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

In this article we examine the effects of impact of slab rocks eclogitization on the subduction regime under the 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 the steep subduction into the deep mantle. The results of numerical petrological-thermomechanical 2D modeling of subduction under the continent in a wide range of eclogitization parameters of oceanic crust rocks (discrete eclogitization) are presented. The effects of a lower kinetic delay of eclogitization in the water-bearing basalt layer, compared to the drier underlying gabbro layer, have been tested. Based on results of 112 numerical experiments with 7 variants of eclogitization ranges (in range 400–650°C for basalt and 400–1000°C for gabbro) at different potential mantle temperatures (ΔT = 0–250°C, above modern value), and steep, flat and transitional subduction regimes were identified. The mode of steep subduction occurs under modern conditions (ΔT = 0°C) with all ranges of eclogitization. Here it is characterised 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 or and then tucking of the slab. Subduction is accompanied by the formation of felsic and mafic volcanics and their plutonic analogues. At elevated temperatures of the mantle (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 episodic, scarce magmatism (from mafic to felsic), which occurs at a significant distance (≥500 km) from the trench. During the transition regime, which is also realised in models with elevated mantle temperatures, there is a characteristic change occurs from flat to steep subduction, resulting in a stepped shape of the slab. As the kinetic shift of eclogitisation increases, flat subduction develops. An increase in the thickness of the continental lithosphere from 80 km to 150 km contributes to the implementation of 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 the thickness of the oceanic crust and the degree of depletion of the oceanic mantle lithosphere. As a result, the less frequent flat movement of slabs is determined by other factors.