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Abstract: We use a coupled petrological–thermomechanical model to investigate the dynamical effects of metamorphic reactions and melting on the seismic structure of thermal–chemical plumes beneath volcanic arcs. Plume generation is driven by the subduction of buoyant crustal rocks and expulsion of aqueous slab fluids that causes hydration and partial melting of the mantle wedge. The model demonstrates two chemically distinct types of plumes. Unmixed plumes initiate from the melting front within the mantle that arises as a consequence of infiltration of slab-derived water-rich fluids, whereas mixed plumes initiate from the slab itself and entrain both slab- and mantle-derived magmas. Mixed plumes explain magmas such as adakites with crustal signatures, while primitive arc tholeiites are attributed to unmixed plumes. As a consequence of the interplay between water content and temperature both positive and negative seismic velocity anomalies are associated with the plumes. Positive anomalies are prevalent close to the slab due to the lowered temperatures associated with regions of cold plume initiation. Negative seismic anomalies develop at shallower depth due to the partially molten rocks that form plume heads. Flat lying partially molten regions that form beneath volcanic arcs as a consequence of cold wet plumes are manifest by >20% variations in the local Poisson ratio, as compared to variations of ∼2% expected as a consequence of temperature variation within the mantle wedge. In contrast to models that attribute a purely thermal origin for mantle wedge seismic anomalies, the petrological–thermomechanical model is able to explain the strong seismic velocity variations existing beneath volcanic arcs. [Copyright &y& Elsevier]

Abstract: We investigate crustal growth processes on the basis of a 2D coupled geochemical–petrological–thermomechanical numerical model of retreating intraoceanic subduction. The model includes spontaneous slab retreat and bending, subducted crust dehydration, aqueous fluid transport, mantle wedge melting, and melt extraction resulting in crustal growth. Our numerical experiments show that the rate of plate retreat influences both the rate of crust formation and composition of newly formed crust. The rate of trench retreat, which is a manifestation of subduction rate, strongly varies with time: retreat rates slow (from 7cm/a to 1cm/a) shortly (in a few Ma) after the beginning of subduction and then increase (up to 4cm/a). Subsequently two different scenarios can be distinguished: (1) subduction rate decay that leads ultimately to cessation of subduction, (2) subduction rate acceleration (up to 12cm/a), which stabilizes subduction. The rate of crust formation positively correlates with rate of trench retreat. Modelled average rates of crustal growth (30–50km3/km/Ma) do not include effects of dry mantle melting and are close to the lower edge of the observed range of rates for real arcs (40–180km3/(kmMa)). The composition of new crust depends strongly on the evolution of subduction. Four major magmatic sources can contribute to the formation of the crust: (1) hydrated partially molten peridotite of the mantle wedge, (2) melted subducted sediments, (3) melted subducted basalts, (4) melted subducted gabbro. Crust produced from the first source is always predominant. In all studied cases it appears shortly after beginning of subduction and is a persistent component so long as subduction remains active. Significant amount of crust produced from other three sources appear (i) in the beginning of subduction due to the melting of the slab “nose” and (ii) at later stages when subduction velocity is low (<1cm/a), which leads to the thermal relaxation of the slab. Both the intensity of melt extraction, which was prescribed by given melt extraction threshold, and the age of subducted plate affect the volume of new crust. On a long time scale the greatest volume of magmatic arc crust is formed with an intermediate melt extraction threshold (2–6%) and medium subducted plate ages (70–100Ma). [Copyright &y& Elsevier]

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]

Abstract: We use a coupled petrological–thermomechanical model of subduction with spontaneous slab bending to investigate magmatic productivity at active continental margins. The model is designed to simulate fossil Pacific-type margins that have a broad well-developed fore-arc accretionary wedge system. The degree of plate coupling strongly depends on the dimensionless ratio () between the plate convergence rate and the water propagation velocity. Delamination of the slab from the overriding plate followed by trench retreat is common for models with relatively slow convergence rates (). In contrast, higher convergence rates () result in continuous plate coupling. The slab bending curvature increases with the length of the subducted plate. Periodic variations of the slab angle with time are observed at the later stages of subduction and become more conspicuous with depth. These variations are favoured by slower subduction rates and stronger oceanic crust. Two fundamentally different regimes of melt productivity are observed in numerical experiments and correlated with natural observations: (1) during continuous convergence with coupled plates (as in the Late Paleozoic margin of central Chile) the largest melt productivity occurs at the onset of subduction due to temporary steepening of the slab, productivity then decays rapidly with time due to flattening of the slab inclination angle, thereby precluding further formation of partially molten mantle wedge plumes. (2) Highest melt productivity is obtained in simulations associated with slab delamination and trench retreat. As a result an extension occurs with decoupling of plates that finally leads to the formation of a pronounced back-arc basin (similar to the Mesozoic margin of southernmost Chile). In this case melt production increases with time due to stabilization of slab inclination associated with upward asthenospheric mantle flow toward the spreading centre, thereby favouring the generation of hydrous partially molten plumes from the slab. [Copyright &y& Elsevier]