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.

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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 metamorphic phase as retrogressive, of amphibolite facies, and associated with migmatisation. 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

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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.

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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 shortening 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 relationships 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 in the Outer Hebrides, areas of low intensity deformation occur in eastern Barra (Francis I969), on Udal Point, North Uist (Graham I969) and in parts of North Harris and Lewis (Myers I969). It should be noted that not all these areas of low intensity deformation lie in the central zone of Dearnley; many fall within the zones which he considered to have suffered more intense Laxfordian reworking.

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Thus, while detailed work has fully sustained the observations ofDearnley (1962), and Dearnley & Dunning (1968) on the heterogeneity of the Laxfordian deformation, it has been found that the structure is far more complex than was previously suggested. There is no single boundary between Scourian and Laxfordian zones in South Uist; instead, a few small pockets of weak f2 Laxfordian deformation are preserved within a much broader area of strongly deformed rocks, and as will be shown later, the variations established during the f9 phase were perpetuated during later phases. None of the structures west of the Outer Isles thrust zone can be considered as unmodified Scourian for all bear some imprint of Laxfordian tectonic reworking.

(A) ORIENTATION OF THE f2 STRUCTURES F, folds are coaxial with structures off1 age and also with earlier structures of Scourian age. F, lineations and fold axes plunge at moderate angles to the northwest in the northwestern part of South Uist and Benbecula and to the southeast in the southeastern parts of the islands. There is thus a plunge culmination of north-northeast trend crossing the centre of South Uist (Fig. 5). From the orientation of the f2 axial planes at the crests of major f3 folds (where the effects off8 are small) the f2 folds would appear to have been fiat lying structures. There appears to be no major f2 fold in South Uist and all the small folds of this generation have an 'M' or 'Z' shape (when plunging to the northwest) and the gneiss pile can be considered as constituting a single major f2 fold 'limb'. There is no evidence of an 'inverted'f2 fold limb on South Uist (though Francis 1969 and Graham 1969, have reported local 'inversions' on Eriskay and in North Uist respectively).

(B) REGIONS OF LOW INTENSITY DEFORMATION Regions of low-intensityf~ deformation crop out as lens shaped zones, ½ to I km across and generally orientated parallel to the trend of the f2 structures. Outlines of these regions of low intensity deformation at Ardivachar and Garry-a-siar are shown in Fig. 2. Within the areas of low intensity deformation, the Scourie dykes are generally discordant, though they may locally be tightly folded and concordant with the gneissose foliation. The deformation is heterogeneous and high intensity f9 deformation is often concentrated in relatively narrow zones, often close to or within a group of Scourie dykes. The borders of areas of low intensity deformation are marked by the sudden change to concordance of the Scourie dykes and by the appearance of a planar gneissose fabric in contrast to the lineated and rodded structure characteristic of the gneiss in the areas of low intensity deformation.

C) SMALL SCALE STRUCTURES Folds of this generation are common throughout South Uist. Where they have a northwesterly plunge, their profiles have a 'Z' or 'M' form; relatively few 'S' shaped folds occur, indicating, as already mentioned, that no major folds are present. On the limbs of moderate sized f2 folds, and even in zones of intense 'M' folding, f~ minor folds show a high degree of flattening; folded quartzofcldspathic veins in

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early amphibolites record flattening strains of over 8"I superimposed on buckle- folds and from more detailed measurements on some of these veins, total two dimensional strain values of 23:I have been obtained. The amount of flattening after buckling measured for 23 f2 folds in northwest Benbecula and 44 folds in central South Uist is given in Figs. 4a and b. All the folds investigated are tight structures, generally of classes I c and 3, but approaching class 2, that is approaching 'similar' folds. There is very little difference between the amount of flattening in the north (i.e. in the central zone of Dearnley I962 ) and in the south of South Uist. Variations in amount of flattening appear to be due to local viscosity contrasts and to the relative position of the folded layers in multilayered fold systems. Within the areas of low intensity f2 deformation, at Garry-a-siar for example, the amount of strain is much lower; folds of class I b or I c with low values for homogeneous flattening have been recorded and some dykes remain unfolded even though lying normal to the orientation of the f2 axial planes. From fold analyses, one can obtain an idea of the relative ductilities of different rock types in the gneiss complex with less ductile (that is, more competent) layers usually forming folds of class I, while more ductile layers form folds of class 3. For folds formed during the early phases of Laxfordian deformation in South Uist, the following ductility sequence holds: (least ductile) pure quartz, pure feldspar, pure pyroxene-rocks < garnet amphibolite (+ pyroxene) < garnet amphibolite < amphibolite (without garnet) < biotite amphibolite < biotite bearing acid gneiss. The presence of hornblende increases the ductility, as does any mixed assemblage, and biotite usually occurs in the most ductile rocks. Hornblendic acid gneiss was sometimes less ductile than normal amphibolite, sometimes more. On the limbs of the f2 folds, the Scourie dykes are transformed to concordant basic sheets and may suffer pinch-and-swell or boudinage. The boudins formed during this phase of deformation are usually blocky or barrel shaped and are of the chocolate tablet kind, indicating extension in two perpendicular directions: this type of structure suggests that the strain had a very low k-value (cf. Flinn 1962). 3" The f3 phase of deformation The most intense 3~ deformation in South Uist is recorded close to a large j~ synform which strikes northwest across the southern part of the island. The intensity off3 deformation decreases northeastwards from this synform and in parts of northeastern South Uist and Benbecula, it is almost negligible. In the south, large scale f3 folds are tight structures on steeply dipping axial planes and often have a well developed fabric parallel to the axial planes. In the northern part of South Uist, the f3 folds are large open inclined structures characterised by a gentle

