deformation of the long valley caldera eastern california from mid 1983 to mid 1988 measurements using a two color geodimeter deformation of long valley caldera in eastern california has been closely monitored since june 1983 with a two color geodimeter which has a precision between 0 12 and 0 20 ppm of the baseline length initial trilateration measurements were concentrated in the south moat of the caldera the source of the january 1983 earthquake swarm since then the network has been expanded from 13 to 42 baselines the additional baselines span the resurgent dome and the western half of the caldera with a combination of measurements made several times per week on 7 11 baselines and monthly or yearly measurements on the remaining baselines we have measured both the temporal and spatial deformation of the caldera although the m 6 4 chalfant valley earthquake was located 35 km from the nearest baseline in the network we observed coseismic strain changes of up to 1 5 ppm on several baselines using fault planes selected to coincide with the hypo centers of the main shock and the m 5 9 foreshock we have inferred a combined moment of 3 0 1025 dyn cm from the two color data this cumulative moment of both earthquakes is equivalent to a magnitude 6 3 shock over the period from mid 1983 to mid 1988 the long term trends determined from the length changes of the baselines within the caldera show a general decrease in the rate of extension during 1983 measurements show a maximum extension rate of 5 0 ppm yr after mid to late 1984 the extension rates decreased by a factor of 2 4 relative to those in the first year with the exception of the coseismic step the extension rates have been fairly constant during the interval between late 1984 and mid 1988 the long term changes in rate can be modeled by using two mogi point sources representing inflation beneath the resurgent dome of the caldera and a dislocation surface representing slip in the south moat modeling indicates that both the rates of slip and inflation have decreased since mid 1983 but these two processes are still occurring at a measureable and constant rate

co2 diffuse emission from maar lake an example in changbai volcanic field ne china abstract numerous maars and monogenetic volcanic cones are distributed in northeast china which are related to westward deep subduction of the pacific ocean lithosphere comprising a significant part of the pacific ring of fire it is well known that diffuse co2 emissions from monogenetic volcanoes including wet e g maar lake and dry degassing systems e g soil diffuse emission fault degassing etc may contribute to budget of globally nature derived greenhouse gases however their relationship between wet e g maar lake and concomitant dry degassing systems e g soil diffuse emission fault degassing etc related to monogenetic volcanic field is poorly understood yuanchi maar one of the typical monogenetic volcanic systems is located on the eastern flank of tianchi caldera in changbai volcanic field of northeast china which displays all of three forms of co2 degassing including the maar lake soil micro seepage and fault degassing measurements of efflux of co2 diffusion from the yuanchi maar system yms indicate that the average values of co2 emissions from soil micro seepage fault degassing and water air interface diffusion are 24 3 23 3 g m 2 d 1 39 2 22 4 g m 2 d 1 and 2 4 1 1 g m 2 d 1 respectively the minimum output of co2 diffuse emission from the yms to the atmosphere is about 176 1 88 3 ton yr of which 80 4 results from the dry degassing system degassing from the fault contributes to the most of co2 emissions in all of the three forms of degassing in the yms contributions of mantle crust air and organic co2 to the soil gas are 0 01 0 10 10 20 32 36 and 48 54 respectively which are quantitatively constrained by a he c isotope coupling calculation model we propose that co2 exsolves from the upper mantle melting beneath the tianchi caldera which migrates to the crustal magma chamber and further transports to the surface of yms along the deep fault system during the transportation processes the emission of gas experiences crustal contamination influence of magma chamber beneath the yms sub surface processes and air dilution

dynamics of carbon dioxide emission at mammoth mountain california mammoth mountain a dormant volcano in the eastern sierra nevada california has been passively degassing large quantities of cold magmatic co2 since 1990 following a 6 month long earthquake swarm associated with a shallow magmatic intrusion in 1989 a search for any link between gas discharge and volcanic hazard at this popular recreation area led us to initiate a detailed study of the degassing process in 1997 our continuous monitoring results elucidate some of the physical controls that influence dynamics in flank co2 degassing at this volcano high coherence between variations in co2 efflux and variations in atmospheric pressure and wind speed imply that meteorological parameters account for much if not all of the variability in co2 efflux rates our results help explain differences among previously published estimates of co2 efflux at mammoth mountain and indicate that the long term annual co2 degassing rate has in fact remained constant since 1997 discounting the possibility of large meteorologically driven temporal variations in gas efflux at other volcanoes may result in spurious interpretations of transients that do not reflect actual geologic processes

correlated increase in co2 fumarolic content and diffuse emission from la fossa crater vulcano italy evidence of volcanic unrest or increasing gas release from a stationary deep magma body 1 the peculiarity of the quiescent la fossa volcano is the occurrence of crises characterized by strong increases of fumarole t and output and by chemical changes indicative of an increasing input of magmatic fluids several surveys carried out during a new crisis began in november 2004 indicate that the total diffuse co2 emission for the crater area increases by one order of magnitude during crises up to 1600 ton d 1 in december 2005 concern exists on the possibility that these crises be related to an unrest process leading to eruption the repetition along decades of the same gas compositional variations during crises their temporal coincidence with increases of the local shallow seismicity and the lack of any significant ground motion rather suggest that they correspond to moments of increasing volatile release from a stationary magma system

fluid flow in the resurgent dome of long valley caldera implications from thermal data and deep electrical sounding abstract temperatures of 100 c are measured at 3 km depth in a well located on the resurgent dome in the center of long valley caldera california despite an assumed 800 c magma chamber at 6 8 km depth local downflow of cold meteoric water as a process for cooling the resurgent dome is ruled out by a peclet number analysis of temperature logs these analyses reveal zones with fluid circulation at the upper and lower boundaries of the bishop tuff and an upflow zone in the metasedimentary rocks vertical darcy velocities range from 10 to 70 cm a 1 a 21 km long geoelectrical profile across the caldera provides resistivity values to the order of 10 0 to 10 3 m down to a depth of 6 km as well as variations of self potential interpretation of the electrical data with respect to hydrothermal fluid movement confirms that there is no downflow beneath the resurgent dome to explain the unexpectedly low temperatures in the resurgent dome we challenge the common view that the caldera as a whole is a regime of high temperatures and the resurgent dome is a local cold anomaly instead we suggest that the caldera was cooled to normal thermal conditions by vigorous hydrothermal activity in the past and that a present day hot water flow system is responsible for local hot anomalies such as hot creek and the area of the casa diablo geothermal power plant the source of hot water has been associated with recent shallow intrusions into the west moat the focus of planning for future power plants should be to locate this present day flow system instead of relying on heat from the old magma chamber

a diagenesis model for geomechanical simulations formulation and implications for pore pressure and development of geological structures forward basin modelling is routinely used in many geological applications with the critical limitation that chemical diagenetic reactions are often neglected or poorly represented here a new temperature dependent kinetic diagenesis model is formulated and implemented within a hydromechanical framework the model simulates the macroscopic effects of diagenesis on 1 porosity loss 2 sediment strength 3 sediment stiffness and compressibility 4 change in elastic properties 5 increase in tensile strength due to cementation and 6 overpressure generation a brief overview of the main diagenetic reactions relevant to basin modelling is presented and the model calibration procedure is demonstrated using published data for the kimmeridge clay formation the calibrated model is used to show the implications of diagenesis on prediction of overpressure development and structural deformation the incorporation of diagenesis in a uniaxial burial model results in an increase in overpressure of up to 9 mpa due to both stress independent porosity loss and overpressure generated by disequilibrium compaction caused by a reduction in permeability finally a compressional model is used to show that the incorporation of diagenesis within geomechanical models allows the transition from ductile to brittle behaviour to be captured due to the increase in strength that results in an over consolidated stress state this is illustrated by comparison of the present day structures predicted by a geomechanical only model where a ductile fold forms and a geomechanical model accounting for diagenesis in which a brittle thrust structure is predicted

