trans tension
TRANSCRIPT
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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
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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
P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 516854
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
P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 516856
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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P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 5168 57
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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
P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 516858
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
P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 516860
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
P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 5168 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 EW trending anticline, indicating that the NS shortening of the
Kvamshesten Basin was partly syndepositional.
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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
P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 516862
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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
P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 5168 63
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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
P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 516864
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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.
P.T. Osmundsen, T.B. Andersen / Tectonophysics 332 (2001) 5168 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 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.
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Acknowledgement
Financial support from NORSK AGIP a/s is greatly
acknowledged.
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