An assessment of dynamically recrystallized grainsize as a palaeopiezometer in quartz-bearing mylonite zones

475

Tectonophysics, 78 (1981) 475-508 Elsevier Scientific Publishing Company,

Amsterdam

-

Printed

in The Netherlands

AN ASSESSMENT OF DYNAMICALLY RECRYSTALLIZED GRAINSIZE AS A PALAEOPIEZOMETER IN QUARTZ-BEARING MYLONITE ZONES

M.A. ETHERIDGE Department (Received

and J.C.

of Earth January

Sciences,

WILKIE

*

Monash

University,

Clayton,

Vie. 3168

(Australia)

26,1981)

ABSTRACT Etheridge, M.A. and Wilkie, J.C., 1981. An assessment of dynamically recrystallized grainsize as a palaeopiezometer in quartz-bearing mylonite zones. In: G.S. Lister, H.-J. Behr, K. Weber and H.J. Zwart (Editors), The Effect of Deformation on Rocks. Tectonophysics, 73: 475-508. Recrystallized grainsize has been measured in quartz and, in some cases, feldspar, in about 200 samples from 22 mylonite zones from 3 discrete tectonic settings: (1) supracrustal, low-temperature, shallow dipping thrusts; (2) deeper level major (> 100 km) thrusts in high-grade metamorphic rocks; and (3) steeply dipping fault zones in granite within a low-grade Palaeozoic fold belt. The concept of a steady-state dynamically recrystallized grainsize largely unaffected by post-creep processes (e.g. static grain growth) receives strong support, especially from the microstructures and the independence of finite strain and grainsize in most mylonite zones. However, the usefulness of this grainsize for predicting stress magnitudes is limited by the apparently independent influence of metamorphic environment on grainsize. The available theoretical and empirical stress/ grainsize relationships do not account for these effects. The concept of a steady-state grainsize and the ease of accurate measurement offer a potentially powerful method for predicting stress intensity during deformation. Predicted deviatoric stresses have been computed using Twiss’ theoretical model for comparison with other crustal stress estimates, and most samples fall in the range from 20 to 150 MPa. In a few cases where discrete high-strain (mylonite) zones occur within contemporaneously regionally deformed rocks, the recrystallized grainsize indicates that stresses were 1.5 to 2 times higher within the zones. There is a clear need for improved theoretical and/ or empirical stress/grainsize relationships for quartz and other minerals before extensive further application of this technique.

INTRODUCTION During high-temperature dislocation materials, ‘a number of microstructural

* Present address: berra, A.C.T. 2600,

Research Australia.

0040-195I/8I/OOOO-0000/$02.50

School

creep of a wide range of crystalline parameters depend upon the devi-

of Chemistry,

@ 1981

Elsevier

Australian

Scientific

National

Publishing

University,

Company

Can-

atoric stress. The most widely measured of these parameters are dislocation density, subgrain or cell size and dynamically recrystallized grainsize (see Bird et al., 1969; Evans and Langdon, 1976; Twiss, 1977; Mercier et al., 1977; White, 1979a, for recent reviews in a range of materials). The realization that dislocation creep is a significant deformation mechanism in rocks has encouraged the use of these parameters to assess palaeostress intensities in a range of deformational environments in the earth. Of the three parameters, dynamically recrystallized grainsize is the most easily measured, and may be less subject to adjustments after the steady-state deformation. For example, the free dislocation density in mylonitic rocks may contain an appreciable proportion of dislocations induced during cooling, uplift, etc, (White, 1979a, b). In addition, subgrain size seems to vary with scale of observation and boundary misorientation, so it is difficult to know what to measure as the steady-state value (White, 1973, 1979a; Bell and Etheridge, 1976; Schmidt et al., 1980). Attention to these parameters as stress indicators in geology has been focussed on the mantle (Mercier et al., 1977; Ross et al., 1977), and mylonite zones (Twiss, 1977; White, 1979b; Weathers et al., 1979). Interest in the latter is partly because evidence of dislocation flow and dynamic recrystallization is common in such zones, but also because they are presumably the downward extension of brittle, seismogenic fault zones (Sibson, 1977). The stress magnitudes obtained from creep parameters in the plastic regime will thus be useful in constraining dynamic models of seismic faulting at higher levels. This paper describes the results of a study of the microstructure and recrystallized grainsize of 22 well-characterized mylonite zones from a range of tectonic settings. The primary aims of the study are: (1) to evaluate the concept of a steady-state dynamically recrystallized grainsize in rocks deformed to a range of finite strains and under various metamorphic conditions; (2) to assess the applicability of the published relationships between stress intensity and grainsize to naturally deformed quartz-bearing rocks; and (3) to compare the stress values obtained with data from other sources. GEOLOGICAL

SETTING

OF MYLONITE

ZONES

The mylonite zones that have been examined fall into three tectonic and geographic groups (Fig. 1): (1) Thrusts and thrust nappes of the northern margin of the intracratonic Amadeus Basin (locaility 1, Fig. 1) (Forman, 1971). These structures occur in regions of intense deformation of an otherwise fairly gently deformed sequence of low metamorphic grade. They are largely supracrustal structures thought to be related to movement on deep-seated basement faults (Marjoribanks, 1974). Thrust zones from a few millimetres to tens of metres wide are best developed in the basal unit of the cover sequence, the Heavitree Quartzite. This unit consists dominantly of relatively pure orthoquartzite, with

Fig. 1. Map of Australia showing the three sampling areas used in this study. Locality 1 = Thrusts and thrust nappes of the northern Amadeus Basin. Locality 2 = WoodroffeDavenport thrust zones, Musgrave Range. Locality 3 = Foliated zones within Palaeozoic granites of the Lachlan Fold Belt.

