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

origin of granulite terranes and the formation of the lowermost continental crust differences in composition and pressures of equilibration between exposed regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust as had been thought but rather middle and lower middle crustal levels application of well calibrated barometers indicate that exposed granulites record equilibration pressures of 0 6 to 0 8 gigapascal 20 to 30 kilometers depth of burial whereas granulite xenoliths which also tend to be more mafic record pressures of at least 1 0 to 1 5 gigapascals 35 to 50 kilometers depth of burial thickening of the crust by the crystallization of mafic magmas at the crust mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep seated mafic crust represented by many xenolith suites

zircon u pb and trace element data from rocks of the huai an complex new insights into the late paleoproterozoic collision between the eastern and western blocks of the north china craton abstract the trans north china orogen tnco is interpreted as a continent continent collisional belt along which the discrete archean eastern and western blocks amalgamated to form the basement of the north china craton the timing of its formation has not been precisely constrained yet six samples including five high grade metamorphic rocks and a pegmatite dike from the huai an complex in the northern segment of the tnco were selected for zircon u pb dating and trace element analysis a tonalite trondhjemite granodiorite ttg gneiss sample and a high pressure hp mafic granulite sample contain magmatic zircon cores yielding protolith formation ages of 2503 17 and 1964 38 ma respectively detrital zircons of magmatic origin in two metasedimentary rocks show concordant or moderately discordant u pb ages of 2683 22 to 1964 13 ma indicating that they were mainly derived from the tnco with some possibly derived from the archean continental rocks of the north china craton metamorphic zircons in the ttg gneiss and two metasedimentary rocks exhibit low hree contents flat hree patterns and negative eu anomalies implying their formation coeval with garnet and feldspar under grannulite facies conditions metamorphic zircons in an hp mafic granulite and a sample of leucosome from a ttg gneiss also have depleted hree contents flat hree patterns but only weak negative eu anomalies which may also indicate formation under granulite facies conditions whereas the weak eu anomalies may be due to the whole rock composition all these metamorphic zircons have formation temperatures of 702 749 c approaching the peak metamorphic temperatures of hp granulite rocks in this area they have identical ages of ca 1845 ma within analytical errors which is the best estimate age for hp granulite facies metamorphism of the huai an complex these results confirm previous models proposing late paleoproterozoic collision between the eastern and western blocks of the north china craton along the tnco on the other hand some metamorphic zircons in the leucosome sample show planar or polygonal zoning high th u contents and high th u ratios high ree contents and steep hree patterns indicating that they crystallized during partial melting with abundant garnet decomposition they yielded a weighted mean age of 1819 13 ma which we interpret to register the time of retrograde metamorphism combined with previously published data there are three age peaks at 1876 6 1849 2 and 1814 4 ma for metamorphic zircons from high grade metamorphic rocks in the tnco these may indicate either a protracted event or at least three peaks of metamorphism a pegmatite dike cutting the foliation of a mafic granulite boudin was dated at 1806 15 ma which signifies termination of collision between the eastern and western blocks in the huai an area

drainage basin responses to climate change recent investigations have shown that the extent of the channel network in some drainage basins is controlled by a threshold for overland flow erosion the sensitivity of such basins to climate change is analyzed using a physically based model of drainage basin evolution the golem model simulates basin evolution under the action of weathering processes hillslope transport and fluvial bedrock erosion and sediment transport results from perturbation analyses reveal that the nature and timescale of basin response depends on the direction of change an increase in runoff intensity or a decrease in vegetation cover will lead to a rapid expansion of the channel network with the resulting increase in sediment supply initially generating aggradation along the main network followed by downcutting as the sediment supply tapers off by contrast a decrease in runoff intensity or an increase in the erosion threshold will lead to a retraction of the active channel network and a much more gradual geomorphic response cyclic changes in runoff intensity are shown to produce aggradational degradational cycles that resemble those observed in the field cyclic variations in runoff also lead to highly punctuated denudation rates with denudation concentrated during periods of increasing runoff intensity and or decreasing vegetation cover the sediment yield from threshold dominated basins may therefore exhibit significant variability in response to relatively subtle environmental changes a finding which underscores the need for caution in interpreting modern sediment yield data

river system discontinuities due to lateral inputs generic styles and controls in alluvial river systems lateral inputs of water and or sediment at junctions or undercut hillsides can disrupt what would otherwise be smooth downstream trends in mainstream bed elevation channel gradient and bed grain size generic styles of mainstream response to lateral inputs are investigated using a one dimensional sediment routing model with multiple grain size fractions numerical experiments isolate the effects of three para meters ratio of tributary to mainstream water flux qr ratio of tributary to mainstream bedload flux fr and ratio of tributary to mainstream bedload diameter dr the findings are not unduly sensitive to the choice of initial conditions or to approximations made in the model the primary distinction is between junctions that aggrade causing local profile convexity with interrupted downstream fining and junctions that degrade the immediate effects of aggradation extend further upstream than downstream whereas degradation is much more subdued and has no upstream impact aggradation is typical of coarse inputs dr 2 and degradation of fine inputs dr 1 but very high ratios of qr to fr also promote degradation both aggrading and degrading junctions can lead to a change in mainstream bed grain size well below the junction with higher ratios of qr to fr producing a coarser distal bed the effect of a tributary reflects the interplay between additional bed load and additional discharge to transport it

an extensive erosion surface of a strongly deformed limestone bed in the gushan and chaomidian formations late middle cambrian to furongian shandong province china sequence stratigraphic implications abstract in order to understand sequence development and sea level fluctuations during the late middle cambrian to early furongian on the north china epeiric platform the present study focuses on a unique subtle erosion surface of an extensive approx 100 km strongly deformed limestone bed in the uppermost part of the gushan formation shandong province china the gushan formation and the overlying chaomidian formation consist mainly of shales and a variety of carbonates that were deposited in subtidal environments e g deep subtidal shallow subtidal shoreface shoal subtidal microbial flat and restricted platform interior three third order depositional sequences s1 3 are identified each of which comprises a thin transgressive systems tract tst and a relatively thick highstand systems tract hst each sequence is bounded by a drowning unconformity sb1 a subaerial unconformity sb2 or a surface of submarine erosion sb3 the upper sequence boundary sb2 of sequence 1 s1 is represented by a subtle erosion surface of an extensive deformed limestone bed with a wide variety of soft sediment deformation structures e g lime mudstone breccias chaotic wacke packstone laminae and fragments homogenized oolites and clastic dykes and is overlain by small sporadic microbial buildups and an extensive bioclastic grainstone bed the deformed limestone was formed during early diagenesis by differential deformation processes brecciation liquefaction fluidization and injection which were most likely induced by pore water overpressure during the period of rapid sea level fall despite the lack of subaerial exposure features e g paleokarst paleosol etc the characteristics of the erosion surface cutting well lithified sediment below the missing of a significant geological record the prochuangia biozone and the worldwide correlatable positive carbon isotope excursion collectively indicate that the erosion surface developed under conditions of subaerial exposure after contemporaneous marine cementation of the deformed sediment the missing of the prochuangia biozone is most likely due to non deposition at a subaerial hiatal surface the erosion surface was submerged as a result of subsequent rise in sea level where sporadic microbial buildups formed under suitable conditions freshly deposited winnowed shell dominated transgressive lag deposits containing chuangia trilobite fragments brachiopod shells and abundant glauconite grains formed with continued rise in sea level which became in turn overlain by shale dominated facies the unique combination of the subtle erosion surface sensu stricto a subaerial unconformity and the underlying deformed limestone bed provides an important criterion for recognizing the subtle changes in relative sea level on shallow epeiric platforms

superposed deformation in the helanshan structural belt implications for mesozoic intracontinental deformation of the north china plate abstract the north china plate ncp underwent multi phase intracontinental deformation during the mesozoic which is a much debated research subject here we present a case study of this deformation inferred from regional scale helanshan superposed folds in the west of the ncp new structural analyses and fault kinematic analyses in the helanshan structural belt hsb indicate a two stage syn folding paleo stress field comprising early nne ssw compression with mean orientations 190 01 plunge direction plunge for 1 283 26 for 2 and 100 63 for 3 and later wnw ese compression with mean orientations of 1 2 and 3 at 124 07 213 04 and 001 75 respectively these results combined with geochronological analysis indicate two episodes of shortening in this belt during the mesozoic late triassic early jurassic nne ssw shortening which produced 8 1 km of horizontal shortening led to the development of open wnw trending fold structures subsequent late jurassic early cretaceous wnw ese shortening which produced 18 7 km of horizontal shortening reshaped the early wnw trending structures and generated close overprinting nne trending folds that resulted in the development of the regional scale superposed folds in this area in terms of geodynamics the early nne ssw shortening is likely to be related to indosinian intercontinental convergence between the north china plate and the yangtze plate and the later wnw ese shortening is likely to have been related to subduction of the paleo pacific plate the regional structural analysis indicates that the whole of the western north china plate experienced two phase contractional deformation during the mesozoic as manifest by the early nne ssw and the late wnw ese shortening that characterized mesozoic intracontinental deformation in the western ncp

tectonothermal history of the basement rocks in the western zone of the north china craton and its tectonic implications the basement of the north china craton can be divided into the eastern central and western zones based on lithological structural metamorphic and geochronological data the western zone comprises two different petrotectonic units archaean tonalitic trondhjemitic granodioritic ttg grey gneisses and metamorphic mafic rocks and palaeoproterozoic khondalite series the former is characterized by isobaric cooling ibc type anticlockwise p t paths in the north northwestern part of the zone and near isothermal decompression itd type clockwise p t paths in the eastern part adjacent to the central zone on the other hand the tectonothermal evolution of palaeoproterozoic khondalite series rocks is characterized exclusively by nearly isothermal decompression following the peak of metamorphism and then cooling defining clockwise p t paths the archaean ttg gneisses and associated mafic rocks with anticlockwise metamorphic p t paths reflects an origin related to underplating and intrusion of mantle derived magmas which may be derived from mantle plumes they represent a late archaean continental block in the western part of the north china craton the palaeoproterozoic khondalite series rocks represent passive continental margin deposits they were metamorphosed and deformed in the late palaeoproterozoic during the amalgamation of the western continental block with another continental block in the east part of the north china craton the itd type clockwise p t t paths of the palaeoproterozoic khondalite series rocks record the tectonothermal histories of the collision of the western and eastern continental blocks which resulted in the final assembly of the north china craton at c 1800 ma

subsidence control on river morphology and grain size in the ganga plain the ganga plain represents a large proportion of the current foreland basin to the himalaya the himalayan sourced waters irrigate the plain via major river networks that support approximately 10 percent of the global population however some of these rivers are also the source of devastating floods the tendency for some of these rivers to flood is directly linked to their large scale morphology in general the rivers that drain the east ganga plain have channels that are perched at a higher elevation relative to their floodplain leading to more frequent channel avulsion and flooding in contrast those further west have channels that are incised into the floodplain and are historically less prone to flooding understanding the controls on these contrasting river forms is fundamental to determining the sensitivity of these systems to projected climate change and the growing water resource demands across the plain here we present a new basin scale approach to quantifying floodplain and channel topography that identifies areas where channels are super elevated or entrenched relative to their adjacent floodplain we explore the probable controls on these observations through an analysis of basin subsidence rates sediment grain size data and sediment supply from the main river systems that traverse the plain yamuna ganga karnali gandak and kosi rivers subsidence rates are approximated by combining basement profiles derived from seismic data with known convergence velocities results suggest a more slowly subsiding basin in the west than the east grain size fining rates are also used as a proxy for relative subsidence rates along the strike of the basin the results also indicate higher fining rates and hence subsidence rates for given sediment supply in the east by integrating these observations we propose that higher subsidence rates are responsible for a deeper basin in the east with perched low gradient river systems that are relatively insensitive to climatically driven changes in base level in contrast the lower subsidence rates in the west are associated with a higher elevation basin topography and entrenched river systems recording climatically induced lowering of river base levels during the holocene

tio2 activity in metamorphosed pelitic and basic rocks principles and applications to metamorphism in southeastern canadian cordillera the activity of tio2 can be precisely defined as a function of pressure temperature and activities of other components for common mineral assemblages in metapelites ilmenite quartz garnet plagioclase al2sio5 and in metabasites plagioclase sphene ilmenite quartzgarnet these mineral assemblages can be modelled by the equilibria 1 3ilmenite al2sio5 2quartz almandine 3tio2 2 anorthite 2sphene grossular 2tio2 quartz 3 3anorthite 3quartz 6ilmenite grossular 6tio2 2almandine these mineral assemblages can be used at a text tio text 2 1 rutile saturation and a given t to get maximum pressure limits of some metapelites and metabasites when electron microprobe analyses of mineral grains adjacent to ti bearing phases are made these data give maximum pressure estimates in reasonable agreement with other geobarometers the activity of tio2 in many metapelites is very near rutile saturation but for metabasites the activity of tio2 in some sillimanite zone rocks is as low as 0 6 the solubility of tio2 in biotite hornblende and garnet is a complex function of t p the activities of components in coexisting minerals and crystal chemical constraints in these minerals at a given p and t the solubility of tio2 in biotite and hornblende does not appear to be strongly dependent upon a text tio text 2 1 for sphene and ilmenite versus rutile bearing assemblages

