Holocene to recent landscape evolution of the Leader Valley, Waiau, North Canterbury

Abstract

The Kaikōura earthquake ripped across the North Canterbury and Marlborough regions of the South Island on 14th November 2016. It initiated on the Humps Fault, near Waiau township and propagated north-eastward. The Leader valley, which is the focus of this study, is located ~16 km northeast of Waiau. During the Kaikōura earthquake, the Humps Fault East (HFE) and South Leader Fault (SLF) ruptured to the surface at Leader valley causing incredible surface deformation. In light of this catastrophic seismic event, this investigation aims to understand the Late Holocene to recent (postKaikōura earthquake) landscape evolution of the Leader valley. Firstly, detailed geomorphological maps of the pre-earthquake and post-earthquake (2016) landscape were produced, supported by aerial imagery and published geological maps of the area. Then Ground Penetrating Radar (GPR) surveys were undertaken across mapped geomorphological features complemented by field truthing (exposure logging). Geomorphological maps and GPR survey results, in support with supplementary data, were used to identify processes driving the landscape evolution of the Leader valley. Geomorphological maps showed that the landscape of the Leader valley changed significantly following the Kaikōura earthquake. The co-seismic Leader landslide, Lake Rebekah and waterfalls are some of the key features observed in the post-earthquake (2016) landscape. These features are not observed in the pre-earthquake setting. The GPR survey results suggest that the gravel terraces were formed in a braided gravel bed river setting, because wide, and flat bedded channels with gravel bars are identified in the radargrams. This interpretation agrees with the current river channel morphometry because geomorphological mapping indicates channels are wide, flat and shallow. The GPR survey results for the steep slopes northwest of the site do not indicate large scale landslides. The debris slumps interpreted on the radargrams do not resonate with the size of the Leader landslide, therefore they are interpreted as debris slumps occurring from small instabilities on these steep slopes. Unfortunately, GPR surveys were not successful in identifying fault expressions at Leader valley. Geomorphological processes at this site can be categorised as: (1) structural controls on geomorphology, (2) hillslope processes and (3) fluvial processes. The evolution of the Leader valley is reduced to four key stages: (1) the original landscape where cover rocks overlaid the basement rock, (2) folding and thrusting in the Northern Canterbury Domain resulting in uplift of the basement, (3) glaciated alluvial greywacke gravel deposition in the basins, and (4) down cutting of the alluvial gravel terraces due to end of glaciation. Since its inception, the key geomorphological processes evolved the Leader valley, and will continue to change and shape the valley in the future as well.