Particle Dynamics Simulations of Fold and Thrust Belt Evolution: Exploring the Effects of Backstops, Cohesive Strengths, and Strain Rates

Discrete numerical simulations using high-resolution particle dynamics (PD) methods offer a unique way to study the structural evolution of fold and thrust belts that develop in many settings (e.g., convergent margins, passive margin, volcano flanks). PD approaches simulate the Coulomb frictional rheology of a granular medium, which is a reasonable analog for the brittle-plastic rheology of sediments and shallow crustal rocks. Material cohesion and tensile strength can be introduced through interparticle bonding; bond breakage and time-dependent healing allow for important strain rate dependent behaviors. Here, convergence is simulated by capturing a “backstop” of particles that plows forward into the strata to produce a distinctive forewedge-retrowedge geometry. A subduction channel beneath the backstop allows for underthrusting and subduction erosion. Décollement planes are constrained as frictionally weak and/or unbonded horizons within the sediments. To first order, the resulting fold and thrust belt geometries compare well to physical analog models and natural systems. A high-strength décollement produces a narrow, high-taper wedge characterized by closely spaced forethrusts and a well-defined deformation front. In contrast, a low-strength décollement favors a broad, low-taper wedge with widely spaced thrust faults and detachment folds. Initial faulting at the deformation front often occurs as backthrusting rather than forethrusting, denoting a strong dependence on basal strength and local stress conditions. A range of backstop geometries and properties are defined to explore their effects on the structural evolution of the wedge. A strong backstop guides backthrusting and formation of a retrowedge; an intermediate strength backstop favors out of sequence forethrusting, and construction of outer and inner wedges distinguished by different tapers, strain rates, and fault activities. With increasing complexity, PD numerical simulations provide important constraints on the mechanical processes and growth histories of fold and thrust belts in a wide range of settings.