Thickening the Altiplano crust by gravity-driven crustal channel flowL. Husson and T. SempereInstitut de Recherche pour le De´veloppement, Lima, Peru´Received 4 January 2003; revised 4 February 2003; accepted 6 February 2003; published 12 March 2003.[1] In the Central Andes, crustal thickness is not wellcorrelated to upper crustal shortening. Only little shorteningis documented in the upper crust of the 60–65 km-thickAltiplano plateau, whose thickening and uplift were delayedwith respect to earlier and greater thickening in the adjacentWestern and Eastern Cordilleras. Because crustal thicknessvariations induce horizontal stress gradients and cause crustalflow, a thickness-dependent channel flow is modeled hereand applied to the Central Andes. In situ thickening isassumed for both cordilleras, while the Altiplano crustalthickening is generated by lateral flow from t heseoverthickened adjacent domains. A 8.1018Pa s viscositychannel is predicted for crustal thicknesses exceeding 45–50km to match the estimated topographic evolution of theCentral Andes. Thickening in the cordilleras was sufficient togenerate a flow of 6.109m3per unit length toward the initially30–35 km-thick Altiplano.INDEX TERMS: 3210Mathematical Geophysics: Modeling; 5104 Physical Properties ofRocks: Fracture and flow; 8164 Tectonophysics: Evolution of theEarth: Stresses—crust and lithosphere; 8102 Tectonophysics:Continental contractional orogenic belts; 8122 Tectonophysics:Dynam ics, gravity and tectonics. Citation: Husson, L., andT. Sempere, Thickening the Altiplano crust by gravity-drivencrustal channel flow, Geophys. Res. Lett. , 30(5), 1243, doi:10.1029/2002GL016877, 2003.1. Introduction[2] The origin of the Altiplano plateau is poorly under-stood. In the Bolivian Orocline, i.e. the central segment ofthe Central Andes, upper crustal shortening estimates do notcorrelate well with total crustal thickness [Kley and Mon-aldi, 1998]. The largest deviation is found for the 60 – 65km-thick [Beck et al., 1996] Altiplano crust: in this domain,the pre-Neogene total crustal thickness was 30– 35 km[Sempere et al., 2002] and only minor (<10–15%) uppercrustal shortening has occurred during the Neogene [Rochatet al., 1999]. This profound discrepancy means that homo-geneous crustal shortening cannot have been responsible forthe Altiplano crustal g rowth. We address this issue bytesting the hypothesis that the Bolivian Orocline has devel-oped heterogeneously from gravity-driven channel flow ofcrustal material injected from overthickened areas of theWestern and Eastern Cordilleras towards the Altiplano.2. Relevant Characteristics of the BolivianOrocline[3] The Bolivian Orocline (BO) is commonly divided intoa number of geomorphic zones (Figure 1), among which theAltiplano plateau is the most characteristic due to its 3600 mmean elevation and 60–65 km crustal thickness [Isacks,1988; Beck et al., 1996]. The Altiplano is bounded by theEastern Cordillera (EC), which originated mainly fromtectonic shortening [e.g., Sheffels, 1990], and by the WesternCordillera (WC), where cluster the volcanoes of the sub-duction arc. Both EC and WC crustal thicknesses are, at leastlocally, over 70 km [Beck et al., 1996], but there is noevidence that the WC , unlike the EC, originated fromtectonic contraction only. West of the WC, which is largelycovered by Late Neogene volcanics, the Coastal Belt dis-plays a west-tapering 65–0 km-thick crust [ANCORP Work-ing Group, 1999] and a complex history. In contrast, the EChas resulted from the Oligocene-Miocene tectonic inversionof a Triassic-Jurassic rift system [Sempere et al., 2002]. Eastof the EC, between 18S and 23S, the Subandean Zone(SAZ) is a Neogene fold-and-thrust belt, the foredeep ofwhich underlies the Chaco plain [e.g., Dunn et al., 1995].[4] Models of Andean uplift [Gregory-Wodzicki, 2000;Kennan, 2000] consider that the onset of the WC upliftoccurred 60 Ma ago, and that the EC uplift developed laterand slower (Figure 2a). This first stage of mountain growthlast until the Late Paleogene, when the WC and EC reachedelevations of 2000 m and 1000 m respectively. Upliftrates of both cordilleras have increased since 20 Ma, leadingto the present-day high elevations. Located between them,the Altiplano uplift mainly occurred during the last 20–10Ma, and thus was delayed with respect to the cordilleras.3. Discrepancies Between Shortening Rates andCrustal Thicknesses[5] The present-day crustal thickness in the EC can beexplained by intense inversion and tectonic contraction ofpreviously thinned crust [Sempere et al., 2002]. Accordingto shortening estimates [e.g., Rochat et al., 1999], bulkstrain in the EC is 0.4, in agreement with its 70 km crustalthickness. In the SAZ, minimum bulk strain is 0.45, but itsoverall crustal thickening has been only moderate due to itsthin-skinned deformation.[6] The high crustal thickness of the WC has not beensatisfactorily explaine d yet: tectonic shortening, magma-tism, and possibly other in-situ crustal growth processes(see, Lamb and Hoke [1997] for a review) have contributedto crustal growth.[7] Figure 3 compares the actual crustal thickness and thecrustal thickness predicted by assuming in situ crustalthickening correlated to upper crustal bulk strain acrossthe BO. The SAZ presents a large excess of crustal volume,whereas the Altiplano is characterized by a significantdeficit (bulk strain 0.12). Because this discrepancy cannotbe explained by homogeneous crustal deformation, weexplore the hypothesis that thickening of the Altiplano crustGEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 5, 1243, doi:10.1029/2002GL016877, 2003Copyright 2003 by the American Geophysical Union.0094-8276/03/2002GL016877$05.0047--1originated by viscous flow from the overthickened WC andEC crusts.4. Viscous Mid/Lower Crustal Channel Flow[8] Laboratory experime nts suggest that the viscosityexponentially decreases with temperature [e.g., Goetze,1978; Kirby, 1983]. Thus viscosity strongly decreases atsome critical depth in thick crusts. This in turns help tounderstand why the upper crust is more likely described byan elastic/brittle behavior and the lower crust by a ductilebehavior. Following Bird [1991], models have demonstratedthat a lateral flow in the middle to lower crust may explaingeological observations [e.g., Royden, 1996; McQuarrieand Chase, 2000; Clark and Royden, 2000]. We modelthe response of a low-viscosity material flowing int o achannel