Strain variations and three-dimensional strain factorization at the transition from the southern to the central Appalachians

Abstract

The transition between the southern and the central Appalachian foreland thrust system is a region subjected to at least two non-coaxial deformations, where typical two-dimensional analyses such as cross-section balancing or plane-strain retro-deformation cannot be applied. Three-dimensional factorization can simulate the non-coaxial strain history. Simulation results for compaction and layer-parallel shortening (LPS) were compared to finite strains of 16 quartz arenite samples from the Tuscarora Sandstone. Successful simulations indicate: in the southern Appalachians—30–35% total compaction by volume loss including a 4.5–9% E-W horizontal shortening during lithification, and 5% LPS along 150–330° by plane strain or axial symmetric flattening; in the central Appalachians—30–35% total compaction by volume loss including a 3–7% N-S horizontal shortening during lithification, and 10% LPS along 120–300° by plane strain or axial symmetric flattening; in the transition zone—30% compaction by volume loss including a 3% E-W horizontal shortening during lithification, 5% ‘southern’ LPS along 150–330° by axial symmetric flattening, and 5–10% ‘central’ shortening along 130° or 140° by plane strain or axial symmetric flattening at 10° to the normal ‘central’ trend. From the simulations, horizontal shortening in the southern Appalachians during lithification in the Mississippian was greater than the later Alleghanian deformation. This shortening probably represents a very early LPS developed in unlithified sand that required less deviatoric stress to deform than did the cemented quartz arenites during later tectonic deformation. Also, the oblique and weaker central tectonic LPS in the transition zone probably represents behavior at the limits of central Appalachian deformation. Finally, the LPS simulations indicate that axial symmetric flattening (oblate strain) is more successful than plane strain as the representative behavior, so caution is recommended in assuming plane strain as the dominant strain behavior in orogenic forelands.