Geology Reference
In-Depth Information
Fig. 5.23.
Alternative structure contour maps from the same data.
a
Control points contoured.
b
Structure contours based on interpretation that the structure is a north-south trending, non-plung-
ing anticline. Contours from
a
are
dashed
5.6
Minor Folds
Map-scale folds may contain minor folds that can aid in the interpretation of the map-
scale structure. The size ranking of a structure is the
order
of the structure. The largest
structure is of the first-order and smaller structures have higher orders. Second- and
higher-order folds are particularly common in map-scale compressional and wrench-fault
environments. The bedding attitudes in the higher-order folds may be highly discordant
to the attitudes in the lower-order folds (Fig. 5.24). Minor folds provide valuable informa-
tion for interpreting the geometry of the first-order structure but, if unrecognized, may
complicate or obscure the interpretation. A pitfall to avoid is interpreting the geometry of
the first-order fold to follow the local bedding attitudes of the minor folds.
If produced in the same deformation event, the lower-order folds are usually coaxial
or nearly coaxial to the first-order structure and are termed parasitic folds (Bates and
Jackson 1987). Plots of the bedding attitudes of the minor folds on a stereogram or
tangent diagram should be the same as the plots for the first-order structures and can
be used to infer the axis direction of the larger folds. Seemingly discordant bedding
attitudes seen on a map can be inferred to belong to minor folds if they plot on the
same trend as the attitudes for the larger structure.
Asymmetric minor folds are commonly termed drag folds (Bates and Jackson 1987).
A drag fold is a higher-order fold, usually one of a series, formed in a unit located
between stiffer beds; the asymmetry is inferred to have been produced by bedding-
parallel slip of the stiffer units. The sense of shear is indicated by the arrows in Fig. 5.24
and produces the asymmetry shown. The asymmetry of the drag folds can be used to
infer the sense of shear in the larger-scale structure. In buckle folds, the sense of shear
is away from the core of the fold and reverses on the opposite limbs of a fold (Fig. 5.24),