Foliation and strain development in ice-mica models

Tectonophysics,

93

92 (1983) 93- 122

Elsevier Scientific

FOLIATION

Publishing

Company,

Amsterdam

- Printed

in The Netherlands

AND STRAIN DEVELOPMENT

IN ICE-MICA

School of Earth Science, University of Melbourne, Parkville,

Vie. 3052 (Australia)

MODELS

C.J.L. WILSON

(Received

September

1, 1982)

ABSTRACT

Wilson, C.J.L., (Editors),

1983. Foliation Deformation

The behaviour and

ice-mica

involves

during

shear

in Tectonics.

experimental

is described

pure

developed

and strain development

Processes

deformation

in order

to replicate

and a component

parallels

the principal

controlled

by the nature

direction.

Where

such

direction,

inhomogeneous

This results in varying distributions

with a March that contains produces

deformation strong

fabric pattern a strong

deformation

mica distribution, parallel

orientation

of ice

The deformation

axis. A foliation

is

of airbubbles

that closely

such as strain,

but is also

fabrics

quicker

orientation

or fabrics

However,

distributions.

parallel

These

grain

features

Values of strain obtained

from

finite strain

in a homogeneous

and are therefore

parallel

to the shortening

finite strain

distribution

plane of the local strain ellipsoid. This mica

to the predictions

of the March model. In contrast,

to the shortening distribution,

direction

will develop

and characteristics

of the initial

Shortening

perpendicular

to

the mica fabric.

is the main means

growth

the over

mica preferred-orientation.

ice matrix

direction,

to the flow direction

of an initial layering

asymmetric

only strengthens

relative to the shortening

to the bulk shortening

the observed

results

than an initial uniform

slip in the ductile

in many quartz-mica

of layering

extension

to the principal

a strong c-axis preferred-orientation.

mica phase.

orientation

observed

rocks.

on a parameter

layering

less than

always persist within the deformed

Intracrystalline

mica

bimodal

orientations

and also produces

and S. Cox

rock models composed

to the shortening

local strain and mica distributions. are always

strengthens

initial preferred

the dispersed

of a primary

values of strain that correspond

of initial

preferred

dependent

with non-uniform

model. However,

a uniform

and the mica distribution distribution

oblique

of micas and a preferred

is not purely

there is some obliquity

mica orientation

incompatible

foliated

in quartz-mica

of the initial mica fabric and the orientation

is markedly

short distances.

shear

In: M. Etheridge

plane of the local finite strain ellipsoid.

The degree of mica re-alignment

deformation

models.

92: 93- 122.

of composite deformation

of simple

which is defined by an alignment

in ice-mica

Tectonophysics,

Grain-boundary during

resemble

of accommodating mobility

and post-dating and

the deformation

in the ice is retarded

deformation

are compared

with

by

does not effect

identical

structures

rocks.

INTRODUCTION

In the study of foliation development, where there is an alignment of platy silicates in the form of a slaty cleavage or schistosity, it is well known that there are

0 1983 Elsevier Scientific

Publishing

Company

94

intrinsic

problems

micas relative important

in the determination

questions

Tullis,

years

1976) to relate

according

mica orientation

there have been phyllosilicate

of mechanical

by the rock. In addition

on which there is a paucity

what is the effect of an initial in the formation of foliation. In recent

of the amount

to the total strain experienced

attempts textures

of experimental

rotation

of

there are two

data. These are: ( 1)

and (2) what is the role of the matrix (Oertel.

1970, 1974; Siddans,

to the finite

to the models of: (1) rigid body rotation

strain

of ellipsoid

states

objects

1976;

in rocks.

proposed

by

Jeffery (1922), (2) March’s (1932) analysis of deformable rods and plates and (3) the displacement model for arbitrary shaped grains of Willis (1977). As many rocks possess significant proportions of minerals that undergo plastic deformation more readily than some phyllosilicates (Wilson, 1980), then the rotation of the phyllosilicates may be controlled by the supporting matrix. The present set of experiments have been designed to investigate the rotation of rigid platelets of muscovite mica in a ductile ice matrix and are therefore comparable to the model proposed by March. The hexagonal polycrystalline ice being used in these experiments can be considered as a good analogue for quartz bearing rocks (Wilson, 1979). Similarly mica bearing rocks are commonly foliated and/or consist of mica-rich layers interlayered with non-foliated rock, generally with a high proportion of quartz. Therefore using ice and ice-mica experiments,

interlayers

it is possible

layers of uniform

initial

to model

thickness

many

natural

of ice versus

rocks. In these

ice-mica,

where the

mica is either weakly or strongly oriented, are alternated to form a composite model rock. Where layers of two kinds are interlayered, the geometry of deformation will be determined by the ratio of their viscosities (Bayly, 1970). However, the internal fabric, particularly the mica fabric within these layers, is generally used to interpret strain and foliation reviews by Siddans,

development relative to the externally applied stress field (see 1972; Wood, 1974). With the inclination of layers at different

angles to the bulk strain direction, the finite strain response in the interlayered material, and hence its final fabric, can be different. The evaluation of the effects of varying

initial

fabric

orientation

on the final

fabric

is also one of the problems

investigated in the set of experiments described in this paper. In this study there is no intracrystalline slip of mica on its basal plane,

instead

intracrystalline deformation mechanisms occur solely in the ice matrix (Wilson, 1979). Therefore the orienting rnec~~n~srn for the mica deformation. in a situation where there is minimal volume change, involves the rigid rotation of tabular mica crystals in a plasticly deforming aggregate, caused by torques exerted on them as a consequence of their inequant shape (March, 1932; Tullis, 1976; Willis, 1977). This mechanism predicts that the basal planes of initially random mica will be statistically pe~endicular to the short axis of the superimposed strain ellipsoid. Other experimental studies modelling foliation development with analogue materials (see review by Means, 1977) involve boundary conditions that approximate either (1) axial shortening, (2) pure shear on a bulk scale, or (3) simple shear. In

95

&Z

B

Fig. 1. Summary

of the experimental

face, i.e. perpendicular platen

A, reaction

move parallel extensional

configuration

to the axis of no strain

platen

B and two constraining

to X. Because of the stiffness

strains.

