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| 2. |
- Bender, Hagen, et al.
(författare)
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Metamorphic Zonation by Out‐of‐Sequence Thrusting at Back‐Stepping Subduction Zones : Sequential Accretion of the Caledonian Internides, Central Sweden
- 2018
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Ingår i: Tectonics. - : John Wiley & Sons. - 0278-7407 .- 1944-9194. ; 37:10, s. 3545-3576
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Tidskriftsartikel (refereegranskat)abstract
- Exhumation of the high‐grade metamorphic Seve Nappe Complex and its emplacement between lower‐grade nappes has been related to wedge extrusion in the central Scandinavian Caledonides. To test this hypothesis, the kinematic evolution of the Caledonian nappe pile is studied by systematic structural mapping in central and northern Jämtland, Sweden. Structural data, combined with petrological and quartz microstructure observations, document pervasive top‐to‐the‐ESE, foreland‐directed shearing under progressively decreasing metamorphic grade across the entire nappe pile. Mylonitic foliation, foliation‐parallel boudinage, and abundant top‐to‐the‐ESE and rare, scattered top‐to‐the‐WNW shear‐sense indicators imply foreland‐directed general shear. This deformation regime caused exhumation by concurrent thrusting and vertical ductile thinning. We propose a specific succession of in‐ and out‐of‐sequence thrusts that generated the metamorphic zonation. Our model envisions in‐sequence propagation of thrusts during exhumation of the Seve Nappe Complex, related to subduction of Baltica beneath a volcanic arc within Iapetus. Concurrently, Iapetus subducted beneath Laurentia farther to the west. When Iapetus was closed, Baltica subduction stepped westward and continued beneath Laurentia. The back stepping of subduction at the onset of continental collision caused out‐of‐sequence propagation of the orogenic wedge. Thrusting cut downsection across the existing tectonostratigraphy, emplacing units of lower metamorphic grade above the high‐grade Seve Nappe Complex. This imbrication generated the present metamorphic zonation of the Caledonian nappe pile during sustained convergence between Laurentia and Baltica.
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| 4. |
- Bergström, Ulf, et al.
(författare)
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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
- 2020
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Ingår i: Sweden. - : Geological Society of London. ; , s. 397-434
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 5. |
- Gee, David G., et al.
(författare)
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Lower thrust sheets in the Caledonide orogen, Sweden: Cryogenian–Silurian sedimentary successions and underlying, imbricated, crystalline basement
- 2020
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Ingår i: Sweden. - : Geological Society of London. ; , s. 495-515
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The Jämtlandian Nappes and their equivalents further north, belonging to the lower thrust sheets in the Caledonide orogen of Sweden, comprise a mega-duplex of Cryogenian–Silurian sedimentary rocks sandwiched between structurally higher allochthons and a basal décollement. Further west towards the hinterland, crystalline basement is increasingly involved in this thrusting, imbricate stacking occurring beneath the décollement in antiformal windows. The sedimentary successions were derived from the Cryogenian rifted margin of Baltica, the Ediacaran–Cambrian drifted margin, and Ordovician and Silurian foreland basins. During the Early–Late Ordovician (Floian–Sandbian), hinterland-derived turbidites were deposited in response to early Caledonian accretion of subducted complexes belonging to the outermost margin of Baltica, now preserved in the higher allochthons. Following a quiescent period during the Late Ordovician (Hirnantian) and early part of the Llandovery, collision of Laurentia and Baltica reactivated the foreland basins, with flysch and molasse deposition during the Llandovery–Wenlock. Collisional shortening during this Scandian orogenic episode continued into the Devonian. High- and ultrahigh-pressure (HP/UHP) metamorphism accompanied Baltica's underthrusting of Laurentia in the deep hinterland, and prominent basement-cored antiforms developed towards the foreland during the advance of the orogenic wedge over the foreland basin onto the Baltoscandian platform.
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| 6. |
- Gee, David G., et al.
