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What drives orogeny in the Andes? S.V. Sobolev GeoForschungsZentrum-Potsdam, Telegrafenberg, 14473 Potsdam, Germany, and Institute of Physics of the Earth, B. Gruzinskaya 10, Moscow, Russia A.Y. Babeyko GeoForschungsZentrum-Potsdam, Telegrafenberg, 14473 Potsdam, Germany ABSTRACT The Andes, the world’s second highest orogenic belt, were generated by the Cenozoic tectonic shortening of the South American plate margin overriding the subducting Nazca plate. We use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening. From the modeling, we infer that the most important factor was accelerated westward drift of the South American plate; changes in the subduction rate were less important. Other important factors are crustal structure of the overriding plate and shear coupling at the plates’ 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 tectonic shortening during 30–35 m.y. and replicates the crustal structure and evolution of the high central Andes. The model with an initially thinner ("40 km) continental crust and lower friction coefficient ("0.015) results in "40 km of South American plate shortening, replicating the situation in the southern Andes. Our modeling also demonstrates the important role of the processes leading to mechanical weakening of the overriding plate during tectonic shortening, such as lithospheric delamination, triggered by the gabbro-eclogite transformation in the thickened continental lower crust, and mechanical failure of the sediment cover at the shield margin. and Molnar, 1987; Somoza, 1998), the beginning of intensive tectonic shortening in the Andes was associated with the major reorganization of the plates, followed by an increase of the Nazca–South American convergence rate ca. 25–30 Ma. Russo and Silver (1996) and Silver et al. (1998) attributed the Andean orogeny to the Cenozoic increase of the westward drift rate of the South American plate. Lamb and Davis (2003) suggested that high shear stress at the interface between Nazca and the South American plate, caused by the Cenozoic climatecontrolled sediment starvation in the Central Andean trench, played a leading role in Andean orogeny. The diversity of the suggested hypotheses reflects the complexity of the deformation processes responsible for the Andean orogeny, but it also indicates the lack of quantitative understanding of these pro- Keywords: Andes, subduction, orogeny, numerical model. INTRODUCTION The Andes Mountains extend along the entire western margin of the South American plate above the subducting Nazca plate. The South American plate is drifting westward at a rate that has increased from 2 to 3 cm/yr during the past 30 m.y. (Silver et al., 1998). There is a dramatic difference in structure and evolution between the central Andes (#17$–27$S) and the rest of the Andes. The Altiplano-Puna plateau of the central Andes is the second highest plateau in the world, after the Tibetan Plateau, with an average elevation of #4 km and an area of !500,000 km2 (Fig. 1A). The plateau was formed in the Cenozoic by as much as 300–350 km of crustal shortening in the western edge of 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 generated unusually thick, hot, and felsic continental crust (Allmendinger et al., 1997; Beck and Zandt, 2002; Yuan et al., 2002). No high plateau exists in the northern and southern Andes (Fig. 1), where only minor ("50 km) tectonic shortening has been reported (e.g., Allmendinger et al., 1997; Lamb et al., 1997; Kley and Monaldi, 1998). Perhaps the key question of the Andean orogeny is why the high plateau developed only in the central Andes and only in Cenozoic time (mostly during the past 30 m.y), although the Nazca plate has been subducted along the entire western margin of the South American plate during 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 were underlain by a flat slab that became steeper ca. 25 Ma, causing thermal weakening and intensive tectonic shortening of the compressed lithosphere of the overriding plate. In another hypothesis (e.g., Pardo-Casas Figure 1. A: Surface topography of Andes with indicated major structural features. Trench adjacent to high central Andes has no sedimentary 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 lithosphere is 100–130 km, with thickest lithosphere in eastern (right) part of model corresponding to Brazilian shield margin. South American plate is drifting to west (left) with velocity increasing from 2 to 3 cm/yr during past 30 m.y. (Silver et al., 1998). Lower end of Nazca plate is pulled down, with velocity changing from 5 to 13 cm/yr (Somoza, 1998). C: Shear stress in subduction channel. ! 