Control of lithosphere rheology on subduction polarity at initiation: Insights from 3D analogue modelling

Abstract : A series of analogue experiments was used to explore how lateral variations in lithosphere rheology may result in a subduction polarity reversal. Lithosphere models made of sand and silicone putty were designed to represent a plate convergence setting varying laterally from continent-continent to ocean-continent. Several experiments were performed to test the effects of strength profile variations in the converging plates. The modelling outcomes categorized into models that develop with and without a subduction polarity reversal. While a subduction polarity reversal was the most prevalent experiment outcome, the results suggest that upper mantle strength is the dominant factor in determining whether a subduction polarity shift occurs, or not. A weak upper mantle layer promotes obduction of the oceanic lithosphere in the western segment of the model. In model iterations where the upper mantle of the continental lithosphere was stronger than the oceanic lithosphere, a reversal in subduction polarity from north-dipping in the east, to south-dipping in the west occurred. The experiment design was inspired by the Iberian-Eurasian plate convergence and may provide a simple regional-scale explanation for the deformation patterns along the Pyrenean and Cantabrian orogen.