trans tension

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The middle Devonian basins of western Norway: sedimentaryresponse to large-scale transtensional tectonics?

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 NordfjordSogn Detachment Zone (NSDZ) and display

extensional half-graben geometries in sections parallel to the local direction of principal extension. Based on overall facies

congurations, paleocurrent patterns and intrabasinal structures, we infer an anticlockwise rotation of the syndepositional

extension direction from NWSE in the south (Solund basin) to WSWENE 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 EW owing paleodrainage. Correspondingly, NWSE trending

folds and reverse faults are superposed by EW 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 reect an anticlockwise

rotation of the regional syndepositional strain eld. The above observations and inferences indicate that the Devonian basins inwestern 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 strikeslip 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

B.V. All rights reserved.

Keywords: Devonian basins; transtensional tectonics; NordfjordSogn 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

sedimentological investigations in the Hornelen

basin, Steel et al. (1977), followed by Steel and Glop-

pen (1980), proposed a strikeslip model for basinformation. During the last 15 years, the late- to post-

orogenic extension of the Caledonian mountain belt in

western Norway has received considerable attention

(Hossack, 1984; Norton, 1986, 1987; Seranne and

Seguret, 1987; Steel, 1988; Andersen and Jamtveit,

1990; Fossen, 1992; Chauvet and Seranne, 1994;

Krabbendam and Dewey, 1998). In particular, work

has been focussed on the large-magnitude extensional

NordfjordSogn Detachment Zone (NSDZ) and on

Tectonophysics 332 (2001) 51 68

0040-1951/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.

PII: S0040-1951(0 0)00249-3

www.elsevier.com/locate/tecto

* Corresponding author. Geological Survey of Norway, 7491

Trondheim, Norway.


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the exhumation of deep crustal rocks in its footwall

(op. cit.). Thus, although the Devonian basins are situ-

ated in the hanging wall of the NSDZ, the present

model framework is largely based on observations

in the footwall. The Devonian basins display consid-

erable variation with respect to overall geometry and

facies distribution (cf. Steel, 1976). This variation is

not easily explained by regional unidirectional, top-

to-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 sedi-

mentary basin formation are invariably recorded by

the basin ll. The location and runoff directions of

major drainage basins are, together with their princi-pal bedrock lithologies, recorded in terms of sedimen-

tary facies conguration, 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, intra-

basinal unconformities and the conguration of

syndepositional intrabasinal structures. A re-investigation 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 sedi-

mentology and structure of the western Norwegian

Devonian basins in terms of the above parameters.

In this paper, we shall discuss formation of the Devo-

nian basins in terms of inter- and intrabasinal varia-

tions 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; Seranne

and Seguret, 1987; Andersen and Jamtveit, 1990;

Swensson and Andersen, 1991). 40Ar/39Ar 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, respec-

tively (Berry et al., 1995; Andersen, 1998). The foot-

wall of the NSDZ is constituted by the Western Gneiss

Region (WGR), which experienced late Caledonian

(ca 400 420 Ma) eclogitefacies metamorphism

(Grifn 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 eclo-

gites occur within 3 km from the Devonian rocks thusdemonstrating a metamorphic gap across the NSDZ

corresponding to 4550 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 else-

where (Osmundsen and Andersen, 1994, in press).

The Caledonian nappe rocks are unconformably over-

lain by Devonian sedimentary rocks. The entire crus-

tal section exposed between Sogn and Nordfjord hasbeen 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 ENE-

plunging 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; Seranne et al., 1991;Chauvet and Seranne, 1994). The later stages of short-

ening 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 Carbonifer-

ous 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) 516852


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P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 5168 53

Fig. 1. Overview map of the SognNordfjord area in western Norway showing main tectonostratigraphic units. Note facies congurations 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.


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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 consti-

tuted either by EW striking segments of the low-

angle normal faults (Kvamshesten basin area) or by

steeper faults with normal separation but abundant

evidence for strikeslip movements (Hornelen basinarea, 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). EW striking faults as well as segments of

low-angle normal faults display evidence for dextral

slip along the northern basin margins while compo-

nents of sinistral slip is observed along the southern

basin margins (Seranne and Seguret, 1987; Steel,

1988; Chauvet and Seranne, 1994).

