amphibole sponge in arc crust pressure temperature time paths followed by arc magmas ascending through the lithosphere dictate the phase assemblage that crystallizes and hence the compositions of liquid fractionates here we use la yb and dy yb versus sio 2 relationships from selected volcanoes to show that amphibole is an important mineral during differentiation of arc magma production of intermediate and silicic arc magmas occurs as magmas stall and cool in the mid lower crust where amphibole is stable because amphibole is rarely a phenocryst phase we term this cryptic amphibole fractionation if this process is as widespread as our investigation suggests then 1 amphibole cumulates may act as an effective filter for water dissolved in mantle derived magmas 2 amphibole cumulates may act as a fertile source of intracrustal melts and fluids and 3 recycling of amphibole cumulates has the potential to return incompatible trace elements and water to the mantle

zircon lu hf isotopic constraints on neoproterozoic subduction related crustal growth along the western margin of the yangtze block south china abstract voluminous neoproterozoic igneous rocks along the western margin of the yangtze block south china provide an ideal opportunity for examining the nature of the mantle source petrogenesis of the felsic plutons and processes and timing of crustal growth the dadukou tongde gaojiacun and shaba mafic plutons and datian and dajianshan adakitic plutons were selected for zircon lu hf isotope analysis zircons from the mafic plutons have hf t values ranging from 1 86 to 11 63 with average hf t values of the different samples ranging from 4 78 0 42 to 7 60 1 1 their single stage model ages range from 0 86 to 1 28 ga the two adakitic plutons have hf t values ranging from 2 26 to 11 68 and single stage model ages from 0 86 to 1 24 ga our data further confirm that the mafic plutons were extracted from depleted mantle sources whereas the datian and dajianshan adakitic plutons were products of melting of a subducted oceanic slab the arc geochemical affinity relatively high 143 nd 144 nd ratios and high zircon hf t values as well as the relatively young hf single stage model ages of these neoproterozoic plutons suggest that the continental crustal growth in the region was related to oceanic subduction rather than upwelling of a mantle plume in a rifting setting the neoproterozoic magmatism along the western margin of the yangtze block lasted more than 200 my not show episodic emplacement and contrasting zircon hf isotopes suggesting that continental crustal growth in south china was continuously resulted from subduction

tracing the nd isotope evolution of north pacific intermediate and deep waters through the last deglaciation from south china sea sediments abstract the intermediate and deep waters of the pacific ocean play a crucial role for regulating global climate changes on the millennium timescale however due to poor preservation of carbonate sediments in the deep pacific little attention has been received to better understand deep ocean circulation and its relationship with abrupt climate events here we present the first authigenic nd isotope e nd record extracted from fe mn oxyhydroxides of the south china sea scs sediments so17940 2 1727 m water depth in order to investigate changes of intermediate and deep water circulations in the pacific ocean since the last glacial maximum lgm in general the seawater e nd record extracted from the scs sediments paralleled the existing nd isotopes of fish teeth debris from the eastern north pacific core during the last deglaciation demonstrating that our authigenic e nd can faithfully record the seawater e nd evolution in the deep pacific ocean at the onset of the deglacial period our data exhibit a clear negative shift in e nd towards a gradually increased component of antarctica intermediate water aaiw and reached a maximal influx during the mid heinrich stadial 1 hs1 16 kyr bp between the hs1 and younger dryas deep water circulation gradually shifted back to the modern condition during the bolling allerod warm interval a pronounced negative e nd excursion had occurred during the pre boreal pb suggesting a rapid reorganization of deep water ventilation in the pacific ocean and was predominated by aaiw this is most likely due to poor ventilation in the western north pacific associated with an intensified summer monsoon during the pb early holocene warm period after the pb period the seawater e nd values were once again dominated by pacific deep water at the study site

does neoproterozoic nam co formation in northwest vietnam belong to south china or indochina abstract the nam co belt is located to the northeast of song ma fault in northwestern vietnam the nature of the precambrian basement rocks in this area and its tectonic attribute have still been controversial in this study zircon u pb hf isotopic compositions and whole rock geochemical compositions of meta sedimentary rocks from the nam co formation in this belt have been integrated to reveal the components and tectonic setting of the formation of the basement rocks in the nam co belt zircon u pb dating results show that the nam co formation is of middle late neoproterozoic age the lower nam co formation probably formed between 821 and 726 ma and the upper formation was deposited later than 726 721 ma the sedimentary rocks from the lower and upper nam co formations are characterized by numerous neoproterozoic 900 800 ma detrital materials but the upper part contains more meso to paleoproterozoic sediments than the lower part detrital zircon age distribution and hf isotope compositions of the nam co formation are similar to those of neoproterozoic sedimentary rocks in the southern yangtze block both zircon u pb hf isotopic and whole rock geochemical data suggest that the source materials of the neoproterozoic nam co formation were dominated by felsic rocks with minor mafic materials and the detrital materials were mainly derived from the jiangnan orogenic belt in the southern yangtze block with minor contribution from the panxi belt in the western yangtze block ailaoshan song hong and phan si pan belts the lower nam co formation contains more sediments derived from the panxi belt than the upper part the middle late neoproterozoic nam co formation probably was deposited in a continental extensional basin and the sedimentation environment in later period

deglacial and holocene evolution of the vietnam shelf stratigraphy sediments and sea level change on the vietnam shelf more than 1000 miles of shallow high resolution seismics were analyzed to unravel post glacial evolution in a tropical siliciclastic environment together with 25 sediment cores from water depths between 21 and 169 m to determine stratigraphy distribution and style of sedimentation fourty seven samples were dated with the ams c 14 technique the shelf was grouped into three regions a southern part a central part and a northern part on the broad southern shelf sedimentation is influenced by the mekong river which drains into the scs in this area here incised valley fills are abundant that were cut into the late pleistocene land surface by the paleo mekong river during times of sea level lowstand those valleys are filled with transgressive deposits the holocene sedimentation rate in this low gradient accommodation dominated depositional system is in the range of 5 10 and 25 40 cm ky at locations sheltered from currents the central shelf is narrow and the sedimentary strata are conformable here numerous small mountainous rivers reach the scs and transport large amounts of detrital sediment onto the shelf therefore the holocene sedimentation rate is high with values of 50 100 cm ky in this supply dominated depositional system the broad northern shelf in the vicinity of the red river delta shows as on the southern shelf incised valleys cut into the pleistocene land surface by paleo river channels in this accommodation dominated shelf area the sedimentation rate is low with values of 5 10 cm ky where applicable we assigned the sampled deposits to different paleo facies the latter are related to certain intervals of water depths at their time of deposition comparison with the sea level curve of hanebuth t j j stattegger k grootes p 2000 rapid flooding of the sunda shelf a lateglacial sea level record science 288 1033 1035 indicates subsidence on the central shelf which is in agreement with the high sedimentation rates in this area in contrast data from the northern shelf suggest tectonic uplift that might be related to recent tectonic movements along the ailao shan red river fault zone data from the southern shelf are generally in agreement with the sea level curve mentioned above c 2004 elsevier b v all rights reserved

petrogenesis and geodynamic implications of neoproterozoic gabbro diorites adakitic granites and a type granites in the southwestern margin of the yangtze block south china abstract the occurrence of a type granites following adakitic granites can provide unique insights into the tectonic evolution of subduction zone this paper presents a comprehensive study of zircon u pb hf isotopes and whole rock geochemistry on the neoproterozoic gabbro diorites biotite granites and k feldspar granites in the southwestern margin of the yangtze block south china zircon u pb dating of three distinctive intrusions yield concordant ages of 810 4 2 0 ma for gabbro diorites 800 3 2 1 ma for biotite granites and 749 1 2 0 ma for k feldspar granites the gabbro diorites are sodic and calc alkaline rocks with low sio2 52 62 53 87 wt medium mgo 2 67 3 41 wt high fe2o3t 7 18 7 49 wt and cao 5 68 7 50 wt contents they display high th zr and rb y ratios but low nb zr and nb y ratios together with their positive whole rock end t 1 0 to 1 5 and zircon ehf t values 3 66 to 8 18 we suggest that these gabbro diorites were derived from the partial melting of depleted lithospheric mantle modified by subduction related fluids the biotite granites display relatively high sr contents 335 395 ppm sr y ratios 38 9 54 3 low y 7 04 9 71 ppm and yb contents 0 78 1 08 ppm resembling the adakitic granites their low mg 36 41 values cr 2 94 3 59 ppm and ni 1 32 1 55 ppm contents as well as positive end t 0 5 to 0 6 and ehf t values 1 62 to 8 07 indicate that they were formed by partial melting of thickened juvenile mafic lower crust probably within the existence of garnet the k feldspar granites have high sio2 76 61 77 14 wt and alkalis na2o k2o 8 55 9 69 wt contents high 10 000 ga al 2 56 2 80 ratios and differentiation index 95 3 96 9 indicating the affinity to highly fractionated a type granites we suggest that these a type granites were mainly generated by the partial melting of felsic crustal rocks under low pressure condition and subsequently extensive fractional crystallization is significant taking into account previous studies from the western yangtze block south china we propose that the gabbro diorites adakitic granites and a type granites in this study were formed in persistent subduction process during the neoproterozoic

initiation of the mekong river delta at 8 ka evidence from the sedimentary succession in the cambodian lowland abstract modern deltas are understood to have initiated around 7 5 9 ka in response to the deceleration of sea level rise this episode of delta initiation is closely related to the last deglacial meltwater events and eustatic sea level rises the initial stage of the mekong river delta one of the world s largest deltas is well recorded in cambodian lowland sediments this paper integrates analyses of sedimentary facies diatom assemblages and radiocarbon dates for three drill cores from the lowland to demonstrate holocene sedimentary evolution in relation to sea level changes the cores are characterized by a tripartite succession 1 aggrading flood plain to natural levee and tidal fluvial channel during the postglacial sea level rise 10 8 4 ka 2 aggrading to prograding tidal flats and mangrove forests around and after the maximum flooding of the sea 8 4 6 3 ka and 3 a prograding fluvial system on the delta plain 6 3 ka to the present the maximum flooding of the sea occurred at 8 0 0 1 ka 2000 years before the mid holocene sea level highstand and tidal flats penetrated up to 20 50 km southeast of phnom penh after a period of abrupt 5 m sea level rise at 8 5 8 4 ka the delta progradation then initiated as a result of the sea level stillstand at around 8 7 5 ka another rapid sea level rise at 7 5 7 ka allowed thick mangrove peat to be widely deposited in the cambodian lowland and the peat accumulation endured until 6 3 ka since 6 3 ka a fluvial system has characterized the delta plain and the fluvial sediment discharge has contributed to rapid delta progradation the uppermost part of the sedimentary succession composed of flood plain to natural levee sediments reveals a sudden increase in sediment accumulation over the past 600 1000 years this increase might reflect an increase in the sediment yield due to human activities in the upper to middle reaches of the mekong as with other asian rivers

