Sea-floor spreading, small-scale convection, and passive margins: Interplay and effect on the driving forces of Plate Tectonics

I present the results of a series of numerical experiments, based on a visco-elasto-plastic rheological model of the lithosphere, aimed at studying the interplay between mantle convection and tectonic processes at continental margins. In these experiments, the reference thermal states of the oceanic and continental lithospheres are described by a plate cooling model and by the solutions of the steady heat equation, respectively, while a small non-adiabatic temperature gradient is assumed for the asthenosphere and transition zone. The resulting thermo-mechanical model incorporates both vertical (Rayleigh-Benard) and horizontal (small-scale) convection and allows to predict the state of stress across continental margins, as well as some tectonic processes that are observed in these regions. Small-scale convection arises from lateral temperature gradients. It always develops along passive margins, where the thermal regime of the oceanic lithosphere meets the downward-dipping isotherms of the continental lithosphere. This form of horizontal convection has the potential to deform the lower part of the continental lithosphere, generate Rayleigh-Taylor instabilities, and produce up to ~50 MPa of compressional stress across continental margins. The formation of Rayleigh-Taylor instabilities is accompanied by lithospheric thinning, which in turn induces negative thermal anomalies that contribute to the maintenance of isostatic equilibrium by increasing the density of the residual lithosphere. These anomalies propagate towards the interior of the continental lithosphere, until the increased rheological strength associated with lower temperatures is sufficient to prevent further delamination. Therefore, the lower continental lithosphere is always colder than predicted by steady-state solutions of the heat equation. Basal landward traction along passive margins, resulting from small-scale convection, is further enhanced when the oceanic lithosphere adjacent to the continental margin is bounded by a spreading ridge. In this instance, numerical experiments consistently show the existence of an active spreading component, up to 5 mm/yr, which generates additional traction below the continental margins and contributes to a compressive stress regime in these regions. Consequently, a net horizontal landward push develops along the continental margins of a tectonic plate, which combines with other driving forces to determine the plate kinematics. Finally, numerical experiments show that non-adiabatic vertical temperature gradients drive the formation of Rayleigh-Benard convective cells with a wavelength of 600-700 km and a height 500-600 km.