Abstract
We combine previous studies, fieldwork, lidar data, and trenching to examine late Quaternary activity of the SE-dipping ≥23 km long Settlement Fault in the southeastern South Island. Trenching of a scarp crossing a small alluvial fan exposed a > 3 m thick sequence of folded sandy to silty fan alluvium. The uniformity of folding and four Optically Stimulated Luminescence dates indicate that these sediments were deposited relatively rapidly at ∼20 ka and subsequently accommodated 2.5 m of throw following ≥1 ruptures on an underlying buried reverse fault. Other key constraints on late Quaternary Settlement Fault activity are: (1) stranding of a shore platform at ∼3.7 ka, due to 1.5–2 m of fault uplift, and (2) a flat bedrock erosion surface overlain by loess and correlated to the last interglacial maximum (∼125 ka), indicating 0–4 m of fault uplift. From incorporating this information into a stochastic model of fault displacement accumulation we derive a slip rate of ∼0.02 mm/yr since 125 ka, and ∼0.27 mm/yr since 20 ka. We link the Settlement Fault's current phase of heightened activity to a similarly timed increase in Akatore Fault activity, despite the tips of these faults being separated by between 9 and 25 km.