theoretical investigation of convective instability in inclined and fluid saturated three dimensional fault zones abstract the convective instability of pore fluid flow in inclined and fluid saturated three dimensional fault zones has been theoretically investigated in this paper due to the consideration of the inclined three dimensional fault zone with any values of the inclined angle it is impossible to use the conventional linear stability analysis method for deriving the critical condition i e the critical rayleigh number which can be used to investigate the convective instability of the pore fluid flow in an inclined three dimensional fault zone system to overcome this mathematical difficulty a combination of the variable separation method and the integration elimination method has been used to derive the characteristic equation which depends on the rayleigh number and the inclined angle of the inclined three dimensional fault zone using this characteristic equation the critical rayleigh number of the system can be numerically found as a function of the inclined angle of the three dimensional fault zone for a vertically oriented three dimensional fault zone system the critical rayleigh number of the system can be explicitly derived from the characteristic equation comparison of the resulting critical rayleigh number of the system with that previously derived in a vertically oriented one has demonstrated that the characteristic equation of the rayleigh number is correct and useful for investigating the convective instability of pore fluid flow in the inclined three dimensional fault zone system the related numerical results from this investigation have indicated that 1 the convective pore fluid flow may take place in the inclined three dimensional fault zone 2 if the height of the fault zone is used as the characteristic length of the system a decrease in the inclined angle of the inclined fault zone stabilizes the three dimensional fundamental convective flow in the inclined three dimensional fault zone system 3 if the thickness of the stratum is used as the characteristic length of the system a decrease in the inclined angle of the inclined fault zone destabilizes the three dimensional fundamental convective flow in the inclined three dimensional fault zone system and that 4 the shape of the inclined three dimensional fault zone may affect the convective instability of pore fluid flow in the system

marine palynological record for tropical climate variations since the late last glacial maximum in the northern south china sea the upper part 191 1439 cm of the marine sediment core md05 2906 from the northern south china sea scs was palynologically investigated the chronology suggested that it covered the record since 19 calendar kiloyears before present cal ka bp and revealed a detailed environmental change history since the late last glacial maximum lgm during the late lgm due to the lowered sea level 100 m lower and the shortened distance from the shore to the study site the pollen concentration was very high the pollen assemblages were dominated by non arboreal taxa especially artemisia pollen before 15 cal ka bp abundant subtropical and tropical pollen taxa were still important components and a south subtropical climate prevailed during the late lgm the coexistent rich artemisia pollen possibly was not derived from near shores but was derived mainly from the northern exposed continental shelf in the east china sea ecs after 15 cal ka bp with the rise in the sea level and enhanced distance from the pollen source areas to the core site pollen concentrations started to decline gradually however during the late deglaciation and early holocene the higher concentrations of many pollen taxa reoccurred which cannot be attributed to the sea level changes pinus pollen deposited in the core which is considered to be mostly water carried based on many modern pollen surveys also started to dramatically increase at the same time therefore the higher pollen concentration with more pinus and typha an aquatic plant pollen indicated a notably enhanced terrestrial runoff and precipitation during the last deglaciation holocene transition 11 3 9 4 cal ka bp we inferred that a strong summer monsoon occurred at this time during the late lgm deglaciation transition period the pollen assemblage reflected a gradually warming climate and the climate fluctuations derived from the high latitudes were not well identified this study suggests that solar insolation played a crucial role in controlling the east asian monsoon and hydrological cycle in the northern scs

seismic reflection evidence for a dangerous grounds miniplate no extrusion origin for the south china sea the collision of india and asia has caused large strike slip faults to form in east asia resulting in the extrusion of crustal blocks toward the southeast since the eocene as a result of the indentation of rigid india into asia it has been suggested that the south china sea opened as a result of relative motion between a rigid indochina sundaland block and china alternative models propose that rifting and seafloor spreading were driven by trench forces to the south we test these competing models by analysis of seismic reflection profiles across the boundary between sundaland and the southern rifted margin known as the dangerous grounds we show that the southern boundary of the dangerous grounds is a subduction zone that jammed in the middle miocene to the west the dangerous grounds are bounded by a strike slip zone also active until similar to 16 ma that becomes diffuse south of the now inactive south china sea seafloor spreading center we place the western edge of the dangerous grounds just to the east of the natuna arch lupar line the west baram line is confirmed as originating as a major strike slip fault within the dangerous grounds and is continuous with the red river fault zone because the dangerous grounds were independent of sundaland until similar to 6 ma its motion cannot have been constrained by motion of this block making extrusion impossible as a mechanism to rift the south china sea se motion by both the dangerous grounds and sundaland suggests subduction forces were the primary trigger for plate motions our reconstruction places a similar to 280 km upper limit on the motion on the red river fault and a similar to 1400 km width to the paleo south china sea

patterns of vegetation and climate change in the northern south china sea during the last glaciation inferred from marine palynological records abstract high resolution marine palynological records from the northern south china sea scs provide new information concerning the regional history of vegetation and climate conditions during the last glaciation 70 19 cal ka bp the preservation of rich pollen from tropical and subtropical broadleaf trees largely composed of evergreen quercus castanopsis and euphorbiaceae suggests that a southern subtropical climate prevailed during the last glaciation including the period of the last glacial maximum relatively higher percentages of such pollen taxa occurred during marine isotope stage mis 3 when phyllocladus pollen from the south also increased indicating a warmer terrestrial climate and a strengthened east asian summer monsoon comparing to mis 4 and 2 moreover the maximum percentages of such pollen occurred during the early mis 3 implying that an optimal climate prevailed in this period the reconstructed terrestrial climate corresponds well to marine evidence such as 18o values from planktonic foraminifer shell and records of sea surface temperature this study confirmed that the regional terrestrial climate could be linked closely to changes in both global ice volume and the east asian monsoon attributable to variations of the earth s orbital parameters modern palynological investigations and statistical analyses of the pollen percentages in the profiles indicate that pinus pollen and spores are easily transported by water flow in the northern scs therefore their relative abundance peaks suggest that several wet intervals occurred during the last glaciation

targeting stratiform zn pb ag massive sulfide deposits in ireland through numerical modeling of coupled deformation thermal transport and fluid flow we explore three and two dimensional regional and local scale numerical models for the development of irish zn pb ag mineralization the calculations examined one of the proposed genetic models for the deposits in which mineralization was associated with mixing of two fluids of different salinities in the hanging walls of faults undergoing active deformation during mineralization the numerical models involve coupling between volume changes arising from elastic plastic deformation single phase fluid flow and mixing of fluids with different salinities together with thermal transport both by conduction and by advection we evaluated critical parameters that could optimize the rate of mineralization in three dimensions at the regional scale and in two dimensions at the local scale topographic relief is proposed as one driver of fluid flow the models indicate a competing process through convection of high salinity fluids mainly within faults and originating as partially evaporated seawater which mix with lower salinity fluids driven by topographic relief at the regional scale although there was substantial fluid flow through the basinal sediments considerable flow of basin derived fluids also occurred through the basement with subsequent focusing through faults the effect was enhanced if the basement permeability was increased by fracturing basinal fluids fluid 1 initially in the basin or introduced from meteoric sources in the adjacent highlands were extensively exposed to the basement and became focused into both northward and southward dipping faults a second more saline fluid fluid 2 was driven down into the rock column by convection it had most exposure to the basinal sediments and in some situations to the basement before mixing with fluid 1 the positions of the fluid mixing sites involved a balance between topographically driven flow and convection in three dimensions at a regional scale changes in porosity and permeability induced by plastic deformation influenced the rates of mineralization at the local scale and local darcy flow rates at mineralizing sites mineralization at the local scale resulted from the interplay of fault dip direction displacement deformation induced dilation seal thickness and integrity the modeling results are compatible with empirical data from the irish ore field suggesting that north facing faults dipping away from the inferred position of topographic highs preferentially host economic deposits there is nothing intrinsic in the modeling that constrains the timing of the mineralization relative to compressional or extensional events although a body of research favors mineralization overlapping with extension as at the navan silvermines and tynagh deposits in contrast at the lisheen deposit mineralization postdates the main extensional fault offsets we suggest that a switch to shortening is a possible driver for this mineralizing system

large scale erosional response of se asia to monsoon evolution reconstructed from sedimentary records of the song hong yinggehai and qiongdongnan basins south china sea

provenance routing and weathering history of heavy minerals from coastal placer deposits of southern vietnam abstract heavy mineral rich sands along the coastal margin of southern vietnam often contain commercial deposits of ilmenite and zircon but their origin is unknown a multi method approach based on petrology geochemistry and detrital zircon geochronology was used to define the provenance and transport history of these mainly quaternary sands a trend of progressive enrichment of ilmenite tio2 content from north to south was observed this reflects increased levels of weathering attributed to a wider coastal margin and shelf in the south combined with a succession of erosion and reburial events associated with interstadial and interglacial sea level changes weathering took place during lowstands detrital zircon u pb age signatures collected from 25 major river outlets along the coast of vietnam helped to locate potential sand sources prominent age groups spanning 90 120 ma and 220 250 ma with a minor group at 400 500 ma are present in all of the detrital zircon u pb age distributions of contemporary beach sands and quaternary coastal dune placer deposits proterozoic grains are also present but constitute pb age signatures from river sands that drain this area are identical to zircon age distributions in placer sands river sands from northern vietnam the mekong and its delta contain abundant paleozoic and proterozoic zircons which are largely absent from the placer sands and so are ruled out as primary sources

mantle and slab contributions in arc magmas destructive plate margins are major sites of terrestrial magmatism that have long had a key role in models for the generation of continental crust and the development of chemical heterogeneities in the upper mantle in these models it is important to evaluate the nature and size of contributions from preexisting crust in the generation of magmas above subduction zones and to constrain the size of the fluxes of different elements from the subducted crust and the overlying mantle wedge in arc magmas kay 1 980 hawkesworth ellam 1 989 hawkesworth et al 1 99 1 a mcculloch gamble 1 99 1 the more mobile components of the subducted oceanic crust are the products of element fractionation processes in the upper continental crust the ocean basins and hydrothermal systems on the ocean floor they therefore have distinctive isotope and even trace element ratios that in principle should be readily distinguished from those in peridotite in the mantle wedge prior to subduction in practice however element ratios are almost certainly fractionated during shallow level dehydration of sub ducted material and at greater depths where material from the slab appears to contribute in the generation of arc magmas moreover the isotope signal of slab derived material may be heavily diluted by exchange with

