Impact of upper mantle convection on lithosphere hyper-extension and subsequent convergence-induced subduction

Abstract

<jats:p>Abstract. We present two-dimensional thermo-mechanical numerical models of coupled lithosphere-mantle deformation, considering the upper mantle down to a depth of 660 km. We consider visco-elasto-plastic deformation and for the lithospheric and upper mantle a combination of diffusion, dislocation and Peierls creep. Mantle densities are calculated from petrological phase diagrams (Perple_X) for a Hawaiian pyrolite. The model generates a 120 Myrs long geodynamic cycle of subsequent extension (30 Myrs), cooling (70 Myrs) and convergence (20 Myrs) in a single and continuous simulation with explicitly modelling convection in the upper mantle. During lithosphere extension, the models generate an approximately 400 km wide basin of exhumed mantle bounded by hyper-extended passive margins. The models show that considering only the thermal effects of upper mantle convection by using an effective thermal conductivity generates results of lithosphere hyper-extension that are similar to the ones of models that explicitly model the convective flow. Applying a lower viscosity limit of 5 × 1020 Pa s suppresses convection and generates results different to the ones for simulations with a low viscosity asthenosphere having minimal viscosity of approximately 1019 Pa s. During cooling without far-field deformation, no subduction of the exhumed mantle is spontaneously initiated. Density differences between lithosphere and mantle are too small to generate a buoyancy force exceeding the mechanical strength of the lithosphere. The extension and cooling stages generate self-consistently a structural and thermal inheritance for the subsequent convergence stage. Convergence initiates subduction of the exhumed mantle at the transition to the hyper-extended margins. The main mechanism of subduction initiation is thermal softening for a plate driving force (per unit length) of approximately 15 TN m−1. If convection in the mantle is suppressed by high effective thermal conductivities or high, lower viscosity limits, then subduction initiates at both margins leading to divergent double-slab subduction. Convection in the mantle assists to generate a single-slab subduction at only one margin, likely due to mantle flow which exerts an additional suction force on the lithosphere. The first-order geodynamic processes simulated in the geodynamic cycle of subsequent extension, cooling and convergence are applicable to orogenies that resulted from the opening and closure of embryonic oceans bounded by magma-poor hyper-extended passive margins, which might have been the case for the Alpine orogeny. </jats:p>

Modeling Lithospheric Deformation Using a Compressible Visco‐Elasto‐Viscoplastic Rheology and the Effective Viscosity Approach

“…In order to demonstrate the practical use of the V‐E‐VP model, we have run long‐term lithospheric deformation simulations (Figures 7–10). The V‐E‐VP model was implemented in the thermo‐mechanical code MDoodz that can handle tectonics deformations (Candioti et al., 2020; Duretz, May & Yamato, 2016; Kiss et al., 2019; Poh et al., 2020) and accounts for composite rheological models (Bessat et al., 2020; Yamato et al., 2019) and a true free surface (Duretz, Petri, et al., 2016). The model domain has a 300 km width and 100 km height.…”