus phase is clinopyroxene excluding fe ti oxides which are strongly dependent on fo and the liquidus temperature of the more evolved cougar point tuff sample bjr 940 1000 c is at least 30 c lower than that of the indian batt rhyolite lava sample ibr2 970 1030 c for the composition bjr the comparison of the compositions of the natural and experimental glasses indicates a pre eruptive temperature of at least 900 c the composition of clinopyroxene and pigeonite pairs can be reproduced only for water contents below 1 5 wt h o at 900 c or lower water contents if the temperature is higher for the composition ibr2 a minimum temperature of 920 c is necessary to reproduce the main phases at 200 and 500 mpa at 200 mpa the pre eruptive water content of the melt is constrained in the range 0 7 1 3 wt at 950 c and 0 3 1 0 wt at 1000 c at 500 mpa the pre eruptive temperatures are slightly higher by 30 50 c for the same ranges of water concentration the experimental results are used to explore possible proxies to constrain the depth of magma storage the crystallization sequence of tectosilicates is strongly dependent on pressure between 200 and 500 mpa in addition the normative qtz ab or contents of glasses quenched from melts coexisting with quartz sanidine and plagioclase depend on pressure and melt water content assuming that the normative qtz and ab or content of such melts is mainly dependent on pressure and water activity respectively the combination of results from the phase equilibria and from the composition of glasses indicates that the depth of magma storage for the ibr2 and bjr compositions may be in the range 300 400 mpa 13 km and 200 300 mpa 10 km respectively

crustal shear wave velocity structure of the western united states inferred from ambient seismic noise and earthquake data 1 surface wave dispersion measurements from ambient seismic noise and array based measurements from teleseismic earthquakes observed with the earthscope usarray transportable array ta are inverted using a monte carlo method for a 3 d vs model of the crust and uppermost mantle beneath the western united states the combination of data from these methods produces exceptionally broadband dispersion information from 6 to 100 s period which constrains shear wave velocity structures in the crust and uppermost mantle to a depth of more than 100 km the high lateral resolution produced by the ta network and the broadbandedness of the dispersion information motivate the question of the appropriate parameterization for a 3 d model particularly for the crustal part of the model we show that a relatively simple model in which vs increases monotonically with depth in the crust can fit the data well across more than 90 of the study region except in eight discrete areas where greater crustal complexity apparently exists the regions of greater crustal complexity are the olympic peninsula the mendocinotriple junction the yakima fold belt the southern cascadia back arc the great central valley of california the salton trough the snake river plain and the wasatch mountains we also show that a strong rayleigh love discrepancy exists across much of the western united states which can be resolved by introducing radial anisotropy in both the mantle and notably the crust we focus our analysis on demonstrating the existence of crustal radial anisotropy and primarily discuss the crustal part of the isotropic model that results from the radially anisotropic model by voigt averaging model uncertainties from the monte carlo inversion are used to identify robust isotropic features in the model the uppermost mantle beneath the western united states is principally composed of four large scale shear wave velocity features but lower crustal velocity structure exhibits far greater heterogeneity we argue that these lower crustal structures are predominantly caused by interactions with the uppermost mantle including the intrusion and underplating of mafic mantle materials and the thermal depression of wave speeds caused by conductive heating from the mantle upper and middle crustal wave speeds are generally correlated and notable anomalies are inferred to result from terrane accretion at the continental margin and volcanic intrusions

yellowstone debate erupts again the heat driving yellowstone s volcanism originates from a depth of at least 700 km according to images of the mantle created using novel seismic methods

stratigraphic record of the yellowstone hotspot track neogene sixmile creek formation grabens southwest montana abstract the sixmile creek formation fills deep grabens in southwest montana and preserves a stratigraphic record of the evolution of the yellowstone hotspot track from 17 ma to 2 ma the ruby beaverhead big hole deer lodge medicine lodge grasshopper three forks canyon ferry jefferson melrose wise river and paradise grabens were active during outbreak of the hotspot they appear to be parts of a radial system of extensional structures that may have formed on a broad dome that was centered on the hotspot outbreak area in southwest idaho and southeast oregon early in the evolution of the grabens massive debris flows surged down paleogene paleovalleys from uplifted and tilted horst blocks and accumulated in the grabens the grabens captured runoff from the hotspot dome with thick deposits of river gravel that appear to have been derived in part from east central idaho as the hotspot track propagated along the eastern snake river plain silicic ash fell into the graben drainage basins and was reworked into thick fluvial beds along graben axes the grabens were cross cut by northwest trending late neogene grabens that propagated on the flanks of silicic volcanic centers along the hotspot track the late neogene grabens diverted the middle miocene drainage patterns in southwest montana

analysis of late holocene faulting within an active rift using lidar taupo rift new zealand abstract high quality light detection and ranging lidar data collected across the rangitaiki plains the fastest extending section of the onshore taupo rift new zealand reveal 122 active fault traces and provide new constraints on displacements displacement rates and paleoearthquakes of the normal faults in the rift the identified lineaments are scarps that record vertical offset of geomorphic surfaces e g beach ridges meander channel floors river terraces etc and trend parallel or sub parallel to other active faults in the rift these lineaments are interpreted to be active faults active fault traces trend 060 and their lengths range from 0 25 to 6 km they mainly traverse and displace a diachronous landscape of

regional conductivity structure of cascadia preliminary results from 3d inversion of usarray transportable array magnetotelluric data 1 in conjunction with the usarray component of earthscope long period magnetotelluric mt data are being acquired in a series of arrays across the continental us initial deployments in 2006 and 2007 acquired data 10 10 000 s at 110 sites covering the us pacific northwest distributed with the same nominal spacing as the usarray seismic transportable array 75 km the most striking and robust features revealed by initial three dimensional inversion of this dataset are extensive areas of high conductivity in the lower crust beneath all of southeastern oregon and beneath the cascade mountains contrasting with very resistive crust in siletzia and the columbia embayment significant variations in upper mantle conductivity are also revealed by the inversions with the most conductive mantle beneath the washington backarc and the most resistive corresponding to subducting oceanic mantle citation patro p k and g d egbert 2008 regional conductivity structure of cascadia preliminary results from 3d inversion of usarray transportable array magnetotelluric data geophys res lett 35 l20311 doi 10 1029 2008gl035326

three dimensional seismic velocity structure of the northwestern united states 1 we present a high resolution three dimensional p wave tomography model for the northwestern united states major features of the model include clear well resolved images of the juan de fuca gorda slab to depths of at least 500 km reduced velocities beneath newberry volcano and north central oregon increased velocities beneath the idaho batholith and a deep slab like eastward dipping feature of increased velocities beneath nevada we find no evidence for a mantle plume in this region and demonstrate that suggested disruption of the juan de fuca slab beneath oregon in the vicinity of newberry volcano is an inversion artifact we interpret the primary features of this new model within the framework of post laramide tectonomagmatism and the continuing tectonic evolution of the cascadia subduction zone a model explaining the regional tectonomagmatic history consistent with the imaged seismic velocity structure is one driven by mantle flow responding to the evolving subduction system

silicic phreatomagmatism in the snake river plain the deadeye member non welded rhyolitic pyroclastic units in the central snake river plain are interbedded with the much better exposed large volume snake river type rheomorphic welded rhyolitic ignimbrites and rhyolite lavas we document one such unit to investigate why it is so different from the interbedded welded ignimbrites the newly recognised deadeye member of southern idaho is a soil bounded eruption unit that comprises ashfall layers and a 4 m thick ignimbrite that extends for 35 km the ignimbrite is non welded lithic clast poor and varies from massive to diffuse low angle cross bedded it contains abundant angular clasts of non vesicular black glass and upper parts contain accretionary lapilli the ashfall layers above it contain coated ash pellets and ash clumps which record moist aggregation of fine ash the magmas of the deadeye eruption were closely similar in composition and temperature to those that generated the intensely welded rheomorphic ignimbrites of the central snake river plain we infer that the marked contrast in physical appearance of the deadeye ignimbrite compared to the other more typical snake river type welded ignimbrites was the result of emplacement at relatively low temperatures during an eruption in a lacustrine environment magmatic volatile driven fragmentation of the rhyolitic magma was influenced by interaction with lake water that also led to cooling the deadeye member is the first recorded example of explosive silicic phreatomagmatism in the central snake river plain

