airborne gravity reveals interior of antarctic volcano abstract understanding antarctic volcanoes is important as they provide a window on magmatic and tectonic processes of the antarctic plate and contain datable records of ice sheet changes we present the results from the first detailed airborne radar and gravity surveys across james ross island northern antarctic peninsula which is dominated by mt haddington an ice covered miocene recent alkaline stratovolcano the surveys provide new insights into the subsurface structure of the volcano and hence its development which are unavailable from the surface geology alone we show that mt haddington is associated with a significant negative bouguer gravity anomaly 26 mgal which suggests that there has not been significant pooling and solidification of a dense shallow level mafic magma chamber during the growth of the volcano over at least the past 6 m y which is consistent with independent geochemical evidence simple flexural isostatic models cannot explain the localised negative bouguer anomaly 3d modelling techniques show that the negative anomaly is best explained by a shallow low density intra crustal body with its top close to or at the surface although comparable gravity anomalies are commonly associated with large 20 km ash filled calderas as seen at yellowstone or toba there is no geological evidence on james ross island for a similar structure we therefore propose that the james ross island volcanic edifice subsided into the thick underlying pile of relatively soft jurassic and cretaceous sediments which were displaced by low density hyaloclastite breccia the type of deformation envisaged is similar to that associated with concepciou or iwaki volcanoes in south america although mt haddington is much larger

tex86 paleothermometer as an indication of bottom water temperature in the yellow sea the tex86 paleothermometer was applied for reconstructing sea surface temperature sst from glycerol dialkyl glycerol tetraethers gdgts in marine sediments it has become clear that gdgt producing archaea live throughout the water column with maximum concentration usually well below the surface mixed layer of the ocean why the tex86 parameter correlates well with sst remains poorly understood here we evaluate the fidelity of the tex86 thermometer using surface sediments and suspended particles from the yellow sea ys a shallow marginal sea between china and the korean peninsula the highest concentration of gdgts in the water column at a site a02 from the middle of the ys occurred in the bottom layer at 70 m this contrasts with phytoplankton lipids which were most abundant near the surface consistent with the maximum abundance of gdgts in bottom water tex86 values in surface sediments correlated better with mean annual bottom water temperature bwt r 2 0 81 than with mean

towards understanding the puzzling lack of acid geothermal springs in tibet china insight from a comparison with yellowstone usa and some active volcanic hydrothermal systems abstract explanations for the lack of acid geothermal springs in tibet are inferred from a comprehensive hydrochemical comparison of tibetan geothermal waters with those discharged from yellowstone usa and two active volcanic areas nevado del ruiz colombia and miravalles costa rica where acid springs are widely distributed and diversified in terms of geochemical characteristic and origin for the hydrothermal areas investigated in this study there appears to be a relationship between the depths of magma chambers and the occurrence of acid chloride rich springs formed via direct magmatic fluid absorption nevado del ruiz and miravalles with magma at or very close to the surface less than 1 2 km exhibit very acidic waters containing hcl and h2so4 in contrast the tibetan hydrothermal systems represented by yangbajain usually have fairly deep seated magma chambers so that the released acid fluids are much more likely to be fully neutralized during transport to the surface the absence of steam heated acid waters in tibet however may be primarily due to the lack of a confining layer like young impermeable lavas at yellowstone to separate geothermal steam from underlying neutral chloride waters and the possible scenario that the deep geothermal fluids below tibet carry less h2s than those below yellowstone

seasonal distribution of archaeal lipids in surface water and its constraint on their sources and the tex86 temperature proxy in sediments of the south china sea the applicability of the temperature proxy based on a tetraether index of 86 carbon atoms tex86 from isoprenoidal glycerol dialkyl glycerol tetraethers isogdgts has increasingly been examined in the south china sea scs views on the proxy differ on whether it is biased to a particular season or to depth in this sea here we studied isogdgts in a large number of suspended particulate matters spm in surface water during summer and winter as well as several surface sediments of the northern scs the integration of these novel data with already published data demonstrated that isogdgts were similar in distribution between surface water spm and shallow shelf sediments they can be characterized by low ratio 4 of gdgt 2 gdgt 3 and low percentage abundance 4 of the crenarchaeol regioisomer however those in deep basin sediments showed higher values for both parameters suggesting a substantial contribution of deep water thaumarchaeota the relationship between tex86 in spm and in situ measured sst differed from a global calibration and showed distinct trends between summer and winter reflecting archaeal seasonality in surface waters by assuming the tex86 temperature signal in shallow sediments as the weighted mean of summer and winter signals we revealed spatial changes in the relative contribution of summer and winter archaeal isogdgts to sediments with a likely higher winter contribution further offshore this occurrence may complicate the interpretation of the shallow sedimentary tex86 record and further investigation is required to fully understand seasonal archaeal lipid production and sedimentation

p wave attenuation of the yellowstone caldera from three dimensional inversion of spectral decay using explosion source seismic data using explosion source seismic refraction data recorded in the 1978 and 1980 yellowstone snake river plain seismic experiments a three dimensional inversion of differential p wave attenuation was used to assess the relative variations in q 1 in and around the volcanically active 45 km by 70 km yellowstone caldera northwestern wyoming differential attenuation was derived from spectral decay of upper crustal pg phases observed from six explosions and recorded at 90 temporary stations because of the relatively short time windows used to determine the spectral content a maximum entropy technique was employed to estimate the spectra that yielded an optimally small variance differential p wave attenuation was calculated from least squares determinations of the spectral ratios corrected for source and path effects the observed differential attenuation parameters were then inverted using a weighted least squares technique for a discretized 70 105 km three dimensional surface and upper crustal q 1 model of the yellowstone caldera and surrounding region results showed that the surface layer to depths of 2 km within the yellowstone caldera is characterized by relatively high attenuation with low q values less than 30 compared to values of 38 to 50 outside the caldera the higher attenuation in the caldera s surface layer is thought to be associated with quaternary lake sediments highly altered rhyolites and the possible influence of steam in areas of hydrothermal activity in the crystalline upper crust at depths of 2 km to 12 km q values of 40 to 70 were observed in areas of thick sedimentary fill northwest of the caldera and in areas of hydrothermal activity within the caldera upper crustal attenuation generally corresponded to q of 200 in areas that are interpreted to be associated with hot but now solidified granitic material in comparison relatively high attenuation q 40 was observed in the upper crust of the northeastern yellowstone caldera in an area that also corresponds to a 20 reduction in pg velocity a prominent negative bouguer gravity low of 20 mgal near yellowstone s largest area of hydrothermal activity and near a quaternary volcanic resurgent dome on the basis of constitutive models of velocity and density this zone of high attenuation is likely a product of unusual hydrothermal activity highly altered upper crustal rocks a shallow magmatic source or a highly porous steam saturated body outside the caldera the upper crust is relatively unaffected by high temperatures and thermal alteration and the corresponding attenuation is low with q values greater than 200

