The following text provides a technical introduction to the Moine Thrust Belt, with links to other parts of this site from the text. If you use these links then to return to this text you should click on the "previous frame" button in your browser to navigate.
The Moine Thrust Belt forms the outer edge of the Caledonian mountain belt in northern Scotland, separating the polydeformed and metamorphosed orogenic interior of Moine and Dalradian rocks from the undeformed foreland of the Lewisian crust with its sedimentary cover of Torridonian and Cambro-Ordovician successions. As such, the thrust belt defines the NW limit of the Ortho-Caledonides. On land it runs from the Scottish north coast near Whiten Head to Sleat on Skye.
Northern continuations have been proposed beneath the West Orkney Basin, on the basis of seismic data. Offshore to the south of Skye the thrust belt presumably lies to the east of Lewisian outcrops on the islands of Coll and Tiree but west of Mull where Moine metasediments crop out. Traditionally it is inferred to lie through the Sound of Iona as Iona island contains probable Lewisian units. However, these could be part of the Moine Thrust Sheet. Regardless of the controversies concerning its offshore trace, the Moine Thrust Belt has achieved worldwide importance for the development of key concepts in structural geology.
Thrust belts are common components of the outer parts of many of the world's orogenic belts. One of the first to be described was the Moine Thrust Belt, following the recognition of large-scale thrust surfaces in the Assynt and Eriboll districts by Callaway (1883) and Lapworth (1883). These discoveries led to a major mapping programme by the Geological Survey, the results of which were eventually published in the epoch-making NW Highlands Memoir (Peach et al., 1907) and, particularly, the special geological map of the Assynt district (Flett, 1923). For the following fifty years, geological research within the thrust belt concentrated to relating minor structures, particularly folds and schistosities, to the regional geology, principally by correlating these from place to place. Up until the mid 1970s this research attempted to find structural links between the Moine and its underlying thrust belt (e.g. Barber, 1965; Soper & Wilkinson, 1975). For example, gently inclined and isoclinal to tight folds were linked to a single mylonite-forming event (termed 'D1'). This type of structural analysis received added impetus with the application of radiometric dating techniques to igneous intrusions within the northern Highlands, these age-dates apparently calibrating the relative structural histories built up from individual outcrop studies (e.g. Wooley, 1970; van Breeman et al., 1979). The second phase of research into the structural evolution of the Moine Thrust Belt began in the late 1970s and was propelled ahead by the application of analytical techniques developed in the foothills of the Rocky Mountains and Appalachians of North America by Elliott & Johnson (1980). The critical conceptual leap was that thrust belts evolve by individual thrusts growing, linking, moving and then dying, forming in a general foreland-propagating sequences so that higher thrust sheets were carried piggyback upon lower ones. In the Appalachians, Mitra & Elliott (1978) showed that folds and deformation fabrics could be explained by local thrusting processes rather than to regional tectonic events, prompting a similar reassessment of minor structures within the NW Highlands. These new approaches led to a major programme of remapping within the thrust belt, in many cases re-examining the relationships between thrusts and the sheets they carry for the first time in a hundred years. Much of this work confirmed the ideas of Elliott & Johnson (1980), particularly that many of the structural complexities and apparently bewildering networks of faults originated from the repetition of individually rather simple geometric elements. However, some parts of the thrust belt show fault geometries that are not predicted by Elliott & Johnson (1980), particularly structures that cut down stratigraphic section and apparently extend bedding (Coward, 1982). Controversy remains as to the larger scale tectonic significance of these features, particularly whether they accommodated crustal extension, gravitational collapse or even if they have purely compressional origins (Coward, 1983; Butler, 1984).