2. Overall basin geometry, facies conguration 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 Lagy

anticline (Fig. 3a). SW of Lagy, however, where

the axial plane trace is deected to a NS trend,

onlap onto basement is apparently towards the NE

in the present conguration (Nilsen, 1968; Steel et

al., 1985, Fig. 3a). The NW continuation of the Solund

basin is exposed in the Vrlandet area, where a thick

succession of breccias and conglomerates are overlain

by uvial sandstones. The basal breccias onlapdepositional basement eastwards and internger with

polymict conglomerates towards the West. In the

Vrlandet area, the Devonian strata display a

pronounced fanning wedge relationship where the

dip of bedding changes from southwards at low strati-

graphic levels to southeastwards in the uvial sand-

stones. The sandstones internger with fanglomerates

and breccias exposed on a SE-trending array of sker-

ries and islands SE of Vrlandet (Fig. 3b). In the

Vrlandet area, an apparent reversal of paleocurrent

direction took place during deposition of the exposed

stratigraphy. Paleocurrent directions inferred fromimbricate 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 conglom-

erates 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


Fig. 2. Schematic, NS cross-section through the NordfjordSogn area (mainly from Andersen, 1998) showing large-scale syn- and antiforms

that deect 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.


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by Nilsen (1968) was re-interpreted by Seranne and

Seguret (1987) to represent a tectonically induced

fabric (see below). In summary, the parts of the

Solund basin exposed in the Solund and Vrlandet

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 Vrlandet and the SW-directed paleocur-

rents recorded in the eastern parts of the uvial sand-

stones indicate that a third depositional system

characterized by SW directed sediment transport

was located along the NE basin margin. In Solund

(Indrevr, 1980; Steel et al., 1985) and particular in

the Vrlandet area (Fig. 3b), onlap relationshipsbetween Devonian strata and basement as well as

interngering relationships between the main sedi-

mentary units indicate that the oldest Devonian

rocks are found in the southwest. The basal unconfor-

mity is thus a diachroneous surface indicating increas-

ing subsidence towards the SW in the exposed parts of

the basin.

2.2. The Hornelen basin

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, south-

ern 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-trans-

ported conglomerates. Thus, the facies distribution in

the Hornelen basin is markedly asymmetric. The stra-

tigraphy 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; Indrevr 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

The Kvamshesten basin (Fig. 5), appears as a SE- to

eastwards rotated half graben basin when viewed in aNWSE 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 inter-

mediate stratigraphic levels (Bryhni and Skjerlie,

1975; Seranne 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) thatreaches a thickness of ca 1 km. Parts of the NMFC

onlaps basement eastwards at low to intermediate

stratigraphic levels. Both fan complexes internger

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 paleo-

current directions while westwards as well as east-

wards 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 sedi-

mentary ll of the Hornelen basin is also clearly

present in the Kvamshesten basin (Osmundsen et al.,

1998, 2000). Additional evidence for a syndeposi-

tional, eastwards tilt direction come from the progres-

sive eastwards migration of uvial facies and from the



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east-stepping, retrogradational stacking of fanglome-

rates shown at high stratigraphic levels in Figs. 5 and

6. An original fanning wedge geometry of the Devo-

nian strata is reected 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 Hasteinen basin

The preserved remnants of the Hasteinen 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 Hasteinen basin is the

southeastwards onlap of the entire basin ll (up to

11 000 m of cumulative stratigraphy) onto Caledo-nian basement with an angle as high as 538 (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 paleotopo-

graphy with a relief of between 5800 and 11 000 m,

an explanation considered unlikely (Vetti and Milnes,

1997). The preserved parts of the Hasteinen 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

In western Norway, the present faulted margins of

the Devonian basins do not correspond directly to the

syndepositional margins as demonstrated by cross-

cutting relationships and by paleomagnetic and radio-

metric 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 identication

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; Seranne and Seguret, 1987;

Chauvet and Seranne, 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 Indrevr and

Steel (1975) as well as by Seranne and Seguret

(1987). Seranne and Seguret (1987) argued that the

pebble fabric passed into a greenschist facies pebblefabric 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 1208, 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 (Osmund-sen, 1996). A number of NW-dipping faults that

cross-cut the basal unconformity have been inter-

preted 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 syn-

sedimentary 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 Lagy 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. Grano-

diorite 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

(Lagy anticline); 8. Low-angle normal fault (Solund fault). (b) Vrlandet 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 (Vrlandet area), 3. Conglom-

erates, 4. Fluvial channel sandstones with overbank intervals.


