The Structure of the West Orkney Basin, Northern Scotland

Journal of rhe Geological Society, London, Vol. 144, 1987, pp. 871-884, 17figs. Printed in Northern Ireland

The Structure of the West Orkney Basin, northern Scotland

M. A. ENFIELD & M. P. COWARD Department of Geology, Imperial College, London SW72BP, UK

Abstract: The West Orkney Basin developed in Devonian times, as the western part of the Orcadian intermontane basin. It has been studied using commercial speculative seismic reflection dataand the MOIST deep seismic data. The NW edge of the West Orkney Basin is formed by listric faults which are also strongly arcuate in plan, while the SE part is composed of straight domino-type faults which formedparallel to earlier (Caledonian) layering in thebasement. Fault restoration and balancing suggest that initial extension in the basin occurred on low-angle reactivated Caledonian thrust faults. Steeper breaching faults cut the low-angleset, forming planar (domino-type) faults in the centreof the basin but listric faults at the NW margin. The maximum extension is about 45% in the basin centre, most of this being taken up on the later breaching fault system. This extension decreases to the SW, where fault tips occur on-shore, but some may transfer to fault systems in the Minches. The faults apparently decouple at a depth of18-20 km and the extension suggests an initial post-Caledonian crustal thickness of up to 40 km. However, the sedimentary thickness is an averageof only 3 km in the basin centre, muchless than would be expected had the lithosphere thinned homogeneously, and there is no evidenceof a thermal subsidence phase to the basin. This suggests that the extension shown by the West Orkney Basin was transferred to lower lithospheric levels to the east along the deep decoupling zone. The Devonian sediments on-land show facies changes and periods of uplift and erosion which may be relatedto extension during basin development. They also show a phase of pre-Late-Permian tectonic inversion where the beds are locally folded and thrusted, probably related to the Hercynian events further south. The West Orkney Basin is capped by Mesozoic sediments and was probably reactivated during Mesozoic basin development in the Minches and Moray Firth. The shape of the faults, their orientation and decoupling levels are strongly controlled by the earlier Caledonian structure, in particular by the layering and crustal anisotropy developed along and above the Moine thrust. The West Orkney Basin with its 20 km deep decoupling level formed by extension of Caledonian thickened crust. It is notablethat the major basin-bounding faults to the NW, the Outer Islesand Flannan faults, whichdeveloped where the crust was thinner and henceless ductile at depth, decoupled at much deeper structural levels, at the Moho or below.

The West Orkney Basin lies off the north coast of Sutherland British Institutions Reflection Profiling Syndicate (BIRPS), and Caithness, northern Scotland, and west of the Orkneys to 15 S TWT (seeSmythe et al. 1982; Brewer & Smythe (Fig. 1). Shallow bore-hole data suggest thatthe basin 1984; Cheadle et al. 1987). The West Orkney Basin overiies carries at least a capping of Permo-Triassic red sandstones the northern continuation of the early Palaeozoic Caledo- (Kirton & Hitchen 1987). However, Devonian age Old Red nian orogen and this paper will discuss aspects of the basin Sandstone sediments are seen on-land on the N Sutherland geometry and its evolution in relation to the structure of the coast (Figs 1 & 14) and more extensively in the Orkneys and Caledonides. Caithness, which representthe emergent portions of the Orcadian basin. These sediments are structurally continuous with those of the West Orkney Basin and indicatea Regional Geology Devonian age of initiation for the West Orkney Basin (see The main features of the Caledonides are shown in Fig. 1. discussion later).These sediments showevidence of both The Moines are Proterozoic metasediments with minor basic normal faulting, producing the basins, and also contractional and acid intrusions which were generally intensely foliated deformation (sensu Norris1958), producing folding and andmetamorphosed to upper greenschist and amphibolite thrusting within the sediments, some of which is strike-slip facies during the Caledonian orogeny. Along the N coast of related. Scotland the Moines show a complex interlayering of This paper aims to describe the structure and structural intensely foliatedsediments, metabasite intrusions and history of the West Orkney Basin and adjacent parts of the earlier basement due to isoclinal folding and closely spaced, Orcadianbasin, using datafrom field work on-landand WNW-directed thrust imbrication (Butler & Coward 1984; fromthe interpretation of offshorecommercial anddeep Barr et al. 1986). The foliation dips E to SE about 15" in the seismic reflection data.The commercial dataare froma western part of the section, but at steeper dips, sometimes speculative survey by Western Geophysical, and consist of nearly vertically, in the eastern part. 48-fold multichannel reflection profiles (Fig. 1) shot using a The Moines were thrust to the WNW over a foreland, 930cm airgun source and 2400 m streamers. Data were consisting of Lewisian (Archaeanto lower Proterozoic) recorded to 6s two-way traveltime (TWT) and all lines basement,overlain by a thick sequence of Torridonian were time migrated. The deep data are from theMoine and (upper Proterozoic)arkosic sandstones, grits andcon- Outer Isles Seismic Traverse (MOIST), undertaken by the glomerates, again overlain by Cambro-Ordovician shelf 871

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I I 5 4 3

Fig. 1. Location map for the Western Geophysical linesof the West Orkney Basin, north of the Scottish coast. MT, Moine Thrust;Ns, Naver slide; SBS, Sgurr Beag slide; OIT, Outer Isles thrust; OIF, Outer Isles fault (possible offshore continuation of the OIT); GGF, Great Glen fault;72/25, BGS shallow borehole number 72/25. Limit of the restored Cambrianof the Moine Thrust Zone after Butler& Coward (1984).

