tectonic and structural evolution of gough volcano a volcanological model abstract morphostructural stratigraphic and tectonic data indicate that the evolution of gough volcano is similar to other oceanic intraplate volcanoes is older than 1 ma and is related to a transform fault at least six evolutionary stages can be distinguished within two major magmatostructural periods dominated by basaltic and trachytic magmas respectively the basaltic shield volcano is characterized by a curved elongated shape in plan and a rift zone with a high density of dykes combined with a radial intrusive system the latter is interpreted as being fed by a magma chamber some 4 km below the surface the activity of the volcano became more centralized at the end of the basaltic period and its slopes became steeper this corresponds to the development of a shallower and narrower central conduit in the edifice the basaltic period was terminated by formation of a shield caldera related to the 4 km deep magma chamber the term shield caldera is used for a collapse structure that is postmagmatic large in comparison with the diameter of the volcano and delimited by normal faults that do not show a closed circular pattern but rather a series of arcs in contrast summit calderas are defined as smaller circular shaped centrally situated synmagmatic features related to a central shallow column during the basaltic period landslides were generated on the flanks of the edifice as a result of slope stability factors which are not easy to determine at present and dynamic factors among which the intrusion of magma along a curved zone certainly played a major role the trachytic period is characterized by comparatively rare pyroclastic deposits and a large volume of thick flows extruded from domes these extrusions as well as plugs formed from vertical cylindrical columns of magma rising from shallow individual magma pockets fed by the main reservoir

deformations and inferred stress field for ellipsoidal sources at long valley california 1975 1982 ground deformation and seismicity observed from 1975 to the present support the existence of magma beneath the central part of long valley caldera california constraints on the source geometry are obtained from deformation modeling of uplift and trilateration data over the period 1975 1982 the deformation data are modeled with an elastic half space incorporating an ellipsoidally shaped magma chamber under uniform pressure the hilton creek fault and the south moat fault the best fit to the deformation data indicates a nearly spherically shaped source beneath the central part of the caldera at a depth of 9 5 1 km however significant deviations of some horizontal displacement data suggest that movements along the faults are probably more complicated than the assumed dislocations the predicted normal stress on the vertical south moat fault plane varies from weak tension above 4 km depth to strong compression below that depth and the shear stress is right lateral in the eastern half and left lateral in the western half of the fault it is unlikely that such a stress field could trigger slip movement on the fault tangential stress at the surface of the magma chamber has a maximum tension near its intersection with the south moat fault it is possible that the 1983 earthquake swarms in the south moat were triggered by shallow magma intrusions beneath the south moat of the caldera a narrow prolate ellipsoidal source which produces about the same uplift as the spherical source model leads to a horizontal tensile stress that is about 1 order of magnitude higher in the region directly above the source however the crack opening location for magma injection is at the top of the source in this case and would not be related to the seismicity in 1983

reanalysis of s to p amplitude ratios for gross attenuation structure long valley caldera california because of the strong interest in the magmatism and volcanism at long valley caldera eastern california and because of recent significant improvements in our knowledge of the caldera velocity structure and earthquake locations i have reanalyzed the local earthquake s to p amplitude ratio data of sanders 1984 for the gross three dimensional attenuation structure of the upper 10 km of long valley caldera the primary goals of the analysis are to provide more accurate constraints on the depths of the attenuation anomalies using improved knowledge of the ray locations and an objective inversion procedure the new image of the high s wave attenuation anomaly in the west central caldera suggests that the top of the principal anomaly is at 7 km depth which is 2 km deeper than previously determined because of poor resolution in much of the region some of the data remain unsatisfied by the final attenuation model this unmodeled data may imply unresolved attenuation anomalies perhaps small anomalies in the kilometer or two just above the central caldera anomaly and perhaps a larger anomaly at about 7 km depth in the northwest caldera or somewhere beneath the mono craters the central caldera s wave attenuation anomaly has a location similar to mapped regions of low teleseismic p wave velocity crustal inflation reduced density and aseismicity strongly suggesting magmatic association

a model for deformation in long valley california 1980 1983 geodetic data collected in long valley california from 1975 through 1983 define a pattern of uplift and strain which is evidently associated with a sequence of earthquakes occurring in may 1980 and subsequent swarm activity continuing until the present we have constructed a model to explain the deformation observed since may 1980 in terms of inflation of two subsurface magma chambers faulting in the south moat region of the caldera and slip on the hilton creek fault the most significant new feature of the model is the shallow magma chamber at 5 km depth located a few hundred meters to the east of the casa diablo hot spring area inflation of this chamber causes stresses which show consistency with various aspects of the seismicity in the south moat of the caldera calculations of stress across vertical planes over the magma chamber can be used together with failure criteria to estimate the inflation volume at which the rock layers intervening between the chamber and the surface will fail by extensional fracture

