genesis of streamlined landforms and flow history of the green bay lobe wisconsin usa abstract the distribution of streamlined landforms in southeastern wisconsin suggests that drumlins and flutes formed during several phases of the green bay lobe between 18 and 14 000 bp the largest group of drumlins formed during a still stand of the ice margin during the johnstown phase presumably about 18 16 000 bp flutes and smaller drumlins are superimposed on larger forms and larger drumlins are remolded this indicates that drumlin modification continued during retreat three fields containing smaller drumlins formed after ice re advanced a short distance or stabilized during the green lake rush lake and st anna phases about 16 14 000 bp the drumlin forming process included erosion and deformation of pre existing ice marginal and proglacial sediments drumlins associated with the johnstown phase show an increase in length up glacier this is probably the result of variations in flow velocity length of time of drumlin formation and sediment availability flutes and small drumlins are associated with retreat moraines and formed near a thin retreating ice margin ice surface profile reconstructions suggest that margins were relatively steep during the johnstown milton green lake and early rush lake phases during the lake mills and late rush lake phases when flutes were forming the ice surface slopes were lower and margins were retreating during retreat minor sublobes developed some perhaps resulting from surges into small proglacial lake basins thus steep ice margins and driving stresses on the order of 15 25 kpa were typical during drumlin formation the formation of flutes and the remolding of larger drumlins into smaller forms took place beneath gently sloping ice of retreating margins associated with lower driving stresses

styles of subglacial glaciotectonic deformation within the context of the anglian ice sheet a substantial proportion of the glacial sediments of east anglia england are chaotic and highly deformed this has been attributed to a direct glaciotectonic deformation or b slumping of glaciomarine sediments we show that interpretation a leads to a classification of subglacial glaciotectonic deposits into sites of constructional deformation where little or no erosion of the preglacial deposits has occurred and styles of deformation are preserved moving up in the sequence and excavational deformation where substantial erosion of preglacial sediments has occurred and deformational styles are superimposed on each other by using glaciological and geotechnical theory we show that the expression of these deformational styles in east anglia is a natural response to the stress fields imposed by the ice sheet and that in consequence their spatial relationships are explicable the depth of deformation is related to the geotechnical properties of till and substrate the stress and effective stress fields and found to be consistent with observations

modes of operation of thermo mechanically coupled ice sheets by richard c a hindmarsh geoffrey s boulton grant institute of geology university of edinburgh west mains road edinburgh eh9 3jw scotland u k and kolumban hutter lnstitut fur mechanik technische hochschule oarmstadt hochschulstrasse i 0 6100 oarmstadt federal republic of germany dimensionless lapse rate a dimensionless model of thermo mechanically coupled ice sheets is used to analyse the operation of the system three thermal processes are identified i dissipation having a maximum time scale of thousands of years ii advection having a time scale of tens of thousands of years and iii conduction having a time scale of loo 000 years kinematical processes occur on two time scales i a marginal advective time scale of thousands of years and ii a diffusive time scale of tens of thousands of years dominant in the divide area

the relationship between drumlins and other forms of subglacial glaciotectonic deformation this paper brings together the field data from 33 drumlins to show the relationship between drumlin formation and other subglacial deforming bed processes it is shown that there is a drumlin structure continuum a depositional which are similar to flutes formed by sediment flowing into the low pressure area behind a large obstacle and these conditions are mostly found within rock cored drumlins b deformational which contain different styles of deformation associated with a relatively weak core including stoss side deformation including both small and large scale brittle and ductile deformation stoss and lee side deformation compressive deformation and extensional deformation c erosional which consist of either truncated stratified sequences or homogeneous tills with distinct ice flow patterns furthermore it is suggested that if more sediment enters the deforming layer than can be removed then there will be net subglacial deposition which will result in the build up of a deforming bed till but if more sediment leaves the deforming layer then net erosion will occur under net erosional conditions any obstacle to flow may be left behind as a drumlin thus although individual drumlins have different internal structures they are formed associated with subglacial net erosion 0 1997 elsevier science ltd all rights reserved

