effective velocities in fractured media a numerical study using the rotated staggered finite difference grid the modelling of elastic waves in fractured media with an explicit finite difference scheme causes instability problems on a staggered grid when the medium possesses high contrast discontinuities strong heterogeneities for the present study we apply the rotated staggered grid using this modified grid it is possible to simulate the propagation of elastic waves in a 2d or 3d medium containing cracks pores or free surfaces without hard coded boundary conditions therefore it allows an efficient and precise numerical study of effective velocities in fractured structures we model the propagation of plane waves through a set of different randomly cracked media in these numerical experiments we vary the wavelength of the plane waves the crack porosity and the crack density the synthetic results are compared with several static theories that predict the effective p and s wave velocities in fractured materials in the long wavelength limit for randomly distributed and randomly orientated rectilinear non intersecting thin dry cracks the numerical simulations of velocities of p sv and sh waves are in excellent agreement with the results of the modified or differential self consistent theory on the other hand for intersecting cracks the critical crack density porosity concept must be taken into account to describe the wave velocities in media with intersecting cracks we propose introducing the critical crack density concept into the modified self consistent theory numerical simulations show that this new formulation predicts effective elastic properties accurately for such a case

calcium carbonate dissolution rates in deep sea bivalve shells on the east pacific rise at 21 n results of an 8 year in situ experiment abstract analysis of shell fragments of two common deep sea hydrothermal vent bivalves the vesicomyid clam calyptogena magnifica and the mytilid bathymodiolus thermophilus deployed more than 100 m from any active hydrothermal vent location at 20 50 n 109 06 w on the east pacific rise indicates significant variation of calcium carbonate dissolution in in situ exposures of more than 8 years shell fragments embedded in epoxy blocks mounted on a buoyed and anchored polypropylene line and deployed by dsv alvin depth 2615 m were continuously exposed to ambient seawater 2 c conditions immediately above the seafloor the mean dissolution rate for the aragonitic shell of c magnifica amounted to 27 7 m yr dissolution rates varied significantly among different shell layers of the clam with the middle fine to irregular complex crossed lamellar shell layer exhibiting the lowest rates mean 22 2 m yr and the irregular prismatic shell layer the highest rates mean 36 9 m yr in the mytilid b thermophilus the dissolution rate of the aragonitic shell layer averaged 41 6 m yr while that of the calcitic shell layer was immeasurable the rates of calcium carbonate dissolution reported here for a ridge crest site remote from any active hydrothermal vent are much lower than those previously documented for active vent sites at 21 n the guaymas basin southern trough and galapagos rift rose garden assuming a constant rate of dissolution we estimate that empty adult shell valves of c magnifica at the experimental site would dissolve completely in a period of 300 years which has important implications for determining the longevity of hydrothermal vent activity along the rise axis

origin of trace gases in submarine hydrothermal vents of the kolbeinsey ridge north iceland two hydrothermal fields of the kolbeinsey ridge area north of iceland show vent gas characteristics which can be related to the subsurface conditions helium isotopes r r air 9 8 10 9 indicate a mantle derived origin and can be considered as a mixture of more helium and a deep mantle plume helium component the carbon isotope composition of co2 ranges between 2 4 and 7 8 parts per thousand the less negative delta 13 c co2 values were found at grimsey the data from grimsey are very similar to those previously published and regarded as being characteristic for the icelandic magmatic source however small amounts of biogenic co2 and or subsurface calcite precipitation are responsible for the lighter isotope values of co2 from kolbeinsey ch4 he 3 ratios which are higher than in morb indicate an additional sedimentary methane source for kolbeinsey and grimsey hydrothermal gases the presence of higher hydrocarbons up to butane together with the carbon isotope values of methane delta 13 c 26 1 to 39 8 parts per thousand suggest a probably high mature organic source within thick sediments of the tjornes fracture zone and smaller depressions on the west side of the kolbeinsey ridge crest geochemical characteristics of hydrocarbons present in kr hydrothermal fluids are however typical for a mixed thermogenic and high temperature hydrothermal e g epr type origin moreover it is likely that secondary processes such as bacterial oxidation and thermal cracking determined the geochemical characteristics of the gases c 1999 elsevier science b v all rights reserved

how many vent fields new estimates of vent field populations on ocean ridges from precise mapping of hydrothermal discharge locations abstract decades of exploration for venting sites along spreading ridge crests have produced global datasets that yield estimated mean site spacings of 12 220 km this conclusion demands that sites where hydrothermal fluid leaks from the seafloor are improbably rare along the 66 000 km global ridge system despite the high bulk permeability of ridge crest axes however to date exploration methods have neither reliably detected plumes from isolated low temperature particle poor diffuse sources nor differentiated individual closely spaced clustered within a few kilometers sites of any kind here we describe a much lower mean discharge spacing of 3 20 km revealed by towing real time oxidation reduction potential and optical sensors continuously along four fast and intermediate rate 55 mm yr spreading ridge sections totaling 1470 km length this closer spacing reflects both discovery of isolated sites discharging particle poor plumes 25 of all sites and improved discrimination at a spatial resolution of 1 km among clustered discrete and diffuse sources consequently the number of active vent sites on fast and intermediate rate spreading ridges may be at least a factor of 3 6 higher than now presumed this increase provides new quantitative constraints for models of seafloor processes such as dispersal of fauna among seafloor and crustal chemosynthetic habitats biogeochemical impacts of diffuse venting and spatial patterns of hydrothermal discharge

crack induced anisotropy in crustal rocks predicted dry and fluid saturated thomsen s parameters abstract in upper crustal conditions elastic anisotropy of rocks is mainly due to cracks as long as the anisotropy of the background matrix remains low because in general rock anisotropy remains weak in the sense that hexagonal symmetry may be sufficient to handle it thomsen suggested that a set of three parameters would be convenient to describe it using effective elasticity we derive thomsen s parameters for the case of dry and fluid saturated cracks in a rock of isotropic background in terms of crack density parameters and fluid compressibility these results can be of interest for the interpretation of seismological and seismic data they provide moreover a way to identify the fluid nature gas or liquid and to extract the crack density tensor from elastic wave data frequency effects are discussed when fluid is present so that frequency dependence can be accounted for

discovery of a magma chamber and faults beneath a mid atlantic ridge hydrothermal field crust at slow spreading ridges is formed by a combination of magmatic and tectonic processes with magmatic accretion possibly involving short lived crustal magma chambers the reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast spreading centres but it has been difficult to image such magma chambers beneath slow spreading centres owing to rough seafloor topography and associated seafloor scattering in the absence of any images of magma chambers or of subsurface near axis faults it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow spreading ridges here we report the presence of a crustal magma chamber beneath the slow spreading lucky strike segment of the mid atlantic ridge the reflection from the top of the magma chamber centred beneath the lucky strike volcano and hydrothermal field is approximately 3 km beneath the sea floor 3 4 km wide and extends up to 7 km along axis we suggest that this magma chamber provides the heat for the active hydrothermal vent field above it we also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward dipping faults cutting through the volcanic edifice suggesting continuous interactions between tectonic and magmatic processes

the quantum event of oceanic crustal accretion impacts of diking at mid ocean ridges seafloor diking eruptive events represent the irreducible quantum events of upper oceanic crustal accretion they record events by which a large portion of the oceanic crust has formed through geological history since 1993 the u s navy s real time sound surveillance system has allowed location of ongoing acoustic signatures of dike emplacement and basalt eruptions at ridge crests in the northeast pacific these diking eruptive events trigger a sequence of related rapidly evolving physical chemical and biological processes magmatic volatiles released during these events may provide nutrients for communities of subseafloor microorganisms some of which thrive in high temperature anaerobic environments many of the organisms identified from these systems are archaea if microorganisms can thrive in the water saturated pores and cracks within deep volcanically active portions of our planet other hydrothermally active planets may harbor similar life forms

fe si oxyhydroxide deposits at a slow spreading centre with thickened oceanic crust the lilliput hydrothermal field 9 33 s mid atlantic ridge diffuse and focused low temperature fluids emanate at 9 33 s mid atlantic ridge and precipitate fe sioxyhydroxides that form chimneys mounds and flat lying deposits this extensive vent field named lilliput lies at the axial zone of a spreading segment with a significantly thickened crust 11 km theoretically much more heat needs to be removed from a thick crust spreading center compared to a spreading center with typical thickness of 6 km therefore settings with thickened crust should be favourable for supporting very powerful hydrothermal systems capable of producing large mineral deposits this is the first report on the composition of seafloor hydrothermal deposits at abnormally thickened oceanic crust due to hotspot ridge interaction our studies revealed that generally the lilliput hydrothermal deposits are very similar in morphology structure composition and lateral extent to other low temperature hydrothermal deposits of mid ocean ridges and intraplate volcanoes deposits at the lilliput vent field are composed of si containing goethite and ferrihydrite have very low contents of a number of transition and rare earth elements and show ree distribution patterns with negative ce and eu anomalies the speciation and precipitation of the main deposit forming elements fe and si at the hydrothermal field appear to be partially controlled by live microbes and exuded organic compounds the 18o values of the precipitated silica containing feoxyhydroxides point to low temperature formation and sr nd pb isotope variations suggest that the hydrothermal precipitates scavenged metals predominantly from the ambient seawater these findings are in agreement with the biogeochemical scenario for their precipitation

reconstruction of continental margin architecture deformed by the contraction of the lagonegro basin southern apennines italy thrust propagation through previously rifted continental margins may result in fold and thrust belts whose structure is strongly controlled by the inherited basin architecture as it occurs in southern italy the lagonegro units of the southern apennines comprise a deformed pelagic basin succession showing variable stratigraphic characteristics mainly lateral variations in both facies and thickness interpreted to be due to a complex basin topography related to a triassic rifting event in contrast to previous studies cross section balancing and restoration indicate that the lagonegro units exposed in the high agri valley area suffered relatively limited internal shortening 8 km i e 35 early deformation of these rocks later incorporated into a large displacement thrust sheet was dominated by folding around present day roughly north south trending axes the attainment of a regional decollement level was favoured by an early mild inversion of the basin producing a roughly similar structural elevation of both hanging wall and footwall successions to mesozoic faults most of the contractional deformation was accommodated by buckling of the mesozoic syn rift strata between synsedimentary faults which represented major mechanical interfaces early strain localization in the lagonegro basin ahead of the active thrust front was most probably mechanically controlled by a faulted crustal segment which originally lay within the continental margin between two massive carbonate platforms