carbon dioxide emissions from vegetation kill zones around the resurgent dome of long valley caldera eastern california usa abstract a survey of diffuse co2 efflux soil temperature and soil gas chemistry over areas of localized vegetation kill on and around the resurgent dome of long valley caldera california was performed to evaluate the premise that gaseous and thermal anomalies are related to renewed intrusion of magma some kill sites are long lived features and others have developed in the past few years total anomalous co2 emissions from the thirteen areas average around 8 7 t per day but the majority of the emissions come from four sites west of the casa diablo geothermal power plant geochemical analyses of the soil gases from locations west and east of the plant revealed the presence of isobutane related to plant operations the 13c values of diffuse co2 range from 5 7 to 3 4 similar to values previously reported for co2 from hot springs and thermal wells around long valley at many of the vegetation kill sites soil temperatures reach boiling at depths 20 cm soil temperature depth profiles at two of the high emissions areas indicate that the conductive thermal gradient in the center of the areas is around 320 c m 1 we estimate total heat loss from the two areas to be about 6 1 and 2 3 mw given current thinking on the rate of hydrothermal fluid flow across the caldera and using the co2 concentration in the thermal fluids the heat and co2 loss from the kill areas is easily provided by the shallow hydrothermal system which is sourced to the west of the resurgent dome we find no evidence that the development of new areas of vegetation kill across the resurgent dome are related to new input of magma or magmatic fluids from beneath the resurgent dome our findings indicate that the areas have developed as a response to changes in the shallow hydrologic system some of the changes are likely related to fluid production at the power plant but at distal sites the changes are more likely related to seismicity and uplift of the dome

near real time field measurements of δ 13 c in co 2 from volcanoes this paper describes the operation and application of a portable cavity ring down spectrometer crds designed to measure the isotopic composition of carbon dioxide the instrument is capable of measuring 13c for co2 concentrations ranging from atmospheric 400 ppm to 100 at precisions and accuracies that are comparable to laboratory based gas source mass spectrometers this flexibility and portability are ideal for applications on active volcanoes and it is now possible to obtain isotopic measurements on a near real time basis we show applications of the crds for soil gases on volcanoes and in calderas for characterizing the isotopic composition of a volcanic plume and for measuring the temporal variability of 13c in the atmosphere future directions hold the potential to use volcanic gas isotopes for monitoring purposes and to combine different isotopic systems to reveal the source or sources of gas

predictive modelling of structure evolution in sandbox experiments in this paper a computational approach is presented that is able to forward model complex structural evolution with multiple intersecting faults that exhibit large relative movement the approach adopts the lagrangian method complemented by robust and efficient automated adaptive meshing techniques a constitutive model based on critical state concepts and global energy dissipation regularized by inclusion of fracture energy in the equations governing state variable evolution the efficacy of the approach is benchmarked by forward simulation of two extensional analogue experiments that exhibit the development of a roll over anticline with a series of superimposed crestal collapse graben systems these sandbox experiments are excellent benchmarks for computational models as both intersecting localisations with different rates of slip and large relative movement on localisations must be represented while other complex phenomena associated with structural evolution over geological timeframes are not present

structural styles in the deep water fold and thrust belts of the niger delta the deep water niger delta includes two large fold and thrust belts products of contraction caused by gravity driven extension on the shelf that exhibit complex styles of thrusting these contractional structures formed above multiple detachment levels in the overpressured shales of the akata formation using the patterns of growth sedimentation fold shapes fault plane seismic reflections and combined conventional and shear fault bend folding theories we describe and model the structural styles and kinematics of the fault related folds and imbricate thrust systems that compose these belts individual fault related folds involving both forethrusts and backthrusts are characterized by long planar backlimbs that dip less than the associated fault ramps with upward shallowing of dips in growth strata above the backlimbs suggesting components of progressive limb rotation forelimbs are short compared to backlimbs but growth strata show more consistent dips that suggest a component of folding by kink band migration thus we employ a combination of classic and shear fault bend fold theories to describe these structures including the influence of a weak basal detachment zone in the overpressured shales we expand upon these theories to model the kinematics of imbricate thrust systems which display a complex history of thrusting related to spatial and temporal variations in deposition across the delta regional patterns of folded growth strata are used to define break forward break backward and coeval thrusting involving single and multiple detachment levels we define two main types of imbricate thrust systems type i system with a single basal detachment level and type ii imbricate system with multiple basal detachment levels which cause massive structural thickening of the akata formation and refolding of shallow thrust sheets through the sequential restoration of two regional cross sections across these systems we resolve the structural styles the timing and sequences of thrusting as well as the regional amounts of shortening all of which have important implications for hydrocarbon maturation and charge in the deep water niger delta

caldera forming processes and the origin of submarine volcanogenic massive sulfide deposits certain volcanogenic massive sulfide vms ore deposits form in submarine calderas this association is well known but the link between caldera formation and the origins of the deposits remains poorly understood here we show that the size and location of a vms deposit within a submarine caldera may be determined by how and when the caldera formed these spatial temporal conditions control development of the hydrothermal system associated with the vms deposit we propose that caldera opening along outward dipping faults promotes magma degassing seawater influx and high temperature leaching resulting in a metal rich hydrothermal fluid these outward dipping faults are considered to provide critical pathways for ore forming fluids responsible for some caldera hosted vms deposits and may also be fundamentally important for the formation of many other caldera hosted ore deposit types

three year decline of magmatic co2 emissions from soils of a mammoth mountain tree kill horseshoe lake ca 1995 1997 we used the closed chamber method to measure soil co2 efflux over a three year period at the horseshoe lake tree kill hltk the largest tree kill on mammoth mountain in central eastern california efflux contour maps show a significant decline in the areas and rates of co2 emission from 1995 to 1997 the emission rate fell from 350 t d 1 metric tons per day in 1995 to 130 t d 1 in 1997 the trend suggests a return to background soil co2 efflux levels by early to mid 1999 and may reflect exhaustion of co2 in a deep reservoir of accumulated gas and or mechanical closure or sealing of fault conduits transmitting gas to the surface however emissions rose to 220 t d 1 on 23 september 1997 at the onset of a degassing event that lasted until 5 december 1997 recent reservoir recharge and or extension enhanced gas flow may have caused the degassing event

summary of recent research in long valley caldera california abstract since 1978 volcanic unrest in the form of earthquakes and ground deformation has persisted in the long valley caldera and adjacent parts of the sierra nevada the papers in this special volume focus on periods of accelerated seismicity and deformation in 1980 1983 1989 1990 and 1997 1998 to delineate relations between geologic tectonic and hydrologic processes the results distinguish between earthquake sequences that result from relaxation of existing stress accumulation through brittle failure and those in which brittle failure is driven by active intrusion they also indicate that in addition to a relatively shallow 7 10 km source beneath the resurgent dome there exists a deeper 15 km source beneath the south moat analysis of microgravimety and deformation data indicates that the composition of the shallower source may involve a combination of silicic magma and hydrothermal fluid pressure and temperature fluctuations in wells have accompanied periods of crustal unrest and additional pressure and temperature changes accompanying ongoing geothermal power production have resulted in land subsidence the completion in 1998 of a 3000 m deep drill hole on the resurgent dome has provided useful information on present and past periods of circulation of water at temperatures of 100 200 c within the crystalline basement rocks that underlie the post caldera volcanics the well is now being converted to a permanent geophysical monitoring station

nested calderas and trapdoor uplift at pantelleria strait of sicily pantelleria is dominated by a caldera 6 km in diameter that formed 55 000 yr ago during eruption of a rheomorphic fall deposit that is zoned from pantellerite to trachyte trachytic lavas then built a shield that nearly filled the caldera magma subsequently reinflated the shallow reservoir and raised the northwest half of the trachyte shield 275 m creating montagna grande by trapdoor uplift the lid eventually failed and pantellerite magma erupted as domes flows and pumice cones along the trapdoor faults and their hingeline we have identified a second caldera 93 000 yr old within which the younger caldera is nested a still older structure may encircle them both replenishment of magma reservoirs beneath calderas can promote dilatant reactivation of older subsidence structures resulting in multicyclic collapses that reuse these structures and produce nested calderas

