The northern Bergen Are Shear Zone an oblique-lateral ramp In the Devonian extensional detachment system of western Norway
OLE PETTER WENNBERG, ALAN GEOFFREY MILNES & INGER WINSVOLD
Wennberg, O. P., Milnes, A. G. & Winsvold, I. The northern Bergen Are Shear Zone - an oblique-lateral ramp in the Devonian extensional detaehment system of western Norway. Norsk Geologisk Tidsskrift, Vol. 78, pp. 169-184. Oslo 1998. ISSN 0029-196X. ISSN 0029-196X.
Structural investigations in the northern part of the Bergen area (western Norway) have been earried out on the major, 2-3 km-thiek, late/post-Caledonian, extensional Bergen Are Shear Zone. This shear zone dips steeply SW with a ehange in strike from 115-120° in the northeast to 150-160° in the southwest, over a distance of 50 km. The footwall eonsists of the autochthonous Western Gneiss Complex and the lowermost Caledonian Nappes. The hanging wall eonsists of strueturally higher Caledonian Nappes uneonformably overlain by eoarse Devonian eonglomerates. The orientation of the mylonitic foliation within the shear zone, in addition to several types of shear-sense indieators, predominantly indieate a dextral shear component. The related stretehing lineation pl unges about 20° W to NW, implying a significant eomponent of normal slip, as indieated by the regional geology. The minimum displacement on the Bergen Are Shear Zone is approximately 16 km, with a normal dip slip eomponent of 8 km, and a dextral strike slip eomponent of 14 km. The Bergen Are Shear Zone post-dates all Caledonian eontraetional deformation in its vieinity. We suggest that the Bergen Are Shear Zone represents a braneh of the Nordfjord-Sogn Detaehment Zone, the main Devonian detaehment, developed as an oblique-lateral ramp.
O. P. Wennberg,• A. G. Mi/nes & l. Winsvold, Geologisk Institutt, Universitet i Bergen, A/legt 41, 5007 Bergen, Norway. • Z & S Geologi ajs. Sverdrupsgate 23, N-4007 Stavanger, Norway.
& Introduction 1988; Seranne et al. 1991; Fossen 1992; Wennberg Milnes 1994). The Nordfjord-Sogn Detachment Zone The Bergen Are Shear Zone (BASZ, Fig. l) has been (NSDZ, Fig. l) and BASZ cut through, on a regional suggested to represent the southward continuation of the scale, the Caledonian Nappe pile, which was formed well-known large-scale (displacement �50 km), Devo during the final collisional event of the Caledonian nian, extensional detachment system in western Norway Orogeny (the Scandian phase), which ended in early - the Nordfjord-Sogn Detachment Zone (Milnes et al. Devonian times (ca. 395 Ma; Milnes et al. 1997). The
Fig. l. Tectonie overview map of SW Norway. Abbreviations: BASZ =Bergen Are Shear Zone; NSDZ =Nordfjord-Sogn Detaehment Zone; HFSD = Hardangerfjord Shear Zone; LGF = Lærdal-Gjende R�f�:..WiJDevonian sediments 1iiiiJC aledonian allochthon Fault; WGC =Western Gneiss Complex. Modified l iiii from Sigmond et al. (1984), Milnes et al. (1997) and Autochthon/Parautochthon ....&....o.L� Late-/post-Caledonian shear zone Wennberg (1996). 1::::::::::J 170 O. P. Wennberg et al. 78 (1998)
NORSK GEOLOGISK TIDSSKR!Ff wsw ENE
km
� Fig. 4b: � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � , , , , , , J'' '''''' ''' '''' '''''''' '''' ''''''' '''''''' 5krn ,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,' ,,,,,,,,,,, ,,, , ,,,,,, ,,, ,,,,,, ,,,,, , ,,,, ,, ,,,,,, , ,,, , , , ,,,,,,,,, , , ,,,,,,,,,,,,,,,, ,,,,,,,,,,,,, lO km , , , ,
HANGING WALL FOOTWALL BASZ BASZ BASZ
E2] Devonlan deposlts
------�------
Fig. 2. Profile aeross the northem Bergen Are Shear Zone showing the main teetonie units. Line of profile is indieated in Fig. l. MaBA = Major Bergen Are zone;
MiBA = Minor Bergen Are zone. downthrown hanging wall of the NSDZ, to the west, lochthons consist of nappe complexes derived from the mainly exposes upper units of the nappe pile (Upper ancient continent Baltica (west of the present Au Allochthon), characterized by ophiolites derived from the tochthonjParautochthon), whereas the overlying nappes Iapetus ocean, and forming the basement to post-oro of the Upper Allochthon contain units with oceanic genic Devonian clastic sequences (Norton 1987; Seranne affinities derived from the Caledonian ocean, Iapetus. & Seguret 1987; Andersen et al. 1990). The NSDZ These tectonostratigraphic units can be identified both footwall, in contrast, mainly exposes deep parts of the within the BASZ and in its footwall and hanging wall orogenic root-poly-deformed high-grade gneisses repre (Fig. 2), and provide a framework for the structural senting the westward continuation of the Baltic Shield analysis. (Milnes et al. 1997 and references therein). The shear zone follows Fensfjorden and is well ex From its regional setting and from an early reconnais posed for structural analysis in two areas, at Byrknesy sance study (Wennberg & Milnes 1994), the steeply in the northwest, and along Osterfjord in the southeast SW-dipping northern BASZ was known to have a large (Fig. 3). Most of the structural data comes from these dextral strike-slip component and a smaller normal com areas, and have been summarized in two profiles, A-A' ponent of movement. Within this tectonic framework, we in the northwest and B-B' in the southeast (Fig. 4). focus on the structural relationships within the northern Profile A-A' exposes the lower part of the BASZ, and part of the BASZ, and their regional implications. profile B-B' represents an almost complete traverse through the shear zone with > 90% exposure. Details from A-A' are presented by Winsvold (1996), and from B-B' will be presented elsewhere (Wennberg & Milnes in Main rock units prep.; Wennberg et al. in prep.). In addition, more regional work has been carried out along the northern In southwestern Norway, the Caledonides consist of a segment of the BASZ. sequence of tectonostratigraphic units, which from the lowest to highest levels have been designated Au tochthonjParautochthon, Lower Allochthon, Middle Al Footwall lochthon and Upper Allochthon (Roberts & Gee 19 85). The AutochthonjParautochthon consists of Precambrian The AutochthonjParautochthon in the footwall of the basement rocks of the Baltic Shield, in addition to BASZ (Fig. 2) is represented by the Western Gneiss Paleozoic cover sediments preserved locally above a Complex (WGC). North of the investigated area, along stratigraphic unconformity. The Lower and Middle Al- Sognefjorden, Precambrian intrusive and metamorphic 78 (1998) Bergen Are Shear Zone, western Norway 171
NORSK GEOLOGISK TIDSSKRIFT Plunge of lineation
0-5°
- 5-15° � - 15-25°
25-35° ._, >35°
._...
