organic matter cracking a source of fluid overpressure in subducting sediments the pressure of deep fluids in subduction zones is a major control on plate boundary strength and earthquake genesis the record by methane rich fluid inclusions of large 50 100 mpa and instantaneous pressure variations in the shimanto belt japan points to the presence of large fluid overpressure at depth 300 500 mpa 250 c to further analyze the connection between methane and fluid overpressure we determined with rock eval the potential for a worldwide selection of deep seafloor sediments to produce methane as a result of organic matter om cracking due to temperature increase during subduction the principal factor controlling the methanogenesis potential of sediments is om proportion while om nature is only a subordinate factor in turn om proportion is mainly controlled by the organic terrigenous input considering a typical sediment from ocean continent subduction zones containing 0 5 wt of type iii om cracking of om has two major consequences 1 methane is produced in sufficient concentration as to oversaturate the pore filling water the deep fluid in accretionary prisms is therefore a mechanical mixture of water rich and methane rich phases 2 ch4 production can generate large fluid overpressure of the order of several tens of mpa the conditions for these large overpressure are a low permeability of the upper plate 10 km where om thermal cracking occurs at these depths om thermal cracking appears as a source of overpressure larger than the last increments of smectite to illite reaction such large overpressures play potentially a role in facilitating slip along the plate interface conversely the scarcity of earthquakes in ocean ocean subduction zones such as marianna or barbados may be related to the low influx of detrital om and the limited methane overpressure generation at depth

use of fracture filling mineral assemblages for characterizing water rock interactions during exhumation of an accretionary complex an example from the shimanto belt southern kyushu japan abstract various fracture filling minerals and secondary minerals in fracture walls were formed by fluid rock interaction during the exhumation of the palaeogene shimanto belt of kyushu japan which is located in an accretionary complex each mineral formed under favourable geological conditions and can be used to estimate the conditions of accretion and formation of the related rock sequences petrographic observations mineralogical and geochemical analyses were made on fracture filling minerals and secondary minerals from boreholes of ca 140 m depth drilled in the shimanto belt results reveal that the secondary minerals were formed in three major stages distinguished by the sequential textural relationships of the minerals and the interpreted environment of mineral formation filling mineral assemblages show that the studied rock formation has been subducted to a depth of several km and the temperature reached was ca 200 300 c after the subduction the rock formation was uplifted and surface acidic water penetrated up to 80 m beneath the present ground surface the acid water dissolved calcite fracture filling minerals to form the present groundwater flow paths which allowed recent wall rock alteration to occur the results shown here imply that filling mineral assemblages can be an effective tool to evaluate the environmental changes during exhumation of an accretionary complex

a new method of reconstituting the p t conditions of fluid circulation in an accretionary prism shimanto japan from microthermometry of methane bearing aqueous inclusions in paleo accretionary prisms and the shallow metamorphic domains of orogens circulating fluids trapped in inclusions are commonly composed of a mixture of salt water and methane producing two types of fluid inclusions methane bearing aqueous and methane rich gaseous fluid inclusions in such geological settings where multiple stages of deformation veining and fluid influx are prevalent textural relationships between aqueous and gaseous inclusions are often ambiguous preventing the microthermometric determination of fluid trapping pressure and temperature conditions to assess the p t conditions of deep circulating fluids from the hyuga unit of the shimanto paleo accretionary prism on kyushu japan we have developed a new computational code applicable to the h2o ch4 nacl system which allows the characterization of ch4 bearing aqueous inclusions using only the temperatures of their phase transitions estimated by microthermometry tmi the melting temperature of ice thyd the melting temperature of gas hydrate and th aq homogenization temperature this thermodynamic modeling calculates the bulk density and composition of aqueous inclusions as well as their p t isochoric paths in a p t diagram with an estimated precision of approximatively 10 we use this computational tool to reconstruct the entrapment p t conditions of aqueous inclusions in the hyuga unit and we show that these aqueous inclusions cannot be cogenetic with methane gaseous inclusions present in the same rocks as a result we propose that pulses of a high pressure methane rich fluid transiently percolated through a rock wetted by a lower pressure aqueous fluid by coupling microthermometric results with petrological data we infer that the exhumation of the hyuga unit from the peak metamorphic conditions was nearly isothermal and ended up under a very hot geothermal gradient in subduction or collision zones modeling aqueous fluid inclusions in the ternary h2o ch4 nacl system and not simply in the binary h2o nacl is necessary as the addition of even a small amount of methane to the water raises significantly the isochores to higher pressures our new code provides therefore the possibility to estimate precisely the pressure conditions of fluids circulating at depth

contrasting variability in foraminiferal and organic paleotemperature proxies in sedimenting particles of the mozambique channel sw indian ocean accurate sea surface temperature sst proxies are important for understanding past ocean and climate systems here we examine material collected from a deep moored sediment trap in the mozambique channel sw indian ocean to constrain and compare both inorganic delta o 18 mg ca and organic u 37 k tex86 temperature proxies in a highly dynamic oceanographic setting for application in paleoceanography high resolution time series current velocity data from long term moorings 2003 present deployed across the mozambique channel reveal the periodic migration of four to six meso scale eddies through the channel per year these meso scale eddies strongly influence water mass properties including temperature and salinity despite the dynamic oceanographic setting fluxes of the surface dwelling planktonic foraminifera globigerinoides ruber and globigerinoides trilobus follow a seasonal pattern temperatures reconstructed from g ruber and g trilobus delta o 18 and mg ca closely mirror seasonal sst variability and their flux weighted annual mean ssts of 28 1 degrees c and 27 3 degrees c are in close agreement with annual mean satellite sst 27 6 degrees c the sub surface dwelling foraminifera neogloboquadrina dutertrei and globigerinoides scitula recorded high frequency temperature variations that on average reflect conditions at water depths of 50 70 m and 200 250 m respectively concentrations and fluxes of organic compounds alkenones and crenarchaeol display no or only moderate seasonality but flux weighted means of the associated temperature signatures u 37 k and tex86h of 28 3 degrees c and 28 1 degrees c respectively also closely reflect mean annual sst we analyzed all time series data using multiple statistical approaches including cross correlation and spectral analysis eddy variability was clearly expressed in the statistical analysis of physical oceanographic parameters current velocity and sub surface temperature and revealed a frequency of four to six cycles per year in contrast statistical analysis of proxy data from the sediment trap did not reveal a significant coupling between eddy migration and organic compound fluxes or reconstructed temperatures this is likely a result of the relatively low resolution 21 days and short 2 5 years duration of the time series which is close to the detection limit of the eddy frequency

long term observations of transport eddies and rossby waves in the mozambique channel data from an array of current meter moorings covering a period of two and a half years are used to estimate the varying transport through the mozambique channel the total transport during this period is small below 1200 m the transport is weak but a prominent deep western boundary undercurrent with cores at 1700 and 2200 m is found that transports 1 5 sv to the north the transport shows a large temporal variability and neither a continuous upper layer western boundary current nor a continuous deep undercurrent is found the variability in the upper layer is dominated by a period of 68 days and results mainly from eddies that migrate southward through the mozambique channel in addition to this southward propagation a westward propagating signal is evident from a space time diagram of the throughflow the signal is interpreted as a mozambique channel rossby normal mode this interpretation is consistent with results from a principal oscillation pattern analysis that estimates normal modes from the data and a quasi geostrophic channel model a detailed inspection of a single eddy event shows that a precursor of an anticyclone is a strong southward current along the madagascar coast that propagates westward to the center of the channel during the westward propagation the current becomes unstable inducing an anticyclone this scenario connects the westward propagating mode with the eddy growth and explains the coincidence of the eddy and rossby mode frequency

