Ductile shear zones related to crustal shortening and domain boundary evolution in the central Fennoscandian Shield

Transcription

1 TECTONICS, VOL. 28,, doi: /2008tc002277, 2009 Ductile shear zones related to crustal shortening and domain boundary evolution in the central Fennoscandian Shield Karin Högdahl, 1,2 Håkan Sjöström, 2 and Stefan Bergman 3 Received 28 February 2008; revised 6 September 2008; accepted 9 October 2008; published 4 February [1] The Paleoproterozoic part of the Fennoscandian Shield is composed of crustal components formed in different tectonic settings and generally separated by well-defined shear zone systems. An anomalous transitional boundary has been investigated by integrating structural analysis and geochronology with published geophysical data. The nature of this boundary is interpreted to be a consequence of an apparent stacking in the lower and middle crust initiating Ga dextral shear along the Gävle- Rättvik Zone (GRZ) and adjacent shear zones, resulting in an arcuate northern boundary of the Bergslagen province. This boundary coincides with geophysical anomalies and temporal and metamorphic breaks. Owing to continuous convergence the pure-shear overprint component increased on the GRZ and caused a shift of dextral shear to the Hagsta Gneiss Zone with recorded shear at 1809 ± 2 Ma. Most likely, both these structures are related to coeval shear zones farther to the east as a part of an 1500 km long crustal, or possibly terrane, boundary. Citation: Högdahl, K., H. Sjöström, and S. Bergman (2009), Ductile shear zones related to crustal shortening and domain boundary evolution in the central Fennoscandian Shield, Tectonics, 28,, doi: /2008tc Introduction [2] The tectonic setting for rocks in deeply eroded ancient shield areas, like the Fennoscandian Shield (Figure 1), is defined mainly by their geochemical and isotopic signatures. In general, the boundaries, between units formed in different tectonic settings including terrane boundaries, are often obscured by subsequent deformation and metamorphism [e.g., Andersson et al., 2002], later intrusions [e.g., King and Barr, 2004], younger cover rocks [e.g., Eglington and Armstrong, 2003; Bogdanova et al., 2006], and their significance is often subjected to reinterpretation [e.g., Ghosh et al., 2004; Mason and Brewer, 2005]. However, a common feature is that these boundaries often coincide with shear zones, shallow or steep, and can be discrete or even cryptic structures [St.-Onge et al., 2001]. 1 Department of Geology, Lund University, Lund, Sweden. 2 Department of Earth Sciences, Uppsala University, Uppsala, Sweden. 3 Geological Survey of Sweden, Uppsala, Sweden. Copyright 2009 by the American Geophysical Union /09/2008TC [3] The west central part of the Fennoscandian Shield is made up by a collage of different Paleoproterozoic plate tectonic elements: a tectonically stacked marine basin (Bothnian Basin), continental margin granitoids (Ljusdal batholith of the Ljusdal domain), a probable fragment of an island arc (Hamrånge fm) and a continental back-arc volcano-sedimentary sequence (Bergslagen province). During the years different criteria have been used to define these tectonic elements, for example, lithology [Gaál and Gorbatschev, 1987], lead isotope compositions [Romer and Wright, 1993, and references therein], lithology in combination with structural form lines [Sjöström and Bergman, 1998], geochronology ( tectonic domains) [Hermansson et al., 2007], and all these criteria integrated with crustalscale geophysical characteristics [Lahtinen et al., 2005]. Regardless of criteria used the boundaries between the individual tectonic elements often coincide and in many cases they are characterized by steeply, and more rarely shallowly dipping ductile shear zones. Commonly these shear zones are well defined, and occasionally they can be traced for hundreds of kilometers in the field. Recently it has been shown that some of the crustal-scale shear zones have experienced long-term activities [Torvela et al., 2008; Högdahl et al., 2008]. [4] The boundary between the Ljusdal domain, roughly equivalent to the Bothnian unit [Lahtinen et al., 2005] and the Bergslagen province is, however, poorly defined. This transitional area including mixed lithologies is bounded by ductile shear zones and has previously been suggested to be a separate geologic domain [Tirén and Beckholmen, 1990]. The transitional nature is enigmatic since the Ljusdal domain and the Bergslagen province show significant differences in metamorphic grade, structural style, and magmatic age of comparable subduction-related granitoids. [5] In this paper we present new structural and geochronological data from the transitional domain and its enveloping shear zones, and new or recently presented data from the neighboring crustal units formed at different tectonic settings; the Ljusdal domain and the Bergslagen province. The studied section provides key data for the western part of a boundary between two major tectonometamorphic units in the Fennoscandian Shield. It is located at a bend of a largescale shear belt and includes three major shear zone systems. The results are integrated with published interpretations of the crustal structure at depth into a temporal and spatial model of the late to postaccretionary evolution of the west central part of the shield. A general outcome is that major shear zones are important structures for the understanding of the tectonic evolution, to link structures at the present surface and in the deep crust, and for identifying 1of18

