Geology Reference
In-Depth Information
Fig. 11.19.
Area-depth relationships in a
fold having displacement on
an upper detachment.
a
Fault-
bend fold model (after Suppe
1983).
b
Area-depth diagram
of
a
(modified from Epard
and Groshong 1993).
D:
dis-
placement on lower detach-
ment;
d
*
:
slope of line for ho-
rizons stratigraphically above
the upper detachment;
d:
dis-
placement on upper detach-
ment;
h
u
:
elevation of upper
detachment
upper detachment. Below the upper detachment, no material is translated out of the
section and the area-depth line is the same as for a locally balanced structure (Eq. 1.23).
For units stratigraphically above the upper detachment, the area-depth line is
h
2
=[1/(
D
-
d
)]
S
2
+
k
,
(11.28)
where
h
2
= the distance from the reference level for units above the upper detachment,
S
2
= the excess area of units stratigraphically above the upper detachment,
d
= displace-
ment on the upper detachment,
k
=
d
h
u
/(
D
-
d
), and
h
u
= the elevation of the upper
detachment. This is the equation of a line of slope
d
*=1/(
D
-
d
) that is valid for
h
>
h
u
.
The excess areas of a fault-bend fold are thus distributed along two line segments
(Fig. 11.19b). The lower-slope segment is valid for
h
<
h
u
and corresponds to Eq. 11.23.
The upper segment is valid for
h
>
h
u
and corresponds to Eq. 11.28. The displacement
on the upper detachment is the difference in the slopes of the two area-depth lines:
d
=
D
-(
d
*)
-1
.
(11.29)
Failure to recognize the presence of an upper detachment and taking into consid-
eration only measurements from above the upper detachment will lead to a significant
overestimation of the depth to detachment (lower dashed projection line on Fig. 11.19b).
If the lower detachment is known to be shallower than the prediction, then an upper
detachment is indicated.
