Geology Reference
In-Depth Information
Fig. 11.10.
Bed length and bed thickness
change in an area-constant
deformation
Fig. 11.11.
Deformation of a bed of origi-
nal thickness
t
0
by oblique
simple shear (after Groshong
1990).
α
:
angle between shear
direction and bedding,
:
angle
of shear,
e
L
:
layer-parallel ex-
tension
ψ
β
t
=
L
0
/
L
1
,
(11.11)
and from Eqs. 11.3, 11.6 and 11.7, 11.8,
e
L
=
β
L
- 1 ,
(11.12)
e
t
=
β
t
- 1 .
(11.13)
Simple shear oblique to bedding (Fig. 11.11) changes the bed length and bed thick-
ness. The layer-parallel strain can be determined from the shear amount and the shear
angle (Groshong 1990) as
e
L
=[sin
α
/sin(
α
-
ψ
)] - 1 , or
(11.14)
e
90
=(1/cos
ψ
) - 1 ,
(11.15)
where
α
= angle between shear direction and bedding and
ψ
= angle of shear. Equa-
tion 11.15 is for vertical simple shear (
= 90°) and Eq. 11.14 is for all other angles. To
find the thickness change from the length change for constant area deformation, sub-
stitute Eq. 11.3 into 11.6 to give
α
e
t
=-
e
L
/(
e
L
+ 1) .
(11.16)
Simple shear parallel to bedding in flexural-slip deformation has no effect on bed
length or bed thickness. If a bed-normal marker is present, or assumed, then its length
strain is given by Eq. 10.15 where the angle of shear is the amount of rotation of the
original bed-normal marker.
Strain is commonly partitioned between deformation features that are visible
at the scale of the map and cross section and deformation at smaller scales, termed
sub-resolution strain. In the low-temperature deformation of lithified rocks (i.e.,
where hydrocarbons can be preserved), the crystal-plastic strains are usually less than
