The Nordfjord-Sogn Detachment, W. Norway

MIKE G. NORTON

Norton, M. G.: The Nordfjord-Sogn Detachrnent, W. Norway. Norsk Geologisk Tidsskrift, Vol. 67, pp. 93-106. Oslo 1987. ISSN 0029-196X.

In western Norway thick continental sediments of Devonian age are found in four basins each bounded to the east by major fault zones. Shallowly dipping fault segments previously identified as thrusts have recently been reinterpreted (Hossack 1984) as parts of a single large extensional fault. Detailed fieldwork along this structure shows it to be a classic detachment (here redefined and renamed as the Nordfjord­ Sogn Detachment) similar to those described from the Basin and Range province of the western USA (e.g. Davis 1983). Most of the features attributed to a late Devonian 'Svalbardian' 'Solundian' orogeny are explained as related to irregularities in the detachrnent surface. Other possible Devonian extensional structures in southern Norway are evaluated.

M. G. Norton, DepartmentofGeology, UniversityofKeele, Keele, Staffs. , ST55BG, UK. Present address: Department of Geology, Royal Holloway and Bedford New College, Egham Hill, Egham, TW20 OEX, UK.

Rocks of Devonian age in western Norway are Range region of the USA. Other possible detach­ found in four major areas interpreted as separate ments within the southern Norwegian Cale­ basins (Fig. 1), from north to south: Hornelen, donides are then evaluated. Håsteinen, Kvamshesten and Solund. These basins have been interpreted as forming in a domi­ nantly strike-slip regime (Steel et al. 1977; Basin development models Roberts 1983; Steel et al. 1985). Syn- to post­ depositional folding has been observed in all the Before considering the main models for basin basins and they all have an unconformable west­ development, details of published sedimentary ern margin and all except Håsteinen a tectonic and tectonic observations are reviewed which pro­ eastern margin. This eastern contact in each case vide constraints on these models. brings the Devonian sediments against a con­ Sedimentation in the preserved Devonian has siderable thickness (>100 m) of mylonites and been related to the development of long-lived the low-angle segments have been interpreted as alluvial fan systems along parts of their present thrusts (Høisæter 1971; Nilsen 1968; Bryhni margins. In Hornelen small debris-ftow fans and 1978; Roberts 1983; Sturt 1983; Torsvik et al. fan deltas formed along the northern margin while 1986); a similar thick mylonite zone is present larger stream-flowfans formed on the eastern and just to the northeast of the Håsteinen outcrop. southeastern margins (Steel et al. 1977; Steel & Hossack (1984) has reinterpreted all the tectonic Gloppen 1980). Apart from consisting mainly of contacts as representing a single irregular exten­ conglomerate, probably of alluvial fan origin, sional fault, which he calls the Måløy Fault. He nothing has been published on the sedimentology identified this structure as an extensional fault of Håsteinen. Kvamshesten has alluvial fan from a consideration of the apparent omission of deposits along its northern (breccia dominated) structural section across it as indicated on pub­ and southern (conglomerate dominated) margins lished geological maps of the area. (Bryhni & Skjerlie 1975). Solund consists almost All existing models for the development of the entirely of alluvial fan deposits (Nilsen 1968) with basins are here reviewed taking account of both at least one major intercalated sedimentary slide sedimentological and tectonic constraints. The unit (M ichelsen et al. 1987). In Hornelen and field characteristics of the extensional structure, in the centre of Kvamshesten the marginal fan here redefined and renamed the Nordfjord-Sogn deposits pass laterally into alluvial plain/flood­ Detachment, are described and compared with basin deposits forming an axial system. Sediment similar structures developed in the Basin and dispersal directions are from south to southwest 94 M. G. Norton NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

l

Upper Plate � Devonian sandstone & tonglomerate �V Devonian volcanics Granodiorite

