Great Glen Fault Research Articles

Lu–Hf and Sm–Nd dating of metamorphic garnet: evidence for multiple accretion events during the Caledonian orogeny in Scotland

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. Supplementary materials: Results of trace element analysis of the garnets by laser ablation inductively coupled plasma mass spectrometry are available at www.geolsoc.org.uk/SUP18583 .

The Dalradian rocks of the central Grampian Highlands of Scotland

The central Grampian Highlands, as defined here, are bounded to the north-west by the Great Glen Fault, to the south-west by Loch Etive and the Pass of Brander Fault and to the south-east by the main outcrop of the Loch Tay Limestone Formation. The more arbitrary northern boundary runs north-west along the A9 road and westwards to Fort William. The detailed stratigraphy of the Dalradian Supergroup ranges from the uppermost Grampian Group through to the top of the Argyll Group, most notably seen in the two classic areas of Loch Leven–Appin and Schiehallion–Loch Tay; Southern Highland Group strata are preserved only in a small structural inlier south of Glen Lyon.Major F1 and F2 folds are complicated by co-axial northeast-trending F3 and F4 folding, as well as by locally important north- or NW-trending folds. In the Loch Leven area, nappe-like F1 folds verge to the north-west, whereas to the south-east the major recumbent F1/F2 Tay Nappe verges to the south-east. The trace of the upright Loch Awe Syncline lies between the opposing nappes, but in this region a large mass of late-Caledonian granitic rocks obscures their mutual relationship. Three tectonic ‘slides’ are identified that are certainly zones of high strain but which in part could be obscuring stratigraphical variations.The regional metamorphism ranges from greenschist facies on the western seaboard of Argyll to amphibolite facies in most of the remainder of the region. The study of garnets, together with kyanite and staurolite in the Schiehallion area, has enabled a detailed history of the metamorphism and structure to be unravelled.Stratabound mineralization occurs in the Easdale Subgroup, where there is also evidence of changes of sedimentary environment associated with volcanicity and lithospheric stretching. The region is dissected by a series of NE-trending, dominantly left-lateral, faults, subparallel to the Great Glen Fault, whose movement history is illustrated here by that of the Tyndrum Fault.

The Dalradian rocks of Scotland: an introduction

The Dalradian Supergroup and its basement rocks, together with younger plutons, underpin most of the Grampian Highlands and the islands of the Inner Hebrides between the Highland Boundary and Great Glen faults. The Dalradian is a mid-Neoproterozoic to early-Ordovician sequence of largely clastic metasedimentary rocks, with some volcanic units, which were deformed and metamorphosed to varying degrees during the Early Palaeozoic Caledonian Orogeny.Sedimentation of the lower parts of the Dalradian Supergroup, possibly commencing about 730 million years ago, took place initially in fault-bounded rift basins, within the supercontinent of Rodinia and adjacent to sectors of continental crust that were later to become the foundations of North America, Greenland and Scandinavia. Later sedimentation reflected increased instability, culminating between 600 and 570 million years ago in continental rupture, volcanicity and the development of the Iapetus Ocean. This left the crustal foundations of Scotland, together with those of North America and Greenland, on a laterally extensive passive margin to the new continent of Laurentia, where turbiditic sedimentation continued for about 85 million years. Later plate movements led to closure of the Iapetus Ocean and the multi-event Caledonian Orogeny. Most of the deformation and metamorphism of the Dalradian strata peaked at about 470 million years ago, during the mid-Ordovician Grampian Event, which has been attributed to the collision of an oceanic arc with Laurentia. The later, mid-Silurian Scandian Event, attributed to the collision of the continent of Baltica with Laurentia and the final closure of the Iapetus Ocean, apparently had little effect on the Dalradian rocks but marked the start of late-orogenic uplift and extensive magmatism in the Grampian Highlands that continued until Early Devonian times.The Dalradian rocks thus record a wide range of sedimentary environments (alluvial, tidal, deltaic, shallow marine, turbiditic, debris flow) and a complex structural and metamorphic history. In areas of low strain, original sedimentary and volcanic structures are well preserved, even at relatively high metamorphic grades. There is convincing evidence for glacial episodes of worldwide importance and economic deposits of stratiform barium minerals are unique. The Grampian Highlands include two of the World's type-areas for metamorphic zonation, Barrovian and Buchan, with spectacular examples of the key metamorphic minerals, and various stages of migmatite development. Polyphase folding is widespread on all scales and gives rise to a range of associated cleavages and lineations. Regional dislocations, both ductile and brittle, are associated with a range of shear fabrics, breccias, clay gouges and veining.

