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

    Crameri F, Tackley P J

    Subduction Initiation, Plate Tectonics, Mantle Convection, Numerical Modelling, Planetary Evolution, Geodynamics

    Free-slip versus free surface steady-state stagnant-lid. Small-scale convection experiments lidC2 with a free-slip surface (left hand side column) and lidC5 with a free surface (right hand side column) both employing a composite yield stress with a yield stress gradient of Δσy = 0.0054 and a dimensional, constant yield stress of σy,const = 8.7 MPa. (a) Surface topography of the free-slip top boundary model that is calculated from the normal stress occurring at the vertically-fixed, top domain boundary and (f) the actual topography that occurs in the model with the free top boundary. (b,g) Effective viscosity and the 700 K contour (black line), (c,h) second invariant of the strain rate, (d,i) second invariant of stress, and (e,j) horizontal velocity are shown for both models, where green colours indicate flow in clockwise direction. If applied, the sticky-air layer is removed visually and replaced with a hatched grey area. Very high values are clipped in regions where they exceed a certain maximum value (black areas). White arrows and grey streamlines indicate the flow direction and pattern.

    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.