Moine Thrust Research Articles

Peach and Horne: the British Association excursion to Assynt September 1912

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.

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 Scotland's 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.

High spatial resolution 40 Ar/ 39 Ar dating of pseudotachylites: geochronological evidence for multiple phases of faulting within basement gneisses of the Outer Hebrides (UK)

Abstract: Pseudotachylites within the Neoarchaean to Palaeoproterozoic basement of the Lewisian Gneiss Complex have been analysed using the high spatial resolution 40 Ar/ 39 Ar UV laserprobe technique. It has been possible to measure 40 Ar/ 39 Ar age and ‘apparent' age variations that would be beyond resolution by lower spatial resolution or bulk 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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. Supplementary material: 40 Ar/ 39 Ar data are available at http://www.geolsoc.org.uk/SUP18372 .

K‐feldspar sand‐grain provenance in the Triassic, west of Shetland: distinguishing first‐cycle and recycled sediment sources?

AbstractSandstone provenance studies can help constrain palaeogeographic reconstructions and ancient drainage system scales and pathways. However, these insights can be obscured by difficulties in geochemically distinguishing or adequately characterizing potential sourcelands, or by failure to identify sedimentary recycling. Triassic basins west of Shetland accumulated ∼2.5 km of sand‐rich sediment. The Middle‐Upper Triassic Foula Formation represents fluvial, aeolian and sabkha facies deposited in the northern interior of the Pangaean supercontinent. Published U‐Pb zircon geochronology and heavy mineral analysis suggest that these sandstones were derived from East Greenland. They contain significant fresh K‐feldspar which is likely to be first cycle and derived directly from its source. Pb isotopic analyses of individual K‐feldspar sand‐grains show a single, unradiogenic Pb population, consistent with the provenance indicated by U‐Pb zircon geochronology. Archaean and Palaeo‐Mesoproterozoic rocks—the Nagssugtoqidian Mobile Belt, the Lewisian Complex or equivalents—are the likely source, with terranes south of the Moine Thrust (Grampian, Caledonian and Variscan) ruled out by both the Pb and U‐Pb data. However, it is not possible to distinguish between rift flank sources to the east and west, as both areas have similar crustal affinity and/or share the same tectonic history. It is possible that the sediment was derived from the West Shetland Platform and not from Greenland. The comparison of provenance signals from robust and less stable mineral phases provides a means of recognizing sedimentary recycling. Robust zircon populations and less stable feldspar in Foula Formation sandstones concur in indicating the same source, suggesting that they are likely to be first cycle. The Triassic sand supply can be contrasted with that in Upper Carboniferous (Namurian) basins in the north of England where a significant zircon population has no corresponding K‐feldspar component. This zircon population is likely to have been recycled from Lower Palaeozoic greywackes from the Southern Uplands Belt or it's along strike extension. Copyright © 2009 John Wiley & Sons, Ltd.

Understanding kinematic data from the Moine thrust zone in terms of a kinematics-based mathematical model of deforming thrust wedges

A salient–recess pair along the Moine thrust zone exhibits significant lateral, kinematic variations. Specifically, the more steeply dipping portion of the thrust surface, associated with the recess, shows evidence for greater strain magnitudes and more out-of-the-plane motion than the adjoining salient. Additionally, a subset of the grain-shape data has grain-shape approximations with their long axes oriented perpendicular to the regional transport direction. To more fully understand these kinematic features, we developed a kinematics-based mathematical model of thrust wedge deformation. Our numerical model predicts an increase in strain magnitude as well as non-plane-strain flattening in the region of a thrust ramp, consistent with the strain patterns observed within the recess along the Moine thrust zone. Furthermore, in the absence of a lateral-confining boundary condition, much of the model thrust wedge experiences transport-perpendicular, maximum extension except for the region closest to the thrust where simple-shear deformation is dominant. Using our numerical model, we predict the amount of lateral-confining strain needed to induce plane-strain deformation ( ɛ ps) as well as the amount of lateral-confining strain necessary to make the long axis of the strain ellipsoid parallel with the transport direction ( ɛ parallel). We have found that the incremental ɛ parallel can be as much as 42% of the incremental strain magnitude. Thus, we consider transport-perpendicular lineations like those seen along the Moine thrust zone to be the result of lateral-confining boundary conditions that are weak enough to allow transport-perpendicular maximum extension.

