scratching technique for the study and analysis of soil surface abrasion mechanism aeolian abrasion is the most fundamental and active surface process that takes place in arid and semi arid environments its nature is a wear process for wind blown grains impinging on a soil or sediment surface which causes particles and aggregates to fracture from the soil surface through a series of plastic and brittle cracking deformation such as cutting ploughing and brittle fracturing using a universal micro tribometer umt a scratching test was carried out on six soil surfaces sandy soil sand loam silt loam loam silt clay loam and silt clay the results indicate that traces of normal and tangential force vs time show a jagged curve which can reflect the plastic deformation and brittle fracturing of aggregates and particles of various sizes fractured from the soil surfaces the jagged curve peaks and the area enclosed underneath may represent the bonding forces and bonding energies of some aggregates and grains on the soil surface respectively connecting the scratching test with an impact abrasion experiment furthermore demonstrates that soil surface abrasion rates are proportional to the square of speeds of impacting particles and to the 2 6 power of mean soil grain size and inversely proportional to the 1 5 power of specific surface abrasive energy or to the 1 7 power of specific surface hardness

the factors influencing the abrasion efficiency of saltating grains on a clay crusted playa the entrainment and subsequent transport of pm10 particulate matter 10 m has become an important and challenging focus of research for both scientific and practical applications arid and semi arid environments are important sources for the atmospheric loading of pm10 although the emission of this material is often limited by surface crusts it has been suggested that the primary mechanisms through which pm10 is released from a crusted surface are abrasion by saltating grains or disturbance by agricultural and recreational activities to examine the importance of saltation abrasion in the emission of pm10 a series of field wind tunnel tests were conducted on a clay crusted surface near desert wells arizona in a previous part of this study it was found that the emission rate varies linearly with the saltation transport rate although there can be considerable variation in this relationship this paper more closely examines the source of the variability in the abrasion efficiency the amount of pm10 emitted by a given quantity of saltating grains the abrasion efficiency was found to vary with the susceptibility of the surface to abrasion the ability of the sand to abrade that surface and the availability of material with a caliper size 10 m within the crust specifically the results of the study show that the abrasion efficiency is related to the crust strength the amount of surface disturbance and the velocity of the saltating grains it is concluded that the spatial and temporal variability of these controls on the abrasion efficiency imposes severe contextual limitations on experimentally derived models and can make theoretical models too complex and impractical to be of use copyright 2001 john wiley sons ltd

the mechanics of continental extension in western north america implications for the magmatic and structural evolution of the great basin abstract a finite element model of continental extension within a rheologically stratified lithosphere is used to examine the structural and magmatic consequences of extensional collapse of thickened crust within the early tertiary north american cordillera crustal thickening creates a weakness in the upper mantle in the model which focuses strain within the great basin during extension marginal highlands corresponding to the sierra nevada and colorado plateau develop near the edges of the weakened region where abrupt changes in the strength of the lithosphere create large gradients in the amount of strain the great thickness of the crust results in widespread ductile flow in the lower crust within the interior of the great basin favoring the formation of metamorphic core complexes during the early stages of extension ductile flow becomes inhibited as the crust thins and cools possibly contributing to the change in the style of extension from low angle detachment faulting to high angle normal faulting during the middle miocene as the lithosphere cools the strength of the uppermost mantle increases after about 10 m y the upper mantle becomes stronger beneath the great basin than beneath adjacent unextended regions strain then begins to migrate outward from the interior of the great basin onto the marginal highlands and their interior edges begin to collapse and are incorporated into the widening great basin after 40 m y of extension the highest rates of strain are localized at the edges of the great basin in positions corresponding to the walker lane tectonic belt of western nevada and the colorado plateau transition zone the weight of the marginal highlands in the model is partially supported by flexure of the strong layer in the uppermost mantle portions of the uppermost mantle which have been flexed upward cool more rapidly than deeper portions creating periodical variations in the strength of the lithosphere which eventually develop into crustal and lithospheric boudinage with a wavelength similar to that observed in regional gravity studies rapid thinning of the model lithosphere during the first 10 m y of extension results in nearly adiabatic decompression of the lithospheric mantle partial melting of the lower lithospheric mantle is likely during this period if volatile phases or mafic lithologies were present this mechanism may account for widespread silicic magmatism in the great basin during the middle tertiary as the region undergoing extension widens the rate of lithospheric thinning decreases and the mantle beneath the great basin begins to cool this leads to a cessation of melting within the lithosphere and waning of silicic volcanism 15 20 m y after the onset of extension the appearance of predominantly basaltic and bimodal volcanic suites in the great basin during the last 5 10 ma correlates well with the onset of decompression melting in the asthenosphere if the mantle potential temperature is greater than about 1350 c approximately 70 c warmer than typical mantle potential temperature

gravitational collapse of orogenic crust a preliminary three dimensional finite element study although gravitational collapse has been widely accepted as a viable explanation for both synorogenic and postorogenic extension its mechanisms have not been well understood previous models of continental deformation that assumed a viscous or viscoplastic rheology are not well suited for simulating stress states and brittle deformation within the upper crust where most extensional collapse occurs here we present preliminary results of a three dimensional 3 d finite element modeling of stress and faulting patterns within the orogenic crust using a viscoelastic rheology major parameters controlling orogenic extension including topographic loading tectonic compression basal shear thermal structure and 3 d tectonic boundary conditions are systematically explored in the model for typical tectonic compression 60 100 mpa and 2 3 km elevation the model predicts synorogenic extension in high plateaus and concomitant thrusting in the lowlands near the foothills viscous relaxation within the ductile crust may amplify deviatoric stresses within the brittle crust by a few times and stress amplification is greater when the crust is hotter and therefore the brittle crust is thinner most synorogenic extension occurs in the direction orthogonal to regional compression while postorogenic extension is more likely to occur normal to the trend of mountain belts we apply the model results to the tibetan plateau and suggest that the age of the north trending grabens is not a reliable proxy for the time when the plateau had reached it present mean high elevation conversely development of the east trending south tibetan detachment system in the miocene by gravitational collapse was more difficult and may require a higher elevation of the himalayas than present or much of the tibetan plateau to have remained low till late miocene

similarities and dissimilarities between the dynamics of sand and dust during wind erosion of loamy sandy soil abstract aeolian dynamics were measured during a wind erosion event on an agricultural field in northern germany because the topsoil was loamy sand particle flow behaved as a sand dust mixture and not as pure sand 63 m or pure dust

the physics of wind blown loess implications for grain size proxy interpretations in quaternary paleoclimate studies abstract loess deposits are recorders of aeolian activity during past glaciations since the size distribution of loess deposits depends on distance to the dust source and environmental conditions at the source during transport and at deposition loess particle size distributions and derived statistical measures are widely used proxies in quaternary paleoenvironmental studies however the interpretation of these proxies often only considers dust transport processes to move beyond such overly simplistic proxy interpretations and toward proxy interpretations that consider the range of environmental processes that determine loess particle size distribution variations we provide a comprehensive review on the physics of dust particle mobilization and deposition furthermore using high resolution bulk loess and quartz grain size datasets from a last glacial interglacial sequence we show that because grain size distributions are affected by multiple often stochastic processes changes in these distributions over time allow multiple interpretations for the driving processes consequently simplistic interpretations of proxy variations in terms of only one factor e g wind speed are likely to be inaccurate nonetheless using loess proxies to understand temporal changes in the dust cycle and environmental parameters requires i a careful site selection to minimize the effects of topography and source distance and ii the joint use of bulk and quartz grain size proxies together with high resolution mass accumulation rate calculations if possible

extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

coupling and decoupling of crust and mantle in convergent orogens implications for strain partitioning in the crust topographic relief strain partitioning and crust mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust this result is obtained by solving analytically for basally driven three dimensional flow within an idealized crust with a laterally invariant depth dependent newtonian viscosity two end member flow states are shown to occur in the first crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust in the second upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust this model is applied to deformation of continental crust above plate like mantle that undergoes oblique convergence across a narrow suture if no significant low viscosity zone develops within the crust crustal deformation is coupled to motion of the underlying mantle and a triangular mountain range develops surface shortening occurs at the foot of the mountains normal to the trend of the range while strike slip and extension are partitioned along the axis of the orogen if a low viscosity zone is initially absent but develops during crustal thickening a steep sided flat topped plateau forms crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau surface shortening normal to the plateau occurs at the foot of the plateau while surface extension normal to the plateau occurs at the plateau edge if a low viscosity zone is present in the lower crust prior to convergence a wide orogen with low topographic relief develops crustal deformation is decoupled from the mantle except at the edges of the deforming region and surface shortening is partitioned uniformly across the belt this study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen similar morphology and strain partitioning can be observed in many real orogenic belts including the eastern alps the tibetan plateau and the lachlan tasman fold belt

