Grampian Highlands Field Guide: Epilogue
|This page is part of a category of pages within the Grampian Highlands Field Guide.
Author: J R Mendum, BGS
Following the excursion to the Dalradian rocks of the Grampian Highlands based at Kindrogan, I feel it is useful to summarise some of the main elements of the geology that were addressed during the week, either directly or indirectly, and comment on the new findings made and on some of the general conclusions presented (modified even). I have also attempted to summarize the tectonic model that I attempted to put across. I must stress that the comments and overall model reflect my personal viewpoint and geological ‘baggage’. You are of course at liberty to disagree with some, or even all of them.
Day 1 started with a study of D1 structures in Dalradian Southern Highland Group turbiditic rocks situated in the wide hinge zone of the ‘Tay Nappe’, close to the Highland Border. These D1 structures lie within the ‘Steep Belt’; note that although bedding may still locally dip at shallow angles, the S1 cleavage is dominantly steeply dipping. The ‘Steep Belt’ is separated from the ‘Flat Belt’ to the northwest by the ‘Downbend’, a monoformal structure, probably formed between the D3 and D4 deformation events and appears to reflect a deeper crustal, possibly basement, step. D4 structures, which typically become relatively abundant close to the ‘Downbend’ hinge, can be traced northwest across the ‘Flat Belt’ almost to the ‘Boundary Slide’. Understanding the bedding-cleavage (S0–S1) relationships enabled us to show that the D1 structures are downward facing in this area. In the afternoon we traversed up the ridge of Benachally across more mixed Southern Highland Group lithologies (gritty quartzose arenites, feldspathic arenites, wackes, semipelitic and pelitic units). The section lay just northwest of the broad ‘Downbend’ hinge, but although we were nominally in the ‘Flat Belt’ the overall bedding and cleavage dips were generally c.40° to the NNW. Some D4 structures were seen in the pelitic rocks close to the ‘Downbend’ and minor F4 and S4 structures were noted in other outcrops, but were not dominant here. The steeply dipping F4 axial planes, local crenulation cleavage and generally open, chevron-style fold profiles are distinctive. The section showed the oncoming of D2 deformation and structures, manifest first in the dominantly pelitic units as open to tight F2 folding with a related penetrative cleavage (S2). S2 was orientated close to the overall bedding, but the folds patently folded an earlier S1 cleavage and bedding (S0). In the thicker gritty quartzose units, D2 deformation effects were muted, and the S1 cleavage still dominated, generally making a shallow angle with the bedding, defined by grading in some units. The beds appeared to young to the northwest and be mainly inverted. Higher up the ridge good examples of ‘classic’ F2 minor folding were found nearly everywhere in the arenites, mainly affecting the earlier spaced S1 cleavage. The folding showed all manner of variation, reflecting the proximity of thicker competent or incompetent units, and the pre-existing S0–S1 geometry. Bedding was commonly difficult to discern. As we traversed up the hill (into deeper structural levels) D2 deformation effects increased, marked by tighter folds, more zones of attenuation, some shear zones, and local development of a second spaced cleavage, S2. In the more pelitic units and some of the arenite units it was difficult to separate the S1 and S2 cleavages; indeed, a composite S1–S2 cleavage was commonly present. On the southwestern flank of Benachally roughly beneath the summit tower thick gritty arenite units showed dips varying form sub-vertical to near-horizontal. Where bedding dips were gentle a steep S1 cleavage could be found indicating we were in an F1 hinge zone (Figure 1.11). Just northwest of the summit crags good exposures showed that D2 deformation had strongly overprinted the earlier D1 structures, with bedding again difficult to define in parts. The overall conclusion is that in this part of the Tay Nappe, although D2 folding is present on a centimetre- to at least decimetre-scale, medium to large scale F1 folds are also present, albeit possibly with dislocations separating major fold structures. The D2 deformation has folded and refolded the earlier D1 structures, but its effects have been variable, dependent on lithology, structural position, etc. Similar relationships were recognised by Crane et al. (2002) across much of the Glen Shee district involving the ‘Green Beds’ and Loch Tay Limestone Formation. The overall structure of the ‘Tay Nappe’ is thus variable along strike, here being represented by a stack of major F1 and F2 folds. Farther southwest it shows a relatively simple nappe form and farther northeast its amplitude diminishes considerably. It is important to decide whether the F1 folds formed as recumbent, relatively flat-lying, major structures or as more upright large-scale structures, subsequently rotated by D2 deformation. The magnitude of the D2 deformation is also critical in defining the overall geometry and determining which model is favoured for the formation of the ‘Tay Nappe’.
