EVOLUTION of the ALTIPLANO BASIN (EOCENE to MIOCENE). Dorothee Mertmann, Ekkehard Scheuber, Patricio Silva-González, Harald Ege, Klaus-J

TOMO 1 - Análisis de Cuencas

EVOLUTION OF THE ALTIPLANO BASIN (EOCENE TO MIOCENE). Dorothee Mertmann, Ekkehard Scheuber, Patricio Silva-González, Harald Ege, Klaus-J. Reutter FR Geologie, Institut für Geologische Wissenschaften, FU Berlin, Malteserstr. 74-100, D- 12249 Berlin [email protected]

The Altiplano is part of the Altiplano-Puna Plateau which, at an average height of 3.7 km (Isacks 1988), extends east of the Western Cordillera, the active volcanic front of the Central Andean Volcanic Zone. The formation of the Andean plateau was linked with the formation of the Altiplano Basin, which reflects major changes in the deformational pattern, magmatism, and the related sedimentary record. Thermochronological (apatite fission track, AFT) and sedimentological data from the central Andean high plateau are relevant to the evolution of the Andean active margin during the Tertiary. The intramontaneous Altiplano Basin covers an area of approximately 110,000 km2. Its main structural elements are the NNE trending Uyuni-Khenayani fault zone (UKFZ) in the center and the N-S to NW-SE trending San-Vicente fault system (SVFS) along the eastern borde. The UKFZ, which is marked morphologically by elongate, N-S to NNE-SSW trending ridges consisting mainly of Ordovician to Silurian sediments, shows dextral transpressional kinematics with thrusting toward E and dextral strike-slip displacements. W of the UKFZ a belt of mainly west-vergent folds and thrusts is developed. To the west these structures are covered by the late Miocene-Quaternary volcanics of the Western Cordillera and exposures of folded older strata. Along the SVFS the Ordovician strata of the Eastern Cordillera were thrust towards W onto Paleogene strata of the Altiplano in early Miocene times (Müller et al. 2002). The rocks underlying the Mesozoic to Cenozoic strata of the Altiplano belong to different basement blocks. In the west evidence for a shallow but hidden Precambrian to Permian basement comes from granitic and gneissic pebbels within the Oligocene-Miocene San Vicente Formation. Comparable rocks are exposed north of the Salar de Uyuni at Cerro Uyarani (Wörner et al. 2001). In the eastern part of the Altiplano the basement is composed of Ordovician to Devonian sedimentary

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rocks exposed along the UKFZ. Deposition started again in the early Cretaceous in a small rift basin near Tupiza in the Eastern Cordillera. During the Maastrichtian and Paleocene former areas of non- deposition were integrated into this basin, which then extended from the Eastern Cordillera over the Altiplano to the Salar de Atacama region in Chile (Sempere 1995, Sempere et al. 1997, Fiedler 2001). The limestones of the El Molino Formation represent a significant marker horizon indicating an elevation near sea level for the entire region by the beginning of the Paleocene. The Altiplano then occupied a back-arc position in relation to the magmatic arc in the Chilean Precordillera. During the Paleocene to Eocene generally fine-grained sediments of playa-mudflat origin were deposited in the basin centre along with coarser fluvial sediments at the basin margins (Potoco formation). Provenance analyses of sandstones have revealed a predominantly recycled orogenic source, probably derived from the Paleozoic basement of the Eastern Cordillera, and locally a magmatic arc source situated in the Chilean Precordillera (Silva-González 2004). Changes in thickness, as revealed by seismic modelling (Elger et al. 2005) across the UKFZ, may indicate local extensional tectonics. AFT data indicate that uplift occurred in the central Eastern Cordillera caused by initial thrusting increments between 40 and 36 Ma (Ege 2004). Around 38 Ma, volcanism ceased in the Chilean Precordillera for some 10 million years. Exhumational cooling started around 32 Ma along basement highs bordered by discrete normal faults in the western Altiplano and along thrusts at the margin of the Eastern Cordillera. These tectonic movements are reflected in the beginning of the coarse grained San-Vicente Formation, which indicates a major change in basin configuration. Around 28 Ma, subsequent to the beginning of exhumation, faulting and sedimentation, magmatism started again with basaltic-andesitic tuffs and later also silicic lavas, and shallow intrusions. The southern Altiplano became part of the Central Andean magmatic arc reaching from the Western Cordillera to the Eastern Cordillera. Widespread magmatic activity is also strongly reflected in the composition of sandstones deposited in smaller intra-arc basins. The San Vicente strata are overlain with an angular unconformity by Late Miocene volcanics and volcaniclastic sediments (11-8 Ma). The uppermost part of the San Vicente Formation growth strata are developed along thrusts indicating a deposition during shortening deformation. East of the San

