landward dipping reflectors in accretionary wedges active dewatering conduits landward dipping seismic reflectors ldrs within the accretionary wedge at convergent plate margins have generally been interpreted as tilted bedding or thrust fault surfaces e g mexico northern japan this interpretation accords with models in which trench axis sediments are scraped off the descending plate to form a series of imbricate thrust sheets however intensely disturbed melanges with a regionally extensive landward dipping foliation are found beneath slope sediments in many ancient and active subduction complexes this observation combined with the fact that large volumes of fluid are released during dewatering of subducting and previously accreted sediments leads to the proposal that some of the seismic reflectors are porous laterally extensive fractureways filled with dewatering fluids derived from deeper underthrust water rich sediments these fractureways trend subparallel to the regional foliation of the accreted melange wedge the contrast in acoustic impedance between the high porosity fluid filled fractureways and the melange wall rocks is enhanced whenever methane and or carbon dioxide exsolve from rising waters ancient ldrs may be recognized where mineralized veins trend subparallel to the regional foliation of melange belts generated during subduction accretion 42 references 2 figures

deep seismic structure of the kaapvaal craton the kaapvaal craton is remarkable for the intensive jurassic cretaceous kimberlite magmatism there are indications that the hot spot heating of the upper mantle in southern africa continued in the cenozoic however surface wave data in spite of their sensitivity to temperature failed to provide any evidence of thermal agitation at depths less than 350 km beneath the kaapvaal craton bloch et al 1969 we investigated the s velocity structure in the depth range of the mantle transition zone with the aid of teleseismic p to s converted phases our analysis suggests that a layer of anomalously low s velocity is present in the 370 470 km depth interval beneath the kaapvaal craton the anomalous layer is most likely of thermal origin its upper boundary may separate the deep mantle root of the craton from the underlying mantle the anomaly could arise beneath the craton already in the mesozoic and melting within this layer could contribute to the kimberlite magmatism

sediment dewatering in the makran accretionary prism abstract sediments within the makran accretionary prism are considerably more compacted than the material entering the subduction zone due to the additional tectonic stresses resulting from plate convergence and accretionary processes tectonic compaction of the offscraped sediments has been documented by deriving in situ porosities in the upper 4 km from their seismic velocities wide angle seismic reflection refraction profiles were analysed from a transect crossing both the makran accretionary prism and the undeformed abyssal plain sediments to the south the 6 7 km thick abyssal plain sediments exhibit normal compaction with depth the sediments incorporated into the accretionary prism have higher seismic velocities at all depths corresponding to reductions in porosity of 30 40 mapping the porosity depth gradient across the prism shows that there is a marked change in porosity at the seaward toe of the prism the frontal fold there is then little further change until further folding begins to occur 70 km landward of the toe the dewatering and consolidation of the sediments is an important control on deformation occurring within the sedimentary prism

estimates of heat flow derived from gas hydrates a gas hydrate phase boundary in marine sediments is often observed as an anomalous reflector in seismic profiling data from the depth of the reflectors heat flow values can be estimated using the phase relation of the gas hydrate system and inferred density and thermal conductivity of sediments this method is applied to the data in the nankai trough around central america and along the blake outer ridge the estimated values consistent with those measured by conventional means suggest that derivation of heat flow by this method is feasible on the landward slope of the nankai trough we find that heat flow increases downslope toward the floor of the trough

seismotectonic and seismicity of makran a bimodal subduction zone se iran abstract the present study combined spatial distribution and mechanism of seismicity seismicity parameters geomorphology in satellite imagery and tectonic features in makran inland to show that the makran low angle thrust fault is divided into two separate segments depth dispersal of earthquakes especially relocated events shows that the sinking plate is steeper in the west than in the east of makran significant facts were identified from the variation of moho depth between west and east slip rates of the inland faults and crustal shortenings in the north of makran the sistan suture zone plays a significant role in dividing the overriding plate into a non deforming helmand block and a less deforming lut block these blocks have different resistances against the northward subduction large and great events such as the 1989 chabahar mw 5 5 and 1945 gwadur mw 8 1 occurred in the south of subduction fronts of the western and eastern makran respectively they occurred in shallow depth and were the pieces of evidence of weak and strong coupling between the oceanic and continental crusts on a low angle and a very low angle reverse faults respectively spectral analysis and depth determination of the instrumental and pre instrumental earthquakes respectively helped to separate some plate boundary and intraplate events this separation geometry and non characteristic behavior of the subduction zone revealed from spatial and temporal distribution of the earthquakes probably dedicate a mature aleutian type for the subduction zone in the east missing great m 6 1 earthquakes less slip rate of the continental faults and steeper dipping of the sinking plate in the west probably indicate a kurile type for the iranian part of subduction makran is a bimodal subduction because geomorphological and seismic pieces of evidence indicate a nnw surficial and right lateral fracture zone in the continental crust and a nw deep and left lateral fracture zone in the oceanic crust respectively the fractures and geometry of subductions showed that the eastern makran has seismotectonically a wider area for potential coupling of a larger earthquake than the western makran

the mechanics of frontal imbrication a first order analysis the detachment and imbrication of thrust slices at the front of a thrust wedge is one of the principle modes by which such wedges grow collapse of the frontal ramp under longitudinal compressional stress cannot explain the regular formation of new slices of finite length unless there are regularly spaced heterogeneities in the footwall layer or the underlying basement surface advance of the thrust wedge over the frontal ramp however increases both the vertical load on the ramp and the traction on the upper flat this will in general produce a peak deviatoric stress in the footwall layer below the leading edge of the thrust wedge failure will occur at this point when the thrust wedge has advanced a distance l such that the deviatoric stress in the footwall layer exceeds its strength l is a function of a rock density b ramp angle c the resistances to motion on the basal detachment the ramp and the upper flat and d the strength and thickness of the footwall layer these mechanical parameters can therefore control the formation of new thrust slices of regular length in the absence of footwall heterogeneities

heat flow and age of the gulf of oman we present the results of twenty heat flow stations in the gulf of oman which are used to infer the first reliable age estimates for the basin a mean surface heat flux of 42 6 3 6 mw m 2 exhibits no significant regional variation after correction for thick and rapidly deposited sediments this yields an age of 70 to 100 ma according to oceanic thermal models this age is also consistent with the sediment corrected basement depths of 5 5 6 0 km and with formation during the cretaceous quiet zone the latter can explain the absence of magnetic sea floor spreading lineations heat flow measurements are also used to confirm the presence of gas hyrdate layers the measured thermal gradient yields a depth for the solid to free gas phase transition which is the same as that deduced from bright spots seen on seismic reflection profiles