FIO. 2 (on facing page) F io. 2. Geological map of South Uist and Benbecula showing the trend of the gneissose foliation round structures off3 andf4 age, west of the Outer Isles thrust.

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+ F~ AXIAL TRACE

F,. AX,AL T.ACE EDGE OF LOW INTENSITY DEFORMATION ZONE

ARDIVACHAR POINT

LOCH C ARRON

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limb dipping from o° to 20 ° to the west, and a steep limb either vertical or relatively overturned and dipping steeply to the east. In the north, there is no new f3 fabric, small scale folds are rare and when developed are open structures. Fig. 2 shows the distribution of the main f3 fold structures. The largest order structures comprise a broad antiformal region to the north and a tight upright synform, southwest of Loch Eynort. This upright tight synform in which f~ deformation is intense, coincides with the zone of intense Laxfordian reworking recognised by Dearnley (i962), while the broad antiformal region to the north coincides with his dominantly Scourian zone.

Strain values indicated by a study of the small scalef, folds show regional variations related to those of the large structures. Small scale folds are poorly developed in the northern part of South Uist and southern Benbecula, and when present are of class I b or class I C characterised by low amounts of homogeneous flattening. In the tight synformal area southwest of Loch Eynort, f3 folds are again relatively rare, but when present they are isoclinal and are of class I c close to class 2, that is, they are almost similar folds. On the 'limb' between the main antiformal region and the main synformal region, small scale folds are more abundant and their variations in profile have been used to trace out the form of the major structures. They have been examined especially in two minor synformal zones within the limb region, one on the northwest coast of Benbecula and the other at Sgeir Dremisdale in South Uist. The minor synformal zone in northwest Benbecula (Fig. 2) lies on the northern flanks of an antiformal structure developed within the Benbeculan region of low intensity f, deformation. Within this antiformal structure the intensity of f3 deformation is also very low, but in the adjacent synformal region, f3 strains are of high intensity, though by no means comparable with the f2 strain values for this region. The f3 folds are fairly tight and are made up of alternating layers exhibiting the forms of classes I C and 3- The flattening calculated from 43 folded layers varies between 1.5"i and 3"I with a mean of 2"25"I (Fig. 4c). The total two dimensional f3 strain estimated from analysis of folded Scourie dykes has values between 7: I and I3.5 : I. The minor synformal zone at Sgeir Dremisdale, poorly exposed except on the coast, lies close to the major synformal region of South Uist. The gneisses here contain abundant folded quartzofeldspathic veins, from which two dimensional strain ratios normal to the fold axis of between 9:I and I4: I have been obtained (Fig. 4e). The flattening strains calculated from 44 multi-layer f3 folds from Sgeir Dremisdale are given in Fig. 4 d. Thus, in the fold zone on the limb between the main antiformal region and the main synformal region, two dimensional strain ratios of between 9"I and I4"I have been obtained from small scale f3 folds. Similar two dimensional strain ratios have been obtained from a minor synformal zone in Benbecula. In the main antiformal region, the strain would appear to be much less and/'3 folds are often only gentle warps. In the major synform, the strain is more intense and the f3 folds are almost similar.

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(B) FANNING OF AXIAL PLANES In the northern part of South Uist, where the f3 folds are open structures, the dip of the f3 fold axial planes is generally between I o ° and 5o°E. As the structures become tighter to the south, the axial planes steepen to the vertical, and on the southern limb of the main synform they dip steeply to the southwest (Fig. 3). The small scale folds also show fanning axial planes, the fan sometimes being through almost I8O °. Both large scale and small scale axial plane fans are probably related to the formation of larger order structures. Ramberg (1963, 1964) has shown that parasitic folds can become asymmetric from later amplification of the structures of larger wavelength with which they are associated. The internal strain produced by the development of the large wavelength folds may further deform the small scale folds and change the orientation of their axial planes. The variation in orientation of axial planes of small scale folds can be computed for large wavelength folds formed either by flexural shear (Donath I962), or by tangential longitudinal strain (Ramsay 1967, pp. 397-4o3). If the large order folding was mainly due to flexural shear, and assuming originally symmetric small scale folds, then the final divergence of the axial planes will be small, and when the major fold is open, the axial planes will remain almost parallel. The small scale folds will become markedly asymmetric and the effect on the fold shape can be considered to be the consequence of simple shear along the limbs of the major fold. Along these limbs, the small scale folds should suffer an increasing amount of flexural shear away from the major fold hinge, and small folds on the limbs will be tighter and may show larger amounts of flattening than those at the crest of the fold. If the large order folding involved tangential longitudinal strain rather than flexural shear, then the final angular divergence of the axial planes will be very large, up to 18o °, but the folds will remain more or less symmetrical throughout.