The geometric

features

pure shear and simple shear. However, The sense of displacement while the material layering

and

is required

to an XZ are a load

rods, C. The constraining

contributing

to the observed

to maintain

ductile continuity.

progressive

at the start of an experiment.

b. The geometry

after 40% shortening.

rods are spring

becomes geometry

are difficult

shortening. These

deformation

processes

and

at larger

are a combination

of

to define on a grain scale.

A ~mpensato~

(Means

loaded

more difficult

set of shears is such as to cause extension

continuous

a. The geometry

high shear strain are rotated

as seen parallel

acting on the specimen

in the spring this movement

along the conjugate

and result in a non-coaxial

geometries

constraints

the shear zone boundaries

as a whole undergoes

distortion

simultaneously

and resultant

Y. External

occur

parallel rotation

continuously

to X, of the and

et al., 1980).

It should be noted that with progressive

shortening

the zones of

closer to the XY plane.

most of these experiments there are regions where the strain history was non-coaxial late in the deformation and generally confined to very localized zones (e.g., Means and Williams, 1972; Tullis, 1976). With this in mind, the present set of experiments was designed with the intention of producing conditions involving a plane strain, progressive bulk inhomogeneous shortening (Bell, 1981) throughout most of the deformation history. Figure 1 shows a two-dimensional representation of the experimental configuration and the imposed strains. EXPERIMENTAL

TECHNIQUES

The apparatus and conditions used in this study are similar to those described by Wilson (198 1). The samples were strained at - 1*C with a constant load of either I kN or 1.5 kN (Table I). The load was applied parallel to the long dimension of the prepared block (Z) and samples underwent expansion parallel to X; steel anvils

10

20

25

20

30

29

40

43

19

33

Ml

M2

M3

M4

A45

M6

M7

M8

M9

Ml0

Shortening

(W)

No.

IlZ

IlZ

65”

65O

20”

IIZ

IlZ

IlZ

IIZ

IIZ

F

F

F

F

F

F

F

Fib

C

C

(Fib)

symmetric symmetric

to

2C 2D

2D

2A

2A

2B

2A

2A

2B

2B

asymmetric

uniform

uniform

bimodal

uniform

uniform

bimodal

bimodal

in Figure.

(C)

coarse-ice

axis. Z fibrous-ice

material

shown

corresponds

Initial mica

fine-ice (F)

of

interlayer.

Nature

shortening

with

respect to

layering

orientation

of

experiments

initial ice

Orientation

of - l°C plane strain ice-mica

Expt.

Summary

TABLE I

32x50~81

31.2~50~83

50X50X53

37x50~63

50X50X70

32.8 x 50 x 70.6

28 x 50 x 64.8

35.8 x 50 x 69.1

37.4 x 5064.6

35 x 50 x 74.4

(mm)

dimensions

specimen

Initial Initial

1.0

1.0

1.5

1.5

1.5

1.0

1.0

1.5

1.5

1.0

(kN)

load

Initial

0.62

0.63

0.60

0.80

0.60

0.60

0.71

0.83

0.79

0.56

(MPa)

stress

axial

Final

0.46

0.58

0.37

0.57

0.46

0.47

0.63

0.66

0.72

0.52

(MPa)

stress

axial

9-l

”

5

709

c-l_ H8

Ao

n

500

M7

Fig. 2. Two-dimensional

frequency

histograms

the trace of (001) and the extension three-dimensional stereographic shortening

orientation

projection direction

of one hundred

are shown

of initial angle between

direction

X, in undeformed

the long axis of muscovites,

ice-mica

samples.

(001) axes plotted on the lower hemisphere

underneath

the histograms.

Z is also shown. The measurements

The orientation

that is

The corresponding of an equal-area

of the superimposed

are taken from sections

cut parallel

to the XZ

face of the sample. A. Uniform

two-dimensional

fabric used in experiments

boundaries

relative to X are shown and the graduations

B. Bimodal

fabric used in experiments

C. Asymmetric

fabric used in experiment

M3, M4, M6 and M7, the initial position illustrated

Ml, M2 and M5, initial layer boundaries M8, the position

of layer

here are used for the other histograms. are parallel

of initial layer boundaries

to Z.

relative to X is also

shown. D. Symmetric

fabric used in experiments

M9 and MlO, initial layer boundaries

are parallel

to Z.

constrained all samples in the Y direction. These three axes are the axes of bulk strain. Two aluminium rods (Wilson, fig. 2, 1981) helped to maintain stability in all samples

and

acted

as a local constraint

in the X direction

in the centre

of the

prepared block. The samples consisted of individual layers (approximately 5 mm wide) of different polycrystalline ice types (Table I), bonded to layers composed of a mixture of ice and mica by a thin coating of ice. Powdered muscovite mica with aspect ratios ranging from 4: 1 to 10: 1 and lengths typically 0.3 to 1 mm was added to fine crushed ice (1 mm diameter). The mixture was then put through a sieve system in order to mix the mica with the ice. The ice-mica mixture was bonded by adding precooled (O’C) distilled water, excess water was expelled by lightly compacting the sample prior to freezing. The resulting samples contained volume concentrations of approximately 10% mica weakly oriented perpendicular to the compaction direction (Fig. 2). The 5 mm thick slabs used to make the multilayers were then cut from such material either, (1) parallel to the compaction direction (experiments Ml-M8) where

9X

the resulting

mica orientations

(Fig. 2B) that lie oblique the direction

are weak (Fig. 2A) or contain

to the bulk shortening

of compaction

The majority

(experiments

of specimens

to the long axis of the prepared

layering

which was inclined

axial shortenings principal grooves,

to Z about

of an experiment

bimodal

concentrations

Z. or (2) perpendicular

to

M9 and M 10).

were deformed

parallel

direction parallel

block.

to the layer boundaries,

However

and

three blocks contained

the Y axis (experiments

MGM8).

are shown in Table I and estimates

a

The total of the liocal

finite strains were obtained from the deformed shape of initially circular 5 mm diameter on 10 mm centres, inserted on the two XZ surfaces of the

experimental block using the technique described by Wilson (1982). Determination of finite strain variation is dependent

and Russell Head on strain marker

preservation with the internal diameter of some grooves being better preserved than the outer diameter or margin. This accounts for some size variation observed between

adjacent

variation

NOTATION

Some of the symbols

in Notation

used in the text to describe

strain

1.

