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

This paper presents a dynamic analysis of boudinage in multilayers of alter nate brittle and ductile layers under layer-normal compression. Based on th e mode of fracturing of individual brittle layers, boudinage is classified into three types: tensile fracture boudinage (Type I), shear fracture boudi nage (Type 2a) and extensional shear boudinage (Type 2b). The layer-thickne ss ratio, T-r (= t(b)/t(d)), and the strength ratio, F (= T/2 eta epsilon), between the brittle and the ductile units are the principal physical facto rs 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 contra st, 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 extensio nal shear fracture boudinage. The square of fracture spacing or boudin widt h in Type 1 boudinage is linearly proportional to layer-thickness, whereas that in Type 2 boudinage shows a non-linear relationship with layer-thickne ss. The aspect ratio (A(r)) of all the types of boudins is inversely propor tional 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. (C) 2000 Elsevier Science Ltd. All rights reserved.