Geology Reference
In-Depth Information
thicknesses on the limbs. If, for example, the layers maintain constant thickness, the
axial surface bisects the hinge. An axial surface may have one or both of the additional
attributes: (1) it may divide the fold into two limbs (Fig. 5.13), and (2) it may be the
surface at which the sense of layer-parallel shear reverses direction (Fig. 5.13). In a
fold with only one axial surface, the axial surface necessarily divides the fold into two
limbs; however, many folds have multiple axial surfaces, none of which bisect the whole
fold (Fig. 5.14). The relationship between a fold hinge and the sense of shear on either
side of it depends on the movement history of the fold. The movement history of a
structure is called its kinematic evolution and a model for the evolution is called a
kinematic model. According to a fixed-hinge kinematic model, the hinge lines are fixed
to material points within the layer and form the rotation axes where the sense of shear
reverses, as shown in Fig. 5.13. Other kinematic models do not require the sense of
shear to change at the hinge (Fig. 5.14b).
Dip-domain folds commonly have multiple axial surfaces (Fig. 5.14) which form
the boundaries between adjacent dip domains. These axial surfaces are not likely to
have the additional attributes described in the previous paragraph. The folds are not
split into two limbs by a single axial surface (except in the central part of Fig. 5.14a).
In the fixed hinge kinematic model, the sense of shear does not necessarily change
across an axial surface although the amount of shear will change (i.e., across axial
surface 4, Fig. 5.14b). Horizontal domains may be unslipped and so an axial surface
may separate a slipped from an unslipped domain (i.e., axial surfaces 2 and 3, Fig. 5.14b),
not a change in the shear direction.
In the strict sense, a round-hinge fold does not have an axial surface because it
does not have a precisely located hinge line. In a round-hinge fold it must be decided
what aspect of the geometry is most important before the axial surface can be defined
Fig. 5.14.
Dip-domain folds.
a
Non-plunging fold showing hinge lines, axial surfaces, and axial surface
intersection lines (modified from Faill 1969). Axial surface intersection lines are horizontal and paral-
lel to the fold hinge lines.
b
Half arrows
indicate the sense of shear in dipping domains. The horizontal
domain does not slip, as indicated by the pin line. Axial surfaces are
numbered
