Moine Thrust Research Articles

Reply to the comments by Alsop and Holdsworth on “Evidence for non-plane strain flattening along the Moine thrust, Loch Strath nan Aisinnin, North-West Scotland”

Reply to the comments by Alsop and Holdsworth on “Evidence for non-plane strain flattening along the Moine thrust, Loch Strath nan Aisinnin, North-West Scotland”

Discussion on “Evidence for non-plane strain flattening along the Moine thrust, Loch Strath nan Aisinnin, North-West Scotland” by Mathew Strine and Steven F. Wojtal

Discussion on “Evidence for non-plane strain flattening along the Moine thrust, Loch Strath nan Aisinnin, North-West Scotland” by Mathew Strine and Steven F. Wojtal

Initiation of basement thrust detachments by fault-zone reaction weakening

Abstract This paper examines how crystalline basement thrust sheets can detach in foreland thrust belts, in terms of the deformation mechanisms and rheological evolution of the detachment fault zones. Basement thrust fault zones of the Moine Thrust Belt and the external Western Alps show relatively narrow thrust zones considering the large displacements accommodated. Microscopic examination of fault rocks from these high strain thrust zones show that syntectonic alteration of fractured feldspars to white mica of strong preferred orientation generated ultramylonites deforming by diffusion creep and other viscous deformation mechanisms, similar to documented basement thrust zones in North America. Motivated by these observations coupled with other published examinations of foreland basement thrust zones, and recent developments in crustal hydrology, a conceptual model is proposed to explain basement detachment formation and evolution. Meteoric fluid that percolated into a previously fractured upper crust is drawn into developing fault zones by dilatancy pumping during the early stages of thrust-related deformation. The generation of cataclastic fault rocks with fresh fracture surfaces by microfracturing enhances the rate of fluid-rock interaction. Syntectonic alteration causes a deformation-mechanism transition to phyllosilicate-dominated ductile fault-rock rheologies, resulting in a large ductility contrast between host rock and fault zone that inhibits growth of the zone into the wall rock and weakens the thrust. Deformation becomes focused into these weakened early thrust zones so that they become zones of high strain, preventing the development of other newer fault zones elsewhere. This model explains the detachment and continued sliding of basement thrust sheets on narrow mica-rich zones of high strain in foreland thrust belts, and suggests that reaction weakening of the basement is important in decreasing the strength of the foreland crust in orogenic wedge evolution.

Late-Proterozoic tectonics in northwest Scotland: one contractional orogeny or several?

The Glenelg-Attadale Inlier of the northwest Highlands of Scotland comprises a Western Unit (WU) containing Trondhjemite-Tonalite-Granodiorite (TTG) gneisses and subordinate mafic bodies and an Eastern Unit (EU) containing abundant mafic rocks (eclogite), minor ultrabasic bodies and manganiferous metasedimentary rocks and marbles intercalated with intermediate to acid gneiss. The boundary between these two units is an amphibolite facies ductile shear zone, which contains kinematic criteria indicating a top-to-the-west (contractional) sense of shear. The EU was exhumed from eclogite facies to amphibolite facies conditions of the mid-crust at ca. 1000 Ma via amphibolite facies ductile shearing. A series of shear zones with identical metamorphic grade and kinematics occur at the contacts of the EU with the WU and the surrounding Moine Supergroup. In this study, an attempt is made to directly date the shear zone between the EU and WU, which also contains Moine Supergroup metasediments. A U–Pb titanite age of 669 ± 31 Ma, from syn-kinematic titanite within a mylonite, indicates a minimum age for movement on this shear zone, demonstrating that it must be Late-Proterozoic in age, and not Caledonian as previously supposed. If the shear zone is Grenvillian (ca. 1000 Ma) in age, this would then require that the Moine be deposited before ca. 1000 Ma in order to undergo burial to the mid-crust at this time. However, the youngest detrital zircons in the Moine indicate a maximum deposition age of 1000 Ma and, therefore, preclude this possibility. An alternative possibility is that movement along this shear zone was punctuated, with an initial phase involving uplift of eclogites to the mid-crust at ca. 1000 Ma, followed by a further period of top-to-the-west shearing, coupling of the Moine Supergroup and EU in the mid-crust during a putative Neoproterozoic “Knoydartian” orogenic event. However, since there are now three contractional “orogenic” events affecting the Moine, ca. 820, 740 and 670 Ma, it is suggested here that the term “Knoydartian” is no longer well-defined and requires re-appraisal. A later phase of extensional ductile shearing and large-scale folding, during a period of lower amphibolite facies metamorphism, is shown to be broadly Caledonian, based on a 437 ± 6 Ma U–Pb titanite age from a pegmatite, that is deformed during this event, and prior to movement along the Moine Thrust Zone. This further suggests an Ordovician “Grampian” orogenic event within the Northwest Highlands of Scotland.

