Subduction Initiation under Unfavorable Conditions and New Fault Formation

How subduction initiates with unfavorable dipping lithospheric heterogeneities is an important and rarely studied topic. We build a geodynamic model starting with a vertical weak zone for the Puysegur incipient subduction zone (PISZ). A true free surface is tracked in pTatin3D, based on the Arbitrary Lagrangian Eulerian (ALE) finite element method, and is used to follow the dynamic mantle-surface interaction and topographic evolution. A simplified surface process, based on linear topography diffusion, is implemented. Density and free water content for different phase assemblages are gained by referring to precalculated 4D (temperature, pressure, rock type and total water content) phase maps using Perplex. Darcy's law is used to migrate free water, and a linear water weakening is applied to the mantle material. A new visco-elastic formulation called Elastic Viscous Stress Splitting (EVSS) method is also included. Our predictions fit the morphology of the Puysegur Trench and Ridge and the deformation history on the overriding plate. We show a new thrust fault forms and evolves into a smooth subduction interface, and the preexisting weak zone becomes a vertical fault inboard of the thrust fault during subduction initiation, which explains the two-fault system at PISZ. Our model suggests that the PISZ may not yet be self-sustaining. We propose that the Snares Trough is caused by plate coupling differences between shallower and deeper parts, the tectonic sliver between two faults experiences strong rotation, and low density materials accumulate beneath the Snares trough. Extended models show that with favorable dipping heterogeneities, no new fault forms, and subduction initiates with smaller resisting forces.