1Department of Earth Sciences, University of Liverpool, Liverpool

Home , Iona, Moine Thrust Belt, Ross of Mull, Staffa

Journal of the Geological Society, London, Vol. 152, 1995, pp. 907-910, 3 figs. Printed in Northern Ireland

metamorphosed late Proterozoic Torridonian mudstones, siltstones and sandstones and Cambrian-early Ordovician Late-orogenic extensional tectonics at the NW siliciclastic and carbonate sediments. The eastern part of margin of the Caledonides in Scotland the Moine Thrust Belt.is dominated by recumbent fold nappes which involve both basement and cover. These nappes were carried and cut by thrusts which are associated G.J. POTTS 1, R.H. HUNTER 1, with mylonite. In general, the mylonitic foliations dip gently A.L. HARRIS 1 & F.M. FRASER 2 towards the ESE and display a down-dip grain shape 1Department of Earth Sciences, University of lineation. Liverpool, Liverpool L69 3BX, UK The Moine Thrust, carrying the Moine Nappe, is the 2Argyll Hotel, Iona, UK westernmost, lowest and youngest of three major thrusts (Fig. 1), the others being the Knoydart and Sgurr Beag (highest and oldest) thrusts, carrying respectively the Knoydart and Sgurr Beag nappes (Barr et al. 1986). Eastwards and upwards across the nappe boundaries A major ductile extensional structure throwing down several metamorphic grade increases such that the different nappes kilometres to the ESE probably marks the southern portion of the NW Caledonian orogenic margin in Scotland. This structure, the have distinguishing metamorphic characteristics. Moine Sound of lona Fault, is probably related to the intensely mylonitized Nappe metasediments are largely at the greenschist to low zones that cut the Lewisian and adjacent lona Group (?Torr- amphibolite facies of metamorphism, the Knoydart Nappe idonian) metasedimentary cover on Iona. The fault-related rocks are mainly at middle amphibolite facies, and the Sgurr mylonites predated the 414 __. 3 Ma Ross of Mull Granite and are Beag Nappe rocks are mainly at middle-to-upper amphibol- inferred to have postdated the c.425Ma Moine Thrust. It is ite facies with widespread migmatization. Whereas the concluded that the Sound of lona Fault is one of the many late Sgurr Beag and Knoydart thrusts are ductile shear zones Caledonian faults in Scotland, but is exposed at a deeper structural marked by intensely flaggy zones of psammite and/or coarse level than normal. Its position at the orogenic margin of the pelitic phyllonites, the Moine Thrust is marked by mylonitic Caledonides is similar to that of the Loch Gruinart Fault on lslay, and/or brittle fault rocks. hinting that (a) major steep fault(s) may be a significant feature of the largely concealed Caledonian margin south of Skye Foreland/hinterland relationships SSW from Skye. Detailed structural mapping of Iona was carried out by G.J.P., Keywords: Caledonides, Inner Hebrides, normal faults, mylonites. R.H.H. and A.L.H., building on earlier work by Bailey & Anderson (1925) and Fraser (1977). This new work permits In Greenland, North America and Scotland the NW margin comparisons with work on the Moine and the Ross of Mull of the Caledonian-Appalachian orogen is marked by late Granite of western Mull (Holdsworth et al. 1987) and draws Ordovician-Silurian thrusting of the internal parts of the on unpublished structural, petrographic and geochemical orogen onto its foreland. In Scotland the foreland and work by R.E. Holdsworth, A.L. Harris, D. O'Halloran and adjacent thrust sheets crop out almost continuously for R.J. Reavy on the Ross of Mull Granite and its envelope. c. 190 km between Skye and the north coast (Fig. 1). SSW Lewisian gneisses on Iona are unconformably overlain from Skye the foreland and orogen are juxtaposed at crop across a contemporary fault scarp by Iona Group siliciclastic only on Iona and the adjacent part of western Mull. Here, sediments (equivalent to Torridonian?), most of which are relationships at the orogenic front can be established and only weakly deformed. Both the Lewisian and Iona Group compared with those farther north. Furthermore, Iona and carry subvertical-to-steep zones of intense mylonitization Mull lie close the projected intersection of the Moine Thrust (Fig. 2); the Lewisian, the Iona Group and the mylonites are and the Great Glen