Stavfjord Nappe, ophiolite

Jotun Nappe, bastment & Eotambrian- Ordovician cover Lowtt Plate

Eocambrian- ?Silurian cover

Anorthosite with granite gneiss

Undifferentiated basement

•' ,. l Unconformable boundary to Devonian � Nordfjord-.Sogn DetQChment 10km .? Thrust ./ High-angle normal fault /�l'Folds affecting Devonian strata NORSK GEOLOGISK TIDSSKRIFT 67 (198) Nordfjord-Sogn detachment 95 along 'northern' margins, north to northwest mentary evidence for faulting active during sedi­ along 'southern' margins and west to southwest mentation near the position of all of the present in the axial systems (Steel et al. 1985). It is clear faulted boundaries. More direct sedimentary evi­ that throughout the sedimentation recorded by dence comes fromthe southwest of Solund in the the Devonian strata strong source area top­ Utvaer region (Indrevaer Steel 1975), where & ography was maintained. NNW-SSE trending extensional faults may have . All of the sediments are organized into coar­ been active forming fault scarps during sedi­ sening upward cyclothems, c. 150 m thick in Hor­ mentation. nelen (Steel et al. 1977), c. 200 m thick in Solund Steel et al. (1985) have recently reviewed the (Steel et al. 1985) and c. 20 m thick in Kvams­ main models proposed for the formation of the hesten (Bryhni Skjerlie 1975) (note that cycles Devonian basins and I will use their nomen­ & of this smaller scale are also recognized in Home­ clature. len). In Hornelen these units are traceable from the marginal fans into the axial sandstones and Model Hinge Faulting. This model is essen­ A. they appear to also be basin-wide in the other tially a propagating half-graben. It explains the sub-basins. This shows clear tectonic control on eastward migration of the depocentre but fails to sedimentation with a sustained, regular, periodic explain (as pointed out by Steel et al. 1985) either rejuvenation of the source region leading to alluv­ the migration of the western end of the basin or ial fan progradation. the eastward tilting of the strata. There is also a clear relative eastward migration of the depocentre with respect to previously Model Strike-slip Faulting. In this model, uplift B. deposited cycles, with progressive overlap of suc­ on a restraining bend on a strike-slip fault pro­ ceeding units. Finally, in some parts of the sub­ vides a sediment source for deposition in small basins there are marked discrepancies between half-grabens formed successively at a releasing clast lithologies in marginal fanglomerates and bend. Despite the observations of Steel et al. adjacent basement rocks across the faulted con­ (1985), the main objection to this model is the lack tacts (Hornelen: Steel Gloppen 1980; Solund: of evidence of a strike-slip continuation of the & Nilsen 1968). steep northern margin contact into basement. In The tectonic constraints show the character of Hornelen, the best described basin, the contact deformation going on during sedimentation. In appears instead to continue across the mouth of most cases it is impossible to prove that faulting Ålfotfjorden, where it enters Nordfjord (Fig. l) or folding were going on during sedimentation directly into the thrust contact as shown by Steel and some published models treat virtually all the et al. (1985). Steel Gloppen (1980) have shown & deformation of the sediments as being a distinct a series of fault lineaments in the adjacent base­ later event (or series of events), e.g. Roberts ment which trend subparallel to this northern Torsvik et al. Other models (Hossack margin and continue to the east of the present 1983; 1986. 1984, Norton 1986 and this paper, also Steel basin margin. However, the age, amount and & Glopen 1980 froma different standpoint) attempt direction of displacement on these zones is not to explain most of the deformation of the sedi­ known. In some cases they are coincident with ments as being a result of the same process as major lithological boundaries; they all follow basin formation. The features that they must basement trends. There is no evidence that any explain are: the overall eastward tilt of the strata, of these fault lineaments are major strike-slip folding or varying style and trend, and the nature faults. of the tectonic margins. This model does attempt to explain many of The only unequivocal evidence of synsedi­ the structural features affecting the basin sedi­ mentary deformation is from the western end ments by appealing to complex transpression and of the K vamshesten Devonian where the basal transtension. unconformity is seen to be progressively rotated during deposition of later sediments (Bryhni Model Listric Normal Faulting. I am in general & C. Skjerlie 1975). There is also the general sedi- agreement with this model as originally proposed

Fig. Geological map of the Sogn og Fjordane area, west Norway. 1. 96 M. G. Norton NORSK GEOLOGISK TIDSSKRIFT 67 (1987) by Hossack (1984), although some detailed criti­ Devonian sediments (Roberts 1983; Torsvik cisms will be made in the next section. The main et al. 1986). objection to this model raised by Steel et al. (4) The formation of a cleavage in the Devonian (1985) is that it does not possess a dynamic hinter­ sediments (Roberts 1983; Torsvik et al. 1986). land. This problem is particularly acute in (5) The types and orientation of magnetic fabric Hossack's (1984) model due to the subhorizontal developed in hanging wall sediments, mylon­ fault geometry on which negligible footwall uplift ites and footwall rocks (Torsviket al. 1986). is predicted, leading to a progressive degradation of fault scarps. However, in a model in which the Before considering these point by point, I will extensional structure is relatively planar and dips present a model of a large-scale extensional struc­ steadily at c.20°-30°, continuous footwall uplift ture (Fig. 2) adapted from Sibson (1977) and would be expected to provide the necessary Davis (1983). Fig. 2(a) shows an extensional fault/ source rejuvenation. shear zone passing through the whole crust at the beginning of its activity. Fig. 2(b) shows the same Model Migrating Listric Fault System. This is D. structure at the end of the extensional phase in a variant of model C but the proposed large­ which a large hanging wall basin has formed, and scale propagation of an extensional fault into its a large amount of footwall material has been footwall seems mechanically unlikely and is not eroded. Notice that the four points A, B, C, D observed in model experiments of extensional which were all at different crustal levels have fault systems (McClay Ellis 1987). & become juxtaposed at the same level due to con­ tinued movement on the zone. Rocks at A started Model C incorporates many aspects of model to be deformed under amphibolite grade con­ B, the difference being one of emphasis in the ditions probably as a mylonitic gneiss and have regional tectonic between dominant strike-slip since been upliftedinto very low grade conditions. with associated extension (B) and extension along Due to heterogeneous strain this area will be a structure with major strike-slip segments (C). variable retrogressed with some areas of medium Both the models fitmost of the constraints and a grade assemblages and fabrics remaining almost choice between them depends critically on the unaffected. A high strain strand of the fault has interpretation of the tectonic contacts as either: juxtaposed A against mylonites (B) which started thrusts which if of relatively small scale are com­ at upper greenschist. Across another high strand patible with the strike-slip model (Steel Glop­ & has experienced only slight upliftwith only pen 1980), or a large-scale extensional fault which area C minimum retrogression and probably consisting forms the basis of model C. The nature of the of both mylonites and cataclasites. Area D, repre­ low-angle tectonic contacts is considered in the senting the first deposited sediments in the half­ following section. graben, has now been juxtaposed against the other areas and has sufferedin con trast a pro grade burial metamorphism almost into lower green­ Low-angle tectonic margins schist facies. At this stage the part of D against the fault zone is being deformed into cataclasites The southeastern margin of Solund, both the and possibly mylonites. Note that sediments at D southern and northern margins of Kvamshesten, are being deformed and metamorphosed simul­ and the eastern margin of Hornelen have, as taneously with the deposition of the last sediment previously indicated, been interpreted as thrusts. in the half-graben E. In terms of the whole basin The lines of reasoning which have been used to this deformation is synsedimentary. support this interpretation can be summarized as:

(l) The presence of mylonites (Høisaeter 1971; Point (1). Keeping the above model in mind, the Bryhni 1964; Nilsen 1968). presence of mylonites says nothing about the type (2) Discordance between structures in the hang­ of fault, merely the condition under which faulting ing wall, especially folded Devonian bedding, occurred, examples of mylonite developed along and the mylonite zone itself (Høisaeter 1971; extensional faults are now becoming widely docu­ Bryhni Skjerlie 1975). mented in the Basin and Range (e.g. Davis 1983) & (3) The metamorphic grade developed in the and the Aegean (Lister et al. 1983). NORSK GEOLOGISK TIDSSKRIFT 67 (1987) Nordfjord-Sogn detachment 97

METAMORPHIC GRAOE FAULT ROCK OEFORMATION TYPE o r------==���iv------_, l o l c l Brittle 150 ------...;_ - - - l l

10 Prehnite - Pumpellyite .ø- : / _l l Transition 350 ------l c-4 Il Lower Greenachist l l Mylonite 450 ------�./: l 20 Upper Greenachist � J -" - Crystal Plastic 550· ------//'-::J:,� B 1 l l møhlboll1e l A /'Æ//>�:;::::;----// 1 1 _ /.

km

Eroded /--- Footwall'-/ /

..- -. . �- --�� 0 ����--����--�--��------��---�--�----- � · .. ·. ·· .. .

Er osion

10 Leve l

450 - -- -

20 U.G.

Fig. Section across a hypothetical extensional fault and associated hanging wall basin at: (a) start of fault activity, (b) near end 2. of fault activity.

