doi:10.1144/gsjgs.149.1.0027 1992; v. 149; p. 27-37 Journal of the Geological Society

 M. SERANNE  

Orcadian basinDevonian extensional tectonics versus Carboniferous inversion in the northern 

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© 1992 Geological Society of London

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 . S E R A N N E 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 contrac- tion. 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- land basins (Fig. 1) was deposited in an extensional setting during the Middle Devonian (e.g. Mykura 1976; Enfield &

100 km SHETLAND

I n

Fig. 1. Map of the main structural features of Northern Scotland. Data from Coward et al. (1989) and BGS 1:250000 Solid Geology Maps of Shetland, Orkney, Caithness. WOB: West Orkney Basin.

27

Coward 1987). These basins form a group of collapse-basins (Seguret et al. 1989) that developed in NW Europe during late- orogenic extensional collapse of the Caledonian orogenic belt (McClay et al. 1986). Later inversion of these Scottish basins resulted in the formation of folds within the Old Red Sandstone and reactivation of basement faults (e.g. Coward et al. 1989). A consensus relates the Late Palaeozoic tectonic evolution of that area to the kinematics of the Great Glen fault which is a major 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 review in 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 Red Sandstone 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 struc- tures 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 Enfield & Coward (1987) and Enfield (1988) have presented evidence for Middle Devonian extensional tectonics in the off- shore West Orkney Basin (Fig. 1). Sediments of Middle

28 M . S E R A N N E

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 struc- tures within the sediments are good evidence for syn-deposi- tional extensional tectonics. Such a simple half-graben geo- metry 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 extensional fault 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 re- gional palaeostress field as they respond to local conditions. Consequently, the Middle Devonian regional extension direc- tion in Caithness and Orkney is best approximated by the geo- metry of the normal faults and associated lateral 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 rocks of Shetland belong to three distinct 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 most of 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 acid volcanic rocks and it is intruded by the Late Devonian Sandsting granite (BGS 1:63650 W Shetland, Sheet No. 127)

(3) The Southeast Shetland basin (including Fair Isle) in which Old Red Sandstone sediments are in contact through unconformity or oblique normal fault with Dalradian gneisses and a ‘tectonic melange’ (BGS 1:63650 S Shetland, Sheet No. 126).

( l ) Melby basin. West of the Melby fault, the Middle Devonian Melby formation consists of sandstones and flagstones that are related to the Old Red Sandstone of Orkney on sedimentological and palaeontological grounds (Mykura 1976; Marshal1 1988). On the island of Foula, structures within the Devonian sediments and underlying basement in- dicate a NE-SW to E-W extension (Norton et al. 1987). In the north, sandstone gives way to volcanic lithologies (Esha Ness). The exposed Melby basin has been juxtaposed to the Walls basin by dextral strike slip along the Melby fault in post Old Red Sandstone time (Donovan et al. 1976). No further evidence of syndepositional tectonic activity has been found.

(2) Walls basin. The Old Red Sandstone of the Walls basin is poorly exposed and complexly folded and faulted. The BGS one inch map provides a good density of bedding dip and strike measurements. Mykura (1976) gave a cartographic interpreta- tion based on the recognition of several marker horizons trace- able throughout the whole basin. Re-mapping of the northern margin of the Old Red Sandstone outcrop and reappraisal of the nature of the Sulma Water fault has led to a simplified interpretation of Mykura’s map in which the late N-S trending folds have been removed for clarity (Fig. 2a).

The northern margin of the Walls basin is an unconformity which suffered minor reactivation as a discontinuous sinistral

a’t

Fig. 2. (a). Simplified map of Devonian bedding in the Walls basin (west Shetland, location on Fig. l), modified from Mykura (1976) and from personal observation on the northern margin. The effects of north trending folding have been removed. Sandsting granite and interbedded volcanic rocks are not represented. Old Red Sandstone series display internal unconformities and progressive syntectonic unconformities that demonstrate syn-depositional folding. Inset map shows the position of Devonian rocks (stippled) in Shetland. MF, Melby fault; SW, Sulma Water fault; WBF, Walls Boundary fault. (b) Section across Walls basin showing the relationship between basement faulting (left-lateral strike-slip) and folding of the sedimentary cover.Vertica1 and horizontal scales are the same.

strike-slip fault. North of the Sulma Water fault and east of the Melby fault the Old Red Sandstone formations preserve at their base ENE-WSW trending synclines which are unconfor- mably overlain by later Old Red Sandstone. Such local un- conformities attest to active tectonics during Old Red Sandstone time. The Sandness formation onlaps eastwards onto Lewisian-type basement. This reflects an eastwards shift of the source area with respect to the locus of sedimentation which, in turn suggests lateral movement of the basement dur- ing Old Red Sandstone deposition.