Fault (Fig. 1) both of which structures all contact metamorphosed by the Ross of Mull granite display large, late Caledonian displacements. The Great (Cunningham-Craig et al. 1911). Glen Fault, which probably has a sinistral strike-slip Only a kilometre or so to the east, across the Sound of displacement (Watson 1984), appears to separate two Iona, lies the main outcrop of the Ross of Mull Granite (Fig. terranes of contrasting basement and late Proterozoic 1), the western margin of which crops out on skerries just off lithostratigraphy. Late Caledonian structures in western the east coast of Iona (Fig. 2). Deformed and regionally Mull and Iona may, therefore, have a bearing on the metamorphosed Moine rocks to the east are cut and tectonic history of the Scottish part of the Caledonian thermally metamorphosed by the granite (Fig. 1). Pelitic orogen, and on the largely unknown relationship between rocks in the Moine are at a metamorphic grade consistent the Great Glen Fault and the Moine Thrust. with one of the higher nappes, either the Knoydart Nappe or parts of the Sgurr Beag Nappe; they are moderately Foreland/interior relationships NNE from Skye. Structural migmatized and locally carry staurolite and kyanite. relationships in the foreland zone NNE from Skye are well Enclaves of such rocks with kyanite pseudomorphed by established (for detailed summary, see Harris & Johnson sillimanite persist throughout the Ross of Mull Granite as 1991). Late Proterozoic Moine metasediments and their far west as the Sound of Iona. basement of late Archaean Lewisian gneisses had already Clough (in Cunningham-Craig et al. 1911) and undergone polyphase deformation at greenschist to am- Beckinsale & Obradovitch (1973) interpreted the structural phibolite facies before they were thrust WNW on the Moine break in the Sound of Iona as the Moine Thrust, but Thrust (Fig. 1) during the late Lower Palaeozoic. They Holdsworth et al. (1987, p.105) inferred that it is a higher came to rest on the Caledonian foreland comprising thrust, the Knoydart or Sgurr Beag Thrust, probably Lewisian basement overlain unconformably by scarcely displaced by a fault of unspecified age, inclination and sense

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j,,l/ / / Fig. 2. (a) Structural map of Iona. Blank: Lewisian ortho- and para-gneisses. (b) Equal area lower hemisphere stereographic projections of poles to mylonitic foliation (solid circles) and the contained grain shape lineation (solid diamonds) in Lewisian gneisses. (e) Equal area lower hemisphere stereographic projections of poles to mylonitic foliation and cleavage planes in Iona Group metasediments (open circles) and their contained grain shape Fig. 1. Major Caledonian faults of Scotland, north of the Great lineation (open diamonds). Glen Fault (GGF). SIF, Sound of Iona Fault; LGF, Loch Gruinart Fault. Inset: General geological map of Mull and Iona. way northwards to a regime in which mylonitization is more pervasive but less intense. It seems likely that displacements of displacement. None of the Moine enclaves in the Ross of across the map-scale mylonite zones are large as they have Mull granite carries fabrics that could be related to a thrust juxtaposed Lewisian rocks of contrasting characteristics; for or fault in the Sound. Rocks of metamorphic grade typical example, in the SW and NW parts of the island, Lewisian of the Moine Nappe do not occur either on the Ross of Mull gneisses which locally contain thin (metres) metasedimen- or Iona. Thus, rocks of the Knoydart or Sgurr Beag Nappe tary units, including magnetite schists, carbonates, and appear to have been juxtaposed at the present level of garnetiferous pelites, and amphibolites give way eastwards erosion against those of Iona, which included mylonites to orthogneisses across the major mylonite zones A-A' and typical of the foreland, prior to invasion and thermal B-B' in Fig. 2. metamorphism by the Ross of Mull Granite. Foliations in the mylonite zones within both Lewisian Any hypothesis that satisfactorily explains the late and Iona Group protoliths, and grain shape fabrics and tectonic evolution of this critical area must: (1) account for penetrative cleavages in rocks of the Iona Group, share a the steep attitude of the mylonite zones, the mylonitic