origin and emplacement of mariana forearc seamounts large seamounts occur on the outer half of the mariana forearc they represent a new class of seamounts consisting of horsts and diapirs of metamorphosed forearc material the degree of metamorphism in this material depends on the amount of water available and the pressure temperature regime of the forearc wedge the major source for the water involved in the metamorphism is most likely the descending slab theoretical models for thermal regimes in convergent margins suggest that the lower grade metamorphic facies will be restricted to the outermost part of a forearc zeolite and chlorite facies rocks predominate in dredge hauls from horsts on the outer 50 km of the mariana forearc thermal models indicate that higher grade greenschist facies should occur farther from the trench seamounts that were probably formed in response to diapiric emplacement of serpentinite predominate from 50 to 120 km from the trench uplift of the horsts and emplacement of the serpentinite diapirs were probably facilitated by vertical tectonic movement in response to subduction of plate seamounts and by fracturing of the mariana forearc

deep slab fluids fuel extremophilic archaea on a mariana forearc serpentinite mud volcano ocean drilling program leg 195 1 as the pacific plate subducts beneath the mariana forearc it releases water that hydrates the overlying mantle wedge converting it to serpentinite that protrudes to form mud volcanoes at the seafloor excess h2o ascends through these mud volcanoes and exits as cold springs at their summits the composition of this deep slab derived water has been determined by drilling on two of these seamounts it has a ph of 12 5 and relative to seawater is enriched in sulfate alkalinity na cl k rb b light hydrocarbons ammonia 18o and deuterium and depleted in chloride mg ca sr li si phosphate and 87sr within the upper 20 m below seafloor at south chamorro seamount a microbial community operating at ph 12 5 made up overwhelmingly of archaea is oxidizing methane from the ascending fluid to carbonate ion and organic carbon while reducing sulfate to bisulfide and probably dissolved nitrogen to ammonia

blueschist metamorphism in an active subduction zone the high pressure low temperature metamorphic rocks known as blueschists have long been considered to form in subduction zones where the descent of a relatively cold slab leads to the occurrence of unusually low temperatures at mantle pressures until now however the link between blueschist facies rocks and subduction zones has been indirect relying on a spatial association of blueschists with old subduction complexes and estimates of the geothermal gradients likely to exist in subduction zones here we strengthen this link by reporting the discovery of blueschist facies minerals lawsonite aragonite sodic pyroxene and blue amphibole in clasts from a serpentinite seamount in the forearc of the active mariana subduction zone the metamorphic conditions estimated from the mineral compositions are 150 250 c and 5 6 kbar 16 20 km depth the rocks must have been entrained in rising serpentine mud diapirs and extruded from mud volcanoes onto the sea floor further study of these rocks may provide new insight into the tectonics of trench forearc systems and in particular the processes by which blueschist facies clasts come to be associated with forearc sediments in ancient subduction complexes

the formation of estuarine turbidity maxima due to density effects in the salt wedge a hydrodynamic process study abstract by means of a numerical model of an idealized flat bottom estuary the paper studies the hydrodynamic control of the turbidity zone by the combined effect of the salt wedge and tidal movements the model is of two dimensional x z finite difference type with high resolution in time and space it computes momentum surface elevation salinity suspended particulate matter spm turbulent kinetic energy and dissipation rate as prognostic state variables at the seaward boundary a tidal forcing is applied at the landward boundary a weir is situated where a constant freshwater discharge is prescribed the initial spm concentration is horizontally homogeneous after simulating a few tidal periods the model results exhibit the evolution of a stable spm peak the estuarine turbidity maximum or etm at the tip of the salt wedge an inspection of the tidal mean velocity profiles around the etm shows that this trapping of spm is due to a residual near bottom upstream current in the region of the salt

