Reaction-enhanced ductility: The role of solid-solid univariant reactions in deformation of the crust and mantle

Abstract

In collision belts and subduction zones, penetrative deformation of thrust sheets may be catalyzed by metamorphic reactions if the reactions have a significant effect on the mechanical properties of rocks. Reaction-enhanced ductility may result if a reaction produces a significant decrease in grain size, thus allowing the rock to deform superplastically. The product grain-size of a reaction is a function of the ratio of nucleation and growth rates. This ratio varies along any pressure-temperature-time ( PTt) path, and only attains a significant value at a certain distance from an equilibrium boundary. A comparison of rates of change of P and T along typical segments of PTt paths for crustal rocks leads to the conclusion that a high nucleation rate, a small grain size and resulting superplastic deformation are more likely to be achieved when reaction boundaries are crossed during underthrusting rather than during the thermal readjustments of a perturbed geotherm following crustal thickening, or during uplift. An example of reaction-enhanced ductility occurs in eclogite-facies metagranitoids in the Sesia-Lanzo Zone, Western Alps, where medium-grained plagioclase reacted to fine-grained Jadeite + quartz during early Alpine underthrusting. Microstructural evidence from shear zones shows that the Jadeite + quartz aggregates were highly ductile and it is inferred that the small grain size allowed superplastic deformation. Further evidence suggests that this process operated during regional deformation of metagranitoids (Rubie, 1982). The reaction of olivine to spinel or the beta phase by metastable overstepping of the two phase stability fields in a subducting plate (Ringwood, 1975) may result in fine-grained products and thus may be analogous to the albite → Jadeite + quartz reaction. The consequent decrease in effective viscosity is likely to have important consequences for plate dynamics at and below the depths of major phase transitions.