OR/15/026 Structure

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Gunn, A G, Mendum, J R and Thomas, C W. 2015. Geology of the Huntly and Turriff Districts. Sheet description for the 1:50 000 geological sheets 86W (Huntly) and 86E (Turriff) (Scotland). British Geological Survey Internal Report, OR/15/026.
Figure 11 Structural and metamorphic features of the Huntly and Turriff districtt.

The regional structure of the Huntly and Turriff districts is dominated by the Portsoy Lineament/Shear Zone whose trace crosses the north-west part of the district (Figure 11). This steep north-north-east-trending structure shows evidence of a long history. It divides complex folded and thrust Appin and lower Argyll group rocks to the west-north-west from upper Argyll and Southern Highland group rocks intruded by mafic-ultramafic plutons of the North-east Grampian Basic Subsuite to the east-south-east.

The rocks occurring west-north-west of the Portsoy Shear Zone (PSZ) show evidence of two main discrete penetrative deformation and metamorphic events, here designated D1 and D2. Note that these designations do not necessarily correlate with similarly labelled events elsewhere in the Dalradian outcrop. Indeed correlation of deformation and metamorphic events even across the PSZ is problematic. The primary D1 folding event and related cleavage formation were accompanied by low pressure–high temperature amphibolite-facies metamorphism. The secondary D2 event resulted in pervasive folding, generation of related cleavages, and formation of south-east dipping shear zones, all accompanied by intermediate to high pressure amphibolite-facies metamorphism. These secondary features dominate the structural pattern at 1:50 000 scale (Figure 11). Later deformation phases are manifest as locally prominent crenulation cleavages and open to close minor folding, but are less widely developed and mainly restricted to the more pelitic lithologies. Such late-stage features are common around the elongate domal structure termed the Ardonald Fold (Figure 11).

East-south-east of the PSZ two main penetrative deformation phases are also apparent in rocks lying below the andalusite isograd. There is evidence of a second deformation that immediately postdates the peak of low pressure Buchan metamorphism, but there is no evidence of an associated high pressure metamorphic event as found to the west of the PSZ. The overall structure in this eastern area is dominated by the late-stage Turriff Syncline, an open structure whose broad axial zone plunges gently north and whose trace is approximately coincident with the western edge of the Devonian Turriff outlier (Figure 11). The syncline is developed largely in the Southern Highland Group turbiditic sequence, but the detailed structural pattern is controlled by the earlier medium-scale folds (F1), and by alternating steep and more gently dipping zones that may reflect an underlying stepped basement profile. The Turriff Syncline folds the metamorphic isograds and exposes shallow structural levels in its broad hinge zone in the Aberchirder–Turriff–Ythanwells area. The rocks here show evidence of only a single fold phase (F1) and cleavage (S1), accompanied by greenschist-facies metamorphic mineralogies (chlorite and biotite zones).

The plutons of the North-east Grampian Basic Subsuite strongly influence the local structural pattern both within and to the east of the PSZ. They cross-cut the early pervasive F1 fold and S1 cleavage pattern, but are postdated by the secondary folding (F2) and related cleavage formation (S2). The Insch Gabbro-peridotite Pluton is bounded by approximately east-trending shear zones and the Huntly and Knock Gabbro-peridotite plutons contain numerous shear zones and exhibit complex relationships with the Portsoy Shear Zone. The Huntly and Turriff districts are also the site of two fault-bounded half-graben structures of Devonian age, the large Turriff outlier and the northern end of the Rhynie outlier (Figure 11).

History of research

Early work in the Huntly and Turriff districts, mainly by Read (1923)[1] focused on the lithostratigraphy, the igneous rocks, and the relationships between the different geological elements, but did not specifically address the structure of the area. Subsequently, Read (1955)[2] and Read and Farquhar (1956)[3] subsequently described the overall structure of north-east Scotland in terms of a Banff Nappe, a large east-facing recumbent fold with gneissose ‘Keith Division’ rocks in its core and less-highly metamorphosed ‘Banff Division’ rocks on its upper limb. Read (1955)[2] proposed that the upper limb was transected by a low-angle extensional structure that he termed the Boyne Line. He interpolated this ‘lag’ structure south from Boyne Bay through unexposed ground on the eastern side of the Cowhythe Gneiss (Cowhythe Psammite Formation) outcrop until it intersected the Huntly Pluton. Read then showed it emerging from the south-west extremity of the Huntly intrusion to link with the prominent shear zone on the south-east side of the Succoth–Brown Hill Ultramafic Intrusion. He did not recognise the Portsoy Shear Zone, but was responsible for defining the Turriff Syncline and farther east the complimentary Buchan Anticline. Johnson and Stewart (1960)[4] discussed the relationship of the mafic-ultramafic plutons to the overall structure of north-east Scotland and concluded that the main elements of the Portsoy, Knock and Huntly intrusions postdated the early deformation phase (Banff Nappe) but were deformed by a secondary phase. Sutton and Watson (1956)[5] carried out a structural traverse of much of the Banffshire coast section and emphasised the importance of a large east-facing monoform, the Boyndie ‘Syncline’, which dominates the local structural pattern. The steep limb of this ‘syncline’ crops out between Whitehills and Macduff. They also concluded that the largely rightway-up Dalradian sequence showed evidence of much deformation but rejected the Read’s hypothesis of the Boyne Line. Johnson (1962)[6] also worked on the Banffshire coast section where he defined four separate deformation phases in the Dalradian rocks and related these to metamorphic porphyroblast growth. The main phases were designated D2 and D3, with the primary D1 being represented by rare tight to isoclinal minor structures and a near bedding-parallel first fabric (S1). Subsequently Fettes (1970[7], 1971[8]) showed that the early S1 cleavage related to the main folding in the greenschist-facies Southern Highland Group rocks, but a secondary crenulation cleavage, which he termed ‘S3’ (here termed S2), was related to the Boyndie ‘Syncline’. He found that farther south the trace of the S1 cleavage was distorted and partly truncated by the Insch Pluton but that the later cleavage (S2) postdated the associated hornfels. Treagus and Roberts (1981)[9] later showed that the early F1 minor folds exhibit consistent upward facing across the Boyndie ‘Syncline’ and hence designated it a D1 structure. Munro (1970)[10] and later Munro and Gallagher (1984)[11] used shallow drilling and geophysical surveys to map out the elements of the North-east Grampian Basic Subsuite between Portsoy and Huntly. They recognised the sheared nature of many of the internal and external boundaries of the Portsoy, Huntly and Knock plutons. Leslie (1984)[12] showed that the north-east part of the Insch Pluton is similarly controlled by and cut by several shear zones. Ashcroft et al. (1984)[13] synthesised this approach by showing how the mafic plutons are closely associated with a network of shear zones in north-east Scotland.

The significance of the Portsoy Lineament/Shear Zone was first argued by Elles (1931)[14], who identified a ductile thrust on the east side of Links Bay, by Portsoy. Ramsay and Sturt (1979)[15] also adopted this interpretation but they resurrected the idea of the Banff Nappe, interpreting the gneissose lithologies in the Dalradian succession as basal lenses of mobilised basement. They used Rb-Sr whole rock isochron ages of 691 ± 39 (674[16]]) and 724 ± 120 (707[17])  Ma from the Inzie Head and Ellon Gneisses respectively, to argue that the nappe had carried an allochthonous succession westwards. Garson and Plant (1973)[18] argued that the distribution of ultramafic rocks along the Portsoy Lineament defined a crustal suture. Fettes et al. (1986)[19] showed how the various lineaments in the Grampian Highlands had influenced both the early sedimentation patterns and the later tectonic development. The Portsoy Lineament is coincident with major lithostratigraphical changes but subsidiary north-west-trending lineaments are also present. Baker (1987)[20] considered the metamorphic patterns of North-east Scotland and suggested that they are best explained by overthrusting of the Buchan block to the west along the Portsoy Shear Zone, with subsequent differential uplift causing the steepening of the structure. Beddoe-Stephens (1990)[21] documented the pressure–temperature variations implied by the differing metamorphic mineral assemblages on either side of the PSZ in the Huntly-Dufftown area. He concluded that pressures had increased, by some 2  kb, on the western side of the PSZ, as a result of loading by westward thrusting of the Buchan block. Goodman (1994)[22] reviewed the evidence for the nature of the Portsoy–Duchray Hill Lineament, noting its complex history and its role in dividing two areas of differing structural and metamorphic history in the Grampian Highlands. Dempster et al. (1995)[23] obtained Rb-Sr mica cooling ages in a traverse across the PSZ on the Banffshire coast section and showed that uplift occurred at around 460  Ma with the PSZ acting as a focus of differential movements. They also concluded that the PSZ had not acted as a major thrust zone and that the pressure overprint was possibly the result of magmatic loading following the injection of mafic and ultramafic intrusions along the Portsoy Lineament at about 470  Ma. Droop and Charnley (1985)[24] concluded that the mineral assemblages from the inner parts of the metamorphic aureoles of the Huntly, Knock, Morven–Cabrach and Belhelvie plutons were all compatible with emplacement of the North-east Grampian Basic Subsuite at pressures of 4 to 5  kb, equivalent to an overburden thickness of 15 to 18.5  km. Droop et al. (2003)[25] presented further detailed mineralogical and geochemical data from the regional and contact metamorphic rocks in and around the Huntly Pluton. They showed that peak metamorphic conditions resulted from intrusion of the mafic and ultramafic pluton and attained temperatures of 900 ± 50°C and pressures of 4.5 ± 1  kb. These conditions resulted in considerable partial melting of the adjacent Dalradian semipelitic country rocks.

