WHAT BRITTLE STRUCTURES CAN TELL US ABOUT THE EMPLACEMENT OF SILICIC LAVA FLOWS AND DOMES

Recent silicic lava eruptions at Chaitén (2008) and Cordón Caulle (2011-2012), Chile, showed a range of simultaneous emplacement processes (endogenous vs. exogenous) and eruptive behaviors (explosive vs. effusive) that were previously regarded as competing end-members. Here we document evidence of a continuum of brittle and brittle-ductile deformation and fracture-induced outgassing during the emplacement of the ~0.17 km³ Obsidian Dome, California, USA. The upper surface is characterized by small (<1 m) mode-I tensile fractures which grow and initiate new cracks which link together to form larger tensile fractures (1-5 m) which penetrate deeper into the lava. Larger fractures bound ogives, which are traditionally interpreted as fold structures resulting from compressional ductile deformation, but have recently been reinterpreted as the resulting from brittle extension (Andrews et al. 2021 EPSL 553: 116643). The largest fractures at Obsidian Dome are crease structures, which develop by a cyclic fracture mechanism in response to lateral spreading (Anderson and Fink 1992 GSA Bull 104: 615-625) in the later stages of the lava's emplacement. Ornamentations preserved on fractured surfaces record degassing and explosive fragmentation away from the vent throughout the lava's emplacement, suggesting explosive activity occurred during the effusive emplacement. At both Obsidian Dome and the ~4 km³ Banco Bonito flow in the Valles Caldera, New Mexico, USA, dissolved water contents of lava samples collected from drill cores are generally at or below the solubility limit for the sample depth. This suggests emplacement in an exogenous or “tank-tread” style. It may also reflect degassing to levels in equilibrium with atmospheric pressure, when fractures propagated downwards into a thick brittle carapace late in the emplacement history. Vesiculation in response to rapid decompression around crack tips may produce “coarse vesicular pumice” (CVP) texture. Crease structures have also been observed in mafic lavas, for example at SP Crater (San Francisco Volcanic Field, Arizona, USA) and Volcán San Pedro (Atacama, Chile). All lavas ultimately cross the glass transition, and similar rheological behavior may produce similar deformational features even in lavas of different composition and/or crystallinity.