Geochronological evidence for repeated brittle reactivations of a pre-existing plastic shear zone: The Himdalen–Ørje deformation zone, Southern Norway

<p>It is well known that faults, once formed, become permanent weaknesses in the crust, localizing subsequent brittle strain increments. The case of repeated brittle reactivations localized along pre-existing plastic shear zones is less recognized, although this situation is frequently observed in many geologically old terranes.</p><p>We have studied the prolonged deformation history of the Himdalen–Ørje Deformation Zone (HØDZ) in SE Norway by combining K–Ar and <sup>40</sup>Ar–<sup>39</sup>Ar geochronology with structural analysis. The HØDZ consists of a large variation of deformation products from mylonites and cataclasites to pseudotachylites and fault gouge. Several generations of mylonites make up the ductile part of HØDZ, called the Ørje shear zone, a km-think SW-dipping shear zone within the late Mesoproterozoic Sveconorwegian orogen. <sup>40</sup>Ar–<sup>39</sup>Ar dating of white mica from one of these mylonites give a plateau age of c. 908 Ma, interpreted to constrain the timing of late-Sveconorwegian extensionial reactivation of the Ørje shear zone.</p><p>This mylonitic fabric is extensively reworked in a brittle fashion along the SW-dipping Himdalen fault, a 10–25 m thick fault zone of cataclasite, breccia, fault gouge and, in places, abundant pseduotachylite veins. <sup>40</sup>Ar–<sup>39</sup>Ar dating of pseduotachylite material gives several small plateaus between c. 375 and 300 Ma, whereas K-feldspar clasts from the cataclasitically deformed host rock carry a Caledonian signal (plateau at c. 435 Ma). K–Ar dating of three fault gouges constrain the timing of gouge development at c. 270 and 200 Ma. Two of the fault gouges also contain protolithic K-bearing mineral phases that overlap in age with the c. 375 Ma pseudotachylite <sup>40</sup>Ar–<sup>39</sup>Ar plateau age, consistent with field observations of the former reworking the latter.</p><p>In sum, the HØDZ records multiple Paleozoic and Mesozoic brittle reactivations of the early Neoproterozoic (and older) mylonitic Ørje shear zone. Most of the brittle deformation is interpreted to have accumulated during development of the Permian Oslo rift and its subsequent latest Triassic evolution. The suggested late Devonian (c. 375 Ma) initiation of brittle deformation does not have a clear tectonic association, but we speculate that it relates to strike-slip displacements caused by the Variscan orogen, as also suggested for the sub-parallel Tornquist zone to the south.</p>

Chapter 1 Introduction to the lithotectonic framework of Sweden and organization of this Memoir

The solid rock geology of Sweden comprises three principal components: (1) Proterozoic and (locally) Archean rocks belonging to the western part of the Fennoscandian Shield; (2) Phanerozoic and (locally) Neoproterozoic sedimentary cover rocks deposited on top of this ancient crust; and (3) the early to mid-Paleozoic (0.5–0.4 Ga) Caledonide orogen. Earlier compilations have applied different principles for the subdivision of the geology in the Fennoscandian Shield and the Caledonide orogen. A uniform lithotectonic framework has been developed here. Crustal segments affected by orogenesis have been identified and their ages determined by the youngest tectonothermal event. Four ancient mountain belts and six orogenies are preserved. Solid rocks outside the orogens have been assigned to different magmatic complexes or sedimentary successions based on their time of formation and tectonic affiliation. This approach allows relicts of older mountain-building activity to be preserved inside a younger orogen – for example, the effects of the Archean (2.8–2.6 Ga) orogeny inside the 2.0–1.8 Ga Svecokarelian orogen and Paleo–Mesoproterozoic (1.7–1.5 and 1.5–1.4 Ga) mountain-building processes inside the 1.1–0.9 Ga Sveconorwegian orogen. Sweden's five largest mineral districts are addressed in the context of this new lithotectonic framework, which forms the architecture to the contents of the chapters in this Memoir.