minor interbedded conglomerate and siltstone, and is Upper Proterozoic in age. The most extensive development of thrusting in the Heavitree Quartzite is found northeast of Alice Springs in the Arltunga nappe complex. In the Atnarpa area of this complex, a number of superimposed shallow dipping thrust sheets have been mapped in detail by Yar Khan (1972). Detailed structural analysis and microfabric studies showed that there was a gradual change from ductile deformation in the north of the area to brittle deformation in the south and west during thrusting. Yar Khan (1972) suggested that this is consistent with lower level, and thus higher temperature rocks being thrust up from the north and east. In order to investigate the effect this had on recrystallized grainsize, several suites of quartzites were collected across thrusts throughout the area. A similar, although less extensive series of thrusts has been sampled in the Ormiston area, 200 km west of Alice Springs (Marjoribanks, 1974, 1976). Immediately north of the Amadeus Basin sediments, extensive, more steeply dipping mylonite zones are exposed in the basement rocks. These are considered to be the deeper equivalents of the more shallow dipping thrusts seen in the overlying sediments (Marjoribanks, 1974). We have sampled across one of these zones where it transects the metamorphosed Chewings Range Quartzite in Jay Creek, in order to compare the microstructure with that in the Heavitree Quartzite. (2) The Woodroffe and Davenport thrusts in the Musgrave Ranges, central Australia (locality 2, Fig. 1) occur within high-grade gneisses and granulites, and are thought to have originated deep in the crust at amphibolite facies

478

conditions (Major et al., 1967; Bell and Etheridge, 1973; Bell, 1978). Previous work in the Woodroffe thrust has suggested that the metamorphic history prior to thrusting affects the microstructural reponse to strain, and, in particular, that the water content of the quartz induced during the highgrade metamorphism controls this reponse (Bell and Etheridge, 1976). The fault zone, up to 1 km wide in outcrop, dips at about 30” south, and the granulites comprise the overthrust block. 10 to 30 km to the south, a parallel structure, the Davenport thrust has thrust, a series of amphibolite to transitional granulite facies gneisses onto the granulites (Collerson et al., 1972). In both cases, mylonitization has been restricted to a narrow (few tens of metres) zone within the granulites, but its effects have been much more widespread (up to several kms) in the originally lower grade gneisses. (3) Steeply-dipping mylonite zones in Pniaeozoic granites are widespread throughout the Lachlan fold belt in southeastern Australia (locality 3, Fig. 1). The Lachlan fold belt is the southernmost tectonic entity within the lower Palaeozoic to Mesozoic Tasman fold belt that occupies much of east-

SYDNEY

r’

Fig. 2. Map showing the major outcropping Palaeozoic granitic bodies in southeastern Australia. The four main sample areas for locality 3 are boxed on this map (Wy = Wyangala granite; Wo = Wondalga granite; Cr = Crackenback fault zone; Co = Coolac Serpentine Belt).

479

ern Australia (Packham, 1969; Solomon and Griffiths, 1972; Scheibner, 1974a, b; and Rutland, 1976). Granitic (sensu l&o) bodies are widespread throughout the fold belt, range in age from Ordovician to Carboniferous (Evernden and Richards, 1962; Brooks and Leggo, 1972; White et al., 1974) and commonly contain deformed zones in which mylonitic rocks are developed. These deformed zones range from narrow (l km), sub-planar regions of foliated granite that generally contain one or more discrete highly deformed zones. We have sampled several examples of each of these types, including one locality (Crackenback) where all are present (Fig. 2). These examples were chosen because they encompass a wide range of tectonic environments, and metamorphic conditions from lowermost greenschist to amphibolite facies. The rock types are primarily pure quartzites or coarse-grained felsic rocks, and they all contain in the unstrained state coarse-grained quartz which has progressively deformed and recrystallized as a single phase. Detailed descriptions and maps of sampling sites may be obtained directly from the writers. MICROSTRUCTURE

Measurement

AND RECRYSTALLIZED

GR 41NSIZE

techniques

Recrystallized grainsize was measured in thin section by the line intercept technique (Exner, 1972; Pickering, 1976). In this technique, the number of grains along a line of known length is counted to give a mean linear intercept (a), which is related to the mean grain diameter (2) by, d=k-Q

(1)

where k is a factor between about 1.5 and 2.1, depending upon the grainshape (Gifkins, 1970). For certain shapes (e.g., square to rectangular grains, which are not uncommon in some mylonites, k will also depend upon grain orientation and the orientation of the reference line. Because of the uncertainty in shape and orientation, we have chosen k = 1.5, the value for equisized spheres, which seems to be the most widely used value in metallography (Cotterill and Mould, 1976), and note that this may lead to a small underestimate of grainsize. We measured approximately 1000 grain intercepts per sample, and this gives rise to relative error of about 2%, based upon the standard deviation of n (Pickering, 1976). In thin sections, as distinct from surface metallographic measurements, there will also be an effect of specimen thickness. It is difficult to quantify this effect, because some grain boundaries that extend only part way through the section will be recorded during measurement, whereas others will not, depending on the relative orientations (i.e., birefringence) of the grains surrounding the boundary, its orientation and its extent. We have

480

tried to minimize this problem by measuring in thin sections of less than normal thickness (15-20 pm). Where possible, measurements were made in areas of the thin section that are monomineralic, or very nearly so. Grainsize seems to be little affected by second phase (usually layer silicate) proportions of less than about 2%, but is noticeably finer where this proportion is exceeded (Etheridge and Wilkie, 1979, fig. 7). Locality 1 Six separate exposures were sampled in the Arltunga nappe complex, comprising the range of ductile to brittle microstructures reported by Yar Khan (1972). In two of the localities (Atnarpa B and C), continuous traverses from undeformed Heavitree Quartzite and gradually increasing strain to totally recrystallized mylonite were sampled. The typical microstructural progression with finite strain at locality B is shown in Fig. 5, and finite strain values are

Fig. 3. Finite strain data for the sampled mylonite zones within the Heavitree Quartzite (locality 1, Fig. 1) plotted on a Flinn diagram. The strains were calculated from Rf/@ plots of deformed elastic grain shapes, with at least 50 grains measured in each section. Even in substantially recrystallized samples original grain outlines are commonly preseved as accumulations of fine opaques and layer silicates. The significant departure from plane strain in most zones suggests that their deformation history departed markedly from a progressive simple shear. The Atnarpa A, B and C localities refer to the Arltunga nappe complex localities described in the text.

