its tectonic evolution. A short comparison with the results of seismic reflection studies and the
tectonic evolution in the Grenville orogen is also presented.
2. Geological setting
The Sveconorwegian orogen in south-western Sweden Fig. 1 is 600 km long and up to 200 km
wide. It continues westwards and northwards into the southern part of Norway and farther north is
buried beneath the Scandinavian Caledonides.
Flat-lying thrusts and major, retrogressive, duc- tile deformation zones with moderate to steep
dips, are conspicuous structural elements in the Sveconorwegian orogen. A complex system of
retrogressive deformation zones, the Sveconorwe-
Fig. 1. Simplified geological map of southern Sweden modified after Wahlgren et al., 1994. DBT, Go¨ta A 8 lv ZoneDalsland
Boundary Thrust; G, Go¨teborg; K, Karlstad; Ka, Karlskoga; Kr, Kristinehamn; MZ, Mylonite Zone; PZ, Protogine Zone; S, Stockholm; SFDZ, Sveconorwegian Frontal Deformation Zone; and V, Va¨rmlandsna¨s.
gian Frontal
Deformation Zone
or SFDZ
Wahlgren et al., 1994, forms the eastern boundary of the orogen and corresponds more
or less
to the
Sveconorwegian front
of Berthelsen 1980. The SFDZ is a tectonic anal-
ogy to the Grenville Front GF in North America Wynne-Edwards and Hasan, 1970;
Gower and Owen, 1984; Gower, 1985. South of Lake Va¨ttern Fig. 1, the frontal deformation
zone is equivalent to the so-called Protogine Zone PZ; Gorbatschev, 1980; see also Larson et
al., 1990, and marks a sharp tectono-metamor- phic break between the Sveconorwegian orogen
to the west and the Svecokarelian orogen to the east.
The Sveconorwegian orogen has been divided into three, major litho-tectonic units, the East-
ern, Median and Western segments Berthelsen, 1980; Larson et al., 1990; Park et al., 1991.
These are separated by ductile deformation zones with approximately N-S strike and are re-
ferred to as the Mylonite Zone MZ between the Eastern and Median segments, and the Go¨ta
A
8 lv ZoneDalsland Boundary Thrust DBT be- tween the Median and Western segments Fig.
1. The litho-tectonic units are dominated by more or less highly deformed and metamor-
phosed intrusive rocks with protolith ages in the time-range c. 1.85 – 1.55 Ga. In the north-eastern
frontal part of the orogen, both supracrustal and intrusive rocks as old as c. 1.9 Ga occur.
Pre-Sveconorwegian intrusive rocks with ages between 1.55 and 1.22 Ga, a prominent, low-
grade, supracrustal sequence, the Dal Group, with an age less than c. 1.22 Ga, and late- to
post-Sveconorwegian, c. 0.92 Ga granites are also conspicuous west of the MZ.
North and east of Lake Va¨nern, the Eastern Segment of the Sveconorwegian orogen is domi-
nated by intrusive rocks belonging to the c. 1.85 – 1.65 Ga Transscandinavian Igneous Belt
TIB. All age-generations of TIB intrusions have been recorded in the area. The older c.
1.85 – 1.78 Ga intrusions dominate in the east- ern, frontal part of the orogen, while U-Pb zir-
con and titanite ages demonstrate that the younger c. 1.71 – 1.66 Ga intrusions dominate
farther west So¨derlund et al., 1999. A signifi- cant lithological component in the TIB-domi-
nated area north of Lake Va¨nern is c. 1.57 and 1.47 Ga dolerite dykes Welin, 1994; Wahlgren
et al. 1996. The youngest rocks which are af- fected by ductile deformation are c. 1.00 – 0.90
Ga dolerite dykes.
During the c. 1.1 – 0.9 Ga Sveconorwegian orogeny, all these rocks were more or less
strongly deformed and metamorphosed. The sig- nificant contrast in trend between pre-Sveconor-
wegian Svecokarelian structures to the east and Sveconorwegian structures with a c. N-S strike
to the west is displayed in the magnetic anomaly map over the area east of Lake Va¨nern Fig.
2a. In the easternmost part of the orogen, the Sveconorwegian structural overprinting is spaced
to semi-penetrative in the east and the deforma- tion becomes more or less penetrative in charac-
ter farther west. The transition from penetrative to
semi-penetrative deformation
occurs near
Kristinehamn, directly east of the PZ Fig. 1 which, in this area, is a brittle fault zone
Wahlgren et al., 1994. In the easternmost part of the orogen, the Sveconorwegian deformation
gave rise to both new fabric development and reorientation of older Svecokarelian structures
into the Sveconorwegian c. N-S structural trend Fig. 2a. The syn-deformational metamorphic
grade increases progressively from low-grade in the east to medium- to high-grade in the west-
ern, penetratively deformed area, which is in ac- cordance with the east to west increase in bulk
strain.
Two phases of Sveconorwegian shear defor- mation have been identified in the area. In the
area immediately north of Lake Va¨nern, be- tween the MZ in the west and the SFDZ in the
east, the older shear foliation constitutes a strongly asymmetric fan-like structure in an
east-west cross-section Wahlgren et al., 1994; Stephens et al., 1996. In the penetratively de-
formed area between the MZ and Kristinehamn, the shear foliation is sub-horizontal. In the
Kristinehamn area, it strikes c. N-S and dips gently to moderately to the east, is vertical far-
ther to the east and dips steeply to the west in the eastern frontal part of the orogen. Dip-slip move-
ments predominate and a kinematic analysis based on several reliable kinematic indicators has
revealed a constant top-to-the-east sense of dis- placement across the entire fan-like structure.
Prominent zones which belong to the younger SFDZ strike NNE-NE, dip westwards or are sub-
vertical, and display dextral and reverse sense of shear Wahlgren et al., 1994.
40
Ar
39
Ar age determinations of hornblende in the penetratively deformed area north of Lake
Va¨nern have yielded ages in the range 1009 – 965 Ma, which has been inferred to be the minimum
age for the crustal thickening event during which the regional shear foliation was developed Page
et al., 1996a. U-Pb ages of c. 976 and 956 Ma for metamorphic titanites from the same area So¨der-
lund et al., 1999 confirm the Sveconorwegian age of the regional foliation. The preservation of ig-
neous titanites in the same rocks indicate that no major tectono-thermal event has affected the
Eastern Segment north of Lake Va¨nern from the time of TIB emplacement until the late Sveconor-
wegian orogenic overprinting.
40
Ar
39
Ar white mica ages in the time range 930 – 904 Ma are
associated with the later, inferred compressional displacements along the SFDZ Page et al.,
1996a.
3. Previous seismic studies
