Devonian Extensional Tectonics Versus Carboniferous Inversion in the Northern Orcadian Basin

Journal of the Geological Society, London, Vol. 149, 1992, pp. 21-31, 14 figs, Printed in Northern Ireland

Devonian extensional tectonics versus Carboniferous inversion in the northern Orcadian basin

M. SERANNE Laboratoire de Gdologie des Bassins, CNRS u.a.1371, 34095 Montpellier cedex 05, France

Abstract: The Old Red Sandstone (Middle Devonian) Orcadian basin was formed as a consequence of extensional collapse of the Caledonian orogen. Onshore study of these collapse-basins in Orkney and Shetland provides directions of extension during basin development. The origin of folding of Old Red Sandstone sediments, that has generally been related to a Carboniferous inversion phase, is discussed: syndepositional deformation supports a Devonian age and consequently some of the folds are related to basin formation. Large-scale folding of Devonian strata results from extensional and left-lateral transcurrent faulting of the underlying basement. Spatial variation of extension direction and distribution of extensional and transcurrent tectonics fit with a model of regional releasing overstep within a left- lateral megashear in NW Europe during late-Caledonian extensional collapse. Later inversion (probably during the Upper Carboniferous)is characterized by E-W to NE-SW contraction. It induced reactivation of extensional faults as thrusts,development of small-scale folds and thrusts, and right lateral transcurrent movement of the major faults such as the Great Glen and Walls Boundary faults

The Old Red Sandstone of the Orcadian and northern Scot- Coward 1987). These basins form a group of collapse-basins land basins (Fig. 1) was deposited in an extensional setting (Seguret et al. 1989) that developed in NW Europe during late- duringthe Middle Devonian (e.g. Mykura 1976; Enfield & orogenic extensional collapse of the Caledonian orogenic belt (McClay et al. 1986). Later inversion of these Scottish basins resulted in theformation of folds within the Old Red Sandstone and reactivation of basement faults (e.g. Coward et al. 1989). A consensus relates theLate Palaeozoic tectonic SHETLAND 100 km In evolution of that area to the kinematics of the Great Glen fault which is amajor tectonic boundary (Flinn 1977). Attempts to decipher directions of extension and subsequent compression have been based on fold geometry (Mykura 1976), orientation of faults and associated lateral ramps (Coward & Enfield 1987), or by the kinematics of the Great Glen fault (Norton & Way 1991). However, the history of movement and amount of offset of this trancurrent fault remain controversial (see reviewin Rogers et al. 1989). In particular, folds and faults may develop oblique to deformation axes, and changes in orientation of the stress field close to major discontinuities may prevent simple approximations of directions of extension or contraction. Field observation and mapping of repetitive small scale structures within the Old RedSandstone of Caithness,the Orkney and the Shetland islands, and along its contacts with basement, together with analyses of published structural and solid geology maps, has led to a re-interpretation of the structures in this area. This paper aims to show that some of the structures previously attributed to compressional tectonics in the northern Orcadian basin were probably formed earlier, during extension. Reappraisal of these structures will eluci- date the kinematics of both the Middle Devonian extensional event and the Carboniferous inversion phase.

Devonian extensional structures Orkney and Caithness

Fig. 1. Map of the main structural features of Northern Scotland. Enfield & Coward (1987) and Enfield (1988) have presented Data from Coward et al. (1989) and BGS 1:250000 Solid Geology evidence for Middle Devonian extensional tectonics in the off- Maps of Shetland, Orkney, Caithness. WOB: West Orkney Basin. shore West Orkney Basin (Fig.1). Sediments of Middle 27

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Devonian age were deposited in the hanging-wall of NNE- trending normal faults associated with ESE-trending transfer faults. Thickness variation across the faults and growth structures within the sediments are good evidence for syn-depositional extensional tectonics. Such a simple half-graben geometry may be observed onshore in the Turiff basin(Norton et al. 1987) and in west Caithness (Enfield 1988) but it is not observed in Orkney, probably due to a lack of continuous exposure. Enfield (1988) analysed outcrop-scale examples of extensionalfault systems that he related to gravitational sliding on top of tilted hanging-walls in half-graben. These slickensided faults do not allow the reconstruction of the regional palaeostress field as they respond to local conditions. Consequently, the Middle Devonian regional extensiondirec- tion in Caithness and Orkneyis best approximated by the geometry of the normal faults and associatedlateral ramps in the West Orkney Basin: a NW-SE direction of extension is sug- gested by the ‘bow and arrow’ rule (Enfield & Coward 1987; Enfield 1988) (Fig. 1).