density and lithospheric strength models of the yellowstone snake river plain volcanic system from gravity and heat flow data abstract the structure and composition of the yellowstone snake river plain ysrp volcanic system were analyzed using gravity data taken at over 30 000 stations in the ysrp and surrounding region additional constraints were provided by tomographic seismic velocity models new heat flow and temperature information gps derived strain rates earthquake locations and chemical analyses of volcanic rocks p wave velocity models and velocity density data based on petrologic information were used to constrain three dimensional density models rheology and strength properties were calculated at selected locations and compared to earthquake focal depths results of this study suggest that the lower crust of the snake river plain srp has been thickened by the addition of an underplated layer composed primarily of clinopyroxene having a density of 3200 kg m3 a mid crustal high density sill is composed of a series of gabbroic lenses inter fingering with the granitic upper crust this geometry yields a bulk composition comparable to diorite and a density of 2900 kg m3 the mid crustal sill varies from 4 to 11 km in thickness resulting in a series of se nw trending gravity highs observed along the axis of the srp the mid crustal sill extends up to 20 km southeast of the srp volcanic field and causes asymmetry of the gravity field the yellowstone plateau volcanic field density model reveals low density partial melt 10 km beneath the caldera that shallows under the northeastern caldera and continues laterally 20 km north of the caldera boundary and notably increases the previously estimated size of the magma reservoir by 20 the caldera melt body has a density of 2520 kg m3 and a significantly lower value of 2470 kg m3 for the northeastern caldera melt body southwest of yellowstone the crustal section occupied by the mid crustal sill in the srp and the partial melt in yellowstone constitute a transition area between the active yellowstone magma system and the now volcanically quiescent srp with a density of 2820 kg m3 strength models show that the crust of the ysrp becomes progressively stronger and cooler with increasing distance from yellowstone and tectonic earthquakes within the yellowstone caldera are unlikely to nucleate below 4 6 km depth thus limiting the maximum magnitude of earthquakes to m 6 5

upper crustal structure of the yellowstone caldera from seismic delay time analyses and gravity correlations the 1978 yellowstone snake river plain seismic experiment provided detailed refraction data that were recorded across a two dimensional array of seismographs in yellowstone national park a delay time analysis was applied to 173 crystalline basement pg arrivals from these data to determine the three dimensional distribution of velocities and the layer configuration of the upper crust beneath the yellowstone caldera the p wave velocity structure of the caldera is characterized by a surface layer of combined sediments and rhyolite flows averaging 2 8 km s that range in thickness from 1 5 to 2 0 km adjacent to the caldera the crystalline upper crustal layer has a velocity of 6 05 0 01 km s but this layer decreases by 6 to 5 70 km s beneath the caldera and extends northeast 15 km beyond the caldera smaller zones of very low p velocity 4 0 km s a 30 velocity reduction compared to the 6 05 km s layer occur in the upper crust beneath the northeastern caldera rim and beneath the southwest caldera in the vicinity of the upper and midway geyser basins a three dimensional gravity interpretation based upon densities derived from the seismic model suggests that the regional gravity low of 60 mgal over the caldera correlates directly with 1 the surface layer of combined sediments and rhyolite flows 2 the low velocity 5 7 km s upper crustal layer and 3 the 4 0 km s low velocity zone beneath the northeastern caldera rim an interpretation of the seismic velocities and densities based on experimental data and theoretical models suggests that the 6 05 km s 2 70 g cm3 regional pg velocity is characteristic of crystalline basement rocks such as the precambrian granitic gneisses that are exposed north and south of the yellowstone caldera the 5 7 km s velocity layer 2 65 g cm3 is interpreted to be a 10 km thick hot body of granitic composition the 4 0 km s low velocity body 2 40 g cm3 beneath the northeastern caldera rim is associated with a local 20 mgal gravity anomaly with a range of interpretations from a large steam dominated system to a felsic body of 10 to 50 partial melt the postulated 4 0 km s low velocity layer beneath the southwestern caldera does not have a corresponding gravity signature and may reflect a zone of high seismic attenuation attributed to a saturated fracture zone this seismic velocity model of the yellowstone caldera suggests that the source of the quaternary silicic volcanic rocks corresponds to the 5 7 km s layer in the upper crust that appears to contribute the principal source of heat to the yellowstone hydrothermal systems the anomalous low velocity body in the northeastern caldera rim area may be a cupola of partial melt with an overlying vapor dominated carapace and may represent the waning stages of the third cycle of quaternary volcanism or the beginning of a new cycle of volcanism that will continue the northeastward progression of the silicic volcanism of the yellowstone snake river plain propagating system

a review of crust and upper mantle structure studies of the snake river plain yellowstone volcanic system a major lithospheric anomaly in the western u s a abstract the snake river plain yellowstone volcanic system is one of the largest basaltic volcanic field in the world here there is clear evidence for northeasterly progression of rhyolitic volcanism with its present position in yellowstone many theories have been advanced for the origin of the snake river plain yellowstone system yellowstone and eastern snake river plain have been studied intensively using various geophysical techniques some sparse geophysical data are available for the western snake river plain as well teleseismic data show the presence of a large anomalous body with low p and s wave velocities in the crust and upper mantle under the yellowstone caldera a similar body in which compressional wave velocity is lower than in the surrounding rock is present under the eastern snake river plain no data on upper mantle anomalies are available for the western snake river plain detailed seismic refraction data for the eastern snake river plain show strong lateral heterogeneities and suggest thinning of the granitic crust from below by mafic intrusion available data for the western snake river plain also show similar thinning of the upper crust and its replacement by mafic material the seismic refraction results in yellowstone show no evidence of the low velocity anomalies in the lower crust suggested by teleseismic p delay data and interpreted as due to extensive partial melting however the seismic refraction models indicate lower than normal velocities and strong lateral inhomogeneities in the upper crust particularly obvious in the refraction data are two regions of very low seismic velocities near the mallard eake and sour creek resurgent domes in the yellowstone caldera the low velocity body near the sour creek resurgent dome is intepreted as partially molten rock together with other geophysical and thermal data the seismic results indicate that a sub lithospheric thermal anomaly is responsible for the time progressive volcanism along the eastern snake river plain however the exact mechanism responsible for the volcanism and details of magma storage and migration are not yet fully understood