Nature of the Moho beneath the Scottish Highlands from a receiver function perspective

The geology of Scotland documents a protracted geological history from Precambrian basement formation through the Caledonian Orogeny and to the Tertiary opening of the Atlantic. A temporary deployment of 22 closely spaced broadband seismic stations in Scotland traverses many of the major terrane boundaries in the region and provides a unique opportunity to place spatial and temporal constraints on variations in crustal structure. With teleseismic P-wave receiver functions, we use the H − κ method to determine variations in bulk crustal parameters: crustal thickness ( H) and V P/ V S ratio. Mean crustal thickness is ~ 28 km, varying from ~ 23 km in the NE highlands and increasing to > 30 km near the Highland Boundary Faultin the southern part of the study area. Mean V P/ V S values of ~ 1.76 show no significant variation across the study area. An abrupt increase in crustal thickness of ~ 4.5 km NW across the Moine Thrust is not easily linked to Tertiary–Recent tectonic activity. Instead it appears that Scotland's crust has retained features for hundreds of millions of years since it was first formed, despite abundant volcanic activity in Tertiary times. Our work favours the view that the Moho is a compositional boundary rather than a mineralogical one defined by a reaction in P– T space.

Eclogite-facies rocks: a window into geodynamics and lithospheric evolution: a celebration of the career of Tony Carswell

This set of six papers arises from the International Eclogite Field Symposium held in Scotland during July 2007 (IEFS2007). A second set of papers from IEFS2007 has been published elsewhere (see Dobrzhinetskaya et al . 2008). The meeting was one of a biennial series organized under the auspices of the International Eclogite Conference Co-ordinating Committee, and marked the 25th anniversary of the first meeting in 1982. Seventy-five delegates from 20 countries gathered in Portree on the Isle of Skye. The field element of the meeting focused upon the nearby Glenelg–Attadale Inlier, a basement unit of the Moine Nappe and part of the classic Moine Thrust system in the western Highlands (see symposium field guide: Storey 2008 a , b ). This was the first place in the UK where eclogites (high-pressure metabasaltic garnet–clinopyroxene rocks) were recognized (Teall 1891), and has attracted several prominent researchers in metamorphic and structural geology (e.g. Tilley 1936; Ramsay 1958; Sutton & Watson 1959; O'Hara 1960). Although one of the smaller eclogite-facies terrains in global terms, the Glenelg–Attadale Inlier is significant in that it is one of the very few examples of Precambrian high-pressure metamorphism (Sanders et al . 1984; Brewer et al . 2003; Storey et al . 2004). IEFS2007 celebrated the life and work of Dennis Anthony (Tony) Carswell, who sadly died on 8 June 2006 while still very much at the height of his intellectual powers. Tony was active in research in eclogite-facies rocks from the early 1960s, and was consistently at the forefront of developments in the field, publishing over 120 papers on the subject with an astonishing total of 85 different co-authors (see Dobrzhinetskaya et al . 2008 for a more extensive appreciation of his career). He edited the book Eclogite Facies Rocks (Carswell 1990), which …

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.

Laurentian palaeostress trajectories and ephemeral fracture permeability, Cambrian Eriboll Formation sandstones west of the Moine Thrust Zone, NW Scotland

Abstract: 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.

Petrogenesis of high Ba–Sr plutons from the Northern Highlands Terrane of the British Caledonian Province

A variety of high Ba–Sr granites and syenites is intruded into the Caledonian terrane of the Northern Highlands of Scotland. Volumetrically rather small, the plutons display a considerable range of compositions from ultramafic and mafic to granitic and syenitic. Most intrude Proterozoic Moine metasediments, though some are exposed within the Moine Thrust Zone in the west, beyond which lies the Archaean Lewisian foreland. Although it is generally agreed that they are related in some way to the subduction of Iapetus Ocean crust beneath Laurentia, their petrogenesis is much debated, especially the enigmatic relationship between alkaline syenites and calc-alkaline granites. New isotope (Sr–Nd–O) data suggest that both may be derived from mantle sources that include small fractions of subducted pelagic sediment. Juvenile material is volumetrically dominant, although subsequent fractionation involved contamination with local continental crust. Direct derivation from a mafic underplate is unlikely given the absence of pronounced HREE and Y depletion, the major element dissimilarity with adakitic magmas, and the presence of coeval and geochemically-contiguous mantle-derived rocks. The parent magmas were derived from a Caledonian Parental Magma Array (CPMA) that extended from isotopically depleted to significantly enriched compositions, with high δ 18O in the latter. The oxygen isotope values and the high Ba–Sr signature in particular strongly suggest that mantle contamination with subducted pelagic carbonates generated the CPMA, and Nd–Sr isotopes constrain the sediment contribution to less than 10%. Differentiation of the parent magmas proceeded by fractional crystallisation and variable concurrent assimilation of small magma batches, involving different crystallising assemblages and different continental reservoirs. Caledonian syenites evolved by extensive pyroxene-dominated fractionation sometimes involving contamination with Lewisian granulites. On the other hand, the granites fractionated amphibole and plagioclase, during which most were contaminated with local Moine metasediment. The extended time interval over which the syenites were intruded (30 Ma) accords with continuing Iapetus subduction beneath the Laurentian margin, while the late pulse of isotopically-variable appinites and granites at about 425 Ma sits comfortably with Scandian slab breakoff. However, the high Ba–Sr granites studied here represent small-volume assimilation-fractionation products of mantle-derived appinitic magmas, rather than the voluminous melts of lamprophyre-underplated crust also predicted by slab breakoff, which are absent from the Northern Highland Terrane.

