mapping upper mantle flow beneath french polynesia from broadband ocean bottom seismic observations upper mantle structures and flow related to the south pacific superswell and to the associated short lived hotspots are investigated using seismic stations deployed in french polynesia beside island stations broadband ocean bottom seismometers bbobs have been deployed between the various polynesian archipelagos to investigate the oceanic upper mantle unperturbed by thickened crust or hotspot activity we investigate seismic anisotropy measured at bbobs stations and found it rather similar to that from island stations this confirms the ability of obs to be used for anisotropy purposes and suggests some homogeneity in the upper mantle pervasive deformation the two obs installed on the head of the society hotspot provide different anisotropy signatures than most of the other french polynesian stations one is isotropic to sks waves and the other provides a fast split direction normal to the average value observed in this area suggesting a more complex mantle flow induced by the plume upwelling

whole layer convection in a heterogeneous planetary mantle 1 geochemical observations demonstrate that earth s mantle is heterogeneous but the sizes forms and characteristics of its reservoirs are not constrained using laboratory experiments we have systematically studied a simple nonhomogeneous system where two layers of miscible fluids with different densities depths and viscosities are subjected to a destabilizing temperature contrast when the buoyancy number b i e the ratio of the stabilizing chemical density anomaly to the destabilizing thermal density anomaly is lower than 0 3 0 5 the whole layer regime develops with a deformed interface and convective patterns over the whole tank depth the system systematically evolves toward one fluid rayleigh benard convection because of stirring however the two isolated fluids can persist for very long time compared to the characteristic timescale of thermal convection and give rise to numerous transient behaviors of particular interest for planetary mantles are the pulsatory dynamics where the interface between the two layers deforms in large domes moving up and down quasiperiodically according to the scaling laws derived from the experiments such a mechanism could indeed provide a simple and single physical explanation for the superswells observed at present on earth and more generally for the long term episodicity in planetary interiors observed in geological records

circulation patterns in the late pennsylvanian north american midcontinent sea inferred from spatial gradients in sediment chemistry and mineralogy abstract the north american midcontinent sea nams reached its maximum extent during glacio eustatic highstands of the late paleozoic ice age during which organic rich core shales accumulated under a stratified water column with anoxic deep waters the core shales of cyclothems are composed of a lower organic rich laminated black shale and an upper bioturbated gray shale the present study investigates circulation patterns within the nams by examining secular variations and spatial gradients in the concentrations and ratios of major elements trace elements and organic carbon as well as carbon and nitrogen isotopes within one cyclothemic core shale the heebner shale the heebner shale represents the core shale member of the upper pennsylvanian lowermost gzhelian virgilian oread limestone formation in kansas and iowa and it is correlative with the teutopolis shale of the mattoon formation in illinois the heebner was sampled at high stratigraphic resolution at five sites on the northern midcontinent shelf and two sites in the illinois basin clay and silt fraction proxies show influences from multiple distinct terrestrial sources the paleosalinity proxy sr ba in combination with spatial patterns of clay and silt proxies implies large scale counterclockwise cyclonic rotation of the nams watermass which was possibly driven by the hydrodynamics of upwelling on the southern midcontinent shelf margin and or riverine discharge into the eastern nams spatial patterns of 13c and 15n are consistent with regionally variable productivity and redox conditions differences in the magnitude of a positive 15n excursion among the study sites record spatially variable water column denitrification rates enrichment of trace metals on the northern midcontinent shelf was a product of intensified water column recycling within an estuarine saltwater wedge trace metal enrichments and fe al ratios are consistent with moderate watermass restriction throughout the nams the results of the present study permit major refinement of the existing superestuarine circulation model for the nams algeo t j et al 2008 modern and ancient epeiric seas and the super estuarine circulation model of marine anoxia in b pratt and c holmden eds dynamics of epeiric seas geol assoc canada spec publ 48 pp 7 38

late pennsylvanian glaciation evidence of icehouse conditions from canyon and cisco units midland basin texas abstract the late paleozoic has long been recognized a time of significant continental glaciation associated with high frequency high amplitude glacioeustasy although precise assessment of glacioeustatic amplitudes are difficult to extract from the stratigraphic record best approximations may be possible by analyzing spatial and temporal distributions of depositional facies utilizing several wells across the study area we used detailed sedimentologic description and depositional facies analysis from 12 wells and applied rigorous cycle stacking pattern analysis exposure surfaces recognition and wireline logs to correlate timelines across the wells on the basis of the high resolution sequence stratigraphic framework built for the scurry area canyon reef operators sacroc field a highly resolved glacioeustatic curve was generated for latest desmoinesian to the latest virgilian our eustatic curve suggests that the missourian and virgilian stages of the late pennsylvanian are characterized by high frequency high amplitude eustatic fluctuations the missourian stage was a period dominated grainstone capped cycles that are isopachous and conformable these cycles reflect a eustatic amplitude of at least 37 m with an average duration of 100 ka suggesting the dominance of short term eccentricity signal the virgilian stage is far more complicated and is characterized by highly shingled cycles compared to the missourian stage deeper water phylloid boundstone and calichified shoal complex grainstones are juxtaposed next to each other which suggests a stronger eustatic amplitude 40 m compared to the missourian stage although the eustatic amplitude of the virgilian cycles is similar to those of the missourian cycles they have a longer frequency 343 ka representative of long term eccentricity we conclude that the late pennsylvanian stratigraphy of the sacroc field in the midland basin records extensive continental glaciation during the missourian stage and increased and reached an acme during the virgilian stage

cyclothem digital correlation and biostratigraphy across the global moscovian kasimovian gzhelian stage boundary interval middle upper pennsylvanian in north america and eastern europe the long standing difficulty of correlating pennsylvanian strata among provincial faunal regions is resolved by effecting digital correlation of major glacial eustatic cyclothems that represent high stands when certain species achieved more global distribution than usual in the late moscovian early gzhelian late desmoinesian early virgilian succession in the midcontinent united states several major cyclothems are correlated by both conodont species in common and cyclothem scale with cyclothems in russia moscow basin and ukraine donets basin and the remaining cyclothems fit into the framework by position and scale in this way the suggested event marker for the global kasimovian gzhelian stage boundary first appearance of idiognathodus simulator is supported while possible event markers for the moscovian kasimovian boundary await further evaluation

on three dimensional gerstner like equatorial water waves this paper reviews some recent mathematical research activity in the field of nonlinear geophysical water waves in particular we survey a number of exact gerstner like solutions which have been derived to model various geophysical oceanic waves and wave current interactions in the equatorial region these solutions are nonlinear three dimensional and explicit in terms of lagrangian variables this article is part of the theme issue nonlinear water waves

upper mantle flow beneath french polynesia from shear wave splitting summary upper mantle flow beneath the south pacific is investigated by analysing shear wave splitting parameters at eight permanent long period and broad band seismic stations and 10 broad band stations deployed in french polynesia from 2001 to 2005 in the framework of the polynesian lithosphere and upper mantle experiment plume despite the small number of events and the rather poor backazimuthal coverage due to the geographical distribution of the natural seismicity upper mantle seismic anisotropy has been detected at all stations except at tahiti where two permanent stations with 15 yr of data show an apparent isotropy the median value of fast polarization azimuths n67 5 w is parallel to the present pacific absolute plate motion direction in french polynesia apm n67 w this suggests that the observed sks fast polarization directions result mainly from olivine crystal preferred orientations produced by deformation in the sublithospheric mantle due to viscous entrainment by the moving pacific plate and preserved in the lithosphere as the plate cools however analysis of individual measurements highlights variations of splitting parameters with event backazimuth that imply an actual upper mantle structure more complex than a single anisotropic layer with horizontal fast axis a forward approach shows that a two layer structure of anisotropy beneath french polynesia better explains the splitting observations than a single anisotropic layer secondorder variations in the measurements may also indicate the presence of small scale lateral heterogeneities the influence of plumes or fracture zones within the studied area does not appear to dominate the large scale anisotropy pattern but may explain these second order splitting variations across the network

quantitative radiometric and biostratigraphic calibration of the pennsylvanian early permian cisuralian time scale and pan euramerican chronostratigraphic correlation a quantitative biostratigraphic and radiometric calibration is presented for the pennsylvanian through early permian global time scale based upon high precision isotope dilution thermal ionization mass spectrometer id tims u pb zircon ages for interstratified ash beds in the parastratotype sections of the southern urals of russia twenty four ash bed ages in three outer ramp and basinal sections of the pre uralian foredeep bracket the biotic definitions of global stages and regional substages from the base of the upper pennsylvanian kasimovian stage to the base of the lower permian artinskian stage four additional ash bed ages in two sections of the eastern slope of the urals constrain the global bashkirian and serpukhovian stages quantitative stratigraphic methods conop9 are applied to a compilation of over 2000 bioevents in 22 stratigraphic sections supplemented by our dated volcanic horizons to refine the pennsylvanian early permian global time scale significant shifts in the duration of several stages are demonstrated ranging from one to six million years compared with prior estimates the unprecedented density of radiometric calibration points for the pennsylvanian permian transition provides a high resolution 0 1 ma global chronostratigraphic standard for testing and improving biostratigraphic correlations across euramerica we integrate radiometric ages biostratigraphic correlation and cyclostratigraphic tuning of major cyclothems to the long period 404 ka eccentricity cycle to elucidate the tempo magnitude and forcing of eustatic changes and cyclothemic deposition associated with the waxing and waning of gondwanan ice sheets and establish a pan euramerican chronostratigraphic framework for most of pennsylvanian and early permian time

new insights on intraplate volcanism in french polynesia from wavelet analysis of grace champ and sea surface data in this paper we discuss the origin of superswell volcanism on the basis of representation and analysis of recent gravity and magnetic satellite data with wavelets in spherical geometry we computed a refined gravity field in the south central pacific based on the grace satellite ggm02s global gravity field and the kms02 altimetric grid and a magnetic anomaly field based on champ data the magnetic anomalies are marked by the magnetic lineation of the seafloor spreading and by a strong anomaly in the tuamotu region which we interpret as evidence for crustal thickening we interpret our gravity field through a continuous wavelet analysis that allows to get a first idea of the internal density distribution we also compute the continuous wavelet analysis of the bathymetric contribution to discriminate between deep and superficial sources according to the gravity signature of the different chains as revealed by our analysis various processes are at the origin of the volcanism in french polynesia as evidence we show a large scale anomaly over the society islands that we interpret as the gravity signature of a deeply anchored mantle plume the gravity signature of the cook austral chain indicates a complex origin which may involve deep processes finally we discuss the particular location of the marquesas chain as suggesting that the origin of the volcanism may interfere with secondary convection rolls or may be controlled by lithospheric weakness due to the regional stress field or else related to the presence of the nearby tuamotu plateau

testing periodicity of depositional cyclicity cisco group virgilian and wolfcampian texas abstract periodicities of 15 cyclic sections of the cisco group are tested through gamma and spectral analyses the sections are composed of marine and nonmarine carbonate and siliciclastic rocks deposited on the eastern shelf north central texas in late pennsylvanian and early permian time gamma analysis generates facies dependent thickness time conversion factors s to construct corrected time series which are more realistic than the uncorrected series obtained by proportional conversion positive and stable s and improved resolution of the corrected spectra indicate successful tuning on the cisco spectra a short eccentricity peak 95 131 ky is the most prominent and persistent long eccentricity 413 ky and obliquity 34 and 43 ky peaks are persistent and precessional index peaks 17 and 21 ky are less persistent a persistent peak of period 50 80 ky may be a combination tone between obliquity and precessional index cycles non milankovitch peaks are sporadic the validity of gamma and spectral results is indicated by consistent estimated ages among the cisco sections and reasonable effective sedimentation rates calculated on the basis of milankovitch spectral calibrations the results combined with geologic evidence suggest that the cisco cyclicity was controlled dominantly by milankovitch climatic forcing

extent of the south pacific superswell 1 the south pacific superswell is known as a broad area beneath french polynesia characterized by numerous volcanic chains and very shallow seafloor compared to the depth predicted for its age by classical seafloor subsidence models so far its exact extent has not yet been established thanks to better bathymetric coverage and a specially adapted filtering method we present here a new complete and precise mapping of the superswell we show that the superswell covers a region broader than previously expected it extends between latitudes 10 n and 30 s and longitudes 130 w and 160 w and has a maximum amplitude of 680 m it is composed of two branches that display different characteristics when compared to other geophysical observations under the southern branch a dip in the geoid is observed which could be linked to upwelling in a convective mantle where the low velocity zone is located immediately below the lithosphere as indicated by tomography models under the northern branch there is a 12 m high in the geoid surface anomaly that may be explained by isostatic compensation of a mass deficiency located in the mantle at 300 km depth the northern branch could also be related to another superplume that is now going down but which was rising up 30 90 m y ago when it initiated the secondary plumes which created the line islands

