estimates of heat flow derived from gas hydrates a gas hydrate phase boundary in marine sediments is often observed as an anomalous reflector in seismic profiling data from the depth of the reflectors heat flow values can be estimated using the phase relation of the gas hydrate system and inferred density and thermal conductivity of sediments this method is applied to the data in the nankai trough around central america and along the blake outer ridge the estimated values consistent with those measured by conventional means suggest that derivation of heat flow by this method is feasible on the landward slope of the nankai trough we find that heat flow increases downslope toward the floor of the trough

the sub himalayan fold thrust belt in the 1905 kangra earthquake zone a critical taper model perspective for seismic hazard analysis 1 we investigate the broader epicentral area of the m 7 8 1905 kangra earthquake s north india affecting the sub himalayan hills the tectonics of the area is characterized by two major rentrants kangra and dehradun interposed by the nahan salient the first order topography between the himalayan frontal thrust and the main boundary thrust shows a marked lateral variation along strike of the mean gradient characterized by a very small mean slope angle 1 in correspondence with the reentrants and higher values 3 in the salient these tectonic and topographic features also show a good correspondence with the peculiar macroseismic field of the 1905 event s which is characterized by two distinct intensity maxima separated by a distance of 150 km clearly overlapping the two major tectonic reentrants in this paper based on available geological and geophysical information and a critical analysis of the general mechanical constraints the seismogenic volume of the external sector of the chain is investigated in terms of critical taper model attempting to clarify the possible correlations between tectonics topography and seismicity in the sub himalayan belt based on different assumptions three possible seismotectonic scenarios are explored in order to constrain their likelihood and therefore to suggest a potential seismic gap in the area corresponding to the nahan salient which may experience an event of significant magnitude in the future

sediment deformation and hydrogeology of the nankai trough accretionary prism synthesis of shipboard results of odp leg 131 the main objective of leg 131 was to provide data on the deformational processes and associated hydrogeology of the nankai prism toe drilling succeeded for the first time in the history of ocean drilling in penetrating the complete sedimentary sequence to basaltic basement reaching 1327 mbsf metres below seafloor with good core recovery 55 excellent correlation of the lithology and structure including the frontal thrust and the decollement with seismic reflection images was also determined bedding dips faults and shear bands analyzed in the cores confirm the pattern of deformation to be mainly due to nw se shortening as expected from the plate tectonic convergence vector below the decollement no significant deformation features were observed indicating that the decollement is a sharp discontinuity in stress transmission physical properties data show major discontinuities at the decollement notably an increase in porosity below the later this may indicate excess pore pressure in the subducted section and decollement zone a less marked increase in porosity below the frontal thrust may reflect the youthfulness of this feature attempts to make downhole measurements were severely hampered by unstable hole conditions but useful constraints have been placed on the thermal regime and some calibration of laboratory physical properties toin situ conditions has been provided andin situ stress and pore pressure were measured in the uppermost sediments evidence of channelized fluid flows is inconclusive no sharp geochemical signatures or unequivocal geochemical anomalies indicative of channelized fluid flow were found thermal measurements are not significantly different from those predicted by a purely conductive heat flow model a signature of low chloride pore water near the decollement may partly be related to smectite diagenesis but may also be due to episodic fluid flow events we conclude that dewatering probably occurred dominantly through diffuse flow throughout the accreted sediments at this site

the vertical chloride ion profile at the iodp site c0002 kumano basin off coast of japan abstract the vertical profiles of chloride ion cl concentration and oxygen and hydrogen isotopic ratios in pore fluids of basin fill sediments in the southern kumano basin were examined a model has been proposed to explain those profiles in conjunction with published data by invoking diffusion and advection of freshwater from methane hydrate dissolution the model requires a low initial cl concentration in the old accretionary prism below the basin before the basin began to fill with sediments it also requires little or no methane hydrate formation in the old accretionary prism given these requirements an upward fluid advection rate of 1 10 12 m s has been estimated which is very low compared to other accretionary prisms our data suggests that the tectonic compaction of the basin filling sediments and fault activity under the basin sediments have not been that active recently in the kumano basin

paleothermal structure of the point san luis slab of central california effects of late cretaceous underplating out of sequence thrusting and late cenozoic dextral offset late cretaceous shale and sandstone turbidites of the point san luis slab are isoclinally folded locally sheared and faulted but their severity of stratal disruption is relatively mild when compared to adjacent polymictic melange of the franciscan complex we tested the interpretation of a trench slope basin origin for these strata by documenting their paleothermal structure including contacts between turbidites and melange values of mean random vitrinite reflectance rm from turbidites are 0 9 1 7 estimates of maximum paleotemperature are 135 200 c melange matrix samples yield rm values of 1 1 2 5 with an average of 1 5 and peak temperatures between 160 and 240 c the turbidite over melange contact is locally cooler over warmer and was folded after peak heating the relatively high paleotemperatures cast doubt on a shallow slope basin model i e 1 2 km burial depth we suggest instead that thermal maturation of the point san luis slab occurred much deeper in an accretionary prism 10 15 km where offscraped trench wedge deposits were faulted against underplated melange the paleothermal structure was offset and tilted after peak heating by two out of sequence faults late oligocene to pliocene strata rest unconformably above the franciscan and there is a significant gap in thermal maturity across this unconformity with no evidence to show that franciscan rocks were reset thermally following the main episode of uplift and erosion three dimensional orientations of isoreflectance surfaces on opposite sides of the san gregorio san simeon hosgri fault system also can be used to test interpretations of strike slip neotectonics the failure to match these geometries among suspected piercing points at point san luis cambria and point sur favors a suggestion that differential post thermal peak dextral offset of franciscan basement has not exceeded 10 15 km

velocity structure of a gas hydrate reflector seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs parallel to the seabed these are often interpreted as the base of a zone in which methane hydrate ice is stable waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a bsr and its thickness a test of this method at a drill site of the ocean drilling program predicts that sediment pores beneath the bsr contain free methane for approximately 30 meters the hydrate and underlying gas represent a large global reservoir of methane which may have economic importance and may influence global climate

heat flow across the toe of accretionary prisms the role of fluid flux compaction driven fluid flow in accretionary prisms is ineffective in changing heat flow across the lower slope even when the rate of fluid flow is greatly enhanced by hydrofracturing as results of numerical analysis show for most faults which are long relative to their thickness conductive heat loss to the wall rock is much more effective in heat transport than advective transport along the faults

kinematic models of dewatering accretionary prisms steady state two dimensional analytical solutions are derived for sediment velocity and rate of fluid expulsion in a thickening accretionary prism with smooth upper and lower surfaces two different kinematic assumptions lead to two similar sets of solutions in one case the sediment flow lines are assumed to diverge uniformly landward in the other case the horizontal velocity of any given sediment column is assumed uniform by further assuming zero horizontal darcy velocity in the prism expressions can be obtained for the approximate vertical fluid velocity and pore fluid pressure fields results from applying these solutions to the cascadia accretionary prism off vancouver island show the importance of modeling the effects of lateral as well as vertical porosity variation on the hydrogeology of the prism these solutions provide a means to estimate fluid flow and pore pressure distribution for prisms with realistic geometry the rate of fluid expulsion can also be incorporated into numerical models as the fluid source term

seismic velocities from the barbados ridge complex indicators of high pore fluid pressures in an accretionary complex three wide aperture 0 5 km offset seismic profiles were shot perpendicular to the strike of the barbados ridge accretionary complex at 16 12 n line 465 14 23 n line 480 and 13 20 n line 484 to image the deep structure of the accretionary complex and obtain seismic velocity data velocity analyses on full fold common depth point cdp gathers were used to provide velocity depth functions at 1 km separation along each line to map the variation in velocity with depth and distance across the accretionary complex the sediments within the complex show consolidation increasing laterally with greater age of the complex as indicated by the increase in seismic velocity and the inferred decrease in porosity the velocity at the base of the wedge along line 480 is 2 4 km s at the toe the velocity at the wedge base at 60 km from the toe where the wedge thickness has increased from 1 8 km to 5 0 km has increased to greater than 3 0 km s however the velocities within the leading margins and within undeithrust sequences in each of the three sections are low as much as 1 0 km s less than velocities found in undeformed sedimentary sequences of equivalent thickness the low velocity regions are interpreted as sediments with high porosity where porosity is inferred to be high pore fluid pressures are also inferred to exceed hydrostatic and pore fluids actually support the increased lithostatic load produced by recent thrusting and thickening of the complex the correlation between the most rapidly thickened regions and the velocity anomalies suggests that a rapid rate of loading relative to the rate of pore fluid expulsion produced these overpressured sediments velocities in the underthrust sequences where the differences between loading histories of wedge sediments and the underthrust sequences are largest are sufficiently low to produce velocity inversions low velocities also occur in regions adjacent to the complex where the pore fluid pressures may be elevated by the pressure produced in the adjacent wedge west of the wedge in the forearc basin along line 480 and near a seismically imaged mud diapir velocities at a depth of 3 5 km below the seafloor are 0 5 km s less than what is expected in a more normal sedimentary section immediately seaward of the complex the deepest undeformed sequences have a velocity anomaly of 0 5 km s velocities remain low for at least 12 km seaward of the complex but they become closer to a more normal sediment section farther from the wedge

fluid rock reaction weakening of fault zones the presence of weak phyllosilicates may explain the low shear strengths of fault zones if they define well developed fabrics the growth of phyllosilicates is favored in meteoric water dominated granitic fault systems where mineral aqueous fluid equilibria predict that modal phyllosilicate will increase via feldspar replacement reactions in deeper more alkaline rock dominated regimes the reactions reverse and feldspars tend to replace phyllosilicates in mg rich mafic rocks however phyllosilicates chlorite biotite can replace stronger framework and chain silicates in both shallower approximately 10 km meteoric h sub 2 o dominated and in deeper alkaline rock dominated regimes where these phyllosilicates precipitate in active fault zones they contribute directly to reaction softening because low temperature deformation of phyllosilicates is not governed by frictional processes alone but can occur by pressure independent dislocation glide the strength of phyllosilicate rich fault rocks can be low at all depths low strain rate creep during interseismic periods can align phyllosilicate grains in foliated gouge and phyllonites where preferred orientations are strong and contiguity of phyllosilicates is large strengths of rocks within fault zones may approach minimum strengths defined by single phyllosilicate crystals fault zones containing localized high concentrations of phyllosilicates with strong preferred orientations in well defined folia can exhibit aseismic slip especially where mafic mg rich rocksmore occur along the fault like parts of the san andreas fault 104 refs 6 figs 1 tab less

