subduction of the daiichi kashima seamount in the japan trench lallemand s culotta r and von huene r 1989 subduction of the daiichi kashimi seamount in the japan trench in j p cadet and s uyeda editors subduction zones the kaiko project tectonophysics 160 231 247 in 1984 1985 the kaiko consortium collected seabeam single channel seismic and submersible sampling data in the vicinity of the daiichi kashima seamount and the southern japan trench we performed a prestack migration of a shell multichannel seismic profile that crosses this area and examined it in the light of this unusually diverse kaiko dataset unlike the frontal structure of the northern japan trench where mass wasting appears to be the dominant tectonic process the margin in front of the daiiclr kashima shows indentation imbrication uplift and erosion emplacement of the front one third of the seamount beneath the margin front occurs without accretion we conclude that the daiichi kashima seamount exemplifies an intermediate stage between the initial collision and subduction of a seamount at a continental margin

fluid venting activity within the eastern nankai trough accretionary wedge a summary of the 1989 kaiko nankai results fluid flow manifestations within the outer 20 km of the eastern nankai accretionary wedge between the water depths of 4000 and 2000 m are concentrated near the outcrop of a shallow detachment fault immediately behind the frontal thrust near a depth of 3800 m and along a probable strike slip fault on the top of a 2000 m deep pop up structure 20 km inside the wedge using subbottom temperature measurements we estimate darcy upward flow velocities below clams colonies at the 3800 m site and show that one can associate a narrow range of velocities with the different types of colonies then combining a statistical survey of the distribution of colonies with the velocity estimates we compute a yearly outflow budget of 200 m 3 per metre of width of the wedge in the toe area this is forty times larger than the amount expected from steady state compactive dewatering we propose however that 80 90 of this flow is produced by shallow seawater convection because variations in sea bottom temperatures were too large we could not estimate fluid outflow budgets in the 2000 m venting area however 20 000 and 130 000 year old shells fossilized within widespread carbonate concretions indicate that fluid flow has been active in the 2000 m venting area since at least 130 000 years ago probably in a sporadic fashion chemical evidence indicates that a significant part of the fluid is coming from a depth of at least several hundred metres subbottom temperature measurements over a period of two months in the 3800 m venting site show fluctuations of 20 30 in the venting velocity below an active colony that have an apparent period of two weeks although the fluid circulation appears to be controlled by active faults at least in the shallow part of the wedge not every active fault corresponds to an aquifer in the absence of good high resolution seismic data and drill holes we cannot describe this plumbing system in any detail and are not able to show why some active faults correspond to fluid outlets and some do not

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

deep scientific dives in the japan and kuril trenches abstract in the summer of 1985 during the french japanese kaiko program ten dives to depths of 6000 m in the japan and kuril trenches were made in the newly launched submersible nautile the sites of the dives were selected on the basis of surface geophysical surveys made during the preceding summer involving seabeam mapping geomagnetic and gravimetric measurements and single channel seismic profiling the results of the dives provide new constraints on the geodynamics of these subduction zones in the japan and kuril trenches huge slump scars were observed on the landward slopes of the trenches slumps produce a typical active erosional morphology with vertical or even overhanging cliffs in poorly consolidated material the slump scars allowed us to observe the internal structure of the margin the monoclinal structure on the northern japan trench margin deduced from the seismic profiles and dsdp drilling was confirmed several dives on kashima seamount confirmed that this volcano has recently been split into two parts by a normal fault system comparisons of lithology and paleontology on the two separated parts of the seamount were made deep sea clams colonies were observed from nearly 6000 m up to 5000 m on the landward slopes of the trenches it can be concluded that the whole margin is venting fluids from depths of 2 3 km which is consistent with the indications of overpressure observed in drill sites on the japan trench margin the fluids probably originate by dewatering of the subducting sediments and then migrate to the seafloor

