interpretation of porosity and lwd resistivity from the nankai accretionary wedge in light of clay physicochemical properties evidence for erosion and local overpressuring in this study we used porosity to assess the compaction state of the nankai accretionary wedge sediments and any implications for stress and pore pressure however hydrous minerals affect porosity measurements and accounting for them is essential toward defining the interstitial porosity truly representative of the compaction state the water content of sediments was measured in core samples and estimated from logging data using a resistivity model for shale we used the cation exchange capacity to correct the porosity data for the amount of water bound to clay minerals and to correct the porosity estimates for the surface conductivity of hydrous minerals the results indicate that several apparent porosity anomalies are significantly reduced by this correction implying that they are in part artifacts from hydrous minerals the correction also improves the fit of porosity estimated from logging while drilling lwd resistivity data to porosity measured on cores low overall porosities at the toe of the accretionary wedge and in the splay fault area are best explained by erosion and we estimated the quantity of sediments eroded within the splay fault area by comparing porosity effective stress relationships of the sediments to a reference curve additionally a comparison of lwd data with core data resistivity and p wave velocity obtained at site c0001 landward of the mega splay fault area suggested a contribution from the fracture porosity to in situ properties on the formation

anisotropy of electrical conductivity record of initial strain at the toe of the nankai accretionary wedge an approach based on march s theory is applied to measurements of the anisotropy of electrical conductivity on samples and is used to quantify initial strain at the toe of the nankai accretionary wedge a quantitative determination of strain is possible from simple assumptions passive reorientation offlat pores forming the porous network and existence of a linear relationship between fabric tensor and electrical conductivity tensor we show that this simple model correctly accounts for the increase of anisotropy with compaction at a reference site located in the trench ocean drilling program drill site 1173 at the toe of the accretionary wedge site1174 development of anisotropy in the horizontal plane and concurrent reduction of vertical plane anisotropy are observed this can be explained by 12 horizontal ductile shortening occurring after decollement initiation but before slip on imbricate thrust faults anisotropy in the underthrust sequence is correctly described by vertical compaction consistent with decoupled stress states across the decollement at site 1174 the magnitude of ductile strain implies at least 75m slip on the decollement ductile shortening is associated with porosity loss implying partly drained conditions above the decollement

brittle ductile transition shear failure and leakage in shales and mudrocks abstract this paper presents an experimental study on brittle ductile transition shear behaviour the formation of shear fractures in shales and mudrocks and the hydraulic properties of shales and mudrocks with shear induced fractures the experimental study is based on triaxial tests on different mudrocks and shales from north sea reservoirs and adjacent areas sheared at different effective confining stresses it is shown that during burial mudrocks behave as normally consolidated materials and exhibit ductile response to increased load however several processes like uplift chemical diagenesis and overpressure build up turn mudrocks into overconsolidated materials which exhibit brittle behaviour during loading triaxial tests with different loading conditions have been performed to establish the brittle to ductile transition of various mudrocks based on the results of the tests it is shown that the brittle to ductile transition can be related to the overconsolidation ratio a relationship between normalized undrained shear strength and overconsolidation ratio is established in combination with this relationship a correlation between compressional wave velocity and apparent pre consolidation stress which accounts for both mechanical and chemical diagenesis may be used as a tool to evaluate possible leakage of hydrocarbon seals

estimating the compressional and shear wave speeds of a shallow water seabed from the vertical coherence of ambient noise in the water column due to the multiple bottom reflections encountered in shallow water environments the spatial structure of the ambient noise field depends strongly on the geoacoustic properties of the seabed which are invariant over time scales associated with most measurements the vertical directionality and coherence are relatively stable features of the noise that are determined primarily by the seabed rather than temporal variations in the surface source distribution in this paper estimates of the compressional and shear wave speeds are determined from ambient noise measurements over shear supporting seabeds using a model of wind generated noise over an elastic seabed it is shown that the noise is sensitive to the compressional and shear wave speeds in the upper few meters of seabed an inversion procedure is developed based on a matched field of the complex broadband coherence from a single hydrophone pair using ambient noise data from two shear supporting sites compressional and shear wave estimates are o

distribution of resistive and conductive structures in nankai accretionary wedge reveals contrasting stress paths in this article we study the characteristics and spatial distribution of the deformation structures along the kumano transect of the nankai accretionary wedge and use this information to interpret the stress path followed by the sediments deformation structures are identified from logging while drilling lwd resistivity images of the materials surrounding the drill hole and from 3 dimensional x ray ct images of cores acquired during the iodp nantroseize project the relative resistivity of the structures identified on logs and the strike dip and density of structures identified on ct scan images are measured the analysis of dip and strike of structures indicates that most of the resistive structures identified on logging data correspond to compactive shear bands results also indicate that conductive structures predominate at the toe of the prism and above the main out of sequence thrust in locations where past and recent erosion occurred we propose several mechanisms that could explain the relation between erosion and the absence of compactive shear bands we conclude that sediments followed different stress paths depending on their location within the wedge and that those differences explain the distribution of deformation structures within the wedge we also show the coexistence of dilatant and compactant structures in fault zones including the frontal thrust and mega splay fault and we interpret the coexistence of these structures as a possible consequence of a transient fluid pressure