phase relations of peraluminous granitic rocks and their petrogenetic implications peraluminous granitic rocks are magmatic rocks that contain quartz po tassic feldspar and or sodic plagioclase and one or more aluminum rich minerals thus these rocks contain more aluminum than could be accom modated by feldspar in a cipw normative calculation and show normative corundum peraluminous granites are defined by their chemical nature but their recognition is often based on petrography this discrepancy thus may lead to inconsistency although peraluminous granitic rocks are com monly true granite streckeisen 1973 they range from tonalite to granite of special interest are strongly peraluminous as defi ned below granite granodiorite or in rare instances trondhjemite these rocks contain alumi nous minerals almandinic garnet cordierite tourmaline muscovite that are thought to be stable in silicate magmas only if these magmas are themselves peraluminous the origin of peraluminous granite and related intrusive rocks is of much current interest many of these form shallow level intrusions and they can reach enormous sizes examples are the batholiths in the lachlan fold belt in southeastern australia white chappell 1983 the cornub ian batholith in england exley stone 1966 the south mountain batholith in nova scotia mckenzie clarke 1975 the manaslu batho lith in the himalayas lefort 1981 much of the idaho batholith

deep water bottom current evolution in the northern south china sea during the last 150 kyr evidence from sortable silt grain size and sedimentary magnetic fabric abstract deep water bottom current dwbc plays a central role in global climate relative to the atlantic due to poor preservation of marine carbonate in the pacific sediments the evolution of pacific deepwater circulation is still unclear the south china sea scs connection to the pacific through luzon strait at a depth of about 2400 m offers a unique opportunity to monitor the changes of western pacific deep water circulation we present a magnetic and grain size analysis of two sediment cores pc111 and pc83 located in xisha trough northern scs in order to reconstruct past changes of dwbc during the last 150 kyrs variations in the mean size of sortable silt are interpreted to indicate past changes of dwbc which suggest abrupt millennial scale increasing dwbc strength during heinrich stadials hs1 hs2 hs3 hs4 hs5 hs6 hs7 hs8 hs9 hs10 hs11 for past 150 kyr the flow orientation of dwbc revealed by the anisotropy of magnetic susceptibility ams are mostly nw se direction in core pc111 and core pc83 which are both parallel to the local topography combined with the published studies we suggest the increase bottom current in northern scs during the heinrich stadials hs1 to hs11 times of weak north atlantic deep water formation demonstrating the tight coupling between the atlantic and pacific oceans

late holocene evolution of the mekong subaqueous delta southern vietnam abstract as asia s third largest river with regard to sediment load the mekong river delivers approximately 160 million tons of sediment per year to the south china sea high resolution seismic profiling and coring during 2006 and 2007 cruises revealed a low gradient subaqueous delta system up to 20 m thick surrounding the modern mekong river delta mrd in the west of the south china sea based on clinoform structure grain size 210pb ams 14c and 13c results the subaqueous delta is divided into four zones defined by different sedimentary processes and depositional features over the past 3000 yr the evolution of the mrd has shown a morphological asymmetry indicated by a large down drift area and a rapid progradation around cape camau 200 km downstream from the river mouth this asymmetric feature is consistent with increased wave influence the strong southwestward coastal current strengthened by the strong ne monsoon plays an important role locally in longshore transport of resuspended sediments into the gulf of thailand a late holocene sediment budget for the mrd has been determined based on the area and thickness of deltaic sediment approximately 80 of mekong delivered sediment has been trapped within the delta area which together with a falling sea level resulted in a rapidly prograding mrd over the past 3000 yr

la icp ms u pb zircon geochronology of early neoproterozoic mafic intermediat intrusions from nw margin of the yangtze block south china implication for tectonic evolution abstract proterozoic magmatism along the margin of the yangtze block is extensive and provides important evidence in the plate tectonic reconstructions for the proterozoic the bikou terrane on the northwestern margin of the yangtze block consists of complex archean to paleozoic metamorphosed sedimentary and volcanic sequences intruded by several mafic intermediate bodies three major intrusions are dated using laser ablation icp ms u pb of zircons this dating yielded ages 884 5 5 ma 884 14 ma and 877 13 ma for the pingtoushan diorite guankouya diorite and liujiping gabbro respectively these are the oldest intrusions found along the margin of the yangtze block geochemical analyses of these mafic intermediate intrusions show they have negative nb ta anomalies that are best interpreted as having been formed in an arc setting the older magmatism and unique rock associations of the bikou terrane suggest that it was an isolated terrane that existed in a marginal sea to the yangtze block the eastward subduction present day orientation resulted in early arc magmatism 880 ma within the bikou terrane and lasted to ca 750 ma this study supports the model that the yangtze block was surrounded by ocean and developed arc magmatism along its western margins in neoproteroic time with continent directed subduction that commenced in the early neoproterozoic the bikou terrane was subsequently accreted onto the northwest margin of the yangtze block at about 750 ma

sedimentary processes on the mekong subaqueous delta clay mineral and geochemical analysis sedimentary processes on the inner mekong shelf were investigated by examining the characteristics of sediments sampled in gravity cores at 15 locations including grain size clay mineralogy sediment accumulation rates and the elemental and stable carbon isotopic composition of organic matter atomic c n ratios and delta c 13 deltaic deposits exhibit contrasting characteristics along different sides of the delta plain south china sea scs hereafter to the east and gulf of thailand got hereafter to the west as well as on and off the subaqueous deltaic system on one hand cores recovered from the subaqueous delta in the scs got are consisted of poorly well sorted sediments with similar different clay mineral assemblage with from mekong sediments excess pb 210 profiles supported by c 14 chronologies indicate either non steady scs side or rapid accumulation got side processes on the subaqueous delta the delta c 13 and c n ratio indicate a mixture of terrestrial and marine sourced organic matter in the deltaic sediment on the other hand cores recovered from areas with no deltaic deposits or seaward of the subaqueous delta show excess pb 210 profiles indicating steady state accumulation with a greater proportion of marine sourced organic matter core analysis s relevance with local depositional environment and previous acoustic profiling are discussed c 2012 elsevier ltd all rights reserved

past environmental and circulation changes in the south china sea input from the magnetic properties of deep sea sediments abstract the south china sea located at the transition between the pacific and the indian ocean receives every year mainly during the rain season enormous amounts of river sediments originating from the erosion weathering of rocks in the catchment basins at sea these sediments are carried by different water masses to their deposition site and they constitute a unique archive for past environmental studies in this region the magnetic fraction of deep sea sediments though forming a minority in volume provides incredibly valuable information for paleoceanographic reconstructions as long as its provenance and source to sink processes are well constrained after a brief description of the climatic sedimentological and oceanographic context of the south china sea scs a review of the information available so far in the literature about the magnetic properties of scs sediments is presented it shows a large variety of interpretations conclusions that finally results in a rather unclear picture because in such a context the characterization of the sediment at the source is critical the magnetic properties recently obtained from a set of samples from rivers and marine surface sediments are summarized to describe the present day situation they are then used to interpret paleorecords from a set of seven marine cores distributed from the southern to the northern basins at different water depths and all covering at least the last climatic cycle the results reported here for the first time suggest that the magnetic mineralogy remains rather stable in time on land and that its time and spatial distribution at sea is an interplay of changes in sea level and deep sea circulation during low sea level periods bottom deep sea circulation is weak and the deposited sediment originates from the proximal rivers on the contrary during high sea level the circulation is enhanced transporting more sediment most likely from taiwan and luzon to the northwestern part of the scs and also in smaller proportion to the southern basin where it mixes with the local river borne sediment by comparing the two longest records we observe that this pattern is repeated over the last 900 ka superimposed to the 100 kyr cyclicity we also observe a longer term evolution with a maximum in the bottom current strength around 500 ka coinciding with global changes in the deep ocean circulation and carbon cycle these new results based on a wide spectrum of magnetic properties of numerous marine sedimentary cores from the scs show that the magnetic fraction yields important insights into past changes of the sedimentary pathways in particular the dynamic of the deep sea circulation depending on the global climatic context

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

biological mixing intensity and rates of organic carbon accumulation in north carolina slope sediments sediment accumulation rates and biological mixing intensities were determined at three sites on the north carolina slope based on profiles of naturally occurring14c 210pb and234th the three sites all were at a water depth of 850 m with a spacing of 150 180 km between sites sediment accumulation rates increase from south to north from values of 7 cm ky 1 at site i to 160 cm ky 1 at site ii to 1100 cm ky 1 at site iii the organic carbon burial rate at these sites also increases in the northward direction from 0 65 site i to 20 site ii to 150 g corg m 2 year 1 site iii these data indicate that continental margin environment can exhibit highly variable carbon fluxes over relatively small distances on the seafloor the rate of organic carbon accumulation at site iii is one of the highest values reported for the marine environment based on these accumulation rates and dissolved inorganic carbon flux estimates from each site the seabed organic carbon preservation efficiency i e the ratio of corg accumulation rate to corg deposition rate times 100 was estimated to vary from 6 0 to 54 to 88 at sites i ii and iii respectively the14c age of organic matter in surface sediments was older at site iii 1800 years bp than at sites i and ii 800 years bp indicating that site iii receives a greater proportion of old sediment from either up slope areas or from terrigenous sources inventories of excess234th half life of 24 days were used as a tracer for particle flux covering the 100 days prior to the october 1989 july august 1990 and august 1991 cruises the mean234th inventories at the three sites were 4 7 1 9 8 4 6 3and23 1 7 3dpm cm 2 for sites i ii and iii respectively profiles of excess234th activity reveal that the biological mixing intensity is greater at site iii meandb 19 11cm2year 1 n 5 than at either site i meandb 6 0 6 2cm2year 1 n 6 or site ii meandb 4 6 5 2cm2year 1 n 9 in addition to the trend in mixing coefficients the depth of particle mixing on a 100 day time scale generally is greater at site iii than at the other two sites these observations of particle mixing intensity are consistent with the northward increase in the mean abundance of macrofauna 300microns from mean values of 9400 m 2 at site i to 21 400 m 2 at site ii to 55 500 m 2 at site iii for the three study sites off north carolina a strong correlation r2 0 99 p 0 06 exists between macrofaunal abundance and the organic carbon deposition rate an equally strong correlation r2 0 99 p 0 04 occurs between macrofaunal abundance and the234th inventories index of 100 day particle flux fine sand size glass tracer beads were dispersed at these three sites by submersible and then the field plots were sampled 1 year later the vertical distributions of beads at the three sites are consistent with a higher mixing intensity at site iii than at the other two sites but the estimated mixing coefficients generally are lower than those determined from the profiles of234th the slower mixing of the glass beads may be the result of their low food value and or their relatively large size compared to the surrounding fine grained sediments