structure of axisymmetric mantle plumes results of high resolution numerical calculations of axisymmetric thermal plumes in an infinite prandtl number fluid with thermally activated viscosity are used to infer the structure of axisymmetric subsolidus thermal plumes in the earth s lower mantle we calculate the velocity and temperature distribution for axisymmetric convection above a heated disk in an incompressible fluid cylinder 2400 km in height and 1200 km in diameter initially at a uniform temperature t the lower boundary representing the plume source region near the core mantle boundary is stress free and maintained at temperature tc t the upper boundary is permeable allowing the plume to escape the cylinder we determine the sensitivity of plume structure to viscosity variations c ranging from 102 to more than 104 and temperature differences t tc t in the range 400 850 k starting plumes consist of a large leading diapir and a trailing conduit connected to the thermal boundary layer above the heated surface the leading diapir expands during ascent to about 400 km diameter at 2200 km height in steady state the plume consists of a narrow high velocity conduit imbedded within a broader thermal halo the width of both these structures is proportional to height above the heated boundary and inversely proportional to t and the viscosity ratio heat transport in the plume is nearly independent of viscosity variations a simplified boundary layer model yields a heat transfer law nu ra where nu is the plume nusselt number and ra is the rayleigh number based on t and the radius of the heated circular region feeding the plume calculated steady state plume heat transports fit this law to within a few percent we present several calculations of plumes with heat transport in the range 100 400 gw similar to the advective heat transport by the hawaiian hotspot at the top of the lower mantle plumes with large viscosity variations have high velocity conduits 50 60 km in diameter and 125 km diameter thermal halos a centerline temperature anomaly of about 500 k and a centerline velocity of about 75 cm yr 1 plumes with weak viscosity variations are approximately twice as broad and have centerline temperature anomalies less than 160 k at the same height and much smaller velocity large viscosity variations and a minimum heat transport are both

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

sequence stratigraphy and onlap history of the donets basin ukraine insight into carboniferous icehouse dynamics abstract the degree to which permo carboniferous cyclothemic successions archive evidence for long term variations in ice volume during the late paleozoic ice age is insufficiently resolved here we develop the sequence stratigraphy and onlap offlap history for a 33 my interval of the carboniferous using the u pb calibrated succession of the donets basin ukraine in order to assess the relationship between sea level high latitude changes in glacial extent and climate integrated subsurface and outcrop data permit meter scale correlation of 242 biostratigraphically constrained limestones and coals and in turn individual cyclothems across 250 km of the donets basin rapid uniform subsidence and basinwide continuity of marker beds indicate pennsylvanian deposition under relatively stable tectonic conditions three scales of sequences avg durations of 140 ky 480 ky and 1 6 my are recognized on the basis of stratigraphic stacking patterns and basinwide architecture of marine to terrestrial facies assemblages the hierarchy of sequences and the geographic and stratigraphic positions of shifts in base level sensitive facies across the donets ramp permit the construction of an onlap offlap history at a sub 400 ky scale major sea level lowstands occur across the mid carboniferous boundary and during the early moscovian these lowstands coincide with glacial maxima inferred from high latitude glacigenic deposits the middle to late pennsylvanian is characterized by a stepwise onlap culminating in an earliest gzhelian highstand suggesting contraction of carboniferous ice sheets prior to the initiation of early permian glaciation the stratigraphic position of climate sensitive facies within individual donets cyclothems indicates a turnover from seasonal sub humid or semi arid climate to everwet conditions during the late lowstand and maximum ice sheet accumulation comparison of the stratigraphic and aerial distribution of coals and evaporites in the donets basin with the onlap offlap history further indicates everwet conditions during lowstands and inferred glacial maxima and drier climate during onlap and inferred ice sheet contraction at the intermediate 0 8 to 1 6 my and long 106 yr time scales taken together the relationship between inferred climate and glacioeustasy suggests a likely teleconnection between high latitude ice sheet behavior and low latitude atmospheric dynamics

distinguishing the roles of autogenic versus allogenic processes in cyclic sedimentation cisco group virgilian and wolfcampian north central texas meter scale transgressive regressive cycles of the subsurface cisco group are composed of marine and nonmarine carbonate and siliciclastic rocks deposited on the eastern shelf of the midland basin during late pennsylvanian and early permian time five cycle types are characterized by thickness magnitude order and principal lithofacies cycle magnitude is defined as the maximum facies shift in a cycle indicating extent of shoreline migration cisco cycles belong to three orders minor intermediate and major and they are superimposed and form a stratigraphic hierarchy each order of cycles has a distinct range of thickness and possibly duration a cycle is also divided into a lower sand poor interval during which coarse siliciclastic supply at the depositional site was diminishing and an upper sand rich interval during which coarse siliciclastic supply was high regional thickness and lithofacies variations of sand rich intervals indicate that progradational infilling at a depositional site lagged marine regression suggesting a delay in sediment supply from the upland source relative to the time of base level fall regional systematic variations in cycle abundance continuity and characteristics along depositional dip and strike record the interplay among regional topography pattern of siliciclastic supply and shelf subsidence which controlled distribution of depocenters and bypass zones and thus stratigraphic completeness and resolution regional persistence of cycles suggests a eustatic control on regional ordered transgressive regressive events in contrast local variations of cycle characters suggest controls by local topography and depositional dynamics which determined depositional loci differential compaction and erosion a predominantly autocyclic cisco record in the upper platform does not imply the absence of allogenic processes an allocyclic cisco record in the lower platform contains abundant autocyclic imprints because allogenic controls on cyclic sedimentation were accomplished through local autogenic processes distinguishing the roles of autogenic versus allogenic processes in cyclic sedimentation is an important step in establishing a high resolution meter scale chronostratigraphy of any sedimentary record 1

the relationship between gravity and bathymetry in the pacific ocean surface ship and satellite derived data have been compiled in new free air gravity anomaly bathymetry and geoid anomaly maps of the pacific ocean basin and its margin the maps are based on smoothed values of the gravity anomaly bathymetry and geoid interpolated on to a 90 x 90km grid the smoothed values of bathymetry and geoid anomaly have been corrected for age the resulting maps show the pacific ocean basin is associated with a number of ene wsw trending geoid anomaly highs with amplitudes of approx or 5m and wavelengths of approx 3000km the most prominent of these highs correlate with the magellan seamounts marshall gilbert islands magellan rise and the hess rise hawaiian ridge regions the correlation between geoid anomaly and bathymetry cannot be explained by models of static compensation but is consistent with a model in which the geoid anomaly and bathymetry are supported by some form of dynamic compensation suggests that the dynamic compensation which characterizes ocean lithosphere older than 80ma is the result of mantle convection on scales that are smaller than the lithospheric plates themselves from authors

small scale sublithospheric convection reconciles geochemistry and geochronology of superplume volcanism in the western and south pacific abstract cretaceous volcanism in the west pacific seamount province wpsp and tertiary volcanism along the cook australs in the south pacific are associated with the same broad thermochemical anomaly in the asthenosphere perhaps related to the pacific superplume abundant volcanism has usually been attributed to secondary plumelets rising from the roof of the superplume the cook australs display distinct geochemical trends that appear to geographically project backward in time to corresponding trends in the wpsp however the implied close proximity of source regions i e 1000 km with very different geochemical fingerprints and their longevity over geological time 100 myrs appear to be at odds with the secondary plumelet hypothesis a mechanism with a typical timescale of 30 myrs moreover ages sampled along the individual volcano chains of the cook australs and of the wpsp violate the predictions of the plumelet hypothesis in terms of linear age distance relationships our numerical models indicate that small scale sublithospheric convection ssc as likely triggered by the thermochemical anomaly of the superplume instead reconciles complex age distance relationships because related volcanism occurs above elongated melting anomalies parallel to plate motion hot lines furthermore ssc melting of a mantle source that consists of pyroxenite veins and enriched peridotite blobs in a matrix of depleted peridotite creates systematic geochemical trends over seafloor age during volcanism these trends arise from variations in the amount of pyroxenite derived lavas relative to peridotite derived lavas along a hot line therefore stretching between the geochemical end members himu and emi these predicted trends are consistent with observed trends in radiogenic isotopic composition from the wakes marshalls gilberts i e the individual volcano groups of the wpsp and the cook australs for increasing mantle temperatures volcanism is further predicted to occur at greater seafloor ages and with a more emi like signature a result that can explain many of the observed systematics thus ssc explains many of the geochemical observations with long term temporal variations in mantle temperature instead of persistent intermediate scale 1000 km compositional heterogeneity

the relationship of conodont biofacies to spatially variable water mass properties in the late pennsylvanian midcontinent sea molybdenum and uranium enrichment factors and nitrogen isotopes suggest that an interplay of open ocean upwelling and riverine runoff led to distinct spatial and secular variations in water mass properties within the epicontinental late pennsylvanian midcontinent sea of north america in particular the intensity of continental runoff influenced the flux of bulk organic matter to the sediment benthic anoxia appears to have been controlled by the vertical density gradient in the water column associated with continental runoff combined with the advection of basinal water anoxic conditions were stronger in proximal i e more shoreward areas of the midcontinent shelf indicating that anoxia did not develop primarily due to upwelling of nutrient rich waters along the southern shelf margin as previously suggested changes in water mass redox conditions not only drove authigenic enrichment of redox sensitive trace elements across the basin but also had a strong effect on the spatial distribution of various conodont taxa our analysis suggests that the widely accepted depth stratification model for the distribution of conodonts is incomplete conodont biofacies distributions seem to have been controlled by physicochemical properties of the water mass e g salinity temperature nutrients turbidity and or dissolved oxygen levels that may correspond less directly to water depth the proximity to terrestrial freshwater influx and the strength of anoxia euxinia in the subpycnoclinal water mass played significant roles in the spatial and temporal distributions of conodont taxa

effects of sedimentation tectonics and glacio eustasy on depositional sequences pennsylvanian minturn formation north central colorado the pennsylvanian minturn formation was deposited adjacent to the ancestral front range on the eastern margin of the actively subsiding central colorado basin during a time of high frequency eustatic sea level changes attributed to glaciations in the southern hemisphere in the 100 km2 study area near mccoy colorado up to nine depositional cycles composed of marine and nonmarine deposits are recognized three tectonic blocks were subsiding at different rates in the study area detailed correlation of measured sections along well exposed outcrop belts has shown that seven of these cycles extend continuously across the field area which includes two delta complexes two cycles are absent on the central block the study area is well suited for examining the role of local tectonic vs eustatic processes in controlling the distribution of sedimentary deposits in the study area cycles are bounded by erosional surfaces with as much as 43 m of relief these surfaces are inferred to be sequence boundaries and are overlain by braided fluvial conglomerates inferred to be incised valley fill deposits marine flooding surfaces overlie the conglomerates the copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received january 17 1996 revised manuscript received october 18 1996 final acceptance march 20 1997 2department of geological sciences university of colorado at boulder campus box 250 boulder colorado 80309 0250 present address department of geology university of colorado at denver campus box 172 p o box 173364 denver colorado 80217 3364 i thank the members of my doctoral dissertation committee at the university of colorado ted walker martin lockley alan scott david budd and john pitlick for their advice and guidance during the course of my research financial support was provided by the rocky mountain section of sepm the colorado mountain club union pacific resources and the department of geological sciences at colorado university boulder charles ross provided fusulinid identifications and biostratigraphic zonation i thank paul weimer colorado university boulder for advice and encouragement and keith miller kansas state university for carefully reviewing a preliminary draft of this paper aapg reviewers william devlin romeo flores denise stone and elected editor kevin biddle provided many suggestions that greatly improved the final manuscript i also thank the residents of mccoy bond and copper spur colorado for granting access to their property and for all their kindness while i was working in the field

