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. 1061 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/156/6/1061/4886891/gsjgs.156.6.1061.pdf by guest on 23 September 2021 1062 A. M. ROBERTS & R. E. HOLDSWORTH 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,