unraveling the evolution of chondrite parent asteroids by precise u pb dating and thermal modeling u th pb isotopic data are reported for mineral fractions individual chondrules and fractions of chondrule fragments from the equilibrated ordinary chondrite richardton h5 chondrules and milligram sized fractions of pyroxene rich chondrule fragments contain highly radiogenic pb and concordant or nearly concordant u th pb isotopic systems and are suitable for precise pb pb age determinations olivine and sulfide have low u concentrations and contain less radiogenic pb the ages of individual chondrules pyroxene rich and phosphate fractions are determined using u pb and pb pb isochron and model date calculations the pb pb isochron date of 4562 7 1 7 ma of the richardton chondrules and chondrule fragments is resolved from the pb pb isochron date of 4550 7 2 6 ma obtained from multiple phosphate fractions possible biases of the isochron dates due to single stage approximation of multi stage evolution contamination with modern common pb and disturbance to the system by reheating are examined and are found to be insignificant the chondrule and phosphate dates are interpreted as the timing of cessation of pb diffusion during cooling following metamorphism in chondrite parent bodies the difference in estimated closure temperatures 950 1150 k for pyroxenes and 700 800 k for phosphates temperature estimates are based on published diffusion rates for pb in pyroxenes and apatite allows evaluation of the average cooling rate at 26 13 k million years for the richardton parent body over the period of 4563 4551 my thermal modeling of the h chondrite parent body which is assumed to be asteroid 6 hebe heated by decay of 26al suggests a scenario in which accretion initiated at 1 7 m y after formation of calcium aluminum rich inclusions and continued for 3 5 m y

hydraulic geometry river sediment and the definition of bedrock channels a growing literature deals with bedrock channels erosion processes within them and their role in landscape evolution formal definitions currently in use classify channels as bedrock when alluvial cover is discontinuous or thin this is equated with a physical property of the flow using up to date erosion laws and descriptions of channel dynamics we show that existing definitions are difficult to apply and may lead to conflicting classifications we propose the definition a bedrock channel cannot substantially widen lower or shift its bed without eroding bedrock applied to channels in taiwan this leads to a different classification than previous definitions by analysing at a station hydraulic geometry of these channels we show that the classification exposes different controls of sediment effects on dynamics of channels classified as alluvial and bedrock

a froude scaled model of a bedrock alluvial channel reach 2 sediment cover previous research into sediment cover in bedrock alluvial channels has focussed on total sediment cover rather than the spatial distribution of cover within the channel the latter is important because it determines the bedrock areas that are protected from erosion and the start and end of sediment transport pathways we use a 1 10 froude scaled model of an 18 by 9 m reach of a bedrock alluvial channel to study the production and erosion of sediment patches and hence the spatial relationships between flow bed topography and sediment dynamics the hydraulic data from this bed are presented in the companion paper in these experiments specified volumes of sediment were supplied at the upstream edge of the model reach as single inputs at each of a range of discharges this sediment formed patches and once these stabilized flow was steadily increased to erode the patches in summary 1 patches tend to initiate in the lowest areas of the bed but areas of topographically induced high flow velocity can inhibit patch development 2 at low sediment inputs the extent of sediment patches is determined by the bed topography and can be insensitive to the exact volume of sediment supplied and 3 at higher sediment inputs more extensive patches are produced stabilized by grain grain and grain flow interactions and less influenced by the bed topography bedrock topography can therefore be an important constraint on sediment patch dynamics and topographic metrics are required that incorporate its within reach variability the magnitude and timing of sediment input events controls reach scale sediment cover

past present and future of thermochronology in one form or another geochronologists have been practicing thermochronology 1 the use of radioisotopic dating to constrain thermal histories of rocks and minerals for over 40 years building from lessons learned over these four decades thermochronology continues to evolve due to technical developments increasingly sophisticated theoretical models and an expanding range of applications in geologic and planetary science most recently interest in earth surface processes and interactions between tectonics erosion and climate has drawn attention to techniques that can address the timing and rates of processes operating at temperatures below about 300 c the purpose of this rimg volume is to assess the current state of thermochronology as of circa 2005 which is coincidentally the 100th anniversary of the first radioisotopic date rutherford 1905 1906 excellent review papers and books on specific topics within this field have been published but no single volume has yet provided a comprehensive review of current practices basic theory and illustrative examples the motivation for this volume stems from these considerations knowing that in a fast developing field a book like this can quickly become dated we tried to include sufficient review of fundamentals and the literature to offer students and new users a useful introduction to thermochronology that may have some staying power in this chapter we first review the salient points of thermochronology s history before assessing our current capabilities and challenges and then taking the risk of suggesting where the field is headed we do not provide a comprehensive history of the method that does full justice to the work of the large and growing cohort of thermochronologists in this short space we instead opted to give our perspectives on where the intellectual and technological roots of the discipline lie which run deeper and go back farther than is sometimes appreciated

constraints on the cooling history of the h chondrite parent body from phosphate and chondrule pb isotopic dates from estacado to constrain the metamorphic history of the h chondrite parent body we dated phosphates and chondrules from four h6 chondritic meteorites using u pb systematics reconnaissance analyses revealed that only estacado had a sufficiently high 206pb 204pb ratio suitable for our purposes the pb pb isochron date for estacado phosphates is measured to be 4492 15 ma the internal residue second leachate isochron for estacado chondrules yielded the chondrule date of 4546 18 ma an alternative age estimate for estacado chondrules of 4527 6 6 3 ma is obtained from an isochron including two chondrules two magnetically separated fractions and four bulk chondrite analyses this isochron date might represent the age of termination of pb diffusion from the chondrules to the matrix from these dates and previously established closure temperatures for pb diffusion in phosphates and chondrules we estimate an average cooling rate for estacado between 5 5 3 2 myr c and 8 3 5 0 myr c using previously published results for ste marguerite h4 and richardton h5 our data reveal that the cooling rates of h chondrites decrease markedly with increasing metamorphic grade in agreement with the predictions of the onion shell asteroid model several issues however need to be addressed before confirming this model for the h chondrite parent body the discrepancies between peak metamorphic temperatures established by various mineral thermometers need to be resolved diffusion and other mechanisms of element migration in polycrystalline solids must be better understood and dating techniques should be further improved

facies architecture and depositional model of a macrotidal incised valley succession qiantang river estuary eastern china and differences from other macrotidal systems filling of the modern coastal zone portion of the qiantang river qr in eastern china was initiated by marine inundation during the transgression after the last glacial maximum and has continued during the holocene sea level stillstand the early part of the fill is transgressive in character while the younger part is regressive this paper deals with the sedimentary facies surfaces architecture and depositional model of the qr incised valley fill based on the detailed analysis of the newly drilled core se2 and its correlation with more than 800 boreholes the incised valley deposits are grouped into five stacked facies amalgamated channel floodplain and channel paleo estuary offshore shallow marine and present day estuary a paleo estuary facies had never been identified before making these observations novel this facies is characterized by a sedimentary succession and sediment distribution that are distinct from those of the present day estuary because of a change in sediment supply from the sea floor beyond the estuary mouth it also contains large numbers of tidal channel sand bodies that are significant reservoirs for shallow biogenic gas since the last glaciation there have been three stages in the development of the qr incised valley fill sequence 1 a formation stage associated with sea level fall 2 a filling stage during the early transgression with rapid sea level rise and 3 a burial stage corresponding to the slowing of sea level rise and the onset of progradation this fill consists of four longitudinal segments each of which is distinguished by a distinct stratigraphic succession and different degrees of marine and fluvial influence the basal erosion surface and sidewalls of the incised valley the top of stiff clay on the interfluves and the top of falling stage fluvial terrace deposits compose the sequence boundary the initial flooding surface and maximum flooding surface are located within the amalgamated channel and offshore shallow marine sediments respectively these observations indicate that 1 relative sea level changes determine the stratal stacking patterns but local environmental factors such as physical processes accommodation sediment supply and coastal configuration control the nature of the facies surfaces and sediment distribution patterns and 2 the tripartite facies organization and wave ravinement surfaces typifying wave dominated or mixed energy wave and tide incised valley fills are absent in the macrotidal qr succession and the tidal ravinement and erosion surfaces of the macrotidal qr incised valley are more extensive and numerous than those of the wave dominated or mixed energy succession

fission track analysis reveals character of collisional tectonics in new zealand during the late cenozoic the pacific plate has been obliquely converging with the australia plate in south island new zealand this has resulted in westward obduction of continental crust of the pacific plate and consequently a major mountain range the southern alps has formed east of the alpine fault and within the leading edge of the pacific plate we have applied the apatite and zircon fission track dating technique to a suite of outcrop samples from three transects across the southern alps starting at the alpine fault to better understand the uplift age and structure of the continental collision the results show the following 1 uplift was underway at 7 ma in the south at haast pass and by 5 ma in the north at arthur s pass 2 profiles of the total amount of late cenozoic uplift show an exponential increase toward the alpine fault adjacent to the alpine fault there has been 14 5 to 21 5 km of uplift in the vicinity of haast pass 14 5 to 21 km near mt cook and 12 5 to 15 5 km in the arthur s pass transect adjacent to the alpine fault in the mount cook region the average uplift rate from 5 5 ma to 1 ma was 2 3 to 2 5 mm yr between 1 ma and the present it has averaged 6 2 mm yr the strongly asymmetric uplift pattern is responsible for the exposure of a 13 25 km wide belt of greenschist to midamphibolite facies rocks immediately east of the alpine fault known as the alpine schists 3 late cenozoic uplift of torlesse greywackes and alpine schists was preceeded during the jurassic early cretaceous by 5 8 km of uplift the otago schist belt to the south was uplifted much more during this interval and in addition western parts of the otago schists were uplifted 5 6 km during the late cretaceous 4 the fission track data do not support the notion of significant late cenozoic shear heating about the alpine fault as previously suggested from spatial variations in k ar ages of the otago and alpine schists

importance of the accretion process in asteroid thermal evolution 6 hebe as an example widespread evidence exists for heating that caused melting thermal metamorphism and aqueous alteration in meteorite parent bodies previous simulations of asteroid heat transfer have assumed that accretion was instantaneous for the first time we present a thermal model that assumes a realistic incremental accretion scenario and takes into account the heat budget produced by decay of 26al during the accretion process by modeling 6 hebe assumed to be the h chondrite parent body we show that in contrast to results from instantaneous accretion models an asteroid may reach its peak temperature during accretion the time at which different depth zones within the asteroid attain peak metamorphic temperatures may increase from the center to the surface and the volume of high grade material in the interior may be significantly less than that of unmetamorphosed material surrounding the metamorphic core we show that different times of initiation and duration of accretion produce a spectrum of evolutionary possibilities and thereby highlight the importance of the accretion process in shaping an asteroid s thermal history incremental accretion models provide a means of linking theoretical models of accretion to measurable quantities peak temperatures cooling rates radioisotope closure times in meteorites that were determined by their thermal histories

