Curved vein fibres: an alternative explanation · 2012. 2. 5. · Tectonophysics. 158 (1989)...
Transcript of Curved vein fibres: an alternative explanation · 2012. 2. 5. · Tectonophysics. 158 (1989)...
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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