Point (2). By the very nature of fault zones strain becoming recognized in extensional systems (McClay Ellis 1987). is discontinuous between hanging wall and foot­ & wall, so folds may develop in the hanging wall and not the footwall and vice versa. Folded hang­ Point (3). Half-graben basin thicknesses of ing wall above unfolded footwall is a relationship greater than 8 km can now be regarded as rela­ better known in thrust systems but is increasingly tive! y common, e.g. the North Celtic Sea Basin 98 M. G. Norton NORSK GEOLOGISK TIDSSKRIFr 67 (1987) and the North Lewis Basin (Cheadle et al. 1987). to support a thrusting hypothesis for the for­ Assuming a standard geotherm of 25°/km, early mation of this contact. sediments in such a basin will undergo a burial metamorphism well into Prehnite-Pumpellyite facies and at rather higher geotherm (35°/km) approach lower greenschist. Extensional fault geometry Hossack (1984) presented a model for a con­ Point ( 4). During such a metamorphism cleavage tinuous extensional fault with a highly irregular formation is only to be expected and is likely to geometry which he calls the Måløy Fault. Several be of two types: compaction cleavage which segments of this fault are high-angle trending should be shallowly dipping and probably dose WSW-ENE including the Bremangerland Fault, to bedding, and a deformation cleavage related the Standale Fault and the pair of faults which to hanging wall deformation whose orientation pass north and south of Florø (Fig. 1). If these would be related to the geometry of the structures faults represent part of the extensional structure developed. there is a major problem in fittingtheir orientation with the very constant lineation direction seen on Point (5). The palaeomagnetic results of Torsvik most other parts of the extensional fault of 260°- et al. 1986 can be summarized as follows: (l) A 3100 (see Fig. 3). Taking this lineation as showing magnetic fabric in the sediments subparallel to the movement direction on the fault these par­ bedding apparently overprinted at the tectonic ticular irregularities Iie at a moderate angle (up margin by a fabric subparallel to the mylonite to 55°) to the movement direction, which would foliation. (2) A pole indicating a middle Devonian cause severe mechanical problems in more than to lower Carboniferous age for the mylonit­ minor extension. ization. (3) Evidence from a fold test of some At least two of the high-angle faults clearly folding after the main magnetization in the west­ continue along strike into basement where the ern part of the Kvamshesten Devonian. Måløy Fault changes direction, the Standale Fault The presence of hanging wall deformation and and that forming the southern boundary to the mylonitization along the contact is not in dispute, Hornelen Devonian. All of these faults are here only the sense of the fault movement. The pole position which probably marks either uplift through a blocking temperature or the cessation of significantdeformation is quite consistent with DETACHMENT MYLONITE UNEATIONS NSD an extensional structure active from lower to • Solund middle Devonian. The local presence of some + Kvamshesten • o folding subsequent to the extension could be Hornelen .,.· related to a post-Devonian inversion event which o o RO will be discussed in a later section. O Roragen The main argument against a thrust origin for the intensely mylonitized contact between Devonian sediments and mylonitic pre-Devonian basement is that a 'special case' model must be developed to allow the observed geometry of younger rock faulted against older (see also Hos­ sack 1984). A second argument concerns the change in Fig. Mineral elongation Iineation data from mylonites in the metamorphic grade across the zone. In thrusting 3. Nordfjord-Sogn Detachment. The Solund data come from the one would expect higher grade hanging wall rocks Hyllestad area, the southeast plunging data are from the hinge undergoing retrogression to be thrust over lower zones of mesoscopic folds. The K vamshesten data points are grade footwall rocks undergoing prograde meta­ representative orientations from southeastern Atløy, two morphism. In extension the reverse situation will localities on the northern side of Dalsfjord, the region of the synclinal axis, and on the south side of Førdefjord. The orien­ apply and this relationship is well seen in the area tation from Hornelen is representative of the data from near studied, as will be shown in the next section. Hyen. Data from the mylonites at Røragen are included for The author knows of no unequivocal evidence comparison. NORSK GEOLOGISK TIDSSKRIFf 67 (1987) Nordfjord-Sogn detachment 99 interpreted as being high-angle extensional faults tensional conglomerates with interbedded of late- or post-Devonian age which post-date volcanics (Sigmond et al. 1984) of Devonian age and cut the main low-angle extensional structure. form the upper plate. Even several kilometres They may have a similar relationship to the pro­ from the detachment the conglomerate is visibly posed detachment as the later high-angle exten­ deformed with the development of a weak cleav­ sional faults have to the low-angle detachments age subparallel to bedding; bedding here dips c. in the Basin and Range (Proffett 1977). 20° towards the southeast. Within a few hundred Approximately 40% of Hossacks's Måløy Fault metres perpendicular distance from the detach­ is therefore interpreted as being non-continuous ment the cleavage becomes more intense; bedding with, and a different age to, the structure which is dips more shallowly (5°-10°) to the southeast. here proposed. In addition, the scale of structure Actually at the contact a few metres of mylon­ being described is quite different between; the itized conglomerates are commonly observed; the Måløy Fault locally reactivating a thrust plane cleavage is seen to rota te into the high strain zone and only a few metres in thickness, the structure so as to indicate an extensional sense of movement described in this paper as kilometric scale exten­ (Fig. 4). sional shear zone. Within the mylonite the conglomerate clasts In this account I have, therefore, chosen the become highly strained.Throughout the whole name Nordfjord-Sogn Detachment (NSD) for the area the conglomerate shows a very strong pebble extensional structure, as this precisely describes fabric with an a-axis preferred orientation NW­ its geographical extent, emphasizes the dif­ SE and a SE dipping imbrication (Nilsen 1968). ferences in geometry from Hossack's Måløy Fault Nilsen considered this fabric to be sedimentary in and indicates its similarity to Basin and Range origin and this fitswell with the humid alluvial fan style detachments (e.g. Davis 1983). In the fol­ environment postulated for these conglomerates. lowing section the Nordfjord-Sogn Detachment Near the detachment, however, this is obviously is described in detail from the Solund and Kvam­ overprinted by the tectonic fabric which is of shesten areas (Fig. 1). similar orientation. The more basic volcanics interbedded in the sediments show that the meta­ morphic grade of the conglomerates reached at The Nordfjord-Sogn Detachment least Prehnite-Pumpellyite. This is the highest This is definedas a zone of high strain which puts grade known to be developed within the upper plate rocks into contact with lower plate Devonian basins and may result from either a rocks. Upper plate rocks consist of three distinct slightly enhanced geotherm related to con­ units: (a) continental margin basement of Pre­ temporaneous vulcanism or a very thick basin fil! cambrian age with late Precambrian to Ordo­ vician sedimentary cover - this unit is considered to equivalent to the main allochthon, the Jotun be Nappe; (b) ophiolitic rocks of the Stavfjord NORDFJORD- SOGN DETACHMENT, SOLUND Nappe thrust onto (a); (c) Devonian coarse clastic sediments unconformable on both (a) and (b). Cleaved Lower plate rocks are Precambrian basement Conglomerate gneisses of the Western Gneiss Region. The mylonites forming the detachment are of meta­ sedimentary type near the contact with the upper 1m plate and of basement type near the lower plate. The line marked on Fig. l represents only the Intense! y top of the proposed detachment whose precise Mylonitised position is easily mappable; the base of the Greenstone detachment zone is gradational and difficult to map precisely.

