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

Abstract

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.

How does orogenic crust deform? Evidence of crustal-scale competent behaviour within the partially molten middle crust during orogenic compression

“…Based on these numbers we can estimate the age of the peak metamorphism (c. 19 km palaeodepth) as c. 1825 Ma, and subsequent exhumation to c. 16 km by c. 1815 Ma, giving an average exhumation rate of 0.03±0.02 cm/year. Similar exhumation rates were implied by Torvela and Ehlers (2010b) in the SW archipelago (SE of the SJSZ; Fig. 2b).…”

“…4; e.g. Patchett & Kouvo, 1986;Bleeker and Westra, 1987;Ehlers et al, 1993;Lahtinen & Huhma, 1997;Väisänen & Hölttä, 1999;Hermansson et al, 2007;Högdahl et al, 2008;Torvela et al, 2008Torvela et al, , 2010aTorvela et al, , 2010bSkyttä and Mänttäri, 2008). This (second) convergence, called 'Fennian orogen' by Lahtinen et al (2005), is the focus of this paper.…”

“…The metamorphic peak conditions in southern Finland have been estimated at c. 750-825°C and c. 4-7 kbar (e.g. Schreurs & Westra, 1986;Väisänen & Hölttä, 1999;Johannes et al, 2003;Mouri et al, 2005;Torvela and Annersten, 2005;Torvela et al, 2010b). Near Sammatti and Turku (Fig.…”

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“…In southern Finland, the metamorphic peak conditions are c. 750−825°C and c. 4−7 kbar (e.g. Schreurs & Westra 1986;Väisänen & Hölttä 1999;Johannes et al 2003;Mouri et al 2005;Torvela & Annersten 2005;Torvela & Ehlers 2010a). This granulitefacies peak metamorphism is mostly undated but some geochronological data exist from migmatite leucosomes near Sammatti and Turku at c. 1825− 1815 Ga Mouri et al 2005).…”

“…Particularly from c. 1825 Ma onwards, the deformation was synchronous with wide-spread low-P, high-T anatectic granite magmatism and migmatisation, metamorphism and formation of steep, local-to crustal-scale, strike-slip shear zones in the Svecofennian domain (e.g. Patchett & Kouvo 1986;Bleeker & Westra 1987;Ehlers et al 1993;Lahtinen & Huhma 1997;Väisänen & Hölttä 1999;Hermansson et al 2007;Högdahl et al 2008;Torvela et al 2008;Torvela & Ehlers 2010a, 2010bSkyttä & Mänttäri 2008). This convergence phase, the 'Fennian orogen' (Lahtinen et al 2005), is the focus of this paper.…”