2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; August 2005; v. 33; no. 8; p. 617–620; doi: 10.1130/G21557.1; 4 figures; Data Repository item 2005118. 617 cesses. Each of these hypotheses is based on analyses of many observations, and all have solid observational grounds. However, none have yet been quantitatively tested by modeling the coupled dynamic and thermal interaction of the overriding and subducting plates. In this study we present such models, focusing on dependence of tectonic deformation of the overriding plate during the past 35 m.y. on (1) convergence rate, (2) overriding rate, (3) strength of mechanical coupling between subducted and overriding plates, and (4) initial lithospheric structure. METHOD AND MODEL Modeling the dynamic interaction between subducting and overriding plates demands realistic rheological models of both plates, including elasticity, plasticity, and temperature- and stress-dependent viscosity. To accomplish this modeling, we use a two-dimensional parallel thermomechanical finite-element/finite-difference code LAPEX-2D (see Babeyko et al. [2002] for the description of the previous version of the code). This code combines the explicit Lagrangian algorithm FLAC (Cundall and Board, 1988; Poliakov et al., 1993) with the particle technique similar to the particle-in-cell method (e.g., Moresi et al., 2003). Particles track material properties and full stress tensor, minimizing numerical diffusion related to remeshing. This method allows the use of realistic temperature- and stress-dependent viscoelastic rheology combined with Mohr-Coulomb plasticity for layered oceanic and continental lithospheres (Fig. 1B). The rheological parameters are taken from published experimental and theoretical rheological studies and are presented in Data Repository Table DR1.1 Because of the presence of the subduction zone, we have used rheological parameters for ‘‘wet’’ rocks everywhere in the model except for the slab and mantle lithosphere of the shield margin. In the crust, we employ friction- and viscosity-strain softening, which is assumed to be more intensive in the Paleozoic sediments in the Subandean zone (Fig. 1B). The viscous deformation in the mantle is considered to be driven by competing dislocation, diffusion, and Peierls creep mechanisms. The numerical method routinely includes shear heating and gabbro-eclogite phase transformation (model details in the Data Repository; see footnote 1). A set of models was run for initial crustal structures expected for the central and southern Andes at 30–35 Ma. Initial crustal structure for the central Andes (Fig. 2A) contains thick felsic upper crust and thinner mafic lower crust, with a total crustal thickness of 40–45 km, assuming that the crust was already significantly shortened by 30–35 Ma (Allmendinger et al., 1997; Lamb et al., 1997). Initial crust for the southern Andes consists of equally thick upper felsic and lower mafic layers and has a total thickness of 35–40 km. The geometry and boundary conditions incorporated in all our models are schematically shown in Figure 1B and Figure 2A. In all models, we explore the interaction of the 45 Ma subducting Nazca plate with the 100–130-km-thick lithosphere of the overriding South American plate during the past 30–35 m.y. We assume low-angle geometry of the subducting plate, consistent with the present-day structure in the Andes. The model box is 1200 km long and 400 km high (Fig. 1B), moving to the left (west) together with the overriding plate. The drift of the overriding plate and subduction are generated by pushing the overriding plate at its right boundary and by pulling the slab from below (Fig. 1B), with the absolute velocities (in hotspot frame) taken from plate tectonics reconstructions (Silver et al., 1998; Somoza, 1998). All other parts of the model box boundary (Fig. 1B) are open for free motion of material. We emphasize that the lower end of the 1GSA Data Repository item 2005118, supplementary data, is available at www.geosociety.org/pubs/ft2005.htm, or on request from editing@geosociety. org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA. 618 Figure 2. Time snapshots of evolution of tectonic shortening for model of central Andes. Positions of snapshots along horizontal axis are their true positions in