Late Svecofennian shear zones in southwestern Finland

Abstract

The bedrock of southwestern Finland is transected by a shear zone network with two dominant directions: ∼E-W and ∼N-S. The shear activity occurred after peak metamorphism and crustal melting at 1840-1810 Ma. The shear zones began to form as a consequence of north vergent oblique continent-continent collision and accommodated the resulting dextral transpression. The 150-200 km long ∼E-W oriented Somero and South Finland Shear Zones acted as dextral strike-slip faults, while the crustal unit between them was transported westward and deformed along the ∼N-S oriented reverse faults. The main deformation within these contractional shear zones took place at 1.81-1.79 Ga. After that, extensional faulting took place, tentatively at 1.79-1.77 Ga, 1.64-1.55 Ga and 1.26 Ga.

Keywords:

• contraction
• extension
• ductile shearing
• pseudotachylite
• shear sense
• strike-slip
• transpression
• Palaeoproterozoic
• Fennoscandian Shield