Geology Reference
In-Depth Information
11.7
Simple-Shear Deformation
According to the simple-shear concept, a cross section deforms as if it were made up
of an infinite number of planar slices that are free to slip past one another (Fig. 11.40).
The shear direction is specified as having a dip
with respect to a regional that does
not change during deformation. The special case for which the shear plane makes an
angle of 90° to the regional is called vertical simple shear because the regional is usu-
ally horizontal (Fig. 11.40a). Oblique simple shear is simple shear along planes at some
angle other than perpendicular to the regional (Fig. 11.40b,
α
α ≠
90°). (Some workers
measure
from the vertical.) Simple-shear restoration is most widely used for exten-
sional structures, in particular the hangingwall rollovers associated with half grabens.
α
11.7.1
Restoration
11.7.1.1
Vertical Simple Shear
Restoration by vertical simple shear involves the differential vertical displacement of
vertical slices of a cross section to restore a reference horizon to a datum (Verrall
1982). This method has its origins in the well-log correlation technique in which
the logs are slipped vertically until aligned side-by-side along an easily recognized
marker horizon. The alignment makes the correlation of nearby markers much easier.
Many computer programs designed for geological or geophysical data interpretation
implement a function of this type. Folds and growth stratigraphy are easily restored
by this method. The restoration of dipping beds by vertical simple shear preserves
vertical (isocore) thicknesses but changes the stratigraphic thicknesses and the bed
lengths.
Begin a vertical simple-shear restoration by picking the reference horizon to be
flattened. In the example of Fig. 11.41a, the top of the sandstone is selected to be the
reference horizon. Define the individual elements to be displaced (lithological logs in
this example). Restore the cross section by shifting the elements vertically until the
Fig. 11.40.
Simple shear oblique to bed-
ding.
a
Vertical simple shear.
b
Oblique simple shear