deforming beds viscous and plastic scales of deformation small scale field and laboratory observations of sub glacial sediment deformation suggest that failure is the predominant method of sediment deformation while large scale modelling studies using a viscous model of till deformation have been reasonably successful in predicting the geological consequences of ice sheet action this suggests that the cross over scale between viscous and plastic deformation occurs at a much larger scale than the grain grain interaction scale previously thought to be appropriate to deforming till sediment deformation and the drainage of sub glacial areas are considered at different scales and how these interact to produce failure and viscous behaviour is discussed a fluctuation length scale is proposed whose horizontal dimensions are of the order of the depth of the base of deformation 1 10 m on this length scale the glacier deforming bed system is highly variable in time as well as in space till deformation occurring at this length scale is plastic happening though failure events three other specific scales are proposed whose different properties depend on the relative spatial variability of static and seepage pressures and topography the key theoretical problem which has yet to be solved is how multiple small scale failure events combine into a viscous type flow this is discussed in the context of self organised criticality it is argued that the length scales appropriate for some drainage features are not appreciably larger than the scale for plastic failure events implying that modelling sub glacial drainage channels by balancing erosion rates against viscous closure of the channel is misconceived the outlines of an alternative model involving percolation theory are presented certain key questions which can only be settled through observations by glaciologists and glacial geologists are discussed it is suggested that the difference between lodgement tills and deformation tills reflects the rate of motion of till by deformation and that lodgement from the ice is unimportant

stick slip statistical mechanics at the bed of a glacier the multiple processes which control motion at the bed of glaciers are too heterogeneous to be modeled directly but a simple stick slip slider block model of a glacier bed allows a statistical mechanical treatment of the physics beneath thin and shallow sloped glaciers the technique may reconcile apparently contradictory field observations of the relationship between glacier sliding and basal water pressure simulations also predict hysteresis in the relationship between water pressure and basal velocities

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

coupled ice till deformation near subglacial channels and cavities previous models of ice till deformation near subglacial channels or cavities neglect the fact that the motions of the two materials are coupled and thus the interface between ice and till may not remain stationary here we analyze in succession two models which address the effect of such coupling via specification of appropriate continuity conditions for stress and velocity across the interface the modelled scenario is that of a shallow channel cavity with its long axis parallel to the principal ice flow direction overlying actively deforming till sediments by applying asymptotic techniques we investigate how the pattern and velocity of the creep flow depend generally on the ratio between the ice and till viscosities and on the deforming till thickness a more sophisticated non linear rheology for till sediments is then introduced it reveals that the two way interaction between water percolation and deformation in the till will enhance the localization of sediment flow near the channel margins the length scale over which transition of effective stress in the till takes place from its relatively high far field value to the low channel value is found to depend critically on a dimensionless permeability parameter a in any case coupled deformation causes sediment and ice flow towards the channel subsidence of the ice till interface just outside the channel and extension of the area over which the ice is in contact with till apart from having direct implications for subglacial sediment transport these results indicate that coupled deformation can contribute significantly to the spatial evolution of stress distribution under a glacier and thus its incorporation into future sliding and drainage theories for a soft bed should be considered essential

evidence from microstructures of deformable bed conditions within drumlins chimney bluffs new york state abstract discussion on the process of drumlin formation has been ongoing for over a century at present two dominant schools of thought prevail viz formation by a subglacial hydraulic mechanisms and b due to subglacial sediment deformation the latter mechanism depends on the existence of a subglacial deformable bed evidence from part of the large central new york drumlin field reveals evidence of syn depositional deformation during the emplacement of diamictons that constitute the major portion of drumlins exposed along shore bluffs in chimney bluffs state park these diamictons cannot be lodgement till facies members but appear to comprise facies indicative of melange style deposition under subglacial deformable bed conditions the evidence obtained from microstructures and plasma fabric within these diamictons supports the existence of a deformable bed during diamicton emplacement it is likely that formation of the drumlins occurred under these same subglacial bed conditions

