TECTONOMAGMATIC CONTROL ON VERTICAL DIP-SLIP BASEMENT FAULTING - AN EXAMPLE FROM THE FENNOSCANDIAN SHIELD

Vertical dip-slip basement faults play an important role in the evolut ion and structuring of the Earth's crust. The Proterozoic anorogenic r apakivi-anorthosite setting of the Fennoscandian Shield in southern Fi nland exhibits a widespread pattern of vertical dip-slip basement faul ts 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 c losely follows the outcrop pattern of Mid-Late Proterozoic anorogenic crustal elements, such as basic dyke swarms, the outline of rapakivi g ranites 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. Successi ve stages of brittle deformation visible in as well the PM-fault and t he Obbnas granite demonstrate that brittle deformation in the PM-fault is coeval with the intrusion of the anorogenic Obbnas rapakivi granit e. Based on the spatial and temporal relationship of anorogenic magmat ism and block faulting, a genetic relationship is proposed.