STYLE AND TIMING OF FOLDS IN SEACOAST NEW HAMPSHIRE: AN APPLICATION OF FOURIER SERIES ANALYSIS

The application of a variant of the Fourier Series Analysis (FSA) to folds identifies a common folding event that affected the Silurian-Ordovician Merrimack Group (MG), the Late Proterozoic (?) Rye Complex (RC), and the boundary between them in the seacoast region of New Hampshire. Regardless of the scale, the FSA requires the values of three amplitudes measured at 30° intervals in each of two “quarter wavelength” units of an antiform, and describes fold styles in terms of asymmetry and shape. These measurements are mathematically sufficient to describe the shape of each “quarter wavelength”. The difference in amplitudes between these units describes the asymmetry, and a combination of them describes the shape of the antiform.

The fold style pertinent to this study has a sinusoidal to chevron shape, and it is moderately to highly asymmetrical. In the MG, these folds have northeast-striking axial planes and plunge mostly to the southwest at a low angle, verge to the southeast, and have associated crenulation-lineations on the limbs. While the RC folds along the coastline are tight and smaller, they also have northeast-striking sub-vertical axial planes parallel to the main foliation but plunge slightly more to the west. Based on published radiometric ages (Lyons et al., 1997) and this folding stage, a relative sequence of events is described as: initial accretion and faulting of the MG against the RC along the Portsmouth Fault Zone, subsequent folding, and the emplacement of Late Silurian to Early Devonian intrusive bodies suggesting Acadian deformation for both blocks.