making geological sense of big data in sedimentary provenance analysis abstract sedimentary provenance studies increasingly apply multiple chemical mineralogical and isotopic proxies to many samples the resulting datasets are often so large containing thousands of numerical values and complex comprising multiple dimensions that it is warranted to use the internet era term big data to describe them this paper introduces multidimensional scaling mds generalised procrustes analysis gpa and individual differences scaling indscal a type of 3 way mds algorithm as simple yet powerful tools to extract geological insights from big data in a provenance context using a dataset from the namib sand sea as a test case we show how mds can be used to visualise the similarities and differences between 16 fluvial and aeolian sand samples for five different provenance proxies resulting in five different configurations these configurations can be fed into a gpa algorithm which translates rotates scales and reflects them to extract a consensus view for all the data considered together alternatively the five proxies can be jointly analysed by indscal which fits the data with not one but two sets of coordinates the group configuration which strongly resembles the graphical output produced by gpa and the source weights which can be used to attach geological meaning to the group configuration for the namib study the three methods paint a detailed and self consistent picture of a sediment routing system in which sand composition is determined by the combination of provenance and hydraulic sorting effects

detrital zircon u pb geochronology of the magog group southern quebec stratigraphic and tectonic implications for the quebec appalachians

distribution of erosion across bedrock channels lateral erosion in bedrock rivers is an important control on the shape of channel crosssections and the coupling of channels and hillslopes recent observations link lateral erosionto the variability of flow we propose two mechanisms to explain this one is based on changing shear stress distributions within the channel with varying flood level the other on the competition between cover and tool effects in fluvial bedrock erosion we assess these processes for the liwu river taiwan and conclude that cover and tool effects dominate the partitioning of lateral and vertical erosion in this case

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

brahmaputra river assam india physiography basin denudation and channel aggradation the brahmaputra river in assam india characterized by high seasonal variability in flow sediment transport and channel configuration experienced a secular period of aggradation from 1971 to 1979 the suspended load budget indicates an overall aggradation of the 607 km assam reach of the brahmaputra by about 16 cm during that period with about 70 of the suspended sediment inflow into the reach being retained in the channel expressed as a percentage of the change in storage for the different reaches computed errors due to sampling variability in sediment discharge generally lie within about 5 15 and do not appear to be large enough to affect the conclusions drawn from the suspended load budget for a 145 km reach of the brahmaputra an alternative method based on measurement of channel cross sections suggests 21 cm of aggradation somewhat more than estimated by the suspended load budget based on the suspended load carried by trans himalayan rivers the present rate of denudation of the eastern himalayas is estimated to be 73 157 cm 103 years the average rate for the last 2 3 million years estimated from the volume of alluvial fill in the brahmaputra valley in assam the sediment yield of the himalayan rivers and assuming a total yield to deposition ratio of 1 4 present study is 3 cm 103 years the current high rate of denudation of the himalayas may be attributed mainly to the rapid uplift of the mountain system recent earthquake activity and high susceptibility of geologic formations to erosion by running water coupled with the effectiveness of the monsoon rainfall regime

progressive incision of the channeled scablands by outburst floods the surfaces of earth and mars contain large bedrock canyons that were carved by catastrophic outburst floods reconstructing the magnitude of these canyon forming floods is essential for understanding the ways in which floods modify planetary surfaces the hydrology of early mars and abrupt changes in climate flood discharges are often estimated by assuming that the floods filled the canyons to their brims with water however an alternative hypothesis is that canyon morphology adjusts during incision such that bed shear stresses exceed the threshold for erosion by a small amount here we show that accounting for erosion thresholds during canyon incision results in near constant discharges that are five to ten fold smaller than full to the brim estimates for moses coulee a canyon in the channeled scablands which was carved during the pleistocene by the catastrophic missoula floods in eastern washington usa the predicted discharges are consistent with flow depth indicators from gravel bars within the canyon in contrast under the assumption that floods filled canyons to their brims a large and monotonic increase in flood discharge is predicted as the canyon was progressively incised which is at odds with the discharges expected for floods originating from glacial lake outbursts these findings suggest that flood carved landscapes in fractured rock might evolve to a threshold state for bedrock erosion thus implying much lower flood discharges than previously thought

