A. G. Leslie

The internal structure of the Moine Nappe Complex and the stratigraphy of the Morar Group in the Fannichs – Beinn Dearg area, NW Highlands

Synopsis The Morar Group, the lowest group of the early Neoproterozoic Moine Supergroup in the Scottish Highlands, forms a >5 km thick metamorphosed siliclastic sequence, recently interpreted to form part of a Grenvillian (c. 1000 Ma) foreland basin. New mapping has elucidated the structure and stratigraphy of the Morar Group in the Fannich–Beinn Dearg area, where the Morar Group occurs in a single coherent thrust sheet (Achness Thrust Sheet), over 70 km long, 20 km wide, and up to 10 km thick. Within this thrust sheet, the strata are folded by two very large, west-vergent and west-facing cylindroidal anticline-syncline pairs that deform the overlying Sgurr Beag Thrust. The lowest long limb is parallel with and grades into the ductile Moine Thrust and Achness Thrust at its base. Low strain zones in steep limbs contain well preserved sedimentary structures. Reconstruction of the stratigraphical architecture shows five formations of metasandstone (psammite), alternating with meta-siltstone (semipelite). Large-scale lateral variations in the lowest metasandstone package are capped by a possible flooding surface of semipelite, followed by more metasandstone. The deformation history shows foreland-propagation of both deformation and metamorphism, from NNW-directed transport on the Sgurr Beag Thrust to WNW-directed transport on the Achness Thrust and Moine Thrust.

Progressive fold and fabric evolution associated with regional strain gradients: a case study from across a Scandian ductile thrust nappe, Scottish Caledonides

Abstract Fold and fabric patterns developed within a major Caledonian thrust nappe in NW Scotland reflect a progressive increase in regional D2 strain towards the basal ductile detachment. Within the upper greenschist to lower amphibolite facies thrust sheet, the main gently east-dipping foliations and SE-plunging transport-parallel lineations maintain a broadly similar orientation over c. 600 km2. Associated main phase, thrust-related folds (F2) are widely developed, and towards the base of the thrust sheet display progressive tightening and increasing curvilinearity of fold hinges ultimately resulting in sheath folds. Secondary folds (F3) are largely restricted to high-strain zones and are interpreted as flow perturbation folds formed during non-coaxial, top-to-the-NW ductile thrusting. These features are consistent with a structural model that incorporates plane strain pure-shear flattening with a superimposed and highly variable simple shear component focused into high-strain zones. The increase in strain over a distance of 30 km across strike is similar to the increasing deformation observed when structures are traced along strike to the north, and which are apparently related to proximity to basement-cover contacts. A U–Pb zircon age of 415±6 Ma obtained from a syn-D2 meta-granite confirms that regional deformation occurred during the Scandian phase of the Caledonian orogeny.

Regional-scale lateral variation and linkage in ductile thrust architecture: the Oykel Transverse Zone, and mullions, in the Moine Nappe, NW Scotland

Abstract Sharp lateral changes in structural geometry of ductile thrust stacks are not widely reported. A regional-scale lateral culmination wall forms the southern boundary of the Cassley Culmination in Moine rocks in the Caledonides of Sutherland, Northern Scotland. This culmination wall is part of the Oykel Transverse Zone (OTZ), a kilometre-scale shear zone characterized by constrictional finite strain fabrics aligned sub-parallel to the regional WNW-directed thrust transport direction. Main phase folds and fabrics in the transverse zone hanging wall are folded by main phase folds and fabrics in the footwall, thus recording foreland-propagating ductile deformation. South of the Cassley Culmination, shortening occurred uniformly, without development of discrete subsidiary thrusts; distributed deformation (fold development) alternated with localized thrusting within the culmination. The classic ESE-plunging mullions at Oykel Bridge are an integral part of the OTZ and were generated by constriction aligned sub-parallel to the transport direction. Constriction is attributed to differential, transtensional movement across the OTZ during culmination development. Subsequent formation of the underlying Assynt Culmination further accentuated upward-bulging of the Cassley Culmination, amplifying the lateral change across the transverse zone. The OTZ aligns with a pronounced gravity gradient on the south-western side of the Lairg gravity low. Interpretive modelling relates this gradient to a buried basement ramp that possibly controlled the location of the transverse zone.

