Tectonophysics 332 (2001) 51±68 www.elsevier.com/locate/tecto
P.T. Osmundsen*,T.B. Andersen
Department of Geology, P.O. Box 1047, University of Oslo, 0316 Blindern, Oslo, Norway
Abstract
The Devonian basins of western Norway were formed during late- to post-orogenic extension of overthickened Caledonian crust. The basins are situated in the hanging wall of the extensional Nordfjord±Sogn Detachment Zone (NSDZ) and display extensional half-graben geometries in sections parallel to the local direction of principal extension. Based on overall facies con®gurations,paleocurrent patterns and intrabasinal structures,we infer an anticlockwise rotation of the syndepositional extension direction from NW±SE in the south (Solund basin) to WSW±ENE in the north (Hornelen basin). The axes of folds that are roughly parallel to the local extension direction are rotated correspondingly. The Kvamshesten basin is located between the Solund and Hornelen basins. Sedimentological and structural data show evidence of an early,southeastwards tilt direction followed by a more eastwards tilt and associated E±W ¯owing paleodrainage. Correspondingly,NW±SE trending folds and reverse faults are superposed by E±W trending ones at low to intermediate stratigraphic levels. The variations in apparent tilt direction for the basins together with variations in intrabasinal structure is interpreted to re¯ect an anticlockwise rotation of the regional syndepositional strain ®eld. The above observations and inferences indicate that the Devonian basins in western Norway formed in a strain ®eld dominated by regional transtension,accommodated by extension along the NSDZ and sinistral strike±slip along orogen-parallel shear zones and faults to the north of the basins; alternatively,NW-directed extension preceded the introduction of a sinistral strike±slip component. The models are in accordance with recent work carried out in the footwall of the NSDZ and illustrates the tectono-sedimentary response to a complex interplay between extension and strike± slip that appears to have been fundamental in the late-stage disintegration of the Caledonian orogen.
q 2001 Elsevier Science
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Keywords : Devonian basins; transtensional tectonics; Nordfjord±Sogn detachment zone
1. Introduction
1.1. The Devonian basins
The middle Devonian basins of western Norway are regarded as classic study areas for tectonically controlled sedimentation (Bryhni,1964a,b; Nilsen,
1968; Bryhni and Skjerlie,1975; Steel et al.,1977,
1985; Steel and Gloppen 1980). Based on detailed
* Corresponding author. Geological Survey of Norway,7491
Trondheim,Norway.
sedimentological investigations in the Hornelen basin,Steel et al. (1977),followed by Steel and Gloppen (1980),proposed a strike±slip model for basin formation. During the last 15 years,the late- to postorogenic extension of the Caledonian mountain belt in western Norway has received considerable attention
(Hossack,1984; Norton,1986,1987; SeÂranne and
SeÂguret,1987; Steel,1988; Andersen and Jamtveit,
1990; Fossen,1992; Chauvet and SeÂranne,1994;
Krabbendam and Dewey,1998). In particular,work has been focussed on the large-magnitude extensional
Nordfjord±Sogn Detachment Zone (NSDZ) and on
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52 P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 51±68 the exhumation of deep crustal rocks in its footwall
(op. cit.). Thus,although the Devonian basins are situated in the hanging wall of the NSDZ,the present model framework is largely based on observations in the footwall. The Devonian basins display considerable variation with respect to overall geometry and facies distribution (cf. Steel,1976). This variation is not easily explained by regional unidirectional,topto-the west extensional faulting. Recent work in the
Kvamshesten basin (Osmundsen et al.,1998,2000) calls for a review of all the basins with respect to local and regional basin-forming mechanisms.
The main controlling mechanisms involved in sedimentary basin formation are invariably recorded by the basin ®ll. The location and runoff directions of major drainage basins are,together with their principal bedrock lithologies,recorded in terms of sedimentary facies con®guration,sediment dispersal patterns and provenance (e.g. Leeder and Gawthorpe,1987).
In the basins,the same parameters betray a record of basin ¯oor tilt directions and differential subsidence, the key aspects in the understanding of tectonic control. Further information regarding tectonic control may be provided by onlap relationships between sedimentary strata and basement,intrabasinal unconformities and the con®guration of syndepositional intrabasinal structures. A reinvestigation of the Devonian basins in western
Norway therefore provides an independent database to be considered in the construction of regional tectonic models. In the following,we review the sedimentology and structure of the western Norwegian
Devonian basins in terms of the above parameters.
In this paper,we shall discuss formation of the Devonian basins in terms of inter- and intrabasinal variations in sedimentary architecture. We furthermore compare our inferences to recent interpretations based on studies within the depositional basement and the footwall of the NSDZ.
1.2. Geological setting
The NSDZ constitutes up to 3 km thick extensional mylonite zone (Fig. 1) with abundant evidence of normal displacement (Norton,1986,1987; SeÂranne and SeÂguret,1987; Andersen and Jamtveit,1990;
Swensson and Andersen,1991).