Shetland The Devonian rocksof Shetland belong to threedistinct basins (Fig. 2): (1) The Melby basin in the west which consists of volcanic rocks (Esha Ness) and sandstones of Middle Devonian age (Foula and Melby) limited by the Melby fault. (2) The Walls basin covers mostof the Walls peninsula.It is limited to the east by the Walls Boundary fault and unconfor- mably overlies Lewisian-type schists and gneisses to the north (Flinn 1985). The basin comprises sandstones and interbedded a’t acid volcanic rocks and it is intruded by the Late Devonian Sandsting granite (BGS 1:63650 W Shetland, Sheet No. 127) Fig. 2. (a). Simplified map of Devonian bedding in the Walls basin (3) The Southeast Shetland basin (including Fair Isle) in (west Shetland, location on Fig. l), modified from Mykura (1976) which Old Red Sandstone sediments are in contact through and from personal observation on the northern margin. The effects unconformity or oblique normal fault with Dalradiangneisses of north trending folding have been removed. Sandsting granite and and a ‘tectonic melange’ (BGS 1:63650 S Shetland, Sheet No. interbedded volcanic rocks are not represented. Old Red Sandstone 126). series display internal unconformities and progressive syntectonic unconformities that demonstrate syn-depositional folding. Inset (l) Melby basin. Westof the Melby fault, the Middle map shows the position of Devonian rocks (stippled) in Shetland. DevonianMelby formation consists ofsandstones and MF, Melby fault; SW, Sulma Water fault; WBF, Walls flagstones that arerelated to the Old Red Sandstoneof Orkney Boundary fault. (b) Section across Walls basin showing the on sedimentological and palaeontologicalgrounds (Mykura relationship between basement faulting (left-lateral strike-slip) and 1976; Marshal1 1988). Onthe island of Foula,structures folding of the sedimentary cover.Vertica1 and horizontal scales are within the Devonian sediments and underlying basement in- the same. dicate a NE-SW to E-W extension (Norton et al. 1987). In the north, sandstone gives way to volcanic lithologies (EshaNess). The exposed Melby basin has been juxtaposed to the Walls basin by dextral strike slip along the Melby fault in post Old strike-slip fault. North of the Sulma Water fault and east of Red Sandstone time (Donovanet al. 1976). No further evidence the Melby fault the Old Red Sandstone formations preserveat of syndepositional tectonic activity has been found. their base ENE-WSW trending synclines which are unconfor- mably overlain by later Old Red Sandstone. Such local un- (2) Walls basin. The Old Red Sandstone of the Walls basin is conformitiesattest to activetectonics during Old Red poorly exposed and complexly folded and faulted. The BGS Sandstonetime. The Sandness formation onlaps eastwards one inch map provides a good density of bedding dip and strikeonto Lewisian-type basement. This reflects an eastwards shift measurements. Mykura (1976) gave a cartographic interpreta- of the source area with respect to the locus of sedimentation tion based on the recognitionof several marker horizonstrace- which, in turn suggests lateral movement of the basement dur- able throughout thewhole basin. Re-mappingof the northern ing Old Red Sandstone deposition. margin of the Old Red Sandstone outcrop and reappraisal of Inthe centre, Walls peninsula is occupied by theWalls the nature of the Sulma Water fault has led to a simplified formation whose sedimentologyis detailed by Mykura (1976) interpretation of Mykura’s map in which the lateN-S trending and Melvin (1985). The formation presently dips towards the folds have been removed for clarity (Fig. 2a). west and appears to have a cumulative stratigraphic thickness The northernmargin of the Walls basinis an unconformity of about12km. It islikely that this largestratigraphic which suffered minor reactivation as a discontinuous sinistral thickness exceeds the final vertical depth of the basin, as a

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result either of westward shifting of depocentres duringdeposi- tion, or deposition above an east-dipping low-angle normal fault. South of the Sulma Water fault Devonian strata arefolded in a manner compatible with left lateral motion on the fault and toward the south they swing into an ENE-trending 2 km wide belt of very steep bedding. The steep belt contains internal angular unconformities (Fig. 2) showing that folding was contemporaneous with deposition. The southern part of the steep belt consistsof an asymmetric syncline (the Walls syncline) whose vertical northern limb recordsa reduced stratigraphic thickness resulting either from faulting or internal unconformities.Older horizons are more tightly folded than younger ones, thus supporting a syn-depositional origin for the Walls syncline. Such a belt of steep bedding can be explained by faulting of the underlying crystalline basement alongfaults parallel tothe SulmaWater fault, the sedimentary cover responding by flexure (Fig. 2b). Significant vertical offset is required to account for thesteep bedding. How- ever, the geometry of strata below and above an internal angularunconformity in the northern limb of the Walls syncline (Fig. 2a) indicates a sinistraloffset of at least 4 km. It is likely that this value represents a minimum offset. Further left lateral movement may have occurred on WSW trending basement faultswithout leaving mappable evidence. Minor reactivation of the basal Old Red Sandstone unconformity by transpressive sinistral strike-slip, observed at various locations along the north shore of Walls Peninsula, suggests that left- lateral shearing continued after the Middle Devonian.