Lateral variations and linkages in thrust geometry : the Traligill Transverse Zone, Assynt Culmination, Moine Thrust Belt, NW Scotland

Abstract Abrupt lateral changes in thrust geometry occur in many fold-and-thrust belts along so-called transverse zones, commonly related to pre-existing basement faults. However, the causative structures are usually concealed. We analyse here the Traligill Transverse Zone in the Assynt Culmination of the Caledonian Moine Thrust Belt, NW Scotland. This transverse zone trends sub-parallel to the WNW transport direction and is associated with en echelon faults cutting thrusts, discontinuity of thrust architecture and oblique fold-and-thrust structures. Thick thrust sheets north of the Transverse Zone contain thick basement slices; thrust sheets to the south are thin and involve a thin-bedded sequence. The Traligill Transverse Zone developed above the Loch Assynt Fault, a basement cross-fault, and reactivated Proterozoic ductile shearzone. Piercing point analysis shows that the cross-fault was active both before and after thrusting. Thrusting thus affected strata that were already disrupted by steep faults. The amplitude of the disturbance in fold-and-thrust architecture along the Traligill Transverse Zone is much greater than the vertical displacement along the fault; this is attributed to localized transpressional thrust-stacking. Other basement cross-faults and their relationship with lateral variations within the Moine Thrust Belt and in other thrust belts are discussed.

Structural setting and petrogenesis of the Ben Vuirich Granite Pluton of the Grampian Highlands: a pre-orogenic, rift-related intrusion

Synopsis The Ben Vuirich intrusion is a small, elongate body of monzogranite that occurs in the Tummel Steep Belt, Perthshire. It was emplaced into Dalradian rocks (Appin Group) at 590 Ma, prior to the D1 phase of the Grampian Event (Caledonian Orogeny), and was strongly deformed during D2. Locally preserved cordierite- and andalusite-bearing hornfelses were altered to garnet ± kyanite-bearing assemblages during post-D2 regional metamorphism. A new study of the lower-grade hornfelses shows that the protolith was an undeformed, fine-grained, parallel-laminated sedimentary unit, confirming that the pluton is pre-orogenic with respect to the Grampian Event. Whole-rock and trace element analyses of 33 samples of the intrusion, together with rare earth elements, Rb–Sr and O-isotope data, show that it is an A2-group monzogranite. This finding supports the hypothesis that the granite, emplaced at a depth of 7–14 km, formed in the same extensional tectonic setting as the Tayvallich lavas at 600 Ma. Geochemical and isotope parameters point to a largely crustal source. The intrusion belongs to a swarm of rift-related, A-type granitoids that originally stretched from the Appalachians to Scotland, and includes foliated granitoids in the Moine. The granitoids formed in response to the early break-up of Rodinia, c. 50 Ma before the development of the Iapetus Ocean.

Structure and stratigraphy of the Morar Group in Knoydart, NW Highlands: implications for the history of the Moine Nappe and stratigraphic links between the Moine and Torridonian successions

Synopsis The Caledonian Orogen in northern Scotland comprises two major thrust nappes: the Moine and the Sgurr Beag Nappe. The Moine Nappe contains early Neoproterozoic Morar Group rocks (Moine Supergroup) and basement inliers. This paper describes the structure and stratigraphy of the Knoydart peninsula, a key area within the southern Moine Nappe. The geology of Knoydart is dominated by a thick internally coherent sequence of Morar Group rocks. This sequence is shown to be deformed by large-scale, west-vergent and west-facing Caledonian (early Palaeozoic) folds that represent D2 within the southern Moine Nappe. Subsequent D3 deformation led to refolding or tightening of F2 folds, so that the major Morar Antiform is, in essence, a composite F2/F3 fold. F2 and F3 folds are broadly co-axial, but F3 folds have steeper axial planes. The F2/F3 folds refold a regional-scale, originally recumbent, isoclinal F1 fold nappe of probable Knoydartian (mid-Neoproterozoic) age. The F1 fold nappe is cored by a thin sliver of basement gneiss; the lower limb comprises migmatitic Morar Group rocks, exposed in the Morar Window. The upper limb of the F1 fold nappe occupies most of Knoydart and is stratigraphically coherent and right-way-up. Within this sequence, the upper unit of the Lower Morar Psammite is barely deformed, preserving trough-cross-bedding and large-scale channels in thick beds. This suggests braided river deposition, similar to the Torridon Group west of the Moine Thrust and the Morar Group in the northern part of the Moine Nappe. On the basis of lithological similarity and stratigraphic disposition, it is suggested that the lowermost part of the Morar Group in Knoydart correlates with the Neoproterozoic Sleat Group on Skye.

Digital surface models and the landscape: interaction between bedrock and glacial geology in the Ullapool area

Synopsis The front cover image for this volume is a hill-shaded digital surface model (DSM) of the Ullapool area, created using NEXTMap Britain elevation data from Intermap Technologies. This is a classic area for bedrock geology, transected by the Moine Thrust Zone, and in recent years it has also been studied in detail for its glacial history. Perhaps equally important, this is one of Scotlands most iconic landscapes. The geology of the area comprises a number of distinct sequences, each of which has a characteristic landscape expression as illustrated by the DSM. This paper considers the influence of the bedrock geology on the glacial geomorphology, and shows that the interplay of the two has led to the development of the different landscape elements of this spectacular area. Surprisingly, it is not always the major geological features – such as the Moine Thrust – that have the strongest topographic expression.

