the nature and distribution of upper cenozoic ignimbrite centres in the central andes abstract a wide variety of upper cenozoic ignimbrite centres have been identified and studied on landsat imagery of the central andes some have been investigated in the field lower miocene ignimbrite centres are distributed along an arc which in north chile diverges markedly from the line of the present trench and parallels an arc of lower miocene plutons in the eastern cordillera of bolivia much of the ignimbrite erupted since the late miocene has been erupted around the faulted margins of the altiplano basin of western bolivia particularly where the western and eastern cordilleras converge typical examples of ignimbrite centres are described some centres take the form of ignimbrite shield volcanoes other ignimbrite sheets are more widely dispersed the degree of dispersal probably relates to the height of the eruption column partial or total caldera formation sometimes took place and a few of these calderas have resurgent centres late stage lava extrusions are common and some of these are located on caldera ring fractures

some unique surface patterns on ignimbrites on earth a bird s eye view as a guide for planetary mappers abstract observations of terrestrial analogs are critical to aiding planetary mappers in interpreting surface lithologies on other planets for instance the presence of ignimbrites on mars has been debated for over three decades and is supported by analogy with deposits on earth critical evidence includes the geomorphic and surface expression of the deposits and those in the central andes of south america are amongst the most cited analogs herein we describe some prominent surface textures and patterns seen in ignimbrites on the scale of high resolution remotely sensed data 10 1 m per pixel these include pervasive joints and fractures that contribute to yardang form and development as well as prominent mounds fissures and fracture networks spiders bugs boxworks on ignimbrite surfaces while all these features are related to intrinsic cooling and degassing processes the involvement of external water buried by hot pyroclastic flows enhances fumarolic activity advective cooling and joint development observations of these geomorphic expressions using remote sensing are only possible with the highest resolution data and limited surface erosion for mars where similarly high resolution datasets are available for example the high resolution imaging sensor experiment or hirise extensive dust cover may limit the recognition of similar features there however significant relief on some of these features on earth indicate they might still be detectable on mars

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

iron rich metalliferous sediments on the east pacific rise prototype of undifferentiated metalliferous sediments on divergent plate boundaries abstract sediments in a zone on the east pacific rise with an especially high spreading rate were studied chemically mineralogically and microscopically they consist of a mixture of metalliferous sediments and plankton tests the metalliferous sediments were formed by an acidic hydrothermal leaching of tholeiitic basalt with seawater and subsequent precipitation in contact with cold near bottom seawater we assume the precipitation from hydrothermal solutions in this part of the east pacific rise to be undifferentiated due to the high spreading rate and the resulting rapid flow on the water through the basalt thus these metalliferous sediments are an initial stage type that have not undergone differentiation mn mo la cu v ni fe zn co and y all of which are leachable in acidic hydrothermal solutions are enriched in the metalliferous sediments in comparison to the tholeiitic basalts zr al and ti on the other hand which under the same conditions are not easily leached are reduced in their concentrations all components of the metalliferous sediments precipitated as hydroxides or as adsorbed ions on the hydroxides of other elements this is due to the oxidizing conditions in the near bottom seawater the sedimentation rate is high the almost 3 m long cores reach only to the late pleistocene the only distinctly observable diagenetic process for this period of time is the formation of goethite from amorphous iron oxides only for na k and rb does it seem possible that a distinct enrichment in the sediments by adsorption from the seawater could have taken place ca sr pb and perhaps sc are primarily bound to the planktonic carbonate part of the sediments

a vegetation history from the arid prepuna of northern chile 22 23 s over the last 13 500 years the quaternary paleoclimate of the central andes is poorly understood due to numerous discrepancies among the diverse proxy records that span this geographically and climatically complex region the exact timing duration and magnitude of wet and dry phases are seldom duplicated from one proxy type to another and there have been few opportunities to compare climatic records from the same proxy along environmental gradients vegetation histories from fossil rodent middens provide one such opportunity on the pacific slope of the andes we previously reported a vegetation history from the upper margin 2400 3000 m of the absolute desert in the central atacama desert of northern chile that record identified a distinct wet phase that peaked between 11 8 and 10 5 ka when steppe grasses and other upland elements expanded as much as 1000 m downslope and a secondary wet period during the middle to late holocene 7 1 3 5 ka the latter wet phase remains controversial and is not as readily apparent in our lowelevation midden record we thus sought to replicate both phases in a midden record from the mid elevations 3100 3300 m of the arid prepuna where slight precipitation increases would be amplified midden records from these elevations identify conditions wetter than today at 13 5 9 6 7 6 6 3 4 4 3 2 and possibly 1 8 1 2 ka dry phases occurred at 9 4 8 4 ka and possibly at ca 5 1 ka present floras and modern hyperarid conditions were established after 3 2 ka the records from the two elevational bands generally match with some important differences these differences could reflect both the discontinuous aspect of the midden record and the episodic nature of precipitation and plant establishment in this hyperarid desert 2003 elsevier science b v all rights reserved

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

yardang geometries in the qaidam basin and their controlling factors abstract the hyperarid qaidam basin features extensive fields of yardangs covering an area of 40 000km2 sculpted in tectonically folded sedimentary rocks we extracted the geometries of 16 749 yardangs such as length to width ratio l w spatial density and spacing from multi source remote sensing data provided by google earth we classified the yardangs into four types based on their l w short axis 1 2 whale back 2 6 hogsback 6 10 and long ridge 10 210 we interpreted the yardang geometries in the context of their geologic setting bedding orientation location along anticline crests or syncline troughs and lithologic heterogeneity our results show that the yardang geometries in the qaidam basin are mainly controlled by the structural geology and rheology of the sedimentary rocks e g strike and dip of bedding the presence or absence of interbedded soft and hard beds and structural position with folds the angle between geomorphically effective wind directions and the strike of bedding and the relative cumulative wind shear force where two geomorphically effective wind directions are present our analysis revealed the following 1 nearly 69 of the yardangs with long ridge and hogsback geometries are distributed in syncline areas whereas 73 of the yardangs with short axis geometries are distributed in anticline areas 2 the l w ratio of yardangs exposed along the windward limbs of anticlines is lower than that of yardangs exposed along the leeward limbs and 3 in the westernmost parts of the basin yardangs are locally sculpted into mounds by two geomorphically effective wind directions

