Tectonophysics. 158 (1989) 311-333

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

311

Curved vein fibres: an alternative explanation

P.F. WILLIAMS 1 and J.L. URAI *

’ Department of Geology, University of New Brunswick, Fredericton, N.B. E3B 5A3 (Canada)

’ Instituut voor Aardwetenschappen, Rijksuniversltelt, Utrecht (The Netherlands)

(Received February 18, 1987; accepted August 18, 1987)

Abstract

Williams, P.F. and Urai. J.L., 1989. Curved vein fibres: an alternative explanation. In A. Ord (Editor), Deformation of

Crustal Rocks. Tectonophysics, 158: 311-333.

Veins occurring at the edge of the dextral Indian Islands Fault (Newfoundland) are folded in response to a

fault-parallel, ductile shear. The veins, which locally constitute a penetrative fabric element at outcrop scale, are

composed mainly of calcite and have a narrow rim of quartz and chlorite. Both the calcite and quartz are generally

fibrous and lattice distortion in the calcite is not commensurate with the observed degree of fibre curvature. This

observation supports the popular view that curved fibres grow curved as they track the vein opening vector. It is

demonstrated, however, that the curvature in the material described here is due to deformation and that the lack of

strong lattice distortion is due, in part, to polygonization and to recrystallization. All observations are consistent with

both the calcite and the quartz fibres having grown perpendicular to the vein walls. The chlorite orientation is

controlled by the orientation of mica grains in the country rock.

In some veins, markers make it possible to define the net opening direction and, in all examples, it is demonstrated

that the direction has not been tracked by the fibres. Since this conclusion may be more generally applicable. caution

should be exercised in interpreting kinematics on the basis of fibre geometry.

Where vein density is high, the veins and country rock screens separating them form a multilayer sequence that is

folded into fairly harmonic folds. The folds appear to have a cleavage transected relationship to the regional cleavage,

but the cleavage is shown to predate the folds.

Introduction northwesterly. They lie on the southeast limb of the Port Albert Synform (Karlstrom et al., 1982)

The veins described in this paper occur in a and cleavage relationships and minor, shallowly sequence of alternating shales and sandstones, in- plunging, asymmetric folds are congruous with the dividual beds varying from a few centimetres to a large structure (Figs. 2, 3 and 4). On Squashberry few decimetres in thickness. The sequence, which Island, horizontally plunging folds outcropping on is rich in cross-bedded ripples and convolute one or other shore do not persist as far as the lamination, is placed in the Wig Warn Formation opposite shore (Fig. 2). of the Silurian Botwood Group by Kean et al. Immediately south of Squashberry Island lies a (1981). The outcrops described occur on the broad ductile fault zone known previously as the southeast side of the Port Albert Peninsula (Fig. 1) Indian Islands Thrust (Eastler, 1971) but referred where the focus of our attention has been on to here as the Indian Islands Fault. On the ex- Squashberry Island (Figs. 1 and 2). Beds generally posed northern edge of this zone there is a rapid dip steeply northwest or southeast and (except in transition, over a few metres, from a transposed the short limbs of minor folds) consistently young lenticular rock, best described as a phyllonite, to

0040-1951/89/$03.50 0 1989 Elsevier Science Publishers B.V.

312

SCALE

KILOMETRES 1 $CHA!,oE

5 1

I i- ,

Fig. 1. Locality map showing the Port Albert Peninsula, Squashberry Island and the Indian Islands Fault. The intensely deformed

area associated with the fault is represented by stippling. The inset map of Newfoundland shows the location of the detailed map.

@ Ktt2i Zding .,

/-- Bedding “k- interpreted

+ Dip of beddmg

\ zx%rY

m Dyke

NN Brittle Fault

, ‘@, Vein domains

_-- Domain boundary

TN

1 I ’ ‘W I I

Fig. 2. Map of Squashberry Island showing form-surface mapping of bedding and other geological data. The distribution of 1

their predominant orientation(s) in each domain are shown diagrammatically by means of parallel lines.

/eins and

313

Fig. 3 Bedding/cleavage relationships viewed looking northeast. Bedding is upright and is steeper than c lea\ rage

bedded rocks in which sedimentary structures are

recognizable. The phyllonite is rich in kinematic

indicators that consistently indicate a dextral

transcurrent movement. The veins described in

this paper occur immediately north of the transi-

tion zone and, as will be demonstrated below, they

also have been subjected to dextral shear, but of

smaller magnitude.

On Squashberry Island and an adjacent head-

land, calcite veins occur in such profusion that

they are locally a penetrative fabric element at

hand specimen scale (Fig. 5). These veins are

0 a 0 b

Fig. 4. Orientation data for Squashberry Island. a. Fifty-two poles to bedding. b. Six minor folds (crosses) and 64 poles to axial plane

cleavage (dots).

314

Fig. 5. Closely spaced calcite veins cutting bedding. Bedding trends from left to right across the photograph and the veins are parallel

to the pocket knife. Later quartz veins trend from top to bottom across the photograph.

recessively weathered and in some outcrops the

weathering is so deep that the veins are not visible

as such, but give the appearance of a second

cleavage (e.g., Fig. 5). In some outcrops the veins

are planar (Fig. 5) whereas in others they appear

strongly folded (Fig. 6.) and it will be shown

below that their form is indeed due to folding.

Domains can be defined on the basis of the form

and orientation of the veins and they can be

matched across the island in a way that indicates

that the domains are bedding controlled. This is

particularly clear on the south side of the island,

where a stratigraphic horizon marked by a distinct

colour change (grey/green) and the boundary be-

tween a planar and a folded domain are almost

exactly the same distance apart on opposite shores

of the island (Fig. 2).

