OBSIDIAN DOME, CALIFORNIA: A COMPLEX RECORD OF EMPLACEMENT OF A YOUTHFUL LAVA DOME

Obsidian Dome is a 1.5 by 1.8 km extrusion of high silica rhyolite situated along the Inyo Craters in eastern California, adjacent to the Long Valley Caldera. Erupted 550-650 years ago, it is one of the youngest centres of silicic volcanism in North America. Obsidian Dome erupted in a semi-arid environment and because of this, much of the dome remains in pristine condition making Obsidian Dome an ideal centre to document a menu of primary structures and flow features that typify silicic domes.

Field observations, coupled with microscopic analysis reveal an ensemble of structures that transcends from macroscopic scale (e.g. compression folds and crease structures 10s of metres in size) to microscopic scale (i.e., deflection of flow banding around phenocrysts) which together, define the flow field of Obsidian Dome. Millimetre-scale flow-banded obsidian, pumice and rhyolite range from planar to chaotically folded, the latter a product of ductile, compressive deformation. Fractures, some of which display en-echelon splitting patterns are a result of brittle deformation. Taken together, these structural features, along with others, result from flow during lava dome growth. Our interpretation based upon the available field evidence suggests a complex process of emplacement that is signified by vesiculation, crystallization and repeated brittle-ductile deformation, owing to episodic crossing of the glass transition.