hotspot frame. Color codes correspond to rock types. Approximately 60% of South American western drift is accommodated by trench rollback and ~40% by tectonic shortening of South American margin. Note that lower end of the slab moves to left >200 km during 35 m.y. and hence slab is not anchored. Note also intensive thickening of felsic upper crust (yellow, orange) and loss of mafic lower crust (green) in South American plate during past 18 m.y. (model times 17–35 m.y.), while mantle lithosphere (light green) in South American plate is becoming thinner during tectonic shortening. At ~25 m.y. modeling time, sedimentary cover of shield margin (red) fails and shield begins to underthrust under growing plateau. slab is not fixed and can be located anywhere between the left and right boundaries of the model. Therefore, the slab is by no means anchored, and retains the full dynamic freedom, e.g., in regard to the retrograde motion. We also note that our boundary conditions do not prescribe tectonic shortening of the upper plate as it was done in the previous models (Wdowinski and Bock, 1994; Pope and Willett, 1998). The interplate interface is modeled as a thin subduction channel, i.e., an #12-km-thick (three finite elements) layer with the special rheology. For each finite element within the channel, we use either a frictional (Mohr-Coulomb, brittle) elastoplastic or a temperaturedependant viscous (Peacock, 1996) rheological model, depending on which of those models requires the lowest shear stress. In this approach, the shallow low-temperature part of the subduction channel has frictional (brittle) rheology with shear stress increasing with depth (Fig. 1C, black curve). At deeper depth and higher temperature, the viscous flow mechanism takes over, and shear stress in the channel decreases with depth (Fig. 1C, red curve). The depth where frictional rheology changes to viscous rheology depends on the friction coefficient, and it is deeper if friction is lower (cf. dashed and solid black curves in Fig. 1C). We consider the friction coefficient in the subduction channel to be a model parameter, and we change it from 0 to 0.15, in agreement with previous estimates (Bird, 1978; Peacock, 1996; Hassani et al., 1997). MODEL FOR THE CENTRAL ANDES We first found the model that best fits the observations for the central Andes for the time between 35 Ma to the present day. To do that, we assumed a thick (40–45 km) crust (Fig. 2A) as a starting model and used kinematic boundary conditions mimicking subduction and overriding velocities according to the plate tectonic reconstructions (Silver et al., 1998; Somoza, 1998). With this setup, we performed GEOLOGY, August 2005 Figure 3. Evolution of surface topography in central Andes model. Note formation of high topography and then plateau during last 10 m.y. (model times, 25–35 m.y.). several numerical experiments, changing the friction coefficient in subduction channel %. All model runs with % ! 0.10 resulted in slab breakoff and termination of the subduction. At % & 0.05–0.10, subduction survived, but large interplate coupling led to too-strong shortening of the overriding plate. The model replicates the case of the central Andes most closely at % # 0.05. In this model, 58% of the westward drift of the South American plate during the past 35 m.y. is accommodated by the Nazca slab trench rollback, and the rest (42%) by tectonic shortening of the South American plate (37%) and subduction erosion (5%) (Fig. 2). During the shortening, the felsic crust thickness almost doubles, while the mafic lower crust and mantle lithosphere actually become thinner. The reason for this is the delamination of the lower crust and mantle lithosphere driven by gabbro-eclogite transformation in the lower crust, first discussed in the Andean context by Kay and Kay (1993). Thickening of the crust to more than 45 km switches on mineral reactions in the mafic lower crust, which increases its density to higher than the density of mantle peridotite (3300 kg/m3). The bodies of the dense lower crust and mantle lithosphere tend to sink into the less dense asthenosphere. While sinking, most of such bodies are moved by the corner flow toward the trench, join the slab, and are then subducted into the mantle (Fig. 2B), somewhat similar to the ablative subduction scenario suggested by Pope and Willett (1998). For more details, see Data Repository Figure DR1 (see footnote 1). After #20–25 m.y. model time, the tectonic shortening generates high