methane derived authigenic carbonates formed by subduction induced pore water expulsion along the oregon washington margin authigenic magnesian calcite dolomite and aragonite are precipitated in the uppermost terrigenous sediments of the washington oregon accretionary prism by subduction induced dewatering these distinctive carbonates are methane derived and occur at sites of concentrated pore water expulsion unique biologic communities that subsist at least indirectly on methane suess et al 1985 are also found at some of these sites the methane which is dominantly biogenic is carried to the uppermost sediments of the prism by fluids and is oxidized by sulfate reducers before being incorporated into a carbonate cement carbonate precipitation occurs below the oxic layer probably no deeper than several centimetres to a few metres below the seabed cementation may be induced by three factors 1 increased carbonate alkalinity resulting from microbial sulfate reduction 2 decreased co 2 solubility resulting from a pressure decrease when the pore water escapes the prism and or 3 the addition of ca 2 and mg 2 ions from sea water near the sediment water interface the convergent margin setting engenders precipitation of authigenic carbonates in several ways compressive stresses induce anomalously rapid compaction and dewatering rates and they may cause overpressuring in migrating pore water thereby delaying precipitation of carbonates until pressure is released near the sediment water interface structural deformation of the accretionary prism creates pathways such as fault zones secondary fracture porosity and dipping permeable layers often exposed by mass movement for efficient advection and expulsion of methane enriched pore water these characteristic conditions which lead to the precipitation of methane derived carbonates may be found at other convergent margins

seismic crustal structure of the limpopo mobile belt zimbabwe abstract a 145 km n s seismic traverse was deployed to determine the crustal structure of the limpopo mobile belt in southern zimbabwe and the nature of its northern boundary with the zimbabwean craton rockbursts from south african gold mines to the south and regional seismicity from the kariba south zambia belt to the north were used as seismic sources p wave relative teleseismic residuals were also measured to assess whether any velocity contrast between the craton and the mobile belt extended into the upper mantle interpretation of reduced travel times from the local buchwa iron ore mine blasts which were broadside to the traverse revealed an upper crustal interface in the limpopo mobile belt at a depth of 5 8 0 6 km dividing material with a velocity of about 5 8 km s from that of about 6 4 km s on the craton arrivals from the same source showed a 4 4 0 5 km thick 5 5 km s layer overlying crust of about velocity 6 5 km s p wave arrivals from the regional seismicity were used to construct a crustal cross section absolute crustal thickness was tentatively estimated from the identification of a moho reflection on the mine blast recordings to the south of rutenga the crust thins from around 34 km to 29 km in association with a positive gravity anomaly centred over the late karoo nuanetsi igneous province and karoo tuli syncline north of rutenga to the boundary with the zimbabwean craton the crust is about 34 km thick the craton boundary was found to be a steeply southerly dipping zone associated with high velocity material which could either be deep seated greenstones or mafic material associated with the margin in the region studied this zone divides cratonic crust which was found to be about 40 km thick from that typical of the mobile belt and implies a step in the moho of around 6 km analysis of relative teleseismic residuals showed that the velocity contrasts are not confined to the crust but extend into the uppermost upper mantle with the cratonic lithosphere being about 4 faster than that of the limpopo mobile belt the resolution of the technique is such that it is difficult to ascertain whether these differences are features of precambrian evolution or are due to reactivation of the upper mantle during karoo igneous and tectonic activity

effect of thermal non equilibrium seafloor topography and fluid advection on bsr derived geothermal gradient abstract the seafloor and bottom simulating reflectors bsrs are interpreted from the 3d seismic data acquired in krishna godavari kg offshore basin in the vicinity of sites drilled cored during national gas hydrate program nghp expedition 01 the shallow structures such as inner toe thrust fault system regional and local linear fault systems and mass transport deposits are inferred from attributes of seafloor time structure as well as from the seismic profiles the geothermal gradient is estimated from the depths and temperatures of the seafloor and the bsr the temperature at the bsr depth is estimated from the methane hydrate and seawater salinity phase boundary assuming that the bsr represents the base of the gas hydrate stability zone the spatial variations in geothermal gradient gtg show a strong correlation with seafloor topography in the kg basin the gtg decreases by 13 30 over the topographic mounds formed due to inner toe thrust faults and recent mass transport deposits the gtg decreases by only 5 10 over the mounds likely due to defocusing of heat flux based on one dimensional topographic modeling hence the gtg perturbation due to topography alone cannot explain the observed gtg anomaly the temperature profile beneath these mounds may not be in equilibrium with the surroundings either due to the recent upliftment of sediments along the inner toe thrust faults or rapid deposition of sediments due to slumping sliding in contrast an increase in gtg by 10 15 is observed in the vicinity of major fault systems we presume that the likely mechanism for the increase in gtg is fluid advection from a deeper part of the basin a detailed thermal modeling involving the effect of surface topography high sedimentation rates fluid advection and sediment thickening due to tectonics is required to understand the thermal profile in kg offshore basin

receiver function structure beneath three southern africa seismic broadband stations abstract the shear wave velocity structure beneath three southern african stations lusaka lsz lobatse lbtb and boshof bosa were estimated using the time domain inversion of stacked teleseismic receiver functions broadband teleseismic 3 component waveform data were used in a source equalisation procedure to estimate radial and transverse receiver functions for each station the radial receiver functions were stacked according to the following criteria an azimuthal interval of 15 similar ray parameter and shape for the shield based stations bosa and lbtb simpler receiver functions than for lsz which is located in the irumide belt were obtained the lateral variation in receiver functions with azimuth as observed at each station stems from lateral heterogeneities beneath each station the velocity models were presented as p wave velocity models from these models the crust mantle transition zones beneath lsz and bosa were determined at depths of about 37 44 km and 34 38 km respectively for lbtb the northeast quadrant velocity model displayed a clear mantle continent transition zone in the depth range 40 45 km whilst the moho depth in the southern quadrants is not as clear beneath all the stations we observed a low velocity zone which appears to correlate with the cratonic velocity structure this feature is consistent with crustal structure results obtained in other cratonic or shield based crustal studies the results in this study contribute crustal structure information which has been lacking at bosa lbtb and lsz

deep seismic profiling in the nosop basin botswana cratons mobile belts and sedimentary basins abstract deep seismic reflection data from the kalahari of western botswana has been interpreted in terms of the tectonics of southern africa the 600 km of 12 fold data exhibit good quality reflections in the range 5 15 s twt the data outline the nosop basin a depositional centre with greater than 10 km of palaeozoic and younger sediments five prominent reflecting horizons are traced in the basement underlying the sediments on the basis of the seismic data the nosop is now interpreted as a single basin the previous multi basin interpretation of the aeromagnetic data esults from features within the magnetic basement the uppermost two basement units are interpreted as proterozoic sediments that rest on archean crust the seismic data show that the crust appears to thicken to the southeast as the kaapvaal craton is approached at the same time the sedimentary cover progressively thins suggesting that the western edge of the kaapvaal is an extended margin interpreted deep crustal shear zones indicate a major terrane boundary separating the proterozoic from the archean depths to basement inferred from aeromagnetic data are consistent with the depth to the seismic boundary associated with the top of the archean some of the aeromagnetic expression may be due to karoo volcanic intrusives in the basement a thrust belt in the northwest of the area now buried in the mid crust is correlated with the nearby ghanzi chobe belt and allows the mid crustal layering to be dated as late proterozoic

a seismic refraction investigation of the archaean kaapvaal craton south africa using mine tremors as the energy source summary the structure of the central kaapvaal craton of southern africa has been investigated by deep seismic sounding using mine tremors as energy sources seismometers were deployed at approximately 10 km intervals on two profiles stretching between major mine tremor source regions mine tremors are rich in shear energy enabling joint interpretation of p and s waves and produce substantial energy at frequencies as low as 1 hz record sections are presented for both p and s waves and the traveltimes and amplitudes interpreted using 2 d ray tracing techniques synthetic seismograms computed for a 1 d velocity model by the reflectivity method compare well with the observed data a generalized seismic model of the kaapvaal craton has the following features supracrustal strata 0 10 km thick upper crystalline basement with p wave velocities of 6 0 6 2 km s the boundary between upper and lower crust at a depth of 14 18 km a lower crust with a relatively uniform seismic velocity in the range 6 4 6 7 kms and the crust mantle transition over 1 3 km with the moho at a depth of about 35 km the lower crust is found to be seismically attenuating and has a poisson s ratio of about 0 28 it is also known to be electrically conductive these observations are in accord with the presence of hydrated mantle rock at the base of the crust the velocity depth model of the kaapvaal craton is similar to models derived for other archaean cratons the proterozoic provinces adjacent to the kaapvaal craton are significantly thicker and have an intermediate to high velocity layer developed at the base of the crust this is interpreted to indicate a change in the process of crustal growth with basaltic underplating becoming more important since the archaean this change is attributed to a change in the composition of the upper mantle the higher temperatures in the archaean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted mantle unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation

