The area under study is located in the southeastern periphery of the Fennoscandian crystalline shield. At present this is a tectonically quiet region without large seismic events. But it is well known that in post-glacial time the Fennoscandian shield was an arena of active postglacial tectonics and large earthquakes. The evidence for such events was found in various parts of Fennoscandia. The traces left by some paleoearthquakes show an undisputed character of large post-glacial faults some tens of kilometres long and of a few meters in displacement. However, some other features left by earthquakes are under discussion. Numerous deformations in bedrock and in soft sediments which can be considered as being due to earthquakes were found in the Russian Karelia. Interpretation of some of these deformation structures can lead to different conclusions about their origin, for example, weathering, cryogenic, glacial, and gravitational factors. One possible way to overcome these difficulties is an integrated study of different types of deformations at key sites, comparison of these with each other and with the tectonic features of the region, and the search for common structural and kinematic features. Another problem is the estimation of parameters of paleoearthquakes. This problem includes the determinations of their location, intensities, magnitudes, and age.

The territory of investigations is located in the SE periphery of the Fennoscandian Shield. It served as an arena of periodic significant restructuring of the hydrographic network associated with the filling and discharge of large late-glacial and Holocene basins during the degradation of the Scandinavian ice sheet and in postglacial time. One such restructuring is a sudden change of the Saimaa Lake direction of flow in the middle Holocene from the west to south to the Lake Ladoga basin via the drainage hollow, inherited by modern Vuoksi River valley. Origin of the Vuoksi River is associated with the catastrophic water breakthrough of the Saimaa Lake across the marginal ridge Salpausselkä I of about 5.7 cal. kyr BP. This event usually connects with water accumulation and overflow due to non-uniform post-glacial uplift according to modern concepts. The authors propose a great earthquake as the immediate cause of the break waters of Saimaa Lake. This suggestion is based on the study of specific deformations of the rocky riverbed in the area of breakthrough and of the loose deposits in the banks of the Vuoksi River valley downstream. Open cracks and horizontally displaced rock blocks were discovered in the area of the former rapids near town Imatra. Their systematic displacements on the both sides of the rocky gorge indicate the shear kinematics of fault zone. Different types of deformations had occurred in loose sediments of the low terraces (3-4 m) in the Vuoksi River valley and 20-30 km below the headwaters. In three studied stratigraphic sections the three cardinal different types of deformations 276

В пределах Онежской структуры Фенноскандинавского (Балтийского) щита Восточно-Европейской платформы обнаружены специфические морфоструктуры, отражающие импульсные сейсмотектонические события. Анализ литературных данных показал, что подобные морфоструктуры выделялись российскими учеными в этих местах уже с XIX века. Изучение их эволюции, кинематики и соотношение морфоструктур с палеопротерозойской геологической структурой региона выяви-ли ряд признаков аккомодации новейших тектонических движений к древней струк-туре. В частности, установлена приуроченность к ней поздне-и послеледниковых разрывов. Обосновывается отнесение большинства из выявленных морфоструктур к дислокациям, сформированным за счет мощных сейсмических воздействий на ряде участков. Подтверждается длительность эволюции палеопротерозойских ин-траконтинентальных морфоструктур и расширяется возможность уточнения оце-нок долговременной сейсмической опасности в области тектонической аккомода-ции Фенноскандинавского щита в пределах флексуры А. А. Полканова. К л ю ч е в ы е с л о в а: Восточно-Европейская платформа; Фенноскандинавский щит; Онежская структура; палеоземлетрясения; тектоническая аккомодация; не-отектоника; палеосейсмодислокации. A. A. Nikonov, A. V. Poleshchuk, D. S. Zykov. ON RECENT FAULTS AND PALEOSEISMIC FRACTURES IN THE PALAEOPROTEROZOIC ONEGA STRUCTURE OF THE FENNOSCANDIAN SHIELDSpecific morphological structures corresponding to impulsive seismotectonic events were identified within the Onega structure of the Fennoscandian (Baltic) shield of the East European platform. Analysis of data from the literature showed that similar morphological structures have been detected by Russian scientists in this area already since the 19 th century. The study of evolution, kinematics, and the correlation of the morphostructures with the region's Palaeoproterozoic geological structure suggest that recent tectonic movements have been accommodated to the ancient structure. In particular, late-and postglacial faults were found to be associated to this structure. We prove that a majority of the identified morphostructures are dislocations caused by strong seismic impacts in a number of areas. The prolonged evolution of the Palaeoproterozoic intracontinental morphostructures is confirmed, and the possibilities for refining the estimates of longterm seismic hazard in the area of tectonic accommodation of the Fennoscandian shield in the central sector of the Polkanov flexure are expanded.

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