Subedifice collapse of an andesitic stratovolcano: The Maitahi Formation, Taranaki Peninsula, New Zealand

Abstract

The Maitahi Formation is a large-volume (>7.5 km 3 ) debris-avalanche deposit that resulted from a mid-Pleistocene (ca. 0.24–0.21 Ma) collapse of Pouakai Volcano, Taranaki Peninsula, New Zealand. It is best exposed at intermediate distances (12–14 km) from its source and consists of heterolithologic megaclast-rich and megaclast-poor facies. There are four megaclast and clast types: (1) volcanogenic sedimentary deposits derived from the cone and ring plain; (2) extrusive and intrusive igneous rocks derived primarily from the cone; (3) Pliocene mudstone-rich, subedifice bedrock; and (4) pyroclastic deposits derived from the cone and ring plain. Transport of intact, weakly to moderately lithified, volcanogenic sedimentary and bedrock megaclasts and megaclast domains, tens of meters in diameter, indicates that the W-NW–directed Maitahi debris avalanche was dominantly viscous and nonturbulent. Normally faulted, stratified megaclasts reflect flow dilation, whereas jigsaw-puzzle fractured megaclasts and clasts are evidence for early-stage compression and limited particle-particle and particle-substrate collisions. Folded and elongated megaclasts and clasts and injected interclast matrix reflect the influences of syn- and postavalanche compression, loading, penetrative shear, and pore-fluid overpressures. A significant subedifice bedrock and ring-plain megaclast and clast content distinguishes the Maitahi Formation from all other Taranaki Peninsula debris-avalanche deposits. Coupled with its large volume, the subedifice bedrock content also suggests that the Maitahi Formation originated from a deep-seated edifice collapse, unlike subsequent, and apparently shallower, edifice collapses at the nearby and genetically similar Egmont Volcano.