A ;:,/ !/ ..... z" 7 " I'/'"'/ ......

- , \ \ / ./ ~i

F xo 3. Structural profiles across thef3 structures in South Uist. The section lines are as shown in Fig. 2.

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The tightness of the minor folds will depend on the tightness of the large order fold and the distance from the neutral surface. It will be greatest at the crest (or trough) on the inner arc of the large order fold structure. It should be noted, that if the folding was of a similar nature i.e. in shear folding, then there should be no variation in orientation of the axial planes of small scale folds throughout the fold system. Any spread of axial planes round a large order fold indicates a com- ponent of buckle folding. It has been noted that the axial planes of the f3 folds of South Uist fan through almost 9 °0 from the trough to the limb of the largest order fold. The antiformal crest in the northern part of South Uist and Benbecula appears to have suffered only weak f3 deformation, while the inner arc, in the synform of the southern region, shows much more intense deformation. The f3 folds therefore appear to have the form of buckle folds in which the internal deformation associated with the largest order fold largely took the form of tangential longitudinal strain.

,9 .8 ,7 .6 .5 .4 .3 .2 .I 0.0 •9 .8 .7 .6 .S .4 . 3 .2 .I 0.0 BI:

--• 4 1_.. 11-1 2

6 .9 .8 .7 .6 .5 .4" .3 .2 .I 0.0

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F t o 4. Histograms showing the amount of post-buckle flattening on" A. 23f2 folds from northwest Benbecula. B. 44f~ folds from central South Uist. (2. 43fa folds from northwest Benbecula. D. 44 f3 folds from Sgeir Dremisdale, South Uist. E. Histograms of two-dimensional natural strain ratios obtained from x5 folded quartzofeldspathic veins from Sgeir Dremisdale, South Uist.

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(C) STRAIN HETEROGENEITY AND THE f3 FOLD SHAPE To recapitulate, the f3 deformation appears to increase from north to south in South Uist. In the north, the f3 folds are usually open structures and the rocks generally retain their earlier fabrics. In the southwest, the f3 folds are much tighter, often isoclinal, and strong new penetrative fabrics have developed. The form of the large order structures is that of a large antiformal domain and a tight, highly strained synform. This form, characterised by tight cuspate synforms separated by broad lobate antiforms is a feature of many of the f3 folds in the southern Hebrides Four major synforms have been recognised, one in Barra, the second in South Uist, described here, a third on the northwest coast of North Uist and a fourth in the Sound of Harris (Coward et al. i97o ). Between these synforms, the foliation curves around open antiformal structures in which smaller f3 folds are open and the amount off3 deformation is low. This is a style of structure which has been recognised elsewhere. Ramberg (I966) and Eskola (I949) considered structures of similar form to have been produced by the gravitational updoming of certain basement areas, while Ramsay (i967) has compared structures of this kind with those produced experimentally by the buckle deformation of an interface between two materials of different competence. Evidence for the origin of the Hebridean folds is given by the fanning of the f3 axial planes and the variations in the amount of strain. These are consistent with an origin by buckling under compressive stresses. Any mechanism invoked for the production of these structures must give a strain pattern similar to that produced by buckle folding involving tangential longitudinal strain, as shown above. Vertical gravitational movements might be expected to produce folds of a similar type, very different from those recorded on South Uist. Further evidence relating to the significance of these structures is given by the petrology of the rocks. Rocks with two pyroxenes and garnet that formed at or soon after the end of the f~ phase have been found by the writer in northeast South Uist and by Francis (I969) in Barra. In northeast South Uist, pyroxene formed in metasediments, in acid gneisses and in basic rocks during an early high grade metamorphism. Similarly in eastern Barra, a Scourian granulite facies metamorphism produced assemblages containing two pyroxenes in acid gneisses and early basic rocks. Elsewhere in western South Uist, western Benbecula, North Uist and western Barra, no pyroxene has been found in metasediments, quartzofeldspathic gneisses or early basic rocks. The early Laxfordian deformation phases were accompanied by the growth of amphibolite facies assemblages in these rocks. Dense pyroxene bearing rocks are thus located in the lower structural levels of the gneiss pile which crop out in northeast South Uist and eastern Barra, in the cores of the f3 antiforms. Parts of these regions are characterised by positive Bouguer anomalies (Bullerwell 1965). The synformal regions are made up of less dense rocks, without pyroxene, from higher structural levels. The f3 structures therefore cannot have been formed by a simple gravitational updoming of the antiformal cores, as these incorporate the most dense material. Thej~ structures have been formed in spite of local gravitational forces, not because of them. There is a strong possibility that gravity affected the shape of the f3