recent drumlins flutes and lineations at vestari hagafellsjokull iceland in the foreland of the vestari hagafellsjokull glacier langajokull central iceland three scales of subglacial bed forms are found drumlins flutes and lineations the smaller bed forms are superimposed upon the larger these three bed forms probably formed as a result of the presence of a rigid subglacial core producing a lee side low pressure area into which sediment flowed and collected the cores of the drumlins are tumuli lava blisters whilst the cores of the flutes and lineations were large clasts differences between the three scales of bed forms are that a the flutes and lineations are far more mobile features as their cores could move within the deforming layer whilst the drumlin bedrock cores were fixed b the lineations formed in association with a clast pavement the change in scale of the bed forms may be related to changes in thickness of the deforming layer and size of the core at this site smaller bed forms were superimposed upon larger bed forms and these may have formed as the deforming layer progressively thinned in association with glacial retreat

origin of the active drumlin field at mulajokull iceland new insights from till shear and consolidation patterns abstract stratigraphic and morphologic data previously collected from the forefield of mulajokull iceland suggest that its recent surge cycles are responsible for the formation of drumlins there and that their relief reflects both deposition on drumlins and erosion between them we have tested these ideas and aspects of leading models of drumlin formation by studying past patterns of bed deformation and effective stress in basal tills of the glacier s forefield patterns of till strain indicated by the anisotropy of magnetic susceptibility ams of 2300 intact till samples indicate that till was deposited during shear deformation with shearing azimuths and planes that conform to the drumlin morphology thus till deposition occurred as drumlins grew in agreement with lidar data indicating that the degree of aggradation of the glacier forefield is largest in areas subjected to the most surges previously described unconformities on the drumlin flanks however indicate that drumlin relief at mulajokull has resulted in part from erosion given that the last surge deposited a till layer both on and between drumlins a reasonable hypothesis is that erosion between drumlins occurred during normal quiescent flow of the glacier between surges densities of till samples analyzed in conjunction with laboratory consolidation tests indicate that effective stresses on the bed during such periods were on the order of 100 kpa larger between drumlins than within them an observation consistent with subglacial channels at low water pressure occupying interdrumlin areas transport of sediment by turbulent flow in these channels or high effective stress adjacent to them causing enhanced till entrainment in ice or increased depths of bed deformation would promote the sediment flux divergence necessary to erode areas between drumlins the observation that effective stresses were higher between drumlins than within them is the opposite of that presumed in leading models of drumlin formation moreover the lack of ams fabric evidence of longitudinal compression in drumlin tills does not support some models of drumlin formation that invoke negative till flux gradients in a deforming bed

reef drowning during the last deglaciation evidence for catastrophic sea level rise and ice sheet collapse elevations and ages of drowned acropora palmata reefs from the caribbean atlantic region document three catastrophic metre scale sea level rise events during the last deglaciation these catastrophic rises were synchronous with 1 collapse of the laurentide and antarctic ice sheets 2 dramatic reorganization of ocean atmosphere circulation and 3 releases of huge volumes of subglacial and proglacial meltwater this correlation suggests that release of stored meltwater periodically destabilized ice sheets causing them to collapse and send huge fleets of icebergs into the atlantic massive inputs of ice not only produced catastrophic sea level rise drowning reefs and destabilizing other ice sheets but also rapidly reduced the elevation of the laurentide ice sheet flipping atmospheric circulation patterns and forcing warm equatorial waters into the frigid north atlantic such dramatic evidence of catastrophic climate and sea level change during deglaciation has potentially disastrous implications for the future especially as the stability of remaining ice sheets such as in west antarctica is in question