40 Ar/ 39 Ar ages on white micas from the detachment zone and the adjacent rocks in the footwall are in the range from
390 to 400 Ma in Nordfjord and Sunnfjord,respectively (Berry et al.,1995; Andersen,1998). The footwall of the NSDZ is constituted by the Western Gneiss
Region (WGR),which experienced late Caledonian
(ca 400±420 Ma) eclogite±facies metamorphism
(Grif®n et al.,1985; Kullerud et al.,1986; B. Tucker in Lutro et al.,1997). Ultrahigh-pressure rocks are present north of the Hornelen basin (Smith,1984;
Wain,1997). The high-pressure metamorphism was most probably a result of A-type subduction of westernmost Baltica underneath the Laurentian craton during the terminal stages of continental collision between Baltica and Laurentia (Andersen et al.,
1991). In the Kvamshesten basin area, $ 16 kbar eclogites occur within 3 km from the Devonian rocks thus demonstrating a metamorphic gap across the NSDZ corresponding to 45±50 km of crust. North of the
Hornelen basin,excision is even more dramatic as
$ 20 kbar eclogites are found within 3 km of the
Devonian sediments (Krabbendam and Wain,1997).
The hanging wall of the NSDZ comprises a suite of
Caledonian nappe rocks described in some detail elsewhere (Osmundsen and Andersen,1994,in press).
The Caledonian nappe rocks are unconformably overlain by Devonian sedimentary rocks. The entire crustal section exposed between Sogn and Nordfjord has been folded in a set of NW- to WSW-trending folds with amplitudes and wavelengths in the order of several kilometers. In the synclines,the Devonian basins and their depositional substrate are preserved while the high-pressure rocks of the WGR crop out in the anticlines (Fig. 2). In the basins,shortening was accommodated by folding around SE,E±W and ENEplunging axes and by top-to-the SW and S reverse faulting (Osmundsen et al.,1998; Braathen,1999).
It has been suggested that shortening commenced during Middle Devonian sedimentation in the basins
(Bryhni and Skjerlie,1975; SeÂranne et al.,1991;
Chauvet and SeÂranne,1994). The later stages of shortening were associated with high anchizone to lower greenschist facies metamorphism (Torsvik et al.,
1986; Svendsen et al.,pers. commun. 1998) and magnetic remanence and fabrics in the sedimentary rocks indicate a Late Devonian to earliest Carboniferous age (Torsvik et al.,1986).
The present eastern margins of the Devonian basins are constituted by semi-ductile to brittle,undulating

P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 51±68 53
Fig. 1. Overview map of the Sogn±Nordfjord area in western Norway showing main tectonostratigraphic units. Note facies con®gurations and main paleocurrent directions (open arrows) in the Devonian basins. Also note orientation of ductile lineations in the footwall of the Nordfjord±
Sogn Detachment Zone.

54 P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 51±68
Fig. 2. Schematic,N±S cross-section through the Nordfjord±Sogn area (mainly from Andersen,1998) showing large-scale syn- and antiforms that de¯ect the entire structural section; the Devonian basins are preserved in the synforms,whereas the eclogite-bearing WGR crops out in the antiforms. Note two sets of extensional detachments beneath the Hornelen and Solund basins: the lower detachment separates the WGR from
Caledonian allochtonous rocks,the upper separates the latter from the Devonian basins in the east,cuts the Devonian unconformity below the basins.
low-angle normal faults that cut folded basin strata.
Segments of these faults accommodated Permian as well as Late Jurassic to Early Cretaceous faulting
(Torsvik et al.,1986,1988,1992; Eide et al.,1997).
The southern and northern basin margins are constituted either by E±W striking segments of the lowangle normal faults (Kvamshesten basin area) or by steeper faults with normal separation but abundant evidence for strike±slip movements (Hornelen basin area,Torsvik et al.,1986; 1988; Andersen et al.,1997;
Braathen,1999). The latter cross-cut the low-angle fault east of the Hornelen basin (Andersen et al.,
1997; Krabbendam and Dewey,1998; Braathen,
1999). E±W striking faults as well as segments of low-angle normal faults display evidence for dextral slip along the northern basin margins while components of sinistral slip is observed along the southern basin margins (SeÂranne and SeÂguret,1987; Steel,
1988; Chauvet and SeÂranne,1994).
2. Overall basin geometry, facies con®guration and sediment dispersal patterns
2.1. The Solund basin
The southeastern parts of the Solund basin are dominated by a several km thick succession of conglomerates banked against the NW-dipping
Solund Fault (Nilsen 1968). Towards the NW,the basin ®ll onlaps the SE-dipping limb of the LaÊgùy anticline (Fig. 3a). SW of LaÊgùy,however,where the axial plane trace is de¯ected to a N±S trend, onlap onto basement is apparently towards the NE in the present con®guration (Nilsen,1968; Steel et al.,1985,Fig. 3a). The NW continuation of the Solund basin is exposed in the Vñrlandet area,where a thick succession of breccias and conglomerates are overlain by ¯uvial sandstones. The basal breccias onlap depositional basement eastwards and inter®nger with polymict conglomerates towards the West. In the
Vñrlandet area,the Devonian strata display a pronounced fanning wedge relationship where the dip of bedding changes from southwards at low stratigraphic levels to southeastwards in the ¯uvial sandstones. The sandstones inter®nger with fanglomerates and breccias exposed on a SE-trending array of skerries and islands SE of Vñrlandet (Fig. 3b). In the
Vñrlandet area,an apparent reversal of paleocurrent direction took place during deposition of the exposed stratigraphy. Paleocurrent directions inferred from imbricate clasts in the basal deposits are mainly
NW-directed while the sandstones display SW- and
SE-directed paleocurrents (Fig. 3b).