effect of thermal non equilibrium seafloor topography and fluid advection on bsr derived geothermal gradient abstract the seafloor and bottom simulating reflectors bsrs are interpreted from the 3d seismic data acquired in krishna godavari kg offshore basin in the vicinity of sites drilled cored during national gas hydrate program nghp expedition 01 the shallow structures such as inner toe thrust fault system regional and local linear fault systems and mass transport deposits are inferred from attributes of seafloor time structure as well as from the seismic profiles the geothermal gradient is estimated from the depths and temperatures of the seafloor and the bsr the temperature at the bsr depth is estimated from the methane hydrate and seawater salinity phase boundary assuming that the bsr represents the base of the gas hydrate stability zone the spatial variations in geothermal gradient gtg show a strong correlation with seafloor topography in the kg basin the gtg decreases by 13 30 over the topographic mounds formed due to inner toe thrust faults and recent mass transport deposits the gtg decreases by only 5 10 over the mounds likely due to defocusing of heat flux based on one dimensional topographic modeling hence the gtg perturbation due to topography alone cannot explain the observed gtg anomaly the temperature profile beneath these mounds may not be in equilibrium with the surroundings either due to the recent upliftment of sediments along the inner toe thrust faults or rapid deposition of sediments due to slumping sliding in contrast an increase in gtg by 10 15 is observed in the vicinity of major fault systems we presume that the likely mechanism for the increase in gtg is fluid advection from a deeper part of the basin a detailed thermal modeling involving the effect of surface topography high sedimentation rates fluid advection and sediment thickening due to tectonics is required to understand the thermal profile in kg offshore basin

advection of fluids at the front of the sicilian neogene subduction complex abstract the sicilian accretionary prism shows a large southward imbricated system of thrust sheets the frontal part of the belt has been tectonically emplaced in plio pleistocene times major decollement surfaces and sole thrusts at the base of and inside the prism consist of melanges with a blocks in matrix fabric in the frontal part two superposed decollement levels are recognized the upper one located at the base of the paleogene neogene sedimentary series and the basal one at the base of duplexes triassic miocene sedimentary series syntectonic dewatering of the sedimentary sections occurred along decollements and thrust faults involving hydrofracture and mineralized vein development inside the deep sole thrusts rooted in the basal decollement hydrofractured blocks show different generations of syntectonic quartz and calcite extensional veins while in the upper decollement only calcite veins have been observed both quartz and calcite veins show aqueous primary fluid inclusions of low salinity with low trapping temperature for fluids in the upper decollement if we assume that at the time of deformation the main heat source was the regional conductive geothermal gradient the trapping temperatures of the fluids inside veins of the deep sole thrusts are higher than those predicted by the pilo pleistocene gradient for this subduction zone and correspond to a thermal anomaly we suggest that the frontal sicilian prism supported localized transient fluid flow of deep freshwater sourced from two different origins a shallow one for fluids now trapped in the upper decollement and a deep internal origin 6 10 km minimum depth for the hot fluids channelized along the basal decollement and now trapped inside the sole thrust of the mt scalpello duplex area

vertical tectonics and the origins of bsrs along the peru margin abstract the distribution of bottom simulating reflections bsrs which mark the base of the gas hydrate zone on marine seismic records across the peru margin is examined with respect to tectonic uplift sedimentation and carbon content of sediment where accretion of only deep ocean pelagic sediment containing less than 1 carbon is seismically imaged no bsr is observed no bsr is apparent on seismic records of carbon rich 4 8 sediment of rapidly subsiding basins bsrs are abundant on records of carbon rich sediment affected by tectonic uplift and or rapid sedimentation in areas of uplift the bottom simulating reflectors contain small amounts of free gas in contact with sediment containing small amounts of gas hydrate during uplift free gas is released from gas hydrates by dissociation as the hydrate phase boundary shifts upward with respect to sea level the strong effects of free gas in reducing seismic velocity and gas hydrate in increasing velocity can explain the relatively small total quantities of gas and hydrate indicated by observations of bsrs off peru

tectonic sediment thickening fluid expulsion and the thermal regime of subduction zone accretionary prisms the cascadia margin off vancouver island tectonic sediment thickening fluid expulsion and the resulting thermal regime of the northern cascadia subduction zone accretionary prism have been described by numerical models both the sediment thickening and the fluid expulsion are found to have important thermal effects constraints on the models are provided by 1 detailed heat flow profiles across the continental slope accretion zone from variations in the thermally controlled depth to a gas hydrate bottom simulating reflector bsr and 2 porosity changes across the accretion zone inferred from the landward variation in the p wave velocity depth profiles the heat flow in the region of rapid prism thickening centered 15 km landward of the deformation front is 20 below that predicted by a larger scale thermal model that ignores the thermal effects of the sediment accretion processes the porosity also is substantially higher in this region compared to at the same depths in the adjacent deep sea cascadia basin the model results are in agreement with both the thermal and velocity data if the incoming sediment section is initially simply tectonically shortened horizontally and thickened vertically generating a high porosity underconsolidated section reconsolidation and fluid expulsion in response to the tectonic thickening is slow occurring mainly 20 30 km landward of the deformation front the maximum predicted fluid expulsion rate is about 4 10 11 m s 1 1 3 mm a 1 the pore pressure distribution across the deformation front region has been estimated from the velocity data using the simplifying assumption that velocity is only a function of effective pressure pore pressures are high reaching 80 of lithostatic 15 to 20 km landward of the deformation front an estimate of the bulk permeability in the seaward tens of kilometers of the prism ranging from 10 16 m2 near the top to about 10 18 m2 near the base has been obtained through matching the pore pressure distribution in the numerical model to that obtained from the velocity data the prism sediments are inferred to be substantially underconsolidated and pore pressures to be high for at least the seaward 30 km of the accretionary prism

water flow in the barbados accretionary complex we present a simple analytic kinematic model of the barbados accretionary complex assuming a porosity depth law and deformation by pure vertical shear the model is applied to two sections a northern one with a 900 m thick basin sedimentary column near 16 n and a southern one with a 4 5 km thick one to the south this simple model is compatible with growth of the barbados accretionary complex since 50 ma assuming a constant subduction rate and a constant basin sedimentary column equal to the present one this conclusion is not affected by reasonable changes in the porosity depth law it suggests that the totality of the entering sedimentary column is either accreted or subcreted to the wedge the accretion velocity decreases dramatically with age and its variation should be taken into account in any quantitative model the average strain rates show remarkably different distributions between the two sections the thinner wedge having a much higher rate in a narrow area at the toe we then use the model to compute the sediment grains flow the fluid flow and their thermal effects these effects are small and do not exceed 5 of the heat flow in the absence of other internal flows in contrast the thermal effect of the burial of the slab can result in a lowering of the surface heat flow by as much as 40 the temperature at the base of the wedge on its arc side is close to 300 c on the southern section and half that amount on the northern one this high temperature could account for the absence of large earthquakes to the south and their presence to the north the amount of water discharged if collected by faults over the whole toe area is unable to account for some of the manifestations observed over the toe or seaward of the toe in the barbados area we suggest two possible other sources one for the southern section is the orinoco fan area the other for the northern section might be seismic pumping at the limit of the seismogenic zone along the backthrusts between the accretionary wedge and the forearc basin

seismic reflection and tomographic velocity model constraints on the evolution of the tofino forearc basin british columbia summary the tofino basin is a sedimentary forearc basin that overlies the continental shelf of the cascadia margin to the southwest of vancouver island the basin which contains up to 4k mo fmarine clastic sedimentary rocks formed following accretion in the early eocene of the crescent and pacific rim terranes and subsequent accretionary wedge basement subduction of the juan de fuca plate has since been the primary tectonic driving force in the development of the basin s structure investigations using coincident seismic reflection profiles tomographic velocity models and recently reassessed biostratigraphic well data show that basement composition has largely controlled deformation of the overlying tofino basin sediments anticlinal folds overlying the accretionary wedge exhibit low p wave velocities at the apex of the fold which may be related to fracturing of older more lithified sediments accompanied by fluid expulsion from the accretionary wedge in contrast the velocity variation across folds over the crescent terrane mimics the fold geometry and does not appear anomalous a sub basin containing up to 3k m ofoligocene to holocene sediment has developed in the central part of the tofino basin at the boundary between the crescent and pacific rim ter ranes seismic interpretation suggests that deposition has increased more rapidly in the late miocene to holocene subsidence within the sub basin is likely to have been controlled by sediment loading flexure and regional tectonic forces localized by pre existing zones of weakness such as the tofino fault the development of the sub basin may also have been influenced by the displacement landward of part of the lower forearc crust during subduction erosion diapiric structures along the axis of the sub basin suggest that fluid expulsion into the tofino basin from the deeper accreted terranes is localized by the terrane bounding fault further seaward fluid expulsion from the accretionary wedge may be more pervasive the seismic data demonstrate that subsidence of the sub basin beneath the inner continental shelf has been occurring relatively late in the history of the tofino basin and that the cascadia accretionary complex cannot be viewed as growing gradually seaward with no inboard deformation

horizontal shortening versus vertical loading in accretionary prisms 1 the analysis of two phased deformation of subducted turbidites within the shimanto belt on kyushu japan shows strongly contrasted kinematics between an earlier noncoxial fold and thrust type stage and a later metamorphic coaxial vertical shortening stage the deep deformation is characterized by the development of a subhorizontal muscovite chlorite metamorphic foliation associated with pure shear deformation which overprinted the structures inherited from the earlier stage we associate the metamorphic deformation stage with the lower levels of accretionary prism landward segment inner wedge which was therefore deformed by vertical loading and horizontal extension much in contrast with prism trenchward segment outer wedge which often shows fold and thrust structures indicative of horizontal shortening a possible explanation based on wang and hu s 2006 model for these longitudinal variations in deformation kinematics of the prism may lie in the contrasted rheology of its basal surface strain weakening in the seismic portion below the inner wedge while strain strengthening in the aseismic portion below the outer wedge the vertical shortening active within the inner wedge revealed by this study possibly represents a large contribution to the exhumation of the deeper levels of accretionary prisms

three dimensional seismic characterization of a venting site reveals compelling indications of natural hydraulic fracturing based on a three dimensional high resolution seismic survey off vancouver island canada we show that natural hydraulic fracturing is an efficient process to create permeable pathways for focused fluid upflow at submarine venting sites the pockmark structure examined in this study is located on top of an accreted ridge where the low permeability base of the gas hydrate stability field is also elevated and fluid overpressure will be induced tectonically elongated hydraulic fractures reveal decreased reflection amplitudes and are oriented along the maximum principal stress direction perpendicular to nearby fault planes a surface expression is created by fluid and material expulsion forming an irregular noncircular rim along the axis of the upflow zone our findings confirm a relationship between lateral variations in seismic reflectivity and sediment distribution regional tectonics episodic fluid flow and the formation and dissociation of gas hydrate they explain the presence of gas hydrate at the top of the upflow zone which requires transport of large volumes of gas through the gas hydrate stability field since hydraulic fracturing is not restricted to compressional regimes it is a likely explanation for the nature shape and orientation of upflow zones as well as for seismic blanking widely observed at both active and passive continental margins

horizontal compressive stress regime on the northern cascadia margin inferred from borehole breakouts during integrated ocean drilling program expedition 311 five boreholes were drilled across the accretionary prism of the northern cascadia subduction zone logging while drilling borehole images are utilized to determine breakout orientations to define maximum horizontal compressive stress orientations additionally wireline logging data at two of these sites and from site 889 of ocean drilling program leg 146 are used to define breakouts from differences in the aperture of caliper arms at most sites the maximum horizontal compressive stress shmax is margin normal consistent with plate convergence deviations from this trend reflect local structural perturbations our results do not constrain stress magnitudes if the margin normal compressional stress is greater than the vertical stress the margin normal shmax direction we observe may reflect current locking of a velocity weakening shallow megathrust and thus potential for trench breaching tsunamigenic rupture in a future megathrust earthquake

