Abstract Subsurface sediment mobilization (SSM) – which includes soft sediment deformations, sand injections, shale diapirs and mud volcanoes – is more widespread than previously thought. The ever-increasing resolution of subsurface data yielded many new observations of SSM, not only from regions obviously prone to sediment remobilization, such as an active tectonic setting or in a region with exceptionally large sediment supply, but also from tectonically quiescent areas. Until now, all the different aspects of SSM have largely been treated as separate phenomena. There is very little cross-referencing between, for example, studies of mud volcanoes and those of sand injections, although both are caused by sediment fluidization. Divisions according to sediment type, mobilization depth or triggering mechanism make little sense when trying to understand the processes of SSM. There is a gradation in mobilization processes that cause considerable overlap between categories in any classification. Hence, it is necessary to integrate our understanding of all types of SSM, regardless of scale, depth, location, grain size or triggering mechanism. In addition, polygonal faults are important in this context, as this non-tectonic structural style is closely associated with sedimentary injections and may also reflect large scale mobilization.

Abstract Geological sediments tend to strengthen during progressive burial but the interplay of porosity and permeability, strain and effective stress gives rise to numerous circumstances in which the strength increase can be temporarily reversed. The sediment becomes capable of bulk movement – sediment mobilization. Most explanations involve overpressuring, which results from additional loading being sustained by pore-fluid that is unable to dissipate adequately, leading to frictional strength reduction. The processes are highly heterogeneous, areally and with depth. The loads can be external ('dynamic') and both monotonic (e.g. a rapidly added suprajacent mass) and cyclic (e.g. the passage of waves), internal (e.g. the result of mineral reactions) and hydraulic (e.g. injection of external fluid). The sediments may become liquidized – that is, lose strength completely and behave as a fluid – through temporary fabric collapse (sensitive sediments) because loads are borne entirely by the pore-fluid (liquefaction), or by the grains becoming buoyant (fluidization), typically due to the ingress of externally derived fluids. In response to hydraulic gradients, buoyancy forces and reversed viscosity or density gradients, the weakened sediment may undergo bulk movement, though this requires failure of the enclosing material and sustained gradients. Mobilized but non-liquidized sediments retain some residual strength but can attain large shear displacements under critical state conditions.

Abstract A numerical code has been used to simulate the flow patterns in geological soft sediments that are driven by buoyancy forces resulting from reverse-density stratification. The aim was to provide a clearer understanding of the different roles of initiating conditions, inertia and rheological behaviour on the morphologies and timing of formation of natural features such as load casts and flame structures. Particular attention was paid to the cuspate form of rising intrusions that is commonly seen in nature but that has proved elusive in most earlier experiments. The numerical results demonstrate that large localised initiating perturbations and inertial influence during flow both tend to cause a decrease in the wavelength of the resulting flow pattern and can, under certain circumstances, serve to promote a cuspate morphology. The use of a relatively low viscosity Newtonian fluid as an approximation of the coarse-grained upper layer coupled with, critically, power-law behaviour in the underlying clayey layer was also found to promote a cuspate form in the rising intrusion.

Abstract The flow of glacier ice can produce structures that are striking and beautiful. Associated sediments, too, can develop spectacular deformation structures and examples are remarkably well preserved in Quaternary deposits. Although such features have long been recognized, they are now the subject of new attention from glaciologists and glacial geologists. However, these workers are not always fully aware of the methods for unravelling deformation structures evolved in recent years by structural geologists, who themselves may not be fully aware of the opportunities offered by glacial materials. This book, and the conference from which it stemmed, were conceived of as a step towards bridging this apparent gap between groups of workers with potentially overlapping interests.

Abstract In this paper we review, and supplement, existing data to investigate the character, origin and deformation of the basal silty ice of the centre of the Greenland ice sheet as revealed in the Dye3, GRIP and GISP2 cores. A major process of basal silty ice formation in the central part of Greenland is incorporation of relict non-glacial ice at the base of the ice sheet during its development. The evidence for this can be found in a stable isotope composition study, both in δD and δ 18 O, in a total gas content and gas composition study, in a comparison of the dielectric conductivity profile and chemical profiles. Ice crystallographic investigations and the study of the isotopic compositions in Nd, Sr and Pb of the mineral particles embedded in the silty ice help to clarify the situation. The processes proposed to explain the stacked sequence developed in the basal silty ice of these central areas are folding resulting in the interbedding of silty ice and glacial ice, and flow-induced mixing related to circular motion of ice in bedrock depressions with flow separation accompanied by some entrainment of the underlying ice.

