Geology of the Aberfoyle district: Caledonian structures in the highland border complex and dalradian supergroup
|This topic provides a summary of the geology of the Aberfoyle district – covered by the British Geological Survey. 1:50k geological map sheet 38E (Scotland).
Authors: C W Thomas, A M Aitken, E A Pickett, J R Mendum, E K Hyslop, M G Petterson, D Ball, E Burt, B Chacksfield, N Golledge and G Tanner (BGS).
The metasedimentary rocks of the Dalradian Argyll and Southern Highland Groups and the Highland Border Complex were deformed and metamorphosed during several Caledonian orogenic events. Incipient deformation probably began sometime in the late Cambrian about 490–500 Ma ago, as the Iapetus Ocean began to close. The Grampian event peaked in the Mid Ordovician, between about 480 and 460 Ma, and continued into the early Silurian (e.g. Soper et al., 1999, Stephenson and Gould, 1995). Orogenesis resulted from orthogonal collision of oceanic island arc systems with the continental margin of Laurentia as the Iapetus Ocean continued to close through Ordovician and Silurian times. Following this, Scandian deformation between about 435 and 425 Ma resulted from oblique, sinistral deformation, as Baltica collided with Greenland on the Laurentian margin to the north (e.g. Strachan et al., 2002). It was during latter part of this phase of deformation that the Grampian Highlands were subject to the major plutonism that resulted in the regionally extensive granite batholiths.
Final closure of the Iapetus Ocean resulted in the docking of Avalonia (England, Wales and Eire) with Laurentia in the latter part of the Early Devonian during the Acadian Orogeny around 395–400 Ma (Soper and Woodcock 2003). This resulted in the reactivation of earlier structures and the initiation of further strike-slip faulting.
Grampian (460–480 Ma) deformation both preceded and continued after accompanying metamorphism that peaked at lower amphibolite facies within the district. This deformation resulted in the development of several distinct sets of structures. At least one, early, large-scale, SE-facing, recumbent nappe formed during D1, together with related minor folds and cleavages. The early structures were subsequently deformed pervasively at lower structural levels during D2 and D3 deformation episodes, coeval with peak metamorphic conditions.
Extension and uplift occurred during the later phases of orogenesis. Within the district, the key element of this later Caledonian deformation was the formation of the Highland Border Downbend (HBDB, Figure 9 — see below), initial formation of which is interpreted to be due to extension (Harte et al., 1984). Compression superimposed on this structure resulted in folds and cleavages assigned to D4. It is possible that at least this compressional phase relates to Scandian orogenesis, rather than being a late phase of Grampian deformation. The monoformal Highland Border Downbend controls the regional dip of the main structural elements within the Dalradian rocks in the district. Its axial plane trace can be followed west-south-west from the north side of Ben Ledi in the east, across Glen Finglas close to the head of the reservoir, on into ground south of Loch Katrine and passing to the north of Ben Venue. It divides the Dalradian outcrop into two domains of differing structural character: the Highland Border ‘Steep Belt’ (HBSB) to the south-south-east and the ‘Flat Belt’ to the north-north-west. The HBSB provides the key to understanding Dalradian structure in the Southern Highlands because it provides a cross-strike section of nearly 9 km through deformed Dalradian rocks between the HBD and the Highland Boundary Fault (Figure 9). Successively lower structural levels are exposed in the ‘Steep Belt’ as the axial trace of the HBD is approached from the south. Within this domain, it is possible to determine the relative effects and timing of pre-downbend phases of deformation, particularly D1 and D2, the two most important phases of deformation in the Southern Highlands resulting from the Grampian Orogeny. The ‘Flat Belt’ is occupied by mainly inverted strata conventionally considered to lie in the inverted limb of the putative ‘Tay Nappe’, the very large scale early fold which dominates the structure of the south-eastern half of the Dalradian outcrop in Perthshire and eastern Argyll (Stephenson and Gould 1995).
Late- and post-orogenic, sinistral/oblique strike slip faulting is important within the Aberfoyle district, displacing the lithostratigraphical units and fold structures, and exposing different structural levels. The Loch Tay Fault and the Duke’s Pass and Creag Innich faults, are particularly important in this regard, controlling the gross distribution of the lithostratigraphical units within the district.
D1 structures comprise kilometre- to metre-scale folds and associated penetrative and spaced cleavages.
