Morphology and slip rate of the Hurunui section of the Hope Fault, South Island, New Zealand

Abstract

The Hurunui section of the Hope Fault is a newly defined, 42 km long geomorphic fault section which extends from Harper Pass to the Hope‐Boyle River confluence. Reconnaissance mapping along the Hurunui section from Hope Shelter to Harper Pass provided new data on its location, geomorphology, displacement, and slip rate. More than 200 previously published field observations of dextrally and vertically displaced landforms along the fault provide data on the distribution of displacement along the fault trace. Five radiocarbon dates found in association with offset geomorphic features are presented and two new measures of dextral slip rate are calculated. At McKenzie Stream, a late Holocene fan complex is cut by the Hope Fault. Young abandoned and active channels on this surface show dextral offsets of up to 22 ± 2 m along a south‐facing scarp with a height of up to 5 m. Woody litter from a unit in this complex has yielded a radiocarbon age of 2331 ±55 yr BP and a corresponding minimum horizontal slip rate of 8.1–11.0 mm/yr. At Macs Knob, large dextral deflections of stream catchments are linked to episodes of glacial resetting of the landscape. Correlation of the offset of “Macs stream” (166 ± 17 m) with a post‐Aranuian age peat (10 782 ± 60 yr BP) yields a maximum horizontal slip rate of 13.0 ± 1.5 mm/yr. The single‐event dextral displacement, based on offset stream channels at McKenzie fan, is 3.2–3.8 m (av. c. 3.4 m). The ratio of dextral to vertical slip is c. 7 ± 2:1, indicating that the Hope Fault has a dominantly strike‐slip sense of motion. The average recurrence interval for the last 5–7 events (i.e., to produce 19–24 m slip at McKenzie fan) is 310–490 yr. The age of the most recent surface‐rupturing earthquake at this site is not known, though felt effects, fault scaling, and landscape arguments indicate it was not the AD 1888 North Canterbury earthquake.