Solund area Foliated Greenstone Fig. Schematic section across the Nodfjord-Sogn Detachment 4. In the Solund area (Fig. l) assumed synex- on Solund. 100 M. G. Norton NORSK GEOLOGISK TIDSSKR!Ff 67 (1987) (c. 10 km) combined with the present erosion strongly mylonitized and the mylonite foliation leve! which exposes the very bottom of the basin. itself is tightly folded. Below the granodiorite The description of the detachment mylonites there follows a further series of metasediments which follows is based on a reconnaissance tra­ and ?metavolcanics. This sequence contains verse from Lifjell to Lavik on the north side of psammite, pelite, marble and amphibolite units. the Sognefjord (Fig. 1). Detailed mapping being The strains are not obviously high in the psam­ carried out by members of the Geological Insti­ mites and pelites, but the amphibolites and mar­ tute, University of Bergen, is expected to lead to bles show intense multiphase folding and the some revision of this part of the section, but not foliation remains subparallel to the rest of the to the general condusions reached (Michelsen sequence so far described. The metamorphic & Tillung, in prep.). grade increases down the section through epidote­ For seven metres immediately below the con­ amphibolite to true amphibolite with a typical tact with upper plate conglomerates greenstones medium grade assemblage being found in the (assumed metavolcanics) are intensely mylon­ pelite (staurolite-almandine-kyanite), although itized with a mylonitic foliation parallel to that in this is partly retrogressed to greenschist. Without the conglomerate dipping between 16° and 25° to detailed mapping it is not dear whether the the northwest. Within the greenstone mylonites changes in metamorphic grade described are a strong mineral elongation lineation is developed abrupt or gradual. plunging 18° to 302°, and the foliation is every­ Continuing down the section the highly de­ where affected by a secondary shear band deav­ formed cover sequence gives way to an equally age (White et al. 1980; Platt Vissers 1981) highly deformed basement sequence consisting of & indicating an oblique extensional sense of mylonitized granite, gabbro, amphibolite and movement. quartzofeldspathic gneisses with edogite pods Beneath this intense shear zone which forms (partly retrogressed). It is instructive to describe the top of the detachment there is c. 100 m of this part of the section moving from east to west. less deformed, though still highly strained, rather At Lavik the basement rocks are strongly de­ homogeneous greenstone with a foliation parallel formed, with a regular southeasterly dipping to the upper contact of the detachment. There mylonitic foliation forming part of the zone of follows several hundred metres of a mixed completely caledonized basement gneisses of sequence of irregularly alternating greenstone, Dietler et al. (1985) containing a lineation c. 120°. pelite, psammite and semi-pelite horizons. Strain Foliation and lineation are both affectedby north­ within this sequence is still moderately high with west verging asymmetric folds, open to dose in the rocks being true mylonites, although the zone style with fairly constant subhorizontal axes is characterized by both occasional higher strain c. 045°. Between Lavik and Leirvik the main foli­ zones and low strain augen. Foliation rotation at ation is affected by a major antiformal structure. the boundaries of these augen consistently indi­ At Leirvik the now northwest dipping mylonites cates extensional sense movement, and shear are affectedby similar northwest verging folds to band deavage developed in favourable lithologies those at Lavik but they are now dose to tight in gives the same sense of movement. Foliation is style, their axes are variable 040°-080° and dearly more variable than before but overall it dips mod­ non-cylindrical. At Rysjedalsvika, just below the erately (c. 30°) to the northwest with a lineation boundary with the mylonitized metasediment the plunging shallowly (5°-15°) to 270°-300° (Fig. 3). same folds are tight to isodinal in style with the On the northern side of the peninsula in this axes now consistently subparallel to the lineation part of the section a large lens of psammite with c. 290°. Both the progressive reorientation of the deformed basic sheets is developed. The meta­ mylonite foliation and the fold axes, and the morphic grade of this part of the section is not tightening up of the folds are consistent with a known in detail but it appears to be all in green­ model of increasing simple shear of extensional schist facies. sense towards the northwest. Continuing eastward along the traverse a major The edogite pods found in the lower margin granodiorite body (c. 500 m thick) of present and footwall of the NSD form part of the suite of sheet-like form is encountered. Although in some edogites found throughout the northwestern part low strain areas the granodiorite is seen to cut an of the Western Gneiss Region. The results of a early foliation in the metasediment it is generally series of studies of P /Tconditions and radiometric NORSK GEOLOGISK TIDSSKRIFT 67 (1987) Nordfjord-Sogn detachment 101 age determinations have recently been reviewed close to the lineation. Throughout the section a by Griffin et al. (1985). Ages duster around 425 secondary shear-band cleavage is developed giv­ Ma and for the eclogites in the footwall of the ing a consistent extensional sense of movement. NSD P/T estimates give P= 14kb, T = 600°C. The base of the extensional shear zone has not K/Ar ages from the Statlandet area (Lux 1985) been studied in this area and no metamorphic suggest that the Western Gneiss Region under­ data are available from lower plate rocks. went continuous uplift from soon after eclogite The proposed detachment structure may then formation to at least the end of the middle be summarized as: a little deformed, weakly meta­ Devonian. The close juxtaposition of basement morphosed upper plate with synextensional sedi­ containing these Silurian eclogites uplifted from ments, the detachment with at its top a c. lOm a depth of c. 50 km against Devonian sediments thick intense shear zone passing down into a thick which have undergone simultaneous subsidence sequence of mylonites derived from cover in the and burial metamorphism represents very strong upper part and basement in the lower part of the indirect evidence for the presence of the proposed section, varying in metamorphic grade from lower extensional structure. greenschist facies to mid amphibolite facies (with part retrogressed eclogites) from top to bottom. The generalized section is shown in Fig. 5; this Kvamshesten area should be compared with the hypothetical section In the Kvamshesten area three units are found in shown in Fig. 2(b). Hossack (1984) suggested that upper plate position immediately above the NSD; the extensional structure formed by the reac­ from west to east these are late Precambrian tivation of the Caledonian thrust structures. The sediments, Precambrian mangerite-syenites and above description, however, strongly impliesthat assumed synextensional Devonian conglomerates the whole of the NSD structure represents a major and sandstones. Preliminary data from Torsvik et extensional shear zone cutting across the pre­ al. (1986) derived from illite crystallinity in the existing tectonostratigraphy. The simplest inter­ Devonian suggest anchizone conditions (c. 200°- pretation of the mylonites is that they represent 3000 C) were developed; they also describe cleav­ a highly attenuated section through the whole age developed in certain fine-grained lithologies. Caledonian thrust pile and part of the autoch­ As in the Solund area the top of the detachment thonous basement, as indicated in Fig. 5. is marked by a zone of intense mylonitization, the uppermost metre of which is clearly derived from the adjacent upper plate rocks. Where mangerite­ Detachment geometry syenite forms the upper plate it is also affected by Although removal of the several WSW-ENE seg­ cataclasite zones and associated pseudotachylyte ments from Hossack's (1984) fault does simplify veins immediately above the detachment. A its geometry, it still remains quite irregular. Many of the corrugations in the can be matched reconnaissance study of the detachment mylonites NSD has shown that they have an overall similarity to with folds in the upper plate such as the overall the section described above in the Solund area synformal structures of the Devonian outcrops of with a few thousand metres of metasedimentary Hornelen, Håsteinen and Kvamshesten. This has mylonites passing down into mylonites of base­ led to the proposal of a late or post-Devonian ment origin. Greenstones, however, form a far fold phase (Sturt 1983; Roberts 1983) correlated less important component of the upper part of with the Svalbardian event on Spitzbergen and the section and no granitic intrusions have been known as the 'Solundian' (Torsvik et al. 1986). described. All of the mylonites are characterized However, one of the major synformal cor­ by a strong mineral elongation lineation plunging rugations in the detachment (in the Solund area) shallowly (10°-25°) to (265°-275°) (Fig. 3). The is not matched with a syncline in the upper plate, detachment here has a synformal character with which in contrast is folded in an anticlinal struc­ the northern margin dipping moderately south ture (Fig. l) about a different axis. In this case (30°-40°) and the southern margin dipping mod­ the synformal corrugation must be an original erately north (20°-30°). The overall westward dip feature of the detachment and the anticline is of the detachment implied is in the range 10°-15°. best explained as an upper plate accommodation In the lower parts of the section tight to isoclinal structure caused by a detachment geometry as northwest verging folds are observed with axes indicated in Fig. 6 (as originally outlined by Hos- 102 M. G. Norton NORSK GEOLOGISK TIDSSKRIFf 67 (1987)