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

geographical distribution of surface wave velocities and 3 d upper mantle structure in africa summary long period records of surface waves from wwssn gdsn ida and geoscope stations are analysed to retrieve purely continental dispersion of the african continent in the period range of 20 200 s for rayleigh waves and 20 125 s for love waves the phase and group velocities are obtained by two station and single station methods with earthquakes within or near the continent a regionalization without any a priori constraints is performed over the whole african continent to retrieve local phase and group velocities phase and group velocity geographical distributions show in the northeast of africa near the red sea a region with lower velocities extending to the west and high velocities in the northwestern and southern parts of africa at long periods there is a general agreement with global models but the resolution is improved in this regional study in the resulting 3 d structure cratonic areas show a fast lithosphere from the nw to the se whereas the high sv velocities of the nw area extend deeper than 200 km lower velocities appear below this depth in the centre in the red sea region low velocities are locally deep rooted with a slow lithospheric layer 150 km thick extending to the west in this region polarization anisotropy is evidenced by love rayleigh incompatibility short period lateral variations are used to model the crustal thickness

the possible role of fluids for the structuring of the continental crust abstract some well known seismic reflectivity patterns of continental crusts characterize specific tectonic terranes the role of fluids in enhancing or destroying such structures is studied seismicity is strongly dependent on the different pore pressure of fluids in intraplate settings in strike slip or extensional areas hydrostatic pore pressure seems to be approached in accordance with standard rheological models and standard seismicity depth distributions in suture zones or zones of developing loads possibly also along zones of near horizontal normal faults extremely high pore pressure develops which approaches lithostatic values they are accompanied by a number of weakening processes with enhanced shear and reduced viscosity these values correlate with low seismic velocities v p in accordance with an empirical v p relationship

porosity of sediments in accretionary prisms and some implications for dewatering processes rapid dewatering largely through loss of porosity is one of the most striking changes affecting deformed sediments in accretionary prisms analysis of porosity from sample and seismic data confirms that porosity in accretionary prisms is significantly less than that in basinal strata of similar lithology more specifically the porosity depth gradient increases arcward across the prism until porosities less than 10 occur within a few kilometers of the surface in deeper parts of the prism dewatering and volume reduction occur not only during loss of porosity but also during dehydration and metamorphic reactions calculations involving volume change lead us to define a function which reflects total volume reduction the distribution of the relative intensity of dewatering within the prism can be calculated from the porosity contours and trajectories of sediment elements an initial attempt indicates that in the toe region the rate of dewatering is accelerated at depth relative to the basinal section and may be greatest near the base of the prism toward the rear of the prism the maximum rate of dewatering occurs at progressively shallower depths several lines of evidence suggest that in addition to gradual diffuse dewatering localized zones of strong dewatering are associated with zones of intense shear this model of shear dewatering based on critical state soil mechanics suggests that sediments adjacent to the basal decollement should be strongly dewatered

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

crustal structure of the arabian plate new constraints from the analysis of teleseismic receiver functions an edited version of this paper was published by elsevier science copyright 2005 elsevier science see also http dx doi org 10 1016 j epsl 2004 12 020 http atlas geo cornell edu saudiarabia publications al damegh 202005 htm

organic carbon cycling and modern phosphorite formation on the east australian continental margin an overview during 1987 the australian bureau of mineral resources conducted a multidisciplinary investigation of the modern phosphorites on the continental margin of southeastern australia between 28 and 32 s the objectives of the work were to examine the processes controlling the cycling of organic carbon and bioactive elements nitrogen phosphorus sulphur and iron in the sediments and to investigate the roles which these processes played in the formation of the modern phosphorites bacterial productivities sulphate reduction rates sedimentary oxygen and pore water concentrations of nitrate ammonia phosphate iron sulphate and fluoride were measured at sea the highest rates of microbial productivity were found in the surficial 0 20 mm sediments of the modern phosphorite zone in 350 460 m water depth these rates were about double those in shallower shelf 300 m sediments and 3 4 fold those rates in mid slope 600 1000 m sediments aerobic and anaerobic oxidation rates of organic matter calculated from sediment oxygen profiles and sulphate reduction rates were highest in the surface sediments in the modern phosphorite zone the recycling of sedimentary iron via reductive dissolution of iron oxyhydroxides and reprecipitation at the oxic anoxic boundary results in a near surface sedimentary trap for iron in the phosphorite zone sediments phosphate released from organic matter in the interfacial sediments and fluoride from seawater are scavenged by iron oxyhydroxides in the top few centimetres of sediment phosphorus in this way is decoupled from organic carbon in the near surface sediments and linked to the redox cycling of iron phosphate and fluoride scavenged onto iron oxyhydroxides and concentrated in the surficial sediments are subsequently released to pore waters in the anoxic sediments when iron oxyhydroxides are buried and dissolve the recycling process releases phosphate and fluoride for incorporation into apatite fluoride is depleted from pore waters at depths 18 cm phosphorite nodules form within anoxic sediments at depths 18 cm and continue to accumulate iron and phosphorus while resident in the mixed layer combinations of rapid sediment mixing rates a slow sedimentation rate and a mixed layer to about 18 cm result in an average particle residence time in the phosphorite zone sediments which is about ten fold that of the mid slope sediments long residence times and rapid mixing promote the oxidation of organic carbon and release of phosphate while the continuous recycling of iron and phosphate concentrates the phosphorus for apatite precipitation and accumulation into phosphorite nodules phosphorite nodules are not found in mid slope sediments probably because of combinations of relatively rapid sedimentation rates ineffective iron phosphorus and fluoride recycling and trapping mechanisms plus dilution and dissemination of any incipient apatite

