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Deformation history of the Palaeoproterozoic Svecofennian supracrustal rocks at Hamrånge, south-central Sweden
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The west-central part of the Palaeoproterzoic Fennoscandian Shield consists of different lithological units: the marine Bothnian basin, granitoids formed in an active continental margin (the Ljusdal Domain, LjD), and a continental back-arc magmatic region (the Bergslagen province). Geochemical signatures indicate that the supracrustal Hamrånge Group (HG) represents an additional unit - a fragment of an oceanic island arc that has been accreted to the continental margin granitoids. Structural data show that the 1.9 Ga supracrustal rocks in the lower part of the stratigraphy of the HG have been affected by three episodes of deformation (D1-D3). On the other hand, the ≤1.86 Ga quartzite in the uppermost part probably has been affected only by the latter two deformational phases. The nature of D1 is poorly understood, but may represent the early accretion of the island arc to the continental margin. A NW-SE compressional phase during D2 resulted in F2-folds and thrusts in the supracrustal rocks, and thrusts are also observed in the footwall granitoids. Microstructures demonstrate that metamorphism outlasted and possibly peaked after D2, and structural data imply a period of flattening subsequent to the D2-thrusting, but prior to D3. During D3 the area was affected by a regional N-S compression causing upright E-plunging F3-folding. This F3-folding matured to steep ductile shear zones, which belong to a system of 1.82-1.80 Ga NW-SE shear zones transecting the central part of the Fennoscandian Shield. The structural evolution recorded in the Hamrånge area, suggesting D2-thrusting towards NW and subsequent N-S (D3) shortening, is in accordance with the accepted tectonic models of the Fennoscandian Shield. The structural configuration of the area is partly comparable, at a smaller scale, to a major tectonic unit in the 0.5 Ga Scandinavian Caledonides, the Lower Seve Nappe (LSN), suggesting similarities in structural evolution of orogenies through time. An important difference is the relative timing of deformation and metamorphism. However, significant differences in tectonostratigraphy suggest that the HG is not far-transported and that the HG and tectonically underlying granitoids of the LjD originally had a close spatial relationship.

National Category
Earth and Related Environmental Sciences
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
URN: urn:nbn:se:uu:diva-103175OAI: oai:DiVA.org:uu-103175DiVA: diva2:310073
Available from: 2010-04-12 Created: 2009-05-15 Last updated: 2010-04-12
In thesis
1. Geological Evolution of the Supracrustal Palaeoproterozoic Hamrånge Group: A Svecofennian Case Study
Open this publication in new window or tab >>Geological Evolution of the Supracrustal Palaeoproterozoic Hamrånge Group: A Svecofennian Case Study
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The work presented in this thesis utilizes several geological methods to investigate the origin and evolution of the supracrustal rocks in the Palaeoproterozoic Hamrånge Group (HG) in the south-central Swedish Svecofennian.

The first paper is based on whole-rock geochemistry to show the plate tectonic setting of volcanic rocks within the HG. This indicates that the environment was probably an oceanic volcanic arc. Geochronology, used in paper two, shows that the volcanism was active at 1888±6 Ma and that the sediments forming the stratigraphically overlying quartzite were deposited after 1855±10 Ma, with provenance ages overlapping both the volcanic rocks and the 1.86-1.84 Ga continental margin Ljusdal granitoids. In the third paper, thermobarometry was applied to samples from the HG, the migmatitic Ockelbo sub-domain to the south, and the 1.81 Ga Hagsta Gneiss Zone (HGZ) that separate these two units. The results show distinct differences in the metamorphic conditions that have affected the HG and the Ockelbo sub-domain, supporting previous interpretations that the HGZ is an important crustal structure, possibly a terrane or domain boundary. Paper four deals with the structural geology of the Hamrånge area. The study shows that the volcanic rocks and the underlying mica schist have been subjected to three deformation episodes (D1-D3), while the uppermost quartzite was most likely only affected by D2 and D3. While structures related to D1 are rarely seen, D2 resulted in a penetrative foliation, strong lineations and NW-vergent folding and thrusting. D3 is a result of a N-S compression that formed regional E-W folds and steep, ca. NW-SE shear zones, e.g. the HGZ.

The results presented in this thesis, integrated with previously published data, outline a model for the geological evolution of the Hamrånge area: At 1.89 Ga a volcanic arc formed that subsequently collided with a continental margin resulting in the first deformation episode, D1, and probably a metamorphic event. This was possibly followed by an extensional period, after 1855±10 Ma, forming a basin that accumulated sediments later to form the quartzite stratigraphically on top of the volcanic rocks. The second deformation episode, D2, formed a fold-thrust belt when the supracrustal HG was thrusted to the NW, on top of the 1.86-1-84 Ga Ljusdal Domain. Flattening and a second metamorphic period followed this thickening of the crust. The last ductile deformation, D3, caused by regional tectonic forces, resulted in F3-folds that matured into ca. 1.8 Ga large-scale, steep shear zones transecting the Fennoscandian Shield.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2010. 53 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 738
Keyword
Hamrånge, Ljusdal, Hagsta Gneiss Zone, geochemistry, geochronology, thermobarometry, metamorphism, fold-thrust belt.
National Category
Earth and Related Environmental Sciences
Research subject
Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
Identifiers
urn:nbn:se:uu:diva-119357 (URN)978-91-554-7795-0 (ISBN)
Public defence
2010-05-12, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2010-04-20 Created: 2010-02-25 Last updated: 2010-04-20Bibliographically approved

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