Journal of the Geological Society, London, Vol. 146, 1989, pp. 369-372, 2 figs. Printed in Northern Ireland

Short Paper: Devonian and later movements on the Great Glen fault system, Scotland

D. A.ROGERS’, J. E. A.MARSHALL’ & T. R.ASTIN3 Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK 2 Department of Geology, The University, Southampton S09 5NH, UK ”Postgraduate Research Institute for Sedimentology, University of Reading, PO Box 227, Whiteknights, Reading RG6 MB, UK

On the Scottish mainland the Great Glen fault (GGF) displaces the the limited provenance data of Mykura & Owens (1983) at Emsian to Frasnian Orcadian Old Red Sandstone (ORS) by only Mealfuarvonie. Mykura (1983) and Donovan et al. (1976) 25-29 km dextrally but net post-ORS dextral offsets in Shetland are noted that the Holgate reconstruction fitted what was known much larger (120 km total). Most of the displacement (15-20 km) of theORS around the SW Moray Firth. However, two occurredbetween Frasnian cessation of Orcadianextension and pieces of Holgate’s evidence are spurious: his matching (1) Permianinitiation of theInner Moray Firth Basin. It probably of the SW limits of the Middle ORS either side of the GGF accompanied transpressional inversion of the Orcadian Basin in the (because of its vertical offset) and (2) of the Middle-Upper mid-late Carboniferous and/or possibly thelate Devonian. ORS boundariesmapped by the Survey; that NW of the Devono-Carboniferoustranstension may also have occurred.The GGF is conformable,whereas to the SE it is probablya earlier history of the GGF includes late Caledonian sinistral motion fault and in any case there are no exposures to indicate its which must have ceased by the late Emsian. course within 13km of theGGF (Rogers 1987). The juxtaposition of two of the three areas of pre-ORS fenite veining known along the GGF is likely to be a coincidence The history of strike-slip onthe Great Glen fault (GGF) since they probably formed during pre-ORS strike-slip (see system has beencontroversial. This paper summarizes the below). constraints on the timing,sense and and amount of GGF motions available from Devonian and later Scottish rocks, Walls boundary fault (WBF). Estimates of c. 95 km dextral particularly the Old Red Sandstone (ORS) of the Orcadian displacement (Astin 1982) are based on (1) the structural Basin. and palaeogeographic continuity of the Walls and Fair Isle Previously published evidence and arguments forthe ORS successions, (2) their biostratigraphic correlation (Fig. history of the GGF system havebeen reviewed by Smith 2) and (3) the presence in the Fair Isle ORS of a pattern of (1977) and Rogers (1987). Four points must be stressed. (1) metamorphism, scapolite veining and basic and acid dykes No proposed match of pre-ORS features across the GGF is indicative, along with a gravity anomaly, of a granite pluton generally accepted. (2) Differences in palaeomagnetic poles to the SW (references in Mykura 1983). Such features are and declinations have been claimed to exist across the GGF seen elsewhere only in the Walls ORSand adjacent indicating post-ORSstrike-slip of several hundred kilo- Devonian granites.Mykura (1983) preferred 60-80km metres, but most workers now agree that the data are not displacement, matching the Fair Isle pluton with the resolvably different. (3)Proven net post-Lower Devonian Sandsting Granitesouth of Walls. Flinn (1977) suggested offsets onmajor Caledonide-parallelfaults in Britain and only 65 km, matching Sandsting with a magnetic anomaly America are small and dextral. (4) Those proposinglarge NW of FairIsle. Matching theFair Isle geophysical post-ORSdisplacements have disbelieved the original phenomena with the plutons north of Walls gives a better fit palaeogeographic contiguity of the Orcadian ORS.The of ORS palaeogeography. evidence cited below should convince them otherwise. Nesting fault. About 16 km dextral movement (Flinn 1977 Original (Emsian to Frasnian) configuration of onshore and references therein) displaces the Graven granite and a Orcadianoutcrops. Figure 1 differs considerably fromthe magnetic anomaly thought to represent buried basic igneous restorations of Donovan et al. (1976) and Mykura (1983 and rocks. The granite is Early Devonian, but we suggest the references therein). The net post-ORS lateral displacements offsetis post-ORS because of its sense; lateCaledonian proposed are as follows. activity on the rest of the GGF system seems to have been sinistral. Great Glenfault. 25-29 km dextraldisplacement (Rogers 1987) offsets NW-SE syndepositionalfault zones which Melby fault. Minor post-ORS strike-slip is proposed. Large bound ‘blocks’ of contrastingMiddle ORS stratigraphy. dextral motions have been proposed (Donovan et al. 1976; Litho- and chronostratigraphy,provenance, palaeocurrents Mykura 1983) to juxtaposethermally metamorphosed and and area1 clast size variations confirm the match of the S deformed ORS of Walls with low grade ORS at Melby, but Black Isle and Cromarty-Nigg blocks NW of the GGF with, Flinn (1977) showed that the fault plane has evidence only respectively, the Foyers-Dores and Inverness-Nairnside for reversed motion. Using the then accepted chronostrati- blocks tothe SE (Fig.2). The reconstruction supports graphy (e.g.Mykura 1983), he proposed thatthe Melby Holgate’s (1969) suggested match of the Loch Oich and ORS post-dates the thermalmetamorphism of Walls. Mealfuarvonie Lower ORS outliers which is consistent with However, Marshall (1988) shows that the Melby ORS is the 369

