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Deformation of glacial materials: introduction and overview

ALEX J. MALTMAN, BRYN HUBBARD & MICHAEL J. HAMBREY

Institute of and Earth Sciences, University of Wales, Aberystwyth, Ceredigion SY23 3DB, UK (e-mail. [email protected]; [email protected]; [email protected])

The flow of ice can produce structures Milnes 1977; Hooke & Hudleston 1978; Lawson that are striking and beautiful. Associated sedi- et al. 1994). However, these concepts remain to be ments, too, can develop spectacular deformation applied to deformation at the scale of ice sheets, structures and examples are remarkably well where analogous structures are at least an order preserved in Quaternary deposits. Although such of magnitude larger. Significantly, these struc- features have long been recognized, they are now tures may well contain information with the the subject of new attention from glaciologists potential for assessing the long-term dynamic and glacial geologists. However, these workers behaviour and stability of their host ice masses. are not always fully aware of the methods for Striking deformation structures are also pro- unravelling deformation structures evolved in duced at a wide variety of scales in the sediments recent years by structural geologists, who them- associated with ice. Fine examples appear, selves may not be fully aware of the opportu- for example, in the works of Brodzikowski (e.g. nities offered by glacial materials. This book, and in Jones & Preston 1989) and in the volumes by the conference from which it stemmed, were Ehlers et al. (1995a, b). In fact, of all the vari- conceived of as a step towards bridging this ous kinds of geological deformation structures, apparent gap between groups of workers with among the very first to be described were fea- potentially overlapping interests. tures ascribed to the action of ice. Two great Glaciologists have long been aware of the geological pioneers were involved: both Sir remarkable structures developed in flowing ice. Charles Lyell (1840) and the visionary Henry Nineteenth century Alpine mountaineers and Sorby (1859) interpreted the disturbance of natural scientists, such as the Swiss naturalist, deposits on the coast of eastern England as Agassiz ('The Father of Ice Ages'), and Forbes being due to the movement of icebergs. Lyell and Tyndall from Britain, described a range of (1840) then expanded his ideas on the deforma- structures in , and were clearly impressed tion of the glacial deposits of Norfolk, and by the by the similarity with deformation structures in end of the century a variety of structures in rocks. The first half of the twentieth century Europe and North America had been interpreted saw few advances in glaciological thinking, but as the result of glacial processes. A history of renewed interest in structures followed the these early studies of deformation of sediments formulation of a flow law for ice (e.g. Nye 1953; by glaciers is given in Aber et al. (1989). Glen 1955) and its application to glaciers (e.g. Despite this long pedigree of research, there is Nye 1957). Since then, as outlined by Hambrey commonly disagreement on the actual mechan- & Lawson (this volume), there have been numer- isms involved in generating glacigenic struc- ous studies of deformation in glaciers. Many of tures. The dominant concept this century, until a these studies link glacier structures to measured decade or so ago, involved the notion of ice deformation rates, notably in the classic case bulldozing into sediments and generating vari- studies of Allen et al. (1960) and Meier (1960) in ous 'ice-thrust' structures as a result. Thus, most North America. However, few glaciologists have deformation of glacial sediment was envisaged applied the structural geological concepts of as being 'made at or near glacial termini' (Flint progressive and cumulative deformation to gla- 1971, p. 121). It was the discovery that some ciers. Where such an approach has been adopted glaciers and parts of major ice-sheets rest not (primarily within glaciers and ice caps), on bedrock, but on a layer of sediment (e.g. new insights have emerged concerning ice defor- Engelhardt et al. 1978; Boulton 1979) that mation in relation to the development of folia- launched the 'deformable bed' hypothesis and tion, folds, and and other faults (e.g. a new significance to deformation structures in Hudleston 1976; Hambrey 1977; Hambrey & glacial sediments.

From: MALTMAN, A. J., HUBBARD, B. & HAMBREY, M. J. (eds) Deformation of Glacial Materials. Geological Society, London, Special Publications, 176, 1-9. 0305-8719/00/$15.00 9 The Geological Society of London 2000. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