provenance record of a foreland basin detrital zircon u pb ages from devonian strata in the north qilian orogenic belt china abstract devonian strata in the north qilian belt of the qilian orogen accumulated in a foreland basin formed through collision between the north china craton and the central qilian belt u pb age data for 393 detrital zircons from four sandstone samples from the lower to middle devonian laojunshan formation and upper devonian shaliushui formation range in age from 3 6 ga to 0 4 ga and indicate input from a multi component source detrital zircons older than the early mesoproterozoic 1 4 ga were likely derived from the north china craton whereas late mesoproterozoic to neoproterozoic zircons were derived from the central qilian belt early paleozoic zircons were derived from the north qilian continental arc and associated ophiolites the occurrence of neoproterozoic zircons indicates that their source the central qilian belt was likely once part of the yangtze block of the south china craton prior to its separation from the craton in the late neoproterozoic

granulite facies metamorphism and crust melting in the huai an terrane at 1 95ga north china craton new constraints from geology zircon u pb lu hf isotope and metamorphic conditions of granulites abstract geological implications of 1 95 ga and 1 85 ga metamorphic ages for hp granulites in the huai an terrane are still controversial which have affected the crustal evolution models for the north china craton this study reports two geological sections in which distinct geological relationships of a quartz dioritic and a tonalitic dike cutting through mafic hp granulite and granulite facies meta sedimentary rocks are described in these two sections magmatic dikes mafic hp granulites and meta sedimentary rocks preserve conformable foliation to indicate their similar deformation history the u pb dating and lu hf isotopic analysis of zircons from metamorphic and meta igneous rocks provide new constraints on the timing and mechanism of thermal events associated with metamorphic and deformation events both magmatic dikes were formed around 1 95 ga and overprinted by later metamorphic event during 1 85 ga the s type granite at geological section gs 1 revealed its formation age was formed around 1959 34 ma n 3 mswd 1 5 and overprinted by later metamorphic event at 1840 8 ma n 10 mswd 1 2 additionally zircons from the mafic hp granulite in the gs2 are dominated by metamorphic ages of 1972 22 ma n 6 mswd 0 03 with few grains recording an age of 1856 29 ma n 3 mswd 0 17 in situ lu hf analyses on zircons from the quartz dioritic dike tonalitic dike mafic hp granulite and s type granite reveal that the magmatic dikes were derived from partial melting of 2 0 2 2 ga meta basic rocks and the s type granite was generated by melting of meta sediments beyond 840 50 c it suggested that those meta sediments were taken into the lower crust in a certain way at granulite facies conditions ca 1 95 ga ago a clockwise p t path acquired from mafic hp granulite revealed peak metamorphic conditions of 880 890 c and 11 13 kbar the temperature results recorded in the 1 95 ga zircons of mafic hp granulite similar to those of high pressure mineral assemblage in matrix suggest that the hp granulite metamorphism was approximately 1 95 ga ago according to the results above this contribution concludes that mafic and meta sedimentary rocks were metamorphosed together under high pressure granulite metamorphic conditions 1 95 ga ago followed by high temperature granulite facies melting event that gave rise to granite and s type granite at 1 95 ga the granulite facies metamorphic rocks and magmatic dikes in the huangtuyao graphite mine experienced the same evolution history after 1 95 ga the mafic and pelitic hp granulites in the huai an terrane are interpreted to be collisional products of the paleoproterozoic cratonization of the ncc

the planform mobility of river channel confluences insights from analysis of remotely sensed imagery river channel confluences are widely acknowledged as important geomorphological nodes that control the downstream routing of water and sediment and which are locations for the preservation of thick fluvial deposits overlying a basal scour despite their importance there has been little study of the stratigraphic characteristics of river junctions or the role of confluence morphodynamics in influencing stratigraphic character and preservation potential as a result although it is known that confluences can migrate through time models of confluence geomorphology and sedimentology are usually presented from the perspective that the confluence remains at a fixed location this is problematic for a number of reasons not least of which is the continuing debate over whether it is possible to discriminate between scour that has been generated by autocyclic processes such as confluence scour and that driven by allocyclic controls such as sea level change this paper investigates the spatial mobility of river confluences by using the 40 year record of landsat imagery to elucidate the styles rates of change and areal extent over which large river confluence scours may migrate on the basis of these observations a new classification of the types of confluence scour is proposed and applied to the amazon and ganges brahmaputra meghna gbm basins this analysis demonstrates that the drivers of confluence mobility are broadly the same as those that drive channel change more generally thus in the gbm basin a high sediment supply large variability in monsoonal driven discharge and easily erodible bank materials result in a catchment where over 80 of large confluences are mobile over this 40 year window conversely this figure is 40 for the amazon basin these results highlight that i the potential areal extent of confluence scours is much greater than previously assumed with the location of some confluences on the jamuna brahmaputra river migrating over a distance of 20 times the tributary channel width ii extensive migration in the confluence location is more common than currently assumed and iii confluence mobility is often tied to the lithological and hydrological characteristics of the drainage basins that determine sediment yield

the measurement of river bank erosion and lateral channel change a review a detailed review and chronological survey is presented of the various techniques which have been used for the measurement of river bank erosion and channel change the techniques are classified according to the time scales involved long intermediate and short and each is discussed with respect to accuracy and repeatability the methods covered include sedimentological evidence botanical evidence historical sources planimetric resurvey repeated cross profiling erosion pins and terrestrial photogrammetry prospects for future developments are also discussed

formation of the maturin foreland basin eastern venezuela thrust sheet loading or subduction dynamic topography 1 the maturin basin in eastern venezuela is considered a good example of a peripheral foreland basin earthquake and tomographic data indicate that eastern venezuela is affected by the oblique subduction of the south american plate underneath the caribbean plate new forward flexural isostatic modeling of eastern venezuela has been carried out in order to determine whether the maturin basin was generated purely by thrust sheet loading from the serrania and monagas foreland thrust belts a sequence of forward models from middle miocene to present was generated for 3 profiles across the serrania del interior thrust belt the monagas foreland thrust belt and the maturin foreland basin the predictions of these models are constrained using seismic reflection and well data the flexural isostatic modeling shows that thrust sheet loading associated with the serrania del interior and monagas foreland thrust belts is insufficient to generate the observed subsidence within the maturin basin dynamic fluid flow modeling of subduction related dynamic topography of eastern venezuela has been used to investigate the influence of south american plate subduction on the generation of the accommodation space observed in the maturin basin fluid flow modeling of subduction related dynamic topography suggests that the subduction of the south american lithospheric mantle caused downward deflection of the south american crust affecting the maturin basin and the serrania thrust belt this modeling suggests that the maturin basin subsidence has two components 55 related to thrust sheet loading and 45 driven by continental subduction

william river an outstanding example of channel widening and braiding caused by bed load addition the lower william river in northwestern saskatchewan canada presents an excellent and unambiguous example of rapid channel adjustment to abrupt additions of sandy bed load a relatively narrow and deep single channel stream as it flows northward to lake athabasca the river picks up a 40 fold increase of bed load over a 27 km reach as it encounters a large dune field just south of the lake as a result of the large infusion of eolian sand the channel develops a thoroughly braided pattern while undergoing a 5 fold increase in width and a 10 fold increase in width depth ratio

dehydration melting of amphibolite at 10 kbar the effects of temperature and time we have simulated the dehydration melting of a natural low k calcic amphibolite 67 4 hornblende 32 5 anorthite in piston cylinder experiments at 10 kbar and 750 1000 c for 1 9 days the solidus temperature is lower than 750 c garnet appears at 850 c the overall reaction is hb p l cpx al hb ca hb ga opx three stages of reaction are 1 melting dominated by the growth of clinopyroxene and garnet with little change in composition of liquid or garnet 2 a reversal of this reaction between 875 c and 900 c with decreases in the amounts of liquid and garnet and 3 a large increase in liquid along with the loss of hornblende and decrease of plagioclase while clinopyroxene and garnet increase garnet is enriched in pyrope and zoned from fe cores to mg edges range 3 mol pyrope liquid composition is enriched first in an to 950 c and then in ab the liquids are more calcic and aluminous than natural tonalites which is attributed to the plagioclase composition an90 the formation of peraluminous liquid from the metaluminous amphibolite is caused by anorthite not h2o saturated conditions the results are consistent with an amphibolite phase diagram with relatively high solidus temperatures in the garnet absent field 900 1000 c but with a solidus backbend at 7 9 kbar coincident with the garnet in boundary hornblende breakdown due to garnet formation in a closed system must make h2o available for h2o undersaturated melting right down to the h2o saturated solidus below 700 c which defines a large low temperature pt area where hydrous granitoid melts can be generated with residual garnet and hornblende

propagation tectonics and multiple accretionary processes of the qinling orogen abstract the qinling orogen was built through collision between the north china and south china blocks previous detailed geological geochemical and geochronological investigations revealed that the mountain range can be divided into four tectonic units with distinct tectono lithostratigraphy which are from north to south the southern sector of the north china block north qinling belt south qinling belt and northern sector of the south china block separated by the kuanping shangdan and mianlue sutures according to the petrology geochemistry and geochronology of ophiolitic melanges and related magmatic rocks as well as the features of sedimentary units we think that the north china block the north qinling belt and the south china block were originally independent continental units while the south qinling belt had been the northern part of the south china block these units experienced three episodes of accretionary tectonic processes and amalgamation from south to north the neoproterozoic accretion took place along the luonan luanchuan fault and kuanping ophiolitic melange belt as a result of southward subduction and subsequent collision between the north qinling and north china blocks during ca 1 0 0 8 ga related to the formation of the supercontinent of rodinia the paleozoic accretion occurred along the shangdan suture resulted from northward subduction of oceanic lithosphere in the early paleozoic and subsequent continental subduction in the late paleozoic late triassic accretion took place along the mianlue suture between the south qinling and south china blocks due to northward subduction of the mianlue oceanic lithosphere during the permian early triassic and subsequent collision in the late triassic after the late triassic collision along the mianlue suture the whole qinling mountain range entered the phase of intense intracontinental deformation

a tale of amalgamation of three permo triassic collage systems in central asia oroclines sutures and terminal accretion the central asian orogenic belt records the accretion and convergence of three collage systems that were finally rotated into two major oroclines the mongolia collage system was a long n s oriented composite ribbon that was rotated to its current orientation when the mongol okhotsk orocline was formed the components of the kazakhstan collage system were welded together into a long single composite arc that was bent to form the kazakhstan orocline the cratons of tarim and north china were united and sutured by the beishan orogen which terminated with formation of the solonker suture in northern china all components of the three collage systems were generated by the neoproterozoic and were amalgamated in the permo triassic the central asian orogenic belt evolved by multiple convergence and accretion of many orogenic components during multiple phases of amalgamation followed by two phases of orocline rotation

provenance of meso to neoproterozoic cover sediments at the ming tombs beijing north china craton an integrated study of u pb dating and hf isotopic measurement of detrital zircons and whole rock geochemistry abstract the north china craton ncc was subjected to an extensional regime after the luliang movement at 1 8 ga and then was covered by an extensive meso to neoproterozoic sedimentary succession namely the changcheng jixian and qingbaikou groups in ascending order we report age spectra for detrital zircons and monazites hf isotopic systematics of detrital zircons and whole rock chemical and nd isotopic compositions for sediments from the succession in the ming tombs area beijing one of the typical meso to neoproterozoic areas in the ncc detrital zircons of six sedimentary samples have two distinct age peaks at 2 52 ga and 1 85 ga there are some detrital zircons at 2 4 2 0 ga but none at 2 3 ga and only a few 2 7 ga the detrital zircon age spectra change with time sediments in the lower succession changcheng group and in the upper successions jixian and qinbaikou groups are dominated by significant detrital zircon populations of late neoarchean and late paleoproterozoic ages respectively the 2 5 ga detrital zircons of the changcheng group have ehf 2 5 ga values and tdm hf model ages mainly ranging from 2 to 7 and 2 8 to 2 7 ga respectively detrital monazites of a sample from the jixian group exhibit a major age peak between 1 95 and 1 80 ga with some data between 2 0 and 1 95 ga the sedimentary rocks of the changcheng group are characterized by high k2o contents mostly 7 09 15 20 and insignificant eu anomalies eu eu 0 71 1 16 they have tdm nd model ages ranging from 2 70 to 2 43 ga being older than the tdm nd ages 2 11 and 1 99 ga of sedimentary samples from the qingbaikou group based on a comparison with ages for the early precambrian 1 8 ga basement of the ncc it can be concluded that 1 the sediments of the meso to neoproterozoic cover were undoubtedly derived from the ncc itself or once neighboring terranes 2 variations in the detrital zircon age spectra from the lower to the upper successions reflect provenance evolution in that the lower crustal late paleoproterozoic rocks were exposed at the surface after the upper crustal late neoarchean rocks had already been eroded

