Paleoseismology of the NW Cardrona Fault, Central Otago

Abstract

This thesis documents a paleoseismic study of the Northwest (NW) Cardrona Fault at the Branch Creek Road site in the Cardrona Valley, Central Otago. The northeast-striking NW Cardrona Fault is a major range-bounding reverse fault with as much as c. 1 km of late Cenozoic, west-side-up, throw. It forms the northwestern component of a significant geological structure, the Nevis-Cardrona Fault System, and defines the western margin of the Otago ‘Range and Basin’ reverse fault province. A pre-existing knowledge of the NW Cardrona Fault was based mostly on mapping, trenching, and dating led by the late Sarah Beanland in the 1980s, as part of seismic hazard investigations for hydroelectric development (Beanland and Barrow-Hurlbert 1988). The Branch Creek Road trench was excavated in 1984 by Sarah Beanland across a scarp of the NW Cardrona Fault that shows east-side-up displacement; the opposite sense to that of the rest of the fault. Beanland’s early interpretations suggested the occurrence of three rupture events at the site; however, age control on the timing of these events was limited by the dating techniques available at the time. This thesis documents a re-excavation of the Branch Creek Road trench and interpretation of fault rupture chronologies via Optically Stimulated Luminescence (OSL) dating. With the aid of nine OSL samples, the timing of these events has been constrained to the following: an antepenultimate event between 37,800 ± 4,000 and 39,300 ± 4,400 years; a penultimate event between 27,200 ± 2,800 and 28,200 ± 2,600 years; and, the most recent event most prudently placed between 3,300 ± 600 and 14,300 ± 1,400 years. The 3.5 m of cumulative dip-slip displacement measured across these three surface rupture events equates to an average dip-slip single-event displacement of c. 1.2 m. Lastly, the opposite sense of fault slip at this site relative to other parts of the fault is interpreted as being a short-lived structural complexity, in that the fault is accommodating surface slip above the blind master NW Cardrona fault. Ruptures on the Branch Creek trace are therefore considered to be directly linked to ruptures on the master NW Cardrona Fault.