Superposed Deformations and Their Hybrid Effects
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Transcript of Superposed Deformations and Their Hybrid Effects
Journal of the Geological Society, London, Vol. 160, 2003, pp. 117–136. Printed in Great Britain.
117
Superposed deformations and their hybrid effects: the Rhoscolyn Anticline
unravelled
SUSAN H. TREAGUS, JACK E. TREAGUS & GILES T. R. DROOP
Department of Earth Sciences, University of Manchester, Manchester M13 9PL, UK (e-mail: [email protected])
Abstract: This study of the controversial structures of the Rhoscolyn Anticline suggests a different result of
two-phase coaxial deformation from Ramsay’s Type 3 interference fold patterns. From detailed field
observations of the sequence of bedded quartzites, psammites, pelites and oblique quartz veins, with their
strong competence contrasts, we conclude that the Rhoscolyn Anticline was an original tight, upright F1
anticline that has undergone modification and distortion in a second deformation (D2). This second
deformation is an oblique, but near-vertical, pure shear, with a quantifiable strain ratio (R ¼ 3) that altered the
Rhoscolyn Anticline and its minor structures into a more open, SE-overturned antiform, with c. 260 m hinge
migration. Refolded folds are rare, but hybrid F1 þ F2 minor folds and their fabrics, especially in the region
between old and new hinges, provide clues to the two-stage history. Oblique distortion of originally NW-
verging F1 minor folds has resulted in their apparent neutral vergence in the present-day hinge of the
Rhoscolyn Anticline. We regard the structures and fabrics in quartzites and psammites as more reliable
indicators of the region’s deformation history than those in pelites or quartz veins, and this may prove true for
other regions of polyphase deformation.
Keywords: Anglesey, polyphase processes, superposed deformation, folds.
Observations of geological structures such as folded fabrics,
crenulation cleavages and folded folds, lead to the conclusion
that a region has undergone polyphase deformation. The nature
of this polyphase deformation can be understood from the 3D
geometry of refolded folds and their sectional interference
patterns (Ramsay 1962; 1967, p. 531; Ramsay & Huber 1987, p.
492), and from the geometry of multiple deformation fabrics in
the field or thin section, such as crenulation cleavages (Passchier
& Trouw 1996, pp. 84–88) or folded lineations (Ramsay &
Huber 1987, pp. 481–484). In this paper, we consider an area of
polyphase deformation whose structures have led to the emer-
gence of many different interpretations, with significant implica-
tions for the tectonic history of the region.
Our study focuses on the Rhoscolyn Anticline located on Holy
Island, Anglesey (Ynys Mon), a 2 km2 coastal area of Monian
Supergroup rocks that is a popular place for teaching elementary
mapping of distinct lithologies around a major fold structure, but is
equally useful for investigating the structures of polyphase defor-
mation from the large to small scale. The area was chosen by Price
& Cosgrove (1990, pp. 482–490) as a case study for structural
analysis of multiple deformation, and by Lisle (1988) to question
the application of vergence principles in refolded regions. The
structures in this area have given rise to many different interpreta-
tions, some of which are illustrated in Fig. 1 (Greenly 1919;
Shackleton 1969; Cosgrove 1980; Phillips 1991); other interpreta-
tions include those of Barber & Max (1979), Lisle (1988), Roper
(1992) and Hudson & Stowell (1997). Many of these workers differ
in their interpretation of the number and significance of the
deformation phases that gave rise to the fabrics and folds. In
essence, these simplify into whether the Rhoscolyn Anticline is a
major first fold, as first proposed by Shackleton (1969) (Fig. 1d),
that is overprinted and modified by later folding and fabrics, or is a
later antiform, that refolds earlier structures, as exemplified by the
other illustrated interpretations (Fig. 1c, e and f).
The Rhoscolyn area provides ample exposure of the follow-
ing types of structural criteria, which have been used variously
to back the different interpretations. Major and minor folds
with clear vergence relationships change their sense around the
Rhoscolyn Anticline. Foliations vary from grain-shape fabrics
in quartzites and psammites, to a dominant crenulation clea-
vage in pelites, with intermediate rocks showing folded
cleavage within a bed, and sometimes two cross-cutting
cleavages. Folded quartz veins are abundant in pelite beds, and
roughly track the first cleavage. Despite all these features,
textbook-style coaxial refolding patterns of Type 3 (Ramsay
1967) are rare, leading us to wonder whether coaxial refolding
is the primary method of superposed deformation in these
rocks. Critical questions in the field, here, are whether folds on
different scales are ‘first’ or ‘second’, whether their vergence
is significant, and how the different cleavages in different rock
types relate to their folds.
We will propose a two-phase model that can account for many
of the ambiguities in this area, and reconcile some of the
differences among previous interpretations outlined more fully
below. Our model and structural observations are restricted to the
tripartite South Stack Group, and we leave investigation of the
differently deformed overlying New Harbour Formation for
inclusion in our continuing investigations of the geology and
structure of NW Anglesey. Our approach begins by reviewing
mechanisms of superposed deformation and polyphase folding,
with special focus on coaxial refolding in rocks with competence
contrasts. This leads to our specific investigation of the structures
associated with the Rhoscolyn Anticline, and the development of
a quantitative model for the two-phase deformation and folding
history. Our model has implications in general for polyphase
deformation in metasedimentary rocks with competence con-
trasts, and for the regional tectonic history of the Monian
Supergroup of Anglesey.
Fig. 1. (a) Regional setting and (b) summary map of the Rhoscolyn Anticline, Holy Island, Anglesey, and the principal previous interpretations and
polyphase history in schematic cross-sectional form of (c) Greenly (1919), (d) Shackleton (1969), (e) Cosgrove (1980) and (f) Phillips (1991). In (c), (e)
and (f), the left-hand diagrams illustrate ‘D1’ stages. B, Borthwen; BD, Bwa Du; CG, Coastguard; PS, Porth Saint; v, volcanic rocks; Q, quartzite.
S .H. TREAGUS ET AL .118
Mechanisms of superposed deformation and coaxialrefolding
Refolded folds are probably the least-disputed evidence for
polyphase deformation. Fold interference patterns in map or
section view (Ramsay 1962, 1967, chapter 10; Ramsay & Huber
1987, chapter 22; see also Thiessen & Means 1980) reveal the
variety of geometry that can arise from two phases of folds,
according to the mutual relationships of their axes and axial
planes. Ramsay’s classification (Ramsay 1962, 1967, chapter 10;
Ramsay & Huber 1987, chapter 22) of fold interference (Types
1–3), is based on two superposed phases of similar folding with
the same wavelength and amplitude, and has been illustrated by
spectacular examples from gneissic rocks.
In this paper, we are interested in the processes of two coaxial
phases of folding, as for the Rhoscolyn structures. This means
that any interference geometry is revealed fully in the shared fold
profile plane of the two phases, and might be expected to show
Ramsay’s Type 3 refolding, or Type 0 (no refolding), according
to the orientation of the superposed shear folds. Ramsay & Lisle
(2000, pp. 885–901) provided a detailed case study of Type 3
interference caused by two perfectly orthogonal superposed
phases of shear folding (Fig. 2a and b). They reveal the complex
fold patterns and strain histories that result, and discuss some of
the anomalies in small-scale structures and fabrics that might
arise. The most significant effect in Fig. 2b is that the first folds
can be clearly traced, with only small differences locally between
original hinges and final hinges, whereas the second folds are
less persistent, with jumps in axial planes. This model does not
consider the complexities that might arise from differences in
scale of first and second folds, or of major and minor folds and
their vergence.
The use of major and minor folds and of vergence of folds and
cleavage has become an established method in geological map-
ping of folded regions, and the use of vergence in areas of
refolding was discussed by Bell (1981) and Weijermars (1982).
Both suggested that in areas of coaxial refolding, the vergence of
first-phase structures would not be changed by second folding,
and so reversals of vergence (vergence boundaries) could be used
to identify early folds. This was tested by Lisle (1988), who
considered the effects of superimposing a set of second similar
(shear) folds on a bed containing a first cleavage, and he
demonstrated that anomalous ‘cleavage vergence’ and vergence
changes can arise. The study by Lisle (1988) is particularly apt
for our paper, as it appears to have been prompted by specific
structures and vergence features of the Rhoscolyn Anticline, and
examples from here were used to illustrate small-scale vergence
reversals that might lead to misinterpretations of the large-scale
structure. Lisle concluded that vergence may be an unreliable
structural tool in areas of polyphase folding.
The models discussed for refolding, and for the case of coaxial
refolding that we focus on in this paper, have so far involved
superposed similar folds, without any concern for their mech-
anics of origin. However, any discussion of folding or refolding
in rocks with competence contrasts, or of minor and major
folding, cannot ignore the origin of first and second folding; that
is, the buckling mechanics. We thus turn to the question of
whether 2D fold interference effects of the kind shown in Fig. 2b
(after Ramsay & Lisle 2000) would be produced by orthogonal
deformations, in layers with competence contrasts.
Many analogue model studies have considered two-phase
folding in layered materials with rheological contrasts
(Watkinson 1981; Ghosh et al. 1992, 1993; Grujic 1993; Johns
& Mosher 1996), but have been principally concerned with
cross-folding (Type 1 or 2 interference patterns) rather than
coaxial refolding (Type 3). These cited studies all reveal the
importance of the first fold geometry on the development of Type
1 or 2 fold interference in mechanically active two-phase folding.
Ghosh et al. (1992, 1993) classified modes of superposed
buckling, and revealed the 3D complexities that arise in interfer-
1 1
2 2
33
44
a b
c d
Fig. 2. Models for two-phase folding with
parallel fold axes and orthogonal shortening
directions. (a) First phase similar-style folds
with four numbered marker layers
(continuous curves), and F1 axial planes
(dashed lines). (b) Classical Type 3
interference, according to Ramsay’s model
of similar refolding, with F2 folds having
the same geometry as the F1 folds, after
Ramsay & Lisle (2000, fig. 35.6). Same
line symbols as in (a), but also showing
discontinuous F2 axial planes (dotted
traces). (c) Reversal of the F1 folds in (a)
by active unfolding, as a result of ‘equal
and opposite’ D1 and D2 deformations (see
text for details). The layers become straight,
and the original F1 axial planes are folded,
and an associated cleavage (S1) would be
folded on orthogonal F2 axial planes
(dotted). (d) An alternative version of equal
and opposite D1 and D2 deformation, but
where D2 is a passive pure shear
modification of the F1 folds shown in (a).
The folds are opened by homogeneous D2
strain, and F1 axial planes undergo
shortening and folding in a similar fashion
to (c).
THE RHOSCOLYN ANTICLINE UNRAVELLED 119
ing folds of different (major, minor) orders. Johns & Mosher
(1996) examined the effects of varying the competence contrasts.
However, we are unaware of any experimental studies that
examined these variables for coaxial superposed buckling, to
investigate whether Type 3 interference patterns are produced in
layered systems with competence contrasts, or what form they
might have. Our discussion must therefore concentrate on theor-
etical principles.
Let us consider a multilayer comprising competent and
incompetent layers in a first phase of layer-parallel compression,
that produced sinusoidal buckles with 608 limb dips (as in Fig.
2a). If we wish to consider the effects of a second deformation,
equal but orthogonal to the first, for comparison with the similar
folding model (Fig. 2b), we need first to estimate the strain
associated with these first folds. If produced by buckling alone,
this would indicate 35% layer shortening, but it allows for no
internal layer shortening before or during folding. From a review
of analogue and experimental models of single and multilayer
folds (Treagus 1997, figs 19.4 and 19.5), a shortening of 47%
seems a more realistic figure, equivalent to a plane strain with
strain ratio of R ¼ 3:5. The example in Fig. 2 can thus be
considered as two phases of pure shear, each with R ¼ 3:5, the
first (D1) parallel to layering, the second (D2) orthogonally, with
shortening parallel to the axial planes of first folds. (The total
strain is therefore zero.) Instead of assuming that D2 causes a
sinusoidal second displacement pattern, we will consider the
likely response of a mechanically active system.