In the centre, Walls peninsula is occupied by the Walls formation whose sedimentology is detailed by Mykura (1976) and Melvin (1985). The formation presently dips towards the west and appears to have a cumulative stratigraphic thickness of about 12km. It is likely that this large stratigraphic thickness exceeds the final vertical depth of the basin, as a

T E C T O N I C S I N T H E N O R T H E R N O R C A D I A N B A S I N 29

result either of westward shifting of depocentres during deposi- tion, or deposition above an east-dipping low-angle normal fault.

South of the Sulma Water fault Devonian strata are folded 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 inter- nal angular unconformities (Fig. 2) showing that folding was contemporaneous with deposition. The southern part of the steep belt consists of an asymmetric syncline (the Walls syncline) whose vertical northern limb records a reduced stratigraphic thickness resulting either from faulting or inter- nal 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 along faults parallel to the Sulma Water fault, the sedi- mentary cover responding by flexure (Fig. 2b). Significant ver- tical offset is required to account for the steep bedding. How- ever, the geometry of strata below and above an internal angular unconformity in the northern limb of the Walls syncline (Fig. 2a) indicates a sinistral offset 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 faults without 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) whose sedimentology has been studied by Mykura (1976) and Allen & Marshall (1981). In the Lerwick area the Old Red Sandstone consists of alluvial-fan conglomerates that grade eastwards into sandstones of braided stream character and are overlain by lacustrine flagstones. Basal breccias overlie an irregular basement unconformity around Easter Quarff (Fig. 3). In the south the Old Red Sandstone consists of fluvial sandstone and pebbly sandstone, interfingering with lacustrine flagstones. Palaeocurrent directions indicate a south to southeasterly flowing drainage system (Allen & Marshall 1981). The lacustrine sediments are bounded to the west along a SSW-trending line roughly parallel to the coast of Main- land. The occurrence of alluvial fans in the north and resedi- mented debris flows in the south are strong evidence for elevated topography and probably active tectonics during sedi- mentation.

The Southeast Shetland basin may be structurally divided into three parts (Fig. 3).

The southern part has 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- mable in the rest of the outcrop, where it is marked by locally derived basal breccia. The core of the syncline is complicated by second order en echelon synclines and by a N-S fault whose splays and parallel synthetic shear planes display left lateral displacements. These observations would indicate that the faulted segments of the basement contact are sinistral strike- slip faults. There is no evidence for normal faulting across this margin as postulated by Norton et al. (1987).

The central part of the Southeast Shetland basin is charac- terized by a discontinuous basal breccia that is unconformable over pre-Devonian basement consisting in this area of the

0 ORS sandstones 8 flagstones

.:.:.:...: ORS conglomerates

a Deformed ORS

D

Basement, tectonic melange

/c ORS bedding trace R dip

6 ORS unconformity

/ Fault

0 10 km L I I I I ~ ~ I I I I

Fig. 3. Simplified map of Southeast Shetland basin. Location on Fig. 1. EQ, Easter Quarff; G, Gulberwick; L, Lerwick; RH, Rova Head; S, Sandwick; SH, Sumburgh Head.

‘tectonic melange’ and of psammites related to the Dalradian (Fig. 3). The tectonic melange comprises lenses of marble and quartzites floating in a matrix of east-dipping foliated phyllites that display sub-horizontal stretching lineations and consis- tent top-to-north ductile shear criteria. The psammites also present penetrative top-to-north shearing and are folded in a north trending syncline, itself truncated by the Old Red Sandstone unconformity. It is likely that the ‘tectonic melange’ was generated by this top-to-north shearing event and thus is pre-Middle Devonian in age.

