Emplacement dynamics of a crystal-rich, highly viscous trachytic flow of the Sancy stratovolcano, France

Emplacement dynamics of highly viscous, silicic lava flows remain poorly constrained due to a lack consideration crystal-rich cases. models mostly apply glassy or microlitic, vesiculated rhyolitic flows. However, crystalline, vesicle-free can flow differently. We studied the Grande Cascade unit, which is vesicle-free, phenocryst-rich, trachytic in Monts Dore massif, France. Field work was carried out define internal structures, and oriented samples were collected for chemical, petrological, anisotropy magnetic susceptibility analyses, allowing us estimate emplacement temperature viscosity. These data allow new subtype that low (800−900 °C), high silica content (up 66.8 wt%), viscosity (109−1011 Pa s), rich phenocrysts (∼35%), lacks vesicles. Brittle deformation occurs upon extrusion, generating cataclasite basal layer thin (3-m-thick) shear zone accommodates all stress, most flow’s volume slide over its base as 40-m-thick plug there no deformation. Blocks are rare, single size (10 ± 1 cm), result from localized break-up zone. different those glassy, pumiceous They closer glacier dynamics, where slides till generated by abrasion milling underlying layer. For flow, means classical blocky crust instead marked fine-grained material. The structures this consistent with ideal, gravity-driven flow. thus argue global reassessment silicic-rich based on crystal using multidisciplinary approach focused well-exposed examples rock record.