Structure within the Northern Highlands Caledonides

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From: Johnstone, G S and Mykura, W. 1989. British regional geology: the Northern Highlands of Scotland (4th edition). (Nottingham: British Geological Survey.)

Structure

Most of the Moine rocks of the Northern Highlands had apparently already been deformed and metamorphosed by 740 Ma, when a suite of concordant pegmatites was emplaced (van Breemen and others, 1974). The pegmatites were evidently formed during a period of migmatisation which produced litparlit gneisses from the more schistose rocks (see Chapter 8). This period was part of a tectonothermal event to which the name Morarian (Lambert, 1969) has been applied. The host rocks of these migmatites, however, may be schists formed during an even earlier episode at about 1100 Ma (Brook and others, 1976) during the period to which the name ‘Grenville Orogeny’ has been given in North America. Much research in recent years has gone into the nature and extent of these pre-Caledonian episodes of the Northern Highlands and there is still some doubt as to whether two such distinct events did in fact take place. The Morarian pegmatites were themselves folded, and in places foliated, by later phases of deformation generally agreed to be Caledonian in age (comparable folds affect the 560 Ma old Carn Chuinneag intrusion, p.94) and the initial pre-Caledonian metamorphic pattern has been oveprinted by Caledonian metamorphism.

The pre-Caledonian structural and metamorphic features seem to be lacking from the Moine rocks of the far north-western part of the area (Soper and Wilkinson, 1975). It has been a matter for conjecture whether this is due either to the presence of a pre-Caledonian tectonic front trending obliquely across the Northern Highlands, to the north-west of which only Caledonian effects are seen, or to the rocks of the north-western area being ‘young’ Moines deposited after the Morarian Orogeny, and thus equivalent to the Lower Dalradian (Grampian Group) south of the Great Glen, which was laid down on a pre-Caledonian basement of ‘old’ Moines. In either case, as no sharp change is readily apparent on the ground, the junction has been concealed or modified by complex tectonism. Recent research seems to indicate that pre-Caledonian structures are present throughout the north-western area.

Despite intensive study of local fold histories and structural styles within the Caledonides of the Northern Highlands, no unified picture of the overall tectonics has emerged. Fold sequences from different areas are numbered differently and have not been satisfactorily correlated; some workers recognise three or four phases of folding, others as many as six. Broadly speaking, however, the fold phases can be related to the sequence given in P915510. This is taken from Mendum (1979), to whose paper reference should be made for more detailed descriptions and further references.

Structural sequence in the Moine
Deposition and formation of the sedimentary structures, local soft sediment folding and boudinage. Minor cleavage formation
D1 Large- to small-scale tight folds. Associated strong axial-planar fabric. Nappe structures in Glenelg-Knoydart area showing E vergence NNE-trending axes. Possible local thrusting. Deformation apparently limited in extent
D2 Ubiquitous thrusting (sliding) to W or WNW. Associated close-to-isoclinal, commonly reclined, large- to small-scale folds. Axes generally plunge gently E to S. Widely developed axial-planar schistosity, dipping moderately to gently E to SE. Mylonite formation in Moine Thrust Zone
D3 Widespread open to tight folds on all scales, best developed in Glenfinnan Division rocks. Axial planes generally trend N, but with marked variation of axial plunge. In pelites, rare axial plane cleavage; more generally, coaxial crenulations. Minor sliding
D4 Large-scale open folds, in parts monoclinal. Crenulations with locally developed cleavage; commonly in conjugate sets. Variable axial planes. In the Moine Thrust Zone box and kink folds developed. Major brittle movements in lower part of the Moine Thrust Zone involving foreland units

In the sequence indicated in P915510, D2 is considered to be the first Caledonian phase, in agreement with the sequence affecting Carn Chuinneag and the Morarian pegmatites of Lochaber. However, in Knoydart this appears to be the D3 of Powell (1974), while in Lochaber Powell and others (1981) also recognise two phases of pre-Caledonian deformation. D2 of the table is a ‘complex and multiple phase’ wherein D2 slide zones can be folded by tight folds similar to D2 structures elsewhere as part of a continuous process; almost collinear fabrics can be superimposed (Johnstone in van Breeman and others, 1974). D1 is everywhere pre-Caledonian. During this episode Lewisian and Moine rocks were repeatedly interleaved in what can be shown in some cases (by symmetrical disposition of strata) to be isoclinal folds. In Glenelg and Morar- Knoydart these apparently face towards the east, contrary to the later Caledonian overfolding and thrusting, which is directed to the W or WNW. However, not every Lewisian inlier lies in the core of a D1 fold and several, whatever their earlier history, are in cores of D2 folds and thrust slices (P915474).

