duality of magmatism in the plutonic sequence of the sumail nappe oman abstract detailed field mapping of the crustal sequence in several areas of the sumail ophiolite nappe mainly in the fizh the salahi and the haylayn blocks and comparisons with other well studied areas have shown that the crustal sequence was constructed by two distinct although nearly contemporaneous magmatic series the first a gabbroic series was emplaced during the accretion stage at an oceanic spreading centre which formed the layered gabbro unit the high level isotropic gabbro unit and their by products the sheeted dyke complex and the lower extrusives fine grained homogeneous planar laminated gabbros commonly occur at the top of the layered gabbro sequence in some specific areas wadi haymiliyah and wadi rajmi these are strongly differentiated fe ti rich gabbronorites and norites which coincide with large scale discontinuities within the plutonic sequence interpreted as axial discontinuities of the spreading magma chambers the second series an ultramafic wehrlitic series is locally rooted in the upper mantle i e the impregnated dunitic transition zone this has intruded the whole crustal sequence up to the pillow lavas countless discordant and concordant wehrlitic bodies intruded the plutonic sequence making up 20 40 of it in volume we describe the various settings and forms of these intrusions which were largely underestimated in previous studies we also describe their mineralogy and textures and show with chemical and mineralogical arguments that these bodies evolved as relatively closed systems independently from each other we infer from the study of the contacts that the gabbros were still hot and consolidated just at the moment of the wehrlitic intrusion at the base of the crust however indications of immiscibility between gabbros and wehrlites are common suggesting that both magmatic series were quite contemporaneous the wehrlites are commonly isotropic and may exhibit an internal layering of the dunite wehrlite clinopyroxenite type sometimes appearing at the upper or lower margins of the intrusions in this case the ultramafic layered cumulates show lobate intrusive contacts with the adjacent layered gabbros these plagioclase free ultramafic layered cumulates therefore are not interbedded within the gabbro cumulates but actually belong to the wehrlitic intrusive series picritic dykes radiating from the uppermost intrusions can be traced up to the upper extrusives which are for the main part cogenetic with the wehrlites the time of this second magmatic event coincides with the cessation of the accretion process and with the beginning of the intraoceanic detachment of the ophiolite at the spreading centre we believe that the compressive regime that initiated the detachment acted as an efficient filter press and helped the extraction and ascent of the wehrlitic magmas towards the uppermost levels of the crust

methane hydrate induced permeability modification for multiphase flow in unsaturated porous media 1 an experimental study was performed using x ray computed tomography ct scanning to capture three dimensional 3 d methane hydrate distributions and potential discrete flow pathways in a sand pack sample a numerical study was also performed to develop and analyze empirical relations that describe the impacts of hydrate accumulation habits within pore space e g pore filling or grain cementing on multiphase fluid migration in the experimental study water was injected into a hydrate bearing sand sample that was monitored using an x ray ct scanner the ct images were converted into numerical grid elements providing intrinsic sample data including porosity and phase saturations the impacts of hydrate accumulation were examined by adapting empirical relations into the flow simulations as additional relations governing the evolution of absolute permeability of hydrate bearing sediment with hydrate deposition the impacts of pore space hydrate accumulation habits on fluid migration were examined by comparing numerical predictions with experimentally measured water saturation distributions and breakthrough curves a model case with 3 d heterogeneous initial conditions hydrate saturation porosity and water saturation and pore body preferred hydrate accumulations best captured water migration behavior through the hydrate bearing sample observed in the experiment in the best matching model absolute permeability in the hydrate bearing sample does not decrease significantly with increasing hydrate saturation until hydrate saturation reaches about 40 after which it drops rapidly and complete blockage of flow through the sample can occur as hydrate accumulations approach 70 the result highlights the importance of permeability modification due to hydrate accumulation habits when predicting multiphase flow through high saturation reservoir quality hydrate bearing sediments

the role of seamount volcanism in crustal construction at the mid atlantic ridge 24 30 n an analysis of approximately 6000 km2 of sea beam swaths indicates that the floor of the median valley of the mid atlantic ridge mar between 24 30 n is strewn with near circular volcanoes we have identified 481 seamounts in the height range 50 650 m because many of the features which appear to be seamounts do not meet our criteria fully they have not been counted and our numbers are likely a minimum estimate of the population the large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the mar the summit height distribution of the mar seamount population is consistent with the exponential frequency size distribution model for off axis eastern pacific seamounts jordan et al 1983 smith and jordan 1987 the model parameters for the mar population yield an average of about 195 seamounts per one thousand square kilometers and a characteristic height of about 60 m shape statistics compiled from the 481 seamounts give parameters that are exactly the same or cover the same range in values as those obtained from a study of pacific seamounts in the height range 140 3800 m smith 1988 implying a universal control on seamount construction based on the volcanic morphology we identify 18 spreading segments along the ridge all except two contain a prominent axial volcanic ridge many of the seamounts identified are associated with the axial ridges from the sea beam swaths we infer that the ridges are composed of piled up seamounts and hummocky flows and interpret them to be the primary sites of crustal construction this style of volcanism contrasts strongly with that observed at the east pacific rise where seamounts are virtually absent at the spreading axis the construction of seamounts and hummocky flows may be directly related to the fact that the mar is fed by magma chambers that are limited in size and frequency we suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows using buoyancy arguments the heights of the mar seamounts are related to the depth of the magma bodies we hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped and then erupt if this hypothesis is correct the thickness of the brittle lid controls seamount height and the exponential distribution of seamount summit heights implies that the thickness of the brittle hd follows an exponential distribution in time and space with characteristic thickness 1 4 1 9 km if seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle ductile transition then the lower crust of the mar will be made up of the products of a multitude of small plutons

visual observation of gas hydrate formation and dissociation in synthetic porous media by means of glass micromodels visual observation of gas hydrates at the microscopic scale in synthetic porous media provides unequivocal visual evidence that clathrates can form in systems without the presence of a free gas phase hydrates were formed from a soluble liquid hydrate former tetrahydrofuran c 4 h 8 o from free gas ch 4 and from dissolved gas co 2 clathrates were found to form within the center of pore spaces rather than on grain surfaces cementation of grains only occurred in regions of a small grain size or where a large proportion of pore space was filled with hydrate however even at high clathrate saturations a thin film of free water persisted on grain surfaces the results have important implications for the potential cementing effect of hydrates on sediments and thus for sediment permeability slope stability and seismic interpretation of hydrate bearing sediments