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(Osmundsen et al., 1998; Bakke 1999). In the Horne-

len basin area, NE- and NW-dipping faults with

normal/oblique separations appear to dominate atlow stratigraphic levels (Hartz et al., 1994; Hartz

and Andresen, 1997).

The SE- to ENE-plunging folds and SE- to EW

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; Seranne et al., 1991; Chauvet

and Seranne, 1994; Osmundsen et al., 1998; Braathen,

1999). The post-Caledonian, NS shortening has been

correlated with the Svalbardian stage of Vogt (1936)

and linked to regional sinistral strikeslip movementsin southern Scandinavia and the British Isles (Vogt,

1953; Seranne et al., 1991; Chauvet and Seranne,

1994).

The basins presently constitute large-scale

synclines with a number of parasitic folds. In the

Solund basin, strains related to NESW directed

shortening are locally high and have given rise to

cleavage formation in the southwesternmost exposed

parts of the basin (Indrevr and Steel, 1975). Large

parts of the basin does, however, appear to be less

folded than the Kvamshesten and Hasteinen basins.

The basal unconformity in the Vrlandet area dipssouthwards at approximately 408 in accordance with

rotation by folding along an E W trending axis, alter-

natively 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. Grndalen syncline

in Fig. 4).

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 EW (Fig. 5). At inter-mediate 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. 4. Map of the Hornelen basin and parts of its substrate (modied 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 (Grndalen syncline) and 9. High-angle brittle faults (southern and parts of northern basin margin).


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Fig. 5. The Kvamshesten basin with main facies distribution, paleocurrent indicators (rose diagrams: readings by present authors, mainly trou

from Asphaug, 1975) and conguration of intrabasinal structures. Note the diachroneity between the southern and northern marginal fanglome

preserved in the basin are located along the southern basin margin. Legend: 1. WGR and detachment mylonites, schematic traces of main

undifferentiated; Devonian sedimentary rocks; 3. Conglomerates and breccias; 4. Floodplain/oodbasin rocks with intercalated channel- and

multistory channel sandstone units separated by subordinate red nes; 6. Multistory channel sandstone units intercalated with plane lami

sandstones and subordinate red nes; 7. Scoop-shaped low-angle normal fault (Dalsfjord Fault), 8. Thrust/reverse fault; 9. Fold axis; 10. Intr


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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 ENEWSW-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. 6. EW cross-section through the Kvamshesten basin (i.e. parallel to fold axis). Note shallow half-graben geometry, onlap/interngering

relationships and eastwards migration of channel sandstones (high stratigraphic levels). The line of prole is generally located along the axial

plane trace of the basin syncline (see Fig. 5).

Fig. 7. Map of the Hasteinen Basin and parts of its substrate (modied 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.


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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 NWSE

trending contractional structures overprinted by

EW trending ones; at high stratigraphic levels, EW

to ENEWSW trending contractional structures

dominate. The latter at least were syndepositional

(Fig. 8).

The Hasteinen basin is deformed into a steeply SE-

plunging syncline where bedding is rotated to more

than 608 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 alluvialfans 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, half-

grabens are closed and transverse systems domi-

nate (e.g. Leeder and Gawthorpe, 1987; Schlische,

1991). In closed basins, the area characterized by

the highest subsidence rates is commonly occupied

by mudat, playa or lacustrine deposits as intra-

basinal drainage tends to converge in this area

(Leeder and Gawthorpe, 1987; Schlische, 1991).

If individual half-grabens link up to form a riftzone, an axial river system usually develops that

ows parallel to the array of basin-bounding faults

(op. cit.). Thus, a variety of paleocurrent direc-

tions may be encountered in continental exten-

sional 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


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 EW trending anticline, indicating that the NS shortening of the

Kvamshesten Basin was partly syndepositional.


