relic magma chamber structures preserved within the mesozoic north atlantic crust the north atlantic transect seismic reflection data collected southwest of bermuda have been reinterpreted following post stack migration and reveal two major intracrustal reflections the shallower of these two events located 1 s below the igneous basement is a subhorizontal undulating surface that in some places is continuous for as much as 10 km on the basis of its position within the section and its laterally discontinuous nature we believe that this upper crustal reflection corresponds to the intermittently sharp contact between the sheeted dikes and the underlying isotropic gabbro a second set of lower crustal reflections dipping 20 40 eastward is also prominent on the migrated profile and terminates downdip against the subhorizontal reflection moho several lines of evidence argue against these features being either artifacts or out of the plane events instead their presence may be ascribed either to crustal penetrating fault zones or to mafic ultramafic cumulate layers frozen into the oceanic crust at the time of formation at the paleo spreading center because of the laminated character of these events and their typical occurrence within 1 0 to 1 5 s of the reflection moho we prefer a compositional versus a structural interpretation for their origin the gradual thinning in the crust approaching the fracture zones is shown to be more complex than was originally inferred although the interpretation that the crust gradually thins toward fracture zones may still apply in a few localities significant departures are recognized elsewhere similarly the improved image on the migrated profile documents an increase in complexity across the localized region directly surrounding the blake spur fracture zone an interpretation advocating crustal thickening in this narrow zone is proposed as an alternative to the crustal thinning model of mutter and others

the bright moho reflection in the 1986 nevada passcal seismic experiment abstract detailed study of the reflected phases from the moho identified in the seismic reflection recordings of the 1986 nevada passcal lithospheric seismic experiment place important constraints on the fine structure and geological nature of the mohorovicic discontinuity two different spectral ratio techniques are used to estimate the reflectivity of the moho we evaluate the ratio of the reflected pmp phase to the incident source amplitude we also calculated the spectral ratio of the pmp phase with a reference reflection reflectivity estimates calculated using both spectral ratio techniques are very consistent high reflectivity within the range 0 28 0 39 was obtained the analysis of the spectral characteristics suggests a layered structure for the moho with an average thickness between 85 and 250 m these results require a layered moho structure with high impedance contrasts amplitude modeling using a reflectivity algorithm demonstrates that constructive interference from a 1 0 1 5 km thick layered structure produces amplitudes and waveforms comparable to the data to achieve such high reflectivities 20 25 of the moho consists of layers with velocities below 6 5 km s the remaining layers have velocities between 7 7 and 8 1 km s geologically the low velocities are consistent with velocities characteristic of basaltic magmas partial melts the high velocities are characteristic of mantle material the partial melts may represent an expression of present day underplating

middle to late holocene decreased fluvial aggradation and widespread peat initiation in the ishikari lowland northern japan this study investigated the influence of sea level and climate changes on the decreased fluvial aggradation and subsequent widespread peat initiation in the middle to late holocene in the ishikari lowland which is a coastal floodplain formed in response to the postglacial sea level change by introducing a new approach to separately evaluate the rates of organic and clastic sediment input we demonstrated that the peat began to form when the fluvial sedimentation rate was significantly decreased less than 0 6 mm yr while plant macrofossil analysis suggested that lowering of water level is also important to the peat initiation such changes in sedimentary environment may be associated with the abrupt abandonment of crevasse splays the concentrated ages of the peat initiation around 5600 5000 4600 4300 and 4100 3600 cal bp suggest that an allogenic control promoted the abandonment of crevasse splays and different onset ages can be explained by different fluvial responses of the ishikari river and i

quantification of holocene asian monsoon rainfall from spatially separated cave records abstract a reconstruction of holocene rainfall is presented for southwest china an area prone to drought and flooding due to variability in the east asian monsoon the reconstruction is derived by comparing a new high resolution stalagmite 18 o record with an existing record from the same moisture transport pathway the new record is from heshang cave 30 27 n 110 25 e 294 m and shows no sign of kinetic or evaporative effects so can be reliably interpreted as a record of local rainfall composition and temperature heshang lies 600 km downwind from dongge cave which has a published high resolution 18 o record wang y j cheng h edwards r l he y q kong x g an z s wu j y kelly m j dykoski c a li x d 2005 the holocene asian monsoon links to solar changes and north atlantic climate science 308 854 857 by differencing co eval 18 o values for the two caves secondary controls on 18 o e g moisture source moisture transport non local rainfall temperature are circumvented and the resulting 18 o signal is controlled directly by the amount of rain falling between the two sites this is confirmed by comparison with rainfall data from the instrumental record which also allows a calibration of the 18 o proxy the calibrated 18 o record provides a quantitative history of rainfall in southwest china which demonstrates that rainfall was 8 higher than today during the holocene climatic optimum 6 ka but only 3 higher during the early holocene significant multi centennial variability also occurred with notable dry periods at 8 2 ka 4 8 4 1 ka 3 7 3 1 ka 1 4 1 0 ka and during the little ice age this holocene rainfall record provides a good target with which to test climate models the approach used here of combining stalagmite records from more than one location will also allow quantification of rainfall patterns for past times in other regions

compositional variation and the origin of deep crustal reflections abstract identifying the sources of crustal reflections is essential for deriving more geological information from deep crustal reflection profiles theoretical and model studies help place constraints on the role of compositional variation in producing deep crustal reflections analysis of laboratory derived velocities and densities of rock types typical for the continental crust indicates that reflection coefficients are generally small but significantly 17 of the possible reflection coefficients have magnitudes between 0 1 and 0 2 comparison between reflection coefficients derived from well logs and reflections observed in associated seismic profiles indicates that constructive interference associated with geological layering is at least as important as the magnitude of the reflection coefficients for producing detectable reflections constructive interference can increase reflection amplitude by two to three times but is limited to a relatively narrow range of layer thickness for a typical 10 40 hz seismic wavelet and typical crustal velocities of about 6 km s constructive interference occurs for layer thickness ranging between about 35 and 80 m layers thinner than 35 m interfere destructively if reflections result from compositional variation seismic models of hypothetical and observed geologic relations provide analogs for interpreting complex reflection patterns observed in deep crustal reflection profiles such models show reflection patterns similar to those observed in the reflection profiles the models indicate that the reflections could originate in the complexly deformed and intruded terranes that are common in the crystalline crust and it may not be necessary to appeal to unobserved phenomena such as special lamellae or fluid filled fractures to explain the reflections

asian summer monsoonal variations during the holocene revealed by huguangyan maar lake sediment record abstract high resolution records of summer monsoonal evolution in the holocene are essential to paleoclimate reconstruction but are hard to obtain here we provide a continuous well dated high resolution record from huguangyan maar lake located in the west pacific climate regime multi proxies including chlorophyll a toc sr rb ratio and magnetic susceptibility have been used to reconstruct the monsoon variations in the holocene higher chlorophyll a values are related with stronger summer monsoon and vice versa high temporal resolution radiocarbon dating enables us to divide summer monsoon evolution stages summer monsoon in zhanjiang area was strongest in the early holocene but began to decline at 6080 yr bp and weakened substantially since about 3600 yr bp the transition around 6080 yr bp supported by our high resolution dating lags the previous estimates by 1700 years the decreasing summer solar radiation at 30 n through the holocene is probably responsible for gradually weakened summer monsoon the anti phased co variation of summer monsoon intensity and enso activity throughout the holocene suggests that they could be interlinked driven together by solar radiation at orbital scale

holocene climate variations from zhuyeze terminal lake records in east asian monsoon margin in arid northern china zhuyeze palaeolake is a terminal lake situated in the arid northern china in the east asian monsoon margin in order to examine the holocene palaeoclimatic change in the east asian monsoon margin qingtu lake section qtl from zhuyeze palaeolake is sampled in high resolution palaeoclimatic proxies such as grain size carbonate toc c n and delta 13 c of organic matter were analyzed eleven 14 c samples and six optically stimulated luminescence osl samples were dated to provide chronological control we also investigated the geomorphic features of lake shorelines in this area the results show that the climate was warm and dry in early holocene 9 5 7 0 cal ka bp cool and humid in mid holocene 7 0 4 8 cal ka bp and increasingly drier in late holocene since 4 8 cal ka bp comparisons of our records with other records in adjacent areas as well as with the records in the asian monsoon areas suggested that changes in effective moisture was synchronous in east asian monsoon marginal zone i e the pattern of dry early holocene humid mid holocene and aridity increasing late holocene and that the moisture optimum during the holocene was out of phase between asian monsoon margin and asian monsoonal dominated region possibly due to the high temperature at that time c 2010 university of washington published by elsevier inc all rights reserved zhuyeze palaeolake is a terminal lake situated in the arid northern china in the east asian monsoon margin in order to examine the holocene palaeoclimatic change in the east asian monsoon margin qingtu lake section qtl from zhuyeze palaeolake is sampled in high resolution palaeoclimatic proxies such as grain size carbonate toc c n and delta 13 c of organic matter were analyzed eleven 14 c samples and six optically stimulated luminescence osl samples were dated to provide chronological control we also investigated the geomorphic features of lake shorelines in this area the results show that the climate was warm and dry in early holocene 9 5 7 0 cal ka bp cool and humid in mid holocene 7 0 4 8 cal ka bp and increasingly drier in late holocene since 4 8 cal ka bp comparisons of our records with other records in adjacent areas as well as with the records in the asian monsoon areas suggested that changes in effective moisture was synchronous in east asian monsoon marginal zone i e the pattern of dry early holocene humid mid holocene and aridity increasing late holocene and that the moisture optimum during the holocene was out of phase between asian monsoon margin and asian monsoonal dominated region possibly due to the high temperature at that time c 2010 university of washington published by elsevier inc all rights reserved

episodic overbank deposition as a dominant mechanism of floodplain and delta plain aggradation the common view that frequent overbank flooding leads to gradual aggradation of alluvial strata on floodplains and delta plains has been challenged by a variety of studies that suggest that overbank aggradation occurs in a strongly episodic fashion however this remains a largely untested hypothesis due to the difficulty in establishing age models with sufficiently high resolution here we use 39 optically stimulated luminescence osl ages from proximal overbank deposits in the mississippi delta to demonstrate for the first time that alluvial aggradation over centennial to millennial time scales is predominantly episodic with aggradation rates of 1 4 cm yr that can persist for centuries osl ages from three separate study areas produce age clusters that are distinctly different yet complement each other these findings suggest that a substantial portion of the continental stratigraphic record consists of patchworks of relatively discrete centennial to millennial scale sediment bodies assembled by autogenic processes