Abstract Ice crystallographic measurements have been made on eight cores retrieved from temperate Glacier de Tsanfleuron, Switzerland. Cores are aligned approximately along a flow-parallel transect, allowing a stratigraphic model of crystallographic evolution at the glacier to be constructed. Results indicate the presence of four crystallographic units at the glacier. Unit 1 composed of homogeneous, fine-grained ice with a uniform fabric, is located within c. 20 m of the ice surface in the accumulation area of the glacier. Crystal growth within this unit occurs in the absence of significant stresses, and its rate is closely described by an Arrhenius-type relationship. Unit 2 ice, characterized by the local development of coarser crystals, forms after some decades of Arrhenius growth, marking the initial influence of processes of dynamic recrystallisation. Unit 3 ice, characterized by an abrupt increase in minimum crystal size, occurs at a depth of c. 33 m throughout the glacier. In the accumulation area, this increase coincides with the first evidence of systematic fabric enhancement, interpreted in terms of dynamic recrystallisation. Unit 4 ice, characterized by large, interlocking grains with a multi-modal girdle fabric, develops within c. 10 m of the glacier bed. Here, the measured minimum crystal size is consistent with a steady-state balance between Arrhenius processes of grain growth and strain-related processes of grain-size reduction. These changes are interpreted in terms of the effects of intense, continuous deformation in this basal zone.

Abstract Sulphuric acid is a naturally occurring contaminant in ice. Recently, it was demonstrated that, during compression tests at –20°C and a strain rate of 1 × 10 -5 s -1 , as little as 0.1 ppm H 2 SO 4 reduced both the peak strength and the subsequent flow stress of ice single crystals that deform primarily by basal slip. In the present work, compression tests were performed at –20°C at a variety of strain rates on both undoped ice single crystals and ice single-crystals containing 6.8 ppm sulphuric acid of various orientations again deforming primarily by basal slip. The results show that sulphuric acid dramatically decreases both the peak stress and the subsequent flow stress of ice single crystals at all strain rates. In contrast, the stress exponent was determined to be 1.89–1.97 and was unaffected by the dopant. This value is similar to values found by previous workers for undoped ice crystals.

Abstract A self-weight stress gradient is developed through the body of a glacier such that strain rates are highest in the lowest few metres as described by Glen's flow law and other stress- and temperature-dependent relationships. Conventional laboratory technology limits the size and complexity of physical models of glacier ice, particularly in the complicated basal ice layers. A geotechnical centrifuge can be used to replicate such stress regimes in a controlled environment using a scaled model of the field ‘prototype’ that is subjected to an accelerational field that is a factor N greater than that of the Earth, g. The development of a technique employing a geotechnical centrifuge as a testbed for such physical models is described. Strain rates of 10 -6 –10 –7 s -1 are calculated for models of low and moderate stress, high temperature ice. Relationships between the physical models and glacial systems suggest a scaling of the effects of transient creep by 1 : N , diffusion creep by 1 : N 2 –1 : N 3 and power law creep by 1:1. Preliminary results demonstrate the potential applications of the technique in the fields of glaciology and glacial geomorphology, in particular where low stresses and high temperatures are key characteristics of a glacial system and in systems containing several stratigraphic units.

Abstract Early structural glaciological research focused on analysis of particular structures or on mapping of structural features at particular glaciers. More recently, glacier structures have been interpreted in the context of deformation rates and histories measured or estimated using a range of techniques. These measurements indicate that glacier ice experiences complex, polyphase deformation histories that can include a wide range of types, rates and orientations of strain. Deformation styles in glacier ice resemble those in rocks, but occur at a much faster rate, allowing direct measurements to be undertaken, and providing potentially useful models of rock deformation. Structural analysis in the context of measured deformation shows that a wide range of structures (e.g. folds, foliations, boudins, shear zones, crevasses and faults) develop in response to complex strain environments, but strain does not necessarily result in the generation of structures. In the future, three-dimensional numerical modelling may be able to interpret and predict deformation histories and structural development.

Abstract Deformation histories of ice exposed at various locations on the centreline in the ablation area of surge-type Variegated Glacier were estimated using a technique based on evaluation of strain-rate fields derived from velocity gradients. The analysis indicates that ice exposed in the upper part of the ablation area a few years after the 1982–83 surge had experienced a relatively simple deformation history that included one surge-related high magnitude deformation event. Ice exposed in the terminal lobe at the same time had experienced six surge phases during its approximately 100 year-long residence time. The histories of accumulation of cumulative strain are complex, and indicate that measures of cumulative strain can mask the effects of large but transient strain events. They also demonstrate that substantial cumulative strain can be ‘undone’ during subsequent deformation to leave a cumulative strain signal that is unrepresentative of earlier cumulative strain states. Structural relationships in general do not reflect the complexity of the deformation histories experienced by the ice. A preliminary analysis of the relationships between the deformation histories and structural assemblages suggests, in particular, that brittle structures are reactivated several times during the history of the ice.