(3) Southeast Shetland basin. Along the SE coast of Mainland extends a discontinuous Old Red Sandstone outcrop (Fig. 3) 0ORS sandstones 8 flagstones whose sedimentology has been studied by Mykura (1976) and D.:.:.:...: ORS conglomerates Allen & Marshall (1981). In the Lerwick area the Old Red Sandstone consists of alluvial-fan conglomerates that grade a Deformed ORS eastwards into sandstones of braided stream character and are Basement, tectonic melange overlain by lacustrine flagstones. Basal breccias overlie an irregular basement unconformity around Easter Quarff /c ORS bedding trace R dip (Fig. 3). In the south theOld Red Sandstone consists of fluvial 6 ORS unconformity sandstone and pebbly sandstone, interfingering with lacustrine flagstones. Palaeocurrentdirections indicate a southto / Fault southeasterly flowing drainage system (Allen & Marshall 0 10 km 1981). The lacustrine sediments are bounded to the west along LIIII~~IIII a SSW-trending line roughly parallel to the coast of Main- land. The occurrence of alluvial fans in the north and resedimented debris flows in the southare strong evidence for elevated topography andprobably active tectonics during sedi- Fig. 3. Simplified map of Southeast Shetland basin. Location on mentation. Fig. 1. EQ, Easter Quarff; G, Gulberwick; L, Lerwick; RH, Rova The Southeast Shetland basin may be structurally divided Head; S, Sandwick; SH, Sumburgh Head. into three parts (Fig. 3). The southern parthas the broad geometry of a 10”S-plung- ing syncline whose west limb only is exposed. The contact with basement is faulted in the Sandwick peninsula and unconfor- ‘tectonic melange’ and of psammites related to the Dalradian mable in the rest of the outcrop, where it is marked by locally (Fig. 3). The tectonic melange comprises lenses of marble and derived basal breccia. The core of the syncline is complicated quartzites floating in a matrix of east-dipping foliated phyllites by second order en echelon synclines and by a N-S fault whose that display sub-horizontal stretching lineations and consis- splays and parallel synthetic shear planes display left lateral tent top-to-north ductile shear criteria. The psammites also displacements. These observations would indicate thatthe present penetrative top-to-north shearing and are folded in a faulted segments of the basement contact are sinistral strike- north trending syncline, itself truncated by the Old Red slip faults. There is no evidence for normal faulting across this Sandstone unconformity. It is likely that the ‘tectonic melange’ margin as postulated by Norton et al. (1987). was generated by this top-to-north shearing event and thus is The central part of the Southeast Shetland basin is charac- pre-Middle Devonian in age. terized by a discontinuous basal breccia that is unconformable The northern part of East Shetland is separated from the over pre-Devonian basement consisting in this area of the central part by a NNW-trending normal faultagainst which an

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Fig. 4. Structures observed in the Rova Head shear zone (location in Fig. 3). Pre-Devonian basement and Old Red conglomerates and sandstones are involved in the shear zone. Horizontal and vertical sections display left-lateral and dip-slip respectively. Stereonet gives poles of intra-cobble tension-gashes (dots) and orientation of shear planes (arrows). Sinistral shear planes are parallel to major contact (bold arrow), dextral ESE strike-slip are conjugate with NNE sinistral ones. Inferred extension direction makes an acute angle with the major contact, in agreement with the observed oblique slip.

open hanging-wall syncline was developed (theGulberwick the rest of the conglomerate, cobbles and pebbles are often syncline). Along the west and northwest margins syntectonic fractured by vertical tension gashes oriented NI40 to N160 alluvial fans were deposited close to the basin bounding fault. indicating an extension oriented parallel to the basin margin. The contactbetween conglomerates and basement is exposed at All observed structures on the NW margin of Southeast Rova Head (Fig. 3). It corresponds toa shear zonethat involves Shetland basin are consistent with an extensional fault with a basement calc-schists of the 'tectonic melange' and Devonian left lateral component that separates basement and Old Red conglomerates(Fig. 4). The basement showsNE-trending Sandstone, and thatwas active during sedimentation. Anorth- shearplanes thatdip 40-70" SE, and they cutthrough east direction of extension is in agreement with the NNW- shallower dipping foliation. Density of shear planes increases trending normal fault bounding thebasin to theSW and sinis- towardthe basement/Old Red Sandstone contact (Fig. Sa). tral movement on the marginal fault which is therefore a lat- Although there are no stretching lineations or slickensides on eral ramp. the shear planes, an oblique movement is inferred as vertical On Mainland (Fig. 6a) bedding dips are consistent with a and horizontal sections yield evidence for normal andsinistral normal fault ramp-flat-ramp geometry at depth developing a offset, respectively. The basement/Old Red Sandstone contact NW-trending hanging-wall syncline (Gulberwick) and ramp is covered by a few metres of sand and drift. For about SO m anticline (Lerwick) as shown in Fig 6a. Close to the lateral abovethe fault Old Red Sandstone conglomerates are de- ramp bounding the basin to the NW, bedding is bowed in a formed by extensive NE-trending shear planes that separate drag-fold consistent with left-lateral movement (Fig. 3). blocks of different lithology, different bedding dip and vari- On each side of Bressay island there are NNE-trending belts able degree of internaldeformation. Highly deformedcon- of deformed Old Red Sandstone that consist of metre scale glomerate has a greenish matrix of fine angular grains with folds and of brecciated sandstones. Breccias are found in exten- some coarserfragments (Fig. Sb). It is affected by cleavage sive outcrops along the eastern and western coast of Bressay developed around undeformed or brittlely stretched pebbles (Fig. 3). It consists of angular clasts of sandstones of varying and cobbles.In horizontal sections, clast long axes appear colour and texture, that have preserved their internal stratifi- parallel to the ENE-trending matrix cleavage and lie at an cation, some may be of ten to several tens of metres large. angle to the NE-trending shear planes that characterizes sinis- According to Mykura(1976), these are thought tobe related to tral shearing (Fig. 5c). Some strips of conglomerate display post-Devonian volcanic activity. However, at the periphery of smaller angular clasts, evidently fragments of larger cobbles, the massive breccias, it canbe observed that finer breccias were clasts are affected by a consistentpattern of fractures normal to formed by disruptions of sandstone bedding in fault zones. In the clast long axis (Figs 4 and Sd). In lenses of weakly de- other places the breccia is found resedimented within the sedi- formedsandstones there are groups of sinistralstrike-slip ment, as rubble beds of thickness varying between several tens faults with centimetre-scale throw that are synthetic to the of centimetres up toseveral metres. This suggests that breccias bordering shear planes. Outside the marginal shear zone, con- are rather the result of tectonic activity and that deformation glomerates are affected by conjugate NNE-trending-sinistral giving rise to folding and brecciation occurred during sedimen- and ESE-trending-dextralshear zones that separate blocks tation. several tens of metres wide (Fig. 4). Some of the shearzones are The kinematics of these sheared belts is unclear. The atti- characterized by passive reorientation of the cobbles parallel tude of bedding close to thebelts suggests drag folding consis- to the shear planes. Thisis a good indication that deformation tent with a downthrow of the eastern block (Figs 3 and 6b). In occurred early in the conglomerate's diagenetic evolution. In addition a numberof independent observations favour strong a

