2D Numerical Simulations of Outer Rise Faulting in the Tonga Subduction System

Abstract:

Deformation patterns in subducting oceanic lithosphere reflect the state-of-stress in the plate: slab-pull forces and bending stresses overcome compressional forces generated at the plate boundary interface, giving rise to outer-rise normal faulting. Recent studies provide extensive constraints on outer-rise faulting patterns, subducting and overriding plate structure and plate kinematics. Here, we use these new observations from the Tonga subduction zone and high-resolution numerical simulations to systematically test how the thickness and strength of the subduction interface affects outer-rise faulting patterns. In contrast to previous numerical models of outer-rise faulting that examine deformation in highly dynamic, time-dependent systems, we construct a 2D thermo-mechanical, elastic-viscous-plastic model based on the present structure of the Tonga subduction system. Thermal and compositional variations in the upper mantle (down to 700 km depth) define a cross-sectional slice of the present day slab structure and drive deformation through slab-pull (no velocity boundary conditions). Bathymetry and oceanic plate ages define the thickness and shape (i.e. curvature) of the subducting and overriding plate lithosphere. The plate boundary interface, accretionary wedge and overriding plate geometry (near the trench) follow structures outlined by a regional, lithospheric-scale tomography study (Contreras-Reyes et al., 2011). High numerical resolution (100-200 m) in the vicinity of the trench permits resolving the plate-boundary interface down to a thickness of 1 km. Consequently, we can systematically test how the thickness and strength of the plate-boundary interface influences outer-rise deformation over short time scales ($< 5$ Myr) and compare the results directly to observations. Preliminary results show deformation near the plate boundary is highly sensitive to the strength of the plate-boundary interface and accretionary wedge. Presented work will focus on the sensitivity of deformation patterns to both the numerical parameters (resolution, viscosity limits) and structure (thickness, strength) defining the plate boundary interface.