effect of fabric on the behaviour of reservoir sandstones a laboratory investigation was undertaken on the mechanical behaviour of two sandstones that are commonly used as analogous hydrocarbon reservoir sandstones using triaxial apparatuses with a capacity of up to 70 mpa confining pressure both materials are lightly cemented but show a mechanical behaviour at large strains that is similar to that seen in rocks with much stronger cementation at small strains however these materials behave more like uncemented sands in that they show a strong stress dependency of the shear modulus and yield takes places at relatively small strains it is believed that fabric plays a fundamental role in the observed behaviour and this is supported by the experimental evidence so that the concept of strong fabric in sandstones is introduced

detecting and characterizing mesoscale and submesoscale structures of mediterranean water from joint seismic and hydrographic measurements in the gulf of cadiz 1 marine seismic and hydrographic data from the go cruise in the gulf of cadiz acquired in april 2007 are analyzed to determine the physical nature and geometric characteristics of acoustic reflectors in the water column seismic data show strong reflectors near the surface above mediterranean water mw undercurrents above a meddy and within a submesoscale mw structure these reflectors are associated with thermohaline layering wavelet analysis of seismic and hydrographic data both in terms of reflectivity shows strong signals with 12 48 m vertical wavelength scales hydrological data with its high vertical resolution capture weaker signals with shorter wavelengths at the edge of a meddy lateral intrusions of fresher water are revealed by reflectors with 25 75 m thickness analyses of marine seismic data allow detection of submesoscale eddies that hydrographic sections identify only occasionally

variations in thickness of layer 3 dominate oceanic crustal structure abstract seismic refraction studies have determined that while much of the crust underlying the world s oceans is about 7 km thick it also occurs in lesser and considerably greater thicknesses here we show that vastly different thicknesses of oceanic crust formed at ridges by seafloor spreading are generally characterized by the two gradient velocity structure recognized as typical of 6 7 km thick oceanic crust adopting a standard we study ninety published seismic velocity structures of oceanic crust ranging in thickness from 2 to 37 km several structures formed in midplate settings ocean islands and seamounts are also considered for comparison regardless of origin ridge or midplate we find that the percentage of the whole crust formed by layer 2 systematically decreases with increasing total crustal thickness while the average velocity of layer 2 varies widely for all crust the average velocity of layer 3 increases systematically with increases in the average whole crustal thickness and velocity crust formed in midplate settings differs in that the thickness of layer 2 velocity crust is generally two to three times that of crust formed at ridges residual depths were calculated for oceanic crust by removing the effects of sediment loading and subsidence due to lithospheric cooling most of the data fall close to a model whereby different crustal thicknesses result from different extents of partial melting of oceanic upper mantle the crustal sections were restored to zero age assuming the average depth of emplacement for 7 km thick oceanic crust is 2 5 km below sea level from this restoration it appears that the depth to the top of layer 3 at the time it was formed is 4 25 1 25 km below sea level regardless of whole crustal thickness or spreading rate these results suggest that the mechanism of construction of oceanic crust is in many ways remarkably uniform despite very large changes in the total thickness of the crust produced very basic questions remain it is for instance not at all clear why layer 2 and layer 3 exist in the proportions they do while neutral buoyancy or melt trapping at the base of a brittle lid may account for local properties of the layer 2 3 boundary they do not seem adequate to account for the global phenomenon