deep biological communities in the subduction zone of japan from bottom photographs taken during nautile dives in the kaiko project abstract twenty seven dives of the submersible nautile in the subduction zone around japan conducted in the french japanese project kaiko proved that fairly luxuriant benthic communities dominated by deep sea giant clams of the genus calyptogena family vesicomyidae were consistently present on the accretionary prism at abyssal depths benthic communities characterized by three hitherto undescribed bivalves of the genus calyptogena were found between depths of about 3800 and 4020 m at the mouth of tenryu canyon and at the top of basement swell of the zenisu ridge both situated in the eastern nankai subduction zone sporadic but discrete patches of organisms characterized by one more undescribed bivalve belonging to the genus calyptogena were observed and collected between depths of 5130 and 5960 m on the landward wall of the japan and kouriles trenches photographic inventories were prepared semiquantitatively using each series of bottom photographs taken in these areas with bow cameras of the submersible nautile observations on the sporadic but dense distribution of the clams and other characteristic associated organisms match well with the scheme that communities sustained by chemosynthetic energy sources can be present at connate water seepages in subduction zones these are to date the deepest record of benthic communities supposedly associated with chemosynthetic processes

geochemistry of temperate water carbonates tasmania australia abstract temperate shelf sediments off tasmania are characterised by high caco 3 contents low concentrations of mg indicate ambient water temperatures of less than 11 c which are in agreement with calculated o 18 water temperatures these mg concentrations also suggest a mixture of low mg and high mg calcites in the sediment in contrast to warm water carbonates which contain high sr moderate na and low mn temperate water shallow marine carbonates are low in sr and high in na mn and fe sr concentrations are low because of the preponderance of calcitic fauna and calcite cement high na is mainly due to biochemical fractionation and some aragonite content high concentrations of mn and fe are also due to the dominantly calcitic mineralogy of skeletal components and marine cements

deep sea submersible survey in the suruga sagami and japan trenches preliminary results of the 1985 kaiko cruise leg 2 abstract nine submersible dives were made in three trenches off central japan between 2990 and 5900 m of water depth our observations confirm the interpretation that daiichi kashima seamount is a cretaceous guyot formed on the pacific plate that has traveled into the japan trench we also confirmed the previous interpretation of a large normal fault that splits the seamount in two halves the lower one being now subducting beneath the japan margin compressional deformation was identified within the lower part of the inner slope in front of the seamount the pattern of deformation that affects quaternary sediments is in agreement with the present kinematics of the convergence between the pacific plate and japan deep water 5700 m clam colonies are associated with advection of fluids driven by the subduction related overpressures in the northern slope of the boso canyon along the sagami trough system philippine sea plate japan boundary the deformation affecting a thick upper miocene to lower pliocene sequence indicates two directions of shortening a n175 e direction which is consistent with the present relative motion along the sagami trough n285 n300 e and a n30 e direction which could be related to a more northerly direction of convergence that occured during the early quaternary and earlier

the eastern and western ends of nankai trough results of box 5 and box 7 kaiko survey abstract seabeam mapping and detailed geophysical surveying have been conducted over the eastern and western ends of the nankai trough the eastern survey covers the transition between the large izu bonin izu ogasawara ridge collision with honshu and the nankai trough subduction it includes a northeast trending basement ridge the zenisu ridge to the southeast of nankai trough as well as two large channel systems one following the trench the nankai channel and the other coming from the north the tenryu canyon the zenisu ridge is a zone of recent intra plate shortening consisting of three distinct segments with an increasing deformation from southwest to northeast we interpret this gradient of deformation as a way to absorb the kinematic discontinuity between the diffuse shortening prevailing over the izu bonin ridge and the concentrated shortening along the nankai trough the shortening axis appears to change from north south along the zenisu ridge to northwest southeast along the nankai trough and finally to east west within honshu the western survey covers the junction between the nankai and ryukyu trenches and the kyushu palau ridge which is subducted without major internal deformation the ridge appears to act as an indenter upon the margin the tip of the indenter is presently situated under the upper accretionary prism