If the layers behave according to Newtonian rheology, and
there are no changes in layer viscosities, a D2 deformation ‘equal
and opposite’ to D1 would simply reverse the folding instability,
and unfold the layers to their initial straightness (Fig. 2c). Such
perfect reversals of deformation can be produced in analogue
fold models, and in finite-element models with Newtonian flow
laws (S. H. Treagus, unpublished data). Within this ‘unfolded
fold’ (Fig. 2c), a hypothetical first cleavage (assumed parallel to
XY for D1) might become crenulated, to develop an orthogonal
second cleavage. Therefore, although the cumulative effects of
folding and unfolding here lead to zero bulk strain in the system,
this would not be obvious if the two-stage effects were preserved
in fabrics within competent and incompetent layers.
An alternative type of superposed D2 deformation, equal and
opposite in strain value to the first, is shown in Fig. 2d. This is
the result of assuming D2 strain is a homogeneous pure shear of
R ¼ 3:5. The effect is also to open up the first folds of Fig. 2a;
but passively, by changing the limb dip from 608 to 268, to create
a distorted hybrid ‘first fold’. As for the previous model, this
superposed pure shear might shorten and crenulate any first
axial-plane cleavage, to produce an orthogonal second cleavage.
In this model, the polyphase effects would be manifested in
deceptively gentle folds of bedding, but potentially stronger folds
of an earlier axial-plane fabric. This quasi-passive model does
not require that the layers maintain the same rheological
contrasts in D2 as in D1.
It would require quantitative models, and adoption of specific
values for layer viscosities, to reveal the true mechanical behav-
iour of superposed deformation on a particular suite of earlier D1
structures. The modelling would need to be two-stage: first to
generate the first folds; then to deform these again, orthogonally,
under the same model conditions. We are unaware of any
modelling of this kind. From theoretical studies of strain
variations across competence contrasts in layered systems or
related to folding (Treagus 1988, 1993, 1997), we think the finite
deformation effects would be complex. The simplest conceptual
model is to assume quasi-passive D2 deformation, of the kind
shown in Fig. 2d. This may be a suitable approximation to the
bulk response, for some patterns of first folding, and will be
pursued below.
The likelihood of developing new second folds by active
folding or refolding in a multiplayer (rather than unbuckling or
passively distorting the earlier folds in either of the manners
shown in Fig. 2c and d), will depend on satisfying two main
requirements.
(1) Are there sufficient ‘straight’ sections of layering around
first folds that remain in compression for second folds to initiate?
Previous analogue models of buckling of layers in oblique
shortening (Beech 1969; Treagus 1972) showed that significant
folds were produced only in layers initially ,258 to principal
shortening. The orientation does not affect the wavelength and
folds should initiate symmetrically (Treagus 1973), but an S or Z
asymmetry would subsequently develop according to the sense of
obliquity to the strain. Applying these results to (second) folding
of layers in hinges or limbs of first folds, we see that no part of
the folds in Fig. 2a would be in suitable orientation for second
folding. Where the two deformations are not orthogonal, or the
first folds are asymmetric, second folds might develop preferen-
tially on alternate limbs of first folds. Only where first folds are
nearly isoclinal and straight-limbed, might significant second
folding be seen on both limbs (not necessarily with the same
symmetry; Ramsay 1967, fig. 10.21).
(2) Does the layering (singly or multiply), in any section
around a first fold, fulfil the mechanical requirements for
buckling (Johnson & Fletcher 1994) into new folds? It is difficult
to explain how a new set of buckles could properly form, having
the same wavelengths as the first (to satisfy the mechanics of the
system), when a set of earlier folds already exists. One explana-
tion is to assume there are reductions of viscosity contrasts in the
second deformation that would reduce the dominant wavelengths
for buckling and allow smaller folds to grow, which would also
reduce the buckling response. Alternatively, a half fold wave or
whole waves (a ‘fold pair’) might be localized on first fold limbs,
especially on ‘long limbs’, so that the wavelengths of the second
folds are controlled by the size of the first fold limbs, rather than
by the buckling mechanics.
From this reasoning, we conclude that development of new
second folds, or proper coaxial refolding of earlier first folds,
should be the exception in mechanically active systems with
competence contrasts subjected to this type of superposed
deformation. Second folds are likely to be on a smaller scale
than first folds, and systematic fold patterns require first folds
that are straight-limbed and tight to isoclinal. These conclusions
highlight the different interplays of mechanics and geometry: the
manner in which the first folds interfere with, or favour, second
folding, for different orientations of superposed deformation, that
potentially give rise to Types 1, 2 or 3 interference. In rocks with
competence contrasts, we consider that Type 3 interference is
likely to be localized and rare, compared with Types 1 and 2.
This theoretical discussion and its conclusions can now be
applied to real rocks, to help unravel the polyphase structures of
the Rhoscolyn Anticline.
The Rhoscolyn Anticline: review and criticalobservations
Following our brief introduction to the Rhoscolyn Anticline
above (Fig. 1), we now consider it in more detail as a case study
for revealing the superposed effects of two coaxial folding
deformations in layered rocks with competence contrasts.
S .H. TREAGUS ET AL .120
Review of geology and previous structural interpretations
The dominant structural feature of the Rhoscolyn area of Holy
Island, Anglesey, is the Rhoscolyn Anticline or Antiform (Fig. 1a
and b), illustrated in our map and profile (Fig. 3); detailed
observations are described in the next section. There have been
several conflicting interpretations of the Rhoscolyn Anticline and
its associated minor structures in these Monian Supergroup
(Precambrian or Cambrian) rocks, as simplified in Fig. 1c–f.
Before reviewing these, we summarize the geometry of structural
elements that is common to them all, regardless of their supposed
age or mechanical significance.
(1) Three lithostratigraphic formations are displayed clearly in
an antiformal fold, the Rhoscolyn Anticline (Figs 1 and 3). The
South Stack, Holyhead Quartzite and Rhoscolyn Formations
collectively comprise bedded quartzites, psammites and pelites.
The South Stack and Rhoscolyn Formations consist of alternating
centimetre- to metre-scale psammites, semipelites and pelites,
which sandwich the Holyhead Quartzite Formation, dominantly a
poorly bedded orthoquartzite. These rocks are succeeded by the
New Harbour Formation, dominantly a distinct finely layered
green semipelite.
(2) The major fold plunges to the NE and has an axial surface
that dips to the NW; it has a ‘long’, flatter, limb that dips gently
to the NW, a broad, rounded, hinge zone, and an apparently
‘short’ limb that dips initially steeply to the SE, becoming locally
vertical and overturned, steeply dipping to the NW.
(3) The bedded units on both limbs are affected by intermedi-
ate-scale folds (tens of metres in wavelength) and abundant
minor-scale folds (metres or less in wavelength), most of which
plunge subparallel to, and are congruent with, the major anti-
form.
(4) In the psammitic rocks, most of the minor- and intermedi-
ate-scale folds of bedding referred to above have a penetrative
Fig. 3. (a) Structural map and field data for the Rhoscolyn Anticline, an abbreviated version from our field maps. Readings related to D2 are not shown.
(b) Downplunge projection of (a), based on the average plunge of fold axes of 248/0568 (profile section plane 1468/668 SW). The nine field localities
described in the text are located on map and profile. H shows the major fold hinge trace; X is discussed in the text.
THE RHOSCOLYN ANTICLINE UNRAVELLED 121
cleavage subparallel to their steep NW-dipping axial surfaces and
to that of the major fold.
(5) The pelites throughout the major fold are dominated by a
crenulation cleavage, which dips at shallow angles to the NW.
They also contain a significant volume of quartz veins, oblique
to bedding, which are folded.
We summarize five previous interpretations of the evolution of
the Rhoscolyn Anticline and its associated minor structures, four
of which are illustrated in schematic form (Fig. 1c–f). Roper
(1992) did not illustrate his interpretation in cross-sectional or
profile view, but we consider it would look very similar to our
own profile (Fig. 3b). We cannot do justice here to the detailed
observations that the previous workers have presented, but have
tried to abstract the essential elements of their work that are
pertinent to their interpretation, and to our reinterpretation.
Readers are referred to these studies for fuller discussion of the
regional geology and tectonics. We omit interpretations (e.g.
Hudson & Stowell 1997) based principally on structures in the
New Harbour Formation, but will address these and their
correlations in another paper.
Shackleton (1969), using sedimentary way-up structures, re-
interpreted the stratigraphic succession and structural interpreta-
tion of the original mapping of the Rhoscolyn area by Greenly
(1919) (Fig. 1c). Shackleton’s four-fold, upward-facing, succes-
sion (Fig. 1d) is accepted by most later workers, and by
ourselves. On the basis of the plentiful minor structures, he
interpreted the Rhoscolyn Anticline as an F1 fold, verging and
facing steeply to the SE. A strong axial-planar penetrative S1
schistosity, fanning around the fold, was observed to be devel-
oped in the quartzites and psammites, parallel to which quartz
veins were segregated in the pelites. Several sets of minor
structures were identified, superimposed upon the Rhoscolyn
Anticline, one of which was said to originate from a vertical
compressional stress field (D2 of this paper).
The view of Shackleton (1969), which we essentially share,
was not challenged until the claim by Cosgrove (1980) that the
Rhoscolyn Anticline was in fact a D2 antiform superimposed on
the flat limb of an earlier major D1 isoclinal fold or kink-band,
facing to the NW (Fig. 1e). This view was based on observations
of the geometry of minor structures (see also Price & Cosgrove
1990, pp. 482–490), which were at variance with those of
Shackleton. Most importantly, an early fabric was observed in
the quartzites and psammites of the South Stack, the Holyhead
Quartzite and the Rhoscolyn Formations, which was folded
around the Rhoscolyn Anticline, although no D1 minor folds
were identified. The well-developed minor folds that verge
towards the major fold were thus identified as D2: In the
quartzites and psammites, a new penetrative S2 cleavage was
developed axial-planar to the minor folds, but fanning around the
major fold; in the steep limb this cleavage was coincident with
S1, but in the hinge zone and flat limb both cleavages were
distinguished. An S2 crenulation cleavage was developed in the
pelites. The geometry of the quartz veins, which were segregated
as planar bodies parallel to S1 in the pelites, was used
particularly by Cosgrove to demonstrate the D2 age of the
Rhoscolyn Anticline and other major folds on Holy Island. His
observed Z-shape geometry of these veins, related to D2 minor
folding in the flat or NW-dipping limbs of the major D2 folds,
was attributed to a top-to-the-NW shear couple set up as a result
of flexural slip between competent beds. The veins on the SE-
dipping limb would have been bodily rotated not folded, and so
the folds with S-shape geometry that were observed in the quartz
veins on the steep limb of the Rhoscolyn Anticline (e.g. Fig. 4b)
were here attributed to a locally developed D3 phase.
Lisle (1988) raised the question of whether the Rhoscolyn
Anticline was a major first or second fold (Fx or Fy in his
terminology), and highlighted its ambiguities, in a discussion of
the nature of structural vergence in refolded regions. As noted in
the preceding section, he showed how a set of shear folds,
superposed on bedding and first cleavage, could produce anom-
alous vergence patterns, and illustrated this with specific exam-
ples from the NW limb of the Rhoscolyn Anticline. Although
presenting evidence that seems more in favour of deducing that
the Rhoscolyn Anticline is a first fold with a related axial-plane
cleavage in the psammites, Lisle concluded that some ambiguity
remained, which could not be solved by vergence information
alone.