Day 2 saw the party ascending a steep burn section at Glenoglehead through the Loch Tay Limestone Formation structurally upwards into the stratigraphically underlying Ben Lui Schist Formation. Lithologically this was transition from banded metalimestones with thin semipelitic beds and some pelitic and cherty interbeds up into a more pelitic succession with thin metalimestones and some psammite beds. The calcareous content reduced upwards until garnetiferous muscovite-bearing semipelite and psammite characteristic of much of the Ben Lui Schist Formation was reached. Amphibolitic mafic sheets were common in the mid and upper parts of the section with individual metadolerite sheets attaining some 10 m in thickness. Contacts were commonly well exposed and although many of the sheets have a sill-like form, in detail the intrusive contacts showed discordance. The main points of the section were the overall weak development of the deformation fabrics, some F2 folding and a related S2 penetrative cleavage were present, but there was little or no evidence of D1 structures and only minor D4 effects. This locality lies in the classic area of inverted rocks that form the inverted ‘limb’ of the ‘Tay Nappe’ within the Flat Belt. The rocks here lie on the SSE limb of the open ENE-trending F4 Ben More Anticline whose trace has been mapped beneath the eastern part of Glen Dochart and Loch Tay.
F2 tight minor folding was also seen in metalimestones of the Loch Tay Limestone Formation in the small disused quarry adjacent to the Lochan na Lairige road, but here the F4 effects were more obvious with open folding and an S4 crenulation cleavage developed locally, particularly in the graphitic pelite interbeds.
At Lochan na Lairige near the dam D4 deformation was dominant in the calcareous semipelitic rocks of the Ben Lawers Schist Formation that contained numerous amphibolite pods and lenses. The steep S4 crenulation cleavage and related open to tight F4 folding were ubiquitous. Earlier F2 folds, fabrics and lineations could be seen in places and some fold interference structures were present. The locality lies close to the wide hinge zone of the Ben Lawers Synform and represents the acme of D4 deformation. Note that the accompanying metamorphic grade was greenschist facies, this retrograde hydrous metamorphism has pervasively overprinted the earlier lower amphibolite facies assemblages.
Our final locality at the north end of Lochan na Lairige involved a traverse through a unit sandwiched between the Ben Lawers and Ben Lui Schist formations. Elsewhere this stratigraphical interval is represented by the Farragon Volcanic Formation, but here a mixed sequence of psammites, semipelites, quartzites (locally gritty), thinly banded amphibolites, graphitic pelites and metalimestones is present. The moderately SSE dipping contact of this sequence with the structurally overlying Ben Lawers Schist Formation is marked by a c.20 to 70 cm-wide zone of laminated mylonites, formed mainly of impure metalimestones and other calcareous rocks. The underlying footwall lithologies vary from graphitic pelites to metalimestones, calc-silicate rocks and quartzites, and appear to be truncated against the overlying calcareous semipelites of the Ben Lawers Schist Formation, whose lithological banding lies near parallel to the contact. This geometry is similar to that of a thrust or extensional ramp. D4 effects were limited below the mylonite zone with tight F4 chevron-style folds dying out over a metre or so. The amount and age of ductile movement along this contact (? towards the NNW) is unclear but the nature of the ductile dislocation is incompatible with it being solely a D4 structure and relative displacement would likely be tens of metres at least. A D2 age is preferred, as this deformation increases rapidly in the underlying succession. The exposed relationships suggest that the beds were probably inverted (by D1?) prior to ductile thrusting or extension, which was focused along this major lithological boundary. Note that Elles (1926) mapped a thrust along this contact!