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NW SE

W E

Fig. 6. Interpretative sections across Southeast Shetland basin (location in Fig. 3). Same legend as in Fig. 3. Horizontal and vertical scales are the same.

More detailed analysis of the structures withinthe sheared belts would be needed to ascertain the nature and magnitudeof movement they underwent but a preliminary interpretation is of a diffuse zoneof shear in the sedimentary cover as a response to sinistralhormal faulting in the basement during sedimentation (Fig. 6b,c)

Origin of folding Folding observed inthe Old Red Sandstoneof Shetland seems to be hard to reconcile with extensionaltectonics. However, in addition to long wavelength compaction synclines, rampsand flats in normal faults profile can produce rollover anticlines, ramp synclines and ramp anticlines in the hanging-wall. The northwest-trending Gulberwick syncline (Fig. 3 and 6) is an example of such a hanging-wall syncline. Forced folds are a response of plastic sedimentary series to movement of under- lyingbasement structures (Steams 1978). Fieldexamples (Chenet et al. 1987) and sand-box models (Vendeville 1987) have shown that folding of sedimentary cover mayresult from Fig. S. Structures in the basin bounding shear zone at Rova Head extensional and strike-slip basement faulting. It is proposed (Fig. 4), that involves basement rocks (a) and Old Red here that large-scalefolds in Shetlandwould result from conglomerates (b, c, d). Strain decreases away from the contact transcurrentmovement along NE-trending basement faults (from b to d). All kinematic indicators (shear bands,mica fishes, and from extension across NW-SE normal faults. The asym- shear planes) are consistent with left-lateral slip. metricWalls syncline (Fig. 2) is bestexplained by sinistral strike slip andior downthrow of the southern basement block (Fig. 7a and b). Folding of the marginal conglomerates near component of lateral movement (coexistence of extensional Lerwick (Fig. 3) results from accommodation of sedimentary and contractional structures, steep fold axes, horizontalslick- strata above an active low-angle normal fault and associated ensides) that would be sinistral in sense (markers sinistrally transfer faults(Fig. 7c). A similar mechanism hasbeen invoked offset, the Bressay sheared belt is colinear with the sinistral for the similar Middle Devonian basinsof Norway (SCranne et strike-slip fault in Sandwick). al. 1989).

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a b C

Fig. 7. Idealized block-diagrams showing how basement faulting (a) normal, (b) strike slip, and (c) low-angle normal associated with transfer faults may induce folding of the overlying sedimentary cover. (1) incipient faulting, (2) after faulting, and (3) after erosion . The complex final geometry is even more difficult to analyse when considering the growth structures that develop in the syntectonic series.

Regional kinematics observed in the northern Orcadian basin is associated with a change of tectonic regime. Extensional tectonics in the West Large- and small scale structures in the Walls basin support a Orkney Basin is accommodated mainly by normal faults and model ofsinistral shearing along ENE-trending basement subordinate transfer faults, that correspond to WNW-ESE ex- faults, and theyounging direction of Old Red Sandstone south tension (Enfield & Coward 1987). In Shetland, trancurrent tec- of Sulma Water fault require synsedimentary relative move- tonics becomes dominant over extension. Inferred directions of ment of the hanging-wall towards theENE, parallel with extension are parallel to the ENE-trending left-lateral trans- basement lineaments. This is in agreement with ENE extension fer faults in the Walls Basin and parallel to the NE-trending parallel to and controlled by ENE-trending transfer or strike basin bounding lateral ramp in the Southeast Shetland Basin slip faults. This interpretation contradicts previous models of (thisstudy). Restoring the Southeast Shetland Basin 70km SE-NW extension across inferred NE-trending basin bounding north (Rogers et al. 1989) results in a spatial distribution of normal faults (Coward& Enfield 1987) which are shown in this extensional and transcurrent tectonics, and variation of exten- study to be an unconformity that suffered only minor post- sion trajectories throughout the Orcadian Basin, that fits with deposition left-lateral slip. In the Southeast Shetland basin, models of ‘horsetailing’ at the extremities of strike-slip faults similar patterns of sinistralshearing along basin-bounding (Granier 1985). According to this model, offset of left-lateral and basement faults, and younging directions in the syntectonic N- to NE-trending strike-slip faults bounding the Devonian sediments argue fora NE-SW oriented extension. Figure 8 is a basin in Shetland is transformed into extension across normal conceptual view of part of the Southeast Shetlandbasin during faults in the West Orkney basin. Recent work (Seranne 1991) the MiddleDevonian that illustratesthe geometrical rela- indicates that a similar regional pattern can be observed in tionship between thespoon-shaped low angle normalfault, Norway where Middle Devonian WNW-trending extension in and the high angle sinistralhormal basement faults that acted the Bergen area(Chauvet & Seranne 1989) progressively as oblique or lateral ramps. rotates northwards until it becomes parallel to the SW-trend- The variation in directions of Middle Devonian extension ing Mare-Trandelag fault zone in the Trondheim region. This