2 A.J. MALTMAN ET AL.

Even so, a common approach today among aware that something of a schism continues. Our Quaternary specialists is to see the structures as aim is that by collecting together papers on an aid to deducing glacial environments and similar subjects from workers with a range of directions of ice movement (e.g. Evans et al. backgrounds and approaches, new possibilities 1999), risky though this is without a knowledge and collaborations may open. We hope the of the geometries, kinematics, and physical mixture presented here will provide a basis for conditions of the deformation. On the other more integrated approaches in the future. hand, understanding such aspects of deforma- tion has been a growing theme of structural Ice deformation over the last 50 years or so, as the subject has become less descriptive (e.g. Twiss The volume opens with four papers concerned & Moores 1992). These efforts, however, have with processes of ice deformation (relation- almost exclusively been restricted to rocks. As ships between ice deformation and structural noted above, few structural geologists have development being considered in the second taken an interest in structures in glaciers. The section). Despite the small number of papers same is largely true for glacial sediments, and in this section, a broad range of approaches concerted efforts at com-bining ideas developed to investigating ice deformation is presented. for rocks with glacial sediments, such as those of These include results from large- and small-scale Banham (1975), Aber et al. (1989), Warren & ice coring projects, laboratory analysis of ice Croot (1994) and Harris et al. (1997), are sparse. character (including isotopic, gas and chemical Yet analysing structures in Quaternary sedi- composition and ice crystallography), and lab- ments can have an important practical advan- oratory analysis of ice rheology, using both tage over studying those in lithified rock: the triaxial deformation apparatus and a novel cen- sediment can readily be scraped away to reveal trifuge apparatus. the full three-dimensional arrangement of the The first paper in the section, by Souchez structures. Indeed, some of the most spectac- et aL, addresses a complex picture of ice for- ular structures appear in working sand and mation by freeze-on and deformation near the gravel pits, where large-scale sections are con- base of the as revealed in a stantly changing. number of basal- sections. Souchez and Because of the importance of sub-glacial his team at Brussels are ideally placed to write deforming sediments to the motion of many ice such a review (based on both published and new masses, it is appropriate to examine the defor- information), since they have been working for mation of ice and glacial sediments together. some years on the physical properties of the In some cases, sediments shearing beneath an ice basal sections of many of the world's most sheet are best regarded as being in continuum important ice cores. The data summarised in this with overlying debris-rich ice (e.g. Hart 1998). contribution relate mainly to the basal sections Thus the division of the papers included in this of 3 cores located in central Greenland (Dye 3 volume into separate sections on ice and GRIP and GISP2). Interpretation of the gas, sediments is largely for convenience. We have stable isotope and chemical composition of these attempted to head each section with a paper that core sections indicates that their silt-laden basal provides some insight and overview. Some ice layers are largely composed of ice that was papers deal with the linkage between ice and formed prior to ice-sheet advance. Souchez et al. sediments, and there are other papers that could argue that such silt-rich ice is incorporated have been placed in more than one group. While without a phase change into the advancing ice some of the papers are explicitly contemporary sheet, and that it is subsequently distributed and reviews, others combine review with new find- mixed tectonically with clean (-derived) ings or point strongly to future work. glacier ice over some metres to tens of metres In many ways, the papers included here con- at the base of the ice sheet. firm the continuing existence of a gap between The second paper in the section, by Tison & different groups of workers - especially in terms Hubbard, presents new information based on a of approaches, methods and terminology. Some series of eight short ice cores recovered from papers that were refereed by, say, a Quaternary along a flow-line at Glacier de Tsanfleuron, specialist and a structural geologist received Switzerland. This paper supplements an earlier conflicting reviews, with contrasting opinions on classification of the ice facies present in these the practices used and the clarity of the terms. cores (Hubbard et al. 2000) with a wealth Hence, a number of articles herein represent a of ice crystallographic data. Significantly, the working compromise. We have attempted to locations of the ice cores allow a glacier-wide produce a balanced volume, but are keenly flow-line model of crystallographic evolution to Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