In the interpretation of Nilsen (1968),the conglomerates in the SE part of the basin were dominated by
NW-directed paleocurrents according to analysis of cross-bedding,pebble roundness distribution and the distribution of pebble lithologies. The extremely consistent orientation of pebble long axes reported

by Nilsen (1968) was re-interpreted by SeÂranne and
SeÂguret (1987) to represent a tectonically induced fabric (see below). In summary,the parts of the
Solund basin exposed in the Solund and Vñrlandet areas were dominated by two main depositional systems; a conglomerate-dominated system sourced in the footwall and a sandy system sourced in the hanging wall of a NW-dipping basin-controlling fault. The SW-wards tapering fanglomerates that occur SE of Vñrlandet and the SW-directed paleocurrents recorded in the eastern parts of the ¯uvial sandstones indicate that a third depositional system characterized by SW directed sediment transport was located along the NE basin margin. In Solund
(Indrevñr,1980; Steel et al.,1985) and particular in the Vñrlandet area (Fig. 3b),onlap relationships between Devonian strata and basement as well as inter®ngering relationships between the main sedimentary units indicate that the oldest Devonian rocks are found in the southwest. The basal unconformity is thus a diachroneous surface indicating increasing subsidence towards the SW in the exposed parts of the basin.
2.2. The Hornelen basin
P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 51±68
The stratigraphy of the Hornelen basin (Fig. 4) comprises a few hundred meters of breccias and conglomerates exposed in the westernmost basin area,fanglomeratic fringes along the northern,southern and parts of the eastern basin margins and a broad central area dominated by ¯uvial sandstones (Bryhni,
1964a,b; Steel et al., 1977; Steel and Aasheim, 1978;
Steel and Gloppen,1980). Steel and Gloppen (1980), followed by Gloppen and Steel (1981),pointed out the difference between the conglomeratic fans on the northern and southern basin margins,respectively.
In their interpretation,the relatively thick,steep fans along the northern basin margin were dominated by debris ¯ow deposits. In the south,individual fans had a larger radius and were dominated by stream-transported conglomerates. Thus,the facies distribution in the Hornelen basin is markedly asymmetric. The stratigraphy of the Hornelen as well as the Solund and
Kvamshesten basins is dominated by coarsening- to
®ning upwards (CUFU) successions at a variety of scales (Steel and Gloppen,1980; Indrevñr and
Steel,1975; Bryhni and Skjerlie,1975; Osmundsen et al.,1998). Steel and Gloppen (1980),followed by
Steel (1988),ascribed this pattern to tectonic control.
Paleocurrent directions in the Hornelen basin is inferred to have been from the margins towards the central basin area (fanglomerates) and to have been mainly W- to WSW-directed in the central basin area
(¯uvial sandstones,Steel and Gloppen,1980). Along the northern basin margin,however,paleocurrents in the ¯uvial sandstones are NW-directed,towards a belt dominated by ¯oodbasin and lacustrine facies (Steel and Gloppen,1980).
2.3. The Kvamshesten basin
55
The Kvamshesten basin (Fig. 5),appears as a SE- to eastwards rotated half graben basin when viewed in a
NW±SE to E±W section (Fig. 6). The southern parts of the Kvamshesten basin (Fig. 5) are dominated by the up to 2 km thick Southern Margin Fan Complex
(SMFC,Osmundsen et al.,1998). The Devonian strata onlap basement towards the NE and E at low to intermediate stratigraphic levels (Bryhni and Skjerlie,
1975; SeÂranne et al.,1991; Osmundsen et al.,1998) so that sandstones rest directly upon the unconformity along the western parts of the northern basin margin.
Large parts of the present northern basin margin are occupied by a fanglomerate complex (NMFC) that reaches a thickness of ca 1 km. Parts of the NMFC onlaps basement eastwards at low to intermediate stratigraphic levels. Both fan complexes inter®nger with a central belt of ¯uvial and ¯oodbasin sandstones and siltsones. The geometry of fan segments indicate that along the basin margins,sediment transport was towards the central areas of the preserved basin. In the lowermost parts of the central sandstones,readings of trough cross-bedding indicate southeastwards paleocurrent directions while westwards as well as eastwards ¯owing paleocurrents have been inferred at intermediate and high stratigraphic levels (Fig. 5).
On the ¯anks of the basin,syncline as well as in the central basin area,readings of trough cross-bedding generally give more northerly and southerly transport directions. The CUFU motif described from the sedimentary ®ll of the Hornelen basin is also clearly present in the Kvamshesten basin (Osmundsen et al.,
1998,2000). Additional evidence for a syndepositional,eastwards tilt direction come from the progressive eastwards migration of ¯uvial facies and from the

56 east-stepping,retrogradational stacking of fanglomerates shown at high stratigraphic levels in Figs. 5 and
6. An original fanning wedge geometry of the Devonian strata is re¯ected by a decrease in the plunge of the main basin syncline from low to high stratigraphic levels in the basin (Osmundsen et al.,1998).
2.4. The HaÊsteinen basin
The preserved remnants of the HaÊsteinen basin
(Fig. 7) are almost entirely conglomeratic with only subordinate sandstone intercalations. No paleocurrent data are available from the basin at present. A striking relationship displayed by the HaÊsteinen basin is the southeastwards onlap of the entire basin ®ll (up to
11 000 m of cumulative stratigraphy) onto Caledonian basement with an angle as high as 53 8 (Vetti,
1996,1997; Vetti and Milnes,1997). One possible mechanism for producing this relationship is onlap onto the ¯ank of a NE-trending rollover anticline
(op. cit.),alternatively onlap onto an inactive fault scarp. The latter would require onlap onto paleotopography with a relief of between 5800 and 11 000 m, an explanation considered unlikely (Vetti and Milnes,
1997). The preserved parts of the HaÊsteinen basin are everywhere in a proximal position with respect to the basin ¯oor and margins,which probably explains the conglomeratic nature of the basin ®ll.
