large scale bedforms sediment distribution and sand mobility in the eastern north sea off the danish west coast abstract two side scan sonar surveys were carried out 9 months apart in a 24 km2 off thorsminde at the danish west coast were water depth is 12 18 m sediment samples were systematically collected in the same area using a 250 m sampling grid the north sea study area is exposed to strong wind induced coastal currents and high waves and thereby characterized by significant sediment transport the seabed data were used to analyse the large scale dynamical bedforms found in the area the mobility of the bedforms the areal variation of the seabed mean grain size and the grain size variation across the large bedforms the compilation of side scan sonar mosaics revealed very large bedforms with a mean wavelength of approximately 500 m and heights of 1 3 m these bedforms indicate a north going net sediment transport caused by the jutland coastal current large dunes l 10 100 m and smaller bedforms were also recorded as well as large shoreface connected ridges with a spacing of 3 4 km the bedform configuration appears to be relatively stable only small changes having been observed between the two surveys even though a 100 yr cyclone and several storms occurred between the two surveys the small changes observed also indicate a net northerly sediment transport the surface sediments reveal a remarkably high positive correlation between the mean grain size and the position across the very large flow transverse bedforms from coarse sand in the troughs to fine sand at the crest and lee side slope of the bedforms

ice dams outburst floods and glacial incision at the western margin of the tibetan plateau a 100 k y chronology from the shyok valley karakoram some of the largest and most erosive floods on earth result from the failure of glacial dams while potentially cataclysmic ice dams are recognized to have repeatedly formed along ice sheet margins much less is known about the frequency and longevity of ice dams caused by mountain glaciers and their impact on landscape evolution here we present field observations and results from cosmogenic nuclide dating that allow reconstructing a 100 k y long history of glacial damming in the shyok valley eastern karakoram south asia our field observations provide evidence that asia s second longest glacier the siachen once extended for over 180 km and blocked the shyok river during the penultimate glacial period leading to upstream deposition of a more than 400 m thick fluvio lacustrine valley fill 10 be depth profile modeling indicates that glacial damming ended with the onset of the eemian interglacial and that the shyok river subsequently incised the valley fill at an average rate of 4 7 m k y 1 comparison with contemporary ice dammed lakes in the karakoram and elsewhere suggests recurring outburst floods during the aggradation period while over 25 cycles of fining upward lake deposits within the valley fill indicate impounding of floods from farther upstream despite prolonged damming the net effect of this and probably earlier damming episodes by the siachen glacier is dominated by glacial erosion in excess of fluvial incision as evidenced by a pronounced overdeepening that follows the glaciated valley reach strikingly similar overdeepened valleys at all major confluences of the shyok and indus rivers with karakoram tributaries indicate that glacial dams and subsequent outburst floods have been widespread and frequent in this region during the quaternary our study suggests that the interaction of karakoram glaciers with the shyok and indus rivers promoted valley incision and headward erosion into the western margin of the tibetan plateau

outburst floods in china a review abstract outburst floods can have disastrous impacts on people and are an important driving force in landscape change and have been studied widely on earth in china although outburst floods have occurred frequently there has been relatively little systematic investigation of the controlling factor here we review outburst floods in china in terms of the characteristics distribution causes of dams and outburst floods in terms of natural dams landslides accounted for the majority 287 cases followed by moraine dams 33 cases which are mainly found on and around the tibetan plateau and although other types such as glacier and volcanic dams were historically rare many examples may be preserved in the geological record in addition there have been thousands of outburst floods from artificial constructed dams the majority of which were from small earth dams the largest reliably recorded peak discharge for an outburst flood was 1 24 105 m3 s which occurred in yigong tibet the peak discharge of the 1975 banqiao collapse was 7 9 104 m3 s the largest outburst flood of a man made dam our recent investigations on the yarlung tsangpo in southeast tibet have identified gravel deposits that probably record megafloods and offer great potential for paleoflood analysis

flow and sediment transport induced by tide propagation 2 the wavy bottom case 1 a model is formulated to describe the flow field locally induced by the propagation of a tidal wave and to evaluate the sediment transport the three dimensional reynolds averaged momentum equations are considered and coriolis terms are retained since they affect the tide propagation turbulent reynolds stresses are described by means of the boussinesq assumption and the eddy viscosity is assumed to be an assigned function of the distance from the sea bed the effects of wind waves on sediment transport are taken into account and both the bed load and the suspended load are computed a comparison of the obtained results with laboratory data and field measurements shows that the model provides a reliable and accurate description of both the velocity field and sediment transport and does not require high computational costs

landslide erosion coupled to tectonics and river incision the steep topography of mountain landscapes arises from interactions between tectonic rock uplift valley incision and landslide erosion on hillslopes an analysis of more than 15 000 landslides in the eastern himalaya mapped from satellite images shows that steep uplands primarily respond to uplift and river incision by increases in landslide erosion rates rather than by steepened hillslope angles

sea ripple formation the heterogeneous sediment case abstract ripple formation beneath sea waves is analyzed both by experimental and analytical means when the bottom is made up of a mixture of sands an oscillatory flow is obtained in a closed duct by the oscillations of two rigidly connected pistons located at the ends of the duct the amplitude and period of the oscillations can be continuously varied a fixed tray located at the bottom of the duct and filled with different types of sediments allows ripple formation to be observed the presence of graded sediments is found to have a stabilizing effect and causes longer ripples to appear moreover a selective sediment transport is observed and quantified which tends to pile up the coarse grains at ripple crests leaving the fine ones in the troughs as in the companion paper the theory is based on a linear stability analysis of a flat sandy bottom subject to an oscillatory flow because of the presence of a mixture a modified version of exner equation is used and an hiding factor should be inserted in the sediment transport rate formula the flow regime in the bottom boundary layer is assumed to be turbulent the conditions for ripple appearance are determined along with their wavelengths as they form good agreement is found between experimental data and theoretical findings

lithospheric scale folding numerical modelling and application to the himalayan syntaxes we describe the eastern and western himalayan syntaxes which are large scale antiforms situated at geodynamically similar locations and the metamorphic evolution of which is coeval in the india asia collisional history to understand the mechanical plausibility of the structural interpretation we present two dimensional finite element modelling of lithospheric folding the models reveal the coeval development of adjacent synformal basins analogous to the peshawar and kashmir basins on both sides of the western syntaxis similarities between geological data and calculated models indicate that lithospheric buckling is a basic response to large scale continental shortening and an efficient mountain building process

rapid exhumation of the upper siang valley arunachal himalaya since the pliocene abstract the work presents in situ low temperature thermochronological apatite fission track aft and u th he zircon zhe ages from the siang valley in the ne himalayan syntaxial zone our results highlight the unique spatial erosion along the axial regions of the siang antiform all the zhe cooling ages from the region range between 14 and 9 ma except for a northernmost sample that is distinctly younger 3 8 ma results from aft data 2 4 ma to present record pleistocene cooling along the siang river valley in the siang antiform and reveal a general northward younging trend of ages with rapid cooling

