JOURNAL OF QUATERNARY SCIENCE (2003) 18(5) 373–383Copyright 2003 John Wiley & Sons, Ltd.Published online 30 May 2003 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jqs.769

Observing artificially induced strain: implicationsfor subglacial deformationJOHN F. HIEMSTRA1* and KENNETH F. RIJSDIJK2

1 Department of Geography and Topographic Science, University of Glasgow, Glasgow G12 8QQ, Scotland2 Nederlands Instituut voor Toegepaste Geowetenschappen (NITG-TNO), P.O. Box 80015, 3508 TA Utrecht, The Netherlands

Hiemstra, J. F. and Rijsdijk, K. F. 2003. Observing artificially induced strain: implications for subglacial deformation. J. Quaternary Sci., Vol. 18 pp. 373–383. ISSN0267-8179.

Received 10 January 2003; Revised 4 March 2003; Accepted 21 March 2003

ABSTRACT: By analysing a series of four successive thin-sections from a ceramic clay that wassubjected to uniaxial compression, we were able to monitor the development of microstructuresin a fine-grained sediment. The artificially induced microstructures, such as unidirectional clayreorientations and linear and circular grain arrangements, are identical to features that have beenobserved in thin-sections of subglacially deformed tills, and therefore may be used as representativeanalogues. We argue that the structures, reflecting slip, planar shear displacements as well asrotational movements, can be explained by assuming a Coulomb-plastic response to imposed shear.We conclude that sediments subjected to subglacial deformation behave as Coulomb materials,at least during the final stages of the deformation. The present study bridges the gap betweenfield studies, experimental studies and theoretical modelling. The microscopic observations assistin visualising inferred subglacial processes and facilitate up- and downscaling between diversemethodological approaches. Copyright 2003 John Wiley & Sons, Ltd.

Journal of Quaternary Science

KEYWORDS: micromorphology; subglacial till; experimental soft-sediment deformation; microstructures; plasmic fabrics.

Introduction

It is now commonly accepted that bed deformation has animportant role in the movement of glacier ice over softsubstrates (Boulton, 1986; Murray, 1997). Support for thisdeforming bed model has accumulated over the years andinvolves both field measurements and theoretical considera-tions (e.g. Alley et al., 1986; Boulton and Hindmarsh, 1987;Boulton, 1996; Clark et al., 1996; Hart and Rose, 2001).Although the deforming bed theory appears to satisfactorilyexplain many aspects of the glacial geomorphological andsedimentary records, there are still certain features that provedifficult to explain in the light of pervasive sediment deforma-tion, leading Piotrowski et al. (2001) to seriously challenge theclaim made by some that the model is widely applicable tothe Laurentide and Fennoscandian ice sheets. Just how gener-ally applicable the model is, and whether it can explain themajority of subglacial phenomena, remains controversial.

Another unresolved issue is how subglacial sedimentsrespond to applied shear stresses, i.e., how they actually

* Correspondence to: Dr John F. Hiemstra, Department of Geography andTopographic Science, University of Glasgow, Glasgow G12 8QQ, UK.E-mail: jhiemstra@geog.gla.ac.uk

Contract/grant sponsor: European Community Marie Curie Fellowship;Contract/grant number: MCFI-2001-494.Contract/grant sponsor: The Netherlands Organisation for Scientific Research;Contract/grant number: 751.495.08.

deform. Boulton and Hindmarsh (1987) launched the conceptthat basal tills behave as viscoplastic fluids. They describedsubglacial tills as Bingham materials, in which strengthdepends largely on the strain rate of the material. Others (e.g.Iverson et al., 1996, 1998; Tulaczyk et al., 2000) describedtills as Coulomb materials, implying that sediment strengthis independent of strain rate but linearly dependent oneffective stresses. There seems to be good grounds for bothviews, and discussion continues on which model provides thebetter fits: only recently Iverson and Iverson (2001) suggestedthat the typical vertical displacement profile in tills (Alley,1989) is not necessarily unique to viscoplastic behaviour (e.g.Boulton and Hindmarsh, 1987; Boulton and Dobbie, 1998),but that it can also be simulated using empirical Coulombmodels.

At this stage, it seems that both laboratory experi-ments and field studies provide useful information whenit comes to understanding till behaviour. Numerical mea-surements relating strain signatures, for example, to mois-ture content and to clay content (Iverson et al., 1997),and also tests showing how porosity and texture of tillsmay change with increasing strain (Hooke and Iverson,1995; Iverson et al., 1996) are of great help in furtherrefining the deforming bed model. Field measurements ofglacier motion and subglacial sediment deformation (Trufferet al., 2000, 2001) and structural observations and measure-ments in sedimentary records (e.g. Benn and Evans, 1996;Phillips et al., 2002) continue to be important for the samepurpose.

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Future deforming bed research should link theoreticalmodelling, laboratory experiments and field studies (seealso Hart and Rose, 2001). At present there seems to bea gap between numerical results from experiments andsimulations on the one hand and visual, structural fieldinformation on the other, and there are hardly any dataavailable that would be capable of effectively linking thetwo. We note that establishing such a connection is notjust a scaling issue. Rather it involves ways of visualisingdeformation processes, learning what certain processes maydo to a sediment structurally, and being able to step acrossboundary conditions and transferring experimental results tofield situations.

