internal tide coherence and decay over a wide shelf sea 1 a quasi synoptic hydrography and velocity section is used to determine the structure and the decay rate of the internal tide it across the broad continental shelf of the celtic sea in these observations the it is coherent over more than 170 km about five wavelengths with an estimated shoreward energy decay scale of 42 km the inferred it wavelength averaged dissipation rate near the shelf edge is estimated as 2 08 10 7 wkg 1 in close agreement with tidally and vertically averaged measurements from the region these results provide the firstin situ evidence of it coherence over many wavelengths in a shelf sea

geochronological and geochemical constraints on mesozoic suturing in east central tibet 1 this paper reports isotopic major and minor element geochemistry of igneous and metamorphic rocks from the kokoxili and yushu regions of central and eastern tibet the first region lies along the kunlun suture which separates the bayan har songpan ganze songpan terrane from the tarim and qaidam blocks two kokoxili granitoids yield u pb zircon dates of 217 10 and 207 3 ma late triassic which represent the time of emplacement and rb sr isochron dates of 195 3 and 190 3 ma early jurassic which are interpreted as cooling ages the geochemical signatures of these granitoids suggest that they are related to subduction continuing into the late triassic in the yushu area three samples help constrain the age of the jinsha suture which separates the songpan terranes from the qiangtang blocks a leucocratic granite and an orthogneiss in the suture zone yield u pb zircon dates of 206 7 and 204 1 ma respectively and a paragneiss south of it a u pb monazite date of 244 4 ma the existence of coeval magmatism in both the jinsha and kunlun sutures suggests that the two subduction zones were simultaneously active combining isotopic dating with structural evidence on subduction polarity and paleomagnetic reconstructions we propose that the kunlun and qinling block boundaries which were distinct in the permian subsequently formed a continuous late triassic northward subducting plate margin our data suggest that the jinsha suture correlates with the benzilan and nan uttaradit sutures which together belong to a major late triassic subduction zone

deep sea records of the continental weathering and erosion response to east asian monsoon intensification since 14 ka in the south china sea we analyzed sediment from ocean drilling program odp site 1144 in the northern south china sea to examine the weathering response of se asia to the strengthening of the east asian monsoon eam since 14 ka our high resolution record highlights the decoupling between continental chemical weathering physical erosion and summer monsoon intensity mass accumulation rates ti ca k rb hematite goethite and sr 87 sr 86 show sharp excursions from 11 to 8 ka peaking at 10 ka clay minerals show a shorter lived response with a higher kaolinite illite chlorite ratio at 10 7 9 5 ka however not all proxies show a clear response to environmental changes magnetic susceptibility rises sharply between 12 and 11 ka grain size becomes finer from 14 to 10 ka and then coarsens until similar to 7 ka but is probably controlled by bottom current flow and sealevel sr and nd isotopes show that material is dominantly eroded from taiwan with a lesser flux from luzon while clay mineralogy suggests that the primary sources during the early holocene were reworked via the shelf in the taiwan strait rather than directly from taiwan erosion was enhanced during monsoon strengthening and caused reworking of chemically weathered pleistocene sediment largely from the now flooded taiwan strait which was transgressed by similar to 8 ka cutting off supply to the deep water slope none of the proxies shows an erosional response lasting until similar to 6 ka when speleothem oxygen isotope records indicate the start of monsoon weakening although more weathered sediments were deposited from 11 to 8 ka when the monsoon was strong these are reworked and represent more weathering during the last glacial maximum lgm when the summer monsoon was weaker but the shelves were exposed c 2012 elsevier b v all rights reserved

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

erosional response of south china to arc rifting and monsoonal strengthening a record from the south china sea abstract ocean drilling program sampling of the distal passive margin of south china at sites 1147 and 1148 has yielded clay rich hemipelagic sediments dating to 32 ma oligocene just prior to the onset of seafloor spreading in the south china sea the location of the drill sites offshore the pearl river suggests that this river or its predecessor may have been the source of the sediment in the basin which accounts for only 1 8 of the total neogene sediment in the asian marginal seas a mean erosion depth of 1 km over the current pearl river drainage basin is sufficient to account for the sediment volume on the margin two dimensional backstripping of across margin seismic profiles shows that sedimentation rates peaked during the middle miocene 11 16 ma and the pleistocene since 1 8 ma nd isotopic analysis of clays yielded nd values of 7 7 to 11 0 consistent with the south china block being the major source of sediment more positive nd values during and shortly after rifting compared to later sedimentation reflect preferential erosion at that time of more juvenile continental arc rocks exposed along the margin as the drainage basin developed and erosion shifted from within the rift to the continental interior nd values became more negative a rapid change in the clay mineralogy from smectite dominated to illite dominated at 15 5 ma synchronous with middle miocene rapid sedimentation mostly reflects a change to a wetter more erosive climate evidence that the elevation of the tibetan plateau and erosion in the western himalaya both peaked close to this time supports the suggestion that the asian monsoon became much more intense at that time much earlier than the 8 5 ma age commonly accepted

the role of fission track dating in discrimination of provenance determination of single crystal ages by fission track dating analysis of zircons and subordinately apatites within a detrital heavy mineral assemblage together with frequency distribution and radial plot analysis of the measured ages permits identification of different age modes synopses are presented of published fission track provenance studies in north central victoria northeastern tasmania barbados the rockall trough the english wealden mexico utah and the central alps from authors

gondwana and cathaysian blocks palaeotethys sutures and cenozoic tectonics in south east asia the triassic indosinian orogeny followed extinction of the palaeotethys ocean resulting in suturing of gondwana affinity and cathaysian blocks

crustal response to continental collisions between the tibet indian south china and north china blocks geochronological constraints from the songpan garze orogenic belt western china we report shrimp u th pb monazite conventional u pb titanite sm nd garnet and rb sr muscovite and biotite ages for metamorphic rocks from the danba domal metamorphic terrane in the eastern songpan garze orogenic belt eastern tibet plateau these ages are used to determine the timing of polyphase metamorphic events and the subsequent cooling history the oldest u th pb monazite and sm nd garnet ages constrain an early barrovian metamorphism m1 in the interval c 204 190 ma coincident with extensive indosinian granitic magmatism throughout the songpan garze orogenic belt a second higher grade sillimanite grade metamorphic event m2 recorded only in the northern part of the danba terrane was dated at c 168 158 ma by a combination of u th pb monazite and titanite and sm nd garnet ages it is suggested that m1 was a thermal event that affected the entire orogenic belt while m2 may represent a local thermal perturbation rb sr muscovite ages range from c 138 100 ma whereas rb sr biotite ages cluster at c 34 24 ma these ages document regional cooling at rates of c 2 3 c myr 1 following the m1 peak for most of the terrane however those parts of the terrane affected by the higher temperature m2 event e g the migmatite zone experienced initially more rapid c 8 c myr 1 cooling after peak m2 before joining the regional slow cooling path defined by the rest of the terrane at c 138 ma regional slow cooling between c 138 and c 30 ma is thought to be the result of post tectonic isostatic uplift after extensive crustal thickening caused by collision of the south and north china blocks the clustering of biotite rb sr ages marks the onset of rapid uplift across the entire terrane commencing at c 30 20 ma this cooling history is shared with many other regions of the tibet plateau suggesting that uplift of the tibet plateau including the songpan garze orogenic belt occurred predominantly in the last c 30 myr as a response to the continuing northwards collision of india with eurasia

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

cenozoic denudation history of southern hainan island south china sea constraints from low temperature thermochronology abstract as a large block of continental crust hainan island located in the western south china sea forms a rift shoulder to the adjacent offshore yinggehai and qiongdongnan basins and lies in the footwall of their main boundary faults we present 11 apatite fission track aft data and 16 u th sm he ages to elucidate the island s cenozoic morphotectonic evolution our data indicate that much of southern hainan island experienced a well defined cooling episode commencing in late eocene oligocene time during this time most samples cooled from 110 c to 55 15 c at a rate of 4 0 c ma following which the cooling rate decreased to 2 5 c ma assuming a constant paleogeothermal gradient of 23 c km similar to that of the present day since oligocene time when most samples cooled into the base of the aft partial annealing zone the total eroded section is estimated at a minimum of 3 5 km about two thirds of which was removed during the oligocene rapid cooling phase this implies that elevation of southern hainan island has decreased since the oligocene and this is supported by independent paleobotanical evidence although a further cooling episode since 5 ma is also suggested by aft thermal history models cooling occurred from relatively low temperatures 45 50 c where annealing kinetics are not well understood so this cannot be rigorously assessed despite evidence for increased sedimentation in offshore basins at this time our data suggest that the no 1 fault of the adjacent offshore yinggehai basin to the west which is thought to be an offshore extension of the red river fault zone was mainly active from late eocene through much of the oligocene but declined rapidly after 22 ma the marked reduction of potential sediment supply rate from hainan island into neighboring basins after that time suggests that miocene to recent sedimentation in the yinggehai basin was mainly supplied by the red river

understanding mesozoic accretion in southeast asia significance of triassic thermotectonism indosinian orogeny in vietnam results from a zircon u pb study of the metamorphic basement of vietnam reveal that a large part of the continental crust was affected by a short lived episode of ductile deformation and high temperature metamorphism between 258 6 ma and 243 5 ma although coincident with final stages of north south china collision qinling orogenesis the thermotectonism in vietnam was caused by accretion of sibumasu to indochina south china this accretion event indosinian orogeny has regional significance because it contributed to the final stages of north south china collision an aspect not explicitly recognized in qinling orogenic models

