Microfabric reconstruction via quantitative digital petrographic image analysis for weakly foliated gneisses

Detection and documentation of petro-structural features such as alignment features of minerals/grains, and extraction of such spatial property data are two fundamental steps for structural geology. Such tasks were mostly carried out manually. However, manual analysis is laborious and potentially biased. These drawbacks are less obvious when the foliation is well developed and the amount of platy mineral is higher. For samples with weakly developed foliation and low platy mineral content, automatic method is required for subjective interpretation. A semi-automatic computerized method of 3D foliation orientation reconstruction via two-dimensional petrographic-shape fabric analysis from serial oriented digital microphotograph has been developed and demonstrated in this study. The foliation is reconstructed by fitting a best fitting plane to the maximum modal peaks of micro textural parameters (SPO) for different mineral groups of platy and granular minerals from each thin section on a stereonet for four coarse grained biotite gneiss samples collected along the Jialie fault, SE Tibet, China. Regardless of platy or granular mineral aggregates, the reconstructed foliations showed similar orientation within 10° angular variation to the field measurement. The 10° angular variation can be maintained if the foliations are reconstructed by consecutive thin section groupings ≦. 50° angular intervals and a horizontal thin section. The angular spread increased to 30° for thin section groupings with >. 50° to 100° angular intervals with a horizontal thin section. Major advantages of the computerized photometric methods demonstrated by this study are: the reduction of human prejudice and obtaining quantified and repeatable data sets.