The nature of ‘roof thrusts’ in the Moine Thrust Belt, NW Scotland: implications for the structural evolution of thrust belts

Buried thrust systems, commonly analysed in terms of duplex structures, are often interpreted as forming in a particular sequence. This model considers that only one thrust is active at a time and the resulting structural geometry is well ordered. However, a reappraisal of structural geometries from the Moine Thrust Belt in NW Scotland has shown that many apparent duplex structures are more complex. Although the upper (roof) thrust of some duplexes may be folded and bulged by underlying imbricate slices (in accordance with simple duplex models), the footwall to these roof thrusts cut up and down stratigraphic section (in conflict with the simple duplex models). Examples include the Glencoul Thrust with its underlying imbricate system, and the Foinaven ‘duplex’. Simultaneous slip on an array of imbricate thrusts can bulge duplex roofs (relationships classically used to infer piggy-back sequences of thrust ‘propagation’). However, activity on more hinterland-ward imbricate thrusts can cause the roof onto which they branch to truncate more foreland-ward structures. In this fashion, the ‘duplex roof’ can cut up and down stratigraphic section, mimicking an extensional fault. By varying thrust activity in three dimensions within an imbricate system, local parts of ‘roof thrusts’ can generate domains of overstep behaviour that have only local significance. Existing interpretations of overstep relationships in the Moine Thrust Belt as representing late-orogenic low-angle extensional faults may need revision. The new interpretations of deformation activity, where displacements in thrust arrays happen synchronously rather than sequentially, are consistent with emergent thrust and fold belts, with analogue experiments and with mechanical models of orogenic wedges.

The minor intrusions of Assynt, NW Scotland: early development of magmatism along the Caledonian Front

AbstractThe Assynt Culmination of the Moine Thrust Belt, in the northwest Scottish Highlands, contains a variety of Caledonian alkaline and calc-alkaline intrusions that are mostly of Silurian age. These include a significant but little-studied suite of dykes and sills, the Northwest Highlands Minor Intrusion Suite. We describe the structural relationships of these minor intrusions and suggest a classification into seven swarms. The majority of the minor intrusions can be shown to pre-date movement in the Moine Thrust Belt, but some appear to have been intruded during the period of thrusting. A complex history of magmatism is thus recorded within this part of the Moine Thrust Belt. New geochemical data provide evidence of a subduction-related component in the mantle source of the minor intrusions.

Lateral ramps and thrust terminations: an example from the Moine Thrust Zone, NW Scotland

Detailed remapping within the Assynt Culmination in NW Scotland shows that the Ben More Thrust joins the Glencoul Thrust along a lateral branch line positioned in Loch Glencoul. This branch line marks the northern lateral termination of the Glencoul Thrust. The thrust on the northern shore of Loch Glencoul (‘the famous view’) is thus the Ben More Thrust. The Ben More Thrust Sheet is therefore much larger than previously thought. The lateral terminations of the Ben More Thrust to the north and the south are of two distinct types. This reinterpretation negates the existence of ‘surge’ zones in Assynt.

