Tectonic evolution of basins in northern Scotland and southern Norway

MIKE G. NORTON, KEN R. McCLAY & NICK A. WAY

Norton, M. G., McClay, K. R. & Way, N. A.: Tectonic evolution of Devonian basins in northern Scotland and southern Norway. Norsk Geologisk Tidsskrift, Vol. 67, pp. 323-338. Oslo 1987. ISSN 0029-196X.

The geometries of the Devonian basins of northern Scotland and southern Norway are analysed to determine the regional tectonic control on their formation. The Orcadian Basin in northern Scotland consists of distinct half-graben sub-basins controlled by separate, closely spaced, sub-parallel arcuate extensional fault elements which are parti y con tro lied by the Caledonian crustal fabric. The unconformity at the base of the middle Devonian in this area is interpreted as being formed by a sudden increase in the rate of extension. In southern Norway, in con trast, extensional structures are more widely spaced and larger in scale. Very deep (>8 km) half-graben basins were developed of which only the bottom parts are now preserved. In the Devonian of Norway the Jack of a major stratigraphic break suggests a constant extension rate. The observed differencesin style are thought to be due to different initial crustal thicknesses.

M. G. Norton, K. R. McCiay and N. A. Way, Department of Geology, Royal Hol/oway and Bedford New College, Egham, Egham Hill, Surrey, TW20 OEX, U.K.

The broad outlines of the Devonian 'Old Red main aspects have been studied; the pattern of Sandstone' basins of northern Scotland, western basin-forming faults and the significance of the Norway and the northern North Sea have been unconformity between lower and middle known for some time (e.g. Ziegler 1982) (Fig. Devonian. In definingthe basin-forming fault net­ la). The tectonic setting of the Devonian sedi­ work, use has been made of the work of Enfield ments has been variously interpreted as related & Coward (1987) in the West Basin. to megashears (Ziegler 1982; Bukovics et al. 1984) Fieldwork has been carried out in the region or regional extension (Dewey 1982; McClay et al. bordering the Moray Firth and in , 1986) (Fig. 1). Large basins like the Orcadian Orkney and . lnterpretation in the Basin of northern Scotland consist of many similar Mora y Firth itself has been carried out using data sub-basins each exhibiting separate tectonic con­ provided by Fina (UK). In the offshore area tro! and distinct sedimentation and subsidence around Shetland the interpretation is based on patterns. This paper provides a detailed analysis the BIRPS SHET survey (McGeary 1987). of some of the individual Devonian sub-basins The identification of faults active during and uses the results to constrain a general model Devonian sedimentation is based on observed or for Devonian tectonics in the north west European inferred faults which correlate well with major continental margin. thickness and facies variations, and published The analysis has been carried out by: the sediment dispersal patterns (Mykura 1983). On reinterpretation of published onshore data, some of the published geological maps faulted fieldwork at various critical localities, the inter­ contacts between Devonian sediments and pretation of offshore geophysical and well data basement are shown to pass laterally into uncon­ together with available structural and sedi­ formable contacts; this is particularly so along the mentological data. north western boundary of the Devonian between Inverness and Helmsdale and in east Shetland. In Northern Scotland most cases unconformity has been inferred along unexposed contacts in which the interface is seen North of the , Devonian to dip at an angle of less than 45°. Now that sediments define the approximate extent of the the existence of low-angle extensional faults has 'Orcadian Basin' (Fig. 2). Within this region two become recognized such contacts can be con- Ul �

HBF - Hlghland Boundary Faun � GGF - Great Glan Faun MF - Mlnch Faull WBF - Wa11a Boundary Faull � MTFZ - More-Trøndelag Fault Zone BF - Blllel)ordafl Fault � � � � � i � () . � ".. X - Wldeapread h.alf-grabena � OEVONIAN - Conaittant eKtenslon direcllonl OISTRIBUTION OF SINISTRAL MEGASHEARS. � . � � SEDIMENT$. - Oreateat eJttenaion in the zone "z o o of greatest cruatal thickening .. ;;; · - Localised øun-aøart basins !:3 · · · �lA-._�. · - Varyitlg extenaion directions �8 .._._.·. · ·J···• • .. ·,· · . aub-parallet to taults .. . . · · . � ��. �0"�.,o� � . æ er "�

..�� ;:: !3 zz o o ,: -�· .{" _)'J ,&.: �j·/ .-/ z o � V> ,/ �\ ::-: o m V o t""' o O 200KM. O 200KM. 0 200KM. '-----' '-----' a '---' b c l V>8 ::-: ::l o V> Fig. l. Northeast Atlantic region in pre-rift configuration showing (a) Distribution of Devonian sediments of northern Scotland, southern Norway, East Greenland and Svalbard, V> ::-: (b) Sinistral Megashear Hypothesis, and (c) Extensional Collapse Hypothesis. (b) and (c) also indicate expected basin characteristics from the two models. �

O>� ...... _ � NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 325 fidently reinterpreted as tectoni'c. In some cases of a significant strike-slip component in the fracture sets are developed sub-parallel to the regional tectonic setting; the Great GJen Fault contact with individual fractures showing exten­ itself has no clear expression in either the sional displacement providing supporting evi­ observed fault network or in the pattern of sedi­ dence for the interpretation of these boundaries mentation (Rogers 1987). However, the inter­ as extensional faults. pretation of widespread unconformities at the The fault pattem established by this study is base of the middle and upper Devonian suc­ shown in Fig. 3. This has been constructed by cessions in the Orcadian Basin has been used assuming the following overall strike-slip move­ as evidence for rnajor periods of upliftwhich have ments since the lower Devonian: Great GJen in turn been used to imply periods of transpres­ Fault - 20 km dextral (Rogers 1987), Walls sion. Boundary Fault - 75 km dextral (Astin 1982), The unconformity at the base of the middle Nestings Fault - 15 km dextral (Astin 1982); no Devonian is best exposed on the southern shore allowance has been made for movement on either of the Moray Firth in the excellent coastal the Helmsdale or Melby Faults whose movements exposures at Pennan (Trewin 1987) within the are insufficientlyconstrained. The overall pattem Turriffhalf-graben sub-basin (Fig. 2). At Pennan would not be greatly affected by movements on lower Devonian conglomeratic sandstones dip these faults of 50 km or less. Three major faults moderate! y steeply (ca. 40°) towards the west and incorporated into the network were also impor­ are overlain by shallowly dipping middle tant structures during the Mesozoic development Devonian conglomerates with sharp angular of the Inner Moray Firth Basin; the Smith Bank unconformity, this is the on! y clear example of Fault, the Lossiemouth Fault and the Buckie angular unconformity in the Orcadian Basin, else­ Fault. The evidence for an earlier phase of move­ where it is marked only by a facies change (Rogers ment on these faults comes from seismic inter­ 1987). The base of the middle Devonian is irregu­ pretation of Devonian sediments in half-grabens lar and strongly erosive. The lower Devonian on their hanging walls and fromwell data showing is affected by a series of small-scale faults with age and facies variations across the faults. Fig. 4 displacements of a few cms up to a few metres shows a generalized geoseismic section across two which do not cut the unconformity and clearly of these faults taken from commercial seismic predate this erosional event. All of the fault zones data (the position of the section is shown in Fig. are extensional in geometry, most of them dipping 2). towards the east. This is the only deformation, One of the most interesting aspects of the fault for which there is evidence, that can explain the network deduced in the Orcadian Basin is the rotation of the lower Devonian strata. The fault presence of several arcuate fault elements which zones themselves are rather diffuse zones in which define the separate sub-basins. These have a no fault rock is developed and appear to have characteristic aspect ratio and size with a strike been formed in only part-consolidated sediments. extent of ca. 120 km. These arcuate fault elements We interpret this unconformity not as a period of are very similar in scale to those observed in the general uplift but as a period of rejuvenation Lake Tanganyika region of the East African Rift along pre-existing extensional faults probably (Rosendahl et al. 1986). It is not clear, however, with the initiation of some new extensional struc­ how all of these faults link together either along tures. In the Turriff region this rejuvenation led strike or at depth. to the progradation of alluvial fans from the new The observed arcuate fault geometry implies fault scarps eroding down into the previously that along much of the fault lengths there is a deposited and on!y partly consolidatedsediments. significant strike-slip component to the dis­ This fits well with the interpretation by Rogers placement. At the ends of some of these faults (1987) of a major change in the drainage pattern the displacement must be almost pure strike-slip at this time. It is possible that both the small-scale in character. The orientation of these high! y faulting and the fan progradation date from this oblique segments can be used to constrain the same extensional pulse. Fig. 5 shows a schematic overall extension direction as approximately cross-section over the Turriff half-graben at end northwest-southeast trending (see also Enfield & middle Devonian times to illustrate the proposed Coward 1987). interpretation. The recognition of the middle The overall structural frameworkshows no sign Devonian basal unconformity throughout the 326 M. G. Norton et al. NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

KEY W

� Mesozoic

U. Oevonian 1 D

[] M. Devonian

� L. Devonian

"'v Volcanics

Irr] Granite

D Basement

"" ;-"' Unconformity

/ Extensional fault

/ Contractional fault

/P Strike-slip fault

/ Han_ging�wall anticline

50 km

.... 'Q MORA Y FIRTH ·,. NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 327

8KM. SW

BANFF F.

l l • l sees � l TWT l

Fig. 4. Schematic geoseismic section ( taken from various com­ mercial surveys) across the Smith Bank and Lossiemouth Faults showing Devonian half-graben basins and reactivation of their marginal faults during the Mesozoic. For approximate location see Fig. 2.