With this in mind we made an attempt to mimic subglacialdeformation processes in fine-grained tills by subjecting anartificially prepared sediment comprising layer-lattice miner-als, silts and sands to compression tests. Instead of measuringphysical parameters or plotting stress–strain relationships, wetook a close look at changes taking place in thin-sectionsof the sediment. By taking ‘snapshot’ samples at succes-sive stages, a ‘motion picture’ of the shearing process wascreated, which enabled us to monitor continuous structuralchanges.

The use of micromorphology follows on from recentapplications of the technique that have demonstrated thatthin-sections can provide impartial and integrative informationfor glacial sedimentary problems (cf. Hiemstra and van derMeer, 1997; Menzies et al., 1997; van der Meer, 1997,2000; Carr, 1998, 2001; van der Meer and Hiemstra,1998; Hiemstra, 1999, 2001; Kluiving et al., 1999; Lachnietet al., 1999; van der Wateren, 1999; Fuller and Murray,2000; Phillips and Auton, 2000; van der Wateren et al.,2000; Khatwa and Tulaczyk, 2001). Another advantage ofmicromorphological analyses is that they allow relatively easyup-scaling from the microscope to the field outcrop as mostof the phenomena that have been identified in thin-sectionsof subglacial sediments (cf. van der Meer, 1993; Menzies,2000) seem to have equivalents at the outcrop-scale (structuralisomorphism).

This paper describes the shear experiment in detail andanalyses results qualitatively and from a mechanical point ofview. In the discussion, artificially induced microstructures arerelated to natural, mostly subglacial analogues. Unlike otherpublications describing shear experiments (e.g. Morgensternand Tchalenko, 1967; Maltman, 1977, 1987b, 1988; van denBerg, 1987; Schokking, 1998), we not only analysed clayfabrics, but also looked into changing grain arrangements inthe test material.

Experimental techniques

The test material was a standard potters clay, readily availablefrom suppliers of ceramic materials (type K122: Vingerling,Haastrecht, Netherlands). Figure 1 shows that the clay contentof this sediment is ca. 53%, its silt content ca. 40%, itssand content ca. 7%, and that the coarsest particles are in theorder of 500 µm (medium sand). X-ray diffraction revealed thatthe clay mineralogy is rather diverse: estimated percentagesare ca. 10–15% for kaolinite, ca. 20–30% for chlorite, ca.20–30% for illite, and ca. 25–35% for vermiculite/smectiteinterstratifications (B. de Leeuw, University of Amsterdam,personal communication, 2000). Initial water content of thesediment is in the order of 26–30%, and the VingerlingLaboratory reports an average water uptake of ca. 13%after baking (ca. 950°C, H. Richter, Vingerling Laboratory,personal communication, 2000), providing an idea of thelikely changes in porosity during the course of the experiment.The sediment was premodified industrially to guarantee auniform quality in the end product; it may be described as‘lightly overconsolidated’ (H. Richter, Vingerling Laboratory,personal communication, 2000).

Four cylindrical specimens (76 mm long, 50 mm in diame-ter) were prepared from a loaf of the K122 sediment. In orderto reduce intersample variations to a minimum, the specimenswere cut as closely to each other as possible, with their longaxes parallel. Three of the specimens were contained in latexsheaths and subjected to the tests reported below. The fourthspecimen was used for reference purposes.

The compression tests were carried out in the laboratory forEngineering Geology at Delft University (Wykeham-Farranceequipment). The tests are standard procedure in soil mechanicsand the experimental set-up that we used is similar to thoseoutlined in Scott (1980) and Maltman (1987a) among others.It is noted that, because we did not use a confining fluidin the pressure cell, our tests should be referred to asuniaxial rather than triaxial (cell pressure σ ′

3 is effectivelyzero; σ ′

1 > σ ′2 = σ ′

3 = 0). The (main) stress component σ ′1 was

in all cases parallel to the long axes of the specimens. Strainwas generated at a low, uniform rate of 1.32 × 10−7 s−1, andwas not driven beyond peak strength of the material. Themain reason for not using pressure cell fluid was to facilitatedrainage of the samples. Total water loss, including actualwater expulsion and desiccation during the test, was in theorder of a few percent.

The ideal situation would have been to perform the teston one single sample, with interruption of the compressionat certain times to examine the strain signatures. However,

Figure 1 Textural distribution of the K122 sediment

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ARTIFICIALLY INDUCED STRAIN AND SUBGLACIAL DEFORMATION 375

Table 1 Specimens in the uniaxial compression test

Specimen Cumulative time (h) Shortening (mm) Strain (%)

I 0 0.00 0.00II 50 1.47 1.93III 89 3.23 4.24IV 191 6.92 9.08

because thin sectioning of soft sediment requires impregnation,we had to work with four different samples, subjected toincreased amounts of strain (Table 1), assuming that thesewere representative of four successive stages of one singledeformation process in one single material.