3. Structural geology
P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 51±68
In western Norway,the present faulted margins of the Devonian basins do not correspond directly to the syndepositional margins as demonstrated by crosscutting relationships and by paleomagnetic and radiometric dating (Torsvik et al.,1986,1988,1992; Eide et al.,1997; Osmundsen et al.,1998; Braathen,1999).
Interpretations concerning syndepositional strain-
®elds must therefore rely on the identi®cation and interpretation of syn-sedimentary intrabasinal structures.
Two main populations of intrabasinal structures affect the Devonian in western Norway. These are
®rstly,extensional/oblique faults dipping towards the W,NW and NE,secondly folds and reverse faults trending NW,W and WSW (Bryhni and Skjerlie,
1975; Roberts,1983; SeÂranne and SeÂguret,1987;
Chauvet and SeÂranne,1994; Osmundsen et al.,
1998; Braathen 1999). In the Solund basin,the strong pebble lineation fabric interpreted as a paleocurrent indicator by Nilsen (1968) was re-interpreted as a tectonically produced lineation by Indrevñr and
Steel (1975) as well as by SeÂranne and SeÂguret
(1987). SeÂranne and SeÂguret (1987) argued that the pebble fabric passed into a greenschist facies pebble fabric close to the NSDZ. Away from the detachment, however,clasts were rotated in a non-consolidated matrix and taken to represent early,soft-sedimentary deformation by the same authors. The elongation direction was NW±SE,trending 120 8 ,at an angle to the WNW-plunging stretching lineation observed in the mylonites of the NSDZ directly adjacent to the basin.
NW-dipping faults are the most prominent extensional/oblique structures in the Caledonian nappe-stack west of the Kvamshesten basin (Osmundsen,1996). A number of NW-dipping faults that cross-cut the basal unconformity have been interpreted as syn-sedimentary and a syn-sedimentary system of NW- and NE-dipping conjugate faults affect high stratigraphic levels (Selsvatn fault system,
Fig. 5; Osmundsen et al.,1998). Evidences for synsedimentary activity include fanglomerate wedges banked against the fault planes,termination of faults upwards in the stratigraphy,stratigraphic climbs displayed by facies boundaries in the hanging walls and outsized clasts and breccia fragments embedded in ¯oodbasin ®nes adjacent to a fault plane
Fig. 3. Map of the Solund Basin. (a) SE parts exposed in the Solund archipelago showing relationship between the Devonian strata,the Solund
Fault and the LaÊgùy anticline. Arrows indicate generalized paleocurrent directions (Nilsen,1968); ®lled arrows represent elongate pebble lineation,open arrows readings of trough cross-bedding. Legend: 1. High-pressure rocks (WGR and HP schists undifferentiated); 2. Granodiorite intruding Caledonian nappe rocks; 3. Caledonian allochton undifferentiated,strongly sheared in the footwall of the Solund fault; 4.
Devonian Conglomerates; 5. Devonian sandstones; 6. Gabbroic bodies interpreted as landslides by Bryhni and Skjerlie (1975); 7. Fold axis
(LaÊgùy anticline); 8. Low-angle normal fault (Solund fault). (b) Vñrlandet area showing main facies distribution and sediment dispersal patterns as inferred from imbricate clasts and from trough cross-bedding (rose diagrams;). Paleocurrent directions displayed in Fig. 2b are from unrestored data. Legend: 1. Caledonian allochton,2. Monomict basal breccias overlying basal unconformity (Vñrlandet area),3. Conglomerates,4. Fluvial channel sandstones with overbank intervals.

P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 51±68 57

58 P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 51±68
Fig. 4. Map of the Hornelen basin and parts of its substrate (modi®ed from Steel and Gloppen (1980),Lutro (1991),Hartz et al. (1994),
Krabbendam and Dewey (1998),Bryhni and Lutro). Open arrows indicate paleocurrent directions in sandy part of basin ®ll (Steel and Gloppen,
1980). Legend: 1. Eclogite-bearing gneisses of the WGR and extensional detachment mylonites; 2. Caledonian allochton; 3. Conglomerates and breccias of Devonian succession; 4. Fluvial channel sandstones; 5. Floodbasin/Lacustrine sandstones,siltstones and mudstones; 6. Top of main detachment zone separating the WGR from overlying allochtonous units; 7. Low-angle brittle normal fault (eastern basin margin) 8. Fold axes (Grùndalen syncline) and 9. High-angle brittle faults (southern and parts of northern basin margin).
(Osmundsen et al.,1998; Bakke 1999). In the Hornelen basin area,NE- and NW-dipping faults with normal/oblique separations appear to dominate at low stratigraphic levels (Hartz et al.,1994; Hartz and Andresen,1997).
The SE- to ENE-plunging folds and SE- to E±W trending reverse faults that deform the Devonian basins are part of a set of contractional structures that deform the entire crustal section exposed in western Norway (Vogt,1936,1953; Roberts,1983;
Torsvik et al.,1986; SeÂranne et al.,1991; Chauvet and SeÂranne,1994; Osmundsen et al.,1998; Braathen,
1999). The post-Caledonian,N±S shortening has been correlated with the Svalbardian stage of Vogt (1936) and linked to regional sinistral strike±slip movements in southern Scandinavia and the British Isles (Vogt,
1953; SeÂranne et al.,1991; Chauvet and SeÂranne,
1994).