an experimental study on the susceptibility of crusted surfaces to wind erosion a comparison of the strength properties of biotic and salt crusts abstract penetrometry has been employed in a number of studies investigating the strength of crusted sediments as related to their stability to particle impact in aeolian systems despite the early success of this procedure questions remain concerning the effect of the angle of penetration the role of fatigue and the relative importance of the ultimate strength as compared to the elasticity of the crust this paper reports on a series of penetrometer and wind tunnel experiments that consider these effects in the context of both salt and biotic crusts the results demonstrate that the angle of penetration is not important but the density of penetrations is the decreased strength observed with high densities of penetration repeated loading and continual impact in wind tunnel experiments provides further confirmation that sedimentary crusts progressively become weaker during mass transport events the elasticity of the surface appears to be at least as important to consider as the ultimate strength although much stronger the salt crusts were found to break down and erode sooner than biotic crusts in wind tunnel studies the notable elasticity of biotic crusts appears to afford them some protection against abrasion

crustal shortening and eocene extension in the southeastern canadian cordillera some thermal and rheological considerations metamorphic core complexes in the southeastern canadian cordillera were formed during eocene crustal extension shortly within a few millions of years after late jurassic paleocene crustal shortening thermal rheological modeling constrained by geological and geochronological studies of the valhalla core complex and other core complexes in this region is used to investigate two major problems concerning the formation of these core complexes 1 the dynamic links between crustal shortening and extension and 2 the cooling history and unroofing rates during extension thermal rheological effects associated with crustal shortening are integrated through the history of crustal compression since crustal shortening in this region was a long and slow process and cannot be treated as an instantaneous event our results suggest that crustal shortening may have played an important role in eocene extension in the southeastern canadian cordillera by 1 producing a thickened and therefore unstable crust and 2 thermally weakening the lithosphere however heat generated by crustal shortening is not enough to account for the thermal state of the valhalla complex and additional heat sources at depth may be necessary we then investigate thermal evolution during extension in both a simple shear model and a progressive pure stretching model we show that the geotherm in an extensional region is time and space dependent and is affected by many variables including the preextensional thermal history and the mode of extension thus caution needs to be exercised when inferring unroofing rates from thermochronologic data the cooling history of the valhalla core complex may be explained by unroofing at rates of 1 2 mm yr

evidence for tibetan plateau uplift before 14 myr ago from a new minimum age for east west extension important changes in south asian climate occurred in the late miocene epoch 8 myr ago 1 2 and these have been attributed by some researchers to uplift of the tibetan plateau at about the same time3 5 unfortunately this link has been difficult to test because the timing of plateau uplift remains poorly constrained by independent evidence one way to determine the minimum age of uplift is to establish the initiation age of the north striking normal fault systems in southern tibet that are widely regarded6 10 as being related to gravitational collapse of the tibetan plateau here we report an40ar 39ar age of 14 myr for hydrothermal mica from an extensional fracture belonging to such a fault system in north central nepal this age implies that east west extension began before 14 myr ago in at least some parts of the tibetan plateau suggesting that the plateau attained its high mean elevation well before late miocene time

evaluation of a wind erosion model in a desert area of northern asia by eddy covariance for the first time vertical fluxes of mineral dust measured by eddy covariance in two desert sites of northern asia have been used to test the performances of a wind erosion model in the field soil parameters required by the model were obtained through field and laboratory determinations model predictions and direct measurements have been compared the main finding was that the direction of the horizontal wind relative to the orientation of nebkhas played a crucial role in determining the emission of particles in one of the investigated sites being unable to simulate such interaction the model generally overestimated the actual emission it provided instead reliable predictions r2 0 87 when the wind direction was suitable in detaching loose erodible elements placed on nebkhas thanks to their normal orientation copyright 2009 john wiley sons ltd

the evolution of deformation and topography of high elevated plateaus 1 model numerical analysis and general results a temperature dependent viscoplastic flow model of continental lithosphere is used to investigate the evolution of deformation and topography of high elevated plateaus such plateaus are products of both continent continent collision tibetan plateau and ocean continent collision the altiplano and develop in the overriding continental plate in this study we emphasize the mechanically simpler case of oceanic collision because it does not involve mass transfer between the two plates the lithosphere is deformed in response to tectonic and buoyancy forces the tectonic forces arise from subduction of an oceanic plate or underthrusting of continental lithosphere that horizontally indents and vertically shears the overriding lithosphere the buoyancy forces arise in response to horizontal density variations and tend to relax existing topography or thick crust the time evolution of the deformation and topography is investigated using a finite element technique that solves for the flow field in the overriding lithosphere the model produces dynamically supported near trench topography and inland mountain topography that is isostatically supported by a thick crust a finite region of localized deformation thick crust and high topography develops only if the model includes a horizontal thermal perturbation or an initially thick crust however only thermally perturbed lithosphere generates a plateau topography the shape and size of the calculated plateau depend on the wavelength of the thermal perturbation grashof number and density contrast between the crust and mantle the time evolution of the deformation shows a significant change in the deformation pattern as the high elevated plateau evolves during early stages compressional deformation of the crust and mantle are localized in the thermally perturbed weak zone at later stages as the crust thickens buoyancy forces of larger magnitude resist further thickening of the crust and the locus of compressional crustal deformation migrates inland this migration does not affect the location of the mantle deformation which remains in the thermally weak region but it is accompanied by a significant shear deformation in the weak lower crust the separation of the crustal locus from the mantle locus of deformation emphasizes the importance of vertically dependent deformation in the formation of high elevated plateaus this demonstrates the limitations of models that ignore changes of deformation with depth such as plane stress or thin sheet models

mechanical model for subduction collision tectonics of alpine type compressional orogens alpine type orogens are characterized by three distinct convergent tectonic phases subduction with deformation that has primarily single vergence a transition from subduction to collision and continental collision with double vergence although the cenozoic history of the european alps has additional complexities a mechanical explanation for these three phases would provide the necessary crustal scale framework in which to develop an understanding of the smaller scale processes we present results from a simple numerical model which explain the mechanics of these three phases as a consequence of the changing buoyancy of the lithosphere subducted beneath the orogen the development and exhumation of a subduction complex suture zone and basement nappe stack piemont suture penninic nappes the presence of a crustal scale back fold and thrust insubric line and uplift of basement on the pro european side of the orogen external basement massifs may be explained as a simple consequence of changing dynamics during the transition from subduction to collision

evolution of lithospheric strength after thrusting modeling of an eroding thrust belt indicates that immediately after thrusting the lithosphere is about twice as strong as unthickened cratonic lithosphere but after 10 to 20 m y it becomes weaker than cratonic lithosphere reaching a minimum of 40 to 70 of cratonic strength 30 to 100 m y after thrusting this may explain why rifting of thrust belts commonly occurs several tens of million years after thrusting ceases relative weakening results from 1 elevation of the geotherm caused by burial of radioisotope enriched surface rocks and 2 replacement of strong mantle rocks by weak crustal rocks in the lithospheric column low denudation rates enhance weakening by allowing radioisotope enriched rocks to remain buried longer our modeling indicates that 1 magmatism after thrusting is favored by high concentrations of heat producing elements in the lower plate and by slow denudation rates and 2 factors that affect denudation rate such as climate and the development of external drainage can significantly influence evolution of a thrust belt

lunar tide in the thermosphere and weakening of the northern polar vortex the lunar tide and its variability are beginning to be realized as elements of ionosphere thermosphere weather herein we demonstrate that an enhancement in m 2 amplitude at 110 km in the dynamo region is related to the timing and intensity of stratospheric polar vortex weakening pvw timing of the pvw which typically occurs during january february changes on the order of 4 7 d yr between 2002 and 2013 and the m2 enhancement changes with it with a time delay of order 2 8 days the m2 temperature amplitude at 110 km as determined from the sounding of the atmosphere using broadband emission radiometry saber measurements on the thermosphere ionosphere mesosphere energetics dynamics timed spacecraft increases linearly from about 20 k to 29 k as the westward zonal mean zonal wind increases from about 10 m s 1 to 80 ms 1 at 70 n and 48 km altitude in accord with pvw events from year to year according to modern era retrospective analysis for research and applications merra data these relationships point to a level of predictability regarding impacts of the lunar tide on thermosphere and ionosphere densities winds and plasma drifts