seismicity and earthquake hazard analysis of the teton yellowstone region wyoming abstract earthquakes of the teton yellowstone region represent a high level of seismicity in the intermountain west u s a that is associated with intraplate extension associated with the yellowstone hotspot including the nearby teton and hebgen lake faults the seismicity and the occurrence of high slip rate late quaternary faults in this region leads to a high level of seismic hazard that was evaluated using new earthquake catalogues determined from three dimensional 3 d seismic velocity models followed by the estimation of the probabilistic seismic hazard incorporating fault slip and background earthquake occurrence rates the 3 d p wave velocity structure of the teton region was determined using local earthquake data from the jackson lake seismic network that operated from 1986 2002 an earthquake catalog was then developed for 1986 2002 for the teton region using relocated hypocenters the resulting data revealed a seismically quiescent teton fault at m l local magnitude 3 with diffuse seismicity in the southern jackson hole valley area but notable seismicity eastward into the gros ventre range relocated yellowstone earthquakes determined by the same methods highlight a dominant e w zone of seismicity that extends from the aftershock area of the 1959 m s surface wave magnitude 7 5 hebgen lake montana earthquake along the north side of the 0 64 ma yellowstone caldera earthquakes are less frequent and shallow beneath the yellowstone caldera and notably occur along northward trending zones of activity sub parallel to the post caldera volcanic vents stress field orientations derived from inversion of focal mechanism data reveal dominant e w extension across the teton fault with a ne sw extension along the northern teton fault area and southern yellowstone the minimum stress axes directions then rotate to e w extension across the yellowstone caldera to n s extension northwest of the caldera and along the hebgen lake fault zone the combination of accurate hypocenters unified magnitudes and seismotectonic analysis helped refine the characterization of the background seismicity that was used as input into a probabilistic seismic hazards analysis our results reveals the highest seismic hazard is associated with the teton fault because of its high slip rate of approximately 1 3 mm yr compared to the highest rate of 1 4 mm yr in southern yellowstone on the mt sheridan fault this study demonstrates that the teton yellowstone area is among the regions highest seismic hazard in the western u s

constraints on north american plate velocity from the yellowstone hotspot deformation field oceanic hotspot tracks generally indicate that the mantle plumes that give rise to them are fixed in position relative to a lower mantle reference frame for timespans of 100 myr1 3 it has not been clear whether continental hotspots are equally stable for example there is evidence that the velocity of the yellowstone hotspot has been quite irregular with respect to the north ameri can plate over the past 16 myr4 7 but measurements of its velocity based on caldera locations have been too uncertain and irregular to be definitive here i describe a new method of obtaining the plume velocity which uses the broader lithospheric effects of the hotspot rather than the locations of individual calderas for the past 10 myr the results indicate a constant north american plate velocity of 2 2 cm yr 1 which agrees with independent estimates from global plate motion inversions8 9 this velocity is considerably lower than previous estimates obtained from caldera locations and clearly differs from the much higher but less well constrained velocities of the previous 6 myr refs 5 7

onset timing and slip history of the teton fault wyoming a multidisciplinary reevaluation the dramatic relief of the teton range in northwestern wyoming results from motion along the teton normal fault new apatite u th he ahe and fission track aft ages of samples collected in the footwall yield fundamental constraints on contrasting models of teton fault activity and consequent relative footwall uplift low elevation samples in the immediate footwall of the fault range from 12 5 to 6 5 ma ahe ages of samples from subvertical transects range from 57 8 to 6 5 ma rendezvous 54 0 to 6 8 ma grand and 20 5 to 7 1 ma moran with all three transects yielding an expected trend of decreasing age with decreasing elevation aft ages obtained from the three transects range from 55 4 to 45 7 ma rendezvous 43 2 to 13 3 ma grand and 38 0 to 11 7 ma moran inverse thermal history modeling indicates that the onset of relatively rapid cooling as a proxy for relative footwall uplift initiated first in the northern part of the range 15 13 ma and then migrated south as fault displacement continued 7 ma to present at rendezvous thus this work suggests that although the teton fault likely initiated due to basin and range extension this structure remains active normal fault displacement models also suggest a minimum of 6 km displacement for footwall uplift of 2 km if evaluated in the context of established normal fault length displacement relationships this yields a length of 180 km substantially longer than current estimates of teton fault length and putting the northern end of the fault well into and potentially beyond the bounds of yellowstone national park

stress triggering of conjugate normal faulting late aftershocks of the 1983 ms 7 3 borah peak idaho earthquake the 1984 devil canyon sequence was a late aftershock sequence of the 28 october 1983 ms 7 3 borah peak idaho earthquake the sequence began on 22 august 1984 with the ml 5 8 devil canyon earthquake which nucleated at a depth of 12 8 0 7 km between the surface traces of two normal faults the challis segment of the lost river fault and the lone pine fault two hundred thirty seven aftershocks were recorded by a temporary array during a 3 week period their focal mechanisms and hypocenter distribution define a cross sectional v pattern whose base corresponds to the ml 5 8 event whose tips correspond to the exposed fault traces and whose sides define two planar fault zones oriented n25 w 75 sw challis fault segment and n39 w 58 ne lone pine fault this pattern describes a graben bounded by conjugate normal faults temporal aspects of the devil canyon sequence provide strong evidence that slip on conjugate normal faults occurs sequentially aftershocks occurred primarily along the challis segment until the occurrence of the 8 september 1984 ml 5 0 earthquake along the lone pine fault after which aftershocks primarily occurred along this fault these observations are consistent with worldwide seismologic and geologic observations and with physical andmore numerical models of conjugate normal faulting aftershocks of the devil canyon sequence occurred immediately northwest of the ml 5 8 devils canyon earthquake which itself was immediately northwest of the thousand springs segment of the lost river fault the fault that slipped in association with the ms 7 3 borah peak earthquake coulomb failure stress analysis indicates that stress increases resulting from both the borah peak mainshock and devil canyon ml 5 8 earthquake were sufficient to induce failure on the lone pine fault these space time patterns suggest that conjugate normal faults may transfer stress or accommodate stress changes at the terminations of major normal faults in the basin and range province less

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

rates of stream incision in the middle part of the arkansas river basin based on late tertiary to mid pleistocene volcanic ash abstract ages and elevations of ash layers correlated with late tertiary and pleistocene eruptions in the western us and present stream elevations are used to calculate net rates of incision by streams in the middle reaches of the arkansas river basin in the south central us the mean of the 23 measurements of rate of stream incision in the study area is 4 2 2 7 cm ky with a range of 0 6 to 9 7 cm ky major influences on rate of stream incision in the study area include the arid to semi arid climate of the region the type of material being incised by streams stream captures and salt dissolution in the bedrock that underlies the region rates of incision exceeded rates of basin filling but significant deposits of unconsolidated late cenozoic sediments occur in the study area basins of streams that have incised the slowest since the late tertiary contain the thickest and most extensive amounts of unconsolidated quaternary sediments rates of incision by streams in the study area are similar to or slower than rates reported for streams elsewhere in the us and the world streams in mountainous regions and areas affected by rapid uplift have incised at rates orders of magnitude faster than streams in the study area