post glacial entrenchment and knickpoint migration of the yarlung tsangpo gorge southeastern tibetan plateau abstract the formation of the yarlung tsangpo river system is commonly thought to be controlled by tectonic processes initiated several million years ago however climate change may also play an important role in guiding drainage evolution here we reconstruct the drainage history of the yarlung tsangpo gorge using field observations age constraints from optically stimulated luminescence and radiocarbon dating paleosalinity reconstruction and grain size analysis of lacustrine deposits and river valley characteristics field observations clay boron derived paleosalinity reconstruction and grain size analysis of lacustrine deposits demonstrate that a closed inland saline lake existed upstream of the yarlung tsangpo gorge implying that cutting of the gorge postdated this paleolake optically stimulated luminescence and radiocarbon dating suggest that the paleolake formed at least 51 ka and terminated after 13 ka with a shift in global climate following the last glacial maximum the paleolake drained by river capture and the gorge entrenchment commenced at the village of jiala northwest of the namche barwa peak approximately 13 10 ka ago the evolution of the yarlung tsangpo occurred on a remarkably short timescale of just tens of thousands of years in response to drainage system reorganization the yarlung tsangpo incised at an average rate of at least 26 mm yr the rapid fluvial incision initiated at jiala 13 ka ago propagated rapidly upstream at 2 12 m yr the yarlung tsangpo knickpoint has not stabilized but will likely continue to migrate upstream

periodic floods from glacial lake missoula into the sanpoil arm of glacial lake columbia northeastern washington at least 15 floods ascended the sanpoil arm of glacial lake columbia during a single glaciation varves between 14 of the flood beds indicate one backflooding every 35 to 55 yr this regularity suggests that the floods came from an ice dammed lake that was self dumping probably the self dumping lake was glacial lake missoula montana because the floods accord with inferred emptyings of that lake in frequency and number apparently entered lake columbia from the east and produced beds resembling backflood deposits of lake missoula floods in southern washington

tracing the distribution of erosion in the brahmaputra watershed from isotopic compositions of stream sediments bank sediments and suspended loads of the brahmaputra river and its important tributaries were collected from the himalayan front to bangladesh along with most of the important tributaries chemical and isotopic compositions of the sediments are used to trace sediment provenance and to understand erosion patterns in the basin overall isotopic compositions range from 0 7053 to 0 8250 for sr and nd from 20 5 to 6 9 this large range derives from the variable proportions of sediments from himalayan formations with high sr isotopic ratios and low nd and transhimalayan plutonic belt with lower sr isotopic ratios and higher nd the latter are exposed to erosion in the tsangpo and in the eastern tributary drainages overall erosion of the himalayan rocks is dominant representing ca 70 of the detrital influx compositions of the brahmaputra main channel are rather stable between 0 7177 and 0 7284 for sr and between 14 4 and 12 5 for nd throughout its course in the plain from the siang tsangpo at the foot of the himalayan range down to the delta this stability despite the input of large himalayan rivers suggests that the siang tsangpo river represents the major source of sediment to the whole brahmaputra geochemical budget implies that erosion of the namche barwa zone represents about 45 of the total flux at its outflow before confluence with the ganga from only 20 of the mountain area higher erosion rates in the eastern syntaxis compared to the other himalayan ranges is related to the rapid exhumation rates of this region possibly triggered by higher precipitation over the far eastern himalaya and the high incision potential of the tsangpo river due to its very high water discharge

tidal sand wave formation influence of graded suspended sediment transport 1 the influence of graded suspended sediment transport on the formation of tidal sand waves is investigated hereto in the framework of a linear stability analysis we examine the time development of small amplitude perturbations superimposed to a flat heterogeneous seabed in a shallow tidal sea dominated by one tidal constituent the heterogeneous bottom material is modeled by a bimodal sediment mixture and the transport of sediment close to the bottom as well as in suspension is considered the active layer concept is employed and hiding exposure effects are taken into account the mechanisms giving rise to the sediment sorting process along the bed forms are found to be unaffected by the fractional transport of sediment in suspension on the other hand the formation of sand waves is quantitatively substantially affected by the graded suspended sediment transport and the wavelength of the most amplified mode is found to depend strongly on the characteristics of the sediment mixture in particular the graded suspended sediment transport largely affects the contribution to the amplification rate related to the sorting process this leads to the growth of bed forms characterized by a smaller wavelength compared with that predicted for a uniform sediment when the mean grain size of the sediment mixture is relatively coarse on the other hand longer bed forms appear when the sediment mixture is characterized by a relatively fine mean grain size moreover a destabilizing effect is identified in the former case while the flat bed configuration is stabilized in the latter case a comparison of the theoretical predictions with field observations seems to support the model even though it cannot be considered conclusive

modelling the formation of shoreface connected sand ridges on storm dominated inner shelves a morphodynamic model is developed and analysed to gain fundamental understanding of the basic physical mechanisms responsible for the characteristics of shorefaceconnected sand ridges observed in some coastal seas these alongshore rhythmic bed forms have a horizontal lengthscale of order 5 km and are related to the mean current along the coast the seaward ends of their crests are shifted upstream with respect to where they are attached to the shoreface the model is based on the two dimensional shallow water equations and assumes that the sediment transport only takes place during storms the flux consists of a suspended load part and a bed load part and accounts for the eects of spatially non uniform wave stirring as well as for the preferred downslope movement of sediment the basic state of this model represents a steady longshore current driven by wind and a pressure gradient the dynamics of small perturbations to this state are controlled by a physical mechanism which is related to the transverse bottom slope this causes a seaward deflection of the current over the ridges and the loss of sediment carrying capacity of the flow into deeper water the orientation spacing and shape of the modelled ridges agree well with eld observations suspended load transport and spatially non uniform wave stirring are necessary in order to obtain correct e folding timescales and migration speeds the ridge growth is only due to suspended load transport whereas the migration is controlled by bed load transport

