Plateau-Style Accumulation of Deformation: Southern Altiplano
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TECTONICS, VOL. 24, TC4020, doi:10.1029/2004TC001675, 2005 Plateau-style accumulation of deformation: Southern Altiplano Kirsten Elger, Onno Oncken, and Johannes Glodny GeoForschungsZentrum Potsdam, Potsdam, Germany Received 5 May 2004; revised 17 December 2004; accepted 23 March 2005; published 31 August 2005. [1] Employing surface mapping of syntectonic during the Paleogene, initially reactivating crustal sediments, interpretation of industry reflection- weak zones and by thermal weakening of the crust seismic profiles, gravity data, and isotopic age dating, with active magmatism mainly in the Neogene stage. we reconstruct the tectonic evolution of the southern Citation: Elger, K., O. Oncken, and J. Glodny (2005), Plateau- Altiplano (20–22°S) between the cordilleras style accumulation of deformation: Southern Altiplano, Tectonics, defining its margins. The southern Altiplano crust 24, TC4020, doi:10.1029/2004TC001675. was deformed between the late Oligocene and the late Miocene with two main shortening stages in the Oligocene (33–27 Ma) and middle/late Miocene 1. Introduction (19–8 Ma) that succeeded Eocene onset of shortening at the protoplateau margins. Shortening [2] Although considerable advance has been made in recent years in understanding the processes involved in rates in the southern Altiplano ranged between 1 and the formation of orogenic plateaus, the precise temporal 4.7 mm/yr with maximum rates in the late Miocene. and spatial patterns of uplift and lateral progradation of Summing rates for the southern Altiplano and the deformation in plateaus beyond the last few million years is Eastern Cordillera, we observe an increase from still shrouded in darkness. This situation is largely due to a Eocene times to the late Oligocene to some 8 mm/yr, lack of systematic data with the potential to date the timing followed by fluctuation around this value during the of deformation and uplift, as well as of detailed structural Miocene prior to shutoff of deformation at 7–8 Ma and data that reveal the internal architecture of the world’s transfer of active shortening to the sub-Andean fold large orogenic plateaus, Tibet and Altiplano [e.g., Dewey and thrust belt. Shortening inverted early Tertiary et al., 1988; Isacks, 1988; England and Molnar, 1997; graben and half graben systems and was partitioned Allmendinger et al., 1997; Shen et al., 2001]. Young, in three fault systems in the western, central, and syntectonic and posttectonic sediments largely cover both plateaus and restrict access to the subsurface. Yet, models of eastern Altiplano, respectively. The east vergent fault large-scale orogeny and plateau formation heavily rely on systems of the western and central Altiplano were assumptions that relate to the early stages of plateau synchronously active with the west vergent Altiplano evolution and the conditions governing these. This is west flank fault system. From these data and from particularly true for the Andean Altiplano, which, due to section balancing, we infer a kinematically linked its setting in an ocean-continent convergence system, has western Altiplano thrust belt that accumulated a been called a plate tectonic paradox [e.g., Allmendinger et minimum of 65 km shortening. Evolution of this belt al., 1997]. Because of the unsatisfactory data situation, contrasts with the Eastern Cordillera, which reached published ideas on accumulation of deformation in the peak shortening rates (8 mm/yr) in between the above Altiplano range from various inferred stages in plateau two stages. Hence local shortening rates fluctuated evolution [e.g., James, 1971; Pardo-Casas and Molnar, across the plateau superimposed on a general trend of 1987; Isacks, 1988; Sempere et al., 1990; Allmendinger and Gubbels, 1996; Kley, 1996; Allmendinger et al., 1997; increasing bulk rate with no trend of lateral Lamb and Hoke, 1997; Lamb et al., 1997; Lamb, 2000; propagation. This observation is repeated at the Hindle and Kley, 2002] to suggestions of continuous east- shorter length and time scales of individual growth ward progradation of deformation, starting at the Western structures that show evidence for periods of enhanced Cordillera, with a propagation style much like that observed local rates at a timescale of 1–3 Myr. We interpret in classic foreland fold and thrust belts [e.g., Horton et al., this irregular pattern of deformation to reflect a 2001; McQuarrie, 2002]. To date, the available data in the plateau-style of shortening related to a self-organized Altiplano do not provide a complete quantitative record of state of a weak crust in the central South American back the spatial and temporal pattern of strain accumulation arc with a fault network that fluctuated around the during plateau formation that also sufficiently resolves the critical state of mechanical failure. Tuning of this state earlier stages. may have occurred by changes in plate kinematics, [3] In the present paper we attempt to provide a