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structures, but the considerations outlined above suggest that it could not have been the main cause. A more reasonable explanation is that the structures were produced by a mechanism similar to that envisaged by Ramsay, involving the buckling of a thick layer or layers of competent material lying beneath South Uist, or by the buckling of an interface between competent rocks at lower structural levels and less competent rocks above. The occurrence of dense pyroxene bearing rocks in the lower structural levels suggests that these may represent the more competent material. The initial form of large open anfiforms and pinched-in synforms could therefore have been instigated by large-scale buckling of the interface between competent, often pyroxene bearing rocks at depth and less competent amphibolites and granite gneisses above. Less competent rocks in the synforms would be sandwiched between the more competent antiformal domains. Further horizontal shortening, not necessarily involving buckle folding, would cause the less competent rocks in the synforms to suffer more intense deformation and produce tighter folds solely because of their lower competence. Thus once irregularities had been set up on the interface between competent and incompetent rocks, any further shortening, whether or not this involved buckle folding, would tend to increase the amount of deformation in the synforms, relative to that in the antiforms.

(D) VARIATION IN PLUNGE OF f8 FOLDS In most localities the f3 structures are coaxial with those offz age, and only rarely is an earlier lineation folded by the f3 folds. The f3 plunge, however, is not constant throughout South Uist. Apart from the areas where it is folded by f4 structures, the f3 plunge shows a systematic variation from a northwesterly area where it plunges at 3o ° to 4O°NW to an area where it plunges at 30 ° to 40°SE. The large f3 folds are in fact periclinal structures, trending broadly northwest and plunging to the northwest and southeast, as shown in Fig. 5. The line which joins the cul- minations of each of the f3 structures, called here the 'plunge culmination line', trends north-northeast. Where the f3 plunge is to the northwest and the f3 axial planes are inclined, the axial planes dip to the northeast and have a west-northwest strike. Where the f3 plunge is to the southeast, the f3 axial planes dip to the east with a north-northeast strike. Thus the orientation of the f3 axial plane changes systematically with the plunge. There is therefore evidence for a variation in orientation of the f3 plunge, the f~ lineations and lineations of earlier structures and also of the axial planes of the inclined f3 folds. This variation in orientation originated at some time prior to the main f4 deformation, as the f3 structures were periclinal before the f4 folds developed. The variation in f3 plunge may reflect a variation in orientation of the foliation prior to f3 deformation; the foliation and f2 structures may have been folded about a north-northeast axis to produce an antiform post dating the fz phase, pre-dating the f3 phase. Alternatively the reorientation may post date the f3 deformation; the orientations of inclined f3 axial planes vary systematically with the fold plunge. The trend of the vertical foliation and the trend of the vertical f3 axial planes, however, remain constant no matter what the plunge, and for the lineation to be folded upon this vertical surface, the mechanism of reorientation

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must be comparable with that which might produce a similar fold (Ramsay 1967, pp. 426 and 473).

4. The f4 phase of deformation The most intense f, deformation is concentrated along the eastern side of the Western Gneiss unit in South Uist where vertical, east-west trending tight to isoclinal folds of many sizes and orders have been mapped (Fig. 2). On the west coast, the effects of the f4 deformation are usually seen only as local warps or monoclinal structures which fold an earlier lineation. From west to east there is a gradual increase in the amount of deformation with an increase in the tightness and number of folds.

(A) FOLDING OF EARLIER STRUCTURES Close to the hinges of large order f3 folds, or on the gently dipping limbs of these folds, closed interference patterns are developed by the superimposition off4 folds. Where the plunge of the f4 folds is steep, (i.e. on steep limbs off3 folds), then open interference patterns are seen. The arrangement of deformed lineations depends on the orientations of the various sets of folds. The f~ and f3 lineations lying on the foliation surfaces were deformed in such a way as to trace out a curved locus in space about the Jr4 fold axis. Where the foliation had a low dip, as on the gently dipping limb of an f3 fold, then the f4 fold axis has a gentle plunge, usually at a high angle to the original orientation of the f2 or f8 lineation, and thus the locus traced out by the deformed lineation lies on a polar great circle (sub-areas VI and IX, Fig. 5). Where the f3 fold limb was steeply dipping, then the deformed lineation traces out a locus about a steeply plunging f4 fold axis. Where little homogeneous flattening has been superimposed on the f4 folds, the deformed lineation traces out part of a small circle or partially modified small circle (sub-areas II and IV, Fig. 5). Around Loch Skiport, however, where the amount off4 deformation was more intense and f~ folds are tighter, the trace of the lineation became further modified by homogeneous deformation superimposed on the folds, and in sub-area X, Fig. 5, the trace is more nearly that of a great circle than a small circle.