on the mesozoic ionian basin summary new seismic reflection profiles of the italian deep crust project crop provide new insights on the structure of the ionian sea in spite of the apennines and hellenides neogene subduction zones two conjugate passive continental margins are preserved at the margins of the ionian sea along the malta escarpment to the southwest and the apulian escarpment to the northeast the ionian sea is likely to be a remnant of the mesozoic tethys ocean confined by these two conjugate passive continental margins the transition from continental to oceanic crust appears sharper to the northeast than to the southwest the basin between southeast sicily and southwest puglia was about 330 km wide and suggests a low spreading rate the inferred oceanic ridge should have been flattened by thermal cooling and buried by later sediments based on stratigraphic and structural constraints to the north in the apennines belt the ocean continued to the northwest this palaeogeography is supported by the seismicity of the apennines slab underneath the southern tyrrhenian sea which implies downgoing oceanic lithosphere the adjacent absence or paucity of deep seismicity does not imply absence of subduction but rather it can be interpreted as due to the more ductile behaviour of the subducted continental lithosphere surprisingly we note that where the oceanic inherited basin is subducting underneath the apennines in the hangingwall of the subduction hinge there are outcropping slices of continental crystalline basement previously deformed by the alpine orogen

evidence from fluid seeps atop serpentine seamounts in the mariana forearc clues for emplacement of the seamounts and their relationship to forearc tectonics abstract the mariana forearc is a non accretionary forearc with numerous seamounts dredge hauls from these seamounts are composed primarily of serpentinite indicating a non volcanic origin in addition dredged carbonate samples reveal 1 the presence of aragonite in water depths greater than the aragonite compensation depth 2 unusual porosity and cement types 3 light carbon and heavy oxygen isotopic compositions and 4 an unusual trace element geochemistry of aragonite and calcite these mineralogic petrologic and geochemical characteristics formed the basis of a prediction that methane bearing fluids are venting from these seamounts subsequent to this prediction submersible dives on a mariana forearc seamount found and sampled chimneys seeping cool water that contains dissolved methane the serpentine substrate beneath the chimneys appears to form from cold gravitational flows of serpentine material fluid inclusion contents of chimney samples also show an earlier history of methane as well as longer chain hydrocarbons aromatics and acetate ions drilling into the flank and the summit of this seamount by the ocean drilling program during leg 125 confirmed the presence of methane other light alkanes and organic acids in the present day interstitial pore fluid of the cores the presence of aromatic compounds and organic acids if derived from the maturation of organic compounds indicates a thermogenic origin for these volatiles strontium isotopic ratios of the aragonitic chimney samples and of the present day interstitial waters range from present day seawater values to significantly lower values the strontium isotopic ratios of the pore waters within the seamount trend toward a relatively less radiogenic composition with increasing depth below the seafloor this strontium isotopic signature is probably not the consequence of near surface processes but more likely a contribution from an igneous source deeper in the lithosphere the lack of a large accretionary prism in the mariana forearc removes the complication of fluid contribution from dewatering of the prism by compression during convergence as in the oregon washington or barbados margins the organic compounds in the fluid inclusions and interstitial pore fluids as well as the isotopic and the geochemical composition of the carbonates and the dissolved constituents suggest that fluids are derived from dewatering compression and dehydration of subducted sediments and the subducting plate furthermore the composition of the fluid is influenced by thermal maturation of organics in subducted sediments as well as by a contribution from a deep igneous source

plumbing accretionary prisms effects of permeability variations fault zones focus fluid expulsion in the muddy northern barbados ridge accretionary prism with fault parallel permeabilities about 1000 times greater than intergranular permeabilities in the adjacent sediment in the oregon prism the low bedding perpendicular permeability due to mudstones inhibits intergranular dewatering however intergranular flow is concentrated where submarine erosion breaches high permeability sandy layers even so faults can capture fluid flow from these exposed sandy layers suggesting the faults have a still higher permeability such observations coupled with laboratory measurements permeabilities suggest that faults off oregon may have fault parallel permeabilities at least 10 10000 times greater than the adjacent sediments results from barbados and oregon suggest fluid flow is concentrated along the most active faults at the toe of prisms the fault zones are being progressively loaded by the thickening wedge and are undergoing compaction preliminary experiments show that permeability decreases relative to the surrounding wall rocks along faults within this compactive deformation regime we believe that these faults must undergo dilation perhaps linked to transient increases in pore pressure if they are to be preferential fluid conduits farther upslope erosion exposes rocks that are more consolidated commonly more cemented and generally of lower intergranular permeability than rocks of equivalent burial further seaward because of their lithification and overconsolidation these rocks dilate during faulting locally enhancing fracture permeability in such dilative regimes faults become evermore focused zones of fluid expulsion relative to occluded intergranular pathways

