A kinematic unifying theory of microstructures in

SEDGEO-05030; No of Pages 14 Sedimentary Geology xxx (2016) xxx–xxx

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A kinematic unifying theory of microstructures in subglacial tills J. Menzies a,⁎, J.J.M. van der Meer b, E. Ravier c a b c

Department of Earth Sciences, Brock University, St. Catharines, Ontario, Canada L2S 3A1 Department of Geography, Queen Mary University of London, Mile End Road, London E1 4NS, UK L.P.G., CNRS UMR 6112, Université du Maine, Avenue Olivier Messiaen, Le Mans Cedex 9 72085, France

a r t i c l e

i n f o

Article history: Received 29 October 2015 Received in revised form 9 February 2016 Accepted 18 March 2016 Available online xxxx Keywords: Till Deformation Glacial geology Micromorphology Microsedimentology Structural geology

a b s t r a c t A key aspect of all subglacial tills is the nature and form of microstructures present. Microstructures are symptomatic of repeated deformation phases prior to, during, and after emplacement. Critical to understanding microstructures in subglacial tills are the probable interrelationship that exists between all of these structures. In analyzing subglacial tills a kinematic deformation relationship can be observed existing between all microstructures. Based upon the rheological conditions at the ice basal interface, a close evolving paleo-strain link can be established that relates levels of deformation to all subglacial till microstructures. As subglacial till undergoes strain during transport and emplacement involving fluctuating conditions of porewater content, percentage of clays present, and changing thermal circumstances, a series of microstructures sequentially evolve. Initially, brittle edge-to-edge grain events occur, followed by grains stack development, often allied closely in time, with microshear formation as the sediment deforms, and is consequently followed by the development of ductile rotation structures. Likewise, deformation bands, shear zone formation, and typically “isolated” domains form. As strain and other factors vary over time so many of these microstructures may be obliterated, altered, or reoriented. Much remains to be understood regarding paleo-strain conditions and subglacial deformation but a first step has been establishing this temporal sequence of microstructure stage development and thus achieving a theory that unifies these disparate microstructures observed in all subglacial tills. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Macro- and microstructures within subglacial tills have long been described and recognized as forms symptomatic of subglacial conditions developing during subglacial till deposition and emplacement. Subglacial till situated beneath an overlying active ice mass, and the underlying bed is analogous to fault gouge, a comparison made in the past but largely under-examined (Eyles and Boyce, 1998; McCarroll and Rijsdijk, 2003). Microstructures within subglacial tills are important geologic structures since they reveal types and phases of deformation, and their kinematic geometries are illustrative of paleo-strain levels. Although these microstructures have been described for some time now, the genesis and development of the structural elements during shear, deposition, and emplacement and their cross-interrelationships remain poorly established. Closely aligned to microstructure development in subglacial tills is the recognition of the subglacial till types themselves. Subglacial till formation and classification remain a contentious issue as a clearly established classification that accepts both macro and micro-sedimentological characteristics has still to be established. With the dominant microstructures now well recognized and mapped within

⁎ Corresponding author. Tel.: +1 9056885550; fax: +1 9056889020. E-mail address: [email protected] (J. Menzies).

subglacial tills, it is timely to re-assess their geologic, structural, and rheological significance within the context of the mechanics of subglacial till deposition. In this paper, we review these subglacial till microstructures and discuss the relationship of these microstructures to one another, to the mechanics of subglacial till formation and present a possible unifying theory of microstructures in subglacial tills. 2. Subglacial till Subglacial till is one of the most pervasive sediments in the world. As a Quaternary sediment whether on land or in the ocean, margins on the continental shelves, tills, or perhaps more accurately glacial diamictons, occur across a very wide sweep of the northern hemisphere in North America and Eurasia. Tills occur on all continents, including mainland Australia and tropical areas like Indonesia. Subglacial tills in modern settings occur in all presently glaciated areas of the world. As to PreQuaternary tillites (diamictites), they were deposited in most geological terrains from the Oligocene through the Permo-Carboniferous to the Archean in most parts of the world (Young, 1996, 2016; Eyles, 2008; Arnaud et al., 2011; Tait et al., 2011; Le Heron et al., 2013). Subglacial tills (glacial diamictons) are sedimentologically complex earth materials. They are formed by accretion in the basal space between ice sheets and glaciers and their beds, by deformation of the bed without input from within the ice or formed at the margins

http://dx.doi.org/10.1016/j.sedgeo.2016.03.024 0037-0738/© 2016 Elsevier B.V. All rights reserved.

Please cite this article as: Menzies, J., et al., A kinematic unifying theory of microstructures in subglacial tills, Sedimentary Geology (2016), http:// dx.doi.org/10.1016/j.sedgeo.2016.03.024

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J. Menzies et al. / Sedimentary Geology xxx (2016) xxx–xxx Table 1 Classification of Till types (modified after Menzies, 2003). Note the various strain levels and styles of deformation for the different subglacial till types.

