2016 PSPA - Mireia Peral – Analogue modelling of double polarity subduction

Reproducing geological processes in the laboratory has been of a great interest among scientists for years. Laboratory experiments, scaled in time and space, may help us to improve our knowledge of some geological processes that are impossible to study in nature. For example, we may reproduce large tectonic processes and study their evolution in the order of millions of years.

This study is based on laboratory simulations of a specific tectonic process, called double polarity subduction. In a subduction system, which occurs on convergent margins of plate boundaries, two plates collide and one moves under the other (overriding plate). In the Mediterranean Vergés and Fernàndez (2012) hypothesized the presence of two subductions retreating in opposite directions since 30 million years.

Our experiments consist in simulating in the laboratory the evolution of a double polarity subduction process, but without taking into account the overriding plate. We realized the experiments in a square Plexiglas tank full of high viscous syrup, representing the mantle. The setup contains two oceanic plates (Figure 1), made with silicone putty, and subduction is started by deflecting manually the leading edge of the plates. Each centimeter corresponds to 60 km. The experiments are monitored with two cameras that take photographs in time intervals of 30 seconds from the top of the experiment and from an oblique position, so we can study the system in function of time. Different setups were designed to test the influence of two variables in the system: i) the width of the plates and ii) the lateral distance between the two subducting plates.

The evolution of the models is characterized by three different phases: (1) initial stage of subduction, corresponding to the evolution of the system until the plates reach the base of the model box; (2) approaching trenches, until they pass each other; (3) diverging trenches, until the retreating of plates is over.

Our results indicate that velocities of the trench (marking the position at which the subducting plate begins to descend) increase during phase 2 and then decrease during phase 3. We explain this trendline as due to the interaction of the mantle flow induced by both plates in the contact area. When plates are wider the same process is active. Nevertheless, when lateral distance between plates increases we do not observe any change of the velocities during the evolution and the interaction of the two plates become negligible when they are 10 cm spaced (600 km in nature).

Figure 1. Top view of a double-polarity subduction experiment. The images correspond to three different stages of the evolution. The plates are 10 cm (600 km in nature) width and initially spaced 0,5 cm (30 km in nature).

This work is supervised by Manel Fernandez (ICTJA-CSIC) and Sergio Zlotnik (UPC, Barcelona). The experiments have been carried out in collaboration with Ágnes Király, Francesca Funiciello and Claudio Faccenna from the Laboratory of Experimental Tectonics (Roma Tre University, Italy). This study is part of the Project “Testing the geodynamic evolution of the Western Mediterranean (We-Me) financed by CSIC (PIE-CSIC-201330E111). We also thank to the project AECT-2016-1-0002 of the Barcelona Supercomputing center (BSC-CNS).