Kinematics of a major fan-like structure in the eastern part of the Sveconorwegian orogen, Baltic Shield, south-central Sweden

Pre(nmbrinn Resenrth ELSEVIER

Precambrian Research 70 (1994) 67-91

Kinematics of a major fan-like structure in the eastern part of the Sveconorwegian orogen, Baltic Shield, south-central Sweden Carl-Henric Wahlgren a, Alexander R. Cruden b, Michael B. Stephens a aGeological Survey of Sweden, Box 6 70, S- 751 28, Uppsala, Sweden bDepartment of Geological Sciences, University of Toronto, Erindale Campus, Mississauga, Ontario L5L IC6, Canada Received September 27, 1993; revised version accepted March 29, 1994

Abstract The N-S-trending so-called Protogine Zone in the Baltic Shield of south-central Sweden is usually considered to mark a tectonic boundary between the rocks of the Transscandinavian Igneous Belt (TIB) in the east and the Sveconorwegian orogen in the west. Detailed structural mapping in the Karlskoga-Kristinehamn area has shown that an anastomosing network of ductile deformation zones with generally N-S strike extends ca. 40 km east of the traditional "Protogine Zone". Furthermore, the western boundary of this ductile deformation and the TIB is not constrained in the Kristinehamn area. Reconnaissance studies indicate that they both extend westwards towards the so-called Mylonite Zone. It is suggested that the eastern limit of the Sveconorwegian orogen is located some 40 km east of the present boundary and that the "Protogine Zone" concept is obsolete. The term Sveconorwegian Frontal Deformation Zone (SFDZ) is proposed as a more appropriate alternative in southern Sweden (south of lake V~ittern) and to correspond to a younger set of oblique ductile deformation zones with reverse and right-lateral components of movement in the easternmost part of the orogen farther to the north. Ductile deformation zones older than the SFDZ in the Karlskoga-Kristinehamn area display a fan-like geometry in an E-W cross-section, with steep westerly dips in the eastern part of the section, vertical dips farther west and moderate easterly dips in the western part of the section. Kinematic analysis indicates that dip-slip movements predominate with a consistent top-to-the-east sense of movement across the entire fan-like structure. In their present orientation, deformation zones are characterized by reverse movements in the eastern part and normal movements in the western part of the structure. Between Kristinehamn and the Mylonite Zone, the main foliation is gently dipping to subhorizontal, indicating that the regional structure is strongly asymmetric, and that the fan-like structure occurs close to the foreland of the orogen. Deformation zones are spaced to semi-penetrative in the eastern part of the fan-like structure, whereas the deformation is more or less penetrative and the TIB rocks are transformed to orthogneisses west of Kristinehamn. This east to west increase in bulk strain is in accordance with an increase in syn-deformational metamorphic grade across the structure. Younger ductile deformation zones belonging to the SFDZ are responsible for a major change in orientation of the older deformation zones in the easternmost part of the structure. The fan-like structure is best explained by models involving the interference of two separate tectonic events. Deformation occurred after ~ 1.57 Ga and prior to deposition of Neoproterozoic and younger cover sedimentary rocks. It is not yet clear whether the initial phase of deformation (early Sveconorwegian or older) was related to the build-up of an imbricate thrust stack in a compressional r6gime, as favoured here, or to regional E-W extension. The younger deformation phase was related to rotation of these older structures into the compressional, late Sveconorwegian SFDZ.

0301-9268/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0301-9268 ( 94 )00035-P

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C.-H. Wahlgren et al. / Precambrian Research 70 (1994) 67-91

1. Introduction The Baltic Shield in Sweden is mainly composed of the Svecokarelian (Svecofennian) and Sveconorwegian orogens, in which the age of latest ductile deformation is constrained to the time intervals ~ 1.78-1.40 Ga and ~ 1.10-0.90 Ga, respectively. The Proterozoic rocks in the Sveconorwegian orogen of southwestern Sweden are transected by major ductile deformation zones, among which the so-called Protogine and Mylonite Zones, the G6ta Alv Zone, and the Daisland Boundary Thrust are the most conspicuous (Fig. 1 ). Although the presence of these major zones has been known for a considerable time, only a few recent studies (e.g., Larson et al., 1986, 1990; Andr6asson and Rodhe, 1990; Park et al., 1991) have addressed their general geometric character and the kinematics of deformation along and between them. This paper presents an analysis of the deformation in the eastern part of the Sveconorwegian orogen, in the area north of lake V~ittern and east of lake V~inern, south-central Sweden (Fig. 1 ). New structural data from four key subareas, lying within a transect from northeast of Karlskoga to Kristinehamn (Fig. 2), are described. In its westernmost part, this transect includes the "Protogine Zone". The regional tectonic implications of this study and alternative models for the development of the observed ductile deformation are discussed. Unless otherwise stated, all ages cited in the text are based on U-Pb multiple zircon determinations. The errors in these ages are provided at the 95% or 2a confidence level.

2. Ductile deformation in the frontal part of the Sveconorwegian orogenmregional setting, timing and previous tectonic models From lake V~ittern to the southernmost tip of Sweden (Fig. 1 ), a 25-30 km wide, steeply dipping zone of high strain and intense schistosity development, referred to as the "Protogine Zone" (Gorbatschev, 1980), marks the eastern limit of the Sveconorwegian orogen and corresponds to

an important tectonometamorphic break. East of this zone, Proterozoic rocks of the Baltic Shield are unaffected by Sveconorwegian deformation and metamorphism while, to the west, they are affected by both older orogenic events as well as Sveconorwegian deformation and high-pressure, granulite metamorphism (Johansson et al., 1991 ). North of lake V/ittern, this deformation zone has been placed between granitoids and mafic intrusive rocks to the east, the ~ 1.85-1.65 Ga Transscandinavian Igneous Belt (TIB) of Patchett et al. ( 1987 ), and a complex suite oforthogneisses to the west (e.g., Gorbatschev, 1980; Gorbatschev and Ga~il, 1987; Lindh, 1987). By contrast, Magnusson et al. ( 1963 ) and Berthelsen ( 1980, 1987) proposed that this ductile deformation continues northwards from lake V~ittern so that it affects a considerable width of the TIB to the east of the zone and eventually disappears beneath the Scandinavian Caledonides, some 300 km to the north-northwest. Several authors have speculated that deformation in the frontal part of the Sveconorwegian orogen originally occurred in connection with the emplacement of the TIB and that this tectonic zone has been repeatedly reactivated during subsequent events (Gorbatschev, 1980; Eriksson and Henkel, 1983; Larson et al., 1986, 1990; Henkel and Eriksson, 1987; Berglund et al., 1992; Henkel, 1992). A long tectonic history for this important belt of deformation has also been emphasized by e.g. Johansson (1990) and Johansson and Johansson (1990). However, the age of syenites, granites and dolerites ( ,,, 1.220.93 Ga) affected by this deformation (Johansson, 1990; Johansson and Johansson, 1990; Jarl, 1992 ) as well as a variety of metamorphic mineral ages (Verschure et al., 1987; Johansson, 1990; Jarl, 1992 ) indicate that it is Sveconorwegian in age. The following tectonic models have been proposed for the deformation in the frontal part of the Sveconorwegian orogen: (1) Compressional structures formed in response to Sveconorwegian collisional tectonics (e.g., Berthelsen, 1980, 1987; Falkum and Petersen, 1980; EUGENO-S Working Group, 1988).