impact of the cracks lost in the imaging process on computing linear elastic properties from 3d microtomographic images of berea sandstone with the current developments in imaging computational techniques and resources computational rock physics has been emerging as a new field of study properties of rocks are examined by carrying out extensive numerical simulations on rocks that have been digitized using high resolution x ray ct scans the ultimate goal of computational rock physics is to supplement the traditional laboratory measurements which are time consuming with faster numerical simulations that allow the parameter space to be explored more thoroughly we applied the finite element method to compute the static effective elastic properties from 3d microtomographic images of berea sandstone saturated with different fluids from the computations we found discrepancies between the numerical results and the laboratory measurements the reason for such a problem is the loss of small features such as fine cracks and micropores in the digitized matrix during the imaging and aggregation process we used a hybrid approach combining the

incremental linear elastic response of rocks containing multiple rough fractures similarities and differences with traction free cracks fractures in the subsurface serve as conduits for fluids and gas connecting remote hydrocarbon reservoir sections to production wells seismic and sonic data are popular sources for information on fracture properties the most commonly used model to extract fracture information from such data is based on the paradigm of the displacement discontinuity interface without a direct link to relevant characteristics such as the surface roughness properties of a fracture indeed fractures can be modeled as displacement discontinuity surfaces and in this sense they resemble traction free cracks in literature cracks and fractures are not always properly distinguished perhaps because the terms are often perceived as synonyms however microstructural parameters that control magnitudes of the discontinuities and thus the effective stiffnesses are entirely different statistics of contacts for fractures versus crack density for traction free cracks we explore the effective elasticity of rocks containing multiple fractures using a model of a fracture as two rough surfaces with isolated contacts this is done in the context of the incremental linear elastic response to small stress changes typical in wave propagation problems fractures are dry or may have diverse orientations and contacts may or may not be hertzian a link exists between contact characteristics and effective stiffness of single and multiple fractures our work examines and accounts for the strong effect of interactions between individual contacts by means of a double sum over mutual positions as well as outlines the differences and similarities between theories for cracks and fractures

seismic characterization of multiple fracture sets does orthotropy suffice as geophysicists rely increasingly on effective media theories to characterize naturally fractured reservoirs they become more and more interested in evaluating the accuracy of different theories estimating their limits of applicability and assessing their usefulness for practice with this in mind we compare two popular seismological theories of hudson and schoenberg with the theory of kachanov developed in the context of mechanics of materials by performing finite element simulations of effective media for models that contain several sets of nonintersecting circular vertical fractures embedded in otherwise isotropic host rock we examine the accuracy of these theories our numeric study reveals that predictions of both the first and second order hudson s theories are typically inferior to those of others especially when the fractures are dry while on average the theories of schoenberg and kachanov fit finite element computations with comparable accuracy the latter appears to be more useful

book reviewthe techniques of modern structural geology vol 1 strain analysis j g ramsay and m i huber academic press london 1983 xiii 307 pp us 55 00 28 00 hardcover

inversion of multicomponent multiazimuth walkaway vsp data for the stiffness tensor vertical seismic profiling vsp an established technique can be used for estimating in situ anisotropy that might provide valuable information for characterization of reservoir lithology fractures and fluids the p wave slowness components conventionally measured in multiazimuth walkaway vsp surveys allow one to reconstruct some portion of the corresponding slowness surface a major limitation of this technique is that the p wave slowness surface alone does not constrain a number of stiffness coefficients that may be crucial for inferring certain rock properties those stiffnesses can be obtained only by combining the measurements of p waves with those of s or ps modes here we extend the idea of horne and leaney who proved the feasibility of joint inversion of the slowness and polarization vectors of p and sv waves for parameters of transversely isotropic media with a vertical symmetry axis vti symmetry we show that there is no need to assume a priori vti symmetry or any other specific typ

strain patterns and shortening in a folded thrust sheet an example from the southern appalachians abstract total strain patterns estimated across the pulaski thrust sheet of the southwest virginia appalachians show an approximately homogeneous plane strain deformation associated with folding and distortion above a subsurface decollement estimated strains are low 1 2 x z

microcrack induced elastic wave anisotropy of brittle rocks the failure of brittle rocks during compression is preceded by the formation growth and coalescence of microcracks elastic wave velocities are reduced in the presence of open microcracks and fractures and may therefore be used to monitor the progressive damage of the rock in general these microcracks are not randomly oriented and the rock displays an elastic anisotropy the elastic anisotropy due to cracks can be expressed in terms of a second rank and fourth rank crack density tensor for open cracks the contribution of the fourth rank crack density tensor to the elastic wave velocities is small these results are compared with recent measurements of the ultrasonic compressional and shear wave velocities for propagation parallel and perpendicular to an increasing axial stress applied at constant confining stress to berea sandstone inversion of the velocity measurements indicates that the microcracks propagate parallel to the maximum compressive stress in agreement with current rock mechanics theory a reasonable fit to the data is obtained using only the second rank crack density tensor even though at high confining stress the cracks are expected to be in partial contact along their length this is consistent with the model of elastic wave propagation in a medium containing partially contacting fractures published by white however measurements of off axis wave velocities are required to fully quantify the contribution of the fourth rank crack density tensor

large volume loss during cleavage formation hamburg sequence pennsylvania green reduction spots in red slate of the hamburg sequence exposed near shartlesville pennsylvania have axial ratios of 1 42 1 0 0 28 on the limbs of near isoclinal folds and 1 0 0 79 0 41 in fold hinge zones conodont cusps and denticles within the reduction spots have been brittlely pulled apart and give independent measures of extension in various directions comparison of conodont extensions with reduction spot shapes on limbs and hinges indicates that sedimentary compaction of 44 preceded the tectonic strain associated with cleavage formation this strain having identical maximum extensions but greater shortening in fold hinges as compared to limbs was characterized by 41 extension in x no change in y 50 to 59 shortening in z and 29 to 42 tectonic volume loss the general lack of directed overgrowths on grains reflects the large volume loss and contrasts with other slates where deformation was an almost constant volume process and extension in x compensated for shortening in z

enriched h2 ch4 and 3he concentrations in hydrothermal plumes associated with the 1996 gorda ridge eruptive event abstract the rapid field response in march and april 1996 to diking eruptive events on the northern gorda ridge allowed the first detailed co measurement of 3 he h 2 and ch 4 in plumes associated with a submarine eruption and examination of the chemical transformation of these volatile species during aging of the plumes the diking eruptive event generated extensive event plumes with a low 3 he heat ratio of 0 4 10 12 cm 3 stp gm 1 c 1 while chronic plumes had 3 he heat ratios of 2 1 6 2 10 12 cm 3 stp gm 1 c 1 hydrogen concentrations in the initial event plume ep96a reached a maximum value of 47 nm 235 times that of ocean background concentration 0 2 nm in contrast roughly one month later h 2 concentrations although still elevated had drastically dropped to 40 times background values methane concentrations in both event and chronic plume fluids were also elevated and similar to stable hydrothermal systems in unsedimented ridge environments reaching a maximum value of 7 nm the intense rapid interaction at very high temperatures of hydrothermal fluids with the host rock during dike intrusion or with seawater during extrusion of the basalt likely generated high localized concentrations of h 2 the data indicate however that maximum rates of h 2 production during event plume formation could have lasted a few days at most

morphology and tectonics of the mid atlantic ridge 7 12 s 1 we present swath bathymetric gravity and magnetic data from the mid atlantic ridge between the ascension and the bode verde fracture zones where significant ridge hot spot interaction has been inferred the ridge axis in this region may be divided into four segments the central two segments exhibit rifted axial highs while the northernmost and southernmost segments have deep rift valleys typical of slow spreading mid ocean ridges bathymetric and magnetic data indicate that both central segments have experienced ridge jumps since 1 ma mantle bouguer anomalies mbas derived from shipboard free air gravity and swath bathymetric data show deep subcircular lows centered on the new ridge axes suggesting that mantle flow has been established beneath the new spreading centers for at least 1 myr inversion of gravity data indicates that crustal thicknesses vary by 4 km along axis with the thickest crust occurring beneath a large axial volcanic edifice once the effects of lithospheric aging have been removed a model in which gravity variations are attributed entirely to crustal thickness variations is more consistent with data from an axis parallel seismic line than a model that includes additional along axis variations in mantle temperature both geophysical and geochemical data from the region may be explained by the melting of small 200 km mantle chemical heterogeneities rather than elevated temperatures therefore there may be no ascension circe plume index terms 1517 geomagnetism and paleomagnetism magnetic anomaly modeling 3010 marine geology and geophysics gravity 3040 marine geology and geophysics plate tectonics 8150 8155 8157 8158

strain analysis in jurassic argillites of the monte sirino area lagonegro zone southern apennines italy and implications for deformation paths in pelitic rocks analysis of strain in jurassic argillites forming part of the folded and thrusted sedimentary succession of the lagonegro basin southern italian apennines has been carried out using ellipsoid shaped reduction spots as strain markers most of the determined finite strain ellipsoids are of oblate type and show a peculiar distribution of the maximum extension direction x with maxima either subparallel or subperpendicular to the local fold axes using the strain matrix method two different deformation histories have been considered to assist the interpretation of the observed finite strain pattern a first deformation history involved vertical compaction followed by horizontal shortening occurring by a combination of true tectonic strain and volume loss whereby all strain is coaxial and there is no change in the intermediate axis of the strain ellipsoid by this type of deformation sequence which produces a deformation path where total strain moves from the oblate to the prolate strain field and back to the oblate field prolate strain ellipsoids can be generated and may be recorded where tectonic deformation has not been large enough to reverse pretectonic compaction this type of deformation history may be of local importance within the study area i e it may characterize some fold hinge regions and more generally is probably of limited occurrence in deformed pelitic rocks a second deformation sequence considered the superposition of pre tectonic compaction and tectonic strain consisting of initial layer parallel shortening followed by layer parallel shear related to flexural folding also in this instance volume change during tectonic deformation and tectonic plane strain have been assumed for geologically reasonable amounts of volume loss due to compaction and of initial layer parallel shortening this type of deformation history is capable of producing a deformation path entirely lying within the oblate strain field but still characterized by a changeover during deformation of the maximum extension axis x from a position parallel to the fold axis to one perpendicular to it this type of deformation sequence may explain the main strain features observed in the study area where most of the measured finite strain ellipsoids determined from the limb regions of flexural folds display an oblate shape irrespective of the orientation of their maximum extension direction x with respect to the local structural trends more generally this type of deformation history provides a mechanism to account for the predominance of oblate strains in deformed pelitic rocks