sediments (Peach et al. 1907; Butler & Coward 1984). A the Middle ORS on Orkney mainland (Wilson et al. 1935), minimum overthrust displacement of 54 km on the Moine where the Lower ORS is considered to have been uplifted thrust is indicated from the restoration of imbricated Middle and eroded prior to Middle ORS deposition. and Upper Cambrian sedimentsin the Eriboll and Foinaven The Middle ORS is strongly diachronous, onlapping to areas(Butler & Coward 1984). Thusthe Moine thrust the west in Caithness and Orkney. The early Middle ORS cannot have cut up through basement or Lower Cambrian succession is a relatively quiescent lacustrine facies, sediments within 54 km of its present outcrop trace in these producing a flagstone sequence 4 km thick in Caithness and areas (Fig. 1). 2 km thick in Orkney. The Upper ORS is separated from Thestrata of the West Orkney Basin have only been the Middle ORS by an unconformity, which apparently testedfrom shallow boreholes off-shore (for example, occurs throughoutthe basin. OnHoy this unconformity borehole 72/25, Dunham 1973; see Fig. 1 for location), but post-dates a set of earlier normal faults (Wilson et al. 1935). thesouthern continuation of the West Orkney Basin Upper ORS sedimentation on Orkney is characterized by sedimentsform aseries of conglomerates and sandstones high energy fluvial deposits, with aeoliandunes locally occupying half-grabens at Tongue, Kirtomy and Poulouris- developed(Astin 1985). On Hoy the development of caig and are considered to be of Old Red Sandstone (ORS) calc-alkaline lavas, dated at about 370Ma (Halliday et al. age (Peach & Horne 1914; Blackbourn 1981). 1977), marks the onset of Upper ORS sedimentation. The ORS deposits of Caithness andthe Orkney Isles unconformablyoverlie an irregularlandscape in the Caledonian rocks and are considered to have been deposited Deep structure as seen on MOIST in a majorinter-montane basin, theOrcadian basin TheMOIST profile (see Smythe et al. 1982; Brewer & (Anderton et al. 1979). Smythe 1984) shows the West Orkney Basin to be comprised The Lower ORS coarselenticular breccias and con- of half-grabens,formed onthe hanging-walls of easterly glomerates, interdigitated with finer bedded sandstones, dippingfaults (see also Brewer & Smythe 1984, 1986; havebeen deposited within small isolated intermontane Blundell et al. 1985; Cheadle et al. 1987; Kirton & Hitchen basins (Mykura1976). Local thick fanglomerates suggest 1987). Beneath the West Orkney Basin, within the lower deposition within basins which are controlled by active fault crust, the MOIST section shows azone of sub-horizontal scarps. An angularunconformity is present at the base of reflectors down to 9 S TWT,where a strongly reflecting zone

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is interpreted as representing the Moho (Smythe et al. 1982; structures onthe map (Fig. l), but may be linked by a Brewer & Smythe 1984). The upper and middle crust in the NW-trending fault N of Lewis. eastern part of the MOIST profile is dominated by strong, The basins appear to be in isostatic equilibrium (Cheadle east-dippingreflectors, which have beeninterpreted as et al. 1987) andthe Moho is at approximately the same lower Palaeozoic sediments overlain by Caledonian foliated TWT throughout (Blundell et al. 1985). The stratigraphy of rocks (Brewer & Smythe 1984) or asCaledonian foliated thesewestern basins is not well known; the uppermost rocks alone (Butler & Coward 1984). The Moine thrust has sediments are of Permo-Triassic to Lower Jurassic age been interpreted as lying within ornear the western and (Binns et al. 1974). Brewer & Smythe (1984) suggested that lower limit of these reflectors (Brewer & Smythe 1984), but the lowermostsediments of the Outer Isles basin may be Butler & Coward (1984) suggested it may lie above and to Torridonian in age, but this is unlikely if the Outer Isles theeast of the mid-crustalreflectors, which could be faultreactivated an earlierCaledonian and hence post- consideredas Caledonianstructures within the basement Torridonian thrust; there may, however, be a considerable and below the Moine thrust. More recently Coward (1986) thickness of Devoniansediments in theOuter Isles and has comparedthe Caledonian structures seen on MOIST Minch basins,as in the West Orkney Basin.These with those in western Shetland (Flinn et al. 1979), where the sediments would have very similar seismic velocities and Moine thrust forms the western and lower boundary of a characteristics to the Torridonian. wide ductile shear zone in Moine and basement schists. To the west of the West Orkney Basin, the Outer Isles and Minch basins lie in the hanging-walls of large normal The geometry of the West Orkney Basin faults, one of which, theOuter Isles fault, hasbeen The detailed analysis of the basin fault systems (Figs 2 & 3) considered to be the reactivated Outer Isles thrust (Brewer is based onthe speculative data shot by Western & Smythe 1984). This fault is approximately planar with a Geophysical andon the MOIST line. The analysis has dip of about 25" and can be tracedinto the lower crust, concentrated on the faults and associated structure at depth, possibly to jointhe Moho. Brewer & Smythe (1984) and beneath the cover sedirnents. The sedimentary rocks of the Peddy (1984) have suggested that the Outer Isles fault may West Orkney Basin produce parallel to sub-parallel cut the Moho, though there is little evidence for any major westward-dipping reflectors, which are locally continuous finite offset of the Moho on the MOIST line. Note that the and well defined. The top to the metamorphic basement is Outer Isles thrust, as seen on land, and the OuterIsles fault, defined by a pronounced change in seismic characteristics as seenon the seismic sections, are widely separated and commonly by astrong reflector. The basement rocks

I l 4 3 NORMAL FAULT

CUT-OUT ZONE FOR TOP OF BASEMENT

Fig. 2. Map of the principal faults in the WOB, showing fault positions (solid lines) and the cut-out zone and contours for the top of metamorphic basement (in TWT). See inset of Fig. 3 for explanation of 'cut-out zone'. Thin dotted lines indicate dip-lines of the Western Geophysical seismic survey grid. Only lines referenced in the text and figures are numbered. The principal faults are numbered 1 to 11. Seismic lines shown in Figs 5, 6, 7 & 16, are from lines WOB 14, 19, 15 and 17. Locations a & b, on the north Scottish coast, refer to Figs 14 & 15, respectively.

