Pinch and swell structures: evidence for strain localisation by brittle–viscous behaviour in the middle crust

Abstract. The flow properties of middle crustal rocks are commonly represented by viscous flow. Examples of pinch and swell structures found in a high strain zone at St. Anne Point (Fiordland, New Zealand) and Wongwibinda (N.S.W., Australia) suggest pinch and swell structures may be initiated by brittle failure of the more competent layer in conjunction with subsequent material softening. On this basis we develop a numerical model where Mohr–Coulomb constitutive strain localising behaviour is utilised to initiate pinch and swell structure development. Results show that pinch and swell structures develop in a competent layer in both Newtonian and non-Newtonian flow, provided the competent layer has sufficient viscosity contrast and can localise strain to form shear bands. The flow regime and strain localising characteristics of the surrounding country rock appear not to impact pinch and swell structure formation. The degree of material softening after the initial strain localising behaviour is shown to impact pinch and swell characteristics, while extensive material softening causes the formation of thick necks between swells by limiting the focused localisation of strain into shear bands. To aid analysis of the structures and help derive the flow properties of rocks in the field, we define three stages of pinch and swell development and offer suggestions for measurements to be made in the field.

Our study suggests that Mohr–Coulomb strain localising behaviour combined with viscous flow is a viable alternative representation of the heterogeneous rheological behaviour of rocks seen in the middle crust. This type of mid-crustal rheological behaviour can have significant influence on the localisation of strain at all scales. For example, inclusion of Mohr–Coulomb strain localising behaviour with viscous flow in just some mid-crustal layers within a crustal-scale model can result in significant strain localisation, extending from the upper crust into the middle crust. This localisation also influences the development of near-surface structures.