venus topography a harmonic analysis a model of venusian global topography has been obtained by fitting an eighteenth degree harmonic series to pioneer venus orbiter radar altimeter data the mean radius is 6051 45 0 04 km the corresponding mean density is 5244 8 0 5 kg m 3 the center of figure is displaced from the center of mass by 0 339 0 088 km towards 6 6 10 1 n 148 8 7 7 e the figure of venus is distinctly triaxial but the orientation and magnitudes of the principal topographic axes correlate rather poorly with the gravitational principal axes however the higher degree harmonics of topography and gravity are significantly correlated the topographic variance spectrum of venus is very similar in form to those of the moon mars and especially earth we suggest that this spectral similarity simply reflects a statistical balance between constructional and degradational geomorphic processes venus and earth are particularly similar and differ from the moon and mars in that the larger bodies both exhibit a significant low degree deficit relative to the extrapolated trend of the higher harmonics

seismic energy releases from volcanoes seismic energy release during the precursory eruptive and declining stages of volcanic activities provides various information about the mechanisms of volcanic eruptions and the temporary developments of their activities hitherto the energy release patterns from precursory earthquake swarms were used to predict the eruption times especially of andesitic or dacitic volcanoes in this paper the discussion is expanded to quantify the total amount of seismic energy released at the threshold of volcanic eruptions with reference to the results observed at several volcanoes the results generally indicate that the cumulative seismic energy release from the precursory earthquake swarms exceed 1017 18ergs before eruptions at any andesitic or dacitic volvanoes this allows the seismic efficiency or the ratio of energy radiated seismically and the energy required for the volumetric expansion to be estimated by incorporating available deformation data with the seismic data the dependency of seismic efficiency on the type of volcanic activity i e non explosive outbreaks phreatic and magmatic eruptions dome formation etc was evaluated from observations at a few volcanoes that provided a variety of examples

mt etna a model for the 1989 eruption abstract during the 1989 eruption of mt etna one of the most important in the last 20 years in terms of effusion rate a pair of fractures formed on the slopes of the southeast crater cone and propagated in a few days trending ca northeast and southeast with large quantities of magma being drained from the northern fracture in contrast the southern fracture after a short initial eruptive phase grew by 5 km in five days without eruptive or seismic activity when the fracture system crossed the southern scarp of the valle del bove wall some 6 km se of the active crater an intense seismic swarm started in the same area this lasted about four days and consisted of several hundred events clustering in a small focal volume although such a sequence of failure events contains elements of compatibility with a shallow dyke intrusion it did not lead to the expected flank eruption which would have threatened settlements just a few kilometres from the distal end of the fracture the question arises therefore whether or not the fracture system was dyke related and if so to what extent we attempt here to answer such a question by integration and interpretation of the available field observation and geophysical data indeed field observation constraints on the southern fracture system and the typical depth 2 km of the swarm do not contain evidence for shallow dyke emplacement beneath the lowermost part of the sse fracture conversely the inferred direction and shape of the tremor source the location and extent of a gravity anomaly observed a few months before the eruption the near horizontal migration of the seismic focii during the swarm and the change with time of related focal mechanisms lead us to hypothesize that the extreme development of the fracture system at its south was due to a deeper intrusion beneath the southern slope of the volcano

survey of hydrothermally generated petroleums from the guaymas basin spreading center abstract the guaymas basin spreading center and hydrothermal system are generating an assortment of petroleum like bitumens by pyrolysis of organic matter in the overlying uncosolidated sediments samples collected by d s v alvin from hydrothermal fields in the southern trough exhibit large variations in the quantities and character of the solvent soluble organic materials various pyrolytic regimes combined with fluctuating thermal gradients and migration velocities may impart a compositional fractionation possibly by differential gaseous solubilization and hydrodynamic alteration of the multi component fluids the data suggest that differential condensation solidification biodegradation and water washing cause selective removal of components at the seabed many hydrothermal oils are unlike normal reservoir petroleums because they contain significant concentrations of polar material due to their rapid genesis and transport and probable solubilization or advection of polar immature components the biomarker distributions confirm high temperature pyrolysis at depth with variable entrainment of less thermally mature bitumens during transport data for shallow core samples from the northern trough indicate that those locations experienced variable influx predominantly volatiles of a low temperature pyrolysate

a model of the onset of the 1991 1993 eruption of etna italy abstract eruptive activity at etna resumed in the early morning of 14 december 1991 preceded by a 9 month inflation and a seismic swarm of appreciable intensity in the early hours of 15 december eruptive vents opened on the western wall of valle del bove at about 2300m elevation and fed the main lava flow the fracturing episode associated with the eruption outburst affected the principal local structural systems trending nne ssw and nnw sse respectively which in recent years have produced several lava flows we propose a model of the eruptive mechanism based on a number of seismological volcanological and field data discussed in the framework of the available information from ground deformation measurements in particular the model hypothesizes the depressurization of a shallow batch of magma owing to the rapid magma withdrawal which caused the downdropping of one side of the eruptive fissure by normal faulting after a momentary stop a shallow inflation drove the restart of the eruption by the opening of the lower edge of the fissure

venus gravity anomalies and their correlations with topography this report provides a summary of the high resolution gravity data obtained from the pioneer venus orbiter radio tracking data gravity maps covering a 70 latitude band through 360 of longitude are displayed as line of sight and vertical gravity topography converted to gravity and bouguer gravity maps are also shown in both systems topography to gravity ratios are made over several regions of the planet there are markedly different ratios for the aphrodite area as compared to the beta and atla areas

