Fringe cracks and plumose structures in layered rocks: stepping senses and their implications for palaeostress interpretation

Many joint surfaces show en échelon arrays of microfractures (fringe cracks, F-joints, twist hackles) along fringe zones in continuity with plumose structures. In layered rocks, joints with two fringe zones showing opposite stepping senses at the top and bottom (type A) are more frequent than those showing a single stepping sense (type P). Consistent with such an arrangement, their curved advancing fronts show continuous, 180°-radial patterns of steps showing a ‘spiral-staircase’ geometry. In double plumose structures diverging from an initiation point, different combinations of symmetry elements allow us to define four basic stepping types: (type PA) single stepping sense on the full joint surface; (type AA) opposite stepping senses at both fringes; (type PP) opposite stepping senses at both sides of the starting point; (type AP) four alternate quadrants with opposite stepping. Most of these types have been observed in limestone beds of the Mequinenza area (Ebro Basin, Spain) and Glamorgan coast (South Wales). According to our observations, the varied stepping arrangements are independent of changes in orientation within the parent joint, the latter showing more persistent strikes than fringe cracks. All these features suggest that fringe crack arrays in continuity with plumose structures are related to local stress redistribution at the advancing front and, as a general rule, do not record remote stresses. After careful observation and discrimination, only those en échelon cracks consistently stepped and showing no geometric continuity with plume barbs should be included in palaeostress analysis.