ИСТИНА

Available results for 12 studied mountain valleys at the Khibinskiye and Lovozerskiye Tundras, Kola Peninsula, suggest that slushflows and, possibly for some valleys, typical debris flows with lower frequency are a leading mechanism of downstream sediment delivery and valley floor topography formation. Fluvial topography in slushflow-affected basins is extremely suppressed or nonexistent under such conditions, as stream channels are unable to rework slushflow deposits and forced to passively adjust. Recovery phase of normal fluvial topography in certain valleys can serve as an indicator of magnitude and time of the last extreme debris flow events. Combination of grain size analysis, radionuclide fingerprinting of 232Th content in the finer-grained sediment matrix (size < 2 cm) and 14C dating were applied to reveal age and common structure of debris flow environments and main factors of their lithodynamics. Those helped to estimate the degree of fluvial reworking of debris flow deposits with time, transportation distances and capacities of flows and to integrate the chronological data acquired by other methods and spatial distribution patterns revealed by geomorphologic mapping and remote sensing data interpretation. Application of radiocarbon dating to determine absolute ages (about 40 dates) of stabilization periods for the mountain fans and valley bottoms allowed distinguishing several stages of increased and decreased activity of debris flows and slushflows in mountain valleys through the second half of the Holocene. In general, debris flow magnitude has significantly reduced since the last deglaciation, hence, reliable chronology of those early stages and events is yet to be obtained and represents the most challenging problem for future research in the area.