thermomechanical milling of accessory lithics in volcanic conduits accessory lithic clasts recovered from pyroclastic deposits commonly result from the failure of conduit wall rocks and represent an underutilized resource for constraining conduit processes during explosive volcanic eruptions the morphological features of lithic clasts provide distinctive textural fingerprints of processes that have reshaped them during transport in the conduit here we present the first study focused on accessory lithic clast morphology and show how the shapes and surfaces of these accessory pyroclasts can inform on conduit processes we use two main types of accessory lithic clasts from pyroclastic fallout deposits of the 2360 b p subplinian eruption of mount meager british columbia as a case study i rough and subangular dacite clasts and ii variably rounded and smoothed monzogranite clasts the quantitative morphological data collected on these lithics include mass volume density 2 d image analysis of convexity c and 3 d laser scans for sphericity and smoothness s shaping and comminution i e milling of clasts within the conduit are ascribed to three processes 1 disruptive fragmentation due to high energy impacts between clasts or between clasts and conduit walls 2 ash blasting of clasts suspended within the volcanic flux and 3 thermal effects we use a simplified conduit eruption model to predict ash blasting velocities and lithic residence times as a function of clast size and source depth thereby constraining the lithic milling processes the extent of shape and surface modification i e rounding and honing is directly proportional to clast residence times within the conduit prior to evacuation we postulate that the shallow seated dacite clasts remain subangular and rough due to short 2 min residence times whereas monzogranite clasts are much more rounded and smoothed due to deeper source depths and consequently longer residence times up to 1 h larger monzogranite clasts are smoother than smaller clasts due to longer residence times and to greater differential velocities within the ash laden jet lastly our model residence times and mass loss estimates for rounded clasts are used to estimate minimum attrition rates due to volcanic ash blasting within the conduit e g 12 cm3 s 1 for 25 cm clasts sourced at 2500 m depth

contrasting seasonal fluxes of planktonic foraminifera and impacts on paleothermometry in the mozambique channel upstream of the agulhas current 1 we assess d 18 o and mg ca based paleothermometry in relation to seasonal variations in the shell fluxes of the surface dwelling planktonic foraminifera g ruber and g trilobus using deep moored sediment traps in the mozambique channel upstream of the agulhas current we find a distinct antiphase response to the annual cycle in sea surface temperature sst in g ruber and g trilobus maximum fluxes of g ruber occur in late austral summer february march when sst ranges between 28 7 c and 30 3 c by contrast g trilobus maxima appear in early winter june july at a lower sst between 25 3 c and 27 0 c cross correlation of the 2 5 year time series data confirmed that the g ruber g trilobus ratio r t ratio closely followed the annual cycle in sst as did their paired d 18 o and mg ca in all proxies g ruber showed a 3 week phase lag against sst and g trilobus of 6 weeks consistent with semi lunar population turnover rates in combination with a 1 week settling time to the 2500 m deep ocean floor after correcting for these lags we derived five independent equations for improved paleothermometry in the southwestern indian ocean that specify flux weighted annual mean sst the offset between flux weighted sst of g ruber and g trilobus and annual mean instrumental sst is about 0 8 c despite their strong seasonality in shell export this is far less than the mean difference in instrumental sst between summer and winter of 3 2 c both species therefore closely reflect annual mean sst in sediments within a margin of 0 5 c for g ruber and 0 3 c for g trilobus w e also find that multiple linear regression of the five proxies r t ratio and the d 18 o mg ca of g ruber and g trilobus strongly reduces the effects of fast rotating eddies and improves the correlation coefficient from an r 2 of 0 4 to an r 2 of 0 8

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

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

in situ measured current structures of the eddy field in the mozambique channel abstract circulation and the related biological production have been studied during five cruises conducted in the mozambique channel mzc between 2005 and 2010 the circulation in the mzc is known to be highly turbulent favouring enhanced primary production as a result of mesoscale eddy dynamics and connectivity throughout the channel due to the variable currents associated with migrating eddies this paper presents the results of in situ measurements that characterize the horizontal and vertical currents in the surface and subsurface layers 0 500 m the in situ data were analysed together with the geostrophic eddy field observed from satellite altimeter measurements different circulation regimes were investigated including the classical anticyclonic eddy generated at the channel narrows 16 s the enhancement of southward migrating eddies by merging with structures both cyclonic and anticyclonic formed in the east of the channel and the presence of a fully developed cyclonic eddy at the channel narrows comparison between in situ measurements s adcp and velocities derived from surface drifters and the geostrophic current derived from sea surface height measurements indicated that the latter can provide a reliable quantitative description of eddy driven circulation in the mzc with the exception that these currents are weaker by as much 30 it is also suggested from in situ observation drifters that the departure from geostrophy of the surface circulation might be linked to strong wind conditions finally our observations highlight that a geostrophic currents need to be considered in future research to facilitate a more comprehensive description of the circulation in this area

the influence of bottom currents on the zambezi valley morphology mozambique channel sw indian ocean in situ current observations and hydrodynamic modelling abstract mixed turbidite contourite systems can be found in oceans where bottom currents and turbidity currents interact the zambezi turbidite system located in the mozambique channel sw indian ocean is one of the largest sedimentary systems in the world in length and area of the related catchments the oceanic circulation in the mozambique channel is intense and complex dominated by eddies flowing southwards and deep currents flowing northwards along the mozambican margin current measurements obtained from moorings at 3400 4050 m water depth in the zambezi and tsiribihina valleys show periods of intense currents at the seafloor with peaks of 40 50 cm s 1 that last up to one month and are not related to turbidity currents these strong bottom current events are correlated with a change in current direction and an increase in temperature the periods of current intensification may be related to eddies since they present similar frequencies around 7 per year moreover modelling results show that during periods of intense deep circulation an anticyclonic eddy is present between the mozambican slope and the centre of the mozambique channel which may block the northward transport of the deep water mass and thus enhance the southward transport along the western slope of madagascar according to our hydrodynamic modelling of the circulation near the seafloor intense currents are often present along the zambezi valley especially along the valley flanks multi channel seismic reflection data show that the zambezi turbidite system does not show the typical characteristics of turbidite systems being dominated by erosional processes which mainly affect the valley flanks levees associated with the valley are absent in the main axis of the system the effect of bottom currents on sedimentation in the basin is evidenced by the low sedimentation rates that witness winnowing in the basin the presence of contouritic sand in the zambezi valley flanks and the abundance of current related bedforms observed in multibeam bathymetry and seismic data the intense oceanic processes observed in the mozambique channel may transport a large part of the fine sediment out of the basin and erode the seafloor even at great depths therefore the zambezi turbidite system could at present be considered as a mixed turbidite contourite system with important implications for source to sink studies

fluid overpressure along an oligocene out of sequence thrust in the shimanto belt sw japan abstract out of sequence thrusts osts exposed in ancient accretionary prisms are considered as fossil analogs of present day megasplay faults in subduction margins and can provide direct information about the conditions of deformation during thrust activity in modern as well as in ancient accretionary prisms first order megasplay faults or osts truncate or merge with faults of lesser importance called second order osts structural analysis of the makinokuchi fault a branch of an oligocene to lower miocene second order ost in the tertiary shimanto belt of central kyushu sw japan brings information about the conditions of deformation at the time of thrusting the studied exposure shows that the fault footwall and to a much lesser extent the fault hanging wall consist of quartz cemented syntectonic dilatant hydraulic breccias testifying to pore fluid pressures larger than the least principal stress component the footwall sandstones are crossed by several centimeters thick quartz veins that merge with the footwall breccias the continuity between the veins and the breccias suggest that the veins acted as conduits which likely collected fluids from the footwall side sandstones upward and toward the fault fluid inclusions indicate that the quartz cementing the breccias and that filling the feeder veins crystallized from similar fluids and under similar pressure and temperature conditions 245 285 c and 5 8 km depth these similarities suggest that the fluids responsible for syn tectonic hydraulic brecciation were collected from the footwall through the conduits the fluid inclusion trapping temperatures are close to the temperatures expected to be reached along the seismogenic zone our analysis shows that fluid overpressures can play a key role in the growth and activity of second order osts in accretionary prisms and suggests that fluids collected along second order osts or splay faults may flow upward along first order osts or megasplay faults