structural petrological and thermal evolution of a tertiary ductile strike slip shear zone diancang shan yunnan the diancang shan a horst massif within the red river fault zone in yunnan people s republic of china preserves a structural petrological and thermal record of two distinct phases of tectonic activity a left lateral ductile shear that terminated between 20 and 17 ma and a ductile to brittle phase of normal faulting which began at 4 7 ma and remains active mylonitic rocks in the core of the range display an early steep high temperature ht schistosity and a horizontal stretching lineation that are both parallel to the trend of the belt kinematic indicators indicate that shear was left lateral the complex shape of the ht schistosity at the southern termination of the massif likely results from a large scale oblique left lateral c shear plane that dismembered the shear zone and separated the ailao shan and the diancang shan as left lateral deformation terminated thermochronological and thermobarometric results suggest that the gneisses were partially unroofed during this event along the eastern edge of the diancang shan the ht fabrics were overprinted by low temperature structures during activation of east dipping normal faults cooling associated with this normal right lateral faulting along the diancang shan and perhaps activation of the right lateral normal movement on the range front fault farther south along the ailao shan began at 4 7 0 1 ma these results tend to support the view that extrusion of indochina occurred along the left lateral red river shear zone between 35 and 19 17 ma initiation of right lateral normal slip during the late miocene may relate to eastward extensional collapse of the thickened tibetan crust or more probably to initiation of the second phase of extrusion

what are the spratly islands abstract seismic records combined with dredged samples and a core indicate that the spratly islands of the dangerous ground province are constructed of presently active carbonate build ups known to extend back continuously at least to the pleistocene and presumed to have initiated in the miocene most likely upon the crests of sea floor cuestas that trend north east south west parallel to the sea floor spreading magnetic anomalies of the contiguous abyssal plain of the southern part of the south china sea the cuestas range from spectacular to subdued constructed of triassic and cretaceous strata and no older rocks have been identified from dredges the cuesta axes plunge towards the south west away from the islands suggesting that the reefs began colonising their more elevated parts but the timing is uncertain the highest seismically recorded cuesta crest is in 440 m of water and the islands and reefs are generally closely surrounded by water deeper than 1500 m since the so called mid miocene unconformity mmu the region has been undergoing post rift thermal subsidence however the nearby seismic lines show no evidence of drowned carbonate reefs it is suggested that the coral algal reefs colonised the crests of the most elevated cuestas that have maintained stability as shown by the 165 m core of one reef indicating periodic exposure with caliche horizons deepening water has protected the build ups from extinction by post rift draping strata in contrast to the central luconia province and the build ups have been able to keep up with regional thermal subsidence the dredged mesozoic strata indicate that the dangerous ground is not exotic and should be interpreted as an integral part of the pre rift sundaland continent that included south china vietnam peninsular malaysia western sarawak and possibly part of sabah igneous and metamorphic samples have been dredged although individual spot k ar dates cannot be accepted at face value such rocks can also be interpreted as an integral part of sundaland post mmu dredged samples are predominantly deep water calcareous mudstones typified by the draping strata of the ocean drilling program odp site 1143 cored from recent to late miocene

lateral dispersion over the continental shelf analysis of dye release experiments lateral dispersion over the continental shelf was examined using dye studies performed as a part of the coastal mixing and optics experiment four experiments performed at intermediate depths each lasting 2 5 5 days were examined in some cases the dye patches remained fairly homogeneous both vertically and horizontally throughout an experiment in other cases significant patchiness was observed on scales ranging from 2 to 10 m vertically and a few hundred meters to a few kilometers horizontally the observations showed that the dye distributions were significantly influenced by shearing and straining on scales of 5 10 m in the vertical and 1 10 km in the horizontal superimposed on these larger scale distortions were simultaneous increases in the horizontal second moments of the dye patches with corresponding horizontal diffusivities based on a fickian diffusion model of 0 3 4 9 m 2 s 1 analysis of the dye data in concert with shear estimates from shipboard acoustic doppler current profiler observations showed that the existing paradigms of shear dispersion and dispersion by interleaving water masses cannot account for the observed diffusive spreading of the dye patches this result suggests that some other mechanisms provided an additional diffusivity of order 0 2 4 6 m 2 s 1 an alternative mechanism dispersion by vortical motions caused by the relaxation of diapycnal mixing events may explain the observed dispersion in some cases

ar ar and fission track ages in the song chay massif early triassic and cenozoic tectonics in northern vietnam the song chay massif is the northeasternmost metamorphic complex in vietnam to the east of the red river shear zone it shows a large antiformal structure involving orthogneisses and migmatites overlain on its northern flank by muscovite bearing marbles an e w striking fault bounds the dome to the south kinematic indicators along a s n section reveal top to the n shear sense along the interface between the orthogneissic core and the overlying metasediments radiometric ages were obtained by the 40ar 39ar method using purified mica separates across the dome ages range from 236 ma at the southern edge to 160 ma in the core attesting to a strong imprint in the early triassic time a clear difference is seen between these mesozoic ages and the eocene to miocene ages from 40 to 24 ma that obtained in the nearby red river shear zone using the same method these data show that the song chay massif was already high in the crust when the high temperature deformation of the red river shear zone took place the final exhumation of the song chay orthogneiss constrained by fission track analysis on samples along the same transect occurred during the early miocene and could be interpreted as the consequence of a first normal sense of motion along the fault which bounds the massif to the south timing is similar to that of exhumation in the red river shear zone

the physical oceanography of jones bank a mixing hotspot in the celtic sea abstract new measurements are presented of the currents and hydrography at jones bank in the celtic sea these measurements collected during the summer of 2008 identify a highly energetic internal wavefield generated by the local interaction of seasonally stratified flow with topography observations close to the bank crest reveal internal waves of up to 40 m amplitude approaching 50 of the local water depth these waves occur during spring periods and are predominantly associated with off bank tidal flow we provide evidence that these waves are the result of hydraulic control of the tidal currents which result in supercritical flow in the bottom mixed layer bml on the upper slopes of the bank the waning tide produces a transition from super to subcritical flow identifiable as a regular hydraulic jump on the bank slope microstructure measurements identify a turbulent bore associated with such jumps that increases bml turbulence by several orders of magnitude and produces a regular burst of enhanced mixing at the base of the ever present thermocline additional mixing in the surface mixed layer is provided during gale force conditions during the first of the two spring tides observed occurring at the start of our three week measurement period the average thermocline mixing rate during stormy spring tide conditions is 1 9 1 0 95 10 3 m 2 s 1 driven by both surface mixing and hydraulic jumps during the later calm spring period the average thermocline mixing rate is 3 3 1 7 10 4 m 2 s 1 dominated by jump associated events by comparison the intervening neap tide period produces no hydraulic jumps and is characterised by relatively calm weather a strong shear layer is however maintained during this quiet period sufficient to enhance thermocline turbulence by a factor 1000 above background levels a short lived peak in thermocline turbulence during the neap period produces diapycnal mixing as high as 1 6 10 2 m 2 s 1 however a more representative background thermocline mixing rate is 2 8 0 6 10 5 m 2 s 1

assessing effective provenance methods for fluvial sediment in the south china sea sediment is delivered by the rivers of se asia to the south china sea where it provides an archive of continental environmental conditions since the eocene interpreting this archive is complicated because sediment may be derived from a number of unique sources and the rivers themselves have experienced headwater capture that also affects their composition a number of methods exist to constrain provenance but not all work well in this area anthropogenic impacts most notably agriculture mean that the modern rivers contain more weathered materials than they did up until about 3000 years ago the rivers have also changed their bulk chemistry and clay mineralogy in response to climate change so that these proxies as well as sr isotopes are generally unreliable provenance indicators nd isotopes resolve influx from luzon but many other sources in se asia have similar values and clear resolution of end members can be difficult instead thermochronology methods are best suited especially apatite fission track which shows more diversity in the sources than either u pb zircon or ar ar muscovite dating nonetheless even fission track is best used as part of a multiproxy approach if a robust quantitative budget is desired

thermochronological evidence for the movement of the ailao shan red river shear zone a perspective from vietnam in order to explore the timing of strike slip movement along the ailao shan red river shear zone an 40ar 39ar thermochronological study of the day nui con voi metamorphic massif in northern vietnam was undertaken the massif exposed in the southeastern segment of the ailao shan red river shear zone reveals a rapid cooling in the early miocene 25 21 ma after a very slow cooling ca 34 25 ma the slow cooling period most likely resulted from a geothermal perturbation in the lithosphere owing to the late paleogene 40 30 ma extension in eastern tibet and western yunnan china the rapid cooling consistent with evidence of a wider rapid cooling span from 27 to 17 ma compiled from the entire ailao shan red river shear zone constrains the duration of left lateral shearing which eventually offset indochina from south china by about 600 km the time constraints we have established reinforce the argument that the onset of the ailao shan red river shear zone postdates the opening of the south china sea that began ca 30 ma this result highlights the necessity for reevaluating the cenozoic tectonic models of southeast asia

