Thin and Thick Skinned Foreland Deformation in the Central Andes: A Numerical Simulation Study

The two main segments of the Central Andean plateau, Altiplano and Puna, demonstrate since the Late Miocene different styles of tectonic shortening. Initially pure shear shortening in the Altiplano plateau switched at 13-9 Ma into the simple shear mode accompanied by formation of one of the world largest thin skinned foreland belt. Further to the south, in the Puna, the pure shear shortening continued until much more recently, gradually transforming into mixed pure and simple shear mode with thick skinned deformation in the foreland (the Santa Barbara System). Through numerical simulation of thermo-mechanical processes we show that different shortening modes - pure and simple shear accompanied by thin or thick skinned tectonics - might be controlled by (i) strength of the foreland uppermost crust and (ii) temperature of the foreland lithosphere. As a numerical tool we use a 2-D parallel thermo-mechanical finite element code LAPEX-2D. The code combines explicit lagrangian finite element FLAC algorithm with particle-in-cell technique. Particles track not only material properties but also full strain and stress tensors minimizing numerical diffusion. We employ Maxwell visco-elastic rheology with temperature- and stress-dependent viscosity, simulating ductile flow, as well as Mohr-Coulomb elasto-plastic rheology, simulating brittle deformation. Both rheological models may experience strain softening. Previous geodynamic models indicated the importance of the lateral temperature variations in the lithosphere on the style of tectonic shortening. However, they failed to reproduce migration of the deformation from the Altiplano plateau into its foreland before the major uplift of the plateau. We show that deformation may easily migrate from the plateau into the foreland by rapidly propagating thin skinned thrust belt as a consequence of dramatic mechanical weakening of the Palaeozoic sediments overlying the cold lithosphere of the Altiplano foreland. The processes in the foreland sediments, which triggered their mechanical failure, remain unclear. We speculate that they might be related to the onset of the hydrocarbon maturation in the foreland sediments after the initial stage of the plateau uplift. Lack of the thick sediments further south, in the Puna foreland, precluded formation of the thin skinned thrusting. If the Puna foreland would have been as cold as the foreland at the latitude of the Altiplano, the Puna plateau would be continuously deformed in a pure shear mode with almost no deformation in the foreland; which is not the case. We show that the existing thick skinned deformation pattern of the Puna foreland might be explained by warmer foreland at the latitude of Puna than that at the latitude of Altiplano, which is in accord with the existing seismic Q-factor observations.