Structure and Cenozoic kinematics of the Eastern Cordillera, southern ...

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unit of the central Andes. In southern Bolivia it is predominantly composed of Ordovician sedimentary rocks as thick as 10 km. Revision of the Ordovician.
TECTONICS, VOL. 21, NO. 5, 1037, doi:10.1029/2001TC001340, 2002

Structure and Cenozoic kinematics of the Eastern Cordillera, southern Bolivia (21°S) Joachim P. Mu¨ller Institut fu¨r Geowissenschaften, Freie Universita¨t Berlin, Berlin, Germany

Jonas Kley1 Geologisches Institut, Universita¨t Karlsruhe, Karlsruhe, Germany

Volker Jacobshagen Institut fu¨r Geowissenschaften, Freie Universita¨t Berlin, Berlin, Germany

Received 14 June 2001; revised 30 November 2001; accepted 6 December 2001; published 5 September 2002.

The Eastern Cordillera is a main morphotectonic unit of the central Andes. In southern Bolivia it is predominantly composed of Ordovician sedimentary rocks as thick as 10 km. Revision of the Ordovician stratigraphy and new structural fieldwork resulted in detailed balanced cross sections, improved estimates of crustal shortening, and a new kinematic model for the Eastern Cordillera. New estimates of total shortening in the Eastern Cordillera are up to 95 km (41 – 44%), higher than previous estimates and sufficient to thicken the Andean crust to its present state between the eastern foreland and the magmatic arc. After initial Tertiary uplift at 58 Ma, first thrusts developed in the middle Eocene to early Oligocene (?40–?30 Ma), exhuming the central part of the Eastern Cordillera. Tectonic shortening culminated in late Oligocene and early Miocene time (25–20 Ma) when west facing thrust systems developed in the central and western part of the Eastern Cordillera. Thrusting terminated in the western part by the end of the early Miocene, and middle Miocene tectonic activity was restricted to minor strike-slip faulting. In the central part, thrusting remained active until the late Miocene. By 10 Ma, shortening in the Eastern Cordillera essentially ceased. Kinematic fault analyses indicate clockwise rotation of the shortening direction, from NNE-SSW or NE-SW in Eocene-early Oligocene time over E-W to WNW-ESE or even NW-SE in late Oligocene to middle Miocene time. Nevertheless, WNW-ESE directed shortening and subvertical extension predominate. Sinistral oblique motion on many thrusts probably reflects plan view flexural slip in the clockwise rotating southern limb of the Bolivian orocline. It is proposed that space problems in the

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1 Now at Institut fu¨r Geowissenschaften, Universita¨t Jena, Jena, Germany.

Copyright 2002 by the American Geophysical Union. 0278-7407/02/2001TC001340

oroclinal hinge slowed the rotation there and caused decoupling of the still rotating south limb along NE INDEX TERMS: 8015 Structural trending transfer zones. Geology: Local crustal structure; 8102 Tectonophysics: Continental contractional orogenic belts; 8107 Tectonophysics: Continental neotectonics; 9360 Information Related to Geographic Region: South America; 9604 Information Related to Geologic Time: Cenozoic; KEYWORDS: kinematics, tectonics, central Andes, back arc, Eastern Cordillera, Bolivia, Cenozoic evolution. Citation: Mu¨ller, J. P., J. Kley, and V. Jacobshagen, Structure and Cenozoic kinematics of the Eastern Cordillera, southern Bolivia (21°S), Tectonics, 21(5), 1037, doi:10.1029/2001TC001340, 2002.

1. Introduction [2] The central Andes are the widest part of the 7500 km long Andean mountain chain. One of their main elements is the central Andean plateau (Figure 1), the second largest high plateau in the world besides Tibet. According to seismic and seismologic data [Wigger et al., 1994; Beck et al., 1996], this plateau is founded on a crust as thick as 75 km. The formation of this thick crustal root has been a long-standing problem in Andean geology. In the back arc region which comprises a large part of the Central Andean plateau (the Altiplano high plains and Eastern Cordillera) as well as the Subandean foothills, crustal shortening and tectonic stacking are probably the main agents in crustal thickening [e.g., Roeder, 1988; Allmendinger et al., 1990; Sheffels, 1990; Schmitz, 1994; Baby et al., 1997; Lamb and Hoke, 1997; Kley and Monaldi, 1998]. Nevertheless, published shortening estimates are insufficient to explain the entire crustal root of the back arc region and the origin of a large volume remains unexplained. One reason for this might be an underestimate of tectonic shortening. Since shortening contributes most to the total crustal volume of the central Andes, errors in the shortening estimates will also have the biggest impact on volume balance calculations. One of the best exposed and most easily accessible transects of the back arc area is in southern Bolivia near 21°S. Well-constrained shortening estimates are available for the eastern part of this transect [Baby et al., 1992; Dunn et al., 1995; Kley, 1996] but not for its western part across