2 Figure 1. Bedrock map of the Fennoscandian Shield, modified from Koistinen et al. [2001], including major shear zones in Sweden and the Baltic and Bothnian Echoes From the Lithosphere (BABEL) profiles C and C1 [Korja and Heikkinen, 2005] as dashed lines. The Paleoproterozoic rocks were mainly formed or reworked during the Svecokarelian orogeny. Numbers indicate the following: 1, Hassela Shear Zone (HSZ); 2, Storsjön-Edsbyn Deformation Zone (SEDZ); 3, Tönnånger SZ; 4, Hagsta Gneiss Zone (HGZ); 5, Gävle-Rättvik Zone (GRZ); 6, Singö Shear Zone (SSZ); 7, Österbybruk-Skyttorp Zone (ÖSZ); 8, Gimo SZ (GZ). BB, Bothnian Basin (approximate northern boundary is marked as a thin dashed line); LjB, Ljusdal batholith; BS, Bergslagen province; CSAC, Central Svecofennian Arc Complex; SSAC, Southern Svecofennian Arc Complex; LSGM, Late Svecofennian Granite-Migmatite Belt. crustal units with different tectonic histories in Paleoproterozoic shield areas. 2. Geological Setting [6] The bedrock in northern and central Sweden belongs mainly to the Paleoproterozoic Svecokarelian orogeny. It has the character of an accretionary orogen with a general crustal growth direction to the south or southwest from an Archean nucleus in the NE [e.g., Gaál and Gorbatschev, 1987; Nironen, 1997; Korja and Heikkinen, 2005]. A recent model explains the Svecokarelian evolution as a result of four different orogenies with separate, but partly overlapping tectonic histories between 1.92 and 1.79 Ga [Lahtinen et al., 2005]. This interpretation is partly based on images of larger mafic bodies and older nuclei identified in the lower crust in seismic reflection data [Korja and Heikkinen, 2005]. The older nuclei are interpreted as remnants of Archean or Paleoproterozoic microcontinents [Lahtinen et al., 2005]. [7] The Bergslagen province (Figure 1) consists of a > Ga volcano-sedimentary sequence and coeval plutonic rocks formed in a back-arc environment inboard an active continental margin [Allen et al., 1996]; late-kinematic to postkinematic granitoids are also abundant. The existence of Ga zircons in 1.89 Ga amphibolites [Andersson et al., 2006] may reflect the existence of a Paleoproterozoic nucleus at depth as suggested by Lahtinen et al. [2005]. To the south and west the Svecokarelian rocks are succeeded by the Andean-type Ga Transscandinavian Igneous Belt (TIB) [e.g., Högdahl et al., 2004]. 2of18

3 [8] The Ljusdal domain, dominated by granitoids formed in an active continental arc setting [Högdahl et al., 2008], is located between the Bergslagen province and the vast Bothnian Basin (Figures 1 and 2). The boundary between the Bothnian Basin and the Ljusdal domain coincides with the crustal-scale Hassela Shear Zone [Sjöström and Bergman, 1998; Högdahl and Sjöström, 2001] and an 50 km wide Ga migmatite belt producing coarse porphyritic granites in the northwestern part [Högdahl et al., 2008]. [9] The Ljusdal domain consists predominantly of Ga K-feldspar megacryst-bearing granitoids of the Ljusdal batholith [Delin, 1993; Welin et al., 1993; Delin and Aaro, 1994; Högdahl et al., 2008], but locally evengrained varieties and minor intrusions of mafic rocks occur. Large proportions of the granitoids were transformed to augen gneisses by subsequent regional metamorphism and deformation. Metavolcanic and metasedimentary rocks are relatively sparse except in the eastern part where they occur mostly in tightly folded (F 1 + F 2 ) migmatitic screens refolded by regional synforms (F 3 ). One of these synforms, the Hamrånge synform (Figure 2), located in the eastern part of the domain, consists of well-preserved amphibolite facies supracrustal rocks including a 1.88 Ga [Bergman et al., 2008] metavolcanic unit [Lundegårdh, 1956; Sukotjo, 1995]. To the west and south supracrustal rocks of more uncertain affinity are abundant in a wedge-shaped area corresponding to the transitional area referred to as the Ockelbo domain by Tirén and Beckholmen [1990] (Figure 2). The southern boundary of this wedge roughly coincides with an area interpreted as a separate tectonic domain [Hermansson et al., 2007] or a kinematically unspecified deformation zone [Tirén and Beckholmen, 1990; Stephens et al., 1994]. [10] In most parts Svecokarelian regional metamorphism is characterized by LP-HT amphibolite facies conditions. Areas affected by greenschist facies exist locally and granulite facies areas occur in SW Bergslagen at the boundary to the TIB [e.g., Andersson et al., 1992; Wikström and Larsson, 1993; Andersson, 1997] and in the central and northeastern parts of Ljusdal domain [e.g., Sjöström and Bergman, 1998; Högdahl et al., 2006]. Granulites are also common in the Southern Svecofennian Arc Complex of southern Finland [Väisänen and Hölttä, 1999]. [11] At least two Svecokarelian metamorphic episodes have been recorded in Sweden. North of the Bothnian Basin the regional metamorphism predates 1.87 Ga [Wikström, 1996; Lundström et al., 1999; Rutland et al., 2001]. In the southern part of the basin, and to the east of the Ljusdal batholith migmatization occurred at Ga [Högdahl et al., 2008]. [12] In the Ljusdal domain south of the HSZ migmatite formation has been dated at 1.82 Ga [Högdahl et al., 2008]. Anatectic melting at Ga [Romer and Smeds, 1994; Claesson and Lundqvist, 1995; Weihed et al., 2002; Stein, 2006; Högdahl et al., 2008] is widespread, and in the Bergslagen province, this metamorphic event is dominating with abundant formation of granites and associated pegmatites [Romer and Smeds, 1994, 1997; Ivarsson and Johansson, 1995; Andersson et al., 2006] overprinting a more discrete, and in general lower grade event at circa 1.87 Ga [Andersson et al., 2006; Hermansson et al., 2007]. However, in the northeastern part of that province higher metamorphic grade and migmatite formation is probably linked to the 1.87 Ga event [Bergman et al., 2004]. 3. Late-Orogenic Shear Zones in Central Sweden [13] Steep ductile shear zones are common in the Paleoproterozoic parts of the Fennoscandian Shield. Many of these zones have developed at the boundaries between tectonic elements. The Hassela Shear Zone (Figure 1) between the Bothnian Basin and the Ljusdal domain is one of the most prominent deformation zones extending from the Phanerozoic Caledonides in the west and possibly to southern Finland [Högdahl and Sjöström, 2001]. It was established at circa 1.87 Ga and localized ductile shear occurred between and Ga [Högdahl, 2000; Högdahl and Sjöström, 2001; Högdahl et al., 2001]. The younger shear episode also resulted in a number of deformation zones within the Ljusdal domain (Figures 1 and 2). Roughly coeval shear zone systems developed at the southern margin of the volcanic arc of the Bergslagen province toward the Andean-type TIB [Beunk and Page, 2001]. A younger episode of shear deformation related to the TIB has affected the western margin of the Ljusdal domain and resulted in the 1.67 Ga Storsjön-Edsbyn Deformation Zone [Bergman et al., 2006]. In the southwestern part of the shield there are numerous large-scale shear zones associated to the Neoproterozoic Sveconorwegian orogeny. [14] The majority of the 1.8 Ga zones have a dextral horizontal component and shallow to moderately plunging stretching lineation indicating a large proportion of strikeslip displacement. A few NE-SW to ENE-WSW relatively younger, conjugate and sinistral zones exist (Figures 1 and 2). Many of these zones were established early during the accretionary history of the Svecokarelian orogeny but the main shear activity is late orogenic and related to roughly N-S to NNW-SSE convergence [Ehlers et al., 1993; Lindroos et al., 1996; Högdahl and Sjöström, 2001; Torvela et al., 2008]. [15] The ductile shear zones studied here at the boundary between the Ljusdal domain and the Bergslagen province show more complex structural characteristics. In addition, they envelop a transitional area between two crustal units above an anomalous structure in the middle crust according to the interpretations of seismic reflection images [Korja and Heikkinen, 2005] The Hagsta Gneiss Zone (HGZ) [16] The HGZ [Bergman and Sjöström, 1994] defines the northern part of an 25 km wide zone of sheared gneisses between the coast in the east and Ockelbo in the northwest (Figure 2). This shear zone system appears as a distinct banded pattern on magnetic anomaly maps. The HGZ is about 2 km wide and truncates the southern margin of the well-preserved rocks of the Hamrånge synform. The shear 3of18

