A Field and Experimental Study of Shear Localisation, Strain Partitioning and Frictional Melting in a Debris Avalanche generated by Volcanic Flank Collapse
Abstract
The Arequipa volcanic landslide deposit to the east of Arequipa (Peru) originated from the andesitic Pichu Pichu volcanic complex, covering a ~200 km2 area of ignimbrite basal rocks. The deposit exhibits preserved internal flow structures, basal shear zones and pseudotachylytes. We present field, microstructural and chemical observations from the slip surfaces identified below and within the deposit. Examination of basal contacts reveals shear zones with varying degrees of strain localisation. In one instance, the shear zone is localised to a 1-2 cm thick layer of ultracataclasite and pseudotachylyte which contains fragments of earlier, solidified frictional melts. Textural and geochemical analysis coupled with rheological modelling of these generations of preserved frictional melts indicates brittle fragmentation of the melt may have occurred due to high strain rates suggesting the debris avalanche experienced slip velocities of >31 m s−1. We also find that frictional melting is unlikely to have provided a mechanism for basal lubrication in this case. Elsewhere, we observe a ~40 cm thick basal shear zone, overprinted by sub-parallel faults that truncate basal asperities to localise strain. We also observe shear zones within the avalanche deposit, some 20 m above the basal contact, suggesting that strain was partitioned within the avalanche. We complement our observations with an experimental investigation of the frictional behaviour (i.e. friction coefficient and wear rates) of faults with dissimilar lithologies and the conditions under which melt is produced by frictional sliding using low-to-high velocity rotary shear experiments. The results of these experiments will be discussed in-terms of the frictional control on large-scale flow behaviour. We conclude that deformation mechanisms at the base of the volcanic debris avalanche fluctuated between cataclasis and frictional melting (and subsequent brittle failure of the melt) during emplacement; exhibiting variable shear localisation and strain partitioning, which, along with underlying topography, modulated the basal resistance to flow and impacted runout distance.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMNH034..04H
- Keywords:
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- 1810 Debris flow and landslides;
- HYDROLOGY;
- 1826 Geomorphology: hillslope;
- HYDROLOGY;
- 4302 Geological;
- NATURAL HAZARDS;
- 4313 Extreme events;
- NATURAL HAZARDS