After the tests, the four samples were air-dried, impregnatedwith synolite (an unsaturated polyester resin), and left to cure.The cylindrical specimens were cut in half (lengthwise) toobtain the largest possible exposure in the thin-sections. Fordetails of the thin-sectioning procedure see Murphy (1986)or van der Meer (1996). The sections were analysed using aLeica optical transmission microscope (magnifications up to35×) and Leica camera equipment.

Micromorphological observations

Observations outlined below are presented in a synthesisedform in Table 2. Descriptive terms are defined in the glossaryin the Appendix.

Specimen I

The unstrained specimen I exhibits a random spatial distribu-tion of the skeleton although elongate grains are preferentiallyaligned in a subvertical microfabric. Overall, the plasma showspervasive, uniform, well developed, near vertical birefrin-gence, which is defined as a masepic plasmic fabric (Fig. 2).The plasma directly adjacent to silt- or sand-grains may showskelsepic plasmic fabrics or pressure shadows, both resulting inaberrations in the continuity of the masepic signal. In pressureshadows, birefringence along opposing sides of the grain isless developed or absent, whereas in skelsepic plasmic fabrics,birefringence seems to be reinforced in striated, subcutanicpatterns.

Planar fractures with indented or serrated walls (crazeplanes) form an irregular dendritic pattern in specimenI (Fig. 3a). The overall fracture density is relatively high.Apparent orientations, as measured from the two-dimensionalthin-section, are variable with no direction preferentiallydeveloped.

(a) (b)

Figure 2 (a) Photomicrograph showing light-coloured subverticalmasepic plasmic fabrics. Cross-polarized light; top of the specimen isto the upper left; represented area 1.5 × 2.4 mm. (b) Interpretedphotomicrograph showing the orientations of masepic plasmic fabrics(fine-lines overlay) and preferred microfabrics (short black lines).Note that plasma around most of the skeleton grains showscircum-grain birefringence

(a) (b)

Figure 3 Fracture patterns in the thin-sections. (a) Detail ofspecimen I in plane light; represented area 10.4 × 18 mm. Theundeformed sediment shows irregular, dendritic fracture patterns.(b) Detail of specimen IV in cross-polarised light; represented area10.4 × 18 mm. After ca. 9% strain, fractures are rectilinear andsystematic. Note that the walls of the cracks are generally smooth

Table 2 Micromorphological observations. Note that the decreasing trend in pre-existing masepicplasmic fabrics (PF) is concealed by the development of new zones of masepic plasmic fabrics in thecourse of the experiment, most notably in specimen IV (see text)

Specimen Masepic PF Unistrial PF Branch-merge Distribution Turbates Lineaments

I • • • A A R A AII •• • • • A R • A/•III •• •• • R/C •• • • •IV • • • • • • •• R/C • • • ••

Key: • • • = abundant; •• = common; • = rare; A = absent; R = random; C = clustered.

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Specimen II

In specimen II (ca. 2% strain), grains are locally organised incircular configurations (turbates; see Fig. 4) or in linear arrange-ments (lineaments; see Fig. 5). The identified lineaments areinvariably short, and observed turbates have diameters that aretypically between 150 and 300 µm. Subvertical microfabricstend to be weaker than in specimen I.

Narrow bands of extinction locally interrupt the masepicplasmic fabrics (Fig. 6). In such bands, where the masepicsignal is less developed or even absent, closely spacedunistrial plasmic fabrics were observed (example in Fig. 7).The unistrials in specimen II are not well developed andmostly discontinuous (the thin, short features are only visibleat 25–35 × magnifications), however, they do show consistentconjugate orientations of a dominant 120° (all orientations aremeasured clockwise with respect to the vertical axis of thesample) and a subordinate 040°.

Overall fracture density is low. Near the top and bottomof the sample, skew and craze planes are predominantlysubhorizontal (090° to 120°), and interconnecting finerfractures are oriented between 040° and 055°. One steeplyinclined planar void (020°) curves around at the upper endto connect with subhorizontal planes. It seems that the mostprominent orientations of planar fractures are compatible withthe conjugate angles of the unistrial signals.

Specimen III

In specimen III (ca. 4% strain), skeleton grains are locallyclustered, and features such as lineaments and turbateswith maximum diameters of ca. 500 µm are common.Microstructures present in specimen III seem to determinethe long-axis orientations of grains to some extent. In turbates,grain axes tend to describe circles, whereas in lineamentselongate grains adopt head-tail positions, rendering a structure-parallel microfabric. As such, microfabrics therefore might bepoorly developed on the sample scale, although they are welldeveloped on a feature-scale.