thermal stresses and the bimodal distribution of elastic thickness estimates of the oceanic lithosphere observations of plate flexure beneath seamounts and at deep sea trenches suggest that the mechanical strength of the oceanic lithosphere increases with age the mechanical thickness h seems to follow the depth to a particular isotherm in the cooling plate model a detailed review of available estimates of elastic thickness te shows that the strength deduced from studies at seamounts is significantly different from the strength inferred at trenches and fracture zones whereas te beneath seamounts approximately follows the 400 c isotherm h at trenches follows the depth to the 700 800 c isotherms the cause of this bimodal distribution is not fully understood thermal bending stresses due to cooling of the lithosphere have been suggested to play an important role in the flexural deformation at transform faults and fracture zones and their distribution with depth is consistent with intraplate seismicity this paper evaluates the effects of thermal stresses on deformation beneath seamounts and at trenches and how their interaction with flexural stresses may modify plate strength the results indicate that thermal bending stresses will bias the moment curvature relationship such that a plate of a certain strength will appear relatively weaker beneath a seamount positive curvature but relatively stronger at a trench negative curvature this systematic bias appears to be in qualitative agreement with published te estimates for old lithosphere the theory predicts a diminishing amount of bias the general lack of sufficient te determinations from old ocean basins and the uncertainties associated with modeling of trenches prohibit a detailed analysis of this part of the te versus age relationship i conclude that the scatter in the elastic thickness estimates appears to be caused by a combination of lithospheric reheating during seamount emplacement bias introduced by thermal stresses and variable amount of yielding of the plate beneath seamounts in proportion to seamount size and lithospheric age at the time of loading

the effect of hydrate saturation on water retention curves in hydrate bearing sediments the experimental measurement of water retention curve in hydrate bearing sediments is critically important to understand the behavior of hydrate dissociation and gas production in this study tetrahydrofuran thf is selected as hydrate former the pore habit of thf hydrates is investigated by visual observation in a transparent micromodel it is confirmed that thf hydrates are not wetting phase on the quartz surface of the micromodel and occupy either an entire pore or part of pore space resulting in change in pore size distribution and the measurement of water retention curves in thf hydrate bearing sediments with hydrate saturation ranging from sh 0 to sh 0 7 is conducted for excess water condition the experimental results show that the gas entry pressure and the capillary pressure increase with increasing hydrate saturation based on the experimental results fitting parameters for van genuchten equation are suggested for different hydrate saturation conditions

deep tow and sea beam studies of dueling propagating ridges on the east pacific rise near 20 40 s nontransform ridge axis discontinuities such as propagating rifts overlapping spreading centers oscs and saddle points occur frequently along the mid ocean ridge system we used sea beam and deep tow to study an example of a relatively large nontransform offset near 20 40 s on the east pacific rise epr the two spreading center tips are 15 km apart and separated by a diffuse shear zone a broad v shaped wake of disturbed topography and magnetics occurs on the flanks of the epr near the offset on a fine scale the offset is tectonically much more complex than either the typical osc or propagating rift structures for two reasons first it is a dueling propagator in which structures produced by successive short term episodes of northward and southward propagation have overprinted each other with migration toward the south prevailing over the long term second its offset distance is such that its tectonic behavior may be transitional from that of a propagating rift to that of an osc system the offset s rate of migration to the south has been 20 mm yr over the last 2 m y far less than total opening rate of 162 mm yr distinct propagation events may have been much more rapid however surging north and south at rates of 160 230 mm yr in the process several fossil spreading ridges 30 50 km long have been abandoned and rafted to the side we propose a fine scale tectonic evolution in which a propagating spreading center either curves inside or runs outside of a soon to be abandoned propagating rift tip we call this process self decapitation dueling propagation at 20 40 s may be caused by competing magmatic sources at shallow points along the epr one to the north near 20 s and one to the south near 21 30 s the 20 40 s dueling propagator occurs at a long wavelength low in the axial depth profile between these competing sources in general the division of spreading centers into segments separated by ridge axis discontinuities occurs at low points along the axis and may be caused by the pattern of upper mantle convection and the factors governing flow of magma along the strike of the ridge

east pacific rise 18 19 s asymmetric spreading and ridge reorientation by ultrafast migration of axial discontinuities a detailed bathymetric side scan and magnetic survey of the east pacific rise out to a seafloor age of 1 ma has been carried out between 18 and 19 s it reveals that some left stepping axial discontinuities have been migrating southward at rates an order of magnitude faster than the spreading rates 1000 mm a or higher these rapid migration events have left on the nazca plate discordant features striking nearly parallel to the ridge axis a discontinuity with an offset of several kilometers has migrated in two stages at around 0 45 and 0 3 ma and has left two large discordant zones consisting of a series of unfaulted hummocky basins bounded to the east by short ridges oriented about n s oblique to the ambient 013 fabric the morphology and reflectivity characteristics of these discordant zones are akin to the overlap basins and abandoned ridge tips which make up the migration trails of large slowly migrating overlapping spreading centers between 18 35 and 19 03 s the ridge axis is flanked a few kilometers to the east by a prominent sedimented ridge previously recognized as a recently abandoned ridge axis the present ridge segment steadily deepens and narrows southward which suggests the abandoned ridge has been rafted onto the nazca plate during the ultrafast southward propagation of the ridge segment rather than by one discrete ridge jump by transferring pacific lithosphere to the nazca plate these migration events account for most of the asymmetric accretion observed faster to the east this process is consistent with the features common to asymmetric spreading namely the sudden onset or demise of asymmetric spreading and the ridge segment to ridge segment variability because the discordant zones left by these rapid migration events are near parallel to the ambient seafloor fabric they are unlikely to be detected by conventional bathymetry or magnetic surveys and so called ridge jumps may actually often represent ultrafast propagation of a ridge segment variations in fault azimuth with age show there has not been any significant change in spreading direction over the past 0 8 m y instead the counterclockwise trend of the east pacific rise relative to the brunhes matuyama reversal 0 78 ma mostly reflects that ultrafast propagation of ridge segments has transferred a larger amount of the pacific lithosphere to the nazca plate at 18 s than at 19 s in keeping with the regional features of the magnetic anomalies we propose that an 8 to 10 km left stepping discontinuity which was located between 17 and 17 30 s at 0 78 ma has been recently redistributed into the present staircase of small left stepping discontinuities between 16 and 19 s this smoothing of the ridge geometry probably occurred through repeated small lateral steps of the ridge segments inside of the discontinuities during ultra fast propagation episodes and may be the consequence of a significant replenishment of the magma reservoir between 17 and 17 30 s during the past 1 m y