Kaolin polytype evidence for a hot-fluid pulse along Caledonian thrusts during rifting of the European Margin

AbstractSedimentary basins developed along the European margin during the earliest, Permian, stage of proto-Atlantic rifting, during a phase of high heat flow. The proximity of some basins to Caledonian thrusts has implied that rifts locally utilized the basement fabric. New mineralogical and palaeomagnetic data show that thrust planes in the Moine Thrust Zone channelled a pulse of hot fluid in Permian time. The fluids precipitated kaolin in fractures in the thrust zone, and with decreasing intensity away from the zone. The high-temperature polytype dickite is largely confined to major thrust planes. Stable H and O isotope analyses indicate that the parent fluid included meteoric water involved in a hydrothermal system. Coeval hydrothermal hematite has a chemical remanence that dates the fluid pulse as Permian. This is direct evidence for post-orogenic activity in the thrust zone, in which the thrusts vented excess heat during regional crustal extension. The example from the European margin exemplifies the importance of deep-seated structures in the release of heat, and the value of kaolinite polytype mapping as a tool to record anomalous palaeo-heat flow.

Evidence for non-plane strain flattening along the Moine thrust, Loch Srath nan Aisinnin, North-West Scotland

We report quartz c-axis patterns, grain-shape fabrics, and microstructures for 11 mylonitic quartzites and quartz-phyllosilicate schists from a transect across the Moine thrust at Loch Srath nan Aisinnin, North-West Scotland. In the footwall samples collected more than 42 m normal distance from the thrust surface, quartz c-axis textures indicate a general flattening strain (i.e. 0< k<1). Samples within 19 m normal distance of the thrust are completely recrystallized and exhibit asymmetric c-axis patterns. Recrystallized hanging wall fault rocks exhibit random c-axis patterns on the scale of a standard thin section. Relict footwall grains provide the closest approximation of finite strain; they have octahedral shear strains ( ϵ s) between 1.10 and 1.47 and exhibit general flattening k-values (0.0524–0.659). The long axis of the mean relict grain shape trends parallel to the regional transport direction and plunges gently to the ESE. In contrast, recrystallized footwall grains have a mean grain shape with the longest axis oriented nearly perpendicular to the transport direction. Furthermore, these samples have grain shape k-value ranges from 0.157 to 0.295. Recrystallized hanging wall grain shapes exhibit the lowest octahedral shear ‘strains’ ( ϵ s=0.532–0.733) and largest mean k-values (0.351–0.961) of this sample set. The long axes of the mean recrystallized hanging wall grain shapes are parallel to transport, similar to that of relict footwall grains. Unrecrystallized quartz overgrowths about opaque mineral grains suggest concurrent elongation in all directions within the mylonitic foliation and support the inference of general flattening deformation. The mylonitic foliation and penetrative lineation are consistent with a WNW shearing direction; however, both were folded during later deformation increments. Recrystallized grains in footwall quartzites suggest a 305–320° azimuth for the shearing direction. The best-fit π-axis of the poles to the foliation is 18,145, close to the mean trend/plunge of mesoscopic fold hinges at this site (18,151). Both suggest a NNW azimuth for the shearing direction. Thus, structural, microstructural and fabric indicators suggest a change in the azimuth of the shearing direction from ∼286 to ∼331°.

U–Pb geochronology of deformed metagranites in central Sutherland, Scotland: evidence for widespread late Silurian metamorphism and ductile deformation of the Moine Supergroup during the Caledonian orogeny

Within the Caledonides of central Sutherland, Scotland, the Neoproterozoic metasedimentary rocks of the Moine Supergroup record NW-directed D 2 ductile thrusting and nappe assembly, accompanied by widespread tight-to-isoclinal folding and amphibolite-facies metamorphism. A series of metagranite sheets which were emplaced and penetratively deformed during D 2 have been dated using SHRIMP U–Pb geochronology. Zircon ages of 424 ± 8 Ma (Vagastie Bridge granite), 420 ± 6 Ma (Klibreck granite) and 429 ± 11 Ma (Strathnaver granite) are interpreted to date emplacement, and hence regional D 2 deformation, during mid- to late Silurian time. Titanite ages of 413 ± 3 Ma (Vagastie Bridge granite) and 416 ± 3 Ma (Klibreck granite) are thought to date post-metamorphic cooling through a blocking temperature of c . 550–500 °C. A mid- to late Silurian age for D 2 deformation supports published models that have viewed the internal ductile thrusts of this part of the orogen as part of the same kinematically linked system of foreland-propagating thrusts as the marginal Moine Thrust Zone. The new data contrast with previous interpretations that have viewed the dominant structures and metamorphic assemblages within the Moine Supergroup as having formed during the early to mid-Ordovician Grampian arc–continent orogeny. The mid- to late Silurian D 2 nappe stacking event in Sutherland is probably a result of the collision of Baltica with the Scottish segment of Laurentia.

