ABSTRACT
The Earth is subject to the secular cooling of a heterogeneous and stratified mantle, and to astronomical tuning. The combination of these two parameters is here inferred as the main controlling factors of plate tectonics. The planet is still hot enough to maintain at about 100 km depth a layer where partial melting determines a low–velocity and low–viscosity layer at the top of the asthenosphere, allowing partial decoupling of the lithosphere with respect to the underlying mantle. The Earth’s rotation and the tidal despinning generate a torque acting on the lithosphere, and producing a net westerly directed rotation of the lithosphere with respect to the underlying mantle, being this rotation decoupled in the low–velocity asthenospheric layer. This polarization controls a diffuse asymmetry along plate boundaries, which are shaped by the “eastward” relative mantle flow with respect to the overlying lithosphere. Velocity gradients are the by–product of the lateral viscosity variations in the low–velocity layer, i.e., the decollement plane. The horizontal component of the solid Earth’s tide pushes plates to the “west”. The brittle–ductile transition is inferred to act as a switch for earthquakes. During the interseismic stage, a dilatational band forms in the brittle upper crust. The stretching is partially recovered during the coseismic stage when the fault hangingwall falls down releasing its gravitational energy along a normal fault. On the contrary, for a thrust fault, during the interseismic stage an over compressed band forms above the brittle–ductile transition, which is opposingly dilated during the earthquake, delivering the elastic energy accumulated in the hangingwall. Therefore, energy storage is different and gravity acts with opposed versus as a function of the tectonic style. Fluids react accordingly, pre, syn and post the earthquake.