(or near marginal surfaces) of ice masses and derived from the glacial system as debris flows or meltout by-products or rainout within bodies of water proximal to ice masses (subglacial rainout in cavities may also contribute to subglacial till emplacement) (Table 1) (Bennett et al., 2006; Livingstone et al., 2012). In terms of subglacial tills where once it was commonplace to think that this subjacent zone between ice and its bed was comparatively simple in a modeling sense, today it has become apparent that this an extremely complex environment, that is dynamic, in relatively constant flux over time and space, enlarging and shrinking over time as an ice mass advances and retreats across the landscape (Alley, 1991; Menzies, 2003, 2012; Piotrowski et al., 2004, 2006; Kjær et al., 2006; Larter et al., 2009; Knight, 2010; Leysinger Vieli and Gudmundsson, 2010; Narloch et al., 2012; Larson et al., 2015; Menzies et al., 2016). Subglacial tills have complex strain histories (Menzies, 2012). Table 1 illustrates that different subglacial till types are emplaced/ deposited in the subglacial environment that are subjected to range of strain conditions in which the tills may deform in brittle and ductile styles. Subglacial tills are repeatedly stressed over time, and these stress conditions can be reflected in the microstructures developed within these tills. It has been previously discussed (Menzies, 2003, 2012) that where tills have undergone subsequent high strain following on from low strain impacts; previous low strain microstructures may be drastically altered or obliterated while the opposite effect of high strain followed by low strain may permit high strain microstructures to survive. Since subglacial tills have likely undergone many repeated strain changes, microstructures in the “final” till “end product” may only allow a glimpse into the chronology of till strain subjected conditions. One aspect in studying till microstructures is to establish strain conditions and a chronology of those strain changes over time. Subglacial tills can be subdivided into four dominant types. Mélange tills are similar to the “subglacial traction” tills of Evans et al. (2006) and form under soft deforming subglacial bed polythermal conditions of high to low strain with evidence of brittle and ductile deformation

styles. These tills contain elements of all other subglacial till types that have been scavenged and incorporated into this “end product.” Lodgement tills that, although perhaps much less common than previously considered (van der Meer et al., 2003; Menzies et al., 2006; van der Meer and Menzies, 2012; Menzies, 2012), are subject to high strain conditions and dominantly, if not completely, ductile deformation. Melt-out tills formed under more passive conditions (Fitzsimons, 1990; Munro-Stasiuk, 2000; Möller, 2010; Pisarska-Jamroży and Zieliǹski, 2012; Larson et al., 2015) in which these tills exhibit low to zero strain effects and brittle deformation conditions. Finally, flow tills formed, likely within subglacial cavities where mass movement has resulted in relatively moderate to low strain conditions and dominantly brittle deformation with occasional ductile deformation where internal porewater content has allowed ductile deformation under low strain to occur (Piotrowski et al., 2006; van der Meer and Menzies, 2011). In volumetric terms it is thought that mélange tills make up the vast majority of subglacial tills in Pleistocene and pre-Quaternary till sediments. Since the mid-twentieth century, the macro-sedimentological aspects of subglacial tills have become well recognized and have been the basis of a well-established subglacial till classification scheme (Dreimanis, 1989; Evans et al., 2006). Many of these established macro-sedimentological aspects, however, are not well understood and assumptions have often been made with comparatively little real understanding of subglacial till depositional mechanics. The science around macro-subglacial till fabrics is a good case in point (Carr and Rose, 2003; Thomason and Iverson, 2006; Phillips et al., 2011). It has generally been accepted that particles within subglacial tills orientate under stress to exhibit an azimuth approximately parallel with the dominant principal stress application that, in turn, is assumed to be the dominant ice direction. Exactly how these particles orient and under what conditions of porewater content and pressure, and overall strain field remains largely unknown or speculated. Various particle

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shapes appear to orient more consistently than others and differing particle sizes equally appear preferential but the actual mechanics remain obscure within the various subglacial till types that are recognized (e.g., Bennett et al., 1999; Evans et al., 2006; Carr and Goddard, 2007; Cook et al., 2011; Gentoso et al., 2012; Haines et al., 2013). The interrelationships between macro and microstructures, specifically clast macro-fabrics, have been discussed by Evenson (1971) and more recently by Neudorf et al. (2015) and Hopkins et al. (2016), suggesting that there is a complex relationship between both sets of fabrics that is a function of particle size, strain level and porewater content that needs new research investigation. It should be noted that the bed of a glacier is in a state of constant flux, strain, effective pressure, porewater content, texture, all are changing continuously by seasonal fluctuations in snow fall, snow melt and incorporation or release of parts of the bed (van der Meer et al., 2003). It is also likely that in some cases porewater is drastically reduced if an ice stream collapses or there is a change in the subglacial bed's thermal regime. At such a stage, soft bed deformation ceases and lodgement till emplacement might occur or subglacial erosion will become dominant. At such a stage, high strain conditions will prevail and the destruction of possible medium and low strain microstructures may occur. 3. Microstructures in subglacial tills As a response to simple shear or pure shear strain, porous granular materials such as subglacial tills, typically fail and in the progressive response to deformation develop a series of microstructures. These microstructures can be pervasive within the failing sediment or occur as isolated “patches.” Likewise, the scale of failure can be pervasive or, more typically, localized within small areas or linearly. Where pervasive strain (distributed as in pure strain) occurs or areally confined strain (simple strain) or in localized patches or zones (relict strain), the spatial relationship is fundamental in discriminating between different shear mechanisms as well as potentially providing a chronology of events between differing modes of deformation. Essentially most subglacial tills, if not all, undergo various levels of cataclasis some under high brittle strain while elsewhere much lower non-cataclastic levels of brittle strain in the subglacial interface (Menzies et al., 2006; van der Meer and Menzies, 2011). As in other forms of cataclasis, strain hardening is likely to take place resulting in bands or zones or microstructures forming in response to continuing strain. Many of the microstructures observed in numerous subglacial tills, from a very wide array of locations and ages, are indicative signatures of this cataclasis. None of these microstructures are formed in isolation and must all be interrelated and limited to the levels of strain, porewater content, the range of particle sizes and shapes, sediment heterogeneity over variable distances and sediment temperatures. All of these parameters fluctuate widely over time and space in the subglacial environment. The microstructures, although some may yet remain to be identified, can be classified as shown in Fig. 1. Microstructures can be subdivided into plasmic, skeletal and, in combination, S-matrix in line with the concepts elucidated by Brewer (1976) and later adapted by van der Meer, 1993; Menzies, 2000; Ravier, 2014). Plasmic fabrics ranging from scattered omnisepic to the strongly aligned unistrial are all considered indicative of degrees of strain (Linch and van der Meer, 2013). The theory, backed up by extensive laboratory testing, is that as strain increases on particles b 30 μm, stronger and stronger alignment of particles occur. The only plasmic fabric that can be attributed to a combination of plasma and skeleton grains form a skelsepic fabric. This fabric is considered the result of either rotation of the skeletal particle and adhesion of plasma to its edges; or possibly porewater elutriation causing plasma to move in the pore space and in the process coat the skeletal particle. S-matrix microstructures (Fig. 1) are subdivided into ductile, brittle, and porewater induced or influenced forms (artifacts). From a

