THERMOMECHANICAL MODELING OF large-scale MELTING IN the middle crust IN THE ALTIPLANO - puna PLATEAU A.Yu. Babeyko, S.V. Sobolev, R.B. Trumbull, L.Lavier, O.Oncken (GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany) Among the first-order observations of the Altiplano-Puna plateau which needs to be explained by any model for plateau formation is the mid-crustal zone of low seismic velocity (Andean Low Velocity Zone -ALVZ) which extends across the entire plateau at ca. 20-25 km depth. The most pronounced part of the ALVZ correlates with the major ignimbrite province of the Altiplano-Puna Volcanic Complex, and there is good evidence that both features reflect crustal melting. Since the ignimbite activity is correlated in space and time with the late Miocene plateau, crustal melting and the ALVZ appear to be related to the plateau-building process. Foremost among the geologic constraints on crustal melting in the ignimbrite province are that well over 10000 cubic kilometers of silicic magma were erupted within 5 My, some 10-15 My after plateau initiation. The ignimbrite magma had pre-eruptive temperatures of at least 800°C and was hybrid, comprising some 70% crustal melt and 30% arc andesite. The problem we address with thermomechanical numerical experiments is how melting temperatures can be achieved in the middle crust within only 10-15 My after initiation of plateau formation. We present model results which explore two potential heat sources for melting in the ALVZ: (1) internal, by shear heating and/or radiogenic heat production related to tectonic shortening; (2) external, by intrusions of arc andesite magmas. We show that scenario 1 results in a temperature rise of less than some tens of degrees within the time constraints. Scenario 2 would require intruding nearly all arc magmas into the mid crust unless ambient temperatures in the melting zone are elevated by other means. Our modeling suggests that the other process could be advection of heat from hot, possibly partially molten, lower crust. The necessary condition for this advection is a low crustal viscosity, which could be achieved by a combination of felsic bulk composition and high strain rates due to deformation. Crustal melting under the Altiplano-Puna plateau developed relatively rapidly because of a combination of tectonic effects of plateau formation and heat input from an active magmatic arc. We suggest that the arc magmatism accelerated a process, which might have occurred later from plateau-building alone. Crustal low-velocity zones like the ALVZ are also observed in the Tibetan plateau, but in that case the process was slower and surface magmatism much less intense than in the Altiplano-Puna.