Journal of the Geological Society, London, Vol. 156, 1999, pp. 1061–1064. Printed in Great Britain.
et al. 1997), but many advocatesof o ffshore regional reacti- vation hypotheses present little or no hard evidence to support such models. In the absence of supporting data we see no reason to accept such reactivation hypotheses carte blanche and thispaper illustratesthatit may be instructiveto look at alternative ideas, i.e. that the younger faults represent import- ant onshore kinematic links between spatially separated off- shore fault sets and that they may form independently of basement structure. What follows is a brief appraisal of relevant examplesfrom the o ffshore basins bordering northern Scotland, at the conclusion of which the possible influence of Mesozoic faults onshore will be discussed.
The Moray Firth Basin. For a long time, following the work of McQuillin et al. (1982), the Moray Firth basin was taken to be a prima facie case of a Mesozoic basin which had formed by reactivation of a Caledonian fault, in this case dextral slip on the Great Glen Fault (Fig. 1). Subsequently, however, Underhill (1991) hasdemonstratedthat the Great Glen Fault wasdormant during the formation of the Moray Firth and that instead, the basin opened as a result of extensional slip on eological relationships in the offshore basins the Helmsdale Fault (Fig. 1), a structure exposed onshore with flanking northern Scotland suggest that a regional syn-depositional Jurassic sediments in its hanging wall. Caledonian control on the structure of these basins is Kinematically the Great Glen story was very convenient, far from proven, but conversely Mesozoic faults of because by transferring displacement along the length of the Gthe offshore set may extend onshore. The Strathconnon fault, Great Glen Fault into the basins of the Scottish west coast long held to be late Caledonian, may be Mesozoic in age, region it allowed the problem of locating the onshore accom- forming the principal kinematic link between the east and west modation of the Moray Firth extension to be circumvented. coast basins of northern Scotland. The numerous outcrops of Opening the basin by extension on the Helmsdale Fault Precambrian basement within the Minch/Hebrides basin nearly doesnot allow thissolution,yet the large extensionon the all lie in the uplifted footwalls of Mesozoic faults. Similar Helmsdale Fault must be kinematically linked to other struc- fault controls on the distribution of onshore outcrop within the tures, otherwise there is a problem in conserving regional strain Precambrian may exist. at the western margin of the basin. The simplest explanation would be if the displacement were transferred to an (as yet Keywords: Scotland, Precambrian, Caledonian Orogeny, Mesozoic, unspecified) en echelon structure (or structures) onshore. basins, reactivation. Offshore north coast, Scotland. Following the advent of deep Both the onshore and offshore regions of northern Scotland seismic reflection profiling offshore UK, perhaps the most cited are classic areas for the study of structural geology and example of evidence for Caledonian control on offshore struc- regional tectonics. The former for its Precambrian and tureshasbeen the correlation of onshorebrittle and ductile Caledonian orogenic (‘basement’) structures, the latter for its thrusts (e.g. Barr et al. 1986) with structures imaged on seismic Mesozoic extensional basins. With some notable exceptions data off the north coast of Scotland (MOIST, Brewer & (e.g. Snyder 1990; Underhill & Brodie 1993; Butler & Hutton Smythe 1984; DRUM, Snyder 1990) (Fig. 1). The recognition 1994) there have, however, been few serious attempts to of west-dipping half-graben (West Orkney basin) formed by integrate the structural evolution of the onshore and offshore the apparent reactivation of older east-dipping thrusts gave rise areas, and the prevalent view remains that these are two to elaborate theories about Devonian post-orogenic collapse distinct structural provinces, with old orogenic structures basins (e.g. Enfield & Coward 1987). More recent work, how- onshore and younger basins offshore. In this paper we advance ever, has demonstrated that the sediment fill of the West the hypothesis that some offshore Mesozoic structures extend Orkney basin is predominantly Permo/Triassic in age, not onshore and that some structures in basement (and tradition- Devonian (e.g. Stoker et al. 1993, figs33 & 34) and that ally regarded asCaledonian) may be younger, or at least onshore extensions of offshore faults demonstrably cut across exhibit a younger component of movement. all basement structures (Holdsworth 1989). This evidence While the existence of onshore Mesozoic structures has doesnot preclude the o ffshore structures being reactivated received little previousattention there hasbeen much specu- Caledonian thrusts, but there remains only a geometric corre- lation about the control exerted by Caledonian and other lation of bright seismic events to the mapped onshore geology. basement structures on the development of offshore Mesozoic The case for such a correlation is weakened still further when it basins (e.g. Snyder 1990; Bartholomew et al. 1993). It hasbeen isrealized that bright seismicreflectionsfrom fault planesare proposed, for example, that many Caledonian brittle and not diagnostic of basement-involved structures, but that any ductile thrusts and steeply-dipping strike-slip faults (e.g. the fault exhibiting a significant velocity contrast will produce a Highland Boundary/Great Glen Fault trends) control bright fault-plane-reflection (e.g. Yielding et al. 1991). None of the location and geometry of Mesozoic offshore structures. these observations preclude reactivation in the West Orkney Reactivation of basement structures is a widely recognized basin, but they reduce the evidence for reactivation to no more phenomenon in the continental lithosphere (e.g. Holdsworth than spatial proximity.