depths of partial crystallization of h2o bearing morb phase equilibria simulations of basalts at the mar near ascension island 7 11 s phase equilibria simulations were performed on naturally quenched basaltic glasses to determine crystallization conditions prior to eruption of magmas at the mid atlantic ridge mar east of ascension island 7 11 s the results indicate that mid ocean ridge basalt morb magmas beneath different segments of the mar have crystallized over a wide range of pressures 100 900 mpa however each segment seems to have a specific crystallization history nearly isobaric crystallization conditions 100 300 mpa were obtained for the geochemically enriched morb magmas of the central segments whereas normal n morb magmas of the bounding segments are characterized by polybaric crystallization conditions 200 900 mpa in addition our results demonstrate close to anhydrous crystallization conditions of n morbs whereas geochemically enriched morbs were successfully modeled in the presence of 0 4 1 wt h2o in the parental melts these estimates are in agreement with direct fourier transform ir measurements of h2o abundances in basaltic glasses and melt inclusions for selected samples water contents determined in the parental melts are in the range 0 04 0 09 and 0 30 0 55 wt h2o for depleted and enriched morbs respectively our results are in general agreement within 200 mpa with previous approaches used to evaluate pressure estimates in morb however the determination of pre eruptive conditions of morbs including temperature and water content in addition to pressure requires the improvement of magma crystallization models to simulate liquid lines of descent in the presence of small amounts of water

young volcanism and related hydrothermal activity at 5 s on the slow spreading southern mid atlantic ridge the effect of volcanic activity on submarine hydrothermal systems has been well documented along fast and intermediate spreading centers but not from slow spreading ridges indeed volcanic eruptions are expected to be rare on slow spreading axes here we report the presence of hydrothermal venting associated with extremely fresh lava flows at an elevated apparently magmatically robust segment center on the slow spreading southern mid atlantic ridge near 5 s three high temperature vent fields have been recognized so far over a strike length of less than 2 km with two fields venting phase separated vapor type fluids exit temperatures at one of the fields reach up to 407 c at conditions of the critical point of seawater the highest temperatures ever recorded from the seafloor fluid and vent field characteristics show a large variability between the vent fields a variation that is not expected within such a limited area we conclude from mineralogical investigations of hydrothermal precipitates that vent fluid compositions have evolved recently from relatively oxidizing to more reducing conditions a shift that could also be related to renewed magmatic activity in the area current high exit temperatures reducing conditions low silica contents and high hydrogen contents in the fluids of two vent sites are consistent with a shallow magmatic source probably related to a young volcanic eruption event nearby in which basaltic magma is actively crystallizing this is the first reported evidence for direct magmatic hydrothermal interaction on a slow spreading mid ocean ridge

a simplified model of effective elasticity for anisotropic shales a simple method to deal with cracklike pores in anisotropic matrix rock such as shales enhances analytical models and their applications actually clayrocks shales in particular are the dominant clastic component in sedimentary basins representing about two thirds of all sedimentary rocks shales are usually assumed to be transversely isotropic ti media they are known to be highly anisotropic because of 1 intrinsic elastic anisotropy of the solid phase matrix forming the rock more or less ordered clay layers and 2 anisotropy induced by the presence of cracklike pores we focus on this second component of anisotropy current analytical models deal with it but they are complex and are restricted in the case of matrix ti symmetry to cracks lying in the symmetry plane we simplify such models within a reasonably good approximation and develop an analysis scheme in which cracklike pore effects are calculated in an equivalent isotropic matrix this simplifies the theoretical approach and potentially broadens its application to any crack and or pore orientation e g damaged shale with horizontal and vertical perpendicular to the bedding plane cracks a high pressure confinement test provides experimental data for checking the proposed tool against a reference model in the case of cracklike pores lying in the bedding plane the results in terms of thomsen parameters are consistent with results from large scale field data

lava flows from a mid 1980s submarine eruption on the cleft segment juan de fuca ridge a series of lava flows with a total volume of 0 05 km3 were erupted in the mid 1980s along 17 km of the northern cleft segment of the juan de fuca ridge observations from camera tows and submersible dives show that the new flows are all similar in appearance and consist entirely of pillow lava with a mixture of smooth and striated surface textures suggesting a relatively uniform eruption rate approaching 1 m3 s at point source vents the flows vary in size from small patches to large steep sided ridges and were probably erupted from a dike intruded along the ridge axis because they are aligned along a linear fissure graben system observations at north cleft show that the physical appearance of new flows changes more rapidly than previously realized and that earlier qualitative dating of young lavas based on sediment cover and glassy surface texture were probably overestimates by an order of magnitude sediment accumulation on the lavas is quite variable and locally surprisingly substantial mainly due to hydrothermal deposits that formed while the lava flows were cooling biological vent communities photographed on the new flows in 1989 show that vent animals can colonize new vent sites rapidly but that warm water was still venting only in a few places nonvent animals are much slower to colonize the new flows and rates of colonization observed at north cleft may be useful for making improved age estimates of young 10 years lava flows elsewhere the north cleft eruption represents about 2 of the estimated average annual volcanic output along the global mid ocean ridge implying that many other submarine eruptions are occurring undetected

effects of the superposition of compaction and tectonic strain during folding of a multilayer sequence model and observations abstract sedimentary rocks deformed at shallow crustal depth can show a wide variety of planar linear and mixed planar linear fabrics depending on their structural position within folds a model has been developed which tries to explain this variation in fabric development in terms of the strain states that it represents we analyse the finite strain states that may develop during compaction and subsequent buckle folding of sedimentary multilayers composed of a regular alternation of competent and incompetent beds the geometric properties of the multilayer during buckling have been analysed from a finite element model characterized by linear viscous material properties a viscosity contrast 1 2 40 and a ratio of incompetent to competent layer thickness d 2 d 1 1 sequential matrix multiplication allows the determination of the finite strain states for all stages of fold development finite strain states predicted from the model appear to be in good agreement with observations of fabric development in naturally deformed rocks from the southern apennines during progressive deformation in the model the finite strain ellipsoid in the limb regions of folded incompetent layers is consistently oblate and the xy plane lies at a small angle to bedding as a consequence of previous compaction natural cleavage consistently lies at a low angle to bedding in folded argillites from the southern apennines and it can be interpreted to represent the xy plane of the total finite strain ellipsoid in strongly compacted sediments and not that of tectonic deformation alone in the hinge regions of open folds finite strains predicted in the incompetent layers of the model are of prolate type and they become oblate as the interlimb angle decreases the presence of a true pencil structure in open folds and of a spaced cleavage associated with a regular pencil structure in more close folds often noted in the case study of the southern apennines most probably reflects this change from prolate to oblate strain in the hinge zones as folding proceeds

low angle normal faulting and focused exhumation associated with late pliocene change in tectonic style in the southern apennines italy in the southern apennines low temperature thermochronometry data indicate that exhumation of previous tectonically buried sedimentary units started at around 10 ma and took place mostly during the last 6 ma relatively high exhumation rates are obtained from apatite fission track aft and u th he ahe analysis pointing to a substantial contribution of tectonic processes to rock exhumation besides erosion exhumation rates derived from new apatite u th he data ahe for the last 3 ma are generally lower than rates determined by aft data and almost in line with erosion rates inferred from cosmogenic nuclides and sediment yield thus suggesting that tectonic exhumation was dominant during the older exhumation stages of this region however younger cooling ages in the monte alpi area from both aft and ahe analyses point out focused exhumation during the last 3 ma structural and morphotectonic analyses indicate that fast exhumation occurred specifically in this area where the apulian platform reservoir carbonates elsewhere buried beneath a several kilometer thick allochthonous cover are exposed at the surface as a result of a complex interplay between steep rooted reverse faulting and shallow low angle extension this deformation involved the development of foreland dipping low angle normal faults affecting the allochthonous cover units during the late stages of reverse fault related anticlinal growth in the underlying buried carbonates extension of the region triggered focused exhumation in the footwall of the extensional low angle faults which was followed by widespread crustal extension and associated development of high angle normal faults leading to surface uplift of monte alpi

seismic characterization of naturally fractured reservoirs using amplitude versus offset and azimuth analysis p wave seismic reflection data with variable offset and azimuth acquired over a fractured reservoir can theoretically be inverted for the effective compliance of the fractures the total effective compliance of a fractured rock which is described using second and fourth rank fracture tensors can be represented as background compliance plus additional compliance due to fractures assuming monoclinic or orthotropic symmetry which take into account layering and multiple fracture sets the components of the effective second and fourth rank fracture compliance tensors can be used as attributes related to the characteristics of the fractured medium synthetic tests indicate that using a priori knowledge of the properties of the unfractured medium the inversion can be effective on noisy data with s n on the order of 2 monte carlo simulation was used to test the effect of uncertainties in the a priori information about elastic properties of unfractured rock two cases were considered with wide azimuth waz and narrow azimuth naz reflection data and assuming that the fractures have rotationally invariant shear compliance the relative errors in determination of the components of the fourth rank tensor are substantially larger compared to the second rank tensor under the same assumptions elastic properties of background media consisting in horizontal layers without fractures do not cause azimuthal changes in the reflection coefficient variation with offset thus due to the different nature of these properties compared to fracture tensor components which cause azimuthal anomalies simultaneous inversion for background isotropic properties and fracture tensor components requires additional constraints singular value decomposition svd and resolution matrix analysis can be used to predict fracture inversion efficacy before acquiring data therefore they can be used to determine the optimal seismic survey design for inversion of fracture parameters however results of synthetic inversion in some cases are not consistent with resolution matrix results and resolution matrix results are reliable only after one can see a consistent and robust behaviour in inversion of synthetics with different noise levels

elastic wave propagation in fractured media using the discontinuous galerkin method abstractwe have formulated and implemented a discontinuous galerkin method dgm for elastic wave propagation that allows for discontinuities in the displacement field to simulate fractures or faults the approach is based on the interior penalty formulation of dgm and the fractures are simulated using the linear slip model which is incorporated into the weak formulation by including an additional term that is similar to the penalty term but uses the fracture compliance instead of an arbitrary penalty parameter we have calibrated our results against an analytic solution of fracture induced anisotropy for a set of elongated horizontal fractures and we have evaluated numerical examples that simulate the reflection and transmission of waves at a fracture and at fracture interface waves this method can further be used with models containing intersecting fractures and multiple fracture sets in 2d or 3d domains