Phillips (1991) postulated that the Rhoscolyn Anticline was a
D2 antiform, but for different reasons from those of Cosgrove
(1980). He considered that D1 was responsible for a top-to-the-
SE shearing of the sedimentary pile, producing a bedding-
parallel (S0/S1) fabric in the pelites and in the finer-grained
psammites and a NW–SE chlorite lineation on S0/S1 surfaces;
no folds or fabrics, oblique to bedding, were produced (Fig. 1f).
D2, a progressive continuation of the SE-directed simple shear,
resulted in several major SE-verging folds on Holy Island, such
as the Rhoscolyn Anticline, as well as the dominant minor folds
and their axial-planar fabrics. These fabrics were pervasive in the
psammites and quartzites but contiguous with a crenulation
cleavage developed in lithologies affected by the D1 bedding-
parallel fabric, especially in the pelites. Phillips differed in his
interpretation of the history of the quartz veins from Cosgrove,
in that the veins did not develop parallel to an S1 fabric, but late
in the D1 event as tension gashes in response to the SE-directed
shear; subsequently they were wrapped around the Rhoscolyn
Anticline and affected by the D2 minor folding, thus accounting
for their S-shaped geometry on both limbs. A localized minor D3
folding with an axial-planar fabric (S3) subparallel to S2 is
recognized in the flat limb and hinge zone of the Rhoscolyn
Anticline.
Roper (1992), using evidence that includes fold vergence,
returned to a first-phase (his Dx) origin for the Rhoscolyn
Anticline, interpreting it as a major upright fold associated with
the dominant minor folds and a penetrative cleavage in all
lithologies in the South Stack, Holyhead Quartzite and Rhosco-
lyn Formations. The quartz veins in the pelites were said to be
produced as tension gashes parallel to the first cleavage, as a
result of stress relaxation late in Dx. The second deformation (his
D y) was responsible for creating NW-verging minor folds
(especially of the first cleavage and the quartz veins) with an
axial-planar crenulation cleavage, imposed across the earlier
structure. The second structures were all attributed to a shear
couple dipping NW, which produced inhomogeneous simple
shear zones (likened to Riedel shears) that rotated the original
steep-dipping first cleavage to become the flat limbs of the
second folds. Although this deformation history differs from our
own, the net product in profile view would probably not be very
different from ours, shown below.
These different conclusions for the age of the Rhoscolyn
Anticline and its structures reflect problems in interpretation of
fabrics, and their correlation with minor and major folds, that are
pertinent to many areas of polyphase deformation. In the case of
Rhoscolyn, these distil into questions regarding the correlation of
folds and fabrics in different rock types; whether single pene-
trative fabrics are first, or second, or combined; and what
information can be gained from fold vergence, first or second. It
will become apparent that our interpretation of the development
of the Rhoscolyn area, in subsequent sections, can reconcile
S.H. TREAGUS ET AL .122
many of the differences among the interpretations reviewed
above, and can explain what might appear to be ambiguous
structural relationships.
Description with key localities
We present our interpretation of the Rhoscolyn structures in
terms of a map, downplunge profile and key localities (Fig. 3),
together with structural sketches and photographs (Figs. 4–8).
The profile section was constructed perpendicular to the mean of
all measured first and second fold axes (248/0568). The major
anticline is apparent in both map and cross-section (Fig. 3): an
asymmetric open major fold with a broad and mildly undulating
hinge region. We have located the fold axial or hinge trace at H
in Fig. 3, in common with the above-cited studies. On the scale
of the map and its profile section (Fig. 3), a few substantial
‘minor’ folds can be seen on each limb, with their vergence
supporting the anticline. From details given below, it will become
clear that we regard these and the major anticline as first folds
(F1). There is plentiful way-up evidence in the area (cross-
bedding, graded bedding, load structures) to confirm the upward-
younging succession, and no evidence for major repetitions to
signify earlier isoclinal structures or major thrust repetitions.
Nine field localities are selected that are accessible and
representative, but also chosen to illustrate characteristic hybrid
effects of the two-phase deformation of the area. For each
locality, we describe structures that occur within c. 100 m of
coastal exposures. Our descriptions are fuller for the central part
of the major structure, in the South Stack Formation, as these
observations are critical in our interpretation. Some are compar-
able with Cosgrove’s localities (Cosgrove 1980, fig. 2; Price &
Cosgrove 1990, fig. 18.49), but his cross-section pays scanter
attention to the SE limb, or the NW limb of the Rhoscolyn
Anticline away from the flat hinge–limb region.
Our traverse of the Rhoscolyn Anticline begins on the SE
‘steep’ limb immediately adjacent to Porth y Hwngan (1),
continuing NW on this limb to (2), then via the Holyhead
Quartzite Formation to exposures of the South Stack Formation
near gullies in the hinge zone (3, 4), a well-visited locality on the
‘flat’ limb (5), and towards Porth Gwalch (6), still in the South
Stack Formation. Via Rhoscolyn Head, where the Holyhead
Quartzite Formation reappears, we then examine the Rhoscolyn
Formation on the NW limb at Porth Saint (7) and nearby (8),
ending at Bwa Du (9). (Note that the section of Cosgrove (1980)
stops at locality 7). Our terms will be F1 for folds we interpret as
first folds, F2 for second folds, and S1 and S2 for their related
cleavages. Thus our notation attributes ‘first’ and ‘second’ to the
folding deformations in these rocks, and will differ from other
workers’ numbering schemes based on fabrics or other criteria.
Locality 1 [26657475] is in the Rhoscolyn Formation on the
steep SE limb of the Rhoscolyn Anticline, where cross-bedding
and graded bedding in psammites provide way-up evidence. F1
fold pairs with several-metre wavelengths can be seen with S
asymmetry and steep NW-dipping axial-plane cleavage (see
profile; Fig. 3b), confirming their position on this steep fold limb.
Examples also occur of local F2 folds cross-cutting F1 folds (Fig.
4a), with a shallower-dipping axial plane parallel to crenulation
cleavage (S2) in semipelites to pelites, and spaced S2 locally in
semipsammites.
Locality 2 [26507485] in the Rhoscolyn Formation shows
characteristic structures in pelite beds and the quartz veins they
contain (Fig. 4b), which occur among psammites on this limb of
the fold, where bedding is generally steeply SE dipping. We
interpret the predominant cleavage in psammites as S1, and
observe quartz veins subparallel to S1 in the pelite. The sense of
S1 cleavage refraction from competent psammite to incompetent
pelite supports its position on the SE limb of a major F1 anticline
(Fig. 4b). The F2 folds of quartz veins and of S1 in semipelites
also show S asymmetry (NW vergence), but with significantly
shallow NW-dipping axial planes, parallel to crenulation cleavage
(S2) developed in the pelite, and to localized cleavage that cross-
cuts S1 in some semipsammites.
The major Rhoscolyn Anticline can be mapped from expo-
sures of the Holyhead Quartzite Formation and pelites within it,
and changes of strike and dip are exemplified by exposures
adjacent to the Coastguard lookout [26327520] close to the crest
of the fold. However, the true nature of the major fold hinge, as
it is recognized today, is better seen in the underlying South
Stack Formation of quartzites, psammites and pelites, both in
detail and across gullies to gain larger-scale downplunge views
of the ‘sheet dip’ (10–258 SE) and the mesoscopic fold
vergence.
Locality 3 [26207510] provides spectacular exposures of
approximately symmetrical cylindroidal folds that cascade with a
sheet dip of c. 258 SE, and is close to the major fold hinge
located in earlier cited studies (Fig. 3, H). The folds can be
examined in three dimensions, and fold axes traceable for many
metres are in detail curved or branching. This is a key locality
for recognizing the hybrid nature of the fold structures. We
interpret most of these folds as originally F1, now significantly
modified and distorted by the second deformation, but there are
also examples of F2 folds of bedding and S1 (Fig. 5a). Here, F1
and F2 folds are not always perfectly coaxial, and an angle of up
to 208 between them may locally occur, giving rise to spiralling
S1 –S0 intersections around F2 folds. We do not agree with
Cosgrove’s interpretation that all the mesoscale folds here are F2
folds that fold an earlier cleavage (Cosgrove 1980, figs 5 and 6;
Price & Cosgrove 1990, figs. 18.51 and 18.52), because many of
the folds have axial-planar S1 in their cores (Fig. 5a). The folds
of quartz veins are clear evidence of a significant D2 shortening,
but in a different orientation to the D1 shortening. The combined
effects of F1 –F2 folding leading to the broadly neutral vergence
at this locality reveal important features that we consider critical
in the two-phase history of the Rhoscolyn Anticline. When
unravelling F2 folds of S1 within cleaved semipsammites (Fig.
5a), we find evidence that the F1 cleavage–bedding vergence was
originally NW. A simple ‘undoing’ of the F2 folds in Fig. 5a
suggests that the F1 fold was originally an asymmetric anticline,
with a short thick NW limb and a much thinner and longer SE
limb (now refolded in the F2 folds); that is, an originally NW-
verging asymmetric F1 fold. If representative, these two lines of
evidence suggest that today’s hinge, H, does not mark the
position of the original F1 axis of the Rhoscolyn Anticline.
Locality 4 [26057510] provides further examples of the two-
phase folding effects in the South Stack Formation, and espe-
cially the variations among the different rock types. The minor
fold geometry in psammites is asymmetric, with Z geometry and
SE vergence, but is the combined effect of F1 and F2 folding
(Fig. 4c). Here, F1 folds with distinct hinges and axial-planar to
convergent S1 cleavage are observed adjacent to F2 folds of both
bedding and S1, with flatter axial planes parallel to the crenula-
tion cleavage (S2) in nearby pelites. Other psammite layers
appear only weakly folded, or include straighter regions with
cleavage fans, which suggest some unfolding of F1 folds. After
removal (by eye) of the F2 folding (e.g. in Fig. 4c), the remaining
F1 folds are found to be NW verging, with Z asymmetry.
Semipelite beds reveal chevron folding of S1 (Fig. 4c), whereas
the pelite beds are dominated by crenulation of S1 to produce S2,
THE RHOSCOLYN ANTICLINE UNRAVELLED 123
and contain strongly folded quartz veins. The effect is that F2
folds of quartz veins and S1 in pelites to semipsammites can
often appear to be tighter than the hybrid F1 –F2 folds of bedding
in psammite to quartzite beds. At this locality there are rare but
important examples of more traditional Type 3 fold interference
in centimetre-thick quartzite beds, where small NW-verging F1
folds and S1 are wrapped around SE-verging F2 folds (Fig. 5b).
This locality also reveals that the minor F1 structures were
initially NW verging, on the original SE limb of the primary
Rhoscolyn Anticline, and that the predominant SE vergence of
the folds is an effect of D2 deformation.
Locality 5 [25957510] exhibits beds with well-developed
slump folding and fluid escape structures (dewatering). Our
structural observations and illustrations (Fig. 4d and e) are taken
S.H. TREAGUS ET AL .124
Fig. 4. Field photographs of critical
structures at some of the numbered
localities (see Fig. 3), all taken downplunge
(NE). Scales: pencil and notebook are
15 cm long; lens cap is 5 cm in diameter.
(a) F1 minor fold in a psammite bed of the
Rhoscolyn Formation (RF) at locality 1,
showing a steep axial plane and S1 (parallel
to pencil), the NW limb refolded by F2
folds, and S2 cleavage (bottom left). (b)
Pelite bed between two cleaved psammite
beds in the Rhoscolyn Formation at locality
2, showing S-shaped geometry of the F2
folds in quartz veins parallel to S1, and the
anticlockwise sense of S1 refraction from
psammites to pelite. (c) Hybrid F1 and F2
folding (hinges labelled) in a pale psammite
bed in the South Stack Formation (SSF) at
locality 4. Annotations show the changing
orientation of S1 within this folded bed.