porosity of sediments in accretionary prisms and some implications for dewatering processes rapid dewatering largely through loss of porosity is one of the most striking changes affecting deformed sediments in accretionary prisms analysis of porosity from sample and seismic data confirms that porosity in accretionary prisms is significantly less than that in basinal strata of similar lithology more specifically the porosity depth gradient increases arcward across the prism until porosities less than 10 occur within a few kilometers of the surface in deeper parts of the prism dewatering and volume reduction occur not only during loss of porosity but also during dehydration and metamorphic reactions calculations involving volume change lead us to define a function which reflects total volume reduction the distribution of the relative intensity of dewatering within the prism can be calculated from the porosity contours and trajectories of sediment elements an initial attempt indicates that in the toe region the rate of dewatering is accelerated at depth relative to the basinal section and may be greatest near the base of the prism toward the rear of the prism the maximum rate of dewatering occurs at progressively shallower depths several lines of evidence suggest that in addition to gradual diffuse dewatering localized zones of strong dewatering are associated with zones of intense shear this model of shear dewatering based on critical state soil mechanics suggests that sediments adjacent to the basal decollement should be strongly dewatered

fold uplift versus regional subsidence and sedimentation rate abstract fold uplifts compete during their development with regional subsidence in the frontal parts of thrust belts and accretionary wedges two cases exist in frontal thrust belts in the first more common case the uplift rate of the external folds is higher than the subsidence rate of the foredeep in the second case the fold uplift is less than the regional subsidence the total fold uplift may be considered as the fold uplift rate minus the regional subsidence rate this value can be either positive or negative positive total fold uplift occurs when folds rise faster than regional subsidence and the envelope of the fold crest rises towards the hinterland of the accretionary wedge in the opposite case negative total fold uplift the envelope dips towards the hinterland because folds rise more slowly than regional subsidence in the first positive case the folds are deeply eroded and the onlap of the growing strata moves away from the fold crest if the sedimentation rate is lower than the fold uplift rate in marine or alluvial environments in the second negative case where folds rise at lower rates than the regional subsidence the onlap moves towards the fold crest if the sedimentation rate is higher than the fold uplift rate this is also the more favorable case for hydrocarbon traps where growth strata can seal the fold moreover the fast subsidence rates in the foredeep provide a higher thermal maturation this second tectonic setting is commonly associated with accretionary wedges forming along westdirected subduction zones which are characterized by high subsidence rates in the foredeep or trench due to the fast eastward roll back of the subduction hinge e g apennines carpathians banda arc

origins of porosity and velocity variations at cascadia accretionary prism at cascadia accretionary prism log based porosities exhibit complex lateral changes in response to variations in cementation horizontal compression and fracturing porosities at the prism toe are lower than in reference sediments whereas porosities landward of the toe are slightly higher than those of reference sediments ideally a porosity velocity relationship can be used to determine two dimensional porosity structure from seismic velocities porosity is the major control on velocity at cascadia but variations in grain framework strength frame bulk modulus cause deformed and undeformed rocks to have quite different velocity porosity relationships within the prism cementation can raise velocities substantially above those for unconsolidated reference sediments of the same porosity also compression induced compaction and fracturing may affect this velocity porosity transform because these factors vary both within and between prisms velocity porosity transforms are local not universal

crustal and lithospheric scale structures of the precambrian superior grenville margin abstract the lithosphere beneath the margin of the archean superior and proterozoic grenville provinces was investigated with a northwest southeast oriented 650 km long profile of 40 magnetotelluric stations dominant geoelectric strike azimuths of n45 e and n85 e were defined for the crust and the lithospheric mantle respectively a 2 d isotropic resistivity model derived using the crustal strike images resistive laurentian margin rocks dipping southeast to the base of the crust bounded to the northwest by the grenville front and to the southeast by the central metasedimentary belt boundary zone the observation is in contrast to conductive mid to lower crust elsewhere in the region a 2 d isotropic resistivity model determined using the lithospheric mantle strike azimuth reveals an extremely resistive region in the upper 100 km of the mantle lithosphere of the northern grenville province the geometry of this body which includes a well defined base and southeast dip suggests that it is superior lithosphere a sub vertical conductor located approximately 50 km along strike from the mesozoic kirkland lake and cobalt kimberlite fields is interpreted to be due to re fertilization of an older mantle scar the resistivity model includes a horizontal conductor at 160 km depth beneath the southern superior province that is possibly the resistivity signature of the lithospheric asthenospheric boundary

seismic structure of the southern cascadia subduction zone and accretionary prism north of the mendocino triple junction four multichannel seismic reflection profiles collected as part of the mendocino triple junction seismic experiment image the toe of the southern cascadia accretionary prism today 250 600 m of sediment is subducting with the gorda plate and 1500 3200 m is accreting to the northern california margin faults imaged west and east of the deformation front show mixed structural vergence a north south trending 20 km long portion of the central margin is landward vergent for the outer 6 8 km of the toe of the prism this region of landward vergence exhibits no frontal thrust is unusually steep and narrow and is likely caused by a seaward dipping backstop close to the deformation front the lack of margin wide preferred seaward vergence and wedge taper analysis suggests the prism has low basal shear stress the three southern lines image wedge shaped fragments of oceanic crust 1 1 7 3 km in width and 250 700 m thick near the deformation front these wedges suggest shortening and thickening of the upper oceanic crust discontinuities in the seafloor west of the prism provide evidence for mass wasting in the form of slump blocks and debris fans the southernmost profile extends 75 km west of the prism imaging numerous faults that offset both the gorda basin oceanic crust and overlying sediments these high angle faults bounding basement highs are interpreted as strike slip faults reactivating structures originally formed at the spreading ridge northeast or northwest trending strike slip faults within the basin are consistent with published focal mechanism solutions and are likely caused by north south gorda pacific plate convergence

fluid flow in accretionary prisms evidence for focused time variable discharge accretionary prisms are wedges of saturated sediment that are subject to intense deformation as a result of lithosphere convergence compressive stress and rapid burial of the accreted deposits result in sediment compaction and mineral dehydration these latter processes in conjunction with fermentation or thermal maturation of entrained organic matter yield hydrocarbon bearing pore fluids that are expelled from the prism regional fluxes of heat and a number of dissolved chemical species most notably carbon are controlled by the advective expulsion of the pore waters numerical modeling observation and monitoring of flow patterns and rates and recent in situ hydrogeological tests quantify the conditions that control rates of fluid flow dispersed intergranular flow 10 8 to 10 11 m s controlled by the vertical permeability of the prism 10 14 to 10 20 m is limited by low permeability lithologies and seems not to vary much from margin to margin focused flow 10 1 to 10 8 m s above the decollement is controlled by fault zones or sedimentary intrusions diapiric structures at low fluid pressures fault zone permeability may be similar to that of adjacent wall rock but as fluid pressure increases from hydrostatic 0 to near lithostatic levels 1 0 fault zones dilate and fracture permeability increases by 2 4 orders of magnitude 10 10 to 10 16 m similarly mud volcanoes and diapirs provide high permeability fluid conduits to the sediment water interface as a result faults and intrusions become primary flow paths and support surface vents at which syntectonic deposits carbonate and gas hydrates accumulate and chemosynthetic organisms cluster models of thermal and chemical anomalies and epigenetic deposits indicate that flow is temporally variable that conclusion has been quantified by extended 1 10 months seafloor and borehole experiments that measured temperature anomalies associated with flow events on the cascadia prism flow estimated velocity 3 10 5 m s 950 m yr confined to a thrust fault that cuts upward through the prism has brought thermogenic hydrocarbons from depths 1 5 km to the surface within the last 400 years

analysis of bubble plume distributions to evaluate methane hydrate decomposition on the continental slope cascadia margin sediments contain a rich reservoir of carbon derived both from terrestrial input and sea surface productivity a portion of this carbon exists as solid gas hydrate within sediment pore spaces which previous studies have shown to be a methane reservoir of substantial size on both the vancouver island and oregon portions of the cascadia margin multichannel seismic reflection profiles on the cascadia margin show the widespread presence of bottom simulating reflectors bsrs within the sediment column indicating the gas hydrate reservoir extends from the deformation front at 3000 m depth to the upper limit of gas hydrate stability near 500 m water depth in this study we compile an inventory of methane bubble plume sites on the cascadia margin identified in investigations carried out for a range of interdisciplinary goals that also include sites volunteered by commercial fishermen high plume density anomalies are associated with both the continental shelf 180 m and the depth of the upper limit of methane hydrate stability depth mhsd that occurs near 500 m in the ne pacific the observed anomalies on the cascadia slope may be due to the warming of seawater at intermediate depths suggesting that modern climate change has begun to destabilize the climate sensitive hydrate reservoir within the cascadia margin sediments reanalysis of similar plume images on the north american atlantic slope suggests a lack of correlation between observed plume depths and the mhsd for much of the latitudinal range

estimates of geothermal gradients and heat flow from bsrs along the western continental margin of india geothermal gradients and heat flow were estimated from the position of bottom simulating reflectors bsrs identified on seismic reflection sections from the western continental margin of india wcmi the estimated geothermal gradients along the wcmi range between 35 to 65 c km and heat flow varies from 50 to 130 mw m 2 the geothermal gradient structure of wcmi inferred from the bsrs shows a landward decrease in heat flow coupled with a high heat flow zone in the north and relatively low heat flow zone at the southern end a decrease of sediment thickness proximal to the ocean continent boundary could explain these high heat flow distribution trends west of the laccadive ridge the bombay saurashtra offshore region emerges as a zone of high heat flow and its proximity to the deccan volcanic activity at 65 ma needs further study

thermal effects of sediment thickening and fluid expulsion in accretionary prisms model and parameter analysis we investigate the thermal consequences of sediment thickening and fluid expulsion in subduction zone accretionary prisms using a model where the rate of fluid expulsion in a uniformly thickening wedge is obtained analytically and the fluid flow and temperature fields are computed numerically variations landward across the wedge in the porosity depth function are included in the model the most important contribution to the thermal regime arises from the thickening of the wedge which can reduce the heat flow at the seafloor relative to the deep lithospheric heat flow by up to a few tens of percent this effect is countered by an opposite but lesser contribution of the heat advected upward by the fluid expelled from the reconsolidating sediments the total thermal perturbation is sensitive to the prism taper angle the incoming sediment thickness and the convergence velocity anomalously low seismic velocities observed in a zone several tens of kilometers wide near the toe of some accretionary prisms suggest that there is a delay between the initial thickening of the sediment section and reconsolidation toward equilibrium porosity at depth the thermal consequences of retarded fluid expulsion are significant and result in locally depressed seafloor heat flow in this region for the full observed range of accretion parameters the rates of fluid expulsion are not sufficient to cause a detectable depth variation in heat flow on a depth scale of less than 1 km