growth of the namche barwa syntaxis and associated evolution of the tsangpo gorge constraints from structural and thermochronological data abstract the eastern syntaxis of the himalaya namche barwa is dominated by a north plunging antiform which began to decompress grow at approximately 4 ma new fission track analyses on both apatite and zircon combined with previous geochronological ages indicate that the namche barwa dome also extended laterally while growing vertically zircon fission track ages range from 17 6 to 0 2 ma and have a strong relationship to the main faults of the region including the tertiary tsangpo suture with the younger ages inside the fault bounds towards the syntaxis core on the indian plate and the older ages away from the fault apatite ages reveal that the dome has grown laterally and now impinges over the older faulted margin onto the asian plate the dome is traversed by the tsangpo which has followed the trace of the suture for over 1300 km from its source to the entrance of the dome near dania as the tsangpo crosses the dome it departs from the suture but rejoins it some 60 km northeastwards we construe that the suture has been displaced by the growing antiform and as a consequence the antecedent river has been dragged in a left lateral sense along the exhuming north plunging dome restoring the suture to its position prior to 4 ma reveals a path of the tsangpo eastwards across the present southwestern position of the namche barwa indentation this geometric reconstrunction implies that the tsangpo and the brahmaputra were always one and the same river in addition the tsangpo was tectonically forced into juxtaposition with a tributary of the jiali parlung which it probably then captured the capture was due to tectonic forcing in the last 4 ma rather than headward retreat of the paleo brahmaputra as has been previously suggested

morpho sedimentary and stratigraphic characteristics of the 2000 yigong river landslide dam outburst flood deposits eastern tibetan plateau abstract sedimentary characteristics of landslide dam outburst flood deposits are distinct from those of rainfall flood deposits due to their very high hydraulic energy and sediment concentration while they have been studied well in the geological record investigations of modern outburst flood deposits are relatively rare well preserved flood deposits and landforms caused by the june 2000 landslide dam outburst flood in the yigong river eastern tibet provide an opportunity to evaluate such sedimentological response to a high energy modern event the sedimentary characteristics of boulder bars and channel evolution over c 17 km downstream of the breach were investigated by field survey and remote sensing interpretation the results show that the post flood channel migrated downstream and increased in curvature the bar shape is relatively stable downstream but the bar area shows a fluctuating decrease poorly sorted accumulations composed of massive granular gravels and boulders near the breach indicate sediment rich bedload depositing processes occurred during rapid rising stage flows in contrast there are eddy bars and deposition of fine gravel with horizontally bedded laminations 17 km downstream of the landslide suggesting suspended sediment dominated deposition with a rapid loss of transport capacity comparison with previous hydrodynamic simulations indicates that the rates of the rising stage and the falling stage strongly affect the sedimentary characteristics of boulder bars and slackwater deposits

u pb zircon constraints on the tectonic evolution of southeastern tibet namche barwa area the eastern syntaxis of the himalayas is expressed in the crust as a pronounced southward bend in the orogen the change in strike of geologic features coincides with the high topography of the namche barwa region the exposure of granulite grade metamorphic rocks and a 180 degree bend in the yalu tsangpo we have conducted a geochronologic and geochemical investigation of several suites of granitoids collected from the namche barwa massif and subjacent terranes of southeastern tibet ranging from cm scale dikes and sills to larger outcrop scale intrusions u pb shrimp rg zircon ages establish at least five magmatic episodes 400 to 500 ma 120 ma 40 to 70 ma 18 to 25 ma and 3 to 10 ma these episodes broadly correlate to spatial patterns in sample localities as follows 400 to 500 ma ages occur in zircon cores collected from within the massif proper 120 ma granites related to early gangdese arc plutonism are primarily located northeast of namche barwa later 40 70 ma gangdese activity is expressed in granites west of namche barwa 18 to 25 ma granites occur both along the suture zone west of gyala peri and directly north of namche barwa along the area of the jiali fault zone and are attributed both to shearing within the jiali fault zone and to an early miocene gangdese thrust event exceptionally young 10 ma zircon ages are clustered near the core of the massif along the yalu tsangpo gorge trace element geochemical data indicates the presence of both fluid present and fluid absent melts with a fluid absent decompression melting regime dominating near the core of namche barwa

evidence for holocene megafloods down the tsangpo river gorge southeastern tibet lacustrineandalluvialterracesandsedimentsrecordtheextentofatleasttwoholoceneglaciallydammedlakesimmediatelyupstreamofthe tsangpo rivergorge at the eastern syntaxis of the himalaya the larger lake covered 2835 km 2 with a maximum depth of 680 m and contained an estimated 832 km 3 of water the smaller lake contained an estimated 80 km 3 of water radiocarbon dating of wood and charcoal yielded conventional radiocarbon ages of 8860 f 40 and 9870 f 50 14 c yr b p for the higher set of lake terraces and 1220 f 40 and 1660 f 40 14 cy r b p forsedimentsfromthelowerterraces catastrophicfailureoftheglacialdamsthatimpoundedthelakeswouldhavereleasedoutburstfloods down the gorge of the tsangpo river with estimated peak discharges of up to 1 to 5 10 6 m 3 s 1 the erosive potential represented by the unit stream power calculated for the head of the gorge during such a catastrophic lake breakout indicates that post glacial megafloods down the tsangpo river were likely among the most erosive events in recent earth history d 2004 university of washington all rights reserved

brahmaputra sediment flux dominated by highly localized rapid erosion from the easternmost himalaya the brahmaputra river slices an exceptionally deep canyon through the eastern himalaya fission track and laser ablation u pb ages of detrital zircon grains from the river document very rapid erosion from this region and its impact on sediment fluxes downstream in the brahmaputra downstream from the canyon 47 of the detrital zircons in the river s modern sediment load comprise a fission track age population averaging only 0 6 ma equally young cooling ages are reported from bedrock in the canyon through the namche barwa gyala peri massif but are absent from riverbank sands of major tributaries upstream simple mixing models of u pb ages on detrital zircons from samples taken above and below this massif independently suggest that 45 of the downstream detrital zircons are derived from the basement gneisses extensively exposed in the massif constraints on the extent of the source area provided by bedrock cooling ages together with sediment flux estimates at pasighat india suggest exhumation rates averaging 7 21 mm yr 1 in an area of 3300 km 2 centered on the massif this rapid exhumation which is consistent with the very young cooling ages of the detrital zircons from this area produces so much sediment that 50 of the vast accumulation in the brahmaputra system at the front of the himalaya comes from only 2 of its drainage this extreme localization of rapid erosion sediment evacuation and bedrock cooling bear on 1 common assumptions in geodynamic and geochemical studies of the himalaya about sources of sediment and 2 plans for hydroelectric development and flood management in southeastern tibet and the heavily populated areas of eastern india

erosion of the tsangpo gorge by megafloods eastern himalaya at the southeastern margin of the tibetan plateau the yarlung tsangpo river plunges through the himalaya to drop 2 km through the tsangpo gorge upstream relict glacial dams and impounded lake terraces suggest that quaternary lakes as large as 800 km 3 catastrophically drained through the gorge as megafloods we report on new megaflood deposits downstream of the gorge and use detrital zircon u pb provenance data to demonstrate that these high magnitude events originated in tibet and more effectively focused erosion in the gorge than both the extremely erosive modern peak flows and one of the largest landslide dam outburst floods ever documented our findings support the proposition that in this steep narrow gorge where hillslope angles are near the threshold angle of bedrock failure megafloods provide a mechanism to rapidly evacuate hillslope material and focus erosion on channel adjacent hillslopes although megaflood frequency remains unconstrained we demonstrate the capability of these events to contribute substantially to rapid exhumation in this region