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structural geological viewpoint each of these subgroups represents common microstructures observed in numerous subglacial tills in thin section. These S-matrix microstructures within ductile shear bands are similar to S-C-C′ microstructures as specified by Berthé et al. (1979). Each characterizes specific strain conditions that develop a distinctive characteristic artifact symptomatic of strain conditions under various rheological sediment states. 4. Significance of microstructures in subglacial tills It has been generally assumed that although microstructures are indicative of various forms of and styles of deformation, they have been regarded as relatively unique and not part of a continuum of strain signatures. However, if a structural geological approach is considered for these various microstructures, a new avenue of comprehension may open. Since microstructures likely develop through a set of processes that often involved complex loading paths that often comprise multiple phases of strain, deposition, and re-entrainment both influenced by ice mass loading and shear stress induced ice flow (Menzies, 2012), some structures may be better developed or evolved than others, while some may be present but in various states of degradation. Until the significance of microstructures in subglacial tills can be fully realized, it is difficult to comprehend the importance and inter-related aspects of these structures. Without that understanding, a more developed group of subglacial till depositional mechanics and subglacial till types cannot be attained. 5. Kinematics of microstructures in non-glacial diamictons Before considering the mechanics of microstructural development in subglacial tills and their possible unifying connectivity, it is useful to review previous non-glacial literature on microstructures and strain conditions. Recent reviews of gouge, deformation bands, fault breccia, and microbreccia (Fossen et al., 2007; Woodcock and Mort, 2008; Kaproth et al., 2010; Rathbun and Marone, 2010; Tembe et al., 2010; Ikari et al., 2011; Ballas, 2013; Haines et al., 2013, 2014; Skurtveit et al., 2014) note the effects of progressive strain, grain size and porewater content on microstructure development. To summarize the work, it has been shown that progressive deformation can be expected as a consequence various microstructures sequentially develop. In experimentally using Caesar subglacial till, Rathbun and Marone (2010) showed that even at low strain, the localization of stress application occurs and that there is an evolutionary trend from Riedel shears to clay foliation (p-shears). Kaproth et al (2013), for example, illustrate that following localized cataclasis strengthening of the “mixture” occurs. Tembe et al. (2010) experimented using 3 levels of shear regimes from low to high, observed that change in sediment rheology occurred at each stage principally noting that from grain-supported matrix to a more clay-supported matrix, microstructures tended to develop due to grain crushing and shear localization. In contrast, Skurtveit et al. (2014) demonstrated that following on from Riedel shear development (brittle deformation), as strain increased, cataclasis occurred in restricted areas as domains and deformation becomes increasingly ductile leading toward grain rotation development. Likewise, Haines et al. (2013, 2014) illustrated that after initial grain rotation, locally elevated porewater pressures tend to develop leading to low friction clay bands and zones. Fossen et al. (2007) showed that deformation in this type of diamictic material was often hierarchical leading from “nested” microstructure fabrics to grain rotation and translocation under strain. It was also demonstrated that rotation and grain sliding required a certain sediment porosity to evolve. Fossen et al. (2007) also suggested that deformation types can be classified into several types from granular flows, to cataclasis, to phyllosilicate smearing. It is instructive to compare these experimental observations with recent work by Narloch et al. (2012) on Polish subglacial tills with the independent lab work completed by Haines et al. (2013), the latter finding similar results that suggest that even at low strains (2–

Please cite this article as: Menzies, J., et al., A kinematic unifying theory of microstructures in subglacial tills, Sedimentary Geology (2016), http:// dx.doi.org/10.1016/j.sedgeo.2016.03.024

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Microfabrics and Mircrostructures within the Plasma and S-Matrix of Tills

Plasma

Skeleton Grains

Plasmic microfabric

S-Matrix Brittle

Ductile

Porewater (induced or influenced)

Lattiseptic plasmic fabric

Faulted domains

Silt and clay coatings

Tectonic fabric

Masepic plasmic fabric

edge to edge grain events

Folded structure Sill and dyke structures

Unistrial plasmic fabric

Discrete shear lines

Crenulation Foliation

Water-escape structures

Reverse fault

Shear zones

Necking structures

Kink bands

Strain shadows and caps

Omnisepic plasmic fabric

Sedimentological fabric: Tectonic and/or incomplete mixing porewater

Kinking plasmic fabric

Skelsepic plasmic fabric

Boundins

Banded plasma

Vertical stress exerted by ice load (pure shear deformation) Horizontal stress exerted by ice flow (simple shear deformation)

Rotational structures

Hydrofractures openings

Water paths

Turbate and Necking structures

Fig. 1. Classification of microstructures (modified from Ravier, 2014; Menzies, 2000; van der Meer, 1993). Note the subdivision of microstructures into ductile, brittle, polyphase, and porewater influenced or induced.

