CURVING CROSS JOINTS AND THE LITHOSPHERIC STRESS-FIELD IN EASTERN NORTH-AMERICA

Cross joints are late-formed nonsystematic fractures that extend across intervals between systematic joints. Traces of such cross joints are seen on bedding-plane surfaces of Devonian Catskill clastic sedimentary rocks of the Appalachian Plateau of western New York State, where the maximum horizontal principal stress (S(H)) is oriented approximately N65-degrees-E, as indicated by in situ stress measurements. Between pairs of closely spaced systematic joints, traces of cross joints are commonly planar and orthogonal to the preexisting joints. However, in the mid-region between some widely spaced systematic joints in western New York, cross joints strike parallel to the S(H) of the present lithospheric stress field, but then curve to abut the preexisting joints at right angles. A curving trace reflects a local perturbation of the regional stress field in the vicinity of preexisting joints, and the perpendicular termination indicates that the preexisting joints were open. Depending on their age, the strike of the mid-region of curving cross joints denotes the orientation of either the neotectonic stress field or its Tertiary predecessor in the North American lithosphere.