range dependent seabed characterization by inversion of acoustic data from a towed receiver array the mapex2000 experiments were conducted in the mediterranean sea in march 2000 to determine seabed properties using a towed acoustic source and receiver array towed systems are advantageous because they are easy to deploy from a ship and the moving platform offers the possibility for estimating spatially variable range dependent seabed properties in this paper seabed parameters are determined using a matched field geoacoustic inversion approach with measured towed array data previous research has successfully applied matched field geoacoustic inversion techniques to measured acoustic data however in nearly all cases the inverted data were collected on moored vertical receiver arrays results here show that seabed parameters can also be extracted by inverting acoustic measurements from a towed array of receivers and these agree with those inverted using data received simultaneously on a vertical array these findings imply that a practical technique could be developed to map range dependent seabed parameters over large areas using a towed acoustic system an example of such a range dependent inversion is given using measurements from the mapex2000 experiments

kinematics and a balanced and restored cross section across the toe of the eastern nankai accretionary prism abstract seismic profiles across the eastern nankai accretionary prism show evidence for diffuse deformation through stratal thickening and uplift of the accreting sediment package thought to reflect the combination of small scale ductile and brittle strains evident within drill cores using a kinematic solution based on changes in stratal thickness and porosities diffuse strains are estimated for a transect across the eastern nankai accretionary prism toe in the vicinity of odp site 808 calculated element displacements are used to reconstruct the undeformed configuration of the prism toe providing a new method for balancing and restoring deformation in accretionary prisms the results of this analysis demonstrate a heterogeneous distribution of strain within the prism toe which appears to correlate with the distribution of brittle deformation structures in drill cores the greatest vertical tectonic thickening and horizontal shortening estimates are obtained within the deepest sediments which also display abundant brittle shears shallower sediments exhibit high volume loss and lower horizontal shortening and in drill cores display very few deformation structures this spatially variable strain distribution may result from inferred high pore pressures near the frontal thrust and decollement inducing a brittle overprint of previous ductile strains

evidence for mechanical decoupling of the upper plate at the nankai subduction zone constraints from core scale faults at nantroseize sites c0001 and c0002 1 detailed analyses of cores from two drill sites landward of the nankai trough provide constraints on the evolution of stress across the sw japan subduction zone core scale faults at sites c0001 and c0002 suggest changing stress geometries that shed light on mechanical coupling across this plate boundary normal faults dominate the record of brittle strain in both the sedimentary cover and the upper accretionary prism and these cut older shear zones and vein structures based on cross cutting relations normal faults postdate thrust faults with the latter interpreted to be inactive the normal faults in contrast are consistent with contemporary stress proxies such as borehole breakout and anelastic strain recovery data stress inversion for faults that are restored to original orientations using paleomagnetic data reveal that thrusting has accommodated trench subperpendicular shortening associated with interplate coupling this phase of deformation is overprinted by normal faulting attributed to mechanical decoupling of the upper plate from the lower plate in response to the underplating of anomalously weak sediments between an out of sequence thrust and the decollement these observations suggest that core scale faults record a stress history that is to date not apparent from in situ stress proxies such as borehole breakout data or anelastic strain recovery data we conclude that the brittle faults provide a unique record of changing stress conditions that can be recognized only through coring

mechanical development of vein structures due to the passage of earthquake waves through poorly consolidated sediments abstract vein structures are typically the earliest expression of brittle deformation within sediments these mud filled veins which characteristically occur regularly spaced within bed parallel arrays form in sediments that possess a strong interlocking particle framework downslope creep has been proposed to explain the origins of vein structures however a recent suggestion that they are generated by the passage of earthquake shear waves through sediments explains aspects of their morphology and their dominant occurrence at active convergent margins their coexistence with less disruptive ghost veins in peru margin sediments and their almost normal attitude to bedding however suggests that vein structures were formed by processes more complex than downslope creep or seismically induced shearing alone experimental earthquake simulation was undertaken by laterally shaking a box containing crushed diatomite fractures were induced almost normal to the horizontal shaking direction and to a lesser extent as antithetic riedel shears both of which closely resembled vein structures the fracturing process during shaking may be viewed as a progressive fragmentation of the diatomite in which new fractures form half way between pre existing ones thus fracture spacings are progressively halved shear zones oriented at a low angle to the shaking direction were also generated combining with the high angle fractures to form structures very similar to those observed in peru margin sediments when shaken fines added to the diatomite segregated into planar zones that resembled ghost veins half way between fractures the alternating pattern of fractures and fines indicated that a standing pressure wave had been created within the box during shaking the fractures were created by alternating compression and extension at the antinodes while the fines concentrated in zones of minimum grain movement around the nodal planes this suggests that vein structures are initiated by the combined action of shear and pressure waves within a sediment the strain waves may be seismic in origin or may also form in downslope movement system

