Transpressive shear related to arc magmatism: The Paleoproterozoic Storsjön-Edsbyn Deformation Zone, central Sweden

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1 TECTONICS, VOL. 25,, doi: /2005tc001815, 2006 Transpressive shear related to arc magmatism: The Paleoproterozoic Storsjön-Edsbyn Deformation Zone, central Sweden Stefan Bergman Geological Survey of Sweden, Uppsala, Sweden Håkan Sjöström Department of Earth Sciences, Uppsala University, Uppsala, Sweden Karin Högdahl 1 Laboratory for Isotope Geology, Swedish Museum of Natural History, Stockholm, Sweden Received 11 March 2005; revised 11 October 2005; accepted 26 October 2005; published 24 January [1] The polyphase, km wide and >200 km long Storsjön-Edsbyn Deformation Zone (SEDZ) apparently separates two major Paleoproterozoic provinces in central Sweden. Four main phases of deformation have been recognized along the zone; associated high-strain rocks are characterized by their mineralogy, microstructures, kinematic patterns, magnetic signatures, and relative ages. The mineral and stretching lineations are consistently steeply to moderately plunging. Analysis of mesoscopic shear zone populations of different generations suggests that most displacements are compatible with north-south to northeast-southwest bulk regional shortening. The structural analysis suggests that the main ductile deformation along the SEDZ was due to dextral transpression with a steep coaxial component. Field evidence combined with published age data constrain the main deformation to the time interval circa Ga. Synkinematic titanite from a ductile protomylonite in the SEDZ yields an U-Pb age of 1674 ± 6 Ma. The combined results imply transpressive deformation broadly synchronous with, and spatially related to, volcanism and plutonism in a continental margin magmatic arc setting. Citation: Bergman, S., H. Sjöström, and K. Högdahl (2006), Transpressive shear related to arc magmatism: The Paleoproterozoic Storsjön-Edsbyn Deformation Zone, central Sweden, Tectonics, 25,, doi: / 2005TC Introduction [2] There are several reasons why deformation zones (ductile shear zones, faults and fracture zones) have been 1 Now at Department of Geology, Lund University, Lund, Sweden. Copyright 2006 by the American Geophysical Union /06/2005TC paid much attention to during the last decades. They may control the formation and geometry of economic mineral deposits and represent potential conduits to syntectonic magmatic rocks. On a larger scale the kinematics of deformation zones help to constrain the conditions during lithospheric deformation. Anisotropies defined by deformation zones on crustal or lithospheric scale are potentially hazardous in the context of nuclear waste disposal as they are apt to be reactivated during future deformation. To define the location, geometry and size of large-scale deformation zones it is necessary to integrate geophysical and geological interpretations. Although geophysical data also give apparent information about kinematics and the structural history, this study demonstrates that complementary structural mapping, microstructural studies and geochronology considerably increase the possibility to integrate shear zone activity into the regional geological evolution. [3] The likely relationships between plutonic rocks and deformation zones have led to new ideas about the mechanisms for magma ascent and emplacement in the crust [e.g., Brown and Solar, 1998a; Petford et al., 2000]. Both strike-slip and extensional deformation zones [e.g., Hutton, 1988], as well as zones characterized by contraction or transpression [D Lemos et al., 1992; Saint Blanquat et al., 1998; Brown and Solar, 1998b] have been proposed to facilitate magma transport. The role of effective stress in the ascent and emplacement of granites in contractional tectonic settings has been discussed by Hutton [1997]. [4] The Storsjön-Edsbyn Deformation Zone (SEDZ), extending 200 km from Lake Storsjön to Edsbyn in central Sweden (Figures 1 and 2), is a major structure in the Paleoproterozoic rocks of the Fennoscandian Shield [Bergman and Sjöström, 1994; Bergman et al., 2004]. The magnetic signature is conspicuous and characterized by a NNW-SSE oriented complex belt discordantly cutting the magnetic pattern in surrounding areas (Figure 3). East-west trending rock units and linear magnetic anomalies to the east rotate clockwise approaching the deformation zone, indicating an apparent dextral, ductile, horizontal shear component. Several narrow, persistent low-magnetic lineaments truncate the ductile structures indicating reactivation 1of16

2 Figure 1. Bedrock map of the Fennoscandian Shield, modified from Koistinen et al. [2001]. Form lines of tectonic foliation in Sweden are from Stephens et al. [1994]. Note that the N-S trending belt of Ga intrusive rocks is discordant to older Svecokarelian structures to the east. Location of Figure 2 is indicated. SEDZ, Storsjön-Edsbyn Deformation Zone; HSZ, Hassela Shear Zone. at shallower crustal levels. On a regional scale (Figure 1), but not in detail, the SEDZ coincides with the boundary between two major Paleoproterozoic provinces, namely the circa 1.7 Ga Dala-Rätan igneous rocks to the west and older rocks (circa Ga) to the east. In this paper we will define the location, structure, kinematics and age of the SEDZ and present a model for its formation during transpression in a magmatic back-arc setting. 2. Regional Geological Framework and Previous Work 2.1. Magnetic Anomaly Patterns [5] On the magnetic anomaly map (Figure 3) the SEDZ is defined by a km wide zone with a pronounced banded or lenticular pattern of straight or slightly curved, commonly 5 10 km long, positive and negative anomalies. The zone strikes NNW SSE in the northern and central parts and N-S in the south. The anomaly pattern indicates a northward and southward widening of the zone, and that the centre of the zone is shifted toward the west. It is truncated by narrow, straight, up to 100 km long negative anomalies. To the south a WNW ESE lineament terminates the banded pattern of the SEDZ in the Edsbyn area. To the north, the magnetic signature of the SEDZ persists to Lake Storsjön where it disappears below the Early Paleozoic Caledonides. [6] To the west of the SEDZ, the relatively smooth, highmagnetic pattern of the circa 1.7 Ga Rätan Batholith is interrupted by low-magnetic fault or fracture zones. However, a few, some kilometers long NE-SW and N-S ductile shear zones (apparent dextral) are indicated within the batholith. As the peak of regional metamorphism and deformation preceded the Rätan Batholith by at least 0.1 Ga, these shear zones probably reflect deformation in the magmatic state. In the circa 1.8 Ga Revsund granitoids and their inclusions of older Svecokarelian rocks, the banded magnetic pattern indicates more pervasive SEDZ deformation (Figure 4). South of the batholith, magnetic 2of16

