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The Moine Thrust Zone forms the Caledonian orogenic thrust front where the Moine Supergroup metamorphic rocks have been thrust westward across the Laurentia plate stable foreland, comprising Archean-Proterozoic granulite and amphibolite facies rocks (Lewisian gneisses), with unconformably overlying Mesoproterozoic and Neoproterozoic Torridonian clastic sediments and Cambrian-Ordovician passive margin sedimentary rocks. Four major thrusts beneath the Moine thrust in the Assynt window include the (i) Ben More Thrust, which places the Loch Ailsh syenite intruded into Lewisian basement and Cambrian-Ordovician sedimentary rocks over the Sole thrust sheet, (ii) Glencoul thrust, which places Lewisian basement and folded cover rocks over Cambrian-Ordovician sedimentary rocks, (iii) Borralan thrust, which carries a large alkaline syenite intrusion beneath the Ben More roof thrust and (iv) the Sole thrust sheet, which carries imbricated Cambrian-Ordovician sedimentary rocks and lamprophyre sills over the stable foreland. Three further thrust sheets within the Lewisian basement gneisses are now recognised through restoration of balanced cross-sections, which were responsible for doming of the Assynt window. Although the Moine thrust is mapped as a single line on the map it encompasses, (a) deep ductile shear zone formed of mylonites derived from hangingwall Moine schists, footwall Cambrian quartzites and Ordovician limestones, and basement Lewisian gneisses, (b) roof thrust of the Glencoul and Ben More Thrust sheets and (c) brittle out-of-sequence motion where the Moine schists have been thrust over mylonites, which directly overlie the stable foreland (Knockan Crag).

The Durness Group of NW Scotland records deposition on the Laurentian margin from the basal Miaolingian (Cambrian, 509 Ma) to the Dapingian–Darriwilian boundary interval (Middle Ordovician, 470.3–468.9 Ma). The 930 m thick succession of peritidal and subtidal carbonates was deposited on the Scottish promontory, a nearly 120° deflection in the Palaeozoic continental margin between the Appalachian and Greenland sectors. These sediments were deposited as part of the Great American Carbonate Bank, a non-uniformitarian, continent-scale carbonate platform developed on the peneplaned craton. Measurement and description of a bed-by-bed composite section through the Durness Group provide a high-resolution reference framework that integrates conodont biostratigraphy, chemostratigraphy and sequence stratigraphy, including correlation with the Sauk megasequence and its subdivisions. The Sauk II–Sauk III sequence boundary marks the base of the group. The top of the group is faulted against rocks of the Moine thrust zone, generated by the Scandian orogeny, but sedimentation was probably terminated by the earlier Grampian arc–continent collision at 470–469 Ma. The highly mature quartz arenites of the underlying Ardvreck Group (Cambrian Series 2) indicate that there was no source-to-sink depositional continuity from the Hebridean foreland to the Dalradian Supergroup, which has coeval clastic sedimentary rocks of contrasting composition.

Caledonian orogenesis in Scotland is currently interpreted in terms of a Mid-Ordovician arc-continent collision (Grampian event) followed by the Silurian collision of Laurentia with Baltica (Scandian event). Lu-Hf and Sm-Nd garnet ages of c. 475-460 Ma obtained from prograde garnets in metasedimentary successions and metabasic intrusions within the Northern Highland and Grampian terranes confirm that the Mid-Ordovician Grampian orogenic event was approximately synchronous in the two terranes. Lu-Hf and Sm-Nd ages of c. 450 Ma obtained from prograde garnets within the Moine Nappe of the Northern Highland terrane provide evidence for a hitherto unrecognized Late Ordovician regional metamorphic event. The existing two-stage Grampian-Scandian model for Caledonian orogenesis in northern Scotland is thus an oversimplification, and the new ages imply a more complex structural evolution. The restriction of the Late Ordovician and Silurian events to the Northern Highland terrane reinforces the suggestion that it was far removed from the Grampian terrane until juxtaposition following major end-Caledonian (Devonian) sinistral displacement along the Great Glen Fault. A similar record of Mid- and Late Ordovician metamorphic events within the Laurentian-derived Uppermost Allochthons of Norway has been attributed to episodic accretion significantly prior to Silurian continent-continent collision and closure of the Iapetus Ocean. d

