1Department of Earth Sciences, University of Liverpool, Liverpool
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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 907 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/152/6/907/4892990/jgs_152_006_0907.pdf by guest on 01 October 2021 908 G. J. POTTS ET A L. N N --- I~'h'l --- Unconformity O -- Geological bound81y -- Fault --.- Intrusive contact z9 T Mylonitic folielia~ 1 ~ialionVeeacalmylonilicz~L., 0 I ,5O km i km St. Mary's -AU~¢ Jetty Cnoe MOINE Scodand earrs~-h / o~ 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.