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Fig. 3. Map of faults and structure within the sedimentary cover. The contours show depth (in TWT)to seismic horizons which can be correlated within individual half- grabens, but there is no certainty of correlation between half-grabens. The ma- i jor faults on shore are shown, along with typical regional bedding dips (degrees) J within the ORS.

vary in seismic characteristics:in the NW continuous or maps and NNE-SSW in the southeastern sector. They have semi-continuousaligned reflectors are almostnon-existent, been numbered 1-11 in Fig. 2 for ease in explanation, but butin the SE thebasement clearly shows gently to thenumbers have no chronological significance. Figure 4 moderately eastward-dippingreflectors, as onthe MOIST shows the cumulativedisplacement diagram for the West line. Some faults are obvious only from the changein Orkney Basin, summing the horizontal displacements for seismic character across them,but there are others in each fault, using the distancebetween footwall and basement and in cover which standout as faultplane hanging-wall cut-offs. Although the displacements vary reflections. considerably along individual faults, the cumulative curve is Figures 2 and 3 show the cut-out zones for the top of the relatively simple.This supports observationsfrom the basement (Fig. 2) and reflectors within the cover (Fig. 3). seismic data that the majority of faults are linked, either at The reflectors within the cover can be traced and correlated depth by frontal branch lines or laterally by steep lateral to alongdip-parallel and strike-parallel seismic sections, but oblique branch lines. Two alternative curves are presented only within individualhalf-grabens, andare of unknown, onthe diagram. The thindashed line is drawn from the but presumed Devonian, age. The cut-out zone marks the seismic data availablefrom this survey,and shows the distance between the hanging-wall and footwall cut-offs, or, extension dying out rapidly tothe SW. However, if the where there has been erosion of the footwall, the distance extension on fault 11 is continued tothe SW into the betweenthe outcrop of the fault andthe cut-off onthe Minches, the curve remains smooth (heavy dashed line). hanging-wall (Fig. 3, inset).The size of the cut-outzone Faultswhere observed in the cover sequence have thus gives a minimum estimate of the amount of horizontal dominantly listric geometries. They occur both as the upper extension; even where the footwall and hanging-wall cut-off continuation of more planar faults within basement and also lines are visible onthe seismic sections, there may have as separate, dominantlyantithetic systems, which detach beenextra small-scale brittleor ductileextension not wholly within the cover sediments and have developed as a detectable from the seismic data. Figures 2 and 3 also show response to local strains in the hanging-walls of basement thedepths to these particularhorizons in TWT. The faults (Fig. 5). difference between the two maps partly reflects the history The extension direction is interpreted as to the SE and is of the faults and partly the different geometries in basement constrained by the orientations of lateralfaults, from the and cover. geometry of the basins on the hanging-walls of faults as seen The faults within the basement dip approximately SE from the contours of depth to particular horizons (Figs 2 & and generally trend NE-SW in the northwestern part of the 3), and from theshape of the cumulative displacement

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NW 4 I 0-

va2 c E 3 CUMULATIVE DISPLACEMENT CURVE FOR THE WEST ORKNEY BASIN

Fig. 4. Simplified map of northern Scotland showing the cumulative displacement curve for the West Orkney Basin andthe principal NW 4 faults onland and offshore. The cumulative displacement curve 0 represents the summed extension for each cut-out zone (see Fig.3, inset) measured parallel to the regional extension direction,which is indicated by arrows. The thin dashed line is constructed onlyfor data shown on the map and assumes that all extension diesout 1 onshore in N Scotland. The heavy dashed line assumes fault 11 continues to the SW, with approximately the same displacement, and the extension links intothe Minch system. The stippledarea is l52 the region of middle crust containing east dipping, strong reflectors. Y c 3 curve, which becomescomplex and asymmetric when c constructedin anorientation oblique to extension. The 3 oblique orientation of the NNE-SSW-trending faults in the southeastern part of the basin could be due to a change in

extensiondirection, a rotation duringmovement on lower 4 faultsduring a sequence of footwall collapse, or by their having initially developed obliquely. We preferthis last alternative, as will be discussed later. Fig. 5. Migrated seismic section of line WOB 14, showing antithetic The faults change dip at depth;faults 5-8 and 10 shallow listric normal faults, which detach wholly within the covet and become low angled at 2.5-3.5s, 6-9 km.However, sediments, developed on the frontal ramp of fault 10. (a) fault 11 shallows below 5 S TWT (c. 15 km), below which uninterpreted and (b) interpreted. The dottedline is the top of depth resolution is lost on the seismic record, and faults 1-3 metamorphic basement (as in all other seismic sections); Reflector and 9 continue as relatively planar structures to below 5 S ‘G’ is a picked horizon used to construct depth contours in Fig. 3, TWT. This deep set of faults is parallel to reflectors in the see text (as in all other seismic sections). basement, which, fromthe MOIST sectionand the constraints imposed by section balancing, are considered to flatten at about 18 km in the crust. These fault blocks appear both above and below the faults on both the speculative to have rotated in a domino fashionduring extension seismic anddeep seismic profiles (MOIST,DRUM), and (Morton & Black 1975; Brun & Choukroune 1983), as are confined to the area of faults in the southeastern part of shown by the absence of any important roll-over fold at the the area. Therefore we feel confident in their interpretation basement/coverinterface (Fig. 9). This contrasts with the as reflections from real structures within the basement. The morepronounced roll-over fold shown by the area showing these strong mid-crustal reflectors (Fig. 4) lies basement/cover contact abovethe listric faults, especially directlyalong strike fromthe on-shoreoutcrops of the faults 7 and 11. Moines and we interpretthe reflectors asrepresenting Structure within the basement is shown by packages of large-scale velocity and density contrasts within the offshore strong mid-crustal reflectors (Figs 4 & 6), some of which can equivalents of the Moine schists. betied betweendifferent seismic lines. They strike The oblique faults in the southeastern part of the basin approximately NNE and dip at moderateangles to the ESE, dip parallel tothe basement reflectors onthe seismic asymptotically to the reflectors in the cover sediments and profiles. They also strike approximately parallel tothe asymptotically to the major faults in the basement. These Caledonian structures on-land, but have a different strike to reflectors could possibly interpretedbe multipleas the normal faults in the NW. Although it is not possible to reflections fromthe major faults. However, packages of determine accurately the strike of the mid-crustal reflectors, reflectors generally show internal structure, oftenchange dip we think it probable that these faults follow the mid-crustal across themajor faults, are traceable for largedistances (Caledonian) reflectors in strike as well as in dip and that it

