Geology Reference
In-Depth Information
compaction the unit thickness will vary as a function of lithology. Deformation-re-
lated thickness changes are usually accompanied by folding, faulting, or both within
the unit being mapped. Deformation-related thickness changes are likely to correlate
to position within a structure or to structural dip.
1.5
Folds
A fold is a bend due to deformation of the original shape of a surface. An antiform is
convex upward; an anticline is convex upward with older beds in the center. A synform
is concave upward; a syncline is concave upward with younger beds in the center.
Original curves in a surface, for example grooves or primary thickness changes, are
not considered here to be anticlines or synclines.
1.5.1
Styles
Folds may be characterized by domains of uniform dip, by a uniform variation of the
dip around a single center of curvature, or may combine regions of both styles. Re-
gions of uniform dip (Fig. 1.12a) are called dip domains (Groshong and Usdansky 1988).
Dip domains are separated from one another by axial surfaces or faults. Dip domains
have also been referred to as kink bands, but the term kink band has mechanical
Fig. 1.11.
Thickness (
t
).
a
Unit of constant thickness.
b
Unit of variable thickness
Fig. 1.12.
Regions of uniform dip properties.
a
Dip domains.
b
Concentric domains separated by a pla-
nar dip domain (
shaded
)
