Research School of Earth Sciences
Evolution of the Svecofennian orogenic province
R.W. Roye Rutland1, Ian. S. Williams1 and Jukka Kousa2
1 Research School of Earth Sciences, Australian National University,
Canberra, ACT 0200, Australia
In several areas of the Svecofennian Province, a major discontinuity has been recognized between Svionian complexes, strongly deformed and metamorphosed at ~1.92 Ga, and overlying post-1.92 Ga Bothnian volcano-sedimentary sequences (Annual reports, 2005, 2006, Skiöld and Rutland, 2008). In Ostrobothnia, Finland (Williams et al., 2008), a similar discontinuity has now been identified within the mostly metasedimentary Western Pohjanmaa Belt and separates two distinct stratigraphic groups. The western Lappfors group, interpreted as a Svionian basement complex, has strong W-trending folding and aeromagnetic signatures that contrast with the unconformably overlying eastern Evijärvi group, interpreted as lower Bothnian, which has more open N-trending folding and magnetic patterns. Several lines of evidence date the unconformity at ~1.92 Ga. Detrital zircons from two samples of Lappfors group metasediment, and a sample of the basal Nivala gneisses in the Eastern Pohjanmaa Belt, have 1.92-1.91 Ga post-depositional low-Th/U metamorphic overgrowths.
The maximum deposition age of the Lappfors sedimentary protoliths, based on detrital zircon ages, is between ~1.97 and ~1.94 Ga. Three samples of Bothnian sediments lack pervasive ~1.91 Ga overgrowths, instead having a variety of detrital zircons as young as ~1.95 - 1.91 Ga, reflecting recycling of the underlying basement complex. The maximum deposition age of the lower Bothnian sedimentary protoliths is inferred to be ~1.91 Ga. The Niska granite, which intrudes the Evijärvi group and is deformed only by the younger tectonic episode affecting that sequence, has a zircon age of 1896 ± 6 Ma. That episode, which established the present relationships between basement and cover, is dated by ~1.88 Ga metamorphic zircon overgrowths in both the Svionian and Bothnian samples, and by 1878 ± 4 Ma metamorphic monazite from a metasediment from the Savo Belt, east of the Nivala district.
Part of the FIRE 3A reflection seismic profile (Kukkonen et al., 2006; Sorjonen-Ward, 2006) ran NW from the western part of the Central Finland Granitoid Complex (CFGC) across the boundary with the Evijärvi group and into the Lappfors group. Our preliminary interpretation of this section (Kousa et al., 2008) suggests that the Svionian Lappfors group is the surface expression of a middle crustal unit that displays a widespread system of E- to SE-dipping reflectors with listric-type geometry, identified by Kukkonen et al. (2006, pp.29-30). This crustal unit may therefore correspond to the accreted Svionian marginal basin, buried to the east beneath the younger, Bothnian, volcano-plutonic complex. As a corollary, the widespread vertical change in reflectivity between upper and middle crust, (op. cit. p.21), and which is present beneath the western CFGC, may be closely related to the change from the overlying Bothnian complexes to underlying Svionian metamorphic complexes.
We now interpret the observed stratigraphic and structural relationships in Ostrobothnia in terms of an extended orogenic evolution, viz.
Kousa, J., Rutland, R.W.R., Sorjonen-Ward, P., Williams, I.S., 2008. Geological
significance of the regional
change in reflectivity between the upper and middle crust in the Svecofennian Province.
SEISMIX 2008, Saariselka, Finland. Abstracts.