Significance of Layer-Parallel Slip During Folding of Layered Sedimentary Rocks
The Greenport Center syncline is an asymmetric fold with a half wave length of about 40 m developed in the Devonian Becraft and Alsen Limestones on Becraft Mountain, New York. Twinning strains have been measured in 19 samples distributed so as to sample all of the structurally significant regions of the fold. The orientation and relative magnitudes of the observed strains are consistent with those expected from the buckling of a thick isotropic layer, but the observed magnitudes are too small by a factor of more than four. Local and nonsystematic variations in strain are much larger than those expected in the buckling of a continuous layer, and the Greenport Center syncline also contains abundant geologic evidence that slip between beds was important.

A theoretical model of the folding of a free, anisotropic, linearly viscous layer of appropriate wave length/thickness ratio is analyzed. Although the smoothly distributed bedding-parallel shear of the model does not adequately represent the process of bedding slip in the real fold, it does permit the estimate of the degree of anisotropy necessary to produce observed limb dips with observed bending strains. If the viscosity coefficient for shear parallel to bedding is 0.03 times that for bending, the bending strains are reduced to the observed value; the required bedding slip could be produced by slip surfaces spaced about 30 cm apart if the average displacement on a slip surface were about 23 cm. The model of inherited asymmetry is both appropriate for the Greenport Center syncline and treatable within the present theoretical framework. Inherited asymmetry is shown to be possible for an anisotropic layer though not for an isotropic one.