topography near the magmatic arc and in the backarc close to the shield margin (Fig. 3, orange curve). Large topographic gradients initiate intensive flow in the lower and middle crust. They even out crustal thickness and surface topography and produce a 4-km-high plateau at 30–35 m.y. (Fig. 3), i.e., during the last 5 m.y., in accord with the timing of the plateau uplift suggested by Gregory-Wodzicki (2000) and similar to the previous modeling results by Wdowinski and Bock (1994). At the same time, the tectonic shortening reaches 300–350 km (curve indicated by filled circles in Fig. 4), in agreement with the geological estimations of the maximal shortening in the central Andes (Kley and Monaldi, 1998). The model also predicts failure of the foreland sediments at #25 m.y. model time, followed by underthrusting of the shield margin and by a switch from a pure-shear to simple-shear mode of shortening, in agreement with the geological model by Allmendinger and Gubbels (1996). WHAT DRIVES ANDEAN OROGENY? Here we examine sensitivity of tectonic shortening to potentially important factors like overriding rate, strength of mechanical coupling between subducted and overriding plates, initial lithospheric structure, and convergence rate. To do that, we modified the central Andes model described here, ‘‘switching off’’ different factors and examining consequences for the tectonic shortening during the 30 m.y. of evolution. First we switched off the acceleration of the South American plate’s westward drift in the central Andes model, leaving all other GEOLOGY, August 2005 Figure 4. Calculated tectonic shortening versus time for different models. Numbers near models indicate subduction channel friction coefficient (first number) and South American western drift velocity (second group of numbers). Time ranges of some critical processes in central Andes model are shown below time axis. parameters unchanged. This modified model (curve denoted by open boxes in Fig. 4) generates only 60% of the shortening achieved in the central Andes model during the 30 m.y. Moreover, if the drift velocity is decreased, to 1 cm/yr, no tectonic shortening happens. These modeling results suggest that the high overriding rate of the South American plate in the Cenozoic may have been a major factor controlling tectonic shortening in the central Andes, in agreement with previous studies (Russo and Silver, 1996; Silver et al., 1998). However, we also conclude that it is unlikely that acceleration of the drift alone could be responsible for the entire observed tectonic shortening. Next we switched off the high subduction channel friction in the central Andes model (setting % to 0.015 instead of 0.05). The resulting model (Fig. 4, curve indicated by open circles) still generates large tectonic shortening, i.e., 74% of the shortening achieved in the central Andes model. However, if we additionally use the mechanically stronger thin-crust (35–40 km) initial model instead of the thick-crust (40– 45 km) model, the tectonic shortening at 30 m.y. model time reduces to "40 km, which matches the situation in the southern Andes well (Fig. 4, curve indicated by solid diamonds). These modeling results suggest that although reducing the subduction channel friction coefficient by four to five times has significant effect on tectonic shortening, only a combination of this factor with another (e.g., initial crustal structure) could have been responsible for the dramatic difference in the amount of tectonic shortening between the central and southern Andes. The shape of the modeled shortening curve for the central Andes model (solid circles in Fig. 4) shows that the maximal convergence rate of 15 cm/yr achieved at 10–15 m.y. model time does not have much effect on the shortening rate. From that, we infer that an increased convergence rate at 25 Ma could not be a reason for the intensive orogeny in the central Andes. In addition to the factors considered here, other processes can also influence the rate of tectonic shortening. Active delamination accompanied by the blocking of the corner flow by the delaminated material (see also Fig. 2B) at 15–20 m.y. model time significantly intensifies shortening as a result of increased coupling between the plates and mechanical weakening of the overriding plate. Another increase in the shortening rate in the central Andes model is associated with the failure of the foreland sediments, followed by underthrusting of the shield 619 margin and by a switch from