481

TAA(3)

60 1

AD00)

J-----LE(5’ 1

2

3

FINITE

4

STRAIN

6

5

RATIO

‘Y,$

Fig. 4. Compilation of finite strain, recrystallized grainsize and degree of recrystallization data for all samples from locality 1 (Atnarpa A to E labelled AA, AB, etc; Damper Gorge = DG. Ormiston Gorge = OG, Chewings Range Quartzite at Jay Creek = CRQ). Samples in which it was not possible to measure strain have their grainsize represented by vertical bars at the right hand side of the diagram, with the number of samples measured and locality written next to each bar. The finite strain ratio Y/Z is plotted for all sample sites except Atnarpa B, where the strain deviates significantly from pure flattening, and the ratio X/Z is plotted. The degree of recrystallization is shown by the symbols (open square = 2.20%; closed square = ZO-40%, open circle = 40-60%; open triangle = 6080%; closed circle = SO-100%).

plotted on a Flinn diagram (Flinn, 1962), in Fig. 3. The recrystallized grainsize does not vary with finite strain, except that there is some coarsening coincident with complete recrystallization (Fig. 4). At two other localities (A and D), tectonic slices of Heavitree Quartzite are bounded on both sides by thrusts against basement lithologies. All samples from these slices are totally recrystallized, and the grainsize is uniform at each locality, even though the finite strain varies across and between slices (Fig. 5). In the south of the Arltunga area (lot. E and F), the fault zones are typically occupied by dark coloured quartzites with a brecciated appearance in outcrop. The microstructures of these rocks are complex, but indicate that a period of plastic deformation was overprinted by a more brittle event. Figure 6a, b illustrates the variation in microstructure with increasing plastic strain from locality E; the relict grains have a cold-worked appearance and the recrystallized grainsize is uniformly fine. Several generations of discrete shear zones, fractures and quartz + hematite veins overprint this microstructure. These structures suggest more brittle behaviour, although fibrous vein filling, deformed veins and strain shadowed grains within shear zones (Fig. 6c) indicate that failure was not catastrophic. The grainsize within the shear zones is the same as that in the remainder of the rock, and insertion of a h-plate

482

shows that both have a similar, strong crystallographic preferred orientation, consistent with the operation of dynamic recrystallization throughout. At the southernmost locality, F, the detrital quartz grains are only slightly elon-

Fig. 5. Photomicrographs illustrating the microstructural transition with increasing finite strain (from a to c) at Atnarpa B. Note that the recrystallized grainsize is essentially independent of finite strain (Fig. 4), except that there is a noticeable increase in grainsize coincident with complete recrystallization. Scale bar is 0.2 mm in all micrographs.

gate and are commonly angular, even close to the thrust contact. Optical evidence of dynamic recovery and recrystallization is very limited, and discrete fractures and veins are common. These southern thrust zones were deforming very close to the brittle-ductile transition, with an increasing tendency for brittle behaviour as thrusting progressed. In Damper Gorge, a complex thrust zone about 100 m wide dips at 40” towards the north. An increasing strain transect has been sampled across these thrusts, and the finite strain data are presented in Fig. 3. The microstructural progression with increasing strain is shown in Fig. 7a, b. Recrystallized grainsize is also independent of finite strain in this zone, although the microstructure differs from the Arltunga localities B and D in the following ways: (1) deformed original grains tend to have strain shadows or deformation bands rather than subgrains on the optical scale; (2) finite strain recorded by original grainshape is higher for a given proportion of recrystallization; and (3) recrystallized grainsize is finer and does not increase in completely recrystallized samples. Ormiston Gorge exposes a shallow dipping thrust plane that truncates the lower limb of a large mesoscopic fold. Again, we have collected a suite of samples from almost undeformed quartzite with increasing strain into the thrust zone. However, in this case, recrystallization is much more limited,

484

finer grained (Fig. 4) and even the most highly deformed samples contain original grain remnants (Fig. 7c, d). In Jay Creek, a mylonite zone approximately 1 m wide and dipping 80”

Fig. 6. Photomicrographs of Heavitree Quartzite within a mylonite zone at Atnarpa E. a. Weakly strained sample showing highly heterogeneous intracrystalline deformation with the mottled extinction and sharp deformation bands typical of low temperature/ fast strainrate conditions. b. More highly strained example from centre of zone; the fine matrix has a pronounced c axis preferred orientation. c. Highly strained sample similar to that in b., but with several generations of quartz ?: haematite veins: (plane polarized light). Scale bar is 0.2 mm in each case.

towards the south, thrusts gneisses over the Chewings Range Quartzite. Outside the fault zone it is a very coarse-grained metamorphic quartzite with 2 to 5% sillimanite as the only significant impurity. The quartz grains are equidimensional, and have the complex grain boundary shapes typical of materials at advanced stages of exaggerated grain growth (Wilson, 1973). The change in microstructure with increasing strain is seen in Fig. 8(a-c). These microstructures are an extension of the trend already noted for the Ruby Gap + Damper Gorge + Ormiston Gorge suites. Original grains are highly flattened without substantial recrystallization, and sharply bounded, highangle deformation bands are common. Recrystallization is confined to grain and deformation band boundaries even at quite high strains although it extends to grain interiors where subgrains are best developed (Fig. 8). Recrystallized grainsize is independent of strain within the mylonite zone, in this case between 7 and 11 pm throughout (Fig. 4). Locality 2 The microstructures

of

the

Woodroffe

thrust

mylonites

have

been

486

described in detail by Bell and Etheridge (1973, 1976). The overthrust granufite facies gneisses ;uzd the underlying amphibslite fxies gneisses reacted quite differently during my~unit~~~t~~n~ despite their similar mineralogy and bulk chemistry (compare figs. 9 and 12 from Bell and Etheridge, 1976). In

487

Fig. 7. Photomicrographs illustrating microstructures at small to moderate (a, c) and large (b, d) finite strains from mylonite zones in Heavitree Quartzite at Damper Gorge (a, b) and Ormiston Gorge (c, d). Scale bar is 0.2 mm.

