Geology Reference
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Fig. 5.11.
Hinge line orientations for the
fold in Fig. 5.10, found from
the bed intersections on a
lower-hemisphere equal-area
stereogram. The primitive
circle that outlines the stereo-
gram is the trace of a horizon-
tal bed. One of the bedding
dip domains has an attitude of
30, 250 and the other of 30, 110.
Numbered arrows give orien-
tations of hinge lines. Hinge
line 1 has the direction 0, 340;
hinge line 2 is 0, 020; hinge
line 3 is 11, 180
Fig. 5.12.
Tangent diagram of the conical
dip-domain fold in Fig. 5.10.
Circles give attitudes of the
three dip domains present
in the fold
the straight line joining the two dip vectors (Fig. 5.12) and plunges 11° to the south, as
observed (Figs. 5.10).
The analytical solution for the azimuth,
, of a hinge line can be
found as the line of intersection between the attitudes of two adjacent dip domains,
from Eqs. 12.4, 12.5 and 12.29-12.31:
θ
', and the plunge,
δ
θ
' = arctan (cos
α
/cos
β
) ,
(5.5)
δ
= arcsin (-cos
γ
) ,
(5.6)
where
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