effects of biological activity on the entrainment of marine sediments nowell a r m jumars p a and eckman j e 1981 effects of biological activity on the entrainment of marine sediments mar geol 42 133 153 the effects of animal tracks and fecal pellet production on the critical entrainment velocity of marine sediments were examined experimentally laboratory measurements in a free surface seawater flume were made using three sediment sizes effects of three species of polychaetes and two species of bivalves were tested boundary shear velocity was calculated from the mean velocity profile in the logarithmic region of the boundary layer measurements were made with a hot film anemometer tracking doubled the boundary roughness and decreased the critical entrainment velocity by 20 ambient or free sediments were more easily entrained than fecal mounds which were restrained from movement by mucous adhesion between the fecal coils isolated pellets such as those egested by amphicteis scaphobranchiata transported readily as bedload over a cohesive sediment surface

a direct determination of the world ocean barotropic circulation abstractthe time mean argo float displacements and the world ocean atlas 2009 temperature salinity climatology are used to obtain the total top to bottom mass transports outside of an equatorial band the total transports are the sum of the vertical integrals of geostrophic and wind driven ekman currents however these transports are generally divergent and to obtain a mass conserving circulation a poisson equation is solved for the streamfunction with dirichlet boundary conditions at solid boundaries the value of the streamfunction on islands is also part of the unknowns this study presents and discusses an energetic circulation in three basins the north atlantic the north pacific and the southern ocean this global method leads to new estimations of the time mean western eulerian boundary current transports maxima of 97 sverdrups sv 1 sv 106 m3 s 1 at 60 w for the gulf stream 84 sv at 157 e for the kuroshio 80 sv for the agulhas current between 32 and 36 s and finally 175 sv for the

interaction of dipole eddies with the western continental slope of the mozambique channel abstract sea level anomaly sla data were used to track a southward propagating eddy dipole along the western slope of the mozambique channel over some 6 months in april 2005 this dipole with the cyclone to the south was close to the continental slope off southern mozambique the contact zone between the contra rotating vortices and the slope was surveyed by ship using onboard s adcp and ctd lines the data showed strong 1 4 m s 1 southward geostrophic currents over the slope adjacent to the anticyclone with horizontal divergence over the shelf edge significant slope upwelling between the dipole and the shelf was evident concomitant with enhanced nutrient and chlorophyll levels enriching shelf near surface waters satellite observations depicted a 300 km long surface chlorophyll filament extending offshore in the frontal zone between the contra rotating vortices a satellite tracked drifter deployed at the coastal base of this filament confirmed the offshore advection of chlorophyll enriched shelf water which ultimately wrapped around the cyclone and filling its centre the slope upwelling was also clearly evident in hourly temperature data collected by a recorder deployed on a nearby reef zambia reef in a depth of 18 m according to the sla data the dipole took several weeks to pass zambia reef causing prolonged bouts of upwelling that finally ceased when it left the continental slope and moved southwards into the open ocean further analysis showed that lone anticyclones and cyclones against the mozambique continental shelf also induce slope upwelling as a result of horizontal divergence created by the radial circulation of the vortex in the case of cyclones the divergence occurs north of the contact zone overall this case study confirms that eddies moving southwards along the western side of the mozambique channel are the main mechanism for pumping nutrients into the otherwise oligotrophic surface waters and moreover provide a vigorous mechanism for shelf open ocean exchange

paleothermal structure of the shimanto accretionary prism shikoku japan role of an out of sequence thrust vitrinite reflectance study on the cretaceous shimanto accretionary prism of southwest japan shows that the vitrinite reflectance varies from 0 6 to 3 0 corresponding to the estimated maximum paleotemperature variation of 90 210 c the thermal structure shows a sharp discontinuity across an out of sequence thrust separating it into two thermal blocks in each block vitrinite reflectance values increase systematically toward the prism younging direction and the thermal structure is clearly overprinted on the tectonic imbrication of turbidite and melange units this provided us with a set of data to calculate the geometry and displacement of the out of sequence thrust the fission track zircon partial annealing ages indicate that this thrust was active about 43 ma probably in response to the accretion of eocene rocks this study shows that initial thickening of the accretionary prism in an order of several kilometres was mainly controlled by out of sequence thrust

tectonic melange as fault rock of subduction plate boundary abstract an assemblage of quantitative data sets is examined to evaluate tectonic melange as a plate boundary fault rocks in subduction zone the research object is the latest cretaceous mugi melange in the shimanto belt southwest japan systematic age younging from pelagic to terrigenous through hemipelagic sediments is well documented even though original stratigraphy is disrupted systematic shear fabric consistent with ancient plate convergence is reconstructed the melange was formed at temperatures of 130 200 c by cataclastic comminution of sandstone layers accompanied by tensile cracking and plastic deformation and the dehydration of clayey shale matrix with subsequent peeling off and underplating of the oceanic basement the temperature setting for the mugi melange indicates around the up dip limit of the seismogenic zone therefore includes various fault rocks suggestive of earthquake fault pseudotachylyte fluidized ultracataclasite with heating evidence amorphous silica and so on these suggest that fluid induced lubrication was dominated localized cataclastic shear which is a candidate of small earthquake or very low frequency earthquake is also recognized especially in sandstone blocks dominated portion in melange these observations are consistent with the melange being a fault rock along the plate boundary that records various types of earthquakes in a subduction zone the quantitative examination of the mugi melange suggests several criteria to define the tectonic melange of the plate boundary fault in subduction zone from other melanges in orogenic belt

wind driven currents on the west florida shelf abstract three weeks of current meter wind and sea level data off cedar key florida are analyzed currents and sea level are found to be coherent with alongshore wind stress in the synoptic band 0 05 0 25 cycle per day and to lag it by approximately half a day little coherence is found with cross shelf wind stress at the inshore mooring 22 m depth currents are nearly barotropic for these winter 1978 data a linear parameterization of bottom stress in the barotropic alongshore current leads to a bottom friction parameter r of 0 01 0 02 cm s 1 using coastal wind stress no significant steady alongshore slope is found during this short interval the dominant momentum balance in the alongshore direction is between wind and bottom stress the offshore frictional length scale csanady 1978 is estimated to be 75 100 km which implies a seaward extent to a depth of about 30 m at the offshore mooring 44 m depth there is vertical shear between the currents at 9 and 39 m the upper cross shelf compon

stability of western boundary currents with an application to pulselike behavior of the agulhas current the northern agulhas current off south africa displays very steady flow characteristics roughly five to six times per year the flow is disrupted by the passage of large solitary meanders propagating downstream observations obtained from moored current meters show that these so called natal pulses extend over the full depth of the current several hypotheses have been proposed for the generation of these pulses here the authors investigate whether they arise as a barotropic instability on the inshore flank of the current itself the stability analysis is performed using a prescribed background state loosely based on agulhas current observations unstable inshore modes are found with propagation speeds close to those of the observed pulses to explain the intermittent temporal behavior of the pulses the interaction of the unstable modes of the western boundary current in the nonlinear regime is investigated the results indicate that the onset of pulselike behavior is found in wave mean flow interaction of unstable inshore modes with the background jet whether the observed timing of pulses is due to internal variability or set by processes external to agulhas current proper remains an open question