......

Heterogeneously Catedonised Basement

- 1km ....___, -

Fig. Generalized section across the Nordfjord-Sogn Detachment in the Solund and Kvamshesten areas. 5.

NW SE

2 km

L-...-1

. Fig. Formation of anticlinal structure on Solund in upper plate due to changes in detachment dip. Line of section passes through 6. Hersvik (Fig. 1). Note that most of the sediment at the northwestern end of the section is derived from the northeast. NORSK GEOLOGISK TIDSSKRIFT 67 (1987) Nordfjord-Sogn detachment 103 sack 1984). Such corrugations are a normal fea­ evaluation of two further possible extensional ture of detachment geometry in the Basin and structures of regional significance is based on Range province (e.g. Frost 1981). reconnaissance fieldwork. It is also possible to relate folding to fault In the Sunnhordaland region an irregular shear geometry along the northern margin of Hornelen. zone cuts through the Sunnhordaland Batholith Sediments in this basin have an overall shallow (late Silurian in age-Brekke et al. 1984) and its (20°-25°) easterly dip but as they approach the envelope. The shear zone is c. 50 m thick and northern margin they become steeply (70°-80°) the associated mylonites have a shallow westerly southerly dipping, forming an overall monoclinal plunging lineation. The mylonite foliation is affec­ structure. The eastern part of the north em margin ted by asymmetric folds and shear band cleavage has a strike of 080° which is slightly oblique to the showing a consistent extensional sense. The apparent movement direction on other parts of amount of movement on this structure (for which the detachment (260°-310°). This obliquity would I propose the name Sunnhordaland Detachment, have been sufficient to cause transpression along Fig. 7) is difficultto estimate, but from the thick­ this margin during extension, and the observed ness of mylonites developed it seems unlikely to monocline is consistent with this. be greater than a few kilometres. If some of the 'Solundian' folds can be The second structure considered forms the explained as upper plate responses to detachment southeastern boundary to the Trondheim Nappe. geometry it seems likely that other major folds in Although interpreted as a thrust it clearly cuts the region have a similar origin (see also Hossack across the Caledonian nappe units with omission (1984). The syncline at Kvamshesten which of the tectonostratigraphy (Sigmond et al. 1984) appears to fold upper plate rocks, the detachment and has an extensional geometry on published and lower plate rocks together is the most difficult sections (Wolff et al. 1980). At the Swedish bor­ to explain in this way. However, if the original der near Meråker (Fig. 7) the Caledonian east geometry of the extensional shear zone in the verging thrust related fabrics are clearly seen to Kvamshesten area was that of a synformal cor­ be reworked by west verging minor structures rugation, gentle near the surface, tightening up through a zone at !east 50 m thick next to an at deeper levels, the upper plate rocks would unexposed zone of a further 50 m thickness mark­ become progressive ly folded as the extension con­ ing the contact itself. Devonian sediments are tinued. This proposed mechanism is in keeping found in dose association with this structure with the observations of progressive overlap in further to the south at Røragen (Fig. 7) (Roberts the Kvamshesten Devonian which led Bryhni 1974; Steel et al. 1985). The mylonites here con­ Skjerlie (1975) to suggest that at !east some tain unequivocal indicators of extensional move­ & of the folding was contemporaneous with ment throughout their thickness as shown by sedimentation. minor fold asymmetry, shear band asymmetry,