plio pleistocene exhumation of the eastern himalayan syntaxis and its domal pop up the eastern termination of the himalayan orogen forms a structural syntaxis that is characterised by young from 10 to 7 km in elevation in the core of the syntaxis very high erosion rates documented in sediment downstream of the gorge at the foot of the himalaya contribute 50 of total detritus to the sediment load of the brahmaputra the initiation of very high rates of exhumation has been attributed either to the extreme erosive power of a river flowing across a deforming indentor corner and the associated positive feedback or to the geometry of the indian plate indentor with the corner being thrust beneath the asian plate resulting in buckling which accommodates shortening both processes may be important the northern third of the syntaxis corresponds to a steep domal pop up structure bounded by the india asia suture on three sides and a thrust zone to the south within the dome greater himalaya rocks equilibrated at 800 c and 25 30 km depth during the miocene with these conditions potentially persisting into the latest miocene and possibly the pliocene with modest decompression prior to 4 ma this domal pop up corresponds to the area of youngest bedrock ages on a wide variety of thermochronometers and geochronometers in this paper we review the extensive scientific literature that has focused on the eastern syntaxis and provide new chronological data on its bedrock and erosion products to constrain the age of inception of the very rapid uplift and erosion we then discuss its cause with the ultimate aim to reconstruct the exhumation history of the syntaxis and discuss the tectonic context for its genesis we use zircon and rutile u pb white mica ar ar and zircon fission track dating methods to extract age data from bedrock brahmaputra modern sediments including an extensive compilation of modern detrital chronometry from the eastern himalaya and neogene palaeo brahmaputra deposits of the surma basin bangladesh numerical modelling of heat flow and erosion is also used to model the path of rocks from peak metamorphic conditions of 800 c to 250 c our new data include u pb bedrock rutile ages as young as 1 4 ma from the namche barwa massif and 0 4 ma from the river downstream of the syntaxis combined with existing data our new data and heat flow modelling show that i the detrital age signature of the modern syntaxis is unique within the eastern himalayan region ii the rocks within the domal pop up were 575 75 c only 1 2 myr ago iii the neogene surma basin does not record evidence of the rise and erosion of the domal pop up until latest pliocene pleistocene time iv pleistocene exhumation of the north easternmost part of the syntaxis took place at rates of at least 4 km myr with bedrock erosion of 12 21 km during the last 3 ma v the inception of rapid syntaxial exhumation may have started as early as 7 ma or as late as 3 ma and vi the yarlung tsangpo is antecedent and subsequently distorted by the developing antiform together our data and modelling demonstrate that the domal pop up with its exceptional erosion and topographic relief is likely a pleistocene feature that overprinted earlier structural and metamorphic events typical of himalayan evolution

sand petrology and focused erosion in collision orogens the brahmaputra case abstract the high relief and tectonically active himalayan range characterized by markedly varying climate but relatively homogeneous geology along strike is a unique natural laboratory in which to investigate several of the factors controlling the composition of orogenic sediments coupling of surface and tectonic processes is most evident in the eastern namche barwa syntaxis where the tsangpo siang brahmaputra river draining a large elevated area in south tibet plunges down the deepest gorge on earth here composition of river sands changes drastically from lithic to quartzofeldspathic after confluence with the lohit river draining the transhimalayan equivalent mishmi arc batholiths sediment composition remains remarkably constant across assam indicating subordinate contributions from himalayan tributaries independent calculations based on petrographical mineralogical and geochemical data indicate that the syntaxis representing only 4 of total basin area contributes 35 6 to the total brahmaputra sediment flux and 20 of total detritus reaching the bay of bengal such huge anomalies in erosion patterns have major effects on composition of orogenic sediments which are recorded as far as the bengal fan in the brahmaputra basin in spite of very fast erosion and detrital evacuation chemical weathering is not negligible sand sized carbonate grains are dissolved partially in mountain reaches and completely in monsoon drenched assam plains where clinopyroxenes are selectively altered plagioclase instead is preferentially weathered only in detritus from the shillong plateau which is markedly enriched in microcline most difficult to assess is the effect of hydraulic sorting in bangladesh where quartz garnet and epidote tend to be sequestered in the bedload and trapped on the coastal plain whereas cleavable feldspars and amphiboles are concentrated in the suspended load and eventually deposited in the deep sea high resolution petrographic and dense mineral studies of fluvial sands provide a basis for calculating sediment budgets for tracing patterns of erosion in mountain belts and for better understanding the complex dynamic feedback between surface processes and crustal scale tectonics

u pb zircon ages as a sediment mixing tracer in the nepal himalaya this paper presents a new approach to quantify sediment mixing based on the mixing of u pb zircon age distributions within sediment two statistical techniques are presented to determine the proportion in which two known age distributions combine to create a known mixed age distribution these techniques are then used to determine relative erosion rates between adjacent drainage basins above and below the main central thrust mct in the central nepal himalaya the mct region is coincident with an abrupt north south change in geomorphic character and mineral cooling ages that are thought to represent an erosional response to higher rock uplift rates north of the mct zone however it is unclear whether the ongoing deformation responsible for the differential uplift rates is 1 focused on the mct 2 at depth along a crustal scale ramp or 3 along newly mapped thrust faults south of the mct our study explores this issue by comparing modern erosion rates with longer term erosion rates determined from mineral cooling ages zircons were separated from modern river sand and dated by la mcicpms before the measured isotopic ratios and ages were used in 1 d and 2 d mixing calculations the 1 d technique creates probability density functions of zircon ages for each sample and then uses both an iterative and inverse approach to estimate mixing between samples in contrast the 2 d technique estimates mixing between probability bfieldsq defined by the measured 238 u 206 pb and 207 pb 206 pb ratios given a finite mixture with perfect sample representation both techniques produce perfect mixing estimates across a range of mixing proportions modeling results demonstrate that given imperfect subsample representation of the complex parent age distribution differing degrees of subsample smoothing may be required to achieve an accurate mixing estimate using mixing of zircon ages as a quantitative proxy for sediment mixing requires a correction for the concentration of zircon in the river sediment two new methods for establishing zircon concentration in river sediment are presented demonstrating the existence of 2 to 5 fold differences in zircon concentration between adjacent drainages relative erosion rates are estimated by determining the zircon mixing ratio between adjacent drainages which are then normalized by the ratio of zircon concentrations and the ratio of drainage areas results show 3 times higher modern erosion rates south of the