high resolution swath sonar investigation of sand ridge dune and ribbon morphology in the offshore environment of the new jersey margin abstract sand ridges on the northeast us atlantic shelf form in the near shore environment most likely in response to storm driven flows as the holocene transgression has continued the ridges have been transferred to an offshore hydrodynamic regime where currents are not constrained by the coast and storms do not influence bottom currents as frequently or as strongly here we qualitatively and quantitatively investigate the morphology of offshore sand ridges and smaller scale features in an effort to place constraints on bedform development in these deeper waters a recent high resolution swath sonar survey mapped a portion of the new jersey shelf from 20 m water depth to the shelf break 120 m imaging both sand ridges and smaller scaled dunes and ribbons in far greater detail than has been previously possible using a robust statistical analysis we find that the gross morphology of ridges height width length does not change with depth beyond 20 m water depth and changes in ridge orientation generally mirror changes in regional contour orientation hence ridges have not continued to grow since transgression has brought them into the offshore hydrodynamic regime however on the inner shelf 20 m water depth to the mid shelf shore we do find evidence in the ridge shape which has an asymmetry opposite to that seen near shore and in the complex backscatter response that some important modifications to ridges are taking place at these water depths in contrast on the mid shelf from the mid shelf shore to the franklin shore ridges tend to have higher backscatter at the crests implying that these are largely winnowed relict features lineated smaller scale 100 500 m wavelength

experimental study of geo acoustic inversion uncertainty due to ocean sound speed fluctuations acoustic data measured in the ocean fluctuate due to the complex time varying properties of the channel when measured data are used for model based geo acoustic inversion how do acoustic fluctuations impact estimates for the seabed properties in may 1999 saclant undersea research center and tno physics and electronics laboratory fel conducted a shallow water experiment on the adventure bank off the southwest coast of sicily italy to assess the effects of a time varying ocean on acoustic propagation and geo acoustic inversion a favorable area for acoustic propagation was identified which had slight internal wave activity and a weakly range dependent bathymetry with sand like bottom properties oceanographic and acoustic measurements were performed continuously over a 3 day period broadband 0 2 3 8 khz acoustic signals from a bottom moored source were transmitted over fixed paths and received on a moored vertical hydrophone array during the transmissions extensive environmental measurements e g sound speed current sea surface waveheight etc were made to correlate the time varying environmental and acoustic data modeled acoustic data show time variability which agrees with the measurements results illustrate severe problems when modeling shallow water acoustic propagation at ranges beyond a few kilometers in the frequency band considered further the acoustic fluctuations in time caused erroneous time variability in inverted seabed properties

perspectives on the roles of melanges in subduction accretionary complexes a review abstract melanges play three principal overlapping roles in the architecture of subduction accretionary complexes sacs during and after sac formation first tectonic melanges serve as zones of concentrated deformation within and below the accreted rocks that are assembled during the subduction accretion process these melanges facilitate preservation of inter melange less deformed accretionary units aus beneath the trench side of the sac the initial deformation zone along the decollement at the top of the down going plate is marked by non melange tectonically dismembered formations or thinner units of scaly rock and breccia that separate accreted rocks above from the subducting rocks below olistostromal rocks may be incorporated into the decollement here in the mid arc to inner arc areas of the sac exotic block bearing melanges develop in zones of tectonic fragmentation and mixing of accreting ocean plate stratigraphy in mud diapirs and along out of sequence faults some of which facilitate uplift of high pressure rocks and serpentinite matrix melanges second after accretion the sedimentary tectonic diapiric and polygenetic melange units serve as single block or sheet architectural aus or subunits within larger accreted aus of the sac diapiric melanges and sedimentary olistostromal melanges formed in and above sac fault blocks respectively may become incorporated into the sac during ongoing deformation and become architectural units as well third melanges serve as post subduction stress guides that focus shear strain during continuing and post accretion deformation of sacs allowing ongoing modification of the sac during progressive deformation of the orogen examples of each type of role reveal the importance of all three processes in the current architecture of outer orogenic belts

relief and shortening in detachment folds we present new thickness relief methods for determining shortening s z mean shortening s z and curvimetric shortening sc z as a function of height in well imaged structures thickness relief measurements allow us to constrain the fractions of the deformation that are layer parallel pure shear simple shear and flexural and to measure the excess area ae of fold cores caused for example by flow of evaporites the key measurements are made in the thickness domain which allows for more certain determination of regional stratigraphic gradients which is required for reliable separation of structural relief from primary stratigraphic thickness variations we apply these methods to a diverse set of active detachment folds from the fronts of the nankai trough japan cascadia accretionary wedge oregon southern tianshan china and agbami anticline deep water niger delta three of these folds can be approximated by pure shear detachment fold models because more than 95 of the shortening is by layer parallel heterogeneous pure shear plus horizontal compaction for this reason curvimetric shortening is one to two orders of magnitude less than total shortening in contrast agbami anticline shows layer parallel stretching above the excess area of the fold core in all examples the layer parallel simple shear component is negligible or absent 2006 elsevier ltd all rights reserved

