mud volcano growth by radial expansion examples from onshore azerbaijan abstract we investigated the mode of growth of flat topped mud volcanoes through the study of three active edifices onshore azerbaijan they are the bozdag guzdek the ayazakhtarma and the akhtarma pashaly mud volcanoes the three edifices are up to 80 m high and 3 km wide and have eruptions with recurrent times from a few months to a few years surface changes during and between eruptions were documented by a combination of mapping from satellite pictures repeated direct observation over five years and structural analysis in addition resistivity profiles and microgravity measurements were used to decipher their subsurface geometries we interpret the flat topped character of the mud volcanoes as the result of rapid isostatic readjustment of a brittle surface crust

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

the nature and hydrogeologic significance of mud diapirs and diatremes for accretionary systems in accretionary wedges often morphologically similar sedimentary intrusions when observed by remote geophysical means may have one of two quite different driving mechanisms and a highly variable significance for the regional hydrogeologic picture for example mud diapirs are driven by buoyancy forces that arise from bulk density contrasts in them mud and pore fluid upwell en masse and fluid migration is a related sometimes important but generally subsidiary process in contrast diatremes contain sediments fluidized during rapid fluid advection and are forcibly and directly driven by the hydrogeologic system the nature of fluid input from local and exotic source regions can therefore strongly affect sedimentary intrusive processes and vice versa this complicates the process of defining the main features of the hydrogeological systems operating in accretionary wedges focused vertical advection through steep sided piercement mud diapirs requires conduit systems otherwise flow will be diffuse and directed more horizontally out of the low permeability mud mass however where the permeability of the overburden is less than that of the diapir the whole diapir may act as a conduit apart from this special case conduits will be associated with highly anisotropic scaly fabrics that can sometimes develop in the marginal shear zone of diapirs scaly fabrics form during deformation and compaction of a mud matrix under conditions of constant or increasing effective stress however the effective stress path can be complex as it is both controlled by the relative rates of upward intrusion and burial by sedimentation and or structural thickening and the hydrogeologic system due to this it appears likely that even in a geographically related group of diapirs effective stress histories will vary widely between intrusions so that some can form advective pathways for fluids and some cannot mixed systems of behavior may also be present with local diatremes developing within diapirs above the terminations of conduit systems and rapidly expanding methane gas pockets the potentially heterogeneous near surface behavior may be why the surface manifestations of sedimentary intrusions are so variable when observed in the field diatremes can also form separately as large primary features above any structural or stratigraphic conduit that rapidly expels water or gas into the base of an unlithified sediment column when active large diatremes require enormous quantities of fluid water or gas to drive them particularly if they are long lived features and hence are a direct indication of at least an episodically vigorous hydrogeologic system

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

osmotic model to explain anomalous hydraulic heads laboratory experiments have shown that compacted clays act as osmotic membranes when they separate aqueous solutions of unequal ionic concentration theoretically osmotically induced differential hydraulic pressure in groundwater systems can be relatively high the magnitude depends primarily upon concentration differences across the membrane type of ions type of clay and pore size in experiments thin compacted clay membranes commonly exhibit varying degrees of osmotic efficiency due to ion leak age through the clay in natural systems the membrane and the solution containers are not as distinct and well defined as they are in the laboratory moreover the membrane is commonly thick inhomogeneous and composite in a buried triassic basin at the savannah river plant near aiken south carolina it is suspected that osmosis causes the saline water in the basin center to be slightly geopressurized in relation to freshwater in the overlying coastal plain aquifer two wells have heads of 7 88 and 12 98 bars 114 3 and 188 3 psi above the head in the coastal plain aquifer the head in each of these wells approximates the osmotic equilibrium head calculated from solution concentration of water produced by each well 12 000 and 18 500 mg l respectively other wells penetrating the top and edge of the triassic basin probably penetrate a zone where ion leakage gives rise to less saline water thus these wells are not geopressurized

