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

easter microplate evolution geochemical evidence confirms that there is current active spreading on both the east and west rift zones of the easter microplate and northwards propagation of the east rift with the active tip at 25 s the east rift is influenced by lateral flow of plume derived material from a hotspot located beneath either sala y gomez or easter island spreading rate variation does not seem to be influential in controlling the composition of erupted basalts

wave mud interactions over the muddy atchafalaya subaqueous clinoform louisiana united states wave supported sediment transport 1 near bottom fluid mud layers were observed during two experiments conducted on the muddy atchafalaya inner shelf subaqueous clinoform louisiana united states on the face of the subaqueous delta 4 7 m water depth first experiment fluid mud layers are produced by seafloor liquefaction and resuspension forced by swells associated with cold front passages and supported by near bed wave induced turbulence the layers are episodic lifetime of 9 12 h form prior to significant postfrontal settling of sediment in the overlying water column and flow seaward downslope at about 5 cm s farther westward on the delta front finer grain size with negligible sand or coarse silt content second experiment similar wave supported fluid mud layers are observed to last longer 2 days show weaker alongshore westward flow of about 1 3 cm s the results suggest a sequence of near bed sediment transport processes triggered by frontal swell activity bed liquefaction resuspension and advection modulated by the bathymetric characteristics of the clinoform that contribute to the formation of clinoform stratigraphy of muddy subaqueous deltas

ocean island densities and models of lithospheric flexure estimates of the effective elastic thickness te of the oceanic lithosphere based on gravity and bathymetric data from island loads are commonly significantly lower than those based on the wavelength of plate bending at subduction zones the anomalously low values for ocean islands have been attributed to the finite yield strength of the lithosphere to erosion of the mechanical boundary layer by mantle plumes to pre existing thermal stresses and to overprinting of old volcanic loads by younger ones a fifth possible contribution to the discrepancy is an incorrect assumption about the density of volcanic loads we suggest that load densities have been systematically overestimated in studies of lithospheric flexure potentially resulting in systematic underestimation of effective elastic thicknesses and overestimation of the effects of hotspot volcanism we illustrate the effect of underestimating load density with synthetic examples and an example from the marquesas islands this effect combined with the other effects listed above in many cases may obviate the need to invoke hotspot reheating to explain low apparent elastic thicknesses

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

a model for the plate tectonic evolution of the east central pacific based on seasat investigations abstract a complete seasat dataset is used to investigate the small scale bathymetry of a 15 by 15 area of the east central pacific northeast of pitcairn island which was involved in the miocene ridge jump and reorientation two fracture zones fz1 and fz2 are mapped along 450 km at an azimuth of n95 e while the signature of the austral fracture zone disappears east of 135 w several seamounts are discovered or confirmed along the oeno henderson ducie lineament and the degree of isostatic compensation of a number of islands at the eastern end of the tuamotu group is studied islands of the northern branch have practically no signatures in the geoid while the southern ones must have formed on lithosphere at least 10 ma old on the basis of these constraints we present a model of the evolution of spreading since 40 ma b p in which the deactivation of the austral fracture zone and the transfer of its offset to fz1 in the late oligocene is interpreted as due to rift propagation triggered by a northern tuamotu hotspot we propose that the oeno ducie chain is the surface expression of a southern tuamotu hotspot deviated 15 in azimuth by the presence of a young and fresh fracture zone thus our model predicts that hotspots and ridge systems can have a moderate influence on each other s kinematics and surface expressions which does not however extend to permanent trapping

thermal stresses and the bimodal distribution of elastic thickness estimates of the oceanic lithosphere observations of plate flexure beneath seamounts and at deep sea trenches suggest that the mechanical strength of the oceanic lithosphere increases with age the mechanical thickness h seems to follow the depth to a particular isotherm in the cooling plate model a detailed review of available estimates of elastic thickness te shows that the strength deduced from studies at seamounts is significantly different from the strength inferred at trenches and fracture zones whereas te beneath seamounts approximately follows the 400 c isotherm h at trenches follows the depth to the 700 800 c isotherms the cause of this bimodal distribution is not fully understood thermal bending stresses due to cooling of the lithosphere have been suggested to play an important role in the flexural deformation at transform faults and fracture zones and their distribution with depth is consistent with intraplate seismicity this paper evaluates the effects of thermal stresses on deformation beneath seamounts and at trenches and how their interaction with flexural stresses may modify plate strength the results indicate that thermal bending stresses will bias the moment curvature relationship such that a plate of a certain strength will appear relatively weaker beneath a seamount positive curvature but relatively stronger at a trench negative curvature this systematic bias appears to be in qualitative agreement with published te estimates for old lithosphere the theory predicts a diminishing amount of bias the general lack of sufficient te determinations from old ocean basins and the uncertainties associated with modeling of trenches prohibit a detailed analysis of this part of the te versus age relationship i conclude that the scatter in the elastic thickness estimates appears to be caused by a combination of lithospheric reheating during seamount emplacement bias introduced by thermal stresses and variable amount of yielding of the plate beneath seamounts in proportion to seamount size and lithospheric age at the time of loading

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

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

two way exchange between the easter mantle plume and the easter microplate spreading axis the conventional model whereby plume volcanism forms linear age progressive volcanic chains with the youngest activity occurring nearest a spreading axis at a hotspot has been challenged for the easter seamount chain1 4 whereas early work suggested the existence of a linear melting anomaly a hotline 1 2 more recent studies3 4 have proposed a hotspot near salas y gomez island connected with the easter microplate spreading system by an 800 km long volcanically active plume channel here we use geochemical geological and geochronological data to argue that the hotspot lies close to easter island moreover new isotopic data for lavas from the seamount chain provide evidence for bidirectional flow between the spreading axis and the plume thus supporting geophysical and fluid dynamical models of mantle flow in a plume spreading axis system5 7 material balance and flux considerations show the easter plume to be weak and cool compared with those beneath larger features such as iceland hawaii and the galapagos islands

relationships between the vredefort structure and the witwatersrand basin within the tectonic framework of the kaapvaal craton as interpreted from regional gravity and aeromagnetic data abstract processed gravity and aeromagnetic data covering the witwatersrand basin and adjacent areas have been used to delineate the boundary of the basin and to investigate other features such as the vredefort structure the bethlehem gravity high and the colesberg magnetic anomaly trend a model is proposed in which the regional aeromagnetic and gravity signatures of these features can be related to the disposition of a semi continuous mid crustal magnetite rich layer which is exposed or has a shallow suboutcrop in three regions in the centre of the vredefort structure along the kaapvaal craton boundary and centrally through the craton northwards from colesberg the bethlehem high is thought to be due to elevation of lower levels of the crust along a nw trending geanticlinal arch the apex of this broad arch is termed the vredefort axis and plunges northwestwards beneath the witwatersrand basin the vredefort structure lies at the intersection of this axis and a major nne trending axis of crustal downwarp which also constitutes the long axis of the witwatersrand basin two contrasting scenarios for the development of these major axes are discussed with either scenario genesis of the vredefort structure should be integrated with the genesis and evolution of the vredefort axis and with the tectonic evolution of the kaapvaal craton

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

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

multiple zircon growth within early archaean tonalitic gneiss from the ancient gneiss complex swaziland abstract u pb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the ancient gneiss complex the oldest episode at3644 4ma 2 produced the dominant type of zircon characteristically purplish brown and massive in texture this probably constitutes unaltered zircon precipitated from the original magma recrystallization accompanied and obscured by early pb loss took place within the oldest grains at3504 6ma and3433 8ma whole new grains grew at these times also we interpret the post 3644 ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss in addition many grains are visibly overgrown by two layers of younger zircon of different colour and texture dated at2986 20ma and2867 30ma euhedral finely zoned whole grains having the 2986 ma age are present also evidently contributed by very thin felsic veins associated with the nearby lochiel granite the age of3644 4ma combined with precise zircon u pb dating of volcanics from the onverwacht group reported elsewhere demonstrates that at least part of the ancient gneiss complex is older than the barberton greenstone belt

on the accuracy of marine gravity measurements we have assessed the accuracy of lamont doherty geological observatory s global marine gravity data bank by examining the crossover errors coes at intersecting ship tracks more than 63 000 coes were found having a standard deviation of 22 43 mgal mgal 10 5 m s 2 we use the coes to find and remove linear drifts and dc shifts present in the data set this adjustment reduces the standard deviation to 13 96 mgal coes generally decrease with latitude which we attribute to uncertainties in the eotvos correction high coes occur in areas of high gravity gradients these two features point to poor navigation as the principal source of error in marine gravity surveys coes have generally been decreasing during the last two decades reflecting improvements in instrumentation and quality of navigation a comparison of the shipboard gravity data to seasat derived gravity revealed a 9 mgal bias in the terrestrial data which probably reflects uncertainties in choice of reference field the adjusted data set was used to generate a gravimetric geoid for the nw atlantic ocean by removing this geoid from the seasat sea surface heights a residual geoid was obtained a special feature of this map is an elongate ene trend that appears to correlate with the edge of the gulf stream