10km
LEGEND
liiillI l Devonian conglomerates
c:=J Lindås. Complex
Major Bergen Are
- Kvalsida Gneiss zone X:X:•: Western Gneiss Region
+ + Ø l + + l ygarden Gneiss Complex
--- Bjørillen syntorm
----- Base Devonian unconform ity
...L- ....&.. Basal thrust zone of the Caledonian allochthon Fig. 3. Tectonic map of northern segment of the BASZ. Arrows represent trend of stretching lineation (from Bøe 1978 and Henriksen 1979, in addition to own data). A1ong most of its 1ength, the northern BASZ is under water. It is best exposed to the northwest (profile A-A'; see Fig. 4A) and to the southeast (profi1e B-B'; see Fig. 48).
relations are preserved in the east, and these are increas lineation under amphibolite facies conditions (Milnes et ing1y reworked and overprinted westwards by Ca1edo al. 1988; Andersen & Jamtveit 1990). nian po1yphase deformation and high-grade meta South of Sognefjord (Fig. 1), the WGC forms the morphism (Milnes et al. 1988). The earliest of these footwall of the BASZ and is composed of granitic, phases is marked by the formation of eclogites which migmatitic and banded gneisses with local quartzites and give Sm/Nd ages in the range of 407-425 Ma (e.g. amphibolites. The gneisses are commonly intruded by Griffin & Bruekner 1985; Mørk & Mearns 1986; Jamtveit granitic and pegmatitic veins, which are correlated with et al. 1991). Exhumation of the eclogite-bearing WGC the youngest Precambrian intrusive activity in the Sogne was accompanied by deformation and metamorphism fjorden and Eksingedalen areas (isotopic intrusion ages related to three processes: ductile flow in the lower crust, of 900-1000 Ma; Gray 1978; Milnes et al. 1988). The localized extensional faulting (e.g. along the Nordfjord degree of Caledonian deformation (i.e. ductile deforma Sogn Detachment Zone, NSDZ), and erosion (e.g. An tion which post-dates the veins and pre-dates the BASZ) dersen & Jamtveit 1990; Osmundsen & Andersen 1994; decreases southeastward in the footwall exposing increas Milnes et al. 1997). The first part of the decompression ingly higher levels. The deformation is characterized by of the high pressure rocks was associated with a slight the development of a foliation and a prominent stretch increase in temperature (Jamtveit 1987; Chauvet et al. ing lineation (Fig. 3). At A-A', a pervasive Caledonian 1992), and the development of a pervasive foliation and foliation is developed, and towards the southeast 172 O. P. Wennberg et al. 78 (1998)
Homogeneous NORSK GEOLOGISKMigmatites TIDSSKRIFT and banded granite foliated gneisses cut by\ Banded gneisses Massive Banded gneisses Eclogite pegmatitic dyke CS-mylonites with deformed with deformed No data and mylonites pegmatites pegmatit(!s / l x � � �
X X X
X X X Zone of new folalion and Zone of reorientation Caledonian foliation Precambrian foliation stretching lineation of Galedonian folialion and stretching lineation Caledonian stretching lineation Fensfjord Earlier fabric obliterated and stretching Anastomosing Caledonian Fault lineation shear zones BASZ
CS-protomylonite Augen gneiss Breccia and fault gauge Anorthosite · Quartzite Migmatitic and Amphibolite Heterogeneous banded gneisses cut Heterogeneous by pegmatitic dykes gneisses "- Mela Augen � gneiss I Sea level X X
Precambrian foliation Caledonian folialion /...,.f------BASZ ------l�/ Caledonian stretcl:ling lineation and/or stretching lineation Strongly reworking and obliteration Locally Caledonian shear zones of earlier fabric New folialion and strechlineation
Fig. 4. Profiles across the BASZ (see Fig. 3 for location). Profile B-B' is part of the regional profile in Fig. 2. LC =Lindås Complex; MaBA =Major Bergen Are zone; 1km KG= Kvalsida Gneiss; WGC=Western Gneiss Complex.
this becomes localized in narrow zones. Overall, the printing similar to the WGC. Its position below the stretching lineation trends E-W with a variable low Caledonian allochthon of the Bergen area, and the struc angle of plunge ( � 15°, Fig. SA). At A-A' (Fig. 4), the tural and lithological similarities, suggest a direct correla Caledonian foliation is folded by open upright folds with tion (Fig. 2). sub-horizontal axes trending E-W and parallel to the The allochthon in the hanging wall comprises primar stretching lineation (Winsvold 1996). Eclogites occur in ily the Lindås Complex and the Minor and Major the footwall of the BASZ only in the northwest at A-A' Bergen Are zones. The Lindås Complex consists of in the core of two less deformed lenses (Winsvold 1996). heterogeneously overprinted Precambrian granulites, To the southeast of the present area, the BASZ trun anorthosites and mafic intrusives. The Precambrian cates the lowermost Caledonian Nappes represented by (Grenvillian) history involved magmatic activity, defor the Bergsdalen Nappes of the Lower Allochthon, which mation and granulite facies metamorphism (Austrheim & experienced lower amphibolite facies Caledonian meta Griffin 1985). This was followed by complex and morphism (Fossen 1993a). Farther east in the footwall is polyphase Caledonian deformation and metamorphism. the Jotun Nappe of the Middle Allochthon (Figs. l, 2). Eclogite facies metamorphism occurred at an early stage and was controlled by shear deformation and fluid mi gration (e.g. Austrheim 1987) with mineral assemblages Hanging wall stable at 700°C at > 18 kbar, corresponding to a crustal The hanging wall of the BASZ comprises mainly of the depth of >55 km (e.g. Jamtveit et al. 1990). U-Ph Autochthon/Parautochthon, Upper Allochthon and De isotopic data of sphene and epidote indicate that the vonian deposits. The Middle Allochthon is apparently eclogites were formed at about 460 Ma (Boundy et al. missing, and the Lower Allochthon is only represented 1997). 40Arj39Ar data of hornblende and muscovite from by thin remnants in the thrust zone at the base of the the eclogites give plateau ages of 450-430 Ma and :::::;430 overlying allochthonous units (Fig. 2). The Autochthon/ Ma, interpreted to represent the time of cooling through Parautochthon is represented by the Øygarden Gneiss :::::;500°C and 325°C respectively (Boundy et al. 1996). Complex, a mass of Precambrian migmatitic and granitic These are significantly older than 40 Ar/39 Ar ages from the rocks with strong but heterogeneous Caledonian over- WGC, which range from 390 to 410 Ma 78 (1998) Bergen Are Shear Zone, western Norway 173
NORSKa) GEOLOGISK TIDSSKRIFT b) c) stretchlng llneatlon (L2) follatlon bedding WGC - footwall BASZ Bjørlllen synform Devonlan strata hanging wall BASZ (Fig.2) hanglng wall BASZ (A-A')
.+ • .
. . · .. � .· ...... o o.o-8.0% .
[IT3B.Q-16.0%
mean269/02 N=l61 • m 16.0-24.0% • Pole to foliatlon - Lindås Complex, Pole to bedding - Devonian deposits, N = 109 N=38 . >24.0% pole to bestfit great circle 150/02 (fold axis of regional synform)
Fig. 5. Stereograms of structural data from the vicinity of BASZ (area0 of Fig. 3). A. Stretching lineations from the WGC in the footwall to BASZ. B. Poles to foliation from the Lindås Complex along the line of section on Fig. 2 (hanging wall). C. P o les to bedding in the Devonian deposits (hanging wall). Equal area, lower hemisphere projections.