measuring flow velocity and sediment transport with an acoustic doppler current profiler an acoustic doppler current profiler adcp measures three dimensional velocity profiles within the water column using the doppler shift principle whilst the bottom tracking function and acoustic backscatter can be used to measure bed load velocity and estimate suspended sediment concentration the adcp offers many advantages over traditional single point current meters and sediment samplers including deployment from a moving launch a single instrument for both velocity and sediment transport measurements profiles of three dimensional velocity and suspended sediment and the ability to map an entire flow field limitations of adcps include a large sampling diameter close to the bed coarse measurement of vertical velocity fragmented bottom track records a poor understanding of the relation between bottom tracking and the mechanisms of sediment transport and sensitivity of the acoustic backscatter to particle size

identifying environmental controls on the shoreline of a natural river delta river deltas form where sediment laden water debouches into a basin the spatial delineation of a delta is nontrivial and yet is fundamental to systematically evaluating fluxes across its surface here we study shoreline dynamics of the wax lake delta wld a naturally developing delta downstream of the atchafalaya river usa we demonstrate the ability to extract hydrodynamic and morphodynamic shorelines from time series of satellite imagery and topography data respectively the hydrodynamic shoreline corresponds to the traditional dry wet interface whereas we introduce the concept of a morphodynamic shoreline demarcating the topset foreset transition of a delta to quantitatively express the degree of inundation of a delta plain these shorelines enable us to assess environmental controls on inundation of the wld delta plain noting the abundance of satellite imagery whereas time series of bathymetric data from delta plains are scarce from the analysis of noaa and u s geological survey environmental data we identify the effects of river discharge tides wind and vegetation on shoreline position in particular using delft3d simulations and a simplified momentum balance we highlight the nonuniform and nonlinear effect of wind on delta plain inundation our analyses reveal that wind riverine discharge and tides significantly contribute to inundation of the wld and hence incorporating their interaction is essential to the accurate modeling of hydrodynamics and ecodynamics of the wld delta plain as well as to the dynamic shape of delta plains in general

peak metamorphic temperatures in type 6 ordinary chondrites an evaluation of pyroxene and plagioclase geothermometry quantifying the peak temperatures achieved during metamorphism is critical for understanding the thermal histories of ordinary chondrite parent bodies various geothermometers have been used to estimate equilibration temperatures for chondrites of the highest metamorphic grade type 6 but results are inconsistent and span hundreds of degrees because different geothermometers and calibration models were used with different meteorites it is unclear whether variations in peak temperatures represent actual ranges of metamorphic conditions within type 6 chondrites or differences in model calibrations we addressed this problem by performing twopyroxene geothermometry using quilf95 on the same type 6 chondrites for which peak temperatures were estimated using the plagioclase geothermometer nakamuta and motomura 1999 we also calculated temperatures for published pyroxene analyses from other type 6 h l and ll chondrites to determine the most representative peak metamorphic temperatures for ordinary chondrites pyroxenes record a narrow overlapping range of temperatures in h6 865 926 c l6 812 934 c and ll6 874 945 c chondrites plagioclase temperature estimates are 96 179 c lower than pyroxenes in the same type 6 meteorites plagioclase estimates may not reflect peak metamorphic temperatures because chondrule glass probably recrystallized to plagioclase prior to reaching the metamorphic peak the average temperature for h l and ll chondrites 900 c which agrees with previously published oxygen isotope geothermometry is at least 50 c lower than the peak temperatures used in current asteroid thermal evolution models this difference may require minor adjustments to thermal model calculations

rapid rotation of southwest japan palaeomagnetism and k ar ages of miocene volcanic rocks of southwest japan summary miocene volcanic rocks have been sampled from the san in district in the central part of southwest japan reliable palaeomagnetic direction and k ar dating data have been obtained from 28 localities a clockwise deflection in declination d 38 5 is observed in the kawai formation having an age of 16 1 1 4 ma whereas a northerly declination is observed in formations younger than 14 2 0 6 ma these data indicate that southwest japan had ceased rotational motion by about 14 ma when compared with the amount of rotation estimated from cretaceous palaeomagnetic data it is apparent that more than 80 per cent of the overall clockwise rotation occurred between about 16 and 14 ma we conclude that the angular velocity of southwest japan about a rotation pole at 129 e 34 n reached 20 myr 1 at about 15 ma the corresponding rate of linear motion of the eastern part of southwest japan was 21 cm yr 1

finite amplitude bars in mixed bedrock alluvial channels we present a nonlinear asymptotic theory of fully developed flow and bed topography in a wide channel of constant curvature to describe finite amplitude perturbations of bottom topography subject to an inerodible bedrock layer the flow field is evaluated at the leading order of approximation as a slowly varying sequence of locally uniform flows slightly perturbed by a weak curvature induced secondary flow using the constraint of constant fluid discharge and sediment flux we calculate an analytical solution for the cross sectional profile of flow depth and bed topography and we determine the average slope in the bend necessary to transport the sediment supplied from a straight alluvial upstream reach both fully alluvial bends and bends with partial bedrock exposure are shown to require a larger average slope than a straight upstream reach the relative slope increase is much larger for mixed bedrock alluvial bends curvature and sediment supply are shown to have a strong effect on the characteristics of the point bars in mixed bedrock alluvial channels higher curvature bends produce bars of larger amplitude and more bedrock exposure through the cross section and increasing the sediment supply leads to taller and wider point bars differences in the relative roughness of sediment and bedrock have a smaller secondary effect on point bar characteristics our analytical approach can potentially be extended to the case of arbitrary yet slowly varying curvature and should ultimately lead to an improved understanding of the formation of meanders in bedrock channels

decadal scale riverbed deformation and sand budget of the last 500 km of the mississippi river insights into natural and river engineering effects on a large alluvial river

sediment transport and topographic evolution of a coupled river and river plume system an experimental and numerical study sediment transfer from rivers to the ocean is the fundamental driver of continental sedimentation with implications for carbon burial land use dynamics and unraveling global climate change and earth history from sedimentary strata coastal rivers are dynamically coupled to their offshore plumes at the river mouth creating regions of nonuniform flow that can dictate patterns of erosion and deposition both onshore and offshore however there are limited experimental and modeling studies on sediment transport and morphodynamics of coupled river and river plume systems and their response to multiple flood events to address this knowledge gap we developed a quasi 2 d morphodynamic numerical model and conducted exploratory flume experiments in a 7 5 m long flume where a 10 cm wide river channel was connected to a 76 cm wide ocean basin both the numerical model and the flume results demonstrate that 1 during low discharge flows backwater hydrodynamics cause spatial flow deceleration and deposition in the river channel and the offshore plume area and 2 during high flows the water surface is drawn down to sea level resulting in spatial flow acceleration and bed scour during high discharge flows we also found that the offshore river plume self channelized owing to both levee formation and bed scour our study suggests that coastal rivers may be in a perpetual state of morphodynamic adjustment and highlights the need to link rivers and river plumes under a suite of flow discharges to accurately predict fluvio deltaic morphodynamics and connectivity between fluvial sediment sources and marine sinks

transcurrent movements in the burma andaman sea region the drag produced by the northward movement of the indian plate has caused the indoburman ranges an accretionary prism to decouple from the burma plate along the kabaw fault and the burma plate itself has decoupled from the eastern highlands of burma along the sagaing fault the indoburman ranges have probably been dragged north slightly ahead of the burma plate although at times the two plates may have locked together to move as one unit it is suggested that the double arc formed by the indoburman ranges and the andaman nicobar sumatra islands was formerly one large single arc northward translation of this larger single arcuate structure has caused the tail end of the arc i e the andaman nicobar sumatra section to rotate into the burma plate causing a bending of the arc to form the present double arc and compression within the burma plate to form the pegu yoma folding northward movement of the burma plate has also resulted in offset by over 400 km of the irrawaddy river from its former connection through the channel of what is now the chindwin river

quaternary geomorphic trends within southwest japan extensive wrench deformation related to transcurrent motions of the median tectonic line abstract large scale transcurrent faults contribute to anelastic deformations along convergent plate boundaries the inner zone of southwest japan has been subjected to shearing deformations during the quaternary associated with dextral motions of the median tectonic line which was activated as a right lateral fault by oblique subduction of the philippine sea plate considering the neotectonic framework the continental crust of the chugoku block in the western part of the inner zone is simplified as a granitic test piece under a simple shear geological and geographical data in the block are incorporated to produce a quantitative analysis of the deformation mode together with an empirical model of the rheology of an orogenic crust the present results indicate that the chugoku block behaves as a newtonian fluid and flows with a strain rate of about 10 14 s 1 under a tectonic stress of 10 mpa the mode of wrenching of the study area is compared with confined deformations of other plate margins in the same tectonic regime unique extensive wrench deformation as wide as 150 km along the median tectonic line is attributed to the absence of weak zones in the continental crust such as ductile melange belt or arc volcanism which is a common phenomenon in the area of active convergence the inner zone of southwest japan is immune from these major factors reducing the shearing strength of the crust because of the intensive intrusions of cretaceous granites and the regional quaternary thermal event

a mechanistic model for river incision into bedrock by saltating bed load 1 abrasion by bed load is a ubiquitous and sometimes dominant erosional mechanism for fluvial incision into bedrock here we develop a model for bedrock abrasion by saltating bed load wherein the wear rate depends linearly on the flux of impact kinetic energy normal to the bed and on the fraction of the bed that is not armored by transient deposits of alluvium we assume that the extent of alluvial bed cover depends on the ratio of coarse sediment supply to bed load transport capacity particle impact velocity and impact frequency depend on saltation trajectories which can be predicted using empirical functions of excess shear stress the model predicts a nonlinear dependence of bedrock abrasion rate on both sediment supply and transport capacity maximum wear rates occur at moderate relative supply rates due to the tradeoff between the availability of abrasive tools and the partial alluviation of the bedrock bed maximum wear rates also occur at intermediate levels of excess shear stress due to the reduction in impact frequency as grain motion approaches the threshold of suspension measurements of bedrock wear in a laboratory abrasion mill agree well with model predictions and allow calibration of the one free model parameter which relates rock strength to rock resistance to abrasive wear the model results suggest that grain size and sediment supply are fundamental controls on bedrock incision rates not only by bed load abrasion but also by all other mechanisms that require bedrock to be exposed in the channel bed

penecontemporaneous metamorphism fragmentation and reassembly of ordinary chondrite parent bodies the occurrence and proportions of petrologic types among ordinary chondrites have been explained by internal heating of three h l ll parent asteroids of 100 km radius e g miyamoto et al 1982 this conventional onion shell model predicts an inverse relation between petrologic type and metallographic cooling rate but none has been observed scott and rajan 1981 devised a metamorphosed planetesimal model to explain this discrepancy whereby metamorphism occurs in planetesimals a few kilometers in radius which then accrete to form r 100 km parent bodies metallographic cooling rates are then controlled by burial depth thermal and collisional constraints on the metamorphosed planetesimal model are examined here and the model is found to be applicable only to highly insulating 26al rich planetesimals that can accrete intact without being shattered by impact or to shattered planetesimals whose fragments are not widely dispersed over the target surface an alternative model is presented here in which onion shell parent bodies are collisionally fragmented during metamorphism and then gravitationally reassembled if reassembly times are short days then metallographic cooling rates would be determined by burial depth in the reaccreted parent body this model unlike previous ones can explain both coherent and incoherent cooling rates of breccia clasts by collisions during or after metamorphism respectively

