No abstract

Observations of seismicity and seismic tomography provide a present‐day constraint on the geometry of slabs within the mantle. However, it is challenging to directly relate the observed shape of slabs to their time evolution. Phase transitions modify the buoyancy forces within slabs, affecting how slabs deform as they sink into the lower mantle and how forces are transmitted to the surface plates. Here we show that if the crustal shear zone is sufficiently weak (1020 Pa s), then the coupled effects of a compositionally dependent phase transition model and a stress‐dependent rheology lead to oscillatory plate speeds and trench motion in response to buckling and folding of the slab. On average, subducting plate velocity is 4 cm/yr and trench motion is ±0.7 cm/yr. However, during folding, subducting plate speed and trench motion increases by a factor of 3 and trench motion switches from advance to retreat. In models with a higher‐viscosity shear zone (1021 Pa s), average plate speed is lower (2 cm/yr) and there is only trench advance. Stress‐dependent weakening due to increasing driving stresses can also cause unexpected changes in plate velocity (i.e., decrease or no change). In addition, due to the added negative buoyancy from the phase transitions, slab breakoff occurs in models with a yield stress ≤500 MPa. Because of the oscillatory motion of the trench, long flat slabs do not form in the transition zone: this suggests that other factors, such as interaction of the slab with deep mantle flow, may be required to create long flat slabs.

Although most present-day subduction zones are in trench retreat, plate reconstructions and geological observations show that individual margins experience episodes of advancing, retreating or stationary trench motion with timevariable subduction rates. However, most laboratory and numerical simulations predict steady plate velocities and sustained trench retreat unless the slab experiences folding in the transition zone. Using 2D dynamical models of subduction with a mobile trench and overriding plate, we find that rapid sinking of the slab during folding causes a reduction in asthenosphere viscosity through the non-linear rheology, which allows the overriding plate to move in the opposite direction of the asthenosphere. This decoupling of the direction of plate and asthenosphere flow allows for episodes of rapid trench advance after each slab folding event. By analyzing the interaction between slab deformation (sinking direction and speed), stress-induced changes in asthenosphere viscosity, asthenosphere flow and plate motions, we show that there are three modes of slab-flow-plate interaction: 1) coupled trench retreat during rapid vertical sinking, 2) coupled trench advance during prograde sinking of the slab, and 3) decoupled, rapid trench advance during folding with prograde motion of the shallow slab and retrograde motion of the deep slab. These results show that non-linear viscosity plays an important role in determining the force balance controlling trench motion and conversely that trench motion can be used as a constraint on the asthenosphere viscosity underlying the overriding plate. In addition, cooling by several hundreds of degrees during episodes of fast subduction could lead to a reduction in slab dehydration and fluid-induced melting in the mantle wedge. Such cold episodes would also likely lead to time-variability in the water content and related geochemical tracers in erupted lavas, as well as the amount of water being transported by slabs into the deep mantle.