(e.g. Chauvet & Dallmeyer 1992; Boundy et al. 1996), high pressure metamorphism and exhumation prior to and also Sm/Nd ages from the eclogites in the WGC of the continent-continent collision (Boundy et al. 1997). 407-425 Ma (see above). Further, the formation and The main Caledonian foliation and lineation in the exhumation of the Lindås Complex eclogites must have Lindås Complex is at amphibolite facies. It post-dates occurred prior to the juxtaposition with the Major and the eclogite formation and may be related to exhumation Minor Bergen Are zones, which contain fossils of of the Lindås Complex and/or thrusting during the Cale Ashgillian to Llandoverian age, and which show no donian collisional contraction. The main foliation and evidence of such high pressure metamorphism. Thus, the lineation are folded in a SE-trending, upright synform: Lindås Complex experienced eclogite facies metamor the Børillen synform (Borthen 1995). The Børillen Syn phism prior to the WGC (Wennberg & Milnes 1994; form curves from a trendjplunge of 135/15° in the NE Boundy et al. 1996). The eclogites of the Lindås Complex (Borthen 1995) to 150/2 at B-B' (Fig. 3, 5B). Extrapolat therefore belong to a group of earlier eclogites situated in ing the axial trace of the synform northwestwards, it the higher tectonostratigraphic levels (e.g. Mørk et al. seems to be cut off by the BASZ and the base Devonian 1988), which were formed and exhumed within Iapetus unconformity, but critical evidence is not exposed. or at its boundaries during pre-collision contraction (cf. The Major Bergen Are zone (MaBA) and part of the Andreasson & Albrecht 1995). Minor Bergen Are zone (MiBA) contain rocks of oceanic Based on lithological similarities, the Lindås Complex affinity. The MaBA can be broadly divided into three has traditionally been correlated with the Jotun Complex units: (l) basic intrusives and extrusives interpreted as a (e.g. Goldschmidt 1916), and consequently has been re dismembered ophiolite fragment, the Gullfjellet Ophiolite garded as part of the Middle Allochthon. However, the Complex (Furnes et al. 1980; Thon 1985) of early Or Lindås Complex is different from the Jotun Complex in dovician age (Dunning & Pedersen 1988); (2) an associ several ways. The former contains evidence of early ated zone of imbricated metasediments (Samnanger Caledonian high-pressure metamorphism, a higher de Complex; Færseth et al. 1977); and (3) conglomerates, gree of internal deformation developed in the retrograde limestones and phyllites of Ashgillian to Llandoverian amphibolite facies foliation, and the occurrence of Cale age (Holdhus and Ulven Groups; Færseth et al. 1977; donian trondhjemitic to granodioritic intrusions. Fur Ryan & Skevington 1976), unconformably overlying the ther, the Lindås Complex Iies above the ophiolitic Major other two. and Minor Bergen Arcs (Fig. 2). This suggests that the In the present area the thickness of the MaBA is Lindås Complex has a higher position in the tectono reduced to less than l km largely due to thinning stratigraphy than the Jotun Complex, and may be corre through the BASZ (Fig. 3), and it is not possible to fit lated with the Seve Nappes in the Upper Allochthon, as the strongly sheared rocks into the general scheme out suggested by Austrheim & Griffin (1985). The Lindås lined above. Here, we subdivide it into an upper part Complex may thus be a remnant of a continental frag (Totland unit) in the hanging wall, and a lower part ment from within Iapetus or the outermost part of the (Dyrsvika unit) which is incorporated into the BASZ Baltic margin, which have experienced subduction with (profile B-B', Fig. 4). 174 O. P. Wennberg et al. 78 (1998)
The Totland unit consists mostly of a garnet-amphi locally, as augen gneisses. RemainsNORSK GEOLOGISK of Precambrian TIDSSKR!Ff peg bole-mica-schist with amphiboles often in characteristic matites occur as coarse-grained lenses parallel to the bow-tie arrangements. Temperature estimates from gar foliation. The mylonites were formed under medium- to net-biotite pairs give temperatures in the range of 550- low-grade metamorphic conditions. 6500C (Wennberg et al. in prep.), which is higher than The Lower Allochthon is interpreted to be represented most earlier works from the MaBA have estimated (max by several (1-6) thin slices of garnet-mica-schist which imum lower amphibolite facies grade; see e.g. Færseth et occur within the WGC close to the contact of the al. 1977). This high metamorphism is restricted to a zone Kvalsida Gneiss (Fig. 4). The garnet-mica-schists are of approximately 300 m thickness below the Lindås often associated with zones of quartzite, and they lie Comp1ex. Fossen (1988) has indicated temperatures in parallel to the mylonitic foliation and wedge out along this same high range further south in the Major Bergen strike. The schists contain mineral assemblages of amphi Are zone, also close to the contact of the Lindås Com bo1ite facies, but are commonly strongly retrogressed. p1ex. This, in addition to the fact that the lowest T-esti Normally, the thickness of the schists is less than 30 m, mate was obtained furthest from the Lindås Complex, is but the maximum thickness is approximately 100 m at taken as indication for an inverse metamorphic gradient profile B-B' (Henriksen 1979; Wennberg & Milnes in the MaBA (Wennberg et al. in prep.). Kinematic 1994). The thickness of the associated quartzite reaches indicators in the Totland unit point to sinistral non-coax its maximum of 150 m NE of B-B'. The associations of ial deformation (top-to-SE), and are interpreted to be garnet-mica-schist ( ± quartzite) we interpret as rem remnants from SE-directed thrusting of the Lindås Com nants of the Lower Allochthon that have been sheared, p1ex on top of the MaBA (Wennberg & Milnes 1994; disrupted and folded together with the WGC during the Wennberg 1996). movement on the BASZ, and possibly also earlier during The western extension of the basal thrust zone of the the Caledonian collisional contraction (Wennberg & Mil Caledonian allochthon (i.e. Lower Allochthon) is not nes in prep.). exposed in the present cross-section. Further south, the The Midd1e Allochthon within the BASZ is repre basal thrust is represented by the strong1y sheared units sented by the K valsida Gneiss, which is a heterogeneous of the Minor Bergen Are zone (MiBA; see Fossen 1989). unit consisting of banded gneisses with granu1ite facies This zone consists of an upper part which represents relics, amphibolites, metagabbros, granitic gneisses, sheared equivalents of the MaBA mafic rocks and a meta-anorthosites and quartzites (Henriksen 1979). The lower part consisting of phyllites, quartzites and granitic Kvalsida Gneiss, which is up to 2.5 km thick, shows gneisses, all strong1y my1onitized. We interpret the latter areas of both strongly foliated mylonitic rocks and lenses as sheared equiva1ents of the Lower Allochthon, which is of undeformed protolithic rocks, and there are transi much hetter preserved in the BASZ footwall. tions between these two end members. As within the Severa1 Devonian basins are preserved a1ong the west WGC, the mylonitization took place under medium- to coast of Norway in the hauging wall of the NSDZ (see low-grade conditions. The unit is tentatively correlated e.g. Norton 1987). Fish fossils indicate a Middle Devo with the Jotun Complex which, in this area, has been nian age of these strata, probably also Early Devonian in dragged into the BASZ and thinned (Fig. 2). the Solund Basin (Kiær 1918; Jarvik 1949). Coarse con Parts of the MaBA (Upper Allochthon) are incorpo glomerates similar to the Solund Devonian Basin further rated into the BASZ, at profile B-B' represented by the north are preserved in the outer parts of Fensfjorden Dyrsvika unit, which mostly consists of phyllites (Fig. 4). (Figs. 3, 4; Kolderup 1927). These are the southernmost Observations of shear sense indicators and stretching exposures of Devonia.n strata in western Norway, and lineations in the outer part of Fensfjorden indicates that may represent a southerly continuation of the Solund the BASZ a1so may cut into the Lindås Complex. How Basin (Steel et al. 1985). Locally, bedding is defined by ever, due to lack of continuous exposure across the lenses of finer material or as a1ternating coarse sand and BASZ here, these exposures may represent distinct, nar conglomerate 1ayers. The bedding is typically oriented row dextral shear zones in the hanging wall, a feature with a shallow (10-20°) northerly dip (Fig. SC). also observed in the same unit along profile B-B'.
