Fault slip-rates and Coulomb stress interactions in the intersection zone of the Hope, Kelly and Alpine Faults, South Island, New Zealand

Abstract

Plate boundary faulting in New Zealand's South Island involves transfer of ∼50% of slip from the largest fault (Alpine Fault) onto the Hope-Kelly Fault system through a structurally complex fault intersection zone. The slip-rate contributions of faults within the Hope-Kelly system and possible role of static stresses in facilitating slip transfer are explored in this study. Lidar-based geomorphic and fault mapping combined with luminescence dating of fault-proximal sedimentary deposits constrain post-last glacial slip-rates on the Hope and Kelly faults. Dextral slip-rates on the central Hope Fault (12–15 mm/yr) decrease westward on the Taramakau section from 5.6 (+2.1/−0.7) mm/yr to 1.7 (+1.0/−0.5) mm/yr. Dextral slip-rates on the Kelly Fault range from 6.2 (+2.7/−1.0) mm/yr to 2.0 (+2.5/−0.7) mm/yr to 6.2 (+7.8/−1.4) mm/yr. Proposed causes of slip-rate spatial variations include (i) complex slip localization and transfer across the deformation zone, (ii) undocumented slip on obscured or unrecognized faults, and (iii) possible transience in slip behaviours. Paleoseismic trenching and radiocarbon ages constrain timing of most recent surface rupture on the western Hope Fault to ca. 1680–1840 CE, with a preferred age of ca. 1800–1840 CE. Coulomb fault stress modelling indicates central Alpine Fault ruptures impart positive stress changes on Hope-Kelly receiver faults >5–10 bars, while Northern Alpine Fault earthquakes reduce Coulomb stresses on Hope-Kelly receiver faults, and vice versa. These results suggest central Alpine Fault earthquakes may propagate onto or trigger ruptures of Hope-Kelly Faults, but Hope-Kelly ruptures reduce stress on the northern Alpine Fault, possibly making ruptures of that fault less likely. This system of stress perturbations provides a mechanism for slip transfer from the central Alpine Fault onto the Hope Fault system.