Geology Reference
In-Depth Information
Fig. 5.1.
Axis of a cylindrical fold.
a
Fold geometry.
A
-
E
are measurement points,
π
E
are poles to bed-
ding.
b
Axis (
π
axis) determined from a stereogram, lower-hemisphere projection. (After Ramsay 1967)
π
A
-
Fig. 5.2.
Fold axis (
δ
3
) found as the
intersection line between two
bedding planes (
δ
1
and
δ
2
)
on a tangent diagram.
(After Bengtson 1980)
The alternative method for finding the axis is to plot the bedding attitudes on a
tangent diagram. The method is based on the principle that intersecting planes have
the same apparent dip in a vertical plane containing their line of intersection (Bengtson
1980). Let
δ
2
are plotted
and connected by a straight line. The perpendicular to this line through the origin,
δ
represent the dip vector of a plane. In Fig. 5.2, planes
δ
1
and
δ
3
,
gives the bearing and plunge of the line of intersection. In a cylindrical fold, all bed-
ding planes intersect in the straight line (
δ
3
) which is the fold axis.
Each bedding attitude is plotted on the tangent diagram as a point at the appropri-
ate azimuth and dip. If the best fit curve through the dip-vector points is a straight line,