common subvertical or steep ESE dip. In addition, foliation, and the slaty cleavage in the Iona Group; (2) point mylonitic foliation and cleavage alike commonly display a to a reasonable conclusion about the sense of displacement grain shape or mineral lineation which plunges approxim- across the mylonite zones; (3) indicate the nature and age of ately down dip (Fig. 2). Based largely on the consistency of the large NNE-trending structure that juxtaposed the these orientations it is concluded that the fabrics are the Knoydart or Sgurr Beag Nappe of the Ross of Mull, and the product of a single, possibly regionally significant, episode Iona Group and the Lewisian of Iona. of deformation. The sub-parallel orientation of the Figure 2 summarizes the structural geology of Iona. The cleavage, the mylonite zones and the foliation within them map shows the distribution of the main units - the Lewisian may be explained in terms of a combination of high and its contained anorthosite ('White Rock'), the Iona strains and marked strain gradients. Group, and the main mylonite zones, which are subvertical Bedding within the Iona Group has a consistent or steeply dipping towards the ESE. The sharply defined, NNE-SSW strike but displays a range of dips varying from discrete, ramifying and coalescing zones of mylonite typical sub-vertical, but just inclined towards the WNW, to steep of the southern part of the Lewisian outcrop (Fig. 2), give towards the ESE. Bedding-cleavage relationships are

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consistent and, together with younging evidence, indicate a consistent model. Normal displacement on steeply ESE synformal syncline to the east of Iona (Fig. 3a). dipping mylonite zones (Fig. 3a) would create the patterns The metamorphic grade of the mylonites is similar to of foliation and lineation on Iona (Fig. 2) and explain the that of the mylonites found at the western margin of the relative orientation of bedding and cleavage within the Iona Caledonian orogen farther north, for example in the Sleat of Group (Fig. 3). Skye and Lochalsh (Fig. 1). The type of foliation and Detailed field studies and thin-section examination of lineation are similar to those in the Moine Thrust Belt and if mylonites from the Iona Group and the Lewisian have failed the foliation and lineation were shallowly inclined or to yield macro- or micro-evidence for sense of shear subhorizontal (e.g. Cheeney & Matthews 1965), there would associated with the episode of mylonitization/cleavage be little hesitation in referring the mylonites of Iona to the formation, and although, from the orientation of the Moine Thrust Belt. The steep attitude of the mylonites can lineation, this must be largely dip-slip, the orientation data be explained by the emplacement of a 'horse' below Iona on do not constrain its extensional or contractional nature. In a lower, later and hypothetical thrust which rotated the the absence of small-scale kinematic indicators, the shape of basement together with an overlying west-facing sub- the partially mylonitized anorthosite is therefore critical in recumbent syncline and shallowly inclined mylonite zones determining the regional sense of displacement. The into their present steep attitude. However, this hypothesis evidence bearing on this is assessed below. seems unlikely as, (a) the mylonites of the southern portion Much of the anorthosite (Fig. 2), comprising strongly of the Moine Thrust Belt are generally associated with the fractured plagioclase feldspar, has sharp western and eastern upper not lower limbs of similar synclines (e.g. Lochalsh subvertical boundaries against gneiss. A zone of mylonite syncline, Barber 1965, fig. 8); (b) it would involve the (c. 50m) impinges on both gneiss and anorthosite to the thrusting of younger (Iona Group) rocks onto older northwestern end of the western margin of the anorthosite; (Lewisian); and (c) it fails to explain the apparent absence a thicker zone of mylonite (c. 200m) impinges on both of the Moine Nappe on both Iona and the Ross of Mull. gneiss and anorthosite towards the southern end of the If the mylonites are in their original attitude most, if not eastern margin (Fig. 2). Relationships between mylonite all, of these field data can be reconciled in a single