thermochronometry reveals headward propagation of erosion in an alpine landscape glacial erosion of mountain ranges produces spectacular alpine landscapes and by linking climate with tectonics influences a broad array of geophysical phenomena although the resultant landforms are easily identified the timing and spatial pattern of topographic adjustment to pleistocene glaciations remain poorly known we investigated topographic evolution in the archetypal glacial landscape of fiordland new zealand using u th he thermochronometry we find that erosion during the past 2 million years removed the entire pre pleistocene landscape and fundamentally reshaped the topography erosion focused on steep valley segments and propagated from trunk valleys toward the heads of drainage basins a behavior expected if subglacial erosion rate depends on ice sliding velocity the fiordland landscape illustrates complex effects of climate on earth s surface morphology

the influence of climate on the tectonic evolution of mountain belts simple physical arguments analogue experiments and numerical experiments all suggest that the internal dynamics of actively deforming collisional mountain ranges are influenced by climate however obtaining definitive field evidence of a significant impact of climate on mountain building has proved challenging spatial correlations between intense precipitation or glaciation and zones of rapid rock uplift have indeed been documented in numerous mountain ranges and are consistent with model predictions more compelling evidence such as tectonic changes in response to rather than just coincident with climate change has however rarely been documented triggered by a climate driven increase in erosion rate friction dominated mountain ranges are expected to show a number of simultaneous responses a decrease in the width of the range a temporary increase in sediment yield a persistent increase in the rate of rock uplift and a reduction in the subsidence rate of surrounding basins the most convincing field evidence for such a coordinated response of a mountain range to climate change comes from the european alps and the st elias range of alaska field studies and experimental research during the past two decades have provided considerable evidence for a significant influence of climate on tectonics recent advances suggest that model predictions can guide future fieldwork aimed at substantiating this view

modeling the tor dotted crests bedrock edges and parabolic profiles of high alpine surfaces of the wind river range wyoming abstract broad low curvature surfaces common to many alpine landscapes and particularly well expressed in the laramide ranges of the western usa appear to be roughly in steady state with respect to the current surface processes uniform thin order 1 m regolith covers the crystalline bedrock with the exception of occasional tors at ridgecrests the surfaces are commonly edged by bedrock lips at the tops of cliffs that bound the intervening glacial troughs regolith production is low order 10 m year on bare bedrock and reaches a maximum under tens of centimeters of regolith all of these characteristics may be accounted for with a numerical model that is faithful to the periglacial surface processes presently operating on these surfaces hillslope transport is modeled assuming the dominant process is frost creep transport efficiency depends linearly upon local slope and appropriately vanishes in the absence of regolith frost frequency scales the rate of motion while the ratio of frost penetration depth to regolith thickness dictates the degree to which the process is curtailed by available regolith initial conditions include initial mean slope and bedrock roughness and a uniform regolith cover boundary conditions are imposed by dictating that the edges of the calculation space be lowered at a rate appropriate for glacial incision broad convex surfaces emerge over time scales of order 1 million years all surfaces rapidly decouple from the bounding glacial troughs as the high negative curvature at the edges disallows accumulation of regolith these bedrock edges lower at a few microns per year while the troughs lower at many times this rate as the bedrock edges therefore serve as the effective boundary condition for the remaining surface the surfaces operate independently of both lowering and widening of the glacial troughs for a reasonable set of transport and weathering rates the final steady state landscape is sensitive to the initial condition in particular the steeper the initial landscape the more likely tors are to develop as the high curvature disallows accumulation of regolith these tors in turn limit the lowering rate of the crest and the landscape retains significant local relief low relief initial conditions more likely lead to parabolic surfaces that lack tors that tors are more likely on the crests reflects the high initial curvature at the crest tors on the side slopes are eliminated the present high surfaces in the laramide ranges of the western usa imply operation of periglacial surface processes over millions of years the occasional tors imply a steeper rather than flatter initial landscape the strong decoupling of surfaces from glacial troughs justifies their use as local geomorphic markers that allow calculation of relief production since glacial incision was initiated on the other hand their present low curvature does not require a landscape history in which these surfaces were once joined as an initially widespread erosion surface

climate tectonics and the morphology of the andes large scale topographic analyses show that hemisphere scale climate variations are a first order control on the morphology of the andes zonal atmospheric circulation in the southern hemisphere creates strong latitudinal precipitation gradients that when incorporated in a generalized index of erosion intensity predict strong gradients in erosion rates both along and across the andes cross range asymmetry width hypsometry and maximum elevation reflect gradients in both the erosion index and the relative dominance of fluvial glacial and tectonic processes and show that major morphologic features correlate with climatic regimes latitudinal gradients in inferred crustal thickening and structural shortening correspond to variations in predicted erosion potential indicating that like tectonics nonuniform erosion due to large scale climate patterns is a first order control on the topographic evolution of the andes

deepening of inner gorges through subglacial meltwater an example from the unesco entlebuch area switzerland this paper explores the mechanisms by which inner gorges in the alps were formed it focuses on the ca 1 5 km long 80 m deep and a few hundred meter wide lammschlucht located in the northern foothills of the central alps we reconstructed the glacial cover using lateral moraines and hanging talus cones that record the elevation of the ice surface at the deglaciation stage of the lgm last glacial maximum we used the reconstructed ice thickness patterns to calculate the erosional potential of the subglacial meltwater the applied model is based on the principle of energy conservation and yields the pattern of downstream changes of the dynamic pressure which is considered a measure for erosion potential the model results suggest a maximum of the dynamic pressure at the end of the inner gorge we interpret therefore that the subglacial meltwater scoured the reach toward the end of the lammschlucht because of the enhanced dynamic pressure which was ultimately controlled by the ice overburden post glacial fluvial erosion then resulted in a readjustment through a regressive shift of the erosional front along the inner gorge farther upstream the current location of this front lies almost in the middle of the lammschlucht inner gorge where a step pool channel changes into a straight plane bed channel flowing on a deeply scoured bedrock

glacier longitudinal profiles in regions of active uplift abstract we combine a simplified equation for ice flow with several proposed glacial erosion laws to solve for longitudinal profiles of glaciers in topographic steady state based on prescribed patterns of rock uplift the solutions produce realistic looking glaciers and are consistent with previous numerical results scaling relationships for the dependence of ice thickness and surface slope on rock uplift and climate in the form of an imposed ice flux emerge from the solutions the general patterns of these dependencies are robust the ice thickness is dependent upon the ice flux and inversely dependent upon the rock uplift while the surface slope is inversely dependent upon the ice flux and dependent upon the rock uplift different erosion laws lead to only subtle differences in these slope and thickness dependencies despite the simplicity of the physics a first order match to an actual over deepened fjord profile can be obtained the results provide a theoretical basis for understanding the shape of glacier profiles in tectonically active landscapes can be used to benchmark numerical models of glacial erosion and can readily be incorporated directly into simple models of orogen dynamics

the role of ocean atmosphere reorganizations in glacial cycles abstract a case is made that glacial to interglacial transitions involve major reorganizations of the ocean atmosphere system such reorganizations constitute jumps between stable modes of operation which cause changes in the greenhouse gas content and albedo of the atmosphere only in this way can the rapidity of glacial terminations the hemispheric synchroneity and symmetry of mountain glaciation and the large polar air temperature and dustiness variations be accounted for if these reorganizations are driven in some fashion by orbitally induced seasonal insolation changes then the connection between insolation and climate is most likely through impacts of fresh water transport on the ocean s salinity distribution

morphometry of glaciovolcanic edifices from iceland types and evolution abstract the morphology of many glaciovolcanoes is considered one of the distinctive characteristics of their ice confining eruption environment however a thorough morphometric analysis of a large number of glaciovolcanic edifices has never been performed based on a semi automatic geomorphometric mapping of glaciovolcanic edifices of the icelandic neovolcanic zones we present a morphometric database of 155 glaciovolcanic edifices formed during the last 0 78 ma this database enables a comprehensive analysis of the morphometric diversity of a large suite of glaciovolcanic edifices sheet like formations however are not considered due to lack of data using three planimetric measurements basal length average basal width and average summit plateau width and their ratios three main morphometric groups can be distinguished in a ternary diagram 1 conical edifices with no or small summit plateaus 2 linear ridges and 3 flat topped edifices subdivided into equidimensional and elongated all three groups contain edifices with and without lava caps these morphometric groups can be fitted to the commonly accepted terminology for cone mound tindar and tuya however since lava caps occur in all morphometric groups a grouping based on its existence is not practical this suggests that by adding the descriptor lava capped to any of the three classes may be a useful way to refine the classification based on the ternary diagram ridges are the most morphometrically distinct glaciovolcanic edifice because of their very extreme elongation followed by flat topped edifices and finally conical edifices however morphometric parameters cannot be used singlehanded to identify glaciovolcanic edifices from other types of volcanic edifices such as composite volcanoes shields or submarine volcanoes and should always be complemented with other observations the glaciovolcanic edifice volumes range from 0 15 10 2km3 to 32 km3 conical edifices are the smallest 1 km3 overall with growth three morphological evolutions can be considered a an initial eruptive fissure concentrates into one vent generating an equidimensional edifice that either can be a conical or a flat topped edifice b the edifice maintains its elongation suggesting that a fissure is the dominating vent structure and during continued eruption develops significant summit plateaus generating elongated flat topped edifices and c the edifice increases in elongation because the fissure lengthens during the eruption and a plateau building stage does not occur producing increasingly elongated ridges