4 Figure 2. Bedrock map of central Sweden. Dashed lines outline the Gävle-Rättvik Zone. Inset map shows the structural domains by Sjöström and Bergman [1998]. Domains B, C, and D correspond to the Bothnian Basin, the Ljusdal domain, and the Bergslagen province, respectively. Grey area represents the Ockelbo domain. Hfm, Hamrånge formation; LSZ, Lindön Shear Zone; GZ, Gimo SZ. 4of18

5 Figure 3. Equal area, lower hemisphere stereograms showing the orientation of the structural elements in the Ockelbo domain, along GRZ and in northern Bergslagen. (a) The northwestern part of the HGZ is steep, but the dip decreases gradually south of the Hamrånge synform; farther southward in the sheared gneisses it becomes steep again. (b) GRZ, east of Gävle graben. (c f) Within the GRZ west of the Gävle graben. (g) Literature data from the northern part of the Bergslagen province [Persson, 1997]. The stars denote the location of rocks dated in this study, and the striped area shows the transition from ms + qz to kfs + sil stability fields. Fine stippled lines outline the GRZ. 5of18

6 Figure 4. (a) Magnetic anomaly map covering the area between the Bergslagen province and the Ockelbo domain [Stephens et al., 2003]. (b) Bouguer anomaly map of the same area [Stephens et al., 2002]. Coordinates referred to in Figure 4a are Swedish National Grid; dots in Figure 4b are measurement localities. Grid size is km. zone is generally steep, but the dip decreases gradually to become shallow NE dipping south of the synform (Figure 3a). The structure thus shows a north-south fanning, which probably is related to the existence of tectonic lenses in the Hamrånge area. A steepening at depth is likely according to the Baltic and Bothnian Echoes From the Lithosphere (BABEL) profiles in the Baltic Sea [Korja and Heikkinen, 2005]. [17] The HGZ is a plane parallel, banded deformation zone with well-developed, shallow east plunging stretching lineation. Rare kinematic indicators (due to recrystallization) exhibit a dextral horizontal component with SW side up that locally results in an extensional component across the zone, contrasting to the regional pattern. Close to the coast the HGZ joins the SW NE trending part of the arcuate, steep, 1 km wide Lindön Shear Zone (Figure 2). This zone truncates and attenuates the supracrustal rocks of the Hamrånge group but also affects the granitoids to the south. The related stretching lineation is pronounced and plunges moderately to the east. Form-line patterns and magnetic data indicate that the Lindön SZ developed along the northern boundary of an approximately 5 15 km tectonic lens bounded to the south by the HGZ (Figure 2). Contradicting kinematic indicators along the Lindön SZ is interpreted to be a result of a large proportion of pure shear across the lens The Gävle-Rättvik Zone (GRZ) [18] On the basis mainly of magnetic and gravity signatures (Figures 4a and 4b), a deformation zone with unknown 6of18