cherts of the barberton greenstone belt interpreted as products of submarine exhalative activity cherts studied from the southern barberton greenstone belt are identified as replacement products of various pillow lavas pyroclastic deposits and sedimentary deposits silica filled fracture systems within the cherts have been found to be associated with this silicification process these systems include 1 silica filled fractures 2 silica vein stockworks 3 silica cemented breccias and 4 pseudoconglomerate bodies some of the silica filled fracture systems can be shown to have formed soon after sedimentation silica vein stockworks can be seen to develop laterally into silica cemented breccias the fragments of which have sometimes become rounded within the fracture systems to form pseudoconglomerate the cherts are interpreted as silicified hemi pelagic sediments that overlie pillow lavas some of which are now also cherts cells of convective sea water interacted with oceanic crust to produce silica rich fluids these fluids silicified lava and sediments on re entering the near surface env

tectonics and evolution of venus the global tectonics of venus differs significantly from that of earth most markedly in that the surface is covered predominately by gently rolling terrain there apparently are no features like ocean rises the gravity is positively correlated with topography at all wavelengths and the few highlands are estimated to be supported or compensated at a depth of approximately 100 kilometers the surface of venus appears to be covered mainly by an ancient crust the high surface temperature making subduction difficult it seems likely that well over 1 billion years ago water was destabilized at the surface and soon after plate tectonics ceased the highlands appear to be actively supported presumably as manifestations of long enduring hot spots

earthquake activity during the 1983 etna eruption seismic stations with automatic p picking and satellite retransmission were set up on mount etna following the eruption started on march 1983 positions of the stations were chosen in order to complement the permanent telemetered network of catania university

venus volcanism and rift formation in beta regio a new high resolution radar image of beta regio a venus highland area confirms the presence of a major tectonic rift system and associated volcanic activity the lack of identifiable impact craters together with the apparent superposition of the theia mons volcanic structure on the rift system suggest that at least some of the volcanic activity occurred in relatively recent geologic time the presence of topographically similar highland areas elsewhere on venus aphrodite terra dali chasma and diana chasma suggests that rifting and volcanism are significant processes on venus

changes in b values before the etnean eruption of march august 1983 the etnean eruption of march august 1983 can be considered among the most important in the last years the associated seismic activity was carefully studied in order to detect possible changes of theb coefficient in the equation relating frequency to magnitude of the seismic events the analysis of 1056 b type earthquakes which occurred during the three months before the eruption was carried out according to the maximum likelihood method a significant increase of theb value was noticed followed by an almost linear decrease from a mean value of 1 7 to 0 8 starting three weeks before the eruption the observed pattern ofb values has been compared with that related to the etnean eruption of march 1981 and some considerations on the volcano dynamics have been made

venus limited extension and volcanism along zones of lithospheric weakness three global scale zones of possible tectonic origin are described as occurring along broad low rises within the equatorial highlands on venus lat 50 n to 50 s long 60 to 310 the two longest of these tectonic zones the aphrodite beta and themis atla zones extend for 21 000 and 14 000 km respectively several lines of evidence indicate that beta and atla regiones located at the only two intersections of the three major tectonic zones are dynamically supported volcanic terranes associated with currently active volcanism rift valleys south of aphrodite terra and between beta and phoebe regiones are characterized by 75 to 100 km widths raised rims and extensions of only a few tens of kilometers about the same magnitudes as in continental rifts on the earth horizontal extension on venus was probably restricted by an early choking off of plate motion by high crustal and upper mantle temperatures and the subsequent loss of water and an asthenosphere

the silica cycle in the precambrian abstract whereas the modem silica cycle is heavily dependent on the biology of silica secreting organisms such as the diatoms the precambrian silica cycle was dominated by inorganic reactions among dissolved silica clay and zeolite minerals and organic matter there is no evidence for deposition of a layered amorphous silica but abundant evidence for diagenetic silicification in the neoproterozoic the time period chosen for this analysis the influx of silica to the neoproterozoic ocean would have been governed as today by the balance among tectonism weathering and hydrothermal input the evidence of neoproterozoic rocks favors tectonic and weathering regimes not very dissimilar to those of the early phanerozoic and hydrothermal inputs significantly altered at certain periods one flux would have been very different the present diffusional influx of silica from interstitial waters into the oceans would have been altered to an efflux from the oceans to interstitial waters reactions of dissolved silica with inorganic phases would have controlled silica concentrations at a level of about 60 ppm this figure is based on experimental determinations of solubility and silica sorption on clay minerals and zeolites as silica in interstitial waters was lowered by opal ct precipitation a diffusional efflux from the ocean into the sediment was set up silicification would have started in early diagenesis as opal ct precipitated from interstitial waters diffusion of silica from overlying seawater declined as the sediment was buried a few hundred meters and the diffusion path lengthened during this still relatively shallow burial silicification continued but this time powered by hydrologie transport of dissolved silica from coastal plain sediments during transgressions and regressions thus the major removal of silica from the neoproterozoic ocean took place by diagenetic reactions