gas hydrate characterization and grain scale imaging of recovered cores from the mount elbert gas hydrate stratigraphic test well alaska north slope abstract using cryogenic scanning electron microscopy csem powder x ray diffraction and gas chromatography methods we investigated the physical states grain characteristics gas composition and methane isotopic composition of two gas hydrate bearing sections of core recovered from the bpxa doe usgs mount elbert gas hydrate stratigraphic test well situated on the alaska north slope the well was continuously cored from 606 5 m to 760 1 m depth and sections investigated here were retrieved from 619 9 m and 661 0 m depth x ray analysis and imaging of the sediment phase in both sections shows it consists of a predominantly fine grained and well sorted quartz sand with lesser amounts of feldspar muscovite and minor clays cryogenic sem shows the gas hydrate phase forming primarily as a pore filling material between the sediment grains at approximately 70 75 saturation and more sporadically as thin veins typically several tens of microns in diameter pore throat diameters vary but commonly range 20 120 microns gas chromatography analyses of the hydrate forming gas show that it is comprised of mainly methane 99 9 indicating that the gas hydrate is structure i here we report on the distribution and articulation of the gas hydrate phase within the cores the grain morphology of the hydrate the composition of the sediment host and the composition of the hydrate forming gas

seamount statistics in the pacific ocean we apply the wide beam sampling technique of jordan et al 1983 to approximately 157 000 km of wide beam profiles to obtain seamount population statistics for eight regions in the eastern and southern pacific ocean population statistics derived from wide beam echograms are compared with seamount counts from sea beam swaths and with counts from bathymetric maps we find that the average number of seamounts with summit heights h h is well approximated by the exponential frequency size distribution h oe h the exponential model for seamount sizes characterized by the single scale parameter 1 is found to be superior to a power law self similar model which has no intrinsic scale in describing the average distribution of pacific seamounts and it appears to be valid over a size spectrum spanning 5 orders of magnitude in abundance large scale regional variations in seamount populations are documented we observe significant differences in seamount densities across the murray fracture zone in the north pacific and the eltanin fracture zone system in the south pacific the eltanin discontinuity is equally evident on both sides of the pacific antarctic ridge in the south pacific regions symmetrically disposed about the ridge axis have very similar seamount densities despite the large difference between pacific plate and antarctic plate absolute velocities evidently any differences in the shear flows at the base of the pacific and antarctic plates do not affect seamount emplacement systematic variations in o and are observed as a function of lithospheric age with the number of large seamounts increasing more rapidly than small seamounts these observations have been used to develop a simple model for seamount production under the assumptions that 1 an exponential size frequency distribution is maintained 2 production is steady state and 3 most small seamounts are formed on or near the ridge axis the limited data available from this study appear to be consistent with the model but they are insufficient to provide a rigorous test of the assumptions or determine accurately the model parameters however the data from the south pacific indicate that the off axis production of large seamounts probably accounts for the majority of seamounts with summit heights greater than 1000 m

seismic images of active magma systems beneath the east pacific rise between 17 05 and 17 35 s seismic reflection data from the east pacific rise between 17 059 and 17 359s image a magma lens that varies regularly in depth and width as ridge morphology changes confirming the notion that axial morphology can be used to infer ridge magmatic state however at 17 269s where the ridge is locally shallow and broad the magma lens is markedly shallower and wider than predicted from regional trends in this area submersible dives reveal recent volcanic eruptions these observations indicate that it is where the width and depth of the magma chamber differ from regional trends indicating an enhanced magmatic budget that is diagnostic of current magmatism

hydrate morphology physical properties of sands with patchy hydrate saturation 1 the physical properties of gas hydrate bearing sediments depend on the volume fraction and spatial distribution of the hydrate phase the host sediment grain size and the state of effective stress determine the hydrate morphology in sediments this information can be used to significantly constrain estimates of the physical properties of hydrate bearing sediments including the coarse grained sands subjected to high effective stress that are of interest as potential energy resources reported data and physical analyses suggest hydrate bearing sands contain a heterogeneous patchy hydrate distribution whereby zones with 100 pore space hydrate saturation are embedded in hydrate free sand accounting for patchy rather than homogeneous hydrate distribution yields more tightly constrained estimates of physical properties in hydrate bearing sands and captures observed physical property dependencies on hydrate saturation for example numerical modeling results of sands with patchy saturation agree with experimental observation showing a transition in stiffness starting near the series bound at low hydrate saturations but moving toward the parallel bound at high hydrate saturations the hydrate patch size itself impacts the physical properties of hydrate bearing sediments for example at constant hydrate saturation we find that conductivity electrical hydraulic and thermal increases as the number of hydrate saturated patches increases this increase reflects the larger number of conductive flow paths that exist in specimens with many small hydrate saturated patches in comparison to specimens in which a few large hydrate saturated patches can block flow over a significant cross section of the specimen

seismic structure of the southern east pacific rise seismic data from the ultrafast spreading 150 to 162 millimeters per year southern east pacific rise show that the rise axis is underlain by a thin less than 200 meters thick extrusive volcanic layer seismic layer 2a that thickens rapidly off axis also beneath the rise axis is a narrow less than 1 kilometer wide melt sill that is in some places less than 1000 meters below the sea floor the small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge crest thermal models however the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate these observations indicate that the paradigm of ridge crest magma chambers as small sill like midcrustal bodies is applicable to a wide range of intermediate and fast spreading ridges

