Field studies, analysis of side-looking airborne-radar data, and examination of proprietary seismic reflection profiles suggest that, although the Eastern Overthrust belt tectonic regime is thin-skinned, the fold plunges, wavelengths, and frequencies appear to be controlled primarily by a Precambrian basement fault system.

Field studies have shown that thrust faults generally predate or are contemporaneous with associated folds. Thus, the proximity of faults controls, to a large degree, the wavelength of the associated folds. In addition, because faults converge downward toward the major decollements, the shorter the distance between the decollement and the ground surface, the more numerous the faults and, consequently, the narrower the associated fold wavelength. Changed decollement levels, in turn, appear to be related to basement faults.

Abrupt changes in wavelength of folds along strike appear to indicate the presence of cross-strike (lateral) ramps that connect decollements at different stratigraphic levels. The position of lateral ramps appears to be controlled in turn by cross-strike faults in the Precambrian basement, as seen on proprietary seismic data.

A map of Mesozoic basins in the eastern United States shows that Precambrian highs between basins and east-west border faults are aligned with lateral ramps. Mesozoic reactivation is therefore indicated. Later reactivation is suggested by the fact that more than 35% of recent earthquakes are coincident with lateral ramps.

Many lateral ramps can be extrapolated seaward and are exactly coincident in strike and nearly coincident in spacing with transform faults. It is hypothesized that the basement faults acted as zones of least resistance along which modern transform faults developed during episodes of seafloor spreading.

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