multiple mafic magmatic and high grade metamorphic events revealed by zircons from meta mafic rocks in the daqingshan wulashan complex of the khondalite belt north china craton abstract meta mafic rocks in the daqingshan wulashan complex in the central segment of the paleoproterozoic khondalite belt of the north china craton are composed predominantly of garnet bearing mafic granulites mafic granulites and amphibolites they occur mainly as irregular lenses and as a set of deformed dyke sill swarms within dioritic granitic orthogneisses and al rich paragneisses an integrated study involving laser raman and scanning electron microscope analyses of mineral inclusions from zircons cathodoluminescence imaging and la icp ms u pb dating of zircons shows that magmatic zircons in these meta mafic rocks record four age groups of 2500 2450 2300 2100 1970 1930 and ca 1850 ma indicating multiple mafic magmatic events occurred within the daqingshan wulashan complex in addition four groups of metamorphic zircons with ages of ca 2450 ca 1950 ca 1900 and ca 1850 ma are present within these meta mafic rocks zircons of the first oldest group contain medium to low pressure mlp granulite facies mineral inclusions of orthopyroxene clinopyoxene amphibole plagioclase quartz and yield ages of ca 2450 ma representing the timing of the early mlp granulite facies metamorphic event within the daqingshan wulashan reworked basement complex in contrast zircons of the second group are free of mineral inclusions and yield ages of ca 1950 ma which is considered to be the timing of the regional high grade metamorphic and structural event probably caused by the collision of the yinshan and ordos blocks zircons of the third group contain mlp granulite facies mineral inclusions of orthopyroxene clinopyoxene amphibole plagioclase quartz similar to the zircons of the first group but yield younger ages of ca 1900 ma interpreted as the timing of the regional mlp granulite facies metamorphism that most likely occurred during the early stage of post orogenic exhumation of the daqingshan wulashan complex zircons of the fourth group contain mlp amphibolite facies mineral inclusions of clinopyoxene amphibole plagioclase quartz and yield the youngest ages of ca 1850 ma which probably represent the timing of the regional amphibolite facies metamorphism during the late stage of post orogenic exhumation of the daqingshan wulashan complex combined with previous geological data of the khondalite belt it is speculated that the multiple mafic magmatism and contemporaneous high grade metamorphism of the daqingshan wulashan complex from the khondalite belt were most likely generated by an orogenic process involving early subduction collision and late exhumation in an active continental margin arc

constraints on timing of peak and retrograde metamorphism in the dabie shan ultrahigh pressure metamorphic belt east central china using u th pb dating of zircon and monazite abstract the dabie shan ultrahigh pressure metamorphic uhpm belt occupies the suture between the yangtze and sino korean blocks in east central china the timing of uhpm in the dabie belt is controversial and most recent data come from dating of zircons monazite has recently been recognized as useful for dating of multiple tectonic events due to its preservation of multiple growth zones and monazite growth has been documented to occur during prograde peak and retrograde metamorphism zircons and monazites from uhp mafic rocks from maowu and a uhp jadeite quartzite from shuanghe were imaged in thin sections and in mineral separates and dated using a high resolution ion microprobe maowu mafic rocks are unique in that they contain high light rare earth element concentrations and therefore contain abundant monazite maowu eclogites and garnet pyroxenites contain zircons with mean 206 pb 238 u age of 230 4 ma and monazites from a clinopyroxenite have 208 pb 232 th ages of 209 4 ma multiple lines of evidence suggest that the measured ages represent the timing of new growth and recrystallization and are not cooling ages i e they do not correspond to the cessation of diffusional pb loss as rocks cooled below the closure temperature during exhumation shuanghe jadeite quartzites contain zircons with cores that define a discordia with an upper intercept age 1921 22 ma and lower intercept age of 236 32 ma interpreted to represent the ages of the source rocks and of peak metamorphism respectively zircon rims contain jadeite inclusions that restrict growth to pressures greater than 1 5 gpa their pooled age of 238 3 ma agrees well with the lower intercept defined by cores suggesting that pb loss from cores and growth of new rims occurred during uhpm at 235 240 ma monazite records multiple events during retrograde metamorphism maowu clinopyroxenite and shuanghe jadeite quartzite experienced monazite growth at 209 ma interpreted to reflect regional amphibolite facies overprinting resulting from pervasive retrograde fluid infiltration only the jadeite quartzite records growth at 223 1 ma estimated exhumation rates for maowu are 7 8 km ma

la icp ms u pb zircon ages of the qianlishan complex constrains on the evolution of the khondalite belt in the western block of the north china craton abstract the qianlishan complex is located in the westernmost part of the khondalite belt a continent continent collisional belt along which the yinshan block in the north and the ordos block in the south amalgamated to form the western block which then collided with the eastern block along the trans north china orogen to form the north china craton the complex is dominated by high grade supracrustal rocks and minor s type granites of which the supracrustals consist of graphite bearing sillimanite garnet gneiss garnet quartzite felsic paragneiss calc silicate rock and marble cl images reveal the existence of detrital and metamorphic zircons in major rocks of the qianlishan complex in most cases detrital zircons occur as either single grains with oscillatory zoning or oscillatory zoning cores typical of igneous origin which are surrounded by metamorphic overgrowth rims that are structureless high bright and low in th u ratio detrital zircons from the qianlishan complex yield nearly concordant 207 pb 206 pb ages ranging from 2 3 to 2 0 ga suggesting that the protoliths of the high grade supracrustal rocks in the qianlishan complex were deposited at some time after 2 0 ga metamorphic zircons yield two age populations with one at 1 95 ga and another at 1 92 ga of which the former is interpreted as the timing of the collision between the yinshan and ordos blocks to form the western block whereas the later is considered to be the age of subsequent post orogenic extensional event minor s type granites were emplaced at 1 88 ga as a result of partial melting of supracrustals at the stage of the exhumation of the qianlishan complex these new zircon ages combined with structural and metamorphic considerations enable resolution of the tectonothermal events involving the collision between the yinshan and ordos blocks to form the western block followed by the post collisional extension and subsequent exhumation of the khondalite belt

cambrian tectonic evolution of the northwestern ordos terrane north china constraints of stratigraphy sedimentology and zircon u pb geochronology the helanshan tectonic belt and its environs within the northwestern ordos terrane western north china craton have been envisaged as aulacogen or plate boundary separating the alxa massif from the ordos block during cambrian however it conflicts with geological and geophysical facts this study presents integrated stratigraphic sedimentary and geochronological data from the cambrian strata in the northwestern ordos terrane to constrain the cambrian tectonic evolutionary process of the western north china craton the cambrian successions comprise in ascending order evaporite platform facies restricted open platform facies and evaporite platform facies featuring a vertical transgression regression sequence an eastward shoaling paleogeography was documented evidenced by shallow water dolomite layers enriched in the east and the detritus derived from northeast ages of detrital zircons span from 2566 to 1649 ma with predominant two peaks around 2500 and 1850 ma suggesting these cambrian detritus were sourced from the archean paleoproterozoic rocks in the yimeng highland comparable fossil associations and unified paleogeography system supported that there was no cambrian plate boundary in the northwestern ordos terrane implying the alxa massif was part of the north china craton during the cambrian the westward deepening facies differentiation with low angle topography contradicts with the tectonic model of helan aulacogen in combination with the relatively low subsidence rate and rare magmatic metamorphic activities we argue that the cambrian shallow marine system in the western north china craton was the continent ward extension of passive continental margin basin system related to the spreading of the paleo asian ocean

channel stability in bed load dominated streams with nonerodible banks inferences from experiments in a sinuous flume 1 experimental and computational evidence shows that bed state adjustments can accommodate a twofold to fourfold change in sediment supply without otherwise affecting the morphology of a sinuous gravel bed stream channel with nonerodible banks in contrast previously published experiments of the same kind but with erodible channel banks suggest that changes in sediment supply are taken up almost exclusively by changes in channel sinuosity and hence the reach average water surface gradient it appears that stream channel response to changes in sediment supply is strongly conditioned by the nature of the stream boundary and that determines which of several potential adjustments are possible

semi determinate hydraulic geometry of river channels south island new zealand data to describe the morphologic hydrologic and sedimentologic characteristics of 72 south island new zealand rivers were collected and analysed nearly 70 per cent of variation in channel morphology is accounted for by differences in cross sectional area slope and cross section shape only 53 per cent of the morphologic variability could be statistically explained by the hydrologic and sediment variables used the level of explanation varied for different morphologic variables nearly 90 per cent of the variability in cross sectional area could be explained but aspect ratio maximum depth divided by hydraulic radius was completely independent apart from the inadequacy of the measured variables as indices of the true underlying controlling factors and the imperfect measurement and sampling procedures the low level of explanation is probably due to the influence of factors such as floodplain vegetation high quasi random variability in bark sediment character boundary effects imposed by bedrock bluffs and the precise sequence of flood events none of which are easily quantified in addition observations indicate that there is a large random variation in channel form which cannot be related to any factor an attempt to relate channel morphology to flow variability using simple indices of the latter was unsuccessful

empirical calibration of the silica ca tschermak s anorthite scan geobarometer the pressure sensitive equilibrium among anorthite quartz and the ca tschermak component in clinopyroxene caal2sio6 cats caal2siocpx6 sioqtz2 caal2si2opl8 scan can be used as a geobarometer in granulites with the proper assemblage and has been calibrated using mineral composition data from partial melting experiments of natural assemblages and from phase equilibrium experiments on the end member cmas system the experimental data cover the p t range 4 32 kbar and 900 1400 c linear least squares regression analysis of the experimental data resulted in the following empirical expressions for pressure in terms of composition and temperature p 5 066 0 760 1300 800 t lnk276 16 t 2 5 kbar orp 6 330 0 116 lnk301 9 t 1 0 kbar where k aplan acpxcats the first equation incorporates an enthalpy term but is less accurate than the second equation in which the enthalpy of reaction is ignored application of these expressions to natural and experimental equilibrium mineral assemblages demonstrates that the empirical barometers are applicable over a wide range of pressures 4 kbar temperatures 700 c and bulk compositions mg 32 5

high pressure granulites in the sanggan area north china craton metamorphic evolution p t paths and geotectonic significance high pressure basic granulites are widely distributed as enclaves and sheet like blocks in the huaian ttg gneiss terrane in the sanggan area of the central zone of the north china craton four stages of the metamorphic history have been recognised in mineral assemblages based on inclusion exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles the p t conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers the low ca core of growth zoned garnet along with inclusion minerals defines a prograde assemblage m1 of garnet clinopyroxene plagioclase quartz yielding 700 c and 10 kbar the peak of metamorphism at about 750 870 c and 11 14 5 kbar m2 is defined by high ca domains in garnet interiors and inclusion minerals of clinopyroxene plagioclase and quartz kelyphites or coronas of orthopyroxene plagioclase magnetite around garnet porphyroblasts indicate garnet breakdown reactions m3 at conditions around 770 830 c and 8 5 10 5 kbar garnet exsolution lamellae in clinopyroxene and kelyphites of amphibole plagioclase around garnet formed during the cooling process at about 500 650 c and 5 5 8 kbar m4 these results help define a sequential p t path containing prograde near isothermal decompression itd and near isobaric cooling ibc stages the clockwise hybrid itd and ibc p t paths of the hp granulites in the sanggan area imply a model of thickening followed by extension in a collisional environment furthermore the relatively high pressures 6 14 5 kbar of the four metamorphic stages and the geometry of the p t paths suggest that the hp granulites together with their host huaian ttg gneisses represent the lower plate in a crust thickened during collision the corresponding upper plate might be the tectonically overlying khondalite series which was subjected to medium to low pressure mp lp 7 4 kbar granulite facies metamorphism with a clockwise p t path including an itd segment both the hp and the mp lp granulite facies events occurred contemporaneously at c 1 90 1 85 ga in a collisional environment created by the assembly process of the north china craton

sedimentary record of triassic intraplate extension in north china evidence from the nonmarine nw ordos basin helan shan and zhuozi shan abstract the helan shan and zhuozi shan of the nw ordos basin china contain thick up to 4 km sequences of nonmarine triassic strata these rocks represent a major intraplate sedimentary basin the paleogeography tectonic setting and provenance of which are poorly understood and controversial studies of the sedimentary geology of the basin supported by new palinspastic reconstruction of younger deformation demonstrate that the basin filled from three sides by fluvial lacustrine deltaic and alluvial fan depositional systems the basin forms a westward thickening wedge that reaches its maximum thickness along the western margin of the helan shan and thins to a relatively constant 600 800 m east of the zhuozi shan the stratigraphy of the basin is strongly asymmetric alluvial fan strata are restricted to the extreme western margin of the basin and interfinger with axial fluvial deposits low in the section and deep lacustrine facies high in the section much of the eastern part of the basin is dominated by west flowing meandering river and deltaic systems large structures of triassic age have not been identified in the helan shan or zhuozi shan but small triassic normal faults have been documented in the western and central helan shan these characteristics most strongly support an extensional origin for the triassic basin in nw ordos the basin is interpreted to have been a north trending half graben bound along its western margin by an east dipping normal fault presently concealed beneath quaternary cover west of the helan shan the eastern margin now found in the zhuozi shan has simple ramp margin geometry driving mechanisms for this extension are not obvious due to limited documentation of triassic structure throughout the region but probably relate to far field stresses from the qinling or jinsha active margins interacting with the stable ordos block

patterns of alluvial rivers the pattern planform of a river can be considered at vastly different scales depending upon both the size of the river and the part of the fluvial system that is under consideration figure 1 for example in the broadest sense river patterns comprise a drainage network dendritic parallel trellis etc figure la the type of pattern is of interest to geomorphologists and geologists who interpret geologic conditions from aerial photographs at another scale a river reach which in figure lb is meandering is of interest to the geomorphologist who is interested in what that pattern reveals about river history and behavior and to the engineer who is charged with maintaining navigation and preventing major instability when a single meander is examined figure 1 c the hydraulics offtow the sediment transport and the potential for bank erosion are of concern in addition the sedimentologist is interested in the distribution of sediment within the bend bed forms within the channel figure ld and sedimentary structures figure ie which also establish a component of roughness for the hydraulic engineer finally the individual grains figure if provide geologic information on the sediment sources the nature of sediment loads and the feasibility of dredging for gravel there is an interaction of hydrology hydraulics geology and geomorphology at all scales which emphasizes the point that the fluvial system as a whole cannot be ignored even though only a component of the system is to be studied in this review only the patterns or planforms of alluvial rivers are discussed although it is apparent that the hydrologic and sediment yield characteristics of the drainage basin figure la as well as its geologic history cannot be ignored in the explanation of the pattern of any river

