Thermo-Mechanical Modeling of Forearc Evolution and Implications for Subduction Zone Earthquakes

Sediment-rich subduction zones, such as the Cascadia and Nankai Margins, are overlain by large subduction wedges that show a distinctive structural morphology defined by four margin-parallel domains. From trench to arc, the domains are (1) the trench slope, with piggy-back style slope basins; (2) the continental shelf, a region with little surface deformation and deposition of up to several km of sediment in local basins; (3) a subaerial forearc high; and (4) a more interior forearc basin, which marks the landward limit of the subduction wedge. We use a thermo-mechanical model with a visco-plastic rheology to explore the conditions needed to develop these structural features, with a focus on implications for decollement strength and great thrust earthquakes. We find that successful models must have the following features to reproduce the observed morphology: a frictional-viscous rheology, a weak basal decollement, flexural compensation and sedimentation. Our modeling suggests the following factors are responsible for each of the structural domains. (1) The trench slope forms as new sediment is frontally accreted above the more seaward and shallowly-dipping part of the subduction thrust. Piggy-back basins are superimposed upon the trench slope as strain is localized in seaward-vergent shear zones. (2) The more steeply-dipping slab landward of the trench slope permits slip on the decollement without upper plate deformation and forms a local topographic high on the shelf edge. Sedimentation in this region forms the continental shelf and forces this region into a mode of stable growth where newly accreted material is underplated beneath the shelf thus preserving the relatively undeformed nature of the shelf basins. (3) The landward termination of the continental shelf coincides with the thermally-activated transition from frictional to viscous deformation at depth. The viscous deformation allows an effective near-horizontal detachment to develop that promotes the growth of a topographic forearc high. (4) A more landward forearc basin forms by flexural loading due to the growth of the forearc high. Other studies argue that subduction zone earthquakes are commonly initiated beneath the shelf region. Our model suggests these earthquakes are starting beneath the stable part of the wedge, where the wedge is strongest relative to the decollement. Also, the limits of the seismogenic zone are thought to be largely temperature controlled. Our model provides an indication of the importance of advective heat transport associated with accretion and within-wedge deformation on estimating temperatures along the subduction thrust.