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

mud volcanoes at the front of the makran accretionary complex pakistan abstract conical mounds 1 1 5 km in diameter and up to 65 m high were mapped at the foot of the active makran continental margin the mounds developed seaward of the accretionary front in a relatively planar zone where the beginning of build up of tectonic pressure initiates deformation based on shallow high resolution 4 khz sediment echosounding the sedimentary sequence in this area is generally well stratified as indicated by closely spaced horizontal reflections however in the vicinity of the mounds the sediment is characterised by many acoustically transparent zones which are 100 300 m in diameter and cut near vertically through the horizontal reflectors two sediment cores from the top of the largest cone and a neighbouring acoustically transparent zone reveal small scale post depositional deformation in a stratified sequence and methane concentrations up to 40 000 ng g this deformation and disruption of potential reflectors provides a clue to explain the acoustic transparency we interpret it as caused by the rise of charged fluids and mud leading initially to the slight disturbance of the generally good acoustic reflectors and eventually to the formation of conical mud mounds mud volcanoes mcs data showing a buried mound in an analogous structural position support the idea of tectonically induced mud fluid expulsion seaward of the accretionary front

mud volcanoes along the inner deformation front of the calabrian arc accretionary wedge ionian sea abstract we present geophysical data integrated with the analysis of well targeted sediment samples in order to contribute to a better understanding of fluid circulation in the calabrian arc accretionary wedge two cores bs81 ii 5 and bs81 ii 10 collected on a swell in the hangingwall of the inner deformation front show upper pleistocene mud breccias that include cretaceous to late miocene rock fragments mechanically incorporated into the eruption deposit by the upward transport of overpressured fluids an additional core cala 21 came from the summit of a topographic high in the footwall of the inner deformation front of the accretionary wedge and contains a mud breccia patchy cloudy facies where sediment disturbance is caused by fluid expulsion integration of the entire data set provides evidences of the interplay among tectonics fluid emissions and sedimentation and enables the identification of two new volcanoes in the accretionary prism where overpressuring due to pliocene and pleistocene sediment accumulation and the evolution of active fault systems triggered fluid circulation and the formation of those structures

submersible study of mud volcanoes seaward of the barbados accretionary wedge sedimentology structure and rheology abstract in 1992 the nautile went to a mud volcano field located east of the barbados accretionary wedge near 13 50n using nannofossil analysis on cores we determined the sedimentation rate and provided a new estimation of the age of the mud volcanoes 750 000 years for the oldest one six structures have been explored with the submersible nautile and manifestations of fluid venting chimneys carbonate cementation and chemosynthetic communities were observed on all sedimentological analysis identifies two sources of diapiric mud most mud volcanoes expel mud containing late miocene to quaternary faunae that have the same composition as sediments drilled above the barbados wedge decollement one volcano also contains older oligocene taxa with a mud composition corresponding to the sedimentary sequence below the decollement we use diving observations to map the fine scale morphology the distribution of chemosynthetic fauna and define two end member types of structures mud pies flat topped mud volcanoes and conical mounds mud pies atalante and cyclops are characterised by the presence of a lake of highporosity mud 70 to 75 in their central parts chemosynthetic benthic communities calyptogena colonies and sponge bushes are concentrated in the outer parts contrasting morphologies of the two mud pies indicate different stages of activity cyclops is growing whereas atalante is collapsing expulsion of water and methane occurs mostly through the mud lake and may be stronger during the collapse phase on conical mounds there are no mud lakes fluid venting concentrates near the summit and occurs through carbonate cemented chimneys which form within the sediment viscosity measurements have been carried out on mud samples from the two mud pies and one conical mound all mud samples have a plastic fluid behaviour the plastic threshold decreases with porosity and thixotropy is observed for a porosity of more than 70 an analogue experiment shows that for this thixotropic mud shearing in the feeding conduit liquefies the mud which then spreads to form a mud pie conical mounds form when the mud remains plastic we show that the dissociation of methane hydrate is the cause of the high porosity in mud pies and confirm that these structures are a consequence of large scale dissociation of methane hydrate at the base of its stability field dissociation of hydrates before and during ascent is only slightly contributing to the pore fluid in conical mounds but solid hydrates still present in the mud may contribute to its buoyancy