polyphase deformation of the fuping complex trans north china orogen structures shrimp u pb zircon ages and tectonic implications abstract the fuping complex is located in the central part of the trans north china orogen tnco a paleoproterozoic collisional belt along which the eastern and western blocks were assembled to form the north china craton three stages of deformation d 1 d 3 have been identified in the fuping complex d 1 formed tight to isoclinal folds f 1 penetrative foliation s 1 and mineral lineation l 1 which resulted from the early nw se orientated thrusting and shearing d 2 progressively overprinted d 1 and was dominated by mostly nw se shortening and top to the se e thrusting under a compressive environment d 1 and d 2 occurred in association with the peak metamorphism m 1 d 3 occurred during the exhumation after the crustal thickening producing regional scale nww see trending open folds f 3 and associated low angle detachment faults and was responsible for the isothermal decompression m 2 metamorphism of the complex following the d 3 deformation the complex was exhumed to shallow crustal levels where the rocks underwent the retrogressive cooling m 3 metamorphism syn tectonic leucocratic dykes are widespread in the fuping complex two leucocratic dykes that did not experience d 1 but underwent d 2 yield shrimp zircon ages of 1843 12 ma and 1844 18 ma which indicate that d 1 must have occurred at some time earlier than 1844 ma whereas d 2 must have occurred at some time later than this age two weakly deformed post d 2 leucocratic dykes yield shrimp zircon ages of 1817 14 ma and 1815 45 ma suggesting that the d 2 deformation occurred in the period 1843 1815 ma the new shrimp data indicate that the collision related deformation of the fuping complex must have occurred in the paleoproterozoic not in the late archean structural and geochronological data presented in this study in association with previous lithological and geochemical data suggest that the fuping complex may have been located in the hinterland of an eastward subduction zone and underwent intensive deformation and metamorphism involved in the final collision between the western and eastern blocks at 1 85 ga

the alashan terrane did not amalgamate with north china block by the late permian evidence from carboniferous and permian paleomagnetic results abstract rock magnetic and paleomagnetic studies have been carried out on the early carboniferous limestones and the late permian purple sandstones sampled in the eastern alashan terrane alt northwest of china two components were isolated from the early carboniferous limestone by thermal progressive demagnetisation a low unblocking temperature component ltc of recent origin a pre folding medium temperature component mtc the paleomagnetic pole is 13 1 n 11 0 e a 95 7 0 that is probably the result of the hydrothermal fluids from the qilian orgenic belt acquired during the late carboniferous early permian also two components were separated from the late permian purple sandstone by thermal progressive demagnetisation the ltc with the recent viscous remanent magnetisation and the higher temperature component htc revealed from three sections which has passed a regional fold test at the 95 probability level and reversal test suggesting a primary characteristic magnetisation the corresponding paleomagnetic pole is 27 2 n 18 8 e a 95 12 0 the apparent polar wander path including early carboniferous late carboniferous early permian late permian and early middle triassic poles of the alt is significantly different with those of the ncb comparison of the apwps between the alt and ncb shows a strong similarity if the apwp of hexi corridor alashan rotated counterclockwise around an euler pole at 44 n 84 e by 32 then the coeval apw path of the alt overlaps to that of the ncb this result indicates that the alt migrated to the ncb after the early middle triassic along a tectonic boundary located between helanshan mountain and zhuozishan mountain and finally amalgamated to the ncb before the early cretaceous

early mesozoic basin development in north china indications of cratonic deformation we integrated a systematic sedimentary data into a regional early mesozoic stratigraphic framework which demonstrated a detailed picture of spatiotemporal variations in basin deposition and formation in the north china craton the early mesozoic basin sedimentary evolution is utilized to interpret polyphase tectonism and to unravel the craton deformation the late triassic nearly wnw trending giant intracratonic ordos basin was widely distributed across most of north china craton with a southern wedge top depozone along the northern east qilian qinling orogenic belt and a northwestern rift depozone along the helanshan the continuous subsidence and deposition within the basin were dominantly related to the thrust load of the east qilian qinling belt and inferred mantle flow effects associated with paleotethys plate subduction and the rift in the northwestern ordos was driven by nearly north vergent compression of the eastern north qilian north qinling active margins with the stable north china craton this intracratonic ordos basin formation initiated the deformation of the north china craton formation of the jurassic nne trending walled intracratonic ordos basin and the broken flexural basins indicates the north china craton underwent the second even more abroad nearly nne trending crustal deformation with lithosphere thickening in the eastern part of the north china craton and dynamic subsidence in the west which may have been driven by nearly northwestward subduction of the izanagi plate and the eastward extrusion and underthrusting of the western north china craton crustal basement

kinematics and geochronology of multistage ductile deformation along the eastern alxa block nw china new constraints on the relationship between the north china plate and the alxa block abstract the ne trending bayanwula shan lang shan is an important tectonic belt lying between the north china plate ncp to the east and the alxa block to the west an understanding of its nature and the timing of deformation are essential to understand the relationship between the ncp and the alxa block two phases of ductile deformation have been observed in this belt large scale top to the west ductile thrusting characterized the early deformation in the bayanwula shan lang shan nearly east west trending quartz stretching lineations and lineations formed by amphibole and biotite are well developed different types of sheath and oblique folds with east west trending fold hinges are also developed in the region the shear strain of this ductile thrust is up to 17 the ductile deformation may have resulted from the top to the west thrusting of the northern part of the ncp over the alxa block and may have occurred ca 351 ma biotite 40 ar 39 ar age later ductile deformation was expressed as ne trending sinistral shear along the entire bayanwula shan lang shan and likely occurred ca 250 ma biotite and muscovite 40 ar 39 ar ages this shear may have resulted from the collision between the yangtze and north china plates to the south during the triassic combined with recently obtained detrital zircon u pb ages for the area the ductile deformation events in the eastern alxa block indicate that the block may not have been part of the ncp at least before the end of the devonian both blocks were located in the paleo asian ocean during the paleozoic and collided or amalgamated with each other at the end of the devonian

sediment supply as a driver of river meandering and floodplain evolution in the amazon basin the role of externally imposed sediment supplies on the evolution of meandering rivers and their floodplains is poorly understood despite analytical advances in our physical understanding of river meandering1 2 the amazon river basin hosts tributaries that are largely unaffected by engineering controls and hold a range of sediment loads allowing us to explore the influence that sediment supply has on river evolution here we calculate average annual rates of meander migration within 20 reaches in the amazon basin from landsat imagery spanning 1985 2013 we find that rivers with high sediment loads experience annual migration rates that are higher than those of rivers with lower sediment loads meander cutoff also occurs more frequently along rivers with higher sediment loads differences in meander migration and cutoff rates between the study reaches are not explained by differences in channel slope or river discharge because faster meander migration and higher cutoff rates lead to increased sediment storage space in the resulting oxbows we suggest that sediment supply modulates the reshaping of floodplain environments by meandering rivers we conclude that imposed sediment loads influence planform changes in lowland rivers across the amazon

anthropic aggradation of the waiho river westland new zealand microscale modelling long term aggradation of the waiho river south westland new zealand has now raised the head of its alluvial fan to unprecedented elevations in its natural state the river would like all other major rivers in the area be somewhat incised into its fanhead the only relevant factor able to account for the aggradation is the presence of control banks stopbanks in local parlance that restrict the ability of the river to move over the whole of its natural fanhead a 1 3333 scale physical hydraulic model a microscale model was used to study this situation an alluvial fan was generated in the model and allowed to develop to equilibrium with steady inputs of water and sediment within boundaries geometrically similar to those of the natural unrestricted waiho river the boundaries were then altered to represent the presence of the stopbanks and the fan allowed to continue evolving under the same water and sediment inputs the model fanhead aggraded in a spatial pattern similar to that recorded on the waiho taking into consideration the limitations of microscale modelling these results indicate that the aggradation in the waiho is a result of the lateral restriction of the river by stopbanks this poses fundamental questions about the variables that control the behaviour of alluvial fans the results also suggest that microscale modelling can be used to make reliable quantitative predictions of the effects of engineering works on rivers in spite of the low level of dynamic similarity with the prototype compared to that in larger scale models copyright 2003 john wiley sons ltd

world wide delivery of river sediment to the oceans new data and new estimates from old data show that rivers with large sediment loads annual discharges greater than about 15 times 10 6 tons contribute about 7 times 10 9 tons of suspended sediment to the ocean yearly extrapolating available data for all drainage basins the total suspended sediment delivered by all rivers to the oceans is about 13 5 times 10 9 tons annually bedload and flood discharges may account for an additional 1 2 times 10 9 tons about 70 of this total is derived from southern asia and the larger islands in the pacific and indian oceans where sediment yields are much greater than for other drainage basins

net local removal of floodplain sediment by river meander migration abstract erosion from the banks of meandering rivers causes a local influx of sediment to the river channel most of the eroded volume is usually replaced in nearby point bars however the typical geometry of river bends precludes the local replacement of all eroded material because i point bars tend to be built to a lower elevation than cutbanks and ii point bars tend to be shorter than the eroding portion of cutbanks because of channel curvature in a floodplain that is in equilibrium i e neither increasing nor decreasing in volume sediment eroded from cut banks must be replaced elsewhere on the floodplain the local imbalance caused by differences in bank height should be balanced primarily by overbank deposition while the local imbalance caused by curvature should be balanced primarily by deposition in abandoned stream courses or oxbow lakes estimates of these local imbalances based on remotely sensed measurements of bank geometry and channel migration have been made on four systems in the united states a 91 km reach of the pearl river in louisiana and mississippi a 62 km reach of the bogue chitto river in louisiana a 35 km reach of the neuse river in north carolina and a 2 7 km reach of the vermillion river in minnesota for these systems the total local imbalance integrated over many bends ranged from 0 36 to 2 27 m 3 yr m of valley length 0 15 to 1 32 m 3 yr m of channel length or 7 to 45 of gross cutbank erosion with a typical value of about 17 when compared with gauged suspended sediment data the measurements provide estimates of the relative importance of floodplains for storing material transported by a river system the data suggest that even if the studied systems are near long term mass balance equilibrium as opposed to undergoing net deposition or erosion almost all of the sand and in some cases much of the silt and clay in transit through these systems is likely to have spent some time stored in an upstream floodplain

decadal scale morphological adjustment of a lowland tropical river abstract compared with temperate regions much less is known about the dynamics of tropical river systems tropical rivers are typically characterised by pronounced seasonal changes in precipitation large sediment loads and high rates of lateral channel migration across often very low gradient and densely populated floodplains understanding the controls on channel migration or change is integral to our ability to fully predict and build resilience against flood risk and wider river related hazards here we analyse channel and confluence migration over the last 40 years along a 85 km reach of the cagayan river and one of its tributaries the pinacanauan de ilagan luzon philippines using optical satellite imagery captured during this period combining this with spatial variations in channel pattern valley width and new bed material grain size data we demonstrate that sediment transport and deposition are key drivers of the observed tropical channel morphodynamics in this region the high sediment supply generated in the catchment headwaters by mass wasting of hillslopes triggered especially in typhoons results in high aggradation rates and channel widening on the lower gradient alluvial plain we suggest that this aggradation enhances local confluence and lateral channel migration rates which can reach 300 m per decade and that lateral migration rates of tropical rivers are typically greater than those of temperate rivers channel morphodynamics have implications for how to best manage these types of tropical river systems where hard bank protection structures may result in a complex geomorphic response and flood risk mapping may need to include assessment of sensitivity to varying channel position and topography

archean paleoproterozoic crustal evolution of the ordos block in the north china craton constraints from zircon u pb geochronology and hf isotopes for gneissic granitoids of the basement abstract the ordos block has been considered as an archean micro block in north china craton however its formation and evolution are poorly understood for a long time due to lack of available basement rocks our la icpms and sims zircon u pb dating of gneissic granitoids from the ordos block basement identified two distinct periods of granitic magmatism at 2 5 ma and 2 2 2 0 ga and two phases of metamorphisms at 1 95 ga and 1 85 ga respectively of which most zircons of 2 5 ga granitoids show positive hf t values ranging from 0 1 to 4 1 with a peak t dm of 2 7 ga minor zircons have negative hf t values from 3 8 to 0 02 and t hf c of 2 8 3 1 ga moreover the 3 40 ga inherited zircon and zircons with t hf c of 3 1 4 2 ga were found from 2 2 ga to 2 0 ga granitoids these suggest that there existed a neoarchean crust in the ordos block which underwent a major crustal growth at 2 7 ga and granitic magmatism at 2 5 ga highly consisting with 2 7 ga crustal growth and 2 5 ga magmatism resulted from juvenile crustal reworking in north china craton the zircons from 2 2 ga to 2 0 ga granitoids in ordos block basement display variable hf isotopic composition of positive to negative hf t values much similar to those of 2 2 2 0 ga granitoids related to continental margin arc setting in the trans north china orogen they were subsequently subjected to 1 95 and 1 85 ga metamorphisms which were widespread throughout the trans north china orogen based on the highly coherent zircon geochronology and hf isotopic composition between ordos block and other micro blocks of north china craton we propose that there is a neoarchean crust in ordos block which underwent a tectonic evolution similar to those in east block in later neoarchean and then experienced 2 2 2 0 ga granitic magmatism related to continental margin arc and 1 95 1 85 ga metamorphism resulted from collision between the east and west blocks along tnco

