Using in situ-produced Be-10 to constrain the age of the latest surface-rupturing earthquake along the Middle Kedrovaya fault (Baikal rift)

The 30-km-long Middle Kedrovaya (MK) fault, on the northwestern flank of Lake Baikal, is one of the most impressive late Quaternary seismogenic structures in the Baikal rift. In places, surface fault scarps expose striated bedrock fault planes and offer some opportunities to examine the faulting history. Using cosmogenic Be-10 exposure dating, we measured the exposure ages of one of the MK scarps (54.44630 degrees N, 108.51524 degrees E, elevation 985 m). This similar to 12-m-high normal fault scarp is composed of cataclasites in early Proterozoic granitoids of the Irel complex. We analyzed 12 of 18 samples, collected every 0.5 m, from 0.45 m above the toe of the exposed fault plane, which dips 55 degrees in its lower half (samples 1-6) and 45 degrees in upper half (samples 7-12). Exposure ages are the following from the toe upward: sample 1, 2.26 +/- 0.53 ka; 2, 1.54 +/- 0.55 ka; 3, 0.80 +/- 0.55 ka; 5, 1.44 +/- 0.55 ka; 6, 2.13 +/- 0.62 ka; 8, 1.45 +/- 0.48 ka; and 9, 1.40 +/- 0.52 ka. The relationship between the oldest calculated ages (samples 1 and 6) and the relative sampling height along the scarp suggests that a recent surface-rupture event with a vertical component of 2.05 m (i.e. net slip of 2.5 m) occurred on the cliff similar to 2.2 +/- 0.6 ka. Striations on the fault plane in the sampling points 1 and 6, as well a close date of 2145-2463 cal years BP calculated from radiocarbon age of a paleoseismic rupture within the Cape Shartly, similar to 62 km further southwestward, support this interpretation. The close ages assume that the dated deformations were related to a single M-w = 7.4 earthquake, the coseismic effects of which extended at least 82 km (the rupture length between the Cape Shartly and the NNE termination of the MK paleoseismic rupture zone). A more concentrated age mode of about 1.46 +/- 0.53 ka (samples 2, 5, 8 and 9), which is in an incorrect stratigraphic order in reference to the older ages, is assumed to result from local erosion or partial shielding. Cosmogenic dating of fault scarps and seismogenic landslide scarps in bedrock potentially offers an independent method of reconstructing the late Quaternary paleoseismology of the northwestern flank of Lake Baikal.