hydrogeochemistry of the northern barbados accretionary complex transect ocean drilling project leg 110 detailed studies of the major element geochemistry oxygen and hydrogen isotope ratios of pore fluids and the 87sr 86sr ratio of dissolved strontium have made it possible to unravel physical and chemical processes that affect the pore fluid chemistry in a transect of drill holes across the northern barbados accretionary complex these processes include 1 alteration of volcanic ash buried in the pleistocene pliocene sediment column 2 alteration of underlying basalts of layer 2 of the oceanic crust 3 movement of fluids from deep in the accretionary complex along fault zones particularly the decollement and minor permeable layers these fluids from deeper in the complex are characterized by low chloride concentrations and increased 18o h2o values presumably as a result of dehydration of smectite interlayers and 4 mixing processes involving the migrating fluids cause incongruities in the geochemical anomalies of these fluids

integrated petrographic and geochemical record of hydrocarbon seepage on the voring plateau authigenic carbonate crusts nodules and chemoherms were sampled from pockmarks and mud diapirs on the southern part of the voring plateau during the ttr 8 and ttr 10 marine expeditions a petrographic and geochemical study was carried out to investigate their possible relationship with the seepage of hydrocarbon fluids all authigenic carbonates are depleted in 13 c 31 6 13 c 18 o 18 o 4 6 18 o 18 o rich fluid the occurrence of different carbonate mineral phases aragonite calcite dolomite is possibly related to varying dissolved sulphate concentrations in the diagenetic environment fluid inclusion microthermometry and raman spectroscopy indicate the presence of an aqueous hydrocarbon mixture inside the inclusions this seepage mixture was almost certainly immiscible resulting in heterogeneous trapping

trinidad mud volcanoes where do the expelled fluids come from abstract in order to establish the origin of fluids expelled from mud volcanoes in trinidad we analyzed their major element trace element and isotopic h o sr compositions the mineralogical chemical and sr isotope compositions of associated expelled muds were also determined on the basis of their chemical and sr isotope compositions the fluids were divided into two groups those southwest and northeast of a major right lateral wrench fault the los bajos fault that both controls the chemical quality of the fluids and acts as a drain strontium isotopes were derived via mixing between a radiogenic source 0 71135 and a nonradiogenic source 0 70671 for both southwest and northeast groups however the nonradiogenic source possibly feeding the northeast group showed a lower sr concentration than that of the southwest group h and o isotope data show that the fluids became enriched in 18 o through interaction with wall rocks the fluids were originally oceanic but their properties were subsequently modified by diagenesis as evidenced by chemical data and mixing between fluids issued from two deep seated reservoirs and surface aquifer end members the gas phase expelled with the mud and the fluid through the mud volcanoes is composed mainly of methane with minor carbon dioxide the amount of expelled gas seems the same in samples from both sides of the los bajos fault the almost unique methane content of the gas phase and the large positive 18 o shifts of the reservoir end member as well as b li and ba contents reveal that the fluids from the deep seated reservoirs inherited their chemical compositions at high temperature fluid rock interactions estimates of equilibrium temperatures of the two deep seated reservoirs suggest that to acquire the estimated higher temperature 150 c the mud volcanoes must have been fed partly by a reservoir located at a depth of more than 3 km this depth corresponds to previous geologic information which located the deep seated reservoir in miocene sediments at such a comparable depth the mixing of the original fluids with a less deep seated reservoir and meteoric waters on the ascending path suggests the presence of a recharge mechanism and contributes to the dynamics of the expulsion itself

comparative characteristics of the black sea and mediterranean ridge mud volcanoes morphological and structural characteristics of the black sea and mediterranean ridge mud volcanoes studied during several unesco esf cruises of the r v gelendzhik 1991 1994 are compared on the basis of single and multi channel seismic and sidescan sonar observations morphologically three types of mud volcanoes were distinguished in the black sea and six types in the mediterranean they differ in size shape and presence of the mud flows some types from both localities are very similar to each other but in any case their deep structure is very distinctive this difference in deep structure is explained by their development in different geological conditions under an extensional regime with high terrigenous input in the black sea and under strong lateral compression on the mediterranean ridge