critical shear stress of cohesive bottom sediments the main purpose of this study is to obtain a simple critical shear stress function for cohesive soft bottom sediments a series of experiments concerning threshold levels of mud transport and two series of tests of their rheological and setting properties were conducted to find some correlation among these indices we defined two threshold levels for mud transport and divided muds into two groups by their characteristics settling type flow curve resuspension behavior etc in discussing the factors determining the characteristics of the two groups we found that natural muds usually belong to the first group in which the two critical shear stresses are given as a function of yield value y1 defined in paper independent of material following a new definition of threshold for sediment detachment from the bed finally the correlation between this yield value and the final settling volume ratio is also discussed

hydrodynamic and sediment transport modelling in the canals of venice italy a framework of numerical models was applied to the canal network of venice in order to simulate hydrodynamics and sediment transport a two dimensional finite element model shyfem was applied to the whole surrounding lagoon using water levels as a forcing the computed levels were extracted along the contour of the city of venice and then a link node model was run in the canal network the simulated variables were calibrated and compared with data from field measurements calculated water elevation displays a good agreement with the measured data and current velocity is well reproduced simulations were initially carried out using a constant manning parameter mc and successively a varying coefficient vmc was adopted in order to account for the different depths of the canals by applying the sediment transport module an evaluation of the sediment accumulation rate in the canal network was obtained permitting an estimation of the yearly silting up of the canal bed different simulations were carried out considering material input from both the city and the lagoon the model is able to reproduce the overall accretion rate of the canal bottom and it is a useful tool for planning the dredging activities in the whole canal network

diverse sources for igneous blocks in franciscan melanges california coast ranges igneous blocks in franciscan melanges are of three chemical petrologic types 1 tholeiitic basalts of both arc and spreading center origin with depletions in light relative to heavy rare earth elements 3 tio 2 1 high y zr and y ti ratios and relict augites that generally have low al and ti and well defined iron enrichment trends 2 basalts of probable seamount origin with marked enrichments in light relative to heavy rare earth elements 5 tio 2 1 lower y zr and y ti than 1 and ti al rich augites showing little if any iron enrichment trends and 3 hypabyssal intrusives having sio 2 52 tio 2 1 flat or only slightly fractionated rare earth abundance patterns and diopsidic augites that are very low in ti and al and show no iron enrichment trends all of the blocks are metamorphosed most are undeformed pumpellyite bearing greenstones and a few contain sodic amphibole lawsonite sodic pyroxene the melanges are probably olistostromal in origin deriving the

emplacement growth and gravitational deformation of serpentinite seamounts on the mariana forearc summary serpentinite seamounts representing some of the first material outputs of the recycling process that takes place in subduction zones are found on the outer mariana forearc multichannel seismic mcs and bathymetric data collected in 2002 image the large scale structures of five seamounts as well as the pre seamount basement geometry and sediment stratigraphy we present data from three edifices that provide insights into seamount growth and internal deformation processes and allow us to support the interpretation that serpentinite mud volcanoes are formed by the episodic eruption of mud flows from a central region the presence of thrust faulting at the base of turquoise and big blue seamounts along with the low surface slopes 5 18 of all the seamounts studied lead us to infer that these edifices spread laterally and are subject to gravitational deformation as they grow numerical simulations using the discrete element method dem were used to model their growth and the origins of features that we see in mcs sections such as basal thrusts inward dipping reflections and mid flank benches the dem simulations successfully reproduced many of the observed features simulations employing very low basal and internal friction coefficients 0 1 and 0 4 respectively provide the best match to the overall morphology and structures of the serpentinite seamounts however the simulations do not capture all of the processes involved in seamount growth such as withdrawal of material from a central conduit leading to summit deflation compaction dewatering and degassing of mud flows mass wasting in the form of sector collapse and growth upon a dipping substrate a strong reflection beneath the summit of big blue the largest serpentinite seamount on the mariana forearc represents the floor of a summit depression that has been partially in filled by younger muds supporting the idea that serpentinite seamounts grow by episodic mud volcanism boundaries of mud flow units are visible in bathymetric data and as normal polarity subhorizontal reflections on seismic profiles big blue seamount displays complex nesting relationships as it merges with other seamounts to form a large composite edifice flank flows of serpentinite muds on big blue and celestial seamounts downlap pre existing forearc substrate the interface between serpentinite seamounts and the underlying forearc sediments is represented by a reverse polarity reflection beneath big blue and celestial seamounts suggesting that the substrate is undercompacted overpressured and may be a zone of fluid migration dem simulations suggest that this boundary represents a distinct decollement along which the seamounts slide laterally in contrast turquoise seamount grows laterally not by stable sliding along the top of forearc sediments but by incorporating them into large basal thrusts

