Abstract
ABSTRACTVertical dip‐slip basement faults play an important role in the evolution and structuring of the Earth's crust. The Proterozoic anorogenic rapakivi‐anorthosite setting of the Fennoscandian Shield in southern Finland exhibits a widespread pattern of vertical dip‐slip basement faults that are deeply eroded. The Porkkala‐Mantsala (PM)‐fault, located c. 30 km W of Helsinki is part of a system of crustal lineaments that closely follows the outcrop pattern of Mid‐Late Proterozoic anorogenic crustal elements, such as basic dyke swarms, the outline of rapakivi granites and remnants of sediment‐filled grabens. These lineaments are formed by low‐grade dip‐slip faults that overprint Svecofennian shear zones. Structural analysis of the PM‐fault supports an interpretation in terms of reactivation of a high‐grade ductile wrench zone. Successive stages of brittle deformation visible in as well the PM‐fault and the Obbnäs granite demonstrate that brittle deformation in the PM‐fault is coeval with the intrusion of the anorogenic Obbnäs rapakivi granite. Based on the spatial and temporal relationship of anorogenic magmatism and block faulting, a genetic relationship is proposed.
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