Bergen Are Shear Zone Structures The AutochthonjParautochthon, in addition to slices The northern segment of the BASZ has a significant belonging to the different levels in the allochthon, are topographic expression where it follows the more than incorporated into the BASZ and strongly sheared during 400 m-deep Fensfjord and much of it is unexposed (Fig. the dextral movements (Fig. 2). The Autochthon/Pa 3). The BASZ has a strike of 115-120° in the northwest rautochthon is represented by the WGC, of which the at A-A', and southeastward it curves over a distance of protolith in the footwall is described above. Within the 50 km to a strike of 150-160° at B-B' (Fig. 3). The BASZ, the WGC typically occurs as quartz/fe1dspar-rich shear zone has a moderate to steep SSW to WSW dip, mylonites with scattered feldspar porphyroclasts and, but the angle of dip is poorly constrained (see below). 78 (1998) Bergen Are Shear Zone, western Norway 175
NORSK GEOLOGISK TIDSSKR!Ff Strike of Strike of foliation BASZ b �
\
c d e
Fig. 6. Diagram of internal structural features in the BASZ. A. Mylonitic foliation oblique to shear zone margins. B. Shear bands (SB) with associated curved/inclined foliation (SI), and mylonitic foliation (SM). C. CS-fabric. D. Asymmetric folds in quartz lenses. E. Block diagram of folds with stretching lineation parallel to fold axis.
Foliation and lineation foliation strikes 140°, whereas the strike of BASZ is 150-160° (Figs. 2, 7). This oblique relationship between The BASZ is characterized by planar dominated fabrics the foliation and the shear zone suggests dextral shear (e.g. S-tectonites) and composite fabric types (e.g. LS (Ramsay 1980), although a sigmoidal shape of the folia tectonites), whereas linear dominated fabrics (L-tec tion is not detectable. The foliation normally dips 40- tonites) are less frequent. Protoliths, i.e. rocks preserved 600 S to SW, with a mean of 48° at A-A' and 54° at from the effects of dextral shearing, are also observed B-B' (Fig. 7). The spread of foliation poles is a response within the BASZ. The protoliths are typically bordered to folding (see below) and to defl.ection around pro by L-tectonites and probably occur as lenses enveloped tolithic lenses, but the pole to the mean orientation plots by the more foliated rocks. el ose to the maximum concentration. At B-B' the di p of The BASZ contains a mylonitic foliation defined by the foliation shows a tendency to decrease SW, i.e. preferred orientation of mineral grains and aggregates structurally upwards through BASZ (Fig. 7C). parallel to a compositional layering, with bands varying Within the BASZ a stretching Jineation is well devel in thickness from millimetres to metres. At the transi oped in the crystalline units, the K valsida Gneiss and the tions from protolithic to mylonitic fabrics, the composi WGC, often developed as LS-tectonites. The MaBA has tional banding shows the effect of dextral shearing of more intense planar fabric development and poor protolithic heterogeneities (details in Wennberg & Milnes stretching lineations. The stretching lineation of the LS in prep.). The strike of the foliation within the BASZ is, tectonites Iies in the foliation plane, and there is no in general, oriented obliquely to the shear zone margins significant change in the orientation of the stretching (Figs. 6, 7A). At A-A', the mean foliation strikes 110° lineation from LS- to adjacent L-tectonites. The trend of and the BASZ margin 115-120°, and at B-B' the mean the stretching lineation changes orientation concomitant 176 O. P. Wennberg et al. 78 (1998)
NORSK GEOLOGISK TIDSSKRIFT foliatlon & shear bonds & folds lineation C-planes a)
� t + +
pole to follatlon. N = poletoshearband. N=6 fold axls, N98 = stretchlng llneatlon. N = 34 poletoC-plane. N=90 poletoaxlalplane, N=31
+ 65 A + b) • "' ' a:l ::i CC ; tA + �·�s � + • A + . . : + + . , . .,...... •· stretchlng llneatlons. N= 179 F1-2- fold axls. poletoSI. N=B6 ' F1-2. N = 46 shear band-SIInter.;ectlon. N = 77: pole to axlalplane. N =36 l • + + c) D O.Q-8.0% � 8.Q-16.Cffo 1116.0-24.0% • >24.0% Major Bergen Are zone (upper part of BASZ) Kvalslda Gnelss (mlddle) Western Gnelss Complex (lower) + 11. c 78 (1998) Bergen Are Shear Zone, western Norway 177 NORSKwith theGEOLOGISK curving TIDSSKRIFf BASZ and the change in strike of the a) foliation (Figs. 2, 7). The mean trend changes from ��t]�����$�������� 270° at A-A' to 296° at B-B'. The plunge varies be- tween 15° and 25°, and there are no marked differ ences along the strike of the BASZ. However, at B-B' the plunge decreases upwards through the BASZ and becomes sub-horizontal in the MaBA (Fig. 7C). Shear hands and CS-fabric Shear hands (banded type CjS fabric of Blenkinsop & Treloar 1995) are best developed in strongly foliated rocks, especially in the MaBA, but also in mica-rich and intensely foliated rocks in the Kvalsida Gneiss and WGC. The vast majority of shear hands (>99%) indi cate dextral sense of shear. The shear hands are devel oped commonly between zones containing a single planar fabric (Sm). Between the shear hands the folia tion is curved and inclined relative to the foliation in the zones of one planar fabric (Figs. 6B, 8A). This curvedjinclined foliation is of two types (Fig. 9): pene trative and composite (Wennberg 1996). The composite one contains two fabric elements: a discrete cleavage and a zonal crenulation cleavage, and is restricted to the uppermost zones of the BASZ. The composite foli ation is interpreted to be developed by dextral shear hands superimposed on earlier sinistral shear hands from the SE-directed thrusting (see Wennberg 1996 for details). The penetrative curvedjinclined foliation is the result of a higher degree of superimposed dextral shear on the earlier SE-directed thrust-related fabric, and is also developed in rock volumes that suffered only slight deformation during the earlier Caledonian con tractional phases. The dextral shear hands are sug gested to have developed at a late stage, post-dating the formation of the foliation (Wennberg & Milnes in prep.). The shear hands result in a late stage thinning of the compositional banding through the BASZ (Fig. 6B). Dextral sense of shear is also indicated by CS-fabrics (Figs. 6C, 8B), which are best developed in larger bod ies of homogeneous granitoids in the Kvalsida Gneiss and in the WGC. 'CS-fabric' is used for structures in mylonitized granitoid rocks, when the C-planes are most intensely developed, and the volumes between the C-planes are characterized by a spaced, diffuse and Fig. 8. Photos of common shear sense indicators from the northem BASZ all undulating S-foliation which envelops feldspar porphy indicating dextral non-coaxial shear. See Fig. 3 for location. A. shear bands. B. roclasts (porphyroclastic type CS-fabric of Blenkinsop CS-fabric. C. u-type asymmetric porphyroclast. D. asymmetric boudins. & Treloar 1995). This indicates that the CS-fabrics de more restricted (Fig. 7). At B-B' most shear hands veloped in relatively weakly deformed zones. Both strike between 145° and 180° and C-planes between shear hands and C-planes have a rather wide range of 135° and 160°. The mean strike of shear hands and orientation. Poles to shear hands show the !argest dis C-planes is dose to the strike of the shear zone (Fig. persion, and poles to C-planes plot within the same 7). The dip of the shear hands at B-B' is normally area of the stereogram as the shear band poles, but are between 40° and 80° south to south- Fig. 7. Structural data from the Bergen Are Shear Zone. A. Profile A-A'. B. Profile B-B'. C. Stereogram of mean structural data from the BASZ at B-B'. Great circle represents the assumed orientation of BASZ. Arrows indicate systematic changes in orientation of mean values upwards in the BASZ. MSL = stretching lineation; SB/SI = shear band/SI intersection; SM = mylonitic foliation; SB = shear band; C= C-planes. Equal area, lower hemisphere projections. 