gas hydrates in ocean bottom sediments gas hydrates belong to a special category of chemical substances known as inclusion compounds an inclusion compound is a physical combination of molecules in which one component becomes trapped inside the other in gas hydrates gas molecules are physically trapped inside an expanded lattice of water molecules the pressures and temperatures beneath arctic water depths greater than 1 100 ft 335 m and subtropical water depths greater than 2 000 ft 610 m are suitable for the formation of methane hydrate theoretical depths to the base of a gas hydrate layer in ocean bottom sediments are determined by assuming 1 a constant hydrostatic pressure gradient 2 two typical hydrothermal gradients 3 variable geothermal gradients and 4 pure methane hydrated with conna e seawater in addition to pressure and geothermal gradient other variables affecting the stability of gas hydrate are examined these variables are hydrothermal gradient sediment thermal conductivity heat flow hydrate velocity gas composition and connate water salinity if these variables are constant in a lateral direction and the above assumptions are valid a local geothermal gradient can be determined if the depth to the base of a gas hydrate is known the base of the gas hydrate layer is seen on seismic profiles as an anomalous reflection nearly parallel to the ocean bottom cross cutting geologic bedding plane reflections and generally increasing in sub ocean bottom time with increasing water depth the acoustic impedance is a result of the relatively fast velocity hydrate layer overlying slower velocity sediments in addition free gas may be trapped beneath the hydrate thereby enhancing the reflection

seismic velocity studies of a gas hydrate bottom simulating reflector on the northern cascadia continental margin amplitude modeling and full waveform inversion on the northern cascadia subduction margin the multichannel seismic amplitude versus offset avo behavior of a bottom simulating reflector bsr suggests a p wave velocity change from high velocity hydrate bearing sediment to lower velocity sediment containing a small amount of free gas the observed nonlinear avo behavior constant or slightly decreasing amplitudes at near to mid offsets and a large amplitude increase at far offsets can be reproduced in the models if an s wave velocity enhancement is assumed as expected from hydrate cementation the avo behavior of the hydrate bsr is found not to be as useful as was earlier thought for determining the amount of free gas below the bsr this is because poisson s ratio change below the bsr due to gas is likely very small in high porosity unconsolidated sediments the uncertainty in the models is large as there is no reliable s wave velocity information for the sediments containing hydrate or gas avo modeling alone is not sufficient to distinguish different velocity models across the bsr our interpretation of the bsr amplitude behavior is that a p wave velocity increase above the bsr is the main cause of the bsr reflection amplitude increase at large incidence angles caution must be taken in applying avo analysis as little is known about s wave velocities however subtle differences in bsr amplitude behavior and reflection waveform can provide constraints through very careful full waveform inversion a well defined reference velocity depth profile is also required to represent water saturated sediment unaffected by either hydrate or free gas using full waveform velocity inversion a high resolution velocity model for the hydrate bsr has been derived the best fit model for the seismic data near the ocean drilling program odp sites 889 890 consists of a high velocity zone above the bsr and a thin low velocity layer below in agreement with the odp downhole velocity data

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

structure of a growing accretionary prism hikurangi margin new zealand the hikurangi margin of eastern north island new zealand represents the feather edge of the indian plate at its convergent boundary with the subducting pacific plate a migrated seismic reflection profile across this margin clearly displays the structural evolution of an accretionary prism a 25 km wide band of protothrusts is delineated between the toe of the slope and a converging seamount this illustrates an early stage in the seaward propagation of a deformation front landward tilted trench slope basins are separated by ridges that have clearly defined thrusts which appear to sole out at a decollement the decollement continues at an angle of only 3 beneath the 150 km wide margin to a depth of 14 km near the coast where it coincides with an onshore zone of high seismicity

weighted stacking for rock property estimation and detection of gas amplitude versus offset concepts can be used to generate weighted stacking schemes here called geo stack which can be used in an otherwise standard seismic data processing sequence to display information about rock properties the zoeppritz equations can be simplified and several different approximations appear in the literature they describe the variation of p wave reflection coefficients with the angle of incidence of a p wave as a function of the p wave velocities the s wave velocities and the densities above and below an interface using a smooth representative interval velocity model from boreholes or velocity analyses and assuming no dip the angle of incidence can be found as a function of time and offset by iterative ray tracing in particular the angle of incidence can be computed for each sample in a normal moveout corrected cmp gather the approximated zoeppritz equation can then be fitted to the amplitudes of all the traces at each time sample of the gather and certain rock properties can be estimated the estimation of the rock properties is achieved by the application of time and offset variant weights to the data samples before stacking the properties which can be displayed by geo stack are p wave reflectivity or true zero offset reflectivity s wave reflectivity and the reflectivity of p wave velocity divided by s wave velocity or pseudo poisson s ratio reflectivity if assumptions are made about the relation between p wave velocity and s wave velocity for water bearing clastic silicate rocks then it is possible to create a display which highlights the presence of gas

estimating the time of pockmark formation in the sw xisha uplift south china sea using reaction transport modeling carbon cycling and fluid seepage in marine sediments over the late quaternary were investigated at a now extinct pockmark located in a mega pockmark field in the sw xisha uplift nw south china sea measured particulate organic carbon poc content and porewater sulfate so4 2 dissolved inorganic carbon dic concentrations and 34s so4 2 distributions were used to constrain a non steady state reaction transport model and quantify poc mineralization rates as well as estimate the time when fluid flow ceased at the investigated pockmark an increase in poc content and 34s so4 2 and a decrease in sulfate concentrations in the upper ca 2m at the pockmark and a reference core implied an increase in the flux and reactivity of organic matter during the early holocene around 10kyrb p caused by enhanced primary productivity during the strengthened southwestern summer monsoon these features were simulated with the model assuming a holocene increase in poc flux and reactivity subsequently starting from an initial condition reminiscent of a modern active cold seep hydrate ridge hindcast simulations showed that fluid seepage at the pockmark ceased ca 39kyr ago this corresponds to a relative sea level high stand which is believed to be associated with gas hydrate stabilization the non steady state model presented in this contribution can also be used to constrain the time when fluid seepage ceased at other presently extinct cold seeps when suitable sediment and porewater data are available

seismic velocity structure at a gas hydrate reflector offshore western colombia from full waveform inversion seismic reflection profiles across many continental margins have imaged bottom simulating reflectors bsrs which have been interpreted as being formed at the base of a methane hydrate stability field such reflectors might arise either from an impedance contrast between high velocity partially hydrated sediments and water saturated sediments or from a contrast with partially gas saturated sediments these alternatives may be hard to distinguish by conventional amplitude versus offset or waveform modeling approaches here we investigate the origin of a high amplitude bsr in the accretionary wedge offshore of western colombia by seismic waveform inversion the inversion procedure consists of three steps firstly determination of root mean square velocities and hence estimates of the interval velocities between major reflectors by a global grid search for maximum normalized energy along elliptical trajectories in the intercept time slowness domain secondly determination of accurate interval velocities between these reflectors by a monte carlo search for maximum energy and thirdly a waveform fit in the frequency slowness domain using differential reflectivity seismograms and a conjugate gradient optimization algorithm to minimize the sample by sample waveform misfit between data and synthetic at two locations near a structural high we find a 30 m thick low velocity zone beneath the bsr with the properties of a partially gas saturated zone while at a third location where the bsr amplitude is lower we find no evidence for anomalously low velocities the preferential development of the bsr in structures that would tend to intercept fluid flow or migrating gas and the presence of free gas beneath the bsr indicate a mechanism of bsr formation in which free methane gas migrates upward into the hydrate stability field or is carried there in advecting pore water