inversion for sources of crustal deformation and gravity change at the yellowstone caldera the 3100 km2 yellowstone caldera yellowstone national park was formed in the latest of three explosive eruptions of rhyolites and ash flow tuffs totalling 3700 km3 at 2 1 2 and 0 6 m y before present its youthful volcanic history widespread hydrothermal activity intense seismicity and extremely high heat flow in excess of 30 times the continental average marks the yellowstone volcanic system as a giant caldera at unrest orthometric height increases of the caldera of up to 76 cm measured from precise leveling surveys from 1923 to 1975 1977 were inverted to determine volume expansion source models for the caldera wide deformation for the 1923 to 1977 uplift episode two regions of expansion were found 1 in the northern part of the caldera near the sour creek resurgent dome of 0 37 km3 and 2 in the southern part of the caldera near the mallard lake resurgent dome of 0 41 km3 both bodies occur in the upper crust from near surface depths to 6 0 km but the largest volume expansions were found in the 3 0 6 0 tan depth range the southern caldera source volume near the mallard lake dome may extend down to 9 0 km the data are however unable to resolve if these two resurgent domes separated laterally by 40 km are connected prom 1976 to 1987 nearly simultaneous measurements of elevation and gravity changes were made on a profile across the northern caldera during a period of net uplift models of the temporal gravity variation infer that the volume increase for the northern caldera source must lie above 9 0 km and involved a density perturbation greater than 0 002 g cm3 the modeled volumetric sources are in the same general locations as bodies of low p wave velocities high seismic attenuation and large negative bouguer gravity anomalies in view of the intense quaternary volcanism the anomalously high heat flow and the correlation of the source volumes with the regional geophysical anomalies it is likely that the modeled volumetric increases were caused by migration of magmas and or the introduction of large volumes of hydrothermal fluids into the upper crust

a deep low velocity body under the yellowstone caldera wyoming delineation using teleseismic p wave residuals and tectonic interpretation in previously published papers iyer 1975 iyer and stewart 1977 iyer 1979 evidence was presented for the presence of a large body of anomalous material with compressional velocities considerably lower than normal from the surrounding rock in the crust and upper mantle under yellowstone national park wyoming in this paper we give a detailed analysis of p residual data from the 26 element telemetered seismic array and three groups of portable stations operated by the u s geological survey in the yellowstone region using a variety of analytical techniques we delineate the shape of the low velocity body determine the velocity structure inside the body and interpret the nature of the material that constitutes the body we will also outline some tectonic implications of our model the spectacular surface geothermal phenomena for which yellowstone is famous result from deep seated volcanism the volcanic evolution of yellowstone is discussed by christiansen and blank 1972 and summarized by eaton and others 1975 briefly the geothermal manifestations in yellowstone are results of three cycles of quaternary volcanism each culminating in a devastating pyroclastic eruption and resulting in large collapse calderas and predominantly rhyolitic flows the oldest cycle ended about 1 9 m y ago forming a caldera from island park to perhaps the central part of the present yellowstone caldera the second cycle climaxed about 1 2 m y ago and was confined to the island park caldera the third cycle began soon after the second and is responsible for the 75 by 45 km yellowstone caldera as it is outlined today this caldera is the result of a catastrophic eruption approximately 600 000 yr ago the most recent episode of volcanism probably began about 150 000 yr ago in the western half of the caldera near old faithful geyser and the youngest flows in yellowstone about 70 000 yr old are probably associated with this episode

nonlinear teleseismic tomography at long valley caldera using three dimensional minimum travel time ray tracing the authors explore the impact of three dimensional minimum travel time ray tracing on nonlinear teleseismic inversion this problem has particular significance when trying to image strongly contrasting low velocity bodies such as magma chambers because strongly refracted and or diffracted rays may precede the direct p wave arrival traditionally used in straight ray seismic tomography they use a simplex based ray tracer to compute the three dimensional minimum travel time ray paths and employ an interative technique to cope with nonlinearity results from synthetic data show that their algorithm results in better model reconstructions compared with traditional straight ray inversions the authors reexamine the teleseismic data collected at long valley caldera by the u s geological survey the most prominent feature of their result is a 25 30 low velocity zone centered at 11 5 km depth beneath the northwestern quandrant of the caldera beneath this at a depth of 24 5 km is a more diffuse 15 low velocity zone in general the low velocities tend to deepen to the south and east the authors interpret the shallow feature to be the residual long valley caldera magma chamber while the deeper feature may represent basaltic magmas ponded in the midcrust the deeper position of the prominent low velocity region in comparison to earliermore tomographic images is a result of using three dimensional rays rather than straight rays in the ray tracing the magnitude of the low velocity anomaly is a factor of approximately 3 times larger than earlier models from linear arrival time inversions and is consistent with models based on observations of ray bending at sites within the caldera these results imply the presence of anywhere from 7 to 100 partial melt beneath the caldera 40 refs 1 fig 1 tab less

