Constraints on rheology of obsidian lavas based on mesoscopic folds

The geometry of mesoscopic single and multiple layer folds in rhyolitic obsidian ̄ows is investigated. Folds are composed of obsidian embedded in a matrix of pumice. Folds form by buckling processes as indicated by discontinuous deformation between obsidian and pumice layers and by the geometries of wavetrains. Buckling occurs through a continuum of styles controlled largely by the thickness ratio of pumice to obsidian (N ). Styles of folds include chevron, harmonic, polyharmonic, disharmonic and single-layer assemblages. Harmonic and chevron folds are observed for small values of N. For large values of N, folds buckle independently of one another and form disharmonic and single-layer assemblages. Wavelength-to-thickness ratios of single-layer folds are compared to theoretically predicted ratios for Newtonian and power law ̄uids as a means of estimating shear viscosity ratios of obsidian and pumice. While all folds indicate that bubble-free rhyolite is more viscous during ̄ow than bubbly rhyolite, estimates of shear viscosity ratio based on Newtonian theory (010± 500), may exceed estimates based on power law rheologies by more than an order of magnitude. Newtonian buckling theory involves a number of simpli®cations and does not account for the possibly complex rheology of bubble-bearing rhyolite. # 1999 Elsevier Science Ltd. All rights reserved.