(författare)
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Middle thrust sheets in the Caledonide orogen, Sweden: the outer margin of Baltica, the continent–ocean transition zone and late Cambrian–Ordovician subduction–accretion
- 2020
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Ingår i: Sweden. - : Geological Society of London. - 9781786204608 ; 50:1, s. 517-548
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Nappes of continental outer and outermost margin affinities (Middle Allochthon) were transported from locations west of the present Norwegian coast and thrust eastwards onto the Baltoscandian foreland basin and platform. They are of higher metamorphic grade than underlying thrust sheets and most are more penetratively deformed. These allochthons are treated here in three groups. The lower thrust sheets comprise Paleoproterozoic crystalline basement (e.g. Tännäs Augen Gneiss Nappe) and greenschist facies, Neoproterozoic, siliciclastic metasedimentary rocks (e.g. Offerdal Nappe). These are overthrust by a Cryogenian−Ediacaran succession intruded by c. 600 Ma dolerites (Baltoscandian Dyke Swarm) with an affinity to mid-ocean ridge basalt containing normal to enriched incompatible element contents (Särv Nappes). The upper sheets are dominated by higher-grade allochthons (Seve Nappe Complex) with similar, mainly siliciclastic sedimentary protoliths, more mafic magmatism and some solitary ultramafic bodies. Within this early Ediacaran continent−ocean transition zone (COT) assemblage, generally metamorphosed in amphibolite facies, some nappes experienced migmatization, and eclogites are present. Evidence of ultrahigh-pressure metamorphism has been obtained from garnet peridotites and eclogites; recently, microdiamonds have been discovered in paragneisses. Subduction of the COT started by the late Cambrian and accretion continued through the Ordovician, prior to the Baltica–Laurentia collision. Thrusting of all these Middle allochthons onto the foreland basin exceeds a distance of 400 km.
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| 7. |
- Gee, David G., et al.
(författare)
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Regional context and tectonostratigraphic framework of the early–middle Paleozoic Caledonide orogen, northwestern Sweden
- 2020
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Ingår i: Sweden. - : Geological Society of London. ; , s. 481-494
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The Scandian mountains in northwestern Sweden are dominated by the eastern part of the Scandinavian Caledonides, an orogen that terminated during the middle Paleozoic with Himalayan-style collision of the ancient continents of Baltica and Laurentia. In this foreland region, far-transported higher allochthons from an exotic continental margin (Rödingsfjället Nappe Complex) and underlying mostly oceanic-arc basin character (Köli Nappe Complex) were emplaced at least 700 km onto the Baltoscandian margin of Baltica. The thrust sheets below the Iapetus Ocean terranes were derived from the transition zone to Baltica (Seve Nappe Complex), comprising mainly siliciclastic metasedimentary rocks, hosting abundant metamorphosed c. 600 Ma mafic intrusions. They preserve evidence of subduction (eclogites, garnet peridotites and microdiamonds in host paragneisses), starting in the late Cambrian; exhumation continued through the Ordovician. Underlying allochthons derived from the outer margin of Baltica are less-metamorphosed Neoproterozoic sandstone-dominated successions, also intruded by Ediacaran dolerite dykes (Särv Nappes); they are located tectonically above similar-aged metasandstone and basement slices, devoid of dykes (Offerdal and Tännäs Augen Gneiss nappes and equivalents). Lowermost allochthons (Jämtlandian Nappes and equivalents), from the inner Baltoscandian margin, provide evidence of Cryogenian rifting, Ediacaran–Cambrian drifting and platformal sedimentation, followed by foreland basin development in the Ordovician and Silurian.
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| 8. |
- Hermansson, Tobias, et al.
