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Pre-Andean and Andean-Age Deformation in the eastern Cordillera of southern Bolivia

Manoutsoglou Emmanouil, Joachim P. Müller Chevron, Jonas Kley Geo, Jacobshagen, Volker

Πλήρης Εγγραφή

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Περίληψη

The Eastern Cordillera of southern Bolivia is situated between the Altiplano high plateau in the west and the thin-skinned Subandean thrust belt in the east. It is mainly built of Ordovician anchimetamorphic sedimentary rocks with a discontinuous, unconformable cover of Cretaceous to Neogene, mostly continental sediments. The eastern margin of the Eastern Cordillera exhibits E-verging basement-involved thrusts linked to the Subandean belt. Its central and western parts, discussed in this paper, represent a province of dominantly W-verging, basement-involved thrusts of Tertiary age superimposed on a deeply eroded pre-Cretaceous fold belt. Several well-preserved remnants of the pre-Cretaceous fold belt show kilometer-scale, W-verging folds with axial plane slaty cleavage. The main activity of Andean (post-Cretaceous) thrusts is bracketed between about 25 and 12 Ma. However, some Andean contractional deformation occurred earlier. The exposed major Andean thrusts dip steep to intermediately at the surface. Cross-section balancing suggests that they merge into a detachment at maximum depths of 9 to 17 km. Neogene shortening is concentrated in an imbricated thrust system at the border of the Eastern Cordillera with the Altiplano in the west, but is relatively moderate (about 10-15%) within the Eastern Cordillera itself. W-verging thrusts in the Eastern Cordillera predate the E-verging thrust belt which underlies the eastern flank of the Andes. Strain rates are comparatively low during these first stages of deformation, but increase markedly at the onset of thin-skinned thrusting farther east. Balancing of a regional cross-section of 150 km present length indicates that the western and central Eastern Cordillera has shortened by about 55-80 km since Cretaceous time. Overall Tertiary tectonic shortening between the deformation front and the magmatic arc is about 215-250 km. This is insufficient to build the entire crustal root of the Andes by thrust thickening alone.

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