Three main, kilometre-scale, F1 folds have been identified in this district: the Aberfoyle Synform (previously termed the Aberfoyle ‘Anticline’), the Ben Ledi Antiform and the Ben Vane Synform (Mendum and Fettes 1985). These three structures form the complex hinge zone of the Tay Nappe. The loci of their axial plane traces are shown in Figure 9. The Aberfoyle Synform is historically important as this was the first of these major structures to be identified (Anderson 1947, Henderson 1938). Henderson recognised that wacke sandstones young away from the Aberfoyle Slate, both to the north and south, and thus proposed that the slates occupied the core of an anticline. Subsequently Shackleton (1957), in his classic paper on the structure of Dalradian rocks adjacent to the Highland Border, recognised that the cleavages associated with the Aberfoyle ‘Anticline’ were downward facing, that is, the beds become younger downwards in the hinge zone of the structure. Thus the Aberfoyle ‘Anticline’ is, in fact, a downward facing structure. This downward facing geometry is characteristic of all the F1 folds in the HBSB and arises from the overturning effects of the Highland Border Downbend. The two other major F1 folds — the Ben Ledi Antiform and the Ben Vane Synform — lie structurally beneath the Aberfoyle Synform. The geometry of both of these lower structures is controlled by the Loch Katrine Volcaniclastic Formation, which acted as a regionally competent unit against which other lithologies deformed.
Overall, the F1 fold axes have shallow to neutral, east-north-east-directed plunge, and axial planar surfaces that dip very steeply north to north-west. Where mixed arenites and pelites are present, the S1 cleavage displays excellent refraction, varying from a steep, slaty cleavage in the pelitic lithologies, to a coarse, spaced, fanning cleavage in coarse-grained metasandstones. Two early cleavages are recognised in the metasandstones: a penetrative to spaced grain-alignment fabric (S1a, cf. Harris et al., 1976) and a later, dominant, strongly spaced and fanning pressure solution cleavage (S1b). It is the latter which is genetically and geometrically related to the F1 folds, thereby determining their downward facing geometry. D1 cleavage–bedding relationships are easily observed at a number of readily accessible localities on the Duke’s Pass, east of Hill Cottage around [NN 516 030], and in outcrops to the west of the Dukes Pass on Creag Innich [NN 492 035]. Abundant younging evidence in the form of graded bedding and scour structures at the base of debris flows and coarse turbidite beds reveals clearly the downward facing character of this early cleavage. Good examples of refracting cleavages and the variations in cleavage style are observed in outcrops along the Duke’s Pass (P643891), particularly on cliffs south-east of Creag a Mhaidaidh [NN 515 035], and west of Aberfoyle around Craig nam Fairenean [NN 508 008]. Other examples of typical F1 folds are shown in (P643909 and P643897).
In the north-east of the district, dominantly semipelitic rocks contain tight F1 folds that commonly have low-angle dislocations along their limbs. In the coarser-grained rocks, the cleavage fans around the fold hinges, but the slaty cleavage in the finer-grained rocks remains more or less planar. Good examples of F1 structures in this part of the district occur in Gleann a’ Chroin between [NN 615 157] to [NN 627 150].
The consistently steep bedding in the HBSB shows that the F1 folds are tight to isoclinal, even at the kilometre scale. However, it is also clear from the high angles between bedding and cleavage, that the F1 folds nearer the Highland Border are dominated by hinges and the short limbs of asymmetrical fold pairs. In the ground south-west of Loch Ard and westwards into the Ben Lomond district, numerous F1 fold closures have been determined in heterolithic units (Shackleton 1957, Figure 2).
D2 deformation was probably coeval with the peak of metamorphism. It imposed variable strain on pre-existing D1 structures, resulting principally in the formation of coarse crenulation to pressure solution cleavage, particularly well developed in arenites; in finer-grained rocks, the cleavage is more penetrative or is a fine crenulation cleavage. Typical D2 structures are shown in P643932, P643895 and P643887.
Within the Aberfoyle district, D2 strain is low or absent at the Highland Boundary, but increases northwards and is first recognisable in coarse-grained rocks in outcrops along the Duke’s Pass around [NN 517 037]. Kilometre-scale, discrete, F2 fold structures are generally absent, though F1 folds within the Ben Ledi Antiform and Ben Vane Synform are modified by D2. To the north along the Duke’s Pass, S1b becomes progressively modified, dominantly by simple shearing acting along or close to bedding, to form the coarse S2 crenulation pressure solution cleavage. The crenulation varies from sinusoidal to chevron in form, modifying both the shape and spacing of the earlier (S1b) fabric (P643932, P643895 and P643887). Minor F2 folds of S1b are developed locally within beds and their hinges are commonly strongly curvilinear, developing, at their most intense, into almost sheath-like fold structures. A key feature of F2 minor and crenulation folds with regard to their genesis is that their sense of vergence is dominantly to the north or north-west; this feature is particularly important to the debate about the tectonic significance of D2, as discussed further below.
The S2 cleavage intensifies northwards to become the dominant fabric in many areas north-west of the Duke’s Pass. It is particularly well developed in coarse quartz arenites to the north-east of Ben Venue [NN 478 061] and between Ben An [NN 505 084] and Meall Reamhar [NN 502 094], where the early cleavage was commonly strongly oblique to bedding.
South-east of the Loch Tay Fault, D2 effects are only observed around Beinn Each [NN 602 158], immediately north of the district. Here, slaty semipelitic rocks of the Ben Ledi Grit Formation display attenuation of S1 and minor F2 folding is present.