thermal constraints on the seismogenic portion of the southwestern japan subduction thrust for coastal cities an important factor in earthquake hazard from subduction zone earthquakes is the landward extent of the seismogenic portion of the subduction thrust fault in this study we test the hypothesis that the maximum downdip extent is defined by a critical temperature we have developed a transient thermal model for the nankai subduction zone of southwest japan to allow comparison of the thermally estimated downdip extent of the seismogenic zone with that from 1 seismicity and tsunami data for two great subduction earthquakes 2 the coseismic faulting extent of these events estimated from geodetic deformation data and 3 the interseismic locked zone determined from interseismic geodetic data the nankai margin has extensive heat flow and heat production data to control thermal models and thus crustal temperatures it has earthquake tsunami and geodetic data that constrain the coseismic rupture portion of the subduction thrust fault for past great earthquakes and the portion of the thrust fault that is locked and storing interseismic elastic strain on the nankaido margin off shikoku island the thermal model indicates that a temperature of 350 c taken to be the limit for seismic initiation from laboratory and field data is reached on the subduction thrust fault 150 km from the trench a transition zone into which rupture may extend with decreasing offset taken to be 450 c extends an additional 45 km downdip the thermal model results are in excellent agreement with the maximum downdip extent of coseismic displacement for the 1946 nankaido ms 8 2 earthquake off shikoku island and with the downdip extent of the present locked zone in the region of the 1944 tonankai ms 8 2 earthquake to the northeast the subduction angle is much steeper and the thermal models indicate a narrower downdip seismogenic extent the seismogenic locked zone from earthquake and geodetic data is also narrower thus our analysis of the southwest japan margin indicates that all three constraints on the downdip extent of the seismogenic zone thermal coseismic and interseismic geodetic data are in general agreement the study also supports the hypothesis that the seismogenic portion of subduction thrust faults is limited primarily by temperature the thermal control implies that subduction thrust faults with shallow dip have wider seismogenic zones compared to those with steep dip the subducting plate age and thus heat flow and the thickness of the insulating sediments on the incoming plate are also very important to the thermal regime and thus to the seismogenic width the relation of the maximum seismic rupture area to the interseismic locked zone is particularly important for earthquake hazard estimation on subduction margins such as cascadia where there have been no great historical events

gas hydrate distribution and volume in canada gas hydrate a solid form of natural gas and water is inferred to widely occur in canadian polar and continental shelf regions and in sediment of outer continental margins although direct indications of hydrate are few and widely separated conditions potentially favorable for gas hydrate formation and stability especially low to moderate temperatures under permafrost or the deep sea combined with favorable geological conditions for gas generation and storage cover vast areas and indicate an immense potential for natural hydrocarbon gas in the upper 2 km of many canadian sedimentary basins we have analyzed the potential of gas hydrates for the vast continental shelves and arctic permafrost regions of canada mackenzie delta beaufort sea and arctic archipelago in the north and davis strait the labrador shelf scotian shelf and grand banks of newfoundland along the canadian atlantic margin and canadian pacific margin our conservative calculation suggests 1010 1012 m3 of gas hydrates in these regions has an associated methane gas potential estimated to be in the range of 1012 1014 m3 the volume of methane in hydrates in canada are geographically distributed in the following regions 0 24 8 7 x 1013 m3 in the mackenzie delta beaufort sea 0 19 6 2 x 1014 m3 in the arctic archipelago 1 9 7 8 x 1013 m3 on the atlantic margin and 0 32 2 4 x 1013 m3 on the pacific margin the total in situ amount of methane in hydrates of canada is estimated to be 0 44 8 1 x 1014 m3 as compared to a conventional canadian in situ hydrocarbon gas potential of approximately 0 27 x 1014 m3 this comparison suggests that gas hydrates represent a possible future assurance of north american energy supply if the gas can be recovered and separated from the hydrate form

identification of influential parameters through sensitivity analysis of the tough hydrate model using lh oat sampling abstract the over parameterisation as a common issue in numerical models may be unavoidable in the tough hydrate t h model therefore a global sensitivity analysis of the influence of main or uneasily measured 26 input parameters on five model outputs i e cumulative gas production gas production rate hydrate saturation gas saturation and aqueous saturation under depressurization and brine inhibitor injection was performed using the lh oat sampling method the 26 parameters were classified as very important important slightly important and unimportant by analyzing their sensitivity indices at different sampling times and sampling positions in the modeling area using the tspac segmentation package the sensitivity of an input parameter to a model output was dependent on sampling time sampling position and the identity of the model output in question cumulative gas production and gas production rate were governed by four permeability related parameters parameter n and residual aqueous saturation in the relative permeability saturation model permeability reduction exponent and critical mobile phase saturation and four inhibitor related parameters inhibitor density the specific enthalpy of the dissolution of the inhibitor in water reference inhibitor mole fraction in the aqueous phase and inhibitor induced reference temperature depression hydrate saturation was governed by two hydrate dissociation related parameters temperature dependent hydrate specific heat and hydrate density and two parameters related to the flow of released fluids in the pore network permeability and pore structure expansion gas saturation was governed by several types of input parameters six parameters related to the flow of pore gas parameter n and residual aqueous saturation in the relative permeability saturation model critical mobile phase saturation intrinsic permeability permeability reduction exponent and pore compressibility three inhibitor related parameters the specific enthalpy of the dissolution of the inhibitor in water reference inhibitor mole fraction in the aqueous phase and inhibitor induced reference temperature depression and two hydrate dissociation related parameters temperature dependent hydrate density polynomial and temperature dependent hydrate specific heat polynomial finally aqueous saturation was governed by five permeability related parameters parameter and residual aqueous saturation in the saturation capillary pressure function model pore expansion grain specific heat and the tortuosity factor model overall the most influential parameter for all five model outputs was the parameter n in the relative permeability saturation function model

sources of methane for marine gas hydrate inferences from a comparison of observations and numerical models the supply of methane for marine gas hydrate is commonly attributed to local conversion of organic material within the zone of stability or to migration of methane bearing fluids from a deeper source area however the relative importance of these methane sources is not well known we use numerical models for the formation of gas hydrate to assess the influence of methane supply on observable features of hydrate occurrences model predictions for the pore fluid chlorinity and the volumes of hydrate and free gas are compared with observations from the blake ridge and cascadia margin we find that the observations from the blake ridge can be explained with a deep methane source if a saturated fluid rises with a velocity of 0 26 mm yr 1 an equally good fit to the observations is obtained by combining in situ production of methane with an upward fluid velocity of 0 08 mm yr 1 which is compatible with estimates of fluid velocity at site 997 observations from the cascadia margin can be explained with a deep methane source if the upward fluid velocity is 0 42 mm yr 1 the predicted volume of hydrate 2 of the pore space is much smaller than some previous estimates and a layer of free gas 1 of the pore space develops below the stability zone allowing for in situ production at the cascadia margin lowers the upward fluid velocity 0 35 mm yr 1 needed to explain the pore water chlorinity

fluid flow and origin of a carbonate mound offshore vancouver island seismic and heat flow constraints a 1 5 km long 1 km wide and 70 80 m high carbonate mound was identified on the mid slope region of the subduction accretionary sedimentary prism offshore vancouver island 3 5 km west of ocean drilling program odp site 889 and integrated ocean drilling program iodp site u1327 seabed video images show the presence of seafloor carbonate as well as chemosynthetic communities a high resolution single channel seismic survey with close line spacing recording coherent reflectivity down to about 400 m beneath the seafloor provided acoustic images of this mound and of the gas hydrate bottom simulating reflector bsr beneath it the mound is interpreted to have developed as a structural topographic high in the hanging wall of a large reverse fault formed at the base of the current seaward slope the fault zone provides pathways for fluids including gas to migrate to the seafloor where diagenetic carbonate forms and cements the near surface sediments to examine the thermal effect of possible upward fluid flow beneath the mound heat flow at the mound and in the neighbouring region was calculated from the depth of the bsr below the seafloor these data were combined with heat flow calculated over a broader region from previous multi channel seismic data heat flow within the flattest portion of the surrounding 4 km by 8 km region averages 74 mw m2 taking this value to represent the regional or background heat flow a simple 2d analytical method was used to calculate theoretical heat flow variations due to topography across the mound most of the variability is explained by topographic effects including a local 6 mw m2 negative anomaly over the central mound and a large 20 mw m2 positive anomaly over the mound steep side slope however just south of the mound there is a 6 7 mw m2 positive anomaly in a 2 km long band that has predominantly flat seafloor most of this anomaly is probably unrelated to topographic effects but rather likely due to warm upward fluid flow along faults or fracture zones

accretion and recent deformation of sediments along the northern cascadia subduction zone new multichannel seismic reflection profiles acoustic imagery and swath bathymetric data from the vancouver island continental margin reveal the surface morphology cross sectional geometry and deformation style of this part of the cascadia accretionary prism the incoming 3 to 5 km thick sediment section fails most commonly along landward dipping thrusts that penetrate to or very close to the top of the underlying oceanic crust the faults are listric with approximately circular cross sections and hanging wall sediments are back tilted in a planar manner high near lithostatic pore pressures in the deeper part of the section are inferred from the flattening of the faults near the decollement and from the long distances over which the decollement has propagated seaward the faults dip more steeply at 40 45 near the surface where pleistocene turbidites are displaced similar folds have developed where the turbidites are particularly thick 4 km and near hydrostatic pore pressures are inferred to occur in the turbidite part of the section an increase in pore pressure with depth is expected as a consequence of sedimentation and tectonic loading particularly because the deeper hemipelagic sediments are probably far less permeable than are the overlying turbidites the top of the downgoing juan de fuca plate which defines the base of the accreted sedimentary wedge beneath the margin is well imaged in the new seismic reflection profiles and is further defined by land reflection data seismic refraction and earthquake hypocenters the landward dipping continental backstop to the prism is also well located if these constraints are used for the geometry of the accretionary prism a general balance is found between the present prism volume and the estimated total sediment supply over the approximately 42 m y accretion history this result along with the observation that deformation front faults penetrate most or all of the sediment section suggests that little or no sediment has been subducted since initiation of the present phase of subduction in the eocene application of the critically tapered wedge model of davis and others to the observed prism geometry suggests that high average pore pressures occur in the prism particularly the seawardmost part beneath the lower slope beneath the continental shelf the dip of the subducting slab increases to greater than 11 a dip that according to the critically tapered wedge model is too steep to allow upward growth of an accretionary prism this provides a simple explanation for ongoing subsidence of the tofino sedimentary basin over the older part of the accretionary complex to the southeast the juan de fuca plate dips much more gently beneath the olympic peninsula of northern washington allowing the accretionary prism to grow upward well above sea level to form the olympic mountains