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NW 6 7 l I

2 T WT (5)

NW 6 7

TW (SI

Fig. 6. Uninterpreted (a) and interpreted (b) migrated seismic section of line WOB 19, showing approximately planar faults in the basement, which are generally parallel to basement reflectors interpreted as Caledonian age structures. The normal faults steepenand become slightly listric above c.2 S TWT.

is thecontrol on their orientation by earliercrustal Orkneyand Caithnesscoastlines, determined fromthe anisotropy which causestheir obliquity to the other West seismic data off shore (Figs 2 & S), are of the same order of Orkney Basin faults. magnitudeas the known ORS thicknesses on land. This indicates an early to middle Devonian age for the initiation Age of basin development of the West Orkney Basin. In the Minch basin to the west andthe Moray Firththeto east thereare thick Many of the faults in the West Orkney Basin appear to have developments of Mesozoic sediments (Binns et al. 1974; been initiated as growth faults (for example, seeFig. 7), that Cheshire et al. 1983) and as Permo-Triassic rocks have been is, they show a thickening of the sedimentary units towards detected from shallow boreholes in the West Orkney Basin, majorfrontal and lateralfaults. The precise age of the there was probably some reactivation of the basin at this sediments and thus of the initiation of the faults is unknown time. owing tothe absence of any deep boreholes within the basin. However, half-grabens with an ORS sedimentary fill (Peach & Horne 1914; Blackbourn 1981) occur along the Fault structure at depthand fault restoration north coast of Sutherlandand represent the lateral Figure 9 shows a series of cross-sections through the West continuations of many of the faults off shore (Figs 2 & 3). Orkney Basin, which have been depth-converted using the Furthermore,the sediment thicknesses approaching the estimated velocity structure from the migrated seismic data.

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NW extension took place by block rotation along slip planes 0 which developed along the earlier Caledonian fabric, even though it was originally oblique to the regional extension direction. Fault 11 flattens at depth, even though it cuts through apparently non-layered crust along most of its length. Faults 7, 8 and 10 are listric but flatten into an area of basement characterized by sub-horizontalreflectors, which are interpreted as Cambriansediments and structures within and immediately below the Moine Thrust Zone. To the E, this fault zone steepens to dips of about 20", parallel to the basement reflectors. Faults 1 to 3 occur on the hanging-wall of fault 11 and appear to bestraight planar structures. If they had developed earlier than, or simultaneously with, the listric faults 7, 8 and 10 they must have had low initial dips andhence bedding on these fault blocks would be impossible to restore without invoking large strains. Instead we propose that the planar faults developed subsequent to movement onthe listric fault system 7,8 and 10. This producesa breaching system, which detachesat a deeper level, onto solea fault whichis seen as the western boundary fault to the West Orkney Basin system. There is no simple method for restoring listric faults, as the original displacementtrajectories and amounts of displacement are unknown. Roll-over folds are a conse- quence of listric fault shape (Gibbs 1983, 1984) and require vertical thinningand extension of thebeds within the roll-over leading to the development of secondary faults or ductilestrains. The only exception to this is where layer-parallel slip occurs,where the upperpart of the hanging-wall is sheared towards the fault plane (see fig. 6 of Fig. 7. Uninterpreted (a) and interpreted (b) migrated seismic section, of line WOB 14. Shows the thickening of the sediment Gibbs 1984 for discussion). The distributionand type of sequence towards fault 11, indicating growth during the initial strain, whetherbedding-parallel extension or bedding- extension. parallel shear, is unknown from seismic data alone and so someapproximations must be made during normal fault restoration. Somefaults are markedly listric, while othersare more In the present analysis markers representing the top of planar. Wehave found faultrestoration techniques the basement are restored back towards their regional level, invaluable for determining fault geometries, fault develop- with the assumption that no, or only slight, change in length mentsequences and, most importantly,for checking the of the top of the basement has occurred and that the rocks seismic interpretation.The restoration techniques involve have deformed by simple shear.Note, however, thatthe bulk line-length and area balancing, as described for thrust sedimentary cover shows evidence of bedding-parallel tectonics regimes by Dahlstrom (1969) and Hossack (1979). extension, in the form of antitheticfaults. Any effects of Some of the problems involved with restoration of the West compactionhave been ignored; it is assumed thatthe Orkney Basin and extensionalregimes in general are basement was fully compacted during the Caledonian discussed below. metamorphic events. Thus, assuming plane strain conditions The planar faults in the West Orkney Basin are parallel in the centre of the basin, area balancing techniques can be tothe earlier Caledonian reflectors,but they steepen applied forthe parts of the section involving basement upwards to produce a listric geometry at shallow depths and rocks. Note that decompaction and backstripping techniques slight roll-over folds andantithetic faults in the cover should be used during restoration of faults in a sedimentary sequence(Fig. 5). Inthe basement, however, the main sequence.