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

the support mechanism of the young foundation seamounts inferred from bathymetry and gravity summary the youngest volcanoes of the foundation volcanic chain were built on a plate less than 5 myr near the pacific antarctic ridge par axis the progressive approach of the spreading ridge to the hotspot during the last 10 myr resulted in the building of the edifices upon a progressively younger plate estimates of the effective elastic thickness te using two independent methods show that the elastic thickness of the plate diminishes towards the spreading ridge from 5 3 km to 2 0 km the spatial variation of te correlates with a significant change in both the morphology and the volume of the volcanoes suggesting an important control of the elastic thickness on the volcanic processes the reduction in te also explains conveniently the reduction in the distance between the north and the south systems the double volcanic line that characterizes the young part of the foundation chain in the direction of young loading ages the south system formed at the top of a flexural bulge created by the emplacement of the north system the main volcanic line the low te estimate for the south system and the shape of the bouguer anomalies confirm this model the effective elastic thicknesses obtained in this study are well matched by isotherms between 200 c and 300 c and support the hypothesis of a weak plate three mechanisms possibly acted to lower the plate rigidity re heating of a normal plate by the hotspot north system local fracturing south system and the creation of a thin lithosphere at the par axis by the coupling of a hotspot and the ridge related melting zones near ridge volcanoes

effects of ambient currents and waves on gravity driven sediment transport on continental shelves abstract observations from several shelf environments show that down slope gravity driven transport may constitute an important mode of suspended sediment dispersal across shelves and highlight the influence of ambient waves and currents on gravity induced sediment flux the phenomena discussed here involve high concentrations of suspended sediment mixed with seawater and thus differ in genesis from hyperpycnal plumes released directly from sediment laden rivers the field sites examined are the gulf of bohai off the mouth of the yellow river huanghe the northern california shelf off the mouth of the eel river and the louisiana shelf west of the mouths of the mississippi river off the yellow river rapid down slope transport over distances of a few km occurred when frictional resistance induced by strong along shelf currents was temporarily relaxed more prolonged down slope motion over longer distances occurred following floods of the eel river when wave and current agitation provided turbulence to sustain gravity driven transport of fluid mud on the louisiana inner shelf the down slope gravity force was much weaker but observations suggest that thin gravity flows may still have occurred in the presence of waves a simple analytical theory is developed that incorporates the influence of ambient shelf currents on gravity driven transport of suspended sediment this theory is quantitatively consistent with the observations from the three sites if the supply of easily suspended sediment is less than the capacity of ambient currents including waves to carry sediment then intense turbulence limits gravity induced sediment transport by increasing the drag at the base of the flow when ambient currents abruptly cease rapid down slope transport can then occur over short distances until the sediment settles such flows do not remain intensely turbulent because the slope of the continental shelf is too gentle to induce shear instability within the gravity flow the maximum sustained rate of gravity induced sediment transport occurs when ambient currents are strong but the supply of easily suspended sediment exceeds the resuspension capacity of the ambient currents feedback then leads to values of the gradient richardson number ri within the flow that are near the critical value of 1 4 this partially damps bottom drag but still allows the generation of sufficient turbulence to maintain sediment in suspension observations also indicate systematic relationships among ri the supply of easily suspended sediment and the bottom drag coefficient acting on the gravity flow

time space mapping of easter chain volcanism abstract new 40ar 39ar and published k ar ages show that the locus of volcanism along the easter volcanic chain evc has shifted systematically from the nazca ridge at about 26 m y to the recently active sala y gomez island easter island region this indicates a plume rather than a hotline i e mantle roll origin for the evc the time space distribution of ages combined with published ages for the galapagos and juan fernandez volcanic chains is used to reconstruct nazca plate velocities over the past 26 m y a plume now located in the region of sala y gomez island is most compatible with these data west of the plume the evc records neither nazca nor pacific plate motions this section of the evc may be related to westward channeling of plume material to the pacific nazca spreading boundary region

modeling of nonlinear wave propagation over fringing reefs abstract the applicability of existing nonlinear triad spectral models for steep slopes 0 1 0 2 characteristic of reef environments was investigated using both deterministic phase resolving and stochastic phased averaged formulations model performance was tested using laboratory observations of unidirectional wave transformation over steep and smooth bathymetry profiles the models developed for mild slopes were implemented with minimal modifications the inclusion of breaking parametrizations and linear steep slope corrections required by laboratory data the deterministic model produced typically more accurate predictions than the stochastic one but the phase averaged formulation proved fast enough to allow for an inverse modeling search for the optimal breaking parametrization the effects of the additional assumptions of the stochastic approach resulted in a slower than observed evolution of the infragravity band despite the challenge posed by the fast wave evolution and energetic breaking characteristic to the steep reef slopes both formulations performed overall well and should be considered as good provisional candidates for use in numerical investigation of wave current interaction processes on steep reefs

water waves over a muddy bed a two layer stokes boundary layer model this study aims to develop an asymptotic theory for the flow kinematics of a thin layer of viscous mud under water surface waves the mud depth the mud stokes boundary layer thickness and the wave amplitude are assumed to be comparable with one another and much smaller than the wavelength by virtue of this sharp contrast in length scales boundary layer equations are used to describe motion of both the mud and the immediately overlying water analytical solutions are sought and explicit expressions are obtained under progressive waves for the fluid velocity fields interface wave characteristics and wave damping rate at the first order and the steady mean discharge rate and mass transport velocity of mud at the second order effects of the mud layer thickness density and viscosity ratios on these kinematic quantities are examined in detail

en echelon volcanic elongate ridges connecting intraplate foundation chain volcanism to the pacific antarctic spreading center abstract the development of volcanic elongate ridges vers could be controlled in many cases by complex processes related to interaction between mantle plumes and spreading centers we report here measured ages for the foundation chain se pacific showing that volcanism along morphologically distinct vers can develop occasionally as rapidly formed continuous lines of coeval volcanism extending from a region of intraplate volcanism to the pacific antarctic spreading center but a significantly more dominant trend is for coeval yet structurally disconnected segments of foundation chain vers to develop in a series of en echelon ne sw elongate zones of coeval hotspot volcanism these elongate zones developed at intervals of approximately 1 myr while maintaining a basically steady state orientation and size as the pacific antarctic spreading center migrated continually closer to the foundation plume hotspot we infer from this new information that foundation chain ver development was controlled by an interplay between short lived local factors e g location of nearest spreading center segment lithospheric stress and long lived attributes of the foundation plume hotspot e g size orientation periodicity

