MAJOR PITFALLS IN THE QUANTITATIVE INTERPRETATION OF MAP-SCALE FOLD-THRUST GEOMETRY

Although it has been common practice for ca. 200 years, constant bed thickness extrapolation in cross-section construction leads to three major pitfalls in the interpretation of kilometric-scale folds: 1. Seemingly applicable kinematic models may fail to correctly predict the structure. 2. Predicted upper detachments may not exist or may have smaller than expected displacement. 3. Bed-length imbalance problems are not necessarily the result of area loss. Pitfalls 1 & 2 are illustrated using simulated outcrop traverses across two seismically well-imaged, area-balanced detachment folds from the deep-water Niger Delta fold belt. One anticline has the limb-dip relationships of a fault-bend fold and the other of a fault-propagation fold. For the fault-bend example, the model provides a reasonable prediction of the lower detachment location but requires significant displacement on a non-existent upper detachment. The deeper predicted upper detachment implies a total displacement that is too large by half whereas the shallower predicted upper detachment gives a displacement that is too small by half. For the fault-propagation-fold style anticline, the model resembles the fold style but provides a poor prediction of the location of lower detachment, the position of the ramp, the location of the ramp tip, and indicates a displacement that is too small by half. Orogenic shortening calculated from kinematic model-based interpretations should be considered to have an error of ± 50%. Pitfall 3 is illustrated by one of the Niger Delta seismic profiles and by an analog model. Line-length restoration of the seismic profile shows significant length discrepancies which might be interpreted to be the result of tectonic compaction of the shorter beds, whereas the area-depth relationship shows the section to be area balanced. The sandbox model is an imbricate stack of thrust-ramp anticlines for which the total restored line lengths are far short of their original lengths, a difference previously inferred to result from tectonic compaction. The calculated horizontal compaction exceeds that possible from grain rearrangement and would require cataclasis. The area-depth relationship shows the section to be balanced with a small dilation. In both examples the apparent excess shortening results from unrecognized bed thickening.