4) microstructure development occurs. Haines et al. (2013) have also noted that grain rotation appears to be facilitated by Riedel shear development, in other words brittle deformation effectively facilitates ductile deformation at a later stage. A common feature of almost all subglacial mélange tills is this rapid change from brittle to ductile and back again repeatedly as the till is being deformed and subglacially transported. The subglacial till essentially behaves in a polyrheological manner. This body of work illustrates the complexity of coarse to fine grained diamicton-like sediments and makes comparison with subglacial tills of considerable value.

6. Kinematics of microstructures in subglacial tills All subglacial tills exhibit evidence of soft sediment deformation both brittle and ductile (van der Meer et al., 2003; Menzies et al., 2006; Menzies, 2012). As Fig. 1 illustrates (see van der Meer et al., 2003, Fig 1), the classification of typical microstructures into ductile, brittle, and porewater influenced or induced. It needs to be pointed out that many microstructures are hybrid types that exhibit over time both brittle and ductile characteristics, suggesting in these cases that as microstructures were being developed the sediment transitioned

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between brittle and became ductile of vice versa. The dominant microstructures are microshears, rotation structures, grain stacks, domains, shear zones, deformation bands, and edge-to-edge (e–e) grain crushing events (Fig. 1) (cf. van der Meer, 1987, 1993; Menzies, 2000; Phillips et al., 2002; van der Meer and Menzies, 2011). It is likely that most deformation microstructures were formed during some phase(s) of emplacement and prior to final sediment immobilization if, as likely, the tills were emplaced within a subglacial soft sediment deforming environment. Clear evidence exists of variations in shear strain rate from comparatively low to high rates reflected in many subglacial tills (Menzies and Reitner, in press). 6.1. Edge-to-edge crushing events (e–e) Edge-to-edge crushing event microstructures are a fairly common feature in many subglacial tills (Fig. 2). It appears that e–e microstructures develop at the earliest of stages of subglacial till formation as soon as the sediment experiences strain and may be the first microstructures to develop, quickly followed by grain stack (gst) particle alignments (Fig. 2a, b). (van der Meer, 1987; Gao et al., 2012; Menzies et al., 2013). However, all subglacial tills do not contain obvious examples of e–e microstructures possibly due to the fact that the grains are crushed or that such microstructures are destroyed as other structures develop. It has been noted in subglacial tills, for example, in W. Antarctica (Reinardy et al., 2011) and in Pliocene subglacial tills in northern Ontario (Menzies et al., 2013) that the lack of e–e microstructures may indicate fast moving ice as typical of an ice stream or, at the very least, very high porewater pressures that act in a dilatant manner to prevent numerous particle to particle collisions (Engelhardt and Kamb, 1998; Tulaczyk et al., 1998; Menzies et al., 2013, Fig. 3). It is possible that in cases where an ice mass has moved across heavily saturated sediments such as glaciolacustrine mud, if they contain rainout coarser sediments or other scavenged coarse units, which are under dilatant

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strain rates, there is a dearth of edge-to-edge grain crushing events. It seems likely this latter hypothesis may explain the paucity of e–e microstructures in many cases and yet may explain the fact that the other microstructures typical of deforming subglacial till are also present. The argument in favor of this hypothesis has been that the influence of high porewater pressure, low effective stress, and possible dilatancy, can all be considered indicative of fast warm basal ice conditions over a rapidly mobile deforming subglacial bed. However, it has become apparent, as will be discussed below, that e–e events may be also a function of high strain related to all of the above subglacial conditions but also sediment texture, specifically clay content varies within the subglacial till matrix. It is apparent that e–e events are a function of high strain but likely within a relatively non-ductile environment in term of porewater pressures being low due to low clay content. 6.2. Grain stacks (gst) During the emplacement of subglacial tills, complicated loading paths can be difficult to identify yet the microstructures extant in these sediments testify to this complexity that involve considerable variations in strain rates, porewater content and effective stress levels, variations in yield strength, rheology of the sediments involved, and large temporal and thermal fluctuations that remain beyond identification in microsedimentological analyses (Fig. 3) (Hodder et al., 2016). In Fig. 3, it is interpreted that grain stacks formed before the rotation structures developed although it is thought the two process occurred rapidly and almost penecontemporaneously). One aspect of these microstructural textures is the development of grain stacks. Typically, these stacks are observed as lines of grains en echelon that many exist as four or five grains of differing sizes and may occur with as many as 8 or 10 grains in length that form as two sets conjugate to one another probably therefore indicative of simple shear. Frequently, grain stacks are linear and of relatively short distance, though on occasion the stacks are distorted

a

dom

dom

gst

domain boundaries

ms 0.1 mm

dom

0.1 mm

b

e-e

Shattered clast 1 mm

Fig. 2. (a) Till from the James Bay Lowlands, Ontario, Canada, showing in plane light examples of e–e grain crushing, also domains (dom), microshear (ms), and grain stacks (gst) (Gao et al., 2012; Menzies et al., 2013). (b) Till from Wijnjewoude, the Netherlands (R.974), in cross-polarized light showing a large example of a crushed grain and associated edge-to-edge grain crushing events (e–e) (van der Meer, 1987).