The basins presently constitute large-scale synclines with a number of parasitic folds. In the
Solund basin,strains related to NE±SW directed shortening are locally high and have given rise to cleavage formation in the southwesternmost exposed parts of the basin (Indrevñr and Steel,1975). Large parts of the basin does,however,appear to be less folded than the Kvamshesten and HaÊsteinen basins.
The basal unconformity in the Vñrlandet area dips southwards at approximately 40 in Fig. 4).
8 in accordance with rotation by folding along an E±W trending axis,alternatively by folding along a SE-plunging axis followed by SE-wards tilt. In the Hornelen basin,folding is particularly well developed along the southern and eastern margins of the basin (e.g. Grùndalen syncline
In the Kvamshesten basin,several SE- to E-striking reverse faults cut the basin ®ll. The SE-striking reverse faults are observed at low stratigraphic levels in the basin while at intermediate to high stratigraphic levels,reverse faults strike E±W (Fig. 5). At intermediate stratigraphic levels,SE-plunging fold trains are cut by a S-dipping reverse fault of unknown displacement. South of Kringlefjellet (Fig. 5),a reverse fault with an inferred displacement of ca
1 km was mapped by Osmundsen et al. (1998). A similar structure with an inferred displacement of minimum 800 m crops out in the NE parts of the basin (Braathen,1997,1999; Osmundsen et al.,
1998,Fig. 5). Both these large reverse faults have

Fig. 5. The Kvamshesten basin with main facies distribution,paleocurrent indicators (rose diagrams: readings by present authors,mainly trough cross beds,arrows: generalized from Asphaug,1975) and con®guration of intrabasinal structures. Note the diachroneity between the southern and northern marginal fanglomerate complexes; the oldest deposits preserved in the basin are located along the southern basin margin. Legend: 1. WGR and detachment mylonites,schematic traces of main foliation; 2. Caledonian allochton undifferentiated; Devonian sedimentary rocks; 3. Conglomerates and breccias; 4. Floodplain/¯oodbasin rocks with intercalated channel- and distal fan deposits,5. Pebbly green multistory channel sandstone units separated by subordinate red ®nes; 6. Multistory channel sandstone units intercalated with plane laminated and low-angle cross-bedded sandstones and subordinate red ®nes; 7. Scoop-shaped low-angle normal fault (Dalsfjord Fault),8. Thrust/reverse fault; 9. Fold axis; 10. Intrabasinal normal/oblique faults.

60 P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 51±68
Fig. 6. E±W cross-section through the Kvamshesten basin (i.e. parallel to fold axis). Note shallow half-graben geometry,onlap/inter®ngering relationships and eastwards migration of channel sandstones (high stratigraphic levels). The line of pro®le is generally located along the axial plane trace of the basin syncline (see Fig. 5).
well-developed anticlines in their hanging walls where bedding is steeply overturned for up to 2 km along strike. Along the northern basin margin,
NW-dipping reverse faults are rotated together with bedding in the footwall of an ENE±WSW-striking reverse fault that places depositional substrate upon the Devonian sedimentary rocks (Fig. 5). Folds in the
Kvamshesten basin display SE plunges at low and intermediate stratigraphic levels and E±W to ENE plunges at intermediate to high stratigraphic levels
(Osmundsen et al.,1998). At high stratigraphic levels in the basin,strata on the ¯anks of an ENE-plunging anticline displays a fanning wedge relationship towards the axial plane trace (Fig. 8). This type of
Fig. 7. Map of the HaÊsteinen Basin and parts of its substrate (modi®ed from Bryhni and Lutro (2000a,b) with additional data from Vetti (1988,
1997)). The basin constitutes a steeply plunging syncline with bedding onlapping basement southeastwards at a high angle (Vetti,1997).
Legend: 1. WGR and detachment mylonites undifferentiated; 2. Gneisses and supracrustals with uncertain tectonostratigraphic position (Lutro,
1991),3. Caledonian allochton undifferentiated; 4. Devonian conglomerates; 5. Faults apparently associated with mylonitic deformation; 6.
High-angle fault associated with mylonitic deformation in the WGR (Standalen Fault). 7. Fold axes.

P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 51±68 61
Fig. 8. Progressive unconformity exposed at high stratigraphic levels in the Kvamshesten Basin. Fine-grained sandstone and siltstone beds display a fanning wedge geometry away from the northern ¯ank of an E±W trending anticline,indicating that the N±S shortening of the
Kvamshesten Basin was partly syndepositional.
relationship has been reported from foreland basins and is typical for syn-sedimentary folds (Burbank et al.,1996).
In summary,low to intermediate stratigraphic levels in the Kvamshesten basin display NW±SE trending contractional structures overprinted by
E±W trending ones; at high stratigraphic levels,E±W to ENE±WSW trending contractional structures dominate. The latter at least were syndepositional
(Fig. 8).
The HaÊsteinen basin is deformed into a steeply SEplunging syncline where bedding is rotated to more than 60 8 on the ¯anks (Vetti,1996,1997; Vetti and
Milnes,1997; Fig. 7).