87sr86sr and ree variations along the easter microplate boundaries south pacific application of multivariate statistical analyses to ridge segmentation abstract we report 87 sr 86 sr ree sc v cr and co analyses on 48 basaltic glasses dredged at 47 different localities along the spreading boundaries of the easter microplate emp the latitudinal 87 sr 86 sr variation along the east and west rifts closely parallels that of the pb isotope variation previously published and confirms the strong abnormality of the mantle underlying this part of the east pacific rise epr the data also further support the binary mantle mixing model previously proposed we evaluate the potential of multivariate statistical analyses in studying this and previously published geochemical data obtained on these very same 48 basaltic glasses the data base includes major and trace elements ranging from compatible e g cr to highly incompatible e g la and the pb and sr isotope ratios multivariate classification of these geochemical variables in q mode sample space allows to clearly distinguish major element grouping from those of moderate to highly incompatible elements and the isotope ratios reflecting mantle source heterogeneities present in the region the power of this multivariate geochemical classification is best represented in two dimensional space by combining principal components analysis pca with cluster analysis ca illustrated in the form of a dendrogram the method is further illustrated by classifying elements of the lanthanide series at different statistical level of affinities using the variance covariance matrix and correlation matrix in r mode we further demonstrate a close relationship between segmentation revealed by the multiple variation of the four isotope ratios and the actual tectonic segmentation both discriminant analysis and pca further indicate a close relationship between the geochemical anomaly located on the east rift of the emp and sala y gomez and easter island to a lesser extent the tuamotu chain whereas there is no close affinity between the emp and the more distant juan fernandez san felix or society islands in the isotopic space considered this provides further support that the geochemical anomaly located on the east rift of the emp may be related to the influence of a plume located either beneath the east rift or in the vicinity of sala y gomez or easter island in the last two models the plume would be laterally deflected at shallow depth towards the epr emp spreading complex an analysis of statistical generalized distances against geographical distances rules out that such influence is caused by radial dispersion of this plume in the upper mantle the flow between the plume and the east rift of the emp must be more confined or channelled the possibility that the plume would be located directly beneath the east rift cannot be ruled out but appears less likely

isostatic compensation of tectonic features of the mid atlantic ridge 25 27 30 s we investigate the isostatic compensation of tectonic features along the mid atlantic ridge 25 27 30 s through a detailed examination of the relationship between bathymetry and gravity anomalies the study area includes three ridge segments spreading at an average full rate of 35 mm yr their flanks out to about 6 m y old crust and the intervening rio grande and moore fracture zones in a three dimensional analysis of gravity and sea beam bathymetry data we focus on crustal thickness variations and mantle density anomalies by removing from the observed fields the predicted contribution of simple crustal and mantle models positive residual gravity anomalies over the northern wall of the rio grande fracture zone indicate that in spots the crust is 2 3 km thinner than average there in contrast to the moore fracture zone where little thinning is observed the greater than average depths of the fracture zones as a whole are not locally compensated by thin crust but the deepest basins within the active transform parts of the fracture zones are partially compensated by thin crust or cooler mantle a narrow linear ridge that crosses one of the inactive branches of the moore fracture zone had been suggested to be the product of an episode of excess volcanism but is found to be underlain by thinner not thicker crust there is no indication of thicker crust beneath topographic highs at the inside corners of ridge transform intersections presumably these highs are dynamically maintained as must be the median valleys characteristic of most the length of the ridge segments judging from the absence of axial residual gravity anomalies there is a residual gravity low associated with an unusually shallow 15 to 20 km section of the ridge segment between the rio grande and moore offsets where the median valley virtually disappears our analysis suggests that while part of the anomalous elevation of this section of the ridge axis is attributable to excess volcanism and a thicker crust much of the elevation contrast is simply caused by the diminution of the dynamic mechanism responsible for median valley formation two dimensional fourier transforms of the gravity and bathymetry fields show that seafloor topography is strongly aligned parallel to either ridges or transforms while density anomalies are more randomly oriented this implies that the tectonic processes that control the gross ridge transform topography are not the dominant control on the magmatic processes that determine the upper mantle and crustal density structure statistical analysis of the coherence between the gravity and bathymetry fields indicates that the average effective elastic plate thickness in the study area is about 6 km

the elastic thickness of the lithosphere in the pacific ocean abstract in this study we present determinations of the effective elastic thickness t e of the oceanic lithosphere along pacific chains or archipelagoes t e is determined by computing the deflection of a continuous elastic plate under the load of volcanoes and constrained by geoid heights provided by seasat in the south central pacific estimates of 14 km for the marquesas and 6 km or less for the pitcairn mururoa gloucester chain are in good agreement with a previous work in this region cook austral and society chains around the line islands chain seasat data reveal that the bathymetry is poorly known preventing fine analysis meanwhile t e looks globally very low 6 km except for three volcanoes but these results may be unreliable the easter chain features low t e values 6 km with no noticeable variation along the chain higher values are found for a samoan island manuae 24 km and along the hawaiian emperor seamounts chain from 32 km at the eastern end of the chain to 21 5 km for the hawaiian volcanoes and from 25 5 to 15 km for the emperor seamounts the large number of t e estimates obtained in this study points out a noticeable difference between north and south pacific results those from the north pacific agree with the general trend increase with the square root of age plate at loading time while those from the south central pacific are much lower according to their plate age these low t e results from the south pacific are only partly explained by taking account of thermal perturbations using the rejuvenation model therefore these results then point out a regional difference in oceanic lithosphere

observations of nearshore wave dissipation over muddy sea beds 1 wave propagation on the louisiana inner shelf is studied using concurrent measurements at two shallow water ocean observatory sites in sedimentary environments dominated by mud at one site and sand at the other contrary to the widely accepted hypothesis that mud induced wave dissipation is important only for long waves observations show significant damping of high frequency short waves which interact weakly with the bottom the mechanism of short wave dissipation is not understood numerical simulations show that other dissipative processes such as refraction or depth limited breaking do not account for the magnitude of the observed effects independent observations of strong sediment re working during storms suggest that these effects are related to sediment resuspension processes

effective elastic thickness of the lithosphere along the easter seamount chain bathymetry and gravity data collected during legs 5 6 and 7 of the 1993 gloria expedition and the recently released 2 min altimetry derived global gravity grid are used to determine the effective elastic thickness of the lithosphere along the easter seamount chain esc forward modeling admittance and coherence methods yield consistent restfits with the exception of the eastern and western ends of the esc the effective elastic thickness along the chain is 1 4 km the thin elastic thickness for the majority of the esc seamounts is compatible with a young seafloor age at the time of loading derived from new radiometric ages of the seamounts along the chain and a magnetic isochron age interpretation of the nazca plate seafloor age the elastic thickness southeast of the nazca fracture zone is 6 km apparently because of the seafloor age discontinuity across the fracture zone the elastic thickness near the san felix island at the eastern end of the esc is even greater 11 km which is compatible with the estimated seafloor age at the time of loading a slight increase in the effective elastic thickness of the far western part of the esc suggests dynamic compensation or less thermal weakening of lithosphere above a plume channel versus directly above the plume center these findings combined with published geochemistry support a hotspot origin for the esc complicated by large scale plate boundary reorganizations and channeling of plume material to the east pacific rise

the fixed hotspot hypothesis and origin of the easter sala y gomez nazca trace reconstructions of the pacific and nazca plates suggest that both the nazca and tuamotu ridges originated from a melting anomaly which existed beneath the pacific farallon ridge during the time interval between anomalies 19 and 11 as would be predicted from the fixed hotspot hypothesis the hotspot hypothesis interpreted according to morphology of the easter sala y gomez nazca trace would further suggest that the hotspot was located entirely beneath the nazca plate during the time interval between anomalies 11 and 2 at the end of this interval the eastern boundary of the easter plate formed centered above the hotspot according to this scheme the present location of the hotspot is about 200 km west of easter island however the easter model is incompatible with the hotspot model for the hawiian emperior chain if the hotspots are fixed relative to one another as the age of the point on the tuamotu ridge which corresponds with the hawaiian emperor bend 53 to 55 m y is significantly greater than the age of the bend 43 m y the fixed hostpot hypothesis can be satisfied by the two traces only if one or a combination of the following conditions is met the kinematic age of the hawaiian emperor bend is substantially greater than the isotopically dated age the magnetic anomaly time scale is in significant error and or magnetic anomaly identifications in the vicinity of the tuamotu ridge are erroneous it might appear that other mechanisms for the origin of hotspot traces are preferable to the hotspot hypothesis the most promising of these involves propagating fractures resulting from intraplate stress this hypothesis has the advantage of accommodating anomalies in some hotspot traces such as persistence of igneous activity long after passage of the trace over the hotspot

modelling of wave damping at guyana mud coast abstract the guyana coastal system is characterized by very thick deposits of amazon mud and high mud concentrations in its coastal waters the mud deposits can be quite soft and may liquefy under incoming waves subsequently the liquefied mud damps the incoming waves effectively this paper presents a simple model to predict wave attenuation over soft fluid mud beds this model is based on the two layer approach by gade gade h g 1958 effects of a non rigid impermeable bottom on plane surface waves in shallow water journal of marine research 16 2 61 82 which is implemented in the standard version of the state of the art wave prediction model swan input to the mud wave damping module consists of the extension thickness density and viscosity of the liquefied fluid mud layer the model is validated against small scale wave attenuation measurements carried out in a laboratory wave flume the model predictions agree favourably with the experimental data next the model is applied to predict wave height and wave attenuation in the guyana coastal system extension and thickness of the liquefiable layer could be assessed from dual frequency echo soundings in the absence of field data the density of the liquefied mud layer is obtained from literature whereas the value of the mud s viscosity had to be established by trial and error the selected value though is in the range of literature values the model predicts significant wave attenuation the computed changes in wave energy spectrum agree qualitatively with measurements in surinam whereas the decrease in significant wave height agrees more or less with historic observations along the guyana coast