Chapter 13 Siliciclastic sedimentation in a foreland basin to the Sveconorwegian orogen and dolerites (0.98–0.95 Ga) related to intracratonic rifting

Sub-ophitic, equigranular or plagioclase-phyric dolerite dykes, referred to as the Blekinge–Dalarna dolerite (BDD) swarm, were emplaced during the time span 0.98–0.95 Ga and trend NNE–NNW in an arcuate fashion, parallel to and east of the Sveconorwegian orogen. Dolerite sills are locally present. These rocks are subalkaline to alkaline with a monzogabbroic or gabbroic composition and show a predominantly within-plate tectonic affinity. ɛNd and ɛHf values fall in the range −2 to +4 and +1 to +5, respectively. Siliciclastic sedimentary rocks (Almesåkra Group) in a small outlier in southern Sweden were deposited in an aeolian to fluviatile or lacustrine environment and an arid or semi-arid warm palaeoclimate, coevally with the dolerite sills. Smaller occurrences of sandstone with peperitic field relationships to the BDD dykes are known from other localities. The spatial distribution, orientation and age of the BDD magmatic suite suggest roughly east–west extension in the eastern, cratonic foreland to the Sveconorwegian orogen during the latest phase of this mountain-building event, the age data tentatively suggesting a younging to the east. The siliciclastic sedimentary rocks represent an erosional relict of a larger and spatially much more extensive early Tonian foreland basin to this orogen, as proposed earlier on the basis of fission-track thermochronology.

Chapter 14 Regional context and lithotectonic framework of the 1.1–0.9 Ga Sveconorwegian orogen, southwestern Sweden

The 1.1–0.9 Ga Sveconorwegian orogen in southwestern Scandinavia belongs to the global system of mountain belts established during the assembly of the supercontinent Rodinia. An overall north–south structural trend and five lithotectonic units bounded by crustal-scale shear zones characterize this orogen. In Sweden, the Eastern Segment abuts the orogen's cratonic foreland eastwards and is separated from the Idefjorden terrane westwards by a ductile shear zone, up to 5 km thick, displaying a sinistral transpressive component. These two lithotectonic units differ on the basis of their pre-Sveconorwegian accretionary tectonic evolution, and the timing of Sveconorwegian high-pressure metamorphism, anatexis and polyphase deformation. High-pressure granulites and migmatites formed at c. 1.05–1.02 Ga in the Idefjorden terrane; eclogites, high-pressure granulites and migmatites at c. 0.99–0.95 Ga in the Eastern Segment. Magmatic activity and crustal extension progressed westwards at c. 0.98–0.92 Ga. Prior to or at 0.93–0.91 Ga, greenschist facies shear deformation with top-to-the-foreland movement affected the frontal part of the orogen. Geodynamic uncertainties concern the affinity of the Idefjorden terrane relative to Fennoscandia (Baltica), the character of the Sveconorwegian orogenesis, and the contiguous or non-contiguous nature of the erosional fronts of the late Mesoproterozoic–early Neoproterozoic orogens in Sweden and Canada.

Chapter 16 Polyphase (1.6–1.5 and 1.1–1.0 Ga) deformation and metamorphism of Proterozoic (1.7–1.1 Ga) continental crust, Idefjorden terrane, Sveconorwegian orogen

Crust generated during an accretionary orogeny at 1.66–1.52 Ga (Gothian), and later during crustal extension at c. 1.51–1.49, c. 1.46, c. 1.34–1.30 Ga and after c. 1.33 Ga, dominate the Idefjorden terrane. Metamorphism under greenschist to, locally, high-pressure granulite facies, emplacement of syn-orogenic pegmatite and granite, and polyphase deformation followed at 1.05–1.02 Ga (Agder tectonothermal phase, Sveconorwegian orogeny). Sinistral transpressive deformation, including foreland-directed thrusting, preceded top-to-the-west movement and large-scale open folding along north–south axial trends during the younger orogeny. Crustal extension with emplacement of dolerite and lamprophyre dykes, norite–anorthosite, and a batholithic granite took place at c. 0.95–0.92 Ga (Dalane phase, Sveconorwegian orogeny). Ductile shear zones divide the Idefjorden terrane into segments distinguished by the character of the Gothian crustal component. Orthogneisses with c. 1.66 and c. 1.63–1.59 Ga protoliths occur in the Median segment; c. 1.59–1.52 Ga gneissic intrusive rocks and 1.6 Ga paragneisses with relicts of Gothian deformation and migmatization at c. 1.59 Ga and at c. 1.56–1.55 Ga occur in the Western segment. Mineral resources include stratabound Cu–Fe sulphides hosted by sandstone deposited after c. 1.33 Ga, and polymetallic quartz vein mineralization locally containing Au.

