1

Moine thrust zone mylonites at the Stack of Glencoul: I - microstructures, strain and 1 influence of recrystallization on quartz crystal fabric development 2

3 R.D. Law1, D. Mainprice2, M. Casey3§, G.E. Lloyd3, R.J. Knipe3, B. Cook1* & J.R. Thigpen1 4

5 6

1. Department of Geosciences, Virginia Tech., Blacksburg, VA 24061, USA 7 (e-mail address: rdlaw@vt.edu) 8

2. Géosciences Montpellier UMR CNRS 5243, Université Montpellier II, 34095 Montpellier, 9 France 10

3. School of Earth and Environment, The University, Leeds LS2 9JT, UK 11 12

§ Deceased 13 * Now at: Department of Earth and Environmental Sciences, University of Kentucky, Lexington, 14

Kentucky 40506, USA 15 16

17 Abstract: Since the early descriptions published by Callaway in 1884, the gently dipping 18

mylonites exposed along the Moine thrust at the Stack of Glencoul have drawn generations of 19

geologists to the northern part of the Assynt district. These mylonites, derived from Cambrian 20

quartzites (footwall) and Moine pelites and psammites (hangingwall), have figured prominently 21

in: a) early research into the influence of crystal plastic deformation and recrystallization on 22

microstructural and crystal fabric evolution, b) debates on the kinematic interpretation of macro- 23

and micro-structures and crystal fabrics and, c) debates on the tectonic significance of such 24

kinematic data. In this paper we first briefly review the historical aspects of this research and 25

then, using both previously published and unpublished data, document the finite strain and quartz 26

fabric development at this classic mylonite locality. Tectonic interpretation of these data, 27

together with quantitative estimates of flow vorticities associated with mylonite formation at the 28

Stack of Glencoul, are presented in a companion paper by Law (this volume). 29

30

end abstract 31

32

The Moine thrust zone (Fig. 1) consists of a series of east-dipping thrusts sheets made up of 33

Archean (Lewisian) gneiss, Proterozoic (Torridonian) metasediments and Lower Cambro-34

Ordovican shelf sediments that were thrust to the west-northwest over a foreland sequence of 35

similar stratigraphic units during the Caledonian orogeny between ca 437-400 Ma (see review by 36

Strachan et al. 2002; Butler 2009). Although there is considerable along strike variation, 37

2

deformation within the thrust zone typical varies from a brittle nature (shallow crustal levels) 38

along the thrusts cropping out in the west (foreland) to a more ductile nature (crystal plasticity of 39

quartz at deeper crustal levels) in the east (see review by Knipe 1990). The Moine thrust (sensu 40

stricto) is structurally the highest thrust within the thrust zone; it crops out furthest to the east, has 41

traditionally been considered to be the oldest of these thrusts (but compare discussions by Peach 42

et al. 1907, p. 471-473; Elliott & Johnson 1980; Butler 2004, this volume) and carries 43

Proterozoic age metasediments of the Moine nappe westwards over rocks of the thrust zone 44

and/or foreland (Fig. 1). Particularly in the northern part of the thrust zone, the Moine thrust is 45

often spatially associated with thick zones of mylonite (meters to tens of meters) in its immediate 46

footwall and/or hangingwall. These mylonites have been the subject of numerous microstructural 47

and crystal fabric studies over the last century, with the majority of studies taking place since the 48

mid-1950s (see review by Law & Johnson this volume). 49

50

Arguably the most spectacular exposures of mylonite found anywhere along the length of the 51

Moine thrust zone are located at the Stack of Glencoul in the northern part of the Assynt region 52

(Fig. 1). Here mylonites are derived from both pelitic Moine metasedimentary rocks and 53

Cambrian quartzites (Fig. 2). These platy or slab-like mylonites are characterized by a strongly 54

developed foliation, which dips gently to the ESE, and a weakly developed grain shape stretching 55

lineation which plunges down the dip of the foliation planes sub-parallel to the Moine thrust zone 56

transport direction. Temperatures of mylonite formation are estimated at 300-350° C (Johnson et 57

al. 1985). Microstructural and crystal fabric data from the Stack of Glencoul mylonites have 58

historically played important roles in debates on: a) deformation mechanisms responsible for 59

mylonite formation, b) kinematic interpretation of fabric symmetry (e.g. the simple shear versus 60

pure shear debate) and, c) if departures from strict simple shear can occur in natural deformation - 61

and the tectonic implications if this does occur. Because of its international significance to 62

research, the Stack of Glencoul is a nationally designated Site of Special Scientific Interest (see 63

review by Mendum et al. 2009), and is one of the most frequently visited sites for undergraduate 64

teaching within the entire Moine thrust zone. Visitors should not hammer these outcrops or 65

collect from either the outcrops or from the mylonite talus. 66

67

3

In this paper we briefly review published microstructural and crystal fabric work on the 68

Stack of Glencoul mylonites and then integrate this work with previously unpublished strain and 69

quartz crystal fabric data (both single grain optical and bulk sample X-ray diffraction), 70

particularly concentrating on the role of dynamic recrystallization in fabric development. In a 71

companion paper (Law this volume) these strain and fabric data are used to quantitatively 72

estimate flow vorticities associated with mylonite formation using analytical techniques that have 73

only been developed over the last 10-15 years and postdate previously published work on the 74

Stack mylonites. This companion paper concludes with a discussion of the structural and 75

tectonic implications of the integrated strain and vorticity data, and is followed by a detailed 76

SEM-based microstructural, petrofabric and seismic anisotropy study of one of our mylonitic 77

Cambrian quartzites samples, using the electron back scattered diffraction (EBSD) technique 78

(Lloyd et al. this volume). 79

80

Historical background to structural/tectonic significance of Stack mylonites 81

The microstructures in the mylonitic Cambrian quartzites at the Stack of Glencoul were first 82

described by Callaway (1883, 1884) who noted both the flattening of detrital grains and 83

distortion/lengthening of "worm burrows" (skolithos) which he perceptively interpreted as being 84

caused by "a force pushing from the east" (Callaway 1884, p. 221). Callaway concluded that 85

these intensely deformed quartzites had "... undergone great compression, the fragments being 86

crushed, flattened out, and 'packed' together as one sees in slates" (Callaway 1884, p. 221). 87

Interestingly, this clear microstructures-based interpretation of deformation processes in the Stack 88

of Glencoul tectonites preceded by one year publication of the internationally far more well 89

known and influential paper by Lapworth (1885) describing similar deformation processes 90

associated with deformation of Lewisian gneiss above the Ben Arnaboll thrust in the Loch Eriboll 91

part of the Moine thrust zone (Fig. 1). It was in this paper that Lapworth coined the name 92

"mylonite" for rocks produced by such grain-scale processes of crushing and fracture (see 93

reviews by Teall 1918, p. 1-3; White 1998, this volume; Law & Johnson this volume). 94

95

Following the early (but very limited) description by Callaway, no further microstructural 96

work was carried out on the mylonites exposed at the Stack of Glencoul until the early 1950s 97

PhD work by John Christie at Edinburgh University on the microstructures and crystal fabrics of 98

4

mylonites in the Assynt region - that included the Cambrian quartzites and overlying Moine rocks 99

at the Stack of Glencoul (Christie et al. 1954; Christie 1956, 1960, 1963). This seminal work 100

was of historical importance for: (a) recognizing that ribbon-like quartz grains - particularly in 101

the Cambrian quartzites at the Stack Glencoul - were due to high strain plastic deformation 102

(Christie 1963, p. 405 and 439), (b) recognizing that the extremely small (typically < 15 micron) 103

equant quartz grains in these mylonites were the result of recrystallization during thrusting 104

(Christie et al. 1954, p. 220; Christie 1960, p. 90; Carter et al. 1964) rather than fracture 105

processes (cataclasis) and, (c) documenting the high degree of quartz crystallographic preferred 106

orientation in the mylonites (Christie 1963) which on microstructural criteria were clearly 107

associated with this plastic deformation and dynamic recrystallization. These papers helped to 108

establish a paradigm shift in the early 1970s from a previous understanding that mylonitization is 109

primarily associated with cataclasis to the present day view (based on microstructural similarities 110

with deformed metals) that mylonites are associated with crystal plastic deformation and dynamic 111

recrystallization (Bell & Etheridge 1973; Tullis et al. 1973; White 1973). 112

113

However, a potential problem existed concerning the timing of crystal fabric development 114

relative to plastic deformation and dynamic recrystallization. This was because the quartz c-axis 115

fabrics which were optically measured by Christie in these mylonites - and particularly those 116

from the large, relatively easily measured, relict detrital grains in the Cambrian quartzites at the 117

Stack Glencoul - displayed a high degree of symmetry with respect to foliation and lineation. 118

Christie (1963, p.405-406) argued that these symmetric fabrics must indicate a late (post 119

thrusting) coaxial flattening superimposed on original asymmetrical fabrics that one would 120

intuitively expect to be associated with thrust-related shearing. Although not explicitly discussed 121

at the time, this interpretation was clearly in conflict with the microstructural evidence for plastic 122

deformation (which must be responsible for the fabrics measured by Christie in the relict detrital 123

grains) and recrystallization being syntectonic (Christie et al. 1954, p. 220, Christie 1960, p. 90). 124

125

Historically, the high symmetry crystal fabrics reported by Christie (1963) from the Moine 126

thrust mylonites have played a prominent role in later debates on whether fabric symmetry can be 127

used as at least a qualitative kinematic (vorticity) indicator (e.g. simple shear versus pure shear 128

deformation) and if departures from strict simple shear occur in natural deformation. For 129

5

example, Johnson (1967) has argued that the symmetric (orthorhombic) fabrics in the Stack 130

mylonites, taken in conjunction with strain measurements on highly deformed Skolithos worm 131

burrows (pipes) in the mylonites, indicate a vertical "flattening" (0<k<1 on the Flinn diagram) 132

deformation that "may not necessarily be related to any large scale horizontal translative 133

movement" (i.e. thrust related shearing). Although the terminology used is different from that 134

employed today, Johnson perceptively argued that, unless mylonitization was accompanied by 135

volume loss, this "flattening" (i.e. non simple shear) deformation would in itself result in 136

displacement of material within the mylonite zone relative to surrounding undeformed rocks 137

(Johnson 1967, p. 247). Such arguments would later be used by other workers both as evidence 138

for and against ductile extrusion as a viable tectonic processes (cf. Ramsay & Huber 1987, p. 139

611-613; Law et al. 2004; Grasemann et al. 2006; Williams et al. 2006; Xypolias & Kokkalas 140

2006; Xypolias et al. this volume). 141

142

A potentially closely related problem here is that the internides of many orogenic belts are 143

characterized by flat-lying or gently dipping foliations oriented sub-parallel to the boundaries of 144

nappes or thrust sheets; this is also true of the mylonite zones associated with the Moine thrust 145

and overlying Moine nappe. As recently discussed by Ring and Kassem (2007), production of 146

flat-lying foliations by horizontal simple shear deformation requires extremely high shear strains 147

throughout the entire thickness of the nappe pile, and yet quantitative strain studies commonly 148

indicate that strain magnitudes are far lower than required to produce such flat-lying foliations in 149

simple shear (Simpson & DePaor 1993, 1997; Kassem & Ring 2004). Additionally, estimated 150

finite strains in such tectonites frequently depart from plane strain (k=1) deformation that should 151

be produced in strict simple shear (Hossack 1968; Dayan 1981; Law et al. 1984, 1986). This 152

suggests that nappe emplacement - at least in the lower ductile crust - most likely occurs by a 153

combination of simple shear with a pure shear component of vertical flattening/shortening. The 154

vertical pure shear component could in theory predate, be synchronous with, or postdate the 155

thrust-related simple shearing. Regardless of timing, however, a component of vertical pure 156

shear shortening must, as pointed out by Johnson (1967) and later workers, result in horizontal 157

stretching (extrusion) with attendant space problems that must be compensated for by processes 158

such as volume loss or extrusion towards the syn-orogenic topographic surface, or by linkage to 159

other structural processes operating at shallower crustal levels. Stated differently, significant 160

6

components of pure shear deformation at deeper crustal levels may act as drivers for synchronous 161

structural processes operating up transport direction at shallow crustal levels. 162

163

This brings us back to the question of when the symmetric c-axis fabrics measured by 164

Christie (1963) on relict detrital quartz grains in the flat-lying Stack mylonites developed relative 165

to thrusting. Here it needs to be emphasised that no account was taken of potential domainal 166

partitioning of fabric development (at either the thin section or outcrop scale) in the seminal 167

paper by Christie (1963). Subsequent X-ray texture goniometry work by Riekels (1973), Riekels 168

& Baker (1977) and Baker & Riekels (1977) on a sample of mylonitic Cambrian quartzite 169

collected by Christie (1956, 1963, his sample 62) from the Stack of Glencoul produced 170

asymmetric crystal fabrics which contrasted with the symmetric fabric measured optically by 171

Christie (1963) on large detrital grains in the same sample. They attributed this discrepancy to 172

the difficulty of measuring c-axes of very small (< 10-15 microns) recrystallized quartz grains 173