crustal structure and isostatic compensation near the kane fracture zone from topography and gravity measurements i spectral analysis approach summary six gravity and bathymetry profiles perpendicular to the kane fracture zone each more than 300 km long were gathered to study the variation in crustal structure in the vicinity of a major fracture zone and the gravitational edge effect at the contact between lithosphere of two different ages a spectral analysis of the gravity and bathymetric series as a function of wavelength shows that the gravitational edge effect is only significant at the longest wavelengths for remaining wavelengths the admittance the ratio of the amplitude of the gravity anomaly to the amplitude of the bathymetry is best explained by a model of isostasy in which topographic loads are partially supported by the flexural rigidity of an elastic plate about 6 km in thickness after subtracting the gravitational attraction of the bathymetry and its compensation substantial isostatic anomalies remain we interpret these anomalies as being caused by variations in crustal thickness which have little correlation with surface topography except at very long wavelengths the apparent crustal thickness varies by as much as a factor of 2 but there is no evidence indicating systematic thinning of the crust beneath the fracture zone our data do suggest that such density variations within the plate are also compensated by the isostatic response of an elastic plate but with very different effect from those at the surface this indicates that there are two different modes of crustal formation with different gravity and topographic signatures effusive volcanism which loads the surface of the elastic plate producing both topographic relief and coherent gravity anomalies and intrusive volcanism or underplating producing gravity anomalies but little topographic relief

evidence for diffuse extension of the pacific plate from pukapuka ridges and cross grain gravity lineations satellite altimeter measurements of marine gravity reveal 100 to 200 km wavelength lineations over a wide area of the pacific plate oriented roughly in the direction of absolute plate motion at least three mechanisms have been proposed for their origin small scale convective rolls aligned in the direction of absolute plate motion by shear in the asthenosphere diffuse n s extension of the lithosphere resulting in lineated zones of extension boudins and minihotspots that move slowly with respect to major hotspots and produce intermittent volcanism recently several chains of linear volcanic ridges have been found to be associated with the gravity lineations following ridgelike gravity signatures apparent in high resolution geosat gravity measurements we surveyed a series of volcanic ridges that extend northwest from the east pacific rise flank for 2600 km onto 40 ma seafloor our survey data as well as radiometric dates on samples we collected from the ridges provide tight constraints on their origin 1 individual ridge segments and sets of ridges are highly elongate in the direction of present absolute plate motion 2 the ridges formed along a band 50 to 70 km wide in the trough of one of the more prominent gravity lineations 3 radiometric dates of the largest ridges show no hotspot age progression moreover the directions predicted for minihotspot traces older than 24 ma do not match observed directions of either the gravity lineations or the ridges based on this last observation we reject the minihotspot model the occurrence of the ridges in the trough of the gravity lineation is incompatible with the small scale convection model which would predict increased volcanism above the convective upwelling we favor the diffuse extension model because it is consistent with the occurrence of ridges in the trough above the more highly extended lithosphere however the multibeam data show no evidence for widespread normal faulting of the crust as predicted by the model perhaps the fault scarps are buried under more than 30 m of sediments and or covered by the elongated ridges finally we note that if ridge push force is much smaller than trench pull force then near the ridge axis the direction of maximum tensile stress must be perpendicular to the direction of absolute plate motion

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

bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry the southern oceans south of 30 s are densely covered with satellite derived gravity data track spacing 2 4 km and sparsely covered with shipboard depth soundings hundreds of kilometers between tracks in some areas flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wave lengths 15 160 km if sediment cover is thin and seafloor relief is moderate at shorter wave lengths the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface at longer wavelengths isostatic compensation cancels out most of the gravity field due to the seafloor topography we combine this theory with wiener optimization theory and empirical evidence for gravity noise to signal ratios to design low pass and band pass filters to use in predicting bathymetry from gravity the prediction combines long wavelengths 160 km from low pass filtered soundings with an intermediate wavelength solution obtained from multiplying downward continued band pass filtered 15 160 km gravity data by a scaling factor s s is empirically determined from the correlation between gravity data and existing soundings in the 15 160 km band by robust regression and varies at long wave lengths we find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99 level and s may be related to the density of seafloor materials in these areas the prediction has a horizontal resolution limit of 5 10 km in position and is within 100 m of actual soundings at 50 of grid points and within 240 m at 80 of these in areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features images of the prediction reveal many tectonic features not seen on any existing bathymetrie charts because the prediction relies on the gravity field at wavelengths 160 km it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation the prediction is available from the national geophysical data center in both digital and printed form

helium isotope ratios in easter microplate basalts abstract 3 he 4 he ratios in easter microplate basalt glasses show clear evidence of the effects of a mantle plume the east rift of the microplate between 26 and 28 s identified by la sm sr and pb isotopes and ridge crest elevation as the region of maximum plume influence has 3 he 4 he ratios spanning the entire range from 7 5 to 11 7 r a the easter microplate is the only section of the entire east pacific rise that is associated with a known hotspot track mantle plume and has elevated 3 he 4 he ratios although most of the west rift basalts contain morb helium 8 0 8 7 r a the basalt closest to the east rift has an elevated 3 he 4 he ratio 11 3 r a consistent with a significant plume component the diversity in isotopic signatures also indicates that homogenization of isotopic anomalies does not occur even in this region of super fast spreading the overall 3 he 4 he 206 pb 204 pb and 3 he 4 he 87 sr 86 sr trends have positive correlations although the high 3 he 4 he 87 sr 86 sr and 206 pb 204 pb component is not a single value but rather a broad band the apparent decoupling between the he and sr isotope distribution is modeled in the context of a plume source migrating ridge sink during channeling of the plume toward the ridge helium is preferentially lost from the center of the channeled plume resulting in lower he pb and he sr concentration ratios in the high 3 he 4 he component mixing trajectories in he sr isotopic space between a lile depleted asthenosphere and a variably degassed plume component provide a reasonably good fit to the data and may explain the isotope systematics of plume ridge interactions in the context of modern theories of plume dynamics

new observations on mid plate volcanism and the tectonic history of the pacific plate tahiti to easter microplate abstract we describe the geology and tectonics of a continuous swathe of seafloor between tahiti and the western edge of the easter microplate imaged by gloria and sea beam on two separate cruise transits in 1987 and 1988 the data reveal that mid plate volcanism is common in this region even on deep seafloor hundreds of kilometres from major lines of seamounts and islands this supports the idea of a thin weak lithosphere over the pacific superswell and the idea that the tops of major mantle plumes may spread out over diameters of the order of 1000 km the mid plate volcanism occurs in two distinct forms over most of our traverse it appears as fields of relatively young and acoustically strongly backscattering lava flows often accompanied by groups of numerous small circular volcanoes east of 122 w about chron 5a however we observed a distinct form major sharp crested constructional volcanic ridges many tens of kilometres long individually trending ene but lying en echelon along an e w regional trend these ridges appear morphologically identical to the cross grain ridges seen elsewhere in the pacific we attribute their formation to magma supplied from the regionally hot mantle leaking along tectonic lines of weakness however although these ridges are parallel to fracture zone trends seen farther west they are morphologically very different from any known fracture zone moreover individual ridges are somewhat oblique to the tectonic spreading fabric around them and so do not seem to follow actual fracture zone traces the whole line of en echelon ridges lies along part of the predicted trace of fracture zone 2 of okal and cazenave 15 and is probably its morphological expression however nowhere did we see a convincing conventional fracture zone trace in or following the predicted position or orientation we suggest instead that magma from an independent source has used lines of weakness along minor fracture zones to produce these en echelon features the austral fracture zone is the only major fracture zone crossed in our transit and here is characterised by four fossil transform strands its marked position on the aapg and gebco maps is found to be in error finally we found that the expected change from nnw to nne trending spreading fabric at chron 6c did not occur in a clear cut way as predicted by earlier tectonic histories of the pacific instead the post chron 6c fabric oscillates in a confused way between nne and nnw suggesting to us that this area has been characterised by an unstable plate boundary probably associated with a succession of propagating rifts or microplates from chron 6c to the present

bottom turbulence on the muddy atchafalaya shelf louisiana usa 1 wave current and sediment observations collected in approximately 5 m depth on the muddy atchafalaya clinoform la usa are used to study the interaction between near bed wave induced turbulent flows and suspended sediment characteristics in a muddy environment low wave bias estimates of near bed reynolds stresses are strongly correlated with flow accelerations and suspended sediment concentration as previously observed on sandy beaches where accelerations have been associated with bed fluidization and sediment transport a detailed numerical analysis of the observations is performed based on a uni dimensional boundary layer model that accounts for the coupling between the fluid and the cohesive sediment phases the numerical simulations suggest that sediment induced stratification effects are of the same order of magnitude as turbulent dissipation and thus play a significant role in the turbulent kinetic energy balance within the tidal boundary layer however inside the wave boundary layer the ratio of stratification to shear induced turbulence production i e gradient richardson number decreases significantly and shear induced turbulence production dominates for these observations the vertical structures of currents and reynolds stresses are relatively insensitive to the exact floc size