3 Figure 2 3 of 16

4 metavolcanic rocks are surrounded by less magnetic metasedimentary rocks and orthogneisses. [7] The area east of the SEDZ (southeastern part of Figures 2 and 3) is dominated by variably magnetic gneissose granitoids and minor supracrustal rocks. An E-W banding and tight to isoclinal folds are visible over large areas. These structures rotate clockwise as the SEDZ is approached, demonstrating that dextral shear is superimposed on the preexisting Svecokarelian deformation. Low-magnetic fault or fracture zones are visible where background magnetization is sufficiently high. [8] In the northeastern part of Figure 3 the magnetic pattern is smooth due to the presence of flat-lying, magnetic, circa Ga dolerite sheets, which efficiently mask any underlying magnetic structures. The boundary between this area and the area to the south is very sharp, and coincides with the Hassela shear zone (Figure 1) Bedrock [9] The Paleoproterozoic Svecokarelian orogeny of central Sweden began with deposition of early Svecofennian sediments and volcanic rocks, and intrusion of coeval mafic to felsic plutonic rocks. The nature of the preexisting crust is still unknown. The stratigraphy of the supracrustal rocks was described by Lundqvist [1987b]. [10] The oldest dated gneissose granitoids (including the Ljusdal Batholith, Figure 2) adjacent to the SEDZ are Ga. These granitoids are partly intensely folded and affected by high-grade low-pressure metamorphism. A minimum age of 1.82 Ga was indicated for Svecokarelian regional deformation and metamorphism in central Sweden by Claesson and Lundqvist [1995]. [11] The Revsund suite extends 350 km from the north into the area of Figure 2. These coarsely porphyritic circa 1.8 Ga [Claesson and Lundqvist, 1995] and similar Ga [Högdahl, 2000; Högdahl and Sjöström, 2001] rocks are generally isotropic outside shear zones, i.e., they postdate the regional Svecokarelian deformation and metamorphism in that area. [12] The Rätan Batholith (Figure 2) has intrusion ages of circa 1.7 Ga [Patchett et al., 1987; Delin, 1996; Ahl et al., 2004]. It predominantly consists of a coarse-grained, K-feldspar porphyritic granite, with subordinate quartz monzodiorite-monzogabbro [Gorbatschev et al., 1997]. The elongate NW-SE batholith is 60 km wide and >100 km long. The northwestern part is covered by Phanerozoic sedimentary rocks and allochthonous Caledonian units. The granites generally have an isotropic appearance, but AMS studies [Mattsson and Elming, 2001a, 2001b] have shown that there is a well-developed magnetic fabric in the batholith. The central part has a subhorizontal magnetic foliation, and a NW-SE, subhorizontal magnetic lineation. Steep magnetic foliation and variable orientations of magnetic lineation characterize the northeastern and southwestern margins. [13] The circa 1.7 Ga [Lundqvist and Persson, 1999] Dala volcanic rocks (southwestern part of Figure 2) form a several thousands of meters thick sequence of ignimbrites, lavas and intercalated conglomerates and sandstones [Lundqvist, 1968]. They rest unconformably on deeply eroded, deformed and metamorphosed Svecokarelian rocks. Lithological, mineralogical, geochemical, and isotopic evidence suggests that the volcanic sequence was deposited in an extensional tectonic setting within a thick active continental margin [Nyström, 2004]. [14] Post-Jotnian circa Ga [Patchett, 1978; Suominen, 1991] dolerite dikes exist near Edsbyn and large volumes are found mainly as flat-lying sheets in the northern and western part of the area (Figure 2). They crosscut the metamorphic pattern and most structures Deformation Zones: Background [15] Very different hypotheses have been presented regarding the location and kinematics of major deformation zones in central Sweden. Most of these zones do not coincide with the SEDZ as defined in this study. In the region thrusts or zones with vertical movements [Magnusson et al., 1957; Lundegårdh, 1960, 1967; Strömberg, 1974, 1976, 1978; Ginet, 1980; Berthelsen, 1987] or strike-slip zones [Gaál and Gorbatschev, 1987; Beunk and Valbracht, 1991; Sturkell and Lindström., 1994] have been proposed. Also deformation zones representing boundaries between geological provinces or crustal blocks [Magnusson et al., 1957; Lundegårdh, 1960, 1967; Berthelsen, 1987; Gaál and Gorbatschev, 1987; Gorbatschev, 1993] have been inferred. One important zone is the Hassela shear zone (Figures 1 and 2) [Bergman and Sjöström, 1994; Sjöström et al., 2000; Högdahl and Sjöström, 2001], which probably is part of a major crustal structure across the Fennoscandian Shield. It defines the boundary between the Ljusdal Batholith and the older Svecofennian metasedimentary rocks (>1.88 Ga [Claesson et al., 1993], Bothnian Basin) to the north. It is a steep, WNW-ESE to NW-SE, dominantly dextral strike-slip shear zone, which formed Ga [Sjöström et al., 2000; Högdahl and Sjöström, 2001]. 3. Early Mylonites (Type 1) [16] Four characteristic groups of high-strain rocks along the SEDZ and adjacent areas have been distinguished (Table 1). The mylonite types 2 (Figures 5a, 5d, 5f, and 5h) and 3 (Figure 5b) are most characteristic for the SEDZ relative to the region, whereas the formation of high-grade mylonites (type 1) and cataclasites (type 4, Figure 5i) are not obviously Figure 2. Bedrock map of the Storsjön-Edsbyn area, compiled from Lundegårdh [1967], Lundegårdh et al. [1984], Lundqvist [1987a], and Delin and Aaro [1992, 1995, 2000a]. Stars indicate sites for age determinations compiled from Welin [1980, 1987], Delin [1993, 1996], Welin et al. [1993], Delin and Persson [1999], Delin and Aaro [1999, 2000b], Lundqvist and Persson [1999], Högdahl [2000], Högdahl and Sjöström [2001], and Ahl et al. [2004]. VFZ, Vikbäcksviken Fault Zone. Location of Figure 4 is indicated. White areas represent lakes. 4of16