(which could form the asymmetrical element of a bulk fabric pattern) with the optical 174

microscope. This opens up the possibility of a thin section to grain scale strain path (and hence 175

fabric) partitioning between the large relict detrital grains and the surrounding fine grained 176

dynamically recrystallized matrix in the mylonites. 177

178

Following these landmark papers critically important evidence for outcrop scale strain path 179

partitioning in the mylonitic Cambrian quartzites at the Stack of Glencoul was described by Law 180

et al. (1986) who in a vertical transect through these mylonites documented a profound change in 181

microstructures and both optically measured c-axis fabrics, and a-axis fabrics measured by X-ray 182

texture goniometry, with distance beneath the overlying Moine thrust (Fig. 3). Close to the 183

thrust, non-coaxial deformation was indicated by asymmetrical c- and a-axis fabrics, the sense of 184

asymmetry in these more highly deformed and recrystallized quartzites being consistent with 185

WNW directed over-thrusting. At distances of greater than 30 cm beneath the thrust, essentially 186

coaxial deformation was qualitatively indicated by c-axis fabrics (measured on detrital [old] 187

grains) and a-axis fabrics which were symmetrical with respect to foliation and lineation. Law et 188

al. (1986) argued that formation of the asymmetrical fabrics (non-coaxial deformation) in the 189

immediate footwall to the thrust must have been either contemporaneous with, or later than, 190

formation of the symmetrical fabrics. This is in marked contrast to the original interpretation by 191

7

Christie (1963, p. 405) that the symmetric fabrics indicated a relatively late period of vertical 192

coaxial shortening overprinting asymmetric fabrics produced during thrust related shearing; see 193

also discussion by Lister & Williams (1979, p. 292-293). Almost simultaneously with 194

publication of these results, similar fabric-based evidence for strain path partitioning within thrust 195

sheets began to be independently recognised in other orogenic belts (e.g. Betic Cordillera of 196

Spain; Platt & Behrmann 1986). 197

198

Although the work reported by Law et al. (1986) arguably represented some advance on the 199

classic paper by Christie (1963), much remained to be done on the Stack mylonites. 1) no 200

quantitative three dimensional strain analysis had been carried out on these mylonites, although 201

strain magnitude estimates had previously been attempted using deformed Skolithos burrows 202

(Pipe Rock) exposed at one horizon in the Cambrian quartzites and assuming either strict pure or 203

simple shear (McLeish 1971; Wilkinson et al. 1975). 2) no microstructural data and only limited 204

crystal fabric data had yet been obtained from the mylonitic Moine rocks which have been thrust 205

over the Cambrian quartzites. 3) although asymmetric crystal fabrics from new (i.e. dynamically 206

recrystallized) grains had been measured in the dominantly recrystallized quartzites located close 207

(< 30 cm) to the overlying Moine thrust, only optically measured c-axis fabrics had been 208

measured on relict detrital grains (symmetric fabrics) in the quartzites at greater distances beneath 209

the thrust. The possibility of thin section scale strain path partitioning between these large relict 210

detrital grains and the surrounding matrix of dynamically recrystallized grains remained to be 211

explored. 4) At that time only qualitative kinematic indicators (fabric symmetry) were available 212

for assessing likely flow vorticities (as potential simple or pure shear end members) and 213

quantitative techniques for determining (at least time averaged) vorticities in terms of vorticity 214

numbers (e.g. Wallis 1992, 1995; Simpson & De Paor 1993, 1997; Wallis et al. 1993) had yet to 215

be developed. These tasks are addressed in this paper and the companion paper by Law (this 216

volume). 217

218

Structural setting and sampling profile of mylonites exposed at Stack of Glencoul 219

The tectonic junction taken to represent the Moine thrust (sensu stricto) at the Stack of Glencoul 220

has been the subject of some controversy. The foliation-parallel ductile contact (Figs. 4a-c; see 221

also photographs in Law et al. 1986, figs. 2 and 3a, b; Law 1998, p. 490-491; Howarth and Leake 222

8

2002, p. 66) between the mylonitic Cambrian quartzite and similarly deformed overlying Moine 223

metasediments (and ?Lewisian rocks) was regarded by Peach et al. (1888, p. 417), C.T. Clough 224

(in Peach et al. 1907, p. 503), Christie (1956, 1963, p. 363; 1965), Weathers et al. (1979), 225

Coward (1983), Law et al. (1986) and Law (1987, 1998) as marking the position of the Moine 226

thrust. However, Johnson (1965) considered the Moine thrust (sensu-stricto) to be a late brittle 227

feature, thus creating the necessity to place the thrust at the base of the mylonitic Cambrian 228

quartzites in the unexposed ground between these strongly deformed tectonites and the 229

underlying, relatively weakly deformed, Cambrian quartzites (Johnson in Macgregor & 230

Phemister 1972, p. 63). This structural position has been followed by McLeish (1971), 231

Wilkinson et al. (1975), Johnson & Parsons (1979) and Elliott & Johnson (1980). Here we will 232

adopt the former (and historically earlier) definition of the Moine thrust at the Stack of Glencoul 233

(Fig. 2). We do agree, however, that a potentially important - but unexposed - gently dipping 234

fault probably marks the base of the mylonitic Cambrian quartzites and separates these intensely 235

deformed rocks from underlying Cambrian quartzites which have only a very weakly developed 236

grain shape fabric (Law et al. 1986, their figs. 4a and b). 237

238

Both the mylonitic Cambrian quartzites and overlying Moine schists at the Stack of Glencoul 239

(Fig. 2) are characterized by a strongly developed foliation, which dips gently (16-30°) to the 240

ESE, and a rather weak grain shape stretching lineation which plunges down the dip of the 241

foliation planes parallel to the transport direction inferred from thrust geometries. These platy or 242

flaggy mylonites are typically S>L tectonites. The oriented samples described in this paper were 243

collected in the early 1980s (before designation as a Site of Special Scientific Interest) in a 244

vertical traverse of mylonitic Moine and underlying Cambrian rocks from the northwestern crags 245

of the Stack of Glencoul (Fig. 2). The mylonitic quartzites contain at one horizon (Fig. 2; 246

specimen SG8) intensely deformed Cambrian Pipe Rock (Law et al. 1986, their fig. 3c). 247

248

Microstructures of the mylonites 249

The microstructures of the mylonitic Cambrian quartzites and Moine rocks at the Stack of 250

Glencoul were first described in detail by John Christie (1956, 1963) in his thesis work on the 251

Assynt mylonites. As outlined above, Christie recognised that the intense internal straining and 252

ribbon-grain development of quartz grains in these mylonites (Fig. 5b & c) was due to crystal 253

9

plastic processes, and was also amongst the first geologists world-wide to recognize that the 254

small (< 15 micron) equant quartz grains (e.g. Fig. 5a) in such tectonites were due to dynamic 255

recrystallization rather than cataclasis (Christie 1960; Carter et al. 1964). 256

257

Details of variation in microstructures of the Cambrian quartzites with depth beneath the 258

Moine thrust have been given by Weathers et al. (1979), Law et al. (1986) and Law (1987, 1998). 259

At distances greater than 40 m beneath the thrust individual detrital quartz grains are slightly 260

flattened and in XZ sections (cut perpendicular to weakly developed foliation and parallel to 261

lineation) display aspect ratios ranging between 2:1 and 4:1 with undulose extinction, 262

deformation bands and sub-basal deformation lamellae. Recrystallization of quartz is of a very 263

minor nature (< 5%), being confined to a few deformation bands and detrital grain margins. 264

265

Due to lack of exposure on the northwestern side of the Stack of Glencoul a sampling gap 266

exists between the weakly deformed quartzites located at greater than 40 m beneath the thrust and 267

the intensely deformed quartzites (Figs. 2 & 4) located at less than 9.6 m beneath the thrust. 268

Within these strongly developed S>L and L-S tectonites mylonitic foliation is defined in thin 269

section by a preferred alignment of flattened relict detrital quartz grains. In XZ sections these 270

flattened grains display variable aspect ratios both within individual thin sections (Figs. 5b & c) 271

and from one specimen to another. Feldspar and epidote clasts are present as a minor grain 272

fraction (< 5%) in these mylonites; both appear to have acted as mechanically rigid phases 273

around which trails of recrystallized quartz anastomose. 274

275

No convincing systematic increase in degree of quartz grain flattening, when traced towards 276

the overlying Moine thrust, has been detected within the quartz mylonites collected at between 277

9.6 m and 0.3 m beneath the thrust. Ribbon-like quartz grains are observed in all these 278

specimens, typically displaying aspect ratios of between 50:1 and 100:1 with long dimensions 279

commonly measuring 2-3 mm in XZ sections. At least some of these ribbon grains may be 280

plastically deformed veins; their cross sectional areas are typically many times larger than 281

surrounding deformed detrital grains (Fig. 5b). In other cases it can be demonstrated that ribbon 282

grains with smaller cross-sectional areas have been produced by kink bands in detrital quartz 283

grains developing parallel to foliation. Preferential recrystallization along the boundaries of these 284

10

deformation bands has then separated the deformation bands in these individual detrital grains, 285

producing foliation-parallel relict grains (ribbons) which give a false impression (overestimate) of 286

the degree of grain flattening (Fig. 5b). Globular quartz grains whose c-axes are either oriented 287

sub-parallel to the sample lineation, or (in rare cases) at a low angle to the foliation pole, have 288

been observed within all these mylonites; more flattened grains anastomose around these globular 289

grains (Fig. 5b & c). These globular quartz grains were interpreted by Law et al. (1986) as 290

indicating that a significant component of coaxial (pure shear) deformation was associated with 291

formation of these mylonites. In contrast, Bell & Johnson (1990, p. 443) argued that the 292

consistently oriented large and rigid quartz grains in the Stack of Glencoul mylonites provided 293

supporting evince for their controversial suggestion that porphyroblasts do not rotate during non-294

coaxial deformation. 295

296

The volume fraction of quartz recrystallization within the mylonitic Cambrian quartzites 297

varies from 40-75%. The recrystallized grain size appears to remain fairly constant at 298

approximately 10-15 microns (at least in XZ thin sections). Recrystallized grains are fairly 299

equant in XZ thin sections, but more elongate (sub-parallel to foliation) in YZ sections, 300

suggesting a tube-like 3D recrystallized grain shape oriented parallel to the sample Y direction. 301

This tube-like grain shape is also indicated by the fact that it is possible to measure optically the 302

c-axis orientation of many 15 micron diameter grains in XZ thin sections of standard 30 micron 303

thickness; see also discussion by Strine & Wojtal (2004) on 3D recrystallized grain shapes in 304

Moine thrust zone mylonites exposed in northern Assynt. 305

306

At distances less than 0.15 m beneath the Moine thrust, quartz recrystallization is more 307

advanced (locally 60-100%) than at greater distances beneath the thrust, with relict detrital grains 308

only locally being preserved. At the contact between the mylonitic quartzites and the overlying 309

Moine rocks (central part of specimen SG-1; Fig. 4b) the quartzite is almost totally recrystallized 310

and mylonitic foliation is defined in XZ thin sections by a preferred alignment of highly elongate 311

domains of recrystallized quartz grains of similar crystallographic orientation. Within these 312

domains, individual elongate recrystallized quartz grains display in XZ thin sections a preferred 313

alignment (Sb) which is oblique to the mylonitic foliation (Sa), the sense of obliquity being 314

compatible with the top to the WNW shear sense inferred from regional thrust geometries (Fig. 315

11

5a). The foliation, as seen in thin section, appears to be parallel (+/- 2°) to both the Cambrian 316

quartzite -Moine contact (i.e. the Moine thrust) and the foliation in the overlying phyllosilicate-317

rich Moine rocks (Fig. 4; see also micrographs in Law et al. 1986, fig. 3b; Knipe 1990, fig. 9.3; 318

Law 1998, p. 491). At this locality the Moine thrust dips at 20° towards 110°, the grain shape 319

lineation within the foliation plunging at 18° towards 118° in both the Cambrian quartzite and 320

Moine rocks. 321

322

Foliation in the overlying mylonitic Moine pelites and psammites is defined by a preferred 323

alignment of phyllosilicates (chlorite and white mica) and minor planar domains of dynamically 324

recrystallized quartz (grain size less than 10 microns). Foliation locally anastomoses around 325

equant-elongate clasts of felspar, epidote and opaques, the clasts occasionally displaying either 326

sigma or delta tails. Within the Moines mylonitic foliation becomes more intensely developed 327

(platy) traced downwards from the summit of the Stack of Glencoul to the underlying Moine 328

thrust plane. At the summit a penetrative foliation is intensely folded about variably oriented fold 329

hinges. Traced downwards these folds tighten and are transposed into the platy mylonitic 330

foliation which is oriented parallel to the underlying thrust plane (Figs. 4b & c). This platy 331

foliation is locally deformed in to WNW verging minor folds (cm scale), picked out by foliation-332

parallel plastically deformed quartz veins, with long limbs oriented parallel to the sheet dip of the 333

foliation (Fig. 6). Alternating successions of sub-horizontal and sub-vertical foliations in the 334

Moine mylonites exposed at the Stack of Glencoul have recently been described by Bell (this 335

volume) and interpreted in terms of successive phases of sub-horizontal and sub-vertical 336

shortening associated with thrusting driven by gravitational collapse (see also Bell & Johnson 337

1989). 338

339

Temperatures of mylonite formation at the Stack of Glencoul were estimated at 300-350 °C 340

by Johnson et al. (1985) based on illite crystallinity; see also review by Johnson & Strachan 341