69

C.-H. Wahlgren et al. /Precambrian Research 70 (1994) 67-91

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Fig. 1. Major tectonic zones in the Proterozoic rocks of southern Sweden.DBT= Dalsland Boundary Thrust, GNC= Glaskogen Nappe Complex, GJI'Z=Grta ~/.lvZone, MZ=Mylonite Zone, SFDZ(PZ)=Sveconorwegian Frontal Deformation Zone (Protogine Zone). G= Grteborg, Gr= Grythyttan, K= Karlstad, M= MalmiS, S= Stockholm. Lines 1, 2 and 3 mark the Kristinehamn-Karlstad, Molkom-Kil and Rgtda-Torsbytransects, respectively. In particular, Berthelsen (1980) interpreted the crustal segment between Karlskoga and Kristinehamn (Fig. 2) to be bounded by a west-dipping thrust in the east and a slightly older eastdipping thrust to the west. (2) Extensional structures related to either late Sveconorwegian uplift of western crustal segments (e.g., Welin and Blomquist, 1966; Patchett and Bylund, 1977 ), or aborted rifting prior to opening of the early Palaeozoic Iapetus Ocean (Andrrasson and Rodhe, 1990).

3. Lithology and deformation in the KarlskogaKristinehamn area 3.1. L i t h o l o g y

In the Karlskoga-Kristinehamn area, Svecofennian metasupracrustal rocks ( ~ 1.89 Ga) are dominated by felsic metavolcanic lithologies with local occurrences of calc-silicate, carbonate and semi-pelitic metasedimentary rocks. An older

70

C.-H. Wahlgren et al. / Precambrian Research 70 (1994) 67-91

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Stretching lineation (L) 1 = Granbergsdal Subarea 2 = Degerfors 3 = Botjtlrnen 4 = Niklasdamm

Fig. 2. Geologicalmap of the Karlskoga-Kristinehamnarea showingsubareas. Based on Wikstr6m ( 1991a), Wahlgren ( 1992, 1993 ) and unpublisheddataby the GeologicalSurveyof Swedennortheastof Karlskoga (geologicalmappingby M.B. Stephens). group of intrusions ( ~ 1.89-1.85 Ga) ranges in composition from granite to gabbro and is dominated by granodiorite-tonalite. Younger intrusions include two separate suites. Suite 1 is composed of granites and pegmatites ( ~ 1.81-1.75

Ga) and forms a significant component in the eastern part of the area (Fig. 2). Suite 2 corresponds to the TIB granitoids and related mafic intrusions which are ~ 1.85-1.65 Ga old (Larson and Berglund, 1992; Persson and Wikstr6m,

C.-H. Wahlgren et al. / Precambrian Research 70 (1994) 6 7- 91

1993) and which dominate the western part of the area (Fig. 2). The oldest TIB granitoid in the area (Stephens et al., 1993 ) is a predominantly equigranular, locally charnockitic granite. Near Karlskoga, this granite is spatially associated with metasupracrustal rocks which display low-pressure, granulite high-grade, contact-metamorphic mineral assemblages (Andersson et al., 1992). Most of the TIB granitoids are finely to coarsely porphyritic with K-feldspar phenocrysts in the coarser ( > 2 cm) varieties often mantled by plagioclase feldspar. They vary in composition from quartz monzodiorite to granite. A subordinate younger suite of TIB granites is equigranular and generally more leucocratic. Scattered mafic intrusions occur in the TIB granitoids with which they commonly display magma-mingling relationships. The youngest rocks affected by ductile deformation in the Karlskoga-Kristinehamn area are various dolerite dykes. Dolerites, which intrude TIB rocks in the westernmost part of the area and which are concordant to the foliation (Fig. 2), have yielded an U-Pb baddeleyite age of ~ 1.57 Ga interpreted as the intrusion age (Wahlgren et al., in prep.). In the metasupracrustal rocks to the east of the TIB, two distinct sets of dolerite dykes are present, trending WNW-ESE and NNW-SSE. These yield Rb-Sr whole-rock/mineral ages of ca. 1.56-1.51 and 1.00-0.90 Ga, respectively (Patchett, 1978). Dykes of both sets locally show deformation and offset along discrete, ductile deformation zones (Fig. 2 ). Pockets of Neoproterozoic sandstone and Cambro-Ordovician sandstone, shale and limestone form the youngest rocks in the area. They lie adjacent to major brittle faults (Fig. 2) and show no evidence of the ductile deformation and metamorphism which affect the older lithologies.

71

fine a network of anastomosing deformation zones, often 50-100 m wide, surrounding less deformed, km-scale tectonic lenses (Fig. 2 ). West and northeast of Karlskoga, ductile deformation zones anastomose in a N-S to NNW-SSE direction through the TIB granitoids and mafic intrusions (Fig. 2 ). The ductile deformation in these rocks is more penetrative than in the area farther east but remains spaced in character; deformation zones up to ca. 500 m wide separate blocks of massive rock up to 2-3 km across. In the eastern part of the TIB, the N-S- to NNW-SSEstriking foliation is reoriented by a younger suite of ductile deformation zones which strike NESW and show reverse dip-slip and right-lateral horizontal components of movement. The most prominent of these zones extends from south of Degerfors to Karlskoga and farther to the northeast (Fig. 2 ). Westwards towards Kristinehamn, the ductile deformation is semi-penetrative and the boundaries of individual deformation zones are difficult to define. West of Kristinehamn, deformation is more or less penetrative (Fig. 2). The prominent structural reworking of the bedrock west of Karlskoga is clearly evident from low-altitude aeromagnetic data (Fig. 3 ).

3.2. Megastructure---ductile deformation zones and younger brittle faults Discrete NE-SW-striking ductile deformation zones with an oblique, reverse sense of shear are prominent east of Karlskoga (Fig. 2). These interact with zones which strike NNW-SSE to de-

Fig. 3. Grey-tone aeromagnetic map of the Karlskoga-Kristinehamn area with flight altitude 30 m and line spacing 200 m. K= Karlskoga. Kr= Kristinehamn.