178 O. P. Wennberg et al. 78 (1998) of 113/52° SW. This is approximatelyNORSK GEOLOGISK 20° TIDSSKRIFTinclined to the BASZ up per margin (B-B ' ) mean foliation and 40° to the mean shear hands. The Quartz systematic change in orientation of the stretching lin Fabrics veins eation at B-B' is associated with a change in the intersec tion between curved/inclined foliation and the shear hands (Fig. 7C). Fo/ds Two sets of folds (F1_1 and F 1_2) developed within the BASZ as a response to the dextral shear. We found no criteria which can establish the relative time sequence between these two sets, mainly because the two sets developed in different units. F1_1 is characterized by fold axes parallel to the stretching lineation (Figs. 60, 7B). F1_1 folds the mylonitic foliation and the compositional o layering, and individual structures are open to isoclinal, (J) which suggests that they were formed with the hinges .s:::. sub-parallel to the stretching lineation. F1_1 occurs be tween areas of unfolded foliation. The poles to the F 1_1 iCl) axial planes show a girdle distribution normal to the "O "O orientation of the fold axes, with most axial planes (J) dipping S to SW, i.e. approximately the same orientation lo B 0.. as the foliation. F 1_1 is best developed in the Kvalsida E -.:::: Gneiss and the WGC. In contrast, F 1_2 is mostly devel (J) 0.. oped in the Oyrsvika unit of the MaBA elose to the upper ::l margin of the BASZ as a set of asymmetric folds with a O> c dextral vergence (Fig. 6E). The orientation of the axial ·u; Cl)o planes of F 1_2 is constant, with a dip of 50-70° SSW, � () whereas the fold axes show a girdle distribution (Fig. 7B). .f; Most of these folds originated as asymmetric boudins and pinch-and-swell structures in quartz during an earlier phase of sinistral shear, and were subsequently folded due to superimposed dextral shear along the BASZ (Fig. 9; Wennberg 1996). A later set of folds (F2), which folds the stretching lineation, is developed at B-B'. F2 has fold axes trending sub-parallel to the BASZ with a shallow plunge SE. Complex fold geometries may be explained by inter Fig. 9. Progressive dextral superimposition on earlier sinistral shear bands and ference between F2 and the earlier folds. asymmetric boudins in quartz across the upper margin of the BASZ at B-B' (Fig. 4B). SB1 =initial sinistral shear band; SI1 =initial curvedjinclined foliation; SC2 =new composite cleavage; SIC2 =new composite curved/inclined foliation; Other features SB2 =new dextral shear band; SIP2 =new penetrative curvedjinclined foliation. Modified from Wennberg (1996). As previously shown, a dextral sense of shear is suggested by the orientation of the foliation, shear hands, CS-fabric westward. Here the shear bands generally steepen down and asymmetric folds (F1_2). Other shear sense indicators ward through BASZ from a di p of 50° in the MaBA to observed within the BASZ are tr-typeasymmetric porphy 60-70° in Kvalsida Gneiss and WGC (Fig. 7C). C-planes roclasts (Passchier & Simpson 1986; Fig. 8C), asymmetric from the Kvalsida Gneiss and the WGC� are even steeper, boudins (see e.g. Hanmer 1986; Fig. 80), microscopic with most dips in the range 70-85°. The variation in the oblique foliation (Simpson & Schmid 1983) and small orientation of shear hands and C-planes can, to some scale shear zones in the protolith lenses, which all show extent, be attributed to local variations in the flow plane the same dextra1 sense of shear. The who1e spectrum of and to syn- and post-shear folding. Local variations in the ductile to semi-ductile shear-sense indicators from early flowplane are caused by the undulating and anastomosing mylonitic (CS-mylonites) to syn-mylonitic (oblique folia geometry on a small scale, and to deflection around tion, asymmetric porphyroclasts) to late mylonitic (shear protolithes and L-tectonites on a larger scale. hands, asymmetric boudins) are thus developed under The curved/inclined foliation associated with the shear dextral non-coaxial flow. hands (here called SI, Fig. 6B) is measured in the relatively Reactivation of the BASZ in the brittle regime has also planar central part, and has at B-B' a mean orientation occurred. The most important late brittle fault is the 78 (1998) Bergen Are Shear Zone, western Norway 179 NORSKFensfjord GEOLOGISK fault TIDSSKRIFT (Fig. 4), which parallels the BASZ and displacements in the order of centimetres to metres. marks the contact between the Kvalsida Gneiss and the Farther into the BASZ, the earlier fabric is obliterated by WGC. This fault is mostly covered by scree or is under a new mylonitic fabric oriented with a dip of ca. 60° S. water, but good exposures of the fault at B-B' reveal a Remnants of the pegmatites are dragged parallel to the zone of brecciation, rebrecciation, intensive veining and mylonitic foliation, and shear sense indicators such as fault gouge development. Several other small-scale faults shear hands, CS-fabric and O"-type porphyroclasts are with a low angle to the Fensfjord fault are observable in developed, all consistently indicating dextral non-coaxial the vicinity. deformation. This obliteration of the earlier fabric takes place over a rather narrow zone of 10-20 m. Shear zone margins The upper margin of the BASZ can be studied in detail Discussion at B-B', where it is situated within the MaBA, and Flow marks the transition from the Totland unit in the hang ing wall to the Dyrsvika unit within the shear zone (see Although the northern segment of the BASZ comprises Wennberg 1996 for details). In the Tot1and unit, sinistral several units, it represents the transition from the Au kinematic indicators such as shear hands and asymmetric tochthonous/Parautochthonous WGC in the footwall to boudins in quartz are preserved. These are interpreted as the Caledonian Nappes in the hanging wall. The BASZ remnants from the SE-directed thrusting of the Lindås at B-B' is a well-defined shear zone with rather sharp Complex on tap of the MaBA. Across the margin, the boundaries and shear sense indicators that show domi sinistral shear bands and asymmetric boudins are nantly dextral movements. The NW-plunging stretching modified, overprinted, and finally ob1iterated by the later lineation has a high angle to shear band/SI-intersections, dextral shear associated with the BASZ (Fig. 9). In addition to this abrupt change in structura1 style, the a) margin of the BASZ is also marked by a lithological and metamorphic change from garnet-amphibole-mica schist of medium to high amphibolite facies in the hang ing wall, to green schistfacies phyllites within the shear zone. The change in metamorphic grade is partly a response to retrogression during the late dextral shear deformation within the shear zone. However, it is likely that the upper margin in this profile is controlled by