488

particular, dynamic recovery and recrystallization took place at lower finite strain, and produced larger grainsizes in the originally amphibolite facies gneisses (Table I). In the Davenport thrust, granuIites are overthrust by

Fig. 8. Microstructural transition from low to very high finite strains (a to c) in the Chewings Range Quartzite within a narrow (“1 m wide) mylonite zone in Jay Creek. Note the transition to more heterogeneous strain on the microscale, finer recrystallized grainsize and lower proportion of recrystallization at a given strain from Atnarpa B (Fig. 5) + Damper Gorge (Fig. 7a, b) + Ormiston Gorge (Fig. 7c, d) + Jay Creek. Scale bar is 0.2 mm.

lower-grade gneisses, but the microstructural response to strain bears the same relationship to pre-mylonite history as in the Woodroffe thrust (Table I). It is thus clear that the microstructure is unrelated to position with respect to the thrust, or any stress variations related to that. Adjacent to the Davenport thrust, there are some rocks of “transitional granulite facies”, intermediate in grade between the amphibolites and granulites at Woodroffe. These rocks also have intermediate mylonitic microstructures and recrystallized grainsizes (Table I), providing further support for the effect of metamorphic pre-history on deformation. Both plagioclase and alkali feldspar have recrystallized in many of these samples, and their grainsize is also given in Table I. Feldspar grainsize is consistently l/5 to l/3 that of quartz in the same specimen and is correlated with pre-mylonite metamorphic history in the same way. Locality 3 We have sampled foliated and mylonitic granites from 9 separate mylonite zones in the Lachlan fold belt. The typical mineralogy of the granites is qtz. + plag. + kspar + bi f hbl + must. In all of the rocks examined, quartz

490

has deformed plastically and has recrystallized readily, even at low strains, whereas feldspar may have been rigid, weakly plastic or essentially brittle in its response. In all cases, feldspar was significantly stronger than quartz.

Fig. 9. a-c illustrate the typical change in microstructure with increasing finite strain in the southeastern Australian granites (locality 3). Note that the recrystallized quartz grainsize is independent of finite strain, as long as single-phase quartz aggregates are retained. d. Recrystallized quartz aggregate showing elongation of recrystallized grains oblique to aggregate elongation due to late stage deformation - Crackenback fault zone. The finergrained material is largely feldspar. Scale bar is 1.0 mm for a-c, and 0.3 mm in d.

492 TABLE I Grainsize data from Locality 2 Sample number *

Grainsize (pm) Quartz

Woodroffe G2 * G3a G3b G3c G4 G5 A5 A4 A3 A2 Al

thrust

Davenport 16 * 18

thrust

10 20 25 26 28 51 71 95 136 79 60 114 207

400 917m

100 105

318 323 472 474 537 6Pl 620 673 882 917a 917b 917k 9171 917p 940 948b 948d 948g 948c

50 66 76 78 44 75 76 62 65 85 89 83 78 58 62 70 86 53 50

531 543 689 698 783 788 900 907b 907f 911 917s 928g 92831

31 46 46 36 47 45 48 33 36 43 50 26 16

Comments Feldspar

5 6 6 8 9 28 22 30 22 14 23 43 -

Granulite Facies

+ Thrust Amphibolite Facies

1. Adamellite 2. Amph-bi ?r ga no px.

27 21 26 21 11 21 -

3. Amph-ga-bi ?: hbl C Opx (Transitional facies)

19 25 26 22 23 18 17 14 20 11 13 11 -

4. Ga-red bi-Opx (Granulite facies)

12 9 .12 12 13

* Sample numbers for the Woodroffe samples are as in Bell and Etheridge (1976).

493

Biotite deforms readily but is slower to recrystallize than quartz, although it tends to disperse throughout the rock at higher strains. In general, the deformational microstructures of these rocks are very similar to those from other mylonitized rocks of the same broad mineralogy (Hobbs, 1966; Bell and Etheridge, 1973, 1976; Burg and Laurent, 1978; Weathers et al., 1979). They are well suited to this study because recrystallized quartz aggregates derived from a single original grain remain coherent and monomineralic to quite high finite strains. Figure 9 shows the typical changes in microstructure with increasing strain from weakly foliated granite to mylonite. Recrystallized grainsize data from four of the localities are given in Table II. In the other localities, grainsizes encompass a similar range, but they were either less extensively sampled or contained a wider variety of premylonite rock types, making analysis of the grainsize data difficult. The Wondalga and Wyangala granites are characteristic of the regionally foliated granites of Valiance (1969) and they contain broad, more highly foliated zones as well as discrete, narrower mylonite zones. The Wondalga granite samples came from four separate localities along the foliated contact between the granite and a belt of medium grade metamorphics (Fig. 2, Table II). In each case, the noticeably finer grainsizes were found in more highly strained rocks from within discrete mylonite zones. Recrystallized feldspar grainsizes (mostly K-feldspar) range from l/6 to l/3 the corresponding quartz grainsize, but the correlation with quartz grainsize is not as good as in the Woodroffe-Davenport area (compare Tables I and II). The Wyangala granite specimens are mostly from the Wyangala Dam spillway mylonites (Hobbs, 1965, 1966) and the Bigga area some 20 km to the southeast (Fig. 2). The grainsize is constant within and between these localities, with a large range of finite strains represented. The Coolac serpentine belt is a narrow, fault-bounded Alpine-type ultramafic belt. It can be traced for about 500 km, but is best exposed at its southern end, between Coolac and Goobarragandra (Golding, 1969; Ashley et al., 1971; Ashley and Chenhall, 1976) (Fig. 2). Along the whole of this section, the ultramafics are bounded on their eastern side by foliated to mylonitized felsic intrusives of the Young Granodiorite (410-420 Ma., Ashley et al., 1971). Ashley and Chenhall (1976) have interpreted the belt as an ophiolite, and attributed its emplacement to abduction during the middleDevonian regional folding event. The microstructure of the mylonites has been described by Ashley and Chenhall (1976), and we have collected specimens from a selection of their localities. The microstructures are, in fact, more complex than they recognized, and the quartz shows the effects of two discrete deformation events. Isolated quartz aggregates generally consist of elongate and variably deformed grains which have themselves undergone some recrystallization (cf. fig. 6 of Ashley and Chenhall, 1976). It is apparent that the quartz has been coarsely recrystallized during an earlier event, and that these recrystallized grains have subsequently been deformed during the mylonitization, that is restricted to the immediate vicinity of the serpen-

94 66 133 212 206 224

93 45 81 83 79 92 54

41255 41277 41278 41279 41309 41310

47360 47361 47362 47363 47364 47365 47372 13 12 21 26

-

27 51 29 38 41 44

28 21 21 26

19 28 31

113 103 130 140 103 52 109 105 56

(pm)

50231 50232 50234 50235 50236 50244 50245 50246 50247

Grainsize Feldspar

3

Quartz

*

Locality

Wondalga batholith

data from

Sample number

II

Locality

Grainsize

TABLE

Crackenback fault -Mt Kosciusko

Coolac serpentine belt

Locality

51108 51110 51111 51114M 51115M 51116 51117 51119 51121

37661 37679 37685 37904

41195 41196 41197

38021 38022 38023 38024

Sample number

**

*

(pm)

