superposed deformations and their hybrid effects

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Journal of the Geological Society, London, Vol. 160, 2003, pp. 117136. Printed in Great Britain.

Superposed deformations and their hybrid effects: the Rhoscolyn Anticline unravelledS U S A N H . T R E AG U S , JAC K E . T R E AG U S & G I L E S T. R . D RO O P Department of Earth Sciences, University of Manchester, Manchester M13 9PL, UK (e-mail: s.treagus@man.ac.uk)Abstract: This study of the controversial structures of the Rhoscolyn Anticline suggests a different result of two-phase coaxial deformation from Ramsays Type 3 interference fold patterns. From detailed eld 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 modication and distortion in a second deformation (D2 ). This second deformation is an oblique, but near-vertical, pure shear, with a quantiable strain ratio ( R 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 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 NWverging 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 regions 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 eld or thin section, such as crenulation cleavages (Passchier & Trouw 1996, pp. 8488) or folded lineations (Ramsay & Huber 1987, pp. 481484). In this paper, we consider an area of polyphase deformation whose structures have led to the emergence of many different interpretations, with signicant implications for the tectonic history of the region. Our study focuses on the Rhoscolyn Anticline located on Holy Island, Anglesey (Ynys Mo n), 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 deformation from the large to small scale. The area was chosen by Price & Cosgrove (1990, pp. 482490) 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 interpretations, some of which are illustrated in Fig. 1 (Greenly 1919; Shackleton 1969; Cosgrove 1980; Phillips 1991); other interpretations 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 signicance of the deformation phases that gave rise to the fabrics and folds. In essence, these simplify into whether the Rhoscolyn Anticline is a major rst fold, as rst proposed by Shackleton (1969) (Fig. 1d), that is overprinted and modied by later folding and fabrics, or is a later antiform, that refolds earlier structures, as exemplied by the other illustrated interpretations (Fig. 1c, e and f).117

The Rhoscolyn area provides ample exposure of the following 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 cleavage 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 rst 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 eld, here, are whether folds on different scales are rst or second, whether their vergence is signicant, 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 specic 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 contrasts, and for the regional tectonic history of the Monian Supergroup of Anglesey.

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S . H . T R E AG U S E T A L .

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.

T H E R H O S C O LY N A N T I C L I N E U N R AV E L L E D

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Mechanisms of superposed deformation and coaxial refoldingRefolded 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. Ramsays classication (Ramsay 1962, 1967, chapter 10; Ramsay & Huber 1987, chapter 22) of fold interference (Types 13), 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 prole plane of the two phases, and might be expected to show Ramsays Type 3 refolding, or Type 0 (no refolding), according to the orientation of the superposed shear folds. Ramsay & Lisle (2000, pp. 885901) 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 signicant effect in Fig. 2b is that the rst folds can be clearly traced, with only small differences locally between original hinges and nal 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 rst 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 mapping 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 rst-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 rst 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 specic 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 mechanics 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 rst 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

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