the yellowstone snake river plain seismic profiling experiment crustal structure of the eastern snake river plain seismic refraction profiles recorded along the eastern snake river plain esrp in southeastern idaho during the 1978 yellowstone snake river plain cooperative seismic profiling experiment are interpreted to infer the crustal velocity and attenuation q 1 structure of the esrp travel time and synthetic seismogram modeling of a 250 km reversed refraction profile as well as a 100 km detailed profile indicate that the crust of the esrp is highly anomalous approximately 3 to 6 km of volcanic rocks with some interbedded sediments overlie an upper crustal layer compressional velocity approx 6 1 km s which thins southwestward along the esrp from a thickness of 10 km near island park caldera to 2 to 3 km beneath the central and southwestern portions of the esrp an intermediate velocity approx 6 5 km s layer extends from approx 10 to approx 20 km depth a thick approx 22 km lower crust of compressional velocity 6 8 km s a total crustall thickness of approx 42 km and a p sub n velocity of approx 7 9 km s is observed in the esrp similar to the western snake river plain and the rocky mountains provinces high attenuation is evident on the amplitude corrected seismic data due to low q values in the volcanic rocks q sub p more 20 to 200 and throughout the crust q sub p 160 to 300 based on these characteristics of the crustal structure and volcanic age progression data it is suggested that the esrp has resulted from an intensitive period of intrusion of mantle derived basaltic magma into the upper crust generating explosive silicic volcanism and associated regional uplift and caldera collapse this activity began about 15 m y ago in southwestern idaho and has migrated northeast to its present position at yellowstone subsequent cooling of the intruded upper crust results in the 6 5 km s velocity intermediate layer crustal subsidence and periodic basaltic volcanism as represented by the esrp complete the sequence of crustal evolution less

the 1978 yellowstone eastern snake river plain seismic profiling experiment data and upper crustal structure of the yellowstone region eleven in line refraction profiles recorded to distances of 300 km and one azimuthal fan plot were constructed from data recorded with a 150 station array in the yellowstone national park area during the 1978 yellowstone snake river plain seismic experiment interpretations of the data suggest that the crustal p wave velocity model for the yellowstone region is characterized by 1 an averaged 10 km thick upper crustal layer v sub p 6 0 km s 2 an average crustal velocity of 6 3 km s and 3 a total crustal thickness of 44 km velocity models are presented for profiles that emphasize the upper crust and show 1 a decrease in the depth to the top of the upper crustal crystalline basement from 5 km in southwestern yellowstone near island park to 1 km at the northeast side of the yellowstone plateau that is interpreted as a progressive thinning of the silicic surface volcanic layer to the northeast and 2 evidence for a large lateral inhomogeneity interpreted to be a low velocity body with a decrease of at least 10 in p wave velocity located beneath the northeast corner of the yellowstone plateau the low velocity zone coincides with a local 30 mgal residual gravity anomaly and is located beneath partmore of the sour creek resurgent dome and part of the hot springs basin the largest hydrothermal system in yellowstone the low velocity body has a maximum depth to the top of 3 km and a minimum depth to the bottom of 9 km and may represent a zone of partial melt in comparison to the thermally undisturbed upper crust of the surrounding rocky mountains the upper crust of the northeastern yellowstone plateau appears laterally inhomogeneous in velocity and layer thickness suggesting effects of thermal and magma intrusion whereas the lower crust appears relatively homogeneous less

analysis of borehole seismograms from long valley california implications for caldera structure a three component seismometer was emplaced in a borehole in long valley caldera for a period of 2 months to record local earthquakes to the south mostly in the sierra nevada the seismic records were plotted in depth versus time sections to produce vertical hypocentral profiles for both the vertical and horizontal components the data collected and displayed in this way provide good resolution of the crustal structure at depth and avoid the attenuation and complications introduced by the near surface caldera fill the record sections were then modeled with two dimensional ray tracing to match the observed travel times although nonreversal of the profiles and uncertainties in hypocentral locations introduce ambiguities the final interpretation reveals two separate low velocity bodies which we have tentatively identified as magma beneath the resurgent dome in long valley the first of these is a small 2 km by 6 km body with a depth to top of approximately 3 7 km beneath the surface at the southern end of the resurgent dome the other body appears larger and lies at a depth of 5 5 km beneath the northern end of the resurgent dome

volatile emissions and gas geochemistry of hot spring basin yellowstone national park usa abstract we characterize and quantify volatile emissions at hot spring basin hsb a large acid sulfate region that lies just outside the northeastern edge of the 640 ka yellowstone caldera relative to other thermal areas in yellowstone hsb gases are rich in he and h2 and mildly enriched in ch4 and h2s gas compositions are consistent with boiling directly off a deep geothermal liquid at depth as it migrates toward the surface this fluid and the gases evolved from it carries geochemical signatures of magmatic volatiles and water rock reactions with multiple crustal sources including limestones or quartz rich sediments with low k u or 40 ar 4 he variations in gas chemistry across the region reflect reservoir heterogeneity and variable degrees of boiling gas geothermometer temperatures approach 300 c and suggest that the reservoir feeding hsb is one of the hottest at yellowstone diffuse co2 flux in the western basin of hsb as measured by accumulation chamber methods is similar in magnitude to other acid sulfate areas of yellowstone and is well correlated to shallow soil temperatures the extrapolation of diffuse co2 fluxes across all the thermal altered area suggests that 410 140 t d 1 co2 are emitted at hsb vent emissions not included diffuse fluxes of h2s were measured in yellowstone for the first time and likely exceed 2 4 t d 1 at hsb comparing estimates of the total estimated diffuse h2s emission to the amount of sulfur as so42 in streams indicates 50 of the original h2s in the gas emission is lost into shallow groundwater precipitated as native sulfur or vented through fumaroles we estimate the heat output of hsb as 140 370 mw using co2 as a tracer for steam condensate but not including the contribution from fumaroles and hydrothermal vents overall the diffuse heat and volatile fluxes of hsb are as great as some active volcanoes but they are a small fraction 1 3 for co2 2 8 for heat of that estimated for the entire yellowstone system