(författare)
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Ar-40/Ar-39 hornblende geochronology from the Forsmark area in central Sweden : constraints on late Svecofennian cooling, ductile deformation and exhumation
- 2008
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 167:3-4, s. 303-315
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Tidskriftsartikel (refereegranskat)abstract
- At Forsmark, ca. 120 km north of Stockholm in central Sweden, ductile high-strain belts with WNW to NW trend anastomose around tectonic lenses with an inferred lower degree of ductile strain. Previous studies of ductile deformation zones with WNW to NW trend, elsewhere in the western part of the Svecofennian orogen in central Sweden, have yielded estimates for the timing of at least one phase of discrete ductile deformation that fall in the time interval 1.82-1.78 Ga. Most of these ages were determined by the U/Pb dating of titanite and, for this reason, provide no information on the thermal evolution. In this paper, we make use of Ar-40/Ar-39 hornblende geochronology to address late Svecofennian cooling, ductile deformation and exhumation. The data demonstrate the presence of three Ar-40/Ar-39 hornblende age generations. All ages have been adjusted to take account of ca. 1% systematic bias between Ar-40/Ar-39 and U/Pb ages recently reported in the literature. The oldest age, ca. 1.87 Ga, and the intermediate age generation, 1.85-1.84 Ga, are spatially restricted to the tectonic lenses. By contrast, the youngest age generation, 1.83-1.81 Ga, occurs both within the tectonic lenses and the enveloping high-strain belts. one explanation for the structurally controlled age distribution involves regional cooling beneath the closure temperature for argon isotopic mobility around or above 500 degrees C by 1.84 Ga, as represented in the oldest and intermediate age generations, followed by resetting of the argon isotope system in hornblende between 1.83 and 1.81 Ga, as represented in the youngest age generation. This resetting occurred in response to retrograde, lower amphibolite- to upper greenschist-facies deformation along discrete high-strain zones within the broader high-strain belts and was associated with regional exhumation. An alternative explanation involves no resetting of the ages. Instead, it is suggested that a period of slow cooling of hornblendes with slightly different closure temperatures, from ca. 1.87 to 1.82 Ga, may have caused the age variation observed within the tectonic lenses, whereas locally maintained higher temperatures, due to activity along the discrete high-strain zones, can explain the consistently younger ages in the broad, enveloping high-strain belts. In this explanation, an increase in cooling rate, in response to regional exhumation, finally closed the argon isotope system in hornblende throughout the area at 1.83-1.81 Ga. It is suggested that the regional exhumation at 1.83-1.81 Ga, which is included in both explanations, is related to far-field effects of the deformation that ended an accretionary tectonic cycle in adjacent tectonic domains. (c) 2008 Elsevier B.V. All rights reserved.
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| 9. |
- Hermansson, Tobias, et al.
(författare)
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Penetrative ductile deformation and amphibolite-facies metamorphism prior to 1851 Ma in the western part of the Svecofennian orogen, Fennoscandian Shield
- 2007
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268. ; 153:1-2, s. 29-45
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Tidskriftsartikel (refereegranskat)abstract
- The Svecofennian orogen in the Fennosandian Shield consists of several Palaeoproterozoic meta-igneous provinces that generally young progressively southwards and westwards. The understanding of the tectonic evolution of these different provinces is linked to the ability to constrain the timing of ductile deformation and metamorphism within them. In the western part of the orogen, in central Sweden, ductile deformation and metamorphism is conventionally thought to have occurred after 1.85 Ga and mainly around 1.82-1.80 Ga. At Forsmark in central Sweden, we have dated two undeformed granite dykes that cross-cut the tectonic fabric in their host rocks to 1851 +/- 15 and 1855 +/- 6 Ma, respectively. Since the former shows an unequivocal field relationship with respect to the tectonic fabric, we establish that penetrative ductile deformation under amphibolite-facies metamorphic conditions occurred prior to 1851 +/- 5 Ma. This U-Pb zircon minimum age for the timing of ductile deformation is supported by a U-Pb titanite age of 1844 +/- 4 Ma. These data as well as the field relationships at Forsmark indicate that an early phase of penetrative deformation and metamorphism affected the Svecofennian bedrock in the western part of the Fennoscandian Shield and was overprinted, after 1851 +/- 5 Ma, by spatially more confined deformational events. In conjunction with earlier results, our data help to establish the presence of tectonic domains with contrasting tectonothermal histories in the western part of the Svecofennian orogen. (c) 2006 Elsevier B.V. All rights reserved.
