Journal of Geodynamics 38 (2004) 117–129 Time evolution of negative buoyancy of an oceanic slab subducting with varying velocity R. Mahatsente, G. Ranalli ∗ Department of Earth Sciences and Ottawa-Carleton Geoscience Centre, Carleton University, Ottawa, Ont., Canada K1S 5B6 Received 11 February 2004; received in revised form 20 April 2004; accepted 17 May 2004 Abstract The negative buoyancy of a subducting oceanic slab is estimated based on thermal models and various kinematic boundary conditions. Both temperature and negative buoyancy are time-dependent and are computed subject to time-varying subduction rates. The magnitude of the negative buoyancy increases with increasing depth of pen- etration of the slab and subduction velocity. For mature oceanic lithosphere, the buoyancy of a slab reaching the base of the mantle transition zone is approximately 5 × 10 13 Nm -1 for a peak subduction rate of 5 cm a -1 . In all cases considered, the rate of increase in negative buoyancy depends on the convergence rate at a particular period of time during subduction. If the velocity increases exponentially in the initial stages of subduction, the maximum rate of increase is observed when the slab subducts with the peak subduction velocity. If subduction continues with a constant velocity, the rate of increase in thermal buoyancy begins to decrease as the slab penetrates the lower mantle, and the buoyancy approaches an asymptotic value for increasing time. If the velocity decreases exponentially in the last stages of subduction, the negative buoyancy begins decreasing while the slab is still sinking, as its thermal anomaly decreases. After subduction stops, the slab achieves thermal equilibrium with the surrounding mantle in approximately 10–20 Ma depending on its past history. © 2004 Elsevier Ltd. All rights reserved. Keywords: Subduction; Oceanic Lithosphere; Temperature in subducting slab; Negative buoyancy 1. Introduction It is widely assumed that the negative buoyancy of oceanic slabs is the primary driving mechanism of plate motion (e.g. Forsyth and Uyeda, 1975; Davies and Richards, 1992). The thermal buoyancy can ∗ Corresponding author. Fax: +1 613 5202569. E-mail address: granalli@ccs.carleton.ca (G. Ranalli). 0264-3707/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jog.2004.05.002