nankai trough and zenisu ridge a deep sea submersible survey abstract eight submersible dives between 3000 and 4200 m water depth were made off southern japan in the eastern nankai subduction zone benthic communities associated with chemosynthetic processes were discovered along the 800 m wide active tectonic zone at the toe of the accretionary prism a benthic community was also discovered along a zone of active compression at the foot of zenisu ridge 30 km south of nankai trough temperature measurements within the sediments below the benthic communities confirm that upward motion of interstitial water occurs there studies of water samples indicate advection of methane and light hydrocarbons specimens of the benthic community have been shown to have included in their shells carbonate resulting from methane consumption thus the benthic communities are related to overpressure driven fluid advection along tectonic zones with active surface deformation a 300 m high active scarp at the toe of the accretionary prism is related to relative motion in a 280 direction which is close to the 305 average direction of subduction in this area the dives establish further that compressive deformation is presently occurring at the foot of zenisu ridge the previous interpretation of the zenisu ridge as a zone of recent north south intraplate shortening 40 km south of the nankai trench is confirmed we conclude that tectonic evolution might well lead to future detachment of the zenisu ridge and overthrusting of this large piece of oceanic crust over the continental margin such a process might be an efficient one to emplace ophiolites over continents

nankai trough japan trench and kuril trench geochemistry of fluids sampled by submersible nautile abstract deep water samples collected during the kaiko project are often associated with biological communities located on geological structures favorable to fluid venting the evidence of fluid venting are the temperature anomalies the decrease in sulfate concentrations the content in methane and the lowc 1 c 2 c 3 ratio of light hydrocarbons because of large dilution by ambiant seawater during sampling it is difficult to compute the composition of the advected end member pore fluid part of this fluid should originate in the petroleum window i e at temperature about 60 c modeling the upward flow of water taking into account the anomalies of temperature measured on the seafloor and the geochemical anomalies leads to non steady state advection of the pore fluid the occurrence of a deep component in the fluid has implications for the geological and tectonic models of the subduction zones off japan

geochemistry of the tenryu canyon deep sea fan biological community kaiko abstract the tenryu canyon deep sea fan biological community is characterized by both reduced and oxidized sediments in the immediate vicinity of the pore water vents the upper sediments in contact with the clams are reduced the organic matter is enriched in sulfur and inorganic sulfides fe cu zn are forming towards the outer fringes of the communities the sediment is oxidized and metals generally associated with ferro manganese oxides are concentrated several metals cd pb mo show distributions which are strongly influenced by the metabolism of the clam colony comparison of water and sediment geochemistry leads to the conclusion that there should be a downward flux of oxygenated seawater on the boundaries of the colony and an upward flux of chemically more reduced deep pore water at the location of the colony trace metals anomalies as well as 15n anomalies of organic matter may be useful to prospect for extinct venting areas in ancient subduction zones

seismic stratigraphy bsr distribution and venting of methane rich fluids west off paramushir and onekotan islands northern kurils abstract single channel seismic recording water chemistry determination and sediment sampling were carried out off the northern kuril islands and the southern tip of kamchatka a bottom simulating reflector bsr was mapped in detail the shipboard analyses of bottom water samples yielded methane concentrations up to 1000 nl 1 west off paramushir island venting of methane rich fluids from the seafloor into the water column is related to unusual trajectories of the bsr which emerges towards the seafloor the destabilisation of gas hydrates and the origin of the vent site may correspond to anomalous high heatflow caused by recent volcanic intrusions into the sediment no venting was found in the areas off the southern tip of kamchatka and off onekotan island

fluid venting along japanese trenches tectonic context and thermal modeling abstract large benthic chemosynthetic communities have been observed at four main locations during the kaiko submersible dives in the japanese trenches they appear to be associated with venting along fractures the first site for our observation was along the japan and kuril trenches where the continental margin is eroded by the subducting plate and collapses into the trench the benthic communities there seem to be related to tension gashes parallel to the subduction vector the other communities were found on the toe of the nankai accretionary prism along the frontal thrust and tension gashes the temperature anomaly associated with one of the communities is modeled to constrain the upward flow of interstitial water as the anomaly has a small spatial extent and as the peak thermal gradient is high the best fitting model is to be found in a vertical upward flow at a velocity of 100 m yr in a cylindrical conduit leading out of an underlying shallow thrust