surface heat flow and co2 emissions within the ohaaki hydrothermal field taupo volcanic zone new zealand abstract carbon dioxide emissions and heat flow have been determined from the ohaaki hydrothermal field taupo volcanic zone tvz new zealand following 20 a of production 116 mw e soil co 2 degassing was quantified with 2663 co 2 flux measurements using the accumulation chamber method and 2563 soil temperatures were measured and converted to equivalent heat flow w m 2 using published soil temperature heat flow functions both co 2 flux and heat flow were analysed statistically and then modelled using 500 sequential gaussian simulations forty subsoil co 2 gas samples were also analysed for stable c isotopes following 20 a of production current co 2 emissions equated to 111 6 7 t d observed heat flow was 70 6 4 mw compared with a pre production value of 122 mw this 52 mw reduction in surface heat flow is due to production induced drying up of all alkali cl outflows 61 5 mw and steam heated pools 8 6 mw within the ohaaki west thermal area ohw the drying up of all alkali cl outflows at ohaaki means that the soil zone is now the major natural pathway of heat release from the high temperature reservoir on the other hand a net gain in thermal ground heat flow of 18 mw from 25 mw to 43 3 5 mw at ohw is associated with permeability increases resulting from surface unit fracturing by production induced ground subsidence the ohaaki east ohe thermal area showed no change in distribution of shallow and deep soil temperature contours despite 20 a of production with an observed heat flow of 26 7 3 mw and a co 2 emission rate of 39 3 t d the negligible change in the thermal status of the ohe thermal area is attributed to the low permeability of the reservoir beneath this area which has limited production mass extraction and sheltered the area from the pressure decline within the main reservoir chemistry suggests that although alkali cl outflows once contributed significantly to the natural surface heat flow 50 they contributed little 2 emissions due to the loss of 99 of the original co 2 content due to depressurisation and boiling as the fluids ascended to the surface consequently the soil has persisted as the major 99 pathway of co 2 release to the atmosphere from the high temperature reservoir at ohaaki the co 2 flux and heat flow surveys indicate that despite 20 a of production the variability in location spatial extent and magnitude of co 2 flux remains consistent with established geochemical and geophysical models of the ohaaki field at both ohw and ohe carbon isotopic analyses of soil gas indicate a two stage fractionation process for moderate flux 60 g m 2 d 1 sites boiling during fluid ascent within the underlying reservoir and isotopic enrichment as co 2 diffuses through porous media of the soil zone for high flux sites 300 g m 2 d 1 the 13 co 2 signature 7 4 0 3 ohw and 6 5 0 6 ohe is unaffected by near surface soil zone fractionation processes and reflects the composition of the boiled magmatic co 2 source for each respective upflow flux thresholds of 2 d 1 for purely diffusive gas transport between 30 and 300 g m 2 d 1 for combined diffusive advective transport and 300 g m 2 d 1 for purely advective gas transport at ohaaki were assigned 13 co 2 values and cumulative probability plots of co 2 flux data both identified a threshold of 15 g m 2 d 1 by which background atmospheric and soil respired co 2 may be differentiated from hydrothermal co 2

a sr isotopic comparison between thermal waters rocks and hydrothermal calcites long valley caldera california abstract the 87 sr 86 sr values of thermal waters and hydrothermal calcites of the long valley caldera geothermal system are more radiogenic than those of young intracaldera volcanic rocks five thermal waters display 87 sr 86 sr of 0 7081 0 7078 but show systematically lighter values from west to east in the direction of lateral flow we believe the decrease in ratio from west to east signifies increased interaction of deeply circulating thermal water with relatively fresh volcanic rocks filling the caldera depression all types of pre syn and post caldera volcanic rocks in the west and central caldera have 87 sr 86 sr m between about 0 7060 and 0 7072 and values for sierra nevada granodiorites adjacent to the caldera are similar sierran pre intrusive metavolcanic and metasedimentary rocks can have considerably higher sr isotope ratios 0 7061 0 7246 and 0 7090 0 7250 respectively hydrothermally altered volcanic rocks inside the caldera have 87 sr 86 sr m slightly heavier than their fresh volcanic equivalents and hydrothermal calcites 0 7068 0 7105 occupy a midrange of values between the volcanic plutonic rocks and the sierran metamorphic rocks these data indicate that the long valley geothermal reservoir is first equilibrated in a basement complex that contains at least some metasedimentary rocks reequilibration of sr isotope ratios to lower values occurs in thermal waters as convecting geothermal fluids flow through the isotopically lighter volcanic rocks of the caldera fill

critical state finite element models of contractional fault related folding part 1 structural analysis abstract fourteen lagrangian finite element models with a critical state mechanics constitutive model illustrate some of the primary controls on the formation of fault propagation fault bend and diffuse folds the models demonstrate how variable mechanical stratigraphy initial fault dip and inter layer detachments affect the way faults propagate and thus exert a significant control on resultant fold layer geometry for example models of uniform sandstone properties exhibit efficient strain localization and clear patterns of fault tip propagation uniform shale properties tend to inhibit fault propagation due to distributed plastic deformation models with mixed inter layered sandstone and shale deform in a disharmonic manner resembling lobate cuspate arrangements that are common to many folds observed in outcrop detachments accommodate shortening by bed parallel slip resulting in fault bend fold kinematics and poorly expressed fault propagation across layers structural analysis of the numerical model results reveals that contractional deformation is a composite of lateral compaction pure shear shortening fault propagation along narrowly localized zones of reverse shear and flexure of layers the relative proportions of these shortening components vary in time and with mechanical properties shale vs sandstone depth to detachment calculations performed on selected numerical models suggest that reasonably accurate predictions can be made for detachment fold thrust belts and toe of slope contractional systems however our study suggests that applications to mid crustal level detachments within the basement beneath a sedimentary cover may be inaccurate

mechanical and chemical compaction model for sedimentary basin simulators abstract this article presents a sediment compaction model for sedimentary basin simulators the concepts previously used in sedimentary basin models are generalized and described in our model based on the formalism specific to rock and soil mechanics sediment compaction is described on a geological time scale by an elastoplastic model in which the elastic modulus and the strain hardening modulus increase when deformation increases the plastic limit is the maximum vertical effective stress reached by the sediment the rheology of the sediment is defined by a relationship that couples the porosity or volume of the sediment with the vertical effective stress assuming uniaxial deformation the model also incorporates a viscoplastic term in the compaction equation this component macroscopically considers viscous compaction phenomena such as pressure solution the viscosity coefficient is considered to be a function of the temperature some theoretical considerations allow us to conclude that the thermal dependency of the viscosity is given with an arrhenius law in which the activation energy ranges from 20 kj mole to 50 kj mole using viscosity coefficients extrapolated from previous laboratory experiments a sensitivity study shows significant effects of viscous deformation on the compaction of basins older than 1 ma in another study the viscosity coefficient is determined by matching the results of numerical simulations with laboratory and borehole data obtained from literature for chalk a constant viscosity coefficient of 2 5 gpa ma 8 1022 pa s has been determined assuming viscosity as a function of temperature with an activation energy of 40 kj mole chalk viscosity at 15 c is calibrated around 25 gpa ma simulations with different thermal gradients show that porosity is a function of the temperature furthermore simulations covering different lengths of time show that porosity is also a function of time