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

geochronology and stratigraphy of the ignimbrites from the 21 30 s to 23 30 s portion of the central andes of northern chile the central volcanic zone cvz of the andes which extends from 14 to 28 s is one of the largest provinces of late tertiary to recent ignimbrite volcanism in the world in the 21 30 s to 23 30 s portion of the cvz in northern chile ignimbrite volcanism was initiated at the beginning of the late miocene 10 4 ma ago and continued until the recent the stratigraphy is dominated by the products of five major dacitic ignimbrite eruptions 100 km3 the artola ignimbrite member 9 4 ma the sifon ignimbrite member 8 3 ma the pelon ignimbrite member 5 5 ma the puripicar ignimbrite member 4 2 ma and the atana ignimbrite member 4 1 ma at least three of these represent only portions of larger ignimbrite sheets erupted from centres of sw bolivia two of the ignimbrites the sifon and atana ignimbrites must represent erupted volumes in excess of 1000 km3 each the major units which represent 90 of the total volume of ignimbrite in this area are intercalated with smaller 50 km3 less extensive volcanic units some of which were erupted from source structures within the study area the upper tertiary to recent volcanic stratigraphy has been revised using the large ignimbrites as the major stratigraphic horizons the ignimbrite formations can be divided into two groups on a geographic basis those in the northern and central portions of the study area are known as the san bartolo group while those in the southern portion belong to the silapeti group these two groups appear to be contemporaneous although the silapeti group may represent a period of ignimbrite volcanism which was initiated later about 6 ma ago the ignimbrite formation of sw bolivia and the punense arancaniano formation of nw argentina are equivalent to the san bartolo and silapeti groups based on correlation of the stratigraphy into adjacent areas in sw bolivia and nw argentina the beginning of the late miocene 10 4 ma marks the onset of the most intense phase ignimbrite volcanism in the cvz this phase of volcanism is suggested to be the result of crustal melting in response to crustal thickening during the quechua phase of deformation and subduction approximately 10 000 km3 of ignimbrite erupted over a period of 9 ma in the 21 30 23 30 s portion and led to the development of a major ignimbrite province

aeolian erosional lineations in the libyan desert dakhla region egypt abstract this paper presents results of a study of aeolian erosion at the landscape scale there have been few such studies in desert geomorphology compared to those focused on individual landforms and on rock surface sculpture the present study area lies in the southern part of the libyan desert in south central egypt between 25 and 27 n 29 and 30 e bedrock comprises paleogene limestone of various lithologies climate is hyperarid the following topics are treated i correspondences between rock outcrop belts and belts of terrain lineated by aeolian erosion and unlineated terrain ii the effect of lithology on aeolian lineation which works through the presence or absence of chert in the limestones lineation is produced by aeolian erosion in chert free poor rocks whereas erosion of chert rich rocks produces a desert pavement that armors the surface suppressing erosion iii the effect of large valleys eroded upwind of lineated terrain which deflect winds and trap sand ending erosion so that downwind lineations are reduced and finally erased by weathering iv sample lineated landscapes in the area show stages of evolution arranged in the time domain into a proposed cycle of aeolian erosion the cycle progresses from initial smooth plain to grooves separating long blade shaped ridges to segmentation of ridges into shorter blades along diagonal joints to streamlining of shorter blades and size reduction to final planation in any one wind parallel swath of lineated terrain stages in the cycle progress downwind in the space domain so that upwind landscapes are more advanced in the cycle v the structure of air flow responsible for lineation is still uncertain but initial grooving may respond to stable longitudinal horseshoe vortices or self organized regularity of erosion by random vortices subsequently the erosion pattern is fixed by evolving relief vi large smooth basins within the lineated terrain were carved by aeolian erosion before oxygen isotope stage ois 5 70 130 ka as were similar basins along the dakhla piedmont below the scarp little geomorphic change has occurred since so in this field of aeolian erosional lineations aels the aeolian erosion cycle may have occupied 10 times as long since continental scale aridity set in at ca 2 4 ma there have been only two cycles in this area a completed cycle earlier than the current incomplete one is indicated by rare small yardangs riding piggyback on larger ones

distinct erosional progressions in the medusae fossae formation mars indicate contrasting environmental conditions abstract the form of erosional remnants of the medusae fossae formation mff on mars provide evidence of their development progression and implicate two spatially distinct environments in the equatorial regions of mars ubiquitous yardangs are clearly the product of strong unidirectional winds acting over time on variably indurated deposits yardang orientation is used as a proxy to map regional and local wind direction at meso scale resolution in other more limited areas not subjected to strong unidirectional winds randomly oriented kilometer scale mesas and buttes are found to be remnants of progressive cliff recession through mass wasting as support is lost from within the mff lithology at its margins the differing processes that dominate the formation of the distinctive landforms have implications for meso scale variations in climate that remain unresolved by current modeling efforts additionally the variability of erosional forms within the deposit emphasizes the overall complexity of this extensive formation