Abstract Localization of deformation in ice is known to be important at all scales in deforming glaciers. However, relatively little is known of the significance of shear localization and the influence of fabric development in anisotropic ice at microscopic scales (<mm–cm). In this experimental study, the effect of initial c -axis preferred orientation and the inclination of the primary layering in anisotropic ice masses, during both plane strain-compression and combined simple shear-compression, have been examined. A series of creep tests in the temperature range of –5 to –1°C over a range of shortening strains varying from 10 to 40% and compressive stresses ranging from 0 to 0.7 MPa have been undertaken. Significant variations in the strain rate and microstructural development have been observed in the plane strain-compression experiments that reflects the varying orientations of the anisotropy and its relationship to easy-glide directions in the ice mass. In the unconfined combined compression and shear experiments minimum shear stress rates vary between the variously oriented anisotropic ice masses and deviate from the normal power flow law for isotropic ice. Where annealing occurs, such ice masses preserve the pre-existing c -axis fabric and hence may reflect a contribution from both the recrystallized and inherited ice components.

Abstract Quantitative stress measurements related to the development of mesoscale structures in rock are difficult, if not impossible. A method for determining the stress distribution and history during the development of mesoscale boudinage structures in ice is introduced here. Boudinage structures in fracture trace ice have been observed in the outlets glaciers of the Framnes Mountains, east Antarctica. Fracture traces are preserved when crevasses fill with surface melt water, which freezes to form coarse-grained columnar ice. A 4.0 km flow-parallel traverse across the Central Ice Stream of the Framnes Mountains is presented to illustrate the boudinage of fracture traces with a mean width of 0.30 m. Field-based measurement of the geometric evolution of boudinage structures has been combined with surface flow rate measurements to quantitatively determine the strain rate at which the boudinage structures formed. The strain rate measurements provide boundary constraints on several two-dimensional finite difference models that have been used to analyse the stress distribution related to the formation of boudinage structures in a visco-elastic solid. The results reveal the development of pressure-shadows during the boudinage of layered rocks, and demonstrate the degree of refraction of stress across rheological boundaries, which have important ramifications for the analysis of planar and linear fabrics in rocks.

Abstract The three-dimensional stress and strain fields derived through high-resolution flow modelling of Haut Glacier d'Arolla, Switzerland are used to predict the generation, passage and surface expression of a variety of structural forms at the glacier. Flow vectors and strain ellipses are computed and illustrated in plan-form and long section. The model is used to predict the formation and orientation of surface crevasses, and, once healed, the downglacier evolution of their traces. Similarly, the evolution of primary stratification where it crops out at the glacier surface is predicted. The resulting stratification pattern compares well with that revealed in aerial photographs of the glacier. Finally, the three-dimensional strain field is used to track the (accumulation area) burial, (englacial) transport and (ablation area) exposure of ice deposited within a pre-defined elevation range. This deposition – transport – exposure tracking allows the location of ice that was initially deposited at any defined location on the glacier to be identified. Such information is of significance in interpreting, for example, the distribution of ash and isotopic horizons within a glacier. We conclude that high-resolution three-dimensional flow modelling has the potential to provide a powerful tool for investigating the genesis and evolution of valley glacier structures.

Abstract An active fold system revealed by interbedded tephra layers is visible on the ablation surface of Johnsons Glacier (Livingston Island, South Shetland Islands, Antarctica). Johnsons Glacier is a cirque-shaped glacier of 5 km 2 area located in a temperate ice cap. Converging flow-lines as a consequence of the reducing channel section extend from ice divides and terminate in a calving ice-cliff. Recent tephra layers from the volcano on Deception Island constitute excellent markers of the internal structure of the glacier and, when dated, provide valuable information about deformation kinematics. Detailed field mapping of the clearly visible tephra markers revealed several folded layers that define a set of folds with sub-horizontal and fan-distributed axes being sub-parallel to the converging flow-lines. Folds become tighter towards the centre of the channel, where a cylindrical anticline is clearly exposed. Related deformational structures, comprising minor folds, foliations, thrust faults and crevasses, are observed in the ablation area. Strain-rates reflect extensional flow and higher deformation values where maximum confluence occurs. We examine how the occurrence of this structure results from the transverse shortening in response to the reducing channel section and the consequent confluence of ice masses, increase in differential flow-rates between the centre of the glacier and the margins, and the development of passive folding processes.

