major types and time space distribution of mesozoic ore deposits in south china and their geodynamic settings the ore deposits of the mesozoic age in south china can be divided into three groups each with different metal associations and spatial distributions and each related to major magmatic events the first event occurred in the late triassic 230 210 ma the second in the mid late jurassic 170 150 ma and the third in the early mid cretaceous 120 80 ma the late triassic magmatic event and associated mineralization is characterized by peraluminous granite related w sn nb ta mineral deposits the triassic ore deposits are considerably disturbed or overprinted by the later jurassic and cretaceous tectono thermal episodes the mid late jurassic magmatic and mineralization events consist of 170 160 ma porphyry skarn cu and pb zn ag vein deposits associated with i type granites and 160 150 ma metaluminous granite related polymetallic w sn deposits the late jurassic metaluminous granite related w sn deposits occur in a ne trending cluster in the interior of south china such as in the nanling area in the early mid cretaceous from about 120 to 80 ma but peaking at 100 90 ma subvolcanic related fe deposits developed and i type calc alkaline granitic intrusions formed porphyry cu mo and porphyry epithermal cu au ag mineral systems whereas s type peraluminous and or metaluminous granitic intrusions formed polymetallic sn deposits these cretaceous mineral deposits cluster in distinct areas and are controlled by pull apart basins along the south china continental margin based on mineral assemblage age and space time distribution of these mineral systems integrated with regional geological data and field observations we suggest that the three magmatic mineralization episodes are the result of distinct geodynamic regimes the triassic peraluminous granites and associated w sn nb ta mineralization formed during post collisional processes involving the south china block the north china craton and the indo china block mostly along the dabie sulu and songma sutures jurassic events were initially related to the shallow oblique subduction of the izanagi plate beneath the eurasian continent at about 175 ma but i type granitoids with porphyry cu and vein type pb zn ag deposits only began to form as a result of the breakup of the subducted plate at 170 160 ma along the nne trending qinzhou hangzhou belt also referred to as qin hang or shi hang belt which is the neoproterozoic suture that amalgamates the yangtze craton and cathaysia block a large subduction slab window is assumed to have formed in the nanling and adjacent areas in the interior of south china triggering the uprise of asthenospheric mantle into the upper crust and leading to the emplacement of metaluminous granitic magma and associated polymetallic w sn mineralization a relatively tectonically quiet period followed between 150 and 135 ma in south china from 135 ma onward the angle of convergence of the izanagi plate changed from oblique to parallel to the coastline resulting in continental extensional tectonics and reactivation of regional scale ne trending faults such as the tan lu fault this widespread extension also promoted the development of ne trending pull apart basins and metamorphic core complexes accompanied by volcanism and the formation of epithermal cu au deposits granite related polymetallic sn w deposits and hydrothermal u deposits between 120 and 80 ma with a peak activity at 100 90 ma

uppermost inner core attenuation from pkp data observed at some south american seismological stations summary qp factor at the top of the inner core ic is derived using the amplitude spectral ratio pkpbc versus pkpdf method applied to the waveforms of suitable strong magnitude 6 0 intermediate depth and deep h 150 km earthquakes recorded at some selected south american stations in most cases the sampled volume of the ic is centred beneath the pacific ocean but some qp values correspond to volumes under south africa and under northern atlantic the obtained value of qp is 323 16 at 95 per cent confidence level close to a normal gaussian distribution the maximum depth of penetration of the pkpdf phase into the ic is roughly 333 km suggesting a possible increase of qp with depth although the large scatter in data prevents a definite conclusion as regards the geographical patterns no variation was observed

a p wave velocity model of earth s core present earth core models derived from the retrieval of global earth structure are based on absolute travel times mostly from the international seismological centre isc and or free oscillation eigenfrequencies many core phase data are left out of these constructions e g pkp differential travel times amplitude ratios and waveforms this study is an attempt to utilize this additional information to construct a model of core p wave velocity which is consistent with the different types of core phase data available in conjunction with our waveform modeling we used 150 differential time measurements and 87 amplitude ratio measurements which were the highest quality observations chosen from a large population of global digital seismograph network gdsn records as a result of fitting these various data sets a one dimensional p wave velocity model of the core prem2 is proposed this model modified from the preliminary reference earth model prem dziewonski and anderson 1981 shows a better fit to the combined data set than any of the existing core models major features of the model include a sharp velocity discontinuity at the inner core boundary icb with a large jump 0 78 km s and a low velocity gradient at the base of the fluid core the velocity is nearly constant over the lower 100 km of the outer core the model features a depth dependent q structure in the inner core such that a constant t for the inner core fits the amplitude ratios and waveforms of short period waves moderately well this means the top of the inner core is more attenuating than the deeper part of the inner core in addition the p velocity in the lowermost mantle is reduced from that of prem as a baseline adjustment for the observed separations of the df and ab branches of pkp at large distances

sedimentary characteristics of the gravel sand transition in the river rhine abstract a gravel bed river often changes abruptly into a sand bed river the causes of such so called gravel sand transitions have been elucidated in recent years but detailed observations of gravel sand transitions in large rivers are scarce the present study fills this knowledge gap by investigating the sedimentary characteristics of the gravel sand transition in the river rhine data on bed composition and flow characteristics are used in combination with a novel method for the evaluation of the bed structure the analysis shows that in contrast to common belief the bed structure in the gravel sand transition zone does not change from fully clast supported to fully matrix supported instead there is a strong cross stream variation in bed structure especially upstream of the transition zone the analysis also shows that the downstream decrease in gravel mobility in the gravel sand transition zone which is usually attributed to the change in bed structure and the so called patchiness effect is at least partly caused by the larger scale bend sorting process which concentrates gravel in outer bends where it is only infrequently mobile dune sorting which has been suggested to have an effect on the gravel mobility similar to that of bend sorting was found to be relatively unimportant in the rhine partly because it is strongly counteracted by armoring processes that affect an uncommonly thick sediment layer the gravel sand transition in the rhine with its length of 50 km is much less abrupt than the gravel sand transitions of smaller rivers reported on in the literature although the dependence of the transition length on river size intuitively is correct the exact causes remain unclear

the equilibrium alluvial river under variable flow and its channel forming discharge when the water discharge sediment supply and base level vary around stable values an alluvial river evolves toward a mean equilibrium or graded state with small fluctuations around this mean state i e a dynamic or statistical equilibrium state here we present analytical relations describing the mean equilibrium geometry of an alluvial river under variable flow by linking channel slope width and bed surface texture the solution holds in river normal flow zones or outside both the hydrograph boundary layer and the backwater zone and accounts for grain size selective transport and particle abrasion we consider the variable flow rate as a series of continuously changing yet steady water discharges here termed an alternating steady discharge the analysis also provides a solution to the channel forming water discharge which is here defined as the steady water discharge that given the mean sediment supply provides the same equilibrium channel slope as the natural long term hydrograph the channel forming water discharge for the gravel load is larger than the one associated with the sand load the analysis illustrates how the load is distributed over the range of water discharge in the river normal flow zone which we term the normal flow load distribution the fact that the distribution of the imposed sediment supply spatially adapts to this normal flow load distribution is the origin of the hydrograph boundary layer the results quantify the findings by wolman and miller 1960 regarding the relevance of both magnitude and frequency of the flow rate with respect to channel geometry

petrogenesis and tectonic implications of early cretaceous s and a type granites in the northwest of the gan hang rift se china the gan hang rift trending at least 450 km in a ne sw direction is a part of a mesozoic basin and range province in southeastern china detailed shrimp zircon u pb chronology major and trace element and sr nd hf isotope data of three granitic plutons and a diabasic dike in the northwest of the gan hang rift are used to explore the origin of these granites and their relationship to the evolution of the gan hang rift shrimp zircon u pb dating shows that the granitic plutons and diabasic dike were emplaced in the early cretaceous 122 129 ma the tongshan and damaoshan plutons close to the gan hang rift consist mainly of weakly peraluminous granitic rocks which show a2 subtype affinity these granites have initial 87sr 86sr ratios of 0 7080 0 7103 end t values of 1 4 to 5 6 and ehf t in situ zircon values of 3 8 to 1 2 detailed elemental and isotopic data suggest that they were formed by partial melting of granulitized mesoproterozoic metamorphic basement including metasedimentary and metaigneous rocks in the shallow 30 km crust at a high temperature 823 c these pure crustal melts underwent crystal fractionation of biotite plagioclase and k feldspar forming the more felsic end member of granitic rocks the ehu pluton more distant from the gan hang rift is composed of two mica granites which are strongly peraluminous and are s type granites these granites have higher initial 87sr 86sr ratios 0 7162 0 7191 and lower end t 7 1 to 7 4 and ehf t 6 8 in situ zircon values than the tongshan and damaoshan granites detailed elemental and isotopic data suggest that the ehu granites were formed by partial melting of paleoproterozoic metasedimentary basement in the deep 40 50 km crust at a relatively low temperature 757 c with no fractional crystallization and mantle derived magma mixing in their evolution the coeval diabasic dike is calc alkaline and has initial 87sr 86sr ratio of 0 7085 end t value of 0 9 and ehf t in situ zircon value of 7 1 suggesting that it was derived from the asthenospheric mantle wedge that had been metasomatised via addition of subducted sediment derived melt with monazite in the residual phase the association of early cretaceous 122 129 ma s and a type granites in the northwest of the gan hang rift marks the onset of back arc extension or intra arc rift with ongoing extension the crust and lithospheric mantle became progressively thinned the upwelling of asthenosphere triggered partial melting of both metasedimentary and metaigneous rocks in the more thinned crust close to the gan hang rift forming the a type granitic magmas such as tongshan and damaoshan whereas partial melting of metasedimentary rocks in the less thinned crust farther from the gan hang rift formed the s type granitic magmas such as ehu the red sediments with the total thickness more than 10 000 m have been successively deposited in the gan hang rift valley since the late early cretaceous 105 ma suggesting that this region experienced the most back arc extension

experimental study of the transport of mixed sand and gravel we measured a wide range of transport rates for five different sand gravel mixtures in a laboratory flume each mixture used the same gravel and sand was added to produce mixtures containing 6 15 21 27 and 34 sand control of other variables allows us to isolate the effect of bed sand content on transport as sand content increases gravel transport rates increase by orders of magnitude even though the proportion of gravel in the bed decreases the increase in gravel transport rate is most rapid over the range of bed sand content between 15 and 27 the increase in transport rate is larger than predicted using standard scaling relations between transport rate and grain size indicating that models of transport and sorting and predictions of stream channel response to sand inputs need to be revised to account for the influence of sand content bed surface grain size was measured at the end of each run surface grain size varied with sand content but showed little or no coarsening with flow strength and transport rate this casts doubt on the idea that armor layers form at small flows and weaken or vanish with increasing flow and transport rate

complex variations in sediment transport at three large river bifurcations during discharge waves in the river rhine river bifurcations strongly control the distribution of water and sediment over a river system a good understanding of this distribution process is crucial for river management in this paper an extensive data set from three large bifurcations in the dutch rhine is presented containing data on bed load transport suspended bed sediment transport dune development and hydrodynamics the data show complex variations in sediment transport during discharge waves the objective of this paper is to examine and explain these measured variations in sediment transport it is found that bend sorting upstream of the bifurcations leads to supply limitation particularly in the downstream branch that originates in the outer bend of the main channel tidal water level variations lead to cyclical variations in the sediment distribution over the downstream branches lags in dune development cause complex hysteresis patterns in flow parameters and sediment transport all bifurcations show evidence of sediment waves which probably are intrinsic bifurcation phenomena the complex transport processes at the three bifurcations cause distinct discontinuities in the downstream fining trend of the river differences among the studied river bifurcations are mainly due to differences in sediment mobility shields value because the variations in sediment transport are complex and poorly correlated with the flow discharge prediction of the sediment distribution with existing relationships for one dimensional models is problematic

grain size sediment transport regime and channel slope in alluvial rivers the general relationship between channel morphology and the grain size of sediment in the channel bed is an important but poorly known aspect of alluvial rivers an analysis of an equation for total sediment flux in the limits of suspension bedload and mixed modes of transport indicates distinct steady state regimes of channel morphology such regimes are readily seen in published data for modern alluvial rivers by way of a conventional shields plot or a plot of channel slope as a function of relative grain size d h and the ratio ws u where d and ws are respectively mean diameter and fall speed of bed sediments and h and u are respectively mean depth and friction velocity of the flow with slope and mode of transport in an alluvial river constrained by grain size and channel depth alone estimates of discharge and sediment flux follow directly introduction of the sediment flux relationship into conventional diffusion models for the evolution of an alluvial system provides nominal estimates of