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TA-GPF TORR ACHILTY-GLAICK-POLINTURK FAULT ZONE Sheet 83). Thisfault has a normal offset which prevents GLFGLEANN LIATH FAULT B BlackIsle precise lateralcorrelation of theORS, but its minor D Dores E Easter Ross N Nigg topographic expression in the adjacentbasement suggests M Mealfuarvonie S Strathpeffer minimal lateral offset. C Crornarty I Inverness Torr Achilityto Glaick-Polinturk fault system. Associated deformation suggests compressional and possibly some .20 km strike-slip reactivation of these ORS syndepositional faults. Contrary to some suggestions, it is concluded that this is not a major strike-slip zone because (1) it does not pass SW into S E Shetland a major fault in the basement, (2) rather than anastomosing, it is offset by highly oblique, syndepositional, transfer faults, (3) the ORS conglomerate richest in Fearn Granite clasts is not offset from its source. Helmsdale fault. There is minor post-ORS strike-slip (in contrast to Flinn's (1977) suggested 50 km dextral offset of magnetic anomalies) because the fault does not continue SW into the basement and so must join the Strath Glass and/or the Polinturk fault (see above). 4 Gleann Liathfault. Post-ORS strike-slip is minor on this RELATIVE syndepositional normal fault because conglomerates are not POSITIONS offset from their sources. y UNCERTAIN

Timing of GGF movements and relationship to Orcadian inversion. Weinfer that in Scotland there has been only limited motion,predominantly dextral and c. 10 km total, onthe GGF S.S. since the Triassic because (1) Tertiary dykes post-date major lateral GGF motion, (2) Jurassic and older rocks in Argyll show minor strike-slip deformation, 50 km some demonstrably dextral, along the GGF and its splays - (Holgate 1969), (3) offshore seismics demonstrate Permian- Triassic normal GGF displacement followed by limited (c. 10 km) netdextral Late Triassic to EarlyCretaceous strike-slip which accommodatedextension in theInner N Moray Firth (McQuillin et al. 1982 and references therein), where minor sinistral motions of theGGF system can explain Late Jurassicand Early Tertiary inversion events (Bird et al. 1987). It was once thought that an offset of zones of maximum density of Permo-Carboniferous dykes in Fig. 1. Reconstruction of the original configuration of onshore Argyll indicatedsubsequent net dextral GGF motion of outcrops of the Orcadian ORS. Only proven offsetsare restored. 8 km, but it has since been shown that some of the dykes Neither post-ORS shorteningor extension obliqueor parallel to the may be of a different age (Morrison et al. 1987). In any case, faults, nor possible slight dextral offsets along subparallel faults are we note the possiblity of intrusion along previously offset taken into account. weaknesses. The GGF system was not active transcurrentlyduring older (Fig. 2) and suggests thatthe fault is an inverted ORS deposition. Figure 2 illustrates the Orcadian Emsian to syndepositionalnormal fault which separatedthe shallow Frasnian history of continuousdeposition and subsidence, Melby succession from the deeply buried Walls ORS at the without the regional unconformities previously attributed to time of its metamorphism and deformation by the Sandsting compressive or transpressive tectonic events. The unconfor- Granite. Thismodel is consistent with syndepositional mities which do exist are local and explainable by normal tectonics in the rest of the Basin (Rogers 1987) and with faulting and tilting during extension (Rogers 1987). There is Walls palaeogeography and early deformation (Astin 1982). no evidence (alluvial fan deposits, lithological or thickness contrasts) onshore for syndepositional activity of any of the GGF splaysinOrkney and Caithness, including the majorpost-ORS strike-slip faults. Indeed, acontinuous suggested, but unprovenSinclair's Bay fault of Donovan et al. Lower toUpper ORS sand-dominatedsequence was (1976): minordextral offsets are implied by associated deposited across the GGF in Ross-shire and the Orcadian deformation, for example on the Latheron fault (Donovan syndepositional fault pattern reflects extension(Rogers et al. 1974), but nomajor offsets are proven orthought 1987). It is inconsistent with basement strike-slip. likely. The following are conclusions of Rogers (1987). Therefore most of the post-ORS strike-slip on the GGF (c. 20 km dextral) occurred between the the latest Frasnian Strath Glass fault. There is minor (a few kilometres or less) and the EarlyPermian. The orientation of compressive post-ORSstrike-slip and