equilibrium line altitudes of the last glacial maximum for the himalaya and tibet an assessment and evaluation of results abstract contemporary equilibrium line altitudes elas for glaciers in the high mountains of the himalaya and tibet have considerable variation because the region is influenced by two major climatic systems the mid latitude westerlies and the south asian summer monsoon and glaciers are subject to strong topographic controls reconstructions of past elas based on the former extent of glaciers are numerous but were estimated using a variety of methods some of which are not appropriate for high altitude glaciers which are strongly influenced by topography and have extensive supraglacial debris cover furthermore few of the reconstructions have adequate chronological control because of the difficulty of dating glacial landforms in high altitude regions where organic matter for radiocarbon dating is sparse or absent cosmogenic radionuclide surface exposure and optically stimulated luminescence dating are providing data to define the timing of glaciation and these together with objective analysis of glacier dynamics may allow better estimates of ela changes in the future however there are currently only two regions the hunza valley and the khumbu himal which are sufficiently well dated to allow reconstructions of last glacial maximum glacier extent and hence elas these two regions cannot provide an adequate assessment of ela changes for the himalayan tibetan region

osl dating of past lake levels for a large dammed lake in southern tibet and determination of possible controls on lake evolution fluvio lacustrine terraces along phung chu river on the central southern tibetan plateau indicate that a large palaeo dammed lake formerly existed in this area based on landscape survey optically stimulated luminescence osl dating and sedimentary analyses this research shows that the phung chu was blocked and a dammed lake over 2500 km in size formed before 30 ka ago osl dating analysis suggests the fluvio lacustrine sediments were well bleached and yield accurate age estimates for two lake drainage events the first drainage event took place after 30 ka resulted in river incision and formed a high terrace at 50 m height from the present river level the second drainage happened after 3 7 ka resulted in further river incision and formed the second terrace at 25 m height from the present river level according to the distribution of the fluvio lacustrine sediments active normal faults particularly the kharta fault in this region and the high gradient slopes after phung chu enters the yo ri gorge seismically induced landsliding is regarded as highly likely to have been the cause of river blockage and associated formation of a dammed lake although glacial damming is also a possible cause the volume of drainages from this dammed lake may have led to catastrophic flooding and analogous modern lakes represent significant geo hazard risks to down river human settlements as dammed lakes are special phases in fluvial evolution often involving river blockage breakthrough and drastic catchment change these processes can reveal how tectonic or climatic events modify landforms however such tectonic derived landform changes can also impact palaeo climate of the region thus this study has added new evidence regarding the evolutionary history of a dammed lake including its formation duration extent and final drainage which is crucial for understanding its general landscape process mechanisms and for better assessing geo hazard risks

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

earth science megafloods downsized a fresh look at the channeled scablands of north america shows that the ancient floods that scarred that landscape were smaller than is commonly assumed this result could revise estimates of similar floods on mars see letter p 229

tourmaline in a low grade clastic metasedimentary rock an example of the petrogenetic potential of tourmaline detrital tourmaline grains and their associated tourmaline overgrowths provide a means to unravel the provenance and petrogenetic history of low grade clastic metasedimentary rocks evidence derives from tourmaline grains found in a lithic wacke metamorphosed to chlorite zone conditions the detrital tourmaline cores are diagnostic indicators of the source rocks of the sediment whereas the overgrowths record both diagenetic and metamorphic reactions in the rock tourmaline grains consist of a detrital core surrounded by asymmetric overgrowths comprised of inner and outer rims abrupt chemical discontinuities between each of these zones implies that volume diffusion within tourmaline was minor under the conditions of formation compositions of the detrital cores vary widely yet can be correlated with source rock types that are consistent with lithic fragments recognizable in the metawacke at either the analogous or antilogous pole inner rim compositions proximal to the detrital cores converge despite the substrate tourmaline composition indicating an approach to chemical equilibrium however significant dufferences in al and x site vacancies at the expense of mg na and ti between the analogous and antilogous poles of the inner rims demonstrate the presence of significant amounts of compositional polarity outer rim compositions at either pole also converge but compositional polarity between the analogous and antilogous poles persists the presence of the inner and outer rims separated by a compositional discontinuity suggests punctuated evolution of the overgrowth this implies that boron was sporadically available during diagenesis and metamorphism based on boron contents of minerals this may correspond to a mechanism such as boron release due to polytypic change of illite or consumption of illite and or muscovite as such tourmaline growth stages may serve as a monitor of chemical reactions in low grade metamorphic rocks

