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

• Published in: Tectonics (Washington, D.C.) D.C.), 2023-08, Vol.42 (8), p.n/a
• Main Authors: Castillo, Enrique M., Ferdowsi, Behrooz, Rubin, Allan M., Schoene, Blair
• Format: Article
• Language: English
• Subjects: Accretion; accretionary wedge; Deformation; Developmental stages; Discrete element method; discrete element method (DEM); Fault lines; Faults; Geological hazards; Kinetic energy; Localization; Modelling; Nucleation; Parameters; Plate boundaries; Propagation; Scale models; Seismic hazard; shear bands; Strain; strain localization; Subduction; Subduction zones; thrusting
• ISSN: 0278-7407; 1944-9194
• DOI: 10.1029/2022TC007707

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 (θR = 45° − Ψ/2) or Arthur orientations (θA = 45° − (ϕ + Ψ)/4), and over greater shortening, rotate into Coulomb orientations (θC = 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. Plain Language Summary Numerical methods that model materials at the individual particle level are used in this study to simulate the propagation of faults in accreted sediment wedges such as those accumulating above a subducting plate in a convergent plate boundary. Our results challenge the traditional geological model predicting upward propagation of faults from the wedge bottom. Specifically, faults do not form clean ruptures but rather first fail in a number of locations independently, ultimately merging into a continuous fault section, with the initial failure points usually occurring near the bottom and top of the wedge. In addition, diffuse material damage and deformation generally precedes the formation of a fault and persists until it gives way to the more permanent and localized fault. These findings have profound implications as