The F2 chevron folds of S1 in the semipelite
in the foreground, with axial planes parallel
to S2 crenulation cleavage, should also be
noted. (d) The variable geometry of F1
folds in the South Stack Formation at
locality 5. The uppermost thick psammite
reveals weak folding, whereas the thinner
quartzite layers in the centre are more
obviously folded, with varying geometry
and overall SE vergence. In the foreground,
subvertical quartz veins show variable to
tight F2 folds. (e) Further detail from
locality 5, showing the steep NW-dipping
S1 in psammite (top), continuing
downwards with slight anticlockwise
refraction into the quartz veins in the
underlying semipelite. The variable
geometry of the F2 folds in this array of
quartz veins should be noted. (f)
Relationships of bedding (S0), S1 and S2 in
the South Stack Formation at locality 6.
The lower semipsammite shows kink-like
folding of S1 within the bed, but semipelite
bed above shows stronger F2 chevron
folding of S1, and the development of a
crenulation cleavage (S2).
THE RHOSCOLYN ANTICLINE UNRAVELLED 125
from cleaved and folded layers stratigraphically between these
sedimentary structures, where the strong asymmetry and SE
vergence of folds would appear to provide support for a position
on the NW ‘flat’ limb of the major Rhoscolyn Anticline. In
detail, however, we consider the asymmetric fold trains seen in
thin psammites to be a combination of F1 and F2 folds (Fig. 4d).
The initially upright F1 folds appear to have been tightened and
rotated into SE vergence by the second deformation, or straigh-
tened out, or have undergone refolding of their limbs, according
to F1 tightness. The final geometry is one of irregular asym-
metric folding on shallow NW-dipping axial planes subparallel to
S2 crenulation cleavage in pelite beds. In contrast, thicker beds
of psammite and semipsammite appear to be only slightly folded
(Fig. 4e), their deformation mainly revealed in the steep axial-
planar S1 cleavage. As for the previous locality, pelite beds
accommodate considerable variations of deformation around the
folds on different scales, and contain a significant volume of
quartz veins of varying continuity, thickness and F2 fold
geometry (Fig. 4e). Any consistent vergence in the folds of
quartz veins is difficult to assess: we find examples of S, M and
Z asymmetry, perhaps reflecting the varying orientations asso-
ciated with S1 cleavage fans around F1 folds, or in localized F2
deformation. We consider the commonest vergence as SE (Z-
shaped) to neutral (M-shaped) (Fig. 4d and e). The subparalle-
lism of quartz veins to S1 cleavage is revealed by a sharp veer
from a variably SE-dipping trend in pelite, to steeply NW-
dipping on approach and entry into psammites (Fig. 4e). This
sense of S1 refraction suggests a position still on the SE limb of
a major F1 anticline. We do not concur with Cosgrove (1980)
that this is evidence that the Rhoscolyn Anticline is a major F2
antiform that refolds F1 structures that are all NW verging.
Instead, we deduce that the original axis of the Rhoscolyn
Anticline is north of this locality, and that the Rhoscolyn
Anticline has been significantly distorted and undergone hinge
migration during an obliquely superposed F2 deformation.
Locality 6 [25807535] is our last site in the South Stack
Formation, still in the broad central part of the Rhoscolyn
Anticline. Successive beds of shallowly NW-dipping psammite
and pelite persist along the cliffs. Psammite beds of .1 m
thickness appear virtually unfolded, but contain a subvertical to
steep NW-dipping S1 cleavage. This is another good locality to
observe the apparent discrepancies in deformation structures in
different rock types and on different scales. Well-cleaved semi-
psammites reveal internal chevron folding of S1 cleavage, on
axial planes at a small angle to bedding (Fig. 4f) or locally
subparallel, and may be accentuated by an S2 cleavage. This type
of refolding feature and vergence effects were described by Lisle
(1988) from this vicinity, and have been discussed above. Some
of the patterns of folded S1 within the beds here are puzzling in
detail, revealing sheaf-like fans (Fig. 5c) that might be preserved
fans related to F1, or F2 geometric effects. These cleaved horizons
show evidence of hybrid folding, as described above, but here
suggestive of overprinting of asymmetric SE-verging F2 folds on
asymmetric SE-verging F1 folds. The like senses of vergence and
asymmetry combine to produce appressed and irregular asym-
metric folds, with lengthened and thinned original F1 long limbs
and a zig-zag shortening of S1 within, and further shortened F1
short limbs (Fig. 5c). Folds of quartz veins in pelite at this
locality are irregular, as before, but we consider the predominant
orientations are steeply NW verging, in S-shaped folds. Taking
the hybrid effects of F1 and F2 folds and the geometry of quartz
veins together, we consider this locality to be on the NW limb of
the initial Rhoscolyn Anticline. Thus, the original axis of the
Rhoscolyn Anticline lies somewhere between localities 5 and 6.
Continuing to traverse northwards, the Holyhead Quartzite
Formation has reappeared (across a fault) around the major
anticlinal closure, and on Rhoscolyn Head (Fig. 3) tight F1 folds
occur in the cliffs [257756], and can be mapped out on the
ground from pelites within the quartzite. The overall vergence is
undisputably SE, on steep NW-dipping axial planes. We then re-
enter the Rhoscolyn Formation, with cliff exposures where
mesoscale folds and fabrics and NW-dipping sheet dips can be
observed from a distance. We interpret these mesoscale folds in
psammite beds to be F1, with strong axial-plane S1 cleavage that
can be clearly observed in local chevron folds (F2) within beds
on F1 long limbs.
Locality 7 is in the bay of Porth Saint [26007585], where three
15 cm quartzite beds in pelite reddened by proximity to faults
reveal a series of F1 folds modified by F2 (Fig. 6). The SE-
Fig. 5. Field sketches of hybrid F1 þ F2 folds and cleavage patterns in
the South Stack Formation, all drawn downplunge (looking NE). Bedding
(S0) and S1 are shown as continuous lines, S2 as dashed lines and quartz
veins in black in (b) and (c). (a) Folded and cleaved psammite layer at
locality 3, the present-day hinge region of the Rhoscolyn Anticline. The
succession of F1 and F2 folds along the layer (revealed by changes of S1
within the bed), which combine to give almost neutral fold vergence,
should be noted. Scale bar represents 1 m. (b) Rare example of F1 folds
refolded by F2 folding and cross-cut by S2, in a thin quartzite bed with
cross-bedding (x) from locality 4. Scale bar represents 10 cm. (c) Detail
of F1 fold pair modified by F2 pair at locality 6, revealed by the F1 axial
traces (dotted lines). In the main psammite layer, S1 is folded (with
rudimentary development of S2) within lengthened flat limbs of F1 folds,
and shows local sheaf-like fans. The hinge region of the syncline appears
much thickened and skewed, as a result of the combination of F1 and F2
folding. Scale bar represents 25 cm.
S.H. TREAGUS ET AL .126
verging folds reveal convergent cleavage fans in the quartzite
fold cores, good axial-plane penetrative cleavage (S1) in the
pelite in inner arcs, and preservation of arcuate cleavage fans and
finite neutral points (Ramsay 1967, p. 417) in outer arcs. The
effects of F2 are seen in a quartz vein that can be traced from
axial planar in a tight inclined syncline, but is folded around an
adjacent anticline, confirming that is has been tightened during
F2. A crenulation cleavage (S2) is developed in the pelite, axial
planar to the quartz vein folds, and crenulating the arcuate S1
cleavage fan of the original F1 anticline (Fig. 6). These folds
clearly reveal the hybrid effects of F1 þ F2 in mechanically
active layers with competence contrasts, and the change from
SE-verging F1 folds on fairly steep axial planes into tighter and
more strongly inclined hybrid F1 –F2 folds whose axial planes are
subparallel to the regional S2.
Locality 8 [25857595] is a distinct site north of Porth Saint,
where a massive quartzite within the Rhoscolyn Formation crops
out in an open upright F1 syncline (Fig. 7a), marked out by a
thin pelite containing folded quartz veins, within the quartzite.
We see different responses to the second deformation in the thick
competent quartzite and thinner incompetent pelite. In the
quartzite, a dominant rough and anastomosing S1 cleavage
reveals the convergent F1 cleavage fan: it can be followed from
NW dipping through subvertical to SE dipping, around the
syncline (northwestwards) as shown in Fig. 7. Locally, this
anastomosing S1 can appear as a lozenge pattern, but we do not
consider this an S1 and S2 effect (Cosgrove 1980). Where the S1
cleavage in the quartzite is strong, it is folded into angular F2
folds with shallow NW-dipping axial planes, and a rough S2
whose trend can be seen to transect the syncline. F2 folding is
more obvious in the pelite, revealed by ptygmatic folding of
subvertical quartz veins without a consistent sense of asymmetry
or vergence, and an axial-planar S2 crenulation cleavage (Fig. 7b
and c). This is a well-preserved F1 fold that has undergone only
mild second deformation, perhaps associated with slight fold
opening. Its approximately vertical axial plane is considered
close to the original orientation of F1 axial planes.
Locality 9 [26007630], at Bwa Du, sees the last exposures of
the Rhoscolyn Formation around the Rhoscolyn Anticline before
a faulted contact with New Harbour Formation. An asymmetric
SE-verging F1 fold pair with clear axial-plane cleavage (S1) can
be seen in quartzite (Fig. 8), and its cross-bedding provides way-
up evidence, confirming a SE-facing of these SE-verging minor
Fig. 7. (a) Large basin-shaped F1 syncline
in thick quartzite units in the Rhoscolyn
Formation at locality 8, viewed downplunge
(NE), showing locations of three detailed
sketches (b–d) traced from field
photographs. Scale bar represents 15 m. (b)
and (c) show the convergent S1 (continuous
lines) in quartzite (stippled), and veins
following S1 in pelites, where the pervasive
fabric is S2 (dashed lines). Scale bars
represent 40 cm. (d) NW inclined bedding
within quartzites; the lower bed (unshaded)
shows a strong crenulation of S1
(continuous lines) and the development of
S2 (dashed lines) cross-cutting the synclinal
structure. Scale bar represents 5 cm.
Fig. 6. Downplunge view (NE) of series of F1 folds with SE vergence, in
layers of quartzite (stippled) and cleaved reddish semipelite of the
Rhoscolyn Formation (RF), at locality 7. The S1 cleavage fans and
refraction from quartzite to semipelite (continuous lines) are well
preserved, and clearly related to the F1 folds, but the tight inclined F1
fold in the upper-right section appears to have been modified by D2
deformation that folded the quartz veins (black) and created the S2
crenulation cleavage (dashed lines). Scale bar represents 1 m.
Fig. 8. Representation of the significant SE-verging F1 fold pair in the
Rhoscolyn Formation at locality 9 near Bwa Du, based on field
photographs taken approximately downplunge (looking NE). The
quartzite bed (stippled) contains a steep axial-planar S1 cleavage
(continuous lines), and the overlying brown semipsammite (annotated) in
the background reveals the steep S1 and also a shallower NW-dipping S2
cleavage (dashed lines) that cuts across the folds. The central unshaded
region is an effect of perspective. Scale bar represents 1 m.
THE RHOSCOLYN ANTICLINE UNRAVELLED 127
F1 folds, synthetic with the major Rhoscolyn Anticline. Just
above the quartzite is a brown semipsammite that exhibits two
cleavages (Fig. 8): an S1 cleavage fanning about the axial plane
of the fold pair; and a shallower NW-dipping S2, which clearly
cross-cuts the limbs of the fold pair. We find no evidence in
these rocks of any flat-lying NW-facing F1 structures, as required
in the interpretation by Cosgrove (1980).