assessing the implications of tectonic compaction on pore pressure using a coupled geomechanical approach abstract overpressure prediction in tectonic environments is a challenging topic the available pore pressure prediction methods are designed to work in environments where compaction is mostly one dimensional and driven by the vertical effective stress applied by the overburden furthermore the impact of tectonic deformation on stresses porosity and overpressure is still poorly understood we use a novel methodology to capture the true compaction phenomena occurring in an evolving 3d stress regime by integrating a fully coupled geomechanical approach with a critical state constitutive model to this end numerical models consisting of 2d plane strain clay columns are developed to account for compaction and overpressure generation during sedimentation and tectonic activity we demonstrate that a high deviatoric stress is generated in compressional tectonic basins resulting in a substantial decrease in porosity with continuing overpressure increase the overpressure predictions from our numerical models are then compared to those estimated by the equivalent depth method edm in order to quantify the error induced when using classical approaches based on vertical effective stress in tectonic environments the stress paths presented here reveal that a deviation from the uniaxial burial trend can substantially reduce the accuracy of the edm overpressure predictions

critical state finite element models of contractional fault related folding part 2 mechanical analysis abstract finite element modeling of contractional fault related folding with critical state mechanics concepts reveals fundamentally different behaviors at two levels depending on 1 mechanical stratigraphy and 2 initial fault geometry deviatoric plastic strain rate and strain patterns show that strong layers like normally consolidated sandstone or overconsolidated shale develop strain localization in form of narrow plastic shear bands soft layers like normally consolidated shale fail to develop localizations and respond by diffuse distributed plastic strain inter layered overburdens display both deformation modes models with listric thrust fault seeds develop basement cored anticlines in the hangingwall that are characterized by locally tensile stress regimes within overall compressive regional stress fields the anticline crests of these models show evidence for strain softening by contrast anticlines emerging in detached fault related fold models are subject to locally compressive stress regimes consistent with the applied far field shortening these anticline crests strain harden during deformation stresses resulting from the finite element models deviate from stress estimates based on integrated densities the discrepancy is small for basement involved models but significant for detached style models suggesting that stress predictions based on integrated densities may be inaccurate for contractional systems first order predictions of plastic deformation suggest that footwall synclines in basement involved fold styles may host reservoirs with relatively poor quality owing to horizontal and vertical compaction anticlines found in detached deformation styles are bounded by compactive domains whereas the crestal regions are less affected by volumetric plastic deformation thus producing relatively better reservoir quality

structural controls on the emission of magmatic carbon dioxide gas long valley caldera usa we present a degassing study of long valley caldera that explores the structural controls upon emissions of magmatic carbon dioxide gas a total of 223 soil gas samples were collected and analyzed for stable carbon isotopes using a field portable cavity ring down spectrometer this novel technique is flexible accurate and provides sampling feedback on a daily basis sampling sites included major and minor volcanic centers regional throughgoing faults caldera related structures zones of elevated seismicity and zones of past and present hydrothermal activity the classification of soil gases based on their 13c and co2 values reveals a mixing relationship among three end members atmospheric biogenic and magmatic signatures dominated by biogenic contributions 4 vol 24 are found on the caldera floor the interior of the resurgent dome and areas associated with the hilton creek and hartley springs fault systems with the introduction of the magmatic component 100 vol 4 5 samples acquire mixing and hydrothermal signatures and are spatially associated with the central caldera and mammoth mountain in particular they are concentrated along the southern margin of the resurgent dome where the interplay between resurgence related reverse faulting and a bend in the regional fault system has created a highly permeable fracture network suitable for the formation of shallow hydrothermal systems this contrasts with the south moat where despite elevated seismicity a thick sedimentary cover has formed an impermeable cap inhibiting the ascent of fluids and gases to the surface

carbon isotope systematics of turrialba volcano costa rica using a portable cavity ring down spectrometer over the past two decades activity at turrialba volcano costa rica has shifted from hydrothermal to increasingly magmatic in character with enhanced degassing and eruption potential we have conducted a survey of the 13c signatures of gases at turrialba using a portable field based crds with comparison to standard irms techniques our 13c results of the volcanic plume high temperature vents and soil gases reveal isotopic heterogeneity in the co2 gas composition at turrialba prior to its recent phase of eruptive activity the isotopic value of the regional fault system falla ariete 3 4 0 1 is in distinct contrast with the central crater gases 3 9 0 1 and the 2012 high temperature vent 4 4 0 2 an indication that spatial variability in 13c may be linked to hydrothermal transport of volcanic gases heterogeneities in the source composition or magmatic degassing isotopic values of co2 samples collected in the plume vary from 13c of 5 2 to 10 0 indicative of mixing between atmospheric co2 9 2 0 1 and a volcanic source we compare the keeling method to a traditional mixing model hyperbolic mixing curve to estimate the volcanic source composition at turrialba from the plume measurements the predicted source compositions from the keeling and hyperbolic methods 3 0 0 5 and 3 9 0 4 respectively illustrate two potential interpretations of the volcanic source at turrialba as of the 29 october 2014 turrialba has entered a new eruptive period and continued monitoring of the summit gases for 13c should be conducted to better understand the dominant processes controlling 13c fractionation at turrialba

geomechanical modeling of stress and strain evolution during contractional fault related folding abstract understanding stress states and rock mass deformation deep underground is critical to a range of endeavors including oil and gas exploration and production geothermal reservoir characterization and management and subsurface disposal of co 2 geomechanical modeling can predict the onset of failure and the type and abundance of deformation features along with the orientations and magnitudes of stresses this approach enables development of forward models that incorporate realistic mechanical stratigraphy e g including competence contrasts bed thicknesses and bedding planes include faults and bedding slip surfaces as frictional sliding interfaces reproduce the overall geometry of the fold structures of interest and allow tracking of stress and strain through the deformation history use of inelastic constitutive relationships e g elastic plastic behavior allows permanent strains to develop in response to the applied loads this ability to capture permanent deformation is superior to linear elastic models which are often used for numerical convenience but are incapable of modeling permanent deformation or predicting permanent deformation processes such as faulting fracturing and pore collapse finite element modeling results compared with field examples of a natural contractional fault related fold show that well designed geomechanical modeling can match overall fold geometries and be applied to stress fracture and subseismic fault prediction in geologic structures geomechanical modeling of this type allows stress and strain histories to be obtained throughout the model domain

geomechanical modeling of stresses adjacent to salt bodies part 1 uncoupled models we compare four approaches to geomechanical modeling of stresses adjacent to salt bodies these approaches are distinguished by their use of elastic or elastoplastic constitutive laws for sediments surrounding the salt as well as their treatment of fluid pressures in modeling we simulate total stress in an elastic medium and then subtract an assumed pore pressure after calculations are complete simulate effective stress in an elastic medium and use assumed pore pressure during calculations simulate total stress in an elastoplastic medium either ignoring pore pressure or approximating its effects by decreasing the internal friction angle and simulate effective stress in an elastoplastic medium and use assumed pore pressure during calculations to evaluate these approaches we compare stresses generated by viscoelastic stress relaxation of a salt sphere in all cases relaxation causes the salt sphere to shorten vertically and expand laterally producing extensional strains above and below the sphere and shortening against the sphere flanks deviatoric stresses are highest when sediments are assumed to be elastic whereas plastic yielding in elastoplastic models places an upper limit on deviatoric stresses that the rocks can support so stress perturbations are smaller these comparisons provide insights into stresses around salt bodies and give geoscientists a basis for evaluating and comparing stress predictions

the 1998 earthquake sequence south of long valley caldera california hints of magmatic involvement a significant episode of seismic and geodetic unrest took place at long valley caldera california beginning in the summer of 1997 activity through late may of 1998 was concentrated in and around the south moat and the south margin of the resurgent dome the sierran nevada block snb region to the south southeast remained relatively quiet until a m 5 1 event occurred there on 9 june 1998 ut a second m 5 1 event followed on 15 july ut both events were followed by appreciable aftershock sequences an additional distinct burst of activity began on 1 august 1998 the number of events in the august sequence over the first week or two was similar to the aftershock sequence of the 15 july 1998 m 5 1 event but the later sequence was not associated with any events larger than m 4 3 all of the summer 1998 snb activity was considered tectonic rather than magmatic in general the snb is considered an unlikely location for future eruptions however the august sequence an aftershock sequence without a mainshock is suggestive of a strain event larger than the cumulative seismotectonic strain release moreover a careful examination of waveforms from the august sequence reveals a small handful of events whose spectral signature is strikingly harmonic we investigate the waveforms of these events using spectral autocorrelation and empirical green s function techniques and conclude that they were most likely associated with a fluid controlled source our observations suggest that there may have been some degree of magma or magma derived fluid involvement in the 1998 snb sequence