relative motions between oceanic plates of the pacific basin appendix tables are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 012 2 50 payment must accompany order relative motion poles describing the displacement histories between the pacific plate and once adjacent oceanic plates farallon kula izanagi i izanagi ii and phoenix were derived for the late mesozoic and cenozoic eras because fracture zone and magnetic anomaly data are generally available from the pacific plate but not from adjacent plates a new method of analysis for onesided data was required this analysis produced stage poles and rates of relative plate motion and estimates of their confidence regions the following are the main conclusions drawn from our analysis 1 for time intervals of the order of 107 years termed stages relative motion poles for plate pairs remained nearly fixed between stages shifts in poles were commonly both large and abrupt within stages rates of plate motion were commonly observed to change markedly indicating that plates changed speed more frequently than they changed direction 2 the relative motions of all of the plates analyzed changed at about chron m11 135 ma chron 34 85 ma and chron 25 56 ma 3 during the early cretaceous there were five oceanic plates in the pacific basin rather than the four recognized by previous workers 4 to determine the number of farallon plates that existed to the east of the pacific plate during the time interval from chron 34 85 ma to chron 25 56 ma fracture zones and magnetic anomalies that record pacific farallon spreading from the northern central and southern pacific plate were analyzed separately and collectively the analysis shows that a single pacific farallon relative motion pole and a single rate are consistent with all of the data 5 spreading rates along the pacific kula ridge decreased markedly between chrons 32b and 25 72 56 ma probably in response to the arrival of buoyant kula lithosphere at a subduction zone northwest of the bering sea 6 soon after chron 25 56 ma a major reorganization is recorded along the pacific farallon boundary the kula farallon boundary is also thought to have changed during this reorganization 7 kula pacific anomalies north of anomaly 25 are modeled by assuming continued kula pacific spreading after chron 25 56 ma 8 probably by chron 18 43 ma pacific kula spreading had ceased and the kula farallon ridge system had evolved into alignment with the pacific farallon spreading direction however the timing of these events is uncertain because pacific kula data younger than chron 25 56 ma are sparse

crustal volumes of the continents and of oceanic and continental submarine plateaus global topographic data and the assumption of airy isostasy have been used to estimate the crustal volumes of the continents and the oceanic and continental submarine plateaus the calculated crustal volumes are 7182 x lo6 km3 for the continents 242 x lo6 km3 for continental submarine plateaus and 369 x lo6 km3 for oceanic plateaus the falkland plateau and the lord howe rise are the two largest continental submarine plateaus with volumes of 48 x lo6 km3 and 47 x lo6 km3 respectively total continental crustal volume is 7581 x lo6 km3 including the volume of continental sediments on the ocean floor 160 x lo6 km3 in good agreement with previous estimates continental submarine plateaus on the seafloor comprise 3 2 of the total continental crustal volume the largest oceanic plateaus in order of decreasing size are the ontong java plateau the kerguelen plateau the caribbean the chagos laccadive ridge the ninetyeast ridge and the mid pacific mountains together they comprise 54 of the total anomalous crustal volume in oceanic plateaus an upper bound to the continental crust addition rate by the accretion of oceanic plateaus is 3 7 km3 yr a value that assumes accretion of all oceanic plateaus with a total volume of 4 9 of the continental crustal volume on a 100 myr time scale even if a substantial fraction of the crustal volume in oceanic plateaus is subducted accretion of oceanic plateaus could make a contribution to continental growth since the upper bound to the addition rate exceeds recent estimates of the island arc addition rate subduction of continental submarine plateaus with the oceanic lithosphere on a 100 myr time scale gives an upper bound to the continental crustal subtraction rate of 2 4 km3 yr much larger than recent estimates of crustal subtraction by subduction of seafloor sediments effective subduction of all oceanic plateaus implies equally effective subduction of continental submarine plateaus a potentially important way to recycle continental crust back into the mantle may be the break off of small fragments from the continents entrapment of the continental fragments in the seafloor and subduction of the fragments with the oceanic lithosphere this process may be occurring in the mediterranean for corsica and sardinia

reconstructed seismic velocity structure of the lewis hills massif and implications for oceanic fracture zones the lewis hills massif of the bay of islands ophiolite complex provides a lower crustal to upper mantle cross section of an oceanic fracture zone segment and adjacent oceanic lithosphere the structural and petrologic character of the fracture zone region is distinctly different from the comparatively simple layered structure of the rest of the bay of islands complex results of detailed field mapping petrography and laboratory seismic velocity measurements have been combined in order to reconstruct the seismic velocity structure of this slice of oceanic lithosphere velocities and velocity anisotropies of each of the major rock units in the massif are restored to their preobduction geometries relative to different stratigraphic levels in the complex and proximity to the fracture zone region although all oceanic fracture zones probably have unique structural and petrologic characteristics in detail from the perspective of the present study it is suggested that in general seismic properties differing from that of normal oceanic crust and upper mantle may be expected within approximately 10 km of the axis of fracture zones in these regions lower crustal as well as upper mantle anisotropy is expected the moho might be expected to shallow due to the presence of abundant ultramafic cumulates resulting in crustal thinning and significant velocity inversions may occur with depth where large cyclic mafic layers occur within cumulate ultramafics in oceanic lithosphere which has passed or is passing through a transform fault domain enhanced mantle anisotropy might occur the middle to lower crust in these regions should be composed of highly anisotropic metamorphic rocks which grade upward into serpentinites and highly fractured gabbros diabases and basalts with relatively low velocities it is suggested that velocities as well as velocity anisotropies produced by these types of structures might be determined in situ by seismic refraction experiments and may be used to map the extent of deformation in the oceanic lithosphere near fracture zones