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rt ms gst

gst 1 mm

ms

rt

1 mm

gst ms Fig. 3. Till from Aberdeen Lake, central Nunavut, Canada (TUR057-J), in plane light, showing grain stacks and associated rotation structures (rt) and microshears (Hodder et al., 2016).

or twisted and may bend or be crooked. In some instances, the stacks parallel microshears that may be symptomatic of preferential shear direction and indicative of simple shear followed by late pure shear (Passchier and Trouw, 2005) (Fig. 4) (Rice et al., 2014), while in other cases, the stacks crosscut microshears and rotation structures (Fig. 5a, b). It is often noticeable that grain stacks appear to occur in local areas or domains of the subglacial tills where clay content is N20% or in a sandy silt clay textures (cf. Lupini et al., 1981; Tembe et al., 2010). Grain stacks appear to be part of an evolving continuum between edge-to-edge grain crushing event and deformation bands, the former with an even lower clay content while deformation bands appear to occur in areas of sediment with a relatively higher clay content (see below). Grain stacks appear similar to what Taboada et al. (2005) have termed “chain forces” being lines of particles aligned in a similar manner to those described above as grain stacks and are likely an important kinematic marker of deformation. 6.3. Microshears (lineations) (ms) Microshears are the commonest microstructure found in subglacial tills. The presence of microshears indicates that the sediment has been differentially “adjusted” (ductile) or brittle sheared though strain partitioning such that a local “tear” occurs within the sediment (Fig. 4a, b) (Menzies and van der Meer, 1998; Rice et al., 2014; Menzies and Reitner, in press). Microshears may be best described as shear localization fabrics (inter alia Tembe et al., 2010; Ballas, 2013). In themselves, microshears indicate that a local readjustment due to shear stress application has occurred. In most instances, this local movement is the result of external stress overcoming local sediment yield strength; however, in some cases, local internal intrinsic adjustments must occur as sediments settle, as porewater causes local changes, and as sediment compaction from loading takes place post-depositionally. Recent work (Narloch et al., 2012) has shown that the orientation of microshears in relation to shear direction and deforming sediment thickness can be perceived as symptomatic of levels of shear stress (Choukroune et al., 1987; Ballas, 2013) (Fig. 4a, b). As Logan et al. (1992) have shown as shear strain rates increase, the angles of the linear shear fracture (the angle is often referred to as the θ angle. i.e., the angle described between the S and C planes. The θ angle allows an estimate to made of the strain intensity and the relative contribution of simple shear stress relative to pure shear stress) reduce and that a value of approximately 25° can be set to differentiate between low strain rates (N25°) (and/or dominant pure shear) and high strain (b25°) (and/or dominant simple shear) (cf. Morgenstern and Tschalenko, 1967; Tschalenko, 1968). Repetitive microshearing at differing orientations and declinations in the form of crosscutting microstructures in

subglacial tills indicate repeated and ongoing phases of deformation at different levels of shear within a soft sediment subglacial mobile zone (Alley, 1991; Piotrowski et al., 2004, 2006; Truffer and Harrison, 2006; Larter et al., 2009; Smith and Murray, 2009; Knight, 2010; Menzies, 2012; Narloch et al., 2012). Likewise, the evidence of sediment consolidation due to internal discontinuities developing indicates localized loading of the subglacial till either by overlying glacial ice or sediment resulting in consolidation effects (Guiraud and Seguret, 1987; Twiss and Moores, 1992; Thomason and Iverson, 2006). Many cross-cutting microshears are indicative of shear fracturing due to compressive shear occurring during emplacement (cf. Engelder, 1987). The orientation of microshears for example in Fig. 4a, b, c are typical of C′ type shear planes or bands. Such shear bands are indicative of oblique foliation (fabrics) and are formed where shear bands may begin nucleating under high differential shear reflecting inhomogeneous simple shear within a nonpervasive deformation zone (typical of subglacial sediment mobilization). In Fig. 4a, a chronology of microstructure formation is likely as follows: first stage (rt and ms simple shear structures), followed by a second stage of conjugate gst structures. Whereas in Fig. 4c, C′ type shear planes or bands are indicative of increasing shear strain. The shear bands generally indicate high strain rates and/or high anisotropy in the sediments. (Choukroune et al., 1987; Passchier and Trouw, 2005). In order for such fractures (microshears) to develop and propagate, it is necessary for the subglacial till to behave in a locally brittle manner either the result of porewater dissipation or possibly short term freezing or clay removal due to translocation leading to localized dilatant zones of clay deficient areas or domains. In losing porewater content via various forms of porewater dissipation the sediment becomes increasingly brittle, effective stress levels rise and deformations will shift to a brittle phase. Likewise, if porewater locally freezes, then again the effective stress levels in the sediment will rapidly increase and so brittle deformation will occur. Finally, in these possible scenarios, where clay content has been removed, axiomatically porewater through porosity decreasing the sediment becomes brittle as effective stress levels rise. These events may have occurred and may be indicative that in the larger system of soft sediment deformation occurring beneath these ice masses, a mosaic of subglacial freezing and thawing and/or of local clay translocation may have occurred repeatedly (Maltman, 1987, 1988; van der Meer et al., 2003; Piotrowski et al., 2004; Passchier and Trouw, 2005; Tylmann et al., 2013; Trommelen and Ross, 2014). It is, however, possible that in shearing under these subglacial conditions, porewater is able to dissipate along shear zones that somehow act as dewatering channels (Menzies and Maltman, 1992; Arch and Maltman, 1990; Van Wateren et al., 2000; Hiemstra and Rijsdijk,

