6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), Extended Abstracts: 297-300 A numerical model testing the raie of climate in high plateau formation Daniel Garcia-Castellanos (1) & Muriel Gerbault (2) (1) Institute of Earth Sciences Jaume AImera, CSIC, Barcelona, Spain; (2) IRD, LMTG, Toulouse, France -sea level detachment leveJ Fig. 1.- Conceptual mode!. Tectonic deformation is calculated with a minimum-work criterion of fault propagation . Erosion and transport is calculated via a stream power law model of fluvial incision and transport. Water flow is calculated assuming a constant evaporation at lakes and a constant precipitation ail over the mode!. that precipitation and basin evaporation are as important as tectonics in controlling the formation and duration of tectonic, internally-drained basins. ln this study, we address the role of thèse c1imatic factors on the development of orogens and intramountain basins to check the hypothesis that climate can be as important as tectonics in determ ining the formation of a closed intra-orogenic basin and in controlling the development of a high-plateau. For this purpose we use a computer cross-section model of orogen and basin evolution that accounts for the interactions 1. Introduction Most investigations regard the formation of high plateaus as the result of tectonic processes disrupting surface mass transport within an orogen. Inherited tectonic structures predating or coeval with compression are often invoked to explain the partitioning of tectonic topography and the hydrological isolation of the plateau from the surrounding areas (internai drainage). Syn- or post-tectonic uplift related to the accommodation of shortening at deep crustal levels is frequently invoked to account for the elevation of the plateau, and open questions remain on whether this deformation consists of a lateral viscous flow in the lower crust (e.g., Isacks, 1988), and/or brittle failure by means of crustal, imbricated megathrusts. In the case of the Andes-Altiplano, measured along-strike variations in tectonic shortening allegedly control the variations in timing and style of deformation (e .g., McQuarrie, 2002). In this context, surface processes have been mostly seen as passively responding to these tectonic events by eroding the orogen and filling the intramountain sedimentary basin. However, modeling studies have shown that erosion has a significant influence on lithosphere dynamics during orogenesis (e.g., Avouac & Burov, 1996). For example, climatic parameters determining the side of the orogen receiving most precipitation seem to be critical in determin ing the later tectonic evolut ion and structure of a rnountain belt (Willet, 1999). Meanwhile, recent numerical modeling techniques (Sobel et al., 2003; Garcia- Castellanos et al., 2003 ; Garcia- Castellanos, 2005) have demonstrated between four processes (Fig . 1): 1) dynamic propagation of thrusting; 2) a basic hydrological balance; 3) surface erosion / sediment transport / sedimentation; and 4) flexural isostasy. 2. Model assumptions and formulation We develop a finite difference code based on tAo (e.g ., Garcia-Castellanos, 2005) that operates in 6 time- iterative steps: 1) Calculation of the formation of new fau Its and the corresponding tectonic deformation; 2) Determination of the changes in topography related to tectonic deformation ; 3) Determination of precipitation and the drainage network following a maximum slope criteria; 4) Determination of the extension of lakes in 297