disparity between measured and bsr heat flow in the xisha trough of the south china sea and its implications for the methane hydrate abstract we calculate the heat ow from the depth of bottom simulating seismic re ectors bsrs on a seismicpro le in the xisha trough of the south china sea and compare them with the probe heat ow measure ments the bsr heat ow turn out to be 32 80 mw m 2 signi cantly lower than the measurements of 83 112 mw m 2 such big disparity cannot be ascribed only to the errors from parameters parameter errors that traditionally believed to in uence the bsr heat ow besides the parameter errors we discussemphatically the errors coming from the theoretical assumption for the bsr heat ow determination theoretical errors which occur when the bsr depth does not coincide with the base of the methanehydrate stability zone mhsz if bsr stays bellow the base of mhsz lying at the top of free gas zone the derived heat ow would be underestimated compared with the parameter errors the theoreticalerrors would be relatively larger in some geological settings the disparity between measured and bsrheat ow in the xisha trough might be mainly due to the theoretical error based on the theoreticalmodel assuming that the bsr lying at the top of the free gas zone the methane ux along the xisha seis mic pro le is estimated and the thickness of the methane hydrate occurrence zone is predicted 2008 elsevier ltd all rights reserved

thermal basin modelling of the arauco forearc basin south central chile heat flow and active margin tectonics abstract the arauco basin is part of the coastal forearc domain in south central chile during its evolution since the late cretaceous it was subject to multiple deposition cycles and the erosion of lower bathyal to beach and lagoon sediments these different environments were established in alternating accretional and erosive subduction tectonic settings along the south andean active margin whereas the general development is well understood inconsistencies arise regarding the origin of the high thermal maturity of eocene coals and the estimates of vertical movements of the whole area during the cenozoic thermal modelling of this forearc basin provides new insights regarding its thermal evolution and evaluation of the magnitudes of subsidence and inversion results are based on the analysis of coal samples from surface outcrops mines and drill cores of ten onshore wells from enap sipetrol newly derived vitrinite reflectance vr r measurements indicated a temperature in the range of 135 150 c for the oldest sediment unit of the arauco basin which was reached in post eocene times furthermore 1d basin modelling techniques indicate scenarios that could explain the coalification values in the basin s sediments the models were calibrated against vr r data from drill core samples supplied by enap sipetrol a miocene and an oligocene subsidence inversion scenario were considered while neither could be securely discarded based on the modelling results furthermore it can be shown that the current thermal maturity was not reached by an increased heat flow hf or a deep subsidence only consequently a structural inversion accompanied by the erosion of 3 0 0 4 km depending on the locality in combination with a high hf of 64 4 mw m 2 is the best explanation of the available data the hf which is high for a forearc setting can be attributed to the increased temperature of the relatively young subducted nazca plate and an additional influence of ascending hot fluids from the subduction zone the maximum temperature gradient inferred is

the effects of fluid escape on accretionary wedges 1 variable porosity and wedge convexity fluid flow out of accretionary wedges is a consequence of the porosity reduction of wedge material both with depth and with distance from the toe of the wedge changes in porosity result in changes in bulk density because bulk density is a function of solid density and porosity these variations in bulk density in turn affect the critical taper of the wedge the critical taper of a wedge with spatially varying strength pore pressure and or bulk density is dependent not only on the value of these parameters but on their first derivatives as well dahlen 1990 using the equations derived by dahlen we find that the arc ward decrease in porosity results in the commonly observed convex wedge surface a hypothetical model using a density depth function generated from a synthesis of accretionary wedge data bray and karig 1985 demonstrates that the surface angle can decrease between 0 5 and 1 in the first 60 km from the toe of the wedge steeper wedges exhibit a greater degree of convexity greater changes in slope the amount of change in slope is relatively insensitive to the internal strength of the wedge wide angle seismic data from the barbados accretionary wedge bangs et al 1990 imaged velocity variations from the surface down to the decollement for a distance of 100 km from the toe these data can be used to map the spatial variations in bulk density within the barbados accretionary wedge holding strength and pore pressure values constant and allowing only bulk density to vary results in a model that closely matches the observed bathymetry both in overall convexity and in shorter wavelength bathymetrie features the model allows a limited range of acceptable basal pore pressure values for the wedge the ability of the model to accurately match the majority of the observed bathymetry suggests that bulk density variations are of first order importance for determining not only the wedge shape but also the shorter wavelength features the model however does not produce the most pronounced changes in slope at the ten kilometers nearest the toe this suggests that other factors such as variations in pore pressure sediment strength or cohesion are more important influences on the wedge taper nearest the toe

syntectonic carbonate veins from the barbados accretionary prism odp leg 110 record of palaeohydrology syntectonic carbonate veins were found along the decollement zone between the atlantic and caribbean plates and in two sites 12 and 17 km landward of the deformation front of the barbados accretionary prism odp leg 110 both calcite and lesser amounts of rhodochrosite mnco3 occur in microcrystalline fibrous and blocky mosaic forms in veins deposited along scaly shear zones descriptions of the occurrences textures mineralogy and stable isotope geochemistry of these veins presented herein help trace the hydrological evolution of the barbados accretionary prism which has seen the impact of more deeply derived fluids on elevated sediment temperatures and altered geochemistry of interstitial fluids at one site for example vein forming fluids have been as much as 10 c warmer and 1 more enriched in 18o than expected carbon isotope ratios of veins indicate a mixture of carbon derived from seawater and from oxidized organic matter during sulphate reduction although dissolution of calcareous skeletons may be an additional component the presence of methane is recorded in one vein sample in the accretionary prism indicating that either the plumbing of the prism has changed with time allowing more fluid to leak into the prism in the past or that materials currently 1 km above the decollement zone were once deformed in the decollement using oxygen isotope analyses of rhodochrosite samples from two sites in conjunction with measurements of fluid temperature and 18o values of interstitial fluids it is proposed that the fractionation of oxygen isotopes between rhodochrosite and water 1000 1n 38 at 25 c 10 greater than the fractionation between calcite and water however it appears that substitution of 10 20 mol ca into rhodochrosite reduces the fractionation factor by 2 9 so impure diagenetic rhodochrosite samples from the deep ocean will rarely be this enriched in 18o

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

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

fluid flow in and around a mud volcano field seaward of the barbados accretionary wedge results from manon cruise a field of mud diapirs and mud volcanoes situated in the barbados trench at 13 50 n and extending along an old oceanic fracture zone mercurus was investigated during the manon cruise using both surface ship and nautile submersible sampling and measurements the entire zone from 13 50 n up to 14 20 n has an anomalously high heat flow which implies that fluids are drained into it from a segment of the accretionary wedge a few hundred kilometers wide two structures interpreted as diatremes atalante and cyclops expell large amounts of water and methane we propose that they were formed from the release of a light fluid when gas hydrates were dissociated in the sediment as the result of the circulation of warm fluid in the area however they expell only a small fraction of the incoming fluid implying that disperse flow is the dominant mode of expulsion in this area the chemoautotrophic communities on the surface of the structures rely mostly on sulfides submersible observations temperature measurements in the sediment and the chemistry of the pore fluid indicate that convection of seawater occurs within the first few meters of sediment through high permeability channels such as cemented carbonate conduits we propose that this convection is driven by the density difference between the pore fluid and seawater but fresh water released by the dissolution of shallow hydrates may also contribute this shallow convection may be a frequent process in cold seep environments

geothermal evidence for fluid flow through the gas hydrate stability field off central chile transient flow related to large subduction zone earthquakes fluids are suspected to play a major role in the nucleation and rupture propagation of earthquakes in chile seismological data were previously interpreted to indicate that fluids captured in the fault zone are released periodically during large underthrust earthquakes leading to post seismic fluid flow in central chile heat flow derived from the presence of a bottom simulating reflector bsr show a smooth trend across the margin bsr derived data are in excellent agreement with thermal subduction zone models over the young accretionary prism both bsr derived and measured surface heat flow support a common trend landwards of the backstop however measured heat flow triples over a distance of 20 30 km producing a profound discrepancy to the bsr derived data we suggest that this disparity is related to transient flow of warm fluids through the gas hydrate stability zone possibly caused by fluids released after large underthrust earthquakes such flow events may inherently affect the distribution of solid gas hydrates between the seafloor and the bsr

pore water anomalies of submarine gas hydrate zones as tool to assess hydrate abundance and distribution in the subsurface what have we learned in the past decade abstract the most significant new contributions to the study of natural gas hydrates in the past decade have come from findings of the ocean drilling program odp notably legs 112 141 146 and 164 and to a lesser extent legs 131 160 170 and 186 leg 164 in particular was dedicated to gas hydrate drilling in the blake ridge gas hydrate field in the west atlantic in an unprecedented multidisciplinary research effort paull et al 1996 proc ocean drill program initial rep 164 623 pp paull c k matsumoto r wallace p j dillon w p eds 2000a proc ocean drill program sci results 164 ocean drilling program college station tx 459 pp most important for the progress of hydrochemical studies related to gas hydrates has been the growing awareness of the significance of diffusion modulated advective processes shaping the chemical and isotopic pore water profiles in hydrate zones this started with qualitative evidence for advective flow from drilling the hydrate bearing peru and hydrate free barbados active margins continued with the nankai trough accretionary prism the japan trench slope and the cascadia and costa rica active margins and culminated in the quantitative advection diffusion model of egeberg and dickens chem geol 153 1999 53 for the passive margin setting of the blake ridge advective flow regimes are different at active and passive margins as there is a tendency for the flow at active margins to be focussed along landward dipping thrust planes and faults in the wedge of imbricated thrust sheets that finds expression in a step pattern of the pore water profiles for passive margins but also for some active margin sites advection is from sources below the drilled section either through subvertical faults passive margins or from the decollement zone active margins we have learned that the well known coupled pore water anomalies that are ascribed to hydrate dissociation downward chlorinity decrease combined with 18 o increase hesse and harrison 1981 earth planet sci lett 55 1981 453 need not occur together in the presence of hydrates because the isotope effect may be overprinted by the effects of other reactions such as volcanic ash alteration or by the advection of low 18 o fluids however if the anomalies show up hydrates are present almost invariably with the exception of advected low cl high 18 o waters coming to terms with the effects of advection and diffusion has allowed successful modeling of the simpler hydrate affected pore water profiles at passive margins instrumental for modeling are chlorine isotopes which provide an effective tool to assess advection rates the egeberg and dickens model allows estimation of hydrate concentration and distribution in the subsurface because it separates the effects of advection diffusion and hydrate dissociation but critically depends on samples taken under in situ pressure and temperature conditions modeling the more complex pore water profiles of active margins is a challenge for the future compared to geophysical methods to estimate hydrate concentration the geochemical method gives minimum amounts