SE West Orkney Basin HOY Brlms-Rlsa l NW Fault

Fig. 8. Section across the eastern edge of the West Orkney Basin along line WOB 14 to Hoy (see Figs 2 & 3 for location). West Orkney Basin fault and sedimentary structure from WOB seismic data, on-land structure V=H projected from maps. I-

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40 11

18 Fig. 9. NW-SE depth-converted sections of lines 109 7 4 3 2 -1 WOB 16, 17 and 18, see text for discussion.

\ representsStippledsedimentsarea coverabove -a\, top of basement. Stacking velocities taken from F10 km ~ 10 km -- the migrated sections have been used for the depth NW SE conversion.

The faults should be restored in reverse sequence to that basement, suggesting local sedimentary onlap. No bedding- in which they developed. In the West Orkney Basin listric parallel extension is assumed. The base of the domino fault fault 11 bounds planar domino-type faults (1 to 3 and 9) to system is takenat between 17 and 20 km, as determined the east, which breach the listric faults (7,8 and 10) in the from the iterativefault restoration procedure anddeep centre of the basin. In thisrestoration fault 11 andthe seismic sections. breachingfaults have beenrestored first, the low-angle During restoration of the dominofaults, if the basal faults later (Fig. 10). detachment level is kept fixed, thenthe basement/cover To restoretheplanar, domino-typefaults, the interface restores to well above the present sea-level (Fig. hanging-wall and footwall cut-off points are brought 11). However, the basal detachment would not remain fixed togetherand the faults and fault blocks rotated so that duringextension, but would rise or fall dueto isostatic reflectors parallel to bedding become horizontal at the base compensation during crustal thinning. This rise or fall will in of the sedimentary sequence and the angles in the footwalls part be related to its initial level relative t3 themantle geoid between faults and the top of the basement are preserved. (Turcotte et al. 1977; Le Pichon & Sibouet 1981; Barr 1987), There is sometimes a slight angular discordance between the the level to which the top of the asthenosphere will rise after lowermostsedimentary reflectors andthe top of the infinite crustal extension. This is normally considered to be at a depth of 3-3.6 km below sea-level. A basal detachment lying beneaththe mantle geoid will rise during homogeneous lithospheric extension. This change in level of the basal detachment will dependon the original crustal thickness andon anyinhomogeneities in lithospheric extension, as well as on the amount of extension. The amount of uplift of the basementlcover interface, necessary duringrestoration of therotated fault blocks, allows a more accurate estimate of the regional level of this marker across the wholebasin and must be used during restoration of the related listric faults, such as fault 11 in the West Orkney Basin. It is gznerally incorrect to assume that restored breachlng faults - the regional level lies parallel to sea level in a section not corrected for isostacy. The isostatic change in regional level is somethinggenerally ignored in fault restoration, often justifiably in thrust zone restoration where the structures are

~ .- -__--_ -10 km T- basaldetachment moslattcally NW compensated SE

Fig. 10. Sequential restoration of line WOB 16 after depth conversion of the migrated time section. (a) deformed section, (b) after the breaching system has been restored. Note topof basement restores to above present sea level due to the sole fault having been kept fixed and horizontal and isostatic uplift having occurred during I extension due to thinningof the upper crust. (c) fully restored ------I---- ORIGINAL DETACHMENT LEVEL section after restoration of early low-angle fault system. The basal detachment must be isostatically restored to allow a reasonable Fig. 11. Schematic figure to demonstrate that the regional levelwill topography of the top of basement regional before extension rise to above sea-levelif the sole detachmentof a fault systemis commenced. kept fixed during restoration.

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I much smaller in scale, but it is important when attempting 5 4. W 3 1 restoration of normal faults at a basin-wide scale. X UNIT LENGTHS The final stage in the restorationprocess (Fig. 10) involves changing the level of the faults and sometimes also 1" their curvature, so that the restored sedimentary markers I20 km overlying thebasement becomehorizontal at close to sea-level. Note that this will partially straighten listric faults at the basin margins.