Day 3 saw a return to Breadalbane to continue the structural traverse northwards into the upper parts of Glen Lyon. The first locality in the upper part of the Allt Bail a’ Mhuilinn showed tightly folded, strongly deformed metalimestone of the Loch Tay Limestone Formation, here overlain by foliated and lineated garnetiferous amphibolite. The amphibolite varied from an L-tectonite (dominated by aligned hornblende forming a pervasive lineation) and an L-S tectonite (both linear and planar features developed).The rock is well-suited to show the variations of the D2 strain ellipsoid, which are mainly dependent on structural position. Typically L-tectonites develop preferentially around fold hinge zones. The metamorphic grade accompanying the strong D2 deformation was certainly amphibolite facies. The structural sequence exposed on this southern flank of Upper Glen Lyon consists of a series of large-scale, gently plunging F2 folds — the Ben Lui Fold Complex. What is unclear is the extent of F1 folding and how that interacted with, or influenced, the pervasive D2 deformation, folding and fabrics.
Lower down in the Allt Bail a’ Mhuilinn, northwest of the Bridge of Balgie Fault, Grampian Group psammites (the Auch Glen Psammite Formation) with some sedimentary structures are exposed. Although definitive examples of cross-bedding were difficult to find, the nature of cut-offs, presence of slump structures, and general bedding features suggest the beds are right way up and show that deformation state is relatively low.
Farther west in the River Lyon section east of Cashlie and immediately north of Meall Ghaordaidh the true scale of the Ben Lui Fold Complex was more apparent. Note that when unfolded the Argyll Group sequence that here defines the fold pattern is still regionally inverted. Exposures of the Loch Tay Limestone Formation, foliated garnetiferous amphibolite and semipelites and wacke psammites of the Southern Highland Group (here termed the Pitlochry Schist Formation) occur in the river where they show abundant minor folding (F2) and are mapped as defining a major fold closure. In the Southern Highland Group rocks some amphibole-bearing units may represent ‘Green Beds’, volcaniclastic arenites and semipelites, commonly found in its lower part.
As time was limited the majority of the party omitted to visit the cross-bedded quartzose arenites of the Beinn a’ Chaisteil Quartzite Formation, exposed by the Giorra Dam, electing instead to walk along the north side of Loch an Daimh to look at exposures of an unusual breccia. Along the track abundant float of calcareous semipelite and psammite with excellent grey-green amphibole porphyroblasts showed we were passing through Lochaber Subgroup rocks (≡ Leven Schist Formation). In a burn section semipelitic and psammitic rocks with abundant both small- and medium-scale amphibolite layers (commonly garnetiferous) showed excellent examples of folding and refolding. The earlier tight to isoclinal folds (?F2) showed a penetrative cleavage (S2) but the later folds (open to tight) typically only showed a related crenulation cleavage (F3 or F4?). Examples of strain shadows were seen on some garnet porphyroblasts testifying to the presence of later penetrative deformation. Although mapped as part of the Lochaber Subgroup succession, the lithologies are atypical and show marked similarities to the Farragon Volcanic Formation (see Slatich locality, Day 6). Their strongly deformed state and situation adjacent to the Ben Lawers Schist Formation suggest that they may lie within a shear zone that here marks the eastern extent of the Argyll Group sequence. This sequence manifestly sits unconformably on the underlying right-way-up Lochaber Subgroup and Grampian Group succession, but may also be the site of a ductile shear zone (the ‘Boundary Slide’).