shear wave anisotropy and stress direction in and near long valley caldera california 1979 1988 we observed shear wave splitting birefringence for two temporary surface deployments of three component digital seismographs that were in place before and after m 6 earthquakes near the long valley caldera california in one of these deployments the data sample precedes the large events of the may 1980 mammoth lakes earthquake sequence by 6 months and cover the two most active months of the may 1980 aftershock sequence one of the stations wit from this deployment was reoccupied with identical instrumentation in 1988 another deployment preceded the 1986 chalfant valley mainshock by 2 days and recorded events for 6 days the polarization of the faster shear wave changes from n30 w at mammoth to due north at chalfant valley and in both cases is parallel to the strike of nearby surface faults and to the mean direction of p axes determined from focal mechanism groupings observations from nearby stations yield fast directions nearly at 23 from each other and time separations of fast and slow shear waves show considerable station to station variation showing no correlation with earthquake station distance or earthquake depth these observations suggest that the observed anisotropy results primarily from near station presumably shallow effects rather than from widespread aligned microcracks the records at long valley station wit show a slight variation of average fast shear wave polarization from 1979 to 1980 but with the limited amount of data available this difference is not statistically significant time separation measurements of up to 0 48 s at station wit yield estimates of 15 30 velocity anisotropy among the highest reported for local earthquakes secular variation in the time separation of fast and slow shear waves between events recorded at station wit in 1979 and 1980 is swamped by the scatter in the data if it is present the five events recorded in 1988 showed similar s wave polarizations and time separations to those in 1980 but there are not enough events for a statistical comparison unlocated events at long valley station mrs show a slight increase in time separation after the may 1980 m 6 0 earthquakes but the increase is not statistically significant

observation of shear wave splitting from natural events south moat of long valley caldera california june 29 to august 12 1982 three component seismograms from local earthquakes recorded by three university of wisconsin stations in the south moat of long valley caldera in 1982 have been analyzed for shear wave splitting the average polarization direction of the first arriving shear waves observed at the two stations south of the resurgent dome is nnw sse time delays between the fast and slow shear waves are proportional to hypocentral distances and the maximum velocity anisotropy inferred is 6 4 the average shear wave polarization observed at the station above the resurgent dome is north south the time delays there are not a function of hypocentral distance and the normalized time delays ms km from events at the southwest boundary of the caldera are larger than those at other azimuths indicating maximum 9 6 velocity anisotropy the east west to ene wsw variation of the tensional stress fields implied by the shear wave polarizations indicates the interaction between the inflating caldera and the sierra nevada range front faults under the generally wnw ese tension expected from the regional tectonics

an attenuating structure beneath the aso caldera determined from the propagation of seismic waves the attenuation of amplitude is seen in seismic waves which pass through the central region of the aso caldera in kyushu japan it is also recognized from spectral analysis of seismic waves that the higher frequencies of the p wave are reduced in the waves which pass through the central region of the caldera it is shown that the relative attenuation increases remarkably for the frequency range of 5 to 10 hz the specific attenuation factor q of the p wave train is about 100 from the surface projection of the ray paths with low q values through the aso caldera to each station the attenuating region is located beneath the center of the caldera extending to the north of the central cones in conjunction with the low q value of the p wave and the decreases of s wave amplitudes the relative p wave residual times have comparatively large values for seismic waves passing through the central region beneath the caldera in order to attempt to provide additional information on the depth configuration of the attenuating material the ray paths of p wave s first arrivals are located in three dimensional space it indicates that the low velocity material is located beneath the center of the caldera at depths of about 6 to 9 km however lowvelocity anomalies above the depth of 6 km and below the depth of 15 km were not able to be detected because most of the available seismic ray paths had crossed the caldera at depths of about 6 to 15 km furthermore the relative residual times have numerous errors resulting from incorrect hypocenter locations origin times inhomogeneities in the structure and uncertainty of the velocity structure at shallow depths in the aso caldera refraction or reflection studies are required for an accurate estimate of the structure and more detailed properties of the attenuating material

seismic evidence for magma in the vicinity of mt katmai alaska p wave traveltime delays of as much as 0 9 sec are consistently observed at one seismic station from local and regional earthquakes 70 to 150 km deep this station is on the southwest flank of mt trident the most recently active volcano within katmai national park alaska delays from local shallow earthquakes are typically less than 0 3 sec suggesting that most of the major delay results from anomalous material at depths of more than a few kilometers this station is located near the center of a bowlshaped low in the bouguer gravity field that is approximately 15 km in diameter and more than 25 mgals deep these anomalies suggest but do not prove the presence of considerable amounts of magma in the shallow part of the crust that could have been the source for all magma erupted in the vicinity of mt katmai and mt trident this century

a systematic approach to the interpretation of magnetotelluric data in volcanic environments with applications to the quest for magma in long valley california magnetotelluric mt data in long valley california are principally sensitive to the complex structure in the rocks filling the caldera while the overall shapes of the sounding curves are set by the deep crustal structure and mantle the lateral variations of that response can be entirely explained using structure of the upper 1600 m a conductive body at depths from 1350 to 1600 m is present in the northern southern and western moats and beneath the resurgent dome this conductor effectively masks structures at intermediate depths we believe that this conductor represents fractured bishop tuff within the caldera and possibly graphitic metasediments beneath it electrical logs from wells penetrating these formations are used to corroborate our interpretation a large conductive 5 ohm m body of magma is precluded by the mt data the response to a spherical chamber of 4 km radius at a depth of 10 km is several times larger than the mismatch between our modeled and observed sounding curves small 2 km radius isolated pockets of magma may be present however we have developed a systematic approach to interpreting mt data in a complex volcanic environment we clustered sites on the basis of similar responses to perceive patterns in spite of severe distortions by local heterogeneity our model was built from the surface downward incorporating other published geophysical surveys and a time domain electromagnetic survey released by unocal three dimensional modeling was employed from the start and our interpretation was based primarily upon the phases this systematic approach of stripping off the known shallow structure in order to study the intermediate and deeper structure was quite successful an important finding of our study is that the phase is sensitive to shallow 1350 m three dimensional structure even at frequencies of 0 01 hz three dimensional modeling simply cannot be avoided in complex geological environments