INTRODUCTION AND OVERVIEW 3 be reconstructed. This progression involves a veins, ultimately giving rise to traces or sequence of four crystallographic units, from even a new foliation if these layers are subject initial ice development within 20m of the sur- to severe cumulative deformation, as below an face in the accumulation area of the glacier, . In compressive flow regimes, thrusts may to strongly metamorphosed ice located within develop, especially in polythermal glaciers. All c. 10 m of the glacier bed. The crystallography of these structures are analogous to those in this basal ice reflects a steady-state balance deformed rocks and can be used to generate between processes of grain growth and strain- models of deformation in fold-and-thrust belts. related processes of grain-size reduction. Like rock structures, those in ice reflect long- Baker et aL report a set of constant strain-rate term deformation (or cumulative strain). Recog- compression tests on individual ice crystals at a nizing the significance of structures in glaciers variety of orientations, comparing the strain has implications for understanding not only ice response of undoped ice with that of ice doped dynamics on all scales, but also how debris is with H2SO4 at 6.8 ppm. While the test corrobo- incorporated and ultimately deposited by the ice. rates earlier results indicating that the presence The papers in this section focus primarily on of very low concentrations of H2SO4 reduces the structures observed englacially and at the both peak stress and subsequent flow stress surface of glaciers, and how they relate to the (Trickett et al. 2000), the present contribution flow of ice. We start with a review of structural goes on to demonstrate that doping does not styles and deformation fields in glaciers by significantly affect the stress exponent in the flow Hambrey & Lawson. This paper outlines the law for that ice. historical development of this topic, beginning Finally, Irving et al. report a series of ice with the nineteenth century pioneers but con- deformation experiments conducted using the centrating on developments since the 1950s, Cardiff Geotechnical Centrifuge in which ice when the physics of glacier flow was elucidated. blocks are rotated at a centripetal accelera- Deformation rates and histories of various tion of 80 g in a beam centrifuge to recreate the glaciers are described, together with measure- true self-weight stress field that drives real ice ments indicating measurements which indicate masses. Even though the laboratory apparatus is that ice experiences a polyphase deformation still being perfected (particularly in terms of history. The importance of cumulative strain in maintaining a constant, controlled temperature discussing structural development is highlighted, through the sample), the preliminary results and comparisons are made between ice struc- presented in this paper are encouraging. These tures and structures in deformed rocks. The indicate that the technique is viable and that it significance of structural to the way may have significant potential in terms of testing in which debris is incorporated, transported and viscosity differences between different ice types. finally deposited by glaciers is also evaluated. In particular, ice containing 10% by volume of Pointers are offered to future work on linking ice sand strained an order of magnitude slower structures to deformation, especially through (10 .7 s -1) than did clean ice (10 -6 s -1) in these modelling approaches. experiments. In a paper on deformation histories and struc- tural assemblages of glacier ice, Lawson et ai. evaluate a range of velocity and strain-rate data Glacier flow and structures from the world's most intensively studied - type glacier, the Variegated Glacier in Alaska. Glacier flow is manifested in a variety of The strain histories of ice at different positions on structures, commonly reflecting deformation on this non-steady-state glacier, following the well- decadal time-scales in temperate valley glaciers documented 1982-83 surge, are compared. The to millenial time-scales in polar ice sheets histories of accumulation of cumulative strain (e.g. Hambrey 1994, chapter 2; Paterson 1994, are complex, and can mask the effects of large, chapter 9). The 'primary structure', sedimentary transient strain events. Substantial cumulative stratification (derived from the accumulation of strain can be 'undone', and the cumulative strain snow and ice) is subject to considerable mod- signal may be unrepresentative of earlier strain- ification or even obliteration during glacier flow. rates. Structural relationships do not always It is commonly overprinted by the 'secondary' reflect the complexity of deformation histories, or deformational structure called foliation, and it is clear that brittle structures in particular either through folding and transposition of the can be reactivated several times as they pass initial layering, or as a completely new structure through the glacier. resulting from shearing. Extensional flow results From the large scale, we turn to a paper by in the development of crevasses or tensional Wilson on how deformation is localized in Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

4 A.J. MALTMAN ET AL. anisotropic ice; this aspect is studied by means of having excellent marker horizons in the form of an experimental study involving a series of creep tephra layers, and a cliff section allowing the tests. The effects of initial c-axis preferred three-dimensional structure to be observed. The orientation and the inclination of primary glacier is characterized by converging flow, and layering, during both plane strain compression several large folds develop axes sub-parallel to and combined simple shear-compression are the flow-lines. These folds thus result from evaluated. The author found that significant transverse shortening in response to reducing variations in both the strain rate and in the channel cross-sectional width, and from an development of microstructures occurred during increase in differential flow-rates between the the plane-strain compression experiments. In the centre of the glacier and the margins. Similar combined compression and shear experiments, structures have been reported from valley the minimum shear strain rates vary according glaciers in Svalbard (Norwegian High-Arctic) to anisotropy in the ice, and deviate from the by Hambrey et al. (1999). normal power flow law for isotropic ice. The final paper in this section is by Herbst & Marmo & Wilson provide an analysis of Neubauer, Who report on research at the well- the stress distribution and deformation history known Pasterze Glacier (Pasterzenkees) in the associated with one type of structure, a set of Austrian Alps. This paper explains how a wide boudins, formed in an outlet glacier of the East range of structures in the glacier, including Antarctic ice sheet. These boudins arise out of foliation, shear zones, folds, thrusts and other stretching of frozen water-filled crevasses. By faults develop, and how they compare with documenting the geometrical evolution of the similar structures in rocks. These structures, boudins and comparing this with surface strain- combined with an understanding of the kine- rates, it can be determined how these structures matics of the glacier, are used to develop the form in relation to the stress distribution, as analogy with a model of an extensional alloch- derived from two-dimensional finite-difference thon, formed on top of an orogenic wedge. The modelling. The demonstration that stress is re- East Carpathian orogen is used as the example. fracted across ice-rheological boundaries has implications for analysing planar and linear fabric in rocks. Subglacial deformation Next, a paper by Hubbard & Hubbard offers a new approach to the understanding of glacier Subglacial sediment deformation probably repre- structures using a high-resolution, three-dimen- sents the single most actively studied glaciologi- sional finite difference model. The model is used cal process over the past 15 years or so. However, to reconstruct the velocity field of Haut Glacier many aspects of the actual mechanisms that d'Arolla in the Swiss Alps. From this field, it is contribute to this sediment deformation are still possible to predict the generation, passage and poorly understood- principally because of the surface expression of a variety of structures. extreme difficulties involved in both observing By means of flow vectors and strain ellipses, the process first-hand and recreating the bound- the evolution of crevasses, crevasse traces and ary conditions of those processes in the labora- stratification (mapped from aerial photographs) tory. Partly as a result of these difficulties, the can be examined. The authors also use the model very nature of the deformation itself remains to track ice that is formed in the accumulation unclear. The debate is principally argued out in area and then subsequently modified by burial terms of whether the deformation can be and englacial transport, before being re-exposed regarded as essentially viscous (where the rate in the area. Although this application of of sediment deformation varies with applied the modelling technique is still at an early stage stress) or essentially plastic (where the sediment of development, it offers considerable scope to is unable to sustain any stress above that at understand better the evolution of a wide variety which the material fails). Recent research by of structures, including foliation, folds, boudins workers such as Iverson (e.g. Iverson et al. 1997, and a variety of fractures, as highlighted in the 1998, 1999; Iverson 1999) and Tulaczyk (Tulac- first paper in this section. zyk et al. 1998, 2000a, b; Tulaczyk 1999) on the Ximenis et al. then report an excellent field details of sediment deformation, and Fischer study of the kind of glacier-wide structural et al. (1998) and Alley (e.g. 1996) on sediment development that fully complements Hubbards' hydrology reflect a shift in the balance of opinion modelling approach. Ximenis et al. deal with away from a simple (commonly viscous-based) folding in , a small - approach towards a more complex (and prob- shaped tidewater glacier in the South Shetland ably realistic) analysis involving multiple plastic Islands of . This glacier is unique in failure, non-steady pore-water pressures, and the Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

