Geology Reference
In-Depth Information
5. Measure all the bed lengths in the hangingwall between the hangingwall pin and
the fault and place them on the restored-state cross section (Fig. 11.36b, bottom).
This defines the shape of the footwall beds in the restored state.
6. Draw the restored-state footwall on the deformed-state cross section (Fig. 11.36c,
top). This gives a new hangingwall cutoff.
7. Continue the cycle of steps 4-6 until the section is complete (Fig. 11.36d).
This predictive technique is sensitive to the constant BLT assumption. Like the clas-
sical depth-to-detachment calculation (Sect. 11.4.2), small bed length changes will have
a significant effect on the predicted depth to detachment. It is always best to confirm
the predicted fault shape with additional information.
11.6.3
Flexural-Slip Kinematic Models
Numerous models have been developed that relate fold shape to fault shape for various
flexural-slip structural styles. Many of these have adopted planar or ramp-flat fault
shapes, leading to dip-domain cross-section styles (Sect. 6.4.1). Models like the fault-
bend fold, fault-propagation fold and fault-tip fault-propagation fold of Fig. 11.37a-c
are based on the assumption of constant bed length and constant bed thickness through-
out and so yield unique relationships between the fold shape and the fault shape. Such
models can be used to predict the entire structure from a very small amount of hard
data. Other models, like the simple-shear, pure shear, and detachment folds in
Fig. 11.37d-f, maintain constant BLT in the upper part of the structure but are area
balanced in the decollement zone at the base. The latter style of model admits a wider
range of geometries and so more must be known before the complete structure can be
predicted. Other variants allow thickness changes in the steep limb, which creates even
more degrees of freedom in the interpretation. A discussion of all the models and their
variants is beyond the scope of this topic. The objective here is to introduce the basic
concepts and provide a guide to additional sources of information.
Fig. 11.37.
Fault-related fold models.
Unshaded:
constant-thickness
units;
shaded:
variable-thick-
ness unit.
a
Fault-bend fold
(after Suppe 1983).
b
Fault-
propagation fold (after Suppe
1985; Suppe and Medwedeff
1990).
c
Fault-propagation fold
at the tip of a long ramp (after
Chester and Chester 1990).
d
Simple-shear fault-bend
fold (after Suppe et al. 2004).
e
Pure-shear fault-bend
fold (after Suppe et al. 2004).
f
Detachment fold (after
Jamison 1987)
