Plateau-style accumulation of deformation: Southern AltiplanoKirsten Elger, Onno Oncken, and Johannes GlodnyGeoForschungsZentrum Potsdam, Potsdam, GermanyReceived 5 May 2004; revised 17 December 2004; accepted 23 March 2005; published 31 August 2005.[1] Employing surface mapping of syntectonicsediments, interpretation of industry reflection-seismic profiles, gravity data, and isotopic age dating,we reconstruct the tectonic evolution of the southernAltiplano (20–22S) between the cordillerasdefining its margins. The southern Altiplano crustwas deformed between the late Oligocene and the lateMiocene with two main shortening stages in theOligocene (33–27 Ma) and middle/late Miocene(19– 8 Ma) that succeeded Eocene onset ofshortening at the protoplateau margins. Shorteningrates in the southern Altiplano ranged between 1 and4.7 mm/yr with maximum rates in the late Miocene.Summing rates for the southern Altiplano and theEastern Cordillera, we observe an increase fromEocene times to the late Oligocene to some 8 mm/yr,followed by fluctuation around this value during theMiocene prior to shutoff of deformation at 7–8 Ma andtransfer of active shortening to the sub-Andean foldand thrust belt. Shortening inverted early Tertiarygraben and half graben systems and was partitionedin three fault systems in the western, central, andeastern Altiplano, respectively. The east vergent faultsystems of the western and central Altiplano weresynchronously active with the west vergent Altiplanowest flank fault system. From these data and fromsection balancing, we infer a kinematically linkedwestern Altiplano thrust belt that accumulated aminimum of 65 km shortening. Evolution of this beltcontrasts with the Eastern Cordillera, which reachedpeak shortening rates (8 mm/yr) in between the abovetwo stages. Hence local shortening rates fluctuatedacross the plateau superimposed on a general trend ofincreasing bulk rate with no trend of lateralpropagation. This observation is repeated at theshorter length and time scales of individual growthstructures that show evidence for periods of enhancedlocal rates at a timescale of 1–3 Myr. We interpretthis irregular pattern of deformation to reflect aplateau-style of shortening related to a self-organizedstate of a weak crust in the central South American backarc with a fault network that fluctuated around thecritical state of mechanical failure. Tuning of this statemay have occurred by changes in plate kinematics,during the Paleogene, initially reactivating crustalweak zones and by thermal weakening of the crustwith active magmatism mainly in the Neogene stage.Citation: Elger, K., O. Oncken, and J. Glodny (2005), Plateau-style accumulation of deformation: Southern Altiplano, Tectonics,24, TC4020, doi:10.1029/2004TC001675.1. Introduction[2] Althoug h considerable advance has been made inrecent years in understanding the processes involved inthe formation of orogenic plateaus, the precise temporaland spatial patterns of uplift and lateral progradation ofdeformation in plateaus beyond the last few million years isstill shrouded in darkness. This situation is largely due to alack of systematic data with the potential to date the timingof deformation and uplift, as well as of detailed structuraldata t hat reveal the internal architecture of the world’slarge orogenic plateaus, Tibet and Altiplano [e.g., Deweyet al., 1988; Isacks, 1988; England and Molnar, 1997;Allmendinger et al., 1997; Shen et al., 2001]. Young,syntectonic and posttectonic sediments largely cover bothplateaus and restrict access to the subsurface. Yet, models oflarge-scale orogeny and plateau formation heavily rely onassumptions that r elate to the early stages of plateauevolution and the conditions governing these. This isparticularly true for the Andean Altiplano, which, due toits setting in an ocean-continent convergence system, hasbeen called a plate tectonic paradox [e.g., Allmendinger etal., 1997]. Because of the unsatisfactory data situation,published ideas on accumulation of deformation in theAltiplano range from various inferred stages in plateauevolution [e.g., James, 1971; Pardo-Casas and Molnar,1987; Isacks, 1988; Sempere et al., 1990; Allmendingerand Gubbels, 1996; Kley, 1996; Allmendinger et al., 1997;Lamb and Hoke, 1997; Lamb et al., 1997; Lamb, 2000;Hindle and Kley, 2002] to suggestions of continuous east-ward progradat ion of deformation, starting at the WesternCordillera, with a propagation style much like that observedin classic foreland fold and thrust belts [e.g., Horton et al.,2001; McQuarrie, 2002]. To date, the available data in theAltiplano do not provide a complete quantitative record ofthe spatial and temporal pattern of strain accumulationduring plateau formation that also sufficiently resolves theearlier stages.[3] In the present paper we attempt to provide afirst nearly complete record of magnitude and timing ofshortening across the southern Altiplano plateau (compareFigure 1) by the mapping of syntectonic sediments, byidentifying structures, their relative ages and magnitudes ofTECTONICS, VOL. 24, TC4020, doi:10.1029/2004TC001675, 2005Copyright 2005 by the American Geophysical Union.0278-7407/05/2004TC001675$12.00TC4020 1of19Figure 1TC4020 ELGER ET AL.: ALTIPLANO DEFORMATION2of19TC4020deformation from the interpretation of industry reflection-seismic profiles, and by isotopic age dating of deformation,including a reevaluation of published ages. Our analysisfocuses on the southern Altiplano at 20–22S. This segmentis characterized by the highest density of geophysical andgeological data in the central Andes, all provided by avariety of recent projects [e.g., Wigger et al., 1994; Becket al., 1996; Swenson et al., 2000; Yuan et al., 2 000;Haberland and Rietbrock, 2001; Brasse et al., 2002; Go¨tzeand Krause, 2002; ANCORP Working Group, 2003]. Inaddition, high-resolution industry reflection seismic sectionscovering the entire plateau were made available to thisproject by the Bolivian national oil industry (courtesy ofYPFB).2. Geological Framework[4] In spite of ongoing subduction for more than 200 Myr,large-scale deformation of the upper plate and formation ofthe Andes high plateau only commenced during early tomid-Tertiary times [Sempere et al., 1990; Lamb and Hoke,1997; Allmendinger et al. , 1997; Horton et al., 2001]. Theresulting central Andean mountain belt is built from a seriesof morphotecto nic units (Figure 1d): the LongitudinalValley in the forearc with