The Kynsikangas shear zone, Southern Finland: paleotectonic role, kinematic partitioning and structural evidence for slow fault slip in the lower continental crust

Detailed field-based structural examination of the Kynsikangas shear zone (KSZ), a prominent structural discontinuity of the Svecofennian Province, allows us to better understand the rheological and kinematic behaviour of localized shearing in the lower continental crust. The analysis of deformation kinematics and mineral shape fabrics on multiple scales revealed that the KSZ formed by pure shear-dominated, left-lateral transpression under NW-SE shortening. A pronounced scale-dependent heterogeneity in the geometry and intensity of mineral shape fabrics as well as in the deformation kinematics are structural hall marks of the KSZ. An important result of our analysis is 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. Furthermore, the ubiquitous presence of mesoscopic shear bands and S-C fabrics, both interpreted as precursor structures of scaly fabrics, known from active crustal shear zones, may constitute slip transients in a frictional-viscous material. Thus, the KSZ features structural evidence for slow-slip seismic events. In terms of paleotectonic significance, the KSZ is likely structurally linked and kinematically compatible with a major shear zone, the Kolinummi reverse shear zone, to the South of the KSZ. The KSZ can, therefore, be considered part of a network of crustal shear zones pervading lithotectonic terranes that amalgamated during the Svecofennian orogeny. Due to its distinct kinematics and orientation, the KSZ most likely served as a tectonic transfer zone among mostly easterly striking oblique reverse shear zones during later stages of the Svecofennian orogeny.