5 Figure 3. Magnetic anomaly map of the deformation zone from Lake Storsjön in the northwest to Lake Stor-Öjungen near Edsbyn in the south. Note how the east-west grain in the southeastern part rotates dextrally into the north-northwest striking deformation zone near the edge of the magnetic Rätan Batholith in the west. Note also dextral offset of the magnetic volcanic rocks near Los. The short edge of the map is 100 km long. The illumination has a declination of 65 and an inclination of 60. No magnetic data are available in the white area. 5of16

6 Figure 4. Interpretation of the magnetic anomaly map in the northwestern part of the area (see Figure 2 for location). Banded magnetic pattern indicates pervasive deformation of Revsund granite and older rocks. Some shear zones are indicated with inferred sense of shear. Note that ductile shear zones are indicated also within the Rätan granite. VFZ, Vikbäcksviken Fault Zone. linked to the SEDZ. The latter are recorded along lowmagnetic faults or fracture zones. They are poorly exposed as they are confined to topographic lineaments. [17] High-temperature deformation zones in the northern part of the area are defined by linear positive magnetic anomalies, which probably reflect the growth of magnetite porphyroblasts recorded in some rocks along such anomalies. Mesoscale, generally 5 20 cm wide, ductile shear zones are very common in the region. Some major zones are several hundreds of meters wide. They formed in both supracrustal and plutonic rocks and contain variable amounts of leucosome. The deformed rocks in the shear zones have grain sizes of mm, lack porphyroclasts and have undergone static recrystallization. Most grains have a strain-free appearance, and quartz grain boundaries are straight or lobate. Several shear zones contain deformed, early pegmatites but are also crosscut by later pegmatite veins. The main foliation is a compositional banding. The stretching lineation is defined by elongate minerals and aggregates, but in some cases there is no linear fabric on the shear plane. The sense of movement is indicated by passively rotated planar markers (e.g., gneiss banding), S-C fabrics, and more rarely, minor shear band-like shear zones. [18] The orientations and kinematics of type 1 shear zones are remarkably consistent in the whole region. Most shear zones have a dominant strike-slip component with dextral displacement on NW-SE trending zones (parallel to long limbs of folds), and sinistral movement on NE-SW trending zones (Figure 6), thus indicating regional northsouth shortening and east-west extension. 4. Character, Orientation and Kinematics of Mylonites Within the SEDZ 4.1. Type 2 Mylonites [19] The dominant mylonites within the SEDZ exhibit a pronounced foliation defined by quartz ribbons, preferred Table 1. Generalized Summary of Characteristic Features of the Different High-Strain Rocks in the Region a Description Temperature/ metamorphic grade Deformation mechanisms, textures Type 1 Type 2 Type 3 Type 4 ductile, synmetamorphic, regionally distributed high, synpeak metamorphism, leucosome formation crystal plasticity, static recrystallization, strain-free grains mostly ductile, retrograde, typical of SEDZ medium-low, postpeak metamorphism dynamic recrystallization, dislocation creep, quartz ribbons, core-andmantle texture brittle-ductile, typical of SEDZ low brittle and intracrystalline deformation, local dynamic recrystallization, quartz porphyroclasts brittle, regionally distributed very low cataclasis Magnetic commonly high commonly high low low susceptibility Common minerals magnetite in leucosome chlorite, magnetite epidote, chlorite, white mica locally laumontite Kinematic indicators sigmoidal foliations, porphyroclast wings sigmoidal foliations, porphyroclast wings, S- C fabrics, shear bands sigmoidal foliations, S-C fabrics, quartz fibers, Riedel fractures displaced markers, Riedel fractures Estimated age Ga ± Ga <1.6 Ga? <1.2 Ga? a Types 2 and 3 are confined to the SEDZ. 6of16