late cenozoic uplift of mountain ranges and global climate change chicken or egg the high altitudes of most mountain ranges have commonly been ascribed to late cenozoic uplift without reference to when crustal thickening and other tectonic processes occurred deep incision and recent denudation of these mountain ranges abundant late cenozoic coarse sediment near them and palaeobotanical evidence for warmer climates where high mountain climates today are relatively cold have traditionally been interpreted as evidence for recent uplift an alternative cause of these phenomena is late cenozoic global climate change towards lower temperatures increased alpine glaciation a stormier climate and perturbations to humidity vegetative cover and precipitation

response of glacial landscapes to spatial variations in rock uplift rate 1 the response of glaciated landscapes to rapid rock uplift driven by tectonic convergence is an important often neglected aspect of proposed interactions between plate tectonic processes and climate change rivers typically respond to more rapid rock uplift in part through increasing channel gradients in contrast the glacial buzzsaw hypothesis suggests that glaciers can erode as quickly as the fastest rock uplift rates 6 10 mm yr without any increase in mean elevations however it has not been established how this is achieved we examined moving window maps swath and longitudinal profiles hillslope relief and hypsometry for glacierized and formerly glacierized basins in areas of spatially variable rock uplift rate in the southern alps new zealand and around nanga parbat pakistan to determine whether glaciers have a specific response to rapid rock uplift the response of these glaciated landscapes to rapid rock uplift 6 10 mm yr comprises 1 modest steepening of the longitudinal profiles in smaller glaciated basins 2 maintenance of shallow downvalley slopes in larger glaciated basins 30 km2 southern alps 100 km2 nanga parbat 3 development of tall headwalls and 4 steepening of the basin as a whole dominated by hillslope lengthening around nanga parbat headwalls several kilometers high constitute 50 of the basin relief at rapid rock uplift rates although glaciers can incise the valley floor swiftly they cannot prevent headwalls from reaching exceptional heights the associated increase in mean distance between cirque heads i e a decrease in drainage density causes regional mean elevation to rise with increasing rock uplift rate however this is much less than the changes in elevation expected in unglaciated ranges

denudation and deformation in a glaciated orogenic wedge the st elias orogen alaska apatite u th he dating from the st elias orogen in southern alaska illustrates a potential association between long term denudation and glacier sliding cooling ages as young as 0 4 ma exhumation 4 5 mm yr are concentrated in a narrow band near the glacier equilibrium line altitude ela front where mean quaternary ela intersects the windward flank of the orogen this band of denudation is not correlated with individual faults structural trends or known concentrations of precipitation and we propose that it is produced by focused glacier sliding at or near the ela front this implies that long term glacial erosion is a maximum at ela which corroborates model predictions that glaciers and climate can control the pattern of crust removal from orogens denudation rates do not covary with fluctuations in glacier size along the ela front suggesting that small glaciers are capable of keeping pace with incision by larger ones and tectonic rock uplift ice discharge may thus play a critical but complex role in excavating glaciated orogens

cirques have growth spurts during deglacial and interglacial periods evidence from 10be and 26al nuclide inventories in the central and eastern pyrenees cirques are emblematic landforms of alpine landscapes the statistical distribution of cirque floor elevations is used to infer glacial equilibrium line altitude and the age of their frontal moraines for reconstructing glacial chronologies very few studies however have sought to measure cirque floor and supraglacial ridgetop bedrock downwearing rates in order to confront these denudation estimates with theoretical models of quaternary mountain landscape evolution here we use 10 be nuclide samples n 36 from moraines bedrock steps and supraglacial ridgetops among a population of cirques in the east central pyrenees in order to quantify denudation in the landscape and detect whether the mountain topography bears any relevance to the glacial buzzsaw hypothesis minimum exposure ages meas obtained for a succession of moraines spanning the oldest dryas to the holocene produced a deglaciation chronology for three different pyrenean ranges maladeta bassies and carlit based on a series of corrections calibrations and chronostratigraphic tuning procedures meas on ice polished bedrock exposures were further used to model denudation depths at nested timescales during the wurm the younger dryas and the holocene results show that subglacial cirque floor denudation was lower during glacial periods wurm 10 mm ka than during deglacial and interglacial periods tens to hundreds of mm ka the relative inefficiency of glacial denudation in the cirque zone during the wurm would have resulted from i cold based and or ii low gradient glaciers situated in the upper reaches of the icefield and or from iii glacier load starvation because of arrested clast supply from supraglacial rockslopes situated in the permafrost zone denudation peaked during the younger dryas and holocene glacial advances a time when cirque glaciers became steeper warmer based and when frost cracking weakened supraglacial ridgetops thus enhancing sub glacial erosion by providing debris to the sliding glacier base cirques therefore grow faster during more temperate periods of cirque glaciation than under full glacial conditions another key finding was the very low rates of ridgetop lowering averaged over the wurm and holocene 10 25 mm ka a comparison of the denudation rates obtained from the cirque zone with regional estimates of crustal uplift indicates that the alpine topography is not in a steady state the low intensity of glacial denudation failed to bring the topography to a buzzsaw equilibrium state

investigating the spatial distribution of summit flats in the uinta mountains of northeastern utah usa isolated laterally extensive gently sloping surfaces known as summit flats are present at high elevations in many laramide ranges and are particularly well developed in the uinta mountains of northeastern utah to investigate the spatial distribution of these surfaces and to consider possible controls on this pattern a map of summit flats in the uintas was developed from digital elevation data summit flats were identified as unglaciated areas of the landscape above an elevation of 3400 m having a slope of less than 0 3 m m 1 and an area greater than 510 2 km 2 as defined summit flats comprise 43 of the unglaciated land area above 3400 m in the uintas with the largest individual flat covering nearly 34 km 2 to quantitatively evaluate the distribution of summit flats in the uintas the area of summit flats was normalized to the total unglaciated area above 3400 m in 10 km wide swaths oriented normal to the range axis values of percent summit flats obtained by this method decrease dramatically westward from a high of more than 60 at the eastern end of the uintas to 0 at the western end given that individual summit flats can be diminished through lateral erosion by surrounding valley glaciers and that the summit flats themselves were apparently never glaciated this result suggests that glacial erosion has been more effective in the western uintas over the course of the quaternary focused glacial erosion at the upwind end of the range is consistent with the hypothesis that the proximity of lake bonneville enhanced precipitation over the western uintas during the last glacial maximum munroe j s and mickelson d m 2002 last glacial maximum equilibrium line altitudes and paleoclimate northern uinta mountains utah u s a journal of glaciology 48 257 266

assessing the relative efficiency of fluvial and glacial erosion through simulation of fluvial landscapes the relative rates of erosion by rivers and glaciers and the topographic effects of these two different styles of erosion remain outstanding problems in geomorphology we use a quantitative description of local fluvial landscapes to estimate how glaciated landscapes might look now had glaciers not developed this indicates the landscape modification attributable to glacial erosion we present examples from the sierra nevada california and the sangre de cristo range colorado in smaller drainage basins glacial modification is focussed above the mean quaternary equilibrium line altitude ela where both ridgelines and valley floors have been lowered as a consequence of glaciation at lower elevations small glaciers have apparently widened valleys without incising the valley floor beyond what a river would have this may reflect the short residence time of the glaciers at their full extent or differences in the subglacial drainage network between the glacier margins and the thalweg in larger drainage basins the pattern of glacial erosion is dramatically different here the glaciers have modified longitudinal profiles as well as valley cross sections far below the mean quaternary ela possible causes of this difference in the larger basins include the larger accumulation area greater shading of the valley floor longer residence times for ice at its full extent and the influence of the shallower valley slope prior to glaciation on the subsequent glacier and subglacial drainage conditions

