Geology Reference
In-Depth Information
Fig. 1.34.
Effect of well orientation on
occurrence of missing or re-
peated section. All units are
right side up and cross sec-
tions are vertical.
a
Wells pen-
etrating a normal fault.
b
Wells
penetrating a reverse fault
Fig. 1.35.
Regional dip of faulted bed-
ding surface, indicated by en-
veloping surfaces is different
than bedding dip
Fig. 1.36.
Map symbols for faults indicating separa-
tion.
a-c
Normal separation,
symbol
on
hangingwall.
d
Reverse separation,
triangle
on hangingwall.
e
Vertical fault, vertical sep-
aration indicated (
U
up;
D
down).
f
Fault of
unknown dip, vertical separation indicated
The median surface of a faulted unit (median line in two dimensions) is the
surface connecting the midpoints of the blocks in the middle of the reference unit
(Fig. 1.35). An enveloping surface is the surface that bounds the high corners or
the low corners on a single unit. Dips within the fault blocks may all be different
from the dip of either the median surface or the enveloping surface. Within a fault
block the original thickness may remain unaltered by the deformation (Fig. 1.35),
although the entire unit has been thickened or thinned as indicated by the changed
thickness between the enveloping surfaces. Common map symbols for faults are given
in Fig. 1.36.
1.6.4
Mechanical Origins
Faults commonly initiate in conjugate pairs (Fig. 1.37). Conjugate faults form at es-
sentially the same time under the same stress state. This geometry has been pro-
duced in countless experiments (Griggs and Handin 1960). The acute angle between
the two conjugate faults is the dihedral angle which is usually in the range of 30 to 60°
but may be significantly smaller if the least principal stress is tensile (Ramsey and
Chester 2004). In experiments the maximum principal compressive stress,
σ
1
, bisects
the dihedral angle. The least principal compressive stress,
σ
3
, bisects the obtuse angle,