a 12 000 year record of vertical deformation across the yellowstone caldera margin the shorelines of yellowstone lake the 600 ka yellowstone caldera exhibits several signs of unrest the most evident of which is historic ground deformation including both uplift and subsidence we document deformation in the area of the southeastern caldera across 12 000 years using the postglacial shoreline terraces of yellowstone lake raised shoreline elevations were interpreted from 230 leveling profiles surveyed across flights of terraces with an accuracy of 0 5 m of about 11 recognizable terraces the five most continuous raised shorelines were correlated around the lake basin to reveal deformation patterns shoreline ages are based on minimum and maximum limiting radiocarbon and obsidian hydration dates each terrace is interpreted as representing an episode of uplift 1 kyr 1 of the caldera interior and subsequent subsidence with little net volume change this cyclic behavior may result from magma emplacement and subsequent withdrawal or cooling and crystallization and or episodic trapping and release of magmatic fluids evolved during cooling and crystallization in a self sealing reservoir as hypothesized by dzurisin et al 1990 early postglacial shoreline deformation reveals substantial intracaldera subsidence possibly reflecting cooling and loss of trapped hydrothermal fluids extension along north south trending normal faults and related structures is also apparent in early to middle postglacial deformation both inside and outside of the caldera margin including downwarping and local faulting of hot weak intracaldera crust net deformation over the past 3 kyr has been dominantly up within the caldera interior and slightly down along the caldera rim relative to the extracaldera region this uplift is roughly similar to the historic pattern and may largely represent the effects of the most recent inflation episode subtraction of the total estimated magnitude of inflation in this episode suggests that the overall trend of postglacial deformation has been subsidence the cause of this trend is undetermined but is most likely related to the effects of regional extension and long term cooling within the yellowstone caldera

the role of a mantle plume in the formation of yellowstone volcanism the origin of the yellowstone volcanic province remains debated proposed hypotheses involve either a mantle plume or not recent tomographic images allow a quantitative evaluation of the plume hypothesis and its interaction with the farallon slabs using 4 d geodynamic models with data assimilation we find that the slab is always in the way of the initially rising plume and that the plume could reach the surface only through the broken slab hinge at 15 ma for most of the time the sinking slabs dominate the mantle flow and prohibit upwelling we find that a plume that satisfies the present mantle image beneath yellowstone fails to predict both voluminous hot materials at shallow depths beneath the western u s and the age migration of the hot spot tracks we suggest that a plume is likely to have much less influence on the yellowstone volcanism than previously thought

shear wave splitting beneath the snake river plain suggests a mantle upwelling beneath eastern nevada usa the snake river plain srp a 90 km wide topographic depression in southern idaho is a topographically anomalous feature in the western u s previous seismic studies focused on the northeastern srp to study its relationship with the yellowstone hotspot we present new teleseismic shear wave splitting data from six broadband seismic stations deployed along the axis of the srp from june 2000 to september 2001 we also analyze splitting at hlid a permanent station of the national seismic network located f100 km north of the plain splitting of individual teleseismic phases is consistent at all stations within 2r errors and we favor the interpretation of anisotropy with a single horizontal fast axis although a dipping axis interpretation is statistically permitted at two of the stations our station fast directions as well as shear wave splitting data from numerous other stations throughout the basin and range are best explained by a lattice preferred orientation of olivine due to horizontal shear along the base of the plate associated with the gravitational spreading of buoyant plume like upwelling material beneath eastern nevada into a southwestward flowing asthenosphere with respect to a fixed hotspot reference frame this parabolic asthenospheric flow paf model for the great basin is attractive because it explains the observed high elevations high mantle buoyancy low velocity anomaly beneath eastern nevada high heat flow and depleted geochemistry of some erupted basalts the lack of pliocene recent major volcanism in eastern nevada suggests that a significant amount of the buoyancy flux is due to compositional buoyancy our splitting station delay times vary in a way not predicted by the paf model and can be explained by a zone of aligned magma filled lenses and or partially molten dikes beneath the srp lithosphere a depleted olivine rich residuum underneath the sides of the eastern srp and or the effect of lateral lower crustal flow from beneath the esrp toward its adjacent flanks d 2004 elsevier b v all rights reserved

palinspastic restoration of navdat and implications for the origin of magmatism in southwestern north america 1 simultaneous palinspastic restoration of deformation and volcanism illuminates relationships between magmatism and tectonics in western north america using arcgis we retrodeformed the navdat north american volcanic database navdat geongrid org using the western north america reconstruction of mcquarrie and wernicke 2005 from these data sets we quantitatively compare rates of magmatism and deformation and evaluate the age composition and migration of cenozoic volcanism from 36 ma to present these relationships are shown in a series of palinspastic maps as well as animations that highlight migrating extension and volcanism with time western north america is grouped into eight different regions with distinct relationships between strain and volcanism to evaluate competing hypotheses regarding the relationship of extension to continental magmatism a first order observation from this study is that magmatism throughout the basin and range appears to be primarily driven by plate boundary effects notably subducting and foundering slabs as well as slab windows exceptions include the yellowstone hotspot system along the northern border of our study area and late stage 8 ma passive extension related asthenospheric upwelling along the eastern and western margins of the basin and range the palinspastic reconstructions presented here highlight that the classic high angle basin and range faulting that comprises most of the physiographic basin and range province commenced during a magmatic lull more broadly with the exception of the rio grande rift we find that pulses of magmatism lag the onset of extension these observations largely contradict the active rifting model where magmatism triggers basin and range extension

evaluation of models for the origin of miocene low δ18o rhyolites of the yellowstone columbia river large igneous province two models have been proposed for the origin of the low 18o rhyolites of the snake river plain yellowstone hotspot trend the first involves a single multicyclic caldera system or multiple nested or overlapping systems in which the early erupted products become hydrothermally altered and are subsequently melted to form low 18o rhyolites bindeman and valley 2001 watts et al 2011 the second model appeals to a crustal magma source which was hydrothermally altered in a thermal event prior to the onset of miocene magmatism boroughs et al 2005 we critically evaluate the plausibility of each and provide thermal temporal and volume constraints which must be included in any model for the petrogenesis of low 18o rhyolites of the central snake river plain although the models are not mutually exclusive we conclude that a component of pre existing low 18o crust is required in the central snake river plain where the greatest volumes of strongly 18o depleted rhyolite occur this component could be provided by meteoric hydrothermally altered granites of the idaho batholith which are cut by the central snake river plain

cosmogenic exposure age chronologies of pinedale and bull lake glaciations in greater yellowstone and the teton range usa abstract we have obtained 69 new cosmogenic 10be surface exposure ages from boulders on moraines deposited by glaciers of the greater yellowstone glacial system and teton range during the middle and late pleistocene these new data combined with 43 previously obtained 3he and 10be ages from deposits of the northern yellowstone outlet glacier establish a high resolution chronology for the yellowstone teton mountain glacier complexes boulders deposited at the southern limit of the penultimate ice advance of the yellowstone glacial system yield a mean age of 136 13 10be ka and oldest ages of 151 157 10be ka these ages support a correlation with the bull lake of west yellowstone with the type bull lake of the wind river range and with marine isotope stage mis 6 end moraines marking the maximum pinedale positions of outlet glaciers around the periphery of the yellowstone glacial system range in age from 18 8 0 9 to 16 5 1 4 10be ka and possibly as young as 14 6 0 7 10be ka suggesting differences in response times of the various ice cap source regions moreover all dated pinedale terminal moraines in the greater yellowstone glacial system post date the pinedale maximum in the wind river range by 4 6 kyr indicating a significant phase relationship between glacial maxima in these adjacent ranges boulders on the outermost set and an inner set of pinedale end moraines enclosing jenny lake on the eastern teton front yield mean ages of 14 6 0 7 and 13 5 1 1 10be ka respectively the outer jenny lake moraines are partially buried by outwash from ice on the yellowstone plateau hence their age indicates a major standstill of an expanded valley glacier in the teton range prior to the younger dryas followed closely by deglaciation of the yellowstone plateau these new glacial chronologies are indicative of spatially variable regional climate forcing and temporally complex patterns of glacier responses in this region of the rocky mountains during the pleistocene