numerical simulations of glacial valley longitudinal profile evolution glaciers shape alpine landscapes they broaden valley bottoms enhance local valley relief generate multiple steps overdeepen valley floors and cause tributary valleys to hang these distinctive glacial signatures result from 10 4 10 5 yr of erosion during which swings in climate drive advances and retreats of alpine glaciers we use a numerical model of glacial erosion to explore the development of the longitudinal profiles of glaciated valleys the model is driven by the past 400 k y of variable climate because both sliding speed which dictates abrasion rate and water pressure fluctuations which strongly modulate quarrying rate should peak at the equilibrium line altitude ela we expect the locus of most rapid erosion to follow the transient ela simulations of a single glacial valley show rapid flattening of the longitudinal profile inclusion of a tributary glacier creates a step immediately downvalley of the tributary junction that persists over multiple glaciations and commonly leaves the tributary valley hanging steps and overdeepenings result from an increase in ice discharge immediately below the tributary junction which is accommodated primarily by increased ice thickness and hence sliding rate the size of the step increases with the ratio of tributary to trunk ice discharge while the height of a hanging valley reflects the difference in the time integrated ice discharge in tributary and trunk valleys and therefore increases as the discharge ratio decreases

rapid glacial erosion at 1 8 ma revealed by 4he 3he thermochronometry alpine glaciation and river incision control the topography of mountain ranges but their relative contributions have been debated for years apatite 4 he 3 he thermochronometry tightly constrains the timing and rate of glacial erosion within one of the largest valleys in the southern coast mountains of british columbia canada five proximate samples require accelerated denudation of the klinaklini valley initiating 1 8 0 2 million years ago ma at least 2 kilometers of overlying rock were removed from the valley at 5 millimeters per year indicating that glacial valley deepening proceeded 6 times as fast as erosion rates before 1 8 ma this intense erosion may be related to a global transition to enhanced climate instability 1 9 ma

mid latitude glacial erosion hotspot related to equatorial shifts in southern westerlies glaciation has affected the shape of mountain ranges and has induced a global increase in erosion rates during the past 2 m y the observed increase in erosion rates appears to vary with latitude reaching a maximum at mid latitudes that is particularly well defined in the southern hemisphere although it is likely that climate played an important role the processes responsible for such latitudinal distribution of erosion are unclear here we exploit the meridional extent of the patagonian andes and identify an erosion hotspot at 44 s using a glacial erosion model and formally inverting the available thermochronometric and geobarometric data we show that this hotspot coincides with the location of maximum precipitation that follows the southern hemisphere westerlies during glacial periods we propose that the increased precipitation rates at 44 s led to greater ice sliding velocities and faster glacial erosion our results imply that the migration of the westerly wind belt toward the equator since 2 3 ma may have played an important role in determining the distribution of mountain erosion in the southern hemisphere

glacial steady state topography controlled by the coupled influence of tectonics and climate glaciers and rivers are the main agents of mountain erosion while in the fluvial realm empirical relationships and their mathematical description such as the stream power law improved the understanding of fundamental controls on landscape evolution simple constraints on glacial topography and governing scaling relations are widely lacking we present a steady state solution for longitudinal profiles along eroding glaciers in a coupled system that includes tectonics and climate we combined the shallow ice approximation and a glacial erosion rule to calculate ice surface and bed topography from prescribed glacier mass balance gradient and rock uplift rate our approach is inspired by the classic application of the stream power law for describing a fluvial steady state but with the striking difference that in the glacial realm glacier mass balance is added as an altitude dependent variable from our analyses we find that ice surface slope and glacial relief scale with uplift rate with scaling exponents indicating that glacial relief is less sensitive to uplift rate than relief in most fluvial landscapes basic scaling relations controlled by either basal sliding or internal deformation follow a power law with the exponent depending on the exponents for the glacial erosion rule and glen s flow law in a mixed scenario of sliding and deformation complicated scaling relations with variable exponents emerge furthermore a cutoff in glacier mass balance or cold ice in high elevations can lead to substantially larger scaling exponents which may provide an explanation for high relief in high latitudes

the influence of alpine glaciation on the relief of tectonically active mountain belts a surface processes model incorporating large scale fluvial processes local hill slope processes and glacial erosion is used to investigate the effect of glaciation on the relief of a fast growing narrow orogen the results of several simulations show that glaciers concentrate erosion near peaks fluvial erosion is reduced downstream of glaciers glaciers with frozen bases can lead to an increase in relief and the net affect of glaciation in the absence of widespread basal freezing is to reduce relief

latitudinal trends in modern fluvial erosional efficiency along the andes abstract latitude is a dominant control on climate influencing both temperature and precipitation leading to potential gradients in erosional efficiency previous work focused on the latitudinal control of glacial erosion on topography however the expected latitudinal trend in erosional efficiency in the absence of glacial processes is unknown we parameterize a suite of numerical models using uniform rock uplift and climate from the andes to explore how latitudinal patterns of rainfall could influence the efficiency of fluvial bedrock erosion we use the child landscape evolution model parameterized with storm duration intensity and frequency statistics from ncep ncar reanalysis data along the andes over 31 years to predict latitudinal trends in topography subject only to fluvial and hillslope erosion modeled elevations as a metric for erosional efficiency vary 2 30 fold depending on the erosional and hydrological model parameterization modeled elevation trends generated using stochastic storm generation vary significantly from those generated using mean annual precipitation map the variation suggests map may not capture the true erosional efficiency of the modern climate in some locations but may in others explaining the discrepancy of conclusions in previous studies erosional efficiency is greatest in the tropics 5 15 s where high intensity and frequent storms occur and mid latitudes 30 35 s where less frequent but intense storms occur erosional efficiency is lowest in the subtropics 20 to 25 s where low intensity and infrequent storms occur and upper mid latitudes 45 50 s where low intensity and somewhat frequent storms occur erosional efficiency varies across the width of the orogen because of across orogen effects the results provide further evidence supporting climate s control on erosion processes and topography highlighting the importance of stochastic storm distributions and suggesting that latitudinal controls on rainfall can explain some observed trends in topography the modern latitudinal trend in topography is frequently used as an argument for a glacial buzzsaw however our results suggest more information on the pre glacial distribution of topography is needed to justify such an argument since latitude dependent rainfall regimes may generate similar topographic trends

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

the importance of pressurized subglacial water in separation and sliding at the glacier bed l effet de l eau sous glaciaire sous pression sur le processus de glissement est examine a partir d un parametre appele indice de decollement du lit qui exprime les contraintes perpendiculaires au lit et la pression d eau les donnees proviennent de trois glaciers de formes differentes une loi puissance du type de weertman avec un exposant voisin de 3 modifiee pour tenir compte de l effet de la pression des eaux sous glaciaires apparait comme etant la meilleure

a new surface processes model combining glacial and fluvial erosion we have developed a new surface processes model incorporating large scale fluvial processes local hill slope processes and glacial erosion ice thickness and velocity are calculated under a shallow ice approximation simulation experiments in fast growing orogens comparing the efficiencies of fluvial and glacial erosion where the two are operating simultaneously over several glacial cycles show that glacial landscapes can support greater ice masses than fluvial landscapes glacial valley and lake shapes create a disequilibrium between landform and land forming process that leads to pulses of high erosion at the end of glacial periods glacial erosion rates can reach a constant value in a uniformly growing orogen and glacial erosion is capable of eroding drainage divides when the ice is moderately thick

earth science glaciers shield mountain tops glaciers frozen to bedrock may have protected the southernmost andes from erosion providing an explanation for the mountains topography and fresh constraints on possible links between climate and tectonics

hillslope glacier coupling the interplay of topography and glacial dynamics in high asia 1 high asian glacial landscapes have large variations in topographic relief and the size and steepness of snow accumulation areas associated differences in glacial cover and dynamics allow a first order determination of the dominant processes shaping these landscapes here we provide a regional synthesis of the topography and flow characteristics of 287 glaciers across high asia using digital elevation analysis and remotely sensed glacier surface velocities glaciers situated in low relief areas on the tibetan plateau are mainly nourished by direct snowfall have little or no debris cover and have a relatively symmetrical distribution of velocities along their length in contrast avalanche fed glaciers with steep accumulation areas which occur at the deeply incised edges of the tibetan plateau are heavily covered with supraglacial debris and flow velocities are highest along short segments near their headwalls but greatly reduced along their debris mantled lower parts the downstream distribution of flow velocities suggests that the glacial erosion potential is progressively shifted upstream as accumulation areas get steeper and hillslope debris fluxes increase our data suggest that the coupling of hillslopes and glacial dynamics increases with topographic steepness and debris cover the melt lowering effect of thick debris cover allows the existence of glaciers even when they are located entirely below the snow line however slow velocities limit the erosion potential of such glaciers and their main landscape shaping contribution may simply be the evacuation of debris from the base of glacial headwalls which inhibits the formation of scree slopes and thereby allows ongoing headwall retreat by periglacial hillslope processes we propose a conceptual model in which glacially influenced plateau margins evolve from low relief to high relief landscapes with distinctive contributions of hillslope processes and glaciers to relief production and decay