heat flow distribution and thermal structure of the nankai subduction zone off the kii peninsula detailed heat flow surveys were carried out in the central part of the nankai trough southeast of the kii peninsula off kumano for investigation of the thermal structure of the subducting plate interface at stations in the kumano trough forearc basin and its vicinity long term monitoring of temperature profiles in surface sediments was conducted because bottom water temperature variations btv significantly disturb subbottom sediment temperatures heat flow values were successfully determined at seven stations by removing the influence of btv from temperature records for 300 to 400 days the surface heat flow data were combined with estimates from depths of methane hydrate bottom simulating reflectors to construct a heat flow profile across the subduction zone heat flow decreases from 90 110 mw m2 on the floor of the nankai trough to 50 60 mw m2 at around 30 km from the deformation front while it is rather uniform 40 60 mw m2 in the kumano trough the values measured on the nankai trough floor are concordant with the value estimated from the age of the subducting philippine sea plate about 20 m y taking into account the effect of sedimentation the obtained heat flow profile was used to constrain thermal models of the subduction zone the subsurface thermal structure was calculated using a two dimensional steady state model in which the frictional heating along the plate interface and the radioactive heat production are treated as unknown parameters comparison of the calculated surface heat flow in the kumano trough with the observed data indicates that the effective coefficient of friction is small about 0 1 or less and thus the shear stress on the plate interface is very low in this subduction zone

on the high heat flow in the nankai trough area a simulation study on a heat rebound process abstract a concentration of high heat flow as high as 120 160 mw m2 has been observed in the nankai trough off southwest japan where the philippine sea plate shikoku basin is subducting beneath the japanese landmass we interpret this high heat flow in the subduction zone as being caused by the recovery of conductive heat flow to the theoretically expected value for the young shikoku basin lithosphere after cessation of hydrothermal heat exchange through sediment the thickening sedimentary cover suppresses the rapid heat exchange between basement and sea water whereas the topmost part of the basement allows a hydrothermal circulation within it to pump up heat from the hot plate to the sediment cover we propose to call this process heat rebound numerical simulation shows that significant heat flow recovery can occur and is largely influenced by the sedimentation rate we believe that the high heat flow in the nankai trough is one of the heat rebound cases however rapid sedimentation in the trough also indicates that heat flow recovery expected by the simulation is not large enough to explain the trough high heat flow by the heat rebound process only

using 3d seismic reflection data to find fluid seeps from the costa rica accretionary prism submersible dives planned using 2d and 3d seismic reflection data off costa rica successfully found numerous sites of fluid expulsion predominantly along surface fault scarps these data were used because more typical data such as deep towed side scan sonar bottom photography and heat flow were unavailable because fluids and fluid pressure distribution profoundly affect the way an accretionary prism deforms detecting fluid pathways and recovering fluids is a key to its deformation processes detailed seismic interpretation of the shallow subsurface identified apparently active faults which are most likely to act as fluid pathways due to a quasi inverse relationship between surface dip and reflection amplitude the amplitude of the seafloor reflection displayed in map view shows the orientation and extent of surface scarps many of which appear related to subsurface faults because the seafloor amplitude response depends on seafloor morphology as well as near surface physical properties removing the effect of the surface dip calculated directly from picked surface structure leaves a residual that may be related to variation in physical properties although the search for fluid seeps by submersible dives over the costa rica prism was sparse and irregular the seeps that have been discovered are all on structural highs of the apron prism boundary

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

gas hydrate stability and the assessment of heat flow through continental margins summary a prominent feature across some continental margins is a bottom simulating reflector bsr this seismic reflection generally coincides with the depth predicted for the base of the gas hydrate stability field because the occurrence of gas hydrates is controlled by temperature and pressure conditions it has been suggested that bsrs mark an isotherm and they have therefore been used to estimate the heat flow through continental margins crucial parameters are the temperature at bsr depth and at the seafloor and the thermal conductivity structure between the bsr and the seabed however very often the required parameters are not available and therefore they have been derived from models for gas hydrate stability and empirical relationships to obtain thermal conductivities from seismic velocities here we use downhole temperature thermal conductivity porosity and logging data from 10 ocean drilling program odp sites drilled into and through the gas hydrate field to investigate the quality of estimates our analyses and application of constraints to the makran margin off pakistan indicate the following i the temperature at bsr depth could be approximated by a seawater methane system although capillary forces chemical impurities or non equilibrium conditions can lower or increase the temperature if calibration by heat probe measurements is possible errors of geothermal gradients are less than 10 per cent otherwise uncertainties of 20 per cent or even higher may arise in addition seasonal variations of bottom water temperature have to be considered because they may affect thermal gradients by up to y10 per cent ii the impact of typical quantities of low thermal conductivity gas hydrate on the bulk thermal conductivity is insignificant iii the thermal conductivity profile between the bsr and the seabed can generally be approximated by a mean value thus iv seabed measurements should be used instead of empirical relationships which may produce errors of 5 30 per cent consequently in addition to high quality seismic data a prerequisite should be a large data set of thermal conductivities and oceanographic data heat probe measurements are recommended to constrain geothermal gradients in this case the uncertainty of heat flow is 5 10 per cent of the estimated heat flow if these data are not available errors uncertainties can reach 50 60 per cent of the calculated value

deformation and fluid flow of a major out of sequence thrust located at seismogenic depth in an accretionary complex nobeoka thrust in the shimanto belt kyushu japan 1 nobeoka thrust in kyushu southwest japan was investigated to understand the relationship between the seismogenic out of sequence thrust ost and fluid flow in accretionary prisms the nobeoka thrust is a fossilized ost being active at seismogenic depth the hanging wall exhibits a penetrative plastic deformation while a brittle cataclastic melange like occurrence characterizes the footwall although both of them have same shale and sandstone dominant protolith vitrinite reflectance analyses indicate that the maximum temperatures of the hanging wall and footwall are approximately 320 and 250 c respectively this thermal gap across the thrust corresponds to 8 6 14 4 km of displacement assuming a 28 47 c km geothermal gradient the brittle damage zone of the thrust is asymmetric only 2 m for hanging wall side and 100 m for footwall three types of mineral veins quartz and carbonate are well developed especially in the damaged footwall the tension crack filling vein the fault filling vein and postmelange one the first is harmonious with fabric perpendicular to the p surface fluid inclusion geothermobarometry indicates the p t of fluid in the intensively damaged zone of the footwall is 300 c 230 250 mpa higher than that from vitrinite reflectance which suggests that hydrothermal fluid flow is associated with deformation the same type vein in the lowest portion of the footwall damaged zone includes a much lower p t fluid this difference suggests that continuous underplating caused the damaged zone to propagate downward with cooling and shallowing which differs from faults characterized by shear localization and might be unique to aquiferous ost in accretionary complexes

thermal effects of hydrothermal circulation and seamount subduction temperatures in the nankai trough seismogenic zone experiment transect japan 1 we examine the thermal effects of seamount subduction seamount subduction may cause transient changes in oceanic crust hydrogeology and plate boundary fault position prior to subduction seamounts provide high permeability pathways between the basaltic crustal aquifer and overlying ocean that can focus fluid flow and efficiently cool the oceanic crust as the seamount is subducted the high permeability pathway is closed shutting down the advective transfer of heat if significant fluid flow occurs it would be restricted after seamount subduction and would result in a redistribution of heat warming the trench and cooling landward parts of the system additionally subducting seamounts can influence the position of the plate boundary fault that has thermal consequences by locally controlling the proportions of incoming sediment that subduct and accrete shifting the decollement to the seafloor at the trench in the wake of seamount subduction causes limited cooling focused at the toe of the margin wedge we apply these features of seamount subduction to a thermal model for the nankai trough seismogenic zone experiment transect on the margin of japan models with hydrothermal circulation provide an explanation for anomalously high surface heat flux observations near the trench they yield temperatures of 100 c 295 c for the rupture area of the 1944 tonankai earthquake temperatures in the region of episodic tremor and slip are estimated at 290 c 325 c 70 c cooler than a model with no fluid circulation

decollement processes at the nankai accretionary margin southeast japan propagation deformation and dewatering the decollement zone expressed on seismic profiles and observed in drill cores from the nankai accretionary margin off the southeast coast of japan reveals several unique characteristics which appear to distinguish it from thrust faults identified in the same setting physical manifestations of these include evidence for the asymmetric distribution of deformation structures about the decollement the extension of this fault zone well in front of the tectonic deformation front and the absence of features indicative of precursory shear for example folded sediments shear bands and penetrative mineral fabrics these characteristics suggest that the mode of formation and evolution of this decollement zone may be unique from that of most thrust faults we propose that the decollement zone propagates not as a shear fracture controlled by tectonic stress conditions but rather as a subhorizontal tension fracture propagating under high pore pressures to test this possibility physical property measurements and clay mineral fabrics were obtained for several samples from the nankai decollement zone using computed tomography and x ray texture methods our findings suggest that deformation within the decollement zone is partitioned into a volumetric component preserved as reduced porosities within coherent fragments and a localized shear component evidenced by mineral preferred orientations along discrete slip surfaces we suggest that the reduced porosities result from the destruction of cementation in the sediments during the early stages of deformation and may arise from cyclic fatiguing of the sediment induced by fluctuating pore pressures the nonpenetrative shear fabrics probably develop as the tectonic deformation front migrates seaward and the weakened protodecollement subsequently accommodates shear displacements along discrete fractures

fluid accumulation and channeling along the northern barbados ridge decollement thrust a volume of three dimensional seismic reflection data acquired in 1992 imaged the decollement beneath the northern barbados ridge accretionary prism revealing reflection amplitude and waveform variations attributed to fluid accumulations along the plate boundary fault we model the seismic reflection by inversion for seismic impedance the product of velocity and density throughout the 5 x 25 km survey area and thus map physical property variations in 1997 ocean drilling program leg 171a penetrated the protodecollement and decollement at five sites with a logging while drilling lwd tool to log density and other physical properties of the decollement we construct a regional map of density and inferred porosity within the decollement from seismic models calibrated with lwd density data in the sediments out in front of the trench the protodecollement forms in a radiolarian rich miocene mudstone with an anomalously high porosity 70 75 that appears as a pervasive inherent characteristic of this interval seaward of the deformation front in the decollement beneath the wedge a consolidation trend of decreasing porosity runs perpendicular to the deformation front with porosity decreasing from 70 at the wedge toe to 50 4 km from the wedge toe a second distinct trend also forms along a 10 km long 1 to 2 km wide ne sw zone in which porosity is 70 as high as it is in the protodecollement this zone can be explained as an area of the decollement where fluid accumulations develop by maintaining high fluid content we postulate that high fluid content is maintained by continuous recharge flowing into and along this channel this porosity distribution within the decollement also strongly influences fluid migration into the overlying accretionary wedge and is directly associated with fluid charging of ramps and out of sequence thrusts above the decollement