palaeoclimatic implications of mio pliocene sedimentation in the high altitude intra arc lauca basin of northern chile abstract this paper examines the late miocene to pliocene sedimentary succession in the high altitude intra arc lauca basin of northern chile and discusses the palaeoclimatic evidence recorded within the basin fill sedimentological and geochronological data allow the reconstruction of four stages of the fluvial to lacustrine basin fill from the late miocene 6 4 ma to the late pleistocene we interpret the sedimentary evolution to reflect changes in precipitation during a time interval of general aridity climatic changes exerted the dominant control on sediment accumulation volcanism tectonism changes in morphology and sediment supply were factors of subordinate importance stage 1 comprises a clastic red bed sequence developed during the late miocene under relatively humid conditions an abrupt change in depositional style occurred around 6 4 ma when ephemeral saline lake sedimentation prevailed stage 2 this change in depositional style is considered to document the begin of a phase of desiccation that lasted until the early pliocene 3 7 ma and which was succeeded by less arid climatic conditions during the late pliocene and pleistocene stages 3 and 4 the transition between stages 1 and 2 and the subsequent dry period is assumed to coincide with a global cooling event during the latest miocene to early pliocene comparable sedimentary responses to this climatic change can be expected in adjacent basins of the central andes

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

distribution des terres rares dans les depots hydrothermaux de la zone famous et des galapagos comparaison avec les sediments metalliferes abstract hydrothermal deposits sensu stricto have been recovered during the famous cruise and leg 54 of the deep sea drilling project near the galapagos spreading centre the studied sediments mainly composed of clay material have very poor ree concentrations below about ten ppm the shale normalized patterns are characterized by a significant enrichment in heavy rare earths and show a negative ce anomaly the magnitude of this anomaly fluctuates but is generally lower than the seawater ce anomaly the geochemical characteristics of these hydrothermal deposits are in contrast with those of metalliferous sediments which are more enriched in trace elements especially in ree

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

investigating error structure of shuttle radar topography mission elevation data product 1 an attempt was made to experimentally assess the instrumental component of error of the c band srtm srtm this was achieved by comparing elevation data of 302 runways from airports all over the world with the shuttle radar topography mission data product srtm it was found that the rms of the instrumental error is about 1 55 m modeling of the remaining srtm error sources including terrain relief and pixel size shows that downsampling from 30 m to 90 m 1 to 3 arc sec pixels worsened srtm vertical accuracy threefold it is suspected that the proximity of large metallic objects is a source of large srtm errors the achieved error estimates allow a pixel based accuracy assessment of the srtm elevation data product to be constructed vegetation induced errors were not considered in this work

assessing global digital elevation models using the runway method the advanced spaceborne thermal emission and reflection radiometer versus the shuttle radar topography mission case the accuracy of global digital elevation models gdems or dems i e the advanced spaceborne thermal emission and reflection radiometer aster and the shuttle radar topography mission srtm has been reassessed using the runway method the runway method uses the vertical profiles of runways as reference data to which corresponding profiles extracted from investigated dems are compared the major advantage of the runway method is that only the source of instrument induced errors of dems is captured this ultimate effect is achieved due to the flatness and the homogeneous materials from which runways are constructed a cross comparison of the two dems has helped to highlight a few important facts both exhibit a negative elevation bias and the aster dem contains outliers that may be relatively easy to detect and correct particularly in flat terrain

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

amplification of bedrock canyon incision by wind water is considered the primary agent that erodes and shapes bedrock canyons analyses of canyon morphology in the central andes suggest that abrasion by wind can amplify canyon incision and reshape canyons on earth and possibly on mars

groundwater recharge through an alluvial fan in the atacama desert northern chile mechanisms magnitudes and causes the chacarilla fan in the atacama desert is one of several formed in the late miocene at the foot of the pre andean cordillera overlying the large complex pampa tamarugal aquifer contained in the continental clastic sediments of the fore arc basin the pampa tamarugal aquifer is a strategic source of water for northern chile but there is continuing doubt over the resource magnitude and recharge during january 2000 a 1 in 4 year storm in the andes delivered a 34 million m3 flash flood to the fan apex where c 70 percolated to the underlying aquifers groundwater recharge through the fan is calculated to be a minimum of 200 l s or 6 of the long term catchment rainfall these figures are supported by hydrochemical data that suggest that recharge may be 9 of long term rainfall isotopic data suggest groundwater less than 50 years old is transmitted westward through the permeable sheetflood sediments of the fan overlying the main aquifer analysis of this and other events shows that the hydrological system is non linear with positive feedback the magnitude of groundwater recharge is dependent on climatic variations antecedent soil moisture storage and changes in channel characteristics long term declines in groundwater level may partly result from climatic fluctuations and the causes of such fluctuations are discussed copyright 2002 john wiley sons ltd

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

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

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

paleo eolian dynamics in the andean plateau inferences from sedimentology of fossil and modern dunes fil lopez steinmetz romina lucrecia universidad nacional de jujuy instituto de ecorregiones andinas consejo nacional de investigaciones cientificas y tecnicas centro cientifico tecnologico conicet salta instituto de ecorregiones andinas argentina universidad nacional de jujuy argentina consejo nacional de investigaciones cientificas y tecnicas argentina

metal variations in divergent plate boundary sediments from the pacific abstract one hundred and three deep sea sediments have been recovered on traverses along the east pacific rise epr between 3 and 20 s and the galapagos rift system grs between 86 and 102 w analyses of 23 elements by instrumental neutron activation revealed that co zn as and sb covary with fe suggesting probable adsorption of these metals from seawater onto fe oxyhydroxides as was found to be significantly higher in epr sediments probably due to hydrothermal as input sb and zn are especially enriched in the sediments of the easternmost traverse of the grs near the hydrothermal mounds area chemical gradients of some metals with depth in the sediments were found in particular u enrichment occurs in sediments from the epr at 13 s and zn and sb enrichment with depth is observed in the sediments near the hydrothermal mounds area rare earth elements ree contents of sediments are low compared to pelagic clay but epr sediments have slightly higher ree than grs sediments trivalent ree correlate with fe and the same is observed for ce grs sediments have however higher ce contents suggesting probably an additional biogenic input for ce in equatorial pacific regions a number of epr and grs sediments have significant au 10 20 ppb and some of the sediments have relatively high ag up to 13 ppm

