Infrared-stimulated luminescence dating of late Quaternary aggradation surfaces and their deformation along an active fault, southern North Island of New Zealand

Loess and silt that overlies and forms the matrix of late Quaternary fluvial aggradation surfaces from the central Wairarapa Valley, southern North Island, New Zealand, have been dated using infrared-stimulated luminescence (IRSL) of K-feldspar in fine grained (4-11 μm diameter) mineral fractions. Three IRSL protocols were used, involving both the additive-dose with thermal transfer correction and regeneration methods, and from four samples using both the additive-dose method and the single aliquot regeneration method. Eight luminescence ages of between and 16.1 ± 1.6 and 10.0 ± 0.9 kyr date the most extensive and youngest fluvial aggradation surface that has been deformed by Wairarapa Fault, the most active fault of the North Island Dextral Shear Zone. The ages provide absolute rates of dextral and vertical slip on the fault over the last ∼ 30 kyr that deforms two aggradation surfaces; an average horizontal slip rate of 11.5 ± 0.5 mm/yr and variable vertical slip rates of between 0.09 and 1.7 mm/yr that correlate with compressional bulges developed on the upthrown side of the fault caused by left-lateral sidesteps, and reversals of the upthrown side of the fault. The horizontal slip rate indicates that one third of the ∼ 33 mm/yr strike-slip motion of oblique convergence between the Australian and Pacific plates is taken up by the Wairarapa Fault. Three younger luminescence ages of < 10 kyr provide evidence for post-youngest aggradation surface fan development and date a possible earthquake.