constraints on plate motions in southern pakistan and the northern arabian sea from the focal mechanisms of small earthquakes the focal mechanism and depth were determined for nine small earthquakes m0 1025 dyn cm m 5 5 that occurred in southern pakistan and the northern arabian sea from an analysis of the vertical component of rayleigh waves in combination with limited first motion data focal parameters were determined from the rayleigh waves by using an event pair method of analysis for earthquakes that are located very close to each other 50 km the event pair method is able to remove a significant proportion of propagation effects at all periods in the range of interest 20 50 s for events separated by more than 100 km the propagation effects are reduced for only the longer periods 40 50 s the earthquakes that were studied provide evidence for a model of plate interactions in the vicinity of the southern pakistan triple junction the owen fracture zone is a transform fault that accommodates right lateral motion between the indian and arabian plates the plate boundary in the vicinity of the murray ridge is also partially made up of transform segments that strike subparallel to the owen fracture zone spreading centers may also exist in the vicinity of the murray ridge but were not documented by seismic or other evidence the slip azimuths for earthquakes along this boundary are significantly more northerly than those predicted by various regional and worldwide models of plate motion the arabian plate is being subducted beneath the eurasian plate along the southern coast of pakistan slip vectors for earthquakes along this boundary trend northnortheasterly in general agreement with predicted directions left lateral motion is documented along the boundary between the indian and eurasian plates in southern pakistan the predicted direction of relative motion between these plates is not significantly different from that observed two of the earthquakes studied appear to be intraplate in nature the depth and focal mechanism of one intraplate event which may have occurred within the accretionary prism along the makran coast however are based on limited data the other intraplate earthquake occurred within the sedimentary deposits along the western edge of the indian subcontinent near the indian eurasian plate boundary appendix is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b83 009 2 50 payment must accompany order

tectonically driven fluid flow in the kodiak accretionary complex alaska sandstone and chert boudins that have syntectonic extensional veins are dominant structural elements in melanges of the kodiak accretionary complex alaska detailed analyses of cogenetic methane rich and water rich fluid inclusions in syntectonic quartz veins indicate that fluid pressures dropped by 20 to 45 during crystal growth this drop occurred faster than uplift of the rock and probably represents the local effect of fluid escaping to shallow levels in the subduction complex during melange deformation possibly along an interconnected fracture network an important implication of channelized fracture flow to higher levels in a subduction complex is that fluids may transport heat and solutes thereby generating thermal and chemical anomalies along shallow faults

average lithospheric structure of southern africa summary we present a new 1 d earth model for the crust and upper mantle beneath the archaean core of southern africa the crustal structure is constrained by published seismic refraction reflection data and by the modelling of teleseismic receiver function data the mantle structure is constrained by traveltime measurements for p and s waves by waveform inversion of multimode surface waves and by forward modelling of p waveforms for regional earthquakes the 42 km thick crust of southern africa consists of four layers a 2 km thick 5 05 km s surface layer a 5 km thick 6 08 km s upper crust a 20 km thick 6 30 km s l mid crust and a 15 km thick 6 73 km s lower crust below the crust there is an 80 km thick upper mantle lid v and v are 8 09 and 4 62 km s l respectively at the top of the lid and the compressional and shearvelocity gradients through the lid are 0 0008 s l and 0 0013 s respectively below the lid there is a substantial shear wave lvz with the shear wave velocity dropping from 4 72 km s l in the lower part of the lid to 4 32 km s at 250 km depth the shear wave lvz is required to fit both the v measurements and the regional surface waveforms forward modelling of p suggests that no upper mantle p wave lvz exists there is an increase in the p wave velocity gradient at 125 km depth to 0 0015 s and a second increase at 250 km depth to 0 0035 s velocities and densities in the seismic model of the lid agree closely with velocity and density estimates from the geochemical analysis of garnet periodotite nodules from kimberlites of the kaapvaal craton implying that the estimates from kirnberlites are representative of a wider region of the archaean core of southern africa not just of the restricted region beneath which the kimberlites are found a comparison of the southern african seismic model with seismic models for other shield regions where nodule data are not available shows that the nodule results may also be representative of the upper mantle in those regions

the use of velocity spectrum for stacking receiver functions and imaging upper mantle discontinuities summary in order to improve the signal to noise ratio of receiver function data it is typical to stack receiver functions calculated from events at similar distances and back azimuths we have adapted the velocity spectrum stacking vss technique used extensively in reflection seismology to the receiver function method in order to stack data with different ray parameters thereby improving further the signal to noise ratio perhaps more importantly by producing the velocity spectrum stacks we take advantage of the differences in the shapes of the moveout curves of converted phases and reverberations to identify and separate the various phases and to infer velocity structure by comparison of velocity spectrum stacks produced from the observed data at the iris ida broad band station at obninsk russia obn with those produced from prem synthetics we have identified ps phases from the 400 and 670 km discontinuities the p to s ps converted phase from the 400 km discontinuity observed at obn is much larger compared with that of the 670 km discontinuity than is predicted by prem this suggests a higher velocity contrast at 400 km than in prem by adapting a bootstrap method to assess the depth and velocity estimates determined by vss we find that the ps phase from the 670 km discontinuity is best imaged with higher than prem upper mantle p and s velocities and at a depth of 663 3 km we find no evidence of a 220 km discontinuity beneath obn in these data

p sv conversions of teleseismic waves beneath the chyulu hills volcanic field kenya observations of p sv converted waves provide new insights on the structure of the lithosphere beneath the chyulu hills a quaternary volcanic field on the se shoulder of the kenya rift east africa a 3d teleseismic delay time tomography and a seismic refraction model revealed a zone of reduced p wave velocity 3 5 extending from the lower crust into the uppermost mantle limited to about 70 km depth in this paper waveform and particle motion analyses are used to identify p sv conversions in teleseismic recordings and to search for lateral variations of seismic discontinuities data from 9 teleseisms contain clear sv phases within the first 8 seconds of the p coda the depth of the generating interfaces is determined with a local velocity depth function based on the seismic refraction model our results show that there are no lateral depth variations of the conrad 19 21 km and moho 43 45 km discontinuities indicating no large scale modification of the crustal structure through the magmatic processes in the time window between the sv arrivals from the conrad and moho discontinuities additional sv energy is observed at some stations but its origin cannot be located

efficient calculation of differential seismograms for lithospheric receiver functions summary a new technique for computing differential seismograms for crustal and upper mantle response to a teleseismic wave receiver functions is developed using the matrix formalism of kennett the work was motivated by the difficulty of modelling teleseismic sv waves and has also proven useful for modelling teleseismic p waves this efficient method for calculating differential seismograms is based on three separate methods for computing synthetic seismograms two of the synthetic seismogram methods save intermediate results then the remaining synthetic seismogram algorithm uses the stored results in an efficient calculation of a new synthetic seismogram for a perturbed velocity model these developments have led to a faster for a 30 layer model a 90 per cent reduction in computation time and more accurate linearized inversion scheme for the determination of velocity models using teleseismic waves

the receiver structure beneath mina nevada abstract receiver functions obtained from broadband teleseismic p waveforms recorded at seismic station mnv near mina nevada are inverted for the vertical shear wave velocity structure southeast of the station the time domain inversion procedure for vertically heterogeneous and laterally homogeneous one dimensional models includes smoothness constraints to minimize model roughness and a pseudo monte carlo approach to investigate the nonuniqueness of the solution models we also report on the potential effects of scattering on a receiver function inversion for a one dimensional velocity structure mnv receiver functions sample the walker belt crust and upper mantle beneath the soda spring valley and pilot mountains to the east and the garfield hills and excelsior mountains to the west of the station the preferred southeast solution models show a smooth positive gradient in the mid and lower crust with the top of the crust mantle boundary at 34 to 36 km depth reaching an upper mantle p velocity of 7 8 to 7 9 km sec 1 between 38 and 40 km these results suggest that the crust beneath mnv may be representative of a transition zone between typical basin and range province crust to the east and a thicker sierran block to the west of the station