holocene yardangs in volcanic terrains in the southern andes argentina yardangs of different sizes were developed in the payun matru volcanic field a semiarid area east of the southern andes mountains yardangs from volcanic terrains have not been described previously from earth although studies from mars interpreted linear ridges found by mariner and viking images as yardangs the payun matru volcanic field is an extensive plateau at 2000 m a s l covered by basaltic lava and ignimbrite flows strong westerly winds affect the extensive plateau micro and mesoyardangs are formed on the ignimbrite rock blanket and macroforms or megayardangs several kilometres in length are developed in the basaltic lava flows as long parallel troughs they all have a distinctive 320 azimuth which is the prevailing wind direction no yardang features are noted in the more recent lava flows younger than 1000 years indicating that their formation needed a longer time or they developed in earlier periods with stronger winds the yardang development is explained by the strong unidirectional winds the poor vegetational cover due to the aridity of the region the available quartz sand and volcanic ash particles as abrasive agents and the volcanic lithology texture and flow structure copyright 2001 john wiley sons ltd

channel bed mobility response to extreme sediment loading at mount pinatubo since the 1991 eruption of mount pinatubo the specific sediment yields from watersheds draining its slopes have been the highest ever recorded in spite of this overwhelming sediment load rivers inundated by pyroclastic flow deposits delivered almost half of the initial deposits to downslope alluvial and or debris fans by 1996 although most of this transport occurred by hyperconcentrated flows and debris flows lahars very high sediment transport rates also characterize low flow measured flow velocities and depths channel properties and the size of both mobile and stable clasts in the sacobia and pasig potrero rivers indicate median bed surface grain sizes of 2 to 8 mm grain size independent dimensionless critical shear stresses c of 0 016 to 0 041 and manning s n values of 0 017 to 0 024 values well below those previously reported for steep mountain channels the dramatic bed surface fining bed mobility and selective transport in these extremely sediment rich channels indicate that changes in grain size c values and bed roughness can increase transport capacity in response to high sediment supply our observations also suggest viewing the apparently contradictory concepts of an equal threshold of bed mobility and grain size dependent selective transport as end member concepts that apply to channels with low or intermittent and high or continuous sediment supply respectively

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

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

deriving high resolution reservoir bathymetry from icesat 2 prototype photon counting lidar and landsat imagery knowledge of reservoir bathymetry is essential for many studies on terrestrial hydrological and biogeochemical processes however there are currently no cost effective approaches to derive reservoir bathymetry at the global scale this study explores the potential of generating high resolution global bathymetry using elevation data collected by the 532 nm advanced topographic laser altimeter system atlas onboard the ice cloud and land elevation satellite icesat 2 the novel algorithm was developed and tested using the icesat 2 airborne prototype the multiple altimeter beam experimental lidar mabel with landsat based water classifications from 1982 to 2017 mabel photon elevations were paired with landsat water occurrence percentiles to establish the elevation area e a relationship which in turn was applied to the percentile image to obtain partial bathymetry over the historic dynamic range of reservoir area the bathymetry for the central area was projected to achieve the full bathymetry the bathymetry image was then embedded onto the digital elevation model dem results were validated over lake mead against survey data results over four transects show coefficient of determination r 2 values from 0 82 to 0 99 and root mean square error rmse values from 1 18 to 2 36 m in addition the e a and elevation storage e s curves have rmses of 1 56 m and 0 08 km3 respectively over the entire dynamic reservoir area the derived bathymetry agrees very well with independent survey data except for within the highest and lowest percentile bands with abundant overpassing tracks and high spatial resolution the newly launched icesat 2 should enable the derivation of bathymetry over an unprecedented number of reservoirs

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

albedo changes milankovitch forcing and late quaternary climate changes in the central andes late quaternary humidity changes resulted in substantial modifications of the land surface characteristics in the altiplano of the atacama desert central andes reconstructions of surface albedo top of atmosphere toa albedo and short wave net radiation in the andes of northern chile for 20 14 10 7 and 0 ka suggest that surface and toa albedo increased substantially during periods of relatively humid environmental conditions i e with large palaeolakes glaciers and dense vegetation the decrease of summer shortwave net radiation and seasonality during the late glacial early holocene humid phase 14 to 10 ka due to earth s surface and atmospheric characteristics added to the effect of orbitally driven negative deviations of southern hemisphere austral summer insolation and minimum seasonality at 20 s therefore in situ radiative forcing is in contrast to the northern hemisphere tropics not a suitable explanation for enhanced convective precipitation and ultimately humid climatic conditions our results suggest that late quaternary humidity changes on the altiplano reflect a collective response to 1 environmental changes in the source area of the moisture e g re expansion of the rain forest and increased release of latent heat over amazonia and the chaco warm sea surface temperatures in the e pacific and 2 large scale circulation patterns and wave structures in the upper troposphere strength and position of the bolivian high divergent flow stimulating convection over the altiplano or that they even reflect a response to 3 interhemispherical teleconnections