generation of short chain acid anions in hydrothermally altered sediments of the guaymas basin gulf of california an initial investigation of the concentration distribution of short chain organic acid anions in hydrothermally altered sediments of the guaymas basin reveals the presence of high concentrations of acetate and propionate relative to normal marine sediments acetate and propionate concentrations appear to vary as a function of sediment temperature and reach concentrations 1100 and 200 m respectively at temperatures 100 c as in previously studied oil field formation waters these acid anions probably play a key role in the generation of co2 and hydrocarbons gases hydrogen ion ph buffering the complexing of aqueous ions and mineral dissolution and precipatation at temperatures 100 c these and other acid anions should be important substrates for heterotrophic bacterial processes including sulfate reduction

support of long wavelength loads on venus and implications for internal structure a thin shell formulation for computing stress distributions on the surfaces of one plate planets is developed the lithospheric model includes laterally varying density anomalies at two depths corresponding to undulations on a crust mantle boundary and variations in upper mantle density for a given set of model parameters values for the vertical displacement crustal thickness anomaly and mantle density anomaly required to satisfy the topographic and gravitational boundary conditions are computed along with the resulting stress field the computed stresses can then be compared to stress directions inferred from observed tectonic features this formulation is used to study loads on venus s lithosphere and it is found that most highland areas appear to have been formed in a state of isostasy or by uplift with a significant portion of their support probably derived from dynamic processes in the mantle a possible exception is ishtar terra whose circumferential mountain belts are most consistent with a radially compressive stress regime generated by flexural loading reasonable lithospheric models predict low density mantle beneath atla beta eisila thetis and tethus regiones and thick crust accompanied by higher density mantle under ishtar terra and ovda regio thus western aphrodite terra appears to be more closely related to ishtar than to the rest of the equatorial highlands these results favor a hot spot model of venus tectonics in which beta eastern aphrodite eisila and tethus are expected to be relatively young and thermally active while ovda and ishtar should be older and less active

thick shell tectonics on one plate planets applications to mars a theory for stress distributions in thick lithospheric shells on one plate planets is developed based on the zero frequency equations of a self gravitating elastic spherical shell overlying a strengthless fluid stress distributions in lithospheres are reviewed for both the compensated and flexural modes in the former case surface stresses only depend on surface topography while for the latter case it is shown for long wavelengths that stress trajectories are mainly dependent on the lithospheric lateral density distribution and not on elastic properties computational analyses are carried out for mars and it is found that isostatically compensated models correctly predict the graben structure in the immediate tharsis region and a flexural loading model is satisfactory in explaining the graben in the regions surrounding tharsis a three stage model is hypothesized for the evolution of tharsis isostasy with north south graben formation on tharsis followed by flexural loading and radial graben formation on the perimeter of tharsis followed by a last stage of loading with little or no regional deformation this model is consistent with the martian lithosphere monotonically thickening over geologic time

geochemistry of shallow sediments in guaymas basin gulf of california hydrothermal gas and oil migration and effects of mineralogy offset of prograde hydrothermal alterations by retrograde reactions is evident in sediments from a piston core pc6 8 m long from the guaymas basin gulf of california geochemical and mineralogical analyses of sediments and pore fluids show superimposed geochemical signals that indicate currently active reactions are modifying previously formed secondary solids hydrothermal barite was formed in a previously active flow channel between 150 and 250 cm depth hydrothermal gypsum is presently dissolving at depths 780 cm below which hydrothermal fluid flow is channeled by a secondary dolomite layer thermal stress of organic matter generates hydrothermal gas and petroleum having wide ranges of compositions and maturities a significant amount of hydrothermally generated oil has been transported laterally through a porous debris flow approx 240 cm subbottom overwhelming the indigenous bituminous matter water soluble petroleum constituents have disseminated throughout the cored sediments from 400 to 700 cm depth the immature organic matter continues to experience thermally enhanced diagenesis and an even higher thermal stress has occurred at the bottom of the core enhanced bacterial degradation of organic matter has led to depletion of pore water sulfate and enrichment of ammonium nh4 in both pore fluid and sediment resulting in formation of nh4 bearing secondary clay minerals at depths below 780 cm

seismic patterns and fluid dynamic features preceding and accompanying the january 15 1990 eruptive paroxysm on mt etna italy abstract on the basis of data collected during a seismological investigation carried out on mt etna italy during a phase of the volcanic activity marked by progressive transition from strombolian bursts to effusive paroxysms at the se crater january 1990 three types of seismic regime have been recognized they are interpreted to be linked to modifications in the regimes of volcanic gases and vapours within the upper levels of the magma column as suggested by recent studies on the dynamics of magmatic fluids our analysis also reveals that the use of usual seismic parameters such as the temporal patterns associated with the occurrence of discrete low frequency events spindles and the coefficient m of the ishimoto iida law can furnish information about variations in the behaviour of the system before the irreversible evolution of the volcanic activity toward eruptive paroxysms

