Synthetic magnetic fabrics in a plasticene medium

Tectonophysics,

164 (1989) 73-78

1Letter Section 1

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Synthetic magnetic fabrics in a plasticene medium G.J. BORRADAILE Department

ofGeology, Lakehead

and M.A. PUUMALA

University,

Thunder Bay, Ont. P7B SE1 (Canada)

(Accepted March 30,1989)

Abstract Borradaile, G.J. and Puumala, M.A., 1989. Synthetic magnetic fabrics in a plasticene medium. Tectonophysics,

164:

73-78. Mixtures of fenimagnetic

grains with a supporting medium of paramagnetic

plasticene permit hypotheses

concerning the interpretation of assemblages of different minerals to be confirmed. The deformation of such plasticene mixtures in pure shear, transpression and simple shear emphasize the dominance of the initial anisotropy over the end-result for low strains (< 25% shortening in pure shear). For minor amounts of multidomainal ferrimagnetic grains the deformation experiments indicate that the magnetic lineation rotates rapidly into the maximum extension direction for the three strain histories tested. Although the magnetic fabric directions are reliable kinematic indicators after modest strain, in simple shear the symmetry of the fabric is not a plane-strain fabric. Great discrepancies occur in the case of assemblages single-domain ferrimagnetic grams in plasticene. In pure shear these produce “inverse” fabrics in which the magnetic lineation spins towards parallelism with the maximum shortening

Introduction Recent work on the anisotropy of magnetic susceptibility (“am,” or “magnetic fabric”) of metamorphic

and deformed

rocks has followed

two interesting directions. Firstly, it has been shown by various methods that there may be several different minerals contributing to the ams of a rock (e.g., Coward and Whalley, 1979; Rochette and Vialon, 1984; Borradaile, 1988). Thus, Henry (1983) considered the effects of different mixtures of minerals on the resultant magnetic fabric. In practice this is very difficult to verify and hitherto the effects have been considered in theoretical models. Practical difficulties arise because real rocks are not homogeneous, nor uniform in mineralogy so that the interfering contributions of variation in mineralogy and variation in intensity of fabric (preferred orientation) cannot be distinguished. 0040-1951/89/$03.50

0 1989 Elsevier Science Publishers B.V.

direction.

Secondly, attention is being paid increasingly to the deformation processes responsible for the development of ams. An early conceptual model, the March model (e.g., see Hrouda,

1987),

has

been used frequently. In this, the rigid-body rotation of non-impinging

magnetic grains in a zero-

viscosity medium is taken as the simplest model of development of preferred orientation in a real tectonite. Owens (1974) emphasized that the actual strain-response model should embody a variety of processes (e.g., Borradaile 1988) to account for the rock fabric which in turn, is (at least partly) expressed in a magnetic fabric. Moreover, strain control may not be the appropriate factor. Stress induced orientation of newly crystallizing phases may be more influential in controlling the magnetic fabric of some metamorphic rocks. For example, traces of metamorphic pyrrhotite (Rochette, 1987a) or matrix-forming metamorphic products such as chlorite (Borradaile et

al., 1986) may appear during syntectonic morphism and dominate the ams. Experimental

deformation,

ture, of synthetic

aggregates

some

information

fabrics

on

(Borradaile these

simulations

at least

an order

than in nature, laboratory changes dent

to produce

strain-rates. in magnetic

(over

at room

tempera-

X ==5.8 Sl/unit

provides

maghemite

magnetic

U.1 microns

and Alford,

stresses,

1987, 1988). Nevrequire

high

of magnitude ductile

flow higher

deformation

Although fabrics

a period

the

at

Induced

are not time-depen-

of 2 years)

the high

flow

stresses probably could have the structure of ferrimagnetic

internal effects on grains which com-

plicate the effects rotation.

attributed

otherwise

Mineral grains used include magnctirc hami uith a grain NIX <)f 1. 0.149 mm. (suxxptibiht~

of minerals

strain-induced

ertheless,

meta-

to grain

mass).

grains

Sljunit

cene and to deform these aggregates in pure shear, simple shear and transpressional shear without stressing

the mineral

grains involved.

The materiab

Our plasticene was a colourless obtained from Mid-Canada Games

(grey) variety Ltd. * Its sus-

ceptibility is 650 X 10e6 SI/unit volume and its density is 1.764 g/cm2. The chief magnetic constituent is montmorillonite with a susceptibility of 2271 X low6 SI/unit mass after it has been degreased with hexane solvents and dried. The bulk of

the

plasticene

is

taken

up

by

portlandite

(Ca(OH),), calcite and oils (taken up by the clay) which have a C: H : N ratio of 28.23 : 3.45 : 0.02. The constituents

other

than montmorillonite

con-

tribute negligibly to the susceptibility. We do not know how variable these parameters are for the various brands of plasticene as the manufacturers would not supply us with information. However, our data are quite consistent for

f‘rom magnetI< solvent

pIat\ long x audio

orthochlorophr-

of the maghemitc

I> (J..Jhl

mabs.

