FRACTURING AND REMOBILIZATION OF OBSIDIAN LAVA FLOW

Flow dynamics of rhyolite lava is not well understood because they occur infrequently. This means that geological investigation is important to clarify the flow dynamics. In this study, we show varied fracturing structures of obsidian lithofacies and discuss about those development. The late Pleistocene Sanukayama rhyolite lava is distributed on Kozushima Island located about 180 km south of Tokyo, Japan. The bulk chemical composition is 76.49 wt.% SiO₂. The exposed thickness is about 130 m, and the section is considered to correspond to the upper half of the lava.

The obsidian lithofacieswith about 40–60 m thick is overlain by the upper pumiceous carapace. Fractures, which are featured by white color, are frequently developed within the obsidian.Microscopic observation shows that the deformed glass fragments filling the fractures are always strongly welded. The geometry of the fracture networks sometimes exhibits typical shear fracture zone characteristics such as Riedel shear structures. The fractures are often elongated with a few meters long.Welding of the fractures indicates that the fracturing occurred above the glass transition temperature.Since Brittle and ductile behaviors are also governed by strain rate (Dingwell, 1996), the obsidian would have fractured under high strain-rate conditions (Dingwell, 1996; Tuffen and Dingwell,2005) induced by the flow. After the strain rate was decreased, the fractures would have easily become welded and remobilized, consequently elongated fractures would be developed.

A nearly horizontally clastic layer <80 cm wide and >20 m long is exceptionally developed. The layer slightly undulates, indicating ductile deformation after the layer formation. The internal part of the layer is composed of angular obsidian clasts and a white-colored welded matrix. The white-colored matrix is composed of abundant broken spherulites. We consider that the layer was formed by in situ shear fracturing within the active lava. Since spherulites are usually developed under large undercooling, broken spherulites means that the clastic layer was formed after most of the lava flow has ceased. These results indicate a probability of remobilization of the obsidian lava after emplacement.