Geoscience Reference
In-Depth Information
8.1 Equipment and measurement
Structural geology is the study of geometrical features (e.g.
planes, lines, surfaces, ellipsoids) produced by deformation.
We describe and measure the orientation and dimensions of
these geometrical elements to better understand how and why
they formed, in terms of rock movements (kinematics) and
stresses (dynamics). Hence, structural fi eldwork may require
some of the optional and specialist equipment (e.g. binoculars,
pocket stereoscope; Table 2.3, p. 6) in addition to the basic
equipment for geological fi eldwork (Table 2.1, p. 4). For
accurate measurement of azimuths, dips and plunges, a
specialist compass-clinometer (e.g. Brunton-type; Section 2.3)
is recommended.
8
8.1.1 Structural measurements and notations
Section 2.3 introduced the compass-clinometer; measurement
of the strike and dip of an inclined plane was covered in
Section 2.3.1, Figures 2.6-2.8. Section 2.3.2, Figures 2.9 and
2.10, described how to measure linear features such as fold
axes, lineations and striations, which are at least as important
as planes in structural geology. Gathering orientation data is a
critical part of structural work, so it is a good idea to develop
consistent, effi cient strategies for measurement in the fi eld.
Because many structural studies require copious measurements
of different lines and planes, it is important to record the data
in a concise, organized and logical manner in your fi eld
notebook (e.g. Figure 4.11c, p. 73). Separate the structural data
from other notes (perhaps even by colour), and arrange them in
tabular form. Abbreviations for different features (e.g. Lc =
crenulation lineation) are a useful shorthand, but ensure that
you are consistent in their usage, and include a key in each
notebook, for instance on the inside cover. As explained in
Section 2.3 it is important to follow the convention for
recording azimuths and dip (or plunge) values:
A long pencil is useful in
some cases as an extension
of the line, if the feature itself
is awkward to measure with
the compass-clinometer.
Azimuth (strike; trend of linear
feature)
three-digit number
076
Dip or plunge (inclination from
horizontal)
two-digit number
76
This convention should avoid confusion between the two
numbers, regardless of the style of notation used (Table 8.1).
This is especially important, because many structural geologists
use a short notation for planes of 'dip angle/dip azimuth',
rather than the longer 'strike azimuth/dip angle + dip
direction'. This saves space, but means that notebook records
of planes and lines look very similar, and therefore should be
