Abstract: Real episodic growth of continental crust or artifact of preservation? A 3-D geodynamic model
U. Walzer, R. Hendel.Real episodic growth of continental crust or artifact of preservation? A 3-D geodynamic model. J. Geophys. Res., 118, doi:10.1002/jgrb.50150, 2013.
Real episodic growth of continental crust or artifact of preservation? A 3-D geodynamic model
Uwe Walzer and Roland Hendel, Institut für Geowissenschaften, Friedrich-Schiller-Universität, Jena, Germany
Abstract
The focus of this paper is the question (1) whether the observed zircon age
distribution of continental crust (CC) is produced by real crustal growth episodes or is
only an artefact of preservation. Question (2): In connection with the second alternative
of (1), it has been proposed that there was little episodicity in the production of
new CC and that modeling corroborates this opinion. In answer to (1), we conclude that
a combination of the two proposals might be possible. In matters of (2), we ascertain
that a dynamic modeling of the convection-differentiation system of the mantle reveals
the high probability of magmatic episodes. We solve the full set of balance equations in
a 3-D spherical-shell mantle. The heat-producing elements are redistributed by chemical
differentiation. A realistic solidus model of mantle peridotite is essential. The solidus
depends not only on depth but also on the variable water concentration. Furthermore,
we introduced realistic profiles of Grüneisen parameter, viscosity, adiabatic temperature,
thermal expansivity and specific heat. Our model automatically produces lithospheric
plates and growing continents. Regarding number, size, form, distribution and surface
velocity of the continents, no rules have been prescribed. Regions of the input parameter
space (Ra, σy, k, f3) which are favorable with respect to geophysical quantities show
simultaneously not only episodicity of CC growth but also a reproduction of the observed
zircon-age maxima referring to the instants of time. Admittedly, we also obtain Archean
events for ages greater than 3000 Ma which are not or scarcely visible in the observed
zircon ages. Sinusoidal parts of the evolution curve of qob, Ur and Ekin are found superposed
with a monotonous decrease. Tmean, however, decreases smoothly and slowly,
nearly without pronounced variations. Therefore, we can dismiss catastrophic mechanisms
which simultaneously incorporate the whole mantle.