connecting the backwater hydraulics of coastal rivers to fluvio deltaic sedimentology and stratigraphy fluvial channels encounter a backwater reach when they approach a standing body of water and recent studies have shown that the transition from normal flow to backwater influenced flow is associated with sediment mass extraction through deposition here we test the hypothesis that systematic changes in the geometry of channel belt deposits and sedimentary architecture occur across this transition using data from the late holocene mississippi southern usa and rhine the netherlands fluvio deltaic systems we use the estimated backwater length and average channel width as characteristic length scales to non dimensionalize the downstream trends in channel belt width for these systems the collapsed data follow similar trends suggesting that the observed variations in channel belt geometry and fluvio deltaic stratigraphy are tied to the location of the backwater transition zone these findings suggest a unifying hydraulic control on fluvio deltaic channel belts and provide a new framework for predicting and understanding the properties of ancient rivers in the coastal zone

the quasi equilibrium longitudinal profile in backwater reaches of the engineered alluvial river a space marching method an engineered alluvial river i e a fixed width channel has constrained planform but is free to adjust channel slope and bed surface texture these features are subject to controls the hydrograph sediment flux and downstream base level if the controls are sustained or change slowly relative to the timescale of channel response the channel ultimately achieves an equilibrium or quasi equilibrium state for brevity we use the term quasi equilibrium as a shorthand for both states this quasi equilibrium state is characterized by quasi static and dynamic components which define the characteristic timescale at which the dynamics of bed level average out although analytical models of quasi equilibrium channel geometry in quasi normal flow segments exist rapid methods for determining the quasi equilibrium geometry in backwater dominated segments are still lacking we show that irrespective of its dynamics the bed slope of a backwater or quasi normal flow segment can be approximated as quasi static i e the static slope approximation this approximation enables us to derive a rapid numerical space marching solution of the quasi static component for quasi equilibrium channel geometry in both backwater and quasi normal flow segments a space marching method means that the solution is found by stepping through space without the necessity of computing the transient phase an additional numerical time stepping model describes the dynamic component of the quasi equilibrium channel geometry tests of the two models against a backwater exner model confirm their validity our analysis validates previous studies in showing that the flow duration curve determines the quasi static equilibrium profile whereas the flow rate sequence governs the dynamic fluctuations

alluvial morphodynamics of low slope bedrock reaches transporting nonuniform bed material research on bedrock rivers primarily focused on bedrock incision and to the best of our knowledge morphodynamic models have not yet considered the variability of sediment grain size and the prese

a theoretical framework for the morphodynamics of bedrock channels 1 bedrock rivers are characterized by conditions of insufficient sediment supply wherein the amount of sediment supplied to the channel is less than the channel s sediment transport capacity consequently bedrock channels generally exhibit discontinuous sediment cover which current approaches to the morphodynamics of alluvial rivers are unable to handle to fill this gap we present a theoretical framework in which local sediment transport rates are proportional to the areal concentration of sediment available for transport on the bed and the sediment continuity equation is reformulated to account for temporal changes in areal concentration of sediment on nonalluviated surfaces we then perform a linear stability analysis that shows that an initially uniform distribution of the areal concentration of sediment on the bed is unstable to small perturbations this suggests that the sediment cover in bedrock channels tends to concentrate into regions which may eventually be alluviated in general agreement with experimental observations

age and isotopic relationships among the angrites lewis cliff 86010 and angra dos reis results of a wide ranging isotopic investigation of the unique antarctican angrite lew 86010 lew are presented together with a reassessment of the type angrite angra dos reis ador the principal objectives of this study are to obtain precise radiometric ages initial sr isotopic compositions and to search for the erstwhile presence of the short lived nuclei 146sm and 26al via their daughter products the isotopic compositions of sm u ca and ti were also measured this allows a detailed appraisal to be made of the relations between and the geneology of these two angrites lew proves to be severely contaminated with modern terrestrial pb which is shown to result from terrestrial weathering nevertheless concordant pb pb model ages of pyroxene separates were obtained 2 4 55784 52 ga for lew and 4 55780 42 ga for ador uranium isotopic compositions are normal within error the inferred initial pb isotopic composition is within error of primordial pb as defined by canon diablo troilite because of the extreme upb ratios of both angrites this places an upper limit of 2 ma on the time between volatile element loss of the angrite parent body or its precursor planetesimals and the final crystallization of the angrites as differentiates initial 87sr86sr ratios were found to be indistinguishable in lew ador and the cumulate eucrite moore county the derived initial 87sr86sr for all three meteorites is 0 698970 15 2 a 147sm 143nd isochron for lew was obtained yielding an age of 4 553 34 ga with an initial 143nd144nd of 0 506682 49 2 146sm142nd isotope systematics were also measured anomalies on 142nd arising from extinct 146sm were clearly resolved resulting in an initial 146sm144sm of 0 0071 17 with nd142 2 57 0 62 at the time of isotopic closure both of these sm nd methods imply derivation of lew from a reservoir with chondritic smnd ratio they are also quite consistent with the previously reported systematics for ador overall the age and isotopic similarities between lew and ador are striking it suggests almost simultaneous production on the same asteroid even though recent experimental studies imply that the two are not comagmatic calcium and titanium do not exhibit enrichments in the n rich isotopes in contrast to cais making it unlikely that the angrite parent body is inherently rich in early condensates from the solar nebula no evidence was found for live 26al in lew when combined with the pb pb age and initial 87sr86sr data this firmly excludes 26al being an important heat source in the early solar system heat supplied for for example 1 the differentiation of the angrite and eucrite parent bodies and 2 metamorphism of the chondrite parent bodies must come from some other source the debate over live vs extinct 26al in the earliest solar system remains unresolved by this new data from lew however both the angrite and eucrite parent bodies have extremely low rbsr and high upb ratios compared to the solar nebula therefore the common initial 87sr86sr of 0 698970 can be firmly associated with the u pb angrite age above and the formation of both bodies this enables an absolute chronology of the early solar system to be established based upon 87sr86sr differences the oldest cais are a minimum of 11 4 ma older than the angrites i e 4 569 5 ga the age and isotopic constraints are discussed with respect to current collapse condensation and accretion timescales calculated for the solar nebula

river discharge influences on particulate organic carbon age structure in the mississippi atchafalaya river system 1 applying ramped pyrolysis radiocarbon analysis to suspended river sediments we generate radiocarbon 14c age spectra for particulate organic carbon poc from the lower mississippi atchafalaya river system mars to better understand a major river system s role in carbon transport ramped pyrolysis 14c analysis generates age distributions of bulk carbon based on thermochemical stability of different organic components our results indicate higher proportions of older material in the poc during higher discharge ages increase throughout the high discharge age spectra indicating that no single component of the poc is responsible for the overall age increases observed instead older material is contributed across the poc age spectrum and unrelated to increased bedload suspension in this comparison of 2 spring discharges less than half of the poc transported during higher discharge is less than 1000 14c years in age constraining of the role of the mars as a flux of atmospheric co2 toward longer term sedimentary sinks in the mississippi delta and the gulf of mexico the results suggest that delta building processes benefit disproportionately from high discharge events carrying larger amounts of sediment because these events involve both a higher proportion of millennially aged carbon from floodplain exchange of poc and a potentially higher proportion of petrogenic carbon 30 530 increase overall an internally consistent picture of po14c age distributions from a major river system emerges as differences in space and time are small compared to the range of ages of poc sources in such a large basin

the role of discharge variability in the formation and preservation of alluvial sediment bodies abstract extant planform based facies models for alluvial deposits are not fully fit for purpose because they over emphasise plan form whereas there is little in the alluvial rock record that is distinctive of any particular planform and because the planform of individual rivers vary in both time and space accordingly existing facies models have limited predictive capability in this paper we explore the role of inter annual peak discharge variability as a possible control on the character of the preserved alluvial record data from a suite of modern rivers for which long term gauging records are available and for which there are published descriptions of subsurface sedimentary architecture are analysed the selected rivers are categorized according to their variance in peak discharge or the coefficient of variation cvqp standard deviation of the annual peak flood discharge over the mean annual peak flood discharge this parameter ranges over the rivers studied between 0 18 and 1 22 allowing classification of rivers as having very low 0 90 annual peak discharge variance deposits of rivers with very low and low peak discharge variability are dominated by cross bedding on various scales and preserve macroform bedding structure allowing the interpretation of bar construction processes rivers with moderate values preserve mostly cross bedding but records of macroform processes are in places muted and considerably modified by reworking rivers with high and very high values of annual peak discharge variability show a wide range of bedding structures commonly including critical and supercritical flow structures abundant in situ trees and transported large woody debris and their deposits contain pedogenically modified mud partings and generally lack macroform structure such a facies assemblage is distinctively different from the conventional fluvial style recorded in published facies models but is widely developed both in modern and ancient alluvial deposits this high peak variance style is also distinctive of rivers that are undergoing contraction in discharge over time because of the gradual annexation of the channel belt by the establishment of woody vegetation we propose that discharge variability both inter annual peak variation and flashiness may be a more reliable basis for classifying the alluvial rock record than planform and we provide some examples of three classes of alluvial sediment bodies representing low intermediate and high very high discharge variability from the rock record that illustrate this point

modeling flow and sediment transport dynamics in the lowermost mississippi river louisiana usa with an upstream alluvial bedrock transition and a downstream bedrock alluvial transition implications for land building using engineered diversions the lowermost mississippi river defined herein as the river segment downstream of the old river control structure and hydrodynamically influenced by the gulf of mexico extends for approximately 500 km this segment includes a bedrock or more precisely mixed bedrock alluvial reach that is bounded by an upstream alluvial bedrock transition and a downstream bedrock alluvial transition here we present a one dimensional mathematical formulation for the long term evolution of lowland rivers that is able to reproduce the morphodynamics of both the alluvial bedrock and the bedrock alluvial transitions model results show that the magnitude of the alluvial equilibrium bed slope relative to the bedrock surface slope and the depth of bedrock surface relative to the water surface base level strongly influence the mobile bed equilibrium of low sloping river channels using data from the lowermost mississippi river the model is zeroed and validated at field scale by comparing the numerical results with field measurements the model is then applied to predict the influence on the stability of channel bed elevation in response to delta restoration projects in particular the response of the river bed to the implementation of two examples of land building diversions to extract water and sediment from the main channel is studied in this regard our model results show that engineered land building diversions along the lowermost mississippi river are capable of producing equilibrated bed profiles with only modest shoaling or erosion and therefore such diversions are a sustainable strategy for mitigating land loss within the mississippi river delta

