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
7
.
1
. The structural model A comparison between the current structural
model for
the Kristinehamn-Karlskoga
area Wahlgren et al., 1994; Stephens et al., 1996
and the present seismic data Fig. 10 shows, in a general sense, good agreement, although there
are differences in detail. A number of factors may have caused these differences. Firstly, the
seismic profile is located south of the structural profile where the zone of Sveconorwegian defor-
mation east of the PZ fault is narrower. If the seismic profile had been located farther north,
the zones of west-dipping reflectivity would
6
.
3
. Comparison with the deep seismic profile north of Lake Va¨nern
The deep
seismic profile
north of
Lake Va¨nern Dahl-Jensen et al., 1991 lies about 70
km to the north of the present study area Fig. 1 and the SFDZ in this area lies approximately
50 km east of the PZ fault implying deforma- tion over a wider area. If the results from the
present profile are superimposed on the deep seismic profile north of Lake Va¨nern Fig. 9
there is good general agreement between the two profiles
even though
the resolution
of the
present study is much greater. The sub-horizon- tal to gently east-dipping reflectivity west of the
PZ fault becomes more steeply east-dipping closer to the fault on both profiles. Farther east,
west-dipping reflectivity is observed. On the deep seismic profile north of Lake Va¨nern, this
west-dipping reflectivity consists of a diffuse band with gentler dip than on the present higher
resolution profile. The gentler dip may be as a result of a broader deformation zone to the
north. Alternatively, the acquisition and process- ing of the deep seismic profile north of Lake
Va¨nern may not have provided sufficient resolu- tion to identify individual reflecting segments in
the diffuse band. However, the change in the dip direction of the reflectivity occurs at about
5 – 10 km to the east of the PZ fault on both
C .
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102 2000
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Fig. 8. Migrated line drawing Fig. 7 merged with surface geology Fig. 2.
C .
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al .
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147 Fig. 9. Migrated line drawing Fig. 7 superimposed on the deep seismic profile from Dahl-Jensen et al. 1991 and the V2 and V8 profiles from Juhlin et al. 1991.
The scales of the profiles are the same and the location of the PZ fault has been used to determine the relative position of the profiles.
probably have been spread out over a wider area. Secondly, the limit of Sveconorwegian
deformation appears to occur farther west on the seismic data than on the surface geology,
even after account is taken of the narrower belt of Sveconorwegian deformation in the area of
the
seismic profile.
However, only
a prolongation of the profile to the east by about
20 km can definitely reveal whether major deformation
zones with
a strong
seismic signature exist east of the ones observed along
the present profile. Finally, together with the deep seismic profile north of Lake Va¨nern, the
seismic data suggest that the eastern limit of the SFDZ dips at a shallower angle than that
indicated in the structural model of Stephens et al. 1996. Furthermore, the seismic data suggest
that the SFDZ extends to about 30 km depth below Lake Va¨nern where it soles into lower
crustal reflectivity Fig. 9. This zone of lower crustal reflectivity may have formed during the
Sveconorwegian orogeny and could represent the eastern
limit of
major Sveconorwegian
deformation at depth. This would explain the lack of any strong crustal reflectivity eastwards
within the undeformed TIB rocks. The results of the reflection seismic surveys
strongly support a two-stage development for the fan-like structure east and north-east of
Lake Va¨nern
Wahlgren et
al., 1994.
Geometrical constraints based on the present seismic profile exclude a single extensional event
Fig. 10a
for the
source of
the fan-like
structure observed
in the
Kristinehamn-Karlskoga area.
The expected
sub-surface geometry of an extensional event followed by later compression Fig. 10b and a
single compressional event Fig. 10c are similar to that of two-stage compression Fig. 10d.
However, both these models preclude a more gradual rollover from east- to west-dipping
structures
than the
two-stage compressional
model. The seismic data from this study show a rapid change in structural dip from east to west
over a few kilometres distance. Furthermore, the high density of both east- and west-dipping
reflectors along the seismic profile and the surface geology indicate that these reflections
represent a
mylonitic foliation
formed in
response to
compressional deformation.