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

heat flow determination in three dsdp boreholes near the japan trench the first deep borehole determinations of temperature gradients and heat flow of the landward wall of the japan trench and forearc were made on ipod dsdp leg 57 these heat flow values are based on temperature logs corrected to equilibrium using a detailed model of the drilling disturbance heat flow values on a deeply submerged terrace landward of the trench slope break are 28 and 32 mw m sup 2 a measurement in the midslope terrace basin on the landward wall of the trench yielded a value of 22 mw m sup 2 the results are in good agreement with earlier seafloor measurements and indicate that most of the forearc area is characterized by heat flow about one half of that over oceanic lithosphere seaward of the trench our observations indicate only a small increase of heat flow from the trench to the volcanic arc in agreement with thermal models which suggests that the subduction of the relatively cold oceanic plate continues to dominate the temperature structure for distances of up to 250 km landward of the trench the temperature profile in the borehole on the midslope terrace indicates possible vertical flow of pore waters hundreds of conductivity determinations were made usingmore a new technique less

cyclic fluid flow through a regionally extensive fracture network within the kodiak accretionary prism two types of periodic textures observed in veins from the kodiak accretionary prism attest to cyclic fluid flow through a regionally extensive fracture network buried at 8 12 km depth 1 crack seal microstructures with bands of mica inclusions and 2 collapse microstructures with jagged bands of residue embedded within euhedral crystals of quartz the difference in texture reflects the closure of cracks crack seal microstructures record the complete chemical sealing of the crack after each fracture event whereas the collapse features record longer fluid filled periods followed by more rapid draining of fractures collapse features consist of pressure solution selvages trapped within veins and in the wall rock adjacent to euhedral growth terminations the high concentrations of immobile elements in these selvages indicate that these fractures closed by collapse and penetration of quartz crystals into wall rock analysis of chemical composition on either side of four large euhedral growth veins and whole rock analysis of slates across the kodiak formation reveal local depletion of silica adjacent to veins but no evidence for long distance silica transport within the system both crack seal and collapse textures are observed in a regionally extensive vein system that displays a regular geometry with thin closely spaced 0 5 3 cm near vertical crack seal veins that connect vertically and laterally with thicker euhedral growth veins arranged in widely spaced 500 mm southeast dipping en echelon sets the mesoscopic distribution and textural variability of the vein network suggests that the development of the vein system involved early nucleation and growth of vertical hydrofractures as the fracture density increased arrays of fractures locally provided zones of weakness and southeast dipping brittle ductile shear zones nucleated these en echelon cracks remained open and provided small reservoirs of fluid textures show that en echelon fractures remain open but periodically grow by upward and downward propagation crack tips are then sealed with locally derived silica and fluid drains back into en echelon fracture arrays this local fluid movement is punctuated by less frequent events where the system links up over a greater distance fractured reservoirs become interconnected and the fluid within reservoirs is drained upward or laterally periodic inflation and deflation of en echelon arrays may reflect periodic slip on crosscutting faults and rupture of the seals that separate reservoirs

hydrate melting in soil around hot conductor there is ice in the gulf of mexico and this type of ice called gas hydrates burns when ignited gas hydrates form slowly within the soil mass when the gas seeping up from offshore oil reservoirs mixes with water under high pressures 500 m of water and low temperatures a few degrees celsius the oil travels from the well to the platform through a conductor pipe the oil and therefore the conductor are very hot and melt the existing hydrates that are within the depth of the foundation piles the melting process generates a large amount of gas that can endanger the stability of the foundation the rate at which the temperature rises around the hot conductor in the hydrate rich soil is studied using the finite element method fem a detailed thermodynamic analysis is performed it includes a laboratory experiment to help validate fem a study of the mesh size the thermodynamic analysis results a study of the conductor size and of the latent heat influence the results can be used to evaluate the temperature rise around a hot pipe buried in soil and therefore the propagation of the hydrate melting front around the conductor and toward the piles

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

dolomite formation in caribbean islands driven by plate tectonics abstract postdepositional dolomite occurs in the shallow subsurface of several caribbean islands in tertiary and pleistocene carbonates e g on barbados grand cayman jamaica and st croix recent investigations have interpreted these dolomites as relatively early diagenetic formations from chemically modified seawater mainly on the basis of petrography stratigraphic association and stable isotope values interpretations of the hydrologic regimes during dolomitization vary and usually include freshwater seawater mixing thermal or density driven convection of seawater or a combination thereof this paper offers two alternative interpretations based on the relatively low sr isotope ratios of the dolomites 0 7085 to 0 7092 and their proximity to active plate margins dolomitization may have been driven by plate tectonics i e either by thermal convection of pore fluids at active spreading centers or by tectonic expulsion of fluids from accretionary prisms the dolomitizing solution was seawater that was enriched in 86sr by magmatic hydrothermal solutions or by interaction with siliciclastics in an accretionary prism respectively

relation between permeability and effective stress along a plate boundary fault barbados accretionary complex in situ bulk permeability was measured in a borehole that intersected the decollement zone a low angle detachment fault between the north american and caribbean plates permeability measurements were made at a variety of fluid pressure conditions defining a quantitative relation between bulk permeability and effective stress for this plate boundary fault zone the bulk permeability in this zone changed by several orders of magnitude as a consistent function of fluid pressure this relation may help to explain the dynamics of fluid fault interactions and the transient nature of hydrologic processes during deformation at convergent margins

source rock and maturation evaluation of the barbados accretionary prism abstract organic material in specimens from surface and downhole sites and samples of oil and gas from barbadian wells were studied for source potential and maturation history the study involved 1 visual examination of the kerogen 2 measurement of total organic carbon content 3 determination of vitrinite reflectance 4 rock eval pyrolysis assay 5 geochemical measurement of pooled and extracted bitumens including carbon isotopic ratios 13 c and 6 gas chromatographic analysis of saturated gaseous hydrocarbons two types of kerogen occur in barbados terrestrial presumably of south american origin and marine planktonic the two types are mixed in most samples the terrestrial kerogens are dominant and occur in all wells in the basal complex and most of its outcrops the planktonic organic matter occurs in exposed melange hemipelagic rocks and rarely in terrigenous turbidites the well specimens do not show significant maturation within the drilled depths suggesting that pooled hydrocarbons originated from deeper levels of the prism and accumulated as a result of migration faults bounding the intraprism thrust sheets probably provided migratory paths for hydrocarbons and water juxtaposed rocks of variable maturity assisted in alteration and biodegradation and led to the formation of structural traps the composition of gases indicates that maturation conditions for gas generation may exist at great depths within the barbados accretionary prism

detection of fluid flow variations at the nankai trough by electric and magnetic measurements in boreholes or at the seafloor detection of changes in the flow rate of expelled fluids in accretionary prisms by monitoring of electric and magnetic fields is discussed a numerical model of the electric and magnetic fields associated with fluid flow variations at the nankai trough is presented which gives a numerical solution of the coupled system of electric convection currents and conduction currents that directly determines the magnetic anomaly itself measurements in a borehole located between two vents are shown to be well adapted to detection of fluid flow variations using the vertical gradient of the electric potential and the horizontal magnetic field the vertical electric field is about 10 mv km up to 500 m depth where there is a lithologic reflector and about 50 mv km below this reflector the horizontal gradient of the magnetic field is 2 nt km at the seafloor modelization with a lower fault conductivity and a larger decollement thickness has also been modeled the vertical gradient of the horizontal magnetic field is 5 to 15 nt km a variation of 3 mv and 1 5 to 3 nt at 600 m depth in a borehole could reveal a fluid flow rate variation of 20 which is a reasonable fluid flow change based on some observations at short scale time since a 1 5 to 3 nt anomaly seems easier to detect than a 3 mv anomaly it is likely that the variation of the magnetic field would more sensitively reveal fluid flow variations when monitoring the magnetic field at the seafloor a change of 0 4 nt km in the horizontal gradient could reveal a fluid flow rate variation of 20

deformation at plate boundaries around the gulf of oman abstract seismic studies during charles darwin cruise 18 86 focused on three zones of deformation the makran accretionary prism the murray ridge owen fracture zone system and the continental margin of east oman a multichannel seismic reflection profile across the makran margin clearly imaged the imbricate thrust slices and their bounding faults a bottom simulating reflector bsr 500 800 m beneath the seabed marking the base of a gas hydrate stability zone may be traced across most of the profile the heat flow variation estimated from changes in the depth of this reflector shows evidence for localised fluid expulsion reflection profiles across the murray ridge show evidence for active extension along a series of major normal faults the rift structure is segmented with the fault polarity changing between segments a combined reflection and wide angle study of the east oman margin shows a deep offshore basin underlain by crust only 5 km thick and bounded by a basement ridge interpreted as a tilted fault block the wide angle data suggest a sleeply dipping moho beneath the margin the large lateral changes in crustal thickness may be explained by episodes of margin parallel shear

control of internal structure and fluid migration pathways within the barbados ridge decollement zone by strike slip faulting evidence from coherence and three dimensional seismic amplitude imaging the application of three dimensional seismic reflection and coherence imaging to the study of the decollement zone of the barbados ridge accretionary complex has provided new insights into the relationships among internal structure fluid flow and previously unrecognized strike slip faulting combined coherence and seismic amplitude imaging of the decollement zone reveal anomalous northeast trending lineaments parallel to and abutting zones of high amplitude negative polarity reflections analysis of these lineaments shows them to be penetrative structures dipping southeast with apparent reverse dip slip offset isopach mapping of the accretionary wedge indicates significant right lateral displacement across these structures these faults apparently channel fluid flow within the decollement zone and the prominent northeast trending conduits so formed are readily visible as high amplitude negative polarity reflections additionally north northeast trending zones of variable coherence and high positive amplitude are inferred barriers to up structure fluid migration pathways movement along strike slip structures probably alternates with displacement along the decollement zone northeast trending strike slip faults extend for 13 km crossing the length of the survey area and into the incoming oceanic plate active arc oblique strike slip faulting of the decollement zone beneath the barbados ridge accretionary wedge implies a stress regime in that 1 is fixed and 2 and 3 either transpose with time or are nearly equal this state of stress may be a common occurrence in forearc tectonism and may have led to the formation of many as yet unrecognized arc oblique strike slip faults at convergent margins