7 kinematics has previously been inferred between Sandviken and Rättvik [Tirén and Beckholmen, 1990] or Gävle and Rättvik [Stephens et al., 1994] (Figures 1 and 2). The spatial discrepancy is related to the ENE WSW Gävle graben, filled with Ga Jotnian sandstones and 1.25 Ga diabase dykes and sills [Söderlund et al., 2006], which obscure both the surface and geophysical expressions of this zone. The magnetic anomaly along GRZ appears as an 25 km wide anastomosing banded pattern (Figure 4a). High strain and steep dips are reflected by long and narrow magnetic connections, which coincide with ductile shear zones that envelope lozenge shaped tectonic lenses. To the south of the GRZ, the large-scale structural and lithological trends rotate clockwise from SW NE toward parallelism with the E W trending deformation zone, indicating apparent dextral shear. Also farther east, a large-scale Z fold north of the Hedesunda Complex indicates dextral shear (Figure 2) as well as steeply plunging, east verging folds affecting tectonic lenses splaying from the Singö Shear Zone System (SSZ) [Persson and Sjöström, 2003]. In contrast, the migmatites to the north of the zone show a less distinct anticlockwise rotation of the trends into the GRZ, indicating a sinistral component (Figure 2). [19] This 2-D apparent information has been constrained and extended to 3-D by field observations. Owing to tectonic lenses the orientation of individual shear zones has a fairly large but consistent variation ranging from NE to NW with steep to moderate dips (Figures 3c, 3e, and 3f). Kinematic indicators are rather common and include S/C fabrics, C 0 -type shear bands, s- and d-porphryoclasts, and stair-stepped objects (Figures 5b 5e and 6e 6g). The stretching lineation shows a large variation in plunge from subhorizontal to vertical but spread on great circles that approximately define the tectonic lens boundaries (Figures 3d 3f). In the shear zones with steep stretching lineations there is no consistency in the horizontal kinematics as the vertical components dominate. However, zones with a north (migmatite side)-up component are approximately twice as common as the opposite (Figures 3e 3g). Owing to the steep dips of the shear zones the amounts of horizontal extension or shortening across the zones are small. Nevertheless, NE SW and NW SE zones have a slight extensional (normal) component and are more common while E W zones record shortening (reverse) (Figure 3e). These structures fit into a pattern with tectonic lenses and the GRZ, like the HGZ, exhibits local extensional components that have not been recorded in the consistent dextral shear zone pattern in the Ljusdal domain to the north. [20] The large variation of the stretching lineation probably represents a mixture of both the well-defined easterly plunging lineations in the migmatites to the north and south of Gävle (Figures 3a and 3b) and in the Ljusdal Domain to the north, and the steeper lineations in the northern (Figure 3g) and northeastern part of the Bergslagen province [Stålhös, 1991; Persson and Sjöström, 2003]. The latter structures have rotated clockwise and have been affected by NW SE and approximately N S shear zones (Österbybruk-Skyttorp (ÖSZ) and Gimo (GZ), respectively) enveloping tectonic lenses (Figure 2). [21] The central part of the GRZ coincides with a significant change in metamorphic grade. In the Bergslagen province to the south, the rocks are in general well preserved, mainly amphibolite facies supracrustal rocks, predominantly felsic metavolcanic rocks and Svecokarelian granitoids [e.g., Allen et al., 1996], in addition to plutonic rocks of the TIB. Steep foliation and lineation is a general feature (Figure 3g). The metamorphic break to the migmatites to the north is accompanied by shallow plunging structures, corresponds to the transition from the muscovite + quartz to the K-feldspar + sillimanite stability fields [Stålhös, 1991]. The change in metamorphic grade across the GRZ is thus in accordance with north-side-up shear sense in the ductile shear zones. 4. The Ockelbo Domain [22] The Ockelbo domain is bounded to the north by the HGZ (Figure 6a) and transitional to the GRZ. There is a large variation of rock types in the domain consisting of a variety of metamorphosed plutonic and supracrustal rocks, stromatic and diatexitic migmatites, and larger bodies of partly pegmatitic younger granites and granitic dykes [Bergman and Söderman, 2005; Delin, 2005; Sukotjo and Sträng, 2005]. In places, the stromatic migmatites have been subjected to more intense in situ melting resulting in schlieren-granites, which appear to be coeval with the pegmatitic granites. [23] East of the Gävle graben, the regional magnetic anomaly outlines a km wide intensely folded migmatite belt truncated by elongated and narrow anomalies (Figure 4a). These consist of both high magnetic and low magnetic anomalies, indicating ductile shear zones and brittle-ductile and brittle zones, respectively. [24] In the field, the corresponding mesoscale folds (F 2 ) are tight, have axial surfaces dipping to the south or SE and refold intrafolial F 1 folds (Figure 6b). The stretching lineation and F 2 fold axes both have, in general, a shallow to moderate plunge to the east (Figure 3b). Long, straight, sometimes sheared limbs and narrow hinges characterize the F 2 folds, which indicates that post-f 2 deformation have flattened the folds (Figure 7). The flattened geometry of the folds is also apparent in magnetic anomaly images. [25] Narrow, granitic dykes and plutons truncate the migmatite (Figure 6c). The dykes locally have a weak boundary parallel biotite foliation and often an additional oblique quartz grain-shape, truncating foliation (Figure 6d). Together these foliations form an S/C fabric indicating dextral shear in and along the boundaries of E W dykes and, in turn, that the dykes have rotated anticlockwise (Figure 7). [26] Irregular granitic bodies that show a faint, but occasionally distinct foliation subparallel to the F 2 axial surface in the migmatite, also truncate the F 2 folds. This is an additional feature indicating that the migmatites and the granites have been affected by post-f 2 deformation. [27] Only a few high-temperature shear zones have been recorded in the southeastern part of the Ockelbo domain. Generally they appear as meter-wide zones of striped gneiss with rare d- and s-porphyroclasts indicating mainly dextral shear. With a south dipping foliation and east plunging 7of18

8 Figure 5. Microstructures from the GRZ and the HGZ. All sections are cut parallel to the stretching lineation and orthogonal to the foliation. (a) Sheared mafic metavolcanic rock from the HGZ collected for U-Pb titanite Thermal Ionization Mass Spectrometry (TIMS) analyses. (b) Sheared amphibolite in the GRZ. Foliation (S) is 311/73, and C 0 /S fabric shows NE-side-up sense of shear. (c) d-porphyroclasts in an amphibole-bearing granitoid, GRZ. Foliation is 131/72, and sense of shear is NE up. (d) s-porphyroclast in the same amphibole-bearing granitoid as that in Figure 5c. (e) Syntectonic garnet in a metasedimentary rock (073/82) indicating north side up with a sinistral horizontal component. (f) A steep, dextral (horizontal component), SW-side-up ultramylonite, east of Gävle. lineation, dextral shear would result in south-side-up component, which is not in accordance with the metamorphic pattern exposing lower grade rocks to the south. This indicates either that this shear episode did not cause the metamorphic break or that the lineation is not parallel to the maximum stretching axis (x) of the migmatites. [28] In the migmatite a few <0.5 m wide ultramylonite zones (Figure 6h) have been recorded indicating reactivation of the GRZ. Microstructures such as subgrains in ribbon quartz and K-feldspar core-and-mantle structures hosted in a fine-grained partially recrystallized matrix (Figure 5f) show that the ultramylonites were formed at medium-grade conditions. 5. Geochronology [29] U-Pb and 40 Ar/ 39 Ar geochronology were applied to constrain the timing of metamorphism in the Ockelbo domain, the ductile shear along the HGZ (Figure 5a), and the minimum age for shear along the GRZ: (1) U-Pb zircon Thermal Ionization Mass Spectrometry (TIMS) geochronology on a synmetamorphic to late-metamorphic schlieren- 8of18

