Geology Reference
In-Depth Information
11.7.5.3
Shear Angle from Strain
The layer-parallel strain is the basis of a third method to determine the best shear
angle. The necessary relationship is derived by solving Eq. 11.14 for
(Groshong 1990).
The result, Eq. 11.48, gives the appropriate shear angle from the layer-parallel strain
and the dip of the median surface of bedding:
α
α
=arctan[( e L + 1) sin
ψ
]/[( e L + 1) cos
ψ
- 1] ,
(11.48)
where
is the angle of rotation of
the median surface of the bed from the regional and e L is the layer-parallel strain as a
fraction. The layer-parallel strain will usually be seen as second-order faults in the
rollover. From measurements of visible bed length and the total extent of the horizon,
e L is determined with Eq. 11.2.
This relationship has been tested on experimental sand and clay models by
Groshong (1990), including the model in Fig. 11.55, and yields predicted faults that
are a close match to the actual faults. The Livingstone Basin (Fig. 11.56a) provides a
large-scale field example illustrating the shear-angle calculation. It is a sub-basin at
α
is the angle of shear measured from the regional,
ψ
Fig. 11.56. Structure of Livingstone basin at the north end of Lake Malawi, East African Rift (after
Wheeler and Rosendahl 1994). a Index map to Livingstone basin, showing the location of line 803. b Cross
section along line 803, Livingstone basin, interpreted from depth-corrected seismic line (Wheeler and
Rosendahl 1994). Darker shaded unit is basement, lighter shaded unit is sedimentary basin fill. No ver-
tical exaggeration
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