heat flow and fluid flow regime in the western nankai accretionary prism the thermal structure of the western nankai subduction zone has been studied through surface heat flow measurements with conventional geothermal probes and estimation of heat flow from depths of gas hydrate bottom simulating reflectors temperature and thermal conductivity measurements at odp site 808 located at the toe of the nankai accretionary prism added important information on any variation in heat flow with depth and thus on thermal effects of pore fluid flow in the prism surface heat flow is generally higher than 130 mw m 2 on the floor of the nankai trough such high heat flow cannot be explained simply by conductive cooling of the subducting shikoku basin lithosphere recent closely spaced probe measurements revealed that the heat flow on the trough floor shows local variation with particularly high values near the deformation front the heat flow distribution suggests that warm fluid flows seaward possibly along the decollement and locally rises toward the seafloor along some channels resulting in both the regional and local high heat flow anomalies contrary to this inference no active localized fluid flow was detected at site 808 the temperatures measured at six depths down to 347 m below seafloor mbsf just below the frontal thrust can be well explained by conductive heat flow of 125 130 mw m2 no geochemical anomaly associated with present day pore fluid movement was observed in cores recovered from the frontal thrust or the decollement on the other hand the high heat flow at site 808 requires some heat source below 350 mbsf there is a possibility that the fluid flow in the nankai accretionary prism is localized and transient and thus was not detected at this single drill site

introduction to special section on the role of fluids in sediment accretion deformation diagenesis and metamorphism in subduction zones accretionary prisms which develop from materials scraped from the subducting oceanic lithosphere form within one of the most dynamic structural environments on earth and constitute critical elements of mountain belts in the marine setting these large accumulations of sediments grow in a water saturated environment consequently their tectonic chemical and thermal evolution is strongly influenced both by the seawater that is incorporated with the sediment as it is accreted and by water that is rendered from the clays and rocks of the prism as they are subjected to increasing pressure and temperatures figure 1 although accretionary prisms are mechanically similar to subaerial fold and thrust belts the latter may develop in consolidated partially saturated sediments with less pronounced hydrogeologic effects

surface expression of fluid venting at the toe of the nankai wedge and implications for flow paths abstract cold seeps have been observed with manned submersibles at many sites on the tokai margin most sites are found on the hanging wall of major thrust faults within 1 2 km of their outcrop few sites relate to purely erosional features this relationship between thrusting and venting may be explained either by fluid flow along fault zones by fluid flow along permeable layers outcropping at the fault scarp or by diffuse upward flow of fluids released during footwall compaction we use deep towed side scan sonar and sediment sounder profiles acquired at the toe of the nankai accretionary wedge to identify the context of seepage at a smaller scale modes of fluid expulsion inferred from observations in this zone include flow channelling along strata along thrusts along gravity faults and along joint fractures as well as diffuse flow we then combine visual observation and monitoring of electrochemical potential eh to assess the relative importance of seepage sites we show that bacterial mats are better visual indicators of active seepage than bivalve beds and that the most active seepage sites correlate with outcrops of coarse turbidites cemented by authigenic carbonates these were sampled and have permeability high enough to channel fluid flow 1 4 10 14 m2 we suggest they are the main conduits near the seafloor however seepage sites are also observed directly on the outcrop of major thrusts indicating that they do play a role as fluid conduits we observe negative polarity seismic reflections on these faults below 1 km depth and wonder about their significance

generation and maintenance of abnormal fluid pressures beneath a ramping thrust sheet isotropic permeability experiments abstract we have investigated the mechanisms responsible for the evolution of excess pore pressures within and beneath a ramping thrust sheet i e fluid flow porosity compression and thermal expansion of water and the sensitivity of pore pressure to a variety of physical parameters e g permeability thrust sheet velocity heat flux coupled pore pressure and temperature equations were solved numerically in two dimensions using a generalized hydrostratigraphy of north american thrust belts because of the lack of either symmetry or a steady state in this problem both deposition and thrust loading stages were simulated the dominant mechanisms controlling pore pressure evolution were fluid flow and compression of pore space by vertical loading thermal expansion of the fluids was found to be insignificant in generating excess pore pressures at common thrust loading rates the results indicate that it is possible to generate high pore pressure to lithostatic pressure ratios within thrust sheets by depositional loading prior to thrusting high values of are generated and maintained during thrust loading for reasonable assumptions about the conditions thought to have existed in thrust belts values of were not constant throughout the model the highest values tended to concentrate near the surface of the model and within and below the toe of the thrust sheet the magnitude and distribution of excess pore pressures and values were found to be especially sensitive to variations in permeability excess pore pressure generation by compression exceeded pore pressure dissipation by fluid flow for permeabilities less than approximately 10 16 m 2 permeabilities greater than approximately 10 16 m 2 produced hydrostatic pore pressure gradients the models demonstrate that permeability inhomogeneity due to lithologic variations may exert a strong control on the magnitude and spatial distribution of excess pore pressures within thrust sheets in addition these models indicate that it is unlikely that fluid pressure is high everywhere in a moving thrust sheet

fluid flow along a decollement layer a model applied to the 16 n section of the barbados accretionary wedge a seismic section across the whole barbados accretionary complex at 16 n recently published by westbrook et al 1988 shows that the reflection marking the decollement fades between 40 and 60 km arcward of the deformation front but is present again further inland this interruption appears to be related to a thrust cutting through the wedge one dimensional steady state models of water flow along the decollement suggest a mechanism responsible for decollement disruption a local increase of the slope either of the wedge or of the decollement causes the subducted layer to compact and locally increases the friction along the decollement if the thrust is used as a conduit for fluids to escape from the decollement the friction along the decollement toeward of the thrust is further increased the model also shows that the permeability of the decollement should be of the order of 10 14 m2 but may be higher near the deformation front for short periods if fluid expulsion mainly occurs in transient bursts associated with hydraulic fracturing as a consequence of the non linearity of the compaction equation pore pressure at depth in the decollement depends on the water flux on the lithostatic pressure gradient along the decollement and on the parameters from which the physical properties of the sediment are calculated however it is almost independent on the pore pressure below the toe

review of major shale dominated detachment and thrust characteristics in the diagenetic zone part ii rock mechanics and microscopic scale abstract characterising large shale thrust zone behaviour down to the diagenetic metamorphic boundary is both simple and complex the task in critical taper analogue and numerical models has been successfully approximated using simple material parameters yet the weakness of shales thrusts on a case by case basis show great variability in key factors across a wide range of scales understanding of these variables and their relationships to different tectonic settings is patchy due to the different objectives of workers studying the various systems e g seismic hazards hydrocarbon exploration structural geology research and the types of available geological and geophysical data early research recognized that megathrusts feature creeping and locked patches which may be described using the rate and state formalism in addition to temperature differential stress and strain rate key controlling factors on thrust weakness are mineralogy amount of weak phase present structure localization mechanisms grain size and shapes porosity permeability and pore fluid pressure the smectite illite transition has been a focus of seismic hazard research due to its coincidence with the top of the seismogenic zone while there may be a relationship other factors such as pore fluid pressure variations or structure localization are also important the critical taper model is a simple means of determining whether basic rock mechanics data e g frictional strength pore fluid pressure is appropriate for natural wedges for example aseismic basal detachments of gravity driven systems smaller critical taper wedges appear to retain higher pore fluid pressures along the basal detachment than seismogenic basal detachments higher critical taper wedges in many continental and deltaic fold and thrust belts high organic carbon content is a very important factor in shale weakness due to 1 the overall more ductile and well cleaved nature of the shale when organic content is high 2 the presence of high overpressures due to maturation of organic material and 3 metamorphism of carbon to low friction graphite this multifaceted influence of organic carbon content is just one example of the diversity of potential influences on shale thrust zone weakness which enable shales to be weak despite considerable lateral deposition related and vertical burial related changes in composition some key variations between different types of fold and thrust belt gravity driven accretionary prism andean himalayan type lie in systematic variations in clay mineralogy magnitudes and origins of overpressures seismic versus aseismic detachments and different structural localization mechanisms further research is required to explore the viability of such distinctions and their impact on structural styles

thermal constraints on basin scale flow systems an important constraint on basin wide flow systems is the thermal regime studies of high temperatures above fault zones which penetrate geopressured sediments and heat flow in orogenic zones have been used to document both pathways and velocities of pore fluid movement similarly the high temperature 100 150 c at shallow depths 1 5km inferred from studies of fluid inclusions associated with late paleozoic mississippi valley type mvt lead zinc deposits places a very restrictive constraint on their formation by regional fluid migration we have used numerical models of fluid and heat transport to evaluate the viability of sediment compaction by overthrusting and topographically driven recharge as driving mechanisms for regional fluid migration sediment compaction by overthrusting leads to darcy velocities on the order of cm yr too low to have significant effects on the temperature field unless flow is channelized topographically driven recharge can lead to fluid velocities on the order of m yr however it is difficult to achieve the extremely high temperatures 100 150 c at shallow depths 1 5km associated with mvt deposits unless heat flow values greater than 100 mw m2 are used thus there are fundamental limitations on the ability of either mechanism to continuously transport large amounts of heat for millions of years on a basin wide scale temporal or spatial focusing of flow is likely to have played a critical role in the genesis of phenomena such as mvt deposits

three dimensional numerical simulation of fluid flow and heat transport within the barbados ridge accretionary complex 1 the thickness of the barbados accretionary prism varies along strike the increased thickness of the prism to the south is believed to affect the transport of heat by fluids resulting in anomalously high heat flow at the deformation front we utilized a coupled three dimensional numerical fluid flow and heat transport model to investigate the effect of variable accretionary prism thickness on fluid flow patterns and heat transport in the decollement zone the variation in sediment thickness produced along strike fluid flow in the more permeable decollement although surface heat flow is higher for simulations where flow occurs along both the decollement and underthrust sediments the resultant advective heat transport was not sufficient to appreciably raise steady state surface heat flow at the latitude of the ocean drilling program transect simulated temperatures in the decollement were consequently much lower than in situ borehole measurements suggesting an additional source of warmer fluid we then investigated the transient hydrologic and thermal response of the system to enhanced fault permeability model results provide insight into the degree to which episodic faulting may drive fluid flow and heat transport within subduction complexes a transient scenario in which decollement permeability was increased two orders of magnitude with fluid flow along both the decollement and underthrust sediments caused the temperature gradient at the location of the drill sites to increase by a factor of two within 7100 years the results of the three dimensional numerical simulations indicate that while both along strike advective heat transport and transient fluid flow events should be considered in the heat flow distribution of the northern barbados accretionary complex the effects of episodic fluid flow are more significant