The Gaick Fold Complex: large-scale recumbent folds and their implications for Caledonian structural architecture in the Central Grampian Highlands

Synopsis New British Geological Survey mapping has examined the stratigraphy and structure of Dalradian strata in the Gaick region of the Central Grampian Highlands of Scotland. In the north of that area, turbiditic strata in the Creag Dhubh Psammite Formation (Corrieyairack Subgroup) pass, via a well-defined sedimentary transition, into the stratigraphically younger Gaick Psammite Formation (Glen Spean Subgroup). This latter formation dominates the lithostratigraphy of the Gaick region, records shallow-water marine shelf conditions throughout, and was probably 1 to 2 km thick prior to deformation. Ordovician (Grampian) orogenesis affected the sedimentary rocks now arranged in a stack of Caledonian recumbent kilometre-scale F2 folds with gently ENE-plunging axes. Regional facing on these recumbent folds is typically sideways to the south. Southeast of the Gaick region, the folds, and thus facing, become progressively inclined and dip to the SE beneath the outcrop of the Appin Group Dalradian. No significant Fl folds, and hence no related facing changes, have been detected within this D2 fold stack. The structure of the Gaick–Drumochter area is therefore essentially a flat belt formed in the D2 deformation event and here named the Gaick Fold Complex. The D2 recumbent structures of this flat belt were rotated and steepened by D3 deformation into the Tummel Steep Belt to the SE, and adjacent to the Glen Banchor High to the NW. Tectonic transport in D2 in the Gaick Fold Complex is interpreted to be oriented on a north–south azimuth, similar to that in the Tay Nappe farther south. Such an interpretation implies that the NE–SW ‘Caledonian trend’ is a consequence of D3 deformation and reorientation, rather than a primary feature of the Grampian Orogeny.

Transverse architecture of the Moine Thrust Belt and Moine Nappe, Northern Highlands, Scotland: new insight on a classic thrust belt

The Caledonian Moine Thrust belt is a world class example of a foreland propagating fold-and-thrust belt. The late 19 th and early 20 th century research in this region was seminal in thrust tectonics, alongside contemporaneous studies of the Alps and Apennines. New British Geological Survey syntheses of the Assynt and Ullapool regions of the Moine Thrust Belt recognise previously unappreciated transverse Zone which accommodate sharp lateral changes in the structural architecture of the brittle-ductile thrust belt, and of the ductile thrust nappes to the east of the Moine Thrust. The Traligill Transverse zones transects the classic Assynt Culmination; the Oykel Tranverse Zone constrains the southern boundary of the Cassley Culmination in Moine rocks east of Assynt. Both transverse structures are oriented sub-parallel to the thrust transport direction and are related to pre-existing faults involving basement.

Caledonian and Knoydartian overprinting of a Grenvillian inlier and the enclosing Morar Group rocks: structural evolution of the Precambrian Proto-Moine Nappe, Glenelg, NW Scotland

The Grenville and Caledonian orogens, fundamental to building Laurentia and Baltica, intersect in northern Scotland. The Precambrian Glenelg Inlier, within the Scottish Caledonides, preserves a record of Grenvillian, Knoydartian and Caledonian orogenesis. Based on new mapping and re-interpretation of previous mapping, we present a structural model for the evolution of the Glenelg Inlier. The inlier can be divided into Western Glenelg gneiss comprising orthogneiss with no record of Grenville-age metamorphism, and Eastern Glenelg gneiss with ortho- and paragneiss, affected by Grenvillian eclogite-facies metamorphism. The basement gneisses and their original cover of psammitic, Neoproterozoic Morar Group (Moine) rocks were deformed by three generations of major ductile folds (F1–F3). In medium-strain areas F2 and F3 folds are broadly coaxial and both face to the west; in higher strain areas F2 and F3 folds are oblique to each other. By restoring post-F1 folds and late faults, the Glenelg gneiss inliers are seen to form the core of a major recumbent SSE-facing F1 isoclinal fold nappe – the Proto-Moine Nappe. The upper limb of this nappe is a thick, right-way-up sequence of moderately strained Morar Group rocks whereas the lower, inverted limb comprises intensely deformed, migmatitic Morar Group rocks. Within the constraints of published geochronology, the Proto-Moine Nappe is likely Pre-Caledonian and may have originated during the early Neoproterozoic Knoydartian Orogeny.