AREA BALANCING AND ITS LARGE-SCALE APPLICATIONS: FROM GRAVITATIONAL COLLAPSE TO OROGENS

Area balancing is a powerful approach for testing models of the large-scale evolution of tectonic systems. In these studies the geometry of individual structures, for example, the interlimb angles, local geometry of fault planes and thrust cut-offs, are of limited importance. Rather, the challenge is to obtain estimates of the integrated datum parallel contraction of extension (the longitudinal strain) and the thickness of units involved in the deformation (either as a depth to detachment or the pre-kinematic thicknesses of the various units). Depending on data quality and setting, obtaining these estimates will carry their own assumptions and uncertainties. Two contrasting examples are presented.

Regional analysis of deep-water fold-and-thrust belts reveals that the minimum values for up-dip extension greatly exceed the maximum values of datum-parallel contraction that can be resolved within the folds and thrusts down-dip. If the implicit assumption that, for gravity-driven tectonics, the extension must balance with the contraction, then additional, unresolved deformation must be present in the section. At present this suggests that in poorly consolidated sedimentary wedges, substantial amounts of lateral contraction can be accommodated by compaction. If true then distributed strain and volume loss deformation must be included in studies of individual fold-thrust structures for their structural models to be valid.

A complementary strategy is needed when applying restoration approaches on a crustal scale to study the large-scale evolution of orogenic systems. In these cases the is significant uncertainty as to the pre-orogenic crustal template, its original thickness and its variations, that existed prior to shortening. A corollary of the popular “depth-dependent stretching” model is that the continental lithosphere entering orogenic belts contains relatively little lower crust. This in turn reduces the amount of crustal subduction required. Testing this notion demands estimates of pre-orogenic rifting, recorded by passive margin stratigraphies. Examples will be drawn from the western Alps and Apennines.

Using of section balancing and restoration in the validation of geometric representations of structural models demands a careful definition of the problem.