A channelized debris-avalanche deposit from Pirongia basaltic stratovolcano, New Zealand

Abstract

AbstractA newly discovered, large volume (3.3 km3) volcanic debris-avalanche is described from the Pirongia Volcano in North Island, New Zealand. Mapping, field surveys and drill core data were used to reconstruct the distribution and facies of the deposit (the Oparau breccia). The debris avalanche was channelized into a lowland graben structure resulting in a prolonged runout distance of ≥20 km and substantial thickness of >200 m in medial areas. The deposit contains block and matrix facies dominated by ankaramite basalt sampled from the oldest parts of the volcanic edifice. The age of deposition of the Oparau breccia is constrained to the period 2.2–1.75 Ma. The collapse source zone is marked by a prominent unconformity on the southwestern flank of the mountain. Movement on faults within the graben is identified as the most likely cause of sector collapse. The collapse scarp is infilled by 5 km3 of post-collapse volcanic material.