Analog Models of Contractional Wedges: Opportunities and Limitations in Testing Theory

There is a long history of using analog models to test theoretical predictions for the growth of contractional mountain belts, as well as to compare them with natural orogens. Such models generally conform to the expectations of critical Coulomb wedge theory, with wedge taper, thrust sequencing and structural style dependent in predictable ways upon yield criteria in the wedge and along its base, and our models are no exception. This remains true despite the fact that analog models appear ill suited to accommodating several aspects of thrust belt mechanics that one generally considers quite important in crustal tectonics and which are treatable using numerical methods. These include flexure, elevated pore fluid pressure and thermal effects on viscosity, and they accommodate erosion and sedimentation only with difficulty. Like numerical models, they have thus far lacked the finer-scale layering in strength and erosional resistance necessary to produce local topographic relief analogous to that found in nature: hence, models typically address larger-scale issues such as bulk taper. It is now possible to monitor the evolution of topography in a model orogen throughout its development and compare it with the distribution of quantitatively determined strain rate and bulk strain. As in nature, early-developing structures sometimes remain important throughout an experiment, as shear dilation and older topography can lead to continued concentration of ongoing strain. We observe the development of a double retrowedge, a phenomenon noted by others in numerical models but typically not observed in nature probably due to some of the natural processes noted above that are not typically encompassed by models. Although model orogens with very different convergence obliquities appear very similar structurally (and require strain analysis across their profiles to differentiate them unambiguously), their margin-normal profiles vary with obliquity as well as material strength. Regardless of obliquity, purely frictional models never undergo margin- normal extension, but they can be demonstrated even at low obliquity to become supercritical.