ИСТИНА

Actual landscapes of the Kola Peninsula Mountains are regarded as products of the expansive Late Pleistocene glaciations followed by an intensive erosion those caused deep troughs and valleys networks. At present, specific slushflow process appears to be one of the essential agents of valley sediment transport alongside the normal fluvial erosion. Despite widespread hazardous manifestations, only the latter is considered when reconstructing the Holocene evolution of initial glacial valley topography. However, detailed study of a series of mountain river basins and adjoining piedmonts revealed a significant role of debris flows and slushflows in their transformation. To reliably distinguish landforms and deposits of different origin we applied a combination of remote sensing data interpretation, small-scale mapping, and field investigation of geomorphic patterns and textural-structural properties of stratigraphic sequences. Grain size analysis, 232Th fingerprinting, and 14C dating of buried organic-rich lenses allowed revealing the age, succession and magnitude of paleogeographical events and factors of valley lithodynamics changes since the Late Pleistocene-Holocene transition. Western and southern parts of the mountains substantially penetrated by the continental ice are filled with corresponding deposits up to 700 m a.s.l. those subsequently became the main source of debris flows. Large (10-20 m high, 4 km2) coarse debris fans are found downstream the deep cuts in glaciolacustrine and glaciofluvial bodies. In contrast, middle reaches of some smaller valleys are completely devoid of any sediment removed by the recent extreme slushflows. In some valleys, initially lacking continental drifts, the position of ice front accumulations at piedmonts controlled the local base levels, distance and lateral distribution of debris flows. Discrete bodies in their middle reaches prove to be ancient tributary debris flow fans. Thus, during the late deglaciation and the Early Holocene, debris flows played a leading role in sediment transport leaving large geomorphic footprints inconsistent with modern runoff conditions. Asynchronous melting of continental and mountain glaciers caused moraine-dammed lakes those breaches led to the largest of debris flow events. Later, distinct decrease of their transportation capacity and frequency occurred due to reduction of both water and sediment sources. They were broadly succumbed by slushflows with much lower clastic content (<12%), which explains smaller (<1-2 m high, 0.1-0.2 km2) actual accumulations. However, higher erosional potential of those phenomena causes direct incising of glacial and debris flow landforms. Integrated 40 14C dates for peats or primitive soils in fans and taluses, attributed to landscape stability periods, indicate several stages of increased slushflow and slope activity over the second part of Holocene. Even low-magnitude but high frequency slushflow episodes almost paralyze normal fluvial process. Disregarding such intensive landscape transformation agents through the Holocene causes difficulties in regional correlations of the postglacial mountain basins development. This work was supported by RFBR project №17-05-00630