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

disequilibrium partitioning of oxygen isotopes associated with sector zoning in quartz fractionation of oxygen isotopes during crystal growth is commonly assumed to be an equilibrium process we present evidence here that large variations in delta 18 o values in a zoned vein quartz crystal are the result of disequilibrium partitioning between fluid and crystal within a single crystal vith well developed concentric and sector zoning delta 18 o values determined with laser extraction techniques on milligram to submilligram sized samples range from 4 7 parts per thousand to 7 3 parts per thousand core to rim variations within single growth sectors fluctuate by 2 6 parts per thousand whereas differences between adjacent sectors along the same growth zone can exceed 2 0 parts per thousand in contrast fluid inclusion homogenization temperatures and salinities are relatively constant and show no effect of grow th or sector zoning differences in delta 18 o values between sectors along the same growth zone cannot result from variations in the oxygen isotope composition of the fluid and strongly suggest disequilibrium partitioning of oxygen isotopes between fluid and the growing crystal the cause of this disequilibrium is believed to be due to differences in surface structure and or growth mechanisms between nonequivalent faces

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

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

stress strain behaviour of a loosely compacted volcanic derived soil and its significance to rainfall induced fill slope failures the stress strain behaviour of a soil is crucial to the recognition of the mechanism of slope failure triaxial tests composed of isotropically consolidated drained icd and undrained compression icu tests and anisotropically consolidated constant deviatoric stress path cqd tests were carried out with the aim of investigating the stress strain behaviour of loosely compacted volcanic derived soils the fact that the critical states are the same for icu icd and cqd tests may show that the critical state is independent of the above three stress paths the critical state line as defined in critical state soil mechanics is obtained from the e log p and q p plots based on the results of the above tests the initial state of the consolidated specimens at initiation of failure may be classified as dilative or contractive in the light of the locations of the soil state relative to the critical state line for contractional soils the increased pore water pressure generated by rainfall infiltration leads to a contractive failure in a drained manner giving rise to high excess pore water pressure the excess pore water pressure caused by contraction cannot be dissipated instantly resulting in a decrease in the shear resistance of the soil the failure process is rapid the failed soil mass is prone to flow after failure under the action of gravity due to its high moisture content and inflow of surface runoff and rainwater for dilational soils the increased pore water pressure resulting from infiltration leads to dilation which reduces pore water pressure and thus increases the shear resistance of the soil however continued rainfall infiltration may be able to equilibrate the reduction in pore water pressure caused by dilation and therefore the dilation or displacement can continue in hong kong volcanic derived soil is characterized by high permeability both the high permeability of volcanic derived soil and a shallow failure surface make it possible for the reduction in pore pressure to equilibrate relatively quickly therefore the failure is also rapid at least for poorly compacted fill slopes c 1999 elsevier science b v all rights reserved

glacier and landslide feedbacks to topographic relief in the himalayan syntaxes despite longstanding research on the age and formation of the tibetan plateau the controls on the erosional decay of its margins remain controversial pronounced aridity and highly localized rock uplift have traditionally been viewed as limits to the dissection of the plateau by bedrock rivers recently however glacier dynamics and landsliding have been argued to retard headward fluvial erosion into the plateau interior by forming dams and protective alluvial fill here we report a conspicuous clustering of hundreds of natural dams along the indus and the tsangpo rivers where these cross the himalayan syntaxes the indus is riddled by hundreds of dams composed of debris from catastrophic rock avalanches forming the largest concentration of giant landslide dams known worldwide whereas the tsangpo seems devoid of comparable landslide dams in contrast glacial dams such as river blocking moraines in the headwaters of both rivers are limited to where isolated mountain ranges intersect the regional snowline we find that to first order high local topographic relief along both rivers corresponds to conspicuously different knickzones and differences in the type and potential longevity of these dams in both syntaxes glacier and landslide dams act as a negative feedback in response to fluvial dissection of the plateau margins natural damming protects bedrock from river incision and delays headward knickpoint migration thereby helping stabilize the southwestern and southeastern margins of the tibetan plateau in concert with the effects of upstream aridity and localized rock uplift