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

zircon u pb and hf isotope constraints on crustal melting associated with the emeishan mantle plume abstract shrimp zircon u pb dates combined with in situ hf isotopic data provide new constraints on the petrogenesis and protolith of peralkaline metaluminous and peraluminous intrusions and rhyolitic tuffs in the emeishan large igneous province with significant bearing on crustal melting associated with mantle plumes syenite and a type granitic intrusions from huili miyi and taihe in the center of this large igneous province yield u pb dates at 260 ma consistent with the ages obtained for mafic layered intrusions in the same province zircon from these rocks exhibits a wide range of initial hf isotope ratios ehf t 1 4 to 13 4 with corresponding tdm1 of 400 900 ma the highest ehf t value is only marginally lower than that of depleted mantle reservoir at 260 ma suggesting that their source is primarily juvenile crust added during emeishan volcanism with incorporation of variable amounts of neoproterozoic crust the trigger of crustal melting is most likely related to advective heating associated with magmatic underplating in contrast the 255 251 ma peraluminous granites from ailanghe and 238 ma rhyolitic tuff from binchuan have negative initial ehf values of 1 3 to 4 4 and of 7 7 to 14 respectively hf isotopic model ages and presence of inherited zircons indicate their derivation from mesoproterozoic and paleoproterozoic crust respectively given the time lag relative to the plume impact 260 ma and insignificant mantle contribution to 255 238 ma magmatism conductive heating is suggested as the trigger of crustal melting that resulted in formation of delayed felsic magmas the involvement of older crust in younger felsic magmas is consistent with upward heat transfer to the lithosphere during plume impregnation if the age of crust is inversely stratified i e changes from paleoproterozoic to mesoproterozoic to neoproterozoic to permian with increasing depth such crust may have resulted from episodic downward crustal growth during the evolution of the western yangtze craton

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

propagation of surface uplift lower crustal flow and cenozoic tectonics of the southeast margin of the tibetan plateau surface uplift of the southeast margin of the tibetan plateau is interpreted to have progressed from the northwest near the tibetan border to the southeast in the red river region of the central yunnan province china this interpretation is based on existing thermochronologic data and new mapping and sedimentologic and paleobotanic data demonstrating incision in the headwaters of the red river in pliocene time or later together with previously published data demonstrating surface uplift and a gradient in crustal thickness in the absence of upper crustal shortening this is strong evidence for growth of the southeast margin of the tibetan plateau through lower crustal flow displacement along the ailao shan red river shear zone slowed or ceased in early pliocene time and the xianshuihe xiaojiang fault system initiated accommodating diffuse deformation and rotation around the eastern himalayan syntaxis we suggest a kinematic link between the change in mode of deformation and the introduction of a weak crustal layer through lower crustal flow

major and trace elements of stream sediments from the lowermost amazon river geochemical results from river bottom sediments taken from the lowermost amazon river are presented bedload sediments were analyzed for 10 major and 38 minor elements mud bulk sediments exhibit a tendency to a slight enrichment in cr mn rb sr zr cs ba and hf pointing to the importance of these elements in the silt fraction generally cu zn rb cs ba and sn are clearly associated with clay minerals while y zr hf th and u are associated with heavy minerals the location of this area in an intensive chemical weathering environment results in the strong modification of sediment composition chemical weathering processes strongly modify original sediment composition and can be recognized apart from provenance processes on the basis of their geochemical signature discriminant analysis makes the detection of less marked differences in sediment characteristics possible thus improving the accuracy of distinctions between groups the amazon mouth and xingu mouth xm are discriminated principally by the textural maturity of their sediments the jari mouth jm shows highest co and fe2o3 contents despite similarities between the amazon north branch anb and amazon south branch asb the anb can be differentiated on the basis of its higher sr p2o5 and na2o contents the asb on the basis of its higher zn mno and cao contents and the estreitos de breves est based on its higher abundance of ni and cu higher contents in p2o5 fe and mn are related to laterite covers

sea level induced variations in clay mineral composition in the southwestern south china sea over the past 17 000 yr variations in clay mineral composition of sediment cores from the margin and continental slope of the sunda shelf southern south china sea se asia covering the past 17 000 yr reflect changing influences of sediment sources together with clay mineral partitioning processes in shallow waters we identify the deglacial sea level rise as the principal factor driving these changes during the late glacial high values of kaolinite are interpreted to reflect a higher contribution of clays from soils that have formed on the exposed sunda shelf and in the southern archipelagos of indonesia at this time core sites were located in close proximity to the mouths of the sunda shelf palaeo drainage systems on the emerged shelf sundaland the progressive landward displacement of the coastline and breakdown of these vast drainage systems during deglaciation led to a decrease in influence of the kaolinite rich southern sources when the coastline had retreated closely to its present day position in mid holocene times the former dominance of southern sources was replaced by a stronger influence of illite rich sources e g borneo the overriding control of sea level changes on the clay mineral distribution patterns precludes a definite climatic interpretation of clay mineral data in terms of climatic monsoonal changes in such highly dynamic sedimentary environment

age and origin of magmatism along the cenozoic red river shear belt china to decipher the geodynamic significance of cenozoic magmatism along the red river shear belt geochemical analyses u pb and rb sr dating and pb sr nd isotope tracing were undertaken zircon monazite titanite and a ti u oxide from foliated granitoid intrusions in the shear belt gneisses yield u pb emplacement ages of 33 1 0 2 2 31 9 0 3 25 8 0 2 and 24 7 0 2 ma and an age of 35 0 0 3 ma was obtained for the roughly 100 km long adjacent jinping phan si pang alkali granite together with our previous data the new ages suggest that magmatism and left lateral strike slip movements occurred coevally during latest eocene oligocene times from 33 to 22 ma the rb sr dating of muscovite and biotite from the northernmost gneisses indicates that cooling to 500 c occurred at 52 6 1 1 ma pre dating the onset of magmatism whereas further cooling to 300 c took place at 28 9 0 6 this shows that unroofing in the north took place almost 9 million years earlier than in the central gneiss segments of the shear zone geochemical data substantiate two types of magmas 1 amphibole bearing intrusions of alkaline trend which are derived from sources with isr 0 7065 0 7089 and i nd 3 7 to 6 6 2 leucogranitic layers and bodies having isr 0 7084 0 7354 and i nd 3 3 to 13 4 the former type of intrusion is found in both the gneisses and the adjacent unmetamorphosed cover rocks whereas leucogranites are restricted to the shear belt gneisses source signatures of the alkaline intrusions lie adjacent to the those of oib plotting at the lower end of the mantle array contamination of these melts by continental material seems to be very limited on the other hand the leucogranitic layers are essentially crustal derived but none of the them has country rock isotope signatures requiring melting of crust different from the actually exposed gneisses magma sources similar to those of ocean island basalt indicate magmatism to involve melting of light rare earth element and large ion lithophile element enriched mantle domains most likely present in the lithosphere underneath the region since lithospheric thickening or subduction can be ruled out to produce both types of magmas the presence of an important thermal anomaly is required which is coevally active with left lateral strike slip shear adiabatic decompression and melting within the rising anomaly is the most plausible mechanism to produce the mantle magmas which successively migrate through the crust to induce anatectic melting at 20 15 km crustal depth alkaline magmas largely dominate the volume of magmatism along the belt being continuously present in the shear zone for millions of years such lubrication potentially explains how very large amounts of displacement can be absorbed in surprisingly narrow shear zones such as the red river belt possibly also playing a role for where and when zones of plate scale lateral extrusion develop

intraplate tectonics in asia a precise age for large scale miocene movement along the ailao shan red river shear zone china tertiary left lateral movement along the 1000 km long ailao shan red river shear zone appears to have played an important role in absorbing post collisional northward penetration of india into asia crustal strike slip shear along this zone caused the formation of a gneiss belt metamorphosed to amphibolite grade including anatectic melting both metamorphism and melting were induced by ductile deformation yielding the possibility to date the major tectonometamorphic event that shaped the ailao shan red river belt 17 u pb isotope analyses were performed on small size fractions of zircon monazite and xenotime extracted from two different leucogranitic layers the two samples are located about 50 km apart in the central segment of ailao shan in structurally well controlled settings where the melts crystallized within the strongly foliated gneisses during late stages of deformation all mineral u pb analyses had to be corrected for excess or deficit amounts of radiogenic 206 pb originating from initial 230 th disequilibrium in the 238 u decay series in monazite such disequilibrium 206 pb reaches 20 of total radiogenic 206 pb the corrected u pb ages of monazite and xenotime lie between 22 1 and 23 9 ma whereas zircon yields significantly discordant u pb ages between 30 5 and 33 9 ma pointing to precambrian material in the magma source region inherited components could also be detected in monazite the set of u pb data shows that monazite and xenotime formed simultaneously in both localities substantiating an early miocene age of 23 0 0 2ma for late kinematic crystallization of anatectic melts in the metamorphic belt of the ailao shan red river shear zone

neogene himalayan weathering history and river87sr86sr impact on the marine sr record abstract clastic sediments in the bengal fan contain a neogene history of erosion and weathering of the himalaya we present data on clay mineralogy major element stable and radiogenic isotope abundances from lower miocene pleistocene sediments from odp leg 116 nd and sr isotope data show that the himalayan provenance for the eroded material has varied little since 17 ma however from 7 to 1 ma smectite replaces illite as the dominant clay while sediment accumulation decreased implying an interval of high chemical weathering intensity but lower physical erosion rates in the ganges brahmaputra gb basin o and h isotopes in clays are correlated with mineralogy and chemistry and indicate that weathering took place in the paleo gangetic flood plain the87sr86sr ratios of pedogenic clays vermiculite smectite record the isotopic composition of sr in the weathering environment and can be used as a proxy for87sr86sr in the paleo gb basin the sr data from pedogenic clays shows that river87sr86sr values were near 0 72 prior to 7 ma rose rapidly to 0 74 in the pliocene and returned to 0 72 in the middle pleistocene these are the first direct constraints available on the temporal variability of87sr86sr in a major river system the high87sr86sr values resulted from intensified chemical weathering of radiogenic silicates and a shift in the carbonate silicate weathering ratio modeling of the seawater sr isotopic budget shows that the high river87sr86sr values require a ca 50 decrease in the sr flux from the gb system in the pliocene the relationship between weathering intensity 87sr86sr and sr flux is similar to that observed in modern rivers and implies that fluxes of other elements such as ca na and si were also reduced increased weathering intensity but reduced sr flux appears to require a late miocene pliocene decrease in himalayan erosion rates followed by a return to physically dominated and rapid erosion in the pleistocene in contrast to the view that increasing seawater87sr86sr results from increased erosion mio pliocene to mid pleistocene changes in the seawater sr budget were the result of reduced erosion rates and sr fluxes from the himalaya

