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Fault displacements measured in coal mines and from seismic data are used to develop a model describing the near-field displacements associated with an ideal, single normal fault. Displacement on a fault surface ranges from a maximum at the center of the fault to zero at the edge or tip-line. The tip-line is elliptical, with the shorter axis of the ellipse parallel to the displacement direction. Contours of equal displacement form concentric ellipses centered on the point of maximum displacement. Displacement gradients vary with fault size and with mechanical properties of host rock; fault radius to maximum displacement ratios range from 5 to 500. Plotting of displacement contour diagrams and knowledge of displacement gradients are useful in interpreting seismic reflectio data, both for quality control of interpretations and for quantitative extrapolation of limited data.
Displacements associated with faulting decrease systematically with increasing distance along the normal to the fault surface; this decrease is seen as reverse drag in both hanging wall and footwall. Hanging-wall rollover and tilting of the reflectors cannot be used to distinguish listric from planar normal faults; even where fault-block rotation can be demonstrated, neither listric fault geometry nor a flat detachment surface is geometrically necessary. Because faulting is accommodated by ductile deformation, rigid fault-bounded blocks cannot exist except in some special circumstances related to a free surface.
The displacements within the rock volume affected by a single fault are not simply related to regional extension. Apparent horizontal extension by faulting varies from one layer to another, and a significant proportion of the extension in a basin may be due to ductile deformation.
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