Subduction initiation from a stagnant lid and global overturn: new insights from numerical models with a free surface

Abstract

Subduction initiation is a key in understanding the dynamic evolution of the Earth and its fundamental difference to all other rocky planetary bodies in our solar system. Despite recent progress, the question about how a stiff, mostly stagnant planetary lid can break and become part in the global overturn of the mantle is still unresolved. Many mechanisms, externally or internally driven, are proposed in previous studies. Here, we present the results on subduction initiation obtained by dynamically self-consistent, time-dependent numerical modelling of mantle convection. We show that the stress distribution and resulting deformation of the lithosphere are strongly controlled by the top boundary formulation: A free surface enables surface topography and plate bending, increases gravitational sliding of the plates and leads to more realistic, lithosphere-scale shear zones. As a consequence, subduction initiation induced by regional mantle flow is demonstrably favoured by a free surface compared to the commonly applied, vertically fixed (i.e. free-slip) surface. In addition, we present global, three-dimensional mantle convection experiments that employ basal heating that leads to narrow mantle plumes. Narrow mantle plumes impinging on the base of the plate cause locally weak plate segments and a large topography at the lithosphere-asthenosphere boundary. Both are shown to be key to induce subduction initiation. Finally, our model self-consistently reproduces an episodic lid with a fast global overturn due to the hotter mantle developed below a former stagnant lid. We conclude that once in a stagnant-lid mode, a planet (like Venus) might preferentially evolve by temporally discrete, global overturn events rather than by a continuous recycling of lid and that this is something worth testing more rigorously in future studies.

The dynamical control of subduction parameters on surface topography

“…As the plate approaches the subduction trench, it is forced to stay flat (due to the fixed surface) instead of creating a fore-bulge, which would reduce the bending radius of the plate. The direct result of that fixation is that the subducting plate bends more strongly at the collision zone and thereby creates artificial stresses within but also on top of the plate as has been shown previously [Crameri and Tackley, 2016]. These artificial normal stresses at the model top boundary cause, for example, yielding in the weak crustal layer on top of the subducting plate close to the trench, but also at shallow depth on the upper plate, which does not have weak crust on top (see Figure 4e).…”

“…Apart from the missing plate‐bending stresses when applying a vertically fixed surface [see Thielmann et al ., ], this must be the result of the high lateral viscosity variations of our model (between strong intact and weak yielding zones of the crustal layer, and between weak subduction channel and strong plate interior), and unnatural plate bending at the subduction trench: As the plate approaches the subduction trench, it is forced to stay flat (due to the fixed surface) instead of creating a fore‐bulge, which would reduce the bending radius of the plate. The direct result of that fixation is that the subducting plate bends more strongly at the collision zone and thereby creates artificial stresses within but also on top of the plate as has been shown previously [ Crameri and Tackley , ]. These artificial normal stresses at the model top boundary cause, for example, yielding in the weak crustal layer on top of the subducting plate close to the trench, but also at shallow depth on the upper plate, which does not have weak crust on top (see Figure e).…”

GHOST: Geoscientific Hollow Sphere Tessellation

“…In the last 40 years, numerical mantle convection studies have improved our understanding of mantle dynamics as a whole (Schubert et al, 2001). While early studies looked at aspects of fluid dynamics (Busse, 1975;Christensen and Harder, 1991), more recent studies have been exploring a wide variety of topics -for example, mantle mixing (van Keken et al, 2002), melting (Tackley, 2012;van Heck et al, 2016;Dannberg and Heister, 2016), the effect of plate motion history on the longevity of deep mantle heterogeneities (Bull et al, 2014) or assimilating lithosphere and slab history in 4-D Earth models (Bower et al, 2015).…”

Analytical solution for viscous incompressible Stokes flow in a spherical shell

“…As a consequence, the numerical modeling of mantle convection has grown in complexity (e.g., Tackley, 2012;van Heck et al, 2016;Dannberg and Heister, 2016) with the advent of ever more refined modeling techniques and powerful computers. It has also been recognized that the mantle exerts a primary control on the evolution of tectonic plates and that both should be simulated together if one is to build a fully dynamic Earth model (e.g., van Hinsbergen et al, 2011;Bull et al, 2014;Bower et al, 2015;Crameri and Tackley, 2016).…”