(2006). The quartz recrystallization mechanism within the detrital grains of these mylonites, 342

however, is dominantly subgrain rotation (Law et al. 1986), referred to as Regime 2 343

recrystallization by Hirth and Tullis (1992). Adopting the microstructural thermometer proposed 344

for quartz by Stipp et al. (2002; p. 175) such Regime 2 recrystallization would indicated 345

deformation temperatures of c. 400-500 °C at natural strain rates. These inferred deformation 346

12

temperatures are clearly higher than the temperatures of metamorphism estimated from illite 347

crystallinity and, because recrystallization regimes are also sensitive to variations in hydrolytic 348

weakening and strain rate, could indicate higher water contents or lower strain rates than are 349

encompassed in the Stipp et al. thermometer. However, recently published SEM-based 350

microstructural analysis by Halfpenny et al. (2006) of a sample of mylonitic Cambrian quartzite 351

from the Stack of Glencoul (equivalent in position to sample SG-10 of this study) has indicated 352

the potentially dominant role of grain boundary bulging recrystallization in formation of the 353

matrix new grains. Adopting the microstructural thermometer proposed by Stipp et al. (2002), 354

such Regime 1 recrystallization would indicated deformation temperatures in the c. 300-400 °C 355

range at natural strain rates. If both subgrain rotation (within detrital grains) and grain boundary 356

bulging (at detrital grain margins) were operating simultaneously this transitional behavior 357

between the two recrystallization mechanisms would indicate deformation temperatures of c. 400 358

°C using the Stipp et al. thermometer. Potential differences in hydrolytic weakening or strain 359

rate between the detrital (old) and matrix of new (recrystallizing) grains would complicate this 360

simple interpretation however. 361

362

Strain symmetry/magnitude of the mylonites 363

Strain analyses were performed on the mylonitic Cambrian quartzites using plastically deformed 364

relict detrital quartz grain shapes imaged on three mutually perpendicular thin sections cut 365

orthogonal to foliation and lineation. A minimum of 150 grains were measured in each section 366

plane. Rf/φ and harmonic mean grain shape/orientation data on individual section planes were 367

analyzed and cross-checked using software packages by Kanagawa (1992) and Chew (2003). For 368

data sets from individual thin sections, closely similar 2D strain estimates were obtained using 369

the Rf/φ and harmonic mean techniques in the two software packages. Best-fit strain ellipsoids 370

for individual samples were calculated using the Kanagawa (1992) software; within observational 371

error (less than 5°) macroscopic foliation and lineation was demonstrated to be parallel to the 372

principal planes of the best fit strain ellipsoids. 373

374

3D strain data from the mylonitic Cambrian quartzites in the immediate footwall to the 375

Moine thrust (Table 1) plot within the general flattening field (Lodes Unit υ of 0.25 - 0.50) (Figs. 376

7 & 8) with the intermediate principal strain axis oriented parallel to orogenic strike. Stretches 377

13

parallel to the X (transport direction parallel lineation) and Y (orogenic strike) principal strain 378

directions are estimated at 170-260% and 26-49% respectively, while shortening perpendicular to 379

foliation is estimated at 72-80%, assuming constant volume deformation (Table 1). This has 380

obvious space problem implications, and to explain as a true plane strain deformation (Ramsay & 381

Wood 1973) with no elongation parallel to orogenic strike would require a 50-70% volume loss 382

(Fig. 8), for which there is no clearly defined microstructural evidence - at least in the optical 383

microscope. 384

385

This general flattening strain is in agreement, however, with both outcrop scale structures 386

such as pinch and swell of quartzite layers exposed in joint planes oriented perpendicular to the 387

stretching lineation, and small circle girdle quartz a-axis fabrics (see below) obtained by X-ray 388

texture goniometry in the mylonitic Cambrian quartzites (Law et al. 1986, Law 1987). These 389

flattening strains are probably of regional extent. For example, both strain and petrofabric data 390

indicating flattening strains have recently been reported from Loch Strath nan Aisinnin (Fig. 1) 391

southwards to the Stack of Glencoul in quartz mylonites from the immediate footwall to the 392

Moine thrust (Strine & Mitra 2004; Strine & Wojtal 2004). 393

394

Arguments could be made for these strain analyses (which use relict detrital grain shapes) 395

either overestimating or underestimating strain magnitude. Inclusion of the ribbon grains in the 396

2D data sets (at least some of which may have originated as either veinlets or deformation bands 397

within larger detrital grains; Fig. 5b) could obviously lead to an overestimation of 2D strain ratio. 398

However, the influence of these ribbon grains on 2D strain estimates seems to be statistically 399

minimized in the software packages used. Similarly, dynamic recrystallization around the 400

margins of elliptical detrital grains (core and mantle structure) will in itself increase the aspect 401

ratio of the relict core (see Dayan 1981, p. 230-232, for numerical treatment), also leading to an 402

overestimation of 2D strain ratio. In contrast, if strain partitioning has occurred between the 403

relict detrital quartz grains and their surrounding matrix of dynamically recrystallizing quartz 404

grains, then the deformed detrital grain shapes may only record part of the total strain, thereby 405

leading to an underestimation of 2D strain ratio. 406

407

14

Our strain analysis of the mylonitic Cambrian quartzites at the Stack of Glencoul using 408

deformed detrital grain shapes has taken a 3D approach and indicates strain ratios in the XZ 409

principal plane within the Rxz = 10.3 - 18.9 range (Table 1), with an arithmetic mean of Rxz = 410

13.6 (nine samples). In general no convincing progressive increase in estimated Rxz values was 411

found traced towards the overlying Moine thrust plane, although the highest Rxz estimate (18.9) 412

was recorded in specimen SG-1 where the relict grain shape data (admittedly from largely 413

recrystallized quartzite) is from within a few centimeters of the thrust plane (Fig. 2). 414

415

Previously published strain analyses on the Stack of Glencoul mylonites have been 416

essentially 2D in nature and have focused on deformation of the Skolithos worm burrows 417

preserved in the single horizon of mylonitic "Pipe rock" in the Cambrian quartzites from which 418

our specimen SG-8 was taken (Fig. 2). Based on the elliptical outline of the deformed pipes 419

exposed on the foliation plane and the assumption that the long axes of the deformed pipes are 420

‘perfectly’ parallel to foliation and lithologic layering (bedding), McLeish (1971, p. 496) 421

estimated X/Y/Z strain ratios of 14.55/1.0/0.32, corresponding to an estimated Rxz value of 422

approximately 45. Wilkinson et al. (1975, their fig. 4) demonstrated that statistically the pipe 423

long axes are actually inclined at a very low angle (c. 2°) to the lithologic banding/foliation, 424

dipping more steeply (presumably to the ESE) than lithologic banding in section planes oriented 425

perpendicular to foliation and parallel to the stretching lineation. This geometric situation was 426

numerically modeled by Wilkinson et al. (1975) assuming simple shearing oblique to original 427

bedding (and orthogonal pipes). Combined data from two samples, incorporating observed 428

angles between lithologic banding/foliation and pipes and the elliptical outline of the deformed 429

pipes exposed on the foliation plane, were interpreted as indicating shear strains (γ) of 9 and 12 430

using their numerical modeling. These shear strains correspond to Rxz strain ratios of 80 and 431

146 respectively, assuming simple shear deformation. Both of these previously published studies 432

using deformed pipes as strain markers clearly predict far larger Rxz strain ratios than indicated 433

by our detrital grain-based strain analyses (Rxz values in the c. 10 - 20 range). The potential 434

importance of incorporating quantitative estimates of flow vorticity in numerical modeling of 435

Pipe rock shearing, and their implication for Rxz strain ratios associated with observed angular 436

shear strains in the pipe rock, will be discussed in the companion paper by Law (this volume). 437

438 Quartz crystal fabrics in the mylonites 439

15

Quartz crystallographic fabrics are exceptionally well developed at the Stack of Glencoul, 440

particularly in the mylonitic Cambrian quartzites. Optically measured c-axis fabrics from the 441

mylonitic quartzites were first described by John Christie in his 1956 University of Edinburgh 442

PhD thesis; these fabrics later became internationally renowned following publication of his 443

seminal 1963 paper by the University of California Press (Christie, 1963). These c-axis fabrics 444

were famous for their high degree of symmetry relative to foliation and lineation and, as outlined 445

above, were regarded by Christie (1963, p. 405) as indicating a relatively late stage period of 446

vertical coaxial shortening overprinting asymmetric fabrics produced during thrust-related 447

shearing. Unfortunately, no records were kept of the outcrop positions of these Cambrian 448

quartzite samples relative to the position of the Moine thrust at the Stack of Glencoul (J. Christie, 449

pers. comm. to R.D. Law in 1988). However, as outlined above, re-sampling of the Cambrian 450

quartzites at the Stack of Glencoul (Law et al. 1986) led to recognition of a major change in 451

quartz fabrics with depth beneath the Moine thrust (Fig. 3). 452

453

All quartz crystal fabrics reported in this paper (Figs. 9-14 and 20-21) are displayed on lower 454

hemisphere projections in which the projection plane is oriented perpendicular to foliation and 455

parallel to the ESE plunging stretching lineation; all fabric diagrams are viewed towards the 456

NNE. Where appropriate the orientation of shear bands (denoted by bars with arrows) and the 457

alignment (Sb) of elongate dynamically recrystallized quartz grains oriented oblique to foliation 458

is indicated on the fabric diagrams (Figs. 9, 10 & 11). 459

460

Fabrics in hangingwall Moine mylonites 461

Optically measured single-girdle c-axis fabrics within quartz-rich layers (veins ?) in the mylonitic 462

Moines pelites and psammites at the Stack of Glencoul consistently indicate a top to the WNW 463

shear sense (Fig. 9). Folded quartz veins in specimen M.3 located 5m above the Moine thrust 464

(Fig.2) yield opposite c-axis fabric asymmetries on adjacent fold limbs (Fig. 6) indicating that the 465

vein was folded by a flexual slip mechanism rather than acting as a passive marker within the 466

thrust zone (Law 1990). The non-uniform density distributions in these fabrics are probably due 467

to an original crystal preferred orientation in the vein material. In cross-polarised light with an 468

analyzer (sensitive tint) plate inserted these contrasting fabrics are indicated by different 469

polarization colors of dynamically recrystallized quartz grains on adjacent fold limbs when the 470

16

thin section is in a constant orientation relative to the polarizers. Opposite senses of obliquity 471

between foliation-parallel vein margins (Sa) and alignment (Sb) of elongate dynamically 472

recrystallized quartz grains within the veins are observed on adjacent fold limbs. By analogy 473

with simulation studies (e.g. Lister & Hobbs 1980) these fabrics, which were measured in 474

sections cut perpendicular to foliation and parallel to lineation, but oblique to fold hinges, also 475

indicate that penetrative deformation is associated with a bulk shearing direction contained in this 476

section plane rather than perpendicular to the fold hinges, as originally proposed by Christie 477

(1963, p. 382-4) for the Moine mylonites exposed at the Stack of Glencoul; see also discussion 478

between Johnson (1965) and Christie (1965) and historical review by Law & Johnson (this 479

volume). 480

481

Quartz grains within the matrix of the pelitic Moine mylonites are too small (< 10-15 482

microns) for optical fabric analysis. Intuitively quartz preferred orientation would be expected to 483

be weak to absent in these phyllosilicate-rich tectonites due to grain boundary sliding. However 484

X-ray texture goniometry of slab from sample M.2 (Fig. 10), situated at c. 10 cm above the thrust 485

plane, reveals that although the strength of quartz c-axis preferred orientation is low (maximum 486

of 1.5 times uniform density), the skeletal outline of the fabric is remarkably similar to fabrics 487

measured in quartz veins from the Moine mylonites (Fig. 9, samples M1 and M2) and 488

particularly the fabric in sample SG-1 (Fig. 10) from the underlying mylonitic Cambrian 489

quartzite. The c-axis fabric for sample M.2 consists of a broad, slightly kinked, single girdle 490

orientated oblique to foliation (sense of obliquity consistent with top to the WNW shearing) 491

oriented at a high angle to shear bands in the phyllosilicate-rich matrix (Fig. 10). The 492

corresponding a-axis fabric for sample M.2 consists of three point maxima, the dominant maxima 493

occupying a pole position to the c-axis girdle (Fig. 10). These X-ray derived fabrics indicate that, 494

although grain boundary sliding is undoubtedly important in the pelitic Moine mylonites, 495

combined dislocation creep and dynamic recrystallization is still capable of producing a preferred 496

crystallographic orientation within these small grains which form thin (less than 100 micron), 497

discontinuous foliation-parallel layers scattered throughout the phyllosilicate-rich matrix. 498

499

Fabrics adjacent to Moine thrust 500

17

Immediately adjacent to the thrust plane both the mylonitic Moines (samples M.1 and M.2 at 1.0 501

and 10 cm above thrust; Figs. 9 & 10) and Cambrian quartzite (SG-1 at 0.5 cm beneath thrust; 502

Fig. 11) are characterized by asymmetric single girdle c-axis fabrics indicating a top to the WNW 503

shear sense. The girdle is fairly straight in sample M.1 (and to a lesser extent in M.2), but 504

distinctly kinked in SG-1. In both samples M.1 and SG-1 there is a vestige of a trailing edge 505

fabric. This trailing edge fabric component strengthens traced downwards in the Cambrian 506

quartzites leading to production of asymmetric Type 1 cross girdle fabric in samples SG-2.1 to 507

