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Geodynamic models incorporating metamorphic phase transformations almost invariably assume the validity of the Boussinesq approximation that violates conservation of mass. In such models metamorphic density changes take place without volumetric effects. We assess the impact of the Boussinesq approximation by comparing models of orogeny accompanied by lower crustal eclogitization both with and without the approximation. Our results demonstrate that the approximation may cause errors approaching 100 per cent in characteristic measures of orogenic shape. Mass conservation errors in Boussinesq models amplify with model time. Mass conservative models of metamorphism are therefore essential to understand long-term tectonic evolution and to assess the importance of the different geodynamic processes. [ABSTRACT FROM AUTHOR]

Abstract: Double subduction is a geodynamic process in which two plates following each other are synchronously subducted. Double subductions are known for both modern (Izu-Bonin-Marianas and Ryukyu arcs) and ancient (West Himalaya collision zone) plate tectonics. However, our knowledge about this process is limited to conceptual schemes and some restricted analogue experiments. In order to fill this gap we performed 2D numerical experiments using a coupled petrological–thermomechanical approach based on finite differences and marker-in-cell techniques combined with thermodynamic database for the mantle. We investigated the influence of convergence rate, intermediate plate length, activation volume of the mantle dislocation creep and age of the lithosphere. Based on these experiments we conclude that: (A) Subduction rates at two zones running in parallel differ and vary in time even when the total convergence rate remains constant. Supremacy of either subduction zone depends on physical parameters such as (i) relative rates of the plates, (ii) slab ages and (iii) length of the middle plate. (B) Subduction dynamics of the double subduction system involves several processes unknown in simple subduction systems, such as (i) eduction (i.e. “un-subduction”), (ii) subduction re-initiation, (iii) subduction flip triggered by shallow slab breakoff and (iv) turn-over of detached slabs to up-side-down attitudes. (C) Simulated tomographic structures related to slab propagation account for both penetration and non-penetration of the 660km discontinuity. Non-penetration is favored by (i) low convergence rate, (ii) faster relative movement of the overriding plate, (iii) young age of the subducting slab and (iv) up-side-down turn-over of detached slab. [Copyright &y& Elsevier]