Boudinage in multilayered rocks under layer-normal compression: a theoretical analysis

Abstract

This paper presents a dynamic analysis of boudinage in multilayers of alternate brittle and ductile layers under layer-normal compression. Based on the mode of fracturing of individual brittle layers, boudinage is classified into three types: tensile fracture boudinage (Type 1), shear fracture boudinage (Type 2a) and extensional shear fracture boudinage (Type 2b). The layer-thickness ratio, T_r (=t_b/t_d), and the strength ratio, F (=T/2η^ε), between the brittle and the ductile units are the principal physical factors determining the type of boudinage. Type 1 boudinage develops rectangular boudins and occurs when T_r is low (<4.5) or F is high (>0.8). In contrast, Type 2a boudinage takes place when T_r is high (>8.5) or F is low (<0.5). The intermediate values of these factors delimit the field of extensional shear fracture boudinage. The square of fracture spacing or boudin width in Type 1 boudinage is linearly proportional to layer-thickness, whereas that in Type 2 boudinage shows a non-linear relationship with layer-thickness. The aspect ratio (A_r) of all the types of boudins is inversely proportional to layer-thickness ratio (T_r). However, Type 1 and Type 2 boudins, have contrasting aspect ratios, which are generally greater and less than 1, respectively.