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Dependence of slab buoyancy on composition and convergence rate: in- sights from kinematic modelling

AutorBoonma, K. ; Kumar, Ajay ; García-Castellanos, Daniel ; Jimenez-Munt, Ivone ; Fernandez, Manel
Fecha de publicación27-ago-2017
CitaciónXV International Workshop on Modelling of Mantle and Lithosphere Dynamics (2017)
ResumenThe peeling o of the lithospheric mantle from the crust and its detachment and sinking into the asthernospheric mantle is known as delamination. Lithospheric delamination is a geodynamic mechanism often invoked for the evolution of collision zones, yet there are still open questions about the conditions under which the mechanism operates. The general condition that was thought to lead to delamination is that the lithosphere must be denser than the asthenosphere (lith > asth) so as it sinks downwards, the buoyant asthenosphere makes contact with the crust, replacing the denser lithosphere. The densities adopted by previous studies have been considered to be temperature dependent only. We adopt here a mineral physics viewpoint, where the density depends on temperature, pressure, and composition, such that the density of the lithospheric mantle can be lower than that of the underlying asthenosphere (lith < asth), posing a serious problem for the initiation of the delamination process. We calculate the density of the upper mantle for given temperature and pressure conditions by computing stable mineral assemblages, using Gibbs free energy minimisation algorithm, for dierent types of lithosphere (Archon, Tecton, and Proton) [1,2]. The mantle mineral assemblages are calculated from major oxides composition based on mantle xenoliths/garnet peridotites in the CaO-FeO-MgO-Al2O3-SiO2 (CFMAS) framework. At a certain convergence rate, plate under-thrusting develops with a downward velocity that is fast enough to prevail on thermal re-equilibration with the surrounding asthenosphere. This facilitates the lithospheric mantle to become heavier due to pressure eect which dominates over the temperature eect. The subducted lithospheric mantle, therefore, acquires negative buoyancy, such that its density is higher than the asthenosphere, and causes it to detach from the crust and sink into the asthenosphere. We present a parametric study on the relationship between slab buoyancy and convergence rate as well as lithospheric mantle composition using a simple 2D kinematic model with thermal advection and diusion.
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