the kinematics of break thrust folds abstract the kinematics of asymmetric open to close often overturned thrust related folds are commonly explained with fault propagation or detachment fold models field observations of thrust related folds exposed in tennessee virginia wyoming and montana that exhibit this geometry indicate that models of fault propagation and detachment folds do not adequately describe the kinematics of these structures existing models of fault propagation and detachment folding employ migrating kink shaped hinges and relate fold geometry entirely to fault geometry slip and to the thickness of the basal detachment layers the models exclude the relation between fold shape and amplitude and they do not predict a correlation between competent layer thickness and fold wavelength as expected from buckling theory thrust related folds examined in this study exhibit unique distributions of fabrics in the hinges and forelimbs which demonstrate a lack of hinge migration furthermore the ratios of dominant member thickness to fold wavelenght agree with those expected for folds controlled by a single layer and they are within the expected range for folds with multiple evenly spaced harmonically deforming layers our analyses suggest these folds evolved from an initial brief stage of sinusoidal buckling to a later stage of fixed hinge kinking and thrusting thrust related folds which exhibit a geometry and mesofabric distribution suggesting this type of evolution are defined as break thrust folds

volume loss during cleavage formation in the meguma group nova scotia canada abstract diffusive mass transfer pressure solution is a widely recognized cleavage forming mechanism under low grade conditions the amount of accompanying volume change during cleavage development however is difficult to quantify and remains controversial the distance between sites of mineral dissolution and reprecipitation and transport mechanisms also are controversial the structural history of the cambrian ordovician meguma group nova scotia canada is well known and abundant strain markers provide an opportunity to assess geometrically volume changes accompanying cleavage development during diagenesis about 70 compaction in muds and 15 in silts and sands occurred the pelitic to psammitic meguma group was cleaved prior to being folded pressure solution was the dominant cleavage forming deformation mechanism under chlorite grade metamorphic conditions during the early devonian acadian orogeny measurement of tectonic strain in principal directions x y z shows that bedding parallel coaxial shortening z is 60 and there is no evidence of fold hinge parallel extension y estimation of tectonic vertical extension x in the cleavage plane is complicated by diagenetic compaction corrections to primary strain markers but is insufficient to compensate for the observed 60 shortening in z indicating 40 60 volume loss during cleavage formation because cleavage formed at least 50 ma after deposition and under approximately 6 km of silurian to early devonian strata porosity of the sediments upon onset of cleavage was likely less than 15 pore space reduction therefore cannot account for the apparent volume loss during cleavage formation cleavage related volume loss applies regionally and to a stratigraphic thickness of several thousand meters implying that a large amount of material was removed and transported large distances when combined with 50 shortening by folding a bulk tectonic shortening of 80 is estimated for the meguma group during the acadian orogeny

generation of spatially correlated fracture models for seismic simulations summary the critical geometrical parameters that quantify the spatial distribution of natural fractures are the orientation length and position of fractures knowledge of their spatial distribution is important as they control the movement of subsurface fluids and also influence seismic waves propagating in the subsurface however generating realistic models of all of these geometrical parameters to use in forward seismic modelling or inversion applications can become very difficult especially when constraints are available only at a few sparse well locations hence this provides strong motivation for applying seismic data to estimate these quantities in field settings and reliable seismic modelling provides important constraints for interpretation and inversion the discrete fracture network dfn approach has been used frequently to generate models with stochastic distributions of fractures based on sparse well and seismic data however most of these studies lack any constraint from physical models of the behaviour of fractured media in this paper we implement and extend an alternative modelling technique to generate several realizations of a fracture model beginning with theoretical results for the strain energy of a fractured material and propose ways to better incorporate geological field observations the method utilizes an elastic energy function that sums the interactions of all pairs of fractures present in the model the energy for each pair depends on the distance between the two fractures their orientations lengths and some material properties this energy function also serves as an objective function for a simulated annealing sa algorithm used to obtain multiple realizations of correlated fracture networks we improve earlier versions of this technique by incorporating periodic boundary conditions including criteria to limit the maximum range of pair wise calculations and suggesting methods to constrain models to match field data assuming that the host rock is isotropic and homogeneous this method produces orthogonal sets of fractures a pattern that is commonly observed during basin formation or subsidence this shows how the method can be utilized to provide constraints with a physical basis to facilitate the development of fracture models we also outline how the fracture model can be converted into an anisotropic seismic velocity model to compute synthetic seismograms for reservoir characterization applications

temporal and spatial variability of seafloor spreading processes in the northern south atlantic the paper reports the results of an aeromagnetic study of the mid atlantic ridge in the northern south atlantic the study consists of 38 000 km of low altitude magnetics tracks centered on the ridge covering nearly 700 000 km2 south of ascension island most tracks extend out to crust of anomaly 5b age large areal coverage coupled with high resolution allows comparison of crustal generation processes over space along ridge strike and time across strike temporal heterogeneities revealed in the data include a pattern of change in spreading rate and direction of plate motion the total spreading rate increased sharply at 10 ma and then decreased between 6 ma and the present the time variation of spreading rate is similar to that seen in other areas such as the north atlantic and the southern and eastern pacific the plate boundary has evolved over time in response to the shift in plate motion direction through fragmentation and rotation of ridge segments between fracture zones in several cases the location of the spreading axis has shifted through jumps or propagation across fracture zones the periods of plate boundary modification correlate with the increase and decrease in spreading rate spatial heterogeneities are caused by the influence of the ascension hotspot on the mid atlantic ridge in the northern portion of the study area and include two apparent seamount chains formed by the hotspot ascension island and several of the seamounts appear to have been produced some distance away from the hotspot location presently about 225 km southeast of ascension island by channeled subaxial magma flow beneath the spreading center the seamount chains therefore do not mark hotspot tracks large local variations in spreading rate and degree of asymmetry along strike are also found

growth processes and melange formation in the southern apennines accretionary wedge abstract the southern apennines owe their present geometry to the subduction of an apulian continental lithospheric slab and are thus best interpreted as an accretionary wedge southward the apenninic thrust front branches on to the still active ionian wedge which is related to the present calabro sicilian subduction the interpretation of conventional seismic reflection profiles and exploration wells combined with geologic field studies biostratigraphy and structural geology provide constraints for a geodynamic model for the post messinian growth of the apenninic prism our model for the apennines is a good analogy for submerged oceanic accretionary prisms in both instances features such as out of sequence thrusts underplating and duplexes are viewed as processes to explain the thickening of accretionary wedges syntectonic terrigenous deposits have accumulated in the synclines developed above the flats of pliocene thrust faults these depocenters represent piggy back basins younger accretion at the eastern border of the accretionary wedge has led to a progressive uplifting and tilting of the already emplaced thrust sheets including a continous westward migration of the depocenters in the piggy back basins during pliocene and early quaternary times a contrary west to east migration of the depocenters locally occurred where preexisting flats of pliocene thrust faults have been refolded rather than only being rotated dewatering processes and effects of overpressures on undercompacted sediment deformations can also be studied in the southern apennines melanges and broken formations form an important component of the trench apparently restricted to the base of tectonic nappes listric normal faults are a major structural element which preserves in combination with the erosion of topographic highs and resedimentation in the piggy back basins a uniform geometric shape critical taper for the prism the age and location of these distensive or gravitational effects are also discussed

seismic anisotropy of fractured rock a simple method for including the effects of geologically realistic fractures on the seismic propagation through fractured rocks can be obtained by writing the effective compliance tensor of the fractured rock as the sum of the compliance tensor of the unfractured background rock and the compliance tensors for each set of parallel fractures or aligned fractures the compliance tensor of each fracture set is derivable from a second rank fracture compliance tensor for a rotationally symmetric set of fractures the fracture compliance tensor depends on only two fracture compliances one controlling fracture compliance normal the other tangential to the plane of the fractures the stiffness tensor which is more useful in the consideration of elastic wave propagation through rocks can then be obtained by inversion the components of the excess fracture compliance tensor represent the maximum amount of information that can be obtained from seismic data if the background rock is isotropic and the normal a

strain variations and three dimensional strain factorization at the transition from the southern to the central appalachians abstract the transition between the southern and the central appalachian foreland thrust system is a region subjected to at least two non coaxial deformations where typical two dimensional analyses such as cross section balancing or plane strain retro deformation cannot be applied three dimensional factorization can simulate the non coaxial strain history simulation results for compaction and layer parallel shortening lps were compared to finite strains of 16 quartz arenite samples from the tuscarora sandstone successful simulations indicate in the southern appalachians 30 35 total compaction by volume loss including a 4 5 9 e w horizontal shortening during lithification and 5 lps along 150 330 by plane strain or axial symmetric flattening in the central appalachians 30 35 total compaction by volume loss including a 3 7 n s horizontal shortening during lithification and 10 lps along 120 300 by plane strain or axial symmetric flattening in the transition zone 30 compaction by volume loss including a 3 e w horizontal shortening during lithification 5 southern lps along 150 330 by axial symmetric flattening and 5 10 central shortening along 130 or 140 by plane strain or axial symmetric flattening at 10 to the normal central trend from the simulations horizontal shortening in the southern appalachians during lithification in the mississippian was greater than the later alleghanian deformation this shortening probably represents a very early lps developed in unlithified sand that required less deviatoric stress to deform than did the cemented quartz arenites during later tectonic deformation also the oblique and weaker central tectonic lps in the transition zone probably represents behavior at the limits of central appalachian deformation finally the lps simulations indicate that axial symmetric flattening oblate strain is more successful than plane strain as the representative behavior so caution is recommended in assuming plane strain as the dominant strain behavior in orogenic forelands

strains and folds within thrust sheets an analysis of the heilam sheet northwest scotland abstract the changes of shape and orientation of fossil worm burrows have been used to carry out a detailed strain analysis of the heilam thrust sheet in the moine thrust zone northwest scotland the strains have been factorised into components of layer parallel shear 1 layer normal shear 2 and layer parallel shortening or elongation variations in the 1 shear are largely lithologically controlled some strain is due to flexural processes as the thrusts and overlying beds change dip over thrust ramps or are tightened by later deformation while there may also be a component of drag which increases towards the overlying thrust this predating the shear due to flexure a contoured map of the values suggests that major folds within the thrust sheet developed as a result of stick or drag on the floor thrust prior to ramp climb this led to differential movement in the transport direction causing layer parallel shortening accompanied or followed by buckling of the layers a map showing layer normal shear strains 2 suggests that the heilam sheet forms a major surge zone the central part having moved further wnw than the northern and southern parts this has led to components of lateral shear along parts of the sheet especially in the north producing curvature of the imbricate faults