marginal basin evolution the southern south china sea abstract the southern south china sea is divided into contrasting morphology by the west baram line to the west is the sundaland extinct passive margin in which rifting began in the eocene 46 ma and ceased at 19 21 ma anomaly 6 where sea floor spreading began much later than along the shelf of china the break up hiatus lasted 3 5 ma marked by the mid miocene unconformity also preserved on land sarawak the post rift strata date from 16 ma and drape over the rifted topography to the east is a convergent margin that became a collision zone in the middle miocene the sunda shelf is of uniform 30 km thickness except in localised deep basins and extends to a water depth of 200 m the continental slope is narrow the continental rise dangerous grounds is covered by water ranging from 500 m to 3 5 km depth at the continent ocean transition its width ranges from 170 to 330 km the rajang delta extends over the shelf and continental slope its post rift sediments drape over the rifted proximal topography of the dangerous grounds to the east the drape is thinner and has not completely buried the rifted topography the cuestas appear to have supported the carbonate build up infrastructures of the spratly islands whose slopes rise abruptly from a sea floor of 2 3 km depth the sabah and brunei margin is a collision zone that was formerly convergent the on land geology indicates a mesozoic ophiolitic basement the main collision feature is the western cordillera constructed mainly of sandy eocene to lower miocene turbidites predominantly oligocene to lower miocene in the west crocker 32 18 ma uplifted episodically throughout the upper miocene and pliocene 14 8 ma the 2 km deep northwest borneo trough may be a relict of the convergence phase but could also be a collisional foredeep the oil prolific baram delta resulting from uplift and erosion of the western cordillera has built out as far as the northwest borneo trough it is suggested that the passive margin continental rise dangerous grounds has been underthrust beneath sabah to cause uplift of the western cordillera the west baram line accordingly abruptly separates the collision zone from the western extinct passive margin and is a now extinct major right lateral transform fault

the persistence of memory the fate of ancient sedimentary organic carbon in a modern sedimentary system the cycle of organic carbon burial and exhumation moderates atmospheric chemistry and global climate over geologic timescales the burial of organic carbon occurs predominantly at sea in association with clay sized particles derived from the erosion of uplifted continental rocks it follows that the history of the fine grained particles on land may bear on the nature of the organic carbon buried in this study the evolution of clay associated organic matter was followed from bedrock source to the seabed in the eel river sedimentary system of northern california using natural abundance 13c and 14c tracers approximately half of the fine grained organic carbon delivered to the shelf is derived from ancient sedimentary organic carbon found in the uplifted mesozoic tertiary franciscan complex of the watershed the short residence time of friable soils on steep hill slopes coupled with rapid sediment accumulation rates on the shelf slope act to preserve the ancient organic carbon a comparable quantity of modern organic carbon is added to particles in the watershed and on the shelf and slope the bimodal mixture of ancient and modern c in soils and sediments may be characteristic of many short mountainous rivers if the eel river chemistry is typical of such rivers more than 40 tg of ancient organic c may be delivered to the world s oceans each year a flux of that magnitude would have a significant influence on marine and global c cycles

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

geological and oceanographic controls on seabed fluid escape structures in the northern zhongjiannan basin south china sea abstract mega pockmarks which can reach a few kilometers in width and a few hundred meters in depth and associated fluid escape features in the northern zhongjiannan basin south china sea have received much attention in the recent years due to their large variety of morphologies and sizes in this work an integrated interpretation of detailed multibeam bathymetry conventional multi channel seismic reflection and seismic oceanography sections covering the northern zhongjiannan basin south china sea are used to image the seafloor morphology the subsurface geology and the water column in order to better understand the geological and oceanographic controls on the formation and evolution of the widespread fluid escape features in this region regional uplift and volcanism took place in this area near the palaeogene neogene boundary and during the neogene quaternary post rift thermal subsidence period differential subsidence and diapirism deformed the overlying kilometer scale successions mainly after the miocene focused flow of fluids originated at depth occurred along gas pipes polygonal faults tapered reflectors and faults into the shallow sediments resulting in intense fluid blowouts and complex fluid escape structures on the seafloor our results show that elongated pockmarks pockmark gullies and gullies in the northern slope are generally controlled by underlying buried channels and gullies however pockmark gullies and gullies in the southern slope are controlled by gravity sliding slumping along with crescent pockmarks developing parallel to water depth contours to varying degrees intense fluid escape activities associated with mud diapirism and gas escape in the study area resulted in mud extrusion and the formation of mud volcanoes complex seabed morphologies and irregular pockmarks mega pockmarks collapse structures and blind valleys have formed at the apexes of tectonic uplifts related to volcanism and above basement highs which suggests that these processes played a role in creating pathways for fluid escape bottom currents associated with the south china sea western boundary current imaged here for the first time with the seismic oceanographic method mainly flow to the south and erode the seafloor making pockmark walls and slopes of gullies and channels in the north steeper than those in the south some fluid escape structures such as the mega pockmarks and pockmark gullies are buried by sediment transported by the bottom currents turbidity currents flowing downslope along the long pockmark gullies also erode the seafloor and make the nw slopes of the mud volcanoes less steep than those in the se eddies imaged by the seismic sections may also play a role in transporting sediment to the deep sea in this area some of the reported fluid escape structures are still active as indicated by the fluid pathways that reach the seafloor and plumes in the water column imaged by the seismic sections

south china sea fluvial sediments an introduction the south china sea offers a special research opportunity for earth scientists because of its location and its well preserved terrigenous sediments wang and li 2009 as the largest one of the marginal seas separating asia from the pacific the south china sea functions as a natural laboratory for study of land sea interactions because of its unique source to sink record of fluvial sediments the south china sea receives more than 600 million tons of detrital sediments annually from numerous rivers including world s largest rivers such as the pearl the red and the mekong fig 1

petrogenesis and ore genesis of the lengshuiqing magmatic sulfide deposit in southwest china constraints from chalcophile elements pge se and sr nd os s isotopes the lengshuiqing magmatic ni cu ore deposit is hosted in four small mid neoproterozoic mafic ultramafic intrusions in the western margin of the yangtze craton in china the ore bearing intrusions were emplaced into early neoproterozoic metamorphosed volcanic sedimentary rocks granites and diorites the intrusions are dominated by lherzolite olivine websterite websterite and gabbro with sulfide mineralization associated with ultramafic rocks in the lower parts of the intrusions the forsterite contents of olivine crystals in the lengshuiqing ore bearing intrusions are up to 86 mol indicating that their parental magma experienced moderate degrees of fractional crystallization olivine crystals in the intrusions are depleted in ca

source to sink transport processes of fluvial sediments in the south china sea the south china sea offers an excellent case for studying source to sink transport processes of fluvial sediments among the global marginal seas this study synthesizes existing clay mineralogical and geochemical data from similar to 1500 samples from the seafloor and surrounding rivers deepwater mooring observation results and high resolution glacial cyclic clay mineralogy records from six high quality sediment cores source to sink sediment transport from the river mouth to the continental shelf and then to the abyssal basin is investigated at two time scales modern process and late quaternary glacial cycles the results firstly show the high diversity of clay mineralogical and geochemical compositions in riverbed surface sediments surrounding the south china sea e g dominant illite and chlorite in taiwan overwhelming majority of smectite in luzon dominant kaolinite in south china and similar amounts of the four clay mineral species in red and mekong river systems the formation of these fluvial sediments through chemical weathering in surrounding drainage systems is controlled principally by the east asian monsoon climate with warm temperature and high precipitation and subordinately by tectonic activity and specific lithological character the basin wide distribution of clay mineral assemblages combined with neodymium and strontium isotopic compositions reflects strong provenance control and differential settling effects the differential settling of kaolinite in slightly saline and proximal regions relative to smectite in more saline and distal regions is well demonstrated in the south china sea through combining clay mineralogical distributions on the seafloor with observed oceanic current systems the modern transport pathways can be well established e g in the northern south china sea illite and chlorite from taiwan are mainly carried by contour currents with the strong influence of mesoscale eddies high resolution sediment dynamic analysis for the late quaternary reveals different sediment transport patterns in the northern western and southern south china sea when the land sea configuration dramatically changed during glacial conditions in the north the terrigenous dispersal is mainly controlled by provenance supply and oceanic current transport in the west the clay mineral assemblage reflects variations of the prevailing surface current influenced by monsoon winds in the south the clay mineral input indicates intensive chemical weathering during interglacial periods and strengthened physical erosion during glacial periods the transport of terrigenous sediments since the last glaciation is quantitatively reconstructed through studying two deepwater sediment cores located in the northern south china sea the relative contributions from three provenances south china luzon and taiwan highlight their distinct variability over the last 28 ka implying that the southward shift of the inter tropical convergence zone itcz at 16 ka bp caused an increased sediment contribution from south china the intensified influence of the kuroshio current intrusion controls more efficient westward transport of luzon sediments and the stronger deepwater current transports taiwan sourced sediments further westward from the last glaciation to the holocene c 2015 elsevier b v all rights reserved

neoproterozoic arc related mafic intrusions along the northern margin of south china implications for the accretion of rodinia abstract south china has been considered as part of the rodinian supercontinent during neoproterozoic time although its paleogeographic position within this supercontinent is still a matter of debate the wangjiangshan and bijigou complexes along the northern margin of south china are among the largest mafic intrusions in china new shrimp zircon u pb results indicate that these two intrusions have crystallization ages of 820 ma and 780 ma respectively enrichment of large ion lithophile and light rare earth elements and depletion of high field strength elements in these intrusions suggest derivation from an active arc along a continental margin this interpretation of these intrusions as part of a continental arc assemblage is in contrast with the previous view that they were products of a neoproterozoic mantle plume that initiated the breakup of rodinia the presence of a neoproterozoic magmatic arc suggests that rodinian oceanic lithosphere was subducted beneath the present northern margin of south

geochronology and geochemistry of the c 80 ma rutog granitic pluton northwestern tibet implications for the tectonic evolution of the lhasa terrane the rutog granitic pluton lies in the gangdese magmatic arc in the westernmost part of the lhasa terrane nw tibet and has shrimp zircon u pb ages of c 80 ma the pluton consists of granodiorite and monzogranite with sio2 ranging from 62 to 72 wt and al2 o3 from 15 to 17 wt the rocks contain 2 33 4 93 wt k2o and 3 42 5 52 wt na2o and have na2o k2o ratios of 0 74 2 00 their chondrite normalized rare earth element ree patterns are enriched in lree la yb n 15 to 26 and do not show significant eu anomalies eu 0 68 1 15 on a primitive mantle normalized trace element diagram the rocks are rich in large ion lithophile elements lile and poor in high field strength elements hfse hree and y their sr y ratios range from 15 to 78 with an average of 30 the rocks have constant initial 87 sr 86 sr ratios 0 7045 to 0 7049 and slightly positive end t values 0 1 to 2 3 similar to i type granites generated in an arc setting the geochemistry of the rutog pluton is best explained by partial melting of a thickened continental crust triggered by underplating of basaltic magmas in a mantle wedge the formation of the rutog pluton suggests flat subduction of the neo tethyan oceanic lithosphere from the south crustal thickening may have occurred in the late cretaceous prior to the india asia collision

