Displacement-distance diagrams illustrate how sedimentary rocks responded to fault propagation, and can be used to distinguish between alternative models for the evolution of fault-propagation folds.
A displacement-distance diagram of a structure at Eagle Rock, Virginia, has a triangular shape (ER in figure). The apex of the triangle is the point of maximum displacement; displacement decreases both up- and down-dip from this point. This can be explained by a fault which propagated in two directions from a central nucleation point. The displacement maximum (nucleation point) lies within a competent sandstone bed that is surrounded by interbedded rocks of moderate ductility and high ductility contrast. Changes in the slope of the profile are produced by lithologic variations between adjacent beds. Fault displacement rate in this and other field examples is interpreted to have decreased relative to fault propagation rate as the fault passed from competent to incompetent units.
The ER profile shape differs from those of fault-propagation fold (fpf, Figure) and fault-bend fold (fbf) models in which the straight-line plots are indicative of a constant rate of fault growth. Such plots are characteristic of natural fault-related folds in low ductility rocks with low ductility contrast.
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