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Cristobal Fault such growth strata were deposited in thrust-top basins to this fault between 13.7 ± 0.4 Ma and 11 ± 0.3 Ma. Around 18 Ma, shortening started in the Altiplano, when the Andean orogen was progressively thrust along the Interandean thrust onto the Brazilian shield. A further change occurred in the Andean system around 10 Ma. Before this time, shortening occurred in the plateau and in the eastern foreland, but after 10 Ma, flat-lying continental deposits indicate tectonic quiescence from the Altiplano to the Eastern Cordillera, whereas in the Chaco the present foreland was created. The termination of deformation within the plateau was accompanied by the underthrusting of cold Brazilian shield lithosphere, causing a narrowing of the magmatic arc and restricting it to the Western Cordillera. The tectonic evolution of the southern Altiplano was linked to very strong vertical crustal movements. The central Altiplano accumulated up to 8 km of sediments during Eocene-late Miozene times, at the same time in uplifted areas Precambrian metamorphics and Paleozoic sediments were eroded. This pattern of strongly contrasting uplift and subsidence is characteristic also for the recent tectonics of the Central Andes, e.g. the Salar de Atacama basin, the Salar de Uyuni basin and others. These basins contain a very thick Neogene fill and are surrounded by regions of erosion. The tectonic significance of these basins still is not clear. Unequivocal indications of a pull-apart mechanism have not yet been described. Possibly subsidence and uplift is controlled by strong density contrasts at depth (as can be inferred from the gravity residual field (Götze & Krause 2002) where areas of dense material are subsiding whereas areas of less dense material are uplifting.

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REFERENCES Ege, H. 2004, Exhumations- und Hebungsgeschichte der zentralen Anden in Südbolivien (21°S) durch Spaltspur- Thermochronologie an Apatit. Digital Dissertation, http://www.diss.fu-berlin.de/2004/64/index.html, 2004-03-24 Elger, K., Oncken, O &. Glodny, J 2005. Plateau-style accumulation of deformation: Southern Altiplano. Tectonics, Vol. 24, TC4020, doi:10.1029/2004TC001675. Fiedler, K. 2001. Die kretazisch-alttertiäre Entwicklung des südlichen Potosí-Beckens (Süd-Bolivien). Berliner geowissenschaftliche Abhandlungen, Vol. A215: 1-185. Götze, H.-J. & S. Krause 2002. The Central Andean Gravity High, a relic of an old subduction complex? Journal South American Earth Sciences, Vol. 14 (8): 799 - 811. Isacks, B.L. 1988. Uplift of the Central Andean plateau and bending of the Bolivian orocline. Journal Geophysical Research, Vol. 93: 3211-3231. Müller, J., Kley, J. & Jacobshagen, V. 2002. Structure and Cenozoic kinematics of the Eastern Cordillera, southern Bolivia (21°S). Tectonics, Vol. 21: 1-1 - 1-24. Sempere, T. 1995. Phanerozoic evolution of Bolivia and adjacent regions. In: Tankard, A. J., Suárez S., R. & Welsink, H. J. (eds.): Petroleum Basins of South America. AAPG Memoir, 62: 207-230. Sempere, T., R. F. Butler, D. R. Richards, L. G. Marshall, W. Sharp & C. C. Swisher III 1997. Stratigraphy and chronology of Upper Cretaceous-lower Paleogene strata in Bolivia and northwest Argentina. Geological Society America Bulletin, Vol. 109 (6): 709-727. Silva-González, P. 2004. Das Süd-Altiplano-Becken (Bolivien) im Tertiär: Sedimentäre Entwicklung und tektonische Implikationen. Digital Dissertation, http://www.diss.fu-berlin.de/2004/125/index.html, 2004-06- 02. Wörner, G., Lezaun, J., Beck, A., Heber, V., Lucassen, F., Zinngrebe, E., Rössling, R. & Wilke, H.G. 2000. Precambrian and early Paleozoic evolution of the Andean basement at Belén (northern Chile) and Cerro Uyarani (western Bolivian Altiplano). Journal South American Earth Sciences, Vol. 13: 717-737.