internally and anorthosite become obscured northwards because of lack of exposure on the Machair and because both mylonite and anorthosite may have been repeated by the fault trending NNW from Cnoc Druidean (Fig. 2). At the W 0 1 km southern end, however, the effect of the mylonite zone in progressively attenuating the anorthosite southwards is unambiguous although interrupted and possibly foreshor- tened by the WNW-trending fault shown on Fig. 2. Not only is the anorthosite strongly mylonitized before it reaches the fault from the north, but to the south of the fault, its attenuation from c. 100m to c. 15m accompanied by progressively intense mylonitization can be traced through well exposed terrain to the coast. Even given the partially obscured relationships to the north, if the anorthosite outcrop is viewed in a plane perpendicular to the mylonitic foliation and parallel to the lineation, its deformed shape is strongly indicative of top-down-to-the-ESE movement a during mylonitization, i.e. extensional displacement. The evidence presented above generally favours the hypothesis that the mylonites of Iona are in their original attitude and are the ductile expression of a major NNE-trending, steeply inclined extensional fault zone throwing down to the ESE. Their age is constrained only by evidence that they postdate the deposition of the Iona Group, the age of which is unknown, and predate the thermal metamorphism imposed on the mylonites and slaty cleavage by the 414 ± 3 Ma Ross of Mull Granite (Halliday et al. 1979). Geometrically this fault zone could be that which has displaced the Moine, Knoydart and possibly Sgurr Beag thrusts down to the ESE bringing the Knoydart or Sgurr Beag Nappe Moine rocks of the Ross of Mull to lie at the same contemporary erosion level adjacent to the Fig. 3. (a) Regional scale diagrammatic cross-section across the deformed foreland rocks of Iona. It seems unlikely to be a Sound of Iona mylonitic zone (extensional model). (b) Regional large wholly post-Caledonian fault, although it is possible scale diagrammatic cross-section across the Sound of Iona mylonitic that further, brittle post-Caledonian displacements did zone (contractional model). Diagonal lines: Lewisian gneisses. occur. Given its relative age and inferred sense of Black, late Proterozoic sediments; squares, Moine Nappe; stipple, displacement it was probably related to the formation of the Knoydart Nappe; regular dots, Sgurr Beag Nappe. Note that the mylonites and the penetrative cleavage on Iona. This fault, extensional model gives the correct bedding-cleavage relationship by implication is likely to be post-425 Ma, in age, the for Iona which the contractional model fails to do. inferred age of the Moine Thrust (Kelley & Powell 1985)

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that it must displace, and pre-414 Ma, the age of the Ross of Relationships between the rocks of Iona and the Ross of Mull Granite. Although we have not succeeded in linking Mull cannot be explained completely by a steep unequivocally the fault in the Sound to the mylonites on top-down-to-the-ESE normal fault; a component of Iona, the balance of the evidence from local and regional thrusting on shallowly dipping faults must have preceded the relationships supports the hypothesis that a major extensional tectonics. Therefore, the suggestion of C.T. mylonite-related fault in the Sound of Iona produced a large Clough (in Cunningham-Craig et al. 1911) that the Moine normal displacement down to the ESE of the Caledonian Thrust continues southwards to Iona is still valid but, rather thrust nappes. than passing at crop down the Sound of Iona, the Moine Thrust (or a comparable Caledonian thrust) lies beneath the Regional implications. The last stages of Caledonian Ross of Mull. Thus, one or more of the main faults of the orogenesis in the Scottish Highlands were marked by the Caledonian thrust system (Fig. 1) must extend to a point no emplacement of granitoid plutons such as the Ross of Mull more than 10-15 km from the projected trace of the Great Granite and related minor intrusions, and by widespread, Glen Fault. extensional and lateral slip faulting some of which was coeval with Old Red Sandstone facies (probably Devonian) sedimentation and volcanicity. Most of