HOLOCENE PALEOSEISMICITY OF THE TUNKA FAULT, BAIKAL RIFT, RUSSIA

The Tunka fault is a major normal-oblique transform fault within the NNE trending Baikal rift that displays geomorphic evidence of recurrent Quaternary movement. A flight of six fanhead terraces of the Kyngarga River is displaced by several parallel faults and has scarps up to 32 m high at Arshan. The main fault zone is exposed in an abandoned gravel quarry about 1 km east of the Kyngarga River. Thirteen radiocarbon dates from the quarry, a shallow trench in a graben, and natural streamcuts constrain the timing of the latest two or three Holocene paleoearthquakes. The latest earthquake is bracketed by the 1024-1315 calendar years (cal. year) B.P. age of an unfaulted terrace and by the 1947-2179 cal. year B.P. age of a soil buried by scarp colluvium in the graben trench. The penultimate earthquake is also relatively well constrained, assuming that the displacement event slightly younger than 7091-7867 cal. year B.P. in the upper quarry exposure is the same as the fissuring event slightly older than 6733-7385 cal. year B.P. in the lower quarry exposure. Evidence for earlier event(s) between 9.2-12.7 ka depends on ambiguous stratigraphic evidence in the lower quarry exposure. On the basis of only the two well-dated earthquakes, the recurrence interval at Arshan may range from 2.9 to 6.8 ka for earthquake displacements of at least 1.3 m or a slip rate of 0.19-0.44 mm/yr. Intermediate-term (100 ka) and long-term (circa 500 ka) slip rates computed from terrace age estimates and fault scarp heights are 0.08-0.16 mm/yr and 0.07-0.11 mm/yr, respectively, rates that are considerably lower than the late Holocene rate and the approximately 0.5 mm/yr that might be inferred from the tectonic geomorphology of the Tunka Range front.