Widths of imbricate thrust blocks and the strength of the front of accretionary wedges and fold-and-thrust belts

Abstract

Besides the large-scale wedge shape itself, the most prominent structural feature of accretionary wedges and fold-and-thrust belts is the common pattern of imbricate thrust faults. This study illuminates the fundamental mechanical processes and material properties controlling the width of the crustal blocks bounded by major thrusts using analytical solutions of stress as well as two-dimensional finite-difference models. The numerical models predict that the initial width w0 of a thrust block is set when that block first forms at the very front of the wedge. The width is found to subsequently decreases approximately in proportion to the mean horizontal strain needed for an ideally triangular-shaped Coulomb wedge with a critical taper. Block width is proportional to the thickness H of the incoming, accreting sediment. A key quantity that influences the normalized initial block width w0/H is the distance L forward of the frontal thrust needed for the net horizontal force from shear on the base of the incoming sediment to balance the net force on the frontal thrust. It is within this distance where stress in the incoming sediment is substantially elevated and thus where the new frontal thrust forms. Results show that L/H and, correspondingly, w0/H increase with increasing sediment friction angle ϕ, cohesive strength C0 and pore-fluid pressure ratio λ, and decrease with increasing basal friction angle ϕb and basal dip β. Normalized width is sensitive to ϕ and relatively insensitive to ϕb and λ. Results for submarine and subaerial wedges follow the same scaling law. The scaling law relates the observables, w0/H and β, to the material properties, ϕ, ϕb, λ, and therefore provides a theoretical relation that can be used independent of, or together with critical Coulomb wedge theory (CWT) to constrain these properties.