geochemical record of holocene to recent sedimentation on the western indus continental shelf arabian sea 1 we present a multiproxy geochemical analysis of two cores recovered from the indus shelf spanning the early holocene to recent 14 ka indus 23 is located close to the modern indus river while indus 10 is positioned 100 km further west the holocene transgression at indus 10 was over a surface that was strongly weathered during the last glacial sea level lowstand lower holocene sediments at indus 10 have higherendvalues compared to those at the river mouth indicating some sediment supply from the makran coast either during the deposition or via reworking of older sediments outcropping on the shelf sediment transport from makran occurred during transgressive intervals when sea level crossed the mid shelf the sediment flux from non indus sources to indus 10 peaked between 11 ka and 8 ka a hiatus at indus 23 from 8 ka until 1 3 ka indicates non deposition or erosion of existing indus shelf sequences higherendvalues seen on the shelf compared to the delta imply reworking of older delta sediments in building holocene clinoforms chemical index of alteration cia mg al and sr isotopes are all affected by erosion of detrital carbonate which reduced through the holocene k al data suggest that silicate weathering peaked ca 4 6 ka and was higher at indus 10 compared to indus 23 fine grained sediments that make up the shelf have geochemical signatures that are different from the coarser grained bulk sediments measured in the delta plain the indus shelf data highlight the complexity of reconstructing records of continental erosion and provenance in marine settings

generation of bulk shear spikes in shallow stratified tidal seas abstract recent finescale observations of shear and stratification in temperate shelf sea thermoclines show that they are of marginal stability suggesting that episodes of enhanced shear could potentially lead to shear instability and diapcynal mixing the bulk shear between the upper and lower boundary layers in seasonally stratified shelf seas shows remarkable variability on tidal inertial and synoptic time scales that has yet to be explained in this paper observations from the seasonally stratified northern north sea are presented for a time when the water column has a distinct two layer structure bulk shear estimates based on adcp measurements show a bulk shear vector that rotates in a clockwise direction at the local inertial period with episodes of bulk shear spikes that have an approximately twice daily period and occur in bursts that last for several days to explain this observation a simple two layer model based on layer averaging of the one dimensional momentum equation is developed

duration of strike slip movements in large shear zones the red river belt china abstract sixty four size fractions of zircon monazite xenotime and titanite were dated by the u pb method to temporally and spatially constrain left lateral movements in the red river shear zone between the depths of 15 and 20 km the shearing led to the formation of the high grade metamorphic gneisses that form the bulk of the 4200 m high diancang shan and 3100 m high ailao shan mountain ranges the omnipresence of melt layers and pockets in these gneisses shows that temperatures associated with tectonometamorphism lie at and above the solidus of continental crust and deformation patterns are very homogeneous over the entire length of the belt north of the gneisses subvolcanic bodies intrude unmetamorphosed cover rocks in particular eocene red beds along the projected trend of the red river shear zone for a leucogranitic layer in the diancang shan gneisses an age of 24 2 0 2 ma 2 is obtained for monazite and 22 4 0 2 ma is obtained for xenotime whereas zircon formed through two growth stages at 24 23 ma these ages record solidification of the anatectic melts over a period of at least 1 8 0 4 ma during which left lateral shear was continuously active in the ailao shan gneisses zircon and titanite ages are 26 3 0 3 and 26 1 0 2 ma for two monzonitic bodies and 24 1 0 2 and 22 4 0 2 ma for two pegmatitic layers that occur within the monzonitic intrusions these results show that zircon monazite titanite and probably also xenotime are closed systems for u and pb under melting conditions 650 c therefore the ages date mineral formation in the melt rather than cooling below certain closure temperatures a significantly older age of 35 0 0 1 ma is obtained for a monzonitic subvolcanic intrusion north of the diancang shan gneisses because these intrusions follow the trend of the shear zone they most likely represent the surface expression of early activity along that zone indicating that strike slip movement had already occurred in latest eocene time the total set of u pb ages shows that magmatic and metamorphic activity occurred from 35 to 20 ma strengthening the idea that the red river shear zone absorbed most of the intraplate movements required to compensate the opening of the south china sea between 32 and 16 ma the existence of monzonitic syenitic intrusions along the red river zone implies that melting affected source rocks other than the diancang shan and ailao shan gneiss protoliths potential sources to produce such rocks lie in the deep crust possibly including melts from the mantle consequently the red river shear zone must be a deep rooted structure along which melts migrated to the surface moreover the contemporaneity of shear motion and magma generation suggests that melting occurred in direct relation to strike slip activity

indosinian nw trending shear zones within the truong son belt vietnam 40ar39ar triassic ages and cretaceous to cenozoic overprints abstract several nw trending ductile and partly mylonitic shear zones cross the truong son belt of central vietnam along the song ma and song ca valleys and north of the kontum block from da nang to aluoi and khe sanh high grade metamorphic rocks of amphibolite facies showing a retrograde evolution and consisting of ortho and paragneisses metavolcanics amphibolites marbles and quartzites are exposed along these structures they display a homogeneous deformation pattern characterized by a generally steeply dipping foliation and a near horizontal to gently plunging mineral stretching lineation indicating a strike slip tectonic regime of deformation along the southernmost fault zone at least various and consistent kinematic indicators including s c structures asymmetric tails of porphyroclasts prove that the strain and metamorphism have been generated by a phase of dominantly non coaxial deformation with a dextral sense of shear 40ar 39ar dating applied on the high grade metamorphic rocks minerals occurring along these zones provide plateau cooling ages closely around 245 ma establishing that this event took place in the lowermost triassic as an early phase of the indosinian orogeny the existence of indosinian movements in vietnam as they have been defined by previous authors during the early century is now accurately confirmed and this is the first insight in the occurrence of ductile strike slip tectonics of indosinian age along nw se fault zones well expressed in central vietnam is a cretaceous thermal and deformational overprint marked by epimetamorphism which took place between 90 and 120 ma as attested by low temperature degassing ages this cretaceous event is not found further north in the song ma zone where younger ages appear as a result of the influence of cenozoic shear movements along the red river fault which displays 20 30 ma ages on the northern flank of song ca in the bu khang phu hoat core complex comparable oligocene lower miocene ages 20 35 ma yielded by biotite and phlogopite reflect a strong overprinting and attest to a rapid uplift of the basement

horizontal advection and dispersion in a stratified shelf sea the role of inertial oscillations abstract the role played by inertial motions in horizontal dispersion within the thermocline of a broad mid latitude shelf sea is examined through the analysis of a deliberately released dye tracer our analysis is of the horizontal and vertical evolution over 40 h of a dye tracer injected into the seasonally stratified thermocline of the celtic sea on the nw european shelf the inferred diapycnal diffusivity was 1 3 1 5 10 5 m2 s 1 and the radial horizontal diffusivities of the depth integrated dye patch ranged from 1 9 to 4 0 m2 s 1 the inferred vertical diffusivity is in agreement with microstructure based estimates and the depth integrated horizontal diffusivity is broadly in agreement with previous dye release derived estimates made over similar scales and time periods asymmetry in the horizontal evolution of the dye patch was evident we argue that mean shear dispersion was responsible for lateral elongation of the dye patch particularly between hours 23 and 35 after release during which time horizontal diffusivity along the major axis ka exceeded that along the minor axis kb by more than a factor of 10 we further show that along patch shear was predominantly a result of differential advection between a deep residual flow to the south east and an oscillating wind driven surface ekman layer in this region of strong low frequency inertial shear a time dependent model of shear dispersion young et al 1982 was able to account for the observed rate of horizontal dispersion calculated on the target isopycnal surface

weathering processes in the ganges brahmaputra basin and the riverine alkalinity budget abstract we present river chemistry data for a network of rivers draining the western and central nepal himalaya our sampling locations cover the system from the sources of rivers in tibet to the gangetic plain water samples were collected throughout the year including the monsoon season for rivers in nepal and for the ganges and brahmaputra in bangladesh rivers draining the north himalaya are characterized by low discharge under a cold and arid climate main stream waters have 13 c dic near 0 and high so42 so42 hco3 ratios xso4 with values around 40 eq and high cl ca is the dominant cation ca2 cations 55 to 75 eq after correction of sodium by chloride dissolved sulfate is produced in waters from the tethian sedimentary series whereas chlorine is related to thermal waters dissolved sulfate is primarily derived from sulfide oxidation rather than evaporite dissolution 13 c dic values of up to 3 9 show that metamorphic co2 is an important weathering agent rivers of the north himalayan basins have about 50 of their alkalinity derived from carbonate dissolution 20 from biogenic activity and 30 from metamorphic co2 on the south flank of the himalaya rivers are more depleted in 13 c and have on average lower xso4 most rivers have 13 c dic and xso4 values compatible with a simple mixing between soil co2 and sulfate derived from sulfide oxidation during the monsoon discharge increases by a factor 20 but the total dissolved concentration is only slightly reduced xso4 and 13 c decrease while ca cations increases implying enhanced dissolution of pedogenic calcite as a whole the g b riverine flux of alkalinity derived from silicate weathering is around 2 7 1011 mol year modest at the global scale sulphuric acid controls 20 30 of the weathering reaction in the brahmaputra and 6 to 9 in the ganges na and k balance 60 to 65 of the silicate derived alkalinity flux and the long term co2 consumption by ganges and brahmaputra is near 6 4 1010 mol year the flux of metamorphic co2 converted to alkalinity via weathering reactions is ca 1 1010 mol year

blueschist bearing metamorphic core complexes in the qiangtang block reveal deep crustal structure of northern tibet a 500 km long belt of metamorphic exposures in the qiangtang block provides an opportunity to study the internal structure of northern tibetan crust metamorphic rocks exposed at two widely separated areas along this belt consist of blueschist bearing melange and are bounded by late triassic early jurassic domal low angle normal faults we propose that this melange was underplated to the qiangtang block and was subsequently exhumed by detachment faulting both the underplating and the exhumation occurred during early mesozoic southward subduction of oceanic lithosphere along the jinsha suture this model predicts that the deeper crust of much of northern tibet consists of accretionary melange in contrast to the continental crystalline crust of southern tibet and may account for north south variations of cenozoic tectonism in tibet