late triassic melting of a thickened crust in southeastern china evidence for flat slab subduction of the paleo pacific plate abstract the dashuang complex in zhejiang province of southeast china is composed of two distinct lithologies syenite in the west and quartz monzonite in the east they record similar zircon u pb ages of 224 3 ma syenite and 226 2 ma and 227 1 ma quartz monzonite respectively but are notably different in petrography magnetic susceptibility whole rock chemistry zircon hf isotope and zircon trace element characteristics the west dashuang syenitic pluton the west body has high modal alkali feldspar high zircon saturation temperatures high whole rock and zircon mree hree ratios low fe mg ti contents and is depleted in ba sr and eu it also has low magnetic susceptibilities belongs to the ilmenite series and is a peraluminous and ferroan granitoid the east dashuang quartz monzonitic pluton the east body has abundant k feldspar megacrysts with hornblende titanite and biotite being the major ferromagnesian minerals in contrast to the west body the east body has lower zircon saturation temperatures lower whole rock and zircon mree hree ratios higher fe mg ti contents and shows no depletion in ba sr or eu the east body has higher magnetite contents high magnetic susceptibilities and belongs to the magnetite series it is a metaluminous and magnesian granitoid of arc affinity zircon hf isotopic data reveal that both bodies were derived from partial melting of paleoproterozoic igneous protoliths in the lower crust but the east body possibly incorporated subducted terrigenous sediments both bodies have higher melting temperatures and pressures than adjacent cretaceous granitoids reflecting their origin in a thickened hotter lower crust the most feasible model to explain their differences is variations in water content during crustal melting resulting in different melting and crystallization behaviors such melting in a triassic thickened crust with variable water involvement followed by cretaceous magmatism in an extensional setting is consistent with the flat slab subduction model proposed for south china the model involves crustal thickening and partial melting with mantle and lower crustal metasomatism during flat slab propagation in the triassic early jurassic and crustal thinning and extension from the mid jurassic to the cretaceous

morphology and controls on the position of a gravel sand transition fraser river british columbia alluvial river channels often exhibit a relatively abrupt transition from gravel to sand bedded conditions the phenomenon is well documented but few prior studies have analyzed the spatial variability through reaches where transitions occur the downstream fining pattern observed in the fraser river is cited as a classic example of an abrupt gravel sand transition in a large alluvial channel however important questions regarding the exact location of the transition its sedimentology and morphology and what controls its location remain unanswered here we present observations of the downstream change in bed material grain size river bed topography and channel hydraulics through the reach within which the transition occurs these observations indicate that the gravel sand transition is characterized by a terminating gravel wedge but there are patches of gravel downstream of the wedge forming a diffuse extension we show that there is a dramatic decrease in shear stress at the downstream end of the wedge and a consequent cessation of general gravel mobility we argue that the patches of gravel observed beyond the wedge are the result of enhanced mobility of fine gravel over a sand bed we also find that sand in suspension declines rapidly at the downstream end of the wedge suggesting that sand is delivered to the bed completing the sedimentary conditions for a gravel sand transition we propose that the break in river slope associated with the transition is a consequential feature of the transition

effects of sand content on initial gravel motion in gravel bed rivers when fine sediments are present in gravel streambeds gravel framework beds the gravel can be more easily removed from its original position as compared with gravel in a streambed without fine sediment but otherwise under the same hydraulic conditions in this study the effect of the presence of sand on the initiation of gravel motion in gravel riverbeds was investigated using flume experiments the relationship between the critical shields stress for gravel motion initiation and the fraction of sand in the bed was determined experimentally the results can be summarized as follows 1 when the fraction of sand in the bed is smaller than about 0 4 the critical shields stress for the initiation of gravel motion decreases with increasing the fraction of sand the critical shields stress increases however with increasing the fraction of sand when it is larger than about 0 4 2 the difference between the value of the critical shields stress predicted by the egiazaroff equation and the value obtained from the experimental data becomes maximum at about 0 4 of the fraction of sand here an empirical relation between the critical shields stress and the fraction of sand is proposed so as to consider the effects of the ratio of the characteristic gravel size to the mean size of the bed material on the critical shields stress 3 gravel in armored beds can be more easily mobilized by supplying sand as part of a sediment augmentation scheme the sand fraction in the subsurface layer of the bed appears to reduce the friction angle of exposed particles sediment augmentation using sand has been recently demonstrated to be a viable alternative for mobilizing gravel for the restoration of gravel bed rivers downstream of dams the quantitative evaluation obtained through the experiments reported here may be useful for the design of augmentation schemes this article is protected by copyright all rights reserved

gold mineralization of the chencai suichang uplift and tectonic evolution of zhejiang province southeast china abstract a metallogenic model is proposed in which gold and base metal mineralization in se china is related to plate interactions along the pacific margin with southeast asia from about 900 to 70 ma all depositional orogenic and metallogenic processes can be explained by subduction events along northeast trending zones with progressive accretion and cratonization along the continental margin these subduction zones moved southeast for a distance of less than 150 km as a result the area is one of crustal reworking in which the rocks and mineral deposits of earlier events have been repeatedly modified the chencai suichang uplift is an uplifted crustal block in central zhejiang province of mainland china in which the proterozoic basement is exposed as tectonic windows in overlying mesozoic volcanics several gold and base metal deposits are located in these proterozoic windows as well as in the younger volcanics most economic deposits were formed during the last major tectono magmatic event affecting the area the yanshanian 160 170 ma event although there is evidence for earlier mineralization that has been remobilized during the yanshanian it is suggested that crustal reworking was particularly important in remobilising and upgrading gold mineralization the model presented is supported not only by local geology and isotopic data but also by the regional distribution of gold deposits which are arranged in a 50 km wide belt parallel to the yanshanian subduction zone three major episodes of mineralization occurred during 1 a subduction related period of metamorphism partial melting and mesothermal fluid generation and mineralization in the lower crust during the caledonian 2 subduction related volcano plutonic activity with epithermal mineralization during the early yanshanian and 3 hydrothermal mineralization during late yanshanian tectono magmatic event in the third two main categories of metallogenic control are recognized namely 1 hydrothermal fluid generation in the lower crust and 2 upper crust structural and lithological controls on mineralization

experiments on downstream fining of gravel ii wide and sandy runs this is the second of two papers about experiments on downstream fining of gravel here we briefly summarize the complete series of runs described in the first paper where the influence of sediment feed rate was evaluated in a narrow channel we then present results for variation in two more conditions sand content and channel width the experiments were carried out in a long flume with an effective 40 m long test section with two different widths of 0 3 and 2 7 m in addition two sediment mixtures were used with different sand contents i e 33 and 55 with the remainder being gravel the following results were obtained the increase in channel width allowed the formation of transverse topography enhanced the development of sediment patches and increased the fining rate of gravel for id d5 d0 and finer percentiles increasing sand content rendered the long profile of the bed much more concave caused marked patch development and most notably increased the fining rate for all percentiles of gravel sized bed material

cretaceous pacific plate movement beneath se china evidence from episodic volcanism and related intrusions abstract extensive late mesozoic igneous rocks in se china have been widely considered to be generated under the paleo pacific tectonic regime the plate subduction model however remains controversial this study focuses on the cretaceous volcanic rocks in northwestern zhejiang province zircon u pb age determination indicates that cretaceous volcanism in northwestern zhejiang took place at three episodes of 140 130 130 127 and 123 118 ma in good agreement with the coeval lower volcanic series in southeastern zhejiang but lacking the episode at 110 88 ma corresponding to the upper volcanic series the cretaceous volcanic rocks in zhejiang therefore show an oceanward younging trend in situ zircon hf isotope analyses of three episodes of volcanics yield e hf t values of 11 2 to 8 7 4 8 to 2 4 and 4 4 to 2 2 respectively the entire sequences display typical isotopic features of magma mixing implying progressive involvement of juvenile component based on systematical researches on the cretaceous volcanic rocks and a series of granitoid plutons in zhejiang it is also identified that the juvenile component involvement gradually occurred from the inland to the coast under an enhanced lithospheric extensional tectonic setting all the observations in this study indicate the northwestward paleo pacific plate subduction with episodic slab rollback which triggered the arc system to retreat towards the pacific ocean rather than the southwestward subduction related to the ridge subduction along the lower yangtze river belt

re os geochronology and systematics in molybdenite from the endako porphyry molybdenum deposit british columbia canada precise re os molybdenite ages for the endako porphyry molybdenum deposit and the nithi mountain occurrence british columbia canada yield new information on the timing of molybdenite deposition three episodes of mineralization are indicated 1 154 ma 2 148 to 146 ma and 3 145 ma re os dates agree well with new u pb and 40 ar 39 ar data from granitoids of the nithi phase 155 2 1 5 ma the endako phase 148 4 1 5 ma and a preore dike 147 4 0 6 ma and the casey phase 145 1 0 2 ma plus hydrothermal biotite 144 5 0 75 ma villeneuve et al 2001 the excellent correlation between the re os dates from molybdenite and the ages of granitoids from the area indicates a direct genetic relationship between these plutons and molybdenite mineralization these re os data also provide further insight into the systematics and robustness of re os in molybdenite the endako molybdenum deposit is the product of two episodes of mineralization both associated with moderate salinity 5 15 wt nacl equiv and high temperature 440 c fluids a third low temperature 190 300 c hydrothermal event is associated with eocene faulting selby et al 2000 the clear distinction between two of the re os molybdenite dates in the deposit and the close agreement of these dates to the ages of granitoid intrusions suggest that the re os system in molybdenite is not reset by high temperature 440 c moderate salinity hydrothermal fluids in contrast magmatic biotite in a 147 ma preore dike has a 40 ar 39 ar age of 145 ma identical to the age of the hydrothermal biotite indicating that 440 c fluids reset the ar ar system in biotite hence we interpret that the re os isotope system in molybdenite is resistant to resetting by high temperature fluids 440 c in contrast to the 40 ar 39 ar system in biotite the re os system in molybdenite is also unaffected by postore hydrothermal fluids these data show that molybdenite can provide critical information in understanding sulfide mineral deposition in ore systems which is not recorded by other isotopic systems in alteration minerals

scatter and bias in differential pkp travel times and implications for mantle and core phenomena differential travel times commonly used to eliminate unwanted near source and near receiver heterogeneity from travel time studies can be affected by slabs and hypocentral errors when phase pair slownesses differ greatly in particular d and core structure studies can be biased if differential pkp times are used here we present differential pkp travel time measurements having significant azimuthal travel time anomalies consistent both in size and pattern with a near source slab effect and explore other sources of error in differential times

observations of pkp df and pkp bc across the united kingdom implications for studies of attenuation in the earth s core summary we demonstrate that pkp df and pkp bc recorded by a network of broad band stations in the united kingdom from earthquakes in the southwest pacific show considerable spatial variation in amplitude and frequency content we suggest that strong heterogeneity in apparent attenuation at the base of the mantle related to ultra low velocity zones ulvz beneath the southwest pacific and or possibly to the east of iceland may explain the anomalous observations of df and bc across the uk the length scale of attenuating ulvzs required to explain the observations is similar to that estimated recently for a southwest pacific ulvz by wen helmberger 1998 that is low wave speed low apparent q regions of 80 km height separated by steep sided valleys of width 40 km with typical lowermost mantle wave speeds high apparent q if the apparent attenuation is due to anelasticity alone disregarding the possible contribution from scattering the minimum ulvz q values required to explain the amplitude of 2 hz bc are about 15 30 consistent with the interpretation of the ulvzs as zones of partial melt at the base of the mantle this suggests that estimates of inner core anelastic attenuation inferred from df bc spectral ratios will be biased if df and or bc cross ulvzs at the base of the mantle under the source and or receiver further the presence of attenuating ulvzs may explain the large scatter in estimates of inner core q from global studies

