Modelling the evolution of asymmetrical basins bounded by high-angle reverse faults with application to foreland basins

Abstract

The evolution of asymmetrical basins bounded by high-angle reverse faults in a 10- and 20-km-thick strong upper crust with a Byerlee-type strength envelope has been modelled by finite element analysis. The substratum is assumed to be inviscid, and the basin is filled to datum by sediments. The results are compared with those of comparable extensional half grabens, as previously studied. As the layer is shortened by progressive increments, zones of plastic failure extend until throughgoing failure occurs, with the basin progressively narrowing as it deepens. At this stage, further subsidence becomes negligible, placing a limit on the depth, which depends on the layer thickness (and sediment density). During the earlier stages of evolution, the flexure profiles are very similar to those of the otherwise equivalent extensional half graben, but during later stages, substantially deeper and narrower compressional basins can form because of the greater crustal strength under compression. Beyond throughgoing failure, further shortening causes plastic compressional folding, which develops on top of the earlier gentle flexural undulations, eventually producing quite strong anticlines and synclines of progressively decreasing wavelength, which decay away from the fault on both sides. Compressional basins as modelled range from under 25 km to over 150 km in width, and they can be up to nearly 8 km deep, with greater widths and depths for a thicker layer. The most important application of the results is to the origin of foreland basins and foredeeps, providing an alternative and possibly complementary mechanism to the supracrustal loading hypothesis for the formation of such basins. In particular, the asymmetrical thrust basin hypothesis explains how the narrow and deep foreland basins characteristic of European mountain belts can form.