Palaeomagnetic dating of fluid flow events along the Moine Thrust Fault, Scotland

Palaeomagnetic results suggest that several fluid flow events caused alteration and hematite authigenesis in the MoineThrust Zone, northwest Scotland, between the Late Palaeozoic and Middle Tertiary. Precambrian Torridonian sandstone and Lewisian gneiss in the fault zone on the Isle of Skye contain a Tertiary magnetization interpreted to be the result of hydrothermal fluids associated with Tertiary intrusions. Samples of the Dumess Formation limestone (Cambrian), Torridon Group and quartzites (Cambrian) in and near the fault zone to the north contain a Late Palaeozoic to Triassic magnetization residing in hematite and interpreted to be related to hydrothermal fluids. This magnetization and associated alteration in the Moine Thrust Zone are direct evidence for post-orogenic activity, in which the thrusts vented excess heat during regional crustal extension.

The Life of Frank Cole Phillips (1902-1982) and the Structural Geology of the Moine Petrofabric Controversy

The Life of Frank Cole Phillips (1902-1982) and the Structural Geology of the Moine Petrofabric Controversy

Introduction

Torridonian is an informal stratigraphic name for the Proterozoic reddish-brown sandstones overlying the Lewisian gneiss complex of the NW Scottish mainland. These sandstones form one of the principal elements of British stratigraphy, comparable in volume (over 150000 km 3 ) to the Lower Old Red Sandstone of eastern Scotland, or the Triassic of England. They form the majestic mountains of NW Scotland, but also extend westwards under the Minch basin (Fig. 1). The subcrop has been identified 20 km north of Cape Wrath on the MOIST seismic reflection profile (Blundell et al . 1985), and beneath Devonian strata in the west Orkney basin (Cheadle et al . 1987). It extends south for 330 km to the latitude of Iona (Binns et al . 1974; Evans et al . 1982). The Torridonian was deposited on the edge of the Laurentian shield, near the roughly contemporaneous Grenville orogenic belt. It lies just outside the Caledonian orogen and has consequently escaped appreciable deformation, except in the Moine Thrust zone. Dips are generally low and the thermal history reflects little more than burial, giving ample scope for studies of the sedimentology, geochemistry, palaeoclimate and palaeomagnetism. Combined investigations of the sedimentology and chemistry of the rocks by several workers over the last ten years, using a total of nearly 600 whole rock analyses, have been particularly fruitful despite the relative neglect of the petrography. The most surprising lacuna in Torridonian studies is the paucity of published work on the micropalaeontology. The main objects of this memoir are to provide a comprehensive

Dating deformation and cooling in the Caledonian thrust nappes of north Sutherland, Scotland: insights from 40 Ar/ 39 Ar and Rb–Sr chronology

40 Ar/ 39 Ar and Rb–Sr mineral ages have been determined from various lithologies exposed in the Caledonian foreland and structurally overlying thrust nappes of north Sutherland, Scotland. Rb–Sr muscovite ages of c . 428, c . 421 and c . 413 Ma obtained from Moine Thrust Zone mylonites are interpreted to date closely regional thrusting during the Late Silurian to Early Devonian. 40 Ar/ 39 Ar muscovite ages within the lower parts of the Moine nappe are mostly anomalously old with respect to Rb–Sr analyses of muscovites from the same samples; it is likely that this discrepancy results from a component of extraneous or ‘excess’ argon. 40 Ar/ 39 Ar hornblende ages and Rb–Sr and 40 Ar/ 39 Ar muscovite ages obtained from structurally higher metamorphic units in the Caledonian thrust nappes generally range between c . 440 Ma and c . 410 Ma. These ages are interpreted to date cooling during and following ‘D 2 ’ regional thrusting and folding within internal sectors of the nappe sequence. A possible tectonic model involves the Silurian collision of Baltica with Scottish segments of Laurentia resulting in the Scandian orogeny and broadly coeval Moine Thrust Zone. D 2 structures were superimposed on structures and metamorphic fabrics formed during a regional Mid-Ordovician tectonothermal event dated previously at c . 470–460 Ma. Syn-D 2 temperatures were generally &gt;600°C and sufficient to achieve more or less complete thermal rejuvenation of Rb–Sr and 40 Ar/ 39 Ar isotopic systems in muscovite and hornblende, even in areas of low D 2 strain.