zircon u pb geochronology and geochemistry of two episodes of granitoids from the northwestern zhejiang province se china implication for magmatic evolution and tectonic transition abstract granitoids 175 80 ma representing a prominent yanshanian jurassic to cretaceous magmatic event in south china widely intrude the precambrian crystalline basement and paleozoic strata here we report zircon u pb age data geochemical characteristics and sr nd isotopes of the late jurassic and early cretaceous granitoids from the northwestern zhejiang province zxb of southeastern china our results reveal two distinct episodes for the yanshanian magmatism the jiemeng and datongkeng granodiorites formed at 148 6 1 1 ma whereas the huangshitan jiuligang and ruhong aluminous a type granites were generated between 129 0 0 6 ma and 126 1 1 1 ma the two magmatic phases represent a tectonic transition from an active continental margin to post orogenic setting during the late jurassic ca 150 ma to early cretaceous ca 128 ma geochemically these intrusions are granodioritic to granitic in composition and show an affinity of s type and a type granitoids respectively the s type granodiorites of jiemeng and datongkeng are characterized by moderate sio 2 65 0 69 6 wt high k 2 o na 2 o 5 0 7 6 wt k 2 o na 2 o 1 2 1 5 zr 31 109 ppm sr 71 190 ppm and high field strength elements low to intermediate mg and moderate nb depletion the a type granites of huangshitan jiuligang and ruhong are characterized by high sio 2 72 7 77 2 wt k 2 o na 2 o 6 9 8 8 wt k 2 o na 2 o 1 3 2 1 fe t fe t mg ga 17 29 ppm 20 ppm commonly zr 96 197 ppm and sr 8 45 ppm with slight nb depletion the s type granodiorites have higher mg a nk sr sr ba sr y la yb n and lree hree and lower sio 2 k 2 o na 2 o ga and zr with weak negative eu anomalies compared to those of the a type granites with negative eu anomalies all these rocks show y nb ratios 1 2 high initial 87 sr 86 sr i sr ratios and low e nd t and are depleted in nb ti and sr indicating crustal origin with subduction zone signatures we suggested that the zxb s type granitic bodies might have been derived from the mesoproterozoic metamorphic basement rocks through partial melting induced by mantle derived magma followed by limited fractional crystallization the zxb aluminous a type granites were also derived from a similar magma source but underwent fractional crystallization at higher crustal levels the a type granites in the zxb correlate with a post orogenic tectonic setting a geological comparison between zxb and adjacent areas indicates that the geochemical features of the zxb a type granites are comparable with the baijuhuajian a type granites and also the adjacent areas a type granites but are distinct from the late cretaceous a type granites 105 90 ma distributed along the southeastern coastal area of south china we correlate the formation of the zxb s type granodiorites to inland compression associated with the subduction and collision of the paleo pacific plate in the late jurassic 170 145 ma in contrast the a type granites formed under a post orogenic setting during the early cretaceous period 145 120 ma resulting from lithospheric thinning and continent extension accompanied by slab roll back of the paleo pacific plate following the subduction collision event

generalized sorting profile of alluvial fans alluvial rivers often exhibit self similar gravel size distributions and abrupt gravel sand transitions experiments suggest that these sorting patterns are established rapidly but how and how fast this convergence occurs in the field is unknown we examine the establishment of downstream sorting patterns in a kilometer scale alluvial fan the sharp transition from canyon to unconfined channelized fan provides a well defined boundary condition the channel changes from deep and entrenched at the fan apex to shallow and depositional over a short distance exhibiting nonequilibrium behavior the resulting gravel fining profile is not self similar the particle size distribution narrows until approximate equal mobility is achieved downfan the gravel sand transition appears to exhibit a self similar form field and laboratory data collapse when downstream distance is normalized by the location of the transition results suggest a generalized sorting profile for alluvial fans as a consequence of the threshold of motion and nonequilibrium channels

emergence of abrupt gravel to sand transitions along rivers through sorting processes gradual downstream fining along gravel bed rivers is often followed by a relatively abrupt change to a sand bed this has usually been explained by the breakdown of pebbles of certain lithologies to sand but it is not restricted to particular rock types and can occur over distances too short for significant abrasion an alternative explanation is that as shear stress declines downstream size sorting is enhanced through nonlinearities and thresholds in bedload transport and deposition mechanisms this hypothesis is tested by numerical modeling of an idealized channel with a mixed gravel and sand bed abrupt and persistent gravel fronts with associated breaks of slope develop from a range of smooth initial states when a new initial motion equation is used but not with a conventional equation the results suggest an emergent phenomenon but one that is sensitive to process specification rather than initial or boundary conditions

human induced changes in bed shear stress and bed grain size in the river waal the netherlands during the past 900 years human impact has changed the flow characteristics and bed sediment characteristics of lowland rivers affecting channel stability flood risk navigability and biodiversity we analysed the effect of human activities on the flow and bed sediment characteristics of the river waal the netherlands the objectives were a to reconstruct the historical change in bed shear stress during the past 900 years b to reconstruct the contemporary change in bed grain size and c to identify the main causes of these changes various data types were used such as borehole descriptions historical river maps and modern hydraulic data it was found that the bed shear stress in the river waal strongly increased during the past 900 years in the same period the gravel content of the sandy river bed increased causing a coarsening of the river bed before ad 1870 the shear stress increase and bed coarsening were mainly due to embankment artificial levee construction after ad 1870 the shear stress increase and bed coarsening were mainly due to river narrowing and dredging the systematic grain size difference between present day and historical rivers should be taken into account in river restoration projects copyright 2009 john wiley sons ltd

magmatism along the southeast margin of the yangtze block precambrian collision of the yangtze and cathysia blocks of china the time and the environment of formation of igneous rocks along the southern margin of the yangtze continental block in southern anhui province and northeastern jiangxi province china are discussed sm nd dating for samples from two ophiolite suites give ages of about 1 ga late proterozoic granodiorites in the study area have ages of about 0 9 ga and have normal intrusive relations against banxi group metamorphic country rocks trace element abundances and initial nd isotope ratios for ophiolite and proterozoic granodiorites are consistent with formation in an island arc environment in contrast sr and nd isotopes for mesozoic granitoids of the region suggest that they are products of remelting of crustal sources similar to the basement of the yangtze block these relations do not support a recently proposed mesozoic overthrust tectonic model for southeast china but rather favor a late precambrian collision history for the yangtze and cathysia blocks

introduction to tectonics of china abstract the continental crust of china is a mosaic of cratonic blocks and orogenic belts containing small cratons and terranes with various tectonic settings they have diverse origins and complex histories of amalgamation and often suffered repeated reworking after multiple episodes of amalgamation in the last three decades extensive geological geochemical and geophysical investigations have been carried out on these cratonic blocks and intervening orogenic belts producing an abundant amount of new data and competing interpretations this provides important insights into understanding the formation and evolution of the chinese continents the papers assembled in this volume present a timely and comprehensive overview on major advancements and controversial issues related to the formation and evolution of continental crust in china complex tectonic histories were experienced not only by the large scale cratonic blocks and orogenic belts but also by small scale terranes and orogens between and inside these blocks nevertheless our understanding of lithotectonic units and geological processes has been greatly advanced by recent studies of zirconology and geochemistry for various rock types from major petrotectonic units in china it has been further advanced from integrated interpretations of geochemical and petrological data for petrogenesis of magmatic rocks an overview of these observations and interpretations provides new insights into understanding the continental plate tectonics and the chemical geodynamics of subduction zones

the gravel sand transition sediment dynamics in a diffuse extension as gravel bedded rivers fine in the downstream direction they characteristically exhibit an abrupt transition from gravel to sand bedded conditions the prevailing theory for why abrupt gravel sand transitions emerge is based on bed load sorting of a bimodal sediment the abruptness is thought to be a consequence of sand overwhelming the gravel sand mixture once it reaches a critical coverage on the bed the role suspension plays in the development of gravel sand transitions has not been fully appreciated the fraser river british columbia is an archetypical abrupt gravel sand transition with a diffuse extension composed of a sand bed with some patches of gravel we examine flow shear stress and suspended sediment flux in the diffuse extension to better understand sediment dynamics where the sand bed emerges sand is carried in suspension upstream of the primary abrupt gravel sand transition but in the diffuse extension sand is moved as both bed load and suspended load we do not observe downstream gradients in shear stress or suspended sand flux through the diffuse extension that would suggest a gradual rain out of sand moving downstream which raises the question how is the sand bed formed sediment advection length scales indicate that with the exception of very fine sand that moves as wash load in the diffuse extension fractions coarser than the median sand size cannot be carried in suspension for more than one channel width this suggests that sand is deposited en masse at the beginning of the diffuse extension forming a sediment slug at low flood flows that is smeared downstream at high flood flows to form the sand reach

distribution of seismic anisotropy in the subduction zone beneath the wellington region new zealand summary shear wave splitting measurements from s arrivals of local earthquakes recorded at the incorporated research institutions for seismology iris broadband sensor snzo are used to determine a basic anisotropic structure for the subduction zone in the wellington region with the use of high frequency filters fast anisotropic polarization w and splitting time dt measurements typical of crustal anisotropy are evident but the larger splitting expected from the mantle is often not resolved the small splitting seen agrees well with the results of previous studies concerning shallow crustal anisotropy with the use of lower frequency filters measurements more consistent with mantle anisotropy are made anisotropy of 4 4 0 9 per cent with a fast polarization of 29 38 is calculated for the subducting slab from 20 to 70 km depth using this result in addition to the results of previous studies a model is proposed the model requires a frequency dependent anisotropy of less than 1 4 per cent when measured with a period of 2 s to be present in the sub slab mantle separate from this population a band of events in northern cook strait with an 86 10 fast polarization is seen this is at about 40 from the strike of the hikurangi margin and suggests a source of shear strain 40 removed from that found in the majority of the region the cause of this is probably a deformation in the subducting slab in this region as it moves towards a greater incline to the south

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

evolution of ore fluids in the donggou giant porphyry mo system east qinling china a new type of porphyry mo deposit evidence from fluid inclusion and h o isotope systematics abstract the donggou mo deposit in the eastern qinling area china is a giant porphyry system discovered based on a targeting concept by using the tectonic model for collisional orogeny metallogeny and fluid flow cmf model mo mineralization is associated with the donggou aluminous a type granite porphyry and was formed during the early cretaceous in a tectonic regime of continental extension the orebodies mainly occur as numerous veinlets in the host rocks hydrothermal ore forming processes include at least three stages characterized by veinlets of 1 quartz k feldspar minor molybdenite 2 quartz molybdenite beryl and 3 quartz carbonate fluorite three types of fluid inclusions fis are distinguished in quartz and beryl in stages 1 and 2 i e aqueous w type carbonic aqueous c type and solid bearing s type with only aqueous fis observed in stage 3 minerals s type fis contain variable daughter minerals including halite chalcopyrite calcite and an unidentified transparent crystal but only halite can dissolve during heating halite bearing s type fis are mainly homogenized by halite dissolution at 182 416 c corresponding to salinities of 30 9 49 2 wt nacl equiv minor halite bearing s type fis are homogenized to liquid at 190 360 c via vapor disappearance with salinities of 29 1 36 2 wt nacl equiv other fis in minerals of stages 1 2 and 3 are homogenized at temperatures of 341 550 c 220 440 c and 125 225 c with salinities of 8 0 18 3 5 3 16 8 and 0 5 7 3 wt nacl equiv respectively the estimated minimum trapping pressures are up to 141 mpa in stage 1 and up to 81 mpa in stage 2 respectively corresponding to an initial mineralization depth of no less than 5 km the quartz in veinlets yields 18 o values of 8 5 10 0 corresponding to 18 o h 2 o values of 2 9 to 5 9 while the d h 2 o values of fluid inclusions range from 59 to 82 these data suggest that the ore fluids forming the donggou deposit changed from high temperature high salinity co 2 rich magmatic to low temperature low salinity and co 2 poor meteoritic fluids via boiling and mixing resembling those of other magmatic hydrothermal systems in qinling orogen and dabie shan this supports the notion that the porphyry systems generated in a post collisional tectonic setting were initially co 2 rich as indicated by abundant c type and co 2 bearing s type fluid inclusions

downstream changes in the lithology and grain size of fluvial gravels the watarase river japan evidence of the role of abrasion in downstream fining two processes have been considered to explain downstream grain size reduction in gravel bed rivers hydraulic sorting by which finer grains are transported faster and farther than coarse grains and abrasion by which individual particles are diminished in size most studies on the cause of downstream fining in rivers emphasize hydraulic sorting as the dominant fining mechanism however broken boulders are often observed in the bed material of japanese rivers also grain size changes may be correlated with the difference in resistance to breakdown among lithologies the purpose of this study is to use the lithologic composition of bed material on the watarase river to demonstrate that some downstream fining cannot be explained by hydraulic sorting alone the study reach on the watarase river flows on a dissected alluvial fan which means that the river is no longer aggrading the study reach is influenced only slightly by input of gravel from tributaries bed gravel in the study reach consists of several lithologies andesite quartz porphyry sandstone hornfels and chert the grain size distribution and lithology of the river bed material were examined at nine sites there are two principal results 1 size distribution of the gravel is strongly related to lithology andesite boulders or large cobbles make up the framework sizes in the upstream part of the study reach while chert pebbles make up the framework sizes in the downstream part there are few andesite pebbles or chert boulders in the river bed 2 longitudinal changes in the composition of each size class show selective transportation by lithology in every gravel size because the mobility of gravel depends mainly on its size hydraulic sorting by lithology does not occur within the same size class these results indicate clearly that particle abrasion occurs in the watarase river and is responsible at least in part for the downstream decrease in particle size of bed material