slope control on the aspect ratio of river basins although it is traditionally thought that drainage basins are geometrically similar we present new results which indicate that the aspect ratio of weakly dissected river basins at large scales 10 103 km2 depends on the surface slope steeper surfaces develop narrower and lengthier basins whereas more gently dipping surfaces develop more equant and wider basins this relationship is interpreted to be related to the nature of water flow over rough surfaces with steeper slopes causing less flow convergence and lengthier and narrower basins we derive an empirical relationship that can be used to infer the slope of a surface on which a river basin acquired its geometry based solely on a measure of its basin form this relation provides a unique means of inferring the relative chronology of river basin development with respect to surface tilting and therefore provides a direct link between river basin morphology and tectonics

sediment supply and channel morphology in mountain river systems 1 relative importance of lithology topography and climate 1 quantifying landscape scale variations in sediment supply to streams and rivers is fundamental to our understanding of both denudational processes and stream channel morpho dynamics previous studies have linked a variety of sediment supply proxies to climatic topographic or geologic factors but few have connected these directly to the characteristics of fluvial systems draining these landscapes here we correlate landscape controls on sediment supply to observed sedimentology and channel patterns through direct measurements of water and sediment fluxes in over 80 drainage basins ranging in area from 1 4 to 35 000 km2 in the northern rocky mountains usa these data show that the relative sediment supply defined by the bankfull sediment concentration is dominated by basin lithology while exhibiting little correlation to factors such as relief mean basin slope and drainage density bankfull sediment concentrations bed load and suspended load increase as much as 100 fold as basin lithology becomes dominated by softer sedimentary and volcanic rocks relative to basins with more resistant lithologies as bed load concentrations increase stream beds become less armored and bed load grain size coarsens at very high sediment concentrations bed surface subsurface and bed load grain sizes converge and a transition from single thread to to braided channel patterns is commonly observed

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

lithosphere dynamics and continental deformation the unifying theme in this section is the remarkable weakness of major faults i will consider the diverse new evidence for weakness and the evidence for high pore pressure localized in faults as a fundamental cause with this background one can better understand why faults remain active even after large rotations with respect to stress i will look at large neogene 23 7 million year old rotations about horizontal axes in the basin and range province and about vertical axes along the pacific margin recent developments will be summarized from studies of neogene tectonics neotectonics in california alaska and the mississippi embayment in the context of a weak north american stress field that results mainly from topographic forces to close i will present new geophysical studies relevant to the continuing controversy over whether the basic structure of the north american mantle lithosphere was altered by an early tertiary episode of flat subduction

strain rates and contemporary deformation in the snake river plain and surrounding basin and range from gps and seismicity we used new horizontal global positioning system gps velocities along with earthquakes faults and volcanic features to assess how strain is accommodated in the northern basin and range province we estimated horizontal velocities for 132 stations within the snake river plain and the surrounding basin and range from gps phase data collected from 1994 to 2007 these velocities show regional scale clockwise rotation suggestive of driving forces beyond those associated with the yellowstone hotspot within the western centennial tectonic belt the gps measurements indicate that the basin and range is extending at a rate an order of magnitude greater than the snake river plain which explains its low seismicity between these two regions we discern the centennial shear zone a ne trending zone of right lateral shear with estimated slip rates that increase northeastward from 0 9 0 3 mm yr in the sw to 1 7 0 2 mm yr in ne we interpret the new gps velocities to indicate 1 right lateral shear may be accommodated by strike slip earthquakes on ne trending faults in the centennial shear zone 2 three basin and range faults lost river lemhi and beaverhead terminate at the snake river plain margin and 3 extension in the snake river plain occurs at a much lower rate than the rate of normal faulting in the western centennial tectonic belt

glacial topographic interactions in the teton range wyoming 1 understanding interactions among tectonics topography climate and erosion is fundamental to studies of mountainous landscapes here we combine topographic analyses with modeled distributions of precipitation insolation and flexural isostasy to present a conceptual model of topographic evolution in the teton range wyoming and test whether efficient glacial relief production has contributed to summit elevations the conceptual model reveals a high degree of complexity inherent in even a relatively small glaciated mountain range back tilting has caused topographic asymmetry with the greatest relief characterizing eastern catchments in the center of the range two high summits grand teton and mount moran rise hundreds of meters above the surrounding landscape their elevations set by the threshold hillslope angle and the spacing between valleys hosting large erosionally efficient glaciers only basins 20 km2 held glaciers capable of eroding sufficiently rapidly to incise deeply and maintain shallow downvalley gradients on the eastern range flank glacial erosion here was promoted by 1 prevailing westerly winds transporting snow to high relief eastern basins leading to cross range precipitation asymmetry 2 the wind blown redistribution of snow from open western headwaters into sheltered eastern cirques with the associated erosion driven migration of the drainage divide increasing eastern accumulation areas and 3 tall steep hillslopes providing shading snow influx from avalanching and insulating debris cover from rockfalls to valley floor glaciers in comparison the topographic enhancement of glacial erosion was less pronounced in western and smaller eastern basins despite dramatic relief production insufficient rock mass is removed from the teton range to isostatically raise summit elevations

anisotropy of the yellowstone hot spot wake eastern snake river plain idaho over the last 10 million years the yellowstone hot spot has passed beneath the eastern snake river plain both magmatically modifying the snake river plain crust and creating a wider wake like tectonic parabola of seismicity and uplift analysis of sks arrivals to a line array of 55 mostly broadband stations distribution across the tectonic parabola reveals a nearly uniform orientation of anisotropy with an average fast axis orientation of n64e the back azimuth of null splitting events is parallel to the measured fast axis suggesting that anisotropic material consists of a single layer splitting parameters are independent of backazimuth suggesting that anisotropy is constant beneath each station thus station averaged split parameters are representative of the anisotropy beneath the station station averaged split times range from 0 6 1 5 s and define a pronounced depression in split times centered about 80 km southeast of the axis of the snake river plain

subsidence of a volcanic basin by flexure and lower crustal flow the eastern snake river plain idaho the eastern snake river plain esrp is a linear volcanic basin interpreted by many workers to reflect late cenozoic migration of north america over the yellowstone hotspot thermal subsidence of this volcanic province with respect to yellowstone has been documented by several workers but no one has characterized subsidence with respect to the adjacent basin and range province this paper documents crustal flexure along the northwest edge of the esrp uses flexure to model the dimensions of a dense load beneath the basin and presents evidence in support of density driven subsidence and lower crustal flow away from the basin crustal flexure adjacent to the esrp is reflected by the attitudes of mesozoic fold hinges and neogene volcanic rocks fold hinges formed with a subhorizontal plunge and a trend perpendicular to the esrp but now show a southward plunge near the esrp of as much as 20 25 we present a contour map of equal fold plunges proximal to the esrp that shows flexure is roughly parallel to and extends 10 20 km north of the average edge of the esrp flexural profiles indicate the minimum amount of esrp subsidence with respect to the basin and range subsidence ranges from 4 5 to 8 5 km the structural contour map and published seismic and gravity data were used to develop and constrain flexural subsidence models these models indicate the flexed crust is very weak flexural parameter of 4 10 km interpreted to be a result of the high heat flow of the esrp assuming subsidence was induced by emplacement of a dense crustal layer beneath the esrp a midcrustal sill identified in previous seismic surveys is too wide and probably too thin to produce the measured flexure new dimensions include a thickness of 17 25 km and a half width of 40 50 km which place the edge of the sill beneath the edge of the esrp the dimensions of the esrp sill are based on isostatic compensation in the lower crust because compensation in the asthenosphere requires an unreasonable sill thickness of 30 km and because esrp seismic gravity and heat flow data support lower crustal compensation density driven lower crustal flow away from the esrp is proposed to accommodate subsidence and maintain isostatic equilibrium timing of subsidence is constrained by esrp exploratory wells where 6 6 ma rhyolites at a depth of 1 5 km indicate most subsidence occurred prior to their emplacement and by strong spatial correlations between plunge contours and quaternary volcanic rift zones two processes interpreted to contribute to the load include an extensive midcrustal mafic load emplaced at 10 ma which provided the heat source for the initial rhyolitic volcanism on the esrp and continuing localized loads from dikes and sills associated with quaternary basalts widespread 10 ma magmatism and subsidence conflicts with simple time transgressive migration of the yellowstone hotspot indicating a need for revision of the hotspot paradigm