first nearly complete record of magnitude and timing of shortening across the southern Altiplano plateau (compare Copyright 2005 by the American Geophysical Union. Figure 1) by the mapping of syntectonic sediments, by 0278-7407/05/2004TC001675$12.00 identifying structures, their relative ages and magnitudes of TC4020 1of19 TC4020 ELGER ET AL.: ALTIPLANO DEFORMATION TC4020 Figure 1 2of19 TC4020 ELGER ET AL.: ALTIPLANO DEFORMATION TC4020 deformation from the interpretation of industry reflection- shortening during the early to middle Miocene [e.g., Kley, seismic profiles, and by isotopic age dating of deformation, 1996; Allmendinger et al., 1997; Lamb and Hoke, 1997; including a reevaluation of published ages. Our analysis Mu¨ller et al., 2002]. The adjoining sub-Andean fold-thrust focuses on the southern Altiplano at 20–22°S. This segment belt accumulated most shortening since the Miocene [Baby is characterized by the highest density of geophysical and et al., 1997; Kley and Monaldi, 1998] and still shows active geological data in the central Andes, all provided by a growth at its eastward deformation front as evidenced from variety of recent projects [e.g., Wigger et al., 1994; Beck active seismicity and GPS data [Lamb, 2000; Hindle and et al., 1996; Swenson et al., 2000; Yuan et al., 2000; Kley, 2002; Bevis et al., 1999]. Haberland and Rietbrock, 2001; Brasse et al., 2002; Go¨tze [6] In contrast to the plateau flanks, the southern Alti- and Krause, 2002; ANCORP Working Group, 2003]. In plano evolution is only recorded by few structural and age addition, high-resolution industry reflection seismic sections data [e.g., Baby et al., 1997; Rochat et al., 1999; Welsink et covering the entire plateau were made available to this al., 1995] (see Figure 1a). Most of the area is covered by project by the Bolivian national oil industry (courtesy of Quaternary fluvial and lacustrine sediments, including sev- YPFB). eral large saltpans (e.g., Salar de Uyuni, Figure 1), and by posttectonic volcanic rocks from the active magmatic arc of 2. Geological Framework the Western Cordillera. Most of the exposed pre-Cenozoic basement is made up of mainly unmetamorphic and mildly [4] In spite of ongoing subduction for more than 200 Myr, folded Ordovician to Devonian clastic sediments while the large-scale deformation of the upper plate and formation of western margin of the plateau is built of deformed Late the Andes high plateau only commenced during early to Paleozoic and Mesozoic igneous and sedimentary rocks. In mid-Tertiary times [Sempere et al., 1990; Lamb and Hoke, southern Bolivia, Cretaceous sediments unconformably 1997; Allmendinger et al., 1997; Horton et al., 2001]. The overlie this basement. These sediments were partly depos- resulting central Andean mountain belt is built from a series ited in a shallow marine environment [Fiedler et al., 2003] of morphotectonic units (Figure 1d): the Longitudinal and mark the last marine ingression prior to plateau forma- Valley in the forearc with an average elevation of some tion. This postrift sequence is related to the Potosi basin 1000 m and thin sedimentary infill; the 4500–5500 m high [Fiedler et al., 2003], a branch of a major rift system in the Chilean Precordillera with Paleozoic and Mesozoic base- south (‘‘Salta Rift’’), the site of which largely matches the ment, overlain by a thin Neogene cover; the Western extent of the later Eastern Cordillera. Cordillera built by the Neogene to recent magmatic arc [7] The establishment of internal drainage early in the with peak elevations of up to 6000 m; the flat Altiplano Andean evolution with two cordilleras confining the proto- Plateau at some 3800 m average elevation forming a plateau area was responsible for storing sediment that 200-km-broad intramontane basin; the Eastern Cordillera reflects the entire deformation history (compare Vilque well reaching 5000–6000 m altitude, a doubly vergent thick- [Welsink et al., 1995]; see Figure 1a). In the southern skinned thrust system; the inter-Andean and sub-Andean Altiplano, the late Paleocene Santa Lucia formation and zones representing an east vergent, ‘‘classical’’ thin-skinned the Eocene/Oligocene Potoco formation [Jordan and foreland fold-thrust belt that accumulated most of the back Alonso, 1987; Sempere et al., 1997; Horton et al., 2001], arc shortening; and finally the down flexed, but otherwise a series of coarse to fine-grained clastics and red beds undeformed Chaco at some 500 m elevation, a 200-km- conformably overlie the Cretaceous/Paleocene deposits wide foreland basin with a wedge-shaped Neogene clastic (Figure 2). Rochat et al. [1999] and Jordan et al. [2001] fill of up to 5 km thickness. suggested that these sediments were partly deposited in [5] Recent work has shown that deformation and uplift of local extensional basins. This sequence is in turn overlain the western flank of the Plateau and Precordillera occurred by the Miocene continental clastic San Vicente Formation between the late Oligocene to late Miocene at very low, [Martı´nez et al., 1994; Mertmann et al., 2003].