compaction of quartz kaolinite mixtures the influence of the pore fluid composition on the development of their microstructure and elastic anisotropy abstract shales play important roles in sedimentary basins acting as both seals and reservoir rocks and knowledge of their anisotropic velocity trends is of practical importance for correct seismic image processing and inversion and for seismic to well tie whereas velocity depth trends are extensively studied for conventional reservoirs and critical factors that control their compaction are well understood little work has been done on shales compaction trends of shaly formations are controlled by a number of parameters such as clay mineralogy silt fraction and depositional environment this large number of parameters which are difficult if not impossible to control in naturally deposited shales make the study of shale compaction a statistically complex multivariate and non unique problem in such a situation laboratory experiments that allow precise planning of mixture mineralogy and chemical compositions of pore fluids as well as an accurate control of microstructural changes seems to be an attractive alternative in this work we have conducted two sets of compaction experiments on quartz kaolinite mixtures with 100 75 and 60 of kaolinite powders dry weight in the first set the use of a kcl brine solution to saturate the mixtures has led to the aggregation of the existing clay particles with each other and with silt particles in the other set the mixtures have been treated with a dispersant to separate existing clay aggregates into individual platelets the mixtures have been further compacted in an oedometric cell at uniaxial stresses up to 30 mpa compressional in vertical and horizontal directions and shear in vertical direction ultrasonic wave velocities have been measured during these compaction experiments and the p wave anisotropy has been estimated the effect of the chemical composition of the pore fluid on shale microstructure has been studied using the micro ct and sem image analysis neutron diffraction experiments have been conducted to understand the orientation of clay platelets at the final stage of the compaction our research shows that the chemical composition of pore fluid significantly affects the microstructure of the clay component and via this the elastic properties of the artificial shales the samples with the dispersed clay microstructure dcm have shown larger elastic anisotropies at the same porosity and significantly larger anisotropies at the same stress the dispersed clay platelets have orientated preferably parallel to bedding plane while the clay platelets in the samples with the aggregated clay microstructure acm show a wider angle distribution in the absence of quartz particles 100 kaolinite samples in the case of 40 of quartz fraction though the clay particles are more aligned in the acm sample than in the dcm one

p and s velocity structure of the yellowstone volcanic field from local earthquake and controlled source tomography the three dimensional p and s velocity from vp vs ratios distribution and improved hypocenters for the yellowstone volcanic field have been determined from inversion of first arrival times from 7942 local earthquakes and 16 controlled source explosions the p velocity model has an rms residual of 0 09 s whereas the vp vs ratio model calculated from 511 earthquakes has an rms residual of 0 29 s high p and s velocities outside the yellowstone caldera represent thermally undisturbed basement and sedimentary rocks a caldera wide 15 decrease from regional p velocities at depths of 6 to 12 km is coincident with a 60 mgal gravity anomaly and is interpreted as a hot subsolidus granitic batholith with a quasi plastic rheology localized 30 reductions from regional seismic velocities and higher vp vs ratios 8 km beneath yellowstone s resurgent domes are interpreted as partial melts and vestigial magma systems associated with youthful less than 2 ma silicic volcanism additional low seismic velocities and vp vs ratios and a 20 mgal gravity low less than 4 km beneath the northeast caldera rim are interpreted as a hydrothermal fracture zone thermally driven by underlying partial melt hypocenters relocated with the three dimensional p velocity model show subparallel alignment with nw se trending postcaldera volcanic vents and normal faults northwest of the caldera focal depths of relocated earthquakes decrease from more than 11 km outside the caldera to less than 6 km within the caldera reflecting thinning and heating of the seismogenic crust

strength and rheology of the western u s cordillera effective elastic thickness te depends primarily on temperature composition and state of stress of the lithosphere in this paper we examine high resolution spectral estimates of te and their relationships to regional heat flow age of the lithosphere seismic properties stress orientations and earthquake focal depths of the western u s cordillera the relationship of te to heat flow indicates that ductile flow accommodates long term 106 to 108 years isostatic response at different levels of the crust and upper mantle depending principally on age and by implication bulk composition of the lithosphere isostatic response is primarily controlled by the upper mantle in archean lithosphere of the middle rocky mountains whereas te depends on lower crustal flow in early proterozoic lithosphere of the colorado plateau the yellowstone snake river plain volcanic field and significantly extended regions in the basin range and northern rocky mountains are associated with latest proterozoic aged lithosphere and indicate middle to upper crustal control of long term te we also show that azimuthal variations of te reflect deviatoric stress in the lithosphere te is found empirically to approximate the 95th percentile focal depth of background seismicity the latter relationship is inconsistent with brittle ductile control of focal depth indicating that another rheological transition e g from stick slip to stable sliding frictional behavior is responsible tectonic and structural relationships expand upon the hypothesis that the geographic distribution of tectonic features depends fundamentally on spatial variations in strength of the lithosphere moreover we find a spatial correlation of the intermountain seismic belt to a marked transition in te implying that forces responsible for this active seismic zone are derived from local buoyancy anomalies rather than from current day plate boundary interactions

monitoring super volcanoes geophysical and geochemical signals at yellowstone and other large caldera systems earth s largest calderas form as the ground collapses during immense volcanic eruptions when hundreds to thousands of cubic kilometres of magma are explosively withdrawn from the earth s crust over a period of days to weeks continuing long after such great eruptions the resulting calderas often exhibit pronounced unrest with frequent earthquakes alternating uplift and subsidence of the ground and considerable heat and mass flux because many active and extinct calderas show evidence for repetition of large eruptions such systems demand detailed scientific study and monitoring two calderas in north america yellowstone wyoming and long valley california are in areas of youthful tectonic complexity scientists strive to understand the signals generated when tectonic volcanic and hydrothermal hot ground water processes intersect one obstacle to accurate forecasting of large volcanic events is humanity s lack of familiarity with the signals leading up to the largest class of volcanic eruptions accordingly it may be difficult to recognize the difference between smaller and larger eruptions to prepare ourselves and society scientists must scrutinize a spectrum of volcanic signals and assess the many factors contributing to unrest and toward diverse modes of eruption

contemporary vertical surface displacements in yellowstone national park relative vertical displacements and average relative vertical velocities have been determined for bench marks located throughout yellowstone national park ynp by comparing geodetic leveling measurements made in 1923 second order 1936 third order 1941 third order 1955 third order 1960 first order and 1975 1977 first order the 1923 1975 data indicate uplift greater than 400 mm 7 7 mm yr of an area approximately 15 20 km wide and at least 40 km long which is centered along the northeast trending axis of the 600 000 year old yellowstone caldera the maximum 1923 1975 vertical displacement in this area is 726 mm 14 0 mm yr the existence of a well defined and untilted yellowstone lake terrace implies that the 1923 1975 average vertical velocities cannot have been maintained for more than a few hundred years the 1955 1977 data from central ynp indicate that the northeastern caldera half has risen relative to the southwestern half at an average velocity of about 8 mm yr a similar bias is present in the 1923 1975 data but it is less pronounced the 1923 1960 and 1960 1975 data from northwestern ynp and a 200 mm depression in the 1923 1975 data near norris junction may be interpreted in terms of deformation associated with the 1959 hebgen lake earthquake and the 1975 yellowstone park earthquake a composite profile of the 1936 1977 and 1941 1977 average vertical velocity data which skirts the edge of the yellowstone caldera in northeastern ynp is most easily explained as the result of random leveling error and or bench mark instabilities the apparent youth of the 1923 1975 yellowstone uplift and its close association with the yellowstone caldera suggest that the most likely cause of the uplift is a recent not more than 500 years ago influx of molten material into the upper crust beneath the caldera