Geology from rail journeys: the Inverness to Wick Line, Scotland

The Inverness to Wick railway line enables travellers to track the geological history of the far northeast of Scotland, spanning about 1000 million years, and starting in the Neoproterozoic. This history includes the Caledonian Orogeny, the deposition of Old Red Sandstone and Mesozoic sediments, the latter along the faulted margins close to the Great Glen Fault system, and ends with Pleistocene and Holocene deposition and erosion.

Re–Os molybdenite dates from the Ballachulish and Kilmelford Igneous Complexes (Scottish Highlands): age constraints for late Caledonian magmatism

Abstract: New Re–Os molybdenite geochronology is presented from the Ballachulish Igneous Complex (433.5 ± 1.8 Ma) and the Kilmelford Igneous Complex (425.8 ± 1.7 Ma) of the SW Scottish Highlands. The sulphide mineralization in the Ballachulish Igneous Complex is related to the latest phase of granite magmatism, and therefore the new Re–Os age provides a minimum crystallization age, c . 3.7 Ma earlier than previous crystallization age estimates. This Re–Os age overlaps U–Pb ages obtained from subduction-related granitic and appinitic magmatism north of the Great Glen Fault, and an origin related to active subduction rather than slab breakoff is proposed for the Ballachulish Igneous Complex. Molybdenite mineralization in the Kilmelford Igneous Complex is spatially and genetically associated with porphyry Cu mineralization, which is consistent with the rapid ascent of volatile-rich magma during early rebound following the breakoff of subducted oceanic lithosphere. The Kilmelford Igneous Complex and the coeval Lorn Lava Pile may represent the earliest of the igneous bodies predicted by slab breakoff, indicating that slab breakoff occurred at c . 426 Ma.

Mid-Devonian sinistral transpressional movements on the Great Glen Fault: the rise of the Rosemarkie Inlier and the Acadian Event in Scotland

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.

Basement-influenced rifting and basin development: a reappraisal of post-Caledonian faulting patterns from the North Coast Transfer Zone, Scotland

Abstract The post-Caledonian development of the West Orkney Basin is regularly cited as a classic example of basement-influenced rifting. This paper presents the first detailed multidisciplinary analysis of the three-dimensional (3D) geometries and distribution of post-Caledonian faults in onshore northernmost Scotland, examining their relationships to basement fabrics and comparing them to rift-related structures developed offshore in the West Orkney Basin. Two phases of rift-related faulting are distinguished: 1) Devonian ENE–WSW extension localized in the east of the basin and related to regional sinistral transtension along the Great Glen Fault; and 2) Permo-Triassic NW–SE extension focused to the west of the basin and probably contemporaneous with movements along the Minch Fault. A complex North Coast Transfer Zone is developed along the northern Scottish coast linking Mesozoic rifts that reactivated Caledonian structures in the West Orkney Basin (Naver Thrust) to those bounding the North Minch Basin (Outer Hebrides Fault Zone). Polymodal faulting patterns are widespread in onshore exposures. Fault patterns vary due to changes in the obliquity between regional rifting vectors and variably orientated pre-existing structures in each basement terrane. The geometric complexity and spatial variations in fault patterns onshore can be correlated with changes in basement structures, despite limited direct reactivation of pre-existing fabrics.

The Cluanie granodiorite, NW Highlands of Scotland: a late Caledonian pluton of trondhjemitic affinity

Synopsis The Cluanie Pluton is a late Caledonian granitoid emplaced into the Glenfinnan Division of the Moine Supergroup in the NW Scottish Highlands. A field investigation of the pluton and its internal facies is presented along with new major- and trace-element whole-rock XRF analyses, and geobarometric and geothermometric studies. Cluanie is predominantly composed of hornblende granodiorite characterized by varying concentrations of distinctive alkali feldspar megacrysts, with minor amounts of biotite granodiorite and rare mingled porphyritic microgranodiorite. The alkali feldspar megacrysts appear to be magmatic in origin. Rare spectacular pegmatitic concentrations most likely represent physical accumulation of the megacrysts. The pluton is geochemically a high Na/K trondhjemite, the only such pluton known among the Newer Granites of Scotland. On the basis of geochemical evidence and a comparison with partial melting experiments, we propose that the magmas were derived by fluid-rich melting of an amphibolitic source leading to relatively low temperature magmas which were significantly contaminated by Moine metasediments. The pluton was emplaced in the mid crust at about 4.3 kbar during an episode of dextral shear on the Glen Glass Fault related to regional strike-slip faulting on the Great Glen Fault system at c . 425 Ma.