glacial advances and stability of the moraine dam on mount namcha barwa since the last glacial maximum eastern himalayan syntaxis abstract the tibetan plateau and indian plain exhibit the largest altitudinal difference between two geographically contiguous areas on earth but the eastern margin of the plateau has not experienced headward erosion of the yarlung zangbo river yzr since 2 5 ma in the past some scientists have argued that moraine dams on mount namcha barwa in the eastern himalayan syntaxis frequently blocked the yzr thus keeping the plateau s margin intact at this point in this study three sets of moraines were identified in the pai valley alongside namcha barwa and were dated using optically stimulated luminescence osl and 10be exposure dating the resultant ages would suggest that three glaciations occurred at 22 2 4 7 and 1 3 ka respectively comparison of osl and 10be ages for glacial advances with local paleoclimatic proxies would suggest that temperature was the dominant factor controlling glacial change because of the paleoglacial advance frequency coupling with rapid fluvial incision moraine dams were cut through by the yzr thus any moraine dams that lost their efficacy in impacting upon knickpoint migration instead similarities between rock uplift and fluvial incision rather than moraine dams might have been the main control impeding headward erosion of the yzr thereby keeping this part of the eastern tibetan plateau margin intact

environmental setting morphology and volumetric evolution of the middelkerke bank southern north sea abstract the middelkerke bank a tidal sandbank located in the southern north sea on the belgian continental platform has been the object of a multidisciplinary and international research project the morphological and hydrodynamical setting of the study area is presented a complete side scan sonar coverage of the bank allowed a detailed analysis of the morphology of the bank and of its superimposed bedforms large dunes cover the flanks and summit of the middelkerke bank they have a constant orientation a height ranging from 0 5 to 5 m a wavelength from 75 to 150 m and in most cases an asymmetrical profile the slopes of both lee and stoss flanks are very low small and medium dune fields occur on the entire bank and in the adjacent swales they are 2 d and present a wavelength ranging from 1 to 15 m flood and ebb oriented small dunes cover respectively the western and eastern flank of the bank bathymetric surveying of the middelkerke bank was carried out to assess the importance of volumetric variations of the bank mass caused by seasonal factors results show that changes in the bank volume can be quite important the bank s volume decreases in periods of heavy weather after which the process of sand uppiling acting during long periods of fair weather conditions causes the bank to restore itself volumetric variations of the total bank volume in a winter summer period can range along certain sections of the bank from 21 to 26 despite these large seasonal volumetric variations the bank shows a long term dynamic equilibrium as proven by the obtained evolution trend

late pleistocene sedimentary history of multiple glacially dammed lake episodes along the yarlung tsangpo river southeast tibet we present a reconstructed lithologic column compiled from a series of lacustrine outcrops along a tributary of the nyang river a major tributary of the yarlung tsangpo in southeast tibet the deposits were preserved between terraces at altitudes of 2950 3100 m asl the stratigraphic record features at least two sets of coarsening upward sequences depicting episodic aggradation and progradation of a glacially dammed lake related delta recognized facies changes illustrate the evolution cycles of depositional environments from pro delta delta front to delta plain radiocarbon and optically stimulated luminescence dates reveal an aging downward trend in stratigraphic order and provide an approximate timeline for the formation of glacially dammed lakes in late pleistocene this result reflects that the zelunglung glacier had progressively advanced to block the yarlung tsangpo river and the dam materials had stepwise stacked up to an altitude of 3095 m asl during marine oxygen isotope stages 4 to 2

provenance and erosional impact of quaternary megafloods through the yarlung tsangpo gorge from zircon u pb geochronology of flood deposits eastern himalaya abstract holocene and upper pleistocene sediments in the eastern himalayan syntaxis represent an extraordinary record of glacial outburst megafloods in one of the most tectonically active landscapes on earth glacial damming and outburst floods in the syntaxis may have focused erosion in the steep tsangpo gorge and inhibited river incision into the margin of the tibetan plateau however few flood slackwater deposits have been studied and it is unknown which of the hundreds of known glacial impoundments in tibet may have sourced the floods here we report n 1438 new detrital zircon u pb data from individual ancient megaflood and historical outburst flood slackwater deposits to examine the provenance and erosive potential of these events detrital zircon u pb geochronology of megaflood deposits show that megaflood provenance is more consistent with impoundment of the yarlung river drainage at or west of the namche barwa massif than impoundment of eastern drainages along the yigong and parlung rivers compared to active bedload in the siang river and historical floods within the same drainage area megaflood samples overall contain a disproportionately large amount of zircons eroded from the tsangpo gorge we interpret this preferential derivation to reflect preferential erosion during large discharge events supporting the hypothesis that quaternary megafloods are a primary contributor to rapid exhumation of the eastern himalayan syntaxis

exhumation and topographic evolution of the namche barwa syntaxis eastern himalaya abstract the namche barwa syntaxis as one of the most tectonically active regions remains an appropriate place to explore the relationship between tectonics surface processes and landscape evolution two leading models have been proposed for the formation and evolution of this syntaxis including the tectonic aneurysm model and the syntaxis expansion model here we use a multi disciplinary approach based on low temperature thermochronometry numerical modeling river profile and topographic analyses to investigate the interactions between tectonics erosion and landscape evolution and to test these models our results emphasize the presence of young cooling ages i e

coupling of rock uplift and river incision in the namche barwa gyala peri massif tibet geodynamic modeling demonstrates the strong potential for erosion to influence the pattern and style of deformation in active mountain belts but field studies yield conflicting views on the importance of erosion in influencing orogenesis here we compare patterns in river power inferred excess fluvial transport capacity topographic relief precipitation and mineral cooling ages to assess the coupling between surface erosion and rock uplift within the vicinity of the namche barwa gyala peri massif an active antiformal structure within the eastern himalayan syntaxis our rich and dense data set reveals a tight spatial correspondence of fluvial incision potential high relief and young cooling ages the spatial coincidence is most easily explained by a sustained balance between rock uplift and denudation driven by river incision over at least the last 1 m y the yarlung tsangpo brahmaputra river is the largest and most powerful river in the himalaya and two lines of evidence point to its active role in the dynamic interaction of local erosion rock uplift thermal weakening of the lithosphere and deformation 1 whereas along the rest of the himalayan front high relief and high rock uplift rates are essentially continuous the high relief and rapid exhumation in the syntaxis is restricted to a bull s eye pattern exactly where the largest river in the himalaya the yarlung tsangpo brahmaputra has the most energy per unit area available to erode its channel and transport sediment 2 the location of rapid incision on the yarlung tsangpo brahmaputra has been pinned for at least 1 m y and without compensatory uplift of the namche barwa gyala peri massif during this time the river would have eroded headward rapidly incising deeply into tibet