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

reconstructing chemical weathering physical erosion and monsoon intensity since 25 ma in the northern south china sea a review of competing proxies reconstructing the changing strength of the east asian summer monsoon has been controversial because different proxies many being indirect measures of rainfall tell contrasting stories about how this has varied over long periods of geologic time here we present new sr isotope grain size and clastic flux data and synthesize existing proxies to reconstruct changing chemical erosion in the northern south china sea since the oligocene using the links between weathering rates and monsoon strength established in younger sediments as a way to infer intensity chemical proxies such as k rb l al and the chemical index of alteration cia together with clay proxies like kaolinite illite chlorite show a steady decline in alteration after a sharp fall following a maximum at the mid miocene climatic optimum mmco 15 5 17 2 ma probably as a result of cooling global temperatures in contrast physical erosion proxies including bulk ti ca and clastic mass accumulation rates mar show peaks at 21 23 ma similar to 19 ma and 15 5 17 2 ma implying faster run off in the absence of drainage capture rates increase again likely driven by slightly increased run off after 13 ma but decrease after 8 ma which is identified as a period of summer monsoon weakening sr isotope composition correlates with hematite goethite and the spectral proxy crat to show stronger weathering linked to more monsoonal seasonality these proxies argue for a strengthening of the east asian monsoon after 22 23 ma followed by an extended period of monsoon maximum between 18 and 10 ma then weakening there is some suggestion that the summer monsoon may have strengthened since 3 4 ma after reaching a minimum in the pliocene c 2014 elsevier b v all rights reserved

late mesozoic and cenozoic thermotectonic evolution along a transect from the north china craton through the qinling orogen into the yangtze craton central china 1 cretaceous and cenozoic reactivation of the triassic qinling dabie orogen between the north china and yangtze cratons resulted from the combined effects of pacific subduction back arc extension in east china and collisions in west china we report new apatite fission track and apatite and zircon u th he data from east qinling along a 400 km long n s transect from huashan through the qinling orogen to huangling the ages show a general pattern of younging northward three major cooling phases are defined by modeling the multiple thermochronologic data sets the first phase occurred locally in the north and south qinling during the late triassic to early jurassic following heating associated with the triassic yangtze subduction and exhumation of the wudang metamorphic core complex on the cratonal edge a second phase represents regional exhumation between 100 and 60 ma coeval with rifting marked by the late cretaceous eocene k2 e red bed deposition in eastern china and possibly indicating a link with pacific subduction back arc extension in eastern china however it may also have been superimposed by eastward tectonic escape resulting from the lhasa west burma qiangtang indochina collision the third cooling phase was initiated at 45 ma exclusively in the north qinling and in the footwall of the graben bounding normal fault of the weihe graben in the lesser qinling we suggest the third phase was related to reactivation of the qinling fault system as a result of eastward tectonic escape imposed by the india asia collision at 50 ma

mixing in a coastal environment 1 a view from dye dispersion 1 dye release experiments were performed together with microstructure profiling to compare the two methods of estimating diapycnal diffusivity during summer and fall stratification on the continental shelf south of new england the experiments were done in 1996 and 1997 as part of the coastal mixing and optics experiment during the 100 hours or so of the experiments the area of the dye patches grew from less than 1 km2 to more than 50 km2 sundermeyer and ledwell 2001 diapycnal diffusivities inferred from dye dispersion range from 10 6 to 10 5 m2 s at buoyancy frequencies from 9 to 28 cycles hour diffusivities estimated from the dye and those estimated from dissipation rates in the companion paper by oakey and greenan 2004 agree closely in most cases estimates of diffusivities from towed conductivity microstructure measurements made during the cruises by duda and rehmann 2002 and rehmann and duda 2000 are fairly consistent with the dye diffusivities the dye diffusivities would be predicted well by an empirical formula involving shear and stratification statistics developed by mackinnon and gregg 2003 from profiling microstructure measurements obtained at the same site in august 1996 all of the measurements support the general conclusion that the diffusivity averaged over several days is seldom greater than 10 5 m2 s in the stratified waters at the site and usually not much greater than 10 6 m2 s severe storms such as a hurricane that passed over the cmo site in 1996 can dramatically increase the mixing at the site however

shear flow dispersion internal waves and horizontal mixing in the ocean abstract two models of advection diffusion in the oscillatory sheared velocity field of an internal wave are discussed our goal is to develop intuition about the role of such currents in horizontal ocean mixing through the mechanism of shear dispersion the analysis suggests simple parameterizations of this process i e those in eqs 7 36 and 42 the enhanced horizontal diffusion due to the interaction of the vertical diffusion and vertical shear of the wave field can be described by an effective horizontal diffusivity which is equal to the actual horizontal diffusivity plus a term equal to the mean square vertical shear of horizontal displacement times the vertical diffusivity provided the vertical length scale of the horizontal velocity field is not too small in the limit of small vertical length scale the expression reduces to taylor s 1953 result in which the effective horizontal diffusivity is inversely proportional to the actual vertical diffusivity the solutions also incidentally

internal wave induced dispersion and mixing on a sloping boundary 1 a fluorescent dye tracer was released in a stratified water column above a sloping boundary revealing profound dispersive asymmetry initially the dye spread symmetrically in the vertical until the deeper portion became influenced by the bottom boundary layer bbl the shallower portion then spread rapidly up slope in a thin boundary layer influenced by highly non linear isopycnal distortions the deeper down slope portion of dye subsequently became detached from the boundary and intruded into the basin interior in three distinct layers along isopycnal surfaces the intrusions result from the gravitational collapse of mixing patches created by baroclinic flow over an irregular boundary the vertical separation of the layers matches the inverse topographic wavenumber up slope displacement of the dye is consistent with an advective diffusive balance near the sloping boundary controlled by slope angle and roughness horizontal diffusivities increased from 1 4 m2s 1 to 18 4 m2s 1 during the 4 day experiment

late cenozoic uplift of southeastern tibet the age of surface uplift in southeastern tibet is currently unknown but the initiation of major river incision can be used as a proxy for the timing of initial uplift the topographically high eastern plateau and gently dipping southeastern plateau margin are mantled by an elevated low relief relict landscape that formed at a time of slow erosion at low elevation and low tectonic uplift rates prior to uplift of the eastern tibetan plateau thermochronology from deep river gorges that are cut into the relict landscape shows slow cooling between ca 100 and ca 10 20 ma and a change to rapid cooling after ca 13 ma with initiation of rapid river incision at 0 25 0 5 mm yr between 9 and 13 ma a rapid increase in mean elevation of eastern tibet beginning at this time supports tectonic climate models that correlate the lateral eastern expansion of high topography in tibet with the late miocene intensification of the indian and east asian monsoons

predominant floodplain over mountain weathering of himalayan sediments ganga basin we present an extensive river sediment dataset covering the ganga basin from the himalayan front downstream to the ganga mainstream in bangladesh these sediments were mainly collected over several monsoon seasons and include depth profiles of suspended particles in the river water column mineral sorting is the first order control on the chemical composition of river sediments taking into account this variability we show that sediments become significantly depleted in mobile elements during their transit through the floodplain by comparing sediments sampled at the himalayan front with sediments from the ganga mainstream in bangladesh it is possible to budget weathering in the floodplain assuming a steady state weathering regime in the floodplain the weathering of himalayan sediments in the gangetic floodplain releases ca 189 92 10 9 and 69 22 10 9 mol yr of carbonate bound ca and mg to the dissolved load respectively silicate weathering releases 53 18 10 9 and 42 13 10 9 mol yr of na and k while the release of silicate mg and ca is substantially lower between ca 0 and 20 10 9 mol yr additionally we show that sediment hydration h2o is a sensitive tracer of silicate weathering that can be used in continental detrital environments such as the ganga basin both h2o content and the d h isotopic composition of sediments increases during floodplain transfer in response to mineral hydrolysis and neoformations associated to weathering reactions by comparing the chemical composition of river sediments across the floodplain with the composition of the eroded himalayan source rocks we suggest that the floodplain is the dominant location of silicate weathering for na k and h2o overall this work emphasizes the role of the gangetic floodplain in weathering himalayan sediments it also demonstrates how detrital sediments can be used as weathering tracers if mineralogical and chemical sorting effects are properly taken into account 2012 elsevier ltd all rights reserved