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

origin and tectonic significance of a mesozoic multi layer over thrust system within the yangtze block south china abstract in the yangtze block south china a well developed mesozoic thrust system extends through the xuefeng and wuling mountains in the southeast to the sichuan basin in the northwest the system comprises both thin and thick skinned thrust units separated by a boundary detachment fault the dayin fault to the northwest the thin skinned belt is characterized by either chevron anticlines and box synclines to the northwest or chevron synclines to the southeast the former structural style displays narrow exposures for the cores of anticlines and wider exposures for the cores of synclines thrust detachments occur along silurian fs and lower cambrian fc strata and are dominantly associated with the anticlines to the southeast this style of deformation passes gradually into one characterized by chevron synclines with associated principal detachment faults along silurian fs cambrian fc and lower sinian fz strata there are however numerous secondary back thrusts therefore the thin skinned belt is like the valley and ridge province of the north american applachian mountains the thick skinned belt structurally overlies the thin skinned belt and is characterized by a number of klippen including the xuefeng and wuling nappes it is thus comparable to the blue ridge province of appalachia the structural pattern of this thrust system in south china can be explained by a model involving detachment faulting along various stratigraphic layers at different stages of its evolution the system was developed through a northwest stepwise progression of deformation with the earliest delamination along lower sinian strata fz analyses of balanced geological cross sections yield about 18 1 21 total 88 km shortening for the thin skinned unit and at least this amount of shortening for the thick skinned unit the compressional deformation from southeast to northwest during late jurassic to cretaceous time occurred after the westward progressive collision of the yangtze block with the north china block and suggests that the orogenic event was intracontinental in nature

preservation of elemental and isotopic source identification of sedimentary organic matter the amount and type of organic matter in the sediments of lakes and oceans contribute to their paleoenvironmental and paleoclimatological records only a small fraction of the initial aquatic organic matter survives destruction and alteration during sinking and sedimentation selective degradation modifies the character of the surviving small fraction of organic matter which becomes incorporated in bottom sediments organic matter alterations can continue to sub bottom depths of hundreds of meters corresponding to millions of years source and paleoenvironmental information nonetheless remains preserved in the molecular elemental and isotopic compositions of organic matter c n and 13c values of total organic matter in particular appear to retain paleoenvironmental information for multi myr time periods

channel related sediment waves on the eastern slope offshore dongsha islands northern south china sea abstract the characteristics and origin of the sediment waves on the eastern slope offshore dongsha islands northern south china sea scs were analyzed by integrating high resolution multibeam bathymetric seismic and piston core data four sediment wave fields were identified which are the west dongsha channel wdsw west taiwan channel wtsw south taiwan channel stsw and west penghu channel wpsw sediment wave fields respectively they are distributed on the right levees looking downstream and adjacent overbank areas of related channels wdsw wtsw and stsw or developed outside the channel mouths wpsw each sediment wave field consists of several to tens of rows of sediment waves these sediment waves are up to 2 8 7 2 km in wavelength and 30 60 m in wave height with wave crests orthogonal to or oblique to the orientation of the channels the sediment waves gradually decrease in dimension both in downslope direction and with the distance increasing away from the channel most sediment waves are asymmetric in cross sections with a thicker upslope flank and a thinner downslope flank and therefore migrate upslope piston cores suggest that sandy turbidites developed with normal size grading dominate the channels while massive muds with thin sandy or silty turbidite interbeds prevail in the inter channel regions on which the sediment waves situated the sediment waves on the channel levees and related overbank areas wdsw wtsw and stwd were presumably built by the turbidity currents overspilled from the channels while those in wpsw located outside the mouth of the west penghu channel are suggested generated by unconfined sheet like turbidity currents outside the channel mouth

subduction related origin of the 750 ma xuelongbao adakitic complex sichuan province china implications for the tectonic setting of the giant neoproterozoic magmatic event in south china abstract the xuelongbao plutonic complex in the eastern margin of the tibetan plateau is dated at 748 7 ma using the shrimp zircon u pb method and represents part of the neoproterozoic igneous assemblage of south china rocks in the complex include tonalite and granodiorite and have sio2 ranging from 62 0 to 74 8 wt and al2o3 from 14 3 to 20 9 wt their na2o contents range from 4 2 to 6 7 wt and k2o from 0 47 to 1 96 wt indicating that they belong to the na series these rocks show chondrite normalized ree patterns depleted in hree and variably enriched in lree they have positive sr and negative nb and ti anomalies in the primitive mantle normalized trace elemental spider diagram their sr contents range from 320 to 780 ppm and y contents are lower than 10 ppm resulting in high sr y ratios 52 320 characteristic of typical adakites their end t and initial sr isotopic compositions range from 0 36 to 2 88 and from 0 7033 to 0 7054 respectively the geochemical features of the xuelongbao plutonic complex are consistent with an origin from adakitic magmas that were likely derived from partial melting of a subducted oceanic slab together with arc signatures of other granites and mafic intrusions in the region the 750 ma xuelongbao adakitic complex provides evidence for a major subduction related neoproterozoic magmatic event of south china

analysis of sediment distribution and transport for mitigation of sand deposition hazard in tam quan estuary vietnam tam quan estuary in the binh dinh province of vietnam provides shelter for about 2000 boats recently erosion and accretion associated with the mouth of the estuary have badly affected marine transportation and economic development an 850 m long jetty was constructed to mitigate the hazard however sand deposition still continues to be an ongoing problem this paper aimed to assess the reasons for sand deposition in the estuary based on sediment distributions and transport pathways following an investigation of topography geological and hydrodynamic conditions and sediment characteristics an analysis of sediment transport trends was undertaken results show that medium sand is distributed along the coast from the shoreline to 2 5 m water depth fine sands are found in deeper areas of 10 13 m where there are also medium sand deposits thought to be of ancient origin sand deposition has intensively occurred in the estuary due to a dominant north to south longshore sediment transport regime sediment from this regime is presently trapped by the jetty and deposited in the navigation channel as a solution it is suggested that a properly designed jetty stemming from the headland on the north side of the estuary could effectively control the patterns of sediment transport enabling the sand to bypass the entrance thereby avoiding entrapment inside its mouth

the influence of amphibole fractionation on the evolution of calc alkaline andesite and dacite tephra from the central aleutians alaska petrologic studies of tephra from kanaga adak and great sitkin islands indicate that amphibole fractionation and magma mixing are important processes controlling the composition of calc alkaline andesite and dacite magmas in the central aleutians amphibole is ubiquitous in tephra from kanaga and adak islands whereas it is present only in a basaltic andesite pumice from great sitkin dacitic tephra from great sitkin do not contain amphibole hornblende dacite tephra contain hb plag ox opx cpx phenocrysts with simple zoning patterns suggesting that the dacites evolved in isolated magma chambers andesitic tephra from adak contain two pyroxene and hornbelende populations and reversely zoned plagioclase indicating a more complex history involving mixing and fractional crystallization mass balance calculations suggest that the andesitic tephra may represent the complements of amphibole bearing cumulate xenoliths both formed during the evolution of high al basalts the presence of amphibole in andesitic and dacitic tephra implies that aleutian cale alkaline magmas evolve in the mid to lower crust under hydrous 4 wt h2o and oxidizing ni nio conditions amphibole bearing andesites and pyroxene bearing dacites from great sitkin indicates fractionation at several levels within the arc crust despite its absence in many calc alkaline andesite and dacite lavas open system behavior involving amphibole fractionation can explain the trace element characteristies of lavas found on adak island neither open nor closed system fractionation involving a pyroxene bearing assemblage is capable of explaining the trace element concentrations or ratios found in the adak suite we envision a scenario where amphibole was initially a liquidus phase in many calc alkaline magmas but was later replaced by pyroxenes as the magmas rose to shallow levels within the crust the mineral assemblage in these evolved lavas reflects shallow level equilibration of the magma whereas the trace element chemistry provides evidence for a earlier amphibole bearing mineral assemblage

suspended sediment dynamics during the inter monsoon season in the subaqueous mekong delta and adjacent shelf southern vietnam land ocean interactions in the coastal zone are severely influenced by tidal processes in regions of high sediment discharge like the mekong river delta in southern vietnam these processes are even more significant three cruises in 2006 2007 and 2008 were carried out to investigate the sediment suspension and their spatial distribution additionally we investigated the influence of the tidal currents in relation to the suspended sediment therefore all cruises took place during the inter monsoon season between march and may where wave and wind influences are not dominant in contrast to the summer monsoon may to early october and winter monsoon season november to early march suspended sediment concentrations sscs in the particle size range between 2 5 and 500 iim were measured with an lisst instrument laser in situ scattering and transmissiometry current velocities and directions were recorded with an acoustic doppler current profiler adcp additionally data of different tidal gauge stations in the mekong river delta were correlated and compared to the mixed semidiurnal diurnal tidal cycle our results show significant areas of sscs greater than 25 mu l l in the mekong river branches and its subaqueous delta during the inter monsoon season 20 of all measured sscs in the subaqueous mekong delta exceed 100 mu l l highest concentrations occur close to the seabed sscs decrease at the transition to the open shelf the shelf region contains only low suspension loads especially on the south eastern shelf 99 of all samples 25 close to the seabed in water depths of 20 25 m two surveys lasting 25 h each were performed on mooring stations in 12 m mooring 1 and 26 m mooring 2 water depth and located 3 2 km apart on the subaqueous delta slope similar patterns of ssc over time show that concentrations of suspension load correlate with the tidal current velocities high tidal current velocities of up to 0 6 m s near the sea bottom generate increasing sscs of more than 25 gl in the water column additionally a significant trend of decreasing ssc from the near seabed to the upper part of the water column can be observed in terms of sediment transport the ebb phase dominates the tidal cycle by its higher tidal current velocities but the flood phase has the longer duration the switch of the tidal current direction from ebb to flood phase occurs rapidly against which the change from flood to ebb phase requires up to 3 h this leads to an asymmetry of the tidal ellipses and may cause a net sediment transport from the shelf into the subaqueous mekong delta in the subaqueous mekong delta and adjacent shelf seven transects show similar patterns of sscs dependent to the tidal phase a hypopycnal sediment plume from the subaqueous mekong delta into the shelf region was not observed our results imply that resuspension by tidal currents dominates the sediment transport in the subaqueous mekong delta and adjacent shelf regions during the inter monsoon season c 2012 elsevier ltd all rights reserved