A little farther west a complex breccia is exposed in a burn over the vertical interval of at least 100 m. It consists of clasts of quartzite, psammite and semipelite in a mostly calcareous semipelite matrix. In parts beds are only slightly dislocated (‘boudinaged’) but in other places large folded ‘clasts’ are present and overall the rock has a massive unstructured appearance. The term ‘melange’ was suggested and seems appropriate. A coherent amphibolite sheet could be traced across the breccia in one place. The features suggest that this lithology formed by mass flow, during, or more probably closely following, sedimentation. The ‘melange’ can be traced down the burn to Loch an Daimh (high water level at time of visit) where it has been mapped as overlying the Beinn Dorain Semipelite Formation (Lochaber Subgroup). However, it is unclear as to whether the Ben Lawers Schist Formation is regionally inverted here and/or whether there are major folds of the Ben Lui Fold Complex present. A similar but thinner breccia has been mapped further south near Pubil in upper Glen Lyon, where it lies within the Auchlyne Formation, structurally beneath but stratigraphically above the Ben Lawers Schist Formation. These features point to the existence of fault scarps, local basins, and major instabilities during deposition of the Auchlyne, Farragon Volcaniclastic and even Ben Lawers Schist formations (upper part of Easdale Subgroup).
Day 4 involved a visit to the Tomintoul area, stopping on the way at Dulnain Bridge to view the gneissose pelite and semipelite of the Grantown Formation, here sculpted by ice into classic roche moutonées. The massive pelitic bedrock contains abundant quartzofeldspathic segregation and veins. The presence of significant kyanite a short distance farther north suggests that the protolith siltstone + mudstone lithology was originally aluminous. As the formation lies close to the base of the Grampian Group sequence its somewhat unusual lithology may reflect uplift and stripping of tropically weathered material, e.g. soils, in the hinterland followed by transport and deposition in the sedimentary basin.
At Bridge of Brown the transition from Grampian Group psammites, quartzites and minor semipelites up into Lochaber Subgroup semipelites and thin psammites, commonly calcareous, is well-exposed in incised sections in the Burn of Brown. The transitional Dalvrecht Slate Formation shows evidence of several cleavages (S2, S3, etc) but despite the enhanced strain there is no evidence for a significant dislocation or an unconformity. The stratigraphy cannot be correlated in detail with Lochaber Subgroup sequences in the Loch Leven or Glen Tilt, Glen Lyon areas, but the lithologies are similar. An unusual but distinctive kyanite-bearing gneissose semipelitic unit, the Fireach Beag Kyanite Gneiss Member, occurs within calcareous semipelites. This aluminous pelitic unit may represent a further input of residual tropical ‘soil material’ into the Dalradian Basin, in this instance shortly after commencement of Appin Group shallow marine sedimentation. In this part of the Grampian Highlands the Dalradian succession is largely right way up, except where kilometre-scale folding is present. The deformation pattern and its timing also differ somewhat from area farther south, probably reflecting tectonic events linked to plate movements involving the Buchan Block. The gravity and magnetic signature of this area of NE Scotland suggest that it is underlain by different basement, and the area shows evidence of a markedly different Grampian tectonic history. Two main deformation events are recognized, termed D1 and D3, but their correlation with similarly annotated events in the southern and southwestern parts of the Grampian Highlands is tenuous at best. The D3 event here appears to link to WNW-directed thrusting across the Portsoy and Keith shear zones, dated by roughly coeval gabbro intrusion at c.474 Ma.
At Bridge of Avon a condensed Ballachulish Subgroup (Appin Group) succession is exposed in the river section. The sequence here is situated above an early basement lineament that influenced the basin geometry and sedimentary facies. It is also the site of a major change in the regional trend of the bedding and main S1 fabrics, informally termed the ‘Knee Bend’. Interestingly, even the Devonian outlier appears to be sited on this feature, suggesting that the fault-bounded basin containing the terrestrial to lacustrine sequence (conglomerate-sandstone-siltstone) may have been controlled by the lineament and related regional change in strike. The overlying dominantly calcareous Blair Atholl Subgroup succession is of more typical thickness, particularly the main Inchrory Limestone Formation. This probably reflects the quiet, stable, shallow marine conditions that persisted in much of the Dalradian Basin in late Appin Group times.