72

C.-H. Wahlgren et al. / Precambrian Research 70 (1994) 67-91

In most regional compilations (e.g., Magnusson, 1958; Gorbatschev, 1980; Gorbatschev and Ga~tl, 1987; Lindh, 1987 ), the "Protogine Zone" is drawn through Kristinehamn and corresponds to the boundary between the TIB and the orthogneisses of southwestern Sweden. A marked boundary between TIB rocks and orthogneisses is also shown on the recently published county map ofV~irmland (Lundeg~trdh et al., 1992 ). The results from this study indicate that there is no lithological break across the "Protogine Zone" (see also below). The latter merely corresponds to the boundary between semi-penetratively and more or less penetratively deformed TIB rocks, i.e., it simply represents a change in the degree of tectonometamorphic reworking of the TIB rocks. This boundary lies close to a younger and kinematically distinct brittle fault system, an important branch of which occurs 2 km west of Kristinehamn (Fig. 2 ). This fault system is prominent on aeromagnetic maps as a low-magnetic lineament and, north of Rhda (Fig. 1 ), follows the Klar~Uven valley northwestwards into Norway where it offsets the Caledonian Front in a manner consistent with an east-side-up sense of movement. Apart from its strike orientation, this fault has no relationship to the older ductile deformation. Several younger brittle faults occur in the Karlskoga-Kristinehamn area of which the most prominent have been marked in Fig. 2. In places (e.g., near Kristinehamn, see Fig. 2 ), these faults strike parallel to the ductile deformation zones; elsewhere (east of the Lokadalen Fault Zone, see Fig. 2), they are highly discordant to these zones. The major fault between Karlskoga and Kristinehamn (Fig. 2) is steeply east-dipping, with a reverse dip-slip sense of movement, indicated by the deflection of the older foliation into the brittle fault surface. On the basis of similar criteria, the fault (Svart~ilven Fault Zone) which strikes northeastwards from Karlskoga and dips steeply to the northwest is also reverse in character (Fig. 2 ). Otherwise, the sense of movement along these brittle structures is poorly constrained. The Lokadalen Fault Zone on the eastern side of lake M/Sckeln (Fig. 2 ) is inferred to have an east-sideup component of movement since pockets of

downthrown Neoproterozoic sandstone occur only on the western side of this structure (Fig. 2). In a similar manner, the occurrence of the Cambro-Ordovician platformal cover rocks on the eastern side of the Kilsbergen Fault Zone (Fig. 2 ) indicates a west-side-up sense of movement along this structure and faulting during or after the Ordovician. The crustal segment between the Lokadalen and Kilsbergen Fault Zones therefore represents part of a major uplifted block. More data on the dip direction and kinematics of the brittle structures are required before it can be inferred with confidence whether this major Phanerozoic structure is a compression-related pop-up or an extension-related horst.

3.3. Mesoscopic structural and kinematic elements Two main types of planar structure related to ductile deformation are present in the Karlskoga-Kristinehamn area: ( 1 ) A main foliation (S) is developed in all rock types. It is defined by the preferred orientation of phyllosilicates, flattened quartz and relic feldspar phenocrysts in the granitoids, and of amphibole, phyllosilicates and deformed feldspar grains in the subordinate mafic to intermediate lithologies. S varies in intensity from very weak at the margin of undeformed areas to protomylonitic approaching high-strain zones; west of Kristinehamn, S is more or less penetrative in character. Except for the westernmost areas, S commonly occurs together with a second planar structure as a composite planar fabric. Since the boundaries of the high-strain zones are often difficult to locate in the field, it is seldom clear whether this second planar structure is a C surface parallel to the boundaries of the high-strain zone as in a composite C/S fabric (Berth6 et al., 1979; Hanmer and Passchier, 1991 ) or an asymmetrical extensional shear band (Platt and Vissers, 1980; White et al., 1980; Hanmer and Passchier, 1991 ) as in a composite C ' / S fabric. (2) A mylonitic foliation (Sm) occurs in the central parts of the high-strain zones or along narrower and discrete high-strain zones which in places cross-cut S. The mylonitic foliation con-

C.-H. Wahlgrenet al. / PrecambrianResearch 70 (1994) 67-91

sists of bands (cm to several m wide) of finegrained mylonite and ultramylonite. It locally contains neocrystallized white mica in addition to other phyllosilicates, quartz and feldspar. Epidote and thin veins of quartz are also conspicuous. A stretching lineation (L) is well developed in the high-strain zones and decreases in intensity towards the undeformed areas. It is defined by a linear preferred orientation of phyllosilicate streaks on S, C and Sm surfaces, and as a shape elongation of quartz grains and feldspar porphyroclasts. Kinematic indicators, indicative of non-coaxial progressive deformation, are commonly observed in high-strain areas. Composite planar fabrics, both C/S and C'/S, are the most common and the most reliable kinematic indicators in the area. Asymmetric, a-type and less commonly in-plane, 6-type tails (Simpson and Schmid, 1983; Passchier and Simpson, 1986; Hanmer and Passchier, 1991 ) of recrystallized material around feldspar porphyroclasts are also present. Tiling structure (Blumenfeld, 1983) in relic feldspar phenocrysts has been observed in the eastern part of the area. A characteristic feature in western areas is asymmetric folding and crenulation of all planar structures and the stretching lineation. 3.4. Style of deformation and kinematics in key subareas

The Karlskoga-Kristinehamn area has been divided into four key subareas in order to illustrate the style of deformation and kinematics along an E-W transect in the eastern part of the Sveconorwegian orogen. They are described separately below, from the low-grade, heterogeneously deformed rocks in the east to the medium-grade, more or less penetratively deformed rocks in the west (Fig. 2 ). 3.4.1. Granbergsdal subarea The southernmost part of the Granbergsdal subarea ( 140 km 2) is situated ca. 5 km northeast of Karlskoga (Fig. 2 ). All lithologies within the subarea (Fig. 4), including dolerite dykes, are

73

affected by non-penetrative ductile deformation. Zones of weak to strong foliation, up to ca. 500 m thick, with local development of highstrain zones containing mylonite and ultramylonite < 1 cm to several dm thick, anastomose around km-scale lenses of massive rock. The subarea is bound on its eastern, southeastern and western sides by topographic linear depressions which correspond to zones of high ductile deformation. On the southeastern and western sides, these depressions are also the locus of zones of younger brittle deformation (Svart~ilven and Lokadalen Fault Zones, Fig. 4 ). By contrast, within the subarea, topographic linear depressions follow younger faults which cut across the structures related to ductile deformation (Fig. 4). S surfaces strike NW-SE and dip steeply and predominantly to the southwest with an average orientation of 326/74°SW (Fig. 5A). Lineations on S surfaces rake to the north with an average orientation of 303/69 ° (Fig. 5A). Along the southeastern boundary of the subarea, the strike of S surfaces changes abruptly into a NESW direction and dip changes steeply to the northwest. This change in orientation is reflected by the spread of S poles into the southeast quadrant of the stereographic projection (Fig. 5A) and is spatially associated with the development of the younger discrete zones of high ductile strain close to and east of Karlskoga. Sm surfaces display a similar orientation pattern as S (Fig. 5B). Furthermore, lineations on Sm surfaces also plunge steeply to the WNW (Fig. 5B). However, several high-strain zones containing Sm cross-cut and deflect the S surfaces. Such zones are younger than and not related to the development of the foliation. Kinematic indicators (composite planar fabrics, tiling structure and a-type tails) are present in several outcrops in the subarea (Figs. 6a, 6b). Virtually all determinations using such indicators (17 of 19) display a top-to-the-east shear sense (Fig. 5C). Assuming that L represents the transport direction, then an oblique, predominantly reverse sense of shear with a subordinate left-lateral horizontal component is inferred. The sense of shear within several of the mesos-

