Microstructures along volcanic avalanche fault zone in French Massif Central

Microscale structures are rarely studied along volcanic avalanche fault zones. Here, the well-exposed basal contacts of the Perrier avalanche units are described in the French Massif Central, which exhibit various deformed sedimentary structures, including planar sheared contacts, asymmetric undulations, schistose bands within a sandy matrix, sigmoids, shears bounding subangular clasts, and cataclastic shear bands. The occurrence of a breaking force chain along clastic veins, antithetic rotational shear, and clastic disaggregation characterize sequential microfabrics. The quartz microstructures observed along the basement of the volcanic avalanche units indicate a combination of syn and post-seismic creep in the low temperature plasticity regime. The microstructural evolution of mica fish records deformation accommodated at variable pressure and thermal conditions along clastic shear bands. Granular segregations with interclastic pumice matrices and cataclastic matrix transformations are related to co-seismic fluids. This volcanic avalanche fault zone has experienced a vertical and lateral gradient of sequential deformations related to lateral spreading in a rift zone that contained a topographic obstacle. •Clastic shear bands are correlated with the avalanche fault zones.•Quartz microstructures are associated with the seismic creep.•Microstructure evolution of mica fish recorded the deformation.