1

List of symbols

XYZ

markers.

are explained

used in text

axes of the bulk strain ellipsoid

RS

finite strain ratio determined

R’,

face finite

strain

ratio

determined

from initially from

circular

strain markers

the deformed

on the XZ

two-dimensional

mica

orientation using the method described by Sanderson (1977). Ri has only been calculated for samples with an initial uniform mica distribution f

mean

E

distance between the two XY faces of the deformed block (Means; i976, p. 133) local finite shortening calculated from the change of the minimum quadratic

e + n

bulk

elongation XZ face orientation orientation number of

shortening,

(Ramsay;

calculated

from

1967, p. 65) measured

the changes

in the straight

from the deformed

line

ellipse on the

of major axis of the finite strain ellipse of (001) mica trace with respect to the extension axis X measurements for mica or ice -axis orientation analysis

Because of the fine-grained nature of the mica and its much greater strength compared with the ice matrix, it was necessary to prepare thicker than normal thin sections; this prevented plucking of micas from a section during the thin-section preparation. In the initial stages of this work, universal stage techniques for

99

measuring decided

mica

crystallographic

to measure

orientation

mica orientations

proved

difficult.

from photomicrographs,

Therefore, recorded

it was from thin

sections cut parallel to the XZ plane. These photomicrographs covered a complete specimen and all identifiable mica grains within a layer or portion of a layer that corresponded

to known

with the XY plane

values of R, were measured.

were therefore

determined

The orientation

and two-dimensional

of (001) traces representations

of the mica fabrics were prepared (e.g., Figs. 2-8). Selected mica rich areas were also prepared for a scanning electron microscope (SEM) investigation. Preparation of SEM samples entailed, (1) ablation of ice from the surface of an ice-mica block, exposing the micas embedded in the ice, (2) covering this surface with a perspex paste and allowing it to harden, and (3) the melting and removal of the remainder of the ice-mica block. This procedure preserves the fine oriented micas in a solid perspex base upon which a layer of carbon prior to the SEM observations.

followed by a plating

of gold is deposited

STRAIN DISTRIBUTION

In these experiments the bulk strain developed in any given sample is physically controlled by the load and reaction platens together with the constraining rods (Fig. 1). One of the most striking features of this experimental the bulk dimensions of a sample will approximate deformation

configuration is that by pure shear with

the principal axis of the strain ellipses, at small strains, oriented subparallel to the XY plane of the sample. However, the samples are in fact undergoing two modes of deformation (1) bulk shortening and (2) a superimposed component of shearing involving two simple shears of equal shear strain and oriented in a conjugate fashion (Ramberg, progressive

1975). Therefore the typical mode of deformation is one of plane strain, bulk inhomogeneous shortening, where the centres of bulk boundaries

are not displaced relative to the average direction of shortening and is coaxial on the bulk scale. This has been referred to as bulk inhomogeneous shortening in the model proposed

by (Bell, 1981) or an inhomogeneous

As these experiments any

discussion

of finite

were undertaken strain

pure shear (Ramsay,

in a plane

can be simplified

strain into

1963).

deformation

apparatus,

a two-dimensional

strain

analysis. The relationship between the observed shortenings (i) and that predicted from dimensional changes of the specimen (c) was found to vary considerably between given areas (Figs. 3-6 and 8). Cumulative strain recorded on trajectories drawn parallel to Z generally approximate the total shortening strains with a mean error of 10%. The orientation of the greatest principal axis of an individual strain ellipse is generally subparallel to X but may vary up to 20’. The variation in strain magnitude in an axial direction can be attributed to the positioning of constraining rods adjacent to the centre of the XZ faces (Fig. 1). Comparing areas on the XZ faces before and after deformation showed that insignificant area changes occurred, the highest loss recorded was 48, and can be

n=1131

M

-

LTot,, I----

M2

(Ez20rn)

Fig. 3. XZ face of deformed blocks shortening

direction

interlayers

are clear.

shortenings

(z) in the 2 direction.

The negative

trace makes with the extension measured

in ice-mica

and M2 showing

Ml

2 is subhorizontal.

The ice-mica numerical

the distribution

rich layers are stippled

values

The histograms

direction

c

B

A

beside

the strain

are frequency-%

E

D

of the strain

ellipses,

and the coarse-grained

ellipses

represent

shown are the totals for a11 mica o~entations

measured

ice

the percent

versus +. the angle the (001) mica

X, M = mean. The data used to compite the histograms

layers within zones A, B, C etc., these are oriented

the

subparahel

in any one sample.

has been

to the XY plane. Also

101

z-

e-2

IY

!

Fig. 4. Strain and mica orientation variation in h43. The ice-mica layers are stippled and the coarse grained ice interlayers are clear. The negative numerical values beside the strain ellipses represent the percent shortenings (2) in the Z direction. The histograms are frequency-% versus rp. the angle the (WI) mica trace makes with the shortening direction X for individual segments of each ice-mica layer, The numerical I, II and III defines the ice-mica layer, whereas zones A, B and C define the length in which mica orientations were recorded. A total diagram for all mica orientations in sample M3 is also shown. iw = mean.

Fig. 5. Strain deformed

and mica orientation

X2 faces are the magnitude

variation

in M4 and M5. The negative

of the principal

shortening

values superimposed

2. Same symbols

as in Fig. 3.

on the

Fig. 6. Strain and mica orientation variation in M6 and M7. The two faces A and B of M6, and one XZ face of M7 are illustrated. Same symbols as in Fig. 3. The shortening direction is horizontal and the orientation of the initial layering (L-L) with respect to 2 being 20” in M6 and 65’ in M7 respectively. Data for the histograms was collected within subparallel zones A, B and C in M6 and in the individual areas A-H indicated in M7.