mantle dynamics and generation of a geochemical mantle boundary along the east pacific rise pacific antarctic ridge a large scale mantle compositional discontinuity was identified along the east pacific rise epr and the pacific antarctic ridge par with an inferred transition located at the epr 23 degrees s 32 degrees s because of the epr easter hotspot interactions in this area the nature of this geochemical discontinuity remains unclear iodp sites u1367 and 1j1368 drilled into the ocean crust that was accreted at similar to 33 5 ma and similar to 13 5 ma respectively between 28 degrees s and 30 degrees s on the epr we use lavas from sites u1367 and u1368 to track this mantle discontinuity away from the epr the mantle sources for basalts at sites u1367 and u1368 represent respectively northern and southern pacific mantle sub domains in terms of sr nd pb hf isotopes the significant isotopic differences between the two iodp sites are consistent with addition of ancient subduction processed ocean crust to the south pacific mantle sub domain our modeling result shows that a trace element pattern similar to that of u1368 e morb can be formed by melting a subduction processed typical n morb the trace element and isotope compositions for site u1368 morbs can be formed by mixing a himu mantle end member with site u1367 morbs comparison of our data with those from the epr par shows a geochemical mantle boundary near the easter microplate that separates the pacific upper mantle into northern and southern sub domains on the basis of reconstruction of initial locations of the ocean crust at the two sites we find that the mantle boundary has moved northward to the easter microplate since before 33 5 ma a model in which along axis asthenospheric flow to where asthenosphere consumption is strongest explains the movement of the apparent mantle boundary crown copyright c 2013 published by elsevier b v all rights reserved

the influence of interior mantle temperature on the structure of plumes heads for venus tails for the earth 1 to form mantle plumes with large heads and narrow trailing conduits requires large 102 viscosity variations within the hot thermal boundary layer at the plume source however new and published data from laboratory experiments show that if a layer of fluid with a temperature dependent viscosity is heated from below and cooled from above convection occurs beneath a stagnant lid and the interior fluid temperature remains close to that of the hot boundary consequently the viscosity reduction across the hot thermal boundary layer is always 10 and upwellings take the form of discrete plume heads thermals subduction of the stagnant layer equivalent to recycling lithospheric plates on the earth will cool the interior of the mantle thus increasing the viscosity ratio hence the existence of earth like mantle plumes may be a consequence of plate tectonics an absence of plate tectonics on venus may lead to upwellings composed of thermals the surface expression of which are coronae as well as a smaller core heat flux than on earth

modal depth anomalies from multibeam bathymetry is there a south pacific superswell a region west of the southern east pacific rise sepr between the marquesas and austral fracture zones has previously been found to exhibit anomalous depth age behavior based on gridded bathymetry and single beam soundings since gridded bathymetry has been shown to be unsuitable for some geophysical analysis and since the area is characterized by unusually robust volcanism the magnitude and regional extent of depth anomalies over the young eastern flank of the so called south pacific superswell are re examined using a mode seeking estimation procedure on data obtained from several recent multibeam surveys the modal technique estimates a representative seafloor depth based on the assumption that bathymetry from non edifice and edifice populated seafloor has a low and a high standard deviation respectively flat seafloor depth values are concentrated in a few bins which correspond to the mode this method estimates a representative seafloor value even on seafloor for which more than 90 of coverage is dominated by ridge and seamount clusters where the mean and median estimates may be shallow by hundreds of meters where volcanism related bias is moderate the mode mean and median estimates are close depth age results indicate that there is only a small anomaly 200 m over is 35 ma pacific plate seafloor with little age dependent shallowing suggesting that the lithosphere east of the main hot spot locations on the superswell is normal an important implication is that in sparsely surveyed areas depths from etopoj are significantly different from true depths even at large scales 1000 km and thus are unsuitable for investigations of anomalies associated with depth age regressions we find that seafloor slopes on conjugate profiles of the pacific and nazca plates from i5 to 35 ma are both slightly lower than normal but are within the global range proximate to the sepr seafloor slopes are very low 218 m myr 2 on the pacific plate o 22 ma and slightly high 410 m myr i21 on the nazca plate o 8 ma slopes for older pacific seafloor 22 37 ma are near normal 399 m myr i seafloor slopes are even lower north of the marquesas fracture zone but are highly influenced by the marquesas swell we find that the low subsidence rate on young pacific seafloor cannot be explained by a local hot spot or a small scale convective model exclusively and a stretching thickening model requires implausible crustal thickness variation 30

thin widespread pennsylvanian black shales of midcontinent north america a record of a cyclic succession of widespread pycnoclines in a fluctuating epeiric sea

low accommodation detrital apron alongside a basement uplift pennsylvanian of midcontinent north america abstract the northern end of the 650 km long nemaha uplift nebraska and kansas usa is an important example of basin margin sedimentation in the north american midcontinent an apron of coarse basal pennsylvanian arkosic clastic sediments bpc was deposited on the flanks of the uplift while marine cyclothems were encroaching from the east small scale fining upward intervals many with demonstrably erosional bases dominate the bpc and are interpreted as overridingly fluvial in origin weak paleosols desiccation cracks and reddened intervals in the bpc record episodic subaerial exposure multiple burrowed horizons and heterolithic strata of probable tidal origin and rare marine fossils also indicate episodic marine influence the bpc appear to have been deposited as a thin apron of coalesced alluvial fans and fan deltas deposition of the bpc occurred during the waning of uplift and subsequent quiescence the comparative thinness and large scale packaging of the bpc are compatible with the controlling effects of relict relief regional subsidence and eustasy rather than ongoing major vertical displacements along active faults a strong autocyclic influence on sedimentation is evidenced by stacked fining upward intervals of poorly sorted conglomerates sandstones and sandy mudstones correlations demonstrate that the accumulation of the bpc took place over more than seven major sea level cycles beginning in cherokee group times middle moscovian middle pennsylvanian and ending only when the eroded uplift was inundated and buried by marine cyclothems on the basis of local correlations with marine cyclothems and using black phosphatic shales so called core shales of heckel p h 1986 sea level surve for pennsylvanian eustatic marine transgressive regressive depositional cycles along midcontinent outcrop belt north america geology 14 330 334 heckel p h 1994 evaluation of evidence for glacio eustatic control over marine pennsylvanian cyclothems in north america and correlation of possible tectonic effects in dennison j m ettensohn f r eds tectonic and eustatic controls on sedimentary cycles sepm concepts in sedimentology and paleontology no 4 pp 5 87 as marker beds we speculate that the bpc exist in backstepping sequences and or parasequences alongside the flanks of the nemaha uplift the bpc are lithologically comparable to contemporaneous deposits alongside the ancestral rockies amarillo wichita uplift and other buried basement highs in north america nonetheless the bpc are an order of magnitude thinner are dominantly retrogradational rather than progradational and their occurrence was not associated with major displacements along basin bounding faults in this manner the bpc are a useful example of low accommodation eustasy dominated coarse grained terrigenous clastic deposition around an uplifted basement block

from cyclothems to sequences the record of eustasy and climate on an icehouse epeiric platform pennsylvanian permian north american midcontinent the stratigraphic succession of the north american midcontinent contains a high resolution 100 000 year record of sea level and climate change spanning the pennsylvanian permian boundary outcrop based sequence stratigraphic analysis on the upper wabaunsee admire and lower council grove groups in kansas and nebraska reveals a hierarchy of fifty one fifth order meter scale cycles grouped into five fourth order composite sequences fifth order cycles are bounded by subaerial unconformities in nearshore settings and correlative conformities in offshore settings and are therefore very thin depositional sequences lowstand systems tracts observed only in nearshore deposits are expressed as sandy incised valley fills in these meter scale cycles transgressive systems tracts which are dominated by carbonate deposition include coastal evaporites indicating arid climatic conditions highstand systems tracts which are dominated by siliciclastic deposition include thin but persistent coals in muddy coastal successions indicating humid climatic conditions paleosols at sequence boundaries evolve from vertic to calcic indicating climate change from relatively humid to relatively arid conditions during subaerial exposure this indicates that relatively arid climate coincided with eustatic lows while relatively humid climate coincided with eustatic highs on this low latitude platform composite sequences are bounded by subaerial angular unconformities and display a transgressive regressive stacking pattern of meter scale cycles the high resolution stratigraphic record of the midcontinent helps constrain the rate and magnitude of environmental change in ancient icehouse conditions and can serve as a baseline of environmental change for comparison with other coeval successions

some nonlinear equatorially trapped nonhydrostatic internal geophysical waves abstractthe author presents an explicit exact solution to the governing equations for geophysical equatorial waves in the plane setting the solution describes equatorially trapped waves propagating eastward above the thermocline and beneath the near surface layer where wind effects are confined at great depths the water is still while the transition toward the large amplitude oscillation of the thermocline is accommodated by an eastward flowing current above the thermocline a flow reversal occurs with the underlying current flowing westward close to the layer where wind effects are confined

seismic azimuthal anisotropy in the oceanic lithosphere and asthenosphere from broadband surface wave analysis of obs array records at 60 ma seafloor we analyzed seismic ambient noise and teleseismic waveforms of nine broadband ocean bottom seismometers deployed at a 60 ma seafloor in the southeastward of tahiti island the south pacific by the tomographic investigation by seafloor array experiment for the society hotspot project we first obtained one dimensional shear wave velocity model beneath the array from average phase velocities of rayleigh waves at a broadband period range of 5 200 s the obtained model shows a large velocity reduction at depths between 40 and 80 km where the lithosphere asthenosphere boundary might exist we then estimated shear wave azimuthal anisotropy at depths of 20 100 km by measuring azimuthal dependence of phase velocities of rayleigh waves the obtained model shows peak to peak intensity of the azimuthal anisotropy of 2 4 with the fastest azimuth of nw se direction both in the lithosphere and asthenosphere this result suggests that the ancient flow frozen in the lithosphere is not perpendicular to the strike of the ancient mid ocean ridge but is roughly parallel to the ancient plate motion at depths of 20 60 km the fastest azimuths in the current asthenosphere are subparallel to current plate motion at depths of 60 100 km additional shear wave splitting analysis revealed possible perturbations of flow in the mantle by the hot spot activities and implied the presence of azimuthal anisotropy in the asthenosphere down to a depth of 190 210 km

late cretaceous sea level from a paleoshoreline the contemporary elevation of an ancient strandline provides a measure of eustasy at a single time in the geologic past if the elevation of a paleoshoreline has not changed since deposition or if it has changed by a known amount then the measured or corrected elevation equals the sea level at that time the ideal location for measuring an ancient sea level is a paleoshoreline deposited high on the shoulder of a stable craton where postdepositional elevation changes through tectonic movement lithospheric compensation to loads and sediment compaction are minimized the contemporary elevation of a late cenomanian 93 ma shoreline was determined at five localities along the tectonically stable eastern margin of the cretaceous western interior seaway north america this shoreline represented by marine to nonmarine facies transitions in strata of the greenhorn sequence uza 2 cycle of haq et al 1987 was identified from outcrop and borehole data biostratigraphic zonations constrained the geologic age at each locality sequence stratigraphic correlations based on identifying discrete progradational units and the surfaces that separate them were used to refine age correlations to better than 100 kyr between localities a single cenomanian shoreline was correlated within a single progradational unit and its elevation was determined at five localities this paleostrandline occurs 265 286 m above present day sea level at an average elevation of 276 m isostatic and flexural corrections were applied to remove the effects of postdepositional vertical movement including sediment compaction by loading uplift due to erosion and glacial loading and rebound errors inherent in each measurement and each correction were estimated corrections and their cumulative error estimates yield a late cenomanian elevation of 269 87 m above present sea level the corrected elevation approximates sea level at 93 ma and provides a measure of late cenomanian eustasy prior to the early turonian highstand establishing the absolute value for eustasy at a single point in geologic time provides a frame of reference for calibrating relative sea level curves as well as constraining the magnitudes of tectonic subsidence sediment flux and othervariables that controlled water depth and relative sea level

mantle transition zone structure beneath the south pacific superswell and evidence for a mantle plume underlying the society hotspot abstract underside reflections of shear waves from the discontinuities at 410 and 660 km depth are used to map lateral variations in the thickness of the mantle transition zone beneath the south pacific superswell and surrounding regions differential travel times of reflected waves indicate that the transition zone is about 25 km thinner than normal and thus hotter than normal over an area 500 km or less in diameter beneath the society hotspot there is no general difference however in transition zone thickness between the superswell area and its surroundings our observations support the inference that the thermal flux from lower to upper mantle beneath the south pacific superswell occur on the lateral scale of a mantle plume several hundred kilometers rather than that of a superplume several thousand kilometers