7 Figure 5 7of16

8 Figure 6. Stereogram (Schmidt projection, lower hemisphere) of poles to boundaries of ductile strike-slip shear zones (type 1). In most parts of the region dextral and sinistral zones separate into quadrants indicating approximately east-west extension or north-south shortening. orientation of recrystallized quartz and mica, and flattened feldspar aggregates. The foliation is locally folded perhaps due to progressive shearing. A strong to weak lineation on the foliation is defined by mineral aggregates and individual grains of quartz, feldspar and mica. The elongate shape of the aggregates and the preferred orientation of minerals indicate that it is a stretching lineation. In addition, intersection lineations between S and C foliations, or between C foliations and shear bands, at a high angle to the stretching lineation, can be seen in places. Fold axes and stretching lineations commonly trend close to the dip direction of the foliation. The magnetic susceptibility in type 2 mylonites is generally high. This suggests that neomagnetization associated with mylonitization accounts for the banded patterns on magnetic anomaly maps. This important finding greatly improved the outlining of the SEDZ Mineralogy and Texture [20] The textures and mineral assemblages suggest that these mylonites formed under greenschist facies conditions. In thin section quartz grains show undulose extinction and feldspars are fractured and sericitized or more rarely have dynamically recrystallized mantles and asymmetric wings. Some rocks contain quartz ribbons. The grains in the quartz ribbons show undulose extinction, subgrains and partly recrystallized fabrics. A grain shape fabric may be present within the quartz ribbons. Posttectonic pinitization of cordierite, growth of white mica and sericitization of feldspar porphyroclasts is common in mylonites in the Haverö area. Biotite is either recrystallized or more or less replaced by chlorite. [21] Secondary magnetite and titanite (see section 5.2) have formed during chloritization. Magnetite appears as euhedral posttectonic porphyroblasts or as porphyroclasts in sheared chlorite. In extreme cases, up to 1 cm thick magnetite (+hematite) veins were formed. Kinematic indicators in type 2 mylonites include sigmoidal foliations, porphyroclast wings, S-C fabrics and shear bands Kinematics [22] In these steep mylonites, both western-side-up and eastern-side-up/reverse movement have been recorded (Figure 7), as clearly shown in the Haverö area (Figure 7c). In other areas this pattern is not as obvious but there is a slight dominance for western-side-up/reverse move- Figure 5. (a) Ductile S-C fabric (type 2) in Revsund granite. A SW (left side) up sense of shear is indicated on the steep WSW (left) to ENE (right) surface. The lens cap is 60 mm in diameter. Lerån, 8 km east of Hackås. (b) Incipient brittleductile (or ductile?) deformation in Revsund granite. The two zones across the picture are the result of intense grain size reduction. Together with the grain shape fabric (lower left to upper right) defined by the feldspar phenocrysts, they make up an S-C fabric (type 3) indicating a dextral strike-slip component. Långviken, 6.7 km east of Hackås. (c) Weakly foliated K-feldspar porphyritic Rätan granite. The coin is 20 mm in diameter. 8 km east of Svenstavik. (d) Same granite as in Figure 5c but with a strong mylonitic fabric (type 2). Porphyroclasts of K-feldspar are still visible. View is toward the north on a steep surface. The coin is 20 mm in diameter. Bingsta is 7 km east of Svenstavik. (e) A post-jotnian 0.5 m thick, subhorizontal dolerite (D) truncates the ductile fabric of the SEDZ, (steep dashed lines) demonstrating that the ductile deformation predates circa Ga. Road cut along road 45, 4 km south of Hackås. (f) Photomicrograph showing the dolerite (black) truncating the subvertical ductile SEDZ-fabric (type 2) defined by grain shape preferred orientation and biotite. Feldspar phenocrysts (white) in the dolerite define a magmatic foliation along the contact. The horizontal length of the picture corresponds to 5.8 mm. (g) Sinistral S-C-mylonite (type 2) overprinted by cataclasite (type 4), in turn cut by a discrete contractional microfault. The protolith is a metaarenite. The vertical length of the photograph corresponds to 18 mm. Shore of lake Havern at Löten, 17 km SSE of Haverö. (h) Close view of central part of Figure 5g. Light bands consist of quartz ribbons and the asymmetric plagioclase porphyroclasts are postkinematically sericitized. The sigmoidal shape of a magnetite grain in the upper left corner corroborates the sinistral movement. Late oxide filled fractures cut the mylonitic foliation at a high angle. (i) Close view of the cataclasite in Figure 5g. Rotated angular to slightly rounded mylonite fragments dispersed in a fine grained matrix of recrystallized fault gouge. (j) Pseudotachylite vein in amphibolite along the VFZ. The lens cap is 60 mm in diameter; 0.5 km west of Långtjärnen, 9 km NNE of Svenstavik. 8of16

9 Figure 7. Results of kinematic analysis of ductile (type 2) shear zones plotted on a map showing mylonite-related, linear, positive magnetic anomalies. Steep motion is clearly more common within the zone while strike-slip motion dominates outside the zone. (a g) Stereograms (Schmidt projection, lower hemisphere) of poles to foliation in dip-slip mylonites within the SEDZ. Note that the strike of measured mylonite foliation deviates from the strike of the SEDZ. Their orientation appears to be controlled by the strike-slip component of dextral transpression while their finite slip direction results from the shortening component across the SEDZ. In the central part (Figures 7b 7d) stretching lineations vary in trend across the zone while in the north and south (Figures 7a and 7f) they vary in plunge along the zone. ment in the data set. The mylonitic foliation generally strikes oblique (anticlockwise) to the main shear zone, which is demonstrated both by the stereograms and the orientations of magnetic lineaments (Figure 7). At the bend of the main shear zone from NNW SSE to N-S near Kårböle there is a corresponding rotation of the mylonitic foliation (compares Figures 7c and 7f), and also a change in the orientation pattern of the generally steep stretching lineations. North of the bend the plunges of the lineations vary mainly across the general strike of the shear zone, in contrast to the south where they vary along the shear zone strike. [23] In the northern part of the SEDZ thin slices of NNW- SSE oriented, steeply dipping Svecokarelian gneissose granitoids and amphibolites, with interleaved minor occurrences of supracrustal rocks, characterize a 10 km wide zone. The geometry indicates a strike-slip duplex or a deep section of a flower structure. In some cases the early Svecokarelian rocks are L tectonites rather than S-L tecton- 9of16