sliding of till over bedrock scratching polishing comminution and kinematic wave theory the observed ratio of removal of material by i scratches gouged by large clasts and ii polishing by debris suggests that the latter is the dominant form of bedrock erosion we examine this with a multi stage breakdown model of glacial debris which includes active erosion by fines these models suggest that a mother clast can through the action of its daughter debris erode several times its own volume of bedrock till sliding over bedrock is likely to be a potent source of scouring by proposing a till bedrock sliding law we investigate the kinematics of sliding till bodies ice flow naturally tends to thin and extend till cover even in the absence of longitudinal gradients in the applied stress thicker till cover has an increased effective pressure at its base a lower sliding velocity and for larger thicknesses a decrease in sediment flux with thickness this implies backward moving kinematic waves and shocks it is suggested that this is related to the blunt upstream faces of drumlins and that drumlinization can be a consequence of debris sliding over bedrock

rheological nonlinearity and flow instability in the deforming bed mechanism of ice stream motion contrary to what has recently been assumed in modeling the proposed deforming bed mechanism for the rapid motion of antarctic ice streams the rheology of water saturated till is probably highly nonlinear according to information from soil mechanics and preliminary experiments on till from the base of ice stream b the equivalent flow law exponent n is probably as high as 100 and the nonlinearities of the shear stress and effective pressure dependences are closely linked the high nonlinearity has important consequences for the deforming bed mechanism a flow system operating by this mechanism can be unstable as a result of feedback from the generation of basal water by shear heating of basal till the short term feedback effect is analyzed for a perturbation in a model ice stream in which the basal meltwater is transported through a distributed system of narrow gap conduits at the ice till interface although the analysis is approximate and some of the system parameters are poorly known the results suggest that the deforming bed mechanism is unstable for n 20 the apparent lack of such an instability in the currently active ice streams implies that their motion is controlled not by the deforming bed mechanism but by some other as yet unidentified mechanism

deformation of the batestown till of the lake michigan lobe laurentide ice sheet deep pervasive shear deformation of the bed to high strains 100 may have been primarily responsible for flow and sediment transport of the lake michigan lobe of the laurentide ice sheet to test this hypothesis we sampled at 0 2 m increments a basal till from one advance of the lobe batestown till along vertical profiles and measured fabrics due to both anisotropy of magnetic susceptibility and sand grain preferred orientation unlike past fabric studies interpretations were guided by results of laboratory experiments in which this till was deformed in simple shear to high strains fabric strengths indicate that more than half of the till sampled has a 5 probability of having been sheared to moderate strains 7 30 secular changes in fabric azimuth over the thickness of the till probably due to changing ice flow direction as the lobe receded indicate that the bed accreted with time and that the depth of deformation of the bed did not exceed a few decimeters orientations of principal magnetic susceptibilities show that the state of strain was commonly complex deviating from bed parallel simple shear deformation is inferred to have been focused in shallow temporally variable patches during till deposition from ice

flow dynamics and till genesis associated with a marine based antarctic palaeo ice stream geophysical and geological data indicate that during the last glacial cycle a palaeo ice stream drained the antarctic peninsula ice sheet apis through marguerite bay to the edge of the continental shelf via a bathymetric trough marguerite trough mega scale glacial lineations msgl in outer marguerite trough are formed in an acoustically transparent sediment unit consisting of a soft deformation till the association of msgl with deformation till indicates that bed deformation occurred beneath the ice stream and the thickness of the subglacial deforming layer was on the order of several metres msgl within outer marguerite trough are related to a pre existing thick sequence of soft sediments on the outer shelf and are inferred to have formed by a combination of subglacial sediment deformation by attenuation from a point source and groove ploughing morphologically the msgl differ in several respects from those documented elsewhere and this variation suggests a polygenetic origin for this subglacial landform an initial advance of the apis across the marguerite bay continental shelf at the lgm deposited a stiff subglacial till following this advance the ice margin within marguerite trough retreated 70 100 km before stabilising and depositing a grounding zone wedge an ice stream then developed in the trough and extended to the shelf edge forming a soft deformation till development of the ice stream may have been a glacio dynamic response to regional deglaciation subsequent retreat of the marguerite trough ice stream was underway by 13 490 uncorrected 14c years before present based on the geophysical and core data ice stream retreat appears to have been rapid and it contrasts with reconstructions of palaeo ice streams from other locations in antarctica this implies marked regional variations in antarctic ice stream dynamics during late quaternary deglaciation

