Geology Reference
In-Depth Information
Fig. 7.31. Displacement along faults oblique to the fold trend. North-south lines are structure contours
labeled with elevations. a Pure vertical slip. The implied strike slip on the fold limbs ( arrows ) is incor-
rect. b Pure strike-slip displacement. The implied vertical slip ( U , D ) is incorrect. For 3-D views of
these maps see Fig. 7.17
produces apparent strike-slip displacements of the fold limbs for which the sense of
slip reverses at the fold hinge line (Fig. 7.31a) although the sense of throw is constant
across the fault. Strike-slip displacement of the folds produces throw that changes sense
along the fault (Fig. 7.31b) but the sense of strike separation is constant. Faults for
which the sense of throw reverses along the trend of the fault are called scissors faults.
Such faults may form by rotation around the points of zero separation or, as in Fig. 7.31b,
may be caused by the strike-slip displacement of folds.
7.7.2
Shape
The inferred shape of a fault is an important criterion in correlating fault cuts and in
validating fault interpretations. A valid fault surface is usually planar or smoothly curved
and has an attitude that is reasonable for the local structural style. Contours on fault surfaces
follow the same rules as contours on bed surfaces (Sect. 3.2) with the exception that the
fault contours can end in the map area where displacement on the fault ends (Bishop
1960). The dip of the fault is obtained from a structure contour map (Sect. 3.6.1).
Smooth contours on the fault surface demonstrate that the correlations between the
fault cuts are acceptable (Fig. 7.32). Because faults are typically planar or gently curved,
it is reasonable to assume that both the strike and dip of the fault are approximately con-
stant. Surface undulations, if present, are most likely to be aligned in the slip direction.
Unfaulted points or unfaulted wells near an inferred fault provide additional constraints
on the geometry because they must lie entirely within the hangingwall or footwall.
Negative evidence, e.g., the absence of a fault cut in a well the penetrates an inferred
fault plane, requires remapping the fault or faults. The dips implied by the contours
must be reasonable for the structural style. Very irregular contours on a fault surface
suggest an incorrect correlation of the control points. Folded faults are possible, espe-
cially thrusts, but must be consistent with the folds in the hangingwall and footwall.
The process of correlation of fault cuts and bed offsets into individual fault planes
based on shape can be made easier and faster with a fault template. A fault template is
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