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uncertainty of age of boundary m Upper Old Red Sandstone marker horizons: time Gap not proven [=1 MiddleoldRed Sandstone -J - John 0’Groats possible intervening strata not exposed m Lower Old Red Sandstone - A- Achanarras base notexposed probable occurrence ------possible

Fig. 2. Chronostratigraphy of the Orcadian basin modified from previous schemes (Mykura 1983) using data of Astin (1982), Marshall (1988), Rogers (1987), references therein and unpublished data. Note thenon-equivalence of the Orcadian ‘Lower, Middle and Upper ORS’ to the threefold Devonian subdivision. The Achanarras and Johno’Groats Horizons are time markers representing particularly widespread lake transgressions.

were associated with basin inversion, which in Orkney NW-trending Tornquist Zone, a line of major Caledonian preceded Permian dyke emplacement (Morrison et al. 1987). and Variscan strike-slip, extendsbetween Orkney and Permo-Triassic palaeomagnetic re-setting in the Middle Shetland (Pegrum 1984) and took up the excess motion or ORS by meteoric diagenesis (Robinson 1985) is consistent offset the faults in Shetlandfrom theirlate Palaeozoic with this timing. Vitrinite reflectance studies near the top of continuations, (3) thatmajor sinistral,as well as dextral, theOrkney ORS (S. J. Hillier,pers. comm.) indicate c. post-ORSmotions occurred onthe GGF s.s., butwere 1km burial and so tend to imply that the main inversion confined to a hypothetical splay east of Shetland. However, followed latest Devonian to Early Carboniferous deposition right along theGGF, post-ORSminor structures imply over the Orcadian area. So Orcadian inversion was probably substantial dextral, not sinistral motion, so a sinistral mid-late Carboniferous,but it was possibly also, or even episode is very unlikely. alternatively, pre-Rosebrae Beds (Fig. 2). Pre-ORSexistence and behaviour of the GGF Large post-ORSstrike-slip in Shetlandversus limited system. The dissimilarity of basement around Walls and &placements in themainland. Over 70 km of dextral Fair Isle proves the pre-existence of the WBF (Flinn 1977). displacement is ‘missing’ on the GGF system s.s compared Evidence for that of the GGFincludes its major topographic to the WBF system (or more if the Melby Fault had any expression,its localisation of late Caledonianfenites and strike-slip, but only 15 km less if the Nesting Fault motion associated phenomena (Garson et al. 1984) and the much was pre-ORS). The solution to this ‘problem’ is uncertain. greater deformation of pre-ORS compared to ORS in the We cannot identify faults in the mainland basement whose faultzone (Mykura 1983; Rogers 1987). Our favoured post-ORSstrike-slip can have been so large. Wedo not hypothesis that post-Emsian strike-slips on the GGF have favour reduced estimates of WBF motion. We can envisage been onlydextral is consistent with Watson’s view(1984) neither sufficient strain between Inverness and Shetland, nor that sinistral NE-SW faults throughout the Highlands were enough motion distributed between known strike-slip faults produced by late Caledonian shearing and therefore that the in the Highlands, but a combination of these may solve the pre-ORS activity of theGGF itselfwas sinistral. The problem. Other possibilities which may contributeto a Orcadian data imply that such motions ceased by the late solution are (1) that major post-ORS, pre-Mesozoic, dextral Emsian. Even earlier, the GGF may have moved dextrally, strike-slip faultsrun from Shetland NE-SW through the during Caledonian compression, during emplacement of the Minch and/or N-S along theNorth Sea, (2) thatthe 435 f 10 Ma Strontian granite (Hutton 1988).

