UNDERSTANDING SPECIFIC PROBLEMS ASSOCIATED WITH OSL DATING IN ACTIVE TECTONIC SETTINGS

Sediment dating based on optically stimulated luminescence (OSL), also termed optical dating, has been applied since the late 1980s. Improvements in precision and accuracy over the last 15 years have stimulated wider recent application. In many cases, OSL dating based on quartz can achieve age estimates as young as a decade or less, extending to an upper limit imposed by signal saturation that may be as high as 1 Myr. The additional use of quartz single grains can often overcome problems imposed by incomplete removal of the OSL signal during deposition, allowing contexts to be dated even when only a fraction of constituent grains received sufficient daylight exposure to fully zero the signal.

However, in several contexts, including many in active tectonic settings, significant limitations to the application of OSL dating based on quartz following conventional approaches are encountered. Specific issues include 1) low quartz OSL sensitivity resulting in signals that are difficult to measure, leading to increased uncertainty in age estimates, 2) increased likelihood and severity of problems associated with thermal transfer OSL signals, 3) other poor OSL characteristics that restrict useful dating, including irregular signal growth, early saturation, failure to “recycle” and additional effects. In these contexts, IRSL (infra-red stimulated luminescence) signals from K-feldspars may offer an improved possibility to derive meaningful age estimates, though problems of both IRSL signal stability and “bleachibility” (sensitivity to daylight) are significant. This paper examines the underlying causes of the issues relating to quartz, methods to identify problem samples and locations, and the potential offered by K-feldspar IRSL. Selection of optimal strategies to derive meaningful age estimates when problems arise will vary depending on location, but an over-arching approach to deal with these contexts will be presented.