stratigraphy and chronology of the periphery of the scandinavian ice sheet at the foot of the ukrainian carpathians abstract the article presents the results of research on the formation conditions and the age of glaciogenic deposits left by the scandinavian ice sheet in the eastern carpathian foreland in its part drained by the dniester river and its tributaries black sea basin these deposits till erratics glaciolacustrine and glaciofluvial sediments preserved only in some places and occurring in the range of altitudes defined by terrace vi early pleistocene and terrace v middle pleistocene are the southernmost traces of the ice sheet in europe they are underlain by fluvial deposits and overlain by loess deposits and locally occur as intraloess layers the following research methods were used geomorphological mapping sedimentological lithological micromorphological palaeopedological and palaeontological analyses as well as luminescence dating the obtained results allowed a reconstruction to be made of the main stages of morphogenesis and the style of the ice sheet advance at its maximum extent defined as the sambor phase the area was glaciated during mis 12 elsterian ii sanian 2 okanian as indicated by the occurrence of a complete middle upper pleistocene loess palaeosol sequence the oldest palaeosol s4 i e sokal mazovian zavadivka soil that developed directly on the glaciogenic deposits or loess l5 is correlated with the holsteinian likhvinian interglacial mis 11

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

mud volcano field seaward of the barbados accretionary complex a deep towed side scan sonar survey a 30 km2 diapiric field has been identified near 13 50 n up to 12 km seaward of the deformation front of the barbados accretionary complex using a deep towed side scan sonar with a 3 5 khz profiler we identified 31 different diapiric structures based on seismic stratigraphy we show that this field has been active for 200 000 years and that it is a transient feature triggered by the seaward propagation of the high pore fluid pressure associated with the decollement beneath the accretionary complex both basement and decollement in this area are anomalously shallow due to the presence of a n110 basement ridge the height of the diapirs above seafloor does not exceed 40 50 m and can be related to the pressure head of the mud below the decollement two types of structures are identified mud volcanoes and massively emplaced diapirs and ridges on the one hand enigmatic circular very flat mud pies and conical mounds on the other hand the second type of structures has steeper slopes and appears to be associated with very active venting as confirmed by a submersible exploration reported in a companion paper le pichon et al this issue b the venting results in the formation of a stiff carbonate crust and of large subsiding basins around the mud pies continuous active fluid expulsion through these structures indicates that an efficient piping system still connects them to the zone of anomalously high pore pressure below the protodecollement

mud diapirism and subcretion in the barbados ridge accretionary complex the role of fluids in accretionary processes mud diapirism has recently been recognized in several modern accretionary wedges it provides an important means of dewatering accretionary wedges and should be regarded as an important process for producing the melanges found in both modern and ancient accretionary terranes mud diapirism affects a large area of the barbados ridge accretionary complex the distribution of the mud diapirs appears to be primarily controlled by the presence of underconsolidated terrigenous submarine fan deposits that are being accreted to the complex the frequency of diapir occurrence decreases northward as the fan becomes thinner mud diapirs are absent from the very eastern most part of the complex formed from sediments accreted at its toe with the exception of a few mud volcanoes on the ocean floor in front of the complex the initiation of diapirism appears to be spatially coincident with the onset of subcretion or underplating of sediment to the base of the complex at a ramp between two levels of decollement it is proposed that the release of mud and pore water from the subcreted sediments is a direct or indirect cause of most of the mud diapirism in the accretionary complex there is a range of diapiric form dependent on the viscosity of the mud from mud volcanoes fed by low viscosity mud to higher viscosity mud ridges the diapirs in the eastern areas of the complex are generally mud volcanoes with narrow conduits feeding a surface mound mud ridges are prominent in the western parts of the complex this is interpreted as reflecting a general westward decrease in the fluid content of the accretionary complex bottom simulating seismic reflectors formed by gas hydrate are commonly developed in the areas of mud volcano occurrences the presence of the hydrate indicates that large volumes of methane are being generated at depth in these regions the generation of methane may be contributing to zones of overpressuring in the wedge methane may also be partly responsible for driving the diapiric material to the surface to form mud volcanoes ridges in the subducting oceanic crust beneath the accretionary complex locally enhance diapirism above their crests and southern flanks faults formed later in the development of the complex are more commonly associated with diapirism than those resulting from accretion at the toe of the wedge these later faults play an important role in controlling the sites of individual mud volcanoes chains of mud volcanoes and mud ridges