Description of veins as seen in outcrop

Six domains of four different types occur on

the island (Fig. 2). One domain (1) is char-

acterized by planar veins that are steeply dipping

and strike approximately east-west. These veins

occupy a series of microfaults that define a

strain-band cleavage. The cleavage is developed in

the regional foliation and locally it overprints

earlier veins that strike in the northeast quadrant.

This domain coincides with the hinges and short

limb of one of the asymmetrical parasitic fold

pairs in bedding. Three domains (2,3 and 6) are

characterized by steeply dipping, planar veins that

strike approximately NE-SW. These domains

coincide with zones of steeply dipping bedding. In

domain 6, which contains some thick shale beds,

the veins refract strongly as they pass through

beds of different grain size. The vein segments in

the incompetent beds are consistently clockwise of

their continuation in the competent beds (Fig. 7)

suggesting dextral shear parallel to bedding. Veins

which make a small angle with bedding are com-

monly boudinaged. Another domain (5) is char-

acterized by folded veins in which the folds are

harmonic through several veins (Fig. 6a, b), are

steeply plunging, and have axial planes approxi-

mately parallel to the steeply dipping bedding.

The remaining domain (4) coincides with the

hinges and short limb of an asymmetric fold pair.

This domain is characterized by two sets of steeply

dipping planar veins that strike approximately

315

Fig.

E- W and NE-SW. These veins intersect one ant Ither and locally a vein of one group appears to be displaced where cut by a vein of the other

grc IUP. However, these possible overprinting rela-

6. Folded veins. Bedding trends from top right to bottom left. Convergent fanning of the regional cleavage can he seen ir

inter-vein lithons in (a) and examples of tight folds can be seen in (b).

tionships do not show any consistency. No fol or boudinaged veins occur in this domain.

The vein o~entations described above are the only ones found in each domain, but are

I the

ded

not the

316

25mm ,

Fig. 7. Field sketch of a horizontal surface showing calcite

veins (black) rotated ductily, and locally boudinaged, in an

incompetent shale bed, and a quartz vein (stippled) offset by

small brittle faults.

dominant orientations. For example, in domain 6

local groups of calcite veins with different orienta-

tions are interspersed with quartz and calcite veins

of yet another orientation. These relationships are

best seen outside the domain, on the headland

immediately southwest of Squashberry Island,

where three groups of calcite veins (V,, V, and & )

are present, each with a distinct orientation (Fig.

8). In this outcrop, the dip of the veins is unusu-

ally shallow and the plunge of the intersection of

bedding and S, is unusually steep. If the block is

rotated, so that the S/S, intersection lineation has

the same plunge as on Squashberry Island, the

vein orientations coincide also. In this outcrop it

can be demonstrated that V, and Vz are older than

V, by their respective ages relative to minor kinks

(Fig. 8). Unfortunately, there is no evidence for

relative ages of V, and V,. In the same outcrop,

there are also quartz rich veins that are vertical

and perpendicular to bedding and that cut calcite

veins of all orientations.

Folds in the veins

In domain 5, where the veins are closely spaced,

the folds are reasonably harmonic through several

veins (Figs. 6 and 9) although in detail the pattern

can be quite complicated, some veins branching

and others crossing. Individual veins are generally

thicker in the fold hinge than on the limbs, but

there is considerable variation (Fig. 9). At one

extreme veins are thinner (measured perpendicular

to the vein wall) in the hinge than on the limb

(class lA, Ramsay 1967, p. 365) and at the other

extreme, veins are several times thicker in the

hinge than on the limb (Fig. 9), even if measured

parallel to the axial plane (class 3, Ramsay 1967,

p. 365). The latter folds are very cuspate in their

outer arc, where they are accompanied by very

strong cleavage refraction, which commonly sug-

gests that vein material has been squeezed out of

the apex of the fold to the point where the walls

0 b

20 mm

Fig. 8. a. Diagrammatic sketch showing the relationship between three vein orientation groups (V,. V2 and V3) and bedding (S), cleavage (S,) and the S/S, intersection lineation. b. Field sketch of a horizontal surface showing the relationships between

representatives of the three groups of veins and a kink. VI and VZ are pre-kink and V, is post-kink.

317

Fig. 9. I ,arge thin section (5 x 7.5 cm) showing bedding and cleavage striking approximately north/south and the morpholog:

folds in the veins. Note the convergent fanning of cleavage and bedding. The arrow indicates a fold that is discussed in the

on op posing limbs, near the hinge, are now in

contac :t with one another (Fig. 10). Such fold

hinges may be so thickened as to have the form of

a lenticular layer, elongate parallel to the axial

plane of the fold and cuspate at one end . This

“layer” commonly shows pinch-and-swell struc-

Fig. 10

stipI

I. I

Microscope sketch of a boudinaged fold closure in a calcite vein. Fibres are well preserved in much of the vein

,led areas represent areas of equigranular calcite and quartz. Note the cleavage convergence. See text for further discus

y of the

text.

but the

318

0 b

Fig. 11. Diagrammatic representation of the development of the relationships between bedding (stippled), veins (heavy black lines)

and cleavage (closely spaced lines); (a) shows original relationships with the veins in the tension gash orientation, (b) shows the

observed relationships with the regional cleavage inclined to the hinges of the folds in the veins (the front surface of the block is

parallel to the vein) but having the appearance of a fanning axial plane cleavage on the horizontal profile plane.

ture, indicating incompetent behaviour in the vein

material, large strains and horizontal extension.