a model for the diffusive growth of hydrate saturation anomalies in layered sediments 1 sediment particles affect the phase behavior of gas hydrates both by increasing the surface energy where pore geometry forces hydrate crystals to attain high curvatures and through wetting interactions that cause aqueous films to coat particle surfaces these effects produce only slight changes to the gas solubility through most of the hydrate stability zone so the particle size has only a modest influence on the rate of hydrate accumulation when the sediments are homogeneous in hydrate reservoirs however discontinuous changes in sediment properties are common and such stratigraphic boundaries often coincide with hydrate anomalies these anomalies are a natural consequence of variations in subsurface sediment properties by accounting for sediment hydrate interactions i show how compositional diffusion supplies the growth of hydrate spikes in coarse grained sediments immediately adjacent to hydrate free regions hfrs in more fine grained sediments where the solubility is slightly elevated over timescales comparable with milankovitch cycles hydrate spikes are typically less than a meter in width and contain essentially all of the hydrate that would have otherwise occupied the much larger adjacent hfr if sediment heterogeneities were absent hydrate can form in the more fine grained sediments only once the spike achieves a sufficiently high saturation level often 90 of pore volume that the solubility is continuous across the stratigraphic boundary the wetting interactions that stabilize much of the residual liquid when hydrate forms an interconnected skeleton spanning many pore diameters can also partially unload sediment particle contacts and lead to the growth of segregated hydrate nodules and lenses

case for very low coupling stress on the cascadia ssubduction fault a fundamental problem in plate tectonics is the shear strength of major plate boundary faults this translates to the question whether the generally observed small earthquake stress drops of 3 10 mpa on major faults release most of the accumulated stress or only a small fraction of it there is strong evidence that the san andreas fault a major transform plate boundary is weak 20 mpa shear resistance it is not yet clear whether subduction thrust faults are also weak we present two types of evidence from the northern cascadia subduction zone that indicate very low coupling shear stress on that plate interface and hence very low strength of the subduction thrust fault comparable to that estimated for the san andreas fault first the well defined surface heat flow and heat generation allow negligible frictional heating on the plate interface the average shear stress on the fault must thus be very low over a time scale of a few million years second focal mechanism solutions for small crustal earthquakes in the southern vancouver island area indicate that the horizontal stress in the direction of plate convergence has a similar magnitude to the vertical stress this inferred stress state requires the present tectonic stress coupled across the subduction thrust fault to be very low one explanation for the weakness of the fault is the presence of near lithostatic pore fluid pressure in the region of the fault zone for which there is independent evidence the conclusion of a weak subduction thrust fault does not conflict with geodetic observations of contemporary surface deformation which indicate that the fault is currently locked accumulating strain energy toward a future great earthquake the surface deformation responds to the small 20 mpa temporal changes of the stress field associated with the subduction earthquake cycle this transient stress is superimposed on the larger background regional stress field in which the maximum compression is parallel to the margin the weakness of the cascadia subduction thrust fault and the unusual stress state of the forearc region have important implications for earthquake hazards for example a subduction earthquake may induce large strike slip earthquakes in the forearc that affect a large area

interseismic uplift rates for western oregon and along strike variation in locking on the cascadia subduction zone 1 we quantify the spatial pattern of uplift rate in western oregon and northernmost california using tidal and leveling records to better understand the pattern of interseismic locking on the cascadia subduction zone we extend relative sea level time series of the six primary noaa tide gauges to include all observations from 1925 to 2006 previously unidentified bench mark instability biases portions of tidal records by 1 6 mm a 1 before correction we determine precise relative uplift rates at the six tidal sites with an adjustment that includes rates of differenced time series as additional constraints our analysis of national geodetic survey leveling data between tide gauges corrects errors in 1941 leveling and 184 secondary ties double the number of highest quality uplift rate estimates relative uplift rates from leveling are adjusted to the tidal rates accounting for uncertainties in both data types tidal and leveling uplift rates agree within error for all but one of the coastal segments where we infer systematic leveling error affects the 1988 line uplift rates are made absolute using an interval and location specific geocentric sea level rise rate of 2 3 0 2 mm a 1 total propagated one sigma errors for the absolute uplift rates of bench marks are 0 4 mm a 1 along strike changes in uplift rate near 45 n and 42 8 n require two distinct changes in locking depth as inferred from elastic dislocation modeling the along strike changes in locking on this portion of the cascadia subduction zone interface correlate to the western and southern extent of the mafic siletzia block in the fore arc

fluid venting in the eastern aleutian subduction zone fluid venting has been observed along 800 km of the alaska convergent margin the fluid venting sites are located near the deformation front are controlled by subsurface structures and exhibit the characteristics of cold seeps seen in other convergent margins the more important characteristics include 1 methane plumes in the lower water column with maxima above the seafloor which are traceable to the initial deformation ridges 2 prolific colonies of vent biota aligned and distributed in patches controlled by fault scarps over steepened folds or outcrops of bedding planes 3 calcium carbonate and barite precipitates at the surface and subsurface of vents and 4 carbon isotope evidence from tissue and skeletal hard parts of biota as well as from carbonate precipitates that vents expel either methane or sulfide dominated fluids a biogeochemical approach toward estimating fluid flow rates from individual vents based on oxygen flux measurements and vent fluid analysis indicates a mean value of 5 5 0 7 l m 2 d 1 for tectonics induced water flow wallmann et al 1997b a geophysical estimate of dewatering from the same area von huene et al 1997 based on sediment porosity reduction shows a fluid loss of 0 02 l m 2 d 1 for a 5 5 km wide converged segment near the deformation front our video guided surveys have documented vent biota across a minimum of 0 1 of the area of the convergent segment off kodiak island hence an average rate of 0 006 l m 2 d 1 is estimated from the biogeochemical approach the two estimates for tectonics induced water flow from the accretionary prism are in surprisingly good agreement

melange and its seismogenic roof decollement a plate boundary fault rock in the subduction zone an example from the shimanto belt japan 1 the mugi melange located in western shikoku of the shimanto belt shows systematic y p deformation fabrics formed by microshear and pressure solution that penetrate throughout the melange pile magnetic susceptibility ellipsoids obtained from the anisotropy of magnetic susceptibility ams are highly oblate maximum and minimum axes of the ellipsoids are consistent with the shear orientation of the melange and the mean pole of p surfaces respectively this agreement suggests that the mugi melange was formed as a result of underthrusting of trench filling sediment vitrinite reflectance ranges from 2 52 to 3 08 which corresponds to a maximum paleotemperature of 180 200 c pseudotachylyte evidence of a seismogenic slip was found in the upper boundary roof fault of the mugi melange however there is not a thermal gap between the melange and the overlying coherent piles and the temperature from vitrinite reflectance gradually rises downward from the coherent piles to the melange beyond the boundary fault which suggests that paleoisotherms parallel the boundary fault orientation the isotherms in the seismogenic zone are estimated as subparallel to the plate boundary decollement therefore the setting of the cataclastic boundary fault which includes pseudotachylyte appears to be a major plate boundary thrust or a subhorizontal splay fault a probable geologic setting that accounts for the mugi melange and the seismogenic roof fault is partitioning of the slip along the plate boundary fault in space and time interseismic slip in the melange and seismic slip along the roof fault

bayesian inversion of marine controlled source electromagnetic data offshore vancouver island canada this paper applies nonlinear bayesian inversion to marine controlled source electromagnetic csem data collected near two sites of the integrated ocean drilling program iodp expedition 311 on the northern cascadia margin to investigate subseafloor resistivity structure related to gas hydrate deposits and cold vents the cascadia margin off the west coast of vancouver island canada has a large accretionary prism where sediments are under pressure due to convergent plate boundary tectonics gas hydrate deposits and cold vent structures have previously been investigated by various geophysical methods and seabed drilling here we invert time domain csem data collected at sites u1328 and u1329 of iodp expedition 311 using bayesian methods to derive subsurface resistivity model parameters and uncertainties the bayesian information criterion is applied to determine the amount of structure number of layers in a depth dependent model that can be resolved by the data the parameter space is sampled with the metropolis hastings algorithm in principal component space utilizing parallel tempering to ensure wider and efficient sampling and convergence nonlinear inversion allows analysis of uncertain acquisition parameters such as time delays between receiver and transmitter clocks as well as input electrical current amplitude marginalizing over these instrument parameters in the inversion accounts for their contribution to the geophysical model uncertainties one dimensional inversion of time domain csem data collected at measurement sites along a survey line allows interpretation of the subsurface resistivity structure the data sets can be generally explained by models with 1 to 3 layers inversion results at u1329 at the landward edge of the gas hydrate stability zone indicate a sediment unconformity as well as potential cold vents which were previously unknown the resistivities generally increase upslope due to sediment erosion along the slope inversion results at u1328 on the middle slope suggest several vent systems close to bullseye vent in agreement with ongoing interdisciplinary observations

high resolution deep towed multichannel seismic survey of deep sea gas hydrates off western canada a multichannel seismic survey was carried out using the high resolution deep towed acoustics geophysics system dtags to image the structure of deep sea gas hydrates on the continental slope off vancouver island and to determine the velocity profile of the hydrated sediments the high frequency dtags data provide the means to estimate the frequency response of the bottom simulating reflector bsr that defines the base of the hydrate stability field in these sediments over a broad frequency band from 15 to 650 hz the dtags sections resolved fine scale layering as thin as a few meters within the hydrated zone and below the bsr and they revealed small scale faults and vertically oriented zones of very low acoustic reflectivity that may represent channels for upward migration of fluids or gas interval velocities determined from the dtags data indicate uniformly low values of about 1500 m s to depths of 100 m below sea floor mbsf increasing to about 1850 m s at the bsr 250 mbsf the reflection from

orogens and slabs vs their direction of subduction abstract subduction zones appear primarily controlled by the polarity of their direction i e w directed or e to nne directed probably due to the westward drift of the lithosphere relative to the asthenosphere the decollement planes behave differently in the two end members in the w directed subduction zone the decollement of the plate to the east is warped and subducted whereas in the e to nne directed it is ramping upward at the surface there are w directed subduction zones that work also in absence of active convergence like the carpathians or the apennines w directed subduction zones have shorter life 30 40 ma than e or ne directed subduction zones even longer than 100 ma the different decollements in the two end members of subduction should control different ptt paths and therefore generate variable metamorphic assemblages in the associated accretionary wedges and orogens these asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast eastward migrating structural wave along w directed subduction zones whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones the magmatic pair calc alkaline and alkaline tholeiitic volcanic products of the island arc and the back arc basin characterise the w directed subduction zones magmatic rocks associated with e or ne directed subduction zones have higher abundances of incompatible elements and mainly consist of calc alkaline shoshonitic suites with large volumes of batholithic intrusions and porphyry copper ore deposits the subduction zones surrounding the adriatic plate in the central mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab the western margin of the adriatic plate contemporaneously overridden and underthrust europe toward the west to generate respectively the alps and the apennines while the eastern margin subducted under the dinarides hellenides these belts confirm the characters of the end members of subduction zones as a function of their geographic polarity similarly to the pacific subduction zones

a seismic study of methane hydrate marine bottom simulating reflectors multichannel seismic reflection data have been analyzed from an area of clear bottom simulating reflectors bsrs on the northern cascadia subduction zone margin off vancouver island the reflector at a depth of about 300 m subbottom is interpreted to represent the base of a layer of methane hydrate or clathrate the shallow water depth of 1300 m and the 3600 m long hydrophone array have allowed bsr amplitude versus offset and high resolution velocity analysis as well as modelling of vertical incidence data the results of all three types of analysis can be best explained by a 10 to 30 m thick high velocity layer located immediately above the bsr about 300 m below the seafloor having a sharp base and transitional top in the layer about one third of the sediment pore spaces must be filled with hydrate ice there is no seismically detectable free gas beneath the bsrs these results put important constraints on models for the distribution and formation of bsr hydrate