2.5 (Fig. 11), but is less obvious traced upwards from the thrust in to the Moine mylonites. All 508

these fabrics are measured on dynamically recrystallized grains and are presumably from 509

tectonites that have undergone high shear strains, given their proximity to the thrust plane (see 510

Law this volume for estimation of γ values). 511

512

Numerical simulations predict that for simple shear the leading and trailing edges of cross-513

girdle quartz c-axis fabrics should develop perpendicular and oblique to the margins of a shear 514

zone at high shear strains (e.g. Lister & Hobbs 1980, Etchecopar & Vasseur 1987; Jessel & Lister 515

1990, but cf. Takeshita et al. 1999). At high strain magnitudes the trailing edge fabric is 516

selectively depopulated by dynamic recrystallization, while the leading edge fabric accordingly 517

strengthens with increasing shear strain and the foliation rotates towards the shear plane (margin 518

of shear zone) and should become perpendicular to the single girdle fabric (Law 1990, his fig. 3). 519

Similar results have been obtained in simple shear experiments on analogue materials (e.g. 520

Bouchez & Duvall 1982; Herwegh & Handy 1996, Herwegh et al. 1997). However, at the Stack 521

of Glencoul the single girdle c-axis fabric is inclined to the thrust parallel foliation even within a 522

few centimeters of the Moine thrust (cf. Figs. 4c, 9, 10 & 11). For example in SG-1 at 0.5 cm 523

beneath the thrust the central section of the optically measured kinked single girdle fabric (Fig. 524

11) is inclined at 83° to foliation (rather than the 90° predicted for high shear strains) near the Y 525

sample orientation (angle ψ in Fig. 15) while the girdle makes an angle of 25° to the foliation 526

pole at the margin of the XZ fabric diagram (Fig. 11; angle C1 in Fig. 15). The corresponding a-527

axis fabric is characterized by a dominant single maximum occupying a pole figure to the single 528

girdle c-axis fabric and inclined at 25° to the lineation (Fig. 14; Law 1987). Once again, 529

numerical modeling would indicate that this a-axis maximum should be parallel to lineation at 530

high shear strains, assuming simple shear deformation. 531

18

532

These observations raise the question of what controls the orientation of the bulk shear plane 533

during large displacement ductile thrusting. The contact between the Cambrian quartzites and 534

overlying Moine schists (Fig. 4) must have been a slip surface during the early stages of 535

thrusting, but over time the bulk shear plane may have been constrained more by the large scale 536

geometry of the thrust wedge, and the lithologic boundary may have become a passive marker 537

within the mylonite zone. In that case the orientation of the foliation may have little relevance to 538

the kinematics associated with later deformation (John Platt, personal communication 2008). 539

Alternatively, the obliquity between finite strain features (thrust-parallel foliation and lineation) 540

and the asymmetric fabrics may indicate that flow has significantly departed from strict simple 541

shear, even within less than a few centimeters of the thrust plane. Heilbronner & Tullis (2006) 542

have described split cylinder experiments on dynamically recrystallizing quartz layers subjected 543

to simultaneously imposed components of simple shear and shear plane normal shortening (i.e. a 544

general shear) in which single girdle c-axis fabrics developed with increasing strain magnitude. 545

Here the relevant observation is that the single girdle fabrics themselves rotated with respect to 546

the experimentally imposed shear couple with increasing shear strain. In these experiments the 547

single girdle fabric was inclined at progressively greater angles to the shear couple with 548

increasing shear strain (26° at a shear strain of 11.5), while still giving the correct shear sense 549

(Heilbronner & Tullis 2006, their fig. 6). In a companion paper (Law this volume) we will 550

attempt to quantify flow vorticities in the Stack of Glencoul mylonites and will compare these 551

data with flow vorticities in the experiments described by Heilbronner & Tullis (2006). 552

553

Fabrics in footwall mylonitic Cambrian quartzites 554

Sampling along a carefully documented vertical transect through the Cambrian quartzites 555

underlying the Moine thrust at the Stack of Glencoul leads to recognition of a profound change in 556

microstructures and both optically measured quartz c-axis fabrics (single grain data), and a-axis 557

fabrics (bulk sample data) measured by X-ray texture goniometry, with distance beneath the 558

Moine thrust (Fig. 3; Law et al. 1986). As described above, close to the thrust, non-coaxial 559

deformation is indicated by asymmetrical c- and a-axis fabrics; the sense of asymmetry in these 560

more highly deformed and recrystallized quartzites is consistent with WNW-directed over-561

thrusting (Figs. 9, 10, 11 & 14). At distances of greater than 30 cm beneath the thrust, essentially 562

19

coaxial deformation is qualitatively indicated by quartz c-axis fabrics (measured on detrital [old] 563

grains) and a-axis fabrics (bulk sample data) which have a general symmetric appearance with 564

respect to foliation and lineation (Figs. 12A-B, 13 & 14). 565

566

Formation of the asymmetrical fabrics (non-coaxial deformation) in the hangingwall and 567

immediate footwall to the thrust must either be contemporaneous with, or later than, formation of 568

the more symmetrical fabrics located at greater distances beneath the thrust (Fig. 3; Law et al. 569

1986). This interpretation, which is based on spatial variation in fabric symmetry, is in marked 570

contrast to the original interpretation by Christie (1963, p. 405) that the symmetric fabrics at the 571

Stack of Glencoul indicated a relatively late stage period of vertical coaxial shortening 572

overprinting asymmetric fabrics produced during thrust-related shearing (see above). 573

Information on strain symmetry may also be inferred from these fabric patterns (Schmid & Casey 574

1986). For example the small circle a-axis fabrics at distances greater than 30 cm beneath the 575

thrust (Fig. 14) indicate deformation within the general flattening field (in agreement with the 576

grain shape analyses - Figs. 7 & 8), while closer to the thrust the point maxima a-axis fabrics 577

indicate an approximate plane strain deformation (sample SG-1 in Fig. 14). 578

579

Quantitative data on more subtle variations in variation in fabric patterns can be obtained by 580

employing the different skeletal parameters used to quantify the degree of c-axis fabric 581

asymmetry (Fig. 15). Information on how the magnitudes of these various fabric parameters 582

change with structural position and recrystallization in the Stack of Glencoul mylonites are 583

summarized in Figures 16-19. Fabric parameter data in these figures are color coded for sample 584

position relative to the Moine thrust - see Figure 16 caption for details. A similar approach has 585

previously been reported by Fernandez-Rodriguez et al. (1994) who included data from the Stack 586

of Glencoul mylonites (taken from Law 1987) in their discussion of statistical methods for 587

quantifying fabric asymmetry. 588

589

Taylor-Bishop Hill simulations of quartz c-axis fabric development (Lister & Hobbs 1980) 590

predict that the fabric half opening angles C1 and C2 should be the same in coaxial deformation 591

but of different values in non-coaxial deformation with the larger angle (C2) lying between the 592

foliation pole and trailing edge of the fabric skeleton (Fig. 15). Observed C2 angles in old and 593

20

new grain fabrics are larger than corresponding C1 angles in many of the Stack samples in 594

agreement with a top to the WNW shear sense (gray shaded areas of Figs. 16a and b) 595

but,particularly in quartzite samples located at greatest distance beneath the thrust (SG-8 to 13), 596

the opposite sense of asymmetry is observed in some old (Fig. 16a) and new grain (Fig. 16b) 597

fabrics. In contrast the difference in relative magnitudes of internal fabric parameters ω1 and ω2 598

are consistent with a top to the WNW shear sense in all old and new grain fabrics (Figs. 17a & b) 599

regardless of sample position relative to the thrust. The external asymmetry parameter ψ (Fig. 600

17f) is also with only one exception (sample SG-9) consistent with a top to the WNW shear sense 601

in all old and new grain fabrics. We view these data as indicating that while external and internal 602

fabric parameters [ψ] and [ω2 - ω1 ] are consistently reliable shear sense indicators, caution 603

should be employed in using the [C2-C1] fabric parameter as a shear sense indicator. 604

605

The greatest degree of skeletal fabric asymmetry is consistently indicated by all external and 606

internal fabric parameters (particularly in new grain fabrics) in those samples located closest to 607

the thrust (Figs. 16b and 17b; samples SG-1, 2.1-2.5, 3 and M.1). Recrystallization commonly 608

appears to lead to a spread in the magnitude range of individual skeletal fabric parameters, 609

relative to their values in the corresponding old grain fabrics (e.g. values of internal fabric 610

parameters ω1 and ω2; Figs. 17c, d and e). However, to what extent this may be simply due to 611

the difficulty of optically measuring the c-axes of very small recrystallized grains is unclear. 612

613

In terms of fabric maxima some of the c-axis fabrics, and particularly new grain fabrics, are 614

defined by maxima close to the periphery and maxima on the central girdle (Figs. 11, 12 & 13). 615

Most of the fabrics have the same maxima, and in the same positions. From this perspective the 616

differences between fabrics close to the thrust plane (e.g. SG-1; Fig. 11) and fabrics at greater 617

distances beneath the thrust (e.g. SG-6; Fig. 12A) become mainly a matter of differences in 618

density distribution with both sharing the same maxima in the same relative positions. These 619

similarly oriented c-axis maxima suggests that all samples have much in common in terms of 620

operative slip systems and recrystallization mechanisms. 621

622

Differences between old and new grain fabrics 623

21

Our previously unpublished analyses of the mylonitic Cambrian quartzites at the Stack of 624

Glencoul indicate significant differences in the optically measured c-axis fabrics of the relict 625

(old) detrital grains and the surrounding much smaller recrystallized (new) matrix grains (Fig. 626

12A-B). Microscope-based analysis yield Type 1 (Lister 1977) cross girdle fabrics for both old 627

and new grains suggesting approximate plane strain deformation, but the small circle element of 628

the fabric (centered about the pole to foliation) in a given sample frequently tends to be more 629

clearly defined in the new grain fabric, while the central bar spanning across the Y sample 630

direction (center of pole figure) is more clearly defined in the corresponding old grain fabric. The 631

small circle element of the optically measured new grain c-axis fabric is particularly clear in 632

samples SG-9 to 13 (Fig. 12b) located at more than 3.5 m beneath the Moine thrust. This small 633

circle fabric element is also clear in the corresponding c-axis fabrics derived from X-ray texture 634

goniometry (Fig. 13; samples SG-8 to SG- 11) where the X-ray beam scans across both old and 635

new grains. 636

637

These subtle differences between old and new grain fabrics may indicate a strain symmetry 638

partitioning with the matrix new grains undergoing a bulk deformation within the general 639

flattening field while the detrital grains deformed (presumably at the same time) at closer to plane 640

strain - but, as indicated by strain analysis (Figs. 7 and 8), still within the general flattening field. 641

A different, but perhaps complementary, form of strain path symmetry partitioning has 642

previously been inferred by Law (1987) in samples SG-1 and SG-10 through analysis of the X-643

ray goniometry derived Orientation Distribution Function (ODF) in which the leading and 644

trailing edges of the cross girdle c-axis fabrics yielded ODF "single crystal" orientations 645

indicative of (presumably synchronous) plane strain (top to WNW shearing) and flattening 646

deformation respectively. These data sets indicate that the strain path followed by individual 647

grains (or groups of grains - e.g. old versus matrix new grains) may significantly depart from the 648

strain path imposed upon the bulk specimen. Potential quartz fabric evidence for strain path 649

partitioning between relict and surrounding recrystallized grains in mylonites from other tectonic 650

environments has also been discussed by Kirschner &Teyssier (1991, 1992) and Hippertt & 651

Borba (1992). 652

653

22

Detailed analysis of skeletal fabric parameters (Fig. 15) on the optically measured old and 654

new grain c-axis fabrics demonstrates that recrystallization always leads to an increase in fabric 655

asymmetry relative to fabrics measured in the adjacent relict detrital grains which have deformed 656

by dislocation glide/climb (Figs. 16-19). This greater fabric asymmetry in the recrystallized 657

grains could be interpreted as indicating a higher degree of vorticity associated with flow of the 658

matrix grains (which might be either contemporaneous with or later than flow in the detrital 659

grains), and has significant implications for using fabric asymmetry parameters (particularly ψ) 660

as input data in quantitative vorticity analyses (see companion paper by Law this volume). For all 661

Cambrian quartzite and Moine samples, a positive correlation is established between degree of 662

internal [ω2−ω1] and external [ψ] skeletal fabric asymmetry (Figs. 18a & b; 19b), with the 663

greatest degree of both internal and external fabric asymmetry recorded in new grains from 664

samples (SG-1 and SG-2.1 to 2.5) located at less than 14.5 cm beneath the Moine thrust plane. A 665

similar, although less clearly defined, positive correlation is also established between degree of 666

asymmetry and recrystallization for external and internal fabric parameters [C2-C1] and [ω2−ω1] 667

(Figs. 18e, f; 19a) and for external fabric parameters [C2-C1] and [ψ] (Figs. 18c, d; 19c). 668

669

Recrystallization also always leads to an increase in c-axis fabric opening angle (C1+C2 in 670

Fig.16e) - as previously demonstrated in numerous studies of experimentally and naturally 671

deformed quartz aggregates (e.g. Tullis et al. 1973; Law 1986). In the Cambrian quartzites at the 672