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a ms

gst

rt rt e-e 1 mm

1 mm

ms

b

sz

ms

clay zones (possibly sheared)

rt

Stress shadow

1 mm

gst

1 mm

ms

S

C

C C’

Shear zone (sz)

c

C’ 0.5 mm

0.5 mm

Fig. 4. (a) Till from Pine Point Mine, NWT, Canada (O-28-0017-5), exhibiting a large set of microshears (in red), rotation structures (rt), e–e grain crushing (e–e), and grain stacks (gst) (Rice et al., 2014). (b) Till from Weissbach, northern Austria (WB-1kp), in plane light, showing microshears (ms), a stress shadow, small shear zones (sz), grain stacks (gst), and rotation structures (rt) (Menzies and Reitner, in press). (c) Till from Moneydie, Perthshire, Scotland (R.756) (in polarized light), showing microshears (ms) within a broad shear zone (Menzies and van der Meer, 1998). Note C, S, and C′ shears.

2003), thus negating the need for either bulk dewatering or freezing or a combination of both (cf. van der Meer and Menzies, 2011; Menzies, 2012). 6.4. Rotation structures (rt) The presence of rotation structures is common in most tills indicative of the plastic deformation that these subglacial tills have undergone most likely within a warm soft subglacial deforming layer (Fig. 5a, b). As subglacial till becomes increasingly sheared it can be noted that rotation of fine and at times larger particles, the latter at times with a halo of smaller surrounding particles develop. Haines et al. (2013, Fig. 17)

demonstrated under experimental conditions that as a sediment begins to deform at the yield strength point particles do begin to rotate after initial Riedel shears form. As deformation continues, rotation becomes increasingly common and pervasive where conditions permit. In subglacial tills that contain interfingers, intraclasts, or small domains of differing lithofacies where often porewater escape structures have occurred, the absence of rotational elements is not uncommon. In fine-grained subglacial tills with relatively high clays contents (N 20%), the presence of rotational structures is commonplace often in close association with clay-rich zones of décollement (Labaume et al., 1997, Lupini et al., 1981; Tembe et al., 2010). In some instances, a sequence of rotational structures that are aligned with each other can be observed

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(cf. Hiemstra and van der Meer, 1997). Hiemstra and van der Meer hypothesize that once rotation begins then the process passes through a sediment leading to a “clock-like gear” process occurring (substantiated

largely by Haines et al.'s, 2013 experimental work). In many cases, the rotational structures are exclusive to certain domains and may be absent in other domains especially where the latter are likely rafted

a

rt

rt gst ms sz

100 µm

100 µm

b rt

sz

rt

gst ms 1 mm

1 mm

c sz

rt

necking structures

rt

gst sz 1 mm

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J. Menzies et al. / Sedimentary Geology xxx (2016) xxx–xxx

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dom

rt

1 mm

ms dom

1 mm

Fig. 6. Till from the James Bay Lowlands, Ontario, Canada (GAO22-13), in plane light, with distinct deformation bands between domains (dom), and a rotation structure (rt) and a microshear (ms) (Gao et al., 2012; Menzies et al., 2013).

fine-grained intraclasts and have, therefore, not undergone deformation and reveal that non-pervasive deformation is the norm under these subglacial conditions, or where particles are larger and motion is restricted due to grain to grain friction (cf. Anthony and Marone, 2005). The presence in many cases of an adjacent, at times almost overlapping, set of rotation structures possibly shows that once a rotational deformation began this “effect” translates to adjacent sediment and sets up additional rotations (cf. Hiemstra and van der Meer, 1997). In many instances, rotations can be observed couched between semi-parallel low angle microshears. The combination of rotations and microshears of this type appear indicative of localized shearing.

indicates that a locally pervasive form of deformation has taken place possibly indicative of deformation at sediment contacts or other rheological “edge” zones. In these tills, shear zones consist of fine-grained sediment often having a high clay content. Any previous relict or remnant evidence of sediment origin has likely been destroyed due to the high strain levels. The presence of these shear zones would appear to be influenced by either simple shear due to ductile shearing associated either with liquefaction and/or clay liquefaction. The width of these zones may be a function of progressive clay translocation to zones of décollement or possibly repeated shearing and décollement resulting in ever increasing zone width over time.

6.5. Deformation bands (db)

6.7. Domains (dom)

These bands are thin zones of fine-grained sediment that reveal distinctive evidence of deformation in relatively clay-rich zones within the subglacial tills (Fig. 6a, b) (Gao et al., 2012; Menzies et al., 2013). Deformation bands are, at times, referred to as shear bands. Often it can be noted that microshears cross cut the deformation bands indicating that two different deformation stages occurred in a subglacial till (Knipe et al., 1991; Włodarski, 2005, 2010; Fossen et al., 2007; Torabi et al., 2007; Torabi and Fossen, 2009; Ballas, 2013). In general, deformation bands tend to occur in subglacial tills that have a N 20% clay content and where zones of relatively clast-free subglacial tills occur locally within the thin section samples. The presence of deformation bands and likely associated plasmic unistrial fabrics are both indicative of relatively high shear strain conditions. In some cases, cataclasis occurs within the band in which edge-to-edge grain crushing events can be observed while in other instances, likely a function of grain size, no cataclasis occurs. It has been observed that low porosities and low confining pressures promote the formation of dilatant bands with no cataclasis, whereas high porosities and high confining pressures promote cataclasis (cf. Antonellini et al., 1994; Fossen et al., 2007; Włodarski, 2010).