A fluvial origin for the Neoproterozoic Morar Group, NW Scotland; implications for Torridon–Morar Group correlation and the Grenville Orogen foreland basin

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.

Sensitivity of shear zones in orogenic wedges to surface processes and strain softening

We investigate the internal deformation of orogenic wedges growing by frontal accretion with a two-dimensional numerical model. Our models are limited to crustal deformation and assume a horizontal detachment as observed for various natural orogens (e.g. Alaska and Costa Rica). The model wedges develop as a result of convergence of a brittle sediment layer in front of a strong backstop. We find that our reference model develops in-sequence forward-thrusts which propagate upward from the basal detachment. For this reference model we investigate the sensitivity of shear zone activity to surface processes and strain softening. Model results show that diffusive or slope dependent erosion enhances material transport across the wedge and slows down forward propagation of the deformation front. Frictional strain softening focuses deformation into narrow shear zones and enhances displacement along them. This has also been postulated for natural thrusts such as the Glarus thrust in the Swiss Alps and the Moine thrust in the Scottish Caledonides. A second series of models investigates the effects of regularly spaced weak inclusions within the sediment layer which simulate remnants of previous deformation phases. These inclusions facilitate and focus internal deformation, influence the thrust dip and thrust vergence and enable thrust reactivation in the internal part of the wedge. Our results show that inactive thrusts in the internal part of the wedges may be reactivated in models with diffusive surface processes, strain softening or weak inclusions. Thrust reactivation occurs as models seek to maintain their critical taper angle. First order characteristics of our numerical models agree well with natural orogenic wedges and results from other numerical and analogue models.

Structural evolution of the Achnashellach Culmination, southern Moine Thrust Belt: testing the duplex model

Abstract The Achnashellach Culmination is one of the major structures of the Moine Thrust Belt. As with other culminations in the belt, it is formed by a stack of imbricate thrusts. Up to 1 km of Torridon Group sediments, together with a further 200–250 m of Cambrian strata, are repeated up to 10 times, but with ramp-on-ramp thrust geometries. Thus structural thickening was chiefly achieved by thick thrust sheets with individually and aggregated displacements that are substantially lower than elsewhere in the thrust belt. The culmination is limited on its flanks by lateral ramps that climb section out of Torridon Group and up into Cambrian strata. To the north the imbricate thrusts may be deduced to branch onto the major Kinlochewe Thrust. To the south the imbricates are represented only by stacked Durness Limestone. The northward-climbing lateral ramp coincides with a major Precambrian structure, the Loch Maree Fault, which controls the thickness of Torridonian strata preserved beneath the sub-Cambrian unconformity, a rare example of basement influence on thrust system geometry within the Moine Thrust Belt. The imbricates of the Achnashellach Culmination show back-steepening and have bulged up the overriding Kishorn and Kinlochewe thrust sheets. However, these structurally higher level tectonic units slice across imbricate structures in their footwalls. Elsewhere high-level thrusts are folded by some parts of underlying imbricates. Collectively these relationships are not compatible with classical duplex models. They are explained better by models of quasi-synchronous slip on imbricate thrusts. Discordant relationships beneath major thrust sheets, including those that cut down stratigraphic section in the transport direction, can be explained by such models without necessitating low-angle extensional faulting within the thrust belt.