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Fig. 1. Location map for the Scottish Highlandsand adjacent o ffshore basins. The main known and postulated Mesozoic faults are shown, highlighting the possibility of structural continuity across the Highlands. The following abbreviationshave been used:CF, Camasunary fault; RF, Raasay fault; AF, Applecross fault; ACF, Achnashellach fault; KF, Kishorn fault; KHF, Kinlochourn fault; LGF Loch Gruinart fault; S, Scalpay; E, Eigg.
Offshore and onshore basement highs in the North Sea and the Precambrian outliers. The largest and easiest to place in its Hebrides regions. Within the offshore area north and east of structural context is the Lewisian outlier forming the Outer Scotland many high-standing horsts and basement highs have Hebrides. Regional cross-sections through the Minch Basin been recognized (e.g. The Halibut horst, Fig. 1; the Shetland (e.g. Morton 1992; Butler & Hutton 1994; Fig. 2) show that the Platform ‘granite’ at 60N; the Tern Eider ridge, Roberts et al. Outer Hebrides sit in the footwall of the large-displacement 1995). Such basement highs are commonly interpreted either as Minch fault, the Mesozoic master fault to the Minch/Sea of the areas of long-lived basement elevation, ‘shielded’ from the Hebridesbasin.The Outer Hebridescan therefore be viewed as surrounding basins by their inherited basement structure, or as a classic footwall high, elevated by footwall uplift (Fig. 2) and ‘pop-up’ flower structures on strike-slip faults reactivating elongate in outcrop along the Minch fault (Fig. 1). older Caledonian structures. In fact there is no need to invoke Next consider two other major Lewisian outliers, the islands any special basement control on these structures, because they of Coll and Tiree (Fig. 1). Morton (1992, fig. 1) showed are all situated in the footwalls of major Mesozoic extensional that these islands sit in the footwall of another large Mesozoic faults(e.g. Roberts& Yielding 1991; Roberts et al. 1995). fault, the Skerryvore fault (see also Fyfe et al. 1993 & BGS Far from being shielded from Mesozoic tectonism, they have 1:250 000 Tiree map sheet). These two islands also therefore participated fully in the formation of the extensional basins, comprise part of an elevated footwall high, in the footwall of having been elevated to their current structurally high positions the next major fault eastwards from the Minch fault (Fig. 1). by syn-extension footwall uplift. The case for invoking Moving northwards, the Skerryvore fault is mapped into the reactivation is unnecessary. Camasunary fault (Morton 1992; Fyfe et al. 1993), which crops On a regional scale comparable footwall highs of elevated out on Skye and passes immediately east of the island of Rum basement comprise the Outer Hebrides (Fig. 1). It has long (Figs1 & 2). Outliersof Precambrian are alsoexposedin the been known that the Minch/Sea of the Hebridesarea westof footwall of the Camasunary fault, although the elevated Scotland is part of a Mesozoic sedimentary basin (e.g. Morton Precambrian rocksare the younger Torridonian sequence 1992 and references therein). Seismic data have shown the (Morton 1992; Fyfe et al. 1993; Butler & Hutton 1994). structure of the basin to be essentially a west-dipping half- Butler & Hutton (1994, fig. 6) suggested that the single graben in the hanging wall of the Minch fault (e.g. Butler strand of the Camasunary fault in SW Skye partitions its & Hutton 1994). In contrast to the North Sea, there are displacement northwards into three discrete faults, the Raasay several large outcrops of Precambrian basement (Lewisian, fault, the Applecross fault and the Kishorn fault. The Raasay Torridonian ) exposed within the basin above sea-level. The fault exposes the Torridonian and Lewisian of Raasay and outcrops in question form all or part of the islands of the Outer Rona in itsfootwall, and in the footwall of the Kishornfault Hebrides, Coll, Tiree, Iona, Skye, Rhum, Raasay, Rona and liesthe Torridonian of Scalpay (Fig. 1). Scalpay (Fig. 1). By now the structural setting of the Precambrian outliers The probable reason for this contrast lies in two separate within the Minch basin is becoming clear; they lie consistently geological processes. First consider the structural setting of all within the footwalls of Mesozoic faults, active during the
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Fig. 2. Schematic cross-section (at true vertical scale) from the Outer Hebrides to the Great Glen Fault (GGF) during the Jurassic, showing how varying components of the Precambrian/Caledonian basement came to be elevated in the footwalls of Mesozoic faults. The present-day erosion level is approximately 1–2 km below the sea-level datum of the section. Stratigraphic thicknesses are not intended to be accurate. See Fig. 1 for location.