Lineaments

As noted above, the main lineament that crosses the Huntly district is the overall north-north-east-trending Portsoy Lineament. No lithostratigraphical units can be traced across the structure, although the well-exposed Banffshire coast section suggests that in broad terms, little, if any, of the stratigraphy of the middle Argyll and lower Southern Highland groups is missing, when compared with the coherent Perthshire succession. The coarse conglomeratic and gritty arenite at the base of the Clashindarroch Formation, lithological variations in the Whitehills Grit Formation, and the local extent and varied lithology of the members of the Blackwater Formation all suggest that there are significant local facies variations adjacent to the lineament. Clean quartzite units (Grumack Hill Quartzite) sit adjacent to graphitic pelite and semipelite (Corinacy Pelite) in which gritty arenite incursions are present. The basal unit of the Southern Highland Group that crops out at the western margin of the Huntly district is a magnetite-rich pelite. These units all suggest deposition within a relatively unstable locally deep-water restricted-basin environment situated roughly along the Portsoy Lineament in late Argyll Group times (c. 620–600 Ma). The lineament likely formed a locus for crustal thinning during the later stages of Dalradian sedimentation with possible local formation of oceanic crust. Intrusion of the Succoth–Brown Hill and related mafic and ultramafic intrusions postdated Argyll Group sedimentation, but these bodies may also be a feature of the slow stretching that seems to have characterised the Laurentian continental margin during Argyll–Southern Highland Group times.

A subsidiary lineament, here termed the Keith Lineament, stretches north-east and south-west from Keith; its outcrop here is restricted to the north-west corner of the Huntly district. The lineament is marked in part by the intrusive sheets and pods of the Portsoy and Keith granites which were emplaced at about 600  Ma. The Keith Lineament also appears to be coincident with considerable thickness variations and facies changes found in several of the Ballachulish and Lochaber subgroup units. The normally persistent Mortlach Graphitic Schist and Corryhabbie Quartzite formations are abnormally thin and show atypical lithologies around Keith possibly reflecting a local depositional high. A thin lens of sparsely vesicular amphibolitic metabasalt, marking the occurrence of local basic volcanism, crops out within the Corryhabbie Quartzite Formation south-east of Keith. It is exposed both at the junction of the Den Burn and Burn of Drum [NJ 4420 4925] and farther west at [NJ 4394 4914]. The locally variable nature of the Keith Limestone Formation (Blair Atholl Subgroup) is also attributed to activity on this lineament. Like the Portsoy Lineament, the Keith Lineament was reworked later during Grampian orogenesis, forming the locus of development of a significant shear zone.

Early deformation structures

The most common early structural feature in most parts of the Huntly and Turriff districts is a pervasive slaty or continuous cleavage or schistosity (S1) that is generally orientated near-parallel to the bedding. In the low-grade, greenschist-facies, turbiditic Southern Highland Group rocks that lie in the broad hinge zone of the Turriff Syncline, S1 is the only penetrative structure present, and in places it is axial planar to open to tight, medium-scale folding (F1) typically with gentle northerly plunging axes. This early folding is best seen on the Banffshire coast between Banff and Gardenstown, but is undoubtedly present in the lower parts of the Deveron valley in the Huntly district. An exception to this F1 fold pattern is found farther south-east in the gorge section of the River Ythan, east of Fyvie where lithologically similar Southern Highland Group mixed arenites and schistose semipelites, have been metamorphosed under lower amphibolite facies conditions to give cordierite and andalusite. A penetrative early cleavage (S1) is prominent, but there is little evidence of F1 folding. In contrast secondary folding is well seen. Farther west to the north of the Insch Pluton, Fettes (1968[26], 1970[7]) documented the regional pattern of the S1 cleavage, and showed that it predated pluton emplacement. In the Aberchirder area S1 strikes approximately north–south, but as the Insch Pluton is approached it swings progressively clockwise, to strike east–west around the northern end of the Rhynie outlier. Farther west again at the western end of the Boganclogh sector of the Insch Pluton the S1 cleavage swings back anti-clockwise to strike north-east to south-west. Typically its dip ranges from about 75° to vertical.

The ‘Slate Hills’ lie along the northern flank of the Insch Pluton and stretch from Hill of Kirkney [NJ 502 315] in the west, via Wishach Hill and Hill of Foudland, to Hill of Tillymorgan [NJ 652 348] in the east. These hills are underlain by dominantly pelitic and semipelitic turbiditic units of the Macduff Formation (Southern Highland Group), termed the Hill of Foudland Pelite Member, and have an outcrop width of 1 to 2.5  km. The bulk of this pelitic unit has been hornfelsed by the Insch Pluton with ‘spotted slates’ typically developed. The ‘spots’ are mostly porphyroblasts of chlorite after biotite. Cordierite, andalusite and fibrolitic sillimanite are also present in the inner parts of the metamorphic aureole. The dominant structure in these pelitic rocks is normally a steep slaty S1 cleavage. Bedding is only seen locally and typically lies subparallel to the cleavage. It is difficult to recognise any small- and medium-scale F1 fold structures in these fine-grained slaty rocks but cleavage-bedding relationships do imply that they may be present in parts (see below for details). A second deformation phase, D2, is superimposed on rocks of the ‘Slate Hills’, overprinting the metamorphic aureole of the Insch Pluton (Fettes, 1970)[7].

In the Appin and Argyll group units west-north-west of the Portsoy Lineament early structures are rarely seen. The complex structural pattern in the Dunnyduff Wood–Mill of Wood Burn area, 1 to 2  km east of Keith, may be partly a result of D1 and D2 folding and deformation, but evidence from outcrop only emphasises the pervasive nature of the D2 folding and cleavage. Also, in the area north of the River Isla only hints of earlier folding are seen. A closed ovoid fold interference pattern about a metre across was noted in close folded, staurolite- and kyanite-bearing graphitic pelites at [NJ 5125 5462] in the Lime Burn section. Metamorphic porphyroblasts, notably kyanite and staurolite here, show evidence of two discrete growth phases with the earlier phase preserving aligned inclusions which commonly lie at a high angle to the later cleavage. It seems that textural and mineralogical evidence of the early deformation phase is not as well preserved west of the Portsoy Lineament as it is in the lower grade areas that lie to the east.

Figure 12 Structural features associated with the Keith and Portsoy shear zones.

Shear zones

The main shear zones in the district are the sinuous, broadly north-north-east-trending, Portsoy Shear Zone (PSZ), and the north-east-trending Keith Shear Zone (KSZ) (Figures 11, 12). Both zones appear to have reactivated pre-existing lineaments, and the PSZ also has a later faulting history that offshore affects rocks as young as Mesozoic.

Portsoy Shear Zone (PSZ)

The PSZ varies from a narrow complex zone of faulted, broken and foliated rocks to a 3 km- wide zone with discrete thrusts, mylonitic microgranite, and lenticular bodies of the older Succoth–Brown Hill type (clinopyroxene-dominated) mafic-ultramafic intrusions and serpentinised dunite and peridotite pods. Unfortunately most of the zone is unexposed in the Huntly district so that its constituent features here are not well known. The main elements of the zone are shown in Figure 11.

A partial cross-section through the PSZ is exposed in the Burn of Cairnie between [NJ 4846 4461] and [NJ 4756 4474]. At the east end of the section, partly garnetiferous, recrystallised striped and foliated amphibolite dips at 60° to 70° to the east-south-east. The amphibolite represents an early lenticular basic (gabbroic) body that has become highly sheared and recrystallised within the PSZ. It is succeeded westwards by schistose garnetiferous semipelite with abundant quartz pods and stringers. Minor quartzose psammite and some garnet-quartz bands are present. At [NJ 4780 4492] blocky, mid grey, fine-grained, mylonitic, highly micaceous psammite and subsidiary semipelite contain pink-orange fine-grained granitoid veins and pods. All these elements are highly deformed and the exposure is dominated by the very strong secondary cleavage, tight minor folding and a pervasive down-dip lineation. The beds have an overall dip of 20° to 30° to the east and east-south-east, with the cleavage near parallel, possibly with a slightly steeper dip. Fold axes are colinear with the lineation and both plunge 20° to 30° to between 093° and 100°. Farther west flaggy quartzites, locally with a pervasive cleavage and lineation are present. These structures and the ductile mylonitic rocks relate to westward thrusting. Evidence of other ductile fabrics linked to the PSZ is not abundant in the Huntly district, but is seen more extensively farther north. Mylonites are present in the Durn Hill Quarry in the Portsoy district and highly deformed ductile fabrics with a prominent, steep, down-dip extension lineation are seen within the PSZ on the coast section immediately west of Portsoy. Here, numerous distinct phases of deformation can be recognised, but they are largely confined to the PSZ and cannot be correlated readily with the regional deformation phases found outwith the shear zone.