Fig. 8. Conceptual view (not to scale) of the Southeast Shetland basin in the Lerwick area showing the relationships between low-angle faulting and sub-vertical strike-slip.

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Carboniferous inversion structures Small scale folds and faults n = 48 The Old Red Sandstoneof Caithness and Orkney are affected by a numberof small scale folds and thruststhat have typically been relatedto post-Devonian inversion (Mykura1976; Enfield 1988; Coward et al. 1989; Norton & Way 1991). A well defined and consistent group consists of folds several tensof centimetres in wavelength and amplitude, often coredby a blind thrust. They are found either isolatedor in bundles. On SW Mainland a b Orkney these folds are roughly orientedN-S with a maximum at N170 (Fig. 9a). They commonly display en echelon tension Fig. 9. (a) Azimuth of horizontal small-scale folds in SW Mainland gashes in sets parallel to foldaxes that imply right-lateral Orkney. (b) Field sketch of en echelon tension-gashes associated with movement along the fold plane (Fig. 9b). This indicates that folding observed in SW Mainland Orkney. They indicate that the direction of contraction was obliqueto fold axes. Thisfirst direction of contraction (white arrows) was oblique to fold axes. group of folds correspondsto a ENE-WSW direction of contraction.North-trending kilometre-scale folds described by fault zone was aleft-lateral strike-slip fault during the Middle Mykura (1976) in theWalls peninsula overprint Devonian Devonian (Seranne 1991) and links up with the NE-trending folding and may be correlated withthis phase of contraction. faults of northern Scotland such as the Great Glen and the On east Mainland Orkneywere found smallscale folds with Shetland Spine faults (Fig. 1) (Skilbrei 1988). It has been sug- steep axes that are clearly associated with strike-slip deforma- gested that extensional Middle Devonian basins in the North tion but whose kinematics and chronology are not known. Sea were formedwithin alarge scale releasing overstep Sandstonesare affected by brittle faults (strike slip or between thesinistral More-Trondelag fault zone and a southern reverse and interbed slippage) withoffsets ranging from tensof limit, probably the Highland Boundary fault, as a result of centimetres up to several metres. Slickensides usually display quartz fibres. Groups of striated faults (5 to different late-Caledonian extensional collapse (Seranne et al. 1991). 20) of strike and dipin a given outcrop allow an approximationto be made of the local directions of extension and compression r'? (Arthaud 1969; Etchecopar 1984). These groups of faults provide a directionof contraction oriented NE-SW (Fig. 10) that N N

a : ORK21 '

b : CAlTHl6 c : SHET27 N YV VI

Fig. 10. Small-scale faults planes and associated slickensides (arrows) at various locations of the Orcadian basin (see locations on Figs 12 and 14). Dots represent the inferred position of the maximum compressional stress (01) for each fault (Arthaud 1969; Etchecopar 1984); their distribution in the stereonet (Schmidt, lower hemisphere) allows an estimate of the local direction of contraction (bold arrows).

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Fig. 11. (a) Fault planes and slickensides in the Sandsting granite (Location on Fig. 14). (b) Inferred directions of principal stresses (01 > 02 > 03). Bold arrows represent the estimated mean orientation of

a a) contraction.

