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- 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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- Hermansson, Tobias, et al.
(författare)
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Migratory tectonic switching, western Svecofennian orogen, central Sweden: Constraints from U/Pb zircon and titanite geochronology
- 2008
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Ingår i: Precambrian Research. - 0301-9268. ; 161:3-4, s. 250-278
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Tidskriftsartikel (refereegranskat)abstract
- The Forsmark area, in the western part of the Svecofennian orogen, central Sweden, is situated between two major Palaeoproterozoic tectonic domains that show contrasting histories with respect to timing of igneous activity, ductile deformation and metamorphism. Geological features common to both the adjacent domains are found at Forsmark, which, consequently, is a key area for understanding the tectonic evolution of the Svecofennian orogen in this region. New SIMS and TIMS geochronological data from several samples in a limited area at Forsmark constrain the timing of igneous activity, ductile deformation and metamorphism in this area. U/Pb zircon dating, in combination with field studies, reveals the existence of two calk-alkaline igneous suites at Forsmark. The older and most voluminous plutonic suite intruded at 1.89–1.87 Ga and is affected by penetrative, yet variably intense ductile deformation, with the development of a ductile grain-shape fabric under amphibolite facies metamorphic conditions and folding. The younger and less voluminous hypabyssal suite intruded at 1.86–1.85 Ga, during the waning stages of this penetrative deformational phase. Furthermore, U/Pb titanite dating shows that the area has been affected by one or more tectonothermal events after regional fabric development, and possibly even after 1.83 Ga. The deformation during these events involved more restricted strain along discrete zones. The Forsmark data, in combination with a compilation and evaluation of available geochronological data from the time interval 1.91–1.84 Ga in central Sweden, points to the existence of at least two major tectonic cycles in this region. Each cycle is characterised by igneous activity associated at least partly with extension, a short interval of compressional deformation and migration of the tectonic activity. In this paper, we discuss two highly contrasting tectonic models that may explain the cyclic tectonic evolution of the western part of the Svecofennian orogen in central Sweden. The favoured model involves continuous subduction, with a constant polarity, beneath a single active continental margin, combined with alternating subduction hinge retreat and advance. The model involves migration of what has been described as tectonic switching in the younger, accretionary orogenic systems of eastern Australia (Palaeozoic Lachlan orogen) and New Zealand.
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- 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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| 8. |
- Hermansson, Tobias
(författare)
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The tectonic evolution of the western part of the Svecofennian orogen, central Sweden : Insight from U/Pb and 40Ar/39Ar geochronology at Forsmark
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Forsmark area, in the western part of the Svecofennian orogen, central Sweden, is situated between two major Palaeoproterozoic tectonic domains that show contrasting histories with respect to timing of igneous activity, ductile deformation and metamorphism. At Forsmark, WNW to NW trending, ductile deformation belts anastomose around tectonic lenses with an inferred lower degree of ductile strain. Geological features common to both of the adjacent tectonic domains are found in the area, which, consequently, is of key importance for the understanding of the tectonic evolution of the Svecofennian orogen in this region. U/Pb zircon dating (6 ages), in combination with detailed field work, have revealed the existence of two calc-alkaline igneous suites at Forsmark. The older and most voluminous plutonic suite intruded at 1.89-1.87 Ga. It is affected by penetrative ductile deformation under amphibolite-facies metamorphic conditions. The younger, less voluminous and hypabyssal suite intruded at 1.86-1.85 Ga, during the waning stages of penetrative deformation and, thus, constrains the main phase of penetrative ductile amphibolite-facies deformation to between 1.87 and 1.86 Ga. Cross-cutting granite dykes, belonging to the younger suite, place an absolute minimum age for this deformational event to c. 1.85 Ga. U/Pb titanite data (4 ages) support the constraints on the penetrative deformation. However, the data also suggest that the Forsmark area has been affected by one or more tectonothermal events after the intrusion of the 1.85 Ga granite dykes. This is confirmed by 40Ar/39Ar hornblende data (16 ages), which demonstrate the existence of two age generations, 1.83-1.82 Ga and 1.81-1.80 Ga, that are suggested to represent resetting of the argon isotope system in response to retrogressive, lower amphibolite- to upper greenschist-facies deformation restricted to discrete high-strain zones within the broader deformation belts. Furthermore, the data suggest that cooling to c. 500 °C took place at around 1.85 Ga and that the area then remained at similar temperatures until the 1.81-1.80 Ga tectonothermal event, during which it was uplifted to higher crustal levels. In addition, 40Ar/39Ar muscovite (5 ages) and biotite (29 ages) data suggest that cooling to 350 °C occurred around 1.75-1.70 Ga, whereas cooling to 300 °C took place at 1.73-1.66 Ga. The estimated uplift rate was at this time c. 22 m/m.y. The Forsmark data, in combination with a compilation of available geochronological data for the time interval 1.91-1.84 Ga in central Sweden, point to the existence of at least two major tectonic cycles. Each cycle is characterised by igneous activity associated with extension, a short interval of compression (c. 10 m.y.), and migration of the tectonic activity. In this thesis, two contrasting conceptual tectonic models, which may explain the cyclic tectonic evolution of the western Svecofennian orogen in central Sweden, are discussed. The favoured model involves continuous subduction beneath a single active continental margin, combined with alternating subduction hinge retreat and advance. This model includes migration of what has been described as tectonic switching in some younger orogenic belts.
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| 9. |
- Jarmar, Tobias, et al.
(författare)
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Injectable bone cements for vertebroplasty studied in sheep vertebrae with electron microscopy
- 2008
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Ingår i: Key Engineering Materials. - 1662-9795.
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Tidskriftsartikel (refereegranskat)abstract
- Vertebral compression fractures were simulated by making a hole into sheep vertebrae and by injecting a stabilizing material. The injectable bio-ceramic Xeraspine™ was evaluated together with a commercially available PMMA (Vertebroplastic™) as the reference material. The Vertebrae were harvested after 7 days and prepared for microscopy. The samples were deposited with gold on the surface and thereafter subjected to SEM and EDX analysis. It was found that the Xeraspine-bone interface was composed of a mixture of elements. The Vertebroplastic implant was embedded in a carbon containing tissue, likely a soft tissue capsule. The Xeraspine sample was subjected to high resolution analysis in the TEM combined with EDX measurements. The TEM sample was prepared with a novel technique for preparation of the tissue-material interface (FIB). In the TEM analysis it was found that the interface region consists of ZrO2 together with a mixture possibly consisting of katoite and apatite formed during setting and/or originating from the boneapatite.
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