OBSERVATIONS AND THEORETICAL BASIS OF LUMINESCENCE THERMOCHRONOLOGY: RELATING TOPOGRAPHIC EVOLUTION TO GEOLOGIC PROCESS

Mountain uplift and valley incision typically involve the progressive cooling of deep-seated rocks as they move towards the Earth’s surface. Knowledge of a rock’s cooling history may be obtained using a range of temperature-sensitive dating systems (thermochronometers), each recording the time since the rock cooled below a certain “closure temperature”. Owing to their low thermal escape barrier energies (typically 1-2 eV), luminescence dating methods (optically stimulated luminescence-OSL, infra-red stimulated luminescence-IRSL, thermoluminescence-TL), based on trapped charge populations, exhibit significant loss of charge over the temperature range of 60 – 0°C. This opens up a new field of low-temperature thermochronology, providing the opportunity to quantify landscape change from a sub-Quaternary timescale (e.g. in response to glacial cycles) to million year timeframe. We explore how limited storage capacity for trapped charge, and the possibility of significant escape at typical surface temperatures, affect the effective closure temperature in trapped charge systems. We present measurements made using IRSL of feldspar from vertical profiles collected up the side of the rapidly exhuming Yukaipa Ridge and within the more stable Big Bear block in the San Bernardino mountains, Southern California. The implications for understanding the interplay between deep-seated processes and fluctuating climates involved in topographic evolution will be examined, in particular, over what timescales and exhumation rates these methods are likely to be useful.