the emission and vertical flux of particulate matter 10 mum from a disturbed clay crusted surface arid and semi arid environments are important sources for the atmospheric loading of pm10 particulate matter 10 lm although the emission of this material is often limited by surface crusts this study investigates the emission and vertical flux of pm10 from a clay crusted playa with and without saltating grains to abrade the surface using a portable field wind tunnel it was found that despite disturbance to the surface the emission of pm10 decays rapidly without abrasion only in the presence of saltating grains was pm10 continuously liberated from the surface such that the emission rate the total amount of pm10 emitted from the surface expressed as a horizontal flux varied linearly with the saltation transport rate although the emission of pm10 was found to depend on saltation abrasion past studies have tended to focus on the relationship between the vertical flux of pm10 the amount of pm10 being transported vertically through the boundary layer and the shear velocity in this study the vertical flux of pm10 was found to vary with the shear velocity to the power of 2ae14 although the vertical pm10 flux is a proportion of the emission rate the horizontal flux no statistically significant relationship was observed between the emission rate and the shear velocity the disparity of these results is explained by the lack of a consistent relationship between the shear velocity and the saltation transport rate in this supply limited environment this suggests that the observed relationship between the vertical pm10 flux and the shear velocity is a spurious correlation resulting from the use of shear velocity to calculate the vertical dust flux it is thus concluded that shear velocity is not an appropriate variable for emission modelling in supply limited environments and that improvements in dust emission modelling will only be realized if the abrasion process is the focus of a concerted research effort

effects of soil moisture and dried raindroplet crust on saltation and dust emission 1 field experiments using a sand particle counter and an optical particle counter clarified the links between saltation and dust emission under wet and weakly crusted conditions in a fallow field previously cultivated with wheat in australia a crust was formed by the impact of raindroplets after small precipitation events a little soil moisture enhanced the strengths of crust and aggregation even though the soil was dried dust concentration was proportional to friction wind velocity but the proportionality was dependent on ground surface conditions such as the minimally dispersed particle size distribution of parent soil and the presence or absence of crust

active deformation of asia from kinematics to dynamics using estimated strain rates across the actively deforming region of asia to infer variations in viscous stress it is shown that gradients of stress point uphill toward the center of tibet where gravitational potential energy per unit surface area reaches a maximum thus the dynamics of the deformation seem to obey the equation of creeping flow which expresses a balance between gradients in stress and the gravitational body force this balance in the region of the tibetan plateau yields an estimate of 10 22 pascal second for the average viscosity of the tibetan lithosphere which is only about 10 to 100 times greater than the viscosity of the convecting upper mantle

aeolian processes and the biosphere ravi s d odorico p breshears d d field j p goudie a s huxman t e li j okin g s swap r j thomas a d pelt s v whicker j j zobeck t m 2011 aeolian processes and the biosphere reviews of geophysics 49 3

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

new views of mars eolian activity materials and surface properties three vignettes from the mars global surveyor mars orbiter camera prior to the mars global surveyor mgs mission a very general view had emerged in which martian surface materials were seen as consisting of a mixture of bright dust dark sand and rocks the configuration of these materials and the winds that transport and deposit sand and dust have been thought to be directly linked to the albedo patterns that have been observed on mars for centuries high spatial resolution images 1 4 20 m pixel obtained by the mgs mars orbiter camera moc between september 15 1997 and july 4 1999 provide new information about the physical nature of the windblown material on the martian surface and show that the pre mgs view was much too simple in addition to bright dust and dark sand moc images show evidence of bright sediment that can be transported by saltation e g sand and dark material that can be transported in suspension e g silt new views of eolian wind streaks in daedalia planum show that part of this region thought to be mantled by bright dust based upon viking and mariner observations is instead covered by a thin veneer of bright windblown sand moc images of sinus sabaeus and parts of syrtis major two regions thought to be sandy based upon viking era observations exhibit thick mantles that are inferred to consist of fine grained sediment deposited from eolian suspension low albedo wind streaks in western arabia terra are also dark mantles and their association with eroded crater floors and megaripples dunes on these floors suggest that these particular wind streaks are deposits of silt sized material that was only briefly suspended before settling to the surface moc images also show evidence that some eolian dunes are active on mars today i e in 1998 and 1999 the evidence for activity is largely based upon wind and avalanche induced streaks superposed on or eroded into seasonal frost on high latitude dune fields moc images show that some other dunes are inactive but the albedo of dunes relative to surrounding terrain is not a good indicator of dune activity because some inactive dunes are not mantled by dust inactive dunes are best identified by superposed features such as impact craters landslide deposits and yardangs

evaluating the role of preexisting weaknesses and topographic distributions in the indo asian collision by use of a thin shell numerical model thin viscous sheet models have proved to be very useful in exploring the interaction between plate boundary and gravitational forces during continental collision however simplifications of these models e g absence of faults planar geometry of the collision zone limit their use in making specific predictions regarding tectonic evolution such as the role of eastward extrusion along major strike slip faults during the indo asian collision this deficiency is overcome by a thin shell finite element model with faults that can assess the effects of preexisting fault configurations and topographic distributions on velocity fields numerical simulations of a palinspastically restored asia at ca 50 ma suggest that preexisting lithospheric weaknesses favor north south shortening during initial collision whereas preexisting high topography in southern asia promotes eastward extrusion these results underscore the first order importance of initial topography and the distribution of preexisting faults in the outcome of geodynamic modeling

a wind tunnel study of particle kinematics during crust rupture and erosion abstract sediments with protective crusts of varying type were subjected to particles fed from an upwind source during wind tunnel experiments carried out to compare their ability to resist erosion and alter the kinematics of the saltation cloud a laser doppler anemometer measured the distribution of particle velocity for saltators impacting each crusted surface and for particles ejected from this same surface inclusive of ricochets biotic crusts grown on sand in an environmental chamber were able to withstand erosion over several hours of continuous particle impact as compared to brittle salt crusts which regardless of wind speed or sodium chloride concentration eroded fully within one half hour despite the appearance of deep pits and grooves on the ruptured crusts loss of mass dominated over particle trapping on these rough surfaces while decreasing salt concentration between 320 g kg 1 and 80 g kg 1 generally was found to be associated with an increase in the mass flux of particles ejected from the surface little to no correlation with either wind speed or particle impact speed was observed the range over which varying salt concentration affects the momentum of ejected particles is rather narrow within 160 g kg 1 temporal changes in the velocity distribution of ejected particles with crust rupture and deflation are complicated by variations in crust strength with depth this is especially true of biotic crusts and weak salt crusts fast moving particles associated with the upper 40 of the cumulative velocity distribution generally demonstrate little variation from the control surfaces and probably represent ricochets in comparison those within the lower 60 of the distribution are significantly affected by crust rupture and erosion and may include low energy rebounds as well as the entrainment of new particles loosened from within the crust while such measurements are exceedingly rare they are needed for the validation of physically based models of crust erosion

meteorological controls on wind erosion during foehn wind events in the eastern southern alps new zealand foehn winds in the lee of mountain barriers create highly favourable meteorological conditions for wind erosion in alpine areas and over adjacent lowlands this paper presents meteorological observations made during winter and summer foehn wind erosion events as part of a 3 year investigation into the physical nature and incidence of eolian processes in the eastern southern alps new zealand foehn winds were observed to initiate eolian activity including dust storm genesis when mean near surface wind speeds exceeded 7 8 m s 1 over a 20 min period in the absence of precipitation wind speeds of 25 30 m s 1 were frequently recorded during typical foehn events and wind speed maxima of 40 50 m s 1 were monitored during severe foehn windstorms saltation clouds sampled at 0 5 and 1 m above glaciofluvial deposits during foehn wind erosion events were found to display a mean grain size of between 300 and 435 m the entrainment of fine grained surface sediments by the airstream appeared to be enhanced in late autumn may and early spring september by freeze thaw cycles that were observed to result in needle ice growth in exposed glaciofluvial and lacustrine deposits the needle ice fragmented such deposits producing surfaces that were aerodynamically rough and highly susceptible to deflation by the airstream following a thaw observations made by this study are thought to be similar to foehn wind erosion events reported in the lee of other substantial mountain ranges such as the rocky mountains andes hindu kush and karakoram