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

millennial scale stratigraphy of a tide dominated incised valley during the last 14 kyr spatial and quantitative reconstruction in the tokyo lowland central japan spatial and quantitative analysis of infilling processes of the tide dominated incised valleys beneath the tokyo lowland during the last 14 kyr was undertaken by using data from 18 sediment cores 467 radiocarbon dates and 6100 borehole logs the post last glacial maximum valley fills consist of braided river meandering river estuary spit and delta systems in ascending order the boundary between the estuary and delta systems is regarded as the maximum flooding surface the maximum flooding surface beneath the tokyo lowland is dated at 8 ka in the arakawa valley and 7 ka in the nakagawa valley this age difference is due to the migration of the tone river from the arakawa valley to the nakagawa valley at 5 ka and suggests that the widely held view that the global initiation of deltas coincided with the abrupt rise of sea level at 9 to 8 ka is true only where there has been steady sediment supply from major rivers the meandering river system is dominated by sheet like sands that were deposited during lateral migration of channels during the younger dryas and isolated vertical sands within muds that reflect vertical aggradation of channels before and after the younger dryas the transition between these channel geometries is controlled by a threshold sea level rise of 4 to 7 mm yr 1 before migration of the tone river at 5 ka the tide dominated bay in the nakagawa valley was filled by upward fining laterally accreting muds the muds accreted from the margin to the axis of the bay such lateral accretion of suspended particles derived from outside the bay has been documented in other tide dominated coastal environments and is probably common in other similar settings after the migration of the tone river the bay was filled by upward coarsening deltaic sediments

evolution of the holocene mississippi river floodplain ferriday louisiana insights on the origin of fine grained floodplains abstract the alluvial architecture and soil characteristics of holo cene mississippi river floodplain deposits in the southern lower mississippi valley provide evidence for significant changes in floodplain development in response to sea level rise floodplain cores acquired near ferriday louisiana show that holocene deposits consist of 15 30 m ave 20 m of sands silts and clays which overlie late wisconsin sands and gravels on the basis of differences in sediment grain size sediment body geometry and the abundance of soil features the holocene deposits are subdivided into lower and upper holocene units lower holocene deposits 5000 yr b p consist of lacustrine and poorly drained backswamp muds that contain authigenic siderite pyrite and vivianite and show little evidence of soil formation muds encase crevasse splay and floodplain channel sand bodies 1 km wide and collectively these deposits represent a mosaic of shallow lakes poorly drained backswamps and multichannel streams similar to modern examples in the atchafalaya basin 100 km south of ferriday upper holocene deposits 5000 yr b p are represented by large mississippi river meander belt sand bodies that are up to 15 km wide and 30 m thick natural levee silts and sands and well drained backswamp muds are present between meander belt sands upper holocene deposits contain abundant soil features and sandy and silty soils are entisols inceptisols and alfisols whereas clayey soils are vertisols the presence of isolated sand bodies surrounded by mud and the scarcity of soil features suggest that lower holocene sediments reflect a period of rapid floodplain aggradation during which crevassing lacustrine sedimentation and avulsion dominated floodplain construction no evidence of large meandering mississippi river channels represented by buried thick tabular sands exists near ferriday and discharge in lower mississippi valley flow was probably conveyed by a network of small multichannel floodplain streams upper holocene sediments record a dramatic change ca 5000 yr b p from rapid to slower floodplain aggradation which was accompanied by extensive lateral channel migration overbank deposition and soil formation on the basis of differences in meander belt dimensions and numbers of abandoned channels upper holocene meander belts are subdivided into simple and complex forms relative age relationships suggest that the smaller and older simple meander belts represent periods of divided mississippi river flow and early attempts to establish a large single channel meandering regime this type of meandering regime is represented by the larger and younger complex meander belts and includes the modern meander belt similarities in the timing of changes in floodplain processes and fluvial style and decreasing rates of holocene sediment accumulation in the southern lower mississippi valley strongly suggest that decelerating holocene sea level rise in the gulf of mexico affected floodplain development at least 300 km inland from the present day coast the alluvial architecture of the lower holocene deposits and the absence of large meandering mississippi river channel deposits older than 5000 yr b p near ferriday indicates that most of the floodplain muds were deposited by avulsion related crevassing and lacustrine sedimentation rather than by overbank flooding of large mississippi river channels similarities between the floodplain history of the mississippi river and those of modern and ancient rivers elsewhere further suggest that avulsion rather than simple overbank deposition contributes to the construction of fine grained floodplains to a greater degree than generally recognized

an improved glacial isostatic adjustment model for the british isles the glacial isostatic adjustment gia of the british isles is of interest due to the constraints that can be provided on key model parameters such as the global meltwater signal local ice sheet history and viscoelastic earth structure a number of recent studies have modelled relative sea level rsl data from this region to constrain model parameters as indicated in these studies the sensitivity of these data to both local and global parameters results in a highly non unique problem this study aims to address this inherent non uniqueness by combining a previously published british irish ice model that is based on the most recent geomorphological data with a new global ice sheet model that provides an accurate prediction of eustatic sea level change in addition constraints from global positioning system gps measurements of present day vertical land motion are considered alongside the entirety of rsl data from both great britain and ireland a model solution is found that provides a high quality fit to both the rsl data and the gps data within the range of earth viscosity values considered the optimal data model fits were achieved with a relatively thin lithosphere 71 km upper mantle viscosities in the range 4 6 1020 pa s and lower mantle viscosities 3 1022 pa s copyright 2011 john wiley sons ltd

reconstructed seismic velocity structure of the lewis hills massif and implications for oceanic fracture zones the lewis hills massif of the bay of islands ophiolite complex provides a lower crustal to upper mantle cross section of an oceanic fracture zone segment and adjacent oceanic lithosphere the structural and petrologic character of the fracture zone region is distinctly different from the comparatively simple layered structure of the rest of the bay of islands complex results of detailed field mapping petrography and laboratory seismic velocity measurements have been combined in order to reconstruct the seismic velocity structure of this slice of oceanic lithosphere velocities and velocity anisotropies of each of the major rock units in the massif are restored to their preobduction geometries relative to different stratigraphic levels in the complex and proximity to the fracture zone region although all oceanic fracture zones probably have unique structural and petrologic characteristics in detail from the perspective of the present study it is suggested that in general seismic properties differing from that of normal oceanic crust and upper mantle may be expected within approximately 10 km of the axis of fracture zones in these regions lower crustal as well as upper mantle anisotropy is expected the moho might be expected to shallow due to the presence of abundant ultramafic cumulates resulting in crustal thinning and significant velocity inversions may occur with depth where large cyclic mafic layers occur within cumulate ultramafics in oceanic lithosphere which has passed or is passing through a transform fault domain enhanced mantle anisotropy might occur the middle to lower crust in these regions should be composed of highly anisotropic metamorphic rocks which grade upward into serpentinites and highly fractured gabbros diabases and basalts with relatively low velocities it is suggested that velocities as well as velocity anisotropies produced by these types of structures might be determined in situ by seismic refraction experiments and may be used to map the extent of deformation in the oceanic lithosphere near fracture zones

archean plate tectonics constraints and inferences the earth has been cooling since archean time the higher temperatures beneath archean ridges resulted in more partial melting which extended down to greater depths than at present the archean oceanic crust was much thicker 20 km than modern crust 5 km this inference is compatible with previous ideas of suspected oceanic crust the thicker oceanic crust in the archean tended to resist subduction similar to modern aseismic ridges but could not prevent it short lived episodes of intra arc spreading followed by cordilleran type compression may have produced archean greenstone belts archean meta tonalites represent a significant part of the continental crust and were probably produced by melting of the subducted slab which was favored by the higher mantle temperatures

seismic stratigraphy of the oceanic moho based on ophiolite models synthetic seismograms that model vertical incidence seismic reflections from the oceanic moho were calculated from detailed laterally varying velocity depth models of two ophiolite complexes the synthetic seismograms reproduce the rapid lateral variability of waveforms amplitudes and two way travel times of reflections from the oceanic moho that are observed in existing mcs data the large variations observed in the synthetic seismograms were caused solely by horizontal changes in the geologic structure along the crust mantle boundary because focusing or defocusing of seismic energy by the upper crustal units was not modeled the petrologic contrast producing the strongest reflector from the oceanic crust mantle transition is highly variable so it is difficult to identify what the particular moho reflections represent geologically although with broad band data velocity gradients can be distinguished from laminated zones more important however is our finding that the seismic refraction oceanic moho and the seismic reflection oceanic moho usually do not coincide and that either or both may cross major geologic contacts

vertical tectonic crustal movements along the japanese coastlines inferred from late quaternary and recent relative sea level changes abstract observed relative sea level rsl changes during the past 130 kyr make it possible to evaluate the vertical crustal movement due to several tectonic processes such as the subduction of oceanic plates here we infer average rates of tectonic crustal movement along the japanese coastlines on three typical timescales of 50 yr 6 kyr and 125 kyr based on tide gauge and holocene rsl observations and the altitudes of marine terraces formed at the last interglacial lig phase at 125 ka the rates on a timescale of 50 yr are derived from tide gauge data thermosteric sea level changes due to thermal expansion of the oceans and predictions due to the glacio hydro isostatic adjustment gia for the last deglaciation and also recent melting of the mountain glaciers and greenland and antarctic ice sheets those for 6 kyr and 125 kyr are based on the rsl observations and the predictions by gia modelling considering uncertainties for temporal changes in eustatic sea level for the mid to late holocene and lig phase the inferred rates for 50 yr are significantly different from those for 125 kyr in most sites particularly for sites along the coastline from eastern hokkaido to northeastern japan shikoku and south kyushu facing the pacific ocean in these regions the rates for 125 kyr and 50 yr are positive uplift and negative subsidence respectively also the observed rsl changes at 6 ka are consistent with the inferred rsl changes using the rates for 125 kyr and gia predictions in many sites but inconsistent with those for 50 yr in most sites except for a few sites these results suggest that the rates on a timescale of 50 yr are not representative of the tectonic crustal movements for timescales longer than 6 kyr in most sites along the japanese coastlines however the inferred rates on these timescales may be useful in discussing the recurrence of megathrust earthquake with its interval of 1 kyr such as the 2011 earthquake off the pacific coast of tohoku

crust mantle boundaries in the taiwan luzon arc continent collision system determined from local earthquake tomography and 1d models implications for the mode of subduction polarity reversal abstract in order to better understand the mode of subduction polarity reversal in the taiwan luzon arc continent collision zone we mapped its crust mantle boundaries using local earthquake tomography by contouring surfaces of constant vp 7 5 km s 1 we identified three moho discontinuities and the plate interface that juxtaposes eurasian lower crust against mantle lithosphere of the philippine sea plate the plate interface dips to the east under southeastern taiwan and steepens progressively towards north until it becomes vertical at 23 7 n from there it continues northward in a vertical orientation until the limit of the tomographic model inhibited further mapping for the moho additional depth constraints were derived from 1d models using p wave arrivals of local earthquakes the mohos of the eurasian and philippine sea plates are disconnected across the plate interface beneath southern taiwan the eurasian moho dips to the east at 50 60 following the orientation of the plate boundary and continuous with the benioff zone towards north the eurasian moho steepens into subvertical again together with the plate boundary the philippine sea plate moho exhibits a synform like crustal root interpreted as the base of the magmatic luzon arc towards the north this root deepens from 30 to 70 km underneath the ryukyu trench in northernmost taiwan the hinge of the vertically subducting eurasian slab steps westward out of the thrust belt leaving the deformation front to the east inactive and giving way to the north dipping philippine sea plate a subhorizontal moho at 30 35 km depth overlies the north dipping philippine sea slab and is interpreted as a newly formed moho established after westward rollback and delamination of the subducting eurasian slab in combination these data support a model of progressive subduction polarity reversal in which a tear within the eurasian lithosphere propagated southwestward deactivating the deformation front

