| Summary: | Does the Mazury dextral shear zone exist?S u m m a r y . Throughout SW Fennoscandia, numerous approximatelyW-E trending faults and ductile shear zones have recently been recognized. The regional-scale zone of strike-slip displacements (Mazury lineament) with a dextral component was referred by many authors as apparently influenced by the distribution of ca. 1.6–1.4 Ga old “anorogenic” intrusions of the Mazury Complex (NE Poland). However, new detailed structural and kinematics studies of drill-cores from 255 boreholes located throughout NE Poland and SW Lithuania (Lazdijai region) indicate that the Mazury Complex, the Suwałki Anorthosite Massif and surrounding lithotectonic domains share a common ductile thrusting deformation, calling into question the concept of a large-scale dextral shear zone and also the “anorogenic” origin hypothesis of the Mazury Complex. One important advance has been the accumulation of structural and kinematics information that allows assessment of the main orogenic transport direction commonly towards the SW, locally to the NE (e.g., in the eastern part of the Suwałki Anorthosite Massif). This assessment has been coupled with the recognition of the role of major and numerous ductile shear zones within the western part of the East European Platform. In general, compressional deformation with ductile thrusting predominated in the western part of the East European Platform. Very probable strike of regional mylonitic foliation (S1) from almost 100 boreholes has been established, based on geophysical measurements of the curvature of boreholeaxis and a few tens oriented core samples (SW Lithuania). It is assumed that the crystalline rocks of NE Poland and SW Lithuania (Lazdijai region) were subjected to similar ductile thrusting to transpressional deformation during the late Gothian (or newly defined Dano-Polonian) orogeny (ca. 1.5 Ga).
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