Structural setting and U–Pb zircon geochronology of the Glen Scaddle Metagabbro: evidence for polyphase Scandian ductile deformation in the Caledonides of northern Scotland

AbstractWithin the Scottish Caledonides, the Glen Scaddle Metagabbro was intruded into the Moine Supergroup of the Northern Highland Terrane after Grampian D2 folding and prior to regional D3 and D4 upright folding and amphibolite-facies metamorphism. A U–Pb zircon age of 426 ± 3 Ma obtained from the metagabbro is interpreted to date emplacement. D3–D4 folding is constrained to have occurred during the Scandian orogenic event. In contrast, polyphase folding and regional metamorphism of the Dalradian Supergroup southeast of the Great Glen Fault is entirely Grampian. These differences are consistent with published tectonic models that invoke a minimum of 700 km of post-Scandian sinistral displacements across the Great Glen Fault to juxtapose the Grampian and Northern Highland terranes.

The brittle evolution of a major strike-slip fault associated with granite emplacement: a case study of the Leannan Fault, NW Ireland

NE–SW-trending sinistral strike-slip deformation is widespread throughout the Highlands of Scotland and NW Ireland and has been interpreted as a response to the subduction of Iapetus and continental collision. The southwestern continuation of the Great Glen Fault zone is marked in NW Ireland by the Leannan Fault. Field evidence derived from the Tullagh Point Granite indicates its intrusion along the fault during ductile sinistral shearing. U–Th–Pb zircon geochronology yields a crystallization age of 422 ± 2 Ma for this granite. Detailed analysis of fracture patterns throughout the Malin Head region demonstrates that sinistral fractures (F 1 ) developed at an acute angle to the main fault strands. These fractures display oblique internal alignment of K-feldspar and are overprinted by dilatational quartz-filled joints at a similar orientation. The trend of these joints is consistent with c . 90° rotation of the σ 1 axes relative to F 1 as a result of intrusion of the Malin Head Granite dated at 412 ± 3 Ma. Subsequent sinistral fractures developed after dissipation of granite-related forces. They define an anticlockwise rotation of the regional σ 1 and moved under a different stress tensor from F 1 . It is concluded that the Leannan Fault moved sinistrally at c . 422 Ma and after c . 412 Ma, and that granite emplacement had pronounced effects on the fault kinematics.

The movement history and fault rock evolution of a reactivated crustal-scale strike-slip fault: the Walls Boundary Fault Zone, Shetland

The Walls Boundary Fault Zone is a crustal-scale strike-slip fault that cuts Precambrian–Caledonian basement terranes in Shetland and has been interpreted as the northern continuation of the Great Glen Fault Zone in Scotland. This paper presents the first detailed account of the kinematic history and fault rock assemblages associated with the onshore evolution of the Walls Boundary Fault Zone. These observations suggest that it initiated as a major late Caledonian (Silurian–Devonian) sinistral strike-slip fault associated with the successive development of mylonites and cataclasites. These fault rocks are preserved only locally, and elsewhere are obscured by the effects of later brittle overprinting and dismemberment of the fault zone during dextral strike-slip reactivation, probably during late Carboniferous inversion of the Orcadian Basin. This led to the development of extensive cataclasite and fault gouge assemblages, which are widely preserved along the Walls Boundary Fault. Narrow zones of post-Triassic dip-slip, and finally sinistral strike-slip, reactivation are localized within earlier-formed gouge-filled fault cores. There are some similarities to the kinematic history of the Great Glen Fault Zone, most notably the recognition of late Caledonian sinistral shear and post-Devonian dextral reactivation, but the post-Triassic reactivation histories appear to differ significantly.