The northern part of East Shetland is separated from the central part by a NNW-trending normal fault against which an

30 M . S E R A N N E

N

open hanging-wall syncline was developed (the Gulberwick syncline). Along the west and northwest margins syntectonic alluvial fans were deposited close to the basin bounding fault. The contact between conglomerates and basement is exposed at Rova Head (Fig. 3). It corresponds to a shear zone that involves basement calc-schists of the 'tectonic melange' and Devonian conglomerates (Fig. 4). The basement shows NE-trending shear planes that dip 40-70" SE, and they cut through shallower dipping foliation. Density of shear planes increases toward the basement/Old Red Sandstone contact (Fig. Sa). Although there are no stretching lineations or slickensides on the shear planes, an oblique movement is inferred as vertical and horizontal sections yield evidence for normal and sinistral offset, respectively. The basement/Old Red Sandstone contact is covered by a few metres of sand and drift. For about SO m above the fault Old Red Sandstone conglomerates are de- formed by extensive NE-trending shear planes that separate blocks of different lithology, different bedding dip and vari- able degree of internal deformation. Highly deformed con- glomerate has a greenish matrix of fine angular grains with some coarser fragments (Fig. Sb). It is affected by cleavage developed around undeformed or brittlely stretched pebbles and cobbles. In horizontal sections, clast long axes appear parallel to the ENE-trending matrix cleavage and lie at an angle to the NE-trending shear planes that characterizes sinis- tral shearing (Fig. 5c). Some strips of conglomerate display smaller angular clasts, evidently fragments of larger cobbles, clasts are affected by a consistent pattern of fractures normal to the clast long axis (Figs 4 and Sd). In lenses of weakly de- formed sandstones there are groups of sinistral strike-slip faults with centimetre-scale throw that are synthetic to the bordering shear planes. Outside the marginal shear zone, con- glomerates are affected by conjugate NNE-trending-sinistral and ESE-trending-dextral shear zones that separate blocks several tens of metres wide (Fig. 4). Some of the shear zones are characterized by passive reorientation of the cobbles parallel to the shear planes. This is a good indication that deformation occurred early in the conglomerate's diagenetic evolution. In

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.

the rest of the conglomerate, cobbles and pebbles are often fractured by vertical tension gashes oriented NI40 to N160 indicating an extension oriented parallel to the basin margin.

All observed structures on the NW margin of Southeast Shetland basin are consistent with an extensional fault with a left lateral component that separates basement and Old Red Sandstone, and that was active during sedimentation. A north- east direction of extension is in agreement with the NNW- trending normal fault bounding the basin to the SW and sinis- tral movement on the marginal fault which is therefore a lat- eral ramp.

On Mainland (Fig. 6a) bedding dips are consistent with a normal fault ramp-flat-ramp geometry at depth developing a NW-trending hanging-wall syncline (Gulberwick) and ramp anticline (Lerwick) as shown in Fig 6a. Close to the lateral ramp bounding the basin to the NW, bedding is bowed in a drag-fold consistent with left-lateral movement (Fig. 3).

On each side of Bressay island there are NNE-trending belts of deformed Old Red Sandstone that consist of metre scale folds and of brecciated sandstones. Breccias are found in exten- sive outcrops along the eastern and western coast of Bressay (Fig. 3). It consists of angular clasts of sandstones of varying colour and texture, that have preserved their internal stratifi- cation, some may be of ten to several tens of metres large. According to Mykura (1976), these are thought to be related to post-Devonian volcanic activity. However, at the periphery of the massive breccias, it can be observed that finer breccias were formed by disruptions of sandstone bedding in fault zones. In other places the breccia is found resedimented within the sedi- ment, as rubble beds of thickness varying between several tens of centimetres up to several metres. This suggests that breccias are rather the result of tectonic activity and that deformation giving rise to folding and brecciation occurred during sedimen- tation.

The kinematics of these sheared belts is unclear. The atti- tude of bedding close to the belts suggests drag folding consis- tent with a downthrow of the eastern block (Figs 3 and 6b). In addition a number of independent observations favour a strong

TECTONICS IN THE NORTHERN ORCADIAN BASIN 31

Fig. S. Structures in the basin bounding shear zone at Rova Head (Fig. 4), that involves basement rocks (a) and Old Red conglomerates (b, c, d). Strain decreases away from the contact (from b to d). All kinematic indicators (shear bands, mica fishes, shear planes) are consistent with left-lateral slip.

component of lateral movement (coexistence of extensional and contractional structures, steep fold axes, horizontal slick- ensides) that would be sinistral in sense (markers sinistrally offset, the Bressay sheared belt is colinear with the sinistral strike-slip fault in Sandwick).