eastern mississippi delta late wisconsin unconformity overlying transgressive facies sea level and subsidence abstract analysis of 92 engineering core logs located in the balize sector of the eastern mississippi delta focuses on the late wisconsin unconformity and lithofacies of strata lying immediately below and above this stratigraphic horizon this major sequence boundary is a key feature used to correlate strata across shelf and slope to the basin observations emphasize lithofacies distributions of the latest pleistocene sediments underlying the unconformity the late pleistocene to early holocene transgressive facies and the immediately overlying deltaic deposits maps and a cross section compiled with this information highlight the critical relation between lithofacies distributions and late wisconsin sea level oscillations core analysis reveals that the transgressive facies comprises distinct environments of deposition offshore to onshore findings identify criteria to determine the approximate position of the shoreline at the late wisconsin maximum sea level lowstand and at the extent of early holocene maximum marine inundation in the balize complex mapping of lithologies along the late wisconsin unconformity serves to improve correlation of sediment facies with changes in acoustic response along high resolution seismic profiles an estimate of long term averaged land subsidence suggests lowering of at least 1 mm year near the shelfedge during the past 18 000 years this mapping also refines sea level response models for the mississippi delta including criteria to locate key paleogeographic features such as shelfedges and maximum flooding surfaces in other recent marine deltas and in older stacked deltaic sequences

characteristics of intermittent turbulent structures for river bank undercut depth increment abstract the complex interplay between the resistive forces of the bank material and reynolds shear stress of the turbulent flow field control the fluvial bank erosion in depth knowledge on the interactions of the flow form at the river bank is a prerequisite to understand erosion mechanism and quantification of the erosion rate measurement at 1 5 m from the bank face as reported in the literature revealed that turbulent lateral flux of stream wise momentum propagated towards the bank face that was held responsible for the removal of material from the bank face further experimental results from the literature depict that the stream wise and lateral fluctuating velocity components deploy a random forcing effect on the river bank face which may be one of the primary agents for sediment particle entrainment from the bank face it is gestated from these previous studies that the momentum flux and turbulent structures of the flow field has a great impact on the river bank erosion nevertheless the previous field and experimental studies do not focus on the flow turbulence and sediment interaction that extracts information on the detailed temporal features of sediment entrainment process thus the present study was carried out to understand the coupled dynamics of the effect of intermittent flow structures on the intermittent removal of river bank materials and for advancing the knowledge on river bank erosion process the present experimental study focuses on the evaluation of the turbulent flow characteristics when the undercut mechanism initiates in a cohesive river bank based on the results it is evident that moderate scale intermittent stream wise flow structures probably imposes a periodic loading on the cohesive aggregates of the bank face this accelerates the entrainment of aggregates from the bank face the study also reveals that the undercut increment is a small scale intermittent process findings of the present study may be helpful for the improved design of bank protection measures

assessment of the pb pb and u pb chronometry of the early solar system abstract an evaluation of early solar system chronometry by the pb pb and u pb methods is provided specifically three consequential factors are examined procedure of age calculation extent of terrestrial pb contamination and initial pb isotopic composition on a pb pb diagram high temperature inclusions of the allende meteorite are tightly organized into a well defined line inside a potentially dispersive mixing field which is consistent with the inclusions containing initial pb that is more primitive than that of canon diablo troilite pat consequences of the possible existence of a pre pat pb to the evolution history of the solar nebula are discussed phosphates from the ordinary chondrite st severin appear to be contaminated by terrestrial pb a condition that renders age calculation based on subtraction of pat inaccurate the pb pb mixing line of these phosphates indicates an age of 4 558 ga interestingly angra dos reis phosphate and pyroxene as well as pyroxene of the other angrite lewis cliff 86010 fall precisely on the line defined by st severin phosphates whole rocks of ordinary chondrites are pictorially and explicitly shown to be seriously contaminated with terrestrial pb thus their single stage u pb ages may not be suitable markers of time because their true crystallization ages are often younger than the whole rocks and because of the possibility of multistage evolution phosphates of ordinary chondrites may yield single stage ages older than their true crystallization ages a hypothetical numerical demonstration is provided on the basis of revised ages and new observations we provide an updated chronometry for the early solar system

innovative collaborative research on specimens from the national meteorite collection of canada summary the national meteorite collection of canada nmtc housed within natural resources canada earth sciences sector ottawa includes 2700 fragments and masses of 1100 different meteorites specimens from the collection have been utilized by local scientists and engineers for hardware software analytical and method development related to space exploration this has led to a recognition of the utility of meteorites to help build a larger skilled community of planetary scientists in canada in anticipation of manned and robotic missions to the moon mars asteroids and comets and sample returns from there the successful collaboration can be linked to initiatives from the nmtc carleton university cu and the neptec design group a nasa prime contractor since 2002 these include use of 3d laser imaging of terrestrial rock samples and meteorites to help develop image libraries for use in recognition of the nature of solid objects in extraterrestrial environments meteorites and igneous sedimentary and metamorphic rocks as outcrops or boulders herd et al 2003 non destructive measurement of petrophysical properties of meteorites especially volume density and magnetic susceptibility the first digitally determined meteorite volume was achieved along with ways to use the magnetic response of stony meteorites to help in their classification smith et al 2006a b

autogenic erosional surfaces in fluvio deltaic stratigraphy from floods avulsions and backwater hydrodynamics erosional surfaces set the architecture of fluvio deltaic stratigraphy and they have classically been interpreted in terms of changes in boundary conditions such as climate tectonics and base level allogenic forces intrinsic dynamics of sedimentary systems autogenic dynamics can also create a rich stratigraphic architecture and a major knowledge gap exists in parsing the relative roles of autogenic versus allogenic processes emerging theoretical and experimental work suggests that backwater hydrodynamics play an important role in driving transient channel incision in river deltas even those experiencing net aggradation here we identify and quantify two autogenic mechanisms that produce broad erosional surfaces in fluvio deltaic stratigraphy namely floods and avulsions using a simple mass balance model for single threaded delta channel systems we show that flood induced scours begin near the shoreline and avulsion induced scours begin at the avulsion site and both propagate upstream over a distance that scales with the backwater length bed slope and bed grain size we also develop scaling relationships for the maximum scour depths arising from these mechanisms which are functions of characteristic flow depth and formative flood variability we test our theoretical predictions using a flume experiment of river delta evolution governed by persistent backwater hydrodynamics under constant relative sea level results indicate that autogenic dynamics of backwater mediated deltas under conditions of constant base level can result in stratigraphic surfaces and shoreline trajectories similar to those often interpreted to represent multiple sea level cycles our work provides a quantitative framework to decouple autogenic and allogenic controls on erosional surfaces preserved in fluvio deltaic stratigraphy

shock metamorphism of ordinary chondrites a revised petrographic classification of progressive stages of shock metamorphism of ordinary chondrites is proposed six stages of shock s1 to s6 are defined based on shock effects in olivine and plagioclase as recognized by thin section microscopy the characteristic shock effects of each shock stage are s1 unshocked sharp optical extinction of olivine s2 very weakly shocked undulatory extinction of olivine s3 weakly shocked planar fractures in olivine s4 moderately shocked mosaicism in olivine s5 strongly shocked isotropization of plagioclase maskelynite and planar deformation features in olivine and s6 very strongly shocked recrystallization of olivine sometimes combined with phase transformations ringwoodite and or phases produced by dissociation reactions s6 effects are always restricted to regions adjacent to melted portions of a sample which is otherwise only strongly shocked in stages s3 to s6 localized melting results from stress and temperature peaks which locally deviate from the equilibration shock pressure due to differences in shock impedance these melting effects are a opaque melt veins shock veins b melt pockets with interconnecting melt veins c melt dikes and d troilite metal deposits in fractures based on a critical evaluation of data from shock recovery experiments a shock pressure calibration for the six shock stages is proposed which defines the s1 s2 s2 s3 s3 s4 s4 s5 and s5 s6 transitions at 5 5 10 15 20 30 35 and 45 55 gpa respectively whole rock melting and formation of impact melt rocks or melt breccias occurs at about 75 90 gpa the symbol for the shock stage may be used in combination with the symbol for the petrologic type to abbreviate the complete classification of a chondrite e g h5 s3 we propose this new shock classification and pressure calibration system to replace previous systems which are out of date with respect to the pressure calibration nominally restricted to l chondrites and based on incomplete and in part illdefined sets of shock effects we have classified seventy six ordinary chondrites using the new classification system and conclude the following 1 1 shock effects and the sequence of progressively increasing degrees of shock metamorphism are very similar in h l and ll groups differences in the frequency distribution of shock stages are relatively minor e g l chondrites appear to have the largest fraction with stages s5 and s6 this suggests that the collisional histories of the h l and ll parent bodies were similar 2 2 petrologic type 3 chondrites are deficient in stages s4 to s6 and with increasing petrologic type the frequency of stages s4 to s6 increases we suggest that the more porous and volatile rich type 3 chondrites are subject to melting at a lower shock pressure than the nonporous chondrites of higher petrologic type volatiles trapped in pores cause shock induced dispersal of the shocked and melted material into small particles which are not expected to survive as meteorites 3 3 stage s3 is the most abundant in nearly all petrologic types 4 4 at shock pressures in excess of about 35 gpa s5 and s6 4he and 40ar are almost completely lost pressures below 10 gpa s1 and s2 do not cause noble gas losses

modulation of outer bank erosion by slump blocks disentangling the protective and destructive role of failed material on the three dimensional flow structure the three dimensional flow field near the banks of alluvial channels is the primary factor controlling rates of bank erosion although submerged slump blocks and associated large scale bank roughness elements have both previously been proposed to divert flow away from the bank direct observations of the interaction between eroded bank material and the 3 d flow field are lacking here we use observations from multibeam echo sounding terrestrial laser scanning and acoustic doppler current profiling to quantify for the first time the influence of submerged slump blocks on the near bank flow field in contrast to previous research emphasizing their influence on flow diversion away from the bank we show that slump blocks may also deflect flow onto the bank thereby increasing local shear stresses and rates of erosion we use our measurements to propose a conceptual model for how submerged slump blocks interact with the flow field to modulate bank erosion

the prospect of high precision pb isotopic dating of meteorites the radiogenic 207pb 206pb ratio is the only extant nuclide chronometer with sufficient time resolution for studies of the solar nebula accretion and early asteroidal differentiation and metamorphism pb isotopic dates can be used to link the dates obtained from extinct nuclide chronometers to the absolute time scale the factors that control precision and accuracy of pb isotopic dates of meteorites instrumental mass fractionation in isotopic analysis mass spectrometer sensitivity removal of common pb multi stage evolution of u pb systems disturbances caused by diffusion alteration and shock metamorphism and uncertainties in decay constants and the natural ratio of the u isotopes are reviewed the precision of pb isotopic dates of meteorites attained with currently available techniques and methodology is 0 5 1 0 myr in favorable cases the accuracy of time interval measurements is approximately the same the most serious limitation on precision and accuracy of pb isotopic dates is placed by the presence of common pb of uncertain and or variable isotopic composition improvement in precision and accuracy of pb isotopic dates would be possible through combined advancement of techniques of isotopic analysis most importantly better control over instrumental mass fractionation and more effective techniques for the removal of common pb together with a better understanding of the effects of thermal metamorphism shock metamorphism and aqueous alteration on the u pb system in meteorites

