A 36Cl age determination for Mystery Creek rock avalanche and its implications in the context of hazard assessment, British Columbia, Canada

摘要

The Sea to Sky Corridor has experienced hundreds of historic and prehistoric landslides. The most common types of historical landslides are rock falls and debris flows, which are relatively small in volume but can be damaging. These types of failures are more common in the southern part of the corridor, between Horseshoe Bay and Porteau, where infrastructure has been built in close proximity to steep slopes. Farther north, fewer landslides have been reported historically, but those that have been recorded are usually large and date to prehistoric time (e. g., Cheekye fan and Mystery Creek rock avalanche). As part of a Geological Survey of Canada surficial geology and landslide inventory mapping study, Mystery Creek rock avalanche, near Whistler, British Columbia, was sampled for 36Cl dating. Samples were collected from three large flat boulders of quartz diorite in the rock avalanche deposit to test a correlation with the previously reported radiocarbon age of 800 ± 100 years BP on charcoal. One sample revealed a mean age of 2,400 years and the other two, 4,300 and 4,800 years, respectively. These new results point to four possible interpretations: (1) Mystery Creek landslide is about 800 years old; (2) Based on the overlapping 2σ uncertainties, the rock avalanche took place between 2,200 and 3,600 years ago; (3) The rock avalanche deposit is 2,400 years old and the other two blocks are too old; and (4) The rock avalanche is between 4,300 and 4,800 years old. Although there is strength in numbers and it is likely that the age varies between 4,300 and 4,800 years, we favor the second interpretation where the age range is broader and statistically significant for all three samples. Moreover, at this time, we favor discounting the radiocarbon age based on a greater number of samples analyzed for 36Cl analysis and lack of detailed information on the charcoal sampling. The causes and triggers of the Mystery Creek rock avalanche remain unknown, but direct glacial debuttressing can be ruled out. Some of the causes are likely a combination of the regional tectonic setting which produced preferential planes of weakness reflected in the trend of major faults, headscarp, and reverse scarps. Yearly cycles of freezing and thawing are considered a plausible cause based on present-day climate records. Finally, a large earthquake still remains a possible trigger because of the active tectonic setting and the presence of potentially contemporaneous landslides in the same area. Mystery Creek rock avalanche and other historic and pre-historic landslides contributed to validation of a heuristic rock fall/rock slide/rock avalanche susceptibility mapping study, in which their headscarps correlated well with medium-high to high susceptibility zones. In terms of hazard assessment, Mystery Creek rock avalanche, although pre-historic in age, occurred in present-day climatic and geological conditions. This poses a threat to infrastructure such as the Sea to Sky Highway, railway, and power line.