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 within the sheared belts would be needed to ascertain the nature and magnitude of movement they underwent but a preliminary interpretation is of a diffuse zone of shear in the sedimentary cover as a response to sinistralhormal faulting in the basement during sedimenta- tion (Fig. 6b,c)

Origin of folding Folding observed in the Old Red Sandstone of Shetland seems to be hard to reconcile with extensional tectonics. However, in addition to long wavelength compaction synclines, ramps and 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- lying basement structures (Steams 1978). Field examples (Chenet et al. 1987) and sand-box models (Vendeville 1987) have shown that folding of sedimentary cover may result from extensional and strike-slip basement faulting. It is proposed here that large-scale folds in Shetland would result from transcurrent movement along NE-trending basement faults and from extension across NW-SE normal faults. The asym- metric Walls syncline (Fig. 2) is best explained by sinistral strike slip andior downthrow of the southern basement block (Fig. 7a and b). Folding of the marginal conglomerates near Lerwick (Fig. 3) results from accommodation of sedimentary strata above an active low-angle normal fault and associated transfer faults (Fig. 7c). A similar mechanism has been invoked for the similar Middle Devonian basins of Norway (SCranne et al. 1989).

32 M . S E R A N N E

a b C

Regional kinematics

Large- and small scale structures in the Walls basin support a model of sinistral shearing along ENE-trending basement faults, and the younging direction of Old Red Sandstone south of Sulma Water fault require synsedimentary relative move- ment of the hanging-wall towards the ENE, parallel with basement lineaments. This is in agreement with ENE extension parallel to and controlled by ENE-trending transfer or strike slip faults. This interpretation contradicts previous models of SE-NW extension across inferred NE-trending basin bounding normal faults (Coward & Enfield 1987) which are shown in this study to be an unconformity that suffered only minor post- deposition left-lateral slip. In the Southeast Shetland basin, similar patterns of sinistral shearing along basin-bounding and basement faults, and younging directions in the syntectonic sediments argue for a NE-SW oriented extension. Figure 8 is a conceptual view of part of the Southeast Shetland basin during the Middle Devonian that illustrates the geometrical rela- tionship between the spoon-shaped low angle normal fault, and the high angle sinistralhormal basement faults that acted as oblique or lateral ramps.

The variation in directions of Middle Devonian extension

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.

observed in the northern Orcadian basin is associated with a change of tectonic regime. Extensional tectonics in the West Orkney Basin is accommodated mainly by normal faults and subordinate transfer faults, that correspond to WNW-ESE ex- tension (Enfield & Coward 1987). In Shetland, trancurrent tec- tonics becomes dominant over extension. Inferred directions of extension are parallel to the ENE-trending left-lateral trans- fer faults in the Walls Basin and parallel to the NE-trending basin bounding lateral ramp in the Southeast Shetland Basin (this study). Restoring the Southeast Shetland Basin 70km north (Rogers et al. 1989) results in a spatial distribution of extensional and transcurrent tectonics, and variation of exten- sion trajectories throughout the Orcadian Basin, that fits with models of ‘horsetailing’ at the extremities of strike-slip faults (Granier 1985). According to this model, offset of left-lateral N- to NE-trending strike-slip faults bounding the Devonian basin in Shetland is transformed into extension across normal faults in the West Orkney basin. Recent work (Seranne 1991) indicates that a similar regional pattern can be observed in Norway where Middle Devonian WNW-trending extension in the Bergen area (Chauvet & Seranne 1989) progressively rotates northwards until it becomes parallel to the SW-trend- 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.

T E C T O N I C S I N T H E N O R T H E R N O R C A D I A N B A S I N 33

n = 48

a b

Fig. 9. (a) Azimuth of horizontal small-scale folds in SW Mainland Orkney. (b) Field sketch of en echelon tension-gashes associated with folding observed in SW Mainland Orkney. They indicate that direction of contraction (white arrows) was oblique to fold axes.

fault zone was a left-lateral strike-slip fault during the Middle Devonian (Seranne 1991) and links up with the NE-trending faults of northern Scotland such as the Great Glen and the Shetland Spine faults (Fig. 1) (Skilbrei 1988). It has been sug- gested that extensional Middle Devonian basins in the North Sea were formed within a large scale releasing overstep between the sinistral More-Trondelag fault zone and a southern limit, probably the Highland Boundary fault, as a result of late-Caledonian extensional collapse (Seranne et al. 1991).

r'?

a : ORK21 '