In the south-west part of the Huntly district extending south-west from Terryhorn Farm [NJ 468 402] a sheet of pink, finely foliated, feldsparphyric microgranite up to 100  m thick forms a distinctive marker within the Beldorney Pelite Formation (see Foliated microgranite section). The microgranite sheet lies near the north-western margin of the PSZ and typically carries a pervasive mylonitic foliation that dips steeply south-east subparallel to the sheet margins. In thin section this fabric is defined by aligned biotite, muscovite and ribbons of fine-grained granular quartz. Tails of recrystallised feldspar and quartz are well developed marginal to the feldspar augen, which in places show a marked asymmetry. The microgranite has apparently acted as a locus for ductile shearing along the PSZ or has possibly been intruded along the shear zone synchronous with thrusting.

In the Huntly and Knock plutons Munro and Gallagher (1984)[27] documented the occurrences of deformed (sheared) mafic and ultramafic rocks. Information from drillholes, geophysical surveys and detailed mapping were used to interpret the pattern of deformation and location of shear zones both in and adjacent to the pluton. Both they and Fletcher and Rice (1989)[28] noted that the mineral assemblages indicated that deformation and shearing had occurred under hydrous amphibolite-facies conditions. Amphibole, biotite, quartz, carbonate, garnet, graphite and sulphide are the characteristic minerals developed. Localised and intense hydrothermal alteration is also focused on the shear zones and late-stage veins contain the assemblages, chlorite-carbonate and serpentine-carbonate-pyrite. Fletcher and Rice (1989)[28] also noted that kinematic indicators in deformed gabbros implied that the sense of movement across the shear zones was top to the west, i.e. at least some acted as thrusts.

Munro and Gallagher (1984)[27] noted that the south-west margin of the Knock Pluton on the west flank of Cuttle Hill between [NJ 4995 4730] and [NJ 4975 4725] consisted of picritic and gabbroic cumulates, which in drillholes show increasing evidence of deformation towards the west. Plagioclase crystals are bent and partly recrystallised to finer-grained aggregates, and olivine and pyroxene almost entirely replaced by amphibole and biotite in the foliated rocks. Narrow steep mylonite zones in gabbros are exposed near Ruthven at [NJ 506 477], and highly foliated ‘flaser gabbros’ recorded from drillholes farther east. One specimen from a drillhole at [NJ 524 478] was identified as a blastomylonitic diorite composed of small andesine megacrysts in a fine-grained quartzofeldspathic matrix. These features are all interpreted as lying within or being linked to the PSZ.

Two boreholes were drilled by BGS adjacent to the serpentinised ultrabasic knoll of Drumna Gorach, close to the north-west margin of the Knock Pluton and just south-east of the north-western margin of the PSZ. The holes NJ 55 SW 13 (Drumnagorrach No.1) and NJ 55 SW 14 (Drumnagorrach No.2) were sited at [NJ 5241 5242] and [NJ 5198 5205] (Figure 12) and reached downhole depths of 151.9m and 88.7  m respectively. They were inclined at 60° towards 295°. Drumnagorrach No.1 penetrated variably foliated and amphibolitised sheets of gabbronorite, granular gabbro, and minor olivine-gabbro. Below 136  m lies a +15  m thick sheet of medium- to coarse-grained amphibolitic gabbro with possible cumulate textures in its upper part. The sheet is only weakly foliated in its upper part but at lower levels contains localised shear zones and the foliation becomes progressively stronger and more penetrative. Interbanded hornfelsed semipelite with andalusite, cordierite and locally sillimanite is present in minor amounts in the upper part of the hole, and beds of calcareous semipelite with some calc-silicate pods and lenses up to 2  m thick are present in the lower parts. Thin serpentinite veins and slivers occur at the boundaries of some of the mafic sheets. Planar biotite-plagioclase-quartz pegmatite veins, up to a metre thick, first occur at about 100  m depth, and are common below 135  m. Where only weakly deformed, they are slightly discordant, but in the lower parts of the amphibolitic gabbro sheet they are highly foliated, recrystallised, and finer grained.

In Drumnagorrach No.2, generally foliated and amphibolitic metagabbro and metanorite containing numerous discrete shear zones and serpentinised ultramafic (dunite) pods and veins are present in the upper 50  m. Below lies an 8  m transitional zone of intermixed contaminated mafic rock and hornfelsed semipelite, which passes downwards into interbanded semipelite (locally with sillimanite and garnet), metalimestone and calc-silicate rock that lies between 70  m and the base of the hole at 88.7 m.

Fletcher and Rice (1989)[28] noted that the Knock Pluton was composed dominantly of Lower and Middle Zone mafic rocks, mainly troctolite and noritic cumulates (see Knock Gabbro-peridotite Pluton section). Variations in plagioclase compositions suggested that the base of the body lies at its western margin. The Drumnagorrach boreholes show that the Knock Pluton adjacent to the PSZ is marked by numerous individual mafic sheets, mainly gabbronorites, biotite norites, granular gabbros, and contaminated mafic rocks. Typically, sheets are a few metres thick, but locally range up to 15  m. They show variable amounts of foliation development, with more intense foliations developed near the margins of individual sheets, particularly adjacent to serpentinite or metasedimentary rocks. In parts unfoliated or weakly foliated mafic sheets are in sharp contact with the adjacent pervasively foliated sheets. Similar lithologies are also typical of the disrupted eastern and sheared central parts of the Huntly intrusion; Fletcher and Rice (1989)[28] argued that such lithologies characterise parts of the roof zone. The combination of Upper and ‘Roof zone’ lithologies, and their apparent intrusion coeval with shearing suggest that the PSZ was active at the time of emplacement of the Huntly and Knock plutons.

On the south-east side of the Knock Pluton the contact zone with gneissose semipelitic and psammitic rocks of the Cowhythe Psammite Formation was intersected in Borehole 81, sited by Claymires Farm [NJ 5330 4944] (Styles, 1992)[29] (Crinan subgroup section). The foliated and amphibolitised gabbros, norites and contaminated mafic rocks of the pluton show local mylonitic zones, some silicification, and late-stage veining. The most deformed mafic rocks occur over the 20  m intersection adjacent to the contact. Munro and Gallagher (1984)[11] interpreted this eastern margin as a shear zone and the information from the boreholes around Claymires Farm supports this iterpretation. The shear zone appears to pass southwards into the Huntly Pluton where it forms a central, steep, north-trending feature.

In the Huntly Pluton there is limited evidence of foliated or mylonitic rocks on the west side of the intrusion. Mylonitic hornfelsed rocks were recorded locally at [NJ 492 445] by Munro and Gallagher (1984)[11], and Munro (1970)[10] noted that cumulate rocks showing evidence of shearing and crushing occurred near Drumdelgie [NJ 484 422], in close proximity to non-hornfelsed metasediments. However, elsewhere, the western boundary of the pluton appears to be sharp, with little sign of contact lithologies, deformed mafic units, or hornfelsed metasediments. This suggests that this margin is marked by a very narrow, ductile shear zone, or in most places, by a late-stage brittle thrust or fault that may cut out or conceal any earlier ductile contact zone.

Munro and Gallagher (1984)[11] and Fletcher and Rice (1989)[28] both presented evidence for a central sheared zone in the Huntly intrusion that corresponds to the 600  m to 2  km wide, poorly exposed north-trending zone of granular orthopyroxene-gabbro and clinopyroxene-leuconorite. Within this zone, mylonitised mafic rocks are recorded locally at a small knoll [NJ 513 412] 1  km north-west of Huntly, and at Upper Robieston Farm [NJ 527 422]. At the former locality the shear zones are steep, but at the latter they form flat-lying zones 3 to 4  cm across in undeformed noritic rocks. More extensively foliated gabbroic rocks are seen in a quarry at [NJ 534 444]. Foliated xenolithic gabbros also crop out near the eastern margin of the Huntly Pluton at [NJ 557 445], and mylonitised mafic rocks were noted adjacent to the southern margin of the pluton at [NJ 512 395] and at [NJ 515 393].

The bulk of the evidence for the late stage movement history of the PSZ comes from the Banffshire coastal section and from the western edge of the Morven–Cabrach Pluton in the Glenbuchat district to the south-west. Steep anastomosing shear zones and the overall geometry of the zone suggest that transcurrent movements, probably dominantly dextral, postdated the earlier thrusting across the PSZ. Such movements probably occurred during the Early or Mid Ordovician, as the metamorphic mineralogies in the steep shear zones remained at amphibolite grade. Some of the steep shear zones in the Huntly and Knock plutons (see above), and those affecting the Succoth–Brown Hill and Portsoy mafic-ultramafic intrusions, may relate to this episode. Some of the smaller serpentinite lenses and pods may also have been emplaced during this phase.

