Naturally deformed polymineralic rocks provide quantitative rheological information

Polyphase rocks are more common than monomineralic rocks, but their rheologies are less well constrained. We utilize a combination of strain markers, natural variations in mineral content, and microstructural observations to quantify the rheology of deformed and metamorphosed turbidite sequences from the Eastern Fold Belt in the Mount Isa Inlier, Queensland, Australia. At the outcrop scale, continuous deflection of quartz-rich veins records changes in shear strain magnitude that correspond to variations in mineralogy. Larger deflections (higher strains) are recorded in biotite- and muscovite-rich (phyllosilicate) domains relative to stratigraphically lower quartz- and feldspar-rich (quartzofeldspathic) domains. X-ray diffraction analyses of cores from a representative turbidite sequence show an increase in mica content (12–52 %) and a decrease in quartz and feldspar content (86–38 %) with increasing stratigraphic height. Electron backscatter diffraction analyses show a measurable crystallographic preferred orientation in quartz, indicating intracrystalline deformation in a quartzofeldspathic domain, which is absent in the adjacent phyllosilicate domain. Back-scattered electron images document evidence of solution-precipitation creep and grain-boundary sliding in a phyllosilicate domain, indicating deformation was accommodated by intercrystalline processes. Despite similar differential stress recorded by both domains, relative strain rates, calculated from ratios of shear strain, increase with mica content. A conceptual model that characterizes strain partitioning in layered systems quantitatively demonstrates the dependence of bulk deformation on the relative thicknesses and viscosity ratios of mineralogical domains.