south pacific hotspot swells dynamically supported by mantle flows the dynamics of mantle plumes and the origin of their associated swells remain some of the most controversial topics in geodynamics here we construct a numerical model of the mantle flow beneath the french polynesia region our study is based on a new regional seismic tomography model which high resolution allows obtaining information at the scale of plumes we find excellent correlations between the observed and the modeled dynamic swells between the modeled flow pattern and the active volcanism and between the buoyancy fluxes obtained from our numerical model and the ones deduced from the swells morphology these outstanding fits reveal for the first time that a direct link exists between the surface observations and mantle flows

observations of the 2004 and 2006 indian ocean tsunamis from a pressure gauge array in indonesia 1 five shallow pressure gauges located in straits in the southern indonesian islands were used to evaluate tsunami signals triggered by the earthquakes off the northwest coast of sumatra in december 2004 and the south coast of java in july 2006 tsunami waves reached the pressure gauges around 5 to 6 hours after the 2004 earthquake the largest waves arrived 10 to 23 hours later with amplitudes ranging from 9 to 25 cm after the 2006 earthquake tsunami arrivals were only evident at the ashmore and roti pressure gauges in timor passage at these two gauges the first waves arrived around 2 25 hours after the earthquake and the largest waves arrived 2 to 3 hours later with amplitudes of 6 and 18 cm spectral analysis shows an increase of energy in the 40 to 80 min period band during the 2004 tsunami and at periods of 10 to 20 min in 2006 a simple ray tracing model of both the 2004 and 2006 events which approximates the tsunami as a shallow water wave was used to evaluate the effect of topography on tsunami propagation in order to provide a physical explanation for the features observed in the pressure gauge data

variations in subsidence rates along intermediate and fast spreading mid ocean ridges summary the nature of subsidence near the ridge crest of the intermediate and fast spreading mid ocean ridges of the indian and pacific oceans is investigated using surface ship bathymetry and magnetics profiles the ridge can be divided into discrete sections apparently bounded by distinct structural features such as major fracture zones in which bathymetry plotted against crustal age forms a well defined envelope with a width roughly the amplitude of the local bathymetry the averaged bathymetry in all of the regions studied follows closely a square root of age subsidence curve which in most regions has a subsidence coefficient c1 in the range of 340 390 m myr 1 2 the best fitting subsidence curve however never reproduces the amplitude of the axial topographic high the most notable region displaying unusual behaviour is the east pacific rise between 9 s and 22 s in this region the western flank of the ridge is subsiding at 200 225 m myr 1 2 while the eastern flank is subsiding at normal rates of 350 400 m myr 1 2 other anomalous areas include the region between the easter island hot spot and the chile rise triple junction in which the ridge crest is shallow and which is subsiding at rates of about 290 m myr 1 2 and the region east of the australia antarctic discordance in which the northern flank is subsiding at 440 m myr 1 2 this area may also be subsiding asymmetrically although there is not much data from the southern flank the asymmetric subsidence in the 9 s 22 s region of the east pacific rise begins immediately at the ridge crest and the low subsidence rates on the west flank continue to at least 12 myr old crust oligocene aged crust on the western flank is subsiding at more normal rates but is 500 m shallow with respect both to the world wide average and to the conjugate crust on the eastern flank the simplest model to explain these observations is that the western flank is underlain by a hotter mantle perhaps as the result of upwelling resulting from the large scale return circulation from the trenches depending on the depth of compensation the observed asymmetry could result from a lateral temperature gradient of 0 05 0 10 c km 1 and a total lateral temperature variation of under 100 c

shallow water equations for equatorial tsunami waves we present derivations of shallow water model equations of korteweg de vries and boussinesq type for equatorial tsunami waves in the f plane approximation and discuss their applicability this article is part of the theme issue nonlinear water waves

brachiopod geochemical records from across the carboniferous seas of north america evidence for salinity gradients stratification and circulation patterns abstract stable isotopic analyses of 100 well preserved carboniferous brachiopod shells representing two time slices from across north america reveal systematic regional changes in environmental conditions for both of the time slices studied the chesterian latest mississippian and the virgilian latest pennsylvanian 18o and 13c show a stepwise decrease moving across the continent from modern day west to east higher 18o in the west reflects greater water depths less freshwater input and more direct upwelling influence from cool open marine waters than in the east modelling based on these regional isotopic data indicates salinity reductions on the order of 1 4 psu in the appalachian basin relative to the midcontinent basin for the virgilian time interval over a range of reasonable thermal gradient and 18ofw assumptions the magnitude of this gradient is similar to those of modern day hudson bay and the panama bight which exhibit permanent haloclines driven by freshwater discharge considering existing paleoecological evidence for freshening the lack of more dramatic benthic freshening suggests that the depth of a regional halocline was shallower than the depths inhabited by thick shelled stenotopic brachiopods although other lines of evidence are necessary to verify the strength of the halocline estimated 18o temperatures for appalachian basin brachiopods are 3 5 c cooler than those of coeval conodonts due to either slightly warmer fresher surface waters or uncertainty in the phosphate 18o paleothermometer despite discrepancies in calculated temperatures the magnitude of the regional isotopic gradient derived from brachiopod and conodont proxies is similar after making appropriate glacioisotopic and water depth corrections a decreasing 13c trend toward the east southern paleolatitude likely reflects greater restriction and terrestrial influence near the sea s tropical shoreline trace element trends are less consistent and are affected by species and metabolism erratic variations in mg ca concentrations even among pristine samples of the same genera with similar 18o values and unrealistic mg ca derived temperatures justify continued caution in applying existing mg ca thermometer equations to paleozoic brachiopods

mifil a method to characterize seafloor swells with application to the south central pacific 1 we propose a filtering method to characterize large scale depth anomalies the mifil method for minimization and filtering requires two stages a first one to roughly remove the volcano component by minimizing the depth anomaly and a second one to smooth the shape and totally remove the small spatial length scale remaining topography using a median filter the strength of this method directly applicable on two dimensional grids is that it does not require any assumption on the location amplitude or width of the large scale feature to characterize except its minimal width we only consider the spatial length scale of the features to remove application to volcanic chains of the south central pacific is presented and the results lead to a better understanding of the tectonics and volcanism emplacement of the zone the society is the only classical hot spot that corresponds to the simple interaction of a plume with the lithosphere and for which a buoyancy flux of 1 58 0 15 mg s 1 is obtained the marquesas volcanic chain although quite comparable presents a swell morphology that prevents such interpretation and quantification for the tuamotu and cook austral volcanic chains no reliable quantification can be made because the depth and geoid anomalies are caused by several phenomena occurring at different depths that cannot be separated

glacial eustatic sea level curve for early late pennsylvanian sequence in north central texas and biostratigraphic correlation with curve for midcontinent north america at least 30 transgressive regressive cycles of deposition are recognized from the upper desmoinesian east mountain shale to the mid virgilian wayland shale in north central texas maximum regressive deposits are typically paleosol mudstones and fluvial sand stones maximum transgressive deposits are typically widespread ammonoid bearing conodont rich dark phosphatic shales in more major cycles and persistent fossiliferous shales or limestones overlying terrestrial deposits in more minor cycles delta complexes dominate the regressive sequences of many cycles using biostratigraphic criteria of first last sole or acme occurrence of ammonoid conodont and fusulinid taxa we correlate 17 cycles in the texas sequence directly with 17 glacial eustatic cycles of similar magnitude in the northern midcontinent this correlation suggests that glacial eustacy was the basic control over the cyclic sequence in texas that tectonic masking of the eustatic signal by nearby orogenic movement in texas was negligible and that delta shifting though conspicuous was only a secondary control over the cyclicity there minor cycles recognized between the correlated cycles also match well enough between texas and the midcontinent to further discount potential tectonic or deltaic masking of glacial eustatic cyclicity this strengthens the likelihood of correlating glacial eustatic events across larger parts of north america and perhaps with other parts of the world

high precision u pb zircon age calibration of the global carboniferous time scale and milankovitch band cyclicity in the donets basin eastern ukraine 1 high precision id tims u pb zircon ages for 12 interstratified tuffs and tonsteins are used to radiometrically calibrate the detailed lithostratigraphic cyclostratigraphic and biostratigraphic framework of the carboniferous donets basin of eastern europe chemical abrasion of zircons use of the internationally calibrated earthtime mixed u pb isotope dilution tracer and improved mass spectrometry guided by detailed error analysis have resulted in an age resolution of 0 05 or 100 ka for these carboniferous volcanics this precision allows the resolution of time in the milankovitch band and confirms the long standing hypothesis that individual high frequency pennsylvanian cyclothems and bundles of cyclothems into fourth order sequences are the eustatic response to orbital eccentricity 100 and 400 ka forcing tuning of the fourth order sequences in the donets basin to the long period eccentricity cycle results in a continuous age model for the middle to late pennsylvanian moscovian kasimovian ghzelian strata of the basin and their record of biological and climatic changes through the latter portion of the late paleozoic ice age detailed fusulinid and conodont zonations allow the export of this age model to sections throughout euramerica additional ages for mississippian strata provide among the first robust radiometric calibration points within this subperiod and result in variable lowering of the base ages of its constituent stages compared to recent global time scale compilations

the evidence and implications of polar ice during the mesozoic abstract the mesozoic perhaps representing the longest period of warmth during phanerozoic earth history contains in general sparse and frequently equivocal evidence for polar ice although this time is undoubtedly punctuated by oscillations in climate whether sufficient to lead to polar cooling and the formation of polar ice an inviting mechanism to account for faunal and floral distribution patterns and large scale sea level change has been widely debated mesozoic evidence for glacial conditions includes abraded rock surfaces generally unsorted stone rich beds and the presence of dropstones rafted by ice within a finer grained host sediment faunal and floral evidence has also been utilised to determine the presence or absence of cold or sub freezing polar conditions together with more indirect evidence for glacial conditions derived from general circulation model gcm simulations of climate the analysis of clay mineral distributions glendonite abundance and palaeontological and sedimentological evidence for globally synchronous sea level change the extent of possible glacial environments during the mesozoic has been established by plotting published reports of glacial sediments on palaeogeographic reconstructions the general clustering of evidence at high palaeolatitudes suggests that the extent of polar ice during the mesozoic is likely to have been approximately one third the size of the present day based on such evidence a number of episodes of cold or sub freezing polar climates during the bajocian bathonian tithonian volgian valanginian and aptian are recognised evidence exists possibly also for a cold episode during the early jurassic pliensbachian although poorly constrained owing to limited biostratigraphical control the longevity of these events may be represented by a cold snap within a stage although undoubtedly the possible smearing of ages may have had the effect of lengthening and hence promoting the importance of these proposed events the climate regime of the earth during these times may be hypothesised to be characterised by a relatively steep pole to equator temperature gradient where low latitude regions are as warm or warmer than today the evidence to support such reasoning includes sharp increases in the areal distribution of both glendonites and deposits with affinities to glacial tillites and dropstones for these times coincidental falls of sea level arid events and an increased bipolarity of faunas at these times imparts further confidence that these events were of a magnitude to effect the earth as a whole

