Comparison of monitoring data with paleo-slip rates; cosmogenic nuclide dating detects acceleration of a rockslide

Instrumental monitoring of displacement rates of slow-moving rockslides is limited to an insufficient length of the pre-catastrophic failure deformation time. Prehistorical slip rates need to be considered to provide context for monitoring data in relation to the length of the failure process. Cosmogenic 10Be exposure ages (n=10) on the backscarp of the Gamanjunni-3 rockslide (northern Norway) are between 5.3+ or -0.5 ka and 1.2+ or -0.1 ka. These ages were adjusted for prefailure nuclide production at depth based on the (super 10) Be concentration in quartz from a stable horizontal bedrock surface above the rockslide. Displacement initiated between 6.6 and 4.3 ka with a relatively fast or instantaneous displacement rate that decelerated a few hundred years after initiation. Slide initiation coincided with the end of the Holocene thermal optimum in northern Norway. The age-height relationship on the backscarp yields an average paleo-slip rate of 28 (-5/+7) mm/yr. This is significantly slower than present-day rates of 54 mm/yr. However, during slide initiation, slip rates were faster than present-day rates. Considering its location within the modern permafrost zone and the observations from instrumental and Holocene displacement rates, we suggest that future global climate changes will influence the Gamanjunni-3 block slip rate.