A global perspective on the Scottish Caledonides

ABSTRACTThe Scottish Caledonides constitute less than 10% of the length of the Caledonian-Appalachian orogen, the rocks of which define one major margin of the Laurentian craton in Neoproterozoic-Palaeozoic times. Scotland was located, however, in a critical position at the tip of a major cratonic promontory bounded by the Caledonian and Appalachian segments of that margin. Isotopic dates from minerals and rocks collected in the Scottish Highlands have been regarded for 40 years as indicating a Neoproterozoic history of compressive orogenesis that is absent in N America and Greenland. They have therefore been taken by some authors to indicate an origin exotic to Laurentia for rocks of the Northern and Grampian Highlands S and E of the Moine thrust belt. An alternative explanation is that the Neoproterozoic rocks in the Scottish Highlands are all related to the two-stage ‘breakout’ of a discrete rift-bounded Laurentian continent from the core of the Rodinian supercontinent, believed to have assembled at the end of the Mesoproterozoic.Traditional reconstructions of the late Neoproterozoic–Early Palaeozoic Earth oppose the proto-Caledonian/Appalachian margin of Laurentia and the W African craton of the newly assembled Gondwanaland. However, consideration of the global inventory of late Precambrian rifted margins, their relation to Grenvillian orogenic belts and of scale, leads to the hypothesis that the conjugate was the proto-Andean margin of S America. Recent recognition that the Cambrian and Lower Ordovician strata of the northwestern Argentine Precordillera and their underlying Grenvillian basement are unquestionably of Laurentian derivation, while not definitive, does point in this direction. If correct, this means that even the presence of Neoproterozoic orogenesis need not imply an exotic origin, as Neoproterozoic orogens are widespread in S America.Traditional models show an Early Ordovician lapetus ocean basin approximately 4500 km wide, but the remarkably synchronous Ordovician collision of arcs and other terranes with the Laurentian and Gondwanan cratons from Argentina to the British Isles, suggests that this premise may be incorrect. The Appalachian–Caledonian orogen may rather have resulted from close and complex tectonic interaction between Laurentia and Gondwana, involving intervening volcanic arcs and other terranes. The interaction may have taken place during a clockwise transit of Laurentia around the proto-Andean margin to its late Caledonian–Scandian collision with Baltica, and the final suturing of Pangaea at the close of the Palaeozoic era. A modern analogue may be the interaction between Australia and Asia, involving intervening volcanic arcs and other terranes of the western Pacific Ocean basin, from ~ 50 Ma through the Present, and into the future.

New high-pressure granulite event in the Moine Supergroup, northern Scotland: Implications for Taconic (early Caledonian) crustal evolution

Relict high-pressure granulite-facies assemblages have been discovered in mafic sheets within Neoproterozoic Moine Supergroup psammites of northern Scotland. Minerals and fabrics associated with the main 440–430 Ma Caledonian deformation and metamorphism (D2) overprint these assemblages, which preserve peak pressure-temperature ( P-T ) conditions of 11–12 kbar at 650–700 °C, indicating burial to ∼35 km. The maximum age of the sheets is constrained by the post-Grenvillian ca. 1000 Ma age of the youngest detrital zircons in the host psammites. The high- P assemblage is constrained by the age of anatexis in the psammites at 467 ± 10 Ma. Assembling these data with P-T and chronological data for the D2, D3, and later cooling events, two partial clockwise P-T-t-D (time-deformation) paths emerge, demonstrating that the high- P event is not part of the evolution of the P granulite assemblages are interpreted to be a relict of an earlier, Taconic 470–460 Ma history. This new evidence suggests that the tectonic evolution of the Caledonian nappes east of the Moine thrust is more complex than previously thought.

