Geology Reference
In-Depth Information
1.6.5
Fault-Fold Relationships
A planar fault with constant displacement (Fig. 1.39a) is the only fault geometry that
does not require an associated fold as a result of its displacement. Of course, all faults
eventually lose displacement and end. A fault that dies out without reaching the sur-
face of the earth is called blind, and a fault that reaches the present erosion surface is
emergent, although whether it was emergent at the time it moved may not be known.
Where the displacement ends at the tip of a blind fault, a fold must develop (Fig. 1.39b).
Displacement on a curved fault will cause the rotation of beds in the hangingwall and
perhaps in the footwall and will produce a fold (Fig. 1.39c). A generic term for the fold
is a ramp anticline. The fold above a normal fault is commonly called a rollover anti-
cline if the hangingwall beds near the fault dip toward the fault.
Fault dips may be controlled by the mechanical stratigraphy to form ramps and
flats, although at the scale of the entire fault, the average dip may be maintained (i.e.,
30° for a reverse fault). A flat is approximately parallel to bedding, at an angle of say,
Fig. 1.39.
Relationships between folds
and faults.
a
Constant slip on
a planar fault does not cause
folding.
b
Slip on either a plane
or curved fault that dies out
produces a fold in the region
of the fault tip.
c
Slip on a
curved fault causes folding in
the hangingwall
Fig. 1.40.
Ramp-flat fault terminology.
HW
is hangingwall;
FW
is
footwall.
a
Before displace-
ment.
b
After reverse displace-
ment.
c
After normal displace-
ment. (After Woodward 1987)