The ‘Boulder Bed’ represents a metadiamictite (‘tillite’) probably deposited mainly under shallow marine conditions. It forms a distinctive chronostratigraphical unit within the Dalradian succession, marking the base of the Argyll Group, and as such its age has been the subject of much speculation. It is also one of the few lithologies that can tell us something about the palaeogeographical conditions and the nature of the basement exposed at the time of sedimentation. The ubiquitous granite clasts in the upper part of the tillite formation along its wide outcrop give U-Pb zircon ages that cluster around 1870 Ma and 1780 Ma (Evans et al., 1998; Hughes and Menuge, 2005). Detrital zircon U-Pb age spectra from the matrix also give similar peaks (Chew et al., 2010). Although the outcrops in the Muckle Fergie Burn are somewhat unprepossessing, the unit’s significance to the understanding of the age, nature and palaeogeography of the Dalradian succession should not be underestimated.
Day 5 visited parts of the Grampian Group in the Loch Laggan area and strayed into the folded Appin Group succession of the Loch Leven–Glen Roy area. It also touched on the role of the exposed basement rocks and their relationships with the Dalradian succession in the central part of the Grampian Highlands. The Strathspey Granite Pluton and its related pegmatitic vein complex, intruded at around 447 Ma also featured prominently. In contrast, the Strath Ossian Granodiorite, intruded at around 420 Ma, underlies areas of generally subdued topography and its aureole effects were not seen. The role of the Main Late Devensian and later Loch Lomond Readvance glaciations were also highlighted, particularly by Loch Treig where relict features of the Treig Delta are prominent.
At Loch Laggan clean exposures (low water conditions) showed excellent examples of Bouma-type sequences in psammites (arenites) and semipelites of the Loch Laggan Psammite Formation indicative of their formation under turbiditic conditions in the Corrieyairack Basin. This basin contains three main facies associations: a lower fluvial-lacustrine facies; an overlying semipelitic unit representing the transition to marine conditions; and an upper mainly turbiditic succession. Calc-silicate lenses and layers of diagenetic origin are common and show an amphibolite-facies metamorphic mineralogy. However, the bulk of the rocks show little evidence of having been metamorphosed under such high grade conditions [T–> 600°C, P–c.8 kb (≡ c.25 km depth)]. Sedimentary features have been recrystallized but their geometry scarcely modified (low strain), except in the pelitic units where recrystallization dominates and a new cleavage has formed. The open nature of the folding and limited deformation (attributed to D2) is in sharp contrast to the complex structural patterns and fabrics recognized in the adjacent Geal Charn–Ossian Steep Belt, in the Loch Treig area to the southwest and farther south in the Tummel Steep Belt. Patently, the Loch Laggan area has been protected from significant Grampian deformation, probably reflecting the nature and thickness of the underlying basement and/or crust. The presence of the Late Ordovician Strathspey Granite and related Loch Laggan Vein Complex, all products of crustal melting, also suggest that the continental crust was thicker here at the time of orogenesis.
Basement areas have been important in defining the margins of the Grampian Group basins with the sedimentary successions showing considerable variations in stratigraphy, thickness, and possibly even in timing of deposition. The nature of the basement and its relationship to the overlying Dalradian succession is not fully known. In places the Dalradian succession appears to onlap upstanding basement blocks, giving rise to gaps in the succession and local unconformities. By Coul Farm gneissose semipelites and psammites attributed to the basement (now termed the Badenoch Group) are in contact with more striped semipelites and psammites of the Lochaber Subgroup. Both rock types are strongly deformed and lie in the northeast part of the Geal Charn–Ossian Steep Belt. In reality there is little apparent lithological difference between the two rock units in outcrop. The definition of the basement here relies on mapping out the distribution of the more distinctive Dalradian lithostratigraphical units and the occurrence of pegmatitic granite pods that give ages of >750 Ma. The shear zones lie in the structurally highest parts of the basement and at the basment-cover contact. The basement psammite-semipelite succession may correlate with the Moine Supergroup of the Northern Highlands, but at present there is a lack of convincing stratigraphical or geochronological evidence to definitely link the two successions.
Unfortunately, as time was limited we failed to visit the ‘Boulder Bed’, exposed rather poorly by the roadside at Kinlochlaggan. The unit has a quartzitic or psammitic matrix (c.f. the Port Askaig and related ‘Boulder Beds’) and is better interpreted as a local slumped unit at the top of the Kinlochlaggan Quartzite Formation within the Lochaber Subgroup succession. It shows no compelling evidence that points to it being of glacial origin.