C.-H. Wahlgren et al. / Precambrian Research 70 (1994) 67-91

74

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copic high strain zones has also been determined on the basis of the progressive curvature of the associated foliation into the mylonitic shear surface (Fig. 6c; viz. Ramsay and Graham, 1970), and in other instances, on the basis of the sense of deflection of an older S surface into a younger shear surface (Fig. 6d). These zones display a

more varied kinematic pattern (Fig. 5D). Virtually all the deformation zones dipping to the northeast, i.e. opposite to the usual dip direction of S and Sin, show a dominant top-to-the-west, i.e. reverse sense of shear with a subordinate right-lateral horizontal component. NNW-SSEtrending ductile deformation zones with a right-

C.-H. Wahlgren et al. / Precambrian Research 70 (1994) 67-91

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Fig. 5. Equal-area, lower-hemisphere, stereographic projections for structural data in the Granbergsdal subarea. (A) Poles to S and L. (B) Poles to Sm and L. (C) Poles to S and L at the localities where the sense of movement has been determined using a composite planar fabric or, less commonly, tiling structure or asymmetric, a-type tails. (D) Poles to Sm and L at the localities where the sense of shear has been determined on the basis of S-surface deflection in mesoscopic high-strain zones.

lateral horizontal shear component are conspicuous along the western boundary of the subarea, more or less along the Lokadalen Fault Zone. Biotite along S surfaces in the TIB granitoids is principally a primary magmatic phase which has been reoriented and partly recrystallized during the deformation. Quartz, particularly in the protomylonites, is completely recrystallized and drawn out into a ribbon structure, and feldspar has deformed by slip along discrete microfractures (Fig. 6a). Blue-green amphibole, biotite and chlorite are conspicuous along S surfaces in the TIB mafic to intermediate intrusions. Both amphibole and chlorite are present in the deformed dolerites. These metamorphic mineral

3.4.2. Degerfors subarea The Degerfors subarea (50 km 2) illustrates characteristic deformation in the east-central part of the transect (Fig. 2). Here, the foliation changes from steep northwesterly dips to vertical across a SE to NW traverse (Fig. 7). The structure of the Degerfors subarea is characterized by several, NE-SW-striking, ductile high-strain zones which are parallel to the elongation direction of the lithological units (Fig. 7 ). The low-strain areas lying between the more pronounced high-strain zones often display a weakly developed foliation which makes it difficult to delimit individual shear zones. However, one well-defined zone occurs along the eastern contact between the locally charnockitic granite and the finely porphyritic granitoid (Fig. 7). This zone is ca. 50-100 m thick and displays a gradual transition from almost massive granitoid to mylonite in its central part. There is a relatively good agreement between the orientation of the S and Sr~ surfaces in the area, with the exception of the somewhat greater scatter in the Sm data (Fig. 8 ). The average strike of these surfaces is 036/69°NW and 050/ 35°NW, respectively. Lineations (L) are also relatively well-clustered with an average trend of 280/55 °. A comparison between the orientation of the S surface and the stretching lineation in individual outcrops indicates a dominant dip-slip sense of movement (Fig. 7). However, the motion is locally oblique with a subordinate horizontal component. Composite planar fabrics (C/ S and C'/S) show a consistent west-side-up sense of movement. Reoriented and partly recrystallized relic magmatic biotite, as well as metamorphic white mica and occasional chlorite are the phyllosilicates which define S surfaces in these rocks. The outer parts of feldspar phenocrysts show dynamic recrystallization. These features suggest that deformation occurred under low- to medium-grade metamorphic conditions. The regionally heterogeneous distribution of high-strain zones around

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C.-H. Wahlgren et al. / Precambrian Research 70 (1994) 67-91

tectonic lenses of more or less massive granitoid also indicates a relatively low-T r6gime (cf. Gapais, 1989). 3.4.3. Botfiirnen subarea

The Botj~irnen subarea (45 km 2) is representative of the west-central part of the transect (Fig. 2). The topography of this part of the transect is characterized by elongate N-S-trending ridges

separated by N-S- to NNE-SSW-trending linear depressions (Fig. 9). Since the higher ground generally corresponds to areas of massive to weakly foliated rock and the lower ground is underlain by strongly foliated equivalents and zones of later brittle deformation, topography is a useful indicator of structural trends in the region. In the Botj/irnen subarea, three N-S- to NNE-SSWtrending ridges of massive to weakly foliated rock

C.-H. Wahlgren et al. / Precambrian Research 70 (1994) 67-91

77

Fig, 6. Mesoscopic structures in the Granbergsdal subarea. Photographs ( a ) - ( c ) are viewed to the SE on steeply dipping joint surfaces; photograph (d) on a top, flat surface with direction 020 ° to the right. (a) Tiling structure in relic K-feldspar phenocryst in foliated, coarsely porphyritic, TIB granitoid with inferred top-to-the-east sense of movement. (b) C/S fabric in same lithology as (a) with inferred top-to-the-east sense of movement. (c) Ductile high-strain zone with ultramylonite development and inferred top-to-the-west sense of movement. (d) Deflection of older S surface into younger ductile high-strain zone in same lithology as (a) and with an inferred right-lateral, horizontal component of movement.

are separated by several zones of more strongly foliated material which extend ca. 1 to 2 km across strike (Figs. 9, 10). The latter are further subdivided by linear zones of higher strain (my-

lonite s.1.) with an across-strike width of 1 to several 100 m. Symmetrical strain gradients (massive granitoid-foliated granitoid-protomylonite-mylonite-ultramylonite), 500 m to < 50

78

C.-H. Wahlgren et al. /Precambrian Research 70 (1994) 67-91

Dolerite

~

Fault

Granite, fine- to medium-grained

[~

Fault, square on downthrown side

[~

Form lines of structures related to ductile deformation (post-1.57 Ga)

Granite to porphyritic

quartz monzonite, finely

Granite, locally charnockitic Monzodiorite to quartz monzodiorite

6~ ~ S, dip in degrees/vertical d0 J Z Sin, dip in degrees/vertical/ dip unknown 60 L, plunge in degrees

Fig. 7. Structural map of the Degerfors subarea.

Degerfors

N

+~+++ +

÷

•

•

S (n = 162)

o +

Sm (n = 78) L (n = 7)

°.