small valley glaciers and the effectiveness of the glacial buzzsaw in the northern basin and range usa the glacial buzzsaw hypothesis suggests that efficient erosion limits topographic elevations in extensively glaciated orogens studies to date have largely focussed on regions where large glaciers tens of kilometres long have been active in light of recent studies emphasising the importance of lateral glacial erosion in lowering peaks and ridgelines we examine the effectiveness of small glaciers in limiting topography under both relatively slow and rapid rock uplift conditions four ranges in the northern basin and range idaho montana and wyoming usa were chosen for this analysis estimates of maximum pleistocene slip rates along normal faults bounding the beaverhead bitterroot mountains similar to 0 14 mm y 1 lemhi range similar to 0 3 mm y 1 and lost river range similar to 0 3 mm y 1 are an order of magnitude lower than those on the teton fault similar to 2 mm y 1 we compare the distribution of glacial erosion estimated from cirque floor elevations and last glacial maximum lgm equilibrium line altitude ela reconstructions and fault slip rate with three metrics of topography in each range the along strike maximum elevation swath profile hypsometry and slope elevation profiles in the slowly uplifting beaverhead bitterroot mountains and lemhi and lost river ranges trends in maximum elevation parallel elas independent of variations in fault slip rate maximum elevations are offset similar to 500 m from lgm elas in the lost river range lemhi range and northern beaverhead bitterroot mountains and by similar to 350 m in the southern beaverhead bitterroot mountains where glacial extents were less the offset between maximum topography and mean quaternary elas inferred from cirque floor elevations is similar to 350 m in the lost river and lemhi ranges and 200 250 m in the beaverhead bitterroot mountains additionally slope elevation profiles are flattened and hypsometry profiles show a peak in surface areas close to the ela in the lemhi range and beaverhead bitterroot mountains suggesting that small glaciers efficiently limit topography the situation in the lost river range is less clear as a glacial signature is not apparent in either slope elevation profiles or the hypsometry in the rapidly uplifting teton range the distribution of elas appears superficially to correspond to maximum topography hypsometry and slope elevations profiles with regression lines on maximum elevations offset by similar to 700 and similar to 350 m from the lgm and mean quaternary ela respectively however grand teton and mt moran represent high elevation teflon peaks that appear impervious to glacial erosion formed in the hard crystalline bedrock at the core of the range glacier size and drainage density rock uplift rate and bedrock lithology are all important considerations when assessing the ability of glaciers to limit mountain range topography in the northern basin and range it is only under exceptional circumstances in the teton range that small glaciers appear to be incapable of imposing a fully efficient glacial buzzsaw emphasising that high peaks represent an important caveat to the glacial buzzsaw hypothesis c 2008 elsevier b v all rights reserved

valley incision and the uplift of mountain peaks simple end member models describe the effect of isostatically compensated valley incision on the elevation of mountain peaks these models allow calculation of the maximum portion of mountain peak elevations attributable to the development of observed relief and predict general downstream patterns of ridgetop uplift in response to erosion by fluvial processes analyses of topographic profiles across major drainages of the central sierra nevada and the tibetan plateau indicate that isostatically compensated valley incision could account for at most 5 10 of the present elevation of mountain peaks in contrast analyses of topographic profiles across the himalaya show that as much as 20 30 of the present elevation of himalayan peaks could be explained by isostatically compensated valley incision the degree to which valley incision is compensated locally however depends on the flexural rigidity of the lithosphere the size of the range and its tectonic boundary conditions these results show that while a tectonic control is required for late cenozoic surface uplift of the sierra nevada a significant portion of the elevation of himalayan peaks may reflect late cenozoic valley incision on the margin of the tibetan plateau

from gullies to mountain belts a review of sediment budgets at various scales this paper reviews the state of the art in the concept as well as in the application of sediment budgets in sedimentary research sediments are a product of mass dispersal at the earth surface and take part in global cycles sediment budgets aim at quantifying this mass transfer based on the principle of mass conservation and are the key to determine ancient fluxes of solid matter at the earth surface this involves fundamental questions about the interplay of uplift climate and denudation in mountain belts and transfer of sediments from the continents to the oceans as well as applied issues such as soil and gully erosion reservoir siltation and coastal protection first after introducing basic concepts relevant scales and methodologies the different components of quaternary routing systems from erosion in headwaters river systems glacial and paraglacial systems lakes deltas estuaries coasts shelves epicontinental seas and deep sea fans are discussed in terms of their sediment budget most suitable are sedimentologically closed or semi closed depositional environments e g alluvial fans lakes deltas and deep sea fans in a second step the dynamics of passive active and collisional tectonic settings and sediment budgets in related sedimentary basins are explored and new concepts of sediment portioning at large geodynamic scales are introduced ancient routing systems are more or less incomplete and may be intensively fragmented or destroyed in active tectonic settings in terms of sedimentary basin types rifts intracontinental and epicontinental settings are preferred objects of sediment budgets because of their persistence and relatively simple overall sedimentary architecture however closing basins such as foreland forearc retroarc piggy back and wedge top basins may provide excellent snapshots of orogenic sediment fluxes in a third step the large long lived routing systems of the amazon the ganges brahmaputra and the rhine are reviewed for each system estimates of either sediment volumes mass or sediment fluxes of continental and marine subsystems have been compiled in order to receive a complete routing in terms of mass conservation for specific time periods since the late glacial maximum as well as the cenozoic following lessons can be taken from these case studies i depositional centers and fluxes show strong shifts in space and time and call for caution when simply looking at subsystems ii the response times of these large systems are within the milankovich time interval thus lower than predicted from diffusion models iii cyclic routing of sediments in continental basins is much more dominated by climate human control than by eustacy and iv at long time scales ultimate sinks win over intermittent storage it is concluded from this review that the quantitative understanding of global sediment cycling over historic and geologic time and its response to allogenic forcing is still in its infancy and further research is needed towards a holistic view of sediment routing systems at various temporal and spatial scales and their coupling with global biogeochemical cycles this includes i to better determine response times of large routing systems by linking quaternary with cenozoic sediment budgets and continental with marine sub systems ii to combine advanced provenance techniques with sediment budgets in order to reconstruct ancient systems iii to study sediment partitioning at the basin scale iv to reconcile continental supply dominated sequence stratigraphy with the eustatic dominated marine concept and iv to account for non actualism of ancient systems with respect to their erosion and transport mode in particular during glaciations and pronounced arid intervals glacial and eolian sediment routing may cross over hydrologic boundaries of drainage basins thus challenging the principle of mass conservation

glacial effects limiting mountain height the height of mountain ranges reflects the balance between tectonic rock uplift crustal strength and surface denudation tectonic deformation and surface denudation are interdependent however and feedback mechanisms in particular the potential link to climate are subjects of intense debate spatial variations in fluvial denudation rate caused by precipitation gradients are known to provide first order controls on mountain range width crustal deformation rates and rock uplift moreover limits to crustal strength are thought to constrain the maximum elevation of large continental plateaus such as those in tibet and the central andes there are indications that the general height of mountain ranges is also directly influenced by the extent of glaciation through an efficient denudation mechanism known as the glacial buzzsaw here we use a global analysis of topography and show that variations in maximum mountain height correlate closely with climate controlled gradients in snowline altitude for many high mountain ranges across orogenic ages and tectonic styles with the aid of a numerical model we further demonstrate how a combination of erosional destruction of topography above the snowline by glacier sliding and commensurate isostatic landscape uplift caused by erosional unloading can explain observations of maximum mountain height by driving elevations towards an altitude window just below the snowline the model thereby self consistently produces the hypsometric signature of the glacial buzzsaw and suggests that differences in the height of mountain ranges mainly reflect variations in local climate rather than tectonic forces