INTRODUCTION AND OVERVIEW 5 role of ploughing of large clasts protruding from layer thickness are likely to exert a stronger the base of the overriding ice (or the roughness of control over the scale of the resulting instability the ice itself). The inclusion of these (perhaps form than are variations in sediment viscosity. non-steady) processes into dynamic models of Fuller & Murray report painstaking field ice-mass motion provides a major stimulus to evidence from the forefield of recently surged current research on subglacial deformation. Hagafellsj6kull Vestari, Iceland. Detailed analy- The papers presented in this section fully sis of macro-scale and micro-scale structures reflect these contemporary issues: a review of in recently exposed flutes and sug- the role of continuity in the presence and char- gests that surge deformation is confined to the acter of a deformable subglacial sediment layer; upper 16 cm of the glacier's subglacial layer. an analysis of the basal sediment-basal ice Detailed field-based evidence also indicates that continuum from an outlet glacier of the East the (surge phase) coupling between the glacier Antarctic ice sheet; a theoretical treatment base and the subglacial sediment layer was strong (based on a viscous deformation model) of the and that ploughing by clasts entrained within, relationships between sediment loading and but protruding from, the ice was widespread. flow-induced landforms; a presentation of field In the penultimate paper in this section, Siegert evidence from Iceland indicating that basal reports evidence from the remarkable SPRI- motion during a surge is concentrated close to TUD airborne radar database of the East the ice-sediment interface; radar evidence from Antarctic ice sheet relating to the nature of the the Antarctic interior that indicates the presence ice-bed interface. Although the presence and of extensive areas of water-saturated subglacial character of now well-documented sub-ice lakes sediments, and a report of a suite of laboratory have been inferred from flat basal radar returns in tests investigating the relationships between earlier papers (e.g. Siegert et al. 1996; Siegert & pore-water pressure, permeability and strength. Ridley 1998), this contact is here supplemented The first of these articles is by Alley, who by a frozen ice-bed interface (weak and scattered presents a broad review of our current under- radar return) and an ice-saturated subglacial standing of the physical character of subglacial sediment interface. The latter radar return is sediments. Alley concludes that many of the similar to, but less flat than, that produced at glaciologically important aspects of deforming an ice-lake interface. This significant develop- subglacial sediments are ultimately controlled by ment points to the potential for ice radar not till production, continuity and, to some extent, only to map areas of presumably deformable pre-existing surface geology, rather than the (and deforming) subglacial sediments, but also finer details of their rheological character. to identify large-scale structures within those Fitzsimons et al. then present investiga- sediments. tions of ice deformation undertaken beneath Finally, Hubbard & Maltman report on a suite Suess Glacier, Antarctica, where a 25m long of laboratory investigations of the dynamic subglacial tunnel has been painstakingly con- permeability of subglacial sediments recovered structed by chainsaw to gain direct access to the from Haut Glacier d'Arolla, Switzerland, and ice-bed interface. This remarkable facility has a (late ) glacierized beach cliff sec- allowed basal ice velocity to be measured over tion in South Wales. Results from these experi- an extended period. Results indicate that fine- ments indicate that, although large variations in grained amber ice containing relatively high sol- permeability did not occur during deformation ute concentrations deforms more readily than (to total strains of <20%), pore-water pressure adjacent clean ice. Further, sediment-laden ice is did exert a major influence over permeability, characterized by more brittle deformation fea- whether dynamic or static. The nature of this tures than cleaner ice, suggesting different modes relationship between permeability and effective of failure in this -17~ ice. pressure is best described by an inverse power Next, Hindmarsh & Rijsdijk evaluate whether relationship above a base permeability such a viscous model of sediment deformation is that permeability increased dramatically (several consistent with field observations of the nature orders of magnitude) at effective pressures of and scale of loading instabilities within glaci- less than c. 150 kPa. genic sediments. The application of Rayleigh- Taylor theory to layered sediments that are Glaciotectonic structures assumed to be characterized by a pre-existing density inversion is found by the authors to Before considering the individual papers in result in features that are consistent with those this section we contrast terminologies and ap- observed in the field. Further, Hindmarsh & proaches used. Since the mention of 'glacial Rijsdijk's model indicates that variations in tectonics' by Slater (1926), the term has evolved Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