The subvertical masepic signal in specimen III is less uniformthan in specimen II. Unistrial plasmic fabrics are well devel-oped and closely spaced, individually up to several millimeterslong and ca. 40 µm in width, and orientated consistently indirections of 140° and 075° (Fig. 8). Locally, unistrials exhibitbranching-and-merging characteristics, a phenomenon thatseems to occur when unistrials split or bifurcate upon grain

(a) (b)

Figure 4 (a) Photomicrograph showing an example of a turbatestructure. Plane light; the top of the specimen is to the top of thephotograph; represented area 4.0 × 4.9 mm. (b) Interpretedphotomicrograph highlighting the circularly arranged cluster of grains

(a)

(b)

Figure 5 (a) Photomicrograph showing an example of a grainlineament. Plane light, the top of the specimen is to the top of thephotograph; represented area 7.5 × 4.9 mm. (b) Interpretedphotomicrograph showing the ‘train’ of three skeleton grains (dashed,black line). The long axes of the grains are parallel to the orientationof the lineament. The two left grains in the lineament have turbatestructures around them: the long axes (short, black lines) of the finesand grains that form the turbate pattern (dashed, white circles) aresubtangential to the outline of the respective core grains

(a) (b)

Figure 6 (a) Photomicrograph showing a shaded area in overalllight-coloured birefringent plasma in specimen II. Cross-polarisedlight; top of the specimen is to the upper left; represented area4.4 × 6.2 mm. (b) Interpreted photomicrograph highlighting thedarker extinction zone (between dashed, black lines) in the bright,homogeneous masepic plasmic fabrics

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ARTIFICIALLY INDUCED STRAIN AND SUBGLACIAL DEFORMATION 377

(a) (b)

Figure 7 (a) Photomicrograph showing an example of distinct,continuous, diagonal unistrial plasmic fabrics. Cross-polarised light;top of the specimen is to the top of the photograph; represented area4.4 × 7.0 mm. (b) Interpreted photomicrograph highlighting theunistrial features (dashed white lines). Microfabrics, indicated byshorter black or white dashed lines, are generally subparallel to theunistrial plasmic fabrics

(a)

(b)

Figure 8 (a) Photomicrograph showing cross-cutting sets of unistrialplasmic fabrics in specimen III. Cross-polarised light; the top of thespecimen is to the top of the photograph; represented area6.7 × 4.0 mm. (b) Interpreted photomicrograph highlighting thesubtle unistrial features (dashed white lines). The sets from upper leftto lower right are better developed

(a)

(b)

Figure 9 (a) Photomicrograph showing an example ofbranching-and-merging characteristics of unistrial plasmic fabrics.Cross-polarised light; top of the specimen is towards the lower left ofthe photograph; represented area 3.1 × 4.9 mm. (b) Interpretedphotomicrograph showing bifurcation and deflection of unistrialfeatures (white dashed lines) (up)on the surfaces of skeleton grains.Anastomosing patterns are formed

surfaces, propagate as two branches for some distance andgrow together as one behind the obstructing grains (Fig. 9).

Fracture density is relatively low and, compared withthe previous samples, the voids in specimen III are mainlyrectilinear and have relatively smooth, parallel walls (jointplanes; see Appendix). Patterns are systematic; measuredorientations are, in order of dominance, 025°, 140° and 095°.

It is noted that the 140° orientation in the unistrials matchesone of the principal joint plane orientations (Fig. 10), andthat the identified grain lineaments are generally aligned(sub)parallel to the 025° or to the 095° fractures.

Specimen IV

Both skeleton and plasma distribution in specimen IV (ca.9% strain) are anisotropic. Local clusters of skeleton-sizedparticles, as well as fine silt concentrations and clay agglomer-ations were observed. Turbate structures, commonly between200 and 400 µm in diameter are abundant and lineaments arecommon.

The masepic plasmic fabric in specimen IV is mostlymoderately developed. There are parts of the sample in whichthe signal is predominant, however, areas where it is poorlydeveloped or absent are quite extensive, particularly whencompared with specimens II and III.

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(a)

(b)

Figure 10 Specimen IV. (a) Photomicrograph showing continuous,unistrial plasmic fabrics that are subparallel to a planar crack.Cross-polarised light with gypsum wedge superposition; the top of thespecimen is towards the top of the photograph; represented area6.2 × 9.5 mm. (b) Interpreted photomicrograph showing that theturbates present (white, dashed circles) are in close proximity to theidentified planar features (void—black, dashed lines; and unistrialplasmic fabrics—white dashed lines)

Unistrial features are well developed, distinct and fairly con-tinuous, and orientations are invariably ca. 125°. Branching-and-merging characteristics are common to abundant. In somecases, bundles of unistrials have concentrated into thickerzones exhibiting fairly uniform birefringence characteristics(Fig. 11).

Fractures form a regular and consistent pattern in specimenIV (Fig. 3b). The fracture density is high. The character of thecracks is similar to that in specimen III: walls are parallel andrelatively smooth. Even the finest cracks are mostly rectilinear.Orientations range between 125° and 150° and between 055°

and 070°. A third set of fractures is nearly vertical.It is noted that orientations of unistrial features are consistent

with the dominant direction of the planar cracks, and thatidentified lineaments are mostly parallel to fractures and/orunistrial plasmic fabrics.