drumlins and catastrophic subglacial floods shaw j kvill d and rains b 1989 drumlins and catastrophic subglacial floods in j menzies and j rose editors subglacial bedforms drumlins rogen moraine and associated subglacial bedforms sediment geol 62 177 202 a number of lines of evidence are shown to be consistent with drumlin formation by catastrophic meltwater floods drumlins are associated with landforms related to such floods and landscapes produced by outburst floods in extragiacial areas contain drumlin like landforms some drumlins which contain mainly residual materials are compared to similar forms covering a wide range of scales and are interpreted to be scour remnant ridges produced by the action of horseshoe vortices on subgiacial materials these vortices are a product of flow separation in turbulent fluids and this interpretation implies drumlin formation by meltwater other drumlin calied cavity fill drumlins have distinctive shapes that are analogous with infilled erosional marks produced by turbulent separated flows these drumlins are interpreted as the infills of cavities eroded upwards into glacier beds by broad subgiacial meltwater sheetflows the sedimentology and internal structure of the cavity fill drumlins indicate rapid deposition from hyperconcentrated flow clast shape surface texture and lithology are consistent with the short distance of giaciofhivial transport predicted by the cavity fill hypothesis estimates of flow discharge 0 6 6 x 107 m3 s total volume of flow 84 1 103 km3 and flow duration 16 162 days are given for the catastrophic flood proposed to have produced the livingstone like saskatchewan drumlin field subgiacially derived floods are used to explain the sudden drainage of extraglacial lakes and major depositional events in proglacial lakes and it is suggested that they might have contributed to the lid of cold freshwater over the north atlantic held to have caused a late glacial cooling event the flood hypothesis requires extensive subgiacial lakes to have occurred beneath the laurentide and fennoscandian ice sheets which if they existed must have influenced the form and behaviour of these major bodies of ice

water pressure coupling of sliding and bed deformation ii velocity depth profilesr b alley 130 139water pressure coupling of sliding and bed deformation iii application to ice stream b antarctica

antarctic palaeo ice streams we review the geomorphological sedimentological and chronological evidence for palaeo ice streams on the continental shelf of antarctica and use this information to investigate basal conditions and processes and to identify factors controlling grounding line retreat a comprehensive circum antarctic inventory of known palaeo ice streams their basal characteristics and minimum ages for their retreat following the last glacial maximum lgm is also provided antarctic palaeo ice streams are identified by a set of diagnostic landforms that nonetheless display considerable spatial variability due to the influence of substrate flow velocity and subglacial processes during the lgm palaeo ice streams extended via bathymetric troughs to the shelf edge of the antarctic peninsula and west antarctica and typically to the mid outer shelf of east antarctica the retreat history of the antarctic ice sheet since the lgm is characterised by considerable asynchroneity with individual ice streams exhibiting different retreat histories this variability allows antarctic palaeo ice streams to be classified into discrete retreat styles and the controls on grounding line retreat to be investigated such analysis highlights the important impact of internal factors on ice stream dynamics such as bed characteristics and slope and drainage basin size whilst grounding line retreat may be triggered and to some extent paced by external atmospheric and oceanic forcing the individual characteristics of each ice stream will modulate the precise timing and rate of retreat through time

physical environment of drumlin formation review of published descriptions of drumlin fields suggests that the following conditions are important to drumlin growth 1 compressive longitudinal and possibly extending transverse strain rates in the ice 2 thin ice such as occurs near the glacier margin and 3 high pore water pressure in the subglacial sediments most drumlin fields display all of these and no fields of well developed drumlins were found that did not on the oilier hand the lithology of drumlin forming sediment appears not to be important in promoting drumlin growth since it varied widely nor are the lithology and large scale topography of the bed