experimental study on the vertical deformation of sand caused by cyclic withdrawal and recharging of groundwater fast urban growth and an ever increasing frequency of extreme weather related events demand a better understanding of land subsidence and the potential measures to control it this paper presents an experimental investigation on the deformation behavior of sand undergoing cyclic withdrawal and recharging of groundwater a sand box model with dimensions 1000 mm 600 mm 892 mm was used to investigate soil deformation following lowering and rising of the water table the sand used was pingtan sand fujian china a quartzitic formation composed of elongated to slightly spherical grains three sand samples with different initial densities i e loose medium dense and dense were tested on repeated cycles the vertical deformation of the sand was measured and the corresponding strain and effective stress were calculated the observations show that initially the sand behaves as an elastic material with the subsidence caused by water withdrawal being recovered as the soil rebounds during recharging with further withdrawal recharging cycles subsidence becomes larger as plastic deformations as well as time dependent deformation occur in the soil the distinct patterns of deformation were identified to be dependent on both the initial density and the number of withdrawal recharging cycle the sand has been subjected to the different behavior of the medium dense sample compared to the loose and dense samples in particular for the first two cycles indicates that the soil response cannot be explained in respect of the initial void ratio alone and further microstructure considerations were discussed it is suggested that the orientation of the grains resulting from the sample preparation technique and the consequent induced anisotropy can be used to explain the results herein presented finally this paper provides the rationale for a micromechanical interpretation of soil deformation when subjected to changes in stress due to rising or lowering of the water table which will help in establishing measures to control land subsidence in particular to assess the efficiency of water recharging following subsidence by withdrawal

ocean internal wave spectra inferred from seismic reflection transects 1 internal waves affect many important dynamical processes in the ocean but in situ observations of internal waves are infrequent and spatially sparse here we show that remote sensing of internal waves by marine seismic reflection methods can provide quantitative information on internal wave energy and its spatial variability at high lateral resolution and full ocean depth over large volumes of the ocean seismic images of the norwegian sea water column show reflections that capture snapshots of finestructure displacements due to internal waves horizontal wave number spectra derived from digitized reflection horizons in the open ocean compare favorably to the garrett munk tow spectrum of oceanic internal wave displacements spectra within 10 km laterally and 200 m vertically of the continental slope show enhanced energy likely associated with internal wave sloping boundary interactions

microstructural study of deformation zones during cone penetration in silt at variable penetration rates during conventional cone penetration testing in silt the soil will normally be partially drained if the penetration rate varies time for drainage is altered and therefore the measured cone resis

seismic reflection imaging of water mass boundaries in the norwegian sea 1 results from the first joint temperature and seismic reflection study of the ocean demonstrate that water mass boundaries can be acoustically mapped multichannel seismic profiles collected in the norwegian sea show reflections between the norwegian atlantic current and norwegian sea deep water the images were corroborated with a dense array of expendable bathythermographs and expendable conductivity temperature depth profiles delineating sharp temperature gradients over vertical distances of 5 15 m at depths over which reflections occur fine structure from both thermohaline intrusions and internal wave strains is imaged low amplitude acoustic reflections correspond to temperature changes as small as 0 03 c implying that seismic reflection methods can image even weak fine structure

