Subduction of continental lithosphere, changes in negative buoyancy, and slab-plume interaction: consequences for slab breakoff

The time evolution of buoyancy of oceanic and continental lithosphere during subduction is estimated on the basis of a kinematic model with both constant and time-varying convergence rates. The negative buoyancy before the arrival of continental lithosphere at the subduction zone increases with increasing depth of penetration of the slab at a rate proportional to the convergence rate. The time required for the negative buoyancy to be reduced to zero by the subduction of positively buoyant continental material depends on the depth reached by the oceanic slab and the convergence rate, but is typically of the order of a few tens Ma. If a rising plume impinges from below on a near-stationary slab in the upper mantle, the corresponding part of the slab is heated and therefore softened. The softening effect is enhanced if the slab includes continental material. The combination of changes in negative buoyancy caused by continental subduction, and softening of a part of the slab caused by slab-plume interaction, may act as a regulator for the time of slab breakoff and consequently for the time and type variations of magmatism in the overriding lithosphere above a subduction zone. A plausible example of this situation may be provided by the Alpine slab subduction beneath the Adria plate at Paleocene time. Here, the Tertiary convergence between Europe and Africa plates was characterized by the consumption of both oceanic and continental European crust. Some million years later (45-30 Ma ago), two contrasting magmatic suites developed in the south-eastern sector of the Alps, partially overlapping in time: a) a calc-alkaline, subduction-related suite, and b) an alkaline, plume-related volcanic suite. On the basis of geological, geophysical and geochemical data we supposed that both the magmatic suites originated by a common and primary deep mantle plume the root of which was located beneath the Cape Verde-Madeira-Canary Islands region, while the head of which was swerved and frayed by the Eurasian plate that from this latitude moved northeastwards. In the Alpine region, the plume head material might have interacted with the Alpine subducting slab causing its heating, softening, and finally its detachment. Ensuing upwelling of plume material through the so formed plate window is supposed to be the responsible for either the partial melting of the lithospheric mantle wedge and for the partial melting of the plume material itself.