detrital zircon u pb geochronological and lu hf isotopic constraints on the precambrian magmatic and crustal evolution of the western yangtze block sw china abstract in situ u pb geochronological and lu hf isotopic studies of detrital zircon from a variety of precambrian strata along the western margin of the yangtze block of south china show that the rocks have maximum depositional ages of ca 1000 740 ma all of the strata probably formed in a subduction related tectonic setting the kunyang group heishantou formation and huili group tongan formation have maximum depositional ages of 960 1000 ma and contain zircons whose ages are dominantly paleoproterozoic 2500 2300 1900 1700 and 1650 1500 ma the yanbian group has a zircon population ranging mainly from 860 to 1000 ma different from that of the bikou group with ages from 745 to 890 ma ages of zircons from the lieguliu and guanyinya formations range from 740 to 900 ma the paleoproterozoic detrital zircons commonly have negative or slightly positive hf values from 8 0 to 2 3 whereas most of the 1000 740 ma zircons have moderately to significantly high positive hf values from 0 3 to 13 0 the new detrital zircon u pb and lu hf isotopic dataset suggests four major episodes of magmatism in the region 2500 2300 1900 1700 1650 1500 and 1000 740 ma and the presence of widespread archean basement of the western yangtze block the data also suggest an important period of juvenile magmatism at 1000 740 ma our results favor a model in which continuous subduction 1000 740 ma along the western margin of the yangtze block lasted for 260 million years

stratigraphic and u pb shrimp detrital zircon evidence for a neoproterozoic continental arc central china rodinia implications abstract the bikou terrane of central china is located within the southwest qinling mountains along the northwest margin of the yangtze plate the terrane consists of a volcanic assemblage the bikou group and a correlative volcaniclastic turbidite sequence the hengdan group u pb shrimp detrital zircon dating of 181 grains collected from the hengdan group indicates ages from ca 850 to 700 ma with a peak near 770 ma given the predominance of volcanic single cycle zircons within the hengdan group and the thick succession of coarsening upward volcaniclastic marine sediments the sequence is best interpreted as a fore arc basin this interpretation is supported by previous u pb shrimp and conventional u pb zircon dating performed within the bikou group that indicates subduction related magmatism persisted from at least ca 850 to as recently as 700 ma together these data indicate that the bikou terrane is a fragment of a neoproterozoic continental arc system and that the yangtze plate was bordered by

pliocene quaternary evolution of the continental shelf of central vietnam based on high resolution seismic data abstract the continental shelf of central vietnam is characterized by complex geological structures in a strike slip setting and thus by unique morphology compared to other parts of the vietnam shelf studying the pliocene quaternary sediments in this area allows the processes that control the stratigraphic evolution to be highlighted in this paper we present new results derived from high resolution seismic data that reveal the geological evolution of the continental shelf of central vietnam during the pliocene quaternary our results demonstrate that the continental shelf of central vietnam is narrow relatively to other parts of the vietnam shelf the evolution of the shelf during the pliocene quaternary was controlled by the pre pliocene rifting process however fault systems below the present shelf were not strongly active during the pliocene quaternary period except for several listric faults in the slope nonetheless quaternary volcanic activities occurred widely both offshore and onshore especially near the major faults the pliocene quaternary sedimentary deposits were subdivided into 7 units based on unconformities as result of marine regression corresponding to glacial cycles sedimentary environments during this period consist of coastal plain fluvial nearshore and shelf environments most of the holocene sediments were trapped nearshore while the outer shelf is characterized by abundant lag gravels as a result of rapid sea level rise between 20 and 8 ka

the yanbian terrane southern sichuan province sw china a neoproterozoic arc assemblage in the western margin of the yangtze block abstract the magmatic and tectonic history of the yangtze block and its possible affinity with other neoproterozoic arc terranes are important elements in the reconstruction of neoproterozoic plate tectonics the yanbian terrane in the western margin of the yangtze block is a typical arc assemblage composed of a flysch type sedimentary sequence intruded by gabbroic and granodioritic plutons the sedimentary sequence consists chiefly of tuffaceous material with interlayered chert sandstone and pillow basalts laser ablation icp ms u pb dating of detrital zircons from the sandstones yield ages as young as 840 ma the gaojiacun and lengshuiqing mafic intrusions are dated at 812 3 ma and 806 4 ma respectively using the shrimp zircon u pb technique geochemical data show that both the gaojiacun and lengshuiqing intrusions have arc signatures with nd t values of 1 5 to 6 0 initial 87 sr 86 sr ratios of 0 705 0 706 and pronounced negative nb ta and zr hf anomalies their geochemical variations are best explained by fractional crystallization without major crustal contamination the yanbian terrane represents a typical arc assemblage formed on the western edge of the yangtze block during neoproterozoic time the sedimentary sequence was deposited in an oceanic setting probably in a back arc basin environment the depleted subduction modified lithospheric mantle wedge above the subduction zone was the source of melts from which the mafic plutons were crystallized this scenario suggests subduction of oceanic lithosphere eastward present day orientation underneath the yangtze block

mangrove bank sedimentation in a mesotidal environment with large sediment supply gulf of papua the most extensive mangrove forests on earth are associated with major river systems the indo pacific region has numerous large rivers that discharge onto broad continental shelves and mangroves occur widely along these coastlines in the gulf of papua gop alone there are 3350 km2 of mangrove forests the majority of which 2900 km2 are associated with the fly kikori and purari river deltas this study examines data from 28 vibracores to describe preserved sedimentary strata and to determine rates of sediment accumulation in this dynamic coastal environment the observations suggest a common stratigraphy is contained in prograding mangrove banks of the mesotidal western gop and that it is similar to mud dominated and physically stratified stratigraphy recorded by other intertidal areas with and without mangroves experiencing large tidal range 2 m and sediment supply e g ganges brahmaputra and yangtze river deltas grain size sedimentary structure radionuclide and remotely sensed data are analyzed and four stratigraphic facies are identified 1 supratidal to high tidal muds 2 mid tidal sandy muds 3 low tidal sand and mud and 4 subtidal channel sands supratidal and high tidal sediments of gop mangrove forests are typically muddy but can have high sand contents 30 in areas exposed regularly to ocean waves these muds can be homogenous but commonly have physical structures laminations and bedding probably reflecting high rates of sediment accumulation relative to bioturbation mangrove muds may have low water contents due to infrequent inundation and because of their cohesive nature and abundant roots can form steep intertidal banks and erosional scarps mid tidal and low tidal sediments are thinly laminated 2 mm laminations likely deposited from fluid mud transport channel sediments have thin and thick mud and sand laminations but are identified by thick 5 cm sand beds a conceptual model of mangrove bank progradation is created using sedimentological and radionuclide data maximum sediment accumulation rates were determined using steady state 210pb profiles and greatest mean rates are found in mid tidal areas 4 4 cm year 3 9 g cm2 year slower accumulation occurs in high tidal and low tidal areas 1 8 and 1 5 cm year respectively 1 2 and 1 0 g cm2 year respectively remotely sensed data indicate that areas of erosion and accretion co exist in the kikori purari region very large 210pb accumulation rates were measured in areas of significant accretion supporting the validity of the calculated rates in areas of rapid accretion cores can have non steady state or event related 210pb profiles reflecting episodic sedimentation or complex radionuclide scavenging dynamics most cores from the fly river delta are of this type sediment trapping in western gop mangroves is estimated to account for 2 14 of the total gop sediment load

evolution of sedimentary mode since pleistocene in the central south yellow sea china based on seismic stratigraphy analysis abstract in order to investigate the distribution development and evolution of the strata in the south yellow sea basin sysb seismic and core data were analyzed fourteen 14 reflecting interfaces and 13 seismic units are identified and a stratigraphic framework since 1 81 ma b p is established the deposition rate has changed since 126 ka b p the stratigraphic model shows that the sysb responded to sea level changes during the same period 126 ka b p which suggests a model change of the development of regional strata between 1 81 ma b p and 126 ka b p the zhejiang minnan uplift zmu obstructed the intrusion of seawater into the sysb which is the main factor responsible for the development of regional strata while the sea level fluctuation and fluvial material input have dominated the formation of the strata since 126 ka b p

magnetic minerals in three asian rivers draining into the south china sea pearl red and mekong rivers the use of the marine sedimentary magnetic properties as tracers for changes in precipitation rate and in oceanic water masses transport and exchanges implies to identify and to characterize the different sources of the detrital fraction this is of particular importance in closed and or marginal seas such as the south china sea we report on the magnetic properties of sedimentary samples collected in three main asian rivers draining into the south china sea the pearl red and mekong rivers the geological formations as well as the present climatic conditions are different from one catchment to another the entire set of performed magnetic analyses low field magnetic susceptibility arm acquisition and decay irm acquisition and decay back field acquisition thermal demagnetization of three axes irm hysteresis parameters forc diagrams and low temperature magnetic measurements allow us to identify the magnetic mineralogy and the grain size distribution when magnetite is dominant some degree of variability is observed in each basin illustrating different parent rocks and degree of weathering on average it appears that the pearl river is rich in magnetite along the main stream while the mekong river is rich in hematite the red river is a mixture of the two compared to clay mineral assemblages and major element contents previously determined on the same samples these new findings indicate that the magnetic fraction brings complementary information of great interest for environmental reconstructions based on marine sediments from the south china sea

east asian monsoon evolution since the late miocene from the south china sea abstract the south china sea scs has become a global focus for paleoclimatic and paleoceanographic studies due to its location which makes it highly sensitive to the east asian monsoon and to pacific ocean changes we present here a refined magnetostratigraphy for the last 6 5 ma for hole u1431d which was recovered during iodp expedition 349 in the central scs magnetic analyses demonstrate that magnetite is the dominant magnetic carrier except for within a greigite bearing layer at 130 5 132 0 meters below sea floor the greigite layer coincides with the timing of intensification of northern hemisphere glaciation and records an oceanographic response to this event environmental magnetic results indicate that the east asian summer and winter monsoon were stable from 6 5 to 5 ma the summer monsoon intensified at 5 ma and then weakened gradually since 3 8 ma in contrast the winter monsoon weakened at 5 ma and has then been enhanced since 3 8 ma and more stable from 0 6 ma spectral analysis indicates that the east asian summer monsoon was driven directly by low latitude insolation changes before 3 2 ma and that it has been affected by both low latitude insolation and high latitude ice volume changes since 3 2 ma