shipboard geophysical indications of asymmetry and melt production beneath the east pacific rise near the melt experiment near the mantle electromagnetic and tomography melt experiment seamounts form and off axis lava flows occur in a zone that extends farther to the west of the east pacific rise than to the east indicating a broad asymmetric region of melt production more seamounts slower subsidence and less dense mantle on the western flank suggest transport of hotter mantle toward the axis from the west variations in axial ridge shape axial magma chamber continuity off axis volcanism and apparent mantle density indicate that upwelling is probably faster and more melt is produced beneath 17 degrees15 s than beneath 15 degrees55 s recent volcanism occurs above mantle with the lowest seismic velocities

modeling of the influence of gas hydrate content on the electrical properties of porous sediments the geophysical investigation of gas hydrate accumulations within marine sediments and the sediments of permafrost regions suffer from a lack of information on the influence of the hydrate content on the physical sediment properties the estimation of the hydrate content using downhole electrical measurements based on archie s law requires the knowledge of the saturation exponent the saturation exponent is an empirical parameter that includes influences from the internal rock structure such as pore shape connectivity and constrictivity of the pore network and the distribution of the conducting phase based on different models that account for the different morphological forms of gas hydrates found during gas hydrate sampling in various research wells the influence of gas hydrate content on the electrical properties of the hydrate bearing sediment was investigated for all studied forms of hydrate occurrence disseminated in the pore space nodular and layered the saturation exponent depends on the sediment properties and on saturation itself the growth of gas hydrate nodules lenses and layers is a process that is assumed to result in the displacement and compaction of the surrounding sediment because of this change of sediment properties during hydrate generation the saturation exponent for these forms of hydrate occurrence depends strongly on the relationship between porosity and formation resistivity factor expressed in the form of archie s cementation exponent for the case that hydrate occurs disseminated in the pore space and the assumption that capillary effects are important for hydrate generation the saturation exponent depends on grain size and grain size sorting for the parameters chosen for these model calculations the saturation exponent varies between 0 5 and 4 the use of a constant mean value for the saturation exponent of approximately 2 can result in both underestimation and overestimation of the hydrate content

the water retention curve and relative permeability for gas production from hydrate bearing sediments pore network model simulation the water retention curve and relative permeability are critical to predict gas and water production from hydrate bearing sediments however values for key parameters that characterize gas and water flows during hydrate dissociation have not been identified due to experimental challenges this study utilizes the combined techniques of micro focus x ray computed tomography ct and pore network model simulation to identify proper values for those key parameters such as gas entry pressure residual water saturation and curve fitting values hydrates with various saturation and morphology are realized in the pore network that was extracted from micron resolution ct images of sediments recovered from the hydrate deposit at the mallik site and then the processes of gas invasion hydrate dissociation gas expansion and gas and water permeability are simulated results show that greater hydrate saturation in sediments lead to higher gas entry pressure higher residual water saturation and steeper water retention curve an increase in hydrate saturation decreases gas permeability but has marginal effects on water permeability in sediments with uniformly distributed hydrate hydrate morphology has more significant impacts than hydrate saturation on relative permeability sediments with heterogeneously distributed hydrate tend to result in lower residual water saturation and higher gas and water permeability in this sense the brooks corey model that uses two fitting parameters individually for gas and water permeability properly capture the effect of hydrate saturation and morphology on gas and water flows in hydrate bearing sediments

laboratory formation of noncementing hydrates in sandy sediments natural hydrate bearing sediment hbs predominantly exists in noncementing habit and its limited availability for use in laboratory studies demands a time effective and repeatable laboratory process for forming representative samples with natural accumulation habit this study reports on a three step laboratory process for forming noncementing methane hydrate in sandy sediments 1 initial hbs formation under excess gas conditions 2 slow saline water 5 wt cacl2 injection under strictly controlled pressure temperature p t conditions and 3 a temperature warming cooling cycle changes in compressional wave velocity vp of sediment as well as p t condition were monitored throughout the tests the evolution of vp in good agreement with rock physics model calculations suggested that the transition from cementing hydrate into noncementing hydrate occurs during saline injection as well as temperature warming cooling cycle the proposed process appeared to be an efficient and consistent substitute for the existing methods to form noncementing hydrate habit in sandy sediments

variation in cross sectional area of the axial ridge along the east pacific rise evidence for the magmatic budget of a fast spreading center along the fast and ultrafast spreading east pacific rise the cross sectional area of the axial ridge varies significantly over length scales similar to its morphologic segmentation using an automated method we measure the ridge area volume per kilometer along axis where there is complete bathymetric coverage along 3500 km of the northern and southern east pacific rise our two study areas encompass numerous transform faults large overlapping spreading centers small overlapping spreading centers and smaller discontinuities first second third and fourth order discontinuities the cross sectional area variation mimics the undulation of the ridge crest depth local area maxima occur toward the middle of segments and the axial area decreases by 40 or more at first and second order discontinuities third order discontinuities are generally marked by smaller disruptions in the ridge area and fourth order offsets do not systematically correspond with features of the cross sectional area profile the correlation between shallower ridges and larger ridge areas breaks down at some locations because axial cross sectional area represents a longer term average of the ridge s magmatic state than axial depth a correlation between large ridge areas and negative residual gravity anomalies indicates that inflated ridges are underlain by low density crust and mantle also a correlation between larger area and higher mgo content of axial basalts suggests that inflated areas generally erupt hotter magmas which are presumably supplied more rapidly to the neovolcanic zone the cross sectional area of the axial ridge appears to correlate with the width of the axis centered low velocity zone in the crust these observations as well as the absence of large relict axial ridges off axis indicate that the axial ridge originates from buoyancy due to thermal expansion and the presence of melt in the crust and mantle within about 10 km of the rise axis portions of the northern east pacific rise underlain by a magma chamber reflector generally occur where the ridge cross sectional area is greatest this supports the connection between processes which inflate the axial ridge and those which heat the crust and upper mantle and produce melt thus while the axial high on fast spreading centers resembles a constructional volcano in cross section it is more like a long narrow balloon whose cross sectional area is a sensitive indicator of magma supply using this relationship we predict with 75 confidence that at least 45 of the unsurveyed northern east pacific rise 18 n to 13 n and 9 n to 5 n and at least 60 of the southern east pacific rise 4 s to 23 s is underlain by a magma chamber reflector