mesozoic cooling across the yidun arc central eastern tibetan plateau a reconnaissance 40ar 39ar study abstract 40 ar 39 ar dating of hydrous minerals from igneous and metamorphic rocks was undertaken with the aim of characterizing the thermal history of the yidun arc eastern tibetan plateau regional metamorphism of palaeozoic rocks of the western yidun arc occurred during the early triassic collision with the qiangtang block along the jinsha jiang suture hornblende muscovite and biotite 40 ar 39 ar ages from syn tectonic igneous rocks within this collision zone suggest that the region had cooled to below 300 c by the late triassic muscovite samples from mica schists within this collision zone yield variably discordant age spectra with generally triassic 40 ar 39 ar ages consistent with cooling of the region to below muscovite closure temperatures by the close of the triassic hornblende and biotite 40 ar 39 ar ages from granite and granodiorite plutons in the eastern yidun arc suggest post emplacement cooling of these plutons occurred rapidly consistent with these granites being high level intrusions in sum these results suggest that by the beginning of the jurassic the yidun arc as a whole had cooled to below 300 c two cretaceous granites in the central yidun arc also experienced rapid cooling suggesting they too were emplaced at shallow crustal depths post triassic deformation of the region must have occurred at or below the brittle ductile transition within the crustal levels now exposed

geochemistry of alkaline earth elements mg ca sr ba in the surface sediments of the yellow sea abstract the concentrations of alkaline earth elements were measured in the surface sediments of the yellow sea in an attempt to establish their sources from horizontal distributions the maximum concentrations of mg are found in the central yellow sea and its horizontal distribution is mainly controlled by quartz dilution the concentrations of ca and sr increase toward the southeastern yellow sea region the relatively higher enrichment of ca and sr in the southeastern yellow sea with sr ca ratios of 0 0053 suggests the presence of carbonates which are comprised of foraminifera and or coccoliths like deep sea carbonates on the other hand the concentrations of ba increase toward the northeastern yellow sea the higher ba enrichment in the sandy region of the northeastern yellow sea is associated with the presence of feldspar as evidenced by excellent correlations among ba mg al mg and sr ca ratios our results suggest that the transgressive and relict sand occurring in the eastern parts of the yellow sea contains al sr and ba enriched feldspars which originated from arprock river korea and thus can be distinguished from sand occurring in the western parts of the yellow sea which originated from yellow and yangtze rivers

the north west borneo trough abstract the north west borneo trough is bordered along its south east margin by a melange wedge that has been the subject of disagreement with insufficient discussion offshore palawan it has been interpreted as an accretionary prism that has been preserved in place when subduction ceased in the middle miocene it is unconformably overlain by undeformed upper miocene to holocene draping strata farther south west along the trough the seismically identical melange wedge has been named a major thrust sheet system which was assumed to have been thrust as a nappe north westwards over the autochthonous dangerous grounds terrane of attenuated continental crust of the south china sea passive margin the accretionary prism model is the simplest resulting in interpretation of the north west borneo trough as a fossil trench subduction was halted by the rifted and attenuated continental crust arriving at the benioff zone choking subduction and causing isostatic uplift of the western cordillera of sabah the subduction system therefore became a collision zone resulting in the term foredeep in which case the actual trench position remains obscure and somewhere towards the south east but shown in unrestrained geological cartoons enigmas remain in the palawan area of the philippines the trough position is bathymetrically obscure in places and its given position makes it impossible to derive the calamian micro continent from continental asia as required from its stratigraphy in the south west the trough terminates abruptly at the west baram line the trough contains several spectacular edifices formerly suggested to be volcanoes or mud volcanoes but herein shown to be carbonate build ups that were drowned indicating that the trough subsided too fast for build up continuation whereas post rift thermal subsidence of the dangerous grounds was slower allowing the spratly islands build ups to continue active

geochemistry and tectonic setting of the luzon arc philippines miocene 10 ma to recent volcanism is associated with eastward subduction along the manila trench for some 1 200 km from the coastal range in taiwan south to min doro we suggest calling this the luzon arc there are five distinct segments along this arc mindoro bataan northern luzon babuyan and taiwan the bataan and mindoro segments in the south are separated by a northeast southwest zone of volcanoes the northern luzon segment consists of pliocene to pleistocene necks and plugs that run along the entire western section of northern luzon the babuyan segment 10 ma to recent is made up of a larger number of active volcanoes than are other segments of the luzon arc the taiwan segment is composed entirely of extinct volcanoes in the coastal range and on the islands of lanhsu and lutao late miocene to 1 8 ma the nature of volcanism throughout the luzon are is clearly subduction related with calc alkaline affinities high th la ratios and negative nb ta and ti anomalies the earliest phase of volcanism so far recorded 10 ma however is tholeiitic island of calayan in the babuyan segment with mid oceanic ridge basalt like morb like tight rare earth element and large ion lithophile element concentrations and low th u ratios there are clear differences in sr and nd isotopic ratios among the samples from the luzon are samples from the southern segments mindoro through bataan fall within the mantle array with higher sr and lower nd isotopic values than those of morb pb isotope data indicate sediment involvement the isotopic values have been interpreted to result from source contamination by slab derived fluids bataan segment and by subducted crustal material from collision of the mindoro palawan terrane mindoro segment with the manila trench in contrast babuyan segment samples north have unusual island arc nd and sr isotopic ratios that fall below the mantle array they appear to be the result of sediment source contaminants or a unique mantle composition incorporation of sediments analyzed from eastern china and taiwan into a morb type mantle can explain the nd and sr isotopic ratios of the babuyan segment

denudation history of onshore central vietnam constraints on the cenozoic evolution of the western margin of the south china sea abstract apatite fission track analysis is used to monitor the timing and rate of upper crustal cooling and denudation across central vietnam in response to rifting of the south china sea results show regional denudation in the syn and post rift phases of the south china sea occurred at a similar rate 40 10 m myr consistent with the sedimentation record in adjacent offshore basins this suggests that extension was not the dominant factor controlling cenozoic denudation probably because the study area was sufficiently distant from the actively rifted continental margin a significant change in regional cooling rate during the upper miocene inferred denudation rates also shift from 34 m myr to 390 500 m myr is linked to enhanced erosion and deposition of prograding sediments in adjacent offshore basins this shift in denudation is associated and contemporaneous with initiation of regional basaltic magmatism and development of topography

mixing of a tracer in the pycnocline a patch of sulfur hexafluoride was released in may 1992 in the eastern north atlantic on an isopycnal surface near 300 m depth and was surveyed over a period of 30 months as it dispersed across and along isopycnal surfaces the diapycnal eddy diffusivity k estimated for the first 6 months was 0 12 0 02 cm2 s while for the subsequent 24 months it was 0 17 0 02 cm2 s the vertical tracer distribution remained very close to gaussian for the full 30 months as the root mean square rms dispersion grew from 5 to 50 m lateral dispersion was measured on several scales the growth of the rms width of the tracer streaks from less than 100 m to approximately 300 m within 2 weeks implies an isopycnal diffusivity of 0 07 m2 s at scales of 0 1 to 1 km larger than expected from the interaction between vertical shear of the internal waves and diapycnal mixing teasing of the overall patch initially about 25 km across into streaks with an overall length of 1800 km within 6 months supports predictions of exponential growth by the mesoscale strain field at a rate of 3 0 5 10 7 s 1 the rms width of these streaks estimated as 3 km and maintained in the face of the streak growth indicates an isopycnal diffusivity of 2 m2 s at scales of 1 to 10 km much greater than expected from internal wave shear dispersion the patch was painted in albeit streakily by 12 months confirming expectations from analytical and numerical models homogenization of the patch continued during the subsequent 18 months while the patch continued to spread with an effective isopycnal eddy diffusivity on the order of 1000 m2 s acting at scales of 30 to 300 km

water and sediment discharge from small mountainous rivers taiwan the roles of lithology episodic events and human activities national science council of taiwan nsc96 2628 m 001 028 u s national science foundation and naval oceanographic office

structural signatures and 40ar 39ar geochronology of the indosinian xuefengshan tectonic belt south china block abstract to advance our understanding of the kinematics of the xuefengshan tectonic belt xfstb and the mesozoic tectonics of the south china block scb this paper presents new data and interpretations from our structural studies and thermochronological dating the xfstb is characterized by greenschist facies metamorphism and development of s and s l tectonic fabrics an asymmetric positive flower structural pattern is composed of nw wnw and se ese dipping cleavages faults and shear zones kinematic indicators indicate a dominant top to wnw thrusting with a sinistral strike slip component on the ese dipping shear zones and top to ese back thrusting on the wnw dipping shear zones the quartz c axis orientations of mylonitic rocks exhibit monoclinic point maximum asymmetry indicative of a sinistral shear sense under the a basal gliding condition the timing of the major deformation event has been constrained to the middle triassic to early jurassic 244 195 ma on the basis of 40ar 39ar geochronology and other geological observations the structural characteristics of the xfstb are probably related to an indosinian oblique convergent zone in combination with tectonic wedging and associated back thrusting all above a low angle se dipping basal detachment oblique northwestward and upward movement along the basal detachment was partitioned into the nw wnw directed thrusting se ese directed back thrusting and subsidiary strike slip movement along the ne and nne trending faults and shear zones the xfstb may represent part of a huge structural fan between the yangtze and cathaysian blocks and can be interpreted as a product of the indosinian intracontinental collision involving a weak zone