Discussion

Mohr–Coulomb

We assume that the standard Mohr–Coulomb model (e.g.Scott, 1980) can approximate the behaviour of the test material

(a) (b)

Figure 11 (a) Photomicrograph showing parallel shear bands inspecimen IV. Cross-polarised light; the top of the specimen is to thetop of the photograph; represented area 2.8 × 4.5 mm. (b) Interpretedphotomicrograph showing that the shear bands (white, dashed lines)are composed of a bundle of individual unistrial features. The coarsergrains in the vicinity of the shear bands form turbate structures (whitedashed circles). Within the upper shear band, some grains arearranged in short lineaments (black short lines show microfabrics)

during compression. The model describes the deformation ofisotropic sediments that are subjected to shear stresses. Accord-ing to the model, the response of the sediment is initially elastic,but with increasing stress, the sediment is said to yield, whichmeans that it starts to deform plastically. As a result, the shearstrength of the sediment increases, a process that is knownas strain hardening. A critical situation is reached once thesediment is no longer capable of accommodating the inducedamount of strain. This leads to eventual failure of the sedimentalong one or more discrete planes at theoretical angle(s) ofπ /4 − ϕ/2 to the vertical, with ϕ the angle of internal friction.

The Mohr–Coulomb model implicitly suggests that theaccommodation of strain that occurs prior to failure ispredetermined by cohesion and internal friction of thesediment. Small-scale shear displacements generated duringplastic deformation of the material are theoretically restrictedto aforementioned theoretical angles. In a two-dimensionalrepresentation of a given isotropic material (in a thin-section)these potential slip planes would constitute a regular networkof small-scale lozenges.

We bear in mind that, as in any other model, Mohr–Coulombdoes not always hold in practice. In quite commonly occurringsituations where sediment is not isotropic or where sedimentstrain involves concurrent volumetric changes (see Verruijt,1983), resulting shear plane geometry is often much morecomplex than is predicted by the model. It also should benoted that, with increasing effective pressures, shear planesmight develop at angles that differ considerably from the theo-retically predicted values. Furthermore, geometry is influencedby the water content of the sediment, and, for example, pri-mary clay mineral fabric plays a role in how and at what angleshear planes are initiated (see Arch et al., 1988).

Circum-grain features

Before attempting to relate the identified trends in theirrespective developments to increasing strain during the

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ARTIFICIALLY INDUCED STRAIN AND SUBGLACIAL DEFORMATION 379

experiment, it is important to define the situation of zerodeformation. The unstrained sediment, as it is represented byspecimen I, shows pervasive (sub)vertical masepic plasmicfabrics and strong (sub)vertical microfabrics, which canonly be explained as being pre-existing. We postulate thatthese features were induced in the industrial premodificationand processing of the sediment (twine pressing: H. Richter,Vingerling Laboratory, personal communication, 2000).Many of the plasmic fabric patterns found in close vicinityof skeleton grains are directly related to pre-existing verticalfabrics and therefore should be considered as processingartefacts themselves. Note that such circum-grain featuresoccur not just in specimen I, but also in specimens II–IV.

However, we also observed circum-grain features thatare obviously related to the experimental shearing itself.In the development of such features, the relative inertiaof larger grains compared with surrounding plasma andsmaller particles plays a role. Skeleton grains may influencethe development of unidirectional plasmic fabrics (masepicplasmic fabrics, unistrial plasmic fabrics) in that they arecapable of obstructing the physical process of progressivereorientation of clay platelets. For example, where shearingdirection is right-lateral, as in Fig. 12a, clay platelets willprogressively align from left to right, which means that onopposing sides of an inert grain, clay platelets are subject tostresses that are quite different from the overall stress field.Obviously, in the lee of the grain (right), stresses are lower,which generally leads to a weaker fabric or a strain shadow.On the left-hand side of the grain, stresses are concentratedat the obstructing surface, which forces clay platelets to takepositions parallel to the surface outline of the grain. This oftenresults in a stronger birefringence characteristic that could evenbe transverse to overall stress directions. This phenomenon ofstresses being different on opposing sides of obstacles is quitecommonly observed in subglacial tills. One example, albeit ata slightly different scale and resulting in a different structuralsignature is provided by Menzies et al. (1997).

In other situations, obstructing grains may be forced toperform rotating movements under the influence of shear-generated torques. In such cases a circular or elliptical haloof skelsepic plasmic fabrics may form around rotating grains(Fig. 12b). Owing to the sweeping motion of the grain, clayplatelets in its direct vicinity take preferred positions parallel oroblique to the surface of the grain. Dependent on the efficacyof the reorienting process, parts of the skelsepic haloes mayactually amplify the overall unidirectional plasmic fabrics.

Skelsepic plasmic fabrics are very common in thin-sectionsof subglacial tills and have been described by many authors.Good examples of the natural version can be found in vander Meer (1993, 1997) and in Khatwa and Tulaczyk (2001).One should be aware though that only skelsepic zones thatare relatively thick, either circular or elliptical in outline, androughly symmetrical with respect to the axis of rotation, maybe associated with rotational movements of grains. The morecommon thin, tight-fit type, that ‘fills in’ all irregularities inthe outline of a grain, should be ascribed, for example, to(differential) compaction of the sediment, or, more generally,to isotropic or non-deviatoric stress fields.

