Geology Reference
In-Depth Information
continues along the plane of the younger fault, between the traces of the cutoff lines
of the older fault against the younger fault (Figs. 8.39b, 8.40). Other horizons will have
their zones of combined separation along the same trend, but offset from one an-
other. Given low bedding dips, a vertical well drilled into the zone of combined sepa-
ration for two normal faults will cut one fault that carries the combined separation
(Fig. 8.40a). A vertical well drilled into the zone of combined separation for two re-
verse faults will have three fault cuts (Fig. 8.40b), the middle one of which will carry
the combined separation.
The amount of the combined separation is given by a relationship from Dickinson
(1954). If the younger fault (B, Fig. 8.40a) is normal,
t
=-
b
+(±
a
) ,
(8.5)
and if the younger fault (B, Fig. 8.40b) is reverse,
t
=+
b
-(±
a
) ,
(8.6)
where
t
= combined stratigraphic separation,
a
= stratigraphic separation on the older
fault,
b
= stratigraphic separation on the younger fault, the “+” sign indicates reverse
separation = thickening, and the “-” sign indicates normal separation = thinning. The
simplicity of this result is due to the fact that all the beds have the same dip. If dip
changes occur across the faults but are small, then Eqs. 8.5 and 8.6 will still provide
good estimates.
The map pattern produced by crossing normal faults depends on the angle of
intersection of the faults, the dip of the faults and the marker horizon, and the
magnitude and sense of slip of the faults. The trace of the fault on the marker hori-
zon depends on the dip of the marker as well as on the attitude of the fault surface.
The following examples illustrate some of the possibilities. In the three parts of
Fig. 8.41, the orientation of the older fault (A) is changed while the orientation of
the younger fault (B) and the attitude of the marker horizon remain constant. The
structure changes from a geometry that could be described as tilted steps (Figs. 8.41a,c)
to a horst that changes into a graben along a northwest-southeast trend (Fig. 8.41b).
Note that the direction of the fault-bedding intersection is not parallel to the fault
strike in any of the examples because the strike of bedding is not parallel to the strike
of the faults.
The width of the zone of combined stratigraphic separation is reduced to a line for
certain combinations of the displacement directions (Fig. 8.42a,b). An apparent strike
separation on the older fault will be caused by slip on the younger fault that is in the
strike direction of the younger fault. In Fig. 8.42c the later, through-going fault (B)
appears to be offset by left-lateral strike-slip on fault A, although it is, in fact, fault B
that displaces fault A.
The development of the map pattern of cross-cutting reverse faults is illustrated
with a forward model (Fig. 8.43). The first fault (A) trends obliquely across the north-
westerly regional dip of the marker (Fig. 8.43a). The trace of the second fault (B) is
found by intersecting the contours of the fault with the displaced marker surface