Y V

N

Carboniferous inversion structures Small scale folds and faults The Old Red Sandstone of Caithness and Orkney are affected by a number of small scale folds and thrusts that have typically been related to post-Devonian inversion (Mykura 1976; Enfield 1988; Coward et al. 1989; Norton & Way 1991). A well defined and consistent group consists of folds several tens of centimet- res in wavelength and amplitude, often cored by a blind thrust. They are found either isolated or in bundles. On SW Mainland Orkney these folds are roughly oriented N-S with a maximum at N170 (Fig. 9a). They commonly display en echelon tension gashes in sets parallel to fold axes that imply right-lateral movement along the fold plane (Fig. 9b). This indicates that the direction of contraction was oblique to fold axes. This first group of folds corresponds to a ENE-WSW direction of con- traction. North-trending kilometre-scale folds described by Mykura (1976) in the Walls peninsula overprint Devonian folding and may be correlated with this phase of contraction.

On east Mainland Orkney were found small scale folds with steep axes that are clearly associated with strike-slip deforma- tion but whose kinematics and chronology are not known.

Sandstones are affected by brittle faults (strike slip or reverse and interbed slippage) with offsets ranging from tens of centimetres up to several metres. Slickensides usually display quartz fibres. Groups of striated faults ( 5 to 20) of different strike and dip in a given outcrop allow an approximation to be made of the local directions of extension and compression (Arthaud 1969; Etchecopar 1984). These groups of faults pro- vide a direction of contraction oriented NE-SW (Fig. 10) that

N

b : CAlTHl6

N

c : SHET27

V I

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).

34 M . S E R A N N E

Fig. 11. (a) Fault planes and slickensides in the Sandsting granite (Location on Fig. 14). (b) Inferred directions of principal stresses (01 > 0 2 > 0 3 ) . Bold arrows represent the estimated mean orientation of

are in agreement with the north-trending folds. The E-W con- traction inferred from a location in the northern end of Sulma Water fault zone (Fig. 1Oc) is interpreted as due to perturba- tion of the stress field close to this major fault. It also allows a right-lateral sense of slip 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 Sandsting granite in Walls Peninsula and they also provide a NE-SW direction of compression (Fig. 11). These faults are interpreted as resulting from the same tectonic event as the compressional one recorded in the Old Red Sandstone.

:ARBONIFEROUS INVER

N

Fig. 12. Structural map of Carboniferous inversion in Orkney and Caithness. Compression directions are given by small-scale folds and thrusts, and slickensides. Each location represents the mean orientation inferred from 5-20 measurements (folds, faults, shear-bands in fault zone). Large scale structures from BGS 1:250000 Solid Geology Map of Orkney and Caithness.

a a) contraction.

Mapping the directions of contraction inferred from small- scale folds and thrusts and striated faults illustrates a general pattern of ENE-WSW to NE-SW contraction expressed in the studied area, from Caithness to Shetland (Fig. 12).

Movement along regional faults Simple cartographic relationships (downthrown block and fault dip) indicate that the East Scapa fault and the Brim-Rissa fault in Orkney-Caithness are reverse faults (Fig. 12). These are interpreted as inverted normal faults by Coward et al. (1989) and Norton & Way (1991).

Brittle fractures in the N060-trending Helmsdale fault zone make a pattern consistent with a dextral strike slip. This fault has had a long history of dextral shearing linked with exten- sion in the Moray Firth, that lasted until the Mesozoic (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 system including the Walls Boundary and Nesting faults. Devonian sandstones are involved in 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 in- dicators of dextral shearing. The Nesting fault is developed mostly within schists and calcschists that are favourable 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 Sulma Water fault has been reactivated in a brittle fashion in its eastern part, where it changes strike and becomes N030-orientated. Analysis of associated Riedel faults provides evidence for 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 may partly be due to super- imposition of dextral shear on the Devonian sinistral shear zone.

Kinematics and age of the inversion All the different types of structure (folds of different scales, faults, shear zones) indicate a mean ENE-WSW direction of compression in the northern Orcadian basin, ranging in direc- tion from E-W in the Orkneys (Fig. 12) to a more northeasterly trend in Shetland (Fig. 14). Similarly, the major strike slip faults system made of the Great Glen-Helmsdale faults and

TECTONICS IN THE NORTHERN ORCADIAN BASIN 35

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.

Fig. 14. Structural map of Carboniferous inversion in Shetland. Same legend as in Fig. 11. Permian contour from BGS 1:250000 Solid Geology Map of Shetland; folds truncated by Permian basal unconformity imaged on seismic profiles provided by Western Geophysical.