cohesive sediment transport model a mathematical model has been developed which simulates the transport of cohesive sediment the model solves the complete three dimensional version of the diffusion advection equation using a splitting method in conjunction with a characteristics technique and a mixed explicit implicit finite difference approach the effects of the flocculation deposition consolidation erosion and slump of the sediment are incorporated in the model which can also handle irregular layouts application of the model to a harbor siltation problem indicated that the large input data requirements can be satisfied economically and that the scheme yields realistic results the application also provided an insight into possible ways of operating the model for different field situations

geophysical evidence of gas hydrates in shallow submarine mud volcanoes on the moroccan margin 1 gas hydrates inside mud volcanoes have been observed in several locations but are generally found at water depths of 1000 m and deeper we present the first observation of the base of a gas hydrate stability zone within a shallow mud volcano in the el arraiche mud volcano field on the moroccan atlantic margin the mud volcano base is located at about 475 m and is over 125 m high on high resolution seismics we observed an anomalous but coherent reflection under the slopes of the mud volcano the event was interpreted as the base of a gas hydrate stability zone because of its inverse polarity and its morphology far from the crater the event is nearly parallel to the seafloor closer toward the crater the event shallows inside the mud volcano crater no event is observed a stability model using thermogenic gas compositions is applied to local p t conditions indicating that thermogenic gas hydrates can be stable at this depth the high modeled heat flow in the crater of the mud volcano indicates a focused flow of warm fluids below the slopes of the mud volcano the inferred heat flow is also elevated but less high in areas of thermogenic gas production gas hydrates can occur at shallow water depths even in areas with high heat flow this also suggests that dewatering of the accretionary wedge complex is mainly focused along fault surfaces and through seafloor structures such as mud volcanoes

dynamic basis of volcanic spreading the established models of the structure and dynamics of a typical hawaiian volcano lead inevitably to a paradox finite element viscoelastic calculations using tecton melosh and raefsky 1980 1981 predict that a compressional stress field should characterize their upper flanks instead extension is observed the paradox is solved by postulating that volcanic evolution is determined by feedback processes related to growth and spreading of the volcanic edifice many examples of the process of spreading are documented in the geologic literature on volcanoes of a very broad size range and very different geologic settings the process is similar across many orders of magnitude in size the deformation of the substratum related to magma intrusion during lava dome building events is at the lower end of the scale the spreading of seafloor at oceanic ridges is at the higher end of the scale volcanoes may evolve from small constructs into huge shield volcanoes and perhaps into ocean plates key elements for volcanic spreading are 1 the existence of a weak basal layer and 2 a sufficiently high mass and magma influx to drive the process as the mass of a volcano grows it passes through five phases which may overlap repeat or be omitted building compressing thrusting intruding and spreading during the building phase the mass of a volcano does not significantly affect the stress field of the substratum primitive magmas are likely to be erupted as the mass increases the volcano subsides and becomes characterized by a compressional stress field which inhibits any further intrusion of magma into the volcanic edifice magma may differentiate in the crust allowing for large caldera forming eruptions to occur in the next phase thrusting begins on a decollement usually located at the foot of the edifice while the stress field on the upper slopes of the volcano remains compressional the stress at the base close to the center becomes extensional allowing the magma to intrude and differentiate at the base of the edifice a basal intrusive complex begins to grow explosive eruptions may be expected finally spreading of this complex creates an extensional stress field on the upper flanks of the volcano confining compression to the lower slopes of the edifice the extension allows primitive magma to erupt again at the summit the process of spreading seems to have a fundamental influence on the structural and magmatic evolution of volcanoes at every scale this process results from coupling and feedback between the gravitational and thermal fields thus studying this process may improve our understanding of the relation between structural dynamics and magma evolution and of the origin and evolution of volcanic seismicity and eruptions

