Geology Reference
In-Depth Information
Fig. 11.58.
Restoration using the constant
fault slip model (after Chai
1994).
d
:
dip separation on
fault;
W:
working distance
between two working lines
Fig. 11.59.
Fault shape prediction using
the constant fault slip model
(after Williams and Vann 1987).
d
:
dip separation on fault;
t
1
: vertical distance between
fault and regional below
B;
HWC:
hangingwall cutoff;
FWC:
footwall cutoff;
thick
dashed line:
predicted fault
5. Sequentially from the fault cutoff, shift the vertical thickness between the regional and
the fault over one working distance in the direction of fault displacement, then move the
thickness down the working line until the top just touches the key bed (for example,
the thickness
t
1
below B moves to below C). The position of the bottom of the line cor-
responds to the location of the fault. Repeat this step until the cross section is complete.
The constant fault slip model predicts the fault shape from the rollover geometry in
a rigid-rotation experiment by Chai (1994). The hangingwall is homogeneous sand
with layers of different colors, the fault is a pre-cut shape in a rigid block, and the
extensional displacement is applied to the entire hangingwall by a flexible sheet on top of
the footwall block, a configuration developed by McClay and Ellis (1987). The critical
boundary condition in this experiment is that the flexible sheet forces the fault to main-
tain constant displacement and so the deformation mechanism is close to rigid rotation
above the listric part of the fault. Characteristic of the model is the formation of a
keystone graben between the rotated block above the listric portion of the master fault
and the translated block above the lower detachment. The graben (Fig. 11.60) repre-
sents the strain required between the two differentially displaced nearly rigid blocks.
The constant fault slip model produces a better match between the rollover shape
and the fault shape (Fig. 11.60) than any of the models described previously. The fit is
close, although not exact. The lack of a perfect fit between the predicted and observed
fault indicates that the best-fitting model does not perfectly duplicate the mechanics of
deformation. Kinematic models are simplifications of the mechanical processes and
so a close (but not necessarily perfect) match validates the interpretation.
