Luminescence dating of Pleistocene alluvial sediments affected by the Alhama de Murcia fault (eastern Betics, Spain) – a comparison between OSL, IRSL and post‐IRIRSL ages

Abstract

The ages of nine alluvial units, identified by the integration of data obtained from five trenches at the southern termination of the Alhama de Murcia Fault (AMF) (eastern Betics, Spain), are constrained using luminescence dating based on the Optically Stimulated Luminescence (OSL) from quartz, Infrared Stimulated Luminescence (IRSL) at 50°C, and post‐IR elevated temperature (225°C) IRSL signals from K‐feldspar. All signals pass the routine tests associated with the Single Aliquot Regenerative (SAR) protocol, including the recycling ratio, recuperation, and dose recovery tests. The equivalent doses (De), residual doses and anomalous fading rates (‘g’‐values) of the IRSL at 50°C (IR50) and post‐IR IRSL at 225°C (pIRIR225) from K‐feldspar are compared for 16 samples. The residual doses in laboratory‐bleached samples suggest that there is no significant unbleachable residual dose using these signals; the residual doses are 0.17±0.15 Gy and 0.93±0.80 Gy, respectively. For both signals, the residual doses appear to depend on the corresponding natural doses; that is, the larger the natural doses, the larger the residuals, an observation made for the first time for IRSL signals. The average fading rate for the pIRIR225 (0.94±0.07%/decade, n = 48) is markedly lower than that for IR50 (2.08±0.16%/decade, n = 48), indicating that the age correction for the pIRIR225 is much smaller than that for IR50. The agreement between the quartz OSL, corrected IR50, and corrected pIRIR225 for the two youngest samples suggests that the pIRIR225 is an accurate dating signal in this age range (<25 ka). The oldest age obtained using the corrected pIRIR225 signal is 320±20 ka for sample 098807; the corrected IR50 age is only 154±15 ka. Although this suggests that the pIRIR225 signal circumvents the effect of anomalous fading to a great extent, the resulting age cannot be regarded as necessarily accurate because of the limitations of the fading correction model used, and the absence of independent age control for the old samples. Nevertheless, our luminescence ages provide the first age constraints on the seismic activity of the southern termination of the AMF.