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Abstract

Large-scale tectonic shortening in the Central Andes and Neogene uplift of the Altiplano-Puna plateau was preceeded by Eocene uplift of the Chilean Precordillera and subsequent rapid eastward expansion of compressional deformation between 40-30 Ma, by the marked decline of the magmatic arc activity at 33-23 Ma and by a strong increase of the convergence rate between the Nazca and South America (SA) plates at 25-20 Ma. We use 2-D numerical thermo-mechanical modelling of the interaction of the subducting Nazca plate and the overriding SA plate during the last 40 Myr to explore the possible relation between these events. The model employs realistic visco-elasto-plastic rheology, shear heating and phase transformations. The interface between the slab and the upper plate is modelled as a few km thick subduction channel with low-friction Mohr-Coulomb rheology, the friction coefficient being the major modelling parameter. We show that variation of this parameter in the plausible range of 0.02-0.07 may dramatically change stress and strain fields in the upper plate. Preliminary modelling results show that at friction coefficient of 0.05-0.07 the stress field in the overriding plate must have changed from minor extension to strong compression in response to the increase of the convergence rate. This could cause extensive tectonic shortening of the overriding plate if it had been rheologicaly weakened beforehand. We quantitatively explore possible mechanisms of such weakening, and currently favour the following scenario yielding the best fit. At ca. 40 Ma an oceanic plateau entered the trench; its motion below the SA plate caused eastward expansion of compressional deformation at 40-30 Ma followed by flattening of the slab and cessation of volcanism. After the plateau was subducted at 25-20 Ma, the slab retreated which intensified the asthenospheric corner flow. This event coincided temporally with the acceleration of the convergence rate inducing increased compression in the upper plate where the subduction-channel friction coefficient was relatively high. After some 10 Myr, required for the corner flow to heat and hence weaken the lithosphere of the upper plate, the latter failed and tectonic shortening accelerated despite slowdown of convergence rate.