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¨ LLER ET AL.: KINEMATICS OF THE EASTERN CORDILLERA, BOLIVIA MU

Figure 1. Tectonic setting of the study area. Modern azimuth and rate of plate convergence after DeMets et al. [1994] and Norabuena et al. [1998]. The region above 3000 m elevation (dark shading except in center, where morphotectonic units are shown) is the central Andean plateau [Isacks, 1988]. Morphotectonic units of the modern arc and back arc regions are only shown in Bolivia and adjacent eastern Chile (modified after Sempere et al. [1988]). the Eastern Cordillera. A thorough determination of Andeanage shortening within the Eastern Cordillera was delayed for several reasons: (1) The Ordovician succession which dominates the Eastern Cordillera in southern Bolivia is very thick and monotonous and was poorly differentiated stratigraphically; (2) these strata were deformed prior to the Andean orogeny (before and in Cretaceous time); and (3) outcrops of Cretaceous and Tertiary strata are scarce, which makes the recognition of Andean-age structures difficult. Because of these difficulties, the published geological maps misinterpret some structures and contain many erroneous correlations of Ordovician strata. Earlier attempts to quantify the amount of post-Cretaceous shortening in the Eastern Cordillera [Kley et al., 1997] were in part based on still vaguely known structure and stratigraphy and accordingly imprecise. [3] The Bolivian orocline is the large bend near 18°S where the entire central Andes change trend from NW in the

north to north in the south. The bend is today widely accepted to have formed during, not after, the Andean orogeny as a result of differential tectonic shortening along strike [Isacks, 1988; Sheffels, 1995; Kley, 1999]. This implies that the orocline did not form by rigid rotation of its limbs and that detailed kinematic histories of the Andean thrust belts are necessary to constrain its evolution. [4] In this paper we present the results of detailed fieldwork by the first author along the 21°S transect in the Eastern Cordillera and new structural interpretations. Many new graptolite finds (J. Maletz and J. P. Mu¨ller, unpublished data, 1997) have much improved the Ordovician stratigraphy. This permitted for the first time the definition of many structures that only involve Ordovician strata, thus providing a sound basis for cross section balancing in the Eastern Cordillera and for a reliable estimate of shortening right across the back arc area at 21°S. Discontinuities in the pre-

¨ LLER ET AL.: KINEMATICS OF THE EASTERN CORDILLERA, BOLIVIA MU

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Figure 2. Geologic structure of the Eastern Cordillera in southern Bolivia and location of the study area. Abbreviations are as follows: MPS, Mal Paso subsegment; TSS, Tupiza subsegment; CT, Chilcobija thrust; CYT, Camargo Yavi thrust; ET, Estarca thrust; NT, Nazareno thrust; SVT, San Vicente thrust; TT, Tocloca thrust; TUT, Tupiza thrust. Andean metamorphic grade of Ordovician rocks [Jacobshagen et al., 2002] were used to identify pre-Tertiary displacements. Fault slip analyses combined with fault timing estimated from the ages of associated Tertiary sediments permit deciphering in detail the kinematic evolution of the Eastern Cordillera in Tertiary times.

2002]. Interandean zone and Subandean Ranges are east facing, thin-skinned fold-thrust belts with two separate west dipping low-angle detachments [Kley, 1996; Schmitz and Kley, 1997]. In the east the Subandean Ranges border the undeformed Chaco foreland basin with up to 4 km of Tertiary sediments [e.g., Horton and DeCelles, 1997].

2. Morphotectonic Units

3. Stratigraphy

[5] East of the presently active volcanic arc, the southern Bolivian Andes comprise five major morphotectonic units: the Altiplano, the Eastern Cordillera, the Interandean zone, the Subandean Ranges, and the Chaco plains (Figure 1). The Altiplano is a 3800 – 4200 m high internally drained basin with the wide salt flat of Uyuni at the latitude of our transect. In the Altiplano as much as 10 km of Upper Cretaceous to Quaternary sediments and volcanics accumulated on top of an Ordovician and Silurian basement. Thrusting and folding affected the Altiplano in Cenozoic time [Baby et al., 1990]. The Eastern Cordillera is the most important mountain chain east of the magmatic arc with elevations peaking at 6429 m (Cerro Nevado Illampu; Figure 1). It consists predominantly of Cambrian to Silurian sediments covered in places by Mesozoic and Cenozoic sediments and volcanics. In terms of structure, it is a bivergent fold-thrust belt, which is thrust toward the west onto the Altiplano and toward the east onto the Interandean zone. In southern Bolivia, major faults subdivide the Eastern Cordillera into three segments of different Paleozoic stratigraphy and metamorphic signature: the Atocha, Mochara´, and Yunchara´ segments (Figure 2) [Erdtmann et al., 1995; Mu¨ller, 2000; Jacobshagen et al.,