the lowermost mississippi river a mixed bedrock alluvial channel in this study the distribution of channel bed sediment facies in the lowermost mississippi river is analysed using multibeam data complemented by sidescan sonar and compressed high intensity radar pulse seismic data as well as grab and core samples of bed material the channel bed is composed of a discontinuous layer of alluvial sediment and a relict substratum that is exposed on the channel bed and sidewalls the consolidated substratum is made up of latest pleistocene and early holocene fluvio deltaic deposits and is preferentially exposed in the deepest thalweg segments and on channel sidewalls in river bends the exposed substratum commonly displays a suite of erosional features including flutes that are quantitatively similar in form to those produced under known laboratory conditions a total of five bed facies are mapped three of which include modern alluvial deposits and two facies that are associated with the relict substratum a radius of curvature analysis applied to the mississippi river centreline demonstrates that the reach scale distribution of channel bed facies is related to river planform from a broader perspective the distribution of channel bed facies is related to channel sinuosity higher sinuosity promotes greater substratum exposure at the expense of alluvial sediment for example the ratio of alluvial cover to substratum is ca 1 5 1 for a 45 km segment of the river that has a sinuosity of 1 76 and this ratio increases to ca 3 1 for a 120 km segment of the river that has a sinuosity of 1 21 the exposed substratum is interpreted as bedrock and given the relative coverage of alluvial sediment in the channel the lowermost mississippi river can be classified as a mixed bedrock alluvial channel the analyses demonstrate that a mixed bedrock alluvial channel boundary can be associated with low gradient and sand bed rivers near their marine outlet

oblique and near collision subduction sagami and suruga troughs preliminary results of the french japanese 1984 kaiko cruise leg 2 abstract leg 2 of the french japanese 1984 kaiko cruise has surveyed the suruga and the sagami troughs which lie on both sides of the northwestward moving and colliding izu bonin ridge the northernmost part of the philippine sea plate the transition from the nankai trough to the suruga trough is characterized by northward decrease in width of the accretionary prism in good agreement with the increasing obliquity between the through axis and the direction of the convergence as the strike of the convergent boundary changes from ene nne to south north south of the area the southern margin of the zenisu ridge shows contractional deformations this supports the interpretation made by the team of leg 1 who studied the western extension of the area we studied that it is an intra oceanic thrusting of the ridge over the shikoku basin in the sagami trough where the relative motion is highly oblique to the plate boundary active subduction is mostly confined in the east west trending portions of the trough located south of the boso peninsula and along the lower boso canyon near the ttt triple junction in between the present motion is mainly right lateral along the northwest trending boso escarpment however an inactive but recent pliocene to lower pleistocene accretionary prism exists south of the boso escarpment which suggests that the relative motion was more northerly than at present before about 1 ma ago

two pool to pool spacing periods on large sand bed rivers mega pools on the madeira and mississippi abstract the madeira river thalweg includes two pool to pool spacing periods first standard pool crossing sequences form 6 to 8 m deep pools these pool crossing sequences recur approximately every six channel widths 9 7 km or 6 8 w on average however the madeira study reach between manicore and porto velho in northern brazil is relatively straight making these pools more frequent than meanders second much deeper pools 13 to 30 m deep also form at relatively regular but much longer spacing 50 60 km about 40 channel widths apart the deeper mega pools occur where the river reaches the edge of the holocene alluvium encountering resistant units e g granite mud clasts and conglomerates in the confining pleistocene units these two pool scales also occur on the lower mississippi river downstream of tarbert landing this study identified pool formation processes responsible for these two pool depth and spacing scales on two large sand bed rivers the analyses computed pool statistics and correlated the deepest pools with planform controls two distinct pool formation hypotheses explain the two pool types the standard pools are rhythmic features developed by macro turbulent feedbacks while the mega pools are forced features imposed by the regional geology the former are fundamentally periodic the latter are not however periodogram analysis indicates that mega pools on the madeira form a second relatively regular pool period the regional geology including the confining pleistocene bluffs and potentially orthogonal neotectonic lineaments imposed periodic regularity to these mega pools on the madeira river that is not observed on the mississippi river a comparably sized sand bed river with mega pools

current subsidence rates due to compaction of holocene sediments in southern louisiana 1 relative contributions of geologic and anthropogenic processes to subsidence of southern louisiana are vigorously debated of these shallow sediment compaction is often considered dominant although this has never been directly observed or effectively demonstrated quantitative understanding of subsidence is important for predicting relative sea level rise storm surge flooding due to hurricanes and for successful wetland restoration despite many shallow borings few appropriate stratigraphic and geotechnical data are available for site specific calculations we overcome this by determining present compaction rates from monte carlo simulations of the incremental sedimentation and compaction of stratigraphies typical of the holocene of southern louisiana this approach generates distributions of present compaction rates that are not expected to exceed 5 mm yr but may locally locations with present subsidence rates greater than the predicted maximum probable shallow compaction rates are likely influenced by additional processes

punctuated sand transport in the lowermost mississippi river 1 measurements of sand flux and water flow in the mississippi river are presented for a portion of the system 35 50 km upstream from the head of its subaerial delta these data are used to provide insight into how nonuniform flow conditions present in the lower reaches of large alluvial rivers affect the timing and magnitude of sand transport near the river outlet field surveys during both low and high water discharge include 1 sequential digital bathymetric maps defining mobile river bottom topography which were used to estimate bed material flux 2 multiple water velocity profiles and 3 multiple suspended sediment profiles collected using a point integrated sampler these data show that total sand transport increases by two orders of magnitude over the measured range in water discharge 11 300 to 38 400 m3 s 1 during low water discharge no sand is measured in suspension and sand discharge via bed form migration is minimal during high water discharge 54 of the sand discharge is measured in suspension while 46 of the sand discharge is part of bed form migration the component of boundary shear stress associated with moving this sediment is estimated using a set of established sediment transport algorithms and values for the total boundary shear stress are predicted by fitting logarithmic velocity functions to the measured profiles the estimates of boundary shear stress using measurements of suspended sand transport bed form transport and downstream oriented velocity profiles are internally consistent moreover the analyses show that boundary shear stress increases by nearly 10 fold over the measured water discharge range we show how this increase in shear stress is consistent with backwater flow arising where the river approaches its outlet the hydrodynamic properties of backwater flow affect the timing and magnitude of sand flux and produce punctuated sand transport through the lowermost mississippi river our field data are used to evaluate the influence of this sand transport style on development of the mixed bedrock alluvial channel for the lowermost mississippi river

experimental constraints on bar migration through bends implications for meander wavelength selection the bed of an initially straight channel often deforms into a series of migrating alternate bars that can cause bank erosion and the development of periodic planform curvature which is often expressed as a meandering channel trace the planform curvature in turn leads to topography similar to alternate bars but stationary with respect to the meander the migration of alternate bars over this steady topography may lead to interactions influencing meander initiation and wavelength selection we studied these interactions in flume channels with bends of various lengths and angles and having various width to depth ratios in channels whose wavelength was equal to the length of bars in straight channels nearly eight channel widths bars migrated through sine generated channels with angles up to 10 the critical bend angle was smaller for channels of longer wavelength and smaller width to depth ratio migration was nonuniform and bars temporarily stalled when in phase with the curvature induced topography leading to amplification of the topography flow against an opposing bank can lead to such a strong boundary shear stress divergence that migration of the bars is prevented because sediment that otherwise would have led to bar migration is scoured from the bar front we propose that the amplification of bar pool topography in bends where free alternate bars are stalled enhances scour of the bank which leads to selection of a meander wavelength equal to that of the alternate bars

metallic minerals thermal histories and parent bodies of some xenolithic ordinary chondrite meteorites we have studied metal grains in the hosts and lithic fragments of widely differing petrologic types in four xenolithic chondrftes using reflected light microscopy and electron probe analysis in weston and fayetteville which both contain solar flare tracks and solar wind gases kamacite taenite and tetrataenite ordered feni and troilite show a variety of textures on a wood plot of central ni content vs dimension taenite analyses scatter as if metal grains cooled at rates of 10 1000 and 1 100 k myr respectively through 700 k although metal in an h6 clast in fayetteville plots coherently with a cooling rate of 50 k myr we propose that metal grains cooled at these rates in chondritic clasts at different locations before host and clasts were compacted and were not subsequently heated above 650 k we predict a similar history for all gas rich ordinary chondrites by contrast metallic minerals throughout bhola and mezo madaras show more uniform textures and plot coherently giving cooling rates in the range 750 to 600 k of 0 1 and 1 k myr respectively we conclude that host and xenoliths in both chondrites were slowly cooled after compaction thus clasts in these chondrites experienced peak metamorphic temperatures and slow cooling through 700 k in different environments according to the conventional onion shell model for h l or ll chondrite parent bodies material of petrologic types 3 5 was arranged in successive shells around a type 6 core prior to catastrophic collisions which mixed all types intimately but if peak metamorphic temperatures were reached during not after accretion as seems plausible maximum metamorphism may have occurred in planetesimals 10 km in radius cooling through 700 k may then have occurred in larger bodies that accreted from these planetesimals iron meteorites mesosiderites and some achondrites may also have experienced melting in planetesimals and slow cooling in larger bodies

pb diffusion in cr diopside augite and enstatite and consideration of the dependence of cation diffusion in pyroxene on oxygen fugacity abstract chemical diffusion of pb under anhydrous po2 buffered conditions has been measured in natural pyroxenes of three different compositions a chromian diopside an augitic clinopyroxene and a near end member enstatite sources of diffusant consisted of pbs powder and ground pyroxene mixed together pre reacted in evacuated silica capsules at 1050 c and re ground prepared sample capsules were annealed for times ranging from several hours to a few months at temperatures from 850 c to 1050 c the pb distributions in the pyroxene specimens were profiled by rutherford backscattering spectrometry rbs the following arrhenius relations are obtained for pb diffusion in the pyroxenes when buffered at qfm d c 8 7 10 7 exp 351 36 kj mol 1 rt m 2 s 1 cr diopside d a 3 8 10 5 exp 372 15 kj mol 1 rt m 2 s 1 augite d e 6 6 10 7 exp 366 29 kj mol 1 rt m 2 s 1 enstatite in all cases diffusion is measured perpendicular to the c axis diffusion in cr diopside does not appear to be strongly anisotropic as diffusion rates for transport parallel to c are similar to the 110 results above diffusion in the augite is faster by nearly an order of magnitude than pb diffusion in the cr diopside diffusion tends to be faster in pyroxenes with higher iron contents as was evident in our earlier work chemical geology 150 1998a 105 but simple correlations are not obvious pb diffusion in enstatite is considerably slower than in the clinopyroxene compositions studied to investigate the fo2 dependence of pb diffusion in pyroxene experiments using the cr diopside augite and enstatite as well as the near end member diopside and a more iron rich clinopyroxene employed in a recent diffusion study chemical geology 150 1998a 105 were run at 950 c under oxygen fugacities ranging from 10 6 to 10 16 all of the pyroxenes exhibited a positive dependence of pb diffusion rate on fo2 with values of m ranging from 0 14 to 0 20 given d fo2 m interestingly the exponential term for diopside is 0 19 in good agreement with the value 3 16 determined from point defect models for diffusion mechanisms controlled by cation vacancies j geophys res 99 1994 9423 the activation energies for pb diffusion in all of the pyroxenes studied are relatively high and pb diffusivities slow when compared with the extant diffusion data for most other minerals it follows as consequence of these comparatively slow diffusivities that pb closure temperatures are higher in pyroxenes than for the majority of minerals with the exception of a few phases such as zircon monazite and garnet the slightly faster diffusivities that characterize fe rich clinopyroxenes will lead to somewhat depressed closure temperatures but pyroxenes in general should be fairly retentive of pb isotopic information when subjected to all but the most extreme thermal events