trace element indicators of crystal accumulation in silicic igneous rocks abstract if fractional crystallization coupled with some crustal assimilation afc is the dominant process for generating evolved magmas in the earth s crust large quantities of igneous cumulates must be present in the rock record however despite some reported occurrences intermediate to silicic rocks characterized by an unambiguous crystal accumulation signature have remained elusive the signature is expected to be subtle in viscous silicic magmas and sensitive criteria need to be developed to test whether these cumulates are truly rare or simply difficult to discern we propose models that use specific trace element concentrations and ratios zr ba eu eu and zr hf within a well constrained framework for the physical extraction of liquid from crystals mostly occurring between 50 and 70 vol crystals to identify silicic cumulates the results of these models indicate that crystal accumulation or melt extraction has occurred in multiple natural examples around the world both in plutonic and volcanic realms however these examples were selected because they represent conditions that are most favorable high amount of melt extraction well constrained initial liquid compositions for unambiguously identifying cumulate residue in many cases we show that the trace element signature for crystal accumulation will remain largely concealed in addition the different liquid lines of descent related to the various tectonic settings on earth dry reduced in hot spots divergent margins and wetter more oxidizing in arc environments will lead to different trace element evolution and each criterion must be applied on a case by case basis despite these limitations the undeniable but subtle presence of silicic cumulates in the mid to upper crust lends support to the idea that igneous differentiation is dominantly driven by melt extraction from partially crystalline reservoirs

partial melting of two amphibolites contrasting experimental results under fluid absent conditions a large portion of the lower continental crust may be amphibolitic in composition and without a free fluid phase as a consequence h2o undersaturated or fluid absent melting of amphibolites may be responsible for the formation of some granites and migmatites produced during major orogenic events in an attempt to determine the systematics of melting under fluid absent conditions a series of piston cylinder experiments was conducted on two natural amphibolites one a meta alkali basalt aba with a total water content of wt contained in hornblende and the other a meta island arc tholeiite iat which has 1 1 3 wt water contained in hornblende cummingtonite and biotite the experimentally determined melting ranges of the two amphibolites showed that the solidus temperatures and sta temperature interval over which amphibole was stable were controlled by the amphibolites different bulk compositions and their resulting metamorphic assemblages the volume of melt produced by melting of the two amphibolites were compared with estimated amounts based on burnham s 1979 water melt solubility model and the fluid absent melting model presented by clemens and vielzeuf 1987 the observed melt volumes were greater than estimated as the water content of melt largely detemines the volume of melt produced independent measurements of the water content of the glass formed during partial melting in the aba were made by thermogravimetric analyses the water content of the aba glass is 2 wt which is less than the assumed melt water content water content of the melt used in previous modeling of fluidabsent anatexis in mafic lithologies as a consequence more melt can be expected during fluid absent partial melting of mafic lower crust as is observed in the experiments a modification of the clemens and vielzeuf 1987 fluid absent melting model for mafic compositions has been made using the experimental data available on melting in basaltic systems and is presented here for pressures of 5 8 and 10 kbar tectonic scenarios in which the crust is thickened i e by collision then undergoes extension or where a previously thinned crust is later rethickened provide enough heat so that amphibolite melting under fluid absent conditions can become importan and hence responsible for some melts produced during post collisional magmatism the results may also have applications to melting in hydrated oceanic crust in subduction zones and in island arc terains

the 860 ma cordilleran type guandaoshan dioritic pluton in the yangtze block sw china implications for the origin of neoproterozoic magmatism abstract the tectonic setting of the widespread neoproterozoic igneous rocks in south china has long been a matter of debate situated in the western margin of the yangtze block of south china the guandaoshan pluton is composed of diorite in the core and quartz diorite in the margin both types contain abundant mafic enclaves and mafic dikes this pluton is dated at documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 858 pm 7 mathrm ma end document by the shrimp zircon u pb method the rocks have sio2 contents of 52 8 63 8

monsoon influenced variations in morphology and sediment of a mesotidal beach on the mekong river delta coast abstract a mesotidal beach on the mekong river delta coast exhibits unique patterns of river sediment discharge and experiences reversals of ocean wave directions in response to the summer and winter monsoons we analyzed long term changes of the shoreline since 1936 from past topographic maps and satellite images and short term changes in geomorphology and sedimentology by undertaking repeated field surveys between november 2005 and february 2008 along six shore normal beach transects the shoreline of the mekong river delta coast has changed asymmetrically over the last 70 years in response to net southwestward sediment transport related to dry northeasterly winter monsoons during the summer rainy season large volumes of sediments are discharged from the river at this time wave direction is reversed in response to the relatively weak southwesterly summer monsoon mud and very fine sand in the surface sediments of the northeastern updrift beach transects tend to be removed during winter suggesting that the sediment supplied from the river during summer is temporarily deposited near the river mouth and later transported southwestward during the winter monsoon the relief of intertidal bars on the beach increased during winter in response to higher waves however previous studies have suggested that higher waves flatten bars we hypothesize that the increase in relief that we observed is related to a wide and shallow subtidal delta front platform that attenuates ocean waves at all times other than at high tide the beach profile varies longshore the accreting to stable beach dips seaward at a gradient of about 1 80 with up to three longshore intertidal bars classified here as either slip face bars or low amplitude ridges in contrast the eroding beach profile is much steeper 1 20 1 40 and linear without intertidal bars and is similar in form to a low tide terrace because there is little variation of wave size and grain size within the study area we consider that the spatial variation of the beach profile is related to the availability of sediment accumulated on the beach

weathering and erosion in central vietnam over the holocene and younger dryas clay mineralogy and elemental geochemistry from the vietnam shelf western south china sea abstract clay mineralogy and elemental geochemistry of core so18401 on the vietnam shelf are used to reconstruct the history of weathering and erosion in central vietnam over the past 13 2 ka the variability of the clay species across the 13 2 ka bp together with the age model allows a subdivision into four temporal stages provenance analysis based on clay mineralogy indicates that small mountainous rivers in southern central vietnam are the major sources of terrigenous sediments on the central vietnam shelf smectite and kaolinite originated from the chemical weathering of mesozoic and neogene quaternary basaltic rocks and paleo mesozoic felsic intrusive rocks respectively under warm and humid climate conditions whereas illite and chlorite were mainly derived from physical weathering of paleo mesozoic felsic intrusive rocks and precambrian metamorphic rocks in central vietnam accordingly smectite illite chlorite ratio together with elemental ratios of al2o3 k2o tio2 al2o3 and sio2 al2o3 can be utilized to reflect the relative importance between chemical weathering and physical weathering erosion of the parent rocks in southern central vietnam higher smectite illite chlorite and al2o3 k2o ratios indicate a more important role of chemical weathering and higher tio2 al2o3 and sio2 al2o3 ratios suggest enhanced contribution of physical weathering and erosion a combination of these mineralogical and elemental proxies allows the distinction of four stages of weathering and erosion evolution in southern central vietnam over the holocene and younger dryas periods enhanced physical weathering and erosion took place during the periods 13 2 11 0 ka bp stage i and 3 9 2 0 ka bp stage iii associated with relatively arid conditions by contrast strengthened chemical weathering occurred during the periods 11 0 3 9 ka bp stage ii and 2 0 ka bp to present stage iv corresponding to more warm and humid climatic conditions our study suggests that the weathering and erosion history of southern central vietnam over the holocene is mainly controlled by the east asian monsoon climate

sediment facies and late holocene progradation of the mekong river delta in bentre province southern vietnam an example of evolution from a tide dominated to a tide and wave dominated delta abstract the mekong river delta southern vietnam is a typical mixed tide and wave energy delta with a wide delta plain formed during the last 6 ka and is one of the largest deltas in the world three cores were taken from bentre province in the lower delta plain with the objective to describe the sediment facies and to clarify the changes from tide dominated to tide wave dominated delta of the mekong river delta during the late holocene three cores bt1 bt2 and bt3 were obtained in 1997 holocene sediments in bt1 and bt3 with thickness ranging from 10 to 20 m were mainly composed of deltaic sediments unconformably overlying the late pleistocene sediments the bt2 core however consisted of transgressive estuarine sediments covered by deltaic sediments infilling the paleo mekong river incised valley which is more than 70 m deep and was formed during the last glacial period the deltaic sediment facies and succession in the bt2 and bt3 cores taken from the outer delta plain with beach ridges were different from those of the bt1 core taken from the inland delta plain the bt2 and bt3 cores provide a good example of tide and wave dominated delta sediments and were characterized by a coarsening upward delta front facies covered by a fining upward subtidal to intertidal flat facies followed by a coarsening upward foreshore dune facies these changes from the inland delta plain to the beach ridge delta plain were accompanied by changes in progradation rates and subaqueous delta topography caused by increased wave influence

development of the east asian monsoon mineralogical and sedimentologic records in the northern south china sea since 20 ma we here reconstruct the past change of the east asian monsoon since 20 ma using samples from ocean drilling program odp site 1146 in the northern south china sea based on a multi proxy approach including a monomineralic quartz isolation procedure identification of clay minerals by x ray diffraction xrd and grain size analysis of isolated terrigenous materials terrigenous supply to odp site 1146 was dominated by changes in the strength of multiple sources and transport processes grain size data modeled by an end member modeling algorithm indicate that eolian dust from the and asian inland and fluvial input have contributed on average 20 and 80 of total terrigenous material to odp site 1146 respectively specifically about 40 53 of the total quartz feldspar and only 6 11 of the total clay is related to eolian supply at the study site detailed analysis of the sedimentary environment and clay minerals combined with previous studies shows that smectite originates mainly from luzon kaolinite from the pearl river and illite and chlorite from the pearl river taiwan and or the yangtze river the proportion and mass accumulation rate mar of the coarsest end member em1 interpreted as eolian dust ratios of illite chlorite smectite quartz feldspar and mean grain size of terrigenous materials at odp site 1146 were adopted as proxies for east asian monsoon evolution the consistent variation of these independent proxies since 20 ma shows three profound shifts in the intensity of east asian winter monsoon relative to summer monsoon as well as aridity of the asian continent occurred at similar to 15 ma similar to 8 ma and the youngest at about 3 ma in comparison the summer monsoon intensified contemporaneously with the winter monsoon at 3 ma the phased uplift of the himalaya tibetan plateau may have played a significant role in strengthening the asian monsoon at similar to 15 ma 8 ma and 3 ma c 2007 elsevier b v all rights reserved

chemical weathering in luzon philippines from clay mineralogy and major element geochemistry of river sediments clay mineralogy and major element geochemistry of 35 surface sediment samples collected in 21 major to moderate rivers of luzon philippines are used to evaluate the present chemical weathering process the clay mineral assemblage consists mainly of smectite average 86 with minor kaolinite 9 and chlorite 5 and very scarce illite 1 and does not show strong island wide differences the major element results of both bulk and clay fraction sediments indicate that the formation of clay minerals is accompanied by leaching of ca and na first and of fe and mn thereafter during the chemical weathering process a low moderate chemical weathering degree of bulk sediments and a moderate intensive degree of clay fraction sediments are obtained in luzon rivers based on proxies of chemical index of alteration cia and smectite crystallinity it is suggested that the majority of andesitic basaltic volcanic and sedimentary rocks along with the tectonically active geological setting and sub tropical east asian monsoon climate are responsible for the predominance of smectite in the clay mineral assemblage

