Comparative strain analyses in a welded ignimbrite: an assessment of welding intensity variables and rankings

Welding is the plastic deformation of pyroclastic material in response to high post-emplacement temperatures, load stresses, and to a lesser extent, retained volatile content. There are few field studies pertaining to the use of physical properties in the quantification of strain in welded ignimbrites, and even fewer that are contemporaneous with modern thermo-mechanical modeling of silicates. Accordingly, this study is a review of several methods for measuring strain in welded ignimbrites applied to the same deposit. The Resting Springs Pass tuff provides an excellent case-study for understanding the distribution of strain in welded ignimbrites. First-order petrofabric (strain measurements at different scales [field and thin section]) and petrophysical data (porosity) were collected to map the vertical distribution of strain throughout a single deposit and to assess the qualities of each of the data-sets and the strength of correlations between them Accurate measurement of strain in the field and laboratory, while possible, is shown to be met with many practical challenges. Results from this study confirm the validity of using pumice clasts in a two-dimensional strain analysis. Volume strain and shear strain measurements are shown to be positively and well-correlated for much of the RSPT and presumably for similarly moderately welded ignimbrites. Only in the most strained, lowest porosity zone is there a significant difference between volume and shear strain: this zone is a black rheomorphic vitrophyre that suggests a different welding history from the rest of the RSPT.