structure of the earth oceanic crust and uppermost mantle the past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere the application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere borehole seismic instrumentation has become more widely employed ocean bottom seismographs while generally decreasing in number have become significantly more reliable and useful and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes the techniques available for the analysis of seismic data have become more sophisticated waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models and travel time data are regularly inverted directly for structure trial and error modeling has become increasingly unimportant marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere this has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves

evidence for variations in magma production along oceanic spreading centers a critical appraisal recent geologic geophysical and geochemical studies of the oceanic lithosphere near fracture zones have resulted in the proposal that the magma budget defined as the amount of magma delivered to magma chambers or conduits beneath a spreading center for a given amount of spreading decreases as fracture zones are approached seismic reflection and refraction studies indicate that the crust becomes anomalously thin near fracture zones but reference must be made to the boundary between residual upper mantle peridotites and overlying cumulate rocks in order to assess potential variations in the magma budget the position of this interface however generally is not constrained by geophysical studies geochemical variations in basaltic glasses collected near fracture zones are consistent with a decrease in partial melting as fracture zones are approached but they could also be produced by variations in open system mag matic processes with no change in the extent of partial melting although a decrease in the magma budget as fracture zones are approached is consistent with these data so are alternative models that incorporate a constant magma budget

holocene floodplain evolution in the shiribeshi toshibetsu river lowland northern japan abstract the influence of sea level and climate changes on the evolution of coastal floodplains is an important problem in fluvial geomorphology and geology however few studies have constructed detailed chronologies of floodplain evolution and the influence of sea level and climate changes at submillennial time scales is not clear this study investigated the holocene evolution of the floodplain in the shiribeshi toshibetsu river lowland hokkaido northern japan based on 13 auger cores 15 radiocarbon ages and 2 cross sections made using existing columnar sections in the study area peat beds 3 6 m thick in the uppermost holocene sediments are underlain by fluvial sediment that mainly consists of sand beds resulting from crevassing or progradational avulsion age elevation plots of the bases of these peat beds suggest that fluvial aggradation was continuous until peat formation began which in turn suggests that peat beds began to form with the cessation of fluvial deposition the chronology of floodplain sediments based on radiocarbon ages indicates that peatlands began to develop locally before ca 6500 cal bp and became moderately widespread before 5600 cal bp peatlands then became more extensive after two periods of rapid expansion during ca 5300 5000 and 4100 3900 cal bp comparison with sea level and regional climate changes suggests that the initiation of these peat beds before 5600 cal bp was associated with the deceleration of sea level rise at ca 7000 cal bp the two later periods of peatland expansion may have been strongly influenced by reduced fluvial activity due to decreased precipitation from a weakened east asian summer monsoon this interpretation suggests that floodplain evolution was controlled by sea level and climate changes and that the response to climate change occurred at submillennial time scales a comparison with the ishikari lowland on hokkaido showed that the two floodplains have slightly different histories possibly because of differences in internal characteristics of the fluvial systems such as the number and size of tributary rivers water discharge and sediment supply although their responses to sea level and climate changes were similar the fluvial response to sea level and climate changes revealed in this study has implications for studies of the holocene evolution of other coastal floodplains

the east pacific rise in cross section a seismic model in 1982 we undertook a seismic refraction experiment known as the magma expedition to examine the detailed structure of the east pacific rise near 12 50 n this segment of the rise where the full spreading rate is about 110 mm yr is near the projected trace of the o gorman fracture zone and is the site of an overlapping spreading center as well as black smoker hydrothermal activity in this paper we describe the analysis of a subset of the travel time data collected during the magma expedition namely the data from profiles which were oriented normal to the rise axis these profiles provide a data set roughly equivalent to those collected on other experiments and sample the cross sectional structure of the rise we have modeled these data using a two dimensional ray tracing program we have found that the seismic velocities in the young oceanic crust are rather high with velocity gradients of 4 0 5 5 s 1 in the uppermost crust the highest velocities at the seafloor occur beneath the rise axis itself and seem to decrease as the crust ages to 0 1 ma this decrease in velocity must result from an increase in porosity in the upper crust and may coincide with the development of abundant surface fissures as the crust spreads the decrease in velocity does not appear to penetrate deeper than about 0 5 km and may reverse itself as hydrothermal alteration fills the pores and cracks the fact that the highest velocities occur under the rise axis suggests that the hydrothermal circulation responsible for the black smokers is confined to seismically unresolved conduits a result consistent with the high temperatures and discrete nature of the vents our best model includes a magma chamber some 4 km wide and extending from the moho to within about 1 1 km of the seafloor this magma chamber is far smaller than many models for the rise axis have predicted but larger than those inferred from seismic refraction experiments at other sites on the east pacific rise these discrepancies probably arise because the magma chamber under the east pacific rise is not a steady state feature but changes with time because of hydrothermal cooling and perhaps because of an episodic supply of magma from the mantle or along the rise axis

the core mantle boundary the largest compositional discontinuity within the earth is the core mantle boundary cmb at a depth of 2889 km this boundary and the adjacent transition zones in the lowermost mantle and outermost core play a critical role in the earth s thermal and chemical evolution estimates of the heat flux out of the core indicate that the 200 km thick d region at the base of the mantle is a major thermal boundary layer the d region like its counterpart boundary layer in the lithosphere is laterally heterogeneous and possibly chemically stratified this complexity obscures the exact dynamic behavior of the d region however it is clear that the large density contrast 4 3 g em 3 across the cmb provides favorable con ditions for density stratification on both sides of the boundary the d region may be the refuse pile for heavy mantle heterogeneities while the outermost core may have a concentration of light components boundary layer instabilities in d have been invoked as sources of both localized mantle plumes and major upwelling currents in whole mantle convection systems dynamically supported topography on the cmb and lateral temperature gradients in d may produce coupling between the core and mantle influencing the excitation of the geodynamo the outer most core is generally believed to be radially and laterally homogeneous although chemical stratification has not been ruled out seismological geodynamical and geomagnetic investigations are unveiling the three dimensional complexity of the deep earth in the vicinity of the cmb

direct high precision u pb geochronology of the end cretaceous extinction and calibration of paleocene astronomical timescales abstract the cretaceous paleogene k pg boundary is the best known and most widely recognized global time horizon in earth history and coincides with one of the two largest known mass extinctions we present a series of new high precision uranium lead u pb age determinations by the chemical abrasion isotope dilution thermal ionization mass spectrometry ca id tims method from volcanic ash deposits within a tightly constrained magnetobiostratigraphic framework across the k pg boundary in the denver basin colorado usa this new timeline provides a precise interpolated absolute age for the k pg boundary of 66 021 0 024 0 039 0 081 ma constrains the ages of magnetic polarity chrons c28 to c30 and offers a direct and independent test of early paleogene astronomical and 40ar 39ar based timescales temporal calibration of paleontological and palynological data from the same deposits shows that the interval between the extinction of the dinosaurs and the appearance of earliest cenozoic mammals in the denver basin lasted 185 ky and no more than 570 ky and the fern spike lasted 1 ky and no more than 71 ky after the k pg boundary layer was deposited indicating rapid rates of biotic extinction and initial recovery in the denver basin during this event

sequence stratigraphic distribution of coaly rocks fundamental controls and paralic examples significant volumes of terrigenous organic matter can be preserved to form coals only when and where the overall increase in accommodation approximately equals the production rate of peat accommodation is a function of subsidence and base level for mires base level is very specifically the groundwater table in paralic settings the groundwater table is strongly controlled by sea level and the precipitation evaporation ratio peat accumulates over a range of rates but always with a definite maximum rate set by original organic productivity and space available below depositional base level groundwater table below a threshold accommodation rate nonzero no continuous peats accumulate due to falling or low groundwater table sedimentary bypass and extensive erosion by fluvial channels this is typical of upper highstand lowstand fan and basal lowstand wedge systems tracts higher accommodation rates provide relatively stable conditions with rising groundwater tables mires initiate and copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received september 7 1995 revised manuscript received november 25 1996 final acceptance may 5 1997 2exxon production research company 3120 buffalo speedway houston texas 77096 3exxon production research company 3120 buffalo speedway houston texas 77096 present address conoco inc p o box 2197 houston texas 77252 we benefited from the input and assistance of many people of special assistance were the teams involved in collaborative studies with esso australia and esso malaysia p moore m sloan j emmett b burns a partridge s creaney hanif hussein r hill r lovell and m feeley we also thank the rock springs team r beauboeuf p mclaughlin w devlin a carroll y y chen g grabowski jr k miskell gerhardt m farley r webster and j schwalbach group members d curry and j yeakel were always helpful we also enjoyed and profited from many discussions of these concepts with our colleagues outside exxon c diessel r boyd k shanley b zaitlin p mccabe m hendricks a cohen and m kirschbaum we thank the reviewers of company reports whose careful comments on several generations of this work improved it s creaney m feeley j van wagoner f wehr j yeakel and a young f weber and j zullig provided extensive management support k linke translated our sketches into the fine figures herein we value all the help

the seismic reflection character of the continental mohorovicic discontinuity the continental mohorovicic discontinuity is most often interpreted as a step function velocity boundary however on deep crustal seismic reflection profiles reflections at depths where refraction data places the moho have laminated character and are laterally discontinuous these observations on continental reflection data point to a model of the moho that has thin layered rather than block structure previous workers who have synthetically generated seismic responses from various crust mantle boundary models have shown that thin layered models of alternating high and low velocity generate responses that best emulate observed moho arrivals geologic interpretations of the thin layering include relatively undeformed metasediments cumulate layering tectonic banding and lenses of partial melt to obtain more direct geologic evidence a synthetic seismogram is generated from geologic cross sections of the ivrea verbano zone an exposed section through the lower crust and upper mantle the synthetic shows laminations similar to those observed on reflection data without more direct evidence no single geologic interpretation of the moho is reasonable indeed the moho may be a laterally variable boundary its composition and structure dependent upon the geologic history of the overlying crust

