Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol102.Issue1-2.2000:

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Seismic imaging in the frontal part of the Sveconorwegian

orogen, south-western Sweden

Christopher Juhlin

_{*, Carl-Henric Wahlgren}

_{, Michael B. Stephens}

a_{Department of Earth Sciences}_,_{Uppsala Uni}

6ersity,Villa6a¨gen16,SE-752 36Uppsala,Sweden

b_{Geological Sur}

6ey of Sweden,Box670,SE-751 28Uppsala,Sweden Received 9 August 1999; accepted 31 January 2000

Abstract

Reflection seismic data have been acquired along a 17 km profile in the eastern frontal part of the Sveconorwegian orogen, south-western Sweden. Receiver spacing was 25 m and nominal shot spacing 100 m. Shots ranging in size from 0.5 to 1.0 kg were fired in 3 m deep shot holes. The data image the structure of the upper part of the crust from depths of a few 100 m down to depths of approximately 10 km. The profile crosses the central axis of an inferred fan-like structure, indicated from earlier, detailed surface mapping results. Deformational fabrics within this structure dip approximately 15 – 40°E in the western part of the profile and 60 – 80°W in its eastern part. The seismic data display a bivergent geometry with the central axis situated approximately 8 km east of a major fault (Protogine Zone in earlier studies). Surface structural data predict the hinge axis to be located about 3 km farther east. Despite this discrepancy, there is good general agreement with the structure inferred from the surface data. The image obtained from the reflection seismic data establishes that the fan-like structure is a major, upper crustal feature. The present data in combination with previous deep seismic data sets exclude a single-stage extensional model, with an east-dipping extensional deformation zone protracting eastwards from the western part of the profile. The combined seismic data provide support for a two-stage compressional model for the development of the fan-like structure, with crustal thickening and stacking prior to 955 Ma and later compression with a stronger horizontal component of displacement during 930 – 905 Ma. The latter gave rise to the development of the complex system of retrogressive deformation zones referred to as the Sveconorwegian Frontal Deformation Zone (SFDZ), a tectonic analogy to the Grenville Front in the eastern part of North America. The SFDZ dips westwards, may become listric at depth and possibly sole in a zone of lower crustal reflectivity west of the present profile. The kinematics and geometry of the SFDZ suggest that reverse displacements (thrusting) may be responsible, at least in part, for the exhumation of the medium- to high-grade rocks in the western part of the fan-like structure. Both the seismic data and the surface structural geology show many similarities to the Grenville Front. © 2000 Elsevier Science B.V. All rights reserved.

Keywords:Shear zone; Migration; Seismic reflection; Upper crust; Compression

www.elsevier.com/locate/precamres

* Corresponding author. Tel.: +46-18-4712392; fax: +46-18-501110. E-mail address:cj@geofys.uu.se (C. Juhlin)

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1. Introduction

Over the last 20 years, the seismic reflection method has proven to be a useful tool for studying the structure of crystalline continental crust in Sweden and elsewhere. Particular cases of interest include definitive identification and mapping of fracture zones in central Sweden (Juhlin et al., 1991; Juhlin, 1995), identification of dolerite and felsic sills or sheets in the Siljan area of central Sweden (Juhlin, 1990), and the identification and mapping of mylonite zones in a number of areas in North America (Hurich et al., 1985; Johnson and Smithson, 1985; Ratcliffe et al., 1986). These studies and others have clearly shown that it is possible to obtain good images of the crust in the depth range of a few hundred meters to 10 km. Normally, the seismic reflection method can image discontinuities in the crust with dips in the range 0 – 45°, but re-cent studies show that it is possible to image discontinuities with dips up to 60 – 70° (Milkereit et al., 1992a). These images demonstrate a geo-metric picture of boundaries in the crust which then need to be interpreted in relation to surface geology and other geophysical information. In addition to the geometric picture, it is also pos-sible in some cases to obtain information on the physical properties of these discontinuities (Goodwin et al., 1989; Juhlin et al., 1991).