inversion for p and s wave attenuation structure long valley caldera california p and s wave amplitude data from digital three component seismograms of local earthquakes recorded on a temporary seismic array have been used to tomographically invert for the three dimensional attenuation structure in the upper crust of long valley caldera california differential attenuation q 1 values are obtained through the use of a spectral ratio technique that minimizes the site effect on the spectral ratios the spectral ratios are inverted for q 1 using the lsqr algorithm zones of high positive p wave q 1 greater p wave attenuation are observed at a depth of 4 5 km beneath the east flank of mammoth mountain and at 6 8 km beneath the southwestern edge of the resurgent dome a region of positive s wave q 1 is seen at 7 8 km beneath the southern edge of the resurgent dome whereas at 6 7 km the s wave results are near average the region of positive s wave q 1 at 7 8 km beneath the resurgent dome may indicate the roof of a magmatic system the positive p wave q 1 at 4 5 km beneath mammoth mountain and 6 8 km beneath the resurgent dome may be due to the presence of a compressible fluid such as supercritical water these zones may represent some of the source regions for the long valley geothermal system

spherical and ellipsoidal volcanic sources at long valley caldera california using a genetic algorithm inversion technique abstract we model the second inflation period at long valley caldera california using a genetic algorithm technique and high quality geodetic measurements of elevation changes and baseline extensions we compare two source inversions for both spherical mogi point sources and the finite prolate ellipsoid of yang and davis a sensitivity analysis for the genetic algorithm is performed based upon synthetic data set inversions on similar sources in order to better constrain the areal location orientation and volume of the potential sources the spherical sources are well constrained the larger located at 9 9 km beneath the resurgent dome with a volume of 0 036 km 3 while the second at only 0 008 km 3 is located at a depth of 7 3 km beneath the south moat the depths to the ellipsoidal sources are switched with the larger source at a depth of 9 6 km and the smaller at 11 8 km with volumes of 0 037 and 0 002 km 3 respectively

analysis of two color geodimeter measurements of deformation within the long valley caldera june 1983 to october 1985 line length changes from several baselines in a trilateration network within the long valley caldera clearly define a decrease in strain rate from june 1983 through october 1985 the data consist of more than 1600 length measurements on 23 baselines using a two color geodimeter which has a precision of 0 2 ppm of the line length initial measurements made during the summer of 1983 show extension rates were as high as 6 ppm yr on some baselines the rates subsequently decreased steadily to less than 1 ppm yr during the summer of 1985 ih contrast the strain rates on a few other baselines were less than 0 2 ppm yr averaged over the 2 3 year interval a model is constructed using these observations as well as yearly observations of a trilateration network within and near the caldera the model contains two points of inflation located at 5 and 10 km beneath the resurgent dome of the caldera plus dextral slip on a fault plane within the south moat within an elastic half space to mimic the observed time dependence in the two color geodimeter data we conclude that the rates of both slip and inflation have steadily decreased over the survey interval because the two color geodimeter network is closer to all three sources than the single color geodimeter network these data are important for defining the factor of 2 3 rate decrease of slip and inflation comparing our estimates of slip and inflation with similar estimates for the year prior to july 1983 we conclude that the rate decrease is probably a result of postseismic and postinflation relaxation following the earthquake swarm in the south moat in january 1983

a magma mass beneath casa diablo further evidence from reflected seismic waves a forty station seismographic network operating in long valley caldera in california during the summer of 1997 recorded a number of earthquakes with unusual secondary phases the phases can be seen on several stations throughout the network allowing the use of particle motions and differential travel times to examine their nature and points of origin based on amplitude frequency and travel time considerations it appears that some of the phases are mode converted s to p reflec tions from a zone below the casa diablo area near mammoth lakes ca the locus of reflection points was imaged using time domain signal migration and stacking under the assumptions of first order scattering theory the results indicate that the reflecting zone is a curved 1 5 km by 2 km surface 7 6 km below and 1 4 km west of the casa diablo geothermal plant previous studies show low p wave velocities and high s wave attenuations in the same location as the reflecting zone an amplitude analysis of the suspected reflec tions suggests they are produced by large contrasts in material properties with the possible range of transitions including solid to partially molten or dry to fluid satu rated rock given these observations plus the volcanic history of the caldera and its recent tectonic activity the most straightforward interpretation of the reflections is that they come from either the top of a sill or intrusion or the overlying hydrothermal system of such structures

velocity structure of the long valley caldera from the inversion of local earthquake p and s travel times a high resolution tomographic study of the long valley region began in 1989 with the installation of a special fan array of eight three component borehole 80 160 m depths receivers on the northwestern rim of the caldera to provide the data necessary for a joint inversion for hypocenters and the three dimensional p and s wave velocity structure in and below the caldera the experiment specifically targeted the subsurface location of the previously inferred magma chamber beneath the resurgent dome additional coverage was provided by existing u s geological survey and university of nevada reno seismographic networks we employed the progressive inversion scheme of thurber 1983 with cubic spline interpolation after michelini and mcevilly 1991 the final model is based on data from 280 well distributed local earthquakes some 6900 arrival times were used including about 700 s wave times from the three component stations the resulting velocity structure reflects the known geology defining the low velocity caldera fill to 2 km depth contrasting markedly with the surrounding higher velocity sierran block and highland terrane no isolated distinct low velocity anomalies are revealed beneath the caldera floor although a diffuse zone of reduced velocity persists to a depth of about 8 km these lower velocities may be related to hydrothermal alteration and or extensive fractures the vp vs structure contains significant lateral variation within and beneath the caldera to about 8 km depth and these variations can be related to accepted models of the active geothermal system the lack of a significant s wave velocity anomaly along with the normal or low values of the vp vs ratio argues against the presence of a sizable and distinct magma body at shallow to midcrustal depth beneath long valley caldera although a very low percentage of partial melt cannot be precluded