formation of the basin. North of the Great Glen fault one The continuation of Mesozoic extension onshore: the remaining ‘island outcrop’ of Precambrian remains to be Strathconnon fault. Having demonstrated that the present day explained, the Lewisian and associated Precambrian cover outcrop of offshore Precambrian is controlled by Mesozoic rocksof Iona (Fig. 1). Potts et al. (1995) have explained the faults, can we extrapolate this relationship (or the faults structures observed on the island of Iona as the result of late themselves) onshore into the northern Highlands? The Caledonian ductile faulting. Regardless of the Caledonian Applecross and Kishorn faults (Fig. 1) are known to extend a extensional history in the Sound of Iona, we believe that the short distance onshore and at the east coast, the Helmsdale geological observation of Precambrian basement exposed on fault passes onshore. Between these structures consider now Iona suggests an analogy with Coll, Tiree, Raasay etc. and that the Strathconnon fault (Fig. 1) (see BGS 1:250 000 Great Glen there islikely to have been Mesozoicmovement and footwall sheet), which exhibits the following features. (1) It shares a uplift on the Iona fault. Two linesof circumstantialevidence common NNE–SSW strike with both the Minch/Hebrides lend support to this idea. First, there is known to have been faultsand the HelmsdaleFault. (2) It hasthe outcrop post-Caledonian fault movement in this area because the geometry to accommodate relay overlap between the Assapol fault on the Ross of Mull has Palaeocene lavas in its Helmsdale Fault and the faults in the Minch. (3) Assuming downthrown hanging wall (Holdsworth et al. 1987). Second, that like the Minch, Camasunary and Helmsdale Faults it dips Potts et al. (1995) suggested a southwards correlation (with to the east at depth then in the Sound of Sleat and nearby which we agree) of the Iona fault (across the Great Glen Fault) onshore, the Strathconnon fault exposes the Lewisian of Skye with the Loch Gruinart fault (Fig. 1). The Loch Gruinart fault and Glenelg (within the Moine Nappe) in itsfootwall (Fig. 2, south of Islay contains Permo-Triassic and Lower Jurassic Fyfe et al. 1993, Great Glen sheet), presenting an analogous rocksin itshanging wall (Fyfe et al. 1993) and thusmust geometry to the Minch fault, Skerryvore fault, etc. (4) Again have accommodated Mesozoic displacement. Where the Loch assuming an easterly dip, the Strathconnon Fault onshore is Gruinart fault traverses the island of Islay it exposes the responsible along much of its length for juxtaposing the Precambrian basementrocksof the Rhinnscomplex in its deeper Morar Group of the Moine in itsfootwall againstthe footwall and between Islay and the Great Glen fault the shallower Glenfinnan Group in its hanging wall, i.e. the Morar Precambrian of the island of Colonsay is likewise exposed. Group may be uplifted in the footwall. (5) If the trace of the Fitches& Maltman (1984) documented the tectonicsof Islay fault in the Sound of Sleat isextrapolated o ffshore beneath the and Colonsay and described the Loch Gruinart fault as Tertiary lavasof Eigg it projectsdirectly into the mapped trace ‘post-Permian’. of the Skerryvore Fault at Coll (Fig. 1). Arising from point 5, In summary, we suggest that from the Rhinns of Islay we suggest that the Skerryvore fault splays northwards into northwardsto the Outer Hebrides,wherever Precambrian the Camasunary Fault and the Strathconnon Fault, perhaps rockscrop out within the Minch/Sea of the HebridesBasin,the explaining why Lewisian is exposed adjacent to the Skerryvore localized outcrop can be explained by footwall uplift adjacent segment (Coll, Tiree), where the summed displacement is to the Mesozoic faults which formed the basin. large, but younger Torridonian isexposedadjacent to the Having outlined the structural setting of the Precambrian Camasunary segment (Rum, Skye), where the partitioned outliers, their presence at the present surface, rather than displacement is smaller (Fig. 2). below a Cretaceous–Recent post-rift sequence (cf. the North If it is accepted that the issue of