high pore pressure generation in sediments in front of the barbados ridge complex dsdp data for the barbados ridge complex show the existence of high pore pressures in sediments both oceanward and arcward of oceanic trenches we model the evolution of pore pressure by using a finite element procedure three consecutive stages are recognized 1 in the oceanic basin sedimentation rate is low there is little excess pore pressure 2 approaching the trench but before being incorporated into the accretionary prism sediments are subjected to increasing tectonic compression as well as greater sediment overburden thus high pore pressure can be created an important result is that the lowest effective stress occurs within the sedimentary layer instead of at its base and a decollement may develop along this weakened zone 3 arcward of the trench sediments above the decollement are offscraped while those beneath it are subducted fast sedimentation combined with tectonic compression can create pore pressures so high as to cause porosity to increase with depth this facilitates the formation of mud volcanoes even oceanward of the deformation front

an evaluation of the accuracy of shallow water matched field inversion results in this article the accuracy of geo acoustic and geometric parameter estimates obtained through matched field inversion mfi was assessed multi frequency mfi was applied to multi tone data 200 600 hz received at a 2 km source receiver range the acoustic source was fixed and the signals were received at a vertical array simultaneously with the acoustic transmissions a ctd conductivity temperature and depth chain was towed along the acoustic track a genetic algorithm was used for the global optimization whereas a normal mode model was applied for the forward acoustic calculations acoustic data received at consecutive times were inverted and the stability of the inverted parameters was determined also the parameter estimates were compared with independent measurements such as multi channel seismic surveys for geo acoustic parameters the obtained uncertainty in the inversion results was assumed to have two distinct origins the first origin is the inversion method itself since each optimization will come up with some solution close to the exact optimum parameter coupling and the fact that some parameters hardly influence the acoustic propagation further contribute to this uncertainty the second is due to oceanographic variability both contributions were evaluated through simulation the contribution of oceanographic variability was evaluated through synthetic inversions that account for the actual sound speed variations as measured by the towed ctd chain

alkaline rocks from continental brazil abstract the present literature review on brazilian alkaline rocks stresses mainly the petrographic and geologic aspects at least eight different rock associations are represented agewise four groups are found among southern brazilian outcrops an older one about 240 m y only that is still poorly known the massifs clustered around jacupiranga about 130 m y old a few occurrences 100 110 m y old tunas itapirapua and the youngest and most important one with an age spread from about 80 m y to about 53 m y recent research shows a clear tectonic control both on local and on regional scales of alkaline occurrences in southern brazil alkaline magmatism was set off by pre drift and drift activity and is at its onset geologically related to basaltic activity emplacement of many massifs is also locally controlled by old lineaments e g in the minas gerais goias belt as is emphasized by geophysical data on a petrographic and chronologic basis several alkaline provinces are distinguished in southern brazil preliminary data on the cambrian precambrian rocks from southern bahia state a province with undersaturated syenites and litchfieldites and from cratonic areas in northern brazil are also presented

deformation at the toe of an active accretionary prism synopsis of results from odp leg 131 nankai sw japan toes of accretionary prisms record the initial deformation of wet sediments during accretion at a convergent plate margin odp leg 131 succeeded in coring the toe of the nankai prism sw japan together with the basal decollement the underthrust sediments and the basaltic ocean basement an unprecedented structural geological inventory has been compiled gravitational deformation is important in the slope apron deposits but tectonic stresses too are transmitted to these levels core scale deformation bands due to heterogeneous bulk shortening of the prism are exceptionally well developed at nankai and range through kink bands shear zones and faults anomalous frequencies indicate localized high strain zones stress inversion analysis gives a principal compression close to the plate convergence vector the frontal thrust of the prism is a 26 m thick zone of breccia scaly fabric with a 309 m net slip it may well be currently active and accommodating almost half of the present convergence strain the basal decollement a 19 m thick breccia zone separates the deforming overconsolidated sediments of the prism from the virtually undeformed materials below these are highly underconsolidated and probably overpressured anomalous fluid pressures localize several of the prism structures unlike some other prisms drainage of the nankai prism is dominantly pervasive rather than controlled by major structures

deformation structures and implications for fluid flow at the costa rica convergent margin odp sites 1040 and 1043 leg 170 abstract during odp leg 170 five sites were drilled and sampled off the costa rica pacific margin two of them site 1040 and 1043 yielded material from a wedge of deformed sediments the main decollement zone and the underthrusting sedimentary sequence of the subducting cocos plate detailed mesoscopic and microscopic analyses of the deformation features characteristic of each domain help to define four different structural hydrologic regimes unexpectedly the wedge of deformed hemipelagic sediments does not represent an accretionary wedge because little or no transfer of material from the subducting plate has occurred the deformed sedimentary wedge records periods of tectonic bulk strain in which fluid pressure intermittently rises and induces fracturing alternating with periods of gravitational bulk strain the last stage of this alternating strain regime is represented by the growth of thick sub horizontal shear zones the sediments caught up in the decollement zone show distinctly different deformation features reflecting different mechanical behaviour and a different response to fluid flow but again transient pulses of increased pore pressure are interpreted as the driving mechanism of dewatering the underthrust pelagic and hemipelagic section is also characterised by dewatering features but only in the uppermost part the basal sediments of this section in contact with underlying gabbro intrusions have a completely different structural history and hydrologic regime structures here are interpreted as related to near ridge processes and the hydrologic system is not linked to the upper sediment dewatering regime but rather seems to have been fed by a seawater source