a fission track and u th he thermochronometric study of the northern margin of the south china sea an example of a complex passive margin zircon fission track zft apatite fission track aft and u th he thermochronometric data are used to reconstruct the cenozoic exhumation history of the south china continental margin a south to north sample transect from coast to continental interior yielded zft ages between 116 6 4 7 ma and 87 3 4 0 indicating that by the late cretaceous samples were at depths of 5 6 km in the upper crust apatite ft ages range between 60 9 3 6 and 37 3 2 3 ma with mean track lengths between 13 26 0 16 m and 13 95 0 19 m whilst ahe ages are marginally younger 47 5 1 9 15 3 0 5 ma these results show the sampled rocks resided in the top 1 1 5 km of the crust for most of the cenozoic thermal history modeling of the combined ft and u th he datasets reveal a common three stage cooling history which differed systematically in timing inland away from the rifted margin 1 initial phase of rapid cooling that youngs to the north 2 a period of relative but not perfect thermal stasis at 70 60 c which increases in duration from the south to the north 3 final stage cooling to surface temperatures that initiated in all samples between 15 and 10 ma the timing and pattern of rock uplift and erosion does not fit with conventional passive margin landscape models that require youngest exhumation ages to be concentrated at or close to the rifted margin the history of south china margin is more complex aided by weakened crust from the active margin period that immediately preceded rifting and opening of the south china sea this rheological inheritance created a transition zone of steeply thinned crust that served as a flexural filter disconnecting the northern margin of the south china block and site of active rifting to the south consequently whilst the south china margin displays many features of a rifted continental margin its exhumation history does not conform to conventional images of a passive margin

kilop cretaceous hardground kale gumushane ne turkey description and origin abstract a hardground surface is well exposed in the kilop area of kale gumushane ne turkey which forms part of the eastern pontides here the hardground is underlain by shallow water lower cretaceous limestones and overlain by upper cretaceous red limestones marls which contains a planktonic microfauna including globotruncanidae in the field the recognition of the hardground is based on the presence of extensive burrows especially vertical burrows the encrusting rudistid bivalve requienia neptunian dykes with infills of pelagic sediments and synsedimentary faults skolithos and thalassinoides type burrows are present some burrow walls show iron hydroxide staining the extensive burrowing occurred prior to lithification on the other hand the neptunian dykes and synsedimentary faults which cut the hard ground occurred after the lithification these features indicate the progressive hardening of the substrate the burrowed limestone consists of an intrabioclastic peloidal grainstone which was deposited in an intertidal to shallow subtidal moderate to relatively high energy environment the peloidal limestone shows little or no evidence of submarine cementation characterized by only scarce relics of isopachous cement rims of bladed calcite spar the grainstone cement is composed predominantly of blocky calcite and overgrowth calcite cements on the echinoid fragments the origin of this cement is controversial biostratigraphic analysis of the limestones demonstrates that there is a marked stratigraphic gap hiatus spanning the aptian to the santonian in the cretaceous of the kilop area the formation of the kilop hardground is related to the break up and subsidence of the eastern pontides carbonate platform during the formation of the black sea backarc basin hardground development was initiated in a shallow marine environment of slow sedimentation and with moderate to high energy indicating slow subsidence later the hardground subsided abruptly as shown by the deposition of pelagic sediments on the hardground surface during drowning the kilop area was converted to a bypass margin where currents were effective the formation of the hardground may also have been associated with an eustatic rise in sea level

formation and fate of sedimentary depocentres on southeast asia s sunda shelf over the past sea level cycle and biogeographic implications abstract sea level variations are the major factor controlling sedimentation as well as the biogeographic patterns at continental margins over late quaternary times fluctuations on millennial time scales produce locally complex deposits in coasts and on shelves associated with short term influence on species development this article reviews the sedimentary and biogeographic history of the tropical siliciclastic sunda shelf as an end member of continental shelves regarding extreme width an enormous sediment supply and highest biodiversity in response to rapid sea level fluctuations we describe particular depositional segments as part of a genetic succession of zones from land to the deep sea based on literature data field observations and calculation of hydro isostatic adjustment effects on changing relative sea level these segments are characterized by individual sedimentary processes and deposits and by a specific potential for material storage and re mobilization long term regressive intervals led to overall sigmoidal promoting extremely thick and wide succeeding units in contrast rapid lateral shifts of defined depocentres over long distances took place in response to short term sea level fluctuations fully isolated small scale sediment bodies formed when sea level changed at exceptionally high rates as a result of the high availability of organic rich sediments mangrove and freshwater peats formed frequently over late quaternary times the appearance of thick massive and widespread peats is mainly linked to time intervals of a sea level rise at slow rates whilst organic matter appears much more dispersely in the sediments during episodes of rapidly changing sea level the preservation potential of the regressive units is generally high due to highest initial sediment supply stabilizing soil formation during exposure and rapid subsidence preservation of depositional elements from other periods is more exceptional and either restricted to local morphological depressions or to episodes of rapid sea level change besides complex channel incision an overall lowering of the sediment surface related to erosion as deep as 20 m or more over wide areas took place mainly during sea level lowering the final export of shelf material is documented by enormous mass wasting packages on the associated continental slope from a palaeogeographic perspective the rapid formation or disappearance of special habitat zones such as mangrove fringes and extended mud flats led to species establishment or truncation in distribution in addition the opening or closure of ocean passages as narrow bridges allowing limited species crossing or as fully colonized corridors had severe impact on eco fragmentation and the expansion or contraction of species independent of such particular conditions sea level changes have been too rapid over the past climatic cycle to allow full regeneration and mature development of coast related ecosystems

evolution of the south china sea revised ages for breakup and seafloor spreading abstract the continental breakup which gave way to the formation of the oceanic south china sea scs basin began in the latest cretaceous in the northeastern scs and propagated in southern and western direction over a long period of time possibly more than 40 m y the seafloor spreading history of the south china sea has been interpreted in different ways in the past and the debate over the correct timing of the major tectonic events continues we review the different models that have been published and present a revised interpretation of seafloor spreading anomalies based on three datasets with documented high quality which cover all of the scs but the northernmost and southernmost parts we can precisely date the onset of seafloor spreading in the central part of the scs at 32 ma after a ridge jump at 25 ma spreading also began in the southwestern sub basin and spreading ended at 20 5 ma in the entire basin followed by a phase of magmatic seamount formation mainly along the abandoned spreading ridge spreading rates vary from 56 mm yr in the early stages to 72 mm yr after the ridge jump to 80 mm yr in the southwestern sub basin we find indications for a stepwise propagation of the seafloor spreading from northeast to southwest in segments bounded by major fracture zones seafloor spreading ended abruptly probably because the subduction zone along the eastern and southern boundary of the scs of which today the manila trench remains was blocked by collision with a continental fragment possibly the northern part of palawan or a part of the dangerous grounds

oligocene miocene bu khang extensional gneiss dome in vietnam geodynamic implications a large oligocene miocene extensional gneiss dome in vietnam has implications for the kinematics of the ailao shan red river shear zone and rifting of the south china sea a major northeast dipping extensional shear zone separates the bu khang dome from less metamorphosed units above shearing deformation occurred during the exhumation of high pressure rocks 40 ar 39 ar ages on micas reveal oligocene miocene ages from 36 to 21 ma after a high pressure episode of probable oligocene age the area was subjected to northeast southwest extension during the opening of the south china sea a kinematic model involving left lateral shear and block rotation within a north south dextral shear zone integrates our observations in the bu khang dome with south china sea rifting

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

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

a high resolution absolute dated late pleistocene monsoon record from hulu cave china oxygen isotope records of five stalagmites from hulu cave near nanjing bear a remarkable resemblance to oxygen isotope records from greenland ice cores suggesting that east asian monsoon intensity changed in concert with greenland temperature between 11 000 and 75 000 years before the present yr b p between 11 000 and 30 000 yr b p the timing of changes in the monsoon as established with 230th dates generally agrees with the timing of temperature changes from the greenland ice sheet project two gisp2 core which supports gisp2 s chronology in this interval our record links north atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer east asian monsoon originates

laser 40ar 39ar dating of single detrital muscovite grains from early foreland basin sedimentary deposits in india implications for early himalayan evolution in india the dagshai and overlying kasauli formations represent the oldest exposed continental foredeep sediments eroded from the himalayan orogen 40ar 39ar dating of individual detrital white micas from these sedimentary units has provided maximum depositional ages of 28 ma for the dagshai formation at one locality and 25 ma at a second locality whereas deposition of the kasauli formation occurred after 28 ma at two localities and after 22 ma at a third locality this timing suggests that in india the start of substantial exhumation and erosion from the rising himalayan orogen was delayed until 28 ma

large scale drainage capture and surface uplift in eastern tibet sw china before 24 ma inferred from sediments of the hanoi basin vietnam current models of drainage evolution suggest that the non dendritic patterns seen in rivers in se asia reflect progressive capture of headwaters away from the red river during and as a result of surface uplift of eastern asia mass balancing of eroded and deposited rock volumes demonstrates that the red river catchment must have been much larger in the past in addition the nd isotope composition of sediments from the hanoi basin vietnam interpreted as paleo red river sediments shows rapid change during the oligocene before similar to 24 ma we interpret this change to reflect large scale drainage capture away from the red river possibly involving loss of the middle yangtze river reorganization was triggered by regional tilting of the region towards the east this study constrains initial surface uplift in eastern tibet and southwestern china to be no later than 24 ma well before major surface uplift and gorge incision after 13 ma