[6] The oldest rocks of southern Bolivia, only exposed near the Argentinian border, are Precambrian to lower Cambrian slates and sandstones. They are covered with marked angular unconformity by thick upper Cambrian sandstones. The conformably overlying Ordovician rocks predominate in the whole Eastern Cordillera [Erdtmann et al., 1995] (Figure 3). On the transect discussed here they consist of a monotonous, up to 10 km thick succession of marine sandstones and shales (Figure 3). The subdivision of Ordovician formations and units is based on the investigations of Rivas et al. [1969], Servicio Geologico de Bolivia (GEOBOL) [1992], Maletz et al. [1995], Erdtmann et al. [1995], and Mu¨ller et al. [1996]. Tremadocian to Caradocian ages are proven by graptolites. Tremadocian and Arenigian rocks are exposed in the Yunchara´ segment and the eastern part of the Mochara´ segment. This succession has been subdivided into the following formations: Iscayachi (950 m), Cieneguillas (1000 m), Obispo (400 – 580 m), Agua y Toro (300 – 530 m), and Pircancha (1800 – 2200 m). Llanvirnian units (Jurcuma, 550– 1500 m, and Potrero, at least 400 m) are exposed in the western part of the Mochara´ segment. Caradocian and probably Ashgillian rocks (Marquina unit, at

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Figure 3. Ordovician stratigraphy in the Eastern Cordillera of southern Bolivia. least 1100 m; Angosto unit, 1250 m; Kollpani unit, 550 m; Tapial unit, at least 1500 m) build up the Atocha segment in the west. A fundamental change in depositional setting occurred in middle to late Ordovician time: Tremadocian to Llanvirnian strata were deposited on a westward prograding shelf, whereas the Caradocian strata were laid down in an eastward prograding foreland basin [Bahlburg, 1991; Mu¨ller et al., 1996]. [7] Upper Jurassic(?) to lower Cretaceous clastic sediments and volcanics (Angostura, Tarapaya, and Aroifilla formations) are only exposed in the central part of the Eastern Cordillera in southern Bolivia (Figure 4). These rocks were deposited with an angular unconformity of 20°– 40° on Llanvirnian strata in the north-south trending San

Lorenzo-Tupiza rift, which connected the Potosi basin (Puca rift) in the north with the Salta rift in the south [Sempere, 1994]. Upper Cretaceous to lower Paleocene clastics and carbonates (Chaunaca, El Molino, Santa Lucı´a, and Impora formations) [e.g., Sempere, 1994; Sempere et al., 1997] were deposited in a wide area of the back arc region. They are not preserved on top of the Lower Cretaceous rocks in Tupiza but overlie Ordovician strata in several other localities, mostly with a low-angle unconformity (10° – 20°). An intra-Paleocene low-angle unconformity locally separates the lower Paleocene Santa Lucı´a formation from the upper Paleocene Cayara formation in the west, at the boundary to the Altiplano [Marocco et al., 1987; Sempere et al., 1997]. By contrast, the equivalent contact is

¨ LLER ET AL.: KINEMATICS OF THE EASTERN CORDILLERA, BOLIVIA MU

Figure 4. Cretaceous and Tertiary stratigraphy in the Eastern Cordillera and Altiplano of southern Bolivia.

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conformable in the Camargo syncline to the east [Mertmann and Fiedler, 1997]. The Cayara formation is overlain by gypsum-bearing mudstones and thin-bedded sandstones of the Eocene Potoco formation. Alluvial conglomerates and conglomeratic sandstones of the Camargo formation overlie the Potoco formation in the Camargo syncline. Ignimbrite and andesite clasts only occur in the uppermost Camargo formation [Sempere et al., 1997], suggesting that most of the Camargo formation was deposited before arc volcanics began to erupt in its source area, i.e., before late Oligocene time. [8] Upper Oligocene to upper Miocene terrestrial rocks accumulated in the Altiplano and in contractional basins of the Eastern Cordillera (Figures 2 and 4). In the Altiplano, alluvial conglomerates to siltstones of the San Vicente formation derived from the nascent Eastern Cordillera were deposited [Welsink et al., 1995b]. Volcanics intercalated in its lower part gave late Oligocene ages (24.0 ± 0.9 Ma, 22.9 ± 0.9 Ma, K-Ar biotite (Bio)) [Kussmaul et al., 1975]. The San Vicente formation was covered with an angular unconformity by uppermost lower to middle Miocene ignimbrites and mudstones of the Chocaya formation (17.2 ± 0.3 Ma, K-Ar Bio [Grant et al., 1979];