post depositional fracturing and subsidence of pumice flow deposits lascar volcano chile unconsolidated pyroclastic flow deposits of the 1993 eruption of lascar volcano chile have with time become increasingly dissected by a network of deeply penetrating fractures the fracture network comprises orthogonal sets of decimeter wide linear voids that form a pseudo polygonal grid visible on the deposit surface in this work we combine shallow surface geophysical imaging tools with remote sensing observations and direct field measurements of the deposit to investigate these fractures and their underlying causal mechanisms based on ground penetrating radar images the fractures are observed to have propagated to depths of up to 10 m in addition orbiting radar interferometry shows that deposit subsidence of up to 1 cm year 1 occurred between 1993 and 1996 with continued subsidence occurring at a slower rate thereafter in situ measurements show that 1 m below the surface the 1993 deposits remain 5 c to 15 c hotter 18 years after emplacement than adjacent deposits based on the observed subsidence as well as estimated cooling rates the fractures are inferred to be the combined result of deaeration thermal contraction and sedimentary compaction in the months to years following deposition significant environmental factors including regional earthquakes in 1995 and 2007 accelerated settling at punctuated moments in time the spatially variable fracture pattern relates to surface slope and lithofacies variations as well as substrate lithology similar fractures have been reported in other ignimbrites but are generally exposed only in cross section and are often attributed to formation by external forces here we suggest that such interpretations should be invoked with caution and deformation including post emplacement subsidence and fracturing of loosely packed ash rich deposits in the months to years post emplacement is a process inherent in the settling of pyroclastic material

geomorphologic environment and age of supergene enrichment of the cuajone quellaveco and toquepala porphyry copper deposits southeastern peru copper ore grades of the cuajone quellaveco and toquepala porphyry cu mo deposits situated at 3 000 to 4 000 m a s l on the pacific slope of the cordillera occidental of southernmost peru lats 17 degrees 02 17 degrees 15 s have been markedly increased by supergene sulfide enrichment following the emplacement of the hypogene mineralization in the early eocene 52 57 ma as the terminal stage in the development of the toquepala group continental volcano plutonic terrane this andean transect was gradually reduced by erosion in a semiarid climate to a low altitude topography unroofing of the deposits had taken place by the mid oligocene the initiation of cordilleran uplift at ca 25 to 26 ma accompanied by the episodic eruption of felsic ash flow tuffs at the oceanward front of the volcanic arc led to more rapid erosion and the conversion of the mid tertiary landscape into the subplanar altos de camilaca surface this regionally extensive pediplain constitutes the major landform component of the present precordillera surrounding the porphyry centers its final configuration was attained at ca 18 to 19 ma ash flow tuff eruption was widespread and frequent at this time supergene enrichment began in the late oligocene during the progressive lowering of topography and continued through the more abrupt water table depression resulting from the latest oligocene to early miocene uplift however the distribution of chalcocite sensu lato in the three porphyry deposits and the local postmineralization landforms and volcanic histories differed significantly during the latter interval at toquepala and to a lesser extent quellaveco the landform regimes were dominated by open valleys and ignimbrite blanketing was short lived whereas the exposed cuajone deposit was both the site of aggressive early miocene valley incision and the deposition of an unusually thick in part welded ash flow tuff at 22 8 or 0 7 ma as a result the enrichment blanket at cuajone remained relatively thin and was even partially eroded in the early miocene whereas the blankets at quellaveco and toquepala were thickened continued strong uplift in the mid miocene ca 8 15 ma generated the array of apron and terrace pediments of the multiple pediment stage which dominates the lower cordilleran slopes and in the mineralized areas led to the deepening of existing valleys and the development of new fluvial channels ignimbrite eruption persisted throughout the miocene again local regimes of erosion and volcanism proved inimical to supergene activity at cuajone but enrichment continued at quellaveco and particularly toquepala where there is no record of later miocene ash flow accumulation uplift in this period was apparently also most extensive in the vicinity of the toquepala deposit which experienced the formation of a deep chalcocite blanket while the extant enrichment zone at quellaveco was thickened the development of enriched assemblages containing massive chalcocite had clearly terminated by 13 1 or 0 4 ma at cuajone and by 9 5 or 0 5 ma at quellaveco the timing of later enrichment at toquepala is less constrained but a mid miocene age is probable supergene upgrading of the three peruvian deposits was contemporaneous with that of copper mineralization in northernmost and northern chile as at chuquicamata and la escondida and in the copiapo mining district in all of which areas a late oligocene to mid miocene age has been inferred for supergene alteration this major and regionally developed metallogenic episode took place during and between two major periods of cordilleran uplift under semiarid climatic conditions the termination of intense enrichment along this 2 000 km stretch of the cordillera occidental in the late miocene was a direct result of marked climatic desiccation which not only reduced the overall supergene activity but focused fluvial erosion leading to the incision of steep walled canyons less favorable for supergene alteration than the earlier subplanar landforms

la pacana caldera and the atana ignimbrite a major ash flow and resurgent caldera complex in the andes of northern chile the recently discovered la pacana caldera 60 35 km is the largest caldera yet described in south america this resurgent caldera of pliocene age developed in a continental platemargin environment in a major province of ignimbrite volcanism in the central andes of northern chile at about 23 s latitude collapse of la pacana caldera was initiated by the eruption of about 900 km3 of the rhyodacitic atana ignimbrite the atana ignimbrite was erupted from a composite ring fracture system and formed at least four major ash flow tuff units that are separated locally by thin air fall and surge deposits all four sheets were emplaced in rapid succession about 4 1 0 4 ma ago caldera collapse was followed closely by resurgent doming of the caldera floor accompanied by early postcaldera eruptions of dacitic to rhyolitic lava domes along the ring fractures the resurgent dome is an elongated asymmetrical uplift 48 5 12 km which is broken by a complex system of normal faults locally forming a narrow discontinuous apical graben later postcaldera eruptions produced large andesitic and dacitic stratocones along the caldera margins and dacitic domes on the resurgent dome beginning about 3 5 ma ago and persisting into the quaternary hydrothermally altered rocks occur in the eroded cores of precaldera and postcaldera stratovolcanoes and along fractures in the resurgent dome but no ore deposits are known a few warm springs located in salars within the caldera moat appear to be vestiges of the caldera geothermal system