The age of intrusion of the North-east Grampian Basic Subsuite is dated at about 471 to 474  Ma (Dempster et al., 2002[30]; Condon and Martin, cited in Oliver, 2008[31] as a personal communication; Carty, 2001; Carty et al., 2012). Pressure (P) and temperature (T) estimates based on the hornfels mineral assemblages (Droop and Charnley, 1985[24]; Droop et al., 2003[25]) imply that emplacement occurred at a crustal depth of 15 to 18.5  km (4 to 5 kb). These P-T conditions are compatible with estimates derived from metamorphic mineral assemblages in some of the sheared zones immediately west of the Huntly intrusion (Beddoe-Stephens, 1990). The distribution of shear zones and the disordered, complex internal structure of the component elements of the Huntly and Knock plutons can be reconciled if their intrusion occurred coeval with shearing and associated deformation. This would explain:

  • the presence of Lower Zone cumulates at both the western and eastern margins of the Huntly Pluton, and the local occurrences of smaller cumulate lenses within the intrusions
  • the variable occurrence of granular gabbros
  • the occurrence of basic sheets more typical of an upper or roof zone at the lower western margin of the Knock intrusion
  • the apparent shallow overall thickness of the Huntly Pluton implied by both the detailed and regional gravity and magnetic data.

Keith Shear Zone (KSZ)

The Keith Shear Zone (KSZ) can be traced north-east across the north-western corner of the district, stretching from the eastern outskirts of Keith [NJ 44 51] to around Clochmacreich [NJ 495 580] (Figure 12). Farther north-east, on Sheet 96W, the zone is overlain unconformably by the Deskford Old Red Sandstone outlier. Between the ORS outlier and the Banffshire coast, exposure is poor, but the variably foliated granite sheets exposed in Boggierow Quarries, about 1.6  km south-west of Portsoy, exhibit similar features to those of the Keith area. They mark the continuation of the KSZ, which here lies within 1  km of the western margin of the Portsoy Shear Zone. The two shear zones possibly coalesce just offshore. South-west of the Huntly district, in Sheet 85E, the KSZ can be traced for 5  km to the south-western outskirts of Keith, but farther south and west its coherence, position, and even its continuation towards Drummuir are uncertain.

In the Huntly district the KSZ is generally poorly exposed. It is defined mainly the limited exposures and inferred outcrops of the foliated Keith–Portsoy Granite (Keith-Portsoy granite section). A few exposures of strained quartzite occur adjacent to the granite, e.g. at Greenbog [NJ 4720 5454], but the only section across the whole shear zone, albeit discontinuous, lies in the Burn of Aultmore between [NJ 4589 5336] and [NJ 4596 5252]. Better sections occur within the town of Keith and the shear zone has been intersected in several boreholes drilled in connection with the proposed bypass route south-west of the town, all in the Glenfiddich district (Sheet 85E). The outcrop width of the KSZ is between 500  m and 1000  m and it dips to the south-east at between 25° and 40°. Hence its true thickness ranges from some 210  m up to over 500  m. Asymmetrical tails on feldspar augen in borehole cores from the Keith area consistently indicate a top-to-north-west thrust sense of movement.

Rocks below the KSZ belong to the Lochaber Subgroup (Findlater Flag and Cairnfield Calcareous Flag formations) and those above range from topmost Lochaber Subgroup to Ballachulish and Blair Atholl subgroups (plus Islay Subgroup at Portsoy). Many of the gneissose semipelitic rocks and metalimestone units within the shear zone are of uncertain affinity, but have been assigned to the Appin Group and are designated as the Keith Limestone Formation. However, despite the local facies changes that occur in the vicinity of the KSZ, and the mapped dislocations and local increase in strain, there is no great disruption of the overall stratigraphy across the shear zone (c.f the PSZ), and there appear to be few significant differences in the metamorphic history or in maximum pressure and temperature conditions experienced on either side of the KSZ (Beddoe-Stephens, 1992[32]).

Figure 13 D2 structural features in the area west of the Portsoy Shear Zone.

Within the KSZ the deformation is concentrated in numerous anastomosing shears, most marked by separate sheets of granite, which exhibit intense planar and linear fabrics. There is an overall foliation defined by muscovite, biotite, and by recrystallised elongate quartz ribbons and feldspar augen, but in many outcrops the dominant fabric is a strong down-dip stretching lineation formed by rodded quartz and feldspar. Microscopic features of the less deformed metagranites include relics of the original igneous texture and textural features which suggest that some deformation possibly occurred during emplacement (Phillips, 1998)[33]. In the most intensely deformed rocks, igneous textures are completely overprinted by the planar to anastomosing foliation, which locally displays S-C geometry. The dominantly semipelitic metasedimentary rocks that separate the sheets and pods of foliated granite are also strongly foliated and show prominent down-dip lineations. In contrast, the metalimestones have been extensively recrystallised and commonly show little fabric. Intersection lineations and minor fold axes are also broadly parallel to the stretching lineation, both in the shear zone and locally in the footwall (Figure 13).

Nature of the PSZ and KSZ and their relation to deformation history

As noted above the Portsoy Shear Zone marks the site of an earlier lineament that has controlled the depositional pattern and thus the nature of the Dalradian stratigraphy, particularly within the Argyll Group. It also marks the western extent of geophysically distinct sub-Dalradian basement that may well reflect a fundamental difference between the Buchan area and the remainder of the Grampian Highlands. The structural and metamorphic history also differs considerably across the PSZ, although in the Huntly and Glenfiddich districts the boundary between the low P–high T Buchan-type metamorphism and the higher P–high T Barrovian-type metamorphism clearly lies to the west of the PSZ (Beddoe-Stephens, 1990[21]). Unfortunately, the lack of inland exposure, complexity of the igneous and sedimentary sequences in the PSZ, and spatially variable nature of the ductile and brittle structures related to the PSZ in the Huntly district, all serve only to pose more questions about the history of this zone and its role in the wider tectonic evolution of the area. The borehole and limited outcrop evidence show that deformation and shearing have accompanied and in part controlled emplacement of the Ordovician North-east Grampian Basic Subsuite, dated at about 474 to 472 Ma.

Major westward shearing on the PSZ was proposed by Baker (1987)[20] and Beddoe-Stephens (1990)[21] to explain the marked change in metamorphic pressures demonstrated across the zone with the increase in pressure to the west of at least 2  kb. This westward translation is accepted here and must have taken place on a gently eastward dipping structure. Movement clearly post-dates the D1 deformation events both in the Buchan area and farther west as the D1 structures and metamorphic pattern are disrupted. If the movement along the PSZ and KSZ are linked, as seems likely, this thrust translation would relate to the main secondary deformation seen to the north-west of the PSZ. The steepening of the PSZ occurred subsequent to the main westward translation. However, a zircon U-Pb TIMS age of 468.5 ± 0.5  Ma from a variably deformed and steeply inclined pegmatitic granite vein at Portsoy Harbour implies that subvertical lateral movements occurred just after 470  Ma, i.e. only shortly after the main deformation. It is unclear as to how (or even whether) the secondary deformation found to the west of the PSZ relates to that to the east.

The discontinuous sheet-like nature of the porphyritic granite intrusions that comprise the approximately 600  Ma Keith–Portsoy Granite cannot be ascribed solely to the subsequent shearing and deformation. Emplacement seems to have been controlled by a pre-existing lineament, along which the lenticular granite sheets and pods were emplaced. The KSZ has subsequently reactivated this structure, which may well have been initiated during deposition of the Argyll Group sediments. This part of the Dalradian succession formed during a period of general crustal extension and localised instability, as shown by the presence of several small but deep rift basins, and the occurrence of widespread basic igneous activity. This was followed by continental break-up and the consequent opening of the Iapetus Ocean. Although emplacement of the Keith–Portsoy Granite was coeval with the extrusion of mafic lavas and intrusion of abundant dolerite sheets farther south-west around Tayvallich, regional considerations and global plate reconstructions suggest that the full rift–drift transition did not occur until around 570  Ma. Any small amounts of granitic magma generated by the crustal melting as continental extension progressively developed would likely migrate along pre-existing or newly formed lineaments.

The KSZ may have been active during early deformation of the Dalradian sequence, but the main foliation and lineation link to top-to-north-west ductile thrusting that formed part of the second major penetrative deformation phase (D2) found in this district, and with major shearing on the PSZ.

Secondary deformation structures

On the Banffshire and Aberdeenshire coastal sections a second main penetrative deformation phase becomes apparent once the andalusite isograd is crossed. Note that this deformation has been termed ‘D3’ on the Banffshire coast but in this memoir it is termed D2. Minor scale secondary folding (F2) and a related crenulation cleavage (S2) are commonly developed, and the earlier F1 structures become progressively tightened. Concomitant with further increases in metamorphic grade to middle amphibolite facies (sillimanite grade) the rocks commonly are typically gneissose with a composite foliation, particularly the more pelitic lithologies. In the Huntly and Turriff districts it is not possible to recognise D1 structures in such high grade rocks, although they can still be identified in parts of the Banffshire coast section, e.g. in the Boyne Limestone and Cowhythe Psammite formations.