borehole image analysis of the nankai accretionary wedge odp leg 196 structural and stress studies electrical images recorded with resistivity at bit rab from two sites drilled during ocean drilling program odp leg 196 were analyzed to study the effects of subduction at the nankai margin for the first time in the history of scientific deep sea drilling in odp in situ complete borehole images of the decollement zone were obtained analyses of all drilling induced fracture data indicated that the maximum horizontal compressive stress shmax axes have an azimuth of 303 and analyses of breakout data from rab images indicated an azimuth of 310 these azimuths approximate the convergence direction of the philippine sea plate towards the eurasian plate the frontal thrust at site 808 was encountered at about 389 mbsf density porosity resistivity and gamma ray data change across the frontal thrust the decollement zone at the deformation front was identified between 937 and 965 mbsf the base of the decollement is sharply defined as the maximum extent of conductive fracturing and is marked by abrupt changes in physical properties mikada h becker k moore j c klaus a austin g l bangs n l bourlange s broilliard j bruckmann w corn e r davis e e flemings p b goldberg d b gulick s s hansen m b hayward n hills d j hunze s ienaga m ishiguro h kinoshita m macdonald r d mcneill l obana s hong o s peacock s pettigrew t l saito s sawa t thaiprasert n tobin h j tsurumi h 2002 proc odp initial rep 196 college station tx ocean drilling program the upper boundary of the decollement is marked by several sets of conductive fractures and by high variability in physical properties the decollement zone is characterized by intense brittle fracturing these fractures are considered to be the consequence of cyclic stresses and high fluid pressures in this zone we analyzed fracture dips and their orientations at both sites and found that they are all consistent with a unique stress field model surrounding the two sites

origin of vein structures in the slope deposits of modern accretionary prisms vein structures that have been described from the slope apron sediments of modern accretionary prisms represent brittle failure caused by an effective tensile stress the effective stress arises from abnormal pore water pressure acting in opposition to the compressional stresses on the sediment thus even though 3 the minimum stress may be compressional if the pore water pressure is of greater magnitude than 3 the resultant effective stress will be tensional and failure opening of the veins will occur in the 1 2 plane as the veins open the pore water can escape into the veins and eventually out of the sediment along more permeable coarser beds the initial movement of pore water into the veins may entrain finer grains from the host rock near the vein walls resulting in the observed finer grained vein filling then as the veins close as pore water escapes the elongate and platy grains may be rotated into near parallelism with the vein walls the tributaries that are observed at the ends of some veins may be a result of continued dewatering after the veins have closed as the pore water is forced to diffuse back into the sediment after being concentrated into an efficient flow by the parallel preferred orientation of the platy mineral grains in the vein

deformation and fluid pressure variation during initiation and evolution of the plate boundary decollement zone in the nankai accretionary prism 1 the plate boundary decollement zone in the muroto region of the nankai accretionary prism records deformation and consolidation histories that have been affected by temporal changes in fluid pressure microstructural observations and chemical analysis demonstrate that the decollement zone initiated in an interval of porous clayey sediments characterized by cementation due to intergranular bonding of authigenic clays crosscutting relations of microstructures indicate that the decollement zone records two compactive deformations the early compactive deformation involved destruction of porous cemented structure probably caused by fluid pressure fluctuation the late compactive deformation was characterized by clay particle rotation and porosity collapse along the sets of slip surfaces resulting in zones of preferred orientation of clay particles these compactive deformations led to significantly higher bulk densities within the decollement zone compared to the compaction trend of the overlying prism sediments elevated fluid pressure following compactive deformations induced an overconsolidated state within the decollement zone with fluid filled dilatant fractures bulk density abruptly decreases at the top of the underthrust sediments but there is no microstructural evidence for cementation fluids in the dilated fractures and underconsolidated underthrust sediments are potential sources for the elevated fluid pressure in and below the decollement zone resulting in mechanical decoupling of the accretionary prism from underthrust sediments the fault fluid interactions in the muroto region may be applicable to other convergent plate margins where high temperature associated with the subduction of a spreading ridge or hot young oceanic crust enhances diagenesis and cementation

in situ stress state in the nankai accretionary wedge estimated from borehole wall failures we constrain the orientations and magnitudes of in situ stress tensors using borehole wall failures borehole breakouts and drilling induced tensile fractures detected in four vertical boreholes c0002 c0001 c0004 and c0006 from nw to se drilled in the nankai accretionary wedge the directions of the maximum horizontal principal stress shmax indicated by the azimuths of borehole wall failures are consistent in individual holes but those in c0002 margin parallel shmax are nearly perpendicular to those in all other holes margin normal shmax constrained stress magnitudes in c0001 and c0002 using logged breakout widths combined with empirical rock strength derived from sonic velocity as well as the presence of the drilling induced tensile fractures suggest that the stress state in the shallow portion of the wedge fore arc basin and slope sediment formations is predominantly in favor of normal faulting and that the stress state in the deeper accretionary prism is in favor of probable strike slip faulting or possible reverse faulting thus the stress regime appears to be divided with depth by the major geological boundaries such as unconformities or thrust faults the margin perpendicular tectonic stress components in the two adjacent sites c0001 and c0002 are different suggesting that tectonic force driven by the plate pushing of the philippine sea plate does not uniformly propagate rather the stress field is inferred to be influenced by additional factors such as local deformation caused by gravitation driven extension in the fore arc and thrusting and bending within individual geologic domains