90

Fig. 7. Mica distribution plotted

against

in M8 displayed

frequency-%

60

30

0

30

60

90

in a thin section and as histograms.

for layers A-F.

The shortening

direction

The angular

Z is horizontal.

attributed to a decrease in volume by the loss of air voids and/or during deformation. Moreover, no significant differences in surface could

be detected

adjacent

when comparing

XZ faces, parallel

the values of

distribution

some ablation strain patterns

R, between equivalent

to the Y axis. In experiments

with layering

+ is

M = mean.

points

parallel

on

to Z,

the mean error of the shortening strains were approximately 10% where e was greater than 20%. However, for experiments involving less bulk shortening, differences of up to 20% are observed, which may reflect the difficulty of measuring axial ratios in poorly preserved strain markers. Where the initial sample contained layers inclined to the shortening direction and involved large strains, the R, variation between the two XZ faces showed more variation. For example, compare r values observed in the two XZ faces of experiment M6 (Fig. 6). Such variations in C can be attributed to different responses of adjacent ice interlayers and ice-mica to deformation and hence can be attributed to differences in their material properties and response to deformation.

n=ss3

-&I!!I!& c

Fig. 8. Strain and mica orient&an

variatian in M9 and IWO. The angular distribution S$is pIott& against

frequency-%, the shortening direction 2 is horizontal. distribution

observed immediately

after deformation

M = mean. In M9 the histograms portray the mica and in portion of the sample arm&d

for 20 days.

In experiments interlayer, the coarse interlayer

where coarse polycrystalline

the strain

recorded

ice. An identical

situation

(e.g. M3). The different

( 1) a greater thickening ice layers. Evidence

ice (M 1 and M2 in Fig. 3) acted as an

in the ice-mica

also exists where

response

of the ice-mica

layers is greater

width, than the central

and more deformed

portion

can be:

to the coarse or fibrous

to the load platens

layer width is thinner,

in

ice is used as an

of the two layers to deformation

layers in comparison

for this can be seen adjacent

(Figs. 3 and 4) where the ice-mica

than that observed

fibrous

in M2 and M3

and reflects the initial layer

of the specimen.

(2) where strain

markers cross the boundaries they seldom possess a perfectly elliptical shape. instead they are sometimes slightly pear-shaped. being more elongate in the ice-mica portion. The latter feature is responsible for quite significant variations in the elongations and shortenings observed in the strain markers, and can be attributed to more deformation occurring in the ice-mica interlayers. The converse situation is evident where the interlayer is composed of fine-grained polycrystalline ice (M4, MS, M9 and MIO). The strain recorded in the fine-ice layers always appears to be greater than that recorderd in the ice-mica layer. Where the interlayers are inclined to the shortening direction (M6-M8) it has not been possible

to assess the differences

in the strength

ice layer. In these cases the strain recorded parallel and perpendicular to the shortening Fig. 6) were recorded

in the centre

rotation of a layer. Deformations involving

shortening

of the ice-mica

versus the fine

in the markers is highly variable both direction. The highest strains (e.g. M7.

of the block where there has been the greatest parallel

to the layering

results in layers being

deflected outwards, in the plane of bulk flow XY, with local barrelling above and below the constraining rod (Figs. 3, 4, 5 and 8) and comparable strain distributions across samples. Layering inclined to Z is rotated, particularly in the more deformed samples (M7 and M8) into orientations subparallel to the XY plane of the bulk strain ellipsoid. In M6 the sense of layer rotation is asymmetrically distributed across the sample, with one end of a layer showing a positive sense of shear changing to a negative in the centre. In M7 and M8 the layer rotations are similar with the highest values of shear strain concentrated MICA PREFERRED

in the central

regions.

ORIENTATIONS

Several problems arise in the handling of data from these experiments, in which the basic observations are directions. The initial two-dimensional orientation of micas in the XZ plane for experiments Ml, M2 and M5 were bimodal, whereas others (Fig. 2A) could be considered uniform in two-dimensions. Observation of mica orientations in the XY and YZ plane show moderate (001) mica-orientation. this is a consequence of the sample preparation techniques. Therefore a three-dimensional plot of initial mica-orientations on the stereographic projection (Fig. 2) shows a concentration of poles to (001) in the peripheral region of the net close to the XY

Fig. 9. SEM micrographs taken adjacent

showing

mica distributions

to an X2 face in experiments

and their position

in photographs

M7 and M9. The bar scale is 0.7 mm.

of thick sections

Fig. 10. Micrographs

illustrating

interlayers

M9 and MlO. The bar scale in all photographs

in samples

A and B. Portions the oriented stronger

comparative

of the deformed

micas

in the ice-mica

mica preferred-orientation

samples

microstructures

between

M9 and Ml0 respectively,

layer and the air-bubble and greater

air-bubble

adjacent

ice-mica

layers

and ice

is 1 mm. in plane polarized

distribution

light, showing

in the ice layer.

There

in the more highly strained

elongation

is a

sample

MlO. C. Undeformed interlayers

portion

of M9 observed

have been joined

under

crossed

nicols.

The ice-mica

using a film of frozen water (a) which also contains

layers

versus

a concentration

the ice of fine

air-bubbles. D. Deformed position However,

area of M9 observed

of boundaries

under

crossed

nicols illustrating

(a) deformed by concentrations

these air-bubbles

are now included

of air-bubbles

within the deformed

the ice grain

shapes.

are still present

and recrystallized

The original

after deformation. ice.