new insights into the mechanics of fluvial bedrock erosion through flume experiments and theory river incision into bedrock drives the topographic evolution of mountainous terrain and may link climate tectonics and topography over geologic time scales despite its importance the mechanics of bedrock erosion are not well understood because channel form river hydraulics sediment transport and erosion mechanics coevolve over relatively long time scales that prevent direct observations and because erosive events occur intermittently and are difficult and dangerous to measure herein we synthesize how flume experiments using erodible bedrock simulants are filling these knowledge gaps by effectively accelerating the pace of landscape evolution under reduced scale in the laboratory we also build on this work by providing new theory for rock resistance to abrasion thresholds for plucking by vertical entrainment sliding and toppling and by assessing bedrock analog materials bedrock erosion experiments in the last 15 years reveal that the efficiency of rock abrasion scales inversely with the square of rock tensile strength sediment supply has a dominant control over bed roughness and abrasion rates suspended sediment is an efficient agent of erosion and feedbacks with channel form and roughness strongly influence erosion rates erodibility comparisons across rock concrete ice and foam indicate that for a given tensile strength abrasion rates are insensitive to elasticity the few experiments that have been conducted on erosion by plucking highlight the importance of block protrusion height above the river bed and the dominance of block sliding and toppling at knickpoints these observations are consistent with new theory for the threshold shields stress to initiate plucking which also suggests that erosion rates in sliding and toppling dominated rivers are likely transport limited major knowledge gaps remain in the processes of erosion via plucking of bedrock blocks where joints are not river bed parallel waterfall erosion by toppling and plunge pool erosion feedbacks between weathering and physical erosion erosional bedforms and morphodynamic feedbacks between channel form and erosion rates despite scaling challenges flume experiments continue to provide much needed tests of existing bedrock erosion theory force development of new theory and yield insight into the mechanics of landscapes

on the formation of sand waves and sand banks a fully three dimensional model is proposed for the generation of tidal sand waves and sand banks from small bottom perturbations of a flat seabed subject to tidal currents the model predicts the conditions leading to the appearance of both tidal sand waves and sand banks and determines their main geometrical characteristics a finite wavelength of both sand waves and sand banks is found around the critical conditions thus opening the possibility of performing a weakly nonlinear stability analysis able to predict the equilibrium amplitude of the bottom forms as shown by previous works on the subject the sand wave crests turn out to be orthogonal to the direction of the main tidal current the present results also show that in the northern hemisphere sand bank crests are clockwise or counter clockwise rotated with respect to the main tidal current depending on the counter clockwise or clockwise rotation of the velocity vector induced by the tide only for unidirectional or quasi unidirectional tidal currents are sand banks always counter clockwise rotated the predictions of the model are supported by comparisons with field data finally the mechanisms leading to the appearance of sand waves and sand banks are discussed in the light of the model findings in particular it is shown that the growth of sand banks is not only induced by the depth averaged residual circulation which is present around the bedforms and is parallel to the crests of the bottom forms a steady drift of the sediment from the troughs towards the crests is also driven by the steady velocity component orthogonal to the crests which is present close to the bottom and can be quantified only by a three dimensional model while the former mechanism appears to trigger the formation of counter clockwise sand banks only the latter mechanism can give rise to both counter clockwise and clockwise rotated sand banks

evidence of episodically accelerated denudation on the tibetan namche barwa massif by megafloods abstract megafloods cause widespread mobilization of sediments that have strong potential to control the geodynamic evolution of the namche barwa nb massif by changing the landscape morphology within a short time however the impact of catastrophic events on the denudation of the massif over the longer time scale has not been explored in detail this contribution assesses the impact of catastrophic floods on the denudation style of the namche barwa nb massif over millennial time scales towards this 21 optical ages and 1700 detrital thermochronometric ages of zircon and apatite minerals extracted from paleoflood sediments downstream of the massif were integrated with decadal sediment load and aggradation data to estimate the minimum denudation rate during paleoflood events the results indicate that the 7 ka flood eroded 71 m of material from the nb massif this denudation is equivalent to the material that would be produced over 8000 years of erosion at a rate of 9 mm a the 3 5 ka flood eroded 24 m of material equaling 2500 years worth of erosion at a rate of 9 mm a such relatively instantaneous denudations are three orders of magnitude higher than averaged denudation rates over million year time scale this study therefore provides the first evidence of the strong interplay of catastrophic flood events and tectonics for the rapid exhumation of the namche barwa massif megafloods from the failure of dammed lakes provide valuable information to understand the flood surge risk for human settlements in the siang and the assam valleys

the namche barwa syntaxis evidence for exhumation related to compressional crustal folding abstract geological observation in the eastern end of the himalayas shows that the asia india suture is folded and continues southward into india and burma where the continuation of the transhimalayan plutonic complex can be identified metamorphic rocks derived from india occur structurally below the suture in the core of a regional antiform isotopic and fission track dating are consistent with the geological interpretation in addition they establish cooling exhumation of rocks from c 30 km depth within the last 4 ma rapid exhumation of deep crustal levels in mountain systems is generally related to extension subsequent to thermal softening of a thickened continental lithosphere however in the eastern himalayas we argue that exhumation is caused by c 10 mm yr 1 erosion coeval with crustal scale folding the general history of this syntaxis resembles the evolution of the western himalayas syntaxis in pakistan

sedimentary records of a palaeo lake in the middle yarlung tsangpo implications for terrace genesis and outburst flooding abstract the middle reaches region of the yarlung tsangpo in the southern tibetan plateau consist of broad valley reaches alternating with gorges which were easily dammed by glaciers and or glacial moraines during the last glacial period we investigated three palaeo lake sedimentary sequences on terraces located in the broad xigaze valley reach and the dazhuka yueju gorge in the middle reaches of the yarlung tsangpo we found that the altitude of the palaeo lake was 3811 m above sea level while its minimum area and volume were 679 km2 and 22 km3 respectively the palaeo lake was formed during the period from about 30 2 to 32 3 cal kyr bp and it failed at about 13 2 cal kyr bp the dam was possibly located at the eastern end of the dazhuka yueju gorge and may have been a glacier and or a moraine during the late part of the last glacial the failure of the dam and related flooding from the dazhuka yueju gorge probably triggered a chain reaction of dam failures downstream forming a megaflood terrace genesis and valley evolution in the investigated region can be divided into four phases phase 32 3 cal kyr bp river occupation phase