(B) THE f4 FABRIC AND METAMORPHISM The f4 deformation phase was accompanied by a phase of amphibolite facies metamorphism and local granitisation which continued in the areas of more intense deformation as an episode of post tectonic recrystallisation. Granitisation and post tectonic recrystallisation were most intense in the east, and here syntectonic f4 fabrics have usually been obliterated. In the west and southwest there is often a penetrative fabric within the acid gneiss and a strong hornblende lineation within the basic rocks. The variations in intensity of this late metamorphism are illustrated schemati- cally in Fig. 6a. The areas distinguished on the map can be briefly described as follows:

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Area I" within this area, the gneisses show few effects of late recrystallisation or granitisation. Earlier Laxfordian fabrics are often well preserved and the main evidence of a late metamorphic phase is given by the presence of large discordant pegmatites. Area 2: within this area the f4 folds usually have an f4 fabric and there is evidence of later recrystallisation or even pegmatite formation in the more deformed portions of these folds. Area 3: in this area, which comprises much of South Uist, the rocks show relics of fabrics associated with the f4 deformation, but many of the fabrics have been obliterated by static recrystallisation. Area 4: here there has been syntectonic granitisation, to the extent that the f4 deformation phase produced disharmonic or 'wild' folds. Patches of gneiss appear to have been transformed into granite or pegmatite. Area 5: within this small area, almost the whole gneiss has been transformed into a granitic mass, the gneissic banding is destroyed and the basic rocks remain only as hornblende and biotite-rich schlieren.

(C) VARIATIONS IN FOLD GEOMETRY AND IN AMOUNT OF FLATTENING A change in style off4 folds from west to east is associated both with the eastward increase in deformation and with the change in syntectonic metamorphic conditions. A rather subjective map (Fig. 63) divides South Uist into several zones based on the field appearance of the folds, supported by a number of fold analyses.

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F I o. 6 A. Map which divides South Uist into five areas, each area having suffered a different intensity off4 metamorphism (see text). B. Map of South Uist to show the structural zones based on the f4 fold geometry. Localities from which analyses were made are shown by solid triangles. C. Map of South Uist showing the variation in orientation off4 axial planes.

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Zone I" within this zone, there are relatively few small scale folds and large scale folds are often no more than gentle warps. The f4 deformation is highly variable and localised and narrow zones of more intense deformation, with fight to isoclinal folds, lie in areas of less intense deformation typified by only gentle warping. Zone 2: small scale folds are locally abundant within this zone, though large order folds form only gentle warps. Within this zone, original variations in layer thickness and position in a multilayer fold system are important in determining the geometry of the folded layer. Generally, the flattening strain superimposed on the folds is fairly low. Most folded layers are of classes I a, i b or zc, with variable flattening strain ratios, alternating with layers of class 3. The amount of flattening revealed by measurement of 58 examples is variable, from zero to 3: I, as shown in Fig. 7 a. Zone 3" structures within this zone show flattening strains similar to those of zone 2 and in fact have been incorporated in Fig. 7a, but are distinguished by a tendency to give disharmonic forms on all scales. Well marked ductility contrasts appear to have been present in the granite gneiss itself, depending on the response of particular rocks to syntectonic granitisation. The quartz feldspar biotite gneisses which are closest to granite in composition acted as the most ductile rocks, while granodioritic and dioritic gneisses and quartz feldspar hornblende gneisses were less ductile.

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Zone 4" the folds of this zone, though comparable with those of zone 3, are usually tighter and show a larger amount of flattening strain. Flattening strains of from 1.5:I to over 8: I have been recorded; the mean is close to 3:1. Fig. 7b gives the amount of flattening for 52 such folded layers. Zone 5" within this zone, the f4 deformation is most intense, producing tight to isoclinal folds. The fold geometry is of classes I c or 3, both approaching that of class 2, and many of the folds in normal quartzofeldspathic gneisses are similar. Fig. 7c gives the amount of flattening measured from x4 folded layers within this zone. Many of the flattening strain ratios are up to IO:I.