Coarse Fine 108 19 109 40 118 23 99 81 112 31 117 23 104 25 132 22

23 44 36 18

8-10 8-10 8-10

15 10-12 6-10 10-12

Quartz

Grainsize

16 11 12

10 13

14

Feldspar

61 69 78 69 76 71 77 80

62 62 65 64 64

58995 58996 58997 58998 58999

70 111

49679 49684

34710 34711 34712 34714 34718 34719 34720 34721

98 71

49639 49646 22 20

14 -

51123M 51131 51135 51142M 51144 51145 51147 51149 51152M 51153M 51157M 51158M 51161 51168M 99 154 138 89 151 156 152 302 80 69 83 61 253 61 19

20

20

16 22

20 20 29 22 27 27

15 42

* Sample numbers from the Wondalga and Wyangala batholith (except Wyangala Dam) are University of Sydney catalogue numbers, and the remainder are Monash University catalogue numbers. ** Sample numbers followed by “M” from Crackenback area were collected from discrete mylonite zones, others are from regionally foliated granites.

Wyangala batholith

106 88

49634 49635

496

tine contact. Very little recrystallization accompanied the second deformation, which is characterized by ribbon-like grains with strong undulose extinction, sharp deformation bands, serrated grain boundaries and few subgrains on the optical scale. Because of the over-printing deformation, the grainsize of the earlier generation of recrystallized grains is difficult to measure but is in the range 80 to 100 pm. These values correspond well with those from the foliated granites in the previous section, and we believe that they have the same origin, that is, they reflect weak to moderate straining of the granodiorite during a regional deformation event prior to the emplacement of the ultramafic belt. The sporadic recrystallization attributable to this later deformation produce grains in the range 6 to 12 pm, which does not vary greatly with finite strain (Table II). The Crachenback fault is traceable for 45 km within the felsic intrusives of the Kosciusko Batholith and the surrounding Ordovician slates and greywackes (White et al., 1977) (Fig. 2). It trends towards 050”, and its straight outcrop trace in rugged terrain suggests that it is steeply dipping. In the Thredbo area, the fault traverses the Mowambah Granodiorite (White et al., 1977) and occupies the valley of the Crackenback River, which separates two distinct topographic levels. The average height of the dissected Miocene peneplain is 300 to 400 m higher on the northwestern side, suggesting that the Crackenback fault is a locus of the Pleistocene to Recent uplift in this area. The recent activity is supported by the tentative epicentre location for an earthquake in this region in 1959 (Cleary et al., 1964). The Mowambah Granodiorite is generally well foliated, even several kilometres from the fault, and narrow (l-10 m) mylonite zones occur throughout. Most importantly, however, both the regional foliation and the mylonitic foliation (they are commonly crosscutting at a low angle) are at 30”-40” to the topographic expression of the fault. The Mowambah Granodiorite is a coarse grained “S-type” intrusive (Chappell and White, 1974) containing quartz + plagioclase + K-feldspar + biotite + minor muscovite. The foliation is defined by flattened quartz grains and more or less well-oriented biotite. Feldspar is only plastically deformed at high bulk strains. In most specimens, there is evidence of two episodes of deformation. The first, which is associated with the regional foliation and the bulk of the mylonitization, has been responsible for a gross microstructure very similar to that shown in Figs. 9 (a-c). In particular, it was accompanied by extensive, relatively coarse recrystallization of quartz (SO to 150 pm, Table II). The narrow ‘mylonite zones which overprint the regional foliation have grainsizes in the lower end of this range (Table II). These coarsely recrystallized grains are quite strongly undulose, invariably have serrated grain boundaries, and are commonly elongate at an angle to the main foliation (Fig. 9d). Recrystallization is not commonly associated with this later deformation, but, where it is, grainsizes range from 15 to 30 Clm (Table II). It is not possible to assign the latest deformation to movement on the Crackenback Fault with certainty, but they appear to be spatially related.

497

DISCUSSION

In all the mylonite zones studied, quartz has deformed by the same general range of dislocation flow, dynamic recovery and dynamic recrystallization mechanisms that have been widely reported elsewhere (White, 1973, 1976, 1977; Bell and Etheridge, 1973, 1976; Nicolas and Poirier, 1976; Etheridge and Wilkie, 1979). A number of consistent relationships have been found between finite strain, microstructure, rock composition and recrystallized grainsize, and as a prelude to discussion we summarize these relationships. (1) Recrystallized grainsize in both quartz and feldspar is largely independent of finite strain on all scales from an aggregate replacing an original grain to a single mylonite zone. (2) The major exception to this constancy of grainsize is that, in a given zone, recrystallized grainsize may vary with parent rock composition and/or pre-mylonite metamorphic history. The compositional effect is most likely to be due to a range of chemical defects in the quartz and feldspar (Hobbs, 1981), and “hydrolitic weakening” (Griggs and Blacic, 1965; Hobbs et al., 1972) is the best documental example of such an effect. (3) The coarser-grained mylonites recrystallize at somewhat lower finite strains and are accompanied by extensive optical evidence of easy recovery. In contrast, rocks with finer recrystallized grainsize have a more “cold optical microstructure, and original grains are highly strained worked” before they substantially recrystallize. (4) Systematic variations in grainsize with tectonic setting are not as clear, but the higher temperature zones are consistently coarser-grained. The Crackenback fault shows evidence of a coarse recrystallized grainsize associated with a regionally developed foliation overprinted successively by individual mylonite zones of intermediate grainsize, and then by a fine grainsize associated spatially with the major fault. This sequence presumably occurs with continuing uplift, and, therefore, decreasing temperature and pressure. (5) Recrystallized feldspar grainsize is generally one-third to one-fifth that of quartz in the same rock, and there is a fair to good correlation between quartz and feldspar grainsize at any locality. This correlation is particularly good in the Woodroffe and Davenport thrusts, suggesting that a hydrolytic or related chemical control on grainsize also operates in feldspar. We have found no significant difference in grainsize between plagioclase and alkali feldspar, although the latter deforms and recrystallizes more readily in the locality 3 granites. The concept

of a steady-state

recrystallized

grainsize

The most impressive microstructural features of all the mylonites examined in this study are: (1) the constancy of recrystallized grainsize within a single thin section (e.g. Figs. 5, 7 and 9); and (2) the consistent indepen-