subglacial bedforms of the zemgale ice lobe south eastern baltic abstract presented in this paper are the results of the mapping of 6600 subglacial bedforms of the zemgale ice lobe zil their composition and internal structure in the south eastern baltic region topographic maps at scale 1 10 000 were mainly used to map bedforms in latvia while a digital elevation model with cell size of 5 m was used for north central lithuania the zil operated during the deglaciation of the late weichselian scandinavian ice sheet sis at least in the middle lithuanian and north lithuanian glacial phases and created a subglacial landform assemblage consisting of glacial lineations such as drumlins mega flutings mega scale glacial lineations msgls and transversal morphologies such as ribbed moraines it is observed that lineations of different scales co exist suggesting subglacial bedform transition and possibly a continuum of bedforms highly elongated lineations with a length of up to 24 km and an elongation ratio of up to 1 50 interpreted as msgls are observed in the ne part of the middle lithuanian plain in the so called zemgale drumlin field 20 of lineations have elongation ratios 10 1 indicating the fastest ice flow in the central part of the main body of the lobe based on test surveys and investigated outcrops the drumlin cores consist of sorted sediments with different levels and depths of glaciotectonic deformation overlain by subglacial till the main drumlin forming ice stress was mostly parallel to the crests of drumlins while some ice stress from inter drumlin depressions acted during the final episodes of their formation the observed sediment structures indicate multiple episodes of the basal ice bed coupling and decoupling which can best be explained by the mosaic ice bed deformation model ribbed moraines are characterized by a complicated structure of multiple till units interbedded with sorted sediments that have been formed by repeated subglacial overthrusting

the stability of a viscous till sheet coupled with ice flow considered at wavelengths less than the ice thickness a turbation method is used to analyse the stability of a thin till layer overlain by a deep ice layer ice is modelled as a linearly viscous fluid while the till viscosity has power law dependence on stress and effective pressure a linearized set of equations yields descriptions of the coupling of the ice flow with the sediment flow and reveals parameter ranges where the till perturbation amplitude can grow this shect flow instability is an essential part of any theory of drumlin formation and shows that viscous models of till have the ability to explain typical deforming bed features this is of great significance for large scale ice shect modelling

on the formation of large subglacial lakes the role of subglacial water storage beneath continental ice sheets is investigated primarily for a deformable bed subglacial ponding is shown to occur under most regions of warm based ice sheets and large subglacial lakes can become established for example in the hudson bay basin the formation of large lakes depends upon the fact that the ice surface gradient is reduced once subglacial ponding occurs and upon the feedback between the reduced ice surface gradient and increased subglacial ponding subglacial ponding likely played a large role in determining the ice sheet topography during late deglaciation and in speeding up the deglaciation process

theory of glacial erosion transport and deposition as a consequence of subglacial sediment deformation a theory of erosion transport and deposition of unlithified sediments by glaciers is presented it predicts the large scale areal distribution of zones and rates of erosion and deposition in time and space through a complete glacial cycle together with the resultant intensity of large scale lincations drumlins which will be incised in the landscape the theory also predicts the dispersal patterns of subglacial lithologies together with the form of dispersal trains derived from distinctive sources and the vertical and horizontal distribution of lithologies within a till it predicts major erosional discontinuities within tills and the formation of boulder pavements it suggests that the dominant proportion of the lowland tills produced by pleistocene mid latitude ice sheets was generated by subglacial deformation and explains why they are predominantly fine grained the theory is based on an analysis of glacier dynamic processes and therefore can be used to infer the dynamic behaviour of former ice sheets from the distribution of tills and their lithologic composition