episodic mantle melting crustal reworking in the late neoarchean of the northwestern north china craton zircon ages of magmatic and metamorphic rocks from the yinshan block abstract the yinshan block in the northwestern north china craton comprises late neoarchean greenstones high grade metamorphic rocks granulite charnockite enderbite and amphibole gneiss diorites sanukitoid and mg adakite and granitoids predominantly trondhjemite tonalite a greenstone terrane comprises a volcanic sequence of predominant basalt and dacite with minor andesite zircons from a high mg andesite and an andesite have weighted mean 207pb 206pb ages of 2533 5 ma and 2510 7 ma respectively which reflect the timing of two distinct volcanic episodes these two either predated or postdated a major volcanic episode that formed a basalt dacite bimodal suite during 2515 10 basalt to 2516 10 ma dacite magmatic zircons from the high grade metamorphic rocks yielded weighted mean 207pb 206pb ages of 2544 5 ma for a mafic granulite 2521 6 ma for a garnetiferous gneiss 2535 5 ma for a charnockite and 2526 5 ma for an amphibole gneiss these ages reflect the minimum emplacement time of rocks that make up the protoliths of the high grade metamorphic complex the metamorphic ages of the high grade complex were determined at 2515 18 ma for mafic granulite and 2503 12 ma for garnetiferous gneiss zircons from a high mg diorite i e sanukitoid have a weighted mean age of 2520 6 ma which is significantly younger than the 2556 14 ma age of a magnesian diorite mg adakite from the same diorite massif these ages reflect distinctive magmatic pulses that generated the diorite massif a tonalite has a zircon age of 2516 7 ma indistinguishable from the 2520 9 ma age of a trondhjemite from the same trondhjemite tonalite dominated batholith a granitic compositional layer in the 2516 7 ma tonalite has zircon ages of 2515 6 ma high u zircon domains and 2512 22 ma low u zircon domains which reflect an intracrustal melting event the presence of an older crustal remnant ca 2600 ma prior to the formation of the yinshan block is indicated by a 2594 4 ma leucogranite xenolith in the diorite massif and by the widespread presence of ca 2583 3485 ma mostly ca 2700 ma zircon xenocrysts in a variety of volcanic and plutonic rocks an undeformed pegmatite dike in high mg andesite has an emplacement age of 2508 17 ma zircon an undeformed trondhjemite dike that cuts the supracrustal greenstone sequence yielded a zircon age of 2502 6 ma similar to that of the pegmatite dike and representing the latest archean magmatic event in the yinshan block these ages of undeformed granitoid dikes delimit the termination of thermo tectonic development of the yinshan block based on these new ages and published geological and geochronological data we conclude that the yinshan block is a typical neoarchean cratonic block not a segment of the so called paleoproterozoic inner mongolia north hebei orogen episodic mantle melting and resultant crustal reworking account for the thermo tectonic development of the yinshan block in a brief period ca 2556 2502 ma in the late neoarchean and based on their regional chronological correlations we argue for their craton scale significance

neoarchean to paleoproterozoic continental evolution and tectonic history of the north china craton a review rocks of archean age are widely distributed in the north china craton ncc and can be divided into high grade regions and granite greenstone belts even though most underwent amphibolite to granulite facies metamorphism quartzo feldspathic gneisses occupy 70 to 85 of the total exposure of archean rocks in the craton supracrustal rocks commonly consist of intercalated volcano sedimentary rocks including metabasites meta intermediate to acid rocks banded iron formations metapelites and minor marbles the oldest rocks outcrop over a small area of 4 km2 and are a quartzite amphibolite sequence that yield 3 8 3 5 ga isotopic ages chemical and geochronological data constrain the earliest main crust forming episode at ca 2 9 2 7 ga when most rocks were metamorphosed to upper amphibolite or granulite facies at moderate pressures mafic granulites can be classified into two textural types referred to as near isobaric cooling ibc and near isothermal decompression itd types high pressure granulites and retrograded eclogites occur mainly near the junction of the provinces of hebei shanxi and inner mongolia in the central ncc and in eastern shandong province of the eastern ncc as well as locally in henan province in the southern ncc these high pressure rocks show clockwise p t paths indicating decompression from 14 kb to 5 6 kb an exposed section interpreted as representing precambrian continental lower crust has also been identified in the northern part of the craton although some mafic ultramafic metamorphic rocks have been interpreted as remnants of ancient oceanic crust and mantle by a number of geologists geochemical and petrological evidence demonstrates that many supracrustal orthogneiss associations have island arc affinities therefore several tectonic models have been proposed including continent continent collision and island arc terrain accretion achieved by arc arc or arc microcontinent collision the ncc underwent multiple high grade metamorphic events of which the two most important took place during the intervals 2600 2450 ma and 1950 1750 ma which are considered to be related to early precambrian supercontinental cycles

the relationship between drainage density erosion rate and hilltop curvature implications for sediment transport processes drainage density is a fundamental landscape metric describing the extent of the fluvial network we compare the relationship between drainage density dd and erosion rate e using the channel hillslope integrated landscape development child numerical model we find that varying the channel slope exponent n in detachment limited fluvial incision models controls the relationship between dd and e with n 1 resulting in increasing dd with e if all other parameters are held constant this result is consistent when modeling both linear and nonlinear hillslope sediment flux we also test the relationship between dd and e in five soil mantled landscapes throughout the usa feather river ca san gabriel mountains ca boulder creek co guadalupe mountains nm and bitterroot national forest id for two of these field sites we compare dd to cosmogenic radionuclide crn derived erosion rates and for each site we use mean hilltop curvature as a proxy for erosion rate where crn derived erosion rates are not available we find that there is a significant positive relationship between dd e and hilltop curvature across every site with the exception of the san gabriel mountains ca this relationship is consistent with an n exponent greater than 1 suggesting that at higher erosion rates the transition between advective and diffusive processes occurs at smaller contributing areas in soil mantled landscapes

how important and different are tropical rivers an overview abstract tropical river systems wherein much of the drainage basin experiences tropical climate are strongly influenced by the annual and inter annual variations of the inter tropical convergence zone itcz and its derivative monsoonal winds rivers draining rainforests and those subjected to tropical monsoons typically demonstrate high runoff but with notable exceptions high rainfall intensities from burst weather events are common in the tropics the release of rain forming aerosols also appears to uniquely increase regional rainfall but its geomorphic manifestation is hard to detect compared to other more temperate river systems climate driven tropical rivers do not appear to transport a disproportionate amount of particulate load to the world s oceans and their warmer less viscous waters are less competent tropical biogeochemical environments do appear to influence the sedimentary environment multiple year hydrographs reveal that seasonality is a dominant feature of most tropical rivers but the rivers of papua new guinea are somewhat unique being less seasonally modulated modeled riverine suspended sediment flux through global catchments is used in conjunction with observational data for 35 tropical basins to highlight key basin scaling relationships a 50 year daily model simulation illuminates how precipitation relief lithology and drainage basin area affect sediment load yield and concentration local sediment yield within the amazon is highest near the andes but decreases towards the ocean as the river s discharge is diluted by water influxes from sediment deprived rainforest tributaries bedload is strongly affected by the hydraulic gradient and discharge and the interplay of these two parameters predicts foci of net bedload deposition or erosion rivers of the tropics have comparatively low inter annual variation in sediment yield

sediment supply and channel morphology in mountain river systems 2 single thread to braided transitions differences in sediment supply between single thread and braided channel types provide a long recognized though difficult to quantify pattern discrimination building on the results from our preceding paper we present a multiscale assessment of the sediment supply geomorphology and sediment transport characteristics of braided gravel bedded reaches in the northern rocky mountains usa first we present a quantitative theoretically based discriminant function that stratifies single thread and braided reaches on the basis of variations in bankfull sediment concentration and dimensionless discharge following the work of millar 2005 and eaton et al 2010 this function correctly classifies 50 of the 53 channel types where bed load concentrations are known second while channel pattern transitions are often linked to changes in slope field studies along sunlight creek wyoming show that downstream transitions between single thread and braided reaches are instead caused by valley constrictions and changes in the grain size of sediment from tributaries finally we demonstrate that the two dimensional variability in flow properties in braided reaches produces locally high values of shear stress and bed load transport yet bed load measurements and sediment transport calculations also show that sediment transport rates between adjoining braided and single thread reaches may be approximately equal where channels are near the pattern threshold and downstream variations in bank fortitude and channel constriction force pattern transitions taken together these results indicate that high bed load concentrations are fundamental to the braided channel planform and that braided channels likely reflect a quasi equilibrium state within watersheds with persistent high sediment supply

the source of granitic gneisses and migmatites in the antarctic peninsula a combined u pb shrimp and laser ablation hf isotope study of complex zircons zircons gneisses and migmatites collected from the antarctic peninsula have different core rim hafnium isotope ratio relationships depending on whether evidence for zircon dissolution is present or absent two samples contain inherited zircon that is partially dissolved in these samples the 176hf 177hf rations of the inherited zircon and new magmatic zircon rims are on average indistinguishable and consistent with in situ melting in such cases the hafnium isotopic composition of the melt was probably strongly influenced by the dissolved zircon component at the source variation in 176hf 177hf within the magmatic zircon rims from grain to grain suggests that hf isotopes were only partially homogenized during melt migration alternatively zircon growth may have taken place within small volumes of partial melt other samples do not preserve textural evidence for zircon dissolution during melt generation in these samples the 176hf 177hf values of the inherited zircon and new magmatic zircon rims are different the zircon rims apparently suggest a source of less evolved hafnium than that contained within the inherited zircon whether this relates to a separate juvenile source or alternatively is derived from minerals other than zircon at the source cannot be resolved inherited zircon irrespective of age has been strongly influenced by the reworking of a juvenile late mesoproterozoic source suggesting that such crust underlies the antarctic peninsula our results therefore suggest that hf isotope analyses provide great potential for future studies investigating the source and processes involved in the generation of crustal melts

bedrock erosion by root fracture and tree throw a coupled biogeomorphic model to explore the humped soil production function and the persistence of hillslope soils 1 in 1877 g k gilbert reasoned that bedrock erosion is maximized under an intermediate soil thickness and declines as soils become thinner or thicker subsequent analyses of this humped functional relationship proposed that thin soils are unstable and that perturbations in soil thickness would lead to runaway thinning or thickening of the soil to explore this issue we developed a numerical model that simulates the physical weathering of bedrock by root fracture and tree throw the coupled biogeomorphic model combines data on conifer population dynamics rootwad volumes tree throw frequency and soil creep from the pacific northwest usa although not hardwired into the model a humped relationship emerges between bedrock erosion and soil thickness the magnitudes of the predicted bedrock erosion rates and their functional dependency on soil thickness are consistent with independent field measurements from a coniferous landscape in the region imposed perturbations of soil erosion during model runs demonstrate that where bedrock weathering is episodic and localized hillslope soils do not exhibit runaway thinning or thickening the pit and mound topography created by tree throw produces an uneven distribution of soil thicknesses across a hillslope thus although episodes of increased erosion can lead to temporary soil thinning and even the exposure of bedrock patches local areas of thick soils remain these soil patches provide habitat for trees and serve as nucleation points for renewed bedrock erosion and soil production model results also suggest that where tree throw is a dominant weathering process the initial mantling of bedrock is not only a vertical process but also a lateral process soil mounds created by tree throw flatten over time spreading soil over bedrock surfaces

partial melting in amphibolites in a deep section of the sveconorwegian orogen sw sweden garnet amphibolite metataxites at the steningekusten nature reserve in southwestern sweden contain tonalitic patches and veins whole rock chemistry suggests that the protoliths were mafic igneous rocks with alkaline affinities orthopyroxene megacrysts are present in leucosome in parts of these garnet amphibolites but absent in others orthopyroxene megacrysts were formed by vapor absent melting initiated by incongruent melting of biotite followed by the breakdown of hornblende the net reaction was bt hbl pi qtz opx melt cpx gt melting occurred at pressures of approximately 1 gpa and temperatures which probably exceeded 800 degrees c pyroxenes are surrounded by hornblende quartz symplectites and hornblende in these coronas has distinctly lower concentrations of na k and ti than that in adjacent mesosorne the hornblende rims formed upon cooling and reaction with crystallizing melt this created a barrier to further reaction thus preserving the orthopyroxene megacrysts garnet amphibolite metatexites lacking pyroxene megacrysts have features characteristic of vapor present melting including lack of peritectic phases predicted by vapor absent melting reactions larger amounts of leucosome 14 versus 7 and less distinct melanosomes the variation in these migmatites reflects open system behavior either on a regional scale with the migration of aqueous fluids into the amphibolites or on a local scale with the migration of melt within the amphibolites zircons from all units have cl dark core domains that are dated at 1415 1390 ma the core zones are cut and overgrown by cl dark and cl bright rims that are dated at 975 965 ma the zircon rims are thin in the mesosome but are thicker in the leucosome suggesting that they formed during migmatization new growth of zircon associated with migmatization at ca 970 ma corresponds to the timing of crustal scale partial melting in the deep regions of the sveconorwegian orogen synchronous with east west extension and the intrusion of mafic dykes if partial melting was driven by an influx of aqueous fluids they were probably derived from a relatively cool source region which would indicate tectonic juxtaposition of hotter and cooler terranes c 2015 elsevier b v all rights reserved less

