Subduction Styles at Different Stages of Geological History of the Earth: Results of Numerical Petrological–Thermomechanical 2D Modeling

In this article we examine the effects of eclogitization of slab rocks on the subduction regime under a continent. Eclogitization of rocks in high-pressure metamorphic complexes occurs only in the areas of penetration of hydrous fluid. In the absence of hydrous fluid, the kinetic delay of eclogitization preserves low-density rocks under P‒T conditions of eclogite metamorphism, delaying the weighting of a slab and reducing the efficiency of the slab-pull mechanism, which contributes to steep subduction into the deep mantle. The results of numerical petrological–thermomechanical 2D modeling of subduction under a continent in a wide range of eclogitization parameters of oceanic crustal rocks (discrete eclogitization) are presented. The effects of a lower kinetic delay of eclogitization in a water-bearing basalt layer, compared to a drier underlying gabbro layer, have been tested. Based on the results of 112 numerical experiments with 7 variants of eclogitization ranges (400–650°C for basalt and 400–1000°C for gabbro) at different potential mantle temperatures (ΔT = 0–250°C, above the modern value), and steep, flat, and transitional subduction regimes were identified. The steep subduction regime occurs under modern conditions (ΔT = 0°C) with all ranges of eclogitization. Here, it is characterized by an increase in the angle of subduction of the slab as the plate descends, and above the boundary of the mantle transition zone there is a flattening and/or tucking of the slab. Subduction is accompanied by the formation of felsic and mafic volcanics and their plutonic analogues. At elevated mantle temperatures (ΔT ≥ 150°С) and discrete eclogitization over a wide range, the flat subduction regime is observed with periodic detachments of its steeper frontal eclogitized part. The flat subduction regime is accompanied by significant serpentinization of the mantle wedge and sporadic, scarce magmatism (from mafic to felsic), which occurs at a significant distance (≥500 km) from the trench. During the transition regime, which is also achieved in models with elevated mantle temperatures, a characteristic change occurs from flat to steep subduction, resulting in a stepped shape of the slab. As the kinetic shift of eclogitization increases, flat subduction develops. An increase in the thickness of the continental lithosphere from 80 to 150 km contributes to steep subduction, while the influence of the convergence rate (5–10 cm/year) is ambiguous. Discrete eclogitization of thickened oceanic crust and depletion of lithospheric mantle in the oceanic plate are the main drivers of flat subduction. In modern conditions, their influence becomes insignificant due to the decrease in thickness of oceanic crust and degree of depletion of the oceanic mantle lithosphere. As a result, less frequent flat movement of slabs is determined by other factors.