Neotectonics of the offshore Cape Egmont Fault Zone, Taranaki Basin, New Zealand

Abstract

The Cape Egmont Fault Zone (CEFZ) is a major structural boundary within the predominantly offshore Taranaki Basin. The northeast‐southwest‐striking principal fault within this zone, the Cape Egmont Fault (CEF), represents the westernmost zone of active deformation associated with the Hikurangi subduction system, and is characterised by normal separation and pronounced surface expression across the Taranaki continental shelf. It has a 53 km long, 1–5 m high seafloor scarp, located 6 km to the east of the Maui‐A production platform, and comprises four segments, each characterised by differences in fault geometry and behaviour. Average slip rates on the CEF for the last 225 000 years range from 0 to 0.8 mm/yr, suggesting concomitant extension rates of 0.1–1.8 mm/yr that are comparable with the deformation rates calculated for onshore active faults in the Taranaki‐Wanganui region. The presence of a seafloor scarp and historic seismicity associated with the CEFZ are considered to be indicative of the recently active nature of the CEF. Analyses of high‐resolution seismic reflection profiles and piston core samples suggest that the most recent movement on the CEF was at least post‐10–11 ka ago. During the late Pliocene‐Quaternary, it is unlikely that the CEFZ has accommodated significant amounts of strike‐slip, as proposed by previous workers. Movement over the last 2–3 Ma is inferred to have been predominantly normal, possibly with an element of dextral oblique‐slip.