Conclusions from field observations
The Rhoscolyn Anticline is a major F1 fold with synthetic
mesoscale folds on each limb, with a sub-penetrative axial-planar
S1 cleavage preserved in psammites, but a predominant crenula-
tion cleavage in pelites (S2). On the small scale, many of the
observable folds are F2 folds of quartz veins and S1 cleavage,
and some folds in thin quartzite beds may also be deduced to be
F2 folds because of inclined axial planes and axial-planar S2.
The ambiguity arises, therefore, as to whether strongly asym-
metric small-scale folds, especially on the NW limb, are F1 or
F2; and if the latter, does this imply that the major structure is an
F2 antiform, as suggested by Cosgrove (1980)? This is not our
conclusion, despite the apparent changes in F2 fold vergence
around the structure.
On detailed investigation, we find that the majority of the folds
that affect bedding in these rocks are original F1 folds that have
been variably modified by the second deformation. According to
the lithology and the initial F1 fold geometry, the effects of F2
are variable. Ambiguous hybrid F1 þ F2 folds occur most in the
open flat hinge region of the Rhoscolyn Anticline (locations 3–
6). At the present-day hinge (locality 3), some F1 structures with
NW vergence appear to be folded around the major structure.
However, on the steep to overturned SE limb (localities 1 and 2),
and the NW-dipping part of the NW limb (localities 7–9),
strongly developed F1 folds have unambiguous vergence in
support of a major F1 fold, and the shallower cross-cutting nature
of the second deformation rules out their interpretation as second
folds synthetic to a major F2 antiform.
Evidence from folded S1 in psammites, especially where the
F2 axial planes and S2 are subparallel to bedding (location 6),
indicates that F2 folding arises from a shortening in a direction
of c. 708 SE in the profile plane (perpendicular to the average S2
trace, 208 NW). This steeply inclined shortening is also deduced
from the range of attitudes of folded quartz veins in pelites
around the major structure. In the next section, we will examine
in more detail the possible effects of a steeply inclined second
shortening on earlier F1 structures, and attempt to remove this
second deformation to reveal the geometry of original F1
structures. However, several lines of field evidence suggest that
the major and minor F1 folds might have been closer to upright
and symmetrical, originally.
We described various structural criteria to suggest that H in
Fig. 3 (including locality 3) is not the major axial trace of the
original Rhoscolyn Anticline. We deduce that this axis was
between localities 5 and 6, placed tentatively at X in Fig. 3, on
the basis of mapped changes in S1 and bedding vergence, and
geometry of folded quartz veins. The c. 260 m distance from X
to H therefore marks the apparent hinge migration of the
Rhoscolyn Anticline, caused by the distortion effected by the
second deformation. This may be considered a quasi-passive pure
shear of the kind shown in Fig. 2d, but here oblique to the F1
axial plane. Thus, the Rhoscolyn Anticline is an F1 anticline, but
has undergone significant modification in its shape and small-
scale structures. It is probably now a more open anticline than
originally, and with a significant section of hybrid structures
where F2 has flattened out, modified or refolded F1 structures on
the original SE limb near the hinge, to make them now part of
the apparent NW ‘flat’ limb of the present-day major structure. If
this region were the main focus of study, and evidence from
further away on the fold limbs disregarded, the major structure
might be deduced to be an F2 fold (antiform).
The overall SE vergence and inclination of the axial planes is
largely a result of the second deformation. We find no field
structural criteria to lead us to interpret the mechanics of either
the first or the second deformation in terms of large-scale simple
shear, in any direction. Instead, the evidence leads us to a simple
model of two phases of pure shear with coaxial intermediate
strain directions parallel to 248/0568 (the average plunge of F1
and F2 folds), and about 708 difference in orientations of
principal strain directions (X and Z) in the profile plane for the
two phases.
Modelling the Rhoscolyn Anticline
Constraining the D1 þ D2 model
The preceding section summarizes the field structural data that
we consider point to a simple two-phase deformation history for
the Rhoscolyn area. These data provide precise constraints for
both deformations, as detailed below, especially in determining
the amount and orientation of the D2 strain. This can then be
removed to reveal the true nature of the D1 structures. For each
deformation, we use X > Y> Z as nomenclature for the principal
axes of strain and their stretch values.
Orientations of principal axes. As F1 and F2 axes are virtually
coaxial (248/0568), we take this as the intermediate axis of strain,
and take Y ’ 1 for both deformations, unless contradictory
evidence emerges. The profile plane of 1468/668SW is thus
common for both deformations, and the two-phase history can be
viewed two-dimensionally, in this plane. The S1 trace is assumed
to reflect the XY plane for D1, but is now modified by D2. The S2
crenulation cleavage is taken as the XY plane of D2 strain. Its
average trace on the profile plane determines the X direction to
be 208 NW (Fig. 9a).
D2 deformation style. The consistency of S2 crenulation
cleavage and F2 folds across the Rhoscolyn Anticline, the
systematic behaviour of specific lithologies, and the absence of
any consistent evidence of any regional shear sense affecting all
the rocks, lead us to reject a model of D2 as simple shear (see
Cosgrove 1980; Phillips 1991; Roper 1992). We consider the
evidence consistent with bulk pure shear (or approximately) for
the D2 deformation; that is, a plane strain pure shear with 208
NW extension (X) and 708 SE shortening (Z) in the profile plane
(Fig. 9). This strain quasi-passively distorted the earlier F1
structures, in the manner of coaxial superposed deformation
discussed above (Fig. 2d), except that the two deformations are
not orthogonal.
Determining the D2 strain from F2 folds. The main constraints
on the D2 strain ratio (X=Z ¼ R), come from the range of
orientations of F2 folds of quartz veins and new F2 folds of
bedding, and their respective shortening values. Buckled quartz
veins in pelites and semipelites across many parts of the
Rhoscolyn Anticline are subparallel to S1, broadly axial-planar
with F1 folds and oriented 60 � 58 NW in profile section (Fig.
3b). Folded veins with S asymmetry have typical shortening
values of c. 20%. Veins that are subvertical to steeply SE dipping
(related to S1 fanning) are more tightly folded into more
symmetrical shapes, with typical shortening values up to 43%. If
S.H. TREAGUS ET AL .128
this is assumed to be measuring the maximum shortening (Z), it
indicates a pure shear of R ’ 3 affecting these rocks.
Determining the D2 strain from orientations of layers with F2
folds. Now let us consider the information from orientations of
veins or layers containing F2 folds. In the D2 pure shear with
strain ratio R, traces at angle j to shortening (Z) change to j9 by
tanj9 ¼ R tanj (1)
(Ramsay 1967, p. 67). For pure shear of R ¼ 2–4, finite short-
ening sufficient to produce measurable folding (20%) requires
j # 208, and so the orientation range for F2 buckling can be set
at 0 � 208 to Z (Fig. 9a). Oriented with respect to Rhoscolyn D2,
this means veins or layers inclined 90-508 SE in profile view (l–
n, Fig. 9a). Testing different R values in Eq. (1), we find that the
vertical orientation (l) deforms to 628 NW, for R ¼ 3, and would
have folds of S asymmetry (l9, Fig. 9b). This agrees with
observations for S1-parallel quartz veins noted in (3). The other
limit (n, 508SE) deforms to 228 SE and folds with Z asymmetry
(n9, Fig. 9b). The direction of greatest folding (m, 708SE) does
not rotate, and would fold with M symmetry. These asymmetry
and vergence relationships are broadly consistent with the
geometry of folded beds and quartz veins at Rhoscolyn, support-
ing our adoption of R ’ 3 for the superposed D2 quasi-passive
pure shear.
Vergence of F2 folds. The vergence of F2 folds of bedding, or
of quartz veins in pelites, or affecting S1 in semipelites to
psammites, can be compared with the relationships noted above
(Fig. 9). These provide additional constraints on the model for
D2 deformation. We have found the geometry and vergence of
folded quartz veins to indicate D2 pure shear with R ¼ 3. The
more variable vein fold geometry seen in pelites in the central
part of the Rhoscolyn Anticline, where shallow SE-dipping veins
with Z asymmetry are locally seen, can be explained in terms of
veins and S1 originally dipping SE, reflecting a divergent fan into
the major hinge zone, or variations around minor F1 fold hinges.
Our observations do not confirm those by Cosgrove (1980) that
quartz veins in pelites have a predominant SE dip with Z folds
throughout the whole of the NW limb of the Rhoscolyn
Anticline. In psammites, the geometry of F2 folds of S1 reveals
the superposition of D2 pure shear on fanning to axial-planar S1
associated with the Rhoscolyn Anticline. The vergence of F2
folds of bedding is more difficult to assess, because of the hybrid
effects of F1 and F2, particularly in the flat hinge region of the
Rhoscolyn Anticline. The dominant SE vergence of minor folds
on the ‘flat’ limb may reflect true F2 vergence for this orientation
of folds of bedding, but also the distortional effects of D2 strain
on F1 folds that produce hybrid F1 þ F2 folds.
Original F1 fold geometry. Today’s Rhoscolyn Anticline has a
SE-leaning geometry (Fig. 3b), but it cannot be assumed that this
reflects the original F1 fold geometry. ‘Removal’ of a D2 strain
of R ¼ 3, as deduced above, and oriented as in Fig. 9, leads to a
conclusion that the original Rhoscolyn Anticline must have been
less obviously SE inclined and asymmetric than it is now. We
concluded that the average S1 and its parallel quartz veins (now
folded and c. 608 NW) must have been approximately vertical
before D2. This leads us to deduce that the Rhoscolyn Anticline
was an upright fold with vertical axial plane, and perhaps
symmetrical. To constrain the original limb dips of the Rhosco-
lyn Anticline, we must consider the evidence from D2 structures
on the NW and SE limbs of the Rhoscolyn Anticline. The ‘steep’
SE limb of the Rhoscolyn Anticline and its F1 minor folds have
been locally folded (refolded) in F2 folds, and so this limb must
fall in the buckling range shown in Fig. 9. Initial dips of .708
SE would steepen and rotate clockwise towards X, whereas dips
of ,708 SE would decrease dip, so not become a ‘steep’ limb.
As a symmetrical fold with both limbs dipping .708 is a very
tight major F1 fold, we take a conservative lowest limit of 708
limb dip. This means that the SE limb (at its steepest) was
subparallel to principal D2 shortening, Z, so underwent no overall
dip change or steepening of this limb during D2. Thus, any
steepening or overturning of this limb is attributed to D2
shortening that tightened or distorted mesoscale F1 folds and
their long limbs. On the other hand, the 708 NW limb will have a
very different history during D2 strain of R ¼ 3, undergoing a
significant clockwise rotation to a shallower dip of 428 NW (Eq.
1), consistent with the constructed fold profile (Fig. 3b). This
limb was thus oriented for D2 extension, potentially to become a
longer ‘flatter’ limb with bed thinning. The evidence from S1,
which is markedly folded within straight psammite beds, together
with observations of possibly unfolded or significantly skewed F1
folds all confirm that this NW limb has indeed been lengthened
by D2. Therefore all this evidence points to an initial upright
symmetrical F1 Rhoscolyn Anticline, with 708 apparent limb dips
(408 interlimb angle), and a vertical axial plane and S1.