9 Figure 6. (a) Hagsta Gneiss Zone. (b) Refolded migmatite east of Gävle. (c) Granitic dyke cutting a stromatic migmatite. (d) Granitic dyke with two foliations; one parallel (white arrow) and the other oblique (black arrow) to the dyke boundary. The dyke crosscuts the migmatite. (e) Dextral CS fabric in a K-feldspar megacryst-bearing granitoid within the GRZ. (f) Sinistral stair-stepped object in a metasedimentary rock within the GRZ viewed parallel to stretching lineation. (g) C 0 /S fabric in an amphibolite indicating NE side up, GRZ. (h) Ultramylonite east of Gävle. Photographs in Figures 6a and 6c 6e represent top surfaces, Figure 6f is viewed on a shallow surface, and those in Figures 6b and 6g 6h are viewed on steep surfaces, roughly perpendicular to foliation. 9of18

10 Figure 7. Cartoon illustrating anticlockwise rotation of a granitic dyke and clockwise rotation and flattening of F 2 folds owing to approximately N S shortening. analysis was conducted on a Micromass 5400 mass spectrometer at the argon geochronology laboratory, Department of Geology, Lund University, by L. M. Page. Prior to analysis the amphibole was irradiated at the Nuclear Research and Consultancy Group (NRG) HRF RODEO facility in Petten, NL, together with the TCR-2 sanidine standard calculated to Ma following Renne et al. [1998]. J values were calculated with a precision of 0.5%. [35] One or two 0.5 mm amphibole grains were loaded into planchettes and step heated using a defocused 50 W CO 2 laser, which was rastered over the sample. For gas purification two Zr-Al getters and a cold finger were used during a clean-up time of 5 min. 40 Ar blank was calculated before the analyses and time zero regressions were fitted to granite (955) from the northeastern part of the Ockelbo domain, (2) U-Pb TIMS on synkinematic titanite from a high-strain metavolcanic rock (15) in the HGZ, and (3) 40 Ar/ 39 Ar geochronology on amphibole from a sheared gabbro (H0498) affected by the GRZ U-Pb [30] The zircon crystals selected for analysis were rounded or stubby with an aspect ratio close to 1. They were divided into six fractions consisting of approximately ten mm crystals each. The rounded crystals were split into four fractions, and the stubby euhedral crystals were separated into two fractions. The zircon was air abraded according to Krogh [1982] and washed in diluted HCl and water. They were placed in Teflon capsules, a 205 Pb U spike was added, and the samples were dissolved in HF:HNO 3 in an autoclave at 205 C for 7 days. After final digestion the remaining acid 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. [31] The titanite in sample 15 appear as lozenge shaped, 100 mm large crystals, which are transparent and have pale brown to more saturated brown colors. The selected titanite was divided, on the basis of the color intensity, into three fractions consisting of crystals each. [32] The titanite samples were washed and 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. [33] The U-Pb analyses were performed on a Finnigan MAT 261 thermal ionization mass spectrometer at the Laboratory for Isotope Geology, Swedish Museum of Natural History in Stockholm, Sweden. Corrected isotope ratios, U and Pb concentrations, and U/Pb ratios were calculated using a program based on the work of Ludwig [1993], and intercept ages were calculated using the program by Ludwig [2003]. Initial lead correction was made according to Stacey and Kramers [1975], and the decay constants applied were those recommended by Steiger and Jäger [1977] Ar-Ar [34] Syntectonic amphibole from a sheared gabbro (H0498) was selected for 40 Ar/ 39 Ar geochronology. The Figure 8. U-Pb concordia diagrams. (a) Zircon data from a schieleren-granite (955) interpreted to reflect the crystallization age of the granite. (b) Titanite data from a highstrain part of the Hagsta Gneiss Zone (15) interpreted to reflect the timing of the ductile shearing. Sample localities are shown in Figures 3 and 10a. MSWD, mean square weighted deviates. 10 of 18

11 Table 1. U-Pb Zircon and Titanite Data 207 Pb/ 206 Pb (Age) r c 2s (%) 207 Pb/ 206 Pb b 2s (%) 207 Pb/ 235 U b 208 Pb 2s (Atm%) b 206 Pb/ 238 U b (%) 207 Pb (Atm%) b 206 Pb (Atm%) b 206 Pb/ 204 Pb a Pbcom (ppm) Pbtot (ppm) U (ppm) Weight (mg) Sample/Mineral t1 HGZ ± t2 HGZ ± t3 HGZ ± z1 955r ± z2 955r ± z3 955r ± z4 955p ± z5 955p ± z6 955p ± a Corrected for mass fractionation (0.1 % per amu). b Corrected for mass fractionation, blank and common lead. c Error correlation. data collected from 10 scans over the mass range Peak heights and background were corrected for mass discrimination, isotopic decay and interfering nucleogenic isotopes derived from Ca, K and Cl. For the cadmium lined position in the Petten reactor the isotopic production values are ( 36 Ar/ 37 Ar) Ca = , ( 39 Ar/ 37 Ar) Ca = , and ( 40 Ar/ 39 Ar) K = The analytical procedure is automated using a laboratory-modified program originally developed at the Berkeley Geochronology Center Results [36] The analyzed rounded and short prismatic zircon from the schlieren-granite in the Ockelbo domain is interpreted to be newly formed and the obtained U-Pb intercept age of 1836 ± 1 Ma (Figure 8a) to be the crystallization age of the granite. Larger prismatic crystals are also present in the zircon population and these were avoided to exclude the possibility of inherited components. Older crystal elements are probably not present in analyzed zircon as the individual analyses are only slightly to moderately discordant and the low mean square weighted deviates (MSWD) value (0.44) indicates a good fit of the regression line. [37] The schlieren-granite is closely related to the stromatic migmatite. In places the granite has consumed the migmatite resulting in very diffuse contacts and ghost structures. The schlieren-granite is interpreted to be related to the metamorphic peak and the obtained age thus approximates the timing of peak metamorphism in the area. [38] The three titanite fractions from the Hagsta Gneiss Zone are all concordant and overlapping and yield a weighted mean age of 1809 ± 2 Ma (Figure 8b). As the titanite is a component in a sheared fabric the age is interpreted to reflect the timing of the ductile deformation of the zone. [39] U and Pb isotopic composition for the zircon and the titanite are given in Table 1. The amphibole from the GRZ yields an 40 Ar/ 39 Ar plateau age of 1797 ± 7 Ma (Figure 9). This only reflects the minimum age of the shear activity, as the PT conditions during the recrystallization of the amphibole in the shear zone are unknown. However, the amphibolite facies mineralogy (am (hbl) + pl) shows that it was higher than the closure temperature (500 C) for the Ar-isotope system in this mineral. Ar analytical data are given in Table Discussion [40] The regional late-orogenic, shear zones to the north of the Ockelbo domain are mainly localized in the limbs of large E W trending open folds (F 3 in that area) [Sjöström and Bergman, 1998], and formed as a result of a N S to NNW SSE convergence. Strain partitioned into the NW SE trending dextral shear zones and less frequently into the NE SW trending sinistral conjugate zones. Stretching lineation and kinematic indicators reveal strike-slip or weakly oblique movements on the shear zones; at the apex of tectonic lenses kinematic indicators are contradicting implying a large component of pure shear at these locations. 11 of 18

