Late-Svecofennian transpressive deformation in SW Finland—evidence from late-stage D 3 structures

This petrographic study of rock samples from the area of a large-scale shear zone, the Sottung-Jurmo shear zone, in SW Finland, illuminates the thermal development of and strain distribution within the rocks during the last development stages of the shear zone. The results indicate relatively high temperatures during the deformation phases that created the gneisses and the mylonites, strain partitioning through time and continued transpression from S-SW until at least c. 1.79 Ga. This has implications to the latest tectonic models since the results together with previous studies suggest a compartmentalisation of regional stresses between the area SW of the shear zone and central Fennoscandia. The results also suggest that the uplift rate increased during the late stage of the transpression.

Section: Geological Settingmentioning

confidence: 99%

Microstructures associated with the Sottunga-Jurmo shear zone and their implications for the 1.83-1.79 Ga tectonic development of SW Finland

Torvela¹,

Ehlers²

2010

“…Previous c. 1830-1815 Ma U-Pb ages were obtained from monazites (Mouri et al, 2005). These monazite ages coincide with the 1824 ± 5 Ma monazite age of metamorphism from Kemiö, 50 km west of Orijärvi (Levin et al, 2005). Ages of crustally derived granites can also be used to deduce the timing of heat flow.…”

Section: Regional Implicationsmentioning

confidence: 99%

U-Th-Pb zircon geochronology on igneous rocks in the Toija and Salittu Formations, Orijärvi area, southwestern Finland: Constraints on the age of volcanism and metamorphism

Väisänen¹,

Kirkland²

2008

Zircons from a felsic volcanic rock in the Toija Formation and a synvolcanic gabbro intrusion in the Salittu Formation within the Orijärvi area were dated by U-Th-Pb SIMS in order to provide depositional constraints on these formations. Zircon crystals from the felsic rock preserve a two-stage crystallisation history with zoned core domains and homogeneous rim domains. Inner domains yield a 1878 ± 4 Ma concordia age, interpreted to determine the crystallisation of this rock. Rims yield a 1815 ± 3 Ma concordia age interpreted to determine the regional metamorphism. Small rounded zircon grains from the Salittu gabbro, located within the Jyly shear zone, yield a concordia age of 1792 ± 5 Ma. We interpret the grain textures to suggest that they recrystallised from inherited zircon seeds during the heat and fluid flow into the shear zone. Although no direct ages for the Salittu Formation have been recovered, field relationships imply that it was deposited between 1878 − 1875 Ma.

“…Generation of the 1.84-1.81 Ga microcline granites is suggested to predate the end of recumbent F 1 folding (Shreurs & Westra, 1986) and the early stage of the recumbent, thrustrelated D 2 of Ehlers et al (1993). Thus, it also predates the peak metamorphic main folding, characterized by E-W trending upright folds (Schreurs & Westra, 1986;Bleeker & Westra, 1987;Ploegsma & Westra, 1990;Levin et al, 2005). In addition, Ploegsma & Westra (1990) recognized the Orijärvi triangle as a non-migmatized low tectonic strain area with preserved early deformation structures.…”

Section: Geological Settingmentioning

confidence: 99%

“…The younger event is characterized by high-temperature, low-pressure (HTLP) conditions (Korsman et al, 1999), P estimates ranging from 3 to 5 kbar and T from 550°C to 825°C (Schreurs & Westra, 1986), thus locally reaching granulite facies conditions. Peak metamorphic conditions of the younger event prevailed at ~1.830-1.815 Ga (Levin et al, 2005;Mouri et al, 2005).…”

Section: Geological Settingmentioning

confidence: 99%

“…described the Orijärvi area as resembling a modern volcanic arc with back-arc basin, and identified the progressive development of geochemical characteristics in the volcanic rocks, starting with bimodal magmatism and evolving to intermediate compositions after 10-15 Ma. Previous structural studies have identified two (Ehlers et al, 1993), three (Shreurs & Westra, 1986;Bleeker & Westra, 1987;Ploegsma & Westra, 1990) or four deformation phases (Verhoef & Dietvorst, 1980;van Staal & Williams, 1983;Kilpeläinen & Rastas, 1990;Levin et al, 2005) in the SSAC. The early deformation phases are suggested to be thrust-related recumbent folds, while the interpreted tectonic transport directions are variable (Ehlers et al, 1993;van Staal & Williams, 1983).…”

Section: Geological Settingmentioning

confidence: 99%

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Supracrustal rocks in the Kuovila area, Southern Finland: Structural evolution, geochemical characteristics and the age of volcanism

Skyttä¹,

Käpyaho²,

Mänttäri³

2005

The supracrustal rocks of the Kuovila area in the Palaeoproterozoic Svecofennian Uusimaa Belt, southern Finland, consist mainly of volcaniclastic rocks associated with banded iron formations (BIFs) and marbles. Small ZnS and PbS mineralizations are occasionally located within the marbles. Some primary features are well preserved in the sedimentary and volcanic rocks, including lamination in tuffites and banded iron formations.Geochemical results show that the volcanism was bimodal and it mainly had volcanic arc affinity. Specific geochemical indicators suggesting a volcanic arc origin for the Kuovila volcanic rocks include: 1) Enrichment of LILE over the HFSE elements and 2) Distinctly low Nb and Ta contents in relation to Th, Ce and LREE. Geochemistry of the Kuovila area volcanic rocks is very similar to those of the Orijärvi and Kisko formations, located ~15 km NE of Kuovila. Felsic tuff in the Kuovila area was dated at 1891±4 Ma by the U-Pb system on zircons. Consequently volcanism was contemporaneous with magmatism in the adjacent Orijärvi area, thus representing the earliest identified volcanic stage in the southern Svecofennian Uusimaa Belt.Early deformation structures within the Kuovila area are suggested to relate to lowmetamorphic or localized low-angle thrusting during D 1 . F 1 folds were recumbent and the S 1 cleavages are generally weak. Thrusting was followed by approximately N-S contraction with upright, peak-metamorphic F 2 folding overprinting D 1 structures and defining the Kuovila synform. Two separate intrusive phases include a synvolcanic granodiorite-dioritegabbro association and a weakly S 2 -foliated syn-D 2 granodiorite. Anatectic granites and associated migmatizing veins are absent, therefore suggesting that D 2 pre-dates the ~1.84-1.82 Ga metamorphic event in the Southern Svecofennian Arc Complex (SSAC). D 2 structures in the Kuovila area are suggested to correlate with the early structures with associated axial planar cleavages in the Orijärvi area. D 2 strain is localized into axial planar high strain zones and the curvilinear patterns of the B 2 fold axes probably result from vertical stretching during D 2 .