Fig. 8. Equal-area, lower-hemisphere, stereographic projection for structural data in the Degerfors subarea.

m wide, are commonly observed on both sides of the high-strain zones. S surfaces show consistent, moderate dips to the ESE (Fig. 11 A) with an average orientation of011/34°E. Sm surfaces are also well-clustered, showing an average orientation of 019/50°E, somewhat steeper than S. Lineations (L) show an average plunge of 38 ° towards 068 ° and, with few exceptions, consistently rake northward with respect to both S and Sin. Kinematic indicators (composite planar fabrics and both a-type and less commonly 6-type tails) show top-to-the-east sense of shear (Figs. 12a, 12c). IfL is taken to be parallel to the transport direction, then all high-strain zones record oblique-slip, normal sense shearing with a leftlateral, horizontal component. As indicated above, it is potentially easy to

C.-H. Wahlgren et al. / Precambrian Research 70 (1994) 67-91

8s72

<125 m

125-150 m

-iilk-m

150-175 m

175-200 m

,,,

>200 m

Fig. 9. Topography of the west-central part of the KarlskogaKristinehamn transect. Note the strong structural control of valleys and ridges. Box corresponds to Fig. 10. Data from the National Land Survey of Sweden, 1:100,000 map sheet 103, Karlskoga.

confuse C surfaces with asymmetrical extensional shear bands. However, the correlation between Sm attitudes (Fig. 11A), which can be assumed to be subparallel to the shear zone boundaries, and C-surface attitudes (Fig. 11B), together with the observation that C and S surfaces attenuate at the same rate going out of highstrain zones (viz. Hanmer and Passchier, ! 991 ) lead us to conclude that the fabric we have described is a composite C/S fabric. Asymmetrical extensional shear bands are locally observed in the Botjiirnen subarea (Fig. 12b). Fig. 11B also illustrates another problem with interpretation of these fabrics: stretching lineations consistently make an acute angle with the intersections between C and S surfaces. This angle should be 90 ° in ideal progressive simple shear zones. This nonorthogonality may suggest that these C/S fabrics were generated during combined progressive pure and simple shear as predicted by Robin and Cruden (1994). Alternatively, this may be due to incipient rotation of this intersection lineation into parallelism with the stretching direc-

79

tion during progressive deformation. Asymmetric crenulations of S, with shallow dipping (15-30°E) axial surfaces and associated minor shear zones which grow out of rootless intrafolial folds (Fig. 12d), also occur locally. Both consistently indicate top-to-the-east transport. Given their orientation and shear sense, they are interpreted to be reverse-slip crenulations (Dennis and Secor, 1987). Such structures form by slip on a foliation initially inclined at an acute angle (measured clockwise ) to the shear zone movement direction. Microstructurally, the deformed granitoids display recrystallized biotite, flattened and dynamically recrystallized quartz, recrystallized mantle texture in feldspar, and bent twinning and recrystallization of plagioclase. Considered in combination with the occurrence of the paragenesis hornblende + epidote_+ garnet along the foliated margins of the diorite body in the western part of the subarea, these features suggest deformation under low- to medium-grade conditions. 3.4.4. Niklasdamm subarea The Niklasdamm subarea (50 km 2) bridges the critical transition zone from discontinuous and semi-penetrative ductile shear deformation in the east to more or less penetrative deformation farther west (Figs. 2, 13 ). The foliation and the lithological boundaries in the Niklasdamm subarea are more or less parallel and strike N-S to NNW-SSE. The structure of the eastern part of the subarea resembles that in the subareas to the east; tectonic lenses of massive to little deformed TIB rocks are surrounded by foliated equivalents (Fig. 13). The average orientations of S and L are 352/29°E and 061/25 °, respectively (Fig. 11C). These mean values also include west-dipping structures related to major folds (Fig. 13 and see below). With a few exceptions, L consistently rakes to the north on the foliation planes. Composite planar fabrics (Fig. 14a) are common and indicate top-to-the-east sense of movement. Taken in combination with the stretching lineation, this indicates an oblique-slip, normal sense of shear with a subordinate left-lateral hor-

80

C.-H. Wahlgren et al. / Precambrian Research 70 (1994) 67-91

~

Granite, fine- to medium-grained Granite to quartz monzonite, finely porphyritic Diorite

~

~

Svecofennian metasupracrustalrocks, older intrusions

rn

F-

I ~

Reversefault, triangle on upthrown hanging wall Form lines of structures related to ductile deformation (post-1.57 Ga)

(2o S, dip in degrees 7o C, dip in degrees / 25 L, plunge in degrees

Fig. 10. Structural map of the Botj~irnen subarea.

izontal component. Asymmetric folding of S, C and L is an important structural feature of the Niklasdamm subarea (Fig. 14b ). These folds are observed on a cm- to m-scale and consist of long, east-dipping and short, west-dipping fold limbs. Parasitic, cm-scale Z-, M- and S-folds are also observed in the hinge zones of larger-scale structures. The existence of large-scale, asymmetric folds is indicated by the local occurrence of a steeply west-dipping foliation (Fig. 13) which represents short, west-dipping fold limbs. The vergence of these asymmetric folds is consistent with a top-to-the-east sense of movement. With the exception of mechanically competent dolerites, all lithologies in the western part of the subarea contain a penetrative foliation ("steady state foliation" of Gapais, 1989).

Composite C/S fabrics are virtually absent. This may be due to parallelism between S- and Cplanes as a function of high bulk strain. A stretching lineation is very conspicuous and most rocks are LS-tectonites. Asymmetric folding of the foliation is also much more pervasive in this part of the subarea. In many places, the stretching lineation has been observed to be folded (Fig. 14c). The high angle between the orientation of the stretching lineation and the fold axes (Fig. 13) suggests that folds developed as late-stage structures during progressive ductile shearing. Locally, this angle is more acute, possibly indicating an incipient rotation of the fold axes into parallelism with the stretching direction, as is commonly observed in high-strain zones. In places, the foliation is strongly crenulated and

C.-H. Wahlgren et al. / Precambrian Research 70 (1994) 67- 91 N

Botj~irnen

N

+

• S (n=134) o Srn (n=40) + L (n=44)

10+00

i~O+

• S (n=23) o C (n=24) + L (n=16)

Niklasdamm

N

o OeO

• S (n=93) o S m (n=13) + L (n=31)

.

41-

e+

N

•

• S (n=169) + L (n=95)

Fig. 11. Equal-area, lower-hemisphere stereographic projections for structural data in the Botj~irnen (A and B) and Niklasdamm (C and D) subareas. (A) Poles to S, Sm and L, Botj~irnensubarea. (B) Poles to C/S fabric elements, including C and S surfaces, related stretching lineation (L) and great circles for the average orientations of C and S surfaces, Botj~irnen subarea. (C) Poles to S, Sm and L in the eastern part of the Niklasdamm subarea, (D) Poles to S and L in the western part of the Niklasdamm subarea.

there is a tendency to develop a gently east-dipping, spaced axial surface cleavage (Fig. 14d). The average trends of the foliation and the stretching lineation in the western part of the subarea are 345/21°NE and 058/23 °, respectively. W- to SW-plunging stretching lineations and NW- to SW-dipping foliation (Fig. 11D) are mainly due to reorientation by asymmetric folding and, more locally, to deflection around more competent dolerite bodies. When unaffected by folding, stretching lineations show consistent rakes to the north and northeast with respect to the foliation plane (Fig. 13 ). The sense of shear,

81

deduced from the vergence of asymmetric folds, indicates the same movement sense as in the eastern part of the subarea, i.e. top-to-the-east. Thus, an oblique-slip, normal sense of shear with a left-lateral, horizontal component is again inferred. Metamorphic conditions during ductile deformation vary within this subarea in accordance with the westward transition from spaced and semi-penetrative to more or less penetrative deformation. In the eastern part, the metamorphic conditions are similar to the Botj~iruen subarea, i.e. low- to medium-grade. By contrast, in the western part of the subarea, syntectonic garnet occurs as an accessory mineral in both the gneissic granitoids and the heterogeneously metamorphosed and deformed dolerite intrusions. The interiors of such bodies vary between completely fresh, olivine-bearing dolerite and amphibolite. This may reflect inhomogeneous flow of synmetamorphic fluids through this mechanically competent lithology. Smaller dolerite intrusions are often completely altered to garnet amphibolite. The original feldspar megacrysts in the granitoids are usually recrystallized and only relic outlines are preserved. South of the Niklasdamm subarea, in a corresponding more or less penetratively deformed area, pargasitic hornblende is present in pegmatitic segregations. The occurrence of prograde accessory garnet and pargasitic hornblende, the recrystallization of the granitoids, and the more or less penetrative character of the foliation indicate at least mediumgrade metamorphic conditions in the western part of the subarea.