Much of the recent work in the Sveconorwe-gian orogen of south-western Sweden has fo-cused on detailed kinematic (e.g. Larson et al., 1986, 1990; Andre´asson and Rodhe, 1990; Park et al., 1991; Wahlgren et al., 1994; Stephens et al., 1996) and metamorphic (e.g. Johansson et al., 1991; Johansson and Kullerud, 1993; Wang and Lindh, 1996; Mo¨ller, 1998, 1999) studies, as well as geochronological work with the aim to constrain the timing of deformation and meta-morphism (e.g. Johansson et al., 1991; Jo-hansson and JoJo-hansson, 1993; Connelly et al., 1996; Page et al., 1996a,b; Wang et al., 1996; So¨derlund et al., 1999). The results of these studies strongly argue for a collisional tectonic scenario to explain the Sveconorwegian orogenic belt (cf. Berthelsen, 1980; Falkum and Petersen, 1980; EUGENO-S Working Group, 1988) which

is similar to that suggested for the Grenville orogen (e.g. Rivers, 1997). Late-Sveconorwegian extension has been suggested to explain decom-pression and exhumation of eclogites in the southernmost part of the orogen (Mo¨ller, 1998, 1999). However, current models and interpreta-tions of the Sveconorwegian orogen are con-strained primarily by surface geological information.

A major question concerns the tectonic devel-opment of the frontal part of the orogen, north and east of Lake Va¨nern (Fig. 1). In order to add a third data dimension, a c.17 km long reflection seismic profile was shot in this area to better constrain the 3D geometry of the defor-mation zones and their defordefor-mational history. The bedrock in and north of the Lake Va¨nern area has previously been explored by seismic methods. These data show that the crust is quite reflective down to Moho depths below Lake Va¨nern (Juhlin et al., 1989) and in the upper 5 – 6 km north of the lake (Dahl-Jensen et al., 1991). However, these experiments were de-signed to study the deeper portions of the crust and little to no information was obtained shal-lower than 1 – 2 km.

Wahlgren et al. (1994) discussed different models-compressional, extensional and various combinations of these-to explain the tectonic evolution in the frontal part of the Sveconorwe-gian orogen, north and east of Lake Va¨nern. A model involving two separate compressional events was favoured, and support for this was obtained in a recent 40

Ar/39

Ar study (Page et al., 1996a). Since the different tectonic models show similar features at the surface, but differ at depth, the sub-surface structural geometry is crucial for the discrimination between the mod-els.

The purpose of this paper is to integrate the results from the reflection seismic study with the surface structural information presented by Wahlgren et al. (1994). It is anticipated that this will lead to a better understanding of the 3D structural geometry in the frontal part of the orogen, and thus provide tighter constraints on

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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 A8lv Zone/Dalsland 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.

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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 A8lv Zone/Dalsland 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

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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).

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.

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Fig.

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has concentrated on determining the large-scale crustal structure. The eastern limit of the wide angle reflection/refraction 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 km/s. 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 km/s 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

The westernmost limit of the seismic profile reported here is located close to the eastern shore of Lake Va¨nern (Fig. 2b), immediately east of the PZ fault. In this area, the tectonic foliation dips about 15 – 40° to the east. The profile extends to the south-east where the surface mapped deforma-tional fabric becomes progressively more steeply dipping, and is near-vertical about 1 km east of Bjo¨rneborg (Fig. 2b). The easternmost 3 km of the profile crosses an area where the surface mapped foliation dips about 60 – 80° to the west. Data were acquired over an 8 day period in August 1996 (Table 1). The eastern half of the profile (station 340 – 681) runs along the main road which connects Kristinehamn with De-gerfors via Bjo¨rneborg. No shots could be fired in Bjo¨rneborg (station 453 – 551) and this section of the profile was undershot. The undershooting im-plies that information in the upper 500 ms, corre-sponding to c. 1500 m depth, was not obtained below Bjo¨rneborg. The western half (station 1 – 340) runs mainly along small roads and across fields.

Noise from the main road degraded the data quality along the eastern half of the profile. Geo-phone stations located in Bjo¨rneborg were also

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Table 1

Acquisition parameters for the seismic reflection survey

End-on and shoot-through Spread type

140–150 No. of channels

100 m Minimum offset

25 m Geophone spacing

Single 28 Hz Geophone type

Nominal shot spacing 100 m

0.5–1 kg dynamite Charge type

3 m Nominal charge depth

Nominal fold 20

SERCEL 348 Recording instrument

2 ms Sample rate

Field low cut Out 250 Hz Field high cut

16 s Record length

Profile length 17 km Total number of shots 144

There are several dips present in the data suggest-ing that dip moveout (DMO) corrections should be made prior to stacking. Attempts to use DMO were made, but the irregular shooting geometry resulted in that DMO was not successful in im-proving the stack. The final stacked section is shown in Fig. 5.