petrogenesis of early cretaceous post collisional granitoids at shapinggou dabie orogen implications for crustal architecture and porphyry mo mineralization abstract the shapinggou molybdenum deposit is located in the qinling tongbai hong an dabie orogen which accommodates the largest molybdenum ore belt in the world new laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb dating geochemical and sr nd hf isotope data for the granitoids at shapinggou are presented in this paper two stages of early cretaceous magmatism 138 128 ma and 116 114 ma took place at shapinggou in a post collisional setting these granitoids can be divided into four groups including the early stage granites 134 129 ma sio2 62 33 to 76 27 and intermediate rocks 129 128 ma sio2 52 41 to 60 72 and the late stage quartz syenite 116 ma sio2 64 09 to 65 12 and mo bearing granite porphyry 114 ma sio2 74 95 to 77 26 the early stage granites have high sr contents sr y and lan ybn ratios but low y yb and mgo contents showing low mg adakitic chemical features they were possibly generated by partial melting of the thickened mafic lower continental crust the early stage intermediate rocks have relatively high mgo contents 2 71 to 4 84 and mg values 46 to 60 but low sr y and lan ybn ratios which were likely derived from a hybrid mantle metasomatized by melts from foundered eclogitic lower continental crust and subsequent fractional crystallization the late stage quartz syenite shows chemical features of metaluminous a type granites including high abundances of alkalis hfse zr nb and trivalent ree the late stage mo bearing granite porphyry displays high sio2 alkalis zr ce contents and a cnk ratios but low ba sr p and ti showing affinity to highly fractionated i type granites the late stage quartz syenite and granite porphyry were possibly produced by partial melting of intermediate compositions of middle crust our results support the conclusion that there were three layer continental crusts in the dabie orogen prior to early cretaceous magmatism and suggest that post collisional granitoids in the dabie orogen were derived from not only lower crust but also middle crust the giant shapinggou porphyry mo deposit was most likely formed by effective transport of volatiles and mo in a convecting granitic magma column rather than by direct melting of mo enriched sources the mo bearing rocks at shapinggou are characterized by a high degree of magma differentiation close associations with a type granite and formation at the end of regional magmatism

timing of the yuchiling giant porphyry mo system and implications for ore genesis the yuchiling mo deposit is a recently discovered giant porphyry system in the east qinling mo belt china its apparent causative intrusion i e the yuchiling granite porphyry is the youngest intrusion phase 4 of the heyu multiphase granite batholith which was emplaced between 143 and 135 ma new robust constraints on the formation of the yuchiling porphyry mo system are provided by combined zircon u pb biotite 40ar 39ar and molybdenite re os dating zircon grains from the mo mineralized granite porphyry yield weighted 206pb 238u age of 134 0 1 4 ma n 19 2 error mswd 0 30 magmatic biotite from the same sample yield a 40ar 39ar plateau age of 135 1 1 4 ma 2 error and an inverse isochron age of 135 6 2 0 ma n 7 2 error mswd 10 8 which are effectively coincident with the zircon u pb age within analytical error three pulses of mineralization can be deduced from the molybdenite re os ages namely 141 137 and 134 ma which agree well with the zircon u pb ages of granitic phases 1 2 and the yuchiling porphyry phase 4 respectively these well constrained temporal correlations indicate that mo mineralization was caused by pulses of granitic magmatism and that the ore forming magmatic hydrothermal activity responsible for the yuchiling porphyry mo system lasted about 8 ma the yuchiling mo deposit represents a unique style of porphyry mo system formed in a post collision setting and associated with f rich high k calc alkaline intrusions which differ from convergent margin associated porphyry mo deposits

petrogenesis of cretaceous adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china implications for geodynamics and cu au mineralization both adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china are associated with cretaceous cu au mineralization the shaxi quartz diorite porphyrites exhibit adakite like geochemical features such as light rare earth element lree enrichment heavy ree hree depletion high al2o3 mgo sr sr y and la yb values and low y and yb contents they have low end t values 3 46 to 6 28 and high 87sr 86sr i ratios 0 7051 0 7057 sensitive high resolution ion microprobe shrimp zircon analyses indicate a crystallization age of 136 3 ma for the adakitic rocks most volcanic rocks and the majority of monzonites and syenites in the luzong area are k rich or shoshonitic and were also produced during the cretaceous 140 125 ma they are enriched in lree and large ion lithophile elements and depleted in ti and nb and ba and exhibit relatively lower end t values ranging from 4 65 to 7 03 and relatively higher 87sr 86sr i ratios varying between 0 7057 and 0 7062 the shoshonitic and adakitic rocks in the luzong area have similar pb isotopic compositions 206pb 204pb 17 90 18 83 207pb 204pb 15 45 15 62 and 208pb 204pb 38 07 38 80 geological data from the luzong area suggest that the cretaceous igneous rocks are distributed along ne fault zones e g tanlu and yangtze river fault zones in eastern china and were likely formed in an extensional setting within the yangtze block the shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of 50 km i e 1 5 gpa possibly leaving rutile bearing eclogitic residue the shoshonitic magmas in contrast originated mainly from an enriched mantle metasomatized by subducted oceanic sediments they underwent early high pressure 1 5 gpa fractional crystallization at the boundary between thickened 50 km lower crust and lithospheric mantle and late low pressure 1 5 gpa fractional crystallization in the shallow 50 km crust the adakitic and shoshonitic rocks appear to be linked to an intra continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along ne fault zones e g tanlu and yangtze river fault zones in eastern china both the shoshonitic and adakitic magmas were fertile with respect to cu au mineralization

geochronology and geochemistry of the giant qian echong mo deposit dabie shan eastern china implications for ore genesis and tectonic setting abstract the qian echong mo deposit is a giant porphyry deposit in the dabie shan of eastern china molybdenum mo mineralization mainly occurs as numerous veinlets in altered schists with the development of potassic phyllic argillic and propylitic alteration assemblages a buried granite porphyry stock and locally exposed dikes are spatially and genetically related to the mo mineralization zircon crystals from exposed quartz porphyry rhyolitic porphyry and granite porphyry dikes as well as a buried granite porphyry dike yield weighted average 206 pb 238 u ages of 128 9 1 1 ma mswd 0 30 2 n 13 127 42 0 94 ma mswd 0 58 2 n 17 127 44 0 98 ma mswd 1 6 2 n 16 and 126 6 1 4 ma mswd 0 67 2 n 15 respectively the buried granite porphyry stock yields a zircon u pb weighted average age of 124 7 1 6 ma mswd 1 5 2 n 16 which is slightly younger than the exposed dikes but within error of the age of the buried granite porphyry dike five molybdenite samples from the ores yield re os isotope ages of 123 31 1 02 to 128 49 1 40 ma which are consistent with the u pb ages for the igneous rocks these age data constrain magmatic hydrothermal activity at the qian echong mo deposit to a period of about six million years from 129 to 123 ma this period overlaps the recognized regional mo mineralization event in the dabie shan of 142 111 ma which took place during the post collisional tectonism subsequent to amalgamation of the yangtze and north china blocks the qian echong granite porphyry stock and dikes have high contents of sio 2 k 2 o and al 2 o 3 and low contents of tio 2 mgo and cao showing a peraluminous high k calc alkaline to shoshonite affinity with obvious lree enrichment and a negative eu anomaly the rocks have a high initial 87 sr 86 sr of 0 70729 to 0 71788 and highly negative e nd t values of 16 2 to 26 1 with t dm2 nd ages of 2 24 to 3 03 ga their 206 pb 204 pb t 207 pb 204 pb t and 208 pb 204 pb t values range from 16 017 to 16 701 15 252 to 15 368 and 37 095 to 37 578 respectively this sr nd pb isotopic signature indicates that the causative granitic intrusions for the qian echong deposit were mainly melts with components of both the northern dabie complex and the taihua and xiong er groups that are part of the precambrian basement of the north china block the melts were generated during the tectonic extrusion of the dabie shan and their emplacement onto the basement of the north china block the qian echong mo deposit is unique based upon the crustal source for the causative porphyry localization of most mineralization in the surrounding schist country rocks and the co 2 rich ore forming fluid thus we propose the qian echong deposit to represent a new sub type of porphyry mo deposit herein termed collisional or dabie type porphyry deposit

controls on gravel termination in seven distributary channels of the selenga river delta baikal rift basin russia the selenga river delta lake baikal russia is 600 km2 in size and contains multiple distributary channels that receive varying amounts of water and sediment discharge the delta is positioned along the deep water 1600 m margin of lake baikal a half graben styled rift basin qualifying it as a modern analogue of a shelf edge delta system this study provides a detailed field survey of channel bed sediment composition channel geometry and water discharge the data and analyses presented here indicate that the selenga delta exhibits downstream sediment fining over tens of kilometers ranging from predominantly gravel coarse pebble and sand near its apex to silt and sand at the delta lake interface we developed an analytical framework to evaluate the downstream elimination of gravel within the multiple distributary channels the findings include the following 1 the selenga river delta consists of at least eight orders of distributary channels 2 with increasing channel order downstream channel cross sectional area width depth ratio water discharge boundary shear stress and sediment flux systematically decrease 3 the downstream elimination of gravel in distributary channels is caused by declining boundary shear stress as a result of water discharge partitioning among the bifurcating channels 4 over longer time scales gravel is contained on the delta topset due to frequent and discrete seismic events that produce subsidence and accommodation so that coarse sediment cannot be transported to the axis of the baikal rift basin the distribution of sediment grain size in deltaic channels as related to hydrodynamics and sediment transport plays a critical role in influencing stratigraphy because the sustained tectonism leads to high preservation potential of the delta topset sedimentary deposits therefore the selenga river delta provides an opportunity to explore the interactions between modern deltaic sedimentation processes and tectonics that affect the production of basin stratigraphy

q in the inner core short period vertical seismograms from ten earthquakes and one nuclear explosion in the distance range of 140 170 degrees are examined for a study of the anelastic properties of the inner core the depth of the earthquake foci varies between 116 and 625 km the phases of interest are pkp bc branch and pkikp with bottoming depths in the upper few hundred kilometers of the inner core the data consist of high quality digital data at sampling rates 20 40 samples per second selected after an extensive search of the 1985 1986 gdsn event tapes for short period p waves with a high signal to noise ratio the results are based primarily upon observations between distances of 147 and 154

porphyry mo and epithermal au ag pb zn mineralization in the zhilingtou polymetallic deposit south china the zhilingtou polymetallic deposit south china provides an excellent opportunity to investigate the magmatic hydrothermal evolution of the porphyry mo to epithermal au ag pb zn ore spectrum new zircon u pb sphalerite 40ar 39ar and pyrite rb sr ages together with previous molybdenite re os ages indicate that the granitic magmatism and mo au ag pb zn mineralization were coeval and cogenetic in the early cretaceous 113 ma fluid inclusions and h o isotope results have established the evolution from an early high temperature h2o nacl magmatic fluid system mo mineralization to a medium temperature mixed fluid system of magmatic and meteoric water au ag mineralization to a peripheral late stage low temperature h2o nacl meteoric water fluid system epithermal au ag pb zn mineralization moreover the sulfur isotopic compositions 34s 2 1 7 8 of the sulfide minerals confirm a genetic link to the granitic magma lead isotope data from pyrite and the granite rhyolite porphyries are similar indicating that they share a common pb source the geochronological geochemical and isotopic lines of evidences consistently suggest that the porphyry mo and epithermal au ag pb zn mineralization formed in one magmatic hydrothermal system the porphyry epithermal mineralization model for zhilingtou has significant importance for prospecting work in south china where mo and associated pb zn ag resources are abundant