sedimentation in a fluvially infilling barrier bound estuary on a wave dominated microtidal coast the oueme river estuary benin west africa the oueme river estuary is located on the seasonally humid tropical coast of benin west africa a striking feature of this microtidal estuary is the presence of a large sand barrier bounding a 120 km2 circular central basin lake nokoue that is being infilled by heterogeneous fluvial deposits supplied by a relatively large catchment 50 000 km2 borehole cores from the lower estuary show basal pleistocene lowstand alluvial sediments overlain by holocene transgressive highstand lagoonal mud and by transgressive to probably early highstand tidal inlet and flood tidal delta sand deposited in association with non preserved transgressive sand barriers the change in estuary mouth sedimentation from a transgressive barrier inlet system to a regressive highstand barrier reflects regional modifications in marine sand supply and in the cross barrier tidal flux associated with barrier inlet systems as barrier formation west of the oueme river led to an increasingly rectilinear shoreline the longshore drift cell matured ensuring voluminous eastward transport of sand from the volta delta in ghana the major purveyor of sand to the oueme embayment 200 km east concomitantly the number of tidal inlets and the tidal flux associated with a hitherto interlinked lagoonal system on this coast diminished complete sealing of lake nokoue has produced a large permanently closed estuary where tidal intrusion is assured through the interconnected coastal lagoon via an inlet located 60 km east since 1885 tides have entered the estuary directly through an artificial outlet cut across the sand barrier although precluding the seaward loss of fluvial sediments permanent estuary mouth closure has especially deprived the highstand estuary of marine sand a potentially important component in estuarine infill on wave dominated coasts in spite of a significant fluvial sediment supply estuarine infill has been moderate because of the size of the central basin estuarine closure has resulted in two co existing highstand sediment suites with limited admixture the marine derived estuary mouth barrier and upland derived back barrier sediments this situation differs from that of mature barrier estuaries characterized by active fluvial marine sediment mixing and facies interfingering

the border ranges ultramafic and mafic complex south central alaska cumulate fractionates of island arc volcanics a discontinuous elongate zone of mafic and ultramafic plutonic rock crops out in south central alaska for a distance of more than 1000 km intermediate and detailed scale geologic mapping petrographic study and compositional data suggest that the plutonic rocks are compositionally petrologically and mineralogically distinct from rocks in mid ocean ridge and back arc basin ophiolites the mafic and ultramafic rocks instead represent part of the plutonic core of an intraoceanic island arc the mafic ultramafic zone referred to as the border ranges ultramafic and mafic complex brumc is composed of ultramafic cumulates gabbronorite cumulates and massive gabbronorites a very minor amount of tectonized ultramafic rock of mantle origin is present in the southern part of the brumc a thick sequence of andesitic volcanic rocks the talkeetna formation of early jurassic age lies to the north of and structurally above the mafic ultramafic zone voluminous calcalkaline plutons composed of quartz diorite

middle to late holocene delta plain evolution of the kimotsuki lowland kyushu southern japan abstract the evolution of delta plains and coastal floodplains is commonly believed to be influenced by sea level and climate changes however regional climate change influences on the evolution of delta plains and floodplains are poorly understood compared with responses to rapid sea level rise since the last deglaciation this study investigated the kimotsuki lowland delta plain evolution kyusyu southern japan including allogenic controls based on 28 auger cores and 28 radiocarbon ages large scale crevasse splay deposition occurred at 7000 6000 cal yr bp in response to sea level rise or increased sediment supply following a volcanic eruption afterwards widespread peat was formed in the absence of substantial fluvial sedimentation decreased precipitation from a weakened east asian summer monsoon may have dried the peatlands around 4000 cal yr bp with increasing precipitation and storminess around 2000 cal yr bp peat formation resumed at areas further from the kimotsuki river while increased overbank flows deposited organic rich mud near the river these changes may reflect an increased frequency of la nina events and higher ocean surface temperatures of the kuroshio extension the increased storminess around 2000 cal yr bp may have produced similar influences on delta plain and coastal floodplain evolution across the east asia region the results of this study suggest that regional climate change in the late holocene has a potential to control the evolution of coastal floodplains and delta plains

thickening of the oceanic crust with age the most widely accepted models for ocean crust formation and composition fail to predict the thickening of the oceanic crust with age yet such thickening has been described in the pacific ocean for years to reconcile this apparent long standing conflict we have reexamined the evidence for the thickening of the pacific crust by using a statistical treatment of a large number of seismic profiles and by doing a detailed reanalysis of several profiles fanning a transect across the southern pacific ocean from the statistical studies we find that crust younger than 30 ma has a mean thickness of 5 67 km whereas crust between 30 and 100 ma has a mean thickness of 6 01 km this 0 34 km difference although statistically significant is far less than that reported in several previous studies our results from the south pacific transect suggest that changes in the crustal thickness are not systematic and from both studies it appears that crustal thickening is not particularly important the small thickening that we do observe is probably the result of isolated processes that are not active under the oceanic crust as a whole as a result arguments favoring a large component of serpentinite in the crust cannot be based upon evidence for crustal thickening beneath the pacific ocean

on eustatic sea level history last glacial maximum to holocene abstract this paper addresses the question of the magnitude and time dependence of the globally averaged eustatic rise of sea level that occurred subsequent to the time of last glacial maximum lgm at approximately 21 000 calendar years before present through the analysis of relative sea level rsl histories predicted by a realistic mass conserving and gravitationally self consistent theory of postglacial sea level change it is demonstrated that there are preferred oceanic locations at which this eustatic function is well approximated by local sea level history one such location is the island of barbados in the caribbean sea a site from which a coral based record exists that extends from mid holocene to lgm because of the sea level ambiguity that is inherent to coral based records however it is important that the global ice equivalent eustatic sea level curve inferred on the basis of the barbados data be tested against observations at other locations from which similarly extensive records are also available but which are derived on the basis of sea level indicators which are not subject to the ambiguities inherent to corals it is shown that when the eustatic function employed in the global theoretical model is tuned so as to enable the model to fit the barbados observations where the maximum relative sea level rsl depression is assumed to be near 120 m then the theory misfits the record from the sunda shelf in the indonesian archipelago as well as the record from j bonaparte gulf in northern australia both of these recently published records appear to constrain the lgm low stand of rsl to a value above 120 m the implications of these results for interpretation of the long coral derived records from the huon peninsula of papua new guinea and the island of tahiti are also discussed

middle and late holocene avulsion history of the river rhine rhine meuse delta netherlands at present little evidence is available on avulsion rates avulsion frequencies and interavulsion periods of aggrading fluvial systems over time scales of millenia the large number of high quality 14c ages from the holocene rhine meuse delta central netherlands enables a detailed assessment of such avulsion parameters and an evaluation of the factors controlling avulsion frequency 14c histogram analysis indicates that instantaneous avulsion coexistence of successive channel belts for less than a few centuries has dominated in this area interavulsion periods were typically 1000 yr although values of 500 or 5000 yr have also been found avulsion frequency was especially large prior to 4300 cal yr b p indicating a strong control by aggradation rate in turn controlled by sea level rise in addition periods with high avulsion frequency appear to correlate with time intervals characterized by large scale transgressive tendencies the time interval with the largest number of coexisting distributaries corresponds to the period with dominance of anastomosing fluvial style

the magnitude symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonites summary seismic anisotropy within the upper mantle originates from the preferred orientation of highly anisotropic single crystals the symmetry and magnitude of anisotropy depend upon 1 the volume percentages of the minerals constituting the upper mantle 2 the degree and symmetry of preferred orientation of each mineral and 3 the alignment of the minerals crystallographic axes relative to one another the nature of upper mantle anisotropy can be examined by studying mineral orientations within ultramafic rocks which were once part of the mantle petrofabric data for olivine and pyroxene have been used to obtain velocity anisotropy patterns over large regions of ultramafic rocks from the samail ophiolite oman the troodos ophiolite cyprus the bay of islands ophiolife newfoundland the twin sisters ultramafic washington usa the dun mountain ophiolite new zealand the red hills ophiolite new zealand and the red mountain ophiolite new zealand the compressional wave anisotropy calculated for these massifs ranges from 3 to 8 per cent in excellent agreement with observed seismic anisotropy in the upper continental and oceanic mantle the symmetry varies from orthorhombic to axial with the axial symmetry axis corresponding to the olivine a axes maxima and subparallel to spreading directions in oceanic upper mantle pyroxene a b and c axes maxima generally parallel olivine b c and a axes respectively and anisotropy decreases with increasing pyroxene content shear wave splitting is predicted for all propagation directions within the upper mantle symmetry is also orthorhombic or axial with the minimum difference in velocity between the two shear waves parallel to the maximum compressional wave velocity

dating the lower crust by ion microprobe ion microprobe uthpb ages of zircons from granulite facies lower crustal xenoliths from north queensland australia correlate well with the ages of major orogenic episodes manifest at the earth s surface about half of the xenoliths contain proterozoic zircons which are similar in age to the episodes of high grade metamorphism of the older surface rocks all the xenoliths contain late paleozoic zircons which show a real 100 ma range in206pb238 u ages from 320 to 220 ma which is attributed to granulite facies metamorphism followed by slow cooling in the deep crust the paleozoic zircon ages coincide in time with the prolonged episode of eruption of voluminous felsic ash flows and intrusion of high level granites in this region 320 270 ma mineral and melt inclusions in the zircons provide clues to the origin of some of the xenoliths and coupled with the age information can be used to infer the geological processes operating in the lower crust the zircons from two mafic xenoliths contain felsic and intermediate melt inclusions implying at least a two stage history for these rocks involving either partial melting of a more felsic protolith or crystal accumulation from an evolved melt some of the zircons from the felsic xenoliths contain co2 rich fluid inclusions indicating that those zircons grew during high grade metamorphism the isotopic and chemical data for the whole rock xenoliths show that they originate from a segment of the lower crust which is a heterogeneous mixture of supracrustal and mafic mantle derived lithologies the major orogenic event responsible for the formation of that crust occurred in the late paleozoic when proterozoic supracrustal rocks were emplaced into the lower crust possibly along thin skinned thrust slices this was accompanied by intrusion of high temperature mantle derived melts which caused partial melting of pre existing crust the most likely setting for such tectonism is a continental margin subduction zone