a 20th century acceleration in global sea level rise 1 multi century sea level records and climate models indicate an acceleration of sea level rise but no 20th century acceleration has previously been detected a reconstruction of global sea level using tide gauge data from 1950 to 2000 indicates a larger rate of rise after 1993 and other periods of rapid sea level rise but no significant acceleration over this period here we extend the reconstruction of global mean sea level back to 1870 and find a sea level rise from january 1870 to december 2004 of 195 mm a 20th century rate of sea level rise of 1 7 0 3 mm yr 1 and a significant acceleration of sea level rise of 0 013 0 006 mm yr 2 this acceleration is an important confirmation of climate change simulations which show an acceleration not previously observed if this acceleration remained constant then the 1990 to 2100 rise would range from 280 to 340 mm consistent with projections in the ipcc tar

mississippi delta subsidence primarily caused by compaction of holocene strata coastal subsidence causes sea level rise shoreline erosion and wetland loss which poses a threat to coastal populations this is especially evident in the mississippi delta in the southern united states which was devastated by hurricane katrina in 2005 the loss of protective wetlands is considered a critical factor in the extensive flood damage the causes of subsidence in coastal louisiana attributed to factors as diverse as shallow compaction and deep crustal processes remain controversial current estimates of subsidence rates vary by several orders of magnitude here we use a series of radiocarbon dated sediment cores from the mississippi delta to analyse late holocene deposits and assess compaction rates we find that millennial scale compaction rates primarily associated with peat can reach 5mm per year values that exceed recent model predictions locally and on timescales of decades to centuries rates are likely to be 10 mm or more per year we conclude that compaction of holocene strata contributes significantly to the exceptionally high rates of relative sea level rise and coastal wetland loss in the mississippi delta and is likely to cause subsidence in other organic rich and often densely populated coastal plains

bedform transport rates for the lowermost mississippi river 1 new methods of data collection and processing are developed to provide quantitative reach scale measurements of bedform transport mass within the tidally influenced mississippi river a multibeam swath profiler was used to collect daily bathymetry over a range of water discharges and bed elevation changes induced by dune migration are measured these values are coupled with bulk physical properties of the bed sediment to constrain mass flux and annual bedform transport is estimated at 2 2 x 10 6 metric tons mt the total annual sand flux from the mississippi river calculated by combining measured bedform transport rates and suspended sediment flux is estimated to be 20 x 10 6 mt survey data also provide information about the spatial distribution of dunes across the channel bottom straight reach segments are commonly mantled by dunes for the entire cross section while bends are typically areas of focused scour devoid of bedforms presumably any sediments associated with migrating dunes are propelled into suspension within bends before redepositing in the subsequent straight reach movement via suspension is therefore an important component for the downriver transport of bed materials in the lower mississippi river

pb isotopic age of the allende chondrules we have studied pb isotope systematics of chondrules from the oxidized cv3 carbonaceous chondrite allende the chondrules contain variably radiogenic pb with a 206pb 204pb ratio between 19 5 268 pb pb isochron regression for eight most radiogenic analyses yielded the date of 4566 2 2 5 ma internal residue leachate isochrons for eight chondrule fractions yielded consistent dates with a weighted average of 4566 6 1 0 ma our best estimate for an average age of allende chondrule formation this pb pb age is consistent with the range of model 26al 26mg ages of bulk allende chondrules reported by bizzarro et al 2004 and is indistinguishable from pb pb ages of ca al rich inclusions cais from cv chondrites 4567 2 0 6 ma amelin et al 2002 and the oldest basaltic meteorites we infer that chondrule formation started contemporaneously with or shortly after formation of cv cais and overlapped in time with formation of the basaltic crust and iron cores of differentiated asteroids the entire period of chondrule formation lasted from 4566 6 1 0 ma allende to 4564 7 0 6 ma cr chondrite acfer 059 to 4562 7 0 5 ma cb chondrite gujba and was either continuous or consisted of at least three discrete episodes since chondrules in cb chondrites appear to have formed from a vapor melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had largely dissipated krot et al 2005 there were possibly a variety of processes in the early solar system occurring over at least 4 5 myr that we now combine under the umbrella name of chondrule formation

longitudinal profile development into bedrock an analysis of hawaiian channels analysis of topographic maps of rivers incised into dated hawaiian lava flows shows that the long term average bedrock erosion rate along certain reaches is linearly related to stream power field observations suggest that two processes may control hawaiian channel downcutting 1 stream power dependent erosion including abrasion of the channel bed by transported particles and 2 step wise lowering caused by knickpoint propagation modeling results indicate that a simple stream power dependent erosion law predicts the straight to weakly convex longitudinal profiles characteristic of kauai channels but is insufficient to predict two other characteristic features the upslope propagation of knickpoints and the straight 5 8 channel slopes below the knickpoints thus more than this single transport law is apparently required to model bedrock channel incision field surveys also indicate that significant portions of the channel lengths below the knickpoints are mantled with large boulders we propose that

age and origin of the new zealand orocline in relation to alpine fault movement the alpine fault and the recurved arc structures orocline in the late palaeozoic mesozoic rangitata orogen are two of the first order structures of the new zealand subcontinent a commonly held view is that the orocline originated as a gigantic drag fold with alpine fault movement in view of the post oligocene age of the alpine fault however this concept involving a single recurved arc cannot reconcile the two published sets of palaeomagnetic data on tectonic rotations within the rangitata orogen and its cover rocks these data indicate at least 62 of dextral rotation during each of the mesozoic rangitata orogeny and the late cenozoic kaikoura orogeny a new model presented here postulates that there are actually two dextral recurved arcs a western arc which lies mainly outside the australia pacific plate boundary zone has a multiple origin having formed chiefly during the late triassic phase of the rangitata orogeny by the subduction accretion of the exotic rakaia terrane an eastern arc coincides with the area of the present plate boundary zone it formed as a mega brittle ductile shear zone with the late cenozoic development of the modern australia pacific plate boundary which includes the alpine fault this solves the problem of the nature and location north of the alpine fault of the relative plate movement evident as 480 km of displacement on the alpine fault a major implication of this model for plate tectonic reconstruc tions of the sw pacific is that only approximately 500 km of dextral displacement has occurred through new zealand during the late cretaceous cenozoic compared with the commonly adopted figure of 1000 km

an andean model of interplate coupling and strain partitioning applied to the flat subduction zone of sw japan nankai trough abstract transcurrent motion is commonly observed in cases of oblique subduction yet the causes remain controversial a model of increased interplate coupling linked to flat subduction is proposed as a mechanism for producing strain partitioning at a great distance several hundred km from the trench the model originally developed for the north andean margin is applied to the geodynamics of sw japan here along the nankai trough and below adjacent sw honshu flat subduction occurs due to the relatively young age m 7 and nine additional m 6 earthquakes in the past 300 years dextral strike slip focal mechanisms for 15 modern and historical events indicate a significant degree of subduction related strain partitioning manifested by right lateral strike slip faulting along the adakitic volcanic arc some 400 km from the nankai trough

active right lateral strike slip fault zone along the southern margin of the japan sea abstract we describe an active right lateral strike slip fault zone along the southern margin of the japan sea named the southern japan sea fault zone sjsfz onshore segments of the fault zone are delineated on the basis of aerial photograph interpretations and field observations of tectonic geomorphic features whereas the offshore parts are interpreted from single multichannel seismic data combined with borehole information in an effort to evaluate late quaternary activity along the fault zone four active segments separated by uplifting structures are identified in this study the east northeast trending sjsfz constitutes paired arc parallel strike slip faults together with the median tectonic line mtl both of which have been activated by oblique subduction of the philippine sea plate during the quaternary they act as the boundaries of three neotectonic stress domains around the eastern margin of the eurasian plate the near trench outer zone and nw se compressive inner zone of southwest japan arc and the southern japan sea deformed under e w compression from south to north

new insights into the mechanics of fluvial bedrock erosion through flume experiments and theory river incision into bedrock drives the topographic evolution of mountainous terrain and may link climate tectonics and topography over geologic time scales despite its importance the mechanics of bedrock erosion are not well understood because channel form river hydraulics sediment transport and erosion mechanics coevolve over relatively long time scales that prevent direct observations and because erosive events occur intermittently and are difficult and dangerous to measure herein we synthesize how flume experiments using erodible bedrock simulants are filling these knowledge gaps by effectively accelerating the pace of landscape evolution under reduced scale in the laboratory we also build on this work by providing new theory for rock resistance to abrasion thresholds for plucking by vertical entrainment sliding and toppling and by assessing bedrock analog materials bedrock erosion experiments in the last 15 years reveal that the efficiency of rock abrasion scales inversely with the square of rock tensile strength sediment supply has a dominant control over bed roughness and abrasion rates suspended sediment is an efficient agent of erosion and feedbacks with channel form and roughness strongly influence erosion rates erodibility comparisons across rock concrete ice and foam indicate that for a given tensile strength abrasion rates are insensitive to elasticity the few experiments that have been conducted on erosion by plucking highlight the importance of block protrusion height above the river bed and the dominance of block sliding and toppling at knickpoints these observations are consistent with new theory for the threshold shields stress to initiate plucking which also suggests that erosion rates in sliding and toppling dominated rivers are likely transport limited major knowledge gaps remain in the processes of erosion via plucking of bedrock blocks where joints are not river bed parallel waterfall erosion by toppling and plunge pool erosion feedbacks between weathering and physical erosion erosional bedforms and morphodynamic feedbacks between channel form and erosion rates despite scaling challenges flume experiments continue to provide much needed tests of existing bedrock erosion theory force development of new theory and yield insight into the mechanics of landscapes

tertiary geothermal events around the japan arc abstract the thermal history through the late tertiary around the japan arc is described by analyzing a number of geological data obtained from boreholes based on an empirical model of time temperature coalification the burial history and coal ranks in a deep offshore borehole miti tottori oki reveal that the western part of the sea of japan was subjected to remarkable thermal events around middle miocene and quaternary times during which voluminous igneous rocks erupted within southwest japan considering the tectonic context around the japan arc it is most probable that the thermal event in the middle miocene was brought about by the extensive continental rifting and formation of the sea of japan the area of higher quaternary temperatures in the upper mantle which has been delineated through heat flow measurements coincides with the distribution of the contemporaneous alkaline volcanics suggesting that the characteristic intra plate volcanism and the latest thermal event can be related to the regional influx of hot asthenosphere beneath southwest japan