Tectonic stratigraphy and structural continuity of the northernmost Moine Thrust Zone and Moine Nappe, Scottish Caledonides

Abstract The Moine Thrust Zone and overlying Moine Nappe represent classic ground in Scottish geology in which Neoproterozoic Moine metasedimentary rocks were translated towards the WNW over the Laurentian foreland comprising Lewisian basement gneisses and a Cambrian cover succession during Caledonian orogenesis at c. 430 Ma. Systematic mapping of both the Moine Nappe and underlying Moine Thrust Zone in the Loch Hope to Whiten Head region of Sutherland extends the tectonic stratigraphy developed in other parts of the thrust zone to the little-studied north coast section. An apparent continuum of deformation is recognized from the Moine Nappe down into the underlying thrust zone with a consistent gently ESE-dipping foliation associated with down-dip mineral and extension lineations. The polyphase fold history present within the Moine Nappe and the thrust zone results from progressive deformation during thrusting. Thrust-related fold axes and bedding-cleavage intersections are typically subparallel to the transport direction in high-strain zones, whereas gently north- or south-plunging attitudes are common in lower strain portions of the thrust zone. Thrusts are commonly deformed by folds developed within their footwalls, consistent with a foreland-propagating sequence of deformation. The newly defined Lochan Riabhach Thrust is interpreted as an ‘out-of-sequence’ structure that sliced at low angles across the thrust zone and toward the WNW after initial thrust stacking. Comparison with the downfaulted segment of the Moine Nappe and thrust zone exposed on the foreland at Faraid Head demonstrates cross-strike continuity of the thrust sheet template and structural history for at least 15 km to the NW.

About this title

This Special Publication, in memory and celebration of the work of Professor Mike Coward, is about the deformation of the continental lithosphere. The collected papers discuss geometry, structural principles, processes and problems in a wide range of tectonic settings and thereby reflect the breadth of Coward's interests. They encompass the evolution of Precambrian basement gneiss terrains, the geometry and evolution of thrust systems, basement involvement and structural inheritance in basins, syn-orogenic extension, salt tectonics, the implication of structural evolution on hydrocarbon prospectivity and structural controls on mineralization. Examples are drawn from the Lewisian and Moine Thrust Belt of NW Scotland, the Italian Apennines, NW Himalayas, the Cyclades, Oman, Zagros Mountains, Colombian Cordillera, Carpathians, North Sea, offshore Brazil, regional studies of the Irumide Belt (central Africa), Taurus Mountains (Turkey), greater South America, and from the Witwatersrand Basin of South Africa and the Antler Orogeny of SW USA.

Fractal Modelling of the Microstructure Property of Quartz Mylonite During Deformation Process

Mylonite is the result of the dynamic metamorphism and minerals in mylonite are deformed gradually with an increase in the degree of metamorphism. Quantifying the degree of deformation including the irregularities of shapes and the frequency distribution of the minerals becomes one of the most challenging efforts in mylonite analysis. Fractal modelling has been demonstrated in this paper to be an effective mean to achieve the above goal. Perimeter-Area fractal model was used to quantify the irregularities in the geometries and Cumulative Number-Area model is used to characterize the irregularities of distribution of quartzs in mylonites, respectively. Examples of quartz from five types of mylonites with different degree of deformation within the foreland of the Moine Thrust Zone in NW Scotland are chosen to study the evolution processes of deformation. As the main mineral component of quartzite mylonite, patterns are extracted from digital photomicrographics of the multiscale-grey image grid data to show quartz grains with different degree of deformation, The areas and perimeters of the quartz grains were calculated by GIS-based image processing technologies. From type one to type five, with an increase in degree of deformation, the corresponding Perimeter-Area exponent \({D_{\rm AP}}\) increases from 1.20, 1.28, 1.38, 1.46, to 1.60, respectively, the fractal dimension \({D_{\rm P}}\) of the perimeter from 1.07, 1.08, 1.17, 1.23, to 1.44, as well as the exponent of Cumulative Number- Area from 0.50, 0.51, 0.58, 0.82, to 0.85, respectively. The result has shown that as increase of the intensity of deformation, the shape of quartz grains tends to be more irregular, grain size tends to be smaller, and the number of grains increases. The results obtained using GSI model has indicated that as an increase in the intensity of deformation, the patterns of quartz grains tends to be more stratified and randomness increases.

