PALEOMAGNETIC DETERMINATIONS OF VERTICAL-AXIS TECTONIC ROTATIONS FROMLATE CRETACEOUS AND PALEOCENE STRATA OF BOLIVIA

Structural development of the Altiplano during the past 10 m.y. involv ed crustal thickening and enhanced curvature of the central Andes. Oro clinal bending of 30 degrees explains reasonably the pattern of vertic al-axis rotations observed from units with ages 10 to 30 Ma.New paleom agnetic data from Late Cretaceous through Paleocene strata of southeas tern Peru and the Eastern Cordillera of Bolivia document rotations tha t are also counterclockwise on the north limb of the orocline and cloc kwise on the south limb. However, the rotations of units with ages 30 to 70 Ma are more variable; nearby areas have undergone differential v ertical-axis rotations of up to 60 degrees. These larger rotations tha t have affected 30-70 Ma units in the central Andes may represent (1) local rotations during late Oligocene to present orogeny, (2) local ro tations during Eocene orogeny, and/or (3) oroclinal bending during Eoc ene orogeny.