grain size sediment transport and alluvial channel pattern abstract the shields parameter a dimensionless bed shear stress which can be expressed as a ratio of the slope depth and characteristic properties of bed material in an alluvial channel has been observed in natural rivers to take on modal values for the regimes of bedload mixed load and suspension transport dade w b friend p f 1998 grain size sediment transport regime and channel slope in alluvial rivers j geology 106 661 675 such conditions correspond to channel geometries associated with conserved sediment flux and thus to a state of approximate equilibrium in which the channels are neither aggrading nor degrading to a significant degree these findings are extended here to accommodate the effects of channel width in the context of a comprehensive data set on stable perennial rivers the new analysis reconciles existing empirical and theoretical approaches to predict alluvial channel geometry

shrimp u pb zircon geochronology of the huai an complex constraints on late archean to paleoproterozoic crustal accretion and collision of the trans north china orogen

mesozoic contractional deformation in the middle of the asian tectonic collage the intraplate western ordos fold thrust belt china abstract intraplate deformation can occur at great distances from synchronous plate boundaries and is most likely the result of strain concentrations into weak zones these weak zones may be pre existing crustal heterogeneities such as faults thermally weakened zones due to magmatism and or the effects of a thick sedimentary cover or mechanical contrasts between adjacent crustal blocks intraplate deformation is not exclusively related to continent continent collision it can be the result of a change in subduction dynamics or the interaction of two convergent plate boundaries asia has many examples of active intraplate deformation that are thought to be related to the cenozoic indo asian collision although much attention is placed on active examples discerning controls on pre cenozoic asian intraplate deformation will lead to not only a better understanding of the processes involved in intraplate orogens but also the tectonic evolution of asia situated on either side of the yellow river along the western margin of the ordos plateau the intraplate western ordos fold thrust belt involves units as old as archean basement and as young as late jurassic the fold thrust belt is spectacularly exposed in two ranges the helan shan and the zhuozi shan cross sections drawn through the north trending fold thrust belt suggest 30 minimum shortening a paleocurrent reversal within the lower middle jurassic section is interpreted to mark the onset of contraction and mountain building a late jurassic synorogenic boulder conglomerate records the final deformation in the fold thrust belt synchronous with contraction and cutting across the fold thrust belt is an e w striking strike slip fault that displays right lateral drag and sub horizontal striae we interpret this fault as an accommodation structure in the thrust belt following e w contraction the thrust belt was dismembered by an n15 w trending left lateral strike slip fault the left lateral fault displaces the frontal portion of the thrust belt the zhuozi shan 62 km to the north relative to a more internal portion the helan shan we propose that the anomalous orientation and intraplate location of the western ordos fold thrust belt is a function of mechanical contrasts between a previously deformed area and a stable crustal block ordos to the east pre existing crustal weakness or anisotropy along the western margin of the ordos block is most likely related to either a late proterozoic early paleozoic aulacogen a triassic fault system or to both although its intraplate location and the complexities of mesozoic asian plate interactions make it difficult to relate deformation in the western ordos fold thrust belt to a specific contemporaneous plate boundary its n s trend may suggest a link with paleo pacific subduction along the eastern margin of asia

prediction of alluvial channel pattern of perennial rivers abstract purely braided meandering and straight channels can be considered as end members of a continuum of alluvial channel patterns several researchers have succeeded in separating channel patterns in fields defined by flow related parameters however the discriminators of the principal channel patterns derived from these diagrams all require some a priori knowledge of the channel geometry in this paper a method is presented which enables prediction of the equilibrium conditions for the occurrence of braided and high sinuosity meandering rivers in unconfined alluvial floodplains the method is based on two almost channel pattern independent boundary conditions median grain size of the river bed material and a potential specific stream power parameter related to bankfull discharge or mean annual flood and valley gradient this can be regarded as a potential maximum of the available flow energy corresponding to the minimum sinuosity condition p 1 based on an analysis of 228 datasets of measurement sites along rivers from many parts of the world an independent discriminating function was found that separates the occurrence of braided rivers and meandering rivers with p 1 5 the function applies to equilibrium conditions of rivers that neither incise nor show rapid aggradation with a bankfull or mean annual flood discharge above 10 m3 s and a median bed material grain size between 0 1 and 100 mm

temporal and spatial adjustments of channel migration and planform geometry responses to enso driven climate anomalies on the tropical freely meandering aguapei river sao paulo brazil

sampling surficial fluvial gravels the precision of size distribution percentile sediments abstract grid by number wolman sampling is commonly used to characterize the texture of surficial fluvial sediments assessments of sampling precision have concentrated on the median grain size but adequate representation of other grain size percentiles is paramount in a number of sedimentological contexts the field effort required to construct percentile sampling distributions by replication is prohibitive and the absence of any general size distribution for fluvial sediments precludes theoretical calculation of percentile standard errors bootstrapping is used in this paper to determine percentile standard errors for large samples from two gravel bar surfaces the improvement of percentile precision with sample size follows the theoretical expectation hence improvement in percentile recision is achieved only at the expense of much greater sampling effort once sample sizes reach about 400 stones however absolute percentile standard errors differ significantly from theoretically derived values of particular interest is the stability of coarser percentile estimates in coarse skewed distributions d95 estimates may be as precise as d50 estimates for a given sample size the sensitivity of the precision of estimated percentiles to distribution shape is examined using synthetic skewed and bimodal grain size distributions the absolute precision of percentile estimates is shown to vary with the grain size distribution of a particular sediment our field results provide a reference for further work

detrital zircon reference for the north china block abstract u pb analyses of 250 single detrital zircons from upper proterozoic to ordovician strata collected from the zhuozi shan in north central china provide a detrital zircon reference for the north china block a major crustal entity in the asian tectonic collage the results which range in age from 1 72 to 2 97 ga shed new light on the age of the crystalline basement in north china much of which is covered by younger sedimentary units in addition this detrital zircon reference can be used to help determine the provenance of clastic sedimentary units and for assessing validity of paleogeographic and regional tectonic models that include the complex history of asian continental amalgamation terrane accretion and subsequent translation that is ongoing today

tropical cyclone driven erosion of the terrestrial biosphere from mountains the transfer of organic carbon from the terrestrial biosphere to the oceans via erosion and riverine transport constitutes an important component of the global carbon cycle measurements of particulate organic carbon load and composition in the liwu river taiwan during cyclone triggered floods suggest that tropical cyclones may facilitate the delivery of non fossil particulate organic carbon to the ocean and its subsequent burial the transfer of organic carbon from the terrestrial biosphere to the oceans via erosion and riverine transport constitutes an important component of the global carbon cycle1 2 3 4 more than one third of this organic carbon flux comes from sediment laden rivers that drain the mountains in the western pacific region3 5 this region is prone to tropical cyclones but their role in sourcing and transferring vegetation and soil is not well constrained here we measure particulate organic carbon load and composition in the liwu river taiwan during cyclone triggered floods we correct for fossil particulate organic carbon using radiocarbon and find that the concentration of particulate organic carbon from vegetation and soils is positively correlated with water discharge floods have been shown to carry large amounts of clastic sediment6 non fossil particulate organic carbon transported at the same time may be buried offshore under high rates of sediment accumulation7 8 9 we estimate that on decadal timescales 77 92 of non fossil particulate organic carbon eroded from the liwu catchment is transported during large cyclone induced floods we suggest that tropical cyclones which affect many forested mountains within the intertropical convergence zone10 may provide optimum conditions for the delivery and burial of non fossil particulate organic carbon in the ocean this carbon transfer is moderated by the frequency intensity and duration of tropical cyclones

assessing channel changes of the ganges padma river system in bangladesh using landsat and hydrological data abstract the ganga ganges 1 is an important river system in south asia which supports the life and livelihoods of millions of people both in india and bangladesh the system has a number of names throughout its length below its confluence with the brahmaputra at aricha it is known as the padma which in turn merges with the upper meghna at chandpur below which the channel is known as the lower meghna there is a growing concern about this large river system because its channels are subject to frequent migration threatening engineering structures and resulting in various environmental and social consequences which may be compounded by climatic variability land use change and agricultural intensification as the basin experiences rapid population growth concerns have been expressed that the construction of a barrage just upstream of the indo bangladesh border has adversely affected the ganges reach in bangladesh partly in order to investigate this the planform changes of the ganges and the padma within bangladesh was analysed over the period 1973 to 2011 using multitemporal landsat images and long term flow data in eight epochs with an average duration of 4 5 years the padma reach is less affected by the barrage and provides a useful control study areas of erosion and deposition were determined from sequential changes in the bankline positions mean channel width sinuosity and braiding index were analysed using a geographic information system gis flood frequency duration and magnitudes were studied using long term discharge records generally channel planform evaluation indicated that both the ganges and the padma experienced contraction expansion and readjustment in configuration over the last 38 years erosion and deposition statistics of the ganges indicate that 57 km2 of land was lost along the right bank whereas around 59 km2 has been gained along the left bank during the assessment period suggesting that the erosion and accretion of both banks is roughly balanced with a general movement towards the right bank the width of the ganges varied from a maximum of 5 36 to a minimum of 3 23 km during the observation period changes to sandbar area are in general much more radical than changes to the overall width and area of the channel measurement of areas of erosion and accretion showed that both banks of the padma experienced considerable loss of land the total net loss for left bank and right bank was 155 and 28 km2 respectively the padma is approximately twice the width of the ganges and the changes to its channel area are not as temporally dynamic as the ganges the relationship between bank curvature and erosion accretion of the river banks for both rivers was analysed and the results contradict established meander theory regression analysis between bank erosion rates annual average discharge and mean flood flow data showed that bank erosion was significantly correlated with annual average discharge for the padma r2 0 6283 and that the ganges bank erosion rate is influenced by mean flood flow r2 0 6738 the flood frequency shows generally good stability across the first eight of the nine epochs for the ganges but for the padma the frequency showed even greater stability we were unable to support the widely held belief that the upstream barrage has a deleterious effect on the ganges but note that there is a slight effect due to the periodic release of sediment through scour sluices

limestone pseudoconglomerates in the late cambrian gushan and chaomidian formations shandong province china soft sediment deformation induced by storm wave loading this paper focuses on the formative processes of limestone pseudoconglomerates in the gushan and chaomidian formations late cambrian of the north china platform shandong province china the gushan and chaomidian formations consist mainly of limestone and shale marlstone interlayers wackestone to packstone grainstone and microbialite as well as numerous limestone conglomerates seventy three beds of limestone pseudoconglomerate in the gushan and chaomidian formations were analysed based on clast and matrix compositions internal fabric sedimentary structures and bed geometry these pseudoconglomerates are characterized by oligomictic to polymictic limestone clasts of various shapes i e flat to undulatory disc blade and sheet marlstone and or grainstone matrix and various internal fabrics i e intact thrusted edgewise and disorganized as well as transitional boundaries limestone pseudoconglomerates formed as a result of soft sediment deformation of carbonate and argillaceous interlayers at a shallow burial depth differential early cementation of carbonate and argillaceous sediments provided the requisite conditions for the formation of pseudoconglomerates initial deformation i e burial fragmentation liquefaction and injection and subsequent mobilization and disruption of fragmented clasts are two important processes for the formation of pseudoconglomerates burial fragmentation resulted from mechanical rupture of cohesive carbonate mud whereas subsequent mobilization of fragmented clasts was due to the injection of fluid materials liquefied carbonate sand and water saturated argillaceous mud under increased stress storm wave loading was the most probable deformation mechanism as an external triggering force subsequent re orientation and rounding of clasts were probably prolonged under normal compactional stress eventually disrupted clasts along with matrix materials were transformed into pseudoconglomerates by progressive lithification soft sediment deformation is prevalent in alternate layers of limestone and mud marl stone and or grainstone regardless of their depositional environments

channel patterns braided anabranching and single thread a new channel pattern classification is presented based on theoretically derived channel pattern discriminant functions the thresholds are formulated as power laws that relate the critical slope associated with a change in channel pattern to dimensionless discharge and relative bank strength one threshold demarcates the boundary between stable single thread channels both straight and meandering and stable multiple thread channels anabranching another threshold separates anabranching from braided channels where braided channels are considered to be fundamentally unstable the exponents of the thresholds are nearly identical to that in the threshold equation originally proposed by leopold and wolman 1957 and the coefficients are quite similar as well an analysis of their data set using our dimensionless approach reveals that no fundamental difference exists between meandering and straight patterns and thus data from both types are grouped together under the more general heading of single thread channels furthermore we demonstrate that over a limited range of conditions an unstable single thread channel can form stable multiple threads but that for systems far from the threshold bounding the single thread channels the number of divisions required to produce stable anabranches grows geometrically this motivates a separation of multiple thread channels into anabranching and braided types our theoretical thresholds are then compared against several large data sets of field data and the results broadly confirm our proposed thresholds

