PALEOMAGNETISM OF UPPER MIOCENE IGNIMBRITES AT THE PUNA - AN ANALYSISOF VERTICAL-AXIS ROTATIONS IN THE CENTRAL ANDES

The origin of the rotations detected paleomagnetically in the Central Andes is controversial. Tectonic models proposed to explain it involve Late Cenozoic oroclinal bending (the Bolivian Orocline), and/or small -block rotations driven by oblique subduction. In this paper, we repor t paleomagnetic data from upper Miocene ignimbrites from the Puna, in the presumed southern limb of the orocline. These rocks showed high un blocking temperature and high coercivity magnetization. The resulting pole position (latitude 85.7 degrees S, longitude 80.5 degrees E, A(95 )=7.9 degrees, K=22, N=17) indicates that the study area has not been significantly affected by vertical-axis rotation since late Miocene on ward. This implies that the whole Puna has not undergone significant r egional rigid-body rotation during the same time span. A farther analy sis based on Neogene paleomagnetic data from 31 Andean localities dist ributed from 11 degrees S to 31 degrees S shows the presence of rotate d and unrotated areas. Systematic counterclockwise rotations in Peru a nd northern Bolivia are observed, whereas clockwise rotations are pres ent in southern Bolivia, northern Chile, and perhaps northwestern Arge ntina. No systematic rotations are found farther south. The overall ti me-spatial distributions of these Neogene paleomagnetic data suggest t hat if orogenic bending occurred, it must have taken place before the middle Miocene. Thus the oroclinal hypothesis can not explain the rota tions observed in middle Miocene rocks or younger ones, which also sho w orocline-like declination anomalies. We suggest that small-block rot ations driven by distributed shear may be a single process that can ac count for the whole rotations in the Central Andes. It is hypothesized that horizontal shear in the Andean crust could be controlled by anci ent structures.