chemostratigraphy of predominantly siliciclastic neoproterozoic successions a case study of the pocatello formation and lower brigham group idaho usa abstract isotopic chemostratigraphy has proven successful in the correlation of carbonate rich neoproterozoic successions in successions dominated by siliciclastic rocks chemostratigraphy can be problematic but if thin carbonates punctuate siliciclastic strata useful isotopic data may be obtained the upper pocatello formation and lower brigham group of southeastern idaho provide an opportunity to assess the potential and limitations of isotopic chemostratigraphy in overwhelmingly siliciclastic successions the 5000 m thick succession consists predominantly of siliciclastic lithologies with only three intervals that contain thin intercalated carbonates its depositional age is only broadly constrained by existing biostratigraphic sequence stratigraphic and geochronometric data the lowermost carbonates include a cap dolomite atop diamictites and volcanic rocks of the pocatello formation the delta 13c values of these carbonates are distintly negative 5 to 3 similar to carbonates that overlie neoproterozoic glaciogenic rocks worldwide stratigraphically higher carbonates record a major positive delta 13c excursion to values as high as 8 8 within the carbonate member of the caddy canyon quartzite the magnitude of this excusion is consistent with post sturtian secular variation recorded elsewhere in the north american cordillera australia svalbard brazil and nambia and exceeds the magnitude of any post varanger delta 13c excursion documented to date in most samples sr isotopic abundances have been altered by diagenesis and greenschist facies metamorphism but a least altered value of approximately 0 7076 supports a post sturtian and pre marinoan varanger age for upper pocatello and lower brigham rocks that lie above the pocatello diamictite thus even though available chemostratigraphic data are limited they corroborate correlations of pocatello formation diamictites and overlying units with sturtian glaciogenic rocks and immediately post sturtian successions in western north america and elsewhere

magnetic anisotropy in rhyolitic ignimbrite snake river plain implications for using remanent magnetism of volcanic rocks for correlation paleomagnetic studies and geological reconstructions some of the field and laboratory expenses for this work were covered by the natural environment research council nerc grant ne g005372 1 the salary and school fees for the first author were covered in part by a national science foundation nsf grant ear 443549 81179 awarded to x zhao

crustal scale recycling in caldera complexes and rift zones along the yellowstone hotspot track o and hf isotopic evidence in diverse zircons from voluminous rhyolites of the picabo volcanic field idaho abstract rhyolites of the picabo volcanic field 10 4 6 6 ma in eastern idaho are preserved as thick ignimbrites and lavas along the margins of the snake river plain srp and within a deep 3 km borehole near the central axis of the yellowstone hotspot track in this study we present new o and hf isotope data and u pb geochronology for individual zircons o isotope data for major phenocrysts quartz plagioclase and pyroxene whole rock sr and nd isotope ratios and whole rock geochemistry for a suite of picabo rhyolites we synthesize our new datasets with published ar ar geochronology to establish the eruptive framework of the picabo volcanic field and interpret its petrogenetic history in the context of other well studied caldera complexes in the srp caldera complex evolution at picabo began with eruption of the 10 44 0 27 ma u pb tuff of arbon valley tav a chemically zoned and normal 18o 18o magma 7 9 unit with high zoned 87sr 86sri 0 71488 0 72520 and low e nd 0 18 and e hf 0 28 the tav and an associated post caldera lava flow possess the lowest end 0 23 indicating 40 60 derivation from the archean upper crust normal 18o rhyolites were followed by a series of lower 18o eruptions with more typical lower crustal sr nd hf isotope ratios and whole rock chemistry the voluminous 8 25 0 26 ma west pocatello rhyolite has the lowest 18o value 18omelt 3 3 and we correlate it to a 1 000 m thick intracaldera tuff present in the inel 1 borehole with published zircon ages 8 04 8 35 ma and similarly low 18o zircon values the significant 4 5 decrease in magmatic 18o values in picabo rhyolites is accompanied by an increase in zircon 18o heterogeneity from 1 variation in the tav to 5 variation in the late stage low 18o rhyolites a trend similar to what is characteristic of heise and yellowstone and which indicates remelting of variably hydrothermally altered tuffs followed by rapid batch assembly prior to eruption however due to the greater abundance of low 18o rhyolites at picabo the eruptive framework may reflect an intertwined history of caldera collapse and coeval basin and range rifting and hydrothermal alteration we speculate that the source rocks with pre existing low 18o alteration may be related to 1 deeply buried and unexposed older deposits of picabo age or twin falls age low 18o volcanics and or 2 regionally abundant late eocene challis volcanics which were hydrothermally altered near the surface prior to or during peak picabo magmatism basin and range extension specifically the formation of metamorphic core complexes exposed in the region could have facilitated the generation of low 18o magmas by exhuming heated rocks and creating the large water rock ratios necessary for shallow hydrothermal alteration of tectonically rift zones and volcanically calderas buried volcanic rocks these interpretations highlight the major processes by which supereruptive volumes of magma are generated in the srp mechanisms applicable to producing rhyolites worldwide that are facilitated by plume driven volcanism and extensional tectonics

a fixed sublithospheric source for the late neogene track of the yellowstone hotspot implications of the heise and picabo volcanic fields the heise and picabo volcanic fields of eastern idaho are part of the more extensive time transgressive yellowstone snake river plain hotspot track calderas associated with these two silicic volcanic fields are buried under 1 to 3 km of younger basalt so their locations and eruption record histories have been based on analysis of silicic units along the margins of the eastern snake river plain along with some limited geophysical data a 1 5 km borehole penetrating through basalt into underlying silicic rocks provides new data we used to reassess caldera locations and the timing of eruptions of these volcanic fields using these new caldera locations we calculate an extension adjusted rate of 2 35 cm yr for the north american plate over the last 6 66 m y and a velocity of 2 30 cm yr over the 10 27 m y recalculation of a previously determined plate velocity based migration of the deformation field surrounding the eastern snake river plain yields an extension adjusted rate of 2 38 0 21 cm yr these migration rates all fall within the previously published range of north american plate velocities of 2 2 0 8 cm yr 2 4 cm yr and 2 68 0 78 cm yr based on a global hot spot reference frame the consistency of these rates suggest that over the last 10 m y the yellowstone hot spot is fixed with respect to the motion of the north american plate and therefore consistent with a classical deep sourced hotspot model

western united states extension how the west was widened abstract cenozoic extension in the western united states presents a complex interrelation of extension volcanism and plate boundary tectonics that defeats simple notions of active or passive rifting forces driving extension can originate at plate boundaries through basal traction basal normal forces or from buoyancy forces internal to the crust and lithospheric mantle the latter two are most responsible for driving extension where it is observed in the basin and range the complex evolution of the northern basin and range probably represents removal or alteration of mantle lithosphere interacting with buoyancy stored in the crust in contrast crustal buoyancy forces combined with a divergent plate boundary between about 28 and 16 ma to drive extension in the southern basin and range the central basin and range most likely extended as a result of boundary forces external to itself but arising from buoyancy forces elsewhere in the western united states

