The Tampere Schist Belt: structural style within an early Proterozoic volcanic arc system in southern Finland

Abstract

The central part of the narrow, E-W trending Tampere Schist Belt comprises volcanic and sedimentary rocks of low metamorphic grade, surrounded by intrusive rocks and migmatites. The first folding event, with subvertical axial surfaces and originally subhorizontal fold axes, dominates the structure of the belt. Well-preserved primary structures indicate a major E-W trending syncline in the northern part of the belt with minor upright, open to tight synclines and anticlines in the limb areas. F 1 fold axes in the limbs have been rotated towards the steeply to vertically plunging stretching lineation. Dextral F 2 folds with subvertical, E-W to NE-SW striking S 2 crenulation cleavage and steeply plunging fold axes overprint penetrative S 1 schistosity. Subhorizontal dextral shear is inferred from the D 2 structures. D 3 kinking and fracturing is restricted to narrow NW-SE to N-S trending zones. The shear zone between the schists and the migmatites to the south is a south dipping thrust or reverse fault with uplift of the higher grade rocks. In the northern part of the Tampere Schist Belt another major fault is postulated. Both shear zones were formed at a late stage of progressive D 1 deformation. On the basis of these structures, and published sedimentological, geochemical and isotopic data, the Tampere Schist Belt is interpreted as an upper part of a thrust sheet within a forearc of an ancient volcanic arc system. The vergence of structures suggests subduction toward the present south.