conserving strain at the Sea), must be explained. Brodie & White (1994) have done so western margin of the Moray Firth requires transfer of dis- by postulating that the basins west of northern Scotland and placement onshore into the northern Highlands and that the the adjacent mainland area were underplated by basaltic melt extension from the Minch basin may also extend onshore, then during early Tertiary opening of the north Atlantic, giving rise we suggest that the Strathconnon fault is the most likely to a permanent uplift. Thisallowsthe uplifted Precambrian structure in the northern Highlands to be the Mesozoic footwalls to be exposed at or above present-day sea-level. Thus kinematic link between the east and west coasts of Scotland. the distribution of Precambrian basement in the Hebrides Of course not all of the extension present in the offshore basins results from fault-controlled footwall uplift during the Meso- can be relayed across the mainland, otherwise the ‘mainland’ zoic, followed by early Tertiary regional uplift asa resultof too would be predominantly below sea-level. We suggest, underplating. however, that a component of extension may be relayed
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between the offshore basins. Following from this suggestion it lineaments. In:P,J.R.(eds)Petroleum Geology of Northwest Europe, is possible, although untested, that the shorter Loch Shiel Proceedings of the 4th Conference. The Geological Society, London, 1109–1122. (Fig. 2), Strathglass and Achnashellach faults (Fig. 1) may be B,J.A.&S, D.K. 1984. MOIST and the continuity of crustal part of the Mesozoic fault set, while the Loch Maree fault reflector geometry along the Caledonian-Appalachian orogen. Journal of (Fig. 1), mapped into the Minch asthe Mid-Minch High, has the Geological Society, London, 141, 105–120. the appropriate orientation (NW–SE) for an extension-parallel B, J.A. & W, N.J. 1994. Sedimentary basin inversion caused by igneous transferfault, astoo doesthe Kinlochourn Fault (Fig. 1). underplating: Northwest European continental shelf. Geology, 22, 147–150. South of the Great Glen thishypothesismaybe developed B , R.W.H. & H , D.H.W. 1994. Basin structure and Tertiary magmatism on Skye, NW Scotland. Journal of the Geological Society, further. Fyfe et al. (1993) have previously suggested that the London, 151, 931–944. Ericht–Laidon and Tyndrum Faults(Fig. 1) in the central E,M.A.&C, M.P. 1987. The structure of the West Orkney Basin, Highlands may respectively connect across the Malin Sea to northern Scotland. Journal of the Geological Society, London, 144, 871–844. the Foyle and Tow Valley Faultsin Northern Ireland. Asthe F,W.R.&M, A.J. 1984. Tectonic development and stratigraphy Foyle and Tow Valley faultsare mapped o ffshore as part of the at the western margin of the Caledonides: Islay and Colonsay, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences, 75, Mesozoic basin complex (Malin 1:250 000 sheet) this would 365–382. imply a Mesozoic history for the Ericht–Laidon and Tyndrum F, J.A., L,D.&E, D. 1993. The geology of the Malin–Hebrides area. faults, comparable with our interpretation of the Strathconnon United Kingdom offshore regional reports. British Geological Survey. fault. H, R.E. 1989. Late brittle deformation in a Caledonian ductile thrust wedge: new evidence for gravitational collapse in the Moine Thrust sheet, Sutherland, Scotland. Tectonophysics, 170, 17–28. Re-examining the orthodoxy. We believe that by integrating ——, B, C.A. & R, A.M. 1997. The recognition of reactivation onshore and offshore geological relationships enough evidence during continental deformation. Journal of the Geological Society, London, can be put forward to make the case for a component of 154, 73–78. Mesozoic extension having traversed the Scottish Highlands ——, H,A.L.&R, A.M. 1987. The stratigraphy, structure and on regionally recognized NNE–SSW-trending extensional regional significance of the Moine Rocks of Mull, Argyllshire, W. Scotland. Geological Journal, 22, 83–107. faults. These faults are traditionally held to be Caledonian, but MQ, R., D,J.A.