Strain Localization Patterns and Thrust Propagation in 3-D Discrete Element Method (DEM) Models of Accretionary Wedges

High-resolution three-dimensional discrete element method (DEM) simulations of sandbox-scale models of accretionary wedges suggest thrusts follow a variety of propagation processes and orientations depending on a number of factors. These include the stage of development of the wedge (precritical vs. critical), basal friction, and type of thrust (forward vs. backward-vergent). In terms of propagation processes, two clear mechanisms are identified. The first involves propagation from the décollement to the wedge top, similar to the standard model of thrust propagation seen in many kinematic models, and in the second, thrusts grow downward from an initial nucleation point just below the top surface of the wedge as well as upward from the décollement joining in the middle. In terms of orientation, forward-vergent thrusts initially form at Roscoe (θ_R = 45° − Ψ/2) or Arthur orientations (θ_A = 45° − (ϕ + Ψ)/4), and over greater shortening, rotate into Coulomb orientations (θ_C = 45° − ϕ/2). To arrive at these results, a wide array of continuum parameters and fields were extracted from the DEM simulations, including stress, strain, strain rate, kinetic energy, Mohr-Coulomb parameters, and proximity to yielding using the Drucker-Prager criterion to visualize thrust nucleation and propagation. Lastly, the advantages and disadvantages of these continuum proxies for discerning failure in the granular assembly are considered, and the spatial and temporal relationship between proximity to yielding and strain localization (both pre-peak and subsequent persistent shear banding) in the granular model of an accretionary wedge is explored.