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

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

controls on spatial variations of glacial erosion in the qilian shan northeastern tibetan plateau abstract understanding interactions between topography and glacial erosion is a key to advancing knowledge of mountain landscape evolution and is often hampered by the lack of quantitative data for glacial erosion here we examine spatial variations in glacial erosion in the qilian shan using a power law model y axb fit to cross profiles of glacial valleys we further analyze the impacts of glacial dynamics topography and lithology on the degree of glacial erosion using qualitative and quantitative methods although the distribution of glacial erosion is controlled by the combined effects of glacial dynamics topography and lithology average glacier size or ice flux and mean slope above the equilibrium line altitude are dominant controls in addition the topographic data analyzed through a b fr diagram suggests two opposite types of glacial valley cross profile development temperate maritime glacier valleys are mainly dominated by overdeepening while continental glacier valleys are mainly dominated by a widening process although this varies with lithology

experimental determination of a double valued drag relationship for glacier sliding the contribution of glaciers to sea level rise and their effects on landscape evolution depend on the poorly known relationship between sliding speed and drag at the ice bed interface results from experiments with a new rotary laboratory device demonstrate empirically for the first time a double valued drag relationship like that suggested by some sliding theories steady drag on a rigid sinusoidal bed increases peaks and declines at progressively higher sliding speeds due to growth of cavities in the lee sides of bed undulations drag decreases with increased sliding speed if cavities extend beyond the inflection points of up glacier facing surfaces so that adverse bed slopes in contact with ice diminish with further cavity growth these results indicate that shear tractions on glacier beds can potentially decrease due to increases in sliding speed driven by weather or climate variability promoting even more rapid glacier motion by requiring greater strain rates to produce resistive stresses although a double valued drag relationship has not yet been demonstrated for the complicated geometries of real glacier beds both its potential major implications and the characteristically convex stoss surfaces of bumps on real glacier beds provide stimulus for exploring the effects of this relationship in ice sheet models

sapropel burn down and ichnological response to late quaternary sapropel formation in two 400 ky records from the eastern mediterranean sea two approximately 400 ky long sedimentary records from the eastern mediterranean sea were used to study the factors determining why some sapropels are more or less completely erased while others are preserved x ray radiographs bulk density organic carbon content fe al and mn al ratios were used to reconstruct initial thickness and burn down ratios of sapropels s1 to s11 the results show that at the low sedimentation rate open ocean settings studied initial carbon content of the sapropel as well as post sapropel sedimentation rate and carbon flux have had only limited influence on the preservation of sapropels in contrast the regional position relative to the deep water source controlling bottom water current intensity and bottom water oxygenation played a major role in the preservation of the sapropels furthermore orbitally controlled variations in the intensity of subsequent summer insolation maxima also seem to play an important role in the preservation potential of sapropel layers subsequent strong insolation maxima resulting in thick sapropels tend to act as a lid preventing further burn down of the older sapropels below the long and well dated records also offer an opportunity to study the response of the benthic fauna to the drastic environmental changes related to the formation of sapropels the sediment in the two cores is generally characterized by mottled burrows with only scarce occurrences of distinct trace fossils such as scolicia thalassinoides chondrites and trichichnus the meager ichnofauna is attributed to the well oxygenated and warm bottom waters in an oligotrophic environment typical for non sapropel times the sparse ichnofauna and the lack of tiered ichnocoenosis render existing models of bioturbational response to changes in bottom water oxygenation unsuitable for this kind of environments however observations on individual traces support earlier studies that suggest that the trace fossils trichichnus and chondrites are the result of chemosymbiotic activities that benefit from reducing conditions in the pore water and that the producers of scolicia have a preference for well oxygenated conditions and sandy substrates c 2006 elsevier b v all rights reserved

geomorphic limits to climate induced increases in topographic relief recognition of the potential for strong dynamic coupling between atmospheric and tectonic processes has sparked intense cross disciplinary investigation and debate on the question of whether tectonics have driven long term climate change or vice versa it has been proposed that climate change might have driven the uplift of mountain summits through an isostatic response to valley incision because isostasy acts to compensate mean elevations the debate hinges on the question of whether climate change can significantly increase topographic relief or more precisely increase the volume of missing mass between summits and ridges here we show that in tectonically active mountain ranges geomorphic constraints allow only a relatively small increase in topographic relief in response to climate change thus although climate change may cause significant increases in denudation rates potentially establishing an important feedback between surficial and crustal processes neither fluvial nor glacial erosion is likely to induce significant isostatic peak uplift

rapid exhumation in the western alps driven by slab detachment and glacial erosion identifying topographic and erosion rate response to tectonic and climatic forcing remains challenging this is in part because of the difficulty in isolating the respective roles of climate and tectonics here we exploit 2500 thermochronometric data points collected over several decades of research using a new inverse technique to image the space time evolution of erosion rate across the european alps over the past 35 m y the most striking feature of our results is a two to three fold increase in erosion rate over the past 2 m y exclusively within the western and central alps this increase appears to be controlled by the inferred high rock uplift rate due to the progressive detachment of the european slab under the western alps the similarity in mean elevation between the western and eastern alps indicates a surprisingly low topographic response to this differential tectonic forcing and points to the role of enhanced glacial erosion in response to surface uplift

numerical modelling of subglacial erosion and sediment transport and its application to the north american ice sheets over the last glacial cycle abstract present day sediment distribution offers a potentially strong constraint on past ice sheet evolution glacial system models gsms however cannot address this constraint while lacking appropriate representations of subglacial sediment production and transport incorporating these elements in gsms is also required in order to quantify the impact of a changing sediment cover on glacial cycle dynamics towards these goals we present a subglacial process model hereafter referred to as the sediment model that incorporates mechanisms for sediment production entrainment transport and deposition bedrock erosion is calculated by both hallet s and boulton s abrasion laws separately and by a novel quarrying law parametrized as a function of subglacial cavity extent these process oriented erosion laws are compared against a simple empirical relationship between erosion rate and the work done by basal stress sediment entrainment is represented by philip s law for regelation intrusion and soft bed deformation is included as a subglacial sediment transport mechanism the model is driven by the data calibrated mun 3d gsm and a newly developed subglacial hydrology module the sediment model is applied to the last north american glacial cycle and predicts sediment thickness and cumulative erosion patterns results are obtained in the context of a sensitivity analysis and are compared against the present day distribution of glacigenic sediment and geological estimates of laurentide ice sheet erosion given plausible ranges for the sensitivity parameters chosen a priori based on available literature or on heuristic arguments the calculated erosion depths overlap with the geological estimates of laurentide erosion most of the runs in the sensitivity set produce unrealistically thick and continuous moraines along the eastern southern and western margins of the north american ice complex which suggests that the model overestimates sediment entrainment and thus englacial transport a realistic sediment distribution is only obtained when the entrainment rate is capped at the average basal melting rate which suggests that modelled entrainment and basal melting rates should be of the same order of magnitude

glacial topographic interactions in the teton range wyoming 1 understanding interactions among tectonics topography climate and erosion is fundamental to studies of mountainous landscapes here we combine topographic analyses with modeled distributions of precipitation insolation and flexural isostasy to present a conceptual model of topographic evolution in the teton range wyoming and test whether efficient glacial relief production has contributed to summit elevations the conceptual model reveals a high degree of complexity inherent in even a relatively small glaciated mountain range back tilting has caused topographic asymmetry with the greatest relief characterizing eastern catchments in the center of the range two high summits grand teton and mount moran rise hundreds of meters above the surrounding landscape their elevations set by the threshold hillslope angle and the spacing between valleys hosting large erosionally efficient glaciers only basins 20 km2 held glaciers capable of eroding sufficiently rapidly to incise deeply and maintain shallow downvalley gradients on the eastern range flank glacial erosion here was promoted by 1 prevailing westerly winds transporting snow to high relief eastern basins leading to cross range precipitation asymmetry 2 the wind blown redistribution of snow from open western headwaters into sheltered eastern cirques with the associated erosion driven migration of the drainage divide increasing eastern accumulation areas and 3 tall steep hillslopes providing shading snow influx from avalanching and insulating debris cover from rockfalls to valley floor glaciers in comparison the topographic enhancement of glacial erosion was less pronounced in western and smaller eastern basins despite dramatic relief production insufficient rock mass is removed from the teton range to isostatically raise summit elevations

an 60 million year periodicity is common to marine 87sr 86sr fossil biodiversity and large scale sedimentation what does the periodicity reflect abstractwe find that the marine 87sr 86sr record shows a significant periodicity of m yr the 87sr 86sr record is out of phase with a 62 3 m yr periodicity previously reported in the record of marine animal diversity these periodicities are close to 58 4 m yr cycles found for the number of gap bounded sedimentary carbonate packages of north america we propose that these periodicities reflect the operation of a periodic pulse of the earth in large scale earth processes these may be linked to mantle or plate tectonic events possibly uplift which affects the earth s climate and oceans and thus the geochemistry sedimentation and biodiversity of the marine realm

