The nature of orogenic crust in the central AndesSusan L. Beck and George ZandtDepartment of Geosicences and Southern Arizona Seismological Observatory, University of Arizona, Tucson, Arizona, USAReceived 22 December 2000; revised 11 December 2001; accepted 16 December 2001; published 16 October 2002.[1] The central Andes (16° –22°S) are part of an active continental margin mountain beltand the result of shortening of the weak western edge of South America between thestrong lithospheres of the subducting Nazca plate and the underthrusting Brazilian shield.We have combined receiver function and surface wave dispersion results from theBANJO-SEDA project with other geophysical studies to characterize the nature of thecontinental crust and mantle lithospheric structure. The major results are as follows: (1)The crust supporting the high elevations is thick and has a felsic to intermediate bulkcomposition. (2) The relatively strong Brazilian lithosphere is underthrusting as far west(65.5°W) as the high elevations of the western part of the Eastern Cordillera (EC) but doesnot underthrust the entire Altiplano. (3) The subcrustal lithosphere is delaminatingpiecemeal under the Altiplano-EC boundary but is not completely removed beneath thecentral Altiplano. The Altiplano crust is characterized by a brittle upper crust decoupledfrom a very weak lower crust that is dominated by ductile deformation, leading to lowercrustal flow and flat topography. In contrast, in the high-relief, inland-sloping regions ofthe EC and sub-Andean zone, the upper crust is still strongly coupled across the basalthrust of the fold-thrust belt to the underthrusting Brazilian Shield lithosphere. Subcrustalshortening between the Altiplano and Brazilian lithosphere appears to be accommodatedby delamination near the Altiplano-EC boundary. Our study suggests that orogenicreworking may be an important part of the ‘‘felsification’’ of continental crust.INDEXTERMS: 7205 Seismology: Continental crust (1242); 7215 Seismology: Earthquake parameters; 7218Seismology: Lithosphere and upper mantle; 7230 Seismology: Seismicity and seismotectonics; 7260Seismology: Theory and modeling; KEYWORDS: continental crust, lower crust, regional waveforms, centralAndes, crustal structureCitation: Beck, S. L., and G. Zandt, The nature of orogenic crust in the central Andes, J. Geophys. Res., 107(B10), 2230, doi:10.1029/2000JB000124, 2002.1. Introduction and Tectonic Setting[2] A major question in Earth science is how continentalcrust forms and how it is modified and reworked throughtime. Mountain belts associated with convergent marginshave long been thought to be sites of both continentalgrowth by addition of material from the mantle and con-tinental recycling or reworking from delamination [Kay etal., 1994; Rudnick, 1995]. However, both of these processesare difficult to document and probably occur at differentstages of mountain belt evolution. Seismological studies arebeginning to give us information about the composition ofthe lower crust, a critical constraint in understanding theprocesses involved in mountain building and crustal recy-cling. The Nazca-South Americ an plate boundary andassociated Andean Cordillera is one of the largest mountainbelts in the world with some of the thickest crust on Earth;hence it is an ideal place to study the nature of tectonicallythickened crust in orogenic systems.[3] The central Andes (between 16°S and 21°S) in north-ern Chile and Bolivia are the highest and widest part of theAndean mountain belt, with deformation extending wellinto the back arc region, more than 800 km inland from thetrench (Figure 1). To first order, the high elevations of theback arc region correspond to large regions of crust inexcess of 60 km thick. The present-day relative p lateconvergence between the Nazca and the South Americanplate is 80 mm/yr [DeMets et al., 1990]. Approximately10–15 mm/yr of that convergence is taken up in a broadzone of deformation comprising the Andean Mountain belt[Norabuena et al., 1998]. Near-orthogonal convergence hasoccurred at a rate of 50 –150 mm/yr since about 50 Ma[Somoza, 1998; Pardo-Casas and Molnar, 1987].[4] The Andean Cordillera is often considered a modernanalog to the older western North America Cordillera. TheAmerican cordilleras have becom e the type examples ofcontinental margin mountain belts. Most workers agree thata large part of the crustal thickening in the back arc of thecentral Andes is due to tectonic shortening (for summaries,see Isacks [1988], Lamb et al. [1997], and Allmendinger etal. [1997]) rather than magmatic addition from the mantle.Magmatic addition may be significant locally in the vol-canic arc regions [Kono et al., 1989]. The geologic record ofthe western margin of South America indicates a changefrom a neutral to a compressional back arc setting during theJOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B10, 2230, doi:10.1029/2000JB000124, 2002Copyright 2002 by the American Geophysical Union.0148-0227/02/2000JB000124$09.00ESE 7 - 1Cretaceous [Coney and Evenchick, 1994]. This is preservedin the geologic record in the back arc as a transition fromplatform deposits to westerly derived foreland deposits[Coney and Evenchick, 1994; Sempere, 1990; Horton andDeCelles, 1998]. Coney and Evenchick [1994] proposedthat the onset of compression in the back arc of the Andeswas triggered by the opening of the South Atlantic at 110–130 Ma, which initiated the westward absolute motion ofthe South American plate.[5] There is still considerable debate on the details of thetiming of deformation and the overall amount of crustalshortening in the central Andes. It is generally thought thatincreased compression in the back arc began in the LateCretaceous, but no significant areas rose above sea leveluntil approximately 60 Ma [Sempere et al., 1997]. Manyworkers argue that a large part of the shortening occurred inthe last 26 m.y. [Allmendinger et al., 1997; Sempere, 1990;Isacks, 1988; Sheffels, 1990], with as much as 2 km of upliftin the last 10 m.y. [Gregory-Wodzicki et al., 1998; Gubbleset al., 1993; Isacks, 1988]. However, other studies suggestthat significant tectonic shortening occurred prior to theNeogene [Horton and DeCelles, 1998; Lamb et al., 1997;Horton et al., 2001; DeCelles and Horton, 2002]. Theminimum upper crustal shortening a mounts during theNeogene determined from geologic s tudies range from240 to 320 km in the vicinity of 20°S and can account forat