observations and numerical modelling of the mediterranean outflow splitting and eddy generation during the canigo project the generation of dipolar eddies in the mediterranean water mw layers off south portugal was recorded by subsurface floats from february to november 1998 the role of the bottom topography on the spreading of the mw in the gulf of cadiz and on the generation of these mw eddies was studied in the framework of the medtop project some of the hydrological and velocity observations collected during medtop in 2001 and 2002 also revealed the association of anticyclones meddies and cyclones in the region just south of the portimao canyon the evolution of these structures is studied with subsurface floats deployed in the vortices the splitting of the mw plume in the gulf of cadiz and the generation mechanisms of the mw vortices in the region from portimao canyon to the estremadura promontory are investigated using a bottom following numerical primitive equations model in a regional ocean configuration incorporating the realistic topography of the iberian continental slope the model reproduces the mw division into different cores in the gulf of cadiz as well as the generation of baroclinic dipoles downstream of portimao canyon the role of the bottom topography in the eddy generation process is discussed

imaging meddy finestructure using multichannel seismic reflection data 1 this work illustrates the great potential of multichannel seismic reflection data to extract information from the finestructure of meddies with exceptional lateral resolution 10 15 m we present seismic images of three meddies acquired in the gulf of cadiz sw iberian peninsula which consist of concentric reflectors forming oval shapes that sharply contrast with the background oceanic structure the seismic images reveal the presence of different regions within the meddies that are consistent with those observed in historical temperature t and salinity s data the core region characterized by smooth t and s variations is weakly reflective the double diffusive upper and lower boundaries and the lateral interleaving outer edges characterized by stronger t and s contrasts display strong reflectivity bands these new observations clearly show differences between layers developed at the upper and lower boundaries that can contribute to the knowledge of mixing processes and layering formation in oceans

quantitative orientation and micro porosity analysis of recent marine sediment microfabric abstract impregnated samples taken from three different post depositional regimes within the mariana mounds hydrothermal area in the mariana trough were observed in the back scattered mode in the sem intensity gradient orientation analysis methods have been adapted to study the orientation patterns within the sediments representing one of the first applications of such quantitative microfabric methods to the study of natural materials orientation was specified by two separate parameters an index of anisotropy and a direction of orientation the index is generally low for any single image but by plotting the direction and anisotropy as a vector centred on the relevant specimen co ordinates consistent patterns of orientation were observed in the samples differences between the orientation patterns from the three environmental regimes were noted a new dispersion ratio parameter was used to assess within sample variations in a complementary study objective thresholding methods based on kohler s relative histogram method were used to segment the images assessments of porosity showed differences which were generally consistent with the trends noted from orientation analysis while this paper uses image analysis techniques to examine deep sea sediments the methods of analysis described are equally applicable to sediments on the continental shelf

identification and quantification of transitional modes of behaviour in sediments of venice lagoon an analysis was made of an extensive series of oedometer and triaxial tests on intact samples of silts and silty clays from the lagoon of venice as well as a more limited series of carefully controlled oedometer tests on reconstituted specimens the soils appear to have transitional modes of behaviour with compression curves that converge only slowly in the v log v plane and critical state lines defined for triaxial compression that are non unique in the v ln p plane two means of quantifying the degree of transitional behaviour are introduced one factor m being based on the oedometer data and the second p on the critical state data the values of m were in general lower than those of p because of the higher stress level considered the values of m were also found to depend on unloading which must be accounted for when comparing data from intact and reconstituted specimens

on the mechanics of structured sands to date the effect of structure on the behaviour of natural sands has focused almost exclusively on the component of bonding and the effect of fabric has been largely overlooked the paper describes a detailed investigation of the behaviour of two natural sands by means of triaxial testing over a wide range of pressures one material had bonding as the principal element of its structure and the other fabric following on from a paper of cuccovillo and coop which examined the influence of the two structural elements on the small strain stiffness the current paper develops a new framework for the yielding and large strain behaviour it is suggested that structure should be considered as an element of the nature of a sand in addition to properties such as mineralogy particle shape and grading the resulting framework is then capable of encompassing the patterns of behaviour seen for both bonding and fabric dominated sands while bonding results in a cohesive mode of shearing it is demonstrated that when

