SLAB GEOMETRY AND TECTONICS OF THE OVERRIDING CONTINENTAL PLATE : INSIGHTS FROM ANALOGUE MODELLING

The shortening of South America and the growth of the Andean Cordillera occurs during episodes of high intensity compression resulting from variations of the boundary conditions applied on the continental plate. To understand the effect of subduction on the overriding plate tectonics, we built analogue experiments to simulate the subduction of an oceanic lithosphere below a continental plate. Lithospheric plates are modelled using silicone putty initially floating above a low viscosity glucose syrup that represents the upper mantle asthenosphere. Plate convergence is imposed by a piston advancing at constant velocity. We observe the evolution of slab geometry, measure slab dip and the upper plate deformation during experiments.

The process of subduction never reaches any steady-state regime, the dip of the slab changing to accommodate slab reorganization close to the upper-lower mantle discontinuity. Following subduction initiation, we observe stages during which the slab advances at depth below the continental plate and the dip of the subduction zone decreases, followed by slab roll-back episodes during which the dip of the slab increases. Upper plate shortening is significantly influenced by these dip variations. We observe that intense shortening occurs when the dip of the slab decreases, i.e. surface tectonics are correlated with slab dip variations rather than with the dip of the slab. These experiments show that the overriding plate shortening may vary despite other parameters influencing the tectonic regime, such as convergence velocity, absolute velocity of the continent, age and buoyancy of the slab, etc…, remain constant. They explain why South America shortening does not always correlate with plates velocity and buoyancy.