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Heterogeneous Deformation in the Development of the Laxfordian Complex of South Uist, Outer Hebrides

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Heterogeneous Deformation in the Development of the Laxfordian Complex of South Uist, Outer Hebrides Heterogeneous deformation in the development of the Laxfordian complex of South Uist, Outer Hebrides MICHAEL P. CO WARD CONTENTS x Methods of strain assessment ........ x42 2 The f2 phase of deformation ........ x42 (A) Orientation of thef~ structures ....... I43 (B) Regions of low intensity deformation ...... x43 (C) Small scale structures ........ x43 3 Thefs phase of deformation ........ 144 (A) Small scale structures ........ I46 (B) Fanning of axial planes _ ...... x47 (C) Strain heterogeneity and thefs foid shape ..... x49 (D) Variation in plunge off3 folds ....... x5o 4 The f4 phase of deformation ........ x5 I (A) Folding of earlier structures ....... x 5 I (B) The f4 fabric and metamorphism ....... x5 x (C) Variations in fold geometry and in amount of flattening . I53 (D) The trend of the f4 axial planes ....... x55 (E) Relationship between intensity off4 deformation and f4 metamorphism x55 5 Discussion ........... x56 6 References ........... x57 SUMMARY Four phases of heterogeneous deformation terised by broad open antiforms and tight (fl t°f4) are distinguished within the Laxford- synforms. These owe their form, in part, to the ian episode of regeneration of the Lewisian strains set up during the buckling of an inter- gneiss complex of South Uist. The variation in face between competent rocks below and less amount of deformation has been examined competent rocks above. Heterogeneities in the with respect to three of the Laxfordian fold f4 deformation are closely related to variations phases, especially fa and f4, and apparent in the intensity of the syntectonic metamor- relationships between the strain heterogeneities phism; intense f4 deformation is closely asso- and both the original nature of the rock and ciated with migmatisation and granitisation. the syntectonic metamorphic conditions have Possibly intense f4 strain may result from been observed. The heterogeneities of the fa localised variations in ductility caused by a deformation appear to be related to the forms local thermal high. of the large scale fa structures that are charac- VARIATIONS IN AMOUNT of deformation have long been recognised in basement regions subjected to tectonic regeneration. In the Precambrian Lewisian rocks of the mainland of northwest Scotland, for example, Peach et al. (19o7) and later Sutton & Watson (I 951) noted that a central block of basement with pyroxenes was bounded to the north and south by zones of reworking. In the Outer Hebrides, Jl. geol. Soc. Lond. vol. x29, x973, pp. x37-x6o, 7 figs. Printed in Northern Ireland. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/139/4884626/gsjgs.129.2.0139.pdf by guest on 30 September 2021 i4 o M. P. Coward Dearnley (I962) and Dearnley & Dunning (I968) recognised variations on all scales in the amount of reworking of the Lewisian gneisses during the Laxfordian cycle of activity. Sometimes variations in deformation state correlate with local structures, thus Clough (I9o7) and later Park (I97 o) showed that in the mainland Lewisian, deformation was concentrated on the southern limb of a Laxfordian antiformal structure, the Tollie antiform. Sutton & Watson (I969) observed that between Gruinard Bay and Torridon, the synformal regions often show the most intense deformation, while parts of the intervening antiformal regions remain relatively unaffected. These observations indicate a structural control on the amount of reworking in this region. In this paper reworked Lewisian gneiss on South Uist is examined so as to relate variations in the amount of deformation to structures produced during the Laxfordian fold phases. On South Uist, a zone of intensely crushed rock and pseudotachylite, the Outer Isles thrust, described by Jehu & Craig (I925) , and Kursten (I957) , divides the island into a broad western portion, the 'Western Gneiss' with which this paper is concerned, and a narrower eastern portion described elsewhere (Coward I972 ). The dominant rock types of the Western Gneiss are hornblende biotite quartzofeldspathic gneisses with interbanded basic rocks of several ages with remnants of metasediments (Coward et al. I969). A distinctive group of basic rocks was correlated with the Scourie dyke suite of the mainland by Dearnley (I962) and used to separate pre-dyke (Scourian) gneiss forming events from post dyke, Laxfordian, events. Dearnley (1962) subdivided the Lewisian of the Outer Hebrides into a central zone showing practically no Laxfordian tectonic effects in which the structure was mainly Scourian, and northern and southern zones in which the Laxfordian reworking was more intense. The northern boundary of this