Analogue modelling of opposite subduction retreating in adjacent plates

Abstract

Opposite subduction retreating in adjacent plate segments has been proposed in several regions of the Earth and particularly in the Westernmost Mediterranean (Verg´es et al., 2012; Casciello et al., 2015). Recent numerical experiments show a strong interaction between the induced mantle flows of each retreating plate (Kir´aly et al., 2016). In this work we show the result of a series of analogue models based on viscous syrup (representing the mantle) and silicone putty (representing the subducting plate), which have been designed to simulate the evolution of a double subduction system. The basic setup contains a pair of plates subducting in opposite directions. The plates are fixed at their trailing edge to enforce slab rollback behaviour and subduction is started by deflecting manually the leading edge of the plate (i.e., initial slab pull, phase 1). Different setups were designed to test the influence of two variables on the system: i) the width of the plates, that varies from 10 cm to 30 cm (1 cm in model corresponds to 60 km in nature) and ii) the lateral distance between the two subducting plates, that varies from 10 to 0.5 cm. Our results show that trench velocities increase during the stage of approaching trenches (phase 2) and then decrease after trenches pass each other (phase 3). On the other hand, the trench curvature increases linearly during the entire evolution whereas the lateral distance increases along time, indicating that effective lateral stresses are produced associates with the asymmetry of toroidal flows. This behaviour indicates a strong interaction between the stresses produced by the two retreating slabs that propagate through the mantle flow, which in turn depends on the initial plate separation.