observed characteristics of mozambique channel eddies 1 the flow in the mozambique channel is dominated by large southward propagating anti cyclonic eddies as opposed to a steady western boundary current these mozambique channel eddies feed their waters into the agulhas current system where they are thought to have a significant influence on the formation of the natal pulse and agulhas ring shedding here we use in situ hydrographic and nutrient data together with satellite altimetry and surface velocity profilers to provide a detailed characterization of the mozambique channel eddies two warm eddies in the channel at 20 s and 24 s had diameters of over 200 km they rotated anti cyclonically with a tangential velocity of over 0 5 m s 1 vertical sections show that the eddies reached to the bottom of the water column relative to the surrounding waters the features were warm and saline the total heat and salt anomalies for the southernmost eddy were computed relative to a reference station close by at 24 s the total anomalies were 1 3 1020 j and 6 9 1012 kg respectively being on par with agulhas rings mozambique channel eddies thus have the potential to form a major contribution to the southward eddy heat flux in the agulhas current system the feature also had positive nutrient and negative oxygen anomalies the large magnitude of the water mass anomalies within the eddy suggests that interannual variability in mozambique channel eddy numbers would have a significant impact on downstream water mass characteristics

sedimentation patterns off the zambezi river over the last 20 000 years marine sediments from continental margins provide high resolution archives of marine and continental climate in particular near large river mouths the zambezi is one of the largest rivers in east africa discharging large amounts of fine grained sediments onto the western margin of the mozambique channel accurate reconstructions of past variations in river discharge can be retrieved from the depositional history of riverine sediments in this study sedimentation patterns along the mozambique margin are inferred from a series of sediment cores spanning the last 20 kyr these cores were retrieved off the zambezi mouth from the shelf and from various depths at the continental slope to provide detailed information on the depositional history of this region high resolution x ray fluorescence and magnetic susceptibility core scanning measurements as well as grain size distributions of the lithogenic sediment fraction and organic matter and carbonate content analyses resolve centennial millennial timescale variations in sediment transport and deposition along the mozambique margin largest changes in lithogenic sediment deposition coincide with the flooding of the mozambique shelf due to deglacial sea level rise and changes in oceanic currents fine grained mainly zambezi sediments were no longer primarily deposited south of the zambezi mouth but increasingly deposited onto the mozambique shelf and transported northwards over the shelf before spilling over to the slope in addition flooding of the shelf initiated remobilization and down slope transport of coarse grained winnowed sediments the heinrich event 1 and younger dryas are clearly indicated as events of increased fine grained sediment deposition along the mozambique margin possibly linked to increased riverine runoff from the zambezi catchment which is in harmony with a southward shift of the intertropical convergence zone itcz and associated african rainfall belts causing wetter conditions this study illustrates that interpreting marine and continental climate proxy records in the vicinity of river deltas in terms of climate change requires proper consideration of sea level and oceanic circulation induced effects on the transport and deposition patterns of fine grained sediments at continental slopes

vitrinite and vitrinite like solid bitumen reflectance in thermal maturity studies correlations from diagenesis to incipient metamorphism in different geodynamic settings abstract vitrinite reflectance vr is a useful and reliable parameter to monitor the level of organic matter maturation from the immature stage to graphite additionally vr is a useful and essential tool in modelling the temperature maturation time path huminite vitrinite bearing sedimentary and meta sedimentary rocks can be used to reconstruct the thermal history of basins or orogenic terranes throughout their geological history the usefulness of vr evidently depends on the presence of huminite vitrinite phytoclasts however pre upper silurian rocks and many organic rich marine rocks e g sapropelic sediments lack of huminite vitrinite particles furthermore vitrinite is sparse in carbonate sediments however the presence of bitumen in rocks lacking type iii kerogen provides a valuable aid via its reflectance vitrinite like solid bitumen reflectance vlbr and or solid bitumen reflectance br to fulfil the above described tasks that vr enables this study presents basically a relationship between br vlbr and vr for various types of geodynamic settings and over the largest possible range of maturation levels so that vlbr may be used as a substitute for vr whereas the effort to correlate vr with br is not novel it is the first time that the vr br correlation is re judged discriminating bitumen from vlb moreover for the first time the correlation is established for various geodynamic settings the investigated geological contexts are 1 sedimentary basins e g the saar ruhr ohio west virginia and petrosani basins 2 metasedimentary terranes occurring as inverted basins in orogenic belts e g from the subalpine molasse helvetic austroalpine and danubian nappe systems and having suffered from very low to low grade metamorphism from anchizone to greenschist facies 3 metasedimentary terranes e g slates and schists from the franciscan and alpine belts that were subducted at anchizone to blueschist facies conditions and 4 metasedimentary rocks affected by contact metamorphism together the frequent geological settings are referred and most of the earth geothermal conditions and heat fluxes considered despite temperature pressure and heating time also heat flow gradient dependences cannot be ignored

fluid rich damage zone of an ancient out of sequence thrust kodiak islands alaska 1 the uganik thrust is a fossil out of sequence thrust fault which was active over a period of 3 ma during the early tertiary until activity ceased with the subduction of the kula farallon spreading ridge at 57 ma during this period the fault experienced at least 1 km of throw and developed a strongly asymmetric damage zone the brittle damage zone in the footwall of the fault acted as a conduit for fluid advection during the active faulting a similar asymmetrical footwall damage zone has been interpreted as a fluid conduit at the nobeoka thrust shimanto belt sw japan thermal indicators in the uppermost footwall give similar maximum paleotemperatures to those in the hanging wall 280 c while previous work elsewhere in the footwall formation suggests maximum burial temperatures of 240 c in this case similar to the irish canyon thrust in the franciscan accretionary complex the location of the thermal anomaly is spatially offset from the structural fault which caused it owing to thermal overprinting in the vicinity of the fault

coastal sea level response to frontal passages on the louisiana texas shelf sea level variations resulting from wind forcing along the louisiana texas shelf are examined on 5 month winter records local weather patterns during the winter are dominated by cold front passages the associated surface wind field is well organized predominantly in the cross shelf direction regional sea level response however shows considerable variability it is mainly a response to alongshore wind at galveston but cross shelf wind at eugene island this spatially nonuniform response appears to be due to the different water depth over the inner shelf which is shallower off eugene island and favors direct cross shelf wind setup a friction model is applied to each region and the results compare well with observations

warm fluid migration along tectonic melanges in the kodiak accretionary complex alaska analyses of methane water fluid inclusions from syntectonic quartz veins in melange boudins of the kodiak accretionary complex alaska indicate fluid temperatures ranging from 215 to 290 c at depths between 10 and 14 km these fluid temperatures are significantly higher than those predicted by conductive heat flow models of subduction zones given the available geological constraints this discrepancy is interpreted as the result of migration of warm fluids from deeper structural levels along active fault zones during subduction related underthrusting of these units this interpretation requires more careful consideration of how migrating fluids affect the general thermal structure of subduction zones

dynamic role of tectonic melange during interseismic process of plate boundary mega earthquakes abstract tectonic melange has a key role in subduction zones because its thick pile forms the plate boundary itself therefore the plate boundary process is nearly identical to the melange forming process we examined three tectonic melanges in the shimanto belt in southwest japan to decipher their progressive deformation process with subduction especially within the seismogenic zone here we report detailed observation of sandstone blocks and the strain history of the shale matrix of a sediment dominated tectonic melange the necessity of tectonic melange accompanying fossil seismogenic decollement is unveiled several deformation processes continue by turn until the depth of the down dip limit of the seismogenic zone is reached the results support the space time partition of deformation in terms of seismic behavior and suggest a possible candidate for a geological consequence of recently observed slow earthquakes