A regional geochemical assessment of some terrane relationships in the British Caledonides

Synopsis Regional geochemical data have been systematically collected across the British Caledonides and interpreted, using single and multielement image analysis, to provide geochemical constraints on terrane relationships. In the Hebridean Terrane, the Lewisian rocks show the characteristics of a large ion lithophile element (LILE) depleted lower crustal complex and seem unlikely to have been the source for the bulk of the overlying Torridonian clastic rocks. The psammite-dominated Moine Supergroup and the Dalradian rocks of the Grampian Group have closely similar evolved chemistries consistent with derivation from a higher crustal level than that of the currently exposed Lewisian complex. Across the Scottish Highland terranes the principal geochemical discontinuities coincide with the Moine Thrust Zone, the boundary between the Grampian Group and the Appin and Argyll Group Dalradian rocks, and the Highland Boundary Fault. The geochemical evidence provides little support for the Great Glen Fault as a significant terrane boundary. South of the Southern Upland Fault, which also coincides with a major geochemical discontinuity, a range of elements indicates division of the Southern Uplands terrane into northern and southern zones. These are transitional between the Orlock Bridge and Moffat Valley faults and reflect a change in sedimentary provenance during the early Silurian. Basic and ultrabasic elements, especially Cr, show exceptionally high levels over early Llandovery strata possibly indicating derivation from Iapetus Ocean crust obducted further NE as the Scandian Orogen was initiated. The geochemical characteristics of the southern part of the Southern Uplands Terrane continue into the Windermere Supergroup of the English Lake District, emphasising the continuity of mid- to late-Silurian deposition across the Laurentia–Avalonia (Iapetus) suture zone.

Regional geochemistry, terrane analysis and metallogeny in the British Caledonides

Abstract The Caledonides of northern Britain are commonly divided into an assemblage of terranes based on the interpretation of geology in relation to major fault structures. This paper incorporates metallogenic concepts into the terrane model and uses systematic regional geochemical data to constrain models of terrane configuration and evolution: analyses of one-, two- and three-component geochemical images are used to identify changes in the levels of single elements and their ratios across the orogen. The distribution of sedimentary exhalative (SEDEX) ore deposits and volcanogenic massive sulphide, and mesothermal and epithermal gold mineralization is shown to reflect the presence of long-lived metallogenic provinces which have distinctive geochemistry reflecting their origin as zones of crustal extension and lithospheric thinning. The geochemistry of the northern Caledonides reflects the predominantly lower crustal nature of the Laurentian continental margin, now exposed in the Lewisian foreland and Caledonized Lewisian inliers in the Moine succession. In contrast, the Avalonian Plate has a more evolved geochemistry, although the ratios of elements associated with basic rocks suggests that they were less fractionated during emplacement into the thinner crust of Avalonia and the Iapetus Suture Zone. The predominantly psammitic Moine Supergroup and the Grampian Group Dalradian have an evolved chemistry consistent with derivation from a higher crustal level than that of the exposed Lewisian complex; basin-scale variations, such as the proportion and composition of arkosic material, are also evident. The regional pattern was intermittently disrupted by the influx of mantle material during deposition of the mineralized Argyll and Southern Highland groups (Dalradian), and later as intrusions were emplaced from early Ordovician (Grampian Orogeny) to early Devonian times. South of the Southern Upland Fault, the geochemical patterns suggest a complex interplay of sedimentary provenance and depositional environments during the final stages of collision between Laurentia and Avalonia. Provenance changes in early Silurian times probably reflect the initiation of major tectonism in the Scandian Orogen to the northeast and, more locally, a change from a mineralized, extensional back-arc- to a foreland-basin environment as the Laurentian plate overrode Avalonia. Very similar geochemical trends extending from the highest Silurian strata in the Southern Uplands into the Windermere Supergroup of the English Lake District are consistent with sequential deposition in the same foreland basin. The principal crustal boundaries indicated by the geochemical data are the Moine Thrust Zone, the Grampian-Appin groups’ boundary, the Highland Boundary Fault and the Southern Upland Fault. The geochemical evidence provides little support for the Great Glen Fault as a significant terrane boundary, while the Iapetus Suture is shown to be a complex transition zone extending across the Southern Uplands and into the south of the Lake District. Within the former, the Moffat Valley Fault is identified as a major structure of regional significance. Throughout the orogen there is a direct relationship between geochemistry, tectonic setting and the presence of significant metalliferous mineralization.