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

impact of longer term modest climate shifts on architecture of high frequency sequences cyclothems pennsylvanian of midcontinent u s a abstract pennsylvanian glacioeustatic cyclothems exposed in kansas and adjacent areas provide a unique opportunity to test models of the impact of relative sea level and climate on stratal architecture a succession of eight of these high frequency sequences traced along dip for 500 km reveal that modest climate shifts from relatively dry seasonal to relatively wet seasonal with a duration of several sequences 600 000 to 1 million years had a dominant impact on facies sediment dispersal patterns and sequence architecture the climate shifts documented herein are intermediate both in magnitude and duration between previously documented longer term climate shifts throughout much of the pennsylvanian and shorter term shifts described within individual sequences climate indicators are best preserved at sequence boundaries and in incised valley fills of the lowstand systems tracts lst relatively drier climate indicators include high chroma paleosols typically with pedogenic carbonates and plant assemblages that are dominated by gymnosperms mostly xerophytic walchian conifers the associated valleys are small 4 km wide and 20 m deep and dominated by quartz sandstones derived from distant source areas reflecting large drainage networks transgressive systems tracts tst in all eight sequences generally are characterized by thin extensive limestones and thin marine shales suggesting that the dominant control on tst facies distribution was the sequestration of siliciclastic sediment in updip positions highstand systems tracts hst were significantly impacted by the intermediate scale climate cycle in that hsts from relatively drier climates consist of thin marine shales overlain by extensive thick regressive limestones whereas hsts from relatively wetter climates are dominated by thick marine shales previously documented relative sea level changes do not track the climate cycles indicating that climate played a role distinct from that of relative sea level change these intermediate scale modest climate shifts had a dominant impact on sequence architecture this independent measure of climate and relative sea level may allow the testing of models of climate and sediment supply based on modern systems

shear wave speed structure beneath the south pacific superswell using broadband data from ocean floor and islands 1 we determined three dimensional shear wave speed structure beneath the south pacific superswell down to a depth of 200 km by analyzing rayleigh wave records from broadband ocean bottom seismograph stations and island stations in the pacific ocean the ocean bottom stations were deployed from 2003 to 2005 on the seafloor in the french polynesian region which enabled us to study the upper mantle structure beneath the superswell with unprecedentedly high resolution we measured the dispersions of fundamental mode rayleigh waves at periods between 40 and 140 seconds by the two station method we found pronounced slow anomalies near the hot spots and in the lau basin the slow anomalies beneath the hot spots have deeper rooted than those associated with the lau basin the slow anomalies near the society macdonald marquesas and pitcairn hot spots continue down to at least 200 km depth

radially anisotropic 3 d shear wave structure of the australian lithosphere and asthenosphere from multi mode surface waves abstract a new radially anisotropic shear wave speed model for the australasian region is constructed from multi mode phase dispersion of love and rayleigh waves an automated waveform fitting technique based on a global optimization with the neighbourhood algorithm allows the exploitation of large numbers of three component broad band seismograms to extract path specific dispersion curves covering the entire continent a 3 d shear wave model is constructed including radial anisotropy from a set of multi mode phase speed maps for both love and rayleigh waves these maps are derived from an iterative inversion scheme incorporating the effects of ray path bending due to lateral heterogeneity as well as the finite frequency of the surface waves for each mode the new s wave speed model exhibits major tectonic features of this region that are in good agreement with earlier shear wave models derived primarily from rayleigh waves the lateral variations of depth and thickness of the lithosphere asthenosphere transition lat are estimated from the isotropic voigt average s wave speed model and its vertical gradient which reveals correlations between the lateral variations of the lat and radial anisotropy the thickness of the lat is very large beneath the archean cratons in western australia whereas that in south australia is thinner the radial anisotropy model shows faster sh wave speed than sv beneath eastern australia and the coral sea at the lithospheric depth the faster sh anomaly in the lithosphere is also seen in the suture zone between the three cratonic blocks of australia one of the most conspicuous features of fast sh anisotropy is found in the asthenosphere beneath the central australia suggesting anisotropy induced by shear flow in the asthenosphere beneath the fast drifting australian continent

borehole petrophysical chemostratigraphy of pennsylvanian black shales in the kansas subsurface pennsylvanian black shales in kansas have been studied on outcrop for decades as the core unit of the classic midcontinent cyclothem these shales appear to be highstand condensed sections in the sequence stratigraphic paradigm nuclear log suites provide several petrophysical measurements of rock chemistry that are a useful data source for chemostratigraphic studies of pennsylvanian black shales in the subsurface spectral gamma ray logs partition natural radioactivity between contributions by u th and k sources elevated u contents in black shales can be related to reducing depositional environments whereas the k and th contents are indicators of clay mineral abundance and composition the photoelectric factor log measurement is a direct function of aggregate atomic number and so is affected by clay mineral volume clay mineral iron content and other black shale compositional elements neutron porosity curves are primarily a response to hydrogen content although good quality logs are available for many black shales borehole washout features invalidate readings from the nuclear contact devices whereas black shales thinner than tool resolution will be averaged with adjacent beds statistical analysis of nuclear log data between black shales in successive cyclothems allows systematic patterns of their chemical and petrophysical properties to be discriminated in both space and time

broadband ocean bottom seismology broadband ocean bottom seismographs bbobss were first developed in the 1980s and have since been used to explore the structure of earth s interior beneath oceanic regions for example mid oceanic ridges subduction zones hot spots and the oceanic lithosphere asthenosphere boundary the best approach for broadband seismic observations in oceanic regions is that of a borehole seismic observatory attached to the ocean floor cable which is realized in several near coast networks considering the high cost of such networks there is still a need to develop autonomous bbobss with a better signal to noise ratio for temporary observations of focused scientific targets far from the coast

the non linear initiation of diapirs and plume heads abstract a simple theory is devised to describe the non linear feedback mechanisms involved in the initial growth of a single diapir or plume head from a low viscosity channel overlain by a much more viscous layer such feedbacks arise primarily from the relation between the growth of a proto diapir i e an undulation on the upper boundary of the low viscosity channel and the draining of the low viscosity channel in the period of time between its initial exponential growth characterized by linear stability analysis and its separation from the low viscosity channel as a fully formed diapir the proto diapir can undergo a significant cessation in its development due to deflation of the low viscosity channel i e the proto diapir s growth can essentially stall for a long period of time before it separates and begins its ascent through the overlying medium the theory is used to determine a criterion for separation of the diapir from the low viscosity channel that is in terms of the geometrical and mechanical properties of the channel instead of the ad hoc volume flux widely used in many models of mantle plumes and plume heads e g whitehead and luther 1975 richards et al 1989 olson 1990 sleep 1990 bercovici and mahoney 1994 from this separation criterion self consistent scaling laws can be formulated to relate the size of the fully developed diapir and its trailing conduit to the properties of the initial channel instead of to the ad hoc volume flux basic laboratory experiments involving highly viscous fluids are presented and demonstrate that the so called stalling period between initial growth and separation does indeed occur these results suggest that nascent mantle plume heads may stall for extended periods at the base of the mantle and thereby contribute to variations in thickness of the d layer

on the vertical extent of the large low shear velocity province beneath the south pacific superswell the three dimensional s wave velocity structure beneath the south pacific superswell is obtained from joint broadband seismic experiments on the ocean floor and islands we collected only approximately 800 relative times of long period teleseismic sh waves by using a waveform cross correlation from 76 events occurring from january 2003 to may 2005 we conducted relative time tomography to obtain a 3d structure to depths of 1600 km in the resultant image we find a characteristic distribution of low velocity regions the most prominent features are a large doughnut shaped low velocity region at 800 km depth and an elongated large low velocity region beneath the society to pitcairn hotspots at 1200 km depth our model suggests that a large low shear velocity province rooted in the d00 extends upwards and culminates near the top of the lower mantle beneath the central part of the south pacific superswell although its perfect continuity is not still confirmed

filtration models guilds and biofacies crinoid paleoecology of the stanton formation upper pennsylvanian midcontinent north america abstract filtration models make several predictions concerning the distribution of crinoids among benthic habitats 1 generally higher energy shoreward settings should contain crinoid assemblages dominated by taxa with dense mesh filtration fans 2 generally lower energy offshore settings should be dominated by crinoids with open mesh filtration fans 3 diversity should be highest in nearshore settings which generally contain low energy microhabitats within high energy settings where both dense fan and open fan forms can co occur whereas offshore assemblages should be dominated by open fan taxa thus open fan forms should be more eurytopic than dense fan forms also previous empirical models for crinoid distribution patterns note that nearshore assemblages are dominated by large forms whereas offshore assemblages are dominated by small forms filtration fan morphology and body size interact significantly to determine how and where crinoids feed and thus are used here as criteria for distinguishing late paleozoic crinoid guilds the distribution patterns of crinoid taxa and guilds among the benthic paleoenvironments of the upper pennsylvanian missourian stanton formation were used to test the predictions of the filtration models the results of quantitative analyses show that large dense fan crinoids are significantly more abundant in shoreward assemblages than smaller open fan crinoids which are more abundant in offshore assemblages these results are consistent with the aforementioned models however diversity patterns are not consistent with the filtration models indicating that mid depth and aerobic offshore assemblages are more taxonomically and trophically diverse than nearshore and dysaerobic offshore assemblages ostensibly due to the overlapping ranges of characteristically nearshore and offshore taxa analysis of guild distribution patterns also indicates some contradictions to predicted patterns of stenotopy and eurytopy indicating additional complexities to late paleozoic crinoid paleoecology five recurrent crinoid assemblages or biofacies have been identified from the stanton formation of midcontinent north america the guild structure and diversity of these biofacies are characterized here as 1 a moderate diversity nearshore biofacies dominated by large calyx dense fan guilds 2 a high diversity midshelf biofacies characterized by equitable contributions of many taxa and guilds 3 a moderate diversity offshore biofacies dominated by non pinnulate and small calyx open fan pinnulate guilds 4 a low diversity offshore dysaerobic biofacies dominated by small calyx pinnulate guilds 5 a depauperate offshore biofacies dominated by a unique stalkless species paragassizocrinus tarri except for biofacies 5 the number of guilds appears to increase offshore however the equitability of taxa among and within the guilds generally decreases offshore this pattern indicates that the heterogeneity of resources or the recognition by taxa of different classes of resources as reflected by the variety of functional types increased from nearshore to offshore but that the abundance or accessibility of these resources and or the relaxation of competition within guilds increased from offshore to nearshore

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

elemental mobility in phosphatic shales during concretion growth and implications for provenance analysis shale normalized rare earth element ree patterns record variable redox conditions in coprolitic phosphorite concretions and their host shales from the pennsylvanian age cyclothems in the u s midwestern states of oklahoma kansas nebraska iowa and ohio as well as the mississippian age fayetteville shale of arkansas shale normalized enrichments in middle ree mree developed as the phosphatic concretions lithified under reducing conditions in pore waters of organic rich muds depletions in mree sm ho in host shales correspond to the mree enrichments in some nearby concretions rare and weak negative cerium depletions in both reworked and unreworked phosphorite concretions suggest exposure to oxygenated waters positive europium anomalies in thick phosphorite coatings on coprolitic phosphorite concretion cores suggest extreme reduction during lithification of the phosphorite coating flat ree patterns in both host shales and phosphorite concretions probably reflect detrital influence phosphate diagenesis can affect the trace element chemistry of black shale enough to alter provenance signals overall enrichment in ree in phosphorite relative to host shale suggests movement of ree as does the mree enriched phosphorite near the mree depleted host shale diagenetically mobile ree can affect la th sc relationships used in provenance analysis the extent of the diagenetic modification of la th sc distribution is related to the amount of p2o5 present typical la th sc distribution in host shale results when p2o5 values are below 0 5 high la abundance substantially skews la th sc distribution when p2o5 is above 5 0 but when p2o5 is between 0 5 and 5 0 the extent of the effect is unclear oxides of aluminum and other major elements may be useful in recording detrital influence and proximity to the shoreline in phosphorite concretions most of the concretions nearest to shoreline sources of detrital input have both flat ree patterns and elevated values of al2o3 al2o3 fe2o3

variation of the subsidence parameters effective thermal conductivity and mantle dynamics abstract the subsidence of young seafloor is generally considered to be a passive phenomenon related to the conductive cooling of the lithosphere after its creation at mid oceanic ridges recent alternative theories suggest that the mantle dynamics plays an important role in the structure and depth of the oceanic lithosphere however the link between mantle dynamics and seafloor subsidence has still to be quantitatively assessed here we provide a statistical study of the subsidence parameters subsidence rate and ridge depth for all the oceans these parameters are retrieved through two independent methods the positive outliers method a classical method used in signal processing and through the mifil method from the subsidence rate we compute the effective thermal conductivity k eff which ranges between 1 and 7 w m 1 k 1 we also model the mantle flow pattern from the s40rts tomography model the density anomalies derived from s40rts are used to compute the instantaneous flow in a global 3d spherical geometry we show that departures from the k eff 3 w m 1 k 1 standard value are systematically related to mantle processes and not to lithospheric structure regions characterized by k eff 3 w m 1 k 1 are associated with mantle uplifts mantle plumes or other local anomalies regions characterized by k eff 3 w m 1 k 1 are related to large scale mantle downwellings such as the australia antarctic discordance aad or the return flow from the south pacific superswell to the east pacific rise this demonstrates that mantle dynamics plays a major role in the shaping of the oceanic seafloor in particular the parameters generally considered to quantify the lithosphere structure such as the thermal conductivity are not only representative of this structure but also incorporate signals from the mantle convection occurring beneath the lithosphere the dynamic topography computed from the s40rts tomography model reproduces the subsidence pattern observed in the bathymetry overall we find a good correlation between the subsidence parameters derived from the bathymetry and the dynamic topography this demonstrates that these parameters are strongly dependent on mantle dynamics