6 A.J. MALTMAN ET AL. into 'glaciotectonics' and become well estab- frictional grain-boundary sliding, sometimes lished. It can, however, cause confusion in called independent particulate flow (e.g. Malt- interdisciplinary work because of the way that man 1994). In this, the mineral particles them- structural geologists use 'tectonics'. In structural selves undergo negligible deformation but simply geology the term has a dual meaning. Originally slide past each other; in most glacial deposits it referred to the 'architecture' (its meaning the mineral grains are stronger than any bond- translated from the original Greek) or config- ing between them. Aggregates of clays may be uration of rock masses, as in papers describing deformed, and, particularly in frozen sediments, the 'tectonics' or 'tectonic style' of a particular any brittle lithic clasts or bedrock that are region. This would seem to be the usage involved may undergo cataclasis (grain break- perpetuated in 'glaciotectonics', but modern age). However, the deformation mechanisms cen- usage in structural geology tends to use 'tec- tral to most crustal structures, that is the various tonic' to refer to the origin of the forces that modes of crystal plasticity and diffusion mass caused the deformation. Tectonic forces origi- transfer, are unlikely to be significant in the nate from physico-chemical changes within the relatively cold and rapid conditions of glaciotec- Earth, contrasting strongly with gravity-based tonic deformation. One significant aspect of this forces that drive (Maltman 1994). difference between glacial sediments and deform- Hence the recent habit of some workers to drop ing rocks is that terms for the resulting structures the prefix glacio- is dangerous if structural have to be used carefully if confusion is to be geologists and glaciologists are to communicate avoided. Mylonite, for example, is a structural clearly. 'Tectonic detachment', for example, has geological term that by definition applies to been reported by glaciologists as a process materials that deformed dominantly by plastic, occurring within ice, but the term might puzzle intracrystalline, processes such as dislocation structural geologists since tectonic forces are creep and dynamic recrystallization (e.g. Hippert unlikely to operate within glaciers. Owen (1989) & Hongn 1998). Yet the term 'mylonitic' has described some structures in sediments in the been applied to glacial deposits that almost cert- that are partly the product of ainly deformed chiefly by grain-boundary sliding tectonic forces associated with crustal uplift, rather than any form of crystal plasticity. and others that are due to ice movement, yet Differences in approach between structural both are referred to as tectonic. To avoid this geology and glacial geology are particularly confusion, all structures directly associated with marked in the microscopic study of deformation glacial processes should be referred to as features, and these differences are reflected in glaciotectonic, a practice we have followed in some of the papers in this section. Even the this volume. subject is named differently: 'micromorphology', 'Glaciotectonic' refers to the direct deforma- a term used in this volume, is foreign to struc- tion and structures resulting from the movement tural geologists, who tend to talk about 'micro- or loading of glacier-ice, outside the ice itself. structure'. Basic working terms such as fabric, The definition and limitations of the term have texture, matrix and structure are minefields of been discussed by Aber et al. (1989). In line with different meanings. their suggestions, the term as employed here These differences in the structural and Qua- does not involve structures within glacier ice, ternary approaches seem fundamentally to result and structures resulting from primary deposi- from their differing heritages. Since Sorby's tional processes (such as till fabric) are excluded, pioneering work in the middle of the nineteenth as are the effects of freeze/thaw and iceberg century (Sorby 1859), structural geology has grounding. Lithospheric adjustments to chan- drawn largely on materials science and metal- ging ice loads can induce deformation, but these lurgy (e.g. Kameyama et al. 1999), and there has effects are also excluded, as the role of ice here is been strong emphasis on the motion geometry of indirect. Glaciotectonic structures include a wide rock particles in response to deforming stresses, range of features conventionally studied by in particular the notion of employing 'shear structural geologists and the larger scale effects sense indicators'. In contrast, the field concerned can grade into major landforms (e.g. Aber et al. with micromorphology of glacial sediments has 1989; Warren & Croot 1994; Van der Wateren existed for little more than about twenty years, 1995). The structures themselves include a wide and is still in the process of cataloguing the variety of folds and faults, on a range of scales, features and evolving the most useful terminol- together with a host of related features. ogy. So far, the subject has drawn more on Almost all glaciotectonic structures, unlike the concepts and terminology of soil science those normally encompassed by structural geol- (e.g. Fitzpatrick 1984) and less on the experi- ogy, involve only one deformation mechanism: ence of deformed rocks, or for that matter the Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