109

plane, with few grains lying parallel to the Y axis. Another factor that complicates any observation of mica-orientation is that some micas, that lie subparallel to the XY plane, may be removed during the thin section preparation of the samples. Such micas have been observed in SEM observations of both the starting material and the deformed material (Fig. 9). SEM (Fig. 9) and optical observations (Fig. 10) of deformed areas suggest that very few individual mica platelets have suffered internal deformation in the form of bending. Where bending is observed it is usually only in the very thin, long and narrow mica platelets, approximately 5 pm wide. The majority of micas after deformation are planar bodies set in the ice matrix and there appears to be little interference between micas. These observations suggest that the micas are appreciably stronger than the ice matrix and are undergoing rigid rotation as envisaged in the models of March (1932) and Jeffery (1922). Unlike the experiments of Tullis (1976) there is apparantly no effect of mutual grain interference during grain rotation, and nor is there a subsequent production of the mica preferred orientation due to mica growth. Two-dimensional mica orientation

In samples shortened parallel to the layering (experiments Ml-M5) the mica preferred orientation has been measured in parallel bands of uniform strain marked A, B, etc. that subparallel the XY plane. In the deformed samples of Ml and M2 the mica orientations, with respect to X (Fig. 3), still reflect the bimodal fabric observed in the initial starting material (Fig. 2B) with a decrease (approximately 4%) in the number of micas lying in a 30” arc from the 2 direction. This suggests that micas oriented subparallel to Z are rotated to a greater extent than other orientations with the enhancement of the bimodality of the initial fabric. In specimen M3 (E = 25%) a significant departure from the initial uniform mica distribution occurs with 80% and 65% of all micas making an angle of less than 45” and 30”, respectively, on either side of the extension direction X (Fig. 4). The strongest mica concentration occurs in a central zone that trangresses the specimen subparallel to the XY plane (areas IB, II& IIIB in Fig. 4). The weakest mica preferred orientation (area IIA, Fig. 4) reflects the initial orientation and is confined to an area immediately adjacent to the reaction anvil; a region that is probably unaffected by any component of shear. Sample M4 and MS (Fig. 5) both have interlayers composed of fine ice, but different initial ice-mica fabrics; M4 was uniform whereas M5 was bimodal. After deformation elements of the bimodal fabric (M5; c = 30%) still persist whereas there is a change in the random fabric (M4; e = 20%) with the development of a strong preferred orientation; 65-75% of all micas lie in a 45” arc subtended by the extension direction X. The relatively small change in mica orientation in M5 contrasts markedly with M4, and this lack of mica fabric change can, in part, be

attributed

to the distribution

of strain

in the sample.

Although

e in M5 (30%) is

higher than M4 (20%) the local magnitude of F in the mica layers is less than e and less than 7 observed in the ice layers. Hence the shortening has occurred primarily in the ice interlayer rather than in the ice-mica, tion of the mica fabric. In the three samples

thereby preventing

M6. M7 and M8, where the initial

extensive

layering

modifica-

was inclined

to

the shortening axis (Figs. 6 and 7) there is a marked change in mica orientation, with the orientations strongly skewed toward the extension direction X. They become more symmetric and unimodal with increasing strain reflecting the development of a strong foliation. This type of distribution is obvious over small areas or within the total mica fabric. Although the local and bulk shortening strains were comparable, the mica fabric in M8 was considerably stronger than that observed in M7. This may reflect the inherited but asymmetric, the alignment

whereas

fabric, with the initial

M7 was uniform

M8 fabric being unimodal

in two-dimensions.

of micas in M8, and their rotation,

appears

greater rate than the uniformly oriented population. The deformation of aggregates with an initial symmetric about

the XY plane

(M9 and MlO) results

in a strong

Upon

deformation,

to have occurred distribution

enhancement

to a

of micas of preferred

orientation with progressive strain (Fig. 8). At 33% shortening, the micas are strongly oriented subparallel to the XY plane over all areas of the sample. These concentrations are skewed from X by approximately 10” and reflect the asymmetry in strain distribution. This asymmetry in preferred orientation can also be correlated with the air-bubble elongation observed in the fine-grained interlayered ice (Fig. 10). With deformation involving bulk shortening up to 20%, the air-bubbles are poorly elongate (Fig. 10A) and it is difficult to correlate their orientation with strain. At higher strain

(Fig. 10B) both isolated

and coalesced

bubbles,

are strongly elongate and subparallel the mica concentration. Static annealing of M9 at - 1°C using the techniques (1982a), inhibited

suggests

by the dispersed

with coarser There

that the growth

grains

mica phase. This leads to substantial change

described

rate of the ice in the ice-mica

in the ice interlayers

is no apparent

often dumbell-shaped

of mica

but little increase orientation

by Wilson

layers is strongly

grain-size

differences

in the ice-mica

or distribution

during

layers. such

annealing (Fig. 8). The reversal in the asymmetry observed in some histograms (Fig. 8) is probably a result of measurement being recorded from an XZ section that is the mirror image of the deformed Three-dimensional

sample.

mica orientations

Representatives of the 3-D mica orientations are seen in samples M5 and M7. In the less deformed portion of MS (area c, Fig. 1lC) the micas still retain elements of the initial fabric (Fig. 2B) with the development of a new concentration oriented 10’ to the XY plane (Fig. 11 B). In the more deformed central portion of the sample (area

ill P&B (001)

Pole Orientations

Ice c-axis

Orientations

Fig. lower

11. Three

dirn~~ion~

hemisphere

portrayed.

equal-area

prefer~d-o~entations projection.

The

observed

in experiment

number of measurementsis

M5. Data shown

is plotted

adjacent

on the

to the area

2 is subhorizontal.

A. Deformed

XZ section showing

location

B and C. Mica (001) pole orientations on a universal-stage

of data sites, areas a to f.

measured

the section was compared

that there were no micas lying parallel

in thick sections. with SEM photos

Before measurements taken from adjacent

were undertaken areas to be certain

to the X.2 plane.

D and E. Ice c-axis orientations.

b, Fig. 11) the initial orientation is not as obvious with a marked concentration subparallel to the XY plane, that is, the poles to (001) are subp~allel to 2. Similar observations can be made in M7 where the more diffuse concentrations (Fig. 12A and C) do not correspond to the highest ellipticity, whereas the well defined concentration of poles to (001) correspond to the areas of highest strain (Fig. 12B, D, and E). The orientation of some mica concentrations in M7 (Fig. 12B and E) are markedly asymmetric with respect to X, the bulk shortening axis, but are locally parallel to the principal axes of the finite strain ellipses. ICE MICROFABRICS

The poly~~stalline ice interlayers were cut from blocks containing equant ice grains with randomly-o~ented c-axes (Wilson and Russell-Head, 1982). The fine-

II?