drumlinization and drumlin forming instabilities viscous till mechanisms glacially induced flow naturally tends to thin and extended till cover through shock formation even in the absence of longitudinal gradients in the applied stress thicker till cover has an increased effective pressure at its surface and base a lower sliding velocity or deformation rate and above a critical thickness a decrease in wave velocity with thickness leading to reverse facing shocks moving downstream for sliding and for some theologies of internal deformation a decrease in sediment flux with thickness occurs implying backward moving kinematic waves and reverse facing reverse moving shock downstream facing shocks are also formed which move upstream if the till is sliding and downstream if the till is deforming internally eventually shocks coalesce leaving an upstream lacing shock for sliding and a downstream facing shock for internal deformation it is observed that some drumlins have downstream blunt ends only fairly realistic three dimensional drumlin shapes can be produced from symmetric sediment bodies and barchan shapes can be produced from linear forms perpendicular to the ice sheet flow the fact that viscous theories produce drumlinoid forms suggests that on this scale till behaves viscously and the the lower length scale for drumlins represents the plastic viscous transition scale

anomalous pn amplitudes through the southeastern tarim basin and western tien shan along two profiles observations and interpretations abstract we observe substantial amplitude anomalies of pn waveforms that propagated through the southeastern tarim basin and western tien shan along the two profiles which are from an earthquake that occurred on the southeastern margin of tarim basin although there is only a 5 difference in azimuth from the epicenter for the two profiles the pn amplitude differences along them are significant the synthetic seismograms in the band of 0 5 1 5 hz for different models along the two profiles are simulated to interpret the observed anomalies the simulation results indicate that the pn anomalies along the two profiles may not result from the earthquake focal mechanism or the average layered mantle lid structure the moho topography along the r a profile may mainly account for the drastic variation of pn amplitudes in 0 5 1 5 hz as the epicentral distance increases from 850 to 1100 km whereas the moho topography and the velocity heterogeneity may together account for the observed pn anomalies in the band of 0 5 1 5 hz along the r b profile

black box modeling of the subglacial water system measurements of water pressure beneath trapridge glacier yukon territory canada yield the following generalizations about subglacial conditions in the studied region 1 even over short distances the subglacial water system is highly heterogeneous 2 the subglacial water system consists of at least two distinct components which we refer to as the connected and unconnected water systems 3 regions of the glacier bed can switch back and forth from being part of the connected or part of the unconnected water system 4 large spatial pressure gradients can exist within the unconnected water system and between the connected and unconnected systems 5 rapid pressure variations can occur in the unconnected water system 6 pressure variations in the unconnected water system do not match those in the connected system and can in fact be strongly anticorrelated with pressure variations in the connected system if the water pressure variations in the connected system are viewed as a forcing and those in the unconnected system as a response to this forcing the input output relation between forcing and response can be efficiently represented as a low order nonlinear ordinary differential equation the response of the unconnected system to forcing from the connected system is governed by time constants having approximate magnitudes of 1 7 hours and 7 4 hours that we believe are associated with process rates for substrate compression and pore water diffusion respectively

the effects of pre existing discontinuities on the surface expression of normal faults insights from wet clay analog modeling abstract we use wet clay analog models to investigate how pre existing discontinuities i e structures inherited from previous tectonic phases affect the evolution of a normal fault at the earth s surface to this end we first perform a series of three reference experiments driven by a 45 dipping master fault unaffected by pre existing discontinuities to generate a mechanically isotropic learning set of models we then replicate the experiment six times introducing a 60 dipping precut in the clay cake each time with a different attitude and orientation with respect to an initially blind 45 dipping master normal fault in all experiments the precut intersects the vertical projection of the master fault halfway between the center and the right hand lateral tip all other conditions are identical for all seven models by comparing the results obtained from the mechanically isotropic experiments with results from experiments with precuts we find that the surface evolution of the normal fault varies depending on the precut orientation in most cases the parameters of newly forming faults are strongly influenced the largest influence is exerted by synthetic and antithetic discontinuities trending respectively at 30 and 45 from the strike of the master fault whereas a synthetic discontinuity at 60 and an antithetic discontinuity at 30 show moderate influence little influence is exerted by a synthetic discontinuity at 45 and an antithetic discontinuity at 60 from the strike of the master fault we provide a ranking chart to assess fault to discontinuity interactions with respect to essential surface fault descriptors such as segmentation vertical displacement profile maximum displacement and length often used as proxies to infer fault properties at depth considering a single descriptor the amount of deviation induced by different precuts varies from case to case in a rather unpredictable fashion multiple observables should be taken into consideration when analyzing normal faults evolving next to pre existing discontinuities

