22 Abstract. The thickened crust of the Central Andes is characterized by several first-order geophysical anomalies that seem to reflect the presence of partial melts. Magnetotelluric and geomagnetic deep- sounding studies in Northern Chile have revealed a high conduc- tivity zone (HCZ) beneath the Altiplano Plateau and the Western Cordillera, which is extreme both in terms of its size and integrated conductivity of > 20 000 Siemens. Furthermore, this region is char- acterized by an extremely high seismic attenuation and reduced seismic velocity. The interrelation between the different petrophysi- cal observations, in combination with petrological and heat-flow density studies, strongly indicates a huge area of partially molten rocks that is possibly topped with a thin, saline fluid film. The aver- age melt fraction is deduced to be ~20 vol.%, which agrees with typical values deduced from eroded migmatites. Based on the dis- tribution and geochemical composition of Pliocene to Quaternary silicic ignimbrites in this area, this zone is thought to be dominated by crustally-derived rhyodacite melts with minor andesitic contri- bution. An interconnected melt distribution – typical for migma- tites – would satisfy both the magnetotelluric and seismic obser- vations. The high melt fraction in this mid-crustal zone should lead to strong weakening, which may be a main cause for the develop- ment of the flat topography of the Altiplano Plateau. 22.1 Introduction The Andes are located at the convergence zone between the oceanic Nazca Plate and the continental South Ameri- can Plate. This Cordilleran-type mountain belt is the larg- est active, subduction-controlled orogen on Earth. It has a length of some 7 500 km and in the central part, between 16° to 25° S (Fig. 22.1), the Andes have extreme dimen- sions and topographic relief; the mountain belt is 800 km wide and elevations reach more than 6 000 m. It has been appreciated for several decades that the crust is also ex- tremely thick, with seismic and gravimetric data indicating a thickness of up to 70 km below the Altiplano and Western Cordillera (James 1971; Ocola and Meyer 1972; Zandt et al. 1994; Wigger et al. 1994). More recent studies have resolved the problem of imaging the Moho discontinuity beneath the volcanic arc by using seismic tomography (Haberland 1998; Rietbrock 1999) and receiver function methods (Yuan et al. 2000, 2002), so that, now, a fairly complete map of Moho depth exists (Yuan et al. 2002). The Bouguer gravity anomaly of this thickened subduction orogen reaches –450 mgal in the magmatic arc (Götze et al. 1994). Chapter 22 Partial Melting in the Central Andean Crust: a Review of Geophysical, Petrophysical, and Petrologic Evidence Frank R. Schilling · Robert B. Trumbull · Heinrich Brasse · Christian Haberland · Günter Asch · David Bruhn · Katrin Mai Volker Haak · Peter Giese · Miguel Muñoz · Juliane Ramelow · Andreas Rietbrock · Edgar Ricaldi · Tim Vietor The prominent landscape, active volcanic chain, world- class ore deposits and the type locality of the Andean orog- eny have motivated a great number of detailed studies which make the Central Andes one of the best, or even the best, studied examples of subduction-related orogeny. Empha- sis has been placed on the strong variations in properties that exist across the Central Andes from west to east, as a consequence of the inherent polarity of the subduction sys- tem. However, it is equally important to consider the strik- ing variations in properties parallel to the active margin. The Central Andes crust is not only thick but is char- acterized by first order, internal geophysical anomalies. One such anomaly is the extreme high conductivity zone (HCZ), revealed by magnetotelluric and geomagnetic deep-sounding studies, beneath the Altiplano Plateau and the Western Cordillera in Northern Chile (Schwarz et al. Fig. 22.1. Morpho-tectonic units of the central Andes modified after Oncken et al. (2006). CC: Coastal Cordillera, PC: Precordillera, WC: Western Cordillera, EC: Eastern Cordillera, SA: Subandean Ranges, SBS: Santa Barbara System, SP: Sierras Pampeanas