We shall not these materials and feldspar)

report (and

all our experiments

others

using chlorite.

but the reader

these experiments

the above information. tion coil instrument This is a low-field

that

reproduced

Our measurements

are made with a Sapphire

with quartz

will appreciate

can be simply

Mixing experiments

ferent susceptibilities and different ams. This has allowed us to mix such mineral grains with plasti-

derived

nol. The susceptibilitv

medium

grains with dif-

0.5 micron>

thick)

strength

mineral

single-domam.

tapes using the dangerous

We have attempted to illuminate both these subjects by using plasticene as a deformable that can support

and

(measuring

instruments

using of ams

Sl-2 induc-

coupled with an IBM PC. * instrument with a r.m.s. field

of 0.6 Oe operating

at 750 HT.

Henry’s hypothesis (1983) hold that susceptibility contributions from different minerals may be simply added so that slight variations in mineralogy in rocks with the same uniform, fabric intensities produce simple linear plots on a k, (i = 1. 2, 3) versus k (average k-value) graph. Plasticene, being a model for a paramagnetic rock matrix, is mixed with different amounts of the magnetite. Ams measurements were made after kneading each sample in the same fashion. Despite the crude attempts to produce a “similar” fabric each time the results were remarkably successful (Fig. 1). Clearly, the three lines (two of which coincide) for the k, values of the specimens are very well defined,

and

they

do intersect

close

to k, = k.

Similarly successful mix-experiments were performed using chlorite grains and Ottawa sand and chlorite-magnetite mixtures. Those experiments confirm relationships inferred from less well-defined information in real rocks concerning the correlations between degree of anisotropy and bulk susceptibility (see Hrouda, 1987, fig. 3) and be-

the batch that we obtained.

* Sapphire * Mid-Canada

Games

Ltd., Downsview.

Ont. M3J 2P5.

2Go.

Instruments.

PO Box 385,

Ruthven.

Ont.

NOP

The impregnated plasticene three different strain histories:

Ki 6.0

shear), rolling shear). into

2 cm x 2 cm segments (carefully

standard

film is used

and prevent

/Vol.)

material

Plastic

the specimens

of the sample-holder.

After one magnetic

measurement

ment of deformation

is applied

is repeated

yet another

to obtain

into the

holder.

to encase

contamination

cut

these can be

orientation)

sample

(simple

is then

so that

preserving

2 cm-cubical

food-wrap

SI

and shearing

The layer of worked

stacked

( units are 10v2

(transpression)

is deformed by flattening (pure

a further

incre-

and the procedure experiment.

Fig. 1. Experiments with varying amounts of magnetite in a plasticene matrix. The principal susceptibilities (k,, i = 1, 2, 3) lie on straight lines (correlation coefficients given) - that intersect at a value corresponding to a theoretical isotropic fabric. k max and kint values coincide because the fabric is nearly perfectly oblate.

tween the proportion and bulk susceptibility

a)

of paramagnetic material (see Rochette 1987b, fig.

3). Deformation

experiments

Deformation experiments were performed on numerous mixtures of plasticene with mutidomainal magnetite sand grains and with single-domain (SD) maghemite grains. The latter is of interest because Rochette (1988, p. 185) and Potter and Stephenson (1988) predict that unusual “inverse” fabrics may develop using SD particles as the dominant source of ams. The ferrimagnetic grains increase the suceptibility of the samples at least a thousand-fold, so that

Flattening magnetite

deformation

kint

=A

in plasticene

kmin

= m

b) I.0

,

I n

n

’

’

the contribution of the montmorillonite-plasticene matrix to the ams can be ignored for practical

purposes.

However,

experiments

on

pure

plasticene produce very similar results to those shown here, due to the alignments imposed upon the paramagnetic montmorillonite. The main difference is that the fabrics are not quite so well defined. In all the experiments shown below the data points on the lower hemisphere, equal area stereonets have 95% confidence limits around the minimum susceptibility directions of the order of 2 or 3O in diameter. On the P’-T, fabric diagrams the error bars are smaller than the symbols used to plot the points.

-1.01

’

I.06

’

’ I-18

’

’ I .30

’

’ I.42

’

I.54

J I.66

P’ Fig. 2. Pure shear deformation of plasticene containing a trace of magnetite sand (grain size < 0.149 mm). (All stereonets are lower hemisphere, equal area.) Successive data points correspond to the.initial fabric and fabrics produced after 25%, 50% and 75% shortening. k_

axes congregate around the shorten-

ing direction (a) and ellipsoid shapes move to oblate forms as the degree of anisotropy (P') increases (b).

76

Pure shear experiments

represents the symmetrv shapes, are represented

These were the most difficult to perform as the of shortening are least accurate in

measurements this

strain

history.

The

agrams for flattening approximately results

data

points

refer, respectively, magnetite

cate that principal

susceptibilities

that kmin becomes

parallel

tion. The magnetic

fabric

these properties ventional

Flinn

to 0% and

diagrams

are compared

The

(Fig. 2) indispin rapidly

so

The results indicate strain.

However,

direc-

son predicted,

ellipsoid

moves

which

far into the field of flattening.