sequence stratigraphy of the upper pennsylvanian cleveland formation a major tight gas sandstone western anadarko basin texas panhandle the upper pennsylvanian lower missourian cleveland formation has yielded 459 bcf of natural gas and 18 6 million bbl of oil from low permeability 0 10 md fluvial and deltaic reservoirs in a seven county tight gas area of the northeastern texas panhandle regional study of the cleveland and underlying desmoinesian marmaton group siliciclastics using tight well control and cores established the sequence stratigraphic framework to clarify the vertical and areal occurrence of cleveland reservoirs seals and possible source rocks regionally distinctive facies stacking patterns in the study interval compose a sequence stratigraphic framework of several westerly sourced systems tracts and three depositional sequences s s1 is characterized by landward and seaward stepping deltaic and strand plain cycles parasequences deposited on the top of oswego type 1 sequence boundary that define in ascending order marmaton late stage lowstand wedge lst pw and transgressive systems tracts tst and a lower cleveland highstand systems tract hst a regionally correlative organic rich marine condensed section at the top of the marmaton tst equivalent to the nuyaka creek black shale bed of mid continent cyclothems represents maximum flooding conditions during eustatic rise a relative sea level drop with the onset f s2 deposition initiated development of a lowstand incised valley system lst iv in the middle to upper cleveland that extended basinward of the lower cleveland depositional shelf edge subsequent coastal onlap by thin deltaic systems of the overlying tst marks the start of decreased sediment influx during late cleveland time resulting in thinning of parasequences and an increase in carbonate beds of high frequency cycles in upper s2 and s3 stratigraphic traps and pinch out of reservoir facies within small southeast plunging anticlines make up most traps in the producing area proximal delta front and fluvial sandstones of the cleveland upper hst and overlying lst iv respectively are the primary reservoirs the high total organic carbon top of marmaton marine condensed section and thick prodeltaic and lower distal delta front shales within the lower cleveland hst are the probable source rocks distal deltaic shales of the middle cleveland tst form most reservoir seals potential new reservoirs should be targeted at the updip terminations of systems tracts at lap out positions of individual sand rich hst and tst parasequences and along lst iv valley margin stratal terminations

modal depths from shipboard bathymetry there is a south pacific superswell since the first quantification 10 years ago of a large regional depth anomaly in french polynesia numerous studies have explored the origin and the geophysical and geochemical consequences of this south pacific superswell these efforts would be widely viewed as a waste of time if this superswell were proven not to exist recently levitt and sandwell 1996 have proposed just that based on a modal analysis of multibeam bathymetric data collected on sea floor aged 15 to 35 ma in eastern french polynesia citing a discrepancy between the etopo5 gridded depths and ship data from 4 expeditions in their study area they suggest that the entire superswell could be an artifact of poor etopo5 sampling and gridding here we present a more comprehensive analysis of original ship soundings from 82 oceanographic expeditions with satellite navigation collected since 1967 the soundings span sea floor from less than 30 to more than 110 ma we confirm the conclusion of levitt and sandwell that depth anomalies calculated from etopo5 slightly overestimate the true values from original ship soundings on sea floor aged 30 to 35 million years but a positive depth anomaly of 250 m exists nevertheless on older sea floor we find that etopo5 slightly underestimates the magnitude of the depth anomaly based on a modal analysis but overall the agreement in the mode of the depth age data between etopo5 and the original ship soundings is quite striking especially given the fact that less than 20 of the ship data used in our analysis had been collected at the time that the global bathymetric grid was prepared there indeed is a south pacific superswell and the magnitude and age dependence of the depth anomaly is as originally calculated from the mode of gridded bathymetry

cordaiteans in paleotropical wetlands an ecological re evaluation abstract cordaiteans in cordaite dominated permineralized peat from pennsylvanian coals in iowa have been reconstructed as mangroves using root anatomy peat taphonomy and geochemical data macrofloral palynofloral and conodont biostratigraphy indicate that these peats come from the latest atokan blackoak coal and earliest desmoinesian cliffland coal mid moscovian both in the kalo formation thus their depositional setting can be used to evaluate the mangrove hypothesis in recent mires thick mangrove peats have accumulated in tropical to subtropical carbonate systems in contrast thick tropical freshwater peats have accumulated in siliclastic systems kalo formation coals which we interpret as freshwater deposits formed in siliciclastic depositional settings similar to those of modern tropical freshwater peat and to other pennsylvanian coals in north america interpreted as freshwater deposits in the late atokan and earliest desmoinesian mid moscovian cordaiteans and tree ferns predominated in the western interior and illinois basins lycopsids and cordaiteans predominated in the appalachian and donets basins the scarcity of lycopsid only mires in north america during the late atokan earliest desmoinesian mid moscovian suggests drier climates than during the mid to late desmoinesian late moscovian rather than indicating mangrove swamps cordaite dominated peat may indicate climates with a low rain season although most plants in cordaite dominated peat probably grew in freshwater coastal mires in climate zones with seasons of low rain may harbor mangrove taxa the changuinola swamp of panama a modern peat accumulating wetland that has a low rain season is a possible analog of ancient cordaite dominated mires in changuinola most plants require freshwater however mangroves sustained by salt water influx into the swamp grow along the seaward edge and along blackwater creeks the low rain season hypothesis has implications for understanding rainfall amount and continuity during pennsylvanian cyclothem deposition the floral succession in diverse cordaite coals from cordaiteans to tree ferns to lycopsids suggests increasingly wet climate during coal accumulation the position of these coals immediately above the sequence boundary suggests humid climate during early glacial melting for these cyclothems

thickness of the mantle transition zone beneath the south pacific as inferred from analyses of scs reverberated and ps converted waves abstract we study the thickness of the mantle transition zone beneath the south pacific in order to determine temperature anomalies associated with presumed hot plumes in the south pacific superswell the scs reverberation method is used to determine the average thickness between an event station pair and the velocity spectrum stacking vss method is used to determine the thickness near each station the thickness obtained by the scs reverberation method shows that the average thickness beneath the superswell is less than the globally averaged thickness by 6 km while the average thickness beneath the tonga fiji subduction is greater than the global average by 10 km the vss method shows nearly normal thickness beneath stations used in the superswell region we conclude that substantial thinning of the transition zone associated with presumed hot plumes occurs locally beneath the society hot spot as revealed by our previous ss precursor study and is not widespread beneath the whole area of the superswell

measurements of mantle wave velocities and inversion for lateral heterogeneities and anisotropy 3 inversion lateral heterogeneity in the earth s upper mantle is investigated by inverting dispersion curves of long period surface waves 100 330 s models for seven different tectonic regions are derived by inversion of regionalized great circle phase velocity measurements from our previous studies we also obtain a representation of upper mantle heterogeneities with no a priori regionalization from the inversion of the degree 6 spherical harmonic expansion of phase and group velocities the data are from the observation of about 200 paths for love waves and 250 paths for rayleigh waves for both the regionalized and the spherical harmonic inversions corrections are applied to take into account lateral variations in crustal thickness and other shallow parameters these corrections are found to be important especially at low spherical harmonic order the trench region and fast velocities down to 250 km under shields below 200 km under the oceans both s velocity and s anisotropy support a model of small scale convection in which cold blobs detach from the bottom of the lithosphere when its age is large enough the spherical harmonic models clearly demonstrate a posteriori the relation between surface tectonics and s velocity heterogeneities in the first 250 km all shields are fast most ridges are slow below 300 km a belt of fast mantle follows the pacific subduction zones however at greater depths large scale heterogeneities that seem to bear no relationship to surface tectonics are observed the most prominent feature at 450 km is a fast velocity region under the south atlantic ocean smaller scale heterogeneities that are not related to surface tectonics are also mapped at shallower depths an anomalously slow region centered in the south central pacific is possibly linked to intense hot spot activity a very fast region southeast of south america may be related to subduction of old pacific plate between 200 and 400 km a belt of sv sh anisotropy follows part of the ridge and subduction systems indicating vertical mantle flow in these regions the spherical harmonic results open new horizons for the understanding of convection in the mantle perspectives for the improvement of the models presented are discussed

three dimensional shear wave structure beneath the philippine sea from land and ocean bottom broadband seismograms 1 we obtained three dimensional 3 d shear wave speed structure beneath the philippine sea and the surrounding region from seismograms recorded by land based and long term broadband ocean bottom seismographic stations the ocean bottom data gave us a better station coverage to obtain a higher spatial resolution about 300 400 km in the philippine sea than in previous studies we employed a new technique of surface wave tomography in which multimode phase speeds are measured and inverted for a 3 d shear wave speed structure by incorporating the effects of finite frequency and ray bending there is a sharp speed contrast along the izu bonin mariana trench across which the philippine sea side has a significantly slower upper mantle than the pacific ocean side in the upper 120 km the shear wave speed structure is well correlated with the age of the provinces at depths greater than 160 km the pattern is dominated by fast anomalies of the subducted slabs of the pacific plate and two slow anomalies to the south of the daito ridge and in the southernmost part of the philippine sea

3d upper mantle structure beneath japan and its surrounding region from inter station dispersion measurements of surface waves abstract three dimensional shear wave speed structure of the upper mantle beneath the japanese islands and the surrounding regions is constructed from the measurements of inter station phase speeds of surface waves we employ the dense broad band seismic network f net deployed throughout japan as well as permanent stations of global seismic network in east asia a temporary broad band seismic network in far east russia which has been constructed as a part of the stagnant slab project since 2004 is also used in combination with the permanent stations using seismic events from 2005 to 2007 with moment magnitude greater than 6 0 and depth shallower than 100 km we measure the phase speeds of the fundamental mode rayleigh and love waves working with the conventional two station technique phase speed maps of the japanese islands and the sea of japan are obtained from the inversions of inter station phase speeds that are measured for a large number of station pairs we have collected over 5300 paths for rayleigh waves and 3800 paths for love waves the phase speed maps of rayleigh waves in the period range from 25 to 140 s are inverted to obtain an isotropic 3d shear wave speed model in the depth range from 40 to 200 km the isotropic s wave model represents prominent fast wave speed anomalies in northeastern japan associated with the subducting pacific plate fast anomalies of the lithosphere beneath the sea of japan are also imaged implying that the average thickness of lithosphere beneath the yamato rise and japan basin is about 60 10 km immediately below this oceanic lithosphere slow anomalies are found in the asthenosphere beneath the entire region of the sea of japan in the depth down to about 200 km in southwestern japan the low angle subduction of the philippine sea plate is mapped clearly indicating that the northern end of the plate extends beyond the northern coast of the chugoku region reaching beneath the oki islands in the sea of japan a strong low velocity anomaly off the west coast of kyushu is found in the depth down to about 130 km which supports the existence of an upwelling flow beneath this area we also invert the phase speed maps of rayleigh and love waves in the period range from 30 to 80 s simultaneously to construct the radially anisotropic shear wave speed model a remarkable anisotropy with sh sv is found in the mantle wedge beneath northeastern japan while only weak anisotropy is seen beneath the sea of japan such anisotropic properties which cannot readily be obtained from body wave studies only will be an important key to understanding the dynamic processes in this tectonically actively region

topography of the mantle discontinuities beneath the south pacific superswell as inferred from broadband waveforms on seafloor we analyzed seismic waveforms recorded by the broadband ocean bottom seismographs deployed in the south pacific superswell to determine the depths of the mantle discontinuities using a receiver function method we estimated the thickness of the mantle transition zone mtz to be 245 km on average beneath the superswell region which is close to the global average the mtz is found to be thinned locally beneath the society hot spot by 30 km temperature anomalies computed from the thinned transition zone and the clapeyron slope of the olivine phase transformations are 150 200 k beneath the society hot spot previous studies using land based data suggested the presence of a hot mtz beneath the pitcairn hot spot the locally hot transition zone beneath the hot spots is attributed to narrow mantle plumes probably rising from the lower mantle the normal average thickness obtained from the present study indicates that there is no broad upwelling across the transition zone beneath the entire superswell region at present