10 Figure 8. (a) Stereographic plot of fabrics in brittle-ductile (type 3) shear zones in the northern part of the area. (b) Principle strain axes from brittle-ductile shear zones in the northern part of the area (constructed from data collected from eight outcrops). (c) Stereographic plots of poles to brittle-ductile shear zones from a locality near Haverö, on a regional NNW lineament. All diagrams are Schmidt projection, lower hemisphere. ites. In this area there is no consistent pattern in the kinematics of type 2 mylonites. Both normal and reverse dip-slip components are indicated, possibly with a dominance of the former (Figure 7a). Stretching and mineral lineations vary in plunge from shallow to steep along the strike of the deformation zone, with a dominance for the latter (Figure 7a). Kinematic indicators are often contradictory, showing west-side-up and east-side-up in XZ sections parallel to the stretching lineation. The symmetry of the kinematic pattern in these sections tends to be orthorhombic and the sections locally contain symmetric boudinage. In YZ sections, however, the kinematic pattern is often more consistent and dominantly monoclinal although the sections are more or less perpendicular to the inferred direction of transport (if defined by the stretching lineation). This difference between XZ and YZ sections either represents pre- and syn-sedz deformation, respectively, or that there was a significant component of pure shear contemporaneously with shearing (i.e., transpression), in agreement with the theoretical strain model by Tikoff and Teyssier [1994], Fossen and Tikoff [1998]. In younger rocks (Revsund granitoids) the fabrics are generally simpler to interpret Displacement [24] The displacement along the SEDZ is difficult to directly estimate due to lack of displaced markers. An exception exists in the Los area where a gently plunging, upright, isoclinal fold in a well-characterized volcanosedimentary sequence is displaced. From the map pattern and with stratigraphic considerations, Lundqvist [1968] suggested a dextral offset of 9 km with a minor vertical component. However, the only mylonites that are exposed in the area are steeply dipping with steeply plunging stretching lineations, and this offset is not satisfactorily explained by movement related to these mylonites Type 3 Mylonites [25] Brittle-ductile mylonites are associated with persistent lineaments characterized by low magnetization, e.g., at the type locality near Haverö. Epidotization gives these rocks a characteristic greenish color. Quartz veins and pods are common along, or at a small angle to, a more or less well developed anastomosing cleavage. The cleavage (pressure solution and/or fracture cleavage?) is heterogeneously developed, and variably curved or offset markers across it suggests mixed brittle and ductile deformation Microstructure [26] In thin section quartz shows evidence for both brittle and intracrystalline deformation. Quartz porphyroclasts are present, and the grains show undulose extinction, formation of subgrains and local dynamic recrystallization. Evidence for grain boundary migration recrystallization is locally found. A well-developed S-C fabric is common. Microfractures are ubiquitous, especially in feldspars. Biotite and feldspar are retrogressed to a chlorite-epidote-carbonate assemblage. [27] Pseudotachylite veins, rarely exceeding 3 cm in thickness, are often spatially associated with type 3 mylonites (Figure 5j). They contain amygdules (up to 5 mm in size), microlites and included fragments of wall rock, and show flow banding and intrusive apophyses. [28] The Vikbäcksviken Fault Zone is a major polyphase deformation zone in the northern part of the area (VFZ, Figures 2 and 4). Along this zone early high-temperature mylonites are overprinted by brittle-ductile mylonites. The latter are epidote bearing and are spatially associated with pseudotachylite Kinematics [29] Brittle-ductile deformation zones show a relatively simple pattern: Preexisting foliations rotate clockwise into dextral, generally steeply dipping NNW-SSE shear zones (Figure 8a). Lineations are gently plunging, indicating dominantly strike slip motion. Measured and constructed (on stereograms) intersection lineations generally have steep plunges, consistent with strike-slip movements (Figure 8a) Strain Orientation [30] In low-grade brittle-ductile mylonite zones, millimeters to centimeters thick conjugate strike-slip shear zones 10 of 16