discrimination of fluvial and eolian features on large ignimbrite sheets around la pacana caldera chile using landsat and srtm derived dem abstract satellite images and ground based observations were used to investigate the morphology and origin of valleys and ridges across large ignimbrite provinces around la pacana caldera n chile we studied 12 separate sites that included five different ignimbrite units that range in age from 5 6 0 5 ma to 1 35 0 15 ma over 400 features from within these study sites were quantified study sites typically have dimensions of 25 35 km measurements were made using landsat tm etm and srtm derived dem and shaded relief images morphological measurements were made of the ridges and channels including the direction of regional slope length orientation and gradient of each feature a classification of the different ignimbrite erosion influences resulting in varying landscape morphologies identified 4 landscape types fluvial eolian modified and complex we conclude that the most prevalent features are fluvial derived channels but the most distinctive are linear wind derived ridges the orientation of these ridges suggests that strong uni directional winds from the northwest have been constant over a long period of time 1 my

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

origin of the medusae fossae formation mars insights from a synoptic approach 1 the geologic origin of the medusae fossae formation mff has remained a mystery despite three decades of research to better constrain its formation an in depth analysis of observations made in the literature was combined with a new survey of over 700 mars orbiter camera narrow angle images of the mff to identify morphologic characteristics and material properties that define this formation as a whole while previous work has identified clear agreement on some characteristics our analysis identifies yardangs collapse features and layering as pervasive features of the mff whereas collapse features and layering may implicate several different physical and chemical processes yardangs provide vital information on material properties that inform about mechanical properties of the mff lithology aspect ratios of megayardangs range from 3 1 to 50 1 and slope analyses reveal heights of up to 200 m with cliffs that are almost vertical other yardangs show lower aspect ratios and topographic profiles these characteristics coupled to the presence of serrated margins suggest that mff lithology must be of weakly to heavily indurated material that lends itself to jointing the characteristics and properties of the mff are inconsistent with those of terrestrial pyroclastic fall deposits or loess but are in common with large terrestrial ignimbrites a hypothesis that explains all key observations with a single mechanism yardang fields developed in regionally extensive ignimbrite sheets in the central andes display morphologic characteristics that correlate with degree of induration of the host lithology and suggest an origin by pyroclastic flow for the mff

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

age of supergene oxidation and enrichment in the chilean porphyry copper province twenty five samples of supergene alunite collected from deeply developed supergene profiles in porphyry copper deposits and prospects between latitudes 20 degrees and 27 degrees s in northern chile yield k ar ages ranging from about 34 to 14 ma therefore supergene oxidation and enrichment processes were active from the early oligocene to the middle miocene a minimum of 20 m y supergene activity at individual deposits lasted for at least 0 4 to 6 2 m y the early oligocene supergene activity affected deposits in the paleocene porphyry copper belt whereas early and middle miocene supergene processes are documented in the early cretaceous paleocene and late eocene to early oligocene porphyry copper belts middle miocene oxidation also affected the oldest epithermal gold silver deposits in the maricunga belt farther east supergene activity commenced no less than 11 m y after generation of each porphyry copper deposit because of the time required to unroof the copper bearing parts of the system supergene activity throughout northern chile ceased at approximately 14 ma the geologic features of deposits and prospects and their morphotectonic positions present latitudes and present elevations display no obvious correlations with the supergene chronology exploration for major cumulative enrichment blankets should not be carried out either beneath thick sequences of piedmont gravels or ignimbrites of oligocene through middle miocene age unless their accumulation is demonstrably late in the documented history of supergene activity or in porphyry copper provinces such as those of central chile and northwestern argentina which formed after approximately 14 ma the uplift responsible for efficient cumulative copper enrichment is difficult to correlate convincingly with the brief pulses of compressive tectonism postulated for northern chile and contiguous areas unless their effects were much more prolonged intensifying aridity is confirmed as the likely reason for the cessation of supergene activity in northern chile and tectonic uplift was its most probable cause however more fundamental global controls producing a period of chemical weathering followed by worldwide dessication also may have played a role

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

late pliocene age for the atacama desert implications for the desertification of western south america the atacama desert forms one of the major hyperarid deserts of the world previous studies suggest that desertification commenced at 14 ma during global climate desiccation sedimentologic data from middle miocene to upper pliocene successions in the modern atacama desert indicate that a semiarid climate persisted from 8 to 3 ma punctuated by a phase of increased aridity at ca 6 ma as such hyperaridity did not commence until the late pliocene implications are 1 that the rain shadow generated by the andean cordillera has had a minor influence on climate change and 2 that the upwelling north flowing cold humboldt current although important in establishing the generally arid climate of western south america did not control the shift to hyperaridity the formation of the hyperarid atacama desert in the late pliocene accompanied the development of the current phase of aridification in the sahara and namib deserts and is attributed to a phase of global climate cooling

late quaternary paleohydrology of the central atacama desert lat 22 24 s chile in northern chile precipitation in the high andes 3500 m recharges groundwater systems that flow down the pacific slope and feed large aquifers in the hyperarid atacama desert wetlands which are often found along the base of the andes mark locations where the water table intersects the land surface at these locations paleo wetland deposits which are present as terraces between 3 and 20 m above modern wetlands record past water table heights along the andean front and are used to reconstruct changes in groundwater discharge paleo wetland deposits in the central atacama desert lat 22 24 s record an episode 15 4 9 ka of high water tables followed by an episode 8 3 ka of moderately high water tables elevated water tables result from increased groundwater discharge and ultimately from enhanced recharge in the andes the concordance of results from three separate hydrologic systems suggests that changes in groundwater discharge and recharge are regional and reflect climatic fluctuations this interpretation is supported by close agreement with other paleoclimatic records in the region the periods of greater groundwater discharge were separated by episodes 9 8 and 3 0 ka of significant groundwater lowering and stream incision implying greatly diminished discharge the central atacama and andes lat 22 24 s receive precipitation mainly from moist air masses transported from the amazon basin by the south american summer monsoon sasm increases in groundwater recharge are therefore thought to reflect an increase in the frequency and or moisture content of sasm air masses crossing the andes fluctuations in sasm precipitation have previously been linked to summer insolation in the southern hemisphere the wettest period in the central atacama 15 4 9 ka however coincides with a minimum in austral summer insolation at 10 ka suggesting that regional summer insolation is not a dominant influence on the sasm instead intensification of the sasm may be linked to extraregional forcings such as the walker circulation