the owens river as a tiltmeter for long valley caldera california in the lower 11 km of its course around the resurgent dome of long valley caldera the owens river displays two parallel meander belts comparable in meander wavelength and amplitude but unequal in age elevation and discharge it appears the two belts take turns carrying the river s flow depending on whether the dome is inflating or subsiding the inboard belt some 200 300 m closer to the dome and now 30 60 cm higher in elevation contains an underfit stream and is now being abandoned the outboard channel formed in a series of avulsions apparently induced by recent uplift of the dome in the upper 4 km of the two channel reach avulsion occurred between 1856 and 1878 as inferred from the original u s coast and geodetic survey mapping in the caldera avulsion had already occurred by 1856 in the lower 4 km of the river suggesting a possible migration of the center of uplift through time muted remains of older oxbows in the outboard meander belt indicate an earlier occupancy by the river and thus an

inversion of teleseismic travel time residuals for velocity structure in the larderello geothermal field italy the analysis of teleseismic travel time residuals recorded on the larderello seismic network of central italy has revealed a sharp low velocity zone lvz in the center of the geothermal production area residuals from 101 teleseismic events are inverted to produce an image of a region of anomalously low velocities which we have interpreted to be a series of intrusive magmatic bodies the top of this anomalous region is constrained to be below 6 km in depth from local earthquake seismicity patterns and extends to a depth of greater than 20 km the lvz broadens with increasing depth and may extend into the upper mantle the observed travel time residual anomaly correlates spatially with observed gravity lows regions of high heat flow and the shallowing of a dominant upper crustal reflection horizon this body has elevated the temperatures in the region to over 350 c at 2 km and appears to be the origin of the geothermal energy

constraints on the upper crustal structure of the long valley mono craters volcanic complex eastern california from seismic refraction measurements during the summers of 1982 and 1983 field work was completed on 10 new seismic refraction profiles in the long valley mono craters region located within the technically active zone between the eastern sierra nevada and the basin and range province the four profiles presented here sample the upper 7 10 km of the crust within the western half of long valley cladera the mono craters ring fracture system and the normal crust north and northeast of long valley the entire area shares a common crystalline basement consisting primarily of mesozoic granitic rocks at a depth of 2 km beneath the topographic surface the p wave velocity is uniformly 5 6 km s beneath which the velocity increases with a gradient of about 0 1 s 1 clear secondary arrivals observed on a profile crossing the west part of the caldera support evidence from 1972 seismic refraction profiles for a reflecting boundary at a depth of 7 8 km beneath the west margin of the resurgent dome which may represent the top of a magma chamber structural details resolved in the upper 2 km of the crust lead to the following conclusions 1 p wave velocities in the crystalline basement at depths shallower than 2 km vary from 3 6 to 5 0 km s this variation reflects differences in the degree of fracturing of the upper portion of the basement 2 an apparently homogeneous basement in the upper 7 10 km beneath the mono craters ring fracture system indicates that any extensive magma chamber must be at least 10 km deep there 3 the downdropped crystalline basement beneath the western half of long valley caldera dips genty 5 10 to the northeast apparent offsets along steeply dipping caldera bounding faults are 1 0 1 7 km along the northern western and southern sides of the caldera with largest offsets in the north 4 rocks forming the caldera fill fall into essentially three p wave velocity groups from the surface downward these velocity groups are 100 400 of 1 2 1 8 km s material corresponding to unconsolidated fluvial or lacustrine deposits or highly fractured rhyolite flows 200 400 m of 2 8 3 1 km s material corresponding to sparsely jointed postcaldera rhyolite rhyodacite and basalt flows and approximately 1000 m of 3 9 4 4 km s material corresponding to deposits of bishop tuff

emplacement related deformation of granitoid magmas southern adamello massif italy the lago della vacca suite in the tertiary adamello massif italian alps is a small 10 km 2 crudely zoned granitoid pluton emplaced at a pressure of 3 5 kbar into folded mesozoic sediments and precursor intrusive rocks field relations and detailed mapping reveal a temporal evolution in the style of emplacement from early passive stoping by the mafic to intermediate composition marginal units to later forceful emplacement of the more silicic core marginal units of the pluton have highly discordant stoped outer contacts against folded triassic carbonates and precursor mafic ultramafic rocks in contrast the core units have steeply inward dipping contacts against these marginal units magmatic foliations defined by the planar alignment of plagioclase amphibole and biotite and by mafic enclaves are well developed in all units they describe steeply inward dipping concentric trajectories that approximately parallel unit contacts and in general increase in intensity toward the pluton margin lineations are rarely present the most intensely foliated rocks occur in the marginal units as anastomosing sinuous bands that wrap around lenses of less intensely foliated granitoid three dimensional strain analysis using mafic enclaves from throughout the pluton indicates that these bands represent zones of very high strain strain ellipsoids are weakly prolate within the core of the pluton becoming increasingly oblate as strain increases toward the pluton margin the short axis z of the strain ellipsoid is uniformly subnormal to the foliation suggestive of radial flattening textures of the marginal rocks which record high strain coupled with amphibole plagioclase thermometry indicate that deformation occurred in the magmatic or submagmatic state 650 780 c and was accompanied by significant migration of interstitial melt the documented transition from passive to forceful emplacement and final geometry of the suite resulted from temporal changes in the rheology of both the magma and adjacent country rocks pulses of low density hydrous magma were initially emplaced by stoping denser country rocks on cooling and solidification these magmas contracted by 15 20 creating an unpressured volume which allowed subsequent magma pulses to be emplaced into the pluton core these pulses distended and flattened their partially molten carapace and those parts of the aureole that had been thermally softened by intrusion of earlier magmas emplacement of the lago della vacca suite was therefore the product of several internal or magma related processes an interaction between stoping radial expansion and volume contraction on solidification of the hydrous magmas the heterogeneous distribution of strain in the marginal units reflects local variations in temperature and most importantly in the distribution of interstitial melt during emplacement

