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
Most previous seismic work in the study area
Fig. 2. a Aeromagnetic anomaly map east of Lake Va¨nern, white rectangle shows location of Fig. 2b. b Geological map of the study area based on Wahlgren, 1993 and location of the seismic profile. B, Bjo¨rneborg; and D, Degerfors.
Fig. 2.
has concentrated on determining the large-scale crustal structure. The eastern limit of the wide
angle reflectionrefraction EUGENO-S profile 3 EUGENO-S Working Group, 1988 is located
immediately west of the Sveconorwegian front, north of the study area. Upper crustal velocities
along the eastern end of the profile range from 6.1 to 6.5 kms. The profile did not continue far
enough to the east to allow the lower crustal velocity structure and depth to Moho to be deter-
mined in the area east of the PZ fault. Just west of this fault, the crustal thickness was estimated to c.
40 km and disruptions in the Moho were inferred farther to the west. The EUGENO-S Working
Group 1988 interpreted both the frontal defor- mation zone and the MZ to be west-dipping
features at depth. These structures were also in- ferred to penetrate the entire crust down to the
Moho
and to
possibly disrupt
the Moho
boundary. In conjunction with the large-scale refraction
profiles of the EUGENO-S project, airgun profi- les were shot on Lake Va¨nern to investigate the
MZ. The airgun shots were recorded by numerous refraction stations south of the lake and on two
refraction stations east and west of the lake Green et al., 1988b. The airgun shots were also
recorded on the Va¨rmlandsna¨s peninsula by a 7 km long multi-channel reflection system Juhlin et
al., 1989.
The refraction station data Green et al., 1988b provided more detailed information on the veloc-
ity structure, dips of interfaces and depth to Moho than the EUGENO-S profile 3. Green et al.
1988b interpreted the Moho to dip about 5° to the east below Lake Va¨nern, reaching a depth of
about 43 km below the eastern part of the lake. Several crustal wide-angle reflections were also
observed. These were interpreted to originate from c. 100 m thick layers with either a positive or
negative velocity contrast of about 0.4 kms and dipping about 10° to the west. The reflection
seismic images from the Va¨rmlandsna¨s survey Juhlin et al., 1989 are consistent with the inter-
pretation of Green et al. 1988b, where reflections from the upper crust are generally sub-horizontal
or dip to the west whereas lower crustal reflectors dip to the east.
About 70 km north of the study area, a 100 km long deep seismic profile was acquired in three
stages in the mid to late eighties Dahl-Jensen et al., 1991. Reflectivity is weak along most of the
profile. However, between the MZ and the PZ fault, strong sub-horizontal to gently east-dipping
reflectivity is observed in the upper 2 s TWT 6 km. Farther east, the reflectivity weakens, but
has a west-dipping pattern. Stephens et al. 1996 used this change in the dip pattern along the
profile as support for the existence of the fan-like structure mentioned earlier.
Earthquake studies using arrival times from local events in the Lake Va¨nern area Slunga,
1985 also indicate that the Moho dips to the east. On the basis of these earthquake studies, the
Moho depth is estimated to be 44 km below the SFDZ.
4. Data acquisition