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

source and tectono metamorphic evolution of mafic and pelitic metasedimentary rocks from the central quetico metasedimentary belt archean superior province of canada abstract a study of the sedimentary rocks in the jean lake area of the quetico metasedimentary belt superior province canada was conducted in order to evaluate the origin source and evolution of rocks including mafic rocks previously mapped as ultramafics rocks bulk compositions of these sedimentary rocks show a mixing between two end members quartzo feldspathic sedimentary rock and komatiitic basalt high cao and mgo contents of the rocks suggest a proximal source of the komatiitic basalt the rocks in the study area record a pressure temperature p t path with three tectono metamorphic stages the first stage formed staurolite 500 700 c under medium p t mp mt metamorphic conditions shortly after the sedimentation the second stage yielded the biotite sillimanite garnet assemblage under the peak conditions of 0 6 0 1 gpa and 700 70 c during transpressional deformation the third stage low p medium t lp mt 0 25 0 11 gpa 540 80 c metamorphism was associated with regional south southeast compression and its timing is constrained by a new u th pb monazite age of 2667 20 ma combining the regional deformation events we suggest the burial metamorphism of sediments up to mp mt conditions during the d1 deformation at 2698 2689 ma they attained the peak metamorphic conditions during the regional transpressive d2 d3 deformation 2689 2671 ma and retrograded to lp mt condition during the south southeast compression regional d4 at 2671 2667 ma a short time span between the sedimentation and mp mt metamorphism accompanied by d1 deformation suggest that the quetico sedimentary rocks formed in an acretionary prism the studied rocks in the jean lake area deposited close to the wabigoon subprovince transported towards the south and buried up to 20 km by rapid underthrusting to amphibolite facies conditions subsequent dextral transpressive deformation d2 d3 during the oblique docking of the quetico belt to the wabigoon subprovince to the north and wawa greenstone belt from the south resulted in the displacement of the sedimentary rocks to the west from the original depositional site the p t time path of the quetico sedimentary rocks is similar to that in modern arc accretion prisms except for a high geothermal gradient of 30 c km recorded in the former compared to 10 c km in modern counterparts the high temperature gradient in the archean accretionary prism explains the lack of high pressure metamorphic rocks such as blueschist that are common in modern accretionary prisms

orogeny and relative plate motions example of the japanese islands abstract the paleogeography of the eastern margin of asia near japan for the past 250 ma has been reconstructed on the basis of the relative motions between the oceanic plates and the eurasian plate engebretson et al 1982 and on land geology the relationships between the plate kinematics and orogeny are as follows calc alkaline magmatism is related to the presence of subduction when a plate boundary is a transform fault there is no volcanism very slow convergence rates do not give rise to magmatism migration of rft type triple junctions or ridge subduction produces forearc volcanism near the trench and also a wider volcanic zone uplift of low p t type regional metamorphic belts is related to collision episodes the uplift of the high p t blueschist belts on the contrary is not contemporaneous with them they appear to be related to the start of the subduction of a young plate after the migration of an rft type triple junction because the formation of a voluminous accretionary complex is contemporaneous with this and thrusting of this young accretionary complex beneath the uplifted metamorphic belt is often observed the underplating of the young accretionary complex beneath the older accretionary complex would be the cause of the uplift of the high p t type metamorphic belt the subduction of a young plate would thus be an important factor causing these orogenic events highly oblique convergence also results in formation of a serpentinite melange zone with fragments of blueschists presumably along a mechanical plate boundary in the forearc

summertime coastal currents in the northeastern gulf of mexico abstract observations of currents and temperature at a mooring on the 18 m isobath 30 km south of the florida shoreline are discussed for the 31 day period 15 august 15 september 1978 tidal currents having average amplitudes of 10 cm s 1 account for 85 of the observed kinetic energy low frequency currents with maximum speeds of 10 cm s 1 appear to be at least partly driven by local wind stress events of magnitude 0 2 dyn cm 2 frictional effects give rise to a veering with depth in the ekman sense of both the low frequency flow and the energetic counterclockwise rotating twice daily tidal currents propagation of both the tidal signal and low frequency coastal sea level fluctuations amplitudes 5 cm is westward through the study area

mesoscale activity in the comoros basin from satellite altimetry and a high resolution ocean circulation model recently it has been shown that anticyclonic eddies are generated in the comoros basin contesting the long held notion of a single large anticyclonic cell the comoros gyre limited knowledge exists about the mesoscale activity within the basin a potential key source of variability for the mozambique channel and subsequently the agulhas current in this paper an automated eddy tracking scheme applied to satellite altimetry data and a high resolution model simulation is used to determine the characteristics of the anticyclonic eddies generated in the comoros basin the generation and characteristics of cyclonic eddies are also investigated the eddy tracking scheme revealed that anticyclonic eddies are primarily generated west of the tip of madagascar due to barotropic instabilities whereas cyclonic eddies are mainly generated along the northwest coast of madagascar as a result of baroclinic instabilities anticyclonic eddies with a mean lifespan of about 3 months reside in the basin for half their lifespan before propagating into the mozambique channel on the other hand the majority of cyclonic eddies with a similar mean lifespan dissipate within the basin initially the anticyclones with translation speeds of 6 8 km d 1 and mean radii of 80 100 km follow the trajectory of the north east madagascar current and turn south upon reaching the african coast the cyclonic eddies tend to be smaller 60 km and have slower translation speeds 2 5 3 5 km d 1 than their anticyclonic counterparts

fluids at crustal pressures and temperatures the pressure volume temperature data of several gases h2 o2 co ch4 n2 and co2 have been used to obtain the following corresponding states equation p 1 kbar t 400 k z 0 0878 a bp r cp r 2 21 where z is the compressibility factor p r reduced pressure p critical p t r reduced temperature t critical t and the coefficients are given by a 1 0 5917 t r 2 b 0 09122 t r 1 c 1 4164 10 4t r 2 2 8349 10 6 ln t r p 1 kbar t 400 k at pressures below 1 kbar z is given by z 0 0761 1 bp r cp r 2 23 where b 0 09827 t r 1 0 2709 t r 3 c 0 01472 t r 4 0 00103 t r 1 5 for water p 1 kbar t 400 k z 0 0209 a bp cp2 22 where a 0 7025 1 16 10 3t 99 6799 t 1b 0 2143 t 1 3 1423 10 14t3c 2 249 10 6t 1 0 1459 t 3 2 1690 10 15t2 at pressure below 1 kbar the equation is p 100 bar t 673 k z 0 0601 1 b v c v2 24 where b 2 20960 t0 5 3 35460 10 8 t3c 3 4569 10 5t2 5 64 9764 ln t fugacities of the gases may be obtained by integrating the z equations separately and combining the results as follows int limits 1 p vdp rt text left int limits 1 1000 z 23 pdp int limits 1000 1 z 21 pdp right 26 the subscript in z denotes the equation which is to be used for calculating z

