Sandbox modelling of thrust wedges with fluid-assisted detachments

Abstract

We have used granular materials to model the development of thrust wedges, where migrating pore fluids assisted in the formation of detachments. The governing equations yield practical scales for linear dimensions, stresses and time. Using compressed air as a pore fluid, models a few centimetres thick were deformed in about half an hour. Model materials were sands of three different grain sizes and a loess. They had suitable values of density, permeability, cohesion and internal friction. Fluid flow obeyed Darcy's law. At yield, the materials satisfied a Coulomb criterion for effective stresses.Models with various sequences of layers were submitted to horizontal shortening in a rectangular box. Compressed air entered through a sieve at the base. The fluid pressure was uniform over the basal boundary. In models made from a single material, the style of deformation depended on the fluid pressure. For no fluid flow, the thrust wedge was short and high, the surface slope attained large angles (30°) and internal structures were mainly forethrusts. For fluid pressures approaching lithostatic values, thrust wedges were longer and lower and surface slopes attained smaller angles. In models containing basal layers of small permeability, detachments formed beneath them and the structural style was dominated by interacting forethrusts and backthrusts. In multilayered models, thin-skinned detachments formed beneath less permeable layers in the sequence.To understand how fluid flow controlled the first stages of detachment, we calculated ideal vertical profiles of fluid pressure, vertical normal stress, effective stress and horizontal shear stress, for multilayered models in the undeformed state. The profiles are segmented, because material properties vary from layer to layer. Sharp drops in shear strength occur at positions where detachments were observed in the sandbox models. We infer that detachments resulted from large fluid pressures beneath relatively impermeable layers.