TURRIFF SUB-BASIN AT END MIODLE DEVONIAN.

PENN"N NEW ABERDO\JR

\ PRESENT EROS ION \ LEYEL /

25 KM. 5. '=====' Fig. Schematic cross-section across the Turriff sub-basin at the end of the middle Devonian as deri ved in this study.

Fig. 3. Devonian fault pattern map of the Orcadian Basin as deduced in this study using data from Enfield & Coward (1987) in .the West Orkney Basin (extensional faults, downthrow on the middle Devonian sub-basins onto the ornament ed side). basement. The marginal fault system to this sub­ basin was still clearly active as a growth fault at this time as shown by thickness and facies Orcadian Basin suggests that this extensional variations across it (Mykura 1983). However, rejuvenation was of regional extent. fault activity appears to be relative!y subdued and I,nthe region lying northwest of the Great GJen much of the upper Devonian sediments represent Fault a series of contractional structures have reworking of the previously deposited Devonian been known for some time, e.g. at Struie Hill (Watson 1984). The unconformity is not well (Armstrong 1964; Mykura 1983), which are prob­ exposed except on where it is associated with ably related to transpression along the fault. the Hoy Javas but no strong discordance between These are best exposed in Iower Devonian strata the middle and upper Devonian has been and a genetic link has been suggested between described. them and the unconformity at the base of the To the south of the Highland Boundary Fault middle Devonian. However, it is clear that some (i.e. south of the area described above) the sedi­ of these structures (in common with those in mentary and tectonic history of the Devonian are Caithness) involve upper Devonian strataas well quite different to that described above. It seems and must be at !east as young as lower Car­ that subduction was still active at the start of the boniferous in age. There seems no reason to Devonian period with eventual closure and small­ suggest a separate period of transpression at the . scale collision (in comparison to the rest of the end of the lower Devonian to explain these Caledonides) marking the start of the middle structures. Devonian. Various lines of evidence have been In the Elgin area the unconformity at the base put forward to show that this convergence was of the upper Devonian involves overstep across associated with significant sinistral strike-slip

Fig. 2. Geological map of the Orcadian Basin modified after various BGS maps showing SHET lines, the Western Geophysical line (WG) as figured by Andrews (1985) and the location of Fig. 4. Key to faults: GGF , HF Helmsdale Fault, NSF North Scapa Fault, WBF Walls Boundary Fault, MF Melby Fault & NF Nesting Fault. 328 M. G. Norton et al. NORSK GEOLOGISK TIDSSKRIFT 67 (1987) motion of the Highland Boundary and other faults (including the Great GJen Fault and faults in the Grampian Highlands) (Bluck 1983; Soper & Hutton 1984). We do not deny the existence of important sinistral strike-slip movements south of the Highland Boundary Fault during late and into lower Devonian times but we suggest that north of this structure such movements were of reduced scale, predated most of the sedi­ mentation (which began in the Siegennian at the earliest) (Mykura 1983), and have very little con­ tro! on basin formation. We suggest that at the KEY beginning of the middle Devonian the ending of Devonlan convergence to the south may have acted as a o Conglomerate . trigger for the pulse of renewed extension to the EjSandatone north.

Dalradlan

Quarff Nappe Shetland The sub-basin geometry both onshore Shetland � Major lntruaion and in the surrounding shelf area is not as clear as that to the south. The results from the SHET / ExtenalonaJ Fault survey (Fig. 2) (McGeary 1987) have been dis­ _/ Strlke-SIIp Fault appointing in delineating Devonian sediments in the shelf area due mainly to unfavourable acqui­ sition conditions, particularly the presence of rela­ tively high velocity rocks (Devonian sediments or Caledonian basement) at the surface in shallow water. Onshore detailed fieldwork has been car­ Fig. 6. Geologica( map of the east Shetland Devonian, modified ried out studying basement-cover contacts and after BGS 250,000 sheet, Shetland. fault patterns within the Devonian particularly in East Shetland and on the island of (Fig. 2). As mentioned in the previous section the western boundary of the East Shetland Devonian is here interpreted as an extensional fault, probably detachment (i.e. as in the model of Davis 1983) active during sedimentation. Observations of wm investigated. Although the high strain zone movement indicators fromthe marginal fault zone is poorly exposed, lineations were found to be and minor faults in the hanging wall indicate mainly sub-parallel to strike (NNW-SSE ), simi­ dominant E-W extensional displacement which lar to that developed in much of the Caledonides supports this interpretation. In the region around of Shetland. Unequivocal sense of shear indi­ Lerwick this extensional fault activity is expressed cators were lacking but as yet we can find no in alluvial fanglomerates but the effect on sedi­ evidence for a genetic link between the high strain mentation dies out towards the south and the rocks and the marginal fault except that the fault throw on the fault is presumed to do the same. has locally reactivated the older structure. The clear unconformity at the base of the On Foula middle Devonian sediments are in Devonian at Quarff (Fig. 6) is explained as repre­ contact with Caledonian basement, the contact senting part of the unconformity on the hanging being interpreted as either a slightly modified wall of the marginal fault. unconformity (Blackbourn & Marshall in prep.) This basin marginal fault follows the line of or as an extensional detachment (Beach 1985). As the 'Quarff Thrust' and 'Quarff Melange' from in East Shetland the boundary between Devonian Lerwick to south of Quarff. The possibility that and basement is parallel to a strong, in this case some of the high-strain rocks in the zone could mylonitic, foliation in the basement (Fig. 7). be related to the observed fault as an extensional Here, however, the mylonites contain unequivo- NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 329