role of oblique convergence in the active deformation of the himalayas and southern tibet plateau noting similarities with subduction along curved oceanic trenches and using a simple block model we show that radial vergence evident in earthquake slip vectors along the himalayan deformation front east west extension on north trending normal faults in the himalayas and southern tibet and right lateral strike slip on the karakorum jiali fault zone can all result from basal shear caused by the indian plate sliding obliquely beneath tibet along a gently dipping arcuate plate boundary within the framework of this mechanism the normal faults in the himalayas and southern tibet are not proxies for the uplift history of tibet the distribution and style of the faults in the himalayas and southern tibet suggest that the basal drag from the underthrusting indian lithosphere extends northward beneath most of southern tibet

a study of particle splash on developing ripple forms for two bed materials a custom designed particle tracking velocimetry ptv system was developed to measure particle splash dynamics in two materials mixed quartz sand and ground acrylic particles approximating the density of snow the strengths and limitations of the ptv system are described in the context of five experiments carried out at friction velocities slightly greater than the threshold friction velocity for particle entrainment and saltation analysis of particle collisions on the bed surface splash demonstrates that the simulated snow particles impact the surface at a smaller angle than quartz sand they also eject a greater number of particles though at lower velocities and angles than observed for the sand particles these relations appear to be highly sensitive to the porosity of the bed surface while particle shape and elasticity play a secondary role with regard to ptv measurements carried out along the wind aligned axis of migrating aeolian ripples particles ejected from the lee slope tend to be slightly larger than those on the stoss slope and in general are ejected with a lower speed and greater angle to the surface irrespective of the type of granular material when the average impact angle is close to or greater than the negative slope of the evolving bedform the particle ejection rate from the stoss slope was measured to be approximately twice that for the lee slope this empirical observation confirms the assumption made in models of ripple development that the ejection flux governs the bedform migration rate however systematic under prediction of this rate by an early model suggests that additional processes inclusive of surface creep must also be accounted for

parallel thrust and normal faulting in peru and constraints on the state of stress abstract we present evidence for recent normal faulting in the cordillera blanca of peru along planes trending approximately n40 w parallel to the chain this type of faulting suggests that there is an important component of extension perpendicular to the chain and contrasts markedly with the underthrusting of the nazca plate on the west side of peru and with the folding and thrusting observed in the sub andes on the eastern flank of the chain these contrasting styles of deformation and associated stress fields can be explained by buoyancy forces due to the high mountains and crustal roots in so far as this explanation is correct the difference in elevation between zones of normal and thrust faulting can be used to place an upper bound of 500 bars on the average stress difference in the crust

when did the roof collapse late miocene north south extension in the high himalaya revealed by th pb monazite dating of the khula kangri granite th pb ion microprobe measurements made on 12 monazite grains from the khula kangri granite tibet bhutan frontier are interpreted to indicate that crystallization occurred at 12 5 0 4 ma the leucogranite is cut by the gonto la detachment part of the southern tibet detachment system that has allowed upper level north directed extension of the himalayan orogen significant orogen normal extension in southern tibet appears to have continued 8 10 m y later than previously recognized this is the first reported crystallization age for a leucogranite east of the yadong cross structure an apparent 70 km offset of the high himalaya and southern tibet detachment west of the yadong cross structure reliable ages for high himalaya events major main central thrust slip granite generation and emplacement attainment of critical topography and major detachment extension group between ca 24 and 19 ma we interpret the west to east change across the yadong cross structure to be due to either 1 an abrupt 10 m y younging of principal high himalayan events or 2 a deeper thus younger exposed part of the footwall of the southern tibet detachment near khula kangri the southern tibet detachment is cut by the highly oblique yadong gulu rift a manifestation of tibet plateau east west extension integrated estimates of magnitude and rate of detachment displacement suggest that the observed postcrystallization north directed extension lasted for 1 3 m y after which time the yadong gulu rift formed this interpretation is consistent with initiation of east west extension of tibet at ca 8 ma

collisions of quartz grains with a sand bed the influence of incident angle the velocities with which grains were observed to emerge from a sand bed after an intersaltation collision at u 40 cm s 1 are reported for four bed attitudes from horizontal bed to adverse bed slope 15 the principal effect of bed angle is to alter the magnitude and direction of the ricochet velocity however emergent velocities of dislodged grains are consistent with reptation path lengths comparable to the length of the upwind face of ripples in the corresponding wind calculations of the loss of forward momentum at collision using the data for the range of bed attitudes studied suggest that creep is most vigorous on the sloping upwind face of the ripple and diminishes at the crest as a result the crest would be expected to accumulate the coarse material which moves predominantly by creep the saltations originating in ricochet from the sloping back of the ripple are more vigorous and more concentrated in plan than are those originating at the crest however the saltation path length is at least an order of magnitude greater than the ripple wavelength and the probability distribution of path lengths is quite dispersed consequently it is very unlikely that these spatial patterns of ricochet are preserved sufficiently distinctly in the saltation cloud and subsequent collision distribution to be the agent of ripple development this study therefore supports a view of moving grain interaction with the bed in which saltation provides the power to mobilize grains but ripple growth is associated with reptation and particularly with a pattern of impact which develops with the bed relief creep is more active on upwind facing slopes than at the crest which therefore is a zone of net creep grain deposition

a wind tunnel study of the resilience of three fungal crusts to particle abrasion during aeolian sediment transport abstract this study is a comparison of the strength stability and resistance to abrasion of monospecific sand crusts formed by three species of free living fungi aureobasidium pullulans trichoderma harzianum and absidia corymbifera compared to three photoautotrophs examined in a previous study the fungal crusts were thicker and stronger in flexure by a factor of two or more in addition to the morphological properties of these fungi the organic substrate required for growth contributes to the total crust strength wind tunnel tests in which the crusts were subjected to particle impact from an upwind sediment source nominally 0 014 kg m 1 s 1 and varied levels of freestream velocity u showed that crusts comprised of trichoderma and absidia were stable at wind velocities under 10 m s 1 crusts of aureobasidium generally ruptured at velocities well below this level and to a much greater extent the distinctive morphologies and metabolic rates of the three fungi appear to explain these differences as compared to the wind tunnel experiments independent tests of crust strength in flexure which were intended to simulate crust flaking were found to be less sensitive indicators of relative crust stability in aeolian settings analysis of the time rate of expansion of the perimeter of crust loss indicates that this relation is approximately exponential with the exception of low wind velocities near the threshold for grain ejection where the momentum of the saltators at impact appears to be of prime importance

tectonomagmatic evolution of cenozoic extension in the north american cordillera the spatial and temporal distributions of cenozoic extension and magmatism in the cordillera suggest that the onset of major crustal extension at a particular latitude was confined to a relatively narrow belt 100 km pre extension and followed the onset of intermediate and silicic magmatism by no more than a few million years extension began in early eocene time in southern british columbia northern washington idaho and montana farther s extension began at about the eocene oligocene boundary in the great basin and slightly later in the mojave sonora desert region the intervening area at the latitude of las vegas remained quiescent until mid miocene time compositional and isotopic characteristics of most pre miocene magmas are consistent with their containing major components of melted continental crust in mid miocene time two major changes occurred widening of the area of extension and the widespread appearance of basaltic magmas the area affected by extension from southwestern montana to the lake mead region widened to several hundred kilometres by this time extension in southern british columbia northern washington and northern montana had ceased probably before the end of the eocene and extension s of lake mead except in the gulf of california had waned regions affected by the broader belt of extension during late miocene pliocene and quaternary time experienced basaltic magmatism which began along a central rift zone in the northern part of the region and which had within a few million years spread to include most of the region later basaltic activity has tended to concentrate in restricted zones especially near the margins of the extended area we recognize a correlation between the amount of earlier crustal thickening and cenozoic extension and between the length of time after shortening but before extension and the degree to which a given region was intruded by late cretaceous plutons the localization of extension in areas of previous crustal thickening and the dependence of the timing of extension on the thermal state of the overthickened crust is consistent with a simple thermal mechanical model developed in a companion paper sonder et al this raises the possibility that stresses inherent in the north american plate dominated over plate interaction forces as controls of the cenozoic tectonomagmatic evolution of the north american cordillera especially in its earlier stages

