Numerical modelling of crustal growth in intraoceanic volcanic arcs

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]