OROGENESIS ABOVE A SUBDUCTION ZONE: EXPLAINING THE FORMATION OF THE ANDES W.P. Schellart*, D.R. Stegman^ and J. Freeman* *Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia ^School of Mathematical Sciences, Monash University, Melbourne, VIC 3800, Australia Most mountain belts on Earth formed as a result of continent-continent collision or due to accretion of arc terranes and plateaus to continental crust. Present-day examples include the Alps, Himalayas, Taiwan and New Guinea, while fossil examples include the Caledonides and Appalachians in Europe and North America, and the Tasman orogen in eastern Australia. The Andean mountain belt in South America is in stark contrast with these collisional orogens, as it formed at a subduction zone. The mountain belt formed in the Late Cenozoic during eastward subduction of oceanic lithosphere (Nazca plate) and its existence remains an enigma. It is the longest and second-highest mountain chain on Earth with a length of ~7500 km and peaks reaching ~7000 m. The Central Andes region has experienced the largest amount of crustal shortening (~350 km) and is currently characterised by a large plateau (Altiplano) at ~4000 m above sea level. Shortening decreases progressively towards the north and south. The formation of the mountain belt has previously been explained by different physical parameters, including trenchward overriding plate motion, fast convergence velocity, strong coupling along the subduction interface due to a sediment-starved trench, the young age of the subducting oceanic lithosphere or flat slab subduction. The problem is that none of the estimated values for these parameters are exceptional when compared to other subduction zones, where overriding plate shortening is absent or negligible. Trenchward overriding plate velocity is ~1-2 cm/yr in South America but can be up to 6 cm at other subduction zones, while the convergence velocity is ~6-8 cm/yr in South America, but can be up to 10 cm/yr elsewhere. In addition, numerous other sediment-starved trenches exist that are characterised by backarc basin formation (e.g. Scotia, Tonga, Mariana). Finally, in the Central Andes (i.e. the region of maximum shortening and plateau formation), subduction of Eocene oceanic lithosphere occurs at a moderate dip angle (25-45º), whilst younger, Oligocene-Present oceanic lithosphere subducts to the north and south with regions of flat slab subduction. From the discussion above it can be concluded that formation of the Andes is at present not satisfactorily explained. We will assess how the South American subduction zone differs from all the other subduction zones on Earth from a global compilation of subduction zone parameters, plate kinematics and trench kinematics. We will demonstrate that the formation of the Andes mountain belt can be ascribed primarily to the size of the subduction zone. Results will be presented from state-of-the-art four-dimensional numerical models of subduction, which reinforce the findings of the global compilation. A new reconstruction of the evolution of the South American subduction zone will also be presented. The reconstruction shows qualitative and quantitative similarities with the numerical models, providing further support that the size of the subduction zone is the controlling parameter in inducing mountain building in the Andes.