impact of bedrock surface topography on spatial distribution of quaternary sediments and on the flow pattern of late weichselian glaciers on the east european craton russian plain this paper presents the first high quality maps of the bedrock surface topography and the quaternary sediment distribution for the late weichselian glaciation area on the east european craton eec and the reasoning behind the estimated sediment distribution geographic information system gis based three dimensional model of the bedrock surface topography the current digital terrain model dtm and the ice flow pattern of the last glaciation provided the main source data for the analyses and interpretations in the current continental area of the western eec which was covered by the last scandinavian ice sheet sis the bedrock lies overwhelmingly 85 above the contemporary sea level with an average altitude of 54 m above sea level the topography of the bedrock surface is very well reflected in the current topography and an 80 m difference in the ice bed surface altitudes results in the topographic focusing of the ice flow and thus all late weichselian ice streams were accommodated to the depressions in the subglacial surface such depressions are characterised by a thin 10 m layer of sediment with one till bed just 50 m upslope from the bottom of the depressions the thickness of the sediment increases two to threefold and comprises five or more till layers indicating relief enhancement through subsequent glaciations the average calculated thickness of the quaternary deposits in the study area was 49 m and the total volume of sediments was 20 280 km3

stress and velocity fields in glaciers part ii sliding and basal stress distribution numerical methods are used to examine the interaction between the spatial distribution of the basal shear traction and the corresponding basal velocity for an inclined slab geometry in our improved treatment we reject the common assumption that basal velocity is a simple function of local variables in favour of a non local treatment that includes normal deviatoric stress and takes basal velocity to be an integrated response to spatially varying influences computationally one must either iterate the basal velocity with a friction parameterization that relates basal shear traction to basal velocity or alternatively prescribe the basal shear traction that results from bed decoupling and substrate deformation the average of basal shear traction over the entire bed of the ice mass is invariant under changes in sliding distribution and thus constitutes a useful reference any local relative reduction of traction leads to basal movement either sliding over the bed or moving with a deforming subglacial layer the local stress reduction is accompanied by a concentration of traction up and down glacier of the moving base growth decay and possible migration ofbasal stress concentrations may be closely related to short lived sliding events and to surges

recent developments in centrifuge modelling of tectonic processes equipment model construction techniques and rheology of model materials abstract the experimental tectonics laboratory at queen s university is equipped with a large capacity centrifuge that is capable of subjecting tectonic models measuring 127 76 mm in plan and up to 51 mm in depth to accelerations as high as 20 000 g this high capacity greatly extends the range of potential model materials and permits the use of relatively stiff and or brittle substances a number of new techniques of model construction have been devised that permit internal and surface strain patterns and kinematic evolution to be monitored in detail one particularly useful technique which will find application in non centrifuged experiments as well allows the preparation of highly uniform anisotropic multilayers composed of alternating layers of plasticine and silicone putty with individual layer thicknesses as low as 20 m and with controllable ratio between thicknesses of the relatively competent and incompetent units examples of models constructed using these new techniques are illustrated one particular type of the commonly used model material silicone putty has been subjected to a series of rheological test the results indicate that at strain rates in the range 10 6 10 3 s 1 applicable to the centrifuge experiments the silicone putty exhibits power law rheology with n 7 2 at higher strain rates the material appears to tend towards linear behaviour available rheological data and dimensional analysis using standard scaling laws and appropriate model ratios suggest that the microlaminated plasticine silicone putty multilayer is a suitable analogue in centrifuged experiments for interbedded sequences of indurate limestone and incompetent shale the excellent degree of dynamic similitude attained is demonstrated by the realistic form of fold and fault structures developed in models constructed of this material