the inverse metamorphic gradient described above. The highest temperature zones along the contact between the Lindås Complex and the MaBA appear to have survived the dextral shearing, while lower-grade rocks farther from the contact were strongly deformed and retro b) C) Sløvåg Sløvåg gressed during the superimposed dextral non-coaxial zone of reorientation zone of obllteration shear. The lower margin of the BASZ is characterized by two transitions best exemplified by the relations at A-A' (Fig. 4). First, the earlier fabrics and structures become reoriented, the Caledonian foliation attains a SW dip, and the stretching lineation and fold axes rotate to a W to WNW plunge of 15-25°. The second transition is the obliteration of all the earlier fabrics by a new mylonitic foliation and stretching lineation. At B-B' (Figs. 3, 4) the zone of reorientation is missing, and the shear zone margin is marked both by obliteration of earlier fabrics • stretchlngllneofton stretchlng llneolion stretchlngllneofton lnshearzones(Lm), pole tomylonltlicfoliatlon(Sm), and a change in orientation of foliation and lineation. A pole to dexfral shearzone, well-exposed profile from the zone of reorientation into (l2), N =21 o (lm), N= 14 o N=3 + N=17 Fig. JO. A. Transition from zone of reorientation to zone of obliteration near the zone of obliteration can be observed close to Sløvåg, + N= 6 S!Øvåg (see Fig. 3 for location). a) Block diagram l - Precambrian foliation, 2 - just southeast of profile A-A' (Fig. l 0). The fabric in the Precambrian dikes with Caledonian stretching lineation, 3 - mafic lenses, 4 - zone of reorientation shows the characteristics of Pre horizontal projection of stretching \ineation with trend, 5 - dextral shear zone, cambrian foliation with discordant pegmatites. A well 6 - dikes dragged into BASZ foliation and boudinaged, 7 - BASZ mylonitic foliation, 8 - BASZ stretching lineation. b) Stereogram of structural data from developed stretching lineation plunges ca. 20° W. the zone of reorientation, c) stereogram of structural data from the zone of Narrow dextral-sense shear zones are developed with obliteration. Equal area, lower hemisphere projections. 180 O. P. Wennberg et al. 78 (1998) in accordance with the shear direction inferred from the pattern, but the dip cannot NORSKbe observed GEOLOGISK directly TIDSSKRIFT and must shear sense indicators. However, the angle between these be inferred from the orientation of minor structures, i.e. two structural elements is ca. 70° and not perpendicular, foliation, CS-fabrics and shear hands. The shear direc possibly because of a change in shear direction with time tions below are determined relative to the present orienta (see below). Observations elsewhere along the investigated tion of the BASZ and do not take in to account an y later part of the BASZ are in accordance with this explanation. rotation or folding of the shear zone (for which, however, This suggests that in general the deformation within the there is no evidence). BASZ approximates plane strain simple shear (bulk sim At A-A' the trend/plunge of the shear direction is ple shear). However, it must be noted that deviations from determined to be 276/20° assuming a strike/dip of the bulk simple shear are to be expected due to the curvature shear zone of 118/55° SW. At B-B' the dip of the BASZ of the BASZ. This curvature results in a higher compo is poorly constrained, as there are considerable differences nent of flattening at A-A' than at B-B', which finds its in dip from C-planes (mean dip 76°) to shear hands (56°) expression in the decrease in the angle between the strike and foliation (54°). The shear plane is assumed to be of the mean foliation and the BASZ from A-A' to B-B'. oriented 155/60° SW which, with the mean stretching Although the deformation approximates simple shear lineation oriented 296/20°, corresponds to a movement on a larger scale, it is heterogeneous on a smaller scale direction of 313/33°. Further, the stretching lineation with local variations in flow pattern in time and space. systematically changes orientation upwards accompanied Most observations indicate non-coaxial dextral shear, but by a change in orientation of the shear band/SI intersec there are also preserved domains which have suffered tion (Fig. 7C). In the higher levels of BASZ (MaBA), the constrictional strain. These are expressed as L-tectonites shallower stretching lineations indicate a movement direc in addition to volumes where fold axes are paraBel to tion of 323/20°, whereas the lowest levels (WGC) have a stretching lineation. Domains of S-tectonites without movement direction of 311/38°. The change in shear kinematic indicators may also be an expression of local band/Sl intersection through BASZ is also consistent with flattening. No observations indicate that the flow may a more strike-slip-dominated shear direction at the higher have approached coaxial plane strain (pure shear). The levels. On the other hand, the angle between the stretching variations are largely a result of deflections around more lineation and the shear band/SI intersection is not perpen rigid protolith lenses, . which have resisted the dextral dicular, i.e. it is approximately 70°, indicating a shallower shearing, and where earlier fabrics are preserved. These shear direction than that inferred from the stretching heterogeneities are considered a result of strain partition lineations (see below). ing within a setting of dextral bulk simple shear deforma tion. Extension elsewhere in the Scandinavian Cale donides has also been related to coaxial plane strain (pure Displacement shear; Andersen & Jamtveit 1990) or general non-coaxial The displacement on the BASZ is best estimated at B-B', shear (Northrup 1996). These examples are from other which is a complete profile through the shear zone, and structural settings in the footwall to the NSDZ and in the where the geometries of the hanging wall and footwall are Caledonian Nappes respectively, and are not comparable well constrained. From profile construction, the throw on with the BASZ, which is a distinct, late shear zone. the BASZ is taken as l O km, a conservative estimate (Fig. 2). Applying the movement direction (313/33°) and shear plane (155/60°) from above results in a rough estimate of Shear direction 16 km displacement, with 14 km dextral strike slip and 8 km normal dip slip components. However, as shown A component of normal movement on the BASZ is above, the shear direction changes through the BASZ. In obvious from the juxtaposition of higher Caledonian the uppermost part of the shear zone (the MaBA lower Nappes in the hanging wall with the Autochthon/Pa part) the shear direction has a shallower orientation. This rautochthon in the footwall. The W- to NW-plunging may contribute to a higher strike slip component and a stretching lineation, in addition to the consistent shear larger total displacement. As a conclusion, the estimate of sense indicators, shows that the BASZ is an oblique shear 16 km displacement on the BASZ is considered a mini zone with a major component of dextral movement and mum. a minor normal component. More accurately, the shear direction may be constrained by the stretching lineation and the orientation of the shear zone or shear plane. The Evolution of the BASZ orientation of the stretching lineation cannot be used directly, since