contribution of volcanism and tectonism to axial and flank morphology of the southern east pacific rise 17 10 17 40 s from a study of layer 2a geometry multichannel seismic images from the east pacific rise 17 10 17 40 s are used to study the geometry of seismic layer 2a in the near axis region wave equation datuming is applied to stacked common midpoint data to remove the distorting effects of seafloor topography on the layer 2a event processed stacks are compared with seafloor morphology and tectonic fabric imaged in side scan sonar data assuming layer 2a corresponds with the extrusive crust these data are used to study the relationship between volcanism and tectonism in the accumulation of the extrusive layer and to assess the contribution of extrusives to ridge crest and ridge flank morphology we find variations in axial thickening of layer 2a which imply twofold variation in the width of the extrusive layer accumulation zone as well as systematic changes in the pattern of accumulation of this layer on the ridge flanks the layer 2a horizon mimics abyssal hill relief consistent with a horst and graben origin for this topography short wavelength variations in the thickness of layer 2a are superimposed on this relief which we attribute to volcanic modification of tectonic topography at the edge of the neovolcanic zone the integrated off axis thickness of layer 2a is correlated with ridge cross sectional area and indicates persistent 100 000 years spatial gradients in extrusive layer thickness systematic changes in the cross axis shape of the ridge observed over distances of 5 10 km cannot be attributed to the extrusive layer and it appears that axial structure beneath the volcanics governs the cross axis morphology of the ridge

practical geological comparison of some seafloor survey instruments seafloor survey instruments are integral to the study of marine geology because understanding their resolution and limitations is critical we compare how different survey systems represent the seafloor coincident data collected at the galapagos propagator gloria seamarc ii sea beam deep tow camera sled and alvin allow comparisons of how well seafloor features e g faults and volcanoes observed and characterized in high resolution data are represented in lower resolution coarser scale data sets our reported values for the minimum sizes of detected and well represented features show that practical geological resolutions are generally 2 10 times lower than theoretical resolutions care must be taken in evaluating which system to use to address a particular problem

trade off in production between adjacent seamount chains near the east pacific rise bathymetric data and side scan sonar images collected near the southern east pacific rise between 15 and 19 s1 2 have revealed many seamounts on the west flank of the rise most of which are organized into chains running perpendicular to the rise axis the number of seamounts of 5 km diameter is at least twice that expected for average east pacific sea floor3 the formation of numerous small 5 myr linear seamount chains and the tendency for fresh lava flows to be found primarily at the near ridge ends of the chains suggest that there are plume like sources in the upper mantle which are relatively stationary active for extended periods and affected by adjacent sources

thermoelastic stress how important as a cause of earthquakes in young oceanic lithosphere we present a series of exploratory models which test the hypothesis that thermoelastic stress is a significant contributor to the state of stress in young oceanic lithosphere less than 35 m y in age in support of this hypothesis is the concentration of seismicity in lithosphere younger than 15 m y where cooling rates are relatively high most near ridge earthquakes have focal depths between the moho and the depth of the 800 c isotherm thrust and strike slip events dominate the shallowest seismicity while the focal mechanisms of the deeper events are generally characterized by normal faulting to assess the importance of thermal stress in the generation of near ridge earthquakes we model the lithosphere as an elastic half space in which the response to cooling is computed using the method of thermoelastic displacement potentials a key assumption in the models is that stress is relieved on time scales generally short compared with the age of the lithosphere a further assumption in some models is that material will not contribute thermal stress until it has cooled below an elastic blocking temperature normal faulting is predicted throughout the lithosphere in thermal stress models based on simple half space cooling models in which a cooled surface layer is incorporated to simulate the effect of hydrothermal circulation on shallow thermal structure predict stresses that can match the locations of both thrust and normal faulting earthquakes near mid ocean ridges the addition of a ridge push stress to the computed thermal stress enhances the likelihood of thrust faulting in the uppermost lithosphere at ages greater than about 15 m y but probably contributes little to the stress field at younger ages though these simple models are limited by several simplifying assumptions particularly in the immediate vicinity of the ridge axis they support the hypothesis that thermoelastic stress can play a major role in the tectonics of young oceanic lithosphere

elastic properties of gas hydrate bearing sediments downhole measured compressional and shear wave velocities acquired in the mallik 2l 38 gas hydrate research well northwestern canada reveal that the dominant effect of gas hydrate on the elastic properties of gas hydrate bearing sediments is as a pore filling constituent as opposed to high elastic velocities predicted from a cementation theory whereby a small amount of gas hydrate in the pore space significantly increases the elastic velocities the velocity increase from gas hydrate saturation in the sediment pore space is small both the effective medium theory and a weighted equation predict a slight increase of velocities from gas hydrate concentration similar to the field observed velocities however the weighted equation more accurately describes the compressional and shear wave velocities of gas hydrate bearing sediments a decrease of poisson s ratio with an increase in the gas hydrate concentration is similar to a decrease of poisson s ratio with a decrease in the sediment porosity poiss

pore scale investigation of permeability evolution during hydrate formation using a pore network model based on x ray ct abstract permeability in hydrate bearing sediment critically governs fluid flow and determines hydrate nucleation growth and distribution making it important to characterize the evolution of permeability with respect to water and gas during hydrate formation this study uses ct scanning of krypton hydrate formation in silica sand using the excess gas method together with a pore network model to investigate variations in hydrate morphology and associated permeation the results show that during hydrate formation the growth habit mainly varies from grain coating to patchy with an increase in hydrate saturation however at relatively low saturation hydrate preferentially grows as a grain cementing habit theoretical models of a capillary tube and of a kozeny grain both predict that permeability in grain coating hydrates will be higher than in patchy hydrates we thus recognise a correlation of permeability and hydrate saturation for multiple growth habits that is of interest for gas production from hydrate reservoirs under lower water saturation there is a decrease in relative permeability to water but an increase in relative permeability to gas due to the reduction in pore shape factors conversely an upward shift in relative permeability to water and a downward shift in relative permeability to gas is found with hydrate formation under higher water saturation owing to the jamin effect steeper curves of relative permeability to gas and water with increasing hydrate saturation resulted in a reduction of the co cementation zone for water and gas even if or though no gas migrates the results suggest that due to the low relative permeability of gas higher water saturation sediments result in an excess water yield accompanied by low gas production that is not desirable for natural gas hydrate production therefore improving the permeability and weakening the jamin effect are critical for gas production in marine hydrate reservoirs especially in low permeability sediments

