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.
Highlights
Earth-like planets lose the heat in their interiors by mantle convection
We present new insights on breaking a stagnant lid by (i) small-scale convection in combination with a free surface and by (ii) global mantle convection in combination with either a lateral gradient in mantle temperature or narrow plumes impinging on the base of the lithosphere
Subduction initiation by small-scale convection A lithosphere below a free surface can be expected to develop a different stress pattern than a lithosphere that is vertically fixed at its top (Thielmann et al 2015)
Summary
Earth-like planets lose the heat in their interiors by mantle convection. Part of mantle convection is the cold top boundary layer: The surface boundary layer is either stagnant on top of a flowing mantle or, which seems less common, it is mobile and part of the vertical flow into and out of the deep mantle (Bercovici 2003; Tackley 2000a,b). Mechanisms like the interaction of thermo-chemical plumes with the lithosphere (Burov and Cloetingh 2010; Burov and Gerya 2014; Ueda et al 2008) or small-scale convection in the sub-lithospheric mantle (Solomatov 2004) might further be able to break an intact lid It is, clear that the question of how subduction might initiate and evolve to its present state remains unclear and elusive (see e.g. Gerya 2011)
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