Fate of sediments on the descending plate at convergent margins

Abstract

The evolution of the continents and of continental crust is strongly dependent on the trajectory of the sedimentary cover on the descending oceanic lithosphere at arc systems. Although direct calculations of accretion are not reliable, indirect evidence strongly suggests that most of the sediment cover is either accreted or underplated (subcreted) to the upper plate. This evidence includes the increased thickening of accretionary prism beneath parts of the inner trench slope that cannot be explained by deformation within the prism and by protolith composition both in subophiolitic metamorphics and in blueschist terrains. That a small fraction of this sediment cover is transported to depths of at least 100 km is demonstrated in several ways. Flux calculations of mass and selected elements through arc systems require addition of a few tens of metres of sediment to the arc magmas. Global correlations of variations between arc magma characteristics and regional geological parameters show: (1) a strong correlation between silica content of average arc magmas and thickness or maturity of crust in the upper plate, attributed to upper plate contamination; (2) regional variations in ⁸⁷ Sr/ ⁸⁶ Sr and Pb isotopic ratios of arc basalts that correlate spatially with isotopic ratios in the non-calcareous components of pelagic sediments. This correlation is argued to reflect the variation of terrigenous material in the basal pelagics that are involved in magma production. Subduction of continental crust to depths of 100 km, either as part of the descending plate or by tectonic erosion of the upper plate is not supported by these correlations. Recycling rates of continental crust by accretion and subcretion and of mantle by subduction of oceanic lithosphere in contemporary arcs are very large compared with the growth rate of continental crust, which appears similar to the magma production rates in arc systems. This present production rate of continental crust is very much smaller than that during early Earth history, and is compatible with Phanerozoic ocean freeboard changes. In addition, average SiO ₂ and K ₂ O contents of contemporary arc magmas are much lower than those estimated for mean continental crust, leading to the conclusion that the magmas being produced at active arcs cannot be used as a model for the development of most of the Earth’s continental areas.