Late Oligocene-early Miocene major tectonic crisis and related basins in Bolivia

Recent advances in biochrology, geochrology, magnetostratigraphy, structural geology, and basin analysis make untenable the traditional correlation of the first major Andean-age deformational episode in Bolivia with the middle Eocene "Incaic tectonic phase" defined in central Peru and intead demostrate that the episode took place in late Oligocene and early Miocene time. This major tectonic crisis resulted in contemporary development of the Subandean external foreland basin and Altiplano intermontana basin, which were separeted by the initiation of thrusting in the present Cordillera Oriental area. The deformation suggests that the Bolivian orocline began to develop at that time. It is likely that this tectonic upheaval is genetically linked to the maked increase in rate of plate convergence produced by the contemporaneus creakup of the Farallon plate. (Résumé d'auteur)

La napa de Calazaya : una prueba de acortamientos importantes controlados por elementos paleoestructurales en los Andes bolivianos

Una napa tectonica de desplazamiento importante se evidencia en el limite Altiplano-Cordillera Oriental de Bolivia (20° S). Su emplazamiento proviene del transporte hacia el Este de un amplio dominio aloctono en el Oligoceno Superior-Mioceno Inferior. El despegue basal del aloctono se ubica en una unidad estratigrafica cuya geometria en la cuenca controlo la de la deformacion. (Résumé d'auteur

Strike-slip faulting, thrusting and related basins in the Cenozoic evolution of the Southern branch of the Bolivian orocline

The evolution of the Cenozoic deformation of the Cordillera Oriental and of the contact zone with the Altiplano in southern Bolivia is well documented in the Tupiza, Nazareno and Estarca basins. The tectonic evolution started at about 29 Ma. The period between 29 and about 22-21 Ma is marked by developement of a pull-apart basin related to N-S trending left-lateral strike-slip faulting. During this period, initial deposition consisted of andesitic lavas and detrital sediments (breccias ; matrix-supported red conglomerats ; sands and silts...) of the Catati and Tupiza Formation, deposited in alluvial fan and flood plain environments. From 21 to 20 Ma, the tectonic setting evolved to N-S oriented thrusts, which had a dextral component. This event caused the Tupiza basin to evolve into a full-ramp basin, and triggered the developement of two piggy-back basins : Nazareno and Estarca. Both basins record detrital deposition (Nazareno Formation) in an alluvial fan environment. At 10-9 Ma, deformation in this area stopped allowing peneplanation during which time the San Juan de Oro erosional surface was formed and the fluvial conglomerates of the Oploca Formation were deposited. This segment of the Andes was then tectonically transported to the east, while uplift due to thrusting continued in the eastern Andean front and in the Subandean zone. (Résumé d'auteur

Long-term denudation rates from the Central Andes (Chile) estimated from a digital elevation model using the black top hat function and inverse distance weighting : implications for the neogene climate of the Atacama Desert

A methodology for determining long-term denudation rates from morphologic markers in a Digital Elevation Model (DEM) is checked by a comparative study of two drainage basins in the Precordillera of the Central Andes. In both cases the initial configuration of an incised pediment surface has been restored by using two different methods: the Black Top Hat (BTH) function and the Inverse Distance Weighting (IDW) interpolation. Where vertical incision and hillslope erosion are recorded, the 1DW appears to be the most adequate to reconstitute the pediment surfaces. Conversely, where only vertical incision is observed, the BTH describes more precisely the former pediment surfaces and it is easier to solve. By subtracting the DEM from the reconstructed marker we calculated an eroded volume, and estimated its uncertainty by considering Root Mean Square Error (RMSE) and DEM grid error. For the last similar to 10 Myr we obtained long-term denudation rates of 7.33 +/- 1.6 m/Myr in the San Andres drainage basin and 13.59 +/- 1.9 m/Myr in the El Salado drainage basin. These estimations are largely in agreement with other reported estimates of long-term denudation rates in the Atacama Desert. Comparison with long-term denudation rates reported in a wide range of climatic regimes suggests that our estimates cannot be explained by the current rainfall in the Precordillera. However they could be explained by a rainfall similar to that reported 40 km to the east in the Puna. This suggests that during the time span concerned the geomorphologic evolution of the study area, this evolution is dominated by an orographically controlled rainfall pattern. The preserved pediment surface and the small long term denudation rates determined in this study also indicate that the Precordillera was never reached by humid tropical air masses and precipitation as currently observed in the Altiplano during the summer months

El altiplano boliviano : una provincia de cuencas intramontanas de antepais ralacionadas con el acortamiento cortical en la region del oroclino boliviano

El altiplano se compone de fajas plegadas y corridas, zonas de transcurrencia y cuencas intramontanas de antepais. Todos estos elementos estuvieron relacionados entre si en el tiempo y en el espacio. Esta complejidad se debe a la posicion del altiplano en el corazon del oroclino boliviano. (Resumé d'auteur