the stream function and gauss principle of least constraint two useful concepts for structural geology abstract to the extent that rock deformation can be approximated by a two dimensional newtonian model a powerful stream function simulation method is applicable the significance of stream functions is that velocity strain stress and energy derived from the same stream function satisfy automatically three basic conditions of dynamics 1 1 the condition of continuity 2 2 the navier stokes equations 3 3 conservation of energy hence we state with jaeger if a stream function can be found which satisfies the boundary conditions of a dynamic model the complete solution follows all pertinent bits of dynamic information are implied in the stream function from which they can be directly derived guaranteed so to speak not to violate the basic conditions of dynamics stream functions useful in structural geology are solutions of 4 4 x 4 2 4 x 2 y 2 4 y 4 0 a double polynomial solution of max degree 14 is developed in which the coefficients are related controlled by the 4 0 constraint and their absolute values are determined by the boundary conditions of specific models and by the condition of maximum rate of energy dissipation or maximum rate of decline of potential energy the polynomial stream function is applied to a collapsing viscous nappe consisting of a thin basal layer with low viscosity on which a thicker layer with high viscosity slides due to gravitational spreading the velocity of forward movement depends upon absolute and relative values of the following parameters viscosity thickness the aspect ratio and density the velocity of a variety of nappes with different thicknesses aspect ratios viscosities and densities is determined

dating ice flow change near the flow divide at roosevelt island antarctica by using a thermomechanical model to predict radar stratigraphy 1 radar detected internal layering contained in some ice divides shows upwarped arches termed raymond bumps the distribution of their amplitude with height can date the onset of divide flow reflecting changes in the basin structure of the ice sheet the distribution depends on rheology surface geometry accumulation rate and temperature conway et al 1999 used an isothermal parameterized ice flow model to estimate a date of 3200 years b p with no error estimate for the onset of divide flow in roosevelt island ross ice shelf which they associated with grounding line retreat of the west antarctic ice sheet no other retreat dating exists for an area of the ross ice shelf distant from geological exposures employing a full thermomechanically coupled transient model we use a direct search to determine which ice rheology best fits the observed bump distribution and surface profile estimating the sensitivity of the dating to model parameters and exploring possible reasons for the observed asymmetry of the surface profile our main results are as follows 1 a best estimate of the date of grounding line retreat near roosevelt island is 3000 years b p bounded by 2300 and 4200 years b p 2 standard rheology power n 3 can only match the observed bump distribution for unrealistic isothermal models 3 for thermomechanically coupled models a high power n 4 in glen s law is required 4 low power rheologies and wind scouring cannot produce correct bump amplitude distributions 5 asymmetric accumulation cannot explain the observed asymmetry of the surface profile 6 modeling isochrones in flanking regions also indicates changes in flow around 3000 years b p

the shallow ice approximation for anisotropic ice formulation and limits ice sheet flow modeling is generally based on the shallow ice approximation sia developed for isotropic ice we extend this approximation to anisotropic ice and check its validity for both the isotropic and anisotropic cases by comparing the results of up to second order sia with those obtained from a model solving the full set of mechanical equations the theory is developed for a constitutive relation describing newtonian and non newtonian behavior but numerical results are shown only for a newtonian behavior the results are compared for plane flow of isothermal ice under steady state conditions sia gives an excellent representation of the newtonian flow of an isotropic ice sheet and a good description for the anisotropic case the zero order approximation is sufficient to describe ice flow precisely over flat bedrock for the anisotropic and isotropic cases even close to an ice divide for uneven bedrock the second order sia gives excellent results for isotropic ice and acceptable results for anisotropic ice finally we quantify the error introduced by using an enhancement factor to represent the anisotropy when longitudinal stresses are taken into account

the frontier mountain meteorite trap antarctica the frontier mountain blue ice field is an important antarctic meteorite trap which has yielded 472 meteorite specimens since its discovery in 1984 remote sensing analyses and field campaigns from 1993 to 1999 have furnished new glaciological data on ice flow ice thickness bedrock topography ice ablation and surface mass transport by wind along with detailed descriptions of the field situation at the trap this solid set of data combined with an updated meteorite distribution map and terrestrial ages available from literature allows us to better describe the nature of the concentration mechanism in particular we observe that the meteorite trap forms in a blue ice field 1 located upstream of an absolute and a shallow sub ice barriers 2 characterized by compressive ice flow with horizontal velocities decreasing from 100 to 10 cm year on approaching the obstacle 3 undergoing mean ablation rates of 6 5 cm year 4 nourished by a limited snow accumulation zone extending 20 km upstream of the blue ice area we also draw the following conclusions 1 the origin of the meteorite trap can be explained according to the present day glaciological situation 2 the meteorite concentration develops according to the general principles of the ice flow model 3 the accumulation model can be described as stagnant ice or slow moving ice against an absolute and submerged barriers according to the descriptive schemes present in literature 4 the frontier mountain ice field is an effective trap for meteorites weighing more than 200 g for smaller masses the combination of wind and glacial drift may remove meteorites in less than a few tens of thousands of years 5 although the activation age of the frontier mountain trap is not yet constrained we infer that one of the most important findsites may be as old as 50 ka predating the last glacial maximum

