Upper plate deformation and trench retreat modulated by subduction-driven shallow asthenospheric flows

Upper plate deformation within a subduction zone depends on the complex relationship between surface plate motions, trench motion, slab pull and asthenospheric flow. Previous modelling studies suggest that trench motion rates should be related to slab buoyancy, but this relationship is neither clear nor verified by observations of natural subduction systems. Trench motion is also thought to induce upper plate deformation; however, no clear correlation has been identified between the direction of trench motion and the mode of upper plate deformation. In this study, we construct 2-D thermo-mechanical models to explore the relationship between slab pull, trench retreat and upper plate deformation, focusing on subduction systems with retreating trenches. We start with quasi-steady-state subduction and introduce a positive density anomaly into the slab to transiently increase slab pull. We vary both the value of the density anomaly and the properties of the upper plate to isolate key controls on trench retreat and upper plate deformation. Our models indicate that asthenospheric flow responds to changes in slab pull and influences both trench retreat and upper plate deformation. We propose that trench retreat depends on the competition between shallow and opposite asthenospheric flows below the subducting and upper plates, and that a fast sub-slab flow can hamper trench retreat even when slab buoyancy is high. After a transient slab pull increase, the mode of upper plate deformation partly depends on the upper plate's ability to translate horizontally: an upper plate with a ridge at its trailing edge deforms by shortening, while a fixed upper plate may deform by extension. Finally, in some cases, upper plate deformation seems to allow trench retreat if the upper plate is weak enough to be deformed by the basal shear from underlying asthenospheric flow. Our results provide insights into retreating subduction systems with contrasting upper plate deformation modes, such as the compressive Andes and the extensional Aegean.