Dynamics of Obsidian Flows Inferred From Microstructures: Insights From Microlite Preferred Orientations

The flow of obsidian lavas leads to crystal alignments that reflect both the accumulated strain and the type of flow across the surface. Microlite preferred orientations were used to investigate the emplacement dynamics, strain history, and the structural evolution of Obsidian Dome, eastern California. Measurements of three-dimensional microlite trend and plunge in samples from the flow front, dike, and conduit show: 1) flow directions along the dome margins 2) the flow type (e.g., pure versus simple shear) at the dome margins and, 3) the variation in strain as a function of position within the system. Microlites form well developed lineations in the plane of flow banding in all samples. Stereographic projections indicate that lineations trend normal to the flow front and shallowly plunge away from the margin. These results highlight a strong correlation between microlite trend the and bulk flow direction inferred from the geometry of the flow front (assumed to be normal to the flow front). The radial flow pattern indicated by measurements suggests that extrusion was from a roughly cylindrical vent. Orientation distributions may further indicate that radial spreading accompanied by flattening was the dominant mechanism for flow emplacement. Comparisons of measured orientation distributions with theoretically predicted distributions (e.g., Blanchard, 1979) suggest that microlite fabrics developed in a pure shear flow. Variance in microlite trend provides a measure of the amount of strain acquired during flow. Standard deviation in trend decreases from the conduit to the flow margins, reflecting progressive alignment of microlites during transport. Pure shear strain inferred from orientation distributions increases from approximately 0.5 in the conduit to about 1.5 at the flow front. The difference between these strains is a measure of the strain associated with flow emplacement. Such strain is similar in magnitude to that estimated (1.6) for horizontal spreading of a fluid whose volume is equal to that of Obsidian Dome. These techniques can be applied to interpret older dissected lavas where erosion has erased much of the original flow front or where larger scale structures indicative of flow directions are poorly preserved.