suspension of bed material over sand bars in the lower mississippi river and its implications for mississippi delta environmental restoration 1 understanding specific pathways for sand transport in the lower reaches of large rivers including the mississippi is a key for addressing multiple significant geologic problems such as delta building and discharge to the oceans and for environmental restoration efforts in deltaic environments threatened by rising sea levels field studies were performed in the mississippi river 75 100 km upstream of the gulf of mexico outlet in 2010 2011 to examine sand transport phenomena in the tidally affected river channel over a range of discharges methods included mapping bottom morphology multibeam sonar cross sectional and longitudinal measurements of water column velocity and acoustic backscatter suspended sediment sampling and channel bed sampling substantial interaction was observed between the flow conditions in the river boundary shear stress channel bed morphology size and extent of sandy bedforms and bed material sand transport quantity transport mode and spatial distribution a lateral shift was observed in the region of maximum bed material transport from deep to shallow areas of subaqueous sand bars with increasing water discharge bed material was transported both in traction and in suspension at these water discharges and we posit that the downriver flux of sand grains is composed of both locally and drainage basin sourced material with distinct transport pathways and relations to flow conditions we provide suggestions for the optimal design and operation of planned river diversion projects

interpretations of a middle miocene and late quaternary steady dextral transpression in sw japan and the opening tectonics for the japan sea abstract an internal rotation hypothesis for the deformation of sw japan for middle miocene and recent times is studied rotations are known to have occurred from paleomagnetic data for the earlier case two geodetic flow field estimates as well as another tectonic study suggest that the rotations are also taking place at present a further examination of the geodetic flow models suggests that the rotations contained in these flow models are intrinsic to the deformation of sw japan which occurs in an e w dextral transpressive simultaneous pure and simple shear tectonic regime maps of rotation rates one half of vorticity and shear gradients the n s variation of the e w velocity field constructed from the geodetic flow models resemble one another in pattern the estimates of the directions of the principal strain rates from the geodetic models are on the whole n s contractional and or e w extensional which match the geodetic reference coordinate system used in the adjustment of the geodetic data these observations suggest that the vortical flow contained in the geodetic flow models is a frame independent shear induced vorticity as the material is rotating with respect to the principal directions of the strain rates and that the oblique motion of the philippine sea plate is partitioned between shear and arc normal convergence components the rotation rate is 10 12 ma furthermore the kinematical vorticity number estimates from the geodetic data correlate well with the first order tectonic fabric patterns in sw japan super simple shear deformation for large parts of sw japan where the median tectonic line and other large strike slip faults are found is also indicated in the internal kinematical vorticity number estimates a new model for the formation of the japan sea is proposed extrusion of japan due to the okhotsk eurasia collision this model agrees with the fundamental observations of the early middle miocene deformation in the region the application of an indentation extrusion model to the okhotsk eurasia collision results in a better agreement between the predicted and observed structures and leads to simpler more reasonable and coherent interpretations for the origin and development of the structures compared with the achievements of the previous models the nne trending early middle miocene rifts lying along the continental margin of western hokkaido formed as pull apart basins along a sinistral fault related to the collision of the okhotsk plate which segmented in a left handed sense because the collision was oblique the n s trending rifts and the normal faults found in the back arc domain of the ne honshu arc formed by eastward extrusion under the influence of the mariana type subduction in the japan trench these rift basins were subsequently left laterally displaced by movement along nw trending sinistral strike slip faults the shikoku basin opened from the east toward the west and dextral shear was imparted to the sw margin from the nankai trough leading to clockwise rotations in the middle miocene in sw japan the paleomagnetic rotations are accommodated as fully internal structural rotations in the new model for the origin of the japan sea the current episode of deformation in sw japan began 1 5 million years ago after a hiatus during a 15 5 ma interval and is likely a continuation of the middle miocene style of tectonics

brief sea level fall event and centennial to millennial sea level variations during marine isotope stage 19 in osaka bay japan detailed sea level variation was investigated for marine isotope stage mis 19 based on diatom and grain size analyses of a marine sequence in a core of the osaka group diatom sea level proxies represent precession related signals correlated with highstands mis 19 3 and 19 1 and lowstand mis 19 2 astronomical tuning shows the marine sequence has a uniform accumulation rate of about 60 cm ka 1 a rapid sea level fall event was found in the earliest mis 19 demonstrated by several independent sea level proxies of diatom and grain size this event began with a rapid sea level drop followed by a gradual recovery at about 783 782 ka a maximum abundance of pelagic diatom taxa at a core depth of 402 20 m evidently shows the highest sea level peak in mis 19 supported by many other proxies based on the diatom data sea level change across mis 19 1 is characterized by centennial to millennial fluctuations the sea level fall event began just after the onset of a cooling event previously reported from the same core observations of a comparable sea level fall signal in many deep sea core records suggest the event is global

active faulting and tectonics of burma and surrounding regions this study of the active tectonics of burma and surrounding regions is based mostly on an interpretation of landsat imagery and on fault plane solutions for shallow and intermediate earthquakes fold axes in the indoburman ranges and an east dipping inclined zone of intermediate depth earthquakes suggest that a slab of oceanic lithosphere was recently subducted to the east under the indoburman ranges fault plane solutions of shallow earthquakes however do not show underthrusting at the present time instead p axes are oriented roughly north south parallel not perpendicular to the fold axes in the ranges p axes for fault plane solutions of intermediate shocks also trend north south parallel to the strike of the subducted slab and may indicate that the hanging slab is being dragged north by india through the surrounding asthenosphere the sagaing fault probably accommodates most of the right lateral slip of india past indochina but india s northward movement also causes internal deformation within burma thailand and yunnan differences between the tectonic fabric seen on the landsat images and the tectonic regime compatible with the fault plane solutions throughout the region imply major changes in the directions of relative motion in the orientations of strain and in the style of deformation in the late cenozoic presumably these changes reflect the transition from subduction to collision and strike slip motion as india penetrated into eurasia past burma large parts of burma and thailand may have rotated several tens of degrees clockwise in this process

the response of subaqueous dunes to floods in sand and gravel bed reaches of the dutch rhine the branches of the river rhine in the netherlands characterized by a sand gravel bed in the upstream part and a sand bed in the downstream part of the river system show migrating dunes especially during floods in the last 20 years these dunes have been studied extensively high resolution echo sounding measurements of these dunes made with single and multibeam equipment were analysed for three different sections of the rhine river system during several floods this analysis was done to quantify the growth decay and migration rates of the dunes during floods in addition the migrating dunes were used to calculate bedload transport rates with dune tracking the results of dune growth and decay and migration rate are shown to be very different for the various sections during the various floods and these differences are related to differences in grain size of the bed and to differences in the distribution of discharge over the main channel and the floodplain the relations are used to show that the growth and migration rate of dunes and the calculated bedload transport rates during the rising stage of a flood wave can be predicted from the mobility of the bed material with simple power relations

backwater controls of avulsion location on deltas river delta complexes are built in part through repeated river channel avulsions which often occur about a persistent spatial node creating delta lobes that form a fan like morphology predicting the location of avulsions is poorly understood but it is essential for wetland restoration hazard mitigation reservoir characterization and delta morphodynamics following previous work we show that the upstream distance from the river mouth where avulsions occur is coincident with the backwater length i e the upstream extent of river flow that is affected by hydrodynamic processes in the receiving basin to explain this observation we formulate a fluvial morphodynamic model that is coupled to an offshore spreading river plume and subject it to a range of river discharges results show that avulsion is less likely in the downstream portion of the backwater zone because during high flow events the water surface is drawn down near the river mouth to match that of the offshore plume resulting in river bed scour and a reduced likelihood of overbank flow furthermore during low discharge events flow deceleration near the upstream extent of backwater causes enhanced deposition locally and a reduced channel fill timescale there both mechanisms favor preferential avulsion in the upstream part of the backwater zone these dynamics are fundamentally due to variable river discharges and a coupled offshore river plume with implications for predicting delta response to climate and sea level change and fluvio deltaic stratigraphy

upb systematics of phosphates from equilibrated ordinary chondrites u pb systematics were determined from fifteen phosphate separates from equilibrated ordinary chondrites and from small bulk fragments of the same meteorites the high 238u204pb ratios of thirteen of these phosphate separates lead to extremely radiogenic pb whose 206pb204pb ratios range from 250 up to 3500 the pb pb model ages for these phosphates range from 4 563 to 4 502 ga with an analytical precision of 106 y and the u pb system is apparently concordant the time interval observed 60 106 y reflects the thermal processing of the equilibrated chondrites and is consistent with that previously derived from the rb sr k ar and pu chronologies the pb pb ages of the phosphates from the seven h chondrites show a negative correlation versus their metamorphic grade this is the first clear relationship ever observed between a long lived chronometer and the intensity of metamorphism as reflected by metamorphic grade assuming that the pb pb age indicates the accurate u pb closure time in phosphates the pb pb chronology is compatible with the model of a layered h chondrite parent body however this interpretation of the u pb systematics is not unique it postulates a slow cooling of the equilibrated materials at high temperature in apparent conflict with petrological observations except for the h chondrites which agree rather well with pu systematics comparison of the pb pb chronology with published radiochronometric data does not reveal simple correlations in the present debate concerning the thermal history of chondrites the chronometric information derived from each isotope system is interpreted as the time of its thermal closure however this basic assumption may not be correct for all the radiochronologies and must be evaluated before the radiochronometric data can be applied as compelling time constraints for the period of 4 56 4 4 ga of proto planetary history

accretionary tectonics of burma and the three dimensional geometry of the burma subduction zone the geometry of the burma wadati benioff zone wbz has been determined by fitting a trend surface parameterized with eight effective degrees of freedom to 184 well located hypocenters the dip of this surface which passes through the middle of the wbz varies from about 50 in the north near the eastern himalayan syntaxis to about 30 in the bay of bengal area the eastern edge of the indo burman ranges closely follows the map projection of the 60 km depth contour of the wbz the curvature of the indo burman ranges is controlled by the geometry of the interface between the more steeply dipping part of the indian plate and the leading edge of the overriding burma platelet shallow earthquakes beneath the indo burman ranges are primarily confined to the underthrusting indian plate their focal mechanisms indicate strike slip faulting and north south shortening parallel to the eastern margin of the indian plate

mitigating land loss in coastal louisiana by controlled diversion of mississippi river sand the bonnet carre spillway diverts floodwaters from the mississippi river to lake pontchartrain and was opened for 42 days during the 2011 flood according to measurements of the newly deposited sediments at least 31 46 of the river s sand load was diverted into the spillway at this time suggesting that such diversions can help mitigate coastal wetland loss

channel dynamics sediment transport and the slope of alluvial fans experimental study a b s t ra c t we present the results of an experimental study of alluvial fan sedimentation under conditions of constant inflow water discharge qw sediment supply qso and median grain size d the study was designed to complement and test a recently formulated model of alluvial fan sedimentation and to emphasize the interactions between and controls on flow channelization and equilibrium fan slope flow channelization and fan sedimentation were studied under conditions of nearly steady uniform aggradation steady conditions were achieved by imposing a steadily rising base level just in balance with the average sediment aggradation rate experimental inflows covered a wide range of conditions allowing examination of the effects of variations in qw q so and d in both bedload and suspension dominated environments experimental results were most consistent with an expanding flow channel model key experimental findings successfully and quantitatively predicted by the expanding flow theory include 1 straight to slightly concave radial profiles of bedload dominated fans 2 distinctly convexo concave profiles of suspension dominated fans 3 a strong inverse relationship between qw and fan slope 4 a strong but secondary relationship between q so and fan slope and 5 near independence of d and fan slope so long as transport stage is high and bedload transport dominant however potential scale effects in the experiments arose from reduced flow reynolds numbers and incorrect geometric scaling of channel widths no confident conclusions regarding the debate over the relative importance of sheetfloods and braided channel flows can be drawn from the experimental data extrapolation to field scale is best accomplished through appropriate application of the theoretical model herein confirmed against experimental data