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

middle miocene climatic change in the atacama desert northern chile evidence from supergene mineralization at la escondida geochronology and paleotopographic reconstruction of the porphyry copper deposit at la escondida chile are used to calculate long term erosion rates and to deduce the timing of tertiary climatic change for a portion of the atacama desert region hypogene hydrothermal alteration and protore mineralization at la escondida took place between 33 7 1 4 and 31 0 1 4 ma based on k ar dating of hydrothermal biotite and sericite supergene weathering and copper sulfide en richment processes were active from 18 0 0 7 to 14 7 0 6 ma based on k ar dating of supergene alunite distinguished from hypogene alunite by grain size color and sulfur isotopic composition reworked lenses of volcanic ash in the vicinity of la escondida provide useful time stratigraphic markers at 8 7 0 4 6 5 0 2 and 4 2 0 2 ma within present soil profiles long term average rates of erosion are determined by these age dates and quantitative calculation of eroded leached capping thickness at la escondida using mass balance analysis of geochemical profiles coupled with an estimate of unmineralized lithocap thickness based on alteration petrology and fluid inclusion geobarometry at similar deposits the observed trend of decreasing long term average erosion rates with time is consistent with arid to semiarid conditions in the early miocene changing to hyperarid conditions during the middle miocene this climatic desiccation caused termination of significant supergene copper suffide enrichment at la escondida and elsewhere in the atacama region and preservation of surficial features including the ash horizons and the leached capping middle miocene climatic desiccation in northern chile and southern peru was probably related to a pronounced decrease in temperature of coastal waters supplied by an ancestral humboldt current and an increase in upwelling intensity as the antarctic ice cap became established at approximately 15 to 13 ma the central andes cordillera which now provides a rain shadow protecting the atacama region from precipitation from the east must have attained at least half its present elevation prior to about 15 ma to have played a similar role in the middle miocene

upper cenozoic volcanism and tectonism in the andes of northern chile abstract recent geological and radiometric information of the upper cenozoic volcanic sequences of northern chile in conjunction with previous evidences point out the complex evolution of the central andes volcanism volcanic activity developed along the andean range during the last 23 m y appears to have been temporally and genetically related to tectonic movements that shaped the major morphostructural features of this region since the middle miocene 12 m y there has been a constant overlap both in time and space between lavas and ignimbrites however ignimbrite eruptions seem to predominate up to the upper miocene while lava eruptions are dominant from the pliocene to the quaternary small scale chronological and volumetric variations of the volcanic activity in the studied areas make difficult the correlations with volcanic episodes through long distances units older than pliocene have been folded and thrusted as a result of compressive movements that occurred ca 7 4 m y ago this deformation is mostly remarkable at the borders of major faulted blocks these blocks were structured along pre existing fractures caused by tectonic movements during the lower miocene pliocene and quaternary block movements developed mainly through n s fault systems these faults together with nw se and ne sw systems control the spatial distribution of most volcanic vents in this region locally the n s faults may correspond to reactivations of previous fault zones this allowed successive effusions of volcanic material at least during the last 17 m y

hydrothermal serpentine in a hess deep sediment core abstract the mineralogy and chemistry of a sediment core from the hess deep is studied in detail the top 46 cm consists of a high charge smectite calcite and feldspar whereas serpentine dominates the lower portion of the core the serpentine was identified as 2m clinochrysotile microscopic and chemical studies suggest that the serpentine has been precipitated from hydrothermal solutions

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

the topsar interferometric radar topographic mapping instrument the authors have augmented the nasa dc 8 airsar instrument with a pair of c band antennas displaced across track to form an interferometer sensitive to topographic variations of the earth s surface during the 1991 dc 8 flight campaign data were acquired over several sites in the us and europe and topographic maps were produced from several of these flight lines analysis of the results indicate that statistical errors are in the 2 4 m range while systematic effects due to aircraft motion are in the 10 20 m range the initial results from development of a second generation processor show that aircraft motion compensation algorithms reduce the systematic variations to 2 m while the statistical errors are reduced to 2 3 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

the emplacement dynamics of pumice lobes ascertained from morphology and granulometry examples from the 1993 deposits at lascar volcano chile abstract the work presented here focuses on lobe shapes and clast populations within lobate termini of the 1993 pumice flow deposits at lascar volcano chile a new method to analyze a coarse tail grain size population with field photographs is presented using this method 33 000 0 5 cm clasts from the pumice lobes of the 1993 pumice flow deposits were measured at 36 sites and the resultant grain size distributions were then related to lobe morphology lobe margins i e levees clefts and snouts were found to contain significantly larger pumice clasts and be more poorly sorted than lobe central channels i e locations away from the margins previous laboratory experiments suggest lobe margins form by the floatation and deflection of larger clasts to the margins of an advancing flow lobe results here indicate that the same sorting process efficiently segregates clasts into two flow regimes 1 a mobile central channel depleted in coarse clasts and 2 friction dominated margins enriched in clasts 15 cm the lobe margins 60 enriched in larger particles with matrix

