Geology Reference
In-Depth Information
Fig. 7.20.
Fault heave (
H
) and throw (
T
) at a fault cut (point
P
1
).
S
d
=
dip separation;
H
=
L
cos
φ
;
T
=
L
sin
φ
;
t
stratigraphic separation = fault cut;
angle between the fault dip and the pole to the
cross-fault bedding attitude at
P
2
. The marker horizon is
shaded
φ
dip of the fault;
ρ
where
H
= heave,
T
=throw,
t
= stratigraphic separation,
φ
= the dip of the fault, and
ρ
= the angle between the cross-fault bedding attitude and the dip vector of the fault.
If bedding is horizontal, its pole is vertical, causing
ρ
to be equal to 90 -
φ
, and Eqs. 7.5
and 7.6 reduce to
H
=
t
/tan
φ
,
(7.7)
T
=
t
.
(7.8)
In the case of zero dip of bedding, and only in the case of zero dip of bedding, the
stratigraphic separation is equal to the fault throw and equal to the vertical separation.
As an example of the calculation, consider a fault having a stratigraphic separation
of 406 m, the cross-fault bedding attitude is
= 37, 220.
The pole to bedding is found to be 80, 360 and the angle between the pole to bedding
and the fault dip is
δ
= 10, 180, and the fault dip is
φ
= 60°. From Eq. 7.5,
H
= 648 m, and from Eq. 7.6,
T
= 489 m. These
values are derived for the fault in Fig. 7.16 and could have been caused by any number
of different combinations of net slip magnitudes and directions.
Neither the stratigraphic thickness nor the attitude of bedding are necessarily the
same on both sides of a fault, which is important when the throw and heave are calcu-
lated from Eqs. 7.5 and 7.6. The appropriate dip and thickness values are always the
cross-fault magnitudes found in the block across the fault from the marker horizon at
the fault cut. In subsurface mapping based only on formation tops and fault cuts, it is
likely that none of the required dips will be known at the early stage of mapping. This
means that throw and heave cannot be calculated accurately at this stage. Throw and
heave can be estimated for preliminary mapping purposes by estimating the dip of the
fault and the dip of cross-fault bedding. If no other information is available, normal-
separation faults can be estimated to dip 60°, reverse separation faults to dip 30°, and
bedding can be assumed to be horizontal. As the required dips are determined by
mapping, the throw and heave magnitudes can be corrected.
ρ
