planetary landscape systems a limitless frontier if it is to be a complete science of landforms and landscapes geomorphology is not appropriately limited geographically to the terrestrial portions of earth s surface various systems of landforms and their generative processes are best understood in a full planetary context moreover by extending its purview to include the nature of landscapes on earth like planets geomorphological inquiry is not appropriately limited in its philosophical presumptions to the reductionist views that have so successfully guided much of physics holistic thinking exemplified by some aspects of evolutionary biology and a systems framework may prove to be particularly fruitful for understanding future extraterrestrial discoveries and the general nature of landforms and landscapes copyright 2008 john wiley sons ltd

rheological properties of water ice applications to satellites of the outer planets 1 abstract the icy moons of the outer solar system have not been quiescent bodies in part because many have a substantial water component and have experienced significant internal heating we can begin to understand the thermal evolution of the moons and the rate of viscous relaxation of surface topography because we now have good constraints on how ice in several of its polymorphic forms flows under deviatoric stress at planetary conditions details of laboratory derived flow laws for pure polycrystalline ice are reviewed in detail one of the more important questions at hand is the role of ice grain size grain size may be a dynamic quantity within the icy moons and it may or may not significantly affect rheology one recent beneficiary of revelations about grain size sensitive flow is the calculation of the rheological structure of europa s outer ice shell which may be no thicker than 20 km

upper limits of power law creep of rocks at depths below a few kilometers most rocks probably flow in the steady state dominantly by diffusion assisted dislocation creep mechanisms whereby strain rate is proportional to a low power of deviatoric stress at high stress levels for metals ceramics and rocks the power law breaks down plb to some other strain rate stress functional relationship and this paper explores four methods by which the breakdown stress b may be estimated for metals alloys and ceramics b 10 3 for non silicate rocks b 5 10 3 and for silicates b 10 2 for silicates the low stress exponent power law breaks down at high stresses to an exponential dependence of strain rate upon stress or to a dorn law the governing flow mechanisms generally being lattice resistance controlled dislocation glide and mechanical twinning plb breakdown stresses for silicates are nearly strain rate insensitive and extrapolated values of b are so high as to suggest that low temperature high stress plasticity may be suppressed entirely in geological deformations plb for olivine rich rocks is discussed in some detail and new three dimensional deformation mechanism surfaces for wet and dry olivine polycrystals are presented

rheology of ice at the bed of engabreen norway a three dimensional finite element model is used to analyze field data collected as dirty basal ice flowed past an instrumented obstacle at the bed of engabreen a temperate glacier in northern norway the ice is modeled as an incompressible power law fluid with viscosity b 2 1 n 2n d where d is the second invariant of the stretching tensor and b and n are two parameters using measurements obtained in 1996 and 1997 two values of b are obtained one using the measured normal stress difference across the obstacle and the other using the measured bed parallel force over the instrument these two values are not equal probably owing to small frictional forces at the bed unaccounted for in the numerical model hence b ranges between 1 9 10 7 and 3 2 10 7 pa s 1 3 in 1996 and between 2 2 10 7 and 4 1 10 7 pa s 1 3 in 1997 these values are smaller than measured elsewhere for clean glacier or laboratory ice field measurements of water content fabric and texture of the basal ice suggest that unbound water between thin sediment layers and lamellae of clean ice may act as a lubricant and significantly weaken the ice near isotropic fabrics indicate that preferred fabric orientation does not enhance the deformation

strain induced phase changes within cold basal ice from taylor glacier antarctica indicated by textural and gas analyses this paper reports detailed textural and gas measurements conducted in cold basal ice 178c from the margin of taylor glacier an outlet glacier of the east antarctic ice sheet the analyzed samples were retrieved from a basal ice sequence excavated at the end of a subglacial tunnel dug near the glacier snout the basal sequence exhibits two contrasting ice facies defined as the englacial and stratified facies on the one hand analysis of ice crystal textures from the basal ice sequence provides evidence for localized ductile deformation especially within the stratified facies where significant dynamic recrystallization was detected on the other hand high resolution gas analyses reveal that strong changes in gas composition occurred at the structural interfaces of the stratified facies these gas composition changes are typical of melting refreezing processes but are not associated with any significant loss of gas volume given the specific subglacial thermal conditions at the margin of taylor glacier we interpret this phenomenon as resulting from microscopic phase changes involving selective gas redistribution through the pre melt phase it is argued that such processes may play an important role in the post genetic geochemical evolution of cold debris laden ice and may be enhanced through intense strain conditions

effects of dispersed particulates on the rheology of water ice at planetary conditions we have investigated the effects of initial grain size and hard particulate impurities on the transient and steady state flow of water ice i at laboratory conditions selected to provide more quantitative constraints on the thermomechanical evolution of the giant icy moons of the outer solar system our samples were molded with particulate volume fractions of 0 001 to 0 56 and particle sizes of 1 to 150 m deformation experiments were conducted at constant shortening rates of 4 4 10 7 to 4 9 10 4 s 1 at pressures of 50 and 100 mpa and temperatures 77 to 223 k for the pure ice samples initial grain sizes were 0 2 0 6 mm 0 75 1 75 mm and 1 25 2 5 mm stress strain curves of pure ice i under these conditions display a strength maximum u at plastic strains e 0 01 after initial yield followed by strain softening and achievement of steady state levels of stress ss at e 0 1 to 0 2 finer starting grain size in pure ice generally raises the level of u petrography indicates that the initial transient flow behavior is associated with the nucleation and growth of recrystallized ice grains and the approach to ss evidently corresponds to the development of a steady state grain texture effects of particulate concentrations 0 1 are slight at these concentrations a small but significant reduction in u with respect to that for pure water ice occurs mixed phase ice with 0 1 is significantly stronger than pure ice the strength of samples with 0 56 approaches that of dry confined sand the magnitude of the strengthening effect is far greater than expected from homogeneous strain rate enhancement in the ice fraction or from pinning of dislocations orowan hardening this result suggests viscous drag occurs in the ice as it flows around the hard particulates mixed phase ice is also tougher than pure ice extending the range of bulk plastic deformation versus faulting to lower temperatures and higher strain rates the high pressure phase ice ii formed in 0 56 mixed phase ice during deformation at high stresses bulk planetary compositions of ice rock 0 4 0 5 are roughly 2 orders of magnitude more viscous than pure ice promoting the likelihood of thermal instability inside giant icy moons and possibly explaining the retention of crater topography on icy planetary surfaces

direct observation of basal sliding and deformation of basal drift at sub freezing temperatures in a tunnel at the base of sub polar urumqi glacier no 1 china three new mechanisms of glacier flow at sub freezing temperatures have been observed taken individually or in combination these modes of flow can account for nearly all 60 80 of the overall glacier motion and yet they act only within the lowermost 1 2 of the effective glacier thickness these mechanisms are 1 enhanced deformation of the frozen and ice laden subglacial drift 2 motion across discrete shear planes or shear bands within the frozen drift or at the ice drift interface and 3 basal sliding at an ice rock interface at a temperature of nearly 5 c the ice laden drift has an effective viscosity of more than one hundred times less than that measured in the overlying ice thus allowing very rapid shear deformation the observed rate or basal sliding at the ice rock interface agrees favorably with that predicted by the recent work of shreve 1984 if proper account is taken of the measured surface roughness and reduced ice viscosity

