Lithospheric aging, instability and subduction

The subduction behaviour of oceanic lithosphere in relation to its age is studied in detail. It is shown that the penetration depth of subducted lithosphere increases with increasing lithospheric age. In all cases where sufficient data are available, the relation proves to be unique. The controlling property appears to be the amount of gravitational instability of the part of the lithosphere concerned with respect to the surrounding upper mantle. The instability depends, through the density and temperature, on the time elapsed between creation and subduction. It is concluded that gravitational instability of the oceanic lithosphere-upper mantle system is a major cause of plate tectonics. The structure of individual subduction zones is interpreted accordingly.