INTRODUCTION AND OVERVIEW 7 approaches of soil (geotechnical) engineering. ments and magnitudes, but here also, the analy- This has the advantage of enabling biological ses are best used in conjunction with other kinds and other high-level constituents that are not of observations. Structural geology has by now normally present in lithified rocks to be dealt evolved methods for dealing with shear sense in with. It also provides a detailed terminology, flowing materials to a considerable degree of based on the term 'plasma' with various prefixes, sophistication (e.g. Passchier 1998), with details for the fine-grained, typically clayey material of the relative roles of different strain symme- that structural geology tends to have neglected. tries being hotly debated (e.g. Lin et al. 1999). However, it does reduce the emphasis on No doubt, particularly complex symmetries exist deformation, not normally an important aspect in deforming glacial materials, through such of agricultural soils. The result is that, at the things as non-planar boundaries and accompa- present time, microstructural studies are rather nying heterogeneous volumetric strains, but at impenetrable to Quaternary geologists, some of this stage of the subject's development in gla- whom use a terminology quite alien to structural cial geology, Van der Wateren et al. felt it geologists. Therefore, there seems a particularly more instructive to restrict their review largely pressing need in the field of microscopic studies to simple shear. of glaciotectonism for greater commonality of A structural geological topic that has by now approach and terminology. However, it seems matured is that of recognising and deciphering premature to attempt a unison here; we have not multiple or polyphase deformation, common, for attempted to lay down yet another set of example, in the rocks that form the mountain working definitions and impose editorial pre- belts of the world. The contribution by Phillips judices. While any particularly arcane terminol- & Auton illustrates how such concepts can be ogy has hopefully been eliminated, authors' used to unravel sequences of events in glacial preferences have been honoured. Consequently, deposits. The paper also shows how a more there is some duality of usage in the following coordinated terminological approach can be papers and the exact meaning has to be deduced achieved. It deals explicitly with 'micromorphol- from the context. ogy' yet employs conventional structural geology The section on glaciotectonics begins with two terminology of S 1, $2, for successive generations reviews that illustrate the differences in approach of fabrics. Structural geological terms such as to microscopic scale analyses and that may help Riedel shears, boudinage, and pressure shadows coordinate them in future. Each uses a simple are used as appropriate alongside terms derived terminology and approach that should make the from Van der Meer (1993) such as lattisepic and reviews fully accessible. Menzies reviews the unistrial plasmic fabrics, quite foreign to struc- state-of-the-art in Quaternary studies that draw tural geology. Such capitalizing on the strengths from the soil science heritage, providing a of the different approaches in order to employ qualitative classification of deformation struc- the most effective terms and concepts must be the tures in glacial sediments and then attempting to way forward if we are to gain before long a link this taxonomy to the different glacial more unified approach to microscopic studies of environments. The review minimizes specialist deformed glacial sediments. terminology and introduces some terms common Structural geological advances in recent dec- to structural geology. Menzies makes it clear that ades on fold-and-thrust belts have already his work is still provisional; deformed glacial been employed in glacial geology (e.g. Croot sediments have not been surveyed comprehen- 1987; Hambrey et al. 1997) and glaciology sively at the microscopic scale and features may (e.g. Hambrey et al. 1999), but the article by remain unrecognized. The ideal goal of this kind Huuse & Lykke-Andersen extends such adapta- of work is to seek criteria at the microscopic scale tions further. All the glaciotectonic structures for recognizing particular lithofacies. However, they report are submarine and are interpreted Menzies emphasises that the complexities of from high-resolution multi-channel seismic data, nature are likely to preclude this in practice. giving unprecedented quality of resolution for Rather, microstructural criteria will have to be such structures. Moreover they are documented used together with other features to diagnose on a scale of kilometres that is normally simply particular glacial settings. too large to observe on land. The profiles they Van der Wateren et aL review aspects of a derive compare closely with on-land fold-and- structural geological topic that has advanced thrust belts (e.g. Macedo & Marshak 1999), and greatly in recent years: that of diagnosing the the rooting of the thrusts into an underlying kinematics of the deformation. They explain detachment zone is comparable to the architec- how various microstructural features can lead ture of accretionary prisms forming at conver- to a better understanding of deformation move- gent plate margins (e.g. Morgan & Karig 1995). Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