observed eddy dissipation in the agulhas current analysing eddy characteristics from a global dataset of automatically tracked eddies for the agulhas current in combination with surface drifters as well as geostrophic currents from satellite altimeters it is shown that eddies from the mozambique channel and south of madagascar dissipate as they approach the agulhas current by tracking the offshore position of the current core and its velocity at 30os in relation to eddies it is demonstrated that eddy dissipation occurs through a transfer of momentum where anti cyclones consistently induce positive velocity anomalies and cyclones reduce the velocities and cause offshore meanders composite analyses of the anti cyclonic cyclonic eddy current interaction events demonstrate that the positive negative velocity anomalies propagate downstream in the agulhas current at 44 km day 23 km day many models are unable to represent these eddy dissipation processes affecting our understanding of the agulhas current

thermal maturity in the shimanto accretionary prism southwest japan with the thermal change of the subducting slab fluid inclusion and vitrinite reflectance study abstract the p t path of the cretaceous shimanto accretionary complex western shikoku southwest japan was determined by a combination of vitrinite reflectance data and homogenization temperatures of fluid inclusion analysis within the complex the p t paths of the upper cretaceous yokonami melange and surrounding coherent strata were compared no p t difference was found between the yokonami melange and surrounding strata at least two types of fluids methane rich and water rich fluids migrated at different stages during metamorphism within the complex the methane rich fluid was initially trapped in vein quartz under pressures of 260 mpa and a geothermal gradient of 24 c km the water rich fluid was included in calcite at 95 125 mpa and a geothermal gradient of 50 c km the complex sequence of occurrence of the vein minerals and radiometric age data suggest that the water rich fluid was trapped during the kula pacific ridge subduction into the shimanto accretionary complex the paleoheat flow values for the two stages of subduction are consistent with the changes in the inferred slab age the metamorphism of the shimanto accretionary complex was predominantly controlled by the thermal condition of the subducting plate

effects of subduction parameters on geothermal gradients in forearcs with an application to franciscan subduction in california geothermal gradients in forearcs are often suppressed below normal values because of the cooling effect of the relatively cold downgoing plate in this paper finite difference thermal modeling is used to evaluate the influence on forearc gradients of variations in six potentially important subduction zone parameters variations in forearc radiogenic heat production over the general range likely in forearc rocks 0 to 1 5 10 6 w m 3 cause only minor changes in temperatures at the base of the forearc about 2 to 35 depending on other parameters and thus have relatively little effect on pressure temperature pt conditions of metamorphism however their effect on gradients at shallow levels and thus on surface heat flows may be greater up to about 100 likely variations in forearc thermal conductivity 1 5 to 3 5 w m 1 c 1 similarly change temperatures at the base of the forearc only modestly 10 25 but change surface heat flows more substantially 1 45 1 75 times decreasing the subduction angle from 27 to 3 6 increases gradients by about a factor of 2 at both deep and shallow levels decreasing the subduction rate by a factor of 8 160 to 20 km m y increases gradients by about a factor of 2 when frictional heating at the plate contact is excluded and less when modest amounts of frictional heating are included forearc gradients increase virtually linearly with the presubduction geothermal gradient in the subducting slab and thus are higher above a younger slab when frictional heating is excluded forearc gradients are 2 0 2 8 times higher above a 10 m y old slab than above a 150 m y old slab again the relative effect is lessened when frictional heating is incorporated the amount of frictional heating at the plate contact potentially has the greatest influence on forearc gradients the frictional heat production h is given by h vs where is the shear stress along the plate contact and vs is the subduction rate the modeling shows that forearc gradients increase linearly with unless plate contact shear stresses are only a small fraction of those expected with frictional sliding of dry rocks outer parts of forearcs would be the sites of pervasive low p t metamorphism greenschist facies rather than high p t metamorphism blueschist facies very high fluid pressures along the plate contact are probably the way shear stresses are reduced pressure temperature conditions of blueschist facies metamorphism in the franciscan subduction complex of california are easily explained with typical subduction rates and slab ages with plate contact shear stresses of the order of 10 mpa equivalent to around 3 of lithostatic pressure but stresses within the range zero to a few tens of megapascals are probably permitted by the thermal constraints speculative application of the modeling results assuming a shear stress of 4 of lithostatic pressure to plate motion reconstructions for the franciscan forearc suggests that forearc gradients were about 8 c km around 85 ma when the subducting slab was perhaps 145 m y old and the subduction rate was perhaps 95 km m y gradients increased moderately through the latest cretaceous to middle tertiary as subduction became slower and the subducting slab became younger reaching about 16 c km at 28 ma when the slab age was about 11 m y and the subduction rate was about 48 km m y the slab age subduction rate and forearc gradient then remained fairly constant until 5 ma when subduction slowed to about 32 km m y and the slab age decreased to about 8 m y causing gradients to rise to about 20 c km

origin of methane in quartz crystals from the tertiary accretionary wedge and fore arc basin of the western carpathians abstract quartz crystals from mineralised joints of the carpathian flysch contain fluid inclusions covering the compositional range from gaseous ch4 co2 c2 mixture through ch4 saturated oil to ch4 saturated aqueous solution the c isotope composition of ch4 extracted from the fluid inclusions 31 and 36 v pdb is indicative of a thermogenic dry gas released from overmature rocks although the maturation level of the surrounding flysch corresponds to a wet gas generating zone trapping pt conditions for the ch4 bearing fluids range between 130 and 205 c and 0 5 and 3 7 kbar the fluid inclusion derived crystallisation temperatures are inconsistent with the vitrinite reflectance data strongly fluctuating fluid pressures during growth of the quartz crystals indicate the existence of a crack seal mechanism with recurrent hydrostatic and lithostatic regimes fluid pressures exceeding the lithostatic load have been recorded in the dukla nappe the overpressures have been generated by fluid volume increase due to thermal decomposition of oil and kerogen to ch4 and pyrobitumen occurrence of oil and condensate in the ch4 bearing inclusion fluids trapped at temperatures considerably above oil window suggest a short termed transient post orogenic heating rather than a prolonged heating related to burial the thermal event giving rise to the quartz crystallisation has been coincidental with regional collapse and uplift of the western carpathians

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

fluid circulation in the depths of accretionary prisms an example of the shimanto belt kyushu japan accretionary prisms constitute ideal targets to study fluid circulation and fluid rock interactions at depths beyond the reach of active margin deep drilling the highest grade rocks from the shimanto belt on kyushu were buried under 3 5 kbars at 300 c toriumi and teruya 1988 they contain abundant quartz veins formed throughout burial and exhumation and variably affected by brittle and ductile deformation cathodoluminescence cl reveals the existence of two distinct types of quartz characterized by a blue and brown color respectively cl blue quartz fills macro veins width 10 m while cl brown quartz is present in micro veins width 1 10 m and ductilely recrystallized domains on the basis of microstructures the fluids associated with the cl blue and cl brown quartz are interpreted as external and local respectively quartz growth rims of alternating cl colors as well as mutually cross cutting veins show that the two fluids cyclically wetted the host rock from fluid inclusions analysis the fluid associated with cl blue quartz has a salinity similar to seawater while the fluid associated with cl brown quartz is less saline in addition cl blue quartz is richer in aluminum than the cl brown one in contrast to the salinity aluminum signature the 18o isotopic signature of both quartz types is similar and buffered by host rock the difference between the preservation of the salinity signature of the fluid and the loss of its 18o signature is explained by quicker exchange kinetics and larger host rock buffering capacity for isotopic reequilibration the local fluid associated with cl brown quartz reflects the dilution of pore water by the pure water produced by prograde dehydration reactions of clay minerals the external fluid associated with cl blue quartz is interpreted as seawater or pore water from shallow depth 1 2 km below seafloor sediments we propose that downward percolation of shallow water to depths 10km is a transient process associated with mega earthquakes