seabed morphology and gas venting in the black sea mudvolcano area observations with the mak 1 deep tow sidescan sonar and bottom profiler abstract the central part of the black sea was investigated with the mak 1 deep tow acoustic system providing sidescan observations and acoustic profiling subsequently underwater tv recordings and bottom sampling was done in an area with extensive mudvolcanism the mud volcanoes can be divided into three types according to their dimensions and morphology the first type has a seabed diameter of more than 2 km reaches a height of 100 150 m has a complex top with a crater like structure and gentle slopes and reflects massive mudflows in the crater and at its flanks the second type has smaller dimensions is more conical in cross section almost without relief on its relatively steep slopes and is characterised by small radial mudflows the third type is made up by the only mud volcano with a partially collapsed rim and with massive and liquid mudflows the first type expectedly is in a mature relatively passive stage the latter two types seem to be younger and more active various types of low relief slides are found in the area and are related to the presence of shallow subbottom gas 40 100 m below the seafloor or mudvolcano activity evidence for the presence of gas in the mudvolcano area is shown by gas venting acoustic turbidity acoustic masking numerous pockmarks acoustic voids and acoustic windows gashydrates do occur associated with some of the mudvolcanoes

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

mantle eduction tectonic fluidisation at depth abstract recent direct observations in trench environments demonstrate that massive volumes of wet sediments are subducted the gas phases in and near trench zones also show the evidence for subduction of organics and ocean fluids serpentine sea mounts also show that large quantities of serpentine are subducted thus in the upper layers of the subducting slab metamorphic processes must generate fluidized beds which may be extruded along the subduction faults or intruded by hydraulic fracture mechanisms into the overlying cover rocks this process which we term eduction may account for some of the exotic assemblages containing high pressure rocks found in subduction assemblages the equivalent of deep mud dykes and volcanoes the same process may be associated with seismic events

the mediterranean ridge and related mud diapirism a background abstract the mediterranean ridge stretching from the calabrian rise to the florence rise is the largest structural unit of the eastern mediterranean sea it is directly related to ongoing convergence and collision of the african and eurasian plates starting in the oligocene and is considered to be a giant accretionary complex consisting of intensively folded and faulted rocks of the african margin since its origin in the late miocene the ridge continued to grow up and outward experiencing more deformation because of the developing collision the mud diapirism and mud volcanism are usual and wide spread phenomena for the mediterranean ridge that developed as a result of an intensive tectonic overburden due to stacking of rock units by thrusting and strong lateral compressional stress pressing up and squeezing plastic sedimentary series out onto the seafloor

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

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

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

chemical and isotopic evidence of gas influenced flow at a transform plate boundary monterey bay california chemical and isotopic compositions of pore fluids document upward flow through communities of vesicomyid clams in monterey bay california within the clam communities the sulfate reduction zone is only 10 cm thick and ca and mg concentrations decrease to values as low as 2 2 mm and 34 5 mm respectively at depths less than 30 cm below the sediment water interface less than 5 m outside the communities the base of the sulfate reduction zone is deeper than the greatest penetration of the cores 30 cm and ca and mg exhibit only minor changes from seawater values the sediment exhibits no significant variation in grain size mineralogy organic carbon nitrogen or carbonate content throughout the region the composition of pore fluid within clam communities results from upward flow of altered fluid rather than different diagenetic reactions within and outside the communities isotopically light dissolved inorganic carbon dic with 13c values ranging from 3 2 to 54 1 could reflect carbon sources from either oxidized thermogenic methane and or a mixture of oxidized microbial methane and solid organic carbon the c1 c2 c3 ratios ranging from 34 to 1142 and the hydrogen and carbon isotopic compositions of methane d values of 109 to 156 13c values of 30 6 to 86 6 suggest that methane is primarily microbial but that a minor component could be thermally generated any thermogenic methane would have migrated from great depths possibly 2 km the presence of methane is likely to contribute to fluid flow by reducing the density of the fluids past fluid migration and venting are reflected by widespread carbonate mineralization at the sediment water interface this mineralization and the geographic distribution and proportions of microbial and thermogenic methane suggest that vent sites migrate when permeability is reduced during carbonate cementation these results demonstrate that along with convergent and divergent plate boundaries transform plate boundaries are characterized by fluid flow and that the flow may be widespread occurring at sites away from fault zones