the aeolian dust accumulation curve this article presents a simple physical concept of aeolian dust accumulation based on the behaviour of the subprocesses of dust deposition and dust erosion the concept is tested in an aeolian dust wind tunnel the agreement between the accumulation curve predicted by the model and the accumulation curve obtained in the experiments is close to perfect and shows that it is necessary to discriminate between the processes of aeolian dust deposition and aeolian dust accumulation two important thresholds determine the accumulation process for wind speeds below the deflation threshold the aeolian accumulation of dust increases linearly with the wind speed for wind velocities between the deflation threshold and the accumulation limit the sedimentation balance is above unity and there is still accumulation though it rapidly drops once the deflation threshold has been exceeded at wind speeds beyond the accumulation limit the sedimentation balance is below unity and there will no longer be an accumulation of dust the thresholds have been determined in a wind tunnel test at friction velocity u 0 34 m s 1 deflation threshold and u 0 43 m s 1 accumulation limit but these values are only indicative since they depend heavily on the characteristics of the accumulation surface and of the airborne grains

mantle plume influence on the neogene uplift and extension of the u s western cordillera espite its highly extended and thinned crust much of the western cordillera in the united states is elevated more than 1 km above sea level therefore this region cannot be thought of as thick crust floating isostatically in a uniform mantle rather the lithospheric mantle and or the upper asthenosphere must vary in thickness or density across the region utilizing crustal thickness and density constraints we modeled the residual mass deficit that must occur in the mantle lithosphere and asthenosphere beneath the western cordillera a major hot spot broke out during a complex series of cenozoic tectonic events that included lithospheric thickening backarc extension and transition from a subduction to a transform plate boundary we suggest that many of the characteristics that make the western cordillera unique among extensional provinces can be ttributed to the mantle plume that created the yellowstone hot spot

threshold friction velocities and rupture moduli for crusted desert soils for the input of soil particles into the air desert soils having clay crusts mostly from the mojave desert were tested for threshold friction velocity the friction velocity at which soil erosion begins with an open bottomed wind tunnel the soils were also tested for content of clay water soluble material calcium carbonate organic material mineralogy of clay and of salts soil moisture modulus of rupture and crust thickness if no loose material existed on the soil surface crusts having modulus of rupture greater than 0 7 bar and crust thickness of 0 7 cm to 0 3 cm were effective in protecting against wind erosion disturbed clay crusts having modulus of rupture before disturbance greater than 2 bar with thickness less than 1 9 cm did not experience significant wind erosion modulus of rupture was related to composition of soil but was shown to depend mostly on clay content soil composition is related to modulus of rupture in an empirical equation

erodibility of some crust forming soils sediments from the southern aral sea basin as determined in a wind tunnel severe dust storms in the southern aral sea basin have become common with the desiccation of the sea the high incidence of dust in the area has had severe ecological consequences within the framework of efforts to reduce this phenomenon deflatability as well as deflatability related characteristics of some prominent soils sediment surfaces in the southern aral sea basin were examined the materials included a salt crust from a developed solonchak a takyr crust and a takyr like soil and salt crusts from undeveloped solonchaks formed on the exposed bottom of the aral sea characteristics determined were particle size distribution dry aggregate size distribution and salt carbonate and organic carbon contents deflatability was examined using a suction type wind tunnel with a sensit type sensor to detect airborne unconsolidated material on materials treated to different moisture levels and with a chemical stabilizer and on restored crusts created from the unconsolidated materials fine sand dominates in the materials and in the takyr crust and takyr like soils is accompanied by significant amounts of silt and clay all materials contain moderate amounts of carbonate and are low in organic matter all soils sediments contain salts but in the salt crusts of the solonchaks the salt fraction dominates they all have more than 50 per cent pm850 particles with diameter 850 m indicating a relatively high deflatability potential the materials from the takyr crusts and takyr like soil with a high proportion of fine aggregates had the lowest threshold friction velocities while the salt crusts of the solonchaks with a high proportion of coarse aggregates had the highest this suggests that takyrs and takyr like soils are the most deflatable and solonchak soils the least deflatable these differences are attributed to the presence of salts that create stable large aggregates in the solonchak crusts wetting of the materials to three moisture levels considerably increased threshold friction velocity the increase was most prominent in the salt rich materials and was attributed to the rapid formation of surface films by drying in the course of the wind tunnel determinations applications of chemical stabilizers at two levels also considerably increased threshold friction velocity on the restored crusts threshold friction velocity dramatically increased occasionally to non recordable values this increase was monitored with both the salt crusts characteristic for the solonchak soils and the fine grained crusts characteristic for the takyr soils the stability was attributed to the tightly packed salt particles in the salt crusts and to the cohesive properties of the fine grained materials in the takyr crusts once the crusts were ruptured however strong deflation commenced these results suggest that by maintaining moisture in the soils sediments for example by maintaining a high water table in the amu darya river flood plain deflation can be reduced by the same means deflation can be reduced by creating new crusts or by preserving existing crusts copyright 2005 john wiley sons ltd

an analysis of soil crust strength in relation to potential abrasion by saltating particles wind erosion depends on the ease with which particles can be detached from the soil surface but suitable tests to characterize this property are not available two possible methods to determine surface soil strength in the field were therefore compared on a range of artificially crusted surfaces these were made by spraying or tension wetting aggregates 10 2 2 0 5 and 0 5 mm from a structurally unstable sandy loam followed by drying each test involved measuring the force exerted on a probe driven at a steady rate into the surface using either a flat tipped 0 6 mm diameter penetrometer or a flat ended cylindrical punch with inner and outer diameters of 5 and 6 mm respectively both probes showed that crusts could be produced reproducibly depending on the probe and aggregate size penetration mainly occurred either as a result of aggregates being deflected out of the pathway of the probe or by genuine rupture of aggregates or of the crusted surface the penetrometer because it was comparable to the size of sand grains gave results that can be used to characterize surface erodibility to saltating particles the punch gave results that would be unsuitable for this purpose as would other strength tests that are on too large a scale penetrometer results were analyzed to calculate the energy required for penetration it was thus possible to demonstrate that only the spray wetted fine aggregates had a surface that could undergo large scale rupture by saltating sand grains for all other surfaces saltating particles would be unable to supply sufficient energy to rupture aggregates or the crusted surface erosion could only occur by a slower process of abrasion in which smaller particles or aggregates are chipped away from the surface however it is shown that saltating particles could rupture the interaggregate bonding in the 2 0 5 mm aggregate surfaces thus permitting creep an alternative and potentially simpler way of characterizing surface erodibility by using a surface modulus of elasticity is also discussed our results demonstrate that the small diameter penetrometer is a promising technique for characterizing erodibility of aggregated and crusted surfaces 1997 by john wiley sons ltd

deformation of leucogranites of the crystalline main central sheet in southern tibet china abstract towards the top of the main central sheet in southern tibet china leucogranites intrude metamorphic rocks related to rocks of the main central thrust in the himalayas our study shows that the leucogranites suffered a northward shear deformation the significance of this event is discussed in terms of gravity driven decollement of the 10 000 m thick tethyan sedimentary pile over its crystalline basement

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

initiation of asymmetric extension in continental lithosphere govers r and wortel m r 1993 initiation of asymmetric extension in continental lithosphere in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 x 96 the physical conditions are investigated under which the lithospheric scale style of extension is pure shear or simple shear we focus on the initial stages of continental extension to monitor how symmetric or asymmetric modes of extension evolve from specific tectonic conditions continental collision magma intrusions and interaction between the lithosphere and the underlying mantle are investigated as sources for extension we use a finite element method to model the thermo mechanical evolution of continental lithosphere experimental flow laws are used to model the elastic hutle power law creep or diffusion creep rheology of lithospheric rocks our results indicate that if in plane forces change from compressive to tensile immediately after a rapid mountain building phase initiation of a lithosphere scale detachment fault is possible we find a strong dependence of the extensional style on the distribution with depth of residual stresses from the collision phase this result is consistent with observations of gravitational collapse in regions like the aegean and the basin and range province where detachment faults have exhumed lower crustal rocks the predicted dip direction of the fault also agrees with observations in these areas intrusion of magma into continental lithosphere which is subject io in plane tensile forces will cause localization of pure shear deformation the style of deformation resulting from mantle plumes impinging to the base of the lithosphere is symmetric delamination of lithospheric mantle may initiate detachment faults if delamination occurs at the end of a collision phase when in plane forces change sign from compressive to tenstle this result also strongly depends on the assumed residual stress distribution if delamination occurs during the mountam huilding phase the style of deformation will be pure shear another interesting outcome from our modeling is that dramatic strain weakening as a result of a deformation mechanism change from dislocation creep to diffusion creep reduces the tendency to strain localization a large volume of the published work on continental extension has been concentrated on quantifying the stretching factor p of the pure shear model of mckenzie 1978a in some basins it was found that the kinematics of deformation could be more accurately described by depth dependent stretching factors royden and keen 19801 which introduced asymmetry near basin edges in the simple shear model of wemicke

