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Figure 15.7 illustrates how surface dip data can be extrapolated to depth and sub-surface fold geometry can be deciphered. The dip data and stratigraphy are given on the left side of the diagram. At locations A and B contact between sandstone and shale are exposed. The dip data shows five planar domains (domain 1-5) suggesting kink fold geometry. At domain boundaries attitude of axial planes are deduced from the bisectors of the adjacent domain dips. The axial surfaces are extrapolated to depth.

Wherever two axial surfaces meet, a new axial surface emerges whose orientation is given by the bisector of the two axial planes. The beds are then extended using the dip domain data and the fold is constructed.

A B

30^o 0^o 40^o 60^o 0^o

Domain 1 Domain 2 Domain 5

Domain 3 Domain 4

Figure 15.7. Extrapolation of surface data to depth using kink method to deduce the geometry of the large fold.

Data

Solution 3 : Maximum ramp height Solution 1 : Maximum depth of fault

Maximum depth of fault Maximum ramp height

Figure 15.8. A hypothetical example of section construction showing range of possible solutions. See text for discussion.

Fig. 15.8 shows an exercise of how surface dip data and known stratigraphy are used to deduce buried thrust. The dip data show five dip domains, three horizontal dip domains are separated by two dip domains where dips are steeper. Overall the fold geometry is that of a flat-crested anticline, so we guess that it is a fault-bend fold. If this is the case there has to be a thrust at depth. Let us suppose we know that the tectonic transport direction is towards left. Note that the axial angle (γ) is known and back limb dip can be used to infer ramp dip or cut-off angle (θ). With available information we cannot deduce the exact location of the fault. However, we can find range of possible solutions. Solution 1 is based on maximum possible depth of the fault and solution 2 gives us maximum ramp height. We recognize that an exact solution is not possible in this example but the range of possible solution can be useful for planning further exploration strategies.

The examples shown in Figs. 15.7 and 15.8 are hypothetical. A real-life example of section construction with limited data is shown in Fig. 15.9 (Suppe 1983). Fig. 15.9a shows the available data near the crest of the Hokou-Yangmei anticline, Taiwan FTB.

The Well A encountered a double thickness of the distinctive Pliocene Chinsui Shale and normal thickness of formations below Chinsui Shale, suggesting that the small fold on which Well A sits does not extend below Chinsui Shale. Two guesses were made, as shown in Fig. 15.9b, both involving a simple step of a thrust fault from one décollement to another in the Chinsui Shale. In solution 1, a thrust steps up to the north and in solution 2 a thrust steps up to the south. The important angular observations are that the dip at the base of the Chinsui Shale is 5° whereas the minimum dip of the Chinsui Shale, between two wells is 32°. Therefore, we choose 32° - 5° = 27° as θ = φ in solution 1 and β in solution 2. Using Suppe's (1983) equations (or graph) we obtain = 34° for solution 1 with 34° - 5° = 29° as the predicted surface dip. This predicted dip is much greater than the observed surface dip of about 16°, so discard this solution was discarded. For solution 2 we obtain θ = φ = 22° and 22° - 5° = 17° as the predicted surface dip, in good agreement with the observation. Therefore solution 2 may be considered viable. We can now compute how the shallow fault in solution 2 will be folded by the deeper anticline (γ = 58°). The cross-cutting fault block is in the footwall,

convex towards the fault; therefore it is a "syncline" and φ = 57° and β = 15°, which are in reasonable agreement with surface dips. The final interpretation of the structural geometry using solution 2 is shown in Fig. 15.9c. In this example, a hypothesis was invoked (fault-bending over a simple step up of décollement) and tested against the available data and a solution was found.

North South

Figure 15.9. Actual example of quantitative section construction, Hokou-Yangmei anticline, Taiwan (Suppe 1983). With available data (a), two solutions are guessed (b). Solution 1 leads to conflict with surface dip data. Solution 2 is in conformity with surface dip data. (c) Final interpretation based on solution 2.

A different approach is illustrated in Fig. 15.10, where the trajectory of Main Frontal Thrust (MFT) and the geometry of the Mohand anticline, Dehra Dun re-entrant have been constrained (Mishra and Mukhopadhyay 2002). The available data are shown in Figs. 15.10a,b; the MFT trajectory and the basal detachment were approximately constrained from published ONGC seismic reflection profile and well data. Several forward models were made, three of which are shown in Figs. 15.10c-e. A model based on multi-bend fault-bend folding with 12% forelimb thinning and uniformly tapering

layers conforms to the surface dip data and interpreted litholog and "best" explains the geometry of the Mohand anticline (Fig. 10e).

The above examples show that it is possible to construct quantifiable structural cross sections. Alluvium Mid. Siwalik Up. Siwalik Dun Gravels

(b)

Multi-bend fault-bend folding model with MFT emergent

4.0 km

Figure 15.10. A actual example of section construction, Mohand anticline, Dehra Dun area Himalayan FTB (Mishra and Mukhopadhyay 2002). (a) and (b) Available data. (c) Solution assuming multi-bend folding model with MFT emergent. (d) Solution assuming multi-bend folding model with MFT blind. (e) Solution assuming multi-bend folding model with two synclinal bends on MFT, uniformly tapering layers, and 12% forelimb thinning. This section is in conformity with surface dip data.