The folds are generally tightest where their

enveloping surface is approximately perpendicular

to bedding. Where the enveloping surface is anti-

clockwise of the normal to bedding, they are con-

sistently more open. Where the enveloping surface

is clockwise of the normal to bedding, however,

there is less consistency. The folds are generally

more open, but locally are just as tight as in the

perpendicular orientation. However, where they

are tight the veins are boudinaged and the boudins

appear to be post-folding. When viewed on near

horizontal outcrop surfaces, the folds have axial-

plane traces that are approximately parallel to the

trace of bedding and cleavage, the latter having

the same strike. In detail, however, the cleavage

fans around the folds in a convergent manner

(Figs. 6, 9 and 10). It is very difficult to measure

the fold plunge, because of the nature of the

outcrop, but wherever fold specimens have been

collected, hinges are found to plunge steeply to-

wards the southwest. Their plunge is steeper than

the plunge of the lineation defined by the intersec-

tion of the vein and cleavage, and is parallel to the

line of intersection of the vein with bedding (Fig.

llb). The folds might therefore be described as

cleavage-transected folds (e.g., Borradaile, 1978) if

the fanning of the cleavage is assumed to indicate

contemporaneity of folds and cleavage. However,

we believe that the relationship is due to over-

printing and that the cleavage is older than the

folds. Several lines of evidence support this con-

clusion: (1) cleavage is a regional foliation that is

consistently overprinted by the phyllonites of the

transcurrent faults; the phyllonites, in turn, are

overprinted by the veins, which are then folded,

indicating that veining and faulting are causally

related. (2) On Squashberry Island the dykes are

only weakly deformed and cut the cleavage. They

are overprinted by the veins. (3) Refraction indi-

cates that the cleavage is at least as old as the

early stages of the folding. Further, fine bedding

lamination within the shales shows exactly the

same fanning as the cleavage (Fig. 9) suggesting

that both have the same deformational history. In

319

addition, microstructures described below indicate

that the veins, and therefore the folds, are younger

than the cleavage.

Interpretation of field data

Wherever consistent overprinting relationships

exist between calcite veins, the older veins can be

said to be clockwise of the younger ones, if the

angle that does not contain the trace of bedding is

considered (e.g., Fig. 8). Also, wherever refraction

of veins is found (e.g., Fig. 7), the veins are rotated

clockwise. This is precisely what is expected if the

veins developed in response to bedding-parallel,

dextral, transcurrent shear, related to the adjacent

fault. Therefore we interpret most of the calcite

veins as tension gashes that developed in an ap-

proximately E-W, vertical orientation and were

rotated clockwise towards an approximately

northeast direction As the veins rotated towards

the normal to bedding they were shortened by

folding. As they rotated beyond the normal they

underwent extension, locally by unfolding and

then boudinage, and elsewhere by boudinage

without unfolding, so that a series of dismembered

folds was preserved.

If the deformation was truly a simple shear,

tension gashes developing at 45” to the shear

plane should not produce folds with enveloping

surfaces perpendicular to bedding that have an

average dihedral angle less than 90 O. Some tighter

folds could be explained by heterogeneity of strain,

but large areas with enveloping surfaces per-

pendicular to bedding are present, in which most

of the folds are tighter than predicted (e.g., Fig. 6).

We interpret this as indicating a situation in which

the deformation combined bed~ng-p~allel simple

shear and bedding-perpendicular shortening com-

ponents; i.e., the vorticity number was less than 1

(Means et al., 1980). The shortening component

could be due to volume loss, but we have no direct

evidence to support this possibility.

Domain 4 does not seem to fit the model

described above. The lack of consistent overprint-

ing relationships suggests that in this domain the

veins are a conjugate pair. It is significant that

conjugate veins should be found in a domain that

coincides with the closure of a shallowly plunging

fold. Because of the folding, this domain lacks the

steeply dipping plane of anisotropy (bedding) that

predominates in the other domains. After folding,

the domain would therefore have been more re-

sistant to the shear component of deformation,

but could be expected to show signs of the bed-

ding-perpendicular shortening. The exact age of

the fold is not known, but it is pre-vein. The

conjugate veins are suitably oriented to have de-

veloped in response to such a shortening and show

signs of a weak dextral rotation as a pair, since

they are not perfectly symmet~cal about the bed-

ding plane (cf. Collins and De Paor, 1986).

As noted above, some quartz veins cut calcite

veins of all orientations and ages and are ap-

pro~mately pe~endicular to bedding in all places

(e.g., Fig. 5). In the rare examples where they are

deformed by bedding-parallel shear, the shear

takes the form of brittle microfaults, so that fault-

terminated vein segments are generally still per-

pendicular to bedding (Fig. 7). All the evidence

therefore suggests that these veins developed late

in the fault history and that they were emplaced

approximately perpendicular to the ductile fault.

They are parallel to a brittle fault direction mapped

throughout the region and they themselves com-

monly give rise to small displacements of the

steeply dipping bedding. We therefore suggest that

they are related to late brittle faulting, rather than

related to the ductile faulting.

