Bed length does not remain constant during deformation: Recognition and why it matters

We apply multiple balancing/restoration methods to three examples of fault-bend folds exhibiting increasing levels of complexity and uncertainty. Three methods (the Chamberlin depth-to-detachment calculation, direct measurement of fault displacement, and flexural-slip restoration/balancing) assume that bed lengths and thicknesses (BLT) remain constant during deformation. The area-depth-strain (ADS) method allows bed lengths and thicknesses to vary during deformation. For a kinematic model, the agreement among methods is exact to within measurement error. For an experimental sand model, the disagreement among methods is substantial. The ADS relationship shows that the sand model has significant layer-parallel shortening and an area increase of ∼4%. A previously published interpretation of a seismically imaged fault-bend fold from the Rosario oil field, Venezuela, is nearly line-length balanced, but the ADS relationship indicates small, but significant, anomalies, including an area deficit for the deeper stratigraphic levels. A revised interpretation with a more internally consistent ADS relationship suggests that much of the footwall uplift is real and not a velocity pull-up. Our comparisons of the results of the various balancing/restoration techniques show the resolving power of the ADS method to detect sub-resolution changes in bed lengths and thicknesses and to identify footwall structures overlooked by the constant BLT methods.