relationship between spreading rate and the seismic structure of mid ocean ridges different parts of the global mid ocean ridge system create oceanic crust at rates that differ by more than a factor of ten the rates of supply of heat to these different systems must also vary substantially and it is this heat supply in the form of hot mantle material rising beneath the ridge axis that determines the nature of the magmatic tectonic and hydrothermal processes that control the production of new oceanic crust here we present evidence suggesting that a systematic relationship exists between the depth to zones of elevated temperature and partial melt beneath mid ocean ridges and their spreading rate recent seismic measurements provide robust determinations at six different locations of the depth to the top of the axial low seismic velocity zone which suggest that as full spreading rate decreases from 150 to 20 mm yr 1 the depth below the sea floor to the top of the lvz increases from 1 to 4 km although this relationship might arise in a number of ways our preferred explanation is that the major faults that extend to greater depths on slower spreading ridges1 allow a vigorous circulation of cooling sea water to reach greater depths blocking the upward migration of large melt bodies

microstructural evolution of gas hydrates in sedimentary matrices observed with synchrotron x ray computed tomographic microscopy the formation process of gas hydrates in sedimentary matrices is of crucial importance for the physical and transport properties of the resulting aggregates this process has never been observed in situ at submicron resolution here we report on synchrotron based microtomographic studies by which the nucleation and growth processes of gas hydrate were observed at 276 k in various sedimentary matrices such as natural quartz with and without admixtures of montmorillonite type clay or glass beads with different surface properties at varying water saturation both juvenile water and metastably gas enriched water obtained from gas hydrate decomposition was used xenon gas was employed to enhance the density contrast between gas hydrate and the fluid phases involved the nucleation sites can be easily identified and the various growth patterns are clearly established in sediments under saturated with juvenile water nucleation starts at the water gas interface resulting in an initially several micrometer thick gas hydrate film further growth proceeds to form isometric single crystals of 10 20 m size the growth of gas hydrate from gas enriched water follows a different pattern via the nucleation in the bulk of liquid producing polyhedral single crystals a striking feature in both cases is the systematic appearance of a fluid phase film of up to several micron thickness between gas hydrates and the surface of the quartz grains these microstructural findings are relevant for future efforts of quantitative rock physics modeling of gas hydrates in sedimentary matrices and explain the anomalous attenuation of seismic sonic waves

synchrotron x ray computed microtomography study on gas hydrate decomposition in a sedimentary matrix in situ synchrotron x ray computed microtomography with sub micrometer voxel size was used to study the decomposition of gas hydrates in a sedimentary matrix xenon hydrate was used instead of methane hydrate to enhance the absorption contrast the microstructural features of the decomposition process were elucidated indicating that the decomposition starts at the hydrate gas interface it does not proceed at the contacts with quartz grains melt water accumulates at retreating hydrate surface the decomposition is not homogeneous and the decomposition rates depend on the distance of the hydrate surface to the gas phase indicating a diffusion limitation of the gas transport through the water phase gas is found to be metastably enriched in the water phase with a concentration decreasing away from the hydrate water interface the initial decomposition process facilitates redistribution of fluid phases in the pore space and local re formation of gas hydrates the observations allow also rationalizing earlier conjectures from experiments with low spatial resolutions and suggest that the hydrate sediment assemblies remain intact until the hydrate spacers between sediment grains finally collapse possible effects on mechanical stability and permeability are discussed the resulting time resolved characteristics of gas hydrate decomposition and the influence of melt water on the reaction rate are of importance for a suggested gas recovery from marine sediments by depressurization this article is protected by copyright all rights reserved

pore scale morphology and wettability characteristics of xenon hydrate in sand matrix laboratory visualization with micro ct abstract the properties of gas hydrate bearing sediments ghbs depend on their changing morphology and wettability characteristics which are in turn important for efficiency and safety issues during production we present the results of xenon hydrate formation experiments using quartz sand packs at different salinities direct 3d visualization of hydrate distribution in the pore space was obtained with the help of high resolution micro computed tomography ct four phases xenon hydrate xenon gas brine and quartz sands were segmented and labelled in the raw micro ct scanned images a 3d reconstruction reveals typical pore scale patterns of hydrate occurrence pore filling grain coating and shows that hydrate preferentially aggregates at the gas water interface reflecting greater local access to xenon gas molecules the hydrate cluster shape factor ranges from 2 to 10 indicating that xenon hydrate forms with a complex and irregular microstructure consistent with random growth nature of hydrate the hydrate saturation is found to decrease with salinity under excess gas condition which we interpret in terms of the hydrate formation kinetics to indicate that gas hydrate prefers to grow into the gas phase at the gas water interface a total of 270 local contact angles of water at the gas and hydrate interfaces in pores were measured with a micro ct image analysis method to obtain information on the wettability variation of hydrate surfaces the average was calculated to be 47 6 indicating that xenon hydrate surface is hydrophilic our analysis of the xenon hydrate morphology provides a new method for revealing hydrate growth behavior at pore scale the wettability characterization offers key parameters for accurate numerical simulation of multiphase flow prediction and seepage behavior in ghbs