Veins in thin section

Typically the veins are composed predomi-

nantly of fibrous calcite and have a narrow rim of

fibrous quartz and chlorite (e.g., Fig. 12). Many

include areas in which large rounded grains of

quartz are surrounded by fairly equant grains of

calcite (stippled areas in Fig. 12a). Elsewhere simi-

lar grains of quartz appear as a line in a fibrous

calcite matrix (stippled lines in Fig. 12a). Such

lines of grains commonly occur along the centre

line of a vein, but they may also run into the vein

boundary (Fig. 12a). The calcite fibres of the

matrix, where unobstructed by a quartz grain, pass

through the line unbroken. These grains do not

apparently represent wallrock fragments (cf.

320

, CALCITE.

MiC A

Fig. 12. Calcite vein approximately in the tension gash orientation. a. Fibres indicated by parallel lines. Stippled lines represent lines

of quartz grains and stippled areas represent areas of approximately equant quartz and recrystallized calcite grains. A thin marker

bed, perpendicular to the section. is represented by the heavy black line. b. Quartz fibres and aggregates of chlorite along the margin

of the vein. See text for further discussion.

Ramsay and Huber, 1983, p. 245) since they are

generally larger than the wallrock quartz grains.

Some veins lack the quartz chlorite rim and

other, very thin veins are composed only of fibrous

quartz and chlorite. Equally thin calcite veins are

rare.

Calcite also occurs as irregular concentrations

in the sandstone beds. Where the concentration is

weak, the calcite occurs as individual grains that

are generally elongate parallel to the strike of

bedding. These grains have aspect ratios of up to 3

or 4 and commonly are terminated at both ends

by detrital quartz or feldspar grains that look as

though they may have originally been single grains

that broke and separated as calcite filled in be-

tween the two fragments.

Where the calcite grains are densely con-

centrated, the concentration may grade (spatially)

into a fibrous vein. Some veins have a gradational

boundary of this type on one side and a sharp

boundary on the other.

Vein calcite microstructures

Since most of the features described below are

not recognizable in 30 pm thin sections, micro-

structural studies have been carried out with dou-

bly polished 2-8 pm sections.

Calcite microstructures in the various fibrous

veins are complex, especially in the folded veins.

Figure 12 is a sketch of a straight vein and demon-

strates many of the common features. Large areas

of the vein are fibrous. The fibres have aspect

ratios in excess of ten and are approximately

equant normal to their long axes. Although in-

dividual fibres cannot be traced from wall to wall

of the vein, their trend can be mapped out, as in

Fig. 12, and they overlap in such a way that no

medial surface is defined by their terminations,

thus indicating an antitaxial origin (Ramsay and

Huber, 1983, p. 262). It can also be seen that the

fibres do not join equivalent points in a thin

marker-bed, across the vein. Even making al-

321

Fig. 13. Vein, approximately in the tension gash orientation, intersecting several beds. Trace of calcite fibres is indicated. Stippled

area represents quartz/chlorite selvage. The section is viewed from below so that the displacement is dextral when viewed from

above as in the field.

lowance for the fact that the fibres may be in-

clined to the surface of the section, it is highly

improbable that they join equivalent points; the

marker-bed is known to be approximately per-

pendicular to the section and fibre aspect ratios

indicate that they can only be inclined to the

section by a small angle. Furthermore, the ob-

servation is true of all six veins in which fibres can

be traced and in which there are suitable markers

(see also Fig. 13).

Locally in the vein shown in Fig. 12 there are

quartz concentrations, as described above. The

zones of equant quartz and calcite grains have a

sharp boundary, which cuts across the vein in a

way that suggests that it may be a late feature, due

to boudinage of the vein during possible anti-

clockwise rotation (see below for discussion of the

rotation). The same microstructure and mixture of

quartz and calcite occurs in the necks of pinch-

and-swell structures in fold hinges (Fig. 10). In

both cases the calcite has been recrystallized to

approximately equant grains that are larger than

the width of the adjacent fibres, but less than the

fibre length.

In veins where the fibres are curved, the fibres

are generally almost optically strain-free (Fig. 14)

or at most only very weakly deformed. However,

several features suggest that they have been quite

strongly deformed and these microstructures are

therefore described in detail.

In some areas of strong curvature, some fibres

do show deformation such that, for example, twin

planes are bent through 15-20”. However, these

fibres are generally sandwiched between two com-

paratively undeformed-looking fibres that show

the same curvature (Fig. 14b). Many fibres show

weak lattice distortion indicative of considerably

less curvature than shown by the fibre boundary.

For example, twins may radiate around a curve

but the dihedral angle between their extreme

orientations may be much less than the angle of

curvature. Though smaller in magnitude, the sense

of curvature of the lattice is generally the same as

that of the fibre. These observations can be inter-

preted in two ways. Either deformation has been

accompanied by recrystallization and the least de-

formed grains are the most recently recrystallized

ones, or all grains showing the same curvature

have undergone the same strain but, because of

variation in orientation, exhibit varying amounts

of lattice distortion (as explained below).

Both mechanisms are reasonable and we be-

lieve that both have operated. There is direct

evidence of recrystallization by grain boundary

migration in some strongly curved areas, in the

form of left-over grains and orientation families

322

Fig. 14. Curved fibres at the margins of veins: (a) is interpreted as having undergone extensive recrystallization by grain boundary

migration; in (b) the fibre overlain by the large bubble shows undulose extinction that is commensurate with its curvature. The grains

to left (at extinction) and right of it appear undeformed despite similar curvature. Crossed nicols. Scale bars 0.2 mm long.

(Urai et al., 1986). This microstructure is generally

found at the margins of veins.

We have no direct evidence of the second

mechanism, but propose it on theoretical grounds.