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

observations of suspended sediment stratification from acoustic backscatter in muddy environments abstract vertical profiles of suspended sediment concentration estimated from acoustic backscatter are used to re examine the relation between sediment stratification and floc size in a cohesive sedimentary environment concentration measurements by optical backscatter sensors at two vertical levels are used to calibrate the acoustic backscatter intensity in spite of the complexities due to the rapidly changing flow and cohesive sediment properties the estimates of sediment concentration reproduce well the observations together with the vertical profiles of suspended sediment concentration measured current profiles are used to calibrate a one dimensional vertical 1dv boundary layer numerical model for combined wave current flow on muddy beds the numerical simulations are used to investigate the effect of the floc size on the vertical structure of the suspended sediment concentration profile for similar flow conditions smaller flocs result in more mixed profiles with higher concentration in the upper water column and lower near bed concentration this is the first time this effect is seen in direct field observations and confirms previously published numerical results

evidence for early proterozoic plate tectonics from seismic reflection profiles in the baltic shield plate tectonics provides the linking framework for all tectonic and magmatic activity seen today but it is not known when plate tectonics first developed on earth new deep seismic reflection and

a history of the easter microplate 5 25 ma to present gloria and other geophysical data are used to interpret the morphology and tectonic pattern of the easter microplate we model magnetic anomaly data and calculate spreading rates for use in euler pole calculations we calculate three plate closure and best fit poles for nazca pacific nazca easter and pacific easter plate motions and compare the results with earthquake focal mechanisms and seismicity we use the three plate closure poles to rotate digital gloria and magnetic isochron data of the microplate and surrounding major plates back in time since the microplate s east rift first started to propagate northward at around 5 25 ma the microplate has evolved in two stages the first is from 5 25 ma to around 2 5 ma and involved continuous northward propagation of the east rift while the west rift became segmented along new transform faults as it adjusted to keep the overall nazca pacific velocities constant the transpressive northern boundary migrated with the east rift tip while the transtensional southern boundary remained more or less still at some time after 2 5 ma the east rift ceased northward propagation and the southwest rift began to open up along the transtensional pacific easter transform a convergent northern boundary formed involving southward thrusting of the nazca plate over the microplate no further nazca plate lithosphere was transferred from north of the microplate to the microplate interior and the pito deep region opened by pervasively rifting apart preexisting nazca plate lithosphere

directional median filtering for regional residual separation of bathymetry 1 we present a new spatial filter designed to separate short length scale bathymetric features from regional bathymetry the directional median dim filter divides a given filter circle into n bow tie sectors allocates data points inside the filter circle to each sector based on their relative position within the circle estimates a median for each sector and returns the lowest of these n medians this approach prevents dim filtering from choosing the biased medians near the features on a sloping regional trend which is a serious artifact of standard median filtering as the bathymetry comprises diverse length scale features the separated results vary with the choice of filter width such variations are spatially distributed using a finite range of filter widths we evaluate the spatial distribution of variations in the dim based separation by estimating their median absolute deviation mad at each data point the distribution of mad values is indicative of uncertainties inherent in the separation we demonstrate the effectiveness of dim filtering with both synthetic and actual bathymetric data

a numerical study on nonlinear surface wave evolution over viscoelastic mud abstract accurate prediction of wave energy dissipation over mud is essential for understanding nearshore circulation sediment transport processes and design of engineering projects along muddy coasts it is desirable to be able to simulate wave propagation over a wide range of mud behaviors as wave evolution is highly dependent on mud rheological characteristics in this study we develop a numerical model to simulate surface wave evolution over viscoelastic muds this new wave mud interaction model is comprised of a frequency domain phase resolving model for wave propagation that explicitly solves nonlinear wave wave interactions and a model for mud induced surface wave dissipation and modulation model results show satisfactory agreement with laboratory measurements of wave height and attenuation rates over mud and shows improvement over the model with a viscous mud mechanism the model is then used to investigate the combined effect of mud viscoelasticity and nonlinear wave wave interactions on surface wave evolution cnoidal and random wave simulations are conducted in general qualitative measures such as shape of cnoidal waves or pattern of variation in h r m s of random waves are dictated by direct mud induced damping which due to resonance effects has a substantially different structure over viscoelastic mud compared to viscous mud nonlinear interactions affect spectral shape and distribution of energy loss across the spectrum subharmonic interactions cause indirect damping of high frequencies but ameliorate damping of harmonics around mud s resonance frequency therefore neglecting mud elasticity can result in significant errors in estimation of bulk wave characteristics and spectral shape

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

wave dissipation by muddy seafloors 1 muddy seafloors cause tremendous dissipation of ocean waves here observations and numerical simulations of waves propagating between 5 and 2 m water depths across the muddy louisiana continental shelf are used to estimate a frequency and depth dependent dissipation rate function short period sea 4 s and swell 7 s waves are shown to transfer energy to long period 14 s infragravity waves where in contrast with theories for fluid mud the observed dissipation rates are highest the nonlinear energy transfers are most rapid in shallow water consistent with the unexpected strong increase of the dissipation rate with decreasing depth these new results may explain why the southwest coast of india offers protection for fishing and for the 15th century portuguese fleet only after large waves and strong currents at the start of the monsoon move nearshore mud banks from about 5 to 2 m water depth when used with a numerical nonlinear wave model the new dissipation rate function accurately simulates the large reduction in wave energy observed in the gulf of mexico

wave mud interaction over the muddy atchafalaya subaqueous clinoform louisiana united states wave processes 1 observations of wave and sediment processes collected at two locations on the atchafalaya inner shelf show that wave dissipation in shallow muddy environments is strongly coupled to bed sediment reworking by waves during an energetic wave event 2 m significant wave height in 5 m water depth acoustic backscatter records suggest that sediment in the surficial bed layer evolves from consolidated mud through liquefaction fluid mud formation and hindered settling to gelled under consolidated mud net swell dissipation increases steadily during the storm from negligible prestorm values consistent with bed softening but shows no correlation with detectable fluid mud layers remarkably the maximum dissipation rate occurs poststorm when no fluid mud layers are present in the waning stage of the storm the contribution of different wave forcing processes to wave dissipation is analyzed using an inverse modeling approach based on a nonlinear three wave interaction model although wave mud interaction dominates dissipative processes nonlinear three wave interactions control the shape of the frequency distribution of the dissipation rate in the wake of the storm the viscosity values predicted by the inverse modeling converge toward measured values characteristic for gelled mud in a trend that is consistent with a fluid mud entering dewatering and consolidation stages

