RETHINKING CROSS-SECTION BALANCING: ARE "BALANCED" CROSS SECTIONS VALID?

Rather than applied to evaluate the validity of cross sections, balancing concepts may be best used to provide alternative interpretations and predict the magnitude and distribution of sub-resolution strain. “Cross-section balancing” has become a standard approach for evaluating the quality and acceptability of structural interpretations, and in the past 30 years the technique has been computerized to aid cross-section construction. The viability of balancing as an analytical tool depends upon a key assumption, no significant volume change during deformation. However, pressure-solution fabrics and tectonic compaction indicate that rock volume is often not conserved, and the distribution and size of petroleum deposits establish that extensive fluid flow and mass transfer have accompanied deformation. Other “sub-resolution” deformation mechanisms, which cannot be resolved on seismic data or quantified in balanced sections, include wedge faulting, intraformational duplexing, small-scale detachment folding, and flowage. Outcrop observations and the measurement of strain in thin-section provide estimates of these sub-resolution strains, but surface exposures comprise only a very small percentage of the rock mass involved in thrust-belt deformation. The vast majority of thrust sheets are buried in the subsurface and unavailable for study. Surface measurements of strain magnitude and distribution may be of little value or even misleading if these strains are unquantifiable in the subsurface. Given these uncertainties, balancing methods may lead to inaccurate interpretations of crustal structure by forcing geologists to honor the invalid underlying assumptions of constant volume or leading them to skew their interpretations on the basis of unevenly sampled strain measurements. A way to avoid such unintended systematic errors is to have a “balanced” section be one of at least two possible interpretations, with the other being the geologist’s best rendering based on available data, field observations, and experience, regardless of balance. Thus, balancing concepts can be applied to establish a range of plausible interpretations, rather than judging the validity of a single interpretation.