The influence of country rock structural architecture during pluton emplacement: the Loch Loyal syenites, Scotland

The late Caledonian Loch Loyal syenites comprise three related, but geographically separate, composite intrusions: the Ben Loyal, Ben Stumanadh and Cnoc nan Cuilean bodies. They were emplaced (c. 426 Ma) into Moine and Lewisian rocks of the Moine and Naver thrust sheets previously deformed by Caledonian NW-directed folding and thrusting at amphibolite facies metamorphic conditions. The intrusions lie mainly within a zone of large-scale cross-folds in which the regional foliation trends NW-SE at high angles to regional orogenic strike (NNE-SSW). The cross folds are underlain by a broad, ESE-dipping zone of high strain known as the Ben Blandy shear zone. The NW basal contact of the Ben Loyal body dips generally SE and strikes sub-parallel to the foliation in the underlying Ben Blandy shear zone. The higher parts of this syenite, together with those of the Ben Stumanadh and Cnoc nan Cuilean intrusions, form a series of variably coalesced dykes trending NW-SE sub-parallel to the foliation in the pre-existing zone of cross-folds. There is little evidence for significant country-rock deformation during emplacement of the syenites. Minor components of top-to-the-SE extension occurred along the NW boundary of the Ben Loyal body, whilst minor dextral shearing is associated with the emplacement of the NW-SE, vertical sheet-like units that are thought to form the SE part of this intrusion and the other Loch Loyal syenites. Generally contact-parallel magma tic-state deformation fabrics are dominant in most of the intrusions and are thought to reflect limited magma pressure-driven, internal deformation of the igneous bodies during and immediately after emplacement. The geometry of these fabrics and the Loch Loyal syenites as a whole has been directly controlled by the pre-existing structural architecture of the surrounding country rocks. This appears to be a previously overlooked feature of many plutons emplaced at mid- to upper crustal depths below the region influenced by the free surface. The siting and emplacement of the Loch Loyal syenites may have been initially facilitated by late Caledonian gravity-driven extension of the Moine Thrust sheets. Following the initial phase of intrusion, it is also possible that the presence of magma actually promoted further nappe collapse in the adjacent country rocks. There is no compelling reason to link the upper crustal extension observed during emplacement to the genesis of the alkaline magmas in the underlying mantle. The siting and ascent of the syenites at depth may be linked to the presence of steeply-dipping structures in the Lewisian autochthon buried beneath the Moine Nappe.

Direct dating of mylonite evolution: a multi-disciplinary geochronological study from the Moine Thrust Zone, NW Scotland

Rb–Sr dating of sub-closure temperature, syn-deformationly crystallized white micas from the Moine mylonites (Knockan and Dundonnell), has yielded ages which vary from 437 Ma to 408 Ma. Morphological and major element analyses of the micas indicate that all the micas within the analysed samples were (re)crystallized during the Moine Thrust shearing. The ages yielded are therefore interpreted as marking the end of crystal plastic deformation associated with shearing on the Moine Thrust. The variation in ages between samples is significantly greater than the individual analytical errors and may be a product of strain localization within the shear zone. Alternatively it may indicate flow of strontium-rich fluids derived from outside the shear zone implying that feldspars may act as potentially unreliable initial isotopic reservoirs even in very high strain samples within the greenschist-facies shear zones. However, the Rb–Sr age are internally consistent and compatible broadly with existing geological and geochronological data for the region suggesting that even if the feldspars have not acted as perfect initial isotopic reservoirs the resulting errors were minimal. The general cessation of ductile deformation at c . 430 Ma probably dates the general initiation of imbrication within footwall to the Moine Thrust. The age of 430 Ma is therefore likely to be a time of significant brittle displacement on the underlying Ben More Thrust. K–Ar dating of the (re)crystallized Moine mylonite micas yielded anomalously old ages in comparison to Rb–Sr values as a result of incorporation of excess Ar during shearing. The degree of incorporated excess Ar decreases away from the base of the shear zone into the hanging wall, becoming indiscernible at c . 1.5 km structurally above the base of the shear zone. The distance ( c . 1.5 km) may represent the distance that fluid can have flowed out of the shear zone during deformation.