K-AR DATES FROM THE ALTIPLANO AND CORDILLERA-ORIENTAL OF BOLIVIA - IMPLICATIONS FOR CENOZOIC STRATIGRAPHY AND TECTONICS

New K-Ar dates from tuffs, lavas and granites, collected in the Bolivian Altiplano and Cordillera Oriental, constrain the evolution of the Bolivian Andes. A phlogopite megacryst from a post-cleavage kimberlite dike near Independencia gave an age of ca, 98 Ma. In the central Altiplano, a widespread suite of basaltic to andesitic lavas, sills and rhyolitic tuffs has yielded dates of 25-22 Ma, coeval with the first major influx of conglomerates in the central Altiplano basin. Rapid early Miocene erosion of the Cordillera Oriental is suggested by biotites from the Quimsa Cruz Granite in the Cordillera Oriental, which vary in age between 32-22 Ma, and also by zircon fission track data. Gentle folding in the Cordillera Oriental near Potosi pre-dates the early Miocene Mondragon Formation which contains a ca. 19 Ma ignimbrite near its base. Further folding here occurred prior to the eruption of the hat-lying ca. 7 Ma ignimbrites of the Condor Nasa Meseta and ca. 12 Ma ignimbrites from the central Los Frailes Meseta. Intense folding in the central Altiplano also occurred between 9 Ma and 5 Ma, and younger tuffs are only slightly tilted. In the Cochabamba region, sedimentary infill of the Parotani Basin contains a ca. 20 Ma tuff horizon, and folded tuffs from the nearby Sacaba Basin have been dated at ca. 2.2 Ma. These ages suggest a protracted history for associated basin-margin ESE sinistral strike-slip and normal faults, However, regional folding pre-dates the San Juan del Oro Surface, which formed an extensive flat-lying peneplain, preserved at ca. 3000 m in the central and eastern Cordillera Oriental. The age of valley-fill tuffs suggest that dissection of this surface had commenced in the Cochabamba region by 6.5 Ma. However, near Sucre, flat-lying tuffs, dated between 3.5 and 1.4 Ma, mantle the surface and predate the deep Pleistocene dissection of the central Cordillera Oriental. Deformation has been concentrated in the Subandean foreland thrust belt since the Pliocene.