temporal aspects of the abrasion of microphytic crusts under grain impact wind tunnel simulations of the response of two moss crusts to grain impact indicate that given sufficient time these surfaces will deteriorate under very low wind velocities only slightly above u t for the loose saltating grains in parallel with these experiments the frequency distributions of ultimate strength and penetration energy were determined for each of the two crust types via penetrometry pohlia was found to be stronger than tortula but even so both of these crusts had ultimate strengths 20 350 times higher than the force delivered by a single grain impacting each surface at a velocity of 1 ms 1 in comparison the modulus of deformation and penetration energy data were very similar for the two surface types especially for the weakest areas of crust development this observation is in accord with the wind tunnel simulations that also found no consistent difference in the response of these two crust types to impact in comparison with crusts formed by clay and salt fibrous microphytic crusts are morphologically complex and typically weak the notable elasticity of these surfaces does reduce the force of grain impact and thereby provides some protection against rupture one of the central conclusions of this study suggests that not only is the particle kinetic energy at impact important in crust breakdown but also tiny fractures at points of localized stress concentration contribute to a progressive reduction in the integrity of the filament net in some of the experiments conducted as part of this study up to 50 or more minutes of constant bombardment was required to produce small abrasion marks on selected areas of the microphytic crust this study prepares a foundation for future experiments needed to examine the breakdown of complex crusts formed in nature copyright 2002 john wiley sons ltd

climate induced sediment palaeosol cycles in a late cretaceous dry aeolian sand sheet marilia formation north west bauru basin brazil aeolian sand sheets which are characterized by low relief surfaces that lack dunes are common in arid and semi arid climatic settings the surface of an aeolian sand sheet can either be stable and subject to pedogenetic effects or unstable such that it is affected by deflation or sedimentation the maro lia formation late cretaceous may be interpreted as an ancient aeolian sand sheet area where alternating phases of stability and instability of the accumulation surface have been recorded detailed field studies were carried out in several sections of the maro lia formation where cyclic alternations of palaeosols and aeolian deposits were evident using palaeopedological and facies analysis methods supported in the laboratory by the analysis of rock samples cut and polished in slabs thin sections scanning electron microscope images and x ray diffraction data from the clay minerals the deposits comprise three lithofacies that in order of abundance are characterized by i translatent wind ripple strata ii flood deposits and iii ephemeral river channel deposits palaeosols constitute on average 65 of the vertical succession three types of palaeosols pedotypes are recognized i aridisols ii entisols and iii vertisols erosional surfaces due to aeolian deflation divide the top of the palaeosol profiles from the overlying aeolian deposits the palaeoenvironmental interpretation of the deposits and the palaeosols allows the depositional system of the maro lia formation to be defined as a flat area dominated by aeolian sedimentation with subordinate ephemeral river sedimentation and characterized by a dry climatic setting with occasional rainfall the climate is the main forcing factor controlling the alternation between episodes of active sedimentation and periods of palaeosol development a climate controlled model is proposed in which i the palaeosols are indicative of a stable surface that is developed during the more humid climatic phases and ii the erosional surfaces and the overlying aeolian sediments attest to periods of deflation and subsequent sedimentation thereby increasing the availability of sediment during the drier climatic phases the ephemeral fluvial deposits mark the more humid climatic conditions and contribute to the lagged sediment influx caused during the drier periods by the erosion of previously stored sediment

did the indo asian collision alone create the tibetan plateau it is widely believed that the tibetan plateau is a late cenozoic feature produced by the indo asian collision however because tibet was the locus of continental accretion and subduction throughout the mesozoic crustal thickening during that time may also have contributed to growth of the plateau this portion of the geologic history was investigated in a traverse through the central lhasa block southern tibet together with earlier studies our mapping and geochronological results show that the lhasa block underwent little north south shortening during the cenozoic rather our mapping shows that 60 crustal shortening perhaps due to the collision between the lhasa and qiangtang blocks occurred during the early cretaceous this observation implies that a significant portion of southern tibet was raised to perhaps 3 4 km elevation prior to the indo asian collision

effect of soil crusting on the emission and transport of wind eroded sediment field measurements on loamy sandy soil field data are reported for the horizontal and vertical flux of wind eroded sediment on an agricultural field in northern germany measurements were made during a windstorm that hit the region on 18 may 1999 the magnitude of both fluxes was significantly affected by the presence of a surface crust covering the test field measuring the physical crust strength at 45 locations with a torvane the relationships between crust strength and the horizontal fh and vertical fv sediment fluxes were investigated both fluxes decreased as the surface crust became stronger the decay behaved as an exponential function for both types of flux the horizontal sediment flux over a crusted surface can be accurately predicted by completing marticorena and bergametti s journal of geophysical research 100 1995 16415 erosion model with a crust function the vertical particle flux over crusted soil can be calculated by adding a similar function to alfaro and gomes s journal of geophysical research 106d 2001 18075 dust production model the study also suggests that the gradual bombardment of a surface crust by impacting particles does not immediately result in a decay of the crust s protective effect provided that the crust has a minimum thickness however once the crust becomes perforated its protective effect disappears very quickly leading to much higher horizontal and vertical sediment fluxes than predicted for undamaged crusted soil

a conceptual model of wind erosion of soil surfaces by saltating particles a conceptual model is described for the prediction of wind erosion rates dependent on the distribution of impact energy delivered to the surface by saltating grains p ei and the distribution of local surface strength p es methods are presented for the measurement of both distributions and consequent loss of material from the bed it is concluded that saltating sand grains can rupture weak crusts under even moderate wind conditions and that the rate of erosion will depend on the shape of the distribution tails copyright 1999 john wiley sons ltd

temperatures heat flux and frictional stress near major thrust faults we examine the two dimensional advective and conductive transport of heat in a region of thrust faulting both simple theoretical considerations and numerical experiments show that the steady state temperatures near the fault are reduced by a divisor s 1 b z vsin below what they would be with the same heat sources but in the absence of advection in this expression z is the depth to the fault v is the slip rate is the component of the dip of the fault in the direction of underthrusting is thermal diffusivity and b is a dimensionless factor that is essentially equal to one for most forms of heating initial changes in temperatures near the fault are given by t z t t z 0 12vsin t z 0 z these simple formulae are successful because of the neglible influence of lateral conduction of heat at least for slip rates of a few mm yr or more two time constants govern the transition from the initial change in temperature to steady state t1 u v where u is the distance along the fault in the direction of underthrusting from the surface to the depth in question and t2 z 2 2 where z is the depth to the fault when elapsed times exceed the sum of these two time constants temperatures differ from their steady state values by only about 10 percent the numerical experiments indicate that the simple formulae are sufficiently accurate that sophisticiated numerical modeling of temperatures in specific regions is unwarranted putain an application of these simple formulae to measurements of conductive heat flow at island arcs implies that shear stresses at island arcs approach 100 mpa and are greater than 30 mpa calculations of temperatures appropriate for the himalaya suggest that shear stresses of 100 mpa on the main central thrust probably are required to account for the tertiary granites of the region if melting took place after slip began on the thrust similarly the cut off in seismicity at a depth of about 15 km in the himalaya if due to temperatures exceeding 350 to 450 c implies a deviatoric stress close to 100 mpa

wind erosion of crusted soil sediments saltating particles increase the rate of dust release from sediments in arid and semi arid areas they also break interparticle bonds in aggregated and crusted soils thereby increasing the number of particles available for entrainment this pilot study examines rates of erosion in relation to the flux of saltating grains for three crusted sediments of different strengths dislodgement of surface particles decreases with increasing crust strength as measured by a cylindrical flat ended penetrometer in addition initial dust release from craters formed by single impactors in unaggregated soil is examined in relation to the associated saltator the volume of material removed depends linearly on the kinetic energy of the abraders