fault type predictions from stress distributions on planetary surfaces importance of fault initiation depth faults on planetary surfaces initiate at depth where stresses due to the weight of overburden are important consequently the prediction of fault type from calculated stresses at depth can yield different faults than those predicted using the surface criteria commonly employed in geophysical models for elastic plate flexure models of mascon loading on the moon stresses calculated at the surface predict that the most likely faulting is strike slip for which there is no direct evidence at a distance from the basin center where grabens are found modifying these plate flexure calculations to account for the weak surface layer of ejecta megaregolith 1 3 km thick that overlies the relatively strong lunar lithosphere results in model stresses in the outer layers of the moon tens of bars that are an order of magnitude lower than in the lithosphere hundreds of bars because faults bounding lunar grabens initiate at the mechanical discontinuity between the megaregolith and lithosphere and because the reverse faults likely responsible for wrinkle ridges probably initiate at the basalt basin floor contact 0 5 2 km deep model stresses must be calculated at these depths superposing nonisotropic stresses due to the weight of the overburden at the depths of these mechanical discontinuities with plate flexure model stresses correctly predicts maximum stress differences in an interior zone of expected wrinkle ridges surrounded by a zone of concentric normal faults or grabens and effectively eliminates the strike slip zone in agreement with observations the absence of strike slip faults from almost all planetary and satellite surfaces may be explainable if lithospheric deformation model stresses are calculated at important shallow crustal mechanical discontinuities where the maximum compressive stress is vertical thereby favoring the formation of normal faults if true mechanical discontinuities in the shallow crusts of planets and satellites play an extremely important and hitherto unrecognized role in the formation of structures observed on their surfaces

continental rifting and the origin of beta regio venus topographic maps based on pioneer venus altimetry suggest that beta regio an elevated feature centered at 27 n 282 e is analogous to domes associated with continental rift systems on earth this interpretation is consistent with the commonly quoted analogy between the east african rift system and the topography of the region from beta regio southward to phoebe regio if beta regio is a dome major structural uplift of the crust of venus is implied suggesting a more dynamic upper mantle than would be the case if beta regio were simply a large volcanic construct

the role of regional tectonics magma pressure and gravitational spreading in earthquakes of the eastern sector of mt etna volcano italy data concerning m 3 3 earthquakes that occurred in the eastern sector of mt etna volcano sicily italy in the period 1984 1989 are here presented and discussed only events with reliable focal mechanisms and detailed macroseismic investigations have been considered instrumental information come from local seismic networks both permanent and temporary run by the university of catania uct the seismological observatory of acireale soacr the cnrs grenoble france and the institut de physique du globe de paris ipg france observations regarding the macroseismic effects have been collected by means of field recognitions mainly carried out by the authors all available data are analyzed in light of the more recent interpretations on the kinematic behaviour of the shallowest structures of etna with particular reference to its eastern flank which according to borgia et al 1992 is likely affected by a seaward gravitational migration driven by the sliding of this sector of the volcanic apparatus over its clay rich substratum the results obtained support the hypothesis that the shallow seismicity of the eastern flank of mt etna is related to a complex stress field due to the combined effects of a the tectonics associated with the interaction between the african and eurasian plates b the rise of magma into the crust and c the gravitative seaward sliding of the eastern sector of the volcano in particular we hypothesize that 1 tectonic forces caused the end of the 1984 eruption by means of a locking mechanism 2 the increment of magma pressure at depth is periodically responsible for local re orientation of the stress field acting on the volcano which also produces earthquakes of high intensity 3 shallow earthquakes z 5 km showing eastward compression mechanism could be related to the gravitational sliding of unbuttressed sectors of the volcanic body accompanying the emplacement of intrusive dykes

isostasy and the ambient state of stress in the oceanic lithosphere two conditions are normally considered to be characteristic of the state of stress in a system in local isostatic equilibrium the first is that the stress is hydrostatic below some depth the level of compensation and the second is that above that depth there are no vertical shear stresses in the case of a planar two dimensional mid ocean ridge these two conditions suffice to determine the stress completely except for the normal component parallel to the ridge but to determine that component an additional assumption is required it is proposed here that an appropriate additional assumption is that in unflexed regions where the stress is due only to the elevation of mid ocean ridges the deviatoric stress in the oceanic lithosphere is a minimum the unique state of deviatoric stress consistent with this assumption is horizontally compressive in a direction perpendicular to the axes of mid ocean ridges with a magnitude in old ocean basins of a few hundred bars in agreement with oceanic intraplate earthquake mechanisms at any given distance from the ridge the stress is maximum at the seafloor and it decreases with depth at about the same rate as the cooling of the lithosphere so that the entire thermal boundary layer may be regarded as acting to support the global square root of age topography of the oceans

textural preservation in siliceous hot spring deposits during early diagenesis examples from yellowstone national park and nevada u s a abstract a petrographic mineralogical and geochemical investigation reveals evidence for selective preservation of microfacies in siliceous sinters plio pleistocene sinters from steamboat springs nevada contain identical but fewer petrographic textures than pleistocene sinters from artist point yellowstone national park wyoming the latter are preserved in altered sediments of unknown origin a sequence of pore filling and mineralogical changes explains the excellent preservation of nine microfacies and two petrographic textures plio pleistocene steamboat springs sinters are preserved in successions of andesitic basalt flows and were likely subjected to a more extreme thermal and a different hydrological environment than those from yellowstone national park most microfacies were obliterated during postdepositional heating of steamboat spring sinters differences in diagenetic histories related to depositional environment water chemistry and or subsequent burial must account for the loss of textural evidence between the diagenetic stage represented by artist point sinters and that of steamboat springs sinters hence early postdepositional history affects both the likelihood and quality of microfossil preservation