East of the PSZ there is evidence for a widespread secondary deformation phase that has resulted locally in open to tight, small- to medium-scale folding, commonly accompanied by a crenulation cleavage (S2). The S2 cleavage is found only in rocks of the andalusite zone and higher metamorphic grades and is mainly defined by growth of biotite. It is notably pervasive in the more pelitic lithologies. Fettes (1970)[7] reported clear overprinting by S2 (N.B. termed ‘S3’ by Fettes and other authors) of both the first slaty cleavage (S1) and of the hornfels mineralogies in the Southern Highland Group rocks for up to 5  km north of the Insch Pluton. Here, S2 strikes generally north-east and dips moderately to steeply, mostly to the south-east; F2 minor fold axes typically plunge gently to moderately to the east (Fettes, 1968)[26]. In contrast, farther north by Aberchirder, both S1 and S2 cleavages strike north–south, and are subvertical. It is clear that D1 deformation predates the intrusion of the North-east Grampian Basic Subsuite but that D2 post-dates or is synchronous with the later phases of intrusion (Fettes, 1970)[7]. Where D2 deformation is well developed, e.g. north of the Boganclogh sector of the Insch Pluton, S2 cleavages and F2 folding are dominant. The S2 cleavage is generally still recognisable as a crenulation cleavage, but where strongly developed it forms the dominant parting in many of the pelitic rocks.

West of the PSZ secondary deformation (D2) structures are more complex. The major fold structures attributable to this deformation in the Huntly district include the Edingight and related folds, the Balloch Folds, and the Ardonald Fold (Figure 13). These structures are generally periclinal with hinge plunges varying from gentle to moderately to the north-east or east-north-east, and to the south-west. In concert with the shear zones they dominate the regional structural pattern. The D2 fold structures are accompanied by a penetrative crenulation or spaced cleavage defined by amphibolite grade mineralogies, including biotite, muscovite, garnet and kyanite. Following on from the work of Chinner (1980)[34] and Harte and Hudson (1979)[35], Beddoe-Stephens (1990)[21] showed that rocks west of the PSZ had experienced a significant increase in pressure (up to 2 kb) at the time of the secondary deformation. This was demonstrable in some specimens where sheaves of kyanite have replaced chiastolite (andalusite), and the mineralogy used to calculate the P-T values formed part of the secondary fabric in the rock. Similar but less clear increases in P-T are also documented by Beddoe-Stephens (1992)[32] across the KSZ. Hence the secondary deformation appears to be intimately related to activity across the major shear zones.

The main elements of the secondary deformation are the crenulation/spaced cleavage (S2), large-scale and minor folding (F2), and a locally prominent lineation (L2). L2 varies from an intersection to a prominent extension lineation (rodding), e.g. in and below the KSZ, and locally it is a mineral lineation. The orientations of these elements are shown in Figure 13.

The cleavage varies from a pervasive crenulation cleavage to a finely spaced (up to 1  mm wide) cleavage defined mainly by aligned biotite and muscovite growth and granular quartz and feldspar. In parts there has been differentiation into quartzofeldspathic and mica-rich lithons, separated by the S2 planes. Chlorite is commonly developed, both within the dominant S2 fabric, and as late porphyroblasts that cross-cut S2. Garnet, kyanite and staurolite growth is also documented in pelitic and semipelitic rocks, coeval with the formation of S2. The S2 cleavage varies in strike from north-north-east to east and generally has moderate to steep dips to the south-east and south respectively. In the area immediately south-east of Keith, in sections along the Burn of Drum and Herricks Burn, and around Denhead [NJ 437 491], S2 strikes east-south-east and dips gently to the south-south-west. The L2 lineations normally have a consistent down-dip plunge of 20° to 40° to the south-east in the foliated Keith Granite within the KSZ. Here, they are readily identified as extension lineations as they relate to the prolate form of the deformed felspar megacrysts. Outwith the shear zone the nature of the lineation is less clear and L2 are more generally intersection lineations linked with F2 folding. F2 fold axes mostly plunge gently to moderately to the north-east or south-west quadrants. A similar pattern is documented farther north-west in the Elgin district (Peacock et al., 1968)[36].

In the Sillyearn Hill–Edingight [NJ 516 556]–Lurg Hill area the structure is dominated by the F2 Edingight fold, a large east-north-east-plunging, tight, north-west vergent antiform developed mainly in Blair Atholl Subgroup semipelites, pelites and metalimestones. Both limbs dip moderately to steeply south-east. Exposures in the fold hinge zone in the Edingight–Whiteley [NJ 524 566] area show minor F2 folds that plunge 20° to 45° to the east-north-east (062° to 078°). The fold is cut by a major north-west-trending fault and on its downthrown south-west side the fold is resolved into a series of anticlines and synclines, exposed in the Burn of Paithnick section. The Corryhabbie Quartzite Formation occurs in its core at Gallow Hill [NJ 483 525]. In this area F2 minor fold axes locally plunge south-west, although the overall structure still retains its regional north-east plunge. The south-east plunging L2 extension lineation is also prominent here. Complementary kilometre-scale Sillyearn and Lurg Hill synclines that preserve the Durnhill Quartzite Formation lie south-east and north-west respectively from the Edingight anticline. Good cleavage (S2)–bedding relationships from the hinge zone are seen in sections in the Lime Burn [NJ 513 550], and on the mainly overturned north-west limb in small stream sections by Westertown [NJ 509 553], Brambleburn [NJ 510 563] and Langley [NJ 516 567]. In some impure limestones the S2 cleavage is a pressure solution striping, and in some psammite units it forms a spaced cleavage. Exposures are poor around the Sillyearn Syncline but in the small quarry at Cairnhill [NJ 5063 5136] an L2 intersection lineation plunges 27° to 063° and S2 (032°/45°SE)–bedding relationships show that the quarry lies on the inverted south-east limb of the syncline. Farther north-east marginal to the flooded Limehillock Quarry [NJ 516 520], the S2 cleavage has a similar orientation to that in Cairdshill Quarry (027°/52°SE) but here a strong L2 lineation plunges approximately 40° towards 130°, i.e. approximately down dip on the S2 cleavage plane. Very tight F2 minor folds are present locally, possibly reflecting their close proximity of the PSZ. The F2 fold axes plunge near-parallel to the finite extension direction. On Lurg Hill the overall synclinal fold structure is complicated by the presence of lenses and sheets of the Keith–Portsoy Granite, and by the proximity of the KSZ to the north-west. F2 axes and L2 intersection lineations plunge 54° to 046° and 35° to 238°, but the strong pervasive S2 cleavage in the more pelitic units dips moderately to steeply south-east. The variable S2–bedding relationships found in the hinge zone of the Lurg Hill Syncline are compatible with the presence of minor F2 folding.

In the north-west corner of the Huntly district west of the KSZ the dominantly flaggy psammites and subsidiary semipelites of the Findlater Flag Formation show a strong S2 cleavage and generally weak, east to south-east plunging L2 extension lineation (commonly down-dip). There are few F2 minor folds, and those recorded are tight and reclined with axes that plunge gently to the east-south-east, e.g. in the Burn of Aultmore section at [NJ 4545 5509]. Later kink bands and minor chevron folds are also locally present.

The Balloch–Mill of Wood area is dominated by tight, kilometre-scale, north-north-west-vergent, overturned folds defined by the thick Corryhabbie Quartzite Formation. On The Balloch itself there are only sparse exposures of quartzite, but a coherent section is seen in the Paties Burn [NJ 483 503]. At the head of Paties Burn an elongate structural outlier of semipelite and psammite with a distinctive bluish grey metalimestone of Blair Atholl Subgroup affinity lies within the quartzite outcrop. This is interpreted as an F2 synclinal outlier. Farther south in the Glen of Coachford [NJ 465 466], Morlach Graphitic Schist Formation crops out in an anticlinal hinge zone in the quartzite. Limited S2–bedding relationships and the presence of anticlinal inliers and synclinal outliers imply that the quartzite is folded into a series of tight F2 periclinal folds in the area around The Balloch. At its western margin the quartzite is foliated and generally highly weathered, and parts of the Ballachulish Subgroup succession appear to have been cut out particularly along its northern part. A thrust is interpreted here on the attenuated overturned limb of a large F2 anticline. Structurally below is a tight syncline, the Dunnyduff Syncline, in graphitic pelite and metalimestone of the Fordyce Limestone Formation. A penetrative S2 slaty to crenulation cleavage is dominant and the F2 axes in the hinge zone plunge 20° to 081°. As the synclinal hinge is traced west-south-west its plunge swings to 42° to 108°, but the S2 cleavage dips relatively uniformly between 45° and 60° to the south-south-east. The hinge zone passes into the Corryhabbie Quartzite in Dunnyduff Wood [NJ 445 495] and here the Dunnyduff Syncline may well refold a tight F1 antiform. A thin antiformal inlier of Mortlach Graphitic Schist lies within the quartzite; its outcrop defining a small ‘hook’ that extends to the south-west. The quartzite is bounded to the north-west by a further thrust, but it is underlain by Lochaber and lowermost Ballachulish subgroup units that are well exposed in sections along the Burn of Drum and the Den Burn, immediately south-east of Keith. Here, the later F3 axes plunge gently to moderately south-east and locally gently north-west, and an S3 cleavage dips between 15° and 50° to the south. The presence of tight F2 folds can be inferred from changes in the S2–bedding relationships. The increasing strain as the KSZ is approached is matched by the rotation of fold axes and intersection lineations such that they plunge gently to moderately south-east. Farther south-east in the Herricks and Birken burn sections, L2 intersection lineations plunge 20° to 25° to the east-south-east, reflecting the increasing distance from the KSZ again.