helium isotopes in geothermal and volcanic gases of the western united states ii long valley caldera abstract systematic trends in helium isotope ratios and gas compositions in long valley caldera hot spring and fumarole gases reflect subsurface processes and conditions in the hydrothermal system high 3 he 4 he ratios to 6 5 atmospheric indicate a magmatic helium source diluted to varying degrees by crustal radiogenic helium as observed in western united states geothermal systems in general helium isotope ratios increase eastward across the caldera which we interpret as an input of magmatic helium to the hydrothermal fluid as it flows eastward over the caldera decreasing he condensible gas he co2 ratios accompanying this trend suggest that co2 addition and or preferential helium loss due to boiling are also occurring although near surface ph related co2 hco3 equilibria obscure the nature of this effect substantial changes in 3 he 4 he ratios were observed in long valley gases between 1978 and 1985 helium isotope ratios increased by 15 in the hot creek thermal vent with maximum ratios achieved in 1983 helium isotope ratios subsequently decreased in this and other hot vents in the caldera these observations indicate that substantial changes in the mixing ratio of radiogenic and magmatic helium may occur over relatively short periods of time in magmatically active systems

crustal refraction profile of the long valley caldera california from the january 1983 mammoth lakes earthquake swarm seismic refraction profiles recorded north of mammoth lakes california using earthquake sources from the january 1983 swarm complement earlier explosion refraction profiles and provide velocity information from deeper in the crust in the area of the long valley caldera eight earthquakes from a depth range of 4 9 to 8 0 km confirm the observation of basement rocks with seismic velocities ranging from 5 8 to 6 4 km sec extending at least to depths of 20 km the data provide further evidence for the existence of a partial melt zone beneath long valley caldera and constrain its geometry this evidence is in the form of pronounced secondary arrivals which we interpret as waves that have propagated through a volume with low q the magma chamber and reflected from the lower boundary of the southern edge of this volume at depths of 18 to 20 km

three dimensional gravity modeling of the geologic structure of long valley caldera a 48 mgal gravity low coincides with long valley caldera and is mainly attributed to low density caldera fill gravity measurements by unocal geothermal have been integrated with u s geological survey data vastly improving gravity station coverage throughout the caldera a strong regional gravity trend is mainly attributed to isostasy a best fitting based on regional control of basement densities airy heiskanen isostatic model was used for the regional correction a three dimensional multiple unit gravity modeling program with iterative capabilities was developed to model the residual gravity the density structure of long valley caldera and vicinity was modeled with 22 discrete density units most of which were based on geologic units information from drill hole lithologies surface geology and structural geology interpretations constrain the model some important points revealed by the three dimensional gravity modeling are that 1 the volume of ejected magma associated with the bishop tuff eruption is greater than previously thought 2 the caldera structure is strongly influenced by precaldera topography and the extensions of major active faults 3 the main west ring fracture is coincident with the inyo domes mono craters fracture system 4 a relatively low density region probably underlies the caldera and 5 a silicic magma chamber may underlie devils postpile

evolution of icelandic central volcanoes evidence from the austurhorn intrusion southeastern iceland the austurhorn intrusive complex in southeastern iceland represents an exhumed tertiary central volcano the geometry of the intrusion and geochemistry of the mafic and felsic rocks indicate austurhorn was a volcanic center analogous to eyjafjallajokull and torfajokull in iceland s eastern neovolcanic zone evz early transitional tholeiitic basalt magmatism at austurhorn formed a shallow crustal chamber 5 km in diameter apparent rhythmic modal layering of and intrusive contacts within the gabbro indicate the mafic chamber was replenished frequently as it cooled and crystallized felsic activity postdated near solidification of the gabbro numerous granitic magmas intruded along gabbro margins and within the adjacent crust field relations indicate that infrequent felsic replenishment prevented convective mixing of the austurhorn chamber during this time although commingled mafic and felsic magmas are observed in an extensive net veined complex late stage mafic dikes intrude the entire complex suggesting that magmatic heat was abundantly available throughout the evolution of the austurhorn system plagioclase and clinopyroxene compositions in mafic through felsic rocks including gabbros support a model of progressive differentiation field relations constrain the felsic magmas to originate at p 1 kbar presumably by fractional crystallization the structure and geochemistry of the austurhorn intrusive complex suggest formation in an immature rift environment similar to the modern evz the proposed rift segment was parallel to the western and eastern neovolcanic zones and probably resulted from a reorganization of plate boundaries 7 ma saemundsson 1979 helgason 1985 jancin et al 1985 triggered by activity of the iceland mantle plume

p to s converted wave observations at long valley caldera we report observations of p to s converted teleseismic body waves recorded with seismic stations located on the resurgent dome of long valley caldera we observe strong conversions occurring at boundaries that could be the tops of small magma bodies at depths of 4 9 kilometers beneath the resurgent dome this technique could prove to be an extremely powerful tool in the search for magma bodies for geothermal energy development our results from a two month deployment of triggered digital teleseismic recorders are discussed