Stack of Glencoul [C1+C2] opening angles for old grain fabrics range from 48°-57°, while the 673

corresponding new grain fabric opening angles range from 61-68°. Within individual samples 674

new grain fabric [C1+C2] opening angles are between 7° and 21° greater than the corresponding 675

old grain fabric opening angle (Fig. 16e). In general, recrystallization leads to a greater increase 676

in the C1 opening angle (relative to the old grain fabric C1 opening angle) than in the C2 opening 677

angle (Figs. 16c, d). 678

679

In addition to changes in skeletal outline, our analyses also indicate that recrystallization 680

leads to significant changes in density distribution on the c-axis fabric diagram. This has been 681

quantified for individual samples using a software package developed by DM at University of 682

Montpellier that counts and compares the number of data points from detrital (old) and 683

recrystallized (new) grains that plot within individual cells of the numerical counting grid used 684

23

for fabric contouring. The results are displayed as fabric density difference plots (Fig. 20A & B) 685

and indicate that, relative to the detrital grains which have deformed by dislocation glide and 686

climb, recrystallization preferentially produces a large number of grains whose c-axes define 687

maxima located on a small circle girdle centered about the foliation pole - as qualitatively 688

observed above from the original pole figures (Fig. 12B). These point maxima are oriented at a 689

small angle (c. 5-20°) to the XZ plane (Figs. 20A & B). In contrast, relative to the recrystallized 690

grain fabrics, the detrital grain c-axes preferentially cluster along a girdle in the YZ plane. The 691

differences between the old and new grain fabrics - in terms of density distribution - are 692

remarkably similar in each sample (see summary fabric diagram in Fig. 21). 693

694

695

Influence of environmental variables and deformation/recrystallization mechanisms on 696

fabric development 697

The observed skeletal and density differences between the old and new grain fabrics in the 698

mylonitic Cambrian quartzites could indicate a difference in operative crystal slip systems 699

between the old and new grains, which are themselves controlled by environmental variables 700

such as deformation temperature, strain rate, deviatoric stress intensity, trace impurity content 701

and degree of hydrolytic weakening (Lister & Dornsiepen 1982; Gordon Lister 2009 personal 702

communication to RDL). Alternatively, the differences between the old and new grain fabrics 703

may reflect a contrast in the relative importance of dislocation glide/climb and recrystallization 704

mechanisms in the interiors of the large old (detrital) quartz grains compared with the 705

surrounding fine grained matrix of dynamically recrystallizing quartz. 706

707

Quartz c-axis fabric opening angles have been proposed as a potential thermometer by Kruhl 708

(1998) with opening angle increasing as the component of prism [c] slip becomes more important 709

with increasing deformation temperatures. In contrast, it could also be argued that where fabric 710

opening angles are controlled by the combination of active slip systems opening angles should 711

change discreetly, rather than progressively, with for example increasing deformation 712

temperature (Gordon Lister 2009 personal communication to RDL). In some tectonic 713

environments a close correlation is found between deformation temperatures indicated by 714

progressively increasing fabric opening angles and temperatures of metamorphism indicated by 715

24

either prograde or retrograde mineral phase equilibria (e.g. Greater Himalayan Slab: Law et al. 716

2004, 2008; contact metamorphism associated with forcibly emplaced plutons: Morgan & Law 717

2004). However, the situation is clearly more complex in the Stack of Glencoul mylonites where 718

old and new grain fabrics from the same sample have different opening angles (48°-57° versus 719

61-68°; Figs. 12A and B) which would indicate deformation temperatures of 390-440 and 475-720

530 +/- 50 °C respectively using the Kruhl (1998) thermometer. Intuitively the deformation 721

temperatures inferred from the recrystallized grain fabrics seem too high for these greenschist 722

(chlorite bearing) mylonites. In addition it seems very unlikely that the matrix recrystallized 723

grains in the Stack samples would record the higher (presumably earlier) temperature part of a 724

deformation path while the adjacent relict detrital grains record a lower (presumably later) 725

temperature part of the deformation path. Certainly differences in deformation temperature 726

cannot be used to explain comparable differences in fabric opening angle of old and new grain 727

fabrics that are also observed in experimental deformation (e.g. Tullis et al. 1973). 728

729

In addition to deformation temperature, opening angle is also a function of the influence of 730

variables such as strain rate and hydrolytic weakening on operative crystal slip systems. The 731

higher opening angles in the new grains fabrics could be interpreted as indicating slower strain 732

rates in the fine grained matrix grains, although one might expect faster strain rates in the matrix 733

grains - particularly if accompanied by grain boundary sliding. However the strong crystal 734

preferred orientation exhibited by these small matrix grains (Figs. 12A and B) would argue 735

against significant grain boundary sliding (see also Lloyd et al. this volume, but c.f. Halfpenny et 736

al. 2006). Alternatively, the larger opening angles in the new grain fabrics could be a reflection 737

of greater hydrolytic weakening in the fine grained dynamically recrystallizing matrix grains 738

promoting a larger component of prism [c] slip - relative to deformation in the interior of the 739

much larger old grains. This seems likely given the greater surface area to volume ratio in the 740

small matrix grains (compared with the larger old grains) and the greater density of grain 741

boundaries (which may act as fluid path ways) per unit sample volume in the fine grained matrix. 742

Hydrolytic weakening and concomitant dynamic recrystallization has been shown by Tullis et al. 743

(1973, p. 304-306) to lead to a dramatic shift from point maxima small circle girdle c-axis fabrics 744

in uniaxial shortening experiments on quartzite. 745

746

25

The differences between old and new grain fabrics could also be due to a contrast in the 747

operation of dislocation glide/climb versus recrystallization mechanisms in the interiors of the 748

large old grains compared with the surrounding fine grained matrix of dynamically recrystallizing 749

new grains. Deformation within the large old grains is clearly dominated by dislocation 750

glide/climb (indicated in the optical microscope by undulatory/patchy extinction and deformation 751

bands) accompanied by subgrain rotatation (Law et al. 1986). Microstructural evidence for 752

operative deformation/recrystallization mechanisms in the surrounding fine grained matrix (less 753

than c. 15 microns) is difficult to resolve in the optical microscope with a standard 30 micron thin 754

section. The detailed features of these grains can. however, be resolved in the scanning electron 755

microscope (SEM). Imaged in the SEM sub grains produced by subgrain rotation within the old 756

detrital grains of sample SG-10 are larger than recrystallized grains in the surrounding matrix, 757

suggesting that the matrix grains may have formed - or at least been subsequently modified - by 758

different recrystallization processes (Lloyd unpublished data; see also Lloyd et al. this volume). 759

Similar observations (subgrains of 16 micron average grain size in interior of old grains, 12 760

micron new grains at margins of old grains) have been reported by Halfpenny et al. (2006) in an 761

SEM electron back scattered diffraction (EBSD) analysis of a sample of mylonitic Cambrian 762

quartzite collected at the Stack of Glencoul in an equivalent structural position to sample SG-10. 763

764

Well developed core/mantle structures within the old grains (particularly the globular old 765

grains) are seen in SEM images of sample SG-10 (Lloyd unpublished data) which we interpret as 766

indicating the progressive rotation of subgrains to form new grains (White 1976, 1977) which 767

were then modified by other cyclic recrystallization processes once they were incorporated into 768

the fine grained matrix. In contrast, Halfpenny et al. (2006) reported that there was no 769

progressive increase in degree of misorientation of subgrains traced outwards from the center to 770

the margins of the old grains as would be expected if the new grains in the surrounding matrix 771

developed by progressive subgrain formation. Halfpenny et al. (2006) argued that whilst the 772

subgrains in the interior of the old grains probably did form by subgrain rotation, new grains at 773

the margins of the old grains may have nucleated by grain boundary bulging and then been 774

modified (together with their crystal preferred orientation) by grain boundary sliding as they 775

became incorporated in to the surrounding matrix of fine grained dynamically recrystallizing 776

quartz. Certainly, grain boundary sliding (with grain rotation about the Y strain axis for at least 777

26

plane strain deformation) does offer one potential mechanism to account the larger c-axis fabric 778

opening angles measured in the matrix recrystallized (new) grains - compared with fabrics 779

measured on detrital (old) grains (Fig. 16). However, as noted above, the strong crystal preferred 780

orientation in the matrix new grains (Figs. 12 & 20) argues against significant grain boundary 781

sliding, unless sliding was accommodated by dislocation flow (Etheridge & Wilkie 1979; 782

Mancktelow 1987; see review by Law 1990). Additionally, straight grain boundary segments 783

and 120° degree triple junctions between matrix grains in sample SG-10 (Lloyd unpublished 784

data) seem more compatible with recrystallization by grain boundary migration (rather than grain 785

boundary bulging) in the fine grained matrix domains. And yet, one would expect grain 786

boundary migration to lead to an overall increase in grain size relative to subgrains within the old 787

grains - which is the opposite of that observed. 788

789

From the above discussion, we have to conclude that while differences between old and new 790

grain c-axis fabrics in the Stack of Glencoul mylonites (Figs. 12 & 20) undoubtedly reflect the 791

relative importance of dislocation glide/climb versus recrystallization processes in the old and 792

new grains respectively - possibly enhanced by hydrolytic weakening in the new grains - the 793

precise recrystallization mechanism(s) responsible for formation of the new grain fabrics remains 794

enigmatic. Recrystallization in these samples always leads to an increase in fabric asymmetry 795

relative to fabrics measured in the adjacent relict detrital (old) grains which have deformed by 796

dislocation glide/climb (Figs. 16-19), indicating a higher degree of vorticity associated with flow 797

of the matrix grains. We speculate that enhanced diffusion of fluids along grain boundaries in 798

these fine grained matrix domains may lead to precipitation of ultra fine-grained second phase 799

particles (e.g. micas) which, by pinning quartz grain boundaries, may then minimize grain growth 800

during continued dynamic recrystallization. 801

802

Summary 803

1. 3D strain analyses indicate that the gently dipping mylonitic Cambrian quartzites located 804

beneath the Moine thrust at the Stack of Glencoul have been deformed in the general 805

flattening field with maximum principal stretching (170-260%) parallel to the WNW thrust 806

transport direction, a significant intermediate principal stretch (26-41%) parallel to orogenic 807

strike, and a sub-vertical principal shortening of 72-80% perpendicular to foliation, assuming 808

27

constant volume deformation. A volume loss of 50-70% would be needed for these data to 809

be compatible with a plane strain deformation and no stretching along orogenic strike. 810

811

2. Optical microscope-based analyses indicate a spatial partitioning of c-axis fabrics at both the 812

thin section and outcrop scale in the Cambrian quartzites. As previously reported, Type 1 813

cross-girdle c-axis fabrics from relict detrital (old) grains display a high degree of symmetry 814

with respect to foliation and lineation, qualitatively suggesting approximate coaxial strain 815

paths. However, analyses of dynamically recrystallized (new) grains in the surrounding 816

quartz matrix yield significantly more asymmetric Type 1 cross girdle fabrics, in which both 817

the external and internal skeletal fabric elements indicate a non-coaxial deformation 818

associated with top to the WNW shearing. 819

820

3. Traced upwards towards the overlying thrust plane the new grain fabrics in the Cambrian 821

quartzites change from subtly asymmetric cross-girdle fabrics at greater than 0.3 m beneath 822

the thrust, to obviously asymmetric cross girdle fabrics at 15 to 1 cm beneath the thrust, to an 823

asymmetric single girdle fabric at less than 1cm beneath the thrust. However, the high angle 824

of obliquity between foliation and the single girdle fabrics indicate that even within a few 825

centimeters of the thrust plane deformation significantly departed from strict simple shear. 826

Within the Cambrian quartzites the ratio of new to old grains in a given thin section is 827

highest in samples located at less than 15 cm beneath the thrust. 828

829

4. In the overlying mylonitic Moine schists (hangingwall to Moine thrust) asymmetric single 830

girdle c-axis fabrics (with only a vestige of a trailing edge) measured on new grains in planar 831

foliation-parallel quartz veins are also compatible with top to the WNW shearing. However 832

in isoclinally folded quartz veins opposite fabric asymmetries are recorded on adjacent fold 833

limbs (indicating top to WNW and ESE shear senses on long and short limbs respectively) 834

suggesting that the folds developed by flexural slip processes during top to the WNW 835

thrusting regardless of the local angle between fold hinges and the WNW thrust transport 836

direction. 837

838

28

5. Both the old and new grain cross girdle c-axis fabrics from the Cambrian quartzites contain a 839

small circle fabric component centered close to the foliation pole, although for any given 840

sample the small circle component is more pronounced in the new grain fabric. This small 841

circle fabric component is particularly pronounced in new grain c-axis fabrics at more than 842

3.0 m beneath the thrust and, in agreement with strain analyses, indicates a component of 843

flattening deformation. A general flattening deformation is also indicated by small circle a-844

axis fabrics obtained by X-ray texture goniometry in these samples. These strain and fabric 845

data suggest that the Type 1 cross-girdle fabrics recorded in these quartzites are not reliable 846

indicators of plane strain deformation, as often assumed in fabric interpretation. 847