climatic and tectonic controls on weathering in south china and indochina peninsula clay mineralogical and geochemical investigations from the pearl red and mekong drainage basins 1 results of clay mineralogy major element geochemistry and sr and nd isotopes in 93 argillaceous samples collected from drainage basins of the pearl red and mekong rivers reveal different degrees of chemical weathering in southeast asia despite similar climate conditions across these regions the kaolinite illite ratio illite chemistry index and illite crystallinity can be used as indicators of chemical weathering intensity these mineralogical proxies combined with the k2o na2o cao molar ratio chemical index of alteration cia and weathering trends observed from major element results indicate intensive silicate weathering in the pearl river basin moderate to intensive in the mekong river basin and moderate in the red river basin although a significant modification of epsilon nd 0 values in our riverine sediments during chemical weathering and transport is unlikely sr 87 sr 86 ratios are controlled by various states of chemical weathering of high sr minerals such as plagioclase rich in na and ca with a linear decrease trend from the pearl mekong to red river basins our results suggest that it is not the warm climate with heavy monsoon precipitation but tectonics playing the most significant role in controlling weathering and erosion processes in south china and indochina peninsula strong physical erosion caused by tectonic activities and river incision along the eastern margin of the tibetan plateau and along the red river fault system is responsible for high contents of primary minerals in the lowlands of red and mekong river basins

neoproterozoic adakitic plutons and arc magmatism along the western margin of the yangtze block south china abstract neoproterozoic adakitic plutons that crop out along the western margin of the yangtze block south china from kangding on the north to panxi on the south provide constraints on the origin of the giant jinningian magmatic event of south china representative plutons include the xuelongbao 750 ma datian 760 ma and dajianshan intrusions the latter two bodies consist mainly of granodiorite with relatively high sio2 51 0 73 4 wt and mg s 0 36 0 55 they have fractionated rare earth element patterns with la yb n ratios ranging from 2 6 to 101 8 and are characterized by high sr 344 1018 ppm and low y 4 3 17 9 ppm yielding sr y ratios ranging from 27 to 111 on primitive mantle normalized trace element diagrams these rocks show enrichment of large ion lithophile elements and depletion of high field strength elements nb ta with positive zr hf and negative ti anomalies consistent with an arc related setting they have relatively constant initial whole rock 87sr 86sr ratios 0 70

late holocene depositional environments and coastal evolution of the mekong river delta southern vietnam abstract the mekong river delta in southern vietnam is a tide dominated delta and has been formed by the mekong river system the sediments are dominantly fine grained and were deposited in the receiving basin with slight inclination of pre existing deposits in the south china sea and gulf of thailand around 6000 to 5000 yr bp maximum holocene transgression occurred and seawaves surrounded uplands of the late pleistocene terrace basement rock and weathered land in the northern parts of the delta double notches were undercut by seawaves on the limestone hill in hatien at about 4 5 and 2 5 m above present sea level marine deposits prevailed throughout the delta during the highstand and regressions of relative sea level over the last 4550 yr bp delta progradation has produced a great flat plain of 62 520 km 2 this extremely fast progradation could be due to a very high sediment supply depositing in the slight inclination of the receiving basin b neotectonic movements and relative sea level changes c widespread mangrove forests playing an important role to enhance sediment accumulation former mangrove marshy deposits were significantly developed in the plain of reeds and longxuyen quadrangle mangrove marshes have also been developing well in the camau peninsula and present coastlines moreover regressions over the last 4550 yr bp have produced many distinct sets of relict beach ridges which are useful in distinguishing the former coastlines and evolutionary history of the delta

water saturated magmas in the panama canal region a precursor to adakite like magma generation amphibole while uncommon as a phenocryst in arc lavas is increasingly recognized as a key constituent in the petrogenesis of arc magmas fractional crystallization of water saturated arc magmas in the lower crust can yield substantial volumes of amphibole cumulates that depending on the pressure of crystallization may also contain garnet fractionation of this higher pressure assemblage has been invoked as a possible mechanism in the production of magmas that contain an adakitic signature this study examines newly dated late oligocene 25 37 0 13 ma hypabyssal amphibole rich andesites from cerro patacon in the panama canal region these andesites contain nodules of amphibole cumulates that are 4 6 cm in diameter and are almost entirely composed of 5 10 mm amphibole crystals dominantly ferri tschermakite geochemical variations optical and chemical zoning of the cerro patacon amphiboles are consistent with their evolution in a crystal mush environment that had at least one recharge event prior to entrainment in the host andesite amphiboles hosted within the cumulate nodules differ from those hosted in the cerro patacon andesite and contain consistently higher values of ti we suggest these nodules represent the early stages of fractionation from a water saturated magma cerro patacon andesites have ree concentrations that plot at the most depleted end of central american arc magmas and exhibit a distinctive depletion in the middle ree these geochemical and petrographic observations strongly support significant amphibole fractionation during formation of the cerro patacon andesite consistent with the petrographic evidence fractionation of water saturated magmas is a mechanism by which adakitic compositions may be produced and the cerro patacon andesites do exhibit adakite like geochemical characteristics e g elevated sr y 28 34 however the relatively elevated concentrations of y and hree indicate garnet was not stable in the fractionating assemblage during this early stage of arc development

convective instability of 3 d fluid saturated geological fault zones heated from below we conduct a theoretical analysis to investigate the convective instability of 3 d fluid saturated geological fault zones when they are heated uniformly from below in particular we have derived exact analytical solutions for the critical rayleigh numbers of different convective flow structures using these critical rayleigh numbers three interesting convective flow structures have been identified in a geological fault zone system it has been recognized that the critical rayleigh numbers of the system have a minimum value only for the fault zone of infinite length in which the corresponding convective flow structure is a 2 d slender circle flow however if the length of the fault zone is finite the convective flow in the system must be 3 d even if the length of the fault zone is infinite since the minimum critical rayleigh number for the 2 d slender circle flow structure is so close to that for the 3 d convective flow structure the system may have almost the same chance to pick up the 3 d convective flow structures also because the convection modes are so close for the 3 d convective flow structures the convective flow may evolve into the 3 d finger like structures especially for the case of the fault thickness to height ratio approaching zero this understanding demonstrates the beautiful aspects of the present analytical solution for the convective instability of 3 d geological fault zones because the present analytical solution is valid for any value of the ratio of the fault height to thickness using the present analytical solution the conditions under which different convective flow structures may take place can be easily determined

provenance and tectonic setting of the neoproterozoic yanbian group western yangtze block sw china abstract tectonic evolution of the yangtze block of south china during neoproterozoic time has been a major focus of debate and is important in the reconstruction of the rodinian supercontinent the yanbian group in the western margin of the yangtze block is a well preserved neoproterozoic volcanic sedimentary sequence and provides a rare opportunity to examine the provenance and tectonic setting it consists of a lower part composed of 1500 m thick basaltic lavas and an upper part of 3500 m thick flysch deposits the flysch sequence contains typical deep marine turbidites consisting mainly of fine grained volcaniclastic sandstones and mudstones indicating a submarine fan depositional system detrital zircon dating yields ages ranging from ca 865 ma to 1000 ma with two peaks at ca 900 ma and 920 ma respectively the yanbian group is intruded by the 860 ma guandaoshan dioritic pluton constraining its depositional age at ca 870 ma the sandstones contain abundant feldspar and lithic fragments with minor amounts of quartz and have an average composition of q16f35l49 suggesting a proximal source and an undissected to transitional arc setting the lithic fragments in the sandstones point to andesitic and felsic volcanic rock sources the sandstones and mudstones have intermediate sio2 al2o3 typically 3 6 high fe2o3 mgo contents 5 12 wt and moderate to high k2o na2o ratios generally 0 1 1 and 1 10 for sandstones and mudstones respectively in comparison with average upper continental crust they show strong negative nb ta anomalies slight depletion of zr hf la and th but moderate enrichment of v cr ni and sc these rocks show lree enrichment la ybn 5 3 7 4 with flat hree and pronounced negative eu anomalies eu eu 0 6 0 74 in chondrite normalized ree patterns similar to post archean shales geochemical data for these rocks suggest an arc setting and intermediate felsic volcanic source consistent with a back arc basin scenario for the basaltic lavas in the lower yanbian group together with regional geological evidence the turbidites of the yanbian group strongly support a model in which the western margin of the yangtze block was a major magmatic arc active probably from 920 ma to 740 ma a period of more than 180 million years

holocene delta evolution and sediment discharge of the mekong river southern vietnam abstract evolutionary changes delta progradation and sediment discharge of the mekong river delta southern vietnam during the late holocene are presented based on detailed analyses of samples from six boreholes on the lower delta plain sedimentological and chronostratigraphic analyses indicate clearly that the last 3 kyr were characterized by delta progradation under increasing wave influence southeastward sediment dispersal decreasing progradation rates beach ridge formation and steepening of the face of the delta front estimated sediment discharge of the mekong river for the last 3 kyr based on sediment volume analysis was 144 36 million t yr 1 on average or almost the same as the present level the constant rate of delta front migration and stable sediment discharge during the last 3 kyr indicate that a dramatic increase in sediment discharge owing to human activities as has been suggested for the yellow river watershed did not occur although southeast asian rivers have been considered candidates for such dramatic increases in discharge during the last 2 kyr the mekong river example although it is a typical large river of this region does not support this hypothesis therefore estimates of the millennial scale global pristine sediment flux to the oceans must be revised

the geochemical characteristics of granitoids in contrasting arcs and comments on magma sources geochemical variations in modern subduction related igneous suites with respect to arc maturity in time and space are illustrated using data for both volcanic suites basalt andesite and dacite and intrusive granitoid suites diorite tonalite granodiorite and granite from circum pacific arcs using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites a subduction zone enrichment of lithospheric mantle locally coupled with crustal assimilation allied with fractional crystallization afc in zones of thickened crust all of which yield magmas with enhanced concentrations of the large ion lithophile lil elements k rb th u lrees etc b with increasing distance from the active trench contributions from within plate sub continental lithosphere producing mantle derived magmas with enhanced levels of high field strength hfs elements among which nb ta hf and y are particularly distinctive thus even for the evolved granitoids of intrusive arc series ratios of hfs lil elements not significantly affected by crystal fractionation such as ta nb k rb la may throw some light on the origin of mafic intermediate precursor magmas in terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites 1 primitive calcic arc granitoids with low lil and hfs element abundances 2 normal calc alkaline continental arc granitoids with enhanced lil element abundances and low hfs lil ratios 3 mature alkali calcic arc granitoids with high levels of lil and hfs elements and higher hfs lil ratios 4 back arc anorogenic alkaline granitoids with the highest levels of hfs elements

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method