bed topography and surges in ice streams 1 extensive bed topography at the scale of the ice thickness occurs frequently in the form of drumlins by analogy with hard bed sliding ice flow over this type of topography leads to the generation of drag on the ice particularly when ice flow is rapid a crucial difference with classical nye kamb sliding is that the upper free surface of the ice has a significant effect on the sliding process through the formation of a standing wave using a theoretical model we demonstrate that the presence of this wave introduces a non linearity into the sliding motion which can lead to multiple sliding velocities for the same large scale ice stream geometry and that switches between these velocities can cause surging behavior

ice flow dynamics and meteorite collection in antarctica abstract typical targets for meteorite searches in antarctica are blue ice fields in front of major bedrock obstacles where the meteorite bearing ice slows down is uplifted and exhumed by ablation however in some areas collection occurs in the lee of topographic barriers as documented in the downstream side of the frontier mountain a major emerged bedrock barrier in this study we present an analogue modelling study of ice flow at frontier mountain which has been chosen as example study area because an exceptionally complete set of glaciological data is available to investigate the causes leading to meteorite concentration results of modelling show how the boundary conditions mainly the bedrock topography control ice flow and how this flow coupled to high ablation may give rise to a stable meteorite trap downstream of major topographic obstacles modelling results support the contention that as erosion is not a mountain builder ablation does not represent a driving force in ice flow dynamics although it plays a significant role in shaving stranding surfaces and exhuming meteorites

catch a falling star meteorites and old ice a model for the process of meteorite concentration in blue ice regions of the antarctic ice sheet is proposed based on data from near the allan hills and the assumptions that both meteorite influx and glacial flow have been constant the meteorite influx is calculated to be 60 x 10 6 kilogram per square kilometer per year and the age of the exposed ice to be 0 to 600 000 years varying with distance from the allan hills these results are in line with other estimates of influx rate and with measurements of the terrestrial ages of the meteorites providing support for the assumption of steady flow and meteorite influx this may be the oldest sequence of ice in stratigraphic order yet discovered and the results imply that this part of the east antarctic ice sheet has been approximately steady during this time interval

a new fully three dimensional numerical model for ice dynamics the problem of describing ice dynamics has been faced by many researchers in this paper a fully three dimensional model for ice dynamics is presented and tested using an approach followed by other researchers ice is considered a non linear incompressible viscous fluid so that a fluid dynamic approach can be used the model is based on the full three dimensional stokes equations for the description of pressure and velocity fields on the saint venant equation for the description of the free surface time evolution and on a constitutive law derived from glen s law for the description of ice viscosity the model computes the complete pressure field by considering both the hydrostatic and hydrodynamic pressure components it is time evolutive and uses high order numerical approximation for equations and boundary conditions moreover it can deal with both constant and variable viscosity three theoretical tests and two applications to priestley glacier antarctica are presented in order to evaluate the performance of the model and to investigate important phenomena of ice dynamics such as the influence of viscosity on pressure and velocity fields basal sliding and flow over perturbed bedrocks all these applications demonstrate the importance of treating the complete pressure and stress fields

transmission of basal variability to a glacier surface 1 transmission of basal variability to a glacier surface is investigated using analytical models for a linearly viscous medium the three dimensional transient response of the surface to both bedrock undulations and spatial variations in basal slipperiness for perturbations of arbitrary wavelengths is determined using perturbation methods both information transfer toward the surface and lateral transmission of horizontal stresses are strongly affected by the slip ratio that is the ratio of basal sliding to deformational velocity for any mean bedrock slope and above a minimum value of slip ratio the amplitude transfer of bedrock undulations toward the surface has a local maximum at undulation span corresponding to about 3 8 times the mean ice thickness the transmission of basal variability to a glacier surface increases quite significantly with increasing slip ratio this explains why the surfaces of fast flowing ice streams are more undulating than the slower moving bordering areas at slip ratios higher than about 100 the flow of glaciers and ice sheets becomes nonlocal in the sense that surface velocities and buildup and propagation of surface undulations cannot be calculated accurately on the basis of local thickness and slope using linearized long wave theories at these slip ratios instead of the more accurate arbitrary wavelength theory gives estimates of decay times that are an order of magnitude too small and phase velocities several times too large the problem of the propagation and decay of small amplitude surface undulations on glaciers in three dimensions is solved small amplitude surface waves on glaciers are strongly diffusive and dispersive redistribution of mass on ice sheets and glaciers is a diffusion process and it is misleading albeit not mathematically incorrect to describe the reaction of glaciers to surface perturbations in terms of a wave propagation