dynamic implications of discontinuous recrystallization in cold basal ice taylor glacier antarctica crystallographic investigations have been conducted of cold 17 c debris bearing ice from the base of an antarctic outlet glacier taylor glacier the 4 m thick sequence studied has been retrieved from a 20 m long tunnel dug from the glacier snout and has been analyzed with an automatic ice fabric analyzer aifa the top and bottom of the sequence consists of clean meteoric ice englacial facies whereas alternating debris rich and clean bubbly ice layers are found in the middle part stratified facies ice from the englacial facies displays a polygonal texture and a strong c axis clustering toward the vertical denoting recrystallization through subgrain rotation sgr in contrast clean ice from the stratified facies shows sgr fabrics which are delimited at the contact with debris rich layers by large interlocking grains organized in ribbons these two distinct textures within the stratified facies are associated with looser c axis patterns at the scale of single thin sections which is interpreted as resulting from migration recrystallization mr the change from sgr to mr trends marks a clear increase in grain boundary and nucleation kinetics hence the term discontinuous recrystallization and may be associated with strain localization at rheological interfaces during basal ice genesis analogies with bottom ice from deep polar ice sheets where temperature is commonly higher than at the studied site are highlighted two recrystallization scenarios are proposed accounting for the development of both types of fabrics it is shown that by controlling the repartition of stress and strain energy within basal ice the rheology of debris bearing ice layers plays a decisive role in recrystallization dynamics at structural interfaces we also demonstrate how the same recrystallization regimes may occur in cold glaciers and temperate ice sheets provided that strain accumulation has been high enough in the former this challenges the common belief that migration fabrics observed in bottom ice from deep ice sheets are exclusive to warm stagnant annealed ice copyright 2008 by the american geophysical union

sand intraclasts as evidence of subglacial deformation of middle pleistocene permafrost north norfolk uk sand intraclasts are common within the bacton green till member a glacitectonic mlange subjected to polyphase deformation during the middle pleistocene in north norfolk uk the intraclasts range from a few tens of centimetres to 10 m in length and have sharp contacts with the surrounding till sand within the intraclasts is unconsolidated and contains well preserved primary stratification the wrapping of glacitectonic foliation around the intraclasts and the development of folds relating to mechanical instabilities indicate that the intraclasts acted as competent masses within a more easily deformable fine grained till that accommodated the majority of the strain sharp contacts and distinctive heavymineral assemblages indicate little intermixing between the sand and till five hypotheses about the entrainment and evolution of the intraclasts are tested against sedimentological structural and mineralogical observations the most reasonable hypothesis attributes the intraclasts to glacitectonic deformation of warm permafrost initial ice advance caused large scale thrusting of proglacial permafrost that led to the stacking of pre glacial and ice marginal sediments that were subsequently deformed sub marginally to generate the intraclasts preservation of primary stratification within the intraclasts is attributed to deformation at temperatures slightly below the pressure melting point when pore ice cemented the intraclasts as rigid bodies at the same time deformation was concentrated into the surrounding finer grained till because of its significant liquid water content and ductile rheology it is concluded that the intraclasts provide a criterion to identify past glacierepermafrost interactions and a potential means of differentiating between subglacial deformation under unfrozen and partially frozen conditions

progressive proglacial to subglacial deformation and syntectonic sedimentation at the margins of the mid pleistocene british ice sheet evidence from north norfolk uk abstract detailed analysis and interpretation of the stratigraphy and structures developed within polydeformed sediments exposed in the coastal section between west runton and sheringham eastern england has allowed the characterisation of the glacitectonic signature associated with the advance of a major mid pleistocene anglian ice sheet the sequence of pre glacial deposits and glacial sediments laid down during earlier southerly directed ice advances d1 d2 is deformed and disrupted by an ice advance from the west southwest that marks a major change in ice flow dynamics in northern east anglia the simplest interpretation of this deformation event d3 is in terms of a progressive proglacial to subglacial deformation model proglacial deformation occurred in advance of the ice margin and was dominated by thrusting positive topographic features which developed above the propagating thrusts controlled sediment dispersal patterns within outwash sandur leading to the formation of small sub basins between these morainic ridges the accretion of the wedge shaped proglacial thrust moraines to the main push moraine formed at the margin of the advancing glacier may have temporarily stalled forward motion of the ice thrusting and large scale folding associated with ice marginal deformation led to the stacking of material excavated from further up ice onto the stoss side of these accreted thrust related moraines that allowed the glacier to override the obstruction subglacial deformation was highly variable in its style and intensity ranging from heterogeneous folding and thrusting through to more pervasive ductile shearing associated with the formation of a subglacial shear zone the thickness and complexity of this shear zone increases up ice where it is characterised by a thick glacitectonic melange the pore water content of the deforming sediments controlled the pattern of deformation within the shear zone however an increase in the efficiency of the drainage system towards the front of the glacier would have led to a rapid thinning of this water enhanced zone of ductile shear evidence from north norfolk suggests that zones of preferential pore water flow within the glacier bed are a major controlling factor on the location of the subglacial shear zone consequently the bulk of the forward motion of the glacier may not be accommodated by shearing within the sediments immediately below its base but occur at a deeper level within the deforming bed

progressive soft sediment deformation within a subglacial shear zone a hybrid mosaic pervasive deformation model for middle pleistocene glaciotectonised sediments from eastern england this paper presents a hybrid pervasive and mosaic model for the progressive deformation of a stratified diamicton complex in eastern england within an active subglacial shear zone sedimentary evidence from undeformed low strain zones towards the base of the sediment pile indicates that the tectonised sediments were originally deposited as a series of subaqueous flows in a glaciolacustrine basin these sediments have subsequently been deformed during a progressive subglacial shearing event this event is divided into three stages ductile folding and sediment remobilisation d1 brittle faulting in the form of reidel shears d2 and hydrofracturing and sediment remobilisation d3 the dominant control on the pattern and style of deformation appears to relate to the rate of thrust induced till accretion and the aquifer properties and pore water content and or pressure of the deforming sediments