the pwaves from the amchitka island explosions summary there is evidence that the equivalent seismic sources of the amchitka island explosions longshot milrow and cannikin depart significantly from the simple model of a point compressional source in a layered elastic medium consequently modelling the observed seismograms using standard source models may not be the most efficient method of determining source properties here an alternative to modelling is used to obtain information on the seismic sources due to the explosions broad band bb estimates of the p signals are obtained from the short period sp seismograms corrected for attenuation and interpreted in terms of p pp and radiation from secondary sources the main conclusions are i bb estimates of the radiated displacement from the explosions can be obtained with only a small reduction in the signal to noise ratio seen on sp seismograms ii observations of differences in pulse amplitudes and spectra are not necessarily due to differences in anelastic attenuation iii p and pp at a given station may differ in shape so that notches in the signal spectrum may not be related to source depth iv there is evidence of arrivals that others have identified as due to slapdown but which could be interpreted as an overshoot to pp v direct interpretation of the estimated ground displacement is a better procedure for determining the seismic source properties of explosions than modelling sp seismograms using idealised models as a starting point

today s sediment budget of the rhine river channel focusing on the upper rhine graben and rhenish massif abstract the river bed of the rhine river is subject to severe erosion and sedimentation such high geomorphological process rates are unwanted for economical ecological and safety reasons the objectives of this study were 1 to quantify the geomorphological development of the rhine river between 1985 and 2006 2 to investigate the bed erosion process and 3 to distinguish between tectonic hydrological and human controls we used a unique data set with thousands of bedload and suspended load measurements and quantified the fluxes of gravel sand silt and clay through the northern upper rhine graben and the rhenish massif furthermore we calculated bed level changes and evaluated the sediment budget of the channel sediment transport rates were found to change in the downstream direction silt and clay loads increase because of tributary supply sand loads increase because of erosion of sand from the bed and gravel loads decrease because of reduced sediment mobility caused by the base level control exerted by the uplifting rhenish massif this base level control shows tectonic setting in addition to hydrology and human interventions to represent a major control on morphodynamics in the rhine the rhine bed appears to be in a state of disequilibrium with an average net bed degradation of 3 mm a sand being eroded from the bed is primarily washed away in suspension indicating a rapid supply of sand to the rhine delta the degradation is the result of an increased sediment transport capacity caused by nineteenth and twentieth century s river training works in order to reduce degradation huge amounts of sediment are fed into the river by river managers bed degradation and artificial sediment feeding represent the major sources of sand and gravel to the study area only small amounts of sediment are supplied naturally from upstream or by tributaries sediment sinks include dredging abrasion and the sediment output to the downstream area large uncertainties exist about the amounts of sediment deposited on floodplains and in groyne fields compared to the natural situation during the middle holocene the present day gravel and sand loads seem to be lower whereas the silt and clay loads seem to be higher this is probably caused by the present day absence of meander migration the deforestation and the reduced sediment trapping efficiency of the floodplains even under natural conditions no equilibrium bed level existed

a new technique for measuring the bed surface texture during flow and application to a degradational sand gravel laboratory experiment we present a new image analysis technique for measuring the grain size distribution texture of the bed surface during flow in a laboratory experiment a camera and a floating device are connected to a carriage used to take images of the bed surface over the entire flume length the image analysis technique which is based on color segmentation provides detailed data on spatial and temporal changes of the areal fraction content of each grain size at the bed surface the technique was applied in a laboratory experiment conducted to examine a degradational reach composed of a well sorted two fraction mixture of sand and gravel the initial bed consisted of an upstream reach that was characterized by an imposed stepwise fining pattern the bimodal reach and a downstream sand reach a lack of sediment supply and partial transport conditions led to the formation of a static armor in the bimodal reach which resulted in a more abrupt spatial transition in the bed surface mean grain size the associated spatial transition in slope led to a backwater effect over the bimodal reach a streamwise reduction in sand mobility and so a static armor that was governed by a downstream fining pattern although a morphodynamic equilibrium state under steady flow is generally characterized by normal flow here the partial transport regime prevented the bed from adjusting toward normal flow conditions and the morphodynamic steady state was governed by a backwater we applied a numerical morphodynamic sand gravel model to reproduce the laboratory experiment the numerical model captured the hydrodynamic and morphodynamic adjustment and the static armor well yet the armoring occurred too slowly although the final configuration of the experiment shows features of a gravel sand transition i e a sudden transition in slope and mean grain size we are hesitant to claim similarities between our results and the physical mechanisms governing a gravel sand transition in the field

kinematics and geochronology of the proterozoic dongxiang shexian ductile shear zone with hp metamorphism and ophiolitic melange jiangnan region south china abstract a kinematic and geochronological analysis of the 10 20 km wide northeast trending dongxiang shexian ductile shear zone that trends along a length of 250 km in the eastern part of the jiangnan belt indicates a sinistral strike slip ductile shear that was formed during the neo proterozoic the zone experienced polyphase deformation related to tectonic collision between the huaiyu and the jiuling terranes geochronological data and kinematic analysis of macroscopic structures microscopic shear indicators and quartz c axes fabrics shows that a ductile southeastward thrusting occurred at about 950 900 ma followed by a sinistral strike slip shearing along a northeast trend at about 770 ma coarse clastic sedimentary rocks of sinian age were deposited unconformably on the entire jiangnan belt including the huaiyu and the jiuling terranes this paper emphasizes that the jiangnan belt in the se margin of yangzi plate is a neo proterozoic orogenic belt with hp metamorphism and ophiolitic melange not a late paleozoic mesozoic collisional orogen

fluvial sediment budget of a modern restrained river the lower reach of the rhine in germany abstract the rhine river is a restrained river which is intensely used for navigation its river bed is subject to human induced erosion and sedimentation processes for river management information on the amount type source transport mode and fate of the sediments moving through the rhine is indispensable the objective of this study was to quantify the downstream fluxes of clay silt sand gravel and cobbles through the rhine between 1991 and 2010 and to identify the sources and sinks of these sediments this was done by analysing a unique dataset containing thousands of sediment transport measurements and by evaluating the sediment budget the river bed of the rhine was found to be subject to a net bed degradation of 3 mm a between 1991 and 2010 bed degradation has been induced by 18th 20th century river training works and nowadays is concentrated in areas with tertiary sands close to the bed surface in areas with mining induced subsidence and in the gravel sand transition zone sediment transport was found to be dominated by suspended clay and silt morphologically relevant however are only the sand gravel and cobble fractions despite the armoured gravely river bed sand is the main morphological agent sediment loads change in the downstream direction sand and fine gravel loads increase due to erosion of the bed whereas coarse gravel and cobble loads decrease due to a reduced sediment mobility caused by the downstream decreasing bed slope approximately one third of the sand and gravel load comes from upstream rhenish massif one third is supplied by bed degradation and one third is supplied artificially by humans for bed stabilisation purposes or as substitute for natural bed load slightly more than one half of the sediment was transported downstream into the north sea basin rhine delta a small amount was lost by abrasion and the remainder must have been deposited in groyne fields on floodplains or in ports the transfer of sand gravel and cobbles from the hinterland towards the rhine delta equalled 0 66 mt a 26 despite the long history of human impact this rate does not differ significantly from the holocene rate of sediment transfer to the rhine delta

petrogenesis and tectonic significance of a mesozoic granite syenite gabbro association from inland south china article i nfo a granite syenite gabbro association from inland south china has been studied for zircon u pb ages and hf isotopic compositions as well as whole rock elemental and sr nd isotopic compositions to constrain their sources and tectonic settings these rocks have distinctive crystallization ages the longyuanba biotite granites have a mean age of 240 ma the longyuanba two mica granites pitou alkali feldspar granites and amphibole bearing alkali feldspar granites tabei and huangbu syenites and chebu and chenglong gabbros have a similar age of 178 ma whereas the dafengnao syenite gives a mean age of 165 ma the longyuanba biotite granites and two mica granites exhibit s type characteristics probably derived from a neoproterozoic crustal source the chenglong gabbro has oib like trace element and highly depleted isotope compositions ehf t 10 0 1 3 end t 5 2 suggesting its parental melt to be of asthenospheric origin with insignificant crustal assimilation however crustal contamination is required to explain the isotopic compositions of the chebu gabbro the syenites are shoshonitic in composition and have depleted sr nd hf isotopic signatures which we interpret to have resulted from a mixed source of asthenospheric mantle and metasomatized lithospheric mantle as expected the signals of crustal assimilation are conspicuous in the petrogenesis of the more evolved syenites the pitou alkali feldspar granite and amphibole bearing alkali feldspar granite exhibit i type and a type characteristics respectively they are isotopically more depleted than the s type longyuanba granites we suggest that they may have formed through magma mixing of predominantly mantle derived melts with the neoproterozoic crust the jurassic granite syenite gabbro association was the product of asthenosphere lithosphere crust interactions which records the primary role of asthenospheric mantle upwelling in magma generation both in the mantle and in the crust in the early yanshanian time in south china they probably occurred in an intraplate rift like environment as a tectonic response to far field stress at plate margins during the early stage of the paleo pacific plate subduction the tectonic transition from the tethys orogenic regime to the paleo pacific regime was accomplished in the early jurassic and the early yanshanian magmatism should be genetically associated with the paleo pacific tectonic regime

the key role of fluvial dunes in transport and deposition of sand gravel mixtures a preliminary note abstract sediment transport and deposition in sand gravel bed rivers is shown to depend on dunes the sediment mixture is vertically sorted in avalanches at the lee side of the dunes part of the resulting upward fining sets of the largest dunes that occur during a discharge wave is preserved in the bed furthermore the sediment is entrained and deposited size selectively in the dune troughs which results in an additional upward fining accumulation of lag deposits this deposit is the source for sediment entrained during the next discharge wave which will depend on the relict vertical sorting and on the depth from which it is entrained the entrainment and deposition depth of the sediment depends on the dune trough level below the average bed level and therefore on the dune height thus subsequent discharge waves of decreasing magnitude will leave the upward fining cross bedded sets at depths below the bed surface related to the concurrent dune height this is demonstrated with flume experiments and with new vibrocores collected from the river waal the netherlands

channel sediment variability along a river a case study of the siret river romania the siret river has the largest drainage basin 42 274 km2 in romania it gathers all the rivers from the eastern part of the eastern carpathians a fact that causes marked asymmetry of the basin this study is principally concerned with changes in the form of the longitudinal profile and the grain size variability introduced by the carpathian tributaries channel sediment analyses considered the petrography granulometry and morphometry of the pebbles relating these to the river bed and floodplain geometry and to some properties of the drainage basin the following conclusions arise the siret river undergoes an intense regrading of its longitudinal profile with marked aggradation between transects 24 and 26 see figures 1 and 2 this reflects selective accumulation of coarse material due to the massive contribution of the carpathian tributaries this phenomenon has been continuous throughout the holocene resulting in the gravel sheet formation of the pericarpathian piedmont

anisotropy in inner core attenuation a new type of data to constrain the nature of the solid core during the last decade the presence of velocity anisotropy inside the inner core with a fast axis parallel to the earth s rotation axis has been well established the quantitative analysis of the amplitudes of waves which sample a particular region of the inner core under africa but exhibit various orientations allows us to document and characterize the presence of anisotropy in attenuation the analysis is based on a comparison of both amplitudes and travel times of the pkp df wave which samples the inner core and the pkp bc wave which has nearly the same path but bottoms inside the liquid core the data reveal that the direction of strong attenuation correlates with that of fast velocity when referred to the same epicentral distance and focal depth the pkp df pkp bc amplitude ratio are about five times lower for paths tilted by 25 with respect to the earth rotation axis than for nearly equatorial paths a clear negative correlation is observed between travel time and amplitude residuals when the angle to earth rotation axis varies this first quantitative analysis combined with experimental results on oriented crystals and mushy media may bring important constraints on the mechanism responsible for anisotropy in the inner core

abrasion model of downstream changes in grain shape and size along the williams river australia 1 modeling pebble abrasion during bed load transport is of fundamental importance in fluvial geomorphology as it may help to understand downstream fining patterns along gravel bed rivers here we review a recently published analytical abrasion model called box equations which can simultaneously track the shape and size evolution of large pebble populations as the cumulative effect of binary collisions between particles the model predicts that pebble shapes move away from the sphere and develop sharp edges due to collisional abrasion by sand we present a field study on the downstream evolution of basalt particle shape and size along the williams river in the hunter valley australia pebbles get flatter and thinner and several aquafacts i e abraded pebbles with sharp edges emerge in the downstream reaches both suggesting the importance of abrasion by sand applying box equations with a few fitted parameters we present a numerical simulation which reproduces both the shape and size evolution of pebbles along the williams river the simulation allows tracking of the shape and size evolution of individual particles as well revealing an interesting phenomenon that particle size controls shape evolution box equations in combination with existing transport concepts provide a framework for future shape and size evolution studies in sedimentary environments in particular they may help to assess the relative importance of size selective transport versus abrasion in causing downstream fining in gravel bed rivers