The Late Proterozoic Torridon Group Applecross Formation in the foreland of the Moine Thrust Belt, NW Scotland, contains deformation bands, three fracture sets (from oldest to youngest A, B, and L) defined by orientation, crosscutting relations, and progressively less quartz cement in younger sets, and joints. Set A crosscuts deformation bands and strikes north-south. Set B has trimodal orientation defining three linked subsets that formed concurrently. Set L strike ranges from NE-SW to ENE-WSW, in parent crack-wing crack arrays that formed progressively; these are more abundant near small-displacement, oblique-slip faults that offset the overlying Cambrian Eriboll Formation and the Moine Thrust Belt. Applecross sandstones have low fracture abundance, possibly a consequence of low elastic moduli (Young's modulus 2.3-17.0 GPa, most values <6.9 GPa) and moderate to high subcritical crack index (45-78), resulting from compacted soft lithic grains and clay-mineral cements. Low abundance contradicts models that postulate persistent incipient failure by subsurface fracture. The fracture sequence resembles that found in the overlying Cambrian Eriboll Formation quartzarenites, implying that no widespread late Proterozoic fracture sets exist in this part of the Applecross Formation, an uneventful record for a rock profoundly resistant to brittle deformation.

Cambrian Eriboll Formation sandstones of the Ardvreck Group that crop out west of the Moine Thrust Zone contain joints and quartz-filled or quartz-lined fractures that resemble cemented joints. Of the fractures containing quartz, five sets strike north, NW to WNW, NE, west and north; according to crosscutting relations this is a progression from the oldest to the youngest set. Sets include opening-mode microfractures, partly visible in transmitted light as fluid-inclusion planes and sharply defined as microveins using SEM-based cathodoluminescence (CL). Dating the oldest north-striking set, using inferred quartz accumulation rates, fluid inclusions and burial history, suggests that these fractures mark a Palaeozoic east-west least horizontal stress trajectory in Laurentia. The youngest two sets of porous fractures are associated with faults that cut and postdate the Moine Thrust Zone. Data indicate that at depth in basins, pervasive fracture systems arising from discrete loading events are ephemeral owing to fracture porosity destruction by cementation.

The Moine Thrust Zone in the Scottish Highlands developed during the Scandian Event of the Caledonian Orogeny, and now forms the boundary between the Caledonian orogenic belt and the undeformed foreland. The Scandian Event, and the formation of the Moine Thrust Zone, have previously been dated by a range of isotopic methods, and relatively imprecise ages on a suite of alkaline intrusions localized along the thrust zone have provided the best age constraints for deformation. Recent British Geological Survey mapping has improved our understanding of the structural relationships of some of these intrusions, and this work is combined with new U-Pb dates in this paper to provide significantly improved ages for the Moine Thrust Zone. Our work shows that a single early intrusion (the Glen Dessarry Pluton) was emplaced within the orogenic belt to the east of the Moine Thrust Zone at 447.9±2.9 Ma. A more significant pulse of magmatism centred in the Assynt area, which temporally overlapped movement in the thrust zone, occurred at 430.7±0.5 Ma. Movement in the thrust zone had largely ceased by the time of emplacement of the youngest intrusions, the late suite of the Loch Borralan Pluton, at 429.2±0.5 Ma, and the Loch Loyal Syenite Complex.

Precambrian sedimentary successions are difficult to date and correlate. In the Scottish Highlands, potential correlations between the thick, undeformed siliciclastic \"Torridonian\" successions in the foreland of the Caledonian Orogen and the highly deformed and metamorphosed siliciclastic Moine succession within the Caledonian Orogen have long intrigued geologists. New and detailed mapping of the Neoproterozoic Altnaharra Formation (Morar Group, lowest Moine Supergroup) in Sutherland has discovered low-strain zones exhibiting well-preserved sedimentary features. The formation comprises 3-5 km of coarse, thick-bedded psammite with abundant nested trough and planar cross-bedding bedforms, defining metre-scale channels. Palaeocurrent directions are broadly unimodal to the NNE-ENE. We interpret the Altnaharra Formation as high-energy, braided fluvial deposits. The Altnaharra Formation and the unmetamorphosed, Neoproterozoic Applecross-Aultbea formations (Torridon Group) are similar in terms of lithology, stratigraphical thickness, sedimentology, geochemistry, detrital zircon ages and stratigraphical position on Archaean basement. Depositional age constraints for both successions overlap and are coeval with late Grenvillean orogenic activity. Detrital zircons imply similar source regions from the Grenville Orogen. The Morar and Torridon groups can thus be correlated across the Caledonian Moine Thrust and are best explained as parts of a single, large-scale, orogen-parallel foreland basin to the Grenville Orogen.

