Long-term crustal movement caused by the Chiuchiungkeng Fault in southwestern Taiwan: Constraints from luminescence dating

摘要

To investigate the long-term crustal movement of the Chiuchiungkeng Fault (CCKF), a proposed active fault located in the southwestern coastal plain of Taiwan, two boreholes and dissected alluvial fans were selected for determination of depositional ages using luminescence. The standard single-aliquot regenerative-dose (SAR) protocol was applied to large aliquots of 90-150 μm quartz, with dose rates based mainly on high-resolution gamma spectrometry. The laboratory growth curves show a relatively large saturation value, which should allow reliable ages to be determined back to ～ 150 ka. To test whether a significant sensitivity change occurred during natural measurement, the single-aliquot regeneration and added dose (SARA) method was also used to estimate equivalent dose; this showed that a sensitivity change of ～4% may be introduced during the routine SAR procedure. The age results, unaffected by significant sensitivity change, all lie within 60 -105 ka, and an 80 ka synchronous surface is used to evaluate the long-term fault behaviour. Based on the offset across the hanging wall and footwall, a long-term fault slip rate of ～1.5m ka~(-1) is estimated for the near-surface CCKF. From the anomalously steep slope of an 80 ka abandoned fan surface observed in the hanging wall, a long-term tilt rate of ～42 m km~(-1) can be also derived based on the assumption that the initial slope is given by the modern river bed. Finally, it is deduced that the slip rate of the CCKF along its deeper part must be greater than ～1.5 m ka~(-1) since the apparent rate is reduced by folding of the hanging wall.