The mode of development of D2 structures, particularly the S2 cleavage and the style of minor folds, has been the subject of considerable debate in recent years. D2 structures were first described and discussed in detail by Harris et al. (1976). These workers ascribed formation of D2 cleavages and folds to south-eastward directed shearing of F1 folds. The early folds have been interpreted to have had an initially upright orientation prior to the onset of D2 (e.g. Krabbendam et al., 1997, and references therein). The inferred south-east sense of shearing (Harris et al. 1976; Krabbendam and Leslie, 1997; Krabbendam et al., 1997) is opposite to the nearly ubiquitous north or north-west sense of vergence of the F2 minor folds. Indeed, Harris et al. (op cit) considered that the formation of F1 folds and their subsequent deformation during D2 was due to south-east-directed shear-dominated deformation which began in D1 and continued through to D3. This interpretation was supported by Krabbendam and Leslie (1996) and Krabbendam et al. (1997), based on work in the Glenshee area, and Rose and Harris (2000). Mendum and Fettes (1985) presented an alternative interpretation, supported more recently by Mendum and Thomas (1997). On the basis of detailed observations in the Aberfoyle district (based, for example, on observations of relationships such as those observed in outcrop in P643895) and adjacent areas of the Southern Highlands, these workers concluded that D2 deformation resulted from north-westerly directed simple shearing with a subordinate, but nonetheless significant component of pure shear (flattening).
Although the S2 cleavage is pervasive at lower structural levels, the intensity of the deformation, in terms of the amount of shearing, is never very great where seen in outcrop in the HBSB. Part of this is a function of lithology, since it is best seen where it interferes with S1b in coarse-grained, quartzose arenites. In the more pelitic rocks, such as those in the Aberfoyle Slate Formation, most of the fabrics are probably composite S1/S2 fabrics, and consequently S1–S2 relationships and the degree of D2 strain are much more difficult to elucidate.
Discrete D3 structures are rare within the Aberfoyle district. However, although F3 folds have been identified with confidence at only one locality on the north side of Loch Katrine (P643943), it is probable that the cleavage at deeper structural levels around the Highland Border Downbend is a composite S1/S2/S3 fabric, since it is approximately axial planar to the minor F3 fold limbs. The F3 folds verge to the south-east and are small-scale, minor, close to tight structures. Where observed in adjacent areas, the F3 folds tend to form in more pelitic units and S3 is manifest locally as a crenulation cleavage. D3 structures developed towards the end of the main metamorphic event and are, therefore, related temporally to D2, although they have a different sense of vergence and different form.
D4 structures and the Highland Border Downbend
Structures nominally assigned to D4 are responsible for the gross structural architecture of the Dalradian, as discussed in the introduction. Thus, although the monoformal Highland Border Downbend (HBD is identified as ‘D4’, in the sense that it postdates D3, it probably formed between D3 and D4 folding. It is considered to have formed as a drape over a basement step, resulting from extension and uplift that occurred mainly during the late stages of the Caledonian orogenic episode. Harte et al. (1984) regarded the structure as forming in the later stages of the Grampian event. However, it is possible that the HBD formed during Scandian uplift and deformation (about 435–425 Ma), during which the major, late Caledonian granite plutons were emplaced. The HBD ‘drape’ was subsequently compressed during D4 deformation, giving rise to the overturned bedding and downward facing structures in the Highland Border Steep Belt (HBSB), as well as the abundant minor folds which are strictly D4 in age. The tectonic regime responsible for the D4 folding is not yet clearly elucidated, but it is distinctly possible that this folding deformation arises from Scandian transpression. If so, D4 deformation will have postdated Grampian D2/D3 deformation by some 35–40 Ma.
F4 folds are the dominant structures developed during D4. They occur on centimetre to decimetre scales and are commonly disharmonic. Plunges are variable, but are mainly to the east-north-east and south-south-west at shallow to moderate angles. S4 crenulation cleavages are locally developed in the hinges of these folds, predominantly in the more pelitic lithologies. They dip gently to steeply, depending on the D4 strain and the orientation of pre-existing structures.
F4 folds are generally open to close structures. They are first seen in the HBSB, on the Duke’s Pass around [NN 52 05] in small outcrops of interbedded wacke sandstone and semipelite. In the HBSB, axial planes and cleavages dip northwards at shallow to moderate, but irregular, angles. Around the HBD, the intensity and scale of the F4 folding increases and such structures are well seen in crags on the north side of Glenfinglas [NN 51 11]. North of the main HBD axis, F4 fold axial planes generally dip more steeply to the north. The Highland Border Downbend itself is a broad fold structure within which Dalradian rocks are overturned over a few hundred metres from their flat- to shallow-dipping attitude in the ‘Flat Belt’, to dip steeply north in the HBSB. It is also responsible for the complex outcrop pattern observed on the Ben Ledi–Ben Vane massif, where Argyll Group strata are preserved in the hinge region, and D4 structures interfere with D1 and D2 structures. D4 deformation was accompanied by retrogressive metamorphism that occurred under lower greenschist facies conditions, giving rise to the growth of new chlorite.
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