Geology Reference
In-Depth Information
dips around a monoclinal dip gives a horizontal distribution of noise. Thus a zero true
dip gives a small false positive average on the azimuth histogram because all dips are
recorded as positive (Fig. 9.4a). As the dip increases the dips form point concentra-
tions that become better defined as the true dip increases (Fig. 9.4b,c). Non-plunging
and uniformly plunging folds give vertical concentrations of points corresponding to
the limbs (Fig. 9.4d,e) and a doubly plunging fold produces an arrow-head-shaped
distribution of points (Fig. 9.4f ). On a tangent diagram a monocline plots as a point
concentration of dips, and a fold (Figs. 5.3, 5.5) will produce a linear or curvilinear
concentration of points.
9.3
Dip Components
A key step in a SCAT analysis is to determine the dip components in the transverse
direction (
T
= transverse = regional dip) and the longitudinal direction (
L
=longi-
tudinal = regional strike) which is at right angles to it. These dip components repre-
sent the dips on vertical cross sections in the
T
and
L
directions and are used to pro-
duce the SCAT histograms (Fig. 9.2) and cross sections (Fig. 9.1) in the
T
and
L
direc-
tions. The
T
and
L
directions are found from the dip vs. azimuth histogram (Fig. 9.4)
or from the plot of bedding dips on a tangent diagram (Fig. 9.5). For monoclinal dip,
the center of the point concentration on the dip-azimuth histogram is the
T
direction
and the
L
direction is 90° away from it (Fig. 9.4b,c). For a fold, the center of the limb
concentrations on an azimuth histogram is the
T
direction and the midpoint between
the concentrations is the
L
direction (Fig. 9.4d-f ). On a tangent diagram (Fig. 9.5), the
orientation of the crest (or trough) line is the
L
direction and the
T
direction is at right
angles to the crest (or trough) line. Both the
T
and
L
lines go through the center of the
tangent diagram regardless of the fold plunge.
The dip components can be found either graphically or analytically. In the graphi-
cal method, the
T
and
L
lines are drawn on a tangent diagram. The
T
and
L
components
are the projections of the dip vectors onto the
T
and
L
axes (Fig. 9.6). The dip com-
Fig. 9.5.
Determination of
T
and
L
di-
rections on the tangent dia-
gram of a fold.
Solid dots
represent dip vectors of
bedding
