According to the plate tectonics scenario, the slab of lithosphere gets denser than the upper mantle by eclogitization, and it sinks, dragging the lithosphere to which it is attached into the mantle in a manner similar to a tablecloth sliding off a table entrained by its own weight. So, as the lithosphere is dragged by the slab the distance between
a and
b decreases. A return flow induces extension in the back arc so that the distance between
b and
c increases. But extension in the back arc is not correlated to the lithosphere displacement toward the trench, and overall, the distance between
a and
c decreases. So there is a net decrease in surface and lithosphere consumption.
According to the flow tectonics scenario, a mantle upwelling spreads at the surface. As the flow progresses (to the left), it overruns the lithosphere on its path, burying it in the mantle. So the arc (
b) and the trench move toward
a but this displacement is correlated to the spreading flow and the back-arc extension, and thus the distance between
b and
c increases of at least the same amount than the distance between
a and
b decreases. So in net, there is no lithosphere consumption or even lithosphere creation.
The later scenario is what is observed in the case of the oriental mediterranean basin: a tectonic flow spreading in the middle of fixed lithosphere, without any net decrease in surface.