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S. J. Hillier, P. F.Friend, M. Armstrong, M. J. Norton, W. S. the biotite granite at Strontian, Scotland. Geological Society of America McKerrow and the late W. Mykura are thanked for advice. We are Bulletin, 100, 1392-1399. grateful for NERC Research Studentships (TU & DAR) and an MARSHALL,J. E. A. 1988. Devonian miospores from Papa Stour, Shetland. 1851 Research Fellowship (DAR). J. York kindly helped with Transactions of the Royal Society of Edinburgh: Earth Sciences, 79, 13-18. drafting. MCQUILLIN,R., DONATO,J. A. & TULSTRUP,J. 1982. Development of basins in the Inner Moray Firth and the North Sea by crustal extension and dextral displacement on the Great Glen Fault. Earth and Planetaly Science Letters, 60, 127-139. References MORRISON,M. A., HENDRY, G.L. & LEAT,P. T. 1987. Regional and tectonic implications of parallel CaledonianPermo-Carboniferousand ASTIN, T. R. 1982. The Devonian geology of the Walls Peninsula, Shetland. lamprophyre dyke swarms from Lisomre, Ardgour. Transactions of the PhD Thesis, University of Cambridge. Royal Society of Edinburgh: Earth Sciences, 77, 279-288. BIRD,T. J., BELL, A., GlBBs, A. D. & NICHOLSON,1987. Aspects of MYKURA,W. 1983. Old Red Sandstone. In: CRAIG,G. Y. (ed.) Geology of strike-slip tectonics in the Inner Moray Firth Basin, offshore Scotland. Scotland, 2nd. edn. Scottish Academic Press, Edinburgh. 205-251. Norsk Geologisk Tidsskrift, 67, 353-369. - & OWENS,B. 1983. The Old Red Sandstone of the Mealfuarvonie DONOVAN,R. N., ARCHER,R., TURNER, P. & TARLING,D. H. 1976. Outlier, Inverness-shire. Institute of Geological Sciences Report, 83/7. Devonian palaeogeography of the Orcadian Basin and the Great Glen PEGRUM, R. M. 1984. Structural development of the southwestern margin of Fault. Nature, 259, 550-551. the Russian-Fennoscandian platform. In: SPENCERet al. (eds) Petroleum -, FOSTER,R. J. & WESTOLL,T. S. 1974. A stratigraphical revision of the Geology of the North European Margin. Norwegian Petroleum Old Red Sandstone of north-eastern Caithness. Transactions of the Royal Society/Graham and Trotman, London. 359-369. Society of Edinburgh, 69, 167-201. ROBINSON,M. A. 1985. Palaeomagnetism of volcanics and sediments of the FLINN,D. 1977. Transcurrent faults andassociated cataclasis in Shetland, Eday Group, southern Orkney. Scottish Journal of Geology, 21, Journal of the Geological Society, London, l33, 231-248. 285-300. GARSON,M. S., COATS,J. S., ROCK,N. M. S. & DEANS, T.1984. Fenites, ROGERS,D. A. 1987. Devonian correlations, environments and tectonics albitites and carbonatitic veins near the Great Glen Fault, Inverness, across the Great Glen Fault. PhD thesis, University of Cambridge. Scotland. Journal of the Geological Society, London, 141, 711-732. SMITH,D. I. 1977. The Great Glen Fault. In: GILL,G. (ed.) Moray Firth area HOLGATE,N. 1969. Palaeozoic and Tertiary transcurrent movements on the geological studies. Special Publication of the Inverness Field Club. 659. Great Glen Fault. Scottish Journal of Geology, 5, 97-139. WATSON,J. V. 1984. The ending of the Caledonianorogeny in Scotland. HUTTON, D.H. W. 1988. Igneous emplacement in a shear-zone termination: Journal of the Geological Society, London, 141, 193-214.

Received 16 November 1988; revised typescript accepted 16 December 1988.

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