4. Discussion
4.1. Tectono-sedimentary development of the
Devonian basins
In continental sedimentary basins,tectonically induced topography exerts a strong control on sediment dispersal patterns (e.g. Leeder and
Gawthorpe,1987; Leeder and Jackson,1993).
Three major depositional systems are commonly observed; out of these,footwall-sourced alluvial fans and hanging wall sourced fans/¯uvial lobes represent drainage that is transverse with respect to the principal basin-bounding fault. In early stages of continental rift development,halfgrabens are closed and transverse systems dominate (e.g. Leeder and Gawthorpe,1987; Schlische,
1991). In closed basins,the area characterized by the highest subsidence rates is commonly occupied by mud¯at,playa or lacustrine deposits as intrabasinal drainage tends to converge in this area
(Leeder and Gawthorpe,1987; Schlische,1991).
If individual half-grabens link up to form a rift zone,an axial river system usually develops that
¯ows parallel to the array of basin-bounding faults
(op. cit.). Thus,a variety of paleocurrent directions may be encountered in continental extensional basins. Paleocurrent data give clues to the syndepositional tilt direction. The tilt direction is often strongly affected by fault shape and may or may not parallel the principal extension direction.
Thus,paleocurrent data must be viewed together

62 P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 51±68 with intrabasinal structural data when addressing the syndepositional strain ®eld.
Although the use of clast long axes as paleocurrent indicators may be disputable in the SE parts of the
Solund basin (SeÂranne and SeÂguret,1987),data independent of clast long axes (cross-bedding,clast roundness distribution,clast lithology distribution) appear to support a NW-directed sediment dispersal (Nilsen,
1968). The NW±SE (ca 120 8 ) trending clast long axis fabric interpreted as produced by tectonic clast rotation under soft-sedimentary conditions (SeÂranne and
SeÂguret,1987) give evidence of an early phase of
NW±SE-directed extension in the basin. The extension direction inferred from the pebble long axis orientation is at a high angle to the NE±SW trending
LaÊgùy anticline (Fig. 3a),which has been interpreted as a rollover anticline (Norton,1986,1987; SeÂranne and SeÂguret,1987). Thus,intrabasinal structure is consistent with NW±SE-directed extension and with sedimentological data that indicate a bulk SE-wards tilt of the basin ¯oor in the Solund area. In the interpretation of Nilsen (1968) followed by Steel (1976), the sedimentary data from the Solund basin re¯ect deposition in a southeastwards tilting,extensional half-graben basin dominated by transverse,NW- and
SE-directed drainage. The basin was probably bounded by a transfer fault along its NE margin
(e.g. Indrevñr,1980; Steel et al.,1985). In the ¯uvial sandstones in northern parts of the basin (Vñrlandet area,Fig. 3b),paleocurrents were SE ¯owing,that is in the direction of the footwall of the basin-bounding fault. SW-¯owing paleocurrents in the same area may represent either a system more axial with respect to the basin-bounding fault or in¯uence from marginal drainage that transported material towards the central basin area.
In the Hornelen basin,sediment entered the basin from the eastern,northern and southern margins and is fed into a ¯uvial channel belt characterized by WSW
(i.e. hanging wall)-directed paleocurrents (Steel and
Aasheim,1978; Steel and Gloppen,1980). In the east, the basin margin was constituted by a W-dipping lowangle normal fault (Cuthbert,1991; Wilks and Cuthbert,1994) that provided a drainage area large enough to supply the basin with large amounts of sand-sized material (cf. Friedmann and Burbank,1995). The main syndepositional tilt direction in the Hornelen basin was towards the east or southeast according to interpretations by earlier workers (Steel et al.,1985;
SeÂranne and SeÂguret,1987; Chauvet and SeÂranne,
1994; Wilks and Cuthbert,1994). As the basin was transported westwards on the detachment,¯anked by oblique/strike±slip fault segments along the northern and southern margins,a shingled arrangement of conglomeratic fan bodies was produced (Steel and
Gloppen,1980; Steel et al.,1985; Steel,1988;
Wilks and Cuthbert,1994). The combination of subsidence and lateral displacements were responsible for the pronounced coarsening- to ®ning upwards grain size motif recognized in all parts of the basin ®ll (op.
cit.). The WSW-¯owing paleocurrents reported from the central basin area by Steel and Gloppen (1980) were thus roughly parallel to the extension direction.
Along the northern basin margin,however,more northerly sediment transport directions indicate increased subsidence along this margin for a large part of the basin history (op. cit.). The WSW-trending folds that deform the basin ®ll are at an angle with the more E±W trending contractional structures in the
Kvamshesten basin.
In the Kvamshesten basin,the thick fanglomerate complex along the southern basin margin resembles that of the Solund basin. The axial belt of ¯uvial sandstones and the paleocurrent directions inferred from them are largely subparallel to the present basin syncline axis similar to the con®guration encountered in the Hornelen basin (Fig. 5). Thickness variations and onlap relationships displayed by the marginal fan complexes are in accordance with development of a
NE-trending rollover anticline±syncline pair during early stages of basin formation (Osmundsen et al.,
1998). East-stepping of fanglomerates along the basin margins and the eastwards migration of the central belt of ¯uvial sandstones give evidence of eastwards migration of the basin's depocentre. This was probably the result of westwards movement of the basin upon the detachment (Osmundsen et al.,2000).