A feature typical of almost all subglacial tills is the presence of multiple domains of often slightly different or in some cases very different lithofacies units that have all been incorporated into the subglacial till (Fig. 8a, b) (Eyles et al., 2011; Menzies and Ellwanger, 2010). In most instances, the domains are either rafted, scavenged units of likely previously deposited sediments “picked up” as the subglacial till was most likely moving within a mobile soft sediment subglacial bed and thus was incorporated prior to emplacement. In some cases, the included units have been drastically modified while in other instances only slightly modified if at all. In most thin sections, domains can be observed that indicate either limited sediment mixing or, subsequent to subglacial till emplacement, some form of clay dispersal via porewater transport under pressure (i.e. elutriation) and/or differential shearing. It would be difficult to assign an origin to the various clay rich domains, but if distance of transport was short (b5 km), these domains might indicate the possibility of limited mixing of various sediment types yet indicate effective subglacial scavenging to “pick up” the sediments (Menzies, 2012; Le Heron, 2015). Given the provenance of each domain, it may be possible to use domain types as discriminating evidence of differing chronologies of incorporation. However, given the repetitive nature of subglacial erosion, transport and emplacement separating chronologies by this means may be virtually impossible.

6.6. Shear zones (sz) In some instances, instead of a relatively thin deformation band a much broader zone of highly deformed ductile sediment can be observed situated between less-strained sediment domains (Fig. 7a, b, c) (van der Meer et al., 1983; Hodder et al., 2016; Menzies and Reitner, in press). Within these zones, due to shear localization, some form of strain softening must have occurred. The evidence of shear zones

7. Interrelationships between microstructures—a unifying theory In developing an understanding of microstructures in subglacial tills, it is apparent that there may exist a continuum of microstructure types related to shear stress and strain rates, porewater, clay content, and rheological and thermal conditions. This implies that there are also

Fig. 5. (a) Till from Greggelberg, northern Austria (G1c), in plane light, exhibiting a set of interacting rotation structures (rt), microshears (in red), a shear zone (sz) and several “twisted” grain stacks (gst) (Menzies and Reitner, 2016). (b) Till from Aschbach, northern Austria (AB-2bp), in plane light, showing large rotation structures (rt) around clast cores with small fragmented shear zones (sz) and grain stacks (gst) (Menzies and Reitner, 2016). (c) Till from Aschbach, northern Austria (A1-10bp), in plane light, showing rotation structures (rt), microshears (ms) (in red), shear zones (sz), grain stacks (gst), and two small necking structures (Menzies and Reitner, 2016).

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a

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sz

rt

gst 1 mm

1 mm

ms

b

rt

gst

sz ms 1 mm

1 mm

c

sz

ne

r zo

a She 3 mm

3 mm

Fig. 7. (a) Shear zones within a till from the Aberdeen Lake, NWT, Canada (TUR057-K), in plane light, note microshears (ms), grain stacks (gst), and a small shear zone (sz) (Hodder et al., 2016). (b) A till from Weissbach, northern Austria (WB-3bdp), in plane light, a distinctive shear zone (sz) with surrounding microshears (ms), grain stacks (gst), and a small rotation structure (rt) (Menzies and Reitner, 2016). (c) A till from Lunteren, the Netherlands (R.745), in cross-polarized light, showing a distinctive shear zone (sz) (van der Meer et al., 1983).

domains without microstructures, not necessarily because they have not been formed, but because they have been erased by liquefaction/ fluidization (Phillips et al., 2013), a reset of microstructures. Experimental lab work on fault gouge (subglacial tills can be considered a form of gouge, cf. Cowan (1985) and Eyles and Boyce (1998)) has demonstrated that various microstructures occur as a function of sediment clast and clay content varying under differing strain conditions (cf. Lupini et al., 1981; Logan et al., 1992; Takahashi et al., 2007; Crawford et al., 2008; Ikari et al., 2007, 2009, 2011; Tembe et al., 2010; Marone and Scholz, 1989; Anthony and Marone, 2009; Haines et al., 2013). It is only once shear localization occurs that edge-to-edge grain crushing events are likely to happen. As subglacial sediment slides beneath an ice mass as a deforming bed it seems that most of the sliding, in the first instance,

will be accommodated along existing lithofacies unit interfaces (domain contacts) developing large shear planes and discontinuities. As shearing progresses smaller shear failures will develop and Riedel and C′ shears and other shearing forms will begin to develop away from these major shear boundaries. Experimental work on gouge show that as shearing progresses large grains tend to migrate into the middle ground between shear planes and likewise take up some of the shearing load resulting in grain stacks and edge-to-edge crushing events (cf. Haines et al., 2013). (It should be noted that shear planes might occur at the ice/bed interface and at the subjacent bedrock surface. However, as more and more subglacial till becomes immobilized the lower shearing interface at one time at the bedrock surface rises within the subglacial till package.) It has also been demonstrated that in sandy gouge