on the stability of isostatically compensated mountain belts the formation of convergent mountain belts is invariably accompanied by an increase in gravitational potential energy due to part of the work done by the forces driving convergence the evolution of potential energy stored in an orogen is dependent on 1 the density structure 2 the thermal evolution and 3 the way convergent deformation is partitioned between crust and mantle lithosphere it is now well recognized that this increase in potential energy associated with the mountain building process raises the possibility that significant extension or collapse may accompany the relaxation of the forces driving convergence provided the lithosphere is thermally weakened e g england 1987 in this paper we evaluate the stability of isostatically compensated mountain belts under the assumption that the strength of continental lithosphere is governed by a combination of frictional sliding and creep processes using the brace goetze model for the rheology of the lithosphere the reference lithosphere defined to be in potential energy and isostatic balance with the mid ocean ridges changes with different thermal parameters of the lithosphere the instantaneous extensional strain rate for thermally mature mountain belts is calculated by balancing the horizontal buoyancy force stored in the mountain belts measured relative to the reference state with the vertically integrated strength of the lithosphere for initial strengths spanning the probable natural range it is shown that horizontal buoyancy forces arising in isostatically balanced mountain belts are sufficiently large to induce the collapse at significant rates greater than a few times 10 16 s 1 and leading to significant finite extension providing the moho temperatures exceed about 650 700 c a condition only likely to be attained if the mantle lithosphere has not been thickened to the same extent as the overlying crust consequently processes that thin the mantle lithosphere as a consequence of convergent deformation such as the convective instability of a thickened lower thermal boundary layer greatly increase the possibility of collapse the calculations presented here suggest that near complete destruction of the mountain system by extensional collapse may be possible if such processes can reduce total lithospheric thickening to less than half the contemporary crustal thickening i e fl fc 2 where fl and fc are the lithospheric and crustal thickening factors

influence of cryptogamic crust disturbance to wind erosion on sand and loam rangeland soils a portable field wind tunnel was used to assess the sediment flux rates of loam and sand textured soils in the mallee region of southeastern australia three levels of crust disturbance nil moderate and severe simulating stock trampling were investigated the results demonstrated the importance of cryptogamic crusts in binding the soil surface and providing roughness after the soil was moderately disturbed on the loamy soil the crust helped maintain sediment flux rates below the erosion control target to 5 g m 1 s 1 for a 65 km h 1 wind measured at 10 m height once the crust was severely disturbed sediment fluxes increased to 1 6 times the erosion target on the sandy soil even with no crust disturbance the sediment flux was 1 6 times the erosion control target disturbing the crust increased sediment fluxes to a maximum of 6 7 times the erosion control target removal of the crust also decreased the threshold wind velocity that resulted in an increase to the ris k of erosion from 5 per cent to 20 per cent 1998 john wiley sons ltd

tertiary structural evolution of the gangdese thrust system southeastern tibet structural and thermochronological investigations of southern tibet xizang suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost tibetan plateau since late oligocene two thrust systems are documented in this study the north dipping gangdese system gts and the younger south dipping renbu zedong system rzt west of lhasa the gangdese thrust juxtaposes the late cretaceous forearc basin deposits of the lhasa block the xigaze group over the tethyan sedimentary rocks of the indian plate whereas east of lhasa the fault juxtaposes the late cretaceous eocene andean type arc the gangdese batholith over tethyan sedimentary rocks near zedong 150 km southeast of lhasa the gangdese thrust is marked by a 200 m thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks a major south dipping backthrust in the hanging wall of the gangdese thrust puts the xigaze group over tertiary conglomerates and the gangdese plutonics north of xigaze and west of lhasa a lower age bound for the gangdese thrust of 18 3 0 5 ma is given by crosscutting relationships the timing of slip on the gangdese thrust is estimated to be 27 23 ma from 40 ar 39 ar thermochronology and a displacement of at least 46 9 km is indicated near zedong the age of the gangdese thrust gt is consistent with an upper age limit of 24 ma for the initiation of movement on the main central thrust in places the younger renbu zedong fault is thrust over the trace of the gt obscuring its exposure the rzt appears to have been active at circa 18 ma but had ceased movement by 8 1 ma the suture between india and asia has been complexly modified by development of the gts rzt and locally strike slip and normal fault systems

forces driving continental collision reconciling indentation and mantle subduction tectonics two end member classes of models indentation and mantle subduction that have been proposed to explain patterns of crustal deformation during continental collision may be reconciled using a model that includes a combination of indentation from the side and basal forcing from underlying subducting layers measurement of the width of deformation across strike can be used to indicate the predominant forcing mechanism and shows that the combined case is controlled by mantle subduction if the crust is well coupled to the underlying mantle lithosphere or by indentation if the crust and mantle lithosphere are decoupled an increase in the width of deformation with convergence as a result of the effect of gravity on crustal thickening can cause a change in the primary control of an orogenic system from mantle subduction to indentation this transfer may be enhanced by diffusional or convective heating at the base of the thickened crust the india eurasia collision is an example where the deformation width may initially have depended on mantle subduction but is now controlled by the indenter limit

dust emission from wet and dry playas in the mojave desert usa the interactions between playa hydrology and playa surface sediments are important factors that control the type and amount of dust emitted from playas as a result of wind erosion the production of evaporite minerals during evaporative loss of near surface ground water results in both the creation and maintenance of several centimeters or more of loose sediment on and near the surfaces of wet playas observations that characterize the texture mineralogic composition and hardness of playa surfaces at franklin lake soda lake and west cronese lake playas in the mojave desert california along with imaging of dust emission using automated digital photography indicate that these kinds of surface sediment are highly susceptible to dust emission the surfaces of wet playas are dynamic surface texture and sediment availability to wind erosion change rapidly primarily in response to fluctuations in water table depth rainfall and rates of evaporation in contrast dry playas are characterized by ground water at depth consequently dry playas commonly have hard surfaces that produce little or no dust if undisturbed except for transient silt and clay deposited on surfaces by wind and water although not the dominant type of global dust salt rich dusts from wet playas may be important with respect to radiative properties of dust plumes atmospheric chemistry windborne nutrients and human health published in 2007 by john wiley sons ltd

quantification of the dry aeolian deposition of dust on horizontal surfaces an experimental comparison of theory and measurements eight techniques to quantify the deposition of aeolian dust on horizontal surfaces were tested in a wind tunnel the tests included three theoretical techniques and five measurement techniques the theoretical techniques investigated were the gradient technique the inferential technique without grain shape correction and the inferential technique corrected for grain shape the measuring techniques included the following surrogate surfaces a water surface a glass surface a metal surface a vertical array of metal plates and an inverted frisbee filled with glass marbles the efficiency of the techniques was investigated for the sediment as a whole all grain sizes together as well as for a large number of grain sizes extending from 1 to 104 m the surrogate surfaces showed more or less comparable catch efficiencies although the water surface nearly always caught the highest quantities of dust and the marble filled frisbee and the vertical array of metal plates the lowest quantities of dust the dust fluxes calculated by theoretical methods were markedly different from those obtained by direct measurements the fluxes calculated by the inferential technique approximated those of the direct measurements only for grain sizes between 30 and 40 m for smaller and coarser grains deviations from the measured fluxes were high the gradient method in its turn provided extremely low calculated fluxes for grains in all size classes investigated the latter technique was not considered very reliable for the dust used in the tests

gravitational spreading of high terrain in ishtar terra venus magellan altimetry measurements indicate that the mountain belts and plateau margins of ishtar terra have topographic slopes in the range 2 30 magellan radar images reveal that numerous sets of narrow closely spaced troughs and lineations which we interpret as graben and normal faults are located in many of these regions of high slope the orientation of many of these graben sets perpendicular to the downslope direction suggests that they formed as a result of gravitational spreading in several of the mountain belts the extensional structures are parallel to the apparent direction of shortening consistent with the interpretation that gravitational spreading occurred while mountain building was still active to explore the implications of this spreading for the tectonic evolution of ishtar terra we model the process by means of a finite element algorithm for viscoelastic deformation in a vertical section of the crust at the margin of a plateau or broad mountain range flow in the crust is assumed to be governed by a nonlinear depth dependent rheology we predict brittle failure and relaxation of topographic relief as functions of time for ranges of crustal thickness 10 30 km and thermal gradient 5 25 k km and for observed ranges in topographic elevation 1 6 km and slope 1 30 this parameter range predicts a large spread in the time scales for initial failure and significant relaxation of relief for a crustal thickness greater than 10 km a thermal gradient of 15 k km or more and average values of relief 3 km and slope 3 the topography relaxes to 25 of its original height in less than 10 m y although initial failure in the form of normal faulting on the highlands and margin and occasional shallow thrusting in the lowlands is predicted to occur much earlier values of horizontal surface strain that are likely to be observable 1 do not accumulate until significant relaxation of relief begins given the predicted rate of topographic relaxation either the crust of ishtar terra must be anomalous in some way either much stronger than predicted by standard flow laws or very thin or cold or the topographic relief and high slopes have been actively built and maintained until times significantly younger than the average crater retention age for venus of 500 ma the large apparent depth of compensation the abundance of volcanism and the uncertainty in the local age favor the view that mountain building in ishtar terra has occurred until times at least as recent as 10 ma