Variation in displacement Variations in amount of displacement on the normal faults within the West Orkney Basin are shown onthe cut-out maps of Figs 2 & 3 andthe cumulativedisplacement diagram in Fig. 4. This shows a variation in extension across the basin, with amaximum of 45% being attained in the centre, decreasing to both the north and south. Figure 12 shows thepattern made by restoringarectilinear grid, drawn parallel and perpendicular to the assumed extension direction. It shows where largecumulative simple shear strains will occur dueto differentialdisplacement of the faults, assuming that all faults have thesame extension direction; the values and orientations of these are shown in Fig. 13. The orientations and strain ratiosof the principal axes of Fig. 13. Locally thesestrains reach ratios of y = 0.75, the bulk deformation related to differential movementof the faults. intenseenough to produce slight cleavage and small-scale The original ratiosof the equidimensional strain ellipse in its structures. The map restoration procedure assumes that all pre-deformation state and orientation are shown in the top right the faults have the same and constant extension direction. corner. The deformed strain ellipses are rotated about a centralaxis However,any movements which may beoblique tothe which corresponds to a line drawn parallel to the extension regional extension would berestored along the assumed direction from the positionof maximum extension within the basin, extension direction and probably could not be distinguished see Fig. 4. The degreeof rotation of the axes of the strain ellipses from seismic dataalone. For afuller discussion of this from an initial45" to the extension direction, and the axial ratioof simple 3D restoration procedure for extensional and thrust the ellipses, is a function of the magnitude of the shear strains. faults and its implications for section balancing andthe location of structuresdeveloped by differential fault movement. Some faults appear to die out at tips,for example fault 4 (Fig. 2), but many probably link by lateral structures, some of which are locally difficult to resolve onthe seismic sections, because of their steep dips. Along some faults (for example fault 10, Fig. 2), displacement dies out rapidly, in the order of 9 km displacement in 5 km. This suggests that the sediments locally suffered a shear strain of y = 1.8, due to this differential movement. It is not known how this relatively high shear strain was accommodated within the sediments, possibly by ductile deformation, possibly by small-scale brittle faults unresolved on the seismic sections. However, we predict,from the y value that within the basement the fault terminates in a strike-slip zone (Fig. 2). Where the faults can be traced onshore, the displacements are observed to die out with only a gentle distance/ displacement gradient, which is illustrated by the decreasing ORS sediment thickness within the half-graben at Tongue (Fig. 14). The faultscan sometimes be traced southwards beyond the limit of ORS infill, where they eventually must N SCOTLAND ) die out in the Moines (Fig. 14). The half-grabens seen on land show someanomalous structures.Figure 15 is adetailed map of one of the Fig. 12. Pattern produced when an originally rectilinear grid is deformed by restoring extensional displacements measured for the half-grabens at Kirtomy. Here the main fault trends NNW top of basement marker, in the extension direction. From the and carries ENE-trendingtear faults on its hanging-wall. change in spacing and relative orientationof the grid lines the TheseENE-trending faults, which were active during reciprocal stretch and simple shear strains can be calculated (Fig. sedimentation, produce a variable downthrow to the basal 13). Note that the strain patternin the southern partof the basin ORS unconformity. This secondary fault geometry suggests assumes that displacement on the fault system dies out onland and a regional extension direction to the ENE, with relatively does not transfer into the Minches. little orno strike-slipdisplacement on the master fault.

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l EILEAH l There must have been a variation in displacement direction forat least this part of the West Orkney Basin, with a localized ENE-directed movementoblique to the regional SE extension.

Structures on the hanging-walls of frontal ramps Antithetic faults on the hanging-walls of large frontal ramps havea pronounced listric geometry (Fig. 5) anddetach within the cover sediments. Generally, however, secondary faults within the West Orkney Basin are synthetic tothe main faults and often cut through into the basement, rather than detaching at shallow levels (Figs 3 & 6). In the half-graben systems the cover sediments show either planar rotation or a roll-over anticline. However, a hanging-wall syncline is developed along fault 11, the sole fault to the West Orkney Basin system (Fig. 16), which is spatially and probably genetically associated with a problematical structure which could be interpreted either as a listric extensional fault with a pronounced roll-over fold, or as a contractional structure. Several reflectors which have characteristic seismic signatures appear to be duplicated by

Fig. 14. Map of Old Red Sandstone outliers at Tongue (see Fig.2 for location). Partially after Blackbourn(1981).

N Old Red c Sandstone TT,- Moinian L,> Basement ,.I. r,, I . ,-l k Faults

Fig. 16. Uninterpreted (a) and interpreted (b) migrated seismic Fig. 15. Detailed map of part of the half-graben adjacent to the section from line WOB 17. The anticlinal structure, in the master fault at Kirtomy showing the main normal faults offsetby hanging-wall of fault 11, is interpreted as a ‘pop-up’ formedby compartmental faults (see Fig.2 for location). inversion of an antithetic normal fault.

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contractionalfaults ratherthan omitted by extension. late Permian(Brown 1975; Rock 1983; Baxter & Mitchell However, reflector 'G' must havea net extensional 1984). However,the dykes are deformed within the fault displacement on the listric, NW-dipping fault. Therefore the zones of themajor faults indicating a later (dextral shortening here is interpreted as having been produced by transtensional)displacement, which is perhapsrelated to inverting an earlierextensional structure. A similar set of Mesozoic strike-slip movements in the Moray Firth. structures occurs onan adjacent line (Fig. 7).Here the Thrust systems and bedding-paralleldecoupling zones, offsets tothe reflectors canbe interpreted asa series of characterized by contractionaldeformation styles, are also E-dippingextensional faults with one W-dipping fault,or present on land. In these contractional structures a transport alternatively as a series of E-dipping extensional faults, one direction dominantly to the WNW is indicated by abrasional of which has beeninverted toproduce duplication of and fibrous slickenside lineations, fault cut-off relationships reflectors. The trend of these faults is shown in Fig. 3; the and small-scale strike-slip faults. The bedding-parallel contractional model is favoured here and the map has been decouplinghorizons appearto berestricted to the lower, drawn accordingly. This model involves contractional thrusts lacustrine facies sedimentaryunits, and are predominantly and folds with an approximately NW-SE shortening seen on the Orkneys. Fault tip propagation folds (cf. Suppe direction. The thrusts on the hanging-wall to fault 11 show a & Medwedeff 1984) and pop-up structures (see Elliott 1981) complex pop-up structure (cf.Elliott 1981) andprobably strike NNEand form on thrustramps which generally branch fromthe main fault. Thismodel involves some branch off the decouplinghorizons at low angles (15" to inversion of the basin. Some of the shortening may be due 30"). The folds and thrust systems are generally developed to bedsbeing moved up acurved master faultduring on a small scale and can be easily mapped on the coastal inversion, this shortening being the inverse to the extension sections. An example is shown in Fig. 17. required when beds movedown a listric masterfault (see The folding and faulting associated with the low angle Gibbs 1983). thrust systems and the sinistral strike-slip faults pre-dates a phase of late Permian dyke intrusion. The folding therefore occurred between late Devonian and late Permian times and Structures seen on land cannot berelated to the Tertiary inversion events of the Themajor fault structures mapped on land are shown in southern North Sea (Zeigler 1982). Fig. 3. Extensionalfaults with associated small-scale structurescan be mapped in eastern Caithnessand the Orkneys but the majority of small- to medium-scale faults on land have a net reverse displacement. The major reverse Discussion and conclusions faultsdip east, between 50" and 85", and include the Brough, Brims-Risa andEast Scapafaults. Asymmetric folds, which generally verge into and strike parallel to these Crustal thinning and isostacy major fault zones, are associated with these faults,as are There is up to 10 km of sediment in the larger of the West rare thrusts which verge away from the faults. The style of Orkney Basin half-grabens, which is developed in the deformation adjacent to andwithin the fault zones indicates hanging-wall of fault 11, but the averagesedimentary that they are sinistralstrike-slip faults, with reversea thickness is in the order of 3 km. Most of the sediments displacement. However, offset regional markers, such as the show evidence of faulting; there seems to have been little early North Scapa fault and the offset of the Eday Group subsidence after the main extension, as would be expected sandstones onSouth Ronaldsay by theEast Scapa fault, for lithospherica stretching model similar tothat of indicatea netdextral strike-slipdisplacement of several McKenzie (1978). kilometres. The asymmetric and open folds associated with All the major normal faults dip to the ESE, but cannot the sinistral displacement predate dykes which are dated as be traced to a depth greater than 18-20 km. This may be the