deep tow survey in the kaiko nankai cold seepage areas deep sea biological communities associated with cold venting were located within the eastern portion of the nankai trough using a deep towed tv system five continuous lines approximately 200 km long were observed bivalve communities were found both in the tenryu canyon outlet region and at the outcrop of the frontal thrusts of the accretionary wedge off cape omaezaki the former is a site at which biological communities had been found during the kaiko dives in 1985 however the second site appears to be associated with larger scale venting and was chosen for the 1989 kaiko nankai submersible cruise three small mud volcanoes with diameters of several hundred meters and heights of several tens of meters are found in the seabeam topography map but the present survey showed neither evidence of recent venting nor biological communities

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

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

tectonic significance of the cold seepage zones in the eastern nankai accretionary wedge an outcome of the 15 years kaiko projects abstract the tectonic significance of the cold fluid venting in the eastern nankai accretionary wedge has been elucidated based upon results of precise mapping and submersible observation executed since 1984 under the french japanese cooperative projects kaiko i ii kaiko nankai and kaiko tokai in conjunction with studies of the japan trench sagami bay and other regions the very sharp boundary of benthic communities associated with temperature anomalies indicates the occurrence of focused upflow of nutrient rich fluids through cracks in the carbonate crust existing on a near surface horizon of permeable sediment the linear alignment of venting zones is a surface expression of the shallow detachment faults methane rich fluids expelled from the conduit along a fault migrate in the overlying permeable sediment and react with circulating sea water containing sulfate to produce hydrogen sulfide and carbonate precipitate en echelon or zigzag patterns of the cracks are developed in the carbonate crust under the influence of tectonic shear stress it has been concluded that bivalve communities grow on a thin veneer of sediment above the cracks of carbonate through which fluid containing hydrogen sulfide diluted with sea water flows out on the other hand bacterial mats seem to flourish either on thin sediment overlying the fault conduit or over a thick cover of sediment without carbonate crust in the former case bivalves cannot grow because venting fluids are too dense with methane and hydrogen sulfide to maintain the oxygen rich condition of the bottom water in the western nankai trough fine grained less permeable sediment cover seems to prevent circulation of fluids whereas methane appears to be widespread in the deeper sedimentary layer there major distinctions in modes of seepage among sites are presumed to be caused by 1 the tectonic regime causing channels of upward fluid venting 2 rates and composition of venting fluids affected by mixing rates of sea water and 3 the permeability and thickness of the near bottom sediment

major cation gradients in dsdp interstitial waters the role of diffusive exchange between seawater and upper oceanic crust abstract interstitial water analyses from dsdp legs 1 53 were surveyed to identify sites at which the dominant process supporting gradients of major seawater cations is alteration of the underlying oceanic crust sites with thin sediment cover are generally without gradients sites with appreciable sediment accumulation are generally with gradients the transition is probably the result of cessation of convective circulation within the sediments a model is developed for the diffusive flux through an accumulating unreactive sedimentary layer supported by continued reaction within the crust the model is consistent with the observations provided that convection within the crust extends to depths 1 km and that convective flow within the sediments ends once 100 150 m of sediment accumulates the model suggests a steady state gradient inversely proportional to the sedimentation rate this dependence is apparent in the data however some variation in the reaction rate is also required fluxes of mg and na are systematically related to the flux of ca which because of its mobility serves as a tracer of hydrolysis of the basalt when the ca flux is low the mg flux serves as the principal sink for alkalinity produced during basalt hydrolysis when the ca flux is high the mg flux becomes diffusion limited and nabearing alteration phases must form the fluxes resulting from these alteration reactions exert little influence on either the chemistry of seawater or that of the upper crust