The postassembly, postrift evolution of passive margins is an essential element of global continental tectonics. Thermal and exhumational histories of passive margins are commonly attributed to a number of drivers, including uplift and erosional retreat of a rift-flank escarpment, intraplate fault reactivation, mantle-driven uplift, and erosional disequilibrium, yet in many cases, a specific factor may appear to dominate the history of a given passive margin. Here, we investigate the complex evolution of passive margins by quantifying exhumation patterns in western Scotland. We build upon the well-studied thermal evolution of the Scottish North Atlantic passive margin to test the importance of spatially heterogeneous factors in driving postorogenic burial and exhumation. Independent investigations of the cooling history from seven different field sites across the western Scottish Highlands using radiogenic apatite helium thermochronometry ([U-Th]/He; n = 14; ca. 31-363 Ma) and thermal modeling confirm that post-Caledonian heating and burial, as well as cooling and exhumation, must have been variable across relatively short distances (i.e., tens of kilometers). Heating associated with Paleogene hotspot activity and rifting locally explains some of this spatial variation, but additional drivers, including margin tilting during rifting, vertical separation along reactivated faults, and nonuniform glacial erosion in the late Cenozoic, are also likely required to produce the observed heterogeneity. These results indicate that passive margins may experience variable burial, uplift, and erosion patterns and histories, without exhibiting a single, dominant driver for behavior before, during, and after rifting.

Basement gneiss inliers within the Scottish Caledonides have been conventionally correlated with the Archaean Lewisian Gneiss Complex of the Caledonian foreland. Alternatively, the inliers could represent allochthonous terranes accreted to Laurentia before or during the Caledonian orogeny. Secondary ionization mass spectrometry U-Pb zircon dating indicates that the Ribigill, Borgie, Farr and Western Glenelg basement inliers are characterized by late Archaean protolith ages, and a period of isotopic disturbance in the late Palaeoproterozoic. The data are broadly consistent with correlation between the inliers and components of the Lewisian Gneiss Complex of the Caledonian foreland. The c. 2900 Ma protolith ages support correlation of the Borgie and Farr inliers with the Assynt terrane, and a younger, c. 2800 Ma age for the Ribigill inlier supports correlation with the Rhiconich terrane. None of the studied inliers shows a complete match of protolith and early metamorphic histories with any of the Lewisian basement terranes, but differences between the inliers and the foreland are no greater than those recorded within the foreland basement terranes themselves. Therefore, it remains probable that the dated inlier gneisses formed a distal part of the Laurentian margin prior to final telescoping during the Caledonian orogeny.

Pseudotachylites within the Neoarchaean to Palaeoproterozoic basement of the Lewisian Gneiss Complex have been analysed using the high spatial resolution 40Ar/39Ar UV laserprobe technique. It has been possible to measure 40Ar/39Ar age and \"apparent' age variations that would be beyond resolution by lower spatial resolution or bulk 40Ar/39Ar dating methods. We report a range of complexities including significantly younger narrow (<1 mm) margins on much older pseudotachylite veins, and heterogeneous excess argon contamination. We also report pseudotachylite veins of varying magnitudes that have been subject to later thermal reheating, whereby there is a relationship between vein thickness and extent of argon loss. In spite of these it is still possible to determine geologically meaningful 40Ar/39Ar ages for pseudotachylite-forming events. In Lewis the c. 1900 Ma event may relate to early stages of Laxfordian reworking. In Barra and South Uist a 1300-1200 Ma event is recorded, whereas in Barra, South and North Uist a second event is recorded at c. 700 Ma, which is likely to be associated with NW- and west-directed thrusting, and could represent far-field effects of tectonic events recorded at this time east of the Moine Thrust in NW Scotland.