fluid evolution of the yuchiling porphyry mo deposit east qinling china the yuchiling mo deposit east qinling china belongs to a typical porphyry mo system associated with high k calc alkaline intrusions the pure co2 pc co2 bearing c aqueous h2o naci w and

grain size sorting in grainflows at the lee side of deltas the sorting of sediment mixtures at the lee slope of deltas at the angle of repose is studied with experiments in a narrow deep flume with subaqueous gilbert type deltas using varied flow conditions and different sediment mixtures sediment deposition and sorting on the lee slope of the delta is the result of i grains falling from suspension that is initiated at the top of the delta ii kinematic sieving on the lee slope iii grainflows in which protruding large grains are dragged downslope by subsequent grainflows the result is a fining upward vertical sorting in the delta systematic variations in the trend depend on the delta height the migration celerity of the delta front and the flow conditions above the delta top the dependence on delta height and migration celerity is explained by the sorting processes in the grainflows and the dependence on flow conditions above the delta top is explained by suspension of fine sediment and settling on the lee side and toe of the delta large differences in sorting trends were found between various sediment mixtures the relevance of these results with respect to sorting in dunes and bars in rivers and laboratory flumes is discussed and the elements for a future vertical sorting model are suggested

metallogenic geochronology of yuanzhuding cu mo deposit in western guangdong eastern guangxi metallogenic belt and its geological significance a large size porphyry cu mo deposit was recently discovered in yuanzhuding area of the western guangdong eastern guangxi metallogenic belt for the purpose of understanding geochronology of cu mo mineralization in this area the authors separated molybdenite samples from two borehole cores in the ore deposit re os dating of these molybdenite minerals from the two borehole cores has yielded eleven model ages ranging from 149 8 to 155 9 ma eight analytical results of borehole zk12 112 samples have yielded a re os isochron age of 157 3 4 3 ma 2 mswd 0 92 eleven analytical results from borehole zk12 112 and zk16 104 samples have yielded another re os isochron age of 155 6 3 4 ma 2 mswd 0 85 the two isochron ages are in accordance with the re os model ages within the range of error which demonstrates that the age results are reliable and the yuanzhuding porphyry cu mo deposit was formed in late middle jurassic the rhenium content of zk16 104 samples is very high suggesting that the metallogenic material of the yuanzhuding ore deposit originated from the mantle and this is further confirmed by the existence of many regional contemporaneous mafic intrusive rocks re os isotopic data of molybdenite from the yuanzhuding ore deposit may imply that izanagi plate with relatively buoyant topographic anomalies was subducted beneath the yuanzhuding area in early jurassic the variation of the subduction angle resulted in the tearing up of the subducting slab and the thickening of the lithosphere as a result a big slab window occurred or the partially over thickened lithospheric keel was delaminated and a series of ne striking extension belts with strong crust mantle interaction were formed which triggered the upwelling of the asthenosphere along the intersection of the nestriking deep fault and the former ew striking fault in mid late jurassic consequently many contemporaneous granitoids and mafic intrusives or enclaves as well as cu pb zn au ag deposits w sn polymetallic deposits and the yuanzhuding porphyry cu mo deposit related to these intrusives were formed in the western guangxi eastern guangdong region

localized boundary layer below the mid pacific velocity anomaly identified from a pcp precursor dense record sections from deep earthquakes in fiji and argentina recorded on hundreds of short period stations in california at distances of 81 to 85 are used to investigate the detailed p wave velocity structure above the core mantle boundary cmb in the fiji data a secondary phase arriving 2 to 4 s after the direct p is identified as a precursor to pcp this phase provides good evidence for a reflection off the top of a thin low velocity layer above the cmb comparisons to synthetic seismograms indicate a layer thickness of 10 km and a velocity reduction of 5 10 compared to the overlying mantle a record section from an argentina event does not show the pcp precursor indicating that the low velocity layer is not a global feature this thin low velocity layer is in the same place as a much larger s wave velocity anomaly in the lower mantle and is probably indicative of a boundary layer just above the cmb under the mid pacific

from source to mouth basin scale morphodynamics of the rhine river abstract in this article we present a unique detailed basin wide morphological analysis of the rhine one of the world s larger rivers the objectives of this study were to characterize the basin scale morphodynamics of the rhine river in the period 1991 2010 by quantifying the downstream fluxes of clay silt sand gravel and cobbles from source to mouth and identifying the sources and sinks of these sediments using sediment budget analyses from source to mouth the rhine traverses four sections with fundamentally different morphodynamic behaviour the alpine impounded free flowing and delta section sediment fluxes are discontinuous primarily because of the presence of a glacial lake trapping all sediments from upstream the lake caused sediment fluxes to be discontinuous already before the anthropocene today s sediment fluxes are strongly influenced by dredging and nourishment operations from a global perspective sediment fluxes in the river s headwaters are large whereas sediment output from the rhine towards the sea is small a special feature of the rhine is the fact that more sediment is transported in upstream direction from the sea into the delta than vice versa on a basin scale nourishment represents the biggest source of gravel and cobbles and tributaries the biggest source of clay silt and sand in the lower rhine delta additional large amounts of clay silt and sand are supplied by the sea dredging represents a main sediment sink for all size fractions for silt and clay also floodplain deposition and deposition in ports represent major sinks the rhine is a typical example of a river in disequilibrium large parts of the river are subject to erosion or sedimentation in contrast to other rivers with erosion upstream and deposition downstream the rhine has net deposition upstream and net erosion downstream although human interventions contributed to the disequilibrium the pristine rhine already was in a state of disequilibrium even today natural factors determine the location of the main sedimentation areas the budget analysis shows that the behaviour of the clay silt sand and gravel cobble fractions strongly differ from each other a particularity is that in many reaches gravel is deposited whereas simultaneously sand is being eroded the budget analysis also shows that sediment dynamics in rivers are much higher than is suggested by echosoundings or transport measurements this study provides valuable insights into the basin scale morphodynamics of the rhine helps to see smaller scale studies of parts of the rhine basin in a correct perspective provides a good data basis to improve numerical prediction models and helps to optimize nourishment dredging and monitoring strategies in the rhine furthermore the rhine represents an excellent reference case for other large river systems for which less data are available knowledge gaps of supra regional importance identified in this study relate to a the fundamental disequilibrium of large river systems b the effect of natural and human factors on the future morphodynamic development of the large river systems c the morphodynamic role of sand in gravel bed rivers and d the long term effects of sediment nourishment

the gravel sand transition along river channels abstract this paper discusses a hitherto neglected aspect of fluvial downstream fining the transition from a gravel bed to a sand bed from the few known examples and by additional study of new sites some general features of such transitions are given these transitions are generally rapid and involve a change in bed texture from unimodal gravels through bimodal sediments to exclusively sand material and are often associated with a break in slope we propose three possible causes local base level control an excess of sand supply and abrasion breakdown of fine gravel we further argue that such transitions occur in a variety of fluvial environments and are geographically more widespread than previously supposed and on this basis we suggest they may also aid in interpreting ancient sediment showing bimodality and rapid proximal to distal changes in grain size

verification of porosity predictors for fluvial sand gravel deposits 1 although porosity is a key property of sediment mixtures little is known of the natural variations in porosity in fluvial systems porosity predictors can help to generate such information the objective of this study was to determine the accuracy of porosity predictors for fluvial sand gravel mixtures in order to do so porosity measurements were done in the rhine river using a diving bell to allow undisturbed sampling under water in addition laboratory experiments were conducted and porosity data from literature were reanalyzed measured porosity values range from 0 06 to 0 48 our study shows that predictors based on the median grain size the deviation from the fuller curve or the sediment standard deviation are unable to reproduce the observed variation in porosity predictors based on the entire grain size distribution perform better but are biased and have a large prediction error this suggests that they do not account correctly for the grain size effect on porosity on the other hand parameters such as grain shape and the mechanism of deposition probably also have a distinct influence on porosity slightly more accurate predictions can be obtained with tailor made predictors for the river under interest multivariate regression analysis on the rhine data set produced a predictor with two independent parameters the sediment standard deviation and the number of grains smaller than 0 5 mm representing grain mixing effects and adhesion effects respectively despite the shortcomings porosity predictors provide valuable insights in the spatial variation in porosity which is demonstrated in a case study for the rhine river

intracontinental porphyry and porphyry skarn mineral systems in eastern china scrutiny of a special case made in china in this contribution we discuss a series of porphyry and porphyry skarn mineral systems which constitute important metallogenic belts along major lithospheric breaks and or collision zones that form the margins of the north china craton and the yangtze craton we describe a selection of porphyry and porphyry skarn deposits in the great xing an range in northeast china the yanshan liaoning xilamulum belts along the northern margin of the north china craton the qinling orogen and the middle lower yangtze river valley along the bonudaries between the north china and the yangtze cratons the occurrence of platform carbonate successions may have played some role in the evolution of ore fluids that resulted in the formation of porphyry skarn deposits the porphyry deposits of eastern china although sharing some common features with classic porphyry systems that are related to convergent margins do exhibit unusual features particularly in terms of ore fluid compositions the common presence of fluorite hydrothermal alteration assemblages which tend to be less hydrous than the porphyry at convergent margins on the other hand the porphyry mineral systems of eastern china have some similarities with the climax type porphyry a major tectonothermal event yanshanian which took place in the jurassic cretaceous affected eastern china and most of the eastern margin of asia this yanshanian event was responsible for the magmatism and associated mineral systems in the region the granitoid intrusions and accompanying porphyry and porphyry skarn deposits were developed along orogenic belts and major structural discontinuities mostly along the tectonic boundaries between the north china and yangtze cratons and between the northern margin of the north china craton and the mongolia siberia plate the porphyry and porphyry skarn deposits herein described include chalukou in the great xing an range orogen jiguanshan in the xilamulun metallogenic belt xiaojiayingzi lanjiagou chehugou yuanbaoshan in the yanshan liaoning belt nanninhu yuchiling jinduicheng lemeingou in the east qinling orogen tongchakoou shaxi from the middle lower yangtze river valley the role of mantle lithosphere dynamics in eastern china which led to the formation of these porphyry and porphyry skarn mineral systems is discussed and reviewed

geochemical and zircon u pb and hf isotopic study of the baijuhuajian metaluminous a type granite extension at 125 100 ma and its tectonic significance for south china abstract the early cretaceous baijuhuajian pluton is an a type granitic intrusion emplaced along the jiangshan shaoxing jssx fault zone in western zhejiang province se south china it intruded into a late jurassic volcanic basin bounded by proterozoic sandstone and siltstone the granite has a coarse grained porphyritic texture and is composed of alkali feldspar phenocrysts in a matrix of k feldspar quartz plagioclase and biotite the pluton has a clear a type geochemical signature e g it is metaluminous a cnk 1 0 and has high sio2 75 to 78 wt total alkalis na2o k2o 4 8 to 8 5 wt rare earth elements total ree 228 to 438 ppm hfse and fe feot feot mgo 0 87 to 0 97 on the other hand it is characterized by low cao 0 51 to 1 01 wt tio2 0 06 to 0 13 wt p2o5 below detection and sr 7 0 to 9 5 ppm the hypersolvus texture and fine grained groundmass indicate emplacement at a shallow depth shrimp u pb zircon dating yielded a weighted mean 206pb 238u age of 126 3 ma and zircons from this pluton give ehf t values between 0 52 and 4 24 suggesting the importance of juvenile material in the magma source most of the plutonic rocks in southeast china are calc alkaline subduction related intrusions of jurassic and cretaceous age previous studies suggested that the sparse a type granites in south china were emplaced in two episodes 190 155 ma in the inland area and 100 ma along the coastal area in se south china our study of the baijuhuajian granite together with the coeval suzhou a type granite 123 ma in jiangsu province defines an important extensional event between 125 and 100 ma considering the available data for the mesozoic igneous rocks in south china we suggest that subduction of the pacific plate in the mesozoic produced voluminous calc alkaline granitoids and volcanic rocks with a younging trend from west to east the a type granites were generated during periods of local extension at 190 155 ma and 125 100 ma perhaps corresponding to localized rollback of the pacific plate