&T, J. 1982. Development of basins in perhapsit istime now to challenge thisassumption.While the the Inner Moray Firth and the North Sea by crustal extension and dextral Great Glen Fault is demonstrably a Caledonian structure displacement of the Great Glen Fault. Earth and Planetary Science Letters, (Stewart et al. 1999) with only minor post-Palaeozoic reacti- 60, 127–139. M, N. 1992. Dynamic stratigraphy of the Triassic and Jurassic of the vation (Underhill 1991), and the Helmsdale Fault is clearly a Hebridesbasin,NW Scotland. In:P, J. (eds) Basins on the Atlantic Mesozoic fault which extends onshore, what of the other Seaboard: Petroleum geology, Sedimentology and Basin Evolution. faults? Could they be Mesozoic-only in age, or have they Geological Society, London, Special Publications, 62, 97–110. demonstrably reactivated older structures? If the onshore P, G.J., H, R.H., H,A.L.&F, F.M. 1995. Late-orogenic faults are predominantly Mesozoic, then the whole assumption extensionaltectonicsat the NW margin of the Caledonidesin Scotland. ff Journal of the Geological Society, London, 152, 907–910. of regional Caledonian control on the development of o shore R,A.M.&Y, G. 1991. Deformation around basin-margin faults basinsbecomesincreasinglyuncertain. in the North Sea/Norwegian rift. In:R, A.M., Y,G.& We do not attempt to provide the definitive answer to these F,B.(eds)The Geometry of Normal Faults. Geological Society, questions here, but rather raise the issue that by attempting to London, Special Publications, 56, 61–78. integrate onshore and offshore structural relationships perhaps ——, ——, K , N.J., W ,I.&D -L , D. 1995. Quantitative analysis of Triassic extension in the Northern Viking Graben. Journal of the we can look at an area of classic structural geology in a new Geological Society, London, 152, 15–26. light. Our discussion also highlights the need for further S, D.B. 1990. The Moine Thrust in the BIRPS data set. Journal of the studies to test this hypothesis. Such studies might include Geological Society, London, 147, 81–86. dating fault movementsfrom a combination of o ffset igneous S, M.A., S,R.A.&H, R.E. 1999. The structure and early kinematic history of the Great Glen Fault Zone, Scotland. Tectonics, features, Ar–Ar dating, and palaeomagnetic data (e.g. Torsvik 18, 326–342. et al. 1992), together with structural studies assessing whether S, M.S., H,K.&G, C.C. 1993. The geology of the Hebrides or not reactivation hasoccurred (e.g. Stewart et al. 1999). and West Shetland shelves, and adjacent deep-water areas. United Kingdom offshore regional report. British Geological Survey. The authorswould like to thank M. Badley and T. Harrisfor T, T.H., S, B.A., S, E., A,T.B.&D, J.F. discussions on new ways to look at old rocks. J. Turner is thanked 1992. Palaeomagnetic dating of fault rocks: evidence for Permian and Mesozoic movements and brittle deformation along the extensional for hisreview and N. Rogersfor hiseditorial comments.The Dalsfjord Fault, western Norway. Geophysical Journal International, 109, template for Fig. 2 was prepared using N. Kusznir’s 565–580. software. U, J.R. 1991. Implicationsof Mesozoic-Recentbasindevelopment in the western Inner Moray Firth, UK. Marine and Petroleum Geology, 8, 359–370. References —— & B, J.A. 1993. Structural geology of Easter Ross, Scotland: implicationsfor movement on the Great Glen fault zone. Journal of the B, D., H,R.E.&R, A.M. 1986. Caledonian ductile Geological Society, London, 150, 515–527. thrusting in a Precambrian metamorphic complex: The Moine of Y, G., B,M.E.&F, B. 1991. Seismic reflections from northwestern Scotland. Geological Society of America Bulletin, 97, 754–764. normal faultsin the northern North sea. In:R, A.M., Y,G. B, M.J., P,J.M.&P, C.M. 1993. Regional structural &F,B.(eds)The Geometry of Normal Faults. Geological Society, evolution of the North Sea: oblique slip and the reactivation of basement London, Special Publications, 56, 79–89.
Received 29 April 1999; revised typescript accepted 3 June 1999. Scientific editing by Nick Rogers.
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