12 Figure Ar/ 39 Ar amphibole step-heating spectrum on sample H0498. The sample is from a ductile shear zone with a completely recrystallized fabric. Sample locality is shown in Figures 3 and 10b. [41] The HGZ fits into this regional shear zone system with respect to strike, shear sense and age but shows a larger variation in dip due to the existence of tectonic lenses. The high-strain parts of HGZ were formed at the northern boundary of an at least 25 km wide shear belt affecting migmatites to the southwest. The ductile shear fabric age of 1809 ± 2 Ma is within the Ga range of other dextral deformation zones within and to the northwest of the Ljusdal domain [Högdahl, 2000; Högdahl and Sjöström, 2001; Högdahl et al., 2001] as well as in southern Finland [Lindroos et al., 1996; Ehlers et al., 2004; Väisänen and Skyttä, 2007; Torvela et al., 2008]. [42] The HGZ merges with the GRZ to the SE and has been recorded in the BABEL seismic reflection images off the coast in the Baltic Sea (Figure 10a) [Korja and Heikkinen, 2005]. In these images the HGZ has been interpreted as a steep 15 km wide, apparently south dipping crustal-scale shear zone system. It is located at the leading edge of a tectonically repeated Paleoproterozoic microcontinent existing in the lower and middle crust Table Ar/ 39 Ar Amphibole Data a Run Pwr/T ( C) Ca/K 36 Ar/ 39 Ar % 36 Ar(Ca) 40 *Ar/ 39 Ar Mol 39 Ar Percent Step Cumulative Percent Percent 40 Ar b Age (Ma) A E ± B E ± C E ± D E ± E E ± F 2.2 b E ± G 2.2 b E ± H 2.3 b E ± I 2.3 b E ± J 2.4 b E ± K 2.7 b E ± 2 a H04:98 hbl, run (J = ± ). Integrated age is 1790 ± 8 Ma. b Plateau age is 1797 ± 7 Ma with percent step of of 18

13 Figure of 18

14 [Lahtinen et al., 2005]. This implies that the HGZ and/or GRZ are the surface expression(s) of a major geological structure at depth below the boundary between the Ljusdal domain and the Bergslagen province. [43] The localized zones of the GRZ have characteristics comparable to shear zones in the Southern Svecofennian Arc Complex (Figure 1) to the east as well as shear zones in the northeastern part of the Bergslagen province. The horizontal component of the anastomosing sets of shear zones belonging to the GRZ is not entirely systematic in relation to the strike of the zones for a conjugate set, but the majority shows a relative north-side-up movements (Figures 3e and 3f). These structural and kinematic variations are inferred to be an effect of strain variation in a transpressive regime with a large component of pure shear during the development of tectonic lenses. The local minor extensional components in roughly NW SE and NE SW direction accommodated by steep zones around tectonic lenses in both the GRZ and the HGZ is compatible with approximately N S shortening as well as tectonic stacking of the lower crust [Korja and Heikkinen, 2005]. [44] N S shortening is also implied by the conjugate relationship between the flattened F 2 folds in the migmatites of the Ockelbo domain and the granitic dykes with shear fabrics indicating anticlockwise rotation (Figure 7). It would also account for the contradictory regional structural break with sinistral rotation of the lithologies in the Ockelbo domain and dextral rotation of lithologies and structures of the northern Bergslagen province (Figure 2). In addition, N S shortening combined with consistent dextral kinematics to the south of the GRZ, also emphasized by the large-scale folding north of the 1.87 Ga Hedesunda Complex and to the south of the SSZ (Figure 2), thus indicates regional transpressive conditions. [45] The surface expression of GRZ coincides with a prominent arcuate Bouguer anomaly pattern (Figures 4b and 10b). This gravity low, although somewhat displaced by the graben, continues to the east of that structure and bends clockwise (dextrally) to join the NW SE trend of the SSZ [Korja et al., 2001; Bergman and Söderman, 2005]. The gravity low implies coherence between the GRZ and the SSZ and also that the GRZ continues to the south of the migmatite belt east of the Gävle graben. A westward arcuate structure of the SSZ north of the Hedesunda Complex is also apparent from the structural grain [Bergman and Söderman, 2005], which contradicts previous interpretations of a NW continuation of the SSZ off coast to the latitude of Gävle [Talbot and Sokoutis, 1995]. [46] The character of the southward splays (Figures 2 and 10) from the prominent, transpressive SSZ [Talbot and Sokoutis, 1995] adds to the arguments of a coherent GRZ- SSZ shear zone system. Although these splays are linked to the SSZ they share several structural features with the GRZ by showing east-up kinematics corresponding to the north up along the GRZ and that they envelop tectonic lenses with different relative vertical movements [Persson and Sjöström, 2003]. In addition, both the GRZ and the splays are accompanied by metamorphic breaks. [47] The SSZ shows early pervasive deformation between 1.87 and 1.86 Ga and subsequent localized deformation [Hermansson et al., 2008]. The early transpression along the zone resulted in dextral horizontal movements (Singö gneiss zone) and later deformation in vertical extrusion of intervening lenses on the Singö mylonite zones [Talbot and Sokoutis, 1995]. In some parts the stretching lineation is plunging moderately to the SE [Stålhös, 1991]. Provided that the lineation shows the shear direction, this would result in SW-side-up kinematics as recorded along a shear zone parallel and adjacent to the SSZ (Figures 2 and 10), that is, opposite to the north-side-up kinematics recorded along the GRZ. However, both the SSZ and the migmatites north of the GRZ show a combination of kinematics and stretching lineation that is not in accordance with the metamorphic variation. A vertical extrusion, on the other hand, is consistent with the north-side-up movements recorded along GRZ. This could be explained by strain partitioning and a perpendicular relationship between fold axes and mylonitic lineations, and the maximum stretching axis, x, which has been inferred for magma flow in migmatites of the Karakoram Shear Zone [Weinberg and Mark, 2008]. [48] In the east, geophysical signatures and structural patterns indicate that GRZ is part of a larger shear zone system including the SSZ and the HGZ but the western continuation is poorly constrained by geological surface data. However, geophysical anomalies turn from E W to NNW SSE north of Rättvik (Figure 10b) and link to the anomalies of the SEDZ (Figure 2). Although the main shear activity along SEDZ was at circa 1.67 Ga [Bergman and Sjöström, 1994; Bergman et al., 2006], an earlier precursor at circa 1.80 Ga has been proposed [Bergman and Sjöström, 1994]. This tentative zone, forming an arc around the southern and western margins of the Ljusdal domain, may Figure 10. (a) Map showing the outline of a metamorphic break from ms + qz to kfs + sil stability fields [Stålhös, 1991; Stephens et al., 2007]. This break roughly coincides with the GRZ. Off the coast the location and vertical interpretation [Korja and Heikkinen, 2005] of the BABEL profiles C and C1 are included. Note that north is facing to the right to facilitate reading of the vertical BABEL profile. Bold line in inset map shows the inferred position of the leading edge of the midcrustal nucleus. HGZ, Hagsta Gneiss Zone; SSZ, Singö SZ; ÖSZ, Österbybruk-Skyttorp SZ; GZ, Gimo SZ; H, Hedesunda Complex. (b) Map showing geochronological results from this study and that of Hermansson et al. [2007], Page et al. [2004, 2007], Delin [2005], and Bergman and Söderman [2005]. Numbers in italic indicate minimum age of metamorphism in different areas, and numbers with asterisks indicate 40 Ar/ 39 Ar amphibole ages. Patterned areas represent postmetamorphic granites, and thin lines contour the gravity low (modified from Sveriges geologiska undersökning [2000]). 14 of 18