95 before intrusion) must be clarified before this geometry can be firmly established. \ The middle Devonian volcanics of Shetland are an apparent anomaly within the basins, whether extensional or transtensional in origin, due to their tholeiitic to calc-alkaline chemistry (Thirlwall 1981). Any interpretation in terms of subduction active at this time is very difficult to fit in with the known geology of the area. One method of generating such magmas during exten­ sion would involve partial melting of previously metasomatized upper mantle (Simpson et al. 1979; Gill 1982) resulting from earlier subduction.

DNOOPSANOSTONfFH EZI ""''" Southern Norway DSHfl..(iSANJSTONEfJ'1 DBLOfli!SBURNftl Devonian sediments in southern Norway (i.e. DsæERLJ(SAI()SfONEFH south of 64°N) are found in three areas (Fig. 8) EEl il'NESSFM (Z::]BASfl"lNT l:::) lt(AiliR.tNITf ---ØIEXT 1'\\ EHSØW.. '"'-''

!KM L.....---'

GEOUlGUL MAP CF FOOLA

Fig. 7. Geological map of Foula, modified after Blackbourn & Marshall (in prep). l ca! sense of shear indicators showing thrust sense displacement towards the northeast. The pattern of minor faults in the Devonian of the hanging wall and in the upper part of the basement of the footwall shows an unequivocal extensional displacement approximately E-W in orientation. It is clear again that the orientation of the exten­ sional fault has been controlled by the pre-existing l _, basement anisotropy. ( ' \ This preliminary work on Shetland has ident­ ' ified a number of extensional faults probably ' active during the Devonian and these have been Oevonlan aedlmenta included in the finalcompilation (see last section). Upper and uppermoat The structure of the west Shetland Devonian '"ochthon Lower and mlddle (Mykura 1976; Astin 1982) with two phases of allochthon Par/autochthon folding and small-scale thrusting is complex and this, together with the emplacement of the tOOKM "\ Extenelonallaull Sandsting Granite Complex, obscures the original basin geometry. The origin and timing of these Fig. 8. Simplified geological map of southern Norway modified structures and their relationship to the granite from various sources including NGU and SGU maps. Key: M Meråker, T Trondheim, B Bergen, R Røragen, NSD (Mykura 1976, 1983: deformation during and Nordfjord-Sogn Detachment, RD Røragen Detachment, SD after intrusion; Astin 1982: deformation during Sunnhordaland Detachment, LGF Lærdal-Gjende Fault, intrusion; Enfield & Coward 1987: deformation MTFZ Møre-Trøndelag Fault Zone. 330 M. G. Norton et al. NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