hypsometry of glaciated landscapes hypsometry frequency distribution of elevations is often used to characterize landscape morphology traditionally in the context of the degree of uvial dissection recently the hypsometry of glaciated regions has been used to infer how rates of glacial erosion compare with tectonic uplift rates however many factors other than tectonics can also exert a major in uence on the hypsometry of a glaciated landscape resulting in a wide variety of hypsometries using examples from the eastern sierra nevada california the western sangre de cristo range colorado and the ben ohau range new zealand we demonstrate that all else being equal the hypsometries of neighbouring basins can indicate the relative degree of glacial modi cation in each a selection of drainage basins from the rocky mountains shows that the position of the equilibrium line altitude ela within the drainage basin relief is a dominant variable in determining the hypsometry of a glaciated basin this is a non linear effect once the ela falls to some critical level the glaciers scour deeply below the ela causing a noticeably different hypsometry the hypsometry of an arbitrary region encompassing many drainage basins can disguise the variation present in the hypsometries and thus landforms of the individual basins unique local circumstances such as the presence of a mountain ice eld waiho basin southern alps substantial hanging valleys avalanche creek glacier national park a narrow outlet canyon sawmill creek sierra nevada and isolated geologic structures baker creek sierra nevada can have a major impact on the hypsometry of an individual basin copyright 2004 john wiley sons ltd

experiments on the dynamics and sedimentary products of glacier slip abstract experimental work in glacial geomorphology is focused almost entirely on processes in the thin shear zone at the beds of sliding glaciers where ice at its pressure melting temperature moves over either rigid rock or deformable till experiments with rotary shear devices illuminate constitutive behavior there central to the dynamics of fast moving glaciers and provide a foundation for interpreting the sedimentary record of glacier slip and associated sediment transport results from experiments designed to study ice sliding over a rigid wavy bed shear deformation of till and plowing of clasts across the surface of a till bed point to a common conclusion drag at the bed can decrease with increasing slip velocity thereby concentrating driving stress elsewhere and promoting rapid glacier flow this rate weakening behavior at glacier beds is in contrast to the viscous slip resistance assumed in ice sheet models and most efforts to determine distributions of basal drag from glacier surface velocities ring shear experiments in which various basal tills and more idealized materials are sheared to high strains provide quantitative insight into grain size evolution mixing at contacts between basal tills microstructure development particle fabric development and development of anisotropy of magnetic susceptibility preferred orientations of principal magnetic susceptibilities provide the most dependable and complete description of till shear patterns when applied to basal tills of the geologic record magnetic till fabrics measured along thick till sections and calibrated experimentally indicate that deformation of the bed by two lobes of the laurentide ice sheet was shallow

influence of a glacial buzzsaw on the height and morphology of the cascade range in central washington state usa abstract analysis of climatic and topographic evidence from the cascade range of washington state indicates that glacial erosion limits the height and controls the morphology of this range glacial erosion linked to long term spatial gradients in the ela created a tilted planar zone of 373 cirques across the central part of the range peaks and ridges now rise 600 m above this zone hypsometric analysis of the region shows that the proportion of land area above the cirques drops sharply and mean slopes 30 indicate that the areas above the cirques may be at or near threshold steepness the mean plus 1 relief of individual cirque basins 570 m corresponds to the 600 m envelope above which peaks rarely rise the summit altitudes are set by a combination of higher rates of glacial and paraglacial erosion above the ela and enhanced hillslope processes due to the creation of steep topography on the high precipitation western flank of the cascades the dominance of glacial and hillslope erosion at altitudes at and above the ela may explain the lack of a correspondence between stream power erosion models and measured exhumation rates from apatite u th he thermochronometry

the glacially overdeepened trough of the salzach valley austria bedrock geometry and sedimentary fill of a major alpine subglacial basin abstract overdeepened valleys are unambiguous features of glacially sculpted landscapes they result from erosion at the bed of the glacier and their size and shape is determined by ice dynamics and the characteristics of the underlying bedrock major overdeepened valleys representing vertical bedrock erosion of several hundreds of meters are characteristic features of many trunk valleys in the formerly glaciated parts of the alpine mountain belt the thick sedimentary fill usually hinders attempts to unravel bedrock geometry formation history and fill characteristics based on reflection seismic data and core logs from multiple deep drillings we construct a detailed bedrock model of the lower salzach valley trough one of the largest overdeepened valleys in the european alps the analysed overdeepened structure characterized by a strongly undulating topography two reaches of enhanced erosion can be identified and are suggested to be related to variations in bedrock erodibility and a triple glacier confluence the sedimentary fill shows clear characteristics of rapid infilling and subaqueous fan delta deposits indicate a strong influence of tributary streams associated surface lowering of the valley floor had a major impact on tributary stream incision but also on the available ice accumulation area at subsequent glaciations the extent to which fills of earlier glaciations have been preserved from erosion during the last glacial maximum remains ambiguous and demands further exploration to our knowledge the presented bedrock model is one of the best defined of any major overdeepened trunk valley

numerically simulating alpine landscapes the geomorphologic consequences of incorporating glacial erosion in surface process models abstract a numerical model that simulates the evolution of glaciated mountain landscapes is presented by employing a popular sliding based glacial erosion model many common glacial landforms are created the numerical model builds on earlier work as it is fully two dimensional and employs the first order forcing on mountain evolution these forcings include tectonic uplift isostasy hillslope processes fluvial processes and glacial processes a climate dependent model of ice dynamics is employed to determine ice coverage and ice flux two simulations are presented one with generic model parameters and a second with parameters that correspond to conditions in the southern alps of new zealand landforms that are produced by the model include climate climate dependent elevation lowering similar to what might be expected by a glacial buzz saw valley overdeepening terminal moraines and valley retreat the model also predicts that current rates of sedimentation are higher than the long term average and that several tens of thousands of years are required for the landscape to adjust to a change in the dominant erosional forcing therefore glaciated orogens are unlikely to achieve topographic steady state over milankovitch timescales

a new bathymetric compilation for the south orkney islands region antarctic peninsula 49 39 w to 64 59 s insights into the glacial development of the continental shelf we present a new high resolution 300 m bathymetric grid of the continental shelf surrounding the south orkney islands northeast of the antarctic peninsula the new grid derived from a compilation of marine echo sounding data improves previous regional bathymetric representations and helps to visualize the morphology of the shelf in unrivalled detail the compilation forms important baseline information for a range of scientific applications and end users including oceanographers glacial modelers biologists and geologists in particular due to the limited understanding of glacial history in this region the bathymetry provides the first detailed insights into past glacial regimes the continental shelf is dominated by seven glacially eroded troughs marking the pathways of glacial outlets that once drained a former ice cap centered on the south orkney islands during previous glacial periods grounded ice extended to the shelf edge north of the islands a large 250 km long sediment depocenter interpreted as a maximum former ice limit of one or more cenozoic glaciations suggests that ice was only grounded to the 300 350 m contour in the south hypsometric analyses support this interpretation indicating that a significant proportion of the shelf has been unaffected by glacial erosion using these observations we propose a preliminary ice cap reconstruction for maximum glaciation of the south orkney plateau suggesting an ice coverage of about 19 000 km2 the timing of maximum ice extent number of past advances and pattern of subsequent deglaciation s remain uncertain and will require further targeted marine geological and geophysical investigations to resolve

a paleomagnetic study of the structural deformation in the yerington district nevada 2 mesozoic basement units and their total and pre oligocene tectonism

signatures of late pleistocene fluvial incision in an alpine landscape uncertainty regarding the relative efficacy of fluvial and glacial erosion has hindered attempts to quantitatively analyse the pleistocene evolution of alpine landscapes here we show that the morphology of major tributaries of the rhone river switzerland is consistent with that predicted for a landscape shaped primarily by multiple phases of fluvial incision following a period of intense glacial erosion after the mid pleistocene transition 0 7 ma this is despite major ice sheets reoccupying the region during cold intervals since the mid pleistocene we use high resolution lidar data to identify a series of convex reaches within the long profiles of 18 tributary channels we propose these reaches represent knickpoints which developed as regional uplift raised tributary bedrock channels above the local fluvial baselevel during glacial intervals and migrated upstream as the fluvial system was re established during interglacial periods using a combination of integral long profile analysis and stream power modelling we find that the locations of 80 of knickpoints in our study region are consistent with that predicted for a fluvial origin while the mean residual error over 100 km of modelled channels is just 26 3 m breaks in cross valley profiles project toward the elevation of former end of interglacial channel elevations supporting our model results calculated long term uplift rates are within 15 of present day measurements while modelled rates of bedrock incision range from 1 mm yr for low gradient reaches between knickpoints to 6 10 mm yr close to retreating knickpoints typical of observed rates in alpine settings together our results reveal approximately 800 m of regional uplift river incision and hillslope erosion in the lower half of each tributary catchment since 0 7 ma