global relationship between oceanic geoid and topography in order to compare model predictions of upper mantle convection with surface observations we have computed global residual depth and geoid anomalies on a 5 5 grid over all oceans after removal of lithospheric cooling effects and long wavelength geoid components 4000 km related to lower mantle sources we have derived the transfer function admittance of geoid over topography as a function of wavelength in the range 1000 4000 km the observed admittance is positive at all and presents a maximum of 2 5 3 5 m km at 2750 km comparing this result with the theoretical admittance computed for the lithospheric cooling plate model we ascribe most of the admittance peak at 2750 km to errors in the depth age and geoid age correction this suggests that a standard age correction applied to all seafloor is not valid at shorter wavelengths the admittance remains roughly constant on the order of 1 5 m km and can be explained in terms of sediment loading and crustal thickening effects comparing spatially the residual geoid and depth anomaly maps we note a positive correlation over a few areas coinciding with hot spot swells volcanic plateaus or seafloor deeps the geoid versus depth ratio amounts then to 6 8 m km in agreement with the values predicted above upwellings and downwellings by models of constant viscosity convection however the global admittance analysis leads to much lower values at all wavelengths suggesting that these areas are too limited in number and in extent to produce any significant signal

a micro finite element model for soil behaviour experimental evaluation for sand under triaxial compression this note evaluates the ability of a combined discrete finite element approach to replicate the experimental response of a dry sand under triaxial compression the numerical sample was created by v

physical chemical and sedimentological aspects of the mediterranean outflow off iberia a multidisciplinary study of the mediterranean outflow in the region west of the strait of gibraltar and off the southern and southwestern coasts of the iberian peninsula was developed in the frame of the canary islands azores gibraltar observations project two high resolution ctd surveys which included water sampling for chemical nutrients and dissolved oxygen and sedimentological analyses took place in september 1997 summer cruise and january 1998 winter cruise in the study region the correspondence between the high salinity mediterranean water mw layer and the low nutrient content and relatively high abundance of particles was a general result further details of the thermohaline analysis and of the geostrophic computations especially for the layer of mw are compared with the results obtained for the chemical properties and the sedimentological characteristics

synchrotron tomographic quantification of strain and fracture during simulated thermal maturation of an organic rich shale uk kimmeridge clay analyzing the development of fracture networks in shale is important to understand both hydrocarbon migration pathways within and from source rocks and the effectiveness of hydraulic stimulation upon shale reservoirs here we use time resolved synchrotron x ray tomography to quantify in four dimensions 3 d plus time the development of fractures during the accelerated maturation of an organic rich mudstone the uk kimmeridge clay with the aim of determining the nature and timing of crack initiation electron microscopy em both scanning backscattered and energy dispersive was used to correlatively characterize the microstructure of the sample preheating and postheating the tomographic data were analyzed by using digital volume correlation dvc to measure the three dimensional displacements between subsequent time heating steps allowing the strain fields surrounding each crack to be calculated enabling crack opening modes to be determined quantification of the strain eigenvectors just before crack propagation suggests that the main mode driving crack initiation is the opening displacement perpendicular to the bedding mode i further detailed investigation of the dvc measured strain evolution revealed the complex interaction of the laminar clay matrix and the maximum principal strain on incipient crack nucleation full field dvc also allowed accurate calculation of the coefficients of thermal expansion 8 10 5 c perpendicular and 6 2 10 5 c parallel to the bedding plane these results demonstrate how correlative imaging using synchrotron tomography dvc and em can be used to elucidate the influence of shale microstructure on its anisotropic mechanical behavior

increased mantle convection during the mid cretaceous a comparative study of mantle potential temperature mantle convection patterns of the past are not well known yet an understanding of changing mantle convection characteristics is fundamental to understanding the evolution of plate tectonics there are very few ways to examine mantle characteristics of the past changes in spreading rate and volcanic activity with time have been used to draw conclusions about historic changes in mantle activity mantle temperature has been found to be related to crustal thickness with this relationship crustal thicknesses may now yield new conclusions about historic changes in mantle characteristics we have inferred changes in mantle convection patterns throughout the last 180 m y by examining variations in assumed crustal thickness within the pacific basin crustal thicknesses were calculated from residual depth anomalies by assuming that residual depth anomalies are the result of isostatic compensation of variations in crustal thickness crustal thickness is determined at the time of crustal formation and is dependent upon the temperature of the mantle source material intraplate hot spot volcanism effects on crustal thickness were not ignored examination of variations in crustal thickness of crust of different ages can reveal information about changing temperatures of the mantle at the ridge through time we have learned that mantle temperatures at the ridge during the mid cretaceous were more variable than those temperatures at the ridge after the mid cretaceous furthermore we have inferred from the data that mantle temperatures at hot spots were higher during the mid cretaceous than those at hot spots existing after the mid cretaceous we suggest that mantle convection at the ridge was more rapid during the mid cretaceous causing a higher variability of temperatures at the ridge we also note that this period of increased mantle convection is concurrent with the increased mantle temperatures at hot spots within the pacific basin