analogue model of gravitational collapse and surface extension during continental convergence most mountain belts occur where continents collide so it is not surprising that the dominant form of surface deformation is shortening but there is also abundant geological evidence for extension in the central parts of many mountain belts at the same time as shortening occurs elsewhere previous models for extension require temporal changes in the thermal structure of the lithosphere1 2 the rate of convergence2 3 the strength of the crust3 or the geometry of accretion4 here we present a simple model in which no such changes are required for surface extension during the convergent thickening of a viscous crusta layer convergence is driven by the motion of two plates at the base of the layer in the area where one plate subducts under the other the surface begins to extend soon after the start of convergence eventually stretching by more than 60 per cent extension occurs because gravity drives horizontal flow faster at the free surface than in the centre of a viscous layer that is fixed at its base in real mountain belts mid crustal weaknesses may allow the depth dependent motion required for surface extension

crust strength a wind tunnel study of the effect of impact by saltating particles on cohesive soil surfaces a wind tunnel study examined the effect of distributions of saltating particles on sediment surfaces which were characterized by distributions of their tensile strength the sediments consisted of varying proportions of large sand sized particles with a fine particle cement the energies of the impacting particles and the surface strengths were compared with the mass of material lost from the surface it is important to consider distributions of parameters rather than mean values only since abrasion and erosion may occur from surfaces not predicted from average strength and saltation velocities at the impact velocities used in this study mean velocity 4 4 m s 1 with standard deviation of 0 51 surfaces containing less than 12 per cent fine material were easily eroded but insignificant erosion occurred when the fine particle content exceeded 60 per cent small amounts of cementing material were easily ruptured allowing the large sand grains to be moved largely in creep by the bombarding particles a significant amount of energy was lost to the bed as the percentage of fine material increased the surface became more difficult to break up and less energy was lost to the bed the probability that erosion will occur for known energy distributions of impacting particles and surface strength can be calculated and the mass loss increases exponentially with a decrease in the percentage of fine cementing particles copyright 2001 john wiley sons ltd

origin of regional rooted low angle normal faults a mechanical model and its tectonic implications rooted listric low angle normal faults 20 of regional extent have been recognized widely in the past few years in the north american cordillera and elsewhere the low angle geometry of these crustal scale normal faults conflicts with anderson s 1942 classic theory of faulting in that theory the orientations of principal stresses are assumed to be vertical and horizontal the predicted dip angle of normal faults is about 60 rather than 20 or less recent geological and geophysical studies in the mid tertiary extensional terrane of southeastern california and western arizona suggest that thick mylonitic gneisses in the lower plates of low angle detachment faults may represent unidirectionally sheared laminar flow in and below the midcrust directed ductile flow possibly related to the gravitational spreading of thickened lower crust may induce a shearing traction on the horizontal or subhorizontal base of the brittle upper crust thus the orientations of the principal stresses can no longer be vertical and horizontal at this interface a simple elastic model incorporates the effect of basal shearing due to gravitational spreading on stress distributions in an elastic upper crust this model shows that parallel belts of compression and extension can be produced if a shearing traction acting on the base of the elastic upper crust is considered in particular appropriate stress conditions for the formation of regional low angle normal faults 20 can be produced by the superposition of two stress fields a basal shear stress field induced by the basal shear traction and a contractional stress field in which the horizontal deviatoric stress is compressional and the vertical gradient of the horizontal normal stress component is constant this superposed stress field may represent a tectonic setting where a stress field with compressional deviatoric stress induced by plate subduction or convergence is superposed on a basal shear stress field induced by gravitational spreading of thickened lower crust these results may explain both puzzling parallel belts of extension and compression and the occurrence of major low angle normal faults in some orogenic systems

grain scale simulations of loose sedimentary beds the example of grain bed impacts in aeolian saltation sediment transport by wind is one of many processes of interest to the geomorphologist in which grain to grain contacts play an important role in order to illustrate the modelling of collections of frictional inelastic sedimentary grains with the particle dynamics method pdm we use the grain impact process in aeolian saltation as a specific example in pdm all the forces on each particle are evaluated at a sequence of small time steps and the newtonian equations of motion are integrated forward in time interparticle forces at grain contacts are treated as springs with prescribed stiffness normal force and by a coulomb friction law tangential force particle inelasticity is represented by spring damping the granular splash resulting from saltation impacts is assessed for sensitivity to the choice of grain properties and the integration time step we find that for the range of impact speeds and impactor masses relevant to aeolian settings grain splashes are relatively insensitive to grain stiffness grain inelasticity and grain friction and that the pattern of ejection from the bed is largely controlled by bed microtopography a large set of impact realizations involving a variety of impact points on a small set of target beds is used to collect the appropriate statistics for describing the stochastic splash process the splash function representing these statistics is then available for use in calculations over longer time scales such as the evolution of the saltation curtain the details given here will enable the interested reader to adapt pdm modelling to other types of clastic sedimentary systems

tectonic evolution of high plateaus abstract the striking contrast in tectonic style between highlands like tibet with its active grabens and low altitude flanks like the thrusting structures in the foothills of the himalayas shows that a disorder of the tectonic stresses correlates with the topography the same is true for gravity in particular for the geoid as clearly observed in the case of the altiplano these observations can be explained physically by means of simple relationships between the dipolar mass distribution of the high topography and its compensating root this analysis leads to a proposed scenario for plateau evolution in three stages 1 1 build up with thrusting and folding 2 2 neutral state or strike slip deformation and finally 3 3 relaxation with normal faulting the basin and range represents a mature illustration of phase 3 its bouguer anomaly suggests the existence of an undulation of the lithosphere astenosphere boundary with a 200 km wavelength physical models of lithospheric boudinage are shown for one to compare with the observed topographic and gravimetric signatures

dust emission and transport in mali west africa vertical dust fluxes were measured in the inland delta region of the niger river mali west africa during april june 1989 and 1990 measurements of dust flux represent for the most part non dust storm conditions or dust haze periods the observed concentration versus height relationships are similar to data presented by other investigators the relationship between wind shear velocity u m s 1 and vertical dust flux f g m 2 s 1 can be described by a relationship in which f is proportional to u 4 however there is considerable scatter within the data set which is attributable to textural controls and surface conditions the vertical dust fluxes measured in mali are compared to dust fluxes measured in texas usa and yukon territories canada the significantly different values for the constant of proportionality a in the f a u 4 relationship for these geographically diverse areas is a function of surficial controls on the release of sediments to the air stream dust concentrations measured in mali were found to be uniformly high and in general exceed who health standards for acceptable total suspended particulate loadings as a result background dust may be considered a long term stress on health for the people of this region of mali

subduction of asian lithospheric mantle beneath tibet inferred from models of continental collision the relative northward motion of the indian subcontinent that followed the onset of continental collision with asia has produced extensive deformation of the earth s crust giving rise to the world s highest mountains in the himalayan chain and the world s largest high elevation region the tibetan plateau the formation of the broad mountain belt implies that contrary to the original tenets of plate tectonics the lithospheric plates have experienced widespread deformation far from the plate boundary1 several models have been proposed2 6 to explain the manner in which this post collisional deformation is distributed within the continental lithosphere of the indian and asian plates here we propose an alternative model in which subduction of the asian lithospheric mantle develops following the collision of india our model is supported by numerical calculations of crustal deformation and thickening and is consistent with available geological and geophysical data7 9 this picture suggests that lithospheric mantle is not deformed along with the crust and would imply that continental collision zones are more analogous to oceanic subduction zones than was previously believed