diminished amplitude reflections fadeout in bermuda rise stratification abstract single and multichannel deep tow seismic records collected on the southwestern bermuda rise reveal areas within the acoustic layering where the intensity of reflectors fade or even disappear over lateral distances of 20 60 m the fadeouts are conspicuously present in a highly stratified interval consisting of lower to middle eocene siliceous turbidites and thin cherty layers interbedded with hemipelagic pelagic clays and oozes acoustic scattering at rough interfaces gas pockets and changes in geoacoustic properties do not readily account for the diminished amplitudes of the reflections detailed examination of the wave coda from selected fadeout areas reveal changes in reflector geometry due to faulting of the sediments strong reflections within the stratified sequence are thought to result from the thin high impedance chert beds that occur within the turbidite layers secondary reflections multiples generated by the hard chert interfaces are thought to interact constructively with primary reflections to generate strong interference reflections but also destructively to cause fadeouts where the faulting has changed the layer configuration alternatively the fadeouts may result from the dissolution of chert beds due to the upward migration of silica deficient pore fluids along faults and fractures in response to excess pore pressures

mapping vertical profile of discontinuous permafrost with ground penetrating radar at nalaikh depression mongolia ground penetrating radar gpr has become an important geophysical tool which can provide a wealth of interpretive information about the vertical profile of discontinuous permafrost a gpr investigation was conducted in october 2006 at the nalaikh site at the southern boundary of the siberian discontinuous permafrost region in mongolia gpr data were collected along four 100 m long profiles to identify the location of the permafrost body which included an in situ drilling borehole and analysis of temperature observations and soil water content measurements from boreholes the gpr interpretation results indicated that the thickness of discontinuous permafrost at the study site was only 1 9 3 0 m and the permafrost is vulnerable to climate change the soil temperature and soil water content data demonstrate the precision of gpr image interpretation this case demonstrated that gpr is well suited for mapping the internal structure of discontinuous permafrost with relatively low soil water content

accretion and erosion in subduction zones the role of fluids convergence occurs today along about 60 000 km of plate boundaries parsons 1981 in this paper we consider frontal subduction which occurs along about 40 000 km of convergent plate boundaries using the kine matic solution of de mets et al 1990 and including 0 5 km 2 yr1 of marginal basin openings otsuki 1989 we find that 2 95 km 2 per year disappear in frontal subduction at an average rate of 7 3 cm yr the upper overriding plate is significantly modified not only through the addition of a large amount of magmatic material markhinin 1968 but also through tectonic processes which result either in outgrowth of the margin through addit ion of sediments silver 1969 or in recession through frontal and subcrustal erosion which appears to be linked to sediment subduction coats 1962 gilluly 1963 the various phenomena observed must in some way be related to variations in mechanical coupling between the two plates uyeda kanamori 1979 had proposed that the transition from erosion to accretion results from increasing mechanical coupling the marianas trench was presented as typical of an crosional margin and the chile trench of an accreting margin ruff kanamori 1980 and kana mori 1986 showed that the magnitude of the subduction earthquakes increases with the convergence velocity and the inverse of the age of the subducted lithosphere this correlation was qualitatively explained by the control of age on the d ip of the platc and of convergence velocity on the

mud volcano field seaward of the barbados accretionary complex a submersible survey we describe a 0 55 km2 semicircular mud pie and three adjacent diapiric mounds explored using the submersible nautile near 13 50 n 12 km seaward of the barbados accretionary complex deformation front this diapiric field had been previously explored with an acoustically navigated deep towed side scan sonar thus providing an accurate base map the mud pie is situated at a depth of 4938 m in addition to visual observations water rock sediment and biological sampling we measured temperature gradients and made geochemical analyses of the samples fluid venting associated with chemosynthetic animal communities is widespread on all four structures but is maximal in the central third of the mud pie the chemosynthetic life distribution on the mud pie displays a concentric zonation which we relate to an increase in venting activity toward the center as evidenced by thermal gradients recent mud flows are present in the centralmost part of the mud pie the simplest interpretation of the structure is that it is a mud lake covered by a mechanically resistant carbonate crust we determine that approximately 106 m3 of fluid are advected upward through the crust of the mud lake every year this large amount of fluid implies large scale lateral transport

