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.

Tectonics of the Taranaki region, New Zealand: Earthquake focal mechanisms and stress axes

“…1) as a means of elucidating the current tectonics. It complements previous work on active faulting (Nodder 1993;Hull 1994;Nicol et al 2005), and geodetic modelling (Wallace et al 2004), and expands significantly on the previously limited use of focal mechanisms to study Taranaki tectonics (Reyners 1980;Cavill et al 1997;Webb & Anderson 1998). It forms part of a multi-faceted study of Taranaki (Sherburn & White 2005;Sherburn et al in press) with the overall aim of understanding the structure and seismicity of the region and using that information to understand better earthquake monitoring data from Mt Taranaki volcano.…”

“…Focal mechanisms from western Taranaki differ from the normal faulting mechanism determined for the 1974 Opunake earthquake (Webb & Anderson 1998), and the normal faulting seen in the surface geology (Hull 1996) and seismic reflection images (Nodder 1993;King & Thrasher 1996;Nicol et al 2005). There are no published stress axes for western Taranaki, but the orientation of active and Quaternary faults suggest that the minimum horizontal compressive stress (Shmin) should be oriented approximately northwest-southeast.…”

“…If focal planes aligned parallel to the common northeast-southwest fault orientation are assumed to be the fault planes, then the sense of motion of many mechanisms would be right-lateral. Previous workers (Hull & Matsuda 1981;Hoolihan & Yang 1991;Nodder 1993;Hull 1994Hull ,1996 have described a small component of right-lateral strike-slip motion in addition to the dominantly normal motion on faults in this area.…”

“…1) northeast of Mt Taranaki shows oblique normal slip with a 3 m scarp and evidence of individual fault offsets of >1 m in the last 13 000 yr (Hull & Dellow 1993;Hull 1994). The offshore Cape Egmont Fault has a 53 km long seafloor fault scarp dominated by normal displacement, and possibly with a small amount of right-lateral slip (Nodder 1993;Nicol et al 2005). Since 225 ka the average rate of dip-slip movement on the fault has been 0.8 mm/yr.…”

CoccolithophoresGephyrocapsa oceanicaandEmiliania huxleyi(Prymnesiophyceae = Haptophyceae) in New Zealand's coastal waters: Characteristics of blooms and growth in laboratory culture

“…18 isotope stage 1) rich in coccoliths of E. huxleyi occur throughout the New Zealand region and include a wide variety of depositional environments ranging from deep sea sites to estuaries and lagoons (Burns 1975;Edwards 1992;Nodder 1993). Almost monospecific assemblages of such coccoliths occur in some highly landlocked Fiordland sediments of younger interglacial age (A. R. Edwards unpubl.…”