(D) THE TREND OF THE f4 AXIAL PLANES The trend of the f4 axial planes in South Uist shows regional strike variations from i4 o° around Loch Bee in the northwest to 9oO-95 ° near Loch Skiport and 7°0 south of Loch Eynort (Fig. 6c). In the southern part of South Uist and on Eriskay, the f4 axial planes trend at 6o°-7 o° (Francis 1969). In the northern part of South Uist, the axial planes are usually vertical or have a steep southerly dip, though occasionally their dips are as low as 3 °0 to 4 o°. In southern South Uist and on Barra, the axial planes are vertical or dip to the north. In South Uist, a pattern therefore emerges in which the axial planes systematically converge towards the east, especially in the Skiport area. From the dips of the axial planes on South Uist and Barra, it can be deduced that they also converge downwards. The XY principal plane of the f4 strain ellipsoid is thought to be orientated almost parallel to the axial planes of f4 folds. This is suggested by the presence of a new planar fabric parallel to the axial planes and also by the occurrence of boudinage on the limbs of tight folds. A plot of the f, axial plane orientations, therefore, gives an idea of the variation in orientation of the f4 XY principal plane. The strain trajectories for the XY plane would appear to converge eastwards and downwards in South Uist, suggesting that the most intense f4 strain was concentrated below the present level of erosion in the eastern part of South Uist.

(E) RELATIONSHIP BETWEEN THE INTENSITY OF f4 DEFORMATION AND f4 METAMORPHISM From the variations in orientation of the f4 axial planes, from the size and number of the f4 folds of various orders, from the geometry of the small scale fold profiles and from the plots of lineations deformed by the f4 structures, the intensity of f4 deformation can be seen to increase from west to east and to be highest in the northeast part of South Uist (in the Skiport region). The area of most intense f4 deformation does not seem to be related to any large scale structural pattern, nor to the previous lithology or structure, but it coincides with the area of most intense f4 metamorphism and granitisation. The problem which now arises is whether intense deformation was responsible for the more intense metamorphism and granitisation, or vice versa. The extensive post tectonic recrystallisation in the eastern area might be due to a high strain energy in the rocks, itself due to more intense deformation. Intense deformation could promote ionic mobility and hence ease the granitisation and migmatisation of the gneisses, some of which were previously 'dry' and in the granulite facies.

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As has been stated, however, pyroxene bearing rocks are often far less ductile than normal hornblende or biotite bearing gneisses, and as the gneisses round Loch Skiport bore more pyroxene than rocks elsewhere in South Uist, it is difficult to see why they should have suffered the more intense deformation. Alternatively, a localised high heat flow in the eastern part of South Uist could give the rocks a higher ductility than that attained elsewhere in South Uist, and hence these rocks would suffer more intense deformation as well as more intense post kinematic f4 metamorphism and granitisation.

5. Discussion The distribution of zones of greater and less Laxfordian reworldng in the Outer Hebrides was interpreted by Dearnley & Dunning (1968) as reflecting the partial uncovering of the undulatory upper surface of a region of intense Laxfordiafl reworking underlying most of northwest Scotland. It has been shown in this paper, however, that the Laxfordian history is complex, and that the distribution of zones of greater and less Laxfordian reworking varies for each Laxfordian phase of deformation and metamorphism. Though it is perfectly true that in certain parts of the Outer Hebrides the Lax- fordian deformation was 'minimal, almost to the point of non-demonstrability' (Dearnley & Dunning I968 , p. 375), these areas are very small, and form less than 1% of the area of exposed rock. Almost all other areas have suffered some degree of Laxfordian deformation. The f~ deformation is usually intense, though localised outcrops show less intense deformation. These areas of low intensity f, deformation fall into no obvious structural pattern. The f3 structures, however, show an arrangement of high intensity deformation in synformal regions with low intensity deformation occurring in the intervening antiforms. From this pattern, from the pattern of the f3 fold axial planes and from the petrology of the rocks, it seems reasonable to suggest that the f8 structures were initiated by compression of an interface between media of different ductilities. The areas of low intensity f3 deformation, therefore, are not islands floating in a sea of reworked rock, but competent rocks from lower levels in the gneiss succession which have been interfolded with the less competent and more deformed rocks from higher in the succession. During the f4 phase, the variations in deformation appear to follow more closely the pattern suggested by Dearnley & Dunning, and the area of high intensity f4 deformation and granitisation at Loch Skiport can be regarded as the high point of an underlying region of late Laxfordian reworking. In the discussion of Park's paper on the Tollie area, (Park I97O), Watson pointed out that in basement areas like the Lewisian complex, many rocks which have suffered low intensity deformation in early stages of reworking also show only low intensity deformation during later events. She suggested that the pattern of low and high intensity deformation in the area between Poolewe and Gairloch on the mainland of northwest Scotland, was set up during the early Laxfordian deformation phases, and continued throughout the whole of the Laxfordian history. A similar point could be made about some of the areas of low intensity deformation