The renewed interest in thrust belt structure, particularly in NW Scotland came when structural geologists began to relate the deformation recorded by mountain belts to plate tectonic processes through integrating surface geology with deep seismic reflection profiles (Soper & Barber, 1982; Brewer & Smythe, 1984). Central to this were attempts to quantify the magnitude of horizontal displacements responsible for stacking up piles of thrust sheets, primarily using so-called balanced cross-sections. These constructions are geological profiles drawn parallel to the inferred direction of displacement and in such a way that the stratigraphy may be graphically restored to a predicted undeformed state. Although simple palinspastic reconstructions have been made of parts of mountain belts for almost a century, balanced sections are a significant improvement because they attempt to quantify the displacements experienced by all layers, an important part of testing models of structural geometry and evolution for internal consistency. This approach provided estimates for the original width of that part of the Cambro-Ordovician shelf now stacked up within the Moine Thrust Belt of almost 60km (Butler & Coward, 1984; Coward, 1985), with the whole belt having formed by perhaps 100km sub-horizontal displacement (Elliott & Johnson, 1980). These movements have carried the Moine rocks and the Ortho-Caledonide orogen by this amount, apparently as a thin sheet across the Lewisian crust (Butler & Coward, 1984). The map-scale structural geometry provided, along with the application of material science methods, a springboard for detailed microstructural studies of fault rocks (e.g. White, 1976; Knipe, 1989). The purpose of these studies was to define better the conditions under which the thrust faults and shear zones developed and thereby understand how the very large tectonic displacements were accumulated on such relatively narrow features.
The Moine Thrust may be defined as the tectonic contact which carries the Moine metasediments together with the Lewisian basement upon which they were deposited. These units were carried onto Cambro-Ordovician and Torridonian sediments which overlie their own Lewisian basement. The thrust is characterised by extensive mylonite development in both its footwall and hangingwall. The mylonite zone is locally several hundred metres thick. In general, where that stratigraphic separation across the thrust is greatest, particularly when the footwall lies in carbonates of the Durness Group, the mylonite zone is carried by a discrete fault zone marked by cataclasites. Such behaviour, indicating a transition from ductile deformation with relatively high temperature crystalline plasticity to brittle faulting and fracture processes, is predicted to occur on fault zones that migrate up through the crust (e.g. Sibson et al., 1981). The transition occurs on all sections across the thrust belt, locally within the deformation zone associated with the Moine Thrust as indicated above and elsewhere within underlying, later thrusts which stack up the Cambro-Ordovician sediments. Thus the Moine Thrust changes its character from place to place. Indeed, the western edge of the Moine outcrop is not everywhere defined by the Moine Thrust. Later thrusts and faults, some associated with much later basin formation characterise the map edge of Moine rocks in much of the Lochcarron area.
The Moine Thrust as defined above, may be shown to predate all Caledonian structures in its footwall. The best illustrations of these relationships are to be found in the Eriboll area and locally in the NE part of the Assynt district (e.g. at the Stack of Glencoul ). However, in south Assynt (e.g. Knockan Crag) the western edge of the Moine outcrop is marked by a low-angle fault, possibly an extensional fault (Coward, 1983), which is demonstrably later than Caledonian thrusts below. In the Lochalsh area it is difficult to trace the Moine Thrust as defined above, largely because of difficulties in evaluating whether highly deformed Lewisian gneisses were once the basement to Cambrian and Torridonian or Moine successions.
There are several different types of thrust-related structures developed beneath the Moine Thrust.
Amongst the most striking structures within NW Scotland are the large thrust sheets which contain Lewisian basement, generally with remnants of the original Cambrian or Torridonian cover. Most famous of these are the Arnaboll sheet at Eriboll and the two in Assynt, the Glencoul and Ben More Thrust Sheets. They are remarkable in that their basal thrusts appear to have cut as discrete planes with only a metre or two of associated tectonite through massive, apparently competent gneisses. The Kinlochewe Thrust at Meall a'Ghiubhais has similar characteristics but also cuts across irregularities on the Lewisian-Torridonian unconformity.