seismic evidence for small scale heterogeneity throughout the earth s mantle details in the amplitude structure of short period precursors to the seismic core phase pkp can be used to constrain the depth extent of mantle scattering the simplest model consistent with the observations invokes small 8 km weak r m s velocity perturbations of 1 random heterogeneities uniformly distributed throughout the mantle the data do not support previously proposed models that place an increase in the concentration of scattering sites near the base of the mantle and instead place an upper limit on the amplitude of any short wavelength topography at the core mantle boundary

observations of pkkp precursors used to estimate small scale topography on the core mantle boundary stacks of global seismic data reveal the time and distance dependence of high frequency 1 hertz precursors to the seismic phasepkkp synthetic seismogram modeling shows that scattering from random small scale topography at the pkkpcore mantle boundary reflection point generates precursory arrivals similar to those seen in the data models with a root mean square core mantle boundary topography of 250 to 350 meters and correlation length of 7 to 10 kilometers explain the main features of the data however a systematic range dependent misfit between observed and predicted precursor power suggests that inner core scattering may contribute to the precursors

zircon u pb geochronology and hf isotopic compositions of the mesozoic granites in southern anhui province china abstract mesozoic magmatism is extensively developed in eastern china along the eastern asian continental margin but the geochronological framework is not well understood for each area in this study seventy eight samples were collected from the mesozoic igneous rocks in south anhui province east central china in order to precisely constrain their formation ages and contributions to crustal growth zircon u pb age determination using ion probe and laser ablation techniques indicate that magmatism in the area can be divided into two separate stages at 150 136 and 136 120 ma respectively the first stage of granitic rocks not documented in other areas of eastern china is mostly composed of hornblende bearing i type granodiorite and monzogranite and is considered to be formed by slab tearing during pacific subduction the second stage of magmatism coeval to that in other areas of eastern china is characterized by the occurrence of a type granite and syenite and their volcanic counterparts although i type granitic rocks are also developed locally these rocks are mostly distributed around and within the fault controlled volcanic basins and some are closely related to detachment shearing indicating their formation in an extensional setting zircon hf isotopic analyses indicate that archean material might be present in the area but that the main episode of crustal growth took place during the mesoproterozoic with very little juvenile material in the mesozoic

re os isotopic dating of molybdenites in the xiaoliugou w mo deposit in the northern qilian mountains and its geological significance abstract the xiaoliugou is a large scale w mo deposit newly explored in the northern qilian mountains caledonian fold belt northwestern china it is a vein and skarn type deposit genetically associated with the xiaoliugou caledonian granodiorite stock re os isotopic dating for the molybdenite in the deposit yields an isochron age of 462 13 ma 2 and model ages of 436 to 496 ma these results suggest that w mo mineralization occurred before the collision of plates and mineralized substances mainly originated from the crust

on why gravel bed streams are paved bedload transport in poorly sorted gravel bed streams downstream of dams is considered bedload and typical bed material subpavement size distributions are observed to be similar it follows that the coarse half of the subpavement moves through a reach at a rate near that of the fine half since coarser grains are intrinsically less mobile than fine grains it follows that some mechanism must act to nearly equalize mobility it is hypothesized that the pavement seen in gravel bed streams at low flow is in fact in place during typical transport events capable of moving all available sizes this pavement can provide the equalizing mechanism by exposing proportionally more coarse grains to the flow field data are used to quantify this concept and to develop a predictive relation for river pavement the model indicates that pavement should be absent in most sand bed streams in agreement with observation

longitudinal variability in hydraulic geometry and substrate characteristics of a great plains sand bed river article i nfo downstream trends in hydraulic geometry and substrate characteristics were investigated along a 200 km reach of the ninnescah river in south central kansas usa the ninnescah river is a large sand bed perennial braided river located in the central plains physiographic province and is a tributary of the arkansas river hydraulic ge ometry characteristics were measured at eleven reaches and included slope sinuosity bankfull channel width and bankfull channel depth results indicated that the ninnescah river followed a predicted trend of decreasing slope and increasing depth and width downstream there were localized divergences in the central tendency most notability downstream of a substantial tributary that is impounded and at the end of the surveying reach where the ninnescah river approaches the arkansas river surface grain size samples were taken from the top 10 cm of the bed at five points across the wetted cross section within each of the 11 reaches sediment analyses demonstrated a significant trend in downstream fining of surface grain sizes d90 and d50 but unlike previous studies of sand bedded rivers we observed coarsening of substrates downstream of the major tributary conflu ence we propose that the overall low discharge from the tributary was the primary reason for coarsening of the beddownstreamof thetributary results of this studyprovidevaluable baseline information thatcanprovide insight in to how great plains sand bed systems may be conserved managed and restored in the future

the gravel sand transition in a disturbed catchment abstract more than 40 million cubic metres of mining waste were supplied to the ringarooma river between 1875 and 1984 leading to successive phases of aggradation and degradation the natural bed material is gravel but given the volume of introduced load and the fact that much of the input was less than 5 mm in diameter the size composition of the bed changed from gravel to sand during the phase of downstream progressive aggradation a very sharp gravel sand transition developed in which median grain size decreased from over 30 mm to under 3 mm in less than 500 m with upstream supplies of mining debris becoming depleted first degradation followed the same downstream progressive pattern as aggradation causing the transition to migrate downstream by 1984 the river could be regarded as a series of zones each characterized by a particular bed condition a natural cobble gravel bed unaffected by mining inputs 0 32 km pre disturbance bed re exposed by degradation over 35 40 years 32 53 km sandy substrate with a gravel armour produced by differential transport during degradation 53 65 km sand dominated but with developing surface patches of coarser material 65 75 km sandy bed reflecting the size composition of the original mining input 75 118 km although the gravel sand transition itself is sharp the transitional zone is lengthy 53 75 km as degradation continues the gravel sand transition is expected to progress downstream but it has remained in a stable position for 12 years indeed two major floods during the period released large quantities of sand from the sub armour layer and newly formed banks of mine tailings causing fining both above and below the transition surface grain size is an adjustable component in the transitional zone as the river strives to recover from a major anthropogenic disturbance

cretaceous tectonic evolution of south china a preliminary synthesis abstract the cretaceous tectonic evolution of south china is characterized by widespread extensional basin and dome generation voluminous magma intrusion eruption and associated polymetallic mineralization all of which are of world wide interests that have stimulated the attention of geologists for more than half a century due to the lack of a comprehensive understanding of regional tectonic evolution and geodynamics of south china many controversies regarding the nature and origin of these features remain this paper attempts to make a review by synthesizing existing structural petrological geochronological and geochemical data of the cretaceous structures and magmas which guides us to propose a three stage tectono thermal evolutionary history of south china during the cretaceous period the earliest cretaceous 145 137 ma tectonic stage was characterized by syn orogenic shortening deformation and metamorphism under a nw se compressional setting which generated voluminous porphyry cu au ore bearing adakitic rocks in the lower yangtze river belt and gneissic granites in the coastal area its geodynamic origin was interpreted as combined effects of the pacific izanagi ridge subduction beneath the lower yangtze river belt and the pacific subduction beneath the cathaysia block this syn orogenic shortening stage was followed by a post orogenic stage 136 86 ma comprising two episodes of alternate extensional and shortening events the extension 136 118 ma in the earlier episode was dominated by a nw se extensional regime it led to significant taphrogenesis manifested by large scale extensional basins and voluminous magma intrusion eruption this extensional event was associated with a combination of the slab window opening during the ridge subduction and the rollback of the subducted pacific slab the subsequent nw se transpressional event led to cessation of active marginal magmatism and resulted in tectonic inversion of previous rift basins this transpression lasted from 117 ma to 108 ma and its driving mechanism was attributed to the collision between the eastern asian margin and the west philippine block the extension 107 86 ma in the later episode was dominated by a wnw ese extensional regime which led to a second phase of basin subsidence and produced numerous a and i type granites and bimodal volcanoes the tectonic regime then changed at ca 85 ma to wnw ese oriented transpression causing inversion of the late cretaceous rift basins and cessation of extension related magmatism the change of stress field from wnw ese extension to wnw ese transpression was related to the variations of the subducted slab dynamics i e the transition from either the ese ward retreat to wnw ward subduction of the pacific slab or the alternate stress during slab break off during the latest cretaceous a drastic change of regional tectonic stress orientation from wnw ese to n s occurred in south china the third stage predominated by n s extension was developed which caused a new phase of regional crustal subsidence along the youngest e w trending extensional structures

cretaceous magmatism and mo mineralization in the south china mo province u pb and re os geochronology constraints from the sanzhishu porphyry mo deposit abstract compared with the mesozoic w sn cu au pb zn ag mineralization in the south china mo province scmp detailed studies on mo deposits especially cretaceous mo deposits are rare due to limited exposures the sanzhishu porphyry mo deposit located in the scmp is characterized by disseminated and veinlet type molybdenite quartz veins within monzogranite zircon sims u pb analysis of the monzogranite yields a concordia age of 112 3 ma indicating that the emplacement of the sanzhishu monzogranite was in the early cretaceous six molybdenite samples yield re os ages between 111 and 112 ma with an isochron age of 111 6 4 ma which is consistent with the zircon age of the ore bearing monzogranite a new compilation of reliable re os isotope age data indicate that cretaceous mo mineralization in the scmp occurred during two major episodes at 130 145 ma and 90 120 ma with most deposits forming in the later episode during the cretaceous a back arc extensional setting in se china is evidenced by the occurrence of numerous high f mo deposits widespread a type granites and the development of pull apart basins the increasing trend in re contents of molybdenite from the early mo mineralization period to the late mo mineralization period in the scmp indicates that the area experienced stronger crust mantle interaction in the later mineralization episode based on geological evidence we propose that the early episode of mo mineralization formed in a tectonic setting associated with a metamorphosed subducted slab that experienced an increase in density and an increase in the gravitational pull on the slab itself which induced rollback of the subducting paleo pacific slab the late episode of mo mineralization developed in an extensional tectonic setting caused by the continued rollback and the eventual slab break off

observations of downstream fining on the north fork toutle river near mount st helens washington we carried out a series of measurements to provide data for testing models of downstream fining in an actively braiding gravel bed river in particular the model developed in the previous paper we sampled a series of nine cross sections for bulk grain size distributions over a downstream distance of 5 4 km on the north fork toutle river near mount st helens washington in addition we sampled two of the nine sections for detailed analysis of surface grain size distributions in the cross stream direction by wolman counting within areas of distinct mean grain size patches our theory predicts that the rate of downstream fining depends on the distribution of deposition and a relative mobility function that depends on the characteristics of the grain size patches application of the theory to the study area shows that most of the observed downstream fining can be explained using patchiness as the dominant mechanism

downstream fining in large sand bed rivers abstract downstream fining of bed sediments is a well known phenomenon in gravel bed rivers but also occurs in large sand bed rivers the underlying processes however are not necessarily the same due to the difference in sediment mobility grain size distribution width and geographical setting downstream fining studies focusing on large sand bed rivers are scarce nevertheless in this paper the processes that affect downstream fining in gravel bed rivers are reviewed and it is evaluated to what extent they are relevant for large sand bed rivers furthermore several new downstream fining processes are discussed ending in a conceptual model of downstream fining in large sand bed rivers in deriving the conceptual model downstream fining processes are divided into three categories abrasion selective transport and sediment addition extraction abrasion rates in large sand bed rivers are small due to the durable lithologies the small grain size the high degree of rounding the dominance of grinding as abrasion process and the dominant suspended transport mode of the bed sediment only in the upstream part of large sand bed rivers abrasion rates may be somewhat higher selective transport rates in large sand bed rivers everywhere overrun abrasion rates but are small compared to selective transport rates in gravel bed rivers because the unimodal sediments in sand bed rivers exhibit stronger hiding exposure effects during threshold of motion conditions but especially because many sand bed rivers though not all are in state of fully mobilised transport making differences in threshold of motion between coarse and fine grains relatively unimportant selective transport in these rivers is probably mainly the result of the presence of suspended load transport in combination with the effects of dune and perhaps bend sorting which are all processes that act on a relatively large scale within sand bed rivers the degree of selective transport probably decreases downstream due to the change in mixture bimodality and sorting efficiency there are indications that coarse grains may sometimes even be more mobile than fine grains in the downstream part of sand bed rivers note that in contrast to abrasion selective transport only produces a stable downstream fining pattern if the river profile is concave generally a stronger profile concavity leads to a higher aggradation rate and a stronger downstream fining trend sediment addition and extraction processes can obscure the effects of abrasion and selective transport in sand bed rivers overbank sedimentation for instance may significantly decrease the downstream fining rate in large sand bed rivers because especially fine grains are removed from the bed dredging can also strongly affect the downstream fining trend depending on the volume and the grain size of the sediment involved meander migration may cause a nett loss of coarse grains from the channel in aggrading circumstances increasing the downstream fining rate river bifurcations can cause a discontinuity in the downstream fining pattern because bend sorting upstream of the bifurcation causes the river branch that originates in the outer bend of the main channel to be much coarser than the other branch all these addition and extraction processes not only have a direct effect on the grain size composition of the river bed but also indirectly influence downstream fining by their effect on the longitudinal bed profile other sediment addition and extraction processes such as tributary confluences non alluvial sediment sources and dumping are less common in large sand bed rivers but can locally have a great effect a full determination of the relative importance of all downstream fining processes in large sand bed rivers requires extensive field measurements the construction of detailed fractionwise sediment balances and the development of numerical downstream fining models that incorporate all processes described above