Syndepositional intrabasinal faults in the Kvamshesten basin comprise NW-dipping faults with normal and sinistral separations at low stratigraphic levels and a conjugate system of NW- and NE-dipping faults at high stratigraphic levels. When the basin syncline and the eastwards tilt of the basin upon the detachment are restored,the NW-dipping faults have separations that are mainly normal while the conjugate faults at higher stratigraphic levels reveal an

basinal extension direction.
P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 51±68 orthorhombic geometry symmetric about N±S and E±
W trending axes (Osmundsen et al.,1998). The E±W trending symmetry axis bisects the obtuse angle between the fault sets and is interpreted to represent the direction of principal elongation (op. cit). Thus, sediment transport directions in the belt of ¯uvial sandstones is commonly parallel to the overall intra-
In the Kvamshesten basin,folds and thrusts display
NW±SE and E±W trends (Fig. 4; Osmundsen et al.,
1998). At low to intermediate stratigraphic levels,
NW±SE trending contractional structures are superposed by E±W trending ones (Fig. 5). At high stratigraphic levels,folds and reverse faults trend E±W and the relations displayed in Fig. 8 indicate that folding probably started during basin sedimentation. The interpretation of a number of other dislocations in the Hornelen and Kvamshesten basins as unconformities (Chauvet and SeÂranne,1994) has,however,been controversial (Wilks and Cuthbert,1994; Osmundsen et al.,1998). The NE-wards onlap onto basement and the inter®ngering relationships observed at low stratigraphic levels in the Kvamshesten and Solund basins may be explained in two ways; ®rstly,onlap may have been onto the SW-dipping ¯ank of a synform that resulted from NE±SW-directed shortening. Alternatively,NE- and eastwards onlap was part of a radial onlap pattern produced by fault growth during early stages of basin formation (e.g. Schlische,1991;
Osmundsen et al.,1998) followed by onlap onto a rollover anticline (Osmundsen et al.,1998,2000)
Both interpretations are compatible with a NW±SE direction of extension.
SE-trending folds and reverse faults superimposed by E±W trending ones at low and intermediate stratigraphic levels in the Kvamshesten basin may indicate that shortening was associated with clockwise rotation of the western parts of the basin; in this scenario, shortening had an overall N±S direction,but reverse faults and folds in the western parts of the basin rotated anticlockwise to more NW±SE orientations and were later overprinted by new E±W trending contractional structures. Alternatively,the strain®eld rotated with time. This would require a change in the boundary conditions where the direction of shortening rotated in an anticlockwise direction and where only the lower parts of the stratigraphy records the early
(NE±SW-directed) shortening. Steep northerly dips recorded at high stratigraphic levels together with the high anchizone/lowermost greenschist facies metamorphism apparently associated with shortening
(Torsvik et al.,1987; SeÂranne and SeÂguret,1987) indicates that much of the shortening post-dates the preserved Devonian stratigraphy (Torsvik et al.,
1986; Osmundsen et al.,1998).
4.2. A model of combined extension and strike±slip for the Devonian basins of western norway
63
In central south Norway as well as in the Bergen arcs area south of the Solund basin,the ®nite streching direction in the ductilely deformed basement is dominantly towards the NW (Fossen,1992,1998; Wennberg et al.,1998; Krabbendam and Dewey,1998;
Andersen,1998). In the Sognefjord±Nordfjord area, streching lineations and fold axes in the footwall of the NSDZ display changes in orientation from NW plunges SE of the Solund basin via E±W and ESE±
WNW beneath the Kvamshesten basin to WSW north of the Hornelen basin (Fig. 1; Chauvet and SeÂranne,
1994; Krabbendam and Dewey,1998). North of the
Devonian basins,lineations and fold axes turn to become parallel with the Mùre±Trùndelag Fault
Zone (MTFZ,Figs. 1 and 9). An important question is whether extension with different (NW,W and SW) orientation in different areas occurred contemporaneously (SeÂranne et al.,1991; Chauvet and SeÂranne,
1994; Krabbendam and Dewey,1998) or if NW and
SW extension directions were separated in time
(extension followed by transtension and orogenparallel strike±slip). The Devonian basins formed in the hanging wall of the NSDZ and would tentatively record large-scale in¯uence of strike±slip during sedimentation. It is also to be expected that this in¯uence would be stronger in the areas close to the MTFZ where kinematic indicators give evidence for Devonian top-SW extension and sinistral strike±slip
(SeÁranne,1992; Robinson,1995).
Of the western Norwegian basins,the Solund basin occupies the position farthest away from the MTFZ.
The consistent SE-wards tilt direction inferred from paleocurrent data and half-graben geometry indicates that the basin formed mainly during NW-directed extension. The onlap relationship towards basement in the Vñrlandet area may indicate that the basin experienced early,NE±SW-directed shortening. The

64 P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 51±68
Hornelen basin occupies the most proximal position with respect to the area affected by strike±slip. The W to WSW direction of extension and the NNW±SSE direction of shortening that can be inferred from the basin geometry is consistent with an anticlockwise rotation of the strain®eld relative to the Solund basin.
Based on the observations and inferences presented above,we infer that the Kvamshesten basin started out as a SE-wards tilting half-graben,similar to the preserved geometry of the Solund basin. The later stages of basin formation conform more closely to that of the Hornelen basin,based on overall con®guration of sedimentary units,paleocurrent data and on the E±W direction of maximum elongation that has been inferred from the Selsvatn fault system
(Osmundsen et al.,1998). That is,while the Solund and Hornelen basins represents con®gurations of extension direction and overall architecture that are separated in space,the Kvamshesten basin constitutes a single basin where both con®gurations have been preserved. In a scenario of regional transtension (i.e.