It has already been pointed out (p.65 ) that the three Moine divisions established by Johnstone and others (1969) do not necessarily represent an upward stratigraphical succession, and that their mutual contacts are tectonically modified. Piasecki and van Breeman (1979a) and Piasecki and Wright (1981) interpret the radiometric ages and structural sequence of the south-western part of the Highlands to indicate that the Morar and Loch Eil divisions never underwent the Grenvillian metamorphism; however, they overlie a basement of Glenfinnan Division rocks which was affected by the Grenvillian episode. The interpretation of the Morar Division dates is somewhat at variance with the findings of Brook and others (1977) and Brewer and others (1979) but the inference concerning the Loch Eil Division is supported by the geochemical studies of Winchester and others (1981). If this basement/cover hypothesis were correct it would greatly extend the tectonic history of the rocks of part of the area. However, others hold that the Loch Eil Division/Glenfinnan Division boundary can be seen in places to be one of stratigraphical passage (BGS studies in Glen Cannich; Roberts and Harris, 1983). Roberts and Harris consider that, while both units have a common structural history, the more intensive deformation of the Glenfinnan Division is due to an early Palaeozoic crustal reworking which dies out eastwards in the Loch Eil Division.

Morar Division

The Morar Division is limited on the west by the Moine Thrust (Chapter 6) and on the east, as far as delineated at present (P915471, P915473) by the Sgurr Beag Slide or Slide Zone (see below). It forms a belt characterised by the presence of infolded or inthrust slices of Lewisian basement, which are best developed in the southern most and northernmost parts of the outcrop.

In the south, the rocks are clearly folded by Caledonian events; a normal stratigraphic succession, based on abundant cross-bedding evidence, is found on the limbs of a large, open, late antiform (the Morar Anticline of Richey and Kennedy, 1939) between the Sound of Mull and Knoydart (P915473, P915474). The rocks on the limbs of this simple structure show evidence of preantiform deformation and, between Loch Ailort and Loch Nevis, a culmination discloses a complex fold pattern at deeper levels. Within its core Lewisian rocks lie infolded (and possibly inthrust) in the Moines, with the folds having an easterly-facing aspect; this interleaving may be of pre-Caledonian age. The Lewisian is considered to be parautochthonous (see Chapter 2), representing infolds of basement, but that basement itself has probably been transported far on the underlying Moine Thrust. Further north, tight infolding of Lewisian and Morar Division rocks can be seen in the Glenelg–Attadale inlier (Chapter 2, P915460, P915471) especially in the Glenelg–Arnisdale section.

Eastwards from the Morar Anticline, Moine rocks are isoclinally folded in early (or pre-) Caledonian structures, and segmented by early Caledonian slides, which themselves become more tightly folded as the limiting Sgurr Beag Slide is approached. The semi-diagrammatic section shown in P915474 represents structural interpretation of the area by Powell (1974). Powell has also produced ‘three- dimensional’ perspective drawings of the highly complex folding.

Eastwards from Glenelg–Attadale, numerous thin strips of Lewisian rocks (in some places only a few centimetres thick) appear as thrust inliers (possibly of F2 age) usually accompanied by migmatisation of the adjacent (overlying) psammite, possibly a result of metasomatism or partial melting related to the thrusting (P915471). Platy zones and migmatisation in the enclosing psammites can be traced beyond the limits of the Lewisian outcrop. Larger areas of interleaved Moine and Lewisian rocks in the Coire nan Gall and Saddle inliers (Simony, 1973) may themselves be folded windows or ‘klippen’ of thrust sheets of deep-level structures of Morar type (P915460, P915471).