evidence for nonlinear diffusive sediment transport on hillslopes and implications for landscape morphology steep soil mantled hillslopes evolve through the downslope movement of soil driven largely by slope dependent transport processes most landscape evolution models represent hillslope transport by linear diffusion in which rates of sediment transport are proportional to slope such that equilibrium hillslopes should have constant curvature between divides and channels on many soil mantled hillslopes however curvature appears to vary systematically such that slopes are typically convex near the divide and become increasingly planar downslope this suggests that linear diffusion is not an adequate model to describe the entire morphology of soil mantled hillslopes here we show that the interaction between local disturbances such as rainsplash and biogenic activity and frictional and gravitational forces results in a diffusive transport law that depends nonlinearly on hillslope gradient our proposed transport law 1 approximates linear diffusion at low gradients and 2 indicates that sediment flux increases rapidly as gradient approaches a critical value we calibrated and tested this transport law using high resolution topographic data from the oregon coast range these data obtained by airborne laser altimetry allow us to characterize hillslope morphology at 2 m scale at five small basins in our study area hillslope curvature approaches zero with increasing gradient consistent with our proposed nonlinear diffusive transport law hillslope gradients tend to cluster near values for which sediment flux increases rapidly with slope such that large changes in erosion rate will correspond to small changes in gradient therefore average hillslope gradient is unlikely to be a reliable indicator of rates of tectonic forcing or baselevel lowering where hillslope erosion is dominated by nonlinear diffusion rates of tectonic forcing will be more reliably reflected in hillslope curvature near the divide rather than average hillslope gradient

the topographic state of fluvially conditioned mountain ranges abstract the topography of mountain ranges reflects the competition of constructive and destructive processes driven by tectonics and climate respectively there is a vital debate whether the topography of individual orogens reflects stages of growth steady state or decay that is fueled by the million year time scales hampering direct observations on landscape evolution the superposition of various process patterns and the complex interactions among different processes hence there is a demand for sophisticated analysis tools to extract constraints on the long term evolution of orogens from their topography we review the field of orogen scale landscape evolution from a numerical perspective summarize the most prominent modelling concepts and their implications for the fluvially driven development of mountain topography and finally evaluate their applicability for understanding real world orogens following the concept of equilibrium a state where uplift rates are balanced by erosion rates and topography remains steady over time the erosional long term response of rivers and hillslopes can be mathematically formalised by the stream power and mass diffusion equations respectively based on a simple 1 dimensional model consisting of two rivers separated by a watershed we explain the influence of uplift rate and rock erodibility on steady state channel profiles and show the time dependent development of the channel drainage divide system dynamic drainage network reorganisation adds additional complexity and its effect on topography is explored on a two dimensional model river capture events and drainage divide migrations as expression of the ongoing drainage network reorganisation cause changes in topography until a stable network topology and hence full topographic steady state is achieved the long time spans needed for this drainage network optimization suggest that orogens on earth may never reach full topographic steady state in real world orogens we find evidence for premature mature and decaying topography as well as relief rejuvenation by analysing slope elevation distributions and trace the expression of crustal strain distorting drainage networks by applying the transform we conclude that modern concepts of landscape evolution allow sophisticated analyses of real world orogens but emphasize that unambiguous results are mostly derived from regions dominated by a limited number of interacting processes such as mountain ranges with limited or no glacial imprint this results in a high demand for techniques to disentangle the complex topography of real world orogens into the signals resulting from the individual land shaping processes

glacial hydrology and erosion patterns a mechanism for carving glacial valleys article i nfo article history most glacial erosion models assume that erosion rates are proportional to ice sliding velocity while recent studies have shown that water plays a major role in modulating sliding velocities the impact it might have on erosion rates is still unclear here we incorporate subglacial hydrology into a glacial erosion model that is based on a sliding rule our results explicitly highlight that adding subglacial hydrology has profound impacts on the temporal and spatial patterns of glacial erosion first it suggests that erosion might mainly occur during melting seasons when subglacial water pressure is large and effective pressure is low i e before the channels fully develop and reduce the water pressure at the ice bedrock interface which promotes sliding and erosion second the distribution of erosion is bimodal with a peak at the equilibrium line altitude and as we demonstrate enhanced erosion at much lower altitudes within the ablation area where water due to melting abounds this has important implications because it explains why glacial erosion can simultaneously

a numerical model of landform development by glacial erosion the jagged peaks knife edged ridges and steep sided valleys of alpine landscapes are well known products of glaciation yet little is known about how the dynamics of ice flow and glacial erosion give rise to such landforms by linking a finite element model for ice flow through a valley with a model for glacial erosion we have simulated the gradual evolution of one of the most striking of glacial landforms the u shaped valley incorporating realistic glacier sliding laws and spatial variations in rock resistance our simulations illustrate how ice flow erosion patterns and topography interact in the long term evolution of glacial landforms in addition our results indicate for the first time that the formation of a steady state u shaped valley takes approximately 105 years

glacial and periglacial buzzsaws fitting mechanisms to metaphors the buzzsaw hypothesis refers to the potential for glacial and periglacial processes to rapidly denude mountains at and above glacier equilibrium line altitudes elas irrespective of uplift rates rock type or pre existing topography here the appropriateness of the buzzsaw metaphor is examined alongside questions of the links between glacial erosion and elas and whether the glacial system can produce low relief surfaces or limit summit heights plateau fragments in mountains on both active orogens and passive margins that have been cited as products of glacial and periglacial buzzsaw erosion instead generally represent dissected remnants of largely inherited pre glacial relief summit heights may correlate with elas but no causal link need be implied as summit erosion rates are low cirque headwalls may not directly abut summits and on passive margins cirques are cut into pre existing mountain topography any simple links between elas and glacial erosion break down on passive margins due to topographic forcing of ice sheet growth and to the km scale vertical swaths through which elas have shifted through the quaternary glaciers destroy rather than create low relief rock surfaces through the innate tendency for ice flow to be faster thicker and warmer along valleys the glacial buzzsaw cuts down

pleistocene relief production in laramide mountain ranges western united states gently sloped summits and ridges collectively referred to as summit flats are abundant in many laramide ranges in the western united states the erosion rate of summit flats is 10 m m y on the basis of the concentrations of cosmogenic radionuclides because erosion rates in valleys between summit flats are an order of magnitude faster relief within these ranges is currently increasing by about 100 m m y if summit flat erosion is slower than rock uplift driven by the isostatic response to valley erosion then this relief production could result in increased summit elevations the mean depth of material eroded from a smooth surface fit to existing summit flats varies from 280 to 340 m in four laramide ranges based on geographic information system gis analyses of digital elevation models this erosion would result in a maximum of 250 300 m of rock uplift assuming airy isostasy however because the laramide ranges examined here are narrow relative to the flexural wavelength of the lithosphere erosionally driven rock uplift is limited to 50 100 m over the past several million years summit erosion would approximately offset this rock uplift therefore we conclude that summit elevations have remained essentially constant even though several hundred meters of relief has been produced on the basis of valley and summit erosion rates and the average depth of erosion we estimate that relief production in laramide ranges began at ca 3 ma we hypothesize that this relief production was climatically driven and was associated with the onset or enhancement of alpine glaciation in these ranges

climatic limits on landscape development in the northwestern himalaya the interaction between tectonism and erosion produces rugged landscapes in actively deforming regions in the northwestern himalaya the form of the landscape was found to be largely independent of exhumation rates but regional trends in mean and modal elevations hypsometry frequency distribution of altitude and slope distributions were correlated with the extent of glaciation these observations imply that in mountain belts that intersect the snowline glacial and periglacial processes place an upper limit on altitude relief and the development of topography irrespective of the rate of tectonic processes operating

