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[1] Subduction zones are not static features, but trenches retreat (roll back) or advance. Here, we investigate the dominant dynamic controls on trench migration by means of two-and three-dimensional numerical modeling of subduction. This investigation has been carried out by systematically varying the geometrical and rheological model parameters. Our viscoplastic models illustrate that advancing style subduction is promoted by a thick plate, a large viscosity ratio between plate and mantle, and a small density contrast between plate and mantle or an intermediate width (w $ 1300 km). Advancing slabs dissipate $45% to $50% of the energy in the system. Thin plates with relatively low viscosity or relatively high density, or wide slabs (w $ 2300 km), on the other hand, promote subduction in the retreating style (i.e., slab roll-back). The energy dissipated by a retreating slab is $35% to $40% of the total dissipated energy. Most of the energy dissipation occurs in the mantle to accommodate the slab motion, whereas the lithosphere dissipates the remaining part to bend and ''unbend.'' With a simple scaling law we illustrate that this complex combination of model parameters influencing trench migration can be reduced to a single one: plate stiffness. Stiffer slabs cause the trench to advance, whereas more flexible slabs lead to trench retreat. The reason for this is that all slabs will bend into the subduction zone because of their low plastic strength near the surface, but stiff slabs have more difficulty ''unbending'' at depth, when arriving at the 660-km discontinuity. Those bent slabs tend to cause the trench to advance. In a similar way, variation of the viscoplasticity parameters in the plate may change the style of subduction: a low value of friction coefficient weakens the plate and results in a retreating style, while higher values strengthen the plate and promote the advancing subduction style. Given the fact that also on Earth the oldest (and therefore probably stiffest) plates have the fastest advancing trenches, we hypothesize that the ability of slabs to unbend after subduction forms the dominant control on trench migration.

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Rheological and mechanical properties of aqueous mono-disperse silica suspensions (Ludox® HS40) are investigated as a function of particle volume fraction (φ p ranging from 0.22 to 0.51) and water content, using shear rate tests, oscillatory methods, indentation and an ultrasonic technique. As the samples are progressively dried, four regimes are identified; they are related to the increasing particle content and the existence and behaviour of the electrical double layer (EDL) around each particle. For 0.22 ≤ φ p ≤ 0.30), the suspensions are stable due to the strong electrostatic repulsion between particles and show Newtonian behaviour (I). As water is removed, the solution pH decreases and the ionic strength increases. The EDL thickness therefore slowly decreases, and screening of the electrostatic repulsion increases. For 0.31 ≤ φ p ≤ 0.35, the suspensions become turbid and exhibit viscoelastic (VE) shear thinning behaviour (II), as they progressively flocculate. For 0.35 ≤ φ p ≤ 0.47, the suspensions turn transparent again and paste-like, with VE shear thinning behaviour and high elastic modulus (III). At higher particle concentration, the suspensions undergo a glass transition and behave as an elastic brittle solid (IV, φ p = 0.51).

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