the gravel sand transition flume study of channel response to reduced slope this paper reports flume experiments with a bimodal gravel sand bed and sediment feed designed to investigate river channel response to declining slope in successive runs an initial near equilibrium channel was subjected to progressively bigger reductions in flume slope this simulated a series of sites along a concave profile approaching a local base level and undergoing a transition from gravel to sand bed it also models the effects of a rise in base level on a fixed site slope reduction led to immediate increase in depth and reduction in velocity and shear stress hence progressive aggradation deposition was size selective and led to the following sequence of bed texture change with increasing severity of slope reduction 1 sand in lee of gravel clusters 2 elongated sandy patches and thin streaks 3 wider sand ribbons covering gravel completely and developing ripples there was feedback from the gradual bed surface changes to hydraulic conditions near bed velocity increased while depth shear stress and roughness height decreased these feedbacks were more pronounced with a bigger reduction in slope these results support the view that the gravel sand transition along river channels with a local base level control is assisted by positive feedback between changes in bed surface texture and near bed flow

fine bed material in pools of natural gravel bed channels natural gravel bed channels commonly contain a fine mode of sand and fine gravel that fills voids of the bed framework of coarser gravel if the supply of fine bed material exceeds the storage capacity of framework voids excess fine material forms surficial patches which can be voluminous in pools during low flow data collected in 34 natural channels in northern california and southern oregon indicate the following 1 fine material on the bed surface can be readily winnowed and transported at high particle velocities much of it in intermittent suspension fine material can dominate the bed material load in gravel bed channels but its abundance on the bed surface is limited by its increasing mobility as hiding places among prominent particles are filled 2 fine material in pools is typically replaced many times per year 3 the proportion v of residual pool volume filled with fine bed material correlates with annual sediment yield in channels whose parent material produces abundant sandy sediment 4 temporal and spatial changes in v appear to correspond to variations in the balance between sediment inputs and water discharge these results suggest that v can be used to monitor and evaluate the supply of excess fine material in gravel bed channels and that samples of fine material in pools can characterize the fine mobile mode of bed material load the sediment load of a stream channel the amount supplied and transported over a period of time can be difficult to evaluate contributions from hillslopes can be quantified with erosion surveys and sediment budgeting but uncertainty in routing makes it difficult to assess the load at any point in a channel network and direct measurement of transport rates is notoriously difficult bed material typically furnishes the bulk of the annual bed load especially in more distal channels but the active volume that contributes to the load is highly variable and difficult to evaluate it may be easier to detect variations in load by examining the mobility of the bed surface of gravel bed channels bed material in gravel bed channels characteristically includes a wide range of particle size and the bed can become

review of the low temperature magnetic properties of magnetite from a rock magnetic perspective the low temperature magnetic properties of magnetite are reviewed and implications for rock magnetism considered the behaviour of fundamental properties of magnetite at low temperatures near the verwey transition tv are documented and attention is given to various verwey transition theories the low temperature behaviour of the magnetic energies that control domain structure is reviewed in detail for the first time in rock magnetic literature the low temperature anomaly in spontaneous magnetization ms is documented and the differences between the saturation magnetization and ms near the verwey transition are discussed it is argued that the low temperature behaviour of the magnetocrystalline anisotropy and in particular the anomaly at tv is most likely to affect multidomain remanence during low temperature cycling for multidomain crystals it is calculated that the large increase in magnetocrystalline anisotropy intensity and reduction in symmetry on cooling through tv is likely to reduce the stability of closure domains

u pb zircon ages of granites from the southern margin of the yangtze block timing of neoproterozoic jinning orogeny in se china and implications for rodinia assembly abstract neoproterozoic jinningian bendong granodiorite and sanfang and yuanbaoshan granites occurred in northern guangxi southern margin of the yangtze block se china and were generally considered to be of orogenic origin that is the collision between the yangtze and huanan blocks during the jinning orogeny these granites were intruded into the mesoproterozoic sibao metamorphic basement and covered unconformably by the neoproterozoic danzhou metasediments they are dominantly peraluminous asi 1 18 1 29 and have low initial end values of 4 8 to 7 6 indicating derivation from a supracrustal source with strong enrichment in rb k and th and relatively low zr hf and higher rare earth elements they are similar to post collision granites zircon u pb ages of 819 9 826 10 and 824 4 ma were obtained for the bendong granodiorite and the sanfang and yuanbaoshan granites respectively by both shrimp and conventional dating methods the age result precludes the existence of any late mesoproterozoic grenville age granitic magmatism in the southern margin of yangtze block together with previous age determinations of ophiolites along the southern margin of the yangtze block the jinning orogeny is referred to the neoproterozoic orogeny which took place between 1 0 and 0 80 ga and likely ended at 0 82 ga the granite formation age of 0 82 ga also supports the basal boundary age of 0 80 ga for the post orogenic sinian system which is probably stratigraphically correlated with the adeladean system in south australia as the link between australia east antarctica and laurentia in the configuration of the supercontinent rodinia the collision between the yangtze and huanan blocks during the neoproterozoic jinning orogeny probably marks the termination of the rodinia assembly

a local crossing path study of attenuation and anisotropy of the inner core 1 we report results from studying a region under the north pacific using 46 ray paths along east west and north south directions to study the nature of inner core anisotropy and find an anisotropy signal in differential pkpbc pkpdf at about the resolution of the data with north south paths faster than east west but no dependence of attenuation on wavespeed or depth in this part of the inner core inner core q is 130 52 255 between 140 340 km depth the observations also provide constraints on the degree of homogeneous meridional anisotropy possibly present in the core to between 0 1 and 0 6 velocity differences in the fast and slow directions significantly smaller than the 2 4 axi symmetric anisotropy in the inner core the small meridional component to anisotropy argues against significant contributions to anisotropy from low order inner core convection or stresses imposed on the inner core by the outer core field

constraints on upper inner core structure from waveform inversion of core phases summary the determination of the inner core boundary structure is key to understanding the mechanisms of inner core growth iron solidification and anisotropy formation in the inner core a waveform inversion of a worldwide data set of pkikp and pkikp core phases is performed to investigate the velocity and the attenuation structure of this region the data set is chosen to sample the top 50 km of the inner core two inversion methods combining time and frequency domain observations are presented synthetic tests are performed in order to evaluate the resolution and the confidence intervals of the model parameters the attenuation of p waves in the uppermost inner core is poorly resolved but appears to be high q 100 the uppermost inner core interface is divided into two regions a region extending from 180 w to 60 e and characterized by negative p wave velocity perturbations dv v 0 per cent and a region extending from 60 e to 180 e and characterized by positive p wave velocity perturbations dv v 1 per cent the results of the inversion do not favour the presence of transverse isotropy aligned along the spin axis of the earth in the uppermost inner core for these two regions however these regions are correlated with those where large scale variations of the inner core anisotropy have been determined previously in the deeper inner core 100 km beneath the inner core boundary

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

u pb zircon geochemical and sr nd hf isotopic constraints on age and origin of jurassic i and a type granites from central guangdong se china a major igneous event in response to foundering of a subducted flat slab abstract the mesozoic geology of se china is characterized by widespread jurassic to cretaceous igneous rocks consisting predominantly of granites and rhyolites and subordinate mafic lithologies however the tectonic regime responsible for the inland jurassic granites remains controversial we report here u pb zircon ages geochemical and sr nd hf isotopic data for the nankunshan alkaline granite and the fogang granitic batholith in central guangdong mineralogical and geochemical features suggest that the fogang and nankunshan rocks are i and aluminous a type granites respectively shrimp u pb zircon analyses yield consistent ages ranging from 159 3 ma to 165 2 ma for four samples from the fogang batholith and an age of 158 5 ma for the nankunshan granite the fogang granites having i sr 0 7098 0 7136 e nd t 4 3 to 12 2 and e hf t 11 5 to 3 1 for the magmatic zircons were derived from paleoproterozoic mafic intermediate igneous protolith with minor addition of mantle derived magmas the nankunshan rocks have relatively lower i sr 0 706 0 708 higher e nd t 0 3 to 2 4 and e hf t 5 7 to 1 1 for the magmatic zircons and some oib like trace element ratios they were likely generated through extensive fractional crystallization of mantle derived alkaline parental magma associated with crustal assimilation these 160 ma i and a type granites in central guangdong were emplaced coeval with the widespread 165 155 ma i and a type granites and syenites following the initiation of intraplate basaltic and or bimodal igneous magmatism at 180 170 ma in the adjacent regions we interpret these jurassic igneous rocks as anorogenic magmatism formed during a major igneous event in response to foundering of an early mesozoic subducted flat slab beneath se china continent

downstream fining through selective particle sorting in an equilibrium drainage network the phenomenon of downstream fining has been attributed to both particle abrasion and selective particle sorting the latter is generally considered to play the dominant role within resistant lithologies it has been recognized that tributaries can disrupt fining patterns however few downstream fining studies have considered the entire fluvial network structure here we combine a theory for selective transport with a model of river basin evolution in order to simulate the dynamics of selective sorting throughout a drainage network previous numerical modeling studies of single thread or braided channels have treated downstream fining as a phenomenon driven by differential deposition rates we show however that in an eroding drainage network downstream fining emerges as a natural dynamic adjustment to variable water sediment and energy inputs even under conditions of uniform size distribution in sediment flux thus although selective deposition and abrasion clearly can and do play a role in some fluvial systems neither is necessary to produce downstream fining within a drainage network

discriminating between pore filling load and bed structure load a new porosity based method exemplified for the river rhine sediments contained in the river bed do not necessarily contribute to morphological change the finest part of the sediment mixture often fills the pores between the larger grains and can be removed without causing a drop in bed level the discrimination between pore filling load and bed structure load therefore is of practical importance for morphological predictions in this study a new method is proposed to estimate the cut off grain size that forms the boundary between pore filling load and bed structure load the method evaluates the pore structure of the river bed geometrically only detailed grain size distributions of the river bed are required as input to the method a preliminary validation shows that the calculated porosity and cut off size values agree well with experimental data application of the new cut off size method to the river rhine demonstrates that the estimated cut off size decreases in a downstream direction from about 2 to 0 05 mm covariant with the downstream fining of bed sediments grain size fractions that are pore filling load in the upstream part of the river thus gradually become bed structure load in the downstream part the estimated mass percentage of pore filling load in the river bed ranges from 0 in areas with a unimodal river bed to about 22 in reaches with a bimodal sand gravel bed the estimated bed porosity varies between 0 15 and 0 35 which is considerably less than the often used standard value of 0 40 the predicted cut off size between pore filling load and bed structure load dc p is fundamentally different from the cut off size between wash load and bed material load dc w irrespective of the method used to determine dc p or dc w dc w values are in the order of 10 1 mm and mainly dependent on the flow characteristics whereas dc p values are generally much larger about 100 mm in gravel bed rivers and dependent on the bed composition knowledge of dc w is important for the prediction of the total sediment transport in a river including suspended fines that do not interact with the bed whereas knowledge of dc p helps to improve morphological predictions especially if spatial variations in dc p are taken into account an alternative to using a spatially variable value of dc p in morphological models is to use a spatially variable bed porosity which can also be predicted with the new method in addition to the morphological benefits the new method also has sedimentological applications the possibility to determine quickly whether a sediment mixture is clast supported or matrix supported may help to better understand downstream fining trends sediment entrainment thresholds and variations in hydraulic conductivity