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Article Dans Une Revue Journal of Volcanology and Geothermal Research Année : 2017

Volcanic avalanche fault zone with pseudotachylite and gouge in French Massif Central

Karine Bernard
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Résumé

Structures and textures with sedimentological variations at different scales of the lithofacies assemblage help us to constrain the basal kinematic transition from non-depositional to depositional conditions during volcanic avalanche emplacement. In the well-exposed impact-sheared contact along volcanic avalanche fault zone in the French Massif Central, we observe how the granular textures of the pseudotachylite and fault gouge have recorded the propagation of shock wave with granular oscillatory stress. Sequential events of basal aggradation along avalanche fault zone have been established related to fractal D-values, temperature pressure regime and oscillatory stress during slow wave velocity. A typical lithofacies assemblage with a reverse grading shows the pseudotachylite and fault gouge. A cataclastic gradient is characterised by the fractal D-values from 2.7 in jigsaw breccias with pseudotachylite partial melt, to 2.6 in the polymodal gouge. Shock, brecciation and comminution produce cataclastic shear bands in the pseudotachylite and quartz microstructures along the basal contact of the volcanic debris-avalanche deposit. Gouge microstructures show granular segregation, cataclasis with antithetic rotational Riedel shear, and an arching effect between the Riedel shear bands. X-ray microtomography provided 3D microfabrics along the clastic vein in the sandy-gouge. From the available statistical dataset, a few equations have been developed implicating the same cataclastic origin with a co-genetic evolution of lithofacies. An impact wave during primary shear propagation may contribute to produce hydroclastic matrix, pseudotachylite partial melt and proximal gouge thixotropy with v ~ 50m/s and a T < 654 °C. The interseismic period with oscillatory stress is related to crushed clasts and basaltic melt around ~ 800 °C, Riedel shear bands with granular segregation along the fault gouge. The secondary shock by matrix-rich avalanche (ΔP = 10GPa, T ≥ 1000–1500 °C) contributes to quartz microstructures along the avalanche basal contact and quartz spheroids in microscopic cataclastic shear bands. Decompression around 654–800 °C is related to tertiary sub-vertical oscillations with a backward moving shock and antithetic rotational fault megablock. Semi-quantitative analyses of seismogenic fault basement contribute to establish the localised conditions related to sequential aggradation along volcanic avalanche fault zone.
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Dates et versions

hal-01708419 , version 1 (13-02-2018)

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Citer

Karine Bernard, Benjamin van Wyk de Vries. Volcanic avalanche fault zone with pseudotachylite and gouge in French Massif Central. Journal of Volcanology and Geothermal Research, 2017, 347, pp.112 - 135. ⟨10.1016/j.jvolgeores.2017.09.006⟩. ⟨hal-01708419⟩
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