STRATIGRAPHIC CHANGES IN BULK ROCK CHEMISTRY MAY EXERCISE CONTROL ON LANDSCAPE EVOLUTION IN AKAROA VOLCANO, NEW ZEALAND

Akaroa Volcano is the largest in a series of eroded Miocene volcanics that form Banks Peninsula in the South Island of New Zealand. Most geologic research on Akaroa has focused on its geochemical evolution and magmatic-tectonic origin, with little understanding of how the modern landscape has developed. Here we present a new geomorphological study emphasizing incipient volcanic controls on modern hillslope gradient. On Akaroa Volcano, cyclic magma batch evolution is the dominant process driving variation in bulk rock chemistry across the volcano. In particular, mafic batches are defined by dramatic shifts from more evolved (trachybasalt) to less evolved (picrite) magmas after a period of gradual evolution. From field observations as well as the preparation of slope and hillshade DEM maps, we focus on inflections in slope gradient (‘benches’ on valley walls) to test whether the composition of lavas is a controlling factor governing modern-day erosion. Using longitudinal profiles extracted from ArcGIS and analyzed with custom-built scripts in R, we compare slope gradients along a sub-vertical transect on the eastern flanks of the volcano with high-resolution chemical stratigraphy of stacked lava sequences. Our model shows that benches in the transect correlate well with sharp changes in lava major element composition from the most evolved magma batch sequence to the least evolved beginning of a new magma batch. The correlation of these benches with chemical cycling in lavas suggests a connection between magmatic processes and long-term landscape evolution. These ‘benches’ are ubiquitous across Akaroa, and understanding their formation may provide insight into the life cycle of continental volcanics. We also hypothesize several physical mechanisms to explain our model. It is possible that a similar incipient control dominates landscape evolution across Banks Peninsula, but only a rigorous and broader future study will shed light on the true origins of this phenomenon.