Alpine-type tectonics in the Paleoproterozoic Lapland-Kola Orogen

Abstract

The Kola region in the northeastern Baltic Shield is characterized by diverse Paleoproterozoic collision processes. The Keivy Terrane is one of the major tectonic units in the northeastern foreland of the Paleoproterozoic Lapland-Kola Collisional Orogen, which markedly differs in a number of parameters from other tectonic units of the Kola region. The study of the Keivy Terrane allowed us to unravel one more basic difference: the large Paleoproterozoic sheath synform of the Serpovidny (Crescentic) Range localized in this terrane. Its core is occupied by volcanic and sedimentary rocks, which correlate with the fill of the Imandra-Varzuga Rift; the limbs are composed of metamorphosed mature sedimentary rocks known as Keivy paraschists of Neoarchean or Paleoproterozoic age. The lower limb of the Serpovidny Synform is strongly squeezed, whereas the upper limb consists of almost undeformed rocks. The deformed rocks underwent ductile flow under conditions of simple or general shear. In the degree of its asymmetry and main parameters, the Serpovidny Synform is similar to the plunging and recumbent anticlines in the Helvetic nappes of the Alps. It is concluded that the Paleoproterozoic core of the Serpovidny Sheath Synform, or plunging anticline, is a fragment of the almost completely eroded deep Serpovidny Nappe of the Helvetic type. During the collision related to the Lapland-Kola Orogeny (1.9–2.0 Ga), this nappe was pushed out northward from the Paleoproterozoic Imandra-Varzuga Rift, which is situated 50 km south of the Serpovidny structure, and thrust over the Keivy paraschists. The latter, together with underlying the Lebyazhka Gneiss, were folded in the process of thrusting and were involved in the structure of the Serpovidny Synform. The Keivy paraschists make up a para-autochthon or a separate nappe of the Pennine type. The Archean Lebyazhka metafelsic volcanics underlie the Keivy paraschists and overlie granitoids of the Archean basement that remained undeformed during thrusting. Most likely, they also belong to the para-autochthon; however, it cannot be ruled out that, like the Keivy paraschists, they occur as a Pennine-type nappe. The large sheath folds known in the Paleoproterozoic and Phanerozoic orogens are genetically related to deep-seated nappes or channel-flow tectonics. Paleoproterozoic and Phanerozoic orogens are similar in this respect.