(exduding the uppermost Silurian ORS Facies here. The strike-slip model fails to explain the Ringerike sandstone which formed in a foreland presence of mylonites along the eastern tectonic basin generated by the advancing Caledonian contact and only in some of the basins have poss­ thrust sheets) (Hossack et al. 1985). These are; on ible strike-slip faults been identified. Many of the the west coast between Sognefjord and features of the strike-slip mod el are, however, Nordfjord, in the outer Trondheimsfjord, and at induded in the extensional model in which the Røragen. The ages of these sedimentary extensional detachments have several higher sequences are not well constrained although the · angle strongly oblique segments (Hossack 1984; dating available suggests that these are mostly Norton 1987; Michelsen & Tillung in prep.). lower to middle Devonian with possibly upper­ Large-scale extension is thought to have been most Silurian on Hitra, outer Trondheimsfjord. generated due to the collapse of overthickened The four west coast basins and Røragen share crust at the end of the Caledonian continent­ the following features: continent collision (Norton 1986). The !argest structure formed during the extensional phase (l) Unconformable boundary to the west onto was the Nordfjord-Sogn Detachment described rocks of the middle or upper allochthon. in detail by Norton (1987). A large half-graben (2) Tectonic boundary to the east consisting of an basin developed in the hanging wall of this struc­ intense mylonite zone, in most cases involving ture filled with the erosional products of the deformed Devonian at the top of the zone, uplifting footwall. The effect of irregularities in against a thick zone of mylonites. the footwall geometry was to give locally distinct (3) Clear tectonic control on sedimentation with depositional systems now recognized as the four alluvial fanglomerates derived frompositions west coast sub-basins Hornelen, Håsteinen, dose to the present tectonic margins. Kvamshesten and Solund; for the larger basin (4) Deformation of the Devonian sediments with unit the name Vestland Basin is proposed. The local formation of deavage, open to dose fault geometry at the surface during sedimen­ folding with local small-scale thrusting tation may have bad similarities to that seen in accompanied by Prehnite-Pumpellyite poss­ the Orcadian Basin (Fig. 3) with the individual ibly into Greenschist facies metamorphism. arcuate fault segments linking together laterally The main models for basin formation are (see and at depth to form the observed detachment. also Steel et al. 1985): Norton (1987) proposed the existence of another major extensional structure in southern Norway, the Røragen Detachment (Fig. 8). In • Extensional half-graben basins with most of this paper we describe in more detail the evidence the deformation and metamorphism caused by hanging-wall accommodation structures for this interpretation. The Røragen Devonian (Fig. 9) consists of ca. 1.5 km preserved strati­ (Hossack 1984; Norton 1986, 1987; Seranne & graphic thickness of sediments interpreted as Seguret 1985, 1987; Michelsen & Tillung pers. comm. 1987). alluvial fan deposits (Roberts 1974; Steel et al. 1985). The overall structure of the sediments is • Transtensional basins formed on releasing simple with a fairly constant 40°-70° dip towards bends on strike-slip faults with deformation the southeast. As indicated earlier an uncon­ caused by local transpression (Steel 1976; Steel formable boundary is present to the northwest et al. 1977; Steel & Gloppen 1980). against rocks of the Levanger-Øyfell Nappe (Koli • Allochthonous basins emplaced into their equivalent (Gee et al. 1985), upper part of upper present position by a major thrusting event with related folding and metamorphism, the allochthon) while the southern and southeastern 'Solundian', of late Devonian age (Sturt 1983; boundaries are apparently tectonic. In the south­ Torsvik et al. 1986); this model does not predict western part of the area an almost monomict any link between basin formation and structure serpentinite conglomerate, the Svartberg Con­ affecting the sediments. glomerate, is in high angle faulted contact with serpentinite of the Skjøtingen-Essandjø Nappe The arguments for and against thrustirrg have (Seve equivalent, lower part of the upper alloch­ been recently critically assessed by Norton (1987) thon, (Gee et al. 1985)). In the northeast of the who found no unequivocal evidence for thrusting area the Devonian is in presumed tectonic contact and the third model is not considered further with a mylonite zone previously interpreted as NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 331

AURSUNDEN

UPPER PLATE � Sandstone Polymict conglomerate Serpentinite conglomerate Upper Allochthon Røros schist Gabbro Serpentin ite Semi-pel i te PETACHMENT

Augen mylonite Q.uartz phyllonite Calcareous phyllonite l Psammite mylonite LOWER PLATE D Eocambrian psammite

Fig. 9. Geological rnap of Røragen di striet showing outcrop with )akes and glacial drift left unornamented, modified afterRoberts (1974).

the thrust at the base of the Trondheim Nappe. aggregate of slightly inequant grains. These define This high strain zone places rocks of the Koli an oblique grain shape fabric consistent with an equivalent against sparagmite arkoses of the extensional sense (i.e. hanging wall displaced Osen-Røa Nappe of the lower allochthon. Along down to NW) to the zone (Lister & Snoke 1984). this structure the whole of the Gula Group is A quartz c-axis fabric measured from a sample missing, as is the Seve equivalent north of taken fromnear the top of the psammite mylonite Røragen, and the middle allochthon is only repre­ shows a strongly asymmetric cross-girdle fabric sented by the ca. 100 m thick augen mylonite implying the same sense of shear (Bouchez et al. presumed to be equivalent to the Tiinniis Augen 1983). Gneiss in Sweden (Gee 1978). Due to rather Above the psammite mylonite there is a ca. patchy exposure, a continuous transect across the 30m thick band of calcareous phyllonite. The mylonite zone is only possible in the northeast of more micaceous parts of this band have a well­ the area shown in Fig. 9 along the section line A. developed secondary shear band cleavage whose The mylonites dip shallowly (15°-30°) towards asymmetry again gives a down to the NW sense WNW and have a tectonostratigraphy as shown of shear; more carbonate rich layers and quartz in Figs. 9 and 10. veins are affected by northwest verging asym­ The lowest exposed mylonite is derived from an metric folds. The origin of this band is not clear arkosic psammite presumed to be the underlying but it could possibly represent a high! y attenuated sparagmite although a transition between them is equivalent of the Cambro-Ordovician shelf not exposed. A strong mineral elongation lin­ sequence of the lower allochthon. Continuing up eation is developed within the mylonite foliation the section there is ca. 50 m of quartz rich phyl­ plunging 28° to 300°. In thin section quartz is lonite whose dominant small-scale structures are seen to be almost completely recrystallized to an northwest verging microfolds. Following the pre- 332 M. G. Norton et al. NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