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

interpretation of gravity data in a complex volcano tectonic setting southwestern nevada this regional gravity study based on an irregular 2 km data grid was conducted during the past few years at yucca mountain southern nye county nevada as part of a program to locate a suitable repository for high level nuclear waste about 100 surface rock samples three borehole gamma gamma logs and one borehole gravity study provide excellent density control a nearly linear increase in density of 0 26 g cm3 per kilometer of depth is indicated in the thick tuff sequences that underlie the mountain isostatic and 2 0 g cm3 bouguer corrections were applied to the observed gravity values to remove regional gradients and topographic effects respectively the bare mountain gravity high with an isostatic anomaly maximum of 48 mgal is connected with a greater gravity high over the funeral mountains to the southwest together these highs result from a continuous block of dense metamorphosed precambrian and paleozoic rocks that stretches across much of the walker lane from the east edge of death valley to bare mountain the calico hills gravity high appears more likely to originate from a northeast trending buried ridge of paleozoic rocks that extends southwestward beneath busted butte 5 km southeast of the proposed repository where two and three dimensional modeling indicates that the pre cenozoic rocks lie less than 1000 m beneath the surface tuff at least 4000 m thick fills a large steep sided depression in the pretuff rocks beneath yucca mountain and crater flat the gravity low and the thick tuff section lie within a large collapse area that includes the crater flat timber mountain silent canyon caldera complexes gravity lows in crater flat itself are interpreted to coincide with the source areas of the prow pass member the bullfrog member and the tram member of the crater flat tuff these source areas add nearly 350 km2 to the previously recognized extent of the local caldera complexes southward extension of the broad gravity low associated with crater flat into the amargosa desert is evidence for sector graben type collapse segments related to the formation of the timber mountain caldera and superimposed on the other volcanic and extensional structures within crater flat

seismic studies of crustal structure during the past four years seismology has continued its role as the key technique for determination of the structure of the continental and oceanic crust by and large however the field and analysis methods used bear little resemblance to those of ten years ago a continued push to increase the number of recording channels has led to the routine use of ocean bottom seismometers sonobuoys and multichannel streamers in oceanic studies and to the use of large sets of portable instruments and multichannel seismic reflection strings on land in studies using earthquakes as sources local and regional arrays provide a capability for obtaining geologically useful two and three dimensional information about the crust

vertical surface displacements at yellowstone caldera wyoming 1976 1986 between first order leveling surveys in 1976 and 1986 the east central floor of yellowstone caldera rose 15 4 1 4 mm yr and the west central floor rose 17 0 2 0 mm yr with respect to points on the south caldera rim the same pattern has prevailed since an initial survey in 1923 except that the western caldera rose 30 faster during 1976 1986 than during 1923 1976 the 1976 1986 uplift extends beyond the northwest caldera rim near the adjacent hebgen lake fault zone site of a magnitude 7 3 earthquake in august 1959 marginally significant displacements in the eastern caldera suggest that uplift continued during 1983 1984 stopped during 1984 1985 and reversed to subsidence during 1985 1986 thus long term uplift may accumulate episodically historical uplift is especially noteworthy because regional crustal extension and thermal contraction during cooling would cause the caldera floor to subside if other processes were not acting to warp it upward the pattern and dimensions of the uplift indicate a source in the upper crust beneath yellowstone caldera probably a reservoir of rhyolite magma within a cooling granitic pluton a simple point source model would suggest a source 10 15 km deep but the reservoir is probably lens shaped and closer to the surface uplift may be caused by basaltic intrusions near the base of the reservoir or accumulation of magmatic fluids during cooling and crystallization within the reservoir subsidence might be a response to regional tectonic extension or release of trapped magmatic fluids magmatic inflation volatile accumulation and crustal extension may culminate eventually in renewed eruptive activity but there is no evidence of an imminent volcanic hazard

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 1978 yellowstone eastern snake river plain seismic profiling experiment crustal structure of the yellowstone region and experiment design in 1978 a major seismic profiling experiment was conducted in the yellowstone eastern snake river plain region of idaho and wyoming fifteen shots were recorded that provided coverage to distances of 300 km in this paper travel time and synthetic seismogram modeling was used to evaluate an average p wave velocity and apparent q structure of the crust from two seismic profiles reversed across the yellowstone national park region this area includes the well known hydrothermal features of yellowstone national park geysers fumeroles etc a large collapse caldera and extensive silicic volcanism of quaternary age features attributed to shallow crustal sources of magma the averaged crustal structure for this region as interpreted from the seismic data consists of 1 a highly variable near surface layer approximately 2 km thick with variable velocities of 3 0 to 4 8 km s and a low apparent q of 30 that is interpreted to be composed of weathered rhyolites and sedimentary infill 2 an upper crustal layer 3 to 4 km thick with variable velocities of 4 9 to 5 5 km s and apparent q of 50 to 200 that is thought to represent the accumulation of the pleistocene quaternary rhyolite flows ash flow tuffs and possible paleozoic and precambrian metamorphic equivalents 3 the crystalline upper crust that is characterized by a laterally inhomogeneous layer that varies in velocity from 4 0 to 6 1 km s averaging 5 km thick with a q of 300 this layer appears to be a cooling but still hot body of granitic composition beneath the yellowstone caldera it is thought to be a remnant of the magma chambers that produced the quaternary silicic volcanic rocks of the yellowstone plateau and may still be a major contributor to the high heat flow 4 a laterally homogeneous intermediate crustal layer 8 to 10 km thick with a velocity of 6 5 km s and apparent q of 100 to 300 5 a homogeneous 25 km thick lower crust with a velocity of 6 7 to 6 8 km s and an apparent q of 300 and 6 a total crustal thickness of 43 km the upper crustal layer 5 5 to 6 0 km s is thought to be the thermally altered equivalent of the continental crystalline basement that is normally 15 to 20 km thick in the surrounding thermally undisturbed archean crust an interpretation from these results suggests that mafic melts from the mantle have penetrated the lower crust without significant variations in the velocity structure but produce the main source of heat that drives the volcanic and hydrothermal systems of yellowstone the high apparent attenuation and large lateral velocity variations in the upper crust are consistent with a model in which partial fractionation partial melting and metamorphism differentiate the original upper crust to produce silicic melts that were extruded as rhyolites and ash flow tuffs across the yellowstone plateau this seismic model is consistent with the evidence for a systematic northeastward propogation of silicic volcanic centers along the eastern snake river plain to their present location beneath the yellowstone hydrothermal system while these findings do not bear directly on the origin and source of the heat i e mantle lumes lithospheric fractures mantle radiogenetic heat basal lithospheric shearing etc they provide a constraint on the configuration and lateral extent of crustal layers that reflect thermal and compositional boundaries