Assume first that deformation occurs by twinning

or dislocation glide parallel to a single lattice

plane that is parallel to the fibre length. As long as

no other mechanism operates, the curvature of the

lattice and the fibre will be the same. On the other

hand, consider a fibre with the twin or glide plane

perpendicular to the fibre length and constrain the

fibre to undergo a simple shear such that the twin

or glide plane is parallel to the shear plane and-in

the case of twinning-the twin and shear senses

are the same, then the fibre can deform without

any permanent distortion of the lattice. Most

grains should be oriented somewhere between these

two extremes and in general more than one mech-

323

Fig. 14 (continued).

anism will operate. The lattice should therefore

generally record less strain than the fibre boundary

and in an aggregate of variably oriented grains,

adjacent grains will show variable amounts of

lattice distortion, even though the fibres all have

undergone the same deformation.

Another feature commonly observed in areas of

curvature is that what appears to be a curved fibre

in plane-polarized light is seen in crossed nicols to

be segmented. Each segment conforms to the gen-

eral fibre shape and is separated from its

neighbours by a boundary that is approximately

perpendicular to the fibre wall (Fig. 15a). The

segments are several times longer than the width

of the fibre and their aspect ratios vary with the

radius of curvature. Where the curved zone is

adjacent to a broader zone of straight fibres, the

fibre segments in the curved zone are noticeably

shorter than segments in the straight zone. The

relative length depends on the curvature and rela-

tive widths of the zones; in one example where the

straight zone is broad and the curved zone has a

324

Fig. 15. Deformed fibres showing core and mantle structure. In (a) the grain boundary migration is well developed but recrystallized

mantles are only locally developed. In (b) the mantles are much better developed. Note also the polygonization shown by the fibres

where they wrap around the sharp curve in (a). Crossed nicols. Scale bars 0.2 mm long.

small radius of curvature, the relative lengths of

the segments are 6 : 1. The segments in both zones

show no, or very little, lattice distortion. Com-

monly, adjacent segments have identical ap-

pearance in terms of twin lamellae, cleavage and

interference colours and differ only by a small

rotation about an axis perpendicular to the sec-

tion, where the section is parallel to the curved

fibre. The amount of rotation is approximately

commensurate with the curvature of the fibre and

it appears, therefore, that the fibre was originally a

single grain that was bent by the deformation and

polygonized into the present string of subgrains,

which locally have become grains.

Core-and-mantle structure is also observed in

many of the more deformed veins and recrystalli-

zation is common along the boundaries of both

curved and straight fibres (Fig. 15). This recrys-

325

Fig. 15 (continued).

tallization may be extensive, but results in a very

small grain-size compared to the large fibrous,

recrystallized grains described above (Fig. 14). It

is associated with fibres that show little or no

curvature or with curved fibres that appear poly-

gonized. It is not found in association with the

large recrystallized fibres.

In summary, the calcite fibres are strongly

curved and, in view of the polygonization and

recrystallization, as well as the sparse strongly

deformed fibres, we believe that the curvature is

due to deformation. However, lattice distortion, is

weak compared to fibre curvature owing to three

factors. First, plastic deformation does not gener-

ally result in lattice curvature that is as strong as

the curvature of the fibre and the relationship

between the magnitudes of the two is generally

not simple. Second, recrystallization by grain

boundary migration has replaced deformed lattice

in some fibres, while preserving a fibrous mi-

326

Fig. 16. Small shear zone cutting fibres on the limb of a fold. Note the curvature of the fibres and note the internal foliation in the

shear zone inclined to the shear zone boundary. Normal thickness thin section; crossed nicols. Scale bar 1 mm long.

crostructure. Third, polygonization has converted

some deformed fibres into strings of subgrains,

some of which have progressed to grains.

It is not clear to us why polygonization, accom-

panied locally by rotational recrystallization to

produce a mantle of small equant grains, has

occurred in some areas of curved fibres, whereas

grain boundary migration, producing large fibres,

has occurred in others. The variation could be due

to differing amounts of strain energy in the two

areas. On the other hand, both processes operated

in equally curved grains, suggesting some other

reason. However, the strain energy factor cannot

be eliminated completely, since we do not have

enough samples to allow us to equate curvature

and magnitude of lattice strain energy. It may be

significant that wholesale recrystallization of fibres

occurs mainly along the margins of veins. Pre-

liminary calcite staining experiments suggest that

the veins are compositionally zoned, and this

321

would give rise to differences in grain boundary

mobility, which may have determined which of the

two processes operated. Alternatively, the larger

grains could have formed early in the deformation

history, when a falling temperature was still high

enough to cause extensive grain boundary migra-

tion; the strings of subgrains and core-and-mantle

aggregates, then formed later at lower tempera-

tures. This alternative requires that early deforma-

tion be localized at the margin of the vein and

later deformation be localized in the centre. We

have no reason to suspect such localization of

deformation, but do have preliminary evidence

that the veins are compositionally zoned. Thus, in

the absence of definitive evidence, we favour the

explanation based on compositional variation.

In local narrow zones of more intense deforma-

tion the fibres are more obviously deformed and

recrystallized. For example, in one folded vein,

there is a narrow zone on a limb of the fold,

approximately parallel to the vein wall, that might

be described as a mylonite zone. It is a zone of

small, slightly inequant grains that show a dimen-

sional preferred orientation, with the long axis

inclined to the zone by approximately 30 O. Fibres

adjacent to the zone are consistently curved (Fig.

16) and clearly show the subgrain structure de-

scribed above. We interpret the microstructure of

the zone in terms of a C/S fabric and both the

C/S relationship and the curvature of the fibres

indicate the sense of movement that is expected

for the position of the vein relative to the fold (i.e.,

it is consistent with flexural slip).