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

the stability of fine grained sediments from the fraser river delta the stability and erodibility of benthic fine grained sediments on the foreshore and upper foreslope of the fraser river delta were measured using thein situbenthic flume sea carousel the erosion threshold c 0 which varied between 0 11 and 0 50 pa was inversely related to erosion rate it was also proportional to sediment wet bulk density b in the form c 0 7 10 4 b 0 47 this trend agreed with those obtained from four other sites across canada that encompassed a wide range in sediment densities 1000 b 2000 kg m 3 erosion threshold shields parameter c was positively related to grain reynolds number over the silt clay size range 0 01 re 5 c 2 86re0 58 this was similar to earlier findings for water saturated sediment this trend is also in continuity with published threshold values measured in biostabilized sand 5 re 50 copyright 1997 academic press limited three erosion types were observed type ia erosion was a surface phenomenon caused by the presence of a thin organic fluff layer it occurred at current speeds less than 0 3 m s 1 type 1b erosion asymptotically decaying with time characterized the mid portions of each erosion time series and occurred at current speeds of 0 3 0 9 m s 1 type i ii erosion transitional was largely found on the foreslope and occurred at current speeds in excess of 0 8 m s 1 the erosion process began in all cases under turbulent transitional flows the change from type i to type ii erosion corresponded to the onset of turbulent rough flows 60 re 70 this is supported by video observations which showed type i erosion to be largely the result of entrainment of small aggregates and flocs whereas type ii erosion was the result of enlargement of surface irregularities and subsequent undercutting and release of large aggregates mean erosion rate em was independent of either azimuthal current speed or bed shear stress it was largely constant throughout each deployment and varied between 1 4 and 7 4 10 4 kg m 2 s 1

modeling diagnosis of suspended sediment transport in tidal estuarine system a three dimensional time dependent hydrodynamic and suspended sediment transport model was performed and applied to the danshuei river estuarine system and adjacent coastal sea in northern taiwan the model was validated with observed time series salinity in 2001 and with salinity and suspended sediment distributions in 2002 the predicted results quantitatively agreed with the measured data a local turbidity maximum was found in the bottom water of the kuan du station the validated model then was conducted with no salinity gradient no sediment supply from the sediment bed wind stress and different freshwater discharges from upstream boundaries to comprehend the influences on suspended sediment dynamics in the danshuei river estuarine system the results reveal that concentrations of the turbidity maximum simulated without salinity gradient are higher than those of the turbidity maximum simulated with salinity gradient at the kuan du station without bottom resuspension process the estuarine turbidity maximum zone at the kuan du station vanishes this suggests that bottom sediment resuspension is a very important sediment source to the formation of estuarine turbidity maximum the wind stress with northeast and southwest directions may contribute to decrease the suspended sediment concentration when the freshwater discharges increase at the upstream boundaries the limits of salt intrusion pushes downriver toward river mouth suspended sediment concentrations increase at the upriver reaches in the danshuei river to tahan stream while decrease at kuan du station

diagnosis of the sediment transport in the belgian coastal zone abstract estimating the age of particles in marine environment constitutes an invaluable tool to understand the interactions between complex flows and sediment dynamics particularly in highly energetic coastal areas such as the belgian coastal zone southern bight in the north sea to this end the constituent age and residence time theory cart introduced by delhez e j m campin j m hirst a c deleersnijder e 1999a toward a general theory of the age in ocean modelling ocean modelling 1 17 27 for passive water constituents is extended to describe the sediment dynamics it is then used in combination with a three dimensional coupled hydrodynamic sediment transport model to investigate sediment processes in the belgian coastal zone focusing on two complementary aspects of the sediment dynamics the internal sediment motion and redistribution within the belgian coast and the horizontal transport

