The Kynsikangas shear zone, Southern Finland: paleotectonic significance, scales of shearing and slow fault-slip in a continental shear zone

Understanding the heterogeneity of deformation in exhumed lower-crustal terranes contributes to elucidating mechanisms of lower-crustal deformation and paleotectonic processes. Here, we focus on the Kynsikangas shear zone (KSZ), a prominent structural discontinuity that is part of a network of crustal shear zones within the southern Svecofennian orogen. The shear zone overprints metagranitic rocks, gneisses and migmatites. A detailed field-based structural examination of the shear zone considered the patterns, intensity, kinematics and rheology of deformation. A pronounced scale-dependent heterogeneity in the geometry and intensity of mineral shape fabrics as well as in the deformation kinematics are structural hallmarks of the KSZ. The analysis of deformation kinematics and mineral shape fabrics revealed that the KSZ formed by strike-slip dominated, left-lateral transpression under NW–SE shortening. Mesoscopic shear bands and S-C fabrics delimit phacoids, which are reminiscent of scaly fabrics, known from active crustal shear zones formed at low metamorphic grade. We, therefore, interpret these structures as high-temperature equivalents of scaly fabrics, which may constitute slip transients in a frictional-viscous material. Our analysis also confirms that small-scale kinematic indicators may not portray the regional sense-of-shear and, thus, questions their use for identifying shear zone kinematics in general. Due to its distinct kinematics and orientation, the KSZ most likely served as a northwest-trending tectonic transfer zone among an anastomosing pattern of mostly easterly striking regional shear zones during later stages of the Svecofennian orogeny.