NW SE

MyaamOrbullen

Fig. JO. Schematic cross-section along line A, Fig. 9, showing small-scale structures developed. The quartz c-axis diagram is contoured at l, 2, 4, 8%, 151 observations. vious logic this layer may represent the Silurian the northeast (Steel et al. 1985), but the observed part of the shelf sequence. clast lithologies of 'Røros Group phyllites and The topmost part of the exposed sequence con­ schists as well as quartzites, chert, sandstone, sists of ca. 100 m of augen mylonite. K-feldspar metagreywacke, vein quartz and other rock-types' porphyroclasts (up to 10 cm in size) form the (Roberts 1974) all indicate derivation from an augen in this rock, the only kinematic indicators area with exposed geology similar to that seen to observed are north west verging asymmetric folds. the west beneath the basal unconformity. This As previously suggested this is thought to rep­ suggests that when sedimentation was initiated resent the lower allochthon. The metamorphic there was no pre-existing major structure sepa­ grade at which mylonitization occurred is not rating the present basin floor from the area clear but the recrystallization of the quartz immediately to the southeast. throughout the section suggests conditions of at Roberts (op. cit.) reported a series of small­ !east greenschist facies. There is an unexposed scale structures developed within the Devonian zone, at its narrowest only a few metres wide, sediments. The most important of these obser­ between the augen mylonite and the Devonian vations were of locally intense deformation in the sediments which is here interpreted as a brittle sandstone/silt layers which are interbedded with fault zone. It is possible to trace this mylonite the fanglomerates. Close, angular northwest verg­ stratigraphy into the poorly exposed ground for ing asymmetric folds are developed. In the exten­ 10 km to the south although no other continuous sional model such deformation is explained in sections were found. terms of hanging wall accommodation. With the The presence of southeasterly dipping geometry as depicted in Fig. 11 hanging wall Devonian sediments in the hanging wall, the omis­ strains would necessarily be developed at the sion or thinning of tectonostratigraphy and the point at which the high angle fault along the south small-scale structures developed in the mylonites of the Røragen Devonian joined the top of the . are all consistent with the interpretation of this detachment. This could be accommodated by structure as an extensional detachment. Sup­ bedding parallel shear which, in the conglo­ porting evidence for this interpretation comes meratic units, would be concentrated in the inter­ fromthe observation of clast populations from the bedded sandstone/siltstone layers. Brekkefjell Conglomerate as reported by Roberts The synclinal closure mentioned by Steel et al. (1974). The alluvial fan system which deposited (1985) observed in the Svartberg Conglomerate this conglomerate is thought to be derived from is probably a result of so-called 'normal fault drag' NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 333

Fig. 11. Schematic cross-section along line B, Fig. 9, showing the assumed relationship between the high angle fault and the mylonite zone.