ar ar muscovite dating as a constraint on sediment provenance and erosion processes in the red and yangtze river systems se asia in this study we applied 40ar 39ar dating methods to muscovite grains in sands from the modern red and yangtze rivers in order to constrain palaeo and modern erosion patterns and sediment transport processes micas in the headwaters of the red river and its southern da river tributary are dominantly cenozoic while the northern lo river tributary shows a typical 160 220 ma population the lower reaches of the red river comprise 53 of such triassic grains indicating that the lo river is the most important net contributor to the modern delta however this contrasts with zircon u pb data indicating that the upper reaches of the red river dominate that mineral group we suggest that the mismatch reflects the rapid transport of mica relative to zircon in the river and the fact that the red river is not in equilibrium we hypothesize that the zircons were eroded 8 ka under a regime of stronger monsoon that enhanced erosion in the northern drainage basin while modern erosion is focused in the south especially in the song chay massif micas from the upper yangtze river are resolvably older than those in the red river 230 250 ma so that this method could be used to test for drainage capture in palaeo delta sediments micas in the modern yangtze delta are generally much younger than those in the upper reaches and indicate that erosion in the longmen shan sichuan and neighbouring regions is more important than the upper yangtze in supplying sediment reflecting the stronger monsoon rains in those areas

clay mineral distribution in surface sediments of the northeastern south china sea and surrounding fluvial drainage basins source and transport clay mineralogy of surface samples throughout the northeastern south china sea scs and surrounding fluvial drainage basins has been investigated to reveal source and transport of detrital fine grained sediments three characteristic end members in clay mineral compositions were observed from three major provenances respectively major kaolinite and moderate illite and chlorite from the pearl river predominant illite and chlorite from taiwan and principal smectite from the luzon arc system the transport of four clay mineral species in the northeastern scs is influenced by various mechanisms smectite derived from the luzon arc system is transported mainly by surface current with significant influence of the kuroshio intrusion illite and chlorite from taiwan are mainly carried by deep water current while kaolinite from the pearl river is affected obviously by the westward guangdong coastal current and longshore current a westerly tongue shaped distribution of high contents of illite and chlorite mainly from taiwan on the eastern shelf may result from the westward guangdong coastal current during the winter the results suggest that clay mineral assemblages in the northeastern scs are mainly controlled by provenance supply and current transport and their downcore records do not bear contemporaneous paleoclimatic features the implication of clay minerals for proxies of the late quaternary east asian monsoon evolution is realized through both the provenance supply and current transport processes c 2010 elsevier b v all rights reserved

geochemical and nd isotopic variations in sediments of the south china sea a response to cenozoic tectonism in se asia secular variations in geochemistry and nd isotopic data have been documented in sediment samples at odp site 1148 in the south china sea major and trace elements show significant changes at ca 29 5 ma and 26 23 ma whereas ond values show a single change at ca 26 23 ma increases in al ti al k rb sr and la lu ratios and a decrease in the th la ratio of the sediments beginning at 29 5 ma are consistent with more intense chemical weathering in the source region the abrupt change in nd isotopes and geochemistry at ca 26 23 ma coincides with a major discontinuity in the sedimentology and physical properties of the sediments implying a drastic change in sedimentary provenance and environment at the drill site comparison of the nd isotopes of sediments from major rivers flowing into the south china sea suggests that pre 27 ma sediments were dominantly derived from a southwestern provenance indochina sunda shelf and possibly northwestern borneo whereas post 23 ma sediments were derived from a northern provenance south china this change in provenance from southwest to north was largely caused by ridge jumping during seafloor spreading of the south china sea associated with a southwestward expansion of the ocean basin crust and a global rise in sea level thus the geochemical and nd isotopic changes in the sediments at odp site 1148 are interpreted as a response to a major plate reorganization in se asia at ca 25 ma b 2003 elsevier science b v all rights reserved

diffusion in irish coastal waters results are presented from dye diffusion experiments performed at a selection of near shore and estuarine sites around the coastline of ireland the data from discrete dye releases have been analysed to derive along and across patch horizontal diffusivities and estimates are presented of both fickian and non fickian diffusion coefficients most of the sites were characterized by strong tidal currents so that the dye became vertically well mixed soon after release consequently the mixing was found to be tidally dominated and to a reasonable approximation the magnitude of the one dimensional radial spreading horizontal diffusivity m 2 s 1 was equal to the tidal current speed m s 1 wind induced mixing was only of secondary importance but this may be a consequence of the experiments having taken place during conditions of generally light winds in general the observed mixing rates agree with those predicted by oceanic diffusion diagrams

fishing and the oceanography of a stratified shelf sea fishing vessel position data from the vessel monitoring system vms were used to investigate fishing activity in the celtic sea a seasonally stratifying temperate region on the shelf of northwest europe the spatial pattern of fishing showed that three main areas are targeted 1 the celtic deep an area of deeper water with fine sediments 2 the shelf edge and 3 an area covering several large seabed banks in the central celtic sea data from each of these regions were analysed to examine the contrasting seasonality of fishing activity and to highlight where the spring neap tidal cycle appears to be important to fishing the oceanographic characteristics of the celtic sea were considered alongside the distribution and timing of fishing illustrating likely contrasts in the underlying environmental drivers of the different fished regions in the central celtic sea fishing mainly occurred during the stratified period between april and august based on evidence provided in other papers of this special issue we suggest that the fishing in this area is supported by 1 a broad increase in primary production caused by lee waves generated by seabed banks around spring tides driving large supplies of nutrients into the photic zone and 2 greater concentrations of zooplankton within the region influenced by the seabed banks and elevated primary production in contrast while the shelf edge is a site of elevated surface chlorophyll previous work has suggested that the periodic mixing generated by an internal tide at the shelf edge alters the size structure of the phytoplankton community which fish larvae from the spawning stocks along the shelf edge are able to exploit the fishery for nephrops norvegicus in the celtic deep was the only one to show a significant spring neap cycle possibly linked to nephrops foraging outside their burrows less during spring tides more tentatively the fishery for nephrops correlated most strongly with a localised shift in the tidal current polarisation suggesting that the muddy seabed required by nephrops is controlled by rotational constraints on the extent of the bottom boundary layer

oligo miocene granitic magmatism in central vietnam and implications for continental deformation in indochina two granitoid intrusions within the bu khang extensional complex in central vietnam have been dated by u pb and rb sr geochronology a monazite u pb age of 26 0 0 2 2 myr was obtained for the bu khang pluton and 23 7 1 6 1 7 myr for monazite allanite and zircon from the dai loc intrusion these ages date crystallization of magmas previously assigned precambrian to devonian rb sr analyses of k feldspar and biotite fractions from the samples yield ages of 19 8 0 6 2 myr and 19 6 0 5 myr respectively the thermal history recorded by the different geochronometers implies an average exhumation rate of 2 mm yr 1 corresponding to 9 km of unroofing magmatism was either i induced passively by lithospheric thinning driven by changes in regional tectonic stresses or ii triggered actively by an ascending plume tertiary exhumation and magmatism documented elsewhere in indochina e g ailao shan red river and wang chao shear zones favours a regional tectonic cause for extension and granitoid magmatism in the bu khang complex on the other hand the presence of an upwelling thermal anomaly since at least 35 ma causing mantle melting below indochina is supported by shear wave velocity variations in the mantle and source geochemistry of both the bu khang plutons and the red river belt intrusions in either case tertiary exhumation of the bu khang complex can account for previously undocumented ne sw directed extension which is required in northern vietnam to account for structural changes related to the opening of the south china sea

multielement geochemistry of offshore sediments in the southeastern yellow sea and implications for sediment origin and dispersal abstract in total 116 surface sediments were collected from an offshore area of the southeastern yellow sea including a large mud belt southeastern yellow sea mud seysm and analyzed for grain size and geochemical composition organic carbon and 42 elements principal component analysis revealed that elemental compositions of the offshore sediments were mostly controlled by common aluminosilicate clay minerals and quartz most elements heavy minerals hf zr u th rare earth elements rees biogenic calcium carbonates ca sr feldspar and mica k ba pb and mn oxides mn oxide coatings on sand grains notably total ree concentrations may be associated with heavy minerals possibly zircon and monazite especially in silty grains even though heavy ree hrees concentrations are more strongly controlled by clay minerals in muddy sediments the diagnostic discrimination plots of geochemical compositions e g fe vs al mg vs al light ree vs hree and la lu ucc vs la y ucc suggested that the offshore sediments especially in seysm deposits were composed of a mixture of korean river derived silts and chinese river derived clays this demonstrated that chinese river sediments especially clay particles largely contributed to the formation of the giant mud belt in the korean coastal area furthermore the observed geochemical end members provide new markers of sediment provenance which can be used to track dispersal patterns and sediment limits from chinese and or korean rivers

age and origin of the tanghu granite southeast china results from u pb single zircon and nd isotopes u pb single zircon and sm nd whole rock isotopic data for the tanghu granite of the zhuguangshan batholith suggest that a basement either composed of or derived from middle proterozoic to late archean rocks underlies southeast china the granite contains two kinds of inherited zircon components whole anhedral zircons and small cores within younger magmatic zircons the u pb data for a single analysis of four magmatic zircons yielded a concordant age of 433 2 ma whereas single zircons yielded two linear arrays with lower intercepts of 435 1 and 434 14 ma and upper intercepts of 1415 3 and 2516 6 ma respectively xenocrystic zircons of archean age had not previously been recognized in any rocks from southeast china two whole rock samples from the granite yielded nd values of 7 1 and 8 3 at 434 ma and nd model ages of 1750 ma relative to a depleted mantle we conclude from this isotope data that the tanghu granite crystallized at 434 1 6 ma weighted average and was derived from a source of unknown age but had an average crustal residence age of 1750 ma and was partially composed of a mixture of continental crustal material of at least two ages 1400 and 2500 ma