flow properties of the ice from the greenland ice core project ice core the reason for folds long term deformation tests on ice from the greenland ice core project grip deep ice core show that ice from the different climate zones in the ice core has flow properties correlated with the concentrations of impurities in the sample the deformation tests are performed by uniaxial unconfined compression at 16 degrees c with an octahedral compression stress of 3 bars the ice samples are compressed for 1 2 to 3 years until the tertiary strain rate is reached it is believed that by the end all downhole flow conditions are forgotten and that the ice sample has settled in a state determined by the applied stress and temperature conditions all samples are tested under the same stress and temperature conditions so the resulting deformation rates and final ice crystal size and fabrics can only differ due to varying impurity concentrations the results show that ice from cold climatic periods with high concentrations of impurities deforms more slowly than ice from warm climatic periods in compression when tertiary creep is reached the crystal size is smaller in the cold ice than in the warm the ice from warmer climatic periods with lower concentrations of impurities deforms at a factor of 2 3 times more rapidly in compression the tertiary steady state crystal size is increased by 50 and the ice crystals have oriented more favorably for the applied compression in the warm ice which is believed to be the reason why the strain rates are greater here than in the cold ice in the bottom 200 m of the grip ice core zones are observed with folds on the scale of 1 8 cm an investigation of the ice layers in and around the folds shows that the layers are composed of ice from different climatic zones the folding is believed to result from the different flow and rheological properties of the layers involved in the folding structures

comment on superplastic deformation of ice experimental observations by d l goldsby and d l kohlstedt knowledge of the rheological properties of ice at devia toric stresses lower than 0 1 mpa is of great interest whenmodeling the flow of glaciers and ice sheets in spite of thedifficulty of obtaining convincing results in the laboratory thereis now agreement on a flow law with a stress exponent lowerthan 2 mellor and testa 1969 duval 1973 alley 1992 duval and castelnau 1995 this result is also supported byborehole deformation measurements dahl jensen and gun destrup 1987 and bubbly ice densification modeling lipenkovet al 1997

the flow of fireweed rock glacier alaska u s a fireweed rock glacier is a large rock glacier in south central alaska u s a it flows relatively fast with velocities up to 3 5 ma 1 and exhibits both seasonal and annual velocity variations some of which are related to periodic terminus calving and increased rainfall our analysis reveals that motion is likely concentrated in a pseudo rectangular channel within the larger parabolic channel with a shear plane at 27 m depth there is likely motion along the shear plane as well as internal deformation above it we estimate that the ice rock mixture is up to seven times softer than clean glacier ice with a temperature of 2 c calving at the terminus is an important component of the mass balance of this rock glacier

how glaciers entrain and transport basal sediment physical constraints abstract simple insights from the physics of ice water and sediment place constraints on the possible sediment transport behavior of glaciers and ice sheets because glaciers concentrate runoff streams generated by glaciers transport much sediment and may erode bedrock rapidly deforming glacier beds also can transport much sediment particularly in marginal regions rapid sediment entrainment producing thick debris rich basal zones may occur by regelation into subglacial materials and by freeze on from rising supercooled waters numerous other mechanisms may be important but primarily near ice margins especially those of advancing or fluctuating glaciers several sediment entrainment mechanisms may be active beneath a single glacier but one process is likely to be dominant at any place and time

in situ creep properties in ice rich permafrost soil an in situ analysis of naturally occurring creep has been carried out at the proposed canadian arctic gas pipeline crossing of great bear river in the northwest territories this is the third of fo

rotating micro structures in antarctic cold basal ice implications for glacier flow and its interpretation structural analyses were conducted in the basal zone of an antarctic glacier the studied basal ice sequence was retrieved from a 20 m long subglacial tunnel dug at the margin of the glacier and is at the temperature of 17 c for the first time rotating clast systems embedded within debris rich ice were thin sectioned using specially designed cutting techniques the observed structures reflect the occurrence of pervasive shearing at the base of the glacier and can be used as shear sense indicators in addition some of these structures provide evidence for the presence of thin liquid films at the time of formation despite the marked freezing temperature of the ice it is showed here that cautious analysis of deformation structures present in debris bearing ice may bring insights not only into the flow dynamics of the embedding matrix but also into the behaviour of the interstitial fluid network at the base of cold glaciers and ice sheets

the shear strength characteristics of frozen coarse granular debris the effect of ice content and normal load on the shear strength characteristics of a frozen coarse granular debris was investigated 31 shear tests were carried out in a modified shearbox allowing a sample temperature of 1 0 0 2 c and a load rate of 9 63 10 4 cm min the tests showed that as the ice content of the frozen debris was increased from 0 under saturated to 25 saturated sample shear strength was markedly increased in contrast sample shear strength was reduced as ice content was increased from 25 saturated to 100 supersaturated the changes in shear strength with increasing ice content were attributed directly to changes in internal friction and the cohesive effects of the pore ice the shear tests also indicate that shear strength increases with increasing normal load up to a critical limit above this limit dilatancy is suppressed causing the shear strength to decrease or remain relatively constant with increased normal load the stress strain curves of the 31 tests indicated that samples with higher ice contents tended to reach peak strength p with less displacement during shear moreover the difference between p and r residual strength was lowest for pure polycrystalline ice and highest for ice saturated samples the mohr coulomb failure envelopes displayed very distinctive parabolic curvilinearity the degree of curvature is thought to be a function of ice creep at low normal loads and particle fracture and crushing at high normal loads

interfacial water in polar glaciers and glacier sliding at 17 c we have observed sliding at a cold 17 oc ice rock interface beneath meserve glacier antarctica and the segregation of ice into clean lenses amidst the dirty basal layers of this glacier we interpret these as manifestation s of thin water films at ice rock interfaces we use shreve s theory for sub freezing sliding to esti mate the nominal film thickness to be at least tens of nanometers such water films should exist around rocks in most polar ices and likely have high solute concen trations due to solute rejection during regelation and due to exchange with veins and grain boundaries where impurities reside

the flow of a polythermal glacier mccall glacier alaska u s a we have analyzed the flow of polythermal mccall glacier in arctic alaska using measurements of surface velocity from the 1970s and 1990s together with measurements of ice thickness and surface slope we have investigated both the present flow and seasonal and long term flow variations our analysis of the present flow reveals that i longitudinal stress coupling is important along the entire length of the glacier and ii there is significant basal sliding beneath a 2 km long section of the lower glacier this sliding exists year round and it accounts for more than 70 of the total motion there we have developed a numerical model which shows that such a sliding anomaly causes an asymmetric decrease in ice thickness accompanying this decrease in thickness is a decrease in surface slope at the center of the anomaly and an increase in slope up glacier from it both effects are reflected in the observed surface profile of mccall glacier the longitudinal stress coupling length of mccall glacier is three times the ice thickness almost twice that typical of temperate glaciers this is a direct effect of lower strain rates which themselves are associated with the smaller mass balance gradients of arctic and continental glaciers long term variations in surface velocity between the 1970s and 1990s are explained solely by the effects of changes in glacier geometry on the deformational flow contribution this means that long term variations in the spatial patterns of longitudinal stresses and basal sliding must have been small seasonally velocities reach their annual minimum in spring and increase during the short summer melt season by up to 75 above mean winter values however the extra motion associated with the period of elevated velocities is only about 5 of the total annual motion the speed up is due to an increase in basal sliding this implies that most of the glacier bed is at the melting point the zone affected by the melt season speed up extends well up glacier of any moulins or other obvious sources for meltwater at the bed