Day 6 saw a return to Glen Lyon, firstly to look at the Roro ultramafic body that sits within the wide outcrop of the Ben Lui Schist Formation on the northern flanks of Ben Lawers. This body has an exposed contact where semipelites, pelites and lenticular locally gritty arenites are interbedded with, or lie adjacent to, the altered ultramafic rocks. Grading in the gritty arenites imply this is an upper contact, in agreement with the interpreted structural pattern. In addition some hard, brittle, graphitic pelite was found in adjacent outcrops suggesting possible marginal contact metamorphism. Note that ultramafic bodies are typically anhydrous and intruded at high temperatures — typically ranging from c.1100°C down to perhaps 800°C dependent on the amount of crystallization in the magma. Sharp eyes in the party spotted a very good example of relict igneous banding, now shown clearly by the corresponding metamorphic mineralogies (garnetiferous amphibolite, amphibole rock, etc), suggesting differentiation in a magma body with ultramafic-mafic-felsic ‘grading’ towards the northwest. Higher up, altered zones rich in actinolite and talc anastomose through the intrusion defining areas of preferential fluid flow. In one place metre-wide talc vein was noted, within which the talc plates were crenulated. Deformation in the ultramafic body was variable as shown by the presence of a prominent foliation in the more mafic sections and a shear zone focused on a rare pegmatitic felsic vein. These observations suggest that the body was originally emplaced as a laccolithic intrusion into the pre-existing Argyll Group succession, probably at shallow crustal levels, rather than representing a ‘cold’ piece of mantle exhumed onto the ocean floor. The presence of other ultramafic bodies in Crinan-Tayvallich subgroup rocks along the length of the Grampian orogen suggest that this event may have links with the mafic volcanism that occurred coeval with deposition of the Tayvallich Subgroup or lower parts of the Southern Highland Group. Their occurrence as intrusive igneous bodies at similar stratigraphical levels has implications for their timing (roughly coeval) and depth of emplacement (?relatively shallow).
After a satisfactory morning a sudden, rather intense, rain + hail shower around lunchtime managed to dampen proceedings and make the high bracken and locally steep slopes at the Slatich locality rather unappealing. This locality was chosen to look at the Farragon Volcanic Formation, here represented by laminated and thinly bedded amphibolite with psammite and quartzite interbeds. The lithologies have been interpreted to represent mafic lavas and tuffs. The outcrop of the formation defines the complex antiformal fold hinge of the eastward plunging D4 Ruskich Antiform, but on a smaller scale evidence of penetrative D1 and D2 deformation and folding is preserved. The upper parts of the structurally overlying Ben Lawers Schist Formation contain psammite and quartzite interbeds and some calc-silicate lenses and bands. These lithologies show good examples of F2 + F4 interference patterns. The structurally underlying Ben Lui Schist Formation was only seen in the narrow fold hinge zone. Note that this stratigraphical interval has been visited on three occasions during this excursion, the lithologies differ considerably, but all suggest that it represents an active environment, marked by volcanism, local rift basin formation, melange generation, etc, indicating considerable tectonic instability on the progressively extending continental margin.
The Lochan an Duim locality at the northwestern end of Strath Fionan exposed Blair Atholl Subgroup metalimestones and graphitic pelite and semipelite. The metalimestones defined tight F2 folds on a small and medium scale and the pelites were suitably kyanite-bearing. The varied Appin Group sequence shows extremely complex fold interference patterns that can be mapped out here, illustrative of very strong ductile D1 and D2 deformation. Exposures of the ‘Boundary Slide’ above Trinafour show the classic ‘tramline’ appearance seen in many ductile shear zones, here attributed mainly to D2. The new road cuts show that the succession is lithologically somewhat variable, but dominated by muscovitic semipelite with some thin metalimestone units, intruded by numerous thin amphibolitic mafic sheets and pods (commonly garnetiferous). Attribution of the lithologies to specific formations in the Lochaber Subgroup seems somewhat problematical. This Schiehallion–Trinafour area has also been the site of later deformations, giving rise to the tight Errochty Synform, itself refolded by the Trinafour Antiform.