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

boron isotope geochemistry of metasedimentary rocks and tourmalines in a subduction zone metamorphic suite abstract in order to understand the behavior of boron b and its isotope fractionation during subduction zone metamorphism b contents and isotopic compositions together with major element compositions were determined for metasedimentary rocks and tourmalines from the sambagawa metamorphic belt central shikoku japan no systematic changes in whole rock b content and isotope composition of the metasediments were observed among the different metamorphic grades indicating the lack of a bulk fluid rock b isotope fractionation as a result of devolatilization both modal abundance and grain size of tourmaline increase with increasing metamorphic grade in contrast b contents in muscovite and chlorite decrease with increasing metamorphic grade these observations combined with mass balance calculations of b suggest the formation of tourmaline during progressive metamorphism from metamorphic fluids containing b mainly derived from muscovite and subordinately from chlorite without allowing significant net removal of b from the metasedimentary rocks tourmalines in the higher grade metasedimentary rocks have zonal structure of b isotope and major element composition with decreasing 11 b and increasing mg mg fe from the inner rim core to the outer rim the change of mg mg fe in the tourmalines with increasing grade is paralleled by similar variation in chlorite these observations suggest that the growing tourmalines record the progressive evolution of the b isotopic composition of the metamorphic fluid in the outermost rims preserving the isotope signature of peak metamorphic p t fluid conditions based on the above observations the 11 b of the tourmaline is thought to have been nearly identical to that of the metamorphic fluid resulting in the apparent b isotopic fractionation factor between metamorphic fluid and whole rock 11 b 10 b fluid 11 b 10 b whole rock which decreases from 1 007 0 003 to 1 001 0 003 from chlorite to biotite zone metamorphism such results together with the formation of tourmaline from and sequestering of b in metamorphic fluids may lead to less b isotopic fractionation as a result of subduction zone devolatilization than noted in suites containing less tourmaline this therefore makes it possible to transport b isotopic signatures which ultimately reflect earth s surface materials to the deep mantle perhaps resulting in mantle b isotope anomalies near convergent margins

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

hourglass sector zoning in metamorphic tourmaline and resultant major and trace element fractionation a new type of sector zoning with an hourglass shape has been identified in metamorphic tourmalines that formed under a wide variety of physical and chemical conditions the two sectors in the c direction are not equivalent due to asymmetry in the crystal structure of tourmaline along the c axis the c sector is characterized by low concentrations of ti ca mg and na although al is high and has a pale commonly blue or pale green color conversely the c sector is low in mg and al and high in ca fe and ti the latter two causing the dark brown color of this sector the a sector has intermediate characteristics and probably approximates a sector free tourmaline thin sectioning of these sector zoned tourmalines perpendicular to the c axis can produce three types of apparent radial zoning patterns blue green cores dark brown cores or no distinct cores these apparent cores will further vary in relative diameter depending on the sectioning level furthermore core boundaries can be straight or ragged depending on whether the relative growth speeds for the different faces was constant or variable these textures have been used to argue for a prograde or detrital origin of tourmaline cores however sector zoning is a more appealing explanation for most of these textures and can further explain the textural resemblance among metamorphic tourmalines from highly variable bulk rock composition metamorphic history and mineral paragenesis the sector zoning that is described here develops by preferential uptake of elements on the r growth plane resulting from a combined effect of differences in surface charge and morphology of this plane in the c and c directions this leads to the preferential incorporation of more positively charged elements in the c direction and a preference for a vacant x site in the c direction because the compositional differences among the sectors are pronounced in both major and trace elements and in the same order of magnitude as growth zoning variability the presence of sector zoning must be established and taken into account when making inferences from tourmaline chemistry