near axis seamounts on the flanks of the east pacific rise 8 n to 17 n with complete seabeam and seamarcii bathymetry and nearly complete seamarcii side scan coverage of the northern east pacific rise and its flanks from 8 to 17 n we characterize the entire population of seamounts on young seafloor 0 to 2 ma formed along this fast spreading center there are 179 seamounts in this area taller than 200 m most of which belong to one of 21 seamount chains these chains are oriented between the relative and absolute plate motion directions of the pacific and cocos plates and most of the larger chains are more voluminous near their centers than near their ends several of the seamount chains contain bends in orientation which are not coincident with each other the two largest seamount chains at 9 55 n lamont seamounts and 15 40 n are aligned with portions of the east pacific rise crest which are among the shallowest and broadest of this area overall the maximum abundance and size of seamounts increase with shallower and broader ridge axes no near axis seamounts form on the summit of the axial ridge nonetheless within 15 km 0 3 ma of the axis the abundance of seamounts achieves its overall average observations of the numbers and cumulative volume of seamount edifices versus distance from the axis are consistent with a model where the majority of near axis seamounts are formed in a narrow zone on crust aged 0 1 0 3 ma 5 15 km from the axis with significant growth in a zone several times wider side scan sonar reflectivity and seamount magnetization analyses are largely consistent with the predictions of this model two exceptions to this model however are seamount chains at 15 40 n and 8 25 n which show evidence for activity over a broader region this may be due to the presence of fracture zones within about 30 km of these chains a compilation of seamount counts from a wide range of spreading centers indicates that seamounts taller than 400 m are more abundant at greater spreading rates and with transitions from a rift valley to an axial ridge the variations with spreading rate of melt availability and of lithosphere penetrability probably combine to produce the observed results although these influences cannot be distinguished in this analysis

a site effect study in acapulco guerrero mexico comparison of results from strong motion and microtremor data abstract the city of acapulco is located near or above the mature seismic gap of guerrero along the mexican subduction zone with the purpose of studying the character of strong ground motion on soft sites four digital accelerographs have been installed in the city on such sites these instruments have been in operation since 1988 two additional instruments part of the guerrero accelerograph array are located on hard sites in the area one of these vnta has been in operation since 1985 and the other acan since 1989 these stations have recorded several earthquakes we use data from eight events 4 2 m 6 9 to study spectral amplification of seismic waves at the soft sites with respect to vnta the s waves are amplified by a factor of 6 to 25 at the soft sites in a fairly broad range of frequencies both the amplification and the frequency band over which it occurs depend upon the site although the largest earthquake in our data set m 6 9 gave rise to a peak horizontal acceleration exceeding 0 3 g at one of the soft sites no clear evidence of nonlinear behavior of the subsoil is found spectral amplifications of s wave coda are very similar to those of s waves we also measured microtremors at the strong motion sites the microtremor spectra were interpreted using reasonable assumptions to test the feasibility of this technique in reproducing the spectral amplifications observed during earthquakes our results show that only a rough estimate of site response can be obtained from this technique at least in acapulco caution is warranted in its use elsewhere

measured acoustic wave velocities of r11 ccl3f hydrate samples with and without sand as a function of hydrate concentration we report acoustic laboratory measurements on initially unconsolidated sand packs with hydrate formation taking place in the pore space both p and s wave velocities were measured hydrate was formed from water and the refrigerant r11 ccl3f at 2 c and atmospheric pressure measurements were performed on two types of initially unconsolidated sand packs with average grain size 100 m and 280 m respectively p wave velocities varied from about 1700 m s at low hydrate content to 3810 m s at a calculated hydrate fraction in the pore space equal to 0 52 s wave velocities were only detectable at hydrate fractions larger than 0 35 where s wave velocities up to 2230 m s were measured our measurements indicate that acoustic measurements are not sensitive to small amounts of hydrate in a system of initially unconsolidated sand the data show a very distinct change in behavior when the hydrate fraction exceeds 0 35 the concentration at which hydrate cementation of the sand grains is believed to occur measurements for samples without sand samples containing water r11 and hydrate are also reported for hydrate without sand p wave velocities ranged from about 1400 m s at very low hydrate content to about 2500 m s at a measured hydrate fraction equal to 0 68 for these samples reliable s wave data were not detected

methane hydrate formation in partially water saturated ottawa sand bulk properties of gas hydrate bearing sediment strongly depend on whether hydrate forms primarily in the pore fluid becomes a load bearing member of the sediment matrix or cements sediment grains our compressional wave speed measurements through partially water saturated methane hydrate bearing ottawa sands suggest hydrate surrounds and cements sediment grains the three ottawa sand packs tested in the gas hydrate and sediment test laboratory instrument ghastli contain 38 1 porosity initially with distilled water saturating 58 31 and 16 of that pore space respectively from the volume of methane gas produced during hydrate dissociation we calculated the hydrate concentration in the pore space to be 70 37 and 20 respectively based on these hydrate concentrations and our measured compressional wave speeds we used a rock physics model to differentiate between potential pore space hydrate distributions model results suggest methane hydrate cements unconsolidated sediment when forming in systems containing an abundant gas phase

pore habit of methane hydrate and its evolution in sediment matrix laboratory visualization with phase contrast micro ct abstract describing the pore habit of methane hydrate in sediment matrices is essential for understanding natural distribution of methane hydrate methane trace transport and solidification in the hydrate stability zone physical properties of hydrate bearing sediments and the associated influence on potential gas production pore habit visualization in natural media at pore scale even with laboratory synthesized cores has been challenging due to the similar densities of methane hydrate from pore liquid in this work we used phase contrast assisted micro ct with potassium iodine doped brine to visualize four phases sand particles pore fluid methane hydrate and methane gas this study visualizes the pore habit of methane hydrate at various stages including during hydrate formation in excess gas systems its evolution after brine injection to replace pore fluid and hydrate formation in excess water systems hydrate tends to adopt round and smooth surfaces when in contact with water while exhibits relatively angular interfaces when in contact with methane gas hydrate formation in excess gas systems results in a partial cementing and partial mineral coating pore habit while hydrate in excess water systems develops mainly as pore filling and locally cementing or mineral coating where big gas pockets exist at the initial state pore liquid replacement from methane gas to brine triggers a shift of hydrate pore habit towards pore filling methane hydrate evolution over time produces bigger hydrate particles but with less contact area with sand particles the effects of hydrate pore habit become less important as hydrate particle size exceeds the pore size additionally hydrate formation could trap residual methane gas and brine as inclusions