incipient extension along the active convergent margin of nubia in sicily italy cefalu etna seismic zone 1 recent geodetic data are compatible with nne ssw tectonic extension at a rate of 5 mm yr in sicily southern italy within a broader region of net active compression along the nubian plate margin northern africa the structures that accommodate such extensional regime and its cause are still unknown from field structural surveys and seismological analyses the geometry kinematics structural architecture and seismic potential of an extensional seismic zone linking cefalu and mount etna in central eastern sicily are defined the zone includes high angle wnw striking normal and right lateral strike slip faults and subordinate north and nne striking strike slip faults either right or left lateral the occurrence of small discontinuous faults and the absence of related depressions and sedimentary basins suggest that the extensional regime is still in an incipient stage the ongoing seismic activity possibly reactivates preexisting faults instrumentally and historically recorded earthquakes are lower than about 6 in magnitude and destructive events are historically unknown since at least 1300 a d this apparent upper bound of earthquake magnitudes is consistent with the maximum magnitude values estimated from the length of the longest mapped faults and sources of seismic swarms which all together suggest a value between 6 and 6 5 as the maximum expected magnitude that can be proposed at the present stage of investigation for earthquakes in the study area lateral extension on preexisting faults and upwelling of melt mantle material beneath mount etna are considered viable processes to explain at least in part the active extensional tectonics along the cefalu etna seismic zone strike slip seismic faulting beneath mount etna may be part of a previously proposed diffuse transfer zone affecting northeastern sicily and including the tindari fault

displacement rates of normal faults previous estimates of displacement rates on individual faults have been limited to neotectonic faults and averaged over time intervals of about 200 kyr or less1 2 3 4 5 these estimates have been highly variable which has led to a belief that longer term displacement rates on individual faults are likely to be variable as well here we report estimates of long term normal fault displacement rates averaged over time intervals ranging from 1 to 40 myr and based on observed decreases in displacement of progressively younger horizons intersected by syn sedimentary faults we find that displacement rates are remarkably stable over these longer time periods and within a given fault system the rates are strongly dependent on the relative size of the fault as measured by cumulative vertical displacement taken together these results indicate that faults become large relative to nearby faults by having higher displacement rates even when small rather than as a consequence of having been active for longer our analyses also show that high regional strain rates tend to be accommodated by high fault displacement rates rather than high fault densities

geochronology of age progressive volcanism of the oregon high lava plains implications for the plume interpretation of yellowstone 1 the high lava plains province hlp is a late cenozoic bimodal volcanic field at the northern margin of the basin and range province in southeastern oregon that hosts a westward younging trend of silicic volcanism that crudely mirrors northeastward migration of silicic volcanism along the yellowstone snake river plain ysrp trend we present 40ar 39ar ages for 19 rhyolite domes 5 rhyolite ash flow tuffs and 34 basaltic lavas from the hlp the previously identified trend of westward migration of hlp rhyolites is confirmed the rate of propagation is 33 km m y from 10 to 5 ma slowing to 13 km m y after 5 ma the duration of silicic volcanism at any locus is 2 m y three older hlp dacite domes yielded ages of 15 5 ma basalts are not age progressive we identify several episodes of increased basaltic activity at 7 5 7 8 5 3 5 9 and 2 3 ma with the younger episode likely continuing into the recent the hlp and ysrp trends emerged from the axis of middle miocene basaltic volcanism of the columbia river and steens basalts we propose a model in which 1 miocene flood basalts and widespread silicic rocks are the result of emplacement of a plume head near the craton margin enhanced by flow up a topographic gradient along the base of the lithosphere at the craton margin 2 the hlp trend is the result of westward flow originating at the craton margin and 3 the ysrp trend is the trace of the motion of the north american plate over the tail of the plume

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

impact of rock uplift on rates of late cenozoic rocky mountain river incision 1 the high relief of the modern rocky mountain landscape formed in the late cenozoic by downcutting of a fluvial network that links a series of easily eroded sedimentary basins across relatively resistant crystalline cores of adjacent ranges using a numerical model of fluvial erosion and the flexural isostatic response to the associated unloading we first calculate the expected pattern and pace of incision caused by rock uplift related to migration of the yellowstone hot spot and to growth of the northern portion of the rio grande rift calculated incision rates are 60 m myr and total depth of erosion of sedimentary basins is 300 m well below the long term incision rates and amounts of erosion interpreted from the geologic record broad scale tilting of the region toward the east accomplished by a gradient in rock uplift of 1 km along the north south axis of the central rockies declining to zero 1000 km to the east can account for the additional erosion needed to match observations in each modeling scenario stream incision is nonsteady with rock uplift outpacing erosion for 1 myr in perimeter basins and 1 5 myr in interior basins three factors dominate the spatial and temporal pattern of regional landscape evolution 1 the time since uplift began 2 the uplift pattern and 3 the distribution of relatively resistant bedrock within the region our results suggest that the spatial variability in late cenozoic exhumation can be explained by a long lived transience in the stream network response to these various late cenozoic geophysical events

mid miocene rhyolite volcanism in northeastern nevada the jarbidge rhyolite and its relationship to the cenozoic evolution of the northern great basin usa we present new physical geochemical geochronologic and oxygen isotope constraints on the mid miocene jarbidge rhyolite in northeastern nevada usa providing new constraints on the tectonomagmatic evolution of the cenozoic northern great basin widespread extension due to rapid collapse of the nevadaplano began at ca 17 16 ma across the northern great basin coeval with this event was compositionally bimodal basalt rhyolite volcanism that is often attributed to the inception of the yellowstone hotspot the most widespread mid miocene volcanic units in northeastern nevada are lavas and domes of the jarbidge rhyolite the thickest and most areally extensive exposures of these lavas include and are found just west of the jarbidge mountains nevada this study focuses on jarbidge rhyolite directly south of the central snake river plain adjacent to the thickest exposures in the vicinity of jarbidge nevada textures on a range of scales indicate that the jarbidge rhyolite consists primarily of phenocryst rich lavas laser 40 ar 39 ar ages for sanidine are consistent with effusive eruption of metaluminous to slightly peraluminous ferroan calc alkalic rhyolite from 16 1 to 15 0 ma prior k ar ages suggest that some activity occurred over a slightly longer duration major and trace element data coupled with new stable and prior radiogenic isotope measurements suggest that jarbidge rhyolite magmas formed primarily via melting of quartzofeldspathic crust the jarbidge rhyolite lavas are geochemically dissimilar from younger snake river plain rhyolites e g lower mgo lower nb higher rb nb and are more similar to coeval rhyolites erupted to the west on or adjacent to the oregon plateau the distribution of the jarbidge rhyolite lavas in northeastern nevada reflects an intimate association with temporally and spatially coincident extension rather than the yellowstone hotspot

models of lithosphere and asthenosphere anisotropic structure of the yellowstone hot spot from shear wave splitting 1 teleseismic shear wave splitting measured at 56 continuous and temporary seismographs deployed in a 500 km by 600 km area around the yellowstone hot spot indicates that fast anisotropy in the mantle is parallel to the direction of plate motion under most of the array the average split time from all stations of 0 9 s is typical of continental stations there is little evidence for plume induced radial strain suggesting that any contribution of gravitationally spreading plume material is undetectably small with respect to the plate motion velocity two stations within yellowstone have splitting measurements indicating the apparent fast anisotropy direction is nearly perpendicular to plate motion these stations are 30 km from stations with parallel to plate motion the 70 rotation over 30 km suggests a shallow source of anisotropy however split times for these stations are more than 2 s we suggest melt filled stress oriented cracks in the lithosphere are responsible for the anomalous orientations within yellowstone stations southeast of yellowstone have measurements of oriented nnw to wnw at high angles to the plate motion direction the archean lithosphere beneath these stations may have significant anisotropy capable of producing the observed splitting

