STRUCTURAL ANALYSIS OF THE MARGINS OF AN OBSIDIAN LAVA: INSIGHTS INTO FLOW MECHANISMS AND THE FORMATION OF FLOW LOBES

Eruptions of silicic lava are rare but two eruptions in Chile between 2008 and 2013 revealed the importance of endogenous growth and the advance by marginal flow lobes. The hazard potential of silicic lavas is largely unstudied, especially compared to basaltic lavas. In contrast, many studies of ancient lavas that have emphasized exogenous processes. Structural analysis of foliations (flow banding, lithological layering) and lineations (stretched vesicles and fold hinges), combined with estimates of finite strain can constrain deformation style, direction, relative timing, and intensity, and therefore, discriminate between endogenous and exogenous emplacement mechanisms. Previous field studies have focused on the mapping and description of surface deformation features rather than that around the margins, and strain analyses only exist from microlites and micro-vesicles that chiefly record conduit flow before effusion. We present structural data from twelve, approximately 30 m-long, vertical transects through the northwestern margins of the 600 year-old Obsidian Dome (Long Valley caldera, eastern California) in order to determine the flow mechanisms responsible for emplacement during, at least, the final stages of advance. We have identified potential flow lobes, elliptical in cross-section, around the margins and use our structural data to reconstruct probable flow paths: where these diverge over short distances likely indicates the boundary between two lobes. Strain markers are ubiquitously non-coaxial or plane strain indicating a strong sub-horizontal shearing component to the deformation and limited vertical flattening; this supports our interpretation of endogenous, non-coaxial flow within flow lobes, rather than ‘tank-track’ overflow and settling during exogenous emplacement. Confirmation of the presence of flow lobes in Obsidian Dome, the best studied silicic lava, will require a reassessment of the hazards posed by some or all silicic lavas that form by endogenous rather than exogenous growth, because flow lobes have the potential for rapid transport of hot, pressurized magma to the flow front leading to marginal explosions and the generation of pyroclastic density currents.