The model for the Grampian Orogeny presented during the week has five main elements, which in turn have consequences for the geological history of the Grampian Highlands.
- Slow stretching of a continental margin coeval with deposition of the Dalradian succession (c.750 Ma to c.510 Ma). Formation of rift basins during the early phases of stretching (Grampian Group — some basin margins defined by the reactivation of basement lineaments. More uniform shallow marine deposition during Appin Group times. Onset of instability, widespread rifting, and the local generation of ocean crust-floored basins during the deposition of the Argyll Group, some rapid facies variations. Intrusion of mafic volcanics and attendant intrusive mafic and ultramafic bodies. Rift- drift transition occurred during deposition of the Southern Highland Group (c.570 Ma). Note that the generation of the Iapetus Ocean probably resulted in a major promontory in Scotland. The Southern Highland Group turbiditic sequence (arenite, wacke, pelitic, and volcaniclastic rocks) was deposited on attenuated, much thinned crust, locally within deep water marine environments (e.g. Leny Limestone trilobites). The Dalradian succession is unique on the Laurentian margin in that its age can be documented up to at least the upper part of the Early Cambrian (Leny Limestone), and it possibly includes rocks deposited as late as the Early to Mid-Ordovician (Arenig).
- Northwest-directed but short-lived convergent subduction of a relatively recently formed oceanic crustal plate occurred beneath the accretionary wedge (prism) at the southeast margin of the Laurentian continent, probably in mid- to late Cambrian times. Note that crustal basement was either very thin or absent at this stage. Subduction was rapid and at a shallow angle (<11°), entraining the overlying attenuated wedge of Dalradian sedimentary and volcanic rocks, thereby thrusting and stacking it beneath itself to generate the ‘Tay Nappe’. Nappe generation involved regional inversion by a rolling hinge mechanism. Oddly, parts of the Argyll and Southern Highland Group succession show little evidence of small-scale penetrative D1 deformation structures (minor folding, cleavage, lineations, etc), yet patently have been tectonically inverted. The nature of the ‘nappe’ structure developed was dependent on the pre-existing lithological and tectonic make-up of the wedge, the efficiency of the frictional coupling during subduction and underthrusting, the speed of subduction, and the nature of the incoming material at the trench, e.g. continental crustal fragments, seamounts, oceanic plateau, older stiffer oceanic crust, spreading ridges, etc. The structures mapped in the present ‘hinge zone’ of the ‘Tay Nappe’ imply that underthrusting became difficult (periodically stalling even), probably due to changing parameters, giving rise to folding, dislocations, and the local generation of additional fold phases (D1a and D1b). It is likely this occurred just prior to the cessation of underthrusting (and subduction?). The D1 deformation features exposed today formed under hydrous greenschist facies metamorphic conditions. It is unclear as to whether formation of the ‘Tay Nappe’ and generation of a backstop then resulted in subduction flip or reversal, or whether such changes in plate tectonic geometry occurred later, on arrival of the Iapetan arc.