avulsion the relative roles of autogenic and allogenic processes abstract avulsion the abandonment of all or part of a channel belt in favor of a new course is controlled by both autogenic intrabasinal and allogenic extrabasinal processes however their relative roles are not known well the rhine meuse delta is without doubt the best studied delta regarding holocene avulsion processes based on studies of this delta an overview is given of 1 allogenic and autogenic processes influencing different aspects of avulsion 2 the relative importance of these processes in time and space and 3 observations that can be used to differentiate autogenic and allogenic causes of avulsion the following avulsion parameters are quantified and their allogenic and or autogenic nature are discussed period of activity of channel belts period between beginning and end of overbank sedimentation by the river channel interavulsion period time period between successive avulsions of a channel belt avulsion frequency number of avulsions per time interval in a given area and avulsion location in the rhine meuse delta the period of activity of the holocene channel belts varies considerably but shows no significant trend over time whereas extrabasinal factors climate base level and tectonics changed this suggests that this parameter is mainly autogenic controlled the average interavulsion period increased from 8000 cal yr bp to 2800 cal yr bp and decreased since then significant fluctuations occurred on a shorter time scale a maximum variability in the interavulsion period occurred between 3200 and 1800 cal yr bp this was a time when large channel belts gradually came into existence during this period the avulsion frequency also reached a maximum the long term increasing trend in interavulsion period is related to the decreasing rate of sea level rise the decreasing interavulsion period since 2800 cal yr bp can be explained by increased discharge and sedimentation resulting in an increase in the number of avulsions seven avulsion sequences as predicted by the mackey and bridge mackey s d bridge j s 1995 three dimensional model of alluvial stratigraphy theory and application journal of sedimentary research b65 1 pp 7 31 model may be recorded in the holocene dataset of the rhine meuse suggesting a periodicity of 500 600 yr each avulsion sequence coincides with a peak in the avulsion frequency the periodicity of 500 yr in the avulsion frequency is proposed to be autogenic however there is also a long term trend in the avulsion frequency that can be related to sea level rise and after 3000 14c yr bp to climate change and human influence avulsion locations are not randomly distributed over the delta and are related to the allogenic factors relative sea level rise local tectonics and changes in discharge and sediment load

natural levees and human settlement in the song hong red river delta northern vietnam the song hong red river delta northern vietnam is characterized by huge natural levees in an area of the delta plain known as the west floodplain where fluvial sedimentation predominates the natural levees along the day river a major distributary of the song hong are comparable in size with those of the main course of the song hong the day river levees are 3 8 km wide and rise 2 5 m above the adjacent backswamps and have played an important role in human settlements since the late metal age we discussed the relationships among the natural levees of the day river delta progradation and the distribution of archaeological sites on the delta plain during the early holocene the accumulation of sediment discharged by the song hong enhanced both aggradation of the floodplain and river mouth progradation within the drowned valley of the song hong radiocarbon dates from cores trench exposures and archaeological sites record a dramatic slowing of aggradation when sea level stabilized during 6 4 cal

on long term thickening of the oceanic crust a number of studies of seismic refraction data have resulted in suggestions that the oceanic crust thickens with age perhaps over a long time span it is difficult to reconcile notions of crustal thickening with many of our ideas about crustal structure and formation in this study we have re evaluated data compiled from a number of sources to obtain crustal thickness and age parameters for 105 different refraction profiles in the pacific an attempt was made to exclude data which might have poorly determined values of crustal thickness the crust does appear to thicken with age to 30 or 40 m y but the effect is very subtle once probable biases are removed from the data crustal thickening is not significant to a 95 confidence limit if ten m y samples are used however if one takes sample intervals of 0 30 m y and 30 100 m y there appears to be some difference in the average crustal thickness even in the latter case the amount of thickening is far less than previous studies have suggested only 45 km thus evidence for the long term addition of mass to the crust remains tenuous

two dimensional seismic reflection modeling of the inferred fossil oceanic crust mantle transition in the bay of islands ophiolite we investigate the origin and character of oceanic moho reflections by computing two dimensional synthetic seismogram profiles of the inferred fossil oceanic crust mantle transition observed in the bay of islands ophiolite to simulate a seismic reflection experiment we calculated near vertical incidence seismograms at a horizontal spacing of 500 m for three separate sections of the ophiolite totaling 64 km in length in the synthetic profiles the moho reflection event varies from a single phase to two or more phases of up to 1 s two way travel time total duration individual phases show lateral variation in amplitude and their two way travel times vary by as much as 0 25 s over horizontal distances as short as 10 km lateral discontinuity of phases results in abrupt variations in the travel times of first arriving high amplitude moho phases the geological structures generating the highest amplitude moho reflections vary from high and low velocity lenses of mafic and ultramafic material in the lower crust and upper mantle to interlayered mafic and ultramafic lithologies in the moho transition zone reflection amplitudes from the residual upper mantle are insignificant and our modeling suggests that using the first arriving high amplitude moho phase to estimate thickness of magmatic material might result in errors of up to 1 s two way travel time 3 4 km multichannel seismic data from both the western pacific and western north atlantic show moho travel time variations similar to those observed in the synthetic profiles the western north atlantic data also show multiphase moho reflection events that are laterally discontinuous on a scale similar to that observed in the synthetic data suggesting that the structures observed in the inferred fossil crust mantle transition of the ophiolite are characteristic of oceanic lithosphere

the moho in the northern basin and range province nevada along the cocorp 40 n seismic reflection transect cocorp seismic reflection profiles across nevada at about 40 n image a prominent essentially continuous band of reflectors at a two way traveltime of 9 to 11 s the approximate correspondence of this reflection time with estimates of the two way traveltime to the moho in this area provided by seismic refraction data suggests that the prominent reflections are from the moho the relief on these reflectors the reflection moho beneath nevada across a latitudinal transect of

the seismic structure of 140 myr old crust in the western central atlantic ocean summary a detailed seismic refraction experiment using explosive sources and ocean bottom hydrophone obh receivers was carried out over mesozoic magnetic anomaly m17 about 300 km south west of bermuda amplitude and travel time interpretations show this 140myr old crust to be uniform on the seismic scale over a lateral distance of almost 1ookm the best estimate of the velocity structure consists of an average 700 m of sediment overlying a total thickness of igneous crust of 7 2 km the principle components beginning with a 5 km s l velocity at the top of layer 2 are a 0 4 km thickness with gradient 1 1 s a 1 9km thickness with gradient 0 64s 1 7 km thickness with gradient 0 1 s a 3 2 km thickness with zero gradient and constant velocity of 7 km s below which lies a 0 5 km thick moho transition zone the uppermost few kilometres of the upper mantle apparently has little or no velocity gradient the normal incidence two way reflection time through this structure agrees to better than 0 1 s with the location of the moho reflection seen on the ipod usgs multichannel reflection profile that passes within 1 km of this experiment the travel times of the converted shear wave arrivals constrain poisson s ratio in the igneous crust to 0 28 k 0 01 a delay time study of a 20km radius circle of shots fired around a small 6 km five element array of ocean bottom hydrophone receivers confirms the uniformity of structure of this old crust on the scale of a few kilometres the observed differences in shallow crustal delay time between the five obh instruments can be wholly attributed to differences in sediment thickness beneath the receivers as determined by a deep towed hydrophone seismic reflection profile these experiments were located clear of fracture zones or other structural anomalies in a region of well defined linear constant amplitude magnetic anomalies we propose that this was the primary cause of our result of lateral uniformity of structure on the scale of a few kilometres we suggest that the reason that such a simple uniform structural model for oceanic crust is not supported by the historical seismic refraction dataset is that a large proportion of the old experiments were poorly located relative to fracture zones and other structural anomalies unknown at the time and the

lithospheric structure in southern sweden results from fennolora abstract the major objective of fennolora the fennoscandian long range seismic project of 1979 was the determination of lower lithospheric and upper mantle structure down to depths in excess of 400 km below the fennoscandian shield in the component represented by this study data recorded at stations in southern sweden from three shotpoints one in northern germany profile wn and two separated by 300 km in southern sweden profiles bn and cs are used to derive a two dimensional lithosphere structure model extending over 600 km from north to south the profile crosses the svecofennides the smaland varmland granite belt paleozoic cover rocks lying unconformably on the fennoscandian shield the baltic sea where there were no stations and into the caledonian tectonic province of northern germany where one shotpoint but no stations were located interpretation of the three record sections was based on a two dimensional ray tracing procedure which included the calculation of asymptotic ray theory synthetic seismograms lack of data from 0 150 km for the shotpoint in germany to the first station in southern sweden precluded derivation of any detailed crustal structure for this region crustal thickness was determined to be 32 km a rapid increase in velocity from 8 0 to 8 35 km s at about a depth of 50 km is underlain by a low velocity zone extending to a depth of 70 km the reversed profiles bn and cs in southern sweden are fundamentally different interpretation shows a difference in crustal thickness of about 12 km with the northern segment having a deeper moho a feature resulting from a thicker lower crust and a crust mantle transition zone rather than a rapid increase in velocity in the southern segment of the reversed profile a crustal low velocity zone of limited extent is indicated the most significant aspect of the interpretation is the suggestion that the change in crustal structure between the two shotpoints in southern sweden occurs within a transition zone of limited lateral extent less than a few tens of kilometers we suggest that the model transition zone is a fundamental lithosphere boundary associated with the juxtaposition of the smaland varmland granite belt and the svecofennides

geologic and seismic velocity structure of the crust mantle transition in the bay of islands ophiolite complex geological investigations of the bay of islands ophiolite complex show that while a typical layered ophiolite suite is present the thickness of major lithologic units is extremely variable from place to place the composition and internal structure of map scale kilometers across lithologic units as well as the contacts that bound them are laterally variable inferred velocity depth functions of the crust mantle moho transition reconstructed for this terrane as oceanic lithosphere suggest an extremely complex internal seismic structure the crust mantle transition from mafic to ultramafic lithologies varies across the ophiolite from a sharp geologic and seismic velocity discontinuity to a complexly interlayered transition zone as much as 3 km thick the mafic ultramafic transition is characterized by laterally discontinuous lithologic units on the order of hundreds of meters to a few kilometers thick and up to several kilometers long these units megalenses may be composed of lithologies with higher or lower seismic velocities than those of the surrounding units solid state deformation has produced seismically anisotropic materials in the upper mantle and lower crustal units substantial relief and slopes of up to at least 12 over lateral distances of about 10 km occur on the top of the crust mantle transition zone as well as other geologic unit contacts although these types of geological features are striking to the field geologist working in ophiolites and are significant to the understanding of crustal accretion processes many of them occur on such a small scale that they might go undetected in different types of seismic experiments in contemporary oceanic lithosphere

sedimentary products of avulsions in the holocene rhine meuse delta the netherlands abstract the main objective of this study was to describe the characteristics of deposits related to avulsion to determine if avulsion locations can be recognized by characteristic sediment architecture in the rhine meuse delta the majority of avulsion sites is accompanied by crevasse splay deposits with a relatively large areal extent these crevasse splay complexes occur near the base of the overbank deposits of the newly formed channel belt and are similar to avulsion belt deposits described by smith et al sedimentology 36 1989 1 can j earth sci 35 1998 453 the lithology of all investigated crevasse splay deposits ranges from coarse sand to silty clay sand is concentrated in crevasse splay channels and silty and sandy clay occurs as distal deposits and as levee deposits between the channels the total volume of sand is small relative to clay and silt of a total of 91 avulsion sites three sites were studied in detail one each representing the sedimentary architecture of a full nodal avulsion a failed avulsion and a crevasse splay complex that did not lead to avulsion it is concluded that dating evidence of the abandoned and newly formed paleo channel is always required to verify if avulsion really occurred although the presence of large scale sandy crevasse splay deposits in combination with two main meandering channels often indicates avulsion based on a 2 d outcrop or just one cross section it is impossible to determine whether a splay complex was associated with an avulsion or not because the architecture and lithology in both cases can be identical not all avulsion locations are characterized by crevasse splay deposits there are two possible explanations for this 1 splay deposits were eroded by the newly formed channel or 2 splay deposits were never formed this may be a result of reoccupation of an old channel in this case avulsive flow is immediately appropriated by the pre existing channel and no large scale splay formation occurs alternatively splay deposits were not formed because avulsion resulted from headward erosion