the northern cascadia subduction zone at vancouver island seismic structure and tectonic history the structure and tertiary tectonic history of the northern cascadia subduction zone have been delineated by a series of new multichannel seismic lines acquired across the continental shelf to the

heat flow variations from bottom simulating reflectors on the cascadia margin abstract a bottom simulating reflector bsr representing the base of the gas hydrate stability field was observed over a closely spaced grid of seismic lines on the continental slope of the northern cascadia margin a simple conductive model was used to calculate heat flow from the depth of the bsr a regional trend was observed in which heat flow decreased landward across the margin from an average of 80 mw m2 at a distance of 15 km from the deformation front to 65 mw m2 at a distance of 40 km this trend reflects the processes of tectonic thickening of accretionary wedge sediments and subduction of the juan de fuca plate and contrasts with the near constant heat flow across the oregon margin where the dip of the subducting plate and thus the rate of sediment thickening are slightly smaller consistent local variations were also observed notably low heat flow values over prominent topographic highs and high heat flow values over the flanks of the topographic highs at some localities over a horizontal distance of 1 2 km heat flow increased by as much as 50 from typical values of 65 to 100 mw m2 much of this variation may be due to focusing and defocusing effects of the topography alone and indicates the importance of carrying out topographic corrections to heat flow in regions of significant relief such as the continental slope however a component of the local heat flow patterns may be due to dynamic effects including the displacement of isotherms by thrust faulting and the upward migration of warm fluids along permeability channels such as faults

large scale liquefaction and fluidization in the cap chat melange quebec appalachians the cap chat melange crops out discontinuously for 200 km along the south shore of the st lawrence river in the gaspe peninsula it is located just south of logan s line the northern limit of the humber zone with the taconian foreland basin this melange is composed of dismembered rocks of the adjacent formations in particular the lower ordovician riviere ouelle and middle ordovician tourelle formations with lesser contributions by the middle ordovician des landes and the cambrian orignal formations blocks in the melange vary in size from a few centimetres to several kilometres with well preserved internal stratigraphy in the larger blocks the distribution of blocks is not uniform and the composition of the surrounding matrix changes with corresponding changes in block composition tectonic processes mostly extensional and compressional faulting are responsible for some of the chaotic aspects of the melange however the main mechanism was as follows i large scale liquefaction of the mudstone rich riviere ouelle formation ii sinking with consequent dismembering of the tourelle formation into this underlying weakened riviere ouelle formation and iii fluidization of the lowermost sand beds of the tourelle formation resulting in abundant sandstone sills and dikes in the riviere ouelle formation

crustal anisotropy in the forearc of the northern cascadia subduction zone british columbia summary this paper aims to identify sources and variations of crustal anisotropy from shear wave splitting measurements in the forearc of the northern cascadia subduction zone of southwest british columbia over 20 permanent stations and 15 temporary stations were available for shear wave splitting analysis on 4500 event station pairs for local crustal earthquakes results from 1100 useable shear wave splitting measurements show spatial variations in fast directions with margin parallel fast directions at most stations and margin perpendicular fast directions at stations in the northeast of the region crustal anisotropy is often attributed to stress and has been interpreted as the fast direction being related to the orientation of the maximum horizontal compressive stress however studies have also shown anisotropy can be complicated by crustal structure southwest british columbia is a complex region of crustal deformation and some of the stations are located near large ancient faults to use seismic anisotropy as a stress indicator requires identifying which stations are influenced by stress and which by structure we determine the source of anisotropy at each station by comparing fast directions from shear wave splitting results to the maximum horizontal compressive stress orientation determined from earthquake focal mechanism inversion most stations show agreement between the fast direction and the maximum horizontal compressive stress this suggests that anisotropy is related to stress aligned fluid filled microcracks based on extensive dilatancy anisotropy these stations are further analysed for temporal variations to lay groundwork for monitoring temporal changes in the stress over extended time periods determining the sources of variability in anisotropy can lead to a better understanding of the crustal structure and stress and in the future may be used as a monitoring and mapping tool

implications for focused fluid transport at the northern cascadia accretionary prism from a correlation between bsr occurrence a d near sea floor reflectivity anomalies imaged in a multi frequency seismic data set a high resolution seismic survey was carried out at the accretionary prism on the continental slope off vancouver island canada two gi gun data sets with different source frequency ranges of 50 150 and 100 500 hz were combined with 4 khz narrow beam echosounding data parasound the data allow spatial correlation between a gas hydrate bottom simulating reflector bsr and distinct areas of high near sea floor reflectivity an integrated interpretation of the multi frequency data set provides insight into the regional distribution of tectonically induced fluid migration and gas hydrate formation in the vicinity of odp leg 146 sites 889 and 890 the bsr at the base of the gas hydrate stability field is observed within accreted and deformed sediments but appears to be absent within bedded slope basin deposits it is suggested that these basin deposits inhibit vertical fluid flow and prevent the formation of a bsr whereas the hydraulic conductivity of the accreted sediments is sufficiently high to allow for pervasive gas migration an elevation of the bsr beneath the flanks of a topographic high is interpreted as an indicator for local upflow of warm fluids along permeable pathways within outcropping accreted sediments parasound data reveal discontinuous zones of high reflectivity at or directly beneath the sea floor which may indicate local cementation of surface sediments in combination with gi gun data the occurrence of these reflective areas can be related to the location of slope sedimentary basins acting as hydraulic seals it is proposed that the seals sometimes fail along faults extending beneath the bsr leading to focused upflow of methane bearing fluid and the formation of carbonate pavements at the sea floor

sediment compaction and fluid migration in the makran accretionary prism the makran continental margin in the gulf of oman forms the seaward extremity of an accretionary sediment prism which extends several hundred kilometers inland a recently acquired multichannel seismic reflection profile shot across the margin imaged the structure of the prism in greater detail than was previously possible and allowed us to investigate the relationship between deformation and pore fluid motion in the region velocity analyses of the common midpoint gathers reveal a marked change in velocity structure at the toe of the accretionary wedge as seen in previous sonobuoy wide angle data accreted sediments show significantly higher vertical velocity gradients than those of sediments entering the prism this change is interpreted as due to porosity reduction as pore fluids are squeezed out of the compacting sediment a prominent bottom simulating reflector appears 500 800 m beneath the sea bed several lines of evidence suggest that this reflector represents the base of a gas hydrate zone underlain by widespread free gas which may be exsolved from pore water migrating from deep within the sediment pile up permeable fault planes imaged in the profile the hydrate reflector appears to shallow in the region of some faults suggesting a temperature anomaly due to the presence of warm pore fluids a heat flow profile derived from the depth of the hydrate reflector does not show the expected landward decrease as the sediment pile thickens simple thermal modeling suggests that advective heat flow within the prism may explain this anomaly the inferred presence of overpressured pore fluids in the makran suggests that accreted sediments have a low permeability the seismic evidence suggests a two stage compaction process with rapid initial dewatering through intergranular permeability as sediment enters the prism followed by a buildup of pore pressure as the permeability decreases and fluid migration is restricted to fault zones

hanging wall deformation of a seismogenic megasplay fault in an accretionary prism the nobeoka thrust in southwestern japan the structure and occurrence of deformation within the hanging wall of the nobeoka thrust in kyushu japan was investigated to understand the dynamic aspects of splay faulting in relation to seismic events from field observations hanging wall is suggested to have undergone four phases of deformation the first phase involved horizontal shortening as documented by folding and thrusting followed by a phase of vertical loading shown by the development of horizontal slaty cleavages pressure solution and cleavage parallel mineral vein precipitation a third phase involved shearing and deformation along cleavage restricted to near the nobeoka thrust while the fourth phase produced widespread brittle fracturing associated with the development of pseudotachylyte bearing faults and tension crack filling veins high angle to cleavage these four phases can be explained as follows during the inter seismic period an extensionally stable taper was maintained in the inner wedge of the accretionary prism by dominant vertical loading 1 in combination with a lesser amount of horizontal compression 2 related to the locking of the mega thrust elastic strain energy in the hanging wall of the inner wedge was co seismically released by slip on the mega thrust and horizontal shortening in the outer wedge associated with dynamic ductile weakening of the fault plane this sudden release of elastic strain caused brittle fracturing with 1 at a high angle to the shear surface of the nobeoka thrust most of the displacement resulting from deformation of the footwall

methane sources and production in the northern cascadia margin gas hydrate system abstract the oceanographic and tectonic conditions of accretionary margins are well suited for several potential processes governing methane generation storage and release to identify the relevant methane evolution pathways in the northern cascadia accretionary margin a four site transect was drilled during integrated ocean drilling program expedition 311 the 13c values of methane range from a minimum value of 82 2 on an uplifted ridge of accreted sediment near the deformation front site u1326 1829 mbsl meters below sea level to a maximum value of 39 5 at the most landward location within an area of steep canyons near the shelf edge site u1329 946 mbsl an interpretation based solely on methane isotope values might conclude the 13c enrichment of methane indicates a transition from microbially to thermogenically sourced methane however the co existing co2 exhibits a similar trend of 13c enrichment along the transect with values ranging from 22 5 to 25 7 the magnitude of the carbon isotope separation between methane and co2 ec 63 8 5 8 is consistent with isotope fractionation during microbially mediated carbonate reduction these results in conjunction with a transect wide gaseous hydrocarbon content composed of 99 8 by volume methane and uniform dch4 values 172 8 that are distinct from thermogenic methane at a seep located 60 km from the expedition 311 transect suggest microbial co2 reduction is the predominant methane source at all investigated sites the magnitude of the intra site downhole 13c enrichment of co2 within the accreted ridge site u1326 and a slope basin nearest the deformation front site u1325 2195 mbsl is 5 at the mid slope site site u1327 1304 mbsl the downhole 13c enrichment of the co2 is 25 and increases to 40 at the near shelf edge site u1329 this isotope fractionation pattern is indicative of more extensive diagenetic alteration at sites with greater 13c enrichment the magnitude of the 13c enrichment of co2 correlates with decreasing sedimentation rates and a diminishing occurrence of stratigraphic gas hydrate we suggest the decreasing sedimentation rates increase the exposure time of sedimentary organic matter to aerobic and anaerobic degradation during burial thereby reducing the availability of metabolizable organic matter available for methane production this process is reflected in the occurrence and distribution of gas hydrate within the northern cascadia margin accretionary prism our observations are relevant for evaluating methane production and the occurrence of stratigraphic gas hydrate within other convergent margins