origin of convex accretionary wedges evidence from barbados the surface slopes of submarine accretionary wedges generally decrease away from the toe or defor mation front toward the arc this paper presents evidence that this characteristic convex cross sectional shape is a direct mechanical response to the lithification of sediments during accretion the recently accreted sediments near the toe which are typically unconsolidated and therefore very weak must deform until a critical taper is attained at which they are stable in the presence of the horizontal tectonic compression this critical wedge taper and thus the bathymetric slope of the accretionary prism is relatively high for the porous weak sediments near the toe in contrast the compacted sediments farther back in the wedge are stronger so they require a relatively modest critical taper and therefore a lower bathymetric slope to be stable under the same tectonic compression the result is a convex cross sectional wedge shape we investigate several possible causes of the increase in sediment strength within the wedge and conclude that the primary cause is an increase in cohesive strength with decreasing porosity a straightforward generalization of the exact critical taper model is combined with empirical cohesion porosity velocity relations to infer the distribution of porosity and p wave velocity within the barbados accretionary wedge we estimate that the porosity decreases from its near toe value of 70 to approximately 30 at a depth of 3 km below the seafloor and a distance of 60 km arcward from the front of the wedge

heat flow variations from bottom simulating reflectors on the cascadia margin abstract a bottom simulating reflector bsr representing the base of the gas hydrate stability field was observed over a closely spaced grid of seismic lines on the continental slope of the northern cascadia margin a simple conductive model was used to calculate heat flow from the depth of the bsr a regional trend was observed in which heat flow decreased landward across the margin from an average of 80 mw m2 at a distance of 15 km from the deformation front to 65 mw m2 at a distance of 40 km this trend reflects the processes of tectonic thickening of accretionary wedge sediments and subduction of the juan de fuca plate and contrasts with the near constant heat flow across the oregon margin where the dip of the subducting plate and thus the rate of sediment thickening are slightly smaller consistent local variations were also observed notably low heat flow values over prominent topographic highs and high heat flow values over the flanks of the topographic highs at some localities over a horizontal distance of 1 2 km heat flow increased by as much as 50 from typical values of 65 to 100 mw m2 much of this variation may be due to focusing and defocusing effects of the topography alone and indicates the importance of carrying out topographic corrections to heat flow in regions of significant relief such as the continental slope however a component of the local heat flow patterns may be due to dynamic effects including the displacement of isotherms by thrust faulting and the upward migration of warm fluids along permeability channels such as faults

the geology of the persian gulf gulf of oman region a synthesis during the mesozoic most of the arabian peninsula persian gulf south western iran and eastern iraq constituted the arabian platform deformation of the musandam peninsula in the late cretaceous and mid tertiary by compression subduction from the east and southwest collision of the arabian platform and eurasian plate along the zagros crush zone during the oligocene or early miocene and emplacement of the zagros mountains by gravitational sliding during the neogene and pleistocene have reduced the platform area to the persian gulf other factors that contributed to the reduction of the arabian platform include the uplift of the arabian peninsula during the opening of the red sea in the tertiary tectonism of the infracambrian hormuz salt upwarp of the platform sediment cover by basement uplift and or salt tectonics and a 600 to 400 m drop in sea level since the cretaceous at present tectonism in the region is restricted to the northern edge of the gulf of oman where the arabian plate is subducting the eurasian plate from the south and to the zagros crush zone where the arabian and eurasian plates are colliding with one another

sediment compaction and fluid migration in the makran accretionary prism the makran continental margin in the gulf of oman forms the seaward extremity of an accretionary sediment prism which extends several hundred kilometers inland a recently acquired multichannel seismic reflection profile shot across the margin imaged the structure of the prism in greater detail than was previously possible and allowed us to investigate the relationship between deformation and pore fluid motion in the region velocity analyses of the common midpoint gathers reveal a marked change in velocity structure at the toe of the accretionary wedge as seen in previous sonobuoy wide angle data accreted sediments show significantly higher vertical velocity gradients than those of sediments entering the prism this change is interpreted as due to porosity reduction as pore fluids are squeezed out of the compacting sediment a prominent bottom simulating reflector appears 500 800 m beneath the sea bed several lines of evidence suggest that this reflector represents the base of a gas hydrate zone underlain by widespread free gas which may be exsolved from pore water migrating from deep within the sediment pile up permeable fault planes imaged in the profile the hydrate reflector appears to shallow in the region of some faults suggesting a temperature anomaly due to the presence of warm pore fluids a heat flow profile derived from the depth of the hydrate reflector does not show the expected landward decrease as the sediment pile thickens simple thermal modeling suggests that advective heat flow within the prism may explain this anomaly the inferred presence of overpressured pore fluids in the makran suggests that accreted sediments have a low permeability the seismic evidence suggests a two stage compaction process with rapid initial dewatering through intergranular permeability as sediment enters the prism followed by a buildup of pore pressure as the permeability decreases and fluid migration is restricted to fault zones

waveform modelling of teleseismic s sp sspmp and shear coupled pl waves for crust and upper mantle velocity structure beneath africa summary we describe a waveform modelling technique and demonstrate its application to determine the crust and upper mantle velocity structure beneath africa our technique uses a parallelized reflectivity method to compute synthetic seismograms and fits the observed waveforms by a global optimization technique based on a very fast simulated annealing vfsa we match the s sp sspmp and shear coupled pl phases in seismograms of deep 200 800 km moderate to large magnitude 5 5 7 0 earthquakes recorded teleseismically at permanent broad band seismic stations in africa using our technique we produce p and s wave velocity models of crust and upper mantle beneath africa additionally our use of the shear coupled pl phase wherever observed improves the constraints for lower crust and upper mantle velocity structure beneath the corresponding seismic stations our technique retains the advantages of receiver function methods uses a different part of the seismogram is sensitive to both p and s wave velocities directly and obtains helpful constraints in model parameters in the vicinity of the moho the resulting range of crustal thicknesses beneath africa 21 46 km indicates that the crust is thicker in south africa thinner in east africa and intermediate in north and west africa crustal p 4 7 8 km s 1 and s wave velocities 2 5 4 7 km s 1 obtained in this study show that in some parts of the models these are slower in east africa and faster in north west and south africa anomalous crustal low velocity zones are also observed in the models for seismic stations in the cratonic regions of north west and south africa overall the results of our study are consistent with earlier models and regional tectonics of africa