flexure modelling at seamounts with dense cores summary the lithospheric response to seamounts and ocean islands has been successfully described by deformation of an elastic plate induced by a given volcanic load if the shape and mass of a seamount are known the lithospheric flexure due to the seamount is determined by the thickness of an elastic plate te which depends on the load density and the age of the plate at the time of seamount construction we can thus infer important thermomechanical properties of the lithosphere from te estimates at seamounts and their correlation with other geophysical inferences such as cooling of the plate whereas the bathymetry i e shape of a seamount is directly observable the total mass often requires an assumption of the internal seamount structure the conventional approach considers the seamount to have a uniform density e g density of the crust this choice however tends to bias the total mass acting on an elastic plate in this study we will explore a simple approximation to the seamount s internal structure that considers a dense core and a less dense outer edifice although the existence of a core is supported by various gravity and seismic studies the role of such volcanic cores in flexure modelling has not been fully addressed here we present new analytic solutions for plate flexure due to axisymmetric dense core loads and use them to examine the effects of dense cores in flexure calculations for a variety of synthetic cases comparing analytic solutions with and without a core indicates that the flexure model with uniform density underestimates te by at least 25 per cent this bias increases when the uniform density is taken to be equal to the crustal density we also propose a practical application of the dense core model by constructing a uniform density load of same mass as the dense core load this approximation allows us to compute the flexural deflection and gravity anomaly of a seamount in the wavenumber domain and minimize the limitations recognized from the analytic tests then the dense core model is applied to predict the lithospheric flexure beneath howland island in the tokelau seamount chain these results are compared with the predictions of the uniform density model based on age dating of howland and the age of the seafloor traditional te versus age curves predict the elastic plate thickness beneath the seamount to be around 20 km which is comparable to the best dense core model of te 26 km however the best uniform density model is found at te 12 km which is significantly less than the predicted from our investigations of synthetic and real seamount cases we conclude that the dense core model approximates the true mass distribution of a seamount better than the uniform density model finally we suggest that the role of underplating in flexure modelling may need to be reexamined because the dense core model predicts substantially less deflections than the uniform density model without requiring additional buoyancy caused by underplated material

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

shear zones of the limpopo belt and adjacent granitoid greenstone terranes implications for late archaean collision tectonics in southern africa abstract granulite metamorphism in the 2 65 ga limpopo belt of southern africa is related to crustal thickening due to late archaean collision tectonics the central zone of the belt is a major thrust sheet of distinctive lithological structural and isotopic character separated from adjacent terranes by crustal scale shear zones and the northern and southern marginal zones are up thrust deep level granulite facies terranes of the adjacent cratons geometrically the marginal zones are mirror images and the limpopo belt is essentially symmetrical in n s section conventionally regarded as a n s collisional orogeny the gross structural and lithological differences between the central zone and the marginal zones kinematic data from shear zones and geophysical data showing the three dimensional geometry of the central zone are here used to suggest that the direction of emplacement of the central zone was also the collision direction this was from northeast to southwest with the upthrust outwardly directed marginal zones developing in mechanical response to this collision

flexural isostasy and loading sequence of the dokdo seamounts on the ulleung basin in the east sea sea of japan abstract the isostatic compensation and loading sequence of the dokdo seamounts on the ulleung basin in the east sea are studied by a flexure model using gravity and bathymetric data the dokdo seamounts comprise dokdo dok island lying as a cluster of emerged small islets surmounting a large submerged volcanic edifice and two voluminous tablemounts simheungtaek and isabu in spite of their similarity in topographic expression a large difference of about 50 mgal between the observed gravity anomaly peaks over dokdo and the isabu tablemount implies different amounts of compensation a comparison between the observed and calculated gravity anomalies of the seamounts shows that the flexural compensation investigated in this study predicts well the observed data the flexure model suggests that the dokdo seamounts might be composed of high density volcanic rocks modeling results show that the flexural rigidity or effective elastic thickness of the lithosphere under dokdo is stronger or thicker than that under the isabu tablemount it implies that the age of lithosphere at the time of loading for isabu was younger than that for dokdo the effective elastic thicknesses associated with the dokdo seamounts approximately follow 200 400 c isotherms in the cooling plate model the results of this study and the former age dating of dokdo suggest that the isabu tablemount was formed first followed by dokdo after the opening of the east sea ceased

observations of flexure and the rheology of oceanic lithosphere the principal evidence for the long term i e 1 myr mechanical behaviour of the oceanic lithosphere has come from studies of how it deforms in response to large loads such as volcanoes and sediments a model widely used to explain the deformation is an elastic plate in which flexural rigidity depends on the thermal age of the lithosphere at the time of loading an elastic model however is time invariant and does not take into account temporal changes that may occur in the flexural rigidity as a consequence of loading there is evidence for example that the flexural rigidity of the oceanic lithosphere also depends on load age being large at short times and small at long times thus competing effects may exist between thermal cooling which strengthens the lithosphere and some form of load induced stress relaxation which weakens it in order to investigate the relative roles of these processes we have developed a multilayered viscoelastic model which is based on the results of experimental rock mechanics that creep in the lithosphere is a thermally activated process and on a thermal structure that is given by the plate cooling model by comparing the predictions of the model with a new compilation of flexural rigidity estimates we have found that if the upper mantle viscosity is 1020 pa s the activation energy that best describes the long term mechanical behaviour of the oceanic lithosphere is 120 kj mol 1 this parameter pair explains the dependence of flexural rigidity on both plate and load age it also helps account for the subsidence and uplift history of oceanic islands and the stratigraphic patterns that develop in the flexural moats that flank them at atolls a multilayered viscoelastic model explains the rapid subsidence that follows shield building and does not require that the age of the lithosphere that supports a volcano is thermally re set to a younger value our studies suggest that while the oceanic lithosphere has viscoelastic properties the viscosity of its upper layers is so much higher than that of its lower layers that in effect it behaves as a thin elastic plate on long timescales there will therefore be a certain permanence to the observations of oceanic flexure such that they may be used with some confidence to evaluate the tectonic setting of individual features of the seafloor and in some cases their age

physical processes and fine grained sediment dynamics coast of surinam south america abstract the prograding holocene mud wedge between the amazon and orinoco rivers in the trade wind belt of northeastern south america provides a modern day example of muds accumulating under moderate wave energy conditions gigantic shore attached mudbanks 10 km 20 km composed partly of thixotropic fluid mud gel front this coast every 30 60 km to form a buffer to wave attack and a temporary storage for fine grained sediments this mesotidal coast tide range 2 0 m with gentle offshore slope 0 0006 allows the exposure twice a day of extensive tidal flat deposits which are backed by mangrove swamps on a well developed chenier plain complex field experiments were conducted in surinam du ing 1975 and 1977 to provide new information on process form relationships in this interesting but unusual muddy environment simultaneous measurements of waves currents tide elevation suspended sediment concentration and variations in mud density show that soft intertidal and subtidal muds are suspended at both tide and wave frequency suspended sediment concentrations typically exceed 1 000 mg l at the surface as incoming solitary like waves partially disperse fluid mud into overlying water on a falling or rising tide redeposition of mud may occur near time of high tide the strong attenuation of shallow water waves by these muds provides conditions that are favorable for further sedimentation high concentrations of suspended fluid mud together with solitary like waves from the northeast throughout the year can lead to extraordinarily high net sediment transport rates in the nearshore zone calculations based on solitary wave theory and on data obtained from this study indicate that 15 65 106 m3 of mud can move along shore each year without involving breaking waves the concept of radiation stress and a nearshore circulation cell or bedload transport farther offshore outside the zone of wave dominance wind driven currents and the guiana current combine to transport muds to the northwest consistent with the observed direction of mudflat migration

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

plate boundary reorganization at a large offset rapidly propagating rift the existence of rapidly spinning microplates along the southern east pacific rise has been documented by geophysical swath mapping surveys1 6 and their evolution has been successfully described by an edge driven kinematic model7 but the mechanism by which such microplates originate remains unknown proposed mechanisms1 10 have generally involved rift propagation11 possibly driven by hotspots or changes in direction of sea floor spreading here we present geophysical data collected over the earth s fastest spreading centre the pacific nazca ridge between the easter and juan fernandez microplates fig 1 which reveal a large offset propagating rift presently reorganizing the plate boundary geometry a recent episode of rapid duelling propagation of the historically failing spreading centre in this system has created a 120 120 km overlap zone between dual active spreading centres which may be the initial stage of formation of a new microplate