hydrothermal activity on the reykjanes ridge the steinaholl vent field at 63 06 n we have completed a systematic survey for hydrothermal activity along the reykjanes ridge from 57 45 n to 63 09 n shipboard determinations of total dissolvable mn tdmn and dissolved si ch4 and h2 have revealed evidence for only one site of hydrothermal activity along this section of ridge crest the steinaholl vent field at 63 06 n 1 our measurements confirm this site to be a stable hydrothermal convection cell of at least two years duration the site is situated in just 250 350 m of seawater and unlike deeper sited vent fields e g tag 3650 m 26 n mar is notable for the formation of bubble rich plumes which have been imaged using a high frequency 38 khz echo sounder high dissolved gas concentrations of up to 18 nmol l ch4 and 30 nmol l h2 coincide with these bubble rich plumes the steinaholl plume is also characterised by high total dissolvable mn tdmn anomalies up to 60 nmol l and dissolved si anomalies 2 mol l yielding molar tdmn ch4 and tdmn si ratios of 3 5 and 0 03 respectively directly comparable to the tag hydrothermal plume the absence of any indication of hydrothermal activity along the remainder of the reykjanes ridge is intriguing between 11 n and 40 n on the mid atlantic ridge previous studies have revealed evidence for hydrothermally active sites approximately every 150 km for the 300 km of ridge crest studied in detail here 750 km total only one site has been found thus despite its proximity to the iceland hot spot the incidence of hydrothermal activity along this section of ridge crest appears to be only 20 50 of that measured elsewhere along the slow spreading mid atlantic ridge

quantifying diffuse and discrete venting at the tour eiffel vent site lucky strike hydrothermal field 1 the relative heat carried by diffuse versus discrete venting of hydrothermal fluids at mid ocean ridges is poorly constrained and likely varies among vent sites estimates of the proportion of heat carried by diffuse flow range from 0 to 100 of the total axial heat flux here we present an approach that integrates imagery video and temperature measurements to accurately estimate this partitioning at a single vent site tour eiffel in the lucky strike hydrothermal field along the mid atlantic ridge fluid temperatures photographic mosaics of the vent site and video sequences of fluid flow were acquired during the bathyluck 09 cruise fall 2009 and the momarsat 10 cruise summer 2010 to the lucky strike hydrothermal field by the rov victor6000 aboard the french research vessel the pourquoi pas ifremer france we use two optical methods to calculate the velocities of imaged hydrothermal fluids 1 for diffuse venting diffuse flow velocimetry tracks the displacement of refractive index anomalies through time and 2 for discrete jets particle image velocimetry tracks eddies by cross correlation of pixel intensities between subsequent images to circumvent video blurring associated with rapid velocities at vent orifices exit velocities at discrete vents are calculated from the best fit of the observed velocity field to a model of a steady state turbulent plume where we vary the model vent radius and fluid exit velocity our results yield vertical velocities of diffuse effluent between 0 9 cm s 1 and 11 1 cm s 1 for fluid temperatures between 3 c and 33 5 c above that of ambient seawater and exit velocities of discrete jets between 22 cm s 1 and 119 cm s 1 for fluid temperatures between 200 c and 301 c above ambient seawater using the calculated fluid velocities temperature measurements and photo mosaics of the actively venting areas we calculate a heat flux due to diffuse venting from thin fractures of 3 15 2 22 mw discrete venting of 1 07 0 66 mw and by incorporating previous estimates of diffuse heat flux density from tour eiffel diffuse flux from the main sulfide mound of 15 6 mw we estimate that the total integrated heat flux from the tour eiffel site is 19 82 2 88 mw and that the ratio of diffuse to discrete heat flux is 18 we discuss the implication of these results for the characterization of different vent sites within lucky strike and in the context of a compilation of all available measurements of the ratio of diffuse to discrete heat flux

uranium series age constraints on lavas from the axial valley of the mid atlantic ridge mark area abstract mass spectrometric measurements of 230 th 226 ra 235 u 231 pa and 238 u 230 th disequilibria are used to determine eruption ages for four mid ocean ridge basalts from the median valley of the mid atlantic ridge south of the kane fracture zone mark area three samples were collected across axis on the axial volcanic ridge i e the neovolcanic ridge near the snake pit hydrothermal mound and one sample was collected near the crest of serocki volcano 50 km south of snake pit 226 ra 230 th and 230 th 238 u activity ratios are low and uniform for all four samples while 231 pa 235 u activity ratios are elevated and somewhat more variable age constraints suggest that these lavas from the most robust volcanic edifices in the mark area are 10 000 20 000 yr old the age data are used to evaluate the efficacy of commonly used age estimate scales based on qualitative indicators e g sediment cover glass quality and to begin to quantify the temporal and spatial dependence of volcanic tectonic and hydrothermal processes at slow spreading oceanic ridges

slow spreading submarine ridges in the south atlantic as a significant oceanic iron source low levels of the micronutrient iron limit primary production and nitrogen fixation in large areas of the global ocean measurements in the south atlantic suggest that slow spreading submarine ridges serve as a significant oceanic iron source in these waters

first evidence for high temperature off axis venting of deep crustal mantle heat the nibelungen hydrothermal field southern mid atlantic ridge during segment scale studies of the southern mid atlantic ridge mar 7 12 s we found evidence in the water column for high temperature hydrothermal activity off axis east of ascension island extensive water column and seafloor work using both standard ctd and deep submergence auv and rov deployments led to the discovery and sampling of the drachenschlund dragon throat black smoker vent at 8 17 87 s 13 30 45 w in 2915 m water depth the vent is flanked by several inactive chimney structures in a field we have named nibelungen the site is located 6 km south of a non transform offset between two adjacent 2nd order ridge segments and 9 km east of the presently active northward propagating a2 ridge segment on a prominent outward facing fault scarp both vent fluid compositions and host rock analyses show this site to be an ultramafic hosted system the first of its kind to be found on the southern mar the thermal output of this single vent based on plume rise height information is estimated to be 60 15 mw this value is high for a single black smoker vent but small for an entire field the tectonic setting and low he content of the vent fluids imply that high temperature off axis venting at drachenschlund is driven not by magmatic processes as at the majority of on axis hydrothermal systems but by residual heat mined from the deeper lithosphere whether this heat is being extracted from high temperature mantle peridotites or deep crustal cumulates formed at the duelling non transfrom offset is unclear in either case the drachenschlund vent provides the first direct observations of how cooling of deeper parts of the lithosphere at least at slow spreading ridges may be occurring

flux and dispersion of gases from the drachenschlund hydrothermal vent at 8 18 s 13 30 w on the mid atlantic ridge hydrothermal emission of mantle helium appears to be directly related to magma production rate but other processes can generate methane and hydrogen on mid ocean ridges in an on going effort to characterize these processes in the south atlantic the flux and distribution of these gases were investigated in the vicinity of a powerful black smoker recently discovered at 8 17 9 s 13 30 4 w the vent lies on the shoulder of an oblique offset in the mid atlantic ridge and discharges high concentrations of methane and hydrogen measurements during expeditions in 2004 and 2006 show that the ratio of ch4 to 3he in the neutrally buoyant plume is quite high 4 108 the ctd stations were accompanied by velocity measurements with lowered acoustic doppler current profilers ladcp and from these data we estimate the methane transport to have been 0 5 mol s 1 in a wsw trending plume that seems to develop during the ebb tidal phase this transport is an order of magnitude greater than the source of ch4 calculated from its concentration in the vent fluid and the rise height of the plume from this range of methane fluxes the source of 3he is estimated to be between 0 14 and 1 2 nmol s 1 in either case the 3he source is significantly lower than expected from the spreading rate of the mid atlantic ridge from the inventory of methane in the rift valley adjacent to the vent it appears that the average specific rate of oxidation is 2 6 to 23 yr 1 corresponding to a turnover time between 140 and 16 days vertical profiles of methane in the surrounding region often exhibited gaussian like distributions and the variances appear to increase with distance from the vent using a gaussian plume model we obtained a range of vertical eddy diffusivities between 0 009 and 0 08 m2m2 s 1 these high values may be due to tidally driven internal waves across the promontory on which the vent is located

dependence of ridge axis morphology on magma supply and spreading rate why do some mid ocean ridge axes have as their topographic expression a median valley 1 2 km deep and 15 30 km wide whereas others have an axial high 100 200 m high and 1 2 km wide in general median valleys exist at the axes of ridges spreading at half rates less than 20 25 mm yr 1 whereas axial highs are found at faster spreading ridges1 2 a recent crustal genesis model3 incorporating hydrothermal cooling and crustal accretion by means of a magma lens agrees well with observations of the spreading rate dependence of axial morphology1 2 the depth of the axial magma chamber3 4 and the thickness of the ridge axis lithosphere5 6 but there are several notable exceptions to the correlation between spreading rate and axial morphology such as the reykjanes ridge which spreads at l0mm yr 1 but has an axial high here we show that by extending the model of ref 3 to include variations in crustal thickness we can explain the observed dependence of axial morphology on crustal thickness and spreading rate our results suggest that the ultimate control on axial morphology is the thermal structure at the ridge axis which is a function of both spreading rate and magma supply

effective elastic properties of randomly fractured soils 3d numerical experiments this paper is concerned with numerical tests of several rock physical relationships the focus is on effective velocities and scattering attenuation in 3d fractured media we apply the so called rotated staggered finite difference grid rsg technique for numerical experiments using this modified grid it is possible to simulate the propagation of elastic waves in a 3d medium containing cracks pores or free surfaces without applying explicit boundary conditions and without averaging the elastic moduli we simulate the propagation of plane waves through a set of randomly cracked 3d media in these numerical experiments we vary the number and the distribution of cracks the synthetic results are compared with several most popular theories predicting the effective elastic properties of fractured materials we find that for randomly distributed and randomly orientated non intersecting thin penny shaped dry cracks the numerical simulations of p and s wave velocities are in good agreement with the predictions of the self consistent approximation we observe similar results for fluid filled cracks the standard gassmann equation cannot be applied to our 3d fractured media although we have very low porosity in our models this is explained by the absence of a connected porosity there is only a slight difference in effective velocities between the cases of intersecting and non intersecting cracks this can be clearly demonstrated up to a crack density that is close to the connectivity percolation threshold for crack densities beyond this threshold we observe that the differential effective medium dem theory gives the best fit with numerical results for intersecting cracks additionally it is shown that the scattering attenuation coefficient of the mean field predicted by the classical hudson approach is in excellent agreement with our numerical results