physical modeling of the influence of bedrock topography and ablation on ice flow and meteorite concentration in antarctica 1 three dimensional laboratory physical experiments have been used to investigate the influence of bedrock topography and ablation on ice flow different models were tested in a plexiglas box where a transparent silicone simulating ice in nature was allowed to flow experimental results show how the flow field in terms of both flow lines and velocity and variations in the topography of the free surface and internal layers of the ice are strongly influenced by the presence and height of bedrock obstacles in particular the buttressing effect forces the ice to slow down rise up and avoid the obstacle the higher the bedrock barrier the more pronounced the process only limited uplift of internal layers is observed in these experiments in order to exhume deep material embedded in the ice ablation simulated by physically removing portions of silicone from the model surface to maintain a constant topographic depression must be included in the physical models in this case the analogue ice replenishes the area of material removal thereby allowing deep layers to move vertically to the surface and severely altering the local ice flow pattern this process is analogous to the ice flow model proposed in the literature for the origin of meteorite concentrations in blue ice areas of the antarctic plateau

a 14 year mass balance record of a blue ice area in antarctica accumulation and ablation rates over an antarctic blue ice area spanning the 14 year period 1988 2002are presented data were obtained by direct stakemeasurements large spatial and temporal variations in the net balance were observed without any clear trend over the entire period there are marginally significant increases in snow accumulation and in ablation in the blue ice area farthest from the equilibrium zone both at the 95 confidence level the snow blue ice transition zone shows no change over the entire period of observation andtheblue ice area near the zone showsno change inablation rate over the 14 year period the mass balance gradient in scharffenbergbotnen may have increased during the period 1988 2002 however the changes are small especially when compared with the changes observed elsewhere in antarctica even relatively close to the blue ice area this may indicate that the blue ice areas are relatively stable to changes in accumulation rate and possibly temperature

on the glaciological meteorological and climatological significance of antarctic blue ice areas this paper presents the current state of our knowledge about antarctic blue ice areas scattered widely over the antarctic continent yet covering only approximately 1 of its surface area blue ice areas have some peculiar characteristics that make them of particular interest for planetary geologists glaciologists meteorologists and climatologists for instance blue ice areas are subject to net ablation mainly by sublimation since any accumulation is easily swept off their smooth surfaces by winds moreover they are significantly warmer than their surroundings and the wind over them is generally more gusty an important feature of blue ice is its relatively low albedo 0 56 compared with that of snow 0 80 blue ice is probably considerably smoother than snow in the aerodynamic sense with roughness lengths as low as 3 10 6 m which indicates that blue ice is probably the only permanent natural surface over which the air flow is mainly in the aerodynamic smooth regime further since it is likely that very old ice is exposed at the surface of some blue ice areas this ice would be of great interest for paleoclimatic studies it has also been suggested that blue ice areas are sensitive to climate change this would make them useful as antarctic climate indicators because they can easily be monitored by satellite finally this paper addresses several gaps in our knowledge about blue ice areas and gives suggestions for future blue ice area research

flow law hypotheses for ice sheet modeling ice flow modeling requires a flow law relating strain rates to stresses in situ but a flow law cannot be measured directly in ice sheets microscopic processes such as dislocation glide and boundary diffusion control both the flow law for ice and the development of physical properties such as grain size and c axis fabric these microscopic processes can be inferred from observations of the physical properties and the flow law can then be estimated from the microscopic processes a review of available literature shows that this approach can be imperfectly successful interior regions of large ice sheets probably have depth varying flow law constants with the stress exponent n for power law creep less than 3 in upper regions and equal to 3 only in deep ice n probably equals 3 through most of the thickness of ice shelves and ice streams