15 define the domain boundary toward the Bergslagen province in that area. Including the GRZ to the coast, this would result in an 300 km long boundary Significance of the Ockelbo Domain and Its Enveloping Shear Zones [49] The kinematics of the HGZ and the GRZ are in conformity with a relative uplift of the Ockelbo domain with respect to the Hamrånge synform and the Bergslagen province. In the Ockelbo domain the metamorphic peak producing migmatites probably occurred close to the formation of the 1836 ± 1 Ma schlieren-granite and definitely after Ga, which is the magmatic age of a metagranite affected by peak metamorphism [Delin, 2005]. The migmatites south of the GRZ are truncated by the 1.87 Ga Hedesunda Complex [Bergman et al., 2004] defining the minimum age for migmatization and peak metamorphism in that area. 40 Ar/ 39 Ar amphibole ages between 1.87 and 1.84 Ga [Page et al., 2007] from the northeastern part of Bergslagen further excludes any subsequent major heat pulses in this part of the province. [50] In a tectonic lens within the SSZ peak metamorphism was reached between 1.87 and 1.86 Ga [Hermansson et al., 2008]. The metamorphic ages in the SSZ are thus similar to those recorded in the northern part of and to the north of the Ljusdal domain [Högdahl et al., 2008]. 40 Ar/ 39 Ar amphibole ages of Ga from low-strain domains in this lens [Page et al., 2004] imply that it passed, but remained close to the 500 C isotherm in this time range as Ga 40 Ar/ 39 Ar amphibole plateau ages are found both outside and locally within the lens, and systematically in the surrounding high-strain rocks, interpreted to be related to shearing in localized zones [Hermansson, 2007]. [51] The ductile deformation along the GRZ resulting in wide high-grade shear zones occurred prior to 1797 ± 7 Ma, constrained by the 40 Ar/ 39 Ar plateau age (Figure 9) of synkinematic amphibole. Although the PT conditions during shearing have not been established the obtained age reflects the minimum age of the ductile deformation. The mineral assemblage in the shear zone (am(hbl) + pl) and a conservative pressure estimate at GPa implies a temperature of C. A similar 40 Ar/ 39 Ar amphibole plateau age (1.79 Ga [Erlandsson, 2007]) has been obtained from an amphibolite xenolith in the 1836 Ma schlieren-granite in the northeastern part of the Ockelbo domain. This indicates that the Ockelbo domain, the SSZ and the GRZ were at the same crustal level at about 1.80 Ga. [52] The strong structural and geophysical links between the GRZ and the SSZ are matched by temporal overlaps. Pervasive ductile shear along the SSZ preceded 1.85 Ga [Hermansson et al., 2007] and occurred between and 1.80 Ga along the GRZ allowing an overlapping time window for deformation (i.e., Ga). On the GRZ a later pure-shear overprint is recorded by steep lineations, rotation and flattening of folds, rotation of granitic dykes and north-up kinematics, possibly related to reactivation of localized shear at Ga along SSZ [Hermansson et al., 2008] and to shear along the HGZ. To the north, in the Ljusdal domain, the regional east-west folds eventually partitioned into Ga dextral shear zones. Also the clockwise rotation of lithologies and structures in the northern part of the Bergslagen province and the anticlockwise rotations in the Ockelbo domain fit in this scenario. [53] There are few previous records of Ga granitoids and migmatites in the western part of the Fennoscandian Shield. Farther to the east, however, potassium granites in this age range [Korsman et al., 1984; Huhma, 1986; Suominen, 1991] are typical in the Late Svecofennian Granite-Migmatite Belt (LSGM) [Ehlers et al., 1993] in the Southern Svecofennian Arc Complex (Figure 1). In that transpressional area the melts either froze as migmatites at depths or moved to form granites in the middle and upper crust [e.g., Stålfors and Ehlers, 2005]. [54] The structural pattern of the LSGM is characterized by a set of steep NNE SSW and NNW SSE mainly reverse dip-slip zones between two major E W dextral strike-slip zones (Southern Finland SZ and Somero SZ, respectively) [Väisänen and Skyttä, 2007] showing striking similarities with the HGZ, GRZ-SSZ and related shear zones in the northeastern part of the Bergslagen province. For instance, the NNW SSE zones show a systematic eastside-up kinematic [Väisänen and Skyttä, 2007] similar to the southward splays of the SSZ, and the ductile shear activity occurred between 1.83 and 1.79 Ga owing to approximately NW SE convergence [Väisänen and Skyttä, 2007] Tectonic Model [55] It has been proposed that dextral lateral movements occurred along a shallow dipping structure in a middle crustal nucleus [Lahtinen et al., 2005]. The leading edge of this structure coincides with the gravity low at the boundary between the Bergslagen province and the Ockelbo domain and we infer that the structural evolution at depth resulted in initiation of the SSZ and the GRZ. Lahtinen et al. [2005] further suggest that this movement occurred at 1.82 Ga, but surface geology and documented activity along adjacent shear zones (i.e., SSZ [Hermansson et al., 2007] and South Finland Shear Zone [Torvela et al., 2008]) imply that it was, at least, initiated already at Ga and related to the oblique accretion of the Bergslagen nucleus to an older Paleoproterozoic or Archean deep-seated crustal nucleus [Lahtinen et al., 2005] to the north below the Ljusdal domain (Figure 11a). Contemporaneously the Ljusdal batholith was emplaced owing to oblique subduction, and a proto-hsz was established at the edge of another crustal nucleus [Högdahl et al., 2008]. [56] Continued convergence flattened folds in the migmatites, rotated granitic dykes and formed large-scale Z fold to the south of the coherent GRZ-SSZ. Transpressive conditions are indicated at Ga with granite formation in the LSGM in the Southern Finland Arc Complex [Ehlers et al., 1993] and in Åland [Ehlers et al., 2004], thus coeval with the 1836 ± 1 Ma schlieren-granites in the Ockelbo domain and indicate that the Ockelbo domain is a pinched-out part of this belt. The formation 15 of 18

