Geology Reference
In-Depth Information
10.5
Unlikely or Impossible Fault Geometries
It is surprisingly easy to create interpretations that include impossible or at least very
unlikely 3-D fault geometries. Faults placed on single-horizon structure-contour maps
or on geological outcrop maps without regard to the implied 3-D fault shape are the most
likely to cause problems. The quality of a fault interpretation can be assessed using the
implied fault shape and the cutoff-line geometry of hangingwall and footwall. A final
quality-control issue is the distinction between a fault and an axial surface.
10.5.1
Fault Shape
Fault surfaces should always be mapped. The shape of an undeformed fault should be
planar, smoothly curved, or have a stair-step shape. A very irregular fault (Fig. 10.19)
probably cannot move and so is unlikely to be a correct interpretation unless the fault
has been folded by a later event.
Structure contour maps of faulted horizons are commonly shown without contours
on the faults. Contouring the implied fault surface, by matching elevations across the
fault, is a very simple and effective test of the interpretation. Incorrect interpretations
of the fault-bedding relationships will show unreasonable or impossible contour pat-
terns for the fault surface. The implied fault surface in Fig. 10.20 has a spiral shape,
possible perhaps, but not very likely. In more complex situations, crossing contours
may occur on the implied fault, an even less likely result.
Implied structure contours may reveal problems in addition to unlikely fault shapes.
In Fig. 10.21a the implied structures on the fault are parallel and so are reasonable. But
once the fault contours are extended across the map (Fig. 10.21b) a problem becomes
Fig. 10.19.
Map of two faults from the Deerlick Creek area (Fig. 7.32) interpreted as being a single fault.
Squares
are the positions of fault cuts,
contours
are on the fault surface.
a
Structure contour map, syncli-
nal shape is unreasonable.
b
3-D view to the NW of the inferred synclinal fault