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

stratigraphic framework for the plume mode of mantle convection and the analysis of interregional unconformities on geological maps abstract mantle convection is a fundamental planetary process its plate mode is established and expressed by plate tectonics its plume mode also is established and expressed by interregional geological patterns we developed both an event based stratigraphic framework to illustrate the surface effects predicted by the plume model of griffiths et al 1989 and griffiths and campbell 1990 and a methodology to analyze continent scale geological maps based on unconformities and hiatuses the surface expression of ascending plumes lasts for tens of millions of years and rates vary over a few million years as the plume ascends its surface expression narrows but increases in amplitude leaving distinct geological and stratigraphic patterns in the geologic record not only above the plume head center but also above its margins and in distal regions a few thousands of kilometers from the center to visualize these patterns we constructed sequential geological maps chronostratigraphic sections and hiatus diagrams dome uplift with erosion engor 2001 and the flood basalts duncan and richards 1991 ernst and buchan 2001a are diagnostic starting points for plume stratigraphic analyses mechanical collapse of the dome results in narrow rifting burke and dewey 1973 drainage network reorganization cox 1989 and flood basalt eruption in the marginal region patterns of vertical movement deformation and surface response are transient and complex at first the plume margin is uplifted together with the central region but then it subsides as the plume ascents farther with plume head flattening the plume margin experiences renewed outward migrating surface uplift erosion broad crustal faulting and drainage reorganization knickpoint migration occurs first inward directed at the rate of plume ascent and later outward directed at the rate of asthenospheric flow interregional scale unconformity bounded stratigraphic successions document the two inversions the distal regions which did not experience any plume related uplift yield complete sedimentary records of the event event related time gaps hiatuses in the sedimentary record increase towards the center but the event horizon is best preserved in the distal region it may be recognized by tracing its contacts from the center outwards we extracted system and series hiatuses from interregional geological maps and built hiatus maps as proxies for paleo dynamic topography and as a basis for comparison with results from numerical models interregional scale geological maps are well suited to visualize plume related geological records of dynamic topography in continental regions however geological records and hiatus information at the resolution of stages will be needed at interregional scales the plume stratigraphic framework is event based interregional but not global with time dependent amplitudes that are significantly larger than those of global eustatic sea level fluctuations global stratigraphic syntheses require integration of plate and plume stratigraphic frameworks before eustatic contributions may be assessed

seismic expression and geological significance of a lacustrine delta in neogene deposits of the western snake river plain idaho high resolution seismic reflection profiles and well data from the western snake river plain basin are used to identify a buried lacustrine delta system within neogene idaho group sediments near caldwell idaho the delta system is detected 305 m 1000 ft deep near the center of the basin by progradational clinoform reflections having dips of 2 5 a slope typical of prodelta surfaces of modern lacustrine delta systems the prodelta slope relief corrected for compaction indicates the delta system prograded northwestward into a lake basin 255 m 837 ft deep resistivity logs in the prodelta mud and clay facies are characterized by gradual upward increase in resistivity and grain size over a thickness of about 100 m 300 ft lithology of the prodelta is mostly c lcareous claystone with several layers of fine sand some of which fine upward indicating a density flow mechanism of transport and deposition delta plain and front sediments are mostly very fine grained well sorted sand separated by thin mud layers these sediments produce several to five cycles of horizontal high amplitude reflections with a toplap relationship to prodelta clinoforms the sands have an abrupt lower contact with prodelta muds and have high resistivity on logs in this study permeable lacustrine sands within a predominantly mud and clay section are located by using high resolution seismic reflection data identification of a delta system in the idaho group provides insight into the history of pliocene lake idaho the present depth of the delta prodelta facies contact of 305 m 1000 ft is 445 to 575 m 1460 1900 ft below the lake deposits on the margins estimated subsidence from compaction is 220 m 656 ft and the remaining 225 to 325 m 740 1066 ft is attributed to tectonic downwarping and faulting the original lake area had been reduced to one third of the original 13 000 km2 5000 mi2 by the time the delta front prograded to the caldwell area the original lake area may have been sufficient to evaporate most of the inflow and the lake may have only occasionally spilled into other basins diminished area for evaporation later in the history of the lake combined with reduced evaporation accompanying onset of the ice ages may have caused the lake to rise and overtop a basin sill about 2 ma and subsequently deepen hells canyon

zircon petrochronology and 40ar 39ar sanidine dates for the mesa falls tuff crystal scale records of magmatic evolution and the short lifespan of a large yellowstone magma chamber

sedimentary response to orogenic exhumation in the northern rocky mountain basin and range province flint creek basin west central montana middle eocene through upper miocene sedimentary and volcanic rocks of the flint creek basin in western montana accumulated during a period of significant paleoclimatic change and extension across the northern rocky mountain basin and range province gravity modelling borehole data and geologic mapping from the flint creek basin indicate that subsidence was focused along an extensionally reactivated sevier thrust fault which accommodated up to 800 m of basin fill while relaying stress between the dextral transtensional lewis and clark lineament to the north and the anaconda core complex to the south northwesterly paleocurrent indicators foliated metamorphic lithics 64 ma 40ar 39ar muscovite grains and 76 ma u pb zircons in a ca 27 ma arkosic sandstone are consistent with oligocene exhumation and erosion of the anaconda core complex the core complex and volcanic and magmatic rocks in its hangingwall created an important drainage divide during the paleogene shedding detritus to the nnw and ese

holocene scarp on the sawtooth fault central idaho usa documented through lidar topographic analysis high resolution lidar data reveal a prominent latest pleistocene holocene scarp on the sawtooth fault central idaho united states the fault scarp marks 55 65 km of the range front and may comprise two segments the scarp is 4 9 m high in latest pleistocene glacial landforms 11 14 ka and 2 3 m high in holocene alluvial landforms implying 2 3 postglacial rupture events patterns of fault scarp continuity coupled with existing gravity data suggest that active faulting may have migrated northward during pleistocene time detailed comparisons of raw lidar digital elevation models dems bare earth lidar dems and field surveys indicate that the bare earth lidar data document the fault scarp morphology accurately and allow for detailed fault analysis where field evaluation is difficult the documentation of holocene motion on the sawtooth fault demonstrates that ene directed extension extends across central idaho and that the fault contributes to seismic hazards

western us volcanism due to intruding oceanic mantle driven by ancient farallon slabs the origin of late cenozoic intraplate volcanism over the western united states is debated one important reason is the lack of a clear understanding of the mantle dynamics during this volcanic history here we reconstruct the mantle thermal states beneath north america since 20 million years ago using a hybrid inverse geodynamic model with data assimilation the model simultaneously satisfies the past subduction kinematics present mantle tomographic image and the volcanic history we find that volcanism in both the yellowstone volcanic province and the basin and range province corresponds to a similar eastward intruding mantle derived from beneath the pacific ocean and driven mostly by the sinking farallon slab below the central eastern united states the hot mantle that forms the columbia river flood basalt and subsequent yellowstone newberry hotspot tracks first enters the western united states through tears within the juan de fuca slab subsequent coexistence of the westward asthenospheric flow above the retreating juan de fuca slab and eastward propagating mantle beyond the back arc region reproduces the bifurcating hotspot chains a similar but weaker heat source intrudes below the basin and range around the southern edge of the slab and can explain the diffuse basaltic volcanism in this region according to our models the putative yellowstone plume contributes little to the formation of the yellowstone volcanic province volcanism in the western us may result from warm oceanic mantle beneath the pacific ocean being drawn eastwards by mantle flow induced by the sinking of farallon slabs according to numerical model simulations

possible regional tectonic controls on mammalian evolution in western north america abstract previous work has suggested that tectonically active regions act as speciation pumps for mammals and plant species but little is known about how fast or widespread tectonism must be in order to directly influence evolution here we use oxygen and hydrogen isotopic data from miocene sedimentary deposits to characterize the topographic evolution of the southern columbia plateau snake river plain and northern rocky mountain regions during the yellowstone hotspot passage with the ultimate goal of understanding whether topographic changes caused by the hotspot influenced mammalian evolution within those regions we conducted oxygen isotope analyses of 130 samples of lacustrine and paleosol carbonate from miocene stratigraphic sections that span much of the northern rocky mountain region and combined these data with previously published isotopic records collectively these isotopic data show that caldera formation associated with the yellowstone hotspot has modified regional topography and rearranged drainages along the track of the hotspot and that the hotspot has left a topographic depression in its wake we explore the extent to which these topographic changes influenced or are decoupled from diversity changes exhibited by the local mammal faunas and conclude that the passage of the hotspot and consequent surface uplift created rainshadows in the lee of high elevation calderas and or generated large volumes of volcanic materials influencing soils and vegetation collectively that may explain a possible rise in mammal diversity in the cp srp region at 14 ma coincident with a drop in diversity in the nrm it is still unclear however how different taphonomic pathways and sample standardization problems are influencing apparent diversity peaks at this temporal and geographic resolution