reply to comment by p duval and m montagnat on superplastic deformation of ice experimental observations 1 in a recent paper we reported the results of a detailed experimental investigation of the rheological properties of ice goldsby and kohlstedt 2001 one of the innovative aspects of our investigation was the use of fine grained samples that enabled us to explore deformation not only by grain size insensitive mechanisms involving grain matrix dislocation processes but also by grain size sensitive flow mechanisms involving grain boundary sliding gbs an exciting discovery in our research was an extensive creep regime in which gbs contributes substantially to the flow of ice most importantly this deformation regime in which basal slip is accommodated by gbs referred to in our paper as the superplastic flow regime governs the flow of ice over a wide range of temperature grain size and stress conditions found in glaciers ice sheets and icy planetary interiors 2 in their comment on our paper duval and montagnat 2002 repeat the objection that they had raised previously montagnat and duval 2000 concerning our conclusion that superplastic deformation controls the flow of glaciers and ice sheets they assert that gbs as a significant creep mechanism is not compatible with observations on the development of fabrics and microstructures in ice sheets special emphasis is placed instead on the role of grain boundary migration as a recovery process in glacier ice they then continue by arguing that basal slip accommodated by grain boundary migration gbm rather than by gbs is the dominant deformation mechanism in ice 3 in this reply we extend the results presented in detail by goldsby and kohlstedt 1997 and goldsby and kohlstedt 2001 to further demonstrate that gbs is a very important deformation process in natural ice bodies our reasoning is based on three points 1 the microstructures reported for samples from large natural ice bodies are remarkably similar to those observed in samples deformed in the laboratory in the superplastic flow regime 2 our constitutive equation for ice is in excellent agreement with the flow behavior determined from field measurements on glaciers and ice sheets 3 contrary to the assertion of montagnat and duval 2000 and duval and montagnat 2001 fabric development is fully compatible with deformation in the superplastic regime described in our paper in addition we note that while gbm associated with dynamic recrystallization is an important recovery mechanism in ice it is not a deformation mechanism and hence cannot accommodate basal slip

structures and fabrics in glacial ice a review abstract glaciers ice sheets and ice caps represent tectonic systems driven by gravity their movement can be studied in real time and the rheological properties and strength of ice determined from laboratory experiments and field measurements all glacial ice has primary stratification exhibited by variations in grain size bubble content and debris content as it deforms with deformation dominated by plastic flow and recrystallization accompanied locally by fracture under tension a suite of structures develops that reflects the primary fabric of the ice and the anisotropy that develops as a result of cumulative deformation initial variations in solid impurity content and strain dependent anisotropy as a result of a crystallographic fabric give rise to effective viscosity increases or decreases compared to isotropic polycrystalline ice of about a factor of ten foliation develops from inherited mostly stratification or introduced mostly ice veins or fracture traces fabric elements and from dynamic recrystallization it is largely dependent on the accumulated strain which is highest at the base and near the margins of glaciers ice sheets and ice streams folds develop largely passively due to initial amplification of irregularities in the primary stratification to variations in flow with time or to inhomogeneous flow associated with shear zones and ductile accommodation around open fractures buckle folds and boudinage mostly on a small scale occur where viscosity contrast is large mostly in basal ice thrusting and wrench faulting are documented in surging glaciers but theoretically most unlikely and rare or absent elsewhere many structures interpreted as faults are not due to shear failure but rather result from shear displacements during opening and closing of tensile fractures

ice flow and rock glaciers on mars several geologic features suggest the presence of rock glaciers on the surface of mars these features include lobate debris aprons concentric crater fill and lineated valley fill the lateral extent of these rock glaciers can range from 5 km to over 20 km a simple time marching model is developed and used here to demonstrate the ability of ice and ice rock mixtures to flow under martian conditions for temperatures lower than about 220 k even pure ice becomes too rigid to flow and a glacier 5 km long could not have formed results from this model indicate temperatures 20 to 40 k higher than present average midlatitude temperatures 210 k an ice content of no less than 80 and a net accumulation rate of at least 1 cm year 1 are required to produce rock glaciers of the size seen on mars

deformation properties of subfreezing glacier ice role of crystal size chemical impurities and rock particles inferred from in situ measurements to improve understanding of the deformation properties of subfreezing polycrystalline glacier ice and in particular the role of crystal size chemical impurities and rock particle impurities we analyze in situ strain rates of the basal layers of meserve glacier antarctica strain rates were monitored on the walls of a subglacial tunnel where down flow shear stress is relatively uniform and ice properties were measured texture fabric and impurity content we propose a simple empirical model describing strain rate variations due to variations in crystal size and impurity content and we use all relevant meserve data to constrain model parameters we conclude that there is a direct dependence of strain rate on crystal size which reflects an important role for a grain size sensitive deformation mechanism such as grain boundary sliding or diffusion chemical impurities are found to enhance the grain size sensitive deformation and are found to be an important control on strain rate variations in the very impure ices of meserve glacier however the per molar sensitivity of strain rate to chemical impurity content is shown to be very low such that in the ice age ices of the greenland ice sheet there is probably an immeasurable contribution of chemical impurities to strain rate enhancement though we cannot exclude chemical enhancements as high as 1 3 there our analyses detect no direct rheologic effect of rock particles in the meserve ices which suggests that rock content is not directly responsible for the low viscosity of dirty basal layers

interrogating the time and processes of development of the las liebres rock glacier central chilean andes using a numerical flow model the las liebres rock glacier is a large 2 2 km long andean rock glacier whose internal composition and kinematics are known from previous studies we investigate its development by posing and testing the following null hypothesis the rock glacier has developed from a constant supply of debris and ground ice in periglacial conditions and resulting creep of the ice rock mixture a rheological model was formulated based on recent advances in the study of ice rock mixture rheology and calibrated on the known surface velocities and internal composition of the rock glacier we show that the rock glacier viscosity is inversely related to both water and debris fractions in agreement with recent field and theoretical studies of ice rock mixture rheology taking into account the possible variations in water fraction the model was used to estimate the time spans of development 0 91 7 11 ka rates of rock wall retreat 0 44 4 18 mm a and rates of ground ice formation 0 004 0 026 m a for the rock glacier these results support the null hypothesis of a periglacial origin of the las liebres rock glacier copyright 2016 john wiley sons ltd

mobility of icy sand packs with application to martian permafrost 1 the physical state of water on mars has fundamental ramifications for both climatology and astrobiology the widespread presence of softened martian landforms such as impact craters can be attributed to viscous creep of subsurface ground ice we present laboratory experiments designed to determine the minimum amount of ice necessary to mobilize topography within martian permafrost our results show that the jammed to mobile transition of icy sand packs neither occurs at fixed ice content nor is dependent on temperature or stress but instead correlates strongly with the maximum dry packing density of the sand component viscosity also changes rapidly near the mobility transition the results suggest a potentially lower minimum volatile inventory for the impact pulverized megaregolith of mars furthermore the long term preservation of partially relaxed craters implies that the ice content of martian permafrost has remained close to that at the mobility transition throughout martian history