Mica

(001)

Pole

Orientations

/(r----

.:. 8.. L . /2:

l

4r. . zt i i:

+Y

0.

A

X

Ice c-axis

Fig.

Orientations

12. Three-dimensional

lower hemisphere

equal-area

sketch of the deformed A-E.

projection.

observed

in experiment

Z is subhorizontal.

The location

M7. Data of data

is plotted

sites is shown

on the in the

XZ section.

Mica (001) pole orientations

projection F-H.

preferred-orientations

measured

is 100 and taken from areas A-E,

Ice c-axis orientations

from areas A-G.

in thick sections.

The number

of measurements

in each

respectively. The number

of c-axes is shown adjacent

to the projection.

113

Fig. 13. A-C. Microstructure observed in X2 sections from the experimentaIIy deformed samples M2, M5 and M7, respectively. The samples are observed under crossed nicols, 2 is horizontal, and the bar scale in all photographs is 5 mm.

grained polycrystalline ice in the ice-mica layers also contained ice with random c-axes prior to deformation; however, grain shapes were not always equant, sometimes being elongate and rectangular with planar boundaries parallel to the dispersed mica phase (Fig. IOC). Ice grain-sizes in the ice-mica mixtures were always smaller than the average ice grain-size in the ice interlayer. After deformation the grains in the interlayers have undergone grain growth with the grain-size increasing with progressive strain (Fig. 13). In all cases grain boundaries are irregular and some grains possess deformation bands. In samples shortened parallel to layering, involving little layer rotation, interlocking ice grains (Figs. 13A and B) with little undulose extinction or deformation banding are present.

With large layer rotations (MGM8) there is the development of elongate that parallel the layer boundaries (Fig. 13C). The ice grains in the ice-mica are always

small (Figs.

IO and

initial size. They seldom by micas. The c-axis patterns deformed ice interlayers

13), and without

overgrow

substantial

Examples

grains

from their bounded

of preferred-oreintations measured in X.2 sections in all were double maxima lying in the 25”-45” small circle girdle

about 2 (unpublished data). These preferred-orientation those recorded in blocks of polycrystalline ice, deformed Russell-Head

increases

the mica. but occur as irregular

grains layers

(1982) where the deformation of the c-axis preferred

involved

orientation

patterns are identical to at - I’C, by Wilson and

a homogeneous

observed

pure shear.

in the ice-mica

interlayers

are typified by M5 and M7. In M5 (Fig. 11D and E) there is a strong suggestion

of a

double maxima preferred”orientation lying between 2 and the 60” small circle. Many grains still have c-axis oriented at a high angle to the shortening axis in areas a-d (Fig. 11D). The pattern is stronger in areas e and f (Fig. 11E): however, the concentrations are asymmetric. In the more deformed sample M7 the patterns of preferred-orientation are better defined (Fig, 12) with the main population of c-axes lying between the 25” and 45’ small-circles. The microstructures observed in all ice areas have features that suggest development by dynamic recovery and recrystallization. Similarly the c-axis patterns of preferred-orientation, developed from an initial random pattern, indicate that the ice c-axes were rotated through intracrystalline slip (Wilson, 1981). The strength of the pattern of preferred-orientation is a reflection of the strain (Wilson and Russell-Head, 1982). The role of the passive micas in modifying

this preferred

orientation

does not

appear to be significant, but may account for part of the orientation spread observed in areas n-d in M5 (Fig. 11) and areas C and D in M7 (Fig. 12). The effect of the conjugate shears superimposed on the pure shear deformation interlayered ice. However, the slight asymmetry of some

is not obvious in the c-axes fabrics in the

ice-mica areas (e.g., areas e and f in M5, Fig. 11; areas A and B in M7, Fig. 12) may be a function of the superimposed shears (cf. Wilson, 1982b). Overall, the patterns recorded here are comparable to those obtained under a pure shear deformation. Any asymmetry could also reflect the presence of the mica platelets. STEIN-HISTORY

CURVES

Typical examples of strain-rate curves monitoring rates, along a line parallel to Z, are shown in Fig. decrease in the strain rate with time ranging between

the longitudinal shortening 14. In all cases there was a 10m7 to lo-’ sec. ‘, and was

more marked in samples loaded parallel to layering. In samples where layering is inclined to 2 (e.g. M6 and M7) there was a marked deceleration of strain rate with time. The layer parallel shortening therefore reflect constant material properties in the ice matrix and the mica fabric. When layering was not parallel to Z, it rotates

115

TIME (days) 1 I

10"'

7 I

3 ,

28 I

14 I

56 '1

us -a.. ..... .. .__.__.__

e

*u t

* ...........................

F L I

,o-’

lz f

-

M4

. ......*.

Mfj

*.-.-A

M6

e---4

M7

-

MS

,.__....* M,O

j

Fig.

14. Log-log

Samples

plot of strain

M6 and M7 carried

rate vs time for representative

ice-mica

an initial load of I .5 kN, ali other samples

samptes carried

deformed

a load of

at - IT

1 kN.

towards the XY plane, and ultimately reaches maximum extensional strain values near X. The change in slope of the strain rate may reflect this change in layer orientation and hence an increase in angular shear. Therefore none of these experiments can be considered to represent a steady state flow. This may also be a consequence of friction between the XZ face of the sample and the constraining anvil or a result of a stress reduction during the deformation history; as a consequence of having a constant load applied to an XY face that is increasing in area during the deformation. DISCUSSION

AND SUMMARY

Mica preferred-orientation versus strain

There have been many attempts to relate three-dimensional mica orientations to the kinematic models of Jeffery (1922) and March (1932). As pointed out by Willis (1977) such comparisons should take into account the shape of the grains and strain history; and in all such models the rigid objects are assumed to have an initial

1 I6

random

orientation.

sistent

aspect

Although

ratio

and

most mica grains

similar

sizes they did

in these experiments not

have

a truly

have a conrandom

initial

preferred-orientation. After many attempts to produce an inital random three-dimensional preferred-orientation it was found that this was impossible using the techniques samples

described with

here. Instead

the closest approximation

a pre-deformational

micas. These micas were sufficiently are not important

and therefore

model. Applying the March symmetric mica distributions

two-dimensional

dispersed

such that material

meet one of the assumptions

model to pre-deformational is of restricted use because

arise from either the irrotational

to this is found in the

uniform

or rotational

distribution

of the

interference

effects

of the March (1932)

bimodal, asymmetric and of the ambiguity that may

strain components

(Owens,

1973).