Histoire de la dénudation dans le corridor du loch Ness (Écosse) : mouvements verticaux différentiels le long de la Great Glen Fault

The Great Glen Fault (GGF) is a major lithospheric strike-slip fault system that cuts across the Caledonian orogenic belt of Scotland. It separates the Northern Highlands terrane from the Grampian terrane. Movement history along the fault extends at least into the Early Palaeozoic. The low-temperature history of the Scottish Highlands covers a significant period from the Late Palaeozoic to the Present, but the magnitude of vertical movements that affected the GGF area during that time is still a matter of debate. Fission track analysis on detailed vertical profiles in the Ballachulish and Strontian complexes, as well as on a profile oriented northeastwards across the GGF, reveals distributed differential vertical movements along the fault during the Late Palaeozoic. The Mesozoic denudation is estimated at a maximum of 2 km before a more recent widespread exhumation event that started at around 40–25 Ma, leading to 1.6 to 2 km of erosion.

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.

Remagnetization and fluid flow in the Old Red Sandstone along the Great Glen Fault, Scotland

The Devonian Old Red Sandstone in the vicinity of the Great Glen Fault (GGF) in Scotland contains two different components residing in hematite: a postfolding Carboniferous CRM1 in the Loch Ness area and a Cretaceous or perhaps Triassic CRM2 near Hilton. The CRM1 could be related to major fluid flow events in the Late Paleozoic which caused hematite authigenesis and remagnetization along other faults in Scotland. The CRM2 near Hilton was also related to a fluid event in the Cretaceous or Triassic which caused hematite authigenesis. The presence of different CRMs residing in hematite along different segments of the GGF is similar to what has been reported for other major faults in Scotland.

The Blackstones Bank igneous complex: geochemistry and crustal context of a submerged Tertiary igneous centre in the Scottish Hebrides

The Blackstones Bank, located about 60 km WSW of the Isle of Mull in Western Scotland, is a submarine plutonic complex in the British Tertiary Igneous Province. Geochemical and isotopic analysis of gabbros, microgabbros and basic dykes shows that the magmas interacted strongly with crustal rocks during their emplacement. The isotopic signature of the contaminated Tertiary intrusions shows no evidence of any interaction with Archaean basement, despite the location of the Blackstones complex to the west of the Great Glen fault. Instead, the Blackstones rocks have crustal signatures resembling the Proterozoic basement and cover rocks of western Islay. It is therefore inferred that Early Proterozoic crust extends to the west of the Great Glen fault at this point on the Scottish continental shelf. In addition, the occurrence of similar isotopic signatures in Tertiary igneous rocks east of the Loch Gruinard fault confirms that Early Proterozoic basement extends under the Grampian block of mainland Scotland. When combined with published evidence from the Rockall bank, the new data constrain the location of an Archaean–Proterozoic crustal suture with a WNW trajectory which cuts across the continental shelf of northwest Britain.

Constraints on early sinistral displacements along the Great Glen Fault Zone, Scotland: structural setting, U–Pb geochronology and emplacement of the syn‐tectonic Clunes tonalite

Abstract: Current tectonic models emphasize the role of crustal‐scale transcurrent faults in controlling the ascent and emplacement of plutons in orogenic belts. The Clunes Tonalite crops out adjacent to the transcurrent Great Glen Fault Zone within the Ordovician‐Silurian Caledonian orogenic belt of Scotland. The tonalite is characterized by a strong fabric defined by aligned plagioclase, hornblende and biotite that is interpreted to be a magmatic state deformation fabric formed during emplacement. Field and microstructural evidence derived from the tonalite and its Moine Supergroup country rocks supports an emplacement model that links intrusion directly to early sinistral displacements along the Great Glen Fault Zone. U–Pb zircon geochronology yields a crystallization age for the Clunes Tonalite of 427.8±1.9 Ma. The age is thus interpreted to date early sinistral displacements along the Great Glen Fault Zone and is identical within error to that of the Ratagain granite emplaced during sinistral displacement along the Strathconon Fault to the NW.

