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Reconstruction of the final effusive and intrusive phase at an open vent system: the case of Lemptégy (Chaine des Puys, France)

Ludovic Chender 1, Andrew J. L. Harris 1

Abstract

Lemptégy is a scoria cone in the Chaîne des Puys (France) which was formed by a complex series of Strombolian, Violent Strombolian and effusive events around 32 ka. Quarrying provides excellent exposure allowing the eruptive history of this open vent system to be reconstructed. Here we focus on the closing, effusive, phase of activity and use high-spatial resolution imaging and textural analysis to recreate the history final effusive event of Lemptégy. In addition, rheological modeling using a temperature-dependent viscosity model, plus calculated strain rates and shear stresses, reveal how void space deformation evolved under changing flow regimes. Initially lava was erupted as sheet flow at ~80 m3/s, with flowing down the flank of the scoria cone to the north. We trace a progression from initial bubble deformation under flow-induced stress, to the formation of fractures around vesicles within brittle layers as cooling advances. As the lava solidified, down-slope creep to the east took over as the main source of stress, with well-formed shear partings developing at high mechanical stresses. In our case, this transition began after a few hours for the flow surfaces, and after a few days for the flow interior, with flow lateral creep resulting from surface up-lift due to emplacement of a cryptodome. We use the deformation characteristics of the void spaces to estimate that the cryptodome began to form ~7 hours after flow emplacement, with intrusion continuing for at-least ~30 days. This study provides insights into processes operating during shutdown of an open-vent system.