Wavelength Selection In 3-D Decollement Folds of the Appalachian Plateau Province with Estimates of Rheological Parameters

Fold arclength, plan form, lithologic make-up, and measured ~10% layer-parallel shortening (LPS) of decollement folds in the Appalachian Plateau Province, Pennsylvania & New York, are interpreted in terms of a model for low limb-dip selective amplification to give estimates of rheological parameters. The large scale folding is treated as an interfacial, rather than internal, instability and mechanical layers are treated as incompressible, isotropic, nonlinear power-law fluids (stress exponent n, effective viscosity ). The 3-layer model consists of a stiff surface layer, a weak layer of decollement with a slip surface at its base, and an underlying layer that contains thick, relatively weak units but undergoes little deformation. Models are tailored for regions about two distinct folds, the Chestnut Ridge Anticline (CRA) & Firtree Point Anticline (FPA). CRA has the longer fold span (16.3 km to 9.4 km), proportionately thicker mechanical units, and thinner Salina Group salt. Fit to a set of criteria including (I) dominant wavelength Ld = mean fold span; (IV) low structural relief at the base of the decollement layer; and (VI) adequately strong folding instability (qd ≥ 20) yield, for the CRA & FPA, stiff layer to underlying layer effective viscosity ratios R1 = 81 and 147, R2 = 19 & 35. Stiff layer stress exponents n = 4.7 & 3.4 imply that the LPS occurs by distributed wedge faulting and pressure solution in ~ equal proportions. The larger R1 for the FPA reflects a decollement layer in a thicker, weaker Salina Group rather than a shale-dominated rocks. We extend a 2-d cylindrical folding model to 3-d to achieve a fit consistent with the plan form of the folds, and discuss stress distribution and its relationship to internal structures.