contrasts in physical properties between the hanging wall and footwall of an exhumed seismogenic megasplay fault in a subduction zone an example from the nobeoka thrust drilling project 1 we examined the physical properties of an exhumed and fossilized subduction zone megasplay fault by analyzing geophysical logging data obtained by the nobeoka thrust drilling project which provide a high resolution transect of properties across the main fault zone the footwall cataclasite exhibits higher averages of neutron porosity 7 6 and lower values of electric resistivity 232 m compared to the hanging wall phyllite 4 8 453 m this clear contrast between the hanging wall and footwall may account for the difference in maximum burial and structural variation despite the contrast observed between the hanging wall and footwall in macroscopic scale the resistivity and porosity data from both the hanging wall and footwall can be fit with a single curve using archie s law suggesting the similarities in microstructures and mineralogy in this low porosity range above the main fault core of the nobeoka thrust a brittle damage zone in the hanging wall contains pseudotachylyte as evidence of the seismogenic slip and does not follow archie s law damage zones in the hanging wall are also observed in the modern splay fault at shallow depth in the nankai trough but with much thicker width whereas the footwall damage zone is more extensive in the nobeoka thrust splay faults may exhibit strong deformation in the hanging wall in the early stage and as fault rocks get buried deeper and as displacement and physical property contrast increase across the fault the damage effect may eventually be enlarged in the footwall

first principles computation of mantle materials in crystalline and amorphous phases abstract first principles methods based on density functional theory are used extensively in the investigation of the behavior and properties of mantle materials over broad ranges of pressure temperature and composition that are relevant a review of computational results reported during the last couple of decades shows that essentially all properties including structure phase transition equation of state thermodynamics elasticity alloying conductivity defects interfaces diffusivity viscosity and melting have been calculated from first principles using mgo the second most abundant oxide of earth s mantle as a primary example and considering many other mantle materials in their crystalline and amorphous phases we have found that most properties are strongly pressure dependent sometimes varying non monotonically and anomalously with the effects of temperature being systematically suppressed with compression the overall agreement with the available experimental data is excellent it is remarkable that the early calculated results such as shear wave velocities of two key phases mgo and mgsio 3 perovskite were subsequently reproduced by experimentation covering almost the entire mantle pressure regime as covered in some detail the defect formation and migration enthalpies of key mantle materials increase with pressure the predicted trend is that partial mgo schottky defects are energetically most favorable in mg silicates but their formation enthalpies are high so the diffusion in the mantle is likely to be in the extrinsic regime preliminary results on mgo and forsterite hint that the grain boundaries can accommodate point defects including impurities and enhance diffusion rates at all pressures the structures are highly distorted in the close vicinity of the defects and at the interface with excess space recent simulations of mgo sio 2 binary and other silicate melts have found that the melt self diffusion and viscosity vary by several orders of magnitudes with pressure temperature and composition the predicted high compressibility and complex dynamical behavior can be associated with structural changes involving non bridging oxygen oxygen tri clusters si o pentahedra etc occurring on compression we envision future prospect for massively parallel distributed computing of unprecedented magnitude and scope in the study of relevant materials for earth super earth and other planets a renewed computational theme perhaps should be the first principles simulations of large systems with long runs that are necessary to explore realistic natural compositions polycrystalline phases multi component melts crystal melt interfaces trace element partitioning etc

north cascadia heat flux and fluid flow from gas hydrates modeling 3 d topographic effects the bottom simulating reflector bsr of gas hydrate is well imaged from two perpendicular seismic grids in the region of a large carbonate mound informally called cucumber ridge off vancouver island we use a new method to calculate 3 d heat flow map from the bsr depths in which we incorporate 3 d topographic corrections after calibrated by the drilling results from nearby integrated ocean drilling program site 889 and site u1327 we then estimate the associated fluid flow by relating it to the topographically corrected heat flux anomalies in the midslope region a heat flux anomaly of 1 mw m2 can be associated with an approximate focused fluid flow rate of 0 09 mm yr around cucumber ridge high rates of focused fluid flow were observed at steep slopes with values more than double the average regional diffusive fluid discharge rate of 0 56 mm yr as well in some areas of relatively flat seafloor the focused fluid flow rates still exceeded 0 5 mm yr on the seismic lines the regions of focused fluid flow were commonly associated with seismic blanking zones above the bsr and sometimes with strong reflectors below the bsr indicating that the faults fractures provide high permeability pathways for fluids to carry methane from bsr depths to the seafloor these high fluid flow regions cover mostly the western portion of our area with gas hydrate concentration estimations of 6 based on empirical correlations from hydrate ridge in south off oregon significantly higher than previously recognized values of 2 5 in the eastern portion determined from site u1327

co seismic frictional melting along an out of sequence thrust in the shimanto accretionary complex implications on the tsunamigenic potential of splay faults in modern subduction zones abstract out of sequence thrusts ost play an important role in the thickening of accretionary prisms formation of forearc basins and tsunami generation in subduction zones by detailed investigations of geological and paleogeothermal structures an ancient seismogenic ost is identified in the shimanto accretionary complex southwest japan this ost consists of a mappable en echelon fault system developed at a late stage of the accretionary process the estimated accumulated displacement of 2 5 8 5 km and formation depth of 2 5 5 5 km based on a thermal analysis indicates much higher fault activities sense of slip with a sinistral slip and the large displacement was distributed among multiple small faults each with a small offset most of the faults in the en echelon fault system recorded repeated brittle failure and one fault preserves pseudotachylyte a fault rock indicative of seismic faulting this is the first report of pseudotachylyte along an ost in an accretionary prism repeated fracturing in the same narrow shear zones each only a few millimeters thick suggests the osts perform as major shear localized zone in the accretionary prism shallow estimated formation depth and consequent low normal stress also support the hypothesis by a weak coupling along this fault zone these findings are in accord with a thrust activity in the shallow portion of the accretionary prism being associated with rupture propagation from the deep seismogenic region this ost showed high activity and repeated faulting in the shallow portion of the accretionary prism comparable to that of the submarine tsunamigenic ost in the present nankai trough

current deformation and the width of the seismogenic zone of the northern cascadia subduction thrust evidence has been obtained for the accumulation of elastic strain across the northern cascadia subduction zone that may be released in a future very large subduction thrust earthquake vertical and horizontal strain rates across the southern vancouver island region have been determined through 1 long term trends in tide gauge data 2 changes in repeated accurate leveling surveys 3 changes in repeated high accuracy gravity profiles and 4 horizontal shortening observed in repeated precise positioning surveys the outer coast is uplifting at a rate of a few millimeters per year decreasing landward and shortening is occurring across the 100 km wide coastal region at a rate of about 0 1 microstrain per year mm km 1yr 1 the results are compared with the distribution of strain accumulation predicted from elastic dislocation and viscoelastic models for a subduction thrust fault the location of the fault as used in the models is well defined by multichannel seismic reflection and other geophysical data most of the observed current deformation can be explained by interseismic strain accumulation associated with the subduction thrust of southern vancouver island and northern washington provided the locked portion is restricted to a 60 km wide band offshore beneath the continental shelf and slope this conclusion also results from modeling the coseismic subsidence on the outer coast of vancouver island about 300 years ago deduced from paleoseismicity data the unusually narrow downdip extent of the subduction thrust seismogenic zone that extends little if at all beneath the coast is a consequence of high temperatures associated with the young age of the subducted oceanic lithosphere and the thick blanket of insulating sediments the high temperatures limit brittle seismogenic behavior downdip to where the thrust fault is at a depth of less than 15 km the distance from the seismic portion of the megathrust limits the estimated ground motion at the major centers of vancouver and victoria from this source the narrow width may also limit the earthquake size however events of magnitude well over 8 are possible

weakening of deforming granitic rocks with layer development at middle crust abstract in the middle crust deformation of granitic rocks is characterized by crystal plastic deformation of quartz and cataclasis of feldspar for common granitic rocks with textures of framework feldspar and interstice filling quartz brittle strength of feldspar is a close approximation of the bulk rock strength because feldspar forms the load bearing framework with deformation these rocks tend to develop localized high strain zones the high strain zones commonly consist of quartz rich layers and feldspar rich layers we have examined how the strength of granitic rocks changes with the volume fraction of these layers by combining the flow laws developed by handy et al handy m r wissing s b streit l e 1999 frictional viscous flow in mylonite with varied bimineralic composition and its effect on lithospheric strength tectonophysics 303 175 191 our results suggest a dramatic weakening of the bulk rock strength at an early stage of the layer development we also found that formation of muscovite in the layer structures by breakdown of feldspar can result in further weakening in the layer structure the bulk rock strength with mica bearing layers is reduced by a significant amount compared with the mica absent layers our analysis suggests that layer development with syntectonic formation of mica can be a major source of weakening for granitic rocks

a mechanism for the formation of methane hydrate and seafloor bottom simulating reflectors by vertical fluid expulsion bottom simulating reflectors bsr are observed commonly at a depth of several hundred meters below the seafloor in continental margin sedimentary sections that have undergone recent tectonic consolidation or rapid accumulation they are believed to correspond to the deepest level at which methane hydrate clathrate is stable we present a model in which bsr hydrate layers are formed through the removal of methane from upward moving pore fluids as they pass into the hydrate stability field in this model most of the methane is generated below the level of hydrate stability but not at depths sufficient for significant thermogenic production the methane is primarily biogenic in origin the model requires either a mechanism to remove dissolved methane from the pore fluids or disseminated free gas carried upward with the pore fluid the model accounts for the evidence that the hydrate is concentrated in a layer at the base of the stability field for the source of the large amount of methane contained in the hydrate and for bsrs being common only in special environments strong upward fluid expulsion into the hydrate stability field does not occur in normal sediment depositional regimes so bsrs are uncommon upward fluid expulsion does occur as a result of tectonic thickening and loading in subduction zone accretionary wedges and in areas where rapid deposition results in initial undercconsolidation in these areas hydrate bsrs are common the most poorly quantified aspect of the model is the efficiency with which methane is removed and hydrate is formed as pore fluids pass into the hydrate stability field the critical boundary in the phase diagram between the fluid plus hydrate and fluid only fields is not well constrained however the amount of methane required to form the hydrate and limited data on methane concentrations in pore fluids from deep sea boreholes suggest very efficient removal of methane from rising fluid that may contain less than the amount required for free gas production in most fluid expulsion regimes the quantity of fluid moved upward to the seafloor is great enough to continually remove the excess chloride and the residue of isotope fractionation resulting from hydrate formation thus as observed in borehole data there are no large chloride or isotope anomalies remaining in the local pore fluids the differences in the concentration of methane and probably of co2 in the pore fluid above and below the base of the stability field may have a significant influence on early sediment diagenetic reactions

tidally controlled gas bubble emissions a comprehensive study using long term monitoring data from the neptune cabled observatory offshore vancouver island long term monitoring over one year revealed high temporal variability of gas emissions at a cold seep in 1250 m water depth offshore vancouver island british columbia data from the north east pacific time series underwater networked experiment observatory operated by ocean networks canada were used the site is equipped with a 260 khz imagenex sonar collecting hourly data conductivity temperature depth sensors bottom pressure recorders current meter and an ocean bottom seismograph this enables correlation of the data and analyzing trigger mechanisms and regulating criteria of gas discharge activity three periods of gas emission activity were observed a short activity phases of few hours lasting several months b alternating activity and inactivity of up to several day long phases each and c a period of several weeks of permanent activity these periods can neither be explained by oceanographic conditions nor initiated by earthquakes however we found a clear correlation of gas emission with bottom pressure changes controlled by tides gas bubbles start emanating during decreasing tidal pressure tidally induced pressure changes also influence the subbottom fluid system by shifting the methane solubility resulting in exsolution of gas during falling tides these pressure changes affect the equilibrium of forces allowing free gas in sediments to emanate into the water column at decreased hydrostatic load we propose a model for the fluid system at the seep fueled by a constant sub surface methane flux and a frequent tidally controlled discharge of gas bubbles into the ocean transferable to other gas emission sites in the world s oceans