South-west of the west-north-west-trending Cairnie Fault in the Huntly district is the eastern termination of a large elongate periclinal structure, termed the Ardonald Fold. This broad anticlinal structure stretches in an arc from Ardonald [NJ 455 442], west, west-south-west and finally south-west, across into the Glenfiddich district, extending to the head of Glen Rinnes. Grampian Group psammites and quartzites crop out in its core and Lochaber and lower Ballachulish subgroup units form the bulk of the exposed units in the antiform. Evidence for its complex history is best seen in the Glenfiddich district, where the Morthlach Graphitic Schist Formation lies in an apparently complementary synform to the north-west of the Ardonald Fold. Within the synform the S2 cleavage is folded and a further crenulation cleavage is present, whereas in the antiformal hinge zone of the Ardonald fold, S2 is axial planar. Hence the apparent complementary structures have at least partly different and probably composite histories. In the area around Whitehillock Farm [NJ 4464 4563] at the eastern end of the Den of Pitlurg, the flaggy, striped calcareous psammites, semipelites and calc-silicate rocks mainly dip moderately to steeply northwards. The S2 cleavage varies from steeper to shallower than bedding, and a prominent subhorizontal L2 intersection lineation is present. Later open folds (F3) and crenulations with a weak north-dipping crenulation cleavage also occur, particularly where the beds have shallower dips. F3 fold axes are coincident with L2. These relationships suggest that tight F2 folding is locally present and that the Ardonald fold is a composite structure with pre-D2, D2, and post-D2 elements. This may explain why its eastern termination is relatively abrupt, yet the recorded dips of the bedding are generally 20° or less. On the southern flank of the Ardonald Fold in the Huntly district exposures are very sparse but the beds dips moderately to steeply south-east. Thrusts related to the PSZ repeat parts of the Appin Group succession and metadolerite intrusive sheets are abundant. No large scale F2 folding appears to be present, although such fold structures do occur farther south-west in the Glenfiddich district.

Late-stage deformation structures

In several areas, coherent deformation features postdate the D2 structures, but these are only of local extent and cannot be correlated across the district. They are preferentially developed in the more pelitic lithologies, in the thinly bedded psammites and semipelites, and in the banded calcsilicate rocks. The most common late-stage structures are crenulation cleavages and associated open to close folding, or kink/chevron folds. Large scale open folds notably affect the S2 cleavage pattern only in the area between Keith, The Balloch, and Ardonald. In the PSZ and the partly sheared intrusions of the North-east Grampian Basic Suite, only minor late stage shear zones and fracture zones, characterised by low grade metamorphic assemblages, are present. In the very poorly exposed pelitic elements of the schistose and gneissose Castle Point Pelite and Cowhythe Psammite formations, late-stage crenulations result in a ‘wavy’ foliation/schistosity; a good example is exposed in a small quarry on Little Brown Hill at [NJ 5722 5129]. The range and orientation of late-stage structures is well seen on the Banffshire coast section on Sheet 96W between Links Bay and Old Hythe Bay (Cowhythe Psammite Formation), and immediately west of Portsoy (Castle Point Pelite Formation).

Figure 14 Stereogram showing the orientation of the D3 structural features in the area west of the Portsoy Shear Zone (Huntly district). (Lower hemisphere, equal area projection).

In the area west of the PSZ structural elements related to an apparently coherent D3 deformation event are present in parts. They are generally manifest as an S3 crenulation cleavage, an L3C lineation, and associated open to close, small- and medium-scale F3 folds. Figure 14 shows the orientation of the D3 elements on a lower hemisphere equal area stereogram. The S3 cleavage generally dips moderately south-east, and L3C plunges gently to moderately east to south-south-east. F3 fold axes mainly plunge towards the south-east quadrant, but both they and the F3 axial planes show considerable local variation. The orientation of the D3 features largely reflects the dominant bedding and S2 cleavage orientations, with the later deformation features developed on the south-east dipping parts of the already folded succession. Good examples of L3 crenulations and F3 chevron folds are seen in thinly banded, finely cleaved, semipelitic units of the Findlater Flag Formation in the lower part of the Cross Burn section [NJ 4433 5276], east of Newmill, and in the Queans Stripe [NJ 4862 5755] near the northern margin of the district. At the first locality the semipelites are interbanded with psammitic units. A somewhat coarser S3 crenulation cleavage is well developed in the schistose, kyanite and staurolite-bearing graphitic pelite of the Fordyce Limestone Formation in the Burn of Braco between [NJ 5058 5373] and [NJ 5056 5361]. In thin section F3 microfolds are seen to postdate the porphyroblasts and fold the S2 biotite fabric. In these pelitic rocks minor recrystallised biotite locally defines the S3 cleavage. Minor open folds are best seen in the Burn of Tarnash and Birken Burn sections near their junction [NJ 4442 4893]. Their axial planes are very steep and no related cleavage is developed. More prominent F3 open to close folds are developed on the north flank of the Ardonald fold in thinly banded calcareous psammites, semipelites and calc-silicate rocks adjacent to the Den of Pitlurg [NJ 439 453]. F3 axes here plunge gently east (085° to 095°) commonly coaxial with the L2 intersection lineation. The related axial planes dip moderately north and commonly have a related weak S3 crenulation cleavage. D3 deformation is much more strongly developed farther west in the adjacent Glenfiddich district. Here, in the Burn of Davidston area [NJ 415 464] a moderately south-south-east-dipping S3 crenulation cleavage and related chevron folding are the dominant structures in the Mortlach Graphitic Schist Formation and abundant F3 minor folding is also seen in the Pitlurg Calcareous Flag Formation. A kilometre-scale synform is developed in the graphitic pelite unit and S2 is tightly folded by F3. Similar relationships continue south-westwards into Glen Rinnes. It is unclear whether this complementary structure to the antiformal Ardonald Fold is a D3 fold or whether D3 deformation has merely tightened a pre-existing F2 structure.

Faulting

The present distribution of rock types in the Huntly and Turriff districts shows significant local fault control. In the Huntly district, the dominant faults are those associated with the PSZ, and those trending approximately north-west that offset the earlier mainly north-east and north-north-east structural trends. In Strathbogie northerly trending faults affect the parts of the Insch Pluton and control the northern end of the Devonian age Rhynie Outlier. In the Turriff district the generally northerly trending faults that bound the Devonian Turriff Outlier dominate the geology. north-north-west- to west-north-west-trending faults are present in the Insch Pluton and offset its northern boundary. Note that over much of the Huntly and Turriff districts the generally sparse bedrock exposure and broad nature of the lithostratigraphical units make it difficult to delineate faults. As a result the geological maps do not fully represent the fault population in the district. The fault history is undoubtedly complex, and the relative ages, periods of reactivation, and interaction of the differently orientated fault sets remain unclear. The more significant mapped faults are shown on Figure 10.

Faulting is known to have locally reactivated the PSZ, and evidence is seen at the margins of the Knock Pluton. The Drumnagorrach boreholes intersect broken and veined zones marked by chlorite, quartz, carbonate (commonly dolomite) and epidote, and in the Claymires boreholes, carbonate veining, zones of silicification, and brecciated carbonate-quartz-pyrite veining are common. Graphite and slickensided serpentinite are also locally present in these faulted zones.

The north-west- to west-north-west-trending faults are generally steep and both normal and reverse geometries are present. They cross-cut the PSZ but their age is unclear. The north-west-trending Cairnie and Isla fault systems may both have earlier histories, as there are distinct changes in the geology across them. The Cairnie Fault system marks the main northern limit of abundant metadolerite sheets and may possibly have reactivated an earlier lineament. Some gold is reported in stream sediment residues from parts of its trace. The Isla Fault also apparently exerts some control on the distribution of mafic-ultramafic elements of the Huntly and Knock plutons. Its trace now forms part of the north-north-east margin to the Huntly Pluton and it steps north-westwards through a complex mineralised zone between the Huntly and Knock plutons around Littlemill [NJ 518 474] and Auchincrieve [NJ 522 480] (Fletcher and Rice, 1989)[28]. Just south of Nethermills [NJ 506 506] its trace changes from north-west to just north of west, and follows the Isla valley until it intersects the north-east-trending Newmill Fault at the western edge of the district.