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in South Uist. Basic rocks which were little deformed during one phase often acted as competent units during later deformation phases. Similarly, areas of low intensity f~ and f~ deformation within the acid gneiss, such as those at Ardivachar and Garry-a, siar, have remained regions of low intensity deformation during all the Laxfordian fold phases and appear to have acted as competent cores to the 10calf3 folds. It would appear from the geology of South Uist that the amount of deformation in basement regions is a function of the nature and composition of the rock and of the metamorphic conditions. Thus, rocks that form discrete competent bodies will tend to act in a similar way through succeeding deformation phases, given the same metamorphic conditions. Changes of metamorphic conditions however may bring about changes in the mineral phases and hence the physical properties of the rocks may change. Amphibolite facies metamorphism of pyroxene bearing metasediments and basic rocks, such as those recognised in the Loch Skiport area, appears to have changed the ductilities of these rocks relative to the adjacent gneisses; the formation of biotite or hornblende instead of pyroxene appears to reduce the competence. During the f4 phase, rocks that had formed part of the competent antiformal core of an f3 fold, changed their properties completely and became more ductile, not only on a small scale to produce syntectonic migmatitic structures, but on a large scale producing a deformation high in a region of once competent rock.

ACKNOWLEDGEMENTS, I thank Drs. P. W. Francis, R. H. Graham and J. V. Watson for helpful discussion, Drs. P. W. Francis and R. H. Graham for allowing me to incorporate unpublished work, Drs. R. H. Graham and J. V. Watson for critically reading the manuscript, and the Natural Environment Research Council for a maintenance grant.

6. References BULL~.RW~LL,W. I965. Section 5 Geophysical Department. In" Mem. geol. Surv. Summ. Prog. I964: 84. CLOUOH, C. T. in Peach et al. x9o7. COWARD, M. P. I972. The Eastern Gneisses of South Uist. Scott. J. Geol. 8, I-I2. , FRA_nCIS,P. W., GRAHAM, R. H., MYERS,J. s. & WATSON,J. V. 1969. Remnants of an early metasedimentary assemblage in the Lewisian complex of the Outer Hebrides. Proc. Geol. Ass. 8o, 387-4o8. , FRANCIS, P. W., GRAHAM, R. H. & WATSON,J. V. 197o. Large-scale Laxfordian structures of the Outer Hebrides in relation to those of the Scottish mainland. Tectonophysics xo, 425-435- D~.~NL~Y, R. 1962. An outline of the Lewisian complex of the Outer Hebrides in relation to that of the Scottish mainland. Q. Jl. geol. Soc. Lond. xx8, i43-I 76. 8r. DUNNING, F. W. 1968. Metamorphosed and deformed pegmatites and basic dykes in the Lewisian complex of the Outer Hebrides and their geological significance. Q. J1. geol. Soc. Lond. x23, 335-378- DE SITTER, L. U. 1958. Structural geology, McGraw-Hill, New York. DONATH, F. A. 1962. Role of layering in geologic deformation. Trans. N.Y. Acad. Sci. Ser. 2, 24, 236--249" ELLIOT, D. x965. The quantitative mapping of directional minor structures. J. Geol. 73, 865-88o. ESKOLA, P. E. x949. The problem of mantled gneiss domes. Q. Jl. geol. Soc. Lond. xo4, 461-476. FLm~, D. I962. On folding during three-dimensional progressive deformation. Q. Jl. geol. Soc. Lond. xxg, 385-433 .

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FRANCIS, P. W. 1969. Unpublished Ph.D. Thesis, University of London. GILETTI, B. J., MOORBATH, S., LAMBERT,R. ST. J. x96I. A geochronological study of the metamorphic complexes of the Scottish Highlands. Q. Jl. geol. Soc. Lond. xx7, 233-264. GRAHAM, R. H. x969. Unpublished Ph.D. thesis, University of London. J~.HU, T.J. & CRAXO,R. M. 1925. Geology of the Outer Hebrides Part II, South Uist and Eriskay. Trans. R. Soc. Edinb. 54, 6 x5-64I. KURSTEN, M. 1957" The metamorphic and tectonic history of parts of the Outer Hebrides. Trans. Edinb. geol. Soc. 17, 1- 3 I. MYERS,J. S. 1969. Unpublished Ph.D. Thesis, University of London. PARX, R. G. 1970. The structural evolution of the Tollie antiformJa geometrically complex fold in the Lewisian northeast of Gairloch, Ross-shire. Q. Jl. geol. Soc. Lond. 125, 3 x9-350. PEACH, B. N. and others 1907. The geological structure of the northwest Highlands of Scotland. Mere. geol. Sum. U.K. RAMBERO, H. 1963. Evolution of drag folds. Geol. Mag. xoo, 97-106. 1964. Selective buckling of composite layers with contrasted theological properties; a theory for simultaneous formation of several orders of folds. Tectonophysics x, 307-341. 1966. The Scandinavian Caledonides as studied by centrifuged dynamic models. Bull. geol. Instn Univ. Upsala 18. RAMSAY, J. G. 1967. Folding andfraauring of rocks. McGraw-Hill, New York. SHERWm, J. & CHAPPLE, W. M. 1968. Wavelengths of single layer folds: a comparison between theory and observation. Am. J. Sci. 266, I67-179. SUTTON, J. & WATSON, J. V. t951. The pre-Torridonian metamorphic history of the Loch Torridon and Scourie areas in the northwest Highlands, and its bearing on the chronological classification of the Lewisian. Q. dl. geol. Soc. Lond. xo6, 241-3o7 . 1969. Scourian-Laxfordian relationships in the Lewisian of northwest Scotland. Spec. Pap. geol. Ass. Can. 5, xx 9-128.

