Age constraints for precarious rocks in a humid-temperate environment

Mark W. Stirling, Albert Zondervan, Richard J. Norris, & D. Ninis

Submitted 2009, SCEC Contribution #1265

We constrain the age of precariously-balanced rocks (PBRs) within a metamorphic terrane and temperate environment using in-situ cosmogenic isotopes and geomorphic interpretation. Previous PBR age-related studies have been almost exclusively restricted to arid environments in the western United States. Cosmogenic in-situ 10Be concentrations from 11 samples of schist and silcrete from one PBR site in the central Otago region of New Zealand imply rock surface erosion rates of the order 0.5cm/kyr, consistent with rates of landform evolution and catchment denundation determined elsewhere in the region. The PBRs may only have reached their precarious state less than 10kyr ago, and may not have experienced any earthquakes on the nearby Dunstan Fault, given the long recurrence interval of the fault (8kyr). Future New Zealand-based efforts should therefore move to studying fragile geomorphic features close to faults with much shorter recurrence intervals (100-1,000 years). The main international contribution arising from our results is that PBRs from metamorphic terranes in humid-temperate environments similar to that of central Otago (e.g. parts of central California and Pacific Northwest) may be considerably younger than counterparts in the arid western United States.

Stirling, M. W., Zondervan, A., Norris, R. J., & Ninis, D. (2009). Age constraints for precarious rocks in a humid-temperate environment. Journal of Geophysical Research: Earth Surface, (submitted).