The Bolivian Orocline

Abstract:

Available paleomagnetic data indicate that vertical-axis rotations have affected Cenozoic rock units in the central Andes as far inboard as the Eastern Cordillera of Bolivia. Structural development of the Altiplano during the past 10 m.y. involved crustal thickening and enhanced curvature of the central Andes. Oroclinal bending of 30¡ reasonably explains the pattern of vertical-axis rotations observed from units with ages 10 to 30 Ma. Our paleomagnetic data from Late Cretaceous through Paleocene strata of southeastern Peru and the Eastern Cordillera of Bolivia document rotations which are also counterclockwise on the north limb of the orocline and clockwise on the south limb. However, these rotations of units with ages 30 to 70 Ma are more complex and interesting than can be accounted for by simple oroclinal bending. In some cases, nearby areas have undergone differential vertical-axis rotations of up to 60¡. The larger rotations that have affected 30-70 Ma units in the central Andes may represent (1) local rotations during late Oligocene to present orogeny, (2) local rotations during Eocene orogeny, and/or (3) oroclinal bending during Eocene orogeny.

We recently submitted an NSF proposal to continue this paleomagnetic study of the Paleocene Santa Lucâa Formation which outcrops over wide areas of the eastern Altiplano, throughout the Eastern Cordillera, and also within the Subandean zone of Bolivia. In addition, we hope to study equivalent age strata in northwest Argentina and southeast Peru. The proposed research will provide detailed regional determinations of vertical-axis rotations which have affected the eastern portion of the central Andes during the Cenozoic. As well as providing important insights into the geodynamic development of the Altiplano, successful completion of this project would add significantly to knowledge of oroclinal structures, rotations associated with boundaries of crustal domains, and the structural development of fold and thrust belts.

1995 R. F. Butler, Richards, D. R., Sempere, T., and Marshall, L. G., Paleomagnetic determinations of vertical-axis tectonic rotations from Late Cretaceous and Paleocene strata of Bolivia, Geology, 23, 799-802.