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| 10. |
- Kampmann, Tobias Christoph, 1987-
(författare)
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Age, origin and tectonothermal modification of the Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, Bergslagen, Sweden
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Falun pyritic Zn-Pb-Cu-(Au-Ag) sulphide deposit, situated in the Palaeoproterozoic (1.9–1.8 Ga) Bergslagen lithotectonic unit in the south-western part of the Fennoscandian Shield, is one of the major base and precious metal sulphide deposits in Sweden. Felsic volcanic rocks and limestone hosting the deposit, as well as their hydrothermally altered equivalents and the mineralization, were affected by heterogeneous ductile strain and metamorphism under low-pressure, lower amphibolite-facies conditions during the Svecokarelian orogeny (2.0–1.8 Ga). These processes reworked the mineral assemblages of the original hydrothermal system and the mineralization, and also reshaped the structural geometry of the deposit.A three-dimensional modelling approach has been used in order to evaluate geometric relationships between lithologies at the deposit. The polyphase character (D1 and D2) of the strong ductile deformation at Falun is apparent. The main rock-forming minerals in the altered silicate-rich rocks are quartz, biotite and anthophyllite with porphyroblasts of cordierite and garnet, as well as retrogressive chlorite. Major static grain growth occurred between D1 and D2, inferred to represent the peak of metamorphism, as well as after D2 with growth (or recrystallization) of anthophyllite. A major shear zone with chlorite, talc and disseminated sulphides bounds the pyritic Zn-Pb-Cu-rich massive sulphide mineralization to the north, the latter being surrounded elsewhere by disseminated to semi-massive Cu-Au mineralization. F2 sheath folding along axes plunging steeply to the south-south-east is suggested as a key deformation mechanism, accounting for the cone-shaped mineralized bodies, which pinch out at depth, and explaining the similar character of intensely altered rocks on all sides of the massive sulphide mineralization. Immobile-element lithogeochemistry suggests that they share a common volcanic precursor. These relationships are consistent with a model in which the pyritic massive sulphide mineralization is located in the core of a sheath fold structure, surrounded by the same altered stratigraphic footwall rocks with Cu-Au mineralization.The geological evolution in the metavolcanic inlier that hosts the Falun deposit, constrained by secondary ion mass spectrometry (SIMS) U–Pb (zircon) geochronology, involved emplacement of a felsic volcanic and sub-volcanic rock suite at 1894±3 Ma, followed by hydrothermal alteration and mineralization. Subsequent burial and intrusion of late- to post-mineralization dykes occurred between 1896±3 Ma and 1891±3 Ma, followed by further burial and emplacement of plutons with variable composition during the time span 1894±3 Ma to 1893±3 Ma. The age determinations for all these magmatic suites overlap within their uncertainties, indicating a rapid sequence of continuous burial and different magmatic pulses. A metamorphic event, herein dated at 1831±8 Ma and 1822±5 Ma (SIMS U–Pb monazite), falls in the age range of a younger Svecokarelian metamorphic episode (M2). U-Th-Pb isotope systematics in monazite was completely reset during this event.During hydrothermal alteration and mineralization, a hot, reducing and acidic fluid carrying metals and sulphur together flowed upward along syn-volcanic faults, leading to intense chloritization, sericitization and silicification of calc-alkaline volcanic rocks in the stratigraphic footwall to the deposit. This resulted in proximal siliceous associations including Fe-rich chlorite, and dominant Mg-rich chlorite and sericite in more peripheral parts. Cu-Au stockwork mineralization formed in the siliceous core of the hydrothermal system as result of fluid cooling. Neutralization of the metal-bearing fluids upon carbonate interaction stratigraphically higher in the sub-seafloor regime led to formation of Zn-Pb-Cu-rich massive sulphide mineralization, the space for which was created by a combination of carbonate dissolution, primary porosity in the overlying volcanic rocks and secondary porosity produced during syn-volcanic faulting. A hybrid model for mineralization is suggested by alteration styles, metal zoning and textures indicating replacement of carbonate rock or highly porous pumice breccia by pyritic massive sulphide. Aspects of a sub-seafloor volcanogenic massive sulphide (VMS) system and carbonate replacement are both present. Partly Zn-Pb-(Ag) mineralized skarns comprise a separate and subordinate type of mineralization, probably formed after burial of the hydrothermal system to the contact-metasomatic regime.Textures and microstructures in the massive sulphide mineralization indicate that the ductile deformation and metamorphism resulted in internal mechanical and chemical remobilization of sulphide minerals. Laser ablation inductively coupled mass spectrometry (LA-ICP-MS) analysis of the main sulphide minerals suggests, for example, that trace elements (including Au) were liberated from pyrite during metamorphism. A system of auriferous quartz veins, affected by D2 ductile strain, occurs in intensely altered and mineralized rocks on the eastern side of the deposit. It is suggested that they formed after the peak of metamorphism and prior to the completion of the D2 tectonic event, as a result of fluid-assisted remobilization of sulphides and Au in the disseminated to semi-massive Cu-Au mineralization and possibly also the massive sulphide mineralization.
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