1 Chapter 1 Deformation of the Central Andean Upper Plate System – Facts, Fiction, and Constraints for Plateau Models Onno Oncken · David Hindle · Jonas Kley · Kirsten Elger · Pia Victor · Kerstin Schemmann Nazca Plate in a convergent continental margin setting – a situation that is unique along the 60 000 km of conver- gent margins around the globe. The key challenge is to understand why mechanical failure of the later plateau extent developed along the central portion of the leading edge of South America only, as well as why and how this feature developed only during the Cenozoic, although the cycle of Andean subduction has been ongoing since at least the Jurassic. Since the 1980s, a plethora of models has been pub- lished that attempt to find a solution to this ‘geodynamic paradox’ (Allmendinger et al. 1997). These fall into dis- tinct classes with respect to the key mechanisms, although some of the models involve a combination of several pro- cesses. Suggested mechanisms include  upper plate deformation related to changes in plate con- vergence parameters and plate reorganization (e.g. Pardo- Casas and Molnar 1987; Coney and Evenchick 1994; Scheuber et al. 1994; Silver et al. 1998; Somoza 1998);  changes in the geometry and properties of the down- going slab, including topographic features of the oce- anic plate (ridges, oceanic plateaux), with both changes affecting the mechanics of the upper plate system (e.g. Gephart 1994; Giese et al. 1999; Gutscher et al. 2000a, 2000b; Yañez et al. 2001);  plate-scale patterns of sub-lithospheric mantle flux related to upper plate motion and deformation (e.g. Russo and Silver 1996; Silver et al. 1998; Marrett and Strecker 2000; Heuret and Lallemand 2005);  mantle-driven thermal processes affecting the upper plate lithosphere (e.g. Isacks 1988; Wdowinski and Bock 1994; Allmendinger et al. 1997; Mahlburg Kay et al. 1999; ANCORP-Working Group 2003; Garzione et al. 2006);  spatial and temporal variations in the strength and properties of the upper plate foreland and the plate interface (e.g. Allmendinger and Gubbels 1996; Hindle et al. 2002; Lamb and Davis 2003); and  climate-related variations in surface erosion affecting deformation through surface material flux (e.g. Masek et al. 1994; Horton 1999; Montgomery et al. 2001; Lamb and Davis 2003; Sobel et al. 2003). Abstract. We quantitatively analyse the spatial pattern of deforma- tion partitioning and of temporal accumulation of deformation in the Central Andes (15–26° S) with the aim of identifying those mechanisms responsible for initiating and controlling Cenozoic pla- teau evolution in this region. Our results show that the differential velocity between upper plate velocity and oceanic plate slab roll- back velocity is crucial for determining the amount and rate of short- ening, as well as their lateral variability at the leading edge of the upper plate. This primary control is modulated by factors affecting the strength balance between the upper plate lithosphere and the Nazca/South American Plate interface. These factors particularly include a stage of reduced slab dip (33 to 20 Ma) that accelerated shortening and an earlier phase (45 to 33 Ma) of higher trenchward sediment flux that reduced coupling at the plate interface, result- ing in slowed shortening and enhanced slab rollback. Because high sediment flux and transfer of convergence into upper plate short- ening constitute a negative feedback, we suggest that interruption of this feedback is critical for sustaining high shortening transfer, as observed for the Andes. Although we show that climate trends have no influence on the evolution of the Central Andes, the position of this region in the global arid belt in a low erosion regime is the key that provides this interruption; it inhibits high sediment flux into the trench despite the formation of relief from ongoing shortening. Along-strike variations in Andean shortening are clearly related to changes of the above factors. The spatial pattern of distribution of deformation in the Central Andes, as well as the synchronization of fault systems and the total magnitude of shortening, was mainly con- trolled by large-scale, inherited upper plate features that constitute zones of weakness in the upper plate leading edge. In summary, only a very particular combination of parameters appears to be able to trigger plateau-style deformation at a convergent continental margin. The combination of these parameters (in particular, differ- ential trench-upper plate velocity evolution, high plate interface coupling from low trench infill, and the lateral distribution of weak zones in the upper plate leading edge) was highly uncommon dur- ing the Phanerozoic. This led to very few plateau-style orogens at convergent margins like the Cenozoic Central Andes in South America or, possibly, the Laramide North American Cordillera. 1.1 Introduction and Geodynamic Framework To this date, the question of why and how a plateau-type orogen formed with crustal thickening at the leading edge of western South America remains one of the hotly de- bated issues in geodynamics. During the Cenozoic, the Altiplano and Puna Plateaux of the Central Andes (aver- age elevation some 4 km, with an extent of 400 × 2 000 km) developed during continuous subduction of the oceanic