New data on tectonics of Onega-Kandalaksha paleorift (The White Sea)

Abstract

The Onega-Kandalaksha paleorift is a part of the White Sea rift system along the northeastern edge of the East-European platform and formed under conditions of horizontal extension of the edge of continental plate in the Middle‐Late Riphean [1]. The structure of the paleorift is divided into three main segments: the Kandalaksha graben in the water area of the White Sea, the Central and Onega troughs [2], and several smaller troughs (Fig. 1). The paleorift segments are semigrabens of alternative polarity separated by dykes representing crests of crystal foundation. On the northwestern edge of the Kandalaksha trough, there is Kolvitsky [3], another relatively small segment of the rift structure of about 80 km long. It is also separated from the major graben by the slanting intertrough dyke, which is a prominent underwater sill of the crystalline basement protruded as islands of the Sredniye Ludy archipelago above the water surface in some places. This segment is a shallow northwestern part (up to 70 m) of Kandalaksha Bay with many small islands composed of foundation rocks. The Onega-Kandalaksha paleorift was active in the Paleozoic, when alkaline magmatism was especially developed and at the end of the Cainozoe when the modern basin of the White Sea was formed. The formation of the White Sea basin was predetermined structurally and tectonically, since the trough of modern Kandalaksha Bay of the White Sea inherits and renews the Riphean graben, which is shown by active lowering of the Kandalaksha graben followed by renewal of most of the faults and the appearance of numerous seismic foci along them. Despite that the White Sea is the only fully interior sea in Russia; its geology has been studied weakly until recent complex geophysical research in the White Sea basin [4, 5]. The results of those and other recent works conducted in that area change the existing opinions about the earth crust structure of the region in question. If previously the depth of sinking of the crystalline basement in the Kandalaksha graben composed of Riphean terrigenous formations was supposed to reach 3‐3.5 km, according to the latest seismic research of the RF CDP in the water area of the White Sea, this depth is up to 8 km. Thus, the thickness of Riphean deposits is increased actually by a factor of 2‐2.5; however, the area of their spreading was not exactly established. For example, the contemporary geological maps [6] show Riphean deposits in Kandalaksha Bay on the islands of the Sredniye Ludy archipelago and towards the northwest of them in the young (Late Cainozoe) Kolvitskii graben, which contradicts the reality.