If the March model applies, then modification of the uniform mica distribution should be related to R,. In experiments M3 and M4 (Table II) it can be seen that

TABLE Strain

II comparison

strain estimates

in samples

that contained

R‘s are based on the “random

a uniform

2D mica distribution

line” method

of Sanderson

strain markers, adjacent to areas where mica orientations were measured. Experiment

Area in sample

M3

M4

M6

M7

R’,

RS

IA

1.9

1.5-d.8

IB

2.1

1.8-2.2

IC

2. I

1.6-1.9

i 1A

1.2

1.3-1.6

11B

1.3

1.4-2.0

IIC

1.4

1.6

1llA

1.2

1.4-1.6

1ilB

1.6

1.6-1.8

1IlC

1.45

1.5-1.8

A

I.1

1.4-1.7

B

1.6

1.4-1.7

C

1.4

t.4-1.54

D

I.3

1.38-1.45

A

1.3

1.2-2.5

B

1.5

1.2-2.3

C

1.5

1.2-2.2

A

1.5

2.8

B

1.1

1.1-4.0

C

2.0

3.8

D

1.35

1.7-2.1

E

1.15

2.9

F G

1.3 2.0

2.6-3.2 _

H

1.1

1.1-2.3

prior to deformation. The

(1977).Values of R s are from

117

this appears to be the case for samples deformed parallel to layering, although there

is a small underestimate of R’, in some areas. In M6, where layering is inclined to 2, R’, is always on the low side of the observed range of R, values (Table II). In M7 it can be seen that the degree of mica orientation suggests that there is a systematic underestimate of large strains by the March model. Therefore, where R, is locally inhomogeneous, both within and between different layers, there appears to be an underestimate of R’, and clearly suggests that the March model is only applicable where strain is homogeneous throughout marker grains and matrix. In all the experiments where the initial layering was inclined to Z and there has been appreciable angular shear of the primary layering, into an orientation subparallel to XY, there is a considerable local variation in the degree of mica re-orientation which can be related to local strain variation. In two-dimensions, for example, areas such as A, E, F and parts of B in experiment M7 (Fig. 6), these is local strengthening of the mica preferred-orientation at rates greater than other areas (see also Fig. 7). This local variation of mica strength is also seen in the three-dimensional mica orientations (Fig. 12) as well as in any optical or SEM micrograph (Figs. 7 and 9). Therefore these experiments suggest that in situations where layering is oblique to the shortening direction and where there is a strong initial fabric there is often no obvious quantitative relationship between strength of preferred-orientation versus observed strain. Where a sample is shortened parallel to the layering and the mica population is bimodal (M 1, M2, M5), hence symmet~cally oriented about the XY plane, the initial pattern of mica preferred-orientation is preserved even up to 30% shortening with only a small degree of fabric strengthening. If the initial mica orientation is asymmetric (M8) the final mica orientation will also have a component of asymmetry. With a two-dimensional uniform pattern (M6, M7) there is a notable increase of micas oriented parallel to the XY plane. The strengthening of the mica orientation in the XY plane is again quite marked where an initial mica orientation existed in this plane (M9, MIO). From field evidence, many workers believe (see reviews by Siddans, 1972; Wood, 1974) that the prefect-o~entation of phyllosi~~tes in slates may be pe~endicular to the direction of greatest shortening, rather than parallel to the direction of maximum shear strain; on the other hand Williams (1977) points out that there could also be a component of shear strain subparallel to the foliation that contributes to the development of cleavage. Such a situation in nature has been convincingly illustrated by Gratier and Vialon (1980) where contemporaneous shortening and shearing coexist, resulting in a bulk inhomogeneous shortening. This has also been modelled in the present set of experiments, with the zones of shear being progressively rotated towards the XY plane (Fig. 1B). Only with very high shortening strains would the shear zones parallel XY. This was not achieved in these experiments. Where there is little dominance of an inherited mica-fabric (e.g., M3, M6, M7 and M8) the mica preferr~-o~entation is always subparallel to the XY plane of bulk

strain, shear

although strains

subparallel

locally has been

there are small deviations. to locally

enhance

The contribution

the preferred-orientation

to the bulk flow plane XY. There is no evidence

along a plane

that contains

mica distribution

the strongly

or orientation

oriented

of the higher distribution

of shear displacement

micas, nor is it possible

to define areas involving

from the

both shear and shortening

versus only shortening. The role of the ice matrix

The ice served as a weak and ductile medium allowing the rotation of the much stronger, rigid, mica platelets. Unlike other analogue experiments, designed to produce a foliation defined by platy minerals (see review by Means, 1977) the ice matrix serves as a good indicator of what may occur in a hexagonal polycrystalline aggregate such as quartz. Although the ice has been deformed close to it’s melting temperature and the strength of ice is much weaker than quartz, the ice deformed by intracrystalline slip on the basal plane and was accompanied by recrystallization processes. greenschist Cutforth.

The ice therefore and amphibolite

closely

models

the behaviour

facies environments

of quartz

(e.g. Wilson,

deformed

1973; Starkey

in and

1978).