11 Table 2. U-Pb Isotopic Data Sample Size, mm Weight, mg U, ppm Pb Total, ppm Common Pb, ppm 206 Pb/ 204 Pb a 206 Pb- 207 Pb- 208 Pb Radiogenic, b at. % 206 Pb/ 238 U b 207 Pb/ 235 U b 207 Pb/ 206 Pb Age, Ma r c Zircon ± ± ± Zircon ± ± ± Zircon 3 < ± ± ± Zircon 4 < ± ± ± Zircon 5 < ± ± ± Zircon ± ± ± Titanite ± ± ± Titanite ± ± ± Titanite ± ± ± Titanite ± ± ± a Corrected for mass fractionation (0.1% per amu). b Corrected for mass fractionation, blank and common Pb. c Error correlation. or shear fractures are common. As for higher-grade zones there is a systematic distribution on stereograms and it is generally straightforward to approximate the orientations of the principal planes that separate the dextral and sinistral groups. [31] In the northern part of the area, a NE trending shortening direction and a NW trending extension direction is indicated (Figure 8b). The Z axis is at a high angle to the SEDZ, which suggests that there was a component of shortening during the strike-slip movement, i.e., transpression. The Z axis bisects the obtuse angle and the X axis the acute angle between conjugate shear zones, i.e., demonstrating conditions within the regime of ductile failure [e.g., Twiss and Moores, 1992]. Farther south, in the Haverö area, the distribution of dextral and sinistral zones together with the distribution of quartz veins suggest N-S to NE-SW shortening (Figure 8c). The field relations of these veins and parallel epidote-filled veins corroborate their intimate relation with the brittle-ductile deformation. The angle between the average sinistral and dextral zones is here close to Age of Deformation 5.1. Relative Deformation Ages [32] The mylonites of the oldest group (type 1) are recognized by their intimate relation to regional folds, their synmetamorphic microstructures and the fact that they are older than or synchronous with pegmatites. Protolith ages [e.g., Delin, 1993] and the age of a postmetamorphic anatectic granite [Claesson and Lundqvist, 1995] bracket these mylonites to the time interval circa Ga. [33] The type 2 mylonites within the SEDZ deform the rocks of the Ljusdal Batholith as well as those in the Revsund Suite. These mylonites locally affect the circa 1.7 Ga Rätan Batholith, and they are older than crosscutting circa Ga [Patchett, 1978] dolerites (Figures 5e and 5f). The type 3 mylonites probably formed soon after the type 2 mylonites within the SEDZ, but their pre-1.2 Ga age can only be inferred. [34] The fact that topographic lineaments along the SEDZ continue into the Caledonides show that brittle reactivation of the deformation zone occurred in the Phanerozoic. It has also been suggested that a large NW-SE trending lineament on the Norwegian shelf is related to the SEDZ [Fichler et al., 1999]. The seismic activity along this lineament is partly increased, which is probably related to the present ridge-push force Geochronology Methods [35] To establish the age of a protolith affected by the SEDZ and the timing of the ductile shearing U-Pb zircon and titanite thermal ionization mass spectrometry (TIMS) analyses were performed. The sampled rock is a K-feldspar megacryst bearing granite with a well developed type 2 shear fabric located within the SEDZ (17 km SSE of Kårböle, see Figure 2, Swedish National Grid RT90: /147905). Approximately 10 kg were collected, and the minerals used for geochronology were recovered by standard enrichment procedure (crushing, milling, Wilfley table, heavy liquids and Franz magnet). [36] Zircon is abundant, and the vast majority are turbid and have a translucent yellowish-white color. Only a minor amount is transparent and suitable for U-Pb analysis. [37] Altogether, the zircons are generally short prismatic with an aspect ratio of 1:3-1 with sharp edges and poorly developed pyramid surfaces. The short prismatic ones have pronounced {001} surfaces. None of the zircons have the high index surfaces, common in metamorphic crystals and crystals with metamophic overgrowths. The transparent crystals range in size from <74 to 150 mm and are pale pink, fairly flawless, but most of them have a faint hematite staining. From this population, two fractions with 1:3 ratio zircons (1 and 2 in Table 2) and four short prismatic fractions (3 6 in Table 2) were selected for U-Pb analysis with crystals per fraction. Under binocular microscope no cores were observed in the selected zircons. 11 of 16

12 Figure 9. U-Pb concordia diagram for titanite and zircon from a sheared granite within the SEDZ. The sample site is shown in Figure 2. The ellipses t3 and t4 represent a young generation of lozenge shaped titanite. The average age 1674 ± 6 Ma is interpreted to reflect the age of ductile type 2 shearing. Two fractions of older, porphyroclastic titanite are shown with grey ellipses. Five zircon fractions (omitting z2, shown in grey) define an upper intercept age of 1830 ± 8 Ma, which is interpreted as a minimum age of the protolith. [38] Two generations of titanite occur: A porphyroclastic older generation which are up to 2 mm large, but generally appears as mm dark brown fragments in the heavy mineral concentrate, and a younger generation consisting of < mm, pale brown and lozenge shaped crystals. In thin section it is apparent that some of the latter have grown adjacent to older titanite and opaque phases as multigrain mantles (Figure 9). Others occur as trails in chloritized biotite bands. [39] Two fractions of each generation were selected for TIMS analysis. The fractions of older titanite contained 10 fragments each, and to the individual fractions of the younger lozenge shaped titanite 25 crystals were selected. [40] The zircon fractions were air abraded according to Krogh [1982], washed in diluted HCl and water. They were placed in Teflon 1 capsules, a 205 Pb U spike was added and they were dissolved in HF:HNO 3 in an autoclave at 205 C for 7 days. After digestion the remaining liquid was evaporated and the samples were redissolved in HCl. U and Pb were separated in standard ion exchange columns with H 2 O and HCl respectively. [41] The same spike was added to the titanite samples and they were dissolved as the zircon samples but for 5 days. U and Pb were separated with HBr and HCl, respectively, and U was eluted by an additional H 2 O step. [42] The U-Pb analysis was conducted on a Finnigan 261 mass spectrometer at the Swedish Museum of Natural History, Stockholm, Sweden. Corrected isotope values, U/Pb ratios and intercept ages were calculated using the programs by Ludwig [1993, 2000]. Initial lead correction was made according to Stacey and Kramers [1975] and the decay constants applied were those recommended by Steiger and Jäger [1977]. Concentrations, isotopic ratios and ages are listed in Table Results [43] Five of the six zircon fractions fall on a discordia line with an upper intercept at 1830 ± 8 Ma and a mean square weighted deviation (MSWD) value of This age must be regarded as a minimum age of the protolith as some lead loss could have occurred during the ductile shearing and during a possible earlier regional metamorphic event. The low MSWD value is partly an effect of the relatively large errors of the individual analyses. One fraction falls to the right of the discordia line which could reflect inherited components in that fraction or be a result of a smaller degree of lead loss during later events. [44] The two fractions of lozenge shaped titanite yield concordant and overlapping ages of 1672 ± 8 Ma and 1676 ± 10 Ma, with an average age of 1674 ± 6 Ma. As these titanites occur as mantles around porphyroclasts or in chloritised biotite bands in the sheared fabric the age obtained is interpreted to reflect the timing of the ductile shearing. [45] The two analyses on the larger porphyroclastic titanite fragments do not give a conclusive result. One fraction yields an almost concordant age of 1702 ± 4 Ma and the other fraction is fairly discordant with a 206 Pb/ 207 Pb age of 1735 ± 3 Ma. It is not likely that either of these ages 12 of 16