Unidirectional features

Table 2 shows that the strength of pre-existing masepic plasmicfabrics gradually declines. In specimen II we only occasionallyobserved zones of lower intensity masepic plasmic fabrics,but overall the signal could, as in the zero strain situation, be

(a)

(b)

Figure 12 Cartoon outlining skeleton-related plasmic fabricdevelopments. (a) Situation of an immobile, i.e. non-rotating grain ina deviatoric stress field. Dashes represent reorientated clay domains.At the up-stress side, individual clay particles are forced against theinert grain leaving a unilateral zone of birefringence. At the lee sideof the grain, plasma is poorly or non-orientated. (b) The elongategrain is forced to rotate. The plasma in the zone around grain that isaffected shows surface-parallel or imbricate orientation patterns

described as pervasive. In the later stages of the experiment,zones with no or hardly any signature became gradually moreimportant, actually culminating in specimen IV where themasepic signal was described as discontinuous. We furtherobserved that wherever masepic plasmic fabrics were poorlydeveloped or absent, unistrial plasmic fabrics were fairlydistinct, which would suggest an inverse relationship betweenthe two types (see Table 2). These observations led us tobelieve that, during the test, unistrials are generated at the costof the pre-existing fabrics.

Next to this fabric conversion, changes in plasma organisa-tion also may have played a role in the deterioration of thepre-existing plasmic fabrics. We noticed an increasing numberof clay agglomerations and fine silt concentrations developingin the course of the experiment. We think that these concen-trations reflect a process in which, as a result of shear-inducedvolumetric changes, the sediment reorganises its plasma bypreferential transport of fines to dilated zones. Although actualdisplacements of this kind are probably very limited, therecurrence of such a process could possibly account for theobserved features. It seems reasonable to assume that in thisprocess pre-existing clay fabrics would be affected.

We would argue that the unistrial features that weregenerated during the experiment represent the network of slipplanes that is predicted by the Mohr–Coulomb model. Weobserved that orientations of the sets are largely compatiblewith theoretical values, and also that continuity as well asstrength of the individual features increase with strain. Wealso noticed that the unistrial features occur in conjugate setsonly in the early stages of the experiment (specimen II). Inspecimens III and IV, strain seems to be taken up by just one,albeit better developed set.

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380 JOURNAL OF QUATERNARY SCIENCE

In fact this is exactly what would be expected lookingat the model: initially, the deforming system will attempt toaccommodate induced strain by cumulative short displace-ments along any plane of weakness that is available in thesediment. This means that at low strains bits and pieces of thepredetermined network of potential slip planes are used forthis purpose, but also pre-existing masepic fabrics might be‘recycled’ (cf. Arch et al., 1988). However, at higher strains,the sediment can no longer maintain the ‘fragmented’ type ofstress relief, which leads to selection and further developmentof just one predominant shear direction by interconnectingshorter, individual slip planes. This whole process closelyresembles the behaviour of metamorphic rocks dissipatingshearing energy (Ramsay, 1980).

The unistrial plasmic fabrics are similar in appearanceto the ‘crenulations and creases’ that were described byMaltman (1977). The tendency of individual crenulationsto die out, and to be replaced by others of differingwavelengths (Maltman, 1977), compares to what we havecalled ‘branching-and-merging behaviour’. As branching-and-merging is more common in the later stages of deformation(see Table 2), we relate it to increasing strain and growingunistrial plasmic fabrics. We would argue that the lenticularpatterns are not an inherent, steady-state-like characteristic ofthis type of microstructure (Maltman, 1977), but are at leastpartly controlled by sediment properties. We observed thatit usually takes small obstructions such as skeleton grains todeflect or split the unistrials. Whether newly formed branchescoalesce upon further development seems largely dependenton distance between them, and thus on the size of theobstruction.

We further infer that merging, which requires tips ofunistrials to curve, may be facilitated by pre-existing fabricsas these could influence the direction of growth. An examplescenario for the development of the anastomosing signalsis depicted in Fig. 13. Natural examples of branching-and-merging behaviour in clayey zones of what are interpreted assubglacially affected diamicts are shown in Hiemstra (1999).

With progressive strain, the branching-and-merging behavi-our of the unistrials could lead to the formation of shear bands,as was locally observed in specimen IV. In such bands allmaterial seems to be reorientated, without loss of cohesion.This observation again fits in the Mohr–Coulomb model,which describes that distinct shear bands develop when thesediment approaches peak strength, i.e. just before failure (seee.g. Maltman 1987b). The formation of the bands can beregarded as an intermediate stage towards a fully developedshear zone (for examples on the sample-scale see Morgensternand Tchalenko, 1967; Maltman, 1977, 1988). Van der Wateren(1999) describes excellent examples of shear bands in thin-section. The thin-sections were taken from a ductile subglacialshear zone at outcrop-scale.

