Dynamics of obsidian flows inferred from microstructures: insights from microlite preferred orientations

The flow of obsidian lava leads to crystal alignments that reflect both the accumulated strain and the type of flow across the surface. Microlite preferred orientations are used to investigate the emplacement dynamics, strain history, and structural evolution of Obsidian Dome, eastern California. Measurements of three-dimensional microlite trend and plunge in samples from the near-vent region, distal flow, and flow front show: (1) the flow directions along the dome margins, (2) the deformation style (e.g., pure versus simple shear) at the dome margins, and (3) the variation in strain as a function of position within the flow. Microlites form well-developed lineations in the plane of flow banding in all samples. Stereographic projections indicate that lineations trend normal to the western flow front and plunge shallowly away from the margin. The radial flow pattern indicated by measurements made along the western margin suggests that extrusion was from a roughly elliptical vent. These results highlight a strong correlation between microlite trend and the bulk flow direction inferred from the geometry of the flow. Along most of the eastern periphery, lineations trend parallel to the margin and likely reflect the local flow direction as influenced by compression against the thickening flow crust, marginal talus piles, and topography. Orientation distributions imply that radial spreading accompanied by flattening was the dominant mechanism for flow emplacement. Comparisons of measured orientation distributions with theoretical predictions suggest that microlite fabrics in flow front and near-vent samples developed in a pure shear flow. Microstructures in a sample from near the distal flow base records a component of simple shear. Variance in microlite trend provides a measure of the amount of strain acquired during flow. Standard deviation in trend decreases from the near-vent region to the flow margins, reflecting progressive alignment of microlites during transport. Pure shear strain inferred from orientation distributions increases from approximately 0.3 near the vent to about 1.1 at the flow front. The difference between these strains (0.8) is an estimate of the strain associated with flow emplacement. Such strain is similar in size to that estimated from mesoscopic structures (∼1) and for horizontal spreading of a fluid whose volume is equal to that of Obsidian Dome (∼1.6). This suggests that flow on the surface was sufficient to produce observed microlite preferred orientations in the flow front. 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.