848

6. The symmetric (old grain) and asymmetric (new grain) c-axis fabrics developed in response 849

to different deformation mechanisms (dislocation glide and climb with some subgrain 850

rotation in old grains, complete recrystallization in matrix new grains). The thin section and 851

outcrop scale partitioning of these symmetric and asymmetric fabrics suggests that they have 852

developed essentially synchronously although, particularly close to the thrust, non-coaxial 853

deformation associated with continuing dynamic recrystallization and asymmetric fabric 854

formation may have locally outlasted formation of the more symmetric fabric in the old 855

grains. 856

857

7. Quantitative estimates of flow vorticities associated with mylonite formation at the Stack of 858

Glencoul are presented in the following companion paper by Law (this volume). Problems 859

posed by thin section scale (old versus new matrix grain) flow partition for vorticity analysis 860

are discussed in this paper and tectonic implications of integrated strain and vorticity data 861

from the Stack mylonites are briefly reviewed. 862

863 864 Acknowledgements 865

We thank John Platt and Paris Xypolias for their in-depth and thought provoking reviews of an 866

earlier version of this manuscript. The data included in this paper have been collected over a 25 867

year period during which many individuals and institutes have generously provided advice and 868

support. Early work by RDL (1982-5) was supported by Natural Environment Research Council 869

grant GR3/4612 to RJK and the late Mike Coward at Leeds University. Field and laboratory 870

29

work on the Moine thrust zone mylonites is currently supported by National Science Foundation 871

grant EAR 0538031 to RDL. RDL thanks Stefan Schmid for hospitality and providing access to 872

X-ray texture goniometry facilities at ETH, Zurich, in the mid-1980s and early 1990s, and for 873

discussion and advice on data interpretation. RDL also thanks all participants on the 2007 Peach 874

and Horne conference field trips to the Stack of Glencoul for their spirited discussion of fabric 875

development. This contribution to the Peach and Horne volume is in memory of John Christie; 876

his pioneering work on the fabrics of the Moine thrust zone mylonites continues to motivate new 877

generations of research. 878

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experimentally deformed quartzites. Geological Society of America Bulletin, 84, 297-314. 1108

Wallis, S. R. 1992. Vorticity analysis in a metachert from the Sanbagawa Belt, SW Japan. 1109

Journal of Structural Geology, 14, 271-280. 1110

Wallis, S. R. 1995. Vorticity analysis and recognition of ductile extension in the Sanbagawa belt, 1111

SW Japan. Journal of Structural Geology, 17, 1077-1093. 1112

Wallis, S.R., Platt, J.P., & Knott, S.D., 1993. Recognition of syn-convergence extension in 1113

accretionary wedges with examples from the Calabrian Arc and the Eastern Alps. 1114

American Journal of Science, 293, 463-495. 1115

37

Weathers, M.S., Bird, J.M., Cooper, R.F. & Kohlstedt, D.C. 1979. Differential stress determined 1116

from deformation induced microstructures of the Moine thrust zone. Journal of Geophysical 1117

Research, 84, 7459-7509. 1118

White, S.H. 1973. Syntectonic recrystallization and texture development in quartz. Nature, 244, 1119

276-278. 1120

White, S.H. 1976. The effects of strain on the microstructures, fabrics, and deformation 1121

mechanisms in quartzites. Philosophical Transactions Royal Society London, A 283, 69-86. 1122

White, S.H. 1977. Geological significance of recovery and recrystallization and recrystallization 1123

processes in quartz. Tectonophysics, 39, 143-170. 1124

White, S.H. 1998. Fault rocks from Ben Arnaboll, Moine thrust zone, northwest Scotland. In: 1125

Snoke, A.W., Tullis, J. & Todd, V.R. (eds) Fault - Related Rocks: A Photographic Atlas. 1126

Princeton University Press, Princeton, New Jersey, p. 382-391. 1127

White, S.H. this volume Mylonites: Lessons from Eriboll. In: Law, R.D., Butler, R.W.H., 1128

Holdsworth, R., Krabendam, M. & Strachan, R. (eds) Continental Tectonics and Mountain 1129

Building - The Legacy of Peach and Horne. Geological Society, London, Special 1130

Publications 1131

Wilkinson, P., Soper, N.J & Bell, A.M. 1975. Skolithos pipes as strain markers in mylonites. 1132

Tectonophysics, 28, 143-157. 1133

Williams, P.F., Jiang, D. & Lin, S. 2006. Interpretation of deformation fabrics of infrastructure 1134

zone rocks inthe context of channel flow and other tectonic models. In: Law, R.D., Searle, 1135

M.P & Godin, L. (eds) Channel Flow, Ductile Extrusion and Exhumation in Continental 1136

Collision Zones. Geological Society London, Special Publications, 268, 221-236. 1137

Xypolias, P. & Kokkalas, S. 2006. Heterogeneous ductile deformation along a mid-crustal 1138

extruding shear zone: an example from the External Hellenides (Greece). In: Law, R.D., 1139

Searle, M.P & Godin, L. (eds) Channel Flow, Ductile Extrusion and Exhumation in 1140

Continental Collision Zones. Geological Society London, Special Publications, 268, 497–1141

516. 1142

Xypolias, P., Spanos, D., Chatzaras, V., Kokkalas, S. & Koukouvelas, I. this volume. Vorticity of 1143

flow in ductile thrust zones: examples from the Attico-Cycladic Massif (Internal Hellenides, 1144

Greece). In: Law, R.D., Butler, R.W.H., Holdsworth, R., Krabendam, M. & Strachan, R. 1145

38

(eds) Continental Tectonics and Mountain Building - The Legacy of Peach and Horne. 1146

Geological Society, London, Special Publications 1147

1148

1149

1150

39

Figure Captions 1151

1152

Fig. 1. Simplified geological map of the northern part of the Moine thrust zone, NW Scotland, 1153

showing location of main mylonites in footwall to Moine thrust. 1154

1155

Fig. 2. Sampling localities within mylonitic Moines (samples M1-M4) and mylonitic Cambrian 1156

quartzites (SG-1 through SG-13; described in Law et al. 1986 and Law 1987, 1998) exposed in 1157

northwest crags of Stack of Glencoul. Sample M5 collected at 12 m above thrust plane (at a 1158

structural position 4 m above M4, but c. 30 m to the SSE of sample M1). Distances of individual 1159

samples above/below thrust plane are given in Table 1. 1160

1161

Fig. 3. Schematic illustration (viewed towards the NNE) of quartz c and a-axis fabric variation 1162

with distance from the Moine thrust at the Stack of Glencoul, NW Scotland. XZ projection plane 1163

used in all fabric diagrams. C-axis fabrics within mylonitic Cambrian quartzites beneath the 1164

thrust range from asymmetrical kinked single girdles (new grains) at 0.5 cm beneath the thrust, 1165

through asymmetrical cross-girdle fabrics to symmetrical cross-girdle fabrics (detrial grains) at 1166

30 cm beneath the thrust. C-axis fabric transition is accompanied by a concomitant transition 1167

from asymmetrical single a-axis point maxima fabrics (0.5 cm beneath thrust) through 1168

asymmetrical two maxima fabrics to symmetrical two point maxima fabrics (Law et al. 1986, 1169

Law 1987). Deformed quartz veins within phyllosilicate-rich mylonitic Moine metasediments 1170

lying above the thrust are all characterised by asymmetrical single girdle c-axis fabrics. From 1171

Law (1990). 1172

1173

Fig. 4. Moine thrust plane at Stack of Glencoul. All photographs and micrographs are viewed 1174

towards the ENE, at right angles to the transport direction. From Law 1998. 1175

1176

A. Exposure of the Moine thrust at the Stack of Glencoul (Law et al. 1986). Mylonitic 1177

Cambrian quartzite (Q) underlying similarly deformed Moine rocks (M); arrow marks position of 1178

Moine thrust. Since 1986 large sections of these spectacular exposures have been destroyed 1179

through thoughtless sample collection by unidentified geologists. 1180

1181

40

B. Contact between Moine rocks (M) and mylonitic Cambrian quartzite (Q) marking position of 1182

the Moine thrust (sample SG-1). Polished surface cut perpendicular to foliation and parallel to 1183

lineation; note ESE dipping listric extensional fault which off-sets mylonitic foliation in the 1184

quartzites. 1185

1186

C. Optical micrograph of thin section cut from the polished surface of sample SG-1 illustrated in 1187

photograph B (Law 1998). Moine thrust plane separates the phyllosilicate-rich Moines (M) of 1188

the hangingwall from the mylonitic Cambrian quartzites of the footwall. Note minor listric 1189

extensional fault zone (e-e*) with displacement down to the ESE which off-sets the mylonitic 1190

foliation in the quartzites. This extensional fault zone, which is marked by extremely fine 1191

grained (< 10 micron) dynamically recrystallized quartz, predates a quartz vein (V-V*) which 1192

truncates the Moine thrust, but is sheared over to the WNW in the Moine rocks. The deformation 1193

of this vein and the extensional fault zone indicates intermittent changes in the strain path and 1194

deformation mechanisms from subhorizontal thrust sheet extension (by dislocation creep and 1195

veining) to WNW directed shearing during the late stages of mylonite evolution (Knipe 1990; cf. 1196

Bell this volume). 1197

1198

Fig. 5. Mylonitic Cambrian quartzites at the Stack of Glencoul (from Law 1998). All thin 1199

sections are oriented perpendicular to foliation and parallel to lineation; micrographs are viewed 1200

towards the NNE, a sinistral shear sense being compatible with WNW directed overthrusting. 1201

1202

A. Quartzite at 0.5 cm beneath the thrust. A preferred alignment (SB) of elongate dynamically 1203

recrystallized quartz grains dips more steeply to the ESE than the mylonitic foliation (SA) 1204

observed in hand sample; both microstructures and crystal fabrics indicate a sinistral shear sense 1205

(sample SG-1). Crossed polarised light. 1206

1207

B. Quartzite at 8.5 m beneath the thrust. Mylonitic foliation (SA) defined by planar zones of 1208

recrystallized quartz and ribbon-like relict detrital grains (sample SG-13). Ribbon grains 1209

anastomose around a 'hard orientation' globular quartz grain (G) of possible detrital origin, 1210

suggesting an approximately coaxial bulk deformation history. Crossed polarised light. 1211

1212

41

C. Quartzite at 4.6 m beneath the thrust (sample SG-10). Large globular quartz grains (G) 1213

aligned parallel to foliation are remnants of a single ribbon-like parent grain which has been 1214

partially destroyed by the processes of lattice kinking, grain boundary migration and subgrain 1215

rotation. These 'hard orientation' globular grains (c-axes perpendicular to foliation) were 1216

interpreted by Law et al. (1986) as indicating that at least the last strain increments may have 1217

been coaxially superimposed. Crossed polarised light. 1218

1219

Fig. 6. Schematic sketch of folded quartz veins within Moine mylonite (sample M.3) located 5.0 1220

m above the Moine thrust at the Stack of Glencoul. XZ surface cut perpendicular to foliation 1221

and parallel to lineation is 30 cm in length. Quartz c-axis fabrics from three small (0.4 x 0.15 1222

cm) domains on adjacent fold limbs are displayed on XZ projection planes containing lineation 1223

(X) and pole (Z) to foliation; note: 1) opposite fabric asymmetries with respect to foliation and 1224

vein margins (Sa) on adjacent fold limbs, 2) opposite senses of obliquity between vein margins 1225

and alignment (Sb) of elongate recrystallized quartz grains on adjacent fold limbs, 3) non-1226

orthogonal relationship between XZ section and fold hinges (orientations indicated by arrows 1227

lying within foliation) defined by quartz veins. Specimen and fabrics viewed towards the NNE; 1228

movement on Moine thrust associated with WNW directed overthrusting (sinistral shear sense). 1229

Adapted from Law (1990). 1230

1231

Fig. 7. Hsu natural strain plots of calculated strain states in mylonitic Cambrian quartzite in 1232

footwall to the Moine thrust at the Stack of Glencoul. 1233

1234

Fig. 8. Natural log Flinn plot of strain data from mylonitic Cambrian quartzites at Stack of 1235

Glencoul, with contours of plane strain deformation for different volume losses (∆), and contours 1236

of constant strain symmetry expressed by Lodes Unit (ν). 1237

1238

Fig. 9. Optically measured quartz c-axis fabrics (viewed towards the NNE) in plastically 1239

deformed foliation-parallel quartz veins from mylonitic Moine rocks above the Moine thrust. 1240

Less than 0.5, 0.5-1.0 and 1.0-2.0 times uniform density distribution indicated by light gray, dark 1241

gray and pink fields, respectively. Distances of individual fabrics above Moine thrust plane are 1242

indicated. 1243

42

1244

Fig. 10. Quartz fabrics from phyllosilicate rich matrix of mylonitic Moine sample M.2 located at 1245

c. 10 cm above the Moine thrust. Orientation of shear bands and elongate recrystallized grains 1246

(Sb) indicated. Fabrics are derived from Orientation Distribution Function (ODF) analysis of X-1247

ray texture goniometry data. 1248

1249

Fig. 11. Transition in quartz c-axis fabrics (viewed towards the NNE) from single girdle to 1250

cross-girdle fabrics measured by optical and X-ray texture goniometry methods in recrystallised 1251

mylonitic Cambrian quartzites close to the Moine thrust. Less than 0.5, 0.5-1.0 and 1.0-2.0 times 1252

uniform density distribution indicated by light gray, dark gray and pink fields, respectively. 1253

