Major Palaeoproterozoic shear zones of the central Fennoscandian Shield

Abstract

Abstract Employing two recently studied crustal-scale shear zones as type examples, this paper summarizes the major Palaeoproterozoic (Svecokarelian) shear tectonics of the central Fennoscandian Shield and demonstrates that this part of the Shield was not as stable during the Svecokarelian Orogeny as commonly assumed. The collision of the Svecofennian island arc with the Karelian Continent first created numerous NW-SE trending folds and thrusts of stages D1 and D2, which were then modified by successive shearing during stages D3 and D4. Stage D3 built up a system of N-S trending shear zones, here named the SavolappiShearSystem, the type example of which is the Hirvaskoski Shear Zone. This is a dextral strike-slip shear zone at least 150 km long and 10–30 km wide, characterized by blastomylonitic fault rocks and various structures such as hook folds, Z-folds and sheath folds associated with the principal displacement zone, synthetic Riedel shears, and pinnate shears. The traces of the axial planes of F3 en-echelon folds deviate 15°–30° anticlockwise from the plane of the principal displacement zone. Other members of the Savolappi Shear System are the Pajala Shear Zone in northern Sweden and the Russian North Karelia Shear Zone in the east. Stage D4 created a conjugate shear system called the Finlandia Shear System, the type example of which is the Oulujarvi Shear Zone. This is a NE-SW trending sinistral strike-slip shear zone more than 250 km long and 20–30 km wide across its southwestern end. It is composed of a NE-SW trending principal displacement zone, synthetic Riedel shears, and pinnate shears with antithetic Riedel shears in a NW-SE direction. Typical fault rocks within these shears are S-C mylonites. The axial-plane traces of F4 folds of all scales diverge by 20°–40° clockwise from the plane of the principal displacement zone. The Kuopio Shear Zone is a conjugate NW-SE trending counterpart of the Oulujarvi Shear Zone. As a whole, the Finlandia Shear System forms a conjugate network of NW-SE and NE-SW trending shear zones which occupies most of the northern and central Fennoscandian Shield.