Revised manuscript received 29 October I97I ; read 26 January 1972.

Michael Peter Coward, PH.D., F.G.S., Department of Earth Sciences, Univer- sity of Leeds, Leeds 2.

DISCUSSION Dr. JANET WATSON, gave the opening address and concluded the discussion of the three papers (in this number of the journal) by Coward, Francis and Simony, dealing with Lewisian geology and crustal regeneration in northwest Scotland: Now that the antiquity of much of the continental crust has been established it is clear that the metamorphic and tectonic processes of reworking at depth have played a major role in the geological evolution of the crust. These processes have provided the means by which old rock complexes have been reshaped and readjusted, without loss of coherence, in response to new crustal regimes; they have enabled the crust to adapt itself repeatedly to global changes in the distribution of stable and mobile regions and in the siting of zones of high temperatures. The papers published in this volume are all concerned with the effects of deep- seated regeneration as displayed by rocks of the Lewisian gneiss complex which underlies most of northwest Scotland. Taken together, they have a bearing on the operation of a major geological process.

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The oldest components of the Lewisian are at least 3000 million years in age. The time-span of the events discussed by each author was well over a thousand million years and because there is evidence that more than one major change in crustal regime took place during the period of regeneration it seems legitimate to think of the evolution of the complex in terms of several stages and to regard the products as polycyclic. The first stage which is of interest in the present context terminated at about 26oo m.y., the second stage at 18oo-14oo m.y. and the third stage at about 4oo m.y. Most of the material of the Lewisian complex already existed in the form of crystalline gneisses and associated metasedimentary and meta-igneous rocks about 29oo-27oo m.y. ago and the earliest structural and metamorphic patterns which are still clearly recognizable were established by this time. In Scotland such patterns (characteristic of the older parts of the Scourian complex) are only locally preserved without modification, but in Greenland they are displayed through much of the Pre-Ketilidian massif that is much more extensive. There is compelling evidence from the Godthab area to show that portions of the massif date back to about 39oo m.y. and had already been reworked at least once by 3ooo m.y. The relationships of certain belts of highly metamorphosed supra- crustal rocks in Scotland suggest that the Lewisian also incorporates a basement of gneisses which were already polycyclic by the time the Scourian complex was completed. The first major change of regime was that which led to the stabilization of large crustal units over the period 29oo-26oo m.y. Active reworking continued or was renewed after this change in the zones bordering the Pre-Ketilidian massif of Greenland and in the Lewisian terrain of Scotland where the products con- stitute the Laxfordian complex. The papers of Coward and Francis are concerned with the tectonic effects of Laxfordian regeneration in the southern islands of the Outer Hebrides. Further changes of crustal regime at 17oo-14oo m.y. led first to stabilization of a large part of the crust in Greenland and Scotland and subsequently to the definition of a new zone of mobility on the site of what are now the Caledonides of Scotland and Greenland. Simony's paper deals with the effects of regeneration of Lewisian gneisses near the western margin of the Scottish Caledo- Irides. All three studies provide striking demonstrations of the inhomogeneity of the effects of tectonic and metamorphic regeneration. It has long been known that kernels of little-altered Scourian gneisses are enveloped in complexes which were strongly deformed, recrystallized and locally migmatized during successive Lax- fordian episodes. The work of Coward and Francis, taken in conjunction with that of Graham, Myers and others elsewhere in the Hebrides, shows that some of these little-modified kernels are systematically arranged within Laxfordian fold- structures some tens of kilometres in wave length. There appears to have been, at an early stage, a differentiation of the reworked complex into zones of extreme mobility and intervening blocks, now often antiformal in character, which were less thoroughly modified. The evidence provided by Simony in his study of reworked Lewisian basement in the Caledonian belt, with that of workers in the neighbouring regions, indicates that upward-moving basement slabs were defined

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during early episodes and subsequently deformed together with the Moinian cover-succession~again, the picture is one of rapid lateral variations in the intensity and nature of reworking linked with the development of structures of a larger order. The discussion that followed the reading of the papers was concerned largely with the order in which successive structures were developed and modified. Important though this chronological aspect is, I think it is desirable to re-empha- size the evidence that major changes in the shapes, internal structures and external relationships of large crustal units have been achieved by regeneration~the end-products, whatever the details of their evolution, have been adapted to new crustal regimes. The discussion of the papers by Coward and Francis is to be found in a com- bined form at the end of the paper by Francis, in this number of the journal.

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