frost susceptibility related to soil index properties the analysis of frost heave data on several soils confirmed that segregation potential hence frost susceptibility of saturated soils was best related to the average size of the fines fraction the specific surface area of the fines fraction and the ratio of the material s water content to its liquid limit the influence of overburden pressure can also be accounted for by an empirical relationship between the segregation potential the average size of the fines fraction and the compressibility index of the soil the segregation potential was also proportional to the relative fines content in soils where the fines do not completely fill the voids of the coarser fraction this study led to the development of a new frost susceptibility assessment methodology based on simple geotechnical routine soil index testing that was validated on a highway site on frost susceptible subgrade till key words frost heave index properties criteria soil segregation potential

the effect of particles on creep rate and microstructures of granular ice the microstructures of particle free granular freshwater ice and ice containing 1 wt of 50 10 mm uniformly distributed particles were investigated before and after compressive creep to 10 strain with stresses of 1 45 mpa at 10 c and 0 4 mpa at 5 c creep rates of particle containing ice were always higher than those of particle free ice for an initial stress of 1 45 mpa at 10 c dynamic recrystallization occurred with new grains nucleating and growing along grain boundaries for both sets of specimens and the ice with particles showed a higher nucleation rate under creep with an initial stress of 0 4 mpa at 5 c dynamic recrystallization also occurred by the nucleation and growth of new grains along the grain boundaries for ice containing particles but recrystallization in the particle free ice occurred through grain boundary migration

glacier permafrost interactions processes products and glaciological implications abstract glaciers and permafrost represent key components of the global cryosphere widely held assumptions that 1 they are largely mutually exclusive and 2 glaciers resting on permafrost are slow moving and geomorphologically ineffectual have meant that glacier permafrost interactions have been given little attention within the research literature recent research however has demonstrated that such interactions are likely to have been more extensive than previously thought particularly during periods of ice sheet growth when glaciers would have advanced over pre existing permafrost work in both modern and ancient environments has revealed that subglacial processes such as basal sliding and subglacial sediment deformation can remain active at temperatures below the pressure melting point due to the persistence of premelted liquid water consequently cold based glaciers resting on permafrost are potentially more dynamic than previously thought and are capable of creating subglacial features typically viewed as only forming beneath warm based ice in addition the active coupling of cold based ice with ice marginal permafrost means such ice masses are capable of deforming sediments and occasionally bedrock to depths of tens or even hundreds of meters and are commonly associated with the development of a range of distinctive ice marginal landforms including push or thrust moraines and hummocky or controlled moraines this reflects the influence of permafrost on the entrainment of debris rich basal ice as well as the hydraulic transmissivity of the groundwater system and the associated porewater pressures within the substrate this review considers the key characteristics of permafrost and its formation likely extent and rheological behaviour within glacial environments traditional conceptions regarding the motion and landscape impact of cold based glaciers resting on permafrost are considered before their re examination in light of recent work demonstrating the operation of basal processes at sub freezing temperatures the implications for our understanding of the dynamics of glaciers and ice sheets as well as landforms and sedimentary sequences indicative of glacier permafrost interactions are explored and exemplified with reference to modern and ancient glacial environments gaps in our existing knowledge are identified and profitable areas for future research suggested

direct shear tests on sand experimental data from unreinforced direct shear tests on sand are presented to show the influence of the boundary conditions on the test measurements the data and the theoretical considerations o

development of sediment landform associations at cold glacier margins dry valleys antarctica hambrey m j fitzsimons s j 2010 development of sediment landform associations at cold glacier margins dry valleys antarctica sedimentology 57 3 857 882 michael hambrey is corresponding author sponsorship marsden fund of new zealand university of otago research committee nerc grant no ner a s 2000 01256

rock glaciers and protalus landforms analogous forms and ice sources on earth and mars 1 the basic features and terminology of terrestrial rock glaciers are reviewed together with associated forms termed protalus lobes and protalus ramparts two basic models of rock glacier formation and flow invoke either the creep of ice derived from permafrost or glacial former glacial activity a third possible mechanism invokes landslide emplacement observations on terrestrial rock glaciers and similar forms suggest that the ice component can be produced via ground ice or from a glacier system finite element modeling of simple slope systems containing 1 a continuous ice layer buried by debris and 2 a mixture of ice and rigid blocks can both show creep but with very low rates especially at low temperatures possible martian examples of rock glacier and protalus lobe features are identified from candor chasma by wide angle mars orbiter camera moc imagery although the topography does not allow unambiguous interpretation some possible ways in which moc and mars orbiter laser altimeter mola and other mars orbiter data could help future interpretation of rock glaciers related protalus landforms and ice presence are discussed

creep and plasticity of glacier ice a material science perspective major advances in understanding the plasticity of ice have been made during the past 60 years with the development of studies of the flow of glaciers and recently with the analysis of deep ice cores in antarctica and greenland recent experimental investigations clearly show that the plastic deformation of the ice single crystal and polycrystal is produced by intermittent dislocation bursts triggered by long range interaction of dislocations such dislocation avalanches are associated with the formation of dislocation patterns in the form of slip lines and slip bands which exhibit long range correlations and scale invariance long range dislocation interactions appear to play an essential role in primary creep of polycrystals and dynamic recrystallization the large plastic anisotropy of the ice crystal is at the origin of large strain and stress heterogeneities within grains the use of full field approaches is now a compulsory proceeding to model the intracrystalline heterogeneities that develop in polycrystals ice is now highly regarded among the materials science community it is considered a model material for understanding deformation processes of crystalline materials and polycrystal modeling

topographic analysis of features related to ice on mars this study analyses the new topographic data from mars global surveyor mgs through the lobate debris aprons of mars these features are usually interpreted as due to the creep of ground ice the comparison between measured topographic profiles and profiles calculated from rheological laws demonstrates that these features are due to the solid state deformation of ice the high volume of such debris aprons can not be explained by rock fall but only by landslides the ice involved in the flow may then come from the subsurface confirming the presence of large amount of ground ice

on the special rheological properties of ancient microparticle laden northern hemisphere ice as derived from bore hole and core measurements on observe des differences dans le fluage des glaces a forte concentration en microparticules wisconsin recent qui sont plus douces que celles de la glace contemporaine ou holocene on analyse les parametres suivants concentration ou microparticules 18 o taille des cristaux axe c ca na conductivite et densite dans des carottes prelevees a agassiz iles ellesmere sur l ile devon et au groenland

air temperature snow cover creep of frozen ground and the time of ice wedge cracking western arctic coast the time of ice wedge cracking is examined for several sites with young and old ice wedges along the western arctic coast the correlation between sharp air temperature drops and ice wedge cracking is highest where the snow cover is thin and least where the snow cover is thick the favoured duration and rate of a temperature drop that results in cracking is about 4 days at a rate of about 1 8 c d such temperature drops have a minimal effect in cooling the top of permafrost wherever there is an appreciable snow cover since short duration temperature drops often result in ice wedge cracking the thermal stresses that trigger cracking probably originate more within the frozen active layer than at greater depth in permafrost although most ice wedges tend to crack during periods of decreasing air temperatures about one third of those monitored have cracked during periods of increasing air temperatures long term measurements show that the active layer and top of permafrost move differentially all year in