against the nearby high angle fault. The other delag Fault Zone but these faults were clearly features of the Røragen Devonian previously active during the Mesozoic and probably did interpreted as related to a Svalbardian defor­ not control Devonian sedimentation. Due to this mation phase (i.e. the presence of a cleavage and later faulting the relationship between the sedi­ very Iow grade to ? low grade metamorphism) ments and the basement is not clear. Detailed have already been considered by Norton (1987). fieldwork onshore combined with seismic inter­ The main importance of these observations is that pretation in the immediate offshore area will be it suggests that the original Devonian basin in this necessary to establish a basin model for this area. region was very much bigger than that presently exposed. It was proba bly 8 km deep and extended along strike for as much as 300 km, the present strike extent of the mylonite zone with its associ­ Devonian fault patterns and ated omission/thinning of tectonostratigraphy, from the Grong Culmination in the north to the palaeogeography culmination near Otta in the south. The network of faults interpreted in northern The Lærdal-Gjende Fault (Fig. 8) and the Scotland and southern Norway have been com­ associated 'Faltungsgraben' (Milnes & Koestler piled together in Fig. 12. In east Greenland the 1985) are also thought to be extensional structures approximate position of some of the faults of Devonian age. It is very likely that a smaU thought to be responsible for Devonian basin basin was developed in the hanging wa11 of the formation are marked along the trend of the zone Lærdal-Gjende Fault which, although compara­ of low-angle extensional faults as reported by tively sha11ow (2-3 km), probably extended along Frank] (1953) ca1led by McClay et al. (1986) the the whole strike length of the fault i.e. ca. 150 km. East Greenland Extensional Zone. These west­ Milnes & Koestler (1985) also report reactivation dipping extensional systems are not seen to be of the thrusts in this area with displacement to directly associated with Devonian sediments but the northwest, movements of probable Devonian are interpreted as Devonian in age (Ha11er 1971, age. 1985). The present outcrop pattern of the Devonian sediments of the outer Trond­ Devonian in east Greenland is contro11ed by east­ heimsfjord are associated with the Møre-Trøn- dipping post-Devonian extensional faults. 334 M. G. Norton et al. NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

reactivation of the thrusts as extensional faults, DEVONIAN although locally this has undoubtedly taken place. BASIN-FORMING Rather it is suggested that the contractional tec­ FAULTS tonics induced an overall anisotropy in the middle to upper crust which had an important con tro! on the direction of di p of the succeeding extensional structures. Fig. 13 shows the palaeogeography as derived from the observed fault network and published sediment dispersal patterns (Mykura 1983; Astin 1985; Steel et al. 1985). In Scotland drainage systems running axially along half-graben sub­ basins interconnect and have an outlet to the sea (as represented by the marine Devonian of the central North Sea (Ziegler 1982)). Such a con­ nection has been suggested to explain features of the Achanarras fish bed (Trewin 1986). Fig. 13 also shows the main areas in which little or no data is available on the nature of Devonian basins, if present. These areas naturally coincide with the areas of greatest Mesozoic and Tertiary extension where any Devonian strata will be too O 200KM. deep to be imaged on standard seismic refiection L-....1 data. The increasing availability of deep seismic data should allow at !east some of these gaps to be filled in. Fig. 12. Pattem of Devonian basin forming faults in northeast Atlantic region on pre-rift configuration.

The main difference between the basins of Discussion northern Scotland and southern Norway is one of scale. The basin forming faults are in both cases The megashear hypothesis was originally devel­ oped to explain apparent palaeomagnetic evi­ arcuate extensional faults but the Norwegian dence of large discrepancies in palaeolatitude for examples have generally longer strike extent, Devonian sediments fromeither side of structures greater spacing between faults, larger inferred such as the Great GJen Fault in Scotland (Van fault displacements and thicker hauging wall basin der Voo & Scotese 1981) and the Billefjorden fills. This is presumed to be a refiection of the Fault in Svalbard (Harland et al. 1974). More much larger extension resulting from the much recent work has east doubt on the significance of greater crustal thickening produced in Norway at these discrepancies suggesting that no resolvable the end of the Silurian. (i.e. >200 km) movement has occurred on these One other change in style of extension is note­ faults since the lower Devonian (Torsvik et al. worthy, the sense of asymmetry. In northern 1985; Lamar et al. 1986; Tarling 1985). This does Scotland the extensional faults are dominantly not, however, preclude movements of this smaller east to southeastward dipping except for a poorly scale which are quite sufficient to generate the defined zone between Orkney and Shetland observed sedimentary basins. where they dip to the west. In southern Norway The main arguments against the dominance of and east Greenland all the extensional structures strike-slip related sedimentation in the Devonian dip to the west. This contrast in asymmetry is also are: shown in Fig. 13 linking the interpreted palaeo­ geography to the underlying structure. In both (l) The very widespread area in which basins cases the direction of dip of the extensional struc­ developed often far from any known strike­ tures matches that of the preceding contractional slip structures e.g. the Turriff and Røragen structures. This is not thought to imply direct basins. MIDDLE-DEVONIAN z o � PALEOGEOGRAPHY & TECTONICS :><: o tT1 o 5 o c;; :><: :l o ��- , Vl Vl � � · � �

� ..... �

� C) Vl � � � ;::· �OKM 3 ...... Fig. 13. End middle Devonian palaeogeography from northern Scotland to central Norway and east Greenland incorporating strike-slip movements as listed in the text. � Allowance has also been made for ca. 100 km extension post-Devonian in the Viking Graben. (Volcanoes indicate areas of middle Devonian magmatism.) ...., ...., Vl 336 M. G. Norton et al. NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