it is developed in the foliation which has an It has been shown above that there is a systematic change oblique strike to the BASZ. Therefore the orthogonal towards a shallower orientation of the shear direction projection of the stretching lineation into the inferred southwestward at B-B', i.e. structurally upwards through shear plane (BASZ) gives a hetter approximation of the BASZ, expressed as a systematic change in orientation of shear direction, as suggested by Lin & Williams (1992). linear features (stretching lineation, shear band /SI inter The strike of the BASZ is well constrained from the map section). This suggests that the most actively deforming NORSK GEOLOGISK TIDSSKRIFT 78 (1998) Bergen Are Shear Zone, western Norway 181 a) b) c) .. shear dlrection + stretching lineation c shear band/Sl intersection [ill] mlnor shear deformation Dl major shear deformation Fig. 11. Schematic development of BASZ at profile B-B'. a) stage l, b) stage 2 and c) stage 3. Numbers on stereogram refer to the different stages. See text for discussion. zone of BASZ becomes narrower with time (Type Il stretching lineation, the latter also with a shallower shear zone; Means 1991), and that the latest movements plunge than in Stage l. The stretching lineation from were concentrated in the upper levels (i.e. in the MaBA). the highest levels corresponds to the same shear direc This cou1d have been associated with a progressive tion, as inferred from the shear-band/SI intersection change in shear direction towards a more strike-slip in the lowest levels. dominated regime. Hence, the following scenario for the - Stage 3 The activity of the higher levels of the shear evolution of BASZ at B-B' is proposed (Fig. 11): zone decreased, and the latest shear hands in the - Stage l During this early stage, the foliation and MaBA were developed with an even shallower plunge lineation, CS-fabric and asymmetric porphyroclasts of the shear direction. were developed in a broad zone with an oblique shear The ductile to semi-ductile deformation was followed direction now preserved in the lowest part of the by brittle deformation along faults parallel to or at a low BASZ. angle to the foliation (of which the Fensfjord Fault is - Stage 2 During continued shearing, the lower parts most important). Kinematic data from these are sparse, gradually became less active, and dextral shear bands and the shear direction could not be determined. were developed at these levels refl.ecting a gradually Rotation of structures developed at an early stage in shallowing plunge of the shear direction. At the same the evolution of the BASZ is also likely to have occurred. time, the high rate of deformation continued at higher C-planes at B-B' have a significantly higher dip angle Ievels (MaBA) with formation of foliation and than the shear bands, and may have been developed at 182 O. P. Wennberg et al. NORSK GEOLOGISK TIDSSKRIFT 78 (1998) an early stage with a lower dip angle, and subsequently expected. These have not been confirmed, but a major rotated as shearing continued in other levels of the E-W tren ding depression in Fensfjorden south of the BASZ. Devonian deposits may be one representative. As described above, the BASZ has a marked curvature The present study has not revealed any observations associated with a change in strike of the my1onitic folia suggesting that an earlier structural grain in the WGC tion and in the trend of the stretching lineation. The controlled the orientation of this segment of the BASZ. hauging wall and footwall, in addition to the shear zone However, the initiation of shear in the investigated seg itself, are not likely to have behaved rigidly during the ment may have been controlled by heterogeneities in the dextral shearing, and some folding probably has resulted higher Caledonian Nappes, e.g. a SW thinning and in modification of the initial geometry of the BASZ. wedging out of the Jotun Nappe. However, folding of an initially sub-planar shear zone to the present geometry of the BASZ gives a steeply W plunging axis. The orientation of the pre-BASZ stretch Regional implications ing lineation in the immediate footwall (Figs. 3, 5A) is not what would be expected for folding with such an Following the strike of the BASZ northwestwards, it has axis. Although there is a considerable variation in the to truncate, to be truncated by, or to merge with the orientation, there is no systematic change along the Nordfjord-Sogn Detachment zone (NSDZ) between northern segment of the BASZ. Therefore, folding of an Fensfjorden and Sognefjorden (Fig. 1). Similarities be initial planar shear zone does not fit with the present tween the two shear zones suggest a similar time and observations, and the BASZ must have originated with a mode of origin, probably as part of the same detachment curved geometry. The inferred shear directions could system (Wennberg & Milnes 1994). Both are kilometre suggest an anticlockwise rotation of the hanging wall, thick ductile-to-brittle shear zones characterized by but this is not in accordance with data presented by shear-sense indicators and associated stretching lin Fossen (1992). He indicates sinistral movements along eations which show that the hauging wall moved to the the southern segment of the BASZ, although detailed W to NW relative to the footwall. Further, the hauging profiles through this segment are lacking. Therefore, the wall to both shear zones comprises Caledonian Nappes hanging wall was translated W to NW relative to the unconformably overlain by Devonian deposits, whereas footwall. This leads to space problems in the hauging the footwall consists of the same eclogite-bearing part of wall in the outer part of Fensfjorden. The angle between the WGC. the strikes of the mean mylonitic foliation and the shear The BASZ cuts through the lowermost Caledonian zone at A-A' (5-10°) is lower than at B-B' (9-21°), Nappes (the Bergsdalen Nappes), which shows lower probably refiecting a higher component of fiattening amphibolite facies Caledonian metamorphism. This is in strain in the western part due to the curvature (Fig. 12). accordance with the general trend of southward-decreas Also, accommodation structures in this area could be ing P-estimates in the footwall to the NSDZ from 25-30 kbar in the Nordfjord region (Jamtveit 1987; Smith & Lappin 1989) to 11-15 kbar in the outer part of Sogne fjorden (Bailey 1989; Chauvet et al. 1992). This shows that the Devonian extensional detachment system of western Norway is, in general, oblique to the isobars of the Caledonian high-pressure metamorphism in the WGC. Eclogites exist also in the hanging wall of the BASZ, but these are thought to be formed prior to the collision between Baltica and Laurentia (Boundy 1997; see discusson above). In general, the NSDZ is a low-angle normal shear zone (Norton 1987), although it appears to be folded on a large scale with E-W -trending subhorizontal axes with fold limbs dipping up to 50° (Chauvet & Seranne 1994; Hartz et al. 1994). Oblique transfer faults in the NSDZ have previously been indicated by Osmundsen (1996) based on asymmetry in sedimentary facies distribution, although structural data supporting such a model is still lacking in that part of the detachment system. The BASZ Devonian sediments in the hanging wall are preserved in the synclines of these major folds, and the Devonian