fracture porosity in the decollement zone of nankai accretionary wedge using logging while drilling resistivity data abstract fracture porosity in the decollement zone of nankai accretionary wedge is estimated by comparison of porosity measured on cores during ocean drilling program leg 131 and porosity calculated from resistivity logs acquired during leg 196 using logging while drilling resistivity is converted to formation factor considering both pore fluid conductivity and surface conductivity of clay particles pore fluid conductivity is calculated from temperature and ion concentration in interstitial water whereas surface conductivity is calculated from cationic exchange capacity data and exchangeable cation concentrations finally the formation factor is converted to porosity using the generalized archie s law the decollement appears as a zone of compacted rock where dilatant fractures have developed the contrast between resistivity porosity and core porosity is used to estimate fracture porosity in the decollement assuming that the total conductivity is the result of fracture network and rock fragment conductivities behaving as resistors in parallel in the direction of the fracture network fracture porosity increases downward in the decollement zone from 1 8 to 8 5 this suggests pore pressure in the decollement zone is higher than the pore pressure estimated from compaction curves excess pore pressure ratio of 0 47 a possible explanation is that dilatancy is associated with a high pressure transient the migration of a pressure wave along the decollement could occur at a velocity of 500 m yr if the permeability of the dilated zone is higher than 10 12 m2 the characteristic time for transient dissipation by diffusion in the footwall and hanging wall of the decollement is estimated to be 100 1000 years coexistence of dilatant and compactive shear localization structures is observed within the wedge and in the main fault zones however only the decollement is currently dilated by fluids we propose that fluids are injected into the decollement zone during or after fracturing and that initial shear localization is always compactive and occurs ahead of the fluid injections this sequence of events could occur during each fluid migration and slip event constituting an increment of decollement propagation

chronology of earth s glacial and tectonic record the glacial record yields a pattern of ice ages that shows distinctive characters for different periods however ages of deposits generally provide constraints on the time sequence which because of uncertainties are still too wide to be used critically for or against any regular or cyclic history it seems better to assume no periodicity until such can be argued models for cyclic mechanisms with a longer period than the milankovitch cycle that have been proposed include major chancge in tilt of earth s spin axis galactic year and tectonic cycles evolutionary tectonic changes or changes in atmospheric co 2 are more likely to have been contributing factors but the late proterozoic vendian varangian ice ages with at least equatorial glaciation remain enigmatic

the impact of ocean sound speed variability on the uncertainty of geoacoustic parameter estimates this paper investigates the influence of water column variability on the estimates of geoacoustic model parameters obtained from matched field inversions the acoustic data were collected on the new jersey continental shelf during shallow water experiments in august 2006 the oceanographic variability was evident when the data were recorded to quantify the uncertainties of the geoacoustic parameter estimates in this environment bayesian matched field geoacoustic inversion was applied to multi tonal continuous wave data the spatially and temporally varying water column sound speed is parametrized in terms of empirical orthogonal functions and included in the inversion its impact on the geometric and geoacoustic parameter estimates is then analyzed by the inter parameter correlations two different approaches were used to obtain information about the variation of the water sound speed one used only the profiles collected along the experimental track during the experiment and the other also included observations collected over a larger area the geoacoustic estimates from both the large and small sample sets are consistent however due to the diversity of the oceanic sound speed more empirical orthogonal functions are needed in the inversion when more sound speed profile samples are used

geoacoustic inversion in time domain using ship of opportunity noise recorded on a horizontal towed array a time domain geoacoustic inversion method using ship noise received on a towed horizontal array is presented the received signal containing ship noise as a source of opportunity is time reversed and then back propagated to the vicinity of the ship the back propagated signal is correlated with the modeled signal which is expected to peak at the ship s location in case of a good match for the environment this match is utilized for the geoacoustic parameter inversion the objective function for this optimization problem is thus defined as the normalized power focused in an area around the source position using a matched impulse response filter a hybrid use of global and local search algorithms i e ga and powell s method is applied to the optimization problem applications of the proposed inversion method to mapex 2000 noise experiment conducted north of the island of elba show promising results and it is shown that the time domain inversion takes advantage of dominant frequencies of the source si

mass extinctions the view of a sceptic the concept of periodic mass extinctions by extraterrestrial impacts has contributed very strongly to the current revival of interest in the problem of mass extinctions in this context several radical claims have been made asserting that the newly emerging picture of mass extinctions as a separate class of biological phenomena caused by a separate class of processes demands a far reaching revision of the the neodarwinian paradigm of evolution and implies a vindication of the cuvierian perspective on earth9s history the validity of these claims however is questionable at all levels from the interpretation of particular events to the concept of mass extinctions as a special class of phenomena to their proposed causation by impacts and to their alleged historical pedigree and implications for current evolutionary theory mass extinctions may actually be clusters of more or less accidentally aggregated in time but causally distinct extinction episodes