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a dom

e-e

rt dom Fine sandy-clay

dom clay-rich

dom

5 mm

5 mm

b rt dom ms

rt dom gst ms

dom

rt 5 mm

5 mm

Fig. 8. (a) Till from Oakville, Ontario, Canada (12D), in cross-polarized light, showing multiple domains (dom), a rotation structure (rt), and an edge-to-edge grain event (e–e) (Eyles et al., 2011). (b) Till from Lichtenegg, Baden-Württemberg, Germany (11-D), in plane light, showing multiple domains (dom), rotation structures (rt), microshears (s), and grain stacks (in dashed black lines) (Menzies and Ellwanger, 2010)

sediments in which the clay content is low (b 20%) (cf. Lupini et al., 1981; Tembe et al., 2010; Haines et al., 2013) clays infiltrate the larger voids thus allowing “network chains” or “bridge networks” to form (Hooke and Iverson, 1995). Shearing movement along these clast networks are likely to set up e–e events. In contrast where clay content is much higher, slip surfaces and foliation (microshears, deformation bands, and shear zones) occur with grains being separated and e–e events becoming a rarity. Fig. 9 illustrates a possible relationship between various microstructures as a function of increasing strain and decreasing effective stress over time. The figure indicates that over time, first with e–e events, then sequentially other microstructures develop such that a set of microstructures typical of subglacial tills are the end-product. It should be pointed out that this is one interpretation others may differ in relation to timing of the timing of when certain microstructure first form (Denis et al., 2010). It is assumed that once a microstructure initiates

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that this type may continue developing within the strain system within the subglacial till and does not cease as another type begins to form. The figure shows that first e–e events are likely to occur almost immediately at low strain (~1) and that quickly thereafter grain stacks should begin to form followed by microshears (Riedel and C′ type) that permit, between the shear discontinuities, rotations to begin to form. At this stage, critical juncture can take place and deformation bands and shear zones will follow at increasingly higher strain levels (3–4). Domains, in contrast, are a post deformational stage where previous units are re-incorporated into the shearing subglacial till unit. It would be a mistake to assume that every type of microstructures will be either developed fully or, if so, survive intact over time. Thus, a “vague” line exists between survival and partial or total destruction or drastic alteration or reorientation of set of microstructures that is likely to take place somewhat randomly in any final subglacial till unit, the randomness being in itself a function of changing porewater and clay particle content, temperature and other rheological vagaries (Menzies, 2012, Fig. 2B). Since subglacial tills pass through multiple stages of deformation, reworking, and re-emplacement, a mélange of microstructures can be expected to exist in a finally emplaced subglacial till. Fig. 10 demonstrates that at various percentages of clay content, porewater, and shear strain rates added to thermal fluctuations, it is possible that, for example, in a clay deficient sediment under shear then e–e events are most likely to develop (A), while as clay content increases grain stacks (B) and subsequently deformation bands (C) and shear zones (D) are likely to evolve. After repeated transport, emplacement, reworking, and likely further shearing and deformation events an emplaced “subglacial till” develops that carries most, if not all of these microstructures, in varying percentages. It is possible that in clay deficit subglacial tills only types A and B might occur and in fast moving dilatant subglacial tills with sufficient clay content only C and D types will evolve. Since most subglacial tills are an amalgam of various sources, deformation events and even different glaciations, it is more than likely that most subglacial tills will be of the A, B, C, and D format. In the final analyses of microstructures in subglacial tills, it should be possible to detect the various levels of strain and other rheological conditions that have permitted the various microstructures types and sets to develop, thus giving a window into the subglacial till forming conditions that occurred prior to the final emplacement of a particular subglacial till. 8. Summary Microstructures are all too clearly identified in subglacial tills and the continuum of forms leads to a discussion of likely structural organization of these forms. Microstructures in subglacial tills form under progressive strain. It is apparent that clay content and porewater play a major role in the form and type of microstructures developed in subglacial tills (cf. Passchier and Trouw, 2005, Fig. 5.1). Likewise, at various rheological stages in subglacial till formation and, at differing strain levels, various differing microstructure forms will develop. It can be seen that as strain increases some microstructures be destroyed, severely altered, and/or re-oriented—the result being that most subglacial tills contain a range of microstructure types that attest to the various phase of strain and deformational history that any subglacial till has likely undergone. Finally, these microstructures indicate a range of strain conditions over time during the formative phases of subglacial till formation. These strain conditions allude to the levels of strain and other subglacial conditions of basal ice velocity, porewater flux, sediment advection and thermal states. Much remains to be understood in a very complex relationship between the various rheological conditions under which subglacial tills are deposited and emplaced. Considerable research is further required, however, in focusing on microstructures in subglacial tills as structural components of a multiple deforming sediment system.

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dom (domains)

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dfb

Time

(deformation bands)

rt

Till Microstructures Sets

(rotational structures)

ms (microshears)

gst (grain stacks)

e-e (edge-to-edge grain crushing)

Increasing strain / Decreasing effective stress Beyond this approximate line microstructures will be potentially destroyed or drastically altered or re-oriented Fig. 9. Progressive development of microstructures in subglacial tills over time under increasing strain.

a Increasing clay content

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Shear stress / Normal stress fluctuations e-e e-e

e-e

e-e (edge to edge grain crushing)

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May contain domains of A, B, C, D or semi or complete homogenization of A, B, C, D

d Shear zone

+

Porewater / Thermal fluctuations

Fig. 10. Sequence of microstructure development in subglacial tills (modified after Tembe et al., 2010) under increasing clay content and variations in strain, porewater content, and thermal fluctuation to produce an end-product subglacial till with characteristic microstructure sets.

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