Migratory tectonic switching, western Svecofennian orogen, central Sweden: Constraints from U/Pb zircon and titanite geochronology

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. MS 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. rsmark 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.