mid ocean ridge sulfide deposits evidence for heat extraction from magma chambers or cracking fronts numerous seafloor observations show that the sizes of high temperature hydrothermal sulfide edifices vary dramatically with spreading rate on fast spreading ridges venting occurs from spindly chimneys which reach heights of about 10 m while on slow spreading ridges vents are frequently located near the tops of large sulfide mounds whose volumes may reach 105 106 m3 we argue that such variations are the result of a profound difference in the nature of hydrothermal heat extraction between fast spreading ridges where spreading occurs predominantly by magmatism and slow spreading ridges where there is a significant component of tectonic extension along fast spreading ridges a steady state axial magma chamber can be insulated by a relatively thick conductive boundary layer because heat extraction is limited by low permeability in such systems episodes of vigorous venting are linked to diking events not only because these introduce a significant heat source into the upper crust but also because the intrusion of a dike is the primary source of increased permeability near the ridge axis over a time scale of the order of a decade mineral deposition tends to clog the reaction and upflow zones and there is a high probability that young sulfide structures will be buried by subsequent eruptions on slow spreading ridges tectonic extension maintains the fluid pathways necessary to support vigorous convection in the waning stages of magmatism hydrothermal circulation driven by a downward migrating cracking front can cool the entire crust leading to the formation of very large sulfide deposits as the depth of circulation increases overburden pressures reduce the permeability and the hydrothermal heat fluxes decrease progressively once hydrothermal fluids penetrate the moho serpentinization clogs the fluid pathways and high temperature venting ceases until it is reactivated by a fresh magmatic intrusion

growth of bivalves at deep sea hydrothermal vents along the galapagos rift direct measurements of shell growth of an unclassified mussel from active hydrothermal vents along the galapagos rift reveal growth rates of approxmately 1 centimeter per year for mature specimens the largest mussel collected with shell length of 18 4 centimeters was estimated to be 19 7 years old at the time of sampling recorded growth rates are among the highest documented for deep sea species

the critical point and two phase boundary of seawater 200 500 c abstract the two phase boundary of seawater was determined by isothermal decompression of fully condensed seawater in the range of 200 500 c the pressure at which phase separation occurred for each isotherm was determined by a comparison of the refractive index of fluid removed from the top and bottom of the reaction vessel the critical point was determined to be in the range of 403 406 c 285 302 bar and was located by the inflection in the two phase boundary and by the relative volume of fluid and vapor as a function of temperature the two phase boundary of 3 2 nacl solution was found to coincide exactly with that of seawater over the range tested in the present study the boundary for both is described by a single seventh order polynomial equation the two phase boundary defines the maximum temperature of seawater circulating at depth in the oceanic crust thus the boundary puts a limit of about 390 c for seawater circulating near the seafloor at active ocean ridges 2 5 km water depth and about 465 c at the top of a magma chamber occurring at 2 km below the seafloor

the liguride complex of southern italy a cretaceous to paleogene accretionary wedge abstract the ophiolite bearing allochthonous flysch liguride complex of the southern apennines italy has traditionally been divided into two tectonic units the metamorphic frido unit and the unmetamorphosed cilento unit the two units have hitherto been considered to derive from two distinct paleogeographic domains however the identification of cilento unit sediments within frido unit successions implies that the sediments of both units were coeval deposits within the same sedimentary basin cilento unit sediments also occur as the sedimentary cover of kilometre scale oceanic slices within the liguride complex indicating deposition on oceanic basement and not continental basement as was previously considered thrust transport directions and sediment provenance indicate that during late jurassic time this ocean basin lay to the east of the calabrian terrain and to the west of apulia calabria and apulia representing the european and african margins of neotethys respectively northwestwards subduction of oceanic crust beneath the calabrian terrain from late cretaceous time onwards produced an accretionary wedge which was later emplaced onto the apulian margin during the burdigalian collision of calabria and apulia the liguride complex represents the obducted remains of this accretionary wedge this interpretation contains three important implications for pre tertiary plate tectonic reconstructions of the western mediterranean region these are 1 1 the calabrian continental terrain formed part of the iberian plate on the north margin of neotethys this may have implications for the former location and continuation of the north pyrenean fault 2 2 the existence of a continuation of the eo alpine belt through calabria and the kabylies is placed in doubt thus requiring only one constant polarity nw dipping subduction direction to explain the structures now seen in these regions 3 3 the existence of a transform fault between the europe vergent structures of corsica and the africa vergent structures of calabria this transform must have been active from at least late cretaceous time in a position now occupied by the tyrrhenian sea

diking young volcanism and diffuse hydrothermal activity on the southern mid atlantic ridge the lilliput field at 9 33 s detailed exploration with remotely operated and autonomous deep submergence vehicles has revealed at 9 degrees 33 s the presence of the southernmost active hydrothermal field known so far on the mid atlantic ridge the size of the hydrothermal field which we have named lilliput is about 1000 m x 250 m it lies in a water depth of 1500 m on a ridge segment segment a3 with considerably thickened crust of 11 km four relatively small diffuse vent sites occur on a large young estimated 100 years old lava flow partly covering the flow with hydrothermal fe oxide hydroxide sediments based on homogeneous major element compositions of ca 25 lava samples this flow covers an area of at least 5 km x 0 6 km the lava flow erupted from a series of parallel fissures at the western edge of the flow and a volcanic ridge consisting of up to 30 m high pillow mounds the volcanic ridge probably represents the surface expression of an underlying dike which fed the flow several drained lava pond structures were observed within the flow but only one shows hydrothermal activity the hydrothermal venting and precipitation of abundant fe oxyhydroxides appear to be related to the young diking and eruption event and the four different hydrothermally active sites of the lilliput field lie along and almost equidistant from the eastern flank of the supposed dike although a hydrothermal plume some 500 m above the seafloor was found in two consecutive years 2005 and 2006 no high temperature venting associated with lilliput has been found in agreement with findings at other ridges with thick crust such as reykjanes high magma supply rate and frequent diking and eruption events may lead to hot hydrothermal vents being rare in slow spreading segments with thick crust whereas diffuse venting is abundant interestingly the fauna at the lilliput vents largely consists of small and apparently juvenile mussels bathymodiolus sp and did not show any signs of growth during the four years of continuing observations possibly reflecting pulsing hydrothermal activity

influence of the iceland mantle plume on crustal accretion at the inflated reykjanes ridge magma lens and low hydrothermal activity 1 a simple ridge model is applied to the hot spot influenced slow spreading reykjanes ridge in the north atlantic to explore the effects of both mantle temperatures and hydrothermal cooling due to the iceland mantle plume on the thermal structure and crustal accretion at this ridge the objective of this study is not only to fit existing observations but also to raise important questions for future field programs to test does a magma chamber similar to that at the east pacific rise exist beneath the inflated reykjanes ridge north of the transition site at 59 n and if it does at what depths modeling results of this study show that with increased magma supply a long lived magma chamber could be present at the lower crust if hydrothermal activity is comparable to elsewhere of the mid atlantic ridge more importantly modeling results predict a shallow magma chamber beneath the inflated reykjanes ridge north of 59 n if it is associated with low hydrothermal activity since the recent observations of the extremely low incidence of high temperature vents along the reykjanes ridge was interpreted as the indication for the diffusive flow being the dominant mode of hydrothermal circulation there not as the evidence for low hydrothermal activity the questions raised here call for more field observations at the inflated reykjanes ridge north of 59 n against these testable model predictions and thus to resolve these important issues

analysis of dynamic fracture compliance based on poroelastic theory part ii results of numerical and experimental tests in part i a dynamic fracture compliance model dfcm was derived based on the poroelastic theory the normal compliance of fractures is frequency dependent and closely associated with the connectivity of porous media in this paper we first compare the dfcm with previous fractured media theories in the literature in a full frequency range furthermore experimental tests are performed on synthetic rock specimens and the dfcm is compared with the experimental data in the ultrasonic frequency band synthetic rock specimens saturated with water have more realistic mineral compositions and pore structures relative to previous works in comparison with natural reservoir rocks the fracture pore geometrical and physical parameters can be controlled to replicate approximately those of natural rocks p and s wave anisotropy characteristics with different fracture and pore properties are calculated and numerical results are compared with experimental data although the measurement frequency is relatively high the results of dfcm are appropriate for explaining the experimental data the characteristic frequency of fluid pressure equilibration calculated based on the specimen parameters is not substantially less than the measurement frequency in the dynamic fracture model the wave induced fluid flow behavior is an important factor for the fracture wave interaction process which differs from the models at the high frequency limits for instance hudson s un relaxed model

effects of fracture intersections on seismic dispersion theoretical predictions versus numerical simulations abstract the detection and characterisation of domains of intersecting fractures are important goals in several disciplines of current interest including exploration and production of unconventional reservoirs nuclear waste storage co2 sequestration and groundwater hydrology among others the objective of this study is to propose a theoretical framework for quantifying the effects of fracture intersections on the frequency dependent elastic properties of fluid saturated porous and fractured rocks three characteristic frequency regimes for fluid pressure communication are identified in the low frequency limit fractures are in full pressure communication with the embedding porous matrix and with other fractures conversely in the high frequency limit fractures are hydraulically isolated from the matrix and from other fractures at intermediate frequencies fractures are hydraulically isolated from the matrix porosity but can be in hydraulic communication with each other depending on whether fracture sets are intersecting for each frequency regime the effective stiffness coefficients are derived using the linear slip theory and anisotropic gassmann equations explicit mathematical expressions for the two characteristic frequencies that separate the three frequency regimes are also determined theoretical predictions are then applied to two synthetic 2d samples each containing two orthogonal fracture sets one with and another without intersections the resulting stiffness coefficients thomsen style anisotropy parameters and the transition frequencies show good agreement with corresponding numerical simulations the theoretical results are applicable not only to 2d but also to 3d fracture systems and are amenable to being employed in inversion schemes designed to characterise fracture systems

book review the techniques of modern structural geology volume 2 folds and fractures john g ramsay and martin i huber academic press london 1987 xi 309 398 pp price stg17 50 us 34 50 paperback