Krabbendam and Dewey,1998),the tectono-sedimentary response in the basin areas would be largely dependent on their distance from the principal zone(s) of strike±slip deformation. Tentatively,the
Hornelen basin would respond more rapidly to the component of strike±slip as it was initiated closer to the MTFZ than the other basins in western Norway.
The Kvamshesten basin would probably experience the effect of strike±slip at a somewhat later stage while the Solund basin was located too far from the
MTFZ to experience signi®cant strain ®eld rotation during deposition.
An alternative scenario is that the early phase of
NW extension and SE-wards tilt pre-dates orogenparallel sinistral strike±slip and that the change in extension direction marks the onset of sinistral deformation. This opens for a model where all the
Devonian basins formed in a strain ®eld characterized by NW-directed extension. This would ®t the apparent change from NW-to W-directed extension in the
Kvamshesten basin,as well as early,NW-directed extension in the Solund basin. The early facies distribution and paleodrainage patterns in the Hornelen basin should,however,resemble those of the Solund basin. From available map and sedimentological data, this is not obvious.
The directions of shortening in the basins show a swing in orientation from NE±SW in the Solund basin to WSW±ESE in the Hornelen basin. In the Kvamshesten basin,shortening was at least in part syndepositional and the direction of shortening apparently changed from NE±SW to N±S with time. The syndepositional shortening was,however,not continuous. As indicated by the orthorhombic Selsvatn fault system,the area experienced periods with extension in both N±S and E±W directions (Osmundsen et al.,
1998). In the Kvamshesten and Hornelen basins,an effect of syndepositional N±S shortening on sedimentation may be re¯ected in the parallelism between paleocurrent directions inferred from the sandy parts of the basin ®lls and the synclinal fold axes. In the
Kvamshesten basin,a belt of red,®ne-grained ¯oodbasin strata are localized along the axial plane trace of the basin syncline at high stratigraphic levels (Fig. 5).
Thus,it is possible that intrabasinal drainage was partly controlled by the evolving fold system. Shortening continued past the time-window represented by the Devonian sedimentary rocks and the folded basins were eventually cut by low-angle normal faults that constitute the present basin margins.
5. Conclusions
Variations in sedimentary and structural architecture indicate that the Devonian basins of western
Norway developed in a strain ®eld characterized by regional transtension. The syndepositional tilt direction inferred from individual basins re¯ect the local direction of extension in the area where each basin formed (Fig. 9). Each basin was bordered by a large, low-to moderate angle normal fault and a steeper transfer fault subparallel to the extension direction.
This con®guration was responsible for the asymmetric distribution of sedimentary facies within each basin and for the difference in fanglomerate architecture on opposing basin margins. The interplay between normal and strike±slip faulting on the basin scale may also have been responsible for the geometry of
CUFU units encountered within all the basins (Steel and Gloppen,1980). The in¯uence of larger-scale, orogen-parallel strike±slip movements is re¯ected in the combined observations from the array of basins in western Norway. When viewed as an array of contemporaneous basins,the syndepositional tilt directions

P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 51±68 65
Fig. 9. Conceptual model for Devonian basin formation in western Norway. Block diagrams are schematic representations of inferred basin geometry while the paleotopography,generalized facies distributions and sediment dispersal patterns are represented above each block diagram. The syndepositional framework was characterized by extension along the Nordfjord±Sogn Detachment Zone and sinistral strike± slip along the Mùre±Trùndelag Fault Zone or its precursor,which may have been a wider zone characterized by SW-directed extension and sinistral strike±slip. This gave rise to a transtensional strain gradient where the principal axis of extension displayed a progressive anticlockwise rotation from NW to E±W northwards in the study area. The response in the basin areas re¯ects the distance from the principal strike± slip shear zone such that the Solund Basin experienced mainly SE-directed tilt during deposition while the Hornelen Basin was characterized by westwards translation during most of its history. The preserved stratigraphy in the HaÊsteinen Basin probably records NW±SE extension due to the strong SE-wards onlap relationship towards basement. In the Kvamshesten Basin,NW±SE extension and SE-directed tilt was followed by
E±W extension,E-directed tilt and generally E±W ¯owing paleocurrents in the central basin area. Shortening of the basins in a direction roughly normal to the principal direction of extension probably started during sedimentation and the evolving folds may have contributed to the control of paleo¯ow patterns in the central basin areas. As the principal direction of extension changed from NW to W (Kvamshesten Basin), the principal direction of shortening changed from NE±SW to N±S. Shortening was probably not continuous in the basins,but interrupted by periods where elongation was positive in the N±S direction.
and facies con®gurations re¯ect the swing in orientation displayed by the ductile extensional lineation in the NSDZ and WGR and thus the transtensional strain gradient towards the MTFZ. While the Solund and
Hornelen basins may be regarded as the preserved geographical and architectural ªend membersº in this con®guration,the Kvamshesten basin constitutes a tectono-sedimentary link between the two former basins. The anticlockwise rotation of the syndepositional strain ®eld inferred from the Kvamshesten basin can be interpreted as a result of gradual entry into the region affected by strike±slip deformation.
Alternatively,it opens for the possibility that the strike±slip component post-dates the NW-directed extension that accompanied the early stages of basin formation.

66
Acknowledgement
Financial support from NORSK AGIP a/s is greatly acknowledged.
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