Geology Reference
In-Depth Information
σ
1
(
σ
vertical)
σ
h
σ
h
(
σ
2
= σ
3
= σ
h
)
Columnar, vertical
discontinuities plot
around
circumference of
stereonet with no
preferential azimuth
North
North
90-
α
x
x
x
x
x
x
x
α
x
x
x
x
x
x
x
x
x
x
Appears as
girdle of
poles
Following tilting through
α°
Horizontal discontinuities
Figure 3.39 Schematic representation of columnar fractures and their representation on a
stereographic projection, assuming one set (e.g. schistosity or cooling surface) is horizontal and
formed in an isotropic stress
field in the horizontal direction. Second diagram shows typical pattern
of poles if the model were tilted, say through folding. The original pattern of vertical fractures,
on the originally horizontal set.
uenced by
any pre-existing fractures (Rawnsley et al., 1990). Examples of major
structures in
resulting in a new episode of fracturing, which will be in
uencing the geometrical development of joints are given
by Rawnsley et al. (1992) and one of these is illustrated in
Figure 3.43.
The joint geometries clearly follow stress trajectories that were
strongly in
uenced by the pre-existing major fault.
