Noncohesive critical Coulomb wedges: An exact solution

TLDR

Fold‑and‑thrust belts and submarine accretionary complexes can be represented as critically tapered wedges poised for Coulomb failure above a sliding basal decollement. A noncohesive critical Coulomb wedge is defined by four parameters: internal and basal friction coefficients μ, μb and internal and basal fluid pressure ratios λ, λb. The study derives an exact relation linking surface slope and basal dip for a uniform noncohesive wedge, shows stress orientation is uniform, finds μ≈1.1 fits Taiwan data, and suggests that small spatial or temporal changes in basal friction or fluid pressure can account for the diverse tectonic styles along convergent margins.

Abstract

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 μ b and its internal and basal Hubbert‐Rubey fluid pressure ratios λ and λ b . 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, μ b = 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 μ b or λ b .