central zone lay in southwest South Harris with the southern boundary across Loch Eynort in South Uist. Although Dearnley recognised local effects of Laxfordian deformation, he considered that most large scale structures within his central zone were of Scourian age, while the conspicuous large scale structures in the northern and southern zones were considered to be of Laxfordian age. Dearnley (I962), and later Dearnley & Dunning (I968), recognised only two phases of Laxfordian deformation and metamorphism. Dearnley regarded the first phase of metamorphism as being of granulite facies and the second metamor- phic phase as retrogressive, of amphibolite facies, and associated with migma- tisation. The observations given later and those of Coward et al. (I969) show that over much of South Uist all stages of the Laxfordian metamorphism were in the amphibolite facies. Around Loch Skiport, however, quartzofeldspathie gneisses and metasediments include two pyroxenes (Coward et al. I969) , and it would appear that the rocks of this region suffered granulite facies metamorphism either early in the Laxfordian or in Scourian episodes. Four phases of Laxfordian deformation (f~ to f4) have been recognised by the present author. Structures of the first phase, f~, can only be demonstrated at a few localities where the finite Laxfordian deformation is of low-intensity. The f~ deformation is heterogeneous, though f~ folds appear to play only a small part in Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/139/4884626/gsjgs.129.2.0139.pdf by guest on 30 September 2021 Heterogeneous deformation in the Laxfordian of South Uist I4I S KH. PABBAY UDAL N NORTH UIST GARRY ~1 °A'.SIAR BENBECULA SOUTH UIST ERISKAY G~' z BARRA FI o. t. Map of the southern part of the Outer Hebrides, showing the area described in this paper. The fine lines mark the trend of the gneissose foliation, solid triangles mark areas of low intensity f~ deformation and the stipple indicates areas where pyroxenes crystallised in the acid gneiss during early Laxfordian or Scourian episodes. The heavy line marks the trend of the Outer Isles thrust. The geology ofNorth Uist is partly after Graham (t969) that of Barra, after Francis (t969). Fig. 2 outlined. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/129/2/139/4884626/gsjgs.129.2.0139.pdf by guest on 30 September 2021 I42 M. P. Coward the development of the Laxfordian structural pattern in the southern Hebrides, and will not be discussed further in this paper. i. Methods of strain assessment Quantitative methods of strain analysis as described by Ramsay (I967) and Huddleston (I969) are used. Folds have been classified into five types I a, I b, I c, 2 and 3 (after Ramsay I967). Folds of class I b are pure parallel folds and can often be regarded as flexural slip folds (Donath I962 ) which have suffered no further shortening. Folds of class I c can be regarded as folds of class i b on which a certain amount of homogeneous flattening has been superimposed. With high amounts of flattening, folds of class I c have geometries similar to those of class 2, pure 'similar' folds. Following methods used by Sherwin & Chapple (1968) and Huddleston (r 969) an idea of the total amount of two dimensional strain along a folded layer, normal to the fold axis, has been obtained. These methods involve calculating the amount of flattening superimposed on the folded layer (Ramsay I967, pp. 4II-415), measuring the amount of shortening due to buckle folding (De Sitter I958 , Ramsay I967, pp. 387 and 403) and estimating the amount of pre-buckle short- ening using the ratio of dominant fold wavelength to layer thickness (Sherwin & Chapple I968, Huddleston I969). Qualitative methods of strain assessment that could be employed in the field include the recording of the degree of regularity of the gneissic banding and also the degree of obliteration of primary discordant relationships between Scourie dykes and gneissose foliation. With an increase in strain, the gneisses often attain a more strongly banded appearance and earlier structures and discordant relation- ships become gradually obliterated. 2. The f2 phase of deformation Throughout most of South Uist and Benbecula, the gneisses have suffered intense f~ deformation. Scourie dykes are tightly folded or boudinaged and a new fabric often induces a strong planar structure in the rocks, and this f, planar fabric is folded and reorientated by the f3 and f4 structures to give the pattern shown in Fig. 2. Small areas in which f~ deformation was less intense can be recognised. Taking South Uist and Benbecula together, the largest of these areas of low intensity f, deformation occur on the west coast of Benbecula, adjacent to Culla Bay, on the east coast of Wiay (Benbecula), on Ardivachar Point (South Uist), on the eastern slopes of Trinival, (South Uist) and on Roneval (South Uist). Elsewhere
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