13 with respect to the main direction of the SEDZ may indicate that the strike of the mylonite zones is controlled by dextral simple shear but the slip recorded on them is a result of pure shear across the zones. The change in the pattern of stretching lineation plunges on either side of the bend near Kårböle, could reflect a change in the relative proportion of simple shear and pure shear due to the change in orientation of the SEDZ. Figure 10. Schematic block diagram (view from the southeast) showing planar and linear fabrics and shear zones in the SEDZ and in the Rätan Batholith. Magmatic fabrics are modified from Mattsson and Elming [2001b]. The SEDZ-parallel foliation within the Rätan granite is mainly recorded by AMS data. reflect geological events, but rather display various degrees of recrystallization and/or lead loss. 6. Discussion 6.1. Evidence for Transpression [46] Our field results show that shear foliations and stretching lineations of type 2 mylonites are steep, and that the shear sense is variably eastern-side-up or westernside-up. The steep dip makes a distinction between normal and reverse movement irrelevant. Obviously, the SEDZ is neither a simple reverse nor a or transcurrent deformation zone, and the recorded dip-slip kinematics along the zone is much more complex than the dextral strike-slip pattern that is apparent on magnetic anomaly maps. [47] The combination of that dextral rotation of foliation in adjacent rocks, and the overwhelming dominance of type 2 dip-slip mylonites recorded in the field is one of the main enigmas with the SEDZ. The dextral brittle-ductile shear zones (type 3) are unlikely to have caused the continuous foliation swing, which means that the dextral swing and dipslip mylonites were either (1) formed at different times (with little mesoscopic record of the former) or (2) formed simultaneously, and are thus the expressions, on different scales, of bulk transpressional deformation. Our results favor the second alternative, which is in agreement with the theoretical results of Tikoff and Teyssier [1994] and Fossen and Tikoff [1998]. [48] The orientation of lineations within transpressional shear zones do not necessarily correlate with the transport direction, but may reflect along-strike variations in finite strain and/or strain partitioning [Tikoff and Greene, 1997]. The oblique strike of individual dip-lineated mylonite zones 6.2. Relation to the Rätan Batholith and the Dala Volcanic Rocks [49] In the Los area a titanite age of 1679 ± 4 Ma was obtained from a 1862 Ma (zircon age) old metarhyolite [Delin and Persson, 1999]. The titanite age was attributed to heating from the Rätan intrusion. Another indication of this thermal event is a U-Pb pitchblende age of 1670 Ma from a calcite vein in the Los cobalt mine [Welin, 1980]. These ages overlap within errors with the titanite age from the SEDZ presented here. [50] The relationship between the SEDZ and the Rätan Batholith is crucial in understanding the significance of the deformation zone, i.e., whether or not it was active during the intrusion of the batholith and its role in the magmatic emplacement processes along the circa 1.7 Ga continental margin. The fact that the SEDZ broadly follows the eastern boundary of the Rätan Batholith, and the SEDZ-parallel elongate shape of the latter, suggests some causal relationship between the two. The centre of the SEDZ is shifted toward the batholith. This asymmetry may either be an effect of the batholith margin acting as a rigid boundary during shearing, i.e., that the magma solidified before shearing, or alternatively, the magma intruded during shearing and therefore also affected it. Both possibilities are consistent with observations of mylonitized Rätan rocks. However, the latter alternative is favored because it provides a heat source that facilitates strain localization, and also allows ballooning as a mechanism for shortening across the zone. It is also supported by the AMS data presented below. [51] The direct dating of the shear fabric in the SEDZ, the dated inferred thermal overprints by the Rätan Batholith and the known age range of its intrusions all indicate a temporal relationship between the SEDZ (type 2) and the magmatism, in the time interval Ga. If so, there ought to be structural links, such as similar structures within the intrusion and the SEDZ. The AMS studies by Mattsson and Elming [2001a, 2001b] add important information in that context. The patterns of magnetic foliations and lineations east of and within the SEDZ correlate very well with our structural data. The AMS results also show that the magnetic foliation along the eastern margin of the Rätan intrusion is steep and conformable to the solid-state fabric in the SEDZ. The fabric in the granite was interpreted as magmatic due to a fairly low degree of magnetic anisotropy and the lack of mesoscopic or macroscopic evidence of ductile deformation. Our interpretation, supported by geochronological data, is that the conformity of magmatic and solid-state fabric records coeval shear and magma emplacement (Figure 10). In contrast to Mattsson and Elming [2001a, 2001b] we do not consider that NE-SW compres- 13 of 16

