Numerical Study of Weakening Processes in the Central Andean Back-Arc

Abstract

We employed a 2D numerical thermo-mechanical modeling technique to study the dynamics of the plateau-foreland system, with particular application to the Central Andean back-arc. Our model back-arc consists of a weak plateau indented by a relatively stronger foreland. A normal shortening scenario implies pure-shear tectonic inflation of the plateau. Forces that drive tectonic shortening encounter major resistance from the growing gravitational potential of the rising plateau. Other resistance factors are the brittle strength of the upper crust and the viscous resistance of ductile lower crust and upper mantle. The overall resistance to shortening is up to 6--8 ‚àöó 1012 N m‚Äöà{\'\i}1 for plateau heights of less than 5 km. Several thermo-mechanical processes within the back-arc have a gross weakening effect on a back-arc scale and, thus, may be responsible for episodes of shortening rate acceleration in the past. These processes include: (i) on-going eclogitization of the lower mafic crust beneath the plateau, (ii) heating and convection in the plateau felsic crust, (iii) mechanical failure of the foreland sediments and (iv) high erosion and exhumation rates at the plateau margin (effective in monsoon areas but apparently not relevant for the Central Andes). The largest reduction in the effective lithospheric strength (‚Äöàº3 ‚àöó 1012 N m‚Äöà{\'\i}1, i.e., 40--50% of the total strength) could result from the mechanical failure of the foreland sediments, leading to the migration of shortening deformation into the Altiplano foreland (Subandean thrust belt). This failure implies a drastic reduction of both sediment cohesion (almost to zero values) and friction angle (to at least a third of the normal value of 30$\,^{\circ}$). The second important factor is on-going eclogitization of the lower crust beneath the plateau, which increases the average density of the crust and thus prevents rising of the plateau. The eclogitization may decrease the effective lithospheric strength by ‚Äöàº2 ‚àöó 1012 N m‚Äöà{\'\i}1 (25--35% of the total strength). The third important factor is intracrustal convection, which follows eclogitization-driven lithospheric delamination and reduces the brittle strength of the uppermost crust (overall effect ‚Äöàº1 ‚àöó 1012 N m‚Äöà{\'\i}1, or 10--15% of the total strength). High erosion rate at the plateau margin may efficiently weaken the orogenic lithosphere but, in the case of the low to moderate erosion rates typical of the Central Andes, its effect is negligible. In general, internal weakening of the overriding plate is, in addition to plate tectonic forces, among the major controls on the Andean orogeny.