Migration of seismic data is necessary to place the reflecting events in their true spatial position. Wavefield migration of discontinuous reflections such as those in the final stack (Fig. 5) is difficult. Therefore, an automatic line drawing routine was

Table 2

Processing parameters for CDP line

Read SEG2 data 1

Spherical divergence correction 2

3 Spike and noise edit

Air blast attenuation: v=340 m/s 4

5 Trace equalisation: 1000–2500 ms 6 Elevation statics

Refraction statics 7

Surface consistent deconvolution 8

Design gate: 1000–3000 ms Type: spiking

Operator length: 200 ms White noise: 0.1% 9 Bandpass filter: zero phase

25–50–120–180 Hz: 0–1000 ms 20–40–100–150 Hz: 500–2000 ms 15–30–80–120 Hz: 1500–3000 ms 10–20–65–90 Hz: 2000–5000 ms 10 Trace equalisation: 1000–2500 ms

AGC: 300 ms window 11

12 FK filter: remove −2500–−4500 m/s

2500–4500 m/s CDP sort 13

14 Velocity analyses 15 Residual statics

Trim statics: maximum 1 ms 16

17 Mute

18 Stack

19 AGC: 300 ms window Dynamic S/N Filtering 20

21 FX Decon: 20–100 Hz 22 Trace equalisation:

1000–3000 ms at CDP 1 0–2000 ms at CDP 1150 noisy as a result of the ironworks plant there. In

addition, wind noise was present during the first days of acquisition along the eastern half of the profile. These factors resulted in generally better data quality along the western half of the profile.

5. Data processing

Data were recorded to 16 s with signal penetra-tion to about 3 – 5 s on the majority of the shots. Therefore, only the upper 5 s of data were pro-cessed and only the upper 3 s of data have adequate signal penetration along the entire profile. The data were processed at Uppsala Uni-versity using a commercial processing package (Table 2). All data were projected onto a straight Common Datapoint line (CDP) for stacking and interpretation (Fig. 2b). Raw and processed shot records show predominantly east-dipping tures in the west (Fig. 3) and west-dipping struc-tures in the east (Fig. 4). Tests of various stacking velocities show that more coherent images were obtained at low stacking velocities in the west and at higher stacking velocities in the east. This is primarily as a result of the steeper dips in the east.

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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 and/or 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 m/s and the resulting section (Fig. 7) forms the basis for the interpretation of the data.

6. Results

6.1. Reflecti6ity patterns

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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.

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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

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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.

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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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Juhlin

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154

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.

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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

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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

Reflection seismic data were acquired along a 17 km long profile east of Lake Va¨nern, south-western Sweden. The data image the upper crust from depths of a few hundred meters down to depths of approximately 10 km. Reflectors dip-ping up to 60 – 70° have been imaged. The data show a bivergent geometry with its central axis about 8 km east of a major fault (PZ fault). Surface geological data indicate the expected hinge axis to be located about 3 km farther east. In spite of this discrepancy, there is good general agreement with the structure as defined by the surface geology and the image obtained by the reflection seismic experiment supports the fan-like structure interpretation (Wahlgren et al., 1994).

The present seismic data, together with the deep seismic profile north of Lake Va¨nern (Dahl-Jensen et al., 1991) and the Va¨rmlandsna¨s seismic data (Juhlin et al., 1989) exclude a sin-gle-stage extensional model, with an east-dipping extensional deformation zone protracting east-wards from the western part of the seismic profile reported in this study. The data are con-sistent with the development of the fan-like structure as a two stage deformational process. The first stage involved crustal thickening and stacking prior to 955 Ma (Page et al., 1996a; So¨derlund et al., 1999). The second compres-sional stage occurred during 930 – 905 Ma (Page et al., 1996a) and resulted in the development of a complex system of retrogressive deformation zones (SFDZ). The observed eastern limit of surface deformation within the SFDZ is located east of the seismic profile implying, together with the seismic data, a decrease in the degree of deformation towards the east. However, this

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can only be verified by continuation of the seismic profile to the east. Deformation zones dipping to the west on the present profile are suggested to become listric at depth and sole into a zone of lower crustal reflectivity below Lake Va¨nern. If this is correct, it follows that this lower crustal reflectivity also developed during the Sveconorwe-gian orogeny. The absence of this lower crustal reflectivity beneath the TIB rocks to the east supports this hypothesis.

Available seismic data and surface geology from the Sveconorwegian orogen of south-western Sweden show many similarities with observations from the Grenville orogen. Hinterland dipping reflectors related to ductile deformation zones are observed in the frontal parts of both orogens. Towards the hinterland, oppositely dipping reflec-tivity has been documented in both cases. Retro-shear transport of material from the hinterland towards the foreland and exhumation of high-grade rocks appear to have occurred in both orogens.

Acknowledgements

Field work and processing costs for this study were funded by the Geological Survey of Sweden (SGU) under the auspices of a SGU external Research and Development grant (PROJECT NUMBER 03-890/94). H. Palm played an impor-tant role in the field acquisition stage of this project and L. Frenje contributed to the initial stages of the seismic data processing. C. Juhlin is funded by the Swedish Natural Sciences Research Council (NFR) and the work by C.-H. Wahlgren and M.B. Stephens has been funded by a SGU internal Research and Development grant. We thank D. Dyrelius and the referees T. Dahl-Jensen and B. Milkereit for their constructive comments on the manuscript.

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Juhlin

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135

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154

149

(2)

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 Gren

6 ille 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 e

6 olution

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

(3)

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

Reflection seismic data were acquired along a

17 km long profile east of Lake Va¨nern,

south-western Sweden. The data image the upper crust

from depths of a few hundred meters down to

depths of approximately 10 km. Reflectors

dip-ping up to 60 – 70° have been imaged. The data

show a bivergent geometry with its central axis

about 8 km east of a major fault (PZ fault).

Surface geological data indicate the expected

hinge axis to be located about 3 km farther

east. In spite of this discrepancy, there is good

general agreement with the structure as defined

by the surface geology and the image obtained

by the reflection seismic experiment supports the

fan-like structure interpretation (Wahlgren et al.,

1994).

The present seismic data, together with the

deep seismic profile north of Lake Va¨nern

(Dahl-Jensen et al., 1991) and the Va¨rmlandsna¨s

seismic data (Juhlin et al., 1989) exclude a

sin-gle-stage extensional model, with an east-dipping

extensional deformation zone protracting

east-wards from the western part of the seismic

profile reported in this study. The data are

con-sistent with the development of the fan-like

structure as a two stage deformational process.

The first stage involved crustal thickening and

stacking prior to 955 Ma (Page et al., 1996a;

So¨derlund et al., 1999). The second

compres-sional stage occurred during 930 – 905 Ma (Page

et al., 1996a) and resulted in the development of

a complex system of retrogressive deformation

zones (SFDZ). The observed eastern limit of

surface deformation within the SFDZ is located

east of the seismic profile implying, together

with the seismic data, a decrease in the degree

of deformation towards the east. However, this

(4)

can only be verified by continuation of the seismic

profile to the east. Deformation zones dipping to

the west on the present profile are suggested to

become listric at depth and sole into a zone of

lower crustal reflectivity below Lake Va¨nern. If

this is correct, it follows that this lower crustal

reflectivity also developed during the

Sveconorwe-gian orogeny. The absence of this lower crustal

reflectivity beneath the TIB rocks to the east

supports this hypothesis.

Available seismic data and surface geology

from the Sveconorwegian orogen of south-western

Sweden show many similarities with observations

from the Grenville orogen. Hinterland dipping

reflectors related to ductile deformation zones are

observed in the frontal parts of both orogens.

Towards the hinterland, oppositely dipping

reflec-tivity has been documented in both cases.

Retro-shear transport of material from the hinterland

towards the foreland and exhumation of

high-grade rocks appear to have occurred in both

orogens.

Acknowledgements

Field work and processing costs for this study

were funded by the Geological Survey of Sweden

(SGU) under the auspices of a SGU external

Research and Development grant (PROJECT

NUMBER 03-890

/

94). H. Palm played an

impor-tant role in the field acquisition stage of this

project and L. Frenje contributed to the initial

stages of the seismic data processing. C. Juhlin is

funded by the Swedish Natural Sciences Research

Council (NFR) and the work by C.-H. Wahlgren

and M.B. Stephens has been funded by a SGU

internal Research and Development grant. We

thank D. Dyrelius and the referees T. Dahl-Jensen

and B. Milkereit for their constructive comments

on the manuscript.

References