On a microscale unistrials can be regarded as manifestationsof brittle deformation. This scale-dependency may seem trivial,but it is touched upon here because it helps to understand howfractures in the sediment become increasingly systematic inthe course of the experiment (see Fig. 3). Although it is quiteobvious that most of the observed fractures have formed as aresult of desiccation, we noticed that they do show preferentialorientations, particularly in specimens III and IV. So, in thelater stages of the experiment, apparent fracture orientationsare often aligned parallel to apparent orientations of theunistrial plasmic fabrics. In fact, we observed that some planarfractures are coincident with individual unistrial features. Weconclude that the development of birefringence creates planesof weakness in the sediment (= plasma separations) that tendto develop into cracks when the sediment dries out. The

Figure 13 Cartoon showing the development ofbranching-and-merging characteristics in unistrial plasmic fabrics.(a) Short, discontinuous unistrials grow together to form continuousfeatures in (b). Where unistrials ‘meet’ grains, they split or bifurcate.In the lee, individual branches may or may not grow together again

initiation of this fracturing process may involve the formationof ‘c-segments’, previously discussed by Will and Wilson(1989).

Grain arrangements

Finally, the occurrence of turbates and lineaments in thespecimens shows that during deformation not only clay-sized material but also coarser particles might be subjectto reorientations and displacements. How turbates form iseasily explained by extrapolating the model presented inFig. 12b from plasma-sized material to the size of fine skeletongrains. Similar to the manner in which a rotating coarser grainreorientates clay platelets to form skelsepic plasmic fabrics, itis also capable of exerting stresses upon fine skeleton grains toform a turbate structure around its outline. The turbates in ourtest are typically between 150 and 500 µm in diameter, andtheir increasing numbers with experimental shearing suggeststhat strain is progressively taken up in rotational movements(see Table 2). Turbates appear to be common to abundantfeatures in subglacial tills and have been reported by van derMeer (1993, 1997), Carr (2001) and Hiemstra (2001) amongothers.

We would go even further by postulating that the majorityof planar and rotational movements taking place in plasticallydeforming sediments are proportionally related. Table 2 showsan apparent relationship between unistrial plasmic fabricsand turbates in that they both seem to intensify withincreasing strain. That this may not be coincidental can beexplained by pointing out that planar movements inducethe torques that are necessary to rotate nearby grains,

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ARTIFICIALLY INDUCED STRAIN AND SUBGLACIAL DEFORMATION 381

and that increasing numbers of planar displacements willconsequently increase the chance that turbate structures areformed (Fig. 14).

The close relationship of planar and rotational move-ments is illustrated in the occurrence of grain lineaments(Fig. 15). These ‘grain trains’ form where three or morenearby grains are ‘interconnected’ and become part ofa shear plane. In this process, both clay particles andskeleton grains take on plane-parallel positions, result-ing in unidirectional plasmic and microfabrics (Fig. 15aand 15b).

The intensity of the fabrics is enhanced with progres-sive shearing, when the sediment continues to ‘shorten’in directions perpendicular to the stretching direction(see Fig. 15b). The combined stretching and shorteninghas a condensing effect, which may eventually leadto the centres of the skeleton grains being aligned.Ideally, the archetype lineament of Fig. 15c may beformed.

The relationship between the development of grain linea-ments and increasing strain is not so obvious. The lengthof the lineaments does not seem to change significantly,and numbers increase only slightly in the course of theexperiment (see Table 2). This may be attributable to thefact that the formation of lineaments involves at least threeparticipating grains. In an essentially matrix-supported sed-iment such as the one used here the lineament is thusmuch more of a stochastic feature than a turbate, the for-mation of which only requires one grain in the vicinityof a shear plane. The structure being more or less coin-cidental may also be the reason for the fact that linea-ments are not commonly reported from subglacially deformedsediments.

It should be kept in mind that lineaments are two-dimensional representations of possible three-dimensionalstructures, and that absolute movements of the individualgrains are minimal. Note also that ‘grain bridges’ as conceivedby Hooke and Iverson (1995) and lineaments are not thesame: grain bridges are thought to form perpendicular to thestress field, whereas lineaments are essentially parallel to themain stress components. The former are basically a meansof supporting stresses developing in a sediment, whereasthe latter may be regarded as a stress relief phenomenon.The lineaments observed in this study do compare to thealignments of silt particles shown in figs 8 and 9 in Dewhurstet al. (1996).

Figure 14 Cartoon showing the possible relationship betweenunidirectional plasmic fabrics, skelsepic plasmic fabrics and turbatestructures. With the development of a planar shear feature, clayparticles reorientate to form unidirectional plasmic fabrics (unistrialor masepic). The central grain starts to rotate owing to lateraldisplacements occurring along the shear, which leads to developmentof circum-grain plasmic fabrics and turbate grain organisations

(a)

(b)

(c)

Figure 15 Cartoon showing a scenario for the development of grainlineaments (in combination with unidirectional plasmic fabrics). Thedashed line in situation (a) represents the position of a shear planebeing developed. Elongate grains at, or in the direct vicinity of theplane tend to alter their positions by rotating until they are alignedplane-parallel. Concurrently, the grains are manoeuvred closer to theshear plane by a contraction or shortening of the sediment in thedirection perpendicular to the stretching (see strain boxes at theright-hand margin of (a) and (b). Figure (c) shows the resultant grainarrangement, the lineament. Note that around individual grains in alineament structure, turbate substructures may develop (see Figs 5and 14)

Conclusions

We have shown that the response of a sediment toimposed shear stresses during uniaxial compression is reflectedmicroscopically in clay fabrics, grain fabrics and grain arrange-ments, and that changes taking place within the deformingmaterial involve small-scale slip or shear displacements aswell as rotational movements. We have explained how rota-tions (circum-grain birefringence and turbate structures) andplanar displacements (unistrial plasmic fabrics and lineaments)may be closely related and that all of these microstructuresand their apparent gradual developments (see Table 2) canbe attributed to a time-progressive, Coulomb-plastic type ofstrain.

