A new driving mechanism for backarc extension and backarc shortening through slab sinking induced toroidal and poloidal mantle flow: Results from dynamic subduction models with an overriding plate

Abstract

[1] We present numerical subduction models to investigate overriding plate deformation at subduction zones. All models show forearc shortening, resulting predominantly from shear stresses at the subduction zone interface and opposite-sense mantle shear stresses at the base of the forearc lithosphere. Models dominated by backarc extension show that it results from trench-normal positive velocity gradients in the mantle below the overriding plate. Such gradients result from toroidal mantle flow induced by slab rollback, with velocities below the leading part of the backarc faster than the overriding plate velocity. The velocity gradients induce basal shear stresses that increase trenchward and cause trenchward overriding plate motion at a velocity (v OP⊥ ) whose spatial average is below the trench retreat velocity (v T⊥ ). The combination of basal shear stresses and average v OP⊥ < v T⊥ causes trench-normal deviatoric tension in the backarc and backarc extension. Models dominated by backarc shortening show that it results from a relatively immobile subduction hinge and trenchward overriding plate motion driven by poloidal mantle flow. The poloidal mantle flow is induced by downdip slab sinking and causes the average v OP⊥ > v T⊥ . This results in trench-normal deviatoric compression and shortening in the leading part of the overriding plate as it collides with the subduction hinge. Ultimately, the geodynamic models demonstrate that backarc extension is favored for narrow slabs and near lateral slab edges and is driven by rollback induced toroidal mantle flow, while backarc shortening is favored for the center of wide slabs and is driven by poloidal mantle flow resulting from downdip slab motion.Citation: Schellart, W. P., and L. Moresi (2013), A new driving mechanism for backarc extension and backarc shortening through slab sinking induced toroidal and poloidal mantle flow: Results from dynamic subduction models with an overriding plate,

Implementing nonlinear viscoplasticity in ASPECT: benchmarking and applications to 3D subduction modeling

“…This leaves the plate stronger and more resistant to folding than for Schellart and Moresi (2013). Note that our viscoplastic SP of model 2 also does not show draping at the 660km boundary.…”

“…Within the first ∼ 7My, the velocity of the plates steadily increases about 1 order of magnitude and reaches several centimeters per year. Similar to Schellart and Moresi (2013), poloidal and toroidal flow can be observed close to the slab; flow into the trench alters the shape of the plates. After 8.5My, plate velocities drop when the slab tip reaches a depth of 660km (bottom of the domain) and the steep slab starts to bend in to accommodate lateral sliding over the 660km discontinuity.…”

“…The evolution of model 1 strongly resembles that of Schellart and Moresi (2013), on whose set-up the model is based: the slab of the slab at the 660km boundary and the differences in timing of the aforementioned events are probably due to the weak, linearly viscous layer of the SP that is left out here (and the lesser extent of the domain in the y-direction). This leaves the plate stronger and more resistant to folding than for Schellart and Moresi (2013).…”

“…17 and 18 clearly differs. Models in the Appendix of Schellart and Moresi (2013) have shown that the addition of adjacent plates in itself does not affect the geometry of the SP over time (although velocities are affected). Therefore, the differences derive from the temperature, pressure, strain rate and composition dependent rheology.…”

“…We discuss two models; the first is an adaptation of the free-plate model of Schellart and Moresi (2013) and considers no temperature effects and features constant viscosities except for a viscoplastic crustal layer. The second model is an extension of the first, where we add a temperature field and a temperature, pressure and strain rate dependent viscosity, resulting in a nonlinear viscoplastic thermomechanically coupled system.…”