Late Cenozoic deformation and uplift of the western flankof the Altiplano: Evidence from the depositional, tectonic,and geomorphologic evolution and shallow seismic activity(northern Chile at 19°300S)Marcelo Farı´as,1,2,3Reynaldo Charrier,1Diana Comte,2Joseph Martinod,3and Ge´rard He´rail4Received 13 April 2004; revised 4 February 2005; accepted 11 March 2005; published 1 July 2005.[1] We analyze the west vergent thrust system (WTS)along the western flank of the Altiplano in northernChile (18S–21S). In our study area (19200S–19500S), the WTS consists of three thrustpropagation monocline folds (flexures) developinggrowth strata. The relative uplift accommodated by theflexures is rapid between 26 and 8 Ma (0.1 mm/yr),diminishing to 0.02 mm/yr after 8 Ma. Approximately2000 m of relative surface uplift was accommodatedby the flexures since the late Oligocene.Sedimentological and geomorphological analysisshows that westward tilting of the forearc occurredafter 10 Ma, coeval with the shifting of deformationfrom the Altiplano to the sub-Andean zone, where theunderthrusting of the Brazilian Craton would haveresulted in crustal thickening, surface uplift in theorogen, and westward ductile subcrustal flow. Forearctilting is accommodated by east vergent thrusts (ETS)issued from the Benioff zone beneath the CentralDepression emerging into the Western Cordillera,contributing 500–1400 m of surface uplift. The WTSconnects the ETS in the b rittle-ductile crustaltransition (25 km depth), continuing farther east asthe Altiplano low-velocity zone, configuring thewestern Altiplano as a crustal-scale fault bend fold.Forearc tilting would be caused by westward ductileflow in the lower crust pushing the rigid forearc in theETS. Meanwhile, between 19S and 21S, the WTSaccommodates dextral strike slip, and 3kmofN-Sshortening occurred in the Coastal Cordillera.Transcurrence and strain partitioning are probablythe result of slight plate convergence obliquity, strongcoupling within the interplate zone, westwardcontinental concavity, and high elevation opposinghorizontal contraction.Citation: Farı´as, M., R. Charrier,D. Comte, J. Martinod, and G. He´rail (2005), Late Cenozoicdeformation and uplift of the western flank of the Altiplano:Evidence from the depositional, tectonic, and geomorphologicevolution and shallow seismic activity (northern Chile at19300S), Tectonics, 24, TC4001, doi:10.1029/2004TC001667.1. Introduction[2] The western margin of South America is one of thelargest and most active plate boundary zones. Here, theoceanic Nazca plate (or formerly Farallon plate) subductsbeneath the South American continent at a rate of 84 mm/yr[DeMets et al., 1994]. The Andes are located along thisborder (Figure 1) and can be subdivided into the northern,central, and southern Andes according to its geologicalfeatures [Gansser, 1973]. The curved portion of the centralAndes in northern Chile, southern Peru, and Bolivia isthe broadest part of the mountain belt (Figure 1). Thissegment of the range includes the Altiplano-Puna, whichis the highest plateau formed on a noncollisional margin(4 km high) [Isacks, 1988], and the second in theworld after the Tibetan plateau.[3] The origin of this mountain belt i s attributed tocontractional strain related to almost uninterrupted subduc-tion since the Jurassic along the western margin of SouthAmerica [Baby et al., 1997; Coira et al., 1982; Jordan etal., 1983; Kay and Abbruzzi, 1996; Kay et al., 1991, 1999;Mpodozis and Ramos, 1989; Ramos, 1988; Sempere et al.,1990]. However, the major present-day features of thecentral And es were formed during the C enozoic , andparticularly during the last 30 Myr [e.g., Allm endinger etal., 1997; Isacks, 1988; Lamb et al., 1997; Rutland, 1971;Sempere et al., 1990]. Since then and after major platereorganization, the Nazca and South American platesincreased their velocity of relative convergence, whichbecame almost orthogonal to the Chilean margin[Pardo-Casas and Molnar, 1987; Somoza, 1998].[4] The present-day crustal thickness (60–80 km) andsurface elevation of the Altiplano is mainly considered to bea consequence of large crustal shortening [Allmendinger etal., 1997; Beck and Zandt, 2002; Haschke and Gu¨nther,2003; Husson and Sempere, 2003; Isacks, 1988; Lamband Hoke, 1997; Lamb et al., 1997; McQuarrie, 2002;McQuarrie and DeCelles, 2001; Reutter e t al., 1988;TECTONICS, VOL. 24, TC4001, doi:10.1029/2004TC001667, 20051Departamento de Geologı´a, Universidad de Chile, Santiago, Chile.2Departamento de Geofı´sica, Universidad de Chile, Santiago, Chile.3Laboratoire des Me´canismes et Transferts en Ge´ologie, Universite´ PaulSabatier, Toulouse, France.4Institut de Recherche pour le De´veloppement, Laboratoire desMe´canismes et Transferts en Ge´ologie, Toulouse, France.Copyright 2005 by the American Geophysical Union.0278-7407/05/2004TC001667$12.00TC4001 1of27Roeder, 19 88; Sch mitz, 1994; Sheffels, 1990]. However,some discrepancy exists between the observed tectonicshortening and the amount of material needed to explainthe entire crustal thickness [Allmendinger et al., 1997;Husson and Sempere, 2003; Kley and Monaldi, 1998; Kleyet al., 1997; Lamb et al., 1997; McQuarrie , 2002; Schmitz ,1994]. In addition, geophysical and petrological studiessuggest that other processes would have been involved incrustal thickening and plateau surrection, such as magmaticaddition [Gill, 1981; Hoke et al., 1994; Kono et al., 1989;Weaver and Tarney, 1984], lithospheric thinning andremoval of the subcrustal lithosphere [Beck and Zandt,2002; Isacks, 1988; Lamb et al., 1997; Whitman et al.,1996], underplating of material removed from the forearcby subduction erosion [e.g., Baby et al., 1997; Schmitz,1994], and ductile mass transfers within the lower crust[Beck and Zandt, 2002; Gerbault et al., 2002; Husson andSempere, 2003; Kley and Monaldi, 1998].[5] Most tectonic studies in the central Andes Altiplanohave been focused on the back arc region, mainly in theEastern Cordillera and sub-Andean Ranges, where a largeupper crustal shortening (200 – 350 km) has been accom-modated since Cretaceous times by the predominantly eastvergent Andean fold-and-thrust belt. Nevertheless, in moststudies referred to the plateau development, the tectonicactivity accommodated along the monocline-shaped westernflank of the Altiplano has been almost neglected, simplybecause the observed Neogene deformation is much lessthan the