splay fault branching along the nankai subduction zone seismic reflection profiles reveal steeply landward dipping splay faults in the rupture area of the magnitude m 8 1 tonankai earthquake in the nankai subduction zone these splay faults branch upward from the plate boundary interface that is the subduction zone at a depth of approximately 10 kilometers approximately 50 to 55 kilometers landward of the trough axis breaking through the upper crustal plate slip on the active splay fault may be an important mechanism that accommodates the elastic strain caused by relative plate motion

distribution of stress state in the nankai subduction zone southwest japan and a comparison with japan trench to better understand the distribution of three dimensional stress states in the nankai subduction zone southwest japan we review various stress related investigations carried out in the first and second stage expeditions of the nankai trough seismogenic zone experiment nantroseize by the integrated ocean drilling program iodp and compile the stress data overall the maximum principal stress 1 in the shallower levels 1 km below seafloor or in underlying accretionary prism with 1 becoming horizontal is also suggested at all deeper drilling sites we also make a comparison of the stress state in the hanging wall of the frontal plate interface between site c0006 in the nankai and site c0019 in the japan trench subduction zone drilled after the 2011 mw 9 0 tohoku oki earthquake in the japan trench a comparison between stress state before and after the 2011 mega earthquake shows that the stress changed from compression before the earthquake to extension after the earthquake as a result of the comparison between the nankai trough and japan trench a similar current stress state with trench parallel extension was recognized at both c0006 and c0019 sites hypothetically this may indicate that in nankai trough it is still in an early stage of the interseismic cycle of a great earthquake which occurs on the decollement and propagates to the toe around site c0006

geoacoustic inversion of ambient noise a simple method the vertical directionality of ambient noise is strongly influenced by seabed reflections therefore potentially geoacoustic parameters can be inferred by inversion of the noise in this approach using vertical array measurements the reflection loss is found directly by comparing the upward with the downward going noise theory suggests that this simple ratio is in fact the power reflection coefficient potentially a function of angle and frequency modeling and parameter searching are minimized and the method does not require a detailed knowledge of the noise source distribution the approach can handle stratified environments and is believed to tolerate range dependence experimental data from five sites four in the mediterranean one on the new jersey shelf are described most of the mediterranean sites had temporally varying noise directionality yet yielded the same reflection properties as one would hope one site was visited in conditions of very low surface noise this paper concentrates on an experimental demonstration of the feasibility of the method and data quality issues rather than automatic search techniques for geoacoustic parameters

inversion for range dependent water column sound speed profiles on the new jersey shelf using a linearized perturbative method the environment of the new jersey shelf is characterized by high spatial and temporal variability of water column properties caused by intrusions of warm salty water from the continental slope these intrusions cause fluctuations in the water column sound speed profile which can have significant effects on acoustic propagation in shallow water in this work a linearized perturbative inverse technique is applied to estimate range dependent water column sound speed profiles this method utilizes estimates of horizontal wave numbers to determine sound speed as a function of depth this technique is appropriate for the range dependent shallow water environment as horizontal wave numbers can be measured semilocally 1 2 km aperture and their values are a direct measurement of the local environmental parameters difficulty is encountered in application of the perturbative inverse technique because the wave number data are insensitive to some portions of the waveguide and as a result the solution can deviat

a statistical approach to relationships between fluid emissions and faults the sea of marmara case the sea of marmara is traversed by the north anatolian fault system and also presents abundant emission sites of methane gas into the water column in order to assess the spatial relationship between gas emissions and active faults the distribution of distances between gas emission sites and the nearest fault is calculated and compared with the distribution of distances between a uniform random distribution of points poisson process representing the null hypothesis of an absence of relationship between gas emissions and faults and the nearest fault interestingly the distance distribution for the poisson process is nearly exponential indicating that the fault map does not have a characteristic scale other than that representing the intensity of the fault network the distance distribution for the observed gas emissions is significantly narrower than that arising from the poisson process with a kolmogorov distance of 0 25 0 02 the crossing point between the two distributions defines the characteristic half width of the swath of gas emission sites around the mapped active faults for the whole sea of marmara data set a characteristic half width of 900 1000 m is found which matches the half width of the seafloor deformation zone observed around the main active fault when the same analysis is applied to zones covering the western high and the central high it is found that the swath of gas emissions is wider on the central high 2 km half width and not clearly related to the seafloor deformation zone there this difference is put in perspective with recent work showing that creep is occurring on the western segment of the main marmara fault this also causing microseismicity while the central istanbul silivri segment may have remained locked since the 1766 magnitude 7 earthquake this suggests that aseismic slip and not only earthquake occurrence effectively maintains high permeability conduits in fault zones in sediments

anelastic strain recovery reveals extension across sw japan subduction zone 1 sediment dominated convergent margins typically record substantial horizontal shortening often associated with great earthquakes the convergent margin south of japan is arguably one of the most extensively investigated margins and previous studies have documented extensive evidence for accretion and horizontal shortening here we show results from anelastic strains recovered from three partially lithified sediment samples 40 porosities across the southwest japan accretionary prism and propose that the margin is dominated by horizontal extension rather than compression the anelastic strain results are also consistent with stress directions interpreted from two independent techniques bore hole breakout orientations and core scale fault data we interpret this unexpected result to reflect geologically recent underthrusting of a thick sediment package and concomitant weakening of the decollement

