Figure 3

a) Second Invariant Deviatoric Strain Rate (high-to-low corresponds to blue-to-red) and Velocity vectors of crust above incipient subduction zone.

b) Second Invariant Deviatoric Stress (high-to-low corresponds to blue-to-red) of lithosphere in the vicinity of the incipient subduction zone.

c) Model geometry and rheological properties of crust (green upper layer), `fluid' mantle (grey) and mantle lithosphere (other colours). Weak zone in lithosphere is blue. All units SI. phi = Coulomb internal angle of friction, c = cohesion, rho = density, µ = viscosity.

Figure 3: High resolution (201 x 601 Eulerian grid) `subduction' model. Figure shows initial responseto lithospheric input flux, v = 1 x 10<sup>-9</sup> m/s (3.15 cm/yr), at left boundary. Left and right boundaries have small `leakage' flux to balance lithospheric influx. Model materials are very simple but the result demonstrates the capability to solve full density visco-plastic flows with a self-consistent treatment of the free surface. Note doubly-vergent nature of deformation above incipient subduction zone and reasonable resolution within the crust (panel a). Model is illustrative only and not intended to demonstrate the physics of subduction initiation.