No obvious retarding effects on the passive rotation of the mica could be attributed to the ice. Instead the micas appear to have played the reverse role by influencing the degree of ice c-axis preferred-orientation and inhibited grain growth of the ice. The ice c-axis preferred-orientation was weaker than that recorded in the ice interlayers. However, the same general pattern of preferred orientation existed which is a double maximum lying in a 25”-40” small-circle girdle about 2. This is therefore comparable to quartz-mica rocks (Wilson, 1973: Starkey and Cutforth, 1978) where observed changes, in the degree of quartz c-axis orientation and not the pattern can be attributed to an increasing phyllosillicate content. The skeletal outline defined by the c-axis preferred-orientation patterns in this study have been described from polycrystatline ice deformed under coaxial conditions and at - 1“C (Wilson, 1981; Wilson and Russell-Head, 1982). However, all the c-axis distributions observed in the ice-mica layers (Figs. 11 and 12) are asy~etric with c-axes of one orientation dominating different areas of the sample; rather than having a uniform distribution through the sample. The superposition of the shear zones on a pure shear deformation is probably responsible for this asymmetry and is identical to the observations of Wilson (1982b). The apparent absence of this asymmetry in the ice interlayers can be accounted for by the large grain-size (Fig. 13). Few c-axes were recorded from any given area of an ice interlayer and the total c-axis fabric is an average for the whole sample. Therefore the c-axes of the fine ice, within the ice-mica layers, does preserve evidence of the non-coaxial nature of the strain and the effect of bulk inhomogeneous shortening. This is not recognised in areas where the ice has undergone appreciable grain growth.

119

Annealing migration tation

of the deformed

ice in experiment

of the ice grain-boundaries,

of the micas. Therefore

quartz in a quartz-mica tions or orientations.

under

M9 (Fig. 8) shows that with the

static conditions,

static grain growth

in a matrix

rock, may not substantially

there is little reorienphase,

alter pre-existing

for example, mica distribu-

The role of anisotropy Layer thickening of the ice-mica layers versus the ice interlayer is controlled by the material properties of the interlayer, that is whether it is a coarse or fine ice type, and hence may reflect the relative competence contrasts between layers. The finer ice interlayer, which also has a slightly higher air-bubble content, deforms more readily than the coarser

ice interlayers

and appears

to be weaker than the ice-mica

layers.

Where layers are parallel to Z, there was greater layer thickening than where layers are inclined to Z. As the angle between Z and the interlayer increases, a semi-symmetric thinning occurs at the intersection of the conjugate shear zones (Figs. 7 and 8) and has been termed internal boudinage (Cobbold et al., 1971). On the other hand, all samples including those displaying thinning exhibit a component of thickening adjacent

to the load and reaction

thicknesses are not dependent on the strain configurations. described parallel

by Wilson

platens.

Therefore,

the relative

changes

in layer

on competence contrasts alone, instead they depend Unlike some of the experiments in multilayered ice

(198 l), there was no evidence

for layer boundary

slip or shear

to the anisotropy.

Strain varies across layers because of the different competencies exhibited by the ice versus ice-mica layering. However, layer orientation has a marked influence on the strain distribution. Shortening parallel to the layers only produces gentle buckling with a small degree of strain variation (Ml -M5, M9-M 10). In this case the layering is always subperpendicular strain distributions are comparable

to the main flow lines of the deformation. Such to the strain observed in any unlayered block of

ice subjected to comparable shortenings 198 1, 1982b; Wilson and Russell-Head,

during 1982).

a pure shear deformation

(Wilson,

Layering inclined to Z (Fig. 6) appears to become unstable, it is progressively rotated towards the orientation of the maximum principal plane of the bulk strain ellipsoid and the resulting finite strain distribution is heterogeneous. In this case the obliquity of the layering to the extension direction and a component of shear controls the buckling. This obliquity is present at the onset of the deformation but diminishes as the layering rotates towards the flow plane. Therefore the orientation of the layering with respect to the bulk shortening direction is important as it will control both the finite strain distribution and the degree of mica reorientation.

Many recent studies of foliation there is commonly

some component

process contributes

to the observed

phic

rocks

from

low-grade

slates

reported here the deformation ing matrix. However, unlike

development

have supported

of rigid rotation phyllosilicate through

the observation

of platy particles

grain alignment

to high-grade

This

in many metamor-

schists.

is accommodated by the ductile many slates there is a notable

involved,

that

In the models

flow of the supportabsense of solution

effects, that is, dissolution and precipitation. This explains why there was no obvious sign of differentiation or redistribution of the ice versus mica. The realignment of these micas occurs where the bulk deformation a pure shear. As a first approximation the results are therefore geological shortening

approximates applicable to

situations such as slate belts, where there has been extensive crustal over large areas (Wood, 1974). These have been interpreted in terms of

progressive rotation of the platy phyllosilicate authors (e.g., Williams, 1977) suggest from component of shear subparallel achieved in these experiments;

minerals towards the XZ plane. Some field data that there may also be a

to the plane of flattening. Such a situation has been and using the mica alignment alone this cannot be

recognised, particularly where the initial layering was parallel to the shortening direction. Where the initial layering is inclined to the shortening direction then the effect of a non-constant extension parallel to the flow direction becomes marked, with variable degrees of mica realignment and strain distribution. If the mica fabric was correlated with strain, then over any given volume it is not possible to recognise the co-existence of both pure shear and simple shear, or in other words a bulk inhomogeneous shortening. These experiments also place important constraints on applying kinematic models to the synchronous development of foliations. These include (1) the effect of an initial

preferred

orientation,

which can govern

the shape of the final mica distribu-

tion and (2) the role of anisotropy. That is whether the rock is isotropic or a layered sequence; the development of some layer obliquity to the bulk shortening direction during a deformation can result in variable strain and mica orientation distribution. ACKNOWLEDGEMENTS

This work was financially supported under the Australian Research Grants Scheme. The Glaciology section of the Australian Antarctic Division provided the coldroom used for examination and preparation of the ice samples. Mr. R. Thwaites is thanked. for his technical help. S.F. Cox, W.D. Means, G.P. Price and M.A. Sandiford

are thanked

for their comments

on the manuscript.

121

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