It is a simplification that allows us to perform strain analysis on brittle structures such as fault populations.įig. In cases where fractures occur in a high number and on a scale that is significantly smaller than the discontinuity each of them causes, the discontinuities are overlooked and the term brittle strain is used. However, modern geologists do not restrict the use of strain to ductile deformation. The opposite, brittle deformation, occurs when rocks break or fracture. Ductile deformation occurs when rocks flow (without fracture) under the influence of stress. deformation where originally continuous structures such as bedding or dikes remain continuous after the deformation. The number of data within each subarea is indicated by “n”.īy definition, strain applies to ductile deformation, i.e. The plots show the variations within each subarea, portrayed by means of poles, rose diagrams, and an arrow indicating the average orientation. Lineation data from subareas defined in the previous figure. Note how the systematics is reflected in the stereonets.įig. (c) Homogeneous subareas due to kink or chevron folding. (b) Systematic variation in layer orientation measurements. (a) Synthetic homogeneous set of strike and dip measurements.
At a later stage it became clear that the folds were in fact non-cylindrical, with curved hinge lines, requiring modification of earlier models.įig 3, Synthetic structural data sets showing different degree of homogeneity.
This model made it possible to project folds onto cross-sections, and impressive sections or geometric models were created. For example, when the Alps were mapped in great detail early in the twentieth century, their major fold structures were generally considered to be cylindrical, which means that fold axes were considered to be straight lines. Our data will always be incomplete at some level, and our minds tend to search for geometric models when analyzing geologic information. Nevertheless, it may be necessary to make geometric interpretations of partly exposed structures. In most cases, however, natural surfaces are too irregular to be described accurately by simple vector functions, or it may be impossible to map faults or folded layers to the extent required for mathematical description. Shapes and geometric features may be described by mathematical functions, for instance by use of vector functions. Orientations of linear and planar structures are perhaps the most common type of structural data.