498

dence of grainsize and finite strain in most zones (Fig. 4, Table II). These features provide some support for the concept of a steady-state dynamically recrystallized grainsize in mylonitic rocks. However, it can be reasoned that the present grainsize is not the one which equilibrated with the steady-state creep stress responsible for the bulk of the finite mylonitic strain. Changes in grainsize could arise from either continual equilibration of grainsize with decreasing stress during the final stages of deformation, or post-deformational, static grain growth driven by the relatively high grain boundary free energy of a fine grained aggregate (Twiss, 19’77; White, 1979a). The importance of continuous equilibration with decreasing stress depends primarily on the kinetics of the equilibration process, the rate at which the differential stress decays, and the stress dependence of the dislocation creep process. There are very few data available which constrain the first two of these, and the stress exponent for dislocation creep may range from 3 (or even somewhat less in fine grained materials) to about 5. However, it is of some concern that Ross et al, (1977) showed that recrystallized grainsize is able to re-equilibrate to a decrease in steady-state flow stress within a few percent strain under laboratory conditions. These results suggest that re-equilibration would take place readily on the geologic time scale. However, the high stress sensitivity of dislocation creep produces large reductions in creep rate for even relatively small reductions in stress. Since re-equilibration of grainsize requires rearrangement of the dislocation substructure, its rate will depend strongly on creep rate, and will therefore be very sensitive to rate of stress decay. It should be noted that Ross et al. (1977) were only able to test small stress increments at fairly high stress magnitudes. Second, if re-equilibration was rapid in the mylonites studied, all grainsizes should have equilibrated to similar, low stress values, and there should be no relationship between recrystallized grainsize and the microstructure formed during the steady-state deformation. Static grain growth can be driven either by a reduction in total grain boundary area, or by absorption of defects into expanding boundaries (Cotterill and Mould, 1976). Because many deformed rocks spend substantial time (106-lo9 years) at temperatures in excess of lOO”C, grain growth seems kinetically feasible. There are, however, a number of arguments and observations that suggest that it has not taken place to a significant degree in these rocks. First, straight grain boundaries, 120” or 90” triple junctions and uniform grainsize are typical of the dynamically recrystallized microstructure, and all of these reduce the tendency for uniform coarsening during annealing (Cotterill and Mould, 1976). Second, there is no evidence of abnormal coarsening of individual grains, even in the coarsest samples, as would be expected if secondary recrystallization had proceeded to any degree. Third, if stored strain energy was significant in driving static recrystallization, the greatest coarsening should have occurred in those rocks with the most “coldworked” microstructures, and partly recrystallized aggregates should be very rare. Since the driving force due to stored strain energy is at least an order of

499

magnitude larger than that available from reducing grain boundary area (Cotterill and Mould, 1976), the retention of these highly deformed relics argues against static coarsening. In summary, our observations indicate that the relatively low stress, steady state dynamic microstructures produced during mylonitization are stable, and that dynamically recrystallized grainsize has suffered little or no modification. Existing stresslgrainsize

relationships

It has been recognized for some time that materials undergoing steady state dislocation creep develop a stable substructure, and that several parameters of this structure are related uniquely to the deforming stress (Bird et al., 1969). In many metals, alloys and ceramics, dynamic recrystallization does not accompany creep, and the stable microstructure consists of subgrains of fixed size and low relative misorientation, with a low density of mobile dislocations within the subgrains (see Mukherjee, 1975; Evans and Langdon, 1976; Takeuchi and Argon, 1976; McQueen, 1977 for recent reviews on this topic). There has been much more emphasis on subgrain size than recrystallized grainsize, and there is extensive evidence for an empirical relationship between stress (a) and subgrain size (d) of the form (T= kd”

(2)

where k and m are constants, with rn generally being close to -1 (Bird et al., 1969). Despite the clear empirical justification for this relationship, there is no theoretical model which has gained widespread support. Holt (1970) suggested that a uniform density of dislocations is unstable, and developed a theory of clustering into cell walls during cold working, somewhat analogous to the spinodal decomposition of chemical solutions. This produced a stress/ cellsize relationship of the correct form, but Holt (1970, p. 3200) suggested that it may not apply to the well-structured, low angle boundaries typical of high temperature creep. More recently, Gittus (1976,1977,1979) has shown 1 that, at a given stress, the sum of the subgrain boundary energy and the elastic strain energy is a minimum for a particular cell size. This low energy cell size is related to the stress by a relationship of the required form which fits the metallurgical data quite closely. However, Gittus’ analysis is based on the assumption that the mean dislocation spacing within the subgrains is equal to the subgrain diameter (i.e. only one dislocation per subgrain), and this is not consistent with observations on a wide range of materials, including quartz (e.g. Bird et al., 1969; Takeuchi and Argon, 1976, fig. 6; White, 1976, fig. 3b, 4b; White, 1979b, fig. 6D; Weathers et al., 1979, fig. 8). He thus ignores the energy contribution due to these free or mobile dislocations in his calculations. Dynamic recrystallization occurs during high-temperature deformation only in those metals and alloys which are characterized by low rates of