The structure and rheological evolution of reactivated continental fault zones: a review and case study

Abstract Repeated reactivation of structures and reworking of crustal volumes are characteristic, though not ubiquitous, features of continental deformation. Reactivated faults and shear zones exposed in the deeply exhumed parts of ancient orogenic belts present opportunities to study processes that influence the mechanical properties of long-lived fault zones at different palaeo-depths. Ancient basement fault systems typically comprise heterogeneous, superimposed assemblages of fault rocks formed at different times and depths for which down-temperature thermal histories are most common. Several lithological and environmental factors influence the evolution of fault rock fabrics and rheology, but most fault/shear zone arrays appear to develop as self-organized deformation systems. Once mature, the kinematic and mechanical evolution of the system is strongly influenced by the rheological behaviour of the interconnected fault/shear zone network. A case study from the crustal-scale Great Glen Fault Zone (GGFZ), Scotland, reveals a complex evolution of mid- to upper-crustal deformation textures formed adjacent to the frictional-viscous transition. Fluid influx in the mid-crust has led to reaction softening of the rock aggregate as strong pre-existing phases such as feldspar are replaced by fine-grained, strongly aligned aggregates of weak phyllosilicates. In addition, a grainsize-controlled switch to fluid-assisted diffusional creep occurs in the highest strain regions of the fault zone. It is proposed that this led to a shallowing and narrowing of the frictional-viscous transition and to long-term overall weakening of the fault zone relative to the surrounding wall-rocks. Cataclasis is particularly important in the deeper part of the frictional regime as it helps to promote retrograde metamorphism and changes in deformation regime, by both reducing grainsize and promoting pervasive fluid influx along fault strands due to grain-scale dilatancy. Equivalent processes are likely to occur along many other long-lived, crustal-scale fault zones.

U–Pb geochronology of the Fort Augustus granite gneiss: constraints on the timing of Neoproterozoic and Palaeozoic tectonothermal events in the NW Highlands of Scotland

The West Highland granite gneiss suite in Inverness-shire, Scotland, represents a series of S-type, anatectic granites formed by partial melting of host Neoproterozoic metasediments of the Moine Supergroup. U–Pb (SHRIMP) dating of zircons from a member of the suite, the Fort Augustus granite gneiss, indicates that the granitic protolith to the gneiss was intruded at 870±30 Ma. This is indistinguishable from the published age determined by the same method for the Ardgour granite gneiss at Glenfinnan, thus supporting the assumption that the various members of the West Highland granite gneiss are part of a single intrusive suite. The spread of ages from the zircon cores (1626–947 Ma) is interpreted to indicate a Proterozoic source terrain for the Moine sediments that were later melted to form the granitic protolith. A U–Pb age of 470±2 Ma obtained for titanite in the Fort Augustus granite gneiss is interpreted to date amphibolite-facies metamorphism during the early to mid-Ordovician Grampian Orogeny. The emerging similarity in the timing of this event either side of the Great Glen Fault implies that this structure does not juxtapose crustal blocks with significantly different histories with respect to the Grampian Orogeny.

Deformation processes and weakening mechanisms within the frictional–viscous transition zone of major crustal-scale faults: insights from the Great Glen Fault Zone, Scotland

The Great Glen Fault Zone (GGFZ), Scotland, is a typical example of a crustal-scale, reactivated strike-slip fault within the continental crust. Analysis of intensely strained fault rocks from the core of the GGFZ near Fort William provides a unique insight into the nature of deformation associated with the main phase of (sinistral) movements along the fault zone. In this region, an exhumed sequence of complex mid-crustal deformation textures that developed in the region of the frictional–viscous transition (ca. 8–15 km depth) is preserved. Fault rock fabrics vary from mylonitic in quartzites to cataclastic in micaceous shear zones and feldspathic psammites. Protolith mineralogy exerted a strong control on the initial textural development and distribution of the fault rocks. At lower strains, crystal-plastic deformation occurred in quartz-dominated lithologies to produce mylonites simultaneously with widespread fracturing and cataclasis in feldspar- and mica-dominated rocks. At higher strains, shearing appears to increasingly localise into interconnected networks of cataclastic shear zones, many of which are strongly foliated. Textures indicative of fluid-assisted diffusive mass transfer mechanisms are widespread in such regions and suggest that a hydrous fluid-assisted, grainsize-controlled switch in deformation behaviour followed the brittle comminution of grains. The fault zone textural evolution implies that a strain-induced, fluid-assisted shallowing and narrowing of the frictional–viscous transition occurred with increasing strain. It is proposed that this led to an overall weakening of the fault zone and that equivalent processes may occur along many other long-lived, crustal-scale dislocations.

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.