Mechanics of Fold-and-Thrust Belts Based on Geomechanical Modeling

We use a large strain geomechanical model and critical state soil mechanics to study the evolution of stress and deformation in an evolving fold-and-thrust belt and its underlying footwall sediments. Both mean effective stress and deviatoric stress contribute to porosity loss within the wedge with 35% of the porosity loss resulting from increased shear. As a result, porosity increases abruptly across the décollement because both mean-effective and shear stresses are much higher inside the wedge than in the footwall. As the basal friction coefficient (μ_b) increases, more shear stress is transmitted across the décollement, resulting in additional compaction of the footwall sediment and decrease in the porosity contrast across the décollement. As the internal friction coefficient (μ_s) increases, the wedge sediment is more compacted because it can withstand higher mean-effective and deviatoric stresses. Inside the wedge, the sediment experiences subhorizontal shortening strain and subvertical elongation strain. We predict a 10–30 km wide “transition zone” in which the shear-stress ratios and compaction curves change rapidly between compressional critical state failure and uniaxial strain (K₀) state. Our model results agree with the taper angles and the stress orientations predicted by critical taper theory. This large-strain drained modeling approach provides first-order insights into the mechanical processes of loading and compaction in fold-and-thrust belts and a foundation for understanding field observations of pressure, stress, and deformation in thrust belt systems.