14 sion [Bergman and Sjöström, 1994] contradicts their conclusion of NW-SE extension during magma flow. On the contrary, they are complementary in a transpressive system dominated by pure shear. More remarkable is the change from dominantly steep lineation and foliation within the SEDZ to subhorizontal fabrics within the intrusion and the margin-oblique magnetic foliation along its western margin. A releasing bend or extensional stepover (to the south) of the deformation zone could have facilitated the magma flow and resulted in the NW-SE extension recorded by the AMS data. [52] The oblique magnetic foliation along the western margin of the batholith (Figure 10) demonstrates different conditions compared to those along the SEDZ in the east. The integrated effects of magma flow and wall rock displacements on fabrics in sheet-like igneous bodies has been studied by, e.g., Correa-Gomes et al. [2001]. By applying results from their study, a scenario with no wall rock shear deformation contemporaneous with magma flow along the western margin of the Rätan Batholith is possible. A northwestward flow of the magma would then result in a sinistral shear, as suggested by Mattsson and Elming [2001b]. However, even if magma flow and wall rock shear deformation had opposite senses along that margin, this would result in sinistral shear, provided that magma flow velocity was larger than wall rock shear velocity [Correa-Gomes et al., 2001]. A northwestward flow of the magma suggests that the feeding zone was located to the southeast possibly indicated by a N-S linear gravity anomaly across the southeastern part of the pluton (Figure 11a). Figure 11. (a) Schematic map showing the Dala volcanic rocks and Dala granitoids (DG) in a pull-apart basin related to the system of deformation zones (see insert) along which the Rätan Batholith (RB) and coeval Värmland granitoids (VG) to the south were emplaced. A possible location for magma conduit is indicated by a linear gravity anomaly (grey dotted line). (b) Cartoon (view from the north) showing the SEDZ forming in a back-arc setting, during transpression due to oblique subduction Tectonic Setting [53] The SEDZ and the Rätan intrusions are coeval with both the Dala volcanic rocks and the comagmatic Dala granite, and the Värmland granitoids to the south (Figure 11a). The geochemistry and texture of the Dala granite indicate that it was emplaced at a shallow crustal level [Ahl et al., 1999] together with the volcanic rocks in an extensional environment [Ahl et al., 1999; Nyström, 2004]. The volcanic sequence was deposited in a subsiding volcanotectonic graben within a thick active continental margin, similar to the central Andes [Nyström, 2004]. The model for the SEDZ-Rätan relationships (presented above) can be extended southward to include the Dala magmatic province, west of the SEDZ termination (Figure 11a). That is, the SEDZ-Rätan-Dala-Värmland igneous rocks constitute a tectonomagmatic province within the thick, active post-svecokarelian continental margin. This implies that the SEDZ formed as a back (magmatic) arc deformation zone (Figure 11b). In an obliquely convergent plate tectonic setting wrench deformation can occur in both the forearc and back-arc regions by mechanical coupling across the magmatic arc [Saint Blanquat et al., 1998]. In such a setting a very small component of tangential plate motion is required for strike-slip tectonics to occur. Magmatism facilitates partitioning of the plate motion into components of tangential and normal motion, even for almost normal subduction [Saint Blanquat et al., 1998]. 14 of 16

15 6.4. Regional Implications [54] The orientation of the SEDZ and the whole circa 1.7 Ga magmatic complex in central and southern Sweden is N-S to NNW-SSE (Figure 1), which is clearly discordant to the mainly E-W to NW-SE Svecokarelian structural trends and associated regional metamorphism, which were formed before circa 1.79 Ga [Stephens et al., 1997; Andersson et al., 2004]. This indicates that a change in tectonic regime occurred in the time interval circa Ga, and not at 1.81 Ga, which has been suggested recently [Åhäll and Larson, 2000]. [55] The activity of SEDZ and related magmatism postdate the peak of Svecokarelian deformation and metamorphism (circa Ga [Stephens et al., 1997]) but overlap with the initial stage of the Gothian event ( Ga [Åhäll and Larson, 2000]) in southwestern Sweden. By extending the relations between SEDZ and the Rätan Batholith farther to the south it is suggested that related deformation fabrics can be expected along the eastern margin of the Ga intrusive rocks in southern Sweden (Figure 1), in a tectonic position similar to the SEDZ. In this area, however, later events (late Gothian, Hallandian Ga, Sveconorwegian Ga [e.g., Christoffel et al., 1999; Söderlund et al., 2002]) of magmatism, intense deformation and/or high-grade metamorphism are likely to have obliterated many such early structures. 7. Conclusions [56] The combined structural data, strain analysis and comparisons with other work suggest that the SEDZ formed due to transpression. U/Pb titanite results show that shearing was contemporaneous with magmatic activity. We conclude that the solid-state fabric in the SEDZ and the magmatic fabric of the Rätan Batholith reflect coeval shearing and magma emplacement, and that the differences in fabrics are due to strain partitioning between the magma and the deformation zone. The location of the Rätan Batholith is a result of a conduit/conduits related to a stepover or releasing bend of the SEDZ to the southwest. [57] The SEDZ and the Rätan Batholith are components of a 1.7 Ga tectonomagmatic province including comagmatic Dala volcanic and intrusive rocks and the Värmland granitoids. With respect to the transpressive conditions recorded in the SEDZ and the orientation of the Dala granite, the contraction was probably NNE-SSW or NE-SW. [58] The SEDZ is suggested to be a back (magmatic) arc deformation zone and related deformation fabrics can be expected to be found along the eastern margin of the Ga intrusive rocks in southern Sweden (Figure 1), in a tectonic position similar to the SEDZ. [59] Acknowledgments. We would like to thank Sven Aaro, Hans Delin, Jan Ehrenborg, Krister Sundblad, and Sam Sukotjo for interesting discussions and/or for providing us with maps. Christina Wernström, Katarina Rosén-Lindberg and Ingemar Källberg helped with drawings and Bertel Giös, Christer Bäck, and Karl-Erik Alnavik produced the photographic work. The manuscript benefited from constructive comments by C. W. Passchier and three anonymous reviewers. Financial support from the Geological Survey of Sweden (4830) and the Swedish Natural Science Research Council (G-GU , G-GU , ) is acknowledged. References Åhäll, K.-I., and S. Å. Larson (2000), Growth-related Ga magmatism in the Baltic Shield: A review addressing the tectonic characteristics of Svecofennian, TIB 1-related, and Gothian events, GFF, 122, Ahl, M., K. Sundblad, and H. Schöberg (1999), Geology, geochemistry, age and geotectonic evolution of the Dala granitoids, central Sweden, Precambrian Res., 95, Ahl, M., R. 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