QUANTITATIVE GEOMORPHOLOGY OF BANK'S PENINSULA PROVIDES INSIGHT INTO FORCING MECHANISMS FOR LANDSCAPE EVOLUTION IN CONTINENTAL EARTHQUAKE ZONES

Akaroa and Lyttleton Volcanos are the main constituents of an eroded Miocene volcanic shield that forms Bank's Peninsula on the South Island of New Zealand. To date, little research on Akaroa has rigorously investigated how the modern landscape has developed. Here we present a quantitative analysis of landscape evolution on Bank's Peninsula, highlighting process controls on fluvial incision. This area provides a unique opportunity to study the relative magnitudes of a few forcing mechanisms on common lithology. Previous research has established a good basis for understanding original topography and volcanology, and in the timescales of interest uplift or subsidence, as well as climate, can safely be neglected.

Field observations, topography, and stratigraphy suggest that following a period of rapid headcutting and stream piracy, a quasi-stable radial drainage pattern formed with a single dominant network claiming the central catchment on each volcano. Subsequent modification of the landscape has been mostly affected by eustatic sea level change, sedimentation, and earthquake-driven sea cliff retreat. For example, using data collected from DEMs in ArcGIS and analyzed in R, we find that the mouths of small streams draining into the ocean have higher slopes than comparable streams connected to the mainland. From anecdotal evidence and other studies, we hypothesize that shaking and rockfall in earthquakes like the 2011 Canterbury sequence are a major factor driving the retreat of sea cliffs. This suggests that earthquakes and sea level rise are forcing the formation of hanging valleys.

Our models and analysis show that interplay of a few driving factors on Bank's Peninsula gives rise to a complex landscape response. Further study of this response may lead to deeper understanding of both the mechanisms themselves and the long-term evolution of eroding volcanic shields.