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

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

receiver structure beneath the southern mojave block california an inversion of a receiver function recorded at lawrence livermore national laboratory nevada test site network station lac reveals a range of 1 d velocity structures that are consistent the observed waveform the shear velocity structure southeast of lac is relatively simple consisting of an approximately 30 to 34 km thick crust all models contain relatively sharp crust mantle boundaries with a change in velocity from about 3 5 km sec to approximately 4 6 km sec over a depth range of 4 to 6 kilometers the range of models can be roughly divided into two groups that differ mostly in the mid to lower crust the first model of the earth structure contains a large low velocity zone in the middle and lower crust shear wave velocities as low as 2 8 km sec the second model contains nearly constant velocities approximately 3 5 km sec through the middle to lower crust in contrast with the simplicity of the southeast receiver function the receiver functions corresponding to the northwest and southwest show the influence of lateral velocity heterogeneity and can at best be discussed qualitatively

receiver function structure beneath four seismic stations in the sumatra region abstract we estimated the velocity structure beneath four three component broad band seismic stations in the sumatra region by the joint inversion of teleseismic receiver functions and surface wave group velocities the stations part of geofon network ge are located in diverse geologic settings including backarc basins forearc basins and the forearc ridge the stations are distributed roughly trench parallel providing samples of conditions along the length of sumatra 143 receiver functions were computed by employing an iterative time domain deconvolution in order to minimize acausal noise the teleseismic observations at each station were grouped by back azimuth and ray parameter and then stacked to increase the signal to noise ratios surface wave group velocity dispersion was measured for paths across the sunda block from events to the northeast of the stations p wave velocity profiles were derived by inverting the stacks and group velocity data using a linearized time domain inversion scheme with an interpolated crust2 0 global model for input the results differ significantly from crust2 0 and reflect the diversity of geologic environments within which the stations are located crustal thicknesses beneath the stations range from 16 0 km at the forearc to 30 0 km at a backarc basin these results add to the available velocity and crustal thickness data for the sumatra region

implications for focused fluid transport at the northern cascadia accretionary prism from a correlation between bsr occurrence a d near sea floor reflectivity anomalies imaged in a multi frequency seismic data set a high resolution seismic survey was carried out at the accretionary prism on the continental slope off vancouver island canada two gi gun data sets with different source frequency ranges of 50 150 and 100 500 hz were combined with 4 khz narrow beam echosounding data parasound the data allow spatial correlation between a gas hydrate bottom simulating reflector bsr and distinct areas of high near sea floor reflectivity an integrated interpretation of the multi frequency data set provides insight into the regional distribution of tectonically induced fluid migration and gas hydrate formation in the vicinity of odp leg 146 sites 889 and 890 the bsr at the base of the gas hydrate stability field is observed within accreted and deformed sediments but appears to be absent within bedded slope basin deposits it is suggested that these basin deposits inhibit vertical fluid flow and prevent the formation of a bsr whereas the hydraulic conductivity of the accreted sediments is sufficiently high to allow for pervasive gas migration an elevation of the bsr beneath the flanks of a topographic high is interpreted as an indicator for local upflow of warm fluids along permeable pathways within outcropping accreted sediments parasound data reveal discontinuous zones of high reflectivity at or directly beneath the sea floor which may indicate local cementation of surface sediments in combination with gi gun data the occurrence of these reflective areas can be related to the location of slope sedimentary basins acting as hydraulic seals it is proposed that the seals sometimes fail along faults extending beneath the bsr leading to focused upflow of methane bearing fluid and the formation of carbonate pavements at the sea floor

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

the effects of sedimentation and compaction on oceanic heat flow summary the estimation of environmental effects forms an important part of the interpretation of oceanic heat flow measurements in particular the perturbations associated with sedimentation and surface temperature changes must be taken into account analytical solutions can be obtained only for individual simplified versions of these problems whereas any real example is complicated by the process of sediment compaction which changes the bulk thermal properties with depth a physical model is developed which uses sediment porosity trends to predict the thermal parameters and material advection rates for an evolving sediment basement system these values are then used in a numerical solution to the heat flow equation to give estimates of the perturbed surface heat flux through time in addition to variations in sedimentation rate sediment type radioactive heat production and surface temperature changes are considered heat flow corrections may vary by up to a factor of 2 according to sediment type while radioactive heat production can offset the effects of sedimentation by as much as 40 per cent the results also indicate that alterations determined from simple analytical models tend to over estimate the true perturbation to the flux

waveform inversion of seismic refraction data and applications to young pacific crust summary we present a method for constraining the velocity depth structure in the earth using seismic refraction waveform data we test the method with synthetic data from known models and apply it to a set of data collected in 1982 june from the east pacific rise at 13 n from the magma expedition in this iterative process wkbj seismograms are computed for a starting model the difference between these and the observed seismograms is used to update the model subject to physical constraints an important first step in the inverse scheme is the linearization of the wkbj seismogram equation allowing us to compute differential seismograms partial derivatives of the synthetic seismogram with respect to specific model parameters this linearization provides the means for estimating required model perturbations based on the misfit in the seismograms the choice of a suitable numerical strategy for computing an updated model is a crucial second step in formulating a working algorithm because the data contain noise synthetic seismograms can only fit the data to this noise level in this case infinitely many models fit the data to this tolerance and some of these estimates are non physical involving negative layer thicknesses a successful strategy must choose from among these possibilities a well defined physically reasonable new model in a commonly used approach to solving non linear problems the perturbation to the starting model is minimized while improving the fit to the data after several iterations the final model which possesses no special properties still tends to resemble the starting model when used with the magma data this technique essentially does not perturb the model at all a method we find much more satisfactory involves solving for the new model directly while applying physically important constraints as constraints we require the velocity gradient remain below a fixed value and penalize the roughness of the new model we thus solve for the smoothest model fitting the data to the specified misfit

heat flow and thermal structure of the washington oregon accretionary prism a study of the lower slope heat flow measurements across the washington oregon convergent margin show several characteristic features a well defined peak at the foot of the overthrust a gradual decrease landward across the lower slope reaching a minimum at the ridge and a gradual rise further landward constrained by geological and seismic evidence on the rate of stacking and the dimension of stacked thrust sheets and by drilling results on sediment properties we use a two dimensional time dependent finite element procedure to model the stacking of overthrust slices of sediments and to calculate the thermal structure beneath the lower slope of the accretionary prism the calculated heat flow profile is in good agreement with the measurements advective heat transfer due to stacking of overthrust sheets and topographic changes are shown to play a dominant role in controlling the surface heat flow and the thermal state of the accretionary prism

a mechanism for the formation of methane hydrate and seafloor bottom simulating reflectors by vertical fluid expulsion bottom simulating reflectors bsr are observed commonly at a depth of several hundred meters below the seafloor in continental margin sedimentary sections that have undergone recent tectonic consolidation or rapid accumulation they are believed to correspond to the deepest level at which methane hydrate clathrate is stable we present a model in which bsr hydrate layers are formed through the removal of methane from upward moving pore fluids as they pass into the hydrate stability field in this model most of the methane is generated below the level of hydrate stability but not at depths sufficient for significant thermogenic production the methane is primarily biogenic in origin the model requires either a mechanism to remove dissolved methane from the pore fluids or disseminated free gas carried upward with the pore fluid the model accounts for the evidence that the hydrate is concentrated in a layer at the base of the stability field for the source of the large amount of methane contained in the hydrate and for bsrs being common only in special environments strong upward fluid expulsion into the hydrate stability field does not occur in normal sediment depositional regimes so bsrs are uncommon upward fluid expulsion does occur as a result of tectonic thickening and loading in subduction zone accretionary wedges and in areas where rapid deposition results in initial undercconsolidation in these areas hydrate bsrs are common the most poorly quantified aspect of the model is the efficiency with which methane is removed and hydrate is formed as pore fluids pass into the hydrate stability field the critical boundary in the phase diagram between the fluid plus hydrate and fluid only fields is not well constrained however the amount of methane required to form the hydrate and limited data on methane concentrations in pore fluids from deep sea boreholes suggest very efficient removal of methane from rising fluid that may contain less than the amount required for free gas production in most fluid expulsion regimes the quantity of fluid moved upward to the seafloor is great enough to continually remove the excess chloride and the residue of isotope fractionation resulting from hydrate formation thus as observed in borehole data there are no large chloride or isotope anomalies remaining in the local pore fluids the differences in the concentration of methane and probably of co2 in the pore fluid above and below the base of the stability field may have a significant influence on early sediment diagenetic reactions