drumlins products of controlled or uncontrolled glaciodynamic response abstract two schools of thought on the process of drumlin formation have recently developed that have created a controversy within this ara of glacial geology one hypothesis contends that drumlins are in the main developed within a soft mobile subglacial debris zone the other school argues that for those drumlins that contain primary deposited stratified sediment formation during catastrophic subglacial flood events is necessary a classification of subglacial bed types is presented that allows for the accommodation of both schools it is proposed that a new paradigm for subglacial research may be needed in order that a closer relationship between glaciodynamics and sedimentological processes be established in future attempts at explaining glacial processes and froms

mega scale glacial lineations and cross cutting ice flow landforms landsat images reveal a previously unsuspected large scale pattern of streamlining within drift that is assumed to reflect former phases of ice flow such a glacial grain can be regarded as a landform assemblage comprised of a number of components drumlins and megaflutes form part of the pattern but in addition there are two previously undocumented ice moulded landform elements streamlined lineations of much greater proportions referred to as mega scale glacial lineations and a distinctive cross cutting topology within the grain the ice moulded landform assemblage is described and illustrated with reference to examples from canada possible modes of genesis of such landforms are discussed and their glaciological implications outlined the discovery of this pattern indicates the pervasive nature of subglacial deformation of sediment and demands a radical re interpretation of ice sheet dynamics

subglacial till a physical framework for its properties and processes the continuum theory of mixtures provides a framework around which present knowledge of subglacial till can be organized the proposed framework is provisional in two senses the constitutive description of till will require improvement and not all potentially significant processes have been included till is treated as a three component mixture of water clasts coarse grained solids and fine grained matrix solids these components obey mass balance equations and their behavior is subject to constitutive relations that describe such processes as darcian water transport fine grained sediment transport consolidation shear deformation dilatancy and comminution five state variables uniquely determine the compressibility permeability shear strength and other physical properties numerical solutions of the state evolutions equations illustrate that the properties of till are highly responsive to changes in the subglacial environment

formation and deformation of basal till during a glacier surge elisebreen svalbard elisebreen a retreating valley glacier in nw svalbard experienced a surge in the past possibly during the little ice age when ice was thicker and the bed warmer the surge produced an assemblage of subglacial landforms and sediments now appearing in front of the downwasting ice margin we have analysed an array of sedimentary properties of basal till from glacial flutings ploughing marks an interflute ground moraine and meandering ridges in order to infer processes of till formation and deformation during the unrecorded surge till in flutings and ploughing marks reveals higher mechanical abrasion than in other landforms but grain size distribution and mineralogy are largely the same everywhere micromorphological characteristics are also very similar and comprise ductile and brittle deformation structures indicating fluctuating porewater pressures frequent micrograin fracturing suggests stress concentration along grain bridges during the surge till was formed by a combination of lodgement and deformation caused mainly by clast ploughing meandering ridges containing till with water escape structures indicate that surge termination was facilitated by rapid water evacuation from the ice bed interface through r type channels micromorphological similarity of till found in flutings ploughing marks and interflute areas suggests that the resolution of the micromorphological signature is insufficient to constrain the different processes that led to the formation of those landforms

flow mechanism of glaciers on soft beds subhourly measurements of bed deformation bed shear strength subglacial water pressure and surface speed at storglaciaren a glacier in northern sweden showed that the shear strain rates of the bed decrease during periods of high water pressure and surface speed high water pressures appear to be accompanied by a reduction in the coupling of ice with the bed that is sufficient to reduce or eliminate shearing the instability of large ice masses may result from similar decoupling rather than from pervasive bed deformation as has been commonly thought