Geology Reference
In-Depth Information
is in undeformed beds adjacent to the area of interest, but such locations may not be
available. Often both the pin line and the loose line must be placed in deformed beds.
Some of the better possibilities are
1. Along an axial surface. This is appropriate for fixed-hinge folds in which shear does
not occur across the axial surfaces.
2. Perpendicular to horizontal beds. This is appropriate if the strain is zero or con-
stant where beds are horizontal.
3. Pin line and loose line located where the dip is the same in amount and direction.
This is appropriate where layer-parallel shear is proportional to dip.
The typical result of an incorrect choice of a pin line or loose line is the systematic
length error shown by a straight but inclined loose line (Fig. 11.30b). A systematic error
might be corrected by a better choice of pin line and/or loose line without any changes
to the cross section itself.
To restore a structure, the pin line and loose line are chosen (Fig. 11.31a), and the
bed lengths are measured between them, stretched out and placed on the restoration
(Fig. 11.31b). Lengths may be measured by a variety of means. Chamberlin (1910) used
a thin copper wire, curved to follow bedding, then straightened. A ruler or a straight
piece of paper can be rotated along the contact, the lengths of many small segments
marked, and the total measured at the end. Computer methods are especially quick
and convenient (Groshong and Epard 1996). Thicknesses in the deformed-state are
preserved on the restored cross sections. Faults are drawn on the restored cross section
in the positions required by the restored bed lengths. The restored fault trajectories
should match those expected for the structural style. Ordinarily faults should restore
to planes, smooth curves, or ramp-flat geometries.
A question that must be addressed with any restoration is how straight must the
loose line be in order to represent a valid cross section? The loose line in Fig. 11.31b is
not perfectly straight, but the cross section can be considered valid for most purposes.
Constant BLT line-length restoration is a robust technique for which small violations
of the constant BLT assumption cause only small effects in the restoration. The lower,
shorter part of the loose line in Fig. 11.31b may be caused by a small amount of layer-
parallel shortening and thickening of the units in the inner arc of the physical model.
Discrepancies that clearly indicate an invalid cross section are large (Fig. 11.32).
Wilkerson and Dicken (2001) provide an excellent review of common problems re-
vealed by the restoration of thrust faults along with appropriate corrective measures.
Fig. 11.31.
Flexural-slip restoration of the cross section of a physical model.
a
Deformed-state section.
b
Restored section. (After Kligfield et al. 1986)
