From ductile to brittle deformation: structural development and strain distribution along a crustal-scale shear zone in SW Finland

Abstract

This study demonstrates the impact of variations in overall crustal rheology on crustal strength in relatively high P-T conditions at mid-to lower mid-crustal levels. In a crustal-scale shear zone, along-strike variations in the rheological competence result in large-scale deformation partitioning and differences in the deformation style and strain distribution. The structural behaviour of the crustal-scale Sottunga-Jurmo shear zone (SJSZ) in SW Finland is described. The shear zone represents a discontinuity between the amphibolite-to-granulite facies, dome-and-basin style crustal block to the north and the amphibolite facies rocks with dominantly steeply dipping structures to the south. The overall deformation style and resulting strains along the shear zone are greatly affected by the local lithology. The results of this study also have implications for the current tectonic models of the Palaeoproterozoic Fennoscandia. The most important implication is that the SJSZ, together with other structurally related shear zones, compartmentalised the far-field stresses, so that the late ductile structures within and south of the SJSZ can be allocated to a convergence from the south as late as *1.79 Ga rather than to the Nordic orogeny from the west-northwest. It is further suggested that at *1.79 Ga the stress regime was still compressive/ transpressive and that the *1.79 Ga magmatism in Å land at least initiated in a compressive setting. No extension or orogenic collapse, therefore, occurred in the Å land area while the rocks still were within the ductile regime.

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

“…Also Korja & Heikkinen (2005) suggest that a terrane boundary exists from central Sweden to Estonia. Recent studies by Hög-dahl et al (2009) and Torvela & Ehlers (2010) support the interpretation of a terrane boundary in this area. Torvela & Ehlers (2010) further suggest that the SJSZ, as a part of this terrane boundary, compartmentalised the orogenic stresses in south-central Fennoscandia around 1.79 Ga.…”

“…2); the total amount of offset is probably larger but is very difficult to quantify due to the high strains and the multi-stage development history of the shear zone. The kinematic interpretation is based on the deflection of the large-scale fold axial traces and foliations south of the shear zone (Ehlers & Lindroos, 1990), on field observations (Torvela, 2007;Torvela & Ehlers, 2010) and on microstructural observations (this paper).…”

“…The main structural development of the SJSZ is relatively well established (Ehlers & Lindroos, 1990;Torvela, 2007;Torvela & Ehlers, 2010). The SJSZ shows dextral overall kinematics with a SWside up vertical component and an apparent minimum offset in the order of some tens of kilometres.…”

“…The shear bands show a significant simple shear component and are some decimetres to 3-4 meters in thickness. They developed dominantly along the limbs of the open folds common on Hellsö, and/or at the contacts between two different rock types, although exceptions do occur (Torvela, 2007;Torvela & Ehlers, 2010). Kinematic indicators with monoclinic symmetry in gneisses, largely absent in W and S Kökar, become more abundant within the high-strain zones of Hellsö.…”

“…Gaál & Gorbatschev, 1987;Sjöström & Bergman, 1998;Högdahl et al, 2009). Another such a postulated boundary is a several kilometres wide shear structure, the Sottunga-Jurmo shear zone (SJSZ) that extends in a NW-SE direction through the Åland and Turku archipelagos (Torvela & Ehlers, 2010;Figs. 1, 2).…”