geophysical constraints on the surface distribution of authigenic carbonates across the hydrate ridge region cascadia margin on active tectonic margins methane rich pore fluids are expelled during the sediment compaction and dewatering that accompany accretionary wedge development once these fluids reach the shallow subsurface they become oxidized and precipitate cold seep authigenic carbonates faults or high porosity stratigraphic horizons can serve as conduits for fluid flow which can be derived from deep within the wedge and or if at seafloor depths greater than 300 m from the shallow source of methane and water contained in subsurface and surface gas hydrates the distribution of fluid expulsion sites can be mapped regionally using sidescan sonar systems which record the locations of surface and slightly buried authigenic carbonates due to their impedence contrast with the surrounding hemipelagic sediment hydrate ridge lies within the gas hydrate stability field offshore central oregon and during the last 15 years several studies have documented gas hydrate and cold seep carbonate occurrence in the region in 1999 we collected deep towed seamarc 30 sidescan sonar imagery across the hydrate ridge region to determine the spatial distribution of cold seep carbonates and their relationship to subsurface structure and the underlying gas hydrate system high backscatter on the imagery is divided into three categories i circular to blotchy with apparent surface roughness ii circular to blotchy with no apparent surface roughness and iii streaky to continuous with variable surface roughness we interpret the distribution of high backscatter as well as the locations of mud volcanoes and pockmarks to indicate variations in the intensity and activity of fluid flow across the hydrate ridge region seafloor observations and sampling verify the acoustic signals across the survey area and aid in this interpretation subsurface structural mapping and swath bathymetry suggest the fluid venting is focused at the crests of anticlinal structures like hydrate ridge and the uplifts along the daisy bank fault zone geochemical parameters link authigenic carbonates on hydrate ridge to the underlying gas hydrate system and suggest that some of the carbonates have formed in equilibrium with fluids derived directly from the destabilization of gas hydrate this suggests carbonates are formed not only from the methane in ascending fluids from depth but also from the shallow source of methane released during the dissociation of gas hydrate the decreased occurrence of high backscatter patches and the dramatic reduction in pockmark fields imaged on the eastern part of the survey suggest gas hydrate near its upper stability limit may be easily destabilized and thus responsible for these seafloor features high backscatter along the left lateral daisy bank fault suggests a long history of deep seated fluid venting probably unrelated to destabilized gas hydrate in the subsurface

the character and distribution of mineralized fractures in the kodiak formation alaska implications for fluid flow in an underthrust sequence the kodiak formation an accreted slate belt in southwest alaska experienced two major episodes of fracturing and mineralization in the late cretaceous early tertiary the earlier of these episodes is related to underthrusting of sediments below a major decollement d1 whereas a later episode is marked by development of fold and thrust structures and a regionally pervasive slaty cleavage within the accretionary wedge d2 d1 veins are restricted to sand layers and are concentrated at the highest structural level of the kodiak formation the most pervasive d1 fractures occur within a melange zone where the kodiak formation is thrust beneath the older uyak complex microstructures in the melange suggest that these veins formed during the later stages of a deformation that involved layer parallel shear in conjunction with progressive lithification d2 veins cut sand and shale layers and are concentrated at the lowest structural levels of the kodiak formation microstructures within these veins indicate that d2 veins are hydrofractures that were either 1 intermittentiy sealed along the length of the crack e g in the smallest veins 0 5 cm width fracturing coincided with development of a slaty cleavage in a regime of progressive simple shear from highest to lowest structural levels there is a gradual increase in measured finite strain the abrupt appearance of pervasive d2 veins and a greater noncoaxiality of strain related to southeast directed thrusting these observations suggest that d2 veins formed above a decollement that was active at the base of a late cretaceous early tertiary accreted package in total the d1 and d2 veins suggest substantial fluid flow within an ancient subduction complex with development of hydrofractures first in the footwall and then in the hanging wall of a basal decollement that stepped from shallow to deeper structural levels

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

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

geochemical and biogeochemical observations on the biological communities associated with fluid venting in nankai trough and japan trench subduction zones we have studied the compositions and structure of the lamellibranchiacalyptogena sp associated with fluid venting in subduction zones off japan the gills of thecalyptogena are the habitat of symbiotic bacteria which develop owing to chemoautotrophy they most probably assimilate hydrogen sulfide methane and or carbon dioxide and reduce nitrogen the isotopic budget of methane uptake enables to establish the thermal diagenetic origin of the light hydrocarbons brought by the upwelling fluids the microscopic examination of thin sections of the bacteria enables to establish that their membranes are compatible with uptake of inorganic nutrients and to establish relations with the metabolism of archeobacteria

