Preferred orientation of planar microstructures determined via statistical best-fit of measured intersection-lines: the ‘FitPitch’ computer program

The ‘FitPitch’ computer program allows characterisation of preferred orientation of planar microstructures in a rock from the orientations of their intersection-lines on different sections. Measured pitch or strike angles of such intersection-lines are fitted to the theoretical intersection-lines of a single model-plane, or a combination of two or three model-planes. The degree of fit is quantified in terms of the deviation between natural and theoretical intersection-lines, normalised by uniform data. A first application is described concerning porphyroblast inclusion trails whose orientation in 28 samples from the Variscan Iberian Massif was measured. Analysis of these data illustrates how the method can be used to quantify preferred orientation of relict foliations contained in a group of porphyroblasts and how this complements existing techniques for determining foliation intersection axes (FIA) in porphyroblasts. A second described application deals with previously published orientation data for microfracture trace lines in quartz grains of a Variscan granite in the Rhine Graben. Qualitatively established best-fit planes derived originally from this data are compared with the numerically established planes using the FitPitch program, and discussed in terms of the advantages and potential limitations of the method.