Wairau Basin and fault connections across Cook Strait, New Zealand: seismic and geological evidence

Abstract

Cook Strait separates North Island and South Island of New Zealand. It contains the Wairau and other sedimentary basins, which have sedimentary thicknesses of up to 4 km. The Strait overlies part of the Australia–Pacific plate boundary where subduction below the North Island changes to strike-slip motion through the South Island. Strike-slip faults are well documented in the South Island and less so in the North Island. Cook Strait has been surveyed by a variety of shallow penetration seismic in the past, and this has resulted in various models and uncertainty about how major faults join across the strait. The shallow-penetration data image numerous fault traces close to the sea bed, most of which can be traced for only a short distance. Gaps between individual fault traces are common. In recent years, a growing body of research, government and oil company seismic data has imaged structures much more deeply and is the subject of this paper. The Wairau Basin structural style is one dominated by normal faulting, with expansion of basin area and progressive eastwards shift in depocentres with time. A steep structural gradient or faulted margin to basement subcrop and outcrop on the North Island margin indicates a significant north-northwest to northwest component of fault control in this area of Neogene development. The seismic interpretation of this deeper data suggests there are continuous seismogenic zones underlying the discontinuous young seafloor fault scarps. Such discontinuities have led to a perception that Cook Strait faults are not connecting through, that fault rupturing across Cook Strait is improbable. Fault connections across Cook Strait along seismogenic zones have recently been interpreted from historical records. These indicate that the October 1848 earthquakes involved mainshock (M_w 7.4–7.7) displacement on the Awatere Fault in the South Island and probable aftershock displacement on the Ohariu and Wellington faults in the North Island. The January–February 1855 earthquakes (mainshock M_w ∼8.2–8.4) occurred on the Wairarapa Fault in southern North Island, Wharekauhau, Nicholson Bank faults in Cook Strait, Vernon and Awatere faults in the South Island, and Needles Fault off the NE Marlborough coast. These interpretations are supported by recent seismic data that indicate underlying basin-defining seismogenic zones connect across Cook Strait, with movements that date back at least to the top Paleogene.

库克海峡分隔新西兰的北岛和南岛，含Wairau盆地和其他沉积盆地，其沉积厚度达4公里。海峡上覆澳大利亚-太平洋板块部分界线，在此处北岛下面的俯冲作用变为通过南岛的走滑运动。走滑断层在南岛有详尽的记录，而在北岛则较少。库克海峡在过去通过各种浅层穿透地震而被调查，这导致了各种关于主要断层如何跨海峡相连的模型和不确定性。根据浅穿透资料将许多贴近海床的断层迹成图，大部分断层迹只可追溯很短的距离。单个断层迹之间的间隔是常见的。在最近几年，越来越多的研究机构、政府和石油公司将构造进行更深的成像，这就是本文的主题。Wairau盆地构造特征是以正断层占主导地位，随时间的推移扩展盆地面积及在沉积中心逐渐向东转移。北岛陡峭的结构梯度或地下露头和露头的断层边缘说明该地区上第三纪地质发展中具有显著的北北西到西北向的断层控制成分。这种更深处资料的地震解释表明，在不连续的年轻海底断层崖之下存在着连续的地震带。这种不连续性导致人们认为库克海峡断层不是完全连接贯通的，跨海峡的断层张裂是不可能的。最近根据历史记录对断层沿地震发生带跨库克海峡的连接进行了解释。这些记录表明，1848年10月的地震包括南岛Awatere断层上的主震（Mw 7.4-7.7）位移和北岛Ohariu和惠灵顿断层上的余震位移。1855年1月至2月的地震（主震Mw约8.2-8.4）发生在北岛南部的Wairarapa断层上、库克海峡的Wharekauhau和Nicholson Bank 断层上、南岛的Vernon和Awatere断层上以及Marlborough东北海岸的Needles 断层上。这些解释由近期的地震资料支持，表明界定盆地的地震发生带随着可以追溯到至少早第三纪末的运移而跨库克海峡连接。

Key Words:

• Cook Strait basins
• seismic interpretations
• faults
• Wairau Basin
• Neogene history

ACKNOWLEDGEMENTS

We wish to acknowledge Tyers Petroleum Pty Ltd for providing the CR3048 tapes and allowing full disclosure of these data and use of confidential client reports. We also acknowledge the help provided to Tyers Petroleum by GNS Science on Cook Strait, and the useful discussions with Chris Uruski, Brad Field and Dave Francis. The New Zealand Ministry of Commerce also provided access to early seismic data. Thanks are due to Martin Norvick who provided the digital terrain and bathymetric map of Cook Strait area, Greg Blackburn who was able to load the CR3048 digital data onto a Kingdom workstation and Tony Brewster who assisted with the GeoDiscovery lines. We also wish to thank valuable reviewer comments from Professor Mike Hall and Dr Chris Uruski.