North-west-trending faults are common north of the River Isla and form prominent features, commonly marked by springs, on the mainly quartzite ridges of Sillyearn Hill [NJ 514 523] and Lurg Hill [NJ 508 573], and on Knock Hill [NJ 537 551]. Minor quartz-breccia float is present on the ridges and fracturing and quartz veining are observed locally, e.g. at [NJ 5295 5530], 300  m north-east of Knockbog Farm. Pyrite mineralisation is common in large metalimestone boulders (Fordyce Limestone Formation) by the Lime Burn east of Mains of Edingight, e.g. at [NJ 5203 5627], close to notable north-west-trending bench features. Similar trending faults in the Portsoy district to the north bound the Deskford Old Red Sandstone outlier, and hence must postdate the Mid Devonian.

North-east-trending faults are largely confined to the Keith area immediately west of the Huntly district. These form part of the Drummuir Fault system that extends from Keith south-west to Dufftown. An extension of this system cuts through the Dunnyduff area and a fault is exposed in a small quarry by the Burn of Tarnash at [NJ 4429 4925]. Here a 1  m to 1.5 m-wide subvertical breccia with some vein quartz cuts broken, closely jointed, and slightly weathered Corryhabbie quartzites. The bedding steepens immediately north-west of the fault breccia zone and becomes shallower again on the south-east side, implying that the fault downthrow is to the south-east.

South-east of Keith, small-scale, brittle, low to moderate angle faults and duplex structures are locally seen in Appin Group rocks. They are recorded from the Corryhabbie Quartzite Formation in the lower Tarnash Burn at [NJ 4426 4923], and from calcareous semipelites, micaceous psammites and calc-silicate rocks of the Tarnash Phyllite and Limestone Formation in the Herricks Burn’ section at [NJ 4449 4913] and [NJ 4467 4910]. The fractures are marked by both green-black shiny chlorite-rich infill and by clay gouge. They generally dip gently to moderately south and south-south-east and are locally cut by small-scale steep faults. Their overall geometry suggests that they are mainly extensional and hence related to the uplift of the area following the Caledonian orogenic events. Such movements may in part be reactivating earlier small ductile to brittle north-north-west-directed thrust planes; such structures have been mapped at the base of the larger quartzite units in this area. Similar extensional structures are seen in the Glenfiddich district to the west, where they occur for several metres beneath the Old Red Sandstone unconformity, particularly in the vicinity of large faults.

A network of faults occurs in the western part of the Glens of Foudland. In the Den of Glennieston, immediately east of Glennieston Farm [NJ 5767 3464] and in the unnamed ‘den’ immediately west of Broomhall the semipelitic bedrock is fractured, iron- and manganese-stained, and strongly weathered. The dens form prominent north-west-trending features that link to the east-north-east-trending Glen Water, itself probably fault-controlled. In the upper part of this burn there are several springs, including Caird’s Well. A little farther east, the Burn of Stodfold drains Stodfold Moss, flowing northwards to its confluence with Glen Water between Stodfold [NJ 5862 3427] and Clinkstone [NJ 5892 3435]. At [NJ 5871 3375], outcrops and large blocks of pebbly grit and conglomerate are fractured, with slickensides visible on fracture surfaces, and vein quartz locally present. Semipelite outcrops at [NJ 5876 3395] are similarly affected. The burn also marks a topographical feature with its eastern bank forming a scarp about 20  m high and lower ground lying to the west.

In the west of Strathbogie around Brown Hill [NJ 440 367], east-north-east-trending faults are present. Faults of this trend are commonly associated with emplacement of the end Carboniferous quartz-dolerite dyke-swarm and reflect north–south tension at this time. The east-north-east faults generally cross-cut the north-westerly trending set, implying that the latter lie within the Mid Ordovican to end Carboniferous age bracket. The north-west-trending faults are probably mainly Silurian–Devonian in age, linked to the widespread uplift and extensional faulting that took place in the Grampian Highlands at the end of the Caledonian Orogeny.

Turriff and Rhynie outliers

Faulting is conspicuously manifest in its control of the margins of the Old Red Sandstone outliers of Turriff and Rhynie (Strathbogie) in the Huntly–Turriff district. Two faults define the narrow (550  m to 2  km wide), north-trending western graben of the Turriff Outlier that extends from Wood of Wrae [NJ 730 525] in the north to Cushnie [NJ 700 410] in the south. This graben is separated from the main part of the outlier by a 1 to 2  km wide horst, upon which are preserved some 40 metres of conglomerates and breccias.

The main part of the Turriff Outlier is an east-facing, half graben that extends northwards to the Banffshire coast. It is 8  km wide at the northern margin of the district, but narrows southwards with Old Red Sandstone rocks cut out at Fyvie. The western bounding fault trends about 010° over much of its length, but in the south, beyond Sillerton [NJ 738 430] the fault trace swings gradually anticlockwise to trend south-east. Here, it delineates the southern margin of the Devonian outlier. In the Fyvie [NJ 765 380] area, the mapped andalusite and cordierite isograds, which here trend north-north-east, are displaced apparently dextrally by an average of four kilometres. Taking the downthrow on the normal fault here as about 1  km to the north-east, as estimated by Ashcroft and Wilson (1976)[37] from detailed gravity and magnetic studies over the Turriff Outlier, this would imply that the metamorphic isograds dip at only around 15° to the west-north-west. Such dips would imply that the isograds lie close to the regional bedding and their dip relates to the geometry of the Turriff Syncline, whose profile is seen on the Banffshire coast section (see Stephenson and Gould, 1995, fig. 21)[38].

On the basis of aligned topographical features, the fault which bounds the western graben of the Turriff Outlier is shown extending south-westwards through Glenmellan [NJ 654 383]. This fault may link up with the east-north-east- to west-south-west-trending fault, which is mapped in the Stodfold [NJ 586 342] to Whinbrae [NJ 595 347] area, at the western end of the Glens of Foudland (see above).

A north-trending fault in Strathbogie area bounds the western side of the Strathbogie Old Red Sandstone Outlier. The fault trace coincides with a marked break in slope at the eastern end of the Hill of Noth. A small excavation in the area at the time of survey at [NJ 5153 3072] revealed ‘thoroughly smashed’ semipelite and brecciated fragments of andesitic lava. A spring occurs some 70  m south of the excavation, but post diggings only 40  m to the south-south-east have unearthed abundant andesitic lava clasts. The evidence for continuation of the fault to the north along the break of slope at the eastern end of Hill of Kirkney is less obvious, but its trace can be gleaned from changes seen in soil colour and clast content. Red till with cobbles occurs beneath the soil in the fields due north of Boggs of Noth [NJ 5177 3131], but to the west the soil is grey brown with clasts of slaty semipelite.

Devonian lavas and volcaniclastic sediments (including Read’s ‘Cork rock’ — a markedly vesicular andesite) are exposed around Kirkney Bridge [NJ 5182 3361], just north of exposures of Macduff Slate. Although no clear break can be identified here at the boundary between the Dalradian and the Devonian sequence, the Macduff Slate is locally brecciated and the lavas are highly weathered; hence, a fault is inferred. South from the bridge to Boggs of Noth, the bedrock is obscured by till and other superficial deposits.

The northern limit to the outcrop of the Dryden Flags Formation is marked by an east-north-east-trending fault whose trace is mapped up the Glen of Cults [NJ 535314]. Most of the outcrops of both the Devonian and Dalradian rocks show intense brecciation and deep weathering here. Exposures just into the field immediately north of Roadside of Cults [NJ 5280 3108] show vertical, anastomosing ‘fractures’ with lozenge-shaped fragments of pelite ‘smeared’ around more competent gritty beds.

Read’s mapping showed that the ORS inlier is displaced by an east-north-east-trending fault in the Burn of Raws, around the Raws of Noth [NJ 5175 3102]. Faulting in this area is indicated by fractured and weathered semipelite at around [NJ 510 308] in the Glen of Noth. These phenomena, together with the strong feature formed by the glen itself, are interpreted as indicating the presence of a significant fault that offsets the Devonian outcrop.

Detailed structure notes

Fyvie Gorge [NJ 780 369 to NJ 820 392]

Good exposure, way-up evidence and cleavage–bedding relationships all assist in the delineation of several major folds in the Fyvie Gorge section (see 1:50 000 Sheet 86E, Turriff). The abundant grading allows the way-up and structural facing to be determined throughout the section. The bedding strikes between 000º to 020º at the western end of the section, but swings clockwise to strike north-east near the Braes of Blairfowl [NJ 805 304] before swinging back to be more northerly again to the east. The rocks generally dip south-east, but at widely varying angles (10º to 90º); there are some westerly dips on the limbs of minor parasitic folds.

The folds, designated F2, are recumbent, upward-facing, close to tight, generally north-west vergent structures whose axial planes dip south-east at moderate to high angles. Upper limbs typically dip at 10º to 40º south-east, whereas the lower limbs dip more steeply (30º and 80º). Fold axes plunge gently north-east or south-west, as indicated by the minor folds. The related penetrative crenulation to continuous S2 cleavage wraps the cordierite and andalusite porphyroblasts, indicating fold development either during the later stages of the main metamorphic event or following the peak of metamorphism.

