The downslope sliding of the lithosphere away from a spreading center is called a

The downslope sliding of the lithosphere away from a spreading centre is called a ridge push.

Ridge push otherwise sliding plate or gravitational sliding force is a force that causes plate tectonics motion at the mid-ocean ridge. Sliding is caused by raised asthenosphere that eventually pushes the lithosphere into the subduction below mid-ocean ridges.  

The force, however, acts on the entire ocean plate and not only on the ridges due to gravitational force. It is called ridge push because earlier models of plate tectonics during simulations showed the upwelling of the magma during spreading of the sea-floor at mid-oceans ridges which cause the plate to diverge.