A Forward Model of Mantle Convection with Evolving Continents and a Model of the Andean Subduction Orogen

Abstract

Some essential features of Andean orogenesis cannot be explained only by a dynamic regional model since there are essential influences across its vertical boundaries. A dynamic regional model of the Andes should be embedded in a 3-D spherical-shell model. Because of the energy distribution on the poloidal and toroidal parts of the creep velocity and because of geologically determined mass transport alongside the Andes, both models have to be three-dimensional. Furthermore, we developed a new viscosity profile of the mantle with very steep gradients at the lithospheric-asthenospheric boundary and at a depth of 410 and 660 km. Therefore, the challenges to the code Terra are now essentially larger. In the last 3 years we have resolved these problems in an international cooperation (see Sect. 2.2). Based on the new viscosity profile and on an improved Terra, we computed a new forward spherical-shell model (Walzer and Hendel, J Geophys Res submitted, 2012b). For this model, we derived also a new extended acoustic Gruneisen parameter, γ ax , new profiles of the thermal expansivity, α, and of the specific heat, c v , at constant volume as well as a solidus depending on both the pressure and the water abundance. These innovations are essential to incorporate a chemical-differentiation mechanism into the model. We arrived at rather realistic episodes of continental growth interrupted by magmatically quiet time spans distributed over the whole time axis. Nevertheless, the model shows a main magmatic event at the very beginning of the Earth’s evolution. Papers on the improvement of Terra (Kostler et al. Comput Geosci submitted, 2012; Muller and Kostler, Int J Numer Methods Eng submitted, 2012)have been written. We conceived a regional model of the Andean Sect. 3.2.1) with the same new viscosity profile. We want to investigate why there is flat-slab subduction in some segments of the Andes and why deformation of the crust and volcanism migrate eastward. The evolution of the abundances of incompatible elements indicate a cycle which was finished by a fast process, perhaps by a large-scale delamination of the lower plate, perhaps also by another type of delamination. In connection with another spherical-shell model (with prescribed plate boundaries), the regional model should numerically explain why a plateau-type orogen evolved at an oceanic-continental plate boundary.