500

recovery (Sellars, 1978). In these cases, an empirical relationship between recrystallized grainsize (D) and stress of the same form as equation 2 is found, but the value of the exponent m is generally between -0.5 and -1 (Sellars, 1978). Twiss (1977) has recently developed stress/grainsize relationships for both subgrains and recrystallized grains that are consistent with most of the available data. However, one of his fundamental assumptions requires that “the total strain energy of dislocations ordered into a closed surface, i.e., either a subgrain or recrystallized grain boundary, must be . . . equal to the total energy of a steady-state density of dislocations within the enclosed volume” (Twiss, 1977, p. 229). Such an assumption is applicable to the static annealing case, where a relatively uniform dislocation distribution is completely replaced by an array of low and/or high angle boundaries. However, it is harder to justify for steady state creep for the following reasons: (1) the transition from uniformly distributed dislocations to a subgrain network generally takes place during primary rather than steady state creep (Takeuchi and Argon, 1976, Fig. 5); and (2) virtually all materials undergoing steady-state creep contain a moderate density of free dislocations within subgrains or recrystallized grains, and it is this dislocation density that is related to the steady-state creep stress by a relationship of the form used by Twiss (Bird et al., 1969). It would thus seem more reasonable to minimize the sum of the boundary energy and free dislocation energy terms, rather than assume that they are equal (Twiss, 1977, eq. 1). Twiss’ subsequent derivation of stresslgrainsize equations also involves the determination of a number of disposable parameters by fitting curves to the experimental data for metals, alloys and minerals. He argues that the values determined in this way are “well within the rather narrow independently expected limits” (p. 231). However, it is noticeable in his fig. 1 that the mineral data (curves 8 and 9, and the very scattered data points for curve 7) fit his curve less closely than the data from metals and alloys. This may simply be due to differences in the values of his parameters (esp. cy and /3) between metals and minerals, or it may reflect fundamental differences in recovery and recrystallization mechanisms. The empirical data for minerals are limited to olivine (Post, 1973; Mercier et al., 1977; Ross et al., 1977) pyroxene (Ross and Neilsen, 1978), calcite (Schmidt et al., 1979) and quartz (White, 1979b, after Ardell et al., 1973; Mercier et al., 1977). All the data, except those for calcite, were obtained from experiments carried out in a solid pressure medium apparatus, in which uncertainties on stress measurements are large, especially at low stresses, and the chemical environment is poorly controlled. The importance of chemical environment is clearly demonstrated by the hydrolytic weakening behaviour of silicates (Griggs and Blacic, 1965; Hobbs, 1968; Hobbs et al., 1972), and its wider implications are discussed by Hobbs (1981). It is interesting to note that Mercier et al. (1977) reported a difference between the stress/grainsize relationships of “wet” and “dry” olivine, especially in view of the data from locality 2.

501

All the published stresslgrainsize relationships that can be readily applied to quartz thus have shortcomings, and it is not clear which of them will produce the most reliable stress magnitudes. In particular, none of them predicts the effect of chemical environments (esp. H20) which is indicated both by our data and by the hydrolytic weakening results. However, we have chosen Twiss’ theoretical relationship, partly because it can be applied to the feldspar data, but also because it produces stress values intermediate between those from the two empirical equations. There is a clear need for a stress/ grainsize relationship based on a realistic model of recrystallization and calibrated by carefully controlled experiments. In fact, until we have a better understanding of the deformation behaviour of quartz, particularly the role of chemical defects, purely empirical stress/grainsize relationships are of limited use. Calculated

stress magnitudes

Stress magnitudes calculated from the quartz grainsize data using Twiss’ (1977) equation 11 are summarized in Fig. 10. However, using the values for p, V, and b in table 2 of Twiss (1977), we obtain B = 6.1 rather than 5.5 as reported by Twiss (p. 236), and we use the higher value in our computations. We also note that the feldspar data give stress magnitudes consistently 3 to 5 times higher than the quartz, and this must reflect a deficiency in the stresslgrainsize model. The calculated stress magnitudes fall into two main groups. The regionally foliated granites (Wyangala, Wondalga, and coarser grainsizes at Crackenback) give lower values, generally in the range 20 to 40 MPa. In contrast,

LOCALITY I

2

3

Olff*~*~tl~l Sfr*Sl

(MPa)

IO

20

Fig. 10. Summary of stress magnitudes computed Fig. 4, Table I and Table II, using the stresslgrainsize

40

60

80

100

from recrystallized grainsize relationship of Twiss (1977).

200

data in

502

most of the mylonites within discrete zones indicate higher stress magnitudes between 50 and 150 MPa, with the exception of the Woodroffe-Davenport data which spans both ranges. The difficulty of interpreting these stress values is clear from the Woodroffe-Davenport data. There is a close correlation between calculated stress and rock composition (esp. degree of pre-mylonite hydration), and there is no correlation between stress and position with respect to the two thrust planes. It thus seems that the variation in recrystallized grainsize in this case is merely reflecting the effect of hydrous species or other chemical defects on the rheology of quartz and feldspar. The effect of rock chemistry can be partly eliminated by comparing thrust zones within the same rock unit, such as the Heavitree Quartzite. Taken at face value, the grainsize data in Fig. 4 indicate that: (1) stress is constant across an individual zone; and (2) calculated stress magnitude range from 40 MPa at Atnarpa A to about 130 MPa at Ormiston Gorge, with even higher, but less reliable values up to 160 MPa at Atnarpa E. The first of these observations is consistent with a model involving relaxation of stress gradients during steady-state plastic flow. However, it does not necessarily follow that the computed stress variation between zones reflect real palaeostress differences, since the impurity (esp. hydrous species) content of the quartz may very from zone to zone during mylonitization. In the Atnarpa area, Yar Khan (1972) has shown that seven thrust sheets of Heavitree Quartzite and associated rocks are superimposed, and our data from localities A to E indicate that there is no correlation between recrystallized grainsize and structural levels within this pile. Yar Khan suggested that the microstructural transition from northeast to southwest (A to E) was due to decreasing grade of metamorphism in that direction. There are no mineral assemblages which enable precise temperature determination, but, since the supposedly highest grade rocks are at most lower greenschist facies, any temperature difference from A to E must not have exceeded about 200°C. The large difference in calculated stresses from A to E would thus require a much higher temperature sensitivity of the stresslgrainsize relationship than is suggested by any of the existing experimental data (cf. Mercier et al., 1977). The total thickness of the thrust pile at Atnarpa is only about 1 km (Yar Khan, 1972), and the independence of calculated stress and position in the pile argues against a simple model of increasing frictional resistance with depth. It is more likely that the response to thrusting was controlled by an aqueous fluid phase, either simply via the effective pressure at the more brittle extreme, or by controlling the rheology of the mylonitic rocks. The systematic increase in calculated stress from Atnarpa (A, B, C, D) to Damper Gorge to Ormiston Gorge can be correlated with an increase in the dip of the thrust zones (Wilkie, 1979). According to Majoribanks (1974), the increase in dip is expected at deeper structural levels during thrusting, suggesting that shear stress may have increased with depth. The finely recrystallized mylonites within the basement Chewings Range Quartzite might thus

