at the transition zone at depth. Flow in the lower mantle isslightly perturbed (Figure 4a), while strain in the upper plateis negligible (Figures 3b and 5b, solid line).[11] In the models allowing for a deep penetration of

the slab, a deeper convective cell in the mantle is induced(Figure 4b). The lower mantle penetration drives largertransient subducting plate velocities (Figures 3a and 5a),+20% to +70% for the values of lower mantle’s densitycontrasts and viscosities used here. However, the upperplate motions are negligible and strain rates are only 2–

3 times higher than those of the basic subduction model(Figure 5b). The upper plates strain rates in these modelsshow a periodicity, which is due to the continued pilingover the transition zone (Figure 3b). The same is valid formodels of different gLM, with minor variations in the strainhistory.[12] In models where far‐field convergence constraints are

included, margins largely rollback, stretching the upperplates (Figure 4c). Velocities and upper plates’ strain ratesequilibrate in 15–20 m.y. after the constraints are applied, inhampered models, and in as long as 30 m.y., in the lockedsubduction (Figure 3), with strain rates up to 3000 timeslarger than the basic models’. In this configuration, largetrenchmotions and strain rates are sustained until upper platesyield, occurring at 50 and 30 m.y. following the constraintsapplication (Figures 3a and 3b), in hampered and lockedsubduction, respectively. At which point the retreating slabrearranges the mantle flow and extending laterally the con-vecting cell (Figure 4c).

5. Discussion

[13] Several published models have investigated the role offar‐field boundary conditions as well as three‐dimensionalcomplexities to the single subducting plate [i.e., Stegmanet al., 2010, and reference therein]. However, we haveshown here that the trench motions, and hence the tectonicstyle of subduction, might be strongly controlled by theupper plate in the system.[14] The long‐term stationary trench in the Hellenic sub-

duction zone can be reconciled with the thickening due tothe Hellenic orogeny during the initial 100–40 Ma phase.

Figure 3. (a) Surface horizontal velocities of differentmodels with a thick upper plate. Vsub is for the velocity ofsubducting plate (thick lines), vup is the velocity of the upperplate (thin lines) and (b) strain rates in the upper plate versustime. Vertical thin line marks where the constraints areapplied to the basic setup (black line) and boundary condi-tions change.

Figure 4. Second invariant of the strain rates and flow streamlines. (a) Free subduction (Basic) at the end of the simulationwith a thick upper plate of 80 km. (b) Slab‐lower mantleDr/r 30% at the end of the simulation. (c) Locked subduction after500 km of trench rollback.

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F. A. Capitanio and S. Zlotnik, School of Geosciences, MonashUniversity, Clayton, VIC 3800, Australia.C. Faccenna, Dipartimento di Scienze Geologiche, Universitá di Roma

Tre, Rome I 00146, Italy.

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