spectral characteristics and source parameters of microearthquakes from the mt cameroon volcanic region west africa summary estimates of source parameters seismic moment source radius and stress drop for a representative sample of 33 mt cameroon region microearthquakes with magnitudes in the range md 0 8 2 8 have been obtained from the analysis of their far field displacement amplitude spectra the spectral character of these events seems to be characterized by a lack of high frequencies the computed seismic moments lie between 10 and 10l6 nm and vary linearly with magnitude albeit with some scatter source radii are relatively large in the range 135 703m the stress drops are small 0 4 19 bar and roughly constant independent of seismic moment thus suggesting that the property of self similarity seems to hold over this moment range for this area

dating antarctic blue ice areas using a novel ice flow model 1 we present a new type of flow model suitable for antarctic blue ice areas with application to dating ice for paleoclimate purposes the volume conserving model uses field data for surface velocities mass balance and ice thickness along a flow line with parameterized variation of ice rheology with depth to produce particle trajectories and isochrones the model is tested on the contrasting allan hills near western ice field and the scharffenbergbotnen blue ice fields in antarctica by comparing predicted ages with ages inferred from meteorites and 14c data during the glacial periods ice surface velocities at the allan hills must have been 25 less and accumulation rates 50 less than present day values in order to match meteorite ages in contrast scharffenbergbotnen velocities have probably been fairly constant over time due to the atypical valley where it resides

meteorites constrain the age of antarctic ice at the frontier mountain blue ice field northern victoria land abstract we show that meteorites can provide chronological constraints upon the age of the ice cropping out at the frontier mountain meteorite trap antarctica when their terrestrial age is placed in a glaciological context amongst the over 700 meteorites found so far frontier mountain fro 84001 99028 93005 and 93054 were most likely not wind drifted across the ice field since their masses 772 1665 g are much heavier than the local 200 g wind transport threshold the four meteorites were found along a stretch of ice where a representative section of the frontier mountain blue ice crops out based on the bedding of englacial tephra layers the structure of the ice along the section appears to be essentially an up glacier dipping monocline the 14 c terrestrial age of fro 8401 99028 and 93005 are 13 2 21 3 and 27 2 ky respectively the 41 ca 36 cl age of fro 93054 is 40 10 ky the terrestrial ages of the four meteorites increase from the top to the bottom layers of the monocline this geographic distribution is best explained by delivery of meteorites at the ice surface through the ice flow model i e englacial transport from the snow accumulation zone and exhumation in the blue ice area through ablation rather than direct fall since the effect of ablation in decoupling terrestrial ages of meteorites and the age of the ice on which they sit must have been minor most likely 7 ky based on the local ice dynamics we conclude that the age of the bulk of the ice body currently under ablation at frontier mountain is up to 50 ky old this result has implications on both the meteorite concentrations mechanism at frontier mountain and the regional ice dynamics

unravelling the high altitude nansen blue ice field meteorite trap east antarctica and implications for regional palaeo conditions abstract antarctic blue ice zones the most productive locations for meteorite recovery on earth contain old ice that is easily accessible and available in large quantities however the mechanisms behind these meteorite traps remain a topic of ongoing debate here we propose an interdisciplinary approach to improve our understanding of a meteorite trap in dronning maud land east antarctica on the nansen blue ice field meteorite trap 2600 3100 m above sea level where more than half of the asuka meteorites have been collected based on 185 surface blue ice samples one of the largest observed spatial patterns in oxygen isotopic variation to date is found relying on meteorites for which the terrestrial ages are determined using 14c and 36cl this surface ice is interpreted to date from the last interglacial up to the present day by combining state of the art satellite derived surface velocities surface mass balance modelling and ice flow modelling we estimate that about 75 85 of the meteorites found on the ice field were supplied by ice flow after entering the ice sheet in an accumulation area of a few hundred square kilometres located south upstream of the ice field less than 0 4 new meteorites per year are supplied to the ice field through ice flow suggesting that the hundreds of meteorites found 25 years after the first visit to this ice field mostly represent meteorites that were previously not found rather than newly supplied meteorites by combining these findings the infall rate of meteorites from space is estimated which is in line with values from the literature but situated at the higher end of the range a comparison of the oxygen isotopic variation of the surface blue ice to that of the european project for ice coring in antarctica epica dronning maud land edml ice core located 750 km to the west at the same elevation suggests that the regional changes in topography have been relatively limited since the last interglacial supporting theories of an overall stable east antarctic ice sheet eais over this time period