are in agreement with the north-trending folds. The E-W con- Mapping the directionsof contraction inferred from small- traction inferred from a locationin the northern endof Sulma scale folds and thrusts and striated faults illustrates a general Water fault zone (Fig. 1Oc) is interpreted as due to perturba- pattern of ENE-WSW to NE-SW contraction expressedin the tion of the stress field close to this major fault. It also allows a studied area, from Caithness to Shetland (Fig. 12). right-lateral sense ofslip to be determined on the Sulma Water fault, contemporaneous with the phase of inversion. A similar group of strike slip and reverse faults affects the Movement along regional faults Sandsting granite in Walls Peninsula and they also provide a Simplecartographic relationships (downthrown block and NE-SW direction of compression (Fig. 11). These faults are fault dip) indicate that the East Scapa fault andBrim-Rissa the interpreted as resulting from the same tectonic event as the fault in Orkney-Caithness are reverse faults (Fig. 12). These compressional one recorded in the Old Red Sandstone. areinterpreted as inverted normal faults by Coward et al. (1989) and Norton & Way (1991). Brittle fractures in the N060-trending Helmsdalefault zone :ARBONIFEROUSINVER make a pattern consistent with a dextral strike slip. This fault N has had a long history of dextral shearing linked with extension in theMoray Firth, that lasted until theMesozoic (Roberts et al. 1990 and references therein). The Walls Boundary fault is the northern continuation of the Great Glen fault. Flinn (1 977) assumed a dextral sense of movement on the basis of the large scale geometry of the fault systemincluding the Walls Boundary and Nesting faults. Devonian sandstones are involvedin the Walls Boundary fault zone; they display dextral faults and cataclastic zones parallel to the major fault plane. Vertical foliation of the basement gneisses intruded by the Sandsting granite bears kinematic indicators of dextral shearing. The Nesting fault is developed mostly withinschists and calcschiststhat arefavourable for the formation of shear bands. These are always in agreement with dextral shear (Fig.13). In the most deformed part a lOcm thick gouge marks a planar (later?) fault. The SulmaWater fault has been reactivated in a brittle fashion in its eastern part, whereit changes strikeand becomes N030-orientated. Analysis of associated Riedel faults provides evidencefor a late dextral strike slip motion.However the northern margin of the Walls basin,with a similar orientation, does not show any dextral reactivation. The very complex fold structures encountered in the N-S- trending deformed belts of Bressay maypartly be due to super- imposition of dextral shear on the Devonian sinistral shear zone.

Kinematics and age of the inversion Fig. 12. Structural map of Carboniferous inversion in Orkney and All the different types of structure (folds of different scales, Caithness. Compression directions are given by small-scale folds faults, shear zones) indicate a mean ENE-WSW direction of and thrusts, and slickensides. Each location represents the mean compression in the northern Orcadian basin, ranging indirec- orientation inferred from 5-20 measurements (folds, faults, tion from E-W in the Orkneys(Fig. 12) to amore northeasterly shear-bands in fault zone). Large scale structures from BGS trend in Shetland(Fig. 14). Similarly,the major strike slip 1:250000 Solid Geology Map of Orkney and Caithness. faults system made of the Great Glen-Helmsdale faults and

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Fig. 13. Shear bands developed in vertically foliated calc-schists in the Nesting Fault zone; they indicate dextral shear. Similar structures are observed at various locations along the Nesting Fault, as well as along the Walls Boundary fault.

facts strongly argue for the inversion being related to right lateralwrenching along the Great Glen-WallsBoundary faults system. The total offset of post Devonian dextral strike slip along the GreatGlen-Walls Boundary faults system hasbeen a mat- ter of much discussion depending greatly on the method involved: Storetvedt (1987) determined 300 km of dextral offset based on palaeomagnetism, Flinn (1969) matching magnetic anomaliesmeasured 70 kmdextral offset on theWalls Boundarymesting faults system but Rogers(1987, in Rogers et al. 1989) mapped palaeogeographic features across the Great Glen near Inverness and found only 25 km offset. The present study cannot quantify theoffset. However, Van der Voo& and Scotese’s (1981) proposition of 2000 km sinistral Hercynian offset may be ruledout asobserved post-Devonian shear zones are clearly dextral in sense and it is very unlikely that a fault with such a large offset would produce no detectable strainin the surrounding rocks. The inverted normal faults in Orkney and Caithness offset the Upper Devonian. The Sandsting granite complexis affected by theENE-WSW compression, and the dextral Walls Bound- ary fault cuts the complexto the east. The granite intrusionis dated by K-Ar method at 371 f lOMa (Miller & Flinn 1966) implying a post Late Devonian age for the inversion. Seismic reflection surveysin the WestOrkney Basin reveal a pre- Permianangular unconformity (Enfield 1988). In addition, Western Geophysical seismic profiles off SW Shetland show folds of Old Red Sandstone markers that are truncated by the basal Permian unconformity which itself is not deformed (see Fig. 14. Structural map of Carboniferous inversion in Shetland. alsoNorton & Way 1991). The inversionphase therefore Same legend as in Fig. 11. Permian contour from BGS 1:250000 appears to be Carboniferous in age. It must be stressed however Solid Geology Map of Shetland; folds truncatedby Permian basal that (1) the age of markers in the West OrkneyBasin are poorly unconformity imaged on seismic profiles provided by Western constrained, (2) syntectonic unconformities within Old Red Geophysical. Sandstone are not to be excluded, and consequently (3) that folds of the Old Red Sandstone may be Devonian in age as shown onshore Shetland in this study. Walls Boundary-Nesting faults show a change in trend from LateCarboniferous inversion of Devonian and early NNE in the southto N in the northern partof the basin(Fig. 1). Carboniferousbasins iswidely documentedin Britain (e.g. The respective orientationsof contraction and of major faults Collier 1989). Benard et al. (1990) have investigated the direc- are in agreement with dextral slip along the fault system. In tions of contraction during the Carboniferous throughout the addition, most of the observed deformation is concentrated British Isles, using a similar approach to that of the present around, and increases towards, the transcurrent faults. These study.They gave evidence fortwo different compressional