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

structure and metamorphism of the archean murchison belt kaapvaal craton south africa critical examination of five popular prejudices applied to greenstone belts in general and the murchison belt in particular show that 1 the term murchison schist belt is preferred to that of murchison greenstone belt which differs significantly from the barberton greenstone belt 2 the murchison belt is tectonized such that no simple stratigraphy may be erected 3 the murchison belt lacks a synclinal keel 4 heterogeneous oblique to dip slip shear characterizes much of the murchison belt but strike slip deformation occurred in the letaba shear zone along the northeastern margin of the belt and 5 contact metamorphism was a limited phenomenon that was not responsible for the highest grade rocks of the murchison belt the belt comprises a number of domains each with its own characteristic lithologies structural style and metamorphic history bounded by tectonic breaks which have juxtaposed rocks from different crustal levels

geochemistry and mineral formation in the earth surface edited by r rodriguez clemente and yves tardy proceedings of the intermational meeting geochemistry of the earth surface and processes of mineral formation held in granada spain 16 22 march 1986 csic madrid 1987 893p 7000 pesetas isbn 84 00 06683 9

geology of young submarine volcanoes west of easter island southeast pacific abstract submarine volcanoes probably related to the easter hotspot were found between the easter microplate and easter island near 26 15 s 111 w hydrosweep swath mapping accompanied by deep towed camera observations and sampling revealed the presence of fresh lava flows confined to two distinct fields the previously known ahu and the newly discovered umu volcanic fields covering a combined area of more than 4300 km 2 these lava fields form constructional ridges with volcanic cones as well as isolated seamounts another large volcanic area probably also belonging to the easter hotspot was found 100 km south of the ahu umu region thus the young hotspot region has a diameter of about 200 km and its centre probably lies underneath the umu volcanic field rather than under ahu volcanic field the fresh volcanics recovered from these areas are olivine and plagioclase phyric enriched and more rarely depleted tholeiites the enriched lavas are comparable to e morb from the easter microplate spreading axes and volcanic ridges and seamounts west of the microplate

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

nonlinear wave dynamics in the presence of mud induced dissipation on atchafalaya shelf louisiana usa abstract the interplay between wave nonlinearity and mud induced dissipation is studied here using wave and sediment transport measurements collected in winter 2008 on the atchafalaya shelf louisiana between the 8 m and the 4 m isobaths this study focuses on the relatively energetic storm that occurred on march 4th 2 m wave height in 8 m water depth which caused significant bed reworking and left in its wake a 20 cm layer of hindered settling fluid mud while the net wave dissipation rate was maximal during the hindered settling phase after the storm consistent with previous observations significant dissipation was observed throughout the storm duration with a secondary maximum associated with the peak of the storm and the maximum bed reworking effects the effects of mud induced dissipation on the nonlinear shoaling process are investigated here using triads a newly developed spectral model for nonlinear shoaling of waves with mud parameter values estimated using a crude inverse modeling approach the model was used to separate the effects of mud induced dissipation from nonlinear transfers during both the erosion and deposition phases of bed reworking numerical simulations show that nonlinear transfers are always active even under strong dissipation regimes remarkably they also suggest that heavy mud induced dissipation causes the nonlinear energy cascade to revert direction with bulk transfers occurring from high frequency to low frequency spectral bands this supports the hypothesis that the interplay between mud induced dissipation which is much more significant than the bottom dissipation over sandy beds and wave nonlinearity can drain energy from the entire spectrum and not just from the frequency bands that interact directly with the seafloor the decay of high frequency variance induced by the reversal of spectral flux in effect reduces the nonlinearity of the wave field

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

wave sediment interaction on a muddy inner shelf during hurricane claudette measurements of wave and suspended sediment concentration ssc were conducted near the 5 m isobath on the muddy inner shelf fronting atchafalaya bay louisiana during hurricane claudette the data show that wave and current activity resuspended large quantities of sediment with ssc r 0 5 kg m 3 throughout the water column in the waning phase of the storm settling generated a suspension layer with concentrations over 1 7 kg m 3 measured as high as 1 m above the bottom numerical simulations of post storm sediment settling showed that observations are consistent with a high density fluid mud layer ssc between 10 and 20 kg m 3 and separated from the upper water column by a lutocline located at about 1 m above the bottom the formation of the fluid mud layer is correlated with strong broad spectrum wave dissipation consistent with the hypothesis that surface interface wave interaction plays an important part in the energy transfer from the surface to the soft bottom 2004 elsevier ltd all rights reserved

evaluation of the bgm 3 sea gravity meter system onboard r v conrad the first bell aerospace bgm 3 marine gravity meter system available for academic use was installed on r v robert d conrad in february 1984 the bgm 3 system consists of a forced feedback accelerometer mounted on a gyrostabilized platform its sensor requiring no cross coupling correction is a significant improvement over existing beam and spring type sea gravimeters such as the gss 2 a gravity survey over the wallops island test range together with the results of subsequent cruises allow evaluation of the precision accuracy and capabilities of the new system over the test range the bgm 3 data were compared directly to data obtained by a gss 2 meter onboard r v conrad the rms discrepancy between free air gravity anomaly values at intersecting ship tracks of r v conrad was 0 38 mgal for bgm 3 compared to 1 60 mgal for the gss 2 moreover bgm 3 s platform recovered from abrupt changes in ship s heading more rapidly than did the platform of gss 2 the principal factor limiting the accuracy of se

evidence from en echelon cross grain ridges for tensional cracks in the pacific plate sea floor topography in the pacific is mainly aligned with original spreading directions1 but is overprinted by alignments created by mid plate processes spreading produces abyssal hills and fracture zones and mid plate volcanism generates seamounts isolated or in chains a different category of topography the cross grain discovered in geoid height data collected by the seasat radar altimeter2 comprises linear troughs and swells spaced 200 km apart oblique to fracture zones and abyssal hills but parallel to the hawaiian chain three models have been proposed for the cross grain small scale convection organized into longitudinal rolls by the shear of the pacific plate2 compressive buckling3 and lithospheric boudinage resulting from plate wide tensile stresses4 5 none of the previously available data ruled out any of these models here we report multi beam bathymetric data revealing long narrow en echelon ridges along the cross grain interpreted as evidence of tension cracks in the pacific plate

observations on cohesive bed reworking by waves atchafalaya shelf louisiana 1 wave current acoustic backscatter and suspended sediment concentration measurements both single point and vertical profiles estimated by conversion of acoustic backscatter data are used to investigate wave current cohesive sediment interaction on the muddy atchafalaya inner shelf during an energetic storm we propose that bed state follows a cycle of dilation due to fluidization erosion deposition with fluid mud formation and consolidation a one dimensional vertical cohesive sediment transport model is calibrated using current and concentration profiles to estimate the physical parameters that could not be measured directly e g bottom stresses estimated bed position and computed bottom stresses suggest that the critical erosion threshold is in the range of 0 3 pa to 0 5 pa the study site is impacted by a sediment laden fresh water plume coming from the atchafalaya river mouth bed density evolution during the storm is estimated from vertical sediment exchange between the water column and the bed excluding the duration of passage of a sediment carrying water front the values are in the range of 1 030 kg m3 to 1 200 kg m3and indicate that the bed density increases during the erosion phase and decreases during deposition at the end of the storm it shows a steady increasing trend during hindered settling and exceeds the space filling value during consolidation both the critical erosion shear stress and bed density values are consistent with the results of laboratory tests on samples from the experimental site

the structure of the nazca ridge and sala y gomez seamount chain from the dispersion of rayleigh waves summary we report single station measurements of group velocity for fundamental mode rayleigh waves that propagated along two aseismic ridges in the eastern pacific ocean basin the nazca ridge and the sala y gomez chain we find that the nazca ridge dispersion curves when regionalized to isolate the ridge contribution indicate group velocities of about 3 5 km s for short periods these values are slower than predicted for the geometry of the propagation paths interpreting this velocity heterogeneity is complicated by the recognition that the nazca ridge is about 1000 km longer than previously thought but models of vyv for the ridge derived by inverting the dispersion curves reveal abnormally thick 18 f 3 km oceanic crust along its longitudinal axis this value compares favourably with previously published results over the tuamotu plateau thus strengthening the hypothesis that the nazca ridge and tuamotu plateau are mirror images formed by a hotspot coincident with the farallon pacific spreading centre before 20 ma in contrast the measurements along the sala y gomez chain are consistent with typical oceanic crust and mantle when these values are inverted they reveal no thick crust 6 km along the sala y gomez chain we conclude from this result that the chain was likely formed as a leaky fracture zone not as a simple hotspot track

