Slip-rate Estimation of Active Fault by Luminescence Dating on Deformed River Terraces at Tsaotun, Central Taiwan

This study carried out optical stimulated luminescence (OSL)
ages of the
tectonic-disrupted terraces at Tsaotun, where the 1999 earthquake fault (i.e.,
Chelungpu) runs through. Over there, a number of investigations have been done,
but it is still difficult to evaluate the slip-rate and recurrence interval without age
control. For the purpose of understanding neotectonic behavior of the Chelungpu
fault, the ages of the terraces are imperative to be determined.
Radiocarbon dating is unlikely applied in fluvial coarse deposits due to the
syndepositional carbonaceous materials are easy to be decomposed. Alternatively,
OSL dating is therefore tried on sandy layers intercalated within massive gravel
deposits found at Tsaotun. All the terraces developed at Tsaotun are identified as
strath terraces, indicating the depositional age is very close to the surface age that is
the best constraint in deriving the long-term slip rate of the related active fault.
Three samples are collected and presented. In this study only the fraction of coarse
quartz, i.e., 90-150 µm, is sieved out and tested by the Single-Aliquot Regenerative
(SAR) dose protocol with 36 aliquots for each. Considering the incomplete
bleaching during quick deposition, the OSL/TL and OSL/OSL ratios are applied to
normalization in each aliquot and to approach the true De. Dose rate is derived by ICP-MS and XRF analyses. For confirmation of the residual luminescence signals
in the fluvial sediment, the modern samples collected in Wuhsi were also measured
using the same protocol.
Based on the results of modern samples, we found that the residuals are
inevitable in younger samples, especially in the river reach where debris flow is
dominant. On the contrary, the samples older than 10 ka will be slightly influenced
because the age error covers the residual uncertainty. The OSL age of the terrace
samples in the hanging wall is dated ca. 24 ka, which has been corrected for
poorly-bleaching problem. Comparing to the ages collected down hole in the
footwalls, we found out vertical displacements of ca. 72 m and 43 m, has been
cumulated by the slips of main and back thrust. Thus, the long-term slip rates of the
main and back thrust are 2.7-3.3 m/ka and 1.8-2.2 m/ka, respectively.