refining the eustatic sea level curve since the last glacial maximum using far and intermediate field sites the eustatic component of relative sea level change provides a measure of the amount of ice transferred between the continents and oceans during glacial cycles this has been quantified for the period since the last glacial maximum by correcting observed sea level change for the glacio hydro isostatic contributions using realistic ice distribution and earth models during the last glacial maximum lgm the eustatic sea level was 125 5 m lower than the present day equivalent to a land based ice volume of 4 6 4 9 107 km3 evidence for a non uniform rise in eustatic sea level from the lgm to the end of the deglaciation is examined the initial rate of rise from ca 21 to 17 ka was relatively slow with an average rate of ca 6 m ka 1 followed by an average rate of ca 10 m ka 1 for the next 10 ka significant departures from these average rates may have occurred at the time of the younger dryas and possibly also around 14 ka most of the decay of the large ice sheets was completed by 7 ka but 3 5 m of water has been added to the oceans since that time

east asian summer monsoon variations in the past 12 5 ka high resolution delta o 18 record from a precisely dated aragonite stalagmite in central china due to possible aragonite to calcite transformation resulting in alteration in isotopic signatures and a bias in age dating aragonite speleothems are often excluded from paleoclimatic archives however aragonite stalagmites contain ppm level uranium content making them much easier to date achieving higher age dating precisions than calcite stalagmites in this regard provided aragonite to calcite transformation did not occur aragonite stalagmites are potentially well suited for holocene climate research given their climate proxies can be placed into a better constrained chronological framework in this paper we present high precision u th dates and o isotopic time series for a 82 cm long continuous growth aragonite stalagmite lh2 from lianhua cave hunan province china and discuss east asian summer monsoon easm variability for the last 12 5 ka bp before present the u th dated delta o 18 sequence with a mean 16 year resolution and the growth rate pattern of lh2 show that easm experienced a strengthening stage a strong stage and a weakening stage during the last 12 5 ka during the yd 12 5 11 5 ka bp heavy delta o 18 values and low growth rate indicate a weak monsoon period during the preboreal from similar to 11 5 to 10 6 ka bp delta o 18 values decreased dramatically similar to 1 94 parts per thousand reflecting abrupt strengthening of the monsoon from 10 6 to 4 2 ka bp the record is characterized by the lightest delta o 18 values and high growth rates suggesting a strong monsoon period the summer monsoon weakened substantially after 4 2 ka bp as inferred from gradually increasing delta o 18 values and decreasing growth rate overall the intensity of the easm is regulated by summer insolation at 30 degrees n during the last 12 5 ka although oxygen isotope fractionation is different between aragonite h2o and calcite h2o because of rayleigh fractionation law the overall temporal pattern of delta o 18 values from aragonite stalagmite lh2 is concordant with other high resolution holocene calcite stalagmite records from south china the comparison among these records shows that the holocene optimum was synchronous across asian continental region influenced by the easm and indian summer monsoon ism lasting from similar to 10 6 to 4 2 ka bp as reflected by consistently lighter delta 18 ovalues among all these records regardless of their latitudinal difference these records do not support previously reported asynchronism between easm and ism in details the long term delta o 18 trend in lh2 is punctuated by a number of centennial fluctuations for instance two weak monsoon events occurred at 9232 57 yr bp and 8137 21 yr bp correlating in time with cooling events in greenland ice cores the latest weak monsoon event centered at 302 8 yr bp which is related to the little ice age lia in addition the monsoon intensity derived from our record also shows a strong connection with latitudinal migration of the inter tropical convergence zone itcz as recorded in the cariaco basin sediments spectral analysis of delta o 18 values shows that significant peaks match with solar periodicities of 208 yr de vries cycle 86 yr gleissberg cycle or related to delta c 14 production suggesting shorter term monsoon variations are forced by solar radiation overall our study suggests that the delta o 18 record in the aragonite stalagmite is highly consistent with those derived from calcite stalagmites suggesting that aragonite stalagmites are suitable for palaeoclimate reconstruction especially for the holocene period as aragonite calcite transformation has not occurred c 2013 elsevier ltd all rights reserved

replicated stalagmite inferred centennial to decadal scale monsoon precipitation variability in southwest china since the mid holocene a lack of agreement between stalagmite records of the centennial to decadal scale asian summer monsoon asm variability since the mid holocene limited our understanding of forcings and the possible impacts on culture evolution here we present replicated high resolution stalagmite 18o profiles from dark cave in guizhou province southwest china established with 512 oxygen isotope data and 28 230th ages providing a continuous history of the asm precipitation from 6100 to 300 years before present yr bp before ad 1950 consistency of stalagmite 18o inferred long term decreasing asm trends since the mid holocene between caves at this site and others in the indian and east asian monsoon realms on orbital timescales supports the effect of primary orbital solar forcings on monsoonal precipitation the dark cave records display significant centennial to decadal scale variations more evident than previous stalagmite records spectral analysis of the dark cave 18o time series yields periodicities of 731

exposed cross sections through the continental crust implications for crustal structure petrology and evolution abstract the continental crust is exposed in cross section at numerous sites on the earth s surface these exposures which appear to have formed by obduction along great faults during continental collision may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle from an examination of five complexes which meet these criteria it is concluded that the most prominent layering in the crust is not compositional but metamorphic the lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks respectively post metamorphic granitic intrusions are common at intermediate to shallow levels position of discontinuities in refraction velocity where present commonly correspond to changes in composition or metamorphic grade with depth the continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by cocorp field observations coupled with geochemical data indicate a complex evolution of the lower crust which can include anatexis multiple deformation polymetamorphism and reworking of older crustal material the complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments

extension of continental crust at the margin of the eastern grand banks newfoundland seismic and gravity observations from the rifted margin of the eastern grand banks newfoundland support a new model for extension of the continental crust from the shelf edge to odp site 1277 where mantle rocks are exhumed we find that the largest decrease in crustal thickness from about 28 km to 6 km occurs beneath the continental slope of the grand banks over a distance of just 20 km this rapid decrease in crustal thickness coincides with anomalously high seismic velocities 7 0 7 2 km s 1 in the lower crust of the shelf edge the thin crust of the continent ocean transition cot in this area has a smooth basement surface void of upper crustal blocks and prerift sediments we compare our geophysical results with a geodynamical model that represents rifting of a relatively hot continental lithosphere and with another numerical model that represents rifting of a cold lithosphere both geodynamic models suggest that crustal thinning beneath the continental slope was achieved by extensional faulting in the upper crust and ductile shear zones in the middle crust the geodynamic models provide an explanation for the formation of distinct continental slopes at rifted margins beneath the continental shelf of the grand banks the moho and the strong lower crust rotated upwards toward to a 50 dip without visible internal deformation the presence of these strong lower crustal rocks at shallow depth in the rift flank subsequently helped to localize the extension farther seaward with ongoing extension some high angle normal faults may have rotated to a sub horizontal orientation which would explain the lack of brittle deformation visible in the seismic reflection data the two geodynamic models produce different amounts of extension of continental crust in the distal margins the hot rifting model localizes strain much more rapidly leaving narrow zones of extended continental crust and it produces a relatively large amount of melt 30 in the final stages of rifting continental breakup may occur rapidly in hot lithosphere 5 myr on the other hand a cold extension model extends the continental crust to a thickness smaller than 10 km over a width of 50 km in the distal margin similar to what we inferred at the eastern grand banks the cold lithospheric model requires about 23 myr of extension before continental breakup and it predicts much less melting in the mantle 13 the long rift duration wide zones of thinned continental crust and small amount of magmatism make the cold rifting model the most applicable to newfoundland iberia rift

holocene avulsion styles and sedimentation patterns of the saskatchewan river cumberland marshes canada abstract this paper describes the styles and sedimentation patterns of holocene avulsions of the saskatchewan river at the cumberland marshes predating the most recent 1870 s avulsion holocene avulsions involved two major styles 1 progradation of lacustrine deltas and splay complexes and 2 local reoccupation or appropriation of preavulsive channels most avulsions were progradational and occurred in floodplain lakes occupying floodbasin areas between elevated alluvial ridges the two most recent avulsions old channel and 1870 s diverted mainly into inundated peatland progradational avulsions were accompanied by deposition of avulsion belt sediments associated with multiple channel networks the characteristics of avulsion belt deposits include 1 sediments dominated by poorly sorted silt and very fine sand 2 lateral continuity in transverse and longitudinal directions with subhorizontal upper and lower boundaries 3 predominantly nonerosional contacts with underlying sediments 4 coarsening upward successions 5 consistent radiocarbon dates for the tops of preavulsive substrates and 6 occurrence beneath levee and overbank deposits of the associated major channel belt indicating that avulsion belt deposition occurs prior to the development of a new master channel the width of ancient avulsion belts reaches 18 km with typical thicknesses of 2 3 m avulsion belt deposition accompanied by many small channels was followed by flow reversion to a few larger channels and eventually to one single channel accompanied by deposition of levee and channel deposits reoccupational avulsions were not accompanied by deposition of widespread avulsion belt sediments and led mainly to the modification of pre existing channel belts the sedimentation patterns of holocene avulsions of the saskatchewan river reveal that avulsion belt sediments represent major components of the floodplain deposits supporting the model developed from observations of the 1870 s avulsion

factors controlling peat compaction in alluvial floodplains a case study in the cold temperate cumberland marshes canada abstract subsidence due to peat compaction may have important implications in alluvial floodplains because it leads to wetland loss land inundation and damage to buildings and infrastructure furthermore it potentially influences spatial and temporal river sedimentation patterns and hence the evolution of alluvial floodplains as the vast majority of peatlands occur in cold temperate regions alluvial plains situated in such areas are most susceptible to these implications to determine which locations within alluvial floodplains are most vulnerable to high amounts of compaction induced subsidence it is necessary to quantify compaction and to identify which factors influence the amount and rate of peat compaction for this we carried out field work in the cumberland marshes east central saskatchewan canada guided by the following two questions 1 how much peat compaction has occurred over the past decades to centuries and 2 which factors control the amount and rate of peat compaction at the same timescale to address these questions we used methods involving construction of stratigraphic cross sections organic matter content and bulk density measurements and radiocarbon dating a new sampling device was devised to sample uncompacted peat in the field results show that peat layers have compacted up to 43 within a few centuries with compaction rates of up to 6 08 mm yr the dominant factors influencing peat compaction are 1 organic matter content 2 stress imposed on a peat layer and to a lesser extent 3 plant species composition in an alluvial setting crevasse splays and natural levees are sites that are most susceptible to high amounts of peat compaction at short timescales 100 102 years sheet like splay deposits initially compact underlying peat uniformly whereas differential compaction commonly occurs beneath natural levees due to lateral variations in sediment thickness subsidence due to peat compaction creates additional accommodation space and hence locally enhances floodplain sedimentation rates

