Late Cenozoic Paleomagnetism and Chronology of Andean Basins of Bolivia: Evidence for Possible Oroclinal Bending

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Abstract
New paleomagnetic and radioisotopic data are reported from two late Teritary high-elevation, nonmarine basins in the eastern Cordillera of Boliva. (1) Quebrada Honda, located at 22.degree. S lat., consists of a 300 m thick section containing abundant Santacrucian or Friasian (middle Miocene) fossil mammals. This locality is constrained by mean 40K/40AR ages of 12.83 .+-. 0.11 Ma and 11.96 .+-. 0.11 Ma, and the local magnetostratigraphy is correlated to chrons C5AA through C5A on the Magnetic Polarity Time Scale (MPTS). The fossil mammals from Quebrada Honda have an extrapolated age of about 13.0 to 12.7 Ma. (2) Micana, located at 17.degree. S lat., consists of a 205 m thick section containing late miocene fossil mammals, including amegatheriid sloth and the tiny mesothere Microtypotherium cf. M. choquecotense. This locality is constrained by a fission-track age determination of 6.9 .+-. 1.1 Ma, and the local magnetostratigraphy is correlated to Chron 7 on the MPTS. The fossil mammals from this section have an extrapolated age of about 7.3 to 7.4 Ma. In conjunction with two other published data sets (Ocros, 13.degree. S, and Salla, 17.degree. S), these late Tertiary Andean localities indicate counterclockwise rotation at 13.degree. S, negligible rotation at 17.degree. S, and clockwise rotation at 22.degree. S. These data could represent local small-block rotations on a scale greater than about 6 km2. However, these data are also consistent with a model of late Neogene bending of the Bolivian Orocline.