a pure-shear to simple-shear mode of shortening at #25 m.y. model time. We conclude that the major factor controlling Andean orogeny is likely accelerating westward drift of the South American plate. However, we also infer that neither this drift nor any other factor was alone responsible for the orogeny in the central Andes. Our modeling suggests that the extreme orogeny in the Andes took place at the time and in the place when and where at least three major conditions were met: (1) a high overriding rate of the South American plate (achieved during the past 30 m.y. and especially during the past 10 m.y.); (2) a thick crust (40–45 km) in the backarc that resulted from tectonic shortening before 30 Ma, including enhanced shortening during the flat-slab episode; and (3) a friction coefficient of #0.05 in the subduction channel (both latter conditions were likely achieved only in the central Andes). The models also demonstrate the important role of several accompanying processes in causing the internal mechanical weakening of the South American plate during the tectonic shortening. The main accompanying processes are lithosphere delamination, triggered by the gabbro-eclogite transformation in the thickened continental lower crust, and mechanical failure of the sediments covering the shield margin. Note, however, that here we do not consider changes in slab geometry in time and space, which could potentially also affect deformation of the overriding plate. Consideration of these effects will require threedimensional modeling, which is beyond the scope of this paper. Based on our modeling results for the central Andes, we can infer some general remarks on the deformation of a continental margin overriding a subducting plate. The fast overriding (!1–2 cm/yr) leads to the low-angle subduction (van Hunen et al., 2004) and associated strong coupling between the plates. The overriding plate undergoes compression with the maximal stresses achieved in regions of highest interplate friction and of increased buoyancy of the slab due to subducted ridges or ocean plateaus. Those are the places where the continental crust first reaches the critical thickness of #45 km when the gabbro-eclogite–driven lithospheric delamination may be activated, intensifying the tectonic shortening. Later, the less compressed parts of the overriding plate also become involved in this process. Hence, a large tectonic shortening of the overriding plate margin is probably an unavoidable consequence of a fast and long overriding. The rate of shortening depends, however, on a series of parameters. A suitable combination of the most important parameters, i.e., high overriding velocity, high interplate friction, and prethickened crust, generates spectacular orogens like the Cenozoic central Andes, while in other cases (like in the southern Andes) the orogeny remains subtle. ACKNOWLEDGMENTS This work is a part of the collaborative research program SFB-267 Deformation Processes in the Andes, supported by the Deutsche Forschungs Gemeinschaft and GeoForschungsZentrum-Potsdam. We thank Onno Oncken, Tim Vietor, and members of the SFB-267 team for fruitful discussions. The paper benefited from the comments of Shimon Wdowinski, Chris Beaumont, and an anonymous reviewer. Note in press: We dedicate this paper to the memory of Peter Giese who initiated the Berlin-Potsdam multidisciplinary project in the central Andes. REFERENCES CITED Allmendinger, R.W., and Gubbels, T., 1996, Pure and simple shear plateau uplift, Altiplano-Puna, Argentina and Bolivia: Tectonophysics, v. 259, p. 1–13. Allmendinger, R.W., Jordan, T.E., Kay, S.M., and Isacks, B.L., 1997, The evolution of the Altiplano-Puna plateau of the central Andes: Annual Review of Earth and Planetary Sciences, v. 25, p. 139–174. Babeyko, A.Y., Sobolev, S.V., Trumbull, R.B., Oncken, O., and Lavier, L.L., 2002, Numerical models of crustal-scale convection and partial melting 620 beneath the Altiplano-Puna plateau: Earth and Planetary Science Letters, v. 199, p. 373–388. 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Application to the central Andes: Journal of Geophysical Research, v. 99, p. 7121–7130. Yuan, X., Sobolev, S.V., and Kind, R., 2002, New data on Moho topography in the Central Andes and their geodynamic implications: Earth and Planetary Science Letters, v. 199, p. 389–402. Manuscript received 31 January 2005 Revised manuscript received 13 April 2005 Manuscript accepted 18 April 2005 Printed in USA GEOLOGY, August 2005