Northwards from Kintail the Morar Division is mainly represented by what is taken to be the continuation of the Lower Morar Psammite, within which Lewisian strips occur only near the Moine Thrust. The succession appears little deformed, but between Attadale and Beinn Dronaig Langford (1980) has recognised zones of platy rock which probably represent the axial regions of the tight D2 folds and slides. Near the south end of Loch Shin Peacock (1975) has also recorded thin Lewisian slivers and platy rocks which probably represent slides; they are possibly analogues to those found in the Kintail area. They lie in approximately the same position relative to the Sgurr Beag Slide (see below) as do the must larger Lewisian outcrops abutting the north coast of Scotland (Borgie, Tongue, ?Strathy). Mendum (1979) and Moorhouse and Moorhouse (1976) consider these to be slices of Lewisian basement emplaced in Moine rocks during F1 folding and sliding. The slices were themselves folded and slid or thrust during the later F2 phase; the tectonic history is therefore analogous to that of the Morar and Kintail areas.

The Lewisian and Moine rocks of the north coast (and, by inference, of the whole Northern Highlands area west of the Sgurr Beag Slide) may represent a zone of large-scale imbrication made up of relatively steeply inclined crustal segments. The crustal segments may, however, be far-travelled units overlying low-angle thrusts or listric surfaces. This matter is considered further in Chapter 5, which deals with the Moine Thrust Zone.

Sgurr Beag Slide

Independent evidence for this major structural break in the Northern Highlands Moines (P915460, P915471) was obtained in the early 1960s by the Geological Survey in the Barrisdale area of Knoydart (GSGB Summaries of Progress, 1962–1964), by Tanner (1965) in the Kinloch Hourn area and by Simony (1963) in Glen Shiel. Its course and significance as a major feature was discussed by Tanner and others (1970), and by Tanner (1971).

As far south as Morvern (and probably Mull) (P915471, P915473), recent studies by Rathbone and Harris (1979), Baird (1982), and Powell and others (1981), have shown that the Slide is a zone of high strain, separating rocks of the Morar and Glenfinnan Divisions. Traced northwards from Morvern, the slide becomes a progressively more important feature in terms of increasing stratigraphical excision or the size and importance of the Lewisian inliers which are associated with it (P915471).

In the area south of Loch Quoich its presence as a slide was not recognised by earlier workers and its course, shown in Tanner and others (1970), was largely interpretative. North of Loch Quoich, however, the presence of the slide is demonstrable by stratigraphical cut-outs and/or the presence of Lewisian inliers. Around the Fannich–Beinn Dronaig area a klippe of Glenfinnan Division rocks, underlain by the slide, lies surrounded by Morar Division rocks to the west of the main outcrop.

The slide’s continuation north of the Dornoch Firth is less clear, but it is thought to be represented by the Navar Slide (P915460) of the north coast area (Moorhouse, in Soper and Barber, 1982). The slide is probably an early Caledonian feature (e.g. Powell and others, 1981) and it has been folded during at least two later generations of deformation. There is some evidence in Glen Shiel that the slide there is not a single plane of movement, but rather a narrow zone with several planes of transport (BGS records). Along the slide, from at least Loch Eilt to Glen Affric, bands of psammitic rock adjacent to it are commonly gneissic (migmatitic psammitic gneisses). It is possible that, like the gneissic rocks overlying the minor thrust inliers of the Kintail area, these are due to the effects of partial melting during movements on the slide.

Glenfinnan Division

The Glenfinnan Division rocks show evidence of greater ductility during folding than the rocks of the Morar and Loch Eil divisions. The Morar Division, containing two major psammite formations, and the Loch Eil Division, essentially made up of variations of one massive psammite, were certainly highly deformed in several stages. However, the Glenfinnan rocks show a degree of polyphase isoclinal folding, the development of a strong penetrative schistosity and, in places, disruption and flowage, of much greater intensity than that seen in the adjacent divisions; this is due to their dominant pelitic and striped-schist lithology and their generally high degree of migmatisation (P219846). The evidence of radiometric dating shows that the earlier phases of folding and metamorphism were pre-Caledonian (van Breemen and others, 1974; Johnson and others, 1979; Tobisch and others, 1970). It has been mentioned earlier (p.75) that there is current discussion whether the rocks of this division form a ‘Grenvillian’ basement to a cover of Morar and Loch Eil division strata (Piasecki and van Breemen, 1979a; Piasecki and Wright, 1981).

The inclination of strata within the division is generally steep (P915474); however, this stratification is composite banding, resulting in part from the repetition of beds in tight to isoclinal folds, and in part from the development of new schistosities parallel to the axial planes of folds of at least two Caledonian generations. The regional attitude of the bedding is almost entirely obscured, and the present steep dip is mainly the result of the Caledonian folding on steep axial surfaces which trend in a generally NNE–SSW direction.