The detailed appearance of fibre boundaries in

the various veins is markedly variable. In some

veins the boundaries are very straight and sharp,

whereas in others they are much more irregular

and slightly fuzzy. Locally they may even have

interlocking protuberances, some of which may

comprise subgrains. Finally, as mentioned above,

some fibres display a core-and-mantle structure

(Fig. 15) the mantles varying in thickness from

one or two grains to many grains. The gradation

from straight, sharp boundaries to the core-and-

mantle structure is complete and we interpret it as

an evolutionary series and as a rough measure of

increasing strain.

It is very noteworthy that there is a qualitative

correlation between vein orientation and the na-

ture of the boundaries. Describing the orientation

with respect to the instantaneous ellipse, for a

bedding-parallel dextral shear, veins in the origi-

nal tension-gash orientation mostly have straight

fibres, the boundaries of which are generally

straight. Straight fibres in the folded veins, where

the enveloping surface is approximately per-

pendicular to the’shear plane, mostly have irregu-

lar boundaries, but locally display core-and-man-

tle structure, whereas veins in the extensional field

may show irregular boundaries, but more com-

monly display core-and-mantle structure. This

correlation is consistent with our interpretation of

the veins as tension gashes in various stages of

rotation towards the shear plane.

Quartz /chlorite veins and selvages

Many of the calcite veins, of all orientations,

have a “selvage” of quartz and chlorite and much

thinner veins are composed solely of quartz and

chlorite (Fig. 17). Both show the same microstruc-

ture, being composed of short quartz fibres and

similarly shaped zones of chlorite in a quartz

matrix. The chlorite-rich zones are elongate paral-

lel to the adjacent quartz fibres and the chlorite is

consistently inclined to the length of the zone

(Fig. 17).

At the vein wall, the chlorite faithfully mimics

the orientation of the mica grains in the country-

rock. The pelitic rocks have a strong preferred

orientation of the mica and an equally strong

preferred orientation of the chlorite. Locally, at

the vein contact there are small domains in which

the mica grains are inclined to the foliation. Such

irregularities are reflected in the orientations of

the adjacent chlorite grains. In some of the sand-

stones, the mica grains have no obvious preferred

orientation, and chlorite grains in veins in such

sandstones also show no preferred orientation.

One specimen has a crenulation cleavage that is

not seen elsewhere. A vein passes through the

crenulated material and the crenulation is recog-

nizable in the chlorite in the vein. The chlorite

microstructure differs in detail from that of the

mica-rich country rock in that, whereas the mica

grains are bent around the crenulations, the chlo-

328

Fig. 17. Quartz vein with inclusions of chlorite. Near top centre the sparse inclusions are concentrated in one of the fibres and extend

almost to the mid point of the vein. Along the lower wall of the vein the chlorite grains are more widespread and are not concentrated

in any particular fibres. In both areas the chlorite grains are inclined to the length of the fibres and at the vein wall they are parallel

to adjacent mica grains. Note the subgrain development in the quartz and the serrated grain boundaries that are evidence of grain

boundary migration and which show pinning by inclusions. Scale bar 0.2 mm long.

rite grains are straight. However, the chlorite grains host rock mica grains, and (2) that the veins, at

are arranged in domains that reflect the orienta- this stage in their development, grew by addition

tions and spatial distribution of the different mica of material at the vein wall. The second conclusion

orientations around the crenulations. is based on the following argument. If a chlorite-

We interpret these observations as indicating rich fibre grows at its end furthest from the vein

(1) that the orientation of the vein chlorite grains wall, the chlorite grains must be in contact for the

was strongly controlled by the orientation of the orientation of the new grains to be controlled by

329

Fig. 18. a. Microscope sketch of a folded calcite vein with walls and screens of slate. b, c and d. Details of chlorite aggregates

interspersed with quartz fibres in the margin of the vein. Light shading represents the distribution and orientation of mica in the vein

wall and heavy shading represents the distribution and orientation of chlorite in the vein. Stippling represents calcite grains and white

areas represent quartz. See text for further discussion,

the orientation of the old grains. However, we

observe en echelon arrays of chlorite grains that

are not in contact (e.g., the bottom right-hand

corner of Fig. 18~) and so this model allows no

explanation for the preferred orientation of the

newest grains. However, if the growth occurs at

the vein wall, there is always an adequate supply

of mica to control the orientation of the chlorite,

which can then be detached and carried away

from its original host. The possibility that the

distal chlorites are in contact in three dimensions

cannot be totally overruled, but their rather large

spacing in areas such as that represented in Fig.

18 suggests that they are not in contact.

Irrespective of the orientation of the vein, the

chlorite fabric always has the same orientation

330

relationship to the length of the fibre in which it

occurs. This point can be demonstrated by refer-

ence to a folded vein (Fig. 18). Figure 18a shows a

folded calcite vein with a screen of country rock.

Fibrous quartz/chlorite aggregates alternate with

quartz fibres in a selvage adjacent to the screen.

The orientation relationship between the chlorite

grains and the length of the aggregate and adjac-

ent quartz fibres is shown (Fig. 18b, c and d) for

the two limbs and the hinge of the fold. The

relationship is the same for all three; i.e., the

chlorite fabric crosses the aggregates from upper

right to lower left (Fig. 18b, c and d).