WN W ESE

Fig. 17. Field sketch of contractional structures observed in Sarclet Haven, related to a basin inversion episode.

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depth of thedetachment or decouplingzone beneaththe Eday Syncline, a large syncline developed on Eday (Wilson West Orkney Basin. The absence of a thermal subsidence et al. 1935), are thought to berelated tothe strike-slip basin suggests that a detachment zone may have transferred reactivation of major Devonian extensional faults. Small- extension from the upper part of the crust beneath the West scale structures within the fault zones indicate a sinistral and Orkney Basin to the lower part of the crust and lithospheric reverse sense of displacement on these faults, which are not mantle to the east,similar to that in the models proposed by related to compressional bends, but occur along the whole Wernicke (1981, 1985a) forthe Basin andRange. The length of the faults. A regional shortening is inferred during region of thinnedlower lithosphere, upwelled hot slip on these faults, perhaps related to Variscan compression asthenosphereand subsequent thermal subsidenceshould furthersouth, in a similar manner to the mid-Tertiary therefore lie east of the West Orkney Basin, beneath the compression which affects the foreland of the Alpine Orkneys, or perhapsbeneath the area now forming the orogeny (Ziegler 1982, 1984). As discussed by Le Pichon et North Sea. al. (1982), intracontinental basins appear especially prone to Using the data from the MOIST line, Wernicke (198%) compressional failure when the stress system changes from suggests an approximate crustal extension off the north coast extensional to compressional. of Scotland of 80-100%. Cheadle et al. (1987) suggest an The West Orkney Basin is capped by Mesozoic extension of 50-60%. Both these sets of figures assume a sediments(Kirton & Hitchen 1987) but their thickness is simple domino faulting model throughout and estimate the unknown. There was presumably some subsidence and fault extension from the relationship between the dip of the fault reactivation in the West Orkney Basin, whichmay and the dip of the sediments (Wernicke & Burchfiel 1982). correspond with the dextral strike-slip faults observed in the Kirton & Hitchen (1987) estimatethe extension tobe c. Orkneys, during Mesozoic times, as a continuation of basin 30%, based onan interpretation of commercial seismic development within thenorthern Minch and Moray Firth data.Our values forthe amount of extension, which are basins. As shown in Fig. 4, where the position of the Minch based on theWestern Geophysicalspeculative dataand fault is taken from the deep seismic lines, it could link with result from detailed fault restoration, vary from a maximum the northwestern faults of the West Orkney Basin, but to of c. 45% in the centre of the basin to almost zero at the check this continuationrequires a study of further northern margin. commercial data (Coward & Enfield 1987). Asthe crust is now approximately 31-32 km thick beneathScotland, and about 30 km thick tothe north (Bamford et al. 1978; Brewer & Smythe 1984), the original The origin of the basins crustalthickness would havebeen over 50 km, assuming The Old Red Sandstone basins of northern Scotland may homogeneous extension. However, as discussed earlier, we haveformed dueto (a) regional crustalextension, or (b) consider the deformation to have been heterogeneous, the thinning due to relaxation of an originally thick Caledonian extension in the upper crust beneath the West Orkney Basin orogenicbelt. Coward (1986) argues that there are being transferred to the lower part of the lithosphere to the differences in Caledonian structure on-shoreand off-shore east. As the extension would have been confined to what is northern Scotland. In the Sutherland area of the Scottish now the upper 20 km of the crust, this suggests an original mainland, the main Caledonian crustal ramp, which carried crustal thickness of just over 40 km. Moinesfrom mid-crustal levels to place them on lower Palaeozoic sediments of the foreland, cannot be placed less than 50 km east of the present outcrop of the Moine thrust Structural history (Butler & Coward 1984; Coward 1986). The major zone of The higher structural levels of the Caledonian orogen, thickened Caledonian crust should have been even further including the foreland fold and thrust belt, are missing and to the east, in Caithness or in the region now formed by the presumably wereeroded away since the termination of Moray Firth.In the region now occupied by the West Caledonian deformationat end-Silurian times. TheORS Orkney Basin,however, the zone of strong mid-crustal was depositedsoon after the termination of thrust reflectors suggests evidence of Caledonianstructures movements onthe Moine thrust,the age of initiation of affecting mid-crustal rocks. Coward (1986) has suggested lower ORS sedimentationin the Orcadian basin being upper that there must be a Caledonian tear fault or lateral ramp Siegenian-Emsian, datedat about 380Ma. Moine thrust along the north coast of Scotland, separating the thickened activity continued, with at least 50 km of displacement, after crust in thenorth from the thin-skinned thrustzone in the intrusion of the Loch Borrolan igneous complex Sutherland. The West Orkney Basin is situated above this (Coward 1985), dated by Van Breeman et al. (1979) as zone of thickened crust. The problem remains, however, as 430 Ma. towhether the localized extension was generated by the The lower ORS was dominated by sedimentationin a thickened crust, or whether it was dueto someexternal rifting environment,but in the middle ORSthe on-land extensionalforces but was localized because of the geology shows aphase of quiescentlacustrine deposition, thickened crust. during which a thick sequence of sandsand organic-rich Several of the Devonian sedimentary basins have formed carbonate flagstones was laid downover much of the by the reactivation of Caledonian thrusts. Thus the Outer Orcadian basin (Mykura1976); a discussion of the Isles and Minch basins, which are interpreted as having been hydrocarbon potential of the basin is given by Marshal et al. initiated in Devonian times, developed on the hanging-walls (1985). The flagstone succession is followed by further of theOuter Isles thrust.In southern Norway, north of alluvial facies deposition (Foster 1972), probably associated Bergen, there is a series of Devonian basins developed by with renewedfault activity. Asymmetricfolds in close considerable (greaterthan 40 km)extension on earlier proximity tothe major strike-slipfaults on land andthe Caledonian low-angle thrusts (Hossack 1984; Seranne & larger open folds which parallel these faults, for example the Seguret 1987). It seems thatmajor thruststructures are