This is shown on a

P’- T diagram (Fig. 2b) in which P’ represents the degree of anisotropy quite separately from T which

GBSDO

-

rapidly

the

diagram

distinguishes than

of structural

the con-

geology. The

by Borradaile

for SD maghemite

(1988 ).

flakes

stmilarly

of the ams ellipsoid

as Rochette, “inverse”

maximum

toward

diagram

successfully

a rapid rotation

to the flattening swiftly

This

more

on the di-

25%, 50% and 75% shortening.

for multidomainal

by 0 < T-C I

shapes

of the ams ellipsoid. Rod by .- 1 -c T-c 0 and disc

with

Potter and Stephen-

fabrics

are produced

susceptibility

(k,,,)

parallelism

with

the

in

spins

shortening

axis (Fig. 3a). Note that this occurs with less than 2.5% shortening despite the unfavourable orientation of the initial susceptibility ellipsoid. Consequently the axial symmetry requires that the ams ellipsoid is rod-shaped although the deformation ellipsoid is disc-shaped (Fig. 3b).

GBSD3

a)

Rolling (“transpressionul”)

experiments

The plasticene-mineral mixtures were rolled with a domestic rolling-pin, to predetermined thicknesses

corresponding

to successive

shorten-

ings of approximately 25%, 50% and 75%. Ams measurements were then made in the manner outsingle - domoin moghemite flottening

deformotion

kmox

=

kint

.A

kmm

=

l n

b, I.0 I

lined previously. The rolling action was hypothesized to produce k max lineations perpendicular to the direction of rolling. However, this is not the case. The magnetic lineation is found repeatedly to lie parallel to the direction of rolling, i.e. the direction of shearing (Fig. 4a). This is taken to indicate the important, perhaps dominant, transpressional component of the shearing action produced during rolling. Again, the principal susceptibilities spin rapidly,

-,.,I

I.02

I.04

I.06

I.06

P’ Fig. 3. Pure shear deformation of plasticene containing a trace

in this case so that k,,,

lies perpendicular

to the shearing/ rolling plane. The fabrics produced are progressively more flattened as deformation proceeds, moving away from the initially unfavourable, neutral fabric shape for the example shown in Fig. 4b.

of single-domain size ( ~1 micron) fenimagnetic grains. k,, axes rapidly spin to align with the shortening direction giving an inverse fabric (a). Successive data points correspond

Simple shear experiments

to

approximately 0%. 25%, 50% and 75% shortening. The initial shape of the fabric is neutral (b) but moves rapidly into the prolate field.

These are more precise experiments since there is better control on the amount of strain. The

77

R.D.

the shear plane in a sense kinematically

e

a)

compati-

ble with an S-fabric (Fig. 5).

PI

One might have hypothesized that the ultimate fabric in a shear zone would be a plane-strain (neutral ellipsoid) fabric showing neither flattening nor constriction. close to plane

Although the initial fabric is

strain,

only the first increment

moves the fabric towards constriction.

Thereafter,

the fabric becomes progressively more flat-shaped. Thus

the magnetic

fabric

simple interpretation

does not reflect

the

that it should be congruent

with plane-strain. ” Rolling

pin”

deformation

kint

.A

kmin

=

n

shear

a) 4T

direction

GBPLO

-

7

fi

0.

- 0.5

-

t

-1.01

’

’

’

I.08

’ II6

’

’ 1.24

’

’ I.32

’

’

’

I.40

P’ Fig. 4. Transpressional deformation produced by a rolling-pin

simple

on plasticene with traces of magnetite sand. Successive data points correspond to OS, 25%, 50% and 75% thinning of the plasticene sample. k,,

aligns strongly with the rolling direc-

shear

for 8 = 0

to 2.8

increments

of

in

0.4

kint

=A

kmin

=

n

tion (R.D.) see (a). The ams fabric becomes oblate.

plasticene-magnetite mixture is shaped into a 2 cm cube. After an initial measurement of ams the specimen is sheared with a shear strain of 0.4 in a shear box. The material is then carefully cut and the parts reassembled to re-form a 2 cm cube, being careful to preserve the orientation of the parts. A further ams measurement is then taken and the experiment required.

repeated as many times as

The k,, axis spins progressively to a position inclined away from the shear direction and at a high angle to the plane of shear. Thus the magnetic fabric plane k,, - kint becomes inclined to

-

I

I.0 I.10

/ I.20

I

i

I.30

P’ Fig. 5. Simple shear deformation of plasticene with a trace of magnetite sand. Successive data points correspond to the initial fabric and increments of shear strain of 0.4 in a shear-box, to a maximum shear strain of 2.8. The principal directions spin so that k,,

defines a direction that is kinematically compatible

with simple shear (a). However, the ams ellipsoid shape is oblate rather than neutral in shape as one might expect.

7x

Conclusions

References

(1) Hem-y’s hypothesis susceptibility

that different

sources of

give rise to characteristic

k, versus k

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ferrimagnetic

“ inverse”

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that

matic indicators. (3) In a rolling-type,

foland

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of SD

lead to mag-

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axes.

Acknowledgments Graham Borradaile acknowledges the support of NSERC grant no. A6861 and the constructive criticism of Mel Friedman.

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