Geology Reference
In-Depth Information
7. Repeat step 6 for the remaining working lines to find the shape of marker B in the
rollover (Fig. 11.49b).
In both the restoration and prediction techniques, the starting locations of the work-
ing lines are arbitrary. The requirement is that a unit thickness marked off along a
working line is always shifted laterally one displacement increment (
D
). The original
working lines may be spaced as closely as desired to retain the detail of the cross
section. This is also true for the oblique simple shear technique, discussed next.
11.7.2.2
Oblique Simple Shear
The oblique simple-shear technique for predicting the fault geometry from the shape
of a hangingwall fold is the same as the vertical simple-shear method but with ob-
lique working lines. The construction steps are as follows (refer to Fig. 11.50 for the
geometry).
1. Draw the regional.
2. From the hangingwall cutoff of the reference bed, draw a line parallel to the
direction of simple shear (
α
from the horizontal) to its intersection with the
regional.
3. The length
D
is the displacement of the hangingwall block necessary to produce
the observed geometry. Mark off equal distances
D
along the regional, starting from
the footwall cutoff.
4. At each displacement increment, draw a working line parallel to the shear direc-
tion from the regional to the vicinity where the fault is expected to be (lines 1-6).
5. Measure the length
t
1
. Shift this length in the displacement direction one
D
incre-
ment and move it down the shear line until the top of the line just touches the
reference bed in the rollover (location
t
1
'). The bottom of the line segment is the
location of the fault.
6. Repeat step 6 for the next line segment (
t
3
=
t
1
'+
t
2
), and continue to repeat in the
direction of displacement until the section is complete.
Fig. 11.50.
Oblique simple-shear prediction of fault shape (
heavy dashed line
) from rollover shape.
D
:
block displacement;
FWC:
footwall cutoff of reference bed;
HWC:
hangingwall cutoff of reference
bed; predicted fault is
heavy dashed line