seafloor nuclear magnetic resonance assay of methane hydrate in sediment and rock 1 proton nuclear magnetic resonance nmr of liquid water can be used to indirectly detect and assay methane hydrate in opaque earth materials it is also potentially useful for quantifying pore size control of hydrate formation and for estimating in situ hydraulic permeability of hydrate affected earth formations the method is quantitative nondestructive and volumetrically averaging over a spatially selected region in the first controlled validation of this measurement we have used nmr to measure changes in the liquid water content of sediment and rock resulting from methane hydrate formation hydrates were artificially formed at the seafloor by introducing methane into tubes containing sediments or rock saturated with seawater after several weeks the samples were visited by a remotely operated vehicle on which nmr equipment was mounted reduction of the nmr signal indicated hydrate had formed in the sample within a well defined measurement volume hydrate content determined by nmr quantitatively predicted the volume of methane gas evolved during subsequent dissociation

distribution of isolated volcanoes on the flanks of the east pacific rise 15 3 s 20 s volcanic constructions not associated with seamount or volcano chains are abundant on the flanks of the east pacific rise epr but are rare along the axial high the distribution of isolated volcanoes based on multibeam bathymetric maps is approximately symmetric about the epr axis this symmetry contrasts with the asymmetries in the distribution of volcano chains more abundant on the west flank the seafloor subsidence rates slower on the west flank and the distribution of plate motion parallel gravity lineaments more prominent on the west flank most of the isolated volcanoes complete their growth within 14 km of the axis on crust younger than 0 2 ma while seamount chain volcanoes continue to be active on older crust volcanic edifices within 6 km of the ridge axis are primarily found adjacent to axial discontinuities suggesting a more sporadic magma supply and stronger lithosphere able to support volcanic constructions near axial discontinuities the volume of isolated near axis volcanoes correlates with ridge axis cross sectional area suggesting a link between the magma budget of the ridge and the eruption of near axis volcanoes within the study area off axis volcanic edifices cover at least 6 of the seafloor and contribute more than 0 2 to the volume of the crust the inferred width of the zone where isolated volcanoes initially form increases with spreading rate for the mid atlantic ridge 4 km northern epr 20 km and southern epr 28 km so that isolated volcanoes form primarily on lithosphere younger than 0 2 ma 4 6 km brittle thickness independent of spreading rate this suggests some form of lithospheric control on the eruption of isolated off axis volcanoes due to brittle thickness increased normal stresses across cracks impeding dike injection or thermal stresses within the newly forming lithosphere

physical properties of hydrate bearing sediments 1 methane gas hydrates crystalline inclusion compounds formed from methane and water are found in marine continental margin and permafrost sediments worldwide this article reviews the current understanding of phenomena involved in gas hydrate formation and the physical properties of hydrate bearing sediments formation phenomena include pore scale habit solubility spatial variability and host sediment aggregate properties physical properties include thermal properties permeability electrical conductivity and permittivity small strain elastic p and s wave velocities shear strength and volume changes resulting from hydrate dissociation the magnitudes and interdependencies of these properties are critically important for predicting and quantifying macroscale responses of hydrate bearing sediments to changes in mechanical thermal or chemical boundary conditions these predictions are vital for mitigating borehole local and regional slope stability hazards optimizing recovery techniques for extracting methane from hydrate bearing sediments or sequestering carbon dioxide in gas hydrate and evaluating the role of gas hydrate in the global carbon cycle

lithospheric age dependence of off ridge volcano production in the north pacific data for numbers of seamounts on north pacific ocean crust of different age indicate that the production rate of new off ridge seamounts volcanoes is proportional to the inverse of the square root of the age of the lithosphere this observation is consistent with several hypotheses which have been offered to explain the origin of off ridge oceanic volcanism and in combination with petrologic and paleomagnetic evidence for small oceanic volcanoes leads to a new self consistent model for off ridge volcanism in this model the production rate of off ridge volcanoes is controlled primarily by the availability of fracture zone conduit systems in the thickening lithosphere in contrast mantle plume or hotspot volcanoes may punch through the oceanic lithosphere decreasing production rate of off ridge volcanoes on old oceanic lithosphere may be associated with decreasing extent of partial melting of chemically and isotopically heterogeneous mantle material

implications of magma convection for the size and temperature of magma chambers at fast spreading ridges we present a new thermal model that resolves the discrepancy between observations of narrow 4 km magma chambers on the northern east pacific rise epr and predictions based on previous thermal models of wide 20 km chambers by including cooling of the lower crust by convection within the magma chamber for this cooling to produce a solid crust within a few kilometers of the axis the temperature of the magma chamber must be significantly below the solidus temperature of the lower crust this model is internally consistent for an average temperature of 1240 c for primary magmaleaving the mantle and an average magma chamber temperature of 1150 c these temperatures imply 10 mgo for primary magma which is at the low mg end of the range of proposed compositions and 6 mgo for magma residing in the chamber which is comparable to the most evolved basalts commonly erupted along the epr there is a trade off in the inputs to the model such that raising the primary magma temperature requires a lower magma chamber temperature to maintain the same maximum temperature of the lower crust it is reasonable for the magma chamber to be more evolved than most erupted lavas if magma mixing exerts substantial control over the composition of the erupted lavas aphyric 10 mgo basalt should be buoyant in 6 mgo basalt therefore the mixing need not involve the entire chamber prior to eruption

water permeability in hydrate bearing sediments a pore scale study permeability is a critical parameter governing methane flux and fluid flow in hydrate bearing sediments however limited valid data are available due to experimental challenges here we investigate the relationship between apparent water permeability k and hydrate saturation sh accounting for hydrate pore scale growth habit and meso scale heterogeneity results from capillary tube models rely on cross sectional tube shapes and hydrate pore habits thus are appropriate only for sediments with uniform hydrate distribution and known hydrate pore character given our pore network modeling results showing that accumulating hydrate in sediments decreases sediment porosity and increases hydraulic tortuosity we propose a modified kozeny carman model to characterize water permeability in hydrate bearing sediments this model agrees well with experimental results and can be easily implemented in reservoir simulators with no empirical variables other than sh results are also relevant to flow through other natural sediments that undergo diagenesis salt precipitation or bio clogging