upper mantle low anisotropy channels below the pacific plate a new 3d anisotropic model has been obtained at a global scale by using a massive dataset of seismic surface waves though seismic heterogeneities are usually interpreted in terms of heterogeneous temperature field a large part of lateral variations are also induced by seismic anisotropy of upper mantle minerals new insight into convection processes can be gained by taking seismic anisotropy into account in the inversion procedure the model is best resolved in the pacific plate the largest and the most active tectonic plate superimposed on the large scale radial parameter and azimuthal anisotropy of vsv velocity within and below the lithosphere correlated with present or past pacific plate motions are smaller scale 1000 km lateral variations of anisotropy not predicted by plate tectonics channels of low anisotropy down to a depth of 200 km hereafter referred to as lac are observed and are the best resolved anomalies one east west channel between easter island and the tonga kermadec subduction zones observed on both radial and azimuthal anisotropies and a second one only observed on azimuthal anisotropy extending from the south west pacific up to south east hawaii and passing through the polynesia hotspot group for plate older than about 40 ma these features provide strong constraints on the decoupling between the plate and asthenosphere they are presumably related to cracking within the pacific plate and or to secondary convection below the rigid lithosphere predicted by numerical and analog experiments the existence and location of these lacs might be related to the current active volcanoes and hotspots possibly plumes in the central pacific lacs which are dividing the pacific plate into smaller units might indicate a future reorganization of plates with ridge migrations in the pacific ocean

crinoid biofacies in upper carboniferous cyclothems midcontinent north america faunal tracking and the role of regional processes in biofacies recurrence abstract coordinated stasis is a pattern of long term multispecies evolutionary and ecological stability bounded by rapid turnover events one class of models proposed to explain the paleoecological pattern of biofacies stability typified by ecological locking states that biotas within habitats become integrated into ecological units that through population and community level processes rebound and persist through short term disturbances this high frequency community resilience allows the units to persist through geologic time tracking their preferred habitats during transgressions and regressions because species are tightly woven into the fabric of the ecosystem there is little opportunity for speciation this also buffers them against extinction this ecological locking by ecosystem homeostatic processes produces long intervals of regionally stable biofacies in the fossil record in these models the primary structuring mechanisms are at the local community level controlled by species autecology species interactions and local disturbance regime an opposing class of models states that species are fundamentally independent and that species interactions are not strong nor spatio temporally persistent enough to forge communities into integrated units with long term persistence community recurrence would be expected as long as the various habitat types recur in time and space species autecologies remain stable the pool of available species remains intact and the recruitment of species into local habitats is not interrupted the focus of these models is dominantly at the regional level controlled by biogeographic history and dispersal among as well as within communities the results of this study of crinoids from the upper carboniferous lansing group of midcontinent north america show that they are distributed among five recurrent paleocommunity types or biofacies which are generally arrayed along an onshore offshore gradient two of these biofacies are restricted to the offshore dysaerobic horizons of the most extensive cyclothem in the group the other three biofacies recur consistently among the cycles however this repeated pattern includes a regionally distributed offshore biofacies unique to late transgressive deposits that is not found in analogous regressive deposits while the distribution of the two shoreward biofacies is consistent with faunal tracking the recurrence of a biofacies unique to late transgressive horizons is not consistent with an integrated paleocommunity that tracks shifting habitats through transgression and regression these results favor the alternative models namely that biofacies develop in place via dispersal from a persistent species pool in response to prevailing environmental conditions the recurrence of apparently ephemeral biofacies through time underscores the importance of regional processes in controlling local paleocommunity composition the establishment of refuges or source populations during environmental bottlenecks i e lowstands is critical to maintaining the regional species pool through time for continued recruitment into local sink paleocommunities during more amenable times i e highstands metapopulation dynamics among habitats are equally important in maintaining species within a region through time conflicting observations of stochastic individualistic and deterministic integrated behaviors of the same ecological systems probably result from differences in the scales of analyses and therefore the processes being studied

three dimensional mantle flow beneath mid ocean ridges we have studied the development of multiscalar mantle flow beneath mid ocean ridges using a three dimensional numerical technique modeling includes various modes of heating and the shear flow induced by plate motion away from a spreading center with and without transform offsets the computational box consists of two layers differing in their viscosities it is shown than when flow is generated by heat sources located for the most part in the lower layer convective cells have horizontal dimensions proportional to the entire thickness of the box it is only when flow is driven by buoyancy forces in the top layer or by transient cooling along the top interface that transverse rolls are found to be confined within the lop low viscosity layer these rolls remain generally located far from the ridge plane except in the case of a viscosity contrast of 3 orders of magnitude between the bottom and top layers we conclude that segmentation beneath mid ocean ridges requires both strong viscosity contrasts and the lack of deep convective flow marine geophysical observations suggest that mantle upwelling beneath mid ocean ridges is two dimensional sheetlike beneath intermediate fast and hotspot influenced spreading centers and three dimensional plumelike beneath slow spreading centers and ridges overlying cold regions of the mantle such as the australian antarctic discordance we propose that these observations can be explained by variations in the viscosity structure and temperature of the mantle along the mid ocean ridge system models including a transform offset show that the active portion of the transform is a region of mantle downwelling the toroidal flow induced by the offset shifts the plumes away from the ridge crests the effects of the toroidal field are strongly magnified when the return flow cannot penetrate in the lower layer due to a large viscosity contrast stresses generated by the flow partly explains the development of inside corner highs and the evolution of transform faults

on the short wavelength stabilities of some geophysical flows this paper is a survey of the short wavelength stability method for rotating flows additional complications such as stratification in the flow or the presence of non conservative body forces are considered too this method is applied to the specific study of some exact geophysical flows for gerstner like geophysical flows one can identify perturbations in certain directions as a source of instabilities with an exponentially growing amplitude the growth rate of the instabilities depending on the steepness of the travelling wave profile on the other hand for certain physically realistic velocity profiles steady flows moving only in the azimuthal direction with no variation in this direction are locally stable to the short wavelength perturbations this article is part of the theme issue nonlinear water waves

transgressive wave ravinement on an epicontinental shelf as recorded by an upper pennsylvanian soil nodule conglomerate sandstone unit kansas and oklahoma u s a abstract a thin 3 25 cm persistent conglomerate sandstone unit occurs at the base of the transgressive systems tract of an upper pennsylvanian cyclothem below a transgressive limestone in outcrop and subsurface in se kansas and ne oklahoma it has an erosional base and a sharp top and is composed of calcitic clasts some shell fragments and quartz sand and rare coal fragments clasts are rounded equant to elongate coarse sand to pebble sized and moderately to well sorted they are micritic to microsparitic or radial fibrous calcite grains and pisoids micritic and microsparitic clasts are heterogeneous and contain rounded to elongate moulds and radiating or concentric spar or micrite filled cracks pisoids have micrite cores and superficial calcite or clay rims the clasts have similar texture and composition to the calcitic nodules and rhizoconcretions in underlying paleosols and thus were probably derived from these soil nodules upper shoreface erosion during shoreline transgression excavated the underlying paleosols and eroded coeval transgressive deposits landwards of the shoreline the eroded sediments were then reworked and transported to the lower shoreface and inner shelf by storm return flows to be deposited as a soil nodule conglomerate sandstone unit its base is an amalgamated surface of wave ravinement and initial transgression as well as the lower sequence boundary the widespread conglomerate sandstone unit suggests extensive transgressive wave ravinement on an epicontinental shelf the transgressive record composed of upward deepening dominantly fine grained lithofacies defines a new type t c4 of transgressive record which is composed of mixed carbonate and siliciclastic lithofacies with a simple transgressive lag on a low relief shelf

geology and geochronology of the line islands geological and geophysical studies along the entire length of the line islands were undertaken in order to test the hot spot model for the origin of this major linear island chain volcanic rocks were recovered in 21 dredge hauls and fossiliferous sedimentary rocks were recovered in 19 dredge hauls volcanic rocks from the line islands are alkalic basalts and hawaiites in addition a tholeiitic basalt and a phonolite have been recovered from the central part of the line chain microprobe analyses of groundmass augite in the alkalic basalts indicate that they contain high tio2 1 0 4 0 wt and al2o3 3 4 9 1 wt and are of alkaline to peralkaline affinities major element compositions of the line islands volcanic rocks are very comparable to hawaiian volcanic rocks trace element and rare earth element analyses also indicate that the rocks are typical of oceanic island alkalic lavas the line islands lavas are very much unlike typical mid ocean ridge or fracture zone basalts dating of these rocks by 40ar 39ar k ar and paleontological methods combined with deep sea drilling project data from sites 165 315 and 316 and previously dated dredged rocks provide ages of volcanic events at 20 localities along the chain from 18 n to 9 s a distance of almost 4000 km all of these dates define mid cretaceous to late eocene edifice or ridge building volcanic events eocene volcanic events took place from 15 n to 9 s and late cretaceous events took place from 18 n to 9 s in the southern line islands both cretaceous and eocene events took place on the same edifice or ridge indicating recurrent volcanism at a single locality the irregular distribution of atolls in the chain the fact that late cretaceous reefs flourished along a distance of approximately 2500 km in the central and southern line islands and the observation that spatially closely related seamounts exhibit different subsidence histories are interpreted as indicating that large segments of the chain have not followed a t1 2 related subsidence path magnetic surveys of 11 seamounts show that four of the seamounts from the central line chain give virtual geomagnetic poles which fall well to the north of virtual geomagnetic poles of cretaceous seamounts these four poles agree with other paleomagnetic data of middle late eocene early oligocene age from the pacific one of these four seamounts yielded a 40ar 39ar total fusion age of 39 ma because the poles of all four seamounts fall into a tight group we infer that they are probably of middle late eocene age to early oligocene age the other three seamounts as well as one seamont from the line islands analyzed previously all give virtual geomagnetic poles which agree with late cretaceous paleomagnetic data from the pacific of these four seamounts three give late cretaceous 40ar 39ar ages ranging from 71 to 85 ma another in the southern line islands is interpreted on paleontological evidence to be of late cretaceous age the origin of the line islands has been ascribed by previous workers to the effects of a single hot spot and to the action of four hot spots the single hot spot model cannot account for all of the volcanic edifices in the line islands although it does explain a general age progression of 9 6 0 4 cm yr from north to south along the chain derived from a number of dated edifices the four hot spot model accounts for more of the dated volcanic edifices but still does not explain all of the available data the petrologic data argue against a mid ocean ridge or transform fault origin proposed by earlier workers the complex volcanic province represented by the line islands remains a challenge to existing models for the origin of midplate volcanism in the pacific an atoll drilling program which could determine edifice building histories paleolatitudes of seamount formation and subsidence rates and patterns in the line islands is needed

a fractal analysis of quaternary cenozoic mesozoic and late pennsylvanian sea level changes sea level changes are related to both climatic variations and tectonic movements the fractal dimensions of several sea level curves were compared to a modern climatic fractal dimension of 1 26 established for annual precipitation records a similar fractal dimension 1 22 based on 18o 16o in deep sea sediments has been suggested to characterize climatic change during the past 2 m y our analysis indicates that sea level changes over the past 150 000 to 250 000 years also exhibit comparable fractal dimensions sea level changes for periods longer than about 30 m y are found to produce fractal dimensions closer to unity and missourian late pennsylvanian sea level changes yield a fractal dimension of 1 41 the fact that these sea level curves all possess fractal dimensions less than 1 5 indicates that sea level changes exhibit nonperiodic long run persistence the different fractal dimensions calculated for the various time periods could be the result of a characteristic overprinting of the sediment record by prevailing processes during deposition for example during the quaternary glacio eustatic sea level changes correlate well with the present climatic signature during the missourian however mechanisms such as plate reorganization may have dominated resulting in a significantly different fractal dimension

dynamic carbonate sedimentation on the northern line islands ridge palmyra basin abstract the line islands ridge lir located south of the hawaiian islands between 7 n and 1 s is one of the few large central pacific regions shallower than the regional carbonate compensation depth thick sequences of carbonate sediments have accumulated around the lir despite it being located in the sediment starved central tropical pacific the lir is an important source of carbonates to the surrounding region and deposition around the lir has expanded the equatorial pacific carbonate sediment tongue by about 5 of its total area furthermore sediments on the ridge are potentially important paleoceanographic archives a recent survey at the crest of the lir finds evidence for high current activity significant erosion but overall net sediment deposition currents are strong enough to form sediment waves and lee drifts in the palmyra basin at the northern terminus of the lir sediments along the lir are pelagic foraminiferal sands that are easily eroded and flow out into the surrounding abyssal plain in active submarine channel systems as channels migrate pelagic sediments fill in the abandoned channel arms despite significant sediment losses from the top of the ridge 1 3 km of sediment has accumulated in the upper palmyra basin over basement formed 68 to 85 million years ago ma late neogene erosion may be more extensive than earlier erosion cycles in response to reduced sediment production as the palmyra basin exited the high productivity equatorial latitudes sediments with good stratigraphic order needed for paleoceanographic study are limited in this dynamic sedimentary environment but can be found with proper survey