the products of the References faults are brittle, comprising breccia, fault gouge, cataclasite and pseudotachylite. Over much of the Scottish Highlands BAILEY, E.B. & ANDERSON, E.M. 1925. The geology of Staffa, Iona and erosion has not reached the deeper levels of these late faults Western Mull. Memoirs of the Geological Survey of the UK. where products similar to the mylonites of the Moine Thrust BARBER, A.J. 1965. The history of the Moine Thrust Zone, Lochcarron and zone might be expected. Lochalsh, Scotland. Proceedings of the Geologists' Association, 76, 215-242. Given the inferred age and sense of displacement of the BARR, D., HOLDSWORTH, R.E. & ROBERTS, A.M. 1986. Caledonian ductile Sound of Iona structure and related mylonite zones on Iona, thrusting in a Precambrian metamorphic complex: the Moine of it seems likely that this part of the orogenic margin has been Northwestern Scotland. Geological Society of America Bulletin, 97, modified by an unusually deeply eroded representative of 754-764. BECKINSALE, R.D. & OBRADOVITCH, J.D. 1973. Potassium-Argon ages for the fault set that accompanied late Caledonian uplift and minerals from the Ross of Mull, Argyllshire, Scotland. Scottish Journal of exhumation in the British Caledonides. Geology, 9, 147-156. An explanation for the localized deep erosion may lie in CHEENEY, R.F. & MATTHEWS D.W. 1965. The structural evolution of the one or a combination of circumstances which could include Tarskavaig and Moine nappes in Skye. Journal of Geology, 1, 256-281. CUNNINGHAM-CRAIG,E.H., WRIGHT, W.B. & BAILEY, E.B. 1911. The Geology abnormal large post Caledonian uplift related to combined of Colonsay, Oronsay with parts of Ross of Mull. Memoirs of the displacements on the Great Glen Fault and Moine Thrust Geological Survey of the UK. near their intersection; this may have been enhanced by the FRASER, F.M. 1977. The Lewisian and Torridonian geology of Iona. PhD buoyancy of the Ross of Mull Granite itself. The ductile thesis, University of St. Andrews. response of the crust to late Caledonian extension could also HALLIDAY, A.N., AFTALION, M., VAN BREEMEN, O. & JOCELYN, J. 1979. Petrogenetic significance of Rb-Sr and U-Pb isotopic systems in the 400 have been partly induced by high heat flow ahead of the Ma old British Isles granitoids and their hosts. In: HARRIS, A.L. ET AL. emplacement of the granite. (eds) The Caledonides of the British Isles--reviewed. Geological Society, The Sound of Iona Fault occupies a structural setting London, Special Publications, 8, 653-661. similar to that occupied by the Loch Gruinart Fault of Islay HARRIS, A.L. & JOHNSON, M.R.W. 1991. Moine In: CRAIG, G.Y. (ed.) Geology of Scotland 3rd Edition. The Geological Society, London. which, trending NNE, separates the Rhinns Complex HOLDSWORTH, R.E., HARRIS, A.L. & ROBERTS, A.M. 1987. The stratigraphy, (1.9-1.7 Ga) basement rocks with its tectonically modified structure and regional significance of the Moine rocks of Mull, Colonsay Group unconformable cover (foreland) from Argyllshire, W. Scotland. Geological Journal, 22, 83-107. Caledonian (Dalradian) metamorphic rocks to the ESE KELLEY, S.P. & POWELL, D. 1985. Relationships between marginal thrusting and movement on major, internal shear zones in the Northern Highland (Muir et al. 1994, fig. 19). The similarity of the setting of the Caledonides, Scotland. Journal of Structural Geology, 7, 161-174. Loch Gruinart Fault and that inferred for the Sound of Iona MUIR, R.J., F~TCHES, W.R., MALTMAN, A.J. & BENTLEY, M.R. 1994. Fault presents the possibilities that these structures are Precambrian rocks of the southern Inner Hebrides-Malin Sea region: comparable or even were originally continuous and that Colonsay, West Islay, Inishtrahul and Iona. In: GIBBONS, W. & HARRIS, A.L. (eds) A revised correlation of Precambrian rocks in the British Isles. post-thrusting, substantial, steeply inclined faults may be a Geological Society, London, Special Reports, 22, 54-58. common feature of the largely concealed Caledonian front WATSON, J.V. 1984. The ending of the Caledonian orogeny in Scotland. to the SSW of Skye. Journal of the Geological Society, London, 141, 193-214.

Received 16 June 1995; revised typescript accepted 24 July 1995.

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