En Echelon Folds: A Case History from Jacksonwald Syncline of Foreland Folds in Newark Rift Basins of Eastern North America: ABSTRACT

Abstract

The Jacksonwald syncline of the Newark rift basin, like other en echelon folds in both the onshore and offshore Triassic-Jurassic basins, has characteristics typically found in foreland fold and thrust belts. The syncline, comprising Carnian to Sinemurian strata, is a shallow plunging synform (15° toward N55°W) with an axial surface dipping 83° toward S35°W. The fold has a subangular hinge with straight planar limbs, typical of buckle folds. Axial planar, spaced solution cleavage is well developed in siltstones. Although early clastic dikes in the border fanglomerates are rotated, the cleavage is unfanned and maintains a constant orientation. Quartz and calcite, dissolved along cleavage seams, were precipitated in extensional veins oriented perpendicul r to the fold axis and to elongate mud cracks. Conjugate shear fractures near the postulated border fault strike N79°W and N5°W, and show sinistral and dextral slip, respectively. Displacement along these shears indicates that sinistral slip is 5 times greater than dextral. Along the basin margin, tectonic layer-parallel shortening by pressure solution exceeds compaction shortening by 2:1. The fold is intruded by an undeformed Sinemurian diabase dike of Rossville composition, parallel to the extension veins. The data show a consistent strain pattern, from the Upper Triassic through the Lower Jurassic, of northeast shortening and northwest extension, and suggest that en echelon folds formed along an east-trending sinistral shear couple. This stress field is consistent with dextral oblique slip along the Flemington fault, observed by Burton and Ratcliffe. End_of_Article - Last_Page 1440------------