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Structures et cinématique de l'Altiplano nord-bolivien au sein des Andes centrales

Abstract : The central Andes are characterized by high reliefs (Eastern and Western Cordilleras) and an high plateau, the Altiplano, with an average altitude of 3900 m above sea level and a thick crust : 55-70 km. below the Altiplano. Field mapping, kinematic data and seismic reflexion analysis were used to study synorogenic sediments and to discriminate spatial, temporal and geometrical patterns of the deformation of the Altiplano. This intermontane basin is characterized by a very thick succession of Cenozoic continental sedimentary units(4-10 km thick) formed by five major depositional sequences. The first one (Eocene-Lower OIigocene) was deposited before the Altiplano and Eastern cordillera structuration. Sedimentological studies and restored cross sections show that the Altiplano coresponded to a large flexural basin filled up by a low detritismmainly coming from the uplified western andean area. Syntectonic Upper Oligocene-Lower Miocene deposits (Sequence 2) provide evidences for the structuration of the Coniri thrust and fold belt on the eastern border of the Altiplano. The central part of the Altiplano (Corque basin) evolved as a local basin when the western border collapsed along the San Andres and Sannta Lucia fault . Middle Miocene times (Sequence 3) wbow the end of the compressional activity and the formation of an extensive and diachronous erosional surface (Chayanta surface) Iinked with the slow continental uplift of the Eastern Cordillera. The Corque basin was collapsing along the Chuquicambi fault in response to the vertical motion and high sedimentation rates allowed the aggradation or the erosional surfaces Iocated on the basin margins. Upper Miocene times (Sequence 4) are characterized by the partial inversion of the Corque basin. Thin skinned tectonic inversion was perturbed by high angle faults (like: the Chuquichambi fault) which involved complexes short cut geometries. The Coniri thrust system was gently reactivated and the Chayanta surface was dissected following the uplift of the Eastern Cordillera. The Lower Pliocene deposits (Sequence 5) overlap the tectonic limits showing the end of thecompression. ln the eastern part of the basin they aggraded the contemporaneous San Juan del Oro surface which was evoluting by processes of erosional smoothing related to the low uplift of the Eastern Cordillera. Upper Pliocene (Sequence 6) shows for the first time a minor uplift oft he central part of the Altiplano and also of the western parts associated with a short reactivation of the tectonic structures Iinked to the Corque basin inversion. This evolution show that the topography of the central part of the Altiplano was achieved mainly by basin filling (verticaI aggradation) and not by tectonic uplift processes. The calculated rate of vertical aggradation indicates that the subsidence was Iesser than attended for a normal thick crust which requires the addition at depth of Iow density material. ln order to illustrate the neogen shortening distributon aIong the CentraI Andes and to estimate the contribution of the sbortening to crustal thickening , ablanced crustal cross section have been constructed and restored from geophysical, geological and kinematical data synthesis. Neogene shortening is accomodated mainly by the west vergent crustal thrusts of the Precordillera and Western Cordillera, but the total Neogene shortening(124km) is insufficient to produce the 70 km of crustal thickness evidenced by geophysical data below the Altiplano. Comparisons beetween the structural evolution of the Chilean rnargin and the Altiplano show that crustal underplating by materiaI tectonically eroded from the continental margin is the best explanation for the anomaIous thickening an can aIso be interpreted as a cause for the Andean orocline.
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Submitted on : Thursday, September 16, 2010 - 2:00:06 PM
Last modification on : Tuesday, March 17, 2020 - 2:03:01 AM
Document(s) archivé(s) le : Friday, December 17, 2010 - 2:52:33 AM

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  • HAL Id : tel-00518070, version 1

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Philippe Rochat. Structures et cinématique de l'Altiplano nord-bolivien au sein des Andes centrales. Géologie appliquée. Université Joseph-Fourier - Grenoble I, 2000. Français. ⟨tel-00518070⟩

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