overpressure development in sedimentary basins induced by chemo mechanical compaction of sandstones abstract mechanisms of overpressure build up resulting from chemically induced compaction are investigated by considering that intergranular pressure solution controlled by temperature and effective stresses is the dominant process of chemical deformation in sandstones the numerical simulations are performed using a thermo poro mechanical tool based on the finite element method specifically devised to deal with sedimentary basin modeling at the material level purely mechanical and chemo mechanical deformations are respectively addressed by means of plastic and viscoplastic constitutive components porosity data of the middle jurassic garn formation from the haltenbanken area of the mid norwegian continental shelf are taken as reference for calibration of the sandstone model to be used on synthetic cases of a siliciclastic basin in oedometric conditions two situations are proposed involving different depositional sequences of sandy and shaly sediments with the aim to assess the permeability effect in the numerical model the results have shown that early overpressure development in low permeability formations preserves sandstone porosities by reducing effective stresses and thus retarding pressure solution compaction whereas higher effective stresses associated with permeable depositional environments may lead to important chemo mechanical deformation resulting in low porosity sandstones and significant overpressure generation at later stages of basin history an additional case is finally analyzed to investigate the consequences of pressure solution inhibition due to diagenetic grain coating in a sandstone reservoir the simulation resulted in porosity preservation and lower overpressure values showing that a sedimentary basin submitted to an important level of chemo mechanical compaction can present substantially higher overpressure distribution than basins where this phenomenon did not take place the paper highlights the importance of integrating stresses fluid pressure and temperature to represent the porous material evolution in order to describe the geological state of sedimentary basins

geomechanical modeling of stresses adjacent to salt bodies part 2 poroelastoplasticity and coupled overpressures we use a fully coupled poroelastoplastic geomechanical model to study how stresses and pore pressures evolve in sediments bounding a spherical salt body drained analyses pore pressures remain hydrostatic demonstrate that sediments yield in response to loading by the salt which leads to a redistribution of stresses and to deformations larger than predicted by poroelastic or solid coulomb plastic models undrained analyses overpressures develop while no dissipation occurs illustrate that salt loading induces pore pressures that extend kilometers away from the salt body we also model the flow and consequent dissipation that occur in the sediments because of this undrained salt loading we show that with time the pressure field dissipates and expands the dissipation process takes millions of years which suggests that pore pressure perturbations caused by salt loading should still be present in mudstones near many salt bodies under drained conditions stress perturbations generate low minimum principal stresses above and below the salt resulting in convergence of pore pressure and minimum principal stress at these locations such conditions are challenging to drill through in undrained systems sharp drops in pore pressure may occur above and below the salt whereas both the pore pressure and the minimum principal stress rise next to the salt in contrast to previous models that do not couple changes in stress to changes in pore pressure the coupled approach presented here has the potential to predict in situ stresses and pore pressures more accurately in a wide variety of geologic settings

sources and fractionation processes influencing the isotopic distribution of h o and c in the long valley hydrothermal system california u s a abstract the isotopic ratios of h o and c in water within the long valley caldera california reflect input from sources external to the hydrothermal reservoir a decrease in d in precipitation of 0 5 km 1 from west to east across long valley is caused by the introduction of less fractionated marine moisture through a low elevation embayment in the sierra nevada mountain range relative to seasonal fluctuations in precipitation 158 to 35 d ranges in hot and cold surface and groundwaters are much less variable 135 to 105 only winter and spring moisture reflecting higher precipitation rates with lighter isotopic signatures recharge the hydrological system the hydrothermal fluids are mixtures of isotopically heavy recharge d 115 18 o 15 derived from the mammoth embayment and isotopically lighter cold water d 135 18 o 18 this cold water is not representative of current local recharge the 13 c values for dissolved carbon in hot water are significantly heavier 7 to 3 than in cold water 18 to 10 denoting a separate hydrothermal origin these 13 c values overlie the range generally attributed to magmatic degassing of co 2 however 13 c values of metamorphosed paleozoic basement carbonates surrounding long valley fall in a similar range indicating that hydrothermal decarbonization reactions are a probable source of co 2 the 13 c and 18 o values of secondary travertime and vein calcite indicate respective fractionation with co 2 and h 2 o at temperatures approximating current hydrothermal conditions

tracing and quantifying magmatic carbon discharge in cold groundwaters lessons learned from mammoth mountain usa abstract a major campaign to quantify the magmatic carbon discharge in cold groundwaters around mammoth mountain volcano in eastern california was carried out from 1996 to 1999 the total water flow from all sampled cold springs was 1 8 10 7 m 3 yr draining an area that receives an estimated 2 5 10 7 m 3 yr of recharge suggesting that sample coverage of the groundwater system was essentially complete some of the waters contain magmatic helium with 3 he 4 he ratios as high as 4 5 times the atmospheric ratio and a magmatic component in the dissolved inorganic carbon dic can be identified in virtually every feature sampled many waters have a 14 c of 0 5 pmc a 13 c near 5 and contain high concentrations 20 50 mmol l of co 2 aq but are otherwise dilute specific conductance 100 300 s cm with low ph values between 5 and 6 such waters have previously escaped notice at mammoth mountain and possibly at many other volcanoes because co 2 is rapidly lost to the air as the water flows away from the springs leaving neutral ph waters containing only 1 3 mmol l hco 3 the total discharge of magmatic carbon in the cold groundwater system at mammoth mountain is 20 000 t yr as co 2 ranging seasonally from about 30 to 90 t day several types of evidence show that this high discharge of magmatic dic arose in part because of shallow dike intrusion in 1989 but also demonstrate that a long term discharge possibly half this magnitude 10 000 t yr predated that intrusion to sustain a 10 000 t yr dic discharge would require a magma intrusion rate of 0 057 km 3 per century assuming complete degassing of magma with 0 65 wt co 2 and a density of 2 7 t m 3 the geochemical data also identify a small

compaction model for quartzose sandstones application to the garn formation haltenbanken mid norwegian continental shelf abstract the compaction of quartzose sandstones is described by a macroscopical visco elasto plastic model derived from microscopical considerations this model considers that the reduction of total porosity the relative pore volume is related to the changes in the relative cemented volume and the relative intergrain volume furthermore we consider that the variations of these two petrophysical parameters result from two physical mechanisms mechanical compaction and closed system chemical compaction the model has been calibrated for the middle jurassic garn formation from the haltenbanken area of the mid norwegian continental shelf around 300 data points from 17 wells have been used in this study the viscous parameters have been optimised against the present day data with total porosity data the best fit is obtained for an activation energy between 14 and 16 kj mole and a macroscopical viscosity at 15 c between 39 and 46 gpa ma 1 210 24 1 410 24 pa s if quartz cement data are used the best fit is obtained for an activation energy between 16 and 18 kj mole and a macroscopical viscosity at 15 c between 47 and 56 gpa ma 1 510 24 17 10 24 pa s the visco elasto plastic compaction model has been tested in the haltenbanken area of mid norway with the result that observed porosity trends and pore pressures can be modelled using multiple 2 d basin modelling simulations the results of this model and its implications for fluid flow and petroleum systems analysis can be seen the compaction method has significant impact on the simulated fluid flow and petroleum migration pattern

