Noncohesive critical Coulomb wedges: An exact solution


  • F. A. Dahlen


Active fold-and-thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of Coulomb failure everywhere, overlying a basal decollement where frictional sliding is occurring. Ignoring cohesion, the four strength parameters needed to describe a critical Coulomb wedge are its internal and basal coefficients of friction μ and μb and its internal and basal Hubbert-Rubey fluid pressure ratios λ and λb. An exact relation between surface slope α and basal dip β of a noncohesive critical wedge with uniform properties is derived. The state of stress within such a wedge has the same orientation everywhere, and α is constant if β is, and vice versa. A coefficient of internal friction μ = 1.1 is consistent with the known surface slope, basal dip, and pore fluid pressures in the active fold-and-thrust belt of western Taiwan, assuming that Byerlee's law, μb = 0.85, is valid on the base. The wide variety of tectonic styles observed to occur along convergent margins, including subduction erosion, active accretion, subduction without accretion, and even extension and normal faulting, may be controlled by relatively small spatial or temporal variations in either μb or λb.