tilt observations using borehole tiltmeters 2 analysis of data from yellowstone national park we have installed borehole tiltmeters at five sites in yellowstone national park wyoming and have used these instruments to measure the spatial variation of the amplitude and phase of the principal semidiurnal tide the measured tides vary both with position and azimuth and differ from the sum of the body tide and the ocean load by up to 50 the difference predicted by a finite element model constructed from seismic refraction and gravity data has a maximum value of only 12 although the discrep ancy between our observations and the model is only marginally significant at some sites the disagreement between the model and our observa tions is much larger than we observed using the same instruments at other sites and cannot be attributed to an instrumental effect we have been unable to modify the model to explain our results while keeping it consistent with the previous observations

constraints on the upper crustal magma reservoir beneath yellowstone caldera inferred from lake seiche induced strain observations 1 seiche waves in yellowstone lake with a 78 minute period and heights 10 cm act as a load on the solid earth observed by borehole strainmeters with subnanostrain sensitivity throughout the yellowstone caldera the far field strain induced by the load of the seiche waves calculated with a homogeneous elastic model representing the upper crust is more than an order of magnitude smaller than the measured strain amplitude 30 km from the lake shore by contrast the observed far field strain amplitudes are consistent with the seiche load on a two layered viscoelastic model representing an elastic upper crust overlying a partially molten body deeper than 3 6 km with maxwell viscosity less than 1011 pa s these strain observations and models provide independent evidence for the presence of partially molten material in the upper crust consistent with seismic tomography studies that inferred 10 30 melt fraction in the upper crust

ascent and emplacement of buoyant magma bodies in brittle ductile upper crust 1 the emplacement of silicic magma bodies in the upper crust may be controlled by density such that there is no buoyancy to drive further ascent or temperature such that surrounding rocks are too cold to deform significantly over geological timescales evidence for the latter control is provided by negative gravity anomalies over many granitic plutons conditions of diapir ascent and emplacement in this case are studied with a numerical model for deformation and heat transport allowing for ductile elastic and brittle behavior a large strain formulation is used to solve for temperature stress strain and strain rate fields as a function of time for a range of diapir sizes density contrasts and background geotherms the method allows for large viscosity contrasts of more than 6 orders of magnitude and determines the dominant deformation mechanism depending on the local values of temperature strain and strain rate emplacement depth and final deformation characteristics depend on diapir size and buoyancy small diapirs less than about 5 km in diameter cannot reach shallow crustal levels and do not involve brittle deformation in the ductile regime the diapir flattens significantly upon emplacement due to stiff roof rocks and to the free surface above late stage deformation proceeds by horizontal spreading with little upward displacement of roof rocks and is likely to be interpreted as ballooning large diapirs more than about 5 km in diameter rapidly rise to shallow depths 1 5 km and induce brittle faulting in the overlying rocks in this regime buoyancy forces may lead to faulting in roof rocks in this case late stage ascent proceeds by vertical intrusion of a plug of smaller horizontal dimensions than the main body buoyant diapirs keep on rising after solidification long after the relatively short lived high temperature magmatic stage this may account for some phases of late caldera resurgence in extinct volcanic systems

magma chamber and eruptive processes at izu oshima volcano japan buoyancy control of magma migration underground magmatic processes are modeled for a sequence of eruptions that started on november 15 1986 at izu oshima volcano it is predicted from the seismic gap the seismic velocity structure the deformation center and the gravity minimum that there should be a magma chamber at a depth of about 4 km beneath the northwestern flank of the volcano the effusion of basalt that began at the summit crater established a well connected conduit system between the magma chamber and the summit crater the following eruption from new fissures a few days later produced more felsic and less homogeneous compositions basaltic andesite to andesite because of cooling and fractional crystallization of ascending magma subsequently four small eruptions were caused by gravitational sliding of sluggish semisolid magma into openings in and along the sides of the shallow conduit as magma was returning to the magma chamber systematic tilt changes due to transmission of pressure pulses from the summit crater to the magma chamber were observed during a small explosion on november 18 1987 and for many periods of episodic volcanic tremor the migration of magma is interpreted to have been driven by a buoyancy force associated with bubbles in the magma in this regard dominant ascent of magma beneath the summit crater and new fissures in november 1986 was triggered by vesiculation of volatiles in the magma and the subsequent return flow of magma resulted from the release of bubbles through degassing the observed response of volcanic processes following earthquake swarms northwest of the island is also explained by buoyancy controlled magma migration that could intensify the effect of small fluctuations in the crustal stress

simultaneous inversion for lateral velocity variations and hypocenters in the yellowstone region using earthquake and refraction data a three dimensional upper crustal p wave velocity structure of the yellowstone region was determined by simultaneous inversion using pg refracted arrival times and earthquake generated direct p wave arrival times the velocity structure was modeled by a two layer block configuration with the velocity of the surface layer fixed the inversion was performed on a 14 4 km thick second layer 5 6 blocks using an optimal block size of 21 2 18 3 km and incorporated the method of separation of parameters results show volumes of higher velocities approximately 6 3 km s north of the yellowstone caldera that correlate with gravity anomaly highs and are interpreted to be associated with near surface crystalline basement the upper crust beneath the caldera showed an average velocity of 5 8 km s with prominent low velocity zones of 4 9 and 5 2 km s in the northeast and southwest portions of the caldera respectively the 4 9 km s low velocity layer in the northeast corner of the caldera coincides with a localized 20 mgal bouguer gravity low and the largest area of hydrothermal activity in yellowstone national park relocating earthquakes with the new velocity model showed a systematic migration of epicenters radially toward the caldera by an average of 1 2 km relative to locations using a homogeneous velocity model

