Geology Reference
In-Depth Information
11.8
Fault-Parallel Simple Shear
Fault-parallel displacement in the hangingwall is modeled with a slight modification
of the vertical simple shear model (Williams and Vann 1987). The method uses a vari-
able distance between working lines so as to maintain constant slip on the fault as it
changes dip. This method approximates the hangingwall geometry developed above a
listric normal fault in which there is nearly rigid rotation above the circular part of the
fault, rigid-block displacement above the bedding-parallel fault segment, and a key-
stone graben between the rotated block and the translated block.
11.8.1
Restoration
The method for restoring a cross section by the constant dip separation model was
developed by Chai (1994). The construction procedure is as follows (refer to Fig. 11.58
for the geometry):
1. Define the regional.
2. Draw a vertical working line through the hangingwall cutoff of the reference bed
(point B).
3. Measure the straight-line dip separation on the fault,
d
(length AB).
4. Swing an arc from B of length
d
until it intersects the fault at C. This point locates
the next vertical working line. Swing the next arc from C to find the location of D
and so on. Draw vertical working lines through each marked point.
5. Measure the vertical distance from the reference bed to the fault, for example,
t
1
the
heavy line above point C, and shift it one working distance (
W
) horizontally and up the
fault so that the base of the line is at the fault. The top of the line is the restored position
of the reference bed. This is done for each vertical line to restore the section.
11.8.2
Fault-Shape Prediction
The construction of the fault geometry from the shape of a reference bed in the rollover
is as follows (refer to Fig. 11.59 for geometry):
1. Define the regional.
2. Measure the straight-line dip separation,
d
, along the fault between the hangingwall
cutoff and the footwall cutoff of the reference bed.
3. Draw a vertical working line through the hangingwall cutoff of the reference bed.
Swing an arc with radius
d
from point A at the intersection of the working line and
the regional. Where the arc hits the reference bed marks the position of the next
vertical working line, which defines point B.
4. Repeat the swinging of arcs of constant length equal
d
from the intersection of each
vertical working line with regional to the reference bed until the rollover returns to
regional.