8 A.J. MALTMAN ET AL.

The two final papers in this section illustrate EHLERS, J., GIBBARD, P. L. & ROSE, J. 1995a. Glacial the interface between glaciotectonic structures deposits of Great Britain and Ireland. A. A. and landforms. Fowler provides a dynamic- Balkema, Rotterdam. numerical analysis of the flow processes that --, KOZARSKI, S. & GIBBARD, P. 1995b. Glacial deposits in north-east Europe. A. A. Balkema, may give rise to that still controversial feature - Rotterdam. drumlins. Graham & Midgley outline a case ENGELHARDT, H. F., HARRISON, W. D. & KAMB, B. study of in North Wales, in which the 1978. and conditions at the glacier interplay between deformation processes and bed as revealed by bore-hole photography. structures, and particularly the role of thrusting, Journal of Glaciology, 20, 469-508. is explored. Their paper also nicely reflects the EVANS, D. J. A., SALT, K. E. & ALLEN, C. S. 1999. pedigree of the studies reported in this volume. Glacitectonized sediments, Barrier Lake, Kanaskis It was in the same locality in North Wales that Country, Canadian Rocky Mountains. Canadian Charles Darwin in 1842 showed that, not only Journal of Earth Sciences, 36, 395-407. FISCHER, U. H., IVERSON, N. R., HANSON, B., HOOKE, must ice have once extended over southern R. L. & JANSSON, P. 1998. Estimation of hydraulic Britain, but also that the associated processes properties of subglacial till from ploughmeter had much influence on today's landscape. measurements. Journal of Glaciology, 44, 517-522. FITZPATRICK, E. A. 1984. Micromorphology of soils. This volume represents the product of an interdisci- Chapman & Hall, London. plinary conference, entitled the 'Deformation of FLINT, R. F. 1971. Glacial and Quaternary Geology. Glacial Materials', held at the apartments of the John Wiley, New York. Geological Society of London in Burlington House in GLEN, J. W. 1955. The creep of polycrystalline ice. September 1999. The editors would first and foremost Proceedings of The Royal Society of London, like to thank the conference participants, who con- Ser. A, 228. tributed so fully to the success of both the conference HAMBREY, M. J. 1977. Foliation, minor folds and and this volume. We would also like to thank the staff strain in glacier ice. Tectonophysics, 39, 397-416. of Burlington House and the staff of the Geological --1994. Glacial Environments. UCL Press, London. Society Publishing House respectively for their assis- -- & MILNES, A. G. 1977. Structural geology of an tance in organizing the meeting and in producing this Alpine glacier (Griesgletscher, Valais, Switzer- volume. In addition, we acknowledge the efforts of the land). Eclogae Geologicae Helvetiae, 70, 667-684. many referees who gave up their time to read and --, BENNETT, M. R., DOWDESWELL,J. A., GLASSER, comment on the manuscripts contained in this volume. N. F. & HUDDART, D. 1999. Debris entrainment Finally, we gratefully acknowledge the support of the and transfer in polythermal valley glaciers. sponsors of the conference- the Tectonics Studies Journal of Glaciology, 45. Group of the Geological Society, the International --, HUDDART, D., BENNETT, M. R. & GLASSER, Glaciological Society, the Quaternary Research Asso- N. F. 1997. Genesis of 'hummocky rrloraines' by ciation, and our commercial sponsors, Lasmo plc and thrusting in glacier ice: Evidence from Svalbard Badley Ashton & Associates Ltd. and Britain. Journal of the Geological Society, London, 154, 623-632. HARRIS, C., WILLIAMS, G., BRABHAM, P., EATON, G. References & MCCARROLL, D. 1997. Glaciotectonized Qua- ternary sediments at Dinas Dinlle, Gwynedd, ABER, J. S., CROOT, D. G. & FENTON, M. M. 1989. North Wales, and their bearing on the style of Glaciotectonic landforms and structures. Kluwer, deglaciation in the eastern Irish Sea. Quaternary Dordrecht. Science Reviews, 16, 109-127. ALLEN, C. R., KAMB, W. B., MEIER, M. F. & SHARP, HART, J. K. 1998. The deforming bed debris-rich basal R. P. 1960. Structure of the lower Blue Glacier, ice continuum and its implications for the Washington. Journal of Geology, 68, 601-625. formation of glacial landforms (flutes) and sedi- ALLEY, R. B. 1996. Towards a hydrological model for ments (melt-out till). Quaternary Science Reviews, computerized ice-sheet simulations. Hydrological 17, 737-754. Processes, 10, 649-660. HIPPERT, J. F. & HONGN, F. D. 1998. Deforma- BANHAM, P. 1975. Glacitectonic structures: a general tion mechanisms in the mylonite/ultramylonite discussion with particular reference to the con- transistion. Journal of Structural Geology, 20, torted drift of Norfolk. In: WRIGHT, A. E. & 1435-1448. MOSELEY, F. (eds) Ice Ages, Ancient and Modern. HOOKE, R. I. & HUDLESTON, P. 1978. Origin of Geological Journal Special Issue. Seel House foliation in glaciers. Journal of Glaciology, 20, Press, Liverpool, 69-94. 285-299. BOULTON, G. S. 1979. Processes of glacier erosion HUBBARD, B., TISON, J.-L. & JANSSENS, L. 2000. Ice- on different substrates. Journal of Glaciology, 23, core evidence for the thickness and character of 15-38. clear facies basal ice: Glacier de Tsanfleuron, CROOT, D. G. 1987. Glacio-tectonic structures-a Switzerland. Journal of Glaciology, 46, 140-150. mesoscale model of thin-skinned thrust sheets. HUDLESTON, P. J. 1976. Recumbent folding in the base Journal of Structural Geology, 9, 797-808. of the Barnes , Baffin Island, Northwest Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021