mariana blueschist mud volcanism implications for conditions within the subduction zone several recently discovered active mud volcanoes on the nonaccretionary mariana convergent plate margin are erupting slab derived fluids serpentine mud and metamorphosed rocks from depths of as great as 25 km blueschist materials from the metamorphosed subducted plate are contained in the muds pore fluids indicate a depth dependence for decarbonation the mud volcanoes record in situ conditions along the decollement including pressure and temperature conditions and physical properties within the subduction zone similar mud flow material occurs worldwide as sedimentary serpentinite deposits in accreted fragments of former convergent margins making this kind of mud volcanism a more important phenomenon in convergent margins than previously recognized

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

extension in the northern mariana inner forearc seismic reflection seamarc ii acoustic imagery and bathymetric data across the northern mariana island arc reveal dense high angle normal faulting of the arc and inner forearc faults with throws up to 780 m cut a thick sequence of stratified but otherwise undeformed sediments the major faults display a sinuous or zigzag trace that is commonly associated with rifting this pattern can be indicative of a single event of triaxial strain that requires two contemporaneous conjugate fault pairs in orthorhombic symmetry interpretation and statistical analyses of the data show activity since the last sedimentary deposition and concurrent fault development of two distinct fault populations one associated with the backarc basin s east boundary fault and the other with extension in the forearc extensional faults along the eastern side of the backarc basin near the active arc at 22 n where active rifting is occurring have approximately the same trend as the east boundary fault in the survey area we estimate the direction of extensional minimum principal strain for the east boundary fault at 22 n to be 39 1 orthogonal to the extensional strain direction measured in the inner forearc the change in strain is distinct and abrupt occurring across a 20 km wide region along the arc massif within this narrow region of transition the intermediate and minimum principal strain directions are nearly equal in magnitude with orientations midway between the dominant forearc and backarc basin principal strain directions statistical analyses show that the strain from either backarc or forearc regime is not transmitted across this narrow region the two opposing strain regimes coexist virtually juxtaposed the coexistence of these two orthogonal extensional fault regimes within the overriding plate between the mariana backarc basin and inner forearc may be explained by assuming that a weak transitional region exists along the east boundary fault zone where stresses can be more easily relieved this allows active basin wide extension in the backarc basin while the forearc data are consistent with models that require radial forearc deformation resulting from increasing arc trench curvature of the northern half of the mariana island arc

vertical sediment distribution so far various calculation models for the vertical distribution of suspended sediment concentration have been produced by several investigators from different theories the limitations of all these models imply that it is possible to find a more reasonable model for which each previous model can be included as special case the formulation of a reasonable general model is the purpose of this paper

the effect of sediment stratification on tidal dynamics and sediment transport patterns 1 the western tip of southwest korea is characterized by a tidally dominated turbid coastal environment there are well developed tidal flats along the coast and around islands together with offshore sand ridges several tens of kilometers long in the west the effects of bottom boundary layer bbl sediment stratification on sediment transport and tidal dynamics in this environment were examined using a sediment transport model coupled with a three dimensional tidal hydrodynamic model model experiments using two scenarios with and without the effect of sediment induced stratification showed that bbl sediment stratification influenced the spatial distribution and reduced the magnitude of net sedimentation the presence of a sediment stratified bbl also led to a reduction in suspended sediment fluxes and an increase in the vertical gradient of sediment concentrations in the water column these variations occurred because sediment induced bbl stratification leads to not only a reduction in bottom shear stress but also a decrease in buoyancy production ofturbulent kinetic energy and an associated dampening ofturbulence the significant reductions in turbulence and bottom shear stress result in changes to the vertical current structure of the m 2 tide including alteration of the tidal ellipse configuration and an increase in vertical shears of the tidal current amplitude and phase these reductions also lead to a slight increase in tidal amplitudes due to the decreased tidal energy dissipation the model results indicate that feedback between the sediment transport dynamics and hydrodynamics is an important factor in modeling sediment transport dynamics in tidally dominated turbid coastal environments such environments include the west coast of korea and the yellow sea a suitable approach to simulating the effects of this feedback may be the use of a modified bottom drag coefficient as a stability function together with the inclusion of the effect of sediment stratification on the hydrodynamics