activity of mount etna 1977 1983 volcanic phenomena and accompanying seismic tremor a period of exceptional eruptive activity for both size and frequency of lateral eruptions and violence of paroxysms at the summit craters characterized etna from 1977 to 1983 the complex fissure eruption of 1971 and the eccentric eruption of 1974 on the w flank appear to be precursors of the present activity which began in july 1977 with the change in style of summit eruptions from moderate persistent activity to intermittent paroxysms the ne crater produced a score of such unusual eruptions up to march 1978 lasting a few days or even a few hours with powerful lava fountaining and flows up to 7 km long with high effusion rates other solely explosive paroxysms occurred in april 1980 in the chasm central crater and later at the se crater a return to a period of gentle persistent activity may august 1980 at the latter vent was interrupted by the recommencement of violent explosive and effusive events at the ne crater in september 1980 and february 1981 particularly remarkable lateral flank eruptions occurred in april june 1978 fracturing of the se slope from 3000 to 2600 m altitude august 1978 se ene 3000 2300 m november 1978 se 3000 1650 m august 1979 se e ne 3000 1600 m march 1981 nnw 2600 1120 m and march august 1983 s 2700 2260 m the first four of these eruptions were preceded and accompanied by violent lava fountaining at the se crater all were accompanied and or followed by phreatic explosions principally from the bocca nuova or w central crater the flow emitted on 4th august 1979 from the eastern flank reached next to the village of fornazzo in half a day that of 17 march 1981 the most destructive cut all communications roads and railways round the northern foot of the mountain in the 14 hours following its appearance it was in places 800 m wide and in less than 5 days 18 106 m3 of lava had been emitted covering 6 km2 of largely agricultural land from 28th march to 6th august 1983 the slow but sustained emission of lava gradually covered and destroyed several tourist installations between 2300 and 1100 m altitude each of these eruptive paroxysms was marked by a substantial increase in the trace amplitude of harmonic seismic tremor the intensity of which being not directly proportional to the observed activity it is likely that turbulence caused by the rising of magma in the main conduits had an important effect in the energy of the tremor this is considerably reduced after the initial paroxysmal phase e g in 1983 yet the lava effusion may continue tremor energy attains relatively high levels from 1 to 5 months before eruptions this may be caused by the recharging of the system of dykes and fissures leading up to an eruptive event after a variable period several distinct periods of recharging may precede one eruption there were three before the 1983 eruption in the few days preceding the start of some of the eruptions aug 1979 sep 1980 feb and mar 1981 a rapid diminution in the amplitude of tremor occurred this may be associated with changes in the stress field caused by the opening of new fractures or the closing opening of pre existing ones

oceanic ridge crest processes oceanic ridge crest processes constitute a vigorous frontier of multidisciplinary research with global ramifications in the earth atmospheric and biologic sciences this deep ocean frontier is gradually opening in response to investigations which are elucidating the role of these processes in the lithosphere hydrosphere atmosphere and biosphere and the application of advanced instrumental and interpretive techniques which are increasing the quality and quantity of data these techniques include crustal drilling with borehole logging deep sea drilling project reports high resolution bathymetry sea beam etc side scan sonar seamarc i and ii gloria etc imagery with video and photography multichannel seismics determination of water column properties ctd with rosette sampler etc and submersible applications

venus ishtar gravity anomaly ishtar has been detected as a positive gravity anomaly verification of this result has been made with two independent data sets isostatic compensation of the ishtar topography using an airy model is achieved at approximately 150km

role of membrane stresses in the support of planetary topography the principal purpose of this paper is to examine whether membrane stresses can support topographic loads on planetary elastic lithospheres it is found that the ability of a spherical shell to support loads through membrane stresses is determined by the nondimensional parameter ed gr2 where d is the thickness of the elastic lithosphere is the density difference between the mantle and crust and r is the radius of the planetary body when this parameter is large membrane stresses can fully support topographic loads without flexure and when it is small the influence of membrane stresses can be neglected solutions of the equation governing the behavior of a spherical shell are obtained for a topographic load expressed in terms of spherical harmonics spherical harmonic expansions of the measured gravity and topography for mars and the moon are compared with the theory it is found that for mars the support of topography is primarily due to membrane stresses for n 8 and for the moon for n 17 for mars the data for 4 n 7 give 0 5 for the moon the data have considerable scatter that is attributed to the mascons but generally correlate with 0 5 if bending stresses are neglected the governing equation for the deflection of the spherical shell is legendre s equation a general solution is obtained for an axisymmetric load this solution is applied to the tharsis region on mars the 60 65 compensation of this region requires that 0 6 the well defined fracture pattern surrounding the tharsis region is attributed to tensional membrane stresses

beryllium isotope systematics of submarine hydrothermal systems abstract both 10 be and 9 be concentrations have been measured in a hydrothermal fluid from the sediment starved east pacific rise at 21 n as well as in fluids and associated sediments from the sediment covered guaymas basin and escanaba trough hydrothermal systems the results show that while the 9 be concentrations in fluids from both types of systems are comparable about three orders of magnitude higher than that of the seawater supplying the systems their 10 be concentrations are significantly different the 10 be concentration in the solution emanating from the studied sediment starved hydrothermal system is at least two times lower than that of the pacific bottom water by contrast the 10 be concentration in hydrothermal solutions from sediment hosted environments are enriched over ambient seawater and are at least one order of magnitude higher than that measured in the 21 n fluid the sedimentary cover being the only available source of 10 be in these systems these data demonstrate that 10 be is an unambiguous tracer of interactions between hydrothermal fluids and recent sediments as an example this study examines hydrothermal solutions from the sediment starved mariana trough back arc basin which have compositions similar in many respects to those from sediment hosted environments however these fluids are depleted in 10 be relative to ambient seawater which precludes interaction with recent sediment as the cause of their peculiar chemical composition