the fountain hills unique cb chondrite insights into thermal processes on the cb parent body we report the results of an extensive study of the fountain hills chondritic meteorite this meteorite is closely related to the cba class mineral compositions and o isotopic ratios are indistinguishable from other members of this group however many features of fountain hills are distinct from the other cb chondrites fountain hills contains 23 volume percent metal significantly lower than other members of this class in addition fountain hills contains porphyritic chondrules which are extremely rare in other cba chondrites fountain hills does not appear to have experienced the extensive shock seen in other cb chondrites the chondrule textures and lack of fine grained matrix suggests that fountain hills formed in a dust poor region of the early solar system by melting of solid precursors refractory siderophiles and lithophile elements are present in near ci abundances within a factor of two related to the enhancement of metal moderately volatile and highly volatile elements are significantly depleted in fountain hills the abundances of refractory siderophile trace elements in metal grains are consistent with condensation from a gas that is reduced relative to solar composition and at relatively high pressures 10 3 bars fountain hills experienced significant thermal metamorphism on its parent asteroid combining results from the chemical gradients in an isolated spinel grain with olivine spinel geothermometry suggests a peak temperature of metamorphism between 535 c and 878 c similar to type 4 ordinary chondrites

the 2000 yigong landslide tibetan plateau rockslide dammed lake and outburst flood review remote sensing analysis and process modelling abstract in april 2000 a large scale rock avalanche dammed the yigong zangpo river forming an extensive rockslide dammed lake the impoundment lasted for 62 days before a catastrophic breaching caused a massive outburst flood in the yarlung zangpo tibet and the dihang rivers india that travelled downstream to the main floodplain of the brahmaputra in northeastern india in response to discrepancies in the published literature on the event we present a review and re evaluation of the characteristics of the rock avalanche and associated landslide dammed lake we use digital topographical data srtm 3 and dynamic landslide modelling dan w and dan3d to determine the salient characteristics of the damming landslide and to characterise its behaviour our analysis indicates that the volume of the damming rockslide was ca 115 mm3 including 91 mm3 from the initial rockslope failure bulked during disaggregation to 109 mm3 and 6 mm3 from entrainment during its 10 1 km travel down zhamulong gully the debris travelled with an average velocity of 15 18 m s and resulted in a landslide dam on the yigong river with a minimum height of about 55 m using landsat 7 imagery obtained before during and after impoundment in conjunction with an srtm 3 dem we reproduced the filling of the lake we determine that the landslide dam formed an extensive reservoir with an impounded volume of 2 015 gm3 and a maximum possible lake level of 2264 m asl rounded to 2265 m asl our figures differ from those previously published but are believed to be well constrained verifiable estimates of the volumes of the 2000 yigong events the outburst occurred after an attempt by army personnel to manually dig a spillway over the landslide debris and resulted in the entire volume of the lake draining in about 12 h the outburst flood travelled over 500 km south into india with a recorded rise in river level of 5 5 m at the pasighat gauging station 462 km downstream in terms of historical outburst volumes from rockslide dammed lakes the volume of the 2000 yigong event is only exceeded by that of the 1841 outburst flood from the indus river rockslide dammed lake northern pakistan

tibetan plateau river incision inhibited by glacial stabilization of the tsangpo gorge much attention has focused on how and when the tibetan plateau formed but far less research has been done on the controls on river incision into the plateau itself oliver korup and david montgomery address this issue which also relates to the question of why the tibetan plateau is still there at all despite hosting one of the deepest gorges on earth the plateau edge is extremely well defined and according to widely accepted stream power theory should be heavily dissected by one of asia s most powerful rivers the yarlung tsangp or yarlung zangbo korup and montgomery propose that the southeastern edge of the plateau was preserved in part by the presence of glaciers that were large enough during the holocene to stall aggressive river downcutting along the plateau margin a considerable amount of research has focused on how and when the tibetan plateau formed in the wake of tectonic convergence between india and asia1 although far less enquiry has addressed the controls on river incision into the plateau itself2 widely accepted theory3 predicts that steep fluvial knick points river reaches with very steep gradients in the eastern himalayan syntaxis at the southeastern plateau margin should erode rapidly4 driving a wave of incision back into the plateau preservation of the plateau edge thus presents something of a conundrum that may be resolved by invoking either differential rock uplift matching erosional decay5 6 7 or other mechanisms for retarding bedrock river incision8 9 in this region where high stream power excludes the potential for aridity as a simple limit to dissection of the plateau10 here we report morphologic evidence showing that quaternary depression of the regional equilibrium line altitude where long term glacier mass gain equals mass loss was sufficient to repeatedly form moraine dams on major rivers such damming substantially impeded river incision into the southeastern edge of the tibetan plateau through the coupled effects of upstream impoundment and interglacial aggradation such glacial stabilization of the resulting highly focused river incision centred on the tsangpo gorge could further contribute to initiating and accentuating a locus of rapid exhumation known as tectonic anaeurysm6

landscape evolution of the eastern himalayan syntaxis based on basin hypsometry and modern crustal deformation abstract basin hypsometry is sensitive to tectonic uplift and erosion an area of intense tectonism the eastern himalayan syntaxis ehs is used to explore the characteristics of basin hypsometry and discuss the dominant variables controlling landscape evolution we selected 137 primary drainage basins and delimited their subordinate basins according to strahler order the hypsometric integrals and corresponding hypsometric curves of catchments are spatially independent of lithologic factors and basin scales but correlate strongly with indices of vertical dimension of drainage basins i e mean altitude and maximum altitude these observations also suggest different relationships between tectonic uplift and surface erosion around the namche barwa syntaxis nbs the area northwest of the nbs the nbs itself and the chayu deformation belt are in stages of sinking approximate steady state and rising respectively combined with gps data and the regional earthquake distribution we suggest that the present tectonic uplift of the ehs is concentrated in the nbs and the chayu deformation belt to the southeast of the nbs while the actual uplift of the earth s surface and significant horizontal crustal shortening is only concentrated within the chayu deformation belt the center of crustal deformation of the ehs therefore is inferred to have migrated southeastward from the nbs to the chayu deformation belt

age and extent of a giant glacial dammed lake at yarlung tsangpo gorge in the tibetan plateau many glacier dams on major rivers at the southeastern edge of the tibetan plateau had been previously determined through remote sensing and glacier terminal position calculation it was hypothesized that such damming substantially impeded river incision into the plateau interior investigation on the large glacial dammed lake at the entrance of tsangpo gorge is critical for understanding this hypothesis so far the issues such as age lake surface elevation and stages of this dammed lake are still in debate our field survey of lacustrine deposits and loess distribution along the middle yarlung tsangpo river and its tributary nyang river suggested that the lake surface elevation was at about 3180 m asl the 23 quartz optically stimulated luminescence osl and 4 organic ams 14 c ages all fall into the last glacial period 41 13 ka the osl and 14 c ages are in general agreement with each other where applicable there might be only one long damming event because the ages of lacustrine deposits from 2970 to 3100 m asl are similar and every lacustrine section is sustained for a long time the estimated lake surface area was 1089 km 2 and the volume was 170 km 3 which differ from previous estimations which suggested two stage about early holocene and 1 5 ka lakes and the largest lake surface elevation reached 3500 m

