Abstract Fractured gneiss lithologies form a basement-cored high, the Rona Ridge in the Faroe–Shetland Basin. Basement structures are known to play an important role in the petroleum system for the overlying giant Clair Field. An onshore analogue exposure in the Lewisian Gneiss Complex at Kinlochbervie in NW Scotland provides an example of a hanging-wall damage zone of a large basement-hosted normal fault. In this study, we used remote sensing (2D), outcrop line sample methods (1D) and a virtual outcrop created by terrestrial laser scanning methods (3D) to characterize spatial variations of the fracture systems. Spacing distributions from 1D line samples collected from exposures and pseudo-wells constructed through the virtual outcrop show power-law distributions. The virtual outcrop data enable us to extend the scale-invariant description from 1 to 3 orders of magnitude. We developed a novel box-counting workflow to provide an assessment of 2- and 3D variations in the fracture properties. Fracture density and fractal dimension are elevated whereas the number of intersections is decreased within a 220 m-wide volume adjacent to the fault. We discuss how the methods and results from this study can aid the development of analogue for basement reservoirs in the offshore UK continental shelf.

Abstract The world's mountain ranges are the clearest manifestations of long-term deformation of the continental crust. As such they have attracted geological investigations for centuries. Throughout this long history of research a few keynote publications stand out. One of the most important is the Geological Survey's 1907 memoir, The Geological Structure of the North-West Highlands of Scotland . The papers in this Special Publication celebrate the 100th anniversary of this remarkable book, placing the original findings in a present-day context by juxtaposing them against modern studies, not only from the NW Highlands, but also from elsewhere around the world. On first sight, the 1907 memoir by B. N. Peach and colleagues is simply a description of regional geology, albeit an exceptionally detailed one. However, the memoir outlines the Geological Survey's chief discoveries, and an abundance of minor ones, resulting from a decade of fieldwork in NW Scotland. At its height this activity occupied many of the most talented field geologists of Peach and Horne's generation and a significant number of the Scottish branch of the Survey. This team performed wondrous deeds, recognizing and interpreting many deformation structures on a variety of scales. The memoir describes these structures in a matter-of-fact manner, with occasional musings on their significance for broader tectonic processes. By focusing on the detail and establishing the supreme importance of getting the basic field relationships correct, the memoir's authors established the NW Highlands of Scotland as a prime location for training. Subsequent generations of geologists, from young students to experienced

Abstract Following years of sporadic debate, in the early 1880s consensus was reached that thrust tectonics explained hitherto controversial geological field relationships in NW Scotland. This spawned a major research effort there by the Geological Survey of Great Britain that culminated in a series of highly detailed geological maps, preliminary research papers and, eventually, the publication of a memoir to the region. These works became highly influential to early-20th century geoscience, especially structural geology. Not only did they provide the first major synthesis of thrust belt structure, they also provided the basis for descriptions of fault and shear zone processes and deductive methods for unravelling tectonic histories in metamorphic basement. A common misconception is that the results arose from mapping alone, without regard to extant models and theory and this approach is held up as an ideal for fieldwork. Yet the notebooks and writings of the surveyors show the application of learning not only from other research groups but also between themselves. As with modern mapping, the Survey team created interpretations that built on contemporary knowledge. This work in turn has driven subsequent research for over 100 years, in the NW Highlands and in deformed rocks throughout the world.

Abstract At the meeting of the British Association held in Dundee in September 1912 a group of eminent European geologists, including most of the leading Alpine tectonic experts of the day, intrigued by the account of the structure of the NW Highlands given in the 1907 memoir, expressed a wish to see these structures for themselves. Peach and Horne were approached, and agreed to lead an excursion to the Assynt area following the meeting. The programme for the excursion followed an itinerary that many geological parties still follow today. On the final evening of the excursion Albert Heim (Zurich), the doyen of Alpine geologists, gave a vote of thanks to the leaders and Maurice Lugeon (Lausanne) composed La Chanson du Moine Thrust which the participants sung with great enthusiasm. Brief biographies are given of the participants, many of whom were already distinguished, while most of the junior participants, particularly those from the British Geological Survey, went on to pursue long and distinguished careers, making major contributions to our knowledge of Scottish geology.

