Tectonics of detached middle crust in the north-eastern foreland of the palaeoproterozoic lapland-kola Collisional Orogen, North-Eastern Baltic shield

The Serpovidny Ridge Fold is located in the north-western Keivy Terrane situated in the north-eastern foreland of the Palaeoproterozoic Lapland-Kola Collisional Orogen, north-eastern Baltic Shield. Its isoclinal core is composed of Palaeoproterozoic rift-related rocks and displays sheath-like morphology (8 × 2 km in size on the surface). Sedimentary structures that indicate the polarity of bedding suggest that this fold is a synformal anticline, with its axial surface dipping northwards. The lower (southern) limb of the fold is strongly thinned (the X/Z ratio up to 25) whereas the upper (northern) limb is almost undeformed. Kinematic indicators suggest that this super-large sheath fold resulted from north-directed movements under amphibolite-facies conditions at the boundary between the middle and lower crust. The core of the Serpovidny Ridge Sheath Fold composed of Palaeoproterozoic rift-related rocks is interpreted as an outlier of a Helvetic-type nappe that came from the Imandra–Varzuga Rift-Belt located ca. 50 km south of the study area. This tectonism is classified as tectonics of detached middle crust in foreland of the Lapland–Kola Collisional Orogen. It seems to be a counterpart of thin-skinned tectonics in foreland of Phanerozoic collisional orogens which operates in the upper crust.