Strategic Considerations

• Line of section: Inherent in the methods of cross-section balancing is plane strain.

For this reason the line of section must be chosen strictly parallel to the tectonic transport direction. However, a line of section within ± 10° of the tectonic transport direction does not result in significant error (Woodward et al. 1989).

The tectonic transport direction can be determined using bow-and-arrow rule, as illustrated in Fig. 15.1a (Elliott 1976). This direction can also be determined from stretching lineations or from the orientations of the axes of small-scale folds. If the fault traces and axial traces are parallel on the map (Fig. 15.1b), then a section line oriented perpendicular to fault/axial traces is acceptable in most situations. In most FTBs, the tectonic transport direction is approximately known.

• Ramp-flat geometry: The thrust faults in fold-thrust faults usually assumed to have ramp-flat trajectory with sharp bends for ease in section construction.

Slight curvature near the bend can be ignored without adding significant error.

Curved faults can be considered to have multiple bends with straight line trajectories in between bends.

Thrust tip

Direction of tectonic transport Direction of

tectonic transport

Axial traces Thrust

(a) (b)

Figure 15.1. (a) “Bow-and-arrow” rule to determine tectonic transport direction, which is given by the perpendicular to a line which connects the two exposures of a thrust tip line in map view and is in the direction from tip connector to the fault (Elliot 1976). (b) The direction perpendicular to the fault and axial traces also gives the tectonic transport direction.

• Depth to Detachment: When we construct deformed-state cross section in fold-thrust belts, what we essentially try to do is fill up the space between the topographic surface and the basal detachment! So it is important to know the depth and dip of this basal detachment. It may be known from seismic reflection profiles or borehole litholog data. In the absence of such data, a commonly used method is depth-to-detachment calculation, which assumes area conservation during deformation. The method of depth-to-detachment calculation is illustrated in Fig. 15.2.

1 2 3 4 5

1'

2' 3'

4' 5'

Area A

Area B

d Area A = Area B = d Sx

S = l - l_o

S = l - lo

Shortening, d = area A/S

• Reference lines: There are two types of reference lines, viz., pin line and loose line (Fig. 15.3). A pin line is a line in the deformed-state cross section from which all restoration measurements are made. Pin lines can be regional or local. Regional pin lines are perpendicular to stratification and are located in the undeformed foreland. Local pin lines are located within the thrust belt. Loose lines are usually marked near the trailing edge of the cross section. A loose line can be considered to be chain of marker points in the layered sequence. It is particularly useful in tracking layer-parallel simple shear, which may not be obvious in deformed-state cross section.

Local pin lines

Regional pin line Loose line

Foreland

Figure 15.3. Different types of pine lines.

• Template: A restoration template shows the assumed pre-tectonic attitudes of the rocks shown in the deformed-state cross section. Restoration is usually done on this template. Template can be constructed if stratigraphy of the area is well known.

• Sequence of thrusting: It used to be assumed that thrusts in fold-thrust belts form in a forward-breaking sequence. But now we know that break-back sequence, out-of-sequence (Morley 1988) and synchronous thrusting could be important.

Knowledge of sequence of thrusting is particularly useful for restoration. Faults should be restored in the reverse order than they formed.

• Slip on a fault: If the slip on a fault surface is assumed to be constant then matching hangingwall and footwall cut-offs is simple. However, for a blind thrust this is not true. The slip should become zero at the fault tip and the shortening is transferred to the overlying fold.

• Balanced forward modelling: It should be an integral part of section construction.

As many balanced forward models as possible should be constructed in the vicinity of each fault. Then, we can attempt to fit these models like a jig-saw puzzle. Sections that contain forward modelled structures have better chance of being restorable.

• Previous sections: Do not throw away previous sections, if available. Modify such sections after proper evaluation in terms of admissibility and viability. One can avoid repeating the same mistake made by previous workers. Of course if there is no previous section available one has to start from scratch with raw data.

• Data: All available data should be gathered and compiled, such as surface geological map, stratigraphy, regional geology and tectonic setting, dip data collected at the surface, dipmeter data from borehole logs, lithologs, seismic reflection profiles etc. Obviously more data we have more confidence we will have in our cross section. Subsurface data, such as, seismic reflection profiles, are not absolutely essential, for balancing a cross section, which can be done from geological map and surface dip data alone. Of course, for an offshore project or if the area is covered by alluvium the first data is commonly seismic reflection profile. It should be remember that even with today's improved data acquisition and processing techniques, seismic data often leave much to be desired.