Field relationships and emplacement of the Caledonian Ross of Mull Granite, Argyllshire

Synopsis The Ross of Mull granite (ROMG) occupies the largest part of the pre-Tertiary inlier of the Ross of Mull peninsula. Previous ideas that the Moine Thrust crops out in the Sound of Iona have now been discounted. Recent models have proposed that: (a) the Sound of Iona Fault and the associated mylonites on Iona are part of a steeply inclined extensional fault throwing down to the ESE, bringing high grade Sgurr Beag or Knoydart Nappe Moine rocks to the same erosion level as the foreland units on Iona; and (b) one of the main Caledonian thrusts must extend to a point no more than 10–15 km from the projected trace of the Great Glen Fault. A hypothetical intersection would occur SW of Mull and it is proposed here that the intersection of these two structures has been the dominant control on the siting of the ROMG. In this model, magmas using this intersection as an ascent site were emplaced eastwards as flattish sheets within an overall east-dipping extensional zone that has high-grade Moine lithologies in its hanging wall. The present erosion level provides a section through a granitoid complex from a roof zone with flattened diorite enclaves and abundant country rock xenoliths to a clearly demonstrable westward-dipping floor; a situation which may be unique in the British Caledonian granitoid complexes.

Thrust sequences and the significance of low-angle, out-of-sequence faults in the northernmost Moine Nappe and Moine Thrust Zone, NW Scotland

In common with most thrust belts, the Moine Thrust Zone, Scotland, preserves a foreland-propagating sequence of thrusting. However, significant examples of local to regional overstep sequences are also recognized and their interpretation remains controversial. Remapping of the northernmost Moine Thrust Zone has defined a thick belt of mylonites in the centre of which lies the ductile WNW-directed Moine Thrust. The base of the mylonite belt, however, is marked by a regionally significant, low-angle, west-directed brittle fault, here termed the Lochan Riabhach Thrust, that truncates thrust-related structures in its footwall. Comparison of the tectonic stratigraphy of the mylonite belt with that preserved in a downfaulted outlier to the west at Durness indicates that the Lochan Riabhach Thrust cuts down-section in its footwall and therefore appears to be extensional. Locally, it is unequivocally breached by underlying imbricate thrusts. We prefer to interpret the Lochan Riabhach ‘Thrust’ as an extensional fault that formed during a hiatus in thrust stacking. We relate this faulting episode to the changes in the stability of the evolving orogenic wedge and reiterate that such extensional faults may be common, although possibly unrecognized, features in many other thrust belts.

Constraining the maximum age of movements in the Moine Thrust Belt: dating the Canisp Porphyry

Synopsis The Moine Thrust Belt of the Northwest Highlands of Scotland represents the front of the Caledonian Orogeny. The exact age of this thrust belt is the source of continuing debate, and has not been fully resolved either by dating of igneous rocks within the thrust belt or by direct dating of mylonites. The Canisp Porphyry sills are porphyritic quartz–microsyenite intrusions, found in the Foreland to the Moine Thrust Belt in the Assynt region. The sills occur close to the lowest thrust, but are never seen within the thrust belt. They are thus considered to predate movement on the lowest thrusts. This paper presents a U–Pb (TIMS) zircon date of 437 ± 4.8 Ma for the Canisp Porphyry, and thus movement on the lower thrusts of the Moine Thrust Belt is constrained as having occurred after that date.

Multiple fluid migration events along the Moine Thrust Zone, Scotland

A palaeomagnetic study of different lithologies exposed along the Moine Thrust Zone from Skye to Durness, Scotland, has identified three chemical remanent magnetizations (CRMs), a potential fourth CRM, and one possible primary component that all reside in hematite. Local and regional fold tests suggest that the CRMs are post-folding and post-thrusting. A contact test in the Torridon Group sandstone indicates that a Permian CRM is localized in and near the fault zone. The Permian CRM, which is also found in the Durness Group, and associated alteration are direct evidence for post-orogenic activity, in which the thrusts vented excess heat during regional crustal extension. On the Isle of Skye, sandstones (Sleat, Torridon and Eriboll Groups) in and around the fault zone contain a Tertiary CRM interpreted to be related to hydrothermal fluids associated with Tertiary intrusions. A possible Mesozoic CRM is also present but it could represent the vector addition of the Permian and Tertiary CRMs. A Devonian CRM is interpreted to be related to hydrothermal fluids associated with Devonian volcanic rocks or to fluids triggered by late stages of the Caledonian orogeny. Geochemical and petrographical studies provide supporting evidence for multiple fluid flow events along the Moine Thrust Zone. These results show that palaeomagnetism can be used to date multiple fluid migration events and that dormant fault zones can act as conduits for flow.