on precisely modelling surface deformation due to interacting magma chambers and dykes combined data sets of insar and gps allow us to observe surface deformation in volcanic settings however at the vast majority of volcanoes a detailed 3 d structure that could guide the modelling of deformation sources is not available due to the lack of tomography studies for example therefore volcano ground deformation due to magma movement in the subsurface is commonly modelled using simple point mogi or dislocation okada sources embedded in a homogeneous isotropic and elastic half space when data sets are too complex to be explained by a single deformation source the magmatic system is often represented by a combination of these sources and their displacements fields are simply summed by doing so the assumption of homogeneity in the half space is violated and the resulting interaction between sources is neglected we have quantified the errors of such a simplification and investigated the limits in which the combination of analytical sources is justified we have calculated the vertical and horizontal displacements for analytical models with adjacent deformation sources and have tested them against the solutions of corresponding 3 d finite element models which account for the interaction between sources we have tested various double source configurations with either two spherical sources representing magma chambers or a magma chamber and an adjacent dyke modelled by a rectangular tensile dislocation or pressurized crack for a tensile okada source representing an opening dyke aligned or superposed to a mogi source magma chamber we find the discrepancies with the numerical models to be insignificant 5 per cent independently of the source separation however if amogi source is placed side by side to an okada source in the strike perpendicular direction we find the discrepancies to become significant for a source separation less than four times the radius of the magma chamber for horizontally or vertically aligned pressurized sources the discrepancies are up to 20 per cent which translates into surprisingly large errors when inverting deformation data for source parameters such as depth and volume change beyond 8 radii however we demonstrate that the summation of analytical sources represents adjacent magma chambers correctly

mid ocean ridge eruptive vent morphology and substructure evidence for dike widths eruption rates and evolution of eruptions and axial volcanic ridges high resolution side scan sonar data of mid atlantic ridge mar inner valley floor axial volcanic ridges reveal details of their architectural elements we develop quantitative models for basaltic eruptions from dikes and compare the predicted products of these eruptions with the observed morphologic features inhibition of gas exsolution lack of magma disruption and hydrothermal effects combine to decrease the rise speed of magma in submarine dikes and enhance cooling leading to more rapid centralization of eruptions along the widest places in the dike dikes are predicted to initially feed eruptions from fissure vents producing lines of hummocky ridges centralization of activity to several adjacent vents produces chains of hummocky bulbous mounds and can enhance the effusion rate at a single vent to produce small seamounts the widest dikes are predicted to produce smooth flows up to several kilometers in length which should pond in adjacent lows edifice sizes predicted on the basis of volume fluxes and flow lengths implied by the widths of dikes observed on the seafloor in iceland and in ophiolite complexes are in quantitative agreement with the dimensions of observed features each ridge is made up of the products of a variety of these individual dike emplacement and extrusive events involving various dike widths cooling times and eruption durations on the basis of typical mar spreading rates about one dike emplacement event would be expected every 40 years since not all dikes reach the surface the actual mean interval between eruptions will exceed this mar dikes should solidify between average dike emplacement events dike emplacement events are more frequent on the east pacific rise epr and thus new dikes are more likely to reoccupy the sites of incompletely solidified older dikes differences between the morphology of the mar axial volcanic ridge and the sheet flow dominated epr are attributed to on average wider dikes erupting with greater frequency along the epr

geochronology of tag and snakepit hydrothermal fields mid atlantic ridge witness to a long and complex hydrothermal history geochronological studies of a large number of precipitates from the tag hydrothermal field and of few samples from snakepit hydrothermal field of the mid atlantic ridge show intermittent repeated hydrothermal events at both sites 210pb pb and 230th 234u measurements of sulfides iron and manganese oxides and 14c measurements of carbonates combined with observations of hydrothermal events recorded as discrete layers in sediment cores provide the basis for unravelling the temporal history of the fields the tag field shows intermittent activity over the past 120 000 years as evidenced by ages of low temperature mn oxides the presently active black smoker mound first formed about 40 000 50 000 years ago with precipitation of massive sulfides it has had intermittent pulsed high temperature activity every 5000 6000 years over the past 20 000 years which may reflect renewed magmatic activity at the ridge axis fluid flow is focussed at the mound site by structural and tectonic control suggested by the intersection of n s ridge parallel lystric normal faults and an e w transform fault periods of inactivity are marked by covering of the mound with pelagic carbonate ooze which is probably partially dissolved and reprecipitated as aragonite at the end of each high temperature event the snakepit field had an initial event about 4000 years ago probably shortly after the eruption of the volcanic ridge on which it sits a recent renewal still presently active was probably initiated by recent fissuring of the volcanic pile

accurate estimation of transport properties from microtomographic images estimating and minimizing several sources of numerical error result in very accurate predictions of transport properties directly from digitized images simulation of electrical conductivity on microtomographic images of fontainebleau sandstone are in excellent agreement with experimental measurements over a wide range of porosity the results show the feasibility of combining digitized images with transport calculations to accurately predict petrophysical properties of individual rock morphologies

effective elasticity of fractured rocks a snapshot of the work in progress exploration and development of naturally fractured reservoirs rely on understanding and interpretation of certain signatures associated with seismic waves propagating through cracked rocks this understanding comes primarily from the effective media theories that predict an overall elastic behavior of a solid containing many inhomogeneities cracks in particular whose sizes are too small to be seen individually by the waves to model seismic responses of fractured formations a geophysicist typically has a choice between the effective media schemes of hudson and schoenberg while the two predictions usually deviate slightly for liquid filled cracks the differences are significant when the fractures are dry explaining the origin of these differences and selecting a more accurate scheme is the first goal of this tutorial our second more challenging task is to prove that simply adding the compliance contributions of cracks as if they were isolated and noninteracting remains sufficiently accurate even for fractures that grossly violate the basic theoretical assumption of penny shaped cracks real fractures have notoriously irregular shapes might be partially closed and often form interconnected networks yet these details of fracture microgeometry turn out to be unimportant for the effective elasticity given a typical noise level in seismic data no closed form theory exists for irregular fracture shapes however take into account finite element simulations on so called digital rocks demonstrate which features of the crack geometry have to be taken into account because they influence propagation of long compared to the size of fractures seismic waves and which features can be ignored

seismic dispersion and attenuation in saturated porous rocks with aligned fractures of finite thickness theory and numerical simulations part 1 p wave perpendicular to the fracture plane when a seismic wave travels through a fluid saturated porous reservoir containing aligned fractures it induces oscillatory fluid flow between the fractures and the embedding background medium although there are numerous theoretical models for quantifying the associated seismic attenuation and velocity dispersion they rely on certain assumptions such as infinitesimal fracture thickness and dilute fracture concentration which usually do not hold in real reservoirs the objective of this work is to overcome some of these limitations and therefore improve the applicability of the available theoretical models to do so we extend existing models to the finite fracture thickness case for p waves propagating perpendicular to the fracture plane using the so called branching function approach we consider three types of fractures namely periodically and randomly spaced planar fractures as well as penny shaped cracks the extended unified model is then tested by comparing with corresponding numerical sim

ridge plume interactions or mantle heterogeneity near ascension island between the ascension and bode verde fracture zones an 200 km length of the axis of the mid atlantic ridge has a large positive residual depth anomaly and an axial high morphology that suggest a plume influence new wide angle seismic and gravity data from this area show that 1 the crustal thickness increases from 6 km at the propagating rift marking the northern limit of the anomalous zone to 10 km at the center of the zone and 2 the residual depth anomalies may be largely explained by airy isostatic support for these crustal thickness variations these observations combined with the lack of long wavelength gravity and residual depth anomalies associated with the proposed ascension plume suggest either the presence of a weak and intermittent plume or melting of a series of small mantle heterogeneities

forward modeling of fracture induced sonic anisotropy using a combination of borehole image and sonic logs we develop a methodology to model and interpret borehole dipole sonic anisotropy related to the effect of geologic fractures using a forward modeling approach we use a classical excess compliance fracture model that relies on the orientation of the individual fractures the elastic properties of the host rock and the normal and tangential fracture compliance parameters orientations of individual fractures are extracted from borehole image log analysis the model is validated using borehole resistivity image and sonic logs in a gas sand reservoir over a 160 ft 50 m vertical interval of a well significant amounts of sonic anisotropy are observed at three zones with a fast shear azimuth fsa exhibiting 60 of variation and slowness difference between 2 and 16 numerous quasivertical fractures with varying dip azimuths are identified on the image log at the locations of strong sonic anisotropy the maximum horizontal stress direction given by breakouts and drilling induced fractures is shown not

hydrothermal activity along the slow spreading lucky strike ridge segment mid atlantic ridge distribution heatflux and geological controls we have reviewed available visual information from the seafloor and recently acquired microbathymetry for several traverses across the lucky strike segment to evaluate the distribution of hydrothermal activity we have identified a new on axis site with diffuse flow ewan and an active vent structure similar to 1 2 km from the axis capelinhos these sites are minor relative to the main field and our total heatflux estimate for all active sites 200 1200 mw is only slightly higher than previously published estimates we also identify fossil sites w of the main lucky strike field a circular feature similar to 200 m in diameter located on the flanks of a rifted off axis central volcano is likely a large and inactive hydrothermal edifice named grunnus we find no indicator of focused hydrothermal activity elsewhere along the segment suggesting that the enhanced melt supply and the associated melt lenses required to form central volcanoes also sustain hydrothermal circulation to form and maintain large and long lived hydrothermal fields hydrothermal discharge to the seafloor occurs along fault traces suggesting focusing of hydrothermal circulation in the shallow crust along permeable fault zones

modelling electrical conductivity for earth media with macroscopic fluid filled fractures modeling electrical conductivity for earth media with macroscopic fluid filled fractures james g berryman 1 and g michael hoversten 2 lawrence berkeley national laboratory one cyclotron road ms 90r1116 berkeley ca 94740 usa e mail jgberryman lbl gov chevron energy technology co 6001 bollinger canyon road san ramon ca 94583 e mail hovg chevron com

