MAJOR PALAEOPROTEROZOIC SHEAR ZONES OF THE CENTRAL FENNOSCANDIAN SHIELD

Employing two recently studied crustal-scale shear zones as type examp les, 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 Svecokarelia n Orogeny as commonly assumed. The collision of the Svecofennian islan d are with the Karelian Continent first created numerous NW-SE trendin g folds and thrusts of stages D-1 and D-2, which were then modified by successive shearing during stages D-3 and D-4 Stage D-3 built up a sy stem of N-S trending shear zones, here named the Savolappi Shear Syste m, 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 s uch as hook folds, Z-folds and sheath folds associated with the princi pal displacement zone, synthetic Riedel shears, and pinnate shears. Th e traces of the axial planes of F-3 en-echelon folds deviate 15 degree s-30 degrees anticlockwise from the plane of the principal displacemen t zone. Other members of the Savolappi Shear System are the Pajala She ar Zone in northern Sweden and the Russian North Karelia Shear Zone in the east. Stage D-4 created a conjugate shear system called the Finla ndia Shear System, the type example of which is the Oulujarvi Shear Zo ne. This is a NE-SW trending sinistral strike-slip shear zone more tha n 250 km long and 20-30 km wide across its southwestern end. It is com posed of a NE-SW trending principal displacement zone, synthetic Riede l 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 F-4 folds of all scales diverge by 20 degree s-40 degrees clockwise from the plane of the principal displacement zo ne. The Kuopio Shear Zone is a conjugate NW-SE trending counterpart of the Oulujarvi Shear Zone. As a whole, the Finlandia Shear System form s a conjugate network of NW-SE and NE-SW trending shear zones which oc cupies most of the northern and central Fennoscandian Shield.