eddy generation in the mediterranean undercurrent abstract in the framework of the european union mast iii project canary islands gibraltar azores observations 24 rafos floats were deployed in the mediterranean water mw undercurrent off south portugal between september 1997 and september 1998 a preliminary analysis of this lagrangian approach complemented with xbt and current meter data show some of the major aspects of the flow associated with the undercurrent as well as associated eddy activity floats that stayed in the undercurrent featured a downstream deceleration and a steering by bottom topography three meddy formations at cape st vincent could be isolated from the float data the dynamical coupling of meddies and cyclones was observed for a considerable period of time the generation of two dipolar structures in the portimao canyon region also was observed with the float data a major bathymetric relief gorringe bank was not only an important constraint to the eddy trajectories and of the flow at the mw levels but also a site for meddy formation

seismic reflection imaging of shallow oceanographic structures 1 multichannel seismic mcs reflection profiling can provide high lateral resolution images of deep ocean thermohaline fine structure however the shallowest layers of the water column z 150 m have remained unexplored by this technique until recently in order to explore the feasibility of shallow seismic oceanography so we reprocessed and analyzed four multichannel seismic reflection sections featuring reflectors at depths between 10 and 150 m the influence of the acquisition parameters was quantified seismic data processing dedicated to so was also investigated conventional seismic acquisition systems were found to be ill suited to the imaging of shallow oceanographic structures because of a high antenna filter effect induced by large offsets and seismic trace lengths and sources that typically cannot provide both a high level of emission and fine vertical resolution we considered a test case the imagery of the seasonal thermocline on the western brittany continental shelf new oceanographic data acquired in this area allowed simulation of the seismic acquisition sea trials of a specifically designed system were performed during the aspex survey conducted in early summer 2012 the seismic device featured i four seismic streamers each consisting of six traces of 1 80 m ii a 1000 j sig sparker source providing a 400 hz signal with a level of emission of 205 db re 1 pa 1 m this survey captured the 15 m thick 30 m deep seasonal thermocline in unprecedented detail showing images of vertical displacements most probably induced by internal waves

mid depth internal wave energy off the iberian peninsula estimated from seismic reflection data energy levels of internal waves are estimated from seismic reflection data three legacy seismic sections from 1993 and 1997 obtained off the iberian peninsula have been analyzed for acoustic reflections within the water column the reflections are aligned continuously for up to several kilometers over large parts of the sections and in the depth interval from 200 to 2000 m depth variations of these reflections are thought to be caused by the background internal wave field from the variations we derive horizontal wave number spectra of normalized internal wave displacement the general slope of the power density spectra is remarkably consistent for all sections and agrees well with model spectra for internal waves significant differences within the sections can be found when sufficiently large subsections are averaged the spatial variation of the energy level indicates increasing internal wave activity with shallower water depths as well as near a subsurface eddy

thermohaline fine structure in an oceanographic front from seismic reflection profiling we present acoustic images of oceanic thermohaline structure created from marine seismic reflection profiles across the major oceanographic front between the labrador current and the north atlantic current the images show that distinct water masses can be mapped and their internal structure imaged using low frequency acoustic reflections from sound speed contrasts at interfaces across which temperature changes the warm cold front is characterized by east dipping reflections generated by thermohaline intrusions in the uppermost 1000 meters of the ocean our results imply that marine seismic reflection techniques can provide excellent spatial resolution of important oceanic phenomena including thermohaline intrusions internal waves and eddies

estimating movement of reflectors in the water column using seismic oceanography the observation of spatial and temporal dynamics of the ocean is fundamental to understand global and regional aspects of water mixing physical oceanography has traditionally observed ocean structures with in situ measurements often limited in temporal and or spatial resolution in exploration seismology a set of techniques has been developed over the last decades to image and characterize the physical properties of sub seafloor structures by inversion methods at high horizontal resolution the two different fields have made contact in seismic oceanography where the well developed methods of marine reflection seismology have been applied to the dynamic ocean however one aspect so far ignored in seismic oceanography is the dynamical temporally varying nature of water structures here we show that it is possible to estimate temporal variations of reflectors in water structures as an inversion parameter the new dynamic property reflector movement velocity gives an additional parameter to characterize ocean water dynamics