Formation of mylonites in a compressional tectonic setting have been documented in a
seismic study in the Grenville orogen in Quebec Ji et al., 1997. Consequently, the present study
together with the deep seismic profile north of Lake
Va¨nern, support
the preferred
model interpretation of Wahlgren et al. Fig. 10d that
the fan-like structure is the result of two pulses of compressional, Sveconorwegian deformation.
The east-dipping deformation zones which display top-to-the-east sense of displacement in
the western part of the fan-like structure Fig. 10 are interpreted to display only an apparent
normal sense of shear. They are interpreted to have originated in a foreland-verging thrust
system, which is inferred to be the initial result of oblique collision and crustal shortening. At
this stage, the shortening was absorbed by thickening of the crust. The MZ overprinted this
thrusting and is suggested to have formed during sinistral transpression in connection with
escape-like tectonics Stephens et al., 1996. In the
frontal part
of the
orogen, the
older compressional structures were rotated clockwise
as a result of late Sveconorwegian compressional deformation
which resulted
in dextral
and reverse
movements along
the SFDZ.
This deformation marked the final expression of the
oblique collision.
7
.
2
. Reflecti6ity in TIB rocks — a S6econorwegian signature
Deep seismic reflection data acquired over un- deformed TIB rocks on land in Sweden primarily
show a transparent lower crust Dahl-Jensen et al., 1991; Juhojuntti and Juhlin, 1998. In these
areas, the Moho boundary has proven to be difficult to identify on the basis of the conven-
tional ’’die out’’ of reflectivity definition. This observation has been explained either by the ex-
tension of the seismically rather homogeneous TIB rocks to Moho depths or that during intru-
sion these rocks obliterated any existing fabric development in the lower crust Dahl-Jensen et
al., 1991; Juhojuntti and Juhlin, 1998. Lack of
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149 Fig. 10. Migrated line drawing Fig. 7 compared with the structural section of Stephens et al. 1996. Possible tectonic models from Wahlgren et al. 1994 are included
with a being the result of one extensional event; b extension followed by compression; c one compressional event; and d two compressional events.
signal penetration does not appear to be the reason for the lack of lower crustal reflectivity
Juhojuntti and Juhlin, 1998. Since U-Pb data So¨derlund et al., 1999 suggest that the rocks
east of the MZ in the western part of Fig. 9 were undeformed TIB rocks prior to the Sve-
conorwegian
orogeny, we
suggest that
the present crustal reflectivity in this area developed
in connection with this orogenic event. In the eastern part of the section, Sveconorwegian de-
formation was insignificant below the SFDZ.
7
.
3
. Comparison with the Gren6ille orogen
7
.
3
.
1
. Seismic studies Major
deep seismic
profiles across
the Grenville Front GF Green et al., 1988a; Pratt
et al., 1989; Culotta et al., 1990 display distinct, moderately east-dipping 30 – 35° reflection se-
quences that are associated with the GF. Corre- lation with geological information implies that
the base of this reflectivity defines the western limit of the GF in North America. The reflectiv-
ity has been interpreted to be as a result of mylonite zones which developed in a compres-
sive, ductile re´gime associated with the Grenvil- lian orogeny Green et al., 1988a. Late- to
post-orogenic collapse and extension are inferred Milkereit et al., 1992b; Martignole and Calvert,
1996 to have followed the Grenvillian orogeny and gave rise to extensional structures. The pri-
mary evidence for this extension is thinned crust where the GF reflection sequence projects into
the Moho Martignole and Calvert, 1996.
The composite seismic section across the East- ern Segment of the Sveconorwegian orogen
north and north-east of Lake Va¨nern Fig. 9 shows some similarities to the seismic images of
the Grenville orogen in North America Green et al., 1988a; Pratt et al., 1989; Martignole and
Calvert, 1996; Hynes and Eaton, 1999. Reflec- tors which dip moderately to steeply towards
the hinterland are conspicuous in the frontal parts of both orogens. Furthermore, this reflec-
tivity is apparently related to ductile deforma- tion
zones. Hynes
and Eaton
1999 also
interpret the GF in eastern Quebec to extend deep into the crust, giving it a similar geometry
to that of the SFDZ in Fig. 9. On the Western Quebec
Seismic Transect
Martignole and
Calvert, 1996, west-dipping foreland-dipping reflectors are observed between the Grenville
Front and the Allochthon Boundary Thrust. This defines a bivergent geometry similar to that
observed in the present study area. This biver- gent geometry is not observed farther south in
the Grenville orogen Green et al., 1988a. How- ever, in the Grenville basement beneath the
Palaeozoic cover in Ohio in the eastern US mid- continent, a pair of oppositely dipping crustal-
scale shear zones, one east-dipping and one west-dipping, have been imaged in a COCORP
deep seismic survey Culotta et al., 1990. The east-dipping shear zone is supposed to corre-
spond to the Grenville Front Tectonic Zone GFTZ, while the west-dipping zone is sug-
gested to mark an intra-Grenville suture.