provenance of ore metals in base and precious metal deposits of central idaho as inferred from lead isotopes sulfide ores containing principally argentiferous galena sphalerite and tetrahedrite occur both in paleozoic sedimentary rocks and in late cretaceous granites along the eastern margin of the idaho batholith in central idaho two genetic models have been proposed for the sediment hosted deposits one involving vein and replacement styles of mineralization during the cretaceous and tertiary and the other representing initial paleozoic sedimentary exhalative sedex style of mineralization and subsequent remobilization during the cretaceous and tertiary there is also uncertainty about the sources of ore components studies based on fluid inclusions and stable isotopes suggest a shallow crustal source whereas the pb isotope studies favor sources deeper than the exposed crustal rocks we present new lead isotope data on 16 sulfide ore samples and 24 whole rock and mineral separate samples and compare them with data from the literature to assess the possible sources of base and precious metals in the deposits of central idaho two potential ore metal sources are recognized a sedimentary source and an igneous source deposits that derived their metals from a sedimentary source are characterized by radiogenic pb and are subdivided into three types triumph 206 pb 204 pb 19 4 20 1 207 pb 204 pb 15 7 15 9 and 208 pb 204 pb 39 0 39 9 minnimoore 206 pb 204 pb 19 9 20 6 207 pb 204 pb 15 8 16 0 and 208 pb 204 pb 40 1 41 3 and pacific 206 pb 204 pb 20 4 21 4 207 pb 204 pb 15 7 15 8 and 208 pb 204 pb 40 8 41 1 the pb isotope compositions of sediment hosted deposits can be explained by the mixing of pb derived from paleozoic sedimentary rocks and early proterozoic crystalline rocks or by the mixing of pb derived from the cretaceous granite and early proterozoic crystalline rocks alternatively the pb in sediment hosted deposits may represent a mixture of pb derived from all three sources in varying proportions none of these scenarios requires pb from deep sources the results of this study and prior fluid inclusion and stable isotope studies support a genetic model involving shallow crustal sources for metals and sulfur mobilized by meteoric water dominated hydrothermal systems deposits in which the metals were derived from an igneous source are subdivided into carrietown and non carrietown types carrietown type ores have relatively low uranogenic pb isotope ratios 206 pb 204 pb 17 3 19 1 and 207 pb 204 pb 15 4 15 7 and variable thorogenic pb isotope ratios 208 pb 204 pb 38 9 41 0 the non carrietown type ores are isotopically similar to their host granites and have higher uranogenic pb isotope ratios 206 pb 204 pb 19 3 20 6 207 pb 204 pb 15 7 15 8 and 208 pb 204 pb 39 3 40 1 compared to the carrietown type ores we propose that the two groups are dominated by pb sources from different depths the carrietown type ores from middle crustal sources uranium depleted and the non carrietown type ores from upper crustal sources enriched in both u and th a magmatic hydrothermal origin for both types of ores is supported by their granitic host rocks hydrothermal alteration of intrusive granite and surrounding rocks high temperatures of ore formation and similar radiometric ages for the granite and mineralized veins

flow law for ice in polar ice sheets theories of glacier flow are based commonly on the assumption that ice is not a newtonian fluid but has a non linear stress dependent viscosity1 5 here we re examine the spreading of antarctic ice shelves and suggest that the data cannot define a unique flow law tilt measurements in four boreholes in both the arctic and antarctic seem to show that a linear flow law e a1 where a1 10 15 s 1 pa 1 at 28 c where e is the effective strain rate and the effective shear stress may be just as appropriate for describing the flow of polar ice sheets

basal perturbations under ice streams form drag and surface expression classical sliding theories consider ice sliding over obstacles which are much shorter than the thickness of overlying ice here we present a theory which considers form drag generated under ice streams by large obstacles such as subglacial bedforms which may have lengths comparable to ice thickness we also investigate how perturbations at the surface of an ice stream can be generated by such bedforms and develop a mathematical framework for separating the effects of such local kilometre scale variations in ice flow from the bulk flow of the ice stream