2 d traveltime and waveform inversion for improved seismic imaging naga thrust and fold belt india summary exploration along the naga thrust and fold belt in the assam province of northeast india encounters geological as well as logistic challenges drilling for hydrocarbons traditionally guided by surface manifestations of the naga thrust fault faces additional challenges in the northeast where the thrust fault gradually deepens leaving subtle surface expressions in such an area multichannel 2 d seismic data were collected along a line perpendicular to the trend of the thrust belt the data have a moderate signal to noise ratio and suffer from ground roll and other acquisition related noise in addition to data quality the complex geology of the thrust belt limits the ability of conventional seismic processing to yield a reliable velocity model which in turn leads to poor subsurface image in this paper we demonstrate the application of traveltime and waveform inversion as supplements to conventional seismic imaging and interpretation processes both traveltime and waveform inversion utilize the first arrivals that are typically discarded during conventional seismic processing as a first step a smooth velocity model with long wavelength characteristics of the subsurface is estimated through inversion of the first arrival traveltimes this velocity model is then used to obtain a kirchhoff pre stack depthmigrated image which in turn is used for the interpretation of the fault waveform inversion is applied to the central part of the seismic line to a depth of 1 km where the quality of the migrated image is poor waveform inversion is performed in the frequency domain over a series of iterations proceeding from low to high frequency 11 19 hz using the velocity model from traveltime inversion as the starting model in the end the pre stack depth migrated image and the waveform inversion model are jointly interpreted this study demonstrates that a combination of traveltime and waveform inversion with kirchhoff pre stack depth migration is a promising approach for the interpretation of geological structures in a thrust belt

a renewed argument for crystal size control of ice sheet strain rates at present it is generally believed that crystal size has no direct influence on strain rate in the ice sheets and that the fraction of strain rate enhancement there which is not ascribable to c axis fabric is due to impurity content here we challenge this view because it is not consistent with recent results from analyses of deformation at meserve glacier and instead ascribe residual enhancement in the ice sheets to variations in crystal size we resurrect the idea that variations of crystal size can be an important part of the total shear enhancement in the ice sheets though agree with paterson that this effect is generally dominated by variations of crystal fabric we propose that the enhanced shear strain rate of ice age ice in southern greenland as inferred from tilt of the dye 3 borehole can be explained as a result of combined fabric variations and crystal size variations with these two ice properties accounting for roughly 70 and 30 of the average enhancement respectively permitting a grain size dependence of ice viscosity also resolves the quandary concerning closure and tilt of the agassiz ice cap borehole

engineering geomorphology of rock glaciers a partnership between geomorphology and engineering is facilitating human development in this harsh environment rock glaciers provide locations for urban water sources construction borrow sources drill sites shaft and tunnel portals ski tower supports and dam abutments rock glaciers as dynamic landforms necessitate proper identification in the field placing structures on in or adjacent to rock glaciers requires an appreciation and understanding of their temporal stability internal and surface characteristics provide important clues to the development and deformation of rock glaciers rock glaciers play a significant role in the alpine debris transport system active movement and mass wasting are perhaps the most obvious geologic hazards affecting engineered works the structure of the rock glacier is conducive to the production of a steady continuous supply of meltwater during summer months thus rock glaciers serve as alpine aquifers consideration of rock glaciers as potential aquifer sources requires caution because of the long term impact of climate change on the temporal nature of the landform from the rock glaciers that we have monitored for water quality characteristics it appears that they provide quality potable water this paper provides a foundation for appreciation and understanding of rock glaciers from an engineering geomorphologic point of view the approach taken in this paper provides practical important information to aid the engineer and engineering geologist in prudent evaluations of rock glaciers as potential sites for human development and uses the bottom line of our paper is rock glaciers must be avoided for essentially all structures

strain patterns in glacitectonically thrusted sediments and conditions during thrusting abstract glacitectonically thrusted blocks of unlithified sediment are commonly inferred to have been frozen during thrusting with implications for paleoclimate and glacier dynamics we inferred strain patterns within thrusted blocks of glaciolacustrine mud in the rubjerg knude and mons klint glacitectonic complexes of coastal denmark to try to assess whether the mud was frozen during thrusting fabrics based on the anisotropy of magnetic susceptibility ams of the muds are interpreted with results of ring shear experiments and models of ams fabric development during layer parallel shortening lps in intact distal stratigraphy fabrics indicate moderate lps fabrics where thrusts have disrupted more proximal stratigraphy indicate rotation of mud sheets during thrusting with insufficient penetrative strain to reset the lps fabric the exception is within decimeters of faults at rubjerg knude where shear overprinted the lps fabric although these data indicate that mud sheets were mostly rigid during thrusting this is not necessarily evidence of permafrost critical taper theory indicates that even a gentle slope of the proglacial surface away from the glacier margin would have allowed slip without internal deformation of thrust sheets requiring only minimal pore water pressures

the basal ice layer of glaciers and ice sheets abstract in many glaciers and ice sheets there is a basal ice layer bil in which the ice is conditioned primarily by processes operating at the bed the bil is chemically and physically distinctive and is characterised by a component of basally derived sediment the bil is a rheological control on ice sheet dynamics an indicator of subglacial conditions and processes an agent of subglacial geologic processes the source of a substantial proportion of glacial sediments a limit to the downward extension of the climate record from deep ice cores if debris characteristics of the bil are preserved in glacial sediments former glacier conditions can be inferred analysis of stable isotopes gas and cation composition ice crystallography debris composition and structural glaciology has elucidated many mechanisms for bil formation common elements can be identified from a range of locations key issues are entrainment of ice and debris from the bed formation of new ice metamorphism of existing ice in the bil and thickening of the bil by deformation a distinction can be made between basal ice formed by accretion of material from the bed stratified facies and basal ice formed by metamorphic processes within the ice close to the bed dispersed facies

small strain behavior of frozen sand in triaxial compression the stress strain behavior of frozen manchester fine sand has been measured in a high pressure low temperature triaxial compression testing system developed for this purpose this system incorporates dc servomotor technology lubricated end platens and on specimen axial strain devices a parametric study has investigated the effects of changes in strain rate confining pressure sand density and temperature on behavior for very small strains 0 001 to very large 20 axial strains this paper presents constitutive behavior for strain levels up to 1 on specimen axial strain measurements enabled the identification of a distinct upper yield stress knee on the stress strain curve and a study of the behavior in this region with a degree of precision not previously reported in the literature the young s modulus is independent of strain rate and temperature increases slightly with sand density in a manner consistent with counto s model for composite materials and decreases slightly with confining p