petrogenesis of late paleoproterozoic liangcheng charnockites and s type granites in the central northern margin of the north china craton implications for ridge subduction abstract the 1930 1890 ma garnet bearing peraluminous liangcheng granitoids are mainly distributed in the fengzhen belt in the north china craton ncc they consist of early porphyritic coarse grained charnockites magmatic orthopyroxene granitic rocks and late medium to coarse grained s type crustal melt granites the charnockites are distinguished from the s type granites by their higher tio 2 al 2 o 3 feo t mgo cao and trace element contents especially the heavy rare earth elements ree the charnockites show varied light ree enrichments la yb n 1 92 18 0 negative eu anomalies eu eu 0 2 0 9 and negative anomalies in high field strength elements hfses e g nb ta ti and in sr compared with the neighbouring elements in spidergrams their 87 sr 86 sr t t 1900 ma ratios vary from 0 704 to 0 707 with one at 0 74 and nd t t 1900 ma values range from 4 to 1 in contrast the liangcheng s type granites show strong light ree enrichments la yb n 13 3 230 positive eu anomalies eu eu 1 1 3 6 negative anomalies in hfses e g nb ta ti and negative or positive anomalies in sr compared with neighbouring elements in spidergrams their 87 sr 86 sr t t 1900 ma ratios vary from 0 703 to 0 712 and nd t t 1900 ma values range from 3 to 1 both the charnockites and s type granites originated by melting of in situ meta sediments with significant charnockites or little s type granites amounts of additional melts from the parental magma of the adjacent xuwujia gabbronorites high mg 19 23 wt samples of the xuwujia gabbronorites have whole rock re contents of 0 16 1 93 ppb os contents of 1 6 9 8 ppb 187 re 188 os ratios of 0 08 1 76 and 187 os 188 os ratios of 0 1168 0 1186 with one exception at 0 1845 these values are comparable with those of typical komatiites and agree with a depleted source for these rocks the charnockites have received 10 30 wt or even larger amounts of melt contribution from the xuwujia gabbronorites whereas the s type granites received only few melts from the gabbronorites and they possibly had lower melting temperature than the charnockites we consider that the charnockites and s type granites had surface counterparts represented by two types of rhyolitic to dacitic volcanics in the halaqin succession which also contains basalts to basaltic andesites that are geochemically comparable to the xuwujia gabbronorites or the charnockites some andesites from their field relations and geochemical and thermal characteristics we propose that the peraluminous charnockites and s type granites were most likely generated by ridge subduction at 1930 ma prior to the final cratonization of the ncc at 1850 ma and that this ridge subduction was also responsible for the regional 1950 1880 ma gabbronorites charnockites s type granites volcanics as well as the 1930 ma ultrahigh temperature metamorphism in the ncc

depositional processes of the zhushadong and mantou formations early to middle cambrian shandong province china roles of archipelago and mixed carbonate siliciclastic sedimentation on cycle genesis during initial flooding of the north china platform to understand the depositional processes and environmental changes during the initial flooding of the north china platform this study focuses on the lower to middle cambrian zhushadong and mantou formations in shandong province china the succession in the jinan and laiwu areas comprises mixed carbonate and siliciclastic deposits composed of limestone dolostone stromatolite thrombolite purple and grey mudstone and sandstone a detailed sedimentary facies analysis of seven well exposed sections suggests that five facies associations are the result of an intercalation of carbonate and siliciclastic depositional environments including local alluvial fans shallowing upward carbonate siliciclastic peritidal cycles oolite dominant shoals shoreface and lagoonal environments these facies associations successively show a transition from an initially inundated tide dominated carbonate platform to a wave dominated shallow marine environment in particular the peritidal sediments were deposited during a large number of depositional cycles these sediments consist of lime mudstone dolomite stromatolite and purple and grey mudstones these shallowing upward cycles generally resulted from carbonate production in response to an increase of accommodation during rising sea level the carbonate production was however interrupted by frequent siliciclastic input from the adjacent emergent archipelago the depositional cycles thus formed under the influence of both autogenetic changes including sediment supply from the archipelago and allogenic control of relative sea level rise in the carbonate factory a low relief archipelago with an active tidal regime allowed the development of tide dominated siliciclastic and carbonate environments on the vast platform siliciclastic input to these tidal environments terminated when most of the archipelago became submerged due to a rapid rise in sea level this study provides insights on how a vast cambrian carbonate platform maintained synchronous sedimentation under a tidal regime forming distinct cycles of mixed carbonates and siliciclastics as the system kept up with rising relative sea level during the early stage of basin development in the north china platform

mesozoic and cenozoic multiple deformations in the helanshan tectonic belt northern china abstract the helanshan tectonic belt htb is located in the tectonic boundary between the alxa massif and the ordos basin and witnessed the tectonic processes between the alxa massif and the ordos basin the deformation structures in the phanerozoic successions are keys to understanding the multiple phases of deformation along the htb together with regional geology our new geological mapping and structural analysis reveal that htb has been formed at least by four phases of deformation during the mesozoic and cenozoic times first phase of deformation d1 is represented by the wnw trending open folds of the paleozoic and triassic strata and formed by nne ssw contraction the angular unconformity between the deformed paleozoic triassic strata and overlying middle jurassic strata constrains that d1 occurred in the early jurassic second phase of deformation d2 is indicated by the ne striking tight folds of the triassic and jurassic strata and the superimposed deformation on d1 structures geometrical and kinematic analysis suggests that d2 was constructed by a phase of nw se contraction the angular unconformity between the middle jurassic and the lower cretaceous strata suggests that d2 was formed during the late jurassic third phase of deformation d3 is characterized by nne striking normal faults on both sides of the htb which separate the htb from the bayanhote basin to the west and yinchuan graben to the east d3 is consistent with the regional ne trending horst graben structural system and correlates to a regional extension in wnw direction according to the contact correlation of the lower cretaceous deposits and these normal faults they are indicated as syndepositional faults during the early cretaceous the fourth phase of deformation d4 was characterized by nw striking open folds and the ne trending strike slip faults in the eocene strata as well as the older strata together with the youngest strata involved into d4 deformation is the oligocene the previous age of 13 4 ma for the reactivation of the zongbieli zhengyiguan fault constrains the deformation time of d4 in the late miocene

assembling north china craton within the columbia supercontinent the role of double sided subduction abstract an evaluation of recent s wave receiver functions s wave velocities and two versions of p wave tomographic images along various transects in the north china craton provides some clues on the subduction collision history of the different crustal blocks and their final amalgamation within the paleoproterozoic columbia supercontinent interpretation of a n s seismic section of the craton suggests thick slab debris sinking to various depths in the mantle the w e seismic corridors show the preservation of a thick 200 km lithospheric root tectosphere beneath the ordos block and its variable and extensive erosion towards the yanliao block eastern block this zone is characterized by layers with marked velocity contrast and suggests repeated stacking of the remnants of underplated and accreted paleoproterozoic oceanic lithosphere the present day lithosphere asthenosphere boundary beneath this region probably marks the erosional plane along which decratonization occurred through subduction erosion from the east and thermal and material erosion by upwelling asthenosphere from below resulting in the partial destruction of the tectosphere and its thinning towards the east within the asthenosphere below the yanliao block younger and thinner slabs predominate in the absence of any prominent thick high velocity layers these younger slabs define a westward polarity and constitute a mega scale duplex formed by underplating through phanerozoic subduction process particularly the pacific plate subduction from the east the lithologic associations within the inner mongolia suture zone dividing the yinshan block to the north and ordos block to the south correspond to an accreted ocean plate stratigraphic sequence with the tonalite trondhjemite granodiorite ttg gneisses charnockites and calc alkaline granites representing a continental arc built up through subduction from the north the seismic transects bring out a contrasting polarity in the subduction regime with an oblique east to southward subduction of the yinshan block and a westward subduction of the yanliao block here i propose a double sided subduction history for the ncc similar to the ongoing subduction process in the western pacific such double sided subduction is considered to promote rapid amalgamation of continental fragments within supercontinents and the subduction polarities and mantle dynamics of ncc are therefore considered to be critical in evaluating the final assembly of the paleoproterozoic supercontinent columbia

stratigraphy and geochronology of permo carboniferous strata in the western north china craton insights into the tectonic evolution of the southern paleo asian ocean abstract the northwestern ordos terrane not in the western north china craton w ncc comprises the northwestern ordos basin in the east and the eastern alxa massif in the west bound by the helanshan tectonic belt htb the key position makes the not crucial for understanding the evolutionary processes of the w ncc and particularly the tectonic relation of the alxa massif with the w ncc in this study petrologic stratigraphic and geochronologic studies were conducted on permo carboniferous successions in the not stratigraphic correlation reveals that carboniferous marine successions display a transgressive sequence with a slight westward deepening facies variation evidenced by the continuous onlap of tidal flat layers toward the east the permian nonmarine strata in the htb and the ordos basin have no substantial facies variation defining an upward regressive sequence from deltaic to fluvial associations while time equivalent units in the eastern alxa massif have been eroded the generally ssw directed paleocurrents suggest that permo carboniferous siliciclastic materials were derived from a highland to the northeast the unified sedimentary system in the not constrains the alxa massif to be part of the w ncc the lower carboniferous sandstone contain zircons with a concentrated age cluster of 1700 2700 ma comparable to archean to paleoproterozoic crystalline basement in the northern w ncc by contrast in addition to zircons of 1700 2700 ma late carboniferous and permian sandstones all contain abundant paleozoic zircons with two age clusters around 300 ma and 420 ma which are similar to age patterns of paleozoic magmatism in the northern w ncc zircon age profile and sandstone modal composition indicate the origin from an andean type continental arc the permo carboniferous tectono sedimentary processes of the not should occur in a marginal basin behind the continental arc along the northern w ncc in response to the southward subduction of solonker ocean southern branch of paleo asian ocean

the late paleoproterozoic orogeny in the north china craton abstract granulite ttg and khondalite formations are the two typical types of precambrian metamorphic basement in the north china craton the former belongs to the basement of the archean craton while the latter is a paleoproterozoic metasedimentary rock series the formation of the ttg rock series took place during the period of 2500 to 2450 ma near the end of the archean and it has undergone multiple times of metamorphism the cooling rate of the ttg lower crust is quite low less than 0 3 ma developed during slow crustal cooling in a rather stable tectonic environment the khondalite rock series underwent amphibolite to granulite facies metamorphism that is isotopically constrained to 2000 to 1800 ma followed by fast crustal cooling from 2 4 to 3 4 ma based on the thermal and dynamic features of different types of metamorphic rocks the tectonic state of the crust and the obvious change of crustal cooling rate from neo archean to paleoproterozoic it is clear that continental orogeny existed in the north china craton between 2000 ma and 1800 ma

channel responses to varying sediment input a flume experiment modeled after redwood creek california article i nfo storage flume aggradation degradation at the reach scale a channel adjusts to sediment supply and flow through mutual interactions among channel form bed particle size and flow dynamics that govern river bed mobility sediment can impair the beneficial uses of a river but the timescales for studying recovery following high sediment loading in the field setting make flume experiments appealing we use a flume experiment coupled with field measurements in a gravel bed river to explore sediment transport storage and mobility relations under various sediment supply conditions our flume experiment modeled adjustments of channel morphology slope and armoring in a gravel bed channel under moderate sediment increases channel bed elevation increased and sediment output increased but channel planform remained similar to pre feed conditions during the following degradational cycle most of the excess sediment was evacuated from the flume and the bed became armored under high sediment feed channel bed elevation increased the bed became smoother mid channel bars and bedload sheets formed and water surface slope increased concurrently output increased and became more poorly sorted during the last degradational cycle the channel became armored and channel incision ceased before all excess sediment was removed selective transport of finer material was evident throughout the aggradational cycles and became more pronounced during degradational cycles as the bed became armored our flume results of changes in bed elevation sediment storage channel morphology and bed texture parallel those from field surveys of redwood creek northern california which has exhibited channel bed degradation for 30 years following a large aggradation event in the 1970s the flume experiment suggested that channel recovery in terms of reestablishing a specific morphology may not occur but the channel may return to a state of balancing sediment supply and transport capacity published by elsevier b v

long term elevated post eruption sedimentation at mount pinatubo philippines post eruption sedimentation rates from two basins draining mount pinatubo philippines are compiled and extended through a d 2009 to compare short term predictions with long term yields the pasig potrero and sacobia rivers were covered by 1 km 3 of thick pyroclastic flow deposits during the 1991 eruption of mount pinatubo blanketing 30 of each basin with highly erodible sediments erosion of these deposits led to some of the highest sediment yields ever recorded yields peaked in 1991 and declined exponentially for the first decade following the eruption deposition on the aggrading alluvial fan shows that sedimentation rates from 2001 to 2009 were almost twice as high as an exponential decay model would predict with rates that have leveled off and are no longer declining exponentially patterns of long term sedimentation are similar to those of the north fork toutle river washington state united states at mount st helens with two phases of erosion and sediment export in the first erosion of hillslope tephra and carving of the initial valley network lead to high sediment yields that decline exponentially over 5 10 yr in the second phase continued valley widening and fluvial instability maintain high sediment yields for at least two decades a comparison of post eruption deposition with pre eruption yields indicates that century to millennial scale sedimentation is dominated by sediment associated with discrete eruptive events

