SOME THERMOMECHANICAL ASPECTS OF THE SUBDUCTION OF CONTINENTAL LITHOSPHERE

On the basis of thermal and mechanical modeling, it is concluded that the subduction of continental lithosphere can lead to its breakup and the formation of a new plate contact within the middle or lower crust. As a result, (part of) the subducting continental crust is transferred to the upper plate. Breakup is caused by the resistive forces acting upon subducting continental crust, due to the buoyancy of crustal material and to friction at the plate contact, as well as the decrease in strength of the subducting crust once it has been subducted to a depth of a few tens of kilometers. The crustal thickness and the thermal and compositional structure of the continental crust just before the onset of subduction have a large influence on the depth to which continental crust can be subducted (prior to its breakup). The depth to which continental crust can be subducted coherently decreases as the surface heat flow or the crustal thickness of the subducting continental plate increases. In many cases, breakup is found to occur at a time when the upper surface of the continental plate has been subducted to a depth of 25-50 km. The subduction of a cold continental shield or of continental lithosphere with a relatively small crustal thickness, on the other hand, may lead to the subduction of both upper and lower crust to mantle depths.