Other folds appear to be related to the cross­ quartz c-axis asymmetry and quartz grain shape cutting WSW-ENE trending faults, e.g. the fabrics. Locally intense deformation in the finer­ Grøndalen syncline whose axis exactly parallels grained Devonian sediments may be related to the basin margin fault to the south. This may irregular footwall geometry as proposed along the indicate a phase of structural inversion along these NSD. In order to bring the interpreted synex­ faults; such phases are recognized in the adjacent tensional sediments into contact with mylonites offshore regions (Pegrum 1984). in the footwall, movement of several kilometres is indicated on this structure (for which I propose the name Røragen Detachment). For more details Other extensional structures in south see Norton et al. (in prep.). Norwegian Caledonides The other structure shown in Fig. 7 is the Apart from the NSD the only major extensional WNW-ENE trending Møre-Trøndelag Fault structure of Devonian age previously described Zone (Gabrielsen et al. 1984). The present out­ from southern Norway is the Laerdal-Gjende crop pattern of the Devonian sediments and rocks Fault (Fig. 7) (Milnes Koestler 1985). Smaller of similar character to the upper plate of the NSD & structures of similar style have, however, been of the outer Trondheimsfjord are controlled by reported from the Hardanger-Ryfylke nappe this fault system. These faults juxtapose the complex (Naterstad et al. 1973). The following unmetamorphosed Devonian sediments with OEVONIAN EXTENSIONAL STRUCTURES

+-t-+ tt-t-+ + .· -rt-t-t+ ++t -t • ++ +-t +t+t . \ "+t++ ++ -t-t- ,. . . 4- +- t + + • RO· \ + • T to •• ++ ++ -t+ + . l . , .

. l . l

l _ ....l \.... \ \

1: od Oevonian sediments

� Main allochthon

O Lower allochthon

100 km O Par l Autochthon NORSK GEOLOGISK TJDSSKRIFf 67 (1987) Nordfjord-Sogn detachment 105 underlying low grade metamorphic and igneous References rocks against rocks of medium to high grade. The Brekke, H., Furnes,H., Nordas, J. Hertogen, J. 1984: Lower implied extensional movements on these faults & Palaeozoic convergent plate margin volcanism on Bømlo, SW are proba bly of Mesozoic age as indicated by their Norway, and its bearing on the tectonic environments of the control on the development of the Møre Basin. Norwegian Caledonides. geo/. Soc. London 1015- J. 141, Devonian tectonics in the southern Scandi­ 1032. navian Caledonides are here interpreted as being Bryhni, I. 1964: Migrating basins on the Old Red continent. Nature Lond. 384-385. 202, dominated by extension. This extension has been Bryhni, I. 1978: F1ood deposits in the Hornelen basin, west achieved partly by the reactivation of the Cale­ Norway. Nor. Geo/. Tiddskr. 273-300. 58, donide thrusts (Milnes Koestler 1985; Nater­ Bryhni, I. Skjerlie, F. J. 1975: Syndepositional tectonism & & stad et al. 1973) but mainly by the development of in the K vamshesten district (Old Red Sands tone) western Norway. Geo/. Mag. 593-600. a series of cross-cutting, NW-dipping extensional 112, Cheadle, M., McGeary, S. Warner, M. R. 1987: Extensional & structures with displacements ranging from sev­ structures on the UK continental shelf: a review of evidence eral hundred metres to c. 100 km (Norton 1986). from deep seismic profiling. In Coward, M. P. Dewey, J. & All of these structures have rather irregular F. (eds.): Continental Extensional Tectonics. Spee. Publ. geometries, which has led in man y cases to locally Geo/. Soc. London (in press). intense deformation within their hanging walls. Davis, G. H. 1983: Shear-zone model for the origin of meta­ morphic core complexes. Geology 342-347. 11, Dietler, T. N., Koestler, A. G. Milnes, A. G. 1985: A & preliminary structural profile through the Western Gneiss Complex, Sognefjord, southwestern Norway. Nor. Geo/. Conclusions Tidsskr. 233-235. 65, Frost, E. G. 1981: Mid-Tertiary detachment faulting in the (l) Small-scale structures within mylonites along Whipple Mtns., Calif., and Buckskin Mtns., Ariz., and its the Nordfjord-Sogn Detachment confirm its relationship to the development of major antiforms and syn­ extensional nature. forms. Geological Society ofAmerica Abstractswith Programs No. 57. (2) The Detachment cross-cuts the Caledonian 13, 2, Gabrielsen, R. H., Faerseth, R., Hamar, G. Rønnevik, H. & nappe structures. 1984: Nomenclature of the main structural features on the (3) The Detachment is similar to those seen in Norwegian Continental Shelf north of the 62nd parallel. In the Basin and Range but is on an unusually Spencer, A.M. et al. (eds.): Petroleum Geology of the North large scale. European Margin. Norwegian Petroleum Society. (4) Many of the folds affecting upper plate rocks Gibbs, A. D. 1984: Structural evolution of extensional basin margins. geo/. Soc. London 609--620. J. 141, are best explained as accommodation struc­ Griffin, W. L., Austrheim, H., Bråstad, K., Bryhni, I., Krill, tures resulting from irregular detachment A. G., Mørk, M. B. E., Qvale, H. Torudbakken, B. 1985: & geometry as originally suggested by Hossack High-pressure metamorphism in the Scandinavian Cale­ donides. In Sturt, B. A. Gee, D. G. (eds.): The Caledonide (1984). Some, however, may be related to & Orogen: Scandinavia and related areas. Wiley. a phase of inversion along the WNW-ESE Hossack, J. R. 1984: The geometry of listric growth faults in trending faults. the Devonian basins of Sunnfjord, Norway. geo/. Soc. W. J. London 629-637. (5) Two other extensional structures of regional 141, significanceare proposed, the Sunnhordaland Høisæter, T. 1971: Thrust Devonian sediments in the Kvam­ shesten area, western Norway. Geo/. Mag. 287-292. and Røragen Detachments. 108, Indrevaer, G. Steel, R. J. 1975: Some aspects of the sedi­ & mentary and structural history of the Ordovician and Acknowledgements. - The work reported here was initiated Devonian rocks of the westernmost Solund Islands, West while I was employed by the University of Bergen, under a Norway. Nor. geo/. unders. 23-32. 317, contract with Elf Aquitaine. More recent fieldwork has been Michelsen, J. K., Hansen, H. J., Lund, T. Nilsen, T. H. & supported by funds from the University of Keele, the receipt of 1987: Devonian catastrophic rocksliding, and associated vol­ which is gratefully acknowledged. I thank Brian Sturt for inspir­ canism in the Solund Devonian sedimentary basin. geo/. J. ing my interest in Devonian tectonics. I also thank my other Soc. London (in press). colleagues at Bergen ( especially H. Brekke and J. K.Michelsen) McClay, K. R, Ellis, P. 1987: Geometries of extensional fault & for many useful discussions systems. Geology (in press). Milnes, A. G. Koestler, A. G. 1985: Geological structure & Manuscript received January 1986 of Jotunheim, southern Norway (Sognefjell-Valdres cross-