compaction fabrics of pelites experimental consolidation of kaolinite and implications for analysis of strain in slate abstract compaction of clay and shale results in large reductions in volume as pore water is expelled preferred orientation of the platy minerals increases with compaction strain and loss of porosity according to the march owens model this relationship has been studied quantitatively by experimentally consolidating kaolinite clay from slurries and analyzing the resulting fabrics with the x ray pole figure goniometer and scanning electron microscope sem initial porosity corresponds to the onset of the strain recorded by the preferred orientation and the values of 0 78 for dispersed slurries and 0 76 for flocculated slurries reflect the electrostatic forces between the clay platelets initial porosities of recently deposited fine silt and clay are in the range of 0 60 0 90 and are a function of grain size and mineralogy loss of this initial porosity has a large effect on the subsequent development of slaty cleavage matrix methods were used to model deformation paths for slates in the welsh slate belt preferred orientation of mica and ellipsoidal shapes of reduction spots were simulated for one locality by loss of a 0 60 initial porosity a 6 tilt of the beds and horizontal shortening involving plane strain strain determinations for shales and slates should include the large reduction in volume

seismic characterization of multiple fracture sets at rulison field colorado conventional fracture characterization methods assume the presence of a single set of aligned vertical cracks in the subsur face we relax this assumption and demonstrate the feasibility of seismic characterization of multiple fracture sets our technique relies on recent numerical findings indicating that multiple differently oriented possibly intersecting planar cracks embedded in an otherwise isotropic host rock result in a nearly orthorhombic or orthotropic effective medium here the governing parameters of crack induced orthotropy are estimated from 3d wide azimuth multicomponent seismic reflection data acquired over the tight gas rulison field in colorado we translate strong azimuthal variations of the normal moveout velocities intointerval crack densities fracture orientations type of fluid infill and velocities of p and s waves in an unfractured rock our inversion procedure identifies a set of cracks aligned in approximately west northwest east southeast direction in the western part

pencil structure and strain in weakly deformed mudstone and siltstone abstract weakly deformed mudstone and siltstone middle ordovician knobs formation of the appalachian valley and ridge province south western virginia u s a show strain dependent transitions between bedding fissility pencil structure and cleavage pencil structures are associated with a bulk inhomogeneous shortening deformation where minimum principal strain z ranges between 9 and 26 shortening assuming a plane strain and constant volume where strains are less bedding fissility dominates pencil fragments are defined by intersecting fracture sets subparallel to the pre existing bedding fissility and cleavage their long axes are both parallel to the bedding cleavage intersection and to the inferred y axis of the tectonic strain ellipsoid pencil development is considered to result from fracturing along both fabric anisotropies during weathering and post tectonic stress relaxation pencils show variations in size and shape depending on lithology grain size and composition degree of initial clay preferred orientation degree of cleavage development type of cleavage total bulk strain and degree of strain homogeneity their shape l w however is a direct measure of total y z strain since strain determinations from chlorite pressure fringes on framboidal pyrite within the pencil fragments give y z 0 913 0 019 l w pencil structure is therefore a potential strain marker in weakly deformed rocks

changes in geophysical properties caused by fluid injection into porous rocks analytical models author s pride sr berryman jg commer m nakagawa s newman ga vasco dw abstract 2016 european association of geoscientists a engineers analytical models are provided that describe how the elastic compliance electrical conductivity and fluid flow permeability of rocks depend on stress and fluid pressure in order to explain published laboratory data on how seismic velocities and electrical conductivity vary in sandstones and granites the models require a population of cracks to be present in a possibly porous host phase the central objective is to obtain a consistent mean field analytical model that shows how each modeled rock property depends on the nature of the crack population the crack populations are described by a crack density a probability distribution for the crack apertures and radii and the averaged orientation of the cracks the possibly anisotropic nature of the elasticity conductivity and permeability tensors is allowed for however only the isotropic limit is used when comparing to laboratory data for the transport properties of conductivity and permeability the percolation effect of the crack population linking up to form a connected path across a sample is modeled however this effect is important only in crystalline rock where the host phase has very small conductivity and permeability in general the importance of the crack population to the transport properties increases as the host phase becomes less conductive and less permeable

enhanced hydrothermal activity on an ultraslow spreading supersegment with a seismically detected melting anomaly abstract seafloor hydrothermal venting fields occur on all ocean spreading ridges osrs regardless of spreading rates however the distribution of seafloor hydrothermal activity such as frequency and spacing on ultraslow spreading osrs are poorly known chinese dayang cruises from 2015 to 2016 conducted detailed water column surveys for seafloor hydrothermal activity using a towed system with an array of turbidity sensors and a near bottom camera along the ultraslow spreading southwest indian ridge here we report the discovery of multiple hydrothermal plumes overlying segments 28 29 and 30 between the indomed and gallieni fracture zones from these data and earlier explorations in segments 25 27 we identify nine active venting sites the spatial density fs sites 100 km of active sites along the 394 km of ridge axis in our study area is thus 2 8 nearly 3 higher than predicted by the global trend of fs for ultraslow osrs in the interridge database previous studies concluded that an enhanced magma supply to the central indomed gallieni supersegment 11 8 ma is now limited to segment 27 our results indicate that although hydrothermal activity may be most concentrated in segment 27 the discoveries of active venting in segments 25 30 implies the presence of additional magma bodies across a broad extent of the indomed gallieni supersegment

mechanisms of thrust propagation some examples and implications for the analysis of overthrust terranes abstract evidence is presented for the existence of three fundamental mechanisms by which thrust sheets move the mechanisms are 1 failure of a stiff layer to form ramp flat geometry imbricated thrust sheets 2 detachment of a layer by folding decollement thrust sheets 3 differential layer parallel shortening lps thrust sheets the mechanisms are independent but may operate interactively to form families of hybrid thrust sheets since lps thrust sheets have not been previously described or documented data from the new york plateau deformed almost exclusively by differential lps is presented to demonstrate the physical characteristics of this thrust mechanism information from the plateau indicates that finite strain behavior closely reflects the geometric boundary conditions and is independent of temperature and depth of burial finite strain data are used to construct a set of iso strain maps in both the deformed and undeformed states the iso strain maps are in turn used to determine the displacement field for the thrust sheet the displacement field allows visualization of the effects of both lagrangian and eulerian transformations on an initially orthogonal grid utilizing sections from a number of overthrust belts it is shown that the three mechanisms occur universally although in various proportions lps is regionally developed throughout the central and northern appalachians the presence of the lps thrust sheets probably accounts for the failure of structural cross sections to bed length balance in these blind thrust terranes finally a series of examples drawn from the field as well as the literature are used to illustrate both the characteristics of hybrid thrust sheet families as well as to indicate how knowledge of their behavior can be utilized in developing strategies for the construction and balancing of structural cross sections

seismic waves in rocks with fluids and fractures seismic wave propagation through the earth is often stronglyaffected by the presence of fractures when these fractures are filledwith fluids oil gas water co2 etc the type and state of the fluid liquid or gas can make a large difference in the response of theseismic waves this paper summarizes recent work on methods ofdeconstructing the effects of fractures and any fluids within thesefractures on seismic wave propagation as observed in reflection seismicdata one method explored here is thomsen s weak anisotropy approximationfor wave moveout since fractures often induce elastic anisotropy due tononuniform crack orientation statistics another method makes use ofsome very convenient fracture parameters introduced previously thatpermit a relatively simple deconstruction of the elastic and wavepropagation behavior in terms of a small number of fracture parameters whenever this is appropriate as is certainly the case for small crackdensities then the quantitative effects of fluids on thesecrack influence parameters are shown to be directly related to skempton scoefficient b of undrained poroelasticity where b typically ranges from0 to 1 in particular the rigorous result obtained for the low crackdensity limit is that the crack influence parameters are multiplied by afactor 1 b for undrained systems it is also shown how fractureanisotropy affects rayleigh wave speed and how measured rayleigh wavespeeds can be used to infer shear wave speed of the fractured medium higher crack density results are also presented by incorporating recentsimulation data on such cracked systems

strain paths during slaty cleavage formation the role of volume loss abstract strain analyses of slates commonly investigate the sequence of shapes that must have been superimposed to produce the observed strained state methods such as rf try to quantify the cleavage forming deformation and see through that component to examine any initial shape an alternative approach is to reverse this process by considering the variation in final shapes derived when plausible cleavage forming strain ellipsoids are superimposed on realistic initial shapes specifically a deformation involving both plane strain and volume loss may be superimposed on an initial shape formed by diagenetic flattening in this case the angle between bedding and the incipient cleavage plane is an important variable in controlling final ellipsoid shapes this approach has been adopted for a data set measured from the ordovician borrowdale volcanics in the english lake district results show that neither successive increments of plane strain without volume loss nor incremental plane strain plus volume loss adequately explain the observed pattern of final shapes one permissible solution is generated if increments of pure volume loss perpendicular to the incipient cleavage plane precedes plane strain distortion initial layer parallel shortening may therefore play an important part during cleavage development in these slates

effective elasticity of rocks with closely spaced and intersecting cracks the noninteraction approximation nia is the simplest effective media theory that describes the overall elasticity of fractured rocks if the nia is used for fracture characterization its accuracy and the range of applicability must be estimated we do it by performing a series of 3d finite element simulations of effective elasticity for models that contain several sets of fractures embedded in otherwise isotropic host rock we intentionally place the cracks close to each other to create strong interactions in their local stress fields in addition we allow the cracks to intersect in such a way that they do not break a rock specimen apart perhaps surprisingly we find that fracture interactions and intersections have little influence on the effective elasticity and the nia performs well in all cases while it has a tendency to slightly underestimate the effective stiffnesses the incurred errors are small their typical magnitudes are just a few percent in the entire range of the crack densities expe

seismic velocity and q anisotropy in fractured poroelastic media a fluid saturated poroelastic isotropic medium with aligned fractures behaves as a transversely isotropic and viscoelastic tiv medium when the predominant wavelength is much larger than the average distance between fractures a planar fracture embedded in a fluid saturated poroelastic background medium can be modeled as a extremely thin and compliant porous layer p waves traveling in this type of medium induce fluid pressure gradients at fractures and mesoscopic scale heterogeneities generating fluid flow and slow diffusion biot waves causing attenuation and dispersion of the fast modes mesoscopic loss a poroelastic medium with embedded aligned fractures exhibits significant attenuation and dispersion effects due to this mechanism which can properly be represented at the macroscale with an equivalent tiv medium in this work we apply a set of compressibility and shear harmonic finite element fe experiments on fractured highly heterogeneous poroelastic samples to determine the five complex and frequency dependent stiffnesses characterizing the equivalent medium the experiments consider brine or patchy brine co 2 saturated samples and a brine saturated sample with a heterogeneous fractal skeleton with fractures we show that fractures induce strong seismic velocity and q anisotropy both for qp and qsv waves enhanced either by patchy saturation or frame heterogeneity