hydrodynamic and sedimentary responses to two contrasting winter storms on the inner shelf of the northern gulf of mexico abstract results are presented from the deployment of three bottom mounted instrumentation systems in water depths of 6 9 m on the sandy inner shelf of louisiana usa the 61 day deployment included nine cold front passages that were associated with large increases in wind speed two of the most energetic cold front passages were characterized by distinct meteorological hydrodynamic bottom boundary layer and sedimentary responses and may potentially be treated as end member types on a continuum of regional cold front passages arctic surges ac storms have a very weak pre frontal phase followed by a fairly powerful post frontal phase when northeasterly winds dominate migrating cyclones mc storms are dominated by a strong low pressure cell and have fairly strong southerly winds prior to the frontal passage followed by strong northwesterly winds on the basis of measurements taken during this study ac storms are expected to have a lower average significant wave height than mc storms and are dominated by short period southerly waves subsequent to the frontal passage currents are weak and northerly during the pre frontal phase but become very strong and southwesterly following the passage sediment transport rate during as storms was not as high as during mc storms and the mean and overall direction tended to be southwesterly to westerly with low frequency flows producing easterly transport and wind wave flows producing southeasterly transport mc storms had the most energetic waves of any storm type with peaks in significant wave height occurring during both the pre and post frontal phases the wave field during mc storms tended to be more complex than during as storms with an energetic northerly swell band gradually giving way to a southerly sea band as the post frontal phase progressed currents during mc storms were moderate and northerly during the pre frontal phase but became much stronger and southeasterly during the post frontal phase shear velocity was high during both the pre and post frontal phases of the storm although sediment transport was highest following the frontal passage mean and overall sediment transport was directed southeasterly during mc storms with low frequency and wind wave flows producing northerly transport in summary the data sets presented here are unique and offer insight into the morphosedimentary dynamics of mid latitude micro tidal coasts during extratropical storms

on sverdrup discontinuities and vortices in the southwest indian ocean abstract the southwest indian ocean is distinguished by discontinuities in the wind driven sverdrup circulation these connect the northern and southern tips of madagascar with africa and the southern tip of africa with south america in an analytical barotropic model with a flat bottom the discontinuities produce intense westward jets those off the northern tip of madagascar and the southern tip of africa are always present while the strength of that off southern madagascar depends on the position of the zero curl line in the indian ocean the jet is strong if the line intersects madagascar but weak if the line is north of the island all three jets are barotropically unstable by the rayleigh kuo criterion the authors studied the development of the instability using a primitive equation model with a flat bottom and realistic coastlines the model produced westward jets at the three sites and these became unstable after several weeks generating 200 300 km scale eddies the eddies generated west of

suspended sediment transport in an estuarine tidal channel within san francisco bay california abstract a recently developed instrumentation system has been used to monitor simultaneously flow conditions and suspended sediment distribution in the bottom boundary layer of a tidal channel within san francisco bay california measurements were made every 15 min over six successive flood and ebb tidal cycles they included mean velocity profiles from four electromagnetic current meters within 1 m of the seabed mean suspended sediment concentration profiles from seven miniature nephelometers placed within 1 m of the seabed near bottom pressure fluctuations vertical temperature gradient and bottom photographs additionally suspended sediment was sampled from four levels within 1 m of the seabed three times during the tidal cycle the instrument system was retrieved during each slack water period to exchange suspended sediment sample bags while the instrument was deployed std nephelometer measurements were made thoughout the water column and water samples were collected each 1 h and bottom sediment was sampled at the deployment site size distributions of the suspended sediment samples estimates of particle settling velocity s friction velocity u and reference concentration c a at z 20 cm were used in the suspended sediment distribution equations to evaluate their ability to predict the observed suspended sediment profiles three suspended sediment particle conditions were evaluated 1 individual particle sizes in the 4 11 62 5 0 5 m size range with the reference concentration c a at z 20 cm c 2 individual particle sizes in the 4 6 size range flocs representing the 7 11 size range with the reference concentration c a at z 20 cm c f and 3 individual particle sizes in the 4 6 size range flocs representing the 7 11 size range with the reference concentration predicted as a function of the bed sediment size distribution and the square of the excess shear stress an analysis was also carried out on the sensitivity of the suspended sediment distribution equation to deviations in the primary variables s u and c a in addition computations of mass flux were made in order to show vertical variations in mass flux for varying flow conditions

field measurements of fairweather bottom boundary layer processes and sediment suspension on the louisiana inner continental shelf field measurements of bottom boundary layer and sediment transport processes were made on the louisiana inner continental shelf in spring 1992 at a depth of 15 5 m and in spring and summer 1993 at a depth of 20 5 m two different wave current boundary layer sediment transport models were applied to the measured near bed flows in addition the log profile method was applied to estimate hydraulic roughness and bed stress consistent with the results of others our measurements show that near bed flows were very weak under non storm conditions bed stresses were typically too low to resuspend bed sediments however the advection of high turbidity layers or plumes past the instrumentation apparently caused a sustained period of high suspended sediment concentration throughout the log layer in spring 1993 in the absence of wave activity or high suspended sediment concentrations boundary layer profiles showed the bed to have been hydraulically very smooth with z o 0 1 cm however wave agitation combined with increased suspended sediment concentration caused hydraulically rough conditions with z o 0 1 cm

thermal modeling of the chugach metamorphic complex heating from hot young subducting oceanic lithosphere and from a subducting spreading center has been proposed to explain the metamorphism and associated plutonism observed in former accretionary prism sedimentary rocks in the gulf of alaska we evaluated the thermal effects on the overlying accretionary wedge by heat conduction from a hot young subducting slab through numerical modeling the temperature limiting parameter in the modeling is the age of the subducted crust the rate of convergence angle of subduction amount of radiogenic heating and values of thermal conductivity are of secondary importance in determining temperatures in the accretionary prism a possible former setting for the subduction of young oceanic crust is recorded in the valdez metasedimentary rocks in the chugach metamorphic complex of the chugach terrane in southern alaska petrologic data indicate that rocks now at the surface were at 10 km depth when they were metamorphosed under greenschist facies conditions 350 400 c and subsequently intruded by felsic plutons using parameter values appropriate for chugach terrane rocks our results indicate that for accretionary prisms less than 20 km thick oceanic lithosphere aged 1 0 ma or less must be subducted in order to obtain temperatures greater than 300 c at 10 km depth within the prism we also modeled the transient heating of the accretionary prism due to the cessation of subduction of young oceanic lithosphere recent seismic refraction surveys from the trans alaska crustal transect have been interpreted to show underplated sedimentary rocks and relict oceanic crust beneath the chugach mountains transient heating due to cessation of subduction or underplating of hot oceanic crust does not appreciably relax the requirement of subducting oceanic crust aged 1 0 ma or less in order to attain temperatures greater than 30o c at 10 km depth however at depths greater than 25 km it may provide the heat necessary to generate hot fluids and felsic magma which can augment the temperatures at shallow depths by advection of heat as well as produce the postmetamorphism or synmetamorphism emplacement of the anatectically derived plutons observed in the chugach mountains our modeling requires subduction of such young crust that subduction of a spreading ridge was probable although underplating of extremely young oceanic crust would also have provided sufficient heat to cause the formation of the chugach metamorphic complex

