CRITICAL WEDGES AND PENETRATIVE STRAIN: HOW DOES PENETRATIVE STRAIN ALTER THE CONCEPT OF A CRITICAL WEDGE?

The Critical Coulomb Wedge Theory attributes the wedge of earth created in a fold-and-thrust belt to be comparable to a wedge of soil that forms in front of a moving bulldozer. It is used to understand the development of fold-and-thrust belts, where variables such as internal and basal friction, surface taper and dip of the décollement are taken into account. Numerical modeling shows that the minimum stable surface taper is dependent on the basal friction and the overburden, but that there are a range of possible tapers for a stable wedge. By definition, a critical wedge shows limited internal deformation; however, the internal deformation that does occur is frequently overlooked due to the calculated critical angle equations being created with the thinnest critical wedge proportions. We here propose that a proportion of the overlooked internal deformation is in fact taken up by penetrative strain. Penetrative strain is the proportion of total shortening that is not accommodated by macroscale folds and thrusts; it is deformation occurring on a microscale within a deforming rock sequence. To further explore this hypothesis, this study presents a series of analog models, where the overburden thickness is systematically varied over a constant basal décollement layer. Models are shortened to 5%, 10%, 15%, 20%, and 25% respectively, creating a total of 14 experiments. Models were photographed from top and side view at each increment (1%) of shortening and side view photographs were used to measure surface taper. We expected that the surface taper would increase with a thinner overburden, and this did occur in early model stages. However, in the latter stages of shortening and in the final configuration, models tended to the same surface taper, within the stable field, accommodated by varying amounts of penetrative strain. A model with a thinner overburden showed an increase in average penetrative strain within the overburden, relative to the comparison model with a thicker overburden from a previous experimental series. These results suggest that whilst critical wedge theory is a valuable construct for understanding the final configuration of a fold-thrust belt, the detailed behavior and development of the wedge cannot be understood without the inclusion of the penetrative strain concept.