Stress Analysis Using Deformation Lamellae in Quartz

Abstract

<p> Orientations of deformation lamellae in quartz and C-axes of quartz grains containing lamellae were measured at seven locations across a gently folded bed of the Oriskany Sandstone, central Pennsylvania. Lamellae so measured have an average inclination of 20°-30° to the basal plane, and are therefore categorized as "subbasal".</p> <p> Fabrics of poles to deformation lamellae and corresponding C-axis orientations suggest a paleostress field for the fold which had the arrangement; σ1 bedding parallel, σ3 bedding perpendicular, and σ2 fold axis subparallel. Relative magnitudes of the stresses were given by the condition σ1 > σ3. σ2 is believed to have varied in magnitude between σ1 and σ3 during fold development.</p> <p> Exact timing of formation of lamellae is uncertain. Textural evidence shows that they predate a later generation of compressional and tensional microfractures associated with the deformation. The best explanation is that they formed in early stages of layer parallel compaction when compressive stresses in all zones of the fold were greatest. Stress refraction may or may not have been operative.</p>