Time dependence of negative buoyancy and the subduction of continental lithosphere

The time evolution of negative buoyancy of a subducting slab is modelled fr om the beginning of subduction under various kinematic conditions (dip angl e and subduction velocity). The calculations take into account the thermal and density effects of the variations of the thermophysical parameters with temperature and pressure, and of phase transitions. The magnitude of the n egative buoyancy increases during subduction of oceanic lithosphere, up to values in the (2-4) x 10(13) N m(-1) range when the tip of the slab reaches a depth of 600-700 km. If continental material arrives at the trench and i s subducted, the downward buoyancy decreases by an amount proportional to t he volume of the subducted continental crust. Assuming that subduction stop s when the buoyancy becomes zero, and that delamination of the continental crust or slab breakoff do not occur, the maximum downdip length of the subd uctable continental crust is estimated as a function of the dip angle, subd uction velocity and geometry of the margin. In most cases, subduction of co ntinental material down to depths of 100-250 km is possible, and continenta l subduction can continue for times up to 10-15 Ma if the velocity is low. These estimates are not significantly affected by the hypothetical occurren ce of a metastable olivine wedge within the slab, and could be lower bounds if the lower continental crust is mafic and transforms to eclogite. (C) 20 00 Elsevier Science Ltd. All rights reserved.