This relationship is readily explained by the

model proposed above for development of the

folded veins by shear rotation of tension gashes. If

it is assumed that quartz fibres grow perpendicu-

Fig. 19. Model for the development of the folded veins and the

fibre microstructures. The stippled bands represent bedding

and the light cross-hatching of the veins, represents the quartz

and calcite fibre orientation. Heavy cross hatching within the

fibres represents the local distribution and orientation of chlo-

rite. The sketches depict horizontal surfaces so that cleavage,

which is not shown, would be parallel to bedding. (a and b)

show possible starting situations and the other diagrams repre-

sent veins modified by deformation. (c, d and e) and (g, h and

i) can be combined to give the microstructure of the fibres in

individual folds. For further discussion see text.

lar to the vein wall and that chlorite mimics the

orientation of the country rock foliation, then in

undeformed tension gashes, inclined to the bed-

ding-parallel (as seen in a horizontal section, Fig.

11) country rock foliation by approximately 45 O,

the chlorite will grow inclined at approximately

45” to the fibre length (Fig. 19a or b). If the vein

is then rotated dextrally, such that it folds (Fig.

11) strain in the fold limbs and hinge will be

partitioned, as shown in Fig. 19~2, e and d, respec-

tively. The relationship between the chlorite fabric

and the fibre length is preserved, although the

angles are changed in magnitude, and the re-

sultant microstructure is the same as that observed

(cf. Fig. 18).

As with calcite, indications of strain within the

quartz fibres are consistent with our model. Where

quartz/chlorite veins occur in the tension gash

orientation and the fibres are perpendicular to the

vein wall, the fibres are composed of single unde-

formed quartz grains. In veins that have been

rotated, where the fibres are inclined to the vein

wall and bent, the fibres display undulose extinc-

tion and subgrain development and when not

strongly deformed are recrystallized to a fine-

grained aggregate of approximately equant grains.

Modelling of complex veins with internally folded fibres

The model proposed above (Fig. 19a-e) is ca-

pable of explaining most of the observed micro-

structures, but does not explain the internal folds

observed in the fibres of some veins. According to

popular interpretation of fibres (e.g., Durney and

Ramsay, 1973; Ramsay and Huber, 1983) the

curvature might be considered a growth feature,

rather than a product of folding. However, we

have demonstrated that the curved fibres that we

observe are in fact deformed and we are unable to

reconstruct feasible opening histories for many of

the veins, if we assume that the fibres grew in their

curved form while tracking the opening direction.

Therefore, in rnodelling the curved fibres, we start

from the hypothesis that the curvature is due to

folding.

We observe veins, that are close to the tension

gash orientation, that make an angle of less than

331

45” with bedding. In such veins the fibres are

inclined to the vein wall at an angle of less than

90” (as in Fig. 19f). Such veins can be interpreted

as tension gashes that have been back-rotated (i.e.,

rotated anticlockwise), due to the bedding-per-

pendicular flattening component of deformation

being more significant than the bedding-parallel

shear component during the early history of the

vein. If such a vein is then subjected to the normal

rotation, due to an increase in the significance of

the shear component, the vein-parallel shear that

accompanies such rotation results in shortening

parallel to the length of the fibres. Thus, as the

vein rotates, the fibres can be expected to fold, as

shown in Fig. 19h and 19i. Axial planes should

develop perpendicular to the shortening direction,

but in order to do so would need to deform the

vein wall. Thus, because of the low competency of

the vein, the folds might be constrained to form

with their axial planes parallel to the vein wall in

some if not all cases (Fig. 12).

We interpret the vein in Fig. 12 as a tension

gash that was initially back-rotated with concom-

itant rotation of the fibres and boudinage of the

vein. Then the upper part of the vein was rotated

clockwise and the fibres folded. This stage would

be intermediate to Fig. 19f and h. Further rotation

should produce folds with microstructures on their

alternate limbs and hinges as shown in Fig. 19g, i

and h, respectively. The complex vein depicted in

Fig. 20 can be explained in this way. The shear is

dextral parallel to the trace of S,. The left limb is

therefore equivalent to Fig. 19i and has well-devel-

oped folds in the fibres. The fibre folds die out

towards the vein hinge and fibres in the right-hand

limb make a very acute angle with the vein wall up

to where they pass into the attenuated part of the

limb; in this narrow, high-strain area the calcite is

completely recrystallized and the fibrous mor-

phology obliterated.

Discussion and conclusions

The project described here started as an at-

tempt to analyse the history of the veins on the

basis of the popular hypothesis (Durney and

Ramsay, 1973; Ramsay and Huber, 1983) that

vein fibres track the direction of opening of the

veins. Patterns in the straight veins can be inter-

preted in this way, but the opening directions in

different parts of even the simpler folded veins

(i.e., folds with a microstructure combining the

geometry of Fig. 19c, d and e), are internally

incompatible. Further, we are able to demonstrate,

wherever suitable markers are available, that the

fibres do not join points on opposite sides of the

veins, that were originally coincident. Not very

many of the veins contain suitable markers, but

the six that do all demonstrate this point. We

therefore conclude that ’ fibres in the veins de-

scribed in this paper did not track the vein-open-

ing direction. We cannot be sure of their original

orientation; however, we interpret the fibres as

having grown perpendicular to the vein wall, since

a common observation is that fibrous grains gen-

erally grow perpendicular to the bounding surface

of cooling bodies, such as undeformed pegmatite

veins, ice and metal ingots. Further, in the rocks

described here, fibres in the veins showing the

least evidence of deformation are inclined to the

vein wall, at angles approaching 90 O. Having in-

terpreted the original microstructure in this way,

we are able to model all the observed structures in

a simple manner that is consistent with the overall

shear. The model is summarized in Fig. 19.