tectonic evolution of the easter microplate the plate tectonic history of the easter microplate has been reconstructed by closing the microplate in a series of steps using the pacific nazca magnetic anomalies north and south of the microplate and the nuvel 1 global plate motion model after each step the easter microplate was rotated rigidly to match the nazca and pacific anomalies gaps and overlaps formed by this kinematic treatment indicate compressional and tensional deformation respectively and show that rigid plate motions are insufficient to explain the complete tectonic evolution analysis of the magnetic anomaly data was guided by contoured seamarc ii sea beam and 3 5 khz bathymetry data and a lineament map derived from seamarc ii side scan and sea beam bathymetry data the patterns of lineaments and bathymetric structures suggest that rotational deformation of the nazca plate is the general mechanism that accommodates the space problems arising from transfer of the nazca plate to the microplate and rapid rotation of the microplate against the nazca plate similar but smaller amounts of deformation are predicted along the southern boundary of the microplate prior to the origin of the microplate the east pacific rise epr was offset in at least two places according to the older magnetic anomalies yet there is no evidence of linear fracture zones within the sparse data set except for occasional small consistent changes in regional depth across these age offsets the magnetic bathymetry and satellite altimetry data indicate that the microplate initially formed at or perhaps southeast of easter island near a left lateral offset of the epr sometime between anomaly 3 and 3 the east rift started propagating north from the present location of easter island at 4 5 ma which is 1 5 m y earlier than previously proposed however the magnetic data that support this interpretation are sparse and complicated by recent volcanic flows and associated rough bathymetry west of easter island the geometry of the microplate changes very rapidly during its evolution at the initial stages of development the microplate resembles a large propagating rift system suggesting that deformation may have been occurring throughout most of its interior up to about 2 47 ma at this time the length to width of overlap ratio of the two rifts reaches a value of 3 the northward propagation slows down the curved opening of the southwest rift becomes well established and rigid rotation of the previously deformed transferred lithosphere probably starts to predominate at this time the offset distance between the two overlapped rifts starts to increase some time after 2 47 ma and before 1 ma the east rift starts propagating northwestward probably in response to the microplate rotation and continues up until present also during this time period the east rift breaks into a series of northward propagating rifts each propagating into the microplate interior thereby transferring lithosphere from the microplate to the nazca plate and reducing the total growth rate of the microplate

the geology and tectonic evolution of the pietersburg greenstone belt south africa abstract the pietersburg greenstone belt occurs near the northern margin of the low grade granite greenstone terrain of the kaapvaal craton to the north lies the high grade granite greenstone terrain which constitutes the southern marginal zone of the limpopo belt the original relationship between the two terrains has been overprinted by the effects of the limpopo orogeny and are therefore not known a better understanding of the tectonic evolution of the pietersburg greenstone belt may help to clarify this relationship the rocks of the southwestern part of the pietersburg greenstone belt have been mapped in considerable detail the rocks can be subdivided into two sequences the first comprises a sequence of massive to pillowed metabasalts metagabbros and metaperidotites as well as bif fine grained mafic tuffs and sediments the sequence was subjected to extensive hydrothermal metamorphism at low pressures and variable temperatures in the greenschist to amphibolite facies this suggests an oceanic like environment the second sequence known as the uitkyk formation comprises a sequence of terrestrial clastics which overlie the first sequence with a profound unconformity sedimentology indicates that the uitkyk formation formed in a prograding alluvial fan or braided alluvial plain the uitkyk sediments were deformed and infiltrated by fluids that precipitated quartz tourmaline gold and siderite soon after deposition age relationships bedding cleavage relationships and younging directions indicate that both the sediments and simatic rocks were involved in a phase of thrust related deformation d2 that resulted in south over north stratigraphic repetition under generally low grade conditions of metamorphism post d2 granites date this thrusting at pre 2 69 ga d2 was probably diachronous and at surface the deformation was accompanied by syntectonic terrestrial sedimentation and placer gold deposition the tectonic environment was probably one of an intermontane accretionary or terrestrial foreland basin within this tectonic environment the basement and cover became tectonically stacked across evolving shear zones combined field and microstructural relationships within a major fault zone have revealed a complex history of progressive d2 deformation metamorphism mineralisation and associated sedimentary processes the zone represents a deformed syn sedimentary duplex that was carried piggy back above an allochthonous thrust sheet of simatic rocks at the deepest levels this fault and other shear zones were intruded by syntectonic granitoids this late archaean northward verging d2 thrust stack model for the greenstone belt with the syntectonic granites intruding the thrusts from beneath the greenstone belt is in good accord with the recent geophysical observations the age of cover sedimentation is older than 2 69 ga but younger than 2 98 ga in age this supports suggestions that these deposits are age equivalent of the upper gold bearing sediments of the witwatersrand basin and that the northwards thrusting in both basins may be of the same age and related to the same regional tectonic forces the simatic rocks of the pietersburg greenstone belt may have been part of a more extensive allochthonous simatic terrain which includes the similar greenstone remnants of the southern marginal zone of the limpopo belt such a terrain must have been disrupted and metamorphosed during the northward directed imbrication and tectonic burial recorded in the pietersburg belt prior to the climax of the limpopo orogeny the latter subsequently severely disrupted the pietersburg greenstone belt during ventersdorp times 2 6 2 7 ga

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

rough versus smooth topography along oceanic hotspot tracks observations and scaling analysis some hotspot tracks are topographically smooth and broad nazca carnegie cocos galapagos walvis iceland while others are rough and discontinuous easter sala y gomez tristan gough louisville st helena hawaiian emperor smooth topography occurs when the lithospheric age at emplacement is young favoring intrusive magmatism whereas rough topography is due to isolated volcanic edifices constructed on older thicker lithosphere the main controls on the balance of intrusive versus extrusive magmatism are expected to be the hotspot swell volume flux qs plate hotspot relative speed v and lithospheric elastic thickness te which can be combined as a dimensionless parameter r qs v 1 2 te which represents the ratio of plume heat to the lithospheric heat capacity observational constraints show that except for the ninetyeast ridge r is a good predictor of topographic character for r 3 they are smooth and dominated by intrusion

speed limit for oceanic transform faults oceanic transform faults with slip rates greater than 145 km m y do not currently exist along the east pacific rise where sea floor spreading rates range from 145 to 160 km m y instead offsets of the the very fast spreading east pacific rise are accommodated by microplates propogating rifts or overlapping spreading centers this suggests that there might be a speed limit above which transform faults do not exist a physical reason for a speed limit is not known but it might be related to unstable stress fields near the rifts tips causing them to episodically propagate and prevent a transform fault from being formed the spreading rates quoted are from our new 0 0 73 ma relative motion model for the pacific and nazca plates

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

multiple episodes of volcanism in the southern austral islands flexural constraints from bathymetry seismic reflection and gravity data 1 at the southeastern end of the cook austral island chain in the vicinity of the currently active macdonald seamount multiple episodes of volcanism have left a diverse population of seamounts multichannel seismic reflection and shipboard gravity data provide observational constraints on the magnitude and wavelength of flexure and multibeam bathymetry obtained by r v ewing and f s sonne within the study area provides full spatial coverage of all volcanic loads three dimensional flexural modeling of closely spaced loads emplaced at discrete times performed both with analytical models of idealized loads and with fourier domain solutions of the observed volcanic loads partitioned into the younger and older seamounts is compared with seismic and gravity data this modeling provides a more complete view of the volcanic history than radiometric dating and geochemical analysis of sparse dredge samples alone wide angle seismic refraction data from ocean bottom hydrophones obhs and expendable sonobuoys are also consistent with the flexural modeling results volume estimates of the different volcanic episodes show that one half to two thirds of the material added to the abyssal seafloor by midplate volcanic processes is due to older volcanism that erupted on young lithosphere while the younger higher seamounts contributed the remainder

the stratigraphy of the 3 5 3 2 ga barberton greenstone belt revisited a single zircon ion microprobe study abstract recent field and geochemical studies indicate a need to test the stratigraphy of the ca 3 5 ga barberton greenstone belt as it is presently adopted 1 2 this work uses the ion microprobe shrimp to attempt such a test results show that 1 volcaniclastic sediments of the theespruit formation

heave of a surficial rock layer due to pressures generated by injected fluids 1 this paper examines the problem of heave developed in a surface or caprock layer of a storage formation due to the injection of pressurized fluids at depth the problem is of interest to the deep geologic disposal of hazardous wastes and the geologic sequestration of carbon dioxide by injection the paper develops a mathematical model that assumes elastic behavior of the storage medium while the surface rock layer is modeled as a thin plate elastic solutions are developed for the axisymmetric flexure deflections of the surface layer during internal pressurization over a flat circular region the model provides a convenient tool for the preliminary estimation of the influence of injection pressures on the integrity of surface rock layers