fluid overpressure in subduction plate boundary caused by mantle derived fluids abstract subduction plate boundaries are commonly weak owing to fluid overpressure the generation of fluid overpressure has been invoked for compaction disequilibrium and mineral dehydration in subducting sediments and altered oceanic crust the subduction melange in the shimanto accretionary complex southwest japan preserves the quartz calcite filled shear veins and sigmoidal extension veins formed under fluid overpressure near the downdip limit of the thermally controlled seismogenic zone the shear veins record the repeated low angle brittle thrusting under fluid overpressure while the extension veins constitute the riedel shear zones developed during subduction under vertical maximum principle stress to examine the origin of overpressured fluids we conducted fluid inclusion and noble gas analyses on the shear and extension veins the result of fluid inclusion analysis shows various fluid pressures between lithostatic to hydrostatic pressures during the vein formation which were associated with warm fluid infiltration 3he 4he values of shear veins and extension veins range 1 6 2 5 ra and 1 6 2 3 ra respectively indicating the presence of mantle helium 40ar 36ar values and 84kr 36ar and 130xe 36ar ratios of the veins are similar to those of serpentinized mantle suggesting that mantle helium originated from serpentinized mantle because the mantle derived rocks such as peridotite and serpentinite are absent in the melange the results of noble gas analyses represent the infiltration of fluids from the serpentinized mantle into the melange shear zone our results show that an infiltration of fluids derived from serpentinized mantle contributed the fluid overpressure in the melange shear zone near the downdip limit of the seismogenic zone

modeling consolidation and dewatering near the toe of the northern barbados accretionary complex at the toe of the northern barbados accretionary complex temperature and pore water chemistry data indicate that fluid flow is channeled along the decollement and other shallow thrust faults we examine mechanisms that may prevent consolidation and maintain high permeability over large sections of the decollement high resolution bulk density data from five boreholes show that the decollement is well consolidated at some sites while other sites remain underconsolidated underconsolidated decollement behavior is associated with kilometer scale negative polarity seismic reflections from the decollement plane that have been interpreted to be fluid conduits we use a coupled fluid flow consolidation model to simulate the loading response of a 10 km long by 680 m thick slice of sediment as it enters the accretionary complex the simulations capture 185 ka 5 km of subduction with a load function representing the estimated effective stress of the overriding accretionary prism 3 8 taper angle simulation results of bulk density in the decollement 3 2 km arcward of the deformation front are compared with observations the results show that persistent high pore pressures at the arcward edge of the simulation domain can explain underconsolidated behavior the scenario is consistent with previous modeling results showing that high pore pressures can propagate intermittently along the decollement from deeper in the complex simulated seaward fluxes in the decollement 1 14 cm yr 1 lie between previous estimates from modeling studies of steady state 1 m yr 1 flow maximum simulated instantaneous fluid sources 2 5 10 13 s 1 are comparable to previous estimates the simulations show minor swelling of incoming sediments fluid sources 3 10 15 s 1 up to 3 km before subduction that may help to explain small scale shearing and normal faulting proximal to the protodecollement

heat transfer in the oceanic crust of the brazil basin a detailed geothermal survey has been made on the western flank of the mid atlantic ridge in the brazil basin where the crust is 20 m y old the area is generally covered by a 150 to 200 m thick layer of foraminiferal ooze but basement outcrops are numerous sedimentary temperature profiles indicate anomalously low temperatures at the base of the sedimentary layer which results from cold bottom water penetrating laterally into the upper igneous crust and absorbing much of the geothermal heat net influx rates of 10 8 m s 1 are implied by a simple aquifer model a negative pressure drop across the sedimentary layer is required by this interpretation refraction studies indicate low seismic velocities 3 3 to 4 5 km s 1 near the top of layer 2 and a large velocity gradient with depth the velocity structure in the upper kilometer of the crust is related to the porosity from which it is inferred that permeabilities there are high many of the 52 temperature profiles show a significant decrease in gradient with depth the curvature can be attributed to one or a combination of the following causes 1 upward advection of pore water at rates of 10 7 s 1 2 bottom water temperature variations 3 variation of conductivity with depth and 4 experimental errors interpretation of curvature in terms of an advective model leads to results hard to reconcile with present knowledge of pressure variations and permeabilities of the oceanic crust

reprint of comparison of modern fluid distribution pressure and flow in sediments associated with anticlines growing in deepwater brunei and continental environments iran abstract differences in fluids origin creation of overpressure and migration are compared for end member neogene fold and thrust environments the deepwater region offshore brunei shale detachment and the onshore arid central basin of iran salt detachment variations in overpressure mechanism arise from a the availability of water trapped in pore space during early burial deepwater marine environment vs arid continental environment and b the depth temperature at which mechanical compaction becomes a secondary effect and chemical processes start to dominate overpressure development chemical reactions associated with smectite rich mud rocks in iran occur shallow 1900 m smectite to illite transformation causing load transfer related moderate overpressures whereas mechanical compaction and inflationary overpressures dominate smectite poor mud rocks offshore brunei the basal detachment in deepwater brunei generally lies below temperatures of about 150 c where chemical processes and metagenesis are inferred to drive overpressure development overall the deepwater brunei system is very water rich and multiple opportunities for overpressure generation and fluid leakage have occurred throughout the growth of the anticlines the result is a wide variety of fluid migration pathways and structures from deep to shallow levels particularly mud dykes sills laccoliths volcanoes and pipes fluid escape pipes crestal normal faults thrust faults and widespread inflationary type overpressure in the central basin the near surface environment is water limited mechanical and chemical compaction led to moderate overpressure development above the upper red formation evaporites only below thick early miocene evaporites have near lithostatic overpressures developed in carbonates and marls affected by a wide range of overpressure mechanisms fluid leakage episodes across the evaporites have either been very few or absent in most areas locations where leakage can episodically occur e g detaching thrusts deep normal faults salt welds are sparse however in both iran and brunei crestal normal faults play an important role in the transmission of fluids in the upper regions of folds

abundance of smectite and the location of a plate boundary fault barbados accretionary prism isolating the respective roles of factors responsible for the initiation and localization of fault zones remains one of the more important goals of research in neotectonics the frontal decollement zone of the northern barbados accretionary prism provides several important clues as to how the interwoven variables of clay mineralogy fluid flow chemical interactions and sediment physical properties affect strain localization this plate boundary fault is centered at a lithologic contact between miocene claystone and fine grained oligocene turbidites the fault zone is nearly 40 m thick at ocean drilling program site 948 and its upper part passes through smectite rich deposits a sharp minimum in percent smectite and a maximum in percent illite mark the base of the decollement there is a consistent increase in percent smectite with distance above the base of the decollement but the top of the decollement is poorly defined by clay mineralogy the intrinsic mechanical weakness of strata with abundant smectite group clays probably influences where the fault tip propagates into the undeformed stratigraphy of the atlantic abyssal plain a second inherited parameter is the local abundance of radiolarians which contribute to higher than normal porosities sediment shear strength also decreases because pore pressure within the fault zone is significantly greater than hydrostatic the principal cause of excess pore pressure seems to be updip fluid advection in theory however decreases in pore fluid salinity and porosity collapse should increase the amount of physicochemical stress generated by expandable clay minerals the imported fluid is unusually low in salinity because it has migrated from zones of deeper seated dehydration reactions if fresher pore water migrates to the propagating tip of the decollement its arrival should increase smectite swelling and reduce the shear strength of the mudstone even more the location and evolution of the decollement therefore are controlled by a complicated interplay of static factors inherited from the abyssal atlantic stratigraphy and dynamic factors associated with episodic fluid flow and changing fluid chemistry

fluid flow paths in the middle america trench and costa rica margin the hydrology of the subducting plate and its dewatering behavior through the shallow subduction zone is linked to the structure and deformation of the forearc prism the nature of the seismogenic zone the composition of seawater for selected elements and the composition of the residual slab subducted to depths of magma generation at the volcanic arc two locally independent systems of fluid flow govern the transport of heat and chemistry through the costa rica subduction complex a dominantly nonaccretionary subduction zone one fluid system is the margin wedge decollement and underthrust sediment section fluid sources include local sediment compaction and mineral dehydration at depth a second flow system occurs in basement beneath the sedimentary sequence on the incoming plate this region is characterized by extremely low conductive heat flow and the sediment overlying basement has pore water geochemistry similar to that of seawater flow nearly parallel to the trench could be directed by permeability associated with faults and driven by a combination of differential heating and earthquake strain cycling

fault system and thermal regime in the vicinity of site nghp 01 10 krishna godavari basin bay of bengal abstract drilling coring activities onboard joides resolution for hydrate resource estimation have confirmed gas hydrate in the continental slope of krishna godavari kg basin bay of bengal and the expedition recovered fracture filled gas hydrate at the site nghp 01 10 in this paper we analyze high resolution multi channel seismic mcs high resolution sparker hrs bathymetry and sub bottom profiler data in the vicinity of site nghp 01 10 to understand the fault system and thermal regime we interpreted the large scale fault system 5 km predominantly oriented in nnw sse direction near nghp 01 10 site which plays an important role in gas hydrate formation and its distribution the increase in interval velocity from the baseline velocity of 1600 m s to 1750 1800 m s within the gas hydrate stability zone ghsz is considered as a proxy for the gas hydrate occurrence whereas the drop in interval velocity to 1400 m s suggest the presence of free gas below the ghsz the analysis of interval velocity suggests that the high concentration of gas hydrate occurs close to the large scale fault system we conclude that the gas hydrate concentration near site nghp 01 10 and likely in the entire kg basin is controlled primarily by the faults and therefore has high spatial variability we also estimated the heat flow and geothermal gradient gtg in the vicinity of nghp 01 10 site using depth and temperature of the seafloor and the bsr we observed an abnormal gtg increase from 38 c km to 45 c km at the top of the mound which remarkably agrees with the measured temperature gradient at the mound nghp 01 10 and away from the mound nghp 01 03 we analyze various geological scenarios such as topography salinity thermal non equilibrium of bsr and fluid gas advection along the fault system to explain the observed increase in gtg the geophysical data along with the coring results suggest that the fluid advection along the fault system is the primary mechanism that explains the increase in gtg the approximate advective fluid flux estimated based on the thermal measurement is of the order of few tenths of mm yr 0 37 0 6 mm yr