Despite the occurrence of oppositely or vari- ably dipping structures in the more internal
parts of the Grenville and Sveconorwegian oro- gens, all profiles over or near the frontal parts
of these
orogens display
hinterland-dipping reflectivity. The latter is interpreted to have de-
veloped, at least initially, in a compressive duc- tile re´gime during thrusting over the stable
forelands.
7
.
3
.
2
. Tectonic e6olution The south-western part of the GFTZ has been
proposed to be a retro-shear which progrades into the stable foreland e.g. Haggart et al.,
1993; Jamieson et al., 1995; Reynolds et al., 1995 according to the geodynamic models for
convergent orogens of e.g. Willett et al. 1993 and Beaumont and Quinlan 1994. The retro-
shear developed in response to north-westerly transport of material from the hinterland of the
Grenville orogen towards the stable foreland. Rapid exhumation of high-grade metamorphic
rocks is suggested to have occurred in the hang- ing wall of the GFTZ see also Bethune, 1997.
A similar model has been presented for the frontal area in the north-eastern part of the
Grenville orogen Rivers et al., 1993.
An equivalent tectonic scenario may be re- sponsible for the development of the SFDZ in
the Sveconorwegian orogen north-east of Lake Va¨nern, i.e. the SFDZ may be an analogue to
the geodynamically modelled retro-shear. The reverse movements in the SFDZ may have ini-
tiated in the westernmost, west-dipping deforma- tion zones displayed in the seismic profile Fig.
7. The deformation then propagated into the stable Svecokarelian foreland, and is represented
by the easternmost deformation zones that over- print Svecokarelian structures and isotropic TIB
rocks.
The kinematics along the SFDZ Wahlgren et al., 1994 suggest that reverse displacements may
be responsible, at least in part, for the exhuma- tion of the medium- to high-grade rocks in the
western part of the fan-like structure, i.e. in the hanging wall of the SFDZ. Consequently, the
metamorphic and structural relationships in the frontal part of the Sveconorwegian orogen in
south-central Sweden support actualistic models, that reverse displacements thrusting in conver-
gent orogens are inferred to be an effective mechanism for exhumation of high-grade meta-
morphic rocks e.g. Burg et al., 1997; Thompson et al., 1997a,b. However, the rate of thrust-re-
lated exhumation is dependent on the obliquity of convergence. Lateral extrusion, as a result of
oblique convergence, will lower the rate of verti- cal exhumation Thompson et al., 1997a,b.
Many of the deformation zones that build up the SFDZ east of Lake Va¨nern display a signifi-
cant component of horizontal displacement. This may explain why the metamorphic break across
the SFDZ in south-central Sweden is not as sharp as in southern Sweden, where high-pres-
sure granulites occur immediately west of the SFDZ PZ.
In the geodynamic modelling by Willett et al. 1993 and Beaumont and Quinlan 1994, the
retro-shear is accompanied by a contemporane- ous pro-shear with opposite sense of vergence.
No such crustal-scale shear zone that could qualify as the postulated pro-shear zone has
been documented in either the Grenville or the Sveconorwegian orogen. The east-dipping defor-
mation zones and associated reflectors in the western part of the fan-like structure in the Sve-
conorwegian orogen display a suitable orienta- tion which could be related to a pro-shear.
However, the kinematics top-to-the-east does not support this interpretation. In large orogens,
the retro- and pro-shear are not necessarily ad- jacent to each other, but may be widely sepa-
rated
Willett et
al., 1993;
Beaumont and
Quinlan, 1994.
8. Conclusions