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episodes during the Carboniferous: ARTHAUD,F. 1969. Mithode de determination graphique des directions derac- courcissement, d’allongement et intermidiaire d’une population de failles. E-W contraction during the Namurian-Westphalian, Bulletin de la Sociiti Giologique de France, 7, 729-737. N-S contractionduring the Westphalian-Stephanian BENARD, F., MAXLE,A., LE GALL,B., DOLIGEZ,B. & Rossr, T. 1990. Paleo- related to Hercynian deformation. stress fields in the Variscan Forelandduring the Carboniferousfrom Microstructural Analysis in the British Isles. In: MATTE, Ph.(ed.) Terranes The dextralsense of motion observed along the NNE-trending in the Variscan belt of Europe and circum-Atlantic Paleozoic orogens. Tec- major strike-slip faults is consistent with regional E-W con- tonophysics, Special Issue, 177, 1-13. BGS 1978. 1:63.360 Solid Edition. Southern Shetland, Sheet No 126. traction,and therefore a Namurian-Westphalian age is in- - 1971 1:63.360 Solid Edition. Western Shetland, Sheet No 127. ferred for the inversion in the northern Orcadian basin. - 1982 1:250,000 Solid Geology Map, Caithness. Sheet 58N04 W. - 1985 1:250.000 Solid Geology Map. Orkney, Sheet 59N04W Conclusions CHAUVET,A. & SERANNE,M. 1989. Microtectonic evidence of Devonian extensional westward shearing in southwest Norway. In: GAYER,R. (ed.) The This study has confirmed the existence of superimposed tec- Caledonide Geology of Scandinavia. Graham & Trotman, 245-254. tonicsin the Orcadian basin. Complex folded and faulted CHENET,P. Y., COLLETTA,B., LETOUZET,J., DESFORGES, G., OUSSET,E. & structures in the Old Red Sandstone typically require several ZAGHLOUL,E. A. 1987. Structures associated with extensional tectonics in the Suez Rift. In: COWARDM. P, DEWEY, J.F. & HANCOCK, P.L. (eds), Con- phases of post-depositional compressive deformation. How- tinental Extensional Tectonics. Geological Society, London, Special Publi- ever, the tectonic evolution that emerges is somewhat simpler cation, B,551-558. than previously assumed. Some large scale folds that had pre- COLLIER,R. E. 1989. Tectonicevolution of theNorthumberland Basin; the viouslybeen related to post-depositionalcompression, are effects of renewed extension upon an inverted extensional basin. Journal of the Geological Society, London, 146, 981-989. shown to have developedduring sedimentation of the Old Red COWARDM. P. & ENFIELD, M. A.1987. The structure of the West Orkney and Sandstones.These structures are interpreted as extensional adjacent basins. In: BROOKSJ. & GLENNIE,K. W. eds Petroleum geology ramp synclines and forced folds of sedimentary cover over- of north west Europe. London, Heyden & Son, 3‘’ Cdnference on Petrol- lying basement faults. eum Geology of North West Europe Proceedings, 687496. --, & FISCHER,M. W. 1989. Devonian basins of Northern Scotland: During the Middle Devonian the northern Orcadian basin extension and inversion related to Late Caledonian-Variscan tectonics. was part of a regional releasing overstep including Norway In: COOPER,M. A. & WILLIAMS,G. D. (eds) Inversion Tectonics. Geo- andthe North Sea that wasassociated with extensional logical Society, London, Special Publication, 44, 275-308. collapseof the Caledonian orogen (McClay et al. 1986; DONOVAN,R. N., ARCHER,R., TURNER,P. & TARLING, D.H. 1976. Devonian Seguret et al. 1989). Thiswould suggest that extensional palaeogeography of theOrcadian Basin andthe Great Glen Fault. Nature, 259, 55G551. collapseof the Caledonides was triggered by left-lateral ENFIELD,M. A. 1988. The Geometry of Normal Faults and Basin development: movement along major inherited tectonic contacts (Seranneet Northern Scotland and Southern France. PhD Thesis, University of al. 1991). London. The basins were probablyinverted during the Upper - & COWARD,M. P. 1987. The West Orkney Basin, northernScotland. Journal of the Geological Society, London, 144, 871-884. Carboniferous. The phase of E-W to NE-SW contraction is ETCHECOPAR,A. 1984. Etude des htats de contrainte en tectonique cassante et related to dextral shearing ofa band centred around the Great simulations dediformations plastiques (approche mathimatique). These Glen/WallsBoundary faults. Dextral movement also oc- d’Etat. Universite de Montpellier (France). curred on discrete faults that represent reactivated Devonian FUNN, D.1969. A Geological interpretation of the aeromagnetic maps of the continental shelf aroundOrkney and Shetland. Geological Journal, 6, (or older) structures in Mainland Scotland. In Shetland how- 279-292. ever the major dextral faults crosscut and offset the Devonian - 1977. Transcurrent faults and associated cataclasis in Shetland. Journal structures. of the Geological Society, London, 133, 231-248. Northern Scotlandis therefore the locusof a long historyof - 1985. The Caledonides of Shetland. In: GEE, D. G. & STURT,B. A. (eds) oblique or lateraldisplacement. Caledonian orogeny was Caledonide Orogen-Scandinavia & RelatedAreas, J. Wiley & Sons, Chichester, U.K., 1159-1172. dominated by left-lateral shearing(Soper & Hutton 1984) GRANIER,T. 1985. Origin, damping and pattern of development of faults in associatedwith N-S convergencealong N-E-trending granites. Tectonics, 4, 721-737. transcurrent faults such as Great Glen or the Walls Boundary McClay, K. R.,Norton, M. G., Coney, P. & DAVIS,G. H.1986. Collapse of faults(transpression). During Middle Devonian extensional the Caledonian orogen and the Old Red Sandstones.Nature, 323, 147-149. MARSHALL,J. E. A. 1988. Devonian miosporesfrom Papa Stour, Shetland. collapse, these faults remained active as left-lateral faults but Transactions of the Royal Society, Edinburgh, 79, 13-18. withinregional a E-W extension(transtension). In MELVIN,J. 1985. Walls Formations, Western Shetland;distal alluvial plain Carboniferoustime, probably following a plate reorganiz- deposits within a tectonically active Devonian basin. Scottish Journal of ation, E-W contraction activated the same faults in dextral Geology, 21, 2340. MILLER,J.A. & FUNN, D. 1966. A survey ofthe age relationsof Shetland strike-slip. rocks. Geological Journal, 5, 95-1 16. MYKURA,W. 1976. British Regional Geology. Orkney and Shetland. H.M.S.O. This study was funded by the Commission of the European Communi- NORTON,M. G., MCCLAY,K. R.,WAY, N. A. 1987. Tectonic evolutionof ties (Contract SCI. 0089) and was undertaken at Imperial College, Devonian basins in Northern Scotland and southern Norway. Norsk Geo- London. I am grateful to M. Seguret for his contribution in the field logisk Tidsskrift, 67, 323-338. and to M. Enfield for discussions. This work profited from the pre- NORTON, M. G.& WAY,N. A. 1991. Carboniferous dextral strike-slip on the Great Glen Fault system. Tectonics, in press. printscommunicated by F. Benard and M. Norton. Western Geo- ROBERTS,A. M., BADLEY, M. E., PRICE,J. D. & HUCK, I.W. 1990. The struc- physical gave access to part of the Fair Isle seismic survey. The final tural history of a transtensional basin: Inner Moray Firth, NE Scotland. version has been improved by the reviews of D. Barr, M. Norton and D. Journal of the Geological Society, London, 147, 87-103. Snyder. Contribution CNRS-INSU Programme DBT ‘Theme Dyna- ROGERS,D. A., MARSHALL, J. E.A. & ASTIN,T. R. 1989. Devonian and Later mique Globale’ No349. movements on theGreat Glen Fault system, Scotland. Journal of the Geological Society, London, 146, 369-372, SEGURET,M., SERANNE, M., CHAUVET,A. & BRUNEL,M. 1989. Collapse- References basins: a new type of extensional sedimentary basins from the Devonian of Norway. Geology, 17, 127-130. ALLEN,P. A. & MARSHALL,J. E. A. 1981. Depositional Environments andPaly- SERANNE,M. 1991. Late Palaeozoic kinematics of the Merre-Trandelag Fault- nology of theDevonian South-east Shetland Basin. Scottish Journal of zone and adjacent areas, central Norway. Norsk Geologiske Tidsskrift, in Geology, 17, 257-273. press.