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

chemical and isotopic evidence for sources of fluids in a mud volcano field seaward of the barbados accretionary wedge chemical isotopic and heat budgets are controlled by fluid flow and venting at convergent margins fluids vent from accretionary complex sediments via numerous pathways including mud volcanoes which are common at the barbados convergent margin chemical and isotopic compositions of fluids collected from four mud volcanoes located east of the barbados deformation front at 13 50 n indicate venting is sufficiently rapid to prevent diffusion from seawater in sediments surrounding the mud volcanoes the rate of nonfocused fluid flow is 0 26 cm yr although this rate is 3 orders of magnitude slower than the flow from the mud volcanoes significantly more fluid vents from the sediments surrounding the mud volcanoes than from the restricted areas of rapid flow from the volcanoes chloride concentrations are depleted to a minimum of 211 mm 45 of seawater value primarily by mixing diagenetically altered pore fluids with an 18o enriched fresh water released from the dissociation of methane hydrate this reaction is accompanied by the release of sufficient methane to form a free gas phase and initiate eruption of the mud volcanoes the geochemical compositions of the diagenetically altered pore fluids reflect the interplay between five major reactions 1 clay mineral dehydration which releases h2o and influences a range of chemical and isotopic compositions 2 organic matter regeneration which increases alkalinity nh4 and br and decreases so42 concentrations 3 cation exchange with clay minerals which increases na and decreases nh4 concentrations 4 carbonate recrystallization and precipitation which decrease ca2 but increase sr2 concentrations and 5 conversion of volcanic ash to clay minerals which utilizes h2o and decreases mg2 concentrations the geochemistry of the pore fluids suggests that these reactions occurred at temperatures ranging from 75 to 115 c thus at the regional geothermal gradients of 27 to 40 c km these temperatures indicate that the source regions are at 2 to 4 5 km depth

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

electrokinetic signature of the nankai trough accretionary complex preliminary modelling for the kaiko tokai program abstract fluid flow in the nankai trough accretionary complex has been observed during the previous phases of the kaiko project to be of order 100 m year 1 in small scale high permeability conduits and 10 m year 1 where shallow detachment faults intersect the seafloor the rate of flow was also variable increasing up to 30 over a month monitoring the fluid flow will be one of the principal objectives of the next phase of the kaiko program known as kaiko tokai one method is to measure the electric and magnetic fields generated by the electrokinetic phenomena or streaming potential resulting from fluid flow in the porous sediments this paper describes modelling of fluid flow in the accretionary prism and the resulting electrokinetic signals which may be observed from sensors on the seafloor or in shallow boreholes a two dimensional finite difference model is used to study the hydrogeology of a realistic section of the nankai trough with constraints from seismic reflection data and ocean drilling program boreholes fluid sources owing to porosity reduction in the prism and of distant external origin are modelled we find that porosity reduction alone is insufficient to reproduced the observational estimates of excess pore pressure and fluid flux additional fluid source terms are therefore required along a low angle decollement or detachment fault which separates the deformed prism sediments from the undeformed underthrust sediments beneath the decollement must be significantly more permeable than the adjacent sediments our models suggest by approximately four orders of magnitude and is probably anisotropic parallel to the main structural trends from the hydrogeological model the electric potential and electric and magnetic fields within the ocean and sediments were calculated from the electrokinetic phenomena seafloor electric potentials are found to be of the order of a few millivolts with seafloor horizontal and vertical electric fields of the order of 0 5 vm 1 close to the deformation front as the electric potential rises significantly with depth through the sediments boreole measurements may be particularly useful we conclude with some comments about noise sources and experimental logistics for the kaiko tokai program