deep sea bottom simulating reflectors calibration of the base of the hydrate stability field as used for heat flow estimates ocean drilling program and deep sea drilling project downhole data from three areas the southwestern japan nankai margin the continental slope off peru and the blake bahama outer ridge provide temperature calibrations for bottom simulating reflectors bsr that mark the base of a clathrate hydrate stability field the inferred temperatures at bsrs provide an important reference for the mapping of geothermal gradient and heat flow from subduction zone accretionary sedimentary wedges the borehole results provide information on which stability field is applicable for the bsrs and thus calibrate the heat flow estimates while an ideal calibration has not been possible the bsr temperatures at the three sites in the temperature range 25 27 c have been estimated with uncertainties of 0 7 to 2 0 c the temperatures correspond closely to the laboratory dissociation temperatures for pure water pure methane hydrate at equivalent pressures no laboratory data are available for seawater salinity and methane at equivalent pressures but extrapolation from lower pressures gives temperatures 1 2 c lower which is just significantly different the data also could be explained by the stability curve for seawater salinity and methane with about 7 co2 or with a small amount of higher hydrocarbons but most hydrate samples that have been recovered by deep sea drilling have contained almost pure methane the uncertainties in the temperature at the bsr should contribute no more than 5 error in heat flow estimates from bsr depths if the pure water methane stability field is used

a simplified analysis of parameters controlling dewatering in accretionary prisms abstract one of the many dynamic geologic processes taking place at subduction zones is the compaction driven dewatering of seafloor sediments as they are accreted to the overriding plate the rate of dewatering is equal to the divergence of the sediment matrix velocity field this rate can be estimated analytically if simplifying assumptions are made about the geometry of the prism and motion of the sediments the analytical expression depends only on the sediment accretion velocity thickness of the accreted section prism taper angle and sediment porosity distribution a sensitivity analysis of the solution shows that the fluid production distribution is relatively insensitive to the sediment porosity distribution however the solution is very sensitive to the taper angle of the wedge high angle wedges expel almost all of the incoming water within 20 km while low angle wedges may retain a significant fraction of the incoming water for 50 or more kilometers thickness of the incoming section is also important dewatering of thin accreted sections is more concentrated near the toe of the wedge analyses of transects through the northern barbados makran vancouver and nankai accretionary prisms illustrate a range of dewatering rates and spatial distributions of dewatering

receiver function study of the cascadia megathrust evidence for localized serpentinization 1 we characterize the crustal and upper mantle structure of the cascadian fore arc using receiver function analysis of data from long running observatories located along the north american margin in oregon and washington we identify coherent sv polarized phases consistent with p s conversion from the top of the subducting juan de fuca plate throughout the area west of the volcanic arc in many instances we also find evidence for deeper features likely associated with the subducted oceanic crust at many locations sh polarized energy in receiver functions is associated with the surface of the subducting plate these sh polarized phases display significant directional variation and show clear polarity reversals consistent with the presence of seismic anisotropy previously a layer of serpentinized material within the megathrust has been proposed to explain an anisotropic low velocity high poisson s ratio layer identified by receiver function analysis at corvallis oregon global seismic network station cor our new results suggest that this feature is widespread in cascadia but the specific receiver function signature varies this anisotropic supraslab layer is supported by receiver function estimates beneath seismic stations which similar to cor are 100 km from the coast and 40 km above the slab surface however the orientation of rock fabric implied by the anisotropy varies greatly from station to station modeling with synthetic seismograms supports the existence of serpentinite beneath station gnw green mountain washington closer to the coast where the slab is shallower an anisotropic layer is likely beneath some stations but the evidence is less conclusive we propose that the anisotropic signature beneath the inland stations is related to serpentinization of the mantle wedge at greater depths best explained by the presence of the polymorph antigorite hypothetically suctioned upward by slab rollback a deformed sliver of serpentinite rich rock above the slab interface would likely display the observed large spatial variation in anisotropic symmetry axis within the layer consistent with our observations

neogene sedimentary and mass wasting processes on the continental margin off south central chile inferred from dredge samples the active continental margin off south central chile 36 to 40 s is transitional between the tectonically erosive empty trench margin north of juan fernandez ridge and the accretionary trench filled margin south of the chile triple junction the small width of the presently active accretionary wedge maximum width of 25 to 50 km argues for past phases of tectonic erosion at present this sector shows indications of contemporaneous accretion subduction and underplating of sediment as well as readjustment of the slope by various mass wasting processes in this context this study aims to examine the neogene sedimentary processes on the continental margin from dredge samples recovered during r v sonne cruise so161 5 within this transitional domain using lithology sandstone petrology shale mineralogy and analysis of sedimentary structures our results yield that the principal transport of material occurs in high energy turbidity currents and debris flows via submarine canyons deeply cutting the continental slope whereas sediment on the shelf is transported by strong coast parallel bottom currents and trapped by submarine canyons cutting into the shelf a wide range of mass wasting processes including slumping debris flows evolving to low density turbidity currents and mud flows rework the slope sediments in contrast thick undisturbed sequences of mostly hemipelagic sediments accumulate in active slope basins which are largely protected from mass movements xrd analyses revealed early diagenetic lithification and overall burial depths of up to 230 mbsf suggesting a shallow subsurface cycle of sedimentation subsidence diagenesis uplift erosion and resedimentation the composition of sandstones is dominated by volcanic rock fragments of andean provenance along strike modal changes reflect a southward increase in glacially denudation and rainfall the combination of which caused more intense erosion of volcanic rocks and exposure weathering and as a result increased fluvial transport of metamorphic and plutonic rocks to the sea

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

a seismic reflection profile across the cascadia subduction zone offshore central oregon new constraints on methane distribution and crustal structure in 1989 we conducted an onshore offshore seismic experiment to image the crustal structure of the cascadia forearc in this paper we discuss the processing and interpretation of a multichannel seismic reflection profile across the continental margin that was collected as part of this effort this profile reveals several features of the forearc that were not apparent in an earlier coincident reflection profile one of the most important of these features is a very strong bottom simulating reflection bsr beneath the midslope region that is nearly continuous from water depths of about 1500 m to 600 m where it appears to crop out on the seafloor the pressure and temperature conditions at the bsr derived from our observations are remarkably consistent with the experimentally determined phase diagram for a methane hydrate seawater system over a broad range of temperatures and pressures assuming hydrostatic pressure and the temperature gradiant measured near the base of the continental slope during ocean drilling program odp leg 146 interval velocities and reflection coefficients derived from the data indicate that the bsr represents a contrast between sediment with a small amount of hydrate overlying sediment containing free gas consistent with results obtained during leg 146 although the regional distribution of the anomalously strong bsr beneath the midslope is poorly known we speculate that it may be related to apparent slope instability the data also provide constraints on the thickness and geometry of the siletz terrane which is the basement beneath the shelf and acts as the subduction zone backstop a deep reflection which might mistakenly be interpreted to be moho if coincident large aperture data were not available is interpreted to be the base of the siletz terrane a recently active strike slip fault zone that overlies the seaward edge of the siletz terrane suggests that the siletz terrane controls the location of decoupling of the subduc tion complex from the rest of the forearc

variations in temperature gradients identify active faults in the oregon accretionary prism abstract a multi channel seismic survey mcs yields high resolution images of geologic structures within the oregon accretionary prism in addition to revealing the internal geometry of the prism a prominent bottom simulating reflector bsr provides indirect estimates of temperature gradients the bsr marks the phase transition within the pore space of the sediments of small amounts of solid gas hydrate above the reflector to free gas below although the absolute value of temperature gradients depends on assumptions of gas hydrate stability conditions velocity functions and seafloor temperature relative differences reflecting regional trends and local anomalies are unaffected by the assumptions involved in calculation of temperature gradient from the position of the bsr broad scale landward decreases in temperature gradient reflect the increasing depth to the oceanic crust across the accretionary prism correlation of anomalously high temperature gradients with two out of sequence thrusts indicates that these structures are currently acting as conduits for warm fluids

seismic and heat flow constraints from the gas hydrate system in the krishna godavari basin india abstract high resolution multi channel seismic reflection data acquired in the offshore domain of the krishna godavari kg basin provide seismic indicators of gas hydrate including bottom simulating reflectors bsr and high reflectivity zones below the bsr as an indication of free gas the recent recovery of gas hydrate by drilling across various sites in the kg basin together with the seismic indicators suggests overall widespread favourable gas hydrate stability conditions the bsrs provide indirect estimates of geothermal gradients and heat flow a simple conductive model is used to calculate heat flow from the depth of the bsr bsr derived heat flow values range from about 30 to 102 mw m2 across the kg basin we also modeled the base of gas hydrate stability zone bghsz to analyse the linkage between gas hydrate occurrences in the basin and its relation to the fault systems in the basin the influence of bottom water temperature pore water salinity and geothermal gradient variation on gas hydrate stability zone ghsz thickness is analysed in relation to both geological formations and tectonics our analysis suggests a highly variable ghsz in the kg basin controlled by local variations in the parameters of stability condition and faulting the bsr derived heat flow increases towards deeper water depths across the basin consistent local variations were also observed with low heat flow values over prominent topographic highs and high heat flow values over the flanks of the topographic highs this variation may be due to focusing and defocusing effects of the topography however local bsr depths patterns may also be due to dynamic effects including the displacement of isotherms by thrust faulting additionally recent mass wasting events and normal faulting resulted in the bghsz being too shallow relative to the predicted model which assumed equilibrium conditions

heat flow and thermal models of the barbados ridge accretionary complex the pattern of heat flow from the barbados ridge accretionary complex has been derived from marine surveys with heat flow probes from measurements in drill holes and from the depths of bottom simulating seismic reflectors caused by gas hydrate the heat flow from the accretionary complex has been simulated using a finite difference model to investigate how heat flow responds to changes in the cross sectional shape of the complex and the rate of convergence and to variations in pore fluid pressure within the complex and along the decollement at its base in the south of the complex heat flow decreases westward from the toe of the wedge towards the island arc because the downward movement of subducting lithosphere beneath the wedge and the active thickening of the accreted sedimentary sequence reduce the geothermal gradient more rapidly than thermal diffusion can maintain it localised high anomalies attributed to the flow of warm fluids along fault zones are superimposed on the conductive heat flow pattern farther west heat flow increases arcward produced partly by thermal diffusion re establishing a steeper thermal gradient where the rate of thickening is decreased but mainly by the arcward increase of frictional heating along the base of the wedge caused by the increase of slip rate and increased normal stress arising from thickening of the wedge in the north of the complex around the odp leg 110 drill sites the conductive model cannot reproduce the anomalously high heat flow measurements which can only be explained by the advection of warm pore fluids