fluid inclusion evidence for basement decompression during permo triassic extension se new england usa co2 bearing fluid inclusions in strongly lineated but weakly foliated late precambrian gneisses within the hope valley shear zone of connecticut and rhode island are of mixed composition xco2 0 1 7 wt nacl equivalent and variable density 0 59 0 86 g ml and occur mainly as isolated inclusions also present are dilute 3 wt nacl equivalent aqueous inclusions which occur on healed fractures related to greenschist facies retrograde metamorphism isochores for dense isolated co2 bearing inclusions indicate pressures of 7 5 9 kbar at 500 600 c the estimated temperature conditions of peak metamorphism published 40ar 39ar hornblende plateau age spectra indicate cooling through about 500 c at 265 5 ma isochores for low density co2 bearing inclusions and aqueous inclusions intersect at the conditions of retrograde metamorphism 325 400 c and indicate pressures of 3 4 kbar published 40ar 39ar biotite plateau ages indicate cooling through about 300 c at 250 5 ma these data define a p t uplift curve for the region which is convex towards the temperature axis and indicate uplift rates between 0 4 and 3 3 mm year in permian time exhumation of basement gneisses was coeval with normal west down motion along the regional basement cover contact honey hill lake char willimantic fault system and is interpreted as due to post orogenic extensional collapse of the alleghanian orogeny

s wave attenuation structure in long valley caldera california from three component s to p amplitude ratio data using s to p amplitude ratios measured from three component seismograms we tomographically imaged spatial variations in s wave attenuation in parts of long valley caldera data from seismograms of local earthquakes in the long valley caldera region that occurred during 1984 were employed with the s to p amplitude ratio technique with corrections for source radiation pattern to determine q s resolution is limited throughout most of the model however significant resolution and acceptable error in parts of the model allow meaningful interpretation of the results the primary result is the image of a low q s anomaly at 7 8 km depth beneath the resurgent dome this is independent evidence for high temperatures and possible magma accumulation at this location other regions of anomalously low q s are marginally significant or poorly resolved of these low q s at 4 5 km depth beneath the inyo domes has the most potential significance the similarity of the results from the three component data and from the vertical component data implies that vertical component data which are generally more abundant can be used with confidence

deformation of long valley caldera between august 1982 and august 1983 geodetic and seismic data collected between july 1982 and august 1983 document an important event in the recent evolution of long valley caldera in january 1983 a substantial change in the pattern of deformation was associated with the most intense seismic swarm since may 1980 the earthquake hypocenters of the january 1983 swarm formed a vertical roughly tabular zone beneath the south moat of the caldera analysis of extensive intracaldera horizontal control data indicates that the shear strain distributed throughout this zone could not all be attributed to slip along any single fault plane the surface deformation of the resurgent dome and caldera floor is temporally associated with the swarm and was caused by an expansion of the upper levels of the residual magma chamber analysis of the 1983 data indicates a pressure distribution 2 5 km shallower than could be fit by analysis of the 1979 1982 caldera deformation the volume of this shallow expansion is 10 15 of the whole chamber expansion that occurred between 1979 and 1982 most of the 1983 deformation occurred between december 14 1982 and january 20 1983 and thus was associated with the january 6 7 1983 swarm though right lateral shear strain continued to accumulate in the south moat through 1983 we conclude that a shallow pressure increase probably occurred within the upper levels of the residual magma chamber of long valley caldera during the 5 week period following december 14 1982 it is not clear however whether this pressure increase preceded or accompanied the january 1983 earthquake swarm

seismic reflection studies in long valley caldera california seismic reflection studies in long valley caldera california indicate that seismic methods may be successfully employed to image certain types of features in young silicic caldera environments however nearsurface geological conditions within these environments severely test the seismic reflection method data quality are degraded by static reve t eration and band limiting problems due to these near surface conditions in long valley seismic reflection and refraction methods were used to image both the shallow and deep geothermal aquifers within the area the deep geothermal aquifer the welded bishop tuff was imaged as a fairly continuous reflector across the western moat of the caldera near surface refraction information indicates that there may be a buffed paleochannel system or horst and graben system that could control the shallow geothermal flow pattem high amplitude events observed in a wide angle survey were offginally interpreted as reflections from a contemporary magma body however a migration of the events utilizing the new generalized cellular migration algorithm indicates that these events are probably reflections from the faults of the caldera ring fracture system the reflections may be caused by the high acoustic impedance contrast associated with the juxtaposition of relatively low velocity low density caldera fill against the granite plutons and metasediments of the sierran basement along this fault system

size depth and related structures of intrusions under stratovolcanoes and associated geothermal systems abstract the subvolcanic structural configuration of faults stress field permeability controls and intrusion s shape size and depth are primary factors controlling the location and quality of a geothermal system subvolcanic stocks under stratovolcanoes are typically 1 to 3 km in diameter and emplaced at depths of 1 to 4 km exposed stocks in the western cascades of oregon are of similar size and are emplaced at depths of 1 to 2 km shallow magma chambers in general are typically 4 to 9 km deep and several times larger than the subvolcanic stocks likely subvolcanic structures are radial compression fractures around the magma chamber and above the chamber extension faulting stockwork fractures and breccia pipes pre existing or regional structures and stress field will influence the location and shape of magma bodies and associated thermal systems this study combines data from areas of active stratovolcanoes and older eroded volcanic areas to formulate a detailed structural model of the plumbing system under a stratovolcano the model is correlated with factors controlling associated geothermal systems a geothermal system may occur at depths of 1 2 km under a stratocone s base deeper systems associated with shallow magma chambers are likely to be present