helium isotope and gas discharge variations associated with crustal unrest in long valley caldera california 1989 1992 the onset of anomalous seismic activity in 1989 beneath mammoth mountain on the southwestern rim of the long valley caldera california was followed within approximately 4 months by a large increase in sup 3 he sup 4 he in vapor discharged from a fumarole on the north side of the mountain the helium isotopic ratio at this vent rose to a maximum of 6 7 r sub a in july 1990 and subsequently declined to values near 5 r sub a potential sources of the sup 3 he rich vapors include degassing of fresh magma degassing from fresh surfaces generated in newly fractured igneous rocks and volatile release from a sup 3 he rich gas chamber situated above previously emplaced intrusives the magnitude of the increase in helium isotopic composition from 3 8 to 6 7 r sub a the persistence of relatively high values 5 r sub a over a period of 3 years the increase in the flux of total he relative to gases in air saturated water and the increases in the rates of discharge of steam and gas from this fumarole indicate that magmatic intrusion did in fact begin in 1989 beneath mammoth mountain seismic activity and limited measurements of extensional deformation at the surface suggest that the depth of intrusion maymore be as shallow as 2 km consistent with the prompt appearance of increased sup 3 he sup 4 he ratios in the fumarolic gas and that the intrusive process may have persisted for approximately 1 year in contrast a similar combination of magmatic intrusion and anomalous seismic activity beneath the resurgent dome south moat region during the 1989 1991 period resulted in at most relatively small changes in sup 3 he sup 4 he in fumarolic discharge at the southern edge of the resurgent dome the more subdued response may result from a combination of greater intrusive depths and greater dilution of sup 3 he rich inputs to thermal fluid reservoirs in the shallow hydrothermal system in this area compared with mammoth mountain 60 refs 11 figs 4 tabs less

predicting pore pressure in active fold thrust systems an empirical model for the deepwater sabah foldbelt abstract measurements related to mudrock shale and siltstone porosity such as acoustic velocity density or electrical resistivity have traditionally been used to predict pore pressures in extensional stress settings the underlying assumption is that burial and vertical effective stress ves which is the overburden minus the pore pressure controls the compaction of these rocks through porosity loss the dataset presented here compares ves and acoustic velocity of similar composition mudrocks in both an extensional and a compressional stress setting in the extensional stress environment the mudrocks follow a typical compaction trend with a porosity loss and increase in acoustic velocity that can be related to ves in an active fold thrust belt the compressive stresses further reduce the porosity and increase the acoustic velocity of the mudrocks first a layer parallel shortening compacts sediments beyond what is observed for the ves this additional compaction is further enhanced near thrust faults and in anticlinal forelimbs presumably due to additional shear stress in these areas the mudrocks located in folds that are buried by additional sedimentation do not compact again until the tectonic compaction is overridden by enough new burial after that the mudrocks follow the observed extensional setting compaction trend in the fold thrust belt the observed reduction in porosity by stresses other than burial leads to an under prediction of pore pressure using traditional methods to account for this we present a correction that can be applied to the acoustic velocity or porosity using two parameters a proximity to thrust faults and anticlinal forelimbs and b the amount of burial after fold formation with these corrections the extensional velocity ves compaction trend can be used to accurately predict pore pressure within the active fold thrust belt the correction is calibrated with well data and is empirical none the less it is a first step toward understanding the magnitudes of the compressive lateral stresses involved in generating the observed porosity loss within the fold thrust belt

co2 rich gases from lakes nyos and monoun cameroon laacher see germany dieng indonesia and mt gambier australia variations on a common theme abstract helium r a 3 0 to 5 6 and carbon 13 c from 7 2 to 3 4 isotopic compositions and relative co 2 ch 4 n 2 he and ar contents of co 2 rich gases from lakes nyos and monoun cameroon laacher see germany dieng volcanic plateau indonesia and a well at mt gambier australia point to a common essentially magmatic origin absorption of the original magmatic gases into deeply circulating groundwater and equilibration of the resulting solutions with crustal rock at temperatures of about 300 c fix co 2 and ch 4 contents on further rise the solutions start to boil separating gas rich vapors which on encountering an impermeable barrier may accumulate to form gas pockets with steadily increasing pressures in the case of sufficiently high gas contents the pressures may exceed lithostatic pressures leading to a blow out or a pneumatic eruption dieng otherwise gas may accumulate to form a stable pocket mt gambier minor leakage from such pockets may lead to surface discharges of co 2 rich gases as at laacher see re absorption into shallow groundwater to the formation of the low salinity co 2 charged waters encountered at lakes nyos and monoun the occurrence of these high co 2 low temperature systems is likely to be favored in tectonically active regions allowing deep possibly mantle gases to rise but with sufficiently low regional heat flows to prevent the establishment of large scale geothermal activity

inferences on the hydrothermal system beneath the resurgent dome in long valley caldera east central california usa from recent pumping tests and geochemical sampling abstract quaternary volcanic unrest has provided heat for episodic hydrothermal circulation in the long valley caldera including the present day hydrothermal system which has been active over the past 40 kyr the most recent period of crustal unrest in this region of east central california began around 1980 and has included periods of intense seismicity and ground deformation uplift totaling more than 0 7 m has been centered on the caldera s resurgent dome and is best modeled by a near vertical ellipsoidal source centered at depths of 6 7 km modeling of both deformation and microgravity data now suggests that 1 there are two inflation sources beneath the caldera a shallower source 7 10 km beneath the resurgent dome and a deeper source 15 km beneath the caldera s south moat and 2 the shallower source may contain components of magmatic brine and gas the long valley exploration well lvew completed in 1998 on the resurgent dome penetrates to a depth of 3 km directly above this shallower source but bottoms in a zone of 100 c fluid with zero vertical thermal gradient although these results preclude extrapolations of temperatures at depths below 3 km other information obtained from flow tests and fluid sampling at this well indicates the presence of magmatic volatiles and fault related permeability within the metamorphic basement rocks underlying the volcanic fill in this paper we present recently acquired data from lvew and compare them with information from other drill holes and thermal springs in long valley to delineate the likely flow paths and fluid system properties under the resurgent dome additional information from mineralogical assemblages in core obtained from fracture zones in lvew documents a previous period of more vigorous and energetic fluid circulation beneath the resurgent dome although this system apparently died off as a result of mineral deposition and cooling and or deepening of magmatic heat sources flow testing and tidal analyses of lvew water level data show that relatively high permeability and strain sensitivity still exist in the steeply dipping principal fracture zone penetrated at a depth of 2 6 km the hydraulic properties of this zone would allow a pressure change induced at distances of several kilometers below the well to be observable within a matter of days this indicates that continuous fluid pressure monitoring in the well could provide direct evidence of future intrusions of magma or high temperature fluids at depths of 5 7 km

the degassing character of a young volcanic center cerro negro nicaragua cerro negro volcano is a young basaltic cinder cone which is part of the nicaraguan volcanic arc eruptive activity at cerro negro is characterized by explosive strombolian to subplinian eruptions driven by volatile rich basaltic magma ascending rapidly from various crustal depths 15 to 6 km resulting in the onset of precursory activity only 30 min before an eruption in this paper we present a comprehensive degassing characterization of the volcano over a 4 year period aimed at improving our understanding of the magmatic plumbing network and its relationship with regional tectonics a total of 124 individual soil gas samples were collected between 2010 and 2013 and analyzed for stable carbon isotopes 13c from co2 high temperature fumaroles were sampled for 18o d and 3he 4he isotope analysis and major degassing zones were mapped using soil co2 flux measurements gases at cerro negro are characterized by a strong 3he 4he mantle signature 6 3 to 7 3 ra magmatic 13c ratios 2 3 to 3 0 meteoric 18o and d ratios and stable co2 fluxes 31 t d 1 the lack of 13c fractionation and an increase in the relative mantle component from 2002 to 2012 suggest that the volatile flux at cerro negro originates from the mantle and ascends to the surface via a series of crustal fractures that act as permeable conduits despite the lack of new eruptions the hydrothermal system of cerro negro continues to evolve due to seasonal inputs of meteoric water slope failures that expose and bury sites of active degassing and bursts of regional seismicity that have the potential to open up new conduits for gas release as well as magma continuing geophysical and geochemical monitoring of the main edifice and the recently formed south zone is essential as the volcano remains overdue to erupt