plate tectonics on venus the high surface temperature of venus implies a permanently buoyant lithosphere and a thick basaltic crust terrestrial style tectonics with deep subduction and crustal recycling is not possible overthickened basaltic crust partially melts instead of converting to eclogite because mantle magmas do not have convenient access to the surface the 40 ar abundance in the atmosphere should be low venus may provide an analog to archean tectonics on the earth

seismic activity preceding the 1983 eruption of mt etna the march august 1983 eruption of mt etna can be considered as one of the most important in the last years the analysis of seismic activity during the three months immediately before the eruption showed interesting variations of theb coefficient in the frequency magnitude relationship that have been linked to possible changes of the stress field in the etnean region the eruption start was also preceded by a strong seismic crisis with epicenters mostly on the southern eastern and southwestern flanks of the volcano and characterized by the shallowness of most of the events h 3 km the data analysis has led to a hypothesis on the eruption occurrence based on a model of dynamic evolution of the stress field acting on mt etna

variation ofb values before the etnean eruption of march 1981 a fissural eruption occurred from the northern flank of mt etna on march 17 1981 and the associated earthquake activity recorded by the seismic network operating on the volcano was carefully examined to detect possible variations ofb values according to the frequency magnitude relationship before the start of the eruption

heat transfer experiment in marine pelagic clay implication for subseabed disposal of high level radioactive waste and contact metamorphism in the guaymas basin gulf of california abstract a heat transfer experiment has been conducted in marine pelagic clay to provide data to model geomechanical and geochemical properties of seabed sediment exposed to a relatively large thermal gradient and sea floor pressure this experiment entailed emplacement of a resistance type heat source in the center of a tank of sediment 1 m3 from the central north pacific and overlain by a reservoir 2 m3 of artificial seawater the system was pressurized to 500 bar pore fluid samples extracted from sediment at 5 150 c and samples of sediment altered at temperatures from 30 to 270 c yield information on mass transport and sediment alteration processes sediment alteration in the vicinity of the heat source caused an increase in sediment shear strength water and na2o contents and formation of hydrated analcime at the expense of kaolinite and quartz this so called baked zone consisted of sediments altered at temperatures of 170 c the high na2o contents of baked zone sediments require additions of na in amounts exceeding that that available in pore fluid alone and suggest convective circulation of sea salt solution along the relatively permeable boundary between the sediment and the heat probe quartz dissolution and removal of sio2 by convectively circulating sea salt solution moreover played an important role in microcrack extension and permeability enhancement in the baked zone itself mineralogic and chemical changes in baked zone sediment provide useful insight into the mechanism of alteration of hemipelagic sediments in contact with dolerite sills in the guaymas basin gulf of california sediment exposed to temperatures of application of results from the heat transfer experiment to the subseabed disposal concept shows that thermal diffusion will dominate near field mass transport provided sediments remain relatively unconsolidated and impermeable and are not affected by convection assuming convection does not occur or if it does is localized and closed to recharge pathways ph at temperatures of 250 c will most probably be mildly acidic the redox intensity fo2 will be constrained by sediment chemistry and mineralogy

tectonic contrasts between venus and the earth the long wave features of the gravity field of venus differ from those of the earth s field not only in their strong positive correlation with topography but also in their gentler spectral slope these properties are inconsistent with generation of the gravity field by plate tectonics or by processes at great depths they are consistent with generation by a mantle convective system supporting the broad features in topography with an effective compensation depth of about 450 km

siliceous sediments of the guaymas basin the effect of high thermal gradients on diagenesis recent siliceous sediments of the guaymas basin a rapid spreading center in the gulf of california are intruded by basaltic sills which induced significant diagenetic and metamorphic changes in the sediments the transformation from opal a to opal ct opal ct to quartz and opal a directly to quartz in these sediments cored on dsdp leg 64 can be used to infer the temperature history and order of emplacement of intrusives at or near contacts with sills of 30 m or greater average thickness opal a inverts directly to authigenic quartz but there is less quartz than would be expected from the amount of opal a dissolution opal ct forms in sediments sandwiched between adjacent sills based on high rates of quartz nucleation and growth at high temperatures 150 c and on considerations of convective solution transfer opal ct is thought to form only where temperatures were lower or at positions between sills an environment which prevents rapid convective dissipation of silica in solution where temper

volcanic tremors and shocks associated with eruptions at etna sicily september 1980 abstract three strong ash eruptions superimposed on an otherwise rather quiet state of volcanic activity took place from the northeast crater of etna during september 1980 the eruptions lasted only a few hours and were associated with an increase in the amplitudes of volcanic tremors by a factor of 10 compared with the mean value of the preceding months special emphasis in the tremor analysis was given to the eruption of september 26 1980 the time function of the amplitude envelope of the tremors follows a mathematical function which resembles the behaviour of a mechanical oscillator with a resonant period of about 300 s a detailed spectral analysis of the tremors before during and after the eruption reveals factors which seem to be associated with the ascent and descent of the tremor sources in the magma columns the turbulent eruptive magma flow apparently is concentrated in a few dykes and pipes their eigenvibrations during an eruption are observed as spectral peaks of small bandwidth superimposed on the smoother spectrum without a volcanic eruption an analysis of seismicity shows a maximum in august with epicenters clustering along a nw se direction

