Natural and Experimental Boudinage and Pinch-and-Swell Structures

TLDR

Boudinage and pinch‑and‑swell structures reveal the relative plasticity, strength, and tension‑driven mineral growth in competent rock layers. The study describes examples of boudinage and pinch‑and‑swell structures and discusses their formation mechanisms. The authors model boudinage as one‑dimensional plastic compression with compensating multidimensional expansion of layered materials, and use mathematical analysis with field data on tensile strength and viscosity to estimate pressure gradients and flow rates. Boudinage is common in gneiss‑crystalline schist regions, indicating that one‑dimensional compression with compensating elongation is a widespread deformation mode in crystalline rocks.

Abstract

Examples of boudinage and pinch-and-swell structures are described and the mechanisms of formation discussed. One-dimensional plastic compression and compensating one- or two-dimensional expansion of layered cakes of putty, plasticene, and cheese result in structures strikingly similar to the natural boudinage and pinch-and-swell structure. Studies of boudinage and related structures give information about relative plasticity and relative strength of rocks. The commonness of boudinage structures in gneiss-crystalline schist regions shows that one-dimensional compression and compensating elongation in other dimensions is a very common mode of deforming crystalline rocks. The tension which this kind of plastic flowage creates in competent rock layers not only produces boudinage, or pinch-and-swell structure, but is also important in localizing the growth of minerals during metasomatism. Mathematical analysis of the boudinage evolution shows that field measurements combined with data for tensile strength of rocks and their viscosity are sufficient to estimate pressure gradients and rate of flowage connected with the formation of boudins.