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
2 Geological Survey of Finland, Regional Office for mid-Finland, P.O. Box 1237 , FIN-70211 Kuopio, Finland

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.

Tight ~1.92 Ga pre-Bothnian folds in the Lappfors group overprinted by open minor folds, crenulations and veins (left to right) of the post-Bothnian ~1.88 Ga episode. Note the glacial striae from top left to bottom right.

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.

1. Deposition of the Lappfors group, with local maximum deposition ages between ~1.97 and ~1.94 Ga as part of an extensive Svionian marginal basin.
2. Early Svecofennian (D1) closure of the Svionan Basin at ~1.92 Ga involving possible subduction zones.
3. An extensional episode during which the Svionian complex was eroded and the lower Bothnian Evijärvi group with its submarine mafic volcanism was deposited at ~1.91-1.90 Ga in a successor marginal basin.
4. Transition from marine to terrestrial deposition at ~1.90-1.89 Ga as bimodal calc-alkaline volcanism developed, the upper Bothnian sequence was deposited, and early plutonism occurred.
5. Continued plutonism and Middle Svecofennian (D2) deformation at ~1.88 Ga, during which D2 folds, faults and shear zones were superimposed on the earlier W-trending penetrative D1 structures in the Lappfors group (Fig. 1). These D2 structures are largely responsible for the present relationship between the Svionian basement and the Bothnian cover.

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.
Kukkonen, I.T., Heikkinen, P., Ekdahl, E., Hjelt, S,-E., Yliniemi, J., Jalkanen, E., and FIRE Working Group, 2006. Acquisition and geophysical characteristics of reflection seismic data on FIRE transects, Fennoscandian Shield. Geological Survey of Finland Special Paper 43;13-43
Skiold, T., Rutland, R.W.R., 2008. "Successive ~1.94 Ga plutonism and ~1.92 Ga deformation and metamorphism south of the Skellefte district": a Reply to H&L. Precambrian Research (in press).
Sorjonen-Ward, P., 2006. Geological and structural framework and preliminary interpretation of the FIRE 3 and FIRE 3A reflection seismic profiles, central Finland. Geological Survey of Geological Survey of Finland Special Paper 43;105-159.
Williams, I.S., Rutland, R.W.R., Kousa, J., 2008. A regional 1.92 Ga tectonothermal episode in Ostrobothnia, Finland: Implications for models of Svecofennian accretion. Precambrian Research 165;15-36