controls on the spacing of first order valleys 1 many landscapes are composed of ridges and valleys that are uniformly spaced even where valley locations are not controlled by bedrock structure models of long term landscape evolution have reproduced this phenomenon yet the process by which uniformly spaced valleys develop is not well understood and there is no quantitative framework for predicting valley spacing here we use a numerical landscape evolution model to investigate the development of uniform valley spacing we find that evenly spaced valleys arise from a competition between adjacent drainage basins for drainage area a proxy for water flux and that the spacing becomes more uniform as the landscape approaches a topographic equilibrium valley spacing is most sensitive to the relative rates of advective erosion processes such as stream incision and diffusion like mass transport such as soil creep and less sensitive to the magnitude of a threshold that limits the spatial extent of stream incision analysis of a large number of numerical solutions reveals that valley spacing scales with a ratio of characteristic diffusion and advection timescales that is analogous to a peclet number we use this result to derive expressions for equilibrium valley spacing and drainage basin relief as a function of the rates of advective and diffusive processes and the spatial extent of the landscape the observed scaling relationships also provide insight into the cause of transitions from rill like drainage networks to branching networks the spatial scale of first order drainage basins the contributing area at which hillslopes transition into valleys and the narrow range of width to length ratios of first order basins

effects of punctuated sediment supply on valley floor landforms and sediment transport large infrequent fluxes of sediment to streams by mass wasting are intrinsic to the erosion regime of mountain drainage basins to elucidate the role of mass wasting in the construction and evolution of steep land channel environments it is crucial that we identify the processes involved and recognize their legacy on the valley floor in the winter of 1996 nine storm triggered debris flows carried 18 000 m 3 of coarse debris into the upper reaches of the south fork of gate creek oregon cascade range during flood flow analysis of resulting channel morphologies and bed textures shows that the sediment moved downstream as a wave like pulse or pulses overwhelming the channel and causing it to braid with flooding and alluvial deposition over the valley floor downstream progression of the sediment wave resulted in vertical accretion of the valley floor with sediment carried as bedload the maximum depth of valley floor burial being set by the amplitude of the wave passage of the wave left a channel incised to bedrock inset between coarse grained alluvial terraces this study examines the genesis of these features at gate creek and points to such terraces as field indicators of massive episodic influxes of sediment and the associated formation of fluvially transported sediment waves

is the ordos block archean or paleoproterozoic in age implications for the precambrian evolution of the north china craton the ordos block is a large but poorly exposed crustal segment in the western part of the north china craton little is known about its precambrian basement rocks and their evolution because of an extensive cover by younger sedimentary rocks we report shrimp zircon ages and hf in zircon isotopic compositions for five samples recovered from drill holes that penetrated into the ordos basement based on these data two age groups of rocks can be distinguished an early paleoproterozoic group that is represented by one sample from the northern margin of the ordos block and a late or middle late paleoproterozoic group consisting of the remaining four samples the early paleoproterozoic sample contains zircon cores and metamorphic rims with ages of 2 4 ga and 2 28 ga respectively the cores have ehf t values of 8 8 to 6 2 and hf crustal model ages of 2606 to 3221 ma the remaining rocks of late or middle late paleoproterozoic age contain zircons that may also show a core rim structure but with the cores having ages of 2 08 ga and the metamorphic rims recording ages of 1 9 ga the zircon cores have ehf t values of 9 1 to 10 1 and hf crustal model ages of 2039 to 3068 ma combined with data from earlier studies we draw the conclusion that late paleoproterozoic metasedimentary rocks are widespread in the ordos basement there is no evidence from our samples of an archean component and this calls into question the prevailing view that the ordos basement is an archean cratonic block the fact that the ordos basement was involved in a widespread late paleoproterozoic tectono thermal event indicates that most current models for the tectonic evolution of the western portion of the north china craton need to be revised

u pb and hf isotopic study of zircons of the helanshan complex constrains on the evolution of the khondalite belt in the western block of the north china craton abstract the helanshan complex is located at the westernmost segment of the khondalite belt a continent continent collisional belt along which the yinshan block in the north collided with the ordos block in the south to form the western block of the north china craton the complex consists of s type granites and high grade pelitic granulite gneiss felsic paragneiss quartzite calc silicate rock and marble together called the khondalite series rocks la q icp ms u pb geochronology and la mc icp ms lu hf isotopic data of zircons combined with cathodoluminescence cl imaging enable the resolution of magmatic and metamorphic events that can be directed towards understanding the formation and evolution of the khondalite belt in the western block of the north china craton cl images reveal the coexistence of magmatic type detrital zircons and metamorphic zircons in most of the khondalite series rocks of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating magmatic type detrital zircon cores la q icp ms u pb analyses on magmatic type detrital zircons reveal two distinct age populations with one in the archaean 2 85 2 53 ga and the other in the palaeoproterozoic 2 2 2 0 ga suggesting that the sedimentary protoliths of the khondalite series rocks in the helanshan complex must have been deposited at some time after 2 0 ga the hf analyses show that the archaean detrital zircons possess negative ehf t values from 7 64 to 0 15 with depleted mantle model ages ranging from 3 34 to 3 10 ga this implies the existence of a paleo mesoarchaean 3 34 3 10 ga crust in the western block which underwent a recycling event in the period 2 85 2 53 ga nearly all palaeoproterozoic 2 2 2 0 ga detrital zircons possess positive ehf t values 0 86 9 38 with depleted mantle model ages of 2 45 2 15 ga suggesting a significant crustal growth event in the western block in the palaeoproterozoic metamorphic zircons yield two groups of age with one at 1 95 ga and the other at 1 87 ga of which the former is considered as the timing of the collision between the yinshan and ordos blocks to form the western block whereas the latter is consistent with the age of 1 86 ga from two s type granites which are considered to have formed by the partial melting of pelitic granulites gneisses at the stage of the exhumation of the helanshan complex

p t t paths and tectonic history of an early precambrian granulite facies terrane jining district south east inner mongolia china abstract the widespread khondalite series of south east inner mongolia consists largely of biotite sillimanite garnet gneiss and quartzo feldspathic gneiss with some marble and mafic granulite layers it has experienced two metamorphic events at c 2500 and 1900 2000 ma a pre peak stage of the first metamorphism at t 600 700 c and p 6 7 kbar is recognized by the relict amphibolite facies assemblage ky grt bt pl qtz and protected inclusions of biotite hornblende sodic plagioclase and quartz in garnet or orthopyroxene the peak stage with t c 800 50 c and p 8 10 kbar is characterized by the widespread granulite facies assemblages sil grt bt kfs pl qtz in gneiss and opx cpx pl hbl grt in granulite the p t t path suggests that the supracrustal sequence was buried in the lower crust by tectonic thickening during d1 d2 the beginning of the second metamorphism is characterized by further temperature rise to 700 c or more at lower pressure this stage is manifested by the appearance of cordierite after garnet fibrolite sil2 after biotite in gneiss and transformation of hbl1 into opx2 and cpx2 in granulite coronas of symplectitic opx2 pl2 surrounding grt1 and cpx1 in mafic granulite are interpreted as products of near isothermal decompression the p t t path may be related tectonically to waning extension of the crust by the end of the early proterozoic

early paleozoic to devonian multiple accretionary model for the qilian shan nw china abstract the qilian shan located in the northern tibetan plateau of nw china has an excellent record of early to middle paleozoic subduction accretion that resulted from convergence between the alax and qaidam blocks but there is no consensus about its detailed tectonic history this paper summarizes its tectonic divisions and discusses its tectonic evolution from the cambrian to devonian the belt has the following tectonic divisions in the far north the southern passive margin of the alax block is juxtaposed against an early cambrian to ordovician marianan type intra oceanic arc north qilian the north qilian arc is separated from a japanese type arc central qilian to the south by a high pressure metamorphic belt composed mostly of ophiolitic slices and oceanic crusts the central qilian is bounded to the south by a wide melange zone south qilian consisting of ophiolitic slices and continental margin sequences farther to the south lies the oulongbuluk microcontinent that is separated from the qaidam block and farther south by an ultrahigh pressure metamorphic belt tectonostratigraphic analysis together with geochemical geochronological and geophysical data indicates a complex evolution by subduction accretion processes from the cambrian to the devonian before final amalgamation and docking to the northern alax block this model solves the long lasting discussion on the polarity of subduction in paleozoic time this multiple subduction accretion history sheds light on the continuity of paleozoic sutures along the qilian shan and the nature of the altyn tagh fault and thus contributes to an improved understanding of the tectonic architecture of central asia

spatial and temporal dynamics of sediment accumulation and exchange along strickland river floodplains papua new guinea over decadal to centennial timescales 1 we investigated the processes of sediment exchange between the strickland river and its lowland floodplain documenting 1 the rates textures and distributions of sediment accumulation 2 temporal variations in these rates 3 the remobilization of sediment by erosive processes and 4 whether net storage is significant over century timescales we used 210pb geochronology of floodplain cores from 11 transects to measure deposition rates over the past 65 years finding a decline from 5 5 cm a 0 10 m distance from the channel to 0 9 cm a 400 m to 0 1 cm a 1 km rates are elevated along curved sections of channel and are stable over time and episodic accumulation is prevalent and may be correlated to floods integrated temporally and spatially the average rate is 1 6 cm a integrating along both sides of 318 river km we studied for the lowland strickland we calculate 12 19 mt of annual sediment accumulation 0 05 mt km or 0 07 of the annual load per km of river length representing 17 27 of the total annual sediment flux we used georeferenced landsat images 1972 2001 to quantify channel migration and the resulting return of sediment to the channel mean lateral migration was 5 1 0 8 m a given the floodplain width of 10 km this implies a waiting time of 1 ka between floodplain formation and subsequent reentrainment of the bank as the channel migrates laterally this raises the possibility that the net return of material from the floodplain due to channel migration could balance the overbank deposition we observed the exchange flux between cut bank erosion and point bar deposition is 20 40 mt highlighting the significance of sediment recycling 50 of the total load such sediment trapping and recycling affects the transport storage and evolution of biogeochemically reactive particles the evolution of the floodplain and the morphodynamics of basin infilling

multiple phases of deformation in the southern helanshan tectonic belt northern china abstract the southern helanshan tectonic belt shtb is surrounded by the alxa massif the ordos basin and the tibetan plateau and its phanerozoic structures records crucial phanerozoic deformation characteristics for studying the multi phased intracontinental deformation processes among these neighboring units combined with regional geology our structural analysis reveals that the shtb has experienced five phases of shortening deformation during the phanerozoic the first phase d1 is represented by e w trending overturned folds in the cambrian and ordovician strata these folds resulted from two episodes of n s shortening during the late cambrian early ordovician and late ordovician early devonian respectively which were attributed to the closure of the qilian ocean the second phase d2 is characterized by e w trending open folds involving the upper devonian strata which was the products of n s shortening caused by the assembly of the qilian ocean and the north china craton ncc at the end of devonian the third phase d3 is indicated by some wnw trending jura type folds in the carboniferous strata formed by nne ssw shortening during the late triassic early jurassic related to the collision between the ncc and the south china block the fourth phase d4 is inferred by nne trending shortening structures in the jurassic and its underlying strata generated by wnw ese shortening in the late jurassic echoing the paleo pacific subduction the fifth stage d5 is characterized by the ne sw shortening in the late miocene leading to the formation of many nw se striking thrust faults in the miocene and its underlying strata because the outward expansion of the tibetan plateau

detrital zircon u pb ages of middle late permian sedimentary rocks from the southwestern margin of the north china craton implications for provenance and tectonic evolution abstract the southwestern margin of the north china craton ncc is located between the alxa terrane to the northwest the north qilian orogen to the west and the north qinling orogen to the south however the paleogeographic and tectonic evolution for the southwestern part of the ncc in the late paleozoic is still poorly constrained in order to constrain the late paleozoic tectonic evolution of the southwestern ncc we carried out detailed field work and detrital zircon u pb geochronological research on middle late permian sedimentary rocks at the southwestern margin of the ncc the u pb age spectra of detrital zircons from six samples are similar showing four populations of 2 6 2 4 ga 2 0 1 7 ga 500 360 ma and 350 250 ma moreover on the basis of the weighted mean age of the youngest detrital zircons 257 4 ma combined with the published results and volcanic interlayers we propose that the shangshihezi formation formed during the middle late permian our results and published data indicate that the detrital zircons with age groups of 2 6 2 4 ga and 2 0 1 7 ga were likely derived from the khondalite belt and yinshan block in the northwestern ncc the junction part between the north qinling and north qilian orogen may provide the 500 360 ma detrital zircons for the study area the 350 250 ma detrital zircons were probably derived from the northwestern part of the ncc the majority of materials from shangshihezi formation within the study area were derived from the northwestern part of the ncc indicating that the northwestern part of the ncc was strongly uplifted possibly resulting from the progressive subduction and closure of the paleo asian ocean a small amount of materials were sourced from southwestern part of the ncc indicating that the north qinling orogen experienced a minor uplift resulting from the northward subduction of the south qinling terrane

modification of river meandering by tropical deforestation tropical forests are the only forest biome to have experienced increased rates of forest loss during the past decade because of global demands for food and biofuels the implications of such extensive forest clearing on the dynamics of tropical river systems remain relatively unknown despite significant progress in our understanding of the role of trees in riverbank stability here we document rates of deforestation and corresponding average annual rates of riverbank erosion along the freely meandering kinabatangan river in sabah malaysia from landsat satellite imagery spanning a d 1989 2014 we estimate that deforestation removed over half of the river s floodplain forest and up to 30 of its riparian cover which increased rates of riverbank erosion by 23 within our study reaches further the correlation between the magnitude of planform curvature and rates of riverbank erosion only became strongly positive and significant following deforestation suggesting an important role of forests in the evolution of meandering rivers even when riverbank heights exceed the depth of root penetration