Walls Boundary-Nesting faults show a change in trend from NNE in the south to N in the northern part of the basin (Fig. 1). The respective orientations of contraction and of major faults are in agreement with dextral slip along the fault system. In addition, most of the observed deformation is concentrated around, and increases towards, the transcurrent faults. These

facts strongly argue for the inversion being related to right lateral wrenching along the Great Glen-Walls Boundary faults system.

The total offset of post Devonian dextral strike slip along the Great Glen-Walls Boundary faults system has been a mat- ter of much discussion depending greatly on the method in- volved: Storetvedt (1987) determined 300 km of dextral offset based on palaeomagnetism, Flinn (1969) matching magnetic anomalies measured 70 km dextral offset on the Walls 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 the offset. However, Van der Voo & and Scotese’s (1981) proposition of 2000 km sinistral Hercynian offset may be ruled out as observed 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 strain in the surrounding rocks.

The inverted normal faults in Orkney and Caithness offset the Upper Devonian. The Sandsting granite complex is affected by the ENE-WSW compression, and the dextral Walls Bound- ary fault cuts the complex to the east. The granite intrusion is dated by K-Ar method at 371 f lOMa (Miller & Flinn 1966) implying a post Late Devonian age for the inversion. Seismic reflection surveys in the West Orkney Basin reveal a pre- Permian angular 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 also Norton & Way 1991). The inversion phase therefore appears to be Carboniferous in age. It must be stressed however that (1) the age of markers in the West Orkney Basin are poorly constrained, (2) syntectonic unconformities within Old Red 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.

Late Carboniferous inversion of Devonian and early Carboniferous basins is widely documented in Britain (e.g. Collier 1989). Benard et al. (1990) have investigated the direc- tions of contraction during the Carboniferous throughout the British Isles, using a similar approach to that of the present study. They gave evidence for two different compressional

36 M . S E R A N N E

episodes during the Carboniferous:

E-W contraction during the Namurian-Westphalian, N-S contraction during the Westphalian-Stephanian related to Hercynian deformation.

The dextral sense of motion observed along the NNE-trending major strike-slip faults is consistent with regional E-W con- traction, and therefore a Namurian-Westphalian age is in- ferred for the inversion in the northern Orcadian basin.

Conclusions This study has confirmed the existence of superimposed tec- tonics in the Orcadian basin. Complex folded and faulted structures in the Old Red Sandstone typically require several phases of post-depositional compressive deformation. How- ever, the tectonic evolution that emerges is somewhat simpler than previously assumed. Some large scale folds that had pre- viously been related to post-depositional compression, are shown to have developed during sedimentation of the Old Red Sandstones. These structures are interpreted as extensional ramp synclines and forced folds of sedimentary cover over- lying basement faults.

During the Middle Devonian the northern Orcadian basin was part of a regional releasing overstep including Norway and the North Sea that was associated with extensional collapse of the Caledonian orogen (McClay et al. 1986; Seguret et al. 1989). This would suggest that extensional collapse of the Caledonides was triggered by left-lateral movement along major inherited tectonic contacts (Seranne et al. 1991).

The basins were probably inverted during the Upper Carboniferous. The phase of E-W to NE-SW contraction is related to dextral shearing of a band centred around the Great Glen/Walls Boundary faults. Dextral movement also oc- curred on discrete faults that represent reactivated Devonian (or older) structures in Mainland Scotland. In Shetland how- ever the major dextral faults cross cut and offset the Devonian structures.

Northern Scotland is therefore the locus of a long history of oblique or lateral displacement. Caledonian orogeny was dominated by left-lateral shearing (Soper & Hutton 1984) associated with N-S convergence along N-E-trending transcurrent faults such as Great Glen or the Walls Boundary faults (transpression). During Middle Devonian extensional collapse, these faults remained active as left-lateral faults but within a regional E-W extension (transtension). In Carboniferous time, probably following a plate reorganiz- ation, E-W contraction activated the same faults in dextral strike-slip.

This study was funded by the Commission of the European Communi- ties (Contract SCI. 0089) and was undertaken at Imperial College, London. I am grateful to M. Seguret for his contribution in the field and to M. Enfield for discussions. This work profited from the pre- prints communicated by F. Benard and M. Norton. Western Geo- physical gave access to part of the Fair Isle seismic survey. The final version has been improved by the reviews of D. Barr, M. Norton and D. Snyder. Contribution CNRS-INSU Programme DBT ‘Theme Dyna- mique Globale’ No349.

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