Abstract The Lewisian Complex is an Archaean/Proterozoic craton fragment found in NW Scotland and throughout the Outer Hebrides. The 1907 memoir recognized, simply from field relationships and petrographic observation, key features of Lewisian evolution. The bulk of the Lewisian is an old, deformed complex consisting mainly of acid igneous rocks, with some basics, ultrabasics and metasediments. In the Central District of the mainland these are pyroxene bearing (now recognized as granulite facies). The Lewisian Complex was intruded by a suite of basic and ultrabasic dykes which show variable states of later deformation, the intensity of strain being correlated with the development of hornblende schist in the dykes and amphibolite facies assemblages in the country rocks. In the Northern and Southern Districts, this deformation is pervasive and the dykes become concordant hornblende schist sheets. The new foliation with transposed dykes and metasediment sheets is then folded around NW–SE axes. Today there is no single agreed model for the evolution of the complex but an outline is as follows. In the pre-dyke (Scourian) history, subduction led to melting of oceanic crust which provided vast volumes of tonalite-trondhjemite-granodiorite in the period 3100–2700 Ma. Ages show geographic variations but it is not proven whether that implies large displacements between pieces of crust or whether it represents intrusions into other intrusions. The subcontinental lithospheric mantle dates from c . 3000 Ma. K, U and other large ion lithophile elements are depleted in the Central District of the mainland; this is due to depletion in the downgoing oceanic slab which in turn is a result of dehydration prior to melting. Other areas are not depleted in such elements, so various tectonic settings were involved. Remnants of metabasic material in the Lewisian may be relics of oceanic crust. Granulite facies metamorphism with, in places, P >10 kb and T >1000 °C occurred a considerable time after intrusion so is not necessarily linked to igneous events. This ‘Badcallian’ episode affected mainly the Central District and a part of the southern Outer Hebrides; other areas show only amphibolite facies. Zircon dating indicates two high-grade events at 2500 and 2700 Ma. During the ‘Inverian’ episode a series of wide amphibolite-facies shear zones affected the granulite-facies Scourian gneiss prior to the intrusion of the Scourie dykes. The Scourie dykes were intruded from 2400–2000 Ma and are largely quartz tholeiites derived from enriched subcontinental lithospheric mantle; there are some picrites which yield the oldest ages but are also seen to crosscut basic dykes. The dykes trend NW–SE and are steep where not affected by later deformation except where they intrude along, and are controlled by, Inverian fabrics. Post-dyke (Laxfordian) history involves the development of calc-alkaline igneous rocks in the Outer Hebrides and mainland ( c . 1900 Ma). Volcanics associated with sediments younger than 2000 Ma comprise an accretionary complex formed in a subduction setting; they are now intercalated between slabs of Archaean basement indicating that the complex was involved in collision with continental crust. Huge strains transposing dykes and country rocks affected almost all of the Outer Hebrides and the mainland except for the Central District. The NW–SE trending lineation indicates the collision direction; the metasediments on the mainland and the South Harris Igneous Complex may mark a folded suture between two continents. Metamorphism was amphibolite facies almost everywhere; in South Harris it was granulite facies at c . 1880 Ma. At 1750–1675 Ma, a distinct event, called late Laxfordian but much younger than earlier Laxfordian metamorphism and with a distinct tectonic setting, caused folding of the previous structures along NW–SE axes, migmatization and renewed amphibolite facies metamorphism.

Abstract Supplementary material: Analytical methods and data tables are available at: http://www.geolsoc.org.uk/SUP18395 Zircons from two granulite facies gneisses from the central region of the Lewisian Complex have been investigated by high spatial resolution ion-microprobe U–Pb dating and laser ablation combined Pb–Hf isotope methods. The ion-microprobe data reveal a complex pattern of zircon ages distributed along the concordia curve between the time of granulite facies metamorphism at c . 2.5 Ga and the oldest zircon in each sample (respectively 2.89 Ga and 3.04 Ga). This Pb-loss pattern complicates assignment of an unambiguous magmatic protolith age to the zircon although cathodoluminescence (CL) imaging is used to suggest a preferred age of c . 2.85 Ga for both samples, with older grains being inherited. In situ Hf isotopes show a larger spread in the sample containing older grains which is also consistent with inheritance and further suggests that several crust extraction events are represented in the inherited population. Comparison of Hf isotope compositions with plausible model evolution curves suggests that crustal precursors to the Lewisian granulites were derived from their mantle source at c . 3.05–3.2 Ga.