beryllium isotope geochemistry of hydrothermally altered sediments profiles of both10be and9be concentrations have been measured in sediments from the dsdp leg 64 site 477 core established in the deep guaymas basin in an area of high heat flow this site offers the opportunity of studying hydrothermal alteration induced by two distinct hydrothermal systems those caused i by the intrusion of hot sills into porous sediments and ii by heat associated with a magma chamber the results show that the distribution of both beryllium isotopes between two operationally defined sedimentary phases material susceptible to hydroxylamine leaching and residual material changes with the degree of hydrothermal alteration a decrease in10be 9be in the leachable phase of altered sediments relative to unaltered sediments indicates that hydrothermal alteration is not limited to a simple chemical leaching but involves redistribution of the beryllium isotopes between metamorphosed sedimentary phases in addition the similarity of the mean10be 9be ratios associated with the leachable and the residual phases in the part of the core which has experienced greenschist facies metamorphism suggests that this process leads in the prevailing geophysical conditions to isotopic homogenization a close relationship exists between10be and mgo concentrations in altered sediments from this core this supports earlier observations that beryllium is not easily removed from the sedimentary column and indicates that this remains the case even under the conditions encountered in submarine hydrothermal systems

gravity topography and crustal evolution of venus abstract the gravity anomalies of venus although small by comparison with those on mars and the moon are still much larger than those on earth for large features on venus even the low degree spherical harmonic terms for venus gravity field indicate a close association of broad positive gravity anomalies with major topographic highs this is striking contrast to the situation on earth where the broad regional gravity anomalies show little correlation with continental masses or plate tectonic features but instead appear to be caused by deep mass anomalies a method for estimating radial gravity anomalies from line of sight acceleration data their interpolation and use of iteration for improved radial anomaly estimates is outlined a preliminary gravity anomaly map of venus at spacecraft altitude prepared using first estimate values is presented a profile across the western part of aphrodite along longitude 85 e was analyzed using time series techniques an elastic plate model would require a plate thickness of about 180 to 200 km to match the general amplitude of the observed gravity anomaly about 33 mgal a thickness much greater than that found for earth structures and because of high surface temperatures unlikely for venus an airy isostatic model convolved with the topography across aphrodite however provides a better match between the predicted and observed gravity anomalies if the nominal crustal thickness is about 70 to 80 km this thickness is over twice that for continental crust on the earth and considerably greater than that of the earth s basaltic ocean crust only 5 km a different differentiation history for venus than that of the earth thus is anticipated high gravity anomalies 110 mgal occur over beta regio and over the topographic high in eastern aphrodite both highs are associated with regions where detected lightning is clustered and thus the topographic features may be active volcanic constructs the large gravity anomalies at these two sites of volcanic activity require an explanation different than that indicated for western aphrodite

gold enrichment and the bi au association in pyrrhotite rich massive sulfide deposits escanaba trough southern gorda ridge high gold contents to 10 1 ppm avg 1 4 ppm n 34 occur in pyrrhotite rich massive sulfide samples from the sediment covered floor of the escanaba trough the slow spreading southernmost segment of gorda ridge these concentrations reflect the presence of primary gold formed during high temperature hydrothermal activity in mounds and chimneys and secondary gold deposited during sea floor weathering of massive sulfide primary gold occurs as fine grained 2 m secondary gold grains have a porous flaky morphology and occur in samples in which pyrrhotite is oxidized and replaced by fe oxyhydroxides fe sulfate and sulfur mounds and chimneys dominated by pyrrhotite and containing lesser amounts of isocubanite chalcopyrite and fe rich sphalerite were formed by high temperature estimated range 325 275 c reduced low sulfur vent fluids the mineral and fluid compositions during this main stage of hydrothermal venting reflect subsurface interaction between circulating hydrothermal fluids and turbiditic sediment containing as much as 1 1 percent organic carbon as the deposition of pyrrhotite cu fe sulfides and sphalerite waned a volumetrically minor suite of sulfarsenide arsenide bi and au minerals was deposited from highly reduced late main stage fluids diffusing through mounds and chimneys the low solubility of au as a bisulfide complex and the absence of fluid mixing during this stage of hydrothermal activity apparently inhibited the precipitation of gold directly from solution instead gold precipitation is thought to be linked to elevated concentrations of bi in the late main stage fluids the textural relationships of au and bi minerals in pyrrhotite rich samples low melting point of native bismuth 271 4 c and recent experimental results on au and bi in hydrothermal fluids contribute to the hypothesis that gold was effectively scavenged from the escanaba trough vent fluids by coexisting droplets of liquid bismuth additional phase relationships of alloys in the au bi system indicate that deposition of native bismuth and maldonite occurred at temperatures as low as 241 c bismuth droplets trapped in void space between main stage mineral grains scavenged gold from ambient hydrothermal fluid to a greater extent than bismuth enclosed by late forming pyrrhotite the limited solid solution of au in bi can explain the apparent exsolution texture in which gold blebs are hosted by native bismuth the electrum native bismuth with gold inclusions and galena represent the last traces of gold mineralization from late main stage fluids during sea floor weathering and the oxidation of pyrrhotite in the mounds and chimneys secondary gold formed as aggregates of colloidal particles along ph gradients between acidic pore waters and ambient seawater gold was mobilized from earlier formed primary gold minerals and transported as aqueous chloride complexes the reduction of au iii by residual fe2 in partly altered pyrrhotite and adsorption of colloids by fe oxyhydroxides may have influenced the location of secondary gold grains within the alteration front solubility differences between gold and silver chloride complexes at low temperature account for the low ag content of secondary gold grains the high concentrations of bi and thus the association of au and bi minerals in pyrrhotite rich massive sulfide can be ascribed to the extensive interaction of hydrothermal fluids with sediment in the escanaba trough in contrast the absence of the au bi association in massive sulfides at other ridges including other sediment covered sites at middle valley and the guaymas basin reflects the predominantly basaltic source rocks