cryostructures in permafrost tuktoyaktuk coastlands western arctic canada the shape and distribution of ice and sediment within frozen ground constitute its cryostructure observations on perennially frozen sediments in the tuktoyaktuk coastlands enable six cryostructures to be identified 1 structureless 2 lenticular 3 layered 4 reticulate 5 crustal and 6 suspended these cryostructures can be transitional or composite five cryofacies types can also be identified according to volumetric ice content and grain size they can be grouped into cryofacies assemblages the utility of the descriptive classification of ice rich sediments is illustrated from a palaeothaw layer at crumbling point

creep of granular ice with and without dispersed particles the effects of silt sized particles average diameter of 50 m on the compressive creep of polycrystalline ice have been studied at stress levels from 0 1 to 1 45 mpa and temperatures of 12 c and 10 c dislocation densities during creep have been estimated using a dislocation based model of anelasticity the results indicate that at low concentrations up to 4wt particles increase the minimum creep rate power law behavior with an exponent of 3 was observed for both particle free ice and ice with 1 wt particles when the stress was 0 3 mpa in contrast linear behavior was observed when the stress was 0 3 mpa calculations show that the linear behavior is associated with a constant dislocation density and the power law behavior is associated with increasing dislocation densities with increasing stress

in situ creep of frozen soil fenghuo shan tibet plateau china field measurements of frozen soil creep in the upper 3 0 m of permafrost indicate that creep occurs in both winter and summer between 1992 and 1993 the mean rate of creep ranged from 0 44 cm at 1 6 m depth to 0 16 cm at 2 8 m depth but there was extreme variability creep parameters n and a as defined by the power flow law were calculated from field data parameter n ranged between 1 96 and 2 29 and increased with depth while a decreased with depth comparisons of creep rates for different permafrost environments suggest that ground temperature largely controls the magnitude of permafrost creep key words permafrost creep parameters tibet plateau

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

creep behaviour of undisturbed clay permafrost an in situ analysis of naturally occurring creep has been carried out at the proposed canadian arctic gas pipeline crossing of great bear river in the northwest territories this is the last of fou

solifluction rates processes and landforms a global review abstract field data on the rates of solifluction and associated parameters are compiled from the literature in an attempt to evaluate factors controlling the spatial variability in solifluction processes and landforms with special attention on the climate solifluction relationship the analyzed data originate from 46 sites over a wide range of periglacial environments from antarctic nunataks to tropical high mountains solifluction broadly defined as slow mass wasting resulting from freeze thaw action in fine textured soils involves several components needle ice creep and diurnal frost creep originating from diurnal freeze thaw action annual frost creep gelifluction and plug like flow originating from annual freeze thaw action and retrograde movement caused by soil cohesion the depth and thickness of ice lenses and freeze thaw frequency are the major controls on the spatial variation in solifluction processes near the warm margin of the solifluction affected environment diurnal freeze thaw action induces shallow but relatively rapid movement of a superficial layer 5 10 cm thick on average often creating the thin stone banked lobes typically seen on tropical high mountains in addition to diurnal movement annual frost creep and gelifluction may occur on slopes with soil climates of seasonal frost to warm permafrost dislocating a soil layer shallower than 60 cm at a rate of centimeters per year and eventually producing medium size solifluction lobes in high arctic cold permafrost regions two sided freezing can induce plug like flow of a soil mass 60 cm or thicker the correlation between process and landform suggests that the riser height of lobes is indicative of the maximum depth of movement and prevailing freeze thaw type climate change may result in new different ground freezing conditions thereby influencing the surface velocity and maximum depth of soil movement soil moisture and topography also control solifluction high moisture availability in the seasonal freezing period enhances diurnal freeze thaw action and subsequent seasonal frost heaving the latter contributes to raising the moisture content of the thawed layer and promotes gelifluction during the thawing period the slope angle defines the upper limit of the surface velocity of solifluction a diagram correlating the potential frost creep with the actual surface velocity permits an inter site comparison of the relative magnitude of solifluction components physically based modelling of periglacial slope evolution requires synthetic and more detailed field monitoring and laboratory simulations of solifluction processes

basal ice facies a review and unifying approach abstract over the past 30 years numerous basal ice facies have been identified named and classified however the resulting facies descriptions and names are inconsistent and no single scheme encompasses all of the different ice types that exist at different ice masses in this paper we review and critique existing basal ice facies names descriptions and classification schemes and propose a new non genetic approach that has the capacity to name and describe all basal ice types we define six fundamental basal cryofacies and a further 12 composite cryofacies which can all be defined on the basis of a cursory evaluation of debris disposition and concentration more detailed cryofacies description is based on characterizing three sets of ice properties that can also be estimated visually in the field a the thickness of the basal ice facies and its constituent sub layers b the concentration and texture of debris included within any or all of those layers and c the concentration and size of bubbles included within any or all of those layers we also propose a shorthand method for the presentation of this descriptive information here codes for the layer thicknesses are presented as standard text and codes for the included debris characteristics and included bubble characteristics are presented as superscripts and subscripts respectively sub layers are characterized similarly but within a nested sequence of brackets we evaluate the effectiveness of these two schemes by using them to rename and reclassify several existing basal ice facies results indicate that the schemes are robust and that they provide a coherent non genetic framework for the effective naming and description of basal cryofacies

the impact of ice on the tensile strength of unconsolidated sand a model for gas hydrate bearing sands abstract tensile strength is an important parameter when it comes to predictions of potential fracturing of sediments by natural processes such as the emplacement of ice or gas hydrate lenses as well as anthropogenic fracturing or else the stability of engineering constructions such as boreholes yet tensile strength measurements of unconsolidated ice bearing or gas hydrate bearing sands are scarce and affected by a large variability in the course of the sugar project we successfully used ice as a model for pore filling and load bearing gas hydrate in sand to determine compressional wave velocity we were thus able to verify comparable formation characteristics and morphologies of ice and gas hydrate within the pore space as these are important values for the tensile strength of ice hydrate bearing sands ice was also used as a model for hydrate bearing sands despite differences in the mechanical behavior and strength of pure ice and gas hydrate water saturated sand cores with ice saturations sice between 0 and 100 were tested at 6 8 c the varying sice were a result of the freezing point depression caused by saline solutions of different concentrations the was directly determined using a sleeve fracturing test with an internal pressure that was created within the frozen samples the setup was also adapted to fit a pressure vessel for tests using confining pressure the correlation of sice shows an exponential increase of with sice whereas at sice 80 in conjunction with the change in strength the viscoelastic behavior changes a clear peak strength occurs at sice 80 we conclude that given 60 80 the frame gains strength while the amount of residual water decreases tensile failure and cracking now exceed grain boundary sliding as the prevailing failure mode the ice morphology in the sand is non cementing and comparable to a gas hydrate sand mixture