Chapter 17 Accretionary orogens reworked in an overriding plate setting during protracted continent–continent collision, Sveconorwegian orogen, southwestern Sweden

The Eastern Segment in the Sveconorwegian orogen, southwestern Sweden, is dominated by 2.0–1.8, 1.7 and 1.5–1.4 Ga crust; and the overlying Idefjorden terrane by 1.6–1.5 Ga crust. Assuming reorganization of a subduction system prior to 1.5–1.4 Ga and applying a sinistral transpressive component of disruption during the subsequent Sveconorwegian orogeny (1.1–0.9 Ga), the Idefjorden terrane is inferred to be indigenous outboard rather than exotic with respect to the continental plate Fennoscandia (Baltica). The geological record then records successive westwards shift of accretionary orogens along a convergent plate boundary for at least 500 million years. Sveconorwegian foreland-younging tectonic cycles at c. 1.05 (or older)–1.02 Ga (Idefjorden terrane) and at c. 0.99–0.95 Ga (Eastern Segment) prevailed. Crustal thickening and exhumation during oblique convergence preceded migmatization, magmatic activity and a changeover to an extensional regime, possibly triggered by delamination of continental lithosphere, in each cycle. Convergence after 0.95 Ga involved antiformal doming with extensional deformation at higher crustal levels (Eastern Segment) and continued magmatic activity (Idefjorden terrane). An overriding plate setting is inferred during either accretionary orogeny or, more probably, protracted continent–continent collision. Continuity of the erosional fronts in the Grenville and Sveconorwegian orogens is questioned.

Chapter 11 Reworking of older (1.8 Ga) continental crust by Mesoproterozoic (1.5–1.4 Ga) orogeny, Blekinge–Bornholm orogen, southeastern Sweden

The Blekinge–Bornholm orogen in southeastern Sweden consists of calc-alkaline to alkali–calcic intrusive rocks, rhyolites and dacites (1.8 Ga) that were structurally reworked under amphibolite facies conditions, affected by migmatization at mid-crustal levels at c. 1.44 Ga and intruded at c. 1.47–1.43 Ga by ferroan alkali–calcic plutons. This Mesoproterozoic orogen is bordered westwards by the Sveconorwegian orogen and northwards, along the boundary with well-preserved 1.8 Ga magmatic rocks in the Svecokarelian orogen, by a stitching c. 1.45 Ga pluton and steeply dipping ductile zones with a south-side-up, dip-slip shear component. A variably developed gneissic fabric (S1) dips gently to moderately northwards and is affected by asymmetrical F2 folds with a southerly vergence. Ductile high-strain zones with top-to-the south shear sense are suggested to correspond at depth to anomalously reflective zones along seismic profile BABEL line A. Open folding of the gneissosity around gently, north-plunging fold axes (F3) completed the ductile deformational evolution. Uncertainty remains about the timing of the amphibolite facies ductile fabric and the D2 folding, which is either late-stage Svecokarelian (c. 1.77–1.75 Ga) or Hallandian (c. 1.47–1.43 Ga). Non-collisional, accretionary orogenic systems are suggested to have operated during both time periods, radical reorganization of the subduction trend accompanying the Mesoproterozoic event.

Chapter 15 Polyphase (1.9–1.8, 1.5–1.4 and 1.0–0.9 Ga) deformation and metamorphism of Proterozoic (1.9–1.2 Ga) continental crust, Eastern Segment, Sveconorwegian orogen

The Eastern Segment in the Sveconorwegian orogen comprises Paleoproterozoic–Mesoproterozoic magmatic suites, which formed along an active continental margin, and Mesoproterozoic suites emplaced during intracratonic extension. Zn–Pb sulphide and Fe oxide mineralizations in 1.9 Ga metavolcanic rocks form a significant mineral resource cluster in the northeastern part. Deformation and metamorphism under low-pressure (≤5 kbar) and variable-temperature conditions, including anatexis and granulite facies, prevailed during 1.9–1.8 Ga (Svecokarelian) and 1.5–1.4 Ga (Hallandian) accretionary orogenies. Sveconorwegian tectonothermal reworking initiated at c. 0.99–0.98 Ga in structurally lower levels. Crustal shortening, underthrusting with eclogite facies metamorphism (18 kbar), exhumation by eastwards thrusting (D1) during continued shortening and high-pressure granulite (8–12 kbar) to upper amphibolite facies metamorphism prevailed. Anatexis and folding around east–west axial surfaces with west-northwesterly constrictional strain (D2) followed at c. 0.98–0.95 Ga, being consanguineous with crustal extension. Structurally higher levels, northwards and eastwards, consist of high-pressure (10–12 kbar) orthogneisses, not affected by anatexis but also showing polyphase deformation. Sveconorwegian convergence ceased with upright folding along north–south axial surfaces and, in the uppermost frontal part, greenschist facies shearing with top-to-the-foreland normal followed by reverse displacement after 0.95 Ga. The normal shearing detached the upper compartment from the underlying gneisses.