Orientation of shear bands (sample SG-2.2) and maximum angle between foliation and elongate 1254

recrystallized quartz grains (Sb) indicated. Distances of individual fabrics below Moine thrust 1255

plane are also indicated. 1256

1257

Fig. 12A and B. Optically measured quartz c-axis fabrics from relict detrital (old) grains and 1258

recystallised matrix (new) grains in mylonitic Cambrian quartzite at the Stack of Glencoul. Less 1259

than 0.5, 0.5-1.0 and 1.0-2.0 times uniform density distribution indicated by light gray, dark gray 1260

and pink fields, respectively. Maximum angle between foliation and elongate recrystallized 1261

quartz grains (Sb) indicated for samples SG-3 and SG-6. Distances (in meters) of individual 1262

fabrics below Moine thrust plane are indicated. 1263

1264

Fig. 13. Comparison between quartz c-axis fabrics (combined old and new grains) measured by 1265

optical microscopy and c-axis fabrics calculated via ODF analysis from X-ray texture goniometry 1266

(reg. – X-ray). Less than 0.5, 0.5-1.0 and 1.0-2.0 times uniform density distribution indicated by 1267

light gray, dark gray and pink fields, respectively. Distances (in meters) of individual fabrics 1268

below Moine thrust plane are indicated. 1269

1270

Fig. 14. Quartz a-axis fabrics measured by X-ray texture goniometry methods within quartz 1271

mylonites from the Stack of Glencoul. Contour intervals for samples SG-1 – SG-4 (Law 1987): 1272

0.5, 1.0, 1.25, 1.5, 1.75 … 3.0, 3.25 times uniform distribution; less than 0.5, 0.5 - 1.0 and 1.0 – 1273

2.0 times uniform indicated by light gray, dark gray and pink fields respectively. Contour 1274

43

intervals for samples SG-6 – SG-13 (Law et al. 1986): 0.5, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 times 1275

uniform distribution; less than 0.5, 0.5 – 1.0 and 1.0 – 2.0 times uniform distribution indicated by 1276

light gray, dark gray and pink fields respectively. 1277

1278

Fig. 15. Parameters used to characterize external and internal asymmetry in quartz c-axis fabrics 1279

(after Law 1987, 1990). External fabric asymmetry characterized by ψ, C1 and C2; internal 1280

fabric asymmetry characterized by ω1 and ω2. Leading and trailing edges of c-axis fabric 1281

skeleton denoted by l.e. and t.e respectively; ω values measured on the ‘top’ and ‘bottom’ of the 1282

fabric skeleton denoted by ω1t / ω2t and ω1b / ω2b respectively. 1283

1284

Fig. 16. Skeletal symmetry data (external fabric parameters C1 and C2) extracted from optically 1285

measured old and new grain quartz c-axis fabrics of mylonitic Cambrian quartzites and deformed 1286

quartz veins in Moine pelites (M) at the Stack of Glencoul. See Fig. 15 for explanation of 1287

parameters. Gray shaded areas in individual plots (a, b and f) indicate angular ranges where 1288

differences between fabric parameters C2 and C1 are compatible with top to WNW shear sense. 1289

Explanation for color coding of data: green circles - samples M.1 to M.5 at 0.01 - 12.0 m above 1290

thrust; red circles - samples SG-1 and SG-2.1 to 2.5 at 0.5 to 14.5 cm beneath thrust; orange 1291

circles - samples SG-3 and SG-4 at 0.3 and 0.7 m beneath thrust; blue circles - samples SG-6 to 1292

SG-13 at 1.9 to 8.5 m beneath thrust (cf. Fig. 2). In sample SG-3 and SG-10 c-axis fabrics were 1293

measured in both thin sections cut parallel to the XZ and YZ planes; data measured on the YZ 1294

plane were rotated on to the XZ plane before skeletal fabric parameters were measured; both data 1295

sets are included here. 1296

1297

Fig. 17. Skeletal symmetry data for internal parameters ω1 and ω2 (a-e) and external parameter 1298

ψ (f) extracted from optically measured old and new grain quartz c-axis fabrics. Gray shaded 1299

areas in individual plots (a, b, e and f) indicate angular ranges where fabric parameters are 1300

compatible with top to WNW shear sense. See Figs. 15 and 16 respectively for explanations of 1301

parameters and color coding of data. 1302

1303

Fig. 18. Relationships between external (ψ, C2 minus C1) and internal (ω2 minus ω1) fabric 1304

asymmetry in optically measured old grain (left hand column) and new grain (right hand column) 1305

44

quartz c-axis fabrics. Linear regression lines through new grain data are shown for each pair of 1306

old and new grain plots (a-b, c-d and e-f). Gray shaded areas in individual plots indicate angular 1307

ranges where fabric parameters are compatible with top to WNW shear sense. See Figs. 15 and 1308

16 respectively for explanations of parameters and color coding of data. 1309

1310

Fig. 19. Differences between external (ψ, C2 minus C1) and internal (ω2 minus ω1) old and new 1311

grain fabric asymmetry in individual samples. Gray shaded areas in individual plots indicate 1312

angular ranges where both new grain fabric parameters plotted for an individual sample are more 1313

asymmetric than the corresponding old grain fabric parameters for that sample. Zero values 1314

indicate where a given fabric parameter has the same magnitude in both old and new grain fabrics 1315

from an individual sample. See Figs. 15 and 16 respectively for explanations of parameters and 1316

color coding of data. 1317

1318

Fig. 20A and B. Differences between optically measured c-axis fabrics (viewed towards the 1319

NNE) in detrital (old) and recrystallized (new) grains fabrics in mylonitic Cambrian quartzites 1320

beneath the Moine thrust. Fabric diagrams in two right hand columns indicate difference in 1321

intensity distribution between old and new grain fabrics in individual samples; see text for 1322

details. Distances (in meters) of individual fabrics below Moine thrust plane are indicated. 1323

1324

Fig. 21. Schematic summary of differences in fabric density distribution between old and new 1325

grain c-axis fabrics in mylonitic Cambrian quartzites at Stack of Glencoul. 1326

Ullapool

KnockanCrag

Allt nan Sleach

Gorm Loch Mor

Stack of Glencoul

Loch Strath

ERIB

OLL

REG

ION

MT

Loch Eriboll

Ben Hutaig

Moine assemblage with Lewisian inliers

Mylonites of Moine thrust zone

Other rocks of Moine thrust zone

Foreland to Moine thrust zone

5 00'o5 30'o

58 30'o

58 00'o

N

0 15 km

Loch More

ASSYNTREGION

nan Aisinnin

Fig. 1 Law et al. - Peach and Horne volume

M4

M3

M2

2

1

mylonitic Moine and Lewisian (?) rocks

Stack of GlencoulNNW SSE

M1

Grid Reference: NC 28882876

Moine thrust

approximate horizontal and vertical scale

0 10 m

6

78

9

10 & 11

12

13

green - black quartz mylonites

mylonitic pipe rockwhite quartz mylonites

white quartz mylonitesphyllosilicate - rich lenswhite quartz mylonites with

phyllosilicate - rich horizonwhite quartz mylonites

white quartz mylonites3

4

5

white quartz myloniteswhite quartz mylonites

phyllosilicate - rich horizon

Fig. 2. Law et al. - Peach and Horne volume

Moine thrust

mylonitic foliationc

c a

c a

Fig. 3. Law et al. - Peach and Horne volume

500 c-axes

401 c-axes500 c-axes

Sb

SbSb

01822

E

1082275

45

c b a

fold hinge in quartz veinplunges towards 184°

hinge plungestowards 154°

WNW ESE

down

a

bc

13 cm

Sa

SaSa

Fig. 6. Law et al. - Peach and Horne volume

+ 500% X

+ 400% X

300% X

200% X

100% X

-80% Z

-70% Z

-60% Z

-50% Z

-40% Z

+20

% Y

+50

% Y

+ 1

50%

Y

+ 1

25%

Y

70%

Y

-20%

Y

-50%

Y

-60

% Y

0+0.5

+1.0-1.0

-0.5

2.0

1.0

Es Es

2.0

1.0

plan

e st

rain

at c

onst

ant v

olum

e

+ 1

00%

Y150% X

104

13

8

116

73

0% Y

1

Fig. 7. Law et al. - Peach and Horne volume

0.0 0.5 1.0 1.5 2.0 2.5ln Y/Z

0.0

0.5

1.0

1.5

2.0

ln X

/Y

= -0

.8

K= 1.

0,=

0.00

= -0

.75

= -0

.7

= -0

.6 =

-0.5

= -0

.4

= 0.75= 0.50

= 0.25

= -0

.25

= -0

.50

= -

0.75

1013

48

6

11

73

= -0

.2

1

Fig. 8. Law et al. - Peach and Horne volume

WNW ESE

down

shear bands

lineationgrain shape foliation

Sb

oblique grain shape

12.0 m

8.0 m

5.0 m

0.1 m

0.01 m

2.0

4.0

6.0

8.0

10.0

12.05

4

3

1, 2

DIS

TAN

CE

AB

OV

E M

OIN

E T

HR

US

T

(in m

eter

s)

thrust

0.5, 1, 2, 3, 4, 5, 6, 7 times uniform

0.5, 1, 2, 3, 4, 5, 6 times uniform

0.5, 1, 2, 3, 4, 5, 6, 7, 8 times uniform

0.5, 1, 2, 3, 4, 5, 6, 7 times uniform

0.5, 1, 2, 3, 4, 5, 6, 7 times uniform

new grains

Sb

Sb

Sb

Sb

605 c-axes

850 c-axes

1001 c-axes

642 c-axes

615 c-axes

M.3

M.2

M.1

M.5

M.4

Fig. 9. Law et al. - Peach and Horne volume

Contours: 0.60, 0.80, 1.00, 1.20 Contours: 0.80, 0.88, 0.96, 1.04,1.12, 1.20, 1.28, 1.36 (m.u.d.)

c a

maxiumum density = 1.56

minimum density = 0.28

maxiumum density = 1.52

minimum density = 0.70

Sb

Fig. 10. Law et al. - Peach and Horne volume

5.0

1

DIS

TAN

CE

BE

LOW

MO

INE

TH

RU

ST

(in

cm

)

thrust

10.0

15.0

20.0

2.1

2.2

2.3

2.4

2.5

Sb

Sb

Sb

Sb

Sb

Sb

615 c-axes 0.5, 1, 2, 3, 4, 5, 0.5, 1.0, 1.5, 2.0,

685 c-axes

6, 7, 8 times uniform 2.5, 3.0 times uniform

new grains reg. - X-ray

0.5, 1, 2, 3, 4, 5 6, 7 times uniform

0.5, 1.0, 1.5, 2.02.5 times uniform

609 c-axes 0.5, 1, 2, 3, 4, 5,6, 7 times uniform

602 c-axes 0.5, 1, 2, 3, 4, 5,

6, 7 times uniform

0.5, 1.0, 1.5, 2.0,

2.5, 3.0 times uniform

607 c-axes 0.5, 1, 2, 3, 4, 5, 6 times uniform

597 c-axes 0.5, 1, 2, 3, 4, 5,6, 7 times uniform

0.5, 1.0, 1.5, 2.0, 2.5, 3.0 times uniform

0.5 cm

SG-2.1

SG-2.2

SG-1

SG-2.3

SG-2.4

SG-2.5

1.0 cm

4.0 cm

8.0 cm

10.0 cm

14.5 cm

Fig. 11. Law et al. - Peach and Horne volume

0.5, 1, 2, 3, 4, 5 times uniform0.5, 1, 2, 3, 4 times uniform612 c-axes 600 c-axes

SG.3 XZ on XZ

0.5, 1, 2, 3, 4, 5 times uniform0.5, 1, 2, 3, 4, 5 times uniform605 c-axes 700 c-axes

SG-4

old grains new grains

0.5, 1, 2, 3, 4 times uniform0.5, 1, 2, 3, 4 times uniform728 c-axes 625 c-axes

SG-6

0.5, 1, 2, 3, 4 times uniform0.5, 1, 2, 3, 4 times uniform629 c-axes 600 c-axes

SG-7

0.5, 1, 2, 3 times uniform0.5, 1, 2, 3, 4, 5 times uniform572 c-axes 600 c-axes

SG-8

0.3 m

0.7 m

1.9 m

2.5 m

2.9 m

Fig. 12-A. Law et al. - Peach and Horne volume

old grains new grains

0.5, 1, 2, 3, 4 times uniform0.5, 1, 2, 3, 4 times uniform639 c-axes 750 c-axes

SG-9

626 c-axes 600 c-axes

XZ on XZ

0.5, 1, 2, 3, 4, 5 times uniform0.5, 1, 2, 3, 4, 5 times uniform618 c-axes 700 c-axes

SG-11

598 c-axes 0.5, 1, 2, 3, 4 times uniform

SG-12

0.5, 1, 2, 3, 4 times uniform0.5, 1, 2, 3, 4, 5, 6 times uniform638 c-axes 700 c-axes

SG-13

3.55 m

0.5, 1, 2, 3, 4 times uniform0.5, 1, 2, 3, 4 times uniform

SG-10

4.6 m

4.6 m

7.3 m

8.5 m

Fig. 12-B. Law et al. - Peach and Horne volume

old + new grains reg. - X-ray old + new grains reg. - X-ray

1389 c-axes

SG-9

1226 c-axes

SG-10

0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5

1318 c-axes

SG-11

0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5

1338 c-axes

SG-13

0.5, 1.0, 1.5, 2.0, 2.5, 3.0

0.5, 1, 2, 3, 4 times uniform

0.5, 1, 2, 3 times uniform times uniform

0.5, 1, 2, 3, 4 times uniform times uniform

0.5, 1, 2, 3, 4, 5 times uniform times uniform

3.55 m

4.6 m

4.6 m

8.5 m

1212 c-axes

SG-3

0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5

1305 c-axes

SG-4

0.5, 1.0, 1.5, 2.0 times uniform

1353 c-axes

SG-6

0.5, 1.0, 1.5, 2.0, 2.5 times uniform

1229 c-axes

SG-7

0.5, 1.0, 1.5, 2.0, 2.5 times uniform

1172 c-axes

SG-8

0.5, 1.0, 1.5, 2.0, 2.5 times uniform

0.5, 1, 2, 3, 4 times uniform

0.5, 1, 2, 3, 4 times uniform

times uniform

0.5, 1, 2, 3, 4 times uniform

0.5, 1, 2, 3 times uniform

0.5, 1, 2, 3, 4 times uniform

0.3 m

0.7 m

1.9 m

2.5 m

2.9 m

Fig. 13. Law et al. - Peach and Horne volume

SG-1 SG-2.1 SG-2.2 SG-2.3

SG-2.4 SG-2.5 SG-3 SG-4

SG-6 SG-7 SG-8 SG-10

SG-11 SG-13

0.5 cm

10.0 cm

1.9 m

4.6 m 8.5 m

2.5 m 2.9 m 4.6 m

14.5 cm 30.0 cm 70.0 cm

1.5 cm 4.0 cm 8.0 cm

lineation

grain shape foliation

WNW ESE

down

Fig. 14. Law et al. - Peach and Horne volume

C1 C2

2t

2b 1b

foliationL

WNW ESE

1t

l.e.t.e.