holocene climate fluctuations in the yangtze delta of eastern china and the neolithic response six major pollen spore assemblage zones can be identified showing alternate warm and cool climate fluctuations on a millennial timescale the early holocene optimum at c 8000 7800 radiocarbon year

anisotropy in the oceanic lithosphere theory and observations from the ngendei seismic refraction experiment in the south west pacific summary p wave travel time data from a seismic refraction experiment on the 1983 ngendei expedition to the south pacific indicate anisotropy at two levels in the oceanic lithosphere in the upper mantle p wave velocities vary between 8 0 and 8 5 km s 1 with the fast direction at n30 e crustal anisotropy within layer 2 is characterized by azimuthal p wave velocity differences of about 0 2 0 4 km s 1 with the fast direction at n120 e orthogonal to the upper mantle anisotropy the observed anisotropy is consistent with a model in which aligned olivine crystals cause anisotropy in the upper mantle and aligned cracks within layer 2 cause anisotropy in the crust we examine a model of p wave propagation from a point source in an isotropic layer above an anisotropic half space and show that under these conditions anisotropy can cause seismic ray paths to deviate from the vertical plane connecting source and receiver this deviation has a negligible effect on seismic travel times but must be taken into account in modelling p wave polarization anomalies p wave polarization anomalies within the anisotropic half space are the sum of the particle motion deviation from the ray path and ray path deviation from the source receiver azimuth and are typically range dependent p wave polarization anomalies will also occur in the overlying isotropic layer purely as a result of the ray path deviation with greater deviations at long ranges a simple model of upper mantle anisotropy 6kms 1 crust above 8 0 8 6 km s 1 anisotropic mantle produces surface polarization anomalies of 1 5 3 5 in practice p wave polarization anomalies are difficult to observe because they are small compared to observed scatter in polarization data and are sensitive to differences in horizontal seismometer gain levels we examine the implications of anisotropic ray path deviations on travel times and p wave polarizations for the ngendei data as well as some previous marine seismic refraction experiments

cocorp deep seismic reflection traverses of the u s cordillera summary cocorp seismic reflection traverses of the u s cordillera at 40 n and 48 5 n latitude reveal some fundamental similarities as well as significant differences in reflection patterns on both traverses autochthonous crust beneath thin skinned thrust belts of the eastern part of the cordillera is unreflective immediately to the west the cordilleran interior is very reflective above a flat prominent reflection moho mesozoic accreted terranes in the western part of the orogen are underlain on both traverses by very complex reflection patterns in constrast to more easily deciphered patterns beneath areas of cenozoic accretion the prominent reflection moho beneath the orogenic interior on both transects probably evolved through a combination of magmatic and deformational processes during cenozoic extension the main differences between the two traverses lie in the reflection patterns of the middle and lower crust in the cordilleran interior these differences are probably related to the way cenozoic extension was accommodated at depth laminated middle and lower crust above the reflection moho in the western basin and range 40 n may be related to magmatism ductile pure shear and large scale transposition during cenozoic extension by contrast beneath the eastern basin and range 40 n and the orogenic interior in the nw united states 48 5 n cenozoic extension was probably accommodated along dipping deformation zones throughout the crust

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

pollen based quantitative reconstruction of holocene climate changes in the daihai lake area inner mongolia china vegetation around the daihai lake northern china is very sensitive to climate changes in this paper pollen based quantitative climate reconstructions using three methods weighted averaging partial least squares method wapls modern analog technique mat and pollen response surface method prs were conducted to obtain robust reconstructions of holocene climate changes in the daihai lake area the result obtained by the three methods all consistently show the annual precipitation to have been 50 100 mm lower in the early holocene 100 200 mm higher in the mid holocene and 50 100 mm lower again in the late holocene than at present the wapls and the mat methods also show quasi synchronous oscillations of the mean annual temperature ta 18 28c lower in the early holocene and 18 38c higher in the midholocene than today the time period from 6200 to 5100 cal yr bp was the wettest and the warmest interval with an annual precipitation pa greater than 550 mm and mean annual temperature ta higher than 6 58c several cold and dry events can be identified to occur about 8200 6000 and 4400 cal yr bp with an annual precipitation less than 400 mm and a mean annual temperature colder than 4 58c respectively the mean temperature of the warmest month tw as reconstructed using both wapls and mat methods was relatively stable during the holocene fluctuating about 628c relative to the present level but the prs method suggests more varied tw values in both amplitude and frequency after 1500 cal yr bp no consistent pattern can be observed from these three different analyses probably because of the impact of intensified human disturbances on the natural vegetation the fluctuations of annual precipitation pa correspond to that observed in dongge cave in southern china the differences might be linked to indian monsoon and east asia monsoon climates or caused by the different degree of dating precision different temporal resolution and different sensitive response of climate proxies to the climate variations

depositional development of the muskeg lake crevasse splay in the cumberland marshes canada crevasse splays are common geomorphological features in alluvial and deltaic floodplains although crevasse splays can develop into full avulsions thereby transforming large areas of floodbasins little is known about their sedimentary and geomorphological development at the decadal scale and their avulsion potential we used aerial photography and lithological cross sections to reconstruct crevasse splay formation in the largely unmanaged floodplain of the saskatchewan river in the cumberland marshes saskatchewan canada based on surface geomorphology and subsurface deposits various stages of crevasse splay development were described which were linked to both external forcing and internal morphodynamics initial splay deposition following a levee breach during a large flood occurred as a broad but relatively thin sandy sheet in a down basin direction in the receiving backswamp area in a next phase these primary crevasse splay deposits blocked local down basin flow thereby forcing the crevasse splay channel in a direction perpendicular to the parent channel and original floodbasin gradient this created an asymmetrical splay sequence composition which differs in appearance from more commonly observed dendritic crevasse splays it is concluded that sedimentation patterns in the splay have been influenced by inherited effects of previously formed deposits feedbacks of the original floodbasin gradient and earlier stages of splay formation are suggested as prominent mechanisms in creating the current morphology orientation and architecture of its deposits

a high resolution absolute dated holocene and deglacial asian monsoon record from dongge cave china abstract we present a continuous record of the asian monsoon over the last 16 ka from 18o measurements of stalagmite calcite over 900 oxygen isotopic measurements providing information on shifts in monsoon precipitation are combined with a chronology from 45 precise 230th dates 18o and therefore asian monsoon intensity generally follows changes in insolation although changes in 18o are generally accommodated in abrupt shifts in contrast to smoothly varying insolation indicating that threshold effects may be important 18o decreased dramatically 3 at the start of the holocene 11 5 ka and remained low for 6 ka four positive 18o events centered at 11225 97 yr bp 1 05 10880 117 yr bp 1 15 9165 75 yr bp 1 4 and a double event centered at 8260 64 yr bp 1 1 and 8080 74 yr bp 1 0 punctuated this period of high monsoon intensity all four events correlate within error with climate changes in greenland ice cores thus the relationship between the asian monsoon and the north atlantic observed during the glacial period appears to continue into the early holocene in addition three of the four events correlate within error with outburst events from lake agassiz the decline of monsoon intensity in the mid late holocene is characterized by an abrupt positive shift in 18o which occurs at 3550 59 yr bp 1 1 in 100 yr in addition the holocene is punctuated by numerous centennial and multi decadal scale events amplitudes 0 5 to 1 up to half the amplitude of the glacial interstadial events seen in the last glacial period thus holocene centennial and multi decadal scale monsoon variability is significant although not as large as glacial millennial scale variability the monsoon shows a strong connection with northern south american hydrological changes related by changes in itcz position spectral analysis of the 18o record shows significant peaks at solar periodicities of 208 yr and 86 yr suggesting variation is influenced by solar forcing however there are numerous other significant peaks including peaks at el nino frequencies observed for high resolution portions of the record between 8110 and 8250 yr which suggest that changes in oceanic and atmospheric circulation patterns in addition to those forced by solar changes are important in controlling holocene monsoon climate in addition for this high resolution portion we observe a distinctive biennial oscillation of the asian monsoon which has been associated with the tropospheric biennial oscillation tbo

new classification systems for tropical organic rich deposits based on studies of the tasek bera basin malaysia most schemes in common use for field and laboratory classification of peats were developed in boreal and humid temperate regions and do not recognize the distinctive features and specific uses of tropical peats such as those of the tasek bera basin in tropical peninsular malaysia the important aspects of peat texture morphology of constituents and their arrangement and laboratory ash content residue after ignition need modification to be valuable for classifying these and other tropical peat deposits in the tasek bera basin most of the deposits would not be considered as peat according to some classification schemes even though most have c contents 25 we propose a new three group fibric hemic sapric field texture classification applicable to tropical organic deposits which is similar to the system of the us soil taxonomy the classification is based on the following factors 1 visual examination of the morphology of the peat constituents texture and 2 estimates of fiber content and matrix finest fraction of peat consisting of highly humified organic matter and inorganic material the classification is applicable to all organic deposits with 35 loss on ignition we also present a new laboratory classification of organic soils based on ash and c content the us soil taxonomy classifies organic soils as having more than 12 18 organic c depending on clay content ash content and these limits for organic soils allow the discrimination of four main groups peat muck organic rich soil sediment and mineral soil sediment peat is defined as having an ash content of 0 55 muck 55 65 organic rich soil sediment 65 80 and mineral soil sediment 80 100 the peat class is further subdivided into very low ash 0 5 low ash 5 15 medium ash 15 25 high ash 25 40 and very high ash 40 55 subclasses

new cocorp profiling in the southeastern united states part i late paleozoic suture and mesozoic rift basin new cocorp profiling in the southeastern united states has revealed a broad zone of dipping reflections that extends downward through the crust beneath the coastal plain in western georgia the zone is over 50 km wide and most of the reflections dip moderately steeply toward the south regional considerations suggest that this feature marks the late paleozoic suture between north america and africa where crossed by the cocorp survey the suture occurs beneath the north flank of the triassic early jurassic south georgia basin the main depocenter of the south georgia basin occurs about 90 km to the south and is formed by a large half graben containing more than 5 km of rift basin fill farther south the paleozoic suwannee basin sequence beneath northern florida is poorly imaged on the cocorp profiles however weak reflections suggest that these strata including basal felsic volcanics may have an aggregate thickness of about 6 km in north central florida at the northwest end of the cocorp traverse a prominent horizon imaged in the upper crust beneath the inner piedmont probably marks the southern appalachian detachment the detachment appears to be cut off by the towaliga fault implying that the towaliga fault is in part a down to the north normal fault intermittent moho reflections occur at 11 12 s two way time along the length of the cocorp survey indicating that the crust in the region has a roughly uniform thickness of about 33 36 km assuming an average crustal velocity of 6 km s