detection of magma bodies beneath krakatau volcano indonesia from anomalous shear waves abstract the seismograms of 14 local earthquakes recorded by a temporary network in the sunda strait were used to construct an image of shear wave attenuating bodies in the vicinity of the krakatau volcanic complex seismograms for all ray paths going beneath the krakatau complex show a diminution of the amplitude of s waves these diminutions cannot be attributed to source mechanisms or station effects combination of all ray paths although in limited number demonstrates that the attenuating body is located in two zones which are probably disconnected although the limited set of data available does not allow us to draw a definite conclusion the upper zone is about 9 km deep and the lower one is at least 22 km deep the upper zone contains probably numerous irregular pockets of magma the deep zone appears to be a very wide body related to the extensional nature of the sunda strait the existence of a large deep reservoir should be taken into account for the mitigation of future hazards

possible tectonomagnetic effect observed from mid 1989 to mid 1990 in long valley caldera california precise measurements of local magnetic fields have been obtained with a differentially connected array of three proton magnetometers in the long valley caldera region since 1984 two magnetometers are located inside the caldera with a third reference magnetometer located 26km southeast of the caldera after correction for secular variation it is apparent that an anomalous 2 nt decrease in the magnetic field occurred from mid 1989 to mid 1990 at the magnetometer located closest to the center of the resurgent dome inside the caldera during this period a significant increase in geodetic strain rate of 8 5 ppm a was observed on the two color geodimeter network within the caldera from october 1989 to mid 1990 and a dramatic increase in seismic activity occurred from december 1989 to july 1990 a simple dilatational point source model with pressure increasing by 52 mpa from october 1989 to august 1990 at a depth of about 7km beneath the center of the resurgent dome can be fit to the strain data if this same model is used to calculate piezomagnetic fields in the caldera the results obtained agree with the observed local magnetic field data provided the curie point isotherm is at a depth of 5km taken together these magnetic seismic and geodetic data suggest that an episode of active magmatic intrusion occurred from late 1989 to mid 1990 at a depth of about 7 8km beneath the resurgent dome within the long valley caldera other indications of this intrusion should be evident in measurements of leveling local gravity and seismic imaging data

contrasting p t t paths thermochronologic evidence for a late paleozoic final assembly of the avalon composite terrane in the new england appalachians strongly contrasting pressure temperature time paths for the avalon composite terrane and the structurally overlying putnam nashoba zone in eastern new england obtained from thermochronologic and thermobarometric data are best explained by a late paleozoic underthrusting of cover rocks by the avalon composite terrane we present new ar and u pb thermochronologic data that show that in the southern hope valley zone permian 280 ma anatectic metamorphic conditions of 700 c and 6 kbar were quenched by relatively rapid cooling 12 c m y and exhumation 0 5 km m y for 40 m y in contrast peak metamorphic conditions in the putnam nashoba zone predate silurian intrusions and slower cooling 3 5 c m y began at about 400 ma one dimensional thermal modeling suggests that these two belts were not in thermal equilibrium during the permian metamorphism of the avalon composite terrane because of the absence of high grade alleghanian metamorphism in rocks overlying the avalon terrane we conclude that high grade alleghanian metamorphism in the avalon terrane occurred east of rocks now overlying it and that significant motion between avalon and this cover occurred after peak alleghanian metamorphism similarly contrasting metamorphic histories between avalon inliers willimantic window massabesic complex gneiss pelham dome and their cover rocks reveals the regional significance of this boundary the core rocks all show permian cooling but the cover rocks show post acadian cooling ages decreasing from east to west to the pelham area where hornblende ages in avalon and cover differ by only 35 rather than 80 m y model calculations show that thermal equilibrium between instantaneously thrusted blocks of rocks is generally obtained in tens of millions of years consequently underthrusting of avalon is constrained to be middle mississippian or younger because the leading edge of the underthrusting block would have been heated the longest and would have most closely approached thermal equilibrium with its cover core rocks of the pelham dome must have been relatively close to this leading edge thus carboniferous to permian underplating from a generally eastward direction best explains these thermochronologic relationships

attenuation of p and s waves in a magma chamber in long valley caldera california shallow earthquakes around the southwest boundary of long valley caldera west of the hilton creek fault are characterized by lack of s waves at regional seismic network stations to the northwest north and northeast and p waves for these same station event combinations are deficient in frequencies higher than about 2 3 hz earthquakes east of the hilton creek fault and southeast of the caldera have normal p and s wave signatures at the same stations these effects are explained by propagation through a magma chamber in the south central part of long valley caldera at depth greater than 7 8 km

tectonic implications of late paleozoic metamorphism in southeastern new england a map showing the distribution of pennsylvanian permian metamorphism in southern new england shows that the center of metamorphism is located substantially south and east of middle devonian acadian high grade metamorphism this suggests that the permian disturbance in southern new england is not related to delayed post acadian uplift the map is based on existing data within the pennsylvania basins and on a combination of published microstructures and apparent mineral ages in the surrounding rocks the distribution of metamorphism is however apparently related to major tectonic boundaries that were active during the late paleozoic and suggests that it may be the result of terrane accretion during the late paleozoic

p sv conversions at a shallow boundary beneath campi flegrei caldera italy evidence for the magma chamber seismograms from an active seismic experiment carried out at campi flegrei caldera near naples italy show a large amplitude 5v polarized shear wave following by less than 1 5 s p waves reflected at wide angle from a deep crustal interface early arriving sv polarized waves with the same delay to direct p waves are also observed in seismograms from a regional 280 km deep magnitude 5 1 earthquake such short delays of s to p waves are consistent with a p sv conversion on transmission occurring at a shallow boundary beneath the receivers the large amplitude of the converted sv phase along with that the p waves are near vertical requires a boundary separating a very low rigidity layer from the upper caldera fill the converted phases are interpreted as a seismic marker of a magma chamber the top of this magma chamber is located slightly deeper than the deepest earthquakes observed during the 1982 1984 unrest of campi flegrei