evidence of crustal thinning beneath the limpopo belt and lebombo monocline of southern africa based on regional gravity studies and implications for the reconstruction of gondwana abstract the 600 km long by 250 km wide limpopo belt forms a wedge shaped structure between the zimbabwe and kaapvaal cratons in southern africa the results of a gravity survey which included the part of the limpopo belt within zimbabwe are incorporated with other data and presented as a regional gravity map three gravity profiles crossing the limpopo belt and one within mozambique have been interpreted in terms of lateral changes in crustal structure these interpretations are controlled by density measurements and seismic refraction data across the northern and central parts of the belt the zimbabwe craton appears from gravity modelling to be characterised by a horizontal moho at 34 km depth the limpopo belt is parallel to the ene trending gravity contours and in the east has a southwest northeast line of gravity highs coincident with the nuanetsi sabi volcanic rocks this study has shown that the regional gravity high associated with these features is consistent with crustal thinning probably caused by the breakup of gondwana the crust thins in a south southeast direction to about 30 km in the central limpopo belt with a dip on the moho which varies from about 4 to 7 the short wavelength positive gravity anomalies occurring on the nuanetsi sabi regional gravity high are due to late karoo igneous intrusions and volcanics a north south similar line of gravity highs exists along the lebombo monocline on the mozambique south africa border beyond the lebombo monocline to the east the gravity anomaly suggests a thickness of 6 km or more of sediments that could represent a paleo deltaic environment prograding over the continental margin the lebombo nuanetsi sabi gravity anomaly that dominates the gravity response in the eastern part of the study area is likely to be a failed arm triple junction and could imply a much tighter fit of gondwana than has been previously suggested as such the geometry of such a margin may provide a major constraint on the geometric fit of east antarctica with africa

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

crustal structure of precambrian terranes in the southern african subcontinent with implications for secular variation in crustal genesis summary new estimates of crustal thickness poisson s ratio and crustal shear wave velocity have been obtained for 39 stations in angola botswana the democratic republic of congo malawi mozambique namibia rwanda tanzania and zambia by modellingp wave receiver functions using the h stacking method and jointly inverting the receiver functions with rayleigh wave phase and group velocities these estimates combined with similar results from previous studies have been examined for secular trends in precambrian crustal structure within the southern african subcontinent in both archean and proterozoic terranes we find similar moho depths 38 39 3k msd standard deviation crustal poisson s ratio 0 26 0 01 sd mean crustal shear wave velocity 3 7 0 1 km s 1 sd and amounts of heterogeneity in the thickness of the mafic lower crust as defined by shear wave velocities 4 0 km s 1 in addition the amount of variability in these crustal parameters is similar within each individual age grouping as between age groupings thus the results provide little evidence for secular variation in precambrian crustal structure including between meso and neoarchean crust this finding suggests that 1 continental crustal has been generated by similar processes since the mesoarchean or 2 plate tectonic processes have reworked and modified the crust through time erasing variations in structure resulting from crustal genesis

mud diapirism within indus fan sediments murray ridge gulf of oman summary diapiric structures showing striking spatial periodicity have been widely observed at shallow levels on single and multichannel seismic reflection profiles within indus fan sediments the study area is some 200 to 400 km from the mouth of the indus at the fan s northern margin in the region of the murray ridge neighbouring diapirs are generally associated with the same acoustically transparent layer they form at depths less than 500 in below the seafloor and have the form of gentle waves on the upper surface of this parent layer the acoustic appearance of the features may be controlled by the presence of methane which has been widely reported in the area the diapirs are formed by gravitational instability of overpressured muds within the rapidly deposited sedimentary sequence simple 2 d two layer gravity instability modelling of diapir wavelengths and growth rates has determined values of approximately 104 to 106 for the ratio of equivalent viscosity of the overburden sediments to that of the diapiric layer and a maximum equivalent viscosity of 1011 to 1012 pa s for the diapiric layer itself

seismic velocity increase and deep sea gas hydrate concentration above a bottom simulating reflector on the northern cascadia continental slope the amount of gas hydrates in the accretionary wedge sediments of the northern cascadia subduction zone off vancouver island has been estimated from multichannel seismic mcs and ocean drilling program odp data derailed semblance velocity analyses and full waveform inversion of mcs data combined with previously published odp sites 889 890 sonic log and vertical seismic profile vsp data show that sediment velocities increase downward more rapidly than the no hydrate no gas reference profile from about 1500 m s near the seafloor to a maximum of 1900 m s just above the bottom simulating reflector bsr at a subbottom depth of 224 m immediately below the bsr the mcs velocities drop to 1700 m s a low velocity of 1500 m s from the vsp data probably represents a thin layer 20 m containing free gas the difference between the reference and observed velocities is used to estimate hydrate concentration which reaches a maximum of 20 30 of the pore space above the bsr a simple interpretation of the drill core chlorinity dilution data at the odp site yields a similar hydrate concentration of 35 the estimated hydrate concentration with subbottom depth represents about 7 m 3 of hydrate per square meter of seafloor the total methane gas at stp including hydrate above the bsr and the small amount of free gas below is about 800 m 3 per square meter of seafloor or 200 tcf trillion cubic feet on the vancouver island continental slope application of the same method in the blake bahama region of the eastern u s margin indicates that the velocity enhancement and inferred hydrate concentration are very similar

episodic methane concentrations at seep sites on the upper slope opouawe bank southern hikurangi margin new zealand along many active and some passive margins cold seeps are abundant and play an important role in the mechanisms of methane supply from the subsurface into seawater and atmosphere with numerous cold seeps already known the convergent hikurangi margin east of north island new zealand was selected as a target area for further detailed multidisciplinary investigation of cold seeps within the new vents and associated projects methane and temperature sensors mets were deployed at selected seep sites on the opouawe bank off the southeastern tip of north island and near the southern end of the imbricate thrust hikurangi margin together with seismic ocean bottom stations they remained in place for about 48 h while seismic data were collected the seeps were associated with seep related seismic structures methane concentrations were differing by an order of magnitude between neighbouring stations the large differences at sites only 300 m apart demonstrate that the seeps were small scale structures and that plumes of discharged methane were very localised within the bottom water high methane concentrations recorded at active seep sites at anticlinal structures indicate focused fluid flow methane discharge from the seafloor was episodic which may result from enhanced fluid flow facilitated by reduced hydrostatic load at low tides the strong semi diurnal tidal currents also contribute to the fast dilution and mixing of the discharged methane in the seawater despite dispersal by currents fluid flow through fissures fractures and faults close to the mets positions and tidal fluctuations are believed to explain most of the elevated methane concentrations registered by the mets small earthquakes do not appear to be correlated with seawater methane anomalies