the galilean satellites nasa s galileo mission to jupiter and improved earth based observing capabilities have allowed major advances in our understanding of jupiter s moons io europa ganymede and callisto over the past few years particularly exciting findings include the evidence for internal liquid water oceans in callisto and europa detection of a strong intrinsic magnetic field within ganymede discovery of high temperature silicate volcanism on io discovery of tenuous oxygen atmospheres at europa and ganymede and a tenuous carbon dioxide atmosphere at callisto and detection of condensed oxygen on ganymede modeling of landforms seen at resolutions up to 100 times as high as those of voyager supports the suggestion that tidal heating has played an important role for io and europa

deformation of debris ice mixtures mixtures of rock debris and ice are common in high latitude and high altitude environments and are thought to be widespread elsewhere in our solar system in the form of permafrost soils glaciers and rock glaciers these debris ice mixtures are often not static but slide and creep generating many of the landforms and landscapes associated with the cryosphere in this review a broad range of field observations theory and experimental work relevant to the mechanical interactions between ice and rock debris are evaluated with emphasis on the temperature and stress regimes common in terrestrial surface and near surface environments the first order variables governing the deformation of debris ice mixtures in these environments are debris concentration particle size temperature solute concentration salinity and stress a key observation from prior studies consistent with expectations is that debris ice mixtures are usually more resistant to deformation at low temperatures than their pure end member components however at temperatures closer to melting the growth of unfrozen water films at ice particle interfaces begins to reduce the strengthening effect and can even lead to profound weakening using existing quantitative relationships from theoretical and experimental work in permafrost engineering ice mechanics and glaciology combined with theory adapted from metallurgy and materials science a simple constitutive framework is assembled that is capable of capturing most of the observed dynamics this framework highlights the competition between the role of debris in impeding ice creep and the mitigating effects of unfrozen water at debris ice interfaces

seismicity of oceanic and continental rifts a geodynamic approach abstract two major kinds of divergent structures oceanic and intracontinental rifts were compared in principal seismic and tectonic characteristics first the role of main components of the mid oceanic ridges mor was estimated for the whole earth we considered two levels of the mor segmentation the first order structures are the segments of mor between triple junctions and the second order structures are a transform faults and rift parts of mor the seismic catalogues neic and cmt were used to assess the seismic moment release the seismic moment release was calculated another way using the global plate tectonic model nuvel 1 and brune s formulae comparison of these two values shows that the seismic coupling coefficient varies from 1 to 10 for most of mor and is always higher for transform faults most of the deformation therefore is aseismic slip most seismicity of mor is confined to transform faults the energy contribution of transform faults is one to two orders magnitude higher than that of the rift and increases with the spreading rate there is a strong correlation between the seismic moment release of strike slip faults and their total lengths the correlation shows that the seismic moment release depends on the total transform area and confirms the simple thermal model of transform seismicity that was given by burr and solomon the seismic moment release and the spreading rate have opposite patterns for the rifts there is an inverse correlation between the seismic moment of normal faults and spreading velocity while it seems for transforms that these parameters are independent finally these results show that the seismicity of transforms and rifts depends first of all on the thermal structure of oceanic lithosphere in the case of continental rifts one can distinguish in the degree of seismic activity depending on the stage of rifting hence analysis of the continental seismicity requires the consideration of factors of a geological evolution that play practically no role in the case of oceanic lithosphere the comparison of geological and seismic data for the east african region has allowed us to outline the regular changes of the seismic regime during development of the rift zone from the stage of incipient rift to mature oceanic rift in the evolutionary series intracontinental incipient rift intracontinental mature rift intercontinental rift oceanic slow spreading rift oceanic fast spreading rift there is a gradual decrease of the role of rifts sensu stricto as tension structures and increase of the role of strike slip faults epicenters concentrate along major faults as well the level of seismic energy becomes lower although the rate of deformation increases

on the initiation of shear faults during brittle compressive failure a new mechanism brittle materials loaded under compression generally fail by shear faulting this paper addresses the initiation of the fault it presents direct observational evidence from ice which is used as a model material for rock and shows that wing cracking and splay cracking are important processes in the localization of deformation both prior to and during fault initiation wing cracks develop at the tips of sliding intergranular cracks and tend to align with the maximum principal stress splay cracks emanate from one side of the sliding parent crack the theme of the paper is that the splay cracks play the dominant role in triggering the fault the central idea is that the slender columns between the splay cracks are more likely to buckle and fail than are the columns between adjacent wing cracks because they do not have two fixed ends instead the end stemming from the inclined parent crack is free a moment is then applied by frictional sliding of the parent inclined crack and this causes the fixed free columns to break at a much lower stress than the fixed fixed columns columns created near a free surface are more likely to fail than those created elsewhere and this explains the observation that shear localization tends to initiate near free surfaces a first order calculation shows that the failure stress of the splay created columns is of the same order of magnitude as the terminal failure stress

creep and strength testing of frozen saline fine grained soils little creep data is available for frozen fine grained saline soils salinity accelerates the creep rate of a frozen soil under load and reduces its shear strength owing to the higher unfrozen water contents in the pore phase creep data for saline fine grained soils are required by engineers and scientists working in areas of coastal or offshore permafrost the data are also used in projects involving artificial freezing of saline soils this paper describes the results of 34 creep tests and 11 time dependent strength tests carried out on saline soils the tests were carried out for two different research programs consequently two different fine grained soil types were used most of the tests were carried out in a constant stress and constant temperature creep test environment some were completed in a triaxial constant strain rate test format in order to extend the data base to an area of higher strain rates the range of temperatures was 2 3 to 25 c the range of stresses in the creep tests was

fast deformation of perennially frozen debris in a warm rock glacier in the swiss alps an effect of liquid water 1 surface movement internal deformation and temperature were monitored over 5 years on buz north rock glacier a small rock glacier located at the lower limit of the permafrost belt in the swiss alps the permafrost in the rock glacier mainly consists of pebbles and cobbles filled with interstitial ice two inclinometers installed at 4 and 5 m depths showed fast deformation with large seasonal and interannual variations while the permafrost temperatures remained almost at the melting point the movement of the inclinometers coincided with changes in the surface velocities the deformation rapidly accelerated during snowmelt periods whereas it gradually decelerated below a dry snow cover in winter the frozen debris was more deformable than typical glacier ice at the melting point these phenomena suggest that the frozen debris is permeable to snowmelt water the fast deformation should result from significant annual relocation of debris particles which probably creates a network of air voids in the frozen debris that eventually allows water infiltration the meltwater infiltration accelerates the deformation by reducing effective stress resulting in the reduced strength of the frozen debris the refreezing of the pore water which depends on the cooling intensity in winter decelerates the deformation the combination of these processes controls the temporal variations in the deformation