Imbricate thrusts are those which form an array, splaying from an underlying fault (the floor thrust). If these imbricates recombine up-dip onto another major fault (the roof thrust), the entire structure is termed a duplex. The Moine Thrust Belt contains many spectacular examples of such structures. The Cambro-Ordovician sediments appears to have been particularly suited to this type of deformation so that individual formations can be repeated many times. In the northern part of the thrust belt the Cambrian quartzites, particularly the Pipe Rock Formation, appear to accommodated many tens of km shortening through the formation of duplexes on scales from cm to 10s m. The most dramatic of these are on Foinaven and on Conamheall. Elsewhere packages of An t-Sron Group strata (Fucoid Beds and Saltarella Grit) are repeated many times over (e.g. north of Hope, Loch Eriboll). South of Assynt, the thrust belt contains comparatively little imbrication. Indeed in many places it consists entirely of the Moine Thrust. However, in the Torridon area (e.g. Meall a'Ghiubhais and Beinn Liath Mor) the imbricates return, with thick slices containing Torridonian and Cambrian strata.
All thrusts climb up-section in their displacement direction but few do so smoothly. More commonly thrust profiles show step-like forms, composed of bed-parallel 'flats' and steeper 'ramps' which cut across bedding. Lateral variations in the location of ramps can generate radical variations in the content of individual thrust sheets and, when many imbricates are involved, complex folds and culminations within the thrust belt. This behaviour is characterised on a large scale by the Assynt culmination but is perhaps better understood on an outcrop scale at Creag Shomhairle and on maps through the northern part of the Kishorn site.
Folds may be associated with thrusts simply as a geometric consequence of movement up ramp-flat thrust profiles. These antiformal, so-called 'fault-bend folds' (e.g. Rich, 1934; Suppe, 1983) only develop in the hanging-wall. However, in many natural examples, folds may be inferred to have initiated as a buckle through which a thrust subsequently cut (e.g. Fischer & Coward, 1982; Williams & Chapman, 1985). This composite behaviour may be distinguished from simple fault-bend folding by identifying deformation in the footwall to thrusts, particularly large-scale synforms which formed the complementary fold pair to the hangingwall antiforms. Large-scale buckle folds can modify into thrust sheets as some limbs shear out, generating tight folds which, in ideal cases, face in the direction of tectonic transport (e.g. Coward & Potts, 1983), carried on thrust-sense shear zones. This type of behaviour appears to characterise deformation in that part of the Moine Thrust Belt on Sleat and in Lochalsh. This style of broadly distributed Caledonian deformation is in marked contrast to the Lewisian-cored thrust sheets of central and northern Assynt. Presumably this variation reflects the great thickness of Torridonian sediments within the southern part of the thrust belt and perhaps indicates that the interface between Torridonian and Lewisian rocks was prone to buckling instabilities while the Cambrian-Lewisian unconformity favoured thrust ramps.
Estimating the timing of displacements within the Moine Thrust Belt relies on radiometric ages from various alkaline igneous intrusives. The Loch Borrolan complex of Assynt, which appears to cut the Ben More Thrust (Parsons, in press) has been dated at 430+/- 4 Ma (e.g. van Breeman et al., 1979). The Loch Ailsh intrusion, which is cut by Caledonian structures, has yielded an age of 439+/- 4 Ma (Halliday et al., 1987). However, direct dating of mylonite formation in the hangingwall to the Moine Thrust suggests movements continued on this until about 410 Ma (Freeman et al., 1998).
The Moine Thrust Belt forms the outer, NW margin to the Scottish Caledonides. As such it provides spectacular examples of the range of structures found with the marginal zone of an orogenic belt. Over 150 years of geological investigation has seen a number of revolutions in the analytical philosophy behind different types of tectonic analysis and interpretation. The geological areas chosen reflect both this history and the range of structures, both in style and scale, that can be found in thrust belts.
Moine Thrust Belt front page || Background information front page