The rapid alternation of psammite and pelite in the striped lithologies has given rise to extreme variations in fold style, even among folds of the same generation. Folds with strongly curvilinear axes, often varying in plunge by more than 90°, are typical. In extreme cases the rocks are so deformed that the striped banding can break up, with the production of disoriented blocks of psammite completely enclosed in pelite. The intricate nature of the outcrop pattern produced by the multiple phases of folding of the Glenfinnan Division is illustrated and described by Brown and others (1970) in the Morar–Loch Eil area and by Tobisch and others (1970) in the Glen Affric–Glen Strathfarrar area.

From Strath Orrin to Glen Affric the outcrop pattern within the Glenfinnan Division ‘swirls’ in large, closed or almost closed antiforms and synforms. Southwards from Glen Affric the folding is tighter, and even compressed, with a very dominant NNE–SSW ‘grain’ of the D2 and D3 phases of P915510. Within this compressed zone of tight upright folds, highly elongate closed structures can be detected (as around Glen Dessarry at the head of Loch Arkaig); they may represent the more open synforms and antiforms seen in the area further north.

Quoich Line

Originally named by T. N. Clifford (1957), the Quoich Line is a rather poorly defined zone (P915471) marking the change from ‘steeply inclined’ strata of the Glenfinnan Division south of Glen Moriston, to the ‘Flat Belt’ of the Loch Eil Division (see below). This change is probably due to the contrasts in ductility between the rocks of the two divisions; the rocks of the Glenfinnan Division are steep because they are affected characteristically by highly-compressive folding described above, and thus contrast with the beds of the massive Loch Eil Psammite which show more open folding.

The ‘enveloping surface’ or overall regional disposition of stratification in the Glenfinnan rocks could be ‘flatter’ than is immediately apparent. The view that the Loch Eil rocks represent post-Grenville cover has already been referred to as has the view of Roberts and Harris (1983) that the Quoich Line represents the easterly limit of strong early Palaeozoic deformation which dies out in the Loch Eil Division. In Glen Garry, Roberts and Harris suggest that the junction between the Loch Eil Psammites and adjacent pelitic and striped schists typical of the Glenfinnan Division was recumbently folded during an early folding episode, and that the interleaved sequence thus produced was itself folded on roughly N–S axes during the early Palaeozoic phase (P915474, inset). On the west (Glenfinnan) side of the zone which makes up the Quoich Line the interleaved folds are highly attenuated, on the east (Loch Eil) side deformation is less intense and the early folds can be made out as a series of arcuate tongues of pelitic rocks extending into the Loch Eil Psammite (P915471).

Around the head of Loch Eil, Strachan (1982) suggests that the Quoich Line is marked by a zone of early thrusting which displaces mappable subdivisions of the Loch Eil Psammite and the adjacent granite gneiss. This could account for the fact that some fairly conspicuous subdivisions recognised by Stoker (1983) further south in Ardgour are not represented at Loch Eil.

Northwards from Glen Moriston the junction of the Loch Eil Psammites and schists of the Glenfinnan Division is not as sharply defined as it is to the south.

Loch Eil Division

Over most of its outcrop the lithological banding of the Loch Eil Division appears to be undulating, but generally flat-lying (P915474 and P217710). This undulating aspect has given rise to the term ‘Flat Belt’ for the area covered by the rocks of the Division (Leedal, 1952) possibly occasioned by the fortuitous sections afforded by the main through road, but this is misleading to a considerable degree.

In western Ardgour, for instance, Stoker (1983) recognises an early phase of tight folding associated with the production of the regional schistosity (D1: probably Precambrian) and a later D2 phase of large folds which vary from open to tight along their axial surfaces, themselves affected by D3 folds which trend NE–SW.

North from Loch Eil the pervasive flagginess of the rocks conceals at least one generation of isoclinal folds (P915474), and over much of the area this gently inclined flagginess is separated by narrower zones of steep rocks which may by an expression of the more open variants of the D2 folds further south. In one upfold, near Clunes, striped migmatised rocks are possibly Glenfinnan Division strata appearing from under the Loch Eil Psammites.

Selected bibliography