Fig. 12. Trace of curved shear zone in map view. Shaded is the zone of higher strata are folded co-axially with the NSDZ. Kinematic component of fiattening strain. A. S. = accommodation structure, filled arrow = local shear direction, open arrow = general translation of hauging wall, a = angle indicators and stretching lineations suggest shear direc between strike of BASZ and strike of its iuternal foliation. tion parallel to the axis of the NSDZ folds, and the NORSK GEOLOGISK TIDSSKRIFT 78 (1998) Bergen Are Shear Zone, western Norway 183 folding is suggested to be synchronous with the exten In the present case, the BASZ crosscuts the Caledo sional movements on the NSDZ (Chauvet & Seranne nian Nappe pile and its basal thrust zone in the footwall, 1994; Osmundsen 1996). In contrast, the steeply-dipping and therefore post-dates the latest movements on the northern segment of the BASZ is suggested to have been tectonic contacts (Fig. 2). Further, overprinting relations initiated with a curved shape (see above), indicating a across the upper margin (Fig. 9) show that dextral major irregularity in the main detachment system. Fur movements on the BASZ post-date earlier sinistra1 top ther, the BASZ seems to cut the Caledonian Nappes with to-SE structures interpreted to represent SE-directed a high angle to the SE, and the shear direction on the Caledonian thrusting (Wennberg 1996). The BASZ also BASZ is oblique with a major dextral and a minor overprints the main Caledonian foliation and lineation normal component. This suggests that the studied seg (which is post-eclogitic) in the WGC by reorientation ment of the BASZ represents an oblique ramp in the and obliteration; it also reorients EW-trending upright Devonian detachment system of western Norway. folds in the footwall. The BASZ is therefore a late Minor open folds with E-W-trending sub-horizontal structure post-dating all structures related to Caledonian axes are also observed in the footwall of the BASZ at contraction in its vicinity. A-A', although it is of minor importance to the south, where most major folds trend parallel to the arcuate Acknowledgements. - We thank Haakon Fossen and Per Terje Osmundsen for helpful comments on an earlier draft of the manuscript. Careful reviews by E. outcrop pattern of the main rock units. However, at Eide, E. Rykkelid and M. Seranne are gratefully acknowledged. The field work A-A' these folds seem to be rotated into (i.e. pre-date) has been supported financially by Saga Petroleum and the University of Bergen. the BASZ (Winsvold 1996). Therefore the timing of J. Ellingsen and E. Lier are thanked for drafting the figures. shearing in BASZ and NSDZ, with respect to the EW Manuscript received February 1997 trending upright folds, is still a problematic feature. Although the BASZ is an important structure onshore in western Norway, it has not yet been clearly identified References offshore in deep reflection seismic profiles. Recently, 1990: Færseth et al. (1995) interpreted the NSDZ on a coast Andersen, T. B. & Jamtveit, B. Uplift of deep crust during orogenic extensional collapse: a model based on field studies in the Sogn-Sunnfjord parallel seismic profile as continuing westward from its region of west Norway. Tectonics 9, l 097-1112. southernmost exposures in Solund. This interpretation Andersen, T. B. , Skjerlie, K. P. & Furnes, H. 1990: The Sunnfjord Melange, evidence of Silurian ophiolite accretion in the west Norwegian Caledonides. would support the idea that the BASZ is a southward Journal of the Geological Society 147, 59-68. branch of the NSDZ rather than its direct continuation. Andersen, T. B. 1993: The role of extensional tectonics in the Caledonides of A culmination in upper and middle crustal levels south south Norway: discussion. Journal of Structural Geology 15, 1379-1980. Andreasson, P. G. & Albrecht, L. 1995: Derivation of 500 Ma eclogites from the of the offshore image of the NSDZ may represent the passive margin of Baltica and a note on the tectonometamorphic heterogeneity branch line. A further indication is that the BASZ seems of eclogite-bearing crust. Geological Magazine 132, 729-738. to have smaller displacement (> 16 km) than the NSDZ Austrheim, H. 1987: Eclogitization of lower crustal granulites by fluid migrations through shear zones. Earth and Planetary Science Letters 81, 221-232. (:::::50 km; Milnes et al. 1997). Austrheim, H. & Griffin, W. L. 1985: Shear deformation and eclogite formation Two different views of the timing of the extensional within granulite-facies anorthosites of the Bergen Arcs, western Norway. deformation in the southern Norwegian Caledonides Chemical Geology 50, 267-281. Bailey, D. E. 1989: Metamorphic evolution of the crust of southwesternNorway: have been proposed. Firstly, Andersen and co-workers an example from Sognefjord. Unpublished Ph. D. thesis, University of Oxford, (e.g. Andersen & Jamtveit 1990; Andersen 1993) argue 192 pp. 1995: that the extension in the hinterland (i.e. on the west Blenkinsop, T. G. & Treloar, P. J. Geometry, classification and kinematics of S-C and S-C' fabrics in the Mushandike area, Zimbabwe. Journal of coast) was late-Caledonian, coeval with thrusting in the Structural Geology 17, 397-408. eastern foreland, and that the extension was caused by Borthen, S. 1995: Børillen synform in the Lindås Complex, western Norway - petrography, structural geology and regional significance. Unpublished Cand. gravitational instabilities due to detachment and sinking Scient thesis, University of Bergen, 143 pp. of mantle lithosphere below the orogenically-thickened Boundy, T. M. , Essene, E. J., Hall, C. M. , Austrheim, H. & Halliday, A. N. 1996: Rapid exhumation of lower crust during continent-continent collision and late continental crust. Coeval thrusting and extension in 39 extension: evidence from 40Ar/ Ar incremental of hornblendes and muscov other places in the Scandinavian Caledonides have been ites, Caledonian Orogen, western Norway. Geological Society of Amererica suggested by Gee et al. (1994) and Northrup (1996). On Bulletin 108, 1425-1437. the other hand, Fossen (1992, 1993b) demonstrated that Boundy, T. M., Mexger, K. & Essene, E. J. 1997: Tempora! and tectonic evolution of the granulite-eclogite association from the Bergen Arcs, western top-to-W or NW extension post-dates SE-directed Norway. Lithos 39, 159-178. thrusting on the basal thrust zone below the Caledonian Chauvet, A. & Dallmeyer, R. D. 1992: Ar/Ar mineral dates related to Devonian extension in the southwestern Scandinavian Caledonides. Tectonophysics 210, allochthons, suggesting that the main extension was due 155-177. to divergence between Baltica and Laurentia caused by Chauvet, A. & Seranne, M. 1994: Extension-parallel falding in the Scandinavian external forces (e.g. plate reorganization). The latter view Caledonides: implications for late-orogenic processes. Tectonophysics 238, 31- 54. has been supported by Seranne et al. (1991), Wilks & Chauvet, A. , Kienast, J. R. , Pinardon, J. L. & Brune!, M. 1992: Petrological Cuthbert (1994), Milnes et al. (1997) and Rey et al. constraints and PT path of Devonian collapse tectonics within the Scandian (1997). However, correlation between biostratigraphy Mountain Belt. Journal of Geological Society of London 149, 383-400. Dunning, G. R. & Pedersen, R. 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