Fig. 3. Raw a; and processed b shot records from the western part of the profile. Processing up to step 12 in Table
2 is shown here.
structural geometry for the reflecting elements with moderate to steep west-dipping reflectors in
the east and more gently east-dipping reflectors in the west. The maximum dip of imaged reflections
is approximately 60 – 70°. Superimposed on this geometry are higher amplitude, sub-horizontal to
moderately dipping reflections in the western part of the section in the depth interval 3 – 7 km
marked A in Figs. 6 and 7.
TIB rocks probably do not contain significantly strong enough lithological contrasts in an unde-
formed state to generate reflections. However, when sheared, lithological variations related to
formation of mylonites may have developed and these variations can include large enough con-
trasts in velocity andor density to generate reflec- tions Hurich et al., 1985; Ratcliffe et al., 1986;
McDonough and Fountain, 1988. Sheared doler- ites within the TIB rocks may also have generated
some of the reflectivity in the western part of the present profile. This interpretation was also sug-
gested by Dahl-Jensen et al. 1991 for the deep seismic profile to the north.
The higher amplitude reflections in the west at depths between 3 and 7 km A in Figs. 6 and 7
appear to transect the easterly-dipping lower am- plitude reflections. However, these reflections may
emanate from out-of-the-plane of the profile. Sev- eral show a diffractive nature on the time section
Fig. 5, indicating they may originate from the northern or southern ends of near-vertical dolerite
sheets situated south or north of the profile, respectively.
6
.
2
. Comparison with surface geology The bivergent structure of the lower amplitude
dipping reflections is in good agreement with sur- face structural information Fig. 8, with the ex-
ception that the hinge line where the dip changes from east to west is displaced c. 3 km to the west
on the seismic profile, relative to the surface geol- ogy. However, the number of observed surface
structures are limited in the region of the hinge axis and west-dipping structures have been ob-
served south of the profile at CDP 800. Surface dips are generally in the order of 15 – 50° west of
the hinge line and 60 – 80° east of it. More steeply
Fig. 4. Raw a; and processed b shot records from the eastern part of the profile. Processing up to step 12 in Table 2
is shown here.
used to pick the strongest reflections in the final stack Fig. 6. This line drawing of the stacked
section was then migrated at a velocity of 6000 ms and the resulting section Fig. 7 forms the
basis for the interpretation of the data.
6. Results
6
.
1
. Reflecti6ity patterns The migrated section Fig. 7 shows a bivergent
Fig. 5. Final stack with residual and trim statics. Reflection F is discussed in the text.
dipping structures east of the hinge line are also observed in the seismic data. The hinge line
defined from the seismic data is located about 8 km east of the PZ fault Fig. 8.
East of the major west-dipping reflectivity, a weak, steeply dipping reflection is observed which
is prominent at about 1.8 s at the eastern end of the profile and extends to 3 s at CDP 800 marked
F in Figs. 5 – 7. Upon migration, this reflection steepens and projects to the surface close to a
major deformation zone which strikes NNE through Degerfors marked D on Fig. 8. This
zone shows both strong ductile deformation, which belongs to the SFDZ system and dips
steeply westwards, and later brittle reactivation along a major fault. The fault has downthrown
Neoproterozoic sandstone on its western side. There remains some uncertainty concerning how
much the relatively weak, but distinctive reflection marked F in Figs. 5 – 7 is related to the older
SFDZ or younger fault deformation.
Wahlgren et al. 1994 suggested a two-phase Sveconorwegian deformation history east of the
MZ. Comparison of the geometry and location of the gently east-dipping reflectivity in the west with
surficial geological data suggests that it developed during the early phase of crustal thickening and
was later modified during the late Sveconorwegian deformation. A similar analysis suggests that the
west-dipping reflectivity along the eastern part of the seismic profile is essentially the result of the
later phase of Sveconorwegian deformation. Be- low this zone of reflections, there is significantly
less west-dipping reflectivity F in Fig. 5 being the most distinct, suggesting that the Sveconorwe-
gian deformation is less intense east of the present profile. However, this hypothesis can only be ver-
ified by continuing the profile eastwards.
Fig. 6. Automatic line drawing of final stack in Fig. 5. Reflections A and F are discussed in the text.
Fig. 7. Migrated line drawing of Fig. 6. Reflections A and F are discussed in the text. Note that the migrated section has
been extended towards the east so that the easternmost, west- dipping reflections are not removed.
profiles. Thus, the results east of the PZ fault in the present study show the same geometry as
the weak reflectivity observed in the upper crust along the deep seismic profile north of Lake
Va¨nern.
Dahl-Jensen et
al. 1991
interpreted the
reflections and zones of reflectivity between 4 and 8.5 s at distances between 30 and 50 km on
the deep seismic profile north of Lake Va¨nern Fig. 9 as major shear zones that may have
developed during the Sveconorwegian orogeny. This interpretation was based on the increased
reflectivity at these depths compared to the nearly transparent crust to the east. The present
study is consistent with the crust east of the SFDZ being undeformed and a limit to major
Sveconorwegian deformation may be roughly drawn based on the two studies Fig. 9. This
limit projects into west-dipping reflections on both the Va¨rmlandsna¨s deep seismic data and
the deep seismic profile north of Lake Va¨nern Figs. 1 and 9, and possibly soles into east-dip-
ping lower crustal reflectivity farther west at 10 km distance at about 10 s c. 35 km depth. The
west-dipping reflections along the eastern part of the deep seismic profile and event F in Fig. 5 in
this study indicate that the surface limit of ma- jor Sveconorwegian deformation lies to the east
of both profiles, in agreement with surface geo- logical observations.
7. Discussion