- Collision by an arc from the southwest or SSW into the already folded and metamorphosed ‘Tay Nappe’ resulting in marked crustal thickening, major and minor folding (F2) and related thrusting of the Dalradian succession, all linked to overall tectonic transport towards the NNW. Some diachroneity seems likely but ages and detrital mineral data from Ireland [South Mayo Trough (Mange et al., 2010), Tyrone Igneous Complex (Cooper et al., 2011)] and the dates of the peak of metamorphism in Scotland are remarkably consistent at around 470 Ma. Data from Mayo suggest that ‘hard’ collision of the arc occurred between 482 Ma and 470 Ma (Chew and Strachan, 2014). D2 deformation gave rise to the main penetrative fabrics in the orogen. This event was accompanied by increased heat flow, with peak metamorphic conditions following shortly after collision and crustal thickening, with later localised granite formation by crustal melting. Indeed, the time interval seems insufficient to generate Barrovian temperatures by c.470 Ma, suggesting that slab tear and/or detachment, corner flow, etc., may have occurred, at least in places. The ‘D1’ structures in the Tayvallich, Appin, Fort William and Glen Roy areas may be a more distant reflection of the arc collision, here affecting a weakened crustal zone and thicker part of the Dalradian basin. Many of the larger F2 folds (up to kilometre-scale), for example, those mapped in the Ben Lui Fold Complex, probably overprint early F1 structures, but the high D2 strain and attendant lower amphibolite facies metamorphism have served to obscure the relative contributions of D1 and D2. However, the beds are regionally inverted at least as far as the ‘Boundary Slide’. This last structure is interpreted here as a major D2 shear zone, sited approximately coincident with the presence of thicker basement at depth, and corresponding with a significant unconformity within the Dalradian succession. Thus, it seems a suitable place to terminate the regional-scale inversion of the ‘Tay Nappe’, even though large-scale F1 (and major F2) folds have been mapped farther north.
- Note that in Northeast Scotland, within and adjacent to the Buchan Block, the nature and timing of both D1 and D2 events, and the character of the metamorphism (higher heat flow), are dissimilar to the areas farther southwest. D1 deformation certainly predated emplacement of the Portsoy Gabbro and related mafic-ultramafic intrusions at around 474–471 Ma, followed rapidly by crustal melting giving rise to several S-type granite plutons, emplaced at around 470 Ma. Significant westward thrusting across the Portsoy Shear Zone occurred roughly coeval with emplacement of these intrusions. This reflected closure of a small linear oceanic basin, probably formed initially in Upper Argyll Group times. The resultant deformation and pressure overprint has been attributed to D3, whose distribution is generally concentrated in areas marginal to the Buchan Block. In the Central Grampian Highlands D3 deformation is more patchy, related mainly to reactivation of lineaments, or locally present in weaker zones within the orogenic edifice. Studies of the metamorphic mineralogies and derived P-T conditions suggest that the rocks remained at pressures and temperatures close to their metamorphic peak implying that D3 followed very closely after D2. Note that D3 was several million years later in the southern part of the Grampian Highlands, occurring at around 465 Ma.
- Uplift and exhumation followed the main period of orogenesis. Some S-type crustal-melt granitic bodies were emplaced, mainly in the central and northern parts of the Grampian Highlands. Their Late Ordovician age is in accord with the expected time gap between orogenesis and crustal thickening, related heat generation, and pluton emplacement. Differential uplift was commonly focussed along inherited lineaments, one such resulting in formation of the monoformal Highland Border Downbend. Subsequent to uplift and related extension, a further regional compressional event occurred, accompanied by hydrous greenschist facies metamorphism. This D4 event was widespread across the ‘Flat Belt’, mainly affecting Argyll and Southern Highland Group rocks southeast of the ‘Boundary Slide’. It was responsible for the formation of the Ben Lawers Synform and related large-scale structures. The age of D4 is not known but it predated the voluminous, dominantly granitoid intrusions of the Scottish Highlands Silurian Suite, intruded between c.428 and 408 Ma (Neilson et al., 2009). This latter event has been attributed to drop-off of a northwest-directed subducting slab following the docking of Eastern Avalonia (England) with the Laurentian margin (Southern Uplands–Scotland) towards the end of the Wenlock (early Silurian). D4 may be a distant expression of the Avalonia-Laurentia docking farther south, or related to Scandian orogenesis that resulted in the formation of the Moine Thrust Zone and related deformation in the Northern Highlands.
* An epilogue, a word also of Greek origin (επίλογος, epilogos), is the peroration of a speech or in literature the final chapter at the end of a story that commonly serves to reveal the fates of the characters. It can be used to hint at a sequel and/or wrap up the loose ends. Note that it can also occur at a significant period of time after the main plot or story has ended. In some cases, the epilogue has been used to allow the main character to “speak freely.” An epilogue generally continues in the same narrative style and perspective as the preceding story, but on occasion it can be drastically different.