north atlantic warming during global cooling at the end of the cretaceous differences in regional responses to climate fluctuations are well documented on short time scales e g el nino southern oscillation but with the exception of latitudinal temperature gradients regional patterns are seldom considered in discussions of ancient greenhouse climates contrary to the expectation of global warming or global cooling implicit in most treatments of climate evolution over millions of years this paper shows that the north atlantic warmed by as much as 6 c 1 5 decrease in 18o values of planktic foraminifera during the maastrichtian global cooling interval we suggest that warming was the result of the importation of heat from the south atlantic decreasing north atlantic 18o values are also associated with increasing gradients in planktic 13c values suggesting increasing surface water stratification and a correlated strengthening of the north atlantic polar front if correct this conclusion predicts arctic cooling during the late maastrichtian beyond implications for the maastrichtian these data demonstrate that climate does not behave as if there is a simple global thermostat even on geologic time scales

rapid forearc basin uplift and megasplay fault development from 3d seismic images of nankai margin off kii peninsula japan abstract offshore kii peninsula japan a large thrust within the overriding forearc the megasplay fault appears to move coseismically during great earthquakes 3d seismic images of the kumano forearc basin that overlies the megasplay correlated with iodp drilling data are a potential record of the history of large scale motion along this structure in the early quaternary uplift occurred in the southwest portion of the basin that may be a preliminary phase of motion along the megasplay more extensive landward tilting of the outer basin sediments across the seismic volume occurred over 300 kyr in the middle to late quaternary 1 3 1 ma this tilting event may represent the major period of motion along the megasplay that formed the modern fault geometry extensive normal faulting that cuts the forearc basin sediments clearly formed subsequent to the late quaternary tilting and in many cases offset the modern seafloor these faults may form either due to gravitational response to the uplift or as a by product of sediment underthusting these results suggest that the megasplay is a recently formed and transient structure and support the idea that out of sequence thrusts serving as the dominant structure for convergence driven shortening in a subduction zone may be short lived geologically but dominate a margin during these intervals

source localization in a time varying ocean waveguide one of the most stringent impairments in matched field processing is the impact of missing or erroneous environmental information on the final source location estimate this problem is known in the literature as model mismatch and is strongly frequency dependent another unavoidable factor that contributes to model mismatch is the natural time and spatial variability of the ocean waveguide as a consequence most of the experimental results obtained to date focus on short source receiver ranges usually 5 km stationary sources reduced time windows and frequencies generally below 600 hz this paper shows that mfp source localization can be made robust to time space environmental mismatch if the parameters responsible for the mismatch are clearly identified properly modeled and time adaptively estimated by a focalization procedure prior to mfp source localization the data acquired during the advent 99 sea trial at 2 5 and 10 km source receiver ranges and in two frequency bands below and above 600 hz provided an excellent opportunity to test the proposed techniques the results indicate that an adequate parametrization of the waveguide is effective up to 10 km range in both frequency bands achieving a precise localization during the whole recording of the 5 km track and most of the 10 km track it is shown that the increasing mfp dependence on erroneous environmental information in the higher frequency and at longer ranges can only be accounted for by including a time dependent modeling of the water column sound speed profile

deformation and in situ stress in the nankai accretionary prism from resistivity at bit images odp leg 196 1 borehole resistivity images from odp leg 196 allow rapid and complete qualitative assessment of deformation within the toe of the nankai prism japan borehole breakouts were common within the prism but prominent in the trench wedge unit around the frontal thrust suggesting reduced sediment strength breakouts indicate consistent 2 orientations 050 compatible with northwesterly convergence deformation is dominated by discrete zones including the frontal thrust and decollement zone prism fractures trend ne sw consistent with convergence the decollement shows minimal deformation and the dominant structural trend is n s prism deformation zones are characterized by high resistivity compaction whereas the decollement is apparently dilated both with conductive fractures distribution of fracture orientations varies between log units confirming lithologic and rheologic influence pore pressure is elevated within the decollement and the misalignment of conductive fractures may reduce permeability

the effects of environmental variability and spatial sampling on the three dimensional inversion problem the overall goal of this work is to quantify the effects of environmental variability and spatial sampling on the accuracy and uncertainty of estimates of the three dimensional ocean sound speed field in this work ocean sound speed estimates are obtained with acoustic data measured by a sparse autonomous observing system using a perturbative inversion scheme rajan lynch and frisk j acoust soc am 82 998 1017 1987 the vertical and horizontal resolution of the solution depends on the bandwidth of acoustic data and on the quantity of sources and receivers respectively thus for a simple range independent ocean sound speed profile a single source receiver pair is sufficient to estimate the water column sound speed field on the other hand an environment with significant variability may not be fully characterized by a large number of sources and receivers resulting in uncertainty in the solution this work explores the interrelated effects of environmental variability and spatial sampling o