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

tectonic context of fluid venting at the toe of the eastern nankai accretionary prism evidence for a shallow detachment fault during the kaiko nankai diving cruise the peak of the venting activity was located near the top of the very first anticline the most prominent morphological feature between the mid slope 3870 m and the apex of the fold 3770 m is a 20 m high cliff cutting through subhorizontal massive mudstones affected by numerous joints the trend of this scarp is oblique to the fold axis and structurally controlled along two sharply defined nne sse and e w directions fresh talus and blocks found locally suggest active tectonics and recent erosion intense deformation is evident from strongly tilted strata restricted to the base of the cliff that we interpret as an upslope thrust at the scale of seabeam mapping this thrust can be followed eastward for more than 5 km along the 3820 m isobath two seismic lines recorded during one of the pre site surveys show deformation at shallow depth including small scale folding and thrusting affecting only the wedge shaped top sequence deeper layers can be traced continuously below this sequence we conclude that the boundary between the piggy back basin and the frontal fold turbidites acts as a shallow detachment fault and interpret the base of the cliff as the outcrop of the fault dense colonies ofcalyptogena clams and strongly nonlinear thermal gradients locate the major peak of fluid activity at the edge of the plateau above the main cliff scattered biological colonies as well as white bacterial mats and cemented chimneys were also found in a narrow belt along the base of the cliff fluid activity is thus closely related to the shallow detachment fault fluid being expelled both at the outcrop of the fault and above it through the overlying strata possibly using the very dense joint network

isotopic evidence for channeled fluid flow in low grade metamorphosed jurassic accretionary complex in the northern chichibu belt western shikoku japan the low grade metamorphosed jurassic accretionary complex of the northern chichibu belt hijikawa area western shikoku is divided into two units the hijikawa and kanogawa units that are separated by an out of sequence thrust oost the ozu kawabegawa fault the kanogawa unit south of the ozu kawabegawa fault consists mainly of sandstone shale broken formation of alternating sandstone and shale greenstone chert and pelitic melange while the hijikawa unit is characterized by a stack of subunits separated by small scale thrusts the subunits are mainly made up of basic pelitic and psammitic semischists schistose pelitic melange and chert petrological and mineralogical constraints suggest peak metamorphic conditions of 204 247 c at 1 3 kbar in the kanogawa unit and 228 289 c at 3 5 6 kbar in the hijikawa unit quartz and quartz calcite veins are widely developed in the study area especially in the hijikawa unit regional variations in stable isotopic data show that the 18oquartz and 18ocalcite values in veins tend to increase towards the ozu kawabegawa fault the 18owhole rock values are remarkably high in some subunits close to oosts within the hijikawa unit oxygen isotopic compositions from vein quartz indicate that a higher 18o fluid migrated upward from depth along the ozu kawabegawa fault within the accretionary prism during prehnite pumpellyite facies metamorphism the fluid source is inferred to be pelitic rocks at higher temperatures and greater depths within the accretionary prism

time variations of fluid expulsion velocities at the toe of the eastern nankai accretionary complex abstract the temperatures in the bottom seawater and down to a depth of 60 cm in the sediment were measured over two active fluid seeps at the toe of the eastern nankai accretionary complex for periods of 12 and 32 days successively at the first station the fluid advection velocity is low less than 10 m a and the sediment temperature fluctuations closely reflect those of seawater at the second location the fluid advection velocity is greater with a darcy velocity of the order of 100 m a and sediment temperature fluctutations mainly reflect variations in the fluid velocity the temperature data suggest amplitude fluctuations of the fluid velocity of the order of 15 over characteristic periods of about 14 days they also suggest a 30 increase in the velocity over the one month period of the observation

implications of measured thermal gradients for water movement through the northeast japan accretionary prism thermal gradients of 24 36 c km have been found in the slope cover sediments of the accretionary prism off northeast japan deep sea drilling project dsdp sites 438a 439 and 440 these gradients lead to the prediction of temperatures in excess of 400 c at depths of 15 km using an increase in thermal conductivity with depth due to the decrease in porosity such temperatures conflict with the predictions of temperatures of 150 c at even greater depths from conductive thermal models and petrologic studies of low temperature blueschists at similar ancient margins one dimensional steady state conduction and advection via darcian water flow of heat up through the prism was simulated numerically the measured thermal gradients can be reconciled with temperatures less than 125 c at the prism base by water flowing upward at rates of the order of 10 10 m s such velocities imply that the total volume of water available from compaction of sediments subducted beneath the northeast japan prism would flow up through a 10 km width of the prism perpendicular to the trench and would require the dsdp sites to have been located fortuitously over the localized zone s of water flow smaller velocities will reproduce the measured gradients and thus greater widths through which water flow occurs are predicted if the temperature at the prism base is higher and or if the prism is thinner and a subduction channel several kilometers high separates the accreted sediments from the descending plate where water flow is significant as likely wherever abundant water rich sediments are subducted thermal gradients below a few kilometers in accretionary prisms may be much lower than near surface values this has profound implications for the thermal structure of the prism off northeast japan and elsewhere

low altitude permafrost research in an overcooled talus slope rock glacier system in the romanian carpathians detunata goală apuseni mountains abstract ground and air temperature monitoring geophysical soundings and dendrological investigations were applied to a basaltic talus slope rock glacier system from detunata site in the apuseni mountains western romanian carpathians to verify the presence of sporadic permafrost at 1020 1110 m asl well below the regional limit of mountain permafrost the near 0 c mean annual ground surface temperatures imposed by the large negative annual thermal anomalies of the ground up to 7 4 c together with the high resistivity values and the occurrence of trees with severe growth anomalies support the presence of permafrost at this location temperature measurements and ground air circulation experiments proved that the so called chimney effect is the main process favoring the ground overcooling and allowed for the construction of a model of ground air circulation in complex morphology deposits the texture and porosity of the debris were quantified along with the local morphology in order to evaluate their role upon the chimney circulation the debris porosity was found to be very high promoting intense ground overcooling during the cold season including the periods of high snow cover due to the development of snow funnels it efficiently reduces the heat transfer during summer thus contributing essentially to permafrost preservation in compound morphologies the depressed and low lying features are the cold zones subjected to winter overcooling and summer chill while the high positioned and convex up landforms become warm air evacuation features with positive thermal anomalies tree ring measurements showed that the growth of cold affected trees is higher during colder intervals years to decades probably as a consequence of the weakened katabatic air outflow during cooler summers the dendrological analysis of multi centennial spruces and their growth rates also provided palaeoclimatic inferences for the last 200 years

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

geophysical survey of a mud volcano seaward of the barbados ridge accretionary complex a detailed geophysical investigation that included multibeam bathymetry 3 5 khz channel and single channel seismic profiles 20 heat flow measurements and four piston cores was made of a mud volcano located about 8 km seaward of the toe of the barbados ridge accretionary complex in the northern guiana basin the material erupted from the volcano covers an oval area about 3 9 km with its long axis oriented e w roughly perpendicular to the deformation front a flat topped mound on the western side of the feature is the currently active center acoustic stratigraphy suggests that mud volcanism began at the site about 200 000 ma a piston core taken on the flank of the active mound sampled deposits erupted 20 000 30 000 ma that contain late miocene nannofossils heat flow on the active mound is 4 times that in the surrounding basin one dimensional modeling of this anomaly supports a low level continuing flux of mud and water at rates of 2 5 cm yr the mud volcanism appears to have arisen from the combined action of overpressured water that comes from beneath the accretionary prism and existing overpressures in the oceanic section large east west trending basement escarpments underlie the two known mud volcanoes seaward of the barbados ridge accretionary complex and they probably play an important role in developing vertical pathways for fluid to escape and in determining the location where mud volcanism starts

contrasting geothermal regimes of the barbados ridge accretionary complex three e w transects of closely spaced heat flow measurements across the barbados accretionary complex are reported one transect about 100 km in length at 14 20 n and a shorter 15 km line at 14 35 n show that heat flow decreases arcward roughly in proportion to the thickening of the wedge over the actively accreting zone this decrease is due to the combined thermal effects of the underthrusting oceanic lithosphere and thickening of sediments that have been ingested into the wedge except for anomalously high heat flow over a mud volcano on the eastern end of the 100 km transect fluid expulsion appears to have only a secondary effect on the seafloor heat flow in this region an increase of heat flow is observed at the western end of the 100 km transect where high angle arcward vergent thrusting is occurring we attribute this increase primarily to a decrease in the arcward thickening of wedge material and heat generated by friction on the decollement in strong contrast a short line of stations at 15 30 n the deep sea drilling project ocean drilling project dsdp odp drilling transect found anomalously high heat flow a 40 increase over the heat flux from the lithosphere with large variability over the most recently accreted material and also in the adjacent basin the thermal regime in the vicinity of the dsdp odp transect is dominated by a strong flow of pore fluids along the decollement and upward flow along fault planes in the toe of the wedge and adjacent basin sediments heat transport by pore fluids also appears to dominate the subseafloor thermal regime in a zone near 13 50 n the thermal effects of dewatering of the sediments by upward diffusive flow were examined using a simple one dimensional model which showed that the resulting effects are too small to produce detectable heat flow anomalies consequently the flow of fluids must be channelized in subseafloor conduits to produce the observed anomalies in heat flow

heat flow tectonics and fluid circulation at the toe of the barbados ridge accretionary prism detailed patterns of near surface tectonics and heat flow were investigated at two locations at the toe of the barbados ridge accretionary prism at the two locations in the thickly sedimented southern portion of the prism near 12 20 n and in the northern area drilled during ocean drilling program odp leg 110 near 15 32 n relative highs of heat flow were associated with zones of thrusting we attribute them to upward advection of water rising from the decollement along faults through the sediments in the odp area a significant heating plate effect from warm fluids flowing along the decollement and thrusts is required to account for the high regional heat flow this effect implies high rates of fluid flow at the decollement which can only be non steady state or would require massive lateral transport of fluids along strike

the tectonic fabric of the barbados ridge accretionary complex abstract a map showing near surface structure and other surficial geological features on the barbados ridge accretionary complex has been constructed from gloria long range side scan sonographs detailed bathymetric surveys using seabeam and single and multichannel seismic reflection sections the map shows major variations in the size orientation and style of structures across the accretionary complex that are principally related to the increasing thickness of the complex and the sediment drape upon it and variations along the complex that derive from the northward thinning of sediment accreted from the ocean floor and the effects of ridges in the oceanic basement the development of crossfaulting is mainly associated with basement ridges mud diapirism is abundant in the southern part of the complex and is characteristic of that part of the complex into which turbidites from the orinoco submarine fan are accreted slumping on a variety of scales is another feature of the complex the map is presented at a scale of 1 1063415 4 inches per degree of longitude and also shows bathymetry at 200 m contour intervals