impact of glacial lake paleofloods on valley development since glacial termination ii a conundrum of hydrology and scale for the lowstand brahmaputra jamuna paleovalley system

paleofloods records in himalaya we use paleoflood deposits to reconstruct a record of past floods for the alaknanda mandakini rivers garhwal himalaya the indus river ladakh nw himalaya and the brahmaputra river ne himalaya the deposits are characterized by sand silt couplets massive sand beds and from debris flow sediment the chronology of paleoflood deposits established by optically stimulated luminescence osl and c 14 ams dating techniques indicates the following i the alaknanda mandakini rivers experienced large floods during the wet and warm medieval climate anomaly mca ii the indus river experienced at least 14 large floods during the holocene climatic optimum when flood discharges were likely an order of magnitude higher than those of modern floods and iii the brahmaputra river experienced a megaflood between 8 and 6 ka magnetic susceptibility of flood sediments indicates that 10 out of 14 floods on the indus river originated in the catchments draining the ladakh batholith indicating the potential role of glacial lake outbursts glofs and or landslide lake outbursts llofs in compounding flood magnitudes pollen recovered from debris flow deposits located in the headwaters of the mandakini river showed the presence of warmth loving trees and marshy taxa thereby corroborating the finding that floods occurred during relatively warm periods collectively our new data indicate that floods in the himalaya largely occur during warm and wet climatic phases further the evidence supports the notion that the indian summer monsoon front may have penetrated into the ladakh area during the holocene climatic optimum c 2016 elsevier b v all rights reserved

downstream evolution of the thermochronologic age signal in the brahmaputra catchment eastern himalaya implications for the detrital record of erosion the namche barwa massif in the eastern himalayan syntaxis is characterized by very rapid exhumation and provides a significant proportion of the sediment flux carried by the brahmaputra river we present new detrital zircon fission track zft and muscovite 40ar 39ar mar data from modern sediments of rivers draining the eastern himalaya the cooling age populations for both thermochronometers contain a characteristic 2 ma signature related to the rapid exhumation of namche barwa which can be traced hundreds of kilometers downstream from their source into the brahmaputra foreland despite dilution from downstream tributary catchments to estimate present day erosion in the catchments we apply a mixing model based on linear inversion of the binned age distributions the inversion predicts relative erosion rates in the syntaxial region that are an order of magnitude higher than those in upstream catchments and about twice as high as those in the southern himalayan catchments consistent to first order with previous estimates of erosion rate patterns in the eastern himalaya a comparison of the observed downstream evolution of age distributions with a sediment flux model suggests that the zft signal can be explained by dilution from himalayan tributaries only whereas the mar signal is also affected by selective sequestering possibly through winnowing of micas as they are transported downstream nevertheless thermochronological ages 2 ma provide a diagnostic signal of syntaxial exhumation in the sedimentary record of the eastern himalaya this study suggests the most robust signal to be recorded in the most proximal deposits with respect to the syntaxis

late quaternary sedimentary record and holocene channel avulsions of the jamuna and old brahmaputra river valleys in the upper bengal delta plain abstract the first holocene stratigraphic record of river channel occupation and switching between the brahmaputra jamuna and old brahmaputra paleovalleys is presented here motivated by the brahmaputra river s historic avulsion from the old brahmaputra channel to its present day jamuna course we have obtained sediment and radiocarbon samples from 41 boreholes along a 120 km transect crossing these two braided river valleys the stratigraphy along this transect reveals sand dominated holocene channel systems each bound by remnant mud capped pleistocene stratigraphy using sediment lithology and bulk strontium concentration as a provenance indicator we define the geometry and channel occupation history of each paleovalley the western brahmaputra jamuna valley is broad and somewhat deeper compared with the old brahmaputra valley the latter actually comprising a composite of two narrower sub valleys bifurcated by an antecedent topographic remnant the gently sloped valley margins slope 0 002 to 0 007 and high width to thickness ratio w t 1000 of the brahmaputra jamuna valley suggest that it was filled primarily through lateral channel migration and the reworking of braidbelt and overbank deposits conversely the two old brahmaputra sub valleys have comparatively steeper valley margins slope 0 007 to 0 022 and lower width to thickness ratios w t 125 and 250 indicating that these were filled primarily through vertical aggradation of channel sands we attribute this disparity in valley geometry and fill processes to the different occupation histories for each valley in this case the much larger brahmaputra jamuna valley represents the principal if not singular river course during the last lowstand of sea level with a prominent gravel lag underlying the valley in contrast the smaller old brahmaputra valleys do not appear to have been present or at least well developed at the last lowstand rather these courses were first occupied during the early holocene transgression and we infer that the river had been previously excluded from this region by the relatively higher elevation between the madhupur terrace and the shillong massif we also demonstrate that the brahmaputra river experienced 3 4 major avulsions during the holocene with considerably longer occupation times within the principal brahmaputra jamuna valley together these observations indicate that occupation history and antecedent topography have been important controls on river course mobility and avulsion behavior

the tectonics and paleo drainage of the easternmost himalaya arunachal pradesh india recorded in the siwalik rocks of the foreland basin the siwalik sedimentary rocks of the himalayan foreland basin preserve a record of himalayan orogenesis paleo drainage evolution and erosion this study focuses on the still poorly studied easternmost himalaya siwalik record located directly downstream of the namche barwa syntaxis we use luminescence palaeomagnetism magnetostratigraphy and apatite fission track dating to constrain the depositional ages of three siwalik sequences the sibo outcrop upper siwalik sediments at ca 200 800 ka the remi section middle and upper siwalik rocks at 0 8 8 8 2 4 ma and the siang section middle siwalik rocks at 9 3 1 5 to 13 5 1 5 ma cretaceous paleogene detrital zircon and apatite u pb ages characteristic of the transhimalayan gangdese batholiths that crop out northwest of the syntaxis are present throughout the sibo remi and siang successions confirming the existence of a yarlung brahmaputra connection since at least the late miocene a ca 500 ma zircon population increases up section most strikingly sometime between 3 6 to 6 6 ma at the expense of transhimalayan grains we consider the ca 500 ma population to be derived from the tethyan or greater himalaya and we interpret the up section increase to reflect progressive exhumation of the namche barwa syntaxis early cretaceous zircon and apatite u pb ages are rare in the sibo remi and siang successions but abundant in modern siang river sediments zircons of this age range are characteristic of the transhimalayan bomi chayu batholiths which crop out east of the syntaxis and are eroded by the parlung river a modern tributary of the siang river we interpret the difference in relative abundance of early cretaceous zircons between the modern and ancient sediments to reflect capture of the parlung by the siang after 800 ka