scaling the teflon peaks rock type and the generation of extreme relief in the glaciated western alaska range 1 parts of the alaska range alaska usa stand in prominent exception to the glacial buzzsaw hypothesis which postulates that terrain raised above the ela is rapidly denuded by glaciers in this paper we discuss the role of a strong contrast in rock type in the development of this exceptional terrain much of the range is developed on pervasively fractured flysch with local relief of 1000 1500 m and mean summit elevations that are similar to modern snow line elevations in contrast cretaceous and tertiary plutons of relatively intact granite support the range s tallest mountains including mt mckinley or denali at 6194 m with 2500 5000 m of local relief the high granitic peaks protrude well above modern snow lines and support many large glaciers we focus on the plutons of the denali massif and the kichatna mountains to the west we use field observations satellite photos and digital elevation data to demonstrate how exhumation of these plutons affects glacier longitudinal profiles the glacial drainage network and the effectiveness of periglacial processes in strong granite steep smooth valley walls are maintained by detachment of rock slabs along sheeting joints these steep walls act as low friction surfaces teflon efficiently shedding snow simple scaling calculations show that this avalanching may greatly enhance the health of the modern glaciers we conclude that in places such as denali unusual combinations of rapid tectonic uplift and great rock strength have created the highest relief in north america by enhancing glacial erosion in the valleys while preserving the peaks

very slow erosion rates and landscape preservation across the southwestern slope of the ladakh range india erosion rates are key to quantifying the timescales over which different topographic and geomorphic domains develop in mountain landscapes geomorphic and terrestrial cosmogenic nuclide tcn methods were used to determine erosion rates of the arid tectonically quiescent ladakh range northern india five different geomorphic domains are identified and erosion rates are determined for three of the domains using tcn 10be concentrations along the range divide between 5600 and 5700 m above sea level asl bedrock tors in the periglacial domain are eroding at 5 0 0 5 to 13 1 1 2 meters per million years m m y principally by frost shattering at lower elevation in the unglaciated domain erosion rates for tributary catchments vary between 0 8 0 1 and 2 0 0 3 m m y bedrock along interfluvial ridge crests between 3900 and 5100 m asl that separate these tributary catchments yield erosion rates 850 ky respectively these results highlight the persistence of very slow erosion in different geomorphic domains across the southwestern slope of the ladakh range which on the scale of the orogen records spatial changes in the locus of deformation and the development of an orogenic rain shadow north of the greater himalaya copyright 2014 john wiley sons ltd

modelling landscape evolution under ice sheets we use an ice sheet model glimmer with an erosion component to examine the evolution of landscapes under ice sheets over long time scales in hypothetical situations glimmer is a fully coupled thermomechanical 3 dimensional ice sheet model with a sliding component the model thus has the ability to predict ice thickness the distribution of areas at pressure melting point and basal velocities the erosion rate is assumed to be a simple linear function of the basal velocity and therefore erosion only occurs where the bed is not frozen and basal sliding is possible in addition sliding velocities are assumed to be related to basal water production thus giving a mechanism for smooth temporal and spatial transitions between eroding and non eroding portions of the bed the eroding glacial landscape system is modelled for long term persistent glaciation assuming a variety of initial landscapes results indicate that without the inclusion of basal water production in the calculation of sliding velocities any thermal feedbacks are exaggerated because of steep horizontal basal temperature gradients additionally bed morphology exerts a greater influence under low basal slip conditions because ice cannot respond readily to thermal instabilities we suggest that erosion and valley overdeepening may aid stabilisation of the thermal regime of an ice sheet and that it is possible to generate glacial signals in previously fluvial systems after only 100 kyr of glaciation at conservative erosion rates

acme a gis tool for automated cirque metric extraction regional scale studies of glacial cirque metrics provide key insights on the palaeo environment related to the formation of these erosional landforms the growing availability of high resolution terrain models means that more glacial cirques can be identified and mapped in the future however the extraction of their metrics still largely relies on time consuming manual techniques or the combination of more or less obsolete gis tools in this paper a newly coded toolbox is provided for the automated and comparatively quick extraction of 16 key glacial cirque metrics including length width circularity planar and 3d area elevation slope aspect plan closure and hypsometry the set of tools named acme automated cirque metric extraction is coded in python runs in one of the most commonly used gis packages arcgis and has a user friendly interface a polygon layer of mapped cirques is required for all metrics while a digital terrain model and a point layer of cirque threshold midpoints are needed to run some of the tools results from acme are comparable to those from other techniques and can be obtained rapidly allowing large cirque datasets to be analysed and potentially important regional trends highlighted

geomorphology and topography of relict surfaces the influence of inherited crustal structure in the northern scandinavian mountains low relief surfaces in northern norway are mapped and analysed to explore 1 whether surfaces were once continuous and possibly correlative recording an ancient relict landscape 2 whether the distribution of now disrupted surfaces reveals a tectonic history related to rifting of the north atlantic and 3 how topography changes across the transition between the north atlantic and barents sea margins elevation contours on surfaces a smoothed fit to mean elevations and histograms of elevation show three distinct coast parallel zones onshore northern norway and a different pattern in coastal lofoten vesteralen a sharp transition from continuous surfaces to lower discretely stepped discontinuous surfaces is located where the crust thins rapidly to the nw from 39 to 100 km north of the senja fracture zone and then decrease gradually further ne the spatial pattern suggests that a continuous low relief relatively low elevation surface formed in late mesozoic time and was preserved despite modification by cenozoic faulting uplift and slow erosion scandinavia9s present day topographic envelope reflects a crustal strength profile set up during earlier hyperextension and maintained by slow neogene erosion rather than resulting from opening of the north atlantic or caused by glacial erosion supplementary material a map of polygons of all surface heights within the study area and data on the location of median centres of polygons assigned attributes and selected zonal statistics are available at https doi org 10 6084 m9 figshare c 3312960

glaciations in response to climate variations preconditioned by evolving topography landscapes modified by glacial erosion show a distinct distribution of surface area with elevation hypsometry in particular the height of these regions is influenced by climatic gradients controlling the altitude where glacial and periglacial processes are the most active and as a result surface area is focused just below the snowline altitude yet the effect of this distinct glacial hypsometric signature on glacial extent and therefore on continued glacial erosion has not previously been examined here we show how this topographic configuration influences the climatic sensitivity of alpine glaciers and how the development of a glacial hypsometric distribution influences the intensity of glaciations on timescales of more than a few glacial cycles we find that the relationship between variations in climate and the resulting variation in areal extent of glaciation changes drastically with the degree of glacial modification in the landscape first in landscapes with novel glaciations a nearly linear relationship between climate and glacial area exists second in previously glaciated landscapes with extensive area at a similar elevation highly nonlinear and rapid glacial expansions occur with minimal climate forcing once the snowline reaches the hypsometric maximum our results also show that erosion associated with glaciations before the mid pleistocene transition at around 950 000 years ago probably preconditioned the landscape producing glacial landforms and hypsometric maxima such that ongoing cooling led to a significant change in glacial extent and erosion resulting in more extensive glaciations and valley deepening in the late pleistocene epoch we thus provide a mechanism that explains previous observations from exposure dating and low temperature thermochronology in the european alps and suggest that there is a strong topographic control on the most recent quaternary period glaciations

late quaternary glaciations in far ne russia combining moraines topography and chronology to assess regional and global glaciation synchrony during various periods of late quaternary glaciation small ice sheets caps fields and valley glaciers occupied the mountains and uplands of far ne russia including the verkhoyansk suntar khayata and chersky mountains the kolyma anyuy and koryak highlands and much of the kamchatka and chukchi peninsulas here the margins of former glaciers across this region are constrained through the comprehensive mapping of moraines from remote sensing data landsat 7 etm satellite images aster global digital elevation model gdem2 and viewfinder panorama dem data a total of 8414 moraines are mapped and this record is integrated with a series of published age estimates n 25 considered to chronologically constrain former ice margin positions geomorphological and chronological data are compiled in a geographic information system gis to produce best estimate reconstructions of ice extent during the global last glacial maximum glgm and to a lesser degree during earlier phases of glaciation the data reveal that much of far ne russia 1 092 427 km 2 preserves a glaciated landscape i e is bounded by moraines but there is no evidence of former ice masses having extended more than 270 km beyond mountain centres suggesting that during the late quaternary the region has not been occupied by extensive ice sheets during the glgm specifically glaciers occupied 253 000 km 2 and rarely extended more than 50 km in length during earlier pre glgm periods glaciers were more extensive though the timing of former glaciation and the maximum quaternary extent appears to have been asynchronous across the region and out of phase with ice extent maxima elsewhere in the northern hemisphere this glacial history is partly explained through consideration of climatic forcing particularly moisture availability solar insolation and albedo though topographic controls upon the former extent and dynamics of glaciers are also considered as are topographic controls upon moraine deposition and preservation ultimately our ability to understand the glacial and climatic history of this region is restricted when the geomorphological record alone is considered particularly as directly dated glacial deposits are few and topographic and climatic controls upon the moraine record are difficult to distinguish 2012 elsevier ltd