Abstract The Lewisian Gneiss Complex of northwestern Scotland consists of Archaean gneisses, variably reworked during the Proterozoic. It can be divided into three districts – a central granulite-facies district between districts of amphibolite-facies gneiss to the north and south. Recent work has interpreted these districts in terms of separate terranes, initiating a controversy that has implications for how Precambrian rocks are understood worldwide. The northern district of the Lewisian Gneiss Complex (the Rhiconich terrane) is separated from the central district (the Assynt terrane) by a broad ductile shear zone known as the Laxford Shear Zone. This paper reviews the geology of the Laxford Shear Zone, clarifying field relationships and discussing other evidence, to consider whether or not it does indeed represent a terrane boundary. A detailed review of field, geochemical and geochronological evidence supports the recognition of the separate Assynt and Rhiconich terranes. Mafic dykes (the Scourie Dyke Swarm) and granitoids, of Palaeoproterozoic age, occur on both sides of the Laxford Shear Zone and thus the terranes were most probably juxtaposed during the late Archaean to early Palaeoproterozoic Inverian event. Subsequently, the less-competent, more-hydrous amphibolite-facies gneisses of the Rhiconich terrane were affected by later Palaeoproterozoic (Laxfordian) deformation and partial melting, to a greater extent than the more-competent granulite-facies gneisses of the Assynt terrane.

Abstract This contribution presents a new model for the Grampian-age tectonothermal development of the Buchan Block and Barrovian-type regions to its west, in the Grampian Terrane, Scotland. The model has drawn on evidence gathered from field mapping, microstructural analysis, metamorphic petrology and mafic magma geochemistry to propose that emplacement of the Grampian gabbros and regional metamorphic heating associated with production of Barrovian- and Buchan-type units occurred during syn-orogenic (Grampian-age), lithospheric-scale extension. Extension followed lithospheric thickening associated with the initiation of Grampian orogenesis and was followed by renewed lithospheric thickening and termination of the extensional heating. Mantle melting to produce the Grampian gabbros of the Grampian Terrane was achieved by extensional thinning of the lithosphere and decompression melting of the asthenosphere at depths of less than 70 km. Advection of heat from the mantle with emplacement of the Grampian gabbros augmented elevated heat budgets associated with attenuation of isotherms during extension. Deposition of the uppermost Dalradian (the Whitehills and Boyndie Bay Groups and the Macduff Slates) occurred during Grampian-age lithospheric extension. A gently-dipping, mid-crustal detachment focused metamorphic heat sources and accommodated significant lithospheric-scale strain, allowing independent thermal evolution of units in its hanging wall (the Buchan Block) and footwall (Barrovian-type units).

Abstract The Rosemarkie Inlier is a small fault-bounded lens of interleaved Moine psammites and possible Lewisianoid orthogneisses with distinctive leucogranite veins and pods that lies adjacent to the Great Glen Fault (GGF). The basement rocks and most of the leucogranites are strongly deformed and tightly folded with foliations generally steeply dipping and a locally well-developed NE-plunging rodding lineation. Mid-Devonian sandstone and conglomerate unconformably overlie the inlier on its western side. Monazite from a deformed leucogranite vein gave a mean ID-TIMS 207 Pb/ 235 U age of 397.6±2.2 Ma and acicular zircons gave a compatible concordant ID-TIMS U–Pb age of 400.8±2.6 Ma, dating emplacement as mid-Devonian. Xenocrystic zircons from the leucogranites and complex zoned zircons from two adjacent tonalitic gneisses gave LA-MC-ICP-MS concordant ages between 2720 and 2930 Ma confirming their Archaean Lewisianoid origin. Leucogranite emplacement is interpreted to mark the onset of Acadian transpression and sinistral strike-slip movement on the GGF that resulted in multi-phase deformation and oblique exhumation of the Rosemarkie Inlier. The sequence and structure of the Early-Devonian Meall Fuar-mhonaidh Outlier, 32 km farther SW along the GGF, are also linked to this tectonic event, which was apparently localized along the main terrane-bounding faults in Scotland.

