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

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. Plain Language Summary: The formation of mountain ranges such as the Alps is associated with large strain flow of fine‐grained rocks even at low temperatures (200–335°C) and shallow depths (5–8 km). Such ductile behavior represents an aseismic alternative (without earthquakes) in highly deformed zones, so it is important to understand and model it in order to predict the mechanical behavior of actively deforming regions. However, the mechanism and the parameters of the deformation remain enigmatic. Relying on grain shape analyses derived from microscopy techniques, we demonstrate that the deformation is accommodated by grain boundary sliding with locally stress‐driven dissolution in zones that oppose grain sliding and re‐deposition in zones that are stress‐relaxed by this sliding. By comparison with laboratory experiments, we show that this ductile process may operate in fine‐grained rocks, at much lower stresses than required for seismic deformation. We establish flow laws that describe the relation between strain rate and the parameters of the deformation, such as thermodynamic conditions, stress, distance and characteristics of mass transfer along grain boundaries. We show that, at low differential stress and slow strain rate, diffusive mass transfer along grain boundaries could be much slower than when measured in laboratory experiments at high stress and strain rate. Key Points: During the Alpine orogeny, calcareous shales transformed to slate schists accommodated large ductile deformation at shallow depth (5–8 km)Microstructural studies reveal near‐equant fine grains and mass transfers implying grain‐boundary sliding mechanism in superplastic flowModeled deformation maps show that diffusive mass transfer properties may be slower in nature than in laboratory experiments [ABSTRACT FROM AUTHOR]