JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 93, NO. B4, PAGES 3211-3231, APRIL 10, 1988 Uplift of the CentralAndean Plateau and Bending of the BolivianOrocline BRYAN L. ISACKS INSTOCCornell Andes Project, Department ofGeological Sciences, Cornell University, Ithaca, New York Thetopography of thecentral Andes can be consideredthe primary tectonic "signal" of lateCenozoic mountainbuilding in anarid region where the effects of upliftand magrnatism arelitfie obscured by denudation. Thespatial coverage ofthe topographic signal is more complete than that for sparsely sampled geological and geophysicaldata. A color-codedimage of digitizedtopography between 12øS and 37øS highlights the Altiplano-Puna,oneof theworld's most remarkable plateaus, and reveals important physiographic clues about theformation ofthat major feature. The topographic data combined with information onstructure, magmatism, seismicity,and palcomagnetism support a simplekinematical model for thelate Cenozoic evolution of the centralAndes. The model does not require collisional effects or enormousvolumes of intrusiveadditions to the crustbut instead calls upon plausible amounts of crustalshortening and lithospheric thinning. The model interrelatesAndean uplift, a changinggeometry ofthe subdueted Nazca plate, and a changingoutline (in map view)of theleading edge of theSouth American plate. Crustal shortening has accommodated convergence betweenthe Chilean-Peruvian forearc and the South American foreland. The Altiplano-Puna plateau can be constructedby a combinationof crustalshortening and thickening and lithospheric thinning above a shallow dipping(20o-30 ø) subductedplate. The seawardlyconcave bend of westernSouth America, the "Bolivian orocline,"was enhanced but not completelyproduced by an along-strikevariation in the amountof late Cenozoicshortening. Maximum shortening in Bolivia both produced the widest part of theplateau and increasedthe seaward concavity of theBolivian orocline. The along-strike variations of shorteningare hypothesizedto resultfrom correspondingalong-strike variations in ihe widthof a weakenedzone in the overridingplate. Weakeningoccurs above the wedge of asthenospherelocated between the subductedand overridingplates; hence the width of the zone of weakening depends upon the dip of the subducted plate. Two typesof shorteningare recognized:(1) a widespread,basin-and-range, Laramide-like shortening that characterizesmodem activity in the Sierras Pampeanas andlate Miocene deformation ofthe Altiplano-Puna and (2) on theeastern side of thecordilleras and plateau, an eastverging foreland fold-thrust belt in whichthe underthrustforeland compresses andthickens the ductile lower crust and produces a plateau uplift of the upper crust.The second type of shorteningcan be applied to Plio-Quatemarydeformations throughout the central Andesbut with a substantialnarrowing ofthe region of plateau uplift in Peru and south of 28øS. A proposed monoclinalflexure of the upper crust on the western side of the plateau uplift explains the remarkably simple andregular morphology ofthe main westem slope of the central Andes. The monocline islocated above the tip of theasthenospheric wedge between the converging plates; it ispostulated to occur above the western limit of lower crustalthickening. In the regionsof horizontalsubduction the monoclinecan be associatedwith a late Mioceneasthenospheric wedge tip. INTRODUCTION evidenceabout uplift and crustal deformation to supporta model The Alfiplano of southernPeru, Bolivia, and northernChile for the late Cenozoicevolution of the centralAndes. The model connectsalong-strike changes in the amountof shorteningin the andthe Puna of northwesternArgentina together form one of the upperplate with the shapeof the subductedNazca plate and with world'sgreat plateaus. Average elevations near 4 km occurover changesin the map view outline of the leadingedge of the an areaabout 300 km wide and2000 km long. Althoughthe overridingcontinental plate. The key ideais thatan along-strike plateauseems clearly associated with the subductionof theNazca variationin the amountof shorteningimplies the possibility of a plate beneathwestern South America, its origin remains changein theshape of themap view outline of thelead'rag edge of controversial.While many studieshave assumedthat modern the upperplate. If late Cenozoicshortening of theupper plate Andeantopography is dueto crustalthickening produced by wereto be a maximumin the regionof the seawardlyconcave magmaticadditions of materialfrom the mantle [e.g., Thorpe et cornernear the Peru-Chile border (near 18øS), then the map view al., 1981], an increasingamount of evidencepoints to the curvatureof the forearcmust have increased.This providesa importanceof compressionalcrustal shortening