INTRODUCTION AND OVERVIEW 9

Territories, Canada. Geological Society of Amer- 1957. The distribution of stress and velocity in ica Bulletin, 87, 1684-1692. glaciers and ice sheets. Proceedings of the Royal IVERSON, N. R. 1999. Coupling between a glacier and a Society of London, A239, 113-133. soft bed: II. Model results. Journal of Glaciology, OWEN, L. A. 1989. Neotectonics and glacial deforma- 45, 41-53. tion in the Karakoram Mountains and nanga , BAKER, R. W., HOOKE, R. L., HANSON, B. & Parbat Himalaya. Tectonophysics, 163, 227-265. JANSSON, P. 1999. Coupling between a glacier and PASSCHIER, C. W. 1998. Monoclinic model shear zones. a soft bed: I. A relation between effective pressure Journal of Structural Geology, 20, 1121-1137. and local sheer stress determined from till PATERSON, W. S. B. 1994. The Physics of Glaciers. elasticity. Journal of Glaciology, 45, 31-40. Pergamon. , -- & HOOVER, T. S. 1997. A ring-shear device SIEGERT, M. J. & RIDLEV, J. K. 1998. An analysis of for the study of till deformation: Tests on the ice-sheet surface and subsurface topography with contrasting clay contents. Quaternary Sci- above the Vostok Station subglacial lake, central ence Reviews, 16, 1057-1066. East Antarctica. Journal of Geophysical Research- , HOOYER, T. S. & BAKER, R. W. 1998. Ring- Solid Earth, 103, 10195-10207. shear studies of till deformation: Coulomb-plastic --, DOWDESWELL, J. A., GORMAN, M. R. & behavior and distributed strain in glacier beds. MCINTYRE, N. F. 1996. An inventory of Antarctic Journal of Glaciology, 44, 634-642. sub-glacial lakes. Antarctic Science, 8, 281-286. JONES, M. E. & PRESTON, R. M. F. (eds) 1989. SEATER, G. 1926. Glacial tectonics as reflected in Deformation of Sediments and Sedimentary Rocks. disturbed drift deposits. Proceedings of the Geological Society, London, Special Publica- Geologists' Association, 37, 392-400. tions, 29. SORBY, H. C. 1859. On the contorted stratification of KAMEYAMA, M., YUEN, D. A. & KARATO, S. I. 1999. the drift of the coast of Yorkshire. Proceedings Thermal-mechanical effects of low-temperature of the Geological and Polytechnic Society, West plasticity (the Peierls mechanism) on the deforma- Riding, Yorkshire, 1849-1859, 220-224. tion of a viscoelastic shear zone. Earth and Planet- TRICKETT, Y. L., BAKER, I. • PRADHAN, P. M. S. 2000. ary Science Letters, 168, 159-172. The orientation dependance of the strength of LAWSON, W., SHARP, M. & HAMBREY, M. J. 1994. The single ice crystals. Journal of Glaciology, 46, 41-44. structural geology of a surge-type glacier. Journal TULACZYK, S. 1999. Ice sliding over weak, fine-grained of Structural Geology, 16, 1447-1462. tills: dependence of ice-till interactions on till LIN, S., JIANG, D., WILLIAMS, P. F., DEWEY, J. F., granulometry. In: MICKELSON, D. M. & ATTIG, HOLDSWORTH, R. E. 8r STRACHAN, R. A. 1999. J. W. (eds) Glacial processes past and present. 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