12/18

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 (Seranne and Seguret, 1987), data inde-

pendent of clast long axes (cross-bedding, clast round-

ness distribution, clast lithology distribution) appear

to support a NW-directed sediment dispersal (Nilsen,

1968). The NWSE (ca 1208) trending clast long axis

fabric interpreted as produced by tectonic clast rota-

tion under soft-sedimentary conditions (Seranne and

Seguret, 1987) give evidence of an early phase of

NWSE-directed extension in the basin. The exten-

sion direction inferred from the pebble long axis

orientation is at a high angle to the NESW trending

Lagy anticline (Fig. 3a), which has been interpretedas a rollover anticline (Norton, 1986, 1987; Seranne

and Seguret, 1987). Thus, intrabasinal structure is

consistent with NWSE-directed extension and with

sedimentological data that indicate a bulk SE-wards

tilt of the basin oor in the Solund area. In the inter-

pretation of Nilsen (1968) followed by Steel (1976),

the sedimentary data from the Solund basin reect

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. Indrevr, 1980; Steel et al., 1985). In the uvial

sandstones in northern parts of the basin (Vrlandet

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 inuence 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 low-

angle normal fault (Cuthbert, 1991; Wilks and Cuth-

bert, 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;

Seranne and Seguret, 1987; Chauvet and Seranne,

1994; Wilks and Cuthbert, 1994). As the basin was

transported westwards on the detachment, anked by

oblique/strikeslip 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 subsi-

dence 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, morenortherly 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 sand-

stones and the paleocurrent directions inferred from

them are largely subparallel to the present basinsyncline axis similar to the conguration 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 anticlinesyncline 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 east-

wards 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 Kvam-

shesten 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 conju-

gate faults at higher stratigraphic levels reveal an



13/18

orthorhombic geometry symmetric about N S and E

W trending axes (Osmundsen et al., 1998). The EW

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-

basinal extension direction.

In the Kvamshesten basin, folds and thrusts display

NWSE and EW trends (Fig. 4; Osmundsen et al.,

1998). At low to intermediate stratigraphic levels,

NWSE trending contractional structures are super-

posed by E W trending ones (Fig. 5). At high strati-

graphic levels, folds and reverse faults trend E W and

the relations displayed in Fig. 8 indicate that folding

probably started during basin sedimentation. Theinterpretation of a number of other dislocations in

the Hornelen and Kvamshesten basins as unconformi-

ties (Chauvet and Seranne, 1994) has, however, been

controversial (Wilks and Cuthbert, 1994; Osmundsen

et al., 1998). The NE-wards onlap onto basement and

the interngering relationships observed at low strati-

graphic 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 NESW-directed shortening. Alterna-

tively, NE- and eastwards onlap was part of a radialonlap 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 NWSE

direction of extension.

SE-trending folds and reverse faults superimposed

by EW trending ones at low and intermediate strati-

graphic 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 NS direction, but reversefaults and folds in the western parts of the basin

rotated anticlockwise to more NWSE orientations

and were later overprinted by new EW trending

contractional structures. Alternatively, the straineld

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

(NESW-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; Seranne and Seguret, 1987) indi-

cates 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 strikeslip

for the Devonian basins of western norway

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 domi-

nantly towards the NW (Fossen, 1992, 1998; Wenn-

berg et al., 1998; Krabbendam and Dewey, 1998;Andersen, 1998). In the SognefjordNordfjord 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 EW and ESE