(2) The lack of a clear sedimentary signature (4) The basement/cover contact on Foula is a within Devonian strata deposited adjacent to reactivated Caledonian structure, not a Basin the postulated major strike-slip faults, e.g. and Range type detachment. along the Great Glen Fault in the Inverness (5) The Røragen Devonian outcrop is an ero­ area (Rogers 1987). Strong evidence exists for sional remnant of a much larger half-graben strike-slip motion along some basin margin basin formed by extensional movement on faults (e.g. the northern margin of Hornelen the Røragen Detachment. (Steel & Gloppen 1980) but these are readily (6) Devonian half-grabens are likely to be explained as oblique segments on arcuate present on most of the Norwegian Con­ extensional structures. tinental Shelf. (3) The consistency of implied extension direc­ (7) Devonian extensional structures, particularly tions at high angles to the postulated strike­ major detachments, may have had a profound slip faults. infiuence on Mesozoic basin geometries.

The extensional model is also favoured by a Acknowledgements. - The work reported in this paper was correlation between the area of greatest crustal carried out as part of the COCO project funded by Fina (UK) thickening with the !argest extensional structures and Fina Exploration Norway. N. A. Way was supported by a and the deepest apparent basin fills. NERC research studentship; earlier fieldwork by MGN was funded by a grant from the University of Keele. We thank the One of the implications of this model is that following for useful discussions: M. Armstrong (BGS Devonian extension and, therefore, basin for­ Edinburgh), B. T. Kneller, N. H. Trewin (Aberdeen), T. R. mation is likely to have occurred throughout the Astin (Reading), D. Rogers (Cambridge), M. Enfield area affected by late Caledonian crustal thick­ (Imperial), S. McGeary (BIRPS), J. E. A. Marshall (Sou­ ening. Devonian half-grabens should be expected thampton) and J. Scallon, J. Staffurth, J. Mathalone (Fina UK). beneath areas of Mesozoic and Tertiary sedi­ mentation. References An aspect of the Devonian extensional struc­ l. tures which has only been touched on is the effect Andrews, J. 1985: The deep structure of the Moine Thrust, southwest of Shetland. Scottish Journal of Geology 21, 213- on subsequent basin formation. We have pro­ 217. posed partial reactivation of Devonian basin mar­ Armstrong, M. 1964: The geology of the region between the ginal faults during the development of the Alness River and the Dornoch Firth. Ph. D. thesis, Newcastle Mesozoic lnner Moray Firth Basin. If such rela­ upon Tyne (unpublished). Astin, T. R. 1982: The Devonian geology of the Walls peninsula, tively minor structures can exhibit such control, Shetland. Ph. D. thesis, University of Cambridge (unpub­ a structure of the scale of the Nordfjord-Sogn lished). Detachment would be expected to have a very Astin, T. R. 1985: The palaeogeography of the middle Devonian marked effect.A study of the published structures Lower Sandstone, Orkney. ScottishJournal of Geology 21, 353-375. offshoresuggests that the Øygarden/Horda Fault Beach, A. 1985: Some comments on sedimentary basin devel­ Zone which forms the eastern margin of the Meso­ opment in the Northern North Sea. ScottishJournal of Geo­ zoic basin off western Norway could represent a logy 21, 493-512. short cut fault into the detachment at depth. Blackbourn, G. A. &Marshall,J. E. A. (in prep.): The Geology of Foula, Shetland. Bluck, B. J. 1983: Rote of the Midland Valley of Scotland in the Caledonian orogeny. Transactions Royal Society of Edinburgh 74, 119-136. Conclusions Bouchez, J-L., Lister, G. S. & Nicholas, A. 1983. Fabric asym­ metry and shear sense in movement zones. GeologischeRund· (l) Devonian sediments were deposited in sets schau 72, 401-420. of linked half-grabens generated during Bukovics, C., Cartier, E. G., Shaw, N.D. &Ziegler, P. A. 1984: northwest-southeast directed regional Structure and development of the mid-Norway continental extension. margin. In: Spencer et al. (eds.), Geology of the North European Margin, Graham and Trotman, London, pp. (2) Sinistral strike-slip had no infiuence on sedi­ 407-424. mentation north of the Highland Boundary Davis, G. H. 1983: Shear-zone model for the origin of meta- Fault after earliest lower Devonian times. morphic core complexes. Geology 11, 342-347. (3) The unconformity at the base of the middle Davis, G. H. & Coney, P. J. 1979: Geologic development of the Cordilleran metamorphic core complexes. Geology 7, Devonian in the Orcadian Basin represents a 120-124. rejuvenation of extension. Dewey, J. F. 1982: Plate tectonics and the evolution of the NORSK GEOLOGISK TIDSSKRIFT 67 (1987) TSGS Symposium 1986 337

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