Quantitative D2 model
To arrive at a quantitative model, we tested the distortional
effects of a D2 pure shear with small variations of R about 3.0,
variations in its orientation, and a variety of initial fold shapes
and limb dips. We also compared the results for R ¼ 3 distortion
of symmetric anticlines with 708 limb dips, in fold shapes that
Fig. 9. Orientation of D2 pure shearing in the profile section: (a) original
circle before D2, showing the orientations for extension (X) and
shortening (Z); (b) the D2 strain ellipse, again showing X and Z, where X
is aligned parallel to regional S2. The shaded region shows the limit for
F2 buckling (see text for discussion), and the different vergence geometry
that would be expected in layers parallel to l, m and n are shown in (b)
(l9, m9, n9).
THE RHOSCOLYN ANTICLINE UNRAVELLED 129
were a sine wave, a circular arc and a parabola (Hudleston
1973). These permutations provide quantitative testing of the fold
shape and D2 strain values that best satisfy all the preceding
properties and constraints for modelling structures at Rhoscolyn.
The deformed shape that is closest to the geometry of the profile
section of the present-day Rhoscolyn Anticline (Fig. 3b) is
obtained from pure shear with R ¼ 3, passively deforming a 708
parabola.
The original parabolic shape of the schematic Rhoscolyn
Anticline is shown in Fig. 10. A parabola with maximum limb
dip, Æ, is expressed in Cartesian coordinates (Hudleston 1973) as
y ¼ tanÆ(�x� x2)=�: (2)
For Æ ¼ 708, the expression becomes
y ¼ 0:875(�x� x2) (3)
as constructed in Fig. 10. A superposed pure shear (R ¼ 3; X
oriented 208 anticlockwise of horizontal) can be written as a
transformation of the (x, y) parabola coordinates to new coordi-
nates (x9, y9) (see Means 1990, eq. 8), which for this deformation
are found to be
x9 ¼ 1:6xþ 0:37y; y9 ¼ 0:37xþ 0:71y: (4)
This is the deformed fold shape (x9, y9) constructed in Fig. 10;
its geometric similarity to our constructed profile of the Rhosco-
lyn Anticline (Fig. 3b) should be noted.
Results
Comparison of the original and deformed fold, aided by the
positions and spacings of graph symbols in Fig. 10, illustrates
the following features of this hybrid antiform, which we relate to
the broad geometry and structures of the Rhoscolyn Anticline.
(1) The deformed fold is more open than the original shape,
strongly asymmetric and SE verging. It has a broader hinge
region, without a distinct axis or hinge point. The original
horizontal is now c. 138 NW.
(2) The fold axis (A) and axial plane (A–P) have deformed to
occupy positions that appear off-centre (A9–P9), not where the
current axial plane or fold hinge might be constructed for the
distorted fold. The current hinge is taken to be where bedding is
perpendicular to S1 (modified) in this deformed fold (Fig. 10,
H), as is the case in the interpretations of the Rhoscolyn
Anticline that we cited above (also Fig. 3, H).
(3) The distance A9–H is thus the effective hinge migration,
approximately halfway round the ‘right limb’ (SE limb). The
broad crest of the distorted fold that is left (NW) of hinge, H,
largely comprises a section of the original right limb (A9–H),
and is thus a hybrid zone of particular significance.
(4) The original right limb (A–C) has become shorter
altogether, but unevenly. Zone A9–H has actually extended,
whereas H–C9 (the current right limb) has shortened by c. 40%,
because its steepest part is subparallel to Z. Thus the current right
limb is a significant short limb for two reasons: it represents only
two-thirds of the original SE limb, and it includes the part of the
original fold that experienced the maximum D2 shortening.
(5) All of the left limb, both its original section (B9–A9) and
its apparent current form (B9–H), has become extended. This is
now a significant long limb. However, its true length is unlikely
to be fully exposed, because a horizontal section (C9 leftwards)
would omit the steeper half of this left limb.
(6) The deformed fold viewed in profile cross-section (Fig. 10)
now comprises three distinct zones that are applicable to the
Rhoscolyn Anticline: (a) a steeply dipping short SE limb that has
shortened; (b) a flat central region, where structures on the
original SE limb have been modified and obliquely stretched,
now to become the hinge to NW-limb region of the hybrid
Rhoscolyn Anticline structure; (c) a long NW limb that has
extended, and its dip decreased.
(7) The original axial plane (A–P) has changed from vertical
to c. 628 NW, and shortened by 20%. Axial-plane structures, such
as cleavage (S1) and S1-parallel quartz veins, would likewise
rotate, and would develop F2 folds with S asymmetry and NW
vergence (compare lines l and l9, Fig. 9).
(8) Any potential F2 folding or refolding of bedding is
restricted to the orientation range for buckling for this D2
deformation (l–n, Fig. 9a); that is, dips of 90–508 SE, in the
vicinity of H–C9. Dips of .708 SE could develop F2 folds with
S asymmetry and NW vergence (l9, Fig. 9b) (as for S1 and quartz
veins). Dips of 50–708 SE could develop F2 folds with Z
asymmetry and SE vergence (n9, Fig. 9b).
This model, with the features listed above, provides a good
simulation of the large-scale geometry of the Rhoscolyn Anti-
cline, as shown in its profile section (Fig. 3b) and the field
descriptions. In the next section, we will build on the simple
model of one fold surface in Fig. 10, to include features to
simulate detailed geological features and mesoscale to small-
scale structures of the Rhoscolyn Anticline.
Fig. 10. Distortion of a parabolic fold with
708 limb dip (x, y coordinates, j) by D2
pure shear with extension X inclined at 208
and strain ratio R ¼ 3, according to the
coordinate transformation in Eq. (4). The
fold becomes the asymmetric curve (x9, y9
coordinates, half-filled squares) with its
inflexion surface (B9C9) dipping 148 to the
left (NW), and the original vertical axial
plane (AP) now A9P9, dipping 628 to the left
(NW). H marks the position that might be
deduced to be the hinge or axis of the
distorted fold, where the fold tangent
surface is perpendicular to A9P9. (H) is its
undeformed position.
S .H. TREAGUS ET AL .130
Developing the model to unravel two-phase structuresof the Rhoscolyn Anticline
The three main formations in the Rhoscolyn Anticline (Fig. 3)
comprise quartzites, psammites and pelites (and intermediaries)
in various combinations and bed thicknesses. These give rise to
folds on a variety of scales, and the vergence of minor–major
folds has played an important part in previous interpretations of
the area. The presence of buckle folding on many scales also
demonstrates the importance of the competence contrasts in these
rocks. Our approach to modelling these features is not to build
an exact replica of the Rhoscolyn Anticline stratigraphy to
produce the large-scale fold structures shown in Fig. 3b; these
are broadly simulated already in Fig. 10. Instead, we present a
model that includes important key elements to simulate structural
variations that can be related to the Rhoscolyn Anticline on a
number of different scales. We consider three components in Fig.
11: (I) a main bed with bulk properties, to simulate pelite to
semipsammite, with axial-planar S1, and some S1-parallel quartz
veins (v); (II) an upper competent bed, to simulate quartzite or
psammite, with a schematic convergent S1 cleavage fan, mod-
elled on fans in sandstone beds (Gray 1981; Treagus 1982); (III)
a thin layer containing schematic F1 minor folds with changing
vergence and asymmetry, based on patterns given by Ramberg
(1964).
We have not included an incompetent layer with a significant
divergent cleavage fan around the Rhoscolyn Anticline, because
field evidence from the pelites and quartz veins suggests a (more
or less) regional axial-planar S1, not a strongly divergent fan on
each major limb. (Compare, for example, locations 2 and 8, Figs.
4b and 7). Furthermore, theoretical modelling of strain partition-
ing in layers and folds (Treagus 1993, 1997) suggests that in
competent–incompetent alternations, the dominant strain parti-
tioning arises by reduced strains in the competent horizons,
rather than a greatly intensified strain in the incompetent
horizons. Thus, the average layer (Fig. 11, I) provides a reason-
Fig. 11. Schematic model of the distortion
of an upright F1 anticline (a), by D2 pure
shear deformation (b), according to the
model in Fig. 10, but including geological
features relevant to the Rhoscolyn
Anticline. Unit I represents an average layer
or the whole structure, in an F1 similar fold.
After D2 deformation, the S1 cleavage and
veins become inclined (628 NW), and
would develop F2 folds with S asymmetry,
with axial plane S2 crenulation cleavage
(208 NW). The original position of the axial
plane is not a recognizable geometric
feature, and the hinge and axial plane of the
distorted fold would be placed at H–H
(crossed lines). Unit II is a schematic
competent layer with convergent S1
cleavage fan. F2 folding of S1 in this layer
would vary around the main fold, not
occurring on the SE limb, where potential
S2 is at a small angle to S1 in the unit. On
the NW limb, the fanning S1 gives rise to a
steady variation in F2 fold geometry, with
important change in vergence at the
position asterisked. Unit III shows a thin
layer with schematic minor F1 folds (a),
and their distortion and changed vergence
when subjected to passive D2 pure shear
deformation (b). The decrease in
asymmetry towards approximate neutral
vergence on the original SE limb, and an
increase in asymmetry on the NW limb,
should be noted. All these distorted F1 folds
are cross-cut by S2. The position of the
original fold axis (A9) would not be
deduced from the fold vergence in (b).
THE RHOSCOLYN ANTICLINE UNRAVELLED 131
able approximation for incompetent pelite beds around the
Rhoscolyn Anticline, except where pelite beds are involved in F1
minor folds. Here, heterogeneous deformation and significant
variations in S1 orientation would arise locally, but have not been
specifically modelled.
All these components are subjected to the same model of
homogeneous D2 deformation as shown in Figs. 9 and 10,
although we relax the homogeneity on the small scale to allow
S1 to crenulate and to develop S2, and for S1-parallel quartz
veins to buckle into F2 folds. We do not attempt to model
mechanically active D2 behaviour of the lithological layering in
Fig. 11: this will be addressed below (see Fig. 12). The purpose
of Fig. 11 is to reveal the geometrical effects that might occur
from entirely passive D2 deformation of rocks with different
orientations of S1, and containing F1 minor folds with different
initial asymmetry. We will discuss these features in detail in the
next section.
Modelling the competent bed with the same amount of D2
strain, applied homogeneously as for the whole structure,
provides only a rough approximation. Competent beds with
convergent S1 cleavage fans reflect their ‘stiffer’ layer behaviour
in D1, and a similarly reduced strain might therefore be expected
during D2. These variations cannot be quantified without adopt-
ing specific values for viscosity ratios among the different rock
layers (e.g. Treagus 1988), and so are not attempted here.
Qualitatively, we might expect the principal axes of D2 strain to
refract across competence contrasts. We have not recorded a
significant degree of S2 refraction from incompetent to compe-
tent beds at Rhoscolyn. However, the development of S2 in
quartzites and purer psammites has been noted to be generally
weaker and more localized. Thick quartzites, particularly, might
experience a weaker D2 deformation than the rocks as a whole,
and this has implications for the distorting effects of D2 shown
in our model. For example, the whole of the Rhoscolyn Anticline
might have distorted from an initial upright form to its present
asymmetric SE vergent geometry, as modelled (Figs. 10 and 11),
whereas the Holyhead Quartzite Formation and some of the thick
quartzites in the Rhoscolyn Formation might have preserved
more of its original upright F1 structure and undergone a lesser
degree of hinge migration. There is local evidence for this at
locality 8 (Fig. 7), where an approximately upright open F1 fold
in quartzite reveals a convergent S1 fan, and appears less
distorted by D2 than other mesoscale F1 folds around the
Rhoscolyn Anticline (see Figs. 6 and 8).