sand as a stable and sustainable resource for nourishing the mississippi river delta dams have starved the lower mississippi river of sediment over recent decades suggesting that the drowning of the delta is inevitable analysis of the rivers suspended sediment load and morphodynamic modelling suggest that the amount of sand essential for land building has not significantly decreased since dam construction with sand remaining available for several centuries

backwater and river plume controls on scour upstream of river mouths implications for fluvio deltaic morphodynamics sediment flux from rivers to oceans is the fundamental driver of fluvio deltaic morphodynamics and continental margin sedimentation yet sediment transport across the river to marine boundary is poorly understood coastal rivers typically are affected by backwater a zone of spatially decelerating flow that is transitional between normal flow upstream and the offshore river plume flow deceleration in the backwater zone as well as spreading of the offshore plume should render rivers highly depositional near their mouths leading to sedimentation and eventual elimination of the backwater zone at steady state this reasoning is counter to observations of riverbed scour erosional bed forms and long lived backwater zones near the mouths of some coastal rivers e g mississippi river united states to explain these observations we present a quasi 2 d model of a coupled fluvial backwater and offshore river plume system and apply it to the mississippi river results show that during high discharge events the normal flow depth can become larger than the water depth at the river mouth resulting in drawdown of the water surface spatial acceleration of flow and erosion of the riverbed as proposed by lane 1957 the transition to drawdown and erosion is ultimately forced by spreading of the offshore river plume this points to the need to model coupled river and river plume systems with a dynamic backwater zone under a suite of discharges to accurately capture fluvio deltaic morphodynamics and connectivity between fluvial sediment sources and marine depositional sinks

origin of a preferential avulsion node on lowland river deltas river deltas are built by cycles of lobe growth and abrupt channel shifts or avulsions that occur within the backwater zone of coastal rivers previous numerical models differ on the origin of backwater scaled avulsion nodes and their consistency with experimental data to unify previous work we developed a numerical model of delta growth that includes backwater hydrodynamics river mouth progradation relative sea level rise variable flow regimes and cycles of lobe growth abandonment and reoccupation for parameter space applicable to lowland deltas we found that flow variability is the primary mechanism to cause persistent avulsion nodes by focusing aggradation within the backwater zone backwater scaled avulsion nodes also occur under less likely scenarios of initially uniform bed slopes or during rapid relative sea level rise and marine transgression our findings suggest that flow variability is a fundamental control on long term delta morphodynamics

the mississippi delta region past present and future the mississippi delta region of south louisiana houses a wealth of resources within a dynamic subsiding landscape foundations for the delta region reflect neogene evolution of the depocenter whereas details of the modern landscape reflect late pleistocene to holocene evolution of the alluvial deltaic plain the holocene delta plain was constructed by cyclical growth of deltaic headlands followed by avulsion and relocation of the fluvial sediment source abandoned headlands were then submerged and reworked while a new headland was created at the site of active fluvial sediment input historic period levees have decoupled the delta plain from its fluvial sediment source at the same time global sea level rise was accelerating which has accelerated delta plain submergence diversions of mississippi river water and sediment are necessary to achieve delta plain sustainability but upstream dams trap 50 of the total sediment load and there is not enough supply to keep pace with subsidence and accelerated

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

sea level variations during marine isotope stage 7 and coastal tectonics in the eastern seto inland sea area western japan relative sea level rsl observations for marine isotope stage mis 7 were obtained from three sites higashinada kakogawa and mitsu of the eastern seto inland sea area western japan we evaluated the magnitude of sea level highstands during mis 7 and the local tectonics based on rsl records of this coastal area for this purpose we analyzed diatom assemblages and sedimentary sulfur and carried out tephra analysis we also constructed a linear age model to determine the depositional ages of the sediments using altitudes of astronomical age control points obtained mainly from the present diatom data at the mitsu site rsls were estimated to be 22 87 0 50 m at the mis 7 5 highstand peak 234 6 ka below 20 17 m at 211 2 ka and below 20 00 m at 209 9 ka with a tectonic subsidence rate of 0 11 0 02 m ka since the last interglacial maximum at the mitsu site the elevation of the tectonically corrected mis 7 5 highstand peak may have been 2 94 5 19 m the tectonically corrected sea level elevations during mis 7 3 are inferred to have been below 3 06 4 72 m at 211 2 ka and below 3 09 4 70 m at 209 9 ka comparisons between the diatom inferred rsls and the isotopically derived sea level curve indicate that the mis 7 3 highstand peak was above 18 m and may have been lower than the mis 7 5 and 7 1 highstands based on the isotopically derived sea level curve and the rsl data from higashinada and kakogawa a tectonic subsidence rate of 0 26 0 32 m ka was derived for higashinada and a tectonic uplift rate of 0 17 m ka and 0 43 m ka and 0 26 m ka and 0 42 m ka these values indicate the intensity of crustal movements associated with active faulting of the rokko awaji fault system along this tectonically active coast

timing and displacement of holocene faulting on the median tectonic line in central shikoku southwest japan abstract the median tectonic line mtl extends about 900 km parallel to the general trend of the southwest japan island arc the central segment of the mtl has been active during the quaternary although the average slip rate along the mtl is up to several mm a 1 there is no historical record of destructive earthquakes along it in order to collect data on the behavior of the mtl during late holocene times especially on the timing of seismic events and the amount of displacement during each event trench studies were conducted on the active segment in central shikoku in 1984 and 1988 the results are summarized as follows 1 the latest two seismic events produced by slip on the mtl in central shikoku occurred during the early 8th century a d and between 3250 and 2820 years b p 2 the amount of displacement during the latest event is inferred to have been about 5 7 m 3 the earthquake magnitude for the latest event is estimated at 7 9 richter scale and a fault segment trace about 70 km long ruptured during that event 4 an average slip rate along the active trace of the mtl in central shikoku during the late quaternary is estimated to be 5 8 mm a 1

geomorphological explanation of the long profile of the lower mississippi river abstract concavity in the long profile of rivers has traditionally been explained through the concept of grade in which the slope declines downstream as a response to changing discharge bed material size and sediment transport applying this concept to particular river systems has however proved problematic the long profile reflects spatially distributed form process feedbacks between all aspects of channel morphology operating at a range of poorly defined time and space scales and in the presence of natural controls in many river systems process form dynamics are further complicated by engineering interventions which add additional extrinsic controls and constrain the range of intrinsic dynamics in this paper the 1974 75 long profile of the lower mississippi river is examined at three scales the regional the reach and the sub reach pool crossing scales a combination of curve fitting zonation algorithms and empirical classification techniques are used to show that although the long profile of the lower mississippi river is concave at the largest scale the profile is characterised by discontinuities shorter trends and zonal variations in the amplitude and wavelength of pool crossing morphology these characteristics are a response to morphological and bed material changes relating to a range of physical geological tectonic tributary input and engineering controls despite its apparent simplicity and correspondence to a graded condition the long profile of the lower mississippi river is actually a complex and scale dependent morphological property at best the concave river profile is therefore a property which emerges from several scales of process form interaction at worst it is no more than an artefact arising from the application of over simplified curve fitting techniques disclosure of the nature of the long profile thus requires the application of a variety of analytical techniques as well as geomorphological explanations which are themselves scale dependent and which consider the interaction of natural processes and the history of engineering intervention

estuarine controls on fine grained sediment storage in the lower mississippi and atchafalaya rivers observations of the lower mississippi river channel in 1999 2003 with side scan and subbottom seismic profilers multibeam bathymetry core radiotracers 7be 137cs 210pb and 234th and water column profiles of fluid and suspended sediment properties are utilized to constrain the timing location and intensity of the seasonal storage of fine grained sediments in the estuarine channel reach below venice louisiana unlike earlier studies that suggested little or no estuarine sedimentation was taking place in the river the present study demonstrates that the river channel above the head of passes seasonally stores as much as 10 of the annual suspended sediment discharge during periods of falling to low mississippi water discharge associated with salt stratification in the channel thalweg mud deposits are up to 3 2 m thick with 7be derived sediment deposition rates of up to 8 9 mm day water column profiling and 234th 7be ratios suggest that 1 particles are transported and settled mainly as flocs and 2 riverine flocs are settling through the strong salt wedge halocline and are protected from re erosion by the salt stratification with onset of the spring freshet and the corresponding seaward retreat of the saltwater wedge the entire volume of fine grained material stored in the lower river except in low freshet years is remobilized and transported to the gulf of mexico in the turbid freshwater plume this tidal hysteresis strongly influences the timing of sediment supply to the ocean high turbidities in the shelf plume during rising water discharge spikes of 1 to 2 week duration are the product of local channel floor remobilization as well as increased supply from the drainage basin pulsed resupply of sediments and the pore water products of early diagenesis potentially have major implications for issues such as continental margin carbon budgets and coastal eutrophication companion surveys conducted in the lowermost 12 km reach of the atchafalaya distributary of the mississippi show no evidence of saltwater wedge intrusion at low discharge or seasonal fine grained sediment storage likely due to the shallow 7 m river mouth sill that impedes the upstream movement of gulf water

paleomagnetic study of western tibet deformation of a narrow zone along the indus zangbo suture between india and asia abstract more than 150 oriented samples were collected along the kaxigal lhasa road in the southwestern part of the tibetan plateau to determine the deformation aspect of the tibetan region from a paleomagnetic point of view the counter clockwise deflection about 60 in declination was revealed at four localities in the narrow zone along the indus zangbo suture zone a north to northeast direction d 9 to 30 was observed at four localities more than 14 km from the suture zone in the western tibetan region significant tectonic deformation has occurred exclusively in a narrow zone whose width is about 40 km spanning the suture zone the crustal blocks of this narrow zone were rotated in a compressive regime accompanied by left lateral shear during the collision of the indo eurasia continents the deformation of the narrow zone behaves as a buffer zone in a tectonically gigantic dimension where the tangential stress is mainly absorbed

experimental evidence for fluvial bedrock incision by suspended and bedload sediment fluvial bedrock incision sets the pace of landscape evolution and can be dominated by abrasion from impacting particles existing bedrock incision models diverge on the ability of sediment to erode within the suspension regime leading to competing predictions of lowland river erosion rates knickpoint formation and evolution and the transient response of orogens to external forcing we present controlled abrasion mill experiments designed to test fluvial incision models in the bedload and suspension regimes by varying sediment size while holding fixed hydraulics sediment load and substrate strength measurable erosion occurred within the suspension regime and erosion rates agree with a mechanistic incision theory for erosion by mixed suspended and bedload sediment our experimental results indicate that suspension regime erosion can dominate channel incision during large floods and in steep channels with significant implications for the pace of landscape evolution