erosion of soft cohesive sediment deposits erosion behavior of soft cohesive sediment deposits has been investigated in laboratory experiments such deposits are representative of the top active layer of estuarial beds an experimental procedure involving layer by layer erosion under a range of bed shear stresses b of successively increasing magnitude was utilized interpretation of the resulting concentrationtime data together with bed density profiles yielded a description of the variation of the bed shear strength s with depth as well as an expression for the rate of surface erosion in general s increased with depth and was also influenced by the type of sediment bed consolidation period and salinity the rate of erosion was found to vary exponentially with b s 1 2 in modeling estuarial bed erosion it is essential to take these characteristics of s and the rate of erosion into account

automated sediment erosion testing system using digital imaging measurement of vertical profiles of the critical shear stress c and the erosion rate e from the same undisturbed sediment core is crucial for modeling the resuspension of fine grained natural sediments the automated sediment erosion testing system asets was developed to determine profiles of c and e with centimeter spatial vertical resolution in an undisturbed shelby tube sediment core whose surface was eroded by steady turbulent flow through a flume the unique feature of asets is that it is a real time imaging method that accurately determines the position of the core surface during erosion for both calculating the vertical profile of e and controlling a motor driver system that automatically pushes up the core to maintain its surface flush with the flume bottom undisturbed field cores were tested over a range of flow average bed shear stress b conditions the amount of eroded sediment from both optical backscattering measurements and the imaging method were in good agreement which validated asets measured vertical profiles of c and e were similar to those reported in literature e correlated well with b c 2 which agrees with previous results in literature

three dimensional modelling of cohesive sediment transport in the loire estuary in the framework of a european research programme processes describing cohesive sediment transport including fluid mud movements and consolidation have been modelled and implemented in the three dimensional finite element model telemac 3d which solves free surface flow problems the model has been applied to several european estuaries and among them the loire estuary in france this application proved the ability of the code to reproduce typical estuarine features such as the formation of a turbidity maximum and the movements of fluid mud copyright 2000 john wiley sons ltd

determination of the critical erosion threshold of cohesive sediments on intertidal mudflats along the dutch wadden sea coast abstract the bed shear strength on an intertidal mudflat bordering salt marshes along the dutch wadden sea coast was determined by means of an in situ erosion flume isef characteristic erosion patterns type i and type ii erosion were observed for most measurements the critical erosion threshold varied between 0 11 0 18 pa for all measurements and erosion rates varied between 5 10 5 and 3 10 3 kg m 2 s 1 no clear relationship was found between distinct parameters like bed density moisture content or biological activity and the critical erosion threshold a more pronounced effect of the erodability of the mudflat was found when the erosion rate was taken into account basically two clusters were distinguished high mean peak erosion rates of the substratum were found at locations which were characterized by a low mud content of the substratum below 20 mud by weight in these situations bed load transport is expected to be the most important process which determined erosion of the bed and erosion of predominantly the sand fraction was found at higher rates of the mud content over 20 mud by weight mean peak erosion rates decreased substantially an order 6 to 10 times lower and both erosion of the mud and sand fraction was found

cohesive sediment transport in the jiaojiang river estuary china extensive data were obtained of the water and fine sediment dynamics during spring tides in the extremely turbid jiaojiang river estuary at spring tides these data enabled the calibration of a two dimensional 2 d width integrated model four processes dominated the mud dynamics these were firstly the formation at slack tide of soft mud deposits with an erosion constant much smaller than that of the underlying compacted sediment secondly the sediment induced buoyancy effects thirdly the collapse of the turbulence by the sediment suspension in the fluid mud range and fourthly the inflow of sediment at the mouth of the estuary these findings demonstrate the necessity to have detailed field data to enable quantitative as opposed to qualitative modelling of mud dynamics in turbid estuaries the estuary is infilling with sediment from the east china sea and not from riverine inflow this sediment presumably originates from the yangtze river located 200 km further north