geochemical and geothermal evidence for fluid migration in the barbados accretionary prism odp leg 110 measured geochemical and geothermal effects of fluid migration in the northern barbados accretionary prism indicate that 1 fluid flows laterally along low angle active faults 2 the entire length of the leg 110 transect is a fluid discharge zone 3 migration results in heating of sediments across the transect but most strongly at the deformation front and 4 migration results in dilution of pore water cl and sup 18 o enrichment more in the arcward than the seaward side of the prism suggesting that the cl poor sup 18 o rich fluids that intrude the prism are derived from smectite illite reactions occurring arcward these conclusions have strengthened those derived from the initial appraisal of shipboard data modified ideas about where fluids are heated most and where the geochemistry is strongly altered and provided new ideas about the sources of fluids and their relative contributions

permeability of a decollement zone results from a two well experiment in the barbados accretionary complex fault zones are hypothesized to play a major role in focusing fluid expulsion from accretionary complexes a small number of previous investigations have examined the hydrogeologic properties of these fault zones using borehole packer or submersible based slug and constant rate flow tests while these single well experiments yielded the first data on the permeability of active fault zones they could only investigate a small area around the borehole and could not ensure reliable determination of formation storage properties recent data from an unplanned experiment in the decollement zone of the barbados accretionary prism demonstrate the potential for multiple well hydrogeologic tests pressure data from a sealed borehole show a clear signal of drilling activity 45 m away analysis of the pressure response suggests that decollement zone permeability lies between 1 0 and 1 2 x 10 14 m 2 this permeability is 2 4 orders of magnitude greater than overlying and underlying sediment and thus appears sufficient to focus fluid flow along the decollement zone this inadvertent test evaluated fault zone hydrogeologic parameters over a larger radius and longer timescale and at pressures closer to in situ than previous single well tests

generation of high pore pressures in accretionary prisms inferences from the barbados subduction complex recent geological and geophysical exploration of several accretionary complexes reveals that pore pressures inside them are much higher than the hydrostatic values indirect inferences further suggest that such high pore pressures may be present inside most accretionary complexes in this study the mechanisms for generating excess pore pressures is investigated by focusing on the barbados ridge complex which is an end member of sediment rich accretionary prisms modeling is carried out using a two dimensional finite element procedure with constraints provided by the following data deep sea drilling project results on stratigraphy and lithology seismic results on sediment structure and thickness regional geology on tectonic history and laboratory measurements on mechanical properties of sediments coupling among the equations for fluid flow heat transfer and sediment deformation is made throughout the computation the results of the modeling predict that the principal decollement beneath the barbados ridge complex is a zone of high pore pressure high porosity and reversal in seismic velocity these characteristics may be responsible for the mechanical weakness of the decollement which allows continuous subduction of sediments below it and for its seismic signatures results of the modeling further suggest that tectonic overburden may be the most important mechanism for the generation of high pore pressures inside accretionary prisms the direct effect of tectonic compression is of secondary importance and the effect due to temperature increase with subduction is of little significance thrust faulting near the toe of accretionary prisms can disturb pore pressure patterns and produce localized highs rich sediment supply low permeability high subduction angle and high subduction velocity are all favorable factors in creating high pore pressures inside accretionary prisms

in situ observations of earthquake driven fluid pulses within the japan trench plate boundary fault zone transient fluid flow within faults is suspected to be an important component of the earthquake cycle and subduction zone evolution however an understanding of the mechanisms and time scales involved has been limited due to a paucity of direct measurements here we report on in situ observations that appear to capture the thermal signature of earthquake driven fluid pulses within the damage zone of the japan trench plate boundary fault the data are from a sub seafloor temperature observatory installed through the fault following the march 2011 mw 9 0 tohoku oki earthquake as part of the integrated ocean drilling program s japan trench fast drilling project jfast high resolution temperature time series data reveal spatially correlated transients in response to earthquakes that are indicative of advection by transient fluid flow we interpret the observed phenomenon as reflecting pressure redistribution in a fault zone and a potential mechanism for earthquake triggering and episodic heat and chemical transport

frontal accretion along the western mediterranean ridge the effect of messinian evaporites on wedge mechanics and structural style in the context of the imerse project several crossings of the deformation front of the western mediterranean ridge were made in the region of the sirte abyssal plain the messina abyssal plain and the intervening region in this paper we present seismic images and interpretations across the deformation front with particular emphasis on the role the messinian evaporites have played in controlling the accretionary tectonics of the thin frontal portion of the wedge the seismic images show that the basal detachment generally is located at the base of the evaporites from a consideration of the mechanics of the wedge for both coulomb and plastic rheologies we show that the low wedge taper c 2 requires that the detachment is characterised by extreme fluid overpressuring within 2 of lithostatic in places and that the basal yield stress less than 1 mpa is lower than that of a wet salt decollement zone this supports the seismic interpretation that the detachment occurs in overpressured sediments beneath the impermeable evaporites lateral variations in the accretionary style can be related to lateral variations in evaporite thickness the effectiveness of the evaporite as an impermeable seal and to local relief on the subducting plate these factors control the escape of fluids from beneath the evaporites and hence fluid pressure and basal yield stresses

continental margin tectonics forearc processes recent studies of convergent plate margins and the structural development of forearc terranes are summarized in a critical review of u s research from the period 1987 1990 topics addressed include the geometry of accretionary prisms coulomb wedge taper and vertical motion in response to tectonic processes offscraping vs underplating or subduction the response to oblique convergence fluids in forearc settings the thermal framework and the effects of fluid advection and serpentinite seamounts also included is a comprehensive bibliography for the period

hydrogeologic properties of a thrust fault within the oregon accretionary prism two sets of hydrogeologic tests conducted at ocean drilling program odp hole 892 on the oregon accretionary prism provided the opportunity to determine hydrogeologic properties of an active accretionary prism fault zone the first set of tests consisted of shipboard packer tests conducted during odp leg 146 fall 1992 while the second set of tests were constant drawdown and constant discharge tests conducted in fall 1993 using the submersible alvin pressure response during the first set of tests suggests that fractures remained open until excess fluid pressure relative to hydrostatic dropped below 0 315 to 0 325 mpa 0 53 to 0 54 where pore pressure hydrostatic lithostatic hydrostatic analysis of the packer test data suggested an apparent background pressure of 0 25 mpa 0 42 to 0 50 because the borehole had been open for 12 hours prior to the packer tests formation pore pressures may have exceeded this value prior to drilling of the borehole these overpressures dissipated by the time the second set of tests were conducted one possible explanation for this decay is that the borehole may provide a vertical conduit between the overpressured zone and overlying or underlying sediments that had previously been hydraulically separated from the overpressured zone the second set of tests were conducted at pressures 0 019 mpa or 0 03 below that estimated to maintain open fractures and yielded transmissivities 1 to 2 orders of magnitude less than estimated for the packer tests when fractures were open constraints on fluid flow rate along the fault are provided by observed displacement in a bottom simulating reflector bsr at its intersection with the fault zone the closed fracture transmissivities are insufficient to produce flow rates capable of displacing the bsr therefore open fracture transmissivities under conditions of elevated pore pressure are inferred to be necessary for the observed bsr displacement in addition calculated rates of specific discharge through the fault zone are 2 to 3 orders of magnitude lower than discharge measured at an associated seafloor vent site fluid flow must become spatially or temporally focused as it moves up the fault zone toward the seafloor

transient fluid flow through the toe of the barbados accretionary complex constraints from ocean drilling program leg 110 heat row studies and simple models thirty four sediment and mud line temperatures were collected from six drill holes on ocean drilling program odp leg 110 near the toe of the barbados accretionary complex when combined with thermal conductivity measurements from sediment cores and results from earlier surveys these data delineate the complicated thermal structure on the edge of this convergent margin heat flow values at the seafloor of 92 192 mw m sup 2 are 80 300 higher than those predicted by standard heat flow versus age models for oceanic crust but are compatible with earlier seafloor measurements made in this area at the same latitude heat flow tends to decrease downhole at four sites suggesting the presence of heat sources within the sediments these results are consistent with the flow of warm fluids through the complex along high permeability conduits including thrust faults the major decollement zone and sandy intervals simple calculations suggest that this fluid flow is transient occurring on time scales of tens of thousands of years fluid flow velocities along the decollement zone estimated with a simple thermal model are about 10 sup minus 7 m s 100 times faster than predicted by numerical steady state analyses the estimated maximum sediment permeability within the decollement zone which was basedmore on the above fluid flow velocity is about 10 sup minus 12 m sup 2 also 100 times higher that that calculated numerically high heat flow in the vicinity of 15 degree 30 prime n and not elsewhere along the deformation front suggests that the leg 110 drill sites may be situated over a prism water discharge zone with dewatering more active here than elsewhere along the accretionary complex less

melange forming processes in the development of an accretionary prism evidence from fission track thermochronology to give thermotectonic constraints on melange forming processes and to understand the significance of melange in the evolution of accretionary complexes fission track analysis was carried out on samples collected from sandstone blocks in melange matrix and coherent sandstone sequences adjacent to melange units in the cretaceous to miocene shimanto accretionary complex southwest japan which is one of the typical accretionary complexes with an accumulation of geological information nineteen zircon ages and 15 track length distributions from melanges indicate with some local variation that materials with higher maximum paleotemperatures are exposed at the rear part of the prism as is the case of the coherent units two apatite ages are contemporaneous with previously reported apatite ages of 10 ma obtained from coherent units in the region emphasizing the regional cooling through the apatite closure temperature of 100 c at that time there is no evidence for substantial relative motion of the melanges with respect to the surrounding coherent units melange formation is explained by processes that exclude large vertical transport i e 5 km of melange forming materials these thermotectonic characteristics observed in the shimanto accretionary complex are maintained during subsequent local internal deformation caused by the successive growth of the wedge

