Rates of active faulting during late Quaternary fluvial terrace formation at Saxton River, Awatere fault, New Zealand

Abstract

A flight of faulted fluvial terraces at Saxton River on the Awatere fault, northeast South Island, New Zealand, preserves the incremental slip history and detailed paleoearthquake chronology of this major strike-slip fault. Here, six fluvial terraces have been progressively displaced across the inland Molesworth section of the fault, with horizontal displacements ranging from -6 m for an ephemeral channel on the youngest terrace to 81 m for the riser above the oldest terrace. New optically stimulated luminescence ages for abandonment of the two oldest terrace treads are 14.5 ± 1.5 and 6.7 ± 0.7 ka. When combined with new measurements of incremental horizontal displacements and previous age data, these new ages indicate that strike-slip on this part of the Awatere fault has been occurring at a near-constant rate of 5.6 ± 0.8 mm/yr since ca. 15 ka. This rate is similar to recent slip-rate estimates for an adjoining section of the same fault to the east, which suggests that | there is near-complete slip transfer across the junction between the two fault strands. Comparison of the magnitudes and ages of the terrace riser displacements to the timing of paleoearthquakes on the Molesworth section allows the mean per event horizontal displacement over the eight most recent surface-rupture events to be estimated at 4.4 ± 0.8 m. Between ca. 5 ka and ca. 2 ka, surface-rupturing earthquakes increased in frequency and decreased in their mean coseismic displacements to <2.6 m. During this time, the sense of local dip slip also shifted from north-side-up to south-side-up. Rapid incision of Saxton River in the mid-Holocene may have caused a perturbation in the near-surface stress directions acting on the fault plane beneath the Saxton River valley, forcing a change in the near-surface fault geometry that resulted in the shift in sense of dip slip.