evidence for variable upper mantle temperature and crustal thickness in and near the australian antarctic discordance abstract the southeast indian ridge seir in and near the australian antarctic discordance aad exhibits at a constant spreading rate almost the full range of the many geophysical and geochemical parameters characteristic of the slow mid atlantic ridge and fast east pacific rise we used satellite derived gravity data in combination with seamarc ii bathymetry in and near the aad to examine regional density variations in the upper mantle beneath the aad through three dimensional gravity analysis we found that at least two end member models satisfy the gravity observations regional crustal thickness variations of at least 3 km along the seir near the aad or a temperature anomaly of the order of 150 c in the upper mantle beneath the seir these new observations combined with other geophysical and geochemical characteristics of the australian antarctic discordance provide further evidence that the temperature structure of a mid ocean ridge is a controlling factor in addition to spreading rate in the crustal accretionary process numerical models of mantle flow beneath mid ocean ridges offer one means of investigating the dynamic effect of a variable upper mantle temperature on the accretionary process our results indicate that temperature is important especially at intermediate and slower spreading rates where thermal effects can dominate mantle flow beneath a mid ocean ridge and result in increasing crustal production with decreasing spreading rate at the constant intermediate spreading rate of 37 mm yr characteristic of the seir in and near the aad our numerical models show that significant crustal thinning 2 4 km can occur with relatively small variations in upper mantle temperature all else being equal thus combined with our end member gravity models these observations and results suggest that both anomalously cool upper mantle and thin crust exist beneath the aad

quantifying the evolution of soil fabric during shearing using scalar parameters the packing density of a soil is typically described in terms of the scalar parameters void ratio specific volume or porosity within a critical state soil mechanics framework the response of a sand is considered to depend on its state that is the combination of void ratio and effective stress when soil is considered from a particle scale perspective the packing density is often described using the coordination number cn that is the average number of contacts per particle using particles with simple geometries discrete element method dem simulations photoelastic tests and experiments on analogue soils e g glass ballotini have all demonstrated a correlation between cn and mechanical response this study investigates whether these correlations apply to real soils using direct measurement of the fabric via micro computed tomography micro ct a variety of scalar fabric descriptors was measured during the triaxial compression of both intact and reconstituted reigate sand samples while ea

dynamic topography compared with residual depth anomalies in oceans and implications for age depth curves ocean depth is affected by dynamic topography caused by mantle flow regional bathymetric deviations from the cooling plate age depth curve called residual depth anomalies can therefore be used as indicators of dynamic topography in oceans in this work we first evaluated the oceanic residual depth anomalies we then compute the dynamic topography for the flow induced by density perturbations converted from seismic tomography models by assuming we found that the predicted dynamic topography correlates with the depth anomalies when the density perturbations in the shallow part of the upper mantle were inferred from slab densities not from tomographic models we estimated a new age depth curve based on depths corrected for dynamic topography this corrected age depth curve shows that the corrected depths for old seafloor 70 110 ma are a few hundred meters deeper than those uncorrected

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

tectonically controlled origin of three unusual rock suites in the woodlark basin we propose alternative mechanisms for the origin of three unusual rock suites high mg andesites nati basalts and arclike rocks that have been dredged from the woodlark basin southwest pacific ocean we show that the high mg andesites and nati basalts are associated with an unusually cool ridge environment the cooling is due to increased hydrothermal circulation stimulated by an unusually high crustal permeability the high permeability is mainly due to cracking of the woodlark basin lithosphere as it passes over the flexural bulge in front of the subduction zone although local processes such as faulting in fracture zones may also make some contribution this increased convective cooling affects magma dynamics and chemistry at the ridge crest the high mg andesites occur where the hydrothermal circulation lowers the temperatures in the upper oceanic crust causing the magma chamber to sit in the mantle rather than in the crust and promoting the interaction of basalt magma with harzburgite the nati basalts are also the indirect result of increased crustal cooling which causes anomalously low degrees of partial melting of their depleted mantle source the arclike rocks are caused by interaction with volatiles from lithosphere that was emplaced beneath the edge of the basin less than 6 m y ago by a now inactive subduction zone to the north of the currently active one simple thermal models indicate that this formerly subducting plate is still cold enough to retain volatiles and to remain seismically active as the old plate loses volatiles they rise into the mantle convection cell which feeds the woodlark basin ridge crest because ridge subduction increases the likelihood of ophiolite obduction these observations and explanations of woodlark basin tectonics are potentially important for ophiolites