Abstract Structures of the Pasterze glacier (Austria) have been studied in detail and interpreted as representing a natural model of an extensional allochthon formed on top of an orogenic wedge, and also a model for raft tectonics at passive continental margins. Structures of the lower, ablation-controlled portion of the glacier, including ductile structures at the base, are forming close to the melting point of ice, with predominantly brittle structures close to the surface of the glacier. Formation of these structures results from self-weight, gravity-driven spreading, similar to extensional allochthons. S-planes are common and clearly related to three individual flow units. They show a typical, spoon-like arrangement. These three flow units are bordered by centimetre- to decimetre-wide shear zones which also include shear folds. Brittle structures include ice-mineralized tension gashes, thrust and normal faults, and hybrid, shear-extensional fractures. Together, these structures show that glacial flow is more rapid in middle to upper sectors of the glacier than along the lateral, and lower margins. The coherent upper sheet is behaving in a brittle manner and is elongating slightly along the flow direction by tensional deformation. The distribution of structures allows three structural domains within the lower, ablation-controlled sector of the glacier to be distinguished: (1) an upper sector with predominant extensional structures due to rapid flow; (2) a lower sector with ductile and brittle thrust faults, penetrating from the ground and dipping strictly opposite to the local flow direction; (3) a few normal faults at the terminus that developed by rapid melting along the steep lower frontal margin of the glacier. These three structural domains are also found within extensional allochthons as exemplified by the Neogene Alpine–Carpathian system where a huge allochthon, partly driven by gravity, extruded from the Eastern Alps towards the Carpathian arc. Three similar structural domains are also found in recent analogue models and field examples of passive continental margins.

Abstract Subglacial till deformation is glaciologically and geologically important, but is difficult to model owing to numerous uncertainties related to till generation and the flow law for till deformation. I review recent results with the hope of at least focusing the uncertainties. Fine-grained subglacial tills often are sufficiently soft that continuity issues are more important than the ‘flow law’ of the till in affecting glacier behavior. Non-steady forcing, coarse clasts and an irregular ice-till contact favour till deformation to significant depths and thus rapid subglacial transport of till. Over long times, site history is important in till continuity and thus in glacier behaviour.

Abstract Tunnels in glaciers offer unique opportunities for examining basal processes. At Suess Glacier in the Taylor Valley, Antarctica, a 25 m tunnel excavated into the bed of the glacier provides access to a 3.2 m thick basal zone and the ice–substrate contact. Measurements of ice velocity over two years together with glaciotectonic structures show that there are distinct strain concentrations, a sliding interface and thin shear zones or shear planes within the basal ice. Comparison of ice composition, debris concentrations and the shear strength of basal ice samples suggest that strength is controlled by ice chemistry and debris concentration. The highest strain rates occur in fine-grained amber ice with solute concentrations higher than adjacent ice. Sliding occurs at the base of the ice that experiences the highest strain rates. The substrate and blocks of the substrate within basal ice are characterized by brittle and slow ductile deformation whereas ice with low debris concentrations behaves in a ductile manner. The range of structures observed in the basal ice suggests that deformation occurs in a self-enhancing system. As debris begins to deform, debris and ice are mixed resulting in decreased debris concentrations. Subsequent deformation becomes more rapid and increasingly ductile as the debris and sedimentary structures within the debris are attenuated by glacier flow. The structural complexity and thickness of the resulting basal ice are considerably greater than previous descriptions of cold glaciers and demonstrate that the glacier is or was closely coupled to its bed.

Abstract This paper models the operation of loading (Rayleigh–Taylor) instabilities in sediments using an effective-pressure-dependent viscosity such has been used to model the deformation of sediment beneath glaciers. A particular feature is a strong increase of viscosity with depth, resulting from the fact that the effective pressure increases with depth. Observations suggest that more than one wavelength is generally present (e.g. diapirism and loadcasting) which requires at least three layers with uniform properties to be present. Three layers permit wavelength growth maxima at two distinct wavelengths. We investigate whether an effective-pressure dependent rheology is consistent with RT instabilities, and whether the non-uniformity it produces is able to increase the number of growth-rate maxima. The investigation starts from the point where sediment in an underlying layer is less dense than the overlying sediment, and the Rayleigh–Taylor instability starts to operate. The mechanics of two layers of finite thickness but infinite extent are modelled by the Stokes equations. The equation set is linearized, and the Fourier transform taken in order to describe the periodic horizontal variation of flow fields at a specified wavelength. The influences of layer thickness and viscosity ratio on the flow fields are considered. It is found that, for a given wavelength, layer thickness has a far stronger influence on flow fields than does viscosity ratio. For all configurations inspected, the dependence of growth rate on wavenumber exhibited one maximum, meaning that a variable viscosity model does not produce multiple wavelengths. Maximum growth rates occur at wavelengths corresponding to the layer thicknesses. We infer that loading instabilities occurring at wavelengths around the layer thicknesses are consistent with the effective-pressure-dependent viscous model.