Geoscience Reference
In-Depth Information
Table 8.1
Notations for structural orientation data.
Description
Notation
Type
Comments
PLANE striking on a bearing
of 047°, and dipping at 23°
towards the SE
047/23 SE
Strike, dip, dip
direction
Less likely to confuse with plunge and
plunge direction measurement of linear
feature. Strike can be plotted directly onto
map sheet
8
23
→
137
or
23/137
Dip, dip azimuth
Shorter, fewer elements to forget or
mis-record. Strike must be calculated to
plot onto map sheet
LINE plunging 23° towards
137° (line of maximum dip
on above plane)
23
→
137
or
23/137
Plunge, plunge
azimuth
Possible to confuse with plane orientation
recorded in dip, dip azimuth notation;
label carefully
labelled carefully to identify what they are. Because the strike
of a plane is easier to measure accurately than the dip direction
with the Silva-type compass-clinometers, the 'strike azimuth/
dip angle + dip direction' notation tends to be more popular
among non-structural geologists.
Remember that a degree sign
(°) for angle or azimuth can
be mistaken in a handwritten
notebook for an 'extra' zero,
leading to confusion - it is
better to omit these
altogether. Clear
identifi cation of the data,
and use of the three-digit
convention for strike or
azimuth and the two-digit
convention for dip or plunge,
will avoid ambiguity.
8.2 Brittle structures: Faults, joints
and veins
During the study of brittle features it is better to focus fi rst on
their orientation, before looking at more detailed clues to the
direction and sense of past movements on those features, and
their relationships with associated structures.
8.2.1 Planar brittle features - orientation
The attitudes of faults are strongly infl uenced by the
orientations of the three principal stresses (σ
max
, σ
int
and σ
min
).
Faults that we defi ne as normal, thrust and strike-slip
commonly have steep, gentle and subvertical dips respectively.
Fault dip can be roughly estimated in the fi eld, or from maps
and satellite images (Figure 8.1, p. 166), by observing whether
the fault trace cuts sharply across topography (steep dip) or
follows the land's contours (gentle dip). Many faults are
obscured in the fi eld by vegetation, soil and other superfi cial
deposits, so look for indirect clues to their presence (Table 8.2
and Figure 8.2, p. 167). General features of faults are shown in
Figure A8.1 (Appendix A8).
The dip and strike of simple, planar fractures are easily
measured using the compass-clinometer. For uneven surfaces a
clipboard or book can be used to smooth out the unevenness
(Section 2.3.1 and Figure 8.3a, p. 168). If the plane is highly
