A photogeologic method for determining the direction of horizontal dilation from patterns of en echelon fracturing

The direction of horizontal dilation in areas characterized by tensional tectonics can be determined from a statistical study of en echelon patterns of fracturing observed on aerial photographs. Relative, to a north-south dilation, nearly all directions of zones of dextral (right-lateral) en echelon fractures lie in the northeast quadrant, while those of sinistral (left-lateral) en echelon fractures concentrate in the
northwest quadrant. Statistically, directions of the two types of zones define unimodal frequency curves that intersect at about 90 degrees to,
and thus define the direction of, applied dilation. The method has been field tested twice and is believed to be suitable for any area
characterized by (1) generally unidirectional horizontal strain and (2) an adequate population of geologically contemporaneous fissures,
including roughly equal development of sinistral and dextral en echelon arrays. Where exposures and aerial photograph coverage are adequate, the direction can be determined quickly to within about 10 degrees accuracy without fieldwork. The method should be useful for preliminary structural studies, especially in inaccessible areas.