Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021 Deformation of glacial materials: introduction and overview ALEX J. MALTMAN, BRYN HUBBARD & MICHAEL J. HAMBREY Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Ceredigion SY23 3DB, UK (e-mail. [email protected]; [email protected]; [email protected]) The flow of glacier 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 glaciers, 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 valley 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 crevasses 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 geology 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 Greenland Ice Sheet as revealed in a stantly changing. number of basal-ice core 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 (firn-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.