deepwater fold and thrust belt classification tectonics structure and hydrocarbon prospectivity a review abstract deepwater fold and thrust belts dwftbs are classified into near field stress driven type 1 systems confined to the sedimentary section and type 2 systems deformed by either far field stresses alone or mixed near and far field stresses dwftbs can occur at all stages of the wilson cycle up to early stage continent continent collision type 1 systems have either weak shale or salt detachments they occur predominantly on passive margins but can also be found in convergent related areas such as the mediterranean and n borneo examples include the niger and nile deltas the west coast of africa and the gulf of mexico type 2 systems are subdivided on a tectonic setting basis into continent convergence zones and active margin dwftbs continent convergence zones cover dwftbs developed during continent arc or continent continent collision and those in a deepwater intracontinental setting e g w sulawesi makassar straits active margins include accretionary prisms and transform margins the greatest variability in dwftb structural style occurs between salt and shale detachments and not between tectonic settings changes in fold amplitude and wavelength appear to be more related to thickness of the sedimentary section than to dwftb type in comparison with shale salt detachment dwftbs display a lower critical wedge taper more detachment folds long and episodic duration of deformation and more variation in vergence structures unique to salt include canopies and nappes accretionary prisms also standout from other dwftbs due to their relatively long continuous duration rapid offshore propagation of the thrust front and large amount of shortening in terms of petroleum systems many similar issues affect all dwftbs these include the oceanward decrease in heat flow offshore increase in age of mature source rock and causes of trap failure e g leaky oblique and frontal thrust faults breach of top seal by fluid pipes one major difference between type 1 and type 2 systems is reservoir rock high quality continent derived quartz rich sandstones are generally prevalent in type 1 systems more diagenetically reactive minerals derived from igneous and ophiolitic sources are commonly present in type 2 systems or many are simply poor in well developed turbidite sandstone units however some type 2 systems particularly those adjacent to active orogenic belts are partially sourced by high quality continent derived sandstones e g nw borneo s caspian sea columbus basin in some cases very high rates of deposition in accretionary prisms adjacent to orogenic belts coupled with uplift due to collision results in accretionary prism related fold belts that pass laterally from sub aerial to deepwater conditions e g s caspian sea indo burma ranges the six major hydrocarbon producing regions of dwftbs worldwide gulf of mexico niger delta nw borneo brazil west africa s caspian sea stand out as differing from most other dwftbs in certain fundamental ways particularly the very large volume of sediment deposited in the basins and or the great thickness and extent of salt or overpressured shale sdetachments

relationships between the stable isotopic signatures of living and fossil foraminifera in monterey bay california 1 fossil foraminifera are critical to paleoceanographic reconstructions including estimates of past episodes of methane venting these reconstructions rely on benthic foraminifera incorporating and retaining unaltered the ambient isotopic compositions of pore fluids and bottom waters comparisons are made here of isotopic compositions of abundant live and fossil foraminifera uvigerina peregrina epistominella pacifica bulimina mexicana and globobulimina pacifica collected in monterey bay ca from two cold seeps clam flats and extrovert cliffs and from sediments 5 m outside of the clam flats seep clam flats has steep d 13 cdic gradients to 45 but dic at extrovert cliffs is less enriched in 12 c to approximately 22 oxygen isotope values of fossil foraminifera at clam flats are 1 5 enriched in 18 o over the living foraminifera as well as those of both live and fossil foraminifera at extrovert cliffs suggesting they may have lived during the last glacial maximum statistical comparisons student s t and kolmogorov smirnov tests of d 13 c and d 18 o values indicate that live and fossil foraminifera come from different populations at both clam flats and extrovert cliffs at clam flats the difference appears to result from alteration enriching some fossil foraminifera in 12 c over live foraminifera at extrovert cliffs the fossil foraminifera are enriched in 13 c over the live foraminifera suggesting they lived prior to the onset of venting and thus that venting began recently the short time of venting at extrovert cliffs may be responsible for the less alteration there compared with clam flats these results indicate that preservation of foraminifera is likely to be poor within long lived cold seeps but that foraminifera living in the surrounding sediment may incorporate and preserve broad basin wide changes in isotopic compositions of the ambient water

geophysical evidence for dewatering and deformation processes in the odp leg 170 area offshore costa rica abstract we use a combination of borehole data from ocean drilling program odp leg 170 and multichannel seismic reflection mcs data to quantify thickness changes in underthrust sediments away from the boreholes sediments thrust beneath the upper plate at convergent margins may be more rapidly loaded than in any other environment depending on the porosity and permeability of the available fluid pathways these sediments can compact and dewater very rapidly as observed in this area offshore the nicoya peninsula costa rica rapid thinning and dewatering was previously interpreted in this area from mcs data but the lack of velocity data in this deep water environment caused ambiguity in the estimates of thickness change we employ a non linear inversion technique using detailed density data primarily logs and some laboratory measurements and coincident mcs data to create 1d synthetic seismograms and detailed velocity functions at three odp drill sites because only a small part of one hole was logged with a sonic tool and the laboratory measurements significantly underestimate in situ velocities these results provide the most accurate estimate of the velocity profiles we used these velocity functions to depth migrate seven mcs lines in the vicinity of the trench and lower slope spanning a distance of 9 km along strike analysis of the depth migrated images shows that there is significant variation along strike in how the underthrust section compacts which appears to be related to the distribution of normal faults on the cocos plate we interpret that preferentially rapid dewatering in the upper part of the underthrust section may lead to deformation below the original decollement and detachments at deeper stratigraphic levels

organic matter in the franciscan and cedros subduction complexes the problems of instantaneous maturation and missing petroleum in accretionary prisms abstract most accretionary complexes are submarine mountain belts consisting predominantly of sedimentary rock past studies of accretionary complexes have concluded that they have minimal petroleum potential although they do contain potential source rocks owing to their great bulk of sedimentary rock large amounts of petroleum must be generated in accretionary prisms and these hydrocarbons must be trapped in the complexes or vented into the ocean or on land to understand the nature and abundance of organic material in accretionary complexes organic matter in the shales limestones and melange matrix of the franciscan and cedros subduction complexes was studied mostly using rock eval pyrolysis the largest amounts of total organic carbon toc were found in the calera limestone from the san francisco area 1 6 toc terrigenous sedimentary rocks of probable trench or trench slope origin contain from

upper ordovician and silurian ash beds in the holy cross mountains poland preservation in mudrock facies and relation to atmospheric circulation in the southern hemisphere numerous volcanic ash beds k bentonite are recognized globally in ordovician to early devonian sedimentary successions and are important for regional stratigraphic correlations relatively little however is understood of the origin and transport of these ash deposits here we examine ash beds preserved in the upper ordovician and silurian dark shales and mudstones of the holy cross mountains poland and discuss their origin in the context of contemporaneous tectonic configurations the ordovician ash beds occur in the upper sandbian and katian mudrock succession whereas the silurian k bentonites form prominent beds in the upper wenlock and lower ludlow graptolite shales their various thickness and irregular distribution reflect fluctuation of volcanic activity and contribution of physical and biogenic factors in their preservation deposition under dysoxic or anoxic conditions and relatively rapid burial by muddy sediment seem to be dominant factors preventing physical and biological reworking and mixing of the volcanic ash the upper ordovician ash beds in the holy cross mountains as well as the baltoscandian k bentonites appear to be accumulated from pyroclastic material delivered by westerlies of the southern hemisphere from the avalonian volcanoes in the silurian the distribution of pyroclastic material was controlled by se trade winds and therefore the source area for the wenlock and ludlow k bentonites in the holy cross mountains appears to have been an active subduction zone developed close to the southeastern side of baltica however the delivery of volcanic ash to the central part of baltica might have been strongly affected by climatically driven latitudinal migration of the intertropical convergence zone

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

interaction between the chile ridge and chile trench geophysical and geothermal evidence geophysical and geothermal data are examined from the three southernmost sections of the chile ridge starting at 44 s and continuing south to the triple junction of the nazca antarctic and south america plates at 47 s these sections represent three progressively younger stages in a ridge trench collision event corresponding to 3 m y before the collision 1 m y before the collision and culminating in an ongoing collision at the triple junction magnetic and bathymetric data across the chile rise indicate that there is little change in the configuration of the spreading center as the ridge approaches and then collides with the trench in the collision zone a normal looking rift valley can be traced for 40 km before it disappears beneath the toe of the landward trench slope there is no evidence for the complex pattern of ridge jumps and spreading center rotations that occurred when the pacific farallon spreading center collided with north america in contrast the overriding plate is greatly affected by the ridge collision the landward trench slope steepens and narrows as the collision zone is approached at the southern end of the collision zone a ridge associated with the taitao fracture zone is colliding with the trench and may be in the process of being obducted onto the landward trench slope geothermal measurements were made along three transects of the margin corresponding to the time 3 m y before the collision during the collision and 6 m y after the collision the heat flow measurements in the collision zone document a large pulse of heat associated with the subduction of the ridge values as high as 345 mw m2 were recorded on the lower trench slope a prominent bottom simulating reflector bsr observed over a wide area of the landward trench slope north of the triple junction and more locally south of the triple junction is used to expand the grid of heat flow observations excellent agreement is found between measured heat flow and estimates of heat flow based on the depth to the bsr the heat flow measurements compare favorably with a theoretical model assuming conductive heat flow we estimate that the accretionary prism has been substantially removed in the collision zone and conclude that the landward trench slope is undergoing an episode of rapid tectonic erosion periodic ridge collisions in the past may account for the apparent truncation of the andean forearc region

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

elevated fluid pressure and fault zone dilation inferred from seismic models of the northern barbados ridge decollement in 1992 a large volume of three dimensional seismic reflection data were acquired in a 5 25 km area across the toe of the barbados accretionary complex that covers the deep sea drilling project leg 78a and ocean drilling program legs 110 and 156 drilling sites these data are used to examine the acoustic character of the decollement seismic reflection and to qualitatively and quantitatively characterize fluid pressures within the fault zone seismic models have been constructed across a 6 km region of the decollement where it has been mapped as a moderate to bright polarity reversed reflection the models show that this segment of the decollement reflection is caused by a low velocity interval usually 12 16 m thick the top of the low velocity interval appears to be a sharp boundary that requires a decrease in velocity from 1 8 km s to between 1 7 and 1 65 km s with some localized bright reflections with an even lower velocity of 1 6 km s the base of the low velocity layer is less certain from modeling the base consists of either a velocity increase that is usually approximately half the velocity contrast at the top of the layer or the velocity increase is equal to the contrast at the top of the layer but distributed over a 10 m thick interval comparison of these results to laboratory experiments on the relationship between fluid pressure and seismic velocity indicates that in this interval of the decollement fluid pressure is at or near lithostatic furthermore the reflection coefficients of the decollement are sufficiently large that some dilation of the fault zone is required the dilation should lead to high fracture zone permeability and explain the observation of a laterally consistent decollement reflection along a 5 km segment of the decollement it is within these segments of the fault that fluid pressure approaches lithostatic and significantly reduces fault strength

tsunami earthquakes as transient phenomena 1 recent modeling of tsunami waveforms caused by tsunami earthquakes indicates that fault slip occurred along the subduction boundary close to the trench axis on the other hand it is well known that seismicity is generally very low within ca 50 km from the trench axis and this has been attributed to the stable frictional sliding properties of the subducted unconsolidated sediments this makes tsunamigenic slip near the trench difficult to explain in order to solve this dilemma i propose that transition from stable sliding to nearly zero friction with a velocity weakening property occurs as a result of areal increase of zones of elevated fluid pressure which may make fairly rapid seismic slip possible following the breakage of asperities at normal seismogenic depths this transition can be identified as seismic reflections with negative polarities which may help to rate hazards for a coming large tsunami earthquake