Deformation Mechanics of Viscous Layered Materials

Viscous layered materials comprise alternating strata of differing mechanical competence that deform predominantly by flow rather than brittle failure. Under long-term loads and elevated temperatures or pressures, such layers accommodate strain through two principal modes: folding under compression and necking (boudinage) under extension. Folding arises from mechanical instabilities when a competence contrast exists between layers, producing buckle geometries whose wavelength and amplitude depend on layer thickness, viscosity ratio and applied stress. Necking involves localisation of extensional strain into pinch-and-swell structures, again controlled by layer thickness, rheology and strain rate. Both processes lead to structural softening, whereby the evolving geometry reduces the effective viscosity of the composite sequence even if the intrinsic material properties remain unchanged. Viscosity contrasts drive strain partitioning, with more competent layers accommodating long-wavelength buckles and less competent layers forming hinge-parallel flows or diffuse shear zones. Numerical models and analogue experiments have clarified how multi-scale interactions between layers give rise to fold refolding, coexisting fold and boudin assemblages, and the transition from viscous to elastoplastic behaviour during cooling and uplift. This framework underpins interpretations of salt tectonics, crustal migmatite domes, deep-sea sediment décollements and engineered multilayer polymers, highlighting the interplay between rheology, layering geometry and boundary conditions in both natural and industrial settings.

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Deformation Mechanics of Viscous Layered Materials publication trend

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Technical terms

Viscosity: Measure of a material’s resistance to flow under applied stress.

Newtonian viscous rheology: Deformation behaviour in which strain rate is directly proportional to applied stress.

Folding: Buckle instability in layered materials subjected to compression, resulting in sinusoidal or chevron geometries.

Necking (boudinage): Localised extensional instability in competent layers, producing pinch-and-swell structures.

Structural softening (geometric weakening): Reduction in effective viscosity of a layered sequence due to evolving fold or neck geometry without intrinsic property changes.

Viscosity ratio: Ratio between the viscosities of adjacent layers, a key control on wavelength and mode of deformation.

References

1. Folding and necking across the scales: a review of theoretical and experimental results and their applications. Solid Earth (SE) (2016).
2. Rheological stratification in impure rock salt during long-term creep: morphology, microstructure, and numerical models of multilayer folds in the Ocnele Mari salt mine, Romania. Solid Earth (SE) (2021).
3. Boudinage and folding of oblique single layers in bulk constrictional strain fields: Results from analogue modelling. Journal of Structural Geology (2024).