Deformation history of the Palaeoproterozoic Svecofennian supracrustal rocks at Hamrånge, south-central Sweden
(English)Manuscript (preprint) (Other academic)
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.
Earth and Related Environmental Sciences
Research subject Earth Science with specialization in Mineral Chemistry, Petrology and Tectonics
IdentifiersURN: urn:nbn:se:uu:diva-103175OAI: oai:DiVA.org:uu-103175DiVA: diva2:310073