Widths Of Imbricate Thrust Blocks And The Strength Of The Front Of Accretionary Wedges And Fold-And-Thrust Belts

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 w(0) 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 w(0)/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, w(0)/H increase with increasing sediment friction angle phi, cohesive strength C-0 and porefluid pressure ratio lambda, and decrease with increasing basal friction angle phi(b) and basal beta. Normalized width is sensitive to phi and relatively insensitive to phi(b) and lambda. Results for submarine and subaerial wedges follow the same scaling law. The scaling law relates the observables, w(0)/H and beta, to the material properties, phi, phi(b), lambda, and therefore provides a theoretical relation that can be used independent of, or together with critical Coulomb wedge theory (CWT) to constrain these properties.