distributed deformation along the subduction plate interface the role of tectonic melanges recent geophysical monitoring of subduction zones has unraveled a complete spectrum of plate coupling behaviors from coupled portions rupturing during earthquakes to decoupled portions slipping aseismically however the deformation mechanisms and the exhumed rock corresponding to these contrasted behaviors are not yet identified tectonic melange zones are thought to play a major role in the deformation of the plate interface as they represent remnants of the subducted plate scraped off by the overriding plate in this work we examine several tectonic melange zones hyuga okitsu mugi from the shimanto belt an accretionary prism in southwest japan connecting to the active nankai subduction zone these tectonic melange zones have a block in matrix structure with lenses of sandstones and basalts within a metapelitic matrix and their deformation is distributed over zones of hundreds of meters in thickness in addition the examples of melange considered here are bounded by sharp faults some of them bearing pseudotachylyte layers so that distributed deformation within the melange and localized deformation on its boundary are juxtaposed distributed deformation involves the development of a foliation as well as of a pervasive network of macroscopic and microscopic shear zones along with slip on this network strain proceeds by fracturing and precipitation of quartz in long veins parallel to the foliation or in smaller cracks perpendicular to stretching and forming in the neck of competent lenses of sandstones or former quartz veins the analysis of shear band kinematics shows in all three examples a dominant top to the trench sense of shear consistent with deformation along the plate boundary during subduction moreover most shear zones when foliation is restored back to syn subduction position are extensional structures finally the geometry and kinematics of the melange bounding faults as well as radiometric constraints show that in most cases the faults localized structures were formed during a later stage than melange internal deformation these findings bear several consequences on the structure and dynamics of the subduction plate boundary at seismogenic depths first there is no support for a model of plate boundary fault zone composed simultaneously of localized slip zones and domains of more distributed deformation second rather than proposed models of underplating where all deformation is localized into the thrusts bounding the tectonic sheets we suggest that underplating was to a large extent accommodated by distributed deformation within the melange sheets this underplating model accounts for i the large amount of strain within the melange ii the absence of contractional structures during underplating iii thinning of the melange required by the network of extensional shear bands and stretched boudins third melanges appear as likely candidates for portions of the plate interface deforming by aseismic slip the seismic vs aseismic character of the plate interface might depend on the ability of sediments on top of the subducting plate to undergo distributed strain which in turn depends on the efficiency of pressure solution to operate

wind forced sea level variability along the west florida shelf winter 1978 abstract coastal tide gage and meteorological records from pensacola to key west for the period january april 1978 have been examined for low frequency fluctuations the dominant 6 day period signals in sea level alongshore wind stress and atmospheric pressure were coherent over the entire shelf and propagated southward consistent with the movement of cold fronts through the area sea level response tagged the local wind stress by 18 h in the north to 9 h in the south in response to a 1 dyn cm 2 alongshore stress sea level amplitudes were largest 60 cm where the shelf is widest 200 km and undergoes an abrupt bend and were 30 cm elsewhere large transient alongshore sea level slopes on the order of 10 6 were thus set up a linear steady state shelf circulation model hsueh 1980 is used to explore the sea level distribution that is in frictional equilibrium with a wind stress of given orientation for a bottom resistance coefficient of 0 014 cm s 1 a value suggested by an analysis of fe

short paper accretion and uplift in the shimanto belt sw japan k ar cleavage dating and fission track cooling ages have been combined with existing stratigraphic data to constrain the evolution of the shimanto belt a cretaceous neogene accretionary complex cleavage ages record two major phases of accretion during the late cretaceous and late eocene early oligocene the minimum time periods between sedimentation and deformation are constrained to 9 5 9 and 0ma intervals within four tectonic units containing youngest sediments of maastrichtian late eocene late eocene and early oligocene ages respectively one phase of middle to late miocene uplift is indicated for the whole study area by the fission track cooling ages frontal underplating combined with out of sequence thrusting are suggested as mechanisms for this regional uplift the cretaceous neogene shimanto belt fig la forms the outermost onshore exposure of accreted material in sw japan the boundaries of constituent accreted units of the shimanto belt are defined by a detailed stratigraphy taira et al 1980taira et al 1988 providing an opportunity to test the application of cleavage dating and fission track studies to accretionary complex evolution the butsuzo tectonic line fig la juxtaposes the shimanto belt to the jurassic sanbosan belt in the north younger accreted material extends offshore to the south as far as the present day boundary of the eurasian plate the nankai trough fig la continued accretion seaward of the shimanto belt has back rotated older accreted units to their present day steeply dipping orientations the type section of the shimanto belt taira et al 1980 lies in the eastern hata peninsula fig 1b which contains the study

high paleogeothermal gradient with ridge subduction beneath the cretaceous shimanto accretionary prism southwest japan paleotemperature in the jurassic chichibu and the cretaceous shimanto accretionary complexes southwest japan increases southward generally without regard to the early phase geologic structure there is an exceptional peak in the okitsu melange and the thermal structure probably formed after accretion of the complex the estimated geothermal gradient is more than 90 c km in the okitsu melange and more than 50 c km elsewhere in the complex the ophiolitic okitsu melange formed concurrently with eocene subduction of the kula pacific ridge under the cretaceous accretionary prism the previous thermal structure was overprinted by this event

bottom currents and fluviomarine sedimentation on the mississippi prodelta shelf february may 1984 surface winds and currents temperature and water transparency at heights of 1 and 5 m above bottom in 60 m of water were measured during the period from february 23 to may 2 1984 at a site on the mississippi delta the objective was to better understand the influence of near bottom flow on the character of sedimentation on the prodelta shelf data analysis showed currents to be parabathic with periodicities of 5 and 20 days flow showed a strong east west polarization with net flow 0 4 cm s onshore at the lower level and 0 5 cm s offshore at the upper with easterly flow water temperature declined indicating upwelling and current veering between 1 and 5 m was 12 15 in the sense of a bottom ekman layer with westerly flow water temperature increased indicating downwelling and veering was of the same magnitude but in the sense of a surface ekman layer flow and suspended sediment data suggest that during the period of study deposition of sediment from the turbid plume of the mississippi river was continuous calculations of sediment settling velocities indicate that the surface plume of turbid river water transported in suspension particles with hydraulic equivalent diameters ranging from clay to fine sand size computations of friction velocity indicate that here on the outer prodelta shelf the observed bottom currents were not competent to erode bottom sediment short period 1 3 days fluctuations in mean bottom turbidity may be due to variations in the supply of suspended sediment as the surface plume moves laterally in response to the wind and in at least some cases to a vertical redistribution of sediment in suspension by currents too weak to initiate resuspension these fluctuations are probably responsible for the silty and sand laminae generally a millimeter thick or less observed in bottom cores the thickness of the annual flood deposit may approach 1 m in the vicinity of the plume

multiple oscillatory modes of the argentine basin part i statistical analysis observations of the sea surface height in the argentine basin indicate that strong variability occurs on a time scale of 2030 days the aim of this study is to determine the physical processes responsible for this variability first results are presented from two statistical techniques applied to a decade of altimetric data a complex empirical orthogonal function ceof analysis identifies the recently discovered dipole mode as the dominant mode of variability a principal oscillation pattern pop analysis confirms the existence of this mode which has a period of 25 days the second ceof displays a propagating pattern in the northern argentine basin plus a rotating dipole in the southwest corner the pop analysis identifies both patterns as individual modes with periods of 30 and 20 days respectively second the barotropic normal modes of the argentine basin are studied using a shallow water model capturing the full bathymetry of the basin coherences between the spatial patterns of these modes and altimeter data suggest that several of the basin modes are involved in the observed variability this analysis implies that the 20 day mode detected by recent bottom pressure measurements is a true barotropic mode however the 25 day variability as found in altimeter data cannot be directly attributed to the excitation of a free rossby basin mode this study indicates that the results of several apparently conflicting observations of the flow variability in the argentine basin can be reconciled by assuming that multiple basin modes are involved