be expected to represent still higher stress intensities. However, preliminary infrared spectroscopic analyses of unmylonitized Heavitree and Chewings Range Quartzites from outside mylonite zones suggests that the latter has a much lower “gel” water content (Kekulawala et al., 1978; D. Mainprice, pers. commun., 1979). The finer grainsize in the Chewings Range Quartzite may thus simply reflect the lower water content, as found in the WoodroffeDavenport thrust zones. The data from the foliated granites also predict systematic changes in stress magnitude with structural setting (Table II, Fig. 10). In most cases, recrystallization associated with the regional foliation suggests lower stresses than that associated with discrete mylonite zones. In some localities, there is no obvious crosscutting relationship or apparent age difference between these, and the predicted stress concentration within discrete zones of 1.5 to 2 times may be real. At Crackenback, both the foliated granite and mylonite microstructures are overprinted by a deformation episode which is associated with higher level, more recent movement. The recrystallization associated with this episode predicts still higher stresses (Fig. lo), but it is difficult to know whether these values are realistic, or whether they reflect the influence of depth (through P, T, aH$), a,, etc.) on recrystallized grainsize. Perhaps the most convincing evidence for a direct stress control on recrystallized grainsize is shown in Fig. 11. A single original quartz grain in one of the foliated granites is increasingly constricted between two large, rigid plagioclase grains. The gradient in finite strain on this scale must surely reflect a gradient in stress, just as the recrystallized grainsize would predict. Finally, it is instructive to compare the range of values predicted by this work with crustal stress magnitudes derived from other sources. McGarr and Gay (1979) have recently reviewed the “direct” measurements of stress in the upper parts of the crust from techniques such as overcoring and hydrofracture. Even though the deepest measurement is only at 5 km, there is some evidence that stress magnitudes are levelling off at 20 to 40 MPa below about 2 to 3 km. This value is consistent with the data from the regionally foliated granites, which are most likely to reflect the effects of “normal” erogenic stresses. It is also significant that the highest values recorded in this study (approx. 100 MPa) are somewhat lower than the widely quoted values for frictional shear resistance during stick-slip on a range of discontinuities. If the calculated stresses are realistic, either (1) the laboratory experiments have so far been unable to reproduce the conditions within seismogenic fault zones (e.g., the role of a fluid phase in diffusional processes), or (2) the stresses in the QP regime (Sibson, 1977) are lower than in the seismogenic regime. There have .been a number of recent stress computations from dislocation density, subgrain size and recrystallized grainsize in quartzbearing mylonite zones (Burg and Laurent, 1978; Weathers et al., 1979; White, 1979a, b, c). In all cases the stress predicted by dislocation density is higher than that derived from grainsize measurements for reasons that are unclear. The range

Fig. 11. Photomicrograph showing a single original granitic quartz grain progressiveI> increasingly constricted (from right to left) between two rigid feldspar grains. The recrystallized grainsize is noticeably finer between the feldspar grains. where the stresss magnitude was presumably higher. Scale bar is 0.2 mm.

of measured recrystallized grainsizes is the same as reported here, and Weathers et al., (1979) also found little or no variation in grainsize across systematically sampled shear zones, provided that the measured aggregates are essentially single phase. However, it is apparent that some shear zones have deformation histories and assemblages of rock types that may be too complex to allow satisfactory interpretation of dislocation creep parameters in terms of stress, and that limited sampling from a single locality may not be sufficient to establish the steady-state parameters that equilibrated with the bulk of the mylonitization (compare Moine Thrust data of Weathers et al., 1979 and White, 197913, c). It is clear from this discussion that stress magnitudes computed from recrystallized grainsize must be used with caution, even where the pre-mylonitic parent rock is common to a number of zones and the structural control is well established. Greater reliability of stress grainsize relationships will only follow from improvements in our understanding of the rheology of the common crustal minerals. However, there are some samples where the predicted variations in stress magnitude are at least consistent with other structural predictions, and the technique is promising, providing that improved stress/grainsize relationships can be formulated.

505

CONCLUSIONS

(1) The concept of a steady-state recrystallized grainsize during deformation of a single phase aggregate by dislocation creep appears to apply to a wide variety of deformed rocks. In most mylonite zones studied, this grainsize is independent of finite strain, although it may not be uniquely related to stress. In particular, it may be sensitive to chemical environment and temperature. (2) The Twiss (1977) models for relating subgrain and recrystallized grainsize to differential stress predict stress magnitudes that are broadly consistent with other constraints, but have some deficiencies. First, they are static models and contain no reference to the roles of chemical environment or temperature, since these variables have largely kinetic effects. However, more mechanistically realistic models may take kinetic effects into account. Second, they do not correctly predict the difference in grainsize between quartz and feldspar in the same rock. (3) A stress/grainsize relationship based either on a realistic model of recrystallization and/or on a range of careful deformation experiments is a potentially powerful tool for determining palaeostress intensity. The uniformity of grainsize within most mylonite zones and the ease with which accurate grainsize measurements can be made emphasize the usefulness of such a relationship, but it seems pointless to make further extensive measurements until its precision is improved. (4) The most serious practical restriction to applying an improved stress/ grainsize relationship is the need to measure grainsize only in single phase aggregates. Rocks that contain more than one phase intermixed on the scale of the recrystallized grains are unsuitable for grainsize measurement. (5) Differential stress magnitudes calculated from the Twiss stress/grainsize equation for quartz are consistent with general crustal stress levels of a few tens of MPa, rising to the order of 100 MPa in mylonite zones of quite different geometries and tectonic settings. In the lower temperature regime (220-35O”C), faults ranging from horizontal to vertical, and having a variety of displacement vectors all predict stresses between 60 and 150 MPa. We content that these values are of the correct order, but that values from individual zones may be significantly in error. ACKNOWLEDGEMENTS

This research project was supported by funds from the U.S. Department of the Interior, Geological Survey, contract no. 14-08-001-16845, as part of the earthquake hazards reduction program. We would like to acknowledge Peter Kolbe (University of Sydney), Ken Collerson (University of Adelaide) and Tim Bell (James Cook University of North Queensland) for providing specimens, and to thank Bruce Hobbs, Graham Edward, Tim Bell, Alec McLaren and Dave Kohlstedt for many fruitful discussions. The study owes a great deal to Rob Twiss, who questioned the significance of the Woodroffe

506

thrust data in discussions in 1977, and thus prompted a broader study of recrystallized grainsize in a variety of mylonite zones. This paper was presented in poster form at the conference on “Deformation in Rocks” at Gottingen, April 1980, and M.A.E. would like to acknowledge travel funds provided by the conference organizers and Monash University. REFERENCES Ardell, A.J., Christie, J.M. and Tullis, yuartz rocks. Cryst. Lattice Defects,

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