major and trace element and sr isotope constraints on fluid circulation in the barbados accretionary complex part ii circulation rates and fluxes abstract 87 sr 86 sr ratios of pore fluids recovered from diapiric structures east of the deformation front of the barbados accretionary prism remain quasi constant from the surface to depths of 8 m the sr isotopic ratios are non radiogenic compared with seawater which implies that non local fluid was expelled at such a high rate that its sr isotopic composition was dominated by the source fluid and not by pore fluid or seawater chloride depletion recorded in the fluids corresponds to a five fold dilution in the deepest sample from the atalante diatreme this also strongly suggests upward flow the comparison of measured chloride contents and sr isotope depth profiles with theoretical values obtained using a steady state advection diffusive model gives some constraints on the fluid circulation rate in the two diatremes atalante and cyclope and in one mud diapir c the velocities are variable but the diatremes which have the largest thermal and chemical anomalies yield the highest estimated advection rates the fluid circulation rate in the centre part of the largest diatreme atalante 0 47 km 2 total surface is estimated at about 1 m yr the fluid flux from the deepest to shallowest levels of the atalante diatreme was estimated on the basis of the fluid circulation rate calculations fluid release is clearly favoured if active conduits are present as they are in the diatremes but not in the mud diapirs however even though this steady state advection diffusion model provides a rough estimation of flow rates modification of the fluid pressure and therefore fluid volume during earthquakes may superimpose large but transient fluid expulsions

sediment hosted zn pb cu deposits in the central african copperbelt abstract stratabound epigenetic sulphide zn pb cu ore deposits of the central african copperbelt in the democratic republic of congo and zambia are mostly hosted in deformed shallow marine platform carbonates and associated sedimentary rocks of the neoproterozoic katanga supergroup economic orebodies that also contain variable amounts of minor cd co ge ag re as mo ga and v occur mainly as irregular pipe like bodies associated with collapse breccias and faults as well as lenticular bodies subparallel to bedding kipushi and kabwe in the democratic republic of the congo and zambia respectively are the major examples of carbonate hosted zn pb cu mined deposits with important by products of ge cd ag and v in the lufilian arc a major metallogenic province famous for its world class sediment hosted stratiform cu co deposits the carbonate hosted deposits range in age from neoproterozoic to early palaeozoic 680 to 450 ma the formation of the relatively older neoproterozoic deposits is probably related to early collision events during the lufilian orogeny whereas the younger palaeozoic deposits may be related to post collisional processes of ore formation fluid inclusion and stable isotope data indicate that hydrothermal metal bearing fluids evolved from formation brines during basin evolution and later tectonogenesis ore fluid migration occurred mainly along major thrust zones and other structural discontinuities such as karsts breccias and faults within the katangan cover rocks resulting in ore deposition within favourable structures and reactive carbonates of the katangan supergroup

structural setting and tectonic control of mud volcanoes from the central mediterranean ridge eastern mediterranean based on a recent marine geophysical data set including swath bathymetry acoustic imagery and six channel seismics recorded over a large area of the mediterranean ridge mr in early 1998 during the prismed 2 survey this paper presents a study of the various relationships observed between tectonic features cutting across the central mediterranean ridge accretionary wedge and massive mud expulsions known as mud volcanoes identified over large areas of the ridge regional mapping of two of the mud volcano fields previously only partly investigated olimpi and united nations rise revealed the presence of many new mud expulsion centres and a third new mud volcano field has been identified all the mud features show great variability in morphology size and backscatter strength of their surrounding mud flows based on their contrasting morpho acoustic characteristics we propose a classification into three main groups of mud construction 1 mud volcanoes these consist of subcircular and prominent reliefs often associated with high backscatter mud flows 2 mud domes similar to mud volcano but smaller these occurrences correspond to weakly reflective mud constructions 3 mud plateaus represent a third category which appears as wide often highly reflective and rather flat mud extrusions from all available data an attempt to explain the different mud ascent processes and driving forces is discussed in relation to the initial collision structural setting of the central mediterranean ridge within this area most of the mud constructions have been observed to be associated with tectonic features and in particular with strike slip faulting for the first time as a hypothesis we propose in this paper two different ascent processes to explain the contrasting mud constructions 1 extrusion for the mud volcanoes and plateaus and 2 intrusion for the mud domes connected to two different mud levels controlled by the crustal geometry of this pre collision area and especially the southwards extension of the cretan continental crust below the mediterranean ridge accreted sediments