TEXTURAL FEATURES AS INDICATORS OF DEBRIS AVALANCHE TRANSPORT AND EMPLACEMENT

Debris Avalanches (DAs) are voluminous, heterogenic mass-flows of poorly sorted sediments (micron—10’s m) that move downslope under the effect of gravity. They travel with extremely high velocity for long distances with very potential high destructive power. These flows may reach initial velocities as high as 100m/s, travel for several tens of kilometers, and spread over broad sectors. They are commonly considered as inertial dry grain flows where particle-particle interaction can be within a frictional and/or collisional regime. But, depending on their water and clay content, these granular and block-sliding flows may transform into a debris flow with distance from source, changing completely their flow behavior and enhancing their run-out and hazard impacts. The Pungarehu DAD (ca. 25 Ka cal.) was emplaced by the largest known collapse of the Taranaki volcano (New Zealand) occurred near the Last Glacial Maximum (LGM), with snow and ice cover, fluids circulation, hydrothermal alteration and substantial groundwater present. This DAD appears to encompass a range of flow behavior from proximal unsaturated and unmixed conditions with chaotically distributed zones of shear developing where softer lithologies occur in the collapsing mass, through to a distal homogenous and well-mixed mud-dominated flow. The aim of this work is to describe the textural features from proximal to distal reaches within the DAD. The textural analysis of the unit from field descriptions, granulometric investigations, and microscopic analysis of particle forms enables a reconstruction of the transitions in flow conditions during the emplacement of a debris avalanche, which are applicable to the understanding of the mobility and hazards associated with large-scale stratovolcanoes.