The features modelled in Fig. 11 are discussed below, by
structural topic, and presented in terms of the structures of the
Rhoscolyn Anticline and field localities described above. We will
add to this quasi-passive D2 model the question of mechanically
Fig. 12. The development of hybrid F1 þ F2 folds. (a) An asymmetric NW-verging minor F1 fold in a schematic competent layer, such as might be
developed on the SE limb (at 408 dip) of the original anticline (see Fig. 11 for key). S1 and schematic veins (v) are generally vertical, but refract
convergently into the competent layer. (b) F2 folding of the F1 long limb, to achieve the required amount of D2 shortening and ‘passive’ rotation of the 408
SE sheet dip to 108 SE. (See text for further explanation of the model.) In this manner, folds can be produced that change, serially, from F1 with axial-
plane S1 and quartz veins, to F2 folds that fold bedding, S1 and veins, and with axial planes close to the regional S2 orientation. (Compare this model with
examples in Figs. 4c and 5a).
S .H. TREAGUS ET AL .132
active buckling, and the development of F2 folds of bedding, and
refolding. This involves developing a model for producing hybrid
F1 þ F2 folds (see Fig. 12), which are some of the most puzzling
structures at Rhoscolyn, and have been open to different previous
interpretations. We can then reveal a possible reason for the
observed changes in F1 and F2 fold vergence in the vicinity of
the present-day hinge of the Rhoscolyn Anticline.
Bedding and cleavage, folded fabrics and quartz veins
The average layer (Fig. 11, I) reveals that the original vertical
axial-plane cleavage (S1) has deformed to 628 NW, close to the
average S1 trace in the profile of the Rhoscolyn Anticline (Fig.
3b). The axial-planar S1 remains visible in some semipelites,
cross-cut by a shallower NW-dipping S2 (Fig. 8). In most pelites,
the dominant foliation is the S2 crenulation cleavage, with
significantly shallower 208 NW trace. However, at locality 7, the
S1 cleavage in pelite around F1 folds in thin quartzites is clearly
preserved (Fig. 6).
Quartz veins subparallel to the axial-planar S1 generally reveal
F2 folds of S asymmetry and NW vergence (Fig. 11b), with S2
axial planar, as shown at locality 2 (Fig. 4b). However, we
observe many local variations in the geometry of these quartz
veins and their folds, particularly in the South Stack Formation
in the central part of the Rhoscolyn Anticline (localities 5 and
6), as described above. This may in part be due to irregular
initial vein geometry, but we consider it mainly reflects veins that
track S1 fans around F1 folds on different scales and are thus in a
variety of orientations for F2 folding. In our strain model (Fig.
11), layers or veins originally inclined 90–708 SE should fold
with S asymmetry, whereas those at ,708 SE should fold with Z
asymmetry. Such variations in vein fold geometry are revealed at
locality 5 (Fig. 4d and e).
The S1 fabric of the Rhoscolyn Anticline is generally best
preserved in quartzites, psammites and semipsammites, but some
of these last rocks reveal a shallower-dipping cross-cutting S2
(e.g. locality 9, Fig. 8). The model in Fig. 11b shows that in
theory, S1 would be folded in a different geometry, in different
rock types, according to its original attitude. Our field observa-
tions reveal this to be the case. It is important to note that our
model and our structural observations show S2 cross-cutting both
limbs of the Rhoscolyn Anticline, and cannot be explained as an
axial-planar structure. The geometry of S2 cross-cutting F1 folds
is seen on different scales, at many localities around the
Rhoscolyn Anticline (e.g. localities 1, 8 and 9) (Figs. 4a, 7
and 8).
Let us consider now the D2 effects on the convergent S1
cleavage in the schematic competent bed (Fig. 11, II). The F2
folding effects are shown to vary around the Rhoscolyn Anticline
(Fig. 11b), and the same geometric variations would be seen
around F1 folds on smaller scales, too. On the SE limb, S1 is
extended and rotated to a shallower NW dip, but remains steeper
than S2, and so the oblique cross-cutting effects of S1 and S2
would be clear. The sense of S1 refraction from axial planar
clockwise into competent beds will preserve the F1 NW vergence
on this limb, and this refraction sense is seen at field localities 1
and 2 (e.g. Fig. 4b). Entering the central hybrid zone of the
Rhoscolyn Anticline (Fig. 11b, A9–H), which our model con-
siders is part of the original SE limb, this same NW-verging
sense of the convergent S1 fan in the competent bed should be
preserved, although diminished by the D2 deformation. This is
the opposite sense of refraction into a competent bed from what
would be expected on this part of a major fold, if it were a
single-phase structure with its hinge at H. Our descriptions of
localities 3–5 (e.g. Fig. 4e) noted just such a sense of S1
refraction, and it was these observations, together with vergence
and S1 relationships in minor folds and the geometry of folded
quartz veins, that led us to conclude that the original axis of the
Rhoscolyn Anticline lay between localities 5 and 6 (Fig. 3, X),
shown as A9 in the model (Figs. 10 and 11). This ‘wrong’ sense
of S1 refraction is probably the main reason for Cosgrove (1980)
to deduce that the Rhoscolyn Anticline was an antiform that
refolded earlier bedding–cleavage relationships, but our own
observations clarify that this relationship is peculiar to this
central hybrid region, rather than persistent on the whole of the
NW limb of the Rhoscolyn Anticline.
Our model reveals very different effects of D2 on S1 in the
competent bed (II) on the original NW limb (Fig. 11b). Although
bedding lengthens, the convergent S1 cleavage is oriented close
to the D2 shortening direction, and should undergo significant F2
folding with axial planes (S2) at a small angle to bedding. This
feature of folded S1 within non-folded beds produces the
abnormal cleavage vergence relationships described by Lisle
(1988) from this part of the Rhoscolyn Anticline, and discussed
above. Chevron-style F2 folds of this kind, sometimes of just one
wavelength within a particular psammite bed, are notable
features in flat-lying rocks on this NW limb of the Rhoscolyn
Anticline, as shown at locality 6 (Fig. 4f).
According to our model, there is a critical orientation in the
convergent S1 fan, with original inclination of 708 SE (Fig. 11,
asterisk) where maximum folding of S1 occurs, to create
symmetrical M-shaped F2 folds. This point marks a ‘structural
divide’ on the deforming F1 fold (whether major or minor),
between two regions: (1) where S1 rotates clockwise during D2,
and develops F2 chevron folds with S asymmetry: (2) where S1
rotates anticlockwise, developing F2 folds with Z asymmetry. At
this divide, an internal fanning of S1 is geometrically created
within the lengthening competent bed, unrelated to F1 or F2
folding, as illustrated (Fig. 11b). We regard this as the explana-
tion for the sheaf-like fanning shown on a fold limb at locality 6
(Fig. 5c).
Distortion of F1 minor folds, and their vergence
We consider now a suite of schematic minor F1 folds and their
passive distortion by the D2 deformation, assuming they undergo
no mechanically active F2 refolding (Fig. 11, III). The original
NW-verging F1 folds on the steeper part of the SE limb (Fig. 10,
H–C9) will undergo body rotation and some fold tightening. The
original short limbs become flatter and extended, and original
long limbs are steepened to overturned, and shortened. The total
effect is a decrease in S asymmetry while retaining the NW
vergence. In contrast, on the lengthened NW limb of the
Rhoscolyn Anticline (Fig. 10, B9–A9), these minor F1 folds
would increase their Z asymmetry and their SE vergence.
The most interesting and potentially ambiguous effects are in
the hybrid hinge zone (Fig. 10, A9–H). Here, D2 passively
distorts NW-verging F1 minor folds towards a more neutral
vergence and near-symmetrical (M) forms (Fig. 11b), but the
finite effect will depend on the original tightness and asymmetry
of the F1 folds compared with the D2 distortion. Tighter F1 folds
could retain their NW vergence and original hinges, whereas
more open F1 folds could become significantly modified to
become SE verging and Z shaped, with hinge migration. At
Rhoscolyn, the average F1 fold geometry shows an overall
neutral vergence at locality 3, which is close to the present hinge
(Fig. 3, H), as suggested by Roper (1992) and others. According
to our model, this is not the original hinge of the Rhoscolyn
THE RHOSCOLYN ANTICLINE UNRAVELLED 133
Anticline, but is a product of the D2 distortion of the original
fold. We consider that the original axis or hinge of the Rhoscolyn
Anticline (see Fig. 11, A), where F1 folds would have changed
their vergence before the second deformation, is somewhere
between localities 5 and 6 (Fig. 3, X). This position is no longer
uniquely revealed by a changing F1 fold vergence, but was
deduced on the basis of other field evidence, such as S1 relation-
ships, as described above.
F2 folding and hybrid F1 þ F2 folds
Observations at localities 3 and 4 reveal that not all the features
can be explained by the passive D2 deformation model shown in
Fig. 11. Some of the features of the minor folds and their fabrics
clearly arise from hybrid F1 þ F2 folding. We thus need to
consider mechanically active behaviour during D2; that is, F2
folding and/or refolding of F1 folds.
Competent and incompetent layers that behaved in a mechani-
cally active way in the first deformation, to develop F1 minor
folds on different scales, would in theory be expected to behave
in a mechanically active manner during D2, and develop new F2
folds. However, we noted in our discussion above of polyphase
deformation and coaxial refolding that mechanically active F2
folding, which either produces new folds in bedding or refolds
F1 folds, is likely to be a complex process in rocks with
competence contrasts that already contain structures on a range
of scales. We can consider two types of mechanical response of
the Rhoscolyn rocks to D2 deformation.
(1) The superposed deformation might simply reactivate the F1
folds, rather than create new F2 folds (see Fig. 2c). Thus, F1
minor folds on the steep SE limb of the Rhoscolyn Anticline,
undergoing shortening in D2, might be reactivated to become
tighter. The lengthening NW limb of the Rhoscolyn Anticline
would undergo the opposite effect: a potential reversal of
buckling that would open up F1 minor folds. There is some
evidence of opened F1 folds at Rhoscolyn, but this can also arise
with the passive deformation model described above, and so
‘active opening’ is rather difficult to prove.
(2) The superposed deformation might initiate F2 folds in
layers that are favourably oriented for buckling, as discussed
already for S1-parallel quartz veins (see also Results, (7)). This
refolding response is the more usual model for polyphase
deformation in the literature, as reviewed above. We have already
discussed F2 folds that affect quartz veins and S1 cleavage in
different orientations. These provide good evidence for the two-
phase deformation history at Rhoscolyn, and allowed us to
quantify the D2 strain in our model. Here, we are interested in
assessing the degree of F2 folding of sedimentary beds, creating
new folds or refolding of F1 folds.
To produce active F2 folds of bedding, layer(s) must be in a
suitable orientation for folding (Fig. 9), be long and straight
enough to accommodate a half or whole fold wavelength, and
satisfy buckling mechanics. Assuming these conditions are all
met, F2 folds would be expected to have different symmetry and
vergence according to bedding orientation around the major fold,
or on subsidiary folds. Steep beds (.708 SE) would develop F2
folds with S asymmetry and NW vergence, as for the axial-plane
F2 structures (Fig. 9b). Such second folds might arise on long
limbs of F1 folds on the SE limb, as described at locality 1 (Fig.
4a). Symmetrical M-shaped F2 folds (neutral vergence) would
theoretically develop on limbs with 708 SE dips. More moder-
ately SE-dipping beds (50–708 SE) would develop F2 folds with
Z asymmetry and SE vergence. These could also arise on long
limbs of F1 folds on the shallower part of the SE limb, or
perhaps on short limbs of F1 folds on the NW limb.
We consider that the most favourable condition for F2 folding
is on long limbs of asymmetric F1 folds on the SE limb. These
are in the D2 shortening direction, and also provide the greatest
layer length to accommodate an F2 fold pair, albeit one that must
have a shorter wavelength than the F1 fold wavelength. This
situation might be further favoured by irregularities in F1 folds
and their wavelengths, so that an F1 fold with an exceptionally
long wavelength could provide an unusually long limb that could
possibly nucleate an F2 fold close to its ideal buckling wave-
length. As noted above, the resulting F2 folds could be NW or
SE verging, or neutral, according to this long-limb orientation.
