Making sense of brittle deformation in rhyolitic lavas: Insights from Obsidian Dome, California, USA

The scarcity of observed active extrusive rhyolitic lava flows has skewed research to extensively focus on prehistoric lavas for information about their eruptive and emplacement dynamics. The first ever witnessed silicic lava eruptive events, Chaiten (2008) and Cordon Caulle (2011-2012) in Chile, were illuminating to the volcanology community because they featured a range of emplacement processes (endogenous versus exogenous), movement limiting modes, and eruptive behaviors (explosive versus effusive) that were often regarded as acting independently throughout an eruptive event. In this study, we documented evidence of a continuum of brittle and brittle- ductile deformation and fracture-induced outgassing during the emplacement of the similar to 600-yr-old silicic lava from Obsidian Dome, California, USA. This study focused on mapping the textural-structural relationships of the upper surface of the lava onto high-resolution (<10 cm(2)/pixel) orthorectified color base maps. We found that the upper surface is characterized by small (<1 m) mode 1 tensile fractures that grew and initiated new cracks, which linked together to form larger tensile fractures (1-5 m), which in turn penetrated deeper into the lava. We recorded ornamentations on these fracture surfaces that allow snapshot views into the rheological and outgassing conditions during the lava's effusion. The largest fractures developed during single, large fracture events in the final stages of the lava's emplacement. Ornamentations preserved on the fractured surfaces record degassing and explosive fragmentation away from the vent throughout the lava's emplacement, suggesting explosive activity was occurring during the effusive emplacement. Field-based cataloguing of the complexities of fracture surfaces provides qualitative constraints for the future mechanical modeling of effusive lavas.