Deformation took place in the margin of the

ductile Indian Islands transcurrent fault. Because

the bedding, which constitutes the major plane of

anisotropy and the fault are approximately paral-

lel, the deformation had a large component of

bedding-parallel simple shear which, like the fault

itself, was dextral. During a progressive deforma-

tion, veins formed in the tension gash orientation

rotated clockwise and folded as they passed

through the normal to bedding. If rotated far

enough, the folded veins underwent stretching,

which resulted in boudinage and the unfolding of

some folds. During this progressive deformation

the internal structure of the veins was modified as

summarized in Fig. 19a-e.

In addition to the bedding-parallel shear, the

deformation had a bedding-normal shortening

component and some veins were rotated anticlock-

wise in response to this shortening before being

rotated clockwise by the shear component. This

means that the two components must have varied

332

the folds. However, we are now convinced that

these microstructures are a product of the fault-re-

lated deformation and folding. We also recognize

that the fibres have been deformed more than is

apparent from casual observation of the lattice

distortion. Lattice distortion, as discussed above,

has been minimized by several factors: (1) Even if

there has been no recovery, lattice curvature will

generally be considerably weaker than the fibre

curvature. (2) Grain boundary migration in some

areas of the veins and polygonization elsewhere

have eliminated the lattice distortion from many

grains whilst preserving their general fibrous form.

(3) Much of the deformation was achieved by

straight fibre segments sliding past one another,

thus concentrating lattice distortion at the fibre

boundaries where, at relatively high local strains,

it gave rise to recrystallization of the fibre mantle

but still preserved the fibrous appearance of the

core. At low strains it only resulted in weak grain

boundary migration, giving rise to a more irregu-

lar boundary, but preserving the overall fibre

shape.

The validity of the last mechanism can be dem-

onstrated using para-di-chloro-benzene in “see-

through deformation experiments” (e.g., Means,

1977; Urai et al., 1980). A suitable starting

material, with fibrous crystals at a high angle to

the shear plane, can be prepared by arranging a

temperature gradient normal to the shear plane.

We have sheared such aggregates through 45 ‘=‘,

without destruction of the fibres, to produce

material very similar to that shown in Fig. 15.

In summary, the fibres described here did not

generally track the opening of the veins. It is

possible that some veins did open parallel to the

direction in which the fibres were growing, but

there was no cause-and-effect relationship be-

tween the two. We suggest that the fibres probably

grew perpendicular to the vein wall. Their curved

appearance is due to later deformation, rather

than to variation in the vein opening direction. It

is possible that the rocks described here are un-

usual and that fibres more generally do track

opening directions. However, it is also possible

that our conclusions are more generally applicable

(see also Cox, 1987). In view of the apparently low

intragranular strains, had we not had the folded

Fig. 20. Microscope sketch of a complex folded calcite vein

with a quartz selvage (stippled) plus several minor quartz veins

(stippled) at the right-hand side. The fibre morphology is

indicated in the calcite vein and in the thick quartz vein.

Stippling in the calcite vein indicates scattered quartz grains

and on the right-hand limb of the fold represents an area of

equant quartz and calcite. This narrow limb is interpreted as a

zone of high shear strain. Less intense shear strain on the

thicker left-hand limb has resulted in folding of the calcite

fibres. See text for further discussion.

in intensity with time at any given point in and

adjacent to the fault zone. It does not necessarily

imply a two-stage history, but could be due to

local heterogeneity during a progressive deforma-

tion. As a result of the double rotation, the vein

fibres became folded independently within the

veins (Fig. 20). The model is summarized in Fig.

19f-i.

The most surprising results of this study are

that the fibres are preserved through much larger

strains than we initially suspected from the ap-

pearance of the fibres. We noted undulose extinc-

tion, bent twins and localised recrystallization,

and initially interpreted these features as the prod-

uct of a late, weak overprint that was unrelated to

333

veins that proved intractable in terms of the track-

ing hypothesis, we would probably never have

realized that our fibre orientations were due to

strain. This may be true of other examples in the

literature and we suggest that all such fibrous

veins need checking very carefully.

Even though the rocks described here may be

unusual, the fact that some fibres have not tracked

the vein opening direction means that all kine-

matic analyses based on that assumption are sus-

pect. Before such an analysis can be considered

valid, the relationship between fibre growth and

opening direction must be demonstrated and it is

not clear to us how this can be done. It is not

sufficient to show that the fibres join originally

coincident markers, since this configuration may

be common even where fibres have grown per-

pendicular to the vein wall (as represented in Fig.

19a).

Finally, we draw attention to the fact that the

rocks described here provide an example of clea-

vage-transected folds, in which the cleavage, de-

spite refraction, which suggests a causal relation-

ship, is totally pre-folding. In fact, the cleavage is

related to a different generation of structures,

which may be separated from the ductile faulting

episode by a considerable time.

Acknowledgements

The writers wish to thank Drs. Mark Jesse11

and Ron Vernon for helpful reviews, Sherri

Townsend and Diane Tabor for word processing

and Paul Chenard and Bob McCullock for pre-

paration of the figures. P.F.W. acknowledges

financial support from NSERC grant number

A7419 and E.M.R. research agreement 71. J.L.U.

acknowledges financial support from NSF grant

# EAR 8306166 and from a Huygens fellowship

of the Netherlands Organization for the advance-

ment of Pure Research (ZWO).

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