RECONSTRUCTING THE GROWTH OF AN ANCIENT VOLCANO FROM ITS EROSIONAL LANDSCAPE USING GEOMORPHIC AND PETROLOGIC TECHNIQUES - AKAROA VOLCANIC COMPLEX, NEW ZEALAND

Heterogeneities in the architecture of constructional volcanoes impart a complex control on how the volcano will ultimately respond to deconstructional forces. Here, we present a geomorphic and petrologic study of the Miocene-aged Akaroa Volcanic Complex on the east coast of the South Island of New Zealand. Landscape features include lava domes, volcanic plugs, dykes, scoria cones and thick packages of stacked lavas that are most prominent in sea cliffs and in the walls of deeply eroded stream valleys. These valleys propagate radially from the inferred central vent region, and conspicuously, both valley walls and intervening ridges have a step-like geomorphology visible as what we refer to as ‘benches’. Benches coincide with contacts between lava flows and/or where there are thin soil/ash horizons between lava flows. In addition, the dip of the benches fit within the range of possible lava flow dip directions as measured from their apparent dips. Lavas were sampled systematically up through stacked sequences (bracketed by benches) for petrographic and bulk-rock geochemical analysis. Using Google Earth Pro, the benches observed in the field were visualized in three dimensions and their traces were drawn. Google files were then exported to ArcMap where benches were color-coded based on elevation above sea level. Lavas sampled above and below benches record variation in porphyritic and aphyric textures, and evolving geochemical trends (i.e. primitive picrites and hawaiites to evolved mugearites). In comparing the geomorphic and rock chemistry data, two important preliminary results have been identified that will guide future research. Firstly, primary volcanic textures and compositions may be an important control on bench formation. Specifically, erosion may occur more laterally on aphyric and compositionally evolved lavas and more vertically on porphyritic and primitive lavas; however, more research will need to focus on the physical rock properties and erodability. Secondly, it may be possible to correlate benches around the volcano and, from bench/lava dip directions, these erosional structures could act as stratigraphic markers to help unravel the evolution of the volcanic complex by identifying relative ages of bracketed lava sequences from one valley to the next.