COMPOSITION AND RHEOLOGY OF OVER-THICKENED LAVA FLOW UNITS ON BANKS PENINSULA, NZ

On Banks Peninsula, New Zealand, exceptional coastal exposures of Miocene lava sequences provide an opportunity to better understand the emplacement processes of over-thickened lava flows. Qualitative observations of three geochemically variable (a nephelinite, a hawaiite, and a mugearite) over-thickened flows reveal 2 m thick lavas on the upper slopes that transition into 50 m cliff-forming compound lavas toward the ocean. These cliff sections, resulting from filling of paleo-valleys, commonly include volcaniclastic debris, indicating remnant drainage networks into which lavas were directed. Within these thickened sequences partially developed columnar joints propagate across discrete internal horizontal flow boundaries with no coexisting breccias. We suggest this to be the result of rapid emplacement of multiple flows to enable cooling as a single unit. To validate this, we evaluate possible eruption rates and reconstruct emplacement mechanisms to determine controls on the over-thickening of the aforementioned lava examples.

Flow compositions modeled against temperature and viscosity indicate that viscosities and compositions did not exert a significant influence on the over-thickened morphologies. For example, calculated viscosities for the nephelinite flow fell between 17 and 130 Pa s and those for the hawaiite between 105 and 740 Pa s, yet these flows expressed very similar morphological features. Further computational analysis of viscosities, coupled with measurements of flow dimensions and crystal content, reveal that high effusion rates may have been a considerable factor in overthickening flows, with rates of between 400 and 1000 m³ s^-1 being likely. Further analyses of flow dimensions and eruption temperatures are currently being investigated in order to reduce uncertainties in the results but our initial field observations and calculations suggest a combination of topography and high effusion rates could explain how over-thickened lavas with similar physical features can form despite compositions with 10% difference in silica content.