high resolution aeromagnetic mapping of volcanic terrain yellowstone national park high resolution aeromagnetic data acquired over yellowstone national park ynp show contrasting patterns reflecting differences in rock composition types and degree of alteration and crustal structures that mirror the variable geology of the yellowstone plateau the older eocene absaroka volcanic supergroup a series of mostly altered andesitic volcanic and volcaniclastic rocks partially exposed in mountains on the eastern margin of ynp produces high amplitude positive magnetic anomalies strongly contrasting with the less magnetic younger latest cenozoic yellowstone plateau group primarily a series of fresh and variably altered rhyolitic rocks covering most of ynp the yellowstone caldera is the centerpiece of the yellowstone plateau part of its boundary can be identified on the aeromagnetic map as a series of discontinuous negative magnetic anomalies that reflect faults or zones along which extensive hydrothermal alteration is localized the large volume rhyolitic ignimbrite deposits of the 0 63 ma lava creek tuff and the 2 1 ma huckleberry ridge tuff which are prominent lithologies peripheral to the yellowstone caldera produce insignificant magnetic signatures a zone of moderate amplitude positive anomalies coincides with the mapped extent of several post caldera rhyolitic lavas linear magnetic anomalies reflect the rectilinear fault systems characteristic of resurgent domes in the center of the caldera peripheral to the caldera the high resolution aeromagnetic map clearly delineates flow unit boundaries of pre and post caldera basalt flows which occur stratigraphically below the post caldera rhyolitic lavas and are not exposed extensively at the surface all of the hot spring and geyser basins such as norris upper and lower geyser basins west thumb and gibbon are associated with negative magnetic anomalies reflecting hydrothermal alteration that has destroyed the magnetic susceptibility of minerals in the volcanic rocks within yellowstone lake which is mostly within the yellowstone caldera aeromagnetic lows also are associated with known hydrothermal activity in the lake many of the magnetic lows extend beyond the areas of alteration and hot springs suggesting a more extensive currently active or fossil hydrothermal system than is currently mapped steep magnetic gradients suggesting faults or fractures bound the magnetic lows this implies that fractures localize the hot springs magnetic gradient trends reflect the mapped basin and range structural trends of north and northwest as well as northeasterly trends that parallel the regional trend of the snake river plain and the track of the yellowstone hot spot which follow the precambrian structural grain these trends are found both at small scales such as in hydrothermal basins and at more regional fault scales which suggests that the regional stress field and reactivated

global omori law decay of triggered earthquakes large aftershocks outside the classical aftershock zone 1 triggered earthquakes can be large damaging and lethal as evidenced by the1999 shocks in turkey and the 2001 earthquakes in el salvador in this study earthquakes with ms 7 0 from the harvard centroid moment tensor cmt catalog are modeled as dislocations to calculate shear stress changes on subsequent earthquake rupture planes near enough to be affected about 61 of earthquakes that occurred near defined as having shear stress change 0 01 mpa the ms 7 0 shocks are associated with calculated shear stress increases while 39 are associated with shear stress decreases if earthquakes associated with calculated shear stress increases are interpreted as triggered then such events make up at least 8 of the cmt catalog globally these triggered earthquakes obey an omori law rate decay that lasts between 7 11 years after the main shock earthquakes associated with calculated shear stress increases occur at higher rates than background up to 240 km away from the main shock centroid omori s law is one of the few time predictable patterns evident in the global occurrence of earthquakes if large triggered earthquakes habitually obey omori s law then their hazard can be more readily assessed the characteristic rate change with time and spatial distribution can be used to rapidly assess the likelihood of triggered earthquakes following events of ms 7 0 i show an example application to the m 7 7 13 january 2001 el salvador earthquake where use of global statistics appears to provide a better rapid hazard estimate than coulomb stress change calculations

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

why is it downhill from tonopah to las vegas a case for mantle plume support of the high northern basin and range a major abrupt southward decline in elevation 800 m with an accompanying increase in bouguer gravity anomaly 80 mgal crosses a seismically active region of southern nevada at about 37 n the steepness of the gravity gradient requires significant crustal density contrast some of which is probably caused by the plutonic roots of voluminous tertiary volcanism to the north but the amplitude of the anomaly requires additional contrast within the mantle the topographic step probably developed in mid miocene time coincident with the arrival of the starting head of the yellowstone thermal plume at the base of the lithosphere a plausible combination of crustal and deep buoyancy sources related to the heat and melt input of an anomalously hot asthenospheric source is consistent with gravity seismic heat flow and isotopic observations and explains the origin of the topographic step

plio pleistocene climatic transition and the lifting of the teton range wyoming abstract fine grained lacustrine riverine and ash fall sediments of the shooting iron formation whose late pliocene age is established by blancan gastropods and vertebrates yield a pollen flora that is essentially similar in composition to the modern pollen rain in the jackson hole area the pliocene assemblage suggests a climate like that of the jackson valley and foothills today these spectra also resemble a pliocene pollen flora from yellowstone park dated at 2 02 ma however the underlying miocene teewinot sediments differ by containing pollen of four exotic deciduous hardwoods tertiary relicts that suggest a summer moist climate unlike that of today the shooting iron sediments lie with an angular unconformity on and above the miocene lake sediments of the teewinot formation both of these deposits probably preceded the main uplift of the teton range based on the absence of precambrian clasts in the tertiary valley deposits because the pliocene floras were modern in aspect a plio pleistocene transition would be floristically imperceptible here the sequence denotes a protracted period of relative stability of climate during teewinot time and a shift in vegetational state summer wet trees drop out sometime between the latest miocene and latest pliocene the pliocene spectra suggest a dry cooler climate toward the end of shooting iron time

evidence of a microbial community associated with rock varnish at yungay atacama desert chile 1 rock varnish is a very slow growing nanostratigraphic coating consisting of approximately 70 fine grained clay and 30 iron and manganese oxides that forms on the surfaces of rocks in arid and semiarid climates the microbial diversity associated with rock varnish collected from the hyperarid yungay region of the atacama desert was investigated using culture independent biomolecular methods and an adenosine triphosphate atp assay the extraction of dna from rock varnish collected at yungay a region in which little to no dna has been extracted from the surface soil 1 cm to date indicates that rock varnish may provide a niche habitat for microbial life where water is essentially absent the clone library constructed suggests the presence of numerous phylogenetically distinct microorganisms ranging in diversity from cyanobacterial to a proteobacteria lineages the findings also show that only a few micrometers of varnish material are enough to shelter microbes like chroococcidiopsis spp from the intense ultraviolet radiation present in the atacama desert whether or not microorganisms are involved in its nucleation and or growth rock varnish appears to provide a microhabitat resembling cryptoendolithic communities seen on a larger scale

exhumation patterns along shallow low angle normal faults an example from the altotiberina active fault system northern apennines italy a multi method approach palaeothermal and thermochronological analyses thermal modelling is applied to reconstruct the exhumation history of the altotiberina fault atf a representative example of crustal scale active low angle normal faulting in the northern apennines italy thermal maturity and thermochronological data yield similar burial histories but different exhumation patterns for the sedimentary successions in the hangingwall and the footwall of the atf since 3 8 ma the atf footwall has exhumed at rates of 0 90 mm a 1 exhumation led to bending and deactivation of the atf uppermost portion as a result of tectonic unloading and isostatic adjustment followed by migration of extension and the development of a set of domino like east dipping normal faults rooting on the buried portion of the atf atf activity and isostatic rebound exhumed triassic rock units from depths of about 4 km we suggest that isostatic instability is accommodated at shallow crustal levels in a similar way to what is observed on larger structures at mid low crustal levels

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