24 Abstract. We employed a 2D numerical thermo-mechanical model- ing technique to study the dynamics of the plateau-foreland sys- tem, 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 shorten- ing encounter major resistance from the growing gravitational po- tential 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 × 10 12 Nm –1 for plateau heights of less than 5 km. Sev- eral 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 pro- cesses 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 (~3 × 10 12 Nm –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°). The second impor- tant 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 ~2 × 10 12 Nm –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 ~1 × 10 12 Nm –1 , or 10–15% of the total strength). High erosion rate at the plateau margin may efficiently weaken the orogenic lithos- phere but, in the case of the low to moderate erosion rates typical of the Central Andes, its effect is negligible. In general, internal weak- ening of the overriding plate is, in addition to plate tectonic forces, among the major controls on the Andean orogeny. 24.1 Introduction The large-scale temporal and spatial evolution of the Cen- tral Andean orogen involves tectonic shortening of the South American plate margin bounded by the Nazca Plate to the west and the indenting Brazilian shield to the east (Fig. 24.1a) (e.g., Isacks 1988; Allmendinger et al. 1997; Lamb et al. 1997). The external far-field forces that drive tectonic shortening in the back-arc encounter major re- sistance from (i) the growing gravitational potential of the rising plateau, (ii) the brittle strength of the upper crust, and (iii) the viscous resistance of the ductile lower crust and upper mantle. Clearly, in the limiting case of homogeneous, pure-shear plateau inflation (‘vise’ plateau model in the terminology of Ellis et al. 1988), the overall resistance against shortening steadily grows as the pla- teau becomes higher with time. Continued shortening in this mode requires a corresponding increase of the far- field driving forces. Alternatively, any thermo-mechani- cal processes in the back-arc that are able to reduce the Chapter 24 Numerical Study of Weakening Processes in the Central Andean Back-Arc Andrey Y. Babeyko · Stephan V. Sobolev · Tim Vietor · Onno Oncken · Robert B. Trumbull Fig. 24.1. a Map showing the major tectonic elements of the Central Andes. White dashed line contours the model area. b Cartoon illus- trating the set-up for the back-arc model. Material parameters are given in Table 24.1. The initial thickness of the lithosphere under the plateau varies between 70 and 90 km