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CU-Boulder GEOL 5700 - What drives orogeny in the Andes?

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! 2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]; August 2005; v. 33; no. 8; p. 617–620; doi: 10.1130/G21557.1; 4 figures; Data Repository item 2005118. 617What drives orogeny in the Andes?S.V. Sobolev GeoForschungsZentrum-Potsdam, Telegrafenberg, 14473 Potsdam, Germany, and Institute of Physics of theEarth, B. Gruzinskaya 10, Moscow, RussiaA.Y. Babeyko GeoForschungsZentrum-Potsdam, Telegrafenberg, 14473 Potsdam, GermanyFigure 1. A: Surface topography of Andes with indicated majorstructural features. Trench adjacent to high central Andes has nosedimentary fill, which may increase friction in subduction channel(Lamb and Davis, 2003). B: Model setup and boundary conditions.Subducting plate is 45 m.y. old. Initial thickness of continental lith-osphere is 100–130 km, with thickest lithosphere in eastern (right)part of model corresponding to Brazilian shield margin. South Amer-ican plate is drifting to west (left) with velocity increasing from 2 to3 cm/yr during past 30 m.y. (Silver et al., 1998). Lower end of Nazcaplate is pulled down, with velocity changing from 5 to 13 cm/yr (So-moza, 1998). C: Shear stress in subduction channel.ABSTRACTThe Andes, the world’s second highest orogenic belt, were gen-erated by the Cenozoic tectonic shortening of the South Americanplate margin overriding the subducting Nazca plate. We use a cou-pled thermomechanical numerical modeling technique to identifyfactors controlling the intensity of the tectonic shortening. Fromthe modeling, we infer that the most important factor was accel-erated westward drift of the South American plate; changes in thesubduction rate were less important. Other important factors arecrustal structure of the overriding plate and shear coupling at theplates’ interface. The model with a thick (40–45 km at 30 Ma)South American crust and relatively high friction coefficient (0.05)at the Nazca–South American interface generates !300 km of tec-tonic shortening during 30–35 m.y. and replicates the crustal struc-ture and evolution of the high central Andes. The model with aninitially thinner ("40 km) continental crust and lower friction co-efficient ("0.015) results in "40 km of South American plate short-ening, replicating the situation in the southern Andes. Our mod-eling also demonstrates the important role of the processes leadingto mechanical weakening of the overriding plate during tectonicshortening, such as lithospheric delamination, triggered by thegabbro-eclogite transformation in the thickened continental lowercrust, and mechanical failure of the sediment cover at the shieldmargin.Keywords: Andes, subduction, orogeny, numerical model.INTRODUCTIONThe Andes Mountains extend along the entire western margin ofthe South American plate above the subducting Nazca plate. The SouthAmerican plate is drifting westward at a rate that has increased from2 to 3 cm/yr during the past 30 m.y. (Silver et al., 1998). There is adramatic difference in structure and evolution between the central An-des (#17$–27$S) and the rest of the Andes. The Altiplano-Puna plateauof the central Andes is the second highest plateau in the world, afterthe Tibetan Plateau, with an average elevation of #4 km and an areaof !500,000 km2(Fig. 1A). The plateau was formed in the Cenozoicby as much as 300–350 km of crustal shortening in the western edgeof the South American plate (Isacks, 1988; Allmendinger and Gubbels,1996; Allmendinger et al., 1997; Lamb et al., 1997; Kley and Monaldi,1998; Lamb and Davis, 2003; Elger et al., 2005). This shortening gen-erated unusually thick, hot, and felsic continental crust (Allmendingeret al., 1997; Beck and Zandt, 2002; Yuan et al., 2002). No high plateauexists in the northern and southern Andes (Fig. 1), where only minor("50 km) tectonic shortening has been reported (e.g., Allmendinger etal., 1997; Lamb et al., 1997; Kley and Monaldi, 1998).Perhaps the key question of the Andean orogeny is why the highplateau developed only in the central Andes and only in Cenozoic time(mostly during the past 30 m.y), although the Nazca plate has beensubducted along the entire western margin of the South American plateduring more than 200 m.y. (e.g., Isacks, 1988; Allmendinger et al.,1997). Several ideas have been proposed to answer this question. Isacks(1988) suggested that before ca. 25–30 Ma, the central Andes wereunderlain by a flat slab that became steeper ca. 25 Ma, causing thermalweakening and intensive tectonic shortening of the compressed litho-sphere of the overriding plate. In another hypothesis (e.g., Pardo-Casasand Molnar, 1987; Somoza, 1998), the beginning of intensive tectonicshortening in the Andes was associated with the major reorganizationof the plates, followed by an increase of the Nazca–South Americanconvergence rate ca. 25–30 Ma. Russo and Silver (1996) and Silver etal. (1998) attributed the Andean orogeny to the Cenozoic increase ofthe westward drift rate of the South American plate. Lamb and Davis(2003) suggested that high shear stress at the interface between Nazcaand the South American plate, caused by the Cenozoic climate-controlled sediment starvation in the Central Andean trench, played aleading role in Andean orogeny.The diversity of the suggested hypotheses reflects the complexityof the deformation processes responsible for the Andean orogeny, butit also indicates the lack of quantitative understanding of these pro-618 GEOLOGY, August 2005Figure 2. Time snapshots of evolution of tectonic shortening formodel of central Andes. Positions of snapshots along horizontalaxis are their true positions in hotspot frame. Color codes corre-spond to rock types. Approximately 60% of South American westerndrift is accommodated by trench rollback and ~40% by tectonicshortening of South American margin. Note that lower end of theslab moves to left >200 km during 35 m.y. and hence slab is notanchored. Note also intensive thickening of felsic upper crust (yel-low, orange) and loss of mafic lower crust (green) in South Americanplate during past 18 m.y. (model times 17–35 m.y.), while mantlelithosphere (light green) in South American plate is becoming thin-ner during tectonic shortening. At ~25 m.y. modeling time, sedimen-tary cover of shield margin (red) fails and shield begins to under-thrust under growing plateau.cesses. Each of these hypotheses is based on analyses of many obser-vations, and all have solid observational


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CU-Boulder GEOL 5700 - What drives orogeny in the Andes?

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