origin of overpressures in shales constraints from basin modeling techniques for detection evaluation and prediction of pore pressures in low permeability rocks and equations for fluid pressure computations in most integrated basin modeling software are based on relationships between porosity and effective stress in shales however recent data show that overpressured shales in the north sea do not exhibit higher porosities than the normally pressured shales of the same formation at similar depths to further evaluate the existence of porosity vs effective stress relationships in shales fluid flow simulations and porosity modeling in a typical high pressure and high temperature well in the north sea were undertaken the parameters in the permeability and porosity equations were adjusted until a satisfactory fit was achieved between the observed and modeled porosity and fluid pressure at present however the modeled porosity and pore pressure vs depth history of the sediments deviated significantly from known porosity and pore pressure vs depth relationships that have been observed in north sea shales and elsewhere today because the results from basin modeling based on porosity stress relationships were unacceptable irrespective of parameter choices and the well data from the north sea show no signs of elevated porosities in the overpressured shales it is inferred that effective stress driven compaction alone has not generated the hard overpressures observed in deeply buried north sea shales these conclusions are suggested to be generally applicable to shales with low porosities and hard overpressures worldwide both because of the physics involved and because similar results can be extracted from published modeling in the niger delta

2d modeling of overpressure in a salt withdrawal basin gulf of mexico usa this study demonstrates the utilization of 2d basin models to address overpressure development due to compaction disequilibrium in supra allochthonous salt mini basins with very high sedimentation rates in the gulf of mexico by properly selecting 2d line sections with moderate stratigraphic resolution it is possible to predict timing of overpressure development and approximate present day overpressure distributions in the mini basin this study shows that even low resolution models with approximate information on the net to gross sand shale ratio can average 0 4 ppg with a maximum error of 1 0 ppg relative to pressure measurements in sandstones the models based on age depth approximate lithology and an interpretation of complicated salt movement are adequate to evaluate pressure to address issues around trap containment and may be used for preliminary well planning this study tested the results of overpressure prediction utilizing different stratigraphic resolutions and shows the sensitivity of overpressure modeling to 2d line selection also three models were built to investigate how the permeability of salt welds affects overpressure development in an adjacent salt mini basin these results indicate that even a salt weld permeability reduction of 1 5 log md results in a pressure difference between neighboring mini basins additionally these models qualitatively reproduced the seismic velocity volume which is supporting evidence that the salt welds in this mini basin are at least partially sealing

integrated charge and seal assessment in the monagas fold and thrust belt of venezuela conventional basin and petroleum systems modeling uses the vertical backstripping approach to describe the structural evolution of a basin in structurally complex regions this is not sufficient if lateral rock movement and faulting are inputs the basin and petroleum systems modeling should be performed using structurally restored models this requires a specific methodology to simulate rock stress pore pressure and compaction followed by the modeling of the thermal history and the petroleum systems we demonstrate the strength of this approach in a case study from the monagas fold and thrust belt eastern venezuela basin the different petroleum systems have been evaluated through geologic time within a pressure and temperature framework particular emphasis has been given to investigating structural dependencies of the petroleum systems such as the relationship between thrusting and hydrocarbon generation dynamic structure related migration pathways and the general impact of deformation we also focus on seal integrity through geologic time by using two independent methods forward rock stress simulation and fault activity analysis we describe the uncertainty that is introduced by replacing backstripped paleogeometry with structural restoration and discuss decompaction adequacy we have built two end member scenarios using structural restoration one assuming hydrostatic decompaction and one neglecting it we have quantified the impact through geologic time of both scenarios by analyzing important parameters such as rock matrix mass balance source rock burial depth temperature and transformation ratio

hydromechanical modeling of stress pore pressure and porosity evolution in fold and thrust belt systems we present coupled critical state geomechanical fluid flow simulations of the evolution of a fold and thrust belt in nw borneo our modelling is the first to include the effects of both syntectonic sedimentation and transient pore pressure on the development of a fold and thrust belt the present day structure predicted by the model contains the key first order structural features observed in the field in terms of thrust fault and anticline architectures stress predictions in the sediments show two compressive zones aligned with the shortening direction located at the thrust front and back limb between the compressive zones near to the axial plane of the anticline the modelled stress field represents an extensional regime the predicted overpressure distribution is strongly influenced by tectonic compaction with the maximum values located in the two laterally compressive regions we compared the results at three notional well locations with their corresponding uniaxial strain models the 2d thrust model predicted porosities which are as much as 7 5 lower at 2 5 km depth and overpressures which are up to 6 4 mpa higher at 3 km depth these results show that one dimensional methods are not sufficient to model the evolution of pore pressure and porosity in contractional settings finally we performed a drained simulation during which pore pressures were kept hydrostatic the predicted geological structures differ substantially from those of the coupled simulation with no thrust faulting these results demonstrate that pore pressure is a key control on structural development

on the behaviour of thanet sand an example of an uncemented natural sand an extensive laboratory investigation was made of the mechanics of thanet sand testing the soils from a number of locations and depths in the triaxial apparatus over a wide range of stress levels comparisons between tests on intact and reconstituted samples and between vertically and horizontally trimmed intact samples highlighted the effects of structure on the behaviour of this uncemented sand in particular a significant effect of anisotropy two distinct substrata have previously been identified with different fines contents and the key difference in their behaviour has been found here to be the location of their critical state line in the v lnp plane which has a significant effect on the engineering behaviour of the two substrata and can explain recent unusual observations of decreasing pile capacity with depth tests in which the glauconite content was removed from the sand showed that it did not affect its behaviour

soil efflux and total emission rates of magmatic co2 at the horseshoe lake tree kill mammoth mountain california 1995 1999 abstract we report the results of eight soil co 2 efflux surveys by the closed circulation chamber method at the horseshoe lake tree kill hltk the largest tree kill on mammoth mountain the surveys were undertaken from 1995 to 1999 to constrain total hltk co 2 emissions and to evaluate occasional efflux surveys as a surveillance tool for the tree kills hltk effluxes range from 1 to 10 000 g m 2 day 1 grams co 2 per square meter per day they are not normally distributed station efflux rates can vary by 7 35 during the course of the 8 to 16 h surveys disturbance of the upper 2 cm of ground surface causes effluxes to almost double semivariograms of efflux spatial covariance fit exponential or spherical models they lack nugget effects efflux contour maps and total co 2 emission rates based on exponential spherical and linear kriging models of survey data are nearly identical similar results are also obtained with triangulation models suggesting that the kriging models are not seriously distorted by the lack of normal efflux distributions in addition model estimates of total co 2 emission rates are relatively insensitive to the measurement precision of the efflux rates and to the efflux value used to separate magmatic from forest soil sources of co 2 surveys since 1997 indicate that contrary to earlier speculations a termination of elevated co 2 emissions at the hltk is unlikely anytime soon the hltk co 2 efflux anomaly fluctuated greatly in size and intensity throughout the 1995 1999 surveys but maintained a n s elongation presumably reflecting fault control of co 2 transport from depth total co 2 emission rates also fluctuated greatly ranging from 46 to 136 t day 1 metric tons co 2 per day and averaging 93 t day 1 the large inter survey variations are caused primarily by external meteorological processes operating on time scales of hours to days the externally caused variations can mask significant changes occurring at depth a striking example is the masking of a degassing event generated at depth and detected by a soil gas sensor network in september 1997 while an efflux survey was in progress thus occasional efflux surveys are not an altogether effective surveillance tool for the hltk and making them effective by greatly increasing their frequency may not be practical

