K. Nakamura Wendell A. Duffield 1973 <p>The direction of horizontal dilation in areas characterized&nbsp;by tensional tectonics can be determined from a statistical study of en&nbsp;echelon patterns of fracturing observed on aerial photographs. Relative,&nbsp;to a north-south dilation, nearly all directions of zones of dextral&nbsp;(right-lateral) en echelon fractures lie in the northeast quadrant, while&nbsp;those of sinistral (left-lateral) en echelon fractures concentrate in the<br />northwest quadrant. Statistically, directions of the two types of zones&nbsp;define unimodal frequency curves that intersect at about 90 degrees to,<br />and thus define the direction of, applied dilation. The method has been&nbsp;field tested twice and is believed to be suitable for any area<br />characterized by (1) generally unidirectional horizontal strain and (2)&nbsp;an adequate population of geologically contemporaneous fissures,<br />including roughly equal development of sinistral and dextral en echelon&nbsp;arrays. Where exposures and aerial photograph coverage are adequate,&nbsp;the direction can be determined quickly to within about 10 degrees&nbsp;accuracy without fieldwork. The method should be useful for preliminary&nbsp;structural studies, especially in inaccessible areas.</p> application/pdf en U.S. Geological Survey A photogeologic method for determining the direction of horizontal dilation from patterns of en echelon fracturing article