Abstract Benjamin Peach, John Horne and their co-workers recognized a century ago that the identical fauna and lithofacies of the lower Palaeozoic strata in the NW Highlands of Scotland and the North American craton could only be explained if they were: ‘part of one and the same geological and zoological province’. In this sense their work provided critical geological underpinning for the subsequent understanding of Mesozoic–Cenozoic seafloor spreading and continental drift. ‘Tectonic tracers’ such as the fragment of Laurentian craton in the NW Highlands of Scotland, provide the strongest evidence available for deciphering pre-Pangaea palaeogeography. The Laurentian craton appears to have left several such tectonic ‘calling cards’ in today's southern continents. Intriguingly, it is the presence in the Andean Precordillera of northwestern Argentina of an early Palaeozoic fauna identical to that of the NW Highlands that provides perhaps the most unequivocal geological clue to pre-Pangaea palaeogeography. Recent work in East Antarctic and Laurentian cratons has provided a positive test of the hypothesis that they were once juxtaposed prior to the Neoproterozoic opening of the Pacific Ocean basin. Geochronology and isotope geochemistry, supported by palaeomagnetic studies indicate that the Coats Land crustal block of East Antarctica at the head of the Weddell Sea is also a fragment of the Laurentian craton. These three ‘tectonic tracers’ permit tracking of the Laurentian craton in relation to the present southern continents from the Neoproterozoic break-up of the Rodinian supercontinent to the late Palaeozoic assembly of Pangaea.

Abstract The Himalaya-Tibet and Caledonide orogens are comparable in scale and are similar in various aspects. Regional suture zones are recognizable in both, although their identification is more problematic in the deeply eroded Caledonide orogen. Crustal-scale thrust belts, regional Barrovian metamorphism characterized by clockwise P – T paths, and migmatitic cores with crustally-derived leucogranite complexes are the dominant structural feature of both orogens. Both orogens also record calc-alkaline magmatism attributed to subduction activity prior to collision. Syn-orogenic extension accompanied crustal thickening in both orogens, however, the Caledonides also have a protracted record of late- to post-orogenic extension that is attributed to lithospheric delamination in combination with oblique plate divergence. The oblique nature of the Caledonian collision is also reflected in the development of regionally significant sinistral strike-slip faults and shear zones, whereas such structures are apparently not as significant within the Himalayan orogen. The major difference between the two orogens relates to their contrasting gross structure: the Caledonides has bivergent geometry with thrust belts developed in the pro- and retro-wedges, whereas the Himalaya has a thrust belt located only in the pro-wedge segment. These differing geometries are probably explicable with reference to pre-collision contrasts in rheology and/or inherited structures. As such, there is no reason to suggest that either example should be viewed as being a ‘typical’ product of collisional orogenesis – they likely represent end-members of a range of possible orogenic profiles.

Abstract The Moine Supergroup of NW Scotland is a thick sequence of early Neoproterozoic sedimentary rocks, with minor igneous intrusions, that display evidence for multiple phases of regional deformation and metamorphism. The descriptions and interpretations of the ‘Moine Schists’ provided by the 1907 memoir ( Peach et al. 1907 ) have been proved to be essentially correct and have laid the groundwork for a century of distinguished and influential research that has reached far beyond the confines of NW Scotland. The Survey workers recognized the sedimentary protoliths of these rocks, realized that they had been deposited unconformably on inliers of reworked basement gneisses that now occupy the cores of major folds, and understood the likely complexity of folding and the kinematic significance of mineral lineations. Further advances in understanding of the Moine rocks were mainly achieved through two techniques that were not available to the Survey workers of 100 years ago – geochronology and palaeomagnetism. Isotopic studies have confirmed the view of the Survey workers that the Moine rocks are of Precambrian age, and furthermore have demonstrated a complex, polyorogenic history.

Abstract Fold and fabric patterns developed within a major Caledonian thrust nappe in NW Scotland reflect a progressive increase in regional D 2 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 km 2 . Associated main phase, thrust-related folds (F 2 ) 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 (F 3 ) 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-D 2 meta-granite confirms that regional deformation occurred during the Scandian phase of the Caledonian orogeny.