the distribution of rare earth elements in manganese micronodules and sediments from the equatorial and southwest pacific abstract thirty three manganese micronodules and 38 associated sediments from selected areas in the equatorial and southwest pacific have been analyzed for ree significant variations in composition of both micronodules and sediments between the areas are observed in general the avarage ree abundance in manganese modules in a given area is higher than that of the associated micronodules but the ce la ratio of the micronodules is in some cases higher than that of the nodules the avarage ce la ratio of the micronodules is always higher than that of the associated sediment but the ree abundance is lower in several cases the ce anomaly is dominantly positive for micronodules and negative for sediments negative ce anomalies are however found in micronodules from the crest of the east pacific rise epr where metallogenesis is taking place positive ce anomalies occur in sediments from the southwestern pacific basin between the epr and new zealand the highest ce la ratios of both micronodules and sediments are found in this region reflecting the well oxidized conditions prevailing there

wind erosion under cold climate a pleistocene periglacial mega yardang system in central europe western pannonian basin hungary abstract mega yardangs are known to form under extremely arid warm or in some instances moderate climates here we report on the mega yardang field of the pannonian basin hungary which was uniquely formed under periglacial probably semi arid conditions and discuss its implications on factors controlling yardang formation in general the yardang field is composed of advanced streamlined forms and of less aerodynamic ridges separated by well developed corridors the length of the largest features exceeds 60 km their height varies around 150 m together with ventifacts deflation hollows and wind blown sand areas the yardang field is part of an integrated eolian feature system driven by northerly to northwesterly winds dating of ventifact surfaces with in situ produced cosmogenic nuclides places the onset of significant wind erosion to the early pleistocene 1 5 ma osl dated young loess from which well developed yardangs were sculpted and wind blown sands show that eolian action continued until the early holocene the long temporal range of wind erosion compared to average yardang formation rates and the rarity of advanced yardang forms indicate intermittent and relatively short most probably millennial scale deflation periods since the early holocene the system has been inactive and demonstrates that yardangs can persist in a recognizable form for several millennia even under less arid conditions the main control on yardang formation appears to be the existence of strong unidirectional winds in the pannonian basin facilitated mainly by the channeling effect of topographic lows in the surrounding mountain chains yardangs can form even under semi arid climates however the rarity of these occurrences suggests that this requires a special association of environmental conditions yardang formation is facilitated by homogeneous lithology namely by widespread weakly consolidated sandy sediments the distribution of yardang fields is controlled by vertical neotectonic movements which provided uplifted areas to be sculptured and topographic obstacles that sheltered downwind sediments faulting can be excluded as a controlling factor in mega yardang formation nor is the role of jointing supported

a major helium 3 source at 15 s on the east pacific rise an extensive plume of water enriched with helium 3 has been discovered in the deep pacific ocean at latitude 15 s on the east pacific rise in the core of the plume at a depth of 2500 meters over the ridge crest the helium 3 helium 4 ratio is 50 percent higher than the ratio in atmospheric helium indicating a strong injection of mantle or primordial helium at the spreading center axis through local hydrothermal systems the helium 3 plume is completely absent east of the rise but it can be traced over 2000 kilometers to the west above a newly observed physical feature a density discontinuity here caled the ridge crest front the injected plume provides a unique deep sea tracer with an asymmetric distribution which shows that the deep circulation across the rise is from east to west the striking intensity and lateral extent of this helium 3 anomaly compared to observations at known oceanic hydrohrmal sites suggest that the largest hydrothermal fields in the ocean are yet to be discovered and that they will be found near 15 s on the east pacific rise

regional variations of ree patterns in sediments from active plate boundaries abstract about 150 sediment samples from mid ocean ridges east pacific rise central indian ocean ridge carlsberg ridge and the red sea and from a back arc spreading environment lau basin were analyzed by instrumental neutron activation a ratio method for rare earth elements involving a plot of elemental ratios of ce la and la yb is proposed to characterize marine sediments in the characterization plot east pacific rise and lau basin sediments occupy distinct fields in the plot suggesting hydrothermal overprint while sediments from the central indian ocean and the carlsberg ridge plot into the field for normal deep sea sediments

discrimination of fluvial eolian and neotectonic features in a low hilly landscape a dem based morphotectonic analysis in the central pannonian basin hungary abstract the godoll hills a low relief terrain within the central pannonian basin in hungary is characterised by moderate tectonic deformation rates although typical tectonic landforms are not clearly recognisable in the study area this paper succeeded in discriminating between tectonically controlled landforms and features shaped by fluvial erosion or deflation with no tectonic control dem based morphometric parameters including elevation slope and surface roughness enabled the delineation of two nw se trending spearhead shaped ridges separated by a wide rectilinear valley of the same strike although directional statistics suggested possible tectonic control of nw se striking landforms precise morphometry completed with an analysis of subsurface structures rejected their tectonic preformation deflation plays a significant role in shaping the area and the presence of two large scale yardangs separated by a wind channel is proposed in temperate continental areas of europe no deflational landforms of such scale have been described so far suggesting that pleistocene wind power in periglacial areas was more significant than it was previously thought characteristic drainage patterns and longitudinal valley profiles enabled the recognition of areas probably affected by neotectonic deformation a good agreement was observed between locations of quaternary warping predicted by the morphometric study and subsurface structures revealed by the tectonic analysis zones of surface uplift and subsidence corresponded to anticlinal and synclinal hinges of fault related folds in low relief and slowly deforming areas where exogenous forces may override tectonic deformation only the integrated application of morphometric and subsurface structural indications could assure correct interpretation of the origin of various landforms while a morphometric study alone could have led to misinterpretation of some morphometric indices apparently suggesting tectonic preformation on the other hand the described morphological expression of subsurface structures could verify quaternary age of the deformation