Constraints on rheology of obsidian lavas based on mesoscopic folds

The geometry of mesoscopic single and multiple layer folds in rhyolitic obs idian flows is investigated. Folds are composed of obsidian embedded in a m atrix of pumice. Folds form by buckling processes as indicated by discontin uous deformation between obsidian and pumice layers and by the geometries o f wavetrains. Buckling occurs through a continuum of styles controlled larg ely by the thickness ratio of pumice to obsidian (N). Styles of folds inclu de chevron, harmonic, polyharmonic, disharmonic and single-layer assemblage s. Harmonic and chevron folds are observed for small values of N. For large values of N, folds buckle independently of one another and form disharmoni c and single-layer assemblages. Wavelength-to-thickness ratios of single-layer folds are compared to theore tically predicted ratios for Newtonian and power law fluids as a means of e stimating shear viscosity ratios of obsidian and pumice. While all folds in dicate that bubble-free rhyolite is more viscous during flow than bubbly rh yolite, estimates of shear viscosity ratio based on Newtonian theory (simil ar to 10-500), may exceed estimates based on power law rheologies by more t han an order of magnitude. Newtonian buckling theory involves a number of s implifications and does not account for the possibly complex rheology of bu bble-bearing rhyolite. (C) 1999 Elsevier Science Ltd. All rights reserved.