Devonian and Later Movements on the Great Glen Fault System, Scotland

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 on the Great Glen fault (GGF) since they probably formed during pre-ORS strike-slip (see system has been controversial. 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 have been 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 proposing large 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 from the 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 juxtapose thermally 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/3/369/4897812/gsjgs.146.3.0369.pdf by guest on 30 September 2021 370 D. A. ROGERS ET AL. 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 continuous deposition 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

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