venus gravity analysis of beta regio radio tracking data acquired over beta regio were analyzed to obtain a surface mass distribution from which a detailed vertical gravity field was derived in addition a corresponding vertical gravity field was evaluated solely from the topography of the beta region a comparison of these two maps confirms the strong correlation between gravity and topography which was previously seen in line of sight gravity maps it also demonstrates that the observed gravity is a significant fraction of that predicted from the topography alone the effective depth of complete isostatic compensation for the beta region is estimated to be 330 km which is somewhat deeper than that found for other areas of venus

venus banded terrain tectonic models for band formation and their relationship to lithospheric thermal structure recent radar images of the mountains of ishtar terra venus at approximately 3 km radar resolution show a series of linear bands of alternatively greater and lesser backscatter the bands are generally aligned with the topographic strike of the mountain ranges we test the hypothesis that these band features are tectonic in origin the product either of folding during lithospheric compression or of block faulting during lithospheric extension tested models for folding include uniform elastic and viscous plates overlying inviscid substrates layered plates and a viscous half space in which viscosity decreases exponentially with depth the uniform plate models can produce folds at a dominant wavelength equal to the characteristic spacing between bands on venus about 15 to 20 km if the elastic or high viscosity layer is at most a few kilometers in thickness though the required compressive stresses are several kilobars in magnitude for the case of an exponentially decreasing viscosity the skin depth plays the role of layer thickness and similar results hold layered elastic or viscous plates however can fold at the required wavelength with subkilobar stresses and therefore are favored over uniform plate models tested extensional models include graben and horst formation in a surface elastic brittle layer imbricate normal faulting and necking of a surficial layer that is everywhere in a state of extensional failure the spacing between adjacent extensional features can match the spacing between bands on venus if the surficial brittle layer is no more than a few kilometers in thickness both compressional and extensional models for band formation are therefore consistent with radar images of venus mountain ranges at the image resolution presently available additional evidence however including the linearity and continuity of individual bands the relationship between band trends and topographic contours in some regions near the ends of mountain ranges and the very different manifestations of lithospheric extension in other highland areas of venus leads us to favor the hypothesis that banded terrain formed contemporaneously with the mountain ranges of ishtar terra as a result of horizontal compression of the venus lithosphere independent arguments on the thermal structure of the venus crust and the mechanical behavior of crustal rocks as a function of temperature indicate that the elastic lithosphere of venus is approximately 1 10 km thick we therefore suggest that the surficial layer of elastic brittle or high viscosity behavior required for either the compressional or extensional models may in fact be the elastic lithosphere of the venus highlands at the time of band formation

a correlation between earthquakes and eruptive phases at mt etna an example and past occurrences a local array was deployed on mt etna to follow precisely the seismic activity occurring at the end of the eruption of the southeast crater in october 1984 swarm activity at sea level beneath the summit was followed by isolated earthquakes a few kilometres deeper and more distant and eventually by destructive earthquakes on the lower slopes there was no continuous migration between these domains of eruptive and seismic activity but the time relation is striking a revised use of the exceptional record over four centuries of eruptions and destructive earthquakes in the mt etna region documents the statistical significance of this type of occurrence of major earthquakes in relation to the end of eruptions

evidence for retrograde hydrothermal reactions in near surface sediments of guaymas basin gulf of california abstract the guaymas basin gulf of california is a sediment covered portion of the east pacific rise where hydrothermal fluids exit to the sea floor via both direct emanation from spires and chimneys as well as by diffuse circulation through organic matter rich sediments thermal alteration of organic matter generates hydrothermal gas and petroleum which have wide ranges of compositions and maturities diffuse fluid discharge reaches the sediment seafloor interface via channelized pathways that change with time mineralogical and geochemical analyses of sediments and pore fluids from an eight meter long piston core pc 6 show superimposed geochemical signals indicating that presently active reactions are modifying previously formed authigenic materials a slump deposit 140 250 cm shows the presence of hydrothermal barite and a substantially changed chemical composition analysis of the extractable organic matter indicates that a significant amount of hydrothermally generated oil has been transported laterally through the 140 250 cm depth interval overwhelming the indigenous bituminous matter below 782 cm depth sandy layers have undergone substantial hydrothermal alteration as is evident from the presence of gypsum dolomite heulandite well crystallized smectite as well as significant changes in the bulk composition of the solid phases the dolomite layers in the bottom part of the core act as a barrier towards upward fluid flow allowing only diffusive communication with fluids of a hydrothermal origin this is evident from the concentration profiles of dissolved chloride and potassium retrograde reactions e g dissolution of gypsum offsetting prograde hydrothermal reactions in pc 6 sediments illustrate the ephemeral nature of fluid flow pathways in near surface guaymas basin sediments