geometry of magma bodies beneath long valley california determined from anomalous earthquake signals one hundred and ninety two small earthquakes which occurred recently near the southern edge of long valley caldera at depths from 1 to 13 km are used to map the subsurface geometry of magma bodies in the caldera seismograms of these events recorded northwest north northeast and east of long valley with ray paths through the caldera are often anomalous in that s wave arrivals are absent and high frequency p wave energy is missing these anomalous ray paths intersect in an area of the south central caldera which we interpret to be a region of molten or partially molten rock this magma body lies between about 4 1 2 and at least 13 km depth and is 10 km long by 5 km wide beneath 7 km the magma appears to be relatively contiguous below 7 km and more dispersed above another magma body may be present in the northwest caldera coincident with an interpreted 7 8 km deep reflection from the top of a magma chamber

deformation near the long valley caldera eastern california 1982 1986 annual surveys of trilateration and leveling networks in and around long valley caldera in the 1982 1985 interval indicate that the principal sources of deformation are inflation of a magma chamber beneath the resurgent dome and right lateral strike slip on a vertical fault in the south moat of the caldera the rate of inflation of the magma chamber seems to have been roughly constant 0 02 km3 yr in the 1982 1985 interval but the slip rate on the south moat fault has decreased substantially in addition there is evidence for a shallow source of dilatation possibly dike intrusion beneath the south moat of the caldera in 1983 and less certain evidence for a deep source possibly magma chamber inflation beneath mammoth mountain in the western caldera in 1983 1985 deformation in the 1985 1986 interval as inferred from trilateration alone seems to be similar to that observed in 1984 1985

applications of attenuation tomography to imperial valley and coso indian wells region southern california spatial variations in seismic wave attenuation were tomographically imaged from observed s to p wave amplitude ratios in the imperial valley and coso indian wells regions of southern california in the coso indian wells region a highly attenuating body s wave quality factor q 30 coincides with a slow p wave anomaly mapped by walck and clayton 1987 this coincidence suggests the presence of a 3 to 5 km depth magmatic or hydrothermal body in the indian wells region in the imperial alley slow p wave travel time anomalies and highly attenuating s wave anomalies were found in the brawicy seismic zone at a depth of 8 12 kin the effective s wave quality factor is very low q 20 and the p wave velocity is 10 slower than the surrounding area these results suggest either magmatic or hydrothermal intrusions or fractures at depth possibly related to active shear in the brawley seismic zone this attenuation tomographic technique is shown to be useful in delineating the spatial variations in seismic wave attenuation and in estimating the effective seismic quality factor of attenuation anomalies

evidence for 600 year old basalt and magma mixing at inyo craters volcanic chain long valley caldera california andesite inclusions are found within the vent areas of two 600 year old rhyolite domes in the inyo craters volcanic chain eastern california both domes lie within the present northwestern sector of 0 73 ma long valley caldera inclusion morphology indicates that they were partially molten and ductile during incorporation within coarsely porphyritic rhyolite the xenocryst assemblage within the inclusions plagioclase hornblende biotite quartz sanidine is identical to the phenocryst assemblage within the host rhyolite microprobe analyses of xenocrysts within the andesite inclusions show that they have compositions similar to phenocrysts in the host rhyolite these data suggest that the andesite inclusions are the result of mixing of the coarsely porphyritic rhyolite with a more mafic end member whole rock geochemistry of the inclusions generally supports a simple mixing model between the coarsely porphyritic rhyolite of the inyo domes and typical postcaldera mafic lavas found throughout the western caldera moat the inclusions provide the first evidence for involvement of basaltic magma in the 600 year old rhyolite eruptions of the inyo craters volcanic chain and suggest the possibility that the rhyolitic eruptions were initiated by injection of basalt into the lower part of the silicic magma system periodic thermal replenishment of the magmatic system through basaltic injection may explain the relatively high temperatures encountered in drill holes within the western caldera moat the inclusions further suggest that although rhyolite eruptions are statistically more likely from the inyo craters volcanic chain future basaltic eruptions cannot be ruled out

location and configuration of magma bodies beneath long valley california determined from anomalous earthquake signals recordings of 281 small earthquakes which occurred recently near the southern edge of long valley caldera at depths from 1 to 15 km are used to map the subsurface geometry of shear wave attenuating bodies in the caldera signatures of these events recorded northwest north northeast and east of long valley with ray paths through the caldera are often anomalous in that s wave arrivals have very low amplitudes and high frequency p and s wave energy is missing for the same station event combinations the volcanic and geothermal history of the region suggests that ray paths for these anomalous signals have passed through magma which absorbed much of the s wave energy and also attenuated the higher frequencies with over 1200 normal and anomalous ray paths through the caldera we have located two apparently massive magma bodies in the central and northwest caldera as well as two more dispersed magma areas in the southern caldera and beneath crowley lake the central magma body extends from 4 5 to at least 13 km beneath the surface and the northwest body is as shallow as 5 5 km these two magma bodies are probably connected below about 8 km secondary arrivals seen on select seismograms are tentatively interpreted as reverberations inside the central magma chamber and are consistent with the interpreted lateral extent of the magma the northwest and central magma bodies lie beneath the northwest and southeast corners of the caldera s resurgent dome and probably connect beneath the dome s center also the shallower parts of these magma bodies and the southern anomalous area lie beneath the surface expression of the medial graben faults suggesting that structural features seen at the surface of the valley may be related to magma location at depth