Pressure Solution Grain Boundary Sliding as a Large Strain Mechanism of Superplastic Flow in the Upper Crust

Abstract A mechanism accommodating large strain in superplastic flow with elongation of 100%–300%, is described for fine‐grained calcareous shales deformed at temperature of 200–335°C and depths of 5–8 km up to slate schists in the Oisans massif, Western Alps, France. Using electron microscopy techniques on thin sections parallel to the principal finite strain axes, we show that the shape ratios of the slightly elongated grains (1.4–1.6), mainly calcite and quartz, do not match the finite strains recorded by the markers of the deformation (truncated belemnites, folded veins) in the maximum elongation and shortening plane (ratio 6.7) or in the maximum and minimum elongations plane (ratio 2.4). Consequently, a grain boundary sliding mechanism is required to explain the measured large finite strains. The most soluble minerals (quartz, calcite, dolomite, and albite), which represent about 95% of the rock, accommodate deformation by pressure solution grain boundary sliding whereas the least soluble minerals (muscovite, chlorite, Fe‐Ti oxides) act as indenters or are passively reoriented. Pressure solution is especially efficient in polymineralic rocks. Soluble minerals, which have been healed together in veins or fossils, are much more resistant to deformation and act as rigid objects. Models with idealized tessellation of hexagonal grains and creep laws derived from pressure solution indentation experiments provide deformation maps. We discuss the main parameters of this ductile deformation in the upper crust (thermodynamic conditions, strain rate, stress, distance of mass transfer) and show possible drastic decrease of mass transfer efficiency with decrease of stress and strain rate.