the significance of large sediment supply active tectonism and eustasy on margin sequence development late quaternary stratigraphy and evolution of the ganges brahmaputra delta abstract borehole data from the ganges brahmaputra delta system unveil an intriguing late quaternary history controlled by immense sediment discharge tectonics and eustasy sea level first intersected a major portion of the lowstand surface 10 000 11 000 cal yr bp marking the onset of sediment trapping and delta growth despite rapid sea level rise 1 cm yr sediment load was sufficient to maintain relative shoreline stability during this time by 7000 cal yr bp accommodation was quickly infilled with slowed sea level rise leading to upstream channel migration and widespread dispersal of sands this forced coastal progradation along the western delta front where the ganges was situated and much of the river s fine grained discharge bypassed the subaerial delta and formed a prograding deltaic clinoform on the shelf concurrently brahmaputra sediments were sequestered to an inland tectonic basin thus starving the adjacent shoreline and leading to transgression along the eastern delta by 5000 cal yr bp the two rivers had migrated or changed course to discharge along the eastern delta shoreline which began to prograde into what is now the modern river mouth estuary present delta configuration appears to have developed by 3000 cal yr bp evolution of the tectonically active ganges brahmaputra g b system reveals important similarities and differences with other deltas overall facies succession follows that of basic models progressing from an alluvial valley to coastal marine delta front to a prograding subaerial delta plain however the timing thickness and controls of these deposits differ immense sediment discharge from the ganges and brahmaputra rivers allowed initial delta growth 2000 3000 yr earlier than the global average despite debouching onto a canyon incised high energy margin subsequently a thick 50 m transgressive systems tract was formed during the early holocene phase of delta aggradation contrasting with extensive shoreline retreat documented along most margins highstand progradation of coastal and shelf sequences ensued by the middle holocene but subsiding inland basins also favored accumulation of a thick to 40 m highstand sequence in the lower floodplains unlike many other delta systems subsidence in the subaerial g b delta is not dominated by compaction but rather by plate driven tectonic processes that generate rates up to 4 mm year overall the huge sediment load tectonic subsidence major seismic events and a nearshore canyon system have led to widespread sediment dispersal and sequence formation across the subaerial delta shelf and deep sea bengal fan throughout the late quaternary

avulsions channel evolution and floodplain sedimentation rates of the anastomosing upper columbia river british columbia canada ages of channels of the anastomosing upper columbia river south eastern british columbia canada were investigated in a cross valley transect by c 14 dating of subsurface floodplain organic material from beneath levees the avulsion history within the transect was deduced from these data and morphological stages in channel development were recognized additionally floodplain sedimentation rates were established the new data demonstrate that the upper columbia river is a long lived dynamic anastomosing system results show that anastomosis at the study location has persisted since at least 2700 cal years bp with avulsions occurring frequently at least nine channels have formed in the studied cross valley transect within the past 3000 years channel lifetimes from formation to abandonment appear to be highly variable ranging from approximately 800 to 3000 years log jams provoking avulsions and or silting up of old channels are proposed as reasons for this variability long term average floodplain sedimentation rates appear to be significantly lower than previously proposed by smith 1983 int assoc sedimentol spec publ 6 155 168 a long term 4550 years average of 1 75 mm per year after compaction was based on c 14 dates whilea short term sedimentation rate of 0 8 mm was determined for a single relatively small seasonal flood in 1994 using sediment traps however short term sedimentation rates vary considerably over the floodplain with levees aggrading up to four times faster than floodbasins channels of the upper columbia river anastomosed reach follow a consistent pattern in their development with each stage being characterized by different morphology and processes channel evolution comprises the following succession 1 avulsion stage in which a crevasse splay channel deepens by scour and levee sedimentation 2 widening and deepening stage in which bank slumping and bed scouring dominates 3 infilling stage in which either channel narrowing bank accretion or channel shallowing bed accretion takes place and 4 abandonment stage in which the residual remnant channel is filled exclusively by silt clay and organic material vertical stacking super imposition of active channels on recent channel fill sand bodies is a notable feature of the upper columbia river which suggests that reoccupation of residual channels is a common process

structure of the earth mantle and core the deep interior structure of the earth has been extensively analyzed using a wide variety of seismic phases and techniques during the last four years most studies have emphasized quantitative three dimensional mapping of the lateral velocity heterogeneity of the mantle and core these aspherical velocity variations are believed to be direct manifestations of thermal and compositional heterogeneity associated with convective processes a first generation of global models for the lateral velocity variations in the deep earth has been produced providing tantalizing images of large scale structures suggestive of a non steady state thermal convection system the upper 400 km of the mantle has the strongest lateral velocity variations of up to 10 for shear velocity surface wave analyses that do not require a priori regionalizations have demonstrated that there is a strong association between surface tectonic provinces and the uppermost mantle velocity variations thus the thermal and convective state of the upper 200 km of the mantle can be reliably interpreted in the context of plate tectonics the upper mantle models support the contention that continents have deep roots with differences in velocity structure from oceanic and active tectonic regions extending as deep as 400 km very long wavelength lateral velocity variations of a few percent have been detected in the transition zone at depths from 400 to 670 km as well as throughout the lower mantle these deep seated variations have little correspondence to surface tectonics and efforts to interpret their nature are just beginning the lowermost 200 km of the mantle d region has lateral velocity fluctuations comparable to those in the upper mantle and evidence has been presented for the presence of a sizable velocity discontinuity at the top of the d layer a combined thermal and compositional boundary layer roughly mirroring the lithosphere is a likely explanation for this anomalous zone the core mantle boundary appears to have significant 10 km long wavelength topography presumably sustained by dynamic stresses from deep mantle convection the inner core may have strong lateral heterogeneity or axially symmetric anisotropy suggesting a complex thermal and compositional state significant progress has been made in characterizing the frequency dependence of anelastic attenuation in the mantle in the short period body wave band models for teleseismic p wave attenuation operators have converged with t values of 0 7 1 0 s appropriate at 1 hz and t values of 0 4 0 6 s appropriate at 4 hz regional variations of attenuation are slowly being mapped out as well

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

some results of cocorp seismic reflection profiling in the grenville age adirondack mountains new york state cocorp deep seismic reflection profiling in the adirondack mountains of northern new york state has revealed a prominent zone of layered reflectors in the lower crust of the east central adirondacks the strong layered reflectors here termed the tahawus complex occur between 18 and 26 km depth beneath the sparsely reflective granulite grade surface terrane which has been uplifted from depths greater than 20 km the tahawus complex apparently represents layered rocks of some type in the lower crust of the adirondacks possibilities include gneissic layering cumulate igneous layering a layered sill complex and underthrust sedimentary strata the tahawus complex may be spatially coincident with a previously detected high conductivity zone in the lower crust suggesting that either unusual mineralogies or interstitial electrolytes are present in the tahawus complex in contrast to layered reflections discovered in the lower crust of the east central adirondacks and southeast of the adirondacks cro

episodic sediment accumulation on amazonian flood plains influenced by el nino southern oscillation continental scale rivers with a sandy bed sequester a significant proportion of their sediment load in flood plains the spatial extent and depths of such deposits have been described and flood plain accumulation has been determined at decadal timescales but it has not been possible to identify discrete events or to resolve deposition on near annual timescales here we analyse 210 pb activity profiles from sediment cores taken in the pristine beni and mamore river basins which together comprise 720 000 km2 of the amazon basin to investigate sediment accumulation patterns in the andean amazonian foreland we find that in most locations sediment stratigraphy is dominated by discrete packages of sediments of uniform age which are typically 20 80 cm thick with system wide recurrence intervals of about 8 yr indicating relatively rare episodic deposition events ocean temperature and stream flow records link these episodic events to rapidly rising floods associated with la nina events which debouch extraordinary volumes of sediments from the andes we conclude that transient processes driven by the el nino southern oscillation cycle control the formation of the bolivian flood plains and modulate downstream delivery of sediments as well as associated carbon nutrients and pollutants to the amazon main stem

sediment facies and holocene deposition rate of near coastal fluvial systems an example from the nobi plain japan abstract floodplains are a major component of present near coastal fluvial systems that have evolved in response to postglacial changes in climate and sea level knowledge of sedimentary facies and deposition rates on a centennial to millennial time scale is required for considering floodplain evolution two cores ap1 and ap2 were acquired from an abandoned channel of the ibi river and its natural levee on the nobi plain central japan sediment facies analysis electrical conductivity and radiocarbon dating of borehole samples showed that in both cores organic rich dark gray floodbasin mud overlies deltaic deposits dating to after approximately 3200 years calibrated radiocarbon age cal bp in relation to delta progradation the accumulation of floodbasin mud continued at the both sites until about 400 cal bp around 400 cal bp the mud was eroded by the overlying channel sand and gravel at ap1 and was covered by fine grained natural levee deposits at ap2 with an abrupt contact this timing is concordant with the historical record of avulsion of the ibi river during the keicho era ad 1596 1615 averaged aggradation rates at the ap1 and ap2 sites were approximately 2 2 and 3 2 mm yr respectively faulting related subsidence along the western edge of the plain has influenced these rates by creating accommodation averaged deposition rates differed greatly between the floodbasin and the levee suggesting that rapid aggradation of the natural levee also occurred on a centennial to millennial scale these empirical data may be useful for testing models of the architecture and evolution of near coastal fluvial systems

holocene sea level change and antarctic melting history derived from geological observations and geophysical modeling along the shimokita peninsula northern japan 1 a mid to late holocene sea level record based on combined geomorphological geological and micropaleontological observations was obtained from well developed wave cut benches subaerially exposed along the shimokita peninsula northern japan results indicate that the benches were formed during mid to late holocene sea level transgressions reaching a maximum highstand level of 2 m above present at about 4 000 years ago this timing corresponds to an abrupt order of magnitude decrease in sedimentation rate as recorded in a core recovered from proximal mutsu bay in addition glacio hydro isostatic adjustment due to crustal deformation in response to postglacial sea level rise was modeled and results are consistent with the reconstructed local 2 m highstand given that meltwater contributions from the major north american and european ice sheets had largely ceased by 7 000 years ago these independent lines of evidence taken together indicate that melting of the antarctic ice sheet ended by 4 000 years ago

lhasa block and bordering sutures a continuation of a 500 km moho traverse through tibet a north south wide angle fan profile through the yarlung zangbo suture across the lhasa block and through the bangong nujiang suture shows several significant and sharp changes in crustal thickness both sutures appear as vertical 20 km steps in the moho and may have been the loci for eastward strike slip motion of the tibetan lithosphere