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382 JOURNAL OF QUATERNARY SCIENCE

Individual microstructures induced in the experiment resem-ble microstructures observed in thin-sections of naturallydeformed sediments, notably those in tills. In fact, the combina-tion of microstructures that was identified in the sediment spec-imens is remarkably similar to the combination of microscalefeatures commonly attributed to subglacial deformation (seevan der Meer, 1993; Menzies, 2000).

When appreciating the microstructural analogy, the out-come of this study indicates that sediments subjected tosubglacial deformation tend to behave as Coulomb materi-als, at least in the final stages of the process. However, itis noted that this conclusion is to be taken with some carebecause the possibility that tills behave as viscous fluids undercertain circumstances (e.g. during early deformation stages)cannot be ruled out. It is also noted that till behaviour may bescale-dependent as was suggested by Hindmarsh (1997), aninference which could not be tested in our experiment.

We conclude that rotational movements are potentiallyvery important in the subglacial deformation of sediments,which supports the postulation by van der Meer (1997) thatin some cases such movements account for the bulk of strainin tills. We would argue, however, that turbates may onlythen be considered diagnostic for a subglacial till where theyare closely related to planar shear structures. Where turbatestructures occur throughout a sediment, without any apparentrelation to shear planes, the sediment under consideration maywell represent a mass movement deposit, in which flow wasthe predominant deformation mode.

Acknowledgements This research was supported by the EuropeanCommunity (Marie Curie Fellowship 2001-494: JFH) and by NWO,The Netherlands Organisation for Scientific Research. Micromorpho-logical analyses were carried out at the University of Amsterdam.We wish to thank Wim Verwaal (Delft University) for the use of thelaboratory equipment and for his assistance. We thank Bert de Leeuwfor carrying out the X-ray diffraction analysis. We thank Simon Carrand John Menzies for their excellent reviews. Stimulating discussionswith Jaap van der Meer and Dave Evans are much appreciated.

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Appendix

Glossary, partly after Brewer (1976).

Skeleton is the generic term used to indicate all grains—bothmineral and organic—that are coarser than ca. 30 µm (= theapproximate thickness of a thin-section). Skeleton grains canbe observed individually in thin-section.

Lineament is a structure composed of three or more skeletongrains that have their centres aligned. Ideally the long axes ofthe grains are coincident with the imaginary line that runsthrough their centres. This structure is probably planar in threedimensions.

Microfabric refers to orientations of long axes of elongatedskeleton grains (compare with plasmic fabric).

Turbate is a circular arrangement of skeleton grains(in a two-dimensional thin-section). It consists of sys-tematically arranged particles that are often positionedaround a relatively coarse grain. Long axes of the par-ticles exhibit a parallel, or a spiralling, imbricate organ-isation with respect to the surface of the core grain(compare skelsepic plasmic fabric). Note that a turbateis a two-dimensional representation of a three-dimensionalphenomenon.

Plasma is the generic term covering all sediment mate-rial—mineral (amorphous and crystalline) as well as organic—finer than ca. 30 µm. Plasma is complementary to the skele-ton fraction. Individual plasma particles are too small to bediscerned in thin-section.

Plasmic fabric is defined as the organisation of theplasma in terms of orientations. Where within a certainarea plasma particles (i.e. micaceous clay minerals) arepreferentially orientated parallel to each other (separations),the plasma obtains optical properties that show up asextinction/illumination patterns (birefringence) under crossedpolarisers on the microscope.

Masepic plasmic fabric refers to plasma separationsthat are mainly orientated in (parallel) zones in onedirection. The striated orientation is elongated parallelto the length of the zones (compare unistrial plasmicfabric).

Skelsepic plasmic fabric refers to plasma separations withstriated orientations that occur subcutanically (and parallel) tosurfaces of skeleton grains (compare turbate).

Unistrial plasmic fabric refers to striated plasma separationstaking the form of thin, distinct lines in one direction. Orien-tations are necessarily—because of their thinness—parallelto the length of the zones (compare masepic plasmicfabric).

Craze plane—an essentially planar void with a highly com-plex conformation of the walls owing to the interconnectionof numerous short, flat and/or curved planes.

Joint plane—a planar void that traverses the sediment insome fairly regular pattern, such as in parallel or subparal-lel sets.

Skew plane—a planar void that traverses the sedi-ment in an irregular manner, having no specific orienta-tion pattern among individuals; walls may be smooth orslickensided.

Copyright 2003 John Wiley & Sons, Ltd. J. Quaternary Sci., Vol. 18(5) 373–383 (2003)