Abstract Although orogenesis commonly lasts 100 million years, a maximum of three foliations are usually preserved within any outcrop. They record little of the total history due to the effects of preferential partitioning of progressive shearing along S 0 . Such reactivation eventually destroys or rotates multiple successive foliations into parallelism with any layering that is present. Plate motion subjects a collisional orogen to spatially partitioned, episodic, but non-stop deformation. The bulk forces operating are horizontal and intermittently vertical (associated with gravitational load due to crustal thickening) forming sub-vertical and sub-horizontal foliations. These orientations are preserved in the strain shadows of anything competent that hinders reactivation of the compositional layering. Deformation partitioning results in portions of rock remaining unaffected for many events. Gravitational collapse of over-thickened orogen cores leads to significant extrusion in orogen rims and a dominance of sub-horizontal foliations in the latter and episodically continues throughout orogenesis. Reactivation causes decrenulation and/or rotation of foliations into parallelism with S 0 , destroying evidence that multiple sub-vertical and sub-horizontal foliations have developed except where preserved as inclusion trails in porphyroblasts and multiply truncated foliations in mylonites. However, the same processes operate in both the cores of orogens and ductile portions of their margins.

Abstract The NW Highlands of Scotland have been an important test-bed for concepts in thrust tectonics. Here, research following the breakthrough publication of the 1907 memoir is reviewed, especially that relating to structural evolution in the Moine Thrust Belt. This belt was WNW-directed, involving cover sediments and thin sheets of crystalline basement. Displacements total 50–100 km within a branching array of thrusts. There are significant lateral variations in imbricate thrust geometry and localization behaviour. Following the application of linked thrust tectonic models in the 1980s significant attention has been directed at deducing thrust sequences, patterns of strain localization, folding styles and the significance of extensional tectonics as part of the structural evolution. The key has lain in deducing the kinematic linkages between thrusts and other structures, tracing displacements and examining the consequences of structural interpretations through geometric restoration. Thrusting models have been up-scaled to the crust. However, these linked kinematic approaches have been applied only hesitantly to the ductile structures of the Moine Thrust Sheet where structural research has focused on outcrop-scale deformation, especially of folds. Consequently, the larger-scale significance for Caledonian tectonics of thrust systems in the NW Highlands of Scotland has yet to be developed fully.

Abstract Descriptions of structural evolution across thrust belts commonly assume a transition from ductile to brittle deformation, reflecting a progressive reduction in temperature accompanying exhumation. The universality of this model is challenged here using field relationships at Ben Arnaboll, in the northern part of the Moine Thrust Belt. Deformation in the Arnaboll Thrust Sheet, an allochthonous basement body of amphibolite-facies gneisses and pegmatite sheets, carried onto Cambrian sediments, includes widely distributed, low-displacement shears developed under greenschist facies with ingress of water. These ductile deformations post-date the emplacement of the thrust sheet as they link kinematically to breaching thrust structures emanating from the footwall of the Arnaboll Thrust. The thrust itself records a transition from mylonitic (ductile) to strongly localized (brittle) deformation that pre-dates the breaching thrusts and therefore the deformation within the thrust sheet itself. The structure of breaching thrusts charts an up-dip transition from localized slip to distributed shearing analogous to the trishear in fold-thrust complexes, Therefore deformation of the Arnaboll Thrust Sheet shows a return from strongly localized translation-dominated brittle deformation to more broadly distributed ductile deformation. This is likely to have been promoted by the ingress of water and the concomitant reaction-enhanced weakening of the basement.

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.

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.

Abstract The Moine Thrust zone of NW Scotland marks the Caledonian orogenic front and in the Assynt region consists of several west-vergent major thrust sheets (Moine, Ben More, Glencoul and Sole Thrust sheets) that place allochthonous rocks onto the Lewisian basement and Torridon Group cover in the west. Here we present two new balanced and restored sections across the Moine Thrust zone in the region of the Loch Ailsh and Loch Borralan alkali intrusions. Syenites and alkaline pyroxenites intrude up to Durness Group carbonates. Syenites are interpreted as cut by the Moine and Ben More Thrusts and therefore their intrusion age ( c . 430 Ma) provides a maximum age constraint on cessation of motion along the Moine Thrust. We review and discuss several controversies, notably: (1) relationships between folding in the Ben More Thrust sheet (Sgonnan Mor folds) and fabric development associated with intrusion of the Loch Ailsh pluton and initiation of the Moine and Ben More Thrusts; (2) structural relationships around the Borralan intrusion, particularly the nature of its basal and lateral margins (intrusive or thrust-ramp); (3) the structural relationships of the Cam Loch and Benn Fhuarain klippen; and (4) the timing of motion of all thrust sheets in the southern Assynt culmination.