WNW beneath the Kvamshesten basin to WSW north

of the Hornelen basin (Fig. 1; Chauvet and Seranne,

1994; Krabbendam and Dewey, 1998). North of the

Devonian basins, lineations and fold axes turn to

become parallel with the MreTrndelag Fault

Zone (MTFZ, Figs. 1 and 9). An important question

is whether extension with different (NW, W and SW)orientation in different areas occurred contempora-

neously (Seranne et al., 1991; Chauvet and Seranne,

1994; Krabbendam and Dewey, 1998) or if NW and

SW extension directions were separated in time

(extension followed by transtension and orogen-

parallel strikeslip). The Devonian basins formed in

the hanging wall of the NSDZ and would tentatively

record large-scale inuence of strike slip during sedi-

mentation. It is also to be expected that this inuence

would be stronger in the areas close to the MTFZ

where kinematic indicators give evidence for Devo-

nian top-SW extension and sinistral strikeslip

(Seranne, 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 Vrlandet area may indicate that the basin

experienced early, NESW-directed shortening. The



14/18

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 NNWSSE

direction of shortening that can be inferred from the

basin geometry is consistent with an anticlockwise

rotation of the straineld 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 cong-

uration of sedimentary units, paleocurrent data and on

the EW direction of maximum elongation that has

been inferred from the Selsvatn fault system

(Osmundsen et al., 1998). That is, while the Solundand Hornelen basins represents congurations of

extension direction and overall architecture that are

separated in space, the Kvamshesten basin constitutes

a single basin where both congurations have been

preserved. In a scenario of regional transtension (i.e.

Krabbendam and Dewey, 1998), the tectono-sedimen-

tary response in the basin areas would be largely

dependent on their distance from the principal

zone(s) of strikeslip deformation. Tentatively, the

Hornelen basin would respond more rapidly to the

component of strikeslip as it was initiated closer tothe MTFZ than the other basins in western Norway.

The Kvamshesten basin would probably experience

the effect of strikeslip at a somewhat later stage

while the Solund basin was located too far from the

MTFZ to experience signicant strain eld rotation

during deposition.

An alternative scenario is that the early phase of

NW extension and SE-wards tilt pre-dates orogen-

parallel sinistral strikeslip 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 characterizedby 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 distri-

bution 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 WSWESE in the Hornelen basin. In the Kvam-

shesten basin, shortening was at least in part synde-

positional and the direction of shortening apparently

changed from NESW to NS with time. The

syndepositional shortening was, however, not contin-

uous. As indicated by the orthorhombic Selsvatn fault

system, the area experienced periods with extension in

both NS and EW directions (Osmundsen et al.,

1998). In the Kvamshesten and Hornelen basins, an

effect of syndepositional N S shortening on sedimen-

tation may be reected 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 ood-

basin strata are localized along the axial plane trace ofthe basin syncline at high stratigraphic levels (Fig. 5).

Thus, it is possible that intrabasinal drainage was

partly controlled by the evolving fold system. Short-

ening 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 architec-

ture indicate that the Devonian basins of western

Norway developed in a strain eld characterized by

regional transtension. The syndepositional tilt direc-

tion inferred from individual basins reect 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 conguration 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 strikeslip 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 inuence of larger-scale,

orogen-parallel strikeslip movements is reected in

the combined observations from the array of basins in

western Norway. When viewed as an array of contem-

poraneous basins, the syndepositional tilt directions



15/18

and facies congurations reect the swing in orienta-

tion 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 end members in

this conguration, the Kvamshesten basin constitutes

a tectono-sedimentary link between the two former

basins. The anticlockwise rotation of the syndeposi-

tional strain eld inferred from the Kvamshesten

basin can be interpreted as a result of gradual entry

into the region affected by strikeslip deformation.

Alternatively, it opens for the possibility that the

strikeslip component post-dates the NW-directed

extension that accompanied the early stages of basin

formation.


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 NordfjordSogn Detachment Zone and sinistral strike

slip along the MreTrndelag Fault Zone or its precursor, which may have been a wider zone characterized by SW-directed extension and

sinistral strikeslip. This gave rise to a transtensional strain gradient where the principal axis of extension displayed a progressive antic-

lockwise 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 Hasteinen Basin probably records NWSE extension due to

the strong SE-wards onlap relationship towards basement. In the Kvamshesten Basin, NWSE extension and SE-directed tilt was followed by

EW extension, E-directed tilt and generally EW 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 paleoow 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 NESW to NS. Shortening was probably not continuous in the basins, but interrupted by

periods where elongation was positive in the NS direction.


16/18

Acknowledgement

Financial support from NORSK AGIP a/s is greatly

acknowledged.

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