hydrothermal petroleums from yellowstone national park wyoming u s a abstract in yellowstone national park oil is actively discharging with thermal waters at several widely separated thermal areas samples from two locations were studied petroleum from calcite springs occurs as vapor condensates in steam vents detailed geochemical analyses show a complex origin for the petroleum fluid deeply buried hydrothermally altered sedimentary rocks are believed to have contributed a high temperature pyrolysate consisting largely of polynuclear aromatic hydrocarbons and heterocyclic compounds shallow sedimentary rocks which are also hydrothermally altered have contributed large amounts of aromatic and polar compounds as well as minor amounts of hydrocarbons the permian phosphoria formation is suggested as a likely source rock for the bulk of the oils autochthonous pyrolysates and lipids from thermally altered surface and near surface bacterial and land plant debris have also been introduced to the vents the latter may have been introduced by downward percolating meteoric water the oil from rainbow springs is a highly paraffinic low s crude oil depleted in light hydrocarbons unlike the calcite springs samples the oil from rainbow springs is believed to have originated from a single sedimentary source rock with little or no hydrocarbon contributions from surface biota the oil contains a unique molecular marker assemblage that suggests both gymnosperm and angiosperm input and places the age of the source unit in the tertiary consideration of the local stratigraphy and water temperatures limits the probable source zone to the eocene aycross formation an intra volcanic lacustrine mudstone calculation of deep water temperatures indicates that this unit is exposed to thermal waters in an extensive near surface aquifer such conditions are consistent with the widespread surface manifestations and biomarker kinetic models consideration of the petroleum chemistry and regional hydrodynamics indicates that the yellowstone park oils did not migrate into the park from distant sources the chemical data are also inconsistent with the oils being generated and trapped prior to tertiary volcanism at both calcite springs and rainbow springs oil generation appears to be contemporaneous and is attributed to thermal alteration of relatively shallow sedimentary rocks by hydrothermal fluids

crust and upper mantle p wave velocity structure beneath valles caldera new mexico results from the jemez teleseismic tomography experiment new results are presented from the teleseismic component of the jemez tomography experiment conducted across valles caldera in northern new mexico we invert 4872 relative ital p wave arrival times recorded on 50 portable stations to determine velocity structure to depths of 40 km the three principle features of our model for valles caldera are 1 near surface low velocities of minus 17 percent beneath the toledo embayment and the valle grande 2 midcrustal low velocities of minus 23 percent in an ellipsoidal volume underneath the northwest quadrant of the caldera and 3 a broad zone of low velocities minus 15 percent in the lower crust or upper mantle crust shallower than 20 km is generally fast to the northwest of the caldera and slow to the southeast near surface low velocities are interpreted as thick deposits of bandelier tuff and postcaldera volcaniclastic rocks lateral variation in the thickness of these deposits supports increased caldera collapse to the southeast beneath the valle grande we interpret the midcrustal low velocity zone to contain a minimum melt fraction of 10 percent while we cannot rule out the possibility that this zone is the remnant 1 2 ma bandelier magma chamber the eruption history and geochemistry of the volcanic rocks erupted in valles calderamore following the bandelier tuff make it more likely that magma results from a new pulse of intrusion indicating that melt flux into the upper crust beneath valles caldera continues the low velocity zone near the crust mantle boundary is consistent with either partial melt in the lower crust or mafic rocks without partial melt in the upper mantle in either case this low velocity anomaly indicates that underplating by mantle derived melts has occurred copyright 1998 american geophysical union less

geothermal gas compositions in yellowstone national park usa abstract gas samples collected between 1974 and 1986 have been analysed for the ten major components samples have been collected almost exclusively from the tops of pools which has degraded the value of the data and limited inter comparisons to the relatively insoluble components ar n 2 ch 4 h 2 and he a general gas distribution pattern in the park in terms of these components shows the major heat source s to underlie the gibbon and mud volcano areas with all other geothermal areas having gas compositions consistent with a general north south water flow shoshone basin gases show a large range of compositions and these are analysed in detail the patterns conform to that which would be expected from an east west flow or fluid with progressive boiling and subsequent dilution

temperature dependent variation in the distribution of tetraether membrane lipids of marine crenarchaeota implications for tex86 paleothermometry 1 recently a new geochemical temperature proxy the tex86 was introduced this proxy is based on the number of cyclopentane moieties in the glycerol dialkyl glycerol tetraethers gdgts of the membrane lipids of marine crenarchaeota which changes as a response to temperature however until now only sediment data have been used to establish this proxy and experimental work is missing we performed mesocosm studies with marine crenarchaeota incubated at temperatures ranging from 5 to 35 c and salinities of 27 and 35 to test the validity of the tex86 proxy growth of marine crenarchaeota in these mesocosms was evident from the substantial increase in the concentration of marine crenarchaeotal membrane lipids with amounts up to 3400 ng l with increasing temperature an increase in the number of cyclopentane moieties in the crenarchaeotal membrane lipids was observed different salinities did not show any effect on the gdgt distribution the tex86 showed a significant linear correlation to incubation temperature tex86 0 015 t 0 10 r2 0 79 this equation has a similar slope to the correlation obtained from core tops but differs in the intersection tex86 0 015 t 0 28 r2 0 92 this difference is mainly determined by the smaller amount of the regioisomer of crenarchaeol in the incubation series compared to core top samples these incubation experiments indicates that water temperature is indeed the major controlling factor for the membrane distribution of marine crenarchaeota and confirms that the tex86 proxy depends on a physiological response to regulate membrane fluidity

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

vp vs ratios in the yellowstone national park region wyoming abstract in this paper we study the variation of v p v s and poisson s ratio in the yellowstone national park region using earthquakes which were well recorded by a local seismic network we find that the average v p v s value within the geothermally active yellowstone caldera is about 7 lower than in the area outside the caldera within the caldera itself there may be a further 2 7 reduction of v p v s in the hydrothermally active norris geyser basin the upper and lower geyser basins and the yellowstone lake and mud volcano regions after considering various possible causes for v p v s changes such as geologic and structural differences thermal effects partial melting and hydrothermal activity we conclude that the most plausible explanation for the observed v p v s reduction is the presence of hot water at temperatures and pore pressures near the water steam transition in the caldera geothermal reservoirs