lagrangian observations of meddy formation during a mediterranean undercurrent seeding experiment mediterranean eddies meddies play an important role in maintaining the temperature and salinity distributions in the north atlantic but relatively little is known about their early life histories including where how often and by what mechanism they form a major field program called a mediterranean undercurrent seeding experiment has been carried out to directly observe meddy formation and the spreading pathways of mediterranean water into the north atlantic between may 1993 and march 1994 49 rafos floats were deployed sequentially in the mediterranean undercurrent south of portugal and tracked acoustically for up to 11 months the float deployments were accompanied by high resolution xbt sections across the undercurrent nine meddy formation events were observed in the float trajectories six near cape st vincent at the southwestern corner of the iberian peninsula and three near the estremadura promontory along the western portuguese continental slope meddy formation thus occurs where the continental slope turns sharply to the right when facing in the downstream direction of the undercurrent after conditionally sampling the float dataset to identify floats that were well seeded in the undercurrent the authors have estimated a meddy formation rate of 15 20 meddies per year the timescale for meddy formation at cape st vincent was found to be 3 7 days shorter than previous estimates based on the volume of larger meddies meddies were observed to form most frequently when the speed of the mediterranean undercurrent was relatively fast the meddy formation process at cape st vincent resembles the conceptual model of e a d asaro whereby anticyclonically rotating eddies are formed by separation of a frictional boundary layer with negative relative vorticity at a sharp corner comparison of the relative vorticity in the anticyclonic shear zone of the undercurrent and that of the newly formed meddies shows that much of the anticyclonic relative vorticity in meddies can be accounted for by the horizontal shear in the undercurrent this confirms earlier work suggesting that the classical mechanism for the generation of submesoscale coherent vortices by collapse and geostrophic adjustment of a weakly stratified fluid injected into a stratified ocean may not be the principle mechanism at work in the formation of meddies at cape st vincent

on the kinematics of shelly carbonate sand using x ray micro tomography abstract shelly carbonate sands are highly compressible soils due to the susceptibility of their loose and intricate fabric to collapse under loading an accurate assessment of the physical phenomena taking place at the grain scale is critical for a better understanding and modelling the mechanical behaviour of this material this paper presents a study on the grain kinematics of shelly carbonate sand through analysis of in situ 4d x ray tomography images acquired during oedometer compression two sands from the persian gulf with coarse and fine grading were investigated an adaptive watershed segmentation technique is used here to identify the grains in the image s prior to loading and a digital volume correlation dvc technique is employed to obtain the displacement field of each grain under loading the displacement fields are used to reconstruct the grains in their new positions and compute the associated translation and rotation an extensive statistical analysis was carried out to demonstrate the effect of grain morphology and local fabric coordination number on grain kinematics the new findings presented here shed light on the mechanisms of grain rearrangement leading to the compressible fabric of shelly carbonate sands and are also critical to better understand other weak grained sands and or silica sands with an open fabric

quantifying the evolution of soil fabric during shearing using directional parameters over the past 50 years experimental studies have repeatedly demonstrated that the mechanical behaviour of sand is sensitive to the material fabric that is the arrangement of the grains up until now there have been relatively few attempts to describe this fabric quantitatively much of our understanding of the link between the particle movements and interactions and the macro scale response of granular materials including sand comes from discrete element modelling and experiments on analogue sands with simple idealised shapes this paper investigates methods of quantifying the directional fabric of a real sand and its evolution under loading statistical analyses of the distribution of fabric directional data in terms of particle contact normal branch vector and void orientations were carried out at different stages of shearing deformation the data show that the initial particle orientation fabric that develops during the deposition of the material tends to persist during shearing while in the