syn diagenetic evolution of shear structures in superficial nappes an example from the northern apennines nw italy abstract in the superficial nappes of the northern apennines meso and microstructures in sheared mudstones and fine grained turbidites consist mainly of networks of striated faults and calcite vein arrays the striated faults correspond to the first generation of structures they form two sets respectively synthetic and antithetic to the general shearing locally a sigmoidal microfabric is associated with the synthetic faults in scaly deformation bands up to a few metres thick late laterally extensive calcite veins are superposed on these structures they result from the opening of multiple releasing oversteps between shear surfaces the crack seal mechanism of deformation played a major role in their formation structures of the first generation result from heterogeneous ductile deformation of initially poorly lithified and water rich sediments the late formation of the calcite veins corresponds to a transitional ductile brittle behaviour in sufficiently lithified sediment evolution of structures probably depended on a strain hardening process and was dependent on the progressive compaction and dewatering of the sediment the gently dipping faults and veins probably formed the main pathway for the interstitial fluids possibly overpressured expelled from the sediment we emphasize that the geometry of the structures described provides important shear sense criteria for the study of superficial nappe tectonics

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

interpretation of temperature measurements from the kaiko nankai cruise modeling of fluid flow in clam colonies during the kaiko nankai detailed submersible survey numerous measurements of the temperature gradient inside the sediment were performed on the deepest active zone of fluid venting which is situated on the anticline related to the frontal thrust using the ifremer t naut temperature probe operated from the submersiblenautile we thus obtained the temperature structure below different types of clam colonies associated with fluid venting we used the finite element method to model the thermal structure and fluid flow pattern of these vents and to determine the velocity of upward fluid flow through the colonies on a biological basis four types of clam colonies are defined each biological type has distinctive thermal characteristics and corresponds to a particular fluid flow pattern darcian flow velocity in the most active type of colony type a is of the order of 100 m a the total amount of fluid flowing through colonies in the studied area is estimated to be 200 m3 a 1 per metre width of subduction zone most of the flow is vented through type a colonies this value is more than one order of magnitude too high to be compatible with the amount of water available from steady state compaction of sediments in the whole wedge thermal arguments suggest that downwelling of seawater occurs around type a colonies and that seawater is then mixed with upcoming fluids at a depth of 1 or 2 metres furthermore finite element modeling shows that a salinity difference of a few parts per mil between the upcoming fluids and seawater is sufficient to drive convection around the colonies as water samples from a few vents indicate that the fluid source should actually be significantly less saline than seawater we propose that the very high fluid flows measured are a consequence of the dilution of the fluid of deep origin with seawater by a factor of 5 to 10

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

zenisu ridge a deep intraoceanic thrust related to subduction off southwest japan abstract during the french japanese kaiko project seabeam seismic and submersible observations were made in the eastern part of the nankai subduction zone close to the area of collision between the izu bonin island arc and the japan margin the most prominent feature is the zenisu ridge an elongated relief of the philippine sea plate running parallel to the trench magnetic anomalies indicate that the crust of the zenisu ridge is a part of the shikoku oceanic basin formed in the early miocene 23 ma ago and presumably uplifted at a later stage structural analysis of seismic data and diving observations lead us to interpret the superficial structure as being due to compressive tectonics mapping the acoustic basement reveals that the southeastern flank of the ridge is bounded by a double thrust both segments being of equal magnitude vertical offset about 1 to 1 5 km geophysical data support the hypothesis of a main thrust cutting through most of the lithosphere and flattening at depth the overall structure of the surrounding area reveals a compressive deformation zone widening toward the east the magnitude of the compressive deformation decreasing westward as well as southward of the zenisu ridge

geometry and quaternary kinematics of fold and thrust units of southwestern taiwan structural and paleostress analyses provide new insights into the quaternary kinematics of the outermost fold and thrust units of southwestern taiwan foothills the frontal folds are interpreted as fault related folds and their tectonic evolution through space and time is tightly constrained fold development is correlated with reef building on top of the anticlines moreover we provide field evidence that nw se fault zones oblique to the structural grain of the belt probably acted as transfer fault zones during the the quaternary fold thrust emplacement two successive quaternary stress regimes are evidenced in southwestern taiwan a nw se compression followed by a recent nearly e w compression the latter shows an along strike change from pure e w contraction to the north to perpendicular n s extension in the south this southward decrease in n s confinement probably represents the on land signature of the incipient quaternary tectonic escape predicted by analogue and numerical modelling and evidenced at present day by global positioning system data