True and apparent geometric variability of en-échelon vein arrays

The wide range of vein-array angles reported for en-échelon vein arrays can be attributed to both true and apparent geometric effects. A survey of published figures supports a continuum of orientations rather than distinct populations related to different fracture mechanisms. The results of the survey indicate a modal veinarray angle of less than 45 ° which indicates area increase as described by a displacement vector oriented outward from the array. A stereometric analysis of the effects of exposure-plane orientation on the apparent vein-array angle is developed. Examples of vein arrays, from the Nambucca Slate Belt of eastern Australia, exposed on two differently-oriented surfaces are shown. These examples include non-profile exposures with apparent vein-array angles which are (1) less than, (2) greater than, (3) approximately equal to, and (4) of opposite shear sense to the true vein-array angle.