in theformation simple mechanismfor the developmentof Carey's [1958] themodem central Andes [e.g., Dalmayrac et al., 1980;Burchfiel "BolivianOrocline," a conceptthat has recently received support et al., 1981; Jordan et al., 1983a;Chinn and Isacks, 1983; frompaleomagnetic studies [e.g., Kono et al., 1985]. The orøCline Allmendingeret al., 1983; Sudrezet al., 1983;Mdgard, 1984; can be viewedas a bendingof a narrowforearc strip that is Lyon-Caenet al., 1985;Sheffels et al., 1986]. accommodatedby a variableamount of shorteningin a mobilebelt In thispaper I explorethe hypothesis that Andean topography between the forearc and the cratonic foreland. is largelya resultof crustalthickening produced by structural The startingpoint of themodel is theassumption that thermal shorteningof thecrust and of upliftdue to thermalthinning of the weakeningis causedby convectiveand magmatic processes in the lithosphere.I use topographic data in combinationwith published wedgeof asthenospherelocated between the upper and subducted plates.Late Cenozoic magmatism, rather than supplying the large Copyright1988 by the American Geophysic• Union. volumeof crustalmaterial needed to supportthe elevated terrain, canbe takenas a signof theprocess of thermalweakening that Papernumber 7B6065. allowedthe compressionalfailure of the overridingSouth 0148-0227/88/007B-60655-5.00 American plate to happen. The along-strikevariation in 3211 3212 Isnc•: shorteningresults from variationsin thehorizontal width of the THEAL•O-PUNA PLATEAU zone of thermal weakeningin the overridingcontinental plate. Sincethe width of the wedge ofasthenosphere isdetermined by The Plateau the dip of the subductedplate, the along-strikevariation in As shownin Plate 1 andFigure 1, a singlebroad plateau is the shorteningis therebyrelated to the geometryof the subducted maincontinental-scale feature of the centralAndes between about plate. Althoughoversimplified and only kinematic,the proposed 15øSand 27øS. North and southof the plateauthe mountain model is an evolutionary,three-dimensional one. Its success system narrows considerably but continues with average emphasizesthe futility of trying to understandthe subductionelevations greater than 3 km alongthe high Andean cordilleras of processwith traditionalsteady state, two-dimensionalcross- Peru and alongthe Chile-Argentineborder (between 27øS and sectionalmodels. about34øS). For convenience,I distinguish these two segmentsof In the arid and largely unglaciatedcentral Andes where the Andeanmountain system located northwest and south of the denudationis minimizedand the tectonicprocesses are youthful plateauas the "Peruviansegment" (5ø-15øS) and the "Pampean andongoing, the topographyis a direct,first-order expression of segment"(27 ø- 34øS),respectively. theendogenetic processes forming the mountainbelt. Early work The hypsometriccurve for the plateau(Figure 2) hasa sharp such as Bowman [1909a, b, 1916] clearly recognizedthe break in slopeat an elevationof 3.65 kin, approximatelythe youthfulnessof central Andean uplift and its plateaulike character. elevation of the extensiveand internallydrained basins of the However,regional physiography has received very little attention Altiplanoand Puna. Muchof theremaining area of theplateau is in the dominantly geochemical, structural and geophysical characterizedby moderaterelief with elevationsin the rangeof approachesthat characterizemost modemstudy of the region about3.5-4.7 kin. As recognizedby Bowman[1909b] many (Coney[1971] is an outstandingexception). I presenthere a new years ago, peneplanation is the dominating fact in the viewof thecentral Andean physiography based on digitizationof physiographyof theregion. The otherdominating factor is the l:l,000,000-scaletopographic maps now available for mostof the pervasivevolcanism. Many of the highestpeaks are volcanic region.The computer-generatedcolor image of thisdigital data conslxuctionallandforms, although areas of highsu'uctural relief set is shownin Plate 1. The imagedepicts the regional-scale arealso found along the eastern edges of thenorthern and southern physiographyin a particularlyclear and accuratefashion. It partsof theplateau. preservesthe detail lost in a similarlyscaled conventional map in Theexu'usive material largely sits on top of theplateau rather which topographywould be shownby only a limited set of than formingthe volumeof the plateauitself. The upper subjectivelysmoothed contour lines. Miocene-Recentexu'usives form only a thincover on a surfaceof I first summarizerelevant information about the plateau,then beveledolder structures,as is clearly shownby the studiesof describea two-dimensionalmodel for shortening,and finally Baker andFrancis [1978] andBaker [1981],by numerousdetailed describe a three-dimensionalmodel