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-, CHAUVET,A. & FAURE,J. L. 1991. Cinematiquede I'extension tardi- STEARNS,D. W. 1978. Faultingand forced folding in theRocky Mountains orogenique (Divonien) dans les Calidonides Scandinaves et Britanniques, foreland. In: MATHENS,V. (ed.), Larumide folding msociuted with basement Comptes rendus de I'Acaddmie des Sciences de Paris, in press. block faulting in the western United States, Geological Society of America, __, , SEGURET,M. & BRUNEL, M.1989. Tectonics of the Devoniancollapse- Memoir, 151, 1-37. basins of western Norway. Bulletin de la Socidtk Geologique de France, 8, STORETVEDT,K. M. 1987. Major late Caledonian and Hercynian shear move- 489499. ments on the Great Glen Fault. Tectonophysics, 143, 253-267. SKILBREI,J. R. 1988. Geophysical interpretation of the Fosen-Namsos Western VANDER Voo, R. & SCOTESE,C. 1981. Paleomagnetic evidence for a large (c. Gneiss Region and the Trondheim Region Caledonides.In: KRISTOFFERSEN, ZOO0 km) sinistral offset along the Great Glen fault during the Carbonifer- Y. (ed.) Progress in studies of the lithosphere in Norway. Norge Geologisk ous. Geology, 9, 583-589. Undersekelse, Special Publication, 3, 7G79. VENDEVILLE,B. 1987. Chumps de failles et tectonique en extension: modelisation SOPER,N. J. & HUTTON, D.H. W. 1988. Late Caledonian sinistraldisplacements in expkrimentale. These de Doctorat, Universite de Rennes. Britain: Implications for athree-plate collision model. Tectonics,3,781-794.

Received 1 February 1991; revised typescript accepted 19 August 1991.

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