Fig. Geological map of the southem Scandinavian Caledonides showing major Devonian structures.MTFZ Møre Trøndelag Fault 7. Zone; NSD Nordfjord-Sogn Detachment, RD Røragen Detachment; LGF Laerdal-Gjende Fault; Sunnhordaland Detachment. B Bergen; T Trondheim. 106 M. G. Norton NORSK GEOLOGISK TIDSSKRIFT 67 (1987)

section).ln Sturt, B. A. Gee, D. G. (eds.) The Caledonide grunnskart over Norge, million. Norges geologiske & l: l Orogen: Scandinavia and related areas. Wiley. undersøkelse. Naterstad, J. Andresen, A. Jorde, K. 1973: Tectonic suc­ Steel, R. J., Mæhle, S. Røe, S. L., Spinnanger, Nilsen, & Å & cession of the Caledonian nappe front in the Hauk:ilisaeter­ H. R. 1977: Coarsening-upward cycles in the alluvium of Røldal area, Southwest Norway. Nor. geo/. unders. 1- Hornelen Basin (Devonian, Norway) - sedimentary response 292, 20. to tectonic events. Bull. Geo/. Soc. Am. 1124--1134. 88, Nilsen, T. H. 1968: The relationship of sedimentation to tec­ Steel, R. J. Gloppen, T. G. 1980: Late Caledonian Devonian & tonics in the Solund Devonian district of southern Norway. basin formation, western Norway: signs of strike-slip tectonics Nor. geo/. unders. 108 pp. during infilling. Spee. Publ. int. Assoc. Sediment. 79--103. 259, 4, Norton, M. G. 1986: Late Caledonide extension in western Steel, R. J., Siedlecka, A. Roberts, D. 1985: The Old Red & Norway: a response to extreme crustal thickening. Tectonics Sandstone basins of Norway and their deformation: a review. 192-204. In Sturt, B. A. Gee, D. G. (eds.): The Caledonide Orogen: 5, & Pegrum, R. M. 1984: Structural development of the south­ Scandinavia and related areas. Wiley. western margin of the Russian-Fennoscandian Platform. In Sturt, B. A. 1983: Late Caledonian and possible Variscan stages Spencer, A.M. et al. (eds.): Petroleum Geology of the North in the orogenic evolution of the Scandinavian Caledonides. European Margin. Norwegian Petroleum Society. The Caledonian Orogen-IGCP Project Symposium de 27, Platt, J. P. Vissers, R. L. M. 1981: Extensional structures in Rabat. (Abstract). & anisotropic rocks. Struct. Geo/. 397-410. Torsvik, T. H., Sturt, B. A., Ramsay, D. M., Kisch, H. J. J. 2, Proffett, J. M., Jr. 1977: Cenozoic geology of the Yerrington Bering, D. 1986: The tectonic implications of Solundian & district, Nevada and implications for the nature and origin of (Upper Devonian) magnetization of the Devonian rocks of Basin and Range faulting. Geo/. Soc. Am. Bull. 247-266. Kvamshesten, western Norway. Earth Planet. Sei. Lett. 88, 80, Roberts, D. 1974: Sedimentary, tectonic and metamorphic fea­ 337-347. lures of the Devonian of Røragen, Sør-Trøndelag. Nor. geo/. White, S. H., Burrows, S. E., Carreras, J., Shaw, N. D. & unders. 89--108. Humphreys, F. J. 1980: On mylonites in ductile shear zones. 311, Roberts, D. 1983: Devonian tectonic deformation in the Struct. Geo/. 175--187. J. 2, Norwegian Caledonides and its regional perspectives. Nor. Wolff, F. C., Roberts, D., Siedlecka, A., Oftedahl, C. & geo/. unders. 85--96. Grenne, T. 1980: Guide to excursions across part of the 380, Sibson, R. H. 1977: Fault rocks and fault mechanisms. geo/. Trondheim region, central Norwegian Caledonides. Nor. J. Soc. London 191-213. geo/. unders. 129--167. 133, 356, Sigmond, E. M. Gustavson, M. Roberts, D. 1984: Berg- 0., &