16 Figure 11. Cartoon illustrating the evolution of shear zones in the central part of the Fennoscandian Shield between 1.86 and 1.80 Ga (modified after Lahtinen et al. [2005]). (a) Formation of GRZ and SSZ as a coherent shear zone system due to approximately N S convergence. (b) Continued convergence caused dextral shifting from GRZ to HGZ at 1.81 Ga. The HGZ-GRZ forms a part of a large-scale shear zone system that possibly represents a terrane boundary developed between major crustal components. At this time a large number of shear zones were established in other parts of the shield. Shaded area is the Archean Karelian craton, striped areas represent Archean or Paleoproterozoic nuclei, and thin lines are inferred terrane boundaries in the model by Lahtinen et al. [2005]. of an antiform above the middle crustal leading edge caused relative uplift of the Ockelbo domain. [57] The final ductile evolution is characterized by the formation of large-scale E W folds in the Ljusdal domain and mainly dextral Ga shear zones. The middle crustal antiform controlled both the localization and the local extension on the HGZ. At this time the GRZ, with an orthogonal orientation to the shortening direction, was affected by a significant pure-shear overprint by the indentation of the crustal nucleus below the Ljusdal batholith into the Bergslagen nucleus. Owing to the unfavorable orientation of the GRZ, strain was partitioned to the HGZ. The approximately N S shortening and apparent thrusting in the lower and middle crust were thus accommodated not only by folding but also by E W extension and lateral escape on steep shear zones at higher crustal levels as well as local extension across the HGZ and GRZ. [58] On a larger scale the GRZ-HGZ belong to a system of coeval shear zones in Åland [Torvela et al., 2008] and southern Finland [Ehlers et al., 2004] (Figure 11b). Applying the model by Lahtinen et al. [2005] this shear zone system would represent an 1500 km long active (plate) boundary initiated at circa 1.86 Ga with continuous or reactivated shear due to approximately N S convergence until circa 1.80 Ga. 7. Summary and Conclusions [59] 1. The GRZ is coherent with the SSZ, including the southward splays in the northeastern part of the Bergslagen province. The coherent GRZ-SSZ was initiated at Ga as part of the arcuate northern boundary of the Bergslagen province, which was linked to incipient stacking structures in the middle and lower crust. Coeval structures in the Åland archipelago are probably an eastern continuation of the structure. [60] 2. North of the GRZ, migmatites are Ga and probably close to 1836 ± 1 Ma, that is, younger than the 1.87 Ga migmatite formation in the northern part of the Bergslagen province. The 1836 ± 1 Ma schlieren-granite suggests that the Ockelbo domain, at least partly, can be correlated with the potassium granites of the LSGM in the southern Finland Arc to the east. [61] 3. The ongoing stacking in the middle and lower crust was responsible for the relative uplift of the Ockelbo domain, and progressive N S shortening was accommodated by regional E W folds and mainly dextral shear zones in the Ljusdal domain, resulting in E W stretching and lateral escape at the present level of exposure. [62] 4. Continued N S shortening caused an antiform above the leading edge in the lower and middle crust and a substantial pure-shear overprint on the GRZ, resulting in shift of dextral shear to the HGZ, dated at 1809 ± 2 Ma. [63] 5. The HGZ and GRZ are related to ductile shear zones in the Åland archipelago and in southern Finland farther to the east and may be part of an 1500 km long domain or even terrane boundary. [64] Acknowledgments. L. M. Page at the Department of Geology, Lund University, kindly ran the argon analysis. We are grateful to H. Delin, Geological Survey of Sweden, for providing us with information about key outcrops and unpublished geochronological data. Constructive comments by the two reviewers, Frank Beunk and Rolf Romer, considerably improved the paper. This project was supported by the Geological Survey of Sweden. 16 of 18