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Criteria for the differentiation of en echelons and hackles in fractured rocks

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DOI: 10.1016/0040-1951(86)90043-0


Abstract Straight en echelons and hackles are two morphological aspects of cracked surfaces in rocks. Although they often resemble each other, they result from different modes of fracture. Eight criteria enabling their differentiation are presented. En echelons commonly maintain consistent orientations of smooth surfaces with respect to the parent fracture, and uniform azimuths in individual outcrops along the rectilinear boundaries of the parent fractures. Hackles, generally appear as rough surfaces that do not follow such constant orientations and uniform azimuths, and they propagate from circular perimeters. Whereas en echelons characteristically alternate intermittently with steps, may have plumose markings on their surfaces, and have consistent length/width ratios, hackles do not show any of these features. Hackles are formed on a planar surface inclined to the parent fracture; such an angular relationship does not occur between en echelons and their parent joints. Fracture surface morphology also allows for differentiating between natural joints and man-made artificial fractures, since in natural joints the ratio mirror radius/average length of hackles is high, whilst it decreases considerably in man-made artificial fractures.

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