P1: MBL/rsk P2: MBLMarch 21, 1997 19:22 Annual Reviews AR029-05 AR29-05Annu. Rev. Earth Planet. Sci. 1997. 25:139–74Copyrightc1997 by Annual Reviews Inc. All rights reservedTHE EVOLUTION OF THEALTIPLANO-PUNA PLATEAUOF THE CENTRAL ANDESRichard W. Allmendinger, Teresa E. Jordan, Suzanne M. Kay,and Bryan L. IsacksDepartment of Geological Sciences and Institute for the Study of the Continents,Cornell University, Ithaca, New York 14853-1504; e-mail: [email protected] WORDS: South America, continental plateau, uplift, timing, magmatismABSTRACTThe enigma of continental plateaus formed in the absence of continental collisionis embodied by the Altiplano-Puna, which stretches for 1800 km along the CentralAndes and attains a width of 350–400 km. The plateau correlates spatially andtemporally with Andean arc magmatism, but it was uplifted primarily becauseof crustal thickening produced by horizontal shortening of a thermally softenedlithosphere. Nonetheless, known shortening at the surface accounts for only 70–80% of the observed crustal thickening, suggesting that magmatic addition andother processes such as lithospheric thinning, upper mantle hydration, or tectonicunderplating may contributesignificantlytothickening. Uplift in the regionoftheAltiplano began around 25 Ma, coincident with increased convergence rate andinferred shallowing of subduction; uplift in the Puna commenced 5–10 millionyears later.INTRODUCTIONThe Altiplano-Puna plateau of the Central Andes (Figure 1) is the highestplateau in the world associated with abundant arc magmatism, and it is secondonly to Tibet in height and extent. Yet, this remarkable feature was upliftedin the absence of continental collision or terrane accretion; in fact, materialhas been removed from the continental margin during and prior to plateauuplift. Because of its obvious association with Andean magmatism, the plateauwas originally thought to be a product of magmatic processes (James 1971b,1390084-6597/97/0515-0139$08.00P1: MBL/rsk P2: MBLJanuary 9, 1998 19:15 Annual Reviews AR029-05c AR29-05140 ALLMENDINGER ET AL15025030010057535050SantaCruzLa PazAricaLimaCuzcoAntofagastaSaltaBoliviaArgentina25°20°15°65°70°30°75°SierrasPampeanasPCSBChileAltiplanoPerúPunaSubandeanFigure 1 Location map showing the extent of the high plateau of the Central Andes. Dark grayshows area above 3 km elevation; the plateau is defined by the wide area above 3 km between13 and 27◦S. The light gray provinces east of the high topography are thin-skinned thrust belts inthe Subandean ranges of Bolivia and the Precordillera (PC) in Argentina. The Sierras Pampeanasand Santa B´arbara System (SB) are thick-skinned foreland provinces. Thin curves are contours ofdepth to the Wadati-Benioff zone in kilometers from Cahill & Isacks (1992). The hachured zonetrending NW-SE across the Argentine-Bolivian border corresponds to a variety of lateral change inAndean and pre-Andean features and is taken here to be the boundary between the Altiplano andPuna.P1: MBL/rsk P2: MBLMarch 21, 1997 19:22 Annual Reviews AR029-05 AR29-05ALTIPLANO-PUNA, CENTRAL ANDES 141Reymer & Schubert 1984, Thorpe et al 1981). However, analyses of the plateautopography and structures on the eastern flank of the plateau carried out duringthe 1980s resulted in the conclusion that crustal shortening could produce most,if not all, of the required crustal thickening and that thickening, combinedwith lithospheric thinning, could account for the plateau elevations (Isacks1988, Roeder 1988, Roeder & Chamberlain 1995, Sheffels 1990). Here, wereview these arguments, as well as more recent results that appear to show thatshortening may not be able to account for all of the crustal thickening.The central Andean plateau must be viewed not just in terms of volumesand magnitudes, but also in light of its evolution. In this review, we focuson the temporal and spatial evolution of the plateau: when it began to lift upand how it varies laterally, as well as the relative importance of magmatism,crustal shortening, and lithospheric thinning. The plateau is composed of twodistinct parts: the Altiplano of Bolivia and the Puna of northwest Argentinaand adjoining parts of Chile. These areas differ in topography, magmatism,and lithospheric structure, and illustrate the range of conditions under which acontinental plateau can develop in a noncollisional orogen.The data that we review here supports and refines Isacks’ (1988) two-stagemodel for the development of the plateau. Stage 1 uplift began around 25 Ma inthe Altiplano segment and between 15 and 20 Ma in the Puna segment, when anepisode of low-angle to, locally, nearly flat subduction (Coira et al 1993, Kayet al 1995) thinned and thermally softened the lithosphere underlying the areathat was to become the plateau. Shortening ceased in the Altiplano and shiftedeastward (Stage 2) beginning between 12 and 6 Ma, but shortening continuedin the Puna until 1–2 Ma.PHYSICAL DESCRIPTION OF THE PLATEAUAND RELATED FEATURESA convenient definition of the high plateau of the Central Andes is provided bythe notable broadening of the area above the 3-km elevation contour (Figure 1).Defined this way, the high plateau of the Central Andes stretches 1800 kmalong the backbone of the range, from southern Peru to northern Argentina,and varies between 350 and 400 km in width. This definition of the plateau,which follows that of Isacks (1988), is considerably broader than the morecommon association of the plateau with the internally draining basins of theAltiplano and Puna.Plate GeometryThe geometry of the Nazca Plate beneath South America is well known(Barazangi & Isacks 1976, Bevis & Isacks 1984, Cahill & Isacks 1992,P1: MBL/rsk P2: MBLMarch 21, 1997 19:22 Annual Reviews AR029-05 AR29-05142 ALLMENDINGER ET ALHasegawa & Sacks 1981, Stauder 1975). Currently, the plateau correlates witha30◦–east-dipping segment of the subducted Nazca Plate (Figure 1). To thenorth and south, where the mountain belt narrows considerably, the subductedplate shallows and is nearly horizontal. Post-Pliocene volcanism follows thiscorrelation: It is absent where the plate is nearly flat and well developed inthe plateau where the plate is steeper. The distribution of Neogene volcanismis virtually identical to the spatial extent of the plateau, both latitudinally andlongitudinally.The subducted plate geometry differs markedly beneath the northern andsouthern ends of the plateau (Figure 1). Beneath southern