Effects of decollement strength and roughness on upper plate deformation at convergent margins: Insights from particle based modeling

Abstract:

Particle-based numerical simulations of cohesive contractional wedges are used to explore the controls of decollement strength and roughness on deformation and mechanical evolution of the upper plate at convergent margins. Particle assemblages, consolidated under gravity and bonded to impart cohesion, are displaced above a weak decollement. Forward propagation of decollement slip occurs in discrete pulses, modulated by heterogeneous stress conditions along the fault. Over time, slip along the basal decollement induces upper plate contraction. When upper plate stresses reach critical strength conditions, new thrust faults break through the upper plate, relieving stresses and accommodating horizontal shortening. Decollement activity retreats back to the newly formed thrust fault. The cessation of upper plate fault slip causes gradual increases in upper plate stresses, rebuilding shear stresses along the base and enabling renewed pulses of decollement slip. Thus, upper plate deformation occurs out of phase with decollement propagation. Decollement roughness, e.g., due to the presence of a seamount, can have profound effects on the evolution of the margin, for example, inducing locally intense deformation near the base of the wedge and delaying forward propagation of the deformation front. As different parts of the wedge are affected by the impinging roughness, both thrust faults and normal faults may form, overprinting each other in time and space and leading to complicated structural geometries. The mechanical evolution monitored throughout the simulation yields local stress paths, which are shown to correlate directly with the final upper plate deformation. Numerically-obtained predictions of the stress and strain histories within deforming upper plates may help to explain the origins of distinctive structures and domains along natural convergent margins.