erosion patterns in the changjiang yangtze river catchment revealed by bulk sample versus single mineral provenance budgets abstract the changjiang the fourth longest river on earth and the largest in eurasia has a complex sediment routing system presently interrupted by the three gorges dam the world s largest hydroelectric engineering project to study sediment generation processes in the huge catchment and compare the different erosion patterns obtained by different methodological approaches high resolution petrographic and heavy mineral analyses were performed on sands from the trunk river and its major tributaries the frequency distributions of diverse groups of detrital amphiboles were also investigated rigorous statistical methods were used to define end members evaluate mineralogical variability assess similarities among samples and eventually calculate the relative contributions from each major tributary to the trunk river by forward end member modelling of integrated compositional data the litho quartzose sand with few heavy minerals generated in tibetan headwaters evolves downstream to feldspatho litho quartzose with medium rank metamorphic rock fragments and moderately rich amphibole epidote suites sand across the sichuan basin and as far as the three gorges dam is enriched in mafic volcanic clinopyroxene and carbonate grains eroded from permian basalts and paleozoic strata of the south china block the final yangtze tract is characterized by litho feldspatho quartzose sand with moderately poor amphibole dominated suites with epidote clinopyroxene and garnet the orogenic compositional signature acquired in the upper part of the basin is thus carried all the way to the chinese passive margin as observed also for the yellow river in the north even after long distance transport across wide continental areas detrital modes thus reveal the tectonic character of the source rather than the geodynamic environment of the sink quantitative provenance analysis indicates that left bank tributaries draining the longmen and qinlin mountains supply most of the sand reaching the east china sea contributions from tibetan headwaters are negligible as those from right bank tributaries in the middle lower part of the basin sediment budgets and erosion patterns inferred from our integrated petrographic and heavy mineral data set contrast with those based on detrital zircon geochronology but agree with stream profile analysis revealing close correspondence with slope steepness precipitation stream power tectonic activity and frequency of major earthquakes sediment budgets based on amphibole groups are poorly constrained we conclude that provenance budgets based on single mineral species are typically unrobust particularly because they suffer from inaccurate estimates of mineral concentration in the end member sources exploring in detail a millesimal part of the sediment typically 1 5000 in the case of zircon is a risky strategy that may return a distorted picture of reality

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

rapid formation of a modern bedrock canyon by a single flood event deep river canyons are thought to form slowly over geological time see for example ref 1 cut by moderate flows that reoccur every few years in contrast some of the most spectacular canyons on earth and mars were probably carved rapidly during ancient megaflood events quantification of the flood discharge duration and erosion mechanics that operated during such events is hampered because we lack modern analogues canyon lake gorge texas was carved in 2002 during a single catastrophic flood the event offers a rare opportunity to analyse canyon formation and test palaeo hydraulic reconstruction techniques under known topographic and hydraulic conditions here we use digital topographic models and visible near infrared aerial images from before and after the flood discharge measured during the event field measurements and sediment transport modelling to show that the flood moved metre sized boulders excavated 7 m of limestone and transformed a soil mantled valley into a bedrock canyon in just 3 days we find that canyon morphology is strongly dependent on rock type plucking of limestone blocks produced waterfalls inner channels and bedrock strath terraces whereas abrasion of cemented alluvium sculpted walls plunge pools and streamlined islands canyon formation was so rapid that erosion might have been limited by the ability of the flow to transport sediment we suggest that our results might improve hydraulic reconstructions of similar megafloods on earth and mars

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

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

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

scales of disequilibrium and rates of equilibration during metamorphism mounting evidence suggests that partial disequilibrium meaning disequilibrium for some elements but not for others may be a common but rarely detected phenomenon during metamorphic mineral growth even in ordinary prograde reactions that progress to completion detailed examination of compositional variations in garnet crystals over a range of metamorphic grade suggests distinctly different scales of equilibration for common elements with strong temperature dependence under lower greenschist facies conditions fe and mg may equilibrate at hand sample scale whereas mn and ca may not equilibrate even at millimeter scale although mn may achieve hand sample scale equilibration under upper greenschist facies conditions ca and many trivalent cations e g rees may not do so until temperatures exceed those of the middle amphibolite facies even in the lower granulite facies some elements e g y yb show indications of disequilibrium at sub centimeter scales during garnet growth analysis and numerical modeling of undisputed disequilibrium textures demonstrate that the most common impediment to equilibration during metamorphism is the sluggishness of intergranular diffusion the same mechanism known to govern porphyroblast crystallization in many metamorphic environments despite this importance to petrology few quantitative determinations exist of inter granular diffusion rates under metamorphic conditions using numerical simulations of coupled intergranular and intracrystalline diffusion processes in coronal textures around partially resorbed garnet crystals from the llano uplift u s a a very precise and relatively accurate estimate is obtained for the rate of intergranular diffusion of al in fluid undersaturated systems this result and an earlier estimate for fluid saturated systems provide bracketing values for al diffusivity during metamorphism

a super large landslide in tibet in 2000 background occurrence disaster and origin on april 9 2000 a super large landslide 3 x 10 8 m 3 occurred along zhamu creek southeast tibet as a result a landslide dam formed and the yigong river was blocked a channelized diversion was constructed for prevention of overtopping two months later on june 10 a catastrophic flood resulting from the failure of the landslide dam caused a serious loss of property this is the second of landslide occasion dam formation and flooding occurrence at the same site since 1900