Localization of plastic zones in rocks around rigid inclusions: insights from experimental and theoretical models

Abstract

Employing analogue and numerical experiments, we investigated the process of plastic creep in the vicinity of stiff inclusions and its role in the formation of shear zones. Analogue experiments were performed on Polymethylmethacrylate (PMMA) models in pure shear ( ε≈10^-4 s^-1), which produced shear zones at a bulk strain >0.05. The geometrical dispositions of the shear zones do not conform to the stress concentration map derived from the plane theory of elasticity. At the initial stage ( ε_b < 0.03), PMMA models began to deform plastically in four discrete strain localizations, tracking the stress concentration map. These incipient plastic locations develop a new stress field, diverting the zone of plastic yield in the form of multiple shear zones. Finite element models were run to demonstrate the formation of shear zones in this mode. The pattern of shear zones varied with the inclusion geometry. Inclusions of low aspect ratio (<1.5) gave rise to multiple sets of shear zones in their neighborhood. The multiplicity of shear zones tends to progressively decrease toward a single set of conjugate zones when the inclusions have relatively high aspect ratio (>2) and are oriented at an angle (> 20°) to the bulk compression direction. Inclusions with a large aspect ratio (> 4) developed a single dominant shear zone. The experimental findings can be compared to inclusion-controlled shear zones from naturally deformed rocks.