Spherical dynamic models of top‐down tectonics

Abstract

We use the Multipole–Boundary Element Method (MP‐BEM) to simulate regional and global geodynamics in a spherical 3‐D setting. We first simulate an isolated subducting rectangular plate with length (Llitho) and width (Wlitho) varying between 0.5 and 2 times the radius of the Earth (REarth) and with viscosity ηlitho varying between 100 and 500 times the upper mantle (ηUM), sinking in a layered mantle characterized by lower‐upper mantle viscosity ratioλ = ηLM/ηUM varying between 1 and 80. In a mantle with small upper/lower viscosity contrast (λ ≅ 1), trench and plate motions are weakly dependent on Wlitho; plate motion is controlled by slab pull if Llitho ≤ REarth, while for longer plates plate speed strongly decreases because of the plate basal friction and flow reorganization. An increasing viscosity ratio λ gradually breaks this pattern, and for λ ≅ 10 combined with Wlitho ≈ REarth(and greater) trench advance and retreat are simultaneously observed. These results offer a first‐order explanation of the origin of the size (Llitho ≈ Wlitho ≈ REarth) of the largest plates observed over the past 150 Myr. Finally, two global plate tectonic simulations are performed from reconstructed plates and slabs at 25 Ma before present and before 100 Ma, respectively. It is shown that MP‐BEM predicts present plate kinematics if plate‐mantle decoupling is adopted for the longest plates (Llitho > REarth). Models for 100 Ma show that the slab‐slab interaction between India and Izanagi plates at 100 Ma can explain the propagation of the plate reorganization from the Indian to the Pacific plate.