PARTICLE DYNAMICS SIMULATIONS OF COHESIVE ACCRETIONARY PRISMS: APPLICATIONS TO CASCADIA

Seismic reflection profiles and seafloor bathymetry across the Cascadia accretionary prism demonstrate along- and across-strike variations in structural geometries, implying temporal and spatial variations in mechanical and kinematic processes. One example is a local tendency for backthrusting, in contrast to more typical forethrust geometries found in most accretionary prisms. Possible explanations for these heterogeneities include variations in pore fluid pressure and/or sediment strength and rheology. Unraveling the origins of such structural complexities in the prism, and determining the long-term mechanical causes, are necessary steps toward understanding seismogenic processes along the margin. In this study, discrete numerical simulations using high-resolution particle dynamics (PD) methods are used to examine the controls on the prism growth and evolution. PD approaches simulate Coulomb frictional rheology of a granular medium. Material cohesion and tensile strength are introduced through interparticle bonding, allowing for mechanically heterogeneous systems; bond breakage and time-dependent healing allow for important strain rate dependent behaviors. To reproduce the convergent margin system, a weak décollement plane is introduced in a sediment package, and convergence initiated by moving a backstop into the pile. A subduction channel beneath the backstop allows for underthrusting and subduction erosion. Décollement strength can vary along strike and through time, an analog for pore pressure variations. Wedge strength can vary with distance from the deformation front, to simulate time-dependent strengthening. Syn-deformational sedimentation yields distinctive growth strata and forearc basin fill that record prism evolution. To first order, the resulting fold and thrust geometries compare well to physical analog models and natural accretionary prisms. Interestingly, initial faulting at the deformation front often occurs by backthrusting rather than forethrusting, denoting a strong dependence on basal strength and local stress conditions. These models demonstrate that with increasing complexity, PD numerical simulations can provide important constraints on the mechanical processes and growth histories of accretionary prism under a wide range of conditions.