Geology Reference
In-Depth Information
Fig. 7.12.
Drag folds caused by permanent bending strain before faulting.
a
Fold before the units reach
their ductile limit.
b
A fault forms after the ductile limit is exceeded, and the permanent strain remains
as a drag fold
an outcrop, in particular, a high concentration of fractures or an abrupt change in li-
thology not part of the normal stratigraphic sequence.
7.2.7
Fault Drag
The systematic variation of the dip of bedding adjacent to the fault shown in Fig. 7.12b
is known as “normal” drag, or simply
drag
, as the term will be used here. A drag fold
gives the sense of slip on the fault. This geometry is probably best interpreted as being
the result of the permanent strain that occurs prior to faulting (Fig. 7.12a), a sequence
of formation well known from experimental rock mechanics. The term “drag” is thus
a misnomer because the fold forms prior to faulting, not as the result of frictional drag
along a pre-existing fault. In fact, the strain associated with slip on a pre-existing fault
tends to produce bending in the opposite direction relative to the sense of slip (Reches
and Eidelman 1995) called “reverse” drag. The reverse drag described by Reches and
Eidelman (1995) is significantly smaller in magnitude than the normal drag produced
before faulting and might not be noticeable at the map scale. Map-scale reverse drag
is also well known as a consequence of slip on a downward-flattening normal fault
(Hamblin 1965). Because the term drag is long established for the fold close to a fault
and because the curvature of drag folds gives the correct interpretation of the sense of
shear, use of the term is continued here. The drag-fold geometry usually extends no
more than a few to tens of meters away from the fault zone. Although the drag geom-
etry is common, not all faults have associated drag folds.
7.3
Unconformities
Unconformities are important surfaces for structural and stratigraphic interpretation.
They provide evidence for structural and sea-level events, and mineral deposits, in-
cluding hydrocarbons, are commonly trapped below unconformities. An unconformity
truncates older geological features and so shares a geometric characteristic with a fault
which also truncates older features (Figs. 7.4 and 7.13). Sometimes the two can be dif-
ficult to distinguish. The flat portion of a fault may cut bedding at a very low angle, just
like the typical unconformity.