Fig. 15. Law et al. - Peach and Horne volume

1050- 5-10

0

5

10

C2 - C1 NEW GRAINS

C2

- C

1 O

LD G

RA

INS

-5

3

46

7

8

9

1113

3-YZ

10, 10-YZ

15° 10° 5° 0°

5°

10°3

46

7

8

9

10

11

13

3-YZ

10-YZ

75 8070656055504545

50

55

60

C1 + C2 NEW GRAINS

C1

+ C

2 O

LD G

RA

INS 15°10°5°0° 20°

25°

15°

10°

0° 5°

C2

OLD

GR

AIN

S

C2 NEW GRAINS

3

4

6

7

8

9

10

11

133-YZ

10-YZ

40383634323022

24

26

28

30

32

15°

10°

0° 5°

C1

OLD

GR

AIN

S

C1 NEW GRAINS

3

4 6

7

8

9

10

11

13

3-YZ

10-YZ

40383634323022

24

26

28

30

32

15° 5°10° 0°

5°

10°

15°

M1

M31

2-12-42-5

2-2

3-YZ7

2-33

4

13

10-YZ

11

8

10

9

C2 NEW GRAINS

C1

NE

W G

RA

INS

403836343230282624222020

22

24

26

28

30

32

34

36

38

15° 10° 5°

0°

10°

8

11

12

3

7

13

64

10

9

5°

34 363230282624222020

22

24

26

28

30

32

34

36

38

C1

OLD

GR

AIN

S

C2 OLD GRAINS

a b

dc

e

f

Fig. 16. Law et al. - Peach and Horne volume

OLD

GR

AIN

S

NEW GRAINS

95908580757080

85

90

95

15° 10° 5° 0°20°

5°

10°

33-YZ

4

67

8

9

10 10-YZ

11

13

33

3YZ3YZ

44

6

6

7

78

8

9

9 101010YZ

10YZ 11

1113

13

0 10 20 30 40 50 60 70 80-10-20-20

-10

0

10

20

30

40

50

2-1

OLD

GR

AIN

S

2- 1 NEW GRAINS

0°20° 20°

40°

60°

80°

40°

80

70

60

50

40

30

20

1010 20 30 40 50 60 70 80 90 100 110

2 O

LD G

RA

INS

2 NEW GRAINS

0° 20°

40° 60°

40° 20°

33

4 4

6

7

7

8

89

910

10

11

11

13

133YZ

13YZ 13YZ

3YZ6

80

70

60

50

40

30

20

1010 20 30 40 50 60 70 80 90 100 110

1 O

LD G

RA

INS

1 NEW GRAINS

3 3

44 6

6

7

7

8

8

9

9

1010

11

1113 133YZ 10YZ10YZ

0° 20°

40° 60°

40° 20°

0 20 40 60 80 100 1200

20

40

60

80

100

1 N

EW

GR

AIN

S

NEW GRAINS

0°

20° 20°

40°

60°

80°M3

M3

1

1

2-1

2-1

2-22-2

2-3 2-3 2-42-4

2-5

2-5

334

4

3YZ

3YZ

6

6

7

7

8

8

9

9

10

10

11

11

13

13

10YZ

10YZ

M1M1

0 20 40 60 80 100 1200

20

40

60

80

100

1O

LD G

RA

INS

OLD GRAINS

0°20° 20°

40°

3

3

446

6

7

7

8

8

9

9

10

1011

1112 1313

a b

c d

ef

Fig. 17. Law et al. - Peach and Horne volume

90 9585807570

- 5

0

5

10

15C

2 -

C1

NE

W G

RA

INS

NEW GRAINS

M31

2-12-2

2-3

2-42-5

3

3-YZ

46

89

10 10 -YZ11

13

7

90 9585807570

- 5

0

5

10

15

C2

- C

1 O

LD G

RA

INS

OLD GRAINS

3

46

7

8

910

11

12

13

2 -

1N

EW

GR

AIN

S

70 75 80 85 90 95-20

-10

0

10

20

30

40

50

60

70

80

NEW GRAINS

M3M3

1

12-1

2-12-2

2-2

2-3

2-3

2-4

2-42-5

2-5

3

3

3YZ3YZ

4

4 6

6

77

8

8

9

910

10

10YZ

10YZ

11

11

13

13

2 -

1O

LD G

RA

INS

70 75 80 85 90 95-20

-10

0

10

20

30

40

50

60

70

80

OLD GRAINS

33

4

4

6

6

7, 78

8

9

910

1011

11

12

12

13

13

2 -

1N

EW

GR

AIN

S

C2 - C1 NEW GRAINS-10 -5 0 5 10 15 20

-20

-10

0

10

20

30

40

50

60

70

80

M3M3

1

12-1

2-1

2-2

2-2

2-3

2-32-4

2-4

2-5

2-5

33

3YZ

3YZ4

4

6

6

7

78

8

9

9 10

10

10YZ

10YZ

11

11

13

13

2 -

1O

LD G

RA

INS

C2 - C1 OLD GRAINS-10 -5 0 5 10 15 20

-20

-10

0

10

20

30

40

50

60

70

80

33

44

66

7

78

8

9

910

1011

11

12

1213

13

a b

c d

e f

Fig. 18. Law et al. - Peach and Horne volume

new fabric progressively more asymmetric than old fabric

old fabric more asym. than new fabric

old fabric progressively more

asymm

etric than new fabric

new fabric m

ore asym.

than old fabric

-10

-5

0

5

10

15

C2-

C1

OLD

MIN

US

NE

W G

RA

INS

3

3YZ

4 6

8

9

10

10YZ

11

13

7

OLD MINUS NEW GRAINS-10 -5 0 5 10 15

new fabric progressively more asymmetric than old fabric

old fabric more asym. than new fabric

new fabric progressively m

ore asym

metric than old fabric

old fabric more asym

. than new

fabric

2 -

1 O

LD M

INU

S N

EW

GR

AIN

S

OLD MINUS NEW GRAINS-10 -5 0 5 10 15

-80

-60

-40

-20

0

20

40

3 3

3YZ

3YZ

4

4

6

6

77

8

8

9

9 10

10

10YZ

10YZ

11

11

13

13

old fabric progressively more asymmetric than new fabric

new fabric more asym. than old fabric

new fabric progressively m

ore asym

metric than old fabric

old fabric more asym

. than new

fabric

-10 -5 0 5 10 15-80

-60

-40

-20

0

20

40

2 -

1 O

LD M

INU

S N

EW

GR

AIN

S

C2-C1 OLD MINUS NEW GRAINS

3

3

4

3YZ

3YZ

6

6

7

714

14

9

910

10

10YZ

10YZ

11

11

13

13

4

a

b

c

Fig. 19. Law et al. - Peach and Horne volume

maximum density contrast = 2.98 times uniform

maximum density contrast

= 2.41 times uniform

maximum density contrast

= 2.04 times uniform

maximum density contrast

= 2.12 times uniform

maximum density contrast

= 1.19 times uniform

maximum density contrast

= 1.86 times uniform

maximum density contrast

= 2.34 times uniform

maximum density contrast

= 2.21 times uniform

maximum density contrast

= 2.55 times uniform

maximum density contrast

= 1.80 times uniform

SG-3

612 c-axes 600 c-axes;

old grains new grains density new > old

605 c-axes 700 c-axes

SG-4

728 c-axes 625 c-axes

SG-6

629 c-axes 600 c-axes

SG-7

572 c-axes 600 c-axes

SG-8

0.5, 1, 2, 3, 4 times uniform 0.5, 1, 2, 3, 4, 5 times uniform

0.5, 1, 2, 3, 4, 5 times uniform 0.5, 1, 2, 3, 4, 5 times uniform

0.5, 1, 2, 3, 4 times uniform 0.5, 1, 2, 3, 4 times uniform

0.5, 1, 2, 3, 4 times uniform 0.5, 1, 2, 3, 4 times uniform

0.5, 1, 2, 3, 4, 5 times uniform 0.5, 1, 2, 3 times uniform

0.3 m

0.7 m

1.9 m

2.5 m

2.9 m

density old > new

Fig. 20A. Law et al. - Peach and Horne volume

maximum density contrast

= 1.84 times uniform

maximum density contrast

= 2.13 times uniform

maximum density contrast

= 2.78 times uniform

maximum density contrast

= 2.44 times uniform

maximum density contrast

= 2.98 times uniform

maximum density contrast

= 3.61 times uniform

maximum density contrast

= 2.95 times uniform

maximum density contrast

= 2.95 times uniform

maximum density contrast

= 3.06 times uniform

old grains new grains

639 c-axes 750 c-axes;

SG-9

626 c-axes 600 c-axes

SG-10

618 c-axes 700 c-axes

SG-11

638 c-axes 700 c-axes

SG-13

0.5, 1, 2, 3, 4 times uniform 0.5, 1, 2, 3, 4 times uniform

0.5, 1, 2, 3, 4 times uniform 0.5, 1, 2, 3, 4 times uniform

0.5, 1, 2, 3, 4, 5 times uniform 0.5, 1, 2, 3, 4, 5 times uniform

0.5, 1, 2, 3, 4, 5, 6 times uniform 0.5, 1, 2, 3, 4 times uniform

3.55 m

4.6 m

4.6 m

8.5 m

density new > olddensity old > new

Fig. 20B. Law et al. - Peach and Horne volume

18 2140 40

° ° ° °

density old > new density new > old

Fig. 21. Law et al. - Peach and Horne volume

Table 1. Strain estimates for mylonitic Cambrian quartzites at Stack of Glencoul.

sample d (m) X Y Z RXY RYZ RXZ ln RXY ln RYZ ln RXZ Flinn K Lode's υ Nadai εs

SG-1 0.005 3.657 1.41 0.194 2.594 7.271 18.857 0.953 1.984 2.937 0.480 0.351 2.119 SG-2.1 0.010 __ __ __ __ __ 12.76 __ __ __ __ __ __ SG-2.2 0.040 __ __ __ __ __ 12.30 __ __ __ __ __ __ SG-2.3 0.080 __ __ __ __ __ 11.40 __ __ __ __ __ __ SG-2.4 0.100 __ __ __ __ __ 13.50 __ __ __ __ __ __ SG-2.5 0.145 __ __ __ __ __ 11.25 __ __ __ __ __ __ SG-3 0.300 2.81 1.41 0.25 1.977 5.594 11.062 0.682 1.722 2.404 0.396 0.433 1.79 SG-4 0.700 3.218 1.294 0.24 2.487 5.392 13.408 0.911 1.685 2.596 0.541 0.298 1.86 SG-6 1.900 2.741 1.366 0.267 2.007 5.116 10.266 0.696 1.632 2.329 0.427 0.402 1.762 SG-7 2.500 2.774 1.45 0.249 1.913 5.823 11.141 0.649 1.762 2.411 0.368 0.462 1.793 SG-8 2.900 2.856 1.265 0.277 2.258 4.567 10.31 0.814 1.519 2.333 0.536 0.302 1.764 SG-9 3.550 __ __ __ __ __ 9.25 __ __ __ __ __ __

SG-10 4.600 3.34 1.29 0.23 2.588 5.543 14.344 0.951 1.713 2.663 0.555 0.286 1.909 SG-11 4.600 3.201 1.494 0.209 2.143 7.148 15.316 0.762 1.967 2.729 0.387 0.442 1.907 SG.12 7.300 __ __ __ __ __ __ __ __ __ __ __ __ SG-13 8.500 3.666 1.287 0.212 2.848 6.071 17.292 1.047 1.803 2.850 0.580 0.265 1.949

Estimates shown are based on Rf/φ analysis of deformed detrital quartz grain shapes in three mutually perpendicular thin sections cut orthogonal to foliation and lineation, using software packages by Kanagawa (1992) and Chew (2003). Estimated stretches parallel to principal strain directions assume constant volume deformation. Only XZ sections were analyzed in sample SG.2.1 - 2.5 and SG.9. Distance (d) in meters of individual samples beneath Moine thrust plane indicated.