The effects of increased sedimentation on orogenic wedges - dynamic model results in comparison with critical taper theory

The structure of collisional orogens exhibits a wide range of deformation styles from narrow, asymmetric doubly-vergent wedges like the Pyrenees to wide, plateau-like orogens such as the Himalayas. Prime factors in controlling the style of deformation are the amount and rate of convergence between the two plates and the thermal and rheological properties of the wedge. Among the secondary factors, inherited crustal structures and surface processes are thought to play a significant role in the evolution of such mountain belts as well. These factors have been studied extensively throughout the last decades, yet questions still remain about their exact effects on the style of orogenic development.

We present two-dimensional thermo-mechanical model experiments designed to explore the role of rapidly changing sedimentation rate in the development of orogenic fold-and-thrust belts. We use the Western Alps of Switzerland and France as a case study, where the transition from an underfilled to an overfilled foreland basin during the Oligocene coincided with transient stalling of the thrust front. Such a scenario is common in orogenic foreland basins; hence our results have broader implications.

Our model results indicate that a sudden increase in sedimentary loading of an orogenic foreland will temporarily disrupt the formation of an otherwise regular, outward-propagating basement thrust-sheet sequence. When the loading is suddenly increased, the outermost basement thrust remains active for a longer time and accommodates more shortening than the previous thrusts. As the propagation of deformation into the foreland fold-and-thrust belt is strongly connected to basement deformation, this transient phase appears as a period of slow migration of the distal edge of foreland deformation.

We quantitatively compare our results to critical-taper theory considerations. After determining α and β values for each model and examining their evolution over time, we conclude that our model predictions are broadly consistent with predictions from critical-taper theory, despite the more complex and realistic rheology included in our models.