However, the discussion so far reveals that many geometrical
conditions need to be met for F2 folding, reinforcing our field
observations that F2 folds of bedding are localized structures, not
a systematic refolding of F1 folds.
We illustrate this process in Fig. 12, using a schematic initially
asymmetric F1 fold pair with NW vergence, limb dips of 658
NW and SE, and overall ‘sheet dip’ of 408 SE. Such a fold might
have been seen in the A–H region of the original Rhoscolyn
Anticline (Figs. 10 and 11), which became the hybrid zone after
D2. We assume that the layer is competent, and folded almost
entirely by buckling in both D1 and D2, to retain the original bed
length. In Fig. 12b, the ‘sheet dip’ has rotated to 108 SE, as for
the passive model, but in this case it is accompanied by ‘active’
F2 folding. An F2 fold pair is constructed on the F1 long limb,
with Z asymmetry and SE vergence to meet the D2 strain
requirements, and with geometry that best satisfies other space
requirements. In deriving this construction, we find that the
interlimb bisector (axial plane) of the F2 buckles cannot be
exactly parallel to S2, and this angular discordance controls the
precise orientations of F2 fold limbs and the angle of the long
limbs to S2 (always a small angle). The solution in Fig. 12b best
meets all the geometric criteria, but was also guided by field
observations of cleavage and bedding orientations in hybrid
folds.
The final product is a series of folds appearing to have Z
asymmetry and SE vergence, with longer ‘flat’ limbs, but in
detail they have the form anticline, synform, antiform, syncline,
as shown (Fig. 12b). This type of hybrid folding has been
described from localities 3 and 4, and the example from locality
4 (Fig. 4c) shows notably similar features to those modelled in
Fig. 12. Most important is the sequence from a fold hinge with
an axial-plane S1 (F1), to the next fold that folds S1 together with
bedding (F2) (e.g. Figs. 4c and 5a). Another diagnostic feature of
the model is the small angle between the long limbs of these F2
folds and the crenulation cleavage (S2), also confirmed by field
observations. In Fig. 12b, we also attempt to reproduce some of
the complexities that could arise in quartz veins that are parallel
to S1, in different parts of the hybrid F1 þ F2 folds. Such changes
from veins axial planar to some folds, and yet folded around
others, are all observed in association with hybrid folds at
localities 3–5.
Similar constructions of F2 folds have been made for other
orientations and degrees of asymmetry of F1 folds. Those with
‘sheet dips’ of 308 SE and 508 SE range (deformed range of 08 to
238 SE, the latter being close to H in Figs. 10 and 11) reveal
similar geometric properties of ‘refolding’, and their structural
implications, to those shown in Fig. 12 for the intermediate
orientation. These two other examples (not illustrated) produce
new F2 folds and hybrid F1 þ F2 folds that both suggest overall
SE vergence. The hybrid fold asymmetry is greatest at the
shallower dip, comparable with the present crest or ‘flat limb’ of
S.H. TREAGUS ET AL .134
the Rhoscolyn Anticline (see locality 5), and least for the model
closest to today’s hinge, H (locality 3), where we produced
almost neutral effective fold vergence in the model.
Active F2 folding is more difficult to explain and model on the
NW limb of the original Rhoscolyn Anticline. Here, the layer
orientations favourable for F2 shortening with possible buckling
are the short limbs of F1 folds. We conclude from field
observations that most of the observed minor folds on this NW
limb are F1 folds that have been modified by D2, quasi-passively,
as described above (Fig. 11). For example, the tight folds at
locality 7 (Fig. 6) appear to have been overprinted by S2, and a
folded quartz vein suggests tightening of one of the F1 folds, but
no significant new F2 folds are recognized. Likewise, the fold
pair at Bwa Du (locality 9, Fig. 8) reflects only the D2
deformation in the superposition of S2 fabrics. These observa-
tions tend to confirm that F2 folding (refolding) of bedding is
restricted, and is probably significant only in the present-day
‘hinge region’ of the Rhoscolyn Anticline. Even there, it rarely
causes coaxial refolding in the traditional sense, with Type 3
cross-sectional patterns (e.g. Fig. 5b), but instead usually gives
rise to compound F1 þ F2 folds (e.g. Figs. 4c and 5a).
The enigma of F1 and F2 vergence changes
Referring back to the mapped structures at Rhoscolyn (Fig. 3),
Cosgrove (1980) considered that F2 folds changed their vergence
at H, supporting his conclusion that the Rhoscolyn Anticline was
an F2 fold. However, Roper (1992) specifically considered F1 and
F2 minor fold vergence, and concluded that there was an F1
vergence change at c. H, leading him to conclude that the
Rhoscolyn Anticline was an F1 structure with its axis at H. Can
both be correct? According to our model, H is not a significant
point on the original Rhoscolyn Anticline, and not where we
would expect to see a change in minor F1 fold vergence around
the major fold. Also, according to our model, H has no structural
significance in D2 deformation, except that it produces the
illusion that there is a major fold hinge here.
We have shown that the combined effect of quasi-passive D2
distortion on mildly NW-verging F1 folds in this region of the
Rhoscolyn Anticline would be to neutralize the F1 fold vergence,
creating an apparent F1 vergence change near H. At the same
time, the change from F2 folds with S asymmetry that formed on
steep F1 long limbs, to folds with Z asymmetry that formed on
less steep F1 long limbs (discussed above), could also be close to
H. Neither of these vergence changes is likely to be a precise
position on the Rhoscolyn Anticline, given (a) the likely
variation in fold geometry of F1 folds, which affects the
neutralizing effect of the D2 deformation on their NW vergence,
and (b) the dependence of F2 fold vergence on whether folding
occurs in beds or F1 limbs with initial dips above or below 708
SE. Thus, today’s hinge (H in Fig. 3) may mark an approximate
‘vergence divide’ for F1 and F2 folds, which could explain why
the Rhoscolyn structures have remained in contention. We
conclude that H is neither the true axis of the Rhoscolyn
Anticline nor a major F2 fold axis.
Calculating the total deformation at Rhoscolyn
From the structures and modelling of the Rhoscolyn Anticline,
we deduce two superposed deformations, with a common inter-
mediate strain axis (Y) subparallel to F1 and F2 fold axes. The
combined effects of these can therefore be analysed in two
dimensions, in the fold profile plane. Our modelling has not
specifically addressed 3D orientations, or reasons for the plunge
of the folds (c. 248), and we have not extended our analyses here
to structures of similar kinds in nearby localities of Monian
Supergroup. Nevertheless, our field studies in South Stack to
Rhoscolyn Formation rocks in other parts of Holy Island support
the deduction that the F1 folds were initially rather tight upright
structures, consistent with the model presented here for the
Rhoscolyn Anticline.
If the total deformation is modelled as two superposed plane-
strain pure shears with a common XZ plane (the profile plane), it
is straightforward to calculate the total finite strain ellipse. From
the symmetrical upright shape of the parabolic F1 fold (Fig. 10a)
in section, the D1 deformation can be deduced to be a pure shear
with horizontal shortening (Z) and vertical extension (X). We do
not have any direct measurement of the D1 strain. A 708
parabolic anticline produced entirely by buckling could be
achieved by a horizontal stretch of 0.58 (horizontal distance/
curve length), but this excludes any layer shortening before or
during the folding, as evidenced by S1 fabrics in all the rocks at
Rhoscolyn. From a recent review of strain associated with
folding (Treagus 1997), we estimate a bulk shortening of c. 0.5
for a multilayer fold of this shape. For plane strain, this would be
a D1 strain ellipse with axial ratio of R ¼ 4.
If this D1 strain ellipse is subjected to the D2 strain of R ¼ 3
deduced above (according to the coordinate transformation; Eq.
4), the resulting finite (D1 þ D2) strain ellipse is found to have
an axial ratio of R ¼ 3:05, with X oriented 558 NW and
Z ¼ 358SE. This total strain is therefore almost the same, in
value, as the D2 strain, but with axes oriented approximately
midway between the D1 and D2 axes. The deformed X direction
for D1 (e.g. a mean S1 trace) ends up only 78 anticlockwise of
the finite X direction. Thus, if a finite D1 þ D2 grain-shape fabric
were to form, it would cross-cut the deformed S1 fabric by only
a small angle, with a slightly shallower NW dip, and might
appear to be axial planar to the deformed fold. However, this
‘total fabric’ would have a significant angle to S2.
Conclusions
(1) Structural observations and modelling demonstrate that the
Rhoscolyn Anticline was an originally upright F1 fold of
parabolic shape with up to 708 limb dip. The second deformation
(D2) has created its present-day asymmetric, SE-facing, more
open form. We estimate that the original anticline axis was c.
260 m NW of the present-day hinge, and that the region between
the old and new hinge is a hybrid zone. Here, original NW-
verging F1 minor folds have been distorted by the D2 deforma-
tion and modified or refolded by F2 folds, to create many of the
ambiguous structures that have given rise to different previous
interpretations.
(2) The structures at Rhoscolyn allow us to quantify the
D1 þ D2 deformation in this part of Anglesey. The original
Rhoscolyn Anticline is consistent with a bulk horizontal short-
ening of 0.5 (R ¼ 4) for the sequence during D1. The D2
deformation is deduced to be a 708 SE-inclined pure shear
shortening with R ¼ 3. This leads to a total deformation of
R ¼ 3:05, with the shortening inclined 358 SE in fold profile
view.
(3) Few studies have addressed the mechanics of superposed
deformation of layered systems with competence contrasts,
where the fold axes are subparallel but the shortening directions
are at a high angle. We provide theoretical and geometrical
arguments why the Type 3 fold interference patterns of Ramsay
(1962) would be unlikely to develop in these rocks, except
locally. Instead, the superposed deformation causes modification
THE RHOSCOLYN ANTICLINE UNRAVELLED 135
and reactivation of first folds, and creates a variety of hybrid
structures and two-phase fabrics.
(4) This study endorses the conclusion by Lisle (1988) that
fold vergence boundaries are an unreliable method of locating
early folds, in regions of polyphase deformation. The vergence
of minor folds around the Rhoscolyn Anticline has been used in
previous studies, to deduce both that it is a major first fold and
that it is a major second fold. We consider neither is the case,
and the neutral vergence in the present hinge region of the
Rhoscolyn Anticline is the result of the superposition of an
oblique D2 deformation on NW-verging F1 folds.
(5) An important aspect of unravelling the polyphase struc-
tures at Rhoscolyn concerns the relative importance attached to
structures and fabrics in different lithologies. One reason for the
different interpretations is the significance attached to the
deformation and fabrics in psammites (S1), versus those in
pelites (S2) and the quartz veins they contain. We conclude that
least deformed lithologies (quartzites and psammites) provide
clearer clues to the region’s deformation history than those in the
more deformed pelitic lithologies.
We dedicate this paper to the memory of Robert M. Shackleton (died 3
May 2001) and Dennis S. Wood (died 27 April 2001), who introduced
J.E.T. to the geology of Anglesey, and who at Rhoscolyn provided an
enthusiastic introduction to field structural geology for so many geolo-
gists and their students. Thanks go also to the many colleagues and
students, on countless field trips that have helped us form this interpreta-
tion. We appreciate the constructive review of J. Gale, and thank R.
Hartley for drafting some figures. S.H.T. acknowledges a NERC Senior
Research Fellowship, which allowed this paper to be completed.
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Received 14 December 2001; revised typescript accepted 24 May 2002.
Scientific editing by Haakon Fossen
S.H. TREAGUS ET AL .136