Why plate and microplate tectonics?

Plate tectonics is the mechanism determining the motion of the lithosphere fragments (plates) with respect to each other and their movement relative to the underlying mantle. The faster-decoupled plate is the Pacific, which is characterized by the presently known lowest viscosity within the top asthenosphere, i.e., the low-velocity zone (LVZ). The existence of a given plate could be explained by the presence of a roughly homogeneous underlying LVZ viscosity, different from the surrounding mantle. The major plates are associated with a number of microplates: why? We infer that microplates exist because they lay over mantle zones with viscosity values higher or lower with respect to the adjacent major bigger plates. In the case of homogeneous LVZ viscosity and lithosphere/asthenosphere decoupling, we could theoretically expect the entire lithosphere to rotate relative to the mantle as a single outer shell, without fragmentation in separate plates. However, mantle tomography and magmatic geochemistry illuminate a heterogeneous mantle vertically and horizontally. Therefore, lateral variation in the geochemical composition, temperature, and fluid content can easily generate viscosity variations in the LVZ, determining the variable decoupling and different velocities among plates, i.e., plate tectonics. A hierarchy in the viscosity within the LVZ could explain the nature of major plates related to the first-order viscosity values in the asthenosphere planform, and microplates could be explained by the occurrence of second-order viscosity values in more restricted upper mantle zones. Alternative formation of a microplate could be ascribed to plates indentation, or backarc basin spreading due to the subduction hinge migrating away relative to the upper plate.