Résumé
If the surface lateral limits of tectonic plates are well mapped by seismicity, the bottom boundary of the uppermost rigid layer of the Earth, comprising both crust and shallow mantle, remain elusive. A lithospheric plate can be viewed as a cold, rigid, undeformed and/or translating block, which relates to different physical fields, i.e. temperature, viscosity, strain rate and velocity. Their variation with depth are here investigated in thermo-mechanical models of plate and upper mantle dynamics, either in a transient subduction or in a steady-state plate-driven set-up with homogeneous mantle rheology derived olivine plastic creeps. We consider three different definitions of lithosphere-asthenosphere boundaries (LAB), associated to either temperature (1300 K), strain rate (10 (super -16) s (super -1) ). or horizontal velocity (0.5 % difference from surface velocity). The depths of these three LABs are distinct from one another and the base of the "constant-velocity" plate (i.e. the material translating at constant horizontal velocity) deforms in continuity with the underlying asthenosphere mantle. The thermal structure has a major control on the three LABs, which all deepen with increasing plate age. However, the surface plate velocity, the asthenosphere flow geometry and magnitude also influence the velocity- and strain rate-defined LABs, as well as the thickness of the deformed region at the base of the "constant-velocity" plate. We interpret the plate thickening with increasing surface velocity as a thicker mantle layer dragged by a larger surface plate motion. The mechanical transition from lithosphere to asthenosphere adjusts when mantle dynamics evolves, and a "constant-velocity" slab, also deforming at its borders, is observed in subduction simulations. The definition of lithosphere as a constant-velocity plate is relevant to quantify mass transport within the Earth's mantle. We finally discuss how geophysical data do not well constrain the vertical variations of velocity or strain rate with depth.