Minor late deformation is present in places. Although generally only giving rise to very open folds and warps, late deformation is more intense towards the east end of the section, notably around [NJ 820 387]. Monoformal folds with limbs dipping very gently west or subvertical deform the main cleavage, producing a strong crenulation. Quartz veins that elsewhere are near-parallel to the S2 cleavage, are also deformed here.

A prominent feature of rocks in the Fyvie Gorge section is development of a strong joint set that shows close and regularly spacing (about 2  cm in places). The joints generally trend at about 110º and dip between about 50º and 70º. These joints may relate to the main fault that bounds the Turriff Outlier (see below) as they show roughly the same trend.

The Slate Hills

Evidence for large-scale fold structures within the Slate Hills is ambiguous. On the Hill of Kirkney, to the west of Strathbogie, bedding dips in opposing directions on the north and south sides of the hill and high bedding–cleavage intersection angles are present on the ridge. In addition the rocks show minor folding and strong deformation at the eastern end of the ridge, all suggestive the presence of a syncline. However, east of Strathbogie, evidence for a syncline is much less clear. Cleavage is consistently steeper than bedding of the north side of the Hill of Corskie, Wishach Hill, Hill of Foudland, and on the Hill of Tillymorgan, but bedding–cleavage relationships are difficult to find on the south side of the hills. At two localities on the south side of Hill of Tillymorgan the cleavage dips at shallower angles than bedding, possibly indicating the presence on an overturned limb.

Smaller scale structures are present, but are difficult to see owing to the generally massive and fine-grained nature of the pelitic rocks. In two quarries on the western flank of the Hill of Foundland, at [NJ 5918 3330] and between [NJ 5963 3327] and [NJ 5957 3337], changes in strike of bedding indicate the presence of moderately steeply plunging recumbent folds. These open structures, to which the main cleavage appears to be axial-planar, plunge between 45 and 55º to the south-west. A similar structure is present on Hill of Tillymorgan [NJ 6557 3478]. Here, the fold is tighter, but it has a similar orientation and relationship to the main S2 cleavage. All these folds are asymmetrical and exhibit a Z-profile geometry, indicating an antiformal closure to the north-west.

Minor late deformation, seen in several outcrops (e.g. [NJ 5912 3338]), gives rise to large crenulations or flexures which plunge eastwards at about 50º to 60º with a very steep, but poorly developed, axial planar fracture cleavage.

A northerly dipping joint set is present in the Folla Rule area [NJ 731 330] along the northern margin of the Insch Mass. They may be associated with the shear/fault zones that are coincident with the northern boundary of the Insch Pluton in this area.

Strathbogie north of the Slate Hills

North of the Hill of Kirkney [NJ 502 315], turbiditic psammites (arenites) and semipelites are deformed by upright tight folds. Dips are generally steep and to the south-south-east or north-north-west. In places relatively intense minor folding occurs over a few metres of outcrop, indicating the presence of hinge zones. Upstream of Spence’s Pot (Kirkney Water) at around [NJ 514 336], a series of small outcrops display irregular folding accompanied by a steeply dipping axial-planar cleavage; the structural data indicate the presence of an anticlinal closure.

In the valley of the Kirkney Water south from Tillyminnate, at around [NJ 498 318] and around [NJ 500 323], the psammitic and pelitic sequence shows small-scale grading and channelling that implies the rocks are the right way up. However, in the lower part of the Kirkney Water around [NJ 513 337], steeply north-dipping gritty and conglomerate beds are overturned to the south. Father north to the east of Hill of Collithie [NJ 5074 3428] dips are consistently to the north and the younging evidence from rippled bed bases at [NJ 5135 3432] shows the beds to be the right way up. Hence, there appears to be a large anticlinal closure partially defined by the gritty units, whose southern limb has been overturned, possibly by later deformation. The intense deformation in the pelitic rocks immediately south of the gritty/conglomeratic units is interpreted as a contact strain effect resulting from strain partitioning.

Glens of Foudland

Outcrops of the Macduff Slate Formation in the Glens of Foundland are characterised by steep dips, generally in excess of 60º, and by tight, generally upright folds. In a temporary exposure at Broomhill [NJ 5888 3455] grading shows that the rocks are inverted, at least locally. The main cleavage is steeper here than elsewhere, dipping to the north at about 85º. In the approximately 50 m-long road-cut adjacent to the A96, about 300  m west of Bainshole around [NJ 6064 3496], the nature of the folding is clearly seen. The folds are slightly recumbent, moderately tight to open F2 structures with axial planes dipping steeply to the north-west. Bedding–cleavage relationships reveal two antiform-synform fold pairs in the western part of the section, with a larger, more open hinge zone with parasitic minor folds exposed in its eastern part. The cleavage here is a strong, spaced S2 fabric.

Logg Wood, Inverkeithny

Logg Wood covers a steep hillside on the south-east side of the River Deveron around [NJ640 470]. There are several roadside quarries that provide good exposures of fine-grained arenites, semipelites, and pebbly grits with pelitic rip-up clasts. Further exposures also occur on the flanks of Logg Hill and on Logg Island in the River Deveron. The rocks here have been metamorphosed only under greenschist-facies conditions (chlorite to biotite grade). Bedding strike is roughly north–south, and dips are moderate, ranging from 35º to 78º E. The cleavage (S1), only poorly developed in parts, generally dips steeper than bedding, in places by up to 30º. Where way-up indicators are present the beds generally young to the east. At [NJ6395 4693] and [NJ6397 4698] the interbedded arenites and pelites show moderately tight, gently northward plunging folds with an S-profile geometry.

Joints are prominent in the Logg Wood sections. Although no single orientation is dominant, the joints mainly dip in excess of 45º to the south-east or north-west quadrants.

Fabrics in the andalusite schists of the Whitehills Group

Schistose pelitic rocks in the upper part of the Whitehills Grit Formation crop out along the north side of the River Deveron near Kinnairdy Castle (around NJ 611 494). In thin section (S 93839, [NJ 6109 4951]; S93840, [NJ 6112 4937]), an S2 spaced cleavage is seen to crenulate an early penetrative fabric and to wrap the andalusite porphyroblasts. It is also axial planar to an open minor fold that possibly refolds an earlier isocline. The later cleavage is the dominant fabric in outcrop. It dips steeply east at the southern end of the section and at about 40º to the north-west, 150  m to the north.

Similarly schistose pelites and semipelites also crop out in the River Bogie at the northern end of Heugh of Bucharn [NJ 5192 3742]. In thin section the strong, spaced fabric is manifest as segregated quartz and mica-rich microlithons. This fabric has no clear precursor, but wraps the andalusite porphyroblasts in outcrop. A conjugate crenulation lineation is locally developed and relates to late-stage kink folds of the dominant S2 cleavage.

Discussion

The most abundant folds observed in the Huntly and Turriff districts belong to the second D2 phase of deformation. The related S2 cleavage wraps andalusite and cordierite porphyroblasts in the Southern Highland Group rocks, showing that this deformation and folding event clearly postdated the peak of metamorphism. This secondary deformation, here termed D2, has been ascribed in the literature generally to a regional D3 event, e.g. see Fettes, 1970[7] and Strachan et al., 2002[39]. An early cleavage (S1) that predates the growth of andalusite and cordierite is present in many samples, but few F1 folds have been recognised per se. This may be due to the generally poor exposure and more uniform semipelitic lithologies in the Southern Highland Group in the Huntly and Turriff districts. F1 folds are widely recognised in other well-exposed sections in the Southern Highland Group, notably where the metamorphic grade is low. F1 folds are upright on much of the Banffshire coast section, but at Fraserburgh (Kneller, 1987[40]) and farther south at Collieston (Mendum, 1987)[41] the early folds are generally recumbent. In the Huntly and Turriff districts F1 folds are largely upright, but locally are inclined. The apparently simple upright nature of most of the F1 folds may reflect either the absence of D2 deformation at shallow crustal levels, or the superimposition of D2 deformation and folding on D1 structures, resulting in tightening of the pre-existing folds and generation of a composite S1+S2 cleavage.

Thin-section studies commonly reveal at least two penetrative planar fabrics. In the finer-grained lithologies, a penetrative cleavage defined by micas is commonly deformed by a second fabric, S2. Characteristically, quartz veins and veinlets are also deformed by S2. The veins define ptygmatic folds, but are disrupted by the second fabric, which is typically axial planar. In some samples, both S1 and S2 fabrics are crenulated or kinked, indicating late-stage local deformation.

Two distinct planar fabrics (S1 and S2) can be distinguished in some outcrops, but in slaty semipelite and pelite outcrops, generally only a single fabric can be discerned in the field. In many instances this fabric has a spaced appearance and is clearly the S2 cleavage. However, the spaced or crenulated nature suggests the existence of a precursor S1 cleavage. Where the two cleavages are oblique, the slates lose their planar fissility and break into lozenge-shaped masses, becoming very tough where the two cleavages intersect at high angles.

References and footnotes

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