radiogenic isotope composition of carboniferous seawater from north american epicontinental seas abstract abundant marine sedimentary deposits accumulated in the epeiric seas of north america during the carboniferous period 359 299 ma when the earth s climate shifted from a global greenhouse to icehouse geochemical records extracted from these deposits are often assumed to reflect global ocean processes and are increasingly being used to infer relationships between tectonism carbon cycling and climate change we analyzed radiogenic isotopes sr nd in biogenic apatite along a north american transect to constrain the degree of coupling between the epicontinental seas and the open ocean in many samples the primary sr signal is altered however the original nd isotopic composition of the biogenic apatite seems to be preserved the end t values range from 8 6 to 4 5 in the us midcontinent basins and from 11 4 to 5 7 at arrow canyon nevada usa indicating the development of an east west geochemical gradient throughout the pennsylvanian this apparent decoupling of the epicontinental seas from the open ocean was established by the latest mississippian thus circulation of waters between epicontinental seaways of western us and the basins to the east and south was periodically restricted shifts toward unradiogenic nd at arrow canyon seem to correspond with glacio eustatic changes across the mid carboniferous boundary these end t variations may have been caused by changes in weathering rates sources marine circulation or a combination of both

rayleigh wave phase velocity measurements across the philippine sea from a broad band obs array summary we obtained phase velocities of fundamental rayleigh waves in the northwestern pacific ocean and the northern philippine sea from seismograms retrieved from a long term broad band ocean bottom seismometer lt obs array deployed in 1999 2000 and from those recorded in japan guam and ponape islands this array observation was one of the first attempts to record continuous seismograms on the seafloor using broad band seismographs on a long term basis the geographical distribution of the lt obs stations and the surrounding land stations enabled us to determine pure path phase velocities in a frequency range from 0 01 to 0 05 hz across the mariana trough the parece vela basin the shikoku basin and the minami daito basin which were found to be systematically lower than those of the pacific seafloor at comparable ages this result along with other geophysical and geological observations in the philippine sea may be explained by an uppermost mantle that is relatively fe rich

three dimensional s wave structure of the upper mantle beneath turkey from surface wave tomography summary a 3 d upper mantle structure beneath turkey is investigated using phase speeds of fundamental mode rayleigh waves employing a conventional two station method with high density seismic networks in turkey we analyse 289 seismic events with moment magnitude 5 5 and greater and with focal depth shallower than 100 km between 2006 and 2008 waveform data are derived from 164 three component broad band seismic stations operated by two national seismic networks at first rayleigh wave phase speed maps are obtained from the inversion of two station phase speeds using about 1000 3000 paths depending on the period of rayleigh waves the three dimensional s wave model is then obtained in the depth range from 40 to 180 km using the phase speed maps in the period range from 25 to 120 s our model reveals the fast anomalies in the north of cyprus associated with the subducted portion of the african oceanic lithosphere from the cyprus trench we identify a vertical discontinuity of the fast anomaly associated with the cyprus slab starting at 60 80 km depth which may represent a minor tear of the cyprus slab we observed that the western part of the cyprus slab is getting closer to the edge of the hellenic slab beneath the isparta angle ia and antalya basin our model also indicates a slow wave speed anomaly beneath the ia and antalya basin probably due to hot materials of asthenosphere rising from a tear of the subducted african oceanic lithosphere that is a slab tear between the cyprus and the hellenic subductions in the eastern part of turkey a widespread slow anomaly appears in the model that corresponds to the eastern anatolian accretionary complex eaac our model shows a fast anomaly beneath the eaac that can be interpreted as the detached portion of the subducted arabian lithosphere

cycle stratigraphy and porosity in pennsylvanian and lower permian shelf limestones eastern central basin platform texas pennsylvanian and lower permian shelfal limestones were studied in core and wireline logs on the eastern side of the central basin platform in west texas sixty three 63 cycles were delineated in the study interval which includes 200250 m of canyon missourian cisco virgilian and wolfcamp strata four general lithofacies are present fossiliferous wackestones and packstones grainstones phylloid algal boundstones and shales these lithologies typically occur in 118 m thick cycles bounded by subaerial exposure surfaces grainstones in the upper part of some cycles indicate a shallowing of environments prior to subaerial exposure many cycles have subaerial exposure surfaces developed on subtidal fossiliferous wackestones or packstones suggesting rapid falls in se level long term transgressive intervals transgressive systems tracts or tst are dominated by thick 4 m cycles whereas long term regressive intervals highstand systems tracts or hst are dominated by thinner cycles stable carbon isotope data suggest that thick cycles in tsts were subjected to short periods of subaerial exposure whereas thin cycles in the hsts were subjected to much longer subaerial exposure porosity is quite variable beneath subaerial exposure surfaces at any given well reservoir grade porosity 4 is present below only 1045 of the identified subaerial exposure surfaces where pres ent reservoir grade porosity occurs in the upper part of cycles 0 35 m below subaerial exposure surfaces porosity in thick cycles of the tst is relatively widespread and not facies selective porosity is less abundant and facies selective in the hst with porosity occurring only in relatively thick 1 54 m grainstones which are concentrated near the shelf margin thin

high resolution geochemistry and sequence stratigraphy of the hushpuckney shale swope formation eastern kansas implications for climato environmental dynamics of the late pennsylvanian midcontinent seaway abstract the hushpuckney shale member of the swope formation missourian stage eastern kansas is the core shale of a kansas type cyclothem formed during the late transgressive to early regressive phases of a late pennsylvanian glacio eustatic cycle coeval high frequency climato environmental dynamics of the midcontinent seaway are preserved in the kgs orville edmonds no 1a study core as centimeter scale variation in major components organic carbon authigenic sulfides and phosphate detrital siliciclastics organic macerals trace element redox proxies mo u v zn and ichnofabric features benthic o 2 levels declined sharply from the base of the black shale 0 cm went sulfidic at 4 cm and reached a redox minimum at 21 cm above this level redox potential gradually rose fluctuating between sulfidic and nonsulfidic conditions from 35 cm to the black shale gray shale contact at 52 cm onset of dysoxic conditions at that contact allowed establishment of a benthic community of soft bodied organisms comprised of deep tiered tracemakers tolerant of low o 2 conditions trichichnus and shallow tiered tracemakers favoring soupground helminthopsis or firmground substrates the zoophycos phycosiphon schaubcylindrichnus planolites association most geochemical records exhibit a low order cycle spanning the full 52 cm thickness of the black shale submember reflecting a dominant glacio eustatic control the base and top of the black shale submember record lateral migration of the pycnocline across the midcontinent shelf during the transgressive and regressive phases respectively the maximum flooding surface mfs is at 17 23 cm an interval containing the euxinic peak and characterized by high concentrations of illite representing a cratonic siliciclastic flux and terrestrial organic macerals the product of transient increases in humidity weathering rates and the export of coal swamp vegetation associated with the interglacial highstand of the swope cyclothem although a transgressive surface of maximum starvation sms cannot be confidently identified a phosphate rich regressive condensation surface at 28 34 cm records pycnoclinal weakening due to increased aridity associated with renewed southern hemisphere icesheet growth a correlative shift in dominance from terrestrial to marine derived organic macerals reflects increased upwelling of nutrient rich deepwaters and enhanced primary productivity penecontemporaneous paleogeographic and climatic factors e g semirestricted circulation monsoonal precipitation predisposed the midcontinent seaway toward sensitivity to high frequency climato environmental fluctuations which are preserved as 2 to 7 cm thick cycles in the study core the 52 cm thick black shale submember contains 12 such high order cycles which have an estimated average duration of 2 to 9 kyr implying a sub milankovitch band i e millennial scale climatic control the results of this study are relevant to the sequence stratigraphic interpretation of kansas type cyclothems the dynamics of gondwanan ice age glacio eustatic cycles and the development of boundary conditions for late pennsylvanian paleoclimate models

characteristics stratigraphic architecture and time framework of multi order mixed siliciclastic and carbonate depositional sequences outcropping cisco group late pennsylvanian and early permian eastern shelf north central texas usa abstract the cisco group on the eastern shelf of the midland basin is composed of fluvial deltaic shelf shelf margin and slope to basin carbonate and siliciclastic rocks sedimentologic and stratigraphic analyses of 181 meter to decimeter scale depositional sequences exposed in the up dip shelf indicated that the siliciclastic and carbonate parasequences in the transgressive systems tracts tst are thin and upward deepening whereas those in highstand systems tracts hst are thick and upward shallowing the sequences can be subdivided into five types on the basis of principal lithofacies and exhibit variable magnitude of facies shift corresponding to variable extents of marine transgression and regression on the shelf the sequence stacking patterns and their regional persistence suggest a three level sequence hierarchy controlled by eustasy whereas local and regional changes in lithology thickness and sequence type magnitude and absence were controlled by interplay of eustasy differential shelf subsidence depositional topography and pattern of siliciclastic supply the outcropping cisco group is highly incomplete with an estimated 6 11 stratigraphic completeness the average duration of deposition of the major third order sequences is estimated as 67 102 ka on the up dip shelf and increases down dip while the average duration of the major sequence boundaries sb is estimated as 831 1066 ka and decreases down dip the nondepositional and erosional hiatus on the up dip shelf was represented by lowstand deltaic systems in the basin and slope

crustal structure in the society and tuamotu islands french polynesia summary we present seismic investigations of the crustal structure of tahiti society islands rangiroa tuamotu islands and neighbouring areas records from a series of 96 explosions at the short period stations of the polynesian seismic network form a seismic refraction dataset from which we recover the crustal structures the crust in the vicinity of tahiti has the classic structure of 70 myr old ocean floor and features an 8 8 km thick crust below sea floor with a 7 64 8 25 km s jump at the mohorovieic moho discontinuity on the other hand the tuamotu plateau shows a very thick crust reaching 31 km below sea floor with a 6 83 8 10 km s l jump at the moho in addition rangiroa atoll itself features a 2 km thick layer of slow material 3 3 km s which we interpret as limestone comparable structures have been reported at enewetak and bikini we further use the short period seismic arrays on rangiroa and tahiti to recover the slowness vectors and hence the dispersion characteristics of kayleigh waves in the 15 40 s period range recorded from teleseismic events rayleigh dispersion under the society islands is compatible with published oceanic models of adequate age we also use long period records in the 20 90s range for the path rangiioa hao orientated along the tuamotu plateau the rangiroa hao dispersion is slower than previously determined in neighbouring oceanic areas and compatible with a crust 22 30 km thick under the tuamotu plateau dispersion under rangiroa is even slower and requires a thicker 35 km crust any crust of standard thickness can be ruled out as it leads to unacceptably low 3 km s crustal s velocities these results are comparable to structures published for the icelandfzroe and walvis ridges and confirm that the tuamotus were formed onridge in agreement with their very weak geoid signature

the seafloor swells and superswell in french polynesia high quality ship soundings obtained since 1967 in french polynesia are used to better define the pattern of depth anomalies in french polynesia and to test the accuracy of previous results based on etopo5 gridded data the modes of the differences between the ship data and etopo5 demonstrate that the latter database is accurate to 250 m for nearly 70 of the observations in the region although there is a tendency for etopo5 to be of the order of 35 m too shallow the ship depth versus age distribution shows that the south pacific s unusually shallow seafloor termed the superswell cannot simply be described as the cooling of a hotter or thinner thermal plate rather the seafloor rises between the ages of 40 and 80 million years before resuming thermal subsidence the superswell appears to be bounded to the north by the marquesas fracture zone and the depth anomaly tapers down to smaller values south of the austral fracture zone its modal depth versus age relation cannot be mimicked easily by classical steady state thermal cooling models leading us to prefer dynamic explanations for the depth anomaly modal depth analysis aids in the isolation of the individual swells superimposed on the superswell whereas the marquesas swell just north of the superswell is similar to other swells worldwide in its width 1000 km and amplitude 1 km both the society and austral swells are narrower and lower than global norms we explain the small size of the society and austral swells as the reduced thermal buoyancy of these hotspots entrained in a broader mantle upwelling