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readily re-usedduring subsequent extension and influence BARR, D. 1987. Lithospheric stretching, detachednormal faulting and subsequentstratigraphy (see also Johnson & Dingwall footwall uplift. In: COWARD, M.P,, DEWY, J. F. & HANCOCK,P. (eds) 1981). Continental Extemional Tectonics. Special Publication of the Geological Society, London, 28, 75-94. The large number of Devonian basins suggests that the -, HOLDSWORTH,R. E. & ROBERTS,A. M. 1986.Caledonian ductile Caledonides as a whole suffered regional crustal extension thrusting in a Precambrianmetamorphic complex: the Moineof following soon afterthe compressionalevents. They are northwestern Scotland. Bulletin of the Geological Society of America, 97, therefore similar to the Basin and Range province, which 754-64. BAXTER, A. N. & MITCHELL,J. G. 1984.Camptonite-monchiquite dyke suffered regional extension following the Laramide orogeny swarms of Northern Scotland; age relationships and their implications. in the western USA. The British Devonian basins developed Scotfish Journal of Geology, 20, 297-308. along with some magmatic activity; alkali basalts occur in BINNS,P. E., MCQUILLIN,R. & KENOLTY,N. 1974. The geology of the Sea of the ORS basins in the Orkneys (Halliday et al. 1977) and the Hebrides. Report of the Institute of Geological Sciences, 73/4. BLACKBOURN,G. 1981. Probable Old Red Sandstone conglomerates around Midland Valley of Scotland, while alkali intrusive rocks Tongueand adjacent areas,north Sutherland. Scottish Journal of occur in the Ben Loyal and Assynt districts. Geology, 17, 103-18. BLUNDELL,D. J., HURICH,C. A. & SMITHSON,S. B. 1985. A model for the Shape of the faults and depth of detachment MOIST seismic reflection profile, N. Scotland. Journal of the Geological Society, London, 142, 245-58. The southeastern part of the West Orkney Basin extended BREWER,J. A. & SMYTHE,D. K. 1984. MOIST and the continuity of crustal by displacement on relatively planar faults which reactivated reflector geometry along the Caledonian-Appalachian orogeny. Journal of the Geological Society, London, 141, 105-20. earlier Caledonian structure, while in the centre of the basin -& -1986. Deep Structure of the Foreland to the Caledonian Orogen, the early fault set (faults 4, 8 and 10) which is strongly listric NW Scotland: resultsfrom the BIRPS WINCH Profile. Tectonics, 5, and flattens at shallow levels, probably also detacheson 171-94. Caledonian structures. However, fault 11, at the NW margin BROWN,J. F. 1975.Potassium-argon evidence of a Permianage for the to the basin, is also curved in plan and section and yet does camptonite dykes: Orkney. Scottish Journal of Geology, 11, 259-62. BRUN,J.-P. & CHOUKROUNE,P. 1983.Normal faulting, blocktilting and not seem to be controlled by any obvious earlier structures. decollement in unstretched crust. Tectonics, 2, 345-56. To some extent its curvature may be necessary to BUTLER,R. W. H. & COWARD,M. P. 1984. Geological constraints, structural accommodate the rotation of fault blocks to the SE, but it evolution and deep geologyof the northwest Scottish Caledonides. certainly seems possible for faults to develop a listric shape Tectonics, 3, 347-65. CHEADLE, M.J., MCGEARY,S., WARNER, M.R. & MATMEWS, D.H. 1987. in apparently homogeneous crust. Extensional structures on the western U.K. continental shelf a review of The faults of the West Orkney Basin detach at a depth of evidence from deep seismic profiling. In: COWARD, M.P., DEWEY,J. F. 18-20 km. This constrasts markedly with the large faults to & HANCOCK,P. (eds) Continental Extensional Tectonics. Special theNW, such as theOuter Isles andFlannan structures. Publication of the Geological Society, London, 28, 445-65. CHESHIRE,J. A., SWE, D. K. & BISHOP,P. 1983. 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Received 13 May 1986; revised typescript accepted 23 February 1987.

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