deep co2 release revealed by stable isotope and diffuse degassing surveys at vulcano aeolian islands in 2015 2018 abstract the partitioning of carbon dioxide co2 released by soils at vulcano island aeolian islands italy was performed by combining the co2 flux and the carbon isotope measurements based on this method the amount of co2 of volcanic origin was quantified six times during the period 2015 2018 the data analysis allowed us to establish the correlation between co2 soil degassing and changes in the contribution of volcanic fluids carbon isotope determinations were performed in situ to enhance the coverage of data collection in space and time these data were combined with both the co2 contents in the ground gases and the soil co2 flux the amount of volcanic co2 was distinguished from that of biogenic origin by implementing a three component mixing model the results of this study indicate that the increase in co2 output in september 2018 reflects the increase in volcanic gas emissions the measurement method and analysis presented in this work are sufficiently general to be applicable to the monitoring programs of active volcanoes

seismological evidence for magmatic and hydrothermal structure in long valley caldera from local earthquake attenuation and velocity tomography we compare the results of six different tomographic inversions for the three dimensional seismic attenuation and velocity structure in long valley caldera california in order to constrain interpretations on the location and character of magmatic and geothermal structure in the caldera crust p and s wave amplitudes and arrival times obtained in previous studies from seismograms of local earthquakes recorded on vertical and three component seismographis were inverted for q sv 1 q sh 1 q s 1 q p 1 and v p v s in the upper 9 km of teh caldera crust detailed comparisons of the results of the inversions suggest significant locations of high temperatures and perhaps magma and hydrothermal zones in the caldera a region of low q s with locally variable q p and v p v s is imaged in the 7 9 km depth range beneath the resurgent dome overlying and adjacent to this anomaly are significant low q p and low v p v s anomalies as well as smaller zones of low q s and high v p v s together these suggest zones of high temperature magma below and adjacent to zones of supercritical hydrothermal fluids at 4 5 km depth along the southern rim of the caldera just east of mammoth mountain a significant anomaly with relatively low q p and relatively low v p v s is imaged and is interpreted as a supercritical hydrothermal zone poorly resolved relatively low q p and relatively high v p v s are seen at 4 5 km depth beneath the inyo craters suggesting possible high temperatures and perhaps magma because of lack of data in the individual inversions the resultant images have spatially variable error and resolution however by comparing multiple images of different seismic parameters we have improved our ability to determine the significance of an constrain interpretation of seismic structure anomalies

analogue models of collapse calderas and resurgent domes collapse calderas and resurgent domes are a common association related to inflation deflation processes in volcanic systems the structure of calderas and domes depends upon the permitted relative movements of crustal volumes at depth the so called space problem in order to study the structures of collapse calderas and resurgent domes and to take the space problem into account several analogue models were made dry quartz sand was used to simulate the rheology of the brittle crust while newtonian silicone putty located at the base of the sand pack simulated the ductile behaviour of the magma a piston moved the silicone putty downward or upward inducing collapse and doming within the sand three separate sets of experiments simulated 1 caldera collapse 2 resurgence and 3 superimposition of resurgence on collapse and vice versa collapse experiments are characterized by the development of two concentric depressions the first formed depression is bordered by outward dipping reverse ring faults the subsequent outer concentric depression is bordered by inward dipping normal ring faults the deformation pattern during resurgence is a function of the overburden thickness t and the dome diameter d for higher t d ratios a dome forms bordered by inward dipping high angle reverse ring faults outward dipping normal ring faults develop at late stages for lower t d ratios the dome shows at late stages a crestal depression accompanied by radial fractures subsequently an apical extrusion of silicone occurs the superimposition of resurgence over collapse and vice versa is characterized by the complete reactivation with opposite kinematics of all the pre existing ring faults during inversion both in caldera and resurgence reverse ring faults form in the early stages due to differential uplift extensional structures subsequently form to accommodate gravitational collapse during the activity of the reverse faults the experiments and the overall similarities with nature suggest that the activity of both reverse and normal faults constitutes a possible solution to the space problem during major collapses or resurgences

effects of early petroleum charge and overpressure on reservoir porosity preservation in the giant kela 2 gas field kuqa depression tarim basin northwest china kela 2 is a giant gas field with a proven reserve of 597 tcf in the kuqa depression northern tarim basin widespread overpressures have been encountered in the eocene and cretaceous sandstone reservoirs of the field with pressure coefficients up to 2 1 from drill stem tests and well log data analysis disequilibrium compaction associated with horizontal tectonic compression may be the dominant overpressure mechanism in the sandstone reservoirs because the overpressured sandstone with a maximum burial depth over 6000 m 19 685 ft displays anomalously high porosity and low density the causes for sandstone reservoirs with anomalously high porosity in the kela 2 gas field were studied based on an integrated investigation of sandstone reservoir characteristics paleo oil water contact petroleum charge history and overpressure evolution collective evidence indicates that early oil charge had retarded the porosity reduction of the reservoir sandstone and resulted in disequilibrium compaction from overburden rocks and overpressure from disequilibrium compaction and horizontal tectonic compression at the beginning of the rapid subsidence and deposition in the kela 2 gas field again contributed to the preservation of the reservoir porosity 1 overpressured mudstones in the kela 2 gas field are characteristic of normal compaction and overpressure was generated by horizontal tectonic compression instead of disequilibrium compaction 2 the reservoir sandstones with high porosity and permeability are associated with high paleo oil saturation as indicated by quantitative grain fluorescence qgf responses and anomalous qgf on extract intensity 3 sandstone units below the paleo oil water contact have very low porosity and permeability and 4 three episodes of oil and one episode of gas charge are identified in the sandstone reservoirs of the kela 2 gas field and the later two episodes of oil charge occurred circa 5 5 4 5 ma which corresponds to the beginning of the rapid tectonic subsidence and deposition in the kuqa depression the initially charged oil in the sandstone reservoirs was subsequently displaced by gas at circa 3 2 ma through fault activation at the edge of the anticline trap the overpressure evolution for the k1bs reservoir sandstone in the kela 2 gas field indicates that the apparent overpressure development in the sandstone reservoir began at 5 ma following the major oil charge and has been maintained to the present overpressure development from 5 ma in the sandstone reservoirs of the kela 2 gas field is believed to be the dominant cause of the porosity preservation

soil δ13c co2 and co2 flux in the h2s rich rotorua hydrothermal system utilizing cavity ring down spectroscopy abstract diffuse soil co2 flux and stable carbon isotope surveys facilitate interpretation of the nature and structure of hydrothermal systems although soil co2 flux measurements are completed in the field isotopic carbon in co2 i e 13c co2 analyses are more commonly performed in a stable isotope mass spectrometry laboratory thereby increasing the effort time and resources required to complete the survey cavity ring down spectroscopy crds overcomes these challenges by enabling both co2 isotopic and flux analyses in field settings however h2s interference has limited crds applications in h2s rich hydrothermal settings here we assess the application of a mobile crds system equipped with a simple cu wire trap to complete 13c co2 and co2 flux surveying in the relatively high h2s rotorua hydrothermal system rhs north island new zealand laboratory experiments demonstrate that the cu trap was able to remove h2s 250 ppmv enabling accurate and precise 13c co2 measurements in the rhs the cu trap successfully removed h2s concentrations during in situ soil gas analyses carbon dioxide fluxes determined using the crds were found be 102 of those values produced by a traditional soil gas flux meter i e licor carbon dioxide flux determined by crds was similar to previous studies in the rhs however zones of high co2 flux in central kuirau park have moved 100 200 m in the past twelve years isoscapes of 13c co2 for the rhs were constructed from our soil gas survey results and were used to assess the apportionment of magmatic hydrothermal versus biogenic co2 release our findings demonstrate that a crds enabled system allows for faster in field survey measurement lower cost and reactive sampling for both 13c co2 and co2 flux measurement in h2s enriched hydrothermal systems