ABSTRACT

The glacio-fluvial sediments of the Whataroa Valley on the west coast of New Zealand’s South Island contain a record of environmental change since the Last Glacial Maximum. The valley is cut by the Australia–Pacific plate-bounding Alpine Fault, the position of which is obscured by recent glacio-fluvial outwash deposits. Five seismic profiles collected across the inferred surface trace of the Alpine Fault between 2011 and 2018, using a variety of source types, are presented here; these profiles provide constraints on the fault’s location in the upper few hundred metres of the near surface. The sedimentary strata in the Whataroa Valley have been classified into seismic facies based on their reflectivity characteristics. Two reflective seismic facies, recognisable in all profiles, have been interpreted to correspond to recent fluvial outwash gravels (seismic facies 1) and older post-glacial glaciofluvial or marine sediments (seismic facies 2). These reflective packages are cut by Alpine Fault rupture surfaces with reverse motion dips of 60° to 80° and total throw of up to 50 m. Faults imaged in three of the profiles are interpreted to correspond to the most recent surface rupture of the Alpine Fault, due to their close proximity to a recently trenched surface scarp.

摘要

新西兰南岛西海岸的 Whataroa 山谷的冰川河流沉积物记录了自末次盛冰期以来的环境变化。该山谷被澳大利亚-太平洋板块边界的高山断层切割，其位置被最近的冰川-河流冲刷沉积物所掩盖。此处展示了在 2011 年至 2018 年间使用各种震源类型在高山断层推断表面轨迹上收集的五个地震剖面；这些剖面提供了对近地表上部几百米断层位置的限制。瓦塔罗河谷沉积地层根据反射率特征被划分为地震相。两个反射地震相，可在所有剖面中识别，已被解释为对应于最近的河流冲刷砾石（地震相 1）和较老的冰川后冰川河流或海洋沉积物（地震相 2）。这些反射包被阿尔卑斯断层断裂面切割，反向运动倾角为 60° 至 80°，总距离高达 50 m。在三个剖面中成像的断层被解释为对应于阿尔卑斯断层最近的地表破裂，因为它们靠近最近挖出的地表陡坡。