The role of shearing in the Proterozoic development of the Åland archipelago, S.W. Finland

BRANIGAN, N. P., 1987: The role of shearing in the Proterozoic development of the Åland archipelago, S.W. Finland. Bull. Geol. Soc. Finland 59, Part 2, 117—128. tion of a regional tectonochronology. Folds with E—W axial traces and associated shear zones which are characteristic of the southern Finland migmatite belt (1800 Ma) were refolded during a late phase of NW—SE shearing. The resulting folds have a N—S or NW—SE axial trace and plunge parallel to lineations developed within the shear zones, i.e. 40—60 to the south or south east. Shearing ceased for a period in which early-postorogenic ring complexes (c. 1815 Ma) were emplaced at high crustal levels. A subsequent succession of conjugate shears, with N—S or NW—SE sinistral and NE—SW dextral displacement, tightened large fold profiles and affected the early-postorogenic intrusions, with the development of mylonites and ultramylonites. A change from a compressional to a tensile stress regime initiated normal faulting parallel to the NW—SE shears permitting the emplacement of the voluminous rapakivi massifs at or near the surface (c. 1500 Ma). The graben which developed were infilled by Jotnian (1300—1200 Ma) sediments. The major NW— SE shear zone, which may extend across Sweden and into Estonia, overprints the southern Finland migmatite belt and has controlled the tectonic development and magmatic activity of the Åland area for some 600 million years.