Chapter 12 - A foreland-type fold and related structures in the Newark rift basin

Abstract

The Jacksonwald synformal syncline at the eastern end of the Narrow Neck (connecting the Newark and Gettysburg basins) folds Carnian to Hettangian synrift strata. Like other en échelon folds in Mesozoic rift basins, the synform has characteristics of folds in the foreland of fold and thrust belts. The shallow plunging (15° towards 305°), upright synform has a subangular hinge and straight planar limbs, typical of chevron folds. Early formed clastic dikes were rotated towards the hinge, while mud cracks were stretched parallel to the hinge during buckling. An axial planar, spaced solution cleavage is well developed in siltstones and maintains a constant orientation throughout the fold, suggesting formation late in the fold history. Fibrous quartz and calcite were precipitated in veins (that consistently cut cleavage) oriented subperpendicular to the fold axis. The structural family associated with the Jacksonwald synform records shortening at a high angle to the basin border. Three sets of steeply dipping, predominantly strike-slip faults (on map to hand-sample scales) also deform the synrift strata. West-northwest-striking sinistral faults along the border and in the basin appear to be coeval with north-striking minor faults (showing both senses of shear). The early fault sets are cut by northeast-striking sinistral and dextral faults, both in and along the basin. The population of minor faults produced bulk plane strain with subhorizontal northeast shortening and northwest extension, essentially coaxial with the fold structures. A wide variety of tectonic models (including sinistral transtension, simple wrenching, and sinistral transpression) are considered which could produce the structural geometries in this part of the Narrow Neck.