Tectonic thickening of hanging-wall units over a ramp

In this article, we use results of analogue models and published numerical models to study the elements that influence the amount of thickening of hanging-wall units during their transport over a ramp. The models, consisting of granular layers, were shortened over a single-pass, rigid ramp dipping at 15, 30 or 45° and friction coefficient (μ) that was large (0.67), intermediate (0.42) or null. Model results show that the amount of thickening depends on the ramp angle and its friction. Thickening increases dramatically with increasing ramp friction and dip. The maximum amount of thickening (80%) is observed over steepest ramps (45°) with largest ramp friction (μ = 0.67), whereas the minimum amount of thickening (2.5%) occurs over frictionless ramps dipping 15°. For the same friction coefficient along the ramp, steeper ramps cause larger amounts of thickening. The tectonic thickening demonstrated by our models was compared with thickening of hanging-wall units of natural examples, and other experimental and model studies. This comparison shows that thickening occurs whether the material considered is brittle, viscous, or plastic. It also confirms the importance to thickening of ramp angle and fault friction. Finally, it reveals also that ramp shape (round or sharp bend), its changes due to footwall deformations, and the topographic relief, are additional boundary conditions for thickening. More generally, our model results and other published model results demonstrate that retardation during the transport of hanging-wall units over a ramp results in hanging-wall thickening.