3.5. Interpretation--a major fan-like structure The regional ductile shear deformation in the Karlskoga-Kristinehamn area has an anastomosing, predominantly N-S trend. The deformation is spaced to semi-penetrative in the eastern and central parts and more or less penetrative and homogeneous in the westernmost part. The foliation dips mainly to the east but, in the eastern part, passes through the vertical and flips over to westerly dips. A fan-like structure can therefore be defined in E-W cross-sections (Fig. 15 ).

Fig. 12. Mesoscopic structures in the Botj~irnen subarea. All photographs show top-to-the-east shear sense. (a) C/S fabric in foliated TIB granitoid. (b) ~/S fabric and asymmetrical extensional shear bands in mylonite from a high-strain zone. Note grain-size reduction and decrease in spacing between C ~urfaces compared to (a). (c) Protomylonitic TIB granitoid from a high-strain zone showing a-type and g-type porphyroclasts and a C/S fabric. (d) Reverse-slip crenulations in protomylonitic TIB granitoid, looking north on vertical face.

%

e~

C.-H. Wahlgren et al. / Precambrian Research 70 (1994) 67-91

~

Dolerite

J - ] Fault

I

Granite,fine- to medium-grained

-~

~

raniteto quartz monzonite, finely porphyritic

~

Granite, locally charnockitic

~

Mafic intrusion

~

vecofennianfelsic metavolcanic rocks

tn F-

83

Form lines of structures related to ductile deformation (post-1.57Ga)

/ 2 o + S, dip in degrees/horizontal 70 1 25

Sm, dip in degrees L, plunge in degrees

1~ 4~ Foldaxis, plunge in degrees/horizontal

Fig. 13. Structural map of the Niklasdamm subarea.

The orientation of the stretching lineation in relation to the foliation indicates oblique-slip movement but with a predominant dip-slip component. Abundant kinematic indicators have revealed a consistent top-to-the-east sense of shear across the entire width of the fan-like structure, independent of the dip of the individual highstrain zones. This implies that, in the current orientation, the dip-slip component of movement varies from reverse in the east, through the vertical, to normal in the west. The subordinate horizontal component is mainly left-lateral. Discrete, mesoscopic high-strain zones are common in the eastern part of the area. These zones are both concordant and discordant with respect to the regional foliation. In the latter case, the regional foliation is deflected into them. On a megascopic scale, the eastern segment is char-

acterized by deflection of the regional N-Strending foliation into NE-SW-trending, relatively narrow and well-defined high-strain zones which are compressional and show a right-lateral horizontal component of movement. This implies that these regional-scale zones are younger than the foliation and it is possible that several of the discrete, mesoscopic deformation zones are related to these structures. These younger zones are an important structural feature since they are the main cause of the undulating N-S trend and the regional NE-SW strike of the ductile shear foliation in the Karlskoga-Degerfors area. Ongoing work has confirmed the presence and importance of these younger structures in the easternmost, frontal part of the Sveconorwegian orogen even farther south, between Degerfors and lake V~ittern.

84

C.-H. Wahlgren et al. / Precambrian Research 70 (1994) 67-91

4. Westward extension of the fan-like structure towards the Mylonite Zone

A reconnaissance survey of the orthogneisses and younger mafic intrusions along a westward continuation of the Karlskoga-Kristinehamn transect towards Karlstad and the Mylonite Zone (Fig. 1 ) has been carried out in order to gain a broader regional overview of the fan-like structure described above. Complementary reconnaissance work along two more transects farther north in these rocks, from east of Molkom to Kil and between Rhda and Torsby (Fig. 1 ), has also been completed. Porphyritic metagranitoid with mafic enclaves, leucocratic and equigranular granite with variable grain size, and metadolerite are the three important lithologies which comprise the orthogneisses and younger intrusions along these three transects. These rocks can be matched with confidence to the porphyritic granite to quartz monzonite, the equigranular, fine- to mediumgrained granite, and the dolerites which comprise the TIB rocks and younger intrusions to the east (see also Lindh and Gorbatschev, 1984; Wahlgren et al., in prep.). Support for this lithological correlation is apparent from radiometric age-dating results. A porphyritic metagranitoid in the orthogneiss belt has been dated at ~ 1.78 Ga (Welin and K~ihr, 1980) and dolerites/metadolerites in both the TIB rocks and the orthogneisses have yielded similar ages of around ~ 1.57 Ga using a variety of techniques (Welin et al., 1980; Johansson and Johansson, 1990; Wahlgren et al., in prep. ) With the exception of the internal parts of some metadolerites, all lithologies contain an homo-

Fig. 14. Mesoscopic structures in the Niklasdamm subarea. All photographs are viewed to the north in subvertical sections. (a-c) Top-to-the-east sense of shear. (a) C/S fabric in a deformed TIB granitoid. (b) Asymmetrically folded foliation in TIB granitoid. (c) Folded stretching lineation in a penetratively deformed and asymmetrically folded TIB granitoid. (d) Strongly crenulated foliation in a penetratively deformed TIB granitoid, with a tendency for the development of an almost horizontal spaced axial surface cleavage. Hinge zone of a larger-scale fold.

C-H. Wahlgren et al. / Precambrian Research 70 (1994) 67-91

85

W

E

Karlstad

Kristinehamn

I ~ ~ _ '

I

Karlskoga

.:,'--. "'~'~

I

~t. I,? iI

, 7" ;

Fig. 15. Schematic structural section across the eastern part of the Sveconorwegian orogen, from Karlskoga to Karlstad. Horizontal and vertical scales identical. The flat-lying structure between Kristinehamn and Karlstad is also documented down to depths of 5-6 km in seismic reflection profile studies (Dahl-Jensen et al., 199 l; Dyrelius et al., 1992).

geneous and penetrative foliation (S), often with an associated conspicuous stretching lineation (L). These structures are deformed by mesoscopic to inferred megascopic asymmetric folds. As a result of the folding, the S surfaces dip gently to moderately both to the north or northeast, and to the southwest (Fig. 16). Furthermore, the stretching lineation also plunges gently to moderately, both to the northeast and to the west or N

•

S (n=20)

+

L (n=15)

Fig. 16. Equal-area, lower-hemisphere stereographic projection for structural data in the orthogneisses (meta-TIB rocks) along the Kristinehamn-Karlstad, Molkom-Kil and R~daTorsby transects.

southwest (Fig. 16). In the western part of the transect, in the gneissic, meta-TIB rocks immediately east of the Mylonite Zone, the stretching lineation plunges gently to the north-northwest (Fig. 16). Reorientation of the gneissic structures in the vicinity of this younger deformation zone is inferred. The structures along the three transects are similar, both in character and orientation, to those observed in the western part of the Niklasdamm subarea (Fig. 11D ). We make the preliminary conclusions that the post-~ 1.57 Ga S and L fabrics, both along the three transects and in the western part of the Niklasdamm subarea, are equivalent to each other and that the S surface in the fan-like structure between Karlskoga and Kristinehamn continues to flatten westwards towards Kadstad and the Mylonite Zone (Fig. 15 ). This is in close agreement with the results of seismic reflection profile studies (Dahl-Jensen et al., 1991; Dyrelius et al., 1992). Since the tectonic foliation is strongly affected by asymmetric folding, the eastward sheet dip of this foliation is probably somewhat steeper than that measured in individual outcrops. The presence of veining and migmatitic structure in the gneissic, meta-

86

C.-H. Wahlgrenet al. / Precambrian Research 70 (1994) 67-91

TIB rocks (Lundeghrdh et al., 1992) suggests syn-deformational metamorphism under medium- to high-grade conditions.

5. Discussion 5.1. Regional tectonic implications

The present study rejects continued usage of the term "Protogine Zone" (Gorbatschev, 1980). North of lake V~ittern, this structure simply corresponds to the boundary between semi-penetratively and more or less penetratively deformed TIB rocks within the eastern part of the Sveconorwegian orogen. It also happens to lie close to a younger and kinematically distinct, late- or postCaledonian brittle fault with which it is often confused (see, e.g., Eriksson and Henkel, 1983; Henkel and Eriksson, 1987; Henkel, 1992). South of lake V/ittern, it is apparently a complex deformation zone which actually marks the Sveconorwegian Front. This zone contains both ductile and brittle components of deformation which, in some studies, have again not been sufficiently well distinguished (e.g., Andrrasson and Rodhe, 1990). We suggest that the term Sveconorwegian Frontal Deformation Zone (SFDZ) is more appropriate and should be used instead of the term "Protogine Zone" south of lake V~ittern. The SFDZ continues north of lake V~ittern towards Degerfors and Karlskoga (Fig. 2 ), in the frontal part of the Sveconorwegian orogen. Here, it is manifested by the, in part demonstrably younger, ductile deformation zones which strike both NNW-SSE and NE-SW and show reverse dip-slip and right-lateral, horizontal components of shear. Inspection of published maps from lake V/inern southwards illustrates a regional rotation of older deformational structures (Svecokarelian in the east as well as early- and/ or pre-Sveconorwegian in the west) into the SFDZ, consistent with a right-lateral, horizontal component of movement (Larson et al., 1990). An important implication of the present work is that the limit of crustal reworking during the Sveconorwegian orogenic event (s) in south-central Sweden is extended at least ca. 40 km to the

east relative to that envisaged by most workers. These results are consistent with the conclusions of Magnusson et al. (1963) and Berthelsen ( 1980, 1987 ), and the occurrence of Rb-Sr metamorphic mineral ages of around 0.90 Ga in the Grythyttan area (Verschure et al., 1987). It is suggested that the conspicuous NNW-SSE structural trend in westernmost Bergslagen is a result of rotation of older, Svecokarelian structures, which commonly strike NE-SW, into the younger, Sveconorwegian, deformational trend. It is also apparent that considerably more attention needs to be paid to the influence of Sveconorwegian crustal reworking on the geometry and orientation of different lithological units within the western part of the TIB rather than on the influence of Svecokarelian structures (Gorbatschev, 1980) or a combination of magmatic flow and Svecokarelian structures (Wikstrrm, 1984, 1991b). The overall top-to-the-east sense of shear along the ductile deformation zones in the fan-like structure and the increase in the grade of syn-deformational metamorphism from east to west across this structure suggest that progressively deeper crustal levels are exposed from the SFDZ around Degerfors and Karlskoga in the east to Kristinehamn in the west. Even deeper crustal levels appear to be exposed west of Kristinehamn towards the Mylonite Zone. This is in agreement with the conclusions of Jarl (1992) based on the interpretation of K-Ar mineral ages. It is suggested that the more heterogeneous deformation in the east and the more penetrative deformation in the west reflect the variable capability of the rock volume to accommodate shear strain at different crustal depths. This tectonometamorphic model provides support for the interpretation (Lindh and Gorbatschev, 1984; Wahlgren et al., in prep.; this study) that the crustal section west of Kristinehamn to the Mylonite Zone is composed predominantly of metaTIB rocks and younger dolerites (i.e. the same rocks as immediately to the east) which were all more or less penetratively reworked under medium- to high-grade metamorphic conditions after ~ 1.57 Ga. In southern Sweden, the orthogneisses be-

C.-H. Wahlgrenet al. / PrecambrianResearch 70 (1994) 67- 91

tween the Mylonite Zone in the west and the SFDZ in the east contain late Sveconorwegian, high-pressure granulites (Johansson et al., 1991; Fig. 1 ). In combination with the results of this study, it appears that the metamorphic grade in the crustal segment between the Mylonite Zone and the frontal part of the Sveconorwegian orogen decreases from SSW to NNE. The deepest crustal levels occur in southern Sweden where the Sveconorwegian metamorphic break across the SFDZ is also most extreme. North of lake Vattern, tectonostratigraphically higher crustal levels in the Sveconorwegian orogen are better preserved. Where the older Svecokarelian metamorphism in this area is low-grade (e.g., around Grythyttan), a separation of this tectonometamorphic event from the Sveconorwegian overprint is considerably less distinct. 5.2. A tectonic model for the fan-like structure

Any tectonic interpretation of the ductile deformation in the eastern part of the Sveconorwegian orogen must take into account the following important observations along the Karlskoga-Kristinehamn transect: ( 1 ) the overall top-to-the-east displacement, (2) the east-towest increase in metamorphic grade, (3) the fanlike geometry of the structure, and (4) the presence of younger ductile deformation zones (SFDZ) in the eastern part of the transect. Two principal explanations for the observations in the Karlskoga-Kristinehamn transect are considered here. 5.2.1. Regional extension and rotation Any model involving a regional extension must take into consideration the time constraints on the deformation. These permit extension after intrusion of the ~ 1.57 Ga old dolerites but prior to development of the Sveconorwegian deformation zones of the SFDZ which are compressional in character. Andrrasson and Rodhe's (1990) tectonic interpretation is considered highly doubtful since it ignores the unconformity (Wikstrrm and Karis, 1993) between the older Proterozoic rocks, affected by both ductile deformation zones and younger brittle faults, and

87

the Neoproterozoic and younger sedimentary rocks affected solely by brittle deformation. Late Sveconorwegian extension (Welin and Blomqvist, 1966; Patchett and Bylund, 1977) is an attractive possibility, bearing in mind the necessity to uplift the Sveconorwegian high-pressure granulites of southern Sweden and the presence of abundant cPolerites ~ 1.00-0.91 Ga in age which strike more or less parallel to this orogen. However, such a tectonic interpretation is not favoured since the Sveconorwegian compressional structures of the SFDZ occurred after the extensional phase. A tectonic scenario which involves regional E-W extension post-l.57 Ga but pre-SFDZ remains. Normal and reverse deformation zones are known to form more or less simultaneously within the hanging-wall block of an undulating (sinuous), oblique-slip, extensional detachment fault (e.g., Gibbs, 1987 ). The equivalent of such a tectonic scenario in the ductile rrgime (Fig. 17a) is considered one possible model for the fanlike structure which can explain the geometry and kinematics of the deformation along the transect. A second, more attractive model emphasizes again the interference of two separate tectonic events for development of the fan-like structure. Deformation zones with normal senses of movement may have initially steepened eastwards. However, they rotated through the vertical to become apparently reverse in character due to later rotation into the younger compressional structures of the SFDZ (Fig. 17b). 5.2.2. Regional compression and rotation The structures to the west of the Mylonite Zone are characterized by eastward thrusting consistent with build-up of a thickened crustal stack in a compressional rrgime (Heybroek and Zwart, 1949; Zeck and Mailing, 1974; Berthelsen, 1987; Gorbatschev, 1988; Park et al., 1991). Sm-Nd mineral isochron ages in the high-pressure granulites between the Mylonite Zone and the SFDZ in southern Sweden are also consistent with considerable crustal thickening in connection with Sveconorwegian collisional processes (Johansson et al., 1991 ). Thus, the regional geological context argues strongly for formation of the ge-

C.-H. Wahlgren et al. / Precambrian Research 70 (1994) 67-91

88

(a)

(b)

Older deformation zones related to extension

(d)

Older imbricate

(C)

Younger

stack ~

Younger SFDZ

Fig. 17. Alternative tectonic models for the fan-like structure between Karlskoga and Kristinehamn, eastern part of the Sveconorwegian orogen: (a) and (c) envisage that this structure formed during one progressive event, while in (b) and (d) the fanning is explained as a result of two separate tectonic events. All models contain a subordinate left-lateral horizontal component of shear which is not indicated on the figures. All sections are viewed to the north.

ometry and kinematics of the ductile deformation zones along the transect within an east-verging, compressional r6gime. Time constraints allow compressional tectonics after ~ 1.57 Ga but prior to deposition of post-Sveconorwegian rocks. The virtually constant sense of shear along the transect also suggests that the fan-like structure may be related to rotation of the ductile deformation zones either during or after their formation. However, the kinematic data do not provide support for the structural interpretation of Berthelsen (1980). One possible model in the compressional r6gime involves back-rotation of an imbricate thrust system towards the hinterland of the Sveconorwegian orogen (Wahlgren and Stephens, 1990). Steepening and back-rotation of hinterland-dipping structures, due to continued con-

vergence and increasing strain, has been documented in accretionary wedges (Davis et al., 1983 ) and, on a scale similar to that in the Karskoga-Kristinehamn area, within crystalline basement massifs ofcollisional orogens (e.g., Aar Massif; Boyer and Elliot, 1982). Such a model envisages the structures along the transect to have formed as an imbricate fan in which listric thrust surfaces root into a gently west-dipping sole thrust (Fig. 17c). This model is attractive in that it is consistent with the key observations documented above. However, a > 90 ° counter-clockwise back-rotation about a horizontal N-Strending axis is required to restore NE-plunging stretching lineations and east-dipping foliations back to their inferred, initial northwest plunges and west dips, respectively. We are not aware of other examples of such extreme back-rotation in

C.-H. Wahlgren et al. / Precambrian Research 70 (1994) 67-91

the literature. Furthermore, this model fails to take into account deformation associated with the SFDZ. A second, more attractive model in the compressional rrgime, and the one favoured overall in this paper, explains the fan-like structure as a combination of two factors--an oblique cut through part of an early Sveconorwegian or older imbricate thrust stack with easterly sense of vergence and later rotation of individual thrust slices into the younger Sveconorwegian ductile deformation zones of the SFDZ (Fig. 17d). This model explains the geometry and kinematics of the fan-like structure as well as the E-W increase in metamorphic grade. Oblique-slip motion on most shear zones in the area can be accounted for if a subordinate left-lateral transcurrent component is allowed to operate during the development of the thrusts (i.e. left-lateral transpression). Such a model attributes the fan-like structure to two separate tectonic events. The present work emphasizes the urgent need for tighter constraints on the metamorphic conditions and the timing of deformation across the fan-like structure. It should then be possible to resolve more confidently the alternative models discussed here and to improve our understanding of the thermochronological history of the Sveconorwegian orogen.

6. Conclusions ( 1 ) The "Protogine Zone" concept is obsolete and, in order to avoid further confusion, the present study rejects continued usage. Based on new findings presented here and the regional structural relationships, the Sveconorwegian Frontal Deformation Zone (SFDZ) is suggested to be the most appropriate term for the ductile deformation zones with a right-lateral horizontal component of shear deformation south of lake V~ittern and for the ductile deformation zones with reverse dip-slip and right-lateral horizontal components of movement in the easternmost part of the orogen farther north (DegerforsKarlskoga). (2) The older ductile deformation zones in the

89

Karlskoga-Kristinehamn area constitute a fanlike configuration in an east-west section and display a consistent top-to-the-east sense of shear across the entire structure. The fan forms the easternmost part of a major asymmetric structure that extends subhorizontally from Kristinehamn and westwards towards the Mylonite Zone. Younger ductile deformation zones belonging to the SFDZ are responsible for the major change in strike orientation of these older deformation zones from NW-SE to NE-SW in the Karlskoga-Kristinehamn area. (3) The fan-like structure can be explained in terms of build-up of an imbricate thrust stack in either an early Sveconorwegian or older (post~ I. 57 Ga), eastward-verging, compressional rrgime followed by rotation of these structures into the late-Sveconorwegian SFDZ. Less confidently, it may be related to early Sveconorwegian or older regional extension in an E-W direction followed by a similar rotation into the younger compressional SFDZ. Both models incorporate the evidence for polyphase shear deformation in the eastern part of the KarlskogaKristinehamn transect. (4) Irrespective of the choice of tectonic model for the fan-like structure, it is clear that the SFDZ is not a Sveconorwegian collisional suture. Apart from intracrustal deformation, the area including the old Svecokarelian orogen in the east up to the Mylonite Zone in the west has constituted a coherent crustal segment at least since the intrusion of the TIB and onwards.

Acknowledgements The input by seasonally employed geologists who have participated in the fieldwork, in particular Peter Rrnnlund (formerly Uppsala University), is gratefully acknowledged. Furthermore, discussions with various colleagues during the duration of this study, in particular Joseph Hull (formerly Uppsala University) and the participants in an excursion across the fan-like structure Spring 1993, have been of considerable help. Laurence Page (Lund University) and